HK1135798B - Nanostructures and methods for manufacturing the same - Google Patents

Description

Nanostructures and methods of making same

The application is a divisional application of Chinese patent application with the application date of 2003, 7 and 8, and the application number of 03821285.4, and the name of the invention is 'nanostructure and manufacturing method thereof'.

Cross Reference to Related Applications

This application claims priority from U.S. provisional application 60/393,835 filed on 8/7/2002 and U.S. provisional application 60/459,982 filed on 4/2003, and the entire contents of which are incorporated herein by reference.

Technical Field

The present invention relates generally to structures that are essentially one-dimensional in form, the width or diameter of which is nanometer in size, and which are commonly referred to as nanowhiskers, nanorods, nanowires, nanotubes, etc.; for ease of description, such a structure will be referred to as a "one-dimensional nanoelement". The present invention relates more particularly, but not exclusively, to nanowhiskers and methods of forming nanowhiskers.

Background

The basic process of forming whiskers on a substrate (substrate) using the so-called VLS (vapor-liquid-solid) mechanism is well known. Particles of catalytic material (e.g., typically gold) on a substrate are heated in a specific gas atmosphere to form a melt. A column is formed below the melt, which rises at the top of the column. Thus, Whiskers of the desired material are obtained in which a melt of solidified particles is located at the top-see "Growth of Whisskers by the vapor-Liquid-solid chemistry", Givardizov, Current Topics in Materials Science, Vol.1, pp.79-145, North Holland Publishing Company, 1978. The size of such whiskers is in the micrometer range.

International patent application WO 01/84238 discloses in figures 15 and 16 a method of forming nanowhiskers wherein nano-sized particles from an aerosol are deposited on a substrate, the particles being used as seeds (seeds) for the production of filaments or nanowhiskers. For ease of description, the term nanowhisker is intended to mean a one-dimensional nanoelement of nanometer dimension diameter, which element is fabricated using the VLS mechanism.

Generally, nanostructures are devices having at least two dimensions that are less than about 1 μm (i.e., nanometer-sized). Generally, layered structures or stacked materials having one or more layers with a thickness of less than 1 μm are not considered nanostructures, although nanostructures may be used in the preparation of such layers, as described below. Thus, the term nanostructure includes free-standing or isolated structures having two dimensions less than about 1 μm, which have different functions and utilities than larger structures and are typically processed by methods different from existing procedures for making slightly larger, i.e., micron-scale, structures. Thus, although the precise boundaries of the nanostructure magnitude are not defined by specific numerical size limits, the term has meant that magnitude as recognized by those skilled in the art. In many cases, the upper limit of the dimensional size characterizing the nanostructures is about 500 nm.

When the diameter of the nano-elements is smaller than a certain value, for example 50nm, quantum confinement occurs, i.e. electrons can only move along the length of the nano-elements; whereas for the diametric plane (radial plane) the electrons occupy quantum mechanical eigenstates.

The electrical and optical properties of semiconductor nanowhiskers are essentially determined by their crystalline structure, shape and size. In particular, small changes in whisker width may provoke large changes in energy state splitting due to quantum confinement effects. It is therefore important that the width of the whisker be freely selectable, and it is also important that the width be constant for an elongated whisker length. This is required if a combination of whisker technology with current semiconductor device technology is to be possible, and also the possibility of positioning the whiskers at selected locations on the substrate. The growth of GaAs whiskers has been studied in several experiments, the most important of which is reported by Hiruma et al. In metalorganic chemical vapor deposition-MOCVD-growth systems, they grown IH-V group nanowhiskers on III-V substrates-see k.hiruma, m.yazawa, k.haraguchi, k.ogawa, t.katsuyama, m.koguchi, and h.kakibayashi, j.appl.phys.74, 3162, 1993; hiruma, m.yazawa, t.katsuyama, k.ogawa, k.haraguchi, m.koguchi, and h.kakibayashi, j.appl.phys.77, 447, 1995; e.i. givargivov, j.crystal.growth 31, 20, 1975; dunn, j.f.wang, and c.m.lieber, appl.phys.lett.76, 1116, 2000; hiruma, h.murakoshi, m.yazawa, k.ogawa, s.fukuhara, m.shirai, and t.katsuyama, IEICE trans.electron.ew77c, 1420, 1994; hiruma et al, "Self-organized growth on GaAs/InAs heterojunction crystalline by organometallic vapor phase epitoxy", J.Crystal growth 163, (1996), 226-. Their approach consists in annealing a thin Au film to form seed particles. In this way they obtain a uniform whisker width distribution, the average size of which can be controlled by the thickness of the Au layer and the way this layer is converted into nanoparticles. The correlation between film thickness and whisker thickness is not direct because the whisker width also depends on the growth temperature, and there is even evidence of temperature dependence of equilibrium size of the Au particles.

If the whisker is to be used as an electronic device, there must be a well-defined electrical junction along the length of the whisker, and to achieve this, much work has been done, see, for example, Hiruma et al, "Growth and characterization of Nanometer-Scale GaAs, AlGaAs and GaAs/InAs Wires," IEICE traces. Electron., Vol.E77-C, No.9, September 1994, pp 1420-. However, significant improvements are needed.

Much work has also been done on Carbon Nanotubes (CNTs). Despite the advances, research efforts have been hampered by the lack of control over the conductivity type of CNTs and the inability to form one-dimensional heterostructures in a controlled manner. Randomly formed interfaces such as kinks between metallic and semiconducting portions of CNTs [ Yao et al, Nature, 1999, 402, 273], doped (pn) junctions in semiconducting CNTs [ deriycke et al, Nano Letters, 2001, 1, 453 ], and transitions between CNTs and semiconducting (Si and SiC) nanowhiskers [ Hu et al, Nature, 1999, 399, 48 ] have been identified and studied.

In one branch of development, since the late twentieth eighties, efforts have been made by Randall, Reed and co-workers in texas laboratories as pioneers to fabricate one-dimensional devices using a top-down method [ m.a. Reed et al, phys.rev.lett.60, 535(1988) ]. Their top-down approach currently still represents the most advanced process technology in the field of quantum devices, based on epitaxial growth of multiple layers defining two barriers and a central quantum well. An e-beam etch is then used to define the edge definition pattern to form the top contact along with the evaporation of the metal layer. The electron beam sensitive resist is then removed from the surface using a lift-off process and all material surrounding the thin pillars that are desired to be formed is removed by reactive ion etching. Finally, the device is contacted through the substrate and from the top thereof with a polyimide layer. In a study of devices fabricated using this bottom-up technique (bottom-up technique), columns of 100-200nm diameter were observed, however, the peak-to-valley current with electrical characteristics and optimal ratio of about 1.1: 1 was quite disappointing. Recently, another method for realizing low-dimensional resonant tunneling devices has been reported, which utilizes the formation of self-assembled quantum dots caused by stress (I.E. Itskevich et al, Phys. Rev.B 54, 16401 (1996); M.Narihiro, G.Yusa, Y.Nakamura, T.noda, H.Sakaki, appl.Phys.Lett.70, 105 (1996); M.Borgstrom et al, appl.Phys.Lett.78, 3232 (2001)).

Disclosure of Invention

The invention includes a method of making nanowhiskers, one-dimensional semiconductor nanocrystals, in which portions (segments) of the whisker are of different composition, for example, indium arsenide whiskers comprise indium phosphide portions, in which growth conditions are such that abrupt interfaces (discontinuous interfaces) and heterostructure barriers of a thickness of a few monolayers to a few hundred nanometers are formed, thus creating a one-dimensional topography along which electrons can move. In a preferred Chemical Beam Epitaxy (CBE) method, the rapid transition of the composition is controlled by feeding excited atoms into a eutectic melt (eutectic melt) of a seed particle (seed particle) and a substrate, the excited atoms being fed as a molecular beam into an ultra-high vacuum chamber. Rapid switching between the different components is achieved via a procedure in which growth is interrupted or at least attenuated to a negligible extent and supersaturation conditions for growth are re-established; at a minimum, the composition and supersaturation change faster than any appreciable growth. For abrupt changes in the whisker material, the stress and strain resulting from lattice mismatch are accommodated (acomod) by the radial outward expansion of the whisker or at least by lateral displacement of atoms in the crystal plane near the junction.

Furthermore, the present invention includes a technique for synthesizing nanowhiskers of selected dimensions grown epitaxially on a crystalline substrate. Gold aerosol particles of a selected size were used as catalysts, which allowed the surface coverage to vary completely independent of the whisker diameter. The whiskers are rod-shaped and have a uniform diameter between 10 and 50nm in relation to the size of the catalyst seeds. By exploiting the nano-manipulation of aerosol particles, individual nanowhiskers can be nucleated in a controlled manner with an accuracy of the order of nm at specific locations on a substrate. The method of the present invention enhances width control of whiskers by selecting nanoparticles. The nanoparticles may be an aerosol or liquid alloy on a substrate, and their preparation may begin with a gold rectangle formed on the substrate that forms a precise diameter sphere when melted. Other materials may be used as seed particles instead of gold, such as gallium.

In many applications it is desirable to have nanowhiskers with substantially constant diameters, while the shape and other characteristics of the whisker can be varied during its formation by selectively varying the diffusion constant (diffusion coefficient) of a group III material, such as Ga. This can be done as follows:

lowering the temperature of the process-this results in whiskers tapering towards their free ends;

increasing the pressure of the group V material;

increasing the pressure of group V and group III materials.

More specifically, the present invention provides a method of forming nanowhiskers comprising:

depositing a seed particle on a substrate and exposing the seed particle to a material under temperature and pressure controlled conditions to form a melt with the seed particle, whereby the seed particle melt ascends over a column, thereby forming a nanowhisker, the column of the nanowhisker having a diameter of nanometer size;

wherein during growth of the pillar, the composition of the material is selectively varied to discontinuously vary the composition of the material at regions of the pillar along its length while maintaining epitaxial growth, thereby forming a pillar having along its length at least first and second semiconductor portions of a material having a different band gap than the material of the second semiconductor portion.

Functional one-dimensional (1D) resonant tunneling diodes and other devices and structures have been obtained by combining previously designed portions of different semiconductor materials from the bottom up in group III/V nanowhiskers. Electronic or photonic devices comprising nanowhiskers have also been made into heterostructures using single crystal formation, in which the length portions of the nanowhiskers are composed of different materials, to produce well-defined junctions (well-defined junctions) in the whisker between the different bandgap materials, to produce devices with the desired function.

Thus, in general terms, the present invention provides a heterostructure electronic or photonic device comprising a nanowhisker having a column with a diameter of nanometric dimensions, the column having a plurality of length portions of different material composition along its length in order for the device to perform a desired function, with a predetermined radial boundary between adjacent portions extending along the column of the nanowhisker by a predetermined length to give a desired band gap variation at the boundary.

In a general aspect, the present invention provides an electronic or photonic device comprising a nanowhisker comprising a column having a diameter of nanometer dimension,

the column comprises along its length at least first and second length portions of different materials with an abrupt (discontinuous) epitaxial composition boundary between the first and second portions, wherein the lattice mismatch at the boundary is adapted by radial outward expansion of the nanowhisker at the boundary.

In another broad aspect, the invention provides an electronic or photonic device comprising a nanowhisker having a column with a diameter of nanometer dimension,

the pillar includes at least first and second length portions of different materials along its length with an abrupt epitaxial radial material boundary between the first and second portions, wherein a transition between the different material compositions of the first and second portions occurs over an axial distance of no more than 8 diametral lattice planes. Preferably, the transition between the components of the first and second portions occurs over an axial distance of no more than 6 crystal planes, more preferably no more than 5 crystal planes, more preferably no more than 4 crystal planes, more preferably no more than 3 crystal planes, more preferably no more than 2 crystal planes, and most preferably no more than 1 crystal plane.

In another aspect, the invention also provides an electronic or photonic device comprising a nanowhisker having a column with a diameter of nanometric dimensions, the column comprising along its length at least first and second length portions of different materials, the first portion having an a_1-xB_xA stoichiometric component in the form of a second part having A_1-yB_yA stoichiometric composition of form, where a and B are selected species and x and y are variables, wherein the boundary of the epitaxial composition between the first and second portions includes a predetermined gradual change from the variable x to the variable y over a predetermined number of radial crystal planes. In a similar embodiment, the components of the first and second portions of the nanowhiskers of the invention may be represented by formula A, respectively_1-xB_xC and A_1-yB_yC represents, wherein A and B represent elements of one group, e.g. group III of the periodic Table, and C represents elements of another group, e.g. of the periodic TableAn element of group V in (1). The variables x and y may take values between 0 and 1 and represent different numbers within the range. Thus, such nanowhiskers are made from compound semiconductors, the composition of which along its length can be varied, thereby incorporating a heterojunction. An example of such a compound semiconductor is Al_xGa_1-xAs. Nanowhiskers of the invention may be constructed having, for example, two length portions, a first portion having the composition Al_1-xGa_xAs, where the variable x is a given value between 0 and 1, the second fraction having the composition Al_1-yGa_yAs, where the variable y is another value than the value of x. Between the two portions is an interface in which the composition changes continuously from that of the first portion to that of the second portion, i.e., the value of the variable x changes continuously and typically monotonically to the value of the variable y. Thus, the interface constitutes a heterojunction. As will be explained in more detail below, this transformation can be made to occur on a predetermined number of diametral crystal planes by adjusting the conditions of whisker growth. Furthermore, the growth conditions may be periodically adjusted to form a plurality of such heterojunctions along the length of the nanowhisker.

The present invention controls the diameter of a nanowhisker such that it is substantially constant along the length of the nanowhisker, or has a defined variation, such as a controlled taper. This ensures a precise electrical parameter for the nanowhisker, the controlled taper being equivalent to the generation of a voltage gradient along the length of the nanowhisker. The diameter may be small enough so that the nanowhisker exhibits quantum confinement effects. Despite the precise control of the diameter, there is a slight variation in the diameter, which results from the processing method, in particular from the bulging of the nanowhiskers radially outward at the composition boundaries in order to adjust the lattice mismatch in the epitaxial structure. Also, due to the difference in lattice size, the diameter of one portion may be slightly different from the diameter of another portion of a different material.

In accordance with the present invention, the nanowhiskers preferably have a diameter of no greater than about 500nm, more preferably no greater than about 100nm, and more preferably no greater than about 50 nm. Moreover, the diameter of the nanowhiskers of the invention may preferably be in the range of not greater than about 20nm, not greater than about 10nm, or not greater than about 5 nm.

The precise formation of nanowhiskers enables the fabrication of devices, particularly resonant tunneling diodes, that rely on quantum confinement effects. Therefore, RTDs have been developed in which the emitter, collector and central quantum dots are made of InAs and the barrier material is made of InP. Ideal resonant tunneling behavior with peak-to-valley ratios as high as 50: 1 is observed at low temperatures.

In a particular aspect, the present invention provides a resonant tunneling diode comprising a nanowhisker having a column with a diameter of nanometer dimension, to exhibit quantum confinement effects,

along its length, the pillar comprises first and second semiconductor length portions forming an emitter and a collector, respectively; third and fourth length portions between the first and second semiconductor portions, their materials having different band gaps than the materials of the first and second semiconductor portions; a fifth central length portion of semiconductor material having a different band gap than the material of the third and fourth portions, between the third and fourth portions and forming a quantum well.

A problem associated with electronic or photonic devices made from nanowhiskers is how to make efficient electrical contacts to the nanowhiskers.

One approach is to remove the nanowhisker from its substrate by a mechanical scraping process and place the nanowhisker on another substrate with the side edges along the length of the nanowhisker on the substrate. Metal pads (metallic bond pads) are then formed on the ends of the nanowhiskers, or the nanowhiskers may be manipulated so as to be positioned on pre-fabricated contact pads.

Alternatively, in a process that may be more suitable for mass production, the nanowhiskers may be left on the substrate with their bottom ends formed on the electrical contacts. Once formed, the nanowhiskers are encapsulated in a resin or glassy substance, and contact pads are then formed on the encapsulation surface in contact with the free ends of the nanowhiskers. To assist in this, the melt of catalytic particles towards the end where the nanowhiskers are formed may have an additional conductive substance injected therein to improve electrical contact with the pads.

More specific devices are set forth in the appended claims and are explained below. In particular, they include heterogeneous bipolar transistors, light emitting diodes and photodetectors.

Since the light emitting diode can be constructed to have an emission wavelength arbitrarily selected from a continuous range covering wavelengths in the ultraviolet, visible and infrared regions, it is very suitable for use in the present invention.

The present invention provides a light emitting diode comprising a nanowhisker having a column with a diameter of nanometer dimension, thereby exhibiting quantum confinement effects,

the pillar comprises, in sequence along its length, first, second and third semiconductor length sections comprising an emitter, a quantum well active section and a collector, respectively, said second section having a different band gap than the first and third sections and forming an active region of the light emitting diode.

One particular application of light emitting diodes is the emission of single photons. This can be used in a variety of applications, particularly in quantum cryptography, where, according to quantum theory, unauthorized interception of a photon stream will inevitably result in the destruction or alteration of that photon, resulting in the destruction of the transmitted signal-see p.michler, a.imaoglu, m.d.mason, p.j.carson, g.f.strouse, s.k.burato, Nature 406, 968 (2000); santori, m.pelton, g.solomon, y.dale, y.yamamoto, phys.rev.lett.86, 1502 (2001).

The invention provides a single photon light source comprising a one dimensional nano-device having disposed along its length a volume of optically active material small enough to form a quantum well with a tunneling barrier formed on either side of the quantum well so that, in use, the quantum well can emit one photon at a time.

A light source according to another form of the present invention is designed for terahertz radiation beyond the far infrared. People have done a lot of work on superlattices, pioneering the Capasso and its partners from the American lucent technologies. Their 'quantum cascade' lasers take advantage of photon emission in the sub-bands in InGaAs/InAlAs/InP heterostructures and achieve room temperature (pulsed mode) operation at wavelengths up to 17 microns. See, for example, IEEE Spectrum, July 2002, pages 23, 24, "Using Unusablefrequency", and F.Capaso, C.Gmachl, D.L.Sivco, and A.Y.Cho, "Quantum cassette lasers", Physics Today, May 2000, pp.34-39.

The invention provides a terahertz radiation source comprising a nanowhisker comprising a column having a diameter of nanometer dimensions, the column comprising a plurality of layers of a first band gap semiconductor interleaved with a plurality of layers of a second band gap material to form a superlattice having dimensions such that electrons can move in a wave vector to emit terahertz radiation.

In devices, structures and processes according to the present invention, an array of a plurality of nanowhiskers extending from a substrate substantially parallel to each other may be formed. There are various methods for forming such arrays, for example: disposing an array of aerosol particles on a substrate to provide catalytic seed particles; the process of growing nanowhiskers is carried out by depositing particles on a substrate using a colloidal solution, or by forming an array of regions of predetermined shape (rectangular or otherwise) and thickness on the substrate by a nanoimprint etching (NIL) process (or by any other etching process such as electron beam etching, ultraviolet light etching or X-ray etching), which array forms spheres of the required volume when heated.

Such arrays can be used as photonic crystals, solar cells consisting of a large number of photodetectors, Field Emission Displays (FEDs), and converters for converting infrared images into visible light images, all as will be explained below. A further application is in polarizing filters.

In the process of the present invention, an array of a large number of nanowhiskers may be used to produce a layer of epitaxial material on a wafer substrate of a relatively inexpensive substance, such as silicon. A long standing problem in the prior art is the formation of single crystal wafers of expensive III-V materials from which chips can be formed. A great deal of research has been done on forming single crystal layers on silicon wafer substrates-see, for example, WO 02/01648. However, further improvements are needed.

According to the invention, a silicon substrate or a substrate of another substance is provided on which a covering material (mask material) is grown, which covering material has a counteracting effect on epitaxial growth, for example, such as SiO₂Or Si₃N₄Etc. dielectric material. For example, an array of nano-sized holes is formed in the cover material by a NIL process, and a catalytic seed-forming material is disposed in the holes. Alternatively, an array of seed-forming material regions may be deposited on a substrate, followed by a layer of capping material deposited over the substrate and seed particle regions. Heating melts the seed particle region, thereby producing a seed particle, which then initiates growth of nanowhiskers of desired III-V and other materials. After nanowhisker growth, growth of the desired material continues using the whisker as a nucleation center until a single continuous layer of the material is formed. The material is single crystal epitaxial. Preferably, the seed particles melted at the end of the nanowhisker are removed at a convenient time to avoid contamination of the epitaxial layer.

In one variation, the seed particle melt is used as a nucleation site to initiate bulk growth of the epitaxial layer prior to nanowhisker formation, while growth beneath the seed particle remains in the liquid phase.

In another variation, a micro-V-shaped recess is formed in the upper surface of the silicon substrate, e.g., a <111> etch is performed on a <100> substrate. Seed particle forming regions are formed on the surfaces of the V-shaped grooves, and therefore, nanowhiskers are grown at an angle to the substrate and intersect each other at the grooves. This allows the epitaxial layer to grow more efficiently from the nano-nucleation centers. Furthermore, grain boundaries between domains with different growth phases, which have been a problem with the prior art processes, are avoided.

Accordingly, in another aspect, the present invention also provides a method for forming an epitaxial layer of a desired material on a substrate of a different material, the method comprising: forming an arrangement of a plurality of regions of seed particle material on a substrate; forming a covering material layer around the seed particle region; growing nanowhiskers of said desired material from the seed particle region; and continuing to grow the desired material using the nanowhisker as a growth point, thereby forming an epitaxial layer of the desired material extending over the substrate.

In accordance with another aspect of the invention, a process for forming nanowhiskers of group III-V materials extending along a <100> direction has also been developed, as opposed to nanowhiskers generally along a <111> direction. This has important applications, especially for nitride materials, which tend to grow in the <111> direction but have many stacking faults due to the alternation between the sphalerite and wurtzite structures.

The invention provides a method of forming nanowhiskers, comprising: providing a substrate; forming an arrangement of a plurality of seed particles on an upper surface thereof; growing nanowhiskers from said seed particles, the nanowhiskers initially extending from the substrate along a <111> direction; and forming a short barrier material portion in the whisker to change the growth direction of the whisker to a <100> direction.

In another aspect, the invention also provides a method of forming nanowhiskers comprising: providing a substrate; forming an arrangement of a plurality of seed particles on an upper surface thereof; growing nanowhiskers from said seed particles, the nanowhiskers initially extending from the substrate along a <111> direction; and changing the growth conditions of the nanowhiskers to change their growth direction to a <100> direction.

The invention also relates to one-dimensional nanoelements incorporated in MEMS-micro-mechanical devices.

In one aspect, a substrate (e.g., a silicon substrate) has a matrix of electrical contact regions formed on one surface thereof. On each contact region, one or more nanowhiskers are formed, for example from gold catalyst particles, upstanding from the substrate surface. Thus, each whisker or group of whiskers can be addressed independently by an electrical signal. Such structures may contact nerve endings or nerve endings in the retina of the eye and may activate the electrodes to provide a prosthetic or artificial function to the nerve. Thus, the structure may overcome certain blindness problems, for example when applied in the retina of the eye.

In another aspect, nanowhiskers are provided that can function as nerve electrodes or in other applications, wherein the whiskers are formed from silicon or a metal that can be oxidized, and the whiskers are oxidized to form an oxide layer along their length. A molten mass of particles comprising gold or other non-oxidizable material at the end of the whisker remains unoxidized and can be used to form an electrical contact. Such an arrangement may provide more precise electrical properties than a nanowhisker having exposed conductive material along its length, and such a nanowhisker may be used as a neural electrode or as a device in which the capacitance of the nanowhisker plays an important role. In addition, other materials may be used for the outer layer, such as a higher band gap sheath, which may be gallium phosphide, for example, when the whisker is made of gallium arsenide.

One important application of nanostructures is in micromechanical cantilever beams (cantilever beams), in which a beam fixed at one end extends into the air and may be subjected to external forces (e.g., electrical, gravitational, foreign object forces, or chemical forces) to bend the cantilever. The bending can be detected, for example, by a change in capacitance of the structure.

In another aspect, the invention provides one or more nanowhiskers which may or may not be oxidised along their length in accordance with the above aspects of the invention, to provide a cantilever or an array of cantilevers formed as a row or parallel beams. Such an arrangement may provide the same order of or more sensitivity than previous arrangements that used an etching process to form the beam.

One application of such cantilevers is in the formation of whiskers from materials with coatings that are sensitive to certain organic or biological molecules and therefore undergo certain chemical reactions when the molecules contact the cantilever. This creates some stress on the cantilevered beam and causes the beam to bend, which can be detected by optical or electrical monitoring.

In another particular aspect, the nanowhiskers are formed on a substrate in a hole projecting up to a layer of substantially insulating material. The upper surface of the insulating layer has a conductive material formed thereon. Starting from the substrate, the conductive material is substantially level with the tips of the nanowhiskers having a melt of conductive seed particles thereon. By appropriate activation of the conductive material, the whisker can be caused to mechanically vibrate within the hole at a certain eigenfrequency (e.g., in the gigahertz range). During a cycle of oscillation, a single electron is transferred from one side of the conductive material to the other via the seed particle melt. This results in a current standard generator in which the current I through the conductive material is equal to the product of the vibration frequency f and the charge e of one electron: i ═ f · e.

If the whisker is made sensitive to attract certain species of molecules, the molecules deposited on the whisker will change the inert character of the whisker and thus its eigenfrequency of vibration. This can therefore be detected by electrical activation of the conductive material. This technique can be used to calculate the weight of a molecule with great accuracy.

The present invention also provides an optoelectronic device, comprising: a substrate provided with a contact region; at least one nanowhisker extending from said contact region, said nanowhisker forming at least part of a p-n junction for light absorption; a transparent electrode extending over and in electrical contact with the free end of each whisker.

The invention also proposes a solar cell comprising a photovoltaic device as described above, wherein said photovoltaic device is adapted to convert sunlight into electrical power.

The invention also proposes a photodetector comprising an optoelectronic device as described above, wherein said optoelectronic device is adapted to detect radiation.

Drawings

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings. In the drawings:

figure 1 is a schematic representation of a fabrication technique for forming nanowhiskers according to the invention: (a) depositing Au aerosol particles with a selected size on the GaAs substrate; (b) an AFM operation for locating the particles of the whiskers; (c) alloying to form a eutectic melt or eutectic between Au and Ga from the surface; (d) and growing the GaAs crystal whisker.

FIG. 2: (a) TEM micrograph of GaAs nanowhiskers grown from 10nm Au aerosol particles; (b) SEM micrograph of GaAs <111> B substrate with GaAs whiskers grown from 40nm Au aerosol particles; (c) 400kV high resolution electron microscopy images of GaAs whiskers grown from Au clusters, with the inset showing the enlarged whisker portion.

FIG. 3 is a schematic diagram of an apparatus for carrying out the method of the present invention.

Figure 4 is a composition distribution of InAs nanowhiskers obtained using cross-space analysis of lattice spacing according to an embodiment of the invention, the whiskers comprising a number of InP heterostructures: (a) high resolution TEM images of whiskers 40nm in diameter; (b) power spectrum of the image in plot (a); (c) an inverse fourier transform of the information closest to the InP portion reflected by split200, with InP (bright portion) in three bands with widths of about 25, 8 and 1.5nm, respectively; (d) overlapping images of the same cover were used on the 200 reflective InP and InAs sections, respectively.

Figure 5 is an analysis of InP heterostructures in InAs nanowhiskers: (a) TEM images of InP barriers (100, 25, 8 and 1.5nm) in InAs nanowhiskers with a diameter of 40 nm; (b) enlargement of the 8nm barrier region, showing the crystalline integrity and interface abrupt changes at the monolayer level; (c) simulating the band structure pattern of an InAs/InP heterostructure, which includes (to the left) an ideal form of ohmic contact to InAs; (d) the ohmic I-V dependence of the homogeneous InAs whiskers is in contrast to the strongly nonlinear I-V behavior seen in InAs whiskers containing an 80nm InP barrier; (e) an Arrhenius (Arrhenius) plot showing the measurement of thermionic excitation across an InP barrier (bias 10mV), yielding a barrier height of 0.57 eV.

Fig. 6 is an evaluation of the transport mechanism for single barriers of different thickness for use in the resonant tunneling diode of the present invention: (A) SEM image of whiskers on growth substrate (scale bar represents 1 μm); (B) an InAs/InP nanowhisker (scale bar represents 2 μm) contacted by two alloyed ohmic contacts; (C) TEM images of InAs whiskers with 8nmInP segments perpendicular to the whisker long axis; (D) current-voltage characteristics for three different barrier cases.

Fig. 7 is a high resolution TEM image: (A) TEM image of InAs whiskers grown along the <111> direction and having two InP barriers to form the first embodiment of the invention (scale bar represents 8 nm); (B) the one-dimensional composite profile of the box area in the graph (a). The width of the barrier is about 5.5nm (16 lattice spacings) and the interface sharpness is of the order of 1-3 lattice spacings, as judged by the change in image contrast.

FIG. 8 illustrates the formation of a Resonant Tunneling Diode (RTD) according to one embodiment of the present invention:

(A) TEM image of the tip of the whisker with clearly visible double barrier, in this case the barrier thickness is about 5nm (scale bar represents 30 nm);

(B) the band diagram principle of the device studied with the characteristic electron states in the emitter region (left) indicated;

(C) the current-voltage data for the same device shown in figures (a) and (B), which characteristically exhibit sharp peaks, reflects resonant tunneling into the ground state E1z with a voltage width of about 5 mV. This width can be converted to a conversion energy width of about 2meV, corresponding to the width of the shaded portion band in the emitter for electron tunneling. The characteristics of the device are shown in the inset, which provides an enlarged view of the formants for increasing and decreasing voltage.

Figure 9 is a schematic diagram of a preferred embodiment of a resonant tunneling diode according to the present invention;

figure 10 is a schematic diagram of another embodiment of the present invention including a wide bandgap insulation portion;

figure 11 is a schematic diagram of another embodiment of the present invention including a hetero-bipolar transistor (HBT);

figure 12 is a bandgap diagram of an HBT associated with the HBT structure;

FIG. 13 is a graph showing the change in band gap with the change in composition of a ternary compound;

FIGS. 14A and 14B show graphs of bandgap versus lattice size for various semiconductor compounds;

FIG. 15 is a schematic diagram of an embodiment of the present invention including a light emitting diode and a laser;

FIG. 16 is a schematic representation of another embodiment of the present invention including the application of laser light to detect individual molecules of a desired element;

FIG. 17 is a schematic diagram of another embodiment of the present invention including the use of a laser array to pattern photoresist in an NIL process;

FIG. 18A is a schematic diagram of another embodiment of the present invention including a photodetector, and FIGS. 18B and 18C are variations thereof;

fig. 19A is a schematic diagram of another embodiment of the present invention including a solar cell, and fig. 19B is a modification thereof;

FIG. 20 is a schematic representation of another embodiment of the present invention including a radiation source of terahertz radiation;

FIGS. 21A-C are schematic diagrams illustrating an embodiment of the invention including a photonic crystal, FIG. 21D being a variation for forming a 3-D photonic crystal;

FIGS. 22A-G are schematic illustrations of another embodiment of the present invention for forming a material layer along a substrate by epitaxial growth, wherein the crystal lattices are mismatched;

FIGS. 23A-C are schematic diagrams illustrating another embodiment of the present invention for forming a material layer along a substrate by epitaxial growth, wherein the crystal lattices do not match each other;

FIGS. 24A-B are schematic diagrams illustrating another embodiment of the invention for forming whiskers other than the usual <111> direction but extending along the <100> direction;

figures 25A-B are schematic diagrams of another embodiment of the invention comprising a field emission display (fed) in which the individual devices of the display are nanowhiskers and addresses are separated;

FIG. 26 is a schematic diagram of another embodiment of the present invention including a layout for converting an image in the infrared region up to the visible region;

FIG. 27 is a schematic view of another embodiment of the present invention including an antenna for infrared radiation;

FIG. 28 is a schematic diagram of yet another layout including ferromagnetic whiskers for spintronics applications;

FIG. 29 is a schematic diagram of another embodiment of the present invention including a selectively addressable electrode array for implantation into a nerve;

figure 30 is a schematic view of another embodiment of the invention including whiskers having an outer surface oxidized along their length;

figure 31 is a schematic diagram of another embodiment comprising an array of whiskers upstanding from a substrate and forming a cantilever arrangement;

figure 32 is a schematic diagram of another embodiment of the present invention including whiskers arranged to oscillate and provide accurate measurements of weight and frequency; and

FIG. 33 is a schematic view of another embodiment of the present invention including a tip of a scanning tunneling microscope.

Detailed Description

The method of making nanowhiskers according to the invention is described below. This method is applicable to the fabrication of resonant tunneling diodes and other electronic and/or photonic devices as will be described below.

Whiskers are highly anisotropic structures that are spatially catalyzed by molten metal droplets that are often inadvertently introduced to the crystal surface as impurities. Gold is often chosen as a catalyst or seed particle because it can form eutectic alloys with semiconductor materials or components such as Si, Ga, and In. These eutectic alloys have melting points below the typical growth temperatures of Si and III-V materials. The molten metal droplets are used as a micro liquid phase epitaxy system, where the motivators enter them in the form of a vapor or in this case a molecular beam in vacuum. This growth is commonly referred to as gas-liquid-solid growth. The electrical and optical properties of semiconductor nanowhiskers are essentially determined by their crystalline structure, shape and size. In particular, small changes in whisker width can provoke large changes in energy state separation caused by quantum confinement effects. It is therefore important that the width of the whisker be freely selectable, and it is also important that the width be maintained constant for the length of the elongated whisker. Together with the possibility of locating whiskers at selected locations on a substrate, it is necessary to combine whisker technology with current semiconductor device technology.

In accordance with the present invention, techniques have been developed for synthesizing nanowhiskers of selected dimensions grown epitaxially on a crystalline substrate. A chemical beam epitaxy apparatus employed in the technique described below is schematically shown in fig. 3.

Chemical Beam Epitaxy (CBE) combines beam epitaxy techniques such as Molecular Beam Epitaxy (MBE) with the use of chemical sources similar to metalorganic chemical vapor deposition (MOCVD). In MOCVD or related laser coating techniques, the pressure in the reaction chamber is typically greater than 10mbar and the gaseous reactants are viscous, meaning that they have a relatively high flow resistance. The chemical species reach the substrate surface by diffusion. CBE reduces pressure to less than 10^-4mbar, so the mean free path of the diffuser becomes longer than the distance between the inlet of the source and the substrate. The transmission becomes collision-free and occurs in the form of a molecular beam. The exclusion of gas diffusion in CBE systems indicates a fast reaction in the flow at the substrate surface, which makes it possible to grow an atomically abrupt interface.

As shown in fig. 3, the CBE apparatus consists of a UHV (ultra high vacuum) growth chamber 1001 in which a sample 1021 is mounted on a metal sample holder 1041 connected to a heater 1061. Around the growth chamber there is a ring 1081, called cryosheath (cryosheath), filled with liquid nitrogen. The low temperature jacket draws away species that do not impinge on or desorb from the substrate surface. This prevents contamination of the growing surface layer and reduces memory effects. A vacuum pump 1101 is also installed.

The sources of CBE 1121 are in the liquid phase and they are contained in bottles that are under overpressure relative to the growth chamber. The sources are typically: TMGa, TEGa, TMIn, TBAs, TBP. The bottle is held in a thermostatic bath and the partial pressure of the vapour above the liquid is adjusted by controlling the temperature of the liquid source. The vapor then enters the growth chamber through the manifold assembly 1141 and is delivered to the source injector 1161 at the end of the manifold just in front of the growth chamber. The source injector is used to inject a gas source into the growth chamber 1001 and to generate a molecular beam of stable and uniform intensity. Group III materials from organometallic compounds TMIn (trimethylindium), TMGa (trimethylgallium) or TEGa (triethylgallium) were injected through low temperature injectors to avoid agglomeration of the growth species. They will decompose at the substrate surface. The group V materials are provided by organometallic compounds TBAs (tert-butylarsenic) or TBP (tert-butylphosphoric). Unlike the decomposition of group III materials, group V materials will decompose in injector 1161 at high temperatures prior to injection into growth chamber 1001. These injectors 1161 are referred to as a cracking tank and their temperature is maintained at around 900 ℃. The source beam impinges directly on the heated substrate surface. The molecule acquires sufficient thermal energy from the substrate surface to completely dissociate its three alkyl groups leaving the essential group III atoms on the surface, or the molecule desorbs in an undissociated or partially dissociated form. Which of these two processes dominates depends on the temperature of the substrate and the rate at which molecules reach the substrate surface. The growth rate is limited at higher temperatures by the feed and at lower temperatures by the desorption of alkyl groups from the plug sites.

The chemical beam epitaxy process results in the formation of heterojunctions within the nanowhisker that are abrupt, i.e., there is a rapid transition from one material to another over several atomic layers.

For ease of description, an "atomic scale abrupt heterojunction" means a transition from one material to another material over two or fewer atomic monolayers, wherein one material on one side of the two monolayers is at least 90% pure and the other material on the other side of the two monolayers is at least 90% pure. Such "atomic scale abrupt heterojunctions" are sufficiently abrupt or discontinuous so that in an electrical device having a series of heterojunctions and associated quantum wells, a heterojunction defining a quantum well can be formed.

For ease of description, a "sharp heterojunction" means a transition from one material to another material over five or fewer atomic monolayers, where one material on one side of the five monolayers is at least 90% pure and the other material on the other side of the five monolayers is at least 90% pure. Such "sharp heterojunctions" are sufficiently sharp to allow the fabrication of electronic devices having one or a series of heterojunctions in a nano-element, where the heterojunctions need to be accurately defined. Such "sharp heterojunctions" are also sufficiently sharp for many devices that rely on quantum effects.

Illustratively, in the compound AB used in the nanowhiskers of the invention, wherein a represents one or more selected elements of the first group and B represents one or more selected elements of the second group, the total ratio of the selected elements of the first group and the selected elements of the second group is predetermined so as to constitute a semiconductor compound capable of providing the desired characteristics. The compound AB is considered to have a purity of 90% when the total proportion of the selected elements of each group is at least 90% of its predetermined proportion.

Example 1

Figures 1 and 3 show whiskers of predetermined size grown from several III-V materials, specifically GaAs whiskers between 10 and 50nm in width. In contrast to the earlier reported tendency of epitaxially grown nanowhiskers to be tapered from bottom to top, these GaAs whiskers can be grown in rod shape with a uniform diameter. Gold aerosol particles of a selected size are used as catalysts, and thus, the surface coverage can be varied independently of the whisker size.

The width of the whisker is typically slightly larger than the diameter of the seed particle. This is mainly due to two factors: first, gold particles combine with Ga from the substrate and possibly also As, which causes the particles to grow; second, the bottom diameter of the liquid cap will be determined by the wetting angle between the alloy and the substrate surface as the particles melt. The simple assumption gives a broadening that is dependent on temperature and particle diameter of up to 50% and introduces a reversible relationship between particle diameter and whisker width.

Using GaAs<111>B substrate 10, HCl: H at 1: 10₂O to remove any native oxide layer and surface impurities prior to aerosol deposition. In a state of having high purity N₂Gold particles 12 of selected size are produced in a locally constructed aerosol apparatus within an atmospheric glove box 14. These particles are formed in a tube furnace 16 at a temperature of about 1750 c using an evaporation/concentration process and are charged by uv light at reference numeral 18. The size of these particles was selected by differential type mobility analyzer DMA 20. By balancing their airThe resistance is related to their mobility in the electric field, and the DMA classifies the size of these charged aerosol particles. After size classification, the particles were heated to 600 ℃ in order to make the particles dense and spherical. This arrangement results in an aerosol flow with a narrow size distribution with a standard deviation of less than 5% of the mean particle diameter. By means of the electric field E, the particles, which are still in the charged state, are deposited onto the substrate 10. The whiskers were grown using aerosol particles of selected size in the range of 10 and 50 nm.

After aerosol deposition, some of the sample was transferred to an AFM topometric probe 24, also located within the glove box and connected to an aerosol preparation device. Thus, during the deposition and treatment stages, these samples were exposed to H on the order of sub-ppm only₂O and O₂In (1). By means of the AFM tip, a particular particle 12 is selected and placed in a predetermined morphology or arrangement to fully control the positioning of the individual seed particles.

The GaAs substrate 10 is then transferred to a Chemical Beam Epitaxy (CBE) chamber along with the disposed or deposited Au aerosol particles 12. In the CBE structure, GaAs growth is initiated under vacuum/molecular beam conditions, in this case the organometallic sources are triethylgallium TEG and t-butylarsenic TBA. Thermal pre-splitting of TBA to predominantly As₂Molecules, while TEGs typically break apart after impinging on the substrate surface. The growth is generally at a slight As₂At overpressure, which means that the flow of Ga determines the growth rate. Immediately before growth, the substrate was heated to 600 ℃ by a heater for 5 minutes and exposed to As₂Under the beam. In this step, the Au droplet may form an alloy with the GaAs composition, and therefore, the Au particle absorbs some Ga from the substrate. The Au/Ga alloy is formed at 339 ℃. However, this step also serves as a de-oxidation step that removes any new native oxide layer that results from the transport to and from the glove box system. It is generally contemplated, although not necessarily absolutely, that the oxide evaporates at 590 ℃. The volatility of the oxide can be analyzed by reflection high energy electron diffraction RHEED. With the success of migration, at temperatures below 500 ℃A striped diffraction pattern indicating a crystalline, reformed surface can be seen below. However, in general, the oxide remains stable up to 590 ℃, and sometimes up to 630 ℃. The whiskers were grown at a substrate temperature between 500 and 560 ℃, a TEG pressure of 0.5mbar and a TBA pressure of 2.0 mbar. After growth, the samples were studied by scanning electron microscopy, SEM, and transmission electron microscopy, TEM.

The resulting whiskers were rod-shaped and, although they varied slightly in length, were fairly uniform in size. The dimensional uniformity is clearly dependent on the volatility of the surface oxide. It can be seen with RHEED that the dimensional uniformity is reduced for the sample with hard oxide. Therefore, to obtain reproducible results, an oxygen-free environment is preferred. At the growth temperature, no whisker tapering was observed, regardless of the size of the particles. However, it is clear that whiskers grown below 500 ℃ have signs of tapering. Depending on the temperature, the growth of rod-like or conical whiskers can be explained by the absence or presence of uncatalyzed growth on the surface parallel to the long axis of the whisker. The simplest surface for this orientation is the <110> crystal plane. Under conditions close to the conventional CBE growth conditions used in these experiments, the <110> crystal plane is the migration surface. However, at lower temperatures, the diffusion coefficient of Ga decreases, which stimulates growth on the <110> crystal plane. In MOCVD growth, Ga migration lengths are even smaller, which explains the typical tapered whiskers obtained by previous workers.

In fig. 2a, a TEM image of a bundle of whiskers 10 ± 2nm wide grown from 10nm particles is shown. The relatively low density of whiskers is illustrated by the SEM image in fig. 2B, which is an image of a GaAs <111> B substrate with GaAs whiskers grown from 40nm Au aerosol particles. In fig. 2c, a high resolution TEM micrograph shows a single 40nm wide whisker. As found in the other groups, the growth direction is perpendicular to the close-packed plane, i.e., the (111) plane of the cubic zincblende structure. Twinning defects and stacking faults are also observed, in which whiskers alternate between cubic and hexagonal structures. Most of the whiskers had an anomalous wurtzite structure W, except for the part closest to the Au catalyst, which was always zincblende Z. SF-stacking faults and T-twin planes. The variation in image contrast at the core is due to the hexagonal cross-section.

This growth method is employed in the methods described below with respect to figures 4-6 for forming whiskers having whisker portions of different compositions. This method is explained by InAs whiskers containing InP portions.

Example 2

The growth conditions of the whiskers allow the formation of abrupt interfaces and heterostructure barriers with thicknesses ranging from a few monolayers to hundreds of nanometers, thus forming a one-dimensional morphology along which electrons move. High resolution transmission electron microscopy illustrates the changes in crystal integrity, interface quality and lattice constant and derives a conduction band shift of 0.6eV from the current caused by thermal excitation of electrons above the InP barrier.

In this method, group III-V whiskers are grown in a gas-liquid-solid growth mode using gold nanoparticles for catalytic induced growth in the manner described above. Growth was carried out in an ultra-high vacuum chamber 100 specifically designed for Chemical Beam Epitaxy (CBE) as shown in fig. 3. The rapid transition of composition is controlled by feeding excited atoms (precorsor atoms) into the eutectic melt, which are fed into the ultra-high vacuum chamber as a molecular beam. Rapid switching between different compositions (e.g., between InAs and InP) is obtained via a procedure in which the indium source (TMIn) is turned off to interrupt growth, followed by a change in the group III source. Finally, when the indium source is re-injected into the growth chamber, the supersaturation conditions as a prerequisite for resumption of growth are reestablished.

For discontinuities or mutations at the interface, fig. 4 shows TEM analysis of InAs whiskers containing several InP heterostructure barriers. The high resolution images recorded with the three highest barriers with 400kV HRTEM (point resolution of 0.16nm) are shown in fig. 4 a. FIG. 4b shows a non-quadratic power spectrum (non-quadratic power spectrum) of the HRTEM image, indicating that the growth direction is along the [001] direction of the cubic lattice. The reflection shows a slight splitting due to the difference in lattice constant between InAs and InP. Fig. 4c shows an inverse fourier transform using a soft edge mask derived from the 200 reflecting part of the InP lattice. A corresponding mask is placed over the portion of reflective InAs. As shown in fig. 4d, the two images are superimposed.

Figure 5a shows a TEM image of an InAs/InP whisker. In fig. 5b, the enlarged 5nm barrier shows the atomic integrity and abrupt change of the heterostructure interface. Alongside the 100nm thick InP barrier are given the results of 1D poisson simulations of the heterostructure 1D energy morphology expected to be experienced by electrons moving along the whisker (neglecting edge quantization which only contributes about 10 meV) (fig. 5c), which gives the expected 0.6eV band shift (q-shift in the conduction band (the region where electrons move in n-type material)) in the conduction band (the region where electrons move)^1/4B). This barrier-like potential structure is very different from that encountered by electrons in homogeneous InAs whiskers, where ohmic behavior (i.e., a linear dependence of current I on voltage V) is expected and indeed observed (curve shown in fig. 5 d). This linear behavior is in sharp contrast to the I-V measurement curve shown for InAs whiskers containing an 80nm thick InP barrier. A strongly non-linear behavior is observed and the bias required to induce current through the whisker exceeds 1V. With increasing bias, the field-induced tunneling current rises sharply due to the narrowing of the effective barrier through which electrons must tunnel. To determine whether the ideal heterostructure band diagram in the 1D whisker is valid, the temperature dependence of the current caused by electrons overcoming the InP barrier via thermionic excitation was measured. The result is shown in FIG. 5e, where the current (divided by T) is measured²) As a function of the inverse temperature in the form of arvens, measured at a small bias (V) of 10mV that minimizes the band bending effect and the tunneling process described above. The effective barrier height q can be deduced from the slope of the line coinciding with the experimental data point^1/4B0.57eV, which corresponds well with simulations.

An additional advantage of this method for achieving heterostructures within 1D whiskers is that it is provided by effective stress relief through the open sides in the adjacent whisker geometry as an advantage for bonding highly mismatched materials. In contrast, only a few atomic layers are epitaxially grown in the transition between materials with different lattice constants, such as InAs and InP, before either island growth or misfit dislocations occur, thereby preventing the formation of an ideal heterointerface.

Resonant tunneling diode and hetero-bipolar transistor

At least in preferred embodiments, the invention also includes functional 1D (one-dimensional) Resonant Tunneling Diodes (RTDs) that are obtained by bottom-up combination of defined portions of different semiconductor materials in group III/V nanowires (nanowires). In order, such RTD includes: an emitter portion, a first barrier portion, a quantum well portion, a second barrier portion, and a collector portion. As is well known to those skilled in the art, the barrier portion in an RTD is made sufficiently thin so that a large amount of quantum tunneling of charge carriers is possible under conditions that satisfy tunneling. In the RTD according to the present invention, the nanowhiskers made in the form of nanowires can be made thin enough so that their central quantum wells are quantum dots in an efficient sense. In a specific example, the emitter, collector and central quantum dot may be made of InAs and the barrier material may be made of InP. In one example, excellent resonant tunneling behavior up to a peak to valley ratio of 50: 1 is observed.

According to the present invention, 1D heterostructure devices are fabricated using semiconductor nanowhiskers. As described in detail in examples 1 and 2 above, the whiskers were grown by a gas-liquid-solid growth mode, the size of which was controlled by and seeded by Au aerosol particles. Growth was carried out in a chemical beam epitaxy chamber under ultra-high vacuum conditions, in which the supersaturation of the eutectic melt between the Au particles and the reactants served as the driving force for whisker growth.

The incorporation of heterostructure moieties into whiskers can be achieved via the following switching sequence (fully described above): the group III source beam is turned off to stop growth, followed by a change in the group V source. Once the group III source is activatedReintroduced into the chamber, supersaturation reestablished and growth continued. In the examples described below, the material systems used are: InAs is used as emitter, collector and quantum dots, and InP is used as barrier material. The aerosol particles are selected so that the final whisker diameter is 40-50 nm. For the production of contact electronic devices with individual nanowhiskers as active elements, the whiskers are transferred from a growth substrate to be covered with SiO₂On which large pads are defined by evaporation of Au metal through a Transmission Electron Microscope (TEM) grid mask. A Scanning Electron Microscope (SEM) image of the nanowire device is shown in fig. 6B, which demonstrates alignment capabilities in an electron beam etching system, allowing positioning of metal electrodes on the nanowires with better than 100nm accuracy. Fig. 6D shows the current-voltage (I-V) characteristics of a set of single barrier devices, where the InP barrier thickness is tapered from 80nm to zero. The thicker InP portion acts as an ideal tunnel barrier for electron transport, allowing only thermal excitation (measured at about 0.6eV (23)) or tunneling above this barrier, which is effectively thinned when a large bias is applied to the sample, thus making this tunneling possible. As can be seen from fig. 6D, almost no current flows through the thick InP barrier. In samples containing a thin single barrier (fig. 2c), quantum tunneling can occur and electrons can penetrate the barrier to a thickness of less than about 10 nm. In the extreme case of zero barrier thickness, the I-V characteristic is excellent in linearity up to a temperature of at least 4.2K. To examine the crystalline quality and evaluate the mutability of the heterointerface, a high resolution TEM study was performed. Shown enlarged in FIG. 7A<111>The 5.5nm thick InP barrier in InAs nanowhiskers, where the (111) crystal plane can be clearly seen. From the composite plot of the regions in fig. 7A, the sharpness of the interface can be determined to be 1-3 lattice spacings. The average spacing between lattice edges in the brighter bands is 0.344nm, which corresponds well to the d of InP₁₁₁0.338 nm. FIG. 7B is a one-dimensional composite of the boxed area of FIG. 7A. The width of the barrier, judged from the jump in image contrast, is about 5.5nm (16 lattice spacings) and the interface sharpness is of the order of 1-3 lattice spacings. The background is not linear due to bending and stress contrast around the interface. The difference in lattice spacing between InP and InAs is3.4%, which corresponds well with the theoretical value of lattice mismatch (3.3%).

Double barrier resonant tunneling devices are contemplated because the heterointerface must be abrupt enough to make high quality quantum devices. A barrier thickness of about 5nm was chosen. A TEM image of such a double barrier device structure formed in a 40nm wide nanowhisker can be seen in figure 8A. On either side of the 15nm thick InAs quantum dots, the barrier thickness is approximately 5 nm. The expected band diagram of the device is shown below the TEM image (fig. 8B), with the longitudinal confinement (z-direction) determined by the length of the quantum dots and the lateral confinement (vertical direction) dependent on the diameter of the whiskers. For this device, only the lowest lateral quantized energy level is occupied (splitting on the order of 5 meV), has the fermi energy shown, and determines the highest occupied and electron filled longitudinal energy state. The full quantisation level of the central quantum dot is shown between the two InP barriers, in the same order as schematically given in the emitter region in the lateral quantisation level, but with a large split (in the order of 100 meV) between the longitudinal quantisation energy state in the quantum dot and the approximate quantisation energy E1z of the ground state, 40 meV. When zero bias is applied, the current should be zero since no electron state in the emitter is juxtaposed to any state in the central spot due to the difference in energy quantization between the spots and the emitter. As the bias is increased, the states in the dots move to lower energies and the current begins to increase as soon as the lowest dot-state is juxtaposed to the fermi level (assuming here that the fermi level is between the two lowest states in the emitter). When the point-state falls below the energy level of the first emitter state, the current drops to zero again, resulting in a characteristic negative differential resistance.

The electrical properties of the one-dimensional DBRT device are shown in fig. 8C, which shows nearly ideal I-V characteristics, as expected for this device. The I-V trace shows that there is no current flow below a bias of about 70mV, corresponding to a bias condition where electrons must be able to penetrate both barriers plus the central InAs portion to move from the emitter to the collector. It can be seen in the I-V characteristic that there is a sharp peak at about 80mV bias, with a full width at half maximum bias of about 5mV (which translates to a resonance energy sharpness of about 1-2 meV). The peak-to-valley ratio of the 80mV peak is extremely high, about 50: 1, and is seen in the different samples studied. After this deep valley, the current again increased for a bias of about 100mV, with some pending shoulder-like features observed on its ramp. Note that the I-V trace for increasing bias is identical to the I-V trace for decreasing bias, indicating that the device is characterized as highly reversible and exhibits negligible hysteresis effects. In addition, the 80mV phenomenon similarly occurs in the reverse bias polarity. In this case, the peak is only slightly shifted (5mV), indicating that the device structure has a high symmetry. These results thus report studies of the material and barrier properties of the single heterostructure barriers in semiconductor nanowires, up to thick barriers (where only thermal excitation above the barrier is possible), down to single barrier thicknesses (where tunneling through the barrier dominates).

A one-dimensional double-barrier resonant tunneling device is prepared by the method, and the device has high-quality device performance, the energy sharpness is about 1meV, and the peak-to-valley current ratio is 50: 1.

Referring now to figure 9, a preferred embodiment of a resonant tunneling diode is shown having nanowhiskers 40 extending between collector contacts 42 and emitter contacts 44 spaced 2 microns apart. The first InAs portion 46 and the second InAs portion 48 of the whisker are in electrical contact with the contacts 42, 44, respectively. The InP barrier portions 50, 52 separate InAs central quantum dots or quantum well portions 54 from emitter and collector portions. The length of the portion 54 is about 30 nm. The exact dimensions will be chosen according to the band gap barrier height etc. in order to obtain a suitable quantum confinement.

The diode operates in the conventional manner of RTD; for an explanation of the principle of operation, see for example the references Ferry and Goldnick, Transport in Nanostructures, CUP 1999, pp 94 and the following.

In the RTD of fig. 9, the portions 50, 52 may be replaced with a wide bandgap insulating material in the manner shown in fig. 10. Fig. 10 shows an embodiment with an insulating part. Germanium whiskers 100 are grown by the above process, having a short silicon portion 102. The lattice mismatch is accommodated by the radially outward expansion of the whiskers. The silicon dots are oxidized by heating to provide large silicon dioxide spacers 104 within the germanium whiskers. This has a very stable large bandgap offset. Aluminum may be used instead of silicon. This embodiment can be used for the example of tunneling shown in the embodiment in fig. 9.

With respect to making electrical contacts to the collector and emitter portions of the embodiment of fig. 9, this can be done in different ways. As shown in fig. 9, whiskers may be placed across a large metallization pad. Alternatively, the nanowhiskers may be placed on a substrate with their position confirmed by appropriate scanning methods, and then pads formed at the ends of the whiskers by a metallization process. Alternatively, the nanowhiskers may be maintained in a state extended from the substrate with the base in contact with an electrical contact at the substrate to encapsulate the whisker in a resin or glassy substance, and then an electrode formed on the encapsulation to make electrical contact with the tip of the whisker. This latter approach is more suitable for incorporation with other electrical devices and circuits.

An embodiment of the present invention is presented below with reference to fig. 11-14 and includes a heterojunction bipolar transistor (hetero-bipolar transistor; HBT) which differs from conventional bipolar transistors in that materials of different band gaps are used in the transistor. For example, the nanowhisker 110 may have a GaP emitter segment 112 connected to a p-doped Si base segment 114, which in turn is connected to an n-doped Si collector segment 116. Metal electrode 118 contacts portions 112, 114 and 116, respectively. Figure 12 shows a bandgap diagram of the HBT. Due to the relatively wide bandgap of the emitter, a small amount of current flowing from the base to the emitter is suppressed. The depletion region between the base and collector is characterized by a gradual transition of the doping from p-type to n-type. Alternatively, the base and collector may be made of a triple or quadruple material as a stoichiometric composition that gradually changes over a large number of crystal planes (e.g., 100 to 1000 crystal planes) to provide the desired depletion region. FIG. 13 shows a triple mixture Al_xGa_1-xWith composition dependent on the energy band gap of AsAnd (4) changing.

Fig. 14 shows the variation of band gap energy and lattice parameter for various III-V materials. It will be appreciated that with the method of forming nanowhiskers according to the invention it is possible to form heteroepitaxial junctions of materials having widely different lattice parameters, for example GaN/AlP, the lattice mismatch of which is adapted by the expansion (bulging) of the whisker in the radial direction.

Photonic device

Fig. 15 schematically shows a very small LED capable of single photon emission. Single photon emission is important for individual molecules such as quantum photography or detection of molecular species. The whisker 150 has anode and cathode outer regions 152 on either side of an inner region 156 made of indium arsenide, the anode and cathode outer regions 152 being made of indium phosphide to define a quantum well. Regions 152 are connected to anode and cathode electrical contacts, respectively, formed as metal regions 158. For planar devices, only certain wavelengths are possible due to the requirements for lattice matching and relieving mismatch stress; in contrast, it is important to this embodiment that the material used to make the diode can be of any desired composition to obtain the desired wavelength emission (see figure 14 discussed above) since the lattice mismatch is accommodated by the radial outward expansion of the whiskers. Thus, the wavelength of the LED is sufficiently variable. Since the material may be a stoichiometric composition, its wavelength may be continuously varied in a range from 1.5eV to 0.35 eV. One-dimensional structures require less processing than prior art layered structures and are made by a self-organizing process, with the entire structure located between electrical contacts. If a laser structure is required, Fabry Perot (FP) cleaved facets 159 are formed to be spaced at an appropriate pitch. Alternatively, the region 159 is formed as a mirror including a superlattice. The superlattice may be formed as an alternating sequence of InP/InAs that alternates over only a portion of several crystal planes, as is well known to those skilled in the art.

LEDs, lasers and other microcavity structures are typically made of gallium nitride (GaN). Nitrides have some optical advantages in particular, while nitrides also have disadvantages: first, they are filled with dislocations; secondly, there is a lack of suitable substrates (sapphire is a commonly used substrate). The whiskers may be made of a defect-free nitride and have no lattice matching problems with the substrate. Conventional FP lasers can be made with the structure shown in fig. 15 having dimensions less than 300nm, preferably on the order of 100 nm. The architecture is a bottom up architecture (bottom up structure) that is well suited for reading and writing DVDs. Nitride systems are well suited for whisker growth.

The light source emission region 156 can be made as small as about 20nm³. This represents an extreme example of a point source of light and, as schematically indicated in FIG. 16, may be used to locally excite individual biological cells 160. Since the physical separation between the light source and the object is a fraction of the wavelength, the light source 156 provides a near field 162 (decaying exponentially) that excites the cell 160. Which may be used in DNA sequencing, and as shown, the source 156 may be mounted in a groove 164 of a glass capillary 166. The cells flow along the capillary as part of the fluid mixture and through the source 156.

Referring to fig. 17, an embodiment of the invention is shown suitable for nanoimprint etching techniques (NIL), wherein an array 170 of whiskers 156 for providing point light sources are individually addressable by a voltage source 172. The array is mounted on a carriage 174 that is movable over the surface of the resist material 176. The carriage is moved in 20nm steps, and in each step, a voltage is selectively applied to the whiskers 156 in order to illuminate the material 176 with near-field light and create a desired developable pattern in the resist material 176.

Referring to fig. 18A, a photodetector according to the present invention is shown. For example, the nanowhiskers 180 may extend between metal contact pads 182. Typically have a high contact resistance of 10K omega to 100K omega, which results from the small contact area between the pad 182 and the whisker 180. The whisker may include an n-type doped indium phosphide portion 184, a p-type doped indium phosphide portion 186, and a p-n junction 188 therebetween, which may be discontinuous or extend across multiple crystal planes. This arrangement is suitable for detecting light of a wavelength of 1.3 microns or 1.55 microns. As shown in fig. 14, any desired composition "match" may be used, and thus the material may be altered to detect any wavelength from 1.55 microns or less. Alternatively, a PIN or schottky diode structure may be used. As shown in fig. 18B, the PIN structure has a portion 188 of intrinsic semiconductor material located between two semiconductor portions 184 and 186. The whiskers are constructed in the manner described with reference to figure 10. As shown in fig. 18C, the schottky diode structure has a base portion 189 formed as a metal contact from which the whisker extends; the interface between the contact and the whisker forms a schottky diode. The lower frequency limit of radiation detection is in the terahertz region of the electromagnetic spectrum.

Referring to fig. 19A, a solar cell application of the photodetector structure of fig. 18 is shown. Millions of whiskers 190 are formed on a p-doped substrate 193, each having a p-type doping 191 and an n-type doping 192. The whiskers are formed by growth using, for example, gold or other nanoparticles deposited from an aerosol on a substrate 193. The whiskers may be encapsulated in plastic 194 and have transparent tin oxide electrodes 196 on their upper surface that contact the free ends of the whiskers, thereby enabling current to flow along the length of the whiskers. Since each whisker is 100% reliable, this structure is extremely efficient at trapping light. The overall efficiency is between 35 and 50% and can be used in multi-bandgap solar cells. In contrast, porous silicon grown at 300 ℃ has an efficiency of about 10% and crystalline silicon has an efficiency of about 15%, and special purpose III-V solar cells suitable for space applications are grown at 400 ℃ and have efficiencies as high as 25%. Adapted for space applicationsSolar cells with titanium dioxide nanoparticles sprayed on the solar panel together with a suitable dye have efficiencies of up to 8%.

Referring to the variation shown in fig. 19B, each whisker of the solar cell array is modified in the form shown at reference numeral 197 to have a different portion 198 of a different material along its length. These materials are selected so that the p-n junction absorbs light of different wavelengths. The point along the whisker at which the whisker is more sensitive to light of a particular wavelength depends on the precise structure of the solar cell and factors such as reflection and refraction within the structure.

The embodiment of fig. 19A-B is inexpensive because the growth conditions are inexpensive and require only a very small amount of expensive material. In another alternative structure, the whiskers may be silicon (which is the least expensive) or germanium. The length of the whisker is 1 or 2 microns. The PN junction is obtained by doping the whisker along part of its length or by forming a schottky barrier at the bottom of the whisker as shown in fig. 18C.

Referring to fig. 20, an embodiment is shown which is a very long wavelength infrared radiation source, such as at terahertz frequencies. Indium phosphide nanowhisker 200 has a series of very thin indium arsenide stripes 202 separated by indium phosphide isolation stripes 204. These streaks are grown by the above process. Each stripe 202, 204 is several facets wide and these stripes form a superlattice 206. By applying a voltage to the electrode contact 208, electrons move through the superlattice. The superlattice produces a series of quantum well bandgaps (potential wells) that give the conduction band in accordance with bloch theory, with an allowed region of electron wavenumbers or momentum k-i.e., an allowed region corresponding to the terahertz frequency-producing terahertz emission.

Fig. 21A-21D illustrate an embodiment of the invention implemented as a photonic crystal. Photonic crystals are well known-see for example pending application WO 01/77726. The main prior art methods of forming photonic crystals involve etching air holes in a substrate according to a predetermined lattice pattern. The idea of this embodiment is to use a patterning technique to define a lattice pattern on the substrate, but instead of etching holes, nanowhiskers are grown to define the crystal. This has a number of advantages, one of which is that the etching technique is not as reliable as the bottom-up technique of growing whiskers (etching can damage the substrate surface). Thus, the whisker technique is more accurate, of higher quality and simple, and is also economical since it requires fewer process steps.

As shown in figure 21A of the drawings,the substrate 210 has a distance of about 300nm and a distance of about 300nm²A triangular lattice pattern of square gold plates 212 formed by electron beam etching, UV etching or nanoimprint etching (NIL) process. The substrate is initially prepared as a clean substrate free of oxide contamination prior to gold deposition. The substrate is heated to melt the gold rectangles so that they form spheres 214 with a diameter of about 100nm, as shown in fig. 21B, and then annealed. Whiskers 216, approximately 100nm wide, were then grown by the process described in example 1, forming photonic crystals, as shown in figure 21C.

According to the present invention, a three-dimensional photonic crystal can be defined by the formation of whiskers. This may be accomplished as shown in figure 21D by making each whisker from a series of sections 217, 218 of different materials, for example, by alternating sequences of III-V materials such as InAs/GaAs or IV materials such as Ge/Si, according to the method of example 2, to provide sections of suitable refractive index at intervals along each whisker to form a photonic bandgap.

Monocrystalline layer of III-V material

Referring to fig. 22A-22G, one embodiment of the present invention is shown for growing an epitaxial layer of a desired material on a substrate. As shown in fig. 22A and B, a silicon or gallium arsenide substrate 220 has formed on its upper surface gold, indium or gallium rectangles 222 that are positioned on the substrate by imprinting 223 in a NIL process or as described in example 1. An epitaxial cap deposition 224 (a dielectric material several nanometers wide, such as silicon dioxide or silicon nitride) is formed on the substrate 220 and around the rectangle 222. Heat is applied to anneal the rectangle, turning it into a ball 226, as shown in fig. 22C. As shown in fig. 22D, whiskers 228 of InP or GaAs as examples are grown. Alternatively, a carbon-based material is used as the deposit 224 (when the spheres are formed by annealing, the dielectric material desorbs, which stabilizes the particles). The sphere is used as the seed start for bulk growth, i.e. growth of a layer of the desired material. The dielectric layer prevents atomic bonding and lattice mismatch effects between the substrate and the crystalline layer. As shown in fig. 22E, the whiskers are grown with a bulk layer 229 of InP or GaAs. There is a gradual change in the growth conditions from whisker to layer. Thus, there will be nucleation on the whiskers without defects. There are small nucleation steps and no stress effects that cause dislocations. Where the substrate is a III-V material, a significant advantage is. A lattice-mismatched layer is formed on a substrate without generating misfit dislocations.

In a variation, as shown in fig. 22F, gold balls 226 are deposited from an aerosol onto the substrate surface according to the method of example 1. An epitaxial cap deposit 224 is formed on the ball. Then, as shown in fig. 22D, whiskers were grown.

In a further development according to the invention it can be seen that, as shown in figure 23A, the whiskers tend to grow preferentially along the <111> B direction, since for gallium arsenide (zinc blende lattice) the arsenic atoms are located at the vertices of the pyramids and the gallium ions are located at the base of the pyramids. Figure 23B shows a preferred embodiment of the present invention in which a silicon substrate 230 has a serrated surface with V-shaped grooves 232 etched to a microscopic size that exposes the <111> plane. Gold particles 234 are deposited on the surface of the V-shaped grooves. GaAs whiskers 236, grown according to example 1 and shown in phantom form in fig. 23C, would extend perpendicular to the walls of the serrations. These whiskers provide nucleation sites for bulk growth of GaAs layer 238. There is a gradual change in the growth conditions from whisker to layer. Thus, there is nucleation on gallium arsenide without creating defects. There are no small nucleation steps and stress effects that lead to dislocation generation. The whisker direction along the <111> direction at an angle to the substrate forces epitaxial growth along a particular direction and eliminates the once-disturbing problem of anti-phase domains. Thus, a way of bonding group III-V compounds to silicon (or other group N) substrates is provided and is less expensive than prior methods-see for example published PCT patent application WO 02/01648.

Another advantage of the substrate with V-shaped grooves, in relation to the solar cell application of fig. 19, is that the serrated substrate provides multiple reflections of the incident light, thus increasing the probability of photon capture.

A preferred embodiment for controlling the orientation of the whiskers is described below with reference to figure 24. Typically, whiskers of III-V compounds grow along the <111> B direction, as described above. The problem here is that the whiskers vary more or less randomly between hexagonal (wurtzite) (fig. 24A) and cubic (zincblende) (fig. 23A) structures. This results in a number of stacking faults. Stacking faults are always a problem, in particular with regard to the optical properties, but also with regard to the electrical properties. By applying stress to the whisker by changing the growth conditions, the growth direction of the whisker can be changed to a <100> direction, which forms a cubic lattice structure (zincblende) without having stacking faults.

In fig. 24B, a silicon substrate 240 having a <100> surface has whiskers 242, e.g., InP, grown thereon. The whisker begins to grow in the <111> direction at 244 and shortly after initial growth, the operating conditions are rapidly changed by increasing the growth rate and raising the temperature and pressure within the CBE apparatus, so the whisker continues to grow in the <100> direction at 246. The point 248 at which the direction changes is the <110> crystal plane. The whiskers at the transition retain their epitaxial crystalline nature. The crystal structure in portion 246 is a hexagonal close-packed structure, which significantly reduces the problem of stacking faults.

In another growth method, a short barrier portion of a wide bandgap material, such as InAs, is grown at point 248, which has the same effect as changing the subsequent orientation of the whisker.

Therefore, this embodiment is particularly suitable for the growth of nitrides, such as GaN, which are preferably grown in a hexagonal lattice and are particularly prone to stacking faults. By "forcing" the nitride crystal to grow in a cubic structure, stacking faults are reduced. Furthermore, a multi-cavity structure for gallium nitride lasers can be developed in a structure made according to example 2 with different material portions along the whisker. The nitride system is well suited for whisker growth. The problem with nitrides is that they are filled with dislocations and lack a suitable substrate. The whiskers can be made of nitride free of defects and without lattice matching problems. Conventional FP lasers can be made with nanowhiskers on the order of 100nm in length, less than 300 nm. It is a bottom-up structure, well suited for reading and writing DVDs.

Reference is now made to the embodiment shown in fig. 25, which relates to a field emission tip or Spindt cathode. They are used in Field Emission Displays (FEDs) and many methods have been proposed to make such displays. One prior art solution, shown in figure 25a, includes a silicon substrate 250 having a surface 252 that is patterned using laser ablation or the like to form a micro-or nano-tip 253. Phosphor screen 254 is disposed adjacent a tip where a voltage between the tip and the screen produces a very high field strength that causes a current to flow into the screen, resulting in visible radiation being emitted from the screen.

An embodiment of the invention comprising an FED is shown in fig. 25B, where the elements of the display are independently addressable. Etched contact metal regions 256 are formed on the silicon substrate 250. Gold seed particles 258 are placed on each metal region by the method described in example 1. These gold particles are used as seeds for whisker growth in order to grow Si whiskers 259, each of which extends away from a respective metal region. As shown, individual whiskers or a group of nanowhiskers forming one display element may extend away from the corresponding metal region. In addition to being independently addressable, this embodiment also has the advantage that the FED is 100% reliable compared to prior art methods, such as Carbon Nanotubes (CNTs).

Fig. 26 shows an embodiment of infrared to visible up-conversion. An image 260 of infrared radiation having a wavelength of 1.55 or 2.5 μm is illuminated on the bottom surface of a gallium arsenide substrate 262-this substrate is a material with a relatively large bandgap that does not interact with the radiation. The other side of the substrate has indium arsenide protruding whiskers 264, which are grown as described in example 1 and have a relatively small bandgap, which will result in absorption of the radiated photons. In contrast to figure 25, however, whiskers 264 are not independently addressable. A voltage of approximately 20-50V is applied between the end of the whisker and the adjacent phosphor screen 266 and electrons are generated from the indium arsenide whisker. Indium arsenide has a bandgap of up to 3 microns and therefore generates electrons in response to radiation having a wavelength shorter than 3 microns. Alternatively, gallium phosphide can be used, but it has a visible band gap. The emitted electrons cause the fluorescent light to emit visible light 268 from the phosphor screen and upconvert the image to a visible wavelength image. The applied voltage can be increased enough to cause an avalanche effect.

Figure 27 shows an embodiment of the invention in which 400nm long GaAs whiskers 270 (made according to example 1) extend away from metal contact regions 272 on a silicon substrate 274. The dimensions are a quarter wavelength of the 1.55 micron radiation, so the whisker provides a λ/4 resonant antenna for the 1.55 micron radiation. The contact region 272 provides a ground plane. The antenna may be configured to receive radiation in free space 276; alternatively, it may also be disposed adjacent to the end of the quartz fiber coupling 278 to detect radiation in the third optical window.

FIG. 28 illustrates an embodiment of the invention used in the field of spintronics. Spintronics is a field of technology in which the properties of electronic devices depend on the transport of electrons autogyres in the device-see, for example, Scientific American, June 2002, pp 52-59, "Spintronics", david. In fig. 28, whiskers 280 of a magnetic or semi-magnetic material such as manganese gallium arsenide (semi-magnetic) or manganese arsenide (ferromagnetic) made by the process in example 1 are formed on a Si substrate 281. Under an applied voltage V, spin-polarized electrons 283 are emitted from the tips of the whiskers, which are in electrical contact with electrical contacts 284 located on a substrate 286. The spin-polarized electrons 283 are used to read from and write to a magnetic memory device 288 mounted on a substrate 286.

In a further development of the invention, the problem is overcome that for ferromagnetic there is usually a lower limit of the ferromagnetic domain width of about 10-15nm below which ferromagnetic properties are converted into super-paramagnetic. However, according to the method in example 1, when incorporated in a whisker, the domain diameter can be reduced because the probability of symmetrical alignment in a one-dimensional system is reduced, making it more difficult for ions of the material to have more than one orientation. The material of the whisker may be iron, cobalt, manganese or an alloy thereof.

Referring now to fig. 29, another embodiment of the present invention is shown which includes a substrate having an electrode array for implantation in a nerve to repair a nerve function, such as the retina of an eye. The electrodes are independently addressable. Etched contact metal region 350 is formed on silicon substrate 352. Gold seed particles 354 are positioned on each metal region by the method described above. These gold particles are used as seeds for whisker growth in order to grow silicon whiskers 358, each of which extends away from a respective metal region. As shown, individual whiskers or groups of nanowhiskers forming one electrode element extend away from the corresponding metal region. In addition to being independently addressable, this embodiment has the advantage that the electrodes are 100% reliable.

Referring now to figure 30, there is shown another embodiment comprising nanowhiskers 360 formed by the method described above. The whisker is made of silicon and has a gold particle melt 362 at one end thereof. After the whisker is formed, the whisker is exposed to an atmosphere at an appropriate temperature to oxidize silicon. This forms a silica sheath 364 around the whisker and extending along its length. The gold particle melt 362 remains unoxidized. This therefore provides a structure well suited to the electrode assembly shown in fig. 29, where the electrodes have very precise electrical characteristics. The silicon material may be replaced by any other material that can be oxidized.

Alternatively, the whisker 360 may also be exposed to an atmosphere of a suitable material to form a high band gap material that can replace the oxide layer 364.

Referring now to fig. 31, there is shown another embodiment of the present invention comprising a silicon base member 370. The base member may be a planar substrate or simply a rod. In either case, a row of nanowhiskers 372 is formed from one edge surface of the rod or substrate. The nanowhiskers are regularly spaced and project into the space. These nanowhiskers may have a coating formed thereon to attract specific molecular structures. In either case, the cantilever arrangement may be employed as any of the well known applications of cantilever arrangements for measuring molecular species and the like.

Referring to FIG. 32, another embodiment of the present invention is shown including a molecular detection device. A substrate 380, such as silicon nitride, has an insulating layer 382 formed thereon and has a conductive surface 384, such as gold. A hole 386 is formed within the layers 382, 384, within which a nanowhisker 388 is formed.

This is essentially done by a self-contained process, since the holes are formed in the insulating layer 382 and a gold layer 384 is subsequently deposited. Thus, gold is caused to deposit at the bottom of the hole, as shown at 389, and by heating a melt of gold particles is formed that enables nanowhiskers to be formed under appropriate conditions. In the completed nanowhisker, the gold particle melt 389 is located at the top of the nanowhisker. The height of the nanowhisker is such that particle melt 389 is at least substantially in the same plane as gold surface layer 384.

The natural elasticity of a nanowhisker means that it has a characteristic frequency of vibration from side to side in a direction perpendicular to its length. The vibration of particle melt 389 can be detected by a voltage or current signal generated in conductive layer 384. This therefore provides a way of detecting the frequency of vibration of the nanowhisker 388.

By appropriate activation of the conductive material with an applied voltage, the whisker can be caused to mechanically vibrate within the hole at an eigenfrequency, for example in the gigahertz range. This is because, from the low dimensions and small currents involved, one single electron migrates from one side of the conductive material to the other via the seed particle melt during one oscillation cycle. This results in a current standard generator in which the current I through the conductive material is equal to the product of the vibration frequency f and the charge e of an electron: i ═ f · e. Thus, a known reference signal is generated, which can be used in a suitable environment.

In addition, particle melt 389 may be coated with a receptor substance, allowing a specific molecular species to be adsorbed onto the surface of particle melt 389. This will result in a change in the characteristic frequency of the nanowhisker. This change in frequency can be detected, which also provides a means for calculating the weight of molecular species adsorbed on the surface of the melt 389.

Figure 33 shows the tip of a scanning tunneling electron microscope (STM) comprising InP nanowhiskers 392 formed on the ends of flexible beams 394 of silicon. The beam 394 is formed from the substrate or rod by etching.

Claims (21)

1. An optoelectronic device, comprising:

a substrate provided with a contact region;

at least one nanowhisker (190) extending from the contact region, the nanowhisker (190) forming at least part of a p-n junction for light absorption;

a transparent electrode (196) extending over and in electrical contact with the free end of each whisker, each nanowhisker (190) comprising a column of nanometric-sized diameter comprising at least one heterojunction between semiconductor length portions (191, 192, 198) of different composition.

2. The optoelectronic device of claim 1, wherein the substrate is electrically conductive at the contact region.

3. The optoelectronic device of claim 1 or 2, wherein the substrate comprises a doped semiconductor material in the contact region.

4. An optoelectronic device according to claim 1, wherein the nanowhiskers are encapsulated in a transparent material (194).

5. An optoelectronic device according to claim 1, wherein the heterojunctions between the semiconductor length sections (191, 192, 198) are abrupt.

6. An optoelectronic device according to claim 1, wherein the heterojunctions between the semiconductor length sections (191, 192, 198) are graded.

7. An opto-electronic device according to claim 1 characterized in that a first semiconductor length section (191) is p-type doped and a second semiconductor length section (192) is n-type doped, said first and second semiconductor length sections (191, 192) having an interface between them forming a p-n junction.

8. The optoelectronic device of claim 7, wherein the pillar comprises a third intrinsic semiconductor length (188) between the first and second semiconductor length to form a PIN diode.

9. A photovoltaic device in accordance with claim 1, wherein the diode is formed at a base portion (189) of the photovoltaic device, the whiskers extending from the base portion (189).

10. The optoelectronic device of claim 9, wherein the base portion (189) is formed as a metal contact, whereby an interface between the metal contact and the whisker forms a schottky diode.

11. An optoelectronic device according to claim 1, wherein each nanowhisker has a plurality of p-n junctions located between semiconductor length sections selected to form p-n junctions that absorb radiation of a plurality of different wavelengths.

12. The optoelectronic device of claim 1, wherein each p-n junction is electrically connected in series with the contact region by means of a tunneling diode.

13. The optoelectronic device of claim 12, wherein at least one tunneling diode is formed by compositional discontinuities between length portions of different semiconductor materials.

14. An optoelectronic device according to claim 1, wherein the nanowhisker (190) comprises a sheath (364) at least partially surrounding the nanowhisker and extending along its length.

15. The optoelectronic apparatus of claim 14, wherein the housing (364) comprises an oxide material.

16. The optoelectronic apparatus of claim 14, wherein the housing (364) comprises a high bandgap material.

17. An optoelectronic device according to claim 14, comprising a plurality of nanowhiskers extending from a substrate, and wherein the shells (364) of each nanowhisker are made to continue to grow together, using the nanowhiskers as growth sites, to form a bulk layer (229) extending over the substrate.

18. An optoelectronic device according to claim 17, wherein the bulk layer (229) is epitaxial.

19. A solar cell comprising the photovoltaic device of any of claims 1-18, wherein the photovoltaic device is adapted to convert sunlight to electricity.

20. The solar cell of claim 19, wherein the plurality of nanowhiskers extend parallel to each other.

21. A photodetector comprising an optoelectronic device according to any one of claims 1 to 18, wherein the optoelectronic device is adapted to detect radiation.