EP1115648A1

EP1115648A1 - Method for producing nanometer structures on semiconductor surfaces

Info

Publication number: EP1115648A1
Application number: EP99955747A
Authority: EP
Inventors: Stefan Fascko; Heinrich Kurz; Clemens Koerdt
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-09-23
Filing date: 1999-09-18
Publication date: 2001-07-18
Also published as: WO2000017094A1

Abstract

The invention relates to a method for producing regular nanometer structures on semiconductor surfaces, especially regular pyramid and wave structures, which comprise a narrow size distribution and dimensions ranging from 2 to 100 nm, especially 10-60 nm, particularly in diameter or width and height. The invention is characterized in that a semiconductor material is used which is comprised of at least two and preferably two components thus forming a compound semiconductor. In addition, optionally neutralized noble gas ions from an ion source comprising an energy ranging from 10 to 50000 eV, especially 50-2000 eV, are directed onto said compound semiconductor material, with which, under a vacuum and by means of ion sputtering, the surface of the material is removed until the nanometer structure is produced.

Description

Name: Process for the production of nanometer structures on semiconductor surfaces

The invention relates to a method according to the preamble of claim 1 for producing regular semiconductor structures with sizes in the range from a few nanometers to a few tens of nanometers. It is referred to as "self-organized ion sputtering", where ion sputtering means removal or sputtering by ion bombardment. The structures produced with it show exceptional electronic and optical properties. The created nanometer structuring / texturing of surfaces is therefore of great interest. Islands or wires made of semiconductor materials of uniform size in the nanometer range, which are produced on the basis of this method, so-called "quantum dots" or "quantum wires" are in particular as an active medium in semiconductor lasers ("quantum dot lasers") or in Tunnel components of importance.

The technical field to which the invention relates relates to semiconductor structures, compound semiconductors, ion sputtering, semiconductor lasers, tunnel components, nanometer structuring, quantum dots.

Relevant prior art: Various lithography processes are used to produce the smallest, regular structures in the nanometer range on substrates. In recent years, new processes have been developed specifically for the production of quantum dots and wires, such as "self-organized growth" or the epi- tactical growth on pre-structured or highly indexed surfaces.

The change in surface morphology due to ion bombardment has been widely studied. It offers approaches to explaining the formation of structures.

Lithographic processes: Nanometer structures can be produced using conventional lithographic processes. For this purpose, multi-quantum wells are structured by means of photolithography or electron beam lithography, thus limiting the charge carriers in three dimensions [12]. The size of the structures produced by lithography can be up to 20 nm. AFM (atomic force microscopy) lithography can further reduce the size of the structures. Disadvantages of the lithographic methods are high technical expenditure and, in the case of electron beam and AFM lithography, a high expenditure of time due to the serial processing.

Self-organized growth: The epitaxial growth of fewer monolayers of a semiconductor on a substrate creates islands of uniform size under certain conditions [11]. The "self-organized" growth is observed in various material systems: e.g. InAs on GaAs [4], GaSb on GaAs [5], Ge on Si [6]. It is based on the difference in the lattice constants of the materials, which leads to elastic tension in a two-dimensional layer. One way of relaxing this layer is to form three-dimensional islands. In the so-called Stranski-Krastanov growth mode, a wetting layer is first created and then islands of the same size and shape. The size of these islands (mostly pyramids) is a few nanometers to a few tens of nanometers. Measurements of photoluminescence and electroluminescence on quantum structures produced in this way clearly show that the electrons are locked in three dimensions [7, 13].

Change in surface morphology: Change in surface morphology Ion bombardment technology has been widely studied and described in the literature in recent years [8-10]. Two approaches are discussed in the literature to explain the structure formation. A common feature of both explanations is the assumption of a locally changed sputter rate, ie a locally changed material removal rate. This can occur for two reasons:

Contamination- or defect-induced masking: Locations where the sputtering rate is locally lower than in the environment due to foreign atoms or defects on or immediately below the surface, result in outward-looking structures. In the case of multi-component materials (e.g. III-V compound semiconductors), an element can be enriched by preferentially sputtering an element on the surface. Diffusion leads to the separation of the atoms present in excess and thus a masking of the surface. Foreign atoms can also be applied in a defined manner by sputter deposition and masking can be achieved in this way.

Microscopic sputtering: The sputtering rate is a function of the angle at which the ions hit the surface. The roughness is increased microscopically by the sputtering probability of individual surface atoms, while the roughness is reduced mesoscopically by sputtering. A steady state is reached when the sputter rate on the textured surface is constant everywhere, which can lead to regular structures with at least one short-range order.

In the case of GaSb and InSb, in which the regular surface structure was observed, the first case is more likely, since the sputtering rates of the elements differ greatly and therefore preferred sputtering of a component occurs.

literature [I] N. Kirstaedter et al., Electronics Letters 30, 1416 (1994)

[2] D.G. Deppe and D.L. Huffaker, Optics & Photonics News, January, 30,

(1998) [3] W. Chen and H. Ahmed, J. Vac. Be. Technol. B 15 (4), (1997) [4] J.M. Moison, F. Houzay, F. Barthe, and L. Leprince, Appl. Phys. Lett 64

(2), (1994)) [5] E.R. Glaser, B.R. Bennett, B.V. Shanabrook, and R. Magno, Appl. Phys.

Lett 68 (25), 3614, (1996) [6] F.M. Ross, J. Tersoff, and R.M. Tromp, Phys. Rev. Lett 80 (5), 984, (1998) [7] D.L. Huffaker, L.A. Graham, and D.G. Deppe, Appl. Phys. Lett 72 (2),

214, (1998) [8] O. Auciello and R. Kelly, Ion Bombardment Modification of Surfaces, Be am Modification of Materials, Beam Modification of Materials 1, Elese- four (1984) [9] R.Behήsch, Sputtering by Particle Bombardment II, Topics in Applied

Physics, 52, Springer Verlag, (1983) [10] O. Auciello, J. Vac. Be. Technol. 19 (4), 841 (1981)

[I I] M. Grundmann and D. Bimberg, Phys. Blätter 53, 517 (1997) [12] Y.S. Yang et al, J. of Electr. Mat. 24 (2), 99 (1995)

[13] M. Grundmann et al, Phys. Stat. Sol. (b) 188, 249 (1995)

The previously known production methods for producing regular structures on solid surfaces in the range of a few tens of nanometers (nanostructures) and for quantum dots or quantum wires are based on lithographic processes (electron beam / photo or AFM lithography), on self-organization in the case of epitaxial growth or on epitaxial growth pre-structured or highly indexed surfaces.

The object of the invention is to provide a further method and a device for producing regular structures in the nanometer range. This task is solved by the method according to saying 1 and the device according to claim 11.

The new process is based on a new self-ordering principle that occurs in ion sputtering if suitable ion energies are used and the surface to be processed is prepared appropriately.

The nanometer structure according to the invention consists of a large number of individual, isolated islands made of a compound semiconductor material, which are located on a substrate. This substrate can be the same compound semiconductor as the compound semiconductor material or another semiconductor material. The individual islands have largely the same shape and configuration as possible, they are largely arranged regularly on the substrate. In general terms, the invention relates to a method for changing the morphology of a semiconductor surface.

To produce the nano structures (e.g. dots, wires) with sizes from a few nanometers to a few tens of nanometers on solid surfaces by means of "self-organized ion sputtering", a system is required to generate a homogeneous ion or neutralized ion beam with a kinetic energy of the ions or atoms from 10 to 50000 eV, in particular 50 - 2000 eV. 300 - 1000 eV have proven to be particularly advantageous for GaSb. This requires a vacuum chamber that can be operated at pressures below normal pressure, in particular <10 " ³ mbar. In the vacuum chamber, in addition to the ion or atom source, a device (holder) is required to hold the material to be processed. This holder is used the solid is brought into thermal contact and cooled on the back via the holder. It is advantageous to regulate the temperature during the ion sputtering. The ion or neutralized ion beam from the ion or atom source is directed perpendicularly or at a different angle onto the surface of the solid whereby atoms are knocked out of the surface of the material (ion tern, ion etching). A device for measuring the ion current before and during the ion bombardment is necessary to determine the rate at which the surface is bombarded and thus the time required. An analysis method that detects the sputtered elements is also of great advantage for determining the end point.

In the process of ion sputtering, there is a continuous erosion of the surface due to the impingement of the ions or atoms, with the "self-organized ion sputtering" conical structures with sizes of a few tens of nanometers forming from the surface. After initial training, these structures are retained even after further sputtering. The material whose surface is to be covered with nanometer structures by means of self-organized sputtering must have the following properties: it must be a compound, the sputtering rate of its elements having to be different. Compounds that show self-organized sputtering are e.g. B. crystalline GaSb and InSb with (100) orientation of the surface.

If the desired material does not show this property of the formation of regular nanometer structures, the surface of this material can also be structured indirectly via an additionally applied GaSb layer. For this purpose, a GaSb layer is applied to the material with a thickness of a few 10 nm to many 100 nm, preferably at least 250 nm, using a suitable deposition method (e.g. molecular beam epitaxy (MBE), vapor phase deposition (CVD), sputter deposition, etc.) The GaSb layer is then treated by the method described above and the morphology formed in the GaSb layer is transferred to the underlying substrate by further ion sputtering.

To create quantum dots, the material must consist of at least two layers. The most suitable materials are usually tactical process. It is important to lock the electrons in the generated quantum dots, ie that islands of the first layer remain isolated on the layer below (eg GaSb islands on AlSb or on GaAs).

For the end point control when removing the layers of a multilayer structure, it is advantageous to use an element-specific detection of the removed particles or an element-specific surface analysis method that can be carried out during sputtering.

The new process allows nanometer structures of high density (eg 5 * 10 ¹⁰ cm " ² ) and narrow size distribution to be produced, with sizes of typically 10-60 nm and with a hexagonal short-range order. The new process can be used in industrial ion etching plants in the Manufacture of homogeneous large-scale nano-structuring or texturing or of quantum dots (up to 50cm x 50cm possible) compared to epitaxially grown nanostructures, eg with molecular beam epitaxy (Molecular Beam Epitaxy, MBE) or metal organic gas phase epitaxy (Molecular Organic Vapor Phase Epitaxy, MOVPE ) the requirements for the vacuum and the purity of the vacuum chamber are much lower, compared to lithographic processes the new technology has the advantage of a reduced expenditure of time.

Further features and advantages of the invention result from the remaining claims and the following description of non-restrictive explanations which are explained with reference to the drawing. In this drawing:

1: a scanning electron image of a nanometer morphology on GaSb (100) at a magnification of 50,000 times,

FIG. 2: the morphology according to FIG. 1 in a magnification of 200,000 times 3 shows a basic illustration of a plasma ion etching system (also: plasma sputter system) with a quadrupole mass spectrometer for the shutdown control,

4: a basic representation of the plasma ion etching system according to 3 shows an enlarged detail from FIG. 3, in particular a holder for the sample,

Fig. 5: a schematic representation of a sputtering system, as for the

Implementation of the method according to the invention can be used,

6: a diagram of the size distribution of the nanometer structures, the structure diameter is plotted against the number,

Fig. 7: a graph corresponding to Fig. 6, after a long sputtering time and

Fig. 8: a graph corresponding to Fig. 7, after a longer sputtering time.

Based on the new method, regular conical structures were produced on GaSb (100) surfaces of different sizes (see FIGS. 1 and 2, diameter: 26 nm). The structures have a hexagonal short-range order with a density of ~ 5 * 10 ¹⁰ dots / cm ² . High-resolution transmission electron micrographs (HTEM) show that the pyramids have the same composition (stoichiometric GaSb) and the same crystalline structure as the substrate.

To produce quantum dots, the charge carriers must also be locked perpendicular to the surface in addition to the lateral extent, which is achieved in that a suitable sequence of epitaxial grown layers is removed until isolated GaSb islands remain on the underlying layer (eg AlSb, GaAs). As a suitable end point control, the removed atoms are detected by mass spectrometry. In this way, for example, the transition from GaSb to AlSb can be determined when the Ga signal disappears. In this way, GaSb quantum dots were produced on AlSb and GaAs substrates.

The ion sputtering plant, acc. FIGS. 3 and 4, which was used to produce the structures, is a mass spectrometry system for depth profile analysis (INA3 from Leybold). The system consists of a plasma chamber 20 and a sample transfer chamber. Both chambers are UHV compatible. The plasma chamber 20 can be filled with argon gas via a metering valve. The valve controls the argon pressure in the chamber with a control mechanism. A plasma is ignited in the plasma chamber via a coil 22 (rf coil) and an HF generator, which plasma is maintained at pressures of 1 * 10 " ³ mbar to 1 * 10" ⁴ mbar. The plasma consists of Ar ions and electrons in Ar gas.

A piece of 8 x 8 mm is cut out of a GaSb wafer ((100) orientation ± 0.5 °, undoped, 50.8 mm diameter, 500 μm thick, front side "Epi-ready", supplied by Crystec) and cut into a sample holder 24 installed (see Fig. 2 left). The sample 26 is pressed onto the copper block 30 by means of a Cu orifice 28 (orifice diameter: 2 mm). During the sputtering process, the copper block of the sample holder is cooled by the continuous flow of water or liquid nitrogen or another coolant which is particularly suitable for generating lower temperatures. By default, sample temperatures from - 80 ° C to + 60 ° C are set. The sample holder is transferred from the transfer chamber to the plasma chamber. The GaSb, ie the sample 26, in the sample holder 24 is separated from the Ar plasma by a front panel (see FIG. 3).

When a voltage ÜB of 500 V is applied, the ion etching see plasma and back copper block 30 of the sample holder. The voltage accelerates Ar ⁺ ions from the plasma to a kinetic energy of 500 eV and hits the sample 26, ie the GaSb piece, perpendicular to the surface. The current that is measured here indicates the total ion current from the plasma to the sample and aperture. This amounts to 0.4 mA when manufacturing the nanostructures on GaSb. The ion current that is responsible for sputtering can be estimated at 0.15 mA, which corresponds to an ion current density of lxlO ¹⁶ cm- ² s ^_1 . For the manufacture of the structures shown in FIG. 1, sputtering takes 300 seconds, which corresponds to a total dose of approximately 3 × 10 ¹⁸ cm ² .

The plasma chamber 20 also has a mass spectrometer 34 in which the sputtered atoms that ionize in the plasma are detected. During the sputtering, the elements present in the material are tracked (e.g. Ga, Sb and Al in the case of a GaSb on AlSb layer structure) in order to determine the time of demolition in the case of multilayer structures made of different materials, so that isolated GaSb islands are formed on the surface.

The production of the nanometer structures on the sample 26 from GaSb was also shown in a commercial sputtering system (PLS 500 P from Balzers, ECR ion source 40 RRISQ 76 ECR from Roth and Rau, sketch see FIG. 3). In this system the sputtering is done by bombarding the sample 26 with a neutralized ion beam. The ion beam is generated in an ECR plasma chamber 20 (ECR = electron cyclotron resonance). The plasma chamber 20 is separated from the process chamber 38 by a grid system 36. The first grid serves to shield the plasma, while the second grid accelerates the ions. The ions extracted from the plasma are neutralized by bombardment with an electron beam from a so-called plasma bridge neutralizer 42. A GaSb sample, cut from a GaSb wafer ((100) orientation ± 0.5 °, undoped, 50.8 mm diameter, 500 μm thickness, front side "Epi ready" po- lated, supplied by Crystec) is applied to a sample holder 24, which is attached at a distance of 10 cm from the accelerating grid of the ion source. A pressure of <10 " ⁵ mbar is in the process chamber 38. Sputtering is carried out under perpendicular incidence with a kinetic energy of the Ar atoms of 500 eV. The sample holder 24 is cooled while the process is being carried out and regulated to room temperature. This is done with this method processed samples show a homogeneous distribution of the quantum dots on an area of 1cm x 1cm.

43 designates a gas inlet which opens into the plasma chamber 20. The systems are located in a vacuum chamber, the walls of which are labeled 44. 46 is a microwave arrangement.

The formation of the morphology was carried out and verified on GaSb and InSb. From the explanatory approaches, the following condition must be placed on the material: the material must consist of at least two components with different sputtering rates. This is the case, for example, with elements with different masses or surface binding energies. In addition, the element enriched on the surface must show a tendency to accumulate, in particular it should not wet the surface. The concentration of the atoms from a catchment area then determines the density and the size of the resulting structures. For the targeted lithography of materials that do not themselves show the self-organized formation of nanostructures in ion sputtering, it is still possible to use a morphology of a layer GaSb applied to this material with a thickness of a few 10 nm to many 100 nm, preferably at least 250 nm ( epitaxially or amorphously) to the underlying substrate by further ion etching. The nanometer structures achieved are largely homogeneously distributed.

The sputter rate (or removal rate) is defined as the number of sputtered (removed) atoms per incident primary ion: SR = removed Particles / incident particles. The sputtering rate of the preferentially sputtered element should be at least 3%, preferably at least 5%, in particular at least 7% and possibly at least 10% greater than that of the other element. This criterion also has an approximate correspondence in the mass numbers of the elements: the atomic masses of the elements of the compound semiconductor differ by at least 10%, preferably by 20% and in particular by 50%.

The average penetration depth of Ar ions with a kinetic energy of 500 eV in GaSb is 2 nm with a maximum range of 5 nm, i.e. smaller than the extension of the nanometer structures. The destruction of the crystal structure by the incident ions is therefore limited to the surface, which means that the structures remain crystalline. This is also the prerequisite for using these structures as quantum dots. If the penetration depth of the ions is greater than the resulting structures, the structures will be amorphized by the destruction of the crystal structure. With GaSb this can already be expected with Ar ions with an energy of 2 keV. In general, the penetration depth and destruction by the primary ions must be determined or estimated for each material. The penetration depth of the ions is preferably chosen to be smaller than the size of the nanometer structures.

In experiments, structures were made on GaSb at two substrate temperatures:

With water cooling, a substrate temperature of 60 +/- 5 ° C was set during the ion removal. The density of the resulting structures is 5 * 10 ¹⁰ cm- ² , which corresponds to an average distance of the quantum dots of 50 nm. The formation of the structures is a continuous process that begins with small structures, whereby the density does not change during the ion etching. The size of the structures depends on the sputtering time and thus on the ion dose or thickness of the removed layer. The maximum size of 50 nm is reached when the structures bump into each other, so they can no longer enlarge. This steady state is reached with a removed layer of 500 nm. The surface morphology is then transferred into the volume to depths of a few μm.

FIGS. 6 to 8 show the size distributions of the nanometer structures after different sputtering times (the ion etching times increase from FIG. 6 to FIG. 9). The narrow size distribution of the structures indicates a self-ordering principle and is better than many size distributions of self-organized quantum structures in epitaxial growth shown in the literature. The narrow size distribution is one of the primary requirements for quantum dots, since it determines the spectral width of the photoluminescence and thus the efficiency of a laser based on these structures.

When cooled with liquid nitrogen, the sample has a temperature of - 80 ° C to -50 ° C. The structures that were produced at these temperatures have a somewhat greater density of 1.4 * 10 ⁿ cm ^-2 with an average distance of 30 nm. Otherwise, the same behavior and the same narrow size distribution are observed.

The structures are very evenly distributed over large areas. You can find the same density of structures on the edge and in the middle of a sputtered surface. The hexagonal short-range order means that the structures each have 6 neighbors, with no perfect hexagonal structure being present, i. H. you can also find structures with 5 or 7 neighbors. If you do an analysis of the order, you will find a clear correlation over a radius of 6 * 50 nm = 300 nm (this corresponds to the so-called correlation length), but no more (similar to a diffraction pattern of a liquid).

Claims

1. A method for producing regular nanometer structures, in particular regular pyramid and wave structures, with a narrow size distribution and dimensions of 2 to 100 nm, in particular 10-60 nm in diameter or width and height, on semiconductor surfaces, thereby characterized in that a semiconductor material is used which consists of at least two and preferably two components, i.e. is a compound semiconductor and - optionally neutralized - noble gas ions from an ion source with an energy of 10 to 50,000 eV, in particular 50-2000 eV are directed to this compound semiconductor material , with which the surface of the material is removed to the extent that the nanometer structure is present under vacuum using ion sputtering.

2. The method according to claim 1, characterized in that the at least two components of the semiconductor material have different removal rates for the used - and optionally neutralized - sputtering, the removal rate of the preferentially sputtered component at least 3%, preferably at least 5%, in particular at least 7% and may be at least 10% larger than that of other Ren component and the component with the lower removal rate, which accumulates on the surface, shows a tendency to accumulate on this surface, in particular only incompletely wetted the surface.

3. The method according to claim 1, characterized in that the at least two components of the semiconductor material have different atomic masses, the atomic mass of one component being at least 10%, preferably 20% and in particular 50% lower than that of the other component and the Component, which accumulates on the surface, shows a tendency to accumulate on this surface, in particular the surface is only incompletely wetted.

4. The method according to claim 1, characterized in that a compound semiconductor, in particular a III-V compound semiconductor and preferably GaSb, GaP, InP or InSb is used as the semiconductor material.

5. The method according to claim 1, characterized in that Ar ⁺ ions are used as the ion source.

6. The method according to claim 1, characterized in that a plasma ion sputtering system is used.

7. The method according to claim 1, which is used in particular for the production of quantum dots, namely insulated islands of a semiconductor material on a lower layer of another semiconductor material, characterized in that a material from at least two layers is used as the starting material for the processing, which in particular by epitaxial Growing a second layer is obtained.

8. The method according to claim 5, which preferably serves to produce quantum dots, characterized in that GaSb, GaP, InP or InSb as the material on a semiconductor material with a larger band gap than this material, in particular GaSb on AlSb, GaSb on GaAs or GaSb GaP is used.

9. The method according to claim 5, in particular for the production of multilayers of quantum dots on a substrate, characterized in that a combination of a system for ion sputtering and deposition is used and, alternately with the system on the substrate, a compound semiconductor material, in particular GaSb material, InSb or InP is removed and deposited.

10. The method according to claim 1, characterized in that for the transfer of the nanostructures to an underlying material, a compound semiconductor material is first applied to the underlying material and the nanometer structure is transferred by machining this compound semiconductor material and by continued removal to the underlying material.

11. The method according to claim 8, characterized in that GaSb with a layer thickness of at least a few 10 nm up to many 100 nm, preferably at least 250 nm is used as the material to be removed and that GaSb is applied epitaxially or amorphously to the underlying material.

12. Use of the method according to claim 1 in the manufacture of optical or electronic components, in particular the use of multilayers of quantum dots in such components.

13. Use of the method according to claim 1 of a nanometer texturing of surfaces to increase the efficiency of light-emitting diodes or Solar cells

14. Device for performing the method according to one of claims 1 to 9, characterized in that a sample holder (24) for a sample (26) with at least one surface layer made of a compound semiconductor material is arranged in a vacuum chamber (wall 44) and that an ion source is provided with which noble gas ions with an energy of 50-2000 eV can be generated and whose ion beam is directed onto the sample (26).