CN1993869A - Optical semiconductor device - Google Patents

Abstract

The present invention provides an optical semiconductor device comprising a semiconductor thin film (4) having photoconductivity and a pair of electrodes (5) and (10) for applying an electric field to the inside of the semiconductor thin film (4) in a direction substantially perpendicular to the surface of the semiconductor thin film (4), wherein the semiconductor thin film (4) generates an electromagnetic wave when light acts on a region of the semiconductor thin film (4) to which the electric field is applied. The electrodes are provided on the front and back surfaces of the semiconductor thin film (4) with the semiconductor thin film interposed therebetween.

Description

Optical semiconductor device

technical field

The present invention relates to an optical semiconductor device used as a device for generating and detecting high-frequency electromagnetic waves mainly in the range from millimeter waves to terahertz waves by light irradiation.

Background technique

In recent years, a nondestructive sensing technology using electromagnetic waves (30 GHz to 30 THz) ranging from millimeter waves to terahertz (THz) waves has been developed. Technologies being developed in the field of application of electromagnetic waves having this frequency band include imaging using a safe radioscopic inspection system instead of an X-ray system for obtaining absorption spectra or complex complexes inside substances. Spectroscopic techniques for assessing the coupling state of atoms, techniques for analyzing biomolecules, and techniques for assessing carrier concentration or mobility.

JP10-104171A discloses a THz generating device suitably employing a photoconductive element in which an antenna serving also as an electrode is provided on a photoconductive thin film deposited on a substrate. Shown in FIG. 10 is a detector 132 as an example structure of a photoconductive element. The substrate 30 has, for example, a radiation-treated silicon-on-sapphire structure in which a silicon film as a photoconductive material is deposited on the sapphire substrate. In general, LT-GaAs grown on a GaAs substrate at a low temperature is also often used as a photoconductive film. The dipole antenna 138 formed on the surface includes a pair of dipole antenna feed lines 138a and 138b and a pair of dipole antenna arm portions 139a and 139b. The light pulses converge at the gap 133 . When a voltage is applied across the gap, a THz pulse is generated. THz pulses can be detected when no voltage is applied to detect the photocurrent. The substrate lens 136 is used to couple the slab mode (substrate mode) of the electromagnetic wave confined in the substrate 130 to the free space radiation mode, and is also used to control the radiation angle in the electromagnetic wave space propagation mode.

The above illustrates by way of example the propagation of electromagnetic waves into space using the photoconductive element alone. On the other hand, on January 7, 2002, "Applied Physics Letters", Vol. "Volume 84, No. 12 (Applied Physics Letters, vol.84, no.12) pp. 2049-2051 discloses a small functional device in which a semiconductor film used as a photoconductive element and used for transport generation The transmission paths of electromagnetic waves are integrated onto a single substrate. This device has a structure in which a thin film including only an epitaxial layer of a photoconductive element formed of LT-GaAs is transferred onto a part of a high-frequency transmission path formed on a substrate. According to Applied Physics Communications, Vol. 80, No. 1, pp. 154-156, Jan. 7, 2002, a microstrip line is formed on a Si substrate with an insulator resin interposed therebetween. A gap is formed in a portion of the wire. The LT-GaAs thin film is disposed only at the bottom of the gap. On the other hand, according to Applied Physics Communications, Vol. 84, No. 12, pp. 2049-2051, March 22, 2004, coplanar strip lines are formed on a quartz substrate. The LT-GaAs thin film is partially arranged to pass through the gap between the two lines.

Each device performs driving so that it can propagate through space, applying a laser beam from the surface side of the substrate to the metal line gap to allow the generated THz electromagnetic wave to propagate through the line.

Contents of the invention

In the above-mentioned integrated device, it is difficult to increase coupling efficiency and maintain stability because the laser beam is aligned through spatial coupling and photoconductive elements. Since the gap portion of the photoconductive element is formed on the substrate surface, it is impossible to apply light laterally. This prevents alignment-free implementation by providing optical waveguides on the substrate. In other words, there are still issues to be solved such as sensing sensitivity, total measurement time, and manufacturing cost.

In addition, since the electric field is applied through the surface electrodes, the internal electric field of LT-GaAs is not uniform and cannot effectively control the behavior of carriers excited by the laser beam. In addition, in terms of gap intervals, precise photolithography is required to form narrow gaps of 1 micron or less with high reproducibility for controlling electric field intensity, thus resulting in an increase in manufacturing cost. Both single photoconductive elements and integrated devices that allow the propagation of electromagnetic waves through space suffer from similar problems.

The present invention provides a photoconductive element and an integrated device including a photoconductive element and a THz transmission path, etc., wherein the gap interval between electrodes and the electric field on the gap can be easily controlled with a simple structure. The present invention also provides a structure for improving and stabilizing the coupling efficiency at the time of coupling with light to be applied to the gap portion.

The present invention provides an optical semiconductor device comprising a semiconductor thin film having photoconductivity and a pair of electrodes for applying an electric field to the inside of the semiconductor thin film in a direction approximately perpendicular to the front surface of the semiconductor thin film, wherein, The semiconductor thin film receives light to generate electromagnetic waves in one region thereof, which is a region to which an electric field is applied.

In addition, the present invention provides a sensing device, which includes an electromagnetic wave generating device and an electromagnetic wave detecting device for detecting electromagnetic waves, the electromagnetic wave generating device has a semiconductor thin film with photoconductivity and a pair of electrodes, and the pair of electrodes is used for An electric field is applied to the inside of the semiconductor thin film in a direction substantially perpendicular to the surface of the semiconductor thin film, and the semiconductor thin film receives light in a region to which the electric field is applied to generate an electromagnetic wave, wherein the sensing is performed by the electromagnetic wave in the transmission path. An object causes a change in the state of electromagnetic wave propagation to obtain information about the state of that object.

According to the optical semiconductor device of the present invention, it is possible to provide a photoconductive element in which the gap pitch between electrodes and the electric field on the gap can be controlled with a simple structure. In addition, it is possible to provide a structure that improves and stabilizes coupling efficiency at the time of coupling with light to be applied to the gap portion. In view of the above aspects, it is possible to provide a terahertz sensing device capable of operating at a low voltage, having a small size, having an easily adjustable sensing optical system, and having high stability and signal-to-noise ratio. With such a sensing device, it is possible to perform analysis of biological materials, organic substances, or semiconductor materials, non-invasive or non-contact sensing of a human body, sensing of a position change of an object, etc., at low cost and with high sensitivity.

Other features and advantages of the present invention will become more apparent from the following description in conjunction with the accompanying drawings, wherein like reference numerals indicate the same or similar components throughout the present invention.

Description of drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

1 is a structural diagram of an integrated device according to a first embodiment of the present invention, and FIG. 1A is a cross-sectional view along 1A-1A in FIG. 1;

2A, 2B, 2C, 2D, 2E and 2F illustrate the method for manufacturing an integrated device, and Fig. 2BT, 2DT, 2ET and 2FT are top views of Fig. 2B, 2D, 2E and 2F, respectively;

Fig. 3 illustrates the driving optical system of the integrated device according to the present invention;

FIG. 4 illustrates the configuration of an integrated device and a driving system according to a second embodiment of the present invention;

5 is a structural diagram of an integrated device according to a third embodiment of the present invention, and FIGS. 5B and 5C are cross-sectional views along 5B-5B and 5C-5C in FIG. 5, respectively;

6 is a cross-sectional view of a photoconductive element portion according to a fourth embodiment of the present invention;

7 is a cross-sectional view of a photoconductive element portion according to a fifth embodiment of the present invention;

8 is a plan view of an integrated device according to a sixth embodiment of the present invention;

FIG. 9 illustrates an optical system according to a sixth embodiment of the present invention;

Fig. 10 illustrates a conventional photoconductive element.

Detailed ways

The present invention is characterized in that an electric field is applied to the inside of the semiconductor thin film in a direction substantially perpendicular to the surface of the semiconductor thin film. It is particularly preferable that electrodes or quasi-electrodes are formed on the front and rear surfaces of the semiconductor thin film sandwiched therebetween as a photoconductive film so that a voltage can be applied in the film thickness direction. In the manufacturing method of the example for realizing this structure, it is preferable to form electrodes on the photoconductive film formed on the first substrate, then leave the photoconductive film when bonding to the second substrate, and remove the first substrate. Electrodes or electromagnetic wave transmission lines simultaneously serving as electrodes are then formed on the surface, resulting in a photoconductive element formed on said second substrate.

The electromagnetic wave generated at the photoconductive element can be transmitted as it is to the electromagnetic wave transmission path which also serves as an electrode. The transmission path may be provided with an antenna radiating into space. Alternatively, a photoconductive element of the same structure can be used as a THz detector. Electromagnetic waves propagating through the antenna and the transmission path can be efficiently guided to the THz detector. The electromagnetic wave detecting part and the generating part can be interconnected with each other through the electromagnetic wave transmission path and integrated on the same substrate.

In addition, an optical waveguide may be integrated to couple light from a side surface of a photoconductive element to which a voltage may be applied vertically. Light beams for photoexcitation can be delivered through optical fibers, where the optical fibers can be secured to aligned optical waveguides. This provides a highly stable THz generator/detector that does not require adjustment of the optical system.

The semiconductor thin film to which voltage is applied vertically may be of a single composition, or may have a multilayer structure. Specifically, a Group III-V semiconductor thin film formed of a Group III-V compound is preferable. In the case of a single component, for example, it is suitable to use LT-GaAs grown at a low temperature (200° C. to 300° C.) by a molecular beam epitaxy method. The distance between the electrodes can be controlled by the thickness of the epitaxially grown film, so that the electric field intensity can be set with high reproducibility and high precision. In the case of a multilayer structure, a heterojunction structure is formed to sandwich a light-absorbing layer whose thickness is reduced to about 100 nm and is sufficiently doped between semiconductor layers having a wide bandgap, thereby achieving high carrier mobility at the same time rate and short lifespan. The present invention is not limited thereto, and semiconductors of different conductivity types or energy bands may be stacked. As described above, by controlling the thickness of the epitaxial film to control the gap to which the electric field is applied at the nanometer level, the problem of surface treatment can be solved in terms of reproducibility, cost, and the like.

Embodiments of a photoconductive element and an integrated device including the photoconductive element as an optical semiconductor device for generating and detecting terahertz are explained below. Materials, structures, devices, etc. are not limited to the descriptions provided here. Furthermore, in addition to the description given here, various uses of components and various characteristics of electromagnetic waves generated can be utilized.

first embodiment

According to the first embodiment of the present invention, the photoconductive element for converting light into terahertz electromagnetic waves, the optical waveguide, and the microstrip line are integrated on the same substrate, as shown in FIG. 1 .

A Ti/Au electrode (or an Al electrode) 7 is formed in a partial region on the Si substrate 1 as a ground potential plane (ground layer). A low temperature (LT) grown GaAs epitaxial film 4 with a thickness of 2 micrometers was transferred near the center. The optical waveguide 3 is formed on the left side of the LT-GaAs so that a light beam entering from the outside is efficiently irradiated with light to the side surface of the LT-GaAs film. On the other hand, a Ti/Au microstrip line 5 is formed on the right side of the top with an insulator 2 sandwiched so that electromagnetic waves generated in LT-GaAs can propagate. This line also serves as the upper electrode of LT-GaAs. A patch antenna 6 is formed at the other end portion of the line so that electromagnetic waves 9 can be radiated through space. This antenna is a broadband type and is tapered in shape to efficiently transmit a pulse waveform.

Referring to the cross-sectional view along 1A-1A in FIG. 1 (FIG. 1A), the insulator 2 has open windows 11 only at the top of the LT-GaAs to access the electrode contacts. The right end portion of the insulator 2 has a cutout 12 to obtain a terminal for applying a voltage 8 to the LT-GaAs. The AuGe/Ni/Au electrode 10 for contact with LT-GaAs is fused to Au on the top surface of the ground plane 7 using AuSn solder (not shown).

The above configuration allows the photoconductive element to operate with a uniform electric field applied in a direction perpendicular to the film thickness of LT-GaAs, as shown in FIG. 1 . Furthermore, the gap pitch can be defined by the thickness of the epitaxial film, thus achieving high accuracy and reproducibility. Typically, the size of the substrate is as small as about 1 square centimeter overall. Although the optical waveguide has a cross-section of about 10 square micrometers, the size of the beam entering part can be increased to 50 square micrometers to improve the coupling efficiency. Conversely, the thickness direction can be reduced to 2 microns near the point for irradiation to LT-GaAs and tapered to adjust dimensions (not shown). The microstrip line width is 15 microns. When the transferred LT-GaAs is driven as a photoconductive element, the width of the upper electrode is limited by the insulator window 11 to 10 micrometers. Benzocyclobutene (BCB), polysilane or polyimide are suitable as insulators, which are easy to handle and have low dielectric loss at high frequencies. Thickness was set to approximately 6 microns. The above dimensions or materials are for example only, and the present invention is not limited thereto.

Although a patch antenna is used as an example as an antenna, a dipole antenna, a helical antenna, an F-antenna, or the like may also be used. In addition, by converting the transmission line into a slot line or a coplanar line, a surface-type bow-tie antenna or a slot antenna can be used. Alternatively, Yagi antennas, horn antennas, etc. can be integrated to control directivity.

Referring now to Figures 2A, 2B, 2C, 2D, 2E, and 2F, a method for fabricating the device of this embodiment will be described below. The same elements as in Figure 1 will be identified with the same numerals. It should be noted that Figures 2BT, 2DT, 2ET and 2FT are top views of Figures 2B, 2D, 2E and 2F, respectively.

In FIG. 2A, a 100-nanometer-thick AlAs layer 21 as a sacrificial layer for removing a substrate and a 2-micron-thick LT-GaAs layer 22 grown at a low temperature of 250° C. are deposited on GaAs by a molecular beam epitaxy (MBE) method or the like. grown on a substrate 20.

In FIG. 2B, AuGe/Ni/Au electrodes 10 are formed on the surface of LT-GaAs and alloyed at 40°C. A mechanical polishing of 20 to 120 microns was performed on the GaAs substrate, resulting in a chip 23 of about 200 square microns. In addition, in order to increase the thickness of the chip, the electrode 10 may be gold-plated to have a thickness of 3 micrometers. AuSn solder is formed in a partial region of the ground potential plane 7 previously formed on the Si substrate 1 . Chip 23 is manipulated to place it in the desired position and then fused by heating. When gold plating is performed on the chip side, Sn can then be provided on the surface by electroplating or deposition. Chips can be manipulated one by one using electrostatic tweezers or the like. Alternatively, at the wafer level, a plurality of chips may be temporarily bonded at a desired pitch on a glass substrate or the like, and then fused together.

In FIG. 2C, the GaAs substrate of the fused chip is removed. In this case, GaAs was etched at a rate of 10 μm/min using a mixed solution of hydrogen peroxide and ammonia water when the periphery of the chip 23 was covered with an organic material or the like. The etch stops at the AlAs layer. The AlAs used as the sacrificial layer was removed using concentrated hydrochloric acid.

In FIG. 2D , photolithography and wet etching were performed in a similar manner to that described above to reduce the size of the transferred LT-GaAs to 100 μm×50 μm. The purpose of this etch is also to remove the damaged layer of the LT-GaAs sidewall. Here, a portion 25 of the electrode 10 left after etching GaAs may be removed by performing metal etching.

In Fig. 2E, optical waveguide 3 and insulator 2 for microstrip lines are fabricated by coating BCB and photolithography. The optical waveguide 3 is embedded by first forming a core layer using a photosensitive BCB and then applying a non-photosensitive BCB on the whole. A photosensitive BCB having a higher refractive index than the non-photosensitive BCB is used as an optical waveguide. After the entire surface was planarized by the non-photosensitive BCB, the window 11 for the electrode of LT-GaAs was formed by photoetching and by reactive ion etching (RIE) using a mixed gas of oxygen and CF ₄ .

In FIG. 2F, a Ti/Au electrode serving as an upper electrode of LT-GaAs, as well as a microstrip line and an antenna are formed by a lift-off method.

In the foregoing description, BCB was used as an example. Polyimides can also be similarly used for fabrication. Alternatively, in a method using photosensitive polysilane, an optical waveguide can be fabricated by controlling the degree of polymerization with light exposure to change the refractive index.

FIG. 3 illustrates a driving system of an integrated vertical electric field application type photoconductive element according to the present invention. When using THz pulses as the generating electromagnetic field, an ultrashort pulse of about 100 femtoseconds (fsec) is generated by a titanium sapphire femtosecond laser (titanium sapphire femto second laser) 30, and then split into pump beams (pump beam) with a half mirror 36 ) and the probe beam. The pump beam is coupled to the beam entry of the integrated component 31 according to the invention in order to generate THz electromagnetic waves 35 . A hemispherical lens (Hemispherical lens) 32 for improving beam directivity is incorporated on the upper portion of the antenna for emitting electromagnetic waves. The THz electromagnetic wave pulses pass through the object 33 to be examined and are received at the antenna on the detection side (facing the rear) of the integrated device 34 . The integrated device 34 may have the same structure as the generation-side device 31 . For use as a detector, the current flowing between the electrodes of the photoconductive element is detected without an applied electric field. By adjusting the timing at which the probe beam enters the delay optical system 38, the electromagnetic wave pulse shape can be obtained without the need for high-speed electronic circuits. Measuring amplitude changes, relative propagation delays, Fourier spectrum changes, etc. by waveform analysis can examine the physical properties of the sample 33 and the like. Generally, the intensity of the laser beam is about several mW on average, repeated at about 80 MHz, and the voltage applied to the photoconductive element is about 10 volts. In the case of vertical voltage application according to the present invention, the gap pitch is 2 micrometers, which is smaller than 5 micrometers of the commonly used surface electrode type. Therefore, the applied voltage for obtaining the same electric field strength can be lowered.

Various objects including organic materials, semiconductor materials, living materials, etc. can be inspected in a non-destructive and non-contact manner. From a two-dimensional scan of the object, distribution images can also be obtained.

By inserting a person's fingertip into the propagation path of electromagnetic waves to detect blood vessel patterns or fingerprints, identification of skin conditions, blood flow (eg, blood pulse), blood components, etc., and personal authentication can also be performed.

In addition, the present invention can also be effectively used for radiographic inspection of the thickness, number of sheets, or inside of paper or plastic embedded in an object.

Such an embodiment described above provides a semiconductor device in which a vertical electric field application type photoconductive element made of LT-GaAs is integrated with a microstrip line, an antenna, and an optical waveguide. However, of course, the photoconductive element alone can also function as an electromagnetic wave generating element. In this case, the beam can be applied from the side or can be applied from the top near the upper electrode. To increase radiation efficiency, the upper electrode can be shaped like an antenna.

Although the GaAs family is described here, other semiconductors such as InP or InAs may be used. Alternatively, an organic semiconductor having photoconductivity may be used.

second embodiment

According to the second embodiment of the present invention, a plurality of elements are arranged on the same semiconductor substrate, as shown in an integrated device 40 in FIG. 4 . In the manufacturing process shown in the first embodiment, elements are formed together on the Si substrate 41 at the wafer level, so that arrayed elements can be freely provided depending on the arrangement of the elements or the manner in which the wafer is cut.

More specifically, two optical waveguides 42a and 42b, photoconductive elements 47a and 47b, microstrip lines 43a and 43b, and printed dipole antennas 44a and 44b are arranged in parallel. Although the arrangement pitch is described here as 1 cm, the present invention is not limited thereto. In this case, the THz electromagnetic wave includes a component propagating through space as indicated by arrow 46, so that the changing state of the electromagnetic wave reflected on the object 45 can be detected.

As for the optical system, although a femtosecond laser as shown in the first embodiment may be used, here two semiconductor lasers 48 are used for cost and size reduction. When the oscillation wavelengths of the single-mode oscillations of the two semiconductor lasers were slightly different, the coupler 49 mixed the laser beams to be applied to the input end of the optical waveguide 42a, so that the photoconductive element 47a could produce a frequency corresponding to the difference frequency (beat, beat) components of electromagnetic waves. By changing the amount of the difference frequency when it is stable, the oscillation frequency of the THz electromagnetic wave of CW can be varied in the range of about 0.1 THz to 3 THz. On the receiving side, the photoconductive element 47b uses its light mixing function to detect the intensity of the electromagnetic wave received through the antenna 44b when it is incident on the optical waveguide 42b through the timing adjuster 50 . The timing adjuster 50 performs phase adjustment of light mixing. Detection while changing the beat frequency can measure the reflection spectrum of the object to identify physical properties as in the first embodiment.

Using reflectance measurements, it is possible to probe the physical properties of a material with a relatively large absorption rate, its film thickness, its surface roughness, etc. Regarding the application to the human body, it is possible to check the skin surface state such as roughness or aging state, pore state, water content, collagen, fat distribution, blood flow or substances in the blood, etc.

Alternatively, by measuring the delay time of the reflected electromagnetic wave, it is also possible to sense the position change of the object 45 . In this case, a semiconductor laser is modulated with a triangular wave of tens of megahertz (MHz) to perform position identification using THz electromagnetic waves according to the known FMCW method, which can lead to high-resolution (<100 micron) position telemetry devices .

Figure 4 schematically illustrates an optical system. When the laser beam source does not deliver ultrashort pulses, however, optical fibers are used for propagation, which is more advantageous in terms of efficiency and space. Coupling to integrated devices can be accomplished by direct butt coupling, inserting lenses, or providing and securing pigtails at the end of the fiber in an alignment that increases coupling efficiency, thus reducing the overall cost of the device including the light source. size. It is also possible not to combine with the optical fiber system, and the semiconductor laser can be installed on the planar optical circuit on the Si substrate, and integrated with the integrated device integrated with the photoconductive element.

third embodiment

According to a third embodiment of the present invention, as shown in FIG. 5, two photoconductive elements are integrated on the same substrate. This allows sensing devices to work on the substrate for sensing objects placed close to high-frequency transmission lines.

The photoconductive element 53a is similar to that in the first embodiment: a vertical electric field is applied between the wire 54 and the lower electrode 52a, and a THz electromagnetic wave excited by light propagating through the optical waveguide 51a propagates through the wire 54. In this embodiment, no ground potential plane is provided and the high frequency transmission line is a single line comprising signal lines 54 embedded upwards and downwards in insulator 55 . Electrodes 52a and 52b are isolated, each having a thickness of about 3 micrometers, and plated with gold for adjusting the height of the photoconductive element.

Therefore, the manufacturing process is almost the same as that of the first embodiment, except that the electrodes 52a and 52b on the sides of the Si substrate 1 are patterned and increased in thickness, and a transmission line 54 also serving as an electrode is formed, which is then formed on the top Covered by the same insulator for about 3 microns. This can be readily understood with reference to the cross-sectional view along 5B-5B (FIG. 5B) and the cross-sectional view along 5C-5C (FIG. 5C). As shown in the cross-sectional view along 5B-5B, since the insulator 55 is embedded after the wire 54 is formed, a via electrode 57a is provided for obtaining an electrode contact. In addition, as shown in the cross-sectional view along 5C-5C, the middle portion of the transmission line is provided with a hole 56 for increasing the penetration of the propagating electromagnetic wave. The material to be inspected is placed in this hole to be inspected efficiently.

The external optical system is similar to that in the first embodiment: the pump beam generated by the Ti:Sapphire femtosecond laser is guided to the THz electromagnetic wave generation side waveguide 51a, and the probe beam is guided to the detection side waveguide 51b through the delay optical system.

In this system, a change in propagation state on the transmission line 54 is detected at the photoconductive element 53b, so that objects on the transmission path can be identified. At the photoconductive element 53b on the detection side, the generated photocurrent can be measured without applying an electric field, as in the first embodiment.

The electromagnetic wave penetrates into the hole 56 containing the sample as an evanescent wave, and the state of propagation changes according to the properties of the contained object. Specifically, as a change in the signal, attenuation of electromagnetic waves, absorption of specific frequency components, or propagation delay occurs. This results in a significant size reduction compared to conventional measurements using space optics, reducing the number of components, improving stability and light coupling efficiency, and increasing the signal-to-noise ratio.

In addition, liquids, powders, etc. that cannot be measured conventionally are simply placed in the hole 56 to be measured. For example, biologically derived materials such as amino acids, deoxyribonucleic acid (DNA) or proteins should be measured in solution for analysis. The present invention provides biosensors that utilize high-frequency dielectric properties in a solution state for evaluations that have traditionally been difficult.

It should be noted that although a femtosecond laser has been described as an optical system, a semiconductor laser and an optical fiber can be used to reduce the overall device size, as shown in the second embodiment.

Fourth Embodiment

According to a fourth embodiment of the present invention, there is provided a structure that allows increased field strength as well as high-speed movement of electrons.

Fig. 6 is a cross-sectional view of a photoconductive element portion of an integrated device like the first embodiment. The structure is the same as in the first embodiment except for the photoconductive element in which the transferred epitaxial film has a multilayer film structure. The figure shows n-type Al0.08 Ga0.92As marked as "65" grown at normal growth temperature (approximately 600°C), and n-type Al0.2 Ga0.8As marked as "66" marked as "67" Undoped GaAs, n-type Al0.08 Ga0.92As marked as "68" and n-type Al0.2 Ga0.8As marked as "69". The n-type layer is highly doped. Specifically, "65" and "69" make the electrode contacts have low absorption for excitation light at 800 nm. To obtain a total thickness of approximately 2 microns, make "66" and "68" 1 micron thick and the others 100 nanometers thick. Absorption of excitation light occurs in the undoped GaAs absorbing layer 67 having a thickness of 100 nm, so that the generated carriers can move rapidly without almost colliding with the crystal lattice. This embodiment uses room temperature growth, which provides better crystallinity compared to LT-GaAs. Therefore, the mobility of the carriers is high and the lifetime of the carriers can be shortened because the generated carriers quickly reach the N-type layer from the absorbing layer. In addition, the absorbing layer used as an electric field applying layer is thin, thus enabling operation at low voltage. In addition, the carrier trapping of the defect layer caused by the accumulation of arsenic (As cluster) is not used, resulting in reduced heat generation and improved film quality. Since there are fewer variations that are common in the case of using LT-GaAs, device characteristics are stable and yields are high. Conventionally, although photoconductive elements for THz generation exhibit excellent characteristics, they cannot enhance excitation efficiency due to thin absorbing layers. This lateral incidence implementation provides longer absorption, substantially increasing efficiency.

It should be noted that although an n-i-n structure is exemplified here, an m-i-n (metal-intrinsic-n type) structure or a p-i-n structure using a Schottky junction may be employed to apply an opposite electric field. The composition and thickness of the layers are not limited to those described here.

Fifth Embodiment

According to a fifth embodiment of the present invention, there is provided a vertical electric field application type photoconductive element without the transfer process in the first embodiment.

Figure 7 illustrates its cross-sectional view. Semi-insulating GaAs70 is used as the substrate. The n-type Al0.08Ga0.92As layer 71 and the LT-GaAs layer 72 used to manufacture electrode contacts are epitaxially grown by MBE method. The insulating layer 73 and the upper electrode 74 are similar to the corresponding elements in the first embodiment.

For the bottom electrode, the n-type layer 71 is highly doped to obtain a metal-like effect, and a buried electrode 75 makes contact. The electrode 75 is configured such that an alloy electrode is composed of AuGe/Ni/Au at a portion in contact with n-type AlGaAs, and its thickness is increased by gold plating. The contact layer is made of AlGaAs to reduce absorption of light used for excitation.

The photoconductive element configured as described above may be used alone. Alternatively, it may be configured as an integrated device as in the first to third embodiments. In the present method, which does not use transfer, the manufacturing process can be greatly simplified.

Sixth Embodiment

8 is a plan view of an integrated device including the photoconductive element according to the sixth embodiment, the electromagnetic wave transmission path, and the antenna according to the present embodiment.

The surface of the substrate 81 was coated with a thin film of Au and BCB serving as a ground electrode, with an overall thickness of 3 micrometers. A portion of the area 80 is free of BCBs to create a ground electrode. A 1 mm square LT-GaAs thin film 89 is transferred as in the first embodiment. A BCB window of 500 square micrometers is provided to the partial region 90 to expose the surface of the LT-GaAs thin film. The upper electrode 88 is provided so that a bias voltage can be applied from the window region 90 to the surface of the LT-GaAs through a Ti-Au line 87 having a width of 20 micrometers. The microstrip line 84 made to have a width of 5 micrometers for transmitting terahertz electromagnetic waves generated by irradiating LT-GaAs with a short-pulse laser beam has a characteristic impedance adjusted to about 70Ω. A patch antenna 82 of approximately 90 square micrometers is provided for radiating terahertz electromagnetic waves propagating through the microstrip line into space. The cutout 83 is formed for impedance matching so that the feed point from the transmission path reaches a depth of about 30 micrometers relative to the end surface of the antenna.

The laser beam was applied to a spot approximately 5 microns near the top electrode of the LT-GaAs, as shown at position 86 in the enlarged view. A low-pass filter 85 may be provided to a part of the upper electrode for preventing high-frequency electromagnetic waves from propagating to the electrode pad 88, as shown in an enlarged view of FIG. 8 . A portion of the Ti/Au wire 87 near the position 86 for laser beam irradiation may have a cutout or a bulge in order to improve generation efficiency.

The method of driving the sensing using terahertz electromagnetic waves can be the same as the above-mentioned embodiment. FIG. 9 illustrates an example optical system using this device. The photoconductive element portion 91 of the integrated device 90 according to this embodiment is irradiated with a beam 97 from a femtosecond laser through a mirror 95 and a lens 96 . The terahertz electromagnetic wave generated from the integrated device 90 propagates through space through two parabolic mirrors 92 and 93 , as shown at 98 , and is then received by a detector 94 . Here, the femtosecond laser beam can be branched and propagated through a delay system (not shown) to a detector 94 for well-known coherent detection (time region spectrum detection, etc.).

Here, impedance matching between the transmission line of the terahertz waveform and the antenna is performed so that electromagnetic waves can be efficiently radiated into space, thus improving sensitivity when sensing objects on the transmission line, as in the first embodiment and the like.

Since many substantially different embodiments can be made without departing from the spirit and scope of the invention, it should be understood that the invention is not limited to the specific embodiments but only by the appended claims.

This application claims the benefit of Japanese Patent Application No. 2004-223656 filed on July 30, 2004 and Japanese Patent Application No. 2005-025210 filed on February 1, 2005, which are hereby incorporated by reference In this application.

Claims (10)

1. optical semiconductor device, it comprises:

Semiconductive thin film with photoconduction; And

Be used for applying the electrode pair of electric field to the inside of this semiconductive thin film in the direction of the front surface that is approximately perpendicular to described semiconductive thin film,

Wherein said semiconductive thin film is applied at it and receives light in zone of electric field to generate electromagnetic waves.

2. optical semiconductor device as claimed in claim 1, wherein said electrode pair are disposed on the front surface and the back side of described semiconductive thin film, and the described semiconductive thin film of sandwich toward each other.

3. optical semiconductor device as claimed in claim 1, wherein said semiconductive thin film is made of the III-V compounds of group.

4. optical semiconductor device as claimed in claim 1 wherein is coated with the semiconductor of different conduction-types at least one surface of described semiconductive thin film.

5. optical semiconductor device as claimed in claim 1 comprises also being used to propagate the electromagnetic transmission path that produces from described semiconductive thin film that wherein said transmission path is electrically connected to one of described electrode.

6. optical semiconductor device as claimed in claim 1 also comprises being used for the electromagenetic wave radiation that will produce from the described semiconductive thin film antenna to the space.

7. optical semiconductor device as claimed in claim 1 also comprises:

Transmission path is used to propagate the electromagnetic wave that produces from described semiconductive thin film; And

Antenna, the electromagenetic wave radiation that is used for producing from described semiconductive thin film be to the space,

Wherein said transmission path and described antenna mutually coupling so that the impedance of the impedance of described transmission path and described antenna each other about equally.

8. optical semiconductor device as claimed in claim 1 further comprises fiber waveguide, and it is used for light is applied to from the side of described semiconductive thin film the described zone that is applied in electric field of described semiconductive thin film.

9. optical semiconductor device as claimed in claim 1, wherein said electromagnetic frequency range at 30GHz between the 30THz.

10. sensing device, it comprises:

Electromagnetic wave generating device is used to generate electromagnetic waves;

Described electromagnetic wave generating device has: semiconductive thin film and the electrode pair that is used for applying to the inside of this semiconductive thin film in the direction on the surface that is approximately perpendicular to described semiconductive thin film electric field that photoconduction is arranged;

Described semiconductive thin film is applied at it and receives light in zone of electric field to generate electromagnetic waves; And

The electromagnetic wave detection device is used to survey electromagnetic wave,

Wherein, the change of the electromagnetic wave propagation state that causes by the object in the electromagnetic wave transmission path of sensing.

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