WO2013175528A1 - Photoconductive substrate and photoconductive element - Google Patents

Photoconductive substrate and photoconductive element Download PDF

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Publication number
WO2013175528A1
WO2013175528A1 PCT/JP2012/003366 JP2012003366W WO2013175528A1 WO 2013175528 A1 WO2013175528 A1 WO 2013175528A1 JP 2012003366 W JP2012003366 W JP 2012003366W WO 2013175528 A1 WO2013175528 A1 WO 2013175528A1
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photoconductive
substrate
layer
insulating layer
gaas
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PCT/JP2012/003366
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French (fr)
Japanese (ja)
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喜彦 加茂
大島　清朗
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パイオニア株式会社
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Priority to PCT/JP2012/003366 priority Critical patent/WO2013175528A1/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/184—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof the active layers comprising only AIIIBV compounds, e.g. GaAs, InP
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
    • H01L31/09—Devices sensitive to infrared, visible or ultraviolet radiation

Definitions

  • the present invention relates to a photoconductive substrate and a photoconductive element used for generation or detection of terahertz waves.
  • a photoconductive antenna element photoconductive element
  • a semi-insulating GaAs gallium arsenide substrate
  • a GaAs layer photoconductive layer formed on a semi-insulating GaAs substrate by low temperature molecular beam epitaxy
  • a GaAs layer A device including a pair of ohmic electrodes formed thereon is known (see Patent Document 1).
  • the pair of ohmic electrodes has an antenna shape and forms a photoconductive antenna portion.
  • a terahertz wave is generated by irradiating the photoconductive antenna portion with light having a predetermined wavelength. Further, the terahertz wave is detected by making the terahertz wave incident on the photoconductive antenna portion.
  • the terahertz wave cannot be effectively used because the transmissivity of the semi-insulating GaAs substrate with respect to the terahertz wave is low (because of high absorption). Specifically, when a semi-insulating GaAs substrate is used, the use efficiency of terahertz waves in generation / detection is lowered, and the S / N ratio (Signal to Noise ratio) or dynamic range of the terahertz wave spectrum is lowered. In addition, since a semi-insulating GaAs substrate hardly transmits a terahertz wave of 5 THz or more, a terahertz wave of 5 THz or more cannot be used.
  • the photoconductive substrate of the present invention is characterized by comprising a substrate, an insulating layer formed on the substrate and made of an insulator, and a photoconductive layer formed on the insulating layer.
  • an insulating layer made of an insulator is interposed between the substrate and the photoconductive layer, so that a current when a voltage is applied or a generated current when a terahertz wave is irradiated leaks to the substrate side. Can be avoided. Therefore, it is possible to prevent the S / N ratio or dynamic range of the terahertz wave spectrum from being lowered. In addition, since the leakage current is suppressed by using an insulating layer made of an insulator, it is not necessary to use a highly toxic semi-insulating GaAs substrate, and the permeability to terahertz waves can be improved.
  • terahertz waves when a Si substrate is used as a substrate and a Si oxide film is used as an insulating layer, high transmission with respect to terahertz waves can be obtained, and terahertz waves of 5 THz or more can be used. As described above, with a simple configuration, the leakage current can be suppressed while improving the permeability to the terahertz wave.
  • the substrate is composed of either Si or Ge.
  • the insulating layer is formed of one of an Si oxide film and a Ge oxide film corresponding to the substrate.
  • the transparency to the terahertz wave is further improved. Can be improved. In addition, the manufacturing cost can be reduced.
  • the insulating layer preferably has a thickness of 10 nm to 3 ⁇ m.
  • the thickness of the insulating layer is set to 10 nm or more, that is, the insulating layer has a thickness equal to the applied voltage to the antenna formed on the photoconductive layer.
  • the thickness of the insulating layer is 3 ⁇ m or less, that is, the thickness of the insulating layer is 1/10 or less with respect to the wavelength of the terahertz wave (specifically around 3 THz) propagating through one of the substrate and the photoconductive layer. By doing so, the influence of multiple reflection can be reduced as much as possible.
  • the photoconductive layer is preferably bonded to the surface of the insulating layer on the insulating layer side in the entire length and width.
  • the bondability can be improved and the heat generated on the photoconductive layer side can be effectively released to the substrate side. be able to. That is, it is possible to use the substrate as a heat sink by avoiding current leakage to the substrate and releasing heat to the substrate.
  • the photoconductive layer is preferably composed of a III-V group compound semiconductor.
  • the photoconductive layer is preferably made of GaAs.
  • a photoconductive layer suitable for generation / detection of terahertz waves can be easily formed.
  • the photoconductive element of the present invention includes the above-described photoconductive substrate and an antenna formed on the photoconductive substrate.
  • This photoconductive element is a semiconductor element that functions as an electromagnetic wave generating element by applying a voltage to the photoconductive element, and also functions as an electromagnetic wave detecting element by connecting an ammeter.
  • the photoconductive element has a configuration capable of suppressing leakage current while improving the transmittance with respect to terahertz waves.
  • the terahertz wave defined here is a concept including not only a narrowly defined terahertz wave (electromagnetic wave of 0.1 THz to 10 THz) but also a broadly defined terahertz wave (electromagnetic wave of several tens GHz to several hundred THz). .
  • the photoconductive element 1 includes a photoconductive substrate 2 serving as an element substrate, and an antenna 3 (parallel transmission line) formed on the photoconductive substrate 2.
  • the photoconductive substrate 2 includes a substrate 11, a thin insulating layer 12 formed on the substrate 11, and a thin photoconductive layer 13 formed on the insulating layer 12. That is, the insulating layer 12 and the photoconductive layer 13 are laminated on the substrate 11 having a predetermined necessary plate thickness.
  • the substrate 11 is made of single crystal Si (silicon).
  • the material of the substrate 11 is not limited to Si, and Ge (germanium) may be used.
  • the material is not limited to Si and Ge as long as the material has high transmissivity to terahertz waves (low absorbency).
  • the photoconductive layer 13 is composed of a thin film of low temperature grown gallium arsenide (LT-GaAs) epitaxially grown at a low temperature. Further, the photoconductive layer 13 is joined to the surface of the insulating layer 12 on the insulating layer 12 side of the photoconductive layer 13 in the entire length and width. Excitation light such as a femtosecond pulse laser is incident on the surface of the photoconductive layer 13 perpendicularly. The incident excitation light generates excitation carriers (electrons) in the photoconductive layer 13.
  • LT-GaAs low temperature grown gallium arsenide
  • the layer thickness of the photoconductive layer 13 is set to 1 ⁇ m or more and 2 ⁇ m or less in consideration of the critical film thickness.
  • the material of the photoconductive layer 13 is not limited to GaAs (LT-GaAs), and at least one semiconductor among III-V group compound semiconductors can be arbitrarily used. Specifically, GaAs, LT-AlGaAs (AlGaAs), InGaP, AlAs, InP, InAlAs, InGaAs, GaAsSb, InGaAsP, LT-InAs (InAs), and InSb can be used.
  • the insulating layer 12 is interposed between the substrate 11 and the photoconductive layer 13 and is composed of a Si oxide film (insulator) formed by thermally oxidizing the Si substrate 11.
  • the layer thickness L of the insulating layer 12 is set to 10 nm or more and 3 ⁇ m or less in consideration of dielectric breakdown and multiple reflection due to voltage application.
  • the insulating layer 12 is not limited to the Si oxide film.
  • the insulating layer 12 may be made of a Ge oxide film.
  • the material is not limited to the Si oxide film or the Ge oxide film as long as it has high permeability (low absorption) to the terahertz wave.
  • the antenna 3 is composed of a pair of dipole antenna portions 20, and each antenna portion 20 has a line electrode portion 21 (antenna main body) extending in a strip shape and a counter electrode extending inward from an intermediate portion of the line electrode portion 21. And an electrode part 22.
  • One end portion of the line electrode portion 21 functions as an input / output electrode pad, and is connected to a power source, a current amplifier, and the like via a cable.
  • the pair of antenna portions 20 are arranged such that the line electrode portions 21 are arranged in parallel, and the opposing electrode portions 22 are arranged to face each other with a predetermined gap. That is, a gap portion 23 having a width of several ⁇ m (for example, 5 ⁇ m) is formed between the opposing end portions.
  • the antenna 3 is not limited to the dipole type, and may be, for example, a bow tie type, a stripline type, or a spiral type.
  • excitation carriers When the gap 23 is irradiated with excitation light such as a femtosecond pulse laser in a state where a voltage is applied to the pair of line electrode portions 21, excitation carriers are generated.
  • a pulsed current flows between the pair of counter electrode portions 22 (gap portion 23), and a terahertz wave is generated by the current.
  • the photoconductive element 1 can also be used as a detection (reception) element because a current is generated between the pair of counter electrode portions 22 when receiving the terahertz wave.
  • a current amplifier or the like for detecting current (terahertz wave) is connected to the pair of line electrode portions 21.
  • a GaAs substrate 31 is prepared (FIG. 3A), and a GaAs buffer layer (not shown) is epitaxially grown on the GaAs substrate 31 using an MBE (molecular beam epitaxy) apparatus.
  • the substrate temperature is set to 500 ° C. to 600 ° C.
  • the growth rate is 1 ⁇ m / h
  • the As / Ga supply ratio is 5 to 30
  • the GaAs buffer layer is grown in the range of 0.1 ⁇ m to 0.5 ⁇ m.
  • the GaAs buffer layer is provided to increase the crystallinity of the photoconductive layer 13.
  • the photoconductive layer 13 (LT-GaAs layer) is epitaxially grown at a low temperature on the GaAs buffer layer (FIG. 3B). Specifically, the substrate temperature is lowered to 400 ° C. or lower, and the photoconductive layer 13 is grown in the range of 1 ⁇ m to 2 ⁇ m at a growth rate of 1 ⁇ m / h.
  • the As / Ga supply ratio when the photoconductive layer 13 is grown is preferably equal to or higher than the As / Ga supply ratio when the GaAs buffer layer is grown.
  • heat treatment is performed on the surface of the photoconductive layer 13 (upper side in the figure). Specifically, with the As molecular beam irradiated, the substrate temperature is set to 600 ° C. and heat treatment is performed in the range of 5 minutes to 10 minutes.
  • the front surface (upper side in the figure) of the photoconductive layer 13 is attached to a hold substrate 32 such as glass by a resin adhesive or the like, and the backside (lower side in the figure) GaAs substrate 31 is back-ground (backside polishing).
  • a hold substrate 32 such as glass by a resin adhesive or the like
  • the backside (lower side in the figure) GaAs substrate 31 is back-ground (backside polishing).
  • the GaAs buffer layer is also removed at the same time.
  • a Si substrate 11 is prepared (FIG. 3D), and the surface of the substrate 11 (upper side in the drawing) is subjected to thermal oxidation to form an insulating layer 12 (Si oxide film) (FIG. e)).
  • the insulating layer 12 is formed in the range of 10 nm to 3 ⁇ m.
  • the bonding method may be a structure in which bonding is performed with a resin adhesive that absorbs little against terahertz waves, or a structure in which bonding is performed by van der Waals force (so-called wafer bonding). Further, in the case of bonding by van der Waals force, it is preferable to perform heat treatment after removing the hold substrate 32 in order to increase the adhesion.
  • the adhesive of the hold substrate 32 is dissolved with a solvent and the hold substrate 32 is removed, whereby the formation of the photoconductive substrate 2 is completed (FIG. 3G).
  • the photoconductive element 1 is completed by forming the antenna 3 on the photoconductive substrate 2 by photolithography or etching technique (FIG. 3H).
  • the photoconductive element 1 and the photoconductive substrate 2 may be manufactured one by one by the above manufacturing process.
  • a plurality of photoconductive elements 1 and a plurality of photoconductive elements 1 may be manufactured by the above manufacturing process. It is also possible to form a plurality of substrates that constitute the photoconductive substrate 2 and to divide (dicing) the substrate to obtain a plurality of photoconductive elements 1 and a plurality of photoconductive substrates 2. In such a case, after the formation of the photoconductive element 1 is completed (FIG. 3G), or after the photoconductive substrate 2 antenna is formed (FIG. 3H), the dividing process is performed. Is preferred.
  • a time domain spectroscopic device 40 will be briefly described as an application example of the photoconductive element 1 with reference to FIG.
  • the time domain spectroscopic device 40 places the measurement sample S to be measured in the path through which the terahertz wave propagates, and transmits the time waveform of the transmitted terahertz wave and the terahertz wave without the measurement sample S.
  • the time waveform is Fourier transformed to obtain information on the amplitude and phase of the terahertz wave.
  • fine physical properties such as the complex refractive index and complex dielectric constant of the measurement sample S are measured.
  • the time domain spectroscopic device 40 includes a laser irradiation device 41 that generates a femtosecond laser (excitation light), a beam splitter 42 that separates the femtosecond laser, an electromagnetic wave generation element 1a, an electromagnetic wave detection element 1b, and an electromagnetic wave detection element 1b.
  • a delay optical system 43 that delays an incident femtosecond laser, various optical systems that reflect and collect the femtosecond laser, and a signal processing device 44 that processes an input signal are provided.
  • the time domain spectroscopic device 40 has a general configuration.
  • the electromagnetic wave detecting element 1b and the electromagnetic wave generating element 1a are a combination of the above-described photoconductive element 1 and hemispherical lens.
  • each substrate 11 of the electromagnetic wave detecting element 1b and the electromagnetic wave generating element 1a and the hemispherical lens installed on the surface of each substrate 11 are made of the same material so that the refractive indexes are equivalent. It is preferable to use it.
  • the femtosecond laser (wavelength 800 nm) emitted from the laser irradiation device 41 is divided into pump light and probe light by the beam splitter 42.
  • the pump light is incident on the electromagnetic wave generating element 1a in a state where amplitude modulation is applied.
  • a terahertz wave is generated from the electromagnetic wave generating element 1 a by applying a voltage between the pair of antenna units 20.
  • the terahertz wave is reflected by the first parabolic mirror 45, condensed by the first lens 46, and irradiated on the measurement sample S.
  • the terahertz wave that has passed through the measurement sample S enters the electromagnetic wave detection element 1 b via the second lens 47 and the second parabolic mirror 48.
  • the probe light divided by the beam splitter 42 is irradiated to the delay optical system 43 by a plurality of reflecting mirrors 49, is given a time delay, and enters the electromagnetic wave detection element 1b.
  • a signal detected by the electromagnetic wave detection element 1 b is input to the signal processing device 44.
  • the signal processing device 44 stores the time waveform of the terahertz wave transmitted through the measurement sample S and the time waveform of the terahertz wave in the absence of the measurement sample S as time-series data, respectively, and performs Fourier transform processing on the frequency space. Convert.
  • the signal processing device 44 stores the time waveform of the terahertz wave transmitted through the measurement sample S and the time waveform of the terahertz wave in the absence of the measurement sample S as time-series data, respectively, and performs Fourier transform processing on the frequency space. Convert.
  • the physical properties and the like of the measurement sample S can be examined.
  • the insulating layer 12 made of an insulator (Si oxide film) between the substrate 11 and the photoconductive layer 13, a current and a terahertz wave at the time of voltage application can be obtained. It can be avoided that the current generated during irradiation leaks to the substrate 11 side. Therefore, it is possible to prevent the S / N ratio or dynamic range of the terahertz wave spectrum from being lowered.
  • the insulating layer 12 is used to suppress the leakage current, it is not necessary to use a highly toxic semi-insulating GaAs substrate, and the permeability to terahertz waves can be improved. As described above, with a simple configuration, the leakage current can be suppressed while improving the permeability to the terahertz wave.
  • the thickness of the insulating layer 12 is 10 nm or more, that is, the thickness of the insulating layer 12 is such that the withstand voltage of the insulating layer 12 is equal to or higher than the voltage applied to the antenna 3 formed on the photoconductive layer 13. Thus, it is possible to avoid dielectric breakdown on the insulating layer 12 and to prevent current from leaking. Further, the thickness of the insulating layer 12 is set to 3 ⁇ m or less, that is, the thickness of the insulating layer 12 with respect to the wavelength of terahertz waves (specifically around 3 THz) propagating through one of the substrate 11 and the photoconductive layer 13 is set. By setting it to 1/10 or less, the influence of multiple reflection can be reduced as much as possible.
  • the bonding property can be improved and the heat generated on the photoconductive layer 13 side can be effectively transferred to the substrate 11 side. I can escape. That is, it is possible to use the substrate 11 as a heat sink by avoiding current leaking to the substrate 11 and releasing heat to the substrate 11.
  • Si (silicon) and Si oxide film that easily transmit (less absorb) terahertz waves for the substrate 11 and the insulating layer 12, it is possible to further improve the permeability to terahertz waves. In addition, the manufacturing cost can be reduced.
  • the photoconductive layer 13 suitable for the generation and detection of terahertz waves can be easily formed.
  • the antenna 3 is formed on the photoconductive substrate 2 after the photoconductive substrate 2 is formed (manufactured) in the manufacturing process.
  • the structure in which the antenna 3 is formed after the heat treatment of the conductive layer 13 and before the photoconductive layer 13 is attached to the hold substrate 32 may be adopted.
  • the GaAs substrate 31 is removed by back grinding in the manufacturing process.
  • the GaAs substrate 31 may be removed by selective etching.
  • the GaAs substrate 31 and the GaAs buffer layer are removed in the manufacturing process.
  • the GaAs substrate 31 and / or the GaAs buffer are removed. It may be configured to leave a predetermined amount of layers. That is, a configuration in which the photoconductive substrate 2 and the photoconductive element 1 in a state having (remaining) a predetermined amount of the GaAs substrate 31 and / or the GaAs buffer layer may be manufactured.
  • a predetermined amount of the AlAs etching stop layer may be left as shown in FIG. 5B.
  • the antenna 3 is formed on the photoconductive layer 13, but the photoconductive layer 13 may be formed only on the gap portion 23.
  • each antenna portion 20 is formed on the insulating layer 12, and the tip portions of the pair of counter electrode portions 22 are formed so as to rise from the insulating layer 12.
  • the photoconductive layer 13 is formed only between the pair of raised counter electrode portions 22 (gap portion 23). In such a case, the S / N ratio or dynamic range of the generated terahertz wave spectrum can be further improved.
  • a Si semiconductor lens (lens substrate) may be used as the substrate 11.
  • a Si semiconductor prism prism substrate
  • a Si semiconductor ATR prism ATR prism substrate
  • two sets of the insulating layer 12, the photoconductive layer 13, and the antenna 3 are provided and function as two photoconductive elements 1 as shown in FIG. It is also possible to make it.

Abstract

The purpose of the present invention is to provide a photoconductive substrate capable of, with a simple configuration, suppressing leakage of a current to the substrate side, while improving transmissivity with respect to terahertz waves, said current being a current flowing when a voltage is applied or a current being generated when irradiated with terahertz waves. A photoconductive substrate (2) of the present invention is characterized in being provided with: a substrate (11), such as a Si substrate; a thin film insulating layer (12), which is formed on the substrate (11), and which is configured of an insulator, such as a Si oxide film; and a thin film photoconductive layer (13), which is formed on the insulating layer (12), and which is configured of LT-GaAs or the like.

Description

光伝導基板および光伝導素子Photoconductive substrate and photoconductive element
　本発明は、テラヘルツ波の発生または検出に用いられる光伝導基板および光伝導素子に関するものである。 The present invention relates to a photoconductive substrate and a photoconductive element used for generation or detection of terahertz waves.
　従来、光導電アンテナ素子（光伝導素子）として、半絶縁性ＧａＡｓ（ｇａｌｌｉｕｍ　ａｒｓｅｎｉｄｅ）基板と、半絶縁性ＧａＡｓ基板上に低温分子線エピタキシーによって形成されたＧａＡｓ層（光伝導層）と、ＧａＡｓ層上に形成された一対のオーミック電極と、を備えたものが知られている（特許文献１参照）。一対のオーミック電極は、アンテナ形状とされ、光導電アンテナ部を形成している。そして、この光導電アンテナ部に所定の波長の光を照射することで、テラヘルツ波を発生する。また、この光導電アンテナ部にテラヘルツ波を入射させることで、テラヘルツ波の検出を行う。 Conventionally, as a photoconductive antenna element (photoconductive element), a semi-insulating GaAs (gallium arsenide) substrate, a GaAs layer (photoconductive layer) formed on a semi-insulating GaAs substrate by low temperature molecular beam epitaxy, and a GaAs layer A device including a pair of ohmic electrodes formed thereon is known (see Patent Document 1). The pair of ohmic electrodes has an antenna shape and forms a photoconductive antenna portion. A terahertz wave is generated by irradiating the photoconductive antenna portion with light having a predetermined wavelength. Further, the terahertz wave is detected by making the terahertz wave incident on the photoconductive antenna portion.
特開２００８－２４４６２０号公報JP 2008-244620 A
　しかしながら、このような構成では、テラヘルツ波に対する半絶縁性ＧａＡｓ基板の透過性が低いため（吸収性が高いため）、テラヘルツ波を有効に利用することができないという問題があった。具体的には、半絶縁性ＧａＡｓ基板を用いると、発生・検出におけるテラヘルツ波の利用効率が低くなり、テラヘルツ波スペクトルのＳ／Ｎ比（Ｓｉｇｎａｌ　ｔｏ　Ｎｏｉｓｅ　ｒａｔｉｏ）またはダイナミックレンジが低下してしまう。また、半絶縁性ＧａＡｓ基板では、５ＴＨｚ以上のテラヘルツ波をほぼ透過しないので、５ＴＨｚ以上のテラヘルツ波を利用することができない。
　これに対し、基板として、Ｓｉ基板を用いることも考えた。しかしながら、これでは、電圧印加時の電流あるいは、テラヘルツ波照射時の発生電流が基板側にリークしてしまうため（ＧａＡｓ層上の励起キャリアが基板側に逃げてしまうため）、テラヘルツ波スペクトルのＳ／Ｎ比またはダイナミックレンジが大幅に低下してしまうという問題がある。 However, with such a configuration, there is a problem that the terahertz wave cannot be effectively used because the transmissivity of the semi-insulating GaAs substrate with respect to the terahertz wave is low (because of high absorption). Specifically, when a semi-insulating GaAs substrate is used, the use efficiency of terahertz waves in generation / detection is lowered, and the S / N ratio (Signal to Noise ratio) or dynamic range of the terahertz wave spectrum is lowered. In addition, since a semi-insulating GaAs substrate hardly transmits a terahertz wave of 5 THz or more, a terahertz wave of 5 THz or more cannot be used.
On the other hand, the use of a Si substrate as a substrate was also considered. However, in this case, since the current at the time of voltage application or the generated current at the time of terahertz wave irradiation leaks to the substrate side (excited carriers on the GaAs layer escape to the substrate side), the S of the terahertz wave spectrum There is a problem that the / N ratio or the dynamic range is greatly reduced.
　本発明は、簡単な構成で、テラヘルツ波に対する透過性を向上しつつ、リーク電流を抑えることができる光伝導基板および光伝導素子を提供することを課題としている。 It is an object of the present invention to provide a photoconductive substrate and a photoconductive element that can suppress leakage current while improving the permeability to terahertz waves with a simple configuration.
　本発明の光伝導基板は、基板と、基板上に形成され、絶縁体で構成された絶縁層と、絶縁層上に形成された光伝導層と、を備えたことを特徴とする。 The photoconductive substrate of the present invention is characterized by comprising a substrate, an insulating layer formed on the substrate and made of an insulator, and a photoconductive layer formed on the insulating layer.
　この構成によれば、基板と光伝導層との間に、絶縁体で構成された絶縁層を介設することにより、電圧印加時の電流やテラヘルツ波照射時の発生電流が基板側にリークしてしまうのを避けることができる。よって、テラヘルツ波スペクトルのＳ／Ｎ比またはダイナミックレンジが低下してしまうのを防止することができる。また、絶縁体で構成された絶縁層を用いてリーク電流を抑える構成であるため、毒性の強い半絶縁性ＧａＡｓ基板を用いる必要がなく、またテラヘルツ波に対する透過性を向上することができる。例えば、基板としてＳｉ基板を用い、絶縁層としてＳｉ酸化膜を用いることで、テラヘルツ波に対する高い透過性を得ることができ、５ＴＨｚ以上のテラヘルツ波も利用可能となる。これらのように、簡単な構成で、テラヘルツ波に対する透過性を向上しつつ、リーク電流を抑えることができる。 According to this configuration, an insulating layer made of an insulator is interposed between the substrate and the photoconductive layer, so that a current when a voltage is applied or a generated current when a terahertz wave is irradiated leaks to the substrate side. Can be avoided. Therefore, it is possible to prevent the S / N ratio or dynamic range of the terahertz wave spectrum from being lowered. In addition, since the leakage current is suppressed by using an insulating layer made of an insulator, it is not necessary to use a highly toxic semi-insulating GaAs substrate, and the permeability to terahertz waves can be improved. For example, when a Si substrate is used as a substrate and a Si oxide film is used as an insulating layer, high transmission with respect to terahertz waves can be obtained, and terahertz waves of 5 THz or more can be used. As described above, with a simple configuration, the leakage current can be suppressed while improving the permeability to the terahertz wave.
　この場合、基板は、ＳｉおよびＧｅのいずれか一方で構成されていることが好ましい。 In this case, it is preferable that the substrate is composed of either Si or Ge.
　また、絶縁層は、基板に対応する、Ｓｉの酸化膜およびＧｅの酸化膜のいずれか一方で構成されていることが好ましい。 Further, it is preferable that the insulating layer is formed of one of an Si oxide film and a Ge oxide film corresponding to the substrate.
　これらの構成によれば、基板や絶縁層に、テラヘルツ波を透過しやすい（吸収が少ない）Ｓｉ（シリコン）、Ｇｅ（ゲルマニウム）またはこれらの酸化膜を用いることで、テラヘルツ波に対する透過性をより向上することができる。また、製造コストを低減することができる。 According to these configurations, by using Si (silicon), Ge (germanium), or an oxide film thereof that easily transmits (less absorbs) the terahertz wave in the substrate or the insulating layer, the transparency to the terahertz wave is further improved. Can be improved. In addition, the manufacturing cost can be reduced.
　一方、絶縁層は、１０ｎｍ以上３μｍ以下の厚みを有していることが好ましい。 On the other hand, the insulating layer preferably has a thickness of 10 nm to 3 μm.
　絶縁層が薄すぎると、電圧印加時に絶縁層上で絶縁破壊が生じ、電流がリークしてしまう場合がある。一方、絶縁層が厚すぎると、多重反射が生じやすく、その影響でテラヘルツ波が減衰してしまう場合がある。
　これに対し、上記の構成によれば、絶縁層の厚みを１０ｎｍ以上とする、すなわち、絶縁層の厚みを、当該絶縁層の絶縁耐圧が、光伝導層上に形成されるアンテナへの印加電圧以上となる厚みに形成されていることで、絶縁層上での絶縁破壊を避け、電流がリークするのを防止することができる。また、絶縁層の厚みを３μｍ以下とする、すなわち、基板および光伝導層のいずれか一方を伝搬するテラヘルツ波の波長（具体的には３ＴＨｚ前後）に対し、絶縁層の厚みを１／１０以下とすることで、多重反射の影響を極力少なくすることができる。 If the insulating layer is too thin, dielectric breakdown may occur on the insulating layer when a voltage is applied, and current may leak. On the other hand, if the insulating layer is too thick, multiple reflection is likely to occur, and the terahertz wave may be attenuated due to the influence.
On the other hand, according to the above configuration, the thickness of the insulating layer is set to 10 nm or more, that is, the insulating layer has a thickness equal to the applied voltage to the antenna formed on the photoconductive layer. By forming the thickness as described above, it is possible to avoid dielectric breakdown on the insulating layer and to prevent current from leaking. Further, the thickness of the insulating layer is 3 μm or less, that is, the thickness of the insulating layer is 1/10 or less with respect to the wavelength of the terahertz wave (specifically around 3 THz) propagating through one of the substrate and the photoconductive layer. By doing so, the influence of multiple reflection can be reduced as much as possible.
　また、光伝導層は、その絶縁層側が縦横全域で当該絶縁層の表面に接合されていることが好ましい。 Further, the photoconductive layer is preferably bonded to the surface of the insulating layer on the insulating layer side in the entire length and width.
　この構成によれば、光伝導層の縦横全域が絶縁層の表面に接合されているため、接合性を向上することができると共に、光伝導層側で生じた熱を効果的に基板側に逃がすことができる。すなわち、基板に電流がリークするのを避けつつ、且つ熱を基板に逃がし、基板をヒートシンクとして利用することができる。 According to this configuration, since the entire length and width of the photoconductive layer are bonded to the surface of the insulating layer, the bondability can be improved and the heat generated on the photoconductive layer side can be effectively released to the substrate side. be able to. That is, it is possible to use the substrate as a heat sink by avoiding current leakage to the substrate and releasing heat to the substrate.
　また、光伝導層は、III－V族化合物半導体で構成されていることが好ましい。 The photoconductive layer is preferably composed of a III-V group compound semiconductor.
　この場合、光伝導層は、ＧａＡｓで構成されていることが好ましい。 In this case, the photoconductive layer is preferably made of GaAs.
　これらの構成によれば、テラヘルツ波の発生・検出に好適な光伝導層を容易に形成することができる。 According to these configurations, a photoconductive layer suitable for generation / detection of terahertz waves can be easily formed.
　本発明の光伝導素子は、上記の光伝導基板と、光伝導基板上に形成されたアンテナと、を備えたことを特徴とする。 The photoconductive element of the present invention includes the above-described photoconductive substrate and an antenna formed on the photoconductive substrate.
　この構成によれば、テラヘルツ波に対する透過性を向上しつつ、リーク電流を抑えることができる光伝導基板を用いることで、安定性の高い光伝導素子を提供することができる。 According to this configuration, it is possible to provide a highly stable photoconductive element by using a photoconductive substrate that can suppress leakage current while improving transparency to terahertz waves.
本実施形態に係る光伝導素子を模式的に示した斜視図である。It is the perspective view which showed typically the photoconductive element which concerns on this embodiment. 光伝導素子を模式的に示した側面図である。It is the side view which showed the photoconductive element typically. 光伝導基板および光伝導素子の製造工程を示した説明図である。It is explanatory drawing which showed the manufacturing process of the photoconductive substrate and the photoconductive element. 光伝導素子を応用した時間領域分光装置を示した概略図である。It is the schematic which showed the time domain spectrometer which applied the photoconductive element. 光伝導素子の第１変形例（ａ）および第２変形例（ｂ）を模式的に示した側面図である。It is the side view which showed typically the 1st modification (a) and 2nd modification (b) of a photoconductive element. 光伝導素子の第３変形例を模式的に示した斜視図（ａ）および側面図（ｂ）である。It is the perspective view (a) and side view (b) which showed the 3rd modification of the photoconductive element typically. 光伝導素子の第４変形例（ａ）、第５変形例（ｂ）および第６変形例（ｃ）を模式的に示した側面図である。It is the side view which showed typically the 4th modification (a), the 5th modification (b), and the 6th modification (c) of a photoconductive element.
　以下、添付の図面を参照して、本発明の一実施形態に係る光伝導基板を用いた光伝導素子について説明する。この光伝導素子は、これに電圧を印加することにより、電磁波発生素子としての機能を奏する一方、電流計を接続することにより、電磁波検出素子としての機能を奏する半導体素子である。特に、本光伝導素子は、テラヘルツ波に対する透過性を向上しつつ、リーク電流を抑えることができる構成を有している。なお、ここで規定するテラヘルツ波は、狭義のテラヘルツ波（０．１ＴＨｚ以上１０ＴＨｚ以下の電磁波）は元より、広義のテラヘルツ波（数十ＧＨｚ以上数百ＴＨｚ以下の電磁波）をも含む概念である。 Hereinafter, a photoconductive element using a photoconductive substrate according to an embodiment of the present invention will be described with reference to the accompanying drawings. This photoconductive element is a semiconductor element that functions as an electromagnetic wave generating element by applying a voltage to the photoconductive element, and also functions as an electromagnetic wave detecting element by connecting an ammeter. In particular, the photoconductive element has a configuration capable of suppressing leakage current while improving the transmittance with respect to terahertz waves. The terahertz wave defined here is a concept including not only a narrowly defined terahertz wave (electromagnetic wave of 0.1 THz to 10 THz) but also a broadly defined terahertz wave (electromagnetic wave of several tens GHz to several hundred THz). .
　図１および図２に示すように、光伝導素子１は、素子基板となる光伝導基板２と、光伝導基板２上に形成されたアンテナ３（平行伝送線路）と、を備えている。 As shown in FIGS. 1 and 2, the photoconductive element 1 includes a photoconductive substrate 2 serving as an element substrate, and an antenna 3 (parallel transmission line) formed on the photoconductive substrate 2.
　光伝導基板２は、基板１１と、基板１１上に形成された薄膜の絶縁層１２と、絶縁層１２上に形成された薄膜の光伝導層１３と、を備えている。すなわち、所定の必要板厚を有した基板１１に対し、絶縁層１２および光伝導層１３を積層した構成を有している。 The photoconductive substrate 2 includes a substrate 11, a thin insulating layer 12 formed on the substrate 11, and a thin photoconductive layer 13 formed on the insulating layer 12. That is, the insulating layer 12 and the photoconductive layer 13 are laminated on the substrate 11 having a predetermined necessary plate thickness.
　基板１１は、単結晶のＳｉ（シリコン）により構成されている。なお、基板１１の材料としては、Ｓｉに限定されるものではなく、Ｇｅ（ゲルマニウム）を用いても良い。また、テラヘルツ波に対する透過性が高い（吸収性が低い）材料であれば、Ｓｉ、Ｇｅに限定されるものではない。 The substrate 11 is made of single crystal Si (silicon). The material of the substrate 11 is not limited to Si, and Ge (germanium) may be used. In addition, the material is not limited to Si and Ge as long as the material has high transmissivity to terahertz waves (low absorbency).
　光伝導層１３は、低温でエピタキシャル成長させた低温成長ガリウム砒素（ＬＴ－ＧａＡｓ）の薄膜で構成されている。また、光伝導層１３は、当該光伝導層１３の絶縁層１２側が縦横全域で当該絶縁層１２の表面に接合されている。フェムト秒パルスレーザ等の励起光は、光伝導層１３の表面に対し垂直に入射する。そして、入射した励起光が、光伝導層１３内において、励起キャリア（電子）を発生させる。 The photoconductive layer 13 is composed of a thin film of low temperature grown gallium arsenide (LT-GaAs) epitaxially grown at a low temperature. Further, the photoconductive layer 13 is joined to the surface of the insulating layer 12 on the insulating layer 12 side of the photoconductive layer 13 in the entire length and width. Excitation light such as a femtosecond pulse laser is incident on the surface of the photoconductive layer 13 perpendicularly. The incident excitation light generates excitation carriers (electrons) in the photoconductive layer 13.
　なお、光伝導層１３の層厚は、臨界膜厚を考慮して、１μｍ以上２μｍ以下に設定されている。また、光伝導層１３の材料は、ＧａＡｓ（ＬＴ－ＧａＡｓ）に限定されるものではなく、III－V族化合物半導体のうち少なくとも１つの半導体を任意に用いることができる。具体的には、ＧａＡｓ、ＬＴ－ＡｌＧａＡｓ（ＡｌＧａＡｓ）、ＩｎＧａＰ、ＡｌＡｓ、ＩｎＰ、ＩｎＡｌＡｓ、ＩｎＧａＡｓ、ＧａＡｓＳｂ、ＩｎＧａＡｓＰ、ＬＴ－ＩｎＡｓ（ＩｎＡｓ）、ＩｎＳｂを用いることができる。 The layer thickness of the photoconductive layer 13 is set to 1 μm or more and 2 μm or less in consideration of the critical film thickness. The material of the photoconductive layer 13 is not limited to GaAs (LT-GaAs), and at least one semiconductor among III-V group compound semiconductors can be arbitrarily used. Specifically, GaAs, LT-AlGaAs (AlGaAs), InGaP, AlAs, InP, InAlAs, InGaAs, GaAsSb, InGaAsP, LT-InAs (InAs), and InSb can be used.
　絶縁層１２は、基板１１と光伝導層１３との間に介設されると共に、Ｓｉの基板１１を熱酸化して形成されたＳｉ酸化膜（絶縁体）で構成されている。また、絶縁層１２の層厚Ｌは、電圧印加による絶縁破壊や多重反射を考慮して、１０ｎｍ以上３μｍ以下に設定されている。 The insulating layer 12 is interposed between the substrate 11 and the photoconductive layer 13 and is composed of a Si oxide film (insulator) formed by thermally oxidizing the Si substrate 11. The layer thickness L of the insulating layer 12 is set to 10 nm or more and 3 μm or less in consideration of dielectric breakdown and multiple reflection due to voltage application.
　なお、絶縁層１２は、Ｓｉ酸化膜に限定されるものではなく、例えば、基板１１がＧｅで構成されている場合には、絶縁層１２をＧｅ酸化膜で構成するものであっても良い。また、テラヘルツ波に対して透過性が高い（吸収性が低い）ものであれば、Ｓｉ酸化膜、Ｇｅ酸化膜に限るものではない。 The insulating layer 12 is not limited to the Si oxide film. For example, when the substrate 11 is made of Ge, the insulating layer 12 may be made of a Ge oxide film. Further, the material is not limited to the Si oxide film or the Ge oxide film as long as it has high permeability (low absorption) to the terahertz wave.
　アンテナ３は、ダイポール型の一対のアンテナ部２０から成り、各アンテナ部２０は、帯状に延在するライン電極部２１（アンテナ本体）と、ライン電極部２１の中間部から内側に延在する対向電極部２２と、を有している。ライン電極部２１の一方の端部は、入出力用の電極パッドとして機能しており、ケーブルを介して電源や電流増幅器等に接続されている。また、一対のアンテナ部２０は、そのライン電極部２１同士が平行に配設され、且つ相互の対向電極部２２が所定のギャップを存して対向配置されている。すなわち、相互の対向端部間には、数μｍ（例えば５μｍ）の幅を有するギャップ部２３が構成されている。なお、アンテナ３は、ダイポール型に限定されるものではなく、例えば、ボウタイ型、ストリップライン型、スパイラル型であってもよい。 The antenna 3 is composed of a pair of dipole antenna portions 20, and each antenna portion 20 has a line electrode portion 21 (antenna main body) extending in a strip shape and a counter electrode extending inward from an intermediate portion of the line electrode portion 21. And an electrode part 22. One end portion of the line electrode portion 21 functions as an input / output electrode pad, and is connected to a power source, a current amplifier, and the like via a cable. Further, the pair of antenna portions 20 are arranged such that the line electrode portions 21 are arranged in parallel, and the opposing electrode portions 22 are arranged to face each other with a predetermined gap. That is, a gap portion 23 having a width of several μm (for example, 5 μm) is formed between the opposing end portions. The antenna 3 is not limited to the dipole type, and may be, for example, a bow tie type, a stripline type, or a spiral type.
　一対のライン電極部２１に電圧を印加した状態で、ギャップ部２３にフェムト秒パルスレーザ等の励起光を照射すると、励起キャリアが発生する。そして、一対の対向電極部２２の間（ギャップ部２３）にパルス状の電流が流れ、この電流によってテラヘルツ波が発生する。また、この光伝導素子１は、テラヘルツ波を受けたときに一対の対向電極部２２間に電流が発生するため、検出（受信）素子としても用いることができる。この場合、電流（テラヘルツ波）を検出するための電流増幅器等を、一対のライン電極部２１に接続しておく。 When the gap 23 is irradiated with excitation light such as a femtosecond pulse laser in a state where a voltage is applied to the pair of line electrode portions 21, excitation carriers are generated. A pulsed current flows between the pair of counter electrode portions 22 (gap portion 23), and a terahertz wave is generated by the current. The photoconductive element 1 can also be used as a detection (reception) element because a current is generated between the pair of counter electrode portions 22 when receiving the terahertz wave. In this case, a current amplifier or the like for detecting current (terahertz wave) is connected to the pair of line electrode portions 21.
　ここで図３を参照して、光伝導基板２および光伝導素子１の製造工程について説明する。まず、ＧａＡｓ基板３１を用意し（図３（ａ））、ＭＢＥ（分子線エピタキシー）装置により、ＧａＡｓ基板３１上にＧａＡｓバッファ層（図示省略）をエピタキシャル成長させる。具体的には、基板温度５００℃以上６００℃以下、成長速度１μｍ／ｈ、Ａｓ／Ｇａ供給比５以上３０以下に設定して、ＧａＡｓバッファ層を０．１μｍ以上０．５μｍ以下の範囲で成長させる。なお、ＧａＡｓバッファ層は、光伝導層１３の結晶性を高くするために設けられる。 Here, with reference to FIG. 3, the manufacturing process of the photoconductive substrate 2 and the photoconductive element 1 will be described. First, a GaAs substrate 31 is prepared (FIG. 3A), and a GaAs buffer layer (not shown) is epitaxially grown on the GaAs substrate 31 using an MBE (molecular beam epitaxy) apparatus. Specifically, the substrate temperature is set to 500 ° C. to 600 ° C., the growth rate is 1 μm / h, the As / Ga supply ratio is 5 to 30 and the GaAs buffer layer is grown in the range of 0.1 μm to 0.5 μm. Let The GaAs buffer layer is provided to increase the crystallinity of the photoconductive layer 13.
　続いて、ＧａＡｓバッファ層上に光伝導層１３（ＬＴ‐ＧａＡｓ層）を、低温でエピタキシャル成長させる（図３（ｂ））。具体的には、基板温度４００℃以下まで降温させると共に、成長速度１μｍ／ｈの設定で、光伝導層１３を１μｍ以上２μｍ以下の範囲で成長させる。なお、光伝導層１３を成長させるときのＡｓ／Ｇａ供給比は、ＧａＡｓバッファ層を成長させたときのＡｓ／Ｇａ供給比以上であることが好ましい。 Subsequently, the photoconductive layer 13 (LT-GaAs layer) is epitaxially grown at a low temperature on the GaAs buffer layer (FIG. 3B). Specifically, the substrate temperature is lowered to 400 ° C. or lower, and the photoconductive layer 13 is grown in the range of 1 μm to 2 μm at a growth rate of 1 μm / h. The As / Ga supply ratio when the photoconductive layer 13 is grown is preferably equal to or higher than the As / Ga supply ratio when the GaAs buffer layer is grown.
　光伝導層１３を形成したら、光伝導層１３の表面（図中上側）に対し、熱処理（アニール処理）を行う。具体的には、Ａｓ分子線を照射した状態のまま、基板温度６００℃に設定して、５分以上１０分以下の範囲で熱処理を行う。 After the photoconductive layer 13 is formed, heat treatment (annealing) is performed on the surface of the photoconductive layer 13 (upper side in the figure). Specifically, with the As molecular beam irradiated, the substrate temperature is set to 600 ° C. and heat treatment is performed in the range of 5 minutes to 10 minutes.
　次に、ガラス等のホールド基板３２に、樹脂接着剤等により光伝導層１３の表面（図中上側）を貼り付け、裏面側（図中下側）のＧａＡｓ基板３１をバックグラインド（裏面研磨）によって除去する（図３（ｃ））。このとき、ＧａＡｓバッファ層も同時に除去される。なお、ＧａＡｓ基板３１を除去した後、当該裏面に対し研磨などを行い、平坦度や清浄度を向上させることが好ましい。 Next, the front surface (upper side in the figure) of the photoconductive layer 13 is attached to a hold substrate 32 such as glass by a resin adhesive or the like, and the backside (lower side in the figure) GaAs substrate 31 is back-ground (backside polishing). (FIG. 3C). At this time, the GaAs buffer layer is also removed at the same time. In addition, after removing the GaAs substrate 31, it is preferable to polish the said back surface etc. and to improve flatness and cleanliness.
　一方で、Ｓｉの基板１１を用意し（図３（ｄ））、基板１１の表面（図中上側）に対し熱酸化処理を行って絶縁層１２（Ｓｉ酸化膜）を形成する（図３（ｅ））。絶縁層１２は、１０ｎｍ以上３μｍ以下の範囲で形成される。 On the other hand, a Si substrate 11 is prepared (FIG. 3D), and the surface of the substrate 11 (upper side in the drawing) is subjected to thermal oxidation to form an insulating layer 12 (Si oxide film) (FIG. e)). The insulating layer 12 is formed in the range of 10 nm to 3 μm.
　絶縁層１２を形成したら、絶縁層１２表面に、上記のホールド基板３２に貼り付けられた光伝導層１３を貼り合わせる（図３（ｆ））。このとき、光伝導層１３の裏面側の縦横全域を、絶縁層１２の表面に貼り合わせる。なお、貼合せ方法は、テラハルツ波に対する吸収が少ない樹脂製の接着剤により貼り合わせる構成であっても良いし、ファンデルワールス力により貼り合わせる構成（いわゆるウェハボンディング）であっても良い。また、ファンデルワールス力により貼り合わせる場合、密着力を高めるために、ホールド基板３２を除去した後に、熱処理を行うことが好ましい。 When the insulating layer 12 is formed, the photoconductive layer 13 attached to the hold substrate 32 is attached to the surface of the insulating layer 12 (FIG. 3F). At this time, the entire length and width of the back side of the photoconductive layer 13 are bonded to the surface of the insulating layer 12. Note that the bonding method may be a structure in which bonding is performed with a resin adhesive that absorbs little against terahertz waves, or a structure in which bonding is performed by van der Waals force (so-called wafer bonding). Further, in the case of bonding by van der Waals force, it is preferable to perform heat treatment after removing the hold substrate 32 in order to increase the adhesion.
　貼合わせ処理が終了したら、溶剤でホールド基板３２の接着剤を溶解してホールド基板３２を取り外すことで、光伝導基板２の形成が終了する（図３（ｇ））。その後、フォトリソグラフィーやエッチング技術により、当該光伝導基板２上に、アンテナ３を形成することで、光伝導素子１が完成する（図３（ｈ））。 When the bonding process is completed, the adhesive of the hold substrate 32 is dissolved with a solvent and the hold substrate 32 is removed, whereby the formation of the photoconductive substrate 2 is completed (FIG. 3G). Then, the photoconductive element 1 is completed by forming the antenna 3 on the photoconductive substrate 2 by photolithography or etching technique (FIG. 3H).
　なお、本実施形態において、上記の製造工程により、光伝導素子１および光伝導基板２を１つずつ製造する構成であっても良いが、上記の製造工程により、複数の光伝導素子１や複数の光伝導基板２を成す多数取り基板を形成し、これを分割（ダイシング）して複数の光伝導素子１や複数の光伝導基板２を得る構成であっても良い。かかる場合、上記の光伝導素子１の形成が終了した後（図３（ｇ））や、光伝導基板２アンテナが形成された後（図３（ｈ））の状態から、分割処理を行うことが好ましい。 In the present embodiment, the photoconductive element 1 and the photoconductive substrate 2 may be manufactured one by one by the above manufacturing process. However, a plurality of photoconductive elements 1 and a plurality of photoconductive elements 1 may be manufactured by the above manufacturing process. It is also possible to form a plurality of substrates that constitute the photoconductive substrate 2 and to divide (dicing) the substrate to obtain a plurality of photoconductive elements 1 and a plurality of photoconductive substrates 2. In such a case, after the formation of the photoconductive element 1 is completed (FIG. 3G), or after the photoconductive substrate 2 antenna is formed (FIG. 3H), the dividing process is performed. Is preferred.
　続いて、図４を参照して、光伝導素子１の応用例として時間領域分光装置４０について簡単に説明する。図４に示すように、時間領域分光装置４０は、テラヘルツ波が伝播する経路中に測定したい測定試料Ｓを置き、透過したテラヘルツ波の時間波形と、測定試料Ｓの無い状態でのテラヘルツ波の時間波形と、をフーリエ変換して、テラヘルツ波の振幅と位相の情報を得る。これにより、測定試料Ｓの複素屈折率や複素誘電率などの細かい物性測定を行うものである。 Subsequently, a time domain spectroscopic device 40 will be briefly described as an application example of the photoconductive element 1 with reference to FIG. As shown in FIG. 4, the time domain spectroscopic device 40 places the measurement sample S to be measured in the path through which the terahertz wave propagates, and transmits the time waveform of the transmitted terahertz wave and the terahertz wave without the measurement sample S. The time waveform is Fourier transformed to obtain information on the amplitude and phase of the terahertz wave. As a result, fine physical properties such as the complex refractive index and complex dielectric constant of the measurement sample S are measured.
　時間領域分光装置４０は、フェムト秒レーザ（励起光）を発生するレーザ照射装置４１と、フェムト秒レーザを分離するビームスプリッター４２と、電磁波発生素子１ａおよび電磁波検出素子１ｂと、電磁波検出素子１ｂに入射するフェムト秒レーザを遅延させる遅延光学系４３と、フェムト秒レーザを反射・集光する各種光学系と、入力信号を処理する信号処理装置４４と、を備えている。また、その他、時間領域分光装置４０として一般的な構成を有している。なお、電磁波検出素子１ｂおよび電磁波発生素子１ａは、それぞれ上述した光伝導素子１と半球レンズとを組み合わせたものである。なお、多重反射の影響を考慮し、電磁波検出素子１ｂおよび電磁波発生素子１ａの各基板１１と、各基板１１面に設置する半球レンズとは、屈折率が同等となるように、同一の材料を用いることが好ましい。 The time domain spectroscopic device 40 includes a laser irradiation device 41 that generates a femtosecond laser (excitation light), a beam splitter 42 that separates the femtosecond laser, an electromagnetic wave generation element 1a, an electromagnetic wave detection element 1b, and an electromagnetic wave detection element 1b. A delay optical system 43 that delays an incident femtosecond laser, various optical systems that reflect and collect the femtosecond laser, and a signal processing device 44 that processes an input signal are provided. In addition, the time domain spectroscopic device 40 has a general configuration. The electromagnetic wave detecting element 1b and the electromagnetic wave generating element 1a are a combination of the above-described photoconductive element 1 and hemispherical lens. In consideration of the influence of multiple reflection, each substrate 11 of the electromagnetic wave detecting element 1b and the electromagnetic wave generating element 1a and the hemispherical lens installed on the surface of each substrate 11 are made of the same material so that the refractive indexes are equivalent. It is preferable to use it.
　まず、レーザ照射装置４１から発せられたフェムト秒レーザ（波長８００ｎｍ）は、ビームスプリッター４２により、ポンプ光とプローブ光とに分けられる。そして、ポンプ光は、振幅変調を掛けた状態で電磁波発生素子１ａに入射する。このとき一対のアンテナ部２０間に電圧を印加しておくことで、電磁波発生素子１ａからテラヘルツ波が発生する。このテラヘルツ波は、第１放物面鏡４５で反射され、第１レンズ４６により集光されて測定試料Ｓに照射される。測定試料Ｓを透過したテラヘルツ波は、第２レンズ４７、第２放物面鏡４８を介して電磁波検出素子１ｂに入射する。 First, the femtosecond laser (wavelength 800 nm) emitted from the laser irradiation device 41 is divided into pump light and probe light by the beam splitter 42. The pump light is incident on the electromagnetic wave generating element 1a in a state where amplitude modulation is applied. At this time, a terahertz wave is generated from the electromagnetic wave generating element 1 a by applying a voltage between the pair of antenna units 20. The terahertz wave is reflected by the first parabolic mirror 45, condensed by the first lens 46, and irradiated on the measurement sample S. The terahertz wave that has passed through the measurement sample S enters the electromagnetic wave detection element 1 b via the second lens 47 and the second parabolic mirror 48.
　一方、ビームスプリッター４２により分けられたプローブ光は、複数の反射鏡４９によって、遅延光学系４３に照射され、時間遅延を与えられて電磁波検出素子１ｂに入射する。電磁波検出素子１ｂで検出された信号は、信号処理装置４４に入力される。信号処理装置４４は、測定試料Ｓを透過したテラヘルツ波の時間波形および測定試料Ｓが無い状態でのテラヘルツ波の時間波形を各々時系列データとして記憶し、これをフーリエ変換処理して周波数空間に変換する。こうして、測定試料Ｓからのテラヘルツ波の強度振幅や位相の分光スペクトルを得ることで、測定試料Ｓの物性等を調べることができる。 On the other hand, the probe light divided by the beam splitter 42 is irradiated to the delay optical system 43 by a plurality of reflecting mirrors 49, is given a time delay, and enters the electromagnetic wave detection element 1b. A signal detected by the electromagnetic wave detection element 1 b is input to the signal processing device 44. The signal processing device 44 stores the time waveform of the terahertz wave transmitted through the measurement sample S and the time waveform of the terahertz wave in the absence of the measurement sample S as time-series data, respectively, and performs Fourier transform processing on the frequency space. Convert. Thus, by obtaining the spectrum of the intensity amplitude and phase of the terahertz wave from the measurement sample S, the physical properties and the like of the measurement sample S can be examined.
　以上のような構成によれば、基板１１と光伝導層１３との間に、絶縁体（Ｓｉ酸化膜）で構成された絶縁層１２を介設することにより、電圧印加時の電流やテラヘルツ波照射時の発生電流が基板１１側にリークしてしまうのを避けることができる。よって、テラヘルツ波スペクトルのＳ／Ｎ比またはダイナミックレンジが低下してしまうのを防止することができる。また、絶縁層１２を用いてリーク電流を抑える構成であるため、毒性の強い半絶縁性ＧａＡｓ基板を用いる必要がなく、またテラヘルツ波に対する透過性を向上することができる。これらのように、簡単な構成で、テラヘルツ波に対する透過性を向上しつつ、リーク電流を抑えることができる。 According to the configuration as described above, by interposing the insulating layer 12 made of an insulator (Si oxide film) between the substrate 11 and the photoconductive layer 13, a current and a terahertz wave at the time of voltage application can be obtained. It can be avoided that the current generated during irradiation leaks to the substrate 11 side. Therefore, it is possible to prevent the S / N ratio or dynamic range of the terahertz wave spectrum from being lowered. In addition, since the insulating layer 12 is used to suppress the leakage current, it is not necessary to use a highly toxic semi-insulating GaAs substrate, and the permeability to terahertz waves can be improved. As described above, with a simple configuration, the leakage current can be suppressed while improving the permeability to the terahertz wave.
　また、絶縁層１２の厚みを１０ｎｍ以上とする、すなわち、絶縁層１２の厚みを、当該絶縁層１２の絶縁耐圧が、光伝導層１３上に形成されるアンテナ３への印加電圧以上となる厚みに形成されていることで、絶縁層１２上での絶縁破壊を避け、電流がリークするのを防止することができる。また、絶縁層１２の厚みを３μｍ以下とする、すなわち、基板１１および光伝導層１３のいずれか一方を伝搬するテラヘルツ波の波長（具体的には３ＴＨｚ前後）に対し、絶縁層１２の厚みを１／１０以下とすることで、多重反射の影響を極力少なくすることができる。 The thickness of the insulating layer 12 is 10 nm or more, that is, the thickness of the insulating layer 12 is such that the withstand voltage of the insulating layer 12 is equal to or higher than the voltage applied to the antenna 3 formed on the photoconductive layer 13. Thus, it is possible to avoid dielectric breakdown on the insulating layer 12 and to prevent current from leaking. Further, the thickness of the insulating layer 12 is set to 3 μm or less, that is, the thickness of the insulating layer 12 with respect to the wavelength of terahertz waves (specifically around 3 THz) propagating through one of the substrate 11 and the photoconductive layer 13 is set. By setting it to 1/10 or less, the influence of multiple reflection can be reduced as much as possible.
　さらに、光伝導層１３の縦横全域が絶縁層１２の表面に接合されていることにより、接合性を向上することができると共に、光伝導層１３側で生じた熱を効果的に基板１１側に逃がすことができる。すなわち、基板１１に電流がリークするのを避けつつ、且つ熱を基板１１に逃がし、基板１１をヒートシンクとして利用することができる。 Further, since the entire length and width of the photoconductive layer 13 are bonded to the surface of the insulating layer 12, the bonding property can be improved and the heat generated on the photoconductive layer 13 side can be effectively transferred to the substrate 11 side. I can escape. That is, it is possible to use the substrate 11 as a heat sink by avoiding current leaking to the substrate 11 and releasing heat to the substrate 11.
　またさらに、基板１１や絶縁層１２に、テラヘルツ波を透過しやすい（吸収が少ない）Ｓｉ（シリコン）、およびＳｉ酸化膜を用いることで、テラヘルツ波に対する透過性をより向上することができる。また、製造コストを低減することができる。 Furthermore, by using Si (silicon) and Si oxide film that easily transmit (less absorb) terahertz waves for the substrate 11 and the insulating layer 12, it is possible to further improve the permeability to terahertz waves. In addition, the manufacturing cost can be reduced.
　また、光伝導層１３を、ＧａＡｓで構成することにより、テラヘルツ波の発生・検出に好適な光伝導層１３を容易に形成することができる。 Further, by configuring the photoconductive layer 13 with GaAs, the photoconductive layer 13 suitable for the generation and detection of terahertz waves can be easily formed.
　なお、本実施形態においては、製造工程にて、光伝導基板２を形成（製造）した後に、光伝導基板２上にアンテナ３を形成する構成であったが、光伝導層１３を形成し光伝導層１３に熱処理を行った後、ホールド基板３２に光伝導層１３を貼り付ける前の状態で、アンテナ３を形成する構成であっても良い。 In this embodiment, the antenna 3 is formed on the photoconductive substrate 2 after the photoconductive substrate 2 is formed (manufactured) in the manufacturing process. The structure in which the antenna 3 is formed after the heat treatment of the conductive layer 13 and before the photoconductive layer 13 is attached to the hold substrate 32 may be adopted.
　また、本実施形態においては、製造工程にて、バックグラインドにより、ＧａＡｓ基板３１を除去する構成であったが、選択エッチングによってＧａＡｓ基板３１を除去する構成であっても良い。かかる場合、ＧａＡｓ基板３１（厳密にはＧａＡｓバッファ層）と光伝導層１３との間に、ＡｌＡｓエッチングストップ層を成長させることが好ましい。これにより、ＡｌＡｓエッチングストップ層でエッチングがストップするので、ＧａＡｓ基板３１だけを選択的に除去することができる。 In the present embodiment, the GaAs substrate 31 is removed by back grinding in the manufacturing process. However, the GaAs substrate 31 may be removed by selective etching. In such a case, it is preferable to grow an AlAs etching stop layer between the GaAs substrate 31 (strictly, the GaAs buffer layer) and the photoconductive layer 13. Thereby, since the etching is stopped at the AlAs etching stop layer, only the GaAs substrate 31 can be selectively removed.
　さらに、本実施形態においては、製造工程にて、ＧａＡｓ基板３１およびＧａＡｓバッファ層を除去する構成であったが、このとき、図５（ａ）に示すように、ＧａＡｓ基板３１および／またはＧａＡｓバッファ層を所定量残す構成であっても良い。すなわち、ＧａＡｓ基板３１および／またはＧａＡｓバッファ層を所定量有した（残した）状態の光伝導基板２および光伝導素子１を製造する構成であっても良い。ひいては、上記のＡｌＡｓエッチングストップ層を除去するときに、図５（ｂ）に示すように、当該ＡｌＡｓエッチングストップ層を所定量残す構成であっても良い。 Furthermore, in this embodiment, the GaAs substrate 31 and the GaAs buffer layer are removed in the manufacturing process. At this time, as shown in FIG. 5A, the GaAs substrate 31 and / or the GaAs buffer are removed. It may be configured to leave a predetermined amount of layers. That is, a configuration in which the photoconductive substrate 2 and the photoconductive element 1 in a state having (remaining) a predetermined amount of the GaAs substrate 31 and / or the GaAs buffer layer may be manufactured. As a result, when the AlAs etching stop layer is removed, a predetermined amount of the AlAs etching stop layer may be left as shown in FIG. 5B.
　また、本実施形態においては、光伝導層１３上にアンテナ３を形成する構成であったが、ギャップ部２３にのみ光伝導層１３を形成する構成であっても良い。具体的には、図６に示すように、各アンテナ部２０を絶縁層１２上に形成し、一対の対向電極部２２の先端部を絶縁層１２から立ち上がるように形成する。そして、立ち上げた一対の対向電極部２２間（ギャップ部２３）にのみ光伝導層１３を形成する。かかる場合、発生するテラヘルツ波スペクトルのＳ／Ｎ比またはダイナミックレンジをより向上させることができる。 In the present embodiment, the antenna 3 is formed on the photoconductive layer 13, but the photoconductive layer 13 may be formed only on the gap portion 23. Specifically, as shown in FIG. 6, each antenna portion 20 is formed on the insulating layer 12, and the tip portions of the pair of counter electrode portions 22 are formed so as to rise from the insulating layer 12. Then, the photoconductive layer 13 is formed only between the pair of raised counter electrode portions 22 (gap portion 23). In such a case, the S / N ratio or dynamic range of the generated terahertz wave spectrum can be further improved.
　さらに、本実施形態において、図７（ａ）に示すように、基板１１として、Ｓｉ製の半導体レンズ（レンズ基板）を用いても良い。また、図７（ｂ）および図７（ｃ）に示すように、基板１１として、Ｓｉ製の半導体プリズム（プリズム基板）やＳｉ製の半導体ＡＴＲプリズム（ＡＴＲプリズム基板）を用いても良い。なお、半導体プリズムおよび半導体ＡＴＲプリズムを用いた場合には、同図に示すように、絶縁層１２、光伝導層１３およびアンテナ３の組を２組配設し、２つの光伝導素子１として機能させることも可能である。 Furthermore, in the present embodiment, as shown in FIG. 7A, a Si semiconductor lens (lens substrate) may be used as the substrate 11. Further, as shown in FIGS. 7B and 7C, a Si semiconductor prism (prism substrate) or a Si semiconductor ATR prism (ATR prism substrate) may be used as the substrate 11. When the semiconductor prism and the semiconductor ATR prism are used, two sets of the insulating layer 12, the photoconductive layer 13, and the antenna 3 are provided and function as two photoconductive elements 1 as shown in FIG. It is also possible to make it.
　１：光伝導素子、　２：光伝導基板、　３：アンテナ、　１１：基板、　１２：絶縁層、　１３：光伝導層 1: photoconductive element, 2: photoconductive substrate, 3: antenna, 11: substrate, 12: insulating layer, 13: photoconductive layer

Claims (8)

  1. 　基板と、
    　前記基板上に形成され、絶縁体で構成された絶縁層と、
    　前記絶縁層上に形成された光伝導層と、を備えたことを特徴とする光伝導基板。     A substrate,
    An insulating layer formed on the substrate and made of an insulator;
    And a photoconductive layer formed on the insulating layer.
  2. 　前記基板は、ＳｉおよびＧｅのいずれか一方で構成されていることを特徴とする請求項１に記載の光伝導基板。 2. The photoconductive substrate according to claim 1, wherein the substrate is composed of one of Si and Ge.
  3. 　前記絶縁層は、前記基板に対応する、Ｓｉの酸化膜およびＧｅの酸化膜のいずれか一方で構成されていることを特徴とする請求項２に記載の光伝導基板。 3. The photoconductive substrate according to claim 2, wherein the insulating layer is composed of one of a Si oxide film and a Ge oxide film corresponding to the substrate.
  4. 　前記絶縁層は、１０ｎｍ以上３μｍ以下の厚みを有していることを特徴とする請求項３に記載の光伝導基板。 4. The photoconductive substrate according to claim 3, wherein the insulating layer has a thickness of 10 nm to 3 μm.
  5. 　前記光伝導層は、その前記絶縁層側が縦横全域で当該絶縁層の表面に接合されていることを特徴とする請求項３に記載の光伝導基板。 The photoconductive substrate according to claim 3, wherein the photoconductive layer is bonded to the surface of the insulating layer on the insulating layer side in the entire length and width.
  6. 　前記光伝導層は、III－V族化合物半導体で構成されていることを特徴とする請求項３に記載の光伝導基板。 4. The photoconductive substrate according to claim 3, wherein the photoconductive layer is composed of a III-V compound semiconductor.
  7. 　前記光伝導層は、ＧａＡｓで構成されていることを特徴とする請求項６に記載の光伝導基板。 The photoconductive substrate according to claim 6, wherein the photoconductive layer is made of GaAs.
  8. 　請求項１に記載の光伝導基板と、
    　前記光伝導基板上に形成されたアンテナと、を備えたことを特徴とする光伝導素子。     A photoconductive substrate according to claim 1;
    A photoconductive element comprising an antenna formed on the photoconductive substrate.
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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
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JP2016192458A (en) * 2015-03-31 2016-11-10 パイオニア株式会社 Photoconductive element, measurement device and manufacturing method
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