WO2013136369A1 - Photoconductive substrate and electromagnetic wave generating apparatus provided with same - Google Patents
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- WO2013136369A1 WO2013136369A1 PCT/JP2012/001823 JP2012001823W WO2013136369A1 WO 2013136369 A1 WO2013136369 A1 WO 2013136369A1 JP 2012001823 W JP2012001823 W JP 2012001823W WO 2013136369 A1 WO2013136369 A1 WO 2013136369A1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01Q—ANTENNAS, i.e. RADIO AERIALS
- H01Q9/00—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
- H01Q9/04—Resonant antennas
- H01Q9/16—Resonant antennas with feed intermediate between the extremities of the antenna, e.g. centre-fed dipole
- H01Q9/26—Resonant antennas with feed intermediate between the extremities of the antenna, e.g. centre-fed dipole with folded element or elements, the folded parts being spaced apart a small fraction of operating wavelength
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
- G01N21/3581—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
- G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
- G01N21/636—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited using an arrangement of pump beam and probe beam; using the measurement of optical non-linear properties
-
- 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
Abstract
The present invention addresses the problem of providing a photoconductive substrate, which can improve the S/N ratio or the dynamic range of electromagnetic waves to be generated, and an electromagnetic wave generating apparatus that is provided with the photoconductive substrate.
A photoconductive substrate (2) is characterized in that: the photoconductive substrate is provided with a substrate (11), and a photoconductive layer (13), which is configured of a III-V compound semiconductor that is epitaxially grown on the substrate (11); and in the photoconductive layer (13), the lifetime of a carrier generated due to photoelectric effects is within a range wherein the lower limit is higher than a subpicosecond, and the upper limit maintains presence of a cluster of the group V atoms, specifically, the carrier lifetime is 2-50 psec.
Description
本発明は、テラヘルツ波等の電磁波の発生に好適な光伝導基板およびこれを備えた電磁波発生装置に関するものである。
The present invention relates to a photoconductive substrate suitable for the generation of electromagnetic waves such as terahertz waves, and an electromagnetic wave generator provided with the same.
従来、半導体単結晶基板と、半導体単結晶基板上にエピタキシャル成長させ、表面に向かうに従ってIII族原子の濃度に対するＶ族原子の濃度の割合を増加させ、表面近傍においてＶ族原子の濃度をIII族原子の濃度より高くしたIII－Ｖ族化合物半導体層と、を備えた光伝導基板が知られている（特許文献１参照）。
この光伝導基板では、表面近傍におけるＶ族原子クラスターが、光励起キャリアの捕捉に大きく寄与することを利用して、フェムト秒レーザパルスの照射によってIII－Ｖ族化合物半導体層内に発生する光励起キャリアのキャリア寿命を短寿命化している。これにより、残留キャリアによる、テラヘルツ波の検出および発生のＳ／Ｎ比が低下することを防止している。 Conventionally, a semiconductor single crystal substrate is epitaxially grown on the semiconductor single crystal substrate, the ratio of the group V atom concentration to the group III atom concentration is increased toward the surface, and the group V atom concentration is increased in the vicinity of the surface to the group III atom. There is known a photoconductive substrate provided with a III-V compound semiconductor layer having a higher concentration than the above (see Patent Document 1).
In this photoconductive substrate, utilizing the fact that group V atom clusters in the vicinity of the surface greatly contribute to the capture of photoexcited carriers, photoexcited carriers generated in the group III-V compound semiconductor layer by irradiation with femtosecond laser pulses are utilized. Carrier life is shortened. This prevents a decrease in the S / N ratio of detection and generation of terahertz waves due to residual carriers.
この光伝導基板では、表面近傍におけるＶ族原子クラスターが、光励起キャリアの捕捉に大きく寄与することを利用して、フェムト秒レーザパルスの照射によってIII－Ｖ族化合物半導体層内に発生する光励起キャリアのキャリア寿命を短寿命化している。これにより、残留キャリアによる、テラヘルツ波の検出および発生のＳ／Ｎ比が低下することを防止している。 Conventionally, a semiconductor single crystal substrate is epitaxially grown on the semiconductor single crystal substrate, the ratio of the group V atom concentration to the group III atom concentration is increased toward the surface, and the group V atom concentration is increased in the vicinity of the surface to the group III atom. There is known a photoconductive substrate provided with a III-V compound semiconductor layer having a higher concentration than the above (see Patent Document 1).
In this photoconductive substrate, utilizing the fact that group V atom clusters in the vicinity of the surface greatly contribute to the capture of photoexcited carriers, photoexcited carriers generated in the group III-V compound semiconductor layer by irradiation with femtosecond laser pulses are utilized. Carrier life is shortened. This prevents a decrease in the S / N ratio of detection and generation of terahertz waves due to residual carriers.
ところで、光伝導基板に一対のアンテナを設けた光伝導アンテナ素子は、一対のアンテナ（電極）間に電圧を印加することで、テラヘルツ波発生装置として利用され、また一対のアンテナ（電極）間に流れる瞬時電流を検出することで、テラヘルツ波検出装置として利用される。
このため、上記従来の光伝導基板等においては、常にテラヘルツ波の発生および検出に好適な兼用（共用）基板を構成すべく、キャリア寿命を短寿命化（例えば、１ｐｓｅｃ以下）を図るようにしている。
この点において、本出願人は、発生専用の光伝導基板、或いは検出専用の光伝導基板の存在を肯定し、その開発過程で、発生専用の光伝導基板においては、キャリア寿命の短寿命化が、必ずしもＳ／Ｎ比またはダイナミックレンジの向上に相関しないことを見出した。 By the way, a photoconductive antenna element provided with a pair of antennas on a photoconductive substrate is used as a terahertz wave generator by applying a voltage between a pair of antennas (electrodes), and between a pair of antennas (electrodes). By detecting the flowing instantaneous current, it is used as a terahertz wave detection device.
For this reason, in the conventional photoconductive substrate and the like, the carrier life is shortened (for example, 1 psec or less) so that a dual-purpose (shared) substrate suitable for generation and detection of terahertz waves is always configured. Yes.
In this regard, the present applicant affirms the existence of a photoconductive substrate dedicated to generation or a photoconductive substrate dedicated to detection, and in the development process, in the photoconductive substrate dedicated to generation, the carrier life is shortened. It has been found that it does not necessarily correlate with an improvement in the S / N ratio or dynamic range.
このため、上記従来の光伝導基板等においては、常にテラヘルツ波の発生および検出に好適な兼用（共用）基板を構成すべく、キャリア寿命を短寿命化（例えば、１ｐｓｅｃ以下）を図るようにしている。
この点において、本出願人は、発生専用の光伝導基板、或いは検出専用の光伝導基板の存在を肯定し、その開発過程で、発生専用の光伝導基板においては、キャリア寿命の短寿命化が、必ずしもＳ／Ｎ比またはダイナミックレンジの向上に相関しないことを見出した。 By the way, a photoconductive antenna element provided with a pair of antennas on a photoconductive substrate is used as a terahertz wave generator by applying a voltage between a pair of antennas (electrodes), and between a pair of antennas (electrodes). By detecting the flowing instantaneous current, it is used as a terahertz wave detection device.
For this reason, in the conventional photoconductive substrate and the like, the carrier life is shortened (for example, 1 psec or less) so that a dual-purpose (shared) substrate suitable for generation and detection of terahertz waves is always configured. Yes.
In this regard, the present applicant affirms the existence of a photoconductive substrate dedicated to generation or a photoconductive substrate dedicated to detection, and in the development process, in the photoconductive substrate dedicated to generation, the carrier life is shortened. It has been found that it does not necessarily correlate with an improvement in the S / N ratio or dynamic range.
本発明は、上記の知見にかんがみて為されたものであり、発生する電磁波のＳ／Ｎ比またはダイナミックレンジを向上させることができる光伝導基板およびこれを備えた電磁波発生装置を提供することを課題としている。
The present invention has been made in view of the above knowledge, and provides a photoconductive substrate capable of improving the S / N ratio or dynamic range of generated electromagnetic waves and an electromagnetic wave generating apparatus including the same. It is an issue.
本発明の光伝導基板は、基板と、基板上にエピタキシャル成長させたIII－Ｖ族化合物半導体から成る光伝導層と、を備え、光伝導層は、光電効果によって生ずるキャリア寿命が、下限においてサブピコ秒を越え、上限においてＶ族原子のクラスターの存在が維持される範囲内であることを特徴とする。
The photoconductive substrate of the present invention comprises a substrate and a photoconductive layer made of a III-V group compound semiconductor epitaxially grown on the substrate, and the photoconductive layer has a carrier lifetime caused by the photoelectric effect of sub-picosecond at the lower limit. And the upper limit is within a range in which the existence of the cluster of the group V atom is maintained.
具体的には、キャリア寿命が、２ｐｓｅｃ以上、５０ｐｓｅｃ以下であることが好ましく、さらにキャリア寿命が、６ｐｓｅｃ以上、３５ｐｓｅｃ以下であることがより好ましい。
Specifically, the carrier life is preferably 2 psec or more and 50 psec or less, and the carrier life is more preferably 6 psec or more and 35 psec or less.
この構成によれば、発生させた電磁時間波形をフーリエ変換して得た電磁波スペクトルにおいて評価される、Ｓ／Ｎ比またはダイナミックレンジが、光伝導層におけるキャリア寿命がサブピコ秒のものに比して、全周波数帯域で向上することが確認された。電磁波の発生において、キャリア寿命が２ｐｓｅｃ以上のものであると、Ｓ／Ｎ比またはダイナミックレンジが向上するが、５０ｐｓｅｃを越えると、Ｖ族原子のクラスターが存在しなくなる。これにより、キァリアー（光励起キャリア）の捕捉ができなくなり、残留キャリアに起因するノイズが大きくなる。また、キャリア寿命が５０ｐｓｅｃを越えると、Ｖ族原子のクラスターが存在しなくなると共に結晶性の高い層となるため、アンテナ（電極）における絶縁破壊が生じ易くなる。したがって、キャリア寿命が２ｐｓｅｃ以上、５０ｐｓｅｃ以下となる光伝導層とすることにより、高い結晶性とＶ族原子クラスターが存在する構成とし、キャリアの高い移動度（高い電磁波発生効率）を実現すると共に、ノイズの抑制と耐絶縁破壊性の向上を図った。
この２ｐｓｅｃ～５０ｐｓｅｃのキャリア寿命は、例えばＭＢＥ（分子線エピタキシー）装置において、III－Ｖ族化合物半導体を低温成長させた後、Ｖ族原子クラスターを析出させるべく、分子線を照射した状態で熱処理を行うことで実現される。なお、この場合の電磁波は、数十ＧＨｚ～数百ＴＨｚの電磁波を対象とするものである。 According to this configuration, the S / N ratio or dynamic range, which is evaluated in the electromagnetic spectrum obtained by Fourier transforming the generated electromagnetic time waveform, is higher than that of the carrier lifetime in the photoconductive layer is sub-picoseconds. It was confirmed that it improved in all frequency bands. In the generation of electromagnetic waves, if the carrier lifetime is 2 psec or more, the S / N ratio or the dynamic range is improved. However, if it exceeds 50 psec, clusters of group V atoms do not exist. As a result, the carrier (photoexcited carrier) cannot be captured, and noise caused by the residual carrier increases. Further, when the carrier lifetime exceeds 50 psec, a cluster of group V atoms does not exist and a layer with high crystallinity is formed, so that dielectric breakdown in the antenna (electrode) is likely to occur. Therefore, by adopting a photoconductive layer having a carrier lifetime of 2 psec or more and 50 psec or less, it is configured to have high crystallinity and a group V atom cluster, realizing high carrier mobility (high electromagnetic wave generation efficiency), Noise suppression and dielectric breakdown resistance were improved.
The carrier lifetime of 2 psec to 50 psec is, for example, in MBE (Molecular Beam Epitaxy) equipment after III-V group compound semiconductor is grown at low temperature and then subjected to heat treatment in the state of irradiation with molecular beam to precipitate V group atomic clusters. It is realized by doing. Note that the electromagnetic wave in this case is an electromagnetic wave of several tens GHz to several hundreds THz.
この２ｐｓｅｃ～５０ｐｓｅｃのキャリア寿命は、例えばＭＢＥ（分子線エピタキシー）装置において、III－Ｖ族化合物半導体を低温成長させた後、Ｖ族原子クラスターを析出させるべく、分子線を照射した状態で熱処理を行うことで実現される。なお、この場合の電磁波は、数十ＧＨｚ～数百ＴＨｚの電磁波を対象とするものである。 According to this configuration, the S / N ratio or dynamic range, which is evaluated in the electromagnetic spectrum obtained by Fourier transforming the generated electromagnetic time waveform, is higher than that of the carrier lifetime in the photoconductive layer is sub-picoseconds. It was confirmed that it improved in all frequency bands. In the generation of electromagnetic waves, if the carrier lifetime is 2 psec or more, the S / N ratio or the dynamic range is improved. However, if it exceeds 50 psec, clusters of group V atoms do not exist. As a result, the carrier (photoexcited carrier) cannot be captured, and noise caused by the residual carrier increases. Further, when the carrier lifetime exceeds 50 psec, a cluster of group V atoms does not exist and a layer with high crystallinity is formed, so that dielectric breakdown in the antenna (electrode) is likely to occur. Therefore, by adopting a photoconductive layer having a carrier lifetime of 2 psec or more and 50 psec or less, it is configured to have high crystallinity and a group V atom cluster, realizing high carrier mobility (high electromagnetic wave generation efficiency), Noise suppression and dielectric breakdown resistance were improved.
The carrier lifetime of 2 psec to 50 psec is, for example, in MBE (Molecular Beam Epitaxy) equipment after III-V group compound semiconductor is grown at low temperature and then subjected to heat treatment in the state of irradiation with molecular beam to precipitate V group atomic clusters. It is realized by doing. Note that the electromagnetic wave in this case is an electromagnetic wave of several tens GHz to several hundreds THz.
この場合、光伝導層は、ＧａＡｓ化合物半導体層であることが好ましい。
In this case, the photoconductive layer is preferably a GaAs compound semiconductor layer.
この構成によれば、光伝導層の高い結晶性が維持されるため、電磁波の発生効率（キャリアの移動度）を高めることができる。
According to this configuration, since the high crystallinity of the photoconductive layer is maintained, electromagnetic wave generation efficiency (carrier mobility) can be increased.
本発明の電磁波発生装置は、上記した光伝導基板と、光伝導基板上に形成された平行伝送線路と、を備えたことを特徴とする。
An electromagnetic wave generator of the present invention is characterized by comprising the above-described photoconductive substrate and a parallel transmission line formed on the photoconductive substrate.
この構成によれば、電磁波の発生において、Ｓ／Ｎ比またはダイナミックレンジを向上させることができ、電磁波の発生に好適な電磁波発生装置を提供することができる。
According to this configuration, the S / N ratio or dynamic range can be improved in the generation of electromagnetic waves, and an electromagnetic wave generator suitable for the generation of electromagnetic waves can be provided.
以下、添付の図面を参照して、本発明の一実施形態の係る光伝導基板および電磁波発生装置を適用した光伝導アンテナ素子について説明する。この光伝導アンテナ素子は、これに電圧を印加することにより、テラヘルツ波を発生させる発生専用の素子として開発されたものである。また、ここで規定するテラヘルツ波とは、狭義のテラヘルツ波（０．１ＴＨｚ～１０ＴＨｚの電磁波）は元より、広義のテラヘルツ波（数十ＧＨｚ～数百ＴＨｚの電磁波）も含む概念である。
Hereinafter, a photoconductive antenna element to which a photoconductive substrate and an electromagnetic wave generator according to an embodiment of the present invention are applied will be described with reference to the accompanying drawings. This photoconductive antenna element has been developed as a generation-only element that generates a terahertz wave by applying a voltage thereto. The terahertz wave defined here is a concept that includes 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 of GHz to several hundred THz).
図１は、光伝導アンテナ素子を模式的に表した斜視図である。同図に示すように、光伝導アンテナ素子１は、主体を為す光伝導基板２と、光伝導基板２上に成膜され、電圧を印加するための平行伝送線路３と、により構成されている。
光伝導基板２は、基板１１と、基板１１上に形成されたバッファ層１２と、バッファ層１２上に形成された光伝導層１３と、を備えている。また、平行伝送線路３は、一対のアンテナ２１，２１を備えている。なお、バッファ層１２は省略してもよい。 FIG. 1 is a perspective view schematically showing a photoconductive antenna element. As shown in the figure, thephotoconductive antenna element 1 is composed of a photoconductive substrate 2 that forms the main body, and a parallel transmission line 3 that is formed on the photoconductive substrate 2 and applies a voltage. .
Thephotoconductive substrate 2 includes a substrate 11, a buffer layer 12 formed on the substrate 11, and a photoconductive layer 13 formed on the buffer layer 12. The parallel transmission line 3 includes a pair of antennas 21 and 21. Note that the buffer layer 12 may be omitted.
光伝導基板２は、基板１１と、基板１１上に形成されたバッファ層１２と、バッファ層１２上に形成された光伝導層１３と、を備えている。また、平行伝送線路３は、一対のアンテナ２１，２１を備えている。なお、バッファ層１２は省略してもよい。 FIG. 1 is a perspective view schematically showing a photoconductive antenna element. As shown in the figure, the
The
基板１１は、化合物半導体の単結晶基板であり、ＳＩ－ＧａＡｓ（半絶縁性ガリウム砒素）により構成されている。基板１１の材料としては、ＳＩ－ＧａＡｓの他、バッファ層１２や光伝導層１３の材料（の格子定数）に応じて、例えばＳｉ、Ｇｅ、ＩｎＰ等の単結晶半導体を用いることもできる。
The substrate 11 is a compound semiconductor single crystal substrate and is made of SI-GaAs (semi-insulating gallium arsenide). As a material of the substrate 11, a single crystal semiconductor such as Si, Ge, InP, or the like can be used according to the material (lattice constant) of the buffer layer 12 and the photoconductive layer 13 in addition to SI-GaAs.
バッファ層１２は、基板１１の格子定数以上、光伝導層１３の格子定数以下もしくは基板１１の格子定数以下、光伝導層１３の格子定数以上となる材料を用いて、基板１１上にエピタキシャル成長させた薄膜であり、実施形態のものは、ＧａＡｓ（ガリウム砒素）により構成されている。バッファ層１２は、この上に積層する光伝導層１３の結晶性を制御するために設けられている。このため、バッファ層１２を構成する半導体材料は、Ｓｉ、ＡｌＧａＡｓ（ＬＴ－ＡｌＧａＡｓ）、ＩｎＧａＰ、ＡｌＡｓ、ＩｎＰ、ＩｎＡｌＡｓ、ＩｎＧａＡｓ（ＬＴ-ＩｎＧａＡｓ）、ＧａＡｓＳｂ、ＩｎＧａＡｓＰ、ＩｎＡｓ（ＬＴ－ＩｎＡｓ）、ＩｎＳｂ等を、基板１１および光伝導層１３の材料（の格子定数）に応じて任意に選択して用いることができる。また、これらを複数層としてもよい。
The buffer layer 12 was epitaxially grown on the substrate 11 using a material having a lattice constant greater than or equal to that of the substrate 11 and less than or equal to that of the photoconductive layer 13 or less than or equal to that of the substrate 11 and greater than or equal to that of the photoconductive layer 13. It is a thin film, and the thing of embodiment is comprised by GaAs (gallium arsenide). The buffer layer 12 is provided for controlling the crystallinity of the photoconductive layer 13 laminated thereon. Therefore, the semiconductor material constituting the buffer layer 12 is Si, AlGaAs (LT-AlGaAs), InGaP, AlAs, InP, InAlAs, InGaAs (LT-InGaAs), GaAsSb, InGaAsP, InAs (LT-InAs), InSb, and the like. Can be arbitrarily selected and used according to the material (lattice constant thereof) of the substrate 11 and the photoconductive layer 13. Moreover, these may be a plurality of layers.
光伝導層１３は、ＬＴ－ＧａＡｓ（低温成長ガリウム砒素）を、バッファ層１２を介して基板１１上にエピタキシャル成長させたものであり、表面（上面）に垂直に入射した励起光（フェムト秒パルスレーザ等）により、キャリア（電子）を発生させる（詳細は後述する）。この場合、光伝導層１３は、キャリアの応答速度（移動度）の高速化に鑑み、III－Ｖ族化合物半導体であるＧａＡｓを材料として低温でエピタキシャル成長（ＬＴ－ＧａＡｓ）させて形成されている。
The photoconductive layer 13 is obtained by epitaxially growing LT-GaAs (low-temperature gallium arsenide) on the substrate 11 via the buffer layer 12, and pumping light (femtosecond pulse laser incident perpendicularly to the surface (upper surface). Etc.) to generate carriers (electrons) (details will be described later). In this case, the photoconductive layer 13 is formed by epitaxial growth (LT-GaAs) at a low temperature using GaAs, which is a III-V group compound semiconductor, in view of increasing the response speed (mobility) of carriers.
そして、光伝導層１３は、このエピタキシャル成長の過程で、光電効果によって生ずるキャリア寿命が、下限においてサブピコ秒を越え、上限においてＶ族原子のクラスターの存在が維持される時間範囲内となるように、構成されている。すなわち、ＬＴ－ＧａＡｓの光伝導層１３は、電圧の印加により発生するキャリア（光励起キャリア）のキャリア寿命が、２ｐｓｅｃ以上、５０ｐｓｅｃ以下となるように、より好ましくはキャリア寿命が、６ｐｓｅｃ以上、３５ｐｓｅｃ以下となるように、構成されている。
In the process of this epitaxial growth, the photoconductive layer 13 has a carrier lifetime caused by the photoelectric effect within a time range in which the lower limit exceeds subpicoseconds and the upper limit maintains the presence of a group V atom cluster. It is configured. That is, the LT-GaAs photoconductive layer 13 has a carrier lifetime of 2 psec or more and 50 psec or less, more preferably a carrier lifetime of 6 psec or more and 35 psec or less. It is comprised so that it may become.
なお、光伝導層１３を構成するIII－Ｖ族化合物半導体は、上記のＬＴ－ＧａＡｓの他、基板１１がＧａＡｓやＧｅの場合には、ＧａＡｓ、ＡｌＧａＡｓ（ＬＴ－ＡｌＧａＡｓ）、ＩｎＧａＰ、ＡｌＡｓであってもよいし、基板１１がＩｎＰの場合には、ＩｎＰ、ＩｎＡｌＡｓ、ＩｎＧａＡｓ（ＬＴ-ＩｎＧａＡｓ）、ＧａＡｓＳｂ、ＩｎＧａＡｓＰであってもよい。さらに、光伝導層１３が、ＩｎＡｓ（ＬＴ－ＩｎＡｓ）、ＩｎＳｂ等であってもよい。
The III-V compound semiconductor constituting the photoconductive layer 13 is GaAs, AlGaAs (LT-AlGaAs), InGaP, or AlAs when the substrate 11 is GaAs or Ge in addition to the above-described LT-GaAs. Alternatively, when the substrate 11 is InP, it may be InP, InAlAs, InGaAs (LT-InGaAs), GaAsSb, or InGaAsP. Furthermore, the photoconductive layer 13 may be InAs (LT-InAs), InSb, or the like.
一方、平行伝送線路３は、光伝導層１３上に形成したダイポール型の一対のアンテナ２１，２１で構成されている。各アンテナ２１は、線状に延びるライン部２２と、ライン部２２の中央から内側に延設した電極部（電極）２３と、を有しており、ライン部２２の少なくとも一方の端部が電極パッド２４として機能する。一対のアンテナ２１，２１は、そのライン部２２，２２同士が平行に配置され、且つ相互の電極部２３，２３が所定のギャップを存して対向配置されている。
On the other hand, the parallel transmission line 3 includes a pair of dipole antennas 21 and 21 formed on the photoconductive layer 13. Each antenna 21 has a line portion 22 extending linearly and an electrode portion (electrode) 23 extending inward from the center of the line portion 22, and at least one end of the line portion 22 is an electrode. It functions as a pad 24. The pair of antennas 21 and 21 have their line portions 22 and 22 arranged in parallel to each other, and the mutual electrode portions 23 and 23 are arranged to face each other with a predetermined gap.
すなわち、相互の電極部２３，２３の対向端部間には、数μｍの幅（実施形態のものは、５μｍ程度）のギャップ部２５が構成されている。実施形態の各アンテナ２１は、Ａｕ（金）で構成されているが、Ａｌ、Ｔｉ、Ｃｒ、Ｐｄ、Ｐｔ、Ａｕ－Ｇｅ合金、Ａｌ－Ｔｉ合金等の導電性材料であってもよい。なお、平行伝送線路（アンテナ２１）３の形式は、ボウタイ型、ストリップライン型、スパイラル型等であってもよい。
That is, a gap portion 25 having a width of several μm (in the embodiment, about 5 μm) is formed between the opposing end portions of the mutual electrode portions 23 and 23. Each antenna 21 of the embodiment is made of Au (gold), but may be a conductive material such as Al, Ti, Cr, Pd, Pt, Au—Ge alloy, Al—Ti alloy. The form of the parallel transmission line (antenna 21) 3 may be a bow tie type, a strip line type, a spiral type, or the like.
このように構成された光伝導アンテナ素子１を、テラヘルツ波発生素子として機能させるには、一対の電極パッド２４，２４を介して、相互の電極部２３，２３間に所定のバイアス電圧を印加しておいて、ギャップ部２５に、フェムト秒パルスレーザ等の励起光（パルス光）を照射する。すなわち、電極部２３，２３間に電界を発生させておいて、励起光によりギャップ部２５の光伝導層１３を励起する。これにより、光伝導層１３にキャリア（電子および正孔）が生成され、且つ電極部２３，２３間の電圧（電界）でキャリアが加速されて瞬時電流が流れる。このパルス状電流の時間変動（超高速電流変調）によりテラヘルツ波（厳密にはテラヘルツパルス波）が発生し、誘電率の大きい基板１１側に強く放射される。
In order for the photoconductive antenna element 1 configured as described above to function as a terahertz wave generating element, a predetermined bias voltage is applied between the electrode portions 23 and 23 via the pair of electrode pads 24 and 24. In this case, the gap portion 25 is irradiated with excitation light (pulse light) such as a femtosecond pulse laser. That is, an electric field is generated between the electrode portions 23 and 23, and the photoconductive layer 13 in the gap portion 25 is excited by excitation light. As a result, carriers (electrons and holes) are generated in the photoconductive layer 13, and the carriers are accelerated by the voltage (electric field) between the electrode portions 23 and 23 so that an instantaneous current flows. A terahertz wave (strictly speaking, a terahertz pulse wave) is generated by the time variation (ultra-fast current modulation) of the pulsed current, and is strongly radiated toward the substrate 11 having a large dielectric constant.
ここで、キャリア寿命が２ｐｓｅｃ～５０ｐｓｅｃの光伝導層１３を有する光伝導アンテナ素子１の製造方法について、簡単に説明する。この製造方法では、先ずＭＢＥ（分子線エピタキシー）装置に基板１１をセットし、このＳＩ－ＧａＡｓの基板１１上に、０．１μｍ～０．５μｍ厚程度のＧａＡｓのバッファ層１２をエピタキシャル成長させる（バッファ層成膜工程）。具体的には、基板１１の温度を５００℃～６００℃、成長速度約１μｍ／ｈｏｕｒ、Ｇａ分子線強度に対するＡｓ分子線強度の比（Ｇａ／Ａｓ供給比）を約５～３０に設定し、バッファ層１２を０．１μｍ～０．５μｍ程度の膜厚にエピタキシャル成長させる。
Here, a method for manufacturing the photoconductive antenna element 1 having the photoconductive layer 13 having a carrier lifetime of 2 psec to 50 psec will be briefly described. In this manufacturing method, first, a substrate 11 is set in an MBE (molecular beam epitaxy) apparatus, and a GaAs buffer layer 12 having a thickness of about 0.1 μm to 0.5 μm is epitaxially grown on the SI-GaAs substrate 11 (buffer). Layer deposition step). Specifically, the temperature of the substrate 11 is set to 500 ° C. to 600 ° C., the growth rate is about 1 μm / hour, the ratio of As molecular beam intensity to Ga molecular beam intensity (Ga / As supply ratio) is set to about 5 to 30, The buffer layer 12 is epitaxially grown to a thickness of about 0.1 μm to 0.5 μm.
次に、バッファ層１２上に、１μｍ～２μｍ厚程度のＬＴ－ＧａＡｓの光伝導層１３をエピタキシャル成長させる（光伝導層成膜工程）。具体的には、ＭＢＥ装置にセットした基板１１の温度を、４００℃以下（実施形態のものは、約３００±５０℃）に降温させ、成長速度をバッファ層１２と同様に約１μｍ／ｈとし、Ｇａ／Ａｓ供給比をバッファ層１２の供給比以上として、光伝導層１３を１μｍ～２μｍ程度の膜厚にエピタキシャル成長させる。これにより、基板１１、バッファ層１２および光伝導層１３から成る光伝導基板２が形成される。
Next, an LT-GaAs photoconductive layer 13 having a thickness of about 1 μm to 2 μm is epitaxially grown on the buffer layer 12 (photoconductive layer forming step). Specifically, the temperature of the substrate 11 set in the MBE apparatus is lowered to 400 ° C. or less (in the embodiment, about 300 ± 50 ° C.), and the growth rate is set to about 1 μm / h similarly to the buffer layer 12. The Ga / As supply ratio is set to be equal to or higher than the supply ratio of the buffer layer 12, and the photoconductive layer 13 is epitaxially grown to a thickness of about 1 μm to 2 μm. Thereby, the photoconductive substrate 2 including the substrate 11, the buffer layer 12, and the photoconductive layer 13 is formed.
次に、この光伝導基板２に対し、熱処理を行う（アニール工程）。具体的には、ＭＢＥ装置内において、光伝導基板２に対しＡｓ分子線を照射した状態のまま、基板１１の温度を約６００℃とし（昇温）、約５分間の熱処理を行う。このアニール工程により、光伝導層１３のＡｓがＡｓクラスターとして、ＧａＡｓの結晶中に析出する。
Next, heat treatment is performed on the photoconductive substrate 2 (annealing step). Specifically, in the MBE apparatus, the temperature of the substrate 11 is set to about 600 ° C. (temperature increase) while the photoconductive substrate 2 is irradiated with the As molecular beam, and heat treatment is performed for about 5 minutes. As a result of this annealing step, As in the photoconductive layer 13 is deposited as As clusters in the GaAs crystal.
このようにして、光伝導基板２が完成したら、光伝導基板２上に真空蒸着により一対のアンテナ２１，２１を成膜する（アンテナ成膜工程）。具体的には、光伝導基板２（光伝導層１３）上にレジスト膜を形成した後、フォトリソグラフィ法により、平行伝送線路（一対のアンテナ２１，２１）３の形状のレジストパターンを形成し、アンテナ材料のＡｕを真空蒸着により成膜し、不要部分を剥離除去する。
Thus, when the photoconductive substrate 2 is completed, a pair of antennas 21 and 21 are formed on the photoconductive substrate 2 by vacuum deposition (antenna film forming step). Specifically, after forming a resist film on the photoconductive substrate 2 (photoconductive layer 13), a resist pattern in the shape of parallel transmission lines (a pair of antennas 21, 21) 3 is formed by photolithography, The antenna material Au is deposited by vacuum deposition, and unnecessary portions are peeled off.
次に、図２ないし図５を参照して、キャリア寿命が２ｐｓｅｃ～５０ｐｓｅｃの光伝導層１３を有する光伝導アンテナ素子１の評価結果等について説明する。この評価は、いわゆる時間領域分光法やポンプ・プローブ法を用い、光伝導アンテナ素子１から発生したテラヘルツ波の波形を、フーリエ変換することにより得られた、テラヘルツ波スペクトルに基づいて、行っている。
Next, the evaluation results of the photoconductive antenna element 1 having the photoconductive layer 13 having a carrier lifetime of 2 psec to 50 psec will be described with reference to FIGS. This evaluation is performed based on the terahertz wave spectrum obtained by Fourier transforming the waveform of the terahertz wave generated from the photoconductive antenna element 1 using so-called time domain spectroscopy or pump-probe method. .
図２は、光伝導層１３のキャリア寿命が１０ｐｓｅｃと１ｐｓｅｃ未満との場合における、テラヘルツ波スペクトルの図である。縦軸はテラヘルツ波の強度、横軸はテラヘルツ波の周波数を表している。同図に示すように、キャリア寿命が１ｐｓｅｃ未満（サブピコ秒）の光伝導アンテナ素子１に対し、キャリア寿命が１０ｐｓｅｃの光伝導アンテナ素子１は、全ての周波数帯域において、ダイナミックレンジの向上が確認された。
FIG. 2 is a diagram of a terahertz wave spectrum when the carrier lifetime of the photoconductive layer 13 is 10 psec and less than 1 psec. The vertical axis represents the intensity of the terahertz wave, and the horizontal axis represents the frequency of the terahertz wave. As shown in the figure, the photoconductive antenna element 1 having a carrier lifetime of less than 1 psec (sub-picosecond) is improved in the dynamic range in the photoconductive antenna element 1 having a carrier lifetime of 10 psec in all frequency bands. It was.
図３は、テラヘルツ波２．５ＴＨｚ付近において、光伝導層１３のキャリア寿命を可変として測定した、キャリア寿命とダイナミックレンジと関係を表した図（図３（ａ））、およびそのキャリア寿命を対数表示とした図（図３（ｂ））である。同図に示すように、規格化したダイナミックレンジ０．６以上を、従来のものに比して明らかに「効果あり」とした場合、キャリア寿命が２ｐｓｅｃ～５０ｐｓｅｃであることが好ましいことが確認される。さらに、測定数や測定誤差等を考慮する（確実性担保）とキャリア寿命が６ｐｓｅｃ～３５ｐｓｅｃであることが好ましく、製品的には、１０ｐｓｅｃ～２０ｐｓｅｃであることが好ましい。
FIG. 3 is a graph showing the relationship between the carrier lifetime and the dynamic range (FIG. 3 (a)) measured with the carrier lifetime of the photoconductive layer 13 being variable near the terahertz wave of 2.5 THz, and the carrier lifetime is logarithmically. It is the figure (FIG.3 (b)) made into a display. As shown in the figure, when the standardized dynamic range of 0.6 or more is clearly “effective” compared to the conventional one, it is confirmed that the carrier life is preferably 2 to 50 psec. The Furthermore, considering the number of measurements, measurement error, etc. (assurance of certainty), the carrier life is preferably 6 psec to 35 psec, and in terms of product, it is preferably 10 psec to 20 psec.
図４は、テラヘルツ波２．５ＴＨｚ付近において、光伝導層１３の成長温度を可変として測定した、成長温度とダイナミックレンジとの関係を表した図である。この場合の横軸は、ＬＴ－ＧａＡｓ（光伝導層１３）の低温成長で使用される、一般的な最低温度を基準温度「０度」としたときの相対温度である。この場合、製造方法で説明した「約３００±５０℃」の「約３００℃」は、横軸の相対温度では約１００度に相当する。同図に示すように、規格化したダイナミックレンジ０．６以上を、従来のものに比して明らかに「効果あり」とした場合、光伝導層１３の成長温度は、相対温度で７５度～１３０度が好ましいことが分かる。より好ましくは、１００度前後である。すなわち、光伝導層１３を３００℃近辺の温度で低温成長させることが好ましい。
FIG. 4 is a diagram showing the relationship between the growth temperature and the dynamic range, measured by varying the growth temperature of the photoconductive layer 13 in the vicinity of the terahertz wave of 2.5 THz. The horizontal axis in this case is the relative temperature when the general minimum temperature used in the low-temperature growth of LT-GaAs (photoconductive layer 13) is the reference temperature “0 degree”. In this case, “about 300 ° C.” in “about 300 ± 50 ° C.” described in the manufacturing method corresponds to about 100 degrees in the relative temperature on the horizontal axis. As shown in the figure, when the normalized dynamic range of 0.6 or more is clearly “effective” as compared with the conventional one, the growth temperature of the photoconductive layer 13 is 75 ° C. or more in relative temperature. It can be seen that 130 degrees is preferable. More preferably, it is around 100 degrees. That is, it is preferable to grow the photoconductive layer 13 at a low temperature around 300 ° C.
図５は、光伝導層１３の成長温度を可変として測定した、成長温度とキャリア寿命との関係を表した図である。すなわち、図５は、成長温度を図４のダイナミックレンジに代えて、キャリア寿命で評価したものである。同図に示すように、キャリア寿命２ｐｓｅｃ～５０ｐｓｅｃを達成するためには、成長温度が相対温度で７０度～１３０度であることが好ましい。この点においても、光伝導層１３を３００℃近辺の温度で低温成長させることが好ましいことが分かる。
FIG. 5 is a diagram showing the relationship between the growth temperature and the carrier lifetime, measured with the growth temperature of the photoconductive layer 13 being variable. That is, FIG. 5 shows the growth temperature evaluated by the carrier life instead of the dynamic range of FIG. As shown in the figure, in order to achieve a carrier lifetime of 2 to 50 psec, the growth temperature is preferably 70 to 130 degrees relative temperature. Also in this respect, it can be seen that it is preferable to grow the photoconductive layer 13 at a low temperature around 300.degree.
以上のように、第１実施形態によれば、テラヘルツ波発生における光伝導層１３を、キャリア寿命２ｐｓｅｃ～５０ｐｓｅｃのものとすることにより、テラヘルツ波の全ての周波数帯域において、ダイナミックレンジの向上を図ることができる。
As described above, according to the first embodiment, the photoconductive layer 13 for generating terahertz waves has a carrier lifetime of 2 psec to 50 psec, thereby improving the dynamic range in all frequency bands of terahertz waves. be able to.
次に、図６を参照して、第２実施形態に係る光伝導アンテナ素子１につき、主に第１実施形態と異なる部分について説明する。この実施形態では、光伝導基板２において、バッファ層１２と光伝導層１３との間に、キャリア移動阻止層１５を有している。この場合の基板１１は、そのバンドギャップが、光伝導層１３のバンドギャップよりも小さいＳｉで構成されている（Ｇｅでもよい）。また、キャリア移動阻止層１５は、そのバンドギャップが、基板１１のバンドギャップよりも大きく且つ光伝導層１３のバンドギャップよりも大きくなる半導体材料、例えばＡｌＧａＡｓ（アルミニウムガリウム砒素）で構成されている。
Next, with reference to FIG. 6, with respect to the photoconductive antenna element 1 according to the second embodiment, parts different from the first embodiment will be mainly described. In this embodiment, the photoconductive substrate 2 has a carrier movement blocking layer 15 between the buffer layer 12 and the photoconductive layer 13. The substrate 11 in this case is made of Si (which may be Ge) whose band gap is smaller than the band gap of the photoconductive layer 13. The carrier movement blocking layer 15 is made of a semiconductor material such as AlGaAs (aluminum gallium arsenide) whose band gap is larger than the band gap of the substrate 11 and larger than the band gap of the photoconductive layer 13.
このような、第２実施形態の光伝導アンテナ素子１では、キャリア移動阻止層１５と光伝導層１３との界面の光伝導層１３側に、２次元電子ガスが発生する。この２次元電子ガスは、光伝導層１３で発生したキャリアを閉じ込め、キャリアの基板１１側への移動を阻止する。このため、発生したキャリアが基板１１に吸収されることがなく、キャリアをテラヘルツ波の発生に効率良く使用することができる。また、Ｓｉの基板１１は、安価で強度が高いだけでなく、発生したテラヘルツ波をほとんど吸収することがなく、この点でもテラヘルツ波の発生効率を高めることができる。
In such a photoconductive antenna element 1 of the second embodiment, a two-dimensional electron gas is generated on the photoconductive layer 13 side of the interface between the carrier movement blocking layer 15 and the photoconductive layer 13. This two-dimensional electron gas confines the carriers generated in the photoconductive layer 13 and prevents the carriers from moving to the substrate 11 side. For this reason, the generated carrier is not absorbed by the substrate 11, and the carrier can be efficiently used for generating the terahertz wave. Further, the Si substrate 11 is not only inexpensive and high in strength, but also hardly absorbs the generated terahertz wave, and in this respect, the generation efficiency of the terahertz wave can be increased.
次に、図７を参照して、第３実施形態に係る光伝導アンテナ素子１につき、主に第１実施形態と異なる部分について説明する。この実施形態は、テラヘルツ応答が、一対のアンテナの電極部２３間における光伝導層１３で行われることに着目したものであり、ギャップ部２５にのみ光伝導層１３を形成している。具体的には、各電極部２３を、光伝導層１３およびバッファ層１２の厚みに相当する分、基板１１から立ち上げるように形成し、この立ち上げた一対の電極部２３間に、バッファ層１２および光伝導層１３を形成（成膜）している（同図（ａ）参照）。
Next, with reference to FIG. 7, a description will be given mainly of the differences from the first embodiment in the photoconductive antenna element 1 according to the third embodiment. In this embodiment, attention is paid to the fact that the terahertz response is performed in the photoconductive layer 13 between the electrode portions 23 of the pair of antennas, and the photoconductive layer 13 is formed only in the gap portion 25. Specifically, each electrode portion 23 is formed so as to rise from the substrate 11 by an amount corresponding to the thickness of the photoconductive layer 13 and the buffer layer 12, and the buffer layer is interposed between the pair of raised electrode portions 23. 12 and a photoconductive layer 13 are formed (film formation) (see FIG. 5A).
同様に、図７（ｂ）の光伝導アンテナ素子１では、各電極部２３を、光伝導層１３の厚みに相当する分、バッファ層１２から立ち上げるように形成し、この立ち上げた一対の電極部２３間に、光伝導層１３を形成（成膜）している。
Similarly, in the photoconductive antenna element 1 of FIG. 7B, each electrode portion 23 is formed to rise from the buffer layer 12 by an amount corresponding to the thickness of the photoconductive layer 13, and this pair of raised A photoconductive layer 13 is formed (deposited) between the electrode portions 23.
このような、第３実施形態の光伝導アンテナ素子１では、発生するテラヘルツ波のＳ／Ｎ比またはダイナミックレンジを向上させることができる。
In such a photoconductive antenna element 1 of the third embodiment, it is possible to improve the S / N ratio or dynamic range of the generated terahertz wave.
１ 光伝導アンテナ素子、２ 光伝導基板、３ 平行伝送線路、１１ 基板、１２ バッファ層、１３ 光伝導層
1 Photoconductive antenna element, 2 Photoconductive substrate, 3 Parallel transmission line, 11 Substrate, 12 Buffer layer, 13 Photoconductive layer
Claims (5)
- 基板と、
前記基板上にエピタキシャル成長させたIII－Ｖ族化合物半導体から成る光伝導層と、を備え、
前記光伝導層は、光電効果によって生ずるキャリア寿命が、下限においてサブピコ秒を越え、上限においてＶ族原子のクラスターの存在が維持される時間範囲内であることを特徴とする光伝導基板。 A substrate,
A photoconductive layer made of a III-V group compound semiconductor epitaxially grown on the substrate,
The photoconductive substrate is characterized in that the carrier lifetime generated by the photoelectric effect is within a time range in which the lower limit exceeds subpicoseconds and the presence of a cluster of V group atoms is maintained at the upper limit. - 前記キャリア寿命が、２ｐｓｅｃ以上、５０ｐｓｅｃ以下であ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 carrier lifetime is 2 psec or more and 50 psec or less.
- 前記キャリア寿命が、６ｐｓｅｃ以上、３５ｐｓｅｃ以下であることを特徴とする請求項２に記載の光伝導基板。 The photoconductive substrate according to claim 2, wherein the carrier lifetime is 6 psec or more and 35 psec or less.
- 前記光伝導層は、ＧａＡｓ化合物半導体層であることを特徴とする請求項２に記載の光伝導基板。 The photoconductive substrate according to claim 2, wherein the photoconductive layer is a GaAs compound semiconductor layer.
- 請求項２に記載の光伝導基板と、
前記光伝導基板上に形成された平行伝送線路と、を備えたことを特徴とする電磁波発生装置。 A photoconductive substrate according to claim 2;
An electromagnetic wave generator comprising: a parallel transmission line formed on the photoconductive substrate.
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| JP2020092265A (en) * | 2018-12-05 | 2020-06-11 | 光寶光電（常州）有限公司 | Light emitting package structure and method of manufacturing the same |
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