教授 河村裕一
Professor YuichiKawa皿ura 講師 森本恵造
Assistant Professor Keizo Morimot1.研究現況(Current
Projects)概要 当研究室では，ナノ構造を有する量子効果光 デバイスの革新のために，新しい半導体などの材料 の結品成長と物性評価、及びそのデバイス加工プロ セスとデバイス特性評価を研究している主なテー マは、次世代光デバイスの提案とその材料の分子線 成長法および有機金属気相成長法による開発と評 価ならびにデバイス試作，ナノメータデ、パイス実現 のための原子レベノレ結晶成長機構の解祈，加工・評 価技術、光デパイスの応用の研究などである 1. 1 分子線エピタキシャ jレ成長による化合物半導 体量子井戸構造の作製と応用 (Growth of
Co皿pound Semiconductor Quantu皿 Well
Structure Grown by Molecular Beam Epitaxy and Device Application)
InP 基板上の InGaAsSbN 系化合物半導体は波長 2
~3μm 帯の中赤外領域における新しい材料系と して注目されており、環境計測・化学分析・医療な どで有用な波長 2~3μm 帯のナノ構造高性能中 赤外発光及び受光テsパイスの材料として期待され ている。本年度は分子線結晶成長法 (MBE 法)に より試作した InP 基板上の InAsSbN 赤外量子井戸レ ーザダイオード及び InGaAsN 赤外光検出器の特性 評価を行った。まず InAsSbN 量子井戸レーザに関し ては、 InAsSbN 量子井戸活性層の最適化を目的とし て、量子井戸幅を 3nm に固定し、成長温度も 4800C 一定とした条件のものとで、 N 導入の効果、 Sb 導 入の効果を調べた。 InAs に N を 1% 導入すること により、格子不整合が減少し、約 2.4μm付近に単 一の PL ピークを観測することが出来た。さらに Sb を 2% 、 4%及び 6% 導入することにより発光 波長を長波長化出来ることが明らかとなった。 Sb が 2% の場合にもっとも発光強度が強くなること を見出し、この条件において単一量子井戸レーザを 作製した。その結果、これまでで最も関値電流密度 でのレーザ発振を得ることが出来た。他方赤外検出 器に関しては、昨年度 InGaAs/GaAsSb 層タイプ II 量子井戸光吸収層を持いた構造において、波長 2μ m 帯の検出器としては最も小さい暗電流を得るこ とに成功したが、今年度は InGaAs/GaAsSb 層タイプ II 量子井戸光吸収層を持いた構造において非冷却
-38 ー
の赤外 2 次元センサアレイを作製し、撮像実験を行 うことに成功した。
1.2 化合物半導体の有機金属気相成長法による作製 とその評価 (Growth of Compound
Semiconductοrs by Metalorganic Vapor Phase Epitaxy and Their Characterization)
有機金属気相成長法 (MOVP E 法)を用いて 窒化物半導体を作製し，その物性制御を試みている。
窒化物半導体は発光素子として近年，多くの照明機 器に応用され，省エネ，超寿命化に寄与している。本 研究では，この物質系でパンドオフセットが大きく 取れることに着目し，短波長サブFバンド間遷移デバ イスへの応用を目指している。窒化物半導体で高品 質な量子構造を作るためには lOOOOC の高温，常圧 下で反応性の高い原料を層流にする必要がある。そ のためにはサセプターの上面温度を一様にし，熱対 流を最小限にしなければならない。ヒータの材質，
形状，サセプターの保持方法を工夫し，長期間に渡 り再現性良く，原料流を層流にすることに成功した。
この成果を高品質量子構造作製に応用していく予 定である。
1. 3 ナノメータデバイス・プロセスその他
Nanophase devices, materials and characterization)
分子線成長法により成長した InP 基板上 InGaAsSbN 層の光学的及び電気的特性に対する Sb 導入効果およびアニール効果を測定した。その結果、
Sb を微量添加した結品においては PR スベクトノレ の半値幅が減少し結晶性が向上していることが明 らかとなった。またアニーノレにより 6500C以上で光 学的特性が改善しバンドギャップが大きくなるこ
とを見出した。また電気的特性に関しでも、 6 5
OOC 以上において移動度が増大することが観測さ れた。
2 研究発表 (Publications)
2.1 学会誌原著論文 (Original ArticIes in Refereed Journals)
I) "Optical characterization of InGaAsN layers grown on InP substrates"
M. Yoshikawa, K. Miura, Y. Iguchi, Y. Kawamura J. Crystal Growth vol.3ll, 1745 (2009)
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(f) (Invited Papers at Conferences and Papers Reviewed and Printed in Conference Proceedings)1) The 36th international conference of InP and related materials, New port, USA
" Low Dark Current SWIR Photodiode with InGaAs/GaAsSb Type 11 Quantum Wells grown on InP Substrate"
H.Inada, K.Miura, Y.Nagai, M.Tsubokura, Y.Iguchi , Y.Kawamura
Low dark current photodiodes (PDs) in the short wavelength infrared (SWIR) region (1.0-2.5~m) are expected for many applications such as environmental gas detection, process check in chemical plants (e.g. pharmacy), and biodiagnostics. Pin-PDs with the cutoff wavelength up to
2.6~m have been fabricated using In'rich lattice-mismatched InGaAs layers on InP substrates. HgCdTe is predominantly used as a focal plane array for imaging applications.
However, because of high dark current, these devices require coo]er which increases power consumption, size and cost of the system. It was reported that InGaAs/GaAsSb quantum wells with type 11 staggered band alignments have optical response up to 2.5~m. This material system is attractive for realizing low dark current PDs in the SWIR region, owing to lattice-matching to InP substrate. In this work, we investigated GaAsSb growth condition and realized InGaAs/GaAsSb type 11-PD with low dark current which is more than one order of magnitude lower than that of conventional HgCdTe detector.
InGaAs/GaAsSb quntum well structures were grown on S-doped (100) InP substrates by solid source molecular beam epitaxy (MBE) method. GaAsSb layers with 1-3~m thickness were also grown for the evaluation of crystal quality of GaAsSb. The growth temperature (Ts) was changed between 450°C and 520°C. The growth rate was 1.8~m/h for InGaAs layer and 0.8~m/h for GaAsSb. Tetramer As, and monomer Sb, were used for group V beam sources. The V IIII flux ratio was changed between 12 and 35. X-ray diffraction (XRD) and room-temperature photoluminescence (PL) were utilized to characterize the as-grown samples. The absorption layers have 250 pair-InGaAs(5nm)/GaAsSb(5nm) quantUm well structures or GaAsSb (2.5~m) layers. The p-n junctions were formed in the absorption layer by the selected thermal diffusion of zinc. SiN and SiON were used for passivation and anti-reflection, respectively. Diameter of light-receiving region was 140~m. Au-Zn' alloy and Au-Ge-Ni alloy were used as a p-electrodes and n-electrodes, respectively.
The PL peak was observed at 2.4 ~m originated from type II quantum wells. Temperature dependence of dark. The factor n calculated from temperature dependence of dark
current was 2.2 and 1.2 for the samples grown with VIIII~12
and 24, respectively. As the factor n approaches unity, the generation-recombination process is suppressed. Therefore, the crystalline quality seems to be improved with increasing the VII11 flux ratio. Dark current of the GaAsSb-PD grown with V/III~24 is lower than that ofPD grown with V/III~12.
The improvement of crystalline quality of GaAsSb by increasing VlIII flux ratio resulted in the reduction of factor n and dark current. Using the growth condition of GaAsSb with V IIII~24 and Ts~480 °C, we fabricated InGaAs/GaAsSb-PDs. Dark current was 140nA at I V reverse bias. Dark current density was 2.8mA/cm'. Although the device structure is different, this value is better than the former reported value. It should be mentioned that dark current of InGaAs/GaAsSbcPD can be further reduced by optimizing the formation of InGaAs/GaAsSb interface, since dark currents of GaAsSb-and InGaAs-PD are much lower than that of InGaAs/GaAsSb-PD.
We have successfully demonstrated planer type InGaAs/GaAsSb type II quantum well PDs. Dark current was 140nA at IV reverse bias for a 140~m-diameter device.
Low dark current was obtained as a result of improvement of GaAsSb crystalline quality. It was shown that InGaAs/GaAsSb type II quantum well PDs have a potential of uncooled operation for imaging applications.
2) The 14'" international conference of modulated semiconductor structures Th-mP9, Kobe, Japan July 24-28
"Growth and characterization of strain-compensated InGaAs/GaAsSb type 11 mUltiple quantum wells on InP Substrate"
Y. Yonezawa, R. Hiraike, K. Miura Y. Iguchi, Y. Kawamura InGaAs/GaAsSb type II multiple quantum wells (MQWs) on InP substrates are very attractive for light emitting diodes and low dark current photodiodes in the 2 ~m wavelength region, which are expected for many applications such as chemical sensing and medical diagnostics. Very recently, we investigated InGaAs/GaAsSb type II-photodiodes with low dark current and cut off wavelength of 2.4 ~m at room temperature by optimizing growth condition. It was proposed that introduction of strain-compensated structure makes it possible to get longer absorption and emission wavelength in InGaAs/GaAsSb type II MQWs [3]. The strain-compensated structure is very useful to get thick absorption layers for photodiodes.
In this work, strain-compensated InGaAs/GaAsSb type II MQWs were grown by molecular beam epitaxy (MBE) and their optical and electrical properties were studied. High quality strain-compensated type II MQWs were successfully grown, which have longer emission wavelength than that of lattice-matched type 11 MQWs. The observed PL peak energy shift is 43 meV at 300K. In addition, the PL intensity and the electron mobility of the strain-compensated MQWs are comparable to those of the lattice-matched MQWs.
Strain-compensated InGaAs(5nm)/GaAsSb(5nm) type II MQW layers as well as lattice-matched type 11 MQW layers were grown on Fe-doped (lOO}lnP substrates by solid source
MBE. The growth temperature was 480°C, which was monitored by a calibrated infrared pyrometer. Tetramer AS4
and Sb4 were used for group V beam sources. Prior to the growth, the InP substrate surface was thermally cleaned at SISoC under Arsenic (As) vapor pressure. The growth rates ofInGaAs and GaAsSb layers. were -l.S/lnvh and -0.7/lnvh, respectively. The lnAlAs buffer layer (ISO nm thickness) was grown for all samples studied here. All epitaxial layers were intentionally un-doped. The As was supplied by a needle-valve cracking cell, where the cracking zone temperature is set to be low at 600°C, so that tetramer AS4
was supplied in this study. The tetramer Sb4 was supplied by a conventional effusion cell. Ga, Al and In were also supplied by conventional effusion cells.
Photoluminescence (PL) and Hall measurements were utilized to characterize the as-grown samples.
Prior to the investigation of the InGaAs/GaAsSb type II MQWs, the optical and electrical properties of un-doped GaAsSb layers grown in this study are described. PL measurements were carried out using a standard lock-in amplifier technique. PL was detected by a cooled InSb photo-detector. A YAG laser (1064 nm) was used as the modulation beam. It is clearly seen that the PL peak wavelength is almost the same and the PL intensity is comparable between these two samples. The FWHM of the GaAsSb layer is very close to that of the InGaAs layer.
Electrical properties were also characterized by using Hall meas,urements, which were carried out using Van-der-Pau method. The Hall measurements show that the electron concentration of the GaAsSb layer at 300K is 2-3xl 0" cm", which is almost the same as that of the InGaAs layer (1-2 x 10" cm·'). These results indicate that the crystal quality of the GaAsSb layer in this study is comparable to that of the InGaAs layer.
Next, the properties of the strain-compensated JnGaAs/GaAsSb type II MQWs are investigated together with those of the lattice-matched type WMQWs. The Ga composition of the InGaAs is 0.47 and the Sb composition of the GaAsSh is 0.49 for the lattice-matched MQWs. On the other hand, the Ga composition of the InGaAs is increased to 0.S4 and the Sb composition of the GaAsSb is increased to 0.S6 for the strain-compensated MQWs, where the InGaAs layer has 0.6% tensile strain and the GaAsSb layer has 0.6%
compressive strain. It was confirmed by X -ray diffraction measurements that the Oth main peak of the strain-compensated type II MQWs is very close to the InP substrate diffraction peak, indicating that the net strain is zero (strain-compensated condition is satisfied).
The peak wavelength of the strain-compensated MQWs is longer than that of the lattice-matched MQWs. The observed peak energy shift is 43 meV at 300K. On the other hand, the calculated energy shift of 31 me V using the model solid theory. The energy difference between the experimental value and the calculated value is 12 me V. As already mentioned, the Ga composition of the InGaAs is 0.47 and the Sb composition of the GaAsSb is 0.49 for the lattice-matched MQWs, while the Ga composition of the InGaAs is 0.S4 and the Sb composition of the GaAsSb is 0.56 for the strain-compensated MQWs. In the calculation,
we neglect the change of the effective band gap at the InGaAs/GaAsSb hetero-interface, because the composition changes of Ga and Sb are the same (0.07). Probably, this assumption causes the difference between the experimental energy shift and the calculated energy shift. In addition, the PL intensity of the strain-compensated MQWs is almost the same level as that of the lattice-matched MQWs, suggesting that the crystal quality of the strain-compensated MQWs is comparable to that of the lattice-matched MQWs. The peak wavelength shifts from 2470 to 2290 nm ~ith decreasing the temperature from 300 to 10K. It was found that the PL peak energy of the strain-compensated MQWs is smaller than that of the lattice-matched MQWs in all temperature range from 10K to 300K. It is noted that at low temperature, the PL peak energy sift is 20 meV, which is smaller than that at 300K.
The reason of this is not clear at present. There is a possibility that at the InGaAs/GaAsSb hetero-interfaces, some kinds of structural fluctuations; such as compositional fluctuation, -exist. Such a structural imperfection at the hetero-interface may cause the complicated temperature dependence of the PL peak energy at low temperature. In fact, a double peak structure was observed for the lattice-matched MQWs at 150 K as shown in Figure S. This suggests the existence of the structural imperfection at the hetero-interface, although such a clear double peak structure was not observed for the strain-compensated MQWs.
Further studies are still necessary to clarifY the mechanism.
Finally, electrical properties were studied by using Hall measurements. It is clearly seen that the electron concentration and the electron mobility are comparable for both samples. These results suggest that impurity andlor defect densities of the strain-compensated type II MQWs are the same level as those of the lattice-matched type II MQWs.
These results indicate that the strain-compensated structure is very effective to get longer wavelength operation of the InGaAs/GaAsSb type II MQW photodetectors.
2.5 <'jt~c9Eil\(~ (Presentations at Meetings etc.) 2.5.1 <'jt~ • OOf!li(~ilI(Presentations at Meetings of Academic Societies and Conference)
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講演予稿集第 l 分冊
5) 第 57回応用物理学関係連合講演会 (3月，神奈川) 吉川真央、三浦広平、猪口康博、河村裕一
flnP基板上のInGaAsSbN層のアニーノレ効果」
講演予稿第 l 分冊
。 第57 回訪問媛群関協藍合誠実会@月，特鋼 ID 三浦広平、森大樹、永井陽 、稲田博史、猪口 康博、河村裕一
flnGaAs/GaAsSb タイプ口量子井戸を用いた 非冷却赤外 2 次元センサアレイ J
講演予稿第 l 分冊
7) 第 57 町間被理学忌明車合言需給@月，耕痢 ID 井上直久、後藤康則、杉山隆英、河村裕一
「低炭素濃度 Si 結晶中の赤外吸収 (7)J
講演予稿第 l 分冊
8) 第 57 巨E部数躍浮諜総量合講演会@月，持家IID 井上直久、須田良幸、菅谷孝夫、河村裕一
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講演予稿第 I 分冊
3 教育(Education)
.3.1 研究科授業科目 (Lectures in Graduate Coureses)
1)工学研究科電子情報専攻修 1 前期 半導体物理特論河村裕一，森本恵造 2) 工学部電子物理学科学 1 後期
物理学基礎実験河村裕一 3.2 学生(Students)
1)西野正嗣大阪府立大大学院工学研究科M2 flnAsSbN 系歪み量子井戸レーザの研究」
2) 吉川真央大阪府立府立大学院工学研究M2 flnGaAsN の光学的評価に関する研究」
3) 平池龍馬 大阪府立大大学院工学研究科MI flnP 基板上の GaAsSb の評価に関する研究」
4) 山本隆子 大阪府立大大学院工学研究科Ml flnGaAsP/lnAIAsP MQW 構造に関する研究J 5) 王野琢也 大阪府立大大学工学部 B4
flnGaAsP/lnAIAs/lnP 非対称量子井戸の研究J 6) 神尾達弥大阪府立大大学工学部 B4
flnGaAsP/lnAIAsPMQW 光変調器の研究」
7) 坂東直人
flnGaAsN/GaAsb タイプ IIMQWの研究1 8) 白藤重俊大阪府立大学工学部学部研究生
「光変調器に関する研究」
4. 各種の活動 (Miscellaneous)
4.1 研究費補助金等‑41
1)共同研究住友電気工業
「中赤外光検出器の開発」河村裕一 2) 科学研究費補助金
flnAsSbN/GaAsSbN 新構造量子井戸を用い た赤外デ、パイスの研究」河村裕一
4.2 海外出張等 (Visits Abroad)
4.3 特許等
4.4学会活動等 (Activitiesin Academic Societies etc
l .
河村裕一応用物理学会会員，日本真空協会会員、
電気学会会員、日本表面科学会会員、応 用物理学会応用電子物性分科会会員、
日本表面科学会関西支部幹事、パワー半 導体レーザ技術調査専門委員会委員 森本恵造:日本物理学会会員，応用物理学会会員