氏名　Joel Tacla Asubar
学位の種類　博士（工学）
学位記番号　博甲第517号
学位授与の日付　平成21年8月31日
学位論文題目　Epitaxy and characterization of ZnSnAs2 thin films on nearly lattice-matched InP substrates with potential room-temperature spintronics applications　（室温半導体スピントロニクス応用に向けたInP格子整合ZnSnAs2薄膜のエピタキシャル成長と評価に関する研究）
論文審査委員
　主査　准教授　内富　直隆
　副査　教授　上林　利生
　副査　教授　打木　久雄
　副査　准教授　加藤　有行
　副査　准教授　石橋　隆幸

• 論文目次
• 論文要旨
• 論文審査の結果の要旨

［平成21（2009）年度博士論文題名一覧］ ［博士論文題名一覧］に戻る．

LIST OF ABBREVIATIONS
CHAPTER ONE　INTRODUCTION
　1　Introduction　p.1
　1.1　Ternary Chalcopyrite Semiconductors　p.1
　1.2　The ZnSnAs2 ternary semiconductor　p.5
　1.3　Spintronics　p.9
　1.4　Diluted Magnetic Semiconductor （DMS）　p.11
　1.5　GaMnAs DMS system　p.13
　1.6　Theories of Ferromagenetism in Mn-doped III-V based DMSs　p.14
　1.7　Chalcopyrite seniconductor as a DMS host propect　p.16
CHAPTER TWO
　2　Experimental Prepararion　p.21
　2.1　Sample Preparation　p.21
　2.1.1　Molecular Beam Epitaxy　p.21
　2.1.2　The Growth Chamber　p.24
　2.2　Sample Characterization　p.26
　2.2.1　RHEED-Reflection High-Energy Electron Difftion　p.26
　2.2.2　XRD-X-ray Diffraction　p.30
　2.2.3　EPMA-Electron Probe MicroAnalysis　p.34
　2.2.4　AFM-Atomic Force Microscopy　p.36
　2.2.5　Raman scattering spectrosopy　p.38
　2.2.6　TEM-Transmission Electron Microscopy　p.40
　2.2.7　Hall Effect Measurements　p.42
　2.2.8　Vander Pauw Resistivity Measuremants　p.45
　2.2.9　SQUID- Superconducting quantum Interference Device　p.54
CHAPTER THREE
　3　MBE growth　p.56
　3.1　Introduction　p.56
　3.2　BEP Measurements　p.56
　3.3　Growth Sequence　p.61
　3.4　RHEED observation　p.64
　3.5　XRD and EPMA　p.66
　3.6　HR-xrd （High-resolution X-ray Diffraction） and RMS （Reciprocal Space Mapping）　p.69
　3.7　Conclusion　p.77
CHAPTER FOUR
　4　Transport Properties　p.78
　4.1　Introduction　p.78
　4.2　van der Pauw and Hall Effect measurement wxperimental procedure　p.79
　4.3　p.80
　4.4　p.81
　4.5　Impurity Band model　p.84
　4.6　Impurity Band modeling of the transport properties　p.85
　4.7　Negative Magnetoresistance　p.91
　4.8　Band gap estimation from the temperature dependence of resistivity at high temperature regime　p.93
　4.9　conclusion　p.95
CHAPTER FIVE
　5　p.97
　5.1　Introduction　p.97
　5.2　MBE growth sequence　p.97
　5.3　RHEED　p.99
　5.4　EPMA analysis　p.101
　5.5　HR-xrd　p.103
　5.6　TEM　p.113
　5.7　AFM　p.119
　5.8　Raman　p.125
　5.9　I-V characterisstics　p.127
　5.1　Conclusion　p.129
CHAPTER SIX
　6　Mn-doped ZnSnAs2 epitaxial films　p.130
　6.1　Introduction　p.130
　6.2　Another look at the III-V DMS theory of Ferromagnetism　p.130
　6.3　zinc co-doped GaMnAs　p.131
　6.3.1　MBE growth　p.131
　6.3.2　RHEED　p.134
　6.3.3　XRD　p.134
　6.3.4　Hall Effect and Resistivity Measurements　p.139
　6.3.5　Anomalous Hall Effect and SQUID magenetic measurements　p.142
　6.4　MBE growth sequence　p.147
　6.4.1　Growth sequence　p.147
　6.4.2　RHEED　p.150
　6.4.3　EPMA　p.152
　6.4.4　HR-xrd　p.154
　6.4.5　SQUID　p.154
　6.5　 7%Mn-doped ZnSnAs2　p.160
　6.5.1　MBE growth　p.160
　6.5.2　EPMA　p.163
　6.5.3　HD-xrd　p.168
　6.5.4　SQUID　p.168
　6.6　Conclusion　p.177
CHAPTER SEVEN
　7　Conclusion and outlook　p.179
Bibliography　p.183
Appendix　p.194
Curriculum Vitae　p.195
Research Publications in Refereed International Journals　p.195
Paper in Japanese Journals　p.197
Research Presentations at International Conferences and Symposia　p.199
Presentation at Japanese Scientific Community Meetings, Conferences and Symposia　p.203
Selected Honors, Awards and Scholarships　p.212

　In the present thesis, growth and characterization of ternary undoped and Mn-doped ZnSnAs2 epitaxial films on nearly lattice matched InP substrates were investigated.　In the introduction, relevant information about ternary semiconductors was reviewed. The ever fast developing new field of spintronics, diluted magnetic semiconductors, GaMnAs DMS system, and theories of ferromagnetism in Mn-doped III-V based DMSs were also presented. The motivation of this work, i.e., is the possible rich new properties of ZnSnAs2 were explained.
　In chapter two, apparatuses used in performing the experiments were discussed. Molecular beam epitaxy （MBE） was used to prepare the samples while probing the layer by layer mode of the actual growth using reflection high energy electron diffraction （RHEED） technique. Crystalline quality was investigated by High resolution x-ray diffraction （HR-XRD）, surface morphology was observed by atomic force microscopy （AFM）, stoichiometry was determined by electron probe microanalysis （EPMA）, transport properties were studied by Hall Effect and van der Pauw resistivity measurements, interfacial quality were probed using transmission electron microscopy （TEM）, the ordering by micro-Raman spectroscopy, and magnetic properties by superconducting quantum interference device （SQUID） magnetometry experiments.
　In chapter three, the procedures for the MBE preparation of stoichiometric ZnSnAs2 epitaxial films were discussed. Optimum substrate temperature was first determined by growing samples at different temperatures of Ts of 265, 280, 300, and 320℃. The sample grown at 300℃ showed the streakiest RHEED patterns. This same sample exhibited XRD peak assignable to ZnSnAs2. Quantitative compositional analysis using EPMA revealed that indeed the sample grown at 300℃ has stoichiometry consistent with the bulk-ZnSnAs2. This suggests that the optimum substrate temperature to grow ZnSnAs2 thin films is 300℃. To investigate further the relatively large lattice constant obtained, RSM was performed, from which it was determined that the ZnSnAs2 epitaxial film was under compressive strain.
　In chapter four ZnSnAs2 thin films were prepared by MBE on semi-insulating （001） InP substrates for the chief purpose of investigating its transport properties using the same growth conditions previously reported. We observed a pronounced peak in the Hall coefficient temperature dependence curve at ~130K. This is the first confirmation of the present of that peak in ZnSnAs2 epitaxial films. We then reviewed and analyzed the transport properties data of the ZnSnAs2 epitaxial film. We have found out that the temperature dependence of Hall coefficient, resistivity, apparent hole concentration and mobility can be well-described by impurity band model proposed by Isomura and Tomioka.　Negative magnetoresistance was also observed at the low temperature regime suggesting that impurity band conduction is predominant at low temperatures. To the best of our knowledge, this was the first observation of such phenomenon in thin film ternary semiconductors. Resistivity measurements at high temperature regime were also performed to estimate the band gap energy. The band gap was calculated to be 0.69 eV, in very good agreement with the results reported earlier from the bulk-ZnSnAs2.
　In chapter five, the ZnSnAs2 thin films of different values of thickness were grown by MBE on n-type InP（001） substrates to fabricate p-ZnSnAs2/n-InP heterojunctions. EPMA studies indicated that all the samples are nominally stoichiometric. Structural characterization was carried out by performing HR-XRD. Based on the assumption that all the samples are grown pseudomorphically, the values of the free-standing lattice constant afs were calculated to be 5.8840A, 5.8840A, 5.8788A, and 5.8809A for samples A, B, C, and D, respectively. AFM investigations revealed good surface morphology for all the samples. TEM studies results confirmed high interfacial quality between the epitaxial films and the InP substrate. Micro-Raman spectroscopy detected the presence of A1 vibrational modes in all the samples indicating the presence of chalcopyrite ordering. I-V characteristic measurements demonstrated the rectifying capability of the p-ZnSnAs2/n-InP heterodiodes for the first time.
On the beginning part of chapter 6, we take a second look on the Zener model theory of ferromagnetism proposed by Dietl for III-V diluted magnetic semiconductor. Based on the model, Zn-codoping of GaMnAs was carried out. The effect is that increasing the Zn incorporation level in GaMnAs epitaxial films increases the lattice constant, transforming the properties into more metallic and effectively decreasing TC. We have then chosen the more radical way of enhancing TC of diluted magnetic semiconductor by venturing into a new class of material, the ZnSnAs2 chalcopyrite whose potential was discussed in chapter 1. Mn-doped ZnSnAs2 was grown on （001） InP substrates by all elemental solid source molecular beam epitaxy using the previously determined optimum substrate temperature of 300oC and Zn:Sn:As4 beam equivalent pressure ratio of 24:1:52. By using a slower growth rate a lattice constant a is 5.867A was obtained. This value is in very good agreement with the reported lattice constant of the bulk ZnSnAs2 chalcopyrite, suggesting that lowering the growth rate leads to a transition to a more chalcopyrite structure. Hysteresis loop in the M-H curve was still observed at temperature as high as 320K suggesting that Tc is above this temperature. The Curie temperature was estimated to be ~330K from the zero field cooled temperature dependence of magnetization and in good agreement with the reported Tc of bulk Mn-doped ZnSnAs2. A scrupulous HRXRD examination, detected a peak, albeit its weak intensity, assignable to MnAs. To meticulously determine of these peaks whether they are from Mn-based secondary phases, a 7% Mn-doped sample was prepared. Careful HR-XRD analyses did not reveal these peaks leading us to conclude that they may not be due to Mn binary compounds. SQUID measurements showed that the 7%Mn-doped sample has higher saturation magnetization but lower Curie temperature. The decrease in the Curie temperature can be explained by the Zener model proposed by Dietl et. al., i.e., the compensation of holes with increasing Mn incorporation leading to reduction of hole carriers that mediate ferromagnetism. These are the first report of above room-temperature ferromagnetism in Mn-doped ZnSnAs2 that may have possible interesting applications to spintronics devices. Other possible origin of ferromagnetism includes MnAs or MnZn compounds. For future work, the author feels that it is rewarding to investigate the properties of the ZnSnAs2 thin films with different amount of Mn-doping concentration.

本論文は「Epitaxy and characterization of ZnSnAs2 thin films on nearly lattice-matched InP substrates with potential room-temperature spintronics applications（室温半導体スピントロニクス応用に向けたInP格子整合ZnSnAs2薄膜のエピタキシャル成長と評価に関する研究）と題し、7章から構成されている。
第1章では、本研究の背景である半導体スピントロニクスについて示している。半導体スピントロニクスデバイスの実現には強磁性半導体が必要であることから、現在まで中心的に研究されているGaMnAsについてその理論的背景を説明し、この論文と関係の深いII-IV-V2多元系磁性半導体に関する研究動向についてまとめている。このような背景から本研究の目的とその重要性について記述している。
第2章では、本研究で用いた実験装置について詳細に記述している。結晶成長には分子線エピタキシー装置（MBE）を用いて、その構造やその場観察できる反射高速電子線回折（RHEED）について示している。薄膜結晶の評価には、高解像度X線回折装置（HR-XRD）や断面透過型電子線顕微鏡（TEM）などを用いたことが示され、作製した磁性半導体の評価には超伝導量子干渉計（SQUID）を用いている。
第3章では、具体的に分子線エピタキシー（MBE）の実験方法について記述している。インジウムリン（InP）基板上に化学量論的ZnSnAs2薄膜を作製するためには、基板温度、Zn,Sn,As4ビームフラックス比（BEPR）、作製された結晶の組成比の関係を明らかにする必要があり、結晶成長パラメータを変えた実験を行った。その結果、最適な基板温度として300℃、BEPRとして24:1:52であることが確認され、HR-XRDによりZnSnAs2単結晶薄膜が確認された。
第4章では、半絶縁性InP基板上に作製されたZnSnAs2薄膜について、その伝導機構について調べている。ZnSnAs2薄膜はp型伝導性を示し、電気伝導率や移動度などの温度特性は、ZnSnAs2バルク結晶で提案されている不純物バンド伝導モデルで説明できることを示した。また、負の磁気抵抗効果を調べることでこのモデルの有効性を明らかにしている。
第5章では、異なる膜厚を有するZnSnAs2薄膜をn型InP基板上に結晶成長させTEM観察により膜厚測定を行い、pseudomorphicに成長していることを確認している。HR-XRDから得られた格子定数から歪みのない格子定数を5.884Åと算出している。次に、p型ZnSnAs2とn型InPから構成されるpn接合ダイオードを作製して良好な整流特性を観測した。これは、初めての観測である。
第6章では、磁性原子MnをZnSnAs2薄膜にMnドーピングする結晶成長とその薄膜特性について記述している。作製された薄膜には金属第2相は存在せず良質な磁性薄膜が作製された。作製したMnドープZnSnAs2薄膜はSQUIDを用いて、磁化の温度特性と磁場特性を測定した。その結果、室温330Kで室温強磁性を示すことが確認され、半導体スピントロニクスデバイス応用の可能性を示した。MnドープZnSnAs2薄膜の強磁性観測は初めてである。
第7章では、本論文が総括され、今後の研究の展望について記述されている。
　以上のように、本論文は多元系半導体ZnSnAs2薄膜のInP基板へのエピタキシャル成長技術を確立し、その研究結果に基づいて室温強磁性を示す磁性半導体MnドープZnSnAs2薄膜を実現している。よって、本論文は工学上及び工業上貢献するところが大きく、博士（工学）の学位論文として十分な価値を有するものと認める。