Recently, Si-based spin electronics has attracted considerable attention, because it can lead to a variety of spin-electronic devices, which utilize spin degrees of freedom and well-established Si technologies. In this respect, recently proposed Si-based spin MOSFETs, which consist of a MOS gate capacitor and ferromagnetic source and drain (S/D), are promising. Previous theoretical studies showed that the spin MOSFETs can be potentially used for reconfigurable logic gates and nonvolatile memory, since the output characteristics can be controlled by both the gate voltage and magnetization of the S/D. One of the most practical devices is a spin MOSFET with ferromagnetic metals for the S/D, which can be simply realized by substituting ferromagnetic metals for the S/D of an ordinary Schottky MOSFET.

　To realize both high electrical performance and spin-dependent characteristics in spin MOSFETs, thermally stable ferromagnetic metal/Si junctions with low Schottky barrier heights are strongly needed. It is well-recognized that spin injection from a ferromagnetic metal into a semiconductor is the most important issue for spin MOSFETs, and that the spin injection efficiency is considerably affected by the ferromagnet/semiconductor interface quality, its Schottky barrier height, and the spin-polarization of the ferromagnet. Furthermore, like the ordinary Schottky MOSFETs, low Schottky barrier height is required for high-performance electrical output characteristics of the spin MOSFETs. From the viewpoint of device fabrication processes, high thermal stability of the ferromagnetic metal/Si junction is also needed, since there are usually thermal annealing processes at a few hundred degree centigrade. CoFe and CoFeB, which are widely used in spin-valve devices, would not be suitable for S/D of spin MOSFETs, since the formation of intermixing layer can proceed easily, in spite of their high spin polarization of 〜 0.5.

　Ferromagnetic MnAs is a good candidate for S/D of spin MOSFETs, because it has high spin-polarization of 〜 0.5, and epitaxial MnAs/Si(001) junctions with an atomically flat interface can be fabricated by molecular beam epitaxy (MBE). On the other hand, there are no reports evaluating the thermal stability and the Schottky barrier height of MnAs/Si junctions. The aim of this thesis is to fabricate Si-based spin MOSFETs with MnAs S/D and clarify their characteristics. Crystalline properties, thermal stability, and the Schottky barrier height of MnAs/Si(001) junctions are evaluated, and their influence on the device characteristics are examined. Spin MOSFETs with MnAs S/D are fabricated on silicon-on-insulator (SOI) substrates, and their transistor performance and spin-dependent transport are investigated.

　In chapter 2, fundamental properties of epitaxial MnAs films were described. Growth was performed by low temperature molecular beam epitaxy (LT-MBE), and Si(001) wafers were used as substrates. In order to obtain good magnetic properties and crystalline quality, the growth condition was optimized by the in-situ reflective high energy electron diffraction (RHEED) observations, the ex-situ atomic force microscopy (AFM) observations, and their ferromagnetic properties were measured by a superconductive quantum interference device (SQUID) magnetometer. Furthermore, in order to examine the thermal stability of the MnAs films, the magnetic properties were measured after post-growth annealing at 400°C, 500°C, and 600°C. The ferromagnetic behavior was preserved when the annealing temperature were 400°C and 500°C, whereas it disappeared when the annealing temperature was 600°C. As a result, MnAs films on Si substrates were found to have sufficiently high thermal stability in thermal processes up to 500°C.

　In chapter 3, the Schottky barrier height of the MnAs/Si junction was evaluated comparing with that of Co(90)Fe(10)/Si, and (Co(90)Fe(10))(70)B(30)/Si junctions. From current-voltage (I-V) measurement in the temperature range of 300 K to 450 K, MnAs/Si junctions found to have a low Schottky barrier height of 0.16 eV for electrons, while the Co(90)Fe(10)/Si, and (Co(90)Fe(10))(70)B(30)/Si junctions have mid-gap Schottky barrier heights of 0.73 eV and 0.75 eV for electrons, respectively. Thus, MnAs is a promising material for ferromagnetic S/D in spin MOSFETs. A further evaluation of the MnAs/Si interface through secondary ion mass spectroscopy (SIMS) revealed that a substantial amount of As atoms (10(18) 〜 10(19) cm(-3)) are likely to be present near the interface. It is a possible origin of the low Schottky barrier height of the MnAs/Si junction.

　In chapter 4, a spin MOSFET with epitaxial MnAs S/D were fabricated on thin-film silicon-on-insulator (SOI) substrates, and measurements of the transistor characteristics were performed. A spin MOSFET with (Co(90)Fe(10))(70)B(30) S/D was also fabricated for comparison. From the drain current - drain voltage (I(DS) - V(DS)) characteristics, the spin MOSFET with MnAs S/D exhibited 〜100 times higher in current drivability than the spin MOSFET with (Co(90)Fe(10))(70)B(30) S/D. From the drain current - gate voltage (I(DS) - V(GS)) characteristics, the spin MOSFET with MnAs S/D showed the high on-off ratio of 〜 108, which is comparable to that in conventional MOSFETs with pn junction S/D. High electrical performances of the spin MOSFET with MnAs S/D, which were expected in the chapter 3, have been demonstrated. Postgrowth annealing effect on the transistor performance was also investigated. It was found that the I(DS) - V(DS) and I(DS) - V(GS) characteristics were not degraded, but rather improved, when the postgrowth annealing temperature was done at 500℃.

　In chapter 5, the spin-dependent transport properties of the spin MOSFET with MnAs S/D was explored in the temperature range of 2.8 - 50 K. At 2.8 K, the drain current - magnetic field (I(DS) - H) characteristics exhibited hysteresis behavior depending on the gate voltage V(GS) and temperature. This phenomenon could be explained by theoretical studies by other reports. The maximum of the obtained magnetocurrent ratio was - 1.6 %, and the hysteresis of I(DS) - H loops were preserved up to 20 K at high V(GS) >100 V. Since the magnitude of the magneto-current signal is larger than that of the estimated signals of various parasitic magnetization-dependent signals, such as anisotropic magneto-resistance (AMR) of the MnAs film, the magneto-resistance of the non-magnetic Si layer, and local Hall effect (LHE), this signal probably originates from the spin-dependent transport of the spin MOSFET.

　本論文は、「Si-based spin MOSFETs with epitaxial ferromagnetic MnAs source and drain: Growth, Schottky junction, transistor, and spin-dependent transport properties.(エピタキシャル強磁性MnAsソース・ドレインを有するスピンMOSFET: 成長、ショットキー接合特性、トランジスタ特性およびスピン依存伝導現象)」と題し、英文で書かれている。本論文は、新しいMOS型スピンデバイスであるエピタキシャル強磁性MnAsソース・ドレインを有するスピンMOSFETについて、 強磁性薄膜MnAsのエピタキシャル成長、MnAs/Siショットキー接合の特性、スピンMOSFET構造の作製プロセス、トランジスタ特性およびスピン依存伝導現象の研究成果を記述しており、全6章から成る。

　第1章は「Introduction」であり、スピン自由度を利用したエレクトロニクスに向けたスピンデバイス研究の状況と背景を述べ、本論文の構成と目的を示している。

　第2章は「Epitaxial growth and fundamental properties of ferromagnetic MnAs thin films on Si Substrates」であり、Si(001)基板上への強磁性単結晶MnAs薄膜の分子線エピタキシー(MBE)法によるエピタキシャル成長とその構造とエピタキシャル関係、磁気特性とその成長条件依存性、およびアニール効果と熱的安定性を実験的に明らかにした結果を記している。

　第3章は「Schottky barrier height of ferromagnet/Si junctions」であり、エピタキシャルMnAs/Si(001)ショットキー接合を形成し、そのショットキー障壁高さを評価した結果、電子に対して0.16 eVと非常に低い値になることを明らかにしている。この値は、比較のために形成したCoFe/Si(001)およびCoFeB/Si(001)ショットキー接合の障壁高さ(0.62 - 0.65 eV)と比べて格段に低い値であり、エピタキシャルMnAs 薄膜はMOSFETおよびスピンMOSFETのソース・ドレイン電極として有望であることを示している。

　第4章は「Fabrication and electrical transport measurements of MnAs source and drain (S/D) spin MOSFETs」であり、エピタキシャルMnAs薄膜をソース・ドレイン電極とするスピンMOSFET構造をSOI(silicon-on-insulator)基板上に作製し、基板側にバックゲート電極を形成することにより、トランジスタ動作を示すとともに電気的特性を評価した。比較のために形成したCoFeB電極を有するMOSFETと比べておよそ100倍のドレイン電流をとれること、ゲート電圧によるオン/オフ比が108程度で通常のpn接合を用いたMOSFETと同程度であること、500℃程度のアニールによって特性が向上すること、などを明らかにした。

　第5章は「Spin dependent transport properties of spin MOSFETs with MnAs source/drain」と題し、まずスピン依存伝導現象について概略を述べた後で、第4章で作製したエピタキシャルMnAs薄膜をソース・ドレイン電極とするスピンMOSFETの磁気輸送特性を測定した結果について述べている。ゲート電圧(VG)が高いときに、ソース・ドレイン間の抵抗(R(DS))の磁場依存性にヒステリシスと正の磁気抵抗が現れることを示し、その温度依存性、磁場方向依存性、また強磁性電極を持たないデバイスとの比較などから、観測された磁気抵抗はMnAs強磁性電極のスピンバルブ効果とSiチャネル中のスピン依存伝導によるものであると結論づけている。

　第6章は「Concluding remarks and outlook」であり、本論文で得られた結果のまとめと今後の展望を述べている。

　以上のように、本論文では、エピタキシャル強磁性MnAsソース・ドレインを有するMOS型スピンデバイスの研究を行い、まずエピタキシャルMnAs/Si(001)接合のショットキー障壁高さを評価してスピンMOSFETのソース・ドレイン電極として適していることを示し、さらに、MnAsをソース・ドレインとするMOSFETの作製プロセスを開発し特定の温度とゲートバイアス条件においてソース・ドレイン間に磁気抵抗効果が生ずることを示すことにより、強磁性電極を有するスピンMOSFETの動作が原理的に可能であることを実証したものであり、電子工学上、寄与するところが少なくない。

　よって本論文は、博士(工学)の学位請求論文として合格と認められる。