To maintain the rapid development of electronics, alternative technologies that go beyond downscaling of the devices are required. Spintronics, in which digital data is represented by the electron spin, may provide a solution for future electronics. In particular, semiconductor spintronics, where the unique features of ferromagnet (FMs) and semiconductors (SCs) are combined, raises the possibility to devise a new spin-based information technology. The injection, control, and detection of spin polarization in SC are the main building blocks of SC spintronics. Notably, significant progress has been recently made in establishing the two building blocks of the spin injection and detection, particularly in silicon, the mainstream SC, by developing ferromagnetic tunnel contacts, which provide a reliable and robust means for the efficient spin injection and detection in SC. Triggered by the progress in silicon, germanium has recently received significant attention because of many merits of Ge: a large spin-diffusion length due to the high carrier mobility, the possibility of spin control via non-negligible spin-orbit coupling, the feasibility of epitaxial FM/MgO tunnel contacts to the Ge employing the lattice matching of Ge with MgO, and optical spin pumping through the quasi-direct band gap of Ge. In order to utilize the merits and potential of Ge for SC-spintronic appplication, the experimental demonstration of spin injection and detection in Ge system and systematic investigation of spin phenomena in this system are crucial.
This dissertation aims to investigate the issues for the development of Ge spintronics, to experimentally demonstrate the spin injection and detection in Ge by developing suitable ferromagnetic tunnel contacts to the Ge, and to provide viable routes for the realization of Ge-based spintronic devices.
We develop the single-crystalline bcc CoFe/MgO(001) tunnel contacts to Ge with the proper resistance-area (RA) product, satisfying the requirements for the electrical spin injection and detection in the Ge. It is demon-strated that, despite the rather large lattice misfit of 5.3 % between the MgO and Ge, the epitaxial growth of MgO and bcc CoFe/MgO on Ge(001) substrates with high quality interfaces can be realized using molecular beam epitaxy (MBE) technique. From the study of the effect of MgO thickness on the Schottky barrier and RA product in the CoFe/MgO/Ge contacts, it is shown that the RA product of the CoFe/MgO/Ge contacts can be tuned into the proper range for efficient spin injection and detection by controlling the MgO thickness. Moreo-ver, the energy-band structure (e.g., valence band offset (VBO), conduction band offset (CBO)) of the MgO/Ge interface is characterized by x-ray photoelectron spectroscopy (XPS).
We experimentally demonstrate the all-electrical spin injection and detection (or electrical spin accumula-tion) in n-type Ge up to room temperature (RT) by employing the developed crystalline CoFe/MgO tunnel con-tact and three-terminal Hanle (TTH) measurements. The electrical injection as well as detection of spin accumu-lation is established in a CoFe/MgO/moderately doped n-Ge contact at RT. The estimation of the spin accumulations, spin life times, and spin diffusion lengths in moderately doped n-Ge from the measured spin signals is carried out. Albeit the successful injection and detection of spin accumulation at RT, the resistive contact at low temperature hinders the study of electrical spin accumulation and spin lifetime in Ge over a wide temperature range. By reducing the contact resistance via the introduction of a heavily doped surface layer, we clearly observe the overall temperature and bias dependence of the Hanle effect in a CoFe/MgO/composite Ge system. It is shown that the obtained spin signals in both contacts are several orders of magnitude larger than the expected values from the existing spin injection and diffusion theory; the extracted spin lifetimes are much shorter than the values of bulk Ge. The possible origins of these discrepancies are discussed.
We investigate the interfacial spin deplorization (ISD) and related Hanle effects in the FM/oxide/SC con-tacts with the same FM but different host SCs. The role of the host SCs and spin lifetime on the ISD in the CoFe/MgO on Si and Ge contacts is studied by using the combined measurement of normal and inverted Hanle effects. It is shown that, despite more or less the same ferromagnetic electrode and the interfacial roughness of the contacts, significant differences of the ISD depending on the host SC is observed. Through the model calculation, considering the spin precession due to the local filed and the spin relaxation in the host SC, we explain the experimental observations. Furthermore, we perform the electrical investigation of the oblique Hanle effect (i.e., the Hanle effect in an oblique magnetic field, OHE) in FM/oxide/SC contacts and their generic features using a three-terminal measurement scheme. It is shown that the electrical OHE signals and their asymptotic values obtained in the CoFe/MgO/Si and CoFe/MgO/Ge contacts are highly consistent with the predictions of spin precession and relaxation model for the electrical OHE. From the comparison study of the OHE and tunneling anisotropy (TA) signals, it is found that the non-negligible TA signals in both contacts are mainly attributed to the anisotropic spin accumulation owing to the misalignment-induced OHE.
We demonstrate the Seebeck spin tunneling (SST) and thermal spin accumulation in n-type Ge by employing CoFe/MgO/Ge contacts with the asymmetry in tunnel spin polarization (TSP) of tunneling electrons. It is shown that the magnitude of thermally injected spin signal scales linearly with the power of local heating of electrodes, and its sign is reversed as we invert the temperature gradient, demonstrating the key features of the SST and thermal spin accumulation. The sign as well as magnitude of the thermally spin accumulation is ana-lyzed by using the spin-dependent tunneling theory.
강자체와 반도체 각각의 고유한 특성을 결합한 반도체 스핀트로닉스의 등장으로 스핀을 기반으로 하는 새로운 정보기술의 고안 가능성이 높아지고 있다. 비자성 반도체 안으로 스핀 분극된 전자의 주입, 제어, 검출은 반도체 스핀트로닉스의 주요한 구성요소이다. 특히 최근에, 강자성 터널접합을 개발하여 주류 반도체인 실리콘으로의 스핀 주입과 검출을 입증하는 현저한 진전이 이루어졌다. 이러한 실리콘의 진전에 힘입어, 반도체 스핀트로닉스의 응용에 많은 장점을 갖고 있는 게르마늄에 대한 관심이 높아지고 있다. 게르마늄은 긴 스핀 확산 길이, 스핀 제어의 가능성, 결정성의 강자성 터널접합 형성, 광학적 스핀 펌핑등의 장점이 있다. 반도체 스핀트로닉스의 응용에 게르마늄이 갖는 장점과 가능성을 활용하기 위해서는, 게르마늄으로의 스핀 주입과 검출의 실험적 증명 및 이 시스템 안에서 스핀 거동에 대한 체계적 연구가 필수적이다.
본 논문은 게르마늄 스핀트로닉스의 개발과 관련된 이슈에 대해 조사하고, 적절한 강자성 터널 접합을 개발하여 게르마늄으로의 스핀 주입과 검출을 실험적으로 증명하므로 서, 게르마늄 기반의 스핀트로닉 소자의 구현에 있어 바람직한 방향을 제시하는 것을 목표로 하였다.
우리는 게르마늄으로의 주입되는 전자들의 높은 스핀 분극을 유도하기 위해 분자선 에피탁시 방법을 이용하여 단결정의 CoFe/MgO 터널 접합을 게르마늄 위에 성장시겼고, 터널 장벽의 두께를 적절히 조절하여 효율적 스핀 주입과 검출을 위해 요구되는 접합 저항 값을 갖도록 하였다.
준비된 결정성의 강자성 터널 접합과 3단자 Hanle 측정법을 이용하여, 게르마늄으로의 전기적 스핀 주입과 검출이 상온까지 가능함을 실험적으로 증명하였다. 또한, 측정된 Hanle 데이터를 바탕으로, 스핀 축척량, 스핀 생존 시간, 스핀 확산 거리를 추출하였고, 이 값들의 온도와 전압/전류 의존성에 대하여 조사하였다.
강자성체/터널장벽/반도체 접합에서 일어나는 계면 스핀 탈분국현상에 대한 연구는 (강자성체 계면에 가까운) 반도체 안에 스핀 축척과 스핀 거동에 대한 완벽한 이해를 위해 필수적이다. 우리는 같은 강자성체(CoFe)와 다른 반도체들(실리콘, 게르마늄)로 구성된 강자성 터널 접합들을 준비하여, 정상 및 역전 Hanle효과의 병행 측정을 통해, 호스트 반도체와 스핀 생존 시간이 계면 스핀 탈분국현상에 미치는 영향에 대하여 체계적으로 조사하였다. 또한, 강자성 터널 접합에서 전기적 사선 Hanle효과의 특성에 대하여 연구하였다.
우리는 비대칭 스핀 분극을 갖는 결정성 강자성 터널 접합을 이용하여 게르마늄안으로의 스핀 주입과 축적을 (전기적 방식이 아닌) 열적인 방법(제벡 스핀 터널링)으로 유도하였다. 측정된 스핀 데이터로부터, 열적인 방법(제벡 스핀 터널링)을 이용한 스핀 주입 및 축적은 매우 효율적임을 확인하였다.
