Study on the Relation between Spin Accumulation and Crystal Structure in the Spin-tunnel Contact with Germanium : Germanium기판 위에 제작된 스핀 터널 컨택의 결정 구조와 스핀 축적의 관계에 대한 연구

Lee, Soogil 연세대학교 일반대학원 2016 국내박사

Spintronics-based future devices have recently been received much attention from many researchers in the fields of science and engineering. Because the “spin” of an electron can provide additional functionality to charge-based electronic devices, excellent properties such as the high speed operation, low power consumption, and non-volatility can be embedded on them with the employment of the spin. These devices are also expected to overcome the limitations of electronic devices caused by scaling. Spintronic devices are controlled by a “spin current”, which is usually generated by the electrical injection of the electron’s spin from a spin-polarized source such as a ferromagnetic spin injector. Thus, efficient spin injection from ferromagnet into non-ferromagnet with high spin-polarization is a prerequisite for the realization of such devices. In this study, the spin injection efficiency is improved by increasing spin polarization of the spin injector as demonstrated in the strongly (001)-textured MgO/Co40Fe40B20 spin-tunnel contact on the n-type Germanium (001) substrate. The unique structure of MgO (001)[110]/body-centered cubic ferromagnet (001)[100] can generate high tunnel-spin polarization at the Fermi level, thus the structural properties of the MgO tunnel barrier and the Co40Fe40B20 ferromagnet electrode are modified by various methods. For instance, an ultrathin (6Å) Mg layer is inserted between the n-type Ge (001) substrate and the MgO tunnel barrier. This Mg insertion layer induces not only the strong (001) texture in MgO but also the smooth interface between n-type Ge and MgO. The chemical state of the Mg-inserted tunnel barrier is investigated by X-ray photoemission spectroscopy and electron energy-loss spectroscopy. It is observed that the inserted Mg layer is fully oxidized during MgO sputtering and following postannealing because the metallic Mg is not found in both spectra. The annealing-induced epitaxial growth of the Co40Fe40B20 electrode on this MgO (001) template is also examined by cross-sectional transmission electron microscopy. The epitaxial relationship between MgO and Co40Fe40B20 is determined to be MgO (001)[110]/Co40Fe40B20 (001)[100], which is expected to generate high tunnel-spin polarization. Moreover, the (001) texture of MgO is largely enhanced after postannealing. The spin-tunnel devices are fabricated by photolithography and Ar ion milling to characterize the charge and spin transports in structurally modified spin-tunnel contacts. The charge transport of all the as-deposited spin-tunnel contacts exhibits a tunneling transport via the MgO tunnel barrier within a small bias range from -0.2 to +0.2 V. The electrical spin injection experiment is conducted by three-terminal Hanle (or localized geometry) measurements. The clear normal and inverted Hanle signals are obtained from all the prepared devices. The saturation behavior of the Hanle signals is observed at the high applied field of ~1 T which is larger than anisotropy field of the Co40Fe40B20 electrode. In addition, there are no three-terminal Hanle signals in the spin-polarization-removed devices. Both results clearly indicate that the observed Hanle signals are indeed induced by spin injection and spin accumulation phenomena from the MgO/Co40Fe40B20 spin injector. The dependence of Hanle signals on the contact resistance (scaling law). The relation between tunnel-spin polarization and spin accumulation (three-terminal Hanle) signals is clarified by both parameters, namely, the (001) texture of the spin-tunnel contact and the crystallization of Co40Fe40B20 after postannealing. The spin accumulation is strongly influenced by both parameters. The largest magnitude of the Hanle signals obtained from the spin-tunnel contact with the strongly (001)-textured MgO tunnel barrier and the epitaxially grown Co40Fe40B20 electrode on it. From these all results, it is concluded that the enhancement of the tunnel-spin polarization for MgO/Co40Fe40B20 greatly increases spin accumulation in the n-type Ge (001) substrate. These results can be applied not only to n-type Ge/MgO/Co40Fe40B20 but also to other systems with combinations of various electrode materials and substrates. These results should inspire other researchers who want to improve the tunnel-spin polarization with structural modifications and to enhance the efficiency of electrical spin injection. 최근 스핀트로닉스(spintronics)를 기반으로 하는 미래 소자들이 물리와 공학분야의 연구자들에게 많은 각광을 받고 있다. 새롭게 제안되는 소자들은 기존의 전자소자들에 비해 고속 동작 특성, 적은 소비 전력, 비 휘발성 등을 달성하거나 지닐 수 있을 것으로 기대되는데, 그 이유는 전자의 전하만을 이용했던 기존 소자와 다르게 스핀트로닉소자에서는 전자의 “스핀”을 이용하는 것을 기본으로 하기 때문이다. 또한, 이러한 신개념의 소자들은 현재 전자소자들이 직면한 스케일링 문제를 해결할 수 있을 것으로 예견된다. 페르미 준위(Fermi level)에서 스핀 분극(spin polarization)이 존재하는 강자성체(ferromagnet)로부터 비강자성체로 전자의 스핀을 주입해서 발생시킨 “스핀 전류”는 이러한 소자들을 작동시키거나 그 내부에서 정보를 전달할 수 있는 핵심 개념이다. 따라서 상기 언급한 소자들의 성공적인 개발을 위해서는 높은 스핀 분극을 갖는 효율적인 스핀 주입기를 연구하는 것이 선행되어야 한다. 본 학위 논문에서는 스핀 분극율이 큰 것으로 알려진 MgO/Co40Fe40B20 스핀 터널 컨택(혹은 스핀 주입기)을 이용해서 표면이 (001)면인 n형 Ge 기판으로 스핀 주입효율을 향상시키는 연구를 진행했다. 우선, 스핀 주입 실험을 진행하기에 앞서 이론적으로 거의 반금속(half-metal)에 가까운 스핀분극율이 얻어진다고 알려진 MgO (001)[110]/체심입방(body-centered cubic) 강자성체 (001)[100] 구조를 제작하기 위해 MgO와 Co40Fe40B20B의 결정성을 다양한 방법으로 조절했다. 먼저 6Å의 두께를 갖는 Mg 박막을 n형 Ge (001)기판과 MgO사이에 삽입해서 MgO 터널배리어의 (001) 텍스처(texture)를 증가시켰고 또한 기판과 MgO 사이의 계면 거칠기 및 MgO의 표면 거칠기를 더 평탄하게 만들었다. 이에 더해 후속 열처리를 통해 MgO의 (001) 텍스처가 더욱 커지는 것을 관찰하였다. 엑스레이 광전자 분광기법(X-ray photoemission spectroscopy)과 전자 에너지 손실 분광기법(electron energy-loss spectroscopy)들을 이용해서 삽입된 금속 Mg 층이 뒤따르는 MgO 증착 및 후속 열처리를 통해 모두 MgO로 변화한 것을 관찰했다. 또한 후속 열처리를 통해 MgO층의 (001) 텍스처 증가뿐만 아니라 그 위에 스퍼터링으로 증착된 비정질 Co40Fe40B20의 에피 성장(epitaxy growth)이 일어난 것을 투과 전자 현미경(transmission electron microscopy)을 이용해 관찰했다. 이를 통해 밝혀낸 두 인접한 박막사이의 에피 관계(epitaxial relationship)는 MgO (001)[110]/Co40Fe40B20 (001)[100]로써 이는 상기 언급한 높은 스핀분극율을 가진다고 밝혀진 구조와 일치한다. 상기 조건들을 이용해서 얻어진 여러 결정성을 갖는 스핀 터널 컨택의 전하수송 특성과 스핀주입효율을 평가하기 위해 포토리소그래피와 Ar 이온 밀링을 이용해서 소자를 제작했다. 소자의 전기적 특성 측정 결과 모든 스핀 터널 컨택들은 상온에서 ±0.2 V의 작은 전압을 인가했을 때 MgO 터널 배리어를 통한 터널링 특성을 보였다. 전기적인 방법을 통한 스핀 주입실험은 three-terminal Hanle 측정을 이용해서 수행했다. 여러 가지 결정성을 갖는 모든 소자들에서 스핀주입신호가 성공적으로 검출되었다. 고자기장 영역에서 스핀주입신호가 Co40Fe40B20의 자화 방향에 따라 포화되는 현상 역시 측정되었다. 또한 Ru 전극을 Co40Fe40B20 전극 대신 적용해서 스핀분극율을 제거한 제어실험에서 위에서 얻어진 스핀주입신호가 나타나지 않는 것을 관찰했다. 상기 언급한 결과들을 토대로 얻어진 스핀주입신호는 다른 효과들에서 기인하는 것이 아니라 MgO/Co40Fe40B20 스핀 터널 컨택에서 n형 Ge 기판으로 전기적인 방법을 통해 주입된 스핀으로부터 유래됨을 증명해냈다. 스핀주입신호가 터널배리어의 저항에 저수함수에 비례해서 변하는 현상 역시 관찰했다. Mg 층의 삽입여부와 열처리 온도에 따라 여러 결정성을 갖는 소자들, 즉 스핀분극율이 다를 것으로 생각되는 소자들을 이용해서 그것이 스핀주입신호에 미치는 변화를 조사했다. 그 결과 Mg층을 삽입해서 만들어진 최고의 스핀분극율을 가질 것으로 예상되는 소자의 스핀주입신호가 Mg를 삽입하지 않은 소자에서 얻어진 값에 비해 6배 가량 증가함을 알아냈다. 이러한 모든 결과들을 종합해 볼 때 MgO/Co40Fe40B20 스핀 터널 컨택의 스핀분극율 증가를 통해 n형Ge으로의 스핀주입효율을 향상시키는 것이 가능하다고 판단된다. 이 결론은 본 연구에서 이용된 n형Ge기판이나 MgO/Co40Fe40B20 스핀 터널 컨택 같은 특정 재료나 환경에 국한되지 않고 다른 스핀주입 제어실험에도 동일하게 적용될 수 있을 것이라 생각된다. 본 논문에서 연구된 계면제어나 결정성장, 그러한 구조들의 소자적용이 서두에 언급한 스핀트로닉소자의 개발에 큰 도움을 줄 것이라 사료된다.

Study on the relation between spin accumulation and crystal structure in the spin-tunnel contact with germanium

이수길 Graduate School, Yonsei University 2016 국내박사

Spin-dependent transport in smmimetallic bismuth

이경일 Graduate School, Yonsei University 2008 국내박사

The spin dependent transport properties in semimetallic Bi thin films and the magnetotransport properties of single-crystalline Bi nanowires have been investigated. The successful electrical spin injection into a semimetallic thin film is demonstrated and the spin diffusion lengths are also measured. The Bi-inserted MTJs have been fabricated and a certain spin dependent tunneling effect provides a long spin decay length. Single-crystalline Bi nanowires have been successfully grown by our novel method and show outstanding transport properties.The effect of substrate biased plasma etching has been investigated on the properties of MTJs. Our newly developed etching technique is easily applicable for the fabrication of any other spintronic devices based on ferromagnetic metal layers.The magnetotransport properties of the electroplated and sputtered Bi thin films have been investigated in the range 4 – 300 K. We find that the room temperature MR in the sputtered films depends on the trigonal-axis oriented microstructures and grain size, in contrast to the electroplated films. Our results demonstrate the very large room temperature MR in the electroplated and sputtered Bi thin films, which can be used for spintronic device applications.Spin accumulation and diffusion are studied in thin film Bi and BiPb samples using electrical spin injection and detection. The spin diffusion length in a semimetallic Bi95Pb5 sample is estimated to be 235 µm at a temperature of 2 K. This is the first demonstration of electrical spin injection and detection in a semimetal material. Also, a clear spin-dependent tunneling effect and quite long spin decay length of 41 Å in the Bi-inserted MTJs were observed at room temperature.The novel growth method of single-crystalline Bi nanowires has been investigated and the structural analysis on the grown Bi nanowires has been conducted. The devices based on an individual nanowire have been fabricated successfully. The magnetotransport properties of an individual Bi nanowire, investigated in the temperature range from 2 K to 300 K, demonstrate that the Bi nanowires grown by the spontaneous growth method are high-quality and single-crystalline.

MBE를 이용하여 성장시킨 GaMnAs의 특성연구

박종훈 충남대학교 대학원 2002 국내석사

In recent years, ferro-magnetism in Ⅲ-V based magnetic semiconductors introduced a new degree of freedom associated with the magnetic cooperative phenomena to the low-dimensional hetero-structures. We grow the GaMnAs by Low-temperature MBE Ts=225~275˚C, P_As=2.5×10^-6~0.8×10^-6 torr and P_Mn≤2×10^-9 torr are the growth condition of GaMnAs with 0.25㎛/hr growth rate. Grown epi-layer has high quality during GaMnAs because RHEED pattern maintain the streaky line RHEED patterns give many growth information during growing and optimize the growth conditions. Grown GaMnAs layer characterize via DC-XRD, Hall effect measurement, SQUID and EPMA. We optimized GaMnAs growth condition and GaMnAs has ferro-magnetism Tc=80K with magneto-transport properties about 5.4% Mn contents.

MBE로 성장한 다결정 Si_(1-x)Mn_(x) 박막의 전·자기적 특성 연구

김한겸 忠南大學校 大學院 2003 국내석사

Si_(1-x)Mn_(x) semiconductor thin films were grown on (100)Si wafer by using molecular beam epitaxy and their magnetic properties were studied. Native oxide layer on (100)Si wafer was not removed intentionally in order to provide enough nucleation sites for various phases appeared in Si-Mn phase diagram. The Si_(1-x)Mn_(x)/SiO_(2)/(100)Si films were grown at the rate of 100Å~200Å/min and final thickness of the films was 3,000Å ~ 10,000Å. Substrate temperature was fixed to 300℃, 400℃. Mn concentration was changed by controlling the flux ratio of Mn and Si. The composition of Si_(1-x)Mn_(x) films were analyzed by using the energy dispersive x-ray spectrometer. Concentration range of Mn is 10.0at%~76.8at%. Hall effect measurement revealed that the Si_(1-x)Mn_(x) semiconductors have p-type carrier at room temperature and carrier concentration is 10^(19)_ 10^(20)/㎤. The electrical resistivity measured by a standard four-probe method is -10^(-4) ohm·cm, and decreases with Mn concentration. The x-ray diffraction(XRD) observations have been done to identify the ferromagnetic phase of Si_(1-x)Mn_(x) thin films. Magnetization behavior of Si_(1-x)Mn_(x) thin films were measured at room temperature by using vibrating sample magnetometer and ferromagnetic resonance and at low temperature using superconducting quantum interference device(SQUID). Ferromagnetic characteristics are seen clearly at 2K eat substrate temperature. XRD analysis and SQUID characteristics show that two magnetic phases are formed in the Si_(1-x)Mn_(x)(x:0.10 ~ 0.77) semiconductors thin films, ferromagnetic SiMn phase and anti-ferromagnetic Si_(3)Mn_(5) phase * A thesis submitted to the committee of the Graduate School, Chungnam National University partial fulfillment of the requirement for the degree of Master of Material Engineering in February 2003.

Study on Fe and Fe3O4 ultra-thin film growth on GaAs(100) at low temperature and its application as efficient spin-injection device

이재민 서울대학교 대학원 2008 국내박사

Electrical spin injection in epitaxial ferromagnet/semiconductor hybrid structures

이태환 고려대학교 대학원 2009 국내석사

The electrical spin injection and detection is observed in epitaxially grown Fe/GaAs hybrid structure. In hybrid spintronic devices such as spin-FETs (Spin-Field-Effect Transistors), it is important to inject spin-polarized electrons into semiconductor and then to detect these spins from semiconductor. The Schottky tunnel barrier which is formed at the interface of Fe/GaAs is used for efficient spin injection into semiconductor and subsequent spin detection. The hysteresis loop in Fe/GaAs thin film grown by MBE (Molecular Beam Epitaxy) is measured by AGM (Alternating Gradient Magnetometer). The result shows that Fe thin film grown on GaAs have a different anisotropic properties from bulk or film Fe. Photolithography and etching (ion milling and reactive ion etching) technic are used for the spin injection device fabrication. The I-V characteristic in the interface between Fe and GaAs is investigated indirectly. AMR (Anisotropic Magneto Resistance) measurement is performed to get the coercive field of two Fe electrodes (spin injector and detector). In the same device non-local spin signal is measured for the evidence of spin injection and detection. The dips in non-local spin signal are nicely matched with the anti-parallel ranges of two Fe electrodes which are acquired by AMR measurement. The local spin valve signal is also measured in the other spin injection device.

Synthesis and applications of III-nitride semiconductor nanostructures

함문호 Graduate School, Yonsei University 2007 국내박사

Studies on electrogneerated chemiluminescence from cyclometalated iridium(III) complexes

신익수 서울대학교 대학원 2007 국내박사