Characterization of Deep Levels in Zn-compound Semiconductors Grown by Molecular-beam Epitaxy : 分子線エピタキシ法で成長した亞鉛化合物半導體中の深い準位に關する硏究

오동철 Graduate School of Engineering, Tohoku University 2005 해외박사

가시·자외선 영역의 광소자(레이저다이오드, LED, 광검출기, 백색광원)는 ZnSe와 ZnO와 같이 500nm 이상의 에너지 밴드갭을 갖고 있는 wide-bandgap semiconductor로 구연가능하다. 그러나, 이러한 화합물반도체는 고온에서 다원계화합물을 단결정으로 성장시킴으로 제작되어진다. 따라서, 이러한 재료들은 자연적으로 불순물·결함에 노출되기쉽다. 이렇게 인위적이지 않고 자연생성된 불순물·결함은 재료의 구조적·광학적·전기적인 물성 및 소자의 특성 악화를 초래한다. 따라서, 고순도 재료의 개발을 위해서는 결함·불순물에 대한 제어가 중요하다. 그런데, 불순물·결함은 반도체의 에너지갭(energy bandgap)에 깊은 준위의 에너지상태인 deep level를 형성함으로 광학적·전기적 특성을 변화시킨다. 따라서, 이러한 deep level를 모니터링함으로 결함·불순물에 대한 제어가 가능하게된다. 본연구는 분자선에피택시법으로 성장한 ZnSe와 ZnO와 같은 화합물반도체에 대해서, 다각적인 분석방법을 이용하여 재료의 광학적·전기적인 특성을 모니터링함으로써 깊은 준위의 메카니즘을 찾아냄으로, 이에 따른 재료의 특성변화를 명확히 할 수 있었다. 이와 아울러 컨택제작기술 확보 및 계면특성을 고찰할 수 있었다. Zn-compound semiconductors such as ZnSe and ZnO are representative Ⅱ-Ⅵ wide-bandgap semiconductors with direct bandgap, which makes the materials most promising for optical devices. The investigation of deep levels in Zn-compound semiconductors is an important issue, because highly-doped impurities degrade electrical and optical properties considerably through the formation of native and/or complex defects which give rise to deep levels in bandgap. The deep levels trap minority/majority carriers and act as nonradiative centers that reduce carrier lifetime, thereby decreasing emission intensity in light-emitting devices. Furthermore, the deep levels may compensate carriers, which limit the maximum attainable carrier concentration below the level required for device fabrication. Such compensation becomes more serious as the bandgap increases and makes amphoteric doping very difficult. Therefore, the control of the deep levels is crucial when applying to optical devices. The purpose of this thesis is to characterize deep levels in Zn-compound semiconductors in particular in ZnSe and ZnO to contribute to the control of deep levels, which will eventually end up with the successful fabrication of optical devices. ZnSe and ZnO layers are grown by molecular-beam epitaxy (MBE)which is considered to be most successful preparation methods. Characterization of the ZnSe and ZnO layers is performed using various methods including photoluminescence (PL), deep-level-transient spectroscopy (DLTS), photocapacitance (PHCAP), and admittance spectroscopy (AS). Only few studies have been reported on deep levels in heavily Al-doped n-type ZnSe, although Al-doped ZnSe is considered to be most promising for device applications. Highly Al-doped ZnSe layers ([Al]〉1019cm-3)show a saturation of electron concentration accompanied by a decrease in near-band-edge emission intensity. The ZnSe : Al layers exhibit various types of deep level : radiative (RD1 and RD2)and nonradiative (ND1, ND2, and ND3) centers. We elucidate that the degradation of optical and electrical properties are ascribed to RDI (Ev+0.83eV), RD2 (Ev+0.55eV), and ND3 (Ev+0.45eV) centers. RD1 and RD2 are expected to be induced by AlznVzn complex defects and Vzn-related defects, respectively. Deep levels in ZnO layers have been investigated by capacitance method for the first time. Large background electron concentration in ZnO layers hampers the formation of good Schottky contact due to leakage current. In order to solve this problem, Au electrode is deposited onto N-doped ZnO layers with reduced background electron concentration to form a Schottky junction. Thus we obtain the barrier height of ΦsB=0.69 eV and the ideality factor of n=1.8 in the Schottky junction. We reveal the existence of 2-dimenional electron gas (2DEG) at ZnO/GaN heterointerfaces, through extensive C-V measurements for the first time. We observe three electron traps ET1 (Ec-40 meV), ET2 (Ec-0.14 eV), and ET3 (Ec-65 meV). ET1 and ET2 exist in all ZnO layers grown under Zn-rich, stoichiometric, and 0-rich flux conditions, while ET3 additionally emerges in ZnO layers grown under Zn-rich flux conditions. ET2 is the dominant trap and its density increases as the flux condition goes to the Zn-rich side. In conclusion, the present thesis studies deep levels in ZnSe and ZnO layers grown by MBE. Based on the present studies, it is expected to find a way to control defects in ZnSe and ZnO layers, which will enable to control both optical and electrical properties of Zn compound semiconductors.

Microstructure and magnetic properties of InAs quantum well and Fe/MgO/InGaAs multilayer structures prepared by molecular beam epitaxy

김경호 Graduate School, Korea University 2013 국내박사

Since Si based complementary metal-oxide semiconductor (CMOS) technology was limited by devices scale, therefore new concept of semiconductor device has been growing. Thus spin polarized field effect transistor (Spin-FET) has been attracted as an alternative idea of next generation semiconductor device. However, it is hard to realize because low out-put signal and spin precession control by gate voltage are main obstacles. One solution of this problem is increasing spin injection efficiency by using surface control between ferromagnetic and semiconductor by introducing thin tunneling barrier. The other one is utilization of the semiconductor channel which has the large spin-orbit coupling and high electron mobility. In this thesis, I studied Rashba spin-orbit interaction parameter, α in double-sided doped InAs quantum well structures of different potential asymmetries created by introducing two separated carrier supply layers. The internal potential asymmetry is manipulated between negative and positive potential gradient by adjusting the relative doping concentrations of the two carrier supply layers. The larger potential asymmetry results the more extensive variation α with respect to the gate electric field. The structures of the negative and positive potential gradients exhibit the opposite variation of α with respect to gate electric field which evidently supports the fact that the sign of α can be changed by the reversed potential asymmetry. I also studied microstructural evolution and the effect on in-plane magnetic properties of the epitaxial Fe/MgO layers grown on InXGa1-XAs (001) substrates have been investigated as a function of the MgO growth temperature and the thickness of Fe layer. The Fe grows three-dimensional islands with two different in-plane textures along [010] and [110] directions on the MgO layers grown below 200 °C in remarkable contrast to two-dimensional Fe layers on the MgO layers grown above 300 °C. As the MgO growth temperature increases, both tensile-strained MgO and the subsequent Fe are simultaneously relaxed, and the distribution of 45°-rotated Fe lattices with [010] texture becomes dominant. The experimental results imply that the microstructural evolution of the Fe is strongly influenced by the underlying misfit strain within the MgO layers grown at different temperatures. The two different epitaxial relationships of the Fe islands lead to no magnetic anisotropy, while the Fe layer with the single epitaxial relationship of Fe[010]//MgO[110]// InXGa1-XAs [110] shows cubic magnetic anisotropy.

MBE growth of Zinc Telluride thin films for neuromorphic device application

김민재 서울시립대학교 일반대학원 2022 국내석사

ZnTe 은 반도체, 광전자 및 전기 스위칭과 같은 기능으로 많은 관심을 받았습니다. 전기 저항 스위칭 현상인 Ovonic threshold switching(OTS) 현상은 비정질 상의 일부 칼코게나이드 물질에서 나타나는 것으로 알려져 있습니다. 최근, ZnTe 의 다결정상에서도 특성이 나타난다는 보고가 있습니다. 그러나 에피택시 성장의 메커니즘과 OTS 특성은 아직 완전히 연구되지 않았습니다. 이 연구에서 우리는 분자살 켜쌓기를 통해 GaAs (001)와 TiN/Si (001)위에 ZnTe 박막을 합성하였습니다. 실시간 반사 고에너지 전자 회절 및 싱크로트론 기반 고해상고 X 선 회절은 표면의 원자 배열과 결정성 모두를 보여주었습니다. 또한, 화학 조성의 변화는 성장 조건에 따라 달라지며, 이를 X 선 광정자 분광법으로 조사하였습니다. 밴드갭 및 내무 밴드갭 상태에 대한 정보는 분광 타원 측정법을 통해 획득했습니다. 또한 전도성 원자 현미경을 통해 박막의 화학적 조성에 따른 전기적 특성을 확인할 수 있었습니다. 우리의 연구는 차세대 크로스 포인트 어레이 장치 응용을 위한 전기 스위칭 장치 개발의 모델로서 에피택시 ZnTe 박막의 잠재력을 시사합니다. Zinc telluride (ZnTe) attracted many attentions for its functionalities such as semiconducting, optoelectronic, and electrical switching. Ovonic threshold switching (OTS) phenomenon, an electrical resistance switching phenomenon, was known to appear in some chalcogenide materials with amorphous phase. Recently, it was reported that ZnTe appears in polycrystalline phase. However, mechanism of epitaxial growth and OTS property are still not yet thoroughly studied. In this work, we synthesized ZnTe thin films on GaAs and TiN/Si by molecular beam epitaxy. In situ reflection high-energy electron diffraction and synchrotron-based high-resolution X-ray diffraction showed that both atomic ordering of surface and crystallinity. Moreover, the variation of chemical compositions depended on the growth conditions, and it was investigated by X-ray photoelectron spectroscopy. Information on the bandgap and in-gap states was acquired through spectroscopic ellipsometry. Also, conductive atomic force microscopy showed the electrical properties of the thin film according to its chemical composition. Our study suggests that the potential of epitaxial ZnTe films as a model in the development of electrical switching devices for next generation cross-point array device applications.

Mn-Ge Magnetic Semiconductor and Intermetallic Alloy Thin Films: Molecular Beam Epitaxy Growth and Structural, Electrical and Mangetic Properties

Dang Duc Dung 울산대학교 2011 국내석사

The spin-injection efficiency in semiconductors is most promising to apply the next generation devices by using the controller spin rather than charge of carrier. There are two main approaches to realize spin injection: (1) use dilute magnetic semiconductors (DMSs) as the spin aligner or (2) fabricate a ferromagnetic metal/semiconductor (MS) heterostructure via an insulator or Schottky barrier. The low Curie temperature (TC) and phase separation occurs that hind the first approach cause the low solute solution of magnetic ions translation in host semiconductor. In addition, the spin orientation of the carrier tends to be quickly lost at a ferromagnet-semiconductor interface via spin-flip scattering due to the dissimilar crystal structure and chemical bonding and the energy difference between the charge carriers in the ferromagnet and semiconductor. Mn-Ge alloy have attracted a great deal of attention because they are compatible with the current Si-based processing technology. In addition, both approach for spin injection could be used by controlling the Mn and Ge composition, such as Mn-doped Ge and Mn3Ge2, Mn3Ge alloys. However, the origin ferromagnetic odering of Mn-doped Ge was still unclear cause by inhomogeneous Mn substituted in Ge host and/or cluster due to low soluted solution of Mn in Ge. For the secondary approach, the magnetism and transport properties of Mn-Ge alloy film were modificed during the current epitaxi cause the affect of strain by lattice mismatch and/or thermal strain due to thermal coefficience difference between film and substrates. For the first appoarch, a ferromagnetic semiconductor (FMS) is a material type that transplants the magnetic properties of ions into the host semiconductor material. For the group IV-based Mn-doped Ge thin films, a Curie temperature (TC) of 116 K and a p-type carrier of 10^20 cm-3 was originally reported by Park et al.. A well established mechanism for the Mn-based diluted magnetic semiconductors (DMSs) is a hole-mediated ferromagnetism based on the RKKY theory. The hole-mediated exchange interaction permit the control of ferromagnetic ordering by controlling the hole concentration. In GaMnAs, substituting Mn into GaAs generates a hole carrier, whose carrier density depends on the Mn concentration. In Mn:Ge, it is well characterized that the TC depends on Mn concentration. However, the carrier density versus TC is more or less complicated in Mn:Ge because the Mn substitution into Ge generates a deep level with the Mn+2, Mn+3 or Mn+4 valence states. The mechanism of the ferromagnetism in Mn:Ge is still not well understood. we reported on the carrier type changes of the p-type for as-grown Mn:Ge films to n-type for post-annealed samples in a hydrogen ambient. The hydrogen-annealed n-type samples exhibited an increased TC, from 165 to 198 K, and an enhanced magnetic moment, from 0.78 to 1.10 mB/Mn. The first principles calculation using the all-electron full-potential linearized augmented plane wave (FLAPW) method indicated that the addition of an electron carrier strengthens the FM coupling between the Mn atoms, while the hole carrier caused it to weaken. We supprised that annealed samples in N2 also shown the n-type carrier while sample annealed in Ar still maintained p-type. Futher more, the p-n junction were fabricated where n-type layer is (Mn,H):Ge and p-type layer is Mn:Ge. The interesting that both layers are ferromagnetic and spin (electron and hole) were polarization at room temperature by cluster (Mn5Ge3), that result shown the V-I curve difference to comprare with only one layer is ferromagnetic. For the secondary approach, in epitaxial thin films on crystalline substrates, various crystallographic and magnetic phases other than those seen in bulk material have been predicted. These are made possible by additional driving forces such as broken bonding at the surface, interface interaction, and lattice mismatch strain between the substrate and film, etc. Strain is known to be a powerful tool in modifying the structural, electronic, and magnetic properties of a material because the energies associated with structural and magnetic changes have a similar order of magnitude (~0.1 eV per atom). In this work, the strain modifies strongly the magnetic properties of epitaxial Mn3Ge2 thin films. The antiferromagnetic state below 150 K in bulk were modification to ferromagnetic state and the saturation magnetization (MS) was strongly enhanced from 0.08 mB/Mn in bulk to 1.32 mB/Mn in thin film on GaAs(001) and 0.23 mB/Mn in thin film on GaSb(001). For the secondary approach, the new materials with high spin polazization also are importance for application. In the binary Mn-Ge system, six intermetallic phases such as Mn3.4Ge, Mn5Ge2, Mn7Ge3, Mn2Ge, Mn5Ge3 and Mn3Ge2, are known to exist in the equilibrium phase diagram. On the other, under the unstable equilibrium (high-pressure/temperature) condition, new Mn:Ge intermetallic phases such as MnGe, Mn3Ge5, MnGe4, and Mn3Ge, etc., have successfully been synthesized. Interestingly, some Mn:Ge alloys have similar chemical formula but different crystal structure, resulting in unique magnetic properties. Takizawa et al. obtained ferromagnetic Mn3Ge with Cu3Au-type structure, which differed from the antiferromagetic with hexagonal DO19-type structure Mn3Ge reported by Kouvel et al.. Matsui et al. reported the room temperature of unknown Mn3Ge phase with has the tetragonal structure. In this work, we report epitaxial growth and ferromagnetic properties of α-Mn structured Mn3Ge films on GaAs(001) substrate by using molecular beam epitaxy. The single Mn3Ge phase was synthesized for growth temperature of 150 oC while the secondary phases such as Mn11Ge8, Mn5Ge3 were observed above the growth temperature of 250 oC. The Mn3Ge films exhibited ferrimagnetism with the Curie temperature of 344 K. The saturation magnetizations were 255.9 and 313.8 emu/cm3 and the corresponding coercive fields were 453 and 1166 Oe at 10 K for the samples grown at 150 and 300 oC, respectively.

CdTe/ZnTe 다층 양자점의 광학적 특성 및 운반자 동역학에 대한 연구

김수환 연세대학교 대학원 2016 국내석사

Quantum dots (QDs) are attracting increased attention due to their promising applications in nanoscale devices such as lasers, optical switch, solar cell, light emitting diodes, single electron transistors, infrared photodetectors. To date, relatively little work has been performed on group II-VI/II-VI quantum dots in comparison with III-V/III-V QDs because of delicate problems encountered during growth. However, II-VI QDs structures are characterized by large excitonic binding energies and CdTe/ZnTe QDs are of great interest because of their potential application in optoelectronic devices operating in the green spectral range. moreover, the buried strain fields form built islands induce vertical alignment in adjacent stacked QD layers, giving rise to a reduction of the QDs size distributions. In addition, advantage of multiple QDs consists in the electronic coupling between adjacent dots when the separation layer thickness is thin enough to allow carrier tunneling. In this work, we investigate the effects of the number of QDs layers and ZnTe separation layer thickness on the carrier dynamics and optical properties in multi-layer CdTe/ZnTe QDs grown on GaAs substrate using molecular beam epitaxy and atomic layer epitaxy. First, we studied the variation of energy gap of single layer CdTe/ZnTe quantum dots and multi-layer CdTe/ZnTe quantum dots structures, by the low-temperature PL measurements. Second, we analyzed the effects of the number of QDs layers and ZnTe separation layer thickness on the activation energy and carrier dynamics in multi-layer CdTe/ZnTe QDs using the temperature-dependance PL and Time-resolved PL measurements. 현재 화합물 반도체 나노구조는 적외선 검출기, 레이저, 발광 다이오드, 단전자 트랜지스터, 태양전지 등과 같은 고효율 광전자 소자에서의 응용을 위해 활발한 연구가 진행 되고 있다. 특히 양자점은 3차원으로 구속되어 있는 상태 밀도를 갖고 있어 레이저 응용 시 낮은 문턱 전류 밀도, 높은 이득, 높은 열적 안정성을 기대하고 있다. 하지만 양자점의 크기가 불규칙적이고 운반자 수집의 한계로 인하여 기대 이하의 온도 안정성을 갖고 있어 이를 극복하기 위해 양자점의 크기와 운반자 수집을 제어하기 위해 다양한 방법이 연구되고 있다. 그 중 다층으로 성장된 양자점 구조는 양자점의 크기 분포 조절이 용이하고 양자점 층간의 전기적 결합력이 강한 특성이 있다. 또한 II-VI 족 화합물 반도체 양자점은 기존의 III-V 족 양자점보다 더 큰 엑시톤 결합에너지(exciton binding energy)를 가지고 있으며, 이러한 특성을 가지는 II-VI 족 화합물 반도체 양자점 중에서도 CdTe 양자점은 높은 엑시톤 결합에너지와 가시광선영역의 광전자 디바이스에 응용 가능성이 높기 때문에 더욱 각광받고 있다. 본 연구는 분자 선속 에피 성장법(Molecular Beam Epitaxy; MBE)과 원자 층 교대 성장법(Atomic Layer Epitaxy; ALE)으로 CdTe/ZnTe 다층 양자점 구조를 성장하여 CdTe 양자점 층수 변화와 ZnTe 장벽층의 두께에 따른 광학적 특성을 연구하였다. 먼저 CdTe 양자점 층수 변화에서 저온 광루미네센스(Photoluminescence; PL) 측정을 통하여 CdTe 양자점의 층수가 증가할수록 양자점의 PL 피크가 높은 에너지로 이동함을 알 수 있었는데, 이는 CdTe의 변형력(strain)에 의해서 ZnTe 장벽층과 혼합(intermixing)현상이 발생했기 때문이다. 그리고 온도 의존 광루미네센스 측정결과 양자점의 층수가 증가할수록 열적 활성화 에너지가 감소하는 것을 확인하였는데, 이는 양자점의 층수 증가에 따라 ZnTe 장벽층의 결함이 증가하였기 때문이다. 또한 시분해 광루미네센스 측정 결과 양자점의 층수가 증가할수록 양자점의 소멸 시간이 길게 측정되었는데, 이는 다층 양자점 구조에서 변형(strain)으로 인해 전자와 정공의 결합이 증가하였기 때문이다. 다음으로 ZnTe 장벽층 두께 변화에서는 저온 광루미네센스 측정을 통하여 ZnTe 장벽층 두께가 증가할수록 양자점의 PL 피크가 높은 에너지로 이동함을 알 수 있었는데, 이는 CdTe 양자점 안에 압축 변형력(Compressive strain)이 증가하였기 때문이다. 그리고 ZnTe 장벽층의 두께가 증가할수록 PL 세기가 커지는 것을 알 수 있었는데, 이는 더 많은 운반자들이 양자점으로 구속되었기 때문이다. 또한 온도 의존 광루미네센스 측정결과 ZnTe 장벽층의 두께가 감소할수록 혼합(intermixing) 현상으로 인해 정공의 수집 능력이 감소하여 열적 활성화 에너지가 감소하였고, 시분해 광루미네센스 측정을 통해 장벽층 두께가 감소할수록 양자점 층간에 전기적 결합력으로 인한 양자점의 소멸 시간 증가를 확인하였다. 이와 같은 결과 다층 양자점에서 양자점 층수 변화와 ZnTe 장벽층 두께 변화가 단층 양자점의 한계인 열적 안정성과 운반자 수집 능력을 향상 시킬 수 있는 좋은 구조임을 제시하고 있다.

Polymorphism and Electronic Structure of van der Waals Layered Indium Telluride

이상민 서울대학교 대학원 2023 국내박사

III-VI metal chalcogenides represent a distinctive class of van der Waals (vdW) layered materials with exceptional physical properties and potential technological applications. The synthesis and unique physical properties of indium telluride (InTe), which belongs to this family, are studied using molecular beam epitaxy (MBE), scanning transmission electron microscopy (STEM) and ab initio density functional theory (DFT). Aberration-corrected STEM is utilized to directly reveal the interlayer stacking modes and atomic structure, leading to a discussion of various forms of polytypes. It is established that MBE is a viable approach for synthesizing novel polymorphs. In addition, the electron beam-induced phase transitions of InTe are discussed. By investigating the extent and mode of these phase transitions under various electron beam conditions, the feasibility of nanoscale fabrication is demonstrated using electron beams. The experimental validation of the InTe polymorphs expands the family of materials in the III-VI metal chalcogenides while suggesting the possibility of new stacking sequences for known materials in this system. Furthermore, the symmetry of InTe is examined with respect to polymorphism, and ab initio DFT calculations are employed to investigate the exotic physical properties of InTe associated with its symmetry. InTe represents a combination of the heaviest elements in the III-VI metal chalcogenide family, resulting in strong spin-orbit coupling and associated distinctive electronic structural features. The trigonal anti-prismatic structure present in III-VI metal chalcogenides is discussed, and the consequences of its inversion symmetry, notably the hidden Rashba (R-2) effects, are investigated. Trigonal anti-prismatic InTe monolayer undergoes band inversion between R-2 bands, exhibiting an anomalous spin texture distinct from conventional R-2 phenomena. In addition, the unique topological properties associated with mirror symmetry are examined. Due to its vdW layered structure, InTe can form various mirror symmetries depending on the stacking configuration, leading to it being a mirror Chern insulator. Therefore, InTe exhibits the characteristics of a strong topological insulator and a topological crystalline insulator, revealing the unique behavior of the accompanying surface states III-VI 금속 칼코게나이드는 층상 구조를 가지는 반데르발스 (van der Waals) 물질로, 우수한 물리적 특성과 잠재적인 기술적 응용 가능성을 가지고 있다. 이러한 물질군에 속하는 인듐 텔루라이드 (InTe)의 합성과 독특한 물리적 성질을 분자선에피택시성장 (MBE), 주사투과전자현미경 (STEM) 및 밀도범함수이론 (DFT)을 사용하여 고찰한다. 수차 보정 STEM을 이용하여 층간 적층 형태와 원자 구조를 직접 확인함으로써 다양한 형태의 다형성에 대해 논의한다. 이를 통해 MBE를 이용한 에피택시 성장이 III-VI 금속 칼코게나이드의 새로운 다형성 합성을 위한 하나의 성장 기법임을 제시한다. 또한, 전자 빔에 의한 InTe의 상전이에 대해 논의한다. 다양한 전자 빔 조건에서 상전이의 정도와 양상을 관찰함으로써, 전자 빔을 이용한 나노 수준 가공의 실행 가능성을 입증한다. InTe 다형성의 실험적 검증은 III-VI 금속 칼코게나이드 물질군의 범위를 확장하면서, 이 물질군의 기존 구성 재료들에 대한 새로운 적층 형태의 가능성을 제시한다. InTe의 다형성과 관련된 대칭성에 대해 논의하며, DFT 계산을 통해 InTe의 대칭성과 관련된 독특한 물리적 특성을 고찰한다. InTe는 III-VI 금속 칼코게나이드 물질군에서 가장 무거운 원소들로 구성되어 있어, 강한 스핀-궤도 결합과 관련된 흥미로운 전자 구조를 가지고 있음을 내포한다. III-VI 금속 칼코게나이드에서 나타나는 삼각 반프리즘 (trigonal anti-prism) 구조에 대해 조사하며, 이 구조와 관련된 숨겨진 형태의 라쉬바 (R-2) 효과에 대해 논의한다. 삼각 반프리즘 구조의 InTe 단일층은 R-2 밴드 사이에서 밴드 역전을 일으키며, 이로 인해 기존의 R-2 현상과 다른 독특한 스핀 텍스처를 나타냄을 밝힌다. 또한, InTe의 거울 대칭과 관련된 독특한 위상학적 물성에 대해 논의한다. InTe는 반데르발스 층상 구조로 인해 적층 구성에 따라 다양한 거울 대칭을 형성할 수 있음을 보이고, 이에 따른 거울 천 절연체 (mirror Chern insulator)의 특성을 조사한다. 따라서 InTe는 강한 위상학적 절연체(strong topological insulator)와 위상학적 결정 절연체(topological crystalline insulator)의 특징을 동시에 갖고 있음을 입증하고, 이에 동반되는 표면 상태의 특이한 성질을 규명한다.

A study on Electrical Transport Properties of ZnTe buffer layer effect on Tellurium doped GaSb:Te Epitaxial Layer

Aung Khaing Nyi 한국해양대학교 2009 국내석사

Among the III-V binary semiconductors, Gallium Antimonide (GaSb) has attracted considerable attention. Many of its interesting properties are directly associated with its very low effective electron mass and high mobility. Consequently, it is an important candidate in high speed applications in transistors and other devices. Undoped GaSb is always p-type conductivity due to the native defect such as Sb vacancy. Therefore, to achieve n-type thin film with higher carrier mobility, high quality film growth is absolutely required. This thesis presents the electrical transport properties for one typical set of Te-doped GaSb layers; the one is normally grown on a GaAs substrate by molecular-beam epitaxy (Type I), and the other includes a ZnTe buffer between the GaSb:Te layer and the GaAs substrate (Type II) with the structural properties and investigated the effect of ZnTe buffer on the Te-doped GaSb epitaxial layers based on the two layer Hall effect model. The five major scattering mechanisms (ionized impurity, dislocation, piezoelectric, deformation potential and polar phonon) effects were considered. By using this method, two types of GaSb:Te layers show extremely different electrical and structural properties; i) Type I has an electron mobility of 250 ㎠/V·s while Type II has 2.5 times larger value,(630 ㎠/V·s); ii) Type I has a X-ray linewidth of 970 arcsec, while Type II has 2 times smaller value (520 arcsec). The increase of electron mobility in Type II is ascribed to the suppression of defect scatterings by point defects and dislocations, which is consistent to the decrease of X-ray linewidth in Type II. The electron transport mechanisms of the two types of GaSb:Te layers can be explained by ionized-impurity scattering and dislocation scattering. Consequently, it is suggested that the ZnTe buffer layers effectively enhance the structural quality and carrier mobility in Te-doped n-type GaSb epitaxial layers, which will improve the fabrication of optoelectronic devices.

A study on Electrical Transport Properties of ZnTe buffer layer effect on Tellurium doped GaSb

나강영 한국해양대학교 대학원 2009 국내석사

Among the III-V binary semiconductors, Gallium Antimonide (GaSb) has attracted considerable attention. Many of its interesting properties are directly associated with its very low effective electron mass and high mobility. Consequently, it is an important candidate in high speed applications in transistors and other devices. Undoped GaSb is always p-type conductivity due to the native defect such as Sb vacancy. Therefore, to achieve n-type thin film with higher carrier mobility, high quality film growth is absolutely required. This thesis presents the electrical transport properties for one typical set of Te-doped GaSb layers; the one is normally grown on a GaAs substrate by molecular-beam epitaxy (Type I), and the other includes a ZnTe buffer between the GaSb:Te layer and the GaAs substrate (Type II) with the structural properties and investigated the effect of ZnTe buffer on the Te-doped GaSb epitaxial layers based on the two layer Hall effect model. The five major scattering mechanisms (ionized impurity, dislocation, piezoelectric, deformation potential and polar phonon) effects were considered. By using this method, two types of GaSb:Te layers show extremely different electrical and structural properties; i) Type I has an electron mobility of 250 ㎠/V·s while Type II has 2.5 times larger value,(630 ㎠/V·s); ii) Type I has a X-ray linewidth of 970 arcsec, while Type II has 2 times smaller value (520 arcsec). The increase of electron mobility in Type II is ascribed to the suppression of defect scatterings by point defects and dislocations, which is consistent to the decrease of X-ray linewidth in Type II. The electron transport mechanisms of the two types of GaSb:Te layers can be explained by ionized-impurity scattering and dislocation scattering. Consequently, it is suggested that the ZnTe buffer layers effectively enhance the structural quality and carrier mobility in Te-doped n-type GaSb epitaxial layers, which will improve the fabrication of optoelectronic devices.

Strain manipulation of magnetic anisotropy in freely-suspended GaMnAs structures using nanomachining technique

양찬욱 서울대학교 대학원 2020 국내석사

Strain, with recent developments of its manipulation, may offer a new degree of freedom in semiconductor spintronics. Strain directly changes the distance between each atoms in diluted magnetic semiconductor facilitating development of semiconductor spintronics, modifying the electronic band structure and influencing the spin-orbit interaction enormously. As a result, small changes in strain can manifest significant changes in magnetic anisotropy in GaMnAs. In order to manipulate strain-induced magnetic anisotropy changes as well as magnetic anisotropy at strain-free state, GaMnAs/AlGaAs heterostructure is grown by UHV-MBE system. Freely suspended GaMnAs structures are fabricated using typical nanomachining techniques. With aid of soft metallic electric leads, relaxed GaMnAs layer is suspended, isolated from the underlying AlGaAs layer. From the measurement of temperature dependent longitudinal resistivity, no signs of degradation in macroscopic properties during fabrication is observed. From anomalous Hall effect and planar Hall effect, in-plane magnetic anisotropy of GaMnAs under built-in compressive strain is reduced in relaxed suspended structure and out-of-plane magnetic anisotropy is enhanced in the relaxed suspended structure. Further investigation is conducted to manipulate local strain-induced magnetic anisotropy changes due to buckling phenomena. Contrary to the relaxed suspended Hall bar structure with a soft metal support, the suspended multi-Hall bar structure with the semi-rigid support, GaMnAs beneath the metal electrode, exhibits buckling phenomena. The buckling suspended structure is no longer free from evolving strains. Lattice restoration in the Hall beam of suspended Hall bar structure cause further compression towards both ends of the Hall beam. As a result, more compressive strain in semi-rigidly supported Hall probes is induced. The local strain is dependent on the mechanical bending shape of buckled multi-Hall bar structure, explained by simple mass spring model. Anomalous Hall responses and longitudinal resistivities in each Hall probes and Hall beams measured by anomalous Hall effect scheme supports the mechanical compression model. Moreover, two-level anomalous Hall response state corresponding to buckled and non-buckled is observed in a buckled simple Hall cross structure. This magnetic anisotropy by nanomachining-based strain manipulation suggests a potential of bi-state mechanical memory elements and dynamic strain manipulation. In addition to the main contents of this dissertation, on-chip ferromagnetic resonance measurement is introduced in Appendix: from the set-up of FMR experiment to the prospects of it. It enables to investigate the precise measurement of magnetic anisotropy field in GaMnAs by observing in frequency domain. 스핀트로닉스(Spintronics)는 전자가 가지고 있는 전하를 이용한electronics에 스핀(spin)의 자유도를 더하여 함께 연구하는 분야로, 전기학과 스핀을 의미하는 영어 단어의 합성어이고, 1980년대 후반에 거대자기효과(Giant Magnetoresistance, GMR)의 발견 이후 크게 발전하여 하드 디스크 드라이브 등 산업적으로도 많은 기여를 하였고, 오늘 날 양자 컴퓨터의 구현을 위한 한 축으로써 연구되고 있다. 이러한 스핀트로닉스에서 연구되어 온 대표적인 물질로 GaMnAs가 있고, 이 물질은 강한 스핀-궤도 상호작용을 가지는 GaAs 에 Mn을 도핑시켜 강자성을 띄는 대표적인 자성 반도체(Diluted magnetic semiconductor)이다. 한편, 변형률(strain)은 응력으로 인한 물질의 기하학적 변형을 나타내는 것으로 물질 내부의 원자들 간의 거리가 직접적으로 변하게 된다. 앞서 언급한 GaMnAs의 숙주 물질인GaAs는 강한 스핀-궤도 상호작용을 가지고 있어 물질 내부의 원자들 간의 거리 변화에 큰 영향을 받게 되며 이로 인해 변형률의 조절은 GaMnAs의 강자성을 변화시키는 요인이 된다. 이러한 변형률의 조절은 반도체 스핀트로닉스에서 새로운 자유도를 주는 역할을 한다. 본 연구에서는 반도체 스핀트로닉스의 대표적인 물질인 GaMnAs를 직접 성장시키고 NEMS 기술을 이용하여 현수된 구조를 만들어 GaMnAs의 변형률을 조작하여 GaMnAs의 강자성 특징인 자기 이방성(magnetic anisotropy)의 변화에 대한 보고를 한다. 변형률을 조절하기 위해 UHV-MBE 시스템을 이용하여 GaMnAs / AlGaAs / GaAs의 heterostructure를 성장한다. 여기서 AlGaAs층은 NEMS 기술을 이용하여 GaMnAs의 현수된 구조를 만들기 위한 희생층으로 사용된다. 전형적인 NEMS 기술인 e-beam lithography와 선택적 식각을 이용하여 GaMnAs의 현수된 구조를 구현한다. 현수된 GaMnAs 구조물은 크게 2가지 형태로 구현하였는데, MBE 성장 과정에서 발생하는 GaAs substrate와 GaMnAs의 격자의 크기 차이에 의한 변형률을 완전히 완화(relaxation)시키는 구조와 선택적 식각에 따른 버클링 현상이 일어나 버클링된 구조체의 모양에 따라 변형률이 다르게 인가되는 구조가 있다. 먼저, 격자 차이에 의한 변형률을 완전히 완화되는 구조에서 비정상 홀 효과(anomalous Hall effect)와 평면 홀 효과(planar Hall effect)를 통해 자기 이방성을 측정하였고, 현수되지 않은 상태와 비교해 보았다. 현수되지 않은 경우는 GaMnAs 격자가 GaAs 격자에 비해 크기 때문에 GaMnAs는 압축적인(compressive) 변형률을 가지고 있는데 반해, 현수된 구조에서는 이 압축적인 변형률이 완전히 완화한다. 이 때, 자기 이방성은 현수되지 않은 경우는 in-plane상의 biaxial의 성향이 강하게 나타나는데 반해, 현수된 경우에는 in-plane 상의 biaxial의 성향이 줄어들게 되고 out-of-plane방향의 uniaxial의 성향이 강화된다. 다음으로는 버클링을 통하여 부분적으로 인가되는 변형률이 다른 멀티 홀바 의 구조에서는 anomalous Hall 효과를 통해 자기 이방성을 확인할 수 있다. 구조체의 모양에 따라 인가되는 변형률이 다른데, 이를 설명하는 메커니즘은 본문을 통해 설명하였다. 좀더 압축적인 변형률 내에 있는 경우 out-of-plane 방향의 uniaxial의 성향이 낮아지는 반면, 좀더 인장의(tensile) 변형률 내에 있는 경우에는 out-of-plane 방향이 강해지는 것을 확인하였다. 마지막으로 버클링이 일어나는 싱글 홀 크로스 구조를 구현하여 다이나믹한 변형률의 조절을 할 수 있는 가능성을 보았다. 싱글 홀 크로스 구조에서 버클링이 일어난 구조가 여러 번의 온도에 따른 수송 실험을 거쳐 완화된 구조로 변한 것을 확인하였고, 이는 두가지 역학적 구조에 따른 자기 이방성이 두가지 상태로 나타나는 이른바 역학적 메모리로써 가능성을 보여준다. 이 논문의 메인 연구에 더해, Appendix에서는 칩 내에서의 강자성 공명 측정법(on-chip ferromagnetic resonance measurement)을 소개하고 실험을 위한 셋업에서부터 GaMnAs 자기 이방성의 측정까지 보고한다.

InN 나노와이어 및 graphene 전도층 기반 1.3 μm 파장에서 동작하는 유연한 광센서 제작

양호현 전북대학교 일반대학원 2020 국내석사

In this thesis, the formation of high-crystalline InN nanowires (NWs) on Si(111) by controlling the initial growth kinetics is discussed. Also, the fabrication and characterization of flexible InN-NW photo-sensors operating at the wavelength of 1.3 μm are reported. In order to achieve high-quality InN NWs, a new growth approach, In pre-deposition method, was used. To optimize the formation of InN NWs, growth conditions such as substrate temperature and V/III ratio were systematically varied. As an example, the InN NWs with symmetric shapes were grown with increasing substrate temperature from 500 to 575℃. In high-resolution transmission-electron microscope images measured along the vertical direction of an InN NW, the wurtzite (WZ) crystal structure was observed. In addition, defects including stacking faults and dislocation, which are typically observed for the III-V semiconductor NWs, were hardly observed except for the interface between the InN NW and the Si substrate. In the photoluminescence spectra of InN NWs at 10K and room temperature, strong free-exciton (FX) peaks were observed at the wavelengths of 1294 and 1295 nm, respectively. It should be noted that the FX peaks are clearly observed from our InN NWs, indicating to the formation of NWs with high-crystal quality. To fabricate flexible photo-sensors, the InN NWs used as light-absorbing media were randomly and horizontally embedded between graphene sandwich structure working as a carrier channel. For the graphene sandwich structure, the number of bottom graphene layers, transferred on an overhead project (OHP) sheet, was varied from one to five. For the top carrier channel of the photo-sensors, the single-layer graphene capped the InN NWs. The photocurrent and photoresponsivity of the photo-sensor with triple-layer bottom graphene were measured as 1.166 mA and 0.486 A/W, respectively, at the light intensity of 60 mW/cm2 and the voltage of 1 V. These values are much higher than those of previous reports. To evaluate the photoresponse characteristics of the flexible InN-NW photo-sensors, the devices were approximated equivalent circuits with resistances and capacitances. The rise times and decay times for the photo-sensor with triple-layer bottom graphene were measured as 55.01 and 54.42 μs, respectively, under the pulse width of 0.1 ms and duty cycle of 50 % (5 kHz). This signal response is much faster than those of previous reports. To analyze the degree of flexibility, InN-NW photo-sensor with triple-layer bottom graphene was subjected to 200 cyclic-bending tests under the different substrate bending radii. The photocurrent of the photo-sensor was measured as 1.157 mA under the substrate bending radius of 3.1 mm, which was 99.12 % compared to that before bending. From these results, the highly efficient flexible photo-sensors with InN NWs and graphene were successfully fabricated.