TiO_(2)계 묽은 자성반도체 박막의 제조 및 특성 = Preparation and properties of TiO_(2)-based diluted magnetic semiconductor thin films

다국어 초록 (Multilingual Abstract)

Spintronics is a rapidly expanding research area because of recent developments in the physics of spin-dependent phenomena. For use as spintronic materials, dilute magnetic semiconductors (DMS) are of considerable interest as spin injectors for spintronic devices. Many researchers have studied DMS, in which transition metal atoms are introduced into the lattice, thus inserting local magnetic moments into the lattice. Among the materials so far studied, Mn-doped GaAs has the highest Curie temperature, Tc, ~ 110 K, and is presumed to be a promising candidate for practical applications. However, the highest Curie temperature for these materials is still far from room temperature required for practical device applications.
Recently, Co-doped TiO_(2) anatase, grown by pulsed laser deposition (PLD), has been demonstrated to be ferromagnetic and semiconducting for doping levels up to around 8 at.％, and temperatures of up to 400 K. Titanium dioxide is wide gap oxide semiconductor and in addition, anatase has high mobility of n-type charge carrier and large thermopower of 200 V/K at 300 K. Co-doped anatase thin films as DMS materials were prepared by pulsed laser deposition, oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE), rf magnetron sputtering, and sol-gel process for epitaxial or polycrystalline growth. For increase of integration levels in Si process, a demand for thin film fabrication methods with precise composition control, conformal step-coverage, good uniformity, and high throughput is increasing. The chemical vapor deposition technique is well-known as the best approach to fulfill these requirements.
In this study, Ti_(1-x)Co_(x)O_(2) (x=0~0.12) anatase and rutile thin films are prepared using liquid-delivery metalorganic chemical vapor deposition. Their ferromagnetic properties at room temperature were investigated through their compositional and microstructural studies. Also, rutile Ti_(1-x)Co_(x)O_(2) (x=00.12) and Sb co-doped into rutile Ti_(1-x)Co_(x)O_(2) films with 200 nm thickness are grown on R-Al_(2)O_(3) (11￣02) substrates at various deposition temperatures by pulsed laser deposition. The ferromagnetic and electrical properties of Co and Sb co-doped into rutile TiO_(2) films were investigated as a function of deposition temperature, and the Sb doped-Ti_(1-x)Co_(x)O_(2) films are compared with nondoped Ti_(1-x)Co_(x)O_(2) films.
Ti_(1-x)Co_(x)O_(2) thin films were prepared by liquid delivery metalorganic chemical vapor deposition using (C_(11)H_(19)O_(2))2(C_(3)H_(7)O)_(2)Ti (Inorgtech Chemicals, Inc.) and Co(C_(11)H_(19)O_(2))_(3) (Strem Chemicals, Inc.) as the source materials for Ti and Co, respectively. The organic precursors were dissolved in a solvent (THF : tetrahydrofran, C_(4)H_(8)O, Sigma-Aldrich Chemical Co., Inc.) to form a source solution of 0.05 mol concentration of (C_(11)H_(19)O_(2))_(2)(C_(3)H_(7)O)_(2)Ti and Co(C_(11)H_(19)O_(2))_(3). Solutions of each precursor were mixed together and single-mixture solutions with various concentrations were used for the preparation of Ti_(1-x)Co_(x)O_(2) thin films.
The polycrystalline Ti_(1-x)Co_(x)O_(2) thin films onto SiO_(2)(200nm)/Si substrates using liquid-delivery MOCVD were successfully prepared and characterized for ferromagnetic properties as a function of Co-doping concentration. The detailed composition of clusters formed in the samples was analyzed scanning Auger microscopy(SAM). Ferromagnetic behaviors of polycrystalline films were observed at room temperature and the magnetic and structural properties depend critically on the Co distribution, which varies widely with Co-doping concentration. The annealed Ti_(1-x)Co_(x)O_(2) thin films at x=0.05 showed a homogeneous structure without any clusters and pure ferromagnetic properties of thin films are only attributed to the Ti_(1-x)Co_(x)O_(2) (TCO) phases. On the other hand, in case,of thin films at x=0.05, Co clusters are formed in homogeneous Ti_(1-x)Co_(x)O_(2) phases and overall ferromagnetic (FM) properties depend on both FMTCO and FMCo. Co elements in Ti_(1-x)Co_(x)O_(2) (x=0.05) films have COO(Ⅱ) formal oxidation state. In case of Ti_(1-x)Co_(x)O_(2) (x=0.12), the films have COO(Ⅱ) formal oxidation state and Co metal state. Co clusters with about 150 nm size decreases the value of Mr(the remanent magnetization) and increases the saturation magnetic field. From a magnetic measurement at room temperature, the Ms and Hc of anatase Ti_(0.97)Co_(0.03)O_(2) thin film are 20 emu/c㎤ and 250 Oe, respectively. On the other hand, the Ms and Hc of rutile Ti_(0.97)Co_(0.03)O_(2) thin film are 10 emu/㎤ and 275 Oe, respectively. The MS value of rutile Ti_(0.97)Co_(0.03)O_(2) films have the smaller than that of anatase Ti_(0.97)Co_(0.03)O_(2) films. The rutile (101) prefered orientation Ti_(0.97)Co_(0.03)O_(2) thin films deposited at 550℃ using by liquid-delivery MOCVD. This films shows the room temperature ferromagnetic properties and the absorption edge shows a blue-shift signifying the influence of incoporated Co on the electronic states. Such shifts have been found in dilute magnetic semiconductor systems.
However, the high resistivity of Ti_(1-x)CO_(x)O_(2) thin films deposited in high oxygen pressure (~0.5 torr) using by MOCVD , have problems for measured about electrical properties. i.e., Anomalous hall effect (AHE) and magnetroresistance (MR). For that reason, pulsed laser deposition (PLD) which was a possible method in low oxygen pressure (100mtorr ~ 10^(-6) torr), was used for rutile Ti_(1-x)CO_(x)O_(2)(x=0-0.15) thin films were grown on R-Al_(2)O_(3)(11￣02) substrates and investigated for the magnetic and electrical properties. PLD using a KrF excimer laser (=248nm) at various temperatures. The films were deposited at a laser repetition rate of 2Hz and a pulse energy density of 1.5 J/㎠. The operating pressure during the deposition was 5 × 10^(-6) Torr~4×10^(-4)torr. The deposition rate was 0.2/shot, and the film thickness was approximately 200 nm.
The reduced rutile (Magneli phase) Ti_(1-x)CO_(x)O_(2) (x= 0~0.15) films exhibit ferromagnetic properties at room temperature. This films have Co cluster. However, Anomalous Hall effect (AHE) of Magnetro-transport phenomena was observed. The saturation magnetizations and anomalous Hall effects increased and decreased, respectively, with increasing deposition temperature, resulting in Co metal segregation in Ti_(0.97)Co_(0.03)O_(2) films at high deposition temperature of 700℃ in 5×10^(-6)torr. The Co segregation in the films increased the saturation magnetizations and decreased anomalous Hall effects. From hall measurement, Ti_(0.97)Co_(0.03)O_(2) films show the n-type behavior by oxygen vacancies, because Co atoms substituting Ti atoms is small. The Ms and Hall resistivity in Ti_(0.97)Co_(0.03)O_(2) films deposited at 500℃ were approximately 8 emu/㎤ and 0.1 μΩ-cm, respectively. This films show the p-type behavior because Co atoms easily substitute Ti atoms at low deposition temperature(=500℃). Hall resistivity was increased with increase of Co content and increase of resistivity with decrease of measure temperature. This increase of Hall resistivity controlled by side-jump scattering in two anomalous hall effect mechanism.
Ti_(0.97)Co_(0.03)O_(2) films at deposition temperature of 500℃ in 4×10^(-4)torr show increase of the size Co nano-cluster. therefore, the films show superparamagnetic properties and n-type behavior. Hall resistivity of Ti_(0.97)Co_(0.03)O_(2) film was decreased, as decrease of resistivity with decrease of measure temperature.
For reduction of Co clustering and increase of Hall resistivity with change of charge carrier, non-magnetic Sb(V) co-doped Ti_(0.97)Co_(0.03)O_(2) films deposited at 500℃. Anomalous Hall effect of Ti_(0.97)Co_(0.03)O_(2) film increased from ρ_(xy)= 0.1 μΩ-cm to ρ_(xy)= 0.14 μΩ-cm by co-doping of Sb. From ZFC-FC measurement, Co cluster size of Ti_(0.97)Co_(0.03)O_(2) films decreased by Sb co-doping. The results of Sb co-doping at Ti_(0.97)Co_(0.03)O_(2) films showed the possibility about increase of Co solubility in p-type Ti_(0.97)Co_(0.03)O_(2) film and reduction of Co cluster size. And, Hall resistivity was increased by reduction Co clusters of Sb co-doping at p-type Ti_(0.97)Co_(0.03)O_(2) film. Hall resistance of n-type Ti_(0.97)Co_(0.03)O_(2) was increased from 0.4 μΩ-cm to 4μΩ-cm, by n-type carrier doping. An anomalous Hall effect controlled by charge carriers in a Co-doped reduced rutile films would be a strong evidence for the intrinsic ferromagnetism at room temperature. p-type Ti_(0.97)Co_(0.03)O_(2) are advantageous for high density device application in ferromagnetic semiconductor fields using junction between n-type ferromagnetic semiconductor.

목차 (Table of Contents)

• 목차 = ⅰ
• Ⅰ. 서론 = 1
• Ⅱ. 이론적 배경 및 문헌조사 = 5
• Ⅱ-1. 자성반도체 (Diluted Magnetic Semiconductor) = 5
• Ⅱ-1-1. Spin injection = 5
• Ⅱ-2. 산화물계 자성반도체 = 7
• Ⅱ-3. TiO_(2)의 특성 = 16
• Ⅱ-3-1. TiO_(2)의 전기적 특성 = 16
• Ⅱ-4. 금속 산화물의 자기적 성질 = 19
• Ⅱ-4-1. Superexchange (SE) 현상 = 19
• Ⅱ-4-2. 이중교환상호작용 (Double Exchange, DE) = 19
• Ⅱ-4-3. RKKY (Ruderman, Kittel, Kasuta, and Yoshida) interaction = 20
• Ⅱ-5. 자성반도체의 전기적 특성 이해 = 25
• Ⅱ-5-1. 비자성체 재료에서 Galvanomagnetic effects = 25
• Ⅱ-5-1-1. 정상 Hall 효과 (Ordinary Hall Effect) = 25
• Ⅱ-5-1-2. 비자성체의 자기 저항 (magnetoresistance) = 26
• Ⅱ-5-2. 강자성 재료에서 Galvanomagnetic effects = 27
• Ⅱ-5-2-1. 비정상 Hall 효과 = 28
• Ⅱ-5-2-2. Planar Hall 효과 = 30
• Ⅲ. Metalorganic chemical vapor deposition 에 의한 Ti_(1-x)Co_(x)O_(2) 박막의 제조 및 특성 = 35
• Ⅲ-1. 실험방법 = 35
• Ⅲ-1-1. 액체운반용 MOCVD장치 = 35
• Ⅲ-1-2. 시약합성, 박막 제조 = 38
• Ⅲ-2. 측정 및 분석방법 = 43
• Ⅲ-2-1. 상분석 및 조성분석 = 43
• Ⅲ-2-2. 전기적 특성분석 = 43
• Ⅲ-2-3. 자기적 특성분석 = 45
• Ⅲ-2-4. XPS(x-ray photoemission spectroscopy) 분석 = 46
• Ⅲ-3. 증착온도와 조성에 따른 박막의 특성 = 47
• Ⅲ-3-1. 상형성 및 미세구조의 변화 = 49
• Ⅲ-3-2. 미세구조 변화에 따른 전기적 특성과 자기적 특성 = 61
• Ⅲ-3-3. Al_(2)O_(3) (1 T 02)위에 성장된 rutile Ti_(0.97)Co_(0.03)O_(2) 박막의 특성 = 68
• Ⅲ-4. 결과 및 요약 = 75
• Ⅳ. Plused laser deposition에 의한 Ti_(1-x)Co_(x)O_(2) 박막의 제조 및 특성 = 77
• Ⅳ-1. Plused laser deposition에 관한 이론적 고찰 = 77
• Ⅳ-1-1. Plused laser deposition의 특징 및 원리 = 77
• Ⅳ-1-2. Plused laser deposition에 의한 박막의 증착 과정 = 78
• Ⅳ-2. 실험방법 (Experimental procedure) = 84
• Ⅳ-2-1. Taget의 제조 = 84
• Ⅳ-2-2. Pulsed laser deposition method = 84
• Ⅳ-3. 증착조건에 따른 Ti_(1-x)Co_(x)O_(2) 박막의 특성 = 88
• Ⅳ-3-1. 증착온도에 따른 박막의 특성 = 88
• Ⅳ-3-2. 증착압력에 따른 박막의 특성 = 107
• Ⅳ-4. Sb가 도핑된 Ti_(1-x)Co_(x)O_(2) 박막의 특성 = 116
• Ⅳ-5 결과 및 요약 = 137
• Ⅴ. 결론 = 139
• 참고문헌 = 142
• ABSTRACT = 147