PLD법으로 제조한 TiO₂계 자성반도체 박막의 성장과 물성 = Preparation and properties of TiO₂-based diluted magnetic semiconductor thin films deposited by PLD method

다국어 초록 (Multilingual Abstract)

Because of recent developments in the physics of spin-dependent phenomena, spintronics has become a subject of considerable interest. For use in spintronic materials, dilute magnetic semiconductors (DMS) are under consideration 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. It was given out that a TiO_(2) anatase doped with Co, that was grown by pulsed laser deposition(PLD) method, is ferromagnetic and semiconductive for doping levels up to around 8 at.%, and temperatures of up to 400 K. The PLD method is applicable for low oxygen pressure(100mtorr~10^(-6)torr) atmosphere.
The thickness of Rutile Ti_(1-x)Co_(x)O_(2) (x=0~0.12) films and rutile Ti_(1-x)Co_(x)O_(2) films doped with Sb is 200nm. They are grown on R-Al_(2)O_(3) (1□02) substrates at various deposition temperatures by pulsed laser deposition. The Ti_(1-x)Co_(x)O_(2) films doped with Sb are compared with Ti_(1-x)Co_(x)O_(2) films nondoped.
Rutile Ti_(1-x)Co_(x)O_(2) (x=0~0.15) thin films were grown on R-Al_(2)O_(3)(1□02) substrates and investigated for the magnetic and electrical properties. The PLD method uses 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 of deposition process was 5× 10^(-6) Torr. The deposition rate was 0.2/shot, and the film thickness was approximately 200 nm.
Magneli phase Ti_(1-x)Co_(x)O_(2) (x=0~0.15) films exhibit ferromagnetic property at room temperature. These films have Co cluster. The saturated magnetization increased with rise of 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. However, a 'Magneto-transport phenomena' anomalous Hall effect was observed. The Co segregations in the films make increase the saturated magnetization. From hall measurement, Ti_(0.97)Co_(0.03)O_(2) films show the n-type behavior by oxygen vacancies, because the number of Co atoms substituting Ti atoms is few. The M_(s) 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.
Non-magnetic Sb(V) doped with Ti_(0.97)Co_(0.03)O_(2) film was deposited at 500℃ to enhance the solubility of Co impurity into TiO_(2) and increase of Hall resistivity with change of charge carrier. So, We reduced the size of Co metal clusters and increased the Anomalous Hall effect of Ti_(0.97)Co_(0.03)O_(2) thin film from p_(xy)= 0.1 μΩ-cm to p_(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 of increasing 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. 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 advantage of 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 = 4
• Ⅱ-1. 자성반도체 (Diluted Magnetic Semiconductor) = 4
• Ⅱ-1-1. Spintronics의 개념 = 4
• Ⅱ-1-2. Spintronics를 이용한 자성반도체(DMS) = 8
• Ⅱ-1-3. Spin injection = 10
• Ⅱ-2. 산화물계 자성반도체 = 14
• Ⅱ-3. 금속 산화물의 자기적 성질 = 17
• Ⅱ-3-1. Superexchange (SE) 현상 = 17
• Ⅱ-3-2. 미세입자에서의 초상자성(Superparamagnetism) = 19
• Ⅱ-3-3. RKKY (Ruderman, Kittel, Kasuya, and Yoshida) interaction = 22
• Ⅱ-4. TiO_(2)의 특성 = 24
• Ⅱ-5. Galvanomagnetic effects = 27
• Ⅱ-5-1. 정상 흘효과(Ordinary Hall Effect) = 27
• Ⅱ-5-1-1. 비자성체의 자기 저항 (Magnetoresistance) = 28
• Ⅱ-5-2. 비정상 흘효과(Anomalous Hall Effect) = 29
• Ⅲ. 실험방법 = 35
• Ⅲ-1. Pulsed Laser deposition의 원리 및 특징 = 35
• Ⅲ-2. 실험방법 (Experimental procedure) = 40
• Ⅲ-2-1. Taget의 제조 = 40
• Ⅲ-2-2. Pulsed laser deposition method = 40
• Ⅲ-3. 분석 및 측정 = 44
• Ⅲ-3-1. 상분석 및 조성분석 = 44
• Ⅲ-3-2. 전기적 특성분석 = 44
• Ⅲ-3-3. 자기적 특성분석 = 45
• Ⅳ. 결과 및 고찰 = 46
• Ⅳ-1. 증착조건에 따른 Ti_(1-x)Co_(x)O_(2) 박막의 특성 = 46
• Ⅳ-1-1. 증착온도와 조성에 따른 박막의 상형성 및 미세구조 = 46
• Ⅳ-1-2. 증착온도 및 조성에 따른 박막의 자기적 특성과 전기적 특성 = 53
• Ⅳ-2. Sb가 도핑된 Ti_(1-x)Co_(x)O_(2) 박막의 특성 = 61
• Ⅳ-2-1. Sb가 도핑된 Ti_(1-x)Co_(x)O_(2) 박막의 미세구조 = 61
• Ⅳ-2-2. Sb가 도핑된 Ti_(1-x)Co_(x)O_(2) 박막의 자기적 특성 및 전기적 특성 = 67
• Ⅴ. 결론 = 79
• 참고문헌 = 81
• ABSTRACT = 85