由於三五族半導體(砷化鎵(GaAs)，磷化銦(InP))具有高的電子遷移率，所以被廣泛應用在高速元件上。另，因為TiO2與GaAs和InP擁有良好的晶格匹配特性以及高的介電常數(k = 35-100)，所以我們選擇TiO2作為閘極氧化層薄膜。
GaAs與InP因其不穩定的原生氧化層(native oxide)使其擁有高的界面能態密度(interface state density, Dit)而影響界面品質，造成C-V曲線有拉伸(stretch-out)的現象以及高的漏電流。使用有機金屬化學氣相沈積(MOCVD)在GaAs基板上生長之二氧化鈦(TiO2)薄膜雖具有較高的介電常數，但由於TiO2薄膜是多晶結構，其晶界具有很多缺陷及懸鍵，故其漏電流非常大。
利用硫化銨((NH4)2Sx)水溶液對GaAs進行表面硫化處理(S-GaAs)可以有效去除原生氧化層以及填補GaAs表面懸鍵，使其界面品質大幅改善。另，利用來自液相沈積法(Liquid Phase Deposition)生長SiO2 (LPD-SiO2)的溶液的氟離子可鈍化MOCVD-TiO2薄膜晶界上之懸鍵及半導體表面，故在漏電流及Dit方面可大為改善。其漏電流在電場±1.5 MV/cm分別為3.41 x 10-7和1.13 x 10-6A/cm2，Dit可達到4.6 x 1011 cm-2eV-1，介電常數可達到71。因此，氟化的MOCVD-TiO2是一種具有高介電常數和低漏電流之薄膜結構。
另一種鈍化MOCVD-TiO2薄膜的方法為金屬熱處理法(post-metallization annealing (PMA))。利用鋁和存在於MOCVD-TiO2的氫氧根(hydroxyl)作用而產生的活性氫離子(active hydrogen)進入TiO2薄膜以及GaAs表面進行鈍化作用，有效改進漏電流以及界面品質。其漏電流在電場±1.5 MV/cm分別為2.5 x 10-7 和 5 x 10-7 A/cm2，其Dit可達到5.96 x 1011 cm-2eV-1，介電常數可達到66。
最後，為了避免MOCVD-TiO2產生的晶界漏電流問題，以及低溫薄膜沉積法可以保持硫化效果避免硫化失效。我們利用液相沉積法成長非晶結構TiO2在S-GaAs上，其漏電流在電場±0.5 MV/cm分別為1.04 x 10-7和1.91 x 10-7 A/cm2，其Dit可達到3.2 x 1011 cm-2eV-1，介電常數可達到48。而將LPD-TiO2沉積在硫化銨處理過後的磷化銦(S-InP)上，我們製作出4 x 100 μm2 的增強型N通道LPD-TiO2/S-InP MOSFET，其具有不錯的特性表現，其最大轉導值gm為43 mS/mm，電子遷移率μFE 為348 cm2/V•s。

Due to the high electron mobility compared with Si, much attention has been focused on III-V compound semiconductors (gallium arsenide (GaAs) and indium phosphide (InP)) high-speed devices. The high-k material TiO2 not only has high dielectric constant (k = 35-100) but has well lattice match with GaAs and InP substrate. Therefore, titanium oxide (TiO2) was chosen to be the gate oxide in this study.
The major problem of III-V compound semiconductors is known to have poor native oxide on it and leading to the Fermi level pinning at the interface of oxide and semiconductor. The C-V stretch-out phenomenon can be observed and the leakage current is high. The higher dielectric constant of poly-crystalline TiO2 film grown on GaAs can be obtained by metal organic chemical vapor deposition (MOCVD). But the high leakage current also occurred due to the grain boundary and defects in the poly-crystalline TiO2 film.
The surface passivation of GaAs with (NH4)2Sx treatment (S-GaAs) could prevent it from oxidizing after cleaning and improve the interface properties of MOSFET. The fluorine from liquid phase deposited SiO2 solution can passivate the grain boundary of poly-crystalline MOCVD-TiO2 film and interface state. The high dielectric constant and low leakage current of fluorine passivated MOCVD-TiO2/S-GaAs can be obtained. The leakage current densities are 3.41 x 10-7 A/cm2 and 1.13 x 10-6A/cm2 at ±1.5 MV/cm, respectively. The Dit is 4.6 x 1011 cm-2eV-1 at the midgap. The dielectric constant can reach 71.
In addition, the post-metallization annealing (PMA) is another efficiency way to improve the MOCVD-TiO2 quality. The mechanism of PMA process is from the reaction between the aluminum contact and hydroxyl groups existed on TiO2 film surface. Then the active hydrogen is produced to diffuse through the oxide and passivate the oxide traps. For PMA (350oC)-MOCVD-TiO2 on S-GaAs MOS structure, the leakage current densities can reach 2.5 x 10-7 and 5 x 10-7 A/cm2 at ±1.5 MV/cm, respectively. The dielectric constant and the Dit are 66 and 5.96 x 1011 cm-2eV-1, respectively.
In order to avoid the leakage current from grain boundary of poly-crystalline TiO2, and liquid phase deposited TiO2 (LPD-TiO2) at low temperature can preserve the function of sulfur passivation. Therefore, the amorphous LPD-TiO2 was deposited on S-GaAs. The leakage current densities are 1.04 x 10-7 and 1.91 x 10-7 A/cm2 at ±0.5 MV/cm, respectively. The Dit is 3.2 x 1011 cm-2eV-1 and the dielectric constant is 48. The LPD-TiO2 film was deposited on (NH4)2Sx treated InP (S-InP), and the 4 x 100 μm2 enhancement mode N channel InP MOSFET with LPD-TiO2 as gate oxide was fabricated, which showed the good characteristic. The normalized maximum gm is 43 mS/mm at VG = 1.3 V for VDS fixed at 1 V. The maximum calculated μFE of 348 cm2/V•s at VDS = 1 V is obtained.

ACKNOWLEDGMENT I
LIST OF FIGURES X
LIST OF TABLES XIV
ABSTRACT XV

1. Introduction 1
1-1 Properties of TiO2 1
1-2 Comparison of deposition methods of TiO2 2
1-3 Advantages of MOCVD 2
1-4 Drawback of TiO2 for electric applications 3
1-5 Motivation of fluorinated MOCVD-TiO2 on (NH4)2Sx treated GaAs structure 4
1-6 Motivation of PMA-MOCVD-TiO2 on (NH4)2Sx treated GaAs structure 6
1-7 Mechanism and the structure model of InP and GaAs with sulfur treatment 6

2. Experiments 16
2-1 Titanium oxide prepared by MOCVD 16
2-1-1 CVD theorem 16
2-1-2 Deposition system of MOCVD 17
2-1-3 Properties of source materials 18
2-1-3.1 Ti metalorganic precursor 18
2-1-3.2 N2O decomposed 18
2-2 Silicon oxide prepared by LPD 19
2-2-1 Deposition system 19
2-2-2 Mechanisms of LPD-SiO2 19
2-2-3 Preparations of deposition solutions 20
2-2-3.1 SiO2 saturated H2SiF6 solution 20
2-2-3.2 Boric acid solution 21
2-3 Deposition procedures 21
2-3-1 GaAs wafer cleaning and sulfidation procedures 21
2-3-2 Aluminum metal and In-Zn alloy cleaning processes 22
2-3-3 Preparations of LPD-SiO2/MOCVD-TiO2 thin film 22
2-3-4 Preparations of PMA-MOCVD-TiO2 thin film 23
2-3-5 Electrodes fabrication 24
2-4 Characterization 24
2-4-1 Physical properties 24
2-4-2 Chemical properties 25
2-4-3 Electrical properties 25

3. Characterization of TiO2 prepared by MOCVD on GaAs MOS structure 32
3-1 Characteristics of MOCVD-TiO2 film on GaAs substrate 32
3-1-1 Thickness of TiO2 film as a function of growth temperature 32
3-1-2 XRD spectra of TiO2 film as a function of growth temperature 33
3-1-3 SEM morphologies of TiO2 film as a function of growth temperature 34
3-1-4 Stoichiometry of TiO2 film as a function of
growth temperature 34
3-1-5 Reflective index of TiO2 film as a function of growth
temperature 35
3-1-6 C-V characteristic at 400oC, the leakage current density, and dielectric constant of TiO2 film as a function of growth
temperature 35
3-1-7 Conclusions 37
3-2 Characteristics of fluorine passivated MOCVD-TiO2 film on (NH4)2Sx treated GaAs 37
3-2-1 Properties of LPD-SiO2/MOCVD-TiO2 film on S-GaAs 37
3-2-2 Electrical characteristics of MOCVD-TiO2 film on GaAs with and without (NH4)2Sx treatment 39
3-2-3 PL spectra of GaAs with and without (NH4)2Sx treatment 40
3-2-4 Electrical characteristics of LPD-SiO2/MOCVD-TiO2 film on S-GaAs 41
3-2-5 SIMS and ESCA depth profiles of LPD-SiO2/MOCVD-TiO2/S-GaAs structure 41
3-2-6 Fluorinated MOCVD-TiO2 film on S-GaAs after LPD-SiO2 removal 42
3-2-7 Flatband voltage and effective oxide charges of different MOS structures 43
3-2-8 Conclusions 43
3-3 Characteristics of post-metallization annealed MOCVD-TiO2 film on S-GaAs 44
3-3-1 Mechanism of PMA-MOCVD-TiO2 film on S-GaAs substrate 44
3-3-2 SEM cross section of MOCVD-TiO2/S-GaAs 45
3-3-3 I-V characteristics of MOCVD-TiO2 on GaAs without and with (NH4)2Sx treatment and PMA-MOCVD-TiO2 on S-GaAs at different PMA temperatures 45
3-3-4 C-V characteristics of MOCVD-TiO2 on GaAs without and with sulfur treatment and PMA-MOCVD-TiO2 on S-GaAs at different PMA temperatures 47
3-3-5 Dielectric constant and effective oxide charges of MOCVD-TiO2/S-GaAs and PMA-MOCVD-TiO2 on S-GaAs at different PMA temperatures 48
3-3-6 C-V hysteresis loops as a function of PMA temperature 49
3-3-7 Oxide trapped density and mobile ion density as a function of PMA temperature 49
3-3-8 Interface state densities of MOCVD-TiO2/S-GaAs and PMA-MOCVD-TiO2/S-GaAs at different PMA temperatures 50
3-3-9 Conclusions 50

4. Characteristics of TiO2 prepared by LPD on S-GaAs MOS Structures 75
4-1 Motivation of LPD-TiO2/S-GaAs structure process 75
4-2 Preparation of LPD-TiO2 film on S-GaAs 76
4-3 Growth rate of LPD-TiO2 film on S-GaAs as a function of H3BO3 volume 77
4-4 Leakage current densities of LPD-TiO2 film on GaAs as a function of H3BO3 volume 77
4-5 Leakage current densities of LPD-TiO2 film on S-GaAs as a function of H3BO3 volume 78
4-6 C-V characteristics of LPD-TiO2 film on GaAs as a function of H3BO3 volume 78
4-7 C-V characteristics of LPD-TiO2 film on S-GaAs as a function of H3BO3 volume 79
4-8 Effective oxide charges as a function of H3BO3 volume 79
4-9 Interface state densities of LPD-TiO2 film on S-GaAs as a function of the H3BO3 volume 80
4-10 Conclusions 80

5. Enhancement-mode N-channel MOSFET with LPD-TiO2 as gate oxide on S-InP 91
5-1 Electric characteristics of LPD-TiO2 on S-InP MOS structure 91
5-2 Fabrication process of enhancement-mode n-channel MOSFET with LPD-TiO2 as gate oxide on S-InP 92
5-3 Electrical characteristics of enhancement-mode n-channel MOSFET with LPD-TiO2 as gate oxide on S-InP 93

6. Conclusions 109

REFERENCES 111

Vita 124

Publication List 125











LIST OF FIGURES

Figure 1-1 Crystal structures of TiO2 (a) Rutile, (b) Anatase, and (c) Brookite. 8
Figure 1-2 Calculated band offsets of oxides on Si by J. Robertson 9
Figure 1-3 Schematic views of the proposed structure models for the InP(001):S surface: a)~d) sulfur-rich structure and e), f) sulfur- poor structures. 10
Figure 1-4 (1) Srivastava structure, (2) Inverse Srivastava structure, (3) Pashley structure and (4) Inverse Pashley structure. 12
Figure 1-5 Three-dimensional atomic structures of GaAs surface before and after (NH4)2S treatment... 13
Figure 2-1 Steps involved in a CVD process 27
Figure 2-2 MOCVD system 28
Figure 2-3 Liquid phase deposition (LPD) system 29
Figure 2-4 Flowchart of LPD-SiO2/MOCVD-TiO2 film 30
Figure 3-1 Thickness of TiO2 films as a function of growth temperature. 52
Figure 3-2 XRD spectra of TiO2 films as a function of growth temperature: (a) 280oC, (b) 300oC, (c) 350oC, (d) 400oC, (e) 450oC, (f) 500oC, and (g) 550oC. R: rutile, A: anatase. 53
Figure 3-3 SEM cross section of TiO2 films as a function of growth temperature: (a) 280oC, (b) 300oC, (c) 350oC, (d) 400oC, (e) 450oC, (f) 500oC, and (g) 550oC. 54
Figure 3-4 SEM surface morphologies of TiO2 films as a function of growth temperature: (a) 280oC, (b) 300oC, (c) 350oC, (d) 400oC, (e) 450oC, (f) 500oC, and (g) 550oC. 55
Figure 3-5 Stoichiometry of TiO2 film as a function of growth temperature. 56
Figure 3-6 Refractive index of TiO2 film as a function of growth temperature. 57
Figure 3-7 C-V characteristics of TiO2 films prepared at the growth temperature of 400oC. 58
Figure 3-8 Dielectric constant of TiO2 film as a function of growth temperature. 59
Figure 3-9 Leakage current densities of TiO2 films as a function of growth temperature 60
Figure 3-10 Flowchart of LPD-SiO2/MOCVD-TiO2 film on S-GaAs structure. 61
Figure 3-11 Leakage current densities of different MOS structures 62
Figure 3-12 Capacitance-voltage characteristics of different MOS structures 63
Figure 3-13 PL spectra of GaAs with and without (NH4)2Sx treatment. 64
Figure 3-14 Interface state densities of (a) MOCVD-TiO2/S-GaAs, (b) LPD-SiO2/MOCVD-TiO2/S-GaAs, and (c) MOCVD-TiO2/S-GaAs after LPD-SiO2 removal 65
Figure 3-15 SIMS and XPS depth profiles of LPD-SiO2/MOCVD-TiO2 /S-GaAs 66
Figure 3-16 SEM cross section of MOCVD-TiO2/S-GaAs. 67
Figure 3-17 SIMS depth profiles for (a) MOCVD-TiO2/S-GaAs, (b) PMA (350oC)-MOCVD-TiO2/S-GaAs. 68
Figure 3-18 Leakage current densities of MOCVD-TiO2/GaAs with and without (NH4)2Sx treatments and PMA-MOCVD-TiO2/S-GaAs at different PMA temperatures. 69
Figure 3-19 C-V characteristics of MOCVD-TiO2/GaAs with and without (NH4)2Sx treatments and PMA-MOCVD-TiO2/S-GaAs at different PMA temperatures. 70
Figure 3-20 (a) Dielectric constant and (b) effective oxide charges of MOCVD-TiO2/S-GaAs and PMA-MOCVD-TiO2/S-GaAs at different PMA temperatures 71
Figure 3-21 C-V hysteresis loops of (a) MOCVD-TiO2/S-GaAs, (b) PMA at 300oC, (c) PMA at 350oC and (d) PMA at 400oC. 72
Figure 3-22 Oxide trap density and mobile ion density as a function of PMA temperature. 73
Figure 3-23 Interface state density of MOCVD-TiO2/S-GaAs and PMA-MOCVD-TiO2/S-GaAs at different PMA temperatures. 74
Figure 4-1 Flowchart of LPD-TiO2 on S-GaAs 81
Figure 4-2 SEM (a) surface morphology and (b) cross section of LPD-TiO2 film on GaAs substrate with (NH4)2Sx treatment at the H3BO3 volume of 4 ml. 82
Figure 4-3 Growth rate of LPD-TiO2 film on GaAs substrate with (NH4)2Sx treatment as a function of H3BO3 volume. 83
Figure 4-4 Leakage current densities of LPD-TiO2 films on GaAs substrate without (NH4)2Sx treatment as a function of H3BO3 volume. 84
Figure 4-5 Leakage current densities of LPD-TiO2 films on GaAs substrate with (NH4)2Sx treatment as a function of H3BO3 volume. 85
Figure 4-6 Capacitance-voltage characteristics of LPD-TiO2 films on GaAs substrate without (NH4)2Sx treatment as a function of H3BO3 volume. 86
Figure 4-7 Capacitance-voltage characteristics of LPD-TiO2 films on GaAs substrate with (NH4)2Sx treatment as a function of H3BO3 volume. 87
Figure 4-8 Effective oxide charge as a function of H3BO3 volume 88
Figure 4-9 Interface state densities of LPD-TiO2 films on GaAs substrate with (NH4)2Sx treatment for the H3BO3 volume of 2, 4 and 8 ml. 89
Figure 5-1 AFM of the surface of 50 nm thick LPD-TiO2 film on S-InP 95
Figure 5-2 Leakage current densities of (a) LPD-TiO2/InP and (b) LPD-TiO2/S-InP. 96
Figure 5-3 C-V characteristics of (a) LPD-TiO2/InP and (b) LPD-TiO2/S-InP. 97
Figure 5-4 Interface state density of LPD-TiO2/S-InP. 98
Figure 5-5 Steps of MOSFET fabrication process 99
Figure 5-6 ID-VD of e-mode S-InP NMOSFET with LPD-TiO2 as gate oxide 106
Figure 5-7 ID-VG as function of VG of e-mode S-InP NMOSFET with LPD-TiO2 as gate oxide 107
Figure 5-8 Gm as function of VG of e-mode S-InP NMOSFET with LPD-TiO2 as gate oxide 108

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