Integration of high k oxides with wide band gap semiconductors

INTEGRATION OF HIGH-K OXIDES WITH WIDE BAND-GAP SEMICONDUCTORS CHEN QIAN (B. Sc., Chong Qing Univ.) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2011 Acknowledgement Acknowledgement I would like to express sincere appreciation to my advisor, Prof. Feng Yuanping from National University of Singapore (NUS) for his strong support and excellent supervision. He not only taught me many about the research but also shared me his wisdom, insight, and humor in the last few years. It is really an honor for me to get the guidance from him. I would also like to show my sincere appreciation to my advisor, Dr. Wang Shijie from Institute of Materials Research & Engineering (IMRE) for his guidance, unwavering support, and encouragement throughout my study. He has constantly provided me with assistance and valuable advice to improve my research work and has always been supportive of my research endeavors. I am truly grateful for all the help and encouragement he has given me during the past four years. Special thanks to Dr. Chai Jianwei and Wong Ten It for their help in experiments throughout the years and I have enjoyed all the helpful discussions with them. They make my Ph.D career a happy memory. Also thanks to other staffs in IMRE Dr. Pan Jisheng, and Dr. Zhang Zheng for their warm help on my research work. Thanks to the student at NUS Dr. Yang Ming for his support on my research work. I acknowledge National University of Singapore for the research scholarship, which enables me to conduct my research project and finish this thesis. Last but not least, I would like to express my deep appreciation to my parents for their unselfish love and constant support throughout my life. Thanks for their love, encouragement, and many sacrifices throughout my life, which have made my education possible. i Table of Context Table of Contents Acknowledgement i Table of Contents ii Abstract vi List of Tables ix List of Figures x Abbreviation xv Publications xvii Chapter Introduction 1.1 Wide band-gap semiconductors . 1.2 High-k dielectrics . 1.3 The integration of high-k dielectric with wide band-gap semiconductors . 11 1.4 Research approaches 14 1.4.1 Nitridation treatment 14 1.4.2 Band offsets at high-k/wide band-gap semiconductor interfaces . 15 1.4.3 Electrical properties of HfO2 gate dielectrics 20 1.5 Objective and significance of the study . 24 Chapter Experimental and Computational Methods 27 2.1 Growth techniques 27 2.1.1 High-k dielectrics deposition techniques . 27 2.1.2 Metal gates deposition techniques . 30 2.1.3 Rapid thermal annealing 31 2.1.4 Nitridation treatment 33 ii Table of Context 2.2 Characterization techniques . 33 2.2.1 X-ray Photoelectron Spectroscopy (XPS) 33 2.2.1.1 Basic principles of XPS . 34 2.2.1.2 Important parameters of XPS 35 2.2.1.3 Data interpretation . 37 2.2.1.4 Instrument and application of XPS . 41 2.2.2 Other characterization techniques 43 2.3 Computational method (First-principles calculations) . 46 Chapter High-k dielectrics/SiC interfaces 52 3.1 Introduction 52 3.2 Surface treatment of SiC substrates 53 3.3 Interfacial characterization of HfO gate dielectric on SiC 58 3.3.1 Band alignment at HfO 2/4H-SiC interfaces . 59 3.3.2 Nitridation of HfO film and its thermal stability 60 3.3.3 Interface properties of HfO 2/6H-SiC stacks and its thermal stability 64 3.3.4 First-principle calculation of electronic structure of nitrogen-doped HfO2 films 66 3.4 Electrical characterization of Ni/HfO 2/SiC MOS capacitors . 69 3.4.1 MOS fabrication process and measurement setup . 69 3.4.2 Frequency dependence of electrical properties 70 3.4.3 Rapid thermal annealing effect on the electric properties 73 3.4.4 Conduction mechanisms in high-k gate dielectrics 74 3.5 Interface characterization of HfO2 dielectric on graphene formed on SiC 79 3.5.1 Growth and electronic properties of graphene on SiC . 80 3.5.2 Growth and band alignment of HfO dielectric on graphene and its thermal stability . 82 iii Table of Context 3.6 Summary 87 Chapter High-k dielectrics/GaN interfaces .89 4.1 Introduction 89 4.2 Surface passivation of GaN 90 4.3 Interface characterization of HfO2/GaN stacks 93 4.3.1 Band alignment at HfO 2/GaN interfaces 93 4.3.2 Post-thermal annealing . 99 4.4 Electrical characterization of Ni/HfO 2/GaN MOS capacitors 100 4.4.1 MOS fabrication process and measurement setup . 101 4.4.2 Frequency dependence of electrical properties 102 4.4.3 Rapid thermal annealing effect on the electric properties 106 4.4.4 Current conduction mechanism . 108 4.5 Summary 109 Chapter High-k dielectrics/ZnO interfaces 110 5.1 Introduction 110 5.2 Interface characterization of HfO2/ZnO stacks 111 5.2.1 Band alignment at HfO 2/ZnO interfaces 112 5.2.2 First-principle calculation on the structure and properties of HfO 2/ZnO interfaces 115 5.3 Interfaces characterization of ZrO 2/ZnO stacks . 119 5.3.1 Epitaxial relationship of ZrO2/ZnO heterostructure . 120 5.3.2 Band alignment at ZrO 2/ZnO interfaces 122 5.3.3 First-principle calculation of electronic structure of ZrO 2/ZnO interfaces . 124 5.4 Thermal stability and band alignment of N-doped ZnO 126 iv Table of Context 5.4.1 Nitridation treatment of ZnO and its thermal stability . 126 5.4.2 First-principle calculation of electronic structure of N-doped ZnO 130 5.5 Summary 133 Chapter Conclusion and future works 135 6.1 Conclusion 135 6.2 Future works . 139 References………………………………………………………………… .…………….141 v Abstract Abstract Silicon-based Metal-oxide-semiconductor field effect transistors (MOSFET) devices have been thrust of research over the years and are the most developed. However, recent development has allowed silicon system technology to approach the theoretical limits, such as higher blocking voltage, switching frequency and large gate leakage current. To overcome these limitations, during the past several years, a new group of materials has emerged as candidates to replace silicon in the near future, which has enabled applications from optoelectronics devices to high-power, hightemperature, and high-frequency microelectronic devices. This group is known as the wide band-gap semiconductors (WBGs), and is led by SiC, ZnO and GaN. These large band-gap materials allow commercialization in power MOSFET devices, where Si cannot be used. Besides the recent progress of wide band-gap semiconductor to replace silicon in MOSFET devices, high-k oxides have been proposed as alternatives to replace conventional silicon dioxide in MOS devices. It is clear that the dielectric layer downscaling in MOSFET device is limited by the leakage current problem and it is an urgent task to introduce new dielectric material with higher dielectric constant (high-k) to replace silicon dioxide as the gate dielectrics. Therefore, in this thesis, integration of high-k dielectric materials with wide band-gap semiconductors was studied by using both experimental and theoretical methods. The growth and characterization (e.g. electronic structure, thermal stability) of HfO2 films on various wide band-gap semiconductor substrates (SiC, GaN and ZnO) were studied by in situ x-ray photoelectron spectroscopy (XPS). The band alignment at the HfO 2/WBGs interface was accurately measured by XPS using a core-level based method. The sufficiently large band offsets between HfO films and various vi Abstract wide band-gap semiconductor substrates (SiC, GaN and ZnO) indicate that HfO dielectric is a promising candidate to be integrated with various wide band-gap semiconductors in the downscaling of MOSFET devices. The effects of interfacial structure on the band alignments and thermal stabilities of HfO films on SiC, GaN and ZnO substrates were also studied. It was found that the interfacial layer changed the band alignments by modifying the interfacial dipoles, which indicates that it is essential to understand and control the interfacial structure to improve the device performance. Ni was chosen as a prototype of metal gates to be grown on HfO to fabricate the Ni/HfO 2/WBGs MOS capacitors. The capacitance and current properties responding to the variation of bias voltage of Ni/HfO2/WBGs MOS gate stacks in comparison with these of gate stacks after rapid thermal process (RTP) in nitrogen and oxygen ambient was investigated. It can be seen that the RTP can effectively reduce oxygen vacancies in the as-deposited HfO2 films. As a result, interface quality of HfO2/WBGs after RTP was improved. The effect of nitridation on the electronic structures and thermal stabilities of high-k dielectrics films (HfO 2) was studied by using in situ x-ray photoelectron spectroscopy (XPS) and First-principles calculations. It was found that nitrogen doping not only can passivate the oxygen vacancies in high-k dielectrics films, but also can change the electronic structure of high-k dielectric films. This work suggests that the nitridation process should be well-controlled to optimize the performance of high-k dielectric films. 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Schematic of band offsets determining carrier injection in oxide band states VBO > 1V WBGs VB Oxide The high- k dielectrics that have VBO or CBO with WBGs smaller than 1 eV will not be considered for further applications because of the large tunneling current However, the investigation on the integration of high- k oxides with wide band- gap 13 Chapter 1 Introduction semiconductors is still lacking Thus,... growth of good quality of HfO2 is still challenging, which is of importance for application since the electronic properties of HfO x is dependent on its oxidation states 1.3 The integration of high- k dielectric with wide band- gap semiconductors Scaling technology plays important roles for further improving the performance and reducing costs of MOS devices, together with the replacement of Si with wide band- gap. .. accurately determine the band offsets at the interfaces of high- k oxides and wide band- gap semiconductors is another important issue which should be addressed 1.4 Research approaches 1.4.1 Nitridation treatment As we have mentioned above, the surface states at the surface of semiconductors and the intrinsic defects in the high- k oxides will affect the interface quality of highk/WBGs Nitridation treatment... constant of SiO 2 (k= 3.9) If we take one of the wide band- gap semiconductors SiC as an example, the low dielectric constant of SiO 2 (k= 3.9) relative to that of SiC (k= 10) results in an electric field in SiO2 is 2.5 times higher than that in SiC This inequity requires device operation at a substrate electric field far below the wide band- gap semiconductor breakdown field to avoid premature SiO2 breakdown... treatment can not only passivate the surface of wide band- gap semiconductors to form a stable nitride layer but also passivate the oxygen defects in high- k dielectrics films Compared with SiO2, most high- k dielectrics suffers from intrinsic issue of high oxygen vacancy density, which will greatly degrade the performance of high- k materials (such as high leakage current, low carrier mobility, threshold... Figure 1.4 Energy band diagram for illustrating the core-level based XPS method to investigate the band offsets at high- k dielectrics/WBGs interfaces Generally, the method of Kraut is based on the assumption that the energy difference between the core-level positions and valence -band maximum (VBM) are fixed both in the bulk If we take the core levels of high- k dielectrics and wide bandgap semiconductors. .. Furthermore, the much wider band- gap of WBGs compared with that of Si indicates that the probability of thermal excitation of carriers from the valence band to conduction is reduced significantly by the large band- gap, and it is beneficial for high temperature application Besides, high- purity SiC material has the highest reported thermal conductivity which is more than three times higher than that of Si (1.5... Thus the high breakdown field of wide band- gap semiconductor is severely underutilized, minimizing one of the material’s major advantages for high- power applications Since the blocking voltage of the power MOS devices scales with the square of the electric field, the device’s blocking voltage capability is dramatically reduced for a given on-resistance Therefore, the relatively thick high- k gate dielectrics... terms of controlling interface defects is an important issue for the integration of high- k oxides on wide band- gap semiconductors Band alignment Band alignment at oxide-semiconductor interface is one of the most interesting and important aspects in the growth and characterization of electronic devices because the transport properties at the hetero-junction interface are determined by the electronic band . INTEGRATION OF HIGH- K OXIDES WITH WIDE BAND- GAP SEMICONDUCTORS CHEN QIAN (B. Sc., Chong Qing Univ.) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF. List of Tables ix List of Figures x Abbreviation xv Publications xvii Chapter 1 Introduction 1 1.1 Wide band- gap semiconductors 1 1.2 High- k dielectrics 5 1.3 The integration of high- k. integration of high- k dielectric with wide band- gap semiconductors 11 1.4 Research approaches 14 1.4.1 Nitridation treatment 14 1.4.2 Band offsets at high- k/ wide band- gap semiconductor interfaces