Acknowledgements ACKNOWLEDGEMENTS Firstly, I would like to express my deepest appreciation to my supervisors, Professor Ong Chong Kim (Department of Physics, National University of Singapore) and Professor Alfred, Huan Cheng Hon (Division of Physics & Applied Physics, Nanyang Technology University and Institute of Materials Research & Engineering), for their excellent guidance, constructive comments, and valuable support during my PhD candidacy Their profound knowledge and rigorous scientific approach have greatly inspired me in the research Secondly, I would like to express my sincere gratitude to my co-supervisor, Dr Wang Shijie, research scientist from Institute of Materials Research & Engineering He has constantly provided me with excellent guidance and support for both of my professional and personal developments His kindness and integrity have always been a good example for me even in my future career Thanks for teaching me many experimental and analytical skills used in this research work I am truly grateful for all his help and encouragement during the course of this research My thankfulness also extends to two great persons in our research group, Dr Dong Yufeng and Dr Mi Yanyu, for their self-giving help and kindly encouragement in the last few years Thanks for the happy time with them during my PhD study i Acknowledgements This research would not have been possible without much assistance from scientists and researchers at NUS and IMRE as well as excellent research equipments provided by NUS and IMRE I would like to specially thank a few more persons here: Prof Feng Yuanping, Dr Pan Jisheng, Dr Chai Jianwei, Mr Lim Poh Chong, Ms Chow Shue Yin, Mr Wang Weide, Dr Li Zhengwen, Dr Kong Lingbin, Dr Yan Lei, Dr Tan Chin Yaw, and Mr Liu Huajun Many thanks also go to Dr Ng Tsu Hau, and his supervisor, Associate Prof Chim Wai Kin, from Department of Electrical and Computer Engineering, NUS, for their technical supports to this work Finally, I would like to dedicate this thesis to my parents, for their consistent encouragement, support and understanding during my study in Singapore Lastly but not least, I wish to express sincere gratefulness and indebtedness to my husband, Mr Guan Zhiyong, for his endless love and strong support If I forgot anybody in this list, it was done by mistake rather than intention ii Table of Contents TABLE OF CONTENTS ACKNOWLEDGEMENTS i TABLE OF CONTENTS .iii SUMMARY v LIST OF FIGURES vii LIST OF TABLES .xi LIST OF PUBLICATIONS xii Chapter Introduction 1.1 Introduction .1 1.2 An overview of device scaling 1.3 Limitations for gate oxide (SiO2) scaling 1.4 Alternative high-k gate dielectrics candidates and materials properties considerations 1.5 Metal gate candidates and materials properties considerations 17 1.6 Band alignments at metal/high-k/semiconductor interfaces 19 1.7 Motivations and scope for present work 23 Reference 26 Chapter Film Deposition, Characterization Techniques and Modeling Methods 33 2.1 Introduction 33 2.2 Film deposition techniques 34 2.3 Characterization techniques 39 2.4 First-principles calculation 46 Reference 49 Chapter Thermodynamics and Thermal Stability Study of HfO2 films in Contact with Si 51 3.1 Introduction 51 3.2 Thermodynamic study of HfO2 films on SiO2-covered silicon 53 3.3 Thermal stability of UHV sputtering HfO2 films by plasma oxidation and low temperature iii Table of Contents annealing 60 3.4 Conclusion 70 Reference 72 Chapter Highly Thermal Stable (HfO2)1-x(Al2O3)x Films 75 4.1 Introduction 75 4.2 (HfO2)1-x(Al2O3)x films fabricated by dual-beam pulse laser deposition 76 4.3 Thermal stability of (HfO2)1-x(Al2O3)x/Si interface 77 4.4 (HfO2)1-x(Al2O3)x/Si interface structure at atomic scale 82 4.5 Electrical characterization of (HfO2)1-x(Al2O3)x/Si stacks 85 4.6 Stress testing of (HfO2)1-x(Al2O3)x films as gate dielectric 88 4.7 Frequency dependent properties of (HfO2)1-x(Al2O3)x films as gate dielectric 93 4.8 Conclusion 95 Reference 96 Chapter Interfaces of Metal Gate/ HfO2/semiconductor System 98 5.1 Introduction 98 5.2 HfO2/Semiconductor interface 100 5.3 Metal gate/HfO2 interface 110 5.4 Band alignments of RuOx on HfO2/Si system 116 5.5 Conclusion 121 Reference 122 Chapter Evolution of Schottky Barrier Heights at Metal/HfO2 Interfaces 126 6.1 Introduction 126 6.2 Evolution of Schottky barrier heights at metal/HfO2 interfaces 127 6.3 Atomistic modeling of Ni/HfO2 interfaces 134 6.4 Conclusion 148 Reference 150 Chapter Conclusions and Future Work 152 7.1 Conclusions 152 7.2 Future work 154 Reference 156 iv Summary SUMMARY High-k dielectrics are proposed to replace traditional SiO2 as gate dielectric layer Meanwhile, conventional poly-silicon electrode should also be replaced by metal gates Metal/HfO2/semiconductor system is very promising to be used in future generation CMOS In this thesis, the physical, electrical and electronic properties of metal/HfO2/semiconductor systems have been systematically studied by combining film fabrication and characterization techniques and first-principles calculations The thermodynamic stability of HfO2 film has been investigated by studying the reactions during the deposition of HfO2 thin films on SiO2-covered silicon substrates in oxygen-deficient conditions Thermodynamic analysis indicates that even if there is a layer of silicate forming at the initial stage of deposition, the silicate layer or the SiO2 layer will be decomposed by metal ions and oxygen in the interfacial layer will be absorbed by HfOx1.0 eV) were obtained for HfO2/semiconductor (Si, Si0.75Ge0.25 and Ge) interfaces, which guarantees HfO2 as an effective carrier barrier for the channels and the most promising high-k gate dielectric candidate on Si and other high performance substrates (Si0.75Ge0.25 and Ge) For metal/HfO2 interfaces, in situ XPS methods were performed to accurately determine the Schottky barrier heights for Ni(Co)/HfO2 stacks The band discontinuities for the vacuum level were found at the interfaces of metal/HfO2 It is believed that this is due to the dipole formed at the metal/oxide interface Detailed studies on the evolution of the band alignment during the formation of metal-dielectric contacts were conducted on Ni/HfO2 stacks to clarify how the interface dipole is formed even for the simple nonreactive metal gate/high-k dielectric interface and how it influences the effective work function of metal gate by in situ XPS and first-principles calculations based on density functional theory This work identified that the interface dipole was induced by the weak interaction of Ni thin film and HfO2 dielectric The energy-band alignments for the RuOx/HfO2/Si stacks and the oxidation-state dependent barrier heights for RuOx in contact with HfO2 dielectrics have been investigated by XPS technique The results in this work imply that the ruthenium oxide, RuO2, is a promising alternative gate electrode to be integrated with high-k gate dielectrics vi List of Figures LIST OF FIGURES Figure 1.1 Gate current density as a function of gate voltage for MOS capacitors with different SiO2 gate dielectric thickness Figure 1.2 Schematic energy band diagram of direct tunneling of electron from the Si substrate to the gate in a turned on n-MOSFET (Si/SiO2/poly-Si gate structure) .7 Figure 1.3 Band offset calculations for a number of potential high-k gate dielectric materials 12 Figure 1.4 Schematic band diagrams for metal/oxide/Si stacks Definitions of band offsets (VBO and CBO) and of SBHs (Φn and Φp) are shown 21 Figure 2.1 Schematic view of the pulse laser deposition system 35 Figure 2.2 Schematic view of e-beam evaporator 39 Figure 2.3 Schematic views of the main basic requirements for XPS system 40 Figure 2.4 Schematic view of photon induced electron emission (photoemission) in XPS (Carbon atom) 41 Figure 2.5 Schematic view of a transmission electron microscope 45 Figure 3.1 (a) Si 2p core-level; (b) O 1s core-level; (c) Hf 4f core-level XPS spectra at different stages of HfO2 deposition on SiO2-covered silicon 55 Figure 3.2 TEM image of 8.75 nm HfO2 film on silicon, free of underlying amorphous SiO2 57 Figure 3.3 XPS depth profile for 8.75 nm HfO2 film on silicon, the sputtering time of 30 s for each level (a) Si 2p; (b) Hf 4f; (c) O 1s core-level spectra 59 Figure 3.4 Si 2p core-level spectra of ~4 nm as-deposited HfO2 film 62 Figure 3.5 Si 2p core-level spectra of ~4 nm as-deposited HfO2 film 62 Figure 3.6 Cross-sectional HRTEM image of ~4 nm as-deposited HfO2 film 63 Figure 3.7 (a) Si 2p (b) Hf 4f core-level spectra of ~4 nm as-deposited HfO2 film and the films with rapid thermal annealing at different temperatures 65 Figure 3.8 Cross-sectional HRTEM images of HfO2 film with different rapid thermal annealing, (a) 500ºC; (b) 700ºC; (c) 900ºC; (d) 1000ºC in N2 for 10 s The thicknesses of interface layers vii List of Figures were labeled 66 Figure 3.9 Capacitance-voltage characteristics of ~4 nm as-deposited HfO2 film and films with rapid thermal annealing at different temperature 67 Figure 3.10 EOT and effective dielectric constant for Al/HfO2/Si capacitors as a function of annealing temperature 69 Figure 3.11 Current-voltage characteristics of ~4 nm as-deposited HfO2 film and films with rapid thermal annealing at different temperature 70 Figure 4.1 Si 2p, Hf 4f, Al 2p and O 1s XPS depth profiling spectra of 10.0 nm HAO film before (a) and after (b) RTA on p-type Si (100) substrate 80 Figure 4.2 SIMS analysis of Hf-Al-O film before RTA (a) and after RTA (b) (Analysis parameters: Ga Gun Energy: 25 KeV Current: 3.00 pA Area: 49.8ì49.8 àm2 Sputter parameters: Ar Gun Energy: 0.50 KeV Current: 3.00 nA Area: 150×150 µm2 ) 81 Figure 4.3 HRTEM images of 10.0 nm HAO film before RTA (a), and after RTA (b) Islands of Hf silicide are formed from interface reaction, and obviously reduced after RTA RTA temperature is 1000ºC in N2 for 10 s The island is indicated by the circle in white 84 Figure 4.4 Capacitance-voltage and current-voltage characteristics of 10.0 nm HAO film before RTA (a) and after RTA (b) The driving frequency is 100 kHz The equivalent oxide thickness is 1.7 nm with dielectric constant at ~22.5 87 Figure 4.5 Flow chart showing the procedure of the measurement and stressing cycles in the developed program 89 Figure 4.6 Schematic diagrams showing the setup for the p-substrate capacitor biased in the (a) inversion and (b) accumulation mode 89 Figure 4.7 Energy band diagram of a capacitor when stressed in the (a) inversion and (b) accumulation mode 90 Figure 4.8 Constant voltage stress induced at accumulation (a) C-V curve, (b) I-V curve (Stress condition: 10 cycles at -0.01 mA followed by another 10 cycles at -1 mA) No major changes in C-V and I-V curves after 10 cycles stress 92 Figure 4.9 (a) Frequency dependence of C-V properties, the diameter of electrode is 200 µm (b) The leakage current density of the HAO film 94 Figure 5.1 Band alignments at HfO2/Si interface 101 Figure 5.2 The valence-band spectra of bulk silicon and HfO2/Si system (left).The Si 2p core-level spectra of HfO2/Si system (right) 103 Figure 5.3 The valence-band and Hf 4f core-level spectra of HfO2/Si system 104 Figure 5.4 The Hf 4f spectra of HfO2/Si, HfO2/Si0.75Ge0.25 and HfO2/Ge interfaces 105 Figure 5.5 The valence-band and Si 2p spectra of Si without and with HfO2 overlayer 107 viii List of Figures Figure 5.6 The valence-band and Si 2p spectra of Si0.75Ge0.25 without and with HfO2 overlayer 108 Figure 5.7 The valence-band and Ge 3p spectra of Ge without and with HfO2 overlayer 109 Figure 5.8 The Δ Gibbs free energy calculation of Co and HfO2 reaction 112 Figure 5.9 Hf 4f (a) and Co 2p (b) core-level XPS spectra at different Co deposition time 113 Figure 5.10 Co 2p3/2 (a) and Co 2p1/2 (b) core-level XPS spectra at the first minutes deposition 113 Figure 5.11 Co 2p3/2 and Hf 4f relative area ratio as a function of Co deposition time 114 Figure 5.12 (a) Valence-band spectra for HfO2; (b) Hf 4f core-level spectra for HfO2; (c) Valence-band spectra for 20 Å Co/HfO2; (d) Hf 4f core-level spectra for Co/HfO2 115 Figure 5.13 The Ru 3d core-level XPS spectra of RuOx/HfO2/Si system under different oxidation-state 118 Figure 5.14 The valence-band and Hf 4f core-level spectra of RuOx/HfO2/Si system 120 Figure 5.15 The schematic view of band alignment at RuO2/HfO2 interface 120 Figure 6.1 Hf 4f and Si 2p core-level spectra for Ni/HfO2/n-Si stack as a function of Ni thickness (for 0.3, 0.6, 0.9, 1.3, 1.6, 1.9, and 2.5 nm Ni thickness) The shifts of the binding energy for Hf 4f 5/2 and Si 2p3/2 are indicated by discontinuity lines 129 Figure 6.2 Valence-band and Ni 2p spectra of the Ni/HfO2/n-Si stack as a function of Ni thickness For the clean HfO2 surface, the VB edge is denoted at 3.80 eV The zero of binding energy corresponds to the Fermi level 129 Figure 6.3 The Fermi level positions with respect to the valence-band maximum of HfO2 as a function of Ni and Co coverage on HfO2/Si The n-type SBHs for Ni and Co on HfO2 are given 132 Figure 6.4 The band diagrams of Ni/HfO2 stacks 133 Figure 6.5 Electronic band structures for c-HfO2 136 Figure 6.6 Total (solid line) and atom-projected (dotted line: O atom; dot-dashed: Hf) density of states (DOS) for c-HfO2 137 Figure 6.7 Relaxed interface structures for (a) 1L, (b) 2L, (c)3L, (d) 4L, and (e) 5L of Ni on HfO2(111) surface (Red atom: O; Deep blue: Ni; Light blue: Hf.) 139 Figure 6.8 PDOS for Ni atoms in different layer (from surface to interface) The Fermi level is at energy zero, denoted by the dotted line 141 Figure 6.9 PDOS for O atoms in different layer (from interface to surface) The Fermi level is at energy zero, denoted by the dotted line 143 Figure 6.10 PDOS for the O atom in the bulk region (O-5), calculated by Gaussian- based method (solid lines) and the tetrahedron method (the dotted lines) 143 Figure 6.11 Atom-projected density of states (PDOS) for different atoms in 5L Ni/HfO2 interface ix List of Figures supercell O-bulk: oxygen atom in the central layer of HfO2; O-inter: oxygen atom at the interface; Ni-bulk: Ni atom in the central layer of Ni thin film; Ni-inter: Ni atom at the interface Accurate PDOS for O-bulk was also shown in dotted line to determine the VB edge of HfO2 The Fermi level is at energy zero 144 Figure 6.12 Penetration of electronic density ρ (z ) of the gap states into the HfO2 for Ni/HfO2 interfaces Position of the first layer of HfO2 (111) is set to z = Å 146 Figure 6.13 Calculation of vacuum work function for Ni(111) surface: (up) supercell (relaxed) for Ni(111) surface with 11 layers of Ni and 15 Å of vacuum; (down) electrostatic potential (ESP) for Ni(111) surface with the Fermi level (EF) at energy zero 147 Figure 6.14 Plane-averaged electrostatic potential (ESP, dotted line) along the interface normal for 5L Ni/HfO2 interface supercell The metal vacuum work function φ Φ φ m,vac , effective work Φ function m,eff , n(p)-type SBH n ( p ), HfO2 electron affinity χ , and the potential drop ΔEvac across the vacuum were shown The position zero is at the interface Values in the figure are in eV 148 x Chapter for O atoms in different layers It is clear that the PDOS for oxygen atoms residing in the central layer recover their characters in bulk c-HfO2 But the interface oxygen anions are noticeably perturbed by the formation of the interface, and gap states appear in the PDOS of interface oxygen ions, as indicated by the arrow in Fig 6.9 These gap states almost disappear from the gap region in the PDOS of the second layer of oxygen It is convenient to consider two sources for the gap states, contribution from the interfacial chemical bonds and contribution from the tails of the metallic wave functions which tunnel into the oxide band gaps or conventional metal induced gap states (MIGS).22,23 For high-k oxide dielectrics (HfO2 or ZrO2), the MIGS is rather weak and the interfacial bonding is the primitive contribution for the formation of the SBH at metal gate/high-k dielectric interface.24,25 The small MIGS were also observed here for Ni on non-polar HfO2 (111) surface, which decay very fast away from the interface and almost disappear in the second layer of O (~1.5 Å away form the interface) In Figure 6.10, the PDOS for the O atom in the bulk region, calculated by Gaussianbased method (solid lines) and the tetrahedron method (the dotted lines) respectively, were shown From the PDOS shown in Fig 6.10, the p-type SBH can be read directly to be 2.27 eV Similarly, the values of SBHs for other thickness (1L － 4L) were also evaluated from respective PDOS The results are summarized in Table 6.3 One can see that SBH was stabilized around 2.3 eV after covering by two layers of Ni 142 Chapter Figure 6.9 PDOS for O atoms in different layer (from interface to surface) The Fermi level is at energy zero, denoted by the dotted line Figure 6.10 PDOS for the O atom in the bulk region (O-5), calculated by Gaussian- based method (solid lines) and the tetrahedron method (the dotted lines) 143 Chapter Table 6.3 Calculated SBHs for Ni/HfO2 interface with different thickness of Ni overlayer Ni1-HfO2 p-SBH (eV) Ni2-HfO2 Ni3-HfO2 Ni4-HfO2 Ni5-HfO2 1.96 2.31 2.28 2.26 2.27 The atom-projected density of states (PDOS) for Ni and O atoms in the bulk and interface region in the 5L Ni/HfO2 interface supercell were redrew in Fig 6.11 It is clear that metal induced gap states (MIGS)23 appear in the PDOS of interface oxygen ions, as indicated by the arrow in Fig 6.11 Figure 6.11 Atom-projected density of states (PDOS) for different atoms in 5L Ni/HfO2 interface supercell O-bulk: oxygen atom in the central layer of HfO2; O-inter: oxygen atom at the interface; Ni-bulk: Ni atom in the central layer of Ni thin film; Ni-inter: Ni atom at the interface Accurate PDOS for O-bulk was also shown in dotted line to determine the VB edge of HfO2 The Fermi level is at energy zero 144 Chapter To closely examine the spatial dispersion of the occupied gap states, we calculated the corresponding charge density profile along the normal direction of the interface.24 ρ ( z ) = A −1 ( E F − EVBM ) −1 ∫ EF EVBM ρ ( x, y, z )dEdxdy (6.3) where the xy plane coincides with the interface and z-direction is along the interface normal, A is the basal area of interface supercell One important feature of the calculated charge density profile shown in Fig 6.12 is its exponential decay into the oxide side The decay length (λ ~1.24Å) is nearly independent of the type of the metal and interface chemistry, and should be viewed as a property of bulk HfO2 It is important to note this decay length (1.24 Å) is much smaller than that in Si (3.0 Å) and GaAs (2.8 Å), but comparable to that in LaAlO3 (1.1 Å), another high-k dielectric candidate Therefore, the small MIGS at metal/HfO2 interface cannot screen the electrostatic effect of interface chemistry efficiently and Fermi level can be strongly modulated by interface chemistry It will be shown later that even the weak interaction of Ni and HfO2 can strongly change the effective WF of Ni on HfO2 Based on the same mechanism, it is also very natural for many researchers observing that interface defects such as oxygen vacancies can change the effective WF significantly To compare with the experimental results, we applied a +1.09 eV quasiparticle correction,24 which consists of 1.23 eV for HfO2 and -0.14 eV for Ni in the calculation to remove the DFT band gap error Then the p-SBH of the Ni/HfO2 interface is 3.36 eV for Ni layer of more than 3L thick, which is in good agreement with the photoemission results 145 Chapter Figure 6.12 Penetration of electronic density ρ ( z ) of the gap states into the HfO2 for Ni/HfO2 interfaces Position of the first layer of HfO2 (111) is set to z = Å To get more accurate results comparable to the experimental results, the calculated SBHs should include quasiparticle and spin-orbital corrections The vacuum WF of Ni (100) surface was calculated using a supercell comprising 11 layers of Ni and 15 Å of vacuum The electrostatic potential along the surface normal direction was shown in Fig 6.13, where the WF was read directly as the energy difference between vacuum level and the Fermi level (EF) For transition metal Ni, -0.14 eV correction (The negative/positive sign means the Fermi level correction will decrease/increase the p-type SBH.) was added by comparing our DFT GGA calculations value (5.21 eV, as shown in Fig 6.13) and the experimental value (5.35 eV) for the work function of Ni (111) surface For oxide, we neglect spin-orbit corrections, as the valence states are oxygen-derived For the quasiparticle corrections, we apply +1.23 eV for HfO2 VBM.27,28 The overall corrections, +1.09 eV, will be used With the many-body correction included, the calculated SBH 146 Chapter (2.26+1.09 = 3.35 eV) for Ni/HfO2 interface is in good agreement with the experimental result (~3.3 eV), considering the accuracy for both methods is around 0.1 eV Figure 6.13 Calculation of vacuum work function for Ni(111) surface: (up) supercell (relaxed) for Ni(111) surface with 11 layers of Ni and 15 Å of vacuum; (down) electrostatic potential (ESP) for Ni(111) surface with the Fermi level (EF) at energy zero The vacuum discontinuity for the formation of Ni/HfO2 interface was also evaluated from the first-principles calculations Figure 6.14 shows the plane-averaged electrostatic potential (ESP) along the interface normal for the 5L Ni/HfO2 interface supercell It is clear that there is a potential drop of 0.3 eV from the vacuum level of Ni(111) surface on one side to that of c-HfO2(111) surface on the other side Notice that the potential drop happens upon the formation of Ni/HfO2 contact This potential drop reveals the interface dipole at the Ni/HfO2 interface and accounts for the difference between Ni vacuum WF (~5.2 eV) and its effective WF (~4.9 eV) on HfO2 with the latter using the vacuum level 147 Chapter of HfO2 as the reference An interface dipole (0.3 eV) is created at the Ni/HfO2 interface through the weak metal-dielectric interaction, namely, the band hybridization (O p and Ni d orbital) and the classical image charge interaction Figure 6.14 Plane-averaged electrostatic potential (ESP, dotted line) along the interface φ normal for 5L Ni/HfO2 interface supercell The metal vacuum work function m,vac , φ Φ effective work function m,eff , n(p)-type SBH Φ n ( p ), HfO2 electron affinity χ , and the potential drop ΔEvac across the vacuum were shown The position zero is at the interface Values in the figure are in eV 6.4 Conclusion In conclusion, the evolution of band alignments at the Ni/HfO2/n-Si stacks with increasing Ni thickness has been studied by in situ XPS.28 The n-SBH at the Ni/HfO2 interface (or the effective WF of Ni on HfO2) increases with Ni thickness and approaches 2.4 eV (or 4.9 eV) when the thickness of Ni overlayer is over 1.6 nm There are 0.3 eV 148 Chapter discontinuities for the vacuum level at the Ni/HfO2 interface First-principles calculations reveal that the interface dipole at the non-reactive Ni/HfO2 interface was created by the weak interaction of Ni thin film and HfO2 dielectric 149 Chapter Reference The International Technology Roadmap for Semiconductor 2003, http://public.itrs.net Y C Yeo, T J King, and C M Hu, J Appl Phys 92, 7266 (2002) D Lim, R Haight, M Copel, and E Cartier, Appl Phys Lett 87, 072902 (2005) Y F Dong, S J Wang, J W Chai, Y P Feng, and A C H Huan, Appl Phys Lett 86, 132103 (2005) J K Schaeffer, L R C Fonseca, S B Samavedam, Y Liang, P J Tobin, and B E White, Appl Phys Lett 85, 1826 (2004) A A Knizhnik, I M Iskandarova, A A Bagatur'yants, B V Potapkin, and L R C Fonseca, J Appl Phys 97, 064911 (2005) Y Liang, J Curless, C J Tracy, D C Gilmer, J K Schaeffer, D H Triyoso, and P J Tobin, Appl Phys Lett 88, 072907 (2006) Q Li, S J Wang, K B Li, A C H Huan, J W Chai, J S Pan, and C K Ong, Appl Phys Lett 85, 6155 (2004) D Sotiropoulou and S Ladas, Surf Sci 408, 182 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conditions has been investigated A series of reactions across the interface were identified by thermodynamic arguments in conjunction with x-ray photoemission spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) analyses It was found that the SiO2 layer was decomposed and the oxygen in the SiO2 layer was absorbed by HfOx1.0 eV) indicate that HfO2 is a promising candidate for use as high-k gate dielectric on Si or high performance substrates (Si0.75Ge0.25 and Ge) For metal/HfO2 interfaces, in situ XPS were performed to accurately determine the Schottky barrier heights for Ni(Co)/HfO2 stacks The band discontinuities for the vacuum level were found at the interfaces of metal/HfO2 It is believed that this is due to the dipole formed at metal/oxide interface In addition, the energy band alignments for RuOx/HfO2/Si stacks and the oxidationstate dependent barrier heights for RuOx in contact with HfO2 films have been investigated by XPS The results in this work imply that the ruthenium oxide, RuO2, is a 153 Chapter promising alternative gate electrode to be integrated with high-k gate dielectrics Finally, detailed studies on the evolution of the band alignment during the formation of metal-dielectric contacts were conducted on Ni/HfO2 stacks to clarify how the interface dipole is formed even for the simple non-reactive metal gate/high-k dielectric interface and how it influences the effective work function of metal gate by in situ XPS In addition, the physical mechanism behind the formation of Ni/HfO2 Schottky barrier at atomic scale were investigated by first-principles calculations based on density functional theory The interface dipole was induced by the weak interaction of Ni thin film and HfO2 dielectric 7.2 Future work Despite the significant amount of efforts devoted in this study, much research is still required to gain a better understanding of metal/HfO2/semiconductor system, as it is clear that HfO2 is still facing formidable challenges to replace traditional SiO2 as gate dielectric With the shrinkage of feature size, deposition and characterization of high quality high-k dielectric materials will continue to be one of the most interesting research topics in the future In our work, thermodynamic and thermal stability of HfO2/Si stacks had been investigated High thermal stability HAO has been demonstrated However, silicide is observed during the formation of thin HfO2 and HAO films, which will seriously degrade the performance of devices Future studies should extend present work to fabricate HfO2 films with silicide-free interfaces Inserting a SiON or SiO2 buffer layer between HfO2 154 Chapter and Si is expected to greatly improve thermal stability without silicide formation at the interface, which would have significant effects on the interface electronic structures and properties.2 Future studies on the physical, electrical and electronic properties of HfO2/SiON/Si or HfO2/SiO2/Si stacks are highly recommended Future work may also be extended to reduce the electrically active defects in high-k metal oxides, which give rise to electronic states in the band gap of the oxide affecting the reliability of oxide It is found that the main defects in high-k metal oxides are oxygen vacancies and interstitials The oxygen vacancies are the major problem as they give rise to defect levels close to the conduction-band of silicon Much research is required on engineering high-k metal oxides to reduce defect densities by processing control and annealing Nitridation of high-k oxides is proposed to reduce the oxygen vacancies by coupling the oxygen vacancies Further studies to clarify the effects of nitrogen incorporation on the properties of HfO2 are recommended Choosing thermal robust HfN metal as a promising gate electrode for advanced MOS device applications would also be an interesting subject Compared to refractory metal nitride such as TiN and TaN, HfN is expected to possess better thermal stability with underlying gate dielectrics (such as HfO2).3 In addition, HfN is proposed to apply in fully depleted silicon-on-insulator4 and/or symmetric double-gate5 MOS device Therefore, future studies may also be extended to the properties of metal nitrides as metal gates, such as thermal stability, effective work function, interfacial electronic structures et al 155 Chapter Reference S J Wang, and C K Ong, Appl Phys Lett 80, 2541 (2002) J H Oh, Y Park, K S An, Y kim, J R Ahn, J Y Baik, and C Y Park, Appl Phys Lett 86, 262906 (2005) H Y Yu, M F Li, and D L Kwong, IEEE Trans on Electron Devices 51, 609 (2004) B Cheng, B Mariti, S Samavedam, J Grant, B Taylor, P Tobin, and J Mogab, IEEE Int SOI Conf 91 (2001) H S P Wong, IBM J Res Dev 46, 133 (2002) 156 ... thicker layer of oxides of higher dielectric constant (k) Intensive research is underway to develop oxides into new high quality electronic materials 1.4 Alternative high- k gate dielectrics candidates... process from gate dielectric to Si channel While SiO2 provides us with the remarkable properties as the gate dielectric, the gate leakage current increases exponentially as the oxide thickness scaled... on most high- k dielectric materials, the use of stable metallic gate electrodes is required to solve the integration problems when the gate dielectric material is replaced by high- k dielectric