STUDY OF METAL GATES AND HIGH-K DIELECTRICS IN NANOELECTRONICS MI YANYU NATIONAL UNIVERSITY OF SINGAPORE 2007 STUDY OF METAL GATES AND HIGH-K DIELECTRICS IN NANOELECTRONICS MI YANYU (B. Sc., M. Sc., Xi’an Jiaotong Univ.) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE Acknowledgement Acknowledgement I would like to express my gratitude my advisors: Prof. Ong Chong Kim from National University of Singapore and Prof. Huan Cheng Hon Alfred from Nanyang Technology University for their support and excellent supervision. It is really an honor for me to get the guidance from them, and I have learnt much from them in the last few years. I want to thank my co-supervisor: Dr. Wang Shijie, the research scientist from Institute of Materials Research & Engineering. He has constantly provided me with assistance and valuable advice to improve my research work and has always been supportive of my research endeavors. Thanks for teaching me hands-on skills on in situ XPS, HRTEM, and giving me ample freedom to learn more techniques used in this research work. I am truly grateful for all the help and encouragement he has given me during the latest four years. This research work was carried out at both NUS and IMRE, where I have met and collaborated with many talented and generous graduate students and colleagues over the last few years. Working with them turns the Ph.D. study to a happy memory for me. Thanks to Ms. Li Qin and Dr. Dong Yufeng from my research group. Thanks for all enlightening scientific and private discussions and their excellent teamwork. We had a good time together. Thanks to the staffs and students at IMRE for their warm help on my research work: Dr. Pan Jisheng, Dr. Foo Yong Lim, Dr. Chai Jianwei, Ms. Chow Shue Yin, Mr. i Acknowledgement Zhang Zheng, Mr. Lim Poh Chong, Mr. Chan Yong Seng Kelvin, Dr. Seng Hwee Leng Debbie, and Ms. Lai Mei Ying Doreen. Thanks to the staffs and students at NUS for their support and encouragement on my research work: Prof. Feng Yuanping, Dr. Gao Xingyu, Dr. Gao Xingsen, Mr. Ning Min, Mr. Yang Ming, Ms. Chen Qian, Ms. Liu Yan, Mr. Liu Huajun, Mr. Chen Shi, Mr. Qi Dongchen, and Dr. Ng Tsu Hau. Last but not least, I would like to thank my parents, without whom I would not be the person I am today. Thanks for their love, encouragement, and support throughout my life. Special thanks go to my husband, Li Qi, for his love and 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 Abstract vi List of Tables viii List of Figures ix Publications xv Chapter Introduction 1.1 MOSFET scaling 1.2 High-k dielectrics 1.3 Metal gates 1.4 Band alignments 1.4.1. Band offsets at high-k dielectrics/semiconductor interfaces 1.4.2. Schottky barrier height at metal gate/high-k dielectrics interfaces 11 1.5 Nitridation treatment 17 1.6 Objective and significance of the study 18 References 21 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. Nitridation treatments 31 2.2. Characterization techniques 2.2.1. X-ray Photoelectron Spectroscopy (XPS) 32 33 2.2.1.1. Principles of XPS 33 2.2.1.2. Data interpretation 35 iii Table of Contents 2.2.1.3. Instrument and application of XPS 2.2.2. High Resolution Transmission Electron Microscopy (HRTEM) 39 41 2.2.2.1. Principle of TEM 41 2.2.2.2. Specimen preparation for HRTEM 43 2.2.2.3. Instrument and application of HRTEM 44 2.2.3. Other characterization techniques 45 2.3. Computational method (First-principles calculations) 48 References 54 Chapter High-k dielectrics/semiconductors Stacks 56 3.1. Introduction 56 3.2. Growth and electronic properties of high-k dielectrics/semiconductor 57 3.2.1. Epitaxial SrTiO3/Si 57 3.2.2. Epitaxial and amorphous LaAlO3/Si 59 3.2.2.1. Epitaxial LaAlO3/Si 59 3.2.2.2. Amorphous LaAlO3/Si 65 3.2.3. LaAlO3/Ge and LaAlO3/GeOxNy/Ge 69 3.2.4. LaAlO3/SiGe/Si and LaAlO3/SiOxNy/SiGe/Si 76 3.3. Thermal stability of high-k dielectrics/semiconductor 81 3.3.1. LaAlO3/Ge and LaAlO3/GeOxNy/Ge 81 3.3.2. LaAlO3/SiGe/Si and LaAlO3/ SiOxNy/Si 91 3.4. Summary 101 References 103 Chapter Metal gate/high-k dielectrics 106 4.1. Introduction 106 4.2. Ni/LaAlO3(001) 107 4.2.1. Growth of epitaxial Ni films on crystalline LaAlO3(001) films 107 4.2.1.1. HRTEM study of Ni films 108 4.2.1.2. Effective work functions of Ni films 109 4.2.2. Evolution of Fermi level position at Ni/LaAlO3 (001) interfaces 4.3. Ni/MgO(001) 114 117 iv Table of Contents 4.3.1. Growth of epitaxial Ni films on MgO(001) 119 4.3.1.1. Surface morphology studied by AFM 120 4.3.1.2. Surface and interface structure studied by HRTEM 123 4.3.2. Schottky barrier height (SBH) at Ni/MgO(001) interfaces 129 4.3.2.1. Evolution of Fermi level position and SBH 130 4.3.2.2. First-principles calculation of SBH 132 4.3.2.3. The effect of interface structure on SBH 135 4.4. Summary 139 References 141 Chapter Nitridation Treatment on High-k dielectrics Films 146 5.1. Introduction 146 5.2. Nitridation treatment on LaAlO3 films 146 5.2.1. Nitridation of LaAlO3/Si and its thermal stability 147 5.2.2. Nitridation of LaAlO3/Ge and its thermal stability 150 5.2.3. Nitridation of LaAlO3/SiGe and its thermal stability 153 5.3. Nitridation treatment on SrTiO3(001) films 157 5.3.1. Introduction 157 5.3.2. Optical and electronic properties of nitrogen-doped SrTiO3(001) 158 films and their thermal stability 5.3.3. First-principles calculation of electronic structure of nitrogen- 172 doped SrTiO3 (001) films References 176 Chapter Conclusions and Future Work 179 6.1. Conclusions 179 6.2. Future work 182 v Abstract Abstract The continual downscaling of complementary metal oxide semiconductor field effect transistors (CMOSFETs) devices not only requires the replacement of SiO2 or SiOxNy gate oxides with high-k dielectrics, but also requires the replacement of conventional poly-silicon (poly-Si) gate with metal gates. To achieve this goal, in this thesis, the integration of metal gates and high-k dielectrics materials with semiconductors was studied by using both experimental and theoretical methods. The growth and characterization (e.g. electronic structure, thermal stability) of LaAlO3 films on various semiconductor substrates (Si, Ge, and Si0.75Ge0.25) were studied by high resolution transmission electron microscopy (HRTEM) and in situ x-ray photoelectron spectroscopy (XPS). The high enough band offsets (> eV) between LaAlO3 films and various semiconductor substrates (Si, Ge and Si0.75Ge0.25) indicate that LaAlO3 dielectrics is a promising candidate to be integrated with various semiconductors in the downscaling of CMOSFETs devices. The effect of interfacial oxynitride layer on the band alignments and thermal stabilities of LaAlO3 films on Ge and Si0.75Ge0.25 substrate was also studied. It was found that the interfacial oxynitride layer changed the band alignments by modifying the interfacial dipoles, which indicates that nitridation treatment not only acts as a surface passivation layer but also changes the interfacial electronic structures. Ni was chosen as a prototype of metal gates to be epitaxially grown on LaAlO3(001) and MgO(001) high-k dielectrics. Good epitaxial structure of Ni films with suitable effective work function indicates that Ni is a promising metal gate candidate integrated with LaAlO3 high-k dielectrics films in PMOS devices. The vi Abstract evolution of Schottky barrier height (SBH) and Fermi level positions at Ni/LaAlO3(001) and Ni/MgO(001) interfaces were studied by in situ XPS, and an upward band bending was found at the initial Ni growth stage. First-principles calculations based on Density Functional Theory (DFT) was employed to understand the underlying physical mechanisms. Adatom-induced states (or interfacial bonding states), metal induced gap states (MIGS) and defects states were used to rationalize the evolution of Fermi level and corresponding SBH in this metal/high-k dielectrics system with various interface structures. This work implies that SBH can be engineered by interface structure control, and is expected to shed light on the effect of interface structures on the formation mechanism of SBH at metal/high-k dielectric oxides interface. The effect of nitridation on the electronic structures and thermal stabilities of high-k dielectrics films (LaAlO3, SrTiO3) films was studied by using in situ XPS, spectroscopic ellipsometry (SE), x-ray absorption spectroscopy (XAS) 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. vii List of Tables List of Tables Chapter Table 4.1 The growth temperature and surface roughness of Ni thin films 122 viii Chapter Nitridation Treatment on High-k dielectrics Films edge is also found in nitrogen-doped HfO2 films.1 Combined with core level spectra, it can be deduced that the initial interstitial N states lie higher above the O 2p valence-band edge than the substitutional nitrogen states do, and post-annealing desorbs the initial interstitial N states and thus upward shifts the valence-band edge of nitrogen-doped SrTiO3 films. This deduction is later confirmed by our theoretical studies, which shows that interstitial N 2p states locate higher above O 2p states in the band gap. Conduction band structure of SrTiO3 films was investigated by O K-edge and Ti L-edge XAS spectra. Figure 5.14(a) shows the O K-edge XAS spectra of different SrTiO3 samples, which presents the transitions from O 1s core level to the unoccupied O 2p level controlled by dipole transition selection rules.29, 30 In Fig. 5.14(a), the unoccupied O p-states hybridized with Ti 3d states just lie above the threshold of spectrum.30 The broader peaks located at 535 and 547 eV are assigned to the transition from O 1s orbitals to the unoccupied O p-states hybridized with Sr 4d and Sr 5sp, respectively. The excitation to the O 2p + Ti 4sp states is located at higher energy position (around 551 eV).31 In specific, at the threshold of spectrum, there are two peaks, in which the sharper one is assigned as t2g sub-bands and the other one is eg sub-band, which are due to the split of Ti 3d states by crystal field effects. The sharp t2g peak at 530.7 eV indicates the Ti 3d states hybridized with O 2pπ states, and small eg peak indicates the hybridization between Ti 3d states with O 2pσ bonds. The crystal field splitting values measured from this figure is 2.4 eV, which is smaller than the reported value (2.8 eV) for SrTiO3 bulk.31 The divergence is probably due to the small 168 Chapter Nitridation Treatment on High-k dielectrics Films difference in the molecular structure between stoichiometric SrTiO3 bulk and SrTiO3 films. We noticed that nitridation did not alter the position of peaks in O K-edge XAS spectra, but the intensity ratio of t2g to eg changed: compared with undoped SrTiO3 films, the intensity ratio of t2g to eg increased in nitrogen-doped SrTiO3 films but decreased a little after thermal annealing. This change is supposed to be related to the change of chemical states of SrTiO3 thin films and the local symmetry around Ti ions. (a) 3d*5/2 eg 3d*3/2 NSTO650 NSTO STO Ti L edge 3d*5/2 3d*3/2 NSTO650 Intensity (arb. units) t2g Intensity (arb.units) (b) O K edge NSTO Sr 4d*, 5s*p* Ti 4sp* STO Ti 3d* L3(p3/2) 528 532 536 540 544 Photo Energy (eV) 548 552 L2(p1/2) 454 456 458 460 462 464 466 468 470 Phote Energy (eV) Figure 5.14 XAS spectra of (a) O-K edge features and (b) Ti-L edge features of undoped SrTiO3 (STO), nitrogen-doped SrTiO3 (NSTO), and annealed N-doped SrTiO3 at 650°C (NSTO650). To investigate the electronic structure of Ti-O bond, we also studied Ti L-edge XAS. Ti L-edge XAS corresponds to the transition from Ti 2p3/2(L3) and Ti 2p1/2(L2) core electrons to unoccupied Ti 3d states, i.e., Ti 2p63d0 (ground state)Æ Ti 2p53d1(excited state).29, 30, 32 Ti L-edge XAS can give the direct information on the unoccupied d-orbitals of transition metal ions where electron transfer occurs. Figure 169 Chapter Nitridation Treatment on High-k dielectrics Films 5.14(b) shows the Ti L-edge XAS spectra for SrTiO3 samples with different treatments. These three plots reveal the similar crystalline structure related to the perovskite structure. Both the Ti L3- and L2- edge represent t2g-like (labeled as 3d*5/2) and eg-like (labeled as 3d*3/2) orbitals, split by ligand field theory under octahedral (Oh) symmetry. It was found that Ti L2-edge is broader than Ti L3-edge, which can be explained by Coster-Kronig decay.29 In the spectra of N-doped SrTiO3 and annealed N-doped SrTiO3 sample, the spectrum of undoped SrTiO3 was inserted as a reference for comparison, denoted as a dotted line. Compared with the spectrum of undoped SrTiO3 films, it was found that, for the N-doped SrTiO3 sample, the eg orbital-derived peaks of both Ti L3- and L2-edge are obviously broadened at the lower energy sides. This change in the shape of eg-derived orbital in Ti L-edge XAS spectra might be due to the change of oxidation states and symmetry of Ti ions upon the nitrogen doping.29 Figure 5.15(a) and 5.15(b) shows the refractive index (n) and extinction coefficient (k) as a function of photo energy for three SrTiO3 samples: undoped SrTiO3 films, nitrogen-doped SrTiO3 films and post-annealed N-doped SrTiO3 films at 650°C. It was found that nitrogen-doped SrTiO3 films has the highest n and k values, while the refractive index n and extinction coefficient k of post-annealed N-doped SrTiO3 are in between of undoped SrTiO3 and nitrogen-doped SrTiO3 films. It is believed that the change of refractive index n and extinction coefficient k of SrTiO3 films is related to the alteration of nitrogen concentration in the films. The nitrogen-doped SrTiO3 films have the highest nitrogen concentration, but post-annealing will remove some unstable interstitial nitrogen atoms, which lead to the decrease of nitrogen concentration. 170 Chapter Nitridation Treatment on High-k dielectrics Films According to this result, we believe that nitrogen incorporation has strong effect on the optical properties of SrTiO3 films. This result is in good agreement with the results of 2.8 2.6 350 (a) 2.4 (C) 300 2.2 STO NSTO NSTO650 1.8 1.6 1.4 0.12 250 0.5 2.0 (αE) Extinction coefficient (k) Refractive index (n) nitrogen-doped ZrO2 films.33 (b) 0.10 200 150 STO NSTO NSTO650 0.08 100 STO NSTO NSTO650 0.06 0.04 50 0.02 0.00 1.0 1.5 2.0 2.5 3.0 Photo Energy (eV) 3.5 1.0 1.5 2.0 2.5 3.0 3.5 Photo Energy (eV) Figure 5.15 (a) The refractive index (n) and (b)extinction coefficient (k) as a function of photo energy for undoped SrTiO3 films (STO), nitrogen-doped SrTiO3 films (NSTO) and annealed nitrogen-doped SrTiO3 films at 650°C (NSTO650); (c) The plot of (αE)1/2 versus photo energy for determining the indirect band gap of these three samples. Figure 5.15(c) shows a plot of (αE)1/2 versus photo energy. The indirect band gap can be evaluated by extrapolating the linear portions of plots. According to the Tauc plots, the indirect optical band gap of undoped SrTiO3 films was measured to be 3.3 eV, which is in good agreement with the reported value. 34 Optical band gap of nitrogen-doped SrTiO3 films and post-annealed N-doped SrTiO3 films were determined to be 2.8 and 3.0 eV, respectively. The possible error should be considered due to the obvious tail at absorption edge induced by the low concentration of nitrogen 171 Chapter Nitridation Treatment on High-k dielectrics Films incorporation. According to the earlier analysis, it was found that the nitrogen-doping reduces the indirect band gap of SrTiO3 films from the UV region (wavelength λ < 387 nm, i.e. Eg > 3.2 eV) to visible light region. This finding could be used to understand the observed enhanced photoactivity of nitrogen-doped SrTiO3 films in visible light range. However, we also noticed that the thermal instability of nitrogen-doped SrTiO3 films may seriously degrade the photoactivity during applications. 5.3.3. First-principles calculation of electronic structure of nitrogen-doped SrTiO3 (001) films Given the importance and interest of this subject, a clear rationalization of the nitrogen species presented in the nitrogen-doped samples is desirable,28 but the explanation is quite controversial. For TiO2 system, based on density functional theory (DFT) calculations, Asahi’s et al. proposed that the mixing of N 2p states with O 2p-derived valence-band contributes to the band gap narrowing.11 However, recent calculations based on DFT indicate that localized N 2p states just lie above valence-band maximum, which can not induce the band gap narrowing.10, 28 In order to provide physical insights for the variation of the electronic structures of SrTiO3 in the presence of nitrogen, we conducted first-principles electronic structure calculations based on DFT. In our first-principles calculation, perovskite SrTiO3 with a 40-atoms supercell with three models were considered: the undoped SrTiO3, the nitrogen-doped SrTiO3 with N substitution (the doping concentration is 1/23, the ratio of the number of N atoms to O atoms), and the nitrogen-doped SrTiO3 with interstitial N states. The charge 172 Chapter Nitridation Treatment on High-k dielectrics Films compensation can be satisfied with charge transfer modified after nitridation. The total energy and electronic structure were calculated using CASTEP code35 with Vanderbilt pseudopotentials and the generalized gradient approximation (GGA). The plane-wave DOS (arb. units) basis cutoff energy is 340 eV. (a) STO-DOS O 2p PDOS (b) STO-Nsub DOS O 2p PDOS N 2p PDOS STO-Nint DOS (c) O 2p PDOS N 2p PDOS -8 -6 -4 -2 Energy (eV) Figure 5.16 The local density of states (DOS) of (a) undoped SrTiO3 (STO), (b) nitrogen-substituted SrTiO3 (STO-Nsub), and (c) SrTiO3 with interstitial N atoms (STO-Nint). Figure 5.16 shows the local density of states (DOS) and atom-projected density of states (PDOS) of undoped SrTiO3 (STO), nitrogen-doped SrTiO3 with nitrogen substitution (STO-Nsub), and SrTiO3 with interstitial nitrogen (STO-Nint), which are aligned by using the PDOS of O atoms far away from the dopant in respective supercell. For undoped SrTiO3 bulk, the valence band edge is mainly derived from the 173 Chapter Nitridation Treatment on High-k dielectrics Films O 2p states, while the hybridization of Ti 3d states with unoccupied O 2p states forms conduction band edge, as shown in Fig. 5.16(a). The calculated band gap of pure undoped SrTiO3 bulk is largely underestimated as 1.8 eV, due to the well-known limitation of DFT. Figure 5.16(b) shows DOS and PDOS of N and O atoms in nitrogen-doped SrTiO3 with nitrogen substitution. Both O 2p and N 2p states form the valence band edge, and the change in the shape of conduction band are mainly due to the hybridization of Ti 3d states with unoccupied both O 2p states and N 2p states. From the valence band spectrum of nitrogen-doped SrTiO3 with nitrogen substitution, it was found that N 2p states is localized at about 0.4 eV above O 2p states, and the mixing of N 2p with O 2p states is too weak to produce a significant band-gap narrowing in SrTiO3. This finding confirmed the recent calculation results of the localized nature of the N 2p states in nitrogen-doped anatase TiO2.28 It was proposed that these localized N 2p states induced the shift of absorption edge and hence, stimulated the photoactivity in visible light region. In our XPS experiments, we found that some N-O bonds formed because of interstitial atoms, so we also investigate the effect of interstitial N on the change of electronic structure, as shown in Fig. 5.16(c). It was found that the interstitial N atoms not only contribute to the valence band states but also the conduction band states below metal Ti 3d states. These theoretical calculations are in agreement with our experimental results of the significant shift of optical absorption to visible light region after the nitridation of SrTiO3. Meanwhile, it was also found that N 2p states in interstitial N model locate at higher binding energy above O 2p state than N 2p states in substitutional N model. This results confirmed 174 Chapter Nitridation Treatment on High-k dielectrics Films out experimental results about the shift of VBM under different process. In summary, the electronic structure and optical properties of nitrogen-doped SrTiO3 films and their thermal stability have been studied by experimental characterizations and first-principle calculations. It was found that N 2p states localized both above O 2p in the valence band edge and below Ti 3d in the conduction band region are attributed to the photoactivity of nitrogen-doped SrTiO3 in the visible light range. Post-annealing will remove some interstitial N states and also produce more oxygen vacancies on the surface of nitrogen-doped SrTiO3. The unexpected thermal instability might seriously degrade the photoactivity of nitrogen-doped SrTiO3 in the visible light range if the thermal process temperature is too high. 175 Chapter Nitridation Treatment on High-k dielectrics Films References: S. J. Wang, J. W. Chai, Y. F. Dong, Y. P. Feng, N. Sutanto, J. S. Pan, and A. C. H Huan, Appl. Phys. Lett. 88, 192103 (2006). C. S. Kang, H. J. Cho, K. Onishi, R. Nieh, R. Choi, S. Gopalan, S.Krishina, J. H. Han, and J. C. Lee, Appl. Phys. Lett. 81, 2593 (2002). N. J. Seong, S. G. Yoon, S. J. Yeon, H. K. Woo, D. S. Kil, J. S. Roh, and H. C. Sohn, Appl. Phys. Lett. 87, 132903 (2005). J. S. Wang, S. Yin, M. Komatsu, Q. Zhang, F. Saito, and T. Sato, Appl. Catal. B: Environ. 52, 11 (2004); S. Yin, H. Yamaki, M. Komatsu, Q. M. Zhang, J. S. Wang, Q. tang, F. Saito, and T. Sato, J. Mater. Chem. 13, 2996 (2003). H. Y. Hwang, Nature Mater. 4, 803 (2005). D. Kan, T. Terashima, R. Kanda, A. Masuno, K. Tanaka, S. Chu, H. Kan, A. Ishizumi, Y. Kanemitsu, Y. Shimakawa, and M. Takano, Nature Mater. 4, 816 (2005); D. Kan, R. Kanda, Y. Kanemitsu, Y. Shimakawa, M. Takano, T. Terashima, and A. Ishizumi, Appl. Phys. Lett. 88, 191916 (2006). M. Itoh, R. Wang, Y. Inaguma, T. Yamaguchi, Y-J. Shan, and T. Nakamura, Phys. Rev. Lett. 82, 3540 (1999). E. Bellingeri, L. Pellegrino, D. Marré, I. Pallecchi, and A. S. Siri, J. Appl. Phys. 94, 5976 (2003). M. Batzill, E. H. Morales, and U. Diebold, Phys. Rev. Lett. 96, 026103 (2006). 10 J. Y. Lee, J. Park, and J. H. Cho, Appl. Phys. Lett. 87, 011904 (2005). 11 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, Science. 293, 269 (2001). 12 W. Choi, A. Termin, and M. R. Hoffmann, J. Phys. Chem. 98, 13669 (1994). 13 R. G. Breckenridge and W. R. Hosler, Phys. Rev. 91, 793 (1953). 176 Chapter Nitridation Treatment on High-k dielectrics Films 14 D. C. Cronemeyer, Phys. Rev. 113, 1222( 1959). 15 S. U. M. Khan, M. Al-Shahry, and W. B. Ingler, Jr., Science. 297, 2243 (2002). 16 H. Irie, Y. Watanabe, and K. Hashimoto, Chem. Lett. 32, 772 (2003). 17 Y. Choi, T. Umebayashi, and M. Yoshikawa, J. Mater. Sci. 39, 1837 (2004). 18 T. Umebayashi, T. Yamaki, and H. Itoh, K. Asai, Appl. Phys. Lett. 81, 454 (2002). 19 T. Yamaki, T. Sumita, and S. Yamamoto, J. Mater. Sci. Lett. 21, 33 (2002). 20 K. B. Sundaram and P. Wahid, Phys. Status. Solidi B. 161, K63 (1990). 21 M. Capizzi and A. Frova, Phys. Rev. Lett. 25, 1298 (1970). 22 N. C. Saha and H. G. Tompkins, J. Appl. Phys. 72, 3072 (1992). 23 T. S. Yang, M. C. Yang, C. B. Shiu, W. K. Chang, and M. S. Wong, Appl. Surf. Sci. 252, 3729 (2006). 24 B. Cord and R. Courths, Surf. Sci. 162, 34 (1985). 25 S. A. Chambers, Y. Liang, Z. Yu, R. Droopad, J. Ramdani, and K. Eisenbeiser, Appl. Phys. Lett. 77, 1662 (2000). 26 R.O’Connor, S. McDonnell, G. Hughes, and K.E. Smith, Surf. Sci. 600, 532 (2006). 27 X. Yu, O. Wilhelmi, H. O. Moser, S. V. Vidyaraj, X. Gao, A. T. S. Wee, T. Nyunt, H. Qian, and H. Zheng, J. Electron. Spectrsc. Relat. Phenom. 144-147, 1031 (2005). 28 C. D. Valentin, G. Pacchioni, A. Selloni, S. Livraghi, and E. Giamello, J. Phys. Chem. B 109, 11414 (2005); C. D. Valentin, G. Pacchioni, and A. Selloni, Phys. Rev. B. 70, 085116 (2004). 29 W. Ra, M. Nakayama, W. Cho, M. Wakihara, and Y. Uchimoto, Phys. Chem. Chem. Phys. 8, 882 (2006). 30 L. Soriano, M. Abbate, A. Fernández, A. R. González-Elipe, and J. M. Sanz, Surf. Interface. Anal. 25, 804 (1997). 177 Chapter Nitridation Treatment on High-k dielectrics Films 31 F. M. F. de Groot, J. Faber, J. J. M. Michiels, M. T. Czyżyk, M. Abbate, and J. C. Fuggle, Phys. Rev. B. 48, 2074 (1993). 32 J. G. Chen, Surf. Sci. Rep. 30, (1997). 33 S. Venkataraj, O. Kappertz, R. Jayavel, and M. Wuttig. J. Appl. Phys. 92, 2461 (2002). 34 M. Capizzi, and A. Frova, Phys. Rev. Lett. 25, 1298 (1970). 35 M. D. Segall, P. J. D. Lindan, M. J. Probert, C. J. Pickard, P. J. Hasnip, S. J. Clark, and M. C. Payne, J. Phys.: Condens. Matter. 14, 2717 (2002). 178 Chapter Conclusions and Future Work Chapter Conclusions and Future Work 6.1. Conclusions In this thesis, we have studied the growth and electronic structures of metal/high-k dielectrics/semiconductor system, including the LaAlO3/semiconductors (Si, Ge, Si0.75Ge0.25), Ni/high-k dielectrics (Ni/LaAlO3, Ni/MgO), and the nitridation of high-k dielectrics films (LaAlO3, SrTiO3). Firstly, the growth and characterization (e.g. electronic structure, thermal stability) of LaAlO3 films on various semiconductor substrates (Si, Ge, and Si0.75Ge0.25) were studied by high resolution transmission electron microscopy (HRTEM) and in situ x-ray photoelectron spectroscopy (XPS). The high enough band offsets ( > eV) between LaAlO3 films and various semiconductor substrates indicate that LaAlO3 dielectrics films is a promising candidate to be integrated with various high performance semiconductors in the new generation CMOSFETs devices. The effect of interfacial oxynitride layer on the band alignments and thermal stabilities of LaAlO3 films grown on Ge and Si0.75Ge0.25 were also studied. It was found that the interfacial oxynitride layer changed the band alignments slightly. The main reason is that oxynitride layer modified the interfacial structures and thus modified the interfacial dipoles, which directly influence the band alignments. This work indicates that nitridation treatment not only acts as a surface passivation layer but also changes the 179 Chapter Conclusions and Future Work interfacial electronic structures. These studies about the effect of interfacial oxynitride layer on the band alignments could directly benefit the application of nitridation treatments in the semiconductor industry. Secondly, Ni metal was chosen as a metal gate grown on LaAlO3 (001) and MgO (001) dielectrics. For Ni/LaAlO3 (001) system, good epitaxial relationship between Ni and LaAlO3 (001) was determined to be Ni(001)||LaAlO3(001) and Ni[001]||LaAlO3[001]. The effective work function of Ni on crystalline LaAlO3 was measured to be 5.15 eV, which is as same as its vacuum work function. Good epitaxial structure of Ni films with suitable effective work function indicates that Ni is a promising metal gates candidate integrated with LaAlO3 high-k dielectric films in PMOS devices. By studying of the evolution of Schottky barrier height (SBH) and Fermi level positions of this system, it was noticed that there is an upward band bending of Fermi level position at the initial Ni growth stage. The underlying mechanism was further understood by studying the interfacial electronic structure of Ni/MgO (001) system. In Ni/MgO (001) system, the growth of Ni films at different growth temperatures has been studied, and the epitaxial relationship at the interface was determined to be Ni(001)‖MgO(001) and Ni[010]‖MgO[010]. Misfit dislocations were found to relax the lattice mismatch. The evolution of Fermi level position and SBH of Ni/MgO with different interfacial structures was studied. It was found that upward band bending occurred at the initial Ni growth stage and Ni bulk properties recovered after 6~8 Å thickness. The measured SBH were strongly interface structure-dependent, with 180 Chapter Conclusions and Future Work variation in the range of 3.1 eV for perfect interface to 1.6 eV for defect-rich interface. Direct comparison between experimental results and theoretical calculation provided the physical mechanisms to understand the influence of interfacial structures on the electronics properties of metal/high-k dielectric oxides systems. Adatom-induced states (or interfacial bonding states), MIGS and defects states were used to rationalize the evolution of Fermi level and corresponding SBH in this metal/oxides system with various interface structures. This work implies that SBH can be engineered by interface structure control, and is expected to shed light on the effect of interface structures on the formation mechanism of SBH at metal/high-k dielectric oxides interfaces. Finally, the effect of nitridation on the electronic structures and thermal stabilities of high-k dielectrics films (LaAlO3, SrTiO3) was studied. The effect of nitridation on the electronic and thermal stabilities of amorphous LaAlO3 films grown on various semiconductors (Si, Ge, and Si0.75Ge0.25) was studied by in situ XPS. It was found that there are three types of bonds with N atoms: metal-N bonds, semi-N bonds and N-O bonds, irrespective of the semiconductor substrates. The chemical states and distribution of N states are strongly influenced by the annealing temperature. To further understand the mechanisms of the effect of nitrogen doping, we chose SrTiO3 films as a prototype to study the nitrogen-doped SrTiO3 (001) films in both experimental and theoretical ways. It was found that nitrogen doping not only can effectively passivate the oxygen vacancies in SrTiO3 films, but also can change the band structures of SrTiO3. There are both substitutional and interstitial N states 181 Chapter Conclusions and Future Work existing in nitrogen-doped SrTiO3 films. First-principles calculation shows that N 2p states localized above O 2p in the valence band edge and below Ti 3d in the conduction band region are attributed to the photoactivity of nitrogen-doped SrTiO3 in the visible light range. This work implies that the nitridation process should be well-controlled to optimize the performance of high-k dielectric films. 6.2. Future Work Currently, for most high-k dielectrics used in the form of amorphous, the crystallization is a serious issue to be solved. In the high performance devices, the gate dielectric scaling may be more aggressively achieved by using the crystalline dielectrics. However, there is still a long way to go to realize the integration of epitaxial high-k dielectrics with semiconductors. In this thesis, LaAlO3 films have been epitaxially grown on Si with an ultrathin epitaxial SrTiO3 layer using molecular beam epitaxy (MBE). In the future work, more work can be attempted to deposit epitaxial LaAlO3 films directly on other semiconductors. Proper growth conditions, such as the pressure of oxygen ambient, the growth temperatures, post-annealing temperature and time, should be chosen carefully to achieve these goals. Further studies on the growth and characterization of epitaxial LaAlO3 films on other semiconductors will be an attractive subject with the further downscaling of CMOSFETs devices. According to our study, it was found that interfacial oxynitride layer not only acts as a passivation layer but also changes the electronic structures of dielectrics. Studies on the effect of the interfacial oxynitride layer on the electronic performance of high-k 182 Chapter Conclusions and Future Work dielectrics are recommended in the future work. To accomplish this, C-V and I-V measurement are highly suggested in metal/LaAlO3/Ge (or SixGey) system with and without interfacial oxynitride layer. Meanwhile, the effect of nitrogen incorporation on the electronic performance of high-k dielectrics (LaAlO3) is also worthy to be studied. The possible investigations on fixed oxide charges and interface trapped charge density of LaAlO3 high-k dielectrics can be achieved by using C-V and I-V measurements. This work will provide comprehensive information about the effect of nitridation on high-k dielectrics films and benefit the application of nitridation treatment in the improvement of electronic performance of high-k dielectrics. Choosing proper metal gate materials integrated with high-k dielectrics, particularly in PMOS devices, is considered as the major challenges in the application of metal/high-k dielectrics. Although some encouraging results were shown in this thesis, we readily acknowledge that our study is only based on prototypes of metal gate and high-k dielectrics materials, e.g. epitaxial Ni metal grown on LaAlO3 (001) and MgO (001) films, which represents ideal situations compared with the much more complicated situations in real metal gates/high-k materials applications. Future work is needed to identify to what extent the results (e.g., the interfacial electronic structures, thermal stabilities et al.) found here can be applied in more metal gate/high-k dielectrics systems. 183 [...]... dielectrics interfaces Studying the formation mechanisms of SBH at metal/ high- k dielectrics on atomic scale is one of the main objectives in this thesis To fully understand the formation mechanisms of SBH at metal/ high- k dielectrics, it is essential to measure the effective work function of metal gates (Φm,eff ) or SBH accurately The most widely used method to extract the effective work function of metal gates. .. level pinning would be found This situation applies to most metal on covalent or small gap semiconductors (Si, Ge, and GaAs).64 However, most high- k dielectrics are large band gap ionic semiconductors or insulators, the decay length of MIGS is rather short In this situation, it is expected that interface bonding instead of MIGS would play a dominant role in determining the SBH of metal/ high- k dielectrics. .. effective work function of metal gates should be tunable in a range of ~ 1.1 eV (the magnitude of Si band gap) 1.4.1 Band offsets at high- k dielectrics/ semiconductor interfaces The potential barrier for the injection of electrons and holes into the oxides side is defined by the conduction band offset (CBO) and valence band offset (VBO) between high- k dielectrics and semiconductor, respectively Band offsets... energy bands are aligned only using the electron affinity of the materials in the two sides, and in this case, vacuum levels are continuous.53 However, this is not common situation in metal/ semiconductors contacts According to Fig 1.2, the effective work function can be directly determined by barrier heights of metal/ high- k dielectrics interface and high- k dielectrics /silicon interface separately, expressed... 13 Chapter 1 Introduction effectiveness of defects states on pinning Fermi level at a metal/ semiconductor interface is based on the density of defects at interfaces.59 In defects-rich interfaces, the defects states will be the dominating factor determining the position of Fermi level In this thesis, to study the effect of the defect states on the electronic properties at metal/ oxides interfaces, we... semiconductors (Si, Ge, SiGe) will be investigated by using XPS methods Meanwhile, the effect of interfacial structures on the band alignment in such systems is also studied It is expected to provide a further understanding of formation mechanism of band offsets at high- k dielectrics/ semiconductor interfaces 1.4.2 Schottky barrier height for metal gate /high- k dielectrics interfaces While metal gate shows its advantages... is negligible within the interface-specific region (several nanometers) For a given high- k dielectrics/ Si interface, the effective work function is solely determined by the Schottky barrier height (SBH) at metal gate /high- k dielectrics interface, which is defined as the energy separation between the Fermi level of metal and the edge of the majority carrier band of oxide right at the interface.54 However,... alignments), the effect of interfacial structures and underlying microscopic mechanisms will be further studied This work would provide useful information for the choice of metal gates on LaAlO3 dielectrics Meanwhile, this work is expected to provide valuable information for better understanding the formation mechanisms of SBH at metal/ high- k dielectrics interfaces To study the effect of nitridation on the... mechanism of SBH at metal gate /high- k dielectrics interface, namely metal/ semiconductor interfaces, has been a subject of debates for decades.49 According to the first model describing the properties of metal/ semiconductor interfaces, which was proposed by Schottky55and Mott56, the SBH is related to the difference between the vacuum metal work function (Φm,vac) and the electron affinity of semiconductor... the intrinsic properties of semiconductor and the 10 Chapter 1 Introduction other is the specific interfacial bonding.48,49 The physical model for describing the intrinsic component of band offset is based on metal induced gap states (MIGS),50,51 which origins from the metal/ semiconductor contacts The details of this model will be introduced in section 1.4.2 This model can be applied to the band offsets . STUDY OF METAL GATES AND HIGH-K DIELECTRICS IN NANOELECTRONICS MI YANYU NATIONAL UNIVERSITY OF SINGAPORE 2007 STUDY OF METAL GATES AND HIGH-K DIELECTRICS IN NANOELECTRONICS. Publications Chapter 1 Introduction 1.1 MOSFET scaling 1.2 High-k dielectrics 1.3 Metal gates 1.4 Band alignments 1.4.1. Band offsets at high-k dielectrics/ semiconductor interfaces 1.4.2. Schottky. requires the replacement of conventional poly-silicon (poly-Si) gate with metal gates. To achieve this goal, in this thesis, the integration of metal gates and high-k dielectrics materials with