First principles calculations and in situ characterizations

Surface passivation and high-κ dielectrics integration of Ge-based FETs: First-principles calculations and in situ characterizations YANG MING (B.Sc., Fujian Normal University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2009 Acknowledgements I would like to thank my advisors, Prof. Feng Yuanping and Dr. Wang Shijie, for their guidance, unwavering support, and encouragement throughout my study and research. Prof. Feng and Dr. Wang taught me many about the research, and also shared me their wisdom, insight, and humor during these years. It has been a great experience to study under their guidance. Special thanks to Dr. Chai Jianwei for his help in experiments. Also to my group members: Dr. Sun Yiyang, Dr. Liu Lei, Dr. Wu Rongqin, Dr. Pan Hui, Dr. Dong Yufeng, and Dr. Peng Guowen (First-principle calculations guidance); Dr. Mi Yanyu (TEM sample preparations); Dr. Lu Yunhao, Shen Lei, Sha Zhendong, Cai Yongqin, Chen Qian, and Deng Wensheng for valuable discussions and happy time spent together. I acknowledge National University of Singapore for the research scholarship, which enables me to conduct my research projects and finish this thesis. Finally, I would like to express my deep appreciation to my parents for their unselfish love and constant support throughout my life. i Table of Contents Acknowledgements i Abstract vi Publications ix List of Tables xii List of Figures xiii Introduction 1.1 Scaling Si-based MOSFETs . . . . . . . . . . . . . . . . . . . . . . 1.2 Ge-FETs and Ge surface passivation . . . . . . . . . . . . . . . . . 1.3 The integration of high-κ dielectrics on Ge-FETs . . . . . . . . . . 10 1.3.1 Introduction to high-κ dielectrics . . . . . . . . . . . . . . . 10 1.3.2 Band offsets at high-κ dielectrics/Ge interface . . . . . . . . 14 Motivations and scope for present work . . . . . . . . . . . . . . . . 17 1.4 Methodology 2.1 20 Thin film growth techniques . . . . . . . . . . . . . . . . . . . . . . 21 2.1.1 21 Atomic source oxidation and nitridation . . . . . . . . . . . ii 2.1.2 2.2 2.3 Pulsed laser deposition . . . . . . . . . . . . . . . . . . . . . 22 Characterization techniques . . . . . . . . . . . . . . . . . . . . . . 24 2.2.1 X-ray photoemission spectroscopy . . . . . . . . . . . . . . . 24 2.2.2 Transmission electron microscopy . . . . . . . . . . . . . . . 28 First-principles calculations . . . . . . . . . . . . . . . . . . . . . . 31 2.3.1 Earlier approximation and density functional theory . . . . . 32 2.3.2 The exchange-correlation functional approximation . . . . . 35 2.3.3 Bloch’s theorem and supercell approximation . . . . . . . . . 36 2.3.4 Brillouin zone sampling . . . . . . . . . . . . . . . . . . . . . 38 2.3.5 Plane-wave basis sets . . . . . . . . . . . . . . . . . . . . . . 40 2.3.6 The pseudopotential approximation . . . . . . . . . . . . . . 40 2.3.7 VASP and CASTEP . . . . . . . . . . . . . . . . . . . . . . 43 Interface properties of GeO2 /Ge (001) 44 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 3.2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.3 Band alignments at GeO2 /Ge interface . . . . . . . . . . . . . . . . 47 3.3.1 High quality GeO2 on Ge (001) substrates . . . . . . . . . . 48 3.3.2 Band alignments at GeO2 /Ge(001) interface . . . . . . . . . 50 3.3.3 Impact of oxide defects on the band alignments . . . . . . . 52 3.4 Effects of nitrogen incorporation on band alignments and thermal stability at GeO2 /Ge interface . . . . . . . . . . . . . . . . . . . . . 56 3.4.1 Incorporated nitrogen into GeO2 57 3.4.2 Effects of nitrogen incorporation on band alignments at GeO2 /Ge . . . . . . . . . . . . . . . interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 iii 3.4.3 3.5 Effects of nitrogen incorporation on thermal stability at GeO2 /Ge interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Chapter summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Electronic and defect properties of Ge3 N4 67 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4.3 Electronic properties of bulk Ge3 N4 . . . . . . . . . . . . . . . . . . 70 4.3.1 Structural properties of Ge3 N4 . . . . . . . . . . . . . . . . . 70 4.3.2 Electronic properties of Ge3 N4 . . . . . . . . . . . . . . . . . 72 4.3.3 Optical properties of Ge3 N4 . . . . . . . . . . . . . . . . . . 76 Intrinsic defect properties of Ge3 N4 . . . . . . . . . . . . . . . . . . 79 4.4.1 Formation energy of defects in β-Ge3 N4 . . . . . . . . . . . . 79 4.4.2 Defect passivation methods . . . . . . . . . . . . . . . . . . 83 Chapter summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 4.4 4.5 Interface properties of Ge3 N4 /Ge(111) 87 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 5.2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 5.3 Experimental study of crystalline Ge3 N4 on Ge (111) . . . . . . . . 90 5.3.1 Growth of crystalline Ge3 N4 on Ge . . . . . . . . . . . . . . 90 5.3.2 Band alignments at crystalline Ge3 N4 /Ge interface . . . . . 92 Theoretical study of β-Ge3 N4 (0001)/Ge (111) interfaces . . . . . . 95 5.4.1 Interface models and energetics of β-Ge3 N4 (0001)/Ge (111) 95 5.4.2 Electronic properties at the β-Ge3 N4 (0001)/Ge (111) interface 98 5.4 iv 5.4.3 The calculated band offsets at β-Ge3 N4 (0001)/Ge (111) interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 5.4.4 5.5 Effects of dangling bonds on interface properties . . . . . . . 103 Chapter summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Interface properties of high-κ dielectric SrZrO3 on Ge (001) 107 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 6.2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 6.3 Experimental study of interface properties of SrZrO3 /Ge . . . . . . 110 6.4 6.5 6.3.1 Growth of SrZrO3 on Ge (001) . . . . . . . . . . . . . . . . . 110 6.3.2 Band alignments at the interface of SrZrO3 /Ge (001) . . . . 112 6.3.3 Thermal stability at the interface of SrZrO3 /Ge (001) . . . . 114 First-principles study of SrZrO3 /Ge interface . . . . . . . . . . . . . 118 6.4.1 Interface structures and energetics . . . . . . . . . . . . . . . 118 6.4.2 Electronic properties at the interface of SrZrO3 /Ge (001) . . 123 6.4.3 Tuning band offsets of SrZrO3 /Ge (001) interface . . . . . . 128 Chapter summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 Conclusion remarks 132 7.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 7.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 References 138 v Abstract Surface passivation and high-κ dielectrics integration are two critical issues for fabricating high performance Ge-based CMOS devices. In this thesis, first-principles calculations and experimental characterizations such as XPS and HRTEM were used to study these two issues. Atomic source oxidation was used to grow stoichiometric and good quality GeO2 dielectrics on Ge (001) substrates. The XPS measured VBO and CBO at this GeO2 /Ge interface are 4.59 and 0.54 eV, respectively. The calculated PDOS indicates that oxygen and Ge vacancies formed at different oxidation stages may cause the reduction of VBO at reduced GeOx /Ge(001) interface, which clarified the large difference of VBO determined by XPS directly and extracted from highκ/GeOx /Ge stacks. In addition, it was found that the VBO at GeOx Ny /Ge interface decreases with increasing doped nitrogen concentrations in GeO2 thin films, while the thermal stability slightly increases. These provide us an effective way to tune band offsets and thermal stability at GeOx Ny /Ge/Ge interface. First-principles calculations were carried out to study electronic, optical, and intrinsic defect properties of bulk Ge3 N4 . It was found that lattice constants of vi β-Ge3 N4 match well with those of Ge (111) 2×2 surface. The calculated band gap and dielectric constant of Ge3 N4 can satisfy the requirements of gate dielectrics as well. Furthermore, it is expected that nitrogen vacancies would be the main source of intrinsic defects in Ge3 N4 due to their low formation energies. These nitrogen vacancies might become charge trapping centers because their energy levels are close to Ge conduction band edge. The calculations suggest that to grow Ge3 N4 in nitrogen rich ambient would reduce nitrogen vacancies in Ge3 N4 thin films. Besides, the calculation results also indicate that to deposit a thin layer of Si on Ge surface before nitridation process is another effective way to decrease nitrogen vacancies. Two interface structures were proposed for β-Ge3 N4 (0001)/Ge (111), and the calculated interface formation energies indicate that the interface structure without dangling bonds are much more energetically favorable. This stable interface structure is contributable to its perfect interface bonding structure and strong Ge-N bonds at the interface. The calculated VBO and CBO at this stable interface are 1.23 and 2.10 eV, respectively. The calculations also indicate that dangling bonds at interface would induce interface gap states, and reduce the VBO. Hydrogen saturated interface exhibits better interface properties, but Ge-H bonds at the interface are unstable due to their low dissociation energies. Experimentally, atomic source nitridation was used to grow crystalline Ge3 N4 on Ge (111) substrate at 400◦ C, which was verified by HRTEM images. The band offsets at this Ge3 N4 /Ge (111) interface determined by XPS are consistent with the theoretical predictions. Experimentally, SrZrO3 thin films were prepared on Ge (001) substrate by using PLD. The corresponding VBO and CBO at this interface were measured by vii XPS to be 3.26 eV and 1.77 eV, respectively. The SrZrO3 on Ge substrate remains stable after annealing at 600◦ C, compatible to Ge fabrication process, but the HRTEM images indicate that the amorphous SrZrO3 thin films would become polycrystalline after annealing. Theoretically, it was found that SrZrO3 (001) surface matches well with that of Ge (001) in terms of surface symmetry and lattice constants, and various interface structures of cubic SrZrO3 (001)/Ge (001) were proposed. The calculated interface formation energies show that Zr-O terminated interface is more stable in oxygen rich ambient. This suggests that to grow SrZrO3 with Zr-O terminated surface on Ge surface in oxygen-rich ambient might be more favorable to realize the epitaxial growth. The calculated band offsets are larger than 1.0 eV, and it was also found that oxygen chemical potential affects the band offsets and interface stability greatly. viii Publications [1] M. Yang, R. Q. Wu, W. S. Deng, Y. P. Feng, S. J. Wang, and C. M. Ng, “Interfacial bonding and energetics at SrZrO3 /Ge (001) interface”, submitted to Appl. Phys. Lett. [2] M. Yang, R. Q. Wu, W. S. Deng, Y. P. Feng, and S. J. Wang, “Nitrogen tuned electronic and thermal properties of GeOx Ny /Ge”, submitted to J. Appl. Phys. [3] M. Yang, W. S. Deng, Q. Chen, Y. P. Feng, L. M. Wong, J. W. Chai, J. S. Pan, S. J. Wang, and C. M. Ng, “Band alignments and thermal stability at the interface of SrZrO3 /Ge (001)”, submitted to Appl. Phys. Lett. [4] M. Yang, R. Q. Wu, Q. Chen, W. S. Deng, Y. P. Feng, J. W. Chai, J. S. Pan, and S. J. Wang, “Impacts of oxygen defects on band alignments at GeO2 /Ge interface”, Appl. Phys. Lett. 94, 142903 (2009). [5] M. Yang, R. Q. Wu, W. S. Deng, L. Shen, Z. D. Sha, Y. Q. Cai, Y. P. Feng, and S. J. Wang, “Electronic structures of β-Si3 N4 (0001)/Si (111) interfaces: Perfect bonding and dangling bond effects”, J. Appl. Phys. 105, 024108 (2009). [6] M. Yang, G. W. Peng, R. Q. Wu, W. S. Deng, L. Shen, Q. Chen, Y. P. Feng, J. W. Chai, J. S. Pan, and S. J. Wang, “Interface properties of Ge3 N4 /Ge (111): ix Chapter 7. Conclusion remarks 7.2 Future work The real interfaces of passivation and high-κ materials on Ge are much more complicated than the prototypes (e.g., the crystalline interface at high-κ dielectrics/Ge) we used in calculations. Thus, further research is needed to explore the extent to which these ideal results (e.g., the dependence of band offsets on atomic interface structures) can be applied to experimental crystalline or even amorphous interfaces. To better understand the amorphous interface of high-κ dielectrics/Ge, calculation work to model the amorphous interface of high-κ dielectrics and Ge is greatly needed. Another direction for future work is to extend the present defect-free interfaces to interfaces with defects such as oxygen vacancies, interstitials, or their complexes considering the fact that there are more defects in high-κ oxides than SiO2 . [3, 6] Moreover, the work of GW calculation [131] to obtain more accurate band gaps is also desirable since the conventional calculations based on LDA underestimates band gaps of semiconductors or insulators to some extent. Besides, from our calculations [132], it can be predicted that nitrogen vacancies would be a problem for application, so further calculations on how to passivate the nitrogen vacancies is also needed. In terms of experimental work, further study on fabricating high quality of Ge3 N4 thin films on Ge and the characterizing there electrical properties such as I-V or CV is desired. Another direction for future work is to study how to passivate nitrogen vacancies in Ge3 N4 thin films. 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Lett. 88, 192103 (2006). 150 Surface passivation and high-κ dielectrics integration of Ge-based FETs: First-principles calculations and in situ characterizations YANG MING NATIONAL UNIVERSITY OF SINGAPORE 2009 [...]... that the band offsets at high-κ dielectrics/Ge interface are asymmetric, and incorporating an interfacial layer at the interface may tune band offsets Theoretically, the procedure of obtaining band offset by using first-principle calculations is actually similar to that of using XPS to determine band offsets mentioned 15 Chapter 1 Introduction above Typically, in XPS, the core level and valence band spectra... different interface net dipoles, they obtained sizable changes of VBO at SrTiO3 /Si interfaces This is very encouraging and has wide applications in engineering high-κ/Si interfaces, as well as high-κ/Ge interfaces However, few studies have been carried on interface engineering at crystalline high-κ dielectrics/Ge interfaces In this thesis, we will show how interface bonding structures and strain of high-κ... (GeO2 and Ge3 N4 ), and to study, both experimentally (using XPS and TEM) and theoretically (using first -principles calculations) , the electronic, intrinsic defect, and interface properties and thermal stability of the surface passivations/Ge stacks (ii) To grow good quality high-κ dielectrics (SrZrO3 ) on Ge surface by pulsed Laser deposition (PLD), and to theoretically and experimentally investigate interface... stability and band offsets at the interface And, the possibility of chemically tuning interface properties of SrZrO3 /Ge was explored by firstprinciples calculations also The efforts of studying GeO2 and Ge3 N4 based Ge surface passivation materials and integrating the high-κ dielectric SrZrO3 on Ge channel should provide a fundamental theoretical understanding of the electronic properties and interface... /Si is insufficient enough, which precludes the possibility of using SrTiO3 as gate dielectric on Si substrate if there were no atomic interface engineering methods to increase the CBO For band offsets at high-κ dielectrics/Ge interface, Afanas’ev et al determined the band alignments at the HfO2 /Ge interface using IP spectroscopy, and the correspond VBO and CBO is 3.0 and 2.0 eV, respectively [29] Band... are measured across the interface; independent measurements on substrate bulk samples are performed to obtain valence-band edge and the core levels, which is then used to line up the valence bands and obtain the band offsets All-electron calculations can actually mimic this approach, and provide information about core-level lineups as well as band offsets Although pseudopotential calculations cannot directly... determine the band offsets at crystalline interfaces, such as ZrO2 /Si [69, 70] From Eq (1.4), the VBO at interface is affected by interface properties also excluding the independent bulk effects, which means that we may engineer the band-offset for some specific materials by using interface strain (uniaxial deformation and/ or lattice distortions) and interface chemical effects (i.e different interface chemical... essential for growing crystalline high-κ dielectrics on Ge with high quality interface or engineering the interface to obtain the desirable properties 13 Chapter 1 Introduction CBM CBO CBM E -oxide E g g VBM Ge VBO Oxide VBM Figure 1.3: Schematic band diagram for high-κ dielectrics and Ge Definitions of band offsets (VBO and CBO) are shown 1.3.2 Band offsets at high-κ dielectrics/Ge interface The band offsets... are still limited, and the optical dielectric and intrinsic defect properties of Ge3 N4 have not been well understood yet Moreover, it is highly favorable to obtain more information about the interface properties of Ge3 N4 /Ge such as atomic interface structures, interface stability, and band alignments 9 Chapter 1 Introduction 1.3 The integration of high-κ dielectrics on GeFETs Scaling technology plays... challenging but highly desirable because the low-κ oxide interfacial layer would be thermally unstable, and also could increase EOT, which will no longer be tolerable for long-term applications The long-standing problem of epitaxially growing high-κ oxides on Si substrate was partially solved by Mckee et al [58] In their pioneering work, alkaline earth and perovskite oxides were grown in perfect crystalline . Surface passivation and high-κ dielectrics integration of Ge-based FETs: First-principles calculations and in situ characterizations YANG MING (B.Sc., Fujian Normal University) A. respectively. The calculations also indicate that dangling bonds at interface would induce interface gap states, and reduce the VBO. Hydrogen saturated interface exhibits better interface properties,. lines) and pseudoelectron (dash lines) potentials and their corresponding wavefunctions. . . . 41 3.1 The XPS survey spectrum of Ge peaks in GeO 2 /p-Ge(001) thin films. 48 3.2 The original and