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Ptse2 and Hfse2: New Transition Metal Dichalcogenides for Switching Device Applications

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Ptse2 and Hfse2: New Transition Metal Dichalcogenides for Switching Device Applications PtSe2 and HfSe2: New Transition Metal Dichalcogenides for Switching Device Applications by AbdulAziz AlMutairi A thesis presented to the University of Waterloo in fulfillment of the thesis requirement for the degree of Master of Applied Science in Electrical and Computer Engineering - Nanotechnology Waterloo, Ontario, Canada, 2018 © AbdulAziz AlMutairi 2018 AUTHOR'S DECLARATION I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis, including any required final revisions, as accepted by my examiners. I understand that my thesis may be made electronically available to the public. ii Abstract Recently, silicon-based complementary metal-oxide-semiconductor (CMOS) technology has been struggling in keeping the continuous improvement predicted by Moore’s Law. Hence, significant efforts have been made to find alternatives to the conventional silicon technology. Nanoelectronics based on two-dimensional (2D) materials, such as black phosphorus (BP) and molybdenum disulfide (MoS2), have demonstrated great potential for electronic devices due to their intriguing mechanical, optical and electrical properties. In this thesis, two of novel 2D materials among the transition metal dichalcogenides (TMDs) family have been explored for the use in nanoelectronic devices – platinum diselenide (PtSe2) and hafnium diselenide (HfSe2). It was reported earlier that PtSe2 and HfSe2 exhibit higher carrier mobilities among PtX2 and HfX2 families. First-principle simulations and atomistic quantum transport simulations based on the non-equilibrium Green’s function (NEGF) method within a tight-binding (TB) approximation are used to study PtSe2 and HfSe2 and their device applications. Despite the fact that PtSe2 has a relatively small electron effective mass (0.21m0), its six conduction valleys in the first Brillouin zone give rise to relativity large density of states (DOS). As a result, compared to its molybdenum diselenide (MoSe2) counterpart, PtSe2 field-effect transistors (FETs) exhibit better on-states characteristics (>30%) while maintaining a near-ideal subthreshold swing (SS) of ~64 mV/dec. The scaling study of the channel length (Lch) and the equivalent oxide thickness (EOT) show that a near ideal SS can be persevered with channel lengths longer than 15 nm or through aggressive scaling of the gate oxide (e.g., EOT = 0.4 nm). On the other hand, HfSe2 FETs exhibit nearly identical symmetric transfer characteristics for n-type (NMOS) and p-type transistors (PMOS) despite its asymmetrical effective mass and DOS in the conduction and the valence band. Both exhibits steep switching (<70 mV/dec) with exceptional on- current (~1 mA/µm). Through the scaling study, it was revealed that HfSe2 FETs exhibit great immunity to short-channel effects (SCE) at Lch ≥ 15 nm, but show significant degradations in subthreshold swing and drain-induced barrier lowering at sub-10 nm channel even with a thin gate dielectric. Finally, both NMOS and PMOS HfSe2 devices exhibited excellent intrinsic device performance, making them promising candidates for future logic device applications. iii Acknowledgements First and foremost, all thanks and graduates are due to Allah for providing me with the strength and blessing to complete the presented work. So, Alhamdulillah for offering me the chance to contribute and enrich the knowledge of mankind. I, also, would like to express my gratitude to my respected supervisor Professor Youngki Yoon for giving me the chance to join his research group, and for his support and help throughout my master’s study. His feedbacks and guidance helped me develop my academic achievement and research to the point it is now. My parents support and prayers in the past two years was essential for me to complete my degree. For that, I would like to thank them for all the supports and prayers. I also want to extend my thanks to my siblings, uncles, aunts and my grandmother for their support and prayers, too. I would like to thank my group members, Gyu Chull Han, Demin Yin, Yiju Zhao and Hyunjae Lee for all the help they provided me during my study for the master’s degree. In addition, I would like to express my sincere gratitude to my friends, Ahmed AlQurashi, Hasaan Waseem, Amr Abdelgawad, Dawood Alsaeidi, Abdulrahman Aloraynan, and Harish Krishnakumar for the insightful helps and discussions. Special thanks are given to my neighbor and friend Khalfan AlMarzooqi for providing me with his knowledge and experience while doing his PhD. He has been and will always be a friend that I cherish. I would also like to thank professors Yuning Li and Chettypalayam Selvakumar for providing me with insightful feedbacks about my work. Lastly, I would like to acknowledge the financial support from Graduate Research Studentship from University and the Nanofellowship from Waterloo Institute for Nanotechnology. iv Dedication This thesis is dedicated to Islam, parents and my siblings. v Table of Contents AUTHOR'S DECLARATION ...................................................................................................... ii Abstract ....................................................................................................................................... iii Acknowledgements ...................................................................................................................... iv Dedication .....................................................................................................................................v Table of Contents ......................................................................................................................... vi List of Figures............................................................................................................................ viii Chapter 1 Introduction ...................................................................................................................1 Chapter 2 Parameterization for Non-Equilibrium Green’s Function (NEGF) Simulation ............... 10 2.1 Density Functional Theory (DFT) ....................................................................................... 10 2.1.1 DFT Simulation for Platinum Diselenide (PtSe2) .......................................................... 20 2.1.2 DFT Simulation for Hafnium Diselenide (HfSe2) ......................................................... 22 2.2 Tight-Binding (TB) Approximation .................................................................................... 23 2.2.1 Slater-Koster Tight-Binding Parametrization................................................................ 23 2.2.2 Maximally Localized Wannier Functions (MLWFs) Tight-Binding .............................. 26 2.2.3 TB parametrization for Platinum Diselenide (PtSe2) ..................................................... 32 2.2.4 TB parametrization for Hafnium Diselenide (HfSe2) .................................................... 34 2.3 Non-Equilibrium Green’s Function ..................................................................................... 36 Chapter 3 Platinum Diselenide (PtSe2) ......................................................................................... 42 3.1 Motivation.......................................................................................................................... 42 3.2 Simulation Approach .......................................................................................................... 43 3.3 Results ............................................................................................................................... 46 3.4 Summary ............................................................................................................................ 49 Chapter 4 Hafnium Diselenide (HfSe2) ......................................................................................... 50 4.1 Motivation.......................................................................................................................... 50 4.2 Simulation Approach .......................................................................................................... 50 4.3 Results ............................................................................................................................... 52 4.4 Summary ............................................................................................................................ 56 Chapter 5 Conclusions and Future Work ...................................................................................... 57 5.1 Conclusions ........................................................................................................................ 57 5.2 Future Work ....................................................................................................................... 58 Bibliography ................................................................................................................................ 59 vi Appendix A PtSe2 Tight-Binding Parameters ............................................................................... 65 Appendix B HfSe2 Tight-Binding Parameters .............................................................................. 97 vii List of Figures Figure 1.1: Transistors number per chip for the past few decays and their clock speeds in MHz. It displays great deal of agreement with Moore’s law. However, clock speeds have been saturated since 2004 [3]. ............................................................................................................................................1
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