First-Principles Study of the Doping Effect on MoS2 from the Impurities and Intrinsic Point Defects in the underlying SiO2 Substrate and the Origin of the Enhanced Carrier Mobility by the Top High-k HfO2 Layers
Date Issued
2015
Date
2015
Author(s)
Ho, An
Abstract
MoS2 is a promising candidate for the new nano-electronic devices primarily due to its outstanding physical and electronic properties. In practical applications, monolayer MoS2 has been successfully integrated into a MOS transistor showing a mobility of >200 cm2/V·s with an on/off current ratio >108. However, the MoS2-based transistors still have some disadvantages that require further improvements to uplift their performance. Therefore, it would be of great interest to develop detailed atomistic understanding of this material system in many fundamental aspects, particularly for the interactions between MoS2 and the insulating dielectric substrates. In the first part of the thesis, we employed first-principles calculations to explore the origins of the n-type doping effect on MoS2 from the underlying SiO2 substrate. We first constructed various structure models of a-SiO2 containing boron, sodium and the relevant Si point defects in the substrates and then investigate the electronic structure changes of the MoS2/SiO2 hybrid system. Our calculated results show that B atoms can form various stable bonding configurations such as •BO-SiO3, •SiO2-BO2, SiO3-BO2 and B2O3 with the associated Si point defects like E’ and S centers in a-SiO2. Furthermore, those dopant configurations and the associated Si point defects can induce the formation of electronic defect states in the band gap of SiO2, some of which can be effective electron donors inducing electron transfer from a-SiO2 to monolayer MoS2. We also found that this n-type doping effect can be much enhanced by the appearance of S vacancies in MoS2 mainly attributed to the induced unoccupied defect states at ~0.7 eV below its conduction band minimum. On the other hand, Na atom in a-SiO2 was found to be an effective electron donor on MoS2 as well. Nevertheless, since Na generally appears as Na+ ions in a-SiO2 glasses, it is not expected to be the major source contributing to the n-type conducting behavior in MoS2 monolayer. We next investigated the effects of the N-, P-, and As-doped SiO2 substrates on the electronic property changes of MoS2 monolayer. Our calculations unambiguously show that some of the bonding configurations of N and As atoms with the associated Si point defects in a-SiO2 can become effective electron acceptors for monolayer MoS2, providing one possible route to fabricate the MoS2-based p-MOSFET. Nevertheless, the P-doped SiO2 is not possible to induce any p-type doping on MoS2 monolayer though As and P are both the Group VA elements in the periodic table. In the second part of the thesis, we intend to reveal the physical origin of the enhanced carrier mobility in MoS2 by the deposition of the top high-k HfO2 layers. We first investigated the influence of HfO2 layer on the electronic property of monolayer MoS2, and then probed for the effect of the induced electronic doping on the dielectric property changes of MoS2 as well as the contact resistance between the S/D metal and MoS2 layer. Our results show that perfect HfO2 layer has no significant effect on the electronic property change of MoS2, but as O vacancy is present in the high-k layer, it was found to induce significant amount of n-type doping on monolayer MoS2. Our calculations further show that the induced n-type doping from HfO2 can effectively reduce the Schottky barrier height between the S/D metal and monolayer MoS2, thereby largely enhancing the electron mobility in the MoS2-based MOS transistor. In addition, our calculations show that the dielectric screening along the z axis (�驔z) of MoS2 is nearly unchanged upon n-type doping. Therefore, the reduction of coulomb scattering arising from the charge traps in SiO2 cannot be used to account for the enhanced electron mobility in the MoS2 channel by the deposited HfO2 layer.
Subjects
MoS2
first-principles calculation
SiO2
HfO2
Schottky barrier
Type
thesis
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