Fabrication and characterization of single-layer graphene transferred on hexagonal boron nitride

Abstract

Graphene, a two-dimensional single atomic layer of graphite, has emerged as a promising material with interesting physical and chemical properties and high potential for various applications such as sensors, transparent electrodes and electronics. Due to its high carrier mobility, graphene has gained much interest as a possible candidate to extend beyond silicon complementary metal-oxide-semiconductor technology for future nano-electronic devices. In this thesis, two different kinds of graphene field-effect transistors (G-FETs) have been fabricated using exfoliated single-layer graphene (SLG). These two kinds of G-FETs are used for transport measurement and capacitance measurement respectively. The electronic properties of graphene have been investigated through measurements of the mobilities, quantum Hall effects, Shubnikov-de-Haas oscillations and quantum capacitance of these G-FETs. Properties of graphene affected by resonant impurities and other different kinds of impurities are first investigated. We have successfully introduced silver adatoms to graphene as resonant impurities and observed the induced midgap states. In order to have a clearer picture of resonant impurities, it is very necessary to minimize or even exclude the effects that come from the induced electron-hole puddles and charged impurities by the substrate SiO₂ and the dielectric layers of Y₂O₃ used for the G-FET devices. Thus h-BN is introduced since h-BN can largely improve the performance of the G-FETs and reflect the intrinsic graphene’s electronic properties. Meanwhile, the technique of transferring h-BN onto graphene opens the possibility to fabricate more complex 3-dimensional structures using 2-dimensional materials, which is a very hot topic recently in the field of 2D material research. I have independently developed three different approaches to fabricate single-layer graphene-hexagonal boron nitride (SLG/h-BN ) structure, namely the wet-transfer approach, the direct-transfer approach and the dry-transfer approach. Through transferring, I fabricated transport G-FETs using h-BN as substrates and capacitance G-FETs using h-BN as the top-gate insulators. Pronounced improvement is observed in the performance of both transport G-FETs and capacitance G-FETs. From the transport measurement of the transferred G-FETs, the mobility of graphene is prominently enhanced compared to that of SLG/SiO₂ samples. From the capacitance measurement of the transferred G-FETs, Landau level quantization is observed clearly. Comparing to the capacitance G-FETs using other top-gate insulator materials such as Y₂O₃ and Al₂O₃, Landau level quantization is normally suppressed by the insulator-induced electron-hole puddles and charged impurities. The improvement in the performance of transferred G-FETs implies that h-BN introduces less electron-hole puddles and charged impurities to graphene in comparison with other materials such as SiO₂, Y₂O₃ and Al₂O₃ . Consequently, it is a good material for the fabrication of future graphene electronics.

Date of Award	2013
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology