Quantum transport in two-dimensional electron system: 1.Transition dynamics in the electrical breakdown of the integer quantum Hall effect 2.Disorder induced scattering in chemical vapor deposited graphene

Abstract

We study the electric transport of two different two-dimensional electron systems: graphene, and two-dimensional electron gas (2DEG) formed between the interface of semiconductor heterostructure. 2DEG has been widely applied to both academic researches and industrial uses due to its high mobility and constructability to lower dimensional electronic system. Graphene is an intrinsic two-dimensional electronic system consisting of a monolayer carbon atoms hybridized in honeycomb lattice. This lattice structure gives rise to many special electronic and optical properties, which raise general interest in the field. This thesis presents two research interests which are the transition dynamics in the electrical breakdown of the integer quantum Hall effect (QHE) in 2DEG system, and the disorder induced scattering in chemical vapor deposited graphene. QHE is an important feature in two-dimensional electronic system, we investigate the dynamic properties of the breakdown of integer quantum Hall states (QHSs) to study the mechanism of the QHE. With systematically study on the critical field of the breakdown with different scan rates of the applied Hall field, we observe bistable nature of the breakdown phenomena. We find the Hall field dependent escape rate between the low-dissipation QHS and the dissipation state ranges from a few seconds to 10 μs in bistable regime. The results are consistent with the simulation based the bootstrap electron heating model. This suggests that the dynamic behaviors are governed by the lifetime and the applied experimental setup condition. Graphene is a novel material and has great potential on application owing to its special transport properties. Nevertheless, graphene is sensitive to the disorder and the environment, which might limit or change its properties. We investigate the effects of random disorders in electric transport on chemical vapor deposited (CVD) graphene. We study the carrier density dependence of conductivity and magneto-conductivity crossing the charge neutral point and compare our data with pervious theories concerning sharp structural disorder and charged disorder. The magneto-conductivity exhibits weak localization behavior and weak localization is enhanced by inter-valley scattering with the presence of the electron hole puddle near charge neutral point (CNP). The electric mobility shows strong correlation with the charged impurity density. We find that the inhomogeneous potential fluctuation induced by charged impurities plays a dominated role in electronic transport especially near the CNP. The study on temperature and carrier density dependent conductivity further suggests that the sharp defect dominate the transport in high carrier density regime, but the charge impurity becomes important in lower density regime.