Vertical Transport of van der Waals Materials and Their Application in Hot Electron Transistors
- Author(s): Zhu, Xiaodan
- Advisor(s): Wang, Kang Lung
- Pei, Qibing
- et al.
Vertical integration of van der Waals (vdW) materials into heterostructures with atomic precision is one of the most intriguing possibilities brought forward by these 2-dimensional (2D) materials. Essential to the design and analysis of these structures is a fundamental understanding of the vertical transport of charge carriers into and across vdW materials. In this dissertation, I explore the important roles of single layer graphene in the vertical tunneling process, both as a collecting electrode and as a vdW tunneling barrier, and explore graphene’s application as the base material of hot electron transistors (HETs).
When graphene comes into contact with highly doped silicon, a fully preserved vdW gap is formed at the interface, which acts effectively as a tunnel barrier. In the scenario where graphene acts as the collecting electrodes, the electrons injected from the highly doped silicon are captured by graphene, and propagate laterally through graphene. Using electron tunneling spectroscopy (ETS), it is shown that this process is limited by the relaxation of carriers into the linear density of states of graphene.
When graphene is sandwiched between two electrodes, the graphene layer together with the vdW gap act as a tunnel barrier that is transparent to the vertically tunneling electrons due to its atomic thickness and the mismatch of transverse momenta between the injected electrons and the graphene band structure. This is accentuated from the ETS showing a lack of features corresponding to the Dirac cone band structure of the graphene. Meanwhile, the graphene acts as a lateral conductor through which the potential and charge distribution across the tunnel barrier can be tuned. These unique properties make graphene an excellent 2D atomic net, which is transparent to charge carriers, and yet it can control the carrier flux via electrical potential at the same time. A new model including the effect of the quantum capacitance of the graphene for vertical tunneling is developed to further elucidate the role of graphene in modulating the tunneling process.
As a result of the unique vertical transport properties of graphene, hot electron transistors with graphene as the base material and the vdW gap as the tunnel barrier can be fabricated, eliminating the need for an additional tunnel barrier. This leads to significantly increased current densities, as well as minimized energy loss for the hot electrons in the tunnel barrier, which in turn leads to lower turn on voltages and higher current gain, compared to previous reports of graphene based HETs.