Anisotropy of impact ionization in WSe2 field effect transistors

Abstract

Carrier multiplication via impact ionization in two-dimensional (2D) layered materials is a very promising process for manufacturing high-performance devices because the multiplication has been reported to overcome thermodynamic conversion limits. Given that 2D layered materials exhibit highly anisotropic transport properties, understanding the directionally-dependent multiplication process is necessary for device applications. In this study, the anisotropy of carrier multiplication in the 2D layered material, WSe2, is investigated. To study the multiplication anisotropy of WSe2, both lateral and vertical WSe2 field effect transistors (FETs) are fabricated and their electrical and transport properties are investigated. We find that the multiplication anisotropy is much bigger than the transport anisotropy, i.e., the critical electric field (ECR) for impact ionization of vertical WSe2 FETs is approximately ten times higher than that of lateral FETs. To understand the experimental results we calculate the average energy of the carriers in the proposed devices under strong electric fields by using the Monte Carlo simulation method. The calculated average energy is strongly dependent on the transport directions and we find that the critical electric field for impact ionization in vertical devices is approximately one order of magnitude larger than that of the lateral devices, consistent with experimental results. Our findings provide new strategies for the future development of low-power electric and photoelectric devices. Graphical Abstract

High-quality HfO2/HfSe2 gate stack for low-power steep-switching computing devices

“…Impact ionization FET fabricated with the HfO2/HfSe2 structure Several approaches have been investigated to achieve an SS < 60 mV/dec, overcoming the fundamental Boltzmann limit, such as tunneling FET 48 , negative-capacitance FET [49][50][51] , electromechanical switches 52,53 , and impact ionization FET (I 2 -FET) [54][55][56] . Recently, based on the unique properties of 2D materials such as in-plane carrier transport and thickness-dependent tunable bandgaps, excellent steep switching behaviors have been reported for various I 2 -FET structures [57][58][59][60][61] . We fabricated HfO 2 /HfSe 2 based I 2 -FET with a separately controlled channel structure, as shown in Fig.…”