Ultimate limit in size and performance of WSe2 vertical diodes

Jonghwa Eom,Dong Hoon Shin,Chanyong Hwang,Muhammad Farooq Khan,Junho Suh,Ghazanfar Nazir,Jun Yeon Hwang,Jihwan Kim,Kyoung Soo Kim,Suyong Jung,Hakseong Kim,Dong Su Lee

doi:10.1038/s41467-018-07820-8

Abstract

Precise doping-profile engineering in van der Waals heterostructures is a key element to promote optimal device performance in various electrical and optical applications with two-dimensional layered materials. Here, we report tungsten diselenide- (WSe2) based pure vertical diodes with atomically defined p-, i- and n-channel regions. Externally modulated p- and n-doped layers are respectively formed on the bottom and the top facets of WSe2 single crystals by direct evaporations of high and low work-function metals platinum and gadolinium, thus forming atomically sharp p–i–n heterojunctions in the homogeneous WSe2 layers. As the number of layers increases, charge transport through the vertical WSe2 p–i–n heterojunctions is characterized by a series of quantum tunneling events; direct tunneling, Fowler–Nordheim tunneling, and Schottky emission tunneling. With optimally selected WSe2 thickness, our vertical heterojunctions show superb diode characteristics of an unprecedentedly high current density and low turn-on voltages while maintaining good current rectification.

Highlights

Precise doping-profile engineering in van der Waals heterostructures is a key element to promote optimal device performance in various electrical and optical applications with twodimensional layered materials
Various concepts of 2D van der Waals (vdW)-based devices have been proposed, with some demonstrating respectable device performances by utilizing a wide range of SC-transition metal dichalcogenides (TMDs) energy band gaps varying from a few hundredth meVs to a few eVs, and a straightforward 2D atomic-crystal assembly in lateral or vertical forms[21]
Numerous reports have claimed that the atomically defined p–n junctions in their vertical heterostructures could be essential for the reported diode characteristics; these claims though are challenged by recent evidence that the Schottky barriers at metal–SC–TMD interfaces and unaccounted lateral transport channels could induce such strong diode-like charge transport characteristics[14]

Summary

Introduction

Precise doping-profile engineering in van der Waals heterostructures is a key element to promote optimal device performance in various electrical and optical applications with twodimensional layered materials. An atomically clean vertical p–n junction was demonstrated in homogeneous molybdenum disulfide (MoS2) films by selective chemical doping to relieve the above-discussed electrical and physical issues in the SC-TMD vdW heterostructure, but the on-current level was still limited by the metal–MoS2 contacts and lower charge–carrier mobility across laterally formed transport channels[33,35]. The rectification behaviors are further improved up to RR ≥ 10,000 for the diodes with thicker films (tWSe2 ≈ 42 nm) with only a modest setback of the on-current values We ascribe these superb diode characteristics to the formation of ideal p–i–n heterojunctions free of crystallographic misalignments, ultra-low metal–WSe2 contact resistances, and staggered energy-band alignments inside the vertical channel due to the limited charge transfer between 2D vdW layers. A high-efficient switch operation in a radio-frequency (RF) domain is demonstrated as a testbed for practical device applications

Methods

Results

Conclusion