Nano-Pattern Oxidation of WSe2 Via Block Copolymer Self-Assembly for Highly Responsive Homojunction Phototransistors

Abstract

Tungsten diselenide (WSe2), one of the two-dimensional (2D) materials, has emerged as a promising optoelectric candidate because of its thickness dependent bandgap, large optical absorption coefficient, good air stability and high carrier mobility. In addition, the electrical properties of the ambipolar WSe2 could be tuned through the electrostatic doping by the gate modulation. However, the low photo responsivity is a critical problem of WSe2-based optoelectric devices. To overcome this limitation, several approaches have been suggested such as stacking different 2D materials and coating with quantum dots. But these conventional methods were focused on the formation of heterojunctions, which requires complicated alignment process and leads to unavoidable defects and residues. Thus, the novel ways to build homojuction should be investigated for fabricating devices with a clean interface in a simple process.In this work, block copolymer (BCP) was used to prepare hexagonally packed cylinder nano-template as an oxidation mask for UV/ozone treatment, which resulted in numerous nanoscale homojunctions on WSe2. To fabricate the mask with hexagonally packed cylinders, asymmetric poly (styrene-block-methyl methacrylate) (PS-b-PMMA) was spin-coated. After thermal annealing for self-assembly, PMMA was selectively removed by reactive ion etching. UV/ozone treatment, one of the oxidation methods of the WSe2, leads to the formation of the the tungsten oxide (WOX) with high electron affinity and induces a p-doping effect. By using BCP as an oxidation mask, the only region of the WSe2 under the removed PMMA blocks could be converted into WOX.The responsivity of the nano-pattern oxidized WSe2 phototransistor manifests a dramatic change depending on the applied gate bias. The device exhibited a responsivity (532 nm, 8.515 mW/cm2) of 27.5 mA/W under applied gate bias of 0 V. However, under the gate bias of +60 V, the device showed an enhanced responsivity of 155.2 mA/W, as strong p-n junctions were induced. In conclusion, the nano-pattern oxidized WSe2 phototransistor presents increased responsivity (155.2 mA/W), external quantum efficiency (36.3%) and faster response time than the pristine WSe2.This work demonstrates that the responsivity of the WSe2 phototransistor can be improved though well-established perpendicular BCP and optimized by effective modulation of gate bias. By implementing homojunctions on the WSe2 via the nano-pattern oxidation method, this work greatly expands the potential applications of 2D TMDs materials for next-generation nanoscale optoelectronic devices.