Photoresponse characteristics of Graphene-Semiconductor Schottky junctions for optoelectronic devices

Abstract

Schottky junction diodes (SJDs) utilize a built-in potential develops at metal–semiconductor or metal–insulator–semiconductor interface. The SJDs are in principle advantageous over standard p–n junction device given that complementary doping is not indispensable, but still remain inferior to p–n junction devices in that the vacuum- deposition of a reliable Schottky contact electrode is costly in the process. Recently, there have been many interests in the SJDs made with graphene electrodes, intended to replace metal Schottky electrodes, due to simple structure and easy fabrication process. Schottky electrode with optically-transparent graphene has additional advantages over conventional opaque metal electrodes since built-in potential developed even just beneath graphene can be optically active. Recent studies on an atomically-thin layer of graphene have also revealed unique characteristics different from other materials, such as a tunable work function and transparencies for reactivity, wetting, and electric potential. These features, together with different interface defective states, such as interface-trapped charges, caused anomalous device characteristics of graphene-based SJDs, which raises a number of fundamental questions. For instance, what would be similar and what would be different in the graphene–semiconductor Schottky contacts compared to conventional metal–semiconductor contacts? How the unique characteristics of graphene-semiconductor junction can be exploited to impose new functions and/or to maximize the device performances? To address these questions, herein we have fabricated a number of SJDs consisting of ZnO or GaN semiconductor and single-layer graphene grown by chemical vapor deposition (CVD). The SJDs were made with particular attention to the interface of the Schottky contact, and their photoresponse characteristics were carefully investigated. The graphene-semiconductor Schottky junction photodetectors exhibited different responses in photocarrier dynamics depending on the applied bias voltage, which is characterized by either a negative or positive change in photocurrent with time. We proposed underlying mechanisms for the anomalous photocarrier dynamics based on the interplay between electrostatic molecular interactions over the one-atom-thick graphene and semiconductor junction and trapped photocarriers at the defect states in semiconductor. The unique characteristics of graphene-a tunable work function and transparencies for electric potential, were further exploited to enhance the performance of SJDs. For this, we have investigated the power conversion efficiency (PCE) of graphene-silicon Schottky junction solar cells (Gr-Si SJSCs), depending on the external gate voltage bias (Vg) applied to the cells. We found that the PCE value continuously increases/decreases with the negative/positive Vg. Moreover, when the graphene was replaced with graphene mesh electrodes, the SJSC showed more rapid increase of PCE from 5.7 % to 8.1, together with a more sensitive response of the open-circuit voltage and fill factor, with Vg varied from 0 V to −1 V. The result can be attributed to the existence of hole array in the graphene mesh that increase the work function and permeability across the electric-field. This study provides insights into the permeation of electrostatic potential involving graphene−gas molecular interactions on the semiconductor surface through one-atom-thick graphene.