The band alignment of nonpolar m-plane ZnO1−xSx/Mg0.4Zn0.6O heterojunctions

Lei Li,Mi Zhang,Qile Wang,Yinmei Lu,Hong Chen,Yunbin He,Pai Li,Mingkai Li

doi:10.1063/1.5093025

Abstract

Heterostructures such as heterojunctions, quantum wells, and superlattices are core components of advanced optoelectronic devices. Herein, we attempted the first investigations on the band alignment of nonpolar m-plane oriented ZnO1−xSx/Mg0.4Zn0.6O heterojunctions by X-ray photoelectron spectroscopy. All the heterojunctions were revealed to show a type-I band alignment, and the valence band offset (VBO; ΔEV) increased significantly, while the conduction band offset (ΔEC) decreased insignificantly with increasing S content in the ZnO1−xSx layer. Specifically, for the ZnO1−xSx/Mg0.4Zn0.6O heterojunctions with x = 0, 0.13, and 0.22, ΔEV (ΔEC) was determined to be 0.24 (0.22), 0.61 (0.17), and 0.79 (0.11) eV, respectively. The VBOs of ZnOS/MgZnO heterojunctions are significantly larger than those of heterojunctions involving only cation-substituted alloys (ZnO/MgZnO or ZnO/CdZnO) due to the opposite shift in the VB maximum of ZnOS and MgZnO with respect to ZnO. Knowing band alignment parameters of the ZnOS/MgZnO interface can provide a better understanding of the carrier transport mechanism and rational design of ZnO-based optoelectronic devices.

Highlights

ZnO is a typical wide bandgap semiconductor suitable for a large variety of device applications.1 The advantageous characters of ZnO that distinguish it from other semiconductors include direct and wide bandgap (Eg ∼ 3.3 eV at room temperature), large exciton binding energy (60 meV), large piezoelectric constants, strong luminescence, large nonlinear optical coefficients, high melting temperature (2248 K), good biocompatibility, low toxicity, and low cost.2–5 In order to optimize the performance of ZnO-based optoelectronic devices, most designs rely on different kinds of heterojunctions for providing carrier and/or optical confinement
A heterojunction is composed of layers of different compositions, whose key parameters are the bandgap of each layer and the valence/conduction band offset between the individual layers
The dynamics of charge carriers in the heterojunction depends on the potential barrier heights, which are quantified by the values of the valence/conduction band offset at the interface between two contacting layers

Summary

INTRODUCTION

ZnO is a typical wide bandgap semiconductor suitable for a large variety of device applications. The advantageous characters of ZnO that distinguish it from other semiconductors include direct and wide bandgap (Eg ∼ 3.3 eV at room temperature), large exciton binding energy (60 meV), large piezoelectric constants, strong luminescence, large nonlinear optical coefficients, high melting temperature (2248 K), good biocompatibility, low toxicity, and low cost. In order to optimize the performance of ZnO-based optoelectronic devices, most designs rely on different kinds of heterojunctions for providing carrier and/or optical confinement. Because MgZnO and ZnOS alloys show opposite variations in the bandgap energy with the substituent (Mg or S) concentration, we proposed the design of ZnOS/ZnMgO heterostructures, such as superlattices (SLs) and quantum wells (QWs), that have a larger barrier height than ZnO/ZnMgO with appropriate S and Mg concentrations.. Because MgZnO and ZnOS alloys show opposite variations in the bandgap energy with the substituent (Mg or S) concentration, we proposed the design of ZnOS/ZnMgO heterostructures, such as superlattices (SLs) and quantum wells (QWs), that have a larger barrier height than ZnO/ZnMgO with appropriate S and Mg concentrations.19 Such cation- and anion-substituted ZnO alloy composed heterostructures hold great potential, in reality, ZnO-based heterojunctions involving anionsubstituted alloys such as ZnOS remain unexplored to date. XPS has been demonstrated to be a direct and powerful tool for characterizing the band alignment of a large variety of heterojunctions, and offers first-hand information on the offsets of both valence and conduction bands between the two layers in the ZnOS/MgZnO heterojunctions

EXPERIMENTAL

RESULTS AND DISCUSSION

CONCLUSIONS