Melioration of Electrical and Optical Properties of Al and B Co-Doped ZnO Transparent Semiconductor Thin Films

Chien-Yie Tsay,Shih-Hsun Yu

doi:10.3390/coatings11101259

Abstract

Undoped, Al-doped and Al-B co-doped ZnO transparent semiconductor thin films were deposited on glass substrates by sol-gel method and spin coating technique. This study investigated the influence of Al (2 at.%) doping and Al (2 at.%)-B (1 or 2 at.%) co-doping on the microstructural, surface morphological, electrical and optical properties of the ZnO-based thin films. XRD analysis indicated that all as-prepared ZnO-based thin films were polycrystalline with a single-phase hexagonal wurtzite structure. The substitution of extrinsic dopants (Al or Al-B) into ZnO thin films can significantly degrade the crystallinity, refine the microstructures, improve surface flatness, enhance the optical transparency in the visible spectrum and lead to a shift in the absorption edge toward the short-wavelength direction. Experimental results showed that the Al-doped and Al-B co-doped ZnO thin films exhibited high average transmittance (>91.3%) and low average reflectance (<10%) in the visible region compared with the ZnO thin film. The optical parameters, including the optical bandgap, Urbach energy, extinction coefficient and refractive index, changed with the extrinsic doping level. Measured results of electrical properties revealed that the singly doped and co-doped samples exhibited higher electron concentrations and lower resistivities than those of the undoped sample and suggested that 2 at.% Al and 1 at.% B were the optimum dopant concentrations for achieving the best electrical properties in this study.

Highlights

Zinc oxide (ZnO) is an abundant raw material
It has been demonstrated that the electrical characteristics and optical parameters of ZnO thin films can be improved by doping with extrinsic dopants such as group II, III and IV elements [9,10]
It is well agreed that ZnO thin films doped with B, Al, Ga and In ions become more conductive, because one excess electron is produced in those thin films [13]

Summary

Introduction

Zinc oxide (ZnO) is an abundant raw material It is a II-VI group wide-bandgap oxide semiconductor and exhibits superior physical properties such as chemical and thermal stability, biocompatibility, high radiation resistance, good optical transparency in the visible spectral range, high quantum yield, a wide range of electrical resistivity and high voltagecurrent nonlinearity [1,2,3,4,5]. Im et al prepared Al and B co-doped ZnO thin films via RF magnetron sputtering and found that substitution of Al and B for Zn of up to 3 wt.% could increase the optical bandgap energy and electron concentration of ZnO thin films [22] They did not present a clear and detailed discussion of why the optical parameters were adjusted and electrical characteristics were improved. The optical parameters, including the optical bandgap, Urbach energy, extinction coefficient and refractive index, are reported in this study

Precursor Solution Synthesis and Sol-Gel Film Deposition

Physical Properties Characterization

Results and Discussion