Growth and characterization of high transmittance GZO films prepared by sol-gel method

Highlights

• High transmittance Ga-doped ZnO (GZO) thin films deposited by sol-gel process.

• At 4 at.% Ga doping level, the highest transmittance of 98.49% with minimum resistivity of 1.12 × 10^− 2 Ω·cm was obtained.

• The optical band gap of GZO films increase with an increase in Ga content.

Abstract

High transmittance Ga-doped ZnO thin films were deposited on glass substrates using sol-gel spin coating technique. Crystallinity levels, microstructures, optical and electrical properties of the thin films were systematically investigated by scanning electron microscope, X-ray diffractometer, UV–visible spectrophotometer, PL spectrometer and four-point probe method, respectively. All GZO thin films exhibited polycrystalline with a hexagonal wurtzite structure and slight (0 0 2) preferred orientation growth, which show excellent transmittance (> 95%) in the 380–780 nm wavelength range. The PL spectra of the GZO films revealed a strong ultraviolet emission peaks at around 393 nm and a weak blue emission peak at around 468 nm. When the Ga doping level was 4 at.%, the minimum resistivity of 1.12 × 10^− 2 Ω·cm with the highest transmittance was reached. The optical band gaps (E_g) of the films were increased from 3.311 eV to 3.329 eV with the increase of Ga dopant concentration.

Introduction

High transparency with superior electrical conductivity transparent conductive oxides thin films have received great attention, since they are extensively used as transparent electrodes in the flat panel displays and thin film solar cells [1], [2], [3], [4], [5]. Recently, Al and Ga-doped ZnO (AZO and GZO) films have attracted much attention as a promising candidate to replace In₂O₃-based TCO films owing to their non-toxicity, high thermal stability, earth-abundance and excellent optoelectronic properties as compared with ITO [6], [7], [8].

Several deposition techniques have been used for the fabrication of AZO and GZO films, such as metal organic chemical vapor deposition, RF magnetron sputtering, atomic layer deposition, pulsed laser deposition, spray pyrolysis, sol-gel methods and the others. As a wet chemical process, the sol-gel method which was firstly employed by L. Spanhel and M. Anderson [9] offers several advantages compared with other deposition techniques, such as large deposition area, simple equipment, low-cost fabrication, and highly homogeneous precursor sol. While, it has found that GZO thin films have more advantages than AZO. The smaller difference between the ionic radius of Ga^3 + (0.062 nm) and Zn^2 + (0.074 nm) than between Al^3 + (0.054 nm) and Zn^2 + allows easier substitution of Ga^3 + for Zn^2 + [10]. What's more, the slightly shorter covalent bond length of GaO (1.92 Å) than that of ZnO (1.97 Å) could reduce lattice deformation in ZnO thin films even if high doping concentrations [11], [12], [13]. Additionally, Ga can't be easily oxidized in ZnO films compared with Al. Therefore, Ga should be the best substitutional dopant in ZnO thin films than the others.

In recent decades, GZO films deposited by sol-gel method have been reported in plenty of literature. Sol-gel processed GZO thin film from low concentration solution had been researched by S. Salari et al. [14], it was found that the structural and optoelectronic properties of GZO films prepared with 2 at.% Ga doping concentration had been improved. The lowest electrical resistivity of 2.8 × 10^− 2 Ω·cm with an average transmittance of 91.5% was obtained by C.Y. Tsay et al. [15] in the GZO films doped with 2 at.% Ga. Higher c-axis-oriented GZO films had been synthesized by Z. N. Ng et al. [16] which have the transmittance of 80–95% and the lowest electrical resistivity of 25 Ω·cm was reached at 2 at.% Ga doping content. However, in the study of K.M. Lin et al. [17], GZO thin films with 1.0 at.% Ga doping content show the best performance with 2.2 × 10^− 3 Ω·cm resistivity and above 80% transmittance. Besides, the effect of different viscosity sols on the properties of GZO thin films has been investigated by Q. Li et al. [18], the average transmittance was higher than 80% except the GZO thin films prepared using 3.0 cps sol. The effect of Ga doping concentration, sintering temperature and thickness of coating layer had been investigated by A. AlKahlout et al. [19], the lowest resistivity about 6.4 × 10^− 2 Ω·cm and transmittance in the visible range as high as 90% were obtained.

However, it is scanty that the effects of Ga concentration on the properties of heavily doped GZO films is systematically studied using sol-gel method. Furthermore, most of the reported GZO thin films show no entirely as desired optoelectronic performance. Thus, it is worthwhile to research the properties of GZO thin films with various Ga doping content and further improve its optoelectronic properties. In this paper, GZO thin films were deposited on glass substrate by sol-gel method with Ga concentration ranging from 2 at.% to 5 at.%. The effects of Ga dopant concentration on the structural, morphological, photoelectric and photoluminescence properties of the GZO films were detailedly investigated. GZO thin films with high transmittance above 95% were obtained and the best performance was reached at 4 at.% Ga doping level.