Effect of NaOH solution on surface textured ZnO: Al films prepared by pulsed direct current magnetron sputtering

Abstract

Transparent conducting Al-doped ZnO (ZnO:Al, AZO) thin films were prepared at substrate temperature of 270 °C by pulsed direct current magnetron sputtering. NaOH solution (5 wt%) was employed to etch the AZO films at room temperature, and the surface textured AZO films were obtained successfully. The relationship between the surface textured structures and the etching process controlled by etching time was discussed. The textured morphology of the etched AZO films became clear as increasing the etching time, and the AZO film etched for 30 min exhibited uniformly and distinctly crater-like surface textured structure. Correspondingly, the haze and the resistivity increased with the increasing etching time. And the resistivity of the AZO film etched for 30 min was 3.2×10⁻³ Ω cm.

Introduction

ZnO, a wide band gap semiconductor, is a promising optoelectronic material [1], [2], [3], [4], [5], [6], [7], [8]. In particular, ZnO:Al (AZO) thin film has attracted much attention as the front electrode in silicon-based thin film solar cells in recent years due to its low cost, low resistivity, high transparency in the visible wavelength range, non-toxicity and stability under hydrogen plasma environment [9], [10], [11]. Moreover, in order to achieve a high efficiency in thin film solar cells, the smooth surface of AZO film deposited by sputtering is generally transformed to the textured surface by wet chemical etching, which can elongate the effective light path by means of light trapping. In most researches, diluted HCl with a concentration of 0.5% was used as the typical etching solution for the formation of surface textured AZO films [12], [13], [14], [15]. However, it is difficult to control the etching process and the surface morphology because of the high etching rate of HCl solution. To slow down the etching rate, NH₄Cl solution and weak acetic acid solution were used to achieve surface textured AZO films [16], [17], but the surface morphology was not quite satisfactory yet. Surface textured structures of AZO films can also be obtained with NaOH solution which can control the etching process and the surface morphology effectively. But up to now, few studies on the surface textured AZO films etched with NaOH solution have been reported [18].

In this paper, we report the preparation of surface textured AZO thin films by etching with NaOH solution. Through the wet-chemical etching process, effective surface textured structures were obtained in NaOH solution (5 wt%) successfully. The optoelectronic properties and the etching behaviors as a function of etching time were studied.