Effects of substrate temperature on the efficiency of hydrogen incorporation on the properties of Al-doped ZnO films

Abstract

Al-doped ZnO (AZO) transparent conducting films were successfully prepared on glass substrates by RF magnetron sputtering at different substrate temperatures in Ar and H₂ + Ar sputtering ambient. The effects of substrate temperature on the effectiveness of hydrogen incorporation in Al-doped ZnO films were investigated. The microstructural, electrical and optical properties of AZO films were systematically analyzed by surface profiler, X-ray diffractometry, scanning electron microscope, four-point probe measurement and UV/vis spectrophotometer. The XRD patterns and SEM pictures indicate that the crystallinity of AZO thin films was markedly improved with hydrogen incorporation at low substrate temperature, while the improvement of crystallinity was not an obvious change at high substrate temperature. The results also indicate that hydrogen incorporation has the stronger effectiveness on the transparent conductive properties of AZO films with the substrate temperature decreasing. The resistivity of the films decreases, especially for lower substrate temperatures, due to the incorporation of hydrogen atoms. These results suggest that substrate temperature should be controlled to the lower level to effectively reduce resistivity without detriment to transmittance of AZO thin films when hydrogen is incorporated.

Introduction

Transparent conductive oxide (TCO) films have been extensively used in a variety of electronic and opto-electronic applications such as displays, solar cells, optoelectronic devices, electrochromic devices, etc. [1]. Al-doped ZnO (AZO) thin films are intensively studied to replace ITO thin films, which are most generally used in transparent conducting films, due to its non-toxic, low cost, with high thermal/chemical stability and resource availability [2], [3], [4], [5], [6]. Compared with various deposition techniques including sputtering, pulse laser deposition, chemical vapor deposition and sol–gel process [7], [8], [9], [10], the RF magnetron sputtering method has the advantages in simplifying fabrication step, lowing production cost and lowing temperature process as well as improving reproducibility. Unfortunately, these deposition techniques required relatively high temperatures [11], [12] to obtain good electrical and optical properties. Therefore, it has limited the transparent electrode application of AZO on plastic substrates or sensitive photoresist as those used in light emitting diodes and solar cells.

Recently, there is a particular interest in the properties of hydrogen in ZnO, because density function theory and total energy calculations predict that H always acts as a donor in ZnO and some other oxides [13], [14], [15], [16]. Unlike in many other materials, H partially compensates the conductivity provided by extrinsic dopants in some other oxides [17]. Nevertheless, there are few reports on the influence of hydrogen on the electrical and optical properties of AZO grown at different substrate temperatures by magnetron sputtering. The role of hydrogen in the growth process of magnetron sputtered AZO films is not completely understood yet. Especially, the effects of substrate temperature on the effectiveness of hydrogen incorporation in Al-doped ZnO films were not reported yet.

In this study, AZO thin films were prepared on glass substrates by RF magnetron sputtering at different substrate temperatures in Ar and H₂ + Ar sputtering ambient. We demonstrate that the effectiveness of hydrogen doping in AZO thin films is dependent on the substrate temperature and ranged from 100 to 250 °C. Effects of the hydrogen incorporation on the electrical, optical, and structural properties of AZO thin films were investigated in depth.