Resistivity reduction in Ga-doped ZnO films with a barrier layer that prevents Zn desorption
Introduction
Transparent conducting thin films based on ZnO with adequate dopants exhibit superior conducting and optical properties in addition to the advantages of less loads on the environment, low toxicity, and abundant resources [1]. Owing to a large number of potential applications, many studies have been conducted on doped ZnO films under various deposition conditions to achieve homogeneous large area transparent electrodes with very low resistivities. Most of these studies describe the relationship between deposition conditions and film properties. These studies have revealed that the electrical properties of doped ZnO films strongly depend on the preparation conditions and equipment used for deposition, even in vacuum processes. This indicates that the deposition process introduces various types of defects in the film, such as interstitials, vacancies, ionized impurities, and grain boundaries that cause significant fluctuations of properties in a seemingly random manner [[2], [3], [4], [5]]. Accordingly, defect-controlling factors are still uncertain.
Another way to control film properties instead of controlling deposition conditions is to anneal the films after deposition. There have been many studies of doped ZnO films annealed not only in vacuum or an inert gas atmosphere [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]] but also in various active atmospheres. Resistivity is increased when annealed in an oxygen-containing atmosphere due to the incorporation of oxygen atoms into the ZnO lattice [9,13,14,16,19]. A hydrogen-containing atmosphere reduces resistivity through the removal of excess oxygen or hydrogen doping of the lattice [9,13,15,[20], [21], [22]]. In general, the defects that exist in as-deposited films are reduced during post-annealing towards the equilibrium defect state determined by the annealing temperature and atmosphere. Therefore, by comparing the as-deposited films with the subsequently annealed films, the non-equilibrated defect states can be distinguished from the equilibrated states. For example, an as-deposited film of Ga-doped ZnO (GZO) deposited on a wide area substrate generally has a large positional distribution of electrical properties and subsequent vacuum annealing homogenizes this distribution [23]. This indicates that the as-deposited films contain inhomogeneous positional distribution of non-equilibrated defects that are generated by the deposition process, and vacuum annealing decreases and homogenizes the defects to the equilibrium state.
Many studies on film annealing focus on the effect of the annealing atmosphere from which extrinsic atoms or elements are incorporated into the doped ZnO films. In these cases, the desorption of constituent atoms from deposited ZnO films has not been considered thoroughly, although oxygen and zinc desorption has been observed from ZnO [[24], [25], [26], [27], [28]] and doped ZnO films by thermal desorption spectrometry [29,30]. Furthermore, the reduction of the resistivity observed for doped ZnO films capped by TiO2 [31] and Si [17] layers is possibly caused by the prevention of Zn desorption from the ZnO lattice at annealing temperatures. To investigate the effect of atomic transport between a GZO lattice and the surrounding atmosphere, this paper focuses on the variation in properties of GZO films during thermal annealing as a function of annealing temperature by means of SiO2 capping layers formed on GZO films as diffusion barriers.
Section snippets
Experimental
GZO films were deposited on SiO2 glass substrates at room temperature by a radio frequency magnetron sputtering method using a target of 5.7 wt% Ga2O3:ZnO. The thickness of the GZO film was 200 nm. SiO2 capping layers 100 nm in thickness were deposited on the GZO films at room temperature by sputtering. Both the GZO and SiO2/GZO films were annealed at temperatures between 200 °C and over 650 °C in vacuum. The vacuum was produced by simple evacuation of air to 10−1 Pa. After annealing for 30 min
Comparison between GZO and SiO2/GZO films
Fig. 1 shows the electrical properties of GZO and SiO2/GZO films after vacuum annealing as a function of annealing temperature. The plots at room temperature in this and subsequent figures represent the as-deposited films. Resistivity decreased and carrier density increased for both GZO and SiO2/GZO films with increasing annealing temperatures up to 350 °C. As the annealing temperature was further increased, the resistivity of the GZO film increased, resulting from a steep decrease in carrier
Conclusions
The resistivity of a GZO film capped by a SiO2 layer decreased from 1.26 × 10−3 Ω cm before annealing to 4.1 × 10−4 Ω cm as the vacuum annealing temperature was elevated to 600 °C, while GZO films without a SiO2 capping layer showed a resistivity upturn at approximately 400 °C. The decrease in resistivity with temperature observed for capped GZO films results from a retention of carrier density and an increase in mobility. Electrical behavior investigated both with and without a SiO2 capping
Acknowledgement
The authors thank Y. Fujita and the Interdisciplinary Center for Science Research, Shimane University for their help with the optical measurements.
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