Elsevier

Thin Solid Films

Volume 609, 30 June 2016, Pages 25-29
Thin Solid Films

Positional variation and annealing effect in magnetron sputtered Ga-doped ZnO films

https://doi.org/10.1016/j.tsf.2016.04.037Get rights and content

Highlights

  • As-deposited films show an exponential-like resistivity change with c-axis length.

  • Thermal annealing changes electrical and structural properties.

  • Three different types of defect exist in the as-deposited films.

Abstract

Positional distribution of electrical resistivity and crystalline lattice constant of Ga-doped ZnO films deposited by the magnetron spattering method at room temperature have been investigated. Electrical resistivity and c-axis lattice constant strongly depended on substrate position and substrate–target distance when the films were deposited without substrate heating. Films deposited at the positions facing erosion region showed higher electrical resistivity, less carrier density and elongated c-axis lattice constant than those deposited at the other positions. These films showed exponential-like change of resistivity and carrier density as a function of c-axis lattice constant in a wide range of substrate–target distance. Thermal annealing decreased resistivity and c-axis lattice constant of the as-deposited films and homogenized the positional distribution of their properties. Comparison of electrical and structural changes between before and after the annealing revealed that there are at least three types of defect in the sputtered Ga-doped ZnO films that affect carrier density and mobility with different annealing behaviors.

Introduction

ZnO is a well-known transparent conducting oxide (TCO) and the films doped with Al or Ga exhibit very high electrical conductivity and optical transparency comparable with the indium tin oxide (ITO). A number of studies about the preparation methods of doped ZnO films have been reported for practical applications and indicated that their properties are quite sensitive to deposition processes and conditions. In a magnetron sputtering process, which is one of the most promising processes for low cost mass production, the electrical resistivity of doped ZnO films varies with substrate position [1], [2], [3], [4], [5]. Films deposited on a substrate that is placed at positions facing erosion area on the target in magnetron sputtering, have lower carrier density and lower mobility than films deposited at other positions. The variation of electrical properties with substrate position is inadequate for fabricating homogeneous films with low resistivity. Therefore, homogenization of the positional distribution induced in the doped ZnO films prepared by the magnetron sputtering method is strongly required in addition to the reduction of electrical resistivity. In order to control the positional variation, the origin of the inhomogeneity should be considered.

The electrical deterioration of the films deposited at the erosion positions in magnetron sputtering process is often ascribed to the incident of high-energy particles [6], [7], [8], [9], [10], [11]. Film degradation by the intentional implantation of atomic oxygen is also reported [12]. The bombardment or implantation of the high-energy particles introduces imperfection in crystal structure of ZnO [13]. Defects in ZnO crystal are studied theoretically [14], [15], [16] and discussed based on experimental data [17], however, there are few reports determining the relationship between electrical properties and corresponding defect types in doped ZnO crystalline imperfection. One of the reasons in regard with experimental aspect is that most of the sputtering depositions were performed at elevated substrate temperatures to obtain high electrical conductivity and consequently, the films were in-situ annealed and the defects in the films were relaxed as the deposition proceeded. Thermal annealing possibly causes dissociation of Zn or O atoms from the films [18], [19], [20]. Therefore, heated deposition contains sophisticated atomic processes of adsorption and desorption to understand the relationship between electrical property and originally introduced defect structure of ZnO films.

In order to separate the annealing effect from the deposition process and investigate the relationship between crystalline defect characteristics and electrical properties, we have focused on the non-heating deposition of Ga-doped ZnO (GZO) films by radio frequency (RF) magnetron sputtering. We measured positional distribution of electrical and structural properties on as-deposited films and found the unique relationship between electrical and structural properties that is not reported on doped ZnO films. Furthermore, we investigated the annealing effect of the as-deposited film to homogenize positional distribution and understand defect properties introduced in the non-heating deposited films by comparing before and after annealing films.

Section snippets

Experimental

GZO films were deposited by conventional planar RF magnetron sputtering. The target was a sintered ZnO disk containing 5.7 wt.% of Gd2O3 with 100 mm in diameter. The deposition was carried out without intentional substrate heating using the working gas of pure Ar of the pressure of 1 Pa with the power of 80 W. Pyrex glass substrates were settled parallel to the target at distances of 50–80 mm from the target surface. The substrates were placed from the position facing the center of the target to the

Results

Electrical resistivity of the as-deposited GZO films is plotted in Fig. 2 as a function of substrate position for various T–S distances from 50 to 80 mm. Resistivity increased as T–S distance increased. For each T–S distance deposition, resistivity was high at the erosion positions (substrate position around 25 mm) and low at both the center (substrate position near 0 mm) and the outer positions. Resistivity changed more than six orders of magnitude depending on substrate position and T–S distance.

Discussion

C-axis length was elongated and carrier density was diminished at the erosion positions in as-deposited films as shown in Fig. 2, Fig. 4. These films exhibited the characteristic relationship between c-axis length and electrical properties as shown in Fig. 5, Fig. 6. Because of the bombardment of high-energy particles in sputtering process, various kinds of defects are introduced in the as-deposited films. It is reasonable to decide that the crystalline defects are introduced more at the

Conclusion

GZO films deposited by sputtering at room temperature were investigated on T–S distance variation, in-plain positional distribution and annealing effect in electrical and structural properties. As-deposited GZO films at erosion positions showed lower carrier density and lower mobility than at other positions. They also showed elongated c-axis lattice constant. Plotting electrical properties with c-axis length revealed that resistivity increased from 10 3 to 10 Ω cm [3] and carrier density

Acknowledgement

This work is partly supported by MEXT of Japan, City Area Program of Shinji Lake & Nakaumi (2009–2012).

References (20)

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