Positional variation and annealing effect in magnetron sputtered Ga-doped ZnO films
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)
- et al.
Improvements of spatial resistivity distribution in transparent conducting Al-doped ZnO thin films deposited by DC magnetron sputtering
Thin Solid Films
(2010) - et al.
Highly transparent and conductive rare earth-doped ZnO thin films prepared by magnetron sputtering
Thin Solid Films
(2000) - et al.
Energetic oxygen particles in the reactive sputtering of Zn targets in Ar/O2 atmospheres
Thin Solid Films
(1999) - et al.
Analysis of relevant plasma parameters for ZnO:Al film deposition based on data from reactive and non-reactive DC magnetron sputtering
Surf. Coat. Technol.
(2003) - et al.
Oxygen bombardment effects on average crystallite size of sputter-deposited ZnO films
J. Non-Cryst. Solids
(2008) - et al.
Effects of atomic oxygen treatment on structures, morphologies and electrical properties of ZnO:Al films
Appl. Surf. Sci.
(2010) - et al.
The effect of target aging on the structure formation of zinc oxide during reactive sputtering
Thin Solid Films
(2007) - et al.
The effects of oxygen partial pressure on local structural properties for Ga-doped ZnO thin films
Thin Solid Films
(2006) - et al.
Thermal stability of ZnO thin film prepared by RF-magnetron sputtering evaluated by thermal desorption spectroscopy
Appl. Surf. Sci.
(2010) - et al.
Effect of substrate bias on crystal structure and thermal stability of sputter-deposited ZnO films
J. Cryst. Growth
(2009)
Cited by (16)
Electrical resistivity reduction and spatial homogenization of Ga-doped ZnO film by Zn layer insertion
2020, Thin Solid FilmsCitation Excerpt :Both explanations are based on the idea that the crystalline defects compensate electron carriers doped in ZnO films. The degradation of resistivity and inhomogeneity are improved by heated or biased deposition [6,20–23]; however, they can be impaired by an unrevealed degradation mechanism. In order to reduce the density of VZn and Oi in a doped ZnO crystal, it is indispensable to control the Zn/O ratio in the film.
Monitoring the characteristic properties of Ga-doped ZnO by Raman spectroscopy and atomic scale calculations
2019, Journal of Molecular StructureCitation Excerpt :Ga-doped ZnO thin films were also studied recently [24–26]. Yamada et al. deposited Ga-doped ZnO thin films using RF magnetron sputtering method and investigated the positional distribution of electrical resistivity and crystalline lattice constant of films [27]. Moreover, the study of optical band gap shift in Ga-doped ZnO thin films based on the combination of experimental and first-principles calculations was presented by Wang et al. [28] The optical properties of Ga-doped ZnO films grown using metal organic chemical vapor deposition were investigated by Ye et al. [29] They reported a study on the Burstein-Moss and band-gap renormalization effects on the near-band-edge transition in films.
Resistivity reduction in Ga-doped ZnO films with a barrier layer that prevents Zn desorption
2018, Thin Solid FilmsCitation Excerpt :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.
Physical properties of TiO<inf>2</inf>-doped zinc oxide thin films: Influence of plasma treatment in H<inf>2</inf> and/or Ar gas ambient
2017, VacuumCitation Excerpt :Our previous study [16] investigated influence of in situ hydrogen doping and substrate temperatures on physical properties of TZO thin films and achieved a low resistivity of 9.2 × 10−4 Ω-cm under the process conditions: the H2/(H2/Ar) flow ratio of 7.5% and the substrate temperature of 373 K. Although these earlier literature indicated that the appropriate amount of Ti and H in TZO thin films and the optimal deposition parameters could enhance the opto-electronic characteristics of TZO thin films, many research efforts for further improvement on characteristics of various TCO thin films have been proposed continuously. A few previous studies [17–24] make use of post-deposition treatments such as annealing and plasma treatment to enhance characteristics of TCO thin films. Fang et al. [17] studied the effects of vacuum annealing on the properties of the ZnO:Al (AZO) films and indicated that the annealing process led to improvement of (002) orientation, increased carrier concentration and band-gap of the AZO films.
Improved sputtering method to deposit high conducting doped ZnO films without substrate heating
2023, Applied Physics Express