The effects of the post-annealing with a Zn cap on the structural and electrical properties of sol-gel derived MgxZn1−xO films

Structural and electrical properties of Al-doped MgxZn1−xO films were improved by post-annealing with supplying Zn vapor. The Al-doped MgxZn1−xO films were deposited on glass substrates by a sol-gel method. The substrates were dip-coated with a precursor solution and were dried on a hotplate at 270 °C for 10 min. This dip-coating and drying process was repeated 10 times, and the Al-doped MgxZn1−xO films were obtained after calcination in air at 500 °C for 1 h. The as-grown films were post-annealed in H2 at 400 °C for 20 min. To supply zinc vapor, a glass slide with a thermally evaporated Zn layer (Zn cap) was put on the sample surface during the post-annealing. The as-grown films had the wurtzite structure with the c-axis perpendicular to the substrate surface, but the intensity of the (002) diffraction peak decreased with increasing Mg content (x). The crystallinity of the films was improved after the post-annealing with a Zn cap, which was observed when x was below 0.1. The resistivity and carrier concentration of the film (x = 0.1) after the post-annealing with a Zn cap was 6.0 × 10−3 Ωcm and 5.7 × 1019 cm−3, respectively. On the other hand, the resistivity of the film (x = 0.1) after the post-annealing without a Zn cap was 6.6 × 102 Ωcm. Transmittance spectra in the visible range were not affected by the post-annealing. The optical bandgap of the film (x = 0.1) after the post-annealing with a Zn cap was 3.41 eV.


Introduction
Transparent conductive oxide (TCO) films are used in the field of optoelectronic devices such as liquid crystal displays and solar cells. Although indium tin oxide (ITO) is known as a typical TCO film, much attention has been focused on zinc oxide (ZnO) [1][2][3][4]. ZnO can be alloyed with MgO, and Mg x Zn 1−x O films are considered as an attractive TCO film due to its tunable bandgap energy (3.4-7.8 eV) [5,6]. Wide bandgap TCO films are important in ultraviolet photodetector and solar cell applications [7][8][9]. Impurities such as Al, Ga, and In work as donors in Mg x Zn 1−x O films [10][11][12]. However, Mg x Zn 1−x O films show relatively poor electrical properties. Electron concentration and mobility decrease with increasing Mg content [13][14][15]. Liu et al reported that selfcompensation by zinc vacancy acceptors, of which formation enthalpy decreases with increasing Mg content, is responsible for the deteriorated conductivity of Mg x Zn 1−x O films [14].
Mg x Zn 1−x O films can be formed by various techniques including chemical vapor deposition (CVD), radio frequency (RF) magnetron sputtering, and pulsed laser deposition [16][17][18][19][20][21]. Sol-gel deposition is a low-cost solution-based method, and the bandgap of Mg x Zn 1−x O films is tuned by changing Mg concentration in a precursor solution [22][23][24]. However, the electrical properties of sol-gel derived Mg x Zn 1−x O films are inferior to those deposited by vacuum processes. The typical resistivity values of Mg x Zn 1−x O films deposited by nonvacuum processes are about 10 −2 -10 1 Ωcm [25,26]. To overcome this drawback, several types of post-annealing processes are used. It is known that post-annealing in a reducing atmosphere is an effective method to improve properties of sol-gel derived TCO films [7,26,27].

Experimental procedures
The Al-doped Mg x Zn 1−x O (x = 0-0.3) films were deposited on glass substrates (soda-lime glass) by a sol-gel method. Prior to the sol-gel deposition, the substrates were cleaned with acetone (ultrasonic bath), piranha solution (H 2 SO 4 /H 2 O 2 ), and deionized water. The precursor solution for the sol-gel deposition was prepared by mixing zinc acetate dihydrate, magnesium acetate tetrahydrate, aluminum nitrate nonahydrate (Al/(Zn + Mg) = 3at%), monoethanolamine (0.50 mol l −1 ), and ethanol. The sum of the zinc and magnesium concentrations in the precursor solution was 0.25 mol l −1 . Firstly, the substrates were dip-coated with the precursor solution and were dried on a hotplate at 270°C for 10 min. This dip-coating and drying process was repeated 10 times, and the Al-doped Mg x Zn 1−x O films were obtained after calcination in air at 500°C for 1 h. The as-grown films were post-annealed in H 2 at 400°C for 20 min. To supply zinc vapor, a glass slide with a thermally deposited Zn layer (Zn cap) was put on the sample surface during the post-annealing. The thickness of the films was about 300 nm, regardless of the post-annealing condition.
Film thickness was determined by a stylus profilometer (Accretech SURFCOM 1400G). Structural properties and surface morphologies were characterized by x-ray diffraction (XRD, Rigaku SmartLab SE) and scanning electron microscope (SEM, JEOL JSM-7001F), respectively. Electrical properties were measured at room temperature by using van der Pauw configuration. Surface compostion was estimated from x-ray photoelectron spectroscopy (XPS, PHI5000 VersaProbe) measurements. The XPS spectra were calibrated to the C 1s peak at 284.8 eV.  crystallinity of the films. The XRD patterns of the films after the post-annealing with a Zn cap are shown in figure 2(b). The intensity of the (002) diffraction peak is increased after the post-annealing with a Zn cap, and the full width at half maximum (FWHM) value of the peak is decreased. The improvement depended on the crystallinity of the as-grown films. For x = 0, the FWHM value of the (002) diffraction peak for the as-grown film was 0.34°and that for the film after the post-annealing was 0.23°. For x = 0.1, the FWHM value was 0.25°after the post-annealing. The improvement of the crystallinity was observed when x was below 0.1. However, further studies are needed to imporove the properties of the Al-doped Mg x Zn 1−x O films with high Mg content.

Results and discussion
Many researchers have reported that electrical properties of ZnO films are improved after post-annealing in a reducing atmosphere [28][29][30]. The resistivity of the Al-doped Mg x Zn 1−x O films was also decreased after the post-annealing without a Zn cap (conventional post-annealing in H 2 ). However, the conventional postannealing was not effective for the Al-doped Mg x Zn 1−x O films. The resistivity and electron concentration of the film (x = 0) after the conventional post-annealing were 7.6×10 −2 Ωcm and 1.9×10 19 cm −3 , respectively. The resistivity of the film (x = 0.1) after the conventional post-annealing was 6.6×10 2 Ωcm. We could not measure The transmittance spectra of the Al-doped Mg x Zn 1−x O films are shown in figure 3(a). The as-grown films showed a sharp absorption edge, and the absorption edge decreased with increasing Mg content. The transmittance spectra in the visible range were not affected by the post-annealing with a Zn cap. The optical bandgap of the films was estimated from the Tauc plot, (αhν) 2 versus hν, by extrapolating the linear portion of the plot to (αhν) 2 =0 ( figure 3(b)). The optical bandgap of the as-grown films was 3.28 eV (x = 0) and 3.43 eV (x = 0.1). These values are consistent with the reported optical bandgap of Mg x Zn 1−x O films [6,19]. The optical bandgap of the films after the post-annealing with a Zn cap was 3.23 eV (x = 0) and 3.41 eV (x = 0.1). The optical bandgap of the films was decreased slightly after the post-annealing with a Zn cap, but still depended on the Mg content in the films. It was found that no additional ZnO layer was grown although Zn vapor was supplied during the post-annealing with a Zn cap.
XPS measurements were performed to evaluate the surface composition of the Al-doped Mg x Zn 1−x O films (x = 0.1). Zinc, oxygen, magnesium, carbon, and chlorine were observed from the as-grown film and the film after the post-annealing with a Zn cap. The origin of chlorine may be hydrochloric acid used to clean the back side of the substrate after the sol-gel process. The Mg/Zn ratio of the as-grown film (x = 0.1) was 0.09, which was consistent with the Mg content in the precursor solution. As the reslt of Zn supply, the Mg/Zn ratio was decreased after the post-annealing with a Zn cap. Therefore, the (Zn+Mg)/O ratio was increased from 0.96 to 1.05. It was found that the film after the post-annealing with a Zn cap was slightly Zn-rich. Zinc vacancies are known as dominant compensation acceptors in n-type ZnO [31]. The decrease in zinc vacancy concentration is possibly related to the increase in electron concentration after the post-annealing with a Zn cap. Figure 4 shows XPS spectra of the Al-doped Mg x Zn 1−x O films (x = 0.1). The Zn 2p 1/2 (∼1044.5 eV) and Zn 2p 3/2 (∼1021.5 eV) XPS spectra were not changed by the post-annealing with a Zn cap. Figure 4(b) shows the O 1s XPS spectra of the films. The asymmetric peaks were deconvoluted into three components. The O1 component (∼530.2 eV) and the O2 component (∼531.3 eV) are attributed to the O 2− ions in ZnO and the O 2− ions in oxygen-deficient ZnO, respectively [32][33][34][35]. The O3 (∼532.3 eV) component is related to adsorbed hydroxides and contaminants [32][33][34][35]. The O2/O1 ratio was decreased after the post-annealing with a Zn cap. It was suggested that crystallinity of the Al-doped Mg x Zn 1−x O films were improved after the post-annealing with a Zn cap.

Conclusion
Al-doped Mg x Zn 1−x O films were deposited by a sol-gel method and post-annealed in H 2 . Structural and electrical properties of the films were improved by the post-annealing with a Zn cap when x was below 0.1. The film (x = 0.1) after the post-annealing with a Zn cap had resistivity of 6.0×10 −3 Ωcm and electron concentration of 5.7×10 19 cm −3 . The film (x = 0.1) after the post-annealing without a Zn cap had resistivity of 6.6×10 2 Ωcm. The electrical properties were improved after the post-annealing with a Zn cap, while the optical bandgap was not significantly changed. These results suggest that H 2 post-annealing with a Zn cap can be used to form low-resistivity Mg x Zn 1−x O films.