The influence of substrate temperature on the properties of aluminum-doped zinc oxide thin films deposited by DC magnetron sputtering

The thin AZO films were deposited at 40 W and 5 mTorr with various substrate temperatures, ranging from R.T. to 500°C. The effects of substrate temperature on the microstructure of AZO thin films were investigated through XRD analysis. Figure 1 shows the X-ray diffraction patterns of the AZO films. As the substrate temperature increased, the peak intensity increased. The peak intensity is the largest at the substrate temperature of 300°C;then, as the substrate temperature increased above 300°C, the peak began to decrease. These trends were caused by a different level of incident energy at the substrate at various temperatures. Also, the vapor pressure of zinc is relatively higher than that of aluminum. This is why the aluminum doping level at a substrate temperature of 300°C was higher than at the other substrate temperatures [8]. Figure 2 shows the full width at half maximum (FWHM) and grain size of the AZO films, determined through the use of Scherrer’s formula. The grain size was inversely proportional to the FWHM. The largest grain size was ∼27 nm at 300°C. AFM images are shown in Fig. 3. The RMS roughness of the AZO thin film, deposited at 300°C, was lower than the other thin films.

Figure 4 shows the electrical properties of the AZO thin films deposited with various substrate temperatures. The resistivity of the films decreased as the substrate temperature increased from R.T. to 300°C. The minimum value for the resistivity of 6.0 × 10⁻⁴Ω cm was obtained at 300°C. A further increase in substrate temperature resulted in a slight increase of resistivity. Both the carrier concentration and Hall mobility increased as the substrate temperature increased from R.T to 300°C, reaching their maximum values of 2.7 × 10²⁰ cm⁻³and 20.4 cm²/Vs, respectively, at 300°C. As the substrate temperature increased further, both the carrier concentration and Hall mobility decreased.

The transmittance spectra, in the range of 300–800 nm, for the AZO thin films deposited with various substrate temperatures are shown in Fig. 5. The film deposited at R.T has a relatively low transmittance, but it still exhibited a visible light transmittance of about 80%. Also, all films, excluding the thin film deposited at R.T., exhibited average transmittances above 90% in the visible region. Figure 6 shows plots of (αhυ)²vs. hυ for the AZO films deposited with various substrate temperatures. The absorption edge for direct interband transition is given by Serpone et al. [9], where α is an absorption coefficient, hυ denotes photon energy, C is a constant for a direct transition, and E _g is the optical energy band gap.

The optical energy gap E _g can be obtained from the intercept of (αhυ)² vs. hυ for direct allowed transition [10]. The optical band gap widths of the AZO films were estimated to be 3.42, 3.44, 3.59, 3.68, 3.62, and 3.60 eV at substrate temperatures of R.T, 100, 200, 300, 400, and 500°C, respectively. The absorption edge of the transmittance shifted to the shorter wavelength (blue-shift) region up to the substrate temperature of 300°C. This result is believed to be due to the Burstein–Moss effect hypothesis. which predicts that the Fermi level inside the conduction band moves upward with increasing donor concentration due to the filling of the conduction band by the increase of electron carriers. It is known that the optical band gap is proportional to the carrier concentration. The optical band gap decreased with increasing substrate temperature above 300°C. These facts correspond with the aforementioned hypothesis.

Table 1 shows the RMS roughness values of the AZO thin films deposited with various substrate temperatures. The RMS roughness values were 11.0, 0.59, and 0.73 nm for the thin films deposited with the substrate temperatures of R.T., 300°C, and 500°C, respectively. The increase in surface roughness of the films promotes oxygen absorption on the surface of the crystallites to form dangling bonds, which then act as electron traps [11]. These electron traps are responsible for the decrease in carrier concentration and Hall mobility. In addition, the rough surface morphology scatters the incident light, thus reducing optical efficiencies. However, the change of morphology with increasing substrate temperature seems to be rather small except for the film deposited at R.T. Therefore, electronic and optical properties of AZO films would not be affected by the surface roughness in the present case.

The compositions of the AZO thin films, determined using SIMS referenced by RBS, are shown in Fig. 6. The analysis shows that AZO films exhibited a homogenous composition, but the ratio of Zn/O is, in general, greater than 1. This suggests non-stoichiometric structural properties of ZnO [12]. The depth profile of the AZO film resulted in an atomic ratio of Zn/O/Al of 62.27/31.46 /5.93%. The ratio of Al/Zn, by weight, was 1.1%, which is greater than that in the target (2 wt%). This may be due to the sputter yield property of Zn and Al atoms [13].