3.1. Morphology and crystal structure of AZO thin films
X-ray diffraction (XRD) analyses were performed on all the AZO thin films using glass substrates at various deposition conditions, as shown in Fig. 1a-c. Here, we set a baseline recipe as: working pressure was 0.2 Pa, RF power was 100 W, and substrate temperature was ambient temperature. It can be seen from Fig. 1a-c that the diffraction peaks of AZO thin films match well with the standard XRD pattern of ZnO (PDF#36-1451), demonstrating that the hexagonal wurtzite crystal structures were formed. It is well know that the sputtering ZnO thin films are highly texture with a preferential growth perpendicular to the substrate [24–26]. Our experiments represent the similar results, especially for the films fabricated in low working pressure (< 0.6 Pa), moderate RF sputtering power (50–100 W), and suitable substrate temperature (100–200°C), which have a strong diffraction peak of (002) at 2θ value of 34.4°.
Figure 1a reveals that the crystallization of AZO thin films becomes worse when the working pressure increases from 0.2 Pa to 1.0 Pa. The diffraction peaks at 2θ value of 36.2° suggest that some grains are crystalline with orientation along (101) plane. It can be ascribed that the higher working pressure may raise the deposition rate and cause the alteration of the mean free path of the sputtered particles [21], resulting in poor crystallization. XRD patterns of AZO thin films deposited at various RF sputtering power are shown in Fig. 1b. The intensity and full width at half maximum of the diffraction peaks indicate that the crystallite size changes with the sputtering power, and the similar experimental phenomenon was also observed in the literature [13].
We can also find that, for AZO thin films deposited at the substrate temperature ranging from ambient temperature to 200°C, the intensity and sharpness of the (002) peak increased, and the other peaks disappeared (see Fig. 1c). That’s to say, higher substrate temperature is conducive to improve the crystallization of the thin films. According to the literature, with the increase of the deposition temperature, the mobility of the growing species on substrate is greatly enhanced [7, 27, 28]. Therefore, the nucleation of the growing species is improved, further promoting the crystallinity and reducing the defects of AZO thin films. As verified with grain size, calculated by the Scherrer formula based on the (002) peak [6, 8], it increases from 32.0 nm to 34.1 nm with the substrate temperature varying from 100°C to 200°C.
The surface morphology of AZO thin films plays a critical role in the photovoltaic device performance. The SEM micrograph of the film, which is deposited at 0.2 Pa, 120 W and 200°C, is presented in Fig. 1d. It can be clearly observed that the film has a compact and homogeneous self-textured surface. For the purpose of absorbing wider wavelength region, the rough transparent conducting oxide (TCO) thin films are used to enhance light scattering effects. Compared to the reference TCO thin films, there has a reduction in parasitic absorption for the textured films because it has better transparency and maintains excellent light trapping qualities [29, 30]. In our experiments, the electrical and optical properties of AZO thin films are also analyzed in the following.
3.2. Electrical properties of AZO thin films
Electrical properties of AZO thin films deposited at different conditions are shown in Fig. 2 and Table 1. We firstly carried out the measurements of the deposition rate of the films under various deposition conditions. From Fig. 2a, it can be found that the substrate temperature and working pressure have a slight effect on the deposition rate, where we set the ambient temperature as 50°C. Unlike the above two parameters, RF sputtering power can strongly affect the deposition rate of AZO thin films. As described in the literature, the faster deposition rate can be achieved by increasing RF sputtering power, further improving crystalline size [9]. This phenomenon is in good agreement with the XRD results (see Fig. 1b).
The effects of the deposition conditions on carrier concentration (N), Hall mobility (µ) and resistivity (ρ) of AZO thin films are demonstrated in Fig. 2b-d. From Fig. 2b, we find that there has a V-shape curve of electrical properties for AZO thin films deposited at the working pressure ranging from 0.2 Pa to 1.0 Pa. When the working pressure increases to 0.6 Pa, the deposited thin films have the highest resistivity of ~ 1.9⊆10− 2 Ω·cm, the lowest carrier concentration (0.8⊆1020 cm− 3) and Hall mobility (4.2 cm2·(V·s)−1). The electrical properties are closely associated with the film crystallinity, which is evident from the XRD results of AZO thin films under the working pressure (see Fig. 1a). At lower working pressure, the film crystallinity is much better than that deposited under higher working pressure, which indicates better electrical properties.
Unlike the deposition parameter of working pressure, the resistivity decreases as the increasing of RF sputtering power and substrate temperature, but the Hall mobility and carrier concentration increase. As shown in Fig. 2c, when PF sputtering power increases from 30 W to 80 W, the resistivity decreases quickly, which may be mainly caused by increment of film thickness [23]. Then, there has a slight decrease as PF sputtering power keeps rising. From Fig. 2d, the resistivity of AZO thin films decreases gradually from 3⊆10− 3 Ω·cm to 0.9⊆10− 3 Ω·cm with the substrate temperature varying from 50°C to 200°C, while the films have higher carrier concentrations and Hall mobilities at higher substrate temperature. According to the reported literature, the reduction in resistivity at higher deposition temperature is related to the improved film crystallinity and the growth of grain size, which will result in reducing the scattering of carrier transport [31–33]. Overall, the minimum resistivity of 0.9⊆10− 3 Ω·cm, the highest carrier concentration of 2.8⊆1020 cm− 3 and the best Hall mobility of 22.8 cm2·(V·s)−1 can be obtained for AZO thin films deposited at the optimum deposition condition of 0.2 Pa, 120 W and 200°C.
Table 1 Electrical properties of AZO thin films varying with deposition parameters.
Deposition Parameters
|
Electrical Properties
|
Power (W)
|
Pressure (Pa)
|
Substrate temperature (°C)
|
Resistivity (⊆10− 4 Ω·cm)
|
Hall mobility (cm2·(V·s)−1)
|
Carrier concentration (⊆1020 cm− 3)
|
100
|
0.2
|
RT
|
48.26
|
8.69
|
1.51
|
100
|
0.4
|
RT
|
80.51
|
8.41
|
0.93
|
100
|
0.6
|
RT
|
190.99
|
4.19
|
0.79
|
100
|
0.8
|
RT
|
74.20
|
5.82
|
1.47
|
100
|
1.0
|
RT
|
63.90
|
5.99
|
1.64
|
30
|
0.2
|
RT
|
366.49
|
2.81
|
0.62
|
50
|
0.2
|
RT
|
127.64
|
4.84
|
1.02
|
80
|
0.2
|
RT
|
57.44
|
8.55
|
1.27
|
100
|
0.2
|
RT
|
48.26
|
8.69
|
1.51
|
120
|
0.2
|
RT
|
31.50
|
12.15
|
1.73
|
100
|
0.2
|
RT
|
48.26
|
8.69
|
1.51
|
100
|
0.2
|
50
|
30.95
|
12.23
|
1.73
|
100
|
0.2
|
100
|
28.64
|
12.68
|
1.80
|
100
|
0.2
|
150
|
18.33
|
16.50
|
2.01
|
100
|
0.2
|
200
|
9.64
|
22.76
|
2.84
|
3.3. Optical properties of AZO thin films
To achieve the optical properties of AZO thin films, we carried out the transmittance measurements. The optical properties of thin films varying with deposition parameters are summarized in Table 2 and Fig. 3, which are discussed in detail in the following sections. Figure 3a shows the transmittance spectra of the films deposited at various RF sputtering power. In the wavelength range of 400–1100 nm, the films are highly transparent and the average transmittance is up to 85%. It can also be found that the spectrums of as-deposited films exhibit different interference fringes pattern, which is related to the film thickness [34]. The film thickness has a strong influence on the optical bandgap. The absorption coefficient (α) can be calculated using the optical transmittance (T) of AZO thin films, which is described by the relation of T = exp(-αd) [7], where d is the film thickness. Then, the optical bandgaps (Eg) of AZO thin films are determined using the relation of (αhυ)2=hυ − Eg by extrapolating (αhυ)2 plot linearly with the incident photon energy (hυ) for direct transition [9].
Table 2 Optical properties of AZO thin films varying with deposition parameters.
Deposition Parameters
|
Optical Properties
|
Power (W)
|
Pressure (Pa)
|
Substrate temperature (°C)
|
Transmittance (400–1100 nm, %)
|
Bandgap (eV)
|
30
|
0.2
|
RT
|
92.54
|
3.28
|
50
|
0.2
|
RT
|
90.71
|
3.32
|
80
|
0.2
|
RT
|
89.08
|
3.36
|
100
|
0.2
|
RT
|
88.90
|
3.38
|
120
|
0.2
|
RT
|
83.92
|
3.39
|
120
|
0.2
|
100
|
85.32
|
3.41
|
120
|
0.2
|
150
|
88.10
|
3.43
|
120
|
0.2
|
200
|
85.76
|
3.52
|
Based on the above calculation method, the optical bandgaps of AZO thin films deposited at different RF sputtering power can be obtained, as shown in Fig. 3b. The bandgap of the films increases from 3.28 eV to 3.39 eV with the increase of RF sputtering power. The widening bandgap may be caused by the increased carrier concentration (see Fig. 3c) in accordance to Burstein-Moss effect [35]. From Fig. 3d, it can be seen that the transmittance of the films deposited at different substrate temperature is also around 85% in the visible region. Compared with the films deposited at various RF sputtering power, the spectrums of as-deposited films exhibit the same interference fringes pattern. But for NIR spectra, there has an obvious drop in the transmittance, and this may be ascribed to the strengthening of light scattering and absorption [36]. The bandgaps of the films, shown in Fig. 3e, are 3.39 eV, 3.41 eV, 3.43 eV and 3.52 eV with substrate temperature of AT, 100 °C, 150 °C and 200 °C, respectively. We assumed that not only the increase of carrier concentration (see Fig. 3f), but also internal stress and phase purity [16, 37], can affect the optical bandgap of AZO thin films.
Dependence of carrier concentration and optical bandgap of AZO thin films on RF sputtering power and substrate temperature indicates that absorption edges of the films are strongly dependent on carrier concentration. According to Burstein-Moss effect, in the doped n-type semiconductor, the optical bandgap Eg has contribution from free charge carriers E0 and the donor atoms ΔEMB. Therefore, considering the single parabolic band model, the energy shift by carrier concentration of AZO thin films can be expressed as [38–40]:
where, m* is the electron effective mass in the conduction band, h is the Planck constant (6.626⊆10− 34 m2·kg·s− 1), and N is the carrier concentration. To further investigate the relationship of N and ΔEMB, the N-ΔEMB curve is plotted in Fig. 4. Note that the solid curve in Fig. 4 is calculated using the above equation. It can be seen that the ΔEMB exhibits a good linear relation with the carrier concentration, which indicates that the Burstein-Moss theory of band filling can fully interpret the bandgap shift of AZO thin films. Similar results are also reported by Zhu et al., and they demonstrate that the increase of optical bandgap for AZO thin films can be mainly attributed to the Burstein-Moss effect, although other effects also appear [15].