As observed, the film thicknesses (t) varied from 139.4 nm to 290 nm with increase in power of RF applied on ZnO target from 150 W to 225 W. Thus, it can be inferred that there is a direct proportionality between the number of sputtered atoms and applied RF power (Fig. 1).
The Xray diffractogram of ZnO films doped with AlN are presented in Fig. 2. The (002) ZnO orientation appeared in all the films with the highest intensity exhibited by the AZO 24 sample. Also, by comparatively analyzing the diffraction spectra obtained for the doped thin films against that of the bare ZnO presented in an earlier study 10, the two theta value and interspacing (d) of the ZnO were found to be equal to 34.1841º and 2.6229Å, respectively 10. The least d-spacing value of 0.26199 nm of the AZO23 sample resulted due to the substitution of Al+ 3 ion, which has a radius of 0.53 Å with Zn+ 2 ion, which has a larger radius of 0.74 Å, leading to a shift in (002) peak position to a larger value. However, the d-spacing values for AZO 22, AZO 24 and AZO 25 surpasses that of the bare ZnO, possibly as a result of the replacement of O-2 ions, which has a radius of 1.40 Å with N-3 ions that has a larger radius of 1.46 Å, resulting in the shift of (002) to a smaller angle. Using the Scherrer's formula [Eq. 1], the X-ray variables and FWHM of ZnO (002) peak were inputted to determine the crystallite size (D) of the samples.
where λ is X-ray wavelength, θ is the Bragg angle,and β is full width at half maximum (FWHM). The lattice parameters come out from XRD spectra of ZnO thin films doped with AlN (002) are shown in Table 2.
Table 2
XRD results of AlN : ZnO thin films
Sample
|
2 ɵ (˚)
|
FWHM(˚)
|
d (Å)
|
c (Å)
|
D(nm)
|
AZO 22
|
33.9036
|
0.5904
|
2.6441
|
5.2882
|
14.70
|
AZO 23
|
34.1750
|
0.4000
|
2.6199
|
5.2399
|
21.71
|
AZO 24
|
34.0410
|
0.3444
|
2.6338
|
5.2675
|
25.21
|
AZO 25
|
34.1799
|
0.2952
|
2.6234
|
5.2467
|
29.42
|
Table 3 presents the EDX elemental composition of ZnO films doped with AlN wherein trace amounts of nitrogen (0.10–1.26%) are observed. Despite fixing the power at 100 W of RF sputtering on AlN, it is imperative to note that the concentrations of N and Al were varied. These variations in concentrations of incorporated N and Al were achieved by applying various RF powers on ZnO target and the inclusion of N in the course of sample preparation and synthesis, leading to the generation of (N)O acceptors or (N2)O donors. Another plausible mechanism is the production of N-Al-N complex pattern NO (N on O) as well AlZn (Al on Zn), which boosts strong interactions between acceptor (N) and donor (Al), stabilizes ionic charges and limits the acceptors interacts. Also, the acceptor and donor's binding energies can be reduced and increased, respectively, thereby the solubility of N-Al-N compounds in the AlN doped ZnO films will be improved 5.
Table 3
EDX analysis of AlN: ZnO films at different RF powers on Si substrates
Sample
|
NK
(at %)
|
O K
(at %)
|
Al K
(at %)
|
Zn L
(at %)
|
AZO22
|
0.48
|
54.13
|
3.42
|
41.98
|
AZO23
|
1.26
|
53.34
|
2.51
|
42.89
|
AZO24
|
0.56
|
51.42
|
1.48
|
46.53
|
AZO25
|
0.10
|
51.38
|
0.60
|
47.97
|
As shown in Figs. 3 and 4, AFM and FESEM were employed to investigate the doped films microtopography and morphological variations. The root mean square (rms) for AZO22, AZO23, AZO24 and AZO25 were 3.05 nm, 3.87 nm, 6.60 nm and 4.89 nm, respectively occure with increasing RF power on ZnO target. Furthermore, it is discernible from the FESEM results that the grain size of the films were enhanced with increasing RF power on ZnO target, which is compatible with the XRD data. As noted in earlier studies, the number of sputtered molecules that arrive at the substrate surface is the prevailing factor that controls the morphology of the films 12,13.
UV-vis transmission spectrometer was utilized to calculate the energy band gap (Eg) of the samples. The photon energy (E) and the absorption coefficients α are given by Eq. (2) below;
where A is a constant and t is the film thickness. Therefore, the plot of energy E against (αE)2 showed a linear line which cutting of the energy axis at the Eg value.
The prepared films showed with an optical transmittance that exceeds than 76% in visible range, as shown in Fig. 5. Plot of (αE)2 against E yielded band gap of 3.56, 3.33, 3.28 and 3.24 eV occur with RF powers of 150, 175, 200 and 225 W respectively, as presented in Fig. 6. The lowering in energy band gap from 3.56 eV to 3.24 eV could be related to the N doping exceeding the valance-band, irrespective of the ZnO films's conduction type 14.
The photoluminescence (PL) spectra of the thin films (Fig. 7) were acquired under ambient conditions. The characteristic UV emission peaks of free exciton recombination are found in all prepared films, indicating good optical properties and good crystalline structure10,15. The relatively low intensity of broad visible emission in the synthesized ZnO samples compared to undoped ZnO indicates a decline in the intrinsic defects effect and the successful doping of ZnO films with AlN 10,16,17. With increasing Al concentration from 0.6–3.42%, there is an observed shift in UV emission band from 3.24 eV to 3.33 eV as a result of the Moss-Burstein effect, in accordance with the transmittance data.
Raman spectra for the prepared films is shown in Fig. 8 (a). The peak at 276 cm-1 denotes nitrogen 18. Compared with the undoped ZnO, there is a discernible shift in the peak denoting A1 (LO) modes [A. Ismail et al., 2013] at 573.72 cm-1 to 578.58 cm-1 for the sample synthesized, indicating the successful doping of the thin films with AlN18,19. There is a linear correlation of the intensities of Raman modes at 57858 cm-1 and 276 cm-1 with N concentrations in the films, which can be utilized as an important parameter for calculating the relatively concentration of N in the films 20. As well as the modes intensity depends on the applied RF power to the ZnO target, as it shown clearly in Fig. 7 (b). The ZnO samples prepared via RF powers at 200 W and 225 W exhibited higher N concentrations as compared to the samples prepared at 150 W and 175 W. This is clearly observed in Fig. 8 (b), where these samples show higher mode intensities at 276 cm-1 and 578.58 cm-1 compared with those samples prepared by ZnO target with RF power of (150 W and 175 W). Besides N concentration, the conductivity type of the films is dependent on the N2 (molecular): N (atomic) ratio, as reported in a previous study 21, and further shown in the analysis of electrical properties of the synthesized samples. Peaks at 301.04 cm-1 and 521.48 cm-1 (Fig. 8-a) denote the Si substrates 22.
Table 4 and Fig. 9 show the electrical properties derived from Hall effect measurement. It is well known that O vacancies (VO), Zn interstitial (Zni), substitutional Zn on O site (ZnO), N interstitial (Ni), N2 on O site (N2)O and Al on Zn site (AlZn) are donors while N on O site (NO), O interstitial (Oi) and Zn vacancy (Vzn) are acceptors. The results of mobility, carrier concentration and resistivity were shown in Table 4. The samples (AZO 23 and AZO 25) display p-type conductivity with hole concentrations of 3.06 ×10+ 16 cm-3 and 1.83 ×10+ 18 cm-3 respectively. This p-type conductivity behavior of AZO 23 sample results from production of N-Al-N pattern that serve as a shallow acceptor, which results from the substitute of Zn+ 2 ions with Al+ 3 ions. This result is appropriate with the XRD result, EDX and Raman investigations, where the AIN doped ZnO sample showed smaller interface (d) of 0.26199 nm in comparison with a bare ZnO (0.26229 nm) in addition to the nitrogen detected in these samples. The behavior of p-type of AZO 25 sample is because of the compensated of O-2 ions (radius of 0.140 nm) in N-3 ions (radius of 0.146 nm) and the generation of (N)O acceptors and (N2)O donors. The effect of (N2)O donors is smaller than that of (N)O acceptors resulting in p-type conductivity in this sample. This deduction is consistent with the compositional and structural data obtained from XRD, Raman, and EDX results, where N was detected in the synthesized samples, and the AIN doped sample showed larger d-spacing values of 0.26234, 0.26441 and 0.26338 nm for AZO 22 and AZO 24 samples as compared with 0.26229 nm for undoped ZnO. AZO 22, AZO23 and AZO 24 samples also displayed n-type conductivity with concentrations of 8.00 ×10+ 17 cm-3 and 6.20 ×10+ 18 cm-3. This implies that N-3 successfully replaced O-2 ions; By any way, the impact of (N)O acceptors is smaller than that of (N2)O donors and the other intrinsic donors, appearing in n-type conductivity of these samples. The variability in mobility for the different samples may arise from the nature of the grain – boundaries in the samples. To calculate the contribution of the grain – boundaries, the mean free paths of the carriers in the films were calculated using Eq. 3 below:
$$\text{l}={\frac{\text{h}}{2\text{e}}\times \left[\frac{3\text{n}}{{\pi }} \right]}^{(1/3)}\times {\mu }$$
3
………………
where Ɩ, n and µ represent mean free path, concentration of carriers, and mobility, respectively. The mean free paths were calculated to be 2.19 nm, 0.75 nm, 0.11 nm and 0.47 nm for AZO22, AZO23, AZO24 and AZO25, films respectively. Also, the ratio of the mean free path to the crystallite size (Ɩ/D) were found to be 16%, 3%, 0.44% and 1.5% for the respective films, which are higher than earlier reported values (e.g. 0.12%). The higher Ɩ/D ratios for AZO22 and AZO23 films compared with AZO24 and AZO25 films contributed to the higher mobility of carriers in the former films (AZO22 and AZO23).
Table 4
the electrical properties of the prepared thin films
Sample
|
Thickness, t (nm)
|
RF power on ZnO (W)
|
Resistivity
(Ω-cm )
|
Mobility
(cm2 V− 1s− 1)
|
Carrier concentration
( cm− 3)
|
AZO22
|
139.4
|
150
|
0.067
|
116
|
-8 × 10+ 17
|
AZO23
|
160
|
175
|
1.75
|
117
|
3.06 ×10+ 16
|
AZO24
|
239.2
|
200
|
0.337
|
2.99
|
-6.20 ×10+ 18
|
AZO25
|
290
|
225
|
0.179
|
19.1
|
1.83 ×10+ 18
|