XRD analysis
From Figure 1(a) we can see clearly that all films exhibit a crystalline hexagonal wurtzite structure (JCPDS card no. 00-036-1451) with the quasi-predominance of (002) peak indicating the preferential growth of undoped and Mg doped ZnO thin films through the c-axis direction. However, weak inten- sity peaks of other crystallographic directions were observed in the following positions 31.96°(100), 36.56°(101), 47.87°(102), 63.17°(103) and 72.85°(004). The peak (002) experienced a shift towards higher angles over Mg doping (Figure 1(b)) which was reported by a number of authors [35–37] and it is due to the difference in the ionic radius between Mg+2 (0.57 Å) and Zn+2 (0.60 Å) [38]. This difference is directly related to the compression strain on the main axes of the wurtzite structure a and
c. Previously, Chang et al. [39] reported also similar compression strain in their sputtered MgZnO
thin films which were interpreted by good substitution between Mg and Zn ions. The grain size can be calculated by Debye-Scherrer formula [35]:
where λ is the wavelength of the incident X-ray, θ is the Bragg’s angle, K is the shape factor (0.9 for gaussian fit) and β is the Full Width at Half Maxima (FWHM) of the peaks. The crystallite size, lattice parameters and strain axes are tabulated in table 1. It should be noted that there was no significant change in the average crystallite size (33.13 nm for Mg0Zn1O film and an average of 33.03 nm for the MgxZn1−xO films) and the minus sign of the strain values (ϵa and ϵc) approves the compression nature of the strain. To calculate lattice parameter c of the thin films we used the Bragg’s law (with n=1) and the d spacing of wurtzite structure.
Table 1: The information obtained and calculated from the XRD spectra.
sample
|
2θ (degree)
|
d spacing (Å)
|
Crystallite size (nm)
|
a(Å)
|
c(Å)
|
ϵa (10−3)
|
ϵc (10−3)
|
Mg0.00Zn1.00O
|
34.75
|
2.580
|
33.13
|
3.2306
|
5.1592
|
-3.4241
|
-5.4746
|
Mg0.01Zn0.99O
|
34.77
|
2.455
|
31.26
|
3.2240
|
5.1560
|
-5.4601
|
-6.0915
|
Mg0.03Zn0.97O
|
34.77
|
2.456
|
31.30
|
3.2256
|
5.1560
|
-4.9665
|
-6.0915
|
Mg0.05Zn0.95O
|
34.78
|
2.577
|
35.83
|
3.2298
|
5.1547
|
-3.6709
|
-6.3421
|
UV-Vis measurements
The transmittance of the thin films was slightly enhanced on average with the introduction of Mg in the visible region (400-800 nm). The limit of the absorption zone was shifted to smaller wavelengths which renders the thin films to be more dielectric as Mg doping is increased, this effect is directly related to the blue shift of the optical band gap and it was approximated for each film using the Tauc’s plot:
αhν = A(hν − Eg)n (2)
where hν is the photon energy, Eg is the optical gap energy, A is constant and n equals ½ since ZnO has a direct band gap.
The widening of the optical band gap could be ascribed to the difference in electronegativity between Zn+2 and Mg+2 ions [40–42]. In similar study, Al-Ghamdi [43] reported this correla- tion between the optical band gap energy and the electronegativity in his work about amorphous Se96−xTe4Agx thin films.
In fact, it should be noted that both the electronegativity and electron affinity are directly related [44]. In this way Figure 4 presents the calculated optical band gap and the electron affinity versus Mg doping. The electron affinity (eχ) was quantitatively calculated using Vegard’s law [45]:
eχ(MgxZn1−xO) = eχ(ZnO) − (eχ(ZnO) − eχ(MgO))x (3)
eχ(ZnO), eχ(MgO) have the value of 4.5 eV [46] and 0.85 eV [47] for the electron affinity of ZnO and MgO respectively. Previously, iskenderoglu et al. [28] approved experimentally by UPS technique the inverse relation between the optical gap energy and the electron affinity of sprayed MgZnO alloy thin film, with Mg doping ranging from 0 to 15%.
Photoluminescence
The room temperature photoluminescence spectra are shown in Figure 5. The undoped thin film had four peaks at 381 (3.25 eV), 416 (2.98 eV), 441 (2.81 eV) and 505 nm (2.46 eV) which were explained by: the emission corresponding to excitons recombination, zinc interstitial (Zni), oxygen vacancies (VO) and donor VO – acceptor VZn recombination respectively [48, 49]. The same peaks were observed also in Mg0.01Zn0.99O except a blue shift of the 380 peak to 381 nm (3.26 eV). The Mg0.03Zn0.97O thin film had three peaks 379 (3.27 eV), 416 (2.81 eV) and 492 nm (2.52 eV) and they were attributed to excitons recombination, zinc interstitial (Zni) [48] and oxygen vacancy (VO)
[50] respectively. Finally the Mg0.05Zn0.95O thin film exhibited two peaks at 378 (3.28 eV) and 503
nm (2.47 eV) and they were manifestations of excitons and donor VO – acceptor VZn recombinations [48, 49]. The blue shift of the near band edge emission from the PL peaks was in consistency with the UV visible calculations of the optical band gap, however, it was at a lower rate which was explained by stokes shift [51].
Electrical measurements
Figure 6 demonstrates the near isotropic electric transportation of the deposited films which could be attributed to the fact that the films have the predominant orientation (002) of the wurtzite structure [52].
The free charge carriers’ density decreased from 3.146 × 1018 for the undoped film to 9.273 ×
1013 cm−3 for Mg0.05Zn0.95O film. In parallel, the resistivity increased from 109 to 1268 Ω cm and the mobility increased from 0.01821 to 53.08 cm2/(V.s). The decreased charge carriers’ density could be attributed to the fact that the conduction band (CB) of pure ZnO consists mainly of O2p and Zn4s states [53, 54], so the introduction of Mg in ZnO thin films will reduce the Zn4s state and introduce Mg3p state which has high energy relative to the Zn4s [55, 56]. Moreover, the widening in the optical bang gap energy discussed previously, could also influence the free charge carriers’ density since the electrons passing from the valance band must requires higher energy to access the conduction band.
Surface morphology
Figure 8 shows the morphology of undoped and 5 at. % doped ZnO thin films. The undoped (Figure 8(a)) thin film shows a dense surface with granular mixture of small grains and large aggregates. The size of the small grains varies from ∼ 20 to 80 nm and that of the aggregates reaches ∼ 200-300 nm. From the doped sample (Figure 8(b)), it is evident that magnesium has a tendency to promote the phenomenon of coalescence. This doped film shows a less dense and relatively homogeneous morphology with large aggregates whose size varies from ∼ 200 to 500 nm.
No cracks nor empty holes were observed on the surface of the films, revealing the high quality of our films.
M-lines measurements
The M-lines measurements demonstrated the guiding modes present in the films. All films had four guiding modes (TE0, TE1, TM0, and TM1) two for each optical polarization (Transverse Electric mode (TE) and Transverse Magnetic mode (TM)) as illustrated in Figure 9. Using the dispersion equations for TE and TM polarizations, the optogeometric parameters can be calculated [57–59]. The effective indices, the films thicknesses and the birefringence values are presented in Table 2.
Figure 10 illustrate the variation of nTE, nTM, optical gap energy and free charge carriers’ density as a function of the Mg concentration. The slight difference between nTM and nTE confirmed the birefringence behavior of our films. This made the guided waves traveling during the TE mode in the plane perpendicular to the c-axis of the wurtzite structure submit to an ordinary refractive index (nTE) and during the TM mode to an extraordinary one (nTM) [22]. Both nTE and nTM were found to decreased over Mg doping. This behavior seems to be in good agreement with the broadening of the optical band gap and the depletion of the conduction band free charge carriers’ (Figure 10) [60]. The birefringence was measured to be positive for all films which is a good indication for the unchanging orientations of the bonds Zn-O with the replacement of Zn by Mg [61, 62].
Table 2: Optogeometric Properties of the deposited thin films.
Sample
MgxZn1−xO
|
Effective index ± 10−4
|
Thickness TE
± 0.1 (nm)
|
Thickness TM
± 0.1 (nm)
|
Refractive index ± 10−4
|
Birefringence
(nTE-nTM)
|
TE0
|
TE1
|
TM0
|
TM1
|
nTE
|
nTM
|
Mg0.00Zn1.00O
|
1.8923
|
1.6565
|
1.8793
|
1.6033
|
436.1
|
452.1
|
1.9699
|
1.9761
|
0.0062
|
Mg0.01Zn0.99O
|
1.8758
|
1.5994
|
1.8539
|
1.5486
|
347.4
|
382.6
|
1.9680
|
1.9682
|
0.0002
|
Mg0.03Zn0.97O
|
1.8738
|
1.6203
|
1.8581
|
1.5647
|
413.1
|
426.3
|
1.9580
|
1.9652
|
0.0072
|
Mg0.05Zn0.95O
|
1.8451
|
1.5981
|
1.8278
|
1.5494
|
417.7
|
433.7
|
1.9278
|
1.9314
|
0.0036
|
The inverse relation between the optical gap energy, the resistivity and the refractive index as Mg content increased in the ZnO thin film was also observed by Kaushal et al. [63]. The same fact has been reported by Teng et al. [27] for Mg-doped ZnO thin films prepared by pulsed laser deposition. In similar study, Sorar et al. [64] found that the ordinary refractive index generally decreases with the increase of the Si doping in ZnO thin films prepared by sol-gel and annealed at 350 °C and 550 °C whereas their optical band gap energy was found to be blue shifted.