4. Data Collection and Analysis
4.2 Data collection
The surface morphology of all the films under study was obtained using a Leo-Zeiss scanning electron microscope while the elemental (chemical) composition of the films was determined by energy dispersive x-ray spectroscope attached to the scanning electron microscope. The scanning electron microscope (SEM) which was used for imaging the films under study consists of an electron column, a sample chamber and a thermal emission source. The overall SEM equipment is connected to a computer monitor. In each case of characterization, the sample was mounted on the sample chamber. High-energy incident electrons were generated in the electron column and are made to interact with the sample under study. As the incident electrons strike the sample, the samples undergo scanning by the electron microscope wherein surface imaging of the film and the chemical composition of the film were generated and displayed on a computer monitor. This information was further used for the analysis of the films under study in this research.
54 3.8.4 Structural studies
The XRD patterns of all the films in this research were recorded using a SmartLab X-ray diffractometer. The glass substrate containing the sample was mounted on a sample stage in the x-ray diffractometer and scanned between 0o and 80o with operating voltage and current of 45 kV and 200 mA respectively with Cu-kα1 radiation and wavelength, λ set at 1.540593Å.
The x-ray diffractometer was connected to a computer monitor. As the films were being scanned, the diffraction patterns were recorded and displayed on the computer monitor.
For an effective understanding of the diffractogram, structural parameters such as the grain size, inter-planar distance, strain, lattice constant, dislocation density and number of crystallites per unit area have been calculated and presented in appendixes A15 and A16.
The grain size, D, was calculated from the Debye-Scherer expression (Gnanamuthu et al., 2015):
D = 0.94𝜆
βcosθ (3.5)
where λ is the diffraction wavelength given by 1.540593Å, β is the Full Width at Half Maximum (FWHM) while θ is the Bragg‟s diffraction angle.
The inter-planar distance, d, was calculated from the expression (Kissinger, 2007):
d = λ
2sin θ (3.6)
The strain, ε, was calculated from the expression (Muthumari et al., 2012):
ε = βCos θ
4 (3.7)
The dislocation density, δ, was calculated from the mathematical expression (Dawood et al., 2014):
55 δ = 1
D2 (3.8)
where D is the grain size.
The lattice constant for the hexagonal phase of ZnSe was calculated from the relation (Mote et al., 2013):
1 d2 = 4
3a2 (ℎ2 + hk + 𝑘2) + 𝑙
2
c2 (3.9)
where a is lattice constant, d is the inter-planar spacing and hkl represents the miller indices.
The number of crystallites per unit area was calculated using the relation (Aly et al., 2015):
N = t
D3 (3.10)
Where t, is the thickness of the ZnSe film and D is the crystallite size corresponding to each plane.
56
CHAPTER FOUR
RESULTS AND DISCUSSIONS 4.1 Thickness variation of pure ZnSe
The varying thickness obtained for the ZnSe films at different substrate temperature and different flow rate are shown in tables 4.1a and 4.1b respectively.
Table 4.1a: Thickness variation of pure ZnSe films with temperature Temperature (oC) Thickness (nm)
300 100
325 105
350 138
375 200
400 150
Table 4.1b: Thickness variation of pure ZnSe films with flow rate Flow Rate
(μL/spray) Thickness (nm)
400 138
480 183
560 210
640 150
720 110
57
From table 4.1a, the thickness of the ZnSe films increases with increase in substrate temperature up to 375 oC and thereafter decreased at 400 oC. This sudden decrease of film thickness at higher substrate temperature may be as a result of reduced transfer of precursor to the substrate. This can be attributed to gas convection emanating from the chamber, hence pushing the droplets away from the substrate leading to the development of crystallites within the vapour (Gnanamuthu et al., 2015).
From table 4.1b, the thickness of the film increases with flow rate up to 560 μL/spray and thereafter decreases.
4.2 Optical studies of the ZnSe films 4.2.1 Spectra absorption of ZnSe films
The spectra absorption curves of ZnSe films deposited at different substrate temperatures and flow rate at 0% doping are presented in figures 4.1a and 4.1b respectively.
Figure 4.1a: Plot of absorption versus wavelength for ZnSe films deposited at different substrate temperatures
Figure 4.1b: Absorption curves of ZnSe films deposited at different flow rates
58
From figure 4.1a, all the ZnSe films show very poor absorption of light in the visible region of the electromagnetic spectrum and moderate absorption in the ultraviolet region with the highest absorption of about 55.17% observed for the film deposited at 325 oC. (Any number can be expressed as a percentage by multiplying that number by 100. Hence, 0.5517 can be expressed as 55.17%).
Figure 4.1b indicates that the ZnSe films deposited at different flow rates exhibited very poor absorption of radiation in the visible region. In the UV region, an absorbance value of about 51.88 % was observed for the film deposited at a flow rate of 400 μL/spray. However, the films deposited at 480 μL/spray, 560 μL/spray, 640 μL/spray and 720 μL/spray exhibited poor absorption within the range of 12.17% to 46.87% in the UV region. The poor absorption exhibited by ZnSe material in the visible region indicates a high transmission in this region.
The result agrees with previous report of Okereke and Ekpunobi (2011) for chemical bath deposited ZnSe Films for buffer layers.
Figures 4.1a and 4.1b reveal a sharp decrease in the absorption spectra for all the films below 400 nm which can be attributed to the onset of electronic transitions between the valence and conduction bands occurring at the band edge. This observation is referred to as fundamental absorption. This result follows the same trend as the report of Ozutok et al., (2012) for Mn-doped ZnS films and Okereke and Ekpunobi (2011) for chemical bath deposited ZnSe films.
The absorption values (appendixes A1 and A2) for zinc selenide films deposited at different substrate temperatures and flow rates are presented.