Optical spectroscopic analysis of annealed Cd_1−xZn_xSe thin films deposited by close space sublimation technique

Fig. 1 shows the transmittance spectra in the wavelength range between 300 and 2500 nm for the as-grown Cd_1−xZn_xSe films on the glass substrates and for those samples after annealing. The spectra show two regions, one in higher wavelength with practically higher transmission and the other in lower wavelength in which transmission decreases steeply. The spectrum shows the interference pattern with a sharp fall at the band edge which is a confirmative evidence of the formation of uniform and good crystalline films [14]. As the x parameter increases, transmission shows an increasing trend with the wavelength, and the fundamental edge is shifted towards shorter wavelength (or higher energy) region indicating an increase in band gap energy. Optical transmission decreases after annealing, which is probably due to the increase in crystallite size and decrease in intercrystalline boundaries [15]. The absorption edge of the films was observed to shift towards longer wavelengths after annealing. In order to show the effect of annealing temperature, the absorption edge was investigated for the films annealed at 400°C. The optical absorption edge was determined from the transmission spectra, which is a simple method that explains the features concerning the band structure of a film.

The optical absorption edge was analyzed by the following relationship [16]: (α<!-- a -->hν)2=A(hν−<!-- - -->Eg) $${(\alpha h\nu )^2} = A (h\nu - {E_g}) $$ (2)

where α is the absorption coefficient, h is the Planck’s constant, A is a constant and E_g is the optical band gap energy. The band gaps estimated from the plots of (αhν)² versus hν are shown in Fig. 1 for the as-deposited and annealed Cd_1−xZn_xSe thin films. The linear nature of the plots at the absorption edge confirms that Cd_1−xZn_xSe is a semiconductor with a direct band gap. The E_g values for our films vary from 1.69 to 2.57 eV for the as deposited and from 1.65 to 2.50 eV for the annealed films when zinc content varies from 0 to 1. The optical band gaps determined from these curves are listed in Table 1. It can be seen that the optical band gap decreases with annealing temperature. The decrease in optical band gap energy is generally observed in the annealed direct-transition type semiconductor films [17]. The shifts in optical band gap may be attributed to the improved crystallite size and morphology of thin films. Moreover, it is understood that the amorphous phase is reduced with annealing temperature, since more energy is supplied for crystallite growth, thus resulting in an improvement in crystallinity of Cd_1−xZn_xSe thin films.

The SE spectra for Cd_1−xZn_xSe thin films annealed at 400 °C are shown in Fig. 2. From the spectra exhibit oscillations, which is only due to interference effect of light. The oscillating frequency depends on the thickness of the film. Generally, the thicker the film, the higher is the frequency of oscillations. The optical constants derived from the ellipsometric parameters psi ( ψ) and delta (Δ) have been analyzed by an effective medium approximation (EMA) model (air/CdSe+ZnSe/substrate). With the help of simulated optimization, the fitting parameters as well as the film thickness were determined by fitting the ellipsometric spectra. Clearly, the fit shown in Fig. 2 is a good fit.

The calculated values of refractive index n and extinction coefficient k were plotted as a function of wavelength, as shown in Fig. 3. It is observed that the refractive index n as well as extinction coefficient k decrease as the zinc concentration increases both before and after annealing. The decrease in refractive index n of the ternary alloy may also be due to the fact that the zinc ionic radius is smaller than that of cadmium; therefore the grain size as well as lattice constant are reduced [18]. Due to the reduction in grain size, crystallinity decreases which results in lowering the density of the alloy. From Fig. 3 it is evident that there is a sharp rise in the refractive index of the films at the absorption edge towards lower wavelength of the spectrum. This sharp rise in refractive index is assigned to Van Hove singularity in the joint density of state in the figure it can be seen that the excitation of transition between two bands [19]. There is a slight increase in optical constants of Cd_1−xZn_xSe thin films after annealing which is a direct consequence of density and band gap of the material. Higher annealing temperature enhances the formation of larger and more closely packed crystals. The increase of the refractive index with annealing temperature can be partly attributed to the improvement in the film quality with the reduction in porosity of the Cd_1−xZn_xSe films. Since the refractive index is strongly correlated to the band gap energy, it can be concluded that the smaller band gap energy material has a larger value of refractive index in Cd_1−xZn_xSe. This behavior is generally found for most common II-VI and III-V semiconductor alloys [20, 21].

The dispersion of refractive index according to the single effective oscillator model proposed by Wemple and Di Domenico [22], which describes the dielectric response for transitions below the interband absorption edge in the low absorption region, is given by expression: n2(hυ)=1+EoEdE2o−<!-- - -->(hυ)2 $${n^2}(h\upsilon ) = 1 + \frac{{ {E_o}{E_d}}}{{{E^2}_o - {{(h\upsilon )}^2}}} $$ (3) where h is Planck’s constant, E_o is the average excitation energy for electronic transition which is empirically related to the optical band gap, while E_d is the dispersion energy which measures the average strength of optical transitions. By plotting the (n²−1)⁻¹ versus (hν)² and fitting a line as shown in Fig. 4 the values of Eo and Ed can be obtained from the slope (E_oE_d)⁻¹ and intercept (E_o/E_d) on the vertical axis. The values of energy dispersion parameters for the as deposited and annealed samples are listed in Table 1. The obtained values of E_o and E_d increase with increasing Zn content and decrease with annealing effect. If the bonds in our samples are supposed to be covalent, then the increase in E_d can be assigned to higher number of nearest neighbors in the alloy with respect to pure compound [23]. The variation in E_o with Zn content and annealing temperature may be due to the behavior of optical band gap. The values of band gap energy E_g which have been obtained by using the single oscillator model agree with those determined by the relation (αhν)² = A(hν-E_g) as in Table 1.