Khan et al [27] studied the electrical and optical properties of

[27] studied the electrical and optical properties of a-Se70Te30nanorod thin film. They reported that the absorption mechanism was due to indirect transition. CX-4945 molecular weight The optical band gap was estimated to be 1.18 eV. Khan et al. [28] observed an indirect band gap in the tellurium-rich Ga10Te90-x

Sb x (x = 5, 10, 20, and 30) thin films. The value of band gap decreased with an increase in Sb content. Ilyas et al. [29] also reported an indirect band gap in the tellurium-rich Ga x Te100-x thin films. Abd-Elrahman [30] studied the effect of composition on the optical constants of Se100-x Te x (x = 30, 50, and 70) chalcogenide thin films. They reported that an increase in Se contents (from x = 30 tox = 70) resulted in an increase in indirect gap from 1.33 to 1.85 eV. They also found that the absorption coefficient, refractive index, extinction coefficient, and dispersion buy MM-102 energy of the films were dependent on the film composition. El-Zahed et al. [31] studied the dependence of optical band gap with the composition

of Se(1-x)Te x (x = 0.2, 0.4, 0.5, and 0.8). They found that the optical gap was a function ARS-1620 of composition and the width of optical gap varied from 1.8 to 1.06 eV. The band gap decreased with increasing Te content. Most of the reports presented above predicted indirect band gap and the compositional and photon energy dependence of optical band gap and optical constants in the chalcogenides, whereas in present work, size reduction to the nanoscale level results in a dramatic change in the optical properties. Therefore, it may be concluded that the results presented in this paper show the effect of size on optical properties, i.e., observation of direct band gap and

enhanced value of band gap and optical constants for the a-Se x Te100-x thin films containing aligned nanorods. Figure 5 ( α hν) 2 against photon energy (hν) in a-Se x Te 100- x thin films composed of aligned nanorods. Table 1 Optical parameters ALOX15 of a-Se x Te 100- x thin films at 600 nm Sample E g(eV) α(cm-1) k n ε r ′ ε r ″ Se3Te97 1.66 8.40 × 105 4.01 11.90 125.58 95.53 Se6Te94 1.59 5.16 × 105 2.47 10.69 88.54 108.72 Se9Te91 1.51 10.6 × 105 5.08 9.08 66.31 72.85 Se12Te88 1.45 6.50 × 105 3.11 5.54 20.98 34.40 It is well understood that the optical absorption is dependent on both the short-range order and defect states observed in amorphous systems. We can employ Mott and Davis’s ‘density of state model’ to explain this decrease in optical band gap with the increase in Se concentration. It was suggested by Mott and Davis [32] that the degree of disorder and defects in the amorphous systems are two major factors affecting the width of the localized states near the mobility edges. For the present case of a-Se x Te100-x thin films, it is proposed that the unsaturated bonds together with some saturated bonds are produced during the deposition of atoms in the present as-deposited films [33].

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