Being a glass, chalcogenides are versatile platform. The freedom allowed in the preparation of glasses in varied compositions brings about changes in their short range order and thus results in variation of their physical properties. Therefore it is possible to tailor their various properties, as desired for technological applications. They have well defined niches in fiber optics, optical recording, switching, regeneration, wavelength conversion, amplification etc. Besides the wide commercial/device applications like switching memory and xerography etc. of Se, it also exhibits a unique property of reversible transformation [1][2][3][4][5]. This very property makes it very useful in optical memory devices. Ge-Se system is a widely studied system. On addition of Ge to Se, Ge atoms act as bond modifiers; they strengthen the average bond by cross linking the Se chain structure, thereby increasing the glass transition temperature and resistivity [6] and also overcome some difficulties of pure Se, like short life time and low sensitivity. The Ge 20 Se 80 glassy alloy lies at the threshold of mode change i.e. floppy to intermediate region [7]. The addition of Indium (In) to Ge 20 Se 80 system in an effective way, control it's electrical and optical properties which leads the system towards intermediate region. Moreover, Ge-Se-In system is of special interest because of the fact that it forms glasses over a wide domain of compositions [8]. The incorporation of In to Ge-Se alloy expands the glass forming area and also creates compositional and configurational disorder in the system. Optical and electrical study of Ge 20 Se 80-x In x (x = 0, 5, 10, 15, 20) thin films has already been reported by Ishu et al. [9,10]. The non-linear and photosensitive properties together offer numerous applications to a number of integrated optical functions. Since Ge 20 Se 70 In 10 system shows maximum value of optical band gap and photosensitivity, so in continuation authors have decided to study the effect of Bi addition to a-Ge-Se-In system for their optical behavior. Since Bi addition to Ge-Se system increases the chemical durability, the replacement of Se with Bi in Ge-Se host matrix leads to the decrease in optical band gap [11,12] and significantly broadened the transparency range towards NIR region. Maximum of Bi content which can be incorporated to Ge-Se matrix (when Ge content is 20 at. %) is only 13 at. % [13]. It is well documented that the incorporation of about 6-9 at. % Bi into the glass matrix of Ge-Se leads to the change of the conductivity from p to n type [14]. Earlier experiments have shown that the addition of Bi in Ge-Se system brings down the optical band gap upto 1.2 eV for n-type conducting samples, resulting in an enhancement of photoconduction for n-type samples [15,16].
Knowledge of the optical properties of these amorphous materials is obviously necessary for exploiting their interesting technological potential. The present work deals with determination of optical band gap ðE opt g Þ, absorption coefficient (a), refractive index (n) and extinction coefficient (k) for Ge 20 Se 70-x In 10 Bi x (x = 2, 4, 6, 8, 10) thin films by analyzing transmission spectra in the wavelength range from 400-1800 nm. The dispersion of refractive index was analyzed with Wemple-DiDomenico single oscillator