Pulse Electrodeposited AgGaS2 Films and Their Optical Properties K.R.Murali
Authors: Murali Kollegal Ramakrishna 
Published: February 2020

AgGaS2 films were deposited for the first time using pulse electrodeposition technique and the optical properties are presented and discussed. AgGaS2 films were deposited on tin oxide coated conducting glass substrates at different duty cycles, in the range of 6 – 50 % at a deposition potential of - 1.14 V (SCE). The total deposition time was 15 min. The precursors used were Analar grade 0.10 M AgCl, 0.20 M InCl3 and 0.15 M sodium thio sulphate. Thickness of the films estimated by Mitutoyo surface profilometer varied in the range of 400 – 900 nm with increase of duty cycle. The films were characterized by Xpertpanalytical x-ray diffraction unit with Cukα radiation. Optical measurements were recorded using an Hitachi UV–Vis-IR spectro-photometer. Composition of the films was estimated by EDS attachment to JOEL SEM.

The X-ray diffraction pattern of AgGaS2 films formed at different duty cycles indicated the films to be polycrystalline exhibiting the peaks corresponding to the single phase AgGaS2. Peaks corresponding to (112), (220), (204), (312) and (116) orientations of the chalcopyrite structure were observed (JCPDS No.75-0118). The evaluated lattice parameters were a = 5.76 Å and c = 10.27 Å with a c/a ratio of 1.78. The crystallite size was calculated from the Full width half maximum of the diffraction profiles using Scherrer’s equation. The crystallite size increased from 12 nm – 30 nm.

Composition of the films was determined by the Energy dispersive x-ray analysis (EDAX) attachment of the Scanning Electron Microscope (SEM). At low duty cycles, more concentration of Ag and S are deposited due to the fact that Ag and S are more noble compared to Ga. As the duty cycle is increased, due to the thermal energy, more gallium is deposited along with Ag and S, hence the gallium concentration increased. Hence, at lower duty cycle, the Ag/Ga ratio is greater than unity. With increase of duty cycle, due to the increase in concentration of Ga, Ag/Ga ratio approaches unity.

 

The transmission spectra of the AgGaS2 films deposited at different duty cycles exhibits interference fringes and the value of the refractive index was estimated by the envelope method. The value of the refractive index at 550 nm, calculated from the above equations was in the range of 2.92 - 2.54 for the samples deposited at different duty cycle. This value is higher than the values  obtained on thermally evaporated AgGaS2 films. The films exhibited a high absorption co-efficient of the order of 104 cm-1. A plot of (αhν)2 against hν, exhibits linear behavior near the band edge, the band gap of the deposited films was determined to be in the range of 2.35 – 2.57 eV. The band gap was found to slightly increase with decrease of duty cycle, due to the small grain size. Extinction coefficient was determined from the absorption coefficient using the following relation. The decrease in extinction coefficient with increase in wavelength shows that the fraction of light lost due to absorbance decreases.

According to the single-effective oscillator model proposed by Wemple and DiDomenico. Plotting (n2 − 1)-1 against E2 gives the oscillator parameters by fitting a straight line. Figure 8 shows the plot of (n2 − 1)-1 vs E2 for the films deposited at different duty cycles. The values of E0 and Ed can then be calculated from the slope (E0Ed)-1 and the intercept on the vertical axis (E0/Ed). The values of the static refractive index (n0) can be calculated by extrapolating the Wemple–DiDomenico dispersion equation to E → 0. The calculated values of n0 are 2.30, 2.43, 2.60 and 2.65 for the films deposited at different duty cycle. The calculated values of n0, E0 and Ed are listed in table 3. In addition, the optical band gap (Eg)  determined from the Wemple–DiDomenico dispersion parameter E0 using the relation Eg = E0/2, are also in good agreement with the band gap values determined from the (αhν)2 vs hν plot..