Elsevier

Ceramics International

Volume 40, Issue 7, Part B, August 2014, Pages 10657-10666
Ceramics International

Composition dependent optical, structural and photoluminescence behaviour of CdS:Al thin films by chemical bath deposition method

https://doi.org/10.1016/j.ceramint.2014.03.050Get rights and content

Abstract

Undoped and Al doped CdS thin films were deposited successfully on glass plates using chemical bath deposition method at 80 °C with different Al concentrations between 0 and 8%. X-ray diffraction spectra revealed that undoped CdS thin film had cubic structure which was turned to mixed phase of cubic and hexagonal by Al doping. The noticed decrease in d-value, cell parameters, bond length and volume up to 4% of Al is due to the substitution of Al3+ into Cd–S lattice whereas the increase of these parameters beyond 4% is due to the formation of defects. Structural, optical and photoluminescence properties of Al doped CdS thin films were studied using X-ray diffraction, energy dispersive X-ray, scanning electron microscopy, UV–Vis absorption, Fourier transform infra red spectroscopy and photoluminescence spectra. The enhanced crystal size at lower Al concentrations is due to the proper substitution of Al3+ and the reduced crystal size after Al=4% is due the interstitial defects of Al3+ in Cd–S lattice. The continuous red shift of energy gap from 2.18 eV (Al=0%) to 1.96 eV (Al=8%) is elucidated by increasing amount of sulfur deficiency by Al doping. The presence of functional groups and chemical bonding were confirmed by FTIR spectra. Room temperature photoluminescence spectra showed the two well distinct and strong bands; blue band around 409 to 415 nm and green band around 538 to 547 nm. The change in intensity and their shift were discussed in detail.

Introduction

Nano-sized semiconductors have attracted much attention in both fundamental research as well as advanced technological applications owing to their unique size dependent optical and electronic properties [1]. Among these, CdS is a widely used substance with many advanced technological application such as photochemical catalysis, gas sensor detectors, solar cells, nonlinear optical materials, various luminescence and opto-electronic devices [2], [3].

The efficiency of CdS film is improved by changing its optical, structural and electrical properties by doping with some foreign elements such as copper and aluminum [4], [5]. Al doped CdS thin film has emerged as an important material due to its applications in photovoltaic cell and optoelectronic devices [6]. Conductivity of CdS thin films is increased by Al doping. During Al doping into CdS, Al3+ ions were properly entered into Cd site at lower concentration and interstitially at high concentration, interstitial ions will act as recombination centers.

The deposition of CdS films has been explored by different techniques such as chemical vapour deposition [7], pulsed laser deposition [8], electrochemical deposition [9], electroplated [10], cathodic reduction [11] and chemical bath deposition [12]. Among the different techniques, chemical bath deposition (CBD) is extremely attractive because of its advantageous features over other thin film deposition techniques such as its simple low deposition temperature, low cost, low evaporation temperature and easy coating of large surfaces. This technology is based on controlled release of the metal ions (M2+) and sulphide ions (S2−) in an aqueous bath.

Though some works have been already carried out on Al doped CdS system [1], [4], [6], [7], the comparative studies of optical, structural and photoluminescence behaviour is still scanty. Since a free electron is created upon the substitution of Al3+ in Cd–S, it is possible to control the morphology, particle size and chemical composition of the film by controlling the level of doping during synthesis. Therefore, in the present investigation, optical and structural properties can suitably be tailored by varying the Al concentrations in CdS films to determine the feasibility of the films for the potential technological application.

Section snippets

Substrate cleaning

Substrate cleaning plays an important role in the deposition of thin films. The contaminated substrate surface provides nucleation sites facilitating the growth, which results in non-uniform film growth. Therefore, the glass slides of dimensions 26×76×2 mm3 were boiled in chromic acid for 2 h and kept in it for 12 h. Then, they were washed with detergent, and again rinsed in acetone before the deposition of the films. The cleaned substrate was kept dipped in de-ionized water before use.

Preparation of Cd1−xAlxS (0≤x≤0.08) thin films

For the

Growth mechanism and thickness

The growth of the film should be heterogeneous to avoid powdery and non-adherence films. The heterogeneous growth can be achieved with the help of fast reaction by adjusting as low reactant concentrations, high aqueous ammonia concentrations, etc. The deposition process is based on the release of Cd2+, Al3+ and S2− ions as follows:CdCl2+4NH3[Cd(NH3)4]2++2Cl[Cd(NH3)4]2+Cd2++4NH3(Cd2+release)(NH2)2+OHCH2N2+H2O+HSHS+OHH2O+S2(S2release)Cd2++S2CdS(CdSformation)

The Al doped CdS is formed

Conclusions

The following are the conclusions drawn from the present investigation:

  • Undoped and Al doped CdS thin films have been deposited successfully on glass plates using chemical bath deposition method at 80 °C with different Al concentrations from 0% to 8%.

  • X-ray diffraction spectra revealed that undoped CdS thin film had cubic structure which was turned to mixed phase of cubic and hexagonal by Al doping.

  • The noticed decrease in d-value, cell parameters, bond length and volume up to 4% of Al is due to

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