Cadmium sulfide thin films were successfully indium doped during the growing process by means of a chemical bath. Hydrated indium nitrate {In(NO3)3.5H2O} in aqueous solution was added to the CdS growing solution at different proportions. X-ray, optical absorption, Raman spectroscopy, Hall effect, photoconductivity and dark-conductivity measurements were performed to characterize the growth and doping processes of the CdS:In layers. The experimental results indicate a successful doping for low indium dosage, a saturation for intermediate doping levels, and a degradation of the doping process for high impurity densities. This n-type doping method can be useful to prepare CdS photovoltaic solar cells.
Keywords

Semiconductors, chemical synthesis, impurities in semiconductors, electronic transport, II-VI compounds
Introduction

Doped CdS thin films are important for the fabrication of gas sensors and photovoltaic devices [1-3]. Indium is the one of the more effective dopants to obtain n-doped CdS because the extra electron coming from the In3+ ion is placed substitutionally in Cd2+ sites. During the past few years CdS:In thin films have been prepared by thermal evaporation [4], spray pyrolysis [5], chemical bath (CB) [6], and chemical bath plus indium diffusion [7] with good results. The most used technique to grow CdS for photovoltaic solar cells fabrication is CB [1]. In this way, it is important to develop an effective technique to n-dope CdS thin films during the growth process. Lokhande and co-workers reported the preparation of CdS:In thin films by CB, but with an amorphous structure [6], which represents a disadvantage for high efficiency solar cells design. In this work our principal concern is to increase the carrier density to reduce the electrical resistivity of the material, with no sensitive reduction of the forbidden energy band gap (Eg). In order to achieve this goal we employed the CB technique, in such a way that the aqueous solution containing Cd2+, In3+ and S2- ions, allows the formation of CdS:In. The doping method is analyzed for different impurity levels to determine the most effective dosage with good physical properties useful for a wide field of applications.
Experimental

The growth of polycrystalline CdS:In films was performed at 80 ± 1°C on glass substrates. Details of the CdS growth process have been previously reported in Ref. 8. Salt reagents (concentrations) used in the CdS:In preparation were: CdCl2 (0.02 M), KOH (0.15 M), NH4NO3 (1.5 M), SC(NH2)2 (0.2 M) and In(NO3)3-5H2O (0.1 M). The total solution (100 ml) for growing CdS was completed with relative volumes (Vr) of In(NO3)3.5H2O in aqueous solution, Vr ranging from 1.0 to 10.0 ml (1 ml = 1% Vr). This allowed us to study of the material with ten different doping In-levels. The samples are denoted as CdS:Inx, where x is the Vr value. The thickness of the layers was measured by a Dektak II profilometer, the X-ray diffraction (XRD) data were obtained using a Siemens D-5000 diffractometer with CuKα radiation. For the calculation of the (111) interplanar distance, the XRD peaks were fitted by using Lorentzian curves. In this way the center of each XRD peak was calculated up to ± 0.002 Å. The UV-Vis optical absorption spectra were recorded using a Unicam 8700 spectrophotometer. The shift Raman spectra were obtained with a Raman spectrometer with double grating SPEX model 14018. The photoconductivity was measured with a lock-in Stanford Research Systems Model SR530 and a monochromator ScienceTech Inc model 9490. Finally the carrier density was determined by measuring the Hall effect with a conventional apparatus from GMW Magnetic Systems model 3472-50.