The band-gap energies of the CdS semiconductor are obtained by a photoacoustic spectroscopy (PAS) technique over a range of temperature of thermal annealing (TTA), in which the evolution of the sample structure is characterized by x-ray diffraction patterns. The PAS experiment gives a set of data for the band-gap shift in the region of the fundamental absorption edge. With increasing TTA the band-gap shift increases up to a critical TTA when its slope decreases in a roughly symmetrical way. It is suggested that at this temperature a cubic to hexagonal-lattice transition occurs.The use of photoacoustic spectroscopy (PAS) has become well established in the past few years mainly due to its importance as a guide in the study of optical properties of semiconductors.'-" PAS can lead, for instance, to the value of the band-gap energy, which is an important parameter in electronic and optoelectronic design.'-" It is worth saying that less attention has been paid to applications of this technique to investigate the band-gap shift (BGS) of intrinsic and extrinsic semiconductors. In particular, the CdS semiconductor, which presents a highly stable hexagonal structure (y_CdS,= can also be obtained in the metastable cubic phase ,8-CdS.'2-'5 Cardona et al. by means of reflectivity measurements at room temperature found the optical band gap of the two phases of CdS thin films, and they could not infer any other conclusions except that the energy difference between the cubic and hexagonal CdS energy gap differs less than 0

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In this letter we report on cubic CdS thin films with low resistivity by chemical bath deposition (CBD) technique and subsequent annealings in S2 and H2+In. Low temperature photoluminescence, x rays, and transmission spectra support the assumption that S2 annealings contribute to fill the vacancies in the as-deposited films leading to an enlargement of the CdS cubic cell. This fact is revealed by an increase in interplanar distances, evanescence of the PL red broad band, and decrease in band-gap energies. Cubic phase remains after H2+In annealing at higher temperatures. A resistivity as low as 11 Ω cm was obtained at an optimum annealing temperature of 350 °C.

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We study by photoacoustic spectroscopy the band-gap shift effect of CdS films. The CdS films were grown by chemical bath deposition and exposed to different annealing atmospheres over a range of temperature in which the sample structure changes. We show the band-gap evolution and resistivity as a function of temperature of thermal annealing and determine the process that produces the best combination of high band-gap energy and low resistivity. Q