Photoluminescence studies of CdS films grown by close-spaced vapor transport hot walls

Semiconducting films of CdS grown by a modified close-spaced vapor transport hot-wall technique were studied by photoluminescence (PL) in the range of 10-300 K. The films were grown with an Ar pressure of 100 mTorr, the substrate temperature (T-su) varied between 500 and 700 degrees C, while the source temperature (T-so) was kept at 750 degrees C. With these parameters it was possible to obtain different mean grain sizes. Several luminescence bands were observed, one located near the band-gap energy around 2.5 eV at 10 K. This band shows a strong temperature dependence and has an excitonic origin. Two other bands were detected around 2.4 and 2.1 eV at 10 K, and are denoted in the literature as green and yellow emission bands, respectively. The lower energy band with a mean width of 150 meV is accompanied by the corresponding optical phonon replica separated by exactly the LO-phonon energy of 38 meV at low temperatures. The nature of these bands and the photoluminescence process as a function of the temperature as well as of the grain size will be analyzed and discussed in this work. (C) 1999 American Institute of Physics. [S0021-8979(99)03718-4].