GROWTH OF CDS BY ATMOSPHERIC-PRESSURE METALORGANIC VAPOR-PHASE EPITAXY AT LOW-TEMPERATURE

Single-crystalline epilayers (epitaxial layers) of undoped, phosphorus-, and iodine-doped CdS have been grown at 250-400-degrees-C on (100)-oriented GaAs substrates by atmospheric pressure metalorganic vapor-phase epitaxy using dimethylcadmium and hydrogen sulfide as source materials. The premature reaction typically encountered with this source combination, as well as in the case of ZnSe epitaxial growth using dimethylzinc and hydrogen selenide, can be eliminated completely even at atmospheric pressure by controlling the respective flow velocities of the carrier gases, including the source gases, independently. The crystallographic structure of CdS epilayers is largely related to the VI/II source gas mole ratio, and changes from hexagonal into cubic (100) structure with decreasing VI/II ratio. From the x-ray-diffraction and photoluminescence measurements, an epilayer grown at 350-degrees-C with a VI/II ratio of 2 has excellent crystalline quality with complete single-cubic (100)-type epitaxial structure. Also in correspondence with such changes of the crystallographic structure, it is clearly observed that the energy positions of the near-band-edge emissions shift 75, 96, and 101 meV, respectively, downwards at 300, 77, and 4.2 K. In addition, the fundamental data on cubic CdS have been obtained for the first time. It is concluded that a P impurity does not form a shallow acceptor level, but instead SA (self-activated) centers. An I impurity effectively forms a shallow donor level while preserving the high crystalline quality, and changes the crystallographic structure from cubic type into hexagonal type without generating SA centers. It is concluded that the doping alters both crystallographic structure and crystalline quality in the epitaxial growth of CdS.