We have studied the room-temperature photophysics of variously doped CdS and Cd(Hg)S powders by the microwave photoconductivity technique. Accordingly, we infer that both Cu and Ag are effective hole traps, the rate of hole trapping being limiting for the first order decay of photoconductivity. In CdS:Cu, states 0.14 and 0.33 eV below the conduction band are important to the photophysics of the powders; these have been demonstrated to be associated with sulfide vacancies. The presence of a (neutral) donor state associated with mercury appears to be a useful working hypothesis in the interpretation of observations made on Cd(Hg)S powders. Combination of donor (Ga or Eu) and acceptor (Cu or Ag) dopants leads to formation of spectroscopically detectable charge-transfer complexes in the CdS lattice; the conduction band can be populated directly from the acceptor state of the complex on irradiation.
It was demonstrated that CdS can be grown epitaxially by exposing opposite faces of a CdS wafer to cadmium and sulfur vapors, respectively, and thus causing countercurrent diffusion of the cadmium and sulfur to the opposite faces where they form CdS by reaction with the vapor. The effects of substituting zinc for the cadmium vapor, and in another experiment selenium for the sulfur vapor, were also investigated. In the two-component system, good quality single crystal was o.btained while in the three-component system void formation was prevalent.In conventional crystal growth the reactants are brought to the substrate either in a gas or in a liquid phase. Some disadvantages inherent in this approach may be overcome by bringing the reactants to the surface by solid-state diffusion through the substrate itself. One can visualize the driving force being a temperature gradient and/or concentration gradients. Due to the widespread interest in CdS and to its ready availability in single crystalline form, this compound was selected as a model crystal for the experiments; cadmium and sulfur were diffused countercurrently through CdS. The above-described growth by countercurrent diffusion was also combined with exchange reactions. In one experiment zinc was substituted for the cadmium, and in another selenium for the sulfur, thus leading to the CdS --> ZnS and CdS --> CdSe reactions. ExperimentalThe substrates were obtained by wafering a 19 mm diameter cylinder of CdS. To allow optimum utilization with the given shape of the boule, the axis of the cylinder was selected to coincide with the <101-3> direction. After etching in concentrated HC1 for 4 rain, the final thickness was 1.4-1.6 ram. On some of the wafers, circular-shaped tantalum markers were deposifed by sputtering. The wafers were placed, as shown in Fig. 1, sandwiched between two reservoirs; one containing the Group II, the other the Group VI element.To decrease the rate of vapor transport of the substrate CdS components to the cool ends, the cross-sections of the reservoirs were decreased by sealing capillary tubes with an about 0.5 mm diameter bore into the neck of each reservoir.Evacuation was performed at room temperature and the ampules were sealed at 1.0-45 ~.A typical temperature profile is shown in Fig. 1. The experimental conditions and thermodynamic data are * Electrochemical Society Active Member.
A two-dimensional scanning laser has been produced by scanning a pumping electron beam across the faces of various II–VI semiconductors. The laser beam emerges coaxial with the electron beam from the excited point on the crystal with a spot size about the same diameter (20 μm) as the electron beam.
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