Photoluminescent properties of vacuum deposited cadmium sulfide films

Green and blue photoluminescence has been observed below 1000K in vacuum deposited, low resistivity CdS films given no post deposition treatment. The films were deposited in a heated chamber inside the bell jar on o fused quartz substrates held at temperatures from 140-180 C. The background pressure in the vacuum system was 10-6 Torr. The evaporant sed was chlorine-doped CdS powder. The chlorine doping activated the luminescence and gave films that had resistivities at 3000K in the range of 1-102 ohm-cm parallel to the substrate. o The green luminescence in the films at 77 K wa.s similar to the emission reported for donor-doped CdS. Donor-doped CdS emission differ8~: from the phonon-assisted "edge emission" starting at 5140 ~ observed in pure CdS in that the emission peak at 5230 ~, corresponding to emission of one k=O LO phonon, is more intense than the zero phonon peak, The relative peak intensities of the zero phonon and phonon replicas in pure CdS can be described by a Poisson distribution in N where N is the mean number of phonons emitted in the transition. ponent peaks also have equal half-w'idths H. These Gaussian shaped compotent peaks pure OdS at 77 oK values found are N :: 0.9-1.0 and H :: 0.03 eV. For the CdS films good agreement could be obtained with N :: 1.1-1.4 and H ;: 0.04-0.06 eV. How'ever, , it is also possible that the film luminescence i.s the superposition of two emission series. The green emission spectra. observed at lOoK became better resolved than at 77 oK, and the peak position shift with temperature was small. The -6 0 decay time of/the green emission was less than 10 sec at 77 K, and thermal quenching of the luminescence began around 100oK, with an activation energy of -0.15 eV. The quenching is due to the thermal ionization of the acceptor center involved in the transition. Baking in saturated cadmium vapor at 500 C quenched the green emission, and for films baked in H2 S at 600 C, emission similar to that observed in pure CdS was obtained. The blue emission peak observed in the films was at 4892 R at 77°K, which is in the fundamental absorption edge. It shifted to 4875 A and became more intense and narrower at lOoK. This emission persists to 3000K with the peak position decreasing in energy similar to the decrease in energy of the band gap. The peak is quenched by baking in H2S but not by baking in saturated Cd vapor. The blue emission is probably associated with a shallow donor or isoelectronic trap near the conduction band edge and could be due to the decay of a bound exciton. Some of the literature relating to the theory and practice of CdS vacuum deposition is reviewed. Also a summary of the properties of the green luminescence of pure and donor-doped CdS is given.U of I OnlyThesi