Photovoltaic effect in polycrystalline ZnTe films

Abstract

Anoinaloris but, wcak photovoltaic cffect hits bceri observed in the case of thin polyerystalline %nTc films obliquely deposited on glass substrate. Photovoltagc o f t h e order of a fcw hiuidred millivolt per centimeter have been observed a t the ambient temperature. Optimized conditions for growing films to obtain maximum photovoltage havc been found. In order to explain the observed photovoltaic effect, a model comprising of a long chain of a,ligned microcells in the form of p-p+ homojunctions has been proposed. The implications are discussed. IiitroductioiiThe anomalous photovoltaic effect observed in thin films has aroused great in1,erest. Extensive studies of a range of materials have revealed beyond doubt that it is absolutely necessary to grow the filnis by oblique deposition in order t o produce the anomalous photovoltaic effect. It is reported in literature that the photovoltaic effect appears to be due to the photoresponse of p-n niicrojunctions (PENSAK, GOLDSTEIN; ADIROVICH et al. ; BRANDHORST et al.) or Dember effect (SCHWABE; PANKOVE) or grain boundaries (HUSTON; MXRZ; NOVIK;EL-SHAZLY et al.). In all the cases it is presumed that a large number of active crystallites arrange themselves in series, which produce the photovoltage.IGNATY UK, N O V~K have reported high photovoltage in epitaxially grown ZnTe films on ionic crystals. However from the point of view of the applications of the photovoltaic effect, it is necessary to grow the filiiis on a good inert substrate rather than ionic crystals. It appears that so far no reports have been published on photovoltaic effect in polycrystalline ZnTe filnis grown on glass substrates. The present paper describes weak anomalous photovoltaic effect observed in the case of ZnTe film grown on glass substrate. The optimized growth conditions in order to get niaxiniuni photovoltaic effect have been investigated and a possible niodel for i t is suggested and discussed. Experiiiierital procedureZnTc films were prepared by thermal evaporation from pure (99.998%) ZnTe powder on ultrasonically cleaned glass substrates in a vacuum of the order of Pa. The glass substrates in a vacuum of tJhe order of l'a. The glass substrates were held inclined to the diroction of the incident b m m of the vapour. The rate of evaporation was 5-6 r i m sec-1 arid was maintained constant for all depositions. The average thickness of the wedge films grown was about 100 nm and was kept the same for all sets of observations. The substrate temperature was varied from room temperature to 675 K. Gold films for making ohmic contact were evaporated on the two ends of the film on the substrate, which worked BS electrodes. For illuminating the films, the tungsten filament incandescent lamp excited by 66 *

Anomalous photovoltage in thin films

“…Under illumination, the potential at the near end of the film (the end which is nearer to the evaporator during deposition) is always found to be negative with respect to the far end potential. Similar observations were reported by Goldstein and Pensak [8] for CdTe films and by Patel and Patel [2] for ZnTe films. No change of polarity has been observed with the variation in the angle of incidence of light.…”

“…Different models [1][2][3][4][5][6][7][8][9] have been suggested by many workers to explain anomalous photovoltage in thin films of different materials. A good overview of the anomalous photovoltaic effect has been given by Pankove [10].…”

Microstructure and growth of ZnTe films

“…Production of ZnTe films having high conductivity and transparency is of current interest due to its potential application in various semiconducting devices (Kisiel et al, 1976;Kobayashi et al, 1980;Moh Khan et al, 1984;Okamoto, 1973;Patel and Patel, 1983, 1985Vincett et al, 1977). Substrate temperature during deposition has been seen to affect the transparency and conductivity critically (Ciorascu et al, 1972;Okamoto and Ugai, 1973).…”