The Development of Infrared Photosensitive Material Based on Polycrystalline PbS Films
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The investigation of the polycrystalline films and the electrical properties of their grain boundaries is very challenging. In the present report we demonstrate the possibility to change the activation energy of the conductivity Eo by introducing controlled impurity atmospheres near grain boundaries and discuss possible applications of these materials.
Fig 1. Energy model of polycrystalline film and separation of electron-hole pairs under illumination. EXPERIMENTAL TECHNIQUES Thin lead sulphide films, 0.5 - 3 [tm thick, were deposited by the hot wall technique (HWT) on glass substrates at the temperature interval 200-350TC. The wall temperature was 4000C and the vacuum was 10-6 Torr. Structural and compositional characterization were performed using Scanning Electron Microscopy and X-Ray Microanalysis. PbS films were doped by oxygen and indium impurities. The oxygen was introduced into the samples by annealing in oxygen or air atmosphere at various temperatures in the range of 250-400 0C for a fixed period of 1 hour. The indium diffused into the PbS from a gaseous phase (In4Te 3 vapor source) as described in [3]. Using this approach, a constant level of surface In concentration was maintained during the entire diffusion anneal, carried out at 4000C for duration of I to 5 hours. At the completion of the diffusion anneals, the samples were rapidly quenched to room temperature. During the diffusion anneal the sample was kept in sealed and evacuated quartz ampoules, 6-8 cm long and 10-12 mm in diameter. The ampoule was built at its end with a round reservoir that contained the In4Te3 powder, used as a source for the gaseous In. The reservoir was connected by a thin neck (- 2mm diameter) to the rest of the ampoule and contained also some PbS powder, from the same source as the sample, to maintain the crystal stoichiometry. Evacuated and sealed ampoules were inserted into a horizontal oven that had a zone of uniform temperature longer than the full length of the ampoule. After doping anneal the samples were examined by optical microscopy for signs of surface damages.
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Room temperature Seebeck coefficient along the sample was measured at 10 jtm. intervals using a 5 jtm diameter "hot" probe. The temperature of the probe was kept constant within ± 0.1 0 C. Electrical conductivity and Hall effect were measured over the range of temperatures between 80 to 300 K. RESULTS AND DISCUSSION The grain size of the studied PbS films varied in wide limits (from 100 to 500 A) by varying the growth rate and the substrate temperature. The surface topography reflected the internal structure, due to disorientation of grains. We estimate that the mean free path in bulk PbS is of the order of 1000 A at 80 K. Therefore the grain boundaries could effect strongly the carrier scattering. If carriers became localized at these boundaries, charged boundaries could then act as potential barriers preventing the flow of the free carriers. The charge density at the boundaries and consequently the barrier height should be a function of temperatur
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