Electrochemical and photoelectrochemical deposition of thallium(III) oxide thin films
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I. INTRODUCTION Thallium(III) oxide is a degenerate n-type semiconductor with a band gap of 1.4 eV.1"4 The oxide has a roomtemperature resistivity of 70 juohm-cm, majority carrier mobility of 105 cm2 V" 1 s"1, and carrier concentration of 7 x 1020 cm"3.2'3 The 1.4 eV optical transition is indirect, and the material does not absorb light appreciably until the first direct transition at 2.2 eV.4 We have previously exploited the high optical transparency and electrical conductivity of thallium(III) oxide in the fabrication of a heterojunction SIS-type thallium(III) oxide/n-silicon photovoltaic cell.5'6 The efficiency of the photovoltaic cell was 11% with 75.3 mW/cm 2 natural sunlight, and the short-circuit quantum efficiency at 800 nm was 97%. The thallium(III) oxide/n-silicon heterojunction was shown to have a barrier height of 1.0 V, and a diode quality factor of 1.2.5 In the present study, the photoelectrochemical "directwriting" of thallic oxide films onto n-silicon is explored in more detail. The effects of concentration, potential, irradiance, and wavelength on the microstructure of electrochemically and photoelectrochemically deposited thallium(III) oxide films are presented. This research provides the foundation for such future work as the room-temperature photoelectrochemical patterning of thallium-based ceramic superconductor films, and the deposition of transparent ohmic and rectifying contacts to III-V compound semiconductors such as GaAs and InP. II. EXPERIMENTAL Current-potential curves and electrodepositions were run with a Stonehart BC 1200 potentiostat/galvanostat, J. Mater. Res., Vol. 4, No. 4, Jul/Aug 1989
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Princeton Applied Research (PAR) Model 175 universal programmer, PAR Model 379 digital coulometer, and Hewlett Packard Model HP 7047A XY recorder. Thallium(I) acetate (Alfa, ultrapure) was used for all of the depositions. CAUTION: thallium salts are extremely toxic. Singlecrystal (100) wafers of semiconductor-grade n-silicon were obtained from Monsanto. The wafers were doped with phosphorus by the manufacturer to a volume resistivity of 2.1— 2.4 fl-cm. Back ohmic contacts were made either with Ga-In eutectic, or by evaporating a thin film of aluminum onto a cooled silicon substrate. The wafers were exposed to the aqueous electrolyte using the PAR Model K105 flat specimen holder, and they were etched in aqueous 10% HF for five minutes prior to photoelectrochemical deposition. Platinum, stainless steel, and silicon working electrode areas were 1.0 cm2 in all cases. The semiconductor electrodes were irradiated with an Oriel Model 6431 250W quartz tungsten-halogen source. Melles Griot 10 nm bandwidth interference filters were used for monochromatic light experiments. Irradiances were measured with an Eppley Model E6 thermopile radiometer. Scanning electron microscopy was done on the JEOL 35CF scanning electron microscope. III. RESULTS AND DISCUSSION A. Electrochemical deposition of thallium(lll) oxide films We have previously deposited ceramic fi
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