Metastable Changes in the Photoconductive Properties of Microcrystalline Silicon Upon Heat Treatment
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METASTABLE CHANGES IN THE PHOTOCONDUCTIVE PROPERTIES OF MICROCRYSTALLINE SILICON UPON HEAT TREATMENT R. Brüggemann Institut für Physik, Carl von Ossietzky Universität Oldenburg, 26111 Oldenburg, Germany, Email: rudi.brueggemann @uni-oldenburg.de
ABSTRACT We demonstrate that metastable changes or instabilities in the dark conductivity of microcrystalline silicon upon heat treatment and ambient conditions, which have been reported in the literature, are accompanied by changes in the photoconductivity or the majority-carrier mobility-lifetime product. The minority-carrier mobility-lifetime product and sub-gap absorption appear to be much less affected by different heat treatment procedures and ambient conditions. The observations can be related to Fermi-level induced change in defect occupancy by which the effective density of recombination centres is reduced for electrons but remains the same for holes. Minority carrier properties seem to be better suited as an indicator for sample quality and for comparison of microcrystalline silicon samples from different laboratories.
INTRODUCTION Adsorbate effects on the dark conductivity in hydrogenated amorphous silicon are well known and have been studied in some detail [1]. Similar results were also reported for microcrystalline silicon (µc-Si:H) for which heat treatment in vacuum at elevated temperatures to remove adsorbates has been shown to result in changes in the dark conductivity [2-7] when different heat treatment procedures at different temperatures had been applied. For microcrystalline silicon both increase and decrease in the room-temperature dark conductivity upon heat treatment have been reported [2-7]. These changes can be large but it is also possible that the dark conductivity hardly changes upon heat treatment [7]. Finger et al. [4] have categorised the findings and also reported additional experimental results from electron spin resonance and infrared spectroscopy that were related to the dark conductivity variations. Broadly speaking, upon heat treatment, samples of Type I character exhibit an decrease in the room-temperature dark conductivity while samples of Type II character show a increase in the room-temperature dark conductivity. This paper focuses on the related changes in the photoconductive properties of µc-Si:H samples upon heat treatment at temperatures in the range between 90 and 190 °C. It is known that the value of the room-temperature photoconductivity depends to a large degree on the roomtemperature dark conductivity in µc-Si:H or, in relation to the occupancy of defect states, on the Fermi level [8,9]. As the photoconductivity is a figure that indicates the photoelectronic quality of the sample, it is important to estimate the reliability of such values for photoconductivity which may, as the dark conductivity, depend on the thermal or ambient history of the sample. The effect of heat treatment on the minority-carrier properties in terms of diffusion lengths from the steady state photocarrier grating (SSPG) technique [10] which sh
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