Intrinsic microcrystalline silicon by postgrowth anneals
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Intrinsic microcrystalline silicon by postgrowth anneals Jong-Hwan Yoona) Department of Physics, College of Natural Sciences, Kangwon National University, Chunchon, Kangwon-do 200-701, Korea (Received 2 Nov 2000; accepted 26 Mar 2001)
Hydrogenated microcrystalline silicon (c-Si:H) grown by a conventional plasma-enhanced chemical vapor deposition from high hydrogen-diluted silane was annealed by increasing the temperature from 25 to 450 °C at a constant rate of 12 °C/min (one annealing cycle). Dark-conductivity activation energy gradually increases with increasing the number of annealing cycle to a saturation value of about 0.6 eV, observed in truly intrinsic c-Si:H films. For the saturated state, the dark conductivity of the order of 10−8 S/cm was obtained. Little or no change in the oxygen content was observed after the annealing.
Hydrogenated microcrystalline silicon (c-Si:H) has a high potential in thin film electronic devices such as solar cells and thin film transistors (TFTs). The stable electronic properties of c-Si:H against light exposure1 and the high mobility,2 compared with amorphous silicon, make it be of growing interest for device applications. However, the strong n-type character, usually observed in undoped c-Si:H films grown by a conventional plasma-enhanced chemical vapor deposition (PECVD) from high dilution of silane in hydrogen, is one of the main properties to be improved for device applications. The n-type character results in a reduction of the photocurrent collection efficiency in a solar cell when used as an absorbing layer and causes the unwanted high offcurrents in a thin film transistor. It has been suggested that the n-type character of undoped c-Si:H may arise from either the rather large content of oxygen impurities or native defects within the films.3,4 It is unclear which affect is dominant for the n-type character. Recently, various preparation techniques, such as microdoping,5 purifying the gas,6 and the use of dichlorosilane,7 have been used for intrinsic microcrystalline silicon films. In this article, we present the experimental results indicating that c-Si:H of intrinsic character can be achieved by postgrowth anneals. The annealing gives rise to a gradual increase in the dark-conductivity activation energy, followed by a saturation value of 0.6 eV. All samples used in this work were grown from high hydrogen-diluted silane by a conventional PECVD using a plasma excitation frequency of 13.56 MHz. The plasma
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e-mail: [email protected] J. Mater. Res., Vol. 16, No. 6, Jun 2001
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reactor consists of two capacitively coupled parallel electrodes and has never been exposed to any doping gases. The hydrogen dilution ratio SiH4/(SiH4 + H2) in the gas flow rate was kept constant at a value of 3.3%. The flow rates for SiH4 and H2 gases were 5 sccm and 145 sccm, respectively. The base pressure before deposition was
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