Metastable changes of the electrical conductivity in microcrystalline silicon
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Metastable changes of the electrical conductivity in microcrystalline silicon N. H. Nickel and M. Rakel Hahn-Meitner-Institut Berlin, Kekuléstr. 5, D-12489 Berlin, Germany. Abstract The temperature dependence of the dark conductivity, σD, of as-grown and H depleted µc-Si was measured. While σD of the H depleted samples did not exhibit any influence of thermal treatment prior to the measurements, in as-grown µc-Si the dark conductivity increased by 2 orders of magnitude below 300 K upon rapid thermal quenching. The frozen-in state is reversible and an anneal at 440 K followed by a slow cool completely restores the initial state. The time and temperature dependence of the relaxation of the quenched-in state reveals two competing processes. At short times σD increases due to the activation of a donor complex and at long times σD decreases due to the dissociation of bond-center H complexes.
1. Introduction Metastability is a well-known phenomenon in amorphous and polycrystalline silicon. It limits the electrical and optical properties of the material and its applications in devices such as solar cells and thin-film transistors. Metastability manifests itself as a change in the electronic transport properties due to the generation of silicon dangling bonds. The most prominent metastable effect is the light-induced defect generation, well known as the Staebler-Wronski effect [1]. The lightinduced metastability is not limited to amorphous semiconductors. It has also been observed in hydrogenated polycrystalline silicon [2]. Silicon dangling-bond defects are also generated by charge injection in pin diodes [3], carrier accumulation in thin-film transistors [4], doping [5], and rapid thermal quenching [6]. Generally, it is believed that the presence of hydrogen in amorphous silicon (a-Si:H) is the cause for metastability. Dersch et al. [7] observed that the light-induced change of the electrical dark conductivity is accompanied by an increase in the silicon dangling bond concentration suggesting that the defect generation is connected with changes of the electronic properties of a-Si:H. In hydrogenated amorphous silicon metastability has been documented extensively. Hitherto a consistent model for the defect generation does not exist. However, a key experiment that directly relates hydrogen motion to the light-induced defect generation mechanism was reported previously. Changes of the spin density due to repeated degradation and annealing cycles and the rejuvenation of the effect upon reexposure of the samples to monatomic H demonstrated the direct participation of hydrogen in the light-induced defect generation mechanism [2]. In addition to the light-induced defect generation mechanism metastable behavior of the temperature dependence of the electrical dark conductivity was observed in hydrogenated poly-Si. Rapid thermal quenching results in an enhancement of the electrical dark conductivity, σD, by up to eight orders of magnitude with an equilibrium temperature of 268 K. The effect originates from the formation and dissoc
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