Carrier Transport through Grain Boundaries in Hydrogenated Microcrystalline Silicon

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CARRIER TRANSPORT THROUGH GRAIN BOUNDARIES IN HYDROGENATED MICROCRYSTALLINE SILICON S. GREBNER, F. WANG, R. SCHWARZ Technical University of Munich, Physics Department E16, James-Franck-Str., W-8046 Garching, FRG.

ABSTRACT To analyse the influence of the grain boundaries (gb) on the transport of carriers in hydrogenated microcrystalline silicon (uc-Si:H) the ambipolar diffusion length (Lbmb) was measured by SSPG. In addition, the films were characterised by photo-conductivity, dark conductivity activation energy, Urbach energy (determined by CPM), hydrogen effusion, Raman spectroscopy, X-ray scattering and optical transmission. The sample series was prepared by PECVD of SiH 4 diluted with increasing H2 content. Taking the structural information by Raman spectra and X-ray into account, we explain our optical and activation energy measurements within a three-phase-model (amorphous phase, crystalline phase, gb) and a Fermi level pinning in uc-Si:H.

INTRODUCTION J/c-Si:H can be prepared simply by varying certain deposition parameters of the fabrication process of a-Si:H [1].It has a of five magnitude higher electronic dark conductivity and also a higher optical band gap. Since it was shown that this material can easily be doped [2] it is appropriate for optoelectronic devices like solar cells and large area flat panel displays [3]. In the literature jc-Si:H was seen as a two phase material consisting of amorphous and crystalline silicon [4]. The crystallites are surrounded by grain boundaries, defect rich amorphous silicon with a high content of SiH 2-bondings. In this paper we discuss jic-Si:H as a three-phase material, in which we examine the gb as the third material with high defect density, pinning the Fermi level as in polycrystalline silicon [5].

SAMPLE PREPARATION The sample series were deposited on #7059 Coming glass and crystalline silicon by decomposition of a SiH 4 and H2 gas mixture with a H2 content from 0 up to 97.5% in a PECVD system at substrate temperatures of 250 C and a power intensity of 30 W/mm 2 . So we are able to compare pure a-Si:H with our 1sc-Si:H samples.

CHARACTERISATION A. Structural Investigations The ratio of the TO phonon intensities of the amorphous (at 480 cm-i) and the crystalline (at 522 cm-1) phase in uc-Si:H shown in the Raman spectra (Fig. 1) offers us a possiblity to get an idea of the volume fraction of the crystallites in this material. An estimation results in volume fraction of the crystallites between 30 and 45% depending on the hydrogen dilution Mat. Res. Soc. Symp. Proc. Vol. 283. ©1993 Materials Research Society

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