Spin Resonance Studies on free Electrons and Defects in Microcrystalline Silicon
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C. MALTEN, F. FINGER, P. HAPKE, T. KULESSA, C. WALKER, R. CARIUS, R. FLUJCKIGER* and H. WAGNER Institut ftir Schicht- und Ionentechnik, Forschungszentrum Jilich, D-52425 Jilich, Germany *Institut de Microtechnique, Universit6 de Neuchatel, CH-2000 Neuchatel, Switzerland
ABSTRACT The effect of micro-doping, defect creation, and non-steady state occupation through optical transitions on the electron spin resonance signals found in undoped and weakly doped microcrystalline silicon with a high degree of crystallinity is investigated. The experimental results are in agreement with the assignment of the resonance at g=1.9983 to conduction electrons in the crystalline grains and the resonanccs around g=2.0052 to dangling bonds in the remaining amorphous phase and at the grain boundaries. The simultaneous presence of both resonances can result from a large conduction band offset between crystalline grains and grain boundaries or the amorphous phase. The presence of conduction electron spin resonance in compensated and even p-type material points also to potential fluctuations. Free electrons in interconnected crystalline grains are in agreement with the weakly activated transport found in [tc-Si:H at low temperatures.
INTRODUCTION The structural inhomogeneities of microcrystalline silicon (ptc-Si:H), which consists of a mixture of variable volume fractions of crystalline grains, grain boundaries, and an amorphous phase have a key effect on the electronic properties of this material. Band-offsets between crystalline and amorphous regions and defects located in the amorphous phase and at the grain boundaries will strongly influence the transport behaviour. To investigate structural defects and their relation to electronic transport the use of electron spin resonance (ESR) and related techniques is of particular value as it allows the microscopic identification of defects and reacts on the charge state of defects whereby it can give information on recombination processes. For amorphous silicon (a-Si:H) as a related material, spin resonance techniques have contributed a lot to the understanding of transport processes. Recently we have started electron spin resonance experiments on itc-Si:H [1]. In undoped and lightly n-type doped material we found resonances of both conduction electrons in crystalline grains and singly occupied dangling bonds. It was concluded that this requires a position of the Fermi level close to the conduction band edge in the crystalline grains and near midgap in the amorphous phase. This could be obtained by a large conduction band offset or strong potential fluctuations. To test this assumption in the present study we investigate the changes in occupation of states giving rise to the ESR signals. The changes in the occupation are induced through shifts of the Fermi level by micro-doping and electron bombardment and through illumination with band gap light. 757 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
EXPERIMENT Microcrystalline silicon samples were prepared by plasma enh
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