Photocarrier Recombination in Microcrystalline Silicon Studied by Light Induced Electron Spin Resonance Transients
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PHOTOCARRIER RECOMBINATION IN MICROCRYSTALLINE SILICON STUDIED BY LIGHT INDUCED ELECTRON SPIN RESONANCE TRANSIENTS J. MULLER, F. FINGER, C. MALTEN, H. WAGNER Forschungszentrum Jilich, ISI, D-52425 Jilich, Germany ABSTRACT To get information on the density of states distribution and the photocarrier recombination in microcrystalline silicon (pc-Si:H), samples with various amounts of n- arid p-type doping are studied with electron spin resonance (ESR) and stationary and time-resolved light induced ESR. The intensity of the dark ESR signals from dangling bonds (DB) and conduction electrons (CESR) is investigated as a function of the doping level. The DB signal has a flat distribution over a wide doping range while the CESR signal strongly increases with n-type doping. Upon illumination with white or infrared light both resonances are enhanced with an intensity that depends on the doping level. The decay of the light induced signal and the dependence in time and intensity of the residual signal on different initial excitation energies and dark/light sequences is studied. The results are discussed with a schematic band diagram for tc-Si:H. The existence of a potential barrier is proposed which spatially separates photogenerated carriers. A large band-offset between crystalline and disordered regions is further suggested. INTRODUCTION Microcrystalline silicon (tac-Si:H) is a phase mixture of amorphous and crystalline regions where the position of the electronic states of both phases and their charge state influence the carrier recombination mechanisms and the electronic transport. It was shown [1,2], that the electron spin resonance (ESR) signal of undoped and slightly doped pc-Si:H material contains contributions from resonances at g=2.0046 and g=2.0052 attributed to dangling bonds (DB) at grain boundaries, in the amorphous phase and in the crystalline grains and a resonance at g=1 .997 attributed to conduction electrons (CESR). The CESR resonance was found to correlate with the n-type character of the material while the DB signal maintains a constant intensity over a fairly wide range of Fermi level positions. At low temperatures (T
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