Direct In-Situ Observation of Charge Carrier Separation During Formation of an Amorphous Silicon / Crystalline Silicon Heterojunction

Abstract

ABSTRACTThe signal of the Transient Microwave Detected Photoconductivity after laser pulse illumination is followed during the PECVD growth of intrinsic amorphous silicon on top of crystalline silicon (c-Si) substrates. Depending on the thickness of the amorphous layer the observed phototransients can be assigned to the excess charge carriers in different regions of the so grown heterojunction. For thin a-Si:H layers the signal amplitude decreases exponentially with deposition time. In the case of thick amorphous silicon the signal reflects only the properties of the amorphous silicon and the signal amplitude is constant. In an intermediate range a large contribution of charge carriers absorbed in the a-Si:H and subsequently transported to the crystalline part of the heterostructure to the measured signal can be observed. Due to their much higher mobility in the crystalline layer these excess charge carriers become visible only after reaching the crystalline silicon. This leads to the observed delay of the TRMC signal amplitude relative to the exciting laser pulse. The experiments show, however, that this charge carrier transfer takes place only in the case of a low defect a-Si:H layer to a measurable extent and is suppressed by the low carrier mobility in highly defective a-Si:H deposited at lower substrate temperatures.