INTERACTION OF PHOSPHORUS AND BORON IN COMPENSATED AMORPHOUS-SILICON FILMS

We report on the characterization and modeling of amorphous silicon fi lms deposited from a mixture of silane and diluted phosphine and dibor ane gases at doping concentrations varying over four orders of magnitu de. At the smallest doping concentrations, the single doping species d o not interact and compensation is achieved for diborane/phosphine rat io equal to the inverse of their doping efficiency. At larger concentr ations, compensation is obtained by using the same amount of diborane and phosphine in the gas mixture with silane. This equality indicates an interaction between the two doping species which leads to formation of boron-phosphorous cluster and a decrease of defects. An increase o f photoconductivity with increasing defect density has been found. We model this behavior by assuming the presence in the gap of the materia l of two Gaussian distributions of shallow states with different captu re cross-sections for the two carriers and we discuss a possible origi n of these defects. (C) 1998 Elsevier Science B.V. All rights reserved .