SCANNING-LASER-MICROSCOPE MEASUREMENTS OF MINORITY-CARRIER DIFFUSION LENGTH AND SURFACE RECOMBINATION VELOCITY IN POLYCRYSTALLINE SILICON

A scanning laser microscope was used to study the electronic and recombination properties at grain boundaries of both n- and p-type Wacker polycrystalline silicon in a spatially resolved photoconductivity experiment. The light energy falling on the samples was varied over five orders of magnitude from 10** minus **1 to 10** minus **6 mw. For p-type material the measured L decreased with beam intensity from 150 to 60 mu m, reaching a constant value at very low beam intensities. The small focal spot of the microscope allowed the measurements to be extended to include n-type samples. For these samples L was found to change from 90 to 18 mu m with decreasing beam intensity. The surface recombination velocity S//G//B was evaluated for both samples. For p-type samples it decreased from 25,000 to 6,000 cm/s and for n-type samples fom 21,000 to 3,000 cm/s with decreasing beam intensity. The quasi-Fermi level separation was determined as a function of the excess minority-carrier-concentration density at the grain boundary and found to increase linearly with beam intensity.