N–type Ga-As polycrystalline layers, grown on Mo substrates by the metal-organic process were investigated using the SEM. Micrographs of charge collection contrast indicate a fairly random distribution of high collection (bright) grains. In a typical cell about 1/3 of its area is bright, with nonuniformities in collection current within and between grains. These results correlate well with Isc measurements, and some of the variations betweencells is explained in terms of insufficient doping of the grain boundaries.
Reducing the penetrating depth of the carriers' excitation volume results in lowering of the collection current. The measurements were normalized for changes in beam and EBIC current by a comparison with a single crystal cell of the same geometry. This degradation is explained in terms of contamination or damage of the layer close to the surface during growth.