Optical absorption associated with grain boundaries in n-type germanium

Abstract

The optical absorption by 40 to 25 0 grain boundaries in germanium bicrystals in the spectral region 0040 ev to Oe80 ev was measured using two modulation techniques. The first technique measured directly the difference in optical absorption between the grain boundary layer and the adjacent single crystal regions. The second technique measured the changes in absorption produced when DC and AC electrical biases were applied across the grain boundary. Grain boundary absorption was found to be composed of two relatively distinct components. One, which varies in magnitude as the bias across the grain boundary is variedp is due electric field-assisted absorption in the grain boundary space charge layer. No other effects on optical absorption due to applied bias were observed. The second component was independent of applied bias. This absorption occurs in the narrow (~ 100 A) deformed region at the grain boundary plane and is probably due to the combined effects of lattice disarray and broken covalent bonds in this region. This bias-independent component has an exponentiallike dependence on photon energy in a spectral region of from 0.60 to 0.75 ev (below the single crystal direct edge). This implies an exponential-like joint density of states for optical transitions which in turn implies exponential-like densities of states for the band edges in the grain boundary layero This exponential absorption structure is similar to that observed for thin films of amorphous germanium and may indicate that the higher angle grain boundaries have an amorphous structure. Observations on a pure 60 twist grain boundary showed no significant difference when compared to measurements of the pure tilt boundaries. This indicates that the optical absorption probably arises from similar structural features in the two types of grain boundary.