Room Temperature Photo-Luminescence Imaging of Dislocations in GaAs Wafers

Abstract

To date, photo-luminescence (PL) imaging of GaAs wafers affords the only non-contact and non-destructive characterization of microscopic material inhomogeneity within a depth of less than one micrometer. Material inhomogeneity refers to spatial variations in the distribution of dopants, and native defects which usually occur unintentionally, and can cause considerable variations in transistor characteristics over a water surface. The major source of this material inhomogeneity is associated with the accumulation or depletion of these point defects around dislocation sites during boule formation and cooling. Therefore dislocation density and distribution have been of primary importance in measuring material inhomogeneity. A common way of determining the dislocation density near the surface of GaAs wafers is to etch the wafer and count pits over regions of the surface. This is laborious, destructive, and not representative of wafers from other positions in the ingot; in fact it has been shown [1] that the dislocation network in semi-insulating LEC GaAs wafers varies dramatically as a function of position along the boule. Infrared transmission images of GaAs wafers are sensitive to the distribution of near-midgap impurity states but they are taken through the entire thickness of the wafer and therefore lack sensitivity to the surface fabrication region of interest. X-ray topography can also be used to examine dislocation networks through the wafer but also lacks surface sensitivity and requires several hours of exposure in order to obtain a full wafer topograph. Neither infrared transmission nor X-ray topography can furnish any information on uniformity of epitaxial layers.