| Popis: |
“Reverse contrast imaging” is a technique that has been developed for imaging dislocation distributions in undoped semi-insulating LEC GaAs. This technique, which relies on the mapping of near-bandgap infrared transmission, is both rapid and non-destructive. Until now, no satisfactory mechanism has been proposed for the images except that it has been concluded that they are a result of an absorption process. In the present work we have investigated the role of scattering in the production of this image. We have previously described an optical apparatus which is sensitive to changes in refractive index and have shown how it can be used to image both growth striations and doping non-uniformities in n-type GaAs. In this method, changes of refractive index which produce small changes in paths of light beams transmitted by the sample, are directly imaged by an infra-red sensitive TV camera. Using this apparatus we have now demonstrated that the images in reverse contrast, which occur only when the sample is cooled below 150 K and when imaged with light of energy within ∽ 50 meV of the bandedge, have two components, one due to absorption and one due to scattering. The wavelength dependencies of these components are different; the scattering image is observable at longer wavelengths than the absorption image. We conclude that the scattering and absorption arise from the same defects. These absorb near bandedge light, are gettered by dislocations and the resulting concentration gradients are the cause of the light scattering. We tentatively associate these defects with arsenic vacancies. |
