Microtomography observation of precipitates in semi-insulating GaAs materials

Defects introduced in GaAs materials during growth and post-growth thermal processes are known to largely influence the specifications of transistors. Various techniques have been improved to detect these defects and obtain images of their organization. Laser scanning tomography provides us with macroscale images of fault structures, the details of which up to now have not been resolved. In this paper we show that extending this technique to a microscopic scale reveals small individual scatterers which are at the origin of the tomography images: these scatterers are microprecipitates located on dislocations or condensed on point defects. Several examples of undoped, In doped, and annealed materials are reported and discussed: apart from the decoration precipitates which look rather large, smaller and abundant microprecipitates are observed, even in dislocation-free materials; their number is found to be consistent with microetched pit density responsible for the ‘‘microroughness’’ of etched surfaces. Ingot annealed materials developing a conventional cell structure are also shown to contain clouds of such microprecipitates concentrated in the central region of the cells. It is likely that the type and the arrangement of these particles are strongly related to the dislocation network and to the thermal history. There is no doubt that the chemical species involved play an important role at a microscale level in the local electronic properties of the material.