We have recently shown that, contrary to the well-reputed opinion, the EL2° centers in GaAs undoped materials are not especially concentrated on dislocations: the cell pattern, in particular, does not disappear after photoquenching at Liquid Nitrogen Temperature. In this paper we shall complete this study with details of the EL2° densities in the vicinity of individual dislocations in GaAs-In materials. Laser Scanning Tomography and Infra Red Transmission images will be analyzed complementarily. Grown-in axial dislocations appear to be surrounded by a wide EL2-rich cylindrical zone, whereas the dislocation itself is not to be considered as a large reservoir of gettered EL2 centers.
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.
Three independent but complementary methods (OSL photoetching combined with etch rate profiling, spatially resolved PL and LST) were employed to Study the distribution of microdefects and electrically active centres in commercially available SI undoped. LEc-grown GaAs after different ingot-annealing treatments. A one-to-one correlation was obtained on comparing the microscale distribution of decoration precipitates (OPS), matrix precipitates (MPI) and microdefects (MMS) by the DSL and LST methods. Clustering of MPS is also revealed by PL intensity profiling and photoetch rate measurements. Two types of matrix microdefects are distinguishable by OSL photoetchlng In Samples after two-stage annealing. A high degree of homogeneity across the dislocation cells (after low-temperature and multiple annealing) was clearly revealed by a drastic decrease in the uniformity parameter (U) after DSL photoetching and by a significant decrease in the x intensity measured at the cell walls with respect to the cell interiors. Using the present results and recent data from the literature, an explanation of phase transitions during ingot annealing is proposed. This is based on the assumption that the final properties and structures after annealing are the result of two competitive processes: (i) formation of decoration and matrix microdefects; (ii) generation of electrically active point defects (clusters), presumably EL2
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