The role of EL2 in the infrared transmission images of defects in semi-insulating GaAs

725 Abstract—Infrared transmission images from GaAs semi insulating wafers were considered for years as directly related to the quantum absorption by electrons on fundamental states of deep centers, especially EL2. The satisfying correspondence of these images with the dislocations revealed by etching or X ray topography or infrared tomography led to the opinion that a strong concentration of EL2 centers was to be expected in the immediate vicinity of the dislocations. More recent work indicates that contrary to the expected behavior the photoquénching of transmission images at T=80K does not appreciably change the image structure itself but more largely the uniform background level of absorption. Such investigations show that the transmission images of isolated dislocations (Indium doped materials) or cell structures of tangled dislocations (undoped materials) can be partly attributed to scattered light; similar operation at T= 10K removes the dark features associated to EL2 but still preserves the skeleton of the pattern which is due to scattering. A result of the measurements is that dislocations must not be considered any longer as inexhaustive EL2 reservoirs. The lifetime of the photoquénching mechanism is shown to vary differently for EL2 centers located close to the dislocations or in the matrix. In this paper we will develop the details of infrared image photoquénching experiments in the vicinity of dislocations; undoped and In doped GaAs materials will be shown. These results will be discussed in the light of surface etching experiments.