DEVELOPMENTS OF IRBIC AND QIRBIC IN DEFECT STUDIES - A REVIEW

Optical injection, as well as the electronic one, is of great use for semiconductor material and component characterization, and in recent years several papers appeared dealing with the analysis of the depth response of scanning light microscopy, the distribution of the charge carriers generated by a focused light beam as well as the photoinduced current dependence on the measurement configuration and the consequent imaging of defects in semiconductors. In these works a laser source is always used because of its features of monochromaticity, focusing and high intensity. Actually the light microscopy has become a widely accepted wafer mapping technique for investigating inhomogeneities and, besides, a method to evaluate diffusion length and surface recombination velocity of minority carriers. We have applied the same principles of the scanning laser microscopy, but substituting filtered light coming from an halogen lamp to the laser source. This expedient, even though it involves some disadvantages about monochromaticity and focusing, allows, however, to easier handle light beam parameters as photon density and energy. We refer to this method as IRBIC (Infra-Red Beam Induced Current) since the photon energy of the scanning beam lies in the infrared portion of the spectrum. Object of our investigations is the current induced by the infrared light beam and, in more detail, the variations in defect imaging observed by changing penetration depth and intensity of the light. In addition, having a scanning light microscope at our disposal, we have modified the basic experimental setup so as to irradiate the back surface of the sample by monochromatic sub-band gap light. When the photon energy of the back surface irradiation coincides with the distance E(t) of a deep level within the forbidden gap from one of the allowed energy bands, a level-band transition occurs and, as a consequence, the concentration of the efficient recombination traps at the level E(t) changes. This process, involving the defect recombination activity, produces strong variations in the current induced by the beam scanning the opposite surface. The above said phenomenon, known for long time in bulk material examinations and named "photoconductivity quenching", has been applied by us to the scanning light microscopy and called "QIRBIC", that is "Quenched Infra-Red Beam Induced Current". Both the methods, IRBIC and QIRBIC, very sensitive to the occupancy factor of the defects, have been used to investigate the electrical activity of extended defects, namely dislocations, and their interaction with impurities and point defects. Besides, investigations have been performed on lamellar structure precipitates and on deformation-induced dislocation loops.