High resolution system for photoresponse mapping of semiconductor devices

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Abstract
A system has been built which employs a cw 6328 Å laser beam to excite the photoresponse of a semiconductor device. The excitation is achieved by focusing the laser beam to a 1 μ spot on the surface of the device under investigation. Heretofore 5−10 μ resolution has been reported in the literature. Further, the optical path of the system is such that the device surface and spot are simultaneously in focus. A Pockel’s cell is used to amplitude modulate the laser beam intensity. The result is a photoresponse at the characteristic amplitude modulation frequency of the Pockel’s cell. A lock−in amplifier, with a reference input at the characteristic amplitude modulation frequency, is used to amplify the photoresponse to the desired level. The result is a versatile, high−resolution system for the study of light−sensitive devices and phenomena. Programmed area scans have given qualitative and quantitative information about semiconductor diode and transistor junction properties near the semiconductor surface. Specifically, areas of surface inversion have been located on transistor structures. Finally, by comparing the device (p−n junction) output to the amplitude modulation (square wave), an effective minority carrier lifetime was measured in the vicinity of a p−n junction.