CATHODOLUMINESCENCE AND ELECTRON-BEAM-INDUCED CURRENT STUDY OF PARTIALLY RELAXED ALGAAS GAAS/INGAAS HETEROJUNCTION PHOTOTRANSISTORS UNDER OPERATING-CONDITIONS/

We have studied time-resolved cathodoluminescence (CL) and electron be am induced current (EBIC) on AlGaAs/GaAs/InGaAs heterojunction phototr ansistors under operating conditions, i.e., at room temperature and un der bias. Devices from four wafers, with a different amount of lattice relaxation, were tested, It is shown that the CL intensity increases more than one order of magnitude as the voltage is Increased and the c urrent gain of the device turns on. The voltage dependence of the CL s ignal is analogous to the current-voltage curve of the transistor, The buildup in CL intensity was found to be much less in devices with low current gain showing that the CL intensity correlates to the electric al gain of the device. Time resolved CL showed two distinct CL decay t imes, one very short, a few nanoseconds, and one long, of the order of microseconds. This indicates that two fundamental recombination proce sses are present, which we attribute to a spatially direct recombinati on between carriers in the base and a spatially indirect recombination . This spatially indirect recombination is believed to come from recom bination of electrons trapped in the notch formed at the conduction ba nd discontinuity and holes in the base. By studying EBIC as a function of beam current for devices from the different wafers we found that r elaxed devices have a complex current-gain relationship. They require higher current densities than nonrelaxed devices to reach high gain, A t low current densities the gain is very low and the ideality factor i s high indicating a high degree of trap related recombination. At high current densities, on the other hand, these traps become filled and t he associated recombination quenched. This results in a gain and an id eality factor comparable to those of nonrelaxed devices. (C) 1997 Amer ican Institute of Physics.