MICROWAVE MODULATED PHOTOLUMINESCENCE AS A CONTACTLESS PROBE OF INTERFACE STATES

Microwave modulated photoluminescence (MMPL) is a developing spectrosc opy in which the sample is subjected to continuous optical pumping and chopped microwave electric fields. The signal detected in an MMPL exp eriment is the change in the photoluminescence spectrum of the sample due to the presence of microwave electric fields, which increase the k inetic energy of the free carriers. In order to investigate the effect s of interfaces on this measured quantity, two types of GaAs samples h ave been compared. The first type of sample was grown such that the Ga As epilayers are exposed, while in the second type the GaAs is ''cappe d'' by a layer of higher band gap material. Several pairs of such samp les have been studied. The MMPL results are consistent with the follow ing mechanism: an increase in the kinetic energy of the free carriers that results from the presence of the microwave fields allows more fre e carriers to reach the interface or surface of the GaAs layer before recombining. The presence of a greater number of nonradiative recombin ation paths in the samples with a bare GaAs surface than at the GaAs/c apping-layer interface is therefore seen experimentally as an increase in the photoluminescence quenching by the microwave electric fields. The potential usefulness of MMPL as a probe of non-radiative recombina tion and as an indicator of interface quality is illustrated by a quan titative estimate of surface/interface non-radiative recombination. (C ) 1997 American Vacuum Society.