Abstract

Optical excitation of semiconductors with femtosecond laser pulses is known to generate highly athermal hot carrier populations. Whereas the bulk relaxation behavior of these carriers is already well understood, little is known about the fate of hot carrier distributions in surface states. Nevertheless the surface induced last carrier recombination plays a dominant role in most optoelectronic semiconductor devices and limits their efficiency. The investigation of the influence of surface stales and defects on carrier relaxation and recombination requires a surface sensitive method with liigh spatial and temporal resolution. The combination of scanning tunneling microscopy and femtosecond laser spectroscopy opens the possibility to achieve both goals. A schematic of the combined femtosecond scanning tunneling microscope is shown in fig. 1. First experiments using this method have shown that the generation of photoelectrons is dominating the measured current at excitation densities above 0.1 mJ cm-²(1). In the present work we investigate the light induced effect on the tunneling contact al lower excitation densities (50 µJcm^-2). In order to gain a sufficiently high sensitivity for light induced surface phenomena we use variable distance scanning tunneling spectroscopy (2).

© 1996 IEEE