TRANSIENT MEASUREMENTS WITH AN ULTRAFAST SCANNING TUNNELING MICROSCOPE ON SEMICONDUCTOR SURFACES

We demonstrate: the use of an ultrafast scanning tunneling microscope on a semiconductor surface. Laser-induced transient signals with 1.8 p s rise time are detected, The investigated sample is a low-temperature grown GaAs layer plated on a sapphire substrate with a thin gold laye r that serves as st bias contact, For comparison, the measurements are performed with the tip in contact to the sample as well as in tunneli ng above the surface, In contact and under bias, the transient signals are identified as a transient photocurrent, An additional signal is g enerated by a transient voltage induced by the nonuniform carrier dens ity created by the absorption of the light (photo Dember effect). The transient depends in sign and in shape on the direction of optical exc itation. This signal is the dominating transient in tunneling mode. Th e signals are explained by a capacitive coupling across the tunneling gap, (C) 1998 American Institute of Physics.