Scanning probe microscopy of a gallium arsenide heterostructure under ambient conditions.

Abstract

We have examined passivated and unpassivated GaAs (110) surfaces under ambient conditions with Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS), and Atomic Force Microscopy (AFM). We have obtained contrast between differently-doped regions of a doping heterostructure using all three imaging methods; we believe that we are the first to have observed AFM contrast on passivated GaAs doping heterostructures, and may be the first to have observed AFM contrast on unpassivated doping heterostructures. The STS results show that, using regular STS I(V) curves taken on the passivated surfaces, it is possible to distinguish doping regions even if the surfaces are topographically rough. We discuss in detail the steps necessary to passivate successfully GaAs (110) surfaces for Scanning Probe Microscopy (SPM) analysis. We are among the first to accomplish this passivation, which is achieved by treating a freshly-cleaved GaAs surface with a heated, dilute $\rm P\sb2S\sb5:(NH\sb4)\sb2S:H\sb2O$ solution. Our surface treatment results compare well with the present literature. We extend the current understanding of these surfaces by comparing characteristics of the passivated and unpassivated surfaces, especially amenability to SPM observation. The passivated surfaces were seen to develop pits where current-voltage characteristics were acquired, an effect which has not been reported previously. We present for the first time current-versus-separation measurements on these passivated surfaces. Finally, we discuss the tunneling barrier itself, comparing square barriers to trapezoidal barriers. To illuminate the differences, we examine calculated transmission coefficients as functions of electron energy, bias voltage, and separation.