Negative Differential Resistance, High Field Domains and Microwave Emission in GaAs Multi-Quantum Wells

Abstract

Experimental results presented in this work are concerned with high longitudinal electric field transport in n-type modulation doped GaAs/AlGaAs multiple quantum wells. Negative differential resistance with accompanying oscillations in the current have been observed at room temperature, and are attributed to real space transfer or Г-L inter-valley transfer within the wells, the frequency of the oscillations is typically of the order of 1GHz. In samples, exhibiting negative differential resistance and current instabilities, high field domains have been observed, using field contrast techniques within a scanning electron microscope. The domains are shown to be either static or slow moving, propagating along the samples and collapsing before reaching the anode, in stark contrast to Gunn domains in bulk GaAs. These samples have exhibited microwave amplification in untuned circuits from 1.2GHz to 3.5GHz. Further experiments are required to determine the performance limits of these structures, but these results show the potential of such devices as high frequency components. In samples where breakdown occurs as a result of impact ionisation scanning electron microscope studies indicate the occurrence of current filamentation. The samples studied also emit light when biased. The threshold field required for light emission is somewhat smaller than that for the current instabilities. The spectral analysis of the electroluminescence indicates that it originates from the quantum wells. The relationship between the current instabilities and light emission is as yet not fully understood.