Assessing thermal Cl2 etching and regrowth as methods for surface passivation

Abstract

The damage layer that results from ion beam processing ultimately limits the smallest size structure which will emit light and hence prevents the realization of low-dimensional quantum size effects. Effective surface passivation of etched structures requires both layer removal and compensation of surface states. Ultimately these two processes must be made compatible. To address this problem, we have coupled an ultrahigh-vacuum (UHV) dry-etching system to a molecular beam epitaxy (MBE) growth chamber. Material is patterned and masked outside of the combined etching/regrowth system using electron beam lithography. Samples are then loaded into the combined UHV system and vertical structures are produced by chemically assisted ion beam etching (CAIBE). These structures are then subjected to a thermal Cl2 gas etch to remove the damage layer created by the ion beam etching. Samples are subsequently transferred directly into the MBE growth chamber, where a cladding layer of Al0.3Ga0.7As is grown over the etched structure to provide passivation. In this way both the damage removal and surface passivation are achieved in-situ without the deleterious effects caused by exposure to air. We find, by low temperature cathodoluminescence, that submicrometer structures subjected to this in-situ processing display fivefold improved luminescence efficiencies. Structures etched and regrown without the thermal Cl2 etching step show no comparable improvement, indicating the damage removal is the key to improved efficiencies.