Gallium nitride (GaN) is a semiconductor material that has great potential for use in optical devices such as green and blue light-emitting diodes. It is also riddled with structural defects. In the 10 August issue of Applied Physics Letters (73, 797-799; 1998), C. Youtsey, L. T. Romano and I. Adesida starkly expose these imperfections in a GaN film, by stripping away the intervening material. Characterization of the flaws may eventually aid the development of structurally pure films.

Resembling stalks of corn in a field, the thread-like defects, or dislocations, can be seen here, surrounded by untreated material. There can be no doubt that these structures coincide with dislocations — other images reveal that each thread stems from a ‘root’ defect in an underlying layer of untreated GaN film. But these delicate filamentary flaws are mere tens of nanometres thick. What kind of eraser could eliminate the material in between with such spatial selectivity?

The answer is a wet chemical etch process. A GaN film is first immersed in an alkaline solution in an electrochemical cell. Under illumination by ultraviolet light, positively charged holes (‘missing’ electrons) are generated, which are involved in the oxidation and disintegration of the film. Yet somehow the defects are impervious to this electrochemical abrasion.

This resilience probably arises from electrical activity in the dislocations, which reduces the concentration of holes in the vicinity, suppressing the destructive oxidation reactions. The electrical activity may be caused by recombination of electrons and holes, or by charges at the dislocations that repel the holes.

Although it is a destructive technique, the erosion process could also be used to creative effect — it may be a way to make useful nanometre-scale structures in GaN films.