Photoelectrochemical Etching of GaN Thin Films With Varying Carrier Concentrations
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1040-Q11-02
Photoelectrochemical Etching of GaN Thin Films With Varying Carrier Concentrations Jacob H. Leach, Ümit Özgür, and Hadis Morkoç Electrical and Computer Engineering, Virginia Commonwealth University, 601 West Main St., Richmond, VA, 23284 ABSTRACT The evolution of the surface morphology of unintentionally doped and Si-doped GaN samples subjected to photoelectrochemical (PEC) etching in the carrier-limited regime in aqueous KOH is reported. It was found that a nanoporous structure precedes whisker formation in samples in which high densities of whiskers ultimately form. Increasing the light intensity accelerated the rate of change of the surface morphology, but increasing the molarity of the KOH had no effect on the etching. Applying biases to the samples during etching also accelerated or decelerated the rates of change of the surface morphology. Thus, the surface morphology in the carrier-limited regime tends to depend on parameters of the starting layers, as well as how much etching in total has occurred. The identification of variations in surface morphology at different times during PEC etching of GaN may have utility in that assorted nanopatterning of the GaN surface can be intentionally achieved in a controllable, large-scale, and inexpensive manner.
INTRODUCTION Photoelectrochemical (PEC) etching of GaN has received much attention because it is a wet etching technique that can serve as an alternative to damage-inducing plasma-based processes [1-3]. PEC etching can produce smooth surfaces [1] and vertical sidewalls [2] for devices, and it is inherently a relatively low cost and simple method with good selectivity. In addition to producing smooth surfaces, PEC etching can also be employed for surface roughening. Roughening can produce layers with very large surface areas, which may be used in the fabrication of gas sensors [4], generate templates for defect reduction in GaN layers through GaN regrowth [5], or utilized as a means to increase the radiation hardness of GaN devices [6]. Additionally, the roughened surfaces can be exploited for increasing light outcoupling from a light emitting diode (LED) structure [7], or fabricating nanostructures, such as nanorods for various optical or electronic devices [8]. Finally, by simply studying the surface morphologies that are produced by PEC etching, we can garner some sense of the mechanisms involved in order to better understand this important fabrication technique. The method of PEC etching of GaN is a two-step oxidation/dissociation process. Abovebandgap light incident on the GaN surface produces electron-hole pairs near the surface. The holes along with hydroxide ions oxidize the GaN surface, and the removal of this oxide will result in a net etching of the GaN. Since holes are necessary, any local variation in the minority carrier hole concentration will cause a variation in the etch rate. Thus, some volume around an electrically active defect would exhibit a decreased etch rate, provided that the reaction is limited by the number of holes produced
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