Ultrasmooth GaN Etched Surfaces Using Photoelectrochemical Wet Etching and an Ultrasonic Treatment

Abstract

We have studied the effect of ultrasonic agitation on the surface of n-type gallium nitride (GaN) layers grown on sapphire that were subjected to a selective photoelectrochemical etch. Solutions of various concentrations of KOH were used along with an ultraviolet lamp to oxidize the exposed GaN surface, resulting in an anisotropic etch. Smooth surfaces with root-mean-square (rms) roughness of ~4 nm were obtained for a narrow range of etching conditions. It was found that this window could be extended by using etch conditions which produced "whisker" growth. Subsequent ultrasonic agitation was then used to remove these whiskers and obtain smoother surfaces with the best rms roughness values being ~0.9 nm.The development of device processing techniques in the IIInitride (i.e., InAlGaN) material system has been crucial to the subsequent rise of blue and ultraviolet light-emitting diodes, laser diodes, photodetectors, and transistors. Traditional dry etching, used in mainstream semiconductor device production and development, has limitations in that surface damage can occur and the etch is not dopant selective, and thus does not provide an etch-stopping mechanism. Wet chemical etching has normally been used to solve these and related problems, but gallium nitride (GaN) does not have a simple wet etch that can be used to reliably and selectively etch smooth layers. 1,2 Recently, a chemical etching technique that goes beyond the simple wet etch has been developed. While some techniques have been used which require high temperature 3 or a more elaborate electrolysis apparatus, 4 the use of a photoelectrochemical (PEC) etch shows promise for the processing of GaN-based devices. This technique, which employs an ultraviolet (UV) light source to enhance the etch rate of GaN films, was first demonstrated by Minsky et al., 5 and subsequently studied by Youtsey et al. 6 From the experimental results, a model for the etching mechanism was proposed based on reaction kinetics and on how differently doped layers were etched. Reports have been made of a wide range of PEC-etched GaN surface morphologies, depending upon the etching conditions employed. Some indicate the formation of "whiskers" in n-type GaN that are thought to relate to threading dislocations, thus revealing the layer defect density. 7,8 Other reports show that smooth surfaces are achievable with this technique. 9,10 Determining the right conditions for a specific GaN sample to achieve smooth, device-quality etched layers is challenging because of the number of variables that determine etch rate and smoothness. These include, for example, (i) sample carrier concentration, (ii) UV light intensity, (iii) solution type, (iv) etchant concentration, (v) temperature, and (vi) agitation. In this work, the parameter space for GaN etching that relates to morphology has been studied, and the wider process latitude of whisker growth was exploited to achieve smooth layers via subsequent ultrasonic agitation.Experimental A series of n + -GaN layers were grown by low-pressure meta...

Gallium-V antireflective nanostructures by metal-assisted chemical etching for photodetector application

“…Therefore, GaN MacEtch is also an effective tool for defect evaluation in GaN epitaxial thin film [104] . Whiskers can be etched away by sonication in KOH solution, making smoother surface condition [111] .…”