A Damage-reduced process revealed by photoluminescence in photoelectrochemical etching GaN
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A damage-reduced process revealed by photoluminescence in photoelectrochemical etching GaN J. M. Hwang, J. T. Hsieh and H. L. Hwang Department of Electrical Engineering, National Tsing-Hua University, Hsin-Chu 300, Taiwan, R.O.C. W. H. Hung Synchrotron Radiation Research Center, Hsin-Chu 300, Taiwan, R.O.C.
Abstract Photoelectrochemical (PEC) etching technique has been proven to be an effective method to etch GaN. Despite its success, investigations on etching-induced damage are still scare. In this work, the damage induced by PEC etching of GaN in KOH electrolyte was studied. Photoluminescence (PL) spectroscopy was used to explore the origin of etching-induced damaged layer. From the variable temperature PL measurements, the origin of etching-induced damage was attributed to be the defect complex of VGa-ON (gallium vacancy bonds to oxygeon on nitrogen antisite). With determination of the defect origin, the electronic transition in the etch damage-related yellow luminescence (YL) band was suggested to be deep donor-like state to shallow-acceptor transition. In addition, a post-treatment method with boiled KOH chemical etching was developed to remove the thin damaged layer. In this method, crystallographic etching characteristics of boiled KOH was observed to assist in the formation of smooth sidewall facets. As revealed by the reduction of yellow luminescence, we propose this novel technique as a near damage-free etching method.
Introduction Since GaN is chemically stable and insoluble in all mineral acid and base solutions at room temperature [1], most processing of III nitrides are currently done by dry plasma etching. High density plasma or energetic ion assisted etching were used to get a smooth etch surface and highly anisotropic sidewalls with high etch rates [2-4] . But there are several disadvantages to use dry etching, including generation of the ion-induced damage and difficulty in obtaining smooth etched sidewalls, which are required for the lasers cavity. The optical damage relative to yellow luminescence (YL) was also extensively investigated using photoluminescence spectroscopy on the etched surfaces. [5] The etching-induced damage will have detrimental effect on electronic or photonic device performance. Therefore, a reliable fabrication process producing low etch damage for GaN-based devices is necessitated. Photoelectrochemical (PEC) etching is an alternative method to produce smooth surfaces and vertical sidewall facets without ion bombardment induced damages.[6-8] As it is well-known that the yellow luminescence commonly existing in epitaxial grown GaN is related to the intrinsic defects or impurities. In this study, we used YL intensity variation to identify the etching-induced damages. A novel boiled KOH post treatment process was
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also developed to remove the etching-induced damages and achieve vertical and smooth facet sidewalls. A near damage-free etching process with vertical sidewalls and smooth etched surface can be realized by this post treatment process. This etching pr
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