Highly selective photoenhanced wet etching of GaN for defect investigation and device fabrication
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Highly selective photoenhanced wet etching of GaN for defect investigation and device fabrication P. Visconti 1, M. A. Reshchikov, K. M. Jones, F. Yun, D. F. Wang, R. Cingolani and H. Morkoç Virginia Commonwealth University, Dept. of Electrical Engineering, Richmond, VA, 23284 1 Also with Istituto per lo Studio di Nuovi Materiali per l’Elettronica, CNR, Lecce, 73100, ITALY C. W. Litton Air Force Research Laboratory, Wright Patterson AFB, OH 45433 R. J. Molnar Massachusetts Institute of Technology, Lincoln Laboratory, Lexington, MA, 02420-9108 ABSTRACT Photoenhanced electro-chemical (PEC) wet etching has been shown to be suitable for dislocation-density estimation in n-GaN films as well as for GaN-based device fabrication. We report on PEC etching of n-GaN samples grown by MBE and HVPE methods in unstirred aqueous KOH solution under He-Cd laser illumination. Characterization of the etched samples was carried out using atomic force microscopy (AFM) in both cross-sectional and plan-view configurations and scanning electron microscopy (SEM). At moderate illumination densities, the SEM and AFM analyses reveal sub-100 nm scale threading vertical wires on the etched surfaces. The calculated density (~1x10 9 cm-2) is in agreement with dislocation density found by transmission electron microscopy. Using cross-sectional AFM, we find that these vertical wires are ~1µm high and are perpendicular to the sapphire surface. Applying a higher illumination density or an external voltage, we obtain a higher etch rate with a smooth free-feature etched surface. Some highly resistive samples that cannot be etched under normal conditions because the band bending is too small to confine the holes to the surface for them to participate in the PEC process, can be etched with the application of a voltage to the sample. In this case, the etch rate depends on both the polarity and the magnitude of the voltage applied. In an MBE-grown sample with an AlN/GaN superstructure inside, we report on high selectivity between AlN and GaN (AlN is an etch stop); the selectivity is due to the etching mechanism of the PEC process. INTRODUCTION Nitride materials are distinguished by their chemical stability, a characteristic that has required unique challenges for device fabrication [1, 2]. Most processing of III-nitrides is conducted by dry plasma etching despite the resulting ion-induced damage and difficulty in obtaining smooth etched sidewalls as required for devices such as lasers [2, 3, 4, 5, 6]. No wet solution has been found to etch significantly the Ga-face of epitaxial GaN at room temperature. Elevation of the temperature (>120 oC) can increase the etch rates but the difficulties associated with finding an effective mask layer make this process unsuitable for nitride-based device processing. Thus, there have been several attempts to utilize assisted wet-etching techniques. Photo-enhanced electro-chemical (PEC) etching has been demonstrated to produce high etch rates and anisotropic etch profiles in GaN films at room temperature [7, 8, 9, 10, 11]. Co
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