Low-Electron-Energy Cathodoluminescence Study Of Polishing And Etching Effects On The Optical Properties Of Bulk Single-
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Low-Electron-Energy Cathodoluminescence Study Of Polishing And Etching Effects On The Optical Properties Of Bulk Single-Crystal Gallium Nitride Lawrence H. Robins and Bruce Steiner National Institute of Standards and Technology, Gaithersburg, MD 20899, U.S.A. Norman A. Sanford National Institute of Standards and Technology, Boulder, CO 80305, U.S.A. Carmen Menoni Colorado State University, Fort Collins, CO 80523, U.S.A. ABSTRACT Low electron energy cathodoluminescence (LEECL) was used to examine polishing-induced damage in a bulk high-pressure grown GaN single-crystal platelet. The Ga-polarity face of the platelet was mechanically polished; chemically-assisted ion-beam etching (CAIBE) to a depth of 200 nm was performed on a portion of this face. Low-temperature (15 K) CL spectra of the polished-only and polished+CAIBE regions of the Ga-face were taken at 2.8 kV, 5.4 kV, and 10.6 kV (corresponding to average electron penetration depths of 19 nm, 56 nm, and 170 nm). The low-temperature CL spectrum of the unpolished, N-polarity face was taken at 10.6 kV. In the near-band-edge region, all the CL spectra from the Ga-polarity face show a narrow peak near 3.47 eV, ascribed to donor-bound exciton recombination, and several overlapping peaks at lower energy (3.1 eV to 3.4 eV), ascribed to defect-related levels or to donor-acceptor pair recombination. Functional curve-fitting analysis enabled deconvolution of the spectra into the sum of an asymmetric peak (the donor-bound exciton) and several symmetric Gaussian peaks (the lower energy, defect-related or donor-acceptor peaks). The linewidth of the donor-bound exciton peak decreased with increasing penetration depth, and also decreased on going from the polished-only to the polished+CAIBE region. The relative intensity of a defect-related peak at ≈3.325 eV showed a similar decreasing trend with increasing penetration depth or with CAIBE treatment. The LEECL results suggest that the thickness of the polishing damage layer is approximately 400 nm; the 200 nm CAIBE step is thus insufficient to completely remove the damage. INTRODUCTION Bulk free-standing high-pressure grown GaN crystals are promising substrates for homoepitaxial growth of GaN and related alloys. Particularly attractive is the reported absence of threading dislocations that may, if present, propagate into epitaxial films.[1] However, thin-film growth on the native {0001} faces of the bulk GaN crystals has proven problematic since the faces, although atomically flat on a scale of roughly 100 µm, are often textured on longer length scales, with hillocks and terraced growth. The {0001} surfaces must therefore be prepared by mechanical or chemical-mechanical polishing before homoepitaxial growth can take place.[2,3] In the present study, a bulk high-pressure grown GaN platelet was examined by low electron energy cathodoluminescence (LEECL) spectroscopy. The LEECL results lend insight into the nature and depth of the subsurface damage induced by mechanical polishing.
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EXPERIMENTAL DETAILS The sample,
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