Strain-free Low-defect-density Bulk GaN with Nonpolar Orientations
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Strain-free Low-defect-density Bulk GaN with Nonpolar Orientations Tanya Paskova1, Plamen P. Paskov2, Vanya Darakchieva2, Roland Kroeger1, Detlef Hommel1, Bo Monemar2, Sebastian Lourdudoss3, Edward Preble4, Andrew Hanser4, Mark N. Williams4, and Michael Tutor4 1 University of Bremen, Bremen, D-28359, Germany 2 Linkoping University, Linkoping, S-58183, Sweden 3 Royal Institute of Technology, Kista, S-16440, Sweden 4 Kyma Technologies Inc., Raleigh, NC, 27617
ABSTRACT Bulk GaN sliced in bars along (11-20) and (1-100) planes from a boule grown in the [0001] direction by HVPE was confirmed as strain free material with a low dislocation density by using several characterization techniques. The high-structural quality of the material allows photoluminescence studies of free excitons, principal donor bound excitons and their twoelectron satellites with regard to the optical selection rules. Raman scattering study of the bulk GaN with nonpolar orientations allows a direct access to the active phonon modes and a direct determination of their strain-free positions. INTRODUCTION The built-in electric fields, typical for the nitride heterostructures grown in the conventional [0001] direction are among the factors hampering the performance of the nitridebased devices. Aiming to rule out this problem, growth in nonpolar [11-20] or [1-100] directions has been intensively investigated during last years, by employing different growth techniques, substrate materials, substrate and layer orientations. However, despite all the growth optimizations the heteroepitaxially grown material on sapphire, SiC and/or LiAlO2 possesses high structural defect density, including dislocations and stacking faults. The latter, being not typical for the polar c-plane nitrides, are found to have pronounced deterioration effect on the optical properties of the material. In addition, strain-related effects play important role, being anisotropically distributed both in growth and in-plane directions. All this makes the growth in nonpolar directions a real challenge with severe problems remained to be solved. Alternative approach is a growth of GaN boules [1-3] in the conventional [0001] direction up to a significant thickness by hydride vapor phase epitaxy (HVPE), that could then be sliced into bars and/or wafers with desired nonpolar orientations [2,3]. EXPERIMENT The material under investigation was produced at Kyma Technologies Inc. Boules with 2” diameter and thickness up to 7-8 mm were grown in the [0001] direction on sapphire.
Rectangular bars with sizes of 10x6 mm2 were then sliced along nonpolar (11-20) a-plane and (1-100) m-plane and were chemo-mechanically polished to produce bulk GaN substrates. The lattice parameters were determined by using high-resolution x-ray diffraction (XRD) Philips MRD diffractometer. Photoluminescence (PL) measurements were carried out at 2 K using cw laser exitation (266 nm line) and 0.55 m monochromator with detection resolution better than 0.2 meV in the range around 3.5 eV. Micro-Raman scattering meas
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