Raman Studies on Oxygen Doped GaN Grown by Molecular Beam Epitaxy
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Raman Studies on Oxygen Doped GaN Grown by Molecular Beam Epitaxy D. Papadimitriou Department of Applied Mathematics and Physical Science, National Technical University of Athens, Athens, Greece A.J. Ptak, D. Korakakis, N.C. Giles, and T.H. Myers Department of Physics, West Virginia University, Morgantown, WV 26506 ABSTRACT High quality Ga-polarity GaN films were grown by plasma-assisted molecular beam epitaxy to study strain effects due to oxygen incorporation. Oxygen concentrations up to 2 x 1022 cm-3 were studied. Layers containing oxygen at levels above 1022 cm-3 exhibit severe cracking while oxygen concentrations less than 1021 cm-3 apparently does not introduce significant strain. Raman spectra of O2-doped films were evaluated with respect to spectra of unintentionally doped GaN films (n=4x1014 cm-3) grown under the same conditions except for the O2-flux. Analysis of the E2 (high frequency phonon mode near 570 cm-1) Raman band indicated the heaviest doped samples exhibit compressive strain. Frequency-shifts between 0.7 and 0.9 cm-1 were observed as a function of the distance from the crack edges in the heavily doped samples, implying a strain (δα/α) of the order of 5x10-4. INTRODUCTION Many groups have studied the role of oxygen in GaN, but few careful studies of intentional oxygen doping have been performed. To date, most studies involving oxygen in GaN have focused on unintentionally doped material with high background levels of impurities, or material with oxygen supplied either by an “uncontrolled” system background or from out-diffusion from the sapphire substrate. A recent study1 investigated controlled oxygen incorporation in GaN using molecular beam epitaxy. Oxygen incorporation in GaN was found to depend strongly on film polarity and III-V flux ratio. These results have been published previously1 and are beyond the scope of this paper. Simply stated, oxygen doping was found to be an effective n-type dopant that is controllable, reproducible, and appears uncompensated up to concentrations of about 1018 cm-3. Samples doped with concentrations higher than about I2.11.1
1020 cm-3 show significant compensation, while samples with oxygen concentrations larger than 5x1021 cm-3 exhibit severe cracking as is shown in Figure 1. Similar cracking, along with associated evidence for strain, has been reported2 for Si-doped GaN samples grown by MOCVD, with Si concentrations ([Si]) of about 2x1019 cm-3. This paper reports the results a Raman spectroscopy study to investigate strain related to oxygen incorporation in Ga-polar GaN. EXPRIMENTAL DETAILS The oxygen doped GaN layers were grown at West Virginia University (WVU) by rf plasma-assisted MBE using procedures described in prior studies.3 Ultra high purity oxygen gas (99.998%) was introduced through a tube directed at the substrate using an ultra-high vacuum leak valve and monitored using a residual gas analyzer (RGA-200, Stanford Research Systems, Sunnyvale, CA). Ga-polarity GaN was grown using either semi-insulating MOCVD GaN templates on A-plane sapphire subs
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