Precipitates in GaN epilayers grown on sapphire substrates
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Precipitates in GaN epilayers grown on sapphire substrates Junyong Kanga) Department of Physics, Gakushuin University, Mejiro, Tokyo 171, Japan, and Department of Physics, Xiamen University, Xiamen 361005, People’s Republic of China
Tomoya Ogawa Department of Physics, Gakushuin University, Mejiro, Tokyo 171, Japan (Received 11 December 1997; accepted 14 February 1998)
Precipitates in GaN epilayers grown on sapphire substrates were investigated by atomic number contrast (ANC), wavelength-dispersive x-ray spectrometry (WDS), energy-dispersive spectrometry (EDS), and cathodoluminescence (CL) techniques. The results showed that the precipitates are mainly composed of gallium and oxygen elements and distribute more sparsely and inhomogeneously in k1120l directions in the sample grown on substrate nitridated for a longer period. Yellow luminescence intensity was imaged to be stronger in the precipitates. The results suggest that the precipitates are formed on dislocations and grain boundaries by substituting oxygen onto the nitrogen site, and result in the formations of deep levels nearby.
I. INTRODUCTION
In spite of the recent success in the growth of GaN on sapphire (Al2 O3 ) substrate by depositing a buffer layer under appropriate conditions,1–3 the defect density is still high in GaN epilayers with good optoelectronic performance.4 The predominant defects in GaN epilayers have been identified as dislocations along the [0001] direction with an edge component.5–7 Another kind of defect has been observed to be coreless and is called “nanopipes.” 8–10 We very recently imaged the plane-views of dislocations with an edge component and observed that the dislocation density is lower in GaN epilayer grown on substrate nitridated for a longer period.11 Information on the precipitates is still sparse, however, and their investigation in GaN epilayers is therefore worthwhile. In this work, precipitates in GaN epilayers grown on sapphire substrate were studied by atomic number contrast (ANC), wavelength-dispersive x-ray spectrometer (WDS), and energy-dispersive x-ray spectrometer (EDS) in a transmission electron microscope (TEM). Yellow luminescence was measured by photoluminescence and cathodoluminescence (CL) techniques. The relationships and physical origins of results are discussed. II. EXPERIMENTAL
GaN epilayers under study were grown on (0001) sapphire substrates by metallorganic vapor phase epitaxy (MOVPE). The substrate surface was first nitridated in a H2 and NH3 mixed gas at 1150 ±C within 20 min a)
Present address: Department of Physics, Xiamen Univ., Xiamen 361005, People’s Republic of China.
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http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 8, Aug 1998
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to prepare different samples. A GaN buffer layer about 20 nm thick was deposited at 550 ±C and then GaN was grown at 1050 ±C by feeding trimethylgallium and NH3 gases into a reactor. As-grown epilayers were transparent with thickness of about 6 mm and different surface morphol
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