Microstructure of GaN Films Grown by RF-Plasma Assisted Molecular Beam Epitaxy
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Microstructure of GaN Films Grown by RF-Plasma Assisted Molecular Beam Epitaxy
Philomela Komninou, Thomas Kehagias, Joseph Kioseoglou, Eirini Sarigiannidou, Theodoros Karakostas, Gerard Nouet1, Pierre Ruterana1, Khalid Amimer2, Spyros Mikroulis2 and Alexandros Georgakilas2 Physics Department, Aristotle University of Thessaloniki, Thessaloniki, GR-54006, Greece 1 ESCTM-CRISMAT, UMR6508-CNRS, ISMRA Caen Cedex, 6 boul. du Marechal Juin, 14050, France 2 FORTH/IESL and Physics Department, University of Crete, Heraklion-Crete, P.O.Box 1527, 71110, Greece ABSTRACT
The influence of the variation of the Ga/N flux ratio during deposition and of the different substrate nitridation temperatures on the microstructure of 2H-GaN films grown on (0001) sapphire, by RF plasma MBE, is investigated by conventional and high resolution Transmission Electron Microscopy (TEM-HREM). The different growth rates of the inverse polarity domains in Ga-rich and N-rich specimens result in film surfaces of different roughness, whereas the stacking fault (SF) content is significantly higher in samples grown under N-rich conditions. Low temperature nitridation of the substrate results in a low density of defects in GaN film. Cubic GaN “pockets”, near the substrate/GaN interface that are present in low temperature nitridated samples are not observed in high temperature nitridated samples.
INTRODUCTION
During the last years, great progresses have been realized in III-V nitride optoelectronics [1]. High brightness visible light emitting diodes [2] and blue laser diodes [3] has been fabricated from GaN alloys and they work in spite of many linear and planar defects [4,5]. The system most commonly used for device material is GaN/sapphire (0001) where the hexagonal wurtzite lattice of GaN (a-GaN) grows with (0001)GaN//(0001)sapphire and [1120]GaN//[1010]sapphire. Identification of structural defects in GaN is important for optimizing growth parameters. In the present study, TEM and HREM are employed in order to investigate changes on the microstructure resulting from the variation of the Ga/N flux ratio during deposition of GaN films. In addition the influence of the different substrate nitridation temperatures on the microstructural features of GaN films is also examined.
EXPERIMENTAL DETAILS
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The GaN layers were grown on the (0001) sapphire surface by RF plasma MBE. Two series of samples were examined. In the first, the sapphire surface was nitridated at high temperature and then a 20-25 nm low temperature buffer of AlN was deposited. The buffer layer was DQQHDOHGDWKLJKWHPSHUDWXUHDQGWKHQD
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In the second series, the substrate was nitridated both in a low (~150o C) and a high ( ~700o C) temperature, before a GaN buffer layer was deposited. Specimens for electron microscopy in cross-section (XTEM) and plan-view geometry were prepared using the standard techniques of mechanical thinning followed by appropriate ion-milling. TEM observations were carried out in a Jeol JEM 120 CX elect
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