Morphology and optical properties of cubic phase GaN epilayers grown on (001) Si
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Internet Journal Nitride Semiconductor Research
Morphology and optical properties of cubic phase GaN epilayers grown on (001) Si M. Godlewski1, E. M. Goldys2, M. R. Philips3, J. P. Bergman4, B. Monemar4, R. Langer5 and A. Barski5 1Institute
of Physics, Polish Academy of Sciences, Science & Technology Laboratory, Macquarie University, 3Microstructural Analysis Unit, University of Technology, 4Department of Physics and Measurement Technology, Linköping University, 5CEA/Grenoble, Département de Recherche Fondamentale sur la Matière Condensée/SP2M, 2Semiconductor
(Received Tuesday, June 23, 1998; accepted Tuesday, November 24, 1998)
Optical properties of GaN epilayers of a cubic phase are studied. We show a strong influence of the sample morphology on intensity of the edge emission. Whereas edge luminescence is reduced at the grain boundaries, red emission is spatially homogeneous.
1
Introduction
Cubic phase GaN shows several encouraging optical properties [1] [2] [3] [4] [5]. In particular, it was recently suggested that ionization energy of shallow acceptors in cubic GaN is smaller than that observed for shallow acceptors in wurtzite phase GaN [6] [7]. If confirmed, this property of cubic GaN can lead to an increased hole concentration in p-type layers and in consequence in more efficient light emitting devices. In this work we discuss properties of photoluminescence (PL) emission transitions observed in cubic GaN epilayers grown by two modifications of molecular beam epitaxy (MBE) on (001) Si substrates. The influence of surface morphology on “edge” and “red” PL is studied. A strong influence of a grain-type structure of epilayers grown by molecular beam epitaxy (MBE) on edge emissions is demonstrated. 2
Samples
0.25 µm thick. GS- and ECR-MBE epilayers were grown on AlN, GaN or SiC buffer layers. RHEED patterns indicated a 3D-growth mode for GS-MBE samples and a 2D-growth mode for ECR-MBE samples. 3
Low temperature PL and time-resolved PL spectra were measured on a conventional set-up. The 351 nm UV line of an Ar+ laser was used for the cw excitation. Pulsed excitation was provided by a mode locked Ti:sapphire solid-state laser with frequency doubling (λ = 340 nm) with a pulse length of 2 ps. Monochromatic cathodoluminescence (CL) and scanning electron microscope (SEM) images were taken at room temperature in a JEOL JSM-35C scanning electron microscope system with a CL attachment in the accelerating voltage of 25 keV. For such energy of electrons, the primary electrons penetrate through whole the sample. 4
Epilayers of cubic GaN were grown in CEA Grenoble in a RIBER 2300 chamber on (001) Si substrates by two modifications of the MBE method. In gas source MBE (GS-MBE), high purity ammonia was supplied directly to the substrate and was thermally cracked in the presence of gallium. The growth rate was about 0.5 µm/h and the substrate temperature was 800 °C with ECR power of 140Watts. GS-MBE samples were 1- 2 µm thick. Other epilayers were grown using electron cyclotron resonance (ECR) nitrogen cell. Here gro
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