The Nature and Impact of ZnO Buffer Layers on the Initial Stages of the Hydride Vapor Phase Epitaxy of GAN (original) (raw)
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The materials quality and availability of large-area bulk GaN substrates is currently considered a key problem for the continuing development of improved GaN-based devices. Since industrial fabrication of bulk GaN substrates with suitable materials quality has proven very difficult, the opto-GaN industry is currently based on heteroepitaxy using either c-sapphire or 6H SiC substrates. ZnO is promising as a substrate material for GaN because it has the same wurtzite structure and a relatively small lattice mismatch ($1.8%).
Journal of Crystal Growth, 2010
A new hydride vapor phase epitaxy (HVPE)-based approach for the fabrication of freestanding GaN (FS-GaN) substrates was investigated. For the direct formation of low-temperature GaN (LT-GaN) layers, the growth parameters were optimized: the polarity of ZnO, the growth temperature, and the V/III ratio. The FS-GaN layer was achieved by gas etching in an HVPE reactor. A fingerprint of Zn out-diffusion was detected in the photoluminescence measurements, especially for the thin (80 mm) FS-GaN film; however, a thicker film (400 mm) was effectively reduced by optimization of GaN growth.
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 2009
This article presents the results of microstructural, compositional, and optical characterization of GaN films grown on ZnO buffered c-sapphire substrates. Transmission electron microscopy showed epitaxy between the GaN and the ZnO, no degradation of the ZnO buffer layer, and no evidence of any interfacial compounds. Secondary ion mass spectroscopy revealed negligible Zn signal in the GaN layer away from the GaN / ZnO interface. After chemical removal of the ZnO, room temperature ͑RT͒ cathodoluminescence spectra had a single main peak centered at ϳ368 nm ͑ϳ3.37 eV͒, which was indexed as near-band-edge ͑NBE͒ emission from the GaN layer. There was no evidence of the ZnO NBE peak, centered at ϳ379 nm ͑ϳ3.28 eV͒, which had been observed in RT photoluminescence spectra prior to removal of the ZnO.
The growth of thick GaN film on sapphire substrate by using ZnO buffer layer
Journal of Crystal Growth, 1993
Sputtered ZnO layers have been used as buffer layers in the growth of GaN by hydride VPE. With these buffers we have not only improved the reproducibility of the growth of GaN but also achieved the preparation of single crystalline GaN films alone by etching buffer layers away. In this paper we have studied the effects of the ZnO buffer layer on GaN films.
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Plasma-assisted molecular beam epitaxy was used to grow ZnO(0001) layers on GaN(0001)/Al 2 O 3 templates and GaN/4H-SiC(0001) layers. The GaN(0001)/Al 2 O 3 template surfaces were subjected to various pre-treatment procedures (Zn, Ga or N pre-exposure or none) prior to the ZnO growth. We studied the impact of these pre-treatment procedures on the initial growth conditions of ZnO(0001). These layers were compared to ZnO layers deposited on 4H-SiC utilizing a GaN(0001) buffer layer that was grown in situ on the 4H-SiC substrate and immediately before the growth of ZnO. The GaN buffer layers were not pre-treated or exposed to ambient. Atomic force and scanning electron microscopy as well as secondary ion mass spectroscopy revealed that the pre-treatment procedures resulted in a very high density of islands. The islands coalesced into films as the growth progressed. In contrast, no ZnO growth occurred on the untreated GaN(0001)/Al 2 O 3 template surfaces. Our main finding is that Ga x O y sub-oxides residing on the surface of the as-received GaN-templates, drastically reduced the ZnO nucleation rate and completely inhibited subsequent coalescence and growth. Our various surface pre-treatment procedures aimed at removing the sub-oxides were necessary for achieving ZnO growth on the GaN-templates. No surface pre-treatment was needed to enable ZnO growth on the in situ grown GaN(0001)/4H-SiC layers.
Hydride vapour phase epitaxy growth and characterisation of GaN layers
Materials Science and Engineering: B, 2001
A vertical hydride vapour phase epitaxial reactor was applied for the growth of GaN layers on (0001) sapphire. In this paper, the growth parameters and the results of optical investigations are reported. It is seen that the layers grown at high temperature have better crystallographic properties than those grown at 600-800°C, although the latter show a smoother surface. The absorption spectra exhibited a tail from the absorption edge down to about 1 eV that may be fitted by the Lukovsky model considering the presence of a level at about 1.2 eV from the conduction band. The cathodoluminescence spectra show three main emissions: the yellow band, a blue band (BB) centred at 2.8 eV and the near band edge recombination. The BB is mostly originated close to the layer/substrate interface and practically extinguished at the layer surface. It is seen that when the intensity of the BB increases that of the fundamental recombination decreases and vice versa. : S 0 9 2 1 -5 1 0 7 ( 0 0 ) 0 0 5 8 4 -5
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Japanese Journal of Applied Physics, 2006
We have found that the surfaces of ZnO(000 1 1) annealed in a box made of ceramic ZnO exhibit atomically flat stepped and terraced structures. Structural and optical properties of ZnO are also improved by this annealing technique. Full width at half maximum of X-ray rocking curves of the ZnO substrates were decreased and green photoluminescence caused by defects were eliminated by the annealing. We have also found that epitaxial growth of GaN on atomically flat ZnO(000 1 1) substrates at room temperature proceeds in a layer-by-layer mode, while growth on the as-received substrates results in formation of amorphous materials. These results indicate that the use of the annealing technique improves, not only crystal quality of ZnO substrates, but also morphology of the GaN films grown on them.