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We have estimated the threading dislocation density and type via X-ray diffraction and Williamson–Hall analysis to elicit qualitative information directly related to the electrical and optical quality of GaN epitaxial layers grown by PAMBE on 4H- and 6H-SiC substrates. The substrate surface preparation and buffer choice, specifically: Ga flashing for SiC oxide removal, controlled nitridation of SiC, and use of AlN buffer layers all impact the resultant screw dislocation density, but do not significantly influence the edge dislocation density. We show that modification of the substrate surface strongly affects the screw dislocation density, presumably due to impact on nucleation during the initial stages of heteroepitaxy.

The growth of GaN by plasma assisted molecular beam epitaxy on GaN template substrates (GaN on sapphire) is investigated with in-situ multi-channel spectroscopic ellipsometry. Growth is performed under various Ga/N flux ratios at growth temperatures in the range 710–780 °C. The thermal roughening of the GaN template caused by decomposition of the surface is investigated through the termporal variation of the GaN pseudodielectric function over the temperature range of 650 °C to 850 °C. The structural, morphological, and optical properties are also discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The immobilization of DNA on passivated n-type InAs (100) surfaces has been studied using X-ray and ultraviolet photoelectron spectroscopy. The benefits of sulfur passivation using ammonium sulfide solution ((NH4)2S) for DNA immobilization were examined. The XPS/UPS data carried out on non-functionalized and functionalized surfaces demonstrate that the DNA probes reacted with the sulfur-passivated InAs surface. The XPS data in combination with fluorescently-tagged DNA indicate that the sulfur passivation process leads to a higher and more uniform attachment of DNA over the surface compared to non-sulfur-passivated InAs surfaces. The XPS data obtained immediately after sulfur passivation clearly observes In-S bonding, with little or no As-S. In addition, the XPS spectra of As 3d core-levels immediately after sulfur passivation shows that there is a negligible amount of As-Ox, but the peak become considerable after exposure to the aqueous DNA probe solution. The increase in As-Ox is likely due to the presence of non-sulfur bonded As atoms present on the surface. The presence of sulfur on the surface does lead to the high areal density of attached ssDNA. This system forms the basis of a DNA sensing system. While chemically passivating the surface against oxidation and facilitating probe attachment, the changes in Fermi level position were also monitored by UPS. UPS spectra show that the Fermi level of a clean InAs surface is located ~0.6 eV above the valence band maximum. The changes in electronic states induced by sulfur passivation and the pinning of EF are discussed.

Spectroscopic ellipsometry has been used to monitor in real time and in situ the molecular beam epitaxial growth of InN on SiC substrates. A three-step growth process consisting of (i) low-temperature (200 degrees C) nitridation of the SiC surface, (ii) low-temperature (350 degrees C) nucleation of a thin InN buffer layer, and (iii) growth of the InN epitaxial layer at 450 degrees C has been applied. The impact of the In flux on the growth kinetics, morphology, and structural and optical properties of InN has been investigated. It is found that independent of the In flux the low-temperature buffer shows In surface accumulation. This In surface accumulation increases during InN growth under In-rich conditions and is depleted during growth under intermediate and N-rich conditions. Better structural, morphological, and optical properties are obtained when the films are grown under In-rich conditions.

We have investigated the growth of InN films by plasma assisted molecular beam epitaxy on the Si-face of 6H-SiC(0001). Growth is performed under In-rich conditions using a two-step process consisting of the deposition of a thin, low-temperature 350 °C InN buffer layer, followed by the subsequent deposition of the InN epitaxial layer at 450 °C. The effect of buffer annealing is investigated. The structural and optical evolution of the growing layer has been monitored in real time using RHEED and spectroscopic ellipsometry. Structural, morphological, electrical and optic properties are discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the impact of the preparation of the Si-face 4H-SiC(0001)Si substrate using a Ga flash-off process on the epitaxial growth of GaN by plasma-assisted molecular beam epitaxy. The nucleation, as well as the resultant structural and morphological properties of GaN grown directly on 4H-SiC(0001)Si are strongly influenced by the chemical and morphological modifications of the SiC surface induced by the Ga flash-off process. Herein we describe the impact of the specific concentration of Ga incident on the surface (quantified in terms of monolayer (ML) coverage): of 0.5 ML, 1ML and 2ML. The residual oxygen at the SiC surface, unintentional SiC nitridation and the formation of cubic GaN grains during the initial nucleation stage, are all reduced when a 2 ML Ga flash is used. All of the above factors result in structural improvement of the GaN epitaxial layers. The correlation between the SiC surface modification, the initial nucleation stage, and the GaN epitaxial layer structural quality has been articulated using x-ray photoelectron spectroscopy, x-ray diffraction, atomic force microscopy and spectroscopic ellipsometry data. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The interaction of InN epitaxial films grown by r.f. plasma assisted molecular beam epitaxy with atomic hydrogen and nitrogen, produced by remote r.f. H2 and N2 plasmas, is investigated. InN strongly reacts with both atomic hydrogen and nitrogen yielding depletion of nitrogen and concurrent formation of In clusters. The impact of hydrogen treatments on the optical properties of InN is assessed using photoluminescence (PL). It is found that hydrogen suppresses the intense PL band peaked at approximately 0.7eV for the as-grown InN epitaxial layers, and results in the appearance of a new PL band whose peak energy and intensity increase with H-dose. The effect of exposure to atomic hydrogen and nitrogen on electrical properties of InN is investigated using Hall effect measurements. Atomic force microscopy is also used for studying the morphological changes of InN upon interaction with atomic hydrogen and nitrogen.

Highly reactive N-polar [000-1] GaN surfaces were analyzed using spectroscopic ellipsometry. Following exposure to air, observed changes in the pseudodielectric function near the GaN band edge indicate that surface contamination reduces the band bending. A subsequent Ga adsorption/desorption experiment on pristine N-polar GaN indicates that it contains a mixture of Ga-terminated and N-terminated surfaces. During deposition, Ga adatoms preferentially bond to the dangling bonds on the N-terminated surface: the measured 3.19 eV desorption activation energy equals the Ga-N decomposition energy. Further deposition forms a 1 ML Ga wetting layer whose 2.78 eV desorption activation energy is comparable to the Ga sublimation energy.

InN epitaxial films were grown by N-2 plasma-assisted molecular beam epitaxy on 4H- and 6H-SiC substrates using low-temperature InN nucleation layers. InN films grown at various In fluxes under the In-rich regime show improved crystal quality, surface morphology, and optical properties, without sizable metallic In incorporation. Photoluminescence measurements show emission up to room temperature, band gap values as low as 0.64 eV at T=10 K, and carrier concentrations of the order of 8x10(17) cm(-3).

Spectroscopic ellipsometry has been used to monitor in real time and in situ the molecular beam epitaxial growth of InN on SiC substrates. A three-step growth process consisting of (i) low-temperature (200°C) nitridation of the SiC surface, (ii) low-temperature (350°C) nucleation of a thin InN buffer layer, and (iii) growth of the InN epitaxial layer at 450°C has been applied. The impact of the In flux on the growth kinetics, morphology, and structural and optical properties of InN has been investigated. It is found that independent of the In flux the low-temperature buffer shows In surface accumulation. This In surface accumulation increases during InN growth under In-rich conditions and is depleted during growth under intermediate and N-rich conditions. Better structural, morphological, and optical properties are obtained when the films are grown under In-rich conditions.

We report the impact of both unintentional and intentional nitridation of 6H-SiC(0001)(Si) substrates on the epitaxial growth of GaN by molecular-beam epitaxy. The unintentional nitridation is dependent upon the details of the pregrowth Ga flash-off process used to remove surface oxides and to achieve a specific pregrowth surface reconstruction. The nucleation process and structural and morphological properties of GaN epitaxial layers are strongly influenced by the modifications of the SiC surface induced by the pregrowth preparation process. We found that residual oxygen at the SiC surface, unintentional SiC nitridation, and the formation of cubic GaN grains at the initial nucleation stage strongly decrease as the concentration of Ga used is increased during the flash cleaning. In addition, recent work has shown that the use of a SiN interlayer for GaN epitaxy on various substrates reduces dislocation density. We observe an improvement in the heteroepitaxy of GaN when the SiC surface is intentionally nitridized at low temperature prior to the initiation of growth.