Influence of Ga/N ratio on morphology, vacancies, and electrical transport in GaN grown by molecular beam epitaxy at high temperature

The effect of Ga/N flux ratio on surface morphology, incorporation of point defects and electrical transport properties of GaN films grown by plasma-assisted molecular beam epitaxy in a recently developed high-temperature growth regime was investigated. The homoepitaxial (0001) GaN films grown at ∼780–790 °C showed smoothest morphologies near the cross-over between N-rich and Ga-rich growth(0.75<Ga/N<1.1) contrasting previous observations for low-temperature growth. The higher-quality growth near Ga/N∼1 resulted from lower thermal decomposition rates and was corroborated by slightly lower Ga vacancy concentrations [VGa], lower unintentional oxygen incorporation, and improved electron mobilities. The consistently low [VGa], i.e., ∼10exp16 cm−3 for all films attribute further to the significant benefits of the high-temperature growth regime.Peer reviewe

Marine Incursion: The Freshwater Herring of Lake Tanganyika Are the Product of a Marine Invasion into West Africa

The spectacular marine-like diversity of the endemic fauna of Lake Tanganyika, the oldest of the African Great Lakes, led early researchers to suggest that the lake must have once been connected to the ocean. Recent geophysical reconstructions clearly indicate that Lake Tanganyika formed by rifting in the African subcontinent and was never directly linked to the sea. Although the Lake has a high proportion of specialized endemics, the absence of close relatives outside Tanganyika has complicated phylogeographic reconstructions of the timing of lake colonization and intralacustrine diversification. The freshwater herring of Lake Tanganyika are members of a large group of pellonuline herring found in western and southern Africa, offering one of the best opportunities to trace the evolutionary history of members of Tanganyika's biota. Molecular phylogenetic reconstructions indicate that herring colonized West Africa 25–50MYA, at the end of a major marine incursion in the region. Pellonuline herring subsequently experienced an evolutionary radiation in West Africa, spreading across the continent and reaching East Africa's Lake Tanganyika during its early formation. While Lake Tanganyika has never been directly connected with the sea, the endemic freshwater herring of the lake are the descendents of an ancient marine incursion, a scenario which may also explain the origin of other Tanganyikan endemics

Self-induced growth of vertical free-standing InAs nanowires on Si(111) by molecular beam epitaxy

&nbsp;We report self-induced growth of vertically aligned (i.e. along the [ 111] direction), free-standing InAs nanowires on Si(111) substrates by solid-source molecular beam epitaxy. Implementation of an ultrathin amorphous SiO(x) mask on Si(111) facilitated epitaxial InAs nanowire growth, as confirmed by high-resolution x-ray diffraction 2 theta-omega scans and transmission electron microscopy. Depending on growth temperature (in the range of 400-520 degrees C) substantial size variation of both nanowire length and diameter was found under preservation of uniform, non-tapered hexagon-shaped geometries. The majority of InAs nanowires exhibited phase-pure zinc blende crystal structure with few defective regions consisting of stacking faults. Photoluminescence spectroscopy at 20 K revealed peak emission of the InAs nanowires at 0.445 eV, which is similar to 30 meV blueshifted with respect to the emission of the bulk InAs reference due to radial quantum confinement effects. These results show a promising route towards integration of well-aligned, high structural quality InAs-based nanowires with the desired aspect ratio and tailored emission wavelengths on an Si platform

In situ characterization of GaN quantum dot growth with reflection high-energy electron diffraction and line-of-sight mass spectrometry

We have investigated the Ga-adlayer mediated growth of GaN quantum dots at 707 degrees C on AlN (0001) by simultaneous use of in situ reflection high-energy electron diffraction and line-of-sight quadrupole mass spectrometry during rf-plasma assisted molecular beam epitaxy (PA-MBE). We have found that the Ga-adsorbate coverage of 1.0 ML (monolayer) is critical in the mediation of a fundamental change in the GaN Stranski-Krastanov (SK) growth mode on AlN. When the Ga-adsorbate coverage was less than 1.0 ML, the SK transition occurred during GaN growth. For larger Ga-adsorbate coverages, the SK transition occurred after the desorption of the Ga coverage in excess of 1.0 ML. We performed variable GaN coverage growth experiments followed by desorption of the Ga adsorbate and subsequent GaN thermal decomposition to determine that the critical GaN coverage for the SK transition was less than 2.0 ML under Ga-droplet PA-MBE growth conditions

Quadrupole mass spectrometry desorption analysis of Ga adsorbate on AIN (0001)

The authors have investigated the adsorption and subsequent desorption of Ga on AlN (0001) with line-of-sight quadrupole mass spectrometry (QMS). The authors present desorption data consistent with a continuous Ga-flux dependent accumulation of a laterally contracted Ga bilayer on AlN (0001) from 0 to 2.7 +/- 0.3 ML GaN equivalent coverage, and further Ga accumulation in macroscopic Ga droplets. The temperature dependence of Ga-adsorbate QMS desorption transients was investigated and the authors determined that the desorption activation energies for individual monolayers of the Ga adsorbate on AlN (0001) were similar to Ga desorption from GaN (0001). For the (first) pseudomorphic Ga-adsorbate monolayer on AlN, the authors measured a maximum Ga coverage of 1.0 +/- 0.1 ML and desorption activation energy of 6.2 +/- 0.3 eV. For the (second) laterally contracted Ga monolayer (1.7 +/- 0.3 ML) the desorption activation energy was 3.8 +/- 0.1 eV. (c) 2006 American Vacuum Society

Ga adsorbate on (0001) GaN: In situ characterization with quadrupole mass spectrometry and reflection high-energy electron diffraction

We have investigated the adsorption and subsequent desorption of Ga on (0001) GaN using simultaneous line-of-sight quadrupole mass spectrometry (QMS) and reflection high-energy electron diffraction (RHEED). The in situ QMS and RHEED desorption transient measurements demonstrate the Ga flux dependent accumulation of the theoretically predicted laterally contracted Ga bilayer [J. E. Northrup , Phys. Rev. B 61, 9932 (2000)] under conditions similar to those used during GaN growth by rf-plasma molecular beam epitaxy. We correlated bioscillatory RHEED desorption transients [C. Adelmann , J. Appl. Phys. 91, 9638 (2002)] to QMS-measured Ga-adsorbate coverage and found both to be consistent with layer-by-layer desorption of the Ga-adsorbate bilayer. The QMS-measured steady-state Ga-adlayer coverage exhibited a continuous increase from 0 to 2.4 ML (monolayer) with respect to impinging Ga flux at substrate temperatures of 640-700 degrees C. We observed an exponential dependence of the Ga flux corresponding to 1.0 ML Ga-adsorbate coverage on substrate temperature and we measured an apparent activation energy of 2.43 +/- 0.11 eV and an attempt prefactor of 6.77x10(12) nm/min (4.36x10(11) Hz) for this transition. (c) 2006 American Institute of Physics

Excitation wavelength dependence of terahertz emission from InN and InAs

The authors report on the excitation wavelength dependence of terahertz emission from n-InN and bulk p-InAs pumped with femtosecond pulses tunable from 800 to 1500 nm. The terahertz amplitude, normalized to pump and probe power, from both narrow band gap semiconductors remains relatively constant over the excitation wavelength range. In addition, terahertz radiation from In- and N-face InN samples with bulk carrier concentrations ranging from 10(17) to 10(19) cm(-3) is also investigated, showing a strong dependence of terahertz emission on bulk carrier concentration. The experimental results agree well with calculations based on drift-diffusion equations incorporating momentum conservation and relaxation. (c) 2006 American Institute of Physics

Optimization of the surface and structural quality of N-face InN grown by molecular beam epitaxy

The authors demonstrate the impact of growth kinetics on the surface and structural properties of N-face InN grown by molecular beam epitaxy. Superior surface morphology with step-flow growth features is achieved consistently under In-rich conditions in a low-temperature region of 500-540 degrees C. Remarkably, off-axis x-ray rocking curve (omega scans) widths are found to be independent of the growth conditions. The band gap determined from optical absorption measurements of optimized InN is 0.651 eV, while photoluminescence peak emission occurs at even lower energies of similar to 0.626 eV. Hall measurements show room temperature peak electron mobilities as high as 2370 cm(2)/V s at a carrier concentration in the low 10(17) cm(-3) region. Analysis of the thickness dependence of the carrier concentration demonstrates a n-type surface accumulation layer with a sheet carrier concentration of similar to 3x10(13) cm(-2). (c) 2006 American Institute of Physics