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ABSTRACT: We report polarization-sensitive solar-blind metal-semiconductor-metal UV
photodetectors based on (11-20) a-plane AlGaN. The epilayer shows anisotropic
optical properties confirmed by polarization-resolved transmission and
photocurrent measurements, in good agreement with band structure calculations.
06/2011;
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ABSTRACT: We present a simple fabrication technique for lateral nanowire wrap-gate
devices with high capacitive coupling and field-effect mobility. Our process
uses e-beam lithography with a single resist-spinning step, and does not
require chemical etching. We measure, in the temperature range 1.5-250 K, a
subthreshold slope of 5-54 mV/decade and mobility of 2800-2500 $cm^2/Vs$ --
significantly larger than previously reported lateral wrap-gate devices. At
depletion, the barrier height due to the gated region is proportional to
applied wrap-gate voltage.
06/2011;
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ABSTRACT: We report epitaxial growth of a -plane (11 2 0) AlInN layers nearly-lattice-matched to GaN. Unlike for c -plane oriented epilayers, a -plane Al <sub>1-x</sub> In <sub>x</sub> N cannot be simultaneously lattice-matched to GaN in both in-plane directions. We study the influence of temperature on indium incorporation and obtain nearly-lattice-matched Al <sub>0.81</sub> In <sub>0.19</sub> N at a growth temperature of 760 ° C . We outline a procedure to check in-plane lattice mismatch using high-resolution x-ray diffraction, and evaluate the strain and critical thickness. Polarization-resolved optical transmission measurements of the Al <sub>0.81</sub> In <sub>0.19</sub> N epilayer reveal a difference in band gap of ∼140 meV between (electric field) E || c [0001] -axis and E ⊥ c conditions with room-temperature photoluminescence peaked at 3.38eV strongly polarized with E || c , in good agreement with strain-dependent band-structure calculations.
Applied Physics Letters 06/2011; · 3.84 Impact Factor
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ABSTRACT: Unlike c-plane nitrides, ``non-polar" nitrides grown in e.g. the a-plane or m-plane orientation encounter anisotropic in-plane strain due to the anisotropy in the lattice and thermal mismatch with the substrate or buffer layer. Such anisotropic strain results in a distortion of the wurtzite unit cell and creates difficulty in accurate determination of lattice parameters and solid phase group-III content (x_solid) in ternary alloys. In this paper we show that the lattice distortion is orthorhombic, and outline a relatively simple procedure for measurement of lattice parameters of non-polar group III-nitrides epilayers from high resolution x-ray diffraction measurements. We derive an approximate expression for x_solid taking into account the anisotropic strain. We illustrate this using data for a-plane AlGaN, where we measure the lattice parameters and estimate the solid phase Al content, and also show that this method is applicable for m-plane structures as well.
06/2010;
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physica status solidi (RRL) - Rapid Research Letters 05/2010; 4(7):163 - 165. · 2.22 Impact Factor
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ABSTRACT: We report epitaxial growth of a-plane (11-20) AlInN layers
nearly-lattice-matched to GaN. Unlike for c-plane oriented epilayers, a-plane
Al_{1-x}In_{x}N cannot be simultaneously lattice-matched to GaN in both
in-plane directions. We study the influence of temperature on indium
incorporation and obtain nearly-lattice-matched Al_{0.81}In_{0.19}N at a growth
temperature of 760^{o}C. We outline a procedure to check in-plane lattice
mismatch using high resolution x-ray diffraction, and evaluate the strain and
critical thickness. Polarization-resolved optical transmission measurements of
the Al_{0.81}In_{0.19}N epilayer reveal a difference in bandgap of ~140 meV
between (electric field) E_parallel_c [0001]-axis and E_perpendicular_c
conditions with room-temperature photoluminescence peaked at 3.38 eV strongly
polarized with E_parallel_c, in good agreement with strain-dependent
band-structure calculations.
03/2010;
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ABSTRACT: A detailed study on the influence of an InGaN interlayer between the GaN buffer and InN epilayer on the microstructural properties of the InN layer is reported. Using high-resolution X-ray diffraction measurements the mosaicity of MOVPE grown InN epilayers deposited directly on c-plane sapphire, on GaN buffer layers, and on different InGaN interlayers have been compared. Generally the angle of tilt and hence the screw dislocation density in the InN epilayers can be controlled by growth conditions, while the angle of twist, corresponding to the edge dislocation density is usually weakly dependent on growth conditions. However, for a given layer thickness the edge dislocation density can be significantly reduced by the insertion of an InGaN interlayer.
Physica Status Solidi (A) Applications and Materials 03/2010; 207(5):1070 - 1073. · 1.46 Impact Factor
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ABSTRACT: We report investigation of electron traps in n-GaN, grown on sapphire by metal organic vapour phase epitaxy, by using charge deep level transient spectroscopy (Q -DLTS). Measurements have been made isothermally by rate window scanning over the temperature range 300–370 K and for rate windows in the range 105 s–1 to 1 s–1. Two traps are observed in this range with (i) activation energies ∼0.58 eV and ∼0.45 eV and (ii) capture cross sections ∼2 × 10–15 cm2 and ∼3 × 10–19 cm2 respectively. The first of these defects has been observed in all of the DLTS investigations reported in the literature. However, the second trap has not been seen in majority of the earlier reports. Possible reasons for this difference are discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b) 07/2008; 245(11):2567 - 2571. · 1.32 Impact Factor
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ABSTRACT: In recent years there have been reports of anomalous electrical resistivity and the presence of superconductivity in semiconducting InN layers. By a careful correlation of the temperature dependence of resistivity and magnetic susceptibility with structural information from highresolution x-ray diffraction measurements we show that superconductivity is not intrinsic to InN and is seen only in samples that show traces of oxygen impurity. We hence believe that InN is not intrinsically a superconducting semiconductor.
03/2008;
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ABSTRACT: We discuss I-V-T, C-V and photoelectric characteristics of Ni-GaN Schottky and n+ InN-GaN heterostructure interfaces. The Schottky barrier heights obtained from the temperature dependence of I-V characteristics, from C-V measurements, and the photoresponse are compared. From internal photoemission measurements, band offsets at the InN-GaN heterointerface are experimentally determined.
Physics of Semiconductor Devices, 2007. IWPSD 2007. International Workshop on; 01/2008
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ABSTRACT: The authors report room temperature reflectance measurements on wurtzite C-plane GaN films grown on sapphire substrates. The spectra clearly show exciton related features near the fundamental bandgap of GaN. The Lorentz oscillator model was used to analyze the excitonic contribution to the spectral lineshape and to determine the transition energies. The results are explained by comparison with electronic band structure calculations which include the effect of strain. The analysis gives an independent estimate of the residual biaxial strain in the films.
Physics of Semiconductor Devices, 2007. IWPSD 2007. International Workshop on; 01/2008
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ABSTRACT: Band discontinuities at the InN–GaN heterointerface are experimentally determined from internal photoemission spectroscopy measurements on n<sup>+</sup> InN on GaN epilayers. The photocurrent shows two threshold energies, one at 1.624 eV and the other at 2.527 eV . From these, we obtain the band offsets ΔE<sub>v</sub>=0.85 eV and ΔE<sub>c</sub>=1.82 eV .
Applied Physics Letters 11/2007; · 3.84 Impact Factor
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ABSTRACT: From a detailed analysis of InN epilayers deposited in a close-coupled showerhead metalorganic vapour phase epitaxy (MOVPE) system under various conditions we investigate the effect of growth parameters on the lattice constants of the InN layer. The layers are under significant internal hydrostatic stress which influences the optical properties. Samples typically fall into two broad categories of stress, with resultant luminescence emission around 0.8eV and 1.1eV. We can correlate the internal stress in the layer and the value of the optical absorption edge, and the PL emission wavelength.
06/2007;
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ABSTRACT: We establish a correlation between the internal stress in InN epilayers and their optical properties such as the measured absorption band edge and photoluminescence emission wavelength. By a careful evaluation of the lattice constants of InN epilayers grown on c-plane sapphire substrates under various conditions by metalorganic vapor phase epitaxy we find that the films are under primarily hydrostatic stress. This results in a shift in the band edge to higher energy. The effect is significant, and may be responsible for some of the variations in InN bandgap reported in the literature. Comment: Submitted to Appl. Phys. Lett
05/2007;
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ABSTRACT: The evolution of InAs quantum dots grown on InP substrates by metal-organic vapor phase epitaxy is studied as a function of InAs coverage. Under specific growth conditions, the onset of the two- to three-dimensional transition proceeds via two distinct pathways: through (i) an abrupt appearance of quantum dots as expected in the usual Stranski-Krastanov growth picture and (ii) a continuous evolution of small surface features into well-developed quantum dots. The average size of the features in both these families increases with coverage, leading to a bimodal distribution in dot sizes at an intermediate stage of growth evolution. On further deposition of material, the two families merge into a single broad unimodal distribution of sizes. Complementary information obtained from independent measurements of photoluminescence spectra and surface morphology is correlated and is found to be independently consistent with the picture of growth proposed.
Journal of Applied Physics 05/2007; 101(9):094303-094303-6. · 2.17 Impact Factor
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ABSTRACT: The authors report the observation of strong polarization anisotropy in the photoluminescence (PL) and the absorption spectra of [110] oriented A-plane wurtzite InN films grown on R-plane (102) sapphire substrates using molecular beam epitaxy. For A-plane films the c axis lies in the film plane. The PL signal collected along [110] with electric vector E⊥c is more than three times larger than for E‖c. Both PL signals peak around 0.67 eV at 10 K. The absorption edge for E‖c is shifted to higher energy by 20 meV relative to E⊥c. Optical polarization anisotropy in wurtzite nitrides originates from their valence band structure which can be significantly modified by strain in the film. The authors explain the observed polarization anisotropy by comparison with electronic band structure calculations that take into account anisotropic in-plane strain in the films. The results suggest that wurtzite InN has a narrow band gap close to 0.7 eV at 10 K.
Applied Physics Letters 10/2006; 89(15):151910-151910-3. · 3.84 Impact Factor
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ABSTRACT: We report the observation of a root-like structure at the interface between GaAs nanowires and the (100) single-crystal GaAs substrate. These nanowires were grown via the vapor-liquid-solid mechanism using metalorganic vapor phase epitaxy. The root-like structure extends from the base of the nanowires into the substrate and has been investigated in detail using transmission electron microscopy and high-resolution electron microscopy. While the nanowires predominantly exhibit the zinc-blende type diamond cubic structure with the growth axis parallel to ⟨111⟩ and growth twins perpendicular to the growth axis, the root regions have a CdTe type orthorhombic structure that has been reported to occur in GaAs only under high-pressure conditions.
Applied Physics Letters 01/2006; 88(3):031919-031919-3. · 3.84 Impact Factor
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ABSTRACT: A systematic manipulation of the morphology and the optical emission properties of MOVPE grown ensembles of InAs/InP quantum dots is demonstrated by changing the growth kinetics parameters. Under non-equilibrium conditions of a comparatively higher growth rate and low growth temperature, the quantum dot density, their average size and hence the peak emission wavelength can be tuned by changing efficiency of the surface diffusion (determined by the growth temperature) relative to the growth flux. We further observe that the distribution of quantum dot heights, for samples grown under varying conditions, if normalized to the mean height, can be nearly collapsed onto a single Gaussian curve.
07/2005;
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ABSTRACT: The evolution of InAs quantum dots grown on InP substrates by metal-organic vapour phase epitaxy is studied as a function of InAs coverage. Under specific growth conditions, the onset of the two- to three-dimensional transition is seen to proceed via two distinct pathways: through (i) an abrupt appearance of quantum dots as expected in the usual Stranski-Krastanov growth picture and (ii) a continuous evolution of small surface features into well developed quantum dots. The average size of the features in both these families increases with coverage, leading to a bimodal distribution in dot sizes at an intermediate stage of growth evolution that eventually becomes a unimodal distribution as more material is deposited. Complementary information obtained from independent measurements of photoluminescence spectra and surface morphology is correlated and is found to be consistent with the picture of growth proposed.
06/2005;
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ABSTRACT: In experiments on electron transport the macroscopic inhomogeneities in the sample play a fundamental role. In this paper and a subsequent one we introduce and develop a general formalism that captures the principal features of sample inhomogeneities (density gradients, contact misalignments) in the magneto resistance data taken from low mobility heterostructures. We present detailed assessments and experimental investigations of the different regimes of physical interest, notably the regime of semiclassical transport at weak magnetic fields, the plateau-plateau transitions as well as the plateau-insulator transition that generally occurs at much stronger values of the external field only. It is shown that the semiclassical regime at weak fields plays an integral role in the general understanding of the experiments on the quantum Hall regime. The results of this paper clearly indicate that the plateau-plateau transitions, unlike the the plateau-insulator transition, are fundamentally affected by the presence of sample inhomogeneities. We propose a universal scaling result for the magneto resistance parameters. This result facilitates, amongst many other things, a detailed understanding of the difficulties associated with the experimental methodology of H.P. Wei et.al in extracting the quantum critical behavior of the electron gas from the transport measurements conducted on the plateau-plateau transitions. Comment: 20 pages, 9 figures
09/2003;
