Showing papers in "Physica Status Solidi (c) in 2003"

Electronics and sensors based on pyroelectric AlGaN/GaN heterostructures – Part B: Sensor applications

Abstract: In the present article recent results concerning sensor applications of AlGaN layers and AlGaN/GaN heterostructures are summarized. The piezoresistive effect in piezoelectric AlGaN layers is investigated and the dependence of the piezoresistive gauge factor on the Al content is attributed to the influence of strain induced piezoelectric fields. An enhancement of this effect is observed in AlGaN/GaN heterostructures resulting in high longitudinal gauge factors. The response of gas sensitive Pt:GaN Schottky diodes to hydrogen and hydrogen containing gases is analyzed up to temperatures of 600 °C and employed to realize gas sensitive field effect transistors which are demonstrated to operate up to 400 °C. In addition, ion sensitive field effect transistors (ISFETs) have also been fabricated on the basis of AlGaN/GaN heterostructures. The GaN surface shows a high pH sensitivity which is attributed to the presence of a thin native metal oxide layer on the surface. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Metalorganic chemical vapor phase epitaxy of gallium-nitride on silicon

Abstract: GaN growth on Si is very attractive for low-cost optoelectronics and high-frequency, high-power electronics. It also opens a route towards an integration with Si electronics. Early attempts to grow GaN on Si suffered from large lattice and thermal mismatch and the strong chemical reactivity of Ga and Si at elevated temperatures. The latter problem can be easily solved using gallium-free seed layers as nitrided AlAs and AlN. The key problem for device structure growth on Si is the thermal mismatch leading to cracks for layer thicknesses above 1 μm. Meanwhile, several concepts for strain engineering exist as patterning, Al(Ga)N/GaN superlattices, and low-temperature (LT) AlN interlayers which enable the growth of device- relevant GaN thicknesses. The high dislocation density in the heteropitaxial films can be reduced by several methods which are based on lateral epitaxial overgrowth using ex-situ masking or patterning and by in-situ methods as masking with monolayer thick SiN. With the latter method in combination with strain engineering by LT-AlN interlayers dislocation densities around 109 cm−2 can be achieved for 2.5 μm thick device structures.

Electronics and sensors based on pyroelectric AlGaN/GaN heterostructures

Abstract: Electronic transport in semiconductors that possess high internal spontaneous and piezoelectric polarization opens up a new field of pyroelectronics and pyrosensors. The pyroelectric character of group-III-nitrides with wurtzite crystal structure yields a novel degree of freedom in designing and tailoring devices for modern micro- and nanoelectronic applications. Furthermore, spontaneous and piezoelectric polarization induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of ions, gases and polar liquids. We present a review of both theoretical and experimental studies of spontaneous and piezoelectric polarization present in AlGaN/GaN heterostructures as well as the electronic transport properties of polarization induced two-dimensional electron gases which are formed at the AlGaN/GaN interface due to the difference in the total polarization of two adjacent III-nitride layers. We demonstrate that the two-dimensional electron gases (2DEGs) achieved without modulation doping are very suitable as channel of high electron mobility transistors optimally suited for high power and high frequency applications (PART A) as well as for various kinds of sensors which can be operated in harsh environments (PART B). (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Freestanding GaN-substrates and devices

Abstract: Various physical aspects and potential applications of the laser-induced separation of GaN epilayers from their sapphire substrate are reviewed. The effect of short laser pulses on the thermal decomposition of GaN and possible applications of the laser-induced dissociation of GaN for fast etching of this material is discussed. Particular emphasis is placed on the defect-free delamination of large area GaN films with thicknesses ranging from 3 to 300 μm from sapphire substrates. The use of the resulting freestanding GaN films in device technology and homoepitaxy of III-nitrides are outlined. Specific examples are the flip-chip bonding of freestanding InGaN/GaN LEDs to a silicon submount and the production of pseudosubstrates for the homoepitaxy of high quality GaN epilayers. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lattice dynamics in GaN and AlN probed with first- and second-order Raman spectroscopy

Abstract: We present a selection of our contributions to basic research on the lattice dynamical properties of groupIII nitrides and their alloys. We used first-order Raman scattering to determine the zone-center phonons and their dependence on structural attributes such as stress, chemical composition, impurities, and doping. Results on the angular dispersion of the polar modes, strain distribution, coupled LO-phonon plasmon modes, multi-mode behavior in AlxGa1–xN, and the quantitative determination of the phase purity of cubic and hexagonal GaN are shown. Second-order Raman-scattering experiments on GaN and AlN provide information on the vibrational states throughout the entire Brillouin zone. Based on a comparison of experimental data and calculated phonon-dispersion curves we assigned the observed structures to particular phonon branches and points in the Brillouin zone. We also discuss the behavior of the optical modes under large hydrostatic pressure.

Hydride vapor phase epitaxy of AlN : thermodynamic analysis of aluminum source and its application to growth

Abstract: A thermodynamic analysis of the generation of gaseous aluminum chlorides by the reaction between aluminum (Al) metal and hydrogen chloride (HCl) gas is described for hydride vapor phase epitaxy (HVPE) of AlN. Regardless of the hydrogen mole fraction in the carrier gas, the major species of aluminum chloride is AlCl when the temperature of the Al metal is above 790 °C and is AlCl3 when the temperature is below 790 °C. Since AlCl3 is less reactive with quartz (SiO2) than AlCl, HVPE of AlN is possible using AlCl3 and NH3 even with a conventional system having a quartz reactor. Successful AlN HVPE on sapphire substrates is also reported. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of native, single‐crystal AlN substrates

Abstract: Aluminum nitride (AlN) is a promising substrate material for emerging wide-bandgap electronic and opto-electronic devices. Although existing manufacturing technology is less mature than for sapphire and silicon carbide substrates, aluminum nitride has been demonstrated to be superior for deep UV light-emitting applications and may ultimately surpass silicon carbide in both cost and performance for high power RF. Advantages include the identical crystal structure and close lattice match to high Al-content nitride alloys. In addition, AlN and GaN have closely matched thermal expansions from room temperature up to typical growth temperatures. The AlN has a band-gap energy of 6.2 eV with an index of refraction of 2.2 which is attractive for the extraction of uv light and is highly insulating which is attractive for high frequency devices. We use a sublimation-recondensation approach to produce large bulk AlN single crystals. Single crystal boules 15 mm in diameter and several cm in length have been grown and native-nitride substrates with dislocation densities below 103 cm−2 and with a thermal conductivity exceeding 3 W/cm-K at 300K, have been obtained from those boules. Crystal IS is currently developing 50mm diameter single-crystal boules. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Current–voltage instabilities in GaN/AlGaN resonant tunnelling structures

Abstract: GaN/AlGaN double barrier resonant tunnelling structures grown by molecular beam epitaxy on GaN templates have been studied. Peaks in the I(V) characteristics are observed, which are similar to resonant peaks seen in conventional III–V based devices. However, the behaviour of the peaks in I(V) depend upon the previous charge-state of the device produced by electrical bias. Current instabilities are also observed at low voltages. The possible origin of the peaks in the I(V) at room temperature and 4 K is discussed. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Surfactants and antisurfactants on group‐III‐nitride surfaces

Abstract: We have studied the effect of various dopants/impurities on structure and stability of group-III-nitride surfaces employing first-principles density-functional theory. For the example of H, Si, and In it is discussed whether and under what conditions impurities can affect the growth morphology and may act as surfactant or antisurfactant. Based on these results surface structures and growth conditions have been identified which allow to overcome intrinsic (thermodynamic) materials properties and limitations. Examples which will be discussed concern the enhancement of the doping efficiency, the formation of novel (tailor-made) alloys, and the self-organized growth of nano-structures. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High-quality AlN epitaxial films on (0001)-faced sapphire and 6H-SiC substrate

Abstract: This paper presents crystal qualities of high-quality AlN epitaxial films on (0001)-faced sapphire and6H-SiC substrates. The AlN epitaxial films are grown using a low-pressure metal organic vapor phase epitaxy (LP-MOVPE) method. 0.5–1 μm-thick AlN films without any cracks are realized on both substrates. Both of AlN films are found to have similar crystal qualities in spite of large difference in in-plane lattice mismatch between AlN and each substrate. The AlN films have an atomically flat surface with clear atomic steps. Results of X-ray rocking curve (XRC) measurement indicate that both of the AlN films have small tilted mosaicity, however relatively large twisted mosaicity. Dislocation density of the AlN films in its surface region is approximately as low as 1 × 1010 cm−2 and most of dislocations consist of edge-type dislocations. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Degradation in AlGaInN lasers

Abstract: Degradation experiments for AlGaInN-based laser diodes were conducted for the purpose of constructing a possible model of the degradation mechanism. Lasers in this experiment were aged under 30 mW continuous-wave operation at 60 °C, and the lifetime was defined as the time at which the operating current increased by 20%. The lifetime was found to be dependent on the dislocation density of the basal ELO-GaN layer, with the degradation rate being almost proportional to the square root of aging time. A model of degradation in which degradation is governed by a diffusion process is proposed based on these results. Although the model describes the experimental findings well, further investigation is considered necessary before the degradation mechanism in AlGaInN lasers can be fully understood. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Indium distribution in epitaxially grown InGaN layers analyzed by transmission electron microscopy

Abstract: An overview is given about microstructure and composition analyses of InGaN quantum wells embedded in Ga(Al)N barriers to study the mechanisms which determine the In distribution in epitaxially grown InGaN layers. The applied technique is transmission electron microscopy (TEM). The main prerequisite for this work was the development of a technique based on high-resolution lattice fringe images that allows quantitative chemical analyses of InGaN on an atomic scale. A large variety of samples was investigated that were produced by molecular beam epitaxy (MBE) and metal-organic vapor phase epitaxy (MOVPE). The effect of the deposition temperature, growth rate, strain and high-temperature annealing treatments on the average In concentration and In distribution was studied to assess the influence of phase separation, In surface segregation and In desorption. Composition fluctuations in InGaN are always observed on two different lateral scales independent of the growth technique and particular set of growth parameters but the strength of the composition fluctuations can be influenced by the details of the growth. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of AlGaN/GaN MIS-HFET using an Al2O3 high k dielectric

Abstract: We report on a metal–insulator–semiconductor AlGaN/GaN heterostructure field-effect transistor (MIS-HFET) using Al2O3 simultaneously for channel passivation layer and as a gate insulator which was deposited by plasma enhanced atomic layer deposition(PE-ALD). Capacitance–voltage measurements show successful surface passivation by the Al2O3 dielectric layer. For a gate length 1.2 μm with 15 μm source-to-drain spacing the maximum drain current was 1.22 A/mm, the maximum transconductance was 166 mS/mm and the gate leakage current was 4 nA/mm at Vgs = −20 V which is at least three orders of magnitude lower than that of conventional AlGaN/GaN HFETs. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Donor‐related density of states and polarizability in a GaAs‐(Ga, Al)As quantum‐well under hydrostatic pressure and applied electric field

Abstract: Theoretical calculations have been used to assess the influence of both an external electric field and hydrostatic stress on the binding energy, impurity polarizability, as a function of the impurity position and density of states for shallow-donor impurities in a GaAs–(Ga, Al)As quantum well. The binding energy maximum is shifted toward the wall at z = –L/2 of the quantum well for increasing values of electric field (keeping a constant pressure) and increasing values of pressure (keeping a constant electric field). The polarizability follows closely the behavior of the binding energy so for smaller binding energies the polarizability is large showing a more delocalized electron cloud. Also, it has been observed that the density of states depends strongly on the applied hydrostatic stress and electric field. In the absence of an electric field the energy level is degenerate for symmetrical positions of the impurities with respect to the center of the quantum well. However, this degeneracy is broken when an electric field is applied in the growth direction of the structure. Associated with this, the density of states becomes richer in structure

Ab initio materials design and Curie temperature of GaN-based ferromagnetic semiconductors

Abstract: Curie temperatures (TC) of the diluted magnetic semiconductors (DMS) of (Ga,Mn)N and (Ga,Cr)N are evaluated from first-principles with the local spin density approximation by using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation (KKR-CPA). The ferromagnetic states are stable in V- and Cr-doped GaN, but Fe-, Co- and Ni-doped GaN show the spin-glass states. In Mn-doped GaN, TC depends on the concentration of Mn due to the competition between the ferromagnetic double-exchange and antiferromagnetic super-exchange interactions. High-TC is expected in those systems if 5 at% concentrations of V, Cr, and Mn in GaN are doped. For (Ga,V)N and acceptor doped (Ga, Cr)N, ferromagnetic super-exchange interaction is responsible for the ferromagnetism, on the other hand, the ferromagnetic double-exchange interaction is dominant for high- TC in (Ga,Cr)N and (Ga,Mn)N. We discuss the acceptor and donor concentration dependence on TC for (Ga,Cr)N and (Ga,Mn)N based upon the magnetic mechanisms. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of sub-50-nm-gate i-AlGaN/GaN HEMTs on sapphire

Abstract: We fabricated sub-50-nm-gate i-AlGaN/GaN high electron mobility transistors (HEMTs) on sapphire and measured their DC and RF characteristics at room temperature. The fabricated HEMTs exhibited true device operation and good pinch-off characteristics down to a gate length Lg of 25 nm. For the HEMTs with a source–drain spacing Lsd of 2 μm, we obtained the Lg dependence of the cutoff frequency fT under a drain–source voltage Vds of 3 V. The peak fT was measured to be 102 GHz at Lg = 35 nm. At Lg = 25 nm, fT started to decrease due to the short-channel effect. The highest fT of 110 GHz was obtained by reducing Lsd from 2 to 1.5 μm and by increasing Vds from 3 to 4 V. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

PVT growth of bulk AlN crystals with low oxygen contamination

Abstract: We have performed growth and characterization of bulk aluminum nitride (AlN). Crystalline boules have been prepared by physical vapor transport (PVT), i.e. sublimation and recondensation of AlN at temperatures exceeding 2000 °C in nitrogen atmosphere. The dense boules are up to 15 mm in height and 2 inch in diameter and consist of high-purity AlN grains up to 5 mm in size. Oxygen was identified to be main impurity, strongly influencing seeding and material transport during growth. Chemical analysis shows that the grown material contains less than 100 ppm wt oxygen. The AlN crystals exhibit high structural quality and a texture in c-axis direction. Optical absorption data taken at different temperatures are discussed in connection with oxygen contamination. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlGaN/GaN HFETs on 100 mm Silicon Substrates for Commercial Wireless Applications

Abstract: GaN grown on Si is currently the only pathway towards high volume manufacturing of GaN based RF devices. In this paper we present the technological status of GaN grown on 100 mm Si substrates. Optimised growth, accounting for the lattice and thermal coefficient of expansion mismatch results in device quality GaN layers that exhibit excellent uniformity over the 100 mm Si substrate. The electrical characteristics of the fabricated devices reflect the high quality of the layers, leading to saturated power levels of 3.3 W/mm, the highest power densities reported to date for GaN on Si. Large periphery devices are shown to achieve up to 27 W of output power.

Electrical and optical characterization of n‐type ZnO thin films

Abstract: N-type ZnO thin films were successfully grown by sol-gel dipping coat method on glass substrates at 300–600 °C under air atmosphere. Poly ZnO thin films were obtained at more than 300 °C. Values of full width at half maximum of (0002) peak at the XRD spectra became small with the increasing the substrate temperatures. Optical transmittances of the ZnO thin films increased with the increasing the substrate temperatures. The optical transmittance of Ga-doped ZnO thin films was larger than In- and Al-doped ZnO films (5 wt%). Moreover, a resistivity of Ga-doped ZnO was smaller than those of In- and Al-doped ZnO films. X-ray photoemission spectroscopy (XPS) results indicated that a chemical shift of oxygen (1s) in Ga-doped ZnO was smaller than those of In- and Al-doped ZnO films. These indicated that Ga atoms were easy to substitute of Zn atoms in comparison with In and Al atoms. This result was clear from the ionic radius and the covalent radius of Ga atoms, which were similar to those of Zn compared with Al and In atoms.

Refractive indices of BxAl1−xN (x = 0–0.012) and ByGa1−yN (y = 0–0.023) epitaxial layers in ultraviolet region

Abstract: Refractive indices of BxAl1−xN and ByGa1−yN epitaxial layers were determined in the ultraviolet (UV) wavelength range from 230 nm to 600 nm by the light reflectance spectrum. The relative refractive index differences of (AlN/B0.01Al0.99N) and (GaN/B0.02Ga0.98N) heterostructures were 17% at 250 nm and 11% at 370 nm, respectively. These values indicate that an optical waveguide can be organized in the UV to deep-UV spectral region using the nitride semiconductors which include a small amount of boron. The relative refractive index difference of 6% was achieved with only 0.2% and 1.1% of boron for (AlN/BxAl1−xN) and (GaN/ByGa1−yN) heterostructures at 250 nm and 370 nm, respectively. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of GaN single crystals by defect-selective etching

Abstract: In this communication it is shown that both “orthodox” etching in molten KOH–NaOH eutectic (E etch and its modification) and photo-etching in aqueous KOH solution (PEC method) permit quick assessment of density, distribution and, after appropriate calibration, type of defects in GaN single crystals and epitaxial layers. Characteristic features of both etching methods are briefly discussed. It is shown that dislocations and micro-defects can be revealed in the form of etch pits (in “orthodox” E etch) and as etch hillocks (PEC method) on both Ga- and N-polar surfaces. The reliability of both methods has been confirmed by direct TEM calibration and by using indentation method. Very low dislocation density (EPD ≤ 2 × 102 cm–2) in the undoped GaN single crystals and MOCVD-grown epitaxial layers has been confirmed by this study.

Admittance characterization and analysis of trap states in AlGaN/GaN heterostructures

Abstract: We present a systematic study on the admittance characterization of trap states in AlGaN/GaN heterostructures. An equivalent circuit model was developed to analyze the spatial location, energy distribution density, and time constant of the AlGaN/GaN trap states. Results show that AlGaN/GaN interface trap states are responsible for the trapping behavior in the 10 kHz–1 MHz frequency range. An increase of AlGaN layer thickness or Al composition leads to greater trap state density at the AlGaN/GaN interface. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Vertical InGaN-based blue light emitting diode with plated metal base fabricated using laser lift-off technique

Abstract: A vertical InGaN-based blue light emitting diode (LED) with a reflective metal electrode and thick gold plated base free from insulating sapphire is demonstrated. A 50 μm-thick gold plated layer on a highly reflective platinum p-side ohmic contact is formed on the InGaN-based LED epi-structure, followed by irradiation with high power ultra-violet pulsed laser from the backside of the original sapphire substrate. The laser processing, so called laser lift-off technique, enables the separation of sapphire by decomposition of interfacial GaN layer. No transparent electrode is necessary for the LED because the current can spread well through the highly conductive n-GaN layer so that uniform blue light is emitted through the n-GaN layer. The electroluminescence (EL) spectra show small multi peaks due to the resonance within the formed vertical cavity. In addition, the measured thermal resistance of the vertical LED is smaller than that of the conventional LED on poor heat-conductive sapphire. The vertical LED would increase the brightness together with smaller chip size and lower series resistance. This would be advantageous espacially for high power and highly efficient LEDs applicable to future solid state lighting. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Correlation between strain, optical and electrical properties of InN grown by MBE

Abstract: The evolution of structural, optical and electrical properties was investigated for thin InN layers on either GaN or AlN buffer layers. Up to a layer thickness around 1 μm a biaxial strain is present in the layers. On AlN buffers the InN layers relax faster than on GaN. Both the interface and the surface affect the electron carrier concentration essentially. Thus, only with layers exceeding a thickness of 1 μm electron densities below 1018 cm−3 can be achieved. Optical spectroscopy indicates a band gap energy below 0.7 eV, however, the position of the absorption edge can be shifted up to 0.5 eV in thin layers. The Moss–Burstein shift due to the high electron concentration has the major influence on these effects. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Al composition on luminescence properties of europium implanted AlxGa1−xN (0 ≤ x ≤ 1)

Abstract: Europium (Eu) ions are introduced into AlxGa1−xN (0 ≤ x ≤ 1) by implantation to investigate the effect of Al composition on the luminescence properties. For all samples with Al contents between 0 and 100%, strong and sharp red emission peaks related to the 4f–4f intra transitions of Eu3+ are observed around 600 ∼ 660 nm at room temperature. The intensity of the Eu3+-related luminescence increases with increasing Al contents, and the photoluminescence of 5D0–7F2 transition related to Eu3+ shows strongest intensity at Al = 50%. The intensity is 100 times stronger than that of Eu in GaN, and several times stronger than that of near-band-edge emission for un-doped GaN. These results suggest that the luminescence property of Eu3+ can be dramatically improved by using Al0.5Ga0.5N as a host material. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier transport and recombination in InGaN/GaN heterostructures, studied by optical four-wave mixing technique

Abstract: Time-resolved four-wave mixing has been performed in InGaN/GaN/sapphire heterostructures using picosecond pulses at 355 nm for carrier excitation. In-plane diffusion and recombination of carriers, confined in the front layer of 50-nm-thick InGaN, were monitored by a delayed probe beam at 1064 nm. Decay times of free carrier gratings with various spatial periods allowed determination of bipolar diffusion coefficient D = 2.1 cm2/s, effective carrier lifetime of 470 ps, and estimate the corresponding hole mobility 40 cm2/Vs at carrier density of about 1018 cm−3. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Increase in luminescence efficiency of GaPN layers by thermal annealing

Abstract: The effect of rapid thermal annealing (RTA) on the luminescence properties of GaPN layers was investigated. GaPN layers were grown on GaP and Si substrates. The luminescence properties were evaluated by cathodeluminescence and low-temperature photoluminescence. It was found that the luminescence intensity was increased and the peak photon energy was shifted to the high-energy side in all samples after RTA. The luminescence intensity of the GaPN layers was increased with increasing the annealing temperature. It was also found that high-temperature RTA can be applied to the GaPN layer grown on the Si substrate with a thin Si cap layer rather than the GaPN layer on the GaP substrate. It was estimated that high-temperature RTA reduces the spatial fluctuation of nitrogen compositions and the density of defects attributed to nitrogen atoms. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular beam epitaxy of cubic III-nitrides on GaAs substrates

Abstract: Molecular beam epitaxy has successfully been used to grow crystalline layers of group III-nitrides (GaN, AlN and InN) with cubic (zinc-blende) structure on GaAs substrates. In this article, we discuss these efforts that, despite inherent difficulties due to the metastability of the c-III nitrides, led to substantial improvements of the structural, electrical and optical quality of these wide gap semiconductors. We review experimental work concerned with the epitaxy of c-GaN and the control of the growth process in-situ, the important issue of p- and n-type doping of c-GaN and investigations of the structural and optical properties of c-InGaN and c-AlGaN.

High-resolution measurements of the bulk dielectric constants of single crystal gold with application to reflection anisotropy spectroscopy

Abstract: Department of Physics, University of Aberystwyth, Ceredigion SY23 3BZ, Wales, UK Received 30 May 2003, revised 4 August 2003, accepted 11 August 2003 Published online 10 November 2003 PACS 07.60.Fs, 78.20.Ci, 78.40.Kc We report measurements of the optical constants of single crystal Au(110) in the range of 1.5 to 5.0 eV using Spectroscopic Ellipsometry. The results are compared with previously published data. In addition we have used the data to carry out a fit to RAS data obtained from the same crystal.

A theoretical investigation of dislocations in cubic and hexagonal gallium nitride

Abstract: In this article we review our theoretical work on dislocations in GaN The methods applied are two distinct approximations to density functional theory: Density functional based tight-binding total energy calculations allow the prediction of low energy core structures and energies of extended defects embedded in larger regions of perfect material However, whenever less approximate electronic structure calculations are required they are obtained in a localised basis pseudopotential approach © 2003 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim