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Positron annihilation spectroscopy is used to study vacancy defects in GaN grown by molecular-beam epitaxy due to different polar directions and varying stoichiometry conditions during oxygen doping. We show that Ga-polar material is free of compensating Ga vacancies up to [O]=1018 cm−3 in Ga stable growth, but high concentrations of VGa are formed in N-stable conditions. We also show that vacancy clusters are formed in N-polar material grown in Ga stable conditions, which may be related to the higher reactivity of the N-polar surface. These clusters have no apparent influence on the electrical properties of the material. We thus infer that their charge state is neutral.

The formation of In vacancies in InN layers grown on Al 2O 3 was discussed. A low energy positron beam was used to identify the vacancies by using molecular beam epitaxy. It was found that the concentration of vacancies decreased with the increasing layer thickness. Results show that the decrease in the vacancy concentration coincided with the increase in the electron Hall mobility.

A method for obtaining the complex permittivity of a Maxwell Garnett liquid with the aid of spectral surface plasmon resonance spectroscopy is presented. The Maxwell Garnett liquid is assumed to consist of small absorbing spherical inclusions in a host liquid. It is shown that the permittivity of the inclusions can be calculated if the optical constants of the host liquid are known. It is believed that such a method will gain increasing importance in the analysis of biological reactions.

Positron annihilation measurements show that negative Ga vacancies are the dominant acceptors in n-type gallium nitride grown by hydride vapor phase epitaxy. The concentration of Ga vacancies decreases, from more than 1019 to below 1016 cm−3, as the distance from the interface region increases from 1 to 300 μm. These concentrations are the same as the total acceptor densities determined in Hall experiments. The depth profile of O is similar to that of VGa, suggesting that the Ga vacancies are complexed with the oxygen impurities.

Carbon-doped GaN layers grown by molecular-beam epitaxy are studied with photoluminescence and positron annihilation spectroscopy. Semi-insulating layers doped with >1018 cm−3 carbon show a strong luminescence band centered at ∼2.2 eV (yellow luminescence). The absolute intensity of the 2.2 eV band is compared with the gallium vacancy concentration determined by positron annihilation spectroscopy. The results indicate that a high concentration of gallium vacancies is not necessary for yellow luminescence and that there is in fact a causal relationship between carbon and the 2.2 eV band. Markedly different deep-level ionization energies are found for the high-temperature quenching of the 2.2 eV photoluminescence in carbon-doped and reference samples. We propose that while the model of Neugebauer and Van de Walle [Appl. Phys. Lett. 69, 503 (1996)] applies for GaN of low carbon concentration, a different yellow luminescence mechanism is involved when the interstitial carbon concentration is comparable to or ...

Positron annihilation has been applied to study thermally oxidized 4H- and 6H-SiC. The SiC/SiO2 interface is found to contain a high density of open-volume defects. The positron trapping at the interface defects correlates with the charge of the interface determined by capacitance–voltage experiments. For oxides grown on n-SiC substrates, the positron annihilation characteristics at these defects are nearly indistinguishable from those of a silicon/oxide interface, with no discernable contribution from C-related bonds or carbon clusters. These results indicate that those defects at the SiC/oxide interface, which are visible to positrons, are similar to those at the Si/oxide interface. The positron annihilation characteristics suggest that these defects are vacancies surrounded by oxygen atoms.

We have applied a low-energy positron beam to study epitaxial Si-doped GaN layers, where the grain size varies from 0.2 to 2–5 μm. Negatively charged Ga vacancies are found in n-type samples. Their concentration is independent of the grain size, suggesting that Ga vacancies exist in the grain interior. Positrons are observed to get trapped also at other negatively charged centers. The positron trapping rate at these defects correlates with the grain-boundary density. We attribute the observed shallow positron traps, which do not contain open volume, to negatively charged edge-type dislocations which define the grain boundaries.

Positron annihilation spectroscopy was used to study GaAsN/GaAs epilayers. GaAsN layers were found to contain Ga vacancies in defect complexes. The density of the vacancy complexes increases rapidly to the order of 1018 cm−3 with increasing N composition and decreases after annealing at 700 °C. The anticorrelation of the vacancy concentration and the integrated photoluminescence intensity suggests that the Ga vacancy complexes act as nonradiative recombination centers.

Gallium vacancies and negative ions are observed in GaN bulk crystals by applying positron lifetime spectroscopy. The concentration of Ga vacancies decreases with increasing Mg doping, as expected from the behavior of the VGa formation energy as a function of the Fermi level. The concentration of negative ions correlates with that of Mg impurities determined by secondary ion mass spectrometry. We thus attribute the negative ions to Mg−Ga. The negative charge of Mg suggests that Mg doping converts n-type GaN to semi-insulating mainly due to the electrical compensation of O+N donors by Mg−Ga acceptors.

Positron annihilation experiments have been performed to investigate the compensating defects in silicon δ-doping superlattices in (001) GaAs. The results reveal vacancies and ion-type defects, which are located between the delta planes in undoped GaAs. The vacancy defect is identified as the Ga vacancy and the negative ion is attributed to the Ga antisite. The concentrations of these defects increase strongly, when the areal concentrations of free carriers are reduced at the delta planes.

We show that positron trapping at negative As vacancies revealed under illumination in bulk semi‐insulating GaAs correlates with a form of near‐band‐edge absorption known as reverse contrast (RC). We conclude that it is the ionization of As vacancies to their negative charge state that gives rise to RC absorption.

High‐efficiency diffractive elements with a graded volume distribution of refractive index are proposed, designed, and fabricated using thermal ion exchange in glass. A deliberate use of isotropic diffusion of ions enables us to generate, even with a single binary mask, a structure that acts like a continuous surface‐relief profile. The advantages of the proposed method include the ease of cleaning and antireflection coating the flat surface, the absence of alignment problems in the fabrication process, and compatibility with stacked planar optics. A grating beam splitter with three equal‐efficiency central orders and 89.7% diffraction efficiency is demonstrated.

We present optically pumped semiconductor disk lasers with a thin dielectric layer placed between the semiconductor distributed Bragg reflector and the metallization interface. The approach is shown to enhance the reflectivity of the semiconductor mirror while introducing a negligible penalty to the thermal resistance of the device. The design has potential for improving the performance of semiconductor disk lasers by avoiding highly pump-absorbing metal layers and allowing thinner mirror structures. The advantages are expected to be especially prominent for material systems that employ thick thermally insulating semiconductor mirrors.

Vertical electrostatic wedge actuators are described that control nanometer-scale gaps between surfaces. Standard parallel-plate electrostatic actuators become difficult to stabilize across extremely small gaps because the nature of the forces and the force laws that describe them often deviate from a Coulomb's law dependence. In this work, a nanometer-scale air gap between a collapsed cantilever structure formed by two facing In0.53Ga0.47As surfaces, with areas of tens of microns, was controlled by a wedge electrostatic actuator. Upon actuation, the gap spacing between the surfaces was tuned over a maximum range of 55 nm with an applied voltage of 60 V.

Bismuth (Bi) is a surfactant which controls the electronic and structural properties of epitaxially grown GaInAs and GaInP alloys. The authors have studied Bi-terminated surfaces of GaInAs(100) layers, deposited onto InP(100) substrates, by means of scanning tunneling microscopy and spectroscopy and x-ray photoelectron spectroscopy. It appears that Bi stabilizes an interesting (2×1) surface reconstruction on GaInAs(100). Under certain conditions, this reconstruction undergoes a phase transition to (2×4), which has about one-half of the Bi content of the (2×1) surface. Based on the experimental findings, they discuss mechanisms leading to this phase transition.

This letter reports optical properties of spherical dielectric particles with alternating layers of high and low refractive indices. The authors “predesigned” and synthesized polymer microspheres with alternating polystyrene and poly(trifluoroethyl methacrylate) layers of various thicknesses. Extinction of the microspheres was examined in a broad spectral range. A multipole expansion of electromagnetic field was applied for calculating the extinction efficiency of the particles. Excellent agreement was obtained between experimental and theoretical extinction curves.

Positron annihilation spectroscopy has been used to study GaN grown by metal-organic chemical vapor deposition on misoriented 4H-SiC substrates. Two kinds of vacancy defects are observed: Ga vacancies and larger vacancy clusters in all the studied layers. In addition to vacancies, positrons annihilate at shallow traps that are likely to be dislocations. The results show that the vacancy concentration increases and the shallow positron trap concentration decreases with the increasing substrate misorientation.

The authors report on the influence of nitrogen incorporation on vacancies in GaAs based superlattices. The samples were molecular beam epitaxy grown on p-type GaAs substrates with the superlattice structure consisting of ten periods of quantum well material separated by GaAs buffers. Three different quantum well compositions were used, Ga0.63In0.37As, Ga0.63In0.37N0.01As0.99, and GaN0.01As0.99. Rapid thermal anneals were performed on each sample set. Positron spectroscopy was used for vacancy detection in the superlattice structure. Annealed GaNAs and GaInNAs superlattice samples were found to contain vacancy-type defects. A comparison with photoluminescence measurements shows that the detected vacancy-type defects are not optically active.

We have applied a slow positron beam to study InN samples grown by metal-organic vapor-phase epitaxy with different V/III molar ratios (3300–24 000) and at different growth temperatures (550–625°C). Indium vacancies were identified in samples grown at V/III ratios below 4000. Their concentration is in the 10exp17cm−3 range. No strong dependence of vacancy concentration on the molar ratio was observed. At low V/III ratios, however, In droplets and vacancy clusters are formed near the substrate interface. The elevated growth temperature enhances the In vacancy formation, possibly due to limited sticking of In on the growth surface close to the decomposition temperature.

Contains fulltext : 32448.pdf (Publisher’s version ) (Open Access) We have used positron annihilation, secondary ion mass spectrometry, and photoluminescence to study the point defects in GaN grown by hydride vapor phase epitaxy (HVPE) on GaN bulk crystals. The results show that N polar growth incorporates many more donor and acceptor type impurities and also Ga vacancies. Vacancy clusters with a positron lifetime tau(D) = 470 +/- 50 ps were found near the N polar surfaces of both the HVPE GaN layers and bulk crystals. (C) 2005 American Institute of Physics.