J. Oila

Ga Vacancies as Dominant Intrinsic Acceptors in GaN Grown by Hydride Vapor Phase Epitaxy

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.

GALLIUM VACANCIES AND THE GROWTH STOICHIOMETRY OF GAN STUDIED BY POSITRON ANNIHILATION SPECTROSCOPY

We have applied positron spectroscopy to study the formation of vacancy defects in undoped n-type metal organic chemical vapor deposition grown GaN, where the stoichiometry was varied. Ga vacancies are found in all samples. Their concentration increases from 1016 to 1019 cm−3 when the V/III molar ratio increases from 1000 to 10 000. In nitrogen rich conditions Ga lattice sites are thus left empty and Ga vacancies are abundantly formed. The creation of Ga vacancies is accompanied by the decrease of free electron concentration from 1020 to 1016 cm−3, demonstrating their role as compensating centers.

Observation of defect complexes containing Ga vacancies in GaAsN

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.

Influence of layer thickness on the formation of In vacancies in InN grown by molecular beam epitaxy

We have used a low-energy positron beam to identify In vacancies in InN layers grown on Al2O3 by molecular beam epitaxy. Their concentration decreases from ∼5×1018 to below 1016 cm−3 with increasing layer thickness (120–800 nm). The decrease in the vacancy concentration coincides with the increase in the electron Hall mobility, suggesting that In vacancies act as electron scattering centers.

Structural, electrical, and optical properties of defects in Si-doped GaN grown by molecular-beam epitaxy on hydride vapor phase epitaxy GaN on sapphire

Molecular-beam epitaxy (MBE) has been utilized to grow Si-doped GaN layers on GaN/sapphire templates prepared by hydride vapor phase epitaxy. An extensive set of characterization techniques is applied to investigate the layers. Positron annihilation experiments indicate that the samples contain open volume defects, most likely clusters of vacancies and possibly Ga vacancy-donor complexes. The number of vacancy clusters decreases, as Si concentration is increased. Photoluminescence spectra show that while the absolute intensity of both the yellow and ultraviolet (UV) band-edge transitions increase with Si doping, the intensity ratio of yellow-to-UV emission is decreased. Secondary ion mass spectrometry indicates that the impurity concentrations are in qualitative agreement with the carrier concentrations determined in electrical experiments. The data suggest further that silicon does not affect the diffusion of oxygen. Moreover, transmission electron microscopy reveals that MBE-grown GaN retains the thread...

Vacancy defects in O-doped GaN grown by molecular-beam epitaxy: The role of growth polarity and stoichiometry

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.

Positron annihilation states at interfaces: evidence of divacancies

We present a method based on positron annihilation to investigate defects at interfaces formed between a thin 10 - 50 A overlayer and a substrate. The method applied to 38 interfaces gives evidence that (i) high concentrations of divacancies exist in Si domains at the interfaces formed with natural oxides obtained after etching and that (ii) positrons see the same annihilation state at various interfaces where the oxides are either commercial, thermally grown, native or obtained after rapid thermal annealing treatments.

Ga vacancies and grain boundaries in GaN

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.

Observation of Ga vacancies and negative ions in undoped and Mg-doped GaN bulk crystals

Abstract 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 V Ga 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.

Target chamber for a slow positron beam: optimization of count rate and minimization of backscattering effects

Positrons, which scatter back from the target and annihilate in chamber walls near the detectors, may cause a significant error in annihilation parameters. We have constructed a new UHV target chamber for slow positron beam studies. In our design special care has been taken to reduce the effect of backscattered positrons. Detector wells are designed for two-detector coincidence measurements and they are situated on both sides of the target. The distance of the wells from the target can be adjusted by simple manipulators. This enables optimization regarding the count rate and the rate of backscattered positrons hitting the detector wells. The magnetic field in front of the target is increased by permanent magnets situated behind the target. The increased magnetic field guides the backscattered positrons effectively away from the detectors. The increased magnetic field also focuses the beam spot strongly.