Evidence of the Zn Vacancy Acting as the Dominant Acceptor in n-Type ZnO

We have used positron annihilation spectroscopy to determine the nature and the concentrations of the open volume defects in as-grown and electron irradiated (Eel=2   MeV, fluence 6×10 exp 17   cm exp −2) ZnO samples. The Zn vacancies are identified at concentrations of [VZn]≃2×10 exp 15   cm exp −3 in the as-grown material and [VZn]≃2×10 exp 16   cm exp −3 in the irradiated ZnO. These concentrations are in very good agreement with the total acceptor density determined by temperature dependent Hall experiments. Thus, the Zn vacancies are dominant acceptors in both as-grown and irradiated ZnO.Peer reviewe

Introduction and recovery of Ga and N sublattice defects in electron-irradiated GaN

We have used positron annihilation spectroscopy to study the introduction and recovery of point defects introduced by 0.45 and 2 MeV electron irradiation at room temperature in n-type GaN. Isochronal annealings were performed up to 1220 K. We observe vacancy defects with specific lifetime of τV=190±15ps that we tentatively identify as N vacancies or related complexes in the neutral charge state in the samples irradiated with 0.45MeV electrons. The N vacancies are produced at a rate Σ0.45N≃0.25 cm exp −1. The irradiation with 2 MeV electrons produces negatively charged Ga vacancies and negative nonopen volume defects (negative ions) originating from the Ga sublattice, at a rate Σ2.0Ga≃5cm exp −1. The irradiation-induced N vacancies anneal out of the samples at around 600 K, possibly due to the motion of the irradiation-induced N interstitials. Half of the irradiation-induced Ga vacancies anneal out of the samples also around 600 K, and this is interpreted as the isolated Ga vacancies becoming mobile with a migration barrier of EV,GaM=1.8±0.1eV. Interestingly, we observe a change of charge state of the irradiation-induced negative ions from 2− to 1− likely due to a reconstruction of the defects in two stages at annealing temperatures of about 600 and 700 K. The negative ions anneal out of the samples together with the other half of the Ga vacancies (stabilized by, e.g., N vacancies and/or hydrogen) in thermal annealings at 800–1100K.Peer reviewe

Mechanisms of arsenic clustering in silicon

A model of arsenic clustering in silicon is proposed and analyzed. The main feature of the proposed model is the assumption that negatively charged arsenic complexes play a dominant role in the clustering process. To confirm this assumption, electron density and concentration of impurity atoms incorporated into the clusters are calculated as functions of the total arsenic concentration. A number of the negatively charged clusters incorporating a point defect and one or more arsenic atoms are investigated. It is shown that for the doubly negatively charged clusters or for clusters incorporating more than one arsenic atom the electron density reaches a maximum value and then monotonically and slowly decreases as total arsenic concentration increases. In the case of doubly negatively charged cluster incorporating two arsenic atoms, the calculated electron density agrees well with the experimental data. Agreement with the experiment confirms the conclusion that two arsenic atoms participate in the cluster formation. Among all present models, the proposed model of clustering by formation of doubly negatively charged cluster incorporating two arsenic atoms gives the best fit to the experimental data and can be used in simulation of high concentration arsenic diffusion.Comment: 13 pages, 4 figures. Revised and shortened version of the paper has been published in Phys. Rev. B, Vol.74 (3), art. no. 035205 (2006

Vacancy-Impurity Complexes in Highly Sb-Doped Si Grown by Molecular Beam Epitaxy

Positron annihilation measurements, supported by first-principles electron-structure calculations, identify vacancies and vacancy clusters decorated by 1–2 dopant impurities in highly Sb-doped Si. The concentration of vacancy defects increases with Sb doping and contributes significantly to the electrical compensation. Annealings at low temperatures of 400–500 K convert the defects to larger complexes where the open volume is neighbored by 2–3 Sb atoms. This behavior is attributed to the migration of vacancy-Sb pairs and demonstrates at atomic level the metastability of the material grown by epitaxy at low temperature.Peer reviewe

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

Abstract 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.

A novel desmoplakin mutation causes dilated cardiomyopathy with palmoplantar keratoderma as an early clinical sign

Background PPKs represent a heterogeneous group of disorders with hyperkeratosis of palmar and/or plantar skin. PPK, hair shaft abnormalities, cardiomyopathy and arrhythmias can be caused by mutations in desmosomal genes, e.g. desmoplakin (DSP). PPK should trigger genetic testing to reveal mutations with possible related cardiac disease. Objectives To report a large multigenerational family with a novel DSP mutation associated with early-onset PPK and adult-onset cardiomyopathy and arrhythmias. Methods A custom-designed in-house panel of 35 PPK related genes was used to screen mutations in the index patient with focal PPK. The identified DSP mutation was verified by Sanger sequencing. DNA samples from 20 members of the large multigenerational family were sequenced for the DSP mutation. Medical records were reviewed. Clinical dermatological evaluation was performed, including light microscopy of hair samples. Cardiac evaluation included clinical examination, echocardiography, cardiac magnetic resonance imaging (CMR), electrocardiogram (ECG), Holter monitoring and laboratory tests. Results We identified a novel autosomal dominant truncating DSP c.2493delA p.(Glu831Aspfs*33) mutation associated with dilated cardiomyopathy (DCM) with arrhythmia susceptibility and focal PPK as an early cutaneous sign. The mutation was found in nine affected family members, but not in any unaffected members. Onset of dermatological findings preceded cardiac symptoms which were variable and occurred at adult age. Conclusions We report a novel truncating DSP mutation causing focal PPK with varying severity and left ventricular dilatation and ventricular extrasystoles. This finding emphasizes the importance of genetic diagnosis in patients with PPK for clinical counselling and management of cardiomyopathies and arrhythmias.Peer reviewe

Vacancy-Impurity Complexes in Highly Sb-Doped Si Grown by Molecular Beam Epitaxy

Positron annihilation measurements, supported by first-principles electron-structure calculations, identify vacancies and vacancy clusters decorated by 1-2 dopant impurities in highly Sb-doped Si. The concentration of vacancy defects increases with Sb doping and contributes significantly to the electrical compensation. Annealings at low temperatures of 400 -500 K convert the defects to larger complexes where the open volume is neighbored by 2 -3 Sb atoms. This behavior is attributed to the migration of vacancy-Sb pairs and demonstrates at atomic level the metastability of the material grown by epitaxy at low temperature. DOI: 10.1103/PhysRevLett.94.165501 PACS numbers: 61.72.-y, 61.82.Fk, 78.70.Bj The interest in highly doped Si is fundamentally related to the miniaturization of field-effect transistors, where increased doping is needed to maintain a sufficient conductance of the source and drain regions The molecular beam epitaxy (MBE) growth at low temperature ( < 600 K) can be applied to achieve metastable doping and free electron concentrations, which become compensated only at 10 21 cm ÿ3 In this work we apply positron annihilation spectroscopy to study vacancies formed in the low-temperature MBE growth of highly Sb-doped Si. Positrons get trapped at open-volume defects. The measured annihilation photons carry information on the electron momentum density, which can be utilized to identify the size of the open volume of the defect and the neighboring dopant atoms. Our results show that the MBE growth creates vacancies and vacancy clusters, which are neighbored by 1-2 Sb atoms. The vacancy concentrations are relevant for the compensation of the Sb doping. We also show that the low-temperature MBE Si is atomically metastable, and annealings at low temperatures of 400 -500 K lead to clustering of vacancies and dopant impurities. We studied Si(100) layers grown by MBE on the Si substrate at 550 K We used a low-energy positron beam to measure the Doppler broadened energy spectrum of the annihilation radiation. The shape of the spectrum was described with conventional S and W parameter

Vacancy-impurity complexes in highly Sb-doped Si grown by molecular beam epitaxy

Positron annihilation measurements, supported by first-principles electron-structure calculations, identify vacancies and vacancy clusters decorated by 1 -2 dopant impurities in highly Sb-doped Si. The concentration of vacancy defects increases with Sb doping and contributes significantly to the electrical compensation. Annealings at low temperatures of 400 -500 K convert the defects to larger complexes where the open volume is neighbored by 2 -3 Sb atoms. This behavior is attributed to the migration of vacancy-Sb pairs and demonstrates at atomic level the metastability of the material grown by epitaxy at low temperature. PACS numbers: 61.82.Fk, 78.70.Bj The interest in highly doped Si is fundamentally related to the miniaturization of field-effect transistors, where increased doping is needed to maintain a sufficient conductance of the source and drain regions The molecular beam epitaxy (MBE) growth at low temperature (< 600 K) can be applied to achieve metastable doping and free electron concentrations, which become compensated only at 10 21 cm −3 In this work we apply positron annihilation spectroscopy to study vacancies formed in the lowtemperature MBE growth of highly Sb-doped Si. Positrons get trapped at open volume defects. The measured annihilation photons carry information on the electron momentum density, which can be utilized to identify the size of the open volume of the defect and the neighboring dopant atoms. Our results show that the MBE growth creates vacancies and vacancy clusters, which are neighbored by 1 -2 Sb atoms. The vacancy concentrations are relevant for the compensation of the Sb doping. We also show that the low-temperature MBE Si is atomically metastable, and annealings at low temperatures of 400-500 K lead to clustering of vacancies and dopant impurities. We studied Si(100) layers grown by MBE on Si substrate at 550