Structural Properties 3 : Vacancy Defects Studied with Positron Annihilation Spectroscopy

Peer reviewe

Vacancy Defects in Bulk and Quasi-Bulk GaN Crystals

In-grown vacancy defects in bulk and quasi-bulk GaN crystals have been extensively studied with positron annihilation spectroscopy. High concentrations of Ga-vacancy-related defects are found irrespective of the growth method used in crystals with a high O contamination or intentional O doping, and they act as the dominant compensating native defect for n-type conductivity. Low-temperature crystal growth also leads to high concentrations of Ga-vacancy-related defects. Ga vacancies are present in the crystals as a part of the different types of complexes with O, H, and/or VN, depending on the growth conditions

Vacancy defect distributions in bulk ZnO crystals

We have used positron annihilation spectroscopy to study vacancy defects in ZnO single crystals grown by various methods from both commercial and academic sources. The combination of positron lifetime and Doppler broadening techniques with theoretical calculations provides the means to deduce both the identities and the concentrations of the vacancies. The annihilation characteristics of the Zn and O vacancies have been determined by studying electron-irradiated ZnO grown by the seeded vapor phase technique. The different ZnO samples were grown with the following techniques: the hydrothermal growth method, the seeded vapor phase technique, growth from melt (skull melting technique), and both conventional and contactless chemical vapor transport. We present a comparison of the vacancydefects and their concentrations in these materials.Peer reviewe

Identifying vacancy complexes in compound semiconductors with positron annihilation spectroscopy: a case study of InN

We present a comprehensive study of vacancy and vacancy-impurity complexes in InN combining positron annihilation spectroscopy and ab-initio calculations. Positron densities and annihilation characteristics of common vacancy-type defects are calculated using density functional theory and the feasibility of their experimental detection and distinction with positron annihilation methods is discussed. The computational results are compared to positron lifetime and conventional as well as coincidence Doppler broadening measurements of several representative InN samples. The particular dominant vacancy-type positron traps are identified and their characteristic positron lifetimes, Doppler ratio curves and lineshape parameters determined. We find that In vacancies and their complexes with N vacancies or impurities act as efficient positron traps, inducing distinct changes in the annihilation parameters compared to the InN lattice. Neutral or positively charged N vacancies and pure N vacancy complexes on the other hand do not trap positrons. The predominantly introduced positron trap in irradiated InN is identified as the isolated In vacancy, while in as-grown InN layers In vacancies do not occur isolated but complexed with one or more N vacancies. The number of N vacancies per In vacancy in these complexes is found to increase from the near surface region towards the layer-substrate interface.Comment: 10 pages, 6 figure

Evidence of a second acceptor state of the E center in Si1-x Gex

We have found evidence of a second acceptor state of the E center in Si1-x Gex by using positron annihilation spectroscopy. To achieve this, we studied proton irradiated n-type Si1−x Gex with a Ge content of 10%–30% and a P dopant concentration of 10 exp 18cm exp −3, in which the number of Ge atoms around irradiation induced E centers was increased by annealing. When measuring the Doppler broadening of the annihilation line, the shape parameter S starts to decrease at 150 K with decreasing measurement temperature. This indicates that a charge transition in the upper half of the Si1−x Gex band gap, above the well known (0/−) level, takes place. Hence, we suggest that the increased concentration of germanium around the E center pulls down the localized second acceptor state into the Si1−x Gex band gap, making the Ge decorated E center a more effective trap for conduction electrons.Peer reviewe

Vacancy defects in semiconductor materials for opto and spin electronics

The vacancy defects in GaN, ZnO and (Ga,Mn)As have been studied by positron annihilation spectroscopy. We show that both the thermodynamical quantities and the kinetics of the growth have a significant impact on impurity incorporation and point defect formation in all three materials. In addition the incorporation and stability of point defects in electron irradiated ZnO is studied. In GaN the Ga vacancies and vacancy clusters are more abundant at the N polar side. The concentrations of oxygen and of acceptor-type impurities are similarly correlated with the polarity. The vacancy concentrations are similar in both HVPE and high-pressure grown GaN in spite of the much higher growth temperature of the latter. This suggests that the thermal stability of the point defect complexes is an important factor determining which defects survive the cooling down from the growth temperature. The Zn vacancy is shown to be the dominant intrinsic acceptor in undoped ZnO. Vacancies on both sublattices and negative ion type defects are produced in electron irradiation, and their concentrations are determined. In addition, the irradiation-induced Zn vacancies have an ionization level close to 2.3 eV, hence it is likely that they are involved in the transition responsible for the green luminescence in ZnO. The irradiation-induced point defects fully recover after the annealing at 600 K. The Zn vacancies anneal out of the material in two stages, indicating that the Zn vacancies are parts of two different defect complexes. The O vacancies anneal simultaneously with the Zn vacancies at the later stage. The negative ion type defects anneal out between the two annealing stages of the Zn vacancies. The Zn vacancies are the dominant defects observed by positrons in thin heteroepitaxial ZnO layers. Their concentration depends on the surface plane of sapphire over which the ZnO layer has been grown. There is a correlation between the misorientation of the sapphire surface planes and the concentration of the vacancies for layers with thickness at most 500 nm. In addition to the misorientation, the defect content in the layer depends on the layer thickness. The concentrations of both native As antisites (donors) and Ga vacancies (acceptors) vary as a function of the Mn content in (Ga,Mn)As, following thermodynamic trends, but the absolute concentrations are determined by the growth kinetics and stoichiometry.reviewe

Positron trapping kinetics in thermally generated vacancy donor complexes in highly As-doped silicon

We have measured positron lifetime and Doppler broadening in highly As-doped silicon containing thermally generated V−As3 defect complexes (vacancy is surrounded by three arsenic atoms). We observe positron detrapping from the V−As3 defect complex and determine the binding energy of 0.27 eV of a positron to the complex. The results explain why 85% of the thermal vacancies formed in highly As-doped Si at temperatures over 700 K are invisible to positron measurements at elevated temperatures.Peer reviewe

Split Ga vacancies : Abundant defects in β-Ga2O3

We have applied positron annihilation spectroscopy to study a wide range of β-Ga2O3bulk crystals and thin films with various doping levels. The Doppler broadening of the 511 keV positron-electron annihilation line exhibits colossal anisotropy compared to other three-dimensional crystalline semiconductors. State-of-the-art theoretical calculations of the positron characteristics in the β-Ga2O3lattice reveal that the positron state is effectively 1-dimensional, giving rise to strong anisotropy. Strongly relaxed split Ga vacancies are found to exhibit even stronger anisotropy and to dominate the positron annihilation signals in almost all experiments. The evidence leads to the conclusion that split Ga vacancies are abundant, with concentration of 1018 cm-3 or more, in β-Ga2O3samples irrespective of conductivity.Peer reviewe

Ga Sublattice Defects in (Ga,Mn)As: Thermodynamical and Kinetic Trends

We have used positron annihilation spectroscopy and infrared absorption measurements to study the Ga sublattice defects in epitaxial Ga1−xMnxAs with Mn content varying from 0% to 5%. We show that the Ga vacancy concentration decreases and As antisite concentration increases with increasing Mn content. This is in agreement with thermodynamical considerations for the electronic part of the formation energy of the Ga sublattice point defects. However, the absolute defect concentrations imply that they are determined rather by the growth kinetics than by the thermodynamical equilibrium. The As antisite concentrations in the samples are large enough to be important for compensation and magnetic properites. In addition, the Ga vacancies are likely to be involved in the diffusion and clustering of Mn at low annealing temperatures.Peer reviewe