Introduction and recovery of point defects in electron-irradiated ZnO

We have used positron annihilation spectroscopy to study the introduction and recovery of point defects in electron-irradiated n-type ZnO. The irradiation (Eel=2MeV, fluence 6×10 exp 17 cm exp −2) was performed at room temperature, and isochronal annealings were performed from 300 to 600 K. In addition, monochromatic illumination of the samples during low-temperature positron measurements was used in identification of the defects. We distinguish two kinds of vacancy defects: the Zn and O vacancies, which are either isolated or belong to defect complexes. In addition, we observe negative-ion-type defects, which are attributed to O interstitials or O antisites. The Zn vacancies and negative ions act as compensating centers and are introduced at a concentration [VZn]≃cion≃2×10 exp 16 cm exp −3. The O vacancies are introduced at a 10-times-larger concentration [VO]≃3×10 exp 17 cm exp −3 and are suggested to be isolated. The O vacancies are observed as neutral at low temperatures, and an ionization energy of 100 meV could be fitted with the help of temperature-dependent Hall data, thus indicating their deep donor character. The irradiation-induced defects fully recover after the annealing at 600 K, in good agreement with electrical measurements. The Zn vacancies recover in two separate 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, with an activation energy of EmV,O = 1.8 ± 0.1 eV. The negative ions anneal out between the two annealing stages of the vacancies.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

Zinc vacancy and oxygen interstitial in ZnO revealed by sequential annealing and electron irradiation

By combining results from positron annihilation and photoluminescence spectroscopy with data from Hall effect measurements, the characteristic deep level emission centered at ∼1.75 eV and exhibiting an activation energy of thermal quenching of 11.5 meV is associated with the zinc vacancy. Further, a strong indication that oxygen interstitials act as a dominating acceptor is derived from the analysis of charge carrier losses induced by electron irradiation with variable energy below and above the threshold for Zn-atom displacement. We also demonstrate that the commonly observed green emission is related to an extrinsic acceptorlike impurity, which may be readily passivated by oxygen vacancies.Peer reviewe

Hardness of ion implanted ceramics

It has been established that the wear behavior of ceramic materials can be modified through ion implantation. Studies have been done to characterize the effect of implantation on the structure and composition of ceramic surfaces. To understand how these changes affect the wear properties of the ceramic, other mechanical properties must be measured. To accomplish this, a commercially available ultra low load hardness tester has been used to characterize Al/sub 2/O/sub 3/ with different implanted species and doses. The hardness of the base material is compared with the highly damaged crystalline state as well as the amorphous material

The Cognitive Change Index as a Measure of Self and Informant Perception of Cognitive Decline: Relation to Neuropsychological Tests

BACKGROUND: The perception of cognitive decline by individuals and those who know them well ("informants") has been inconsistently associated with objective cognitive performance, but strongly associated with depressive symptoms. OBJECTIVE: We investigated associations of self-report, informant-report, and discrepancy between self- and informant-report of cognitive decline obtained from the Cognitive Change Index (CCI) with cognitive test performance and self-reported depressive symptoms. METHODS: 267 participants with normal cognition, mild cognitive impairment (MCI), or mild dementia were included from a cohort study and memory clinic. Association of test performance and self-rated depression (Geriatric Depression Scale, GDS) with CCI scores obtained from subjects (CCI-S), their informants (CCI-I), and discrepancy scores between subjects and informants (CCI-D; CCI-S minus CCI-I) were analyzed using correlation and analysis of covariance (ANCOVA) models. RESULTS: CCI-S and CCI-I scores showed high internal consistency (Cronbach alpha 0.96 and 0.98, respectively). Higher scores on CCI-S and CCI-I, and lower scores on the CCI-D, were associated with lower performance on various cognitive tests in both univariate and in ANCOVA models adjusted for age, gender, and education. Adjustment for GDS slightly weakened the relationships between CCI and test performance but most remained significant. CONCLUSION: Self- and informant-report of cognitive decline, as measured by the CCI, show moderately strong relationships with objective test performance independent of age, gender, education, and depressive symptoms. The CCI appears to be a valid cross-sectional measure of self and informant perception of cognitive decline across the continuum of functioning. Studies are needed to address the relationship of CCI scores to longitudinal outcome

A survey of partial differential equations in geometric design

YesComputer aided geometric design is an area where the improvement of surface generation techniques is an everlasting demand since faster and more accurate geometric models are required. Traditional methods for generating surfaces were initially mainly based upon interpolation algorithms. Recently, partial differential equations (PDE) were introduced as a valuable tool for geometric modelling since they offer a number of features from which these areas can benefit. This work summarises the uses given to PDE surfaces as a surface generation technique togethe

Neither bones nor feet: track morphological variation and “preservation quality”

As purely sedimentary structures, fossil footprints are all about shape. Correctly interpreting the significance of their surface topography requires understanding the sources of morphological variation. Differences among specimens are most frequently attributed to either taxonomy (trackmaker) or to preservation quality. “Well-preserved” tracks are judged more similar to pedal anatomy than “poorly preserved” ones, but such broad-brush characterizations confound two separate episodes in a track’s history. Current evaluations of track quality fail to distinguish among behavioral, formational, intravolumetric, and post-formational sources of variation. Based on analogy with body fossils, we recommend restricting assessments of track preservation quality to modifications that take place only after a track is created. Ichnologists need to try to parse the relative influence of factors affecting disparity, but we currently lack an adequate vocabulary to describe the overall shapes and specific features of formational variants

Ion Implantation and Annealing of Oxides

Ion implantation damage and annealing results are presented for a number of crystalline oxides. In Al2O3, the amorphous phase produced by ion bombardment of the pure material first crystallizes in the (crystal-line) Y phase. This is followed by the transformaiton of Y-Al2O3 to a-Al2O3 at a well defined interface. The activation energy for of a alumina from y is 3.6 eV/atom. In CaTiO3, the implantation-induced amorphous phase transforms to remain crystalline even after high implantation doses at liquid nitrogen temperatures. The near surface of KTaO3 is transformed to a polycrystalline state after inplantation at room temperature or liquid nitrogen temperature.Physic