Ion implantation and low-temperature epitaxial regrowth of GaAs

Channeling and transmission electron microscopy have been used to investigate the parameters that govern the extent of damage in ion‐implanted GaAs and the crystal quality following capless furnace annealing at low temperature (∼400 °C). The implantation‐induced disorder showed a strong dependence on the implanted ion mass and on the substrate temperature during implantation. When the implantation produced a fully amorphous surface layer the main parameter governing the regrowth was the amorphous thickness. Formation of microtwins after annealing was observed when the initial amorphous layer was thicker than 400 Å. Also, the number of extended residual defects after annealing increased linearly with the initial amorphous thickness and extrapolation of that curve predicts good regrowth of very thin (<400 Å) GaAs amorphous layers produced by ion implantation. A model is presented to explain the observed features of the low‐temperature annealing of GaAs

Pulsed electron beam induced recrystallization and damage in GaAs

Single-pulse electron-beam irradiations of 300-keV 10^(15)Kr+/cm^2 or 300-keV 3×10^(12)Se+/cm^2 implanted layers in unencapsulated GaAs are studied as a function of the electron beam fluence. The electron beam pulse had a mean electron energy of ~-20 keV and a time duration of ~-10^(–7) s. Analyses by means of MeV He + channeling and TEM show the existence of narrow fluence window (0.4–0.7 J/cm^2) within which amorphous layers can be sucessfully recrystallized, presumably in the liquid phase regime. Too high a fluence produces extensive deep damage and loss of As

Epitaxial regrowth of thin amorphous GaAs layers

Channeling and transmission electron microscopy have been used to investigate the parameters that govern the crystal quality following capless funace annealing at low temperature (~ 400 °C) in ion-implanted GaAs. From the results obtained, we concluded that the crystal quality after annealing depends strongly on the thickness of the amorphous layer generated by ion implantation and the number of residual defects increases linearly with the thickness of the implanted layer. Single-crystal regrowth free of defects detectable by megaelectron volt He + channeling was achieved for a very thin amorphous layer (<~ 400 Å)

Steady-state thermally annealed GaAs with room-temperature-implanted Si

Semi-insulating Cr-doped single-crystal GaAs samples were implanted at room temperature with 300-keV Si ions in the dose range of (0.17–2.0)×1015 cm–2 and were subsequently steady-state annealed at 900 and 950°C for 30 min in a H2 ambient with a Si3N4 coating. Differential Hall measurements showed that an upper threshold of about 2×1018/cm3 exists for the free-electron concentration. The as-implanted atomic-Si profile measured by SIMS follows the theoretical prediction, but is altered during annealing. The Cr distribution also changes, and a band of dislocation loops ~2–3 kÅ wide is revealed by cross-sectional TEM at a mean depth of Rp~3 kÅ. Incomplete electrical activation of the Si is shown to be the primary cause for the effect

The heteroepitaxy of Ge on Si: A comparison of chemical vapor and vacuum deposited layers

Epitaxial growth of Ge on Si has been investigated by two techniques: vacuum deposition and chemical vapor deposition (CVD). Vacuum-deposited Ge layers (physical vapor deposition, PVD) on heated Si substrates (≤ 500 °C) have smooth surface morphologies with a surface crystalline quality which improves with Ge layer thickness. Layers prepared by the CVD technique at 500–600 °C are comparable with the PVD prepared layers. Main defects in both PVD and CVD layers are dislocations initiating at the Ge/Si interface. Chemical vapor-deposited Ge layers grown at a substrate temperature of 700–800 °C exhibit poor crystalline quality and often are polycrystalline. Chemical vapor-deposited layers grown at a substrate temperature of 900 °C, again are good quality epitaxial layers. In this case, in addition to dislocations, stacking faults are present. All the studied layers are highly conductive and p-type. The conduction and valence band discontinuities determined from electrical measurements are 0.05±0.04 eV and 0.39±0.04 eV, respectively

Measuring functional ability in healthy ageing: a nationwide cross-sectional survey in the Philippine older population

OBJECTIVES: To analyse the empirical support of the functional ability concept in the healthy ageing framework developed by the WHO in a sample of the Philippine older population. According to this framework, environmental factors may enhance or hinder functional ability, which is the person's ability to do what they value, broadly represented by subjective well-being. Moreover, this network of relationships may be moderated by personal characteristics such as gender. DESIGN: Cross-sectional observational study. SETTING: Philippines, general population. PARTICIPANTS: Respondents of the 2016 National Disability Prevalence Survey/Model Functioning Survey aged 50+ (N=2825). PRIMARY AND SECONDARY OUTCOME MEASURES: Latent (unobserved) measures of functional ability, environmental factors (physical environmental factors and social network and support) and subjective well-being (positive affect, negative affect and evaluative well-being) were obtained from different items from the survey questionnaire using a SEM framework. RESULTS: We found that the relationship between environmental factors and the three components of subjective well-being considered in this study was partially explained by differences in functional ability. The portion of those effects accounted for by functional ability was comparatively larger for the physical than for the social environmental factors. We found no evidences of gender differences in this network of relationships. CONCLUSIONS: These findings suggest the relevance of functional ability at explaining the relationship between environmental factors and subjective well-being in older adults. Future studies may replicate these findings longitudinally and including other relevant measures as the person's objective level of intrinsic capacity

A comparison of morphometric traits of sheep breeds of Karnataka in the farmers' flocks

The study revealed that the 4 sheep breeds of Karnataka differed significantly with respect to body weight and other physical traits. Kenguri and Bellary breeds of sheep were larger and heavier than Hassan and Mandya sheep breeds. Amongst them Kenguri rams were heaviest followed by Bellary, Mandya and Hassan but in ewes the above order was reversed in breeds of southern Karnataka. The overall difference in body weights of Kenguri and Bellary rams was 9.66 kg whereas in Hassan and Mandya rams it was 4.37 kg. The corresponding values in ewes were 4.06 kg and 1.5 kg. In ewes, the increase in magnitude of a morphometric trait from a lower age group to next higher age group was marginal. All the sheep breeds of Karnataka attained maximum weight at 8-tooth age

Building capacity without disrupting health services: public health education for Africa through distance learning

The human resources crisis in Africa is especially acute in the public health field. Through distance education, the School of Public Health of the University of the Western Cape, South Africa, has provided access to master's level public health education for health professionals from more than 20 African countries while they remain in post. Since 2000, interest has increased overwhelmingly to a point where four times more applications are received than can be accommodated. This home-grown programme remains sensitive to the needs of the target learners while engaging them in high-quality learning applied in their own work contexts