Substantial P-Type Conductivity of AlN Achieved via Beryllium Doping.

Beryllium has long been predicted by first principle theory as the best p-type dopant for GaN and AlN. But experimental validation of these theories has not, until now, borne out the original predictions. A key challenge is the dopant-induced strain leading to Be rejection from substitutional sites in favor of interstitial sites, leading to self-compensation. More flexible growth methods like metal modulated epitaxy (MME) that can operate at substantially lower temperatures than traditional approaches, can more effectively place Be into the proper substitutional lattice sites. MME grown Be-doped AlN shows substantial p-type conductivity with hole concentrations in the range of 2.3 × 1015 -3.1 × 1018  cm-3  at room temperature. While others have achieved sizable carrier concentrations near surfaces via carbon doping or Si implantation, this is the only known demonstration of substantial bulk p-type doping in AlN and is a nearly 1000 times higher carrier concentration than the best previously demonstrated bulk electron concentrations in AlN. The acceptor activation energy is found to be ≈37 meV, ≈8 times lower than predicted in literature but on par with similar results for MME p-type GaN. Preliminary results suggest that the films are highly compensated. A p-AlN:Be/i-GaN:Be/n-GaN:Ge pin diode is demonstrated with substantial rectification.

Comprehensive Analysis of Metal Modulated Epitaxial GaN.

While metal modulated epitaxy (MME) has been shown useful for hyperdoping, where hole concentrations 40 times higher than other techniques have been demonstrated, and the ability to control phase separation in immiscible III-nitrides, the complexity of the dynamically changing surface conditions during the cyclic growth is poorly understood. While MME is capable of superb crystal quality, performing MME in an improper growth regime can result in defective material. These complications have made the transfer of MME knowledge challenging. This work provides a comprehensive study of the conditions necessary for achieving the benefits of MME while avoiding undesirable defects. The effects of growth temperature, Ga/N ratio, and excess Ga dose per MME growth cycle on the morphological, structural, electronic, and optical properties of unintentionally doped (UID) MME grown gallium nitride (GaN) have been investigated. Optimal structural and electrical quality were achieved for GaN films grown at ∼650 °C, at pre-bilayer Ga coverage and at the moderate droplet regime. However, high defect concentrations were observed at the lowest growth temperatures, and counter to traditional MBE, as the excess Ga dose transitioned from bilayer coverage to the low droplet regime. Optoelectronic properties were optimal for films grown at intermediate growth temperatures, an excess Ga dose condition just before the droplet formation, and, at a III/V ratio of 1.3. Optimization of growth temperatures, Ga/N ratios, and excess Ga dose results in a range of growth conditions achieving smooth surfaces, step-flow surface morphology, and high crystalline quality films with low threading dislocation densities, allowing researchers to utilize the extensive advantages of MME.

Self-rated health in a population of expatriate workers and partners in Riyadh, Saudi Arabia.

BACKGROUND: Anthropological theory suggests that expatriate workers progress through a sequence of adaptive stages during their adjustment to a new environment. The psychological and physiological effects of this adaptation process may be reflected in changes in self-rated general health. AIMS: To explore the relationship between self-rated general health, duration of expatriate assignment and two health-related behaviours: physical exercise and cigarette smoking. METHOD: A self-administered questionnaire recorded the demographics, self-rated general health, exercise and smoking behaviours of the adult non-Saudi residents of an expatriate compound in Riyadh, Saudi Arabia. RESULTS: The mean self-rated general health of the study group was better than comparable UK and New Zealand population norms. Self-rated general health was not associated with duration of assignment, but was associated with physical exercise, including a dose-response effect. Middle Eastern expatriates had lower self-rated health scores and a higher prevalence of cigarette smoking than other expatriates. CONCLUSIONS: While the sampling frame limits generalization, physical exercise may promote expatriates' general health. Middle Eastern expatriates may be a target group for smoking health education.