Evolution of impurity incorporation during ammonothermal growth of GaN

Ammonothermally grown GaN is a promising substrate for high-power optoelectronics and electronics thanks to its scalability and high structural perfection. Despite extensive research, ammonothermal GaN still suffers from significant concentrations of impurities. This article discusses the evolution of impurity incorporation during growth of basic ammonothermal GaN, in specific whether the impurity concentration changes temporally along the growth direction and how the autoclave influences the impurity concentration. The effect of the impurities on the structural, electrical and optical properties of the grown crystal is also discussed. The chemical analysis is carried out by time of flight secondary ion mass spectroscopy (ToF-SIMS) and laser-ablation inductively-coupled plasma mass spectroscopy (LA-ICP-MS). Strain and dislocation generation caused by impurity concentration gradients and steps are studied by synchrotron radiation x-ray topography (SR-XRT). Fourier transform infrared (FTIR) reflectivity is used to determine the effect of the impurities on the free carrier concentration, and the luminescent properties are studied by low temperature photoluminescence (PL). The influence of the autoclave is studied by growing a single boule in multiple steps in several autoclaves. LA-ICP-MS and ToF-SIMS ion intensities indicate that the impurity concentrations of several species vary between different autoclaves by over an order of magnitude. SR-XRT measurements reveal strain at the growth interfaces due to impurity concentration gradients and steps. Oxygen is determined to be the most abundant impurity species, resulting in a high free carrier concentration, as determined by FTIR. The large variation in Mn concentration dramatically affects PL intensity

Structural and electrical characterisation of ion-implanted strained silicon

The production of low resistance ultra-shallow junctions for e.g. source/drain extensions using low energy ion-implantation will be required for future CMOS devices [1]. This architecture will require implants which demonstrate high electrical activation and nm range depth profiles. We investigate the properties of Sb implants in tensile strained silicon due to their potential to satisfy these criteria, and the carrier mobility enhancements associated with tensile strained silicon. Low energy (in this case 2 keV)implants coupled with Sb’s large atomic radius are capable of providing ~ 10 nm implant depths. In addition to this, Sb, in the presence of tensile strain demonstrates higher electrical activation when compared with the more traditional n-type dopant As [2]. We now report on the initial results of an ongoing systematic study over a wide silicon tensile strain range (from 0.4 to 1.25 % strain) in order to establish clear strain-related trends. Graded Si1-xGex virtual substrates (VS) with are used as template substrates, upon which tensile strained Si layers are grown. Prior to implantation the 0.1 ≤ x ≤ 0.3 quality of the strained layer and SiGe buffer is assessed using UV micro-Raman spectroscopy (μRS), synchrotron x-ray topography (SXRT) and high resolution x-ray diffraction (HR-XRD). For measurements of strain following implantation, HR-XRD is found to be more useful than μRS due to additional carrier-concentration induced Si Raman peak shifts in the Raman spectra , these obscure small changes in the strain state, and result from the degenerate doping levels achieved in these samples (~7x1020 cm-3). Using x-ray techniques, we find clear evidence of tilt in the SiGe VS at Ge concentrations > 23% (i.e. ε > 0.9 %), this tilt impacts on the quality of the strained Si. In addition to this, stacking faults have been detected non-destructively in the higher strain samples (ε = 1.25%, VS = Si0.7Ge0.3) using SXRT in transmission mode

Genome-wide association identifies nine common variants associated with fasting proinsulin levels and provides new insights into the pathophysiology of type 2 diabetes

OBJECTIVE - Proinsulin is a precursor of mature insulin and C-peptide. Higher circulating proinsulin levels are associated with impaired b-cell function, raised glucose levels, insulin resistance, and type 2 diabetes (T2D). Studies of the insulin processing pathway could provide new insights about T2D pathophysiology. RESEARCH DESIGN AND METHODS - We have conducted a meta-analysis of genome-wide association tests of ;2.5 million genotyped or imputed single nucleotide polymorphisms (SNPs) and fasting proinsulin levels in 10,701 nondiabetic adults of European ancestry, with follow-up of 23 loci in up to 16,378 individuals, using additive genetic models adjusted for age, sex, fasting insulin, and study-specific covariates. RESULTS - Nine SNPs at eight loci were associated with proinsulin levels (P < 5 × 10-8). Two loci (LARP6 and SGSM2) have not been previously related to metabolic traits, one (MADD) has been associated with fasting glucose, one (PCSK1) has been implicated in obesity, and four (TCF7L2, SLC30A8, VPS13C/ C2CD4A/B, and ARAP1, formerly CENTD2) increase T2D risk. The proinsulin-raising allele of ARAP1 was associated with a lower fasting glucose (P = 1.7 3 10-4), improved b-cell function (P = 1.1 × 10-5), and lower risk of T2D (odds ratio 0.88; P = 7.8 × 10-6). Notably, PCSK1 encodes the protein prohormone convertase 1/3, the first enzyme in the insulin processing pathway. A genotype score composed of the nine proinsulin-raising alleles was not associated with coronary disease in two large case-control datasets. CONCLUSIONS - We have identified nine genetic variants associated with fasting proinsulin. Our findings illuminate the biology underlying glucose homeostasis and T2D development in humans and argue against a direct role of proinsulin in coronary artery disease pathogenesis