MEL LOCH ET AL quality single crystals. These materials are arsenides such as GaAs, AIGaAs, and InGaAs containing arsenic clusters. The composites are formed by incorporating excess arsenic in the semiconductor, which per cipitates in the anneal. The incorporation of the excess arsenic is accomplished by molecular beam epitaxy at low substrate temperatures. The cluster density can be controlled with the coarsening annealing. The positioning of the clusters can be controlled with heterojunctions and doping. These composites exhibit several interesting properties, including high-resistivity, appreciable optical absorption below the band gap of the semiconductor matrix material, a large electro-optic effect, and very short carrier lifetimes.
The photoexcited carrier lifetimes in ex situ-annealed low temperature growth GaAs are measured with a femtosecond transient absorption experiment. The study encompassed two low temperature growth GaAs films with approximately 0.3% and 0.9% excess arsenic incorporated during growth. The observed lifetimes are found to be a function of the spacing of arsenic precipitates formed during the 30 s anneals to temperatures between 650 and 1000 "C!. The carrier lifetime for unannealed films was found to be less than-200 fs. The carrier lifetimes increased from-2 to-10 ps 'as the average precipitate spacing was increased from-400 to-900 A. These results are in sharp contrast to recent reports of subpicosecond lifetimes in similar GaAs annealed at 600 "C.
In this paper, we report on the investigation of silicon avalanche photodiodes (APDs) for high-energy photon imaging applications. This includes a new APD design that provides X-ray and-ray imaging with significant reduction in electronic readout requirements. This new APD design, referred to as position-sensitive avalanche photodiode (PSAPD), involves charge sharing amongst the electrodes that enable determination of position of interaction. PSAPDs with 14 14 mm 2 area have been fabricated using planar processing. The performance of these devices has been evaluated for energy resolution, timing resolution (4 ns full-width at half-maximum), and spatial resolution (300 m intrinsic spatial resolution). The potential of these APDs in high-energy physics and medical imaging is addressed.
GaAs epilayers were grown with a wide range of excess arsenic concentrations and subjected to various anneals to study the role of the point defects and arsenic precipitates in carrier trapping and recombination. Prior to anneal, the point defects rapidly trap photogenerated electrons and holes—usually on subpicosecond time scales. However, full electron-hole recombination occurs on a significantly longer time scale. After anneal, the full electron-hole recombination lifetime appears to be greatly reduced, indicating that the arsenic precipitates play a significant role.
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