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Register a new userSpace charge formation in chromium compensated GaAs radiation detectors
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Space charge formation in chromium-compensated GaAs sensors is investigated by the laser-induced transient current technique applying pulsed and DC bias. Formation of non-standard space charge manifested by an appearance of both negatively and positively charged regions in DC biased sensors was revealed during 5 ms after switching bias. Using Monte Carlo simulations of current transients we determined enhanced electron lifetime {tau} = 150 ns and electron drift mobility {mu}d = 3650 cm2/Vs. We developed and successfully applied theoretical model based on fast hole trapping in the system with spatially variable hole conductivity.
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Thin coatings of Chromium oxide have been used for applications as absorbing material in solar cells, as protections for magnetic data recording devices and as shields in flexible solar cells. Thin coatings of pure chromium were vacuum deposited on a glass substrate using hot electrons from tungsten filament. These coatings were then treated with a nanosecond and femtosecond laser in ambient conditions. The microstructure, morphology and the color of the coatings treated with laser sources were modified and there was a formation of an oxide layer due to the heat dissipation on the chromium coating from the energetic photons. High-resolution scanning electron microscope studies showed the morphological evolution that are directly correlated with the laser fluence of both the nanosecond and femtosecond lasers. This morphological evolution was accompanied by the microstructural change as observed from the x-ray diffraction patterns, the chromaticity response of the coating was studied by UV-Vis spectrometer and the response of the coating in the visible region evolved with the laser fluences. The Rutherford backscattering depth profiling of the laser treated coatings revealed the diffusion of oxygen atoms in the coating as a result of laser treatment fluence.
We report here a general theory describing photoelectron transportation dynamics in GaAs semiconductor photocathodes. Gradient doping is incorporated in the model through the inclusion of directional carrier drift. The time-evolution of electron concentration in the active layer upon the injection of an excitation pulse is solved both numerically and analytically. The predictions of the model are compared with experiments via carrier-induced transient reflectivity change, which is measured for gradient-doped and uniform-doped photocathodes using femtosecond pump-probe reflectometry. Excellent agreement is found between the experiments and the theory, leading to the characterization of key device parameters such as diffusion constant and electron decay rates. Comparisons are also made between uniform doping and gradient doping for their characteristics in photoelectron transportation. Doping gradient is found to be able to accelerate electron accumulation on the device surface. These results offer new insights into the dynamics of III-V photocathodes and potentially open a new avenue toward experimental characterization of device parameters.
The motivation for investigating the use of GaAs as a material for detecting particles in experiments for High Energy Physics (HEP) arose from its perceived resistance to radiation damage. This is a vital requirement for detector materials that are to be used in experiments at future accelerators where the radiation environments would exclude all but the most radiation resistant of detector types.
The layers of a high-temperature novel GaAs:Fe diluted magnetic semiconductor (DMS) with an average Fe content up to 20 at. % were grown on (001) i-GaAs substrates using a pulsed laser deposition in a vacuum. The transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy investigations revealed that the conductive layers obtained at 180 and 200 C are epitaxial, do not contain any second-phase inclusions, but contain the Fe-enriched columnar regions of overlapped microtwins. The TEM investigations of the non-conductive layer obtained at 250 C revealed the embedded coherent Fe-rich clusters of GaAs:Fe DMS. The X-ray photoelectron spectroscopy investigations showed that Fe atoms form chemical bonds with Ga and As atoms with almost equal probability and thus the comparable number of Fe atoms substitute on Ga and As sites. The n-type conductivity of the obtained conductive GaAs:Fe layers is apparently associated with electron transport in a Fe acceptor impurity band within the GaAs band gap. A hysteretic negative magnetoresistance was observed in the conductive layers up to room temperature. Magnetoresistance measurements point to the out-of-plane magnetic anisotropy of the conductive GaAs:Fe layers related to the presence of the columnar regions. The studies of the magnetic circular dichroism confirm that the layers obtained at 180, 200 and 250 C are intrinsic ferromagnetic semiconductors and the Curie point can reach up to at least room temperature in case of the conductive layer obtained at 200 C. It was suggested that in heavily Fe-doped GaAs layers the ferromagnetism is related to the Zener double exchange between Fe atoms with different valence states via an intermediate As and Ga atom.
Experimental observations have long-established that there exists a smooth roll-off or knee transition between the temperature-limited (TL) and full-space-charge-limited (FSCL) emission regions of the emission current density-temperature J-T (Miram) curve, or the emission current density-voltage J-V curve for a thermionic emission cathode. In this paper, we demonstrate that this experimentally observed smooth transition does not require frequently used a priori assumptions of a continuous distribution of work functions on the cathode surface. Instead, we find the smooth transition arises as a natural consequence of the physics of nonuniform thermionic emission from a spatially heterogeneous cathode surface. We obtain this smooth transition for both J-T and J-V curves using a predictive nonuniform thermionic emission model that includes 3-D space charge, patch fields (electrostatic potential nonuniformity on the cathode surface based on local work function values), and Schottky barrier lowering physics and illustrate that a smooth knee can arise from a thermionic cathode surface with as few as two discrete work function values. Importantly, we find that the inclusion of patch field effects is crucial for obtaining accurate J-T and J-V curves, and the further inclusion of Schottky barrier lowering is needed for accurate J-V curves. This finding, and the emission model provided in this paper have important implications for modeling electron emission from realistic, heterogeneous surfaces. Such modeling is important for improved understanding of the interplay of emission physics, cathode materials engineering, and design of numerous devices employing electron emission cathodes.
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