Low-temperature positron transport in semi-insulating GaAs

Positron diffusion and drift in semi-insulating (SI) GaAs in the temperature range of 50-300 K were studied by the slow-positron beam technique. Both the temperature-dependent positron diffusion coefficient and positron mobility were measured independently using the method reported recently [Y. Y. Shan et al., Phys. Rev. B 54, 1982 (1996)]. The experimental results are consistent with the Einstein relation. The diffusion coefficient and mobility approximately follow D +(T)=9400T -β cm 2 s -1, and μ +(T)=10 8×T -σ cm 2 V -1 s -1, with β=1.5±0.1, and σ=2.5±0.2, respectively in the temperature range of 50-300 K. The results are consistent with scattering from optical-phonon modes as the dominant scattering process for positron transport in GaAs (SI) in this temperature range. No trapped positron states were observed to 50 K.published_or_final_versio

Field effect on positron diffusion in semi-insulating GaAs

An energy-tunable monoenergetic positron beam was used to study positron diffusion in the space-charge region of an Au/GaAs(SI) (semi-insulating) Schottky contact, where the electric field reaches ∼105 V cm-1 by reverse biasing the diode. An analytical solution of the time-dependent positron drift-diffusion model under an electric field was obtained for the case of a semi-infinite body with a capturing boundary, and explains the experimental results well. A positron diffusion coefficient of 1.8±0.2 cm2 s-1, and a positron mobility of 70± 10 cm2 V-1 s-1 in GaAs(SI) at 300 K, were obtained independently. This result is consistent with the Einstein relation. The dependence of the positron current density at the Au/GaAs interface on the electric field shows that GaAs(SI) is a possible candidate for the fabrication of the field-assisted positron moderator.published_or_final_versio

Microscopic structure of DX centers in Cd 0.8Zn 0.2Te:Cl

Photoexcitation of chlorine DX centers induces a transition of the Cl atoms to the shallow-donor state and persistent photoconductivity at low temperature in Cd 0.8Zn 0.2Te:Cl. The relaxation of the substitutional Cl atoms to the DX state at 140 K is coincident with a decrease of the positron line-shape parameter and an increase of annihilation with high-momentum core electrons. The results indicate positron trapping and annihilation at DX centers and at chlorine A centers. The data support the bond breaking model of the DX centers and the outward relaxation of the Cl and Cd(Zn) atoms alone the [111] direction. The thermal barrier for the shallow-deep transition was found to be 0.44 eV.link_to_subscribed_fulltex

Visible light emission from silicon implanted and annealed SiO2 layers

Silicon implanted and annealed SiO2 layers are studied usin g photoluminescence (PL) and positron annihilation spectroscopy (PAS). Two PL emission bands are observed. a band centered at 560 nm is present in as-implanted samples and it is still observed after 1000 degrees C annealing. The emission time is fast. A second band centered at 780 nm is detected after 1000 degrees C annealing. The intensity of the 780 nm band further increased when hydrogen annealing was performed. The emission time is long (1 mu s to 0.2 ms). PAS results show that defects produced by implantation anneal at 600 degrees C. Based on the annealing behavior and on the emission times, the origin of the two bands is discussed

The microscopic structure of DX centers in Cd 0.8Zn 0.2Te:Cl

The microscopic structure of chlorine DX centers was studied in Cd0.8Zn0.2Te:Cl by positron annihilation and thermoelectric current measurements. Photoexcitation of the DX centers induces a transition of the Cl atoms to the shallow-donor state and persistent photoconductivity (PPC) at low temperature. The DX state is recovered by annealing at 140 K where the relaxation of the substitutional Cl atoms to the DX state was observed. The shallow-deep transition is coincident with a sharp decrease of the positron line-shape parameter and a significant increase of annihilation with high-momentum core electrons. The results indicate positron trapping and annihilation at chlorine A centers after photoexcitation and at DX centers after annealing. Among the three configurations proposed for Cl DX centers in Cd 1-xZn xTe the data support the bond-breaking geometry and the relaxation of the Cl and Cd(Zn) atoms along the [111] direction. The thermal barrier for the shallow-deep transition was found to be 0.44 eV.link_to_subscribed_fulltex

A 0. 5 to 3. 0 MeV monoenergetic positron beam

An adjustable, 0.5--3 MeV monoenergetic positron beam has been constructed at Brookhaven. Currently a /sup 22/Na source with a W(100) foil transmission moderator produces a 1.1 mm FWHN beam with an intensity of 3/times/10/sup 5/ e/sup +//sec at a target located downstream from the accelerator. The divergence of the beam is less than 0.1/degree/ at 2.2 MeV energy. A SOA gun with 2 lens transport system brings the beam to a focus at the entrance of an electrostatic 3 MeV Dynamitron accelerator. The post acceleration beam transport system comprises 3 focusing solenolds, 4 sets of steering magnets and a 90/degree/ double focusing bending magnet. The beam energy spread at the target is <1 keV FWHN deduced from the beam size. Below we describe the positron extraction optics and acceleration, the construction of the beamline and the beam diagnostic devices. The salient beam parameters are listed at the end of this paper. 2 refs., 3 figs., 1 tab