Study of defect annealing by supercurrent proton beam irradiation and of radiation defect profiles in GaAs by the positron annihilation method

The annealing of radiation defects in GaAs by a supercurrent proton beam with currents up to 200 A cm^?2 and 4.8 MeV energy was studied. The positron annihilation method (measurement of angular distribution of annihilation photons) has been used to study the depth profiles of radiation defects in solids. Profiles of vacancy-type defects in GaAs irradiated by a supercurrent proton beam have been measured.     

Effect of doping on positron annihilation in GaAs

The effect of doping on positron annihilation was studied in GaAs single crystals. The positron lifetimes in Si- and Ge-doped crystals (n-type) are similar to that in an instrinsic crystal. On the other hand, the lifetimes in Zn-, Cd-, Cr- and Mn-doped crystals (p-type) are shorter than that in an intrinsic one. The difference is small but significant.     

The study of native defects in as-grown GaAs by positron annihilation

A mono-energetic positron beam was applied for a study of native defects in GaAs. From measurements of Doppler broadening profiles and lifetime spectra of positrons, it was found that Ga-vacancies were introduced by Si-doping. On the other hand, for p-type GaAs, interstitial clusters were found to be introduced by Zn-doping. The observed species of defects are in agreement with those expected from the Fermi level.     

Defect study on electron irradiated GaAs by means of positron annihilation

Measurements of the positron lifetime and Doppler-broadened annihilation-radiation have been performed in electron-irradiated GaAs. The positron lifetime at the irradiation induced defects was 0.250 ns at 300 K. The defect clustering stage was found to occur at around 520–620 K, and the coarsening and annealing stage is believed to be above 620 K. Similar annealing stages were also observed in GaAs lightly doped with Si (0.2×10¹⁸ cm^–3). Both the lifetime and the S-parameter in the irradiated GaAs were found to decrease with temperature from 300 K to 100 K, suggesting the coexistence of shallow traps in electron irradiated GaAs.     

Analysis of semi-insulating GaAs and the role of positron annihilation

The uses of Positron Annihilation (PA) techniques for the analysis of native defects in bulk GaAs are reported. PA has allowed the structure of the metastable state of the important EL2 defect to be examined and has demonstrated that a vacancy is present in the complex. PA has also provided strong evidence that the centre responsible for minority-carrier lifetime degradation in this material is the arsenic vacancy.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Different annealing behaviour of the metastable state in GaAs

We have studied the temperature dependence of the stable state (EL2) recovery after photoquenching in two different materials, grown by the LEC (material A) and the horizontal gradient freeze (B) techniques, by means of optical absorption measurements. The temperature of annealing of the metastable state is 110–120 K in material A and 40–50 K in material B, suggesting the existence of two different defects belonging to the EL2 family. Furthermore, one sample from the B ingot, which has been platically deformed, shows the two thresholds of temperature recovery (50 and 120 K) and also an increase of the EL2 concentration.     

Review of defect investigations by means of positron annihilation in II-VI compound semiconductors

1-x Cd_xTe after post-growth annealing. The experiments on defect generation and annihilation after low-temperature electron irradiation of II-VI compounds are also reviewed. The characteristic positron lifetimes are given for cation and anion vacancies. Received: 19 November 1997/Accepted: 20 November 1997     

Optical potential approach for positron scattering by metastable 2~3S state of helium

The momentum space coupled channels optical(CCO) method for positron scattering has been extended to study the scattering of positrons by metastable helium for impact energies in the range from the positronium threshold up to high energies. Both the positronium formation and ionization continuum channels are included in the calculations via a complex equivalent local potential. The positronium formation, ionization, elastic and 2~3S–2~3P excitation, and total scattering cross sections are all presented and compared with the available information.     

Effect of vacancy charge state on positron annihilation in silicon

The charge-state-dependent lattice relaxation of mono-vacancy in silicon is studied using the first-principles pseu- dopotential plane-wave method. We observe that the structural relaxation for the first-neighbor atoms of the mono-vacancy is strongly dependent on its charge state. The difference in total electron density between with and without charge states in mono-vacancy and its relevant change due to the localized positron are also examined by means of first-principles simu- lation, demonstrating the strong interplay between positron and electron. Our calculations reveal that the positron lifetime decreases with absolute charge value increasing.