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Abstract. Positron lifetime spectroscopy measurements have been carried out for semi-insulating GaAs with applied electric fields in the samples directed towards, and away from the positron injecting contact. The lifetime spectra have been decomposed into two components, the longer of which (~ 400 ps) is characteristic of open volume defects at the metal-semiconductor interface through which positrons are injected. The interesting feature of these experiments is the large increase in the intensity of this interface component as the field is directed towards the contact. We show that this increase is caused by a significant fraction of implanted positrons drifting under the influence of a strong electric field produced by a layer of space-charge formed adjacent to the positron injecting contact. The general trend of the intensity variation is well explained by the proposed model. Experiments involving the application of an ac bias to the samples strengthen the suggestion that the space charge region is largely formed from ionized EL2 donors. The results of the present work indicate that semi-insulating GaAs possesses properties that make it a suitable material for the fabrication of a high efficiency (~ 10%) room-temperature field-assisted positron moderator. The extraction of positrons from the GaAs substrate into the vacuum through a thin metalization is discussed based upon available positron affinities for the GaAs and various elemental metals. These data suggest that a few monolayers of a strongly electronegative metal such as Au or Pd may allow vacuum emission through quantum tunneling. These studies are important in their own right as a means of checking mechanisms of carrier transport in materials. They also have more direct technological relevance in that materials exhibiting large positron diffusivities/mobilities can often be employed as efficient moderators for low energy positron beams [7]. The motivation behind the present work on SI (Semi-Insulating) GaAs was initially in the former category, the intention being to make #+ measurements using the lifetime spectroscopy technique previously used for Si [8]. However, the unexpected results obtained, together with their lack of sensitivity towards #+, have led us to conclude that the relevance of the present study may be more in the second category; namely that SI-GaAs may be a good material from which to form an efficient FA (Field-Assisted) moderator of positrons.FA moderation is the name given to a particular type of process for slowing fi+ radiation, by which a significant fraction of positrons are, after implantation into either an insulator or semiconductor stressed with an electric field, enabled to avoid annihilation by drifting through the material into the vacuum. Since positron drift distances in an electric field can exceed typical thermal diffusion distances by approximately two orders of magnitude, the FA moderator has ultimately greater potential for super-efficient generation of slow positrons [9]. Calculations show that such moderators c...

Positron lifetime measurements performed on Au/GaAs samples at room temperature with an applied square-wave ac bias show a frequency dependent interface related lifetime intensity that peaks around 0.4 Hz. The observation is explained by the ionization of the deep-donor level EL2 to EL2 ϩ in the GaAs region adjacent to the Au/GaAs interface, causing a transient electric field to be experienced by positrons drifting towards the interface. Without resorting to temperature scanning or any Arrhenius plot the EL2 donor level is found to be located 0.80Ϯ0.01Ϯ0.05 eV below the conduction-band minimum, where the first error estimate is statistical and the second systematic. The result suggests positron annihilation may, in some instances, act as an alternative to capacitance transient spectroscopies in characterizing deep levels in both semiconductors and semi-insulators.

An energy-tunable monoenergetic positron beam was used to study positron diffusion in the space-charge region of an Au/GaAs͑SI͒ ͑semi-insulating͒ Schottky contact, where the electric field reaches ϳ10 5 V cm Ϫ1 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 1.8Ϯ0.2 cm 2 s Ϫ1 , and a positron mobility of 70Ϯ10 cm 2 V Ϫ1 s Ϫ1 in GaAs͑SI͒ 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 GaAs͑SI͒ is a possible candidate for the fabrication of the field-assisted positron moderator. ͓S0163-1829͑96͒04428-1͔