Semi-insulating GaAs: A possible substrate for a field-assisted positron moderator

Abstract

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...

Plasma and trap-based techniques for science with positrons

“…Moderation efficiencies are at most ∼ 10 −3 with positron energy spreads of < 1eV. Other materials that have been discussed as positron moderators include diamond (Al-Qaradawi et al, 2002;Brandes et al, 1997;, silicon carbide (Stormer et al, 1996), gallium arsenide (Shan et al, 1994) and gallium nitride (Jørgensen and Schut, 2008).…”

EL2Deep Donor State in Semi-Insulating GaAs Revealed by Frequency Dependent Positron Mobility Measurements

“…where Pb; is the intrinsic band bending (built in potential) of the contact, V(t) is the applied bias, 1(t) is the residual current fiowing through the sample, R is the bulk series resistance, and e is the permittivity of the material [15,18]. According to (4), if the applied bias is fixed or slowly varying, w(t) will collapse onto the positron injecting contact as s(t) increases.…”

“…According to (4), if the applied bias is fixed or slowly varying, w(t) will collapse onto the positron injecting contact as s(t) increases. Within the same depletion approximation [15,18] the electric field E(x, t) inside our samples, both of width d, for~~d is given by:…”

“…In (3) nT has been taken as the EL2 concentration which is 1.5 X 10'6 cm 3. In (4) e has been taken as 12.4ep, Pb;, as 0.3 eV [18],and 1(t) was obtained from dc experiments at various temperatures. In (5) the bulk resistance R is estimated to be 1.3 X 10 0 from the sample resistivity and dimensions.…”

“…In (5) the bulk resistance R is estimated to be 1.3 X 10 0 from the sample resistivity and dimensions. In (6) the mean implantation depth xp for GaAs is taken as 40 pm [18]. In (7), since we are not aware of any theoretical v(E) relation for polaroptic phonon scattering, the relationship v+(E) is taken as the full Bardeen The reasonable fits to our data indicate the basic correctness of our interpretation, namely that the rise in positron mobility just below room temperature is an artifact produced by the enhancement of the mean-electric field experienced by positrons in the samples.…”