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 mobility measurements carried out on semi-insulating GaAs, using the Doppler shift in annihilation radiation technique, show a sharp transition from a high mobility value -120 cm'-V ' s ' to a lower value -45 cm' V ' s ' just below room temperature. The temperature of the transition is found to be dependent on the frequency of the applied AC bias. We show that this effect is an artifact due to the thermal ionization of the EL2 deep donor state, which in its ionized state forms a positive
Positron annihilation spectroscopy provides a new probe to study the properties of interface traps in metal-oxide semiconductors (MOS). Using positrons, we have examined the behavior of the interface traps as a function of gate bias. We propose a simple model to explain the positron annihilation spectra from the interface region of a MOS capacitor.
The I-V characteristics of Aulsemi-insulating InP (100) under reversed bias have been measured between 230 K and '290 K. A simple model based on the thermionic field emission theory (TFE) is proposed to describe this system. Based on this model, the value of the bandgap obtained from the C V characteristics is in reasonable agreement with the literature. At room temperature, the corresponding fitted values of the barrier height Qb and the series bulk resistance R, are 0.68 0.05V and 1.32 x IO'R respectively.
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