Hole Trapping Sites Research Articles

Quasi-nondestructive readout of holographically stored information in photorefractive Bi12SiO20 crystals

The hologram fixing process in a Bi12 SiO20 crystal arises due to the formation of a complementary grating of positive charges localized in shallow traps. Uniform illumination of the crystal with blue or green light erases the electronic charge pattern but leaves the positive charge grating undisturbed. Due to the smaller mobility lifetime product of holes, this grating decays at room temperature with a time constant that is much longer than that of the electronic grating. We show in this letter that the readout time constant can be further increased considerably by cooling the crystal. Images retrieved from a crystal kept at 0 °C temperature and under continuous illumination for a few hours are presented. The energy levels of the hole trapping sites involved in this process are found to be situated at 0.56 and 1.1 eV above the valence band.

Relaxation of Trapping Sites in the ZnSe Films of AuZnSeSiO2Si and AuZnSeGe Structures

A thermodepolarization analysis is made of trapping sites in the ZnSe layer of multi-layer AuZnSeSiO2Si and AuZnSeGe structures. A hole trapping site with a cross-section typical of neutral centers, is found. The TS activation energy is found to increase with TS filling, and not to decrease, as is always the case. This fact is explained in terms of a Schottky barrier model, where the semiconductor layer has a thickness comparable to the depth of the spatial charge region. According to the proposed model, the energy level, wich corresponds to the trapping site, is distanced from ZnSe valent band bottom by no less than 0.91 eV. [Russian Text Ignored].

Further consideration of the track−interaction model for thermoluminescence in LiF(TLD−100)

At the Second International Conference on Luminescence Dosimetry in Gatlinburg in 1968 Claffy, Klick, and Attix introduced the track−interaction model in an attempt to explain the phenomena of supralinearity and sensitization generally observed in the thermoluminescence of LiF(TLD−100). Their proposal in general terms was that the electron−hole traps are located near the paths of the energetic charged particles (primary or secondary) which traversed the crystal during ionizing irradiation, and that this nonrandom spatial distribution affects the thermoluminescence process both with respect to its dependence upon dose and upon linear energy transfer. Moreover the observed sensitization of the phosphor, after receiving a large radiation dose followed by a partial annealing, was attributed to the survival of luminescence centers. In the present work a very simplistic model has been adopted for purposes of estimating how the average distance between luminescence centers along a charged−particle track compares with the average distance between tracks. Magnesium centers, occurring at ?68−Å intervals, are assumed to provide trapping sites both for electrons and holes. The luminescence centers are taken to be F centers associated in some way with the titanium centers which occur at ?127−Å intervals. For the Compton secondary electrons resulting from 60Co γ−ray interactions, the average separation of ionizing events along the track is ?350 Å, and the average separation of F centers is ?2300 Å, resulting from about every seventh ionizing event. The average separation of tracks for a γ−ray exposure of 103 R is ?660 Å, which is roughly the same as the average separation of trapped charge carriers along a given track from their nearest F center in the same track. This is the same exposure level at which supralinearity of TL response vs exposure begins to appear for 60Co γ irradiation. The more densely ionizing tracks of heavy charged particles are shown to be much more widely separated relative to the luminescence center intervals in each track; thus supralinearity should not be expected in that case, in agreement with observations. An important feature of this paper is the estimation of an energy budget for LiF(TLD−100), beginning with the deposition of 60Co γ−ray dose in the crystal, and terminating in the emission of 0.04% of that energy as thermoluminescence. Reasonable avenues are suggested by which the energy losses may occur, and estimates are made of the energy fraction in each category, based on available evidence.

Analysis of thermal glow curves in rare-earth-doped CaF2 crystals

The main parameters, that is, thermal activation energies, frequency factors, and order of kinetics, are calculated for some glow peaks in rare-earth-doped CaF2. From the results, it is inferred that the hole trapping sites are in close proximity to the rare-earth ions.