We had investigated effects of the high-energy proton irradiation on the properties of radiation detectors fabricated as
Schottky diodes on 4H-SiC. The doses of 24 GeV protons ranged from 1013 cm-2 up to 1016 cm-2. Numbers and activities
of radionuclides and isotopes produced after the irradiation were analysed. Activities of 7Be and 22Na were found to be
proportional to the irradiation dose and ranged from 1.3 up to 890 Bq and from 1.9 up to 950 Bq, respectively. The
contact properties were investigated by means of the current-voltage analysis. At lower irradiation doses a slight
decrease of the effective potential barrier height from about 0.75 eV down to < 0.7 eV took place. The reverse current of
the diodes grew by up to one order of magnitude. At the doses above 3x1015 cm-2 opposite changes were observed.
Irradiation by up to 1x1016 protons/cm-2, resulted in the increase of the potential barrier height up to ~ 0.85 eV, followed
by the drop of the reverse current by up to two orders of magnitude. The observed effects were explained by the
appearance of the disordered material structure because of the high-energy particle bombardment.
We have investigated properties of poly(9-vinylcarbazole) (PVK) doped with 30% wt 4-dibutylamino-4'-nitrostilbene
(DBANS), depending on orientation of the polar DBANS molecules. Appearance of the orientation-induced built-in
electric field was proven optically by the solid electric field induced second harmonic generation and electrically by the
current-voltage characterization. Modification of optical properties was evidenced by the spectral dependencies of the
absorption coefficient. The thermally stimulated current spectra had demonstrated that carrier transport and trapping are
affected, too.
We had investigated photoelectrical and current transient properties of TlBr in the temperature region from -20 C up to
+20 C, in which the effect of ionic conductivity changes significantly. The evaluated thermal activation energy value of
conductivity of about 0.78 eV could stand for several mechanisms, e.g., electronic and ionic conductivity. From the
spectral dependencies of photocurrent several defect-related maxima in the region between 1 and 2.5 eV were identified,
being dependent on sample prehistory. Maximum at about 1.2 eV had appeared if the spectra were scanned from the
high to the low quantum energies. Meanwhile the height of the maximum at about 2.1-2.2 eV used to decrease if the
sample was kept biased for several hours. The transient photocurrent kinetics had demonstrated a complex behaviour
that could be explained either by the combined trapping and/or recombination of light-generated carriers to the defect
centres associated with potential inhomogeneities, followed by the growing ionic conduction or by a diffusion-related
scattering variation in time. The evaluated thermal activation energy of the time constants of both processes was found to
be 0.24 - 0.27 eV. The obtained results indicate that ionic conductivity can take place also at -20 C, though its influence
is less because of the thermally activated character.
Earlier we reported the investigation of the electrical properties of selectively doped and degenerate CdS/ZnSe quantum heterostructures grown by molecular beam epitaxy. The maximum Hall mobilities in these heterostructures were nevertheless still inferior to 400 cm2/Vs. The purpose of the present work was to optimize these quantum structures in order to increase the carrier mobility and to analyze in detail the scattering mechanisms. We demonstrate that the Hall mobility in the CdS quantum well (QW) can reach 2800 cm2/Vs for slightly doped structures at low temperatures and that it is mostly limited by interface alloying scattering.
We investigated carrier transport and capture in poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) Schottky diodes by thermally stimulated currents and current-voltage characteristics. Experimentally in the region from 80 K up to 450 K two distinct current peaks were found after the white light excitation. Their maxima were located in the temperature regions 214 - 244 K and 304 - 394 K respectively. The detailed numerical modeling revealed that the full TSC is a superposition of at least six traps and/or other thermally stimulated processes with different parameters. We observed effect of oxygen on these traps that was never reported before. The filling of three of these levels could be increased significantly by exposing the sample to the air. The two deepest traps with activation energies at about 0.76 - 0.8 eV and 0.76 - 0.9 eV are likely located nearby surface. Meanwhile the mid-deep trap with an activation energy 0.45 - 0.55 eV is most probably distributed over the sample depth. As far as these traps are related to the oxygen they could be identified as electron traps. In contrast none of the traps could be recharged by applied voltage. Instead the injected carriers created a long-living sample polarization. The non-exponential depolarization lasted for several thousands seconds and was not thermally activated even above the glass transition temperature. These facts make it necessary to include into analysis other possible physico-chemical mechanisms, e.g., reversible chemical reactions or chain structure reorganization induced by electric field.
We investigated by the thermally stimulated currents (TSC) high purity high resistivity GaAs films, grown by vapor phase epitaxy (VPE). To reveal the single levels the thermal emptying of the defects by fractional heating was used. We also investigated the thermally stimulated depolarization currents (TSD) in samples, which were excited and polarized by light in the presence of an electric field. The following prevailing levels below the conductivity band were found: 0.21-0.23, 0.31-0.33, 0.35-0.38, 0.45-0.49 and 0.54 eV. We demonstrate that the TSC and TSD current spectra depend sensitively on the excitation conditions. So light excitation with quantum energy higher than 0.83 eV reveals the level with an activation energy of 0.21-0.23 eV, which dominates over the temperature range from about 95 to 200 K. Most probably these defects are located within the VPE layer, and have relatively small effective electron capture and generation coefficients. Moreover, at low temperatures their effective ionization energy is supposed to increase because of the existence of potential barriers. By contrast, if the quantum energy is 0.5-0.83 eV the influence of this level diminishes, and the contribution of donor levels at about 0.13 and 0.17 becomes visible. This is evidence of carrier redistribution between different defects. Furthermore, peculiarities of the TSC were observed which could not be explained by a homogeneous semiconductor model. The existence of different polarization sources in different temperature ranges is demonstrated by TSD. In particular, the inhomogeneous sample polarization causes the scattering of the activation energy values. Our results prove the influence of the potential relief of the band gap, appearing due to microinhomogeneities of the samples. Furthermore, excitation by 0.5-0.83 eV light could lead to the formation of different (p-type) conductivity channels around dislocations through the metastable transformation of the EL2 level.
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