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Investigation of Carrier Transport in GaN Single Crystals and Radiation Detectors by Thermally Stimulated Methods

100%
Kažukauskas V., Kalendra V., Vaitkus J.
Acta Physica Polonica A
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2005
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vol. 107
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issue 2
340-345
EN
We investigated single crystals of GaN and thin film GaN radiation detectors by thermally stimulated currents and thermally stimulated depolarization methods in order to characterize carrier transport properties as influenced by material defect structure. In thick GaN no expressed structure of the thermally stimulated current spectra was observed in the temperature range from 100 K up to 350 K, which could be characteristic of the thermal carrier generation from trap levels. The experimental facts imply that the thermally stimulated current spectra might be caused not by carrier generation, but it could be due to thermal mobility changes. Therefore we had applied the numerical analysis by taking into account carrier scattering by ionized impurities and phonons. It was found that mobility limited by ionized impurities varies as T^{2.8} and lattice scattering causes the dependence T^{-3.5}. The highest mobility values were up to 1550 cm^2/(V s) at 148-153 K. Such high values indicate relatively good quality of the single GaN thick crystals. In high resistivity GaN detectors irradiated by high doses of high-energy neutrons and X-rays current instabilities were observed which could be caused by the change of carrier drift paths in a highly disordered matter. A model of carrier percolation transport is presented.
2
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Effect of Trap Levels and Defect Inhomogeneities on Carrier Transport in SiC Crystals and Radiation Detectors

100%
Kažukauskas V., Kalendra V., Vaitkus J.
Acta Physica Polonica A
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2005
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vol. 107
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issue 2
333-339
EN
We present investigation of carrier transport and trapping in 4H-SiC single crystals and high-energy radiation detectors. SiC detectors were produced from bulk vanadium-compensated semi-insulating single crystal 4H-SiC and provided with nickel ohmic and titanium Schottky contacts. The prevailing defect levels were revealed by means of thermally stimulated current and thermally stimulated depolarization methods and their advanced modification - multiple heating technique. From I-V measurements a Schottky barrier height of≈1.9 eV was found. In 4H-SiC:Va the following thermal activation values were deduced: 0.18-0.19 eV, 0.20-0.22 eV, 0.3-0.32 eV, 0.33 -0.41 eV, and 0.63 eV. The maximum with activation energy of 0.33-0.41 eV appears below 125 K and most probably is caused by thermal carrier generation from defect levels. In contrast, the first three maxima with lowest activation energies, which appear at higher temperatures, are likely associated with material inhomogeneities causing potential fluctuations of the band gap. The existence of different polarization sources in different temperature ranges is also demonstrated by thermally stimulated depolarization.
3
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Influence of Irradiation by High-Energy Protons on GaN Detectors

100%
Kažukauskas V., Kalendra V., Vaitkus J., Jasiulionis R.
Acta Physica Polonica A
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2008
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vol. 113
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issue 3
1013-1016
EN
We had investigated effects of the irradiation by 24 GeV protons with doses ranging from 1×10^{14} up to 1×10^{16} p/cm^2 on the properties of GaN ionising radiation detectors. In theγ-spectra of the samples radiation of ^7Be, ^{22}Na, and other long-lived radionuclides with A <70 was identified. Their activities were proportional to the irradiation dozes. Device contact properties were analysed by current-voltage I-V dependences. Created defects were revealed by the thermally stimulated defect spectroscopy. In the less irradiated samples the following values of the effective thermal activation energies were found: 0.12-0.16 eV, 0.18-0.22 eV, 0.35-0.42 eV, and 0.84-0.94 eV. Meanwhile, in the detectors irradiated with the highest doses only current growth with the activation energy of about 0.8-1.0 eV could be identified. Effects of percolation transport in disordered media were proved in the irradiated material.
4
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Charge Carrier Transport in Non-Homogeneous MDMO-PPV Polymer

100%
Kažukauskas V., Pranaitis M., Janonis V.
Acta Physica Polonica A
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2011
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vol. 119
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issue 2
128-130
EN
By thermally stimulated currents we have investigated carrier transport and trapping in [poly-(2-methoxyl, 5-(3,77dimethyloctyloxy)] paraphenylenevinylene (MDMO-PPV). To assure selective excitation of the defect states the spectral width of the exciting light was varied from 1.77 eV up to 3.1 eV. The thermally stimulated current curves were shown to be a superposition of carrier generation from trapping states and thermally stimulated mobility growth. The extrinsic excitation resulted in 0.16 eV photoconductivity effective activation energy values, which decreased down to 0.05 eV for the intrinsic excitation. The deeper states with activation energies of 0.28-0.3 eV and 0.8-0.85 eV were identified, too. The results are direct indication of distributed in energy trapping and transport states with the standard deviation of the density of states of about 0.015 eV.
5
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Photoelectrical Behaviour of DNA:PEDT-PSS Functionalised Films

100%
Kažukauskas V., Pranaitis M., Krupka O., Sahraoui B.
Acta Physica Polonica A
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2011
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vol. 119
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issue 2
151-153
EN
We report investigations of functionalized DNA:PEDT-PSS films. The thermal activation energy of the conductivity near the room temperature was about 0.033 eV. The weak carrier trapping was identified by the thermally stimulated current method, proving the fast recombination of light-generated carriers. A "bistable" photoconductivity below the room temperature was evidenced upon the white light excitation. By cooling the samples down to 145-155 K the photoconduction was small. Below this temperature sudden increase of the photoconductivity was observed. Meanwhile by heating the photosensitivity remained enhanced up to 235-245 K. The slow relaxations of the current after the light excitation took place, the time constant of which reached several hundreds of seconds. Such phenomenon could presumably be attributed to the light-induced changes of the sample material morphology and/or associated variation of carrier transport conditions.
6
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Long-Lasting Current Transient Phenomena in TlBr

76%
Kažukauskas V., Ziminskij A., Vainorius N., Gostilo V., Shorohov M., Bozhko V.
Acta Physica Polonica A
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2011
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vol. 119
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issue 2
268-270
EN
The persistent conductivity effects in TlBr were evidenced at the temperatures below about 200 K after the intrinsic light excitation. They were associated with the filling of the trapping states having thermal activation energy of about 0.08-0.12 eV and accompanied by the appearance of the pronounced maximum of the thermally stimulated currents. In this state the enhanced photoconductivity was observed that could be thermally quenched above about 180 K. The quenching is initiated by the emptying of the trapping states having activation energy of 0.63-0.65 eV.
7
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Effect of Molecular Orientation on Photovoltaic Efficiency and Carrier Transport in a New Semiconducting Polymer

64%
Kažukauskas V., Pranaitis M., Sentein C., Rocha L., Raimond P., Duyssens I., Van Severen I., Lutsen L., Cleij T., Vanderzande D.
Acta Physica Polonica A
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2008
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vol. 113
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issue 3
1009-1012
EN
New functionalized soluble poly(p-phenylene vinylene) derivative bearing polar molecules was designed and synthesized in order to investigate effects of molecular orientation in polymer photovoltaic devices. The active polar molecule is the 4-(N-butyl-N-2-hydroxyethyl)-1- nitro-benzene group. The grafting of the push-pull molecule with a donor/transmitter/acceptor structure, possessing a large ground state dipole moment, enables the molecular orientation by a dc electric field. An internal electric field stored in such system facilitates exciton dissociation and improves charge transport in single-layer devices. In our systems an increase in the external quantum efficiency by a factor of about 1.5 to 2 is estimated. The associated effects of orientation on the carrier injection and transport properties were evidenced.
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