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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.
2
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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.
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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Investigation of Carrier Recombination in Si Heavily Irradiated by Neutrons

100%
Gaubas E., Kadys A., Uleckas A., Vaitkus J.
Acta Physica Polonica A
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2008
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vol. 113
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issue 3
829-832
EN
Variations of recombination lifetime, with fluence of the reactor neutrons from 10^{12} to 3×10^{16} n/cm^2, in the magnetic field applied Czochralski grown Si samples are examined by the contactless transient techniques of the microwave probed photoconductivity and dynamic gratings. A nearly linear decrease in lifetime from few microseconds to about 200 ps within the examined range of neutron irradiation fluences was obtained. This dependence persists under relatively low (≤80°C) temperature heat treatments. Also, cross-sectional scans of lifetime depth-profiles were examined, which show rather high homogeneity of lifetime values within wafer thickness.
5
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Photoluminescence and Photoconductivity Dynamics in Semi-Insulating Epitaxial GaN Layers

76%
Gaubas E., Juršėnas S., Kazlauskas K., Miasojedovas S., Vaitkus J., Žukauskas A.
Acta Physica Polonica A
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2005
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vol. 107
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issue 1
215-219
EN
The transients of fast free-carrier recombination and of multi-trapping processes due to different species of defects have been investigated by photoluminescence and by contact and microwave photoconductivity. Three distinct stages of relaxation, namely, of stimulated emission, of recombination due to point defects and capture into trapping centers associated with dislocations, and a non-exponential stage with a stretched-exponent asymptotic decay ascribed to dislocations mediated multi-trapping were distinguished by correlated examination of time-resolved photoluminescence and photoconductivity transients.
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