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2 Opto-Electronics Review

2 2004

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Influence of defect traps and inhomogeneities of SiC crystals and radiation detectors on carrier transport

Kažukauskas V., Vaitkus J. V.

Opto-Electronics Review

2004

Vol. 12, No. 4

377-382

We present investigation of carrier traps and their transport in 4H-SiC single crystals and high energy radiation detectors. SiC detectors have been produced from bulk vanadium-compensated semi-insulating 4H-SiC single crystal. They were supplied with a nickel ohmic contact on the back surface and titanium Schottky contact on the front surface. The prevailing defect levels were revealed by means of thermally stimulated current (TSC) and thermally stimulated depolarization (TSD) methods and their advanced modification ń multiple heating technique. From I-V measurements of the samples a 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, 0.33-0.41 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 the thermal carrier generation from defect levels. In contrast, the first two maxima with the lowest activation energies, which nevertheless 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 TSD.

Carrier transport in GaN single crystals and radiation detectors investigated by thermally stimulated spectroscopy

Kažukauskas V., Vaitkus J. V.

Opto-Electronics Review

2004

Vol. 12, No. 4

399-403

We investigated single crystals of GaN and thin film GaN radiation detectors by thermally stimulated currents (TSCs) and thermally stimulated depolarization (TSD) methods in order to characterize carrier transport properties as influenced by material defect structure. In thick GaN, no expressed structure of the TSC spectra was observed in the temperature range from 100 K up to 350 K that could be characteristic for thermal carrier generation from trap levels. The experimental facts imply that TSC spectra might be caused not by carrier generation from traps, 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 by phonons. It was found that mobility limited by ionized impurities varies as ~ T2.8 and lattice scattering causes the dependence ~ T -3.5. The highest mobility values were up to 1550 cm² /Vs 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, the instabilities were observed that could be caused by the change of carrier drift paths in a highly disordered mater. A model of carrier percolation transport is presented.