Gangqiang Zha

Study on temperature dependent resistivity of indium-doped cadmium zinc telluride

Indium-doped CdZnTe crystals, grown by the modified vertical Bridgman method, were characterized by temperature dependent resistivity measurements in the range from 220 to 320 K. The Fermi level, pinned near the midgap, was confirmed by fitting ln(ρ) versus 1/k0T plots, giving energies of 0.63 eV and 0.72 eV above the valence band for the high resistivity samples, with doping levels of 5.0 × 1016 at cm−3 and 4.8 × 1017 at cm−3, respectively. Different dominant deep level defects or complexes for pinning the Fermi level, and hence producing high resistivity, were expected when comparing the charge transport behaviours of the materials and the dopant concentration. However, two energies of 0.24 eV and 0.33 eV, below the conduction band for the Fermi level, were calculated at positive and negative bias voltages, respectively, for a low resistivity sample doped by 9.7 × 1017 at cm−3 In.

Characterization of CdZnTe Crystals Grown Using a Seeded Modified Vertical Bridgman Method

In this paper, several 60 mm diameter CdZnTe crystal ingots, containing large-size single crystals, were grown using (111) or (211) orientation seeds by the modified vertical Bridgman method. The Zn concentration distribution along the axial direction of the ingots was measured by near-infrared transmission spectroscopy at room temperature. The partition coefficient of Zn during the growth was calculated to be about 1.2. Zinc distribution uniformity measurements were carried out using photoluminescence mapping measurement at 80 K, which showed that the band gap variation in the CdZnTe wafers is less than 0.004 eV. IR microscopy showed that the diameters of the Te inclusions present in the material are in the range 6-9 mum, and the density of the inclusions is 1~3times105 cm-3. IR transmission measurements in the wave number region from 500 to 4000 cm-1 demonstrate that the IR transmittance of CdZnTe wafers is higher than 60%. Current-voltage measurements were performed on test structures fabricated using thermally evaporated Au contacts deposited on as-grown crystals, which revealed bulk resistivity values of 2~5times1010 Omegamiddotcm. Typical leakage currents for the planar devices were ~ 4 nA at a field strength of 1500 Vcm-1. The electron and hole mobility-lifetime products were evaluated using alpha particle irradiation. The obtained typical (mutau)e and (mutau)h values for the as-grown CdZnTe were 2.3times10-3 cm2V-1 and 1.5times10-4 cm2V-1, respectively.

Photoluminescence analysis on the indium doped Cd0.9Zn0.1Te crystal

Photoluminescence spectra are used to characterize high resistivity of In-doped CdZnTe crystal. Shallow level donor-acceptor pair (DAPs) peak at 1.6014eV is found due to shallow donor and acceptor compensation related defects, which forms the [VCd–InCd]− singly negative complex and the neutral complex [VCd–2InCd]. Cd vacancy acceptors are compensated dominantly due to indium doped [InCd]+ donor and Te antisite [TeCd]4+ donor, which improves the resistivity of CdZnTe:In crystal. Energy level model diagram of CdZnTe:In crystal is discussed. Current-voltage measurement results confirm that Cd vacancy compensation by doped indium improves the resisitivity of CdZnTe crystal.

Dependence of film texture on substrate and growth conditions for CdTe films deposited by close-spaced sublimation

The texture of CdTe films deposited via close-spaced sublimation (CSS) was studied. Different substrates were used, including Si(100), fluorine-doped SnO2 (FTO), and CdS, and different growth conditions were applied. The texture behaviors of each sample were evaluated based on its XRD spectrum and are found to be dependent on both the substrate and the growth conditions. The texture strength is found to be, in order, Si(100) > FTO > CdS at a substrate temperature of 763 K under 100 Pa Ar, which is the opposite of the order of the surface roughness of the substrates. The textures of the films on FTO and chemical bath deposition (CBD)-CdS substrates, especially on CBD-CdS, are very sensitive to the growth conditions, whereas those on Si(100) are not. It is found that the texture is strengthened at elevated temperatures and suppressed under decreased ambient pressure. It is also found that the textured films are composed of both (111) and (511) texture components, which is believed to be the result of the tw...

Investigation on defect levels in CdZnTe : Al using thermally stimulated current spectroscopy

Defect levels present in as-grown semi-insulating CdZnTe : Al samples, taken from the top, middle and tail of the same ingot, have been investigated by thermally stimulated current (TSC) spectroscopy. Their trap signatures, particularly the thermal activation energy, capture cross section and concentration, were characterized and discussed, respectively, by fitting the measured TSC spectra with the simultaneous multiple peak analysis method. Furthermore, the deep donor levels EDD from the top, middle and tail were found to be positioned at 0.692 eV, 0.659 eV and 0.618 eV below the conduction band, respectively, by fitting the ln(I) versus 1/kT plots above room temperature. The Fermi level was positioned at 0.716 ± 0.05 eV by fitting the linear plots of the temperature dependence of resistivity, which was pinned by the EDD level near the middle of the band gap, which in turn caused the observed high resistivity in the as-grown CdZnTe : Al ingot.

The effects of deep level traps on the electrical properties of semi-insulating CdZnTe

Deep level traps have considerable effects on the electrical properties and radiation detection performance of high resistivity CdZnTe. A deep-trap model for high resistivity CdZnTe was proposed in this paper. The high resistivity mechanism and the electrical properties were analyzed based on this model. High resistivity CdZnTe with high trap ionization energy Et can withstand high bias voltages. The leakage current is dependent on both the deep traps and the shallow impurities. The performance of a CdZnTe radiation detector will deteriorate at low temperatures, and the way in which sub-bandgap light excitation could improve the low temperature performance can be explained using the deep trap model.

Radiation damage on CdZnTe:In crystals under high dose 60Co γ-rays

A radiation damage mechanism of CdZnTe:In crystals under 60Co γ-rays with a cumulated radiation dose of ~2.4 kGy was proposed and discussed. Thermally stimulated current (TSC) measurements were carried out to characterize the γ-ray induced radiation damage. Four main trap levels (T1–T4) originating at four main microscopic defective states were identified. We attributed traps T1 and T2 to shallow donor impurities and shallow acceptor A-centers, respectively. Trap T3 was ascribed to dislocations and a great increase in defect concentration occurred after radiation. Trap T4 originated at deep acceptor Cd vacancies and vacancy related defect complexes and a considerable increase in the trap density, after radiation, lead to the conversion of conduction type from n to p in the Hall measurements and a Fermi level shift in the temperature-dependent resistivity analyses. Radiation induced electrically active defects contributed to the variation of the electrical compensation conditions and the worsening of the charge transport properties, which were consequently reflected by a significant deterioration in CZT detector performance.

The study on the work function of CdZnTe with different surface states by synchrotron radiation photoemission spectroscopy

The work functions of CdZnTe single crystal with different surface states were measured using synchrotron radiation photoemission spectroscopy. The work function of CdZnTe after mechanical and chemical polishing is higher than that of clean and ordered surface. The damage layer deduced by Ar ion sputtering increases the work function. The Te adatoms of the CdZnTe (111)B (2×2) reconstruction form the interface dipole and increase the work function. The effects of H2O adsorption and Au evaporation on the work function of CdZnTe were investigated. The relationships among work function, surface dipole, valence band bending, ionization energy, and electron affinity of CdZnTe with different surface states were discussed.

Influence of deep level defects on carrier lifetime in CdZnTe:In

The defect levels and carrier lifetime in CdZnTe:In crystal were characterized with photoluminescence, thermally stimulated current measurements, as well as contactless microwave photoconductivity decay (MWPCD) technique. An evaluation equation to extract the recombination lifetime and the reemission time from MWPCD signal is developed based on Hornbeck-Haynes trapping model. An excellent agreement between defect level distribution and carrier reemission time in MWPCD signal reveals the tail of the photoconductivity decay is controlled by the defect level reemission effect. Combining 241Am gamma ray radiation response measurement and laser beam induced transient current measurement, it predicted that defect level with the reemission time shorter than the collection time could lead to better charge collection efficiency of CdZnTe detector.

Effects of sub-bandgap illumination on electrical properties and detector performances of CdZnTe:In

The effects of sub-bandgap illumination on electrical properties of CdZnTe:In crystals and spectroscopic performances of the fabricated detectors were discussed. The excitation process of charge carriers through thermal and optical transitions at the deep trap could be described by the modified Shockley-Read-Hall model. The ionization probability of the deep donor shows an increase under illumination, which should be responsible for the variation of electrical properties within CdZnTe bulk materials with infrared (IR) irradiation. By applying Ohms law, diffusion model and interfacial layer-thermionic-diffusion theory, we obtain the decrease of bulk resistivity and the increase of space charge density in the illuminated crystals. Moreover, the illumination induced ionization will further contribute to improving carrier transport property and charge collection efficiency. Consequently, the application of IR irradiation in the standard working environment is of great significance to improve the spectroscopic characteristics of CdZnTe radiation detectors.