Cl-doped CdTe Research Articles

Defect equilibrium in semi-insulating CdTe(Cl)

Abstract The Hall coefficient R H and conductivity σ were measured in the temperature range 250–450 K in semi-insulating Cl-doped CdTe crystals. Due to the high electron mobility, a reversal of sign of R H appears at 370 K. A simultaneous analysis of R H ( T ) and gs ( T ) was performed with the help of theoretically determined mobilities and Hall factors of both electrons and heavy holes. It yields a position of the deep level connected with the second charge state of a divalent cadmium vacancy E a2 = 0.69 eV above the valence band and the position of Fermi level E F = − E g + E a2 + 0.72 k 0 T pinned to E a2 . Using these values total concentrations of defects and the energy necessary for the creation of the neutral complex (V Cd Cl TE ) x ≈ 1.06 eV were determined on the basis of detailed point defect model, which describes the association of cadmium vacancies and chlorine atoms. The energy liberated by the compensation of chlorine donors by these complexes (≈ 1.20 eV) is sufficient for the generation of these complexes so that a strong compensation occurs at a growth temperature of 450°C for Te-rich conditions.

Chlorine-related photoluminescence lines in high-resistivity Cl-doped CdTe

High-resistivity Cl-doped CdTe crystals grown by the gradient-freeze method were studied by high-resolution photoluminescence (PL) and electrical measurements. A sharp PL line ascribed to a Cl-related defect was observed at 1.5903 eV in all of the high-resistivity crystals doped with different amounts of chlorine. A broad line around 1.586 eV was clearly observed in highly doped crystals. The intensities of these lines increased with the amount of chlorine. Annealing under Cd-saturated atmosphere eliminated the 1.5903 eV line and intensified the neutral donor-bound exciton line together with its two-electron transition. The high-resistivity crystals were also converted to n-type with low resistivity by the Cd annealing. These results show that the 1.5903 eV line is a key emission line for the high-resistivity, and suggest that the line is associated with Cd-vacancy/chlorine complex, probably in the form of V CD-Cl.

γ-ray detection stability under total depletion in CdTe surface barrier detectors

The paper presents an interesting way of overcoming the well-known "polarization effect" in Cl-doped CdTe detectors. The detection efficiency is kept constant as long as the sample remains totally depleted, even though hole detrapping may occur from deep levels. Excellent stability has been checked for periods of time up to half an hour.

Characterization of the Transport Properties of Halogen-Doped CdTe Used for Gamma-Ray Detectors

The mobilities, trapping times, activation energies, and trap concentration have been measured for both holes and electrons in Br and Cl-doped CdTe using the time-of-flight technique. Two electron traps 25 and 50 meV below the conduction band and two hole traps 140 and 350 meV above the valence band have been found. The 10 times larger concentration of levels found in the Br-doped CdTe can be explained using a model that describes the association of cadmium vacancies and substitutional halogens. The physical interpretation of the μι+ product when two levels are present is discussed for this case where the mobility is reduced and the lifetime is increased by trapping-detrapping phenomena. The measurements demonstrate that the material has excellent potential for γ-ray detectors that do not polarize with the proper surface preparation and make good detectors (6 keV FWHM for 122 keV γ-ray).

Self-compensation in CdTe

Using the time-of-flight technique, the drift mobilities, trapping levels, and trap concentrations were measured for both holes and electrons in Cl- and Br-doped CdTe grown by the Travelling Heater Method. Electron traps 25 and 50 MeV below the conduction band and hole traps 140 and 350 MeV above the valence band were found in Cl-doped material. The 25 MeV electron and 140 MeV hole traps were also found in the Br-doped CdTe, but with a concentration nearly 10 times that of the Cl-doped CdTe. A model based on the mass action approach of Kroeger which describes the association of cadmium vacancies and halogen atoms substitutionally located on tellurium sites has been used to interpret the different behavior of Br and Cl in CdTe. The stronger bonding of the Cl increases the tendency to form associates, thus decreasing the number of unassociated species (cadmium vacancies and isolated Cl) which act as trapping-centers in CdTe.