M.E. Özsan

Drift mobility and mobility-lifetime products in CdTe:Cl grown by the travelling heater method

We report the electron and hole charge transport properties of semi insulating CdTe:Cl grown by the Travelling Heater Method (THM). An alpha-particle Time of Flight (TOF) method was used to measure electron and hole drift mobility, with room temperature values of 880 cm2 /Vs for electrons and 90 cm2/Vs for holes. The variation in mobility was also investigated as a function of temperature, with electron and hole mobilities at 190 K of 1150 cm2 /Vs and 20 cm2/Vs respectively. Using a Hecht analysis the electron and hole mobility-lifetime products were also measured over the same temperature range, with values at room temperature of 8times10-4 cm2/V and 7times10-5 cm 2/V respectively. Time-resolved ion beam induced charge (IBIC) imaging was used to produce micrometer resolution maps of electron drift mobility and signal amplitude, which showed excellent spatial uniformity

Investigation of the internal electric field distribution under in situ x-ray irradiation and under low temperature conditions by the means of the Pockels effect

The internal electric field distribution in cadmium zinc telluride (CdZnTe) x-ray and γ-ray detectors strongly affects their performance in terms of charge transport and charge collection properties. In CdZnTe detectors the electric field distribution is sensitively dependent on not only the nature of the metal contacts but also on the working conditions of the devices such as the temperature and the rate of external irradiation. Here we present direct measurements of the electric field profiles in CdZnTe detectors obtained using the Pockels electo-optic effect whilst under in situ x-ray irradiation. These data are also compared with alpha particle induced current pulses obtained by the transient current technique, and we discuss the influence of both low temperature and x-ray irradiation on the electric field evolution. Results from these studies reveal strong distortion of the electric field consistent with the build-up of space charge at temperatures below 250 K, even in the absence of external irradiation. Also, in the presence of x-ray irradiation levels a significant distortion in the electric field is observed even at room temperature which matches well the predicted theoretical model.

Correlation of chemical and structural parameters with non-uniform cell performance in CdS/CdTe solar cells

Abstract Poor performance of CdS/CdTe material near the edges of 30×30 cm cells has been observed. This was correlated to poor recrystallisation of the ‘edge material’ upon annealing. Various techniques — XRD and GAXRD, TEM, SIMS and XPS — have been used in order to carefully map differences in as-deposited material across the cell surface that could be responsible for this effect. The results revealed that there is a subtle difference in the structure of the CdTe near the cell edges, with the edge material actually being in a slightly better state of crystallisation and material from the cell centre being very defective. Recrystallisation is essential for achieving good cell operation and the driving force behind CdTe recrystallisation upon annealing is probably this high defect density.

IBIC characterization of charge transport in CdTe:Cl

AbstractsStudies of charge transport uniformity in bulk CdTe:Cl have been carried out using ion-beam-induced charge (IBIC) imaging. High resolution maps of charge collection efficiency, mobility-lifetime product (μτ), and drift mobility (μ) were measured using a scanning microbeam of 2 MeV protons focused to a beam diameter of ∼3 μm. Excellent charge transport uniformity was observed in single crystal CdTe:Cl, with electron μτ values of up to 5 × 10−3 cm2/V s. The presence of extended defects such as tellurium inclusions was also studied using IBIC, and their influence on the charge transport performance of CdTe detector structures is discussed.

CdZnTe Schottky diodes for radiation spectroscopy

Based on raw CdZnTe material provided by eV Products, a division of II-VI Inc., we have established a programme of device fabrication and characterisation. Schottky barrier type devices were fabricated on low grade, 5/spl times/5/spl times/5 mm/sup 3/, CdZnTe (CZT) crystals for radiation spectrometers. Ohmic contacts were applied by alloying indium metal onto CZT surfaces. Vacuum evaporated gold metal provided the rectifying contact. All diodes showed good rectification. Schottky barrier heights were calculated using IN measurements and barrier heights were determined to be 0.99 eV for air cleaved and 1.04 eV for methanol/bromine etched surfaces. Photoresponse measurements revealed bulk band gap value around E/sub g/=1.51 eV corresponding to a Zn molar percentage of 6.3% in the CZT. Free carrier concentration was measured using C-V measurements on Schottky diodes and found to vary between mid 10/sup 11/-10/sup 12/ cm/sup -3/ in the bulk CZT. Radiation spectra were collected with Schottky diodes, at 100 V bias, using an Am-241 source. Schottky diodes prepared on low grade CZT material showed reasonable detector performance with photo peak resolution of 19% and a charge collection efficiency of 31%. Comparison is drawn between Au-CZT-Au and Au-CZT-In devices. It is found that the operation of these detectors is sensitive to chemical surface preparation of the raw CZT material prior to contact application, and to exposed surface passivation.