A. Lohstroh

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

Effect of dislocations on charge carrier mobility–lifetime product in synthetic single crystal diamond

The authors report correlations between variations in charge transport of electrons and holes in synthetic single crystal diamond and the presence of nitrogen impurities and dislocations. The spatial distribution of these defects was imaged using their characteristic luminescence emission and compared with maps of carrier drift length measured by ion beam induced charge imaging. The images indicate a reduction of electron and hole mobility–lifetime product due to nitrogen impurities and dislocations. Very good charge transport is achieved in selected regions where the dislocation density is minimal.

Mapping of polarization and detrapping effects in synthetic single crystal chemical vapor deposited diamond by ion beam induced charge imaging

Diamond has been regarded as a promising radiation detector material for use as a solid state ionizing chamber for decades. The parameters degrading the charge transport from what is expected from an ideal crystal are still not completely understood. Recently, synthetic chemical vapor deposited (CVD) single crystal diamond has become available, offering the opportunity to study the properties of synthesized material independent of grain boundaries. We have studied the charge transport of a synthetic single crystal diamond with α-particle induced charge transients as a function of temperature and established the presence of a shallow hole trap with an activation energy of 0.29±0.02eV in some parts of the detector. Ion beam induced charge imaging has been used to study the spatial variations of the charge transport in a synthetic single crystal diamond. Pulses influenced by the shallow hole trap had their origin close to the substrate∕CVD interface of the sample. They could be clearly distinguished from pul...

Development of large area polycrystalline diamond detectors for fast timing application of high-energy heavy-ion beams

We have studied the effects of electrode fabrication and detector capacitance on the time resolution of large area electronic grade polycrystalline chemical vapour deposited diamond sensors that are suitable for time of flight measurements of heavy ions at relativistic velocities. Sensors were prepared both in house, with Al or Au metal contacts, and commercially fabricated with Au/diamond-like carbon contacts. He, Ar and a mixture of Ne and O beams at 16.3, 33.5 and 21-23 MeV/u, respectively were used on these devices whilst arranged in transmission geometry. Signal processing only began over one meter away from the sensors. The present approach, where we have large-area/large capacitance multi-strip detectors with processing electronics at some distance from the target, is compatible with anticipated space limitations in particle- identification and tracking setups at existing and planned nuclear fragmentation facilities. In a systematic study under these conditions, we demonstrate that the time resolution is limited by detector capacitance and energy deposition in the sensors. An intrinsic time resolution σ = (44±5) ps was achieved for a diamond detector of ∼ 14 pF capacitance. We conclude that, once further refinements are made, a large area time of flight detection system using polycrystalline diamond detectors would be able to provide time resolutions better than 40 ps, approaching the requirement for particle-identification in relativistic fragmentation experiments, such as those at the facility for antiproton and ion research, FAIR.

High-resolution mapping of the mobility–lifetime product in CdZnTe using a nuclear microprobe

We present frontal ion beam induced charge imaging on a cadmium zinc telluride device using a 2.05 MeV He microbeam. Two sets of voltagedependent 3 mm × 3m ms cans over the device cathode were acquired at room temperature and 250 K respectively and the corresponding charge collection efficiency (CCE) images extracted. The reduction of CCE due to electron transport with reduced bias voltage can be described using a simplified Hecht equation. This allows us to measure the mobility–lifetime product for electrons (µeτe )a nd so produce high-resolution µeτe images at 296 and 250 K. At 296 K, CCE values up to 96% were observed for an electric field of 3570 V cm −1 .I n general, the CCE values at 250 K are lower than the comparable values at 296 K with µeτe decreasing from 4.7 × 10 −4 to 1.2 × 10 −4 cm 2 V −1 ,a bout 1/ 4o fits original value. Additionally, we observe an increase in CCE at 250 K due to previous irradiation of the material, caused by partial filling of electron traps by the ion beam.

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.

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.

Stability of Silicon Carbide Particle Detector Performance at Elevated Temperatures

The alpha spectroscopy performance and electric current stability of 4H-silicon carbide Schottky devices with 50 μm epitaxial layer was examined at temperatures between 300 to 500 K at 50 K intervals. An activation energy of 5.98 ±0.64 meV was extracted from temperature dependent resistivity measurements. The Schottky barrier height decreases from 1.33 eV at 300 K to 1.11 eV at 500 K and the ideality factor increases from 1.17 at 300 K to 1.79 at 500 K. The reverse bias leakage currents stabilizes faster at higher temperatures. The charge collection efficiency is above 90% for temperatures up to 500 K. Pulse height spectra collected for 24 hours at constant voltage and temperature show improvements with time within the first 8 hours and remained stable for the remainder of the acquisition time. The peak width of the alpha spectra reduces significantly with increasing temperature at applied bias voltages below 50 V, which indicates that leakage currents are not the limiting factor in those conditions even at 500 K in our set up. So far, the devices indicate reasonable stability for extended periods of operation and highlight possible applications in harsh radiation media.

Digital pulse height correction in HgI2 γ-ray detectors

We report on the application of digital pulse processing algorithms to improve the spectroscopic performance of a 1.2 mm thick planar HgI2 ?-ray detector. We have used offline processing of pulses which were recorded using a high resolution waveform digitizer. The recovery processes include long duration shaping to avoid ballistic deficit in the case of slow pulses, and the application of biparametric correction techniques to compensate for charge loss. Pulses of duration as long as 100 ?s were recorded to facilitate long duration shaping. Two different pulse processing algorithms, viz. semi-Gaussian and moving window deconvolution, were applied and their performance was compared. The application of long duration shaping and digital charge-loss correction improved the energy resolution at 662 keV by more than 20% and the peak to background ratio by a factor of two. The resolution and the peak to background ratio were further seen to improve drastically upon rejection of counts with very slow rise-time. A 2.6% energy resolution at 662 keV with 14:1 peak to background ratio was obtained.

Ion Beam Induced Charge Studies of CdZnTe Grown by Modified Vertical Bridgman Method

Ion beam induced charge (IBIC) and time resolved digital IBIC techniques have been used to map the electronic properties of CdZnTe manufactured by Yinnel Tech Inc. The 2 MeV proton scanning microbeam at the University of Surrey Ion Beam Centre was used to map the charge transport properties of both holes and electrons at room temperature. The electron response of the detector showed good uniformity whereas the hole response showed significant variations across the bulk.