Diamond is known for its extreme hardness which may allow it to operate as a particle detector in high fluence environments even after absorption of large radiation doses. We present a study of the deterioration of the charge collection efficiency (CCE) due to neutrons produced by (235)U fission, with irradiation fluences up to 1 x 10(16) n cm(-2). The planar devices were fabricated by thermal evaporation of Au onto approx. 300 mu m thick high purity chemical vapour deposited diamond produced by Element Six Ltd., UK. The detector performance was investigated as a function of bias voltage at room temperature using (241)Am alpha-particles and minimum ionising particles (MIPs) of a (90)Sr source. At low fluences up to 2 x 10(13) n cm(-2), the detectors reach the initial saturated signal amplitude after irradiation. However, the signal is less stable and deteriorates due to polarisation. This effect can be reduced by initial priming with X-rays. No peak could be distinguished in the detector response in the unprimed state after 10(16) n cm(-2) with bias voltages up to 1000 V (equivalent to 32 kV cm(-1)). However, a peak at about 18% CCE could be recovered after priming. (C) 2010 Elsevier B.V. All rights reserved.

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.

CdZnTe (CZT) is commonly used as a radiation detector material and the surface properties are important as they influence detector performance. The surface chemistry is generally controlled through chemical etching and oxidation processes. In this paper, X‐ray photoelectron spectroscopy (XPS) is employed to investigate changes in the surface composition of single‐crystal Cd0.95Zn0.05Te samples after exposure to bromine in methanol (BM) chemical etching treatments and subsequent oxidation in air or 30% H2O2. BM treatment of CZT is found to result in a graded Te‐rich surface layer which increases in thickness as a function of BM concentration. Room‐temperature air oxidation of 0.2% BM‐treated CZT follows a logarithmic rate law. BM‐treated CZT exposed to 30% H2O2 for 30 s shows a linear increase in the TeO2 oxide thickness with increasing BM concentration up to a BM concentration of 1.5%. Above 2% BM concentration, the Te enrichment in the CZT surface region has reached saturation and is effectively pure Te. Copyright © 2010 John Wiley & Sons, Ltd.

Ion beam induced charge (IBIC) imaging is a powerful technique for quantitative mapping of the charge transport performance of wide bandgap semiconductor materials. In this paper we present results from a study of electron and hole mobility–lifetime product and drift mobility in CdTe:Cl and CdZnTe, which are semiconductor materials used for radiation detector applications. IBIC imaging has been used to produce mobility–lifetime product maps in CdTe:Cl and CdZnTe, revealing the influence of extended defects and tellurium inclusions and assessing the large area response uniformity of the materials. The recent extension of this method in the form of digital time-resolved IBIC is also discussed and time of flight maps are presented which give quantitative images of electron and hole drift mobility.

The properties required for γ-ray detection of single-crystal Cadmium Manganese Telluride (Cd1−xMnxTe) are investigated and samples are evaluated as room temperature radiation detectors. Undoped Cd1−xMnxTe crystals are characterised using sub band gap IR microscopy mapping of extended defects, room temperature photoluminescence band-gap measurements, IV measurements and α particle spectroscopy. The band gap energy calculated from room temperature near-band-edge luminescence was found to be 1.70±0.01eV, corresponding to a manganese fraction in the sample of x=0.13. The hole mobility-lifetime product was measured at 150K and was found to be of the order of 1×10−5cm2/V.

Studies 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.

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

In this paper, we report the influence of room temperature post-treatment using an oxygen and carbon tetrafluoride plasma on the photoresponsivity of chemical vapour deposited (CVD) polycrystalline diamond UV photodetectors. X-ray diffraction (XRD) measurements show that the plasma-treatment preferentially passivates the defects at the (220) and (311) facets of diamond film. Photoluminescence (PL) and Raman measurements carried out on these diamond surfaces indicate that the plasma can effectively etch the sp2-bonded carbon impurities and passivate the silicon-vacancy defects. Plasma post-treatment employed on devices resulted in more than two orders of magnitude increase in the photoresponsivity discrimination between the UV (∼225 nm) and visible light (∼700 nm) regions.

Electron charge transport in high resistivity CdTe was investigated in terms of drift mobility, charge collection efficiency, and mobility-lifetime product. CdTe devices were produced from material grown by the Travelling Heater Method. Infrared microscopy was used to assess the quality of CdTe wafers, which showed a concentration of bulk defects and tellurium precipitates around the edges of the wafers. Laser-induced time of flight was used to measure the electron drift velocity, which was linear with respect to electric field at field strengths up to 200V/cm. The measured electron drift mobility was 1040±20cm2/Vs. Ion-beam induced charge (IBIC) imaging of the device cathode was carried out to produce high resolution maps of signal amplitude and electron drift time. Excellent spatial uniformity was observed in the sample, and a value of 6×10−3cm2/V was measured for the electron mobility-lifetime product.

We present results from a characterisation of bulk defects in CdTe wafers, and their role in the degradation of charge transport performance of CdTe radiation detectors. Sub-bandgap IR microscopy and X-ray Lang topography have been used to characterise material quality prior to device processing. IR microscopy clearly identifies extended defects such as tellurium precipitates in the material bulk, whilst Lang topography characterises stacking faults, crystallite boundaries and other crystallographic features in the near-surface region. After fabrication of contacts onto the material, ion beam induced charge imaging is used to investigate the correlations between material defects and charge transport. Digital ion beam induced charge imaging is used to produce high resolution maps of charge signal amplitude, carrier drift time, and carrier drift mobility.

We report a study of pulse shapes of a radiation detector with a sandwich structure fabricated from chemical vapor deposited (CVD) polycrystalline diamond. The pulse shapes were recorded at room temperature using 5.486 MeV alpha particles from 241Am source. Only “fast” component was observed in the electron predominated pulses, whereas both “fast” and “slow” components were obtained in the hole predominated pulses, suggesting that electron charge drift is prompt and no detrapping occurred. In contrast, hole charge drift is slower than expected and trapping-detrapping took place during hole travel process.

Using a focussed and pulsed beam of 850 nm laser radiation, it is possible to assess a number of bulk properties of CdZnTe. The sub-bandgap pulsed radiation (pulse width 80 ns) excites a detectable signal from defects near the band edges in CdZnTe. This is detected using a conventional charge sensitive preamplifier. Further information is obtained by observing the transmitted optical beam simultaneously with the detected photo-induced current from the CdZnTe detector. The intensity of the laser beam is adjusted such that an average of 157 keV is deposited in the CdZnTe detector per optical pulse. Using optical polarisers, electric field distribution has been measured without the space-charge induced distortion of electric field normally experienced when using higher intensity d.c. light sources, and the results correlated with charge collection efficiency measurements. By varying the pulse repetition frequency, information is obtained about the lifetime of shallow defects at room temperature.

Diffusion of atoms across the CdS/CdTe interface during post‐deposition annealing is thought to have an important effect on the performance of CdS/CdTe‐based solar cells. Thin‐film solar cell samples were annealed at 400°C for three different times. The samples were etched sequentially in a bromine/methanol solution and then examined with Auger depth profiling and X‐ray diffraction. Profiling showed the as‐deposited samples to have little or no interfacial mixing, but annealing even for a short time caused diffusion of sulphur into the CdTe material. As the annealing times lengthened, the concentration of sulphur in the CdTe increased. X‐ray diffraction measurements indicated that the CdS layer rapidly crystallized into its hexagonal Greenockite modification, but with lattice parameters increased by 0.8%. This changed little with annealing time. The CdTe near the interface appeared to have a modified but well‐defined cubic structure with respect to reference CdTe, as indicated by splitting of the diffraction peaks, denoting the growth of a CdTe x S y interfacial layer. This layer increased in thickness with annealing time and most probably has a stoichiometry within the range CdTeS 0.04 –CdTe 0.88 S 0.12 . Copyright © 2002 John Wiley & Sons, Ltd.

Complete thin film solar cells have been fabricated by physical vapour deposition of CdTe onto chemical-bath-deposited CdS supported on a commercial SnO2/glass substrate. Cell I–V characteristics were measured under AM 1.5(G) illumination. Energy dispersive X-ray analysis was used to measure the average stoichiometry. Nuclear Reaction Analysis was used to measure the degree of S diffusion within the CdTe thin films. X-ray diffraction was used to determine the lattice parameter of the CdTe. Results showed the CdTe films deposited at low deposition rates (<2 nm s−1) exhibited greater S diffusion after a partial recrystallisation during annealing. Higher CdTe deposition rates correlated to increased Te concentration for the as-deposited films, significantly reduced during annealing.

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, 5x5x5 mm(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 0.99eV for air cleaved and 1.04eV for methanol/bromine etched surfaces. Photoresponse measurements revealed bulk band gap value around E-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(11) - 10(12) cm(-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.

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.

As the material structures of CdS-CdTe heterojunction solar cells have a significant effect on cell efficiency, there is a requirement to investigate new methods for thin film, structural, depth profiling. In an attempt to characterise structural details such as stress, stoichiometry and texture, we have developed a novel method of depth profiling, This is based upon a chemical etch bevel followed by spatially resolved Xray diffraction and Rutherford backscattering spectrometry. We show that the method provides a depth resolution of better than 0.1 mu m.We have used this method to examine CdTe thin films ( similar to 1.8 mu m) produced by electrodeposition supported upon CdS ( < 0.1 mu m). We present the results of these studies and use the method to investigate the: effect upon the structures of a type conversion anneal. The data is compared to previous studies using different depth profiling methods. The results indicate the formation of an spatially limited, intermixed CdTe(1-x)Sx layer and the conversion of the whole CdS film into a CdS(1-x)Tex layer. The structural characteristics are correlated to optical and electrical properties of the films. (C) 2000 Elsevier Science S.A. All rights reserved.

The results of room temperature photoluminescence spectroscopy and decay time measurements carried out on CdTe/CdS solar cells are reported. The as-grown structures were annealed in air at temperatures in the range 350–550 °C. For excitation via the CdTe/CdS interface, longer photoluminescence decay times were observed as the anneal temperature was increased, this is attributed to localized passivation of nonradiative states possibly due to the effect of S interdiffusion. When the photoluminescence is excited via the CdTe free surface, the decay curves consist of a fast and slow component. The fast component (<130 ps) of the photoluminescence is attributed to nonradiative recombination at grain boundaries or the CdTe free surface. The slow component is attributed to the effects of carrier drift and diffusion and subsequent recombination at the CdTe/CdS interface. Modeling of the transport process has led to the extraction of a value of 0.20±0.03 cm2 s−1 for the minority carrier diffusion coefficient of the CdTe for the sample annealed at 450 °C. These results are correlated with improvements in device efficiency determined from illuminated current density–voltage measurements.