Hole Mobility-lifetime Products Research Articles

CdZnTe像素阵列探测器成像评价模型及实验

A flexible induced charge model of CdZnTe pixellated detector is presented. This model takes into account the charge trapping effect in the detection process. The distribution of the trapped carriers in CdZnTe crystal is derived based on the continuity equations. In consequence, an evaluation model based on the presampling modulation transfer function (presampling MTF) is obtained. As shown through the numerical simulation, the detector imaging quality decreases obviously with the increase in incident photon energy. When the electron- and hole-mobility lifetime products are 0.510-3 to 5.010-3 cm2/V and 2.010-5 to 7.510-5 cm2/V, respectively, the model signal is almost due to the electron

Fast High-Flux Response of CdZnTe X-Ray Detectors by Optical Manipulation of Deep Level Defect Occupations

We experimentally investigate the possible correlation between high hole-trap concentrations in wide-bandgap semiconductors and delayed temporal response of high-flux x-ray detector devices to changing photon fluxes. We show that fast photo-current response can be achieved with (1) CdZnTe detectors with high hole mobility-lifetime products, (2) temperature increased detrapping, and (3) constant below-bandgap energy light illumination that modifies the dark defect occupation towards a steady-state with a reduced concentration of active hole traps. This way, the detector signal stabilizes immediately upon flux onset, independent of details of the semiconductor's point defect structure. Quasi-instantaneous response stabilization (<; 3 ms) to x-ray flux changes > 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> photons mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> is demonstrated.

Sensitizer Effects on the Transport Properties of Polymer:Sensitizer Organic Blend

AbstractIn this work we studied the effect of sensitizer concentration on a mobility-lifetime product μτ, on photoconductivity response time ph τph, and on drift mobility μ of the majority carriers in an organic polymer:sensitizer blend. The intensity modulated photocurrent and photo-EMF technique were used as experimental tools for this purpose. The studied material consists of a mixture of the novel non-conjugated main chain hole-transporting polymer PFO6:PDA (Poly(N,N'-bis(4-hexyloxyphenyl)-N'-(4-(9-phenyl-9H-fluoren-9-yl)phenyl)phenylen-1,4- diamine) sensitized with the highly soluble C60 derivative PCBM (phenyl-C61-butyric acid methyl ester) in the range from Z = 1 to 40 wt.-%. It was experimentally observed that (1) at the increasing sensitizer concentration the overall photoconductivity increases; (2) the majority carrier type switches from holes to electrons at approximately 2:1 polymer:sensitizer ratio; (3) the holes response time becomes shorter at the decreasing polymer fraction, while the electrons lifetime is only slightly dependent on sensitizer concentration; (4) the hole mobility-lifetime product decreases at the decreasing concentration of hole transporting component (polymer), while the electrons mobility-lifetime product increases at the increasing concentration of the electron transporting component (sensitizer); (5) the same is true for the carriers mobilities.

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 1~3times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . IR transmission measurements in the wave number region from 500 to 4000 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> 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 2~5times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> Omegamiddotcm. Typical leakage currents for the planar devices were <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">~</i> 4 nA at a field strength of 1500 Vcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . The electron and hole mobility-lifetime products were evaluated using alpha particle irradiation. The obtained typical (mutau) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> and (mutau) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</sub> values for the as-grown CdZnTe were 2.3times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and 1.5times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , respectively.

Ion beam induced charge imaging of charge transport in CdTe and CdZnTe

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.

Characterization of Low-Defect ${\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te}$ and CdTe Crystals for High-Performance Frisch Collar Detectors

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Low dislocation density, high-purity, and low inclusion concentration <formula formulatype="inline"><tex>${\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te}$</tex></formula> (CZT) and CdTe crystals were grown by a vertical Bridgman technique using in-house zone refined precursors. The grown crystals were sequentially processed using optimized chemo-mechanical processes to fabricate planar and Frisch collar detectors. Infrared transmission and scanning electron microscopy studies have shown that EIC grown CZT and CdTe crystals have significantly lower Te inclusions and defect densities than commercially available spectrometer grade crystals. The charge transport properties (electron and hole mobility-lifetime products, <formula formulatype="inline"><tex>$\mu \tau _{e}$</tex></formula> &amp; <formula formulatype="inline"><tex>$\mu \tau _{h}$</tex></formula>) of various detectors have been evaluated by Hecht analysis. The detectors have been tested for spectral response using 59.5 and 662 keV <formula formulatype="inline"><tex>$\gamma $</tex></formula>-ray sources. The CZT detectors with planar electrodes showed 2.6% FWHM at 662 keV. By adding a Frisch collar, the detectors' spectra improved significantly. The Frisch collar detectors proved to be very promising for assembling large-area arrays with excellent energy resolution at relatively low manufacturing cost. </para>

Temperature Dependence of Spectroscopic Performance of Thallium Bromide X- and Gamma-Ray Detectors

Thallium bromide (TlBr) is a compound semiconductor characterized by wide band-gap energy (2.68 eV), high atomic numbers (Tl: 81, Br: 35) and high density (7.56 g/cm3). Mobility-lifetime products for electrons and holes in TlBr crystals are 1times103 cm2/V and 3times10-4 cm2/V, respectively, which are comparable to for carriers in CdTe crystals. Therefore, TlBr is a suitable material for fabrication of X- and gamma-ray detectors. In this study, TlBr detectors have been fabricated from crystals grown by the traveling molten zone method. The detectors have thicknesses of approximately 0.4 mm, and have gold electrodes (3 mm in diameter) on both surfaces of the crystals. In order to evaluate the temperature dependence of the charge transport properties of the detectors, (mutau)e and (mutau)h in the detectors have been estimated by the Hecht analysis at various temperatures of the detectors (-40, 20, 0, 20 and 40degC). The (mutau)h in the detectors was of the order of 10-5 cm2/V at -40degC, and increased with increasing temperatures. The maximum value of (mutau)h the was obtained at 20degC, and was about ten times larger than the value at -40degC. On the other hand, the (mutau)e was approximately 1times10-3 cm2/V between -40degC and 20degC. The temperature dependence of spectral response of the detectors has been evaluated by acquiring energy spectra of gamma-rays from 241Am at -40, 20, 0, 20 and 40degC. At -40degC, the full-energy peak corresponding to 59.5 keV gamma-rays has been obtained from the detectors, but the energy resolution of the peak was degraded due to the hole tailing on the low-energy side of the peak. The energy resolution of the detectors gradually improved with increasing temperatures of the detectors. The best FWHM energy resolution was measured to be 5.4 keV (9.1%) at 20degC.

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.

Crystal Growth and Characterization of CdTe and Cd0.9Zn0.1Te for Nuclear Radiation Detectors

AbstractCdTe and Cd0.9Zn0.1Te (CZT) crystals have been studied extensively at EIC Laboratories, Inc. for various applications including x- and γ-ray imaging and high energy radiation detectors. The crystals were grown from in-house zone refined ultra pure precursor materials using a vertical Bridgman furnace. The growth process has been monitored, controlled and optimized by a computer simulation and modeling program (MASTRAPP). The grown crystals were thoroughly characterized after sequential surface passivations and post-growth annealing treatments with and without component overpressures. The infrared (IR) transmission images of the post-treated CdTe and CZT crystals showed average Te inclusion size of ∼10 μm for CdTe crystal and ∼8 μm for CZT crystal. The etch pit density was ≤ 5×104 cm−2 for CdTe and ≤ 3×104 cm−2 for CZT. Various planar and Frisch collar detectors were fabricated and evaluated. From the current-voltage measurements, the electrical resistivity was estimated to be ∼1.5×1010 Ω·cm for CdTe and 2-5×1011 Ω·cm for CZT. The Hecht analysis of electron and hole mobility-lifetime products (μτe and μτh) showed μτe=2×10−3 cm2/V (μτh=8×10−5 cm2/V) and μτ3-6×10−3 cm2/V (μτh=4-6×10−5 cm2/V) for CdTe and CZT, respectively. Final assessments of the detector performance have been carried out using 241Am (60 keV) and 137Cs (662 keV) energy sources and the results are presented in this paper.

α Particle response of undoped CdMnTe

The properties required for γ-ray detection of single-crystal Cadmium Manganese Telluride (Cd 1− x Mn x Te) are investigated and samples are evaluated as room temperature radiation detectors. Undoped Cd 1− x Mn x Te 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±0.01 eV, corresponding to a manganese fraction in the sample of x=0.13. The hole mobility-lifetime product was measured at 150 K and was found to be of the order of 1×10 −5 cm 2/V.

Local photocurrent mapping as a probe of contact effects and charge carrier transport in semiconductor nanowire devices

Three types of two-terminal CdS nanowire devices with distinct current versus voltage characteristics were fabricated by forming Schottky and/or Ohmic contacts in a controlled manner. Argon ion bombardment of CdS nanowires increased the carrier concentration allowing the formation of Ohmic Ti–CdS contacts. Scanning photocurrent microscopy (SPCM) was used to explore the influence of the contacts on the spatially resolved photoresponse in two-terminal devices and to analyze charge carrier transport processes. Modeling of the spatial profiles of the local photocurrent images enabled the quantitative extraction of electron and hole mobility-lifetime products in Ohmic devices and the hole mobility-lifetime product in Schottky devices. Analysis of the evolution of SPCM images with bias suggests that the electric field is localized to the optical generation region in the Ohmic devices and localized beneath the contacts in the Schottky devices.

Characterization of Cd1−xZnxTe crystals grown from a modified vertical bridgman technique

Cd1−xZnxTe (CZT) crystals grown from a modified vertical Bridgman technique were characterized by means of an optical polarized transmission technique using the Pockels effect, low-temperature direct current (DC) photo-conductivity technique, low-temperature photoluminescence (PL) spectroscopy, room-temperature PL mapping technique, and detector performance measurements. Electric field mapping indicates that an approximation of a uniform electric field distribution approximation is generally satisfied for CZT detectors operated at room temperature under typical working conditions. A nonuniform electric field distribution is observed under intense infrared (IR) light illumination, and a model is proposed based on charge generation of defects, trapping, and space-charge effects. The largest hole mobility-lifetime product (μτ)h of the CZT detector measured by DC photoconductivity is 7.0 × 10−4 cm2/V. The detector treated with 2% bromine in methanol chemical etch has a relatively small surface recombination velocity at room temperature, which was obtained from DC photocurrent and detector performance tests, as measured by irradiation of 5.5-MeV α particles and 59.6-keV γ-rays, respectively. We have clearly shown the equivalence of charge collection efficiency results measured by both DC photocurrent and α particle response. Low-temperature DC photocurrent measurements show that surface recombination velocity increases significantly with decreasing temperature from 300 K to 250 K. The effective electron mobility-lifetime product—combination effects of bulk and surface of CZT crystal—increases with increment of temperature. Room-temperature PL mapping measurements indicate uniformity of zinc concentration within CZT crystals. Low-temperature PL spectroscopy shows that the dominant emission peaks are excitons, which are bound to either shallow neutral donors (D0, X) or neutral acceptors (A0, X), depending on the temperature, concentration of donors and acceptors, and the incident light intensity. It was found that the luminescence of (D0, X) depends linearly on the incident laser intensity, while (A0, X) has a nonlinear dependence.

The Influence of Deposition Conditions on the Electronic Properties of a-Si:H Prepared in Expanding Thermal Plasma

AbstractA number of a-Si:H samples prepared in an expanding thermal plasma under varying conditions were examined by means of time-of-flight transient photocurrent measurements. A high deposition temperature allows high deposition rates while achieving high hole mobility and mobility-lifetime product. Lower hole mobility but higher μτ product result from slower deposition at lower temperature. Electron μτ products are uncharacteristically low for all samples due to pronounced deep trapping. RF biasing of the substrate does not improve the results.

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

Influence of light-soaking and annealing on electron and hole mobility–lifetime products in a-Si:H

Light-soaking and annealing effects on majority and minority carrier properties in undoped a-Si:H and simultaneous changes in subgap optical absorption are investigated. Subgap absorption was obtained by constant photocurrent method (CPM). Electron mobility–lifetime product was deduced from photoconductivity. Hole mobility–lifetime product was estimated from measurements of ambipolar diffusion length by steady state photocarrier grating technique (SSPG). Hysteresis-like behaviour is found in the relationship between mobility–lifetime products of electrons and holes during photodegradation and isothermal annealing which is a signature of distinct evolutions of the gap-states density along the different sections of the metastability cycle. Analysis of changes of the sub-gap absorption coefficient at different photon energies and numerical simulations with a recombination model suggest the consideration of two species of metastable states with different sensitivities to light-exposure and annealing.

Characterisation of single-crystal mercuric iodide

The mobility-lifetime product of electrons and holes in single-crystal mercuric iodide for detector applications was measured to be 4×10 −5 and 3×10 −5 cm 2/V respectively. The charge carriers were optically induced by a near band gap excitation using a GaP (560 nm) light emitting diode. Optical Deep Level Transient Spectrometry measurements of trapping states showed three dominant energy levels at 0.26, 0.8 and 1.4 eV. There is little correlation between trapping levels reported in the literature.

Parameters for photoelectronic characterisation and the Fermi level in amorphous silicon

We discuss the application of the so-called parameter b for the characterisation of hydrogenated amorphous and microcrystalline silicon samples. This parameter which reflects the ratio of the electron to hole mobility-lifetime products and results from excess-carrier properties. It was suggested that it is useful because b also monitors the Fermi level position, which is a thermal-equilibrium property. Our study for both doped and undoped amorphous silicon samples confirms the trend that b increases when the Fermi levels moves towards the conduction band. Despite this general trend we do not find a one-to-one relationship between b and the Fermi level. A determination of the Fermi level from the dark conductivity as the corresponding equilibrium property appears to be more reliable.

The X-ray response of TlBr

We present the results of a series of X-ray measurements on several prototype TlBr detectors. The devices were fabricated from mono-crystalline material and were typically of size 2.7×2.7×0.8 mm3. The material is extremely pure, having impurity concentrations <100 ppm. The measured electron and hole mobility–lifetime products were found to be 3×10−4 and 1×10−5 cm2 V−1, respectively, which are about an order of magnitude higher than previously reported values. Three detectors were fabricated and extensively tested over the energy range 2.3–100 keV at three synchrotron radiation facilities: the Physikalisch-Technische Bundesanstalt (PTB) laboratory at the Berliner Elektronenspeicherring fur Synchrotronstrahlung (BESSY II), the European Synchrotron Radiation Research Facility (ESRF) and the Hamburger Synchrotron-strahlungslabor (HASYLAB) radiation facility. Room temperature energy resolutions under full-area illumination of 1.8 and 3.3 keV FWHM have been achieved at 5.9 and 59.95 keV, respectively. At reduced detector temperatures of −30°C, these fall to 800 eV and 2.6 keV FWHM, respectively. Under monochromatic pencil beam illumination, the measured energy resolutions at 6 and 60 keV were 664 eV and 3 keV FWHM at the same temperature. For energies <20 keV, the measured spectra display symmetric photopeaks. However, the peaks become increasingly tailed at higher energies. At the highest energies, the energy-losses due to the electrons and holes are clearly separated. Whilst the detectors gave reproducible results over 12 months of operation, it was observed that for synchrotron beam measurements above 45 keV, they were unstable, showing rate dependent gain shifts and polarization effects. These were not observed at lower energies. The spatial uniformity of the detectors was measured using a 50×50 μm2, 12 keV mono-energetic X-ray beam, raster scanned over the forward active area. Whilst two detectors were spatially uniform to a level commensurate with statistics, the third was not. In all cases, evidence was found for charge collection problems caused by field fringing.

Light-soaking and annealing kinetics of majority and minority carrier mobility–lifetime products in a-Si:H

A study on undoped a-Si:H of the light-soaking/annealing process of majority as well as minority carrier properties and their relations to defect density is presented. Defect density was measured by the constant photocurrent method (CPM), electron mobility–lifetime product was deduced from photoconductivity and hole mobility–lifetime product was obtained from measurements of ambipolar diffusion length by steady-state photocarrier grating technique (SSPG). We confirm the already known hysteresis behaviour in the dependence of electron mobility–lifetime product on defect density, and report a new and opposite hysteresis-like dependence of the hole mobility–lifetime product during the light-soaking/annealing process. The changes in the mobility–lifetime products of both carriers and concomitant changes in the defect density and in the Fermi level are discussed on the basis of recombination kinetics and structural order effects.

Recent progress in n-type CdZnTe arrays for gamma-ray spectroscopy

This study focuses on the characterization of n-type CdZnTe gamma-ray spectrometers, which are designed for pixelated imaging array. The intentionally doped n-type CdZnTe crystals are grown by IMARAD Imaging Systems by a modified horizontal Bridgman technique. Imaging arrays, which are produced by IMARAD with ohmic contacts are compared with arrays with different types of contacts, fabricated at Technion-Israel Institute of Technology. We investigate the performance of n-type MSM spectrometers fabricated with several types of contacts: ohmic anodes and cathode, rectifying anodes and cathode as well as mixed (i.e., ohmic anodes and rectifying cathode). The DC characteristics are correlated with the detector dark noise and the spectral performance. The study presents the significant recent advance in the technology of room-temperature semiconductor gamma-ray spectrometers. It also gives physical insight to the parameters that determine the performance of n-type CdZnTe spectrometers. The comparison between spectrometers with different contacts may indicate whether the high performance of IMARAD arrays depends upon electron injection from the cathode, as previously suggested, or to adequate hole mobility-lifetime products in the n-type CdZnTe material, as suggested here.