Electron Mobility-lifetime Product Research Articles

Characterization of pure organic NTI crystals for direct x-ray detection

Accurate detection of x-ray radiation is essential for tissue equivalence detection and safety control during medical radiotherapy. Here, a promising radiation detector based on organic 1,8-naphthalimide (C12H7NO2, NTI) single crystals is proposed. NTI single crystals with dimensions of 8 × 2 × 1 mm3 were synthesized by an optimized solvent-cooling method, and the crystal structure was characterized to elucidate the anisotropy mechanism. The coplanar detector made by NTI has a leakage current of less than 0.1 pA at an electric field strength of 1 kV cm−1 and provides excellent detection of 5.49 MeV α-particles (241Am) with a full-energy peak resolution of up to 32.3%. The calculated electron mobility (μe) and electron mobility-lifetime product (μτ)e were 7.9 cm2 V−1s−1 and 1.56 × 10−6 cm2 V−1, respectively. Additionally, the detection limit of the NTI detector was 0.35 µGy s−1 in the electric field strength range of 0.2–1.0 kV cm−1 under a 20 kV x-ray beam with a high sensitivity of 56.2 μC Gy−1cm−2. Consequently, the NTI detector achieves an excellent spatial resolution of 0.8–0.9 lp mm−1 in terms of x-ray imaging capability.

Characterization of a 5 mm thick CZT-Timepix3 pixel detector for energy-dispersive γ-ray and particle tracking

The present manuscript describes a comprehensive characterization of a novel highly segmented 5 mm CZT sensor attached to Timepix3. First, the sensor’s IV curve was measured and basic sensor characterization was done with laboratory γ-radiation sources. The sensor resistivity was determined to be (0.155± 0.02) GOhm · cm. The sensor showed decent homogeneity, both for the per-pixel count rate and electron mobility-lifetime product μ e τ e. The latter was measured to be = 1.3 × 10−3 cm2/V with a standard deviation σ = 0.4 × 10−3 cm2/V describing the dispersion of values for different pixels. The basic sensor characterization is complemented by measurements at grazing angle in a 120 GeV/c at the CERN’s Super Proton Synchrotron. The penetrating nature of these particles together with the pixelation of the sensor allows for a determination of the charge collection efficiency (CCE), as well as charge carrier drift properties (drift times, lateral charge cloud expansion) as a function of the interaction depths in the sensor. While CCE drops by 30%–40% towards the cathode side of the sensor, from the drift time dependency on interaction depth, the electron mobility μ e was extracted to be (944.8 ± 1.3) cm2/V/s and τ e = (1.38 ± 0.31) μs. The spectroscopic performance was assessed in photon fields and extracted from energy loss spectra measured at different angles in the pion beam. While at photon energies below 120 keV incomplete charge collection leads to an underestimation of the photon energy when irradiated from the front-side, at higher energies the relative energy resolution was found to be ∼4.5%, while a relative energy resolution of ∼7.5% was found for the particle energy loss spectra. It is shown that the drift time information can be used to reconstruct particle interactions in the sensor in 3D, providing a spatial resolution of σ xyz = 241 μm within the sensor volume and a particle trajectory measurement precision Δxyz = 100 μm, at a distance of 1 m from the sensor. We demonstrate by measurement with a 22Na source, that the energy resolution combined with the 3D reconstruction allows for detection of γ-ray source location and polarity using Compton scattering within the sensor (Compton camera and scatter polarimeter).

Large-volume CdZnTe bar detectors characterized by laser-induced transient currents

In this work, we studied the free-carrier transport properties and space-charge formation/polarization in a CdZnTe bar-shaped radiation detector using the Laser-Induced Transient Current technique. We found out that a steady-state space charge ranging from 8 × 108 to 2.1 × 109 cm−3 is formed throughout the detector at about 0.5 s following biasing. The measured current waveforms were modeled by Monte Carlo simulations after taking into account the electric and weighting fields inside the detector and shielding box. Modeling of the unpolarized waveforms revealed an exceptionally high electron mobility-lifetime product μeτe≥0.095cm2V−1. The observed formation of positive space charge in the biased detector and linear scaling of the current waveform shapes on applied bias revealed that the detector polarization is attributed to carrier injection at the anode. Measurements in pulsed and DC bias in modified electrode geometry proved the surface-charge formation.

Photoluminescence of CdTe and CdZnTe compounds doped with 2% selenium

The addition of 2 % of selenium in CdTe and CdZnTe (CZT) exhibited an additional acceptor-bound exciton in photoluminescence (PL). From the PL and inductively coupled plasma mass spectrometer (ICP/MS) analyses, the additional acceptor-bound exciton emission is associated with a Cu impurity in the relatively low-purity (5N) precursor CdSe starting material. In addition, thermal quenching of the acceptor/donor bound exciton emissions do not require a shift to any higher excited states or to the free exciton state. ICP/MS analysis also showed a higher concentration of Sn in CdZnTeSe (CZTS) compared to CdTe and CZT which is known to form a deep level trap at around EC – 0.83 eV. The trapping and de-trapping times indicate that the Sn-related deep trap acts as an electron trapping center. The relatively low electron mobility-lifetime product of CZTS might be correlated with the concentration of Sn in the impure CdSe material. Pulse height spectra using an Eu-152 gamma-ray source was taken for a 2-mm-thick planar CZTS detector. The spectra clearly showed all the expected gamma peaks with energy lower than 344.3 keV.

Enhanced hole mobility-lifetime product in selenium-added CdTe compounds

An improved detector performance for 2% selenium-added CdZnTe(CZT) was observed and explained by the reduced concentration of a deep-level hole trap, which was confirmed by photoluminescence measurements of CdZnTeSe(CZTS) and CdTeSe(CTS). The hole lifetime in CZTS was found to be 2.18μs in the selenium-containing material, which is much longer than that typically measured by 10–100 times. The prolonged lifetime of hole carriers is likely caused by the disappearance of hole trapping at a specific binding energy of 1.1-eV below the conduction band. Also, the enhanced pulse height spectra of CZTS gamma-ray detectors were explained in terms of the ratio of electron and hole mobility-lifetime products. Cu-migrated patterns are used for the first time to reveal sub-grains and their networks in CZT and CZTS. Based on qualitative analysis, the Cu-migrated patterns in CZT and CZTS showed similar results.

Development of Single Crystal CsPbBr3 Radiation Detectors from Low-Cost Solution Synthesized Material

The all-inorganic perovskite CsPbBr3 is a strong candidate for room-temperature, semiconducting radiation detecting applications. With a high stopping power, a large bandgap, and a high mobility-lifetime (μτ) product for both holes and electrons, CsPbBr3 contains all the desirable properties of a room temperature radiation detector. Unfortunately, the production of detector-grade single crystal samples requires high quality starting materials, which must be further processed to achieve the desired purity for semiconducting operation. We have developed a modified zone refining method combining the continuous purification of ternary CsPbBr3 and the subsequent crystal growth step. Taking advantage of this technique, low-cost polycrystalline CsPbBr3 synthesized via solution chemistry processes can be directly used in the production of high-purity, detector-grade crystals. Semi-cylindrical CsPbBr3 single crystal ingots up to 120 mm in length and 22 mm in diameter were obtained. The final product exhibited good chemical stoichiometry and high trace metal purity (2.34 ppm across 73 elements). Detector devices fabricated from the crystal wafers displayed resistivities > 2.0 × 108 Ω·cm and high photocurrent responses. The radiation detectors were able to produce spectroscopy responses to 241Am α-particle. The hole mobility-lifetime (μτH) product of the detectors was determined to be in the range of 1.45 × 10−3 cm2/V.

Gel growth and characterization of Cs3Bi2Br9 perovskite single crystals for radiation detection

Metal halide perovskites have been sought for ionizing radiation detection due to their tunable bandgap, high quantum yield, high absorption coefficient, excellent charge transport properties, flexible chemistry synthesis and cost-effective manufacturing. Among the family of perovskites, bismuth-based halide perovskites have attracted a rapidly growing interest as possible alternatives to lead-based halide perovskites in the development of nontoxic perovskites for opto-electronic devices. Herein, bismuth-based inorganic perovskite Cs3Bi2Br9 single crystals were successfully grown using an innovative dual-diffusion gel growth technique for the first time. Silica gel has been developed as an enabling medium for gel growth of single crystals due to its transparency and easy control of nucleation sites. The UV-vis transmission spectrum was recorded using a light source of deuterium and halogen lamps and a Tauc plot was obtained, which gave an estimate of the bandgap energy, 2.54 eV. Silver electrodes were used on the top surface and the bottom surface of Cs3Bi2Br9 single crystals for material characterization and detector tests. Current-voltage (I-V) measurements gave a room-temperature resistivity of 1.79×1011 Ω•cm. The Cs3Bi2Br9 single crystals were then tested for X-ray response using ON\OFF testing which revealed attractive responsiveness for X-ray photons (rise and fall time: ≈0.3 s ). Using the net current (IXray−Idark), which can be extracted from the X-ray response measurements at varied applied voltages, a modified Hecht fitting was applied to estimate the mobility-lifetime product of electrons, μτ=5.12×10−4 ±6.70×10−5 cm2/V for a Ag/Cs3Bi2Br9/Ag device. This study shows that our innovative crystal growth method, enabled by the unique gel growth process, can be used as an appealing technique to grow functional crystals for opto-electronic devices. Meanwhile, Cs3Bi2Br9 has shown great potential as a promising candidate for X-ray detection applications. The efforts in this work will serve as a metric for growing halide perovskites in the gel for opto-electronic devices.

Effects of annealing in Te2 atmosphere on photoelectric properties and carrier transport properties of CdZnTe films

CdZnTe (CZT) epitaxial films were deposited on GaAs substrates by closed spaced sublimation. In order to reduce the adverse effects of point defects and dislocations in the film on its properties, CZT films were annealed at 400 °C in Te2 atmosphere at different times. The I–V test and energy spectrum show that the resistivity of CZT films increased from 109 Ω cm to 1010 Ω cm, and the electron mobility lifetime product (μτ)e increased from 10−5 cm2 V−1 to 10−4 cm2 V−1. According to the etch pit density and photoluminescence tests, the results show that annealing reduced the dislocation density from 3.28 × 105 cm−2 to 2.52 × 105 cm−2 in the CdZnTe films. In this study, the annealing process which can stably improve the properties of CdZnTe films is obtained, that is, annealing at 400 °C in Te2 atmosphere for 2–4 h.

Vertical gradient freeze growth of detector grade CdZnTeSe single crystals

We report the growth of detector grade Cd0.9Zn0.1Te0.97Se0.03 (CZTS) single crystals, a recently discovered quaternary semiconductor for room temperature radiation detection, by a vertical gradient freeze (VGF) method. VGF is a comparatively low-temperature growth method and avoids relative motion between the heater and the ampoule containing the precursor materials which minimizes any thermal drift or temperature fluctuations. As a result, CZTS single crystals with superior charge transport properties has been obtained. Growth of detector-grade CZTS single crystals using VGF method has not been reported yet. X-ray spectroscopy based elemental analysis showed that the grown crystals demonstrated the desired stoichiometry required for high resolution radiation detection. Planar detectors fabricated by deposition of gold contacts (∼0.07 cm2) demonstrated high bulk resistivity ∼1010 Ω-cm and a very low leakage current density of 2.8 × 10−8 A/cm2 at a bias of 100 V when measured at room temperature. The detectors showed excellent radiation response with 100 % charge collection efficiency when exposed to 5486 keV alpha particles. The electron mobility-lifetime (μτ) product was measured to be 3 × 10−3 cm2/V using a single polarity Hecht analysis which is at par with the recently reported values measured in CZTS grown using conventional Bridgman or travelling heater method. The electron mobility has been calculated to be 964 cm2V−1s−1 using a time-of-flight (TOF) method, a substantial improvement over that obtained from conventionally grown CZTS single crystals.

Effect of low-temperature rapid annealing on structural and electrical properties of electroless platinum contacts on CdZnTe detectors

In this work, platinum (Pt) electrodes were deposited on CdZnTe wafers by electroless H2PtCl4 solution and subsequently annealed at different temperature from 323 K to 473 K. Focused ion beam (FIB) imaging, transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), current-voltage (I–V) analysis and energy spectra under 241Am irradiation were adopted to investigate the effect of low-temperature rapid annealing on interface structure, electrical and carrier transport properties of Pt/CdZnTe samples. Single-crystal PtTe layer with [100] orientation and thickness of 20 nm was found in the interface between electroless Pt and CdZnTe wafer. The leakage current of the Pt/CdZnTe detectors at 200 V decreased from 212 nA cm−2 to 101 nA cm−2 after low-temperature rapid annealing, due to a decrease of voltage drop across the PtTe interface layer. The energy spectra and carrier transport performance of Pt/CdZnTe detectors suggested that the optimal annealing temperature was 423 K. The energy spectra of Pt/CdZnTe detector under 241Am showed that the FWHM was improved from 23.3% to 14.4%, while the full charge collection and mobility lifetime product of electrons increasing from 611 to 625 and 3.17 × 10−3 to 3.76 × 10−3 cm2 V−1 s−1, respectively.

Growth and characterization of detector-grade CdMnTeSe

The Cd0.95Mn0.05Te0.98Se0.02 (CMTS) ingot was grown by the vertical Bridgman technique at low pressure. All wafers showed high resistivity, which suggests potential as a room-temperature semiconductor detector. The resistivity of the CMTS planar detector was 1.47 × 1010 Ω· cm and mobility lifetime product of electrons was 1.29 × 10−3 cm2/V. The spectroscopic property with Am-241 and Co-57 was evaluated. The energy resolution about 59.5 keV gamma-ray of Am-241 was 11% and the photo-peak of 122 keV gamma-ray from Co-57 was clearly distinguished. The result shows the first detector-grade CMTS in the world and proves CMTS’s potential as a radiation detector operating at room temperature.

Minority carrier decay length extraction from scanning photocurrent profiles in two-dimensional carrier transport structures

Carrier transport was studied both numerically and experimentally using scanning photocurrent microscopy (SPCM) in two-dimensional (2D) transport structures, where the structure size in the third dimension is much smaller than the diffusion length and electrodes cover the whole terminal on both sides. Originally, one would expect that with increasing width in 2D transport structures, scanning photocurrent profiles will gradually deviate from those of the ideal one-dimensional (1D) transport structure. However, the scanning photocurrent simulation results surprisingly showed almost identical profiles from structures with different widths. In order to clarify this phenomenon, we observed the spatial distribution of carriers. The simulation results indicate that the integrated carrier distribution in the 2D transport structures with finite width can be well described by a simple-exponential-decay function with the carrier decay length as the fitting parameter, just like in the 1D transport structures. For ohmic-contact 2D transport structures, the feasibility of the fitting formula from our previous 1D analytical model was confirmed. On the other hand, the application of a simple-exponential-decay function in scanning photocurrent profiles for the diffusion length extraction in Schottky-contact 2D transport structures was also justified. Furthermore, our simulation results demonstrate that the scanning photocurrent profiles in the ohmic- or Schottky-contact three-dimensional (3D) transport structures with electrodes covering the whole terminal on both sides will reduce to those described by the corresponding 1D fitting formulae. Finally, experimental SPCM on a p-type InGaAs air-bridge two-terminal thin-film device was carried out. The measured photocurrent profiles can be well fitted by the specific fitting formula derived from our previous 1D analytical model and the extracted electron mobility-lifetime product of this thin-film device is 6.6 × 10–7 cm2·V−1. This study allows us to extract the minority carrier decay length and to obtain the mobility-lifetime product which can be used to evaluate the performance of 2D carrier transport devices.

In and Al Schottky Contacts Comparison on P-Type Chlorine-Doped CdTe.

The performance of the CdTe radiation detectors heavily relies on the method of contact preparation. A convenient research method addressing this problem is the laser-induced transient current technique. In this paper, we compare the performance of two CdTe crystals which underwent different metallization processes. We showed that appropriately designed Au/Al contacts induce much less bulk polarization than commercial Pt/In electrodes under the same working conditions and can thus provide a convenient alternative to the industry standard. The comparison was based on the monitoring of the time-dependent sensor polarization measuring transient currents excited by above-bandgap laser illumination complemented by the Am 241 gamma spectroscopy. The theoretical analysis of current waveforms and radiation spectra enabled us to determine the charge carrier mobility, mobility-lifetime products of electrons and holes, and temporal and bias dependence of the space charge formation.

Solution-Grown Formamidinium Hybrid Perovskite (FAPbBr3) Single Crystals for α-Particle and γ-Ray Detection at Room Temperature.

Compared with the widely reported MAPbBr3 single crystals, formamidinium-based (FA-based) hybrid perovskites FAPbBr3 (FPB) with superior chemical and structure stability are expected to be more efficient and perform as more reliable radiation detectors at room temperature. Here, we employ an improved inverse temperature crystallization method to grow FPB bulk single crystals, where issues associated with the retrograde solubility behavior are resolved. A crystal growth phase diagram has been proposed, and accordingly, growth parameters are optimized to avoid the formation of NH4Pb2Br5 secondary phase. The resulting FPB crystals exhibit a high resistivity of 2.8 × 109 Ω·cm and high electron and hole mobility-lifetime products (μτ) of 8.0 × 10-4 and 1.1 × 10-3 cm2·V-1, respectively. Simultaneously, the electron and hole mobilities (μ) are evaluated to be 22.2 and 66.1 cm2·V-1·s-1, respectively, based on the time-of-flight technique. Furthermore, a Au/FPB SC/Au detector is constructed that demonstrates a resolvable gamma peak from 59.5 keV 241Am γ-rays at room temperature for the first time. An energy resolution of 40.1% is obtained at 30 V by collecting the hole signals. These results demonstrate the great potential of FAPbBr3 as a hybrid material for γ-ray spectroscopy and imaging.

Charge collection in h-BN neutron detectors at elevated temperatures

Many of the neutron detector applications are in the environments with harsh conditions such as high temperatures. We report here the measurements of charge collection parameters of hexagonal boron nitride (h-BN) detectors at elevated temperatures, including the electron mobility-lifetime product (μτ) and surface recombination field (s/μ). It was found that μτ is increased, while s/μ is decreased as temperature increases. The temperature dependence of the surface recombination field (s/μ) revealed that electrons trapped in the surface states tend to thermally activate to the bulk region at higher temperatures with an activation energy of about 0.22 eV, leading to a reduction in the density of the charged surface states at elevated temperatures. Consequently, the charge collection efficiency is enhanced at elevated temperatures due to a reduced surface recombination field and increased electron mobility-lifetime product. The results suggested that h-BN neutron detectors are favorable for high temperature operation.

Defect levels characterized by photoconductivity and thermally stimulated current in CdZnTe crystals

Deep-level defects in CdZnTe (CZT) crystals were studied by combining photoconductivity (PC) and thermally stimulated current (TSC) measurements. The stretched-exponential function could well describe the time-dependent photocurrent decay kinetics, and the decay time constant was fitted to be ~54 s for CZT1 and ~98 s for CZT2, respectively. TSC spectra were analyzed through SIMPA fitting, and the total defect density was calculated to be about 1.14 × 1016 cm−3 in CZT1 and 1.80 × 1016 cm−3 in CZT2, respectively. Deep donor (Te antisites) could be considered as dominating the photoconductivity decay process. The electron mobility was fitted in Time-of-Flight (TOF) spectra to be about 783 cm2/Vs in CZT1 and 717 cm2/Vs in CZT2, respectively. Approximately the same electron mobility of the two CZT crystals corresponds to similar concentration of all the defect traps. The mobility-lifetime (μτ) product for electrons could be fitted by Hecht equation to be about 1.61 × 10−3 cm2/V in CZT1 and 3.01 × 10−4 cm2/V in CZT2, respectively. Higher concentration of deep donor (Te antisites) in CZT2 compared to that in CZT1 will lead to lifetime reduction and the resultant lower (μτ)e product.

Hydrogen passivation of defect levels in the annealed CdZnTe:In crystals

Abstract

The mechanism of defects effect on carrier transport by employing multi-wavelength sub-bandgap lights on CdZnTe crystal

The effect of different point defects in CdZnTe (CZT) crystals on carrier transport performance were discussed. The energy level and concentration of the traps were determined by the photo-induced current transient spectroscopy (PICTS). By adding the infrared light illumination into the PICTS experiment, the effect of sub-bandgap light on defects was observed. Meanwhile, several DC photoconductive experiments were done under different wavelength lights were used to explore the effect of the different defects on carrier transport properties of CZT crystals. Furthermore, the energy spectrum test was used to investigate the effect of sub-bandgap light on the energy resolution of CZT detectors. The data results showed that sub-bandgap light could effectively change the ionization state of defects. The doubly ionized Te antisite (TeCd2+) had the greatest effect on mobility-lifetime product(μτ) of electrons owing to its stronger ability of capturing electrons and energy level located around the midgap. While the secondary ionized Cd vacancy (VCd2−), as considered as a hole trap, had the greatest effect on μτ-product of holes. Consequently, choosing suitable infrared light can improve the detection performance of CZT crystal effectively.

Thallium Bromide Gamma-Ray Spectrometers and Pixel Arrays

Thallium bromide (TlBr) is a compound semiconductor with a band gap of 2.68 eV making it ideal for room temperature radiation detection. The high atomic numbers, 81 and 35, and the high density of 7.56 g/cm3 give it excellent gamma-ray stopping power. TlBr is a cubic material that melts congruently at a relatively low temperature (~ 480 C). These properties make it relatively easy to grow good quality crystals with high yield. As a result of improvements in the purification of TlBr mobility-lifetime product of electrons, µτe, is now on the order of 10-2 cm2/V, which is similar to that of CZT. High µτe enables the fabrication of thicker detectors with good charge collection and energy resolution. The properties of TlBr make it ideal for use in room temperature gamma radiation operation [Smith 2013]. Single carrier devices such as small pixel arrays [O'Neal 2018a] and Frisch collar devices [Hitomi, et al.2013] which were developed for CZT can also been applied to TlBr. For example, better than 1% FWHM at 662 keV has been obtained for single pixel events with small (e.g. 3 x 3 pixels, 1-mm pitch, 5-mm thick) arrays.

Defects, electronic properties, and α particle energy spectrum response of the Cd0.9Mn0.1Te: V single crystal

Cadmium manganese telluride is a promising material for fabricating room-temperature nuclear radiation detectors widely used in medical imaging, environmental protection, nuclear security detection, astrophysics, and so on. The Cd0.9Mn0.1Te: V (V: CMT) crystal examined in this work was grown through the Te solution (10% excess) vertical Bridgman method. The low-temperature photoluminescence (PL) spectra indicated that the grown crystal has good quality. A simultaneous thermal excitation current spectrum was used to characterize the effect of vanadium doping on the level defects in the crystal. The current–voltage and Hall test results showed that the crystal resistivity was (3.781–6.185) × 1010 Ω cm. The conductivity was of n type. The carrier concentration was (1.69–9.94) × 106 cm−3. The Hall mobility was (3.08–9.29) × 103 cm−2 V−1 s−1. The maximum measured ratio of the light and dark currents, when the crystal was exposed to 5 mW white light, was 11. In addition, the room-temperature electron mobility-lifetime product of the middle sample was 6.925 × 10−4 cm2 V−1 using the 241Am@5.48 MeV α particle source.