Hole Mobility-lifetime Products Research Articles

Selenophosphate Pb2P2Se6 Single Crystals Growth by Chemical Vapor Transport (CVT) Method for Radiation Detection.

The heavy metal selenophosphate Pb2P2Se6 emerges as a promising room-temperature X-ray/γ-ray detectors due to its high resistivity, robust radiation-blocking capability, and outstanding carrier mobility-lifetime product, etc. However, the high activity of phosphides poses significant impediment to the synthesis and single crystal growth. In this work, we have prepared high-quality Pb2P2Se6 single crystals with using the chemical vapor transport (CVT) method. The XRD analysis combined with EDS result confirmed the uniform composition of the resulting as-grown single crystals, while UV-Vis-NIR transmittance spectra revealed the bandgap of 1.89 eV. Selected area electron diffraction patterns indicated the crystal belonged to the P21/c(14) space group. Additionally, the Au/Pb2P2Se6/Au device is fabricated, which exhibits a robust X-ray response with a sensitivity of 648.61 μC Gy-1 cm-2 at 400 V mm-1 under 50 kVp. Notably, the device also excels in alpha particle detection, boasting a resolution of ~14.48 % under a bias of 400 V bias. The hole mobility-lifetime product (μτ)h of Pb2P2Se6 is estimated to be ~2.58×10-5 cm2 V-1. The results underscore potential applications of Pb2P2Se6 crystal is in the field of the semiconductor radiation detectors.

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.

Incomplete Charge Collection at Inter-Pixel Gap in Low- and High-Flux Cadmium Zinc Telluride Pixel Detectors.

The success of cadmium zinc telluride (CZT) detectors in room-temperature spectroscopic X-ray imaging is now widely accepted. The most common CZT detectors are characterized by enhanced-charge transport properties of electrons, with mobility-lifetime products μeτe > 10−2 cm2/V and μhτh > 10−5 cm2/V. These materials, typically termed low-flux LF-CZT, are successfully used for thick electron-sensing detectors and in low-flux conditions. Recently, new CZT materials with hole mobility-lifetime product enhancements (μhτh > 10−4 cm2/V and μeτe > 10−3 cm2/V) have been fabricated for high-flux measurements (high-flux HF-CZT detectors). In this work, we will present the performance and charge-sharing properties of sub-millimeter CZT pixel detectors based on LF-CZT and HF-CZT crystals. Experimental results from the measurement of energy spectra after charge-sharing addition (CSA) and from 2D X-ray mapping highlight the better charge-collection properties of HF-CZT detectors near the inter-pixel gaps. The successful mitigation of the effects of incomplete charge collection after CSA was also performed through original charge-sharing correction techniques. These activities exist in the framework of international collaboration on the development of energy-resolved X-ray scanners for medical applications and non-destructive testing in the food industry.

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.

Demonstration of Energy-Resolved γ-Ray Detection at Room Temperature by the CsPbCl3 Perovskite Semiconductor.

The detection of γ-rays at room temperature with high-energy resolution using semiconductors is one of the most challenging applications. The presence of even the smallest amount of defects is sufficient to kill the signal generated from γ-rays which makes the availability of semiconductors detectors a rarity. Lead halide perovskite semiconductors exhibit unusually high defect tolerance leading to outstanding and unique optoelectronic properties and are poised to strongly impact applications in photoelectric conversion/detection. Here we demonstrate for the first time that large size single crystals of the all-inorganic perovskite CsPbCl3 semiconductor can function as a high-performance detector for γ-ray nuclear radiation at room temperature. CsPbCl3 is a wide-gap semiconductor with a bandgap of 3.03 eV and possesses a high effective atomic number of 69.8. We identified the two distinct phase transitions in CsPbCl3, from cubic (Pm-3m) to tetragonal (P4/mbm) at 325 K and finally to orthorhombic (Pbnm) at 316 K. Despite crystal twinning induced by phase transitions, CsPbCl3 crystals in detector grade can be obtained with high electrical resistivity of ∼1.7 × 109 Ω·cm. The crystals were grown from the melt with volume over several cubic centimeters and have a low thermal conductivity of 0.6 W m-1 K-1. The mobilities for electron and hole carriers were determined to ∼30 cm2/(V s). Using photoemission yield spectroscopy in air (PYSA), we determined the valence band maximum at 5.66 ± 0.05 eV. Under γ-ray exposure, our Schottky-type planar CsPbCl3 detector achieved an excellent energy resolution (∼16% at 122 keV) accompanied by a high figure-of-merit hole mobility-lifetime product (3.2 × 10-4 cm2/V) and a long hole lifetime (16 μs). The results demonstrate considerable defect tolerance of CsPbCl3 and suggest its strong potential for γ-radiation and X-ray detection at room temperature and above.

CsPbBr3 perovskite detectors with 1.4% energy resolution for high-energy γ-rays

Halide perovskite semiconductors are poised to revitalize the field of ionizing radiation detection as they have done to solar photovoltaics. We show that all-inorganic perovskite CsPbBr3 devices resolve 137Cs 662-keV γ-rays with 1.4% energy resolution, as well as other X- and γ-rays with energies ranging from tens of keV to over 1 MeV in ambipolar sensing and unipolar hole-only sensing modes with crystal volumes of 6.65 mm3 and 297 mm3, respectively. We report the scale-up of CsPbBr3 ingots to up to 1.5 inches in diameter with an excellent hole mobility–lifetime product of 8 × 10−3 cm2 V−1 and a long hole lifetime of up to 296 μs. CsPbBr3 detectors demonstrate a wide temperature region from ~2 °C to ~70 °C for stable operation. Detectors protected with suitable encapsulants show a uniform response for over 18 months. Consequently, we identify perovskite CsPbBr3 semiconductor as an exceptional candidate for new-generation high-energy γ-ray detection. Energy resolution of high-energy photon detectors is desired for applications ranging from biomedical imaging to homeland security. In this work, perovskite-based γ-ray detection with 1.4% energy resolution is demonstrated.

Material properties and device performance of CdSe radiation detectors

CdSe single crystals are considered as promising candidates for radiation detectors, however, the progress is limited by the fabrication of high-quality CdSe single crystals. Herein, we reported a CdSe-based radiation detector, exhibiting superior detection performance for α-particles and γ-rays. The FWHM of the X-ray rocking curve of CdSe (100) plane was determined to 0.03°. The as-grown CdSe single crystal exhibited a high room-temperature resistivity of 1.83×109Ω cm. Moreover, the detector exhibited an energy resolution of ∼13.22% and ∼11.21% for α-particles and γ-rays, respectively. The hole mobility-lifetime product (μτ(h)) in experimental of the detector up to 2.20 × 10−4 cm2 V−1 was reported by using the Hecht equation.

Low defects density CsPbBr3 single crystals grown by an additive assisted method for gamma-ray detection

Metal halide perovskites have arisen as a new family of semiconductors for radiation detectors due to their high stopping power, large and balanced electron–hole mobility-lifetime (μτ) product, and tunable bandgap.

Effect of Ga substitution for In in LiInSe2 crystals on carrier transport behaviors and alpha particles detection

Due to high density of 6Li isotope and the ability to respond to ionizing radiation through direct charge carrier transport, Li-containing chalcogenides are expected to be the replacement of 3He-tube or neutron reactive material coated semiconductor films for thermal neutron detection. However, the neutron detection efficiency of 6LiInSe2 is limited to 82% ascribed to that 115In isotope captures neutrons and generates gamma rays, which deteriorated the neutron response. Neutron detection efficiency could be improved by gallium substitution since its low neutron capture cross-section. In this work, the electron and hole mobility (μ) of LiIn1−xGaxSe2(x = 0, 0.25, 0.5, 0.75, 1) were studied by first-principles density functional theory (DFT) calculation, which demonstrated the electron mobility (μ e) of LiIn0.5Ga0.5Se2 is close to that of LiInSe2 while the hole mobility (μ h) is much improved. To further certify, both LiIn0.5Ga0.5Se2 and LiInSe2 crystals were grown under similar condition by vertical Bridgman method, exhibited comparable bandgap and bulk resistivity at room temperature. The charge transport behaviors were evaluated under 241[email protected] alpha particles. The full energy peak is clearly resolved in the pulse height spectra. The electron and hole mobility-lifetime products of LiIn0.5Ga0.5Se2 are 1.5×10−5 cm2/V and 6.9×10−5 cm2/V, respectively. The resulting μ e of LiIn0.5Ga0.5Se2 and LiInSe2 crystals are 270 cm2V−1s−1 and 180 cm2V−1s−1, respectively, fitted by time of flight (TOF) technique. In addition, the μ h of LiIn0.5Ga0.5Se2 is obtained with the value of 110 cm2V−1s−1, but the hole response is too weak to be resolved for LiInSe2.

Perovskite CsPbBr3 single crystal detector for alpha-particle spectroscopy

Here we report the first spectroscopic alpha particle detection based on CsPbBr3 detectors with asymmetric contacts. The CsPbBr3 single crystal was grown from the melt using Bridgman method and then fabricated into detectors with different contacts. The In/CsPbBr3/Au detector presented a low dark current density (∼100 nA/cm2) and temporal stable performance under high electric field (1000 V/cm). Such detector demonstrated excellent gamma ray resolving capability with a full-width at half maximum (FWHM) of ∼5.9 keV for the57Co 122 keV γ ray. The CsPbBr3 detector was capable of simultaneously resolving both the alpha particle (5.5 MeV) and γ ray (59.5 keV) peaks from 241Am radioactive isotope. The transport properties of CsPbBr3 were then determined based on the alpha particle spectra and corresponding rise time distributions. The equivalent values of electron and hole mobilities were indicated as 63 and 49 cm2/(V ⋅ s) respectively. The calculated electron and hole mobility-lifetime products were 4.5 × 10−4 and 9.5 × 10−4 cm2/V, respectively, demonstrating superior transport properties of holes over electrons in CsPbBr3. This work widens the scope of perovskite detectors to encompass charged radiation as well as high energy X/γ rays, and will significantly promote and guide further studies on perovskite materials for radiation detection applications.

Electrical parameters retrieval of carbon nanoparticle-based metal semiconductor metal structure by standard methods and beta-ray-induced charge

ABSTRACTSemiconducting carbon nanoparticles (CNPs) represent various applications in sensing systems with exceptional electronic properties. The extraction of electronic parameters of a sensor is very important to interpret the sensing characteristics of the electronic devices. This work is concerned with the extraction of electronic parameters such as the ideality factor, the barrier height, the series resistance, and some other diode parameters of a CNP-based metal semiconductor metal structure. The parameters are determined from the experimental data and physical model using the standard current–voltage (I–V) analysis in the frame of the thermionic emission theory, impedance spectroscopy, and other methods. The mobility-lifetime products (μτ) for electrons and holes in CNP micro-wire were determined by beta-ray-induced charge with Schottky contacts.

Induced Charge Fluctuations in Semiconductor Detectors with a Cylindrical Geometry

Now, compound semiconductors are very appealing for hard X-ray room-temperature detectors for medical and astrophysical applications. Despite the attractive properties of compound semiconductors, such as high atomic number, high density, wide band gap, low chemical reactivity and long-term stability, poor hole and electron mobility-lifetime products degrade the energy resolution of these detectors. The main objective of the present study is in development of a mathematical model of the process of the charge induction in a cylindrical geometry with accounting for the charge carrier trapping. The formulae for the moments of the distribution function of the induced charge and the formulae for the mean amplitude and the variance of the signal at the output of the semiconductor detector with a cylindrical geometry were derived. It was shown that the power series expansions of the detector amplitude and the variance in terms of the inverse bias voltage allow determining the Fano factor, electron mobility lifetime product, and the nonuniformity level of the trap density of the semiconductor material.

Detection of charged particles with a methylammonium lead tribromide perovskite single crystal

Methylammonium lead tribromide (MAPbBr3) perovskite crystals have attracted significant attention due to their attractive performance in various optoelectronic applications such as solar cells, light-emitting devices, photodetectors, and recently in X-ray detectors. In this study, we demonstrate a possible use of perovskite-based devices for detection of charged particles (which can be applied in basic scientific research, health physics, and environmental analysis) and investigate the mechanism of fundamental charge transport inside perovskite crystals. It was found that inexpensive MAPbBr3 single crystals could be used for measuring the energy spectrum of charged particles through direct collection of the produced charge. After fitting the plot of the centroid peak position versus voltage with the Hecht equation for single-polarity charge transport, the obtained hole mobility-lifetime product was in the range of (0.4–1.6)×10−3cm2/V.

Fabrication of double-sided thallium bromide strip detectors

Double-sided strip detectors were fabricated from thallium bromide (TlBr) crystals grown by the traveling-molten zone method using zone-purified materials. The detectors had three 3.4-mm-long strips with 1-mm widths and a surrounding electrode placed orthogonally on opposite surfaces of the crystals at approximately 6.5×6.5mm2 in area and 5mm in thickness. Excellent charge transport properties for both electrons and holes were observed from the TlBr crystals. The mobility-lifetime products for electrons and holes in the detector were measured to be ~3×10−3cm2/V and ~1×10−3cm2/V, respectively. The 137Cs spectra corresponding to the gamma-ray interaction position were obtained from the detector. An energy resolution of 3.4% of full width at half maximum for 662-keV gamma rays was obtained from one “pixel” (an intersection of the strips) of the detector at room temperature.

Characterization of Screen-Printed Mercuric Iodide Photoconductors for Mammography

We describe energy-dependent incomplete signal generation in semiconductor detectors with a simple planar geometry using deep charge trapping and depth-of-interaction models. Based on this formalism, we have characterized mercuric iodide ( ${\rm HgI}_2$ ) photoconductors by extracting performance parameters from x-ray-induced signals under mammographic imaging conditions. The ${\rm HgI}_2$ sample photoconductors were prepared using a simple screen-printing method. For an x-ray tube output from tungsten target and 30-kV setting, the quantum absorption efficiency of samples with a thickness of approximately 0.1 mm was measured to be $ \sim 70\%$ . X-ray sensitivity was measured to be $0.4~\hbox{nC cm}^{ - 2} {\rm mR}^{ - 1}$ . Mobility-lifetime products for electrons and holes were measured as $ \sim 2 \times {10^{ - 6}}~\hbox{cm}^2~\hbox{V}^{ - 1}$ and $ \sim 0.9 \times {10^{ - 6}}~\hbox{cm}^2~\hbox{V}^{ - 1}$ , respectively. Considering incomplete charge collection, the W-value (or $w$ ) was estimated to be $ \sim 27 ~\hbox{eV}$ . Since the characterization method described in this study is simple and does not require sophisticated equipment, it can be useful for the characterization of photoconductor materials.

Long Term Stability of Thallium Bromide Gamma-Ray Spectrometers

Thallium bromide (TlBr) has been under development for room temperature gamma ray spectroscopy. As a result of improvements in material processing and detector fabrication over the last several years, the electron and hole mobility-lifetime products of TlBr are close to those of CdTe. This has allowed us to fabricate 10 mm thick arrays. Without any correction, 4.1% FWHM energy resolution was measured from one pixel of 3×3 array at 662 keV at room temperature. To minimize the polarization effect, lowering the operation temperature was applied to 13 mm thick array and the stable operation beyond 8 months could be seen without any significant degradation of detector performance. Much improved energy resolution of 2.2% or less at 662 keV has been also obtained from one pixel with depth correction. In this paper improved charge collection with Cr contacts and our recent progress made in long term stability of TlBr detectors at room temperature by applying post annealing are presented.

CsHgInS3: a New Quaternary Semiconductor for γ-ray Detection

The new layered compound CsHgInS3 was synthesized using solid state and flux synthesis techniques. The compound is a semiconductor and shows promising properties for X-ray and γ-ray detection. It features a layered structure that crystallizes in the monoclinic space group C2/c with cell parameters: a = 11.2499(7) Ǻ, b = 11.2565(6) Ǻ, c = 22.146(1) Ǻ, β = 97.30(5)°, V = 2781.8(4) Ǻ3, and Z = 8. CsHgInS3 is isostructural to Rb2Cu2Sn2S6, where the Hg, In, and Cs atoms occupy the Cu, Sn, and Rb sites, respectively. Large single crystals with dimension up to 5 mm were grown with a vertical Bridgman method as well as a horizontal traveling heater method. CsHgInS3 has a γ-ray attenuation length comparable to commercial Cd1–xZnxTe and a band gap value of 2.30 eV. The electrical resistivity of CsHgInS3 is anisotropic with values of 98 GΩ cm and 0.33 GΩ cm perpendicular and parallel to the (001) plane, respectively. The mobility-lifetime product (μτ) of electrons and holes estimated from photoconductivity measureme...

A New Approach for Evaluating the Mobility-lifetime Products of Electron-hole Pairs in Semiconductor Detectors

By developing the special relation between the electron and the hole mobility-lifetime products ((µ )e and (µ )h), we propose a new approach to evaluate each parameter for gamma-rays instead of particles. We assumed that the incident gamma-ray was absorbed by only one reaction within the semiconductors because the cross-section of the photoelectric eect is much higher than there of other reactions at low energies. For the planar CZT detector manufactured by eV Products, the values of (µ )e = 1.44 ◊ 10 3 cm 2 /V and (µ )h = 7.56 ◊ 10 5 cm 2 /V were simply derived by using a modified single-parameter Hecht equation and 59.5 keV gamma-rays emitted from the 241 Am isotope.

Measurement of the drift mobilities and the mobility-lifetime products of charge carriers in a CdZnTe crystal by using a transient pulse technique

In this work we present results on the measurement of the drift mobility and the mobility-lifetime product of charge carriers in a 16-pixellated CdZnTe detector. For the determination of an interaction position based on the pulse rise-time method in a CZT detector, it is necessary to characterize the transport properties governed by drift mobility and lifetime for electrons and holes. In order to extract the transport properties of an electron and a hole, we bombarded 5.5-MeV alpha particles from a 241Am source and 81-keV gamma rays emitted from a 133Ba source on the negatively biased contact of the CZT detector. A time-of-flight (TOF) method was used to measure the electron drift mobility at room temperature whose value turned out to be 906.4 cm2/V⋅s. With the Hecht's equation, the electron mobility-lifetime product was also determined from the bias-dependent alpha response and was equal to (9.88 ± 2.33) × 10−3 cm2/V. On the other hand, the hole mobility-lifetime product was evaluated by a model based on the average charge collection efficiency which accounts for the absorption probability with a given photon energy. By using a single parameter fitting of the model, we obtained the hole mobility-lifetime product of (8.28 ± 0.17) × 10−4 cm2/V.