Carrier Mobility-lifetime Product Research Articles

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

The heavy metal selenophosphate Pb2P2Se6emerges 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.

Lead-Free Bismuth-Based Perovskite X-ray Detector with High Sensitivity and Low Detection Limit.

Bismuth-based halide perovskites have shown great potential for direct X-ray detection, attributable to their nontoxicity and advantages in detection sensitivity and spatial resolution. However, the practical application of such materials still faces the critical challenge of combining both high sensitivity and low detection limits. Here, we report a new type of zero-dimensional (0D) perovskite (HIS)BiI5 (1, where HIS2+ = histamine) with high sensitivity and a low detection limit. Structurally, the strong N-H···I hydrogen bonds between HIS2+ cations and inorganic frameworks enhance the rigidity of the structure and diminish the intermolecular distance between adjacent inorganic [Bi2I10]4- dimers. By virtue of such structural merits, single crystal 1 exhibits excellent physical properties perpendicular to both the (001) and (010) faces. Perpendicular to the (010) face, 1 exhibited a high electrical resistivity (2.31 × 1011 Ω cm) and a large carrier mobility-lifetime product (μτ) (2.81 × 10-4 cm2 V-1) under X-ray illumination. Benefiting from these superior physical properties, it demonstrates an excellent X-ray detection capability with a sensitivity of approximately 103 μC Gyair-1 cm-2 and a detection limit of 36 nGyair s-1 in both directions perpendicular to the (001) and (010) crystal faces. These results provide a promising candidate material for the development of new, lead-free, high-performance X-ray detectors.

Stable photocurrent–voltage characteristics of perovskite single crystal detectors obtained by pulsed bias

Photocurrent–voltage characterization is a crucial method for assessing key parameters in x-ray or γ-ray semiconductor detectors, especially the carrier mobility lifetime product. However, the high biases during photocurrent measurements tend to cause severe ion migration, which can lead to the instability and inaccuracy of the test results. Given the mixed electronic–ionic characteristics, it is imperative to devise novel methods capable of precisely measuring photocurrent–voltage characteristics under high bias conditions, free from interference caused by ion migration. In this paper, pulsed bias is employed to explore the photocurrent–voltage characteristics of MAPbBr3 single crystals. The method yields stable photocurrent–voltage characteristics at a pulsed bias of up to 30 V, proving to be effective in mitigating ion migration. Through fitting the modified Hecht equation, we determined the mobility lifetime products of 1.0 × 10−2 cm2⋅V−1 for hole and 2.78 × 10−3 cm2⋅V−1 for electron. This approach offers a promising solution for accurately measuring the transport properties of carriers in perovskite.

Halide Ordering Enables Superior Charge Transport in 3D (NMPDA)Pb2 I4 Br2 Perovskitoid Single Crystal.

Halide composition engineering has been demonstrated as an effective strategy for optical and electronic properties modulation in 3D perovskites. While the impact of halide mixing on the structural and charge transport properties of 3D perovskitoids remains largely unexplored. Herein, it is demonstrated that bromine (Br) mixing in 3D (NMPDA)Pb2 I6 (NMPDA = N-methyl-1,3-propane diammonium) perovskitoid yields stabilized (NMPDA)Pb2 I4 Br2 with specific ordered halide sites, where Br ions locate at the edge-sharing sites. The halide ordered structure enables stronger H-bonds, shorter interlayer distance, and lower octahedra distortion in (NMPDA)Pb2 I4 Br2 with respect to the pristine (NMPDA)Pb2 I6 . These attributes further result in high ion migration activation energy, low defect states density, and enhanced carrier mobility-lifetime product (µτ), as underpinned by the electrical properties investigation and DFT calculations. Remarkably, the parallel configured photodetector based on (NMPDA)Pb2 I4 Br2 single crystal delivers a high on/off current ratio of 3.92×103 , a satisfying photoresponsivity and detectivity of 0.28 AW-1 and 3.05×1012 Jones under 10.94 µWcm-2 irradiation, superior to that of (NMPDA)Pb2 I6 and the reported 3D perovskitoids. This work sheds novel insight on exploring 3D mixed halide perovskitoids toward advanced and stable optoelectronic devices.

X-Site Substituted 2D Cs2 Pb(SCN)2 Br2 Perovskites for X-Ray Detection.

2D Ruddlesden-Popper (RP) perovskites have been intensively investigated due to their superior stability and outstanding optoelectrical properties. However, investigations on 2D RP perovskites are mainly focused on A-site substituted perovskites and few reports are on X-site substituted perovskites especially in X-ray detection field. Here, X-site substituted 2D RP perovskite Cs2 Pb(SCN)2 Br2 polycrystalline wafers are prepared and systematically studied for X-ray detection. The obtained wafers show a large resistivity of 2.0 × 1010 Ω cm, a high ion activation energy of 0.75eV, a small current drift of 2.39 × 10-6 nA cm-1 s-1 V-1 , and charge carrier mobility-lifetime product under X-ray as high as 1.29 × 10-4 cm2 V-1 . These merits enable Cs2 Pb(SCN)2 Br2 wafer detectors with a sensitivity of 216.3 µC Gyair -1 cm-2 , a limit of detection of 42.4 nGyair s-1 , and good imaging ability with high spatial resolution of 1.08 lp mm-1 . In addition, Cs2 Pb(SCN)2 Br2 wafer detectors demonstrate excellent operational stability under high working field up to 2100V cm-1 after continuous X-ray irradiation with a total dose of 45.2 Gyair . The promising features such as short octahedral spacing and weak ion migration will open up a new perspective and opportunity for SCN-based 2D perovskites in X-ray detection.

Ultrasound-Assisted Crystallization Enables Large-Area Perovskite Quasi-Monocrystalline Film for High-Sensitive X-ray Detection and Imaging.

In recent years, halide perovskites have shown great application potential in X-ray detection due to their superior optoelectronic properties and high X-ray attenuation coefficient. However, large-area perovskite fabrication for high performance X-ray detectors remains extremely challenging. Herein, ultrasound-assisted crystallization combined with the hot-pressing method is proposed to prepare large-area (10cm × 10cm) and high-quality quasi-monocrystalline thick film of a mixed-cation perovskite MA0.42 FA0.58 PbI3 . The rapid ultrasound-assisted crystallization provides more homogeneous nucleation, which is essential to the fabrication of large-area and uniform perovskite microcrystalline film. Furthermore, the post hot-pressing treatment is implemented to fuse the crystal boundaries, rearrange the crystal grains, and eliminate the voids between crystals, resulting in a quasi-monocrystalline film. After the hot-pressing treatment, the carrier mobility and the carrier mobility-lifetime product increased about 13-fold (from 1.8 to 23.5 cm2 s-1 V-1 ) and 18 times (from 8.4 × 10-6 to 1.5 × 10-4 cm2 V-1 ), respectively. As a result, a high-performance MA0.42 FA0.58 PbI3 quasi-monocrystalline X-ray detector is achieved with an impressively high sensitivity (1.16 × 106 µC Gyair -1 cm-2 ) and low detection limit (37.4 nGyair s-1 ), demonstrating the potential of the ultrasound-assisted crystallization and hot-pressing strategy from an industrial perspective.

Direct Fast-Neutron Detection by 2D Perovskite Semiconductor.

Fast-neutrons play a critical role in a range of applications, including medical imaging, therapy, and nondestructive inspection. However, direct detecting fast-neutrons by semiconductors has proven to be challenging due to their weak interaction with most matter and the requirement of high carrier mobility-lifetime (µτ) product for efficient charge collection. Herein, a novel approach is presented to direct fast-neutron detection using 2D Dion-Jacobson perovskite semiconductor BDAPbBr4 . This material features a high fast-neutron caption cross-section, good electrical stability, high resistivity, and, most importantly, a record-high µτ product of 3.3 × 10-4 cm2 V-1 , outperforming most reported fast-neutron detection semiconductors. As a result, BDAPbBr4 detector exhibited good response to fast-neutrons, not only achieving fast-neutron energy spectra in counting mode, but also obtaining linear and fast response in integration mode. This work provides a paradigm-shifting strategy for designing materials that efficiently detect fast-neutrons and paves the way toward exciting applications in fast-neutron imaging and therapy.

Numerical modeling based on the analytical solution of the steady-state small signal photocarrier grating equations and analysis of field-dependent measurements for noncrystalline Si: H samples

The available experimental data of field-dependent steady-state photocarrier grating (SSPG) are numerically modeled employing the derived formula of [Formula: see text] (where [Formula: see text] represents the ratio of coherent to incoherent currents in SSPG technique) obtained from the analytical solution of the small signal steady-state grating problem by applying Galerkin’s method. This numerical analysis shows the extent of validity and success of using the derived formula of [Formula: see text] [R. I. Badran, Results Phys. 17, 103079 (2020)] in reproducing the field-dependent data for noncrystalline Si: H samples. The best quality of fittings is reproduced when [Formula: see text] value is minimum. This analysis provides us with approximate and corrected values for each of transport quantities, namely, the small-signal response lifetime, electron and hole mobilities that are found in agreement with the values obtained from the applications of other methods. For two polymorphous (pm) Si: H samples, the average estimated, corrected values for the carrier density and mobility-lifetime product, namely, [Formula: see text][Formula: see text]cm[Formula: see text] and [Formula: see text][Formula: see text]cm[Formula: see text] and ([Formula: see text][Formula: see text]cm2V[Formula: see text] and [Formula: see text][Formula: see text]cm2V[Formula: see text], respectively, are obtained. The analysis is also extended to analyze the field-dependent data for nanocrystalline (nc) Si: H samples at low temperatures. The extracted values of [Formula: see text] and ([Formula: see text] are [Formula: see text][Formula: see text]cm[Formula: see text] and [Formula: see text][Formula: see text]cm2V[Formula: see text].

High-Performance and Self-Powered X-Ray Detectors Made of Smooth Perovskite Microcrystalline Films with 100-μm Grains.

Perovskite single crystals and polycrystalline films have complementary merits and deficiencies in X-ray detection and imaging. Herein, we report preparation of dense and smooth perovskite microcrystalline films with both merits of single crystals and polycrystalline films through polycrystal-induced growth and hot-pressing treatment (HPT). Utilizing polycrystalline films as seeds, multi-inch-sized microcrystalline films can be in-situ grown on diverse substrates with maximum grain size reaching 100 μm, which endows the microcrystalline films with comparable carrier mobility-lifetime (μτ) product as single crystals. As a result, self-powered X-ray detectors with impressive sensitivity of 6.1 × 104 μC Gyair-1 cm-2 and low detection limit of 1.5 nGyair s-1 are achieved, leading to high-contrast X-ray imaging at an ultra-low dose rate of 67 nGyair s-1. Combining with the fast response speed (186 μs), this work may contribute to the development of perovskite-based low-dose X-ray imaging.

Investigation of defects in melt and solution grown perovskite CsPbBr3 single crystals

Recently, there has been considerable interest in x-ray and gamma ray detectors with large volume and high energy resolution that operate at room temperature. To improve detector energy resolution, the carrier mobility-lifetime product needs to be increased, and the electronic trap state concentration needs to be minimized. Defect concentrations in the part per billion range can alter the charge transport and carrier recombination lifetime. In this work, thermally stimulated current spectroscopy measurements were systematically carried out in bulk halide perovskite single crystals of CsPbBr3 over a temperature range of 80–320 K. The origins and trap parameters of CsPbBr3 crystals from the solution growth and melt growth procedures were determined and compared. Trap concentrations were ranged from 1 × 1011 to 1 × 1016 cm−3. Appreciable detector performance was observed for CsPbBr3 crystals with trap concentrations less than 1 × 1014 cm−3. The comparison of spectral responses of crystal samples grown using two different methods shows that, after purification, solution-grown crystals are comparable to melt-grown crystals in terms of low defect concentration and improved detector performance. For an improved mobility-lifetime product and enhanced spectral response to high energy radiation from fissile materials, trap states in either type of a crystal ingot must be reduced closer to 1011 cm−3.

MAPb(Br1–xClx)3 Hybrid Perovskite Materials for Direct X-ray Detection

The study and development in recent years of hybrid (organic–inorganic) halide perovskite materials have given them an unprecedented opportunity for direct ionizing radiation detection, given their large attenuation coefficient and sufficient charge carrier mobility lifetime product. The use of single crystals, considered as model materials, allows us to investigate their intrinsic properties. Characterizations under X-ray illumination of detector devices based on methylammonium lead tribromide (MAPbBr3) single crystals, obtained by optimized growths, show good sensitivity but high dark current density. To improve this critical parameter, while using MAPbBr3 as the base material, we employ anion engineering within the halide elements. We present here mixed halide perovskite crystals, with bromide partially replaced with chloride, obtained through optimized growths using modified inverse temperature crystallization in dimethylformamide, leading to high-quality single crystals of the general formula MAPb(Br1–xClx)3. Six chlorine contents are targeted and carefully determined experimentally via energy-dispersive X-ray analysis and X-ray powder diffraction. For each composition, several crystals are synthesized and used to prepare X-ray detection devices. Their optoelectronic properties are determined under standard X-ray medical conditions and hint at the existence of an optimal composition. MAPb(Br0.85Cl0.15)3 exhibits the best sensitivity with a value of S ≈ 3 μC mGyair–1 cm–2 for RQA5 spectral quality and the lowest dark current density with a value of Jdark ≈ 22 nA mm–2, both recorded at a 50 V mm–1 electric field. This sensitivity value doubles our own MAPbBr3 single crystal device and is higher than that of CsI(Tl)- or a-Se-based flat panels. The present work broadens the benefits and drawbacks of employing halide engineering in perovskite materials to improve the optoelectronic performance under high-energy radiation.

X-ray Detectors Based on Halide Perovskite Materials

Halide perovskite has remarkable optoelectronic properties, such as high atomic number, large carrier mobility-lifetime product, high X-ray attenuation coefficient, and simple and low-cost synthesis process, and has gradually developed into the next-generation X-ray detection materials. Halide perovskite-based X-ray detectors can improve the sensitivity and reduce the detectable X-ray dose, which is applied in imaging, nondestructive industrial inspection, security screening, and scientific research. In this article, we introduce the fabrication methods of halide perovskite film and the classification and progress of halide perovskite-based X-ray detectors. Finally, the existing challenges are discussed, and the possible directions for future applications are explored. We hope this review can stimulate the further improvement of perovskite-based X-ray detectors.

Effects of electrode fabrication on electrical properties of CdMgTe room temperature radiation detectors

Cd1-xMgxTe (CdMgTe) is an ideal alternative material for room temperature radiation detectors. In this paper, the effects of different electrode structures on the performance of CdMgTe detectors were studied. The electrodes were prepared by vacuum evaporation and chemical precipitation method using Au and Al materials. The results indicated that CdMgTe detectors with Au/CdMgTe/Al electrode structure prepared by vacuum evaporation method had the best electrical properties. For the crystals grown by Cd-excess and Te-rich conditions, the resistivity were 2.89 × 1011 Ω cm and 3.8 × 1011 Ω cm, the leakage current (under 100 V bias voltage) were 4 nA and 3 nA, and the Hall mobility were 2.73 × 103 cm2 V−1 s−1 and 3.69 × 102 cm2 V−1 s−1, respectively. The conductive type of the crystals were weak n-type. 241Am @ 5.48 MeV α particle spectra of the detectors showed that the optimal energy resolution were 8.66% and 8.62%, and the carrier mobility-lifetime products (μτ)e were 1.47 × 10−3 cm2/V and 1.03 × 10−3 cm2/V, respectively. Moreover, the carrier mobility obtained by time-of-flight (TOF) test were 15.01 cm2/(V·s) and 77.79 cm2/(V·s), respectively.

Performance optimization of CZT thick films by CdTe buffer layers for solar-blind ultraviolet photoelectric detectors

CdZnTe (CZT) thick films with CdTe buffer layers (CZT/CdTe) were studied for high-performance ultraviolet photoelectric detector. CZT thick films were prepared on AlN/Si substrates via close-spaced sublimation (CSS) and CdTe buffer layers were introduced to optimize the interface quality of CZT thick films. Results of x-ray diffraction showed that the full width at half maximum (FWHM) and dislocation density (δ) of interface of optimized samples were 0.156 ° and 2.543 × 1014 lin/m2, respectively. Interface detect analysis of free-standing CZT and CZT/CdTe thick films was studied by x-ray photoelectron spectroscopy (XPS) and low-temperature photoluminescence (PL) spectroscopy, and mechanism of in-situ annealing in CdTe atmosphere to reduce interfacial defect states was discussed. Moreover, the photo-dark current ratio of the CZT/CdTe thick film metal-semiconductor-metal (MSM) structural photodetector was as high as 5530, which was nearly double that of the CZT thick film detector, and the carrier mobility lifetime product could reach 5.59 × 10−4 cm2/V at 100 V bias, confirming the improvement of carrier transport capacity and detection efficiency. CZT-based detectors have inestimable prospect in the field of solar-blind ultraviolet detection with the participation of CdTe buffer layers.

Self-Powered X-ray Detection and Imaging using Cs2AgBiCl6 Lead-Free Double Perovskite Single Crystal

Despite the high performance of lead-halide perovskite-based X-ray detectors, the toxicity and instability of lead require the development of lead-free perovskites for X-ray detection applications. Here, we demonstrate lead-free and environmentally friendly, all-inorganic millimeter-sized Cs2AgBiCl6 double perovskite single crystal (SC) for X-ray detection and imaging. The high dark resistivity (3.1 × 1010 Ω cm), high carrier mobility-lifetime product (5.36 × 10–4 cm2 V–1), and lower trap density (1.18 × 109 cm–3) in these double perovskite SCs render them a potential material for X-ray detection. The fabricated vertical structured X-ray detector exhibits a sensitivity of 325.78 μC Gy air–1 cm–2 and a limit of detection of 241 nGy s–1. Additionally, the Cs2AgBiCl6 SCs exhibit self-powered X-ray detection at zero bias with a sensitivity of 7 μC Gy air–1 cm–2. Moreover, the fabricated perovskite X-ray detector exhibits a stable and robust performance under continuous X-ray irradiation and long-term ambient storage. Further, we demonstrated the imaging capability of the Cs2AgBiCl6 X-ray detector using a metal test object and obtained a distortion-free image. Our findings demonstrate that the Cs2AgBiCl6 single crystal-based X-ray detectors have great potential as practical X-ray detectors and imaging for medical radiography.

Surface Recombination and Space-Charge-Limited Photocurrent-Voltage (PC-V) Measurements in (Cd,Mn)Te Samples-Kinetics of Photocurrent (PC).

Photocurrent-voltage characteristic (PC-V) is a method of determining the critical parameter in X-ray and gamma-ray detector plates, i.e., the carrier mobility-lifetime product, μτ. We show for the (Cd,Mn)Te samples that the measurement results depend strongly on the surface treatment and the space charge distribution. The PC-V characteristics obtained for ħω > Eg and ħω ~ Eg indicated that etching with 20% HCl caused an appearance of a significant concentration of very shallow surface traps at the (Cd,Mn)Te sample surface. These traps seriously changed the results of measurements of PC-V characteristics and PC kinetics. We also noticed a small contribution of holes to photoconductivity in the PC kinetics. The measurements of PC-V characteristics for ħω > Eg may test the detector plate surface quality.

Boosting the Optoelectronic Performance by Regulating Exciton Behaviors in a Porous Semiconductive Metal–Organic Framework

Exciton behaviors including exciton formation and dissociation dynamics play an essential role in the optoelectronic performance of semiconductive materials but remain unexplored in semiconductive metal-organic frameworks (MOFs). Herein, we reveal that the exciton behaviors in semiconductive MOFs can be regulated by framework-guest interactions, a feature often not achievable in traditional inorganic or organic semiconductors. Incorporation of the electron-deficient molecule within the pores of a terbium-based semiconductive MOF (Tb2L2·4H2O·6DMF, L = TATAB3-, 4,4',4″-s-triazine-1,3,5-triyltri-p-aminobenzoate, DMF = N,N-dimethylformamide) results in efficient energy transfer from the MOF skeleton to molecular acceptors, with a yield of up to 77.4%. This interaction facilitates distinctive exciton type conversion, giving rise to modified conductivity and photoelectric performance. We further fabricated a MOF-based X-ray detection device to demonstrate how the new architecture bolsters the optoelectronic efficiency, which outperforms the properties of parent semiconductive MOFs, with more than 60 times and 40 times enhancement of the photocurrent on-off ratio and detection sensitivity, respectively. With judiciously optimized exciton behaviors, the detection device exhibits a high sensitivity of 51.9 μC Gyair-1 cm-2 and records a charge carrier mobility-lifetime product of 1.12 × 10-3 cm2 V-1 among MOF-based X-ray detectors, which are competitive with values for commercially available detectors. These findings demonstrate a rational synthetic approach to designing exciton arrangements to improve the optoelectronic efficiency of semiconductive MOFs.

Fine-control-valve of halide perovskite single crystal quality for high performance X-ray detection

Halide perovskite single crystals (HPSCs) provide a unique platform to study the optoelectronic properties of such emerging semiconductor materials, while the temperature induced crystal growth method often has an increased solute integration speed and/or unavoidable solute consumption, resulting in a soaring or slumping crystal growth rate of HPSCs. Here, we developed a universal and facile solvent-volatilization-limited-growth (SVG) strategy to finely control the crystal growth rate by the fine-control-valve for high quality crystal grown through solution processes. The grown HPSCs by SVG method exhibited a record low trap density of 2.8 × 108 cm−3 and a high charge carrier mobility-lifetime product (μτ product) of 0.021 cm2/V, indicating the excellent crystal quality. The crystal surface defects were further passivated by oxygen suppliers as Lewis base, which led to a reduction of surface leakage current by two times when using for low dose rate X-ray detection. Such HPSC X-ray detector displayed a high sensitivity of 1274 µC/(Gyair cm2) with a lowest detectable dose rate of 0.56 μGyair/s under 120 keV hard X-ray. Further applications including alloy composition analysis and metal flaw detection by HPSC detectors were also demonstrated, which not only shows the bright future for product quality inspection and non-destructive materials analysis, but also paves the way for growing high quality single crystals and fabricating polycrystalline films.

Halide-modulated self-assembly of metal-free perovskite single crystals for bio-friendly X-ray detection

Halide-modulated self-assembly of metal-free perovskite single crystals for bio-friendly X-ray detection

Device performance of chemical vapor deposition monocrystal diamond radiation detectors correlated with the bulk diamond properties

Diamond radiation detectors (DRDs) based on type-IIa chemical vapor deposition (CVD) monocrystal diamond are more suitable for engineering applications with high consistency requirements due to the lower cost and property controllability. However, their saturated charge collection efficiencies (CCEs) have huge differences. Six type-IIa CVD monocrystal diamond plates were analyzed by the Fourier transform infrared spectrometer, high resolution x-ray diffraction, Raman spectroscopy, photoluminescence and secondary ion mass spectroscopy. Then DRDs were fabricated by them and the CCEs were measured under the irradiation of 241Am source. The results show that the most important factor restricting CCEs are the impurities in the diamond plates, while dislocations with a density <1 × 107 cm−2 for all samples have weaker impact in our case. The reason is that the carrier mobility-lifetime (μτ) product of diamond is more strongly influenced by impurities in this dislocation density range. Thinning diamond plate, if the thickness is kept above 100 μm, is not a good means to obtain high performance DRDs. Therefore, to further improve the detector performance, ultra-high purity diamond growth is most important. Moreover, a diamond test and selection scheme for high performance DRDs is also demonstrated.