High Dark Resistivity Research Articles

Direct & efficient detection of x-ray radiation using thermally evaporated 2D hybrid lead halide perovskite (BA2PbBr4)(BA=n-Butylammonium=C4H9NH3) film

This report demonstrates fast response and direct detection of x-ray radiation by thermally evaporated 2D hybrid halide perovskite (BA2PbBr4)(n-BA=Butylammonium=C4H9NH3) based film. A highly efficient pin-hole free/compact film (thickness∼1 μm), via a controlled evaporation process, has been grown to fabricate solid-state x-ray radiation detector film (typical dimension ∼1 × 1 cm2) of 2D hybrid halide perovskite BA2PbBr4 ( BAPB) material. Direct detection of x-rays is possible using this detector by simple electrical readout method, where a change in resistance occurs while exposed to x-ray radiation at ambient temperature. The detector current increases when an x-ray impinges on it and exhibits the highest sensitivity ∼1000 μC/Gy/cm2. The detector film shows fast response, exhibiting quick response-recovery time ∼ ms towards x-ray. The detector film exhibits reasonably high dark resistivity ∼ 1010Ω−cm and good mobility-lifetime ( μτ) product 3.308×10−6cm2/s. The 2D hybrid halide perovskite (BA2PbBr4) based detector can sustain an effective high electric field ∼5000 V cm−1 corresponding to 40 V operational bias. It shows very good radiation resistant towards x-ray and retains its crystal structure, as confirmed from structural data under sustained x-ray exposure. The 2D halide perovskite detector (BAPB) shows long-term stability or shelf life (∼4 months) and response is highly repeatable with less than 5% fluctuation, which is an important key performance metric for any radiation detector. This detector has good application potential in several areas like, medical imaging, security checking, etc.

Bismuth oxysulfide thin films for light and humidity sensing

Bismuth oxysulfide (BOS) films were formed on dielectric glass substrates by the chemical bath deposition. They have a high sensitivity to moisture content and demonstrate the decrease in the electrical resistance up to three orders of magnitude when the relative humidity reaches 85% and more. The high sensitivity of resistive structure representing 0.65 µm thick film between two Ag contacts is associated with randomly oriented thin nanoplate crystals. The presence of a great number of intergrain boundaries restricts electron transport (both in the dark and under illumination), which is desirable for resistive humidity sensors. The BOS films possess a high surface-to-volume ratio making them ultra-sensitive to adsorption of water. It is supposed that ionic conductivity in a thin layer of adsorbed water plays a crucial role in fast response of sensing structure (1–3 s) to adsorption-desorption cycles. High dark resistance and low photoconductivity of BOS films make them practically insensitive to processes of molecular oxygen adsorption and action of light when they are used as humidity sensing structure.

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.

Material properties and performance of ErAs:In(Al)GaAs photoconductors for 1550 nm laser operation

ErAs:In(Al)GaAs photoconductors have proven to be outstanding devices for photonic terahertz (0.1–10 THz) generation and detection with previously reported sub-0.5 ps carrier lifetimes. We present the so far most detailed material characterization of these superlattices composed of ErAs, InGaAs, and InAlAs layers grown by molecular beam epitaxy. The variation of the material properties as a function of the ErAs concentration and the superlattice structure is discussed with focus on source materials. Infrared spectroscopy shows an absorption coefficient in the range of 4700–6600 cm−1 at 1550 nm, with shallow absorption edges toward longer wavelengths caused by absorption of ErAs precipitates. IV characterization and Hall measurements show that samples with only 0.8 monolayers of electrically compensated ErAs precipitates (p-delta-doped at 5×1013 cm−2) and aluminum-containing spacer layers enable high dark resistance (∼10–20 MΩ) and high breakdown field strengths beyond 100 kV/cm, corresponding to >500 V for a 50 μm gap. With higher ErAs concentration of 1.6 ML (2.4 ML), the resistance decreases by a factor of ∼40 (120) for an otherwise identical superlattice structure. We propose a theoretical model for calculation of the excess current generated due to heating and for the estimation of the photocurrent from the total illuminated current. The paper concludes with terahertz time-domain spectroscopy measurements demonstrating the strengths of the material system and validating the proposed model.

Photoconductive mechanism of IR-sensitive iodized PbSe thin films via strong hole-phonon interaction and minority carrier diffusion.

Photoconductive PbSe thin films are highly important for mid-infrared imaging applications. However, the photoconductive mechanism is not well understood so far. Here we provide additional insight on the photoconductivity mechanism using transmission electron microscopy, x-ray photoelectron microscopy, and electrical characterizations. Polycrystalline PbSe thin films were deposited by a chemical bath deposition method. Potassium iodide (KI) was added during the deposition process to improve the photoresponse. Oxidation and iodization were performed to sensitize the thin films. The temperature-dependence Hall effect results show that a strong hole-phonon interaction occurs in oxidized PbSe with KI. It indicates that about half the holes are trapped by KI-induced self-trapped hole centers (Vk center), which results in increasing dark resistance. The photo Hall effect results show that the hole concentration increases significantly under light exposure in sensitized PbSe, which indicates the photogenerated electrons are compensated by trapped holes. The presence of KI in the PbSe grains was confirmed by I 3d5/2 core-level x-ray photoelectron spectra. The energy dispersive x-ray spectra obtained in the scanning transmission electron microscope show the incorporation of iodine during the iodization process on the top of PbSe grains, which can create an iodine-incorporated PbSe outer shell. The iodine-incorporated PbSe releases electrons to recombine with holes in the PbSe layer so that the resistance of sensitized PbSe is about 800 times higher than that of PbSe without the iodine-incorporated layer. In addition, oxygen found in the outer shell of PbSe can act as an electron trap. Therefore, the photoresponse of sensitized PbSe is from the difference between the high dark resistance (by KI addition and iodine incorporation) and the low resistance after IR exposure due to electron compensation (by electron traps at grain boundary and electron-hole recombination in KI hole traps).

Improved InGaAs and InGaAs/InAlAs Photoconductive Antennas Based on (111)-Oriented Substrates

The terahertz wave generation by spiral photoconductive antennas fabricated on low-temperature In0.5Ga0.5As films and In0.5Ga0.5As/In0.5Al0.5As superlattices is studied by the terahertz time-domain spectroscopy method. The structures were obtained by molecular beam epitaxy on GaAs and InP substrates with surface crystallographic orientations of (100) and (111)A. The pump-probe measurements in the transmission geometry and Hall effect measurements are used to characterize the properties of LT-InGaAs and LT-InGaAs/InAlAs structures. It is found that the terahertz radiation power is almost four times higher for LT-InGaAs samples with the (111)A substrate orientation as compared to (100). Adding of LT-InAlAs layers into the structure with (111)A substrate orientation results in two orders of magnitude increase of the structure resistivity. The possibility of creating LT-InGaAs/InAlAs-based photoconductive antennas with high dark resistance without compensating Be doping is demonstrated.

Avalanche Gain in Metal–Semiconductor–Metal Ga2O3 Solar-Blind Photodiodes

Metal–semiconductor–metal structured photodetectors based on β-Ga2O3 thin films were fabricated. Because of the high dark-resistance and considerable photoconduction, extremely uneven distribution ...

Toward Ultrahigh Sensitivity and UV–Vis–NIR Broadband Response of Organolead Halide Perovskite/Tin–Phthalocyanine Heterostructured Photodetectors

Organolead halide perovskite has recently emerged as a star material for various photoelectronic devices owing to its excellent optical and electronic properties. However, it is challenging to develop a perovskite-based photodetector with response across the UV–vis–NIR spectrum due to the lack of absorption in the NIR region for perovskite materials. Herein, we report a broadband photodetector based on the MAPbI3/SnPc heterostructure with mutual-complementary absorption spectra, extending the photoresponse from UV–vis to the NIR region because of the dominant absorption of the SnPc. The ultralow dark current of ∼0.01 nA is obtained and the ultrahigh photosensitivity is up to 105, which is attributed to the high dark resistance and the high electron injection barrier of the heterostructure, and the enhanced photoresponse by the heterostructure architecture. The outstanding sensitivity of our broadband photodetector can be comparable or superior to that of some inorganic, organic, and perovskite photodetectors reported previously. This work provides a route to designing high-performance broadband photodetectors with low noise and high sensitivity.

Efficient photoconductive terahertz detector with all-dielectric optical metasurface

We designed an optically thin photoconductive channel as an all-dielectric metasurface comprising an array of low-temperature grown GaAs nanobeams and a sub-surface distributed Bragg reflector. The metasurface exhibited enhanced optical absorption, and it was integrated into a photoconductive THz detector, which showed high efficiency and sensitivity as a result. The detector produced photocurrents over one order of magnitude higher compared to a similar detector with an unstructured surface with only 0.5 mW of optical excitation while exhibiting high dark resistance required for low-noise detection in THz time-domain spectroscopy and imaging. At that level of optical excitation, the metasurface detector showed a high signal to noise ratio of 106. The detector showed saturation above that level.

ε-Ga2O3 epilayers as a material for solar-blind UV photodetectors

Electrical and optical properties of undoped single-phase ε-Ga2O3 epitaxial films prepared by MOCVD are reported. It is shown that this still unexplored polymorph of gallium oxide possesses wide bandgap and very high dark resistivity, thus allowing the design and fabrication of solar-blind UV photodetectors. Simple cost-effective photoresistors, fabricated by direct deposition of the epilayers on c-oriented sapphire substrates, exhibited good performance. The physical properties and the photoresponse of ε-Ga2O3 make this material very interesting in view of novel applications.

Study on ultrafast dynamics of low-temperature grown GaAs by optical pump and terahertz probe spectroscopy

Low-temperature-grown GaAs (LT-GaAs) possesses high carrier mobility, fast charge trapping, high dark resistance, and large threshold breakdown voltage, which make LT-GaAs a fundamental material for fabricating the ultrafast photoconductive switch, high efficient terahertz emitter, and high sensitive terahertz detector. Although lots of researches have been done on the optical and optoelectrical properties of LT-GaAs, the ultrafast dynamics of the photoexcitation and the relaxation mechanism are still unclear at present, especially when the photocarrier density is close to or higher than the defect density in the LT-GaAs, the dispersion of photocarriers shows a complicated pump fluence dependence. With the development of THz science and technology, the terahertz spectroscopy has become a powerful spectroscopic method, and the advantages of this method are contact-free, highly sensitive to free carriers, and sub-picosecond time resolved. In this article, by employing optical pump and terahertz probe spectroscopy, we investigate the ultrafast carrier dynamics of photogenerated carriers in LT-GaAs. The results reveal that the LT-GaAs has an ultrafast carrier capture process in contrast with that in GaAs wafer. The photoconductivity in LT-GaAs increases linearly with pump fluence at low power, and the saturation can be reached when the pump fluence is higher than 54 J/cm2. It is also found that the fast process shows a typical relaxation time of a few ps contributed by the capture of defects in the LT-GaAs, which is strongly dependent on pump fluence: higher pump fluence shows longer relaxation time and larger carrier mobility. By employing Cole-Cole Drude model, we can reproduce the photoconductivity well. Our results reveal that photocarrier relaxation time is dominated by the carrier-carrier Coulomb interaction: under low carrier density, the carrier-carrier Coulomb interaction is too small to screen the impurity-carrier scattering, and impurity-carrier scattering plays an important role in the photocarrier relaxation process. On the other hand, under high pump fluence excitation, the carrier-carrier Coulomb interaction screens partially the impurity-carrier scattering, which leads to the reduction of impurity-carrier scattering rate. As a result, the photocarrier lifetime and mobility increase with increasing pump fluence. The experimental findings provide fundamental information for developing and designing an efficient THz emitter and detector.

Growth and characterization of ErAs:GaBixAs1−x

We explore the growth and characterization of ErAs:GaBiAs as a candidate material for terahertz generation and detection via photoconductive switches. Spectrophotometry shows that the incorporation of small amounts of bismuth causes a reduction in the band gap, making these materials compatible with fiber-coupled lasers. ErAs pins the Fermi level within the band gap, causing high dark resistance while maintaining high mobility, shown by Hall effect measurements. Finally, transient absorption (optical pump, optical probe) measurements show that the ErAs provides a carrier recombination pathway, causing short carrier lifetimes. These material properties make ErAs:GaBiAs an interesting choice for fiber-coupled photoconductive switches.

Significantly enhanced photoresponse in carbon nanotube film/TiO2 nanotube array heterojunctions by pre-electroforming

Traditional TiO2 based photodetectors (PDs) suffer from high dark resistance, which increases loss of photoexcited charge carriers. Here, we report a new and simple way to improve the performance of PDs based on double-walled carbon nanotube (DWCNT)/TiO2 nanotube heterojunctions. Highly ordered TiO2 nanotube arrays were fabricated using a two-step anodic oxidation method, and coated with a DWCNT film, which functioned as a semitransparent electrode and a photoactive layer. Via pre-electroforming, the device was switched from a high resistance state (HRS) to a low resistance state (LRS). At an applied bias of 1 V, the dark resistance was reduced from 926 to 0.67 kΩ, as a result of the formation of oxygen vacancy related conducting filaments. The photoresponse (ΔI = Ip − Id, where Ip and Id represents photocurrent and dark current, respectively) of the PD in LRS reached 816.76 μA W−1 under 532 nm laser illumination and 802.89 μA W−1 under 1064 nm laser irradiation, which is 965 and 3980 times higher, respectively, than those obtained from the HRS device under the same conditions. This strategy for enhancing the photoresponse of TiO2 based PDs may have applications in further improving the power conversion efficiency of dye-sensitized solar cells.

Towards optical‐quality nanocrystalline diamond with reduced non‐diamond content

AbstractOur nominally undoped nanocrystalline diamond (NCD) films were deposited on fused silica substrates by the microwave plasma enhanced chemical vapor deposition (MW CVD) at a relatively low temperature below 600 °C. They show high dark resistivity and measurable photosensitivity after surface oxidation. We present the “true” optical absorptance spectra calculated from transmittance T and reflectance R measurements corrected on the surface scattering and compare them with the normalized photocurrent spectra. The optical scattering does not allow to evaluate the small optical absorption in visible and near IR range from the T and R spectra. The photocurrent spectra were measured in the ultraviolet, visible, and near infrared optical range using the dual beam photocurrent spectroscopy (DBP) under constant UV illumination. Previously, NCD films often showed non‐diamond content with the photo‐ionization threshold at 0.8 eV increasing significantly the optical absorption in near IR and visible region. Here, we show that the non‐diamond content can be reduced by several orders of magnitude by depositing NCD on the carefully selected UV‐grade fused silica substrates under the optimized growth conditions followed by the post‐deposition chemical etching and cleaning. Unlike the NCD layers with high non‐diamond content, the NCD layers with reduced non‐diamond content are stable up to 450 °C.

On the reduction of the non-diamond phase in nanocrystalline CVD diamond films

We present photocurrent spectra of nominally undoped nanocrystalline diamond (NCD) films grown on glass substrates by hot filament (HF) and microwave (MW) plasma enhanced chemical vapor deposition (CVD). The spectra were measured in a broad optical range (200–2000 nm) by dual-beam photocurrent spectroscopy (DBP) and Fourier-transform photocurrent spectroscopy (FTPS) in amplitude modulated step scan mode. The NCD films with carefully oxidized surface show photosensitivity and high dark resistivity. Unlike single crystal type IIa diamond with the photonization threshold at 5.5 eV, the photocurrent spectra of NCD films are dominated by the “non-diamond phase” with the photo-ionization threshold at about 0.8 eV. Some HF CVD samples have lower sub-band gap absorption (non-diamond phase contamination). The non-diamond phase content increases after annealing at elevated temperature. The non-diamond phase content can be reduced by exposing NCD to hydrogen plasma at temperature below 350 °C.

Simulation, modeling, and crystal growth of Cd0.9Zn0.1Te for nuclear spectrometers

High-quality, large (10 cm long and 2.5 cm diameter), nuclear spectrometer grade Cd0.9Zn0.1Te (CZT) single crystals have been grown by a controlled vertical Bridgman technique using in-house zone refined precursor materials (Cd, Zn, and Te). A state-of-the-art computer model, multizone adaptive scheme for transport and phase-change processes (MASTRAP), is used to model heat and mass transfer in the Bridgman growth system and to predict the stress distribution in the as-grown CZT crystal and optimize the thermal profile. The model accounts for heat transfer in the multiphase system, convection in the melt, and interface dynamics. The grown semi-insulating (SI) CZT crystals have demonstrated promising results for high-resolution room-temperature radiation detectors due to their high dark resistivity (ρ≈2.8 × 1011 Θ cm), good charge-transport properties [electron and hole mobility-life-time product, μτe≈(2–5)×10−3 and μτh≈(3–5)×10−5 respectively, and low cost of production. Spectroscopic ellipsometry and optical transmission measurements were carried out on the grown CZT crystals using two-modulator generalized ellipsometry (2-MGE). The refractive index n and extinction coefficient k were determined by mathematically eliminating the ∼3-nm surface roughness layer. Nuclear detection measurements on the single-element CZT detectors with 241Am and 137Cs clearly detected 59.6 and 662 keV energies with energy resolution (FWHM) of 2.4 keV (4.0%) and 9.2 keV (1.4%), respectively.

Layered Compound Semiconductor GaSe and GaTe Crystals for THz Applications

AbstractThe single crystal growth of layered semiconductors GaSe and GaTe by vertical Bridgman technique using zone refined selenium (Se), tellurium (Te) and high purity (HP) gallium (Ga) have been described. The grown crystals (2.5 cm diameter and ∼10 cm long) have demonstrated efficient broadband tunable THz emission and as sensitive THz detectors. The crystals have shown promising characteristics with good optical quality, high dark resistivity, wide band gap (GaSe-2.01 eV and GaTe-1.66 eV at 300 K), good anisotropic (parallel, p & perpendicular, pa) electrical properties (σ∥ vs σ⊥ and μ∥ vs σ⊥) and long term stability. Different steps involved in processing GaSe and GaTe crystals as THz sources and sensors are described.

Photo-Hall effect measurements in P, N and B-doped diamond at low temperatures

Resistivity and Hall effect measurements are applied to understand the electronic properties of homoepitaxially grown n- and p-type CVD diamond in the wide temperature range 10–900 K. To overcome the high dark resistivity at low temperature, carrier concentration is enhanced by photo-excitation. It is found that the lifetime of photo-excited carriers decreases significantly at low temperature whereas their mobility increases. It is suggested that the short lifetime and the continuous photo-excitation can lead to the non-equilibrium stationary distribution of the photo-excited carriers at low lattice temperature. The enhanced average kinetic energy of the photo-excited carriers can effectively reduce ionized impurity scattering hence increasing the carrier mobility.

Semiconductor Materials for Ultrafast Photoswitches

This paper gives a review of semiconductor materials that are used to fabricate ultrafast photoswitches. The optoelectrical response of the switches is first described with simple models, from which the material requirements are deduced. The basic principles of the required material properties - ultrashort free carrier lifetime and high mobility, high dark resistivity, and high field breakdown - are explained. Then, the most popular ultrafast semiconductors are listed, together with their characteristics. A special emphasis is put on low-temperature grown GaAs. Finally, two applications of these ultrafast materials are presented, namely antennae for terahertz radiation and all-optical nonlinear devices.

Effects of Deposition Parameters and Conditions on the Physical and Electro-Optical Properties of Buffer Solution Grown CdS Thin Films

CdS films were grown by a buffer solution growth technique using CdSO 4 as the cadmium source, thiourea as the sulphur source, and (NH 4 ) 2 SO 4 as the buffer. It has been shown that by varying the ratio (R c ) of molar concentration of (NH 4 ) 2 SO 4 to that of CdSO 4 in the solution, with other deposition parameters fixed, the growth rate of CdS films, and hence, its thickness can be adjusted. For [CdSO 4 ] 0.002 M, a CdS film of maximum thickness of 120 nm was obtained for R c = 10, above which the film gradually became thinner because of the slow release of Cd 2+ into the solution to participate in the heterogeneous CdS film deposition, and below which thin films (50 to 80 nm) were grown because of the undesirable homogeneous precipitation of CdS particles in the solution. When the deposition solution was not covered during CdS film deposition, a loss of deposition solution via evaporation was observed causing a degradation of the film properties in the form of reduced CdS film thickness, relatively non-uniform film surface, high dark resistivity and optical energy gap.