Photoconductive Structures Research Articles

Self-powered, ultraviolet-to-near infrared broadband photodetector based on Ag-doped CsPbI3/PEDOT:PSS heterojunction.

Metal halide perovskites are studied for photodetection applications because of their outstanding optical and electrical properties. A self-powered ultraviolet-to-near infrared broadband photodetector based on a Ag-doped CsPbI3/PEDOT:PSS heterojunction was investigated. The photodetector using a CsPbI3:Ag/PEDOT:PSS heterostructure with a planar photoconductive structure operated over a broad 355-1560 nm wavelength range in self-powered mode. A terahertz signal was modulated with the CsPbI3:Ag/PEDOT:PSS structure at low optical excitation intensity to investigate its photodetection mechanism. The experimentally designed detector can present images of the letters "C", "N" and "U" in the visible and near-infrared wavelengths, indicating a potential broadband imaging application.

A new approach to electrically detected magnetic resonance: Spin-dependent transient spectroscopy

Electrically detected magnetic resonance (EDMR) is arguably the most sensitive method available to study electrically active point defects in semiconductor devices. Most EDMR studies have utilized spin-dependent recombination current and, thus, require p–n junctions or a photoconductive structure. Some time ago, Chen and Lang proposed and demonstrated EDMR via spin-dependent deep level transient spectroscopy in metal–oxide–semiconductor capacitors. We report on a similar and significantly simpler technique: spin-dependent transient spectroscopy (SDTS). We show that the sensitivity of this technique is independent of the resonance field and frequency. Through capacitance–voltage analysis, combined with our SDTS results, this technique can (crudely) provide information about the density of states of defects with a broad distribution of energy levels. In addition, we show that SDTS can be readily adapted to near-zero-field magnetoresistance effect measurements.

Comparative Analysis of Multilayer Lead Oxide-Based X-ray Detector Prototypes.

Lead oxide (PbO) photoconductors are proposed as X-ray-to-charge transducers for the next generation of direct conversion digital X-ray detectors. Optimized PbO-based detectors have potential for utilization in high-energy and dynamic applications of medical X-ray imaging. Two polymorphs of PbO have been considered so far for imaging applications: polycrystalline lead oxide (poly-PbO) and amorphous lead oxide (a-PbO). Here, we provide the comparative analysis of two PbO-based single-pixel X-ray detector prototypes: one prototype employs only a layer of a-PbO as the photoconductor while the other has a combination of a-PbO and poly-PbO, forming a photoconductive bilayer structure of the same overall thickness as in the first prototype. We characterize the performance of these prototypes in terms of electron–hole creation energy (W±) and signal lag—major properties that define a material’s suitability for low-dose real-time imaging. The results demonstrate that both X-ray photoconductive structures have an adequate temporal response suitable for real-time X-ray imaging, combined with high intrinsic sensitivity. These results are discussed in the context of structural and morphological properties of PbO to better understand the preparation–fabrication–property relationships of this material.

Dark Current Modeling for a Polyimide-Amorphous Lead Oxide-Based Direct Conversion X-ray Detector.

The reduction of the dark current (DC) to a tolerable level in amorphous selenium (a-Se) X-ray photoconductors was one of the key factors that led to the successful commercialization of a-Se-based direct conversion flat panel X-ray imagers (FPXIs) and their widespread clinical use. Here, we discuss the origin of DC in another X-ray photoconductive structure that utilizes amorphous lead oxide (a-PbO) as an X-ray-to-charge transducer and polyimide (PI) as a blocking layer. The transient DC in a PI/a-PbO detector is measured at different applied electric fields (5–20 V/μm). The experimental results are used to develop a theoretical model describing the electric field-dependent transient behavior of DC. The results of the DC kinetics modeling show that the DC, shortly after the bias application, is primarily controlled by the injection of holes from the positively biased electrode and gradually decays with time to a steady-state value. DC decays by the overarching mechanism of an electric field redistribution, caused by the accumulation of trapped holes in deep localized states within the bulk of PI. Thermal generation and subsequent multiple-trapping (MT) controlled transport of holes within the a-PbO layer governs the steady-state value at all the applied fields investigated here, except for the largest applied field of 20 V/μm. This suggests that a thicker layer of PI would be more optimal to suppress DC in the PI/a-PbO detector presented here. The model can be used to find an approximate optimal thickness of PI for future iterations of PI/a-PbO detectors without the need for time and labor-intensive experimental trial and error. In addition, we show that accounting for the field-induced charge carrier release from traps, enhanced by charge hopping transitions between the traps, yields an excellent fit between the experimental and simulated results, thus, clarifying the dynamic process of reaching a steady-state occupancy level of the deep localized states in the PI. Practically, the electric field redistribution causes the internal field to increase in magnitude in the a-PbO layer, thus improving charge collection efficiency and temporal performance over time, as confirmed by experimental results. The electric field redistribution can be implemented as a warm-up time for a-PbO-based detectors.

Oxide perovskite Ba2AgIO6 wafers for X-ray detection.

X-ray detection is of great significance in biomedical, nondestructive, and scientific research. Lead halide perovskites have recently emerged as one of the most promising materials for direct X-ray detection. However, the lead toxicity remains a worrisome concern for further commercial application. Great efforts have been made to search for lead-free perovskites with similar optoelectronic properties. Here, we present a lead-free oxide double perovskite material Ba2AgIO6 for X-ray detection. The lead-free, all-inorganic nature, as well as the high density of Ba2AgIO6, promises excellent prospects in X-ray applications. By employing the hydrothermal method, we successfully synthesized highly crystalline Ba2AgIO6 powder with pure phase. Furthermore, we prepared Ba2AgIO6 wafers through isostatic pressure and built X-ray detectors with Au/Ba2AgIO6 wafer/Au photo-conductive structure. The as-prepared X-ray detectors showed a sensitivity of 18.9 µC/(Gyair·cm2) at 5 V/mm, similar to commercial α-Se detectors showcasing their advantages for X-ray detection.

2D-SnS$_2$ Nanoflakes Based Efficient Ultraviolet Photodetector

This paper reports a SnS $_2$ nanoflakes based UV photodetector having high sensitivity and thermal stability upto 120 $^{\circ}$ C. Simple and low cost solvothermal technique has been used to synthesize SnS $_2$ nanoflakes of close to hexagonal shapes. A simple photoconductor structure on SiO $_{2}$ /Si substrate is fabricated by using Ag as contact material. The resultant device has good sensitivity ( $\sim$ 400), responsivity ( $\sim$ 5.5 A/W), EQE ( $\sim$ 1868%), detectivity ( ${\sim} \text{1.72}\times \text{10}^{13}$ Jones) and low response time ( $\sim$ 2.2 s). The reported characteristics are superior to many other UV photodetectors utilizing complex hybrid structure of the device, either in form of additional filter layer or nanostructural light sensitive material.

Investigation of Porous Silicon Photoconductive Structures

Investigation of Porous Silicon Photoconductive Structures

Investigation of porous silicon photoconductive structures

Investigation of porous silicon photoconductive structures

Ultraviolet detectors based on (GaIn)_2O_3 films

We demonstrated a (GaIn)2O3 films based UV photodetector with a planar photoconductor structure, and investigated the photoresponse properties of the fabricated devices. The (GaIn)2O3 films are of ohmic contact with a Au/Ti electrode. The fabricated photodetectors show relatively high photoresponsivity of more than 0.1 A/W. The turn-on wavelength of the photodetectors varied from 285 to 315 nm with the increase of the indium content from 0.25 to 0.49. The properties of the films were also further investigated. The films are of a (−201) oriented monoclinic phase with a high transmittance of more than 90% in the visible region and smooth surfaces without phase separation. The absorption edges of the films shift toward a longer UV wavelength region with the increase of indium content. The above results suggest that wavelength selective UV detectors can be realized based on these films.

The effect of Ar plasma etching time on the microstructure, optical and photoelectric properties of CdZnTe films

Abstract In this work, Ar plasma etching was performed to polycrystalline CdZnTe thick films grown from close-spaced sublimation method. The effect of Ar plasma etching time on the microstructure, optical and photoelectric properties of CdZnTe films were investigated by using AFM, XRD, UV–visible spectrophotometer and current-voltage characterization system. The results showed that proper plasma etching time can significantly improve the surface roughness and passivate the CdZnTe film surface, leading to less surface leakage current and higher photo-response of the Au/CdZnTe/FTO photo-conductive structure. Its photo-response sensitivity under 281 nm UV irradiation increases with one order of magnitude after 5 min' etching.

Development of solar-blind photodetectors based on Si-implanted β-Ga(2)O(3).

β-Ga(2)O(3) films grown on Al(2)O(3) by a metalorganic chemical vapor deposition technique were used to fabricate a solar-blind photodetector with a planar photoconductor structure. The crystal structure and quality of the β-Ga(2)O(3) films were analyzed using X-ray diffraction and micro-Raman spectroscopy. Si ions were introduced into the β-Ga(2)O(3) thin films by ion implantation method and activated by an annealing process to form an Ohmic contact between the Ti/Au electrode and the β-Ga(2)O(3) film. The electrical conductivity of the β-Ga(2)O(3) films was greatly improved by the implantation and subsequent activation of the Si ions. The photoresponse properties of the photodetectors were investigated by analyzing the current-voltage characteristics and the time-dependent photoresponse curves. The fabricated solar-blind photodetectors exhibited photoresponse to 254 nm wavelength, and blindness to 365 nm light, with a high spectral selectivity.

A novel intermediate layer for Au/CdZnTe/FTO photoconductive structure

In this work, graphene is tried to use to improve the performance of polycrystalline CdZnTe high-energy radiation and photon detectors. A graphene intermediate layer is prepared by spin-coating process on the surface of polycrystalline CdZnTe film, which forms a photoconductive Au/graphene/CdZnTe/FTO structure. XRD, Raman, photoelectric response and other characterisation methods are adopted to investigate the effect of graphene layer on the electrical characteristics and UV photo-response performance of CdZnTe photoconductive structure. It is demonstrated that graphene layer can significantly improve the contact property of Au/CdZnTe structure, and obviously enhance its UV photo-response and the UV sensitivity increased with one order of magnitude.

Fast, Air‐Stable Infrared Photodetectors based on Spray‐Deposited Aqueous HgTe Quantum Dots

The ability to detect near‐infrared and mid‐infrared radiation has spawned great interest in colloidal HgTe quantum dots (QDs). In contrast to the studies focused on extending the spectral range of HgTe QD devices, the temporal response, another figure of merit for photodetectors, is rarely investigated. In this work, a single layer, aqueous HgTe QD based photoconductor structure with very fast temporal response (up to 1 MHz 3 dB bandwidth) is demonstrated. The device is fabricated using a simple spray‐coating process and shows excellent stability in ambient conditions. The origin of the remarkably fast time response is investigated by combining light intensity‐dependent transient photocurrent, temperature‐dependent photocurrent, and field‐effect transistor (FET) measurements. The charge carrier mobility, as well as the energy levels and carrier lifetimes associated with the trap states in the QDs, are identified. The results suggest that the temporal response is dominated by a fast bimolecular recombination process under high light intensity and by a trap‐mediated recombination process at low light intensity. Interestingly, it was found that the gain and time response of aqueous HgTe QD‐based photoconductors can be tuned by controlling the QD size and surface chemistry, which provides a versatile approach to optimize the photodetectors with selectable sensitivity and operation bandwidth.

All‐in‐Fiber Chemical Sensing

A new all-in-fiber trace-level chemical sensing approach is demonstrated. Photoconductive structures, embedded directly into the fiber cladding along its entire length, capture light emitted anywhere within the fiber's hollow core and transform it directly into an electrical signal. Localized signal transduction circumvents problems associated with conventional fiber-optics, including limited signal collection efficiency and optical losses. This approach facilitates a new platform for remote and distributed photosensing.

Influence of the alignment surface on the characteristics of optically addressed liquid-crystal modulators

Two optically addressed transmission-type liquid-crystal modulators based on a nematic liquid crystal-a-Si: C: H photoconductor structure are studied. The modulators differ in the initial tilt angle of the director and in the barrier height at the interface. It is shown that these parameters influence the dependence of the diffraction efficiency on the feed conditions, recording pulse repetition rate, and spatial frequency of the recorded grating. The spatial resolution of the modulators at a level of half the diffraction efficiency is equal to 75 mm−1. The maximal diffraction efficiency is high, 69%, owing to the asymmetric profile of the grating, which arises at a large initial tilt angle of the liquid crystal director.

Silicon-on-Insulator Photoconducting Mesas for High-Speed Detection of Laser Speckle Motion

Silicon-on-insulator (SOI) photoconductor structures show promise in making real-time laser speckle monitoring useful for high-speed applications, such as acoustic and ultrasonic surface vibration detection. For small detection areas below 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ¿m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and at low optical intensities, the internal gain mechanism of these mesa structures can provide better SNR ratios than similarly sized photodiodes while still operating in the megahertz regime. Small-area detectors, in turn, improve the sensitivity of speckle monitoring systems, suggesting that these simply constructed SOI photoconducting mesas are a good choice of detector element for high-speed speckle monitoring applications.

Ge 1 − y Sn y photoconductor structures at 1.55μm: From advanced materials to prototype devices

Prototype detector structures were fabricated on Si substrates using Ge1−ySny as active material for the first time. This alloy system covers the entire near-IR telecommunication spectrum and grows at a low temperature of 350°C, compatible with complementary metal-oxide-semiconductor (CMOS) Si technology. Processing protocols were developed for photolithography-based patterning and subsequent etching, CMOS compatible metallization, and for the formation of low-resistivity Ohmic contacts. A first generation of devices based on as-grown Ge1−ySny layers was followed by a second generation incorporating ex situ rapid thermal annealing for defect reduction, as well as additional growth and processing improvements, leading to enhanced mobilities and simultaneous reduction in intrinsic carrier concentrations. While both device generations show a significant photoconductive response at 1.55μm, the thicker second-generation samples yield improved performance due to better confinement of deleterious defects near the interface, which increases the optically active fraction of the film.

Dependence of the electrical properties of stabilized a-Se on the preparation conditions and the development of a double layer X-ray detector structure

Abstract Stabilized a-Se films deposited at sufficiently low substrate temperatures are n-like in which electrons can drift but holes are deeply trapped. Such layers can be conveniently incorporated in a multilayer a-Se detector structure to block the injection of holes from the positive electrode. We have shown that a simple double-layer detector structure based on a cold deposited n-layer (which is then annealed) on which an i-like layer is grown can have dark current densities lower than 10−10 A cm−2 at a field of 10 V/μm. The dark current depends on the thickness of the n-like layer. An a-Se X-ray detector for slot scanning was fabricated by having the i–n a-Se photoconductor structure coated onto a CCD chip. The latter detector was shown to have excellent resolution with a modulation transfer function remaining above 0.5 up to a spatial frequency of 11–14 lp mm−1.

UV Schottky photodiode on boron-doped CVD diamond films

We report on experimental study of photosensitivity and Q-DLTS spectra of polycrystalline CVD diamond UV photodetectors. The measured characteristics of Schottky photodiode on boron-doped diamond films are compared with those obtained for planar photoconductive structures (photoresistor type) based on undoped CVD diamond. The Schottky photodiode exhibited a sharp cut-off in photoresponse with spectral discrimination ratio (between wavelengths of 190 nm and 700 nm) as high as 5 · 10 5 at zero bias voltage (at zero dark current). The photodiode showed the maximum of photoresponse at wavelength < 190 nm, and a low density of trapping and recombination centers as evaluated with the Q-DLTS technique. The devices demonstrated the photoresponsivity at 190 nm from 0.03 to 0.1 A/W with quantum yield of 0.20 to 0.67 in closed circuit, while the photovoltage ≥ 1.6 V was measured in open circuit regime. Another type of UV detector, the planar photoconductive structures with interdigitizing ohmic electrodes fabricated on undoped diamond film and operated under a bias voltage, revealed a higher density of (surface) defect centers and the maximum photoresponse at ∼ 210 nm wavelength. A strong influence of UV light illumination on the Q-DLTS spectra of the planar photoconductive structures was observed. This effect can be used for development of new UV detectors and dosimeters based on the Q-DLTS signal measurements.

Synthesis and Characterization of 1,3,4-Oxadiazoles Triazolopyridinone and Carbazole as the Potential Bluish-Green Electroluminescent Materials for the Single Layer Device

1,3,4-Oxadiazoles carrying triazolopyridinone and carbazole were synthesized and characterized for the electroluminescent materials in single layer devices. Carbazole and triazolopyridinone were introduced into 2,5-centers of 1,3,4-oxadiazole core to assist the control of fundamental photolytic process due to their electron-donating nature, excellent photo-conductivity, and flexible structure properties. Study from spectrometry and cyclic voltammetry revealed that the 1,3,4-oxadiazole derivatives carrying these two heterocyles are able to be used as the potential bluish-green electroluminescent materials.