Photoconductivity Research Articles

Charge Carrier Dynamics in Co-evaporated MAPbI3 with a Gradient in Composition

Co-evaporation of metal halide perovskites by thermal evaporation is an attractive method since it does not require harmful solvents and enables precise control of the film thickness. Furthermore, the ability to manipulate the Fermi level allows the formation of a graded homojunction, providing interesting opportunities to improve the charge carrier collection efficiency. However, little is known about how these properties affect the charge carrier dynamics. In this work, the structural and optoelectronic properties of co-evaporated MAPbI3 films varying in thickness (100, 400, and 750 nm) with a gradient in composition are analyzed. The X-ray diffraction patterns show that excess PbI2 is only present in the thick layers. From X-ray photoelectron spectroscopy depth analysis, the I/Pb atomic ratio indicates methylammonium iodide deficiencies that become more prominent with thicker films, resulting in differently n-doped regions across the thick MAPbI3 films. We suggest that due to these differently n-doped regimes, an internal electric field is formed. Side-selective time-resolved microwave photo conductivity measurements show an elongation of the charge carrier lifetimes on increasing thickness. These observations can be explained by the fact that excess carriers separate under the influence of the electric field, preventing rapid decay in the thick films.

Enhanced optoelectronic performance and photogating effect in quasi-one-dimensional BiSeI wires

Quasi-one-dimensional (quasi-1D) materials are a newly arising topic in low-dimensional research. As a result of reduced dimensionality and enhanced anisotropy, the quasi-1D structure gives rise to novel properties and promising applications such as photodetectors. However, it remains an open question whether performance crossover will occur when the channel material is downsized. Here, we report on the fabrication and testing of photodetectors based on exfoliated quasi-1D BiSeI thin wires. Compared with the device on bulk crystal, a significantly enhanced photoresponse is observed, which is manifested by a series of performance parameters, including ultrahigh responsivity (7 × 104 A W−1), specific detectivity (2.5 × 1014 Jones), and external quantum efficiency (1.8 × 107%) when Vds = 3 V, λ = 515 nm, and P = 0.01 mW cm−2. The conventional photoconductive effect is unlikely to account for such a superior photoresponse, which is ultimately understood in terms of the increased specific surface area and the photogating effect caused by trapping states. This work provides a perspective for the modulation of optoelectronic properties and performance in quasi-1D materials.

Transient analysis of photomultiplication-type organic photodiodes

Photomultiplication-type organic photodetectors have emerged as a class of next generation solution-processed photodetectors with high gain. Despite this promising feature, the reported photodectors still suffer from relatively large dark currents at high bias voltages. To overcome this drawback, a mechanistic understanding of the photomultiplication effect in organic photodiodes is required. In this work, we advanced the performance of photomultiplication-type organic photodetectors by tuning the active layer composition and interfacial layers. The optimized devices exhibit small dark currents and flat dark current–voltage curves under the reverse bias condition up to −10 V. The optimized photodetectors also reached an ultra-high responsivity of 23.6 A/W and the specific detectivity of 1.04 × 1012 Jones at −10 V. More importantly, we investigated the photomultiplication process with multiple transient techniques and revealed that the photoconductive gain effect is a slow process, which relies on the photo-Schottky effect enabled by charge carrier tunneling and the accumulation of holes. Furthermore, we also demonstrated prototypical pulsed-light detection based on the optimized devices, which showed great potential for real applications.

Non-Layered Te/In2 S3 Tunneling Heterojunctions with Ultrahigh Photoresponsivity and Fast Photoresponse.

A photodetector based on 2D non-layered materials can easily utilize the photogating effect to achieve considerable photogain, but at the cost of response speed. Here, a rationally designed tunneling heterojunction fabricated by vertical stacking of non-layered In2 S3 and Te flakes is studied systematically. The Te/In2 S3 heterojunctions possess type-II band alignment and can transfer to type-I or type-III depending on the electric field applied, allowing for tunable tunneling of the photoinduced carriers. The Te/In2 S3 tunneling heterojunction exhibits a reverse rectification ratio exceeding 104 , an ultralow forward current of 10-12 A, and a current on/off ratio over 105 . A photodetector based on the heterojunctions shows an ultrahigh photoresponsivity of 146 AW-1 in the visible range. Furthermore, the devices exhibit a response time of 5ms, which is two and four orders of magnitude faster than that of its constituent In2 S3 and Te. The simultaneously improved photocurrent and response speed are attributed to the direct tunneling of the photoinduced carriers, as well as a combined mechanism of photoconductive and photogating effects. In addition, the photodetector exhibits a clear photovoltaic effect, which can work in a self-powered mode.

Optically Activated In-Waveguide Semiconductor Attenuators for the Controllable Isolation of Ka-Band Microwaves

Three different isolator topologies utilizing photoconductive (PC) elements are explored for their application as a controllable attenuator for a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ka</i> -band radar system. Network analyzer measurements are reported for each geometry in the unilluminated case, while a high-speed, high dynamic range heterodyne detection apparatus is used to measure the transient attenuation behavior of the isolators when illuminated. The electromagnetic characteristics of the illuminated isolators are demonstrated to be in good agreement with COMSOL Multiphysics simulations. Two of the isolator topologies rely on the PC element becoming highly reflective to achieve high isolation, which in turn requires high optical power and charge carrier density (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-3}$ </tex-math></inline-formula> ). For the optical power available here (100 W), the first device demonstrated a peak attenuation of 53 dB, while the second device achieved only 33 dB. In the third topology, RF propagation is parallel to the major dimensions of the PC element. As a result, superior isolation is achieved with the PC element in the primarily absorbing state, associated with significantly lower carrier concentration (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-3}$ </tex-math></inline-formula> ). This device achieved 63 dB of attenuation for only 3 W of optical power, demonstrating that PC technologies may be competitive with other isolator technologies with some notable advantages.

Strain-balanced InAs/GaSb superlattices used for the detection of VLWIR radiation

This work reports on the impact of defects on the parameters of type II InAs/GaSb superlattices (SLs) and photoconductors (PCs) from the very long wavelength infrared region. SLs were grown by means of molecular beam epitaxy and characterised using microscopes, high-resolution X-ray diffractometry (HRXRD) and low-temperature photoluminescence (LT-PL). PCs were investigated using Fourier-transform infrared spectroscopy and a current–voltage measurements. The basic technological parameters were optimised using 30-period SLs. Tests on the growth temperature showed that different defect types dominated in SL samples of different thicknesses. The diffuse scattering in reciprocal space maps (RSMs) taken for 200-period SLs was the mirror image of that for 30-period SLs. Scanning electron microscope (SEM) images of 200 period SLs revealed three types of defects: holes, slits and others not belonging to the first two groups. Slits were the predominant type. The diameter of the defects determined by SEM and HRXRD was about 2.5 μm. The cross section along the defects was made using focus-ion beam technique. The investigations using high-resolution transmission electron microscopy revealed the origins of the defects: holes were formed at the SL/buffer interface and propagated throughout entire SL, while slits were created in the volume of the SL without disturbing the surrounding crystal lattice. The lowest defect density was found for an SL grown at 425 °C. The highest overall crystal quality, measured using an HRXRD rocking curve, was obtained for an SL deposited at 405 °C, while the highest optical quality was determined for an SL at 445 °C by means of LT-PL. The best parameters were achieved for PCs grown at two lower temperatures. The difference in the effective bandgaps of the PCs made it difficult to determine the optimal value of the growth temperature. The effective bandgap for three PCs was determined based on the spectral dependences of the photoresponse measured at 20 K. Activation energies for high (150–300 K) and low (<50 K) temperature ranges were estimated from R(1000/T) dependence.

Ultrasensitive, Ultrafast, and Gate-Tunable Two-Dimensional Photodetectors in Ternary Rhombohedral ZnIn2S4 for Optical Neural Networks.

The demand for high-performance semiconductors in electronics and optoelectronics has prompted the expansion of low-dimensional materials research to ternary compounds. However, photodetectors based on 2D ternary materials usually suffer from large dark currents and slow response, which means increased power consumption and reduced performance. Here we report a systematic study of the optoelectronic properties of well-characterized rhombohedral ZnIn2S4 (R-ZIS) nanosheets which exhibit an extremely low dark current (7 pA at 5 V bias). The superior performance represented by a series of parameters surpasses most 2D counterparts. The ultrahigh specific detectivity (1.8 × 1014 Jones), comparably short response time (τrise = 222 μs, τdecay = 158 μs), and compatibility with high-frequency operation (1000 Hz) are particularly prominent. Moreover, a gate-tunable characteristic is observed, which is attributed to photogating and improves the photoresponse by 2 orders of magnitude. Gating technique can effectively modulate the photocurrent-generation mechanism from photoconductive effect to dominant photogating. The combination of ultrahigh sensitivity, ultrafast response, and high gate tunability makes the R-ZIS phototransistor an ideal device for low-energy-consumption and high-frequency optoelectronic applications, which is further demonstrated by its excellent performance in optical neural networks and promising potential in optical deep learning and computing.

The Negative Photoconductivity of Ag/AgO Grown by Spray-Pyrolysis

The main goal of this work is to provide a general description of the negative photoconductivity effect observed in Ag/AgO films grown by the spray-pyrolysis technique. X-ray diffractograms display hybrid films with high texturized AgO and metallic Ag phases. Scanning electron microscopy images show small Ag particles on the surface. Due to its surface nature, X-ray photoelectron spectroscopy revealed the predominance of the metallic character of Ag 3d spectra as compared to Ag2+. Negative photoconductivity with photoresponse in the order of seconds is observed under several wavelengths of excitation. We found that the amplitude of the negative photoresponse is strongly dependent on the optical absorbance and enhanced by surface plasmon resonance. The low-cost technique employed and the special features regarding negative photoconductivity provide an exciting platform for developing optical-electronic devices with low power consumption.

An Optoelectronic Reservoir Computing for Temporal Information Processing

The implementation of 5G increases the demand for data acquisition, thus increasing the pressure of data processing. Although artificial neural network shows great potential in processing big data, efficient neuromorphic visual system is desired due to the waste of computation resources when processing non-structural visual data. Although reservoir computing (RC) has advantages in temporal information processing, the separation of sensors and RC results in addition cost. Here, an optoelectronic RC system is proposed for temporal information processing in sensors. The reservoir is built on photodetectors based on a non-uniform MoS₂ film. The persistent photoconductivity effect of the photodetectors enables mapping different temporal inputs into corresponding reservoir states. The readout layer could be simply trained to identify different reservoir states. As a proof of concept, different classification tasks of numbers are demonstrated. The proposed optoelectronic RC system provides a low training cost strategy for intelligent edge machine visual system to process temporal information.

Hysteresis-free Ga2O3 solar-blind phototransistor modulated from photoconduction to photogating effect

High tunability of photoresponse characteristics under work conditions is desired for a single solar-blind photodetector to be applied in multifarious fields. Three-terminal metal–oxide–semiconductor field-effect phototransistors have shown excellent controllability of performance, but the hysteresis issue impedes their stable operation. In this work, the metal–semiconductor field-effect phototransistor based on the exfoliated Ga2O3 microflake and graphene thin film is demonstrated. The high-quality quasi-van der Waals interface between Ga2O3 and graphene eliminates the hysteresis issue and generates a subthreshold swing as low as 69.4 mV/dec. By regulating gate voltage (Vg), the dominated mechanism of photocurrent generation in the device can be tuned continuously from the fast photoconduction effect to photogating effect with high photogain. Accordingly, the responsivity, dark current, detectivity, rejection ratio, and decay time of the device can be well adjusted by the Vg. At Vg = −1 V and a source to drain voltage of 2 V, the device shows excellent performance with a responsivity of 2.82 × 103 A/W, a rejection ratio of 5.88 × 105, and a detectivity of 2.67 × 1015 Jones under 254 nm illumination. This work shows the possibility of realizing highly tunable solar-blind photodetectors to meet the requirements for different application fields by introducing gate voltage modulation.

Highly Sensitive Direct‐Conversion Vacuum Flat‐Panel X‐Ray Detectors Formed by Ga2O3‐ZnO Heterojunction Cold Cathode and ZnS Target and their Photoelectron Multiplication Mechanism

AbstractX‐ray detectors are significantly applied in medical diagnoses, industrial inspections, and scientific researches, but the detection sensitivity is limited by the material, device structure, and detection mechanism. This article proposes a direct‐conversion flat‐panel X‐ray detector formed by Ga2O3‐ZnO heterojunction cold cathode and ZnS target to achieve highly sensitive X‐ray detection. The field emission efficiency of Ga2O3‐ZnO heterojunction cold cathode is improved by optimizing the spacing between the adjacent circular pattern in patterned ZnO nanowire arrays. The electron–hole pairs generated by X‐ray photons are multiplied in ZnS target by the electron bombardment induced photoconductivity effect, resulting in a high internal gain (1 × 104%) and a high X‐ray detection sensitivity (7.1 × 102 μCGyair−1 cm−2) for 6 keV X‐ray at 9.2 V μm−1 electric field. In addition, the proposed detector has low dark current density (50 pA mm−2 at 9.2 V μm–1 electric field), good stability (0.2% fluctuation for 60 s), and fast response speed (rise time: 5.3 ms) owing to a built‐in electric field in Ga2O3‐ZnO heterojunction, a ballistic electron transportation in vacuum gap, and a high resistance in ZnS thin film.

X-ray Photoelectric and magnetic properties of chemically synthesized Cd–Ni Ferrite nanomagnetic particles

Cadmium nickel (Cd–Ni) ferrite samples have been successfully synthesized via chemical co-precipitation technique. The structural analysis revealed the formation of FCC framework and Fe-phase in a trivalent state. The crystallite size is decreased with increasing Cd2+ ion composition whereas the lattice constant is increased. SEM was used to obtain the surface morphology and average grain size of the microstructure. The FTIR shows the formation of metal oxide, hydroxyl and carboxylic groups. EDX revealed the formation of Ni2+, Cd2+, Fe3+, and O2− ions in proper stoichiometric composition. Large optical losses were revealed by Cd2+ poor-NiFe2O4 samples whereas Cd2+ rich-CdxNi1−xFe2O4 samples revealed low optical losses and showed enhanced photoconductivity and photoelectric effect. Result from optical analysis showed that Cd2+ rich-CdxNi1−xFe2O4 nanoparticles can be used as infrared (IR) detector, ultraviolet (UV) filter and in optoelectronics device applications. VSM measurement showed an increase in saturation magnetization and decrease in coercivity as Cd2+ ion content is increased. The remanance magnetization and magnetic anisotropy were also examined. Photoluminescence (PL) spectroscopy examined the nature of the light emission of the samples at the excitation wavelength 380 nm and emission of series of colours such as red, green, yellow, orange and violet light at different wavelengths were found.

Photoinduced carrier transport mechanism in pn- and nn-GaN/GaON heterojunctions

The unexpected high persistent photoconductivity effect in Ga2O3 material hinders the application of deep-ultraviolet photodetectors, while GaON can effectively avoid the effect caused by oxygen vacancies through anion engineering. As the behaviors of the carrier’s transport are crucial and essential to the photoelectric conversion processes, analysis of the carrier transport mechanism is helpful to propose feasible and effective strategies for high-performance photodetectors. In this work, GaN/GaON pn- and nn-heterojunctions with various thicknesses of GaON thin films were obtained by oxidizing the n-GaN and p-GaN films, and their photoinduced carrier transport mechanism has been comprehensively investigated. At a low bias, as the electric field is limited in the GaON layer and only the carriers generated in the GaON layer can be collected by the electrodes for both GaN/GaON pn- and nn-heterojunctions, the current increases linearly with an increase in the voltage. At a high bias, the electric field can affect the GaN/GaON heterojunction interface. For the GaN/GaON nn-heterojunction, the current continues to increase with increasing voltage as a small potential barrier is created between GaON and n-GaN to separate and transport the photogenerated carriers. However, for the GaN/GaON pn-heterojunction, the current increases slowly and then rapidly with an increase in the high voltage, because the electric field is not strong enough to help the carriers cross the potential barrier caused by the reverse GaN/GaON pn-heterojunction first and then overcome the barrier with a higher voltage.

An ultraviolet-visible distinguishable broadband photodetector based on the positive and negative photoconductance effects of a graphene/ZnO quantum dot heterostructure

Microstructures is an international peer-reviewed, open access, online journal. The journal of Microstructures publishes articles covering new research and developments, perspectives, and reviews related to the microstructures of materials, including, but not limited to metals and alloys, ceramics, polymers, composites, crystals, glasses, biomaterials, interfaces and nanomaterials.

Compensation method for performance degradation of optically addressed spatial light modulator induced by CW laser

Abstract In this paper, we propose an effective method to compensate for the performance degradation of optically addressed spatial light modulators (OASLMs). The thermal deposition problem usually leads to the on-off ratio reduction of amplitude OASLM, so it is difficult to achieve better results in high-power laser systems. Through the analysis of the laser-induced temperature rise model and the liquid crystal layer voltage model, it is found that reducing the driving voltage of the liquid crystal light valve and increasing the driving current of the optical writing module can compensate for the decrease of on–off ratio caused by temperature rise. This is the result of effectively utilizing the photoconductive effect of Bi12SiO20 (BSO) crystal. The experimental results verify the feasibility of the proposed method and increase the laser withstand power of amplitude-only OASLM by about a factor of 2.5.

Inverse photoconductivity effect in triple cation organic–inorganic hybrid perovskite memristors with various iodine concentrations, electrodes, and modified layers

A novel inverse photoconductivity horizon in perovskite photo-memristor applications with various iodine concentrations, electrodes, and modified layers.

Solution-processed flexible broadband ZnO photodetector modified by Ag nanoparticles

In this work, we present flexible broadband photodetectors (PDs) fabricated by a deposition of nanostructured zinc oxide (ZnO) films on polyimide (PI) substrates by using cheap and scalable aqueous method Successive Ionic Layer Adsorption and Reaction (SILAR). In order to increase the long-wavelength absorption of the nanostructured ZnO layer, we created its intrinsic defects, including oxygen vacancies by post-treatment at 300 °C in vacuum and thus the light-sensitive material ZnO/PI was obtained. Then we applied silver nanoparticles (Ag NPs) from a silver sol onto a nanostructured ZnO film, which were visualized using SEM in the form of spheres up to 100 nm in size that increased the photocurrent and figures of merit of thus obtained light-sensitive material ZnO_Ag/PI due to localized surface plasmon resonance and double Schottky barriers at the Ag-ZnO interface. To fabricate photodetectors based on a photoconductive effect, these ZnO/PI and ZnO_Ag/PI materials were equipped with ohmic aluminum contacts. The spectral responsivity (Rλ up to 275 A/W to UV light) of solution-processed flexible broadband photodetector based on ZnO_Ag/PI material at different wavelengths of light and light power densities is better than Rλ of the ZnO/PI photodetector, and at least an order of magnitude higher than Rλ of photodetectors based on nanostructured zinc oxide described in recent articles. The external quantum efficiency (EQE) of both PDs in this study in UV–Vis-NIR spectra is very high in the range from 1∙102 to 9∙104 % and is better or of the same order of magnitude as the EQE data of modern flexible broadband high-sensitivity PDs based on nanostructured heterostructures containing ZnO. The specific detectivity in UV–Vis-NIR spectra is large for ZnO/PI (from 3.5∙1010 to 1∙1012 Jones) and especially for ZnO_Ag/PI (from 1.6∙1011 to 8.6∙1013 Jones), which indicates the ability of the PDs based on light-sensitive materials ZnO/PI and ZnO_Ag/PI to recognize a very weak light signal.

Influence of Pb2+ doping in the optical and electro-optical properties of SnO2 thin films

Different types of doping incorporated to tin dioxide (SnO2) have been reported in order to control its bandgap aiming optoelectronic applications, such as transparent electrodes, solar cells and displays. In this work, the doping of SnO2 with lead in the oxidation state 2+ (Pb2 +) is investigated, in conjunction with the influence on the optical and electro-optical properties of thin films, deposited by sol-gel-dip-coating. It was observed that for an insertion up to 25 at% of lead, there is no formation of a secondary crystalline phase beyond the rutile phase of SnO2. Optical characterization data indicate two behavior tendencies: for doping level up to 1 at%, Pb2+ enters preferentially as interstitial impurity, whereas for higher doping it enters substitutional to Sn4+. For doping up to 1 at% the bandgap increases due to the Burnstein-Moss effect, in addition to a decrease in the capture energy for metastably photoexcited electrons. On the other hand, for doping above 1 at%, there is an increase in the Urbach energy, shifting the energy level of the valence band, leading to a decrease in the bandgap from 3.45 to 2.89 eV, which shows potential applicability in bandgap engineering. The higher doping level also increases the capture energy for photoexcited electrons and gives rise to the persistent photoconductivity effect at low temperatures, related to a large lattice relaxation around the dominant photoexcited defect.

Gate tunable photoresponse of a two-dimensional p-n junction for high performance broadband photodetector

Optoelectronic devices based on two-dimensional (2D) van der Waals (vdWs) materials and their heterostructures are promising owing to their intriguing properties. However, the achievement of high-performance photodetectors over a wide detection range is still limited. In particular, detection in the near-infrared (NIR) region has not been attained successfully because of the relatively low responsivity to date. Herein, we report an efficient photodetection from visible to NIR ranges using a ReS2/Te vdWs heterostructure. The dependence of the photocurrent on the incident power reveals that the dominant operating mechanism can be switched between the photogating effect and photoconductive effect by applying the back-gate voltage. The ReS2/Te photodetector exhibits remarkable responsivities of 3 × 106 A W‒1 at 458 nm and 1 × 106 A W‒1 at 1062 nm laser irradiations, and an ultrahigh photoconductive gain (up to ∼108) resulting from the strong photogating effect. These outstanding performances demonstrate that the ReS2/Te vdWs heterostructure is suitable for high-performance broadband photodetectors in practical applications.

Enhancing the Sensitivity of GaN High Electron-Mobility Transistors-Based pH Sensor by Dual Function of Monolithic Integrated Planar Multi-Channel and Ultraviolet Light

In order to realize high-sensitivity real-time monitoring of acid–base neutral solutions in the industrial field, it is very important to study how to improve the sensitivity of a pH sensor. Herein, the sensitivity of the pH sensor was improved by using both monolithic integrated planar multi-channel and photoconductive properties of gallium nitride (GaN) high electron-mobility transistors (HEMTs) as a core of the pH sensor. The sensitive area of aluminum gallium nitride/gallium nitride (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</sub> N/GaN) heterojunction HEMT channel surface was increased, and the channel effective resistance was reduced by planar multi-channels. And the photogenerated current was generated by 365-nm ultraviolet (UV) light based on the photoconductive effect. This investigation mainly shows as the sensitivity of the tri-channel was 37.04 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{A}$ </tex-math></inline-formula> /pH, which increased 30 times compared with that of the single-channel device under the dual action of planar multi-channel and 365-nm UV light coupling in potassium hydrogen phthalate, mixed phosphate, and borax pH correction buffer solutions. The concept of using the GaN HEMT-based pH sensor for direct quantification of pH values in waste water provides a simple and high-performance method for industrial applications.