Enhancement Of Photoconductivity Research Articles

Pushing the Limits of Photoconductivity via Hot Electrons in Deep Trap States in Plasmonic Architectures.

Although extensive research on the mechanisms of photoconductivity enhancement in plasmonic Schottky structures has been conducted, the photoconductive interplay between hot electrons and trapping states remains elusive. In this study, we explored the photoconductive relationship between plasmonic hot-carriers and defect sites present in plasmonic architectures consisting of N-face n-GaN and Au nanoprisms. Our experimental results clearly verified that the plasmonic hot-electrons generated by interband transitions preferentially occupied deep trap levels in n-GaN, thereby considerably enhancing the photoconductivity through the combination of photogating and photovoltaic effects. Our quantitative evaluation demonstrated that a mere 63% increase in hot-electron trapping leads to a 1.7-fold increased photocurrent under localized surface plasmon resonance (LSPR) excitation compared to the figure of photocurrent under non-LSPR stimulus. Our findings provide novel insights into the mechanisms of photoconductive enhancement for advanced plasmonic applications.

Radical n-p Conduction Switching and Significant Photoconductivity Enhancement in NbOI2 via Pressure-Modulated Peierls Distortion.

The absence of intrinsic p-type 2D layered semiconductors has hampered the development of 2D devices, particularly in complementary metal-oxide-semiconductor (CMOS) devices and integrated circuits. Developing practical p-type semiconductors and advanced modulation techniques for precise carrier control is paramount to advancing electronic devices and systems. Here, by applying pressure to continuously tune the Peierls distortion in NbOI2, we effectively control the polarity and concentration of carriers and significantly enhance its photoelectric properties. The results demonstrate that by suppressing the off-center displacement of Nb atoms along the in-plane b direction under pressure, NbOI2 undergoes a semiconductor-to-semiconductor phase transition from C2 to C2/m, leading to a significant transition from n-type to p-type carrier behavior. Additionally, the gradual inhibition of internal interactions within Nb-Nb dimers along the in-plane c direction under high pressure facilitates electron delocalization, substantially enhancing the photoelectric properties. The photocurrent is increased by more than 3 orders of magnitude under xenon irradiation, and the spectral response range is continuously red-shifted and extended to 1450 nm. These findings highlight the potential of pressure engineering to adjust photoelectric properties effectively and flexibly, offering valuable insights for designing high-performance p-type two-dimensional semiconductors.

Photoexcited charge carrier dynamics and electronic properties of two-dimensional MXene, Nb2CT x

Two-dimensional, 2D, niobium carbide MXene, Nb2CT x , has attracted attention due to its extraordinarily high photothermal conversion efficiency that has applications ranging from medicine, for tumor ablation, to solar energy conversion. Here, we characterize its electronic properties and investigate the ultrafast dynamics of its photoexcitations with a goal of shedding light onto the origins of its unique properties. Through density functional theory, DFT, calculations, we find that Nb2CT x is metallic, with a small but finite DOS at the Fermi level for all experimentally relevant terminations that can be achieved using HF or molten salt etching of the parent MAX phase, including –OH, –O, –F, –Cl, –Br, –I. In agreement with this prediction, THz spectroscopy reveals an intrinsic long-range conductivity of ∼60 Ω−1 cm−1, with significant charge carrier localization and a charge carrier density (∼1020 cm−3) comparable to Mo-based MXenes. Excitation with 800 nm pulses results in a rapid enhancement in photoconductivity, which decays to less than 25% of its peak value within several picoseconds, underlying efficient photothermal conversion. At the same time, a small fraction of photoinjected excess carriers persists for hundreds of picoseconds, and can potentially be utilized in photocatalysis or other energy conversion applications.

Plasmons Enhancing Sub-Bandgap Photoconductivity in TiO2 Nanoparticles Film.

The coupling between sub-bandgap defect states and surface plasmon resonances in Au nanoparticles and its effects on the photoconductivity performance of TiO2 are investigated in both the ultraviolet (UV) and visible spectrum. Incorporating a 2 nm gold nanoparticle layer in the photodetector device architecture creates additional trapping pathways, resulting in a faster current decay under UV illumination and a significant enhancement in the visible photocurrent of TiO2, with an 8-fold enhancement of the defects-related photocurrent. We show that hot electron injection (HEI) and plasmonic resonance energy transfer (PRET) jointly contribute to the observed photoconductivity enhancement. In addition to shedding light on the below-band-edge photoconductivity of TiO2, our work provides insight into new methods to probe and examine the surface defects of metal oxide semiconductors using plasmonic resonances.

Suppression of persistent effect and enhancement of photoconductivity in PbTe:CaF2 epitaxial films

This work presents the investigation of photoconductivity effect on CaF2 doped PbTe films, revealing a strong effect of the doping in the electrical and photoconductivity properties. For the electrical properties, Hall effect measurements were carried out under dark and illuminated conditions. We show that the photoconductivity amplitude suffers a huge increase for the doped samples at low temperatures while the persistent photoconductivity, commonly observed at room temperature for PbTe compounds, is suppressed. A simple classical Drude model for photoconductivity amplitude describes the observed effects.

Strain-enabled defect migration and defect activation in monolayer MoS2

Two-dimensional (2D) materials possess remarkable strain tolerance and exhibit strain-tunable properties, making them highly promising for flexible device applications. Defects within these materials significantly impact their optoelectronic response to strain. In this study, we investigate the influence of strain on the electrical properties of monolayer MoS2, emphasizing the pivotal role played by intrinsic defects in shaping the material’s electrical and optoelectronic response under strain. We observed an enhancement in photocurrent and persistent photoconductivity at specific strains, indicating the activation of defects at these strain values, thus enhancing the photoresponse. Moreover, our device exhibits diodic behavior at specific strain values after prolonged measurements under a static field, suggesting a reduction in the migration energy of defects caused by the applied strain. This finding holds significant implications for memory, logic, and flexible devices. Additionally, we observe an increase in electron mobility under tensile strain, with our flexible field-effect transistor exhibiting higher mobility (∼38 cm2 (V·s)−1) at 0.4% strain. Our study provides insight into the role of strain in the activation and migration of defects in monolayer MoS2 and opens up new avenues for the development of multifunctional ultra-thin flexible devices and memory applications.

Photoconductivity Enhancement in Atomically Thin Molybdenum Disulfide through Local Doping from Confined Water.

Two-dimensional transition metal dichalcogenide (TMDC) materials have shown great potential for usage in opto-electronic devices, especially down to the regime of a few layers to a single layer. However, at these limits, the material properties can be strongly influenced by the interfaces. By using photoconductive atomic force microscopy, we show a local enhancement of photoconductivity at the nanoscale in bilayer molybdenum disulfide on mica, where water is confined between the TMDC and the substrate. We have found that the structural phase of the water influences the doping level and thus the tunneling barrier at the nanojunction. This leads to an increase in photocurrent and enhanced photopower generation.

Breakdown and Photoconductivity Enhancement by Mixed Reflective Al-Doped ZnO/Ag Electrode in Vertical SiC Photoconductive Switch

an axial-triggering vertical SiC photoconductive switch with Al-doped ZnO placed under the Ag electrode is developed to enhance breakdown strength and photoconductivity. Traditional and new devices with the AZO/SiC/Ag and AZO/SiC/AZO/Ag structures, respectively, were prepared. Test results show that the conductivity of the new device is approximately 1.2 times higher than that of the old one, attributable to the diffuse reflection effects of the AZO/Ag mirror. Under high bias voltages of up to 16.5 kV/mm and a 30-ns pulse width laser input, the new device shows significantly enhanced breakdown strength. The new device has a lifetime of >1E4 shots, whereas the old device has a maximum lifetime of approximately 1E2 shots. The new device can continuously output electrical pulses for 10 minutes at a 100 Hz repetition rate, 30-ns pulse width, and 100 A peak current without breakdown.

Enhancement of Light Induced Transport Properties in Beetroot Dye-Based Biological Schottky Device Using Titanium Dioxide (TiO2) Nanoparticles

In response to the growing demand for electronics, scientists are actively investigating alternative nonconventional, bio-compatible electronic materials suitable for electronic and optoelectronic device applications. This study focuses on enhancing the photoconductivity and photosensitivity of a beetroot (BR) dye-based Schottky device through the incorporation of Titanium dioxide (TiO2 ) nanoparticles. To achieve this, ITO(Indium tin oxide)/BR/Al (Aluminium) and ITO/BR+TiO2 /Al devices were fabricated through the spin coating technique, and their light-induced charge transport properties were examined. The thin films were also characterized through UV-Vis and FESEM analyses. Initial measurements of photoconductivity, sensitivity, and photoresponsivity of the device yielded unsatisfactory results, indicating the need for improvement to enable effective device applications. To address this, we incorporated Titanium dioxide nanoparticles with the dye by making nano-composite, leading to a remarkable enhancement in photoconductivity, increasing from 2.78×10 -8 S/m to 1.75×10 -7 S/m, and a significant boost in photosensitivity, rising from 1131.03 to 2157.67. Furthermore, we estimated other light transport properties, including diffusion length, transit time, and effective mobility, for a comprehensive analysis of the device's performance. These additional parameters also exhibited notable enhancements in the presence of TiO2 nanoparticles. This research contributes valuable insights into the application of TiO2 nanoparticles in enhancing the performance of organicbased optoelectronic devices, and it offers potential avenues for further exploration and refinement of bio-compatible electronic materials in electronic and optoelectronic applications.

Pressure-induced photoconductivity enhancement and positive–negative switch in bulk silicon

Silicon is a long-standing photosensitive material because of its unique photoelectronic properties and mature manufacturing technology. However, silicon photodetectors are generally limited by weak photoresponse in the near-infrared region. In this work, pressure is used as an effective means of tuning the photoresponse of silicon, specifically in the near-infrared region. Silicon has two different types of photoresponse under pressure. In the pressure range from 1 atm to 10 GPa, huge pressure-enhanced photocurrent is observed under illumination by a xenon lamp and near-infrared light (1064 nm). At 10 GPa, the photocurrent density (Jph), responsivity (R), and external quantum efficiency are increased 40-fold from those at 1.2 GPa. Interestingly, above 10 GPa, a unique pressure-induced positive–negative photoresponse switch is found along with the phase transformation from the semiconductive phase (Si I) to the metallic phase (β-tin). Further experiments show that the photothermal effect is the main factor for negative photoresponse. All these pressure-induced properties give silicon more possibilities in the further design of visible and infrared photodetectors.

Electrostatic gating enhanced persistent photoconductivity at the LaAlO3/SrTiO3 interface

The two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 (LAO/STO) interface exhibits many emergent properties, such as high mobility, giant photoconductivity, the light-enhanced gating effect, etc., which are invaluable for basic research and potential applications. In this work, we report an unusual enhancement of persistent photoconductivity (PPC) by electrostatic gating (EG) at the LAO/STO interface. Under the influence of pre-EG processing, the 2DEG at the LAO/STO interface is triggered into a resistance state showing a more pronounced optical response and greatly enhanced PPC under light illumination (LI) at room temperature. These observations are found to be attributed to EG-induced interfacial polarization and the subsequent LI-accelerated depolarization at the interface. Based on these findings, a nonvolatile memory device made by this interface is proposed. Our work offers a new perspective for tuning the photoelectrical properties at the oxide interface, which is helpful for designing advanced photoelectric devices with high performance and multifunctionality.

Role of Cobalt Doping in CdS Quantum Dots for Potential Application in Thin Film Optoelectronic Devices

Colloidal quantum dots (QDs) are promising materials for optoelectronic devices. In this paper, monodispersed and environment stable cobalt (Co)-doped CdS QDs have been synthesized and characterized for potential application in thin film optoelectronic devices. The Rietveld refinement profiles of X-ray diffraction data reveal that both undoped and Co-doped CdS QDs exhibit a zinc blende structure without any impurity phase. X-ray photoemission spectroscopy has been used for electronic structure and valence state analysis. The detailed information about the doping, coordination number, and local geometry has been studied with XANES and EXAFS measurements. Analysis of Raman spectra reveals that the intensity of longitudinal optical (LO) modes varies considerably due to short-range structural disorder. Absorption spectra also show the creation of a new doping band (DB) near the NIR region in Co-doped CdS QDs which is not observed for doping of many other transition metals. The width of this DB increases with an increase in the doping concentration, and enhancement of photoconductivity of the thin film heterojunction of the samples has been obtained. Evolution of the new DB and enhancement of the photocurrent upon Co doping make the prepared quantum dots very promising materials to exploit for fabricating UV–vis/NIR thin film optoelectronic devices.

Modification of the photoconducting properties of ZnO thin films via low-temperature annealing and air exposure

A simple thermal annealing at 150 °C followed by exposure to air ambient conditions in epitaxial ZnO thin films produces a photoconductivity enhancement and a reduction of the energy gap. The first effect is related to a release of carriers from bulk traps while the second is caused by a gradual adsorption of species on the film surface which increases the band bending, as x-ray photoemission spectroscopy (XPS) shows. An observed drift of the photoconductivity and the energy gap over the days is connected to this adsorption kinetics. These findings have a potential application in ZnO based optoelectronic devices.

Enhanced UV photodetection behavior of Cr doped wurtzite ZnO crystalline nanorods

Abstract The influence of Cr doping on optical, morphological and electron transport properties of ZnO nanorods were studied. High crystalline wurtzite ZnO nanorods were prepared by the reflux method and the structural properties were studied from X-Ray diffraction (XRD) pattern. Morphological analysis was done by field emission scanning electron microscopy (FESEM), transmission electron microscopy (HRTEM). The FESEM and HRTEM results show the size reduction from micrometer to nanometer regime with the addition of Cr doping. The optical properties were studied from absorption spectra. The enhancement in absorption band with the addition of Cr doping is observed. The enhancement of NBE and Green emission of ZnO with the addition of Cr doping was analyzed from fluorescence spectra. The electronic state of Zn, O was confirmed from X-Ray photoelectron spectroscopy. The additional peak around 570–590 eV confirms the chromium doping in the Zn sites. The enhancement in photoconductivity of ZnO nanorods with the addition of Cr doping is observed from the IV measurements.

Terahertz photoconductivity enhancement in graphene in magnetic fields

The terahertz photoconductivity of epitaxial graphene grown on a SiC substrate is studied in magnetic fields. Under a magnetic field applied perpendicular to the sample’s plane, a strong increase in the photoconductivity signal was detected due to suppression of electron-electron scattering. The photoconductivity mechanism based on the heating of electrons by terahertz radiation explains the experimental results.

Dosimetric investigation of a new quantum dots/nanocomposite (CdTe QDs/PVK) sensor for real-time gamma radiation detection

The design of organic/inorganic polymer nanoparticles-based hybrids is promising for gamma ray sensor systems. Here, we design semiconducting nanocomposite system for efficient gamma ray harvesting system where cadmium telluride (CdTe) quantum dots act as dopant and poly(N-vinyl carbazole) (PVK) polymer acts as host. Highly crystalline 1-dodecanethiol (1-DDT) decorated CdTe quantum dots with average diameter of ~ 3 nm were synthesized and characterized with high-resolution transmission electron microscopy (HR-TEM) analysis. Nearly monodispersed CdTe/PVK nanocomposite was subsequently prepared by addition of CdTe quantum dots (QDs) in poly(N-vinyl carbazole) matrix. The photophysical interactions between polymer host and QDs have been investigated by preparing an ohmic contact detector using a conductive cell with two gold-coated walls. Gamma detecting properties of the CdTe QDs/PVK nanocomposite such as sensitivity, repeatability, and the linearity of dose rate response were assessed. Significant enhancement in photoconductivity induced by the gamma radiation in CdTe QDs/PVK nanocomposite has been demonstrated.

Oxygen vacancy induced photoconductivity enhancement in Bi1-xCaxFeO3-δ nanoparticle ceramics: A combined experimental and theoretical study

Based on experimental and density functional studies, we show that tailoring of oxygen vacancies (OV) leads to large scale enhancement of photoconductivity in BiFeO3 (BFO). The OV concentration is increased by substituting an aliovalent cation Ca2+ at Bi3+ sites in the BFO structure. Furthermore, the OV concentration at the disordered grain boundaries can be increased by reducing the particle size. Photoconductivity studies carried out on spark plasma sintered Bi1-xCaxFeO3-δ ceramics show four orders of enhancement for x = 0.1. Temperature dependent Nyquist plots depict a clear decrease in impedance with increasing Ca2+ concentration which signifies the role of OV. A significant reduction in photoconductivity by 2 to 4 orders and a large increase in impedance of the air-annealed (AA) nanocrystalline ceramics suggest that OV at the grain boundaries primarily control the photocurrent. In fact, activation energy for AA samples (0.5 to 1.4 eV) is larger than the as-prepared (AP) samples (0.1 to 0.5 eV). Therefore, the room temperature J-V characteristics under 1 sun illumination show 2–4 orders more current density for AP samples. Density-functional calculations reveal that, while the defect states due to bulk OV are nearly flat, degenerate, and discrete, the defect states due to surface OV are non-degenerate and interact with the surface dangling states to become dispersive. With large vacancy concentration, they form a defect band that enables a continuous transition of charge carriers leading to significant enhancement in the photoconductivity. These studies reveal the importance of tailoring the microstructural features as well as the composition-tailored properties to achieve large short circuit current in perovskite oxide based solar cells.

Nanoscale enhancement of photoconductivity by localized charge traps in the grain structures of monolayer MoS2

We report a method for mapping the nanoscale anomalous enhancement of photoconductivity by localized charge traps in the grain structures of a molybdenum disulfide (MoS2) monolayer. In this work, a monolayer MoS2 film was laterally scanned by a nanoscale conducting probe that was used to make direct contact with the MoS2 surface. Electrical currents and noise maps were measured through the probe. By analyzing the data, we obtained maps for the sheet resistance and charge trap density for the MoS2 grain structures. The maps clearly show grains for which sheet resistance and charge trap density were lower than those of the grain boundaries. Interestingly, we found an unusual inverse proportionality between the sheet resistance and charge trap density in the grains, which originated from the unique role of sulfur vacancies acting as both charge hopping sites and traps in monolayer MoS2. In addition, under light illumination, the larger the trap density of a region was, the larger the photocurrent of the region was, indicating anomalous enhancement of the photocurrent by traps. Since our method provides valuable insights to understand the nanoscale effects of traps on photoconductive charge transport, it can be a powerful tool for noise studies and the practical application of two-dimensional materials.

Enhancement of Photoconductivity by Carrier Screening Effect in n-GaSb/InAs/p-GaSb Heterostructure with Single Deep Quantum Well

n-GaSb/n-InAs/p-GaSb heterostructure with a single InAs QW was grown for the first time by MOVPE. Photocurrent spectra were obtained at reverse bias in the range from 0 to 0.8 V. It was shown that the photocurrent increases nonlinearly. The maximum of differential photoconductivity is archived at low applied voltage up to 0.2 V. This effect was explained by electrostatic screening of electrons localized in QW.

Enhancement of photoconductivity by carrier screening effect in n-GaSb/n-InAs/p-GaSb heterostructure with single deep quantum well

Abstractn -GaSb/ n -InAs/ p -GaSb heterostructure with a single InAs QW was grown for the first time by MOVPE. Photocurrent spectra were obtained at reverse bias in the range from 0 to 0.8 V. It was shown that the photocurrent increases nonlinearly. The maximum of differential photoconductivity is archived at low applied voltage up to 0.2 V. This effect was explained by electrostatic screening of electrons localized in QW.