Photoconductive Gain Research Articles

Out‐of‐Plane Bi2O2Se Nanoflakes for Sensitive Gate‐Tunable Phototransistors

AbstractPhototransistors based on 2D materials exhibit gate‐voltage‐tunable performance, by which the key device parameters such as the photoresponsivity and photoconductive gain could easily surpass the photodetectors without the photomultiplication effects. Phototransistors with the light‐absorption material of 2D Bi2O2Se are rising due to its desirable material properties, such as small effective electron mass, high carrier mobility, and ultrafast intrinsic non‐equilibrium carrier recombination. The in‐plane growth of Bi2O2Se is prevailing on mica substrate, where the interfacial electrostatic interaction exists in between. Crystal quality degradation caused by the organic contaminants and corrosive acids, nevertheless, is unavoidable during transfer procedures to other substrates. In this work, different out‐of‐plane growth modes of Bi2O2Se are achieved, more specifically, whisker‐like, square‐ and rectangular‐shaped flakes are obtained with optimized growth parameters. The pristine interfaces are preserved in the phototransistors with the clean transfer of Bi2O2Se nanoflakes and the electrodes. A photoresponsivity of over 105 A W−1 and a record‐high response speed of 0.32 ms are achieved for the phototransistors fabricated based on the out‐of‐plane Bi2O2Se nanoflakes. Furthermore, trap state occupation with photoinduced carriers is modulated with gate voltages and explored under cryogenic conditions for improving the device performance.

Quantum Efficiency Gain in 2D Perovskite Photo and X‐Ray Detectors

AbstractThe perovskite polycrystalline thin film detector fabricated by solution method is a promising low‐cost, scalable technology for radiation imaging, but its thin volume limits the sensing efficiency for high‐energy X‐ray photons. This work reports 2D perovskite thin film photo‐diodes with a detection gain when sensing visible and X‐ray photons. Detailed power and temperature‐dependent device characterizations reveal a charge multiplication effect to be responsible for the observed high gain. This is caused by a disparity in electron and hole transport, where electron transport is retarded by a trap/de‐trap process via shallow trap states, whereas the hole transport is fast enough to produce a photoconductive gain in satisfying the charge neutrality. The 2D perovskite made with butylamine spacers is also found to exhibit a larger efficiency gain than those made with phenylethylamines because of the higher likelihood of forming shallow traps in the former. The thin film diodes feature a high temporal response over 1 MHz due to the fast charge collection across a thin volume, and the discovery provides device physics mechanisms in connection to material structure‐function relationship for future optoelectronics design that can boost the efficiency of X‐ray sensing and dim light detection.

Printable Organic Photodetectors With Gain Toward High-Performance Vertically Stacked Color Sensors

Vertically stacked photodetectors based on printable organic semiconductors have considerable potential to deliver high-performance, low-cost, filterless color sensors, which are highly sought after for a host of emerging applications. To realize the full potential of this technology, however, it is critical to minimize the impact of the optical losses occurring at the electrodes, which reduce the photon flux reaching deep into the device stack. To this end, here we propose the use of organic photodiodes with photoconductive gain within a vertically stacked device architecture. We first realize polymer-based green- and red-sensitive photodiodes with external quantum efficiencies (EQEs) greater than 100 % and optimize their processing and spectral properties for color sensing. We then analyze their integration in a vertical device stack atop a blue-selective photodetector to achieve red-green-blue (RGB) color sensing. Finally, we examine the ultimate limit of this approach for multispectral/hyperspectral light sensing. The resulting large and balanced photoconversion efficiencies for all RGB color bands and beyond demonstrate the potential of vertically stacked organic photodetectors with gain for high-performance, low-cost color/multispectral/hyperspectral sensors and imagers.

Dependence of persistent photoconductivity on the thickness of β-Ga2O3 thin film photodetectors on c-plane sapphire via magnetron sputtering

β-Ga2O3 is a next-generation, ultra-wide bandgap semiconductor with intrinsic solar-blindness having the potential to replace Si for photodetection applications especially for the UV-C range. The material itself shows excellent photoconductive gain but is quite prone to the menace of the persistent photoconductivity, or the PPC. The fabricated devices become slower because of PPC and it also leads to reliability issues for photodetection logic. Herein, we report the dependence of the PPC effect on the different thickness of β-Ga2O3 thin film based solar-blind photodetectors. The polycrystalline films are grown on c-plane sapphire via RF magnetron sputtering at an elevated temperature of 500 °C. Optical bandgap of the films decreases with increasing thickness while their grain size increases. The oxygen-related defects studied using x-ray photoelectron spectroscopy are responsible for the observation of the enhanced PPC effect for the thinner films. The device performance is intimately connected with the quality of the thin film, its stoichiometry and the amount of oxygen defects present in the system. Better quality films with lower amount of oxygen vacancies show an improved performance with the least amount of PPC. This work shows that oxygen vacancies play an important role in determining the ultimate device performance and need to be engineered for high performance photodetectors.

Ultra-high photoresponsive photodetector based on ReS2/SnS2 heterostructure

Photodetectors based on two-dimensional materials have attracted much attention because of their unique structure and outstanding performance. The response speed of single ReS2 photodetector is slow exceptionally, the heterostructure could improves the response speed of ReS2-based photodetector, but the photodetectors responsivity is reduced greatly, which restricts the development of ReS2. In this paper, a vertically structured ReS2/SnS2 van der Waals heterostructure photodetectors is prepared, using ReS2 as the transport layer and SnS2 as the light absorbing layer to regulate the channel current. The device has an ultra-high photoconductive gain of 1010, which exhibits an ultra-high responsivity of 4706 A/W under 365-nm illumination and response speed in seconds, and has an ultra-high external quantum efficiency of 1.602×106% and a high detectivity of 5.29×1012 jones. The study for ReS2-based photodetector displays great potential for developing future optoelectronic devices.

High-mobility organic semiconducting crystal for direct X-ray detection

Tissue-equivalent pure organic semiconducting crystals (OSCs) have unique advantages in direct X-ray detectors (DXDs), especially for biological tissue fluoroscopy, but their low carrier mobility and inherent weak absorption restrict the limit-of-detection (LoD) and sensitivity of DXDs. High-mobility OSCs theoretically facilitate charge collection and performance leaps, however, they are rare and suffer from poor solubility due to high conjugation, leading to nearly impossible crystal growth from solution and subsequent device applications. Here we report an ingenious solution-processed crystal-growth and high-performance DXD using 2,6-diphenylanthracene (2,6-DPA), a high-mobility OSC we developed recently. In contrast to previous OSCs, high-mobility 2,6-DPA exhibits low dark current density and large photoconductive gain, resulting in record-breaking LoD of <85 nGyair s−1 and sensitivity of >103 μC Gyair−1 cm−2. Compared with semiconductors containing high-Z atoms (Z is atomic number), the accuracy of 2,6-DPA based DXDs is not affected by near-edge absorption effects. Moreover, the non-linear relationship between irradiation current and dose rate is confirmed as a high injection effect. High mobility and ingenious crystal growth strategy make 2,6-DPA an ideal active material for DXDs and also provide the possibility for more optoelectronic applications.

All-solution-processed perovskite/gallium nitride particles hybrid visible-blind ultraviolet photodetectors

Last decades have witnessed the rapid development of ultraviolet (UV) photodetectors in diversity of applications. The III-nitride semiconductor and metal halide perovskite have both performed promising UV-sensing optoelectronic properties. However, they are still suffering from either the high temperature epitaxial-growth or low photocurrent generated in UV range. In this work, we demonstrate an innovative MAPbCl3/GaN particle hybrid device with all-solution-processed deposition methods. Comparing to the control MAPbCl3 photoconductors, the photo-sensing ability of the hybrid device with the optimal concentration of GaN particles is more than one order of magnitude enhanced, and report a responsivity of 86 mA W−1, a detectivity of 3.1 × 1011 Jones and a rise/fall time of 1.1/10.7 ms at 360 nm. The photocurrent increment could be attributed to the enhanced UV absorption of GaN particles and facilitated charge separation and photoconductive gain at MAPbCl3/GaN heterojunction. This work paves a pathway towards the large-scale low-cost UV photodetectors in versatile applications.

High-responsivity dual-band ultraviolet photodetector based on Ga2O3/GaN heterostructure.

Ultraviolet photodetectors have aroused wide concern based on wide-band-gap semiconductors, such as GaN and Ga2O3. Exploiting multi-spectral detection provides unparalleled driving force and direction for high-precision ultraviolet detection. Here we demonstrate an optimized design strategy of Ga2O3/GaN heterostructure bi-color ultraviolet photodetector, which presents extremely high responsivity and UV-to-visible rejection ratio. The electric field distribution of optical absorption region was profitably modified by optimizing heterostructure doping concentration and thickness ratio, thus further facilitating the separation and transport of photogenerated carriers. Meanwhile, the modulation of Ga2O3/GaN heterostructure band offset leads to the fluent transport of electrons and the blocking of holes, thereby enhancing the photoconductive gain of the device. Eventually, the Ga2O3/GaN heterostructure photodetector successfully realizes dual-band ultraviolet detection and achieves high responsivity of 892/950 A/W at the wavelength of 254/365 nm, respectively. Moreover, UV-to-visible rejection ratio of the optimized device also keeps at a high level (∼103) while exhibiting dual-band characteristic. The proposed optimization scheme is anticipated to provide significant guidance for the reasonable device fabrication and design on multi-spectral detection.

Ultrasensitive Near-Infrared InAs Colloidal Quantum Dot-ZnON Hybrid Phototransistor Based on a Gradated Band Structure.

Amorphous metal oxide semiconductor phototransistors (MOTPs) integrated with colloidal quantum dots (QDs) (QD-MOTPs) are promising infrared photodetectors owing to their high photoconductive gain, low off-current level, and high compatibility with pixel circuits. However, to date, the poor mobility of conventional MOTPs, such as indium gallium zinc oxide (IGZO), and the toxicity of lead (Pb)-based QDs, such as lead sulfide and lead selenide, has limited the commercial applications of QD-MOTPs. Herein, an ultrasensitive QD-MOTP fabricated by integrating a high-mobility zinc oxynitride (ZnON)-based MOTP and lead-free indium arsenide (InAs) QDs is demonstrated. A new gradated bandgap structure is introduced in the InAs QD layer that absorbs infrared light, which prevents carriers from moving backward and effectively reduces electron-hole recombination. Chemical, optical, and structural analyses confirm the movement of the photoexcited carriers in the graded band structure. The novel QD-MOTP exhibits an outstanding performance with a responsivity of 1.15 × 105 A W-1 and detectivity of 5.32 × 1016 Jones at a light power density of 2 µW cm-2 under illumination at 905nm.

Identification and Mitigation of Transient Phenomena That Complicate the Characterization of Halide Perovskite Photodetectors.

Halide perovskites have shown promise to advance the field of light detection in next-generation photodetectors, offering performance and functionality beyond what is currently possible with traditional inorganic semiconductors. Despite a relatively high density of defects in perovskite thin films, long carrier diffusion lengths and lifetimes suggest that many defects are benign. However, perovskite photodetectors show detection behavior that varies with time, creating inconsistent device performance and difficulties in accurate characterization. Here, we link the changing behavior to mobile defects that migrate through perovskites, leading to detector currents that drift on the time scale of seconds. These effects not only complicate reproducible device performance but also introduce characterization challenges. We demonstrate that such transient phenomena generate measurement artifacts that mean the value of specific detectivity measured can vary by up to 2 orders of magnitude even in the same device. The presence of defects can lead to photoconductive gain in photodetectors, and we show batch-to-batch processing variations in perovskite devices gives varying degrees of charge carrier injection and photocurrent amplification under low light intensities. We utilize the passivating effect of aging to reduce the impact of defects, minimizing current drifts and eliminating the gain. This work highlights the potential issues arising from mobile defects, which lead to inconsistent photodetector operation, and identifies the potential for defects to tune photodetection behavior in perovskite photodetectors.

Auto-Calibrated Charge-Sensitive Infrared Phototransistor at 9.3 µm.

Charge-sensitive infrared photo-transistors (CSIP) are quantum detectors of mid-infrared radiation (λ=4 µm-14 µm) which have been reported to have outstanding figures of merit and sensitivities that allow single photon detection. The typical absorbing region of a CSIP consists of an AlxGa1-xAs quantum heterostructure, where a GaAs quantum well, where the absorption takes place, is followed by a triangular barrier with a graded x(Al) composition that connects the quantum well to a source-drain channel. Here, we report a CSIP designed to work for a 9.3 µm wavelength where the Al composition is kept constant and the triangular barrier is replaced by tunnel-coupled quantum wells. This design is thus conceptually closer to quantum cascade detectors (QCDs) which are an established technology for detection in the mid-infrared range. While previously reported structures use metal gratings in order to couple infrared radiation in the absorbing quantum well, here, we employ a 45° wedge facet coupling geometry that allows a simplified and reliable estimation of the incident photon flux Φ in the device. Remarkably, these detectors have an "auto-calibrated" nature, which enables the precise assessment of the photon flux Φ solely by measuring the electrical characteristics and from knowledge of the device geometry. We identify an operation regime where CSIP detectors can be directly compared to other unipolar quantum detectors such as quantum well infrared photodetectors (QWIPs) and QCDs and we estimate the corresponding detector figure of merit under cryogenic conditions. The maximum responsivity R = 720 A/W and a photoconductive gain G~2.7 × 104 were measured, and were an order of magnitude larger than those for QCDs and quantum well infrared photodetectors (QWIPs). We also comment on the benefit of nano-antenna concepts to increase the efficiency of CSIP in the photon-counting regime.

Reversible Electrical Control of Interfacial Charge Flow across van der Waals Interfaces.

Bond-free integration of two-dimensional (2D) materials yields van der Waals (vdW) heterostructures with exotic optical and electronic properties. Manipulating the splitting and recombination of photogenerated electron-hole pairs across the vdW interface is essential for optoelectronic applications. Previous studies have unveiled the critical role of defects in trapping photogenerated charge carriers to modulate the photoconductive gain for photodetection. However, the nature and role of defects in tuning interfacial charge carrier dynamics have remained elusive. Here, we investigate the nonequilibrium charge dynamics at the graphene-WS2 vdW interface under electrochemical gating by operando optical-pump terahertz-probe spectroscopy. We report full control over charge separation states and thus photogating field direction by electrically tuning the defect occupancy. Our results show that electron occupancy of the two in-gap states, presumably originating from sulfur vacancies, can account for the observed rich interfacial charge transfer dynamics and electrically tunable photogating fields, providing microscopic insights for optimizing optoelectronic devices.

Origami-inspired perovskite X-ray detector by printing and folding

X-ray detectors are of pivotal importance for the scientific and technological progress in a wide range of medical, industrial, and scientific applications. Here, we take advantage of the printability of perovskite-based semiconductors and achieve a high X-ray sensitivity combined with the potential of an exceptional high spatial resolution by our origami-inspired folded perovskite X-ray detector. The high performance of our device is reached solely by the folded detector architecture and does not require any photolithography. The design and fabrication of a foldable perovskite sensor array is presented and the detector is characterized as a planar and as a folded device. Exposed to 50 kVp−150 kVp X-ray radiation, the planar detector reaches X-ray sensitivities of 25−35 μC/(Gyaircm2), whereas the folded detector achieves remarkably increased X-ray sensitivities of several hundred μC/(Gyaircm2) and a record value of 1409 μC/(Gyaircm2) at 150 kVp without photoconductive gain. Finally, the potential of an exceptional high spatial resolution of the folded detector of more than 20 lp/mm under 150 kVp X-ray radiation is demonstrated.

Thickness dependent ultraviolet photoconductivity studies on sol-gel derived zinc oxide (ZnO) films

ZnO films of varying thicknesses (77–231 nm) were deposited using the sol-gel method for ultraviolet (UV) photodetection applications. The dependence of the photoconductivity parameters of the photodetector on the thickness of the ZnO films has been investigated for UV radiation having 365 nm wavelength and intensity of 24 µW/cm2. The deposited ZnO films exhibited poly-crystallinity with significant surface roughness (30–92 nm) and oxygen-related defects (Oi, Ozn, Vo), which were found beneficial for obtaining enhanced UV photosensitivity. The photoconducting mechanism has been studied in detail using the statistical interpretation of defects observed from the deconvolution of photoluminescence spectra. The 221 nm thick ZnO film showed improved photoconducting parameters (photoconductive gain, current responsivity, rise time and fall time) which are attributed to the increased density of all the defects under deep-level emission.

High-Performance and Polarization-Sensitive Imaging Photodetector Based on WS2 /Te Tunneling Heterostructure.

Next-generation imaging systems require photodetectors with high sensitivity, polarization sensitivity, miniaturization, and integration. By virtue of their intriguing attributes, emerging 2D materials offer innovative avenues to meet these requirements. However, the current performance of 2Dphotodetectors is stillbelow the requirements forpractical application owing to the severe interfacial recombination, the lack of photoconductive gain, and insufficient photocarrier collection. Here, a tunneling dominant imaging photodetector based on WS2 /Te heterostructure is reported. This device demonstrates competitive performance, including a remarkable responsivity of 402 A W-1 , an outstanding detectivity of 9.28 × 1013 Jones, a fast rise/decay time of 1.7/3.2ms, and a high photocurrent anisotropic ratio of 2.5. These outstanding performances can be attributed to the type-I band alignment with carrier transmission barriers and photoinduced tunneling mechanism, allowing reduced interfacial trapping effect, effective photoconductive gains, and anisotropic collection of photocarriers. Significantly, the constructed photodetector is successfully integrated into a polarized light imaging system and an ultra-weak light imaging system to illustrate the imaging capability. These results suggest the promising application prospect of the device in future imaging systems.

Developing low-cost nanohybrids of ZnO nanorods and multi-shaped silver nanoparticles for broadband photodetectors.

Photodetectors are essential elements for various applications like fiber optic communication systems, biomedical imaging, and so on. Thus, improving the performance and reducing the material costs of photodetectors would act as a motivation toward the future advancement of those applications. This study introduces the development of a nanohybrid of zinc oxide nanorods (ZnONRs) and multi-shaped silver nanoparticles MAgNPs through a simple solution process; in which ZnONRs are hybridized with MAgNPs to enable visible absorption through the surface plasmon resonance (SPR) effect. The photodetector based on ZnONRs/MAgNPs is responsive to visible light with representative wavelengths of 395, 464, 532 and 640 nm, and it exhibits high responsivity (R), photoconductive gain (G) and detectivity (D). The maximum R is calculated from the fitting curve of the responsivity-power relation with the value of 5.35 × 103 (mA W-1) at 395 nm excitation. The highest G and D reach 8.984 and 3.71 × 1010 Jones at that wavelength. This reveals the promise of our innovative broadband photodetector for practical usage.

ReS2 on GaN Photodetector Using H+ Ion-Cut Technology.

The wafer-scale single-crystal GaN film was transferred from a commercial bulk GaN wafer onto a Si (100) substrate by combining ion-cut and surface-activated bonding. Well-defined, uniformly thick, and large-scale wafer size ReS2 multilayers were grown on the GaN substrate. Finally, ReS2 photodetectors were fabricated on GaN and sapphire substrates, respectively, and their performances were compared. Due to the polarization effect of GaN, the ReS2/GaN photodetector showed better performance. The ReS2/GaN photodetector has a responsivity of 40.12 A/W, while ReS2/sapphire has a responsivity of 0.17 A/W. In addition, the ReS2/GaN photodetector properties have reached an excellent reasonable level, including a photoconductive gain of 447.30, noise-equivalent power of 1.80 × 10-14 W/Hz1/2, and detectivity of 1.21 × 1010 Jones. This study expands the way to enhance the performance of ReS2 photodetectors.

Carrier Recirculation Induced High-Gain Photodetector Based on van der Waals Heterojunction.

Two-dimensional (2D) materials have attracted great attention in the field of photodetection due to their excellent electronic and optoelectronic properties. However, the weak optical absorption caused by atomically thin layers and the short lifetime of photocarriers limit their optoelectronic performance, especially for weak light detection. In this work, we design a high-gain photodetector induced by carrier recirculation based on a vertical InSe/GaSe heterojunction. In this architecture, the photogenerated holes are trapped in GaSe due to the built-in electric field, suppressing the recombination rate of photocarriers, so the electrons can recirculate for multiple times in the InSe channel following the generation of a single electron-hole pair, resulting a high photoconductive gain (107). The responsivity and detectivity of the InSe/GaSe heterojunction can reach 1037 A/W and 8.6 × 1013 Jones, which are 1 order of magnitude higher than those of individual InSe. More importantly, the InSe/GaSe heterojunction can respond to weaker light (1 μW/cm2) compared to individual InSe (10 μW/cm2). Utilizing GaSe as the channel and InSe as the electrons trapped layer, the same experimental phenomenon is achieved. This work can provide an approach for designing a highly sensitive device utilizing a 2D van der Waals heterojunction, and it also possesses wide applicability for other materials.

Photoresponse of solution-processed transparent heterojunction ultraviolet photodetectors composed of n-type ZTO and p-type NiO-based semiconductor thin films

The n-type zinc-tin oxide (ZTO) and p-type nickel oxide (NiO) semiconductor thin films were prepared by the sol-gel spin coating process. The influence of 4 at% Mg and Cu doping on the microstructural, optical, and electrical characteristics of NiO thin films and physical properties of ZTO thin film were investigated. In addition, bi-layered ZTO/NiO thin films were grown onto Corning glasses to develop a visible light transparent p-n heterojunction ultraviolet (UV) photodetector. Mg doping slightly enhances the visible transparency and enlarges the optical bandgap energy, while Cu doping decreases the visible transparency and narrows the optical bandgap energy. The electrical properties of NiO sol-gel thin films were improved through Mg and Cu doping. According to the current-voltage (I–V) characteristic curves, the n-ZTO/p-NiO:Mg photodetector had the highest rectification ratio of 11.7 and on-to-off current ratio of 18.3 at 5 V bias. Under UV illumination, the three kinds of devices all exhibited a self-powered nature, while the Mg- and Cu-doped NiO devices had better photoresponse characteristics than the undoped NiO device. It was found that the Mg-doped NiO devices displayed the best photoconductivity gain and sensitivity, and acceptable response speed and responsivity, with applied reverse bias of 1 V.

Investigation of RF sputtered, n-Bi2Se3 heterojunction on p-Si for enhanced NIR optoelectronic applications

A significant deal of interest has been generated in heterojunctions made of 2D Bi-Chalcogenides and semiconducting materials due to the wide range of unique properties and functionalities that they exhibit. In this work RF magnetron sputtering technique was used to fabricate the high-quality n-Bi 2 Se 3 /p-Si heterojunction device. The structural properties of the grown Bi 2 Se 3 thin films were examined by XRD, SEM, Raman and Energy dispersive spectroscopy. The current-voltage (I–V) characteristic revealed the development of effective p-n junction with a greatest rectification ratio (RR) of 724 and significant figure of merit (FOM) of 1.11. A visible to near-infrared photodetector was successfully constructed using n-Bi 2 Se 3 /p-Si heterostructure exhibiting a strong response to 1100 nm irradiance and demonstrated outstanding photo-responsivity of 43.3 A/W, an excellent detectivity of 2.08 × 10 12 Jones and convincing photoconductive gain of 48.71. The fabricated n-Bi 2 Se 3 /p-Si heterojunction offers inexpensive optical detection, high performance broadband detector and tremendous potential for superior electrical and optoelectronic applications due to its convenient device fabrication and compatibility with silicon technology. • Using RF sputtering technique with high-quality epitaxial film offering low-cost optical detection. • n-Bi 2 Se 3 /p-Si heterojunction device for ultrafast optoelectronic devices for the future prospectus. • Excellent Responsivity (43A/W) and high Detectivity (2.08 × 10 12 ) from visible to near infrared region making it efficient for broadband detector.