Persistent Photoconductivity Research Articles

Photoelectric properties and persistent photoconductivity of GaN-based ultraviolet photodetectors

Persistent photoconductivity deeply affects the performance of the photodetector, and has been studied in a variety of semiconductor optoelectronic devices. In this work, AlGaN/GaN metal-heterojunction-metal (MHM) ultraviolet (UV) detectors and GaN metal-semiconductor-metal (MSM) ultraviolet detectors were prepared using transverse Schottky contacts, and characterized at different temperatures, light intensity and bias voltage. The two-dimensional electron gas (2-DEG) at the Schottky heterojunction of AlGaN and GaN is used to generate high-speed electron mobility, which significantly improves the response speed and responsivity of photogenerated carriers. Under the illumination of 365 nm center ultraviolet light with an intensity of 4.4 mW/cm2, the photocurrent can reach 434 μA-MHM and 38 μA-MSM (25 °C), and the attenuation rate increase by 2.8 times, 5 times and 3.7 times with temperature (25 °C-250 °C), bias voltage (5/10/15 V), and light intensity (0.33/1.1/4.4 mW/cm2), respectively, and the attenuation rate reaches 50∼80 times.

Cap Layer Effect on Key Features of Persistent Photoconductivity Spectra in HgTe/CdHgTe Double Quantum Well Heterostructures

Persistent photoconductivity (PPC) spectra of HgTe/CdHgTe heterostructures with double quantum wells with different cap layers have been studied in the radiation excitation range 0.62–3.1 eV. We have shown that the material of the cap layer defines key features of the PPC spectra—local extrema—and their origin. An unusual oscillatory behavior of the PPC spectra is demonstrated. Such a behavior is shown to be independent of both cap and barrier layers.

Influence of White Light on the Photoelectric Characteristics of UV Detectors Based on β-Ga2O3

In this report, we consider the effect of radiation with a radiation energy below the β-Ga2O3 band-gap on the photoelectric characteristics of ultraviolet (UV) radiation detectors based on gallium oxide thin films. The gallium oxide film was deposited by the radio frequency magnetron sputtering. Interdigital Ti/V electrodes were formed with an inter electrode distance d = 30 μm. The structures exhibit high value of photocurrent under irradiation with wavelength of λ = 254 nm and intensity of 780 μW/cm2. The detectors exhibit persistent photoconductivity after exposure to UV radiation. The presence of persistent conductivity is explained by the high concentration of traps in the Ga2O3 film and their recharging under UV irradiation. Radiation with an energy quantum below the the β-Ga2O3 band-gap changes the charge state of trap centers in a gallium oxide film. This effect leads to an increase in photocurrent under UV exposure and increases the stability of detectors.

Tin Doping Induced High‐Performance Solution‐Processed Ga2O3 Photosensor toward Neuromorphic Visual System

AbstractGa2O3 is an emerging wide‐bandgap semiconductor with high deep ultraviolet absorption, tunable persistent photoconductivity, and excellent stability toward electric fields, making it a promising component for neuromorphic visual systems (NVSs). However, Ga2O3‐based photosensors with high responsivity and long response decay times are required for efficient NVSs. A solution‐processed doping strategy for fabrication of Ga2O3 is proposed with tin foil as a dopant source. Tin‐doped Ga2O3 (Ga2O3:Sn) photosensors are obtained with ultrahigh responsivity and extremely long response decay times. These behaviors are attributed to substitutional tin and oxygen vacancies that modulate defect‐related hole trapping. High‐performance Ga2O3:Sn photosensors can mimic photonic synaptic behaviors and image pre‐processing functions. NVSs based on a Ga2O3:Sn photonic synapse array perform pattern recognition with an accuracy of 97.3% under an unprecedented low‐light pulse stimuli of 0.5 µW cm−2. This work provides a low‐cost solution‐processed approach to ultrasensitive Ga2O3:Sn NVSs and will facilitate developments in artificial intelligence technology.

Reversible Photomodulation of Two-Dimensional Electron Gas in LaAlO3/SrTiO3 Heterostructures.

Long-lived photoinduced conductance changes in LaAlO3/SrTiO3 (LAO/STO) heterostructures enable their use in optoelectronic memory applications. However, it remains challenging to quench the persistent photoconductivity (PPC) instantly and reproducibly, which limits the reversible optoelectronic switching. Herein, we demonstrate a reversible photomodulation of two-dimensional electron gas (2DEG) in LAO/STO heterostructures with high reproducibility. By irradiating UV pulses, the 2DEG at the LAO/STO interface is gradually transformed to the PPC state. Notably, the PPC can be completely removed by water treatment when two key requirements are met: (1) the moderate oxygen deficiency in STO and (2) the minimal band edge fluctuation at the interface. Through our X-ray photoelectron spectroscopy and electrical noise analysis, we reveal that the reproducible change in the conductivity of 2DEG is directly attributed to the surface-driven electron relaxation in the STO. Our results provide a stepping-stone toward developing optically tunable memristive devices based on oxide 2DEG systems.

Emulating Synaptic and Nociceptive Behavior via Negative Photoconductivity of a Memristor

In comparison to electrical memristive synapses, photoelectric synapses have the capability for ultrafast computing with less power consumption. In this work, a photoelectric synaptic memristor (PSM) based on Zn <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{\text{2}}}$</tex-math> </inline-formula> SnO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{\text{4}}}$</tex-math> </inline-formula> (ZSO)/cuprous oxide (Cu <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{\text{2}}}$</tex-math> </inline-formula> O) heterostructure is fabricated. This two-terminal PSM device with a simple structure exhibits primary synaptic functions such as long-term potentiation (LTP)/long-term depression (LTD), and spike-timing-dependent plasticity (STDP) by electrical stimulation. Under the exposure of violet light, the observed negative persistent photoconductivity (NPPC) of our PSM is associated with inhibitory synaptic plasticity, which leads to synaptic performances such as short to long-term memory (STM-LTM) transition, learning experience behavior, and a transition from post-tetanic potentiation (PTP) to post-tetanic depression (PTD). Furthermore, this PSM mimics the five essential features of a nociceptive receptor (nociceptor) like threshold, relaxation, no-adaptation, hyperalgesia, and allodynia in response to violet light pulse-induced NPPC phenomenon. These results provide a new path to propel the development of neuro-inspired perceptual systems.

Efficient Suppression of Persistent Photoconductivity in β-Ga2O3-Based Photodetectors with Square Nanopore Arrays.

In this work, square nanopore arrays were developed on the surface of β-Ga2O3 microflakes using focused ion beam (FIB) etching, and solar-blind photodetectors (PDs) were fabricated based on the β-Ga2O3 microflakes with square nanopore arrays. The β-Ga2O3 microflake-based device was transformed from a gate voltage depletion mode to an oxygen depletion mode by FIB etching. The developed device exhibited excellent solar-blind PD performance with extremely high responsivity (1.8 × 105 at 10 V), detectivity (3.4 × 1018 Jones at 10 V), and light-to-dark ratio (9.3 × 108 at 5 V) as well as good repeatability and excellent stability. The intrinsic mechanism responsible for this performance was then systematically discussed. This work opens up a new avenue for the fabrication of high-performance β-Ga2O3-based low-dimensional PDs with high reproducibility by employing the FIB etching process.

Photoinduced doping in hexagonal boron nitride

Hexagonal boron nitride is shown to exhibit very significant persistent photoconductivity after UV illumination. This behavior can be initiated by sub-bandgap or close to bandgap illumination. Neither temperature nor pressure affects the buildup of photoinduced carriers. The effect persists at least for months at room temperature and is maintained significantly after heating up to 300 °C. Up to six orders of magnitude increased conductivity has been durably established in the devices, and the effect is reproducible. Double exponential fitting gives time constants up to 4600 days. Irradiation after having saturated the devices is shown to drastically reduce the decay rate. The bulk origin of such effect has been demonstrated. p-hBN/n-AlGaN diodes based on magnesium doped h-BN have been used to determine the type of conductivity through studies of junction capacitance variations under UV irradiation. Depending on illumination wavelength, both n- and p-type durable photoinduced carriers can be produced. These results are of interest for UVC LEDs in which the usual conductive AlGaN layers are still a hurdle toward efficient UV emitters.

Multifunctional Crystalline InGaSnO Phototransistor Exhibiting Photosensing and Photosynaptic Behavior Using Oxygen Vacancy Engineering.

A multifunctional optoelectronic device implementing photodetector, photosynapse, and photomemory is of increasing attention for neuromorphic system. This enables multiple devices to be replaced with a single device, which simplifies the structure of complex, highly integrated electronics. Here, a multifunctional c-axis-aligned crystalline indium gallium tin oxide thin-film transistor (TFT) optoelectronic device is demonstrated. The photodetecting and photosynaptic behaviors could be demonstrated by tuning of gate pulse. The device shows a high responsivity of 1.1×106 AW-1 to blue light (467nm) and cutoff frequency (f-3dB ) of 2400Hz exhibiting high frequency switching using a gate reset pulse. It is possible to implement photosynaptic behavior using persistent photoconductivity effect by applying a gate bias to make the TFT depletion mode. When potentiation and depression of synaptic weight are implemented with light pulse and gate voltage pulse, respectively, 64-state potentiation-depression curves are demonstrated with excellent nonlinearity of 1.13 and 2.03, respectively. When an artificial neural network is constructed with this device for the Modified National Institute of Standards and Technology training pattern recognition simulation, it shows a high pattern recognition accuracy of 90.4%.

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.

Comprehensive enhancement of ZnGa2O4-based solar blind photodetector performance by suppressing defects in oxygen-rich atmosphere

ZnGa2O4 is a potential candidate material for next-generation solar-blind UV photodetectors (PDs) owing to its intrinsically wide bandgap and various favorable physical properties. The photoelectric properties of metal oxides are susceptible to oxygen. This study investigated the effect of oxygen flow on the properties of ZnGa2O4 thin films. Furthermore, photovoltaic devices with Au as Schottky contacts were fabricated to compare the performance. The ZnGa2O4 films grown in an oxygen-rich atmosphere exhibit better crystallinity and fewer oxygen vacancies than those grown in an oxygen-poor atmosphere, and the corresponding device demonstrates excellent performance parameter at a bias voltage of 10 V: a responsivity (R), external quantum efficiency (EQE) and detectivity (D*) up to 15.62 A/W, 7626%, ∼1015 Jones, respectively. Additionally, the suppression of oxygen vacancies prevents persistent photoconductivity under low light irradiation. The device possesses satisfactory rise/decay times of 0.125/0.025 s and maintains excellent stability and repeatability. Our study confirms that sputtering in an oxygen-rich atmosphere is an effective strategy for producing ZnGa2O4 thin films that can be applied for fabricating ZnGa2O4 solar-blind PDs with excellent comprehensive performance.

Preparation and photodetection performance of high crystalline quality and large size β-Ga2O3 microwires

Ultrawide band gap semiconductors are promising solar-blind ultraviolet (UV) photodetector materials due to their suitable bandgap, strong absorption and high sensitivity. Here, β-Ga2O3 microwires with high crystal quality and large size were grown by the chemical vapor deposition (CVD) method. The microwires reach up to 1 cm in length and were single crystalline with low defect density. Owing to its high crystal quality, a metal–semiconductor–metal photodetector fabricated from a Ga2O3 microwire showed a responsivity of 1.2 A/W at 240 nm with an ultrahigh UV/visible rejection ratio (R peak/R 400 nm) of 5.8 × 105, indicating that the device has excellent spectral selectivity. In addition, no obvious persistent photoconductivity was observed in the test. The rise and decay time constants of the device were 0.13 and 0.14 s, respectively. This work not only provides a growth method for high-quality Ga2O3 microwires, but also demonstrates the excellent performance of Ga2O3 microwires in solar-blind ultraviolet detection.

P‐7: Enhanced Visible Light Response of Amorphous InZnO Thin‐Film Transistors by Hydrogen Doping via Al2O3/SiO2 Gate Dielectric

Enhancement of visible light response in InZnO (IZO) thin‐film transistors (TFTs) by hydrogen doping via Al2O3/SiO2 dielectric layer is investigated. The IZO TFTs with Al2O3/SiO2 dielectric show better visible light detection performance due to the introduction of subgap states by hydrogen doping. The photocurrent, photoresponsivity, detectivity, and external quantum efficiency of the device to blue light are respectively improved from 4.94 × 10−7 A to 1.97 × 10−6 A, 4.93 A/W to 19.71 A/W, 2.70 × 1012 Jone to 5.77×1012 Jone, and 13.60 to 54.32 after hydrogen doping. The persistent photoconductivity effect is also eliminated by applying a positive gate pulse voltage.

Light-matter interaction of the polar-polar interface LaVO3-KTaO3 (111)

KTaO3 (KTO) has emerged as a potential candidate in the spintronics block showing the emergent phenomena in its bulk form as well as thin films, especially in KTO-based heterostructures and interfaces. In this work, we have grown a thin film of polar Mott insulator LaVO3 (LVO) on polar band insulator KTO (111) using the pulsed laser deposition technique. This interfce shows the insulator-to-metal transition. We have studied the photoelectrical properties by using a blue laser light of wavelength 405 nm for illumination in temperatures ranging from 76 K and 300 K. Under blue light illumination, the interface shows enhancement in conductivity, for both 76 K to 300 K. This system has shown the persistent photoconductivity as well. Further, electrostatic gating has also been applied to tune the charge carrier density and the conductivity is increased by around 23 % under the effect of light and electrostatic gating.

Synthesis and characterization of sulfate precursor based SnS thin films using SILAR technique at elevated solution temperature

In this study, a microprocessor-controlled SILAR technique was used to prepare SnS thin films on glass substrates using equimolar SnSO4 and Na2S precursors. The combination of sulfate precursor and increase in the solution temperature lead to the dilution of phase purity. The effects of these heterogeneous phase blend on the structural, optical, electrical and photo-response properties of the films were studied in detail. The coexistence of the SnS and SnO2 phases was confirmed by XRD and Raman analysis. The energy band structure of the films was significantly affected by the presence of SnO2. The optical band gap of the films shifted from the mid-visible to the UV region of the electromagnetic spectrum as the amount of mixed phases in the films increased. The photoluminescence spectra also showed prominent emission in the blue and green regions arising from the phase coexistence. Marginal increase in the electrical conductivity was observed for higher solution temperature. The rise-time of the photocurrent was found to increase significantly (nearly 80 %) owing to the presence of heterogenous phase blend. This prolonged decay time suggests the possibility of persistent photoconductivity (PPC) in the films. Data availability statementThe data cannot be made available at this point as the results presented here are part of ongoing research work.

Giant Switchable Persistent Photoconductivity in Soft Chemistry Reduced SrTiO3

AbstractHigh tunability of photoconductivity is highly desired for applications in optical memories, sensors, and bioelectronics. Recently, room temperature persistent photoconductivity (PPC) in SrTiO3 (STO) has been revealed and has attracted great attention. However, reversible switching of the PPC in STO with a large on/off ratio remains challenging to date. Here, a giant switchable PPC in soft chemistry reduced STO is reported. An initial insulator‐to‐metal transition with on/off ratio up to 7 orders of magnitude is observed and about 5 orders of magnitude transition is found to be reversible. Via nuclear magnetic resonance measurements, it is uncovered that such unusual PPC is driven by the generation of excess carriers accompanied with a configuration evolution of the incorporated hydrogen from hydridic HO+ to protic Hi+ upon illumination. The work demonstrates giant switchable PPC transition in soft chemistry reduced perovskite oxides, providing a new platform for pursuing high performance sensors and nonvolatile optoelectronic memory devices.

Optoelectronic memory in 2D MoS2 field effect transistor

2D layered materials with their tunable bandgap and unique crystal structures are excellent candidates for 2D optoelectronic memories. In this work, we present a simple approach for the realization of a nonvolatile optoelectronic memory device based on a MoS2 transistor with light induced charge storage capability. The MoS2 transistor shows 108 on/off current ratio and hysteresis width modulation by air pressure under normal and quiet measurement conditions. Moreover, the device shows persistent photoconductivity and exhibits excellent photo responsive memory performance with a current switching ratio of two orders of magnitude and a photocurrent that increases linearly with the incident light power. We show that a combination of gate voltage and light can be used to control the transistor current and increase the memory window by two orders of magnitude. The obtained results are a significant step toward the improvement of optoelectronic devices, showing that the combination of gate voltage and light can enable a multilevel memory device.

Effect of surface potential pinning on strain behavior of AlGaN/GaN device structures

Understanding the varied strain effects in AlGaN/GaN devices is crucial for realizing optimized flexible electronics systems and strain sensors. Here, we report on the effects of surface potential pinning, altered by the deposition of device-relevant SiNx passivation and Ni gate layers, on the strain-dependent carrier density, ns, of AlGaN/GaN two-dimensional electron gas structures. Flexible van der Pauw samples were made by separating AlGaN/GaN layers from the sapphire growth substrate using a two-dimensional boron nitride van der Waals release layer and transferring them to flexible substrates. For bare surface samples, we observed relatively large decreases in ns with tensile strain (Δns of −2 × 1011 cm−2 at 0.1% uniaxial strain), indicating an unpinned AlGaN surface potential. For the SiNx and Ni covered samples, the ns-strain trends were nearly flat, indicating a more pinned surface potential. Additionally, sub-bandgap 400 nm light is shown to effectively pin the surface potential as evidenced by flattening the ns-strain trend, the mechanism of which we explain in terms of the persistent photoconductivity effect. These observations could have important implications in tuning strain sensors and minimizing device variability in flexible electronics.

Type-II Bi2O2Se/MoTe2 van der Waals Heterostructure Photodetectors with High Gate-Modulation Photovoltaic Performance.

In recent years, two-dimensional (2D) nonlayered Bi2O2Se-based electronics and optoelectronics have drawn enormous attention owing to their high electron mobility, facile synthetic process, stability to the atmosphere, and moderate narrow band gaps. However, 2D Bi2O2Se-based photodetectors typically present large dark current, relatively slow response speed, and persistent photoconductivity effect, limiting further improvement in fast-response imaging sensors and low-consumption broadband detection. Herein, a Bi2O2Se/2H-MoTe2 van der Waals (vdWs) heterostructure obtained from the chemical vapor deposition (CVD) approach and vertical stacking is reported. The proposed type-II staggered band alignment desirable for suppression of dark current and separation of photoinduced carriers is confirmed by density functional theory (DFT) calculations, accompanied by strong interlayer coupling and efficient built-in potential at the junction. Consequently, a stable visible (405 nm) to near-infrared (1310 nm) response capability, a self-driven prominent responsivity (R) of 1.24 A·W-1, and a high specific detectivity (D*) of 3.73 × 1011 Jones under 405 nm are achieved. In particular, R, D*, fill factor, and photoelectrical conversion efficiency (PCE) can be enhanced to 4.96 A·W-1, 3.84 × 1012 Jones, 0.52, and 7.21% at Vg = -60 V through a large band offset originated from the n+-p junction. It is suggested that the present vdWs heterostructure is a promising candidate for logical integrated circuits, image sensors, and low-power consumption detection.

High-sensitivity Solar-blind photodetector based on Ga2O3 films through manipulating oxygen vacancies

Oxygen vacancies (VO’s) play an extremely important role in the internal gain and the persistent photoconductivity (PPC) effect of metal–semiconductor-metal (MSM) Ga2O3 photodetectors (PDs). The VO’s of bulk phase and interfacial can be manipulated by the annealing process. Herein, the annealing of amorphous-Ga2O3 (a-Ga2O3) film can significantly reduce the VO’s of the bulk phase, so the PDs can exhibit faster response speed due to the suppression of the PPC effect. The VO’s at the interface of M/S can be introduced by the device post-annealing. As a result, responsivity (R) and specific detectivity (D*) were greatly improved, reach to 54.0 A/W, and 2.17 × 1014 Jones, respectively. The increased specific internal gain at the interface of M/S should be responsible for the improvement of sensitivity.