Persistent Photoconductivity Behavior Research Articles

Investigation on room-temperature persistent photoconductivity in γ-InSe layered semiconductor

We have presented the room-temperature persistent photoconductivity (PPC) effect in γ-InSe layered semiconductors. The PPC effect can be observed in the temperature range from room temperature down to 200 K, and the PPC decay is well described by a stretch-exponential function. The temperature-dependent PPC and performance of PPC decay indicate that the random local-potential fluctuations (RLPF) mechanism is responsible for PPC effect inγ-InSe. In addition, the PPC behavior at different temperature regions can be well explained by the percolation model associated with the RLPF mechanism.

Dual-mode Conversion of Photodetector And Neuromorphic Vision Sensor via Bias Voltage Regulation on A Single Device.

For optoelectronic devices that convert optical signal to electrical signal, they can have a fast photoresponse speed to function as a photodetector, or possess the ability to perceive, memorize and process images to act as a neuromorphic vision sensor (NVS). However, the simultaneous implementation of photodetector and NVS on a single device faces a great challenge, due to the contradiction of photoresponse speed. In this work, to simultaneous achieve this complex function in a single device, trench-bridged GaN/Ga2 O3 /GaN back-to-back double heterojunction array device is fabricated. Interestingly, the device shows fast photoresponse and persistent photoconductivity behavior at low and high voltages, respectively, through the modulation of oxygen vacancy ionization and de-ionization processes in Ga2 O3 . Consequently, the role of the optoelectronic device can be altered between photodetector and NVS by simply adjusting the bias voltage. As a photodetector, the device is able to realize fast optical imaging and optical communication functions. On the other hand, the device exhibits outstanding image sensing, image memory, and neuromorphic visual pre-processing as a NVS. The utilization of NVS for image pre-processing leads to a noticeable enhancement in both the recognition accuracy and efficiency. The results presented in this work not only offer a new avenue to obtain complex functionality on a single optoelectronic device, but also provide the opportunities to implement advanced robotic vision systems and neuromorphic computing. This article is protected by copyright. All rights reserved.

An organic memory phototransistor based on oxygen-assisted persistent photoconductivity

Persistent photoconductivity (PPC) behavior in organic phototransistors has fascinating potentials in applications of photoelectronic devices. A key issue is how the presence of air affects the PPC behavior. Here, combining with the theoretical and experimental results, the PPC behavior is associated with photogenerated electrons trapped in oxygen atom-induced the reduced Lowest Unoccupied Molecular Orbitals or oxygen molecule-induced new trap state within energy bandgap of organic semiconductor. Inspired by the potential applications arising from the PPC behavior, organic memory phototransistors (OMPTs) are achieved by light programming and electrical erasing. The OMPTs show bistable current states as well as long retention times. Our results suggested that oxygen in air plays a key role in PPC behavior and provides a guidance for controlling the PPC behavior toward integrated multifunctional optoelectronic devices. • 2DMCs are used as OSCs layer to rule out the influence of grain boundaries on OPTs. • PPC behavior can be obtained when OPTs measured in air compared with OPTs measured under vacuum. • Oxygen-induced trap states play a critical role in PPC behavior. • Memory characteristics arising from PPC behavior are investigated.

Synergistic Modulation of Synaptic Plasticity in IGZO-Based Photoelectric Neuromorphic TFTs

Neuromorphic devices are of great significance for next generation energy-efficient brain-like computing and perception. Oxide-based photoelectric neuromorphic transistors are very promising due to the proton-related electric-double-layer (EDL) effect and persistent photoconductivity behavior. In this study, indium-gallium-zinc-oxide (IGZO)-based photoelectric neuromorphic thin-film transistors (TFTs) with tunable synaptic plasticity are proposed. Some important synaptic functions, such as excitatory postsynaptic current, multipulse facilitation, and long-term plasticity, can be effectively tuned by the photoelectric synergistic modulation. More importantly, under the photoelectric synergistic modulation, the temporary short-term memory can be converted into permanent long-term memory. And the voltage coordinated modulation can enhance the image in the pixel and the ability of real-time data processing of the input visual information. Our results are very important for the development of photoelectric neuromorphic devices with configurable dynamic functions.

Tin sulphide prepared by sulphurisation process using metallic tin film precursor obtained from dc magnetron sputtering method

Thin films of tin sulphide were prepared by sulphurisation process of metallic Sn precursor at temperature of 300-500°C in excess ambient of sulphur using a graphite box in N2 atmosphere. Sulphurisation temperature at 300°C, the films were formed in SnS phase with orthorhombic structure. In contrast, the films were formed in SnS2 phase with hexagonal structure when sulphurisation temperature higher than 300°C. From absorption spectra, direct band gap increased from 1.70 to 2.45 eV with increasing sulphurisation temperature. From the transient photoconductivity measurements, persistent photoconductivity behavior was observed in the films sulphurised at 300 and 350°C. The decay current data are better fitted with multiple exponential function resulting in the several slow decay times. Density of trap states corresponding to its decay time was also evaluated from the decay current data.

Brain-Inspired Photonic Neuromorphic Devices using Photodynamic Amorphous Oxide Semiconductors and their Persistent Photoconductivity.

The combination of a neuromorphic architecture and photonic computing may open up a new era for computational systems owing to the possibility of attaining high bandwidths and the low-computation-power requirements. Here, the demonstration of photonic neuromorphic devices based on amorphous oxide semiconductors (AOSs) that mimic major synaptic functions, such as short-term memory/long-term memory, spike-timing-dependent plasticity, and neural facilitation, is reported. The synaptic functions are successfully emulated using the inherent persistent photoconductivity (PPC) characteristic of AOSs. Systematic analysis of the dynamics of photogenerated carriers for various AOSs is carried out to understand the fundamental mechanisms underlying the photoinduced carrier-generation and relaxation behaviors, and to search for a proper channel material for photonic neuromorphic devices. It is found that the activation energy for the neutralization of ionized oxygen vacancies has a significant influence on the photocarrier-generation and time-variant recovery behaviors of AOSs, affecting the PPC behavior.

Oxide Semiconductor Phototransistor with Organolead Trihalide Perovskite Light Absorber

A hybrid phototransistor is developed with solution‐processed organolead trihalide perovskite (MAPbI3) capping indium gallium zinc oxide (IGZO), which well fuses the properties of the two materials in sensitive photodetecting and high‐mobility charge transporting, respectively. The MAPbI3‐capped IGZO phototransistor demonstrates excellent responsivities of over 25 mA W−1 for lights with photon energies above the bandgap of perovskite light absorber. Besides the high sensitivity to light in both ultraviolet and visible regions, hybrid phototransistor maintains a fair on/off ratio of over 106 in the dark, and a field effect mobility of 12.9 cm2 V−1 s−1. The perovskite light absorber also obviates the long‐standing problem for metal oxide phototransistor, the persistent photoconductivity behavior. Furthermore, fast transient response has been achieved by showing rise‐time and fall‐time within tens of milliseconds. The newly developed device opens variable optic‐electric sensing applications for the integrated oxide–perovskite hybrid phototransistors.

Observation of persistent photoconductivity in 2H-MoSe2 layered semiconductors

We report the observation of persistent photoconductivity (PPC) effect in 2H-MoSe2 layered semiconductors. The decay behavior of PPC can be well described by a stretch-exponential function. Experimental results indicate that the lattice relaxation of DX-like impurity is responsible for PPC in MoSe2. In addition, the small capture barrier created by lattice relaxation and the temperature-dependent resistance measurement exhibit that the magnitude of PPC effect in MoSe2 is small, consistent with the PPC behavior in indirect gap materials.

Persistent photocurrent and surface trapping in GaN Schottky ultraviolet detectors

GaN-based Schottky detectors were implemented and their photoresponse as a function of the incident power and time was measured. The measured photoresponse shows gain saturation and persistent photoconductivity behavior. These effects are shown here to be related to each other, arising from a nonideal semiconductor surface. A microscopic model of the gain mechanism to explain these observations is presented. Trap density at the semiconductor metal interface, characteristic lifetime, and carrier capture coefficient are extracted from our measurements.

Persistent photoconductivity in Ho-doped InGaAsP epitaxial layers

Persistent photoconductivity (PPC) was investigated in Ho-doped InGaAsP epilayers with Ho concentrations in the range of 0–0.15 wt%. The decay behavior of PPC resembles that of Al x Ga 1− x As and can be well described by a stretch-exponential function. The temperature-dependent PPC studies show that the lattice relaxation of DX-like centers is responsible for the PPC effect. As the Ho doping increases, the decay-time constant and the electron-capture barrier were found to decrease. We suggest that the introduced Ho elements may chemically react with donor impurities, suppressing lattice relaxation and hence reducing the electron-capture barrier. Also, rare earth doping is demonstrated to be an effective method of improving the quality of InGaAsP epilayers.

Visible-light induced persistent photoconductivity in trilayered films of perovskite manganites

The persistent photoconductivity (PPC) effect has been observed in trilayered films made of perovskite manganites La2/3Ca1/3MnO3/(La0.3Nd0.7)2.3Ca1/3MnO3La2/3Ca1/3MnO3(LNL) induced by He–Ne laser with wavelength of 632.8 nm. According to the result obtained in the thin film of (La0.3Nd0.7)2/3Ca1/3MnO3(LNCMO), which the PPC effect is also observed below ∼50 K, the PPC effect observed in trilayered film LNL should originate from the middle layer LNCMO. Compared with the thin film of LNCMO, the PPC effect of the trilayered film LNL appears at ∼86 K, which is higher than that of LNCMO at ∼50 K. The PPC effect of LNL can be quenched on thermal cycling in the vicinity of 98 K, which is also higher than that of LNCMO at ∼77 K. The difference of PPC behavior between the thin films of LNCMO and LNL can be attributed to the variation of cluster-glass state in the trilayered films of LNL caused by the strong coupling of interlayer between the middle layer LNCMO and the top/bottom layers LCMO.

Yellow luminescence and persistent photoconductivity of undoped n-type GaN

Deep-level defect-related optical properties of undoped n-type GaN grown by metalorganic chemical vapor deposition are investigated using photoluminescence (PL), optical absorption (OA), photoconductivity (PC), and persistent photoconductivity (PPC) measurements. From the temperature dependence of the PL and OA, we find that the yellow luminescence (YL) is due to shallow-to-deep donor recombination. PL, PC, and PPC results manifest a strong correlation in properties related to deep levels. Samples which emit YL exhibit a PC peak at 1.9 eV due to the photoionization of deep levels as well as to the persistent photoconductivity effect, whereas samples with no YL have no PC peak in the forbidden gap and no PPC at any photon energy, suggesting a common origin. Furthermore, two types of PPC behavior were observed depending on the sample quality: typical stretched exponential decay in relatively thick samples and photocurrent quenching and a subsequent reduction of the dark current in thin samples. An explanation of the latter phenomenon based on photoinduced metastable electron traps in a highly defective layer near the interface is suggested from the temporal behavior of the PC. These traps seem to disappear slowly after the illuminating light is turned off.

A research on the persistent photoconductivity behavior of GaN thin films deposited by r.f. magnetron sputtering

The persistent photoconductivity (PPC) behavior has been characterized in sputtered GaN thin films using the room illumination with a wavelength of 254 nm under a 5-V bias. It was found that the N 2 partial pressure in the sputtering atmosphere has an evident effect on the PPC behavior. The obtained data show that the nitrogen vacancy is the candidate for PPC effect in the sputtered GaN film. At lower N 2 partial pressures, the nitrogen vacancy can be induced and resulted in GaN films with more donor levels as compared with those of samples deposited at higher N 2 contents. An energy band model that can account for the experimental observation of PPC behavior is proposed.

The origin of persistent photoconductivity in compensated hydrogenated amorphous silicon

The possible origins of persistent photoconductivity (PPC) in compensated hydrogenated amorphous silicon ( a-Si : H) are reexamined by the numerical calculation. PPC behavior with the variation of defect density and active dopants concentration is compared with the previous experimental data. The results show that PPC cannot be induced by the defect generation only, but the deactivation of active boron should be included. The variation of PPC with light soaking is well explained by the deactivation of a boron acceptor and a defect generation induced by a hole capture. This result supports the thermal equilibrium, or the charge trapping model rather than the model based only on Staebler-Wronski effect.

Deep levels and persistent photoconductivity in GaN thin films

Photocurrent decay in GaN thin films was studied in the time span from a few seconds to several days. The persistent photoconductivity (PPC) behavior was observed not only in Mg-doped p-type GaN films but also in undoped n-type GaN films. The photoconductivity spectra and the photocurrent response time were measured using a weak probe light at several times after the samples had been kept in the dark. During the relaxation, the photocurrent due to the subband-gap probe light decreased more than the photocurrent due to the UV probe light. It is suggested that metastable centers at 1.1, 1.40, and 2.04 eV above the valence band edge are responsible for the PPC behavior in Mg-doped GaN, and that Ga vacancy is the candidate for PPC effect in n-type GaN.

Anomalous temperature dependence of persistent photoconductivity in C60 single crystal

The temperature dependence of persistent photoconductivity (PPC) has been investigated in high-quality C60 single crystals. We found that the rotational order and disorder phase transition (Tc≊260 K) can significantly change the PPC behavior. At temperatures above Tc, the PPC relaxation rate increases as temperature increases. Quite surprisingly, in the region T<Tc, the PPC relaxation rate decreases with increasing temperature. Our result clearly indicates that the underlying mechanism of the PPC effect involves the motion of the C60 molecule.