Negative Persistent Photoconductivity Research Articles

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.

Optogenetics-inspired manipulation of synaptic memory using all-optically controlled memristors.

Memristive synapses compatible with optogenetic techniques allow for the fast and low-power manipulation of memory activities using light in artificial neural systems. However, most of the optoelectronic memristors operate in the hybrid optic-electric mode; the reversible regulation of memristive states solely using light for optogenetic emulation is difficult. In this work, an all-optical controlled optoelectronic memristor (Au/Cs2AgBiBr6/Au) is developed for mimicking optogenetics-tuned memory formation and erasing behaviors in biological synapses. We show that the memristor exhibits positive and negative persistent photoconductivity effects under different light wavelengths, attributed to light-regulated carrier de-trapping/trapping at the Au/Cs2AgBiBr6 interface. This device can emulate both excitatory and inhibitory synaptic plasticity and associated learning and memory effects under light illumination. We constructed a prototype optoelectronic synaptic array and implemented the all-optically controlled memory implantation, erasing, and modification, demonstrating the light-reconfigured cognition capabilities. Our findings will inspire the development of all-optically controlled artificial neural systems with good reconfigurability for efficient neuromorphic computing and machine vision.

Scandium Nitride as a Gateway III‐Nitride Semiconductor for both Excitatory and Inhibitory Optoelectronic Artificial Synaptic Devices

AbstractTraditional computation based on von Neumann architecture is limited by time and energy consumption due to data transfer between the storage and the processing units. The von Neumann architecture is also inefficient in solving unstructured, probabilistic, and real‐time problems. To address these challenges, a new brain‐inspired neuromorphic computational architecture is required. Due to the absence of resistance–capacitance delay, high bandwidth, and low power consumption, optoelectronic artificial synaptic devices are highly attractive. Yet, stable, scalable, and complementary metal–oxide–semiconductor (CMOS)‐compatible materials exhibiting both inhibitory and excitatory optoelectronic synaptic functionalities have not been demonstrated. Here, epitaxial CMOS‐compatible scandium nitride (ScN) optoelectronic artificial synaptic devices that emulate both inhibitory and excitatory biological synaptic activities are presented. The negative and positive persistent photoconductivity of undoped and magnesium‐doped ScN is equated to the inhibitory and excitatory synaptic plasticity, respectively, which leads to functionalities like learning–forgetting, frequency‐selective optical filtering, frequency‐dependent potentiation and depression, Hebbian learning, and logic‐gate operations. Temperature‐dependent photoresponse and photo‐Hall measurements reveal that scattering of photogenerated carriers from charged defect centers results in negative photoconductivity in undoped degenerate ScN. This work opens up the possibility of utilizing a group‐III epitaxial semiconducting nitride material with inhibitory and excitatory optoelectronic synaptic functionalities for practical neuromorphic applications.

Bipolar Persistent Photoconductivity in HgTe/CdHgTe (013) Double Quantum-Well Heterostructures

The effects of the residual photoconductivity in HgTe/CdHgTe (013) double quantum-well heterostructures are studied at T = 4.2 K. It is shown that the residual photoconductivity in this system has a bipolar character, i.e., both positive and negative persistent photoconductivity is present depending on the illumination wavelength.

Биполярная остаточная фотопроводимость в гетероструктурах HgTe/CdHgTe (013) с двойными квантовыми ямами

AbstractThe effects of the residual photoconductivity in HgTe/CdHgTe (013) double quantum-well heterostructures are studied at T = 4.2 K. It is shown that the residual photoconductivity in this system has a bipolar character, i.e., both positive and negative persistent photoconductivity is present depending on the illumination wavelength.

Cyclotron resonance in InAs/AlSb quantum wells in magnetic fields up to 45 T

Electron cyclotron resonance in InAs/AlSb heterostructures with quantum wells of various widths in pulsed magnetic fields up to 45 T are investigated. Our experimental cyclotron energies are in satisfactory agreement with the results of theoretical calculations performed using the eight-band kp Hamiltonian. The shift of the cyclotron resonance (CR) line, which corresponds to the transition from the lowest Landau level to the low magnetic-field region, is found upon varying the electron concentration due to the negative persistent photoconductivity effect. It is shown that the observed shift of the CR lines is associated with the finite width of the density of states at the Landau levels.

Crossover of persistent photoconductivity in a phase-separated La0.325Pr0.3Ca0.375MnO3 thin film

A crossover from positive to negative persistent photoconductivity (PPC) has been observed in the thermal hysteresis region of a La0.325Pr0.3Ca0.375MnO3 thin film. In the cooling process, the resistance shows downward relaxation in darkness. Light illumination induces a resistance drop and positive PPC is observed. However, upon warming, the resistance exhibits upward relaxation without illumination. Moreover, the response of the film to light illumination shows distinct behavior. The resistance decreases to a lower value when the illumination is switched on and then shows upward relaxation during illumination. It recovers to a higher value when the illumination is stopped, which characterizes negative PPC. The PPC ratio is −30.2% at 125 K with a light intensity of 3.06 mW mm−2. These results are discussed based on the phase competition and phase stability switch between ferromagnetic metal and charge/orbital-ordered insulator states. The results may be important for practical applications in photoelectric devices.

Exchange enhancement of the g factor in InAs/AlSb heterostructures

The evolution of the Shubnikov-de Haas oscillations in InAs/AlSb heterostructures with twodimensional electron gas in InAs quantum wells 12–18 nm wide with considerable variation in the electron concentration (3–8) × 1011 cm−2 due to the effect of negative persistent photoconductivity is studied. The values of the effective Lande factor for electrons g* = −(15–35) are determined. It is shown that the value of the g* factor increases as the quantum well width increases.

ANOMALOUS QUANTUM HALL RESISTANCE IN Al0.6Ga0.4As0.1Sb0.9/InAs QUANTUM WELL WITH TUNABLE CARRIER DENSITY

We have investigated magneto-resistance in InAs/Al 0.6 Ga 0.4 As 0.1 Sb 0.9 quantum well (QW), where the lattice mismatch at the interface is less than 2 %. The carrier concentration can be tuned by using positive and negative persistent photoconductivity effect, selected by the wavelength (λ=1.6μ m or 0.95 μm) of illumination. The electrons and holes coexist in the QW structure, and the contribution of both carriers should be taken into account. The observed deviation of Hall plateaus from the quantized value can be understood by considering the carrier transport in both electron and hole edge channels, and the equilibrium of chemical potential at the all electrodes in the sample. The transport properties in quantum Hall regime measured in the samples with the QW thickness of 15, 50 and 150 nm are reported.

Negative and positive persistent photoconductivity effects in Al xGa 1−xAs ySb 1−y/InAs quantum wells

Both negative and positive persistent photoconductivity (NPPC and PPPC) effects on the low-temperature transport properties were studied in the Al x Ga 1− x As y Sb 1− y /InAs quantum wells with a well width of L w=15 and 50 nm. The irradiation of light with λ<1.0 μm causes the NPPC, while the light with λ=1.6 μm causes the PPPC. The Hall coefficient R H( B) in the dark or after illumination considerably increases with increasing B, indicating the coexistence of majority electrons and minority holes and allowing to analyze it by means of the two-carrier model. The PPPC effect leads to an increase in both density (n and p) and mobility of electrons and holes, while the NPPC inversely leads to their decrease, which was confirmed by the PPC effects on the SdH oscillations and the Hall resistance in the regime of quantum Hall effect in high magnetic fields. We have found the experimental relation, p μn 3, so that the minority holes increases (decreases) much more rapidly than the majority electrons increases (decreases) in the PPPC (NPPC).

Persistent photoconductivity effect on weak localization in AlxGa1–xAsySb1–y/InAs quantum wells

AbstractWeak‐field magnetoconductance (MC) caused by the weak localization (WL) in AlxGa1–xAsySb1–y/InAs quantum wells (QWs) has been studied by means of tuning the carrier density n via the positive and negative persistent photoconductivity effects in order to explore the origin of spin‐orbit interaction (SOI). We have found the weak antilocalization (WAL) in extremely weak magnetic fields developing with increasing n. The MC has been fitted by taking account of the spin‐Zeeman effect on the SOI arising from the spin splitting in asymmetric systems. The SO field (Bso) inferred has been found to be almost independent of n, thereby confirming the dominant role of the Rashba field as the cause of SOI in our InAs QWs. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Large negative persistent photoconductivity in InAs∕AlSb quantum wells

Negative persistent photoconductivity in backgated InAs∕AlSb quantum wells under red and blue light emitting diode illumination is observed to depend on both illumination energy and intensity. During these experiments the electron sheet density is varied by one order of magnitude from 5.6×1011 to 5.8×1010cm−2. This behavior is attributed to the role of optically excited holes in the AlSb barrier layers near the InAs quantum well. Furthermore, a long relaxation time in the electron sheet density is observed and attributed to the slow movement of electrons from the InAs quantum well to ionized deep levels in the AlSb barriers.

Photocurrent in epitaxial GaN

A simple kinetic model concerning photocurrent in epitaxially grown GaN is presented. Utilizing a minimal set of rate equations and kinetic parameters, it is shown that in the presence of hole centers with small probabilities of electron-hole recombinations, the time dependence of photocurrent is ruled by competition between capture of conduction band electrons by deep electron traps and electron-hole recombinations. If the probability of electron capture exceeds that of recombination, the decay of current after excitation is turned off shows the usual persistent photocurrent trend. If, on the contrary, the probability of recombination is larger than that of electron capture, a slow photocurrent quenching, past a maximum, can be observed. In some circumstances, after excitation is turned off, the current drops below the steady dark current, at which point the negative persistent photoconductivity effect comes into play.

Unusual Persistent Photoconductivity in the InAs∕AlSb Quantum Well

Unusual features of persistent photoconductivity are reported for the InAs/AlSb quantum-well (QW) structure with a backgate. A negative persistent photoconductivity made it possible to decrease the electron concentration by an order of magnitude from 6 × 1011 cm−2. This is the largest variation in the electron concentration for this effect. In addition to a pronounced negative persistent photoconductivity, the relaxation of the structural resistance was bistable under exposure of the structure to visible light. These phenomena are attributed to the effect of a thin Ge film deposited on the structure surface prior to photolithography. This film forms a region in the GaSb layer in which the holes are accumulated from the sequence of the Ge/GaSb/AlSb layers located above the QW. IR radiation initiates beats of Shubnikov-de Haas oscillations in the region of weak magnetic fields. These beats are believed to be caused by spin splitting in a zero magnetic field due to the asymmetry of a potential profile of the QW. This asymmetry is induced by prolonged illumination of the structure.

Spectra of Persistent Photoconductivity in InAs∕AlSb Quantum-Well Heterostructures

Persistent photoconductivity at T = 4.2 K in AlSb/InAs/AlSb heterostructures with two-dimensional (2D) electron gas in InAs quantum wells is studied. Under illumination by IR radiation (ℏω = 0.6–1.2 eV), positive persistent photoconductivity related to the photoionization of deep-level donors is observed. At shorter wavelengths, negative persistent photoconductivity is observed that originates from band-to-band generation of electron-hole pairs with subsequent separation of electrons and holes by the built-in electric field, capture of electrons by ionized donors, and recombination of holes with 2D electrons in InAs. It is found that a sharp drop in the negative photoconductivity takes place at ℏω > 3.1 eV, which can be attributed to the appearance of a new channel for photoionization of deep-level donors in AlSb via electron transitions to the next energy band above the conduction band.

Physical origin of negative persistent photoconductivity in a GaAs–AlAs/GaAs single heterojunction

When a Si-doped GaAs–AlAs/GaAs heterostructure is illuminated with 1.98 eV light at 4.2 K, a reduction in the two-dimensional (2D) electron density (n2D) is observed. This reduction is followed at times by a small increase in n2D even after the illumination has been switched off. This change is observed on the time scale of minutes and can be explained based on the band bending that results after a reduction in n2D. The negative persistent photoconductivity (NPPC) effect characterized by a persistent reduction in n2D, a postillumination change in n2D, and a long persistence time for T⩽40 K has been investigated. I have used Shubnikov–de Haas oscillations and time-resolved, as well as temperature-dependent, Hall-effect measurements to investigate the origin of this phenomenon. The illumination generates electron–hole (e–h) pairs in the superlattice, where the electrons are trapped into the shallow donor state (SDS) of Si and the holes drift to the two-dimensional channel to recombine with the 2D electrons. All the trapped electrons can be recovered by heating the sample to 60 K. The temperature dependence of the NPPC effect is determined only by the binding energy of the SDS of Si, which is found to be about 5 meV. The e–h recombination in the 2D channel is caused by negatively charged defects, which temporarily bind the holes. This fact is manifested also in the optical quenching of this effect by photons with 1.41 eV or larger energy. The saturation values of n2D(n2Dsat) obtained for 0.8, 1.41, or 1.98 eV illumination at 4.2 K have been investigated and the results confirm the presence of these fixed negative charges (FNCs) near the 2D channel. The change in n2Dsat for 0.8 eV illumination, caused by 1.98 eV illumination, also confirms the presence of FNCs.

Wavelength dependent negative and positive persistentphotoconductivity in Sn δ-doped GaAsstructures

The photoconductivity of GaAs structures δ-doped by Sn has been investigated for wavelengths λ = 650-1200 nm in the temperature interval T = 4.2-300 K. Theelectron densities and mobilities, before and afterillumination, have been determined by magnetoresistance,Shubnikov-de Haas effect and Hall effect measurements, in highmagnetic fields. For the heavily doped structures (Hall densitynH>2×1013 cm-2) we observe underillumination by light with wavelengths larger than the bandgapwavelength of the host material (λ = 815 nm atT = 4.2 K) first positive (PPPC) and then negative (NPPC)persistent photoconductivity. The NPPC is attributed to theionization of DX centres and PPPC is explained by the excitationof electrons from Cr impurity states in the substrate. Forλ<815 nm, in addition, the excitation of electronsover the bandgap of GaAs contributes to the PPPC. For thelightly doped structures (nH⩽2×1013 cm-2) the photoconductivity effect is always positive.

Negative persistent photoconductivity in GaAs (δ-Sn) structures

The effect of illumination with various wavelengths λ (770 nm<λ<1120 nm) on the conductivity of GaAs structures with tin δ-doping of the vicinal faces was investigated in the temperature range 4.2–300 K. Negative persistent photoconductivity was found in strongly doped samples. It was shown on the basis of the results of investigations of the Hall and Shubnikov-de Haas effects that the negative photoconductivity is due to a large decrease in the electron mobility with increasing electron density. The decrease of electron mobility is explained by ionization of DX centers, which destroys the spatial correlation in the distribution of positively charged donors and negatively charged DX centers.

Positive and negative persistent photoconductivity in a two-side-dopedIn0.53Ga0.47As/In0.52Al0.48Asquantum well

We present detailed studies of the persistent photoconductivity effect in ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}{\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}}_{0.52}{\mathrm{Al}}_{0.48}\mathrm{As}$ quantum well, including wavelength, temperature, and time dependencies. We found conclusive results that show competition between the positive and negative persistent photoconductivity effects. We suggest that a complete understanding of the decay and buildup kinetics in the entire temperature region must incorporate both the positive and negative effects. We conclude that the major positive effect is due to the band-to-band electron-hole generation in the well layer followed by the spatial charge separation and the negative effect is related to the pumping of the two-dimensional electrons into the doped barrier layer followed by the decrease of mobility.

Positive and negative persistent photoconductivities in semimetallic AlxGa1−xSb/InAs quantum wells

We report a detailed investigation of the “positive” persistent photoconductivity (PPC) and “negative” persistent photoconductivity (NPPC) in semimetallic AlxGa1−xSb/InAs quantum wells. The studies of the NPPC and PPC effects have been performed under various conditions, such as different photon energy of excitation, different temperature, different Al composition x, and different well width. It is found that all the previously proposed mechanisms fail to explain several of our observations. We suggest that the NPPC and PPC effects are produced principally by two competing processes. At a high temperature, the photoconduction is dominated by the photogenerated electrons in the InAs well, in which the relaxation of the excess electrons is prohibited by an energy barrier due to the trapping of photoexcited holes by deep defects in the InAs well. As a result, the PPC is observed. At low temperature, electrons in the InAs layer are photoexcited into the local potential minima induced by compositional fluctuations at the AlGaSb and InAs interface, the number of electrons in the InAs well decreases, thus the NPPC occurs.