Negative Persistent Photoconductivity Effect Research Articles

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.

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.

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.

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 & Co. KGaA, Weinheim)

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.

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.

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.

Negative persistent photoconductivity in an InAs/GaSb quantum well

We performed magneto-resistance measurements on a semi-metallic InAs–GaSb quantum well. By shining visible light on the sample, a Negative Persistent Photoconductivity effect (NPPC) was observed. Overnight illumination reduced the electron density of the sample from 9.54×10 11 cm −2 to 9.04×10 11 cm −2. A possible explanation for this effect is being sought in interface defect states.

Study of persistent photoconductivity in semimetallic Al0.2Ga0.8Sb/InAs quantum wells

The “positive” persistent photoconductivity (PPC) and “negative” persistent photoconductivity (NPPC) have been observed in Al0.2Ga0.8Sb/InAs quantum wells. The decay kinetics of the PPC and NPPC effects can be well described by stretched-exponential functions and the activation energy of the thermally activated relaxation was obtained. Through the studies of the NPPC and PPC effects under various conditions, such as different temperature and different photon energy of excitation, we suggest that the NPPC and PPC effects are caused by two competing processes. At T≤40K, the electrons in the InAs wells are excited into the valence band of the AlGaSb layer, the recovery to the original state is prohibited by the barrier due to interface band bending and hence NPPC effects occur. At T≥140K, the photoexcited holes are trapped by the deep defects in the InAs well and the remaining electrons contribute to the observed PPC. For 40K<T<140K, these two mechanisms compete with each other. The previously proposed model that describes the NPPC resulting from the capture of the photoexcited electrons by the DX-like deep donors in AlGaSb fails to explain our observation here.

Effect of well thickness on the two-dimensional electron-hole system inAlxGa1−xSb/InAs quantum wells

We have studied the effect of well thickness on the two-dimensional electron-hole system in semimetallic ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Sb/InAs quantum wells by Shubnikov--de Haas (SdH) measurements. The number of hole carriers in the ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Sb barriers was changed by a negative persistent photoconductivity effect. From the amplitude of SdH oscillation for the sample with the thinnest well, we found that the interface roughness scattering dominated when the number of hole carriers is small. After the number of holes is increased by the negative persistent photoconductivity effect, the electron-hole scattering becomes more important, and results in a reduction of the electron quantum lifetime. The competition between electron-hole scattering and interface roughness scattering depends on the amount of holes in the barriers.

Observation of electrically resettable negative persistent photoconductivity in InAs/AlSb single quantum wells

The mechanism of the negative persistent photoconductivity (NPPC) in InAs/AlSb single quantum wells is discussed. The molecular beam epitaxy grown single InAs quantum well sample is made into Hall bars with the substrate as the backgate. Using the newly designed buffer, the gate bias can deplete or enhance the two-dimensional electrons in the InAs quantum well without substantial gate leakage current. Based on the 4.2 K magnetoresistance data, and the fact that the trapped electrons can be redistributed by gate bias, we conclude that the NPPC effect at low temperatures is a result of the capture of photoexcited electrons by ordinary, deep donors in AlSb. Numerical modeling using physical assumptions can quantitatively explain our experimental observation, and the calculated AlSb donor energy is 0.41±0.05 eV above the AlSb valence band maximum, with 4×1016/cm3 to 1017/cm3 in concentration. The previously discussed DX-center-like characteristic of deep levels in AlSb, i.e., lattice-relaxation with a relatively high activation energy, is not evidenced in this work.

Observation of a negative persistent photoconductivity effect in In0.25Ga0.75Sb/InAs quantum wells

We have observed a negative persistent photoconductivity effect in In0.25Ga0.75Sb/InAs quantum wells with Shubnikov–de Haas measurements. The saturated reduction of the electron density in the InAs well was about 10%. The electron effective mass was found to be (0.048±0.004) m0 for an electron density of 18.0×1011 cm−2. The electron quantum lifetime decreased as the electron density was reduced by the negative persistent photoconductivity effect due to electron-hole interaction.

Competition between negative and positive photoconductivity in silicon planar-doped GaAs

Photo-Hall free electron concentrations were measured on molecular beam epitaxy grown silicon planar-doped GaAs samples, with silicon nominal concentration ranging from 1.4×1012 to 8.8×1013 cm−2, as function of temperature. We found conclusive results showing competition between positive photoconductivity and negative persistent photoconductivity effects. At temperatures below a critical Tc, the negative photoconductivity effect is dominant, while above Tc the positive effect dominates. We also found some evidence that the positive effect is related to spatial charge separation and that the negative effect is related to the DX center.

Observation of negative persistent photoconductivity in an n-channel GaAs/AlxGa1−xAs single heterojunction

We report the first observation of negative persistent photoconductivity at 4.2 K in an n-channel modulation doped GaAs/Al0.33Ga0.67As single heterostructure, where two-dimensional electrons have a mobility of ∼550 000 cm2/V s when density is ∼3.0×1011 cm−2. Based on extensive magnetotransport measurements, we conclude that the negative persistent photoconductivity effect comes from the time dependence of (1) the annihilation of two-dimensional electrons by photoexcited holes, and (2) the trapping and de-trapping of photoexcited electrons by shallow donors in doped Al0.33Ga0.67As. A model that quantitatively explains the nonexponential recovery time is presented.

Negative persistent photoconductivity in the Al0.6Ga0.4Sb/InAs quantum wells

We have measured the Shubnikov–de Haas (SdH) effect in the Al0.6Ga0.4Sb/InAs quantum wells under the negative persistent photoconductivity (NPPC) conductions. By illuminating the sample at low-temperature, the carrier concentration of the two-dimensional electron gas in the InAs well was reduced from 5.8 to 3.6×1011 cm−2 and the corresponding quantum lifetime increases from 0.16 to 0.21 ps. The electrons which escaped from the InAs well were captured by the ionized deep donors in the Al0.6Ga0.4Sb layers. The effective mass is equal to (0.0317±0.0005)m0. We also propose, based on the SdH data, that the illumination of the sample with the ionized deep donors at low temperature will exhibit the NPPC effect.

The Negative Persistent Photoconductivity in the Deep Quantum Wells

ABSTRACTWe have measured the Shubnikov-de Haas effect and Quantum Hall effect on the AlxGa1−xSb/InAs quantum wells for the A1 composition (x) from 0.1 to 1.0 under the negative persistent photoconductivity (NPPC) conditions. It confirmed the prediction of ionized deep donor model that the NPPC effect is a general property for the materials containing ionized deep donors at low temperatures. The time-dependent recombination (electron capture) of the ionized deep donors has the similar property to that of DX centers. The saturated reduction of carrier concentration in the InAs well increases with increasing x, and rises steeply at about x=0.4. By comparing with the concentration of the DX center-like deep donor in the bulk AlxGa1−xSb, we believe that the ionized deep donors which cause the NPPC in the AlxGa1−xSb,/InAs QW's are the DX center-like deep donors in the AlxGa1−xSb layers.

Effect of Al Composition on the Deep Level Donors of AlxGa1-xSb/lnAs Single Quantum Wells

ABSTRACTWe have performed Shubnikov-de Haas (SdH) measurements on the AlxGa1-xSb/InAs QW's with x from 0.1 to 1.0 under the negative persistent photoconductivity (NPPC) conditions and confirmed the prediction of Ionized Deep Donor model that the NPPC effect is a universal property for the materials containing ionized deep donors at low temperatures. The time-dependent recombination (electron capture) of the ionized deep donors is similar to that of DX centers. The saturated reduction of carrier concentration in the InAs well increases with increasing x, and rises steeply at about x=0.4. The concentration of deep donors which cause the NPPC in the AlxGa1-xSb/InAs QW's increases with the Al composition.