Photoinduced Resistivity Research Articles

A hybrid optoelectronic Mott insulator

The coupling of electronic degrees of freedom in materials to create “hybridized functionalities” is a holy grail of modern condensed matter physics that may produce versatile mechanisms of control. Correlated electron systems often exhibit coupled degrees of freedom with a high degree of tunability which sometimes lead to hybridized functionalities based on external stimuli. However, the mechanisms of tunability and the sensitivity to external stimuli are determined by intrinsic material properties which are not always controllable. A Mott metal-insulator transition (MIT) is technologically attractive due to the large changes in resistance, tunable by doping, strain, electric fields, and orbital occupancy but not, in and of itself, controllable with light. Here, an alternate approach is presented to produce optical functionalities using a properly engineered photoconductor/strongly correlated hybrid heterostructure. This approach combines a photoconductor, which does not exhibit an MIT, with a strongly correlated oxide, which is not photoconducting. Due to the intimate proximity between the two materials, the heterostructure exhibits giant volatile and nonvolatile, photoinduced resistivity changes with substantial shifts in the MIT transition temperatures. This approach can be extended to other judicious combinations of strongly correlated materials.

Features of the Photoinduced Electrical Resistivity of Ba0.8Sr0.2TiO3/LaMnO3 Heterostructures

Features of the Photoinduced Electrical Resistivity of Ba0.8Sr0.2TiO3/LaMnO3 Heterostructures

Critical fluctuations upon photoinduced phase transition in manganite strips

Light, acting as an external stimulus to induce various intriguing phenomena ranging from photovoltaics to photoinduced catalysis, exerts prominent effects in strongly correlated systems. It would be of particular interest to investigate photon-induced emerging phenomena in spatially confined strongly correlated systems, which are important for applications of these materials in future electronic devices. Colossal magnetoresistive manganites materials offer an ideal platform for such study due to their sensitivity to photo-excitation. Here, we fabricated 900 nm wide La0.325Pr0.3Ca0.375MnO3 strips, whose width is comparable to the size of the electronic phase separation (EPS) domains in this system. We observed the photoinduced critical fluctuations in the strips, where abrupt resistivity jumps occurred upon photoinduced phase transition depending sensitively on the light intensity. Based on the microscopic views of the EPS domains under photoexcitation, we conclude that such photo-induced resistivity fluctuations originate from the photoinduced phase fluctuations of individual EPS domains when their size becomes comparable to the strip width.

Photoinduced Kondo effect inCeZn3P3

The Kondo effect, which originates from the screening of a localized magnetic moment by a spin-spin interaction, is widely observed in nonartificial magnetic materials, artificial quantum dots, and carbon nanotubes. In devices based on quantum dots or carbon nanotubes that target quantum information applications, the Kondo effect can be tuned by a gate voltage, a magnetic field, or light. However, the manipulation of the Kondo effect in nonartificial materials has not been thoroughly studied; in particular, the artificial creation of the Kondo effect remains unexplored. Per this subject study, however, a route for the optical creation of the Kondo effect in the nonartificial material $p$-type semiconductor ${\mathrm{CeZn}}_{3}{\mathrm{P}}_{3}$ is presented. The Kondo effect emerges under visible-light illumination of the material by a continuous-wave laser diode and is ultimately revealed by photoinduced electrical resistivity, which clearly exhibits a logarithmic temperature dependency. By contrast, a La-based compound (${\mathrm{LaZn}}_{3}{\mathrm{P}}_{3}$) displays only normal metallic behavior under similar illumination. The photoinduced Kondo effect, which occurs at higher temperatures when compared with the Kondo effect in artificial systems, provides a potential range of operation for not only quantum information/computation devices but also for operation of magneto-optic devices, thereby expanding the range of device applications based on the Kondo effect.

Deposition and photo-induced electrical resistivity of dip-coated NiO thin films from a precipitation process

Thin films of the semiconductor NiO are deposited using a straightforward combination of simple and versatile techniques: the co-precipitation in aqueous media along with the dip- coating process. The obtained material is characterized by gravimetric/differential thermal analysis (TG–DTA) and X-ray diffraction technique. TG curve shows 30 % of total mass loss, whereas DTA indicates the formation of the NiO phase about 578 K (305 °C). X-ray diffraction (XRD) data confirms the FCC crystalline phase of NiO, whose crystallinity increases with thermal annealing temperature. UV–Vis optical absorption measurements are carried out for films deposited on quartz substrate in order to avoid the masking of bandgap evaluation by substrate spectra overlapping. The evaluated bandgap is about 3.0 eV. Current–voltage (I–V) curves measured for different temperatures as well as the temperature-dependent resistivity data show typical semiconductor behavior with the resistivity increasing with the decreasing of temperature. The Arrhenius plot reveals a level 233 meV above the conduction band top, which was attributed to Ni2+ vacancy level, responsible for the p-type electrical nature of NiO, even in undoped samples. Light irradiation on the films leads to a remarkable behavior, because above bandgap light induced a resistivity increase, despite the electron–hole generation. This performance was associated with excitation of the Ni2+ vacancy level, due to the proximity between energy levels.

Characterization of metallic electrical contacts to SnO2 thin films lightly doped with Eu3+ ions, and photo-induced resistivity

Lightly Eu3+-doped (0.05%) SnO2 thin films are deposited by the sol–gel-dip-coating technique, topped by alternative metallic layers of Al, Sn or In, arranged in a parallel layout on the thin film surface, and deposited by the resistive evaporation technique. Electrical characterization results show that the sort of deposited metal strongly modifies the device resistance, besides thermally treated metallic layers decreases the device resistivity, which may be associated with increased tunneling probability. Current as function of applied voltage show a good linear symmetry contacts for a large temperature range (30–320 K). However, this feature is due to the MSM (metal–semiconductor–metal) structure of the device, because the conduction through the reversed-biased junction is the main mechanism of electrical transport at Schottky potential barriers. The barrier height evaluation is also presented, considering that the dominant mechanism is the thermionic emission through the reversed-biased junction, yielding values in the range 124 meV for annealed In contacts to 187 meV for untreated Sn contacts. This paper also shows results of photo-induced electrical characteristics under irradiation with below bandgap (450 nm) as well as above bandgap (266 nm) light on the SnO2 thin films, where the surface is coupled with untreated Sn contacts.

Large photoinduced conductivity reduction in thin films of metallic ferromagnetic manganites

This paper reports on a study of photoinduced resistivity changes in thin films of ferromagnetic metallic (FMM) manganites. We have observed a significant increase of resistance in La0.7Ba0.3MnO3 thin film under continuous wave argon ion laser illumination presumably associated with photoinduced demagnetization. Strong dependence of resistance on thermal/illumination history is consistent with the coexistence of two phases: ferromagnetic metallic phase and photoinduced less conductive phase.

Effects of oxygen depletion on photoinduced and transport properties of thin films of charge-ordered manganites

In this paper, we report a study of conductive and photoinduced properties of charge-ordered Bi0.4Ca0.6MnO3 thin films with different oxygen contents. The oxygen content of the films was varied following different annealing or deposition protocols. Change in film’s resistivity during annealing reflects oxygen dynamics at high temperatures. It was found that oxygen-deficient films exhibit a significant increase in the photoinduced resistivity changes and the lifetime of photoinduced conductive phase. Possible origin of this effect is discussed.

Photoinduced resistivity change of electron-doped La0.8Te0.2MnO3 film

In this paper, electron-doped La0.8Te0.2MnO3 film has been prepared on SrTiO3 (100) substrate by pulsed laser deposition method, and the photoinduced resistivity change and colossal magnetoresistance effect have been investigated. The results show that the film has a high metal-insulator transition temperature TMI of ∼283 K and the maximum magnetoresistance ratio is ∼24% at 0.7 T. Under laser irradiation, TMI varies from 283 to 243 K and resistivity increases below TMI, while it decreases above TMI. The maximum photoinduced resistivity change in resistivity (LR%) is 134.8%. The photoinduced relaxation character of the film indicates that the relaxation of photoinduced character relates to the excitation of small polarons.

An annealing study of an oxygen vacancy related defect in SrTiO3 substrates

The study addresses the stability of point defects in SrTiO3 (STO) during thin film processing using electron paramagnetic resonance (EPR) spectroscopy. In particular, the intensity of the Fe3+VO EPR signal is monitored after various steps during the growth of STO films on STO substrates. Controlled O2 and vacuum heat treatments are also performed to clarify the fundamental mechanisms responsible for the effects of different processing steps. Comparison of results from film fabrication with those obtained during exposure to the control ambient shows that the presence of oxygen in the pretreatment growth atmosphere decreases the amount of the Fe3+VO complex, but exposure to the low pressure environment of the growth chamber returns the signal to the original intensity. These results are consistent with accepted theories of oxygen vacancy diffusion. However, an unexpected decrease in the oxygen vacancy related signal is also observed during vacuum treatment of an as-received sample. Furthermore, the decrease occurs over the same temperature range as seen for an O2 anneal. The difference between the O2 and vacuum treatments is revealed in postannealing photoinduced EPR and resistivity measurements, which indicate that vacancy related centers change charge state during the O2 anneal and are not removed by oxygen. The effect of the vacuum treatment, though different from that of oxygen, is not yet clear as no charge state changes were induced after exposure to visible or ultraviolet radiation, but the conductivity of the samples changed.

Photoinduced phase separation inBi0.4Ca0.6MnO3thin films

In this paper, we report a study of photoinduced and current-induced resistivity changes in ${\mathrm{Bi}}_{0.4}{\mathrm{Ca}}_{0.6}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films. The photoinduced effects were investigated using both far-field and near-field optical techniques. It was found that substrate-induced strain affects the stability of the charge ordering. The magnitude of the energy barrier separating the metastable conducting phase and the charge-ordered insulating phase estimated from the temperature dependence of the lifetime of the photoinduced conducting phase was found to be large, of the order of the charge-ordering temperature.

Enhancement of photoinduced effects in Bi1−xCaxMnO3 thin films

A considerable increase of the photoinduced resistivity changes and the lifetime of these changes were found in the Bi0.4Ca0.6MnO3 thin films after depositing gold nanoparticles on the surface. A possible interpretation of this effect may be the enhancement of local electromagnetic field at the interface between the film and the metal nanoparticles due to the plasmon resonance, which may promote formation of larger volume of conducting phase producing larger magnitude and the lifetime of photoinduced resistivity changes.

Comparative study of vertical gradient freeze grown CdTe with variable Sn concentration

Four CdTe ingots with gradually increased concentration of the Sn impurity have been grown by the vertical gradient freeze method and were characterized with glow discharge mass spectroscopy, photoinduced current transient spectroscopy, resistivity, photoconductivity, and photoluminescence techniques. It was shown that the Sn impurity strongly influences resistivity and photoconductivity of the material. Concentration of Sn must be higher than the total concentration of residual acceptors to reach strong compensation. The middle-gap donor level pins the Fermi-level. Photoconductive high resistivity material can be prepared with Sn concentrations in the melt in the range 1018–1019 cm−3. In total, 6 electron traps and 3 hole traps were identified in the band gap by several complementary techniques.

Charge order in photosensitive Bi0.4Ca0.6MnO3 films

We report the structural and electronic characterization of the charge order phase inBi0.4Ca0.6MnO3 films, in which photoinduced resistivity changes have been observed at temperaturesapproaching room temperature. Lattice distortions associated with the charge order areobserved in all films, and both the wavevectors and displacements of the distortions are inthe plane of the film. Films under compressive and tensile strain are observed to havedifferent resonant x-ray scattering characteristics—a result that may shed light onthe mechanism responsible for the photosensitivity exhibited by this material.

Photoinduced resistivity changes in Bi0.4Ca0.6MnO3 thin films

We report charge-ordered Bi0.4Ca0.6MnO3 thin films with charge-ordering temperature near room temperature, and observation of large photoinduced resistivity changes in these films associated with melting of the charge ordering by visible light. Films grown under small compressive strain exhibit the largest photoinduced resistivity changes. The lifetime of the photoinduced low-resistance state is on the order of half a minute. These photoinduced resistivity changes in thin films of Bi0.4Ca0.6MnO3 make them very promising for photonic device application.

The change of photo-induced resistivity properties in La2/3Ca1/3MnO3 thin films

Photo-induced resistivity change p has been studied in perovskite manganite La2/3Ca1/3MnO3 thin films with a continous wave laser and modulated laser pulses in this paper. Experimental results show that in the sample films, the maximum of photo-induced resistivity change（ΔR/R）max can reach 43.5%,which is a very exciting figure in this research field. Modulated laser pulse induced signal intensity has a highly nonlinear relation with the applied current and temperature. The maximum of photo-induced resistivity increase is a second power function of the applied current, while the temperature at which the maximum appears is proportional to the bias current. There exist an optimal bias current and a temperature for optical response in this film. These results are attributed to the optical excited eg↓ carriers and polarons.

Photoinduced spin-disorder in a ferromagnetic maganite film

Photoinduced effects in a La 0.6Sr 0.4MnO 3 film ( T c∼315 K) have been investigated in the temperature range 290–470 K by means of a pump-probe method. Below T c, we have observed transient transmission changes Δ T/ T induced by pump pulse excitations at 2.54 and 1.55 eV, which have been interpreted in terms of the photoinduced spin-disorder in the ferromagnetic phase. We further have compared the photoinduced Δ T/ T with the photoinduced resistivity change Δ R/ R.

Photoinduced resistivity change in layered manganite La 2−2 xCa 1+2 xMn 2O 7 ( x=0.3)

Photoinduced effect on the resistivity in layered perovskite manganite La 2−2 x Ca 1+2 x Mn 2O 7 ( x=0.3) has been studied by using a He–Ne laser (632.8 nm) in the temperature range from 20 to 300 K and an applied magnetic field of 5000 G. In both cases of zero magnetic field and applied magnetic field, the resistivity peaks of the sample shift to lower temperatures slightly under the irradiation of the laser. The photoinduced resistivity is negative and positive for the paramagnetic semiconductive regime and the ferromagnetic metal regime, respectively. The variation of the resistivity and magnetoresistance induced by laser irradiation is explained in terms of the effect of the low-dimensional composite polarons.

Photoinduced superconductivity and structural changes in high temperature superconducting films

The illumination of PryGd1−yBa2Cu3Ox semiconducting and superconducting thin films increases their critical temperatures and decreases their normal state resistivities if and only if the films are oxygen deficient. Moreover, these changes are enhanced near the Pr-induced metal-insulator transition. Light also causes a contraction of the c-axis in YBa2Cu3Ox which is correlated with the observed photoinduced resistivity changes. These changes are similar to those observed when oxygen-deficient YBa2Cu3Ox is enriched with oxygen or annealed at room temperature after quenching from high temperatures.