Observation of persistent photoconductivity in conducting polyaniline thin films

Conducting polyaniline thin films have been deposited by a laser-based vaporization technique. The films have been characterized by infrared spectroscopy, UV-Vis spectroscopy, four-point conductivity measurements, thermogravimetric analysis, and fluorescence measurements. In addition, the films have been characterized with respect to their photoconductive response to 532-nm laser light. It is found that the films exhibit persistent photoconductivity and it is proposed that defect base sequences may be responsible for the charge localization that results in such a photoconductive response. PACS 78.66.Qn; 81.15.Fg; 72.80.Le     

Electrical transport and persistent photoconductivity in quantum dot layers in InAs/GaAs structures

The conductivity of quantum dot layers is studied in InAs/GaAs structures in the temperature range from 300 to 0.05 K in the dark and using two types of illumination in magnetic fields up to 6 T. Depending on the initial concentration of current carriers, the conductivity of the structures varied from metallic (the Shubnikov-de Haas effect was observed) to hopping conductivity. At low temperatures, the temperature dependence of the resistance changed from the Mott dependence to the dependence described by the Shklovskii-Efros law for hopping conductivity in the presence of the Coulomb gap in the density of states. The conductivity of samples was studied upon their illumination at λ = 791 nm and λ > 1120 nm. All the samples exhibited a positive persistent photoconductivity at T < 250 K. The structures were also studied using photoluminescence and an atomic force microscope.     

Temperature dependence of photoconductivity and persistent photoconductivity of single ZnO nanowires

Photoelectrical properties of single ZnO nanowires have been investigated using photocurrent–voltage characteristics measurements varying with excitation photon energy and temperature. It is found that persistent photoconductivity (PPC) exists, and the PPC decreases with decreasing temperature. The temperature dependence of the PPC effect indicates that thermally activated return of electrons from shallow traps is responsible for the PPC phenomenon. The photosensitivity is found to be linear with the applied voltage, and it increases with decreasing temperature. A temperature dependence of photoconductivity gain was introduced to explain the experimental results.     

Longitudinal photoconductivity in metal-amorphous silicon-metal structures

Longitudinal photoconductivity has been examined for Pt--SiH-Ni structures (with the light propagating along the field), with forward bias applied to the structure. There is a difference from transverse photoconductivity, where recombination is important, in that the longitudinal conductivity spectra are determined by generation and drift, where the carriers generated at various depths contribute unequally. The longitudinal-conduction spectra have been calculated for the following cases: 1) constant field in a uniform structure, 2) inhomogeneous field due to change in conductivity on illumination, and 3) inhomogeneous field due to doping or a potential barrier. The comparison with experiment indicates that the last case applies for Pt--Si:H structures. The longitudinal conductivity provides information on the electrical homogeneity in wafers and films and on the field distribution.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 57–61, January, 1988.     

Epitaxial periodic p-n structures in PbS

The electrical and photoelectrical properites of periodic p-n structures in an epitaxial film of PbS are studied. The plane of the junctions is perpendicular to the plane of the film. The possibility of using such structures in the photodiode mode as low inertia detectors of IR radiation is shown. Qualitative agreement of the dark characteristics (volt-ampere curve, temperature dependence of resistance) of the p-n structure and photosensitive polycrystalline films was noted. This confirms the hypothesis that in sensitization of polycrystalline films, barriers of the p-n junction type are formed.     

Deposition of hybrid structures of reduced graphene oxide and tin dioxide thin films,and persistent photoconductivity observation

Reduced graphene oxide (rGO) is deposited on glass substrate by dripping and sol-gel-coating methods giving rise to nanostructures. When in combination with thin films of SnO₂, they form a heterostructure SnO₂:2 at% Eu/rGO, which alters the surface electrical conductivity. SnO₂ and rGO were also combined as a composite, with conductivity strongly affected by ultraviolet excitation, and shows persistent photoconductivity (PPC) phenomenon even very close to room temperature. Both sort o hybrid structures can be applied in electronic devices. The SnO₂ films are deposited via chemical route by sol-gel or by a mixed technique that combines powders generated by drying the sol-gel solution with resistive evaporation of this powder. Resistivity measured as a function of temperature show that the SnO₂:2 at%Eu sample behaves very similarly to the SnO₂:2 at%Eu/rGO heterostructure sample, with the same energy level for the dominant defect, 172 meV, coincident with ionization of oxygen vacancies. Despite not changing the position of this level, the presence of rGO on the surface of the SnO₂ film induces a decrease in conductivity in vacuum, demonstrating the surface interaction.     

Observation of persistent photoconductivity at room temperature in ferrocene-doped poly(methyl methacrylate) thin films containing chloroform molecules

Persistent photoconductivity has been observed at room temperature (300 K) in solution cast ferrocene-doped poly(methyl methacrylate) thin films, prepared from chloroform and the mixture of solvents chloroform and benzene. The maximum photoinduced current, the buildup time and decay time of persistent photocurrent increases with the increasing amount of chloroform present in the film as well as the duration of photoexcitation. The buildup and decay kinetics have been studied. Our results indicate that the photooxidation of ferrocene in the presence of chloroform in the film leads to a photocurrent persistent for a longer period at room temperature.     

ZnO nanorod-based UV photodetection and the role of persistent photoconductivity

We report on the substantial persistent photoconductivity (PPC) response exhibited by the zinc oxide (ZnO) nanorod-based ultraviolet (UV) photodetection system. An increase in photocurrent and, hence, rise in PPC was observed for larger UV exposure times at regular intervals. Triggered by quantum efficiency, the increment in sustained conduction band electrons is proposed as the main reason behind the increased photocurrent response. In contrast, the trap centers located below the conduction band are expected to slow down the recombination rate, which accounts for the rise in PPC. The lowering of PPC upon annealing suggests the surface dependent nature of the PPC. The growth and decay mechanism of PPC has a direct relevance while assessing figure of merit of prototype nanostructure-based optical sensor and UV photodetectors.     

Observation of optical Shubnikov de Haas effect in photoconductivity

We have observed a new type of oscillatory magnetophotoconductivity effect. It occurs in highly degenerate materials at high power levels of incident radiation. The effect is an analog of the d.c. Shubnikov de Haas effect and caused by an electron temperature modulation due to free carrier absorption in high magnetic fields. The degree of modulation of the photoconductive signal can even exceed 50% and thus is conveniently observable in III–V and IV–VI compounds.     

Suppression of persistent photoconductivity in high gain Ga2O3 Schottky photodetectors

The defect-related photoconductivity gain and persistent photoconductivity (PPC) observed in Ga₂O₃ Schottky photodetectors lead to a contradiction between high responsivity and fast recovery speed. In this work, a metal-semiconductor-metal (MSM) Schottky photodetector, a unidirectional Schottky photodetector, and a photoconductor were constructed on Ga₂O₃ films. The MSM Schottky devices have high gain (> 13) and high responsivity (> 2.5 A/W) at 230-250 nm, as well as slow recovery speed caused by PPC. Interestingly, applying a positive pulse voltage to the reverse-biased Ga₂O₃/Au Schottky junction can effectively suppress the PPC in the photodetector, while maintaining high gain. The mechanisms of gain and PPC do not strictly follow the interface trap trapping holes or the self-trapped holes models, which is attributed to the correlation with ionized oxygen vacancies in the Schottky junction. The positive pulse voltage modulates the width of the Schottky junction to help quickly neutralize electrons and ionized oxygen vacancies. The realization of suppression PPC functions and the establishment of physical models will facilitate the realization of high responsivity and fast response Schottky devices.     

Far infrared photoconductivity studies in silicon blocked impurity band structures

We report two novel features in the photoconductive response of a boron doped Blocked Impurity Band (BIB) structure. In this structure which comprises a pure and a very doped photoconductive layer between ohmic contacts, spectra of impurities at an interface with the pure layer were evidenced in a process involving field assisted hopping among excited states. In addition, a peculiar photovoltaic response, with open circuit photovoltage surprisingly independent of the incident photon flux was measured and analysed.     

Conductivity and persistent photoconductivity in GaAs epitaxial films containing single and double delta-doped layers

Electrophysical parameters of single and double delta-doped layers in GaAs epitaxial films grown by the metal-organic chemical vapor deposition have been systematically investigated in the temperature range of 4.2 to 300 K. The 2D electron gas density distribution is affected by the overlap of wave functions in neighboring quantum wells, as a result of which the peak on the curve of the Hall mobility in the 2D electron gas versus the separation between the quantum wells shifts. The persistent photoconductivity in delta-doped layers is due to the change in the surface potential caused by the neutralization of the negative charge of surface states by photoexcited holes. A method for comparing delta-doped layers grown under different conditions at different depths from the sample surface has been suggested. Zh. éksp. Teor. Fiz. 113, 693–702 (February 1998)     

Study of persistent photoconductivity effect in n-type selectively doped AlGaAs/GaAs heterojunction

A detailed study of the persistent photoconductivity effect (PPE) at selectively doped Al_0.3Ga_0.7As/GaAs interface was carried out at low (4.2 and 77 K) temperatures on samples with different original channel concentrations and mobilities. The observed selectiveness of the PPE to the photon energy allowed us to identify two independent mechanisms of the PPE making almost equal contributions to the total effect. These two mechanisms are: (i) electron photoexcitation from DX centers in AlGaAs layer, (ii) electron—hole generation in bulk GaAs with a charge separation at the interface. It has been found that the behavior of the mobility as a function of the channel concentration (altered by light) depends on a setback thickness d. For a sample with small d a marked mobility drop has been found. The well-resolved structure in the dependence of the electron mobility on the channel concentration has been observed. The first peculiarity is explained by free electron population of AlGaAs layer due to the electron photorelease from DX-centers. The second feature, occurring at higher charge densities in the channel is attributed to the effect of intersubband scattering arising due to the electron occupation of an excited subband at the interface.