Steady-state Photoconductivity Research Articles

Photoconduction in bacteriorhodopsin/GaAs heterostructures

Observation of transient and steady-state photoconduction across an indium tin oxide (ITO)/bacteriorhodopsin(bR)/GaAs heterostructure, with the bacteriorhodopsin acting as the light-sensitive material, is reported. The protein is dissolved in polyvinyl alcohol and the film dried after deposition. The steady state photoconduction is probably caused by the pH change at the bR/ITO interface owing to proton release from the bR upon photoexcitation, and is therefore an electrochemical effect.

Steady State Photoconductivity in Conjugated Polymers

ABSTRACTVisible and ultraviolet photoconductivity (PC) measurements have been performed on thick (7[.proportional]m) and thin (170nm) films of poly[2-methoxy, 5 ethyl (2' hexyloxy) paraphenylenevinylene] (MEH-PPV) sandwiched between indium tin oxide (ITO) and semitransparent gold (Au) electrodes. PC spectra were obtained for samples under different bias polarities and for illumination through the ITO electrode and through the Au electrode. The results obtained indicate that the absorbed photons form excitons that dissociate extrinsically at or near the electrodes to form hole polarons or intrinsically at higher photon energies in the bulk polymer.

Photoconductivity and Electrical Properties of Pb 1 - x Sn x Te thin films

PbSnTe can be used in the fabrication of IR photoresistors and IR photodiodes. PbSnTe films are economic, if compared with bulk materials. Due to the importance of this material, \( Pb_{1 - x} Sn_x Te \) thin films were prepared with x=0.0, 0.1, 0.2, 0.3 and 0.4 by a flash evaporation technique. The interplanar spacing d, and consequently the reflecting planes (h k l) were identified, from the powder X-ray diffraction analysis. The as-deposited thin films of \( Pb_{1 - x} Sn_x Te \) were polycrystalline. As tin substitutes for lead in the prepared films, the interplanar spacing d decreases and the unit cell dimension a also decreases. Energy dispersive X-ray analysis (EDX) was carried out. From the composition of these films determined by EDX excess Te was detected for x=0.0 and 0.1, while a slight Te deficiency was observed for x=0.2, 0.3 and 0.4. Besides a slight Sn deficiency was found for films of x=0.1, 0.3 and 0.4. On the other hand a slight excess of Sn was present for x=0.2. The electrical properties, example, the electrical resistivity, the Hall coefficient and Hall mobility, were studied in the temperature range 100–450 K. The electrical transport properties of the investigated films are governed by scattering mechanisms, associated with the intergrain barrier height. A.c. photoconductivity measurements were carried out by using the frequency-dependence method. The carrier lifetime was determined for all the investigated films at temperatures of 100, 200, 300 and 400 K. This ranged from 12–20 ms at 100 K. The steady-state photoconductivity, photoconductivity, photosensitivity and the carrier lifetime were studied in the temperature range 100–450 K. The photosensitivity increases sharply at a temperature ∼100 K. Combining the results of the photoconductivity and the electrical measurements, it appears that x=0.1 is a suitable composition for further study to prepare IR device, since it has the best photosensitivity and good electrical properties.

Mechanism of Carrier Photoexcitation in Semiconducting Polymers: The Role of Electron Photoemission in “Photoconductivity” Measurements

ABSTRACTUltrafast photoinduced absorption by infrared-active vibrational modes (IRAV) is used to detect charged photo-excitations (polarons) in solid films of conjugated luminescent polymers. Experiments, carried out in zero applied electric field, show that polarons are generated within 100 fs with quantum efficiencies of approximately 10%. The ultrafast photoinduced IRAV Absorption, the weak pump-wavelength dependence, and the linear dependence of charge density on pump intensity indicate that both charged polarons and neutral excitons are independently generated even at the earliest times. Measurements of the excitation profile of the transient and steady-state photoconductivity of poly(phenylene vinylene) and its soluble derivatives over a wide spectral range up to hν = 6.2 eV indicate an apparent increase in the “photoconductivity” at hν > 3-4 eV that arises from external currents generated by electron photoemission (PE). After quenching the PE by addition of CO2+SF6 (90%:10%) into the sample chamber, the bulk photoconductivity is nearly independent of photon energy in all polymers studied, in a good agreement with the IRAV spectra. The single threshold for photoconductivity is spectrally close to the onset of π-π* absorption, behavior that is inconsistent with a large exciton binding energy.

Energy-resolved photon flux dependence of the steady state photoconductivity in hydrogenated amorphous silicon: implications for the constant photocurrent method

We have studied the photon flux dependence of the steady state photoconductivity in intrinsic and lightly boron-doped hydrogenated amorphous silicon samples illuminated with monochromatic light of sub-bandgap energy. We also report measurements of the sub-bandgap absorption coefficient carried out by using the dc constant photocurrent method (CPM) under different constant values of the photocurrent. We show that the exponent of the power law relation between the photocurrent and the photon flux (which we name λ) depends on the photon energy. This result contradicts one basic hypothesis of the CPM. As a consequence of this fact, the absorption coefficient measured with the CPM is dependent on the constant photocurrent chosen to perform the measurement. Computer simulations based on a complete model for the absorption process reproduce both of these experimental results. We conclude that this model can be used to handle CPM data in order to obtain the actual sub-gap absorption spectra.

Photoconductivity of ZnSe crystals under high excitationrates

The generation-recombination processes in high-resistivity, undopedZnSe single crystals as well as in ZnSe:Mg and ZnSe:Cu crystals werestudied on the basis of an investigation of the photoconductivity kinetics onexcitation with nanosecond laser pulses of an under-bandgap photon energy.The parameters of the recombination centres are estimated. It was establishedthat the laser-stimulated increase in the steady-state photoconductivity ofundoped ZnSe crystals with a low accidental impurity concentration was mainlydue to formation of the slow recombination centres. The effect of accumulationof laser-induced point defects and its role in the observed photosensitizationof highly pure ZnSe single crystals are discussed. The present results can beused in the design and analysis of various optoelectronic and optical devicesusing this compound.

Electronic transport in hydrogenated microcrystalline silicon: similarities with amorphous silicon

Undoped hydrogenated microcrystalline silicon (μc-Si:H) layers were grown by the very high frequency glow discharge (VHF-GD) technique under various deposition conditions. The electronic transport properties under illumination were investigated by means of steady-state photoconductivity and steady-state photocarrier grating methods. Similarly to hydrogenated amorphous silicon (a-Si:H), power law dependencies as a function of the generation rate are observed for the photoconductivity, for the ambipolar diffusion length, and for the parameter b (indicating the Fermi level). For μc-Si:H, as for a-Si:H, nearly constant product of (mobility × recombination time) of majority and minority carriers is observed as a function of the parameter b. Based on these similarities, we assume that the electronic transport model developed for a-Si:H remains valid for μc-Si:H.

Charge carrier generation in a dye sensitized liquid crystal

The photocarrier generation and recombination mechanism in double layer cells of dye sensitized liquid crystalline hexahexylthiotriphenylene (HHTT) have been studied by steady-state photoconduction measurements. From the dependence of the photocurrent on light intensity we could determine the charge carrier generation mechanism in an azo pigment 4,4’-[(2,5-diphenyl1,3,4-oxadiazol)-bis(azo)]-bis[3-hydroxy-(2H-benzimidazo[2,1-a]benz[d,e]isoquinolin-7-one)] as surface enhanced whereas in titanylphthalocyanine charge generation takes place in the bulk of the dye. We investigated the actionspectra of a single HHTT cell and an azo sensitized cell in the different phases of HHTT to show the influence of azo on the spectral response of the photocurrent. The quantum efficiency in a single HHTT cell as well as in a double layer cell with azo as charge generation layer was determined. By solving the diffusion equation for excitons we carried out an estimation of the exciton diffusion length in azo.

Photoconductivity of Ge1-xSix single crystals in the range 115–300 K

The steady-state photoconductivity ofn- andp-type Ge1-xSix single crystals was measured in the range 115–300 K. The results were interpreted using a model considering two types of centers with markedly different capture cross sections.

Protonic Conductivity and Photoconductivity Studies on H3PW12O40x21H2O Single Crystals

Protonic conductivity measurements are reported for H3PW12O40x21H2O single crystals in the temperature range 77 to 303 K. At room temperature, the conductivity is 0.18 Sm-1 and falls to a minimum of 0.26×10-3 Sm-1 at 188 K. An anomalous behavior in conductivity observed in the temperature range 263 to 283 K is reported and it is essentially due to the disordered structure of water molecules. The activation energy determined from the least squares analysis in the temperature range 278 to 303 K and 188 to 273 K are 0.38 and 0.15 eV respectively. The observed conductivity parameter results support the vehicle mechanism as the proton conduction mechanism in this single crystal. Using the Nernst-Einstein relation, the proton diffusion coefficient is calculated and found to be 1.29×10-11 m2s-1 at room temperature. Steady state photoconductivity is measured at room temperature for various intensities and the material is found to be photosensitive. The variation of photocurrent with different illumination levels is found to be linear in these single crystals. The transient photoconductivity measurement shows that the photo-induced responses are moderate in the beginning and slow during decay process with respect to time.

Microcrystalline silicon deposited by the hot-wire CVD technique

Hot-wire chemical vapour deposition (HW-CVD) is a well-known technique to deposit amorphous silicon with high deposition rates from the decomposition of silane and hydrogen gases. By changing the hydrogen and silane flow rates, it is possible to observe a transition from amorphous silicon (a-Si:H) to microcrystalline silicon (μc-Si:H). In this study, structural and electrical properties of layers deposited as a function of silane concentration in the gas are presented. Ellipsometry and X-ray diffractometry have been used to assess the structure of the films. Steady-state photoconductivity, steady-state photocarrier grating, and modulated photocurrent experiments have been carried out to characterise both majority and minority carrier transport properties. Finally, the addition of phosphine and diborane in the reactor allows the deposition of n- and p-type layers with conductivities up to 10 and 1 s cm −1, respectively, making possible the realisation of HW-CVD solar cells.

Photogeneration and transport of charge carriers in hybrid materials of conjugated polymers and dye-sensitized TiO2

Steady state photoconductivity and current–voltage (I–V) experiments are performed on solid films of organic/inorganic composites of dye-sensitized TiO2 in combination with poly(N-vinylcarbazole) (PVK), a ladder-type PPP and a soluble PPV derivative. The I–V characteristics of the composites in the dark are explained by the formation of percolation networks of nanoparticles between the electrodes. Photoaction spectra of the devices prove that the photogeneration of charge carriers is significantly enhanced and spectrally broadened only if electron transfer from the polymer to the dye is possible. Increasing the concentration of the nanoparticles in the hybrid materials changes the spectral shape of the photoresponse. For high TiO2 contents signatures due to the absorption of the Ruthenium dye can be observed. The different electronic properties of anatase/brookite TiO2 and rutile TiO2 have only minor effects on the generation of charge carriers and on the shape of the photoaction spectra. Indeed, photocurrents are more related to the Brunauer—Emmett–Teller surface area of the nanoparticles and thus, to the amount of dye adsorbed. Charge collection efficiencies exceeding 100%, observed for both bias directions, can be explained by photoconductivity gain. It is proposed that recombination is reduced by transport of the oppositely charged carriers in two different phases.

Steady-state and time-resolved photoconductivity measurements of minority carrier lifetime in ZnTe

Minority carrier lifetime in ZnTe has been determined from steady-state and time-resolved photoconductivity (PC) measurements. Three types of single crystal p-ZnTe (EG=2.26 eV) grown by the Bridgman technique were studied: (i) as-grown, before and after hydrogen passivation, (ii) Zn annealed, and (iii) In-doped semi-insulating. Steady-state photoconductivity was studied between 80 and 300 K and showed that for as-grown samples, the lifetime went through a sharp maximum of 4.5×10−7 s at 220 K, decreasing to 2.5×10−8 s at 300 K. For hydrogen passivated samples, the low temperature behavior was similar but the lifetime remained high at 4.5×10−7 s at 300 K, due to passivation of deep acceptors OTe. Time-resolved photoconductivity measurements gave a value of 4.6×10−8 s for as-grown ZnTe in reasonably good agreement and 3.2×10−7 s for Zn annealed and 3.1×10−7 s for SI-ZnTe. The radiative recombination constant B was thus found to be 1.4×10−9 and 4×10−10 cm3 s−1, respectively, at 300 K for as-grown and Zn-annealed samples. PC spectral response studies showed a maximum at 2.41 eV at 300 K corresponding to the main valence–conduction band transition as well as a feature near 3.2 eV corresponding to the splitoff band. Slight shift in peak energy to 2.43 eV occurred on H surface passivation due to reduction of surface recombination velocity.

Steady-state photoconductivity of a supramolecular assembly of nonlinear optical molecules showing evidence of rectifier properties

The steady-state photoconductivity σ ‖ along and σ ⊥ perpendicular to aligned chains of nonlinear optical and other molecules included into parallel channels of perhydrotriphenylene (PHTP) inclusion compounds has been investigated. An anisotropy of σ ‖/ σ ⊥ ∼10 2 supports an 1D-transport of charge carriers along (i) polar and (ii) non-polar chains of guest molecules. For (i) 4-iodo-4′-nitrobiphenyl (INBP) σ ‖ is ∼4 (1)×10 −14 S/cm, whereas a symmetrical guest, (ii) 4,4′-diiodobiphenyl (IIBP), shows a σ ‖ of ∼2 (1)×10 −13 S/cm. A significant asymmetry in the I(U)-curve for PHTP–INBP crystals is in agreement with the preferred direction of, e.g., electron transport along polar chains of INBP. This behaviour is typical for a molecular rectifier system, although the measured effect was small, presumably because of a relatively low electrical field strength applicable ( E≤10 6 V/m) to macroscopically large crystals.

Space charge effects in carrier escape from single quantum well structures

Recently published data on the variation with applied bias and temperature of steady-state photoluminescence and photoconductivity from a series of GaAs/AlGaAs single quantum well p-i-n structures are subjected to detailed theoretical analysis, using phenomenological variables introduced in connection with these results. The data are interpreted as revealing the presence in the well of a space charge, which causes band bending and hence indirectly modifies carrier escape lifetimes. It is shown that the thermionic escape of holes can affect the electron tunneling escape lifetime so that the latter displays a thermal activation energy which is quantitatively similar to the hole well depth.

DC photoconduction action spectra in thin film polymer crystals of 3 and 4BCMU polydiacetylene

Thin polydiacetylene single crystals with alkyl-urethane sidegroups, 3 and 4BCMU PDA, of macroscopic dimensions, have been prepared by surface electron beam polymerisation of bulk monomer crystals. Steady state photoconduction action spectra have been obtained for both the 3 and 4BCMU thin film polydiacetylene crystals. Analysis of this data has allowed determination of the band gap energy, E G for the two crystals. The analysis suggests an indirect, forbidden, band to band transition that is buried under a strong excitonic transition. It is found that the indirect gap, E G, is 2.23 eV±0.01 eV for 3BCMU and 2.21 eV±0.01 eV for 4BCMU. A bandwidth of approximately 1 eV may be inferred.

Photosensitization of InSb crystals by pulsed laser irradiation

The photosensitization of n-InSb single crystals after irradiation with nanosecond ruby laser pulses was studied. Based on a study of the photoconductivity spectra it is established that the surface recombination rate decreases in the samples subjected to subthreshold irradiation. The steady-state photoconductivity, non-equilibrium carrier lifetime and resistivity increases. Changes in the photo-electric properties of InSb crystals are attributed to the cleaning of the surface and to the gettering of electrically active point defects by extended growth defects. The role of laser-induced stress and shock waves in these processes is discussed. The method for changing the surface state and modifying the photo-electric parameters of InSb crystals and for improving the stability of their properties by means of pulsed laser irradiation is advanced.

Deep levels in GaN epilayers grown on sapphire substrates

Steady state and transient extrinsic photoconductivity (PC) measurements on unintentionally doped GaN epilayers grown on sapphire substrates by using metalorganic chemical vapor deposition were performed to investigate deep electron levels. The photoionization cross section and the concentration of the deep levels were determined from the dependence of the rise and the decay times of the extrinsic PC response on the illumination intensity. The concentrations on the deep levels, located at 2.9 and 1.42 eV below the conduction band minimum of the GaN were found to be 2×10 13 and 1.6×10 11 cm −3, respectively. The typical relaxation time of the excess carriers controlled by the deep levels was approximately 10 −3 s. The thermal activation energies of the deep levels were determined from the temperature dependence of the relaxation time of the extrinsic PC response. These results can help improve understanding for potential applications in optoelectronic devices based on GaN epilayers.

Midgap density of states in hydrogenated polymorphous silicon

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to that of the formation of powder, a new type of material, named polymorphous silicon (pm-Si:H) is obtained. pmSi:H exhibits enhanced transport properties as compared to state-of-the-art hydrogenated amorphous silicon (a-Si:H). The study of space-charge-limited current in n+-i-n+ structures along with the use of the modulated photocurrent technique, of the constant photocurrent method and of steady-state photoconductivity and dark conductivity measurements allows us to shed some light on the origin of these improved properties. It is shown that the midgap density of states in the samples studied here is at least ten times lower than in a-Si:H, and the electron capture cross section of deep gap states is also expected to be lower by a factor of 3–4 to account for photoconductivity results. An interesting field of theoretical research is now open in order to link these low densities of states and capture cross sections to the peculiar structure of this new material.

Polarization dependence of the photoconductivity of stretch-oriented poly(p-phenylenevinylene) films

The polarization dependence of the photoconductivity (PC) is studied in stretch-oriented poly(p-phenylenevinylene) films. Differences in the recombination dynamics and spectral responses of the photocurrent are observed for different polarization directions. The steady-state PC spectra show well-resolved vibronic structures with a PC onset at about 2.2 eV in the oriented PPV. The transient subnanosecond PC shows larger photocurrent and lifetime for carriers generated by light polarized perpendicular to the orientation axis.