Spectral Response Of Photoconductivity Research Articles

GaAs/AlGaAs Nanowire Photodetector

We demonstrate an efficient core-shell GaAs/AlGaAs nanowire photodetector operating at room temperature. The design of this nanoscale detector is based on a type-I heterostructure combined with a metal-semiconductor-metal (MSM) radial architecture, in which built-in electric fields at the semiconductor heterointerface and at the metal/semiconductor Schottky contact promote photogenerated charge separation, enhancing photosensitivity. The spectral photoconductive response shows that the nanowire supports resonant optical modes in the near-infrared region, which lead to large photocurrent density in agreement with the predictions of electromagnetic and transport computational models. The single nanowire photodetector shows a remarkable peak photoresponsivity of 0.57 A/W, comparable to large-area planar GaAs photodetectors on the market, and a high detectivity of 7.2 × 10(10) cm·Hz(1/2)/W at λ = 855 nm. This is promising for the design of a new generation of highly sensitive single nanowire photodetectors by controlling the optical mode confinement, bandgap, density of states, and electrode engineering.

Physical properties of ZnSe thin films deposited on glass and silicon substrates

ZnSe thin films were deposited onto Corning glass and silicon substrates using thermal evaporation. The samples were prepared at different substrate temperatures. The thin films’ surface chemical composition was determined through Auger electron spectroscopy (AES). AES signals corresponding to Zn and Se were only detected in AES spectra. The samples’ crystallographic structure was studied through X-ray diffraction. The material crystallised in the cubic structure with preferential orientation (111). Optical properties of the ZnSe films were studied over two energy ranges via electron energy loss spectroscopy (10–90eV) and spectral transmittance measurements (0.4–4eV). In both cases, the spectral variation of the refractive index and the absorption coefficient were determined by fitting the experimental results with well-established theoretical models. Experimental values for the material’s gap were also found, and photoconductivity (PC) measurements were carried out. Transitions between bands, usually labelled ΓV8 → ΓC6 and ΓV7 → ΓC6, were found in the optical and PC responses. A wide spectral photoconductive response between 300 and 850nm was found in the ZnSe/Si samples prepared at 250°C substrate temperature.

Photoconductivity measurements of the electronic structure of organic solar cells

Experimental and theoretical studies of the electronic structure of bulk heterojunction (BHJ) organic solar cells are reported. The photoconductivity spectral response of the solar cells has a weak absorption band extending from the band-gap energy down to 1 eV due to charge-transfer optical excitation at the interface between the polymer and the fullerene. The low-energy absorption indicates an exponential band tail of localized states and an absorption model based on the one-electron joint density of electronic states accounts for the data. Transient photoconductivity measurements of the carrier mobility exhibit a temperature-dependent carrier dispersion. Data analysis for the particular case of transport in the BHJ structure is developed. A multiple trapping model of the dispersive transport is consistent with localized band tail states having a comparable density-of-states distribution to those observed by optical absorption. Theoretical calculations of the density of states including disorder in the \ensuremath{\pi}-\ensuremath{\pi} spacing of the polymer chains also shows exponential band tailing. A density-of-states model is developed from the data and is discussed.

Analysis of the shape of spectral dependence of absorption coefficient and stationary photoconductivity spectral response in nanocrystalline bismuth(III) sulfide thin films

Some aspects regarding the optical and photoelectrical properties of bismuth(III) sulfide nanocrystals deposited in thin film form were studied. The influence of electrostatic interaction between photogenerated charge carriers on the shape of the spectral dependence of absorption coefficient for as-deposited and thermally treated nanocrystalline Bi 2S 3 thin films was investigated. The experimentally obtained spectral dependencies of the optical absorption coefficient were analyzed using the models of Elliott and Toyozawa. It was shown that the sub-band gap optical absorption of the studied films is of exponential type. A modified Urbach rule function was employed to calculate the relevant parameters which determine the value of critical energy at which a transition between the two main absorption regimes occurs in studied semiconductor. Experimentally obtained changes of Urbach energies ( i.e. tailing parameters) upon thermal treatment of the films were explained in terms of the differences in the degree of structural disorder of the as-deposited and annealed films according to the model of Cody. The experimental optical absorption data were also used to model the photoconductivity spectral response of thermally annealed films using various approaches.

Photophysics, photoelectrical properties and photoconductivity relaxation dynamics of quantum-sized bismuth(III) sulfide thin films

Electrical and photoelectrical properties (including both the stationary photoresponse and the photocarriers' relaxation dynamics) of nanocrystalline semiconducting bismuth(III) sulfide thin films were investigated. The experimental design of photoelectrical properties was achieved by controlling the chemistry of the deposition process (varying the reagent concentration in the reaction system) and also by physical means (controlling the crystal dimensions by post-deposition annealing). The band gap energy of thin films characterized by most pronounced photoelectrical properties was calculated, on the basis of measured photoconductivity spectral response curves, by several approaches. All of the obtained values are in very good agreement with the corresponding ones obtained from optical spectroscopy data within the framework of parabolic approximation for dispersion relation. On the basis of measured temperature dependence of dark electrical resistivity of nanocrystalline bismuth(III) sulfide films, the thermal band gap energy and the ionization energy of the impurity level (of donor type) were calculated. The corresponding values are 1.50 and 0.42 eV. Dynamics of non-equilibrium charge carriers' relaxation processes was studied with the oscilloscopic method. By analysis of the photoconductivity decay kinetics data it is found that recombination of non-equilibrium charge carriers is carried out according to the linear mechanism. The calculated relaxation time of photoexcited charge carriers is 1.58 ms, the relaxation processes occurring via local trapping centers. Recombination processes occurring via a single-type trapping center can be described within the framework of the Schockley–Read model. The practically linear regime detected in the measured lux–ampere characteristics of the studied films ( Δ σ ∼ Φ 0.98 ) indicate as well a linear recombination mechanism of the photoexcited charge carriers.

Optical alignment of Cu2+ axial centers in KTaO3: Spectral dependence of the effect

The CuTa2+-VO axial centers in crystalline KTaO3 were found to undergo alignment under the action of polarized light. The sign of the effect is shown to change depending on the wavelength of the aligning light. A parallel study of the spectral response of photoconductivity of the same samples led to the conclusion that the alignment of the copper centers is driven not by reorientation but rather by an anisotropic recharging of the centers, which involves both the conduction and valence bands of the crystal. This interpretation was supported by a study of the kinetics of thermal destruction of the copper center alignment.

Photoconductivity, low-temperature conductivity, and magnetoresistance studies on the layered semiconductor GaTe

Single crystals of $p\ensuremath{-}\mathrm{GaTe}$ were grown by the Bridgman technique and characterized through x-ray diffraction, energy-dispersive x-ray analysis, x-ray photoemission spectroscopy, and transmission electron microscopy studies. The photoconductivity spectral response for in-plane conduction showed a peak at 747 nm (1.66 eV). Photoconductivity gain was determined in two orthogonal directions from which the majority carrier (hole) lifetimes were found to be $3.43\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ and $2.03\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\mathrm{s},$ respectively, parallel and perpendicular to the layer planes. Studies of the temperature dependence of conductivity in the directions along and perpendicular to the layer planes were carried out between 10 and 80 K. Along the layer planes the conductance ${G}_{\ensuremath{\parallel}}$ varied as $\mathrm{ln}T$ between 12 and 20 K, characteristic of weak localization, while between 20 and 50 K the conductivity ${\ensuremath{\sigma}}_{\ensuremath{\parallel}}$ varied as ${T}^{1/2}.$ In the perpendicular direction the conductance ${G}_{\ensuremath{\perp}}$ varied as $\mathrm{exp}{(T/T}_{0}{)}^{1/3}$ between 9 and 20 K and the conductivity ${\ensuremath{\sigma}}_{\ensuremath{\perp}}$ varied as $\mathrm{exp}{(T/T}_{0}{)}^{1/4}$ between 20 and 50 K, characteristic of hopping conduction in two and three dimensions, respectively. Negative transverse magnetoresistance was observed at 10 K for conduction in both directions for magnetic fields $Hl0.4\mathrm{T},$ the increase in conductance being found to be proportional to ${H}^{2}.$ Band conduction with positive magnetoresistance was observed for both current directions at $Tg70\mathrm{K}.$ The $I\ensuremath{-}V$ characteristics at 10 K showed quantized behavior due to electron tunneling across potential barriers caused by stacking faults between layer planes.

Characterization of CdS films prepared by chemical-bath deposition

Thin films of cadmium sulphide were deposited on glass substrates by the chemicalbath deposition method. The results of measurements on the spectral responseof photoconductivity, the optical absorption coefficient and the x-raydiffraction were used for the characterization of films. This study revealedthat CdS films have tensile strain along the hexagonal (002) planes. Thedependence of strain on the grain size is responsible for the variation ofenergy bandgap with the film thickness. Annealing in air, and at a temperaturegreater than 150 °C, relaxes the strain, reduces the extent of disorder ingrain boundaries, reduces the energy bandgap and improves themobility-lifetime product for photogenerated carriers.

Structure and photoconductive properties of dip-deposited SnS and SnS 2 thin films and their conversion to tin dioxide by annealing in air

SnS and SnS 2 thin films have been prepared by the dip technique. In this technique, a substrate was dipped into an alcoholic solution of the corresponding chloride and thiourea and then withdrawn vertically at a controlled speed, and finally baked in a high temperature furnace at atmospheric condition. XRD and SEM data suggest that good quality SnS and SnS 2 films are obtained at a baking temperature of 300 and 360°C, respectively. Values of band gap for SnS and SnS 2 obtained from spectral response of photoconductivity are 1.4 and 2.4 eV, respectively. The indirect allowed band gap values for SnS 2 film obtained from optical absorption measurements are 1.95 and 2.05 eV. Open-air annealing of both SnS and SnS 2 films at 400°C converts them to transparent conducting SnO 2.

Deposition and characterization of Zn xCd 1− xS thin films prepared by the dip technique

Zn x Cd 1− x S (0≤ x≤1) thin films have been deposited by the dip technique on glass substrates. In this method, a clean substrate was dipped into an alcoholic solution of the corresponding nitrates and thiourea and then withdrawn vertically at a controlled speed, and finally baked in a furnace. X-ray diffractometric study suggests that for zinc atomic fraction x≤0.6 films prepared at a baking temperature of 500°C are homogeneous with a hexagonal (wurtzite) structure. Increase in the proportion of zinc in the starting solution is found to produce a decrease in the lattice parameter. SEM studies reveal an increase in grain size with x up to a value of 0.6. For x>0.6, the films appear to have an amorphous character, as no distinguishable peaks can be seen in the X-ray diffractograms. The SEM micrographs also do not show any clearly defined grains over this range. Values of bandgap obtained from optical absorption measurements as well as from spectral response of photoconductivity are in good agreement with each other and vary monotonically from 2.30 eV (CdS) to 2.69 eV (Zn 0.6Cd 0.4S) over the range 0≤ x≤0.6.

Photoconductivity of amorphous GaAs

Abstract The spectral response of photoconductivity has been measured in samples of amorphous gallium arsenide deposited by rf sputtering of monocrystalline GaAs targets with and without hydrogen. The maximum of the normalized spectral photoconductivity (hν ≅ 2.0 eV) as a function of hydrogen pressure, pH2, increases when pH2 ≤ 0.1 Pa and then tends to a constant value. The values of the Tauc optical gap, Eg, and those of the photoconductive threshold, Et,ph are similar. Their difference for the same sample tends to decrease with increasing pH2. The same trend is shown by the difference between the Urbach energy computed either by photothermal deflection spectroscopy, E0, or by photoconductivity absorption coefficient data, E0,ph. These results and the dependence of other quantities, such as the activation energy of the conductivity, Eσ, on pH2, are interpreted in terms of the possible effects that hydrogenation can induce in a compound semiconductor. They are: a less disordered network reducing the band tail depth; decrease of the bond-angle fluctuation and ineffectiveness of hydrogen on the density of wrong bonds. The first two contributions are strictly linked to the deposition conditions of the films which determine, for example, the deposition rate of the film.

Photoconduction of synthetic pyrite FeS2 single crystals

The photoconduction of synthetic pyrite FeS2 single crystals with low impurity concentration prepared by chemical vapor transport method is reported over a temperature range from 15 K to 300 K. The energy gap is determined from the photoconductive spectral response by the Moss rule. At room temperature the indirect energy gap is found to be 0.83±0.02 eV. According to the experimental evidence for a sharp antibonding p-like state above the Fermi level of FeS2 reported by Folkerts et al. [J. Phys. C 20, 4135 (1987)], the indirect gap is assigned as the transition between Fe 3d t2g and S 3p σ* states. The temperature variation of the indirect band gap shows a linear dependence between 100 K and 300 K with a negative temperature coefficience equal to 1.0±0.1 meV/K. A distinct feature at higher energy side of the spectrum is observed and attributed to the direct band-gap transition. At temperature lower than 60 K, a very sharp peak around 0.9 eV emerged below the absorption edge. This feature might be associated with the excess carriers induced by the photothermal ionization between the impurity levels and conduction band.

Dark- and photoconductivity in doped and undoped zinc telluride films

Thin films of ZnTe are prepared under different conditions by a vacuum evaporation technique. Dark- and photoconductivity and measured at different temperatures. The effect of doping on conductivity is studied for BaF2 (7 wt.%), PbCl2 (6 wt.%) and In (6 wt.%) dopants. The spectral response of photoconductivity are intensity variation of photocurrent in ZnTe films of different thickness is studied. The mechanism of conduction in those films is properly explained by considering different models. The variation of drift barrier height with film thickness, and hence the particle size, is studied. An exponential distribution of traps or impurity states within the bands is suggested for the films.

Persistent photoconductivity in poly(p-phenylenevinylene): Spectral response and slow relaxation.

We report the spectral response and slow decay of the steady-state photoconductivity in poly(p-phenylenevinylene) (PPV) films. The spectral response of the photoconductivity is in good agreement with that calculated from the absorption data with the assumption of rapid recombination at the surface of the film; the results indicate direct photogeneration of free charge carriers via an interband transition. The photoconductivity is, therefore, consistent with a description of the electronic structure of PPV in terms of a semiconductor band model (rather than an exciton model). The very slow stretched-exponential relaxation of the photoinduced conductivity is reminiscent of the persistent photoconductivity observed in inorganic semiconductors. By assuming that the photocurrent is carried predominantly by mobile polarons near the surface, one can construct a model for the persistent photoconductivity in which the recombination of long-lived bipolarons is inhibited in the bulk where bipolarons have a lower free energy than polarons. The persistent photoconductivity, therefore, is caused by the slow dispersive diffusion of photogenerated bipolarons to the surface where they dissociate into polarons and where both polaron transport and recombination occur.

Photoconductivity of Ion Pair Charge-Transfer (IPCT)

Abstract The results of photoelectric measurements in salts of planar complexes of mono- and dianionic metal dithiolenes with a variety of cations, such as 2, 2′ ‐ and 4,4′-bipyridinium derivatives, are described. It is shown that by specific combinations of ligands, metals, anionic charges and cations photoconductivity gain and spectral response to specific wavelength regions can be tailored. On the basis of ion pair charge-transfer (IPCT) mechanism a model is given according to which photoconductivity of metal dithiolene complexes may be attributed to the photoinduced “reverse” charge‐transfer between anionic and cationic sites resulting in conducting anionic stacks of mixed‐valence.

Structural and energy gap variations in (ZnTe) x(CdSe) 1− x thin films

Thin films (~200 nm thick) of (ZnTe) x (CdSe) 1− x in the entire range of composition (0 ⩽ x ⩽ 1) were prepared by thermal vacuum evaporation maintaining the glass substrates at 303 and 553 K. XRD studies showed that the films were polycrystalline. Wurtzite structure was found in compositions up to x = 0.5. Beyond this composition the zincblende structure was observed. A structural discontinuity was found in a very narrow range of x. Energy gaps estimated from photoconductivity spectral response and optical transmittance spectra were nearly the same and varied non-linearly with composition. The bowing parameters for films prepared at 303 and 553 K were found to be 1.03 and 1.31 eV, respectively. The structural discontinuity and bowing parameter are discussed in detail. Crystallinity (grain size), lattice parameters and bandgap were found to increase with T s .

Photoconductivity in tin sulphide single crystals

The photoconductivity spectral response was measured for SnS single crystals at different temperatures with polarized light with the plane of polarization parallel to the a- and b-crystallographic axes which lie in the plane of cleavage. The photoconductivity energy gap was determined from the spectral response by the Moss rule. The room temperature photoconductivity energy gap was found to be (1.61 ± 0.031) eV and (1.139 ± 0.015) eV for the a- and b-axes, respectively. The temperature dependence of the photoconductivity energy gap was found to be linear between 91 and 300 K with a negative temperature coefficient equal to -(0.37 ± 0.01) meV/K for the b-axis. The results for the a-axis showed large discrepancy which was attributed to crystal lattice bending which was introduced during the cleavage process. The minority carriers lifetime was determined from the photoconductivity frequency response which was found to be characterized by two different lifetimes.

Photoconductivity in single-crystal germanium sulphide

The photoconductivity spectral response was measured for germanium sulphide (GeS) single crystals at different temperatures from room temperature to near liquid-nitrogen temperature. Measurements were carried out with plane-polarised light with the plane of polarisation parallel to the a and b crystallographic axes, which lie in the plane of cleavage. The photon energy range of measurements extended from 1.0 to 3.5 eV. The photoconductivity energy gap was determined from the spectral response by the Moss rule. The room-temperature photoconductivity energy gap was found to be 1.613+or-0.010 and 1.642+or-0.028 eV for the a and b axes, respectively. The temperature dependence of the photoconductivity energy gap was also deduced. It was found that it is linear between 92 and 300 K with a negative temperature coefficient equal to -0.43+or-0.01 meV K-1 for the a axis. The results for the b axis showed a large discrepancy, which was attributed to crystal lattice bending introduced during the cleavage process. The minority-carrier lifetime was determined from the photoconductivity frequency response, which was found to be characterised by two different lifetimes.

Degradation of mercuric iodide crystals on aging

Mercuric iodide single crystals were grown and characterized by studying their spectral response of photoconductivity, thermally stimulated current, dielectric constant and di-electric loss as a function of frequency. The as-grown crystals have high resistivity, good photo-sensitivity, low dielectric constant and loss at low frequencies, etc. On aging, the crystals show degradation in their properties when they were stored in air, whereas the crystals stored in an iodine chamber do not exhibit any degradation. The degradation of the properties have been interpreted in terms of surface evaporation of HgI2 and consequently formation of a surface layer rich in mercuric atoms. The effect of a dc bias on aged (degraded) samples shows restoration of the properties. The results could be interpreted in terms of polarization of mobile defects and formation of space charge layers which enhance a high internal field in the crystals, hence the restoration of the properties.

Electrical and photoconducting properties of vacuum evaporated pure and silver-doped CdSe films

Thin films of pure and silver-doped CdSe deposited by vacuum evaporation on Corning glass substrates in the temperature range 303–648 K were polycrystalline in nature with zincblende structure. The electrical conductivity of the films was in the range 1.3–12Ω −1 cm −1 and 4–6Ω −1 cm −1 respectively for the pure and silver-doped films. After the films were baked in air, the dark conductivity decreased because oxygen is incorporated as an acceptor impurity in the films. The asdeposited films were not photosensitive. The photosensitivity increased with increase in the baking temperature. The highest photosensitivities were 2.4 × 10 2 and 3.6 × 10 3 respectively for the pure and silver-doped films baked in air at 648 K. The photoconductive spectral response peak was shifted towards longer wavelengths for the silver-doped films. The trap depths evaluated from thermally stimulated currents are in agreement with the results obtained from the electrical conductivity.