Steady-state Photoconductivity Research Articles

Photo-carrier transport in disordered organic TPD films

Steady state and transient photo-carrier transports in disordered tri-methylphenyl diamine (TPD) films have been studied at relatively high temperatures (200–300K). Hopping transport of holes occurs through localized centers with a Gaussian energy distribution. The steady state photo-conductivity is proportional to Gγexp[−(σ/kT)2/2] (with γ≈0.5), where G is the number of absorbed photons and σ (≈0.07eV) the standard deviation of the Gaussian distribution. The residual decay of photo-conductivity is nearly proportional to t−β (β≈0.25). We have formulated the hopping photo-conductivity and reasonable physical parameters, such as hopping relaxation time, recombination time, and diffusion coefficient (and hence mobility) of charge carriers, are estimated for the TPD films.

Photoconductivity in Thin Film of a-(Ge20Se80)0.90Sn0.10

Steady state and transient photoconductivity measurements have been done on thin film of a-(Ge20Se80)0.90Sn0.10 as a function of temperature and intensity. Dark conductivity (σd) and photoconductivity (\(\sigma_{\rm ph}\)) measurements show that the conduction in this glass is through an activated process having single activation energy in the temperature range 283–350 K. The intensity dependence of steady state photoconductivity (\(\sigma_{\rm ph}\)) follows a power law with intensity (F), \(\sigma_{\rm ph} \propto F^{\gamma}\), where the power γ has been found between 0.5 and 1.0, suggesting bimolecular recombination. Rise and decay of photocurrent at different temperatures and intensities show that photocurrent (Iph) rises monotonically to the steady state value and the decay of photocurrent is quite slow. A detailed analysis of photoconductive decay shows that the recombination within localized states may be predominant recombination mechanism in this glassy alloy.

Nonequilibrium Green's function theory of resonant steady state photoconduction in a double quantum well FET subject to THz radiation at plasmon frequency

Recent experimental observations by X.G. Peralta and S.J. Allen, et al. of dc photoconductivity resonances in steady source-drain current subject to terahertz radiation in a grid-gated double-quantum well FET suggested an association with plasmon resonances. This association was definitively confirmed for some parameter ranges in our detailed electrodynamic absorbance calculations. In this paper we propose that the reason that the dc photoconductance resonances match the plasmon resonances in semiconductors is based on a nonlinear dynamic screening mechanism. In this, we employ a shielded potential approximation that is nonlinear in the terahertz field to determine the nonequilibrium Green's function and associated density perturbation that govern the nonequilibrium dielectric polarization of the medium. This ‘‘conditioning’’ of the system by the incident THz radiation results in resonant polarization response at the plasmon frequencies which, in turn, causes a sharp drop of the resistive shielded impurity scattering potentials and attendant increase of the dc source-drain current. This amounts to disabling the impurity scattering mechanism by plasmon resonant behavior in nonlinear screening.

Numerical simulation of the steady state photoconductivity in hydrogenated amorphoussilicon including localized state electron hopping

Numerical simulation of the steady state photoconductivity in hydrogenated amorphoussilicon over a wide temperature range (25–500 K) is extended, to include previouslyneglected carrier transitions between localized states. In addition to free carrier capture(emission) transitions into (from) localized states, we include the process of electronhopping in conduction band tail states. Exponential distributions are assumed for bothconduction and valence band tail states, while the dangling bond defect distribution iscalculated in accordance with the defect pool model. Localized to extended statetransitions follow the Simmons and Taylor statistics, and localized to localized statetransitions involve electron hopping between nearest neighbour sites. Comparisonwith simulations in the absence of electron hopping reveals a smooth transitionaround 110 K, between regions of (high temperature) extended state conductionand (low temperature) hopping conduction. A hopping transport energy level isidentified as the peak of the energy distribution of the hopping photocarriers, andshows a temperature dependence in agreement with existing theoretical work.

Determination of the density of states of semiconductors from steady-state photoconductivity measurements

We discuss a method to obtain the density of states of photoconductive semiconductors from the light-intensity-dependence of the steady-state photoconductivity. Considering a material having different species of gap states – i.e., with different capture coefficients – we deduce a simple expression relating the defect density to measurable quantities. We show that the relevant capture coefficient appearing into the formula is that of the states that control the recombination. We check the validity of the approximations and the applicability of the final expression from numerical calculations. We demonstrate the usefulness of the method by performing measurements on a standard hydrogenated amorphous silicon sample.

Photoconductivity of amorphous As–Se–Sb thin films

The present paper reports the effect of replacement of selenium by antimony on the steady state and the transient photoconductivity in vacuum evaporated amorphous thin films of As 30Se 70− x Sb x ( x = 2.5, 5, 7.5, 10, 12.5, 15 and 17.5 at.%). The composition dependence of the steady state photoconductivity at room temperature shows that the photoconductivity increases while the photosensitivity decreases with the increase in antimony content. The transient photoconductivity shows that the lifetime of the carrier decreases with increasing the light intensity. This decrease suggests that the photoconductivity mechanism in our samples was controlled by the transition trapping processes. Replacement of selenium by antimony results in a monotonic decrease in the band gap of As 30Se 70− x Sb x thin films. This behavior was interpreted on the basis of the chemical bond approach.

Recombination traffic in highly crystallized undoped microcrystalline Si films studied by steady state photoconductivity

The dependences of photoconductivity ( σ ph) of highly crystallized dense undoped hydrogenated microcrystalline silicon (μc-Si:H) thin films on the temperature and the intensity of light were measured in a temperature range 15–325 K. The variation in light intensity exponent ( γ) is found to be 0.5 ≤ γ ≤ 0.9 for thin samples, and 0.15 ≤ γ ≤ 0.8 in a thick fully crystallized sample. Based on our simulation and experimental results, we have been able to construct density of state map for both the samples of different microstructure. In particular, we found that the fully crystallized thick sample has a very steep conduction band tail compared to thin sample. The valence band tails (VBT) are classified into two parts: one with sharper tail near the edge originating from grain boundary defects and other one with less steep tail associated with the defects in columnar boundary regions. Capture cross section for the deeper tail is smaller than the shallower one. The shallower VBT states extend up to more depth in thin sample. Our simulation results for thick sample show the transformation of recombination traffic taking place from shallower states to deeper tail states in the temperature region where γ < 0.5. Therefore, these deeper tail in thick sample works as a “safe hole traps”.

Influence of gas atmosphere and temperature on the conductivity and the photoconductivity of a TiO2single crystal in the surface region

The electrical photoconductivity and conductivity at (and near) the surface of a TiO(2) single crystal (rutile) was studied in a range of temperatures between 300 and 573 K and under different ambient gases (oxygen and nitrogen) by means of impedance spectroscopy. The long times required (many hours) to reach steady state photoconductivity can be explained by the reduction of the material upon illumination. At about 475 K a maximum is observed in the equilibrium photoconductivity and a minimum in the rate constants of the rise and decay after switching on and off, respectively, the light. After switching off the light a fast decay takes place during the first milliseconds followed by a slow exponential decay. The first one is related to recombination through defects, while the latter is due to re-oxidation processes of the material. The results are correlated with measurements of photocatalytic activity.

Phase Transformations in Bulk (Ge2Se7)88Bi5Sb7

AbstractAnnealing at 150 °C induces phase separation in amorphous (Ge2Se7)88Bi5Sb7 bulk samples. Spectrally resolved steady-state photoconductivity measurements indicate the presence of crystalline Bi2Se3 clusters in the annealed material, but also the subsequent gradual disappearance of this microstructure at room temperature. Similar annealing-induced metastable changes are observed in other elements of a (Ge2Se7)88BixSb12-x sample series.

Effect of lead additive on photoconductive properties of Se–Te chalcogenide films

Steady state and transient characteristics of photoconductivity have been studied in amorphous thin films of Se85Te15−xPbx (x=0, 2, 4 and 6). The studies of temperature dependence of photoconductivity show that photoconduction is a thermally activated process. The value of activation energy of photoconduction is, however, smaller as compared to activation energy in dark. The results of intensity dependence of steady state photoconductivity indicate that bimolecular recombination is predominant in these materials. The Se85Te15−xPbx is found highly composition dependent as lead impurity drastically changes the conduction parameters. The incorporation of Pb impurity is found to affect the transient photoconductivity properties drastically. A spike is observed in the rise curve of photocurrent in undoped a-Se85Te15, which disappears in Pb doped samples. The lead incorporation is found to delay the onset time of recombination in the rise and decay of photocurrent which is attributed to the trapping of charge carriers in deep localized states produced by lead impurity. The photocurrent rise and decay is explained by the trap-controlled recombination model proposed by Iovu et al. and the dispersion parameter α of localized state energy distribution is determined from the experimental results.

Light-intensity dependence of the steady-state photoconductivity used to estimate the density of states in the gap of intrinsic semiconductors

We present a method to obtain the density of states of photoconductive semiconductors based on the light-intensity dependence of the steady-state photoconductivity. A simple expression—relating the density of states at the electron quasi-Fermi level to measurable quantities—is deduced by performing suitable approximations from the analytical solution of the generalized equations that describe the photoconductivity of semiconductors. The validity of the approximations and the applicability of the final expression are verified from numerical simulations of the process. The usefulness of the method is demonstrated by performing measurements on a standard hydrogenated amorphous silicon sample.

Photoconductivity excitation spectrum for stretch-oriented PPV

We have obtained the photoconductivity (PC) excitation spectrum for a stretch-oriented poly(paraphenylene vinylene) film over a wide spectral range (up to 5 eV). The measurements were performed in the surface cell configuration with the electric field parallel or perpendicular to the stretch direction. Although the sample had a stretch ratio of ∼4, the dark conductivity and the steady-state photoconductivity were both about 40 and 20 times higher with the electric field parallel to the average chain direction, respectively. However, the shape of the PC excitation spectrum was independent of field direction and did not show a significant rise in the ultraviolet, as is usually observed for measurements in the photodiode configuration. The implications of these results to the charge photogeneration mechanism in conjugated polymers are discussed.

Diffusion length measurements of microcrystalline silicon thin films prepared by hot-wire/catalytic chemical vapor deposition (HWCVD)

Hydrogenated microcrystalline silicon (μc-Si:H) films prepared by using the hot-wire/catalytic chemical vapor deposition (HWCVD) technique at low substrate temperatures between 185 °C and 220 °C with different silane concentrations (SC) were investigated using steady-state photocarrier grating (SSPG) and the steady-state photoconductivity methods (SSPC). Crystalline volume fractions ( I C RS) obtained from Raman spectroscopy change from 0.22 to 0.77. The diffusion length ( L D) is measured at generation rates between G = 10 19 and 10 21 cm − 3 s − 1. L D changes from 27 nm to 270 nm, with maximum values around SC = 5%. The dependence of L D on SC is similar to that observed for similar quality microcrystalline silicon films prepared using the VHF-PECVD technique. The grating quality factor, γ 0, drops from about 0.9 to 0.5 after transition to the microcrystalline regime as indication of scattering from surface patterns.

Effect of light intensity and temperature on the recombination mechanism in a-(Ge20Se80)99.5Cu0.5 thin film

The temperature dependence of electrical conductivity measurements has been studied in the dark as well as in the presence of light in vacuum evaporated thin films of (Ge20Se80)99.5Cu0.5. The dark conductivity increases exponentially with temperature. The temperature dependence of steady state photoconductivity at different intensities indicates that photoconductivity is also thermally activated. These measurements ensure the presence of mono- and bimolecular recombination regions. Transient photoconductivity measurements at different temperatures and light intensities show that the photocurrent passes through a maximum before attaining the steady state. The results have been explained in terms of the multiple-trapping model with photoconductivity kinetics determined by trap-controlled recombination.

Characterization of magnetic Czochralski silicon radiation detectors

Silicon wafers grown by the Magnetic Czochralski (MCZ) method have been processed in form of pad diodes at Instituto de Microelectrònica de Barcelona (IMB-CNM) facilities. The n-type MCZ wafers were manufactured by Okmetic OYJ and they have a nominal resistivity of 1 kΩ cm. Concentrations of oxygen and carbon in MCZ wafers were measured by Secondary Ion Mass Spectroscopy (SIMS) technique to be compared to standard and oxygenated float zone (FZ) silicon material. The diodes were characterized by leakage current and capacitance measurements. The average full depletion voltage, V FD, of the pad detectors is 290±10 V, while the leakage current density at V FD+20 V is 7±3 nA/cm 2. Minority carrier lifetime of MCZ, standard FZ and oxygenated FZ substrates was measured by using a quasi steady-state photoconductance technique. The uniformity of the MCZ wafers was studied before the fabrication by mapping the resistivity of the wafer and after the fabrication process by measuring the full depletion voltage and leakage current as a function of diode position on the wafer. It was found that the fabrication process did not change the resistivity of the material although no donor-killing heat treatment was performed on the wafers.

Cd1 - x ZnxTe crystals for gamma detectors: Evaluation of quality from photoconductivity measurements

Data are presented on the spectral dependence of steady-state photoconductivity for Cd1 - xZnxTe (x = 0.12–0.15) crystals grown under different conditions. The results indicate that the full width at half maximum of the near-edge photoconductivity peak is sensitive to the state of native structural defects and can be used to assess the quality of Cd1 - xZnxTe crystals and their applicability in high-resolution gamma detectors.

Deposition of highly photoconductive wide band gap a-SiO:H thin films at a high temperature without H-dilution

Abstract Electrical and optical characterisation of hydrogenated amorphous silicon–oxygen alloy thin films (a-SiO x :H, x 2 ) grown in a single chamber radio frequency plasma enhanced chemical vapour deposition (PECVD) system at a high substrate temperature of 300 °C is presented. The samples were investigated by Fourier transform infrared spectroscopy (FTIR), optical transmission, the constant photocurrent method (CPM), conductivity and steady-state photoconductivity measurements. With increasing oxygen concentration, the Tauc gap increases from 1.69 to 2.73 eV. The sample with an oxygen concentration of 26.2 at% and a reasonably high bandgap of 2.18 eV shows photoconductivity comparable to that of pure a-Si:H films. The Urbach parameter ( E 0 ) increases almost linearly with oxygen concentration whereas the dangling bond defect density is found to be saturating at a value of about 7.1×10 16 cm −3 . One of the highly alloyed samples with E 0 = 123 meV exhibited a detectable photosensitivity.

Synthesis and Photoresponse Properties of Tricyanovinylized Polysilane

To broaden its photoresponse window, the new polysilane 2 with a pendant (tricyanovinyl)aniline group has been prepared by a facile post-tricyanovinylation reaction of the N,N-dialkylaniline-substituted polysilane precursor 1. A steady-state photoconductivity study showed that polysilane 2 has an extended photoresponse region from the traditional ultraviolet to the visible region compared with polysilane 1.

Electronic density of states in amorphous selenium

Steady-state and transient photoconductivity methods are used to investigate the electronicdensity of states in evaporated layers of amorphous selenium. From the temperaturedependence of the steady-state photocurrent and, independently, from an analysis ofthe post-transit currents of time-of-flight transients, energy levels in the gap at1.43 ± 0.02 eVand 0.40 ± 0.02 eV above the valence band have been determined for the occupied state of thenegative-U centres. An absorption band around 1.50 eV is seen in the spectraldistribution of the photocurrent. The distribution of tail states may—to firstapproximation—be described by a steep exponential with a characteristic width of meV at the valence band and a more steeply declining functional of similar width at theconduction band.

On the role of magnetic field spin effect in photoconductivity of composite films of MEH-PPV and nanosized particles of PbS

The photoconductivity of thin films consisting of a conjugated polymer poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV) and dispersed nanosized particles of an inorganic semiconductor lead sulphide (PbS) was investigated using steady-state photoconductivity and magnetic field spin effect (MFSE) techniques. It was shown that the addition of small amount of PbS nanoparticles leads to the reduction of the photocurrent while higher concentration of PbS increases the photocurrent.