Showing papers on "Photoconductivity published in 1998"
TL;DR: In this paper, a photoelectrochemical cell was formed that consisted of p-type InP QDs loaded on TiO2, which was immersed in a I-/I3- or hydroquinone/quinone acetonitrile solution, and a Pt counter electrode.
Abstract: Quantum dots (QDs) of InP strongly adsorb onto transparent, porous, nanocrystalline TiO2 electrodes prepared by sintering 200−250 A diameter TiO2 colloidal particles. The interparticle space of the TiO2 electrodes is large enough to permit deep penetration of 65-A InP QDs into the porous TiO2 film. The absorption of light increases linearly with the thickness of the TiO2 film indicating that the InP QDs are adsorbed homogeneously on the TiO2 surface. We found that large particles adsorb better than smaller ones probably due to less hindrance by the stabilizer. The solid films exhibit strong photoconductivity in the visible region indicating photosensitization of TiO2 by InP QDs. The photocurrent action spectrum of the TiO2/InP QD film at a potential of +1 V is consistent with the absorption spectrum of the InP QDs. A photoelectrochemical cell was formed that consisted of p-type InP QDs loaded on TiO2, which was immersed in a I-/I3- or hydroquinone/quinone acetonitrile solution, and a Pt counter electrode....
556 citations
TL;DR: In this paper, the effect of the growth temperature on the structural, optical, electrical, and relaxation properties of zinc oxide films has been studied and it was found that there was a critical temperature Tc=180°C below which the thermal decomposition to ZnO did not occur or was incomplete.
Abstract: Undoped ZnO films were deposited by spray pyrolysis using aqueous zinc nitrate solution at different substrate temperatures. The effect of the growth temperature on the structural, optical, electrical, and relaxation properties has been studied. It was found that there was a critical temperature Tc=180 °C below which the thermal decomposition to ZnO did not occur or was incomplete. Films grown above Tc showed strong preferred orientation of polycrystals along the c-axis, while the films grown at Tc or below showed a powder-like, non-oriented polycrystalline structure when they were converted afterwards to zinc oxide by annealing. A slight increase of the optical band gap was observed for as-prepared films as the substrate temperature was decreased near the critical temperature. Annealing brought all the samples to the same band gap 3.30 eV measured at a half height of the maximum absorption. After illumination, the steady-state photoconductivity decayed very slowly with a time constant of about a week for...
275 citations
TL;DR: In this article, the infrared photodetectors were implemented, composed of ten layers of self-assembled InAs dots grown on GaAs substrate, and the electronic level structure was determined, based on polarization, bias, and temperature dependence of the transitions.
Abstract: Thermal generation rate in quantum dots (QD) can be significantly smaller than in quantum wells, rendering a much improved signal to noise ratio. QDs infrared photodetectors were implemented, composed of ten layers of self-assembled InAs dots grown on GaAs substrate. Low temperature spectral response shows two peaks at low bias, and three at a high one, polarized differently. The electronic level structure is determined, based on polarization, bias, and temperature dependence of the transitions. Although absorbance was not observed, a photoconductive signal was recorded. This may be attributed to a large photoconductive gain due to a relatively long lifetime, which indicates, in turn, a reduced generation rate.
256 citations
TL;DR: In this paper, the authors reported far-infrared photoconductivity in self-organized InAs/GaAs quantum dots grown by molecular beam epitaxy using a Fourier transform infrared spectrometer.
Abstract: We report far-infrared photoconductivity in self-organized InAs/GaAs quantum dots grown by molecular beam epitaxy. Through use of a Fourier transform infrared spectrometer, a photoconductivity signal peaked at 17 μm is observed from a n–i–n detector structure with doped InAs quantum dots in the intrinsic region. Comparison of photoluminescence and band-to-band photocurrent absorption spectra suggests the far-infrared response is due to intersubband transitions in the quantum dots.
243 citations
TL;DR: In this article, the authors measured the voltage at which the current under illumination in poly[2-methoxy, 5-(2-ethylhexoxy)-1,4-phenylene vinylene] based light emitting diodes is equal to the dark current.
Abstract: We measure the voltage at which the current under illumination in poly[2-methoxy, 5-(2-ethylhexoxy)-1,4-phenylene vinylene] based light emitting diodes is equal to the dark current. At low temperatures, this voltage, which we term the “compensation” voltage, is found to be equal to the built-in potential, as measured with electroabsorption on the same diode. Diffusion of thermally injected charges at room temperature, however, shifts the compensation voltage to lower values. A model explaining this behavior is developed and its implications for the operation of organic light emitting diodes and photovoltaic cells are briefly discussed.
206 citations
TL;DR: In this paper, a self-assembled InGaAs quantum dot infrared photodetector (QDIP) consisting of self assembled InGaA quantum dots has been demonstrated with a response of 325 mA/W at 92 μm.
Abstract: A quantum dot infrared photodetector (QDIP) consisting of self-assembled InGaAs quantum dots has been demonstrated Responsivity of 325 mA/W at 92 μm was obtained for nonpolarized incident light on the detector with a 45° angle facet at 60 K The QDIPs exhibit some unique electro-optic characteristics such as a strong negative differential photoconductance effect and blueshift of the response peak wavelength
164 citations
TL;DR: In this paper, experimental and theoretical evidence is presented for the metastability of oxygen donors in AlxGa12xN, showing that as aluminum content increases, Hall effect measurements reveal an increase in the electron activation energy, consistent with the emergence of a deep DX level from the conduction band.
Abstract: Experimental and theoretical evidence is presented for the metastability of oxygen donors in AlxGa12xN. As the aluminum content increases, Hall effect measurements reveal an increase in the electron activation energy, consistent with the emergence of a deep DX level from the conduction band. Persistent photoconductivity is observed in Al0.39Ga0.61N:O at temperatures below 150 K after exposure to light, with an optical threshold energy of 1.3 eV. A configuration coordinate diagram is obtained from first-principles calculations and yields values for the capture barrier, emission barrier, and optical threshold which are in good agreement with the experimental results. [S0031-9007(98)05950-X]
159 citations
TL;DR: In this paper, the optimum growth conditions were studied to obtain uniform InGaAs quantum dots constructed in an InGaP matrix, and normal incidence photoconductivity was observed at a peak wavelength of 5.5 μm with a high responsivity of 130 mA/W and a detectivity of 4.74×107 cm/H1/2/W at 77 K.
Abstract: We report InGaAs quantum dot intersubband infrared photodetectors grown by low-pressure metalorganic chemical vapor deposition on semi-insulating GaAs substrates. The optimum growth conditions were studied to obtain uniform InGaAs quantum dots constructed in an InGaP matrix. Normal incidence photoconductivity was observed at a peak wavelength of 5.5 μm with a high responsivity of 130 mA/W and a detectivity of 4.74×107 cm H1/2/W at 77 K.
147 citations
TL;DR: In this paper, a large number of crystals of cuprous oxide (Cu2O) were prepared by various procedures, including X-ray analysis, inductively coupled plasma spectroscopy analysis, optical absorption, photoluminescence, photoconductivity and cyclotron resonance absorption to characterize their optical and electrical qualities.
Abstract: We have prepared a large number of crystals of cuprous oxide (Cu2O) by various procedures. Photoluminescence spectra of these crystals were studied to examine the concentration of defects, especially copper vacancy VCu to seek favourable conditions for growing Cu2O crystal. High-quality single crystals of Cu2O were prepared by the floating-zone melting method in air. Several synthetic crystals (specimens FA, FZ and GZ) and also a natural crystal were studied by X-ray analysis, inductively coupled plasma spectroscopy analysis, optical absorption, photoluminescence, photoconductivity and cyclotron resonance absorption, photoluminescence, photoconductivity and cyclotron resonance absorption to characterize their optical and electrical qualities. The best values of mobility and scattering time of photocarriers at T = 4.2 K are estimated to be μh≈1.8 × 105 cm2 V−1 s−1 and τh≈60 ps for positive holes, and μ•≈1.3 × 105 cm2 V−1 s−1 and τ•≈70 ps for electrons in Cu2O. Further, we report preliminary experimental results on transport property of crystals also of cupric oxide (CuO) purified by the floating-zone melting method.
113 citations
Book•
01 Jun 1998
TL;DR: An extensive collection of research papers on the properties, preparation and exploitation of amorphous silicon can be found in this paper, including discussion of PECVD growth, transient and steady state photoconductivity, solar cells, photodetectors, LEDs, xerographic applications, and macroscopic data.
Abstract: An extensive collection of research papers on the properties, preparation and exploitation of amorphous silicon, which includes discussion of PECVD growth, transient and steady state photoconductivity, solar cells, photodetectors, LEDs, xerographic applications, and macroscopic data.
109 citations
TL;DR: In this paper, the results of persistent photoconductivity and photoluminescence measurements made on radio-frequency plasma assisted molecular beam epitaxy grown, undoped, GaN are reported.
Abstract: The results of persistent photoconductivity (PPC) and photoluminescence measurements made on radio-frequency plasma assisted molecular beam epitaxy grown, undoped, GaN are reported in this work. Hexagonal GaN (h-GaN) epilayers grown on sapphire and cubic GaN (c-GaN) epilayers grown on GaAs and cubic SiC substrates, are employed in this study. Three clear experimental evidences are reported to claim that the commonly seen persistent photoconductivity and yellow luminescence (YL) are related to each other through the same defect. First, PPC is observed only in those samples which show YL. Second, the threshold (the minimum photon energy required) to observe PPC is determined as 1.6±0.2 eV, which is almost at the same energy at which the YL band starts raising. Third, the photocurrent increases monotonically from 1.8 to 2.2 eV, which is consistent with the broad nature of YL band.
TL;DR: In this paper, deep traps in undoped n-GaN layers grown by organometallic vapor phase epitaxy on sapphire substrates were studied by temperature dependent conductivity, photoinduced current transient spectroscopy (PICTS), thermally stimulated current, electron beam induced current (EBIC), and band edge cathodoluminescence (CL) methods.
Abstract: Deep traps in undoped n-GaN layers grown by organometallic vapor phase epitaxy on sapphire substrates were studied by temperature dependent conductivity, photoinduced current transient spectroscopy (PICTS), thermally stimulated current, electron beam induced current (EBIC), and band edge cathodoluminescence (CL) methods. Presence of electron traps with energy levels 0.1–0.2 eV below the conduction band and hole traps with energy levels of about 0.25, 0.5, and 0.85 eV above the valence band edge was detected. CL and EBIC measurements show that the deep recombination centers in GaN are distributed inhomogeneously with well defined cellular pattern. Both carrier lifetime and luminescence intensity are enhanced at cell walls indicating lower density of recombination centers. However, the density of main hole trap (0.85 eV) is enhanced in these regions as determined by local PICTS measurements. Photoconductivity in many GaN samples exhibits very long decay times at temperatures between 100 and 300 K. The effec...
TL;DR: In this article, a characteristic, reversible metastability is observed for Cu(In,Ga)Se2 thin films and ZnO/CdS/Cu(In-Ga,Se2) heterojunctions, and it is shown that annealing at 80°C leads to a decrease of the dark conductivity by up to a factor of 2 at room temperature and several orders of magnitude when measured at lower temperatures.
Abstract: A characteristic, reversible metastability is observed for Cu(In,Ga)Se2 thin films and ZnO/CdS/Cu(In,Ga)Se2 heterojunctions. Annealing at 80 °C leads to a decrease of the dark conductivity of the thin films by up to a factor of 2 at room temperature and several orders of magnitude when measured at lower temperatures. By exposure to light, the initial state can be re-established. This reenhancement of the dark conductivity can be looked at as persistent photoconductivity. Admittance measurements at Cu(In,Ga)Se2 heterojunction solar cells display a reversible shift of the activation energy of a distinct dielectric loss peak ranging from 70 to about 160 meV upon illumination or annealing at 80 °C, respectively. We propose that both phenomena as well as commonly observed light-soaking effects of Cu(In,Ga)Se2 solar cells have a common origin.
TL;DR: In this paper, the structural properties of the germanium alloys have been investigated by infrared, Raman, and secondary ion mass spectroscopy, and it is found that there is a preferential incorporation of Germanium into the film relative to silicon and the films with high germane gas phase composition Xg>0.4 tend to oxidize in atmosphere.
Abstract: Hydrogenated amorphous silicon germanium alloys (a-SiGe:H) have been prepared by rf glow discharge of silane, germane, and hydrogen gas mixture at substrate temperature of 200 and 250 °C. The structural properties of the films have been investigated by infrared, Raman, and secondary ion mass spectroscopy. It is found that there is a preferential incorporation of germanium into the film relative to silicon and the films with high germane gas phase composition Xg>0.4 tend to oxidize in atmosphere. Besides, polysilane is enhanced in the films with low germane gas phase composition. The electrical properties including dark, photo conductivities, and conduction activation energy are measured. As for the optical properties, optical transmission is adopted to determine the optical gap while photoluminescence spectra together with temperature variation are used to study the band tail states of the films. By applying Brodsky’s quantum well model, the various optical and electrical properties could be explained suc...
TL;DR: In this article, an analysis of internal quantum efficiency data obtained on both conventional and thin-film c-Si solar cells has been performed with the aim of describing the light diffusing behaviour of porous Si as well as investigating the surface passivating capabilities.
Abstract: Crystalline silicon (c-Si) is the dominant semiconductor material in use for terrestrial photovoltaic cells and a clear tendency towards thinner, active cell structures and simplified processing schemes is observable within contemporary c-Si photovoltaic research. The potential applications of porous silicon and related benefits are reviewed. Specific attention is given to the different porous silicon formation processes, the use of this porous material as anti-reflection coating in simplified processing schemes and for simple selective emitter processes and its light trapping and surface passivating capabilities, which are required for advantageous use in thin active cell structures.
Our analysis of internal quantum efficiency data obtained on both conventional and thin-film c-Si solar cells has been performed with the aim of describing the light diffusing behaviour of porous Si as well as investigating the surface passivating capabilities. An effective entrance angle of 60° is derived, which corresponds to totally diffuse isotropic light, and the importance of a correction for absorption losses in the porous layer is illustrated. Furthermore, photoconductivity decay measurements of freshly etched porous Si on float-zone p-type Si indicate a strong bias-light dependency and a fast degradation of the surface recombination velocity. © 1998 John Wiley & Sons, Ltd.
TL;DR: In this paper, the defect levels with excitation energies lower than the bandgap energies are found in both Al0.15Ga0.85N and GaN layers, and evidence is observed that these levels contribute significantly to the aforementioned persistent photoconductivity effects.
Abstract: Persistent photoconductivity effects have been characterized in n-type Al0.15Ga0.85N/GaN heterostructures using both monochromatic light and room light illumination. Time constants of ∼1×104 s have been observed, and measurements of photocurrent specta performed using various illumination geometries and techniques have shown that defect levels exist in both the Al0.15Ga0.85N and GaN layers. Broad distributions of defect levels with excitation energies lower than the bandgap energies are found in both Al0.15Ga0.85N and GaN, and evidence is observed that these levels contribute significantly to the aforementioned persistent photoconductivity effects. The photocurrent spectra also reveal the presence of a level with an excitation energy of 3.36 eV that contributes to the persistent photoconductivity in the heterostructure.
TL;DR: In this paper, the density of paramagnetic defects in hydrogenated tetrahedral amorphous carbon and the Urbach slope of the optical absorption edge can be reduced by annealing at 300°C, with little effect on the optical gap.
Abstract: Electronic applications of diamond-like carbon have been limited by its relatively high disorder and defect density. We find that the density of paramagnetic defects in hydrogenated tetrahedral amorphous carbon and the Urbach slope of the optical absorption edge can be reduced by annealing at 300 °C, with little effect on the optical gap. This leads to a reduction in the dark conductivity and an increase in the photosensitivity. The effect is attributed to the migration of hydrogen through the C–C network, to allow better passivation of dangling bonds and a modification of the more weakly bonded sp2 clusters with narrower local band gaps.
TL;DR: In this article, the authors used Bi4Ti3O12 (BiT) ferroelectric thin films as photoconductive or photovoltaic ultraviolet detectors and obtained a specific detectivity of 5.5×108 cm, W−1.1/2 at a modulation frequency of 20 Hz.
Abstract: Bi4Ti3O12 (BiT) ferroelectric thin films were used as photoconductive or photovoltaic ultraviolet detectors. The maximum of the spectral distribution lies around 370 or 390 nm, depending on whether the incident light is continuous or modulated. The maximum current responsivity obtained for the films annealed at 700 °C is about 0.01 A/W. The specific detectivity is 5.5×108 cm W−1 Hz1/2 at a modulation frequency of 20 Hz. The frequency characteristics of the photovoltaic signal measured in modulated light suggests a 1/f frequency dependence of this signal.
TL;DR: In this paper, the authors used an ultrafast photoconductive sampling detector to detect terahertz radiation emitted by coherent phonons in Te, PbTe, and CdTe.
Abstract: Terahertz radiation emitted by coherent phonons in Te, PbTe, and CdTe has been investigated by using an ultrafast photoconductive sampling detector. Pronounced coherent radiation originating from the longitudinal optical (LO) phonon oscillations of infrared-active modes was observed for all samples, irrespective of the different crystal structures. In addition, spectral dips at the transverse optical (TO) phonon frequencies, which could not be explained by absorption in the emitting volume, were observed for all samples. The model calculations indicate that the emission rate of the radiation into the air to that into the dielectric (semiconductor) side is scaled by 1/{1+(nd2+κd2)nd3} (nd and κd are the real and imaginary part of the complex refractive index, respectively). Thus, the enhanced emission of radiation by the coherent LO phonons and the spectral dips at the TO phonon frequencies can be explained by the respective increase and reduction of the emission efficiency of the radiation to the air due to the small and large value of the dielectric constant |ed(ω)|=nd2+κd2 near the LO and TO phonon frequencies, respectively.
TL;DR: The intrinsic photoionization yield increases strongly with electric field but is virtually independent of temperature, at variance with the prediction of the conventional Onsager theory of geminate pair dissociation as discussed by the authors.
TL;DR: In this article, the main manifestations of polaron pairs in the generation of free charge carriers, delayed luminescence, intra-chain triplet exciton generation, recombination of free carriers, persistence of the photoconductivity, displacement current in the transient photoconductivities experiments, and spin-dependent reactions are discussed.
TL;DR: The extent of orientational order which could be photoinduced in the films was found to decrease with increasing spacer length, as did the photoconductivity and the photorefractive two-beam optical coupling gain.
Abstract: A series of amorphous azobenzene and carbazole-containing polymers was prepared, which incorporates both electrooptic activity and photoconductivity into a single multifunctional structural unit. The polymers were cast as thin films and were shown to be suitable for photoinducing birefringence reversibly with polarized light, as well as for the inscription of photorefractive diffraction gratings after electric field poling. Since the polymer series encompasses a range of spacer lengths (from 3 to 10 methylene groups) between the multifunctional side chains and the polymer backbone, these materials are suitable for study of the influence of chromophore mobility on these optical phenomena. The extent of orientational order which could be photoinduced in the films was found to decrease with increasing spacer length, as did the photoconductivity and the photorefractive two-beam optical coupling gain. In thin films of polymers with the highest glass transition temperature, a birefringence of 0.065 could be pho...
TL;DR: Polycrystalline ZnO:Al thin films have been prepared by the (Sol-gel) chemical deposition method as mentioned in this paper, and they are very transparent (∼90%) in the near UV, VIS and IR regions.
TL;DR: In this paper, the slow relaxation of the photoconductivity over a period of days was studied in polycrystalline ZnO and TiO2 films prepared by spray pyrolysis.
Abstract: Slow relaxation of the photoconductivity over a period of days was studied in polycrystalline ZnO and TiO2 films prepared by spray pyrolysis. The phenomenon is described by a model involving deep sensitizing hole traps in the forbidden gap. The trap state distribution based on this model was calculated using an improved Laplace transform method. For ZnO and TiO2 films, the density of states was shown to have a peak-like distribution with a maximum near the lower third of the energy gap.
TL;DR: In this article, a single-chamber radio frequency (13.56 MHz) plasma enhanced chemical vapour deposition system with a carbon dioxide, silane and hydrogen gas mixture was used to obtain a variation in optical gap from 1.96 to 2.12 eV.
Abstract: Hydrogenated amorphous silicon oxide (a-SiO:H) films have been prepared in a conventional single-chamber radio frequency (13.56 MHz) plasma enhanced chemical vapour deposition system with a carbon dioxide , silane and hydrogen gas mixture. Variation in optical gap from 1.96 to 2.12 eV has been obtained by controlling the deposition pressure and radio frequency (rf) power. The photoconductivities of films in each set are similar to each other for similar optical gaps. High rf power does not have any remarkable deteriorative effect on material but the rate of deposition of film and optical gap are enhanced. At rf power density the and are and 2.11 eV respectively whereas under similar other conditions but with rf power density the corresponding and are and 1.96 eV respectively. By lowering the deposition pressure the optical gap of films has also been enhanced. At 1.5 Torr pressure, the optical gap is 1.96 eV and at 0.13 Torr it becomes 2.12 eV. Here variation of the dilution of hydrogen is adopted in order to look into the role of hydrogen atoms in plasma and characteristics of the deposited films. A model in which incorporation of oxygen is described through plasma kinetics is proposed in this regard. With a higher optical gap the photoconductivity reduces but this reduction is mostly due to there being a lower optical absorption coefficient , partly due to there being a lower value of the product of the mobility , lifetime and quantum efficiency and also partly due to the mid-gap defect density . For the power variation set of samples, with optical gap up to about 1.99 eV, and the photoconductivity remain around and respectively and, with this optical gap, the dark conductivity is . When the material is degraded by soaking in light, a modification of mid-gap states and also of the valence band tail state takes place and falls, together with the photoconductivity. For material of 1.95 eV optical gap the initial and are reduced to and respectively after 3 h of soaking in light.
TL;DR: In this paper, the optical, electrical and photovoltaic properties of pyronine G (Y) have been investigated in the form of thin film devices by measuring the J-V characteristics in dark as well as under illumination.
TL;DR: In this paper, a long-lived residual photoconductivity in low-temperature-grown (LT) InGaAs was observed, and the results have significant consequences for devices comprised of LT-InGaAs.
Abstract: We have observed a long-lived residual photoconductivity in low-temperature-grown (LT) InGaAs. These results have significant consequences for devices comprised of LT-InGaAs, other defect moderated materials, and standard-temperature-grown InGaAs. Our investigation utilizes time-resolved terahertz conductivity to quantify the trapping and recombination rates of LT Be-doped In0.53Ga0.47As/In0.52Al0.48As multiple quantum wells and bulk InGaAs. It is found that Be doping reduces the residual photoconductivity and increases the initial electron trapping rate. These results are in contrast to those observed via transient absorption studies, which suggest that these systems have returned to equilibrium after the initial transient. Furthermore, a 600 °C anneal increases both the trapping and recombination rate in all Be-doped samples.
TL;DR: In this paper, the authors demonstrated that the lifetime of high gain GaAs photoconductive semiconductor switches (PCSSs) can be extended to well over ten million pulses by reducing the density of carriers at the semiconductor to metal interface.
Abstract: The longevity of high gain GaAs photoconductive semiconductor switches (PCSSs) has been extended to well over ten million pulses by reducing the density of carriers at the semiconductor to metal interface. This was achieved by reducing the density in the vertical and lateral directions. The latter was achieved by varying the spatial distribution of the trigger light thereby widening the current filaments that are characteristic of the high gain switches. We reduced the carrier density in the vertical direction by using ion implantation. These results were obtained for currents of about 10 A, current duration of 3.5 ns, and switched voltage of /spl sim/2 kV. At currents of /spl sim/70 A, the switches last for 0.6 million pulses. In order to improve the performance at high currents, new processes such as deep diffusion and epitaxial growth of contacts are being pursued. To guide this effort we recorded open shutter, infra-red images, and time-resolved Schlieren images of the current filaments, which form during high gain switching. We measured, under varying conditions, a carrier (electrons or holes) density that ranges from 3/spl times/10/sup 17/ cm/sup -3/ to 6/spl times/10/sup 18/ cm/sup -3/.
TL;DR: The uncertainties inherent in the normalization of subgap photoconductivity spectra to the optical absorption spectra in a-Si:H based films have been addressed in this article, which is based on optical transitions of constant dipole matrix element between parabolic distributions of extended states and exponential distributions of localized tail states.
Abstract: The uncertainties inherent in the normalization of subgap photoconductivity spectra to the optical absorption spectra α(hv) in a-Si:H based films have been addressed. An analysis is presented which is based on optical transitions of constant dipole matrix element between parabolic distributions of extended states and exponential distributions of localized tail states. This analysis has been used to normalize the two sets of results accurately, as verified by photothermal deflection spectroscopy measurements, and is shown to be useful in the commonly encountered cases, in which the two spectra do not overlap over an extended region. Improved quantitative fits of α(hv), for photon energy from ∼1.5 to 2.4 eV, obtained on different a-Si:H based films indicate that the localized exponential band tail regions extend ∼60–70 meV above the optical gap.