Optical Absorption Of Thin Films Research Articles

Novel BixLa1-xMnyCo1-yO3 nanocrystalline perovskite based MIS Schottky UV photodetector device

This study introduces the fabrication of MIS junction for UV photodetection applications employing BixLa1-xMnyCo1-yO3 as an insulating layer (I-layer). The structure refinement of the synthesized perovskite powder is performed. The perovskite thin films' structural, topographical, and compositional features are inspected. The measured UV-visible optical absorption of thin films is discussed, where the energy gap and Urbach energy are estimated to be ∼3.09 eV and 93.4 meV, respectively. The MIS configuration was built employing p-silicon and Ag-electrode. The microelectronic parameters of the designed junction are calculated. The charge transport mechanism is interpreted in detail in the light of trap states. We evaluated the UV photodetection performance of the implemented device under different intestines. Some photodetection parameters have been determined, such as responsivity, detectivity, and external quantum efficiency, which yielded ∼8.01 mA/W, 1.23 × 109 cm.Hz0.5/W, and 4.32%, respectively. The engineered device showed a stable and fast response.

Simultaneous electrical impedance and optical absorption spectroscopy for rapid characterization of oxygen vacancies and small polarons in doped ceria.

Mixed ionic-electronic conductors (MIECs) play a central role in emerging energy conversion and energy efficient computational technologies. However, it is both challenging and resource demanding to characterize MIECs over the broad range of experimental conditions of interest, thereby significantly limiting their study and applications. Here, a novel method of a simultaneous measurement of electrical conductivity and optical absorption of thin films in out-of-equilibrium state, i.e. during a reduction process, is employed for a comprehensive study of a MIEC oxide, PrxCe1-xO2-δ (PCO). It enables, orders of magnitude faster than by established techniques, characterization of the oxygen vacancy and small polaron formation and transport as a function of temperature (demonstrated here down to 200 °C), in a wide range of deviation from stoichiometry, δ. For instance, at 600 °C the PCO properties were obtained during a ten minute reduction process, in the pO2 range from 1 to 10-13 bar. The experimental results show that the oxygen vacancy mobility is constant while the small polaron mobility is linear in δ, in the whole pO2 range, which yields the total conductivity quadratic in δ. Furthermore, the method was applied to study the modification of PCO's transport properties with composition change. It was shown that increasing x from 0.1 to 0.2 suppresses the ionic mobility and, at the same time, enhances the small polaron mobility. Finally, the optically determined δ was used to define an instantaneous oxygen activity in PCO that can be accessed in the out-of-equilibrium experiments. This work opens up new possibilities to study the effects of microstructure, strain and other applied external stimuli on the transport and thermodynamic properties of PCO and similar types of MIEC materials.

Optical Absorption Enhancement in CdTe Thin Films by Microstructuration of the Silicon Substrate.

In this work, the reflectance, optical absorption, and band gap have been determined for CdTe thin films grown on planar and microstructured substrates. The treated surface was prepared by laser ablation of a silicon wafer, forming holes in a periodic arrangement. Thin films were grown by pulsed laser ablation on silicon samples kept at 200 °C inside a vacuum chamber. The presence of CdTe was verified with X-ray diffraction and Raman spectroscopy indicating a nanocrystalline zinc blended structure. The optical absorption of thin films was calculated by using the Fresnel laws and the experimental reflectance spectrum. Results show that reflectance of 245 nm films deposited on modified substrates is reduced by up to a factor of two than the obtained on unchanged silicon and the optical absorption is 16% higher at ~456 nm. Additionally, it was determined that the band gap energy for planar and microstructured films is about 1.44 eV for both cases.

Influence of pH on structural, optical and electrical properties of solution processed Cu2ZnSnS4 thin film absorbers

Cu2ZnSnS4 (CZTS) films were obtained by the dip-coating method. The effect of different pH values 4.0, 4.5, and 5.0 on the structural, morphological, optical and electrical properties of samples was investigated by XRD, SEM, UV–vis and Hall Effect measurements. The XRD spectra showed that the characteristic peak intensity of CZTS semiconductor increased with increasing pH value of the precursor solution. It was also observed that increased pH values resulted in a significant reduction in the amount of impurity phases of the films. The UV–vis studies revealed a significant increase in the optical absorption of thin films in the visible region as the pH value of the solution was increased. The band gap of the samples shifted from 2.0 to 1.38eV by increasing the pH value. The electrical resistivity of the films was found to vary from 2.8×10−2 to 3.1Ωcm, depending on its pH value.

Role of Hydrogen in Variation of Electrical, Optical and Magnetic Properties of ZnSe-Fe Bilayer Thin Films Structure

This paper is reporting the role of hydrogen in preparation and characterization of dilute magnetic semiconductor of ZnSe-Fe bilayer thin film structure. These films are hydrogenated at different pressure to see the effect of hydrogen on electrical, optical, magnetic and structural properties of bilayer structure. Optical absorption in thin films is found to be decrease with hydrogen absorption. It may be due to interaction of hydrogen with bilayer structure and takes electron from the conduction band of thin film structure. The current-voltage characteristic of these films shows the variation in conductivity with hydrogenation due to decrease in electron density. Atomic Force Microscopy and scanning electron Microscope recorded to see the surface topography of bilayer thin films. It has been observed that deposited film have nano size structure that is favorable for hydrogen absorption having higher surface to volume ratio. The Electron diffraction X-ray analysis gives the information about composition of films. Raman spectra have used to see the presence of hydrogen. Super-conducting Quantum Interference Device gives the information of confirmation of diluted magnetic semiconductors and variation of magnetic momentum with hydrogenation.

Optical absorption of radio frequency sputtered GaAs(Ti) films

Composition and optical absorption of thin films of GaAs(Ti) and GaAs, deposited by sputtering on glass substrates under different process conditions, have been investigated. The thin films obtained are typically 200 nm thick. ToF–SIMS measurements show a quite constant concentration and good uniformity of Ti profiles along the GaAs(Ti) layers in all cases and EPMA results indicate that Ti content increases with the substrate temperature in the sputtering process. Measurements of the transmittance and reflectance spectra of the GaAs and GaAs(Ti) thin films have been carried out. In the optical characterization of the films it is found that optical absorption is enhanced in all samples containing Ti. The determination of the optical gap from the optical absorption, shows optical gap variations from 1.15 to 1.29 eV in the GaAs thin films, and from 0.83 to 1.13 eV in the GaAs(Ti) thin films. The differences in absorption and EgTAUC observed between samples of GaAs and GaAs(Ti) are consistent with the presence of an intermediate band.

Enhancement of the optical absorption of thin-film of amorphorized silicon for photovoltaic energy conversion

Abstract In this study we report for the first time a method for simple, precise and single-step generation of thin film of amorphous silicon (a-Si) on silicon substrate induced by laser pulses with the frequency of megahertz under ambient conditions for solar cell fabrication. Also, the effect of laser parameters such as pulse width is investigated by developing an analytical model for the calculation of the non-dimensional surface temperature with various pulse widths; it was found from experimental and analytical results that for a constant power and repetition rate, an increase in the pulse duration corresponds to a significant increase in the surface temperature which results in an increase in the amount of amorphorized material as well as improvement of light absorption. A Scanning Electron Microscope (SEM), scanning near-field optical microscope (SNOM), a Micro-Raman, Energy Dispersive X-ray (EDX) spectroscopy and an optical spectrometer were used to analyze the optical and material properties of the amorphorized thin layer on Si-substrate.

RF magnetron sputter의 분위기에 따른 Tio2박막의 특성

RF 마그네트론 스퍼터링으로 가스조성비와 기판의 온도를 변화시키면서 <TEX>$Tio_2$</TEX> 박막을 성장하였다. XRD, SEM, AFM 및 분광 광도계를 이용하여 박막의 구조와 광학적 특성을 고찰하였다. 박막에는 아나타제 결정성만 관찰되었으며 온도가 높아질수록 회절 피크의 강도가 증가하였다. 산소농도가 증가함에 따라 기둥구조의 결정성장률이 감소되었으며 굴절률과 흡수율은 감소하였다. <TEX>$Tio_2$</TEX> 박막은 300~400oC의 기판온도와 10 %의 <TEX>$O_2$</TEX> 분위기에서 성장한 박막의 표면이 매끄럽고 투과특성이 우수한 <TEX>$300{\sim}400^{\circ}C$</TEX> 박막을 얻을 수 있었다. The <TEX>$Tio_2$</TEX> films were prepared on glass, silicon and quartz substrate at different temperature by radio frequency reactive magnetron sputtering under different flow ratios of Ar and O2 gases. The films were characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM), atomic force microscope (AFM) and UV-VIS spectrophotometer. Only the anatase phase was observed in films and their diffaction peaks increased with temprature of substrate. The size of crystallites decreased with higher concentration of oxygen. Refractive index and optical absorption of thin films decreased with higher concentration of oxygen. The thin films which have good transmittance spectra and smooth surface, deposited in the sputtering ambient with 10 % of <TEX>$O_2$</TEX> at the temperature from <TEX>$400{\circ}C$</TEX> to <TEX>$300{\circ}C$</TEX>.

Mechanical, Electrical, and Optical Properties of (Pr,Ce)O2 Solid Solutions: Kinetic Studies

Praseodymium doped cerium oxide (PCO) shows mixed ionic and electronic conducting (MIEC) characteristics at relatively high pO2 (e.g. air) and enhanced oxygen storage capacity (OSC), of interest for solid oxide fuel cell (SOFC) cathodes and three way catalysts, resulting from the ready reduction of Pr to its trivalent state. This reduction also results in a change of lattice parameter, known as chemical expansion, and changes in optical absorption related to the formation of a Pr impurity band. These properties can be measured as a function of time, yielding oxide ion diffusion and surface exchange coefficients. In this manuscript, the diffusivity and/or surface exchange of PCO is explored using TGA and dilatometry of bulk specimens and HTXRD and optical absorption of thin films. The HTXRD and optical absorption measurements promise the ability to monitor both thermodynamic and kinetic properties of thin film materials exhibiting wide swings in oxygen stoichiometry.

Optical absorption of the hydrogenated evaporated amorphous silicon

This study focuses on the study characteristic of the optical absorption of thin films of evaporated hydrogenated amorphous silicon (a-Si: H) prepared in a built Ultra-High Vacuum (UHV). Atomic hydrogen is produced using a plasma discharge tube directed towards the substrate holder. The optical absorption coefficient was obtained in the range of 0.5 to 2.5 eV using optical transmission measurements and Photothermal Deflection Spectroscopy (PDS). The results obtained are comparable to those of films prepared by decomposition of silane. The refractive index of the films decreases when the concentration of hydrogen increases. EGbandwidth increases (1.45 to 1.60 eV), the parameter threshold Urbach E0decreases (from 149 to 62 meV) and finally the number of spins (4 x 1019 to 6.5 x 1016 cm-3) depending on the concentration of hydrogen increases. Key words: Hydrogenated amorphous silicon, optical absorption, density of electronic states, urbach absorption.

Optical Monitoring of Nanocrystalline Diamond with Reduced Non-diamond Contamination

AbstractPreviously, the nanocrystalline grain boundaries were often contaminated by the “non-diamond phase” with the photo-ionization threshold at 0.8 eV. Here, we present the optical spectra of the NCD films grown on transparent substrates by the microwave plasma enhanced chemical vapor deposition (CVD) at a relatively low temperature below 600°C. The transmittance and reflectance spectra are useful to evaluate the film thickness, the surface roughness and the index of refraction. The direct measurement of the optical absorptance by the laser calorimetry and photothermal deflection spectroscopy (PDS) provides high sensitive methods to measure the weak optical absorption of thin films with rough surface. The optical measurements indicate the high optical transparency of our standard, nominally undoped 0.2-0.3 μm thick NCD film with low non-diamond content. However, the optical scattering is rather high in UV and needs to be reduced.

Origin of the optical absorption of In2O3 thin films in the visible range

Optical absorption of thin films of In2O3 obtained by spray pyrolysis was studied in the visible part of the spectrum, using a numerical procedure for extraction of different contributions in the absorption coefficient. One contribution from a direct forbidden (3.29 eV) and three indirect allowed transitions (2.09 eV, 3.42 eV and 3.58 eV) were identified. They were attributed to the transitions from the valence band to the two lowest minima in the conduction band separated from the valence band by gaps ≈2 eV and 3.3 eV large, in agreement with the results of ab intiio calculations done in Erhart et al (2007 Phys. Rev. B 75 153205). The fundamental absorption edge that has been observed at ≈4 eV in many earlier experimental works is attributed to the transitions Γ8−Γ1, where Γ1 is the lowest minimum in the conduction band at Γ point and Γ8 is a maximum substantially lower than the highest maximum in the valence band (Γ4) (Walsh et al 2008 Phys. Rev. Lett. 100 167402). Therefore, the high conductivity of In2O3 is due to the generation of carriers by transitions from the valence band in the two lowest maxima of the conduction band, going along with a high transmission in the visible range, up to the photon energies of ≈4 eV, at which direct allowed transitions occur, leading to strong absorption.

In situ optical spectroelectrochemistry of single-walled carbon nanotube thin films

A detailed study is presented on the optical absorption of thin films of single-walled carbon nanotubes (SWNT) under electrochemical conditions. The procedure for the preparation of free-standing semitransparent films of SWNT is used for the fabrication of a working electrode for transmission optical spectroelectrochemistry. The analysis of the potential dependent spectroscopic response of the SWNT film benefits from the widest possible electrochemical window, in which the charging of SWNT can safely be investigated. This electrochemical window is not limited by parasitic electrochemistry and/or galvanic breakdown reactions occurring at supporting electrode materials such as indium–tin oxide conducting glass or semitransparent Pt film, which were employed in earlier studies. Electrochemical doping of SWNT is observable at the optical absorptions, which are assigned to allowed electronic transitions between van Hove singularities in the density of states of SWNT. Furthermore, the spectral response of counterions, balancing the charging of the nanotube skeleton, is traceable at certain conditions. The latter effect is monitored here through the overtones of C–H stretching vibrations from tetrabutylammonium cations.

Microstructure and optical properties of photoactive TiO 2:N thin films

In this work, we have chosen oxidation of TiN thin films as a feasible method for preparation of nitrogen-doped titanium dioxide thin films, TiO 2:N, for photocatalytic applications. DC reactive magnetron sputtering with the plasma emission control was used for deposition of stoichiometric TiN thin films. The microstructure and chemical composition of films before and after oxidation were investigated by means of RBS, X-ray diffraction (XRD) in grazing incidence diffraction (GID) configuration, AFM and XPS techniques. The electrical conductivity was measured by the van der Pauw method as a function of the oxidation temperature. The optical transmittance and reflectance spectra of the films were measured over the visible and UV ranges of the light spectrum. GID diffraction patterns of as-sputtered TiN thin films and those after oxidation indicate that TiO 2 rutile is formed at around 300 °C. Nitrogen is still present as indicated by XPS studies even when XRD detects the rutile only. Optical absorption of thin films oxidized at 450 °C is shifted towards the visible range of the light spectrum.

The Influence of Molecular Structure Modification on the Photoluminescence and Optical Absorption of Thin Copper Phthalocyanine Films in the Near-IR Range

We have studied the luminescence and optical absorption of thin films of copper phthalocyanine (CuPc) with a modified molecular structure of the peripheral fragments. The coefficient of absorption in the near-IR and visible range (absorption by defects) and the photoluminescence spectra exhibit correlated changes depending on the modification of the CuPc structure.

Defect-related photoinduced absorption in amorphous silicon

Photoinduced optical absorption in thin films of hydrogenated amorphous silicon (a-Si : H) is studied over the energy range related to electronic transitions involving the silicon dangling-bond defects, by using the dual-beam Constant Photocurrent Method (CPM). The subgap absorption in the range 0.8–1.4 eV is observed to increase with bias light intensity. Since CPM measures only transitions contributing to the photocurrent, the subgap absorption spectrum in undoped a-Si:H is dominated by transitions between singly occupied D 0 or doubly occupied D − defect states and conduction band extended states. The photoinduced changes in the subgap absorption are consistent with numerical calculations with a recombination model for a-Si : H which yield dangling bond electronic occupations as a function of photogeneration rate. The results suggest that the observed changes in the optical absorption spectra are mainly determined by changes in the occupation of defects induced by the bias-light.

Optical Absorption in AlQ

In this paper, we report on optical absorption in thin films of tris(8-hydroxyquinoline)aluminium (AlQ) between 0.7 and 5 eV. We report on the first measurement of the sub-bandgap absorption in AlQ using Photothermal Deflection Spectroscopy (PDS) for different deposition conditions. The sub-bandgap absorption is not very sensitive to sample deposition temperature but increases on exposure to atmosphere. We present a simple model to explain the absorption data. We calculate the density of states in the gap to be about 6.10 17/cm 3 and present evidence that the fermi level lies in the upper half of the band gap close to the LUMO level in AlQ.

Depth Profiling of Optical Absorption in Thin Films via the Mirage Effect and a New Inverse Scattering Theory. Part I: Principles and Methodology

Impulse mirage effect spectroscopy is developed in this work as a nondestructive method for depth profiling the optical properties of samples which are nearly thermally homogeneous with depth. Both a theory and an experimental methodology are presented. An inverse scattering theory of the experimental photothermal deflection signal is derived, based on a previous theory of the impulse mirage effect, which takes into account the effect of Fresnel diffraction on the probe beam. To reconstruct the depth profile of heat source density generated by light absorption in an unknown sample, we have applied our inverse theory to the experimental impulse response, using a regularized minimum square error reconstruction algorithm based on our previously published expectation minimum principle. Because this reconstruction problem is ill posed, it was necessary to identify and compensate for all experimental bias errors significantly affecting the fidelity of the depth profiles. A procedure for obtaining the overall best-fit model of the depth profile given the minimum prior experimental information is presented. These procedures have produced an agreement between the experimental and theoretically predicted mirage effect response to within typical root-mean-square error levels of 0.5% or less.

Depth Profiling of Optical Absorption in Thin Films via the Mirage Effect and a New Inverse Scattering Theory. Part II: Experimental Reconstructions on Well-Characterized Materials

Mirage effect spectrometry is experimentally evaluated in this work as a technique of optical depth profiling in thin films where no prior information is available about the sample properties. An apparatus suitable for performing quantitative measurements is described. High-precision experimental alignment procedures are introduced along with a new method for precise optical correction of the detector signal for experimental frequency response nonuniformities. Reconstructions were made of the heat source density and absorption coefficient depth profile in materials with known depth dependence. These included samples approximating weighted delta function arrays, and depth-continuous media known to obey Beer's law to a good approximation. The properties of these samples were examined independently by using a technique of depth-sensitive light microscopy. Mirage effect depth profiles reconstructed on samples containing discrete absorbers were effectively regularization limited, indicating that resolution is limited by random error in the data rather than experimental bias. Depth profiles obtained in continuously absorbing media show a good agreement with those obtained by reference methods.

Study of optical absorption in thin films of a-As2Se3 by photocapacitance spectroscopy

Photocapacitive spectroscopy is used for the first time to study optical absorption in thin films of a-As2Se3 for optical photons with energies below the gap width. Photocapacitance spectra are obtained over energies of 0.83–1.94 eV and used to characterize the density of localized states in the tail of the valence band and in deep states of charged defects. Aging and illumination of the film are found to affect the level of optical absorption by deep centers.