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Thermally stimulated techniques—thermally stimulated current (TSC) spectroscopy and thermally stimulated depolarization current (TSDC) spectroscopy—were used to comparatively study the electrical properties and deep level defects in β-Ga2O3 pulsed laser deposited thin films and Czochralski-grown bulk crystals. It was found that the samples are highly resistive and each sample may have different dark current activation energy. Deep level defects revealed by the thermally stimulated techniques vary from sample to sample. In addition to the common traps E1 (∼0.56 eV), E2 (∼0.84 eV), and E3 (∼0.99 eV), reported in the literature and revealed by DLTS studies of Ga2O3 bulk crystals, that were also found in our samples by the thermally stimulated techniques, a trap at ∼110 meV and several other traps are revealed specifically by TSDC between 105 and 225 K. [ABSTRACT FROM AUTHOR]

We report annealing induced sign reversal of dispersive optical nonlinearity in ion beam sputtered NiO thin films deposited at 30% and 70% oxygen partial pressures. In the Ultraviolet-visible spectra of the samples, the transmission peak corresponding to d-d transitions is observed near 2 eV. A shift in this peak towards higher energy was observed when the same films were annealed at 523 K. The near resonant photoinduced transitions produced giant nonlinear optical susceptibilities of both third- and fifth- orders when the annealed film was irradiated by a continuous wave 632.8 nm He-Ne laser. The role of the thermo-optic effect has been examined critically. Experimental studies further reveal that the oxygen partial pressure influences the growth direction of the grains in the thin films. The well known Z-scan experimental procedure has been followed for measurements of optical nonlinearities in all the NiO films. The nonlinear refractive indices of both the as-deposited and annealed NiO thin films are defined in terms of the thermo-optic coefficients (dn/dT)T=T0 and (d²n/dT²)T=T0. [ABSTRACT FROM AUTHOR]

High-quality single-crystalline Bi6FeCoTi3O18 and LaBi5FeCoTi3O18 thin films were prepared by pulsed laser deposition. X-ray diffraction characterizations indicate a more disordered lattice structure of the LaBi5FeCoTi3O18 film. The magnetic measurement results demonstrated significantly enhanced ferromagnetism in the LaBi5FeCoTi3O18 film. The modulation of oxidation and hybridization states caused by substituting Bi with La was studied using the soft X-ray absorption spectroscopy. The spectroscopic results revealed the reduced concentration of oxygen vacancies and the more distorted lattice structure in the LaBi5FeCoTi3O18 film, which explained the enhanced ferromagnetism. [ABSTRACT FROM AUTHOR]

Cu2ZnSnS4 is a promising low-cost, nontoxic, earth-abundant absorber material for thin-film solar cell applications. In this study, combinatorial coevaporation was used to synthesize individual thin-film samples spanning a wide range of compositions at low (325 °C) and high (475 °C) temperatures. Film composition, grain morphology, crystalline-phase and photo-excitation information have been characterized by x-ray fluorescence, scanning electron microscopy, x-ray diffraction, Raman spectroscopy, and photoluminescence imaging and mapping. Highly textured columnar grain morphology is observed for film compositions along the ZnS-Cu2ZnSnS4-Cu2SnS3 tie line in the quasi-ternary Cu2S-ZnS-SnS2 phase system, and this effect is attributed to structural similarity between the Cu2ZnSnS4, Cu2 SnS3, and ZnS crystalline phases. At 475 °C growth temperature, Sn-S phases cannot condense because of their high vapor pressures. As a result, regions that received excess Sn flux during growth produced compositions falling along the ZnS-Cu2ZnSnS4-Cu2SnS3 tie line. Room-temperature photoluminescence imaging reveals a strong correlation for these samples between film composition and photoluminescence intensity, where film regions with Cu/Sn ratios greater than ~2 show strong photoluminescence intensity, in comparison with much weaker photoluminescence in regions that received excess Sn flux during growth or subsequent processing. The observed photoluminescence quenching in regions that received excess Sn flux is attributed to the effects of Sn-related native point defects in Cu2ZnSnS4 on non-radiative recombination processes. Implications for processing and performance of Cu2ZnSnS4 solar cells are discussed. [ABSTRACT FROM AUTHOR]

Femtosecond pump-probe spectroscopy is applied to identify transient optical signatures of phase transitions in nearly optimally doped YBa2Cu3O7-δ films. To elucidate the dynamics of superconducting and pseudogap phases, the slow thermal component is removed from the time-domain traces of photo-induced reflectivity in a high-flux regime with low frequency pulse rate. The rescaled data exhibit distinct signatures of the phase separation with abrupt changes at the onsets of TSC and TPG in excellent agreement with the transport data. Compared to the superconducting phase, the response of the pseudogap phase is characterized by the strongly reduced reflectivity change accompanied by a faster recovery time. [ABSTRACT FROM AUTHOR]

The temperature, Fermi-level, and bias dependencies of the inter-surface tunneling current in thin-film topological insulators show unique, identifying signatures of the surface states and their opposite chiralities. The opposite chiralities of the surface states limit the tunneling to the band edges of the gapped Dirac cones. As a result, the tunneling conductance is sensitive to the temperature, the Fermi level, and the surface-surface potential difference. The temperature dependence of the tunneling conductance changes sign as the Fermi level scans through the Dirac point. The tunneling transmission is a minimum when the opposing surface Dirac cones are perfectly aligned in energy. This minimum state of the tunneling channel can result in negative differential resistance (NDR) in the presence of a built-in Rashba-like splitting. The unique thermal response of the tunneling conductance and the existence of NDR suggest a tunneling spectroscopy experiment to demonstrate the opposite chiralities of the opposing surface states. [ABSTRACT FROM AUTHOR]

We have performed Raman-scattering measurements under high hydrostatic pressure on CdO thin films grown by metal-organic vapor phase epitaxy on sapphire substrates. The pressure dependence of the second-order Raman bands is discussed in terms of ab initio lattice-dynamical calculations, which allow us to obtain mode Grüneisen parameters for the zone-center TO and LO modes of CdO. Our experiments and calculations suggest that at low pressures (<4 GPa) the Raman spectra are dominated by second-order modes, while at higher pressures (>4 GPa) the spectra mainly display contributions from disorder-activated first-order modes. [ABSTRACT FROM AUTHOR]

The defect-related levels of CuInS2 thin films prepared by sulfurization using ditertiarybutylsulfide [(t-C4H9)2S:DTBS] have been investigated by photoluminescence (PL) spectroscopy. The PL spectra exhibit four peaks at 1.43, 1.27, 1.21, and 1.17 eV. On the basis of the PL spectra observed at various excitation intensities and temperatures, the peak emissions are attributed to donor-acceptor pair transitions. The ionization energies of donors in CuInS2 thin films are determined to be 66, 330, 300, and 150 meV, and the ionization energies of acceptors are estimated to be 100, 170, and 390 meV, respectively. As a result of the different sulfurization conditions and times, the donors corresponding to these ionization energies are due to indium interstitial (Ini), indium atoms-occupied sulfur vacancies (InS), sulfur atoms-occupied copper vacancies (SCu), and sulfur vacancies (VS), and the acceptors are determined to be due to copper vacancies (VCu), sulfur interstitial (Si), and sulfur atoms-occupied indium vacancies (SIn), respectively. Using these data, a band diagram for the defect levels of CuInS2 thin films has been proposed. [ABSTRACT FROM AUTHOR]

This paper presents an ultrasonic technique for simultaneous determination of the complete set of acoustical and geometrical properties of a film or a thin layer in a stratified material embedded between two known materials using point-focus ultrasonic spectroscopy, which provides a high lateral resolution. The theoretical model of the two-dimensional spectrum Rt(θ,ω) of the stratified material is calculated as a function of six parameters of the unknown layer: longitudinal and transverse velocities cl, ct, attenuation αl, αt, density ρ, and thickness h, which fully determined the properties of the film. The experimental spectrum Re(θ,ω) can be measured by V(z,t) technique. A two-step algorithm is presented to decompose the searching process of parameters from one six-dimensional to two three-dimensional spaces. The sensitivity of the two-dimensional spectrum to individual properties and its stability against experimental noise are studied. The full set properties of a 250 μm thick stainless steel film and a 930 μm thick SiO2 thin layer of a three layered stratified material immersed in water are determined. The proposed technique used a point-focus transducer, which makes the setup simple and reliable. It allows measurement of the local properties of the film and enables precision material characterization. [ABSTRACT FROM AUTHOR]

We have characterized amorphous In2O3:H (H : ∼4 at.%) transparent conducting films by Rutherford backscattering spectrometry (RBS), thermal desorption spectroscopy, spectroscopic ellipsometry, and Hall measurements. The amorphous In2O3:H films have been fabricated at room temperature by sputtering of an In2O3 ceramic target under Ar, O2, and H2O vapor with variation of a flow ratio r(O2) = O2/(O2+Ar). We observe (i) signals originating from Ar in RBS spectra for all the films and (ii) desorption of H2O and Ar gases during post thermal annealing of the films. Furthermore, O2 desorption together with H2O and Ar is observed for the films grown at r(O2) > 0.375%, whereas In desorption together with H2O and Ar is observed for the films grown at r(O2) < 0.375%. These results suggest that the films have void and/or multi-vacancy rich structures inside the amorphous network, and the variety of atoms, such as Ar, H2O, and weakly bonded O and In, is present in the void structures for the films grown at O2-rich and O2-poor conditions, respectively. Corresponding to the structural changes, optical and electrical properties also change at r(O2) = 0.375%. For the films grown at r(O2) < 0.375%, we observe a broad absorption in the visible wavelengths that cannot be explained by free carrier absorption. In this film, the carrier mobility reduces rapidly with increasing carrier density. Analysis of spectroscopic ellipsometry and Hall measurements reveals that a large decrease in mobility is due to a large increase in carrier effective mass, in addition to the effect of ionized impurity scattering. In this article, we discuss the optical and transport properties with the variation of oxygen stoichiometry and microscopic structures in the amorphous In2O3:H films. [ABSTRACT FROM AUTHOR]

The composition and structure of an ultra-thin tin oxide film on Rh(100), prepared by the deposition of a submonolayer of tin followed by annealing in an O2 atmosphere, were examined by a combination of low-energy electron diffraction (LEED), Auger electron spectroscopy, X-ray photoemission spectroscopy (XPS), scanning tunneling microscopy (STM), and ab initio calculations based on density functional theory (DFT). Although the LEED pattern exhibited c(2 × 8) spots clearly, a uniform periodicity of the c(2 × 8) unit cell was not observed in the STM images. The bright dots that were observed periodically in the STM image were similar to those of the ultra-thin Sn2O3 film on Rh(111) and formed a zigzag arrangement with the numerous point and line defects. The XPS study revealed that the Sn 3d5/2 peak of the tin oxide film on Rh(100) showed a metallic state as well as an oxide state that was between the SnO2 and SnO states. The structural models, which were based on the Sn2O3 structure on Rh(111), were determined using DFT total energy calculations. The simulated STM images of the two slightly different honeycomb-chain models well reproduced the zigzag arrangement in the STM image. The STM image and XPS spectrum were interpreted using a combination of the two models. [ABSTRACT FROM AUTHOR]

In this work, the optical nonlinear absorption characteristics of the crystalline Ge2Sb2Te5 thin films were investigated by performing z-scan measurements with nanosecond laser pulse. The experimental results showed that the nonlinear saturable absorption coefficient was as large as -10-2 m/W for the excitation intensity lower than 0.17 GW/m2. The nonlinear saturable absorption changed to the nonlinear reverse-saturable absorption with a coefficient of about + 10-2 at the excitation intensity of above 0.17 GW/m2. To explore the internal mechanisms, the first-principle theory was employed to calculate the electronic structure, and a five-level structure suitable for explanation of nonlinear absorption reversal was suggested. The theoretical calculation and analysis indicated that for the excitation intensity smaller than 0.17 GW/m2, the contribution to the nonlinear saturable absorption is mainly from band-filling effect; for the excitation intensity larger than 0.17 GW/m2, the nonlinear reverse saturable absorption results from the thermal-induced nonlinearity, which is further confirmed by picosecond laser pulse z-scan measurement and the variable-temperature spectroscopy ellipsometric analysis. [ABSTRACT FROM AUTHOR]

An attempt to improve the reflow characteristics of sputtered Cu films was made by alloying the Cu with various elements. We selected Y, Sb, Nd, Sm, Gd, Dy, In, Sn, Mg, and P for the alloys, and 'the elasto-plastic deformation behavior at high temperature' and 'the filling level of Cu into via holes' were estimated for Cu films containing each of these elements. From the results, it was found that adding a small amount of Sb or Dy to the sputtered Cu was remarkably effective in improve the reflow characteristics. The microstructure and imperfections in the Cu films before and after high-temperature high-pressure annealing were investigated by secondary ion micrographs and positron annihilation spectroscopy. The results imply that the embedding or deformation mechanism is different for the Cu-Sb alloy films compared to the Cu-Dy alloy films. We consider that the former is embedded by softening or deformation of the Cu matrix, which has a polycrystalline structure, and the latter is embedded by grain boundary sliding. [ABSTRACT FROM AUTHOR]

The oxygen content and the local oxygen arrangement of laser processed YBa2Cu3O7-δ (YBCO) thin films grown on LaAlO3 substrates have been probed by micro-Raman spectroscopy On these films, channels for easy vortex motion have been laser written (LW) into the bridge constrictions by heat-induced, partial depletion of oxygen from the laser-illuminated areas of these films. Raman microscopy has been used to investigate and characterize the oxygen content and structure of the laser written channels. Direct evidence for local micro-structural changes in the film during the LW process is presented and the heat affected zone, surrounding the channel borders, has been mapped. [ABSTRACT FROM AUTHOR]

Scanning near-field photoluminescence spectroscopy has been applied to evaluate bandgap fluctuations in epitaxial AlGaN films with the AlN molar fraction varying from 0.30 to 0.50. A dual localization pattern has been observed. The potential of the small-scale (<100 nm) localization, evaluated from the width of the photoluminescence spectra, is between 0 and 51 meV and increases with increased Al content. These potential variations have been assigned to small-scale compositional fluctuations occurring due to stress variations, dislocations, and formation of Al-rich grains during growth. Larger area potential variations of 25-40 meV, most clearly observed in the lower Al-content samples, have been attributed to Ga-rich regions close to grain boundaries or atomic layer steps. The density, size, and bandgap energy of these domains were found to be composition dependent. The lower bandgap domains were found to be strongly correlated with the regions with efficient nonradiative recombination. [ABSTRACT FROM AUTHOR]

The surface treatment with boiling hydrogen peroxide (H2O2) solution on the surface of ZnO nano-columnar film was investigated. Field emission-SEM and TEM analysis revealed that amorphous ZnO2 layer covers the ZnO nano-column surface through the H2O2 treatment at 100°C for 1 min. X-ray photoemission spectroscopy (XPS) has been conducted on the H2O2 treated ZnO surface. The surface exhibits high resistive conductivity after the H2O2 treatment, suggesting that the treatment promotes a compensation effect. We demonstrate that dramatic improvement in the rectifying behavior on the Schottky diodes can be achieved by inserting a ZnO2 interface layer between the Pt Schottky electrode and the ZnO nano-column film. The ZnO2 interface layer promotes surface passivation and suppresses the surface leakage current. This is expected to increase the Schottky barrier height to 0.78 eV. The H2O2 treated Schottky diode showed five orders of magnitude in current rectification between forward and reverse bias at 3 V. [ABSTRACT FROM AUTHOR]

The optical constants n and k of holmium (Ho) films were obtained in the 3-1340-eV range from transmittance measurements performed at room temperature. Thin films of Ho with various thicknesses were deposited by evaporation in ultra high vacuum conditions and their transmittance was measured in situ. Ho films were deposited onto thin C-film substrates supported on high transmittance grids. Transmittance measurements were used to obtain the extinction coefficient k of Ho films. The refractive index n of Ho was calculated with Kramers-Krönig analysis; in order to do this, k data were extrapolated both on the high and on the low energy parts of the spectrum by using experimental and calculated k values available in the literature. Ho, similar to other lanthanides, has a low-absorption band below the O2,3 edge onset; the lowest absorption was measured at ∼22 eV. Therefore, Ho is a promising material for filters and multilayer coatings in the energy range below the O2,3 edge in which most materials have a large absorption. Good consistency of the data resulted from the application of f and inertial sum rules. [ABSTRACT FROM AUTHOR]

We report on a new coupling scheme for high resolution terahertz spectroscopy of microcrystalline films using parallel-plate waveguides. Metal flares are used to couple the terahertz radiation into and out of the waveguide. Very good coupling ratios as high as 35% at 1 THz from a collimated free-space beam into a subwavelength gap are obtained. This microwave approach is compared in terms of coupling ratio and spectral characteristics to the established technique of quasioptic coupling to parallel-plate waveguides using silicon lenses. Various samples at room and cryogenic temperatures are measured to show the capabilities of flare coupling for high resolution terahertz spectroscopy. [ABSTRACT FROM AUTHOR]

Considerable ambiguity and controversy exist concerning the defect signatures (H1, H2, and H3) frequently observed in admittance spectroscopy of thin-film CdTe solar cells. We prove that the commonly labeled H1 defects, observed in all devices in this study, are actually due to the freeze-out of the majority carriers in the neutral CdTe absorber. This freeze-out is evident in the temperature dependencies of capacitance, carrier concentration, and depletion region width. Contrary to intuitive expectation, the activation energy of freeze-out is less than, not identical to, that of the conductivity. In some other cases, H2 or H3 are observed and attributed to the back-contact potential barrier, rather than to the carrier emission from the traps. We extract the back-contact barrier height from the activation energy of the saturation current determined from the temperature-dependent current-voltage curves using the back-to-back diode model. The back-contact barrier height agrees well with the H2 or H3 energy determined by admittance spectroscopy. We present a more comprehensive and realistic equivalent circuit that includes the admittances from both the back-contact and the neutral absorber. [ABSTRACT FROM AUTHOR]

The optical constants n and k of lutetium (Lu) films were obtained in the 3-1800 eV range from transmittance measurements performed at room temperature. These are the first experimental optical constant data of Lu in the whole range. Thin films of Lu with various thicknesses were deposited by evaporation in ultrahigh vacuum conditions and their transmittance was measured in situ. Lu films were deposited onto grids coated with a thin, C support film. Transmittance measurements were used to obtain the extinction coefficient k of Lu films. The refractive index n of Lu was calculated with Kramers-Krönig analysis. k data were extrapolated both on the high and on the low-energy sides by using experimental and calculated k values available in the literature. Lu, similar to other lanthanides, has a low-absorption band below the O2,3 edge onset; the lowest absorption was measured at ∼25.1 eV. Therefore, Lu is a promising material for filters and multilayer coatings in the energy range below the O2,3 edge in which most materials have a large absorption. Good consistency of the data was obtained through f and inertial sum rules. [ABSTRACT FROM AUTHOR]

The transmittance of dc magnetron-sputtered Mg thin films was measured in the 25-1300 eV spectral range. Freestanding Mg films protected with Al layers were characterized ex situ. Transmittance measurements were used to obtain the extinction coefficient k of Mg films. The obtained k values along with the data available in the literature, and with interpolations and extrapolations for the rest of the spectrum, were used to obtain the real part of the index of refraction n by the Kramers-Krönig analysis. Sum-rule tests indicated a good consistency of the data. [ABSTRACT FROM AUTHOR]

We have studied the modification of electrical properties in poly(3-hexylthiophene) (P3HT) thin film by means of illumination and annealing. The hole mobility of P3HT was monitored by impedance spectroscopy and the density of localized states in the band gap near the Fermi level was calculated using the step-by-step method based on the space-charge-limited-current model. With the illumination, we found that the bulk trap density increased from 2.51 to 2.85×1016 cm-3 eV-1 and the hole mobility of P3HT decreased from 1.88×10-4 cm2 V-1 s-1 to 4.22×10-5 cm2 V-1 s-1. With the annealing at 70 °C for 30 min, it is found that the mobility then partially recovers to 1.18×10-4 cm2 V-1 s-1. [ABSTRACT FROM AUTHOR]

We investigate the film-growth process of the inhomogeneous sputtered TiO2 thin films by the in situ real-time spectroscopic ellipsometer. The growth process of the film is analyzed by both the uniform and the island film growth models. Based on the analyses from the Ψ-Δ trajectories, the initial thin-film growth corresponds to the island film growth model for a single-layer film. As the film grows, the microstructural phase changes cause the transition from the single-to the double-layer physical model, because of the development of the inhomogeneity in the TiO2 thin film. The dynamic fits with the double-layer physical model and the Cody–Lorentz optical model indicate three different stages of the film growth: the nucleation stage, the coalescence stage, and the continuous-layer stage. Although our presented model works well for most of the experimental data, the determination of the refractive index at the ultrathin thickness may be problematic. [ABSTRACT FROM AUTHOR]

We investigated the Schottky barrier height (SBH) behavior of binary alloy Schottky contacts on n-type zinc oxide (n-ZnO) single crystals. Pt–Ru alloy electrodes were deposited on the Zn-polar and O-polar faces of ZnO substrates by combinatorial ion-beam deposition under identical conditions. The crystal structures of the Pt–Ru alloy film changed from the Pt phase (cubic structure) to the Ru phase (hexagonal structure) in the Pt–Ru alloy phase diagram with decreasing Pt content. The SBH, determined from current–voltage measurements, decreased with decreasing Pt content, indicating that the SBH behavior also followed the Pt–Ru alloy phase diagram. The alloy electrodes on the Zn-polar face showed better Schottky properties than those on the O-polar face. Hard x-ray photoelectron spectroscopy revealed a difference in the interface oxidization of the Pt–Ru alloy: the interface of the O-polar face and Pt–Ru mixed phase with poor crystallinity had a more oxidized layer than that of the Zn-polar face. As a result of this oxidization, the O-polar face, Pt–Ru mixed, and Ru phases showed poor Schottky properties. [ABSTRACT FROM AUTHOR]

This paper reports incredibly large and rapid evolution of elastic constants in deposited copper and silver films observed by the resonant-ultrasound spectroscopy. The evolution begins just after stopping the deposition with the temperature dependent recovery rate. To explain the mechanism, we propose a model, where the elastic constants at grain boundary regions increase by 67% at least. Diffusion of atoms along the grain boundary region is a possible reason, and we confirm that the activation energy is much smaller than that for grain-boundary diffusion in bulk materials. These results are explained by drastic structure change at grain boundaries, being similar to phase transition from liquid into solid phase. [ABSTRACT FROM AUTHOR]

Hydrogenated amorphous carbon nitride (a-C:N:H) films were synthesized with CH3CN dielectric barrier discharges (DBD) plasmas. The effects of varying the CH3CN pressure (p) and the frequency of the power supply (f) on the film growth and film properties were studied. The deposited films were characterized using Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy (AFM), and AFM-based nanoindentation. p and f were found to significantly influence the structures, compositions, deposition rates, surface roughness, and nanohardess of deposited a-C:N:H films. The experimental results indicate that dense a-C:N:H films with extremely low surface roughness (rms<1.0 nm) can be deposited with CH3CN DBD plasmas at f=1 kHz and p=∼100 Pa. The deposition systems were in situ characterized by means of optical emission spectroscopy. The emission intensities of major radicals, such as CN (B 2Σ→X 2Σ) and NH (A 3Π→X 3Σ) significantly increased with increasing f or decreasing p. N2 molecules were formed in the residual gas as a stable product, which leads to a decrease in the N/C ratio in deposited a-C:N:H films. The rotational and vibrational temperature of N2 C 3Π states in CH3CN and N2 DBD plasmas were examined and the N2 molecules produced in CH3CN DBD plasmas had the rotational temperature of ∼2000 K and vibrational temperature of ∼500 K. In the N2 DBD plasma, the rotational and vibrational temperature of the N2 molecules were 470±10 and 2850±50 K, respectively. The basic chemical reactions in the gas phase are presented and correlations between the film properties, the gas-phase plasma diagnostic data, and the film growth processes are discussed. [ABSTRACT FROM AUTHOR]

ZnO thin films and (Bu4N)2(Ru)(dcbpyH)2(NCS)2 (called N719) sensitized ZnO thin films are grown on fluorine-doped tin oxide (FTO) conducting glass substrates using laser molecular beam epitaxy. Ultraviolet-visible absorption, photoluminescence (PL), surface photovoltage spectroscopy, and Raman scattering are employed to probe into the transition process of photogenerated charges and the interaction between ZnO and N719. The experimental results indicate that there is a significant electronic interaction between N719 and ZnO through chemiadsorption. The interaction greatly enhances the photogenerated charge separation and thus the photovoltaic response of the ZnO film but remarkedly weakens its radiative recombination, i.e., PL, implying strong energy and charge transfer occurring between N719 and ZnO. In addition, a new PL peak observed at about 720 nm in N719 sensitized ZnO/FTO is attributed to the electron-hole recombination of N719. [ABSTRACT FROM AUTHOR]

This work demonstrates a method to optimize the indium tin oxide (ITO) thin films as front transparent electrode to maximize the efficiency of substrate type amorphous silicon (a-Si) based thin film solar cells. It shows that the total light intensity absorbed by the a-Si layer can be predicted by combining a multilayer optical simulation with the nonuniform solar spectrum and the spectroscopic response of the absorption coefficient of the a-Si film. Consequently, an optimized ITO film can be identified. The photovoltaic performances of experimentally obtained a-Si single junction solar cells confirm the simulation results, indicating an ITO film about 56 nm thick leads to the highest efficiency. Furthermore, it is shown that the ITO films should be deposited at relatively low temperature around 132 °C to avoid damage to the a-Si top p-layer and p-i-n junction. It is found that introducing a small fraction, ∼0.61% flow ratio, of O2 in the sputtering Ar gas reduces the sheet resistivity of the ITO film and improves its transmittance, leading to higher efficiency a-Si solar cells. [ABSTRACT FROM AUTHOR]

We have characterized amorphous to crystalline transformation of hydrogen (H)-doped In2O3 (In2O3:H) films by transmission electron microscopy, thermal desorption spectroscopy, spectroscopic ellipsometry, and Hall measurements. The In2O3:H films that show a mixed-phase structure embedded with small density of crystalline grains in a large volume fraction of amorphous phase have been fabricated at room temperature by the sputtering of an In2O3 ceramic target with introduction of H2O vapor, and the films have been postannealed in vacuum to crystallize the amorphous phase. With increasing annealing temperature up to 200 °C, the film shows a large increase in Hall mobility (μHall) from 42 to 110 cm2/V s and a decrease in carrier density (NHall) from 4.6×1020 to 2.1×1020 cm-3 with slight decrease in resistivity. The change in μHall and NHall with annealing temperature is strongly correlated with the volume fractions of the amorphous and crystalline phases in the films. Analyses of dielectric functions of the films using the Drude model revealed that the high electron mobility in the crystallized films is attributed mainly to longer relaxation time rather than smaller effective mass, as compared with as-deposited films. Temperature-dependent Hall analysis, relationship between NHall and μHall, and comparison between μHall and optical mobility showed that (i) scattering processes inside amorphous and/or crystalline matrices limit the mobility, (ii) doubly charged ionized impurity scattering is reduced by crystallization, and (iii) phonon scattering becomes dominant after crystallization in the In2O3:H films. The above results suggest that H-doping reduces carrier scattering in the crystallized In2O3:H and structural rearrangements during crystallization eliminate oxygen deficiency and generate H+ that acts as a singly charged donor. In this article, we discuss the transport properties with the variation in microscopic and chemical structures in the In2O3:H films. [ABSTRACT FROM AUTHOR]

A series of Cu(In,Ga)Se2 (CIGS) thin film solar cells with differently prepared heterojunctions has been investigated by admittance spectroscopy, capacitance-voltage (CV) profiling, and temperature dependent current-voltage (IVT) measurements. The devices with different CdS buffer layer thicknesses, with an In2S3 buffer or with a Schottky barrier junction, all show the characteristic admittance step at shallow energies between 40 and 160 meV, which has often been referred to as the N1 defect. No correlation between the buffer layer thickness and the capacitance step is found. IVT measurements show that the dielectric relaxation frequency of charge carriers in the CdS layers is smaller than the N1-resonance frequency at low temperatures where the N1 step in admittance is observed. These results strongly contradict the common assignment of the N1 response to a donor defect at or close to the heterointerface. In contrast, an explanation for the N1 response is proposed, which relates the admittance step to a non-Ohmic back-contact acting as a second junction in the device. The model, which is substantiated with numerical device simulations, allows a unified explanation of characteristic admittance, CV, and IVT features commonly observed in CIGS solar cells. [ABSTRACT FROM AUTHOR]

The high-frequency properties of La0.67Ca0.33MnO3 [LCMO(113)] and La0.67Sr1.33MnO4 [LSMO(214)] thin films were investigated by complex impedance spectra (CIS) at various temperatures. The CIS were then analyzed by using an equivalent circuit model to determine the correlation of the magnetic phase transition to the dielectric and transport properties. The dielectric properties of both systems are insensitive to temperature, revealing that the dielectric behavior is independent of magnetic phase transition but strongly associated with the transport properties. The results provide insight into dielectric variation during the phase transition from a ferromagnetic metallic phase and/or an antiferromagnetic insulating phase to a paramagnetic insulating phase. [ABSTRACT FROM AUTHOR]

We have investigated the transport properties and electronic states of W-doped anatase TiO2 (Ti1-xWxO2) fabricated by pulsed-laser deposition. Based on transport and photoemission spectroscopy (PES) measurements, we discuss the conduction mechanism of Ti1-xWxO2, focusing on the valence state of W. The Ti0.95W0.05O2 film deposited under optimized conditions showed a resistivity of 2×10-3 Ω cm at room temperature, which is approximately ten times higher than that of Ti0.94Nb0.06O2. This is mainly due to the lower carrier density in Ti1-xWxO2 films. From PES measurements of Ti0.91W0.09O2 films, we observed the finite density of states originating from O 2p hybridized with W near the top of the valence band. However, we could not find any states at the same position in the Ti0.94Nb0.06O2 films. In addition, the density of states near the Fermi level [N(EF)] was found to be quite low in the Ti0.91W0.09O2 films. Indeed, this is in sharp contrast to Ti0.94Nb0.06O2, which indicates a remarkably high N(EF) value assigned to the bottom of the Ti 3d conduction band. The difference in transport properties between Ti1-xWxO2 and Ti1-xNbxO2 can be attributed to the existence of impurity states that trap carriers associated with doped W atoms. [ABSTRACT FROM AUTHOR]

1% Ni-doped barium strontium titanate (BST) thin film deposited at room temperature reveals the significantly enhanced leakage current performance which is extraordinarily effective for low temperature applications. Significant leakage current suppression of >2 orders was achieved for electric fields from 0.25 to 2 MV/cm in Pt/Ni-doped BST/Pt metal-insulator-metal (MIM) capacitor cells compared to undoped BST. For Ni doping at the 1% level, the spectral dependence of (i) the imaginary part of the complex dielectric constant, [variant_greek_epsilon]2, obtained from the rotating compensator enhanced spectroscopic ellipsometry and (ii) OK1 absorption spectra obtained from synchrotron x-ray absorption spectroscopy shows significant differences (0.26±0.15 eV) in the conduction band edge trap depth relative to undoped BST. The valence band (VB) edge x-ray photoelectron spectroscopy analysis reveals the Fermi energy level downshift of 0.4 eV for Ni-doped BST toward the VB edge. There is a direct correlation between these changes in band edge states of BST thin films with Ni doping and the improved electrical performance in MIM capacitors led by the qualitatively different charge injection mechanism. The proposed transition metal doping process and analysis approach provide a pathway for charge injection control driven by band edge state changes in other perovskite oxides for low temperature (i.e., room temperature) applications. [ABSTRACT FROM AUTHOR]

Field emission performance of boron nitride (BN) island films is studied in terms of cubic phase evolution in plasma-enhanced chemical vapor deposition. Fine-grained island films with large surface roughness can be grown for initial sp2-bonded BN and subsequent cubic BN (cBN) phases by using low-energy (∼20 eV) ion bombardment. Ultraviolet photoelectron spectroscopy reveals that the electron affinity is as low as 0.3 eV for both sp2-bonded BN and cBN phases. The evolution of cBN islands reduces the turn-on field down to around 9 V/μm and increases the current density up to 10-4 A/cm2. The emission is facilitated by the larger field enhancement due to the larger roughness and the higher conduction of cBN islands. The potential barrier height is estimated to be about 3.4 eV for emission from the Fermi level, while it is only about 0.3 eV for “conduction band emission.” [ABSTRACT FROM AUTHOR]

The local structure of a Zn, Sn codoped In2O3 thin film grown on c⁁-plane sapphire by pulsed-laser deposition was examined by polarization-dependent x-ray absorption spectroscopy. The bixbyite film structure is both out-of-plane and in-plane oriented, and the structural results show that both Zn and Sn dopants occupy In sites. The In–O bond length is comparable to that in powder In2O3. However, both Sn–O and Zn–O bonds have two distinct distances in the first shell. Some of the Zn dopants are undercoordinated and, accordingly, some isovalent Sn dopants are overcoordinated for charge balance. In addition, the results suggest that the aliovalent Sn dopants form Frank–Köstlin clusters, (2SnIn•Oi″)x, which provide enough charge carriers to explain the Hall measurements. [ABSTRACT FROM AUTHOR]

The application of ion scattering spectroscopy (ISS) to the in situ measurement of the surface potential developed on an insulator surface under positive ion irradiation was investigated. The ISS spectra measured for a MgO film of 600 nm thickness on a Si substrate by the irradiation of 950 eV He+ ions revealed that the surface is positively charged by approximately 180 V. For accurate measurement of the surface potential, a correction to take into account the angular deflection of primary ions induced by the high surface potential is required. The dependence of the surface potential on the sample temperature revealed that no charging is induced above 700 °C, indicating that accumulated charges can be removed by heating to 700 °C. From the measurement of the ion-induced secondary electron yield using a collector electrode located in front of the sample surface, the surface potential and ion-induced secondary electron yield were found to be strongly affected by the experimental setup. Secondary electrons produced by the impact of slow positive secondary ions, the maximum energy of which corresponds to the surface potential, play an important role when the bias voltage applied to the collector electrode is positively high for the present experimental setup. The surface potential developed on the surface of MgO films of 600 and 200 nm thickness was measured in situ, revealing that the amount of accumulated charges and the time required to attain the steady state of charging are slightly dependent on the beam current of primary ions and strongly dependent on the thickness of the MgO film. The present results confirmed that the application of ISS has high potential for investigating charging phenomena and the secondary electron emission from insulator surfaces under positive ion irradiation. [ABSTRACT FROM AUTHOR]

The valence band offset of the heterojunction between CuInS2 (CIS) and chemical bath deposited CdS has been determined both by means of combined x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) and by near-UV constant final state (CFS) yield spectroscopy. The use of the latter shows that this spectroscopic method is indeed suitable for the investigation of chalcopyrite thin films. The higher information depth due to the low excitation energies (7.5–4.0 eV) used in CFS makes it possible to obtain a signal from the valence band of the CIS substrate even after a relatively thick (∼5 nm) CdS layer has been deposited. The number of samples needed to determine the band offset is reduced from three to one and the effects of chemical changes on the CIS surface can be accounted for because the valence band edges of both materials are measured directly. The valence band offsets with the XPS/UPS and CFS methods were determined to be 1.25±0.20 and 1.45±0.20 eV, respectively. [ABSTRACT FROM AUTHOR]

The atomic-scale interactions of H atoms with hydrogenated amorphous carbon (a-C:H) films were identified using molecular dynamics (MD) simulations and experiments based on surface characterization tools. Realistic a-C:H films developed using MD simulations were impinged with H atoms with a kinetic energy corresponding to a temperature of 700 K. The specific chemical reactions of the H atoms with the a-C:H surface were identified through a detailed analysis of the MD trajectories. The MD simulations showed that hydrogenation occurs primarily at the sp2 sites and converts them to sp3-hybridized C atoms. Depending on the hybridization of the next-nearest neighbor, a dangling bond may or may not be created. The hydrogenation reaction is highly exothermic, >2.5 eV, and proceeds with a negligible activation energy barrier via a mechanism similar to Eley–Rideal. In certain cases hydrogenation may also cleave a C–C bond. The reaction events observed through MD simulations are consistent with the surface characterization of D-exposed a-C:H films using Raman spectroscopy, spectroscopic ellipsometry, and in situ attenuated total reflection Fourier-transform infrared spectroscopy. [ABSTRACT FROM AUTHOR]

We present in this study a spectroscopic investigation of spontaneous delamination of amorphous carbon nitride (a-CNx) films deposited on silicon substrates (Si) by rf magnetron sputtering of a graphite target in pure Ar/N2 gas mixture. The microstructural properties of the films were determined in their as-deposited state using infrared absorption and Raman spectroscopy experiments. Various shapes of debonding patterns have been observed. Based on the present experimental evidence, a description of nitrogen incorporation and the resulting hybridization states of the CN bonds in the a-CNx films prior and after delamination are proposed to elucidate the microstructural origin of this delamination occurring along the a-CNx/Si interface and the physicochemical mechanism that governed its dynamic. [ABSTRACT FROM AUTHOR]

CdS thin films of variable thickness (between 160 and 1200 nm) were prepared using rf magnetron sputtering. X-ray diffraction measurements showed that the films have hexagonal structure and that the crystallites are preferentially oriented with the <002> axis perpendicular to the substrate surface. The results of electrical conductivity measurements as a function of film thickness and of temperature provide evidence that the conductivity is controlled by a thermally activated mobility in the presence of an intergrain barrier. The room temperature barrier height [lowercase_phi_synonym] decreases with the increase in film thickness. Values of [lowercase_phi_synonym] between 0 and 0.25 eV were determined. Photoinduced current transient spectroscopy performed on five samples having different thicknesses showed the presence of 11 traps with activation energies in the range 0.08–1.06 eV; deeper traps being observed on thinner films. By comparison with literature results, seven traps are attributed to native defects and foreign impurities (mainly Cu, Au, and Ag). Four other traps, not previously observed, are attributed to residual defects. The observation that deeper traps are detected in samples with larger barrier heights has been discussed and interpreted in terms of the energy band profile near the grain boundary. [ABSTRACT FROM AUTHOR]

Amorphous erbium-doped silicon oxide (SiyO1-y:Er, y≥1/3) thin films are currently under investigation as a luminescent material system for complementary metal-oxide semiconductor compatible light emitters. We have grown films with y≈1/3 and investigated their properties using both positron annihilation and photoluminescence (PL) spectroscopies. Films were characterized “as deposited,” following irradiation with 1 MeV Si+ ions and after isochronal annealing. The PL yield from both Er3+ ions and sensitizing defects is reduced by irradiation, depending strongly on the irradiation fluence and reaching saturation at ∼4×1013 Si+/cm2. Higher implantation fluences result in an open-volume defect structure in the film that persists after annealing. This annealing behavior is similar to that of an unrecoverable quenching effect on Er3+-related PL near 1540 nm, and we suggest that these open-volume defects may cause a decoupling of the Er3+ ions from sensitizing oxide point defects that form as a result of the film deposition process. [ABSTRACT FROM AUTHOR]

In this work, we use ultrafast optical pump-optical probe spectroscopy to probe the polaron dynamics in La0.7Ca0.3MnO3 (LCMO) thin films. The temporal evolution in transient reflectivity change ΔR/R exhibits two relaxing components: a fast component with a time constant of subpicosecond and a slow component with time constant ranging from tens of picoseconds to hundreds of picoseconds. The amplitude of the fast component, though has been seldom discussed before, exhibits the similar temperature dependence with that of the resistivity and the neutron scattering intensity due to nanoscale correlated polarons. The results strongly suggest that the fast photoinduced reflectivity change may have been due to the photoexcitation and trapping process of correlated Jahn–Teller polarons in the paramagnetic and ferromagnetic phases and, thus, implies the presence of electronic inhomogeneity in LCMO manganites. [ABSTRACT FROM AUTHOR]

The time-resolved optical emission studies of laser ablated carbon plasma in nitrogen ambient are carried out to understand the dynamic of formation of C2 and CN for depositing quality thin films. An optimum formation of C2 is observed at 3 mm from the target surface and 350 ns after the initiation of plasma. However, CN is formed at comparatively larger distance, 4 mm, and larger delay (850 ns). The expanding plasma plume splits/bifurcates into slow and fast moving two distinct components, with the faster components further splitting up at later times (>350 ns) into two components. The estimated plume front velocity from plume imaging in the nitrogen ambient of 1.2 mbars at a delay of 100 ns is 3.8×106 cm/s, consistent with spectroscopic measurement ∼3.0×106 cm/s. The CNx thin films deposited by pulsed laser deposition technique are characterized by x-ray diffraction, atomic force microscopy, and Raman spectroscopy. The high ID/IG ratio and peak position shift of G band to lower values in the films deposited for longer time indicates the high incorporation of nitrogen and increasing CN concentration within the film. [ABSTRACT FROM AUTHOR]

Thickness-modulated optical dielectric constants and band alignments of HfOxNy films grown by sputtering have been investigated by spectroscopic ellipsometry (SE) and x-ray photoelectron spectroscopy. Based on SE measurements, it has been noted that an increase in optical dielectric constant and band gap has been observed as a function of the film thickness. Analyses of thickness-dependent band alignment of the HfOxNy/Si system indicate that the valence band offset increases, but only slight change in the conduction band offset, resulting from the thickness-induced change in the structure. The suitable optical dielectric constants and band offsets relative to Si make sputtering-derived HfOxNy film a promising candidate for high-k gate dielectrics. [ABSTRACT FROM AUTHOR]

Epitaxial thin films of LaMnO3 were grown by pulsed laser deposition in identical deposition conditions but in situ annealed in different oxygen environments. All the films showed an insulator to metal transition. It also showed a magnetic order that correlates well with an insulator to metal behavior. The transition temperature critically depends on annealing oxygen pressure and increases with increasing oxygen content. It is observed that instead of strain, oxygen content plays a major role in deciding the transport properties. X-ray diffraction and Raman spectroscopy measurements revealed that the increasing oxygen content results in a decrease in unit cell volume along with a reduction in Jahn–Teller distortion. It is shown that the intensities of the two Jahn–Teller activated modes in Raman spectra are highly sensitive to the oxygen stoichiometry of the films and that the intensity ratio can be effectively used to characterize oxygen content in these films. [ABSTRACT FROM AUTHOR]

Copper selenide, CuxSe(x∼2), was examined as a hole-injection layer for low-temperature organic devices. Crystalline CuxSe films grown at room temperature with atomically flat surfaces exhibited metallic conduction with a high electrical conductivity of 4.5×103 S/cm, a hole concentration of 1.4×1022 cm-3, and a mobility of 2.0 cm2/(V s). Analysis of the free carrier absorption using the Drude model estimated the effective mass of a hole as 1.0me. Photoemission spectroscopy measurements of the interfaces between CuxSe and organic hole transport layers, N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine (NPB) and copper phthalocyanine (CuPc), verified that the hole-injection barriers of these interfaces (0.4 eV for NPB and 0.3 eV for CuPc) are smaller than that of a conventional indium tin oxide (ITO) hole-injection electrode/NPB interface (0.6 eV) but are comparable to that of an ITO electrode/CuPc interface (0.3 eV). Hole-only devices using the CuxSe layer as a hole-injection anode exhibited very low threshold voltages (0.4–0.5 V) and nearly Ohmic characteristics. The NPB layer on the CuxSe layer was found to be highly doped at 1017–1019 cm-3, probably due to copper diffusion, while the CuPc layer is nearly intrinsic with a doping concentration lower than 1015 cm-3. These results indicated that a CuxSe film combined with CuPc is a promising candidate for a low-voltage hole-injection anode or a buffer layer in low-temperature devices such as organic light-emitting diodes and thin film transistors. [ABSTRACT FROM AUTHOR]

Variable temperature photoluminescence of ZnO thin films deposited by a reactive laser ablation of metallic zinc was investigated. Free and bound exciton emissions are absent at cryogenic temperature, and the near band edge (NBE) emission is independent of measurement temperature for the ZnO thin film deposited at room temperature. Annealing at 700 °C results in the removal of defects, reappearance of exciton emission, and a temperature dependent NBE emission. The experimental data suggest that defects play an important role in the band edge emission in terms of both spectra shape and temperature dependence. Our observations will have an impact on device applications using ZnO, especially for optoelectronics that utilizes the exciton emission. [ABSTRACT FROM AUTHOR]

We studied the optical properties of as-prepared (amorphous) and thermally crystallized (fcc) flash evaporated Ge2Sb2Te5 thin films using variable angle spectroscopic ellipsometry in the photon energy range 0.54–4.13 eV. We employed Tauc–Lorentz (TL) model and Cody–Lorentz (CL) model for amorphous phase and TL model with one additional Gaussian oscillator for fcc phase data analysis. The amorphous phase has optical bandgap energy Egopt=0.65 eV (TL) or 0.63 eV (CL) slightly dependent on used model. The Urbach edge of amorphous thin film was found to be ∼70 meV. Both models behave very similarly and accurately fit to the experimental data at energies above 1 eV. The CL model is more accurate in describing dielectric function in the absorption onset region. The thickness decreases ∼7% toward fcc phase. The bandgap energy of fcc phase is significantly lower than amorphous phase, Egopt=0.53 eV. The temperature dependent ellipsometry revealed crystallization in the range 130–150 °C. The bandgap energy of amorphous phase possesses temperature redshift -0.57 meV/K (30–110 °C). The crystalline phase has more complex bandgap energy shift, first +0.62 meV/K (150–180 °C) followed by -0.29 meV/K (190–220 °C). The optical properties (refractive index, extinction coefficient, and optical bandgap energy) of as-prepared and fcc flash evaporated Ge2Sb2Te5 thin films are very similar to those values previously reported for sputtered thin films. [ABSTRACT FROM AUTHOR]

The complex permittivity spectra and conductivity of graphite nanoplatelet (GNP)/epoxy composites were measured to assess the application potential of these materials as conductive coatings and/or microwave absorbers with high structural integrity. The frequency used ranged from 0.05 to 18.0 GHz. The measured static conductivity was in fairly good agreement with the prediction from the Neelakanta equations. For the complex permittivity, however, the measured values were much higher than those predicted. A better prediction required the use of the order function as a semiempirical parameter to account for the particle loading effect. The surface treatment using nitric acid was found to increase both the conductivity and permittivity of the GNP/epoxy composites although the effect was small, which led to the decrease in the matching thickness of microwave absorbers. [ABSTRACT FROM AUTHOR]

The electronic and structural properties of copper phthalocyanine deposited onto multiwalled carbon nanotubes were studied as a function of the deposition thickness. The valence band and core level spectra were investigated using photoemission spectroscopy. A shift of the organic highest occupied molecular level toward the Fermi level was observed for very thin film, together with a small shift of the nitrogen and copper core level peak positions. Scanning electron microscope images showed the formation of small particles around the nanotubes, forming a thin overlayer decorating the nanotube walls. An x-ray diffraction analysis reported that these particles present both the α and β crystalline phases. [ABSTRACT FROM AUTHOR]