Optical Conduction Research Articles

Dielectric and Optoelectronic Properties of InSe/CdS/CdSe Heterojunctions

The effect of an InSe substrate on the structural, optical and dielectric properties of CdS/CdSe heterojunctions prepared by physical vapor deposition technique under vacuum pressure of 10−8 bar are reported. The structural analysis carried out by x-ray diffraction revealed a strained type of growth of the CdS/CdSe heterojunction onto the InSe along the $$ a \;{\hbox{and}}\; c $$ -axis of the hexagonal lattice. The lattice mismatches and strained nature of the heterojunctions associated with the InSe participation causes a quantum confinement that results in a red shift in the energy band gap, enhanced near infrared (IR) light absorbability, and valence band offsets of 0.62 eV and 0.53 eV for the InSe/CdS and CdS/CdSe interfaces, respectively. In addition, a pronounced enhancement in the real part of the dielectric constant by 2.5 times is observed at 1.25 eV. Furthermore, the Durde–Lorentz modeling of the optical conductivity of the CdS/CdSe and InSe/CdS/CdSe reveals significant increases in the drift mobility values from 43.8 cm2/Vs at the CdS/CdSe interface to 100.0 cm2/Vs upon replacement of glass by an amorphous InSe substrate. The other optical conduction parameters including the free carrier scattering time at the femtosecond level, the plasmon frequency and the free carrier density are also improved accordingly. The photocurrent illumination intensity dependence for the studied system showed that the presence of InSe increases the photocurrent values and changes the recombination mechanism from sublinear at the surface to trap-assisted recombination. The smart feature of the InSe/CdS/CdSe system is that the structurally controlled quantum confinement results in having mobile photocarriers arising from the enhanced absorbability and large dielectric response in the IR region.

Investigation of the structural and optoelectronic properties of the Se/Ga2S3 heterojunctions

In the current study, the structural and optical properties of the Se/Ga2S3 heterojunctions are investigated by means of X-ray diffraction and ultraviolet–visible light spectrophotometry techniques. The optical interface which was prepared by the physical vapor deposition technique, comprises a polycrystalline orthorhombic selenium layer of thickness of 500 nm coated with amorphous layer of 200 nm thick Ga2S3. The top layer is observed to cause yield stress on the Se layer leading to strained type interface. Optically, the evaporation of Ga2S3 onto selenium blue shifted the energy band gap of Se. The conduction and valence band offsets exhibited values of 1.28 and 0.20 eV, respectively. On the other hand, the optical conductivity spectra which were studied and modeled by the Drude-Lorentz approach in the terahertz frequency domain of 275–675 THz revealed enhanced optical conduction parameters. The use of Se as substrate to Ga2S3 enhanced the drift mobility and plasmon frequency of the Ga2S3. The value of the drift mobility reached 64 cm2/Vs at plasmon frequency of 2.04 GHz. In addition, the Se/Ga2S3 interface are observed to exhibit high biasing dependent photosensitivity to visible light irradiation. Such properties of this interface nominate it for use in optoelectronics including visible light communications.

Dielectric dispersion in InSe/CdS bilayers

In the current study, the effect of the amorphous InSe thin film substrate on the structural, optical and dielectric properties of CdS are investigated. The structural analysis of the bilayers indicated a strained growth of CdS onto InSe leading to decrease in grain size and increase in the dislocation density. The optical measurements have shown that the InSe/CdS exhibits two direct allowed transitions energy band gap values of 2.04 and 1.38 eV, in the high and low absorption regions, respectively. On the other hand, the detailed analysis of the dielectric spectra for the InSe, CdS and InSe/CdS layers has shown that the presence of the InSe substrate significantly improves the optical conduction parameters. Particularly, the Drude-Lorentz modeling for these dielectric systems revealed a drift mobility value of 329 cm2/V for the InSe/CdS bilayer. The deposition of the CdS onto InSe is also observed to shift the plasmon frequency of CdS from 2.49 to 0.77 GHz. The general features of the InSe/CdS as plasmon cavities are promising as it shows its usability for production of optoelectronic devices that exhibit high performance at very high frequencies.

Optical dynamics in the Ag/[formula omitted]-Ga2S3 layer system

In this work, thin films of Ga2S3 are deposited onto 150 nm thick transparent Ag substrate by the physical vapor deposition technique under vacuum pressure of 10−5 mbar. The films are studied by the X-ray diffraction and optical spectrophotometry techniques. It is found that the Ag substrate induced the formation of the monoclinic α-Ga2S3 polycrystals. The transparent Ag substrate also changed the preferred optical transition in Ga2S3 from direct to indirect. It also increased the light absorption by 79 and 23 times at incident light energies of 2.01 and 2.48 eV, respectively. In addition, a red shift in all types of optical transitions is observed. Some the extended energy band tails of Ag appears to form interbands in the band gap of Ga2S3. These interbands strongly attenuated the dielectric and optical conduction parameters. Particularly, an enhancement in the dielectric constant values and response to incident electromagnetic field is observed. The Drude-Lorentz modeling of this interface has shown that the free carrier density, drift mobility, plasmon frequency and reduced electron-plasmon frequency in Ga2S3 increases when the Ag substrate replaced the glass or other metals like Yb, Al and Au. The nonlinear optical dynamics of the Ag/Ga2S3 are promising as they indicate the applicability of this interface for optoelectronic applications.

Dynamic conductivity and partial ionization in dense fluid hydrogen.

A theoretical description for optical conduction experiments in dense fluid hydrogen is presented. Different quantum statistical approaches are used to describe the mechanism of electronic transport in hydrogen's high-temperature dense phase. We show that at the onset of the metallic transition, optical conduction could be described by a strong rise in atomic polarizability, due to increased ionization, whereas in the highly degenerate limit, the Ziman weak scattering model better accounts for the observed saturation of reflectance. The inclusion of effects of partial ionization in the highly degenerate region provides great agreement with experimental results. Hydrogen's fluid metallic state is revealed to be a partially ionized free-electron plasma. Our results provide some of the first theoretical transport models that are experimentally benchmarked, as well as an important guide for future studies.

Temperature and Stress Simulation of 4H-SiC during Laser-Induced Silicidation for Ohmic Contact Generation

We report here on the simulation of temperature and stress evolution of 4H-SiC during laser-induced silicidation to locally generate ohmic contacts between the semiconductor and nickel metallization. The simulation is based on optical free carrier absorption, thermal conduction, and thermal radiation. Our results show that, during laser irradiation, similar temperatures and correspondingly similar contact resistances, as compared to conventional oven-driven annealing processes, are achievable, yet with the advantageous potential to limit the temperature treatment spatially to the desired regions for electrical contacts and without the necessity of heating complete wafers. However, due to temperature gradients during local laser silicidation, thermal induced stress appears, which may damage the SiC wafer. Based on the simulated results for temperature and stress increase, we identify an optimized regime for laser-induced local silicidation and compare it to experimental data and observations.

“Synthesis, Spectral Characterisation, Theoretical Investigations and Nonlinear Optical Properties of Ethyl 4‐(3‐(1, 3‐dimethyl‐1H‐pyrazole‐5‐carboxamido)phenyl)‐1,2,3,4‐tetrahydro‐6‐methyl‐2‐oxopyrimidine‐5‐carboxylate”

AbstractNewly synthesized nonlinear optical material is ethyl4‐(3‐(1,3‐dimethyl‐1H‐pyrazole‐5‐carboxamido)phenyl)‐1,2,3,4‐tetrahydro‐6‐methyl‐2‐oxopyrimidine‐5‐carboxylate (DMPC). It has been characterized experimentally by FT‐IR, FT‐Raman, 1H and 13C‐NMR spectrum and theoretically by DFT method using B3LYP/6‐31G(d,p) level of theory. A detailed interpretation of the vibrational spectrum was comparable with theoretical wavenumber using single scaling factor. The intra‐molecular charge transfers occurring in the molecule have been analyzed by donor‐acceptor interaction from natural bonding orbital analysis. The nonlinear optical activity of the compounds has been calculated, and found that the molecule is a good candidate of nonlinear material. The optical response and conduction nature of the title molecule have band gap of frontier edges were calculated by TD‐DFT calculation and these results studied with UV‐Visible spectrum. Especially the π‐π* bonding interactions were interested to study conjugative property of the molecule. Moreover, molecular electrostatic potential surface of DMPC are useful in investigating the stability and reactivity of the molecule

Effect of fuels on the autocombustion reaction synthesis of nanocrystalline gadolinium sesquioxide (Gd2O3) powder: evaluation of structure, morphology, optical and electrical properties

Nanocrystalline gadolinium sesquioxide (Gd2O3) powders were prepared by the autocombustion technique using three different fuels (citric acid, urea, and oxalyl dihydrazide). The influence of thermodynamic parameters and nature of fuel on structural transformation, phase formation, morphological characteristics, optical and electrical distinctive were comparatively studied for the first time. Gd2O3 prepared using citric acid and urea were well crystalline with cubic structure, whereas oxalyldihydrazide fuel resulted mixed cubic and monoclinic phase. Obtained structural information were further analyzed by computation of reflection intensities from a structural model, often referred to as the Rietveld method. The formation and purity of Gd2O3 nanoparticles were further confirmed by from the Fourier transform infrared (FTIR) spectra. Surface morphology of Gd2O3 favored the formation of large aggregates, instead of loose and porous particles as observed for citric acid and oxalyl dihydrazide. Optical absorption measurements were recorded in the UV-vis-NIR wavelength region and the optical band gap variations with fuel type were discussed. Electrical conductivity mechanisms in the prepared materials were identified using band gap, nearest neighbor hopping, and variable range hopping models. The citric acid- and urea-derived powders only have optical band and nearest neighbor hopping conduction. Magnetization measurements were performed at 300 K, the saturation magnetization, coercivity, and remanence of each sample was calculated and they showed paramagnetism. Photoluminescence spectra of Gd2O3 nanoparticles have been studied and observed spectroscopic features have been correlated with various transitions of Gd3+ ions.

Nanocrystalline copper sulfide and copper selenide thin films with p-type metallic behavior

Copper chalcogenide materials are interesting for multiple applications due to the feasibility of suiting their optical absorption and electrical conduction by the creation of copper vacancies. Here, Cu x S and Cu x Se nanocrystalline films with p-type conductivity have been obtained by heating evaporated copper layers of various thicknesses with elemental sulfur or selenium, at temperatures ranging from 250 to 400 °C. These preparation parameters determine the composition and the crystalline structure of the samples, which in turn control their morphology, optical and electrical properties. Thus, the surface roughness increases with the mean crystallite size, whereas the hole concentration increases as the copper atomic proportion (or x value) decreases. Owing to the high carrier densities achieved, around 1022 cm−3, the samples show a metallic behavior with plasmonic absorption in the near infrared and electrical transport dominated by phonon scattering. Apart from such common behavior, some differences have been established between the sulfide and selenide films. One is the superior thermal stability of hexagonal CuS, present in all the temperature range, with respect to hexagonal CuSe, which evolves to cubic Cu1.8Se above 300 °C. Other is about the bandgap, wider for the sulfide than selenide samples.

Surface-modified TiO2 powders with phenol derivatives: A comparative DFT and experimental study

The charge transfer complex formation between TiO2 powder and variety of phenol derivatives (phenol, 4-nitrophenol, 4-bromophenol, 4-tert-butylphenol, hydroquinone) was achieved. The red-shift of optical absorption was observed upon surface modification of TiO2 powders with phenol derivatives. The influence of substituent functional groups in para position on the optical band gap and conduction band edge of inorganic/organic hybrids was studied using reflection spectroscopy and cyclic voltammetry. The experimental findings were supported by density functional theory calculations. The measured reflection spectra of surface-modified TiO2 powders with phenol derivatives were compared with calculated electronic excitation spectra of corresponding model systems.

Growth and characterization of InSe/Ge/InSe interfaces

In the current study, we report the effect of insertion of a 200nm thick Ge film between two layers of InSe. The Ge sandwiched InSe films are studied by means of X-ray diffraction technique, energy dispersion X-ray spectroscopy attached to a scanning electron microscope, optical spectrophotometry and light power dependent photoconductivity. It was observed that, The InSe prefers the growth of InSe monophase when deposited onto glass and the growth of γ−In2Se3 when deposited onto InSe/Ge substrate. The three layers interface (InSe/Ge/γ−In2Se3) exhibits a Ge induced crystallization process at annealing temperature of 200°C. The optical analysis has shown that the InSe films exhibit a redshift upon Ge sandwiching. In addition, the conduction and valence bands offsets at the first interface (InSe/Ge) and at the second (Ge/γ−In2Se3) interface are found to be 0.55eV and 1.0eV, and 0.40eVand 1.38eV, respectively. Moreover, the photocurrent of the Ge sandwiched InSe exhibited higher photocurrent values as compared to those of InSe. On the other hand, the dielectric spectral analysis and modeling which lead to the identifying of the optical conduction parameters presented by the plasmon frequency, electron scattering time, free electron density and drift mobility have shown that the Ge sandwiching increased the drift mobility values from 10cm2/Vs to ∼42cm2/Vs. The main plasmon frequency also increased from 1.08 to 1.68GHz.

Plasmon-electron dynamics at the Au/InSe and Y/InSe interfaces designed as dual gigahertz-terahertz filters

In this work, the X-ray diffraction, the Scanning electron microscopy, the energy dispersive X-ray, the Raman, The UV–vis light and the impedance spectral techniques are employed to explore the structural, vibrational, optical and electrical properties of the Au/InSe and Y/InSe thin film interfaces. It was shown that with its amorphous nature of crystallization, the InSe thin films exhibited n-type conductivity due to the 3% excess selenium. For this form of InSe, the only active Raman spectral line is 121 (cm−1). In addition to the design of the energy band diagram, the analysis the dielectric spectra and the optical conductivities were possible in the frequency range of 270–1000THz. The modeling of the optical conductivities of the Au, Y, Au/InSe and Y/InSe with the help of Lorentz approach for optical conduction, assured that the conduction is dominated by the resonant plasmon-electron interactions at the metals and metals/semiconductors interfaces. It also allowed tabulating the necessary parameters for possible applications in terahertz technology. These parameters are the electron effective masses, the free electron densities, the electron bounded plasmon frequencies, the electron scattering times, the reduced resonant frequencies and the drift mobilities. On the other hand, the impedance spectral analysis of the Y/InSe/Au interfaces in the frequency range of 0.01–1.80GHz, revealed negative capacitance effect associated with band filter features that exhibit maximum transition line at 1.17GHz. This value nominates the interface as a member of filter classes in the gigahertz technology.

Remarkable change of structural, optical, photoluminescence and electrical properties of chemically prepared nanocrystalline films of PbS with concentration of triethanolamine (TEA)

Although triethanolamine (TEA) is widely used as complexing agent in the preparation of nanocrystalline PbS thin film by chemical bath deposition method, not enough studies have been reported on how its concentration affects various properties of the films. In the present work, we studied the effect of concentration of TEA on structural, morphological, optical, photoluminescence and electrical conduction properties of the films. We observed that with the increase of molar concentration of TEA—the crystallinity deteriorates with reduced crystallite size and increased strain and dislocation density; the densities of the films have been reduced; the films become non stoichiometric; optically these become more transparent but less reflecting showing prominent blue shift in the optical band gap which increased with the concentration of TEA; the width of the localized states in the band gap has been increased. The refractive index and extinction coefficient of the films are observed to be wavelength dispersive. The refractive index decreases with the increase of TEA complexation. In the NIR region, the extinction coefficient also decreases as concentration of TEA increases. The real and imaginary parts of dielectric constant replicates the refractive index and extinction coefficient curves respectively against wavelength. The PL spectra reveal conduction band to valence band transition with reduced intensity at higher molarity of TEA. The current–voltage measurement shows that the conductivity decreases considerably with the increase of TEA concentration. Our measurement estimated the crystallite size in the range of 29–37.49 nm, strain ~10−3, dislocation density ~1011 cm−2, band gap 0.8–1.55 eV, bandtail width 0.27–0.57 eV, the refractive index 2.3–5.8 and conductivity 0.0011–0.20 (Ωcm)−1.

Study on optical investigations and DC conduction mechanism in polycrystalline chalcogenide (Cd, Zn) semiconductor films grown by screen-printing method

The use of thin films of polycrystalline chalcogenide semiconductors has attracted much interest in an extensive variety of applications in various electronic, optoelectronic and photovoltaic devices due to their favorable electrical and optical properties. In this paper II–VI compound (cadmium and zinc chalcogenide) binary and ternary semiconductor thick films were deposited on ultraclean glass substrates by screen-printing method followed by sintering process. The polycrystalline nature of these films was confirmed by X-ray diffraction studies. The stoichiometry of the films was confirmed using energy dispersive X-ray analysis. The optical band gap and absorption coefficient of the films were determined by reflectance measurement in wave-length range 350–900 nm using UV/Vis spectrophotometer. The conduction mechanism in these films were studied by measuring the dark DC electrical conductivity in 300–400 K temperature range by using standard two probe method in vacuum. It was observed that conductivity of these films increases with increase in temperature. The linear nature of plot ln σDC against 1000/T for these films indicates that conductivity of films is in close agreement with the Arrhenius relation and conduction in these films is through thermally activated process.

Optical and electrical conduction mechanisms of [N(CH3)3H]CdCl3

The X-ray powder diffraction patterns shows that at room temperature [N(CH3)3H]CdCl3 crystallizes in the orthorhombic system with the Pbnm space group. The analysis of the data revealed the existence of optical allowed direct transition mechanisms with the band gap energy equal to 5.3eV. The temperature dependences of the real part of dielectric permittivity show a relaxation process at high temperature that can be explained by the reorientational motion of alkyl chains. The alternative current (AC) electrical conduction in compound is governed by three processes, which can be attributed to several models: the correlated barrier hopping (CBH) model in phases I and II, the non-overlapping small polaron tunneling (NSPT) model in phases III and IV.

Band offsets and optical conduction in the CdSe/GaSe interface

In this work, the design and characterization of CdSe/GaSe heterojunction is considered. The CdSe/GaSe thin film interface was prepared by the physical vapor deposition technique. Systematic structural and optical analysis were performed to explore the crystalline nature, the optical band gaps, the conduction and valence band offsets, the dielectric spectra, and the frequency dependent optical conductivity at terahertz frequencies. The X-ray diffraction analysis revealed a polycrystalline interface that is mostly dominated by the hexagonal CdSe oriented in the (002) direction. It was also found that the CdSe/GaSe interface exhibits conduction and valence band offsets of 1.35 and 1.23/1.14 eV, respectively. The dielectric spectra displayed two dielectric resonance peaks at 530 and 445 THz. Moreover, the computational fittings of the optical conductivity of the interface revealed a free carrier scattering time of 0.41 (fs) for a free carrier density of 7.0 × 1018 (cm−3). The field effect mobility for the CdSe/GaSe interface was found to be 5.22 (cm2/Vs). The remarkable features of this device having large band offsets and qualitative optical conduction dominated by a scattering time in the order of femtoseconds in addition to the dielectric property nominate the device to be used in optoelectronic technology.

Assessment of structural, optical and conduction properties of ZnO thin films in the presence of acceptor impurities

The structural, optical and electrical conduction properties of (Li/Cu,N):ZnO codoped thin films synthesized by the sol–gel method were investigated by field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), transmission and absorption, photoluminescence (PL) and I–V measurements in order to bring evidence of the formation of acceptor centers by dual-acceptor codoping processes. The (Li 3%,N 5%):ZnO films consist of crystallites with average size of 15 nm, show 95% transmission in the visible region, and an optical band gap of 3.22 eV. The PL spectra show emission maxima at 3.21 and 2.96 eV which are related to the emission of acceptor centers and the presence of defects, respectively. Li occupies interstitial sites and may form Lii–N(O) defect complexes that act as acceptor centers. The (Cu 3%,N 5%):ZnO films consist of crystallites with average size of 12 nm, and exhibit 90% transmission in the visible region. The PL spectra reveal band edge emission at 3.23 eV and defect related emission at 2.74 eV. In the (Cu,N) codoped films, copper substitutes zinc and adopts mainly the Cu1+ state. A possible defect complex involving Cu and N determines the transition from n- to p-type conductivity. These findings are in agreement with results of electronic structure calculations at the GGA-PBE level.

Optical conduction in amorphous GaSe thin films

In this work, the optical conduction mechanism in GaSe thin films was explored by means of dielectric spectral analysis in the 270–1000THz range of frequency. The GaSe films which are found to be amorphous in nature are observed to follow the Lorentz approach for optical conduction. The modeling of the optical conductivity which takes into account the damped electronic motion resulting from the collision of photogenerated carriers with impurities, phonons and other damping sources allowed determining the optical conduction parameters. Particularly, an average carrier scattering time, a free carrier density, a reduced resonant frequency, a field effect mobility and an electron bounded plasma frequency of 0.142 (fs), 1.7×1019 (cm−3), 875.8 (THz), 1.25 (cm2/Vs) and 82.8 (THz), respectively, were determined. These parameters are promising as they indicate the applicability of GaSe in the technology of mid-infrared plasmonic nanoantennas. In addition, the dielectric optical signal which displayed a resonance peak at 500THz seems to be attractive for use in passive modes operating optoelectronic devices like field effect transistors as they exhibit an increasing signal quality factor with decreasing incident light frequency

An Evaluation of Potential Yield of Indium Recycled from Waste Mobile Phone in China

In recent years, users replace their mobile phone more and more frequently, therefore, the number of waste mobile phones has been increasing year by year. In 2014, the number was more than 1 billion units. Indium, that has unique optical transparency and electrical conduction properties, is a necessary element in the screen of mobile phone. The potential yield of indium recycled from waste mobile phone is considerable. Recycling indium from waste mobile phones can alleviate the constraints of indium resources while gaining great economic benefit. However, there is no relevant research work to evaluate the potential yield of indium in the waste cell phone. This paper explores the main factors that constraint the efficiency of recycling by predicting potential yield of indium recycled from waste mobile phone, and puts forward the corresponding policy recommendations. The results are based on the Logistic model and Stock-base model and a survey. The results show that the potential yield of indium recycled from waste mobile phone from 2015 to 2035 will increase from 8.7 tonnes to 10.2 tonnes in China. However, the recovery rate of waste mobile phone is very low, the actual available amount is far lower than the potential yield of indium. The main factors that constrain the efficiency of recycling are: (1) it is difficult to guarantee information security; (2) the existing recycling system is not perfect; (3) the technology of recycling indium needs to be improved.

Facile synthesis, and the optical and electrical properties of nanocrystalline ZnFe2O4thin films

Nanostructured zinc ferrite (ZnFe2O4) thin films with uniform grain size distribution have been synthesized using a citrate–nitrate sol–gel and spin coating method. The formation and impact of intrinsic oxygen vacancies in all of the grown samples have been explained with various structural, optical and electrical properties. Low temperature four probe resistivity measurements imply that the conduction mechanism involved impurity and valance band contributions. The high donor density and oxygen vacancies have improved the photocurrent density of the optimized sample from 36.85 to 65.87 mA cm2. The optimized sample showed a reduced band gap of 2.316 eV and electrochemical impedance spectroscopy under dark and visible light illumination conditions revealed significantly improved charge transfer properties of the samples. These samples have improved optical absorption and electrical conduction with enhanced photocurrent density and have promise for solar photoelectrochemical applications.