Adachi's Model Research Articles

Charge Transport Mechanism in the Forming-Free Memristor Based on PECVD Silicon Oxynitride

A memristor is a new generation memory that merges dynamic random access memory and flash properties. In addition, it can be used in neuromorphic electronics. The advantage of silicon oxynitride, as an active memristor layer, over other dielectrics it is compatibility with silicon technology. It is expected that SiNxOy-based memristors will combine the advantages of memristors based on nonstoichiometric silicon oxides and silicon nitrides. In the present work, the plasma-enhanced chemical vapor deposition (PECVD) method was used to fabricate a silicon oxynitride-based memristor. The memristor leakage currents determine its power consumption. To minimize the power consumption, it is required to study the charge transport mechanism in the memristor in the high-resistance state and low-resistance state. The charge transport mechanism in the PECVD silicon oxynitride-based memristor in high and low resistance states cannot be described by the Schottky effect, thermally assisted tunneling model, Frenkel effect model of Coulomb isolated trap ionization, Hill–Adachi model of overlapping Coulomb potentials, Makram–Ebeid and Lannoo model of multiphonon isolated trap ionization, Nasyrov–Gritsenko model of phonon-assisted tunneling between traps, or the Shklovskii–Efros percolation model. The charge transport in the forming-free PECVD SiO0.9N0.6-based memristor in high and low resistance states is described by the space charge limited current model. The trap parameters responsible for the charge transport in various memristor states are determined. For the high-resistance state, the trap ionization energy W is 0.35 eV, and the trap concentration Nd is 1.7 × 1019 cm−3; for the low-resistance state, the trap ionization energy W is 0.01 eV, and the trap concentration Nt is 4.6 × 1017 cm−3.

Study of the Thermal Properties and Lattice Disorder Effects in CdTe–Based Crystals: CdBeTe, CdMnTe, and CdZnTe

Mixed semiconductor ternary crystals were grown using the Bridgman–Stockbarger method. This is a high–temperature and high–pressure crystal growth method. Cd1–xBexTe crystals were grown in the range of composition 0 < x < 0.1, such as 0.00, 0.01, 0.03, 0.05, and 0.1. The main goal of this paper was to compare the thermal properties of CdBeTe with previously grown CdMnTe and CdZnTe–mixed ternary crystals. The photopyroelectric technique was applied to examine the thermal properties. The thermal diffusivity and effusivity values were obtained after testing all the samples, and the thermal conductivity was calculated then. As such, a complete thermal characterization of the crystals was carried out. For further characterization, the thermal conductivity versus composition was checked by applying the Sadao Adachi model. Thanks to that, we were able to determine the total thermal resistivity of the crystals and the additional resistivity which arises from the lattice disorder. As such, the disorder effects arising from substituting the native atom with a foreign one were characterized for all crystals. We were looking for the best substitution of the Cd atom in the CdTe matrix based on the compounds’ thermal properties. It turned out that Zn and Mn introduce a similar disorder, with Be being the highest one.

Spectroscopic ellipsometry study of Cu2Zn(GexSi1-x)Se4 bulk poly-crystals

The present study addresses to the synthesis and determination of the dielectric function of Cu2Zn(GexSi1-x)Se4 solid solutions with x = 0.4 and 0.8 over the range 1–4.5 eV by spectroscopic ellipsometry analysis, with the aim to achieve a suitable band gap tuning. The dielectric function of the samples is determined using the Adachi model. From the analysis the lowest E0 transition and high energy E1A and E1B transitions are identified. It is found that the band gap varies nonlinearly on composition in the Cu2Zn(GexSi1-x)Se4 alloys and band gap values as large as 1.87 eV are obtained. These results are essential for the design of efficient tailored photovoltaic solar cells and show the high potential of the kesterite compounds for the development of low-cost sustainable future solar cells.

Charge transport mechanism in the forming-free memristor based on silicon nitride

Nonstoichiometric silicon nitride SiNx is a promising material for developing a new generation of high-speed, reliable flash memory device based on the resistive effect. The advantage of silicon nitride over other dielectrics is its compatibility with the silicon technology. In the present work, a silicon nitride-based memristor deposited by the plasma-enhanced chemical vapor deposition method was studied. To develop a memristor based on silicon nitride, it is necessary to understand the charge transport mechanisms in all states. In the present work, it was established that the charge transport in high-resistance states is not described by the Frenkel effect model of Coulomb isolated trap ionization, Hill–Adachi model of overlapping Coulomb potentials, Makram–Ebeid and Lannoo model of multiphonon isolated trap ionization, Nasyrov–Gritsenko model of phonon-assisted tunneling between traps, Shklovskii–Efros percolation model, Schottky model and the thermally assisted tunneling mechanisms. It is established that, in the initial state, low-resistance state, intermediate-resistance state and high-resistance state, the charge transport in the forming-free SiNx-based memristor is described by the space charge limited current model. The trap parameters responsible for the charge transport in various memristor states are determined.

Bandgap and refractive index estimates of InAlN and related nitrides across their full composition ranges

III-Nitride bandgap and refractive index data are of direct relevance for the design of (In, Ga, Al)N-based photonic and electronic devices. The bandgaps and bandgap bowing parameters of III-nitrides across the full composition range are reviewed with a special emphasis on InxAl1−xN, where less consensus was reached in the literature previously. Considering the available InAlN data, including those recently reported for low indium contents, empirical formulae for InAlN bandgap and bandgap bowing parameter are proposed. Applying the generalised bandgap data, the refractive index dispersion data available in the literature for III-N alloys is fitted using the Adachi model. For this purpose, a formalism involving a parabolic dependence of the Adachi parameters on the dimensionless bandgap {xi }_{{E}_{mathrm{g}}}=left({E}_{mathrm{g}, {mathrm{A}}_{x}{mathrm{B}}_{1-x}mathrm{N}}-{E}_{mathrm{g},mathrm{ BN}}right)/left({E}_{mathrm{g}, mathrm{AN}}-{E}_{mathrm{g},mathrm{ BN}}right) of the corresponding ternary alloys is used rather than one directly invoking the alloy composition.

Spectroscopic ellipsometry study of Cu2ZnSn(SxSe1-x)4 bulk polycrystals

The pseudo dielectric function of Cu2ZnSn(SxSe1-x)4 [x = 0.35, 0.62, 0.81] bulk polycrystals is determined over the range 1.1–4.6 eV at room temperature from the analysis of spectroscopic ellipsometry data using the Adachi model. From the analysis, the lowest E0 transition and high energy E1A and E1B transitions are clearly identified, and used to follow the evolution of the pseudo dielectric function as a function of the composition. It is shown that the fundamental E0 and high energy E1A transitions can be tuned by increasing the sulfur content over a range of 0.3 eV. These results show the potential of the kesterite compounds for the design of efficient tailored photovoltaic solar cells.

Measurement and Modeling of the Solubility of Dodecylcyclohexane in Supercritical Carbon Dioxide

In developing the application of using supercritical carbon dioxide (sc-CO2) to extract fresh base oil from waste lubricant oil, the equilibrium solubility data is very valuable. In this study, the equilibrium solubilities for one of the single lubricant oil component, dodecylcyclohexane, were determined in the temperature T = 313.2 K, 328.2 K, 343.2 K, 358.2 K, 373 K and at pressures up to 17.76 MPa. Phase equilibrium solubility measurements were made in a high-pressure visual cell with variable volume, based on the static-analytic method. Oil solubility is strongly dependent on the density of carbon dioxide (CO2), which increased with pressure at constant temperature while decreased with temperature at same pressure. The solubility data were correlated using three density-based models: Mendez-Santiago and Teja (MST) equation, del Valle and Aguilera equation, Adachi and Lu equation, the results showed that all of these models can represent the experimental data well. Since the Adachi and Lu model provided the best regression analysis (R2) and the lowest average absolute relative deviation (AARD) than the other two models in the examined experimental range, indicating the nonlinear equation of Adachi and Lu model best agreement with the experimental data.

Collective effects and optical characteristics of CdSexTe1-x

Collective effects and some response characteristics such as piezoelectric stress and strain constants, Fröhlich coupling parameter, optical electronegativity and exciton properties of CdSexTe1-x common-cation ternary alloys have been investigated. The calculations are carried out using a pseudopotential approach under a modified virtual crystal approximation that takes into account the alloy disorder effect, combined with the Adachi model. Generally, a good accord is obtained between our results and those available in the literature. The composition dependence of the features of interest shows that the Fröhlich coupling parameter remains almost constant, the piezoelectric properties vary almost linearly and the optical properties exhibit a non-linear behavior.

Piezoelectric and electromechanical coupling constants for GaxIn1-xSb semiconducting alloys

ABSTRACTThe compositional dependence of piezoelectric stress and strain constants and electromechanical coupling coefficient of GaxIn1-xSb have been investigated. The calculations were performed using a pseudopotential approach combined with the Adachi model where different concepts of ionicity are considered. Our results are compared with the available data and found to be in good accord with each other. Our findings show that as the Ga content varies, the piezoelectric and related constants and transport properties of the alloy system of interest are significantly affected.

Solubility Modelling pf Butterfly Wing Leaves (Christia Vespertilionis) in Supercritical Carbon Dioxide

Medicinal herb Christia vespertilionis oil has been claimed to possess anti-cancer and anti-plasmodial properties and show interest in food and pharmaceutical industries. Being an important alternative medicine plant, solubility data of Christia vespertilionis oil is demanded in order to understand the separation process and is crucial for designing purposes. In this work, extraction and determination of the oil’s solubility were carried out using a green technique of supercritical carbon dioxide at a temperature range of 40 to 60℃ and 276 to 414 bar of pressure. The results demonstrated that the highest solubility was obtained at the highest temperature and pressure of 60℃ and 414 bar, respectively. For solubility prediction, experimental data were modelled using four empirical density-based models: Chrastil, del Valle and Aguilera, Adachi and Lu, and Sparks et al. models. In general, all the models were able to predict the solubility of Christia vespertilionis oil in supercritical carbon dioxide. The best fitting showed that Adachi and Lu model gave the best correlation with the lowest %AARD value of 1.61%.

Optimization of the Electronic Band Structure and the Lattice Thermal Conductivity of Solid Solutions According to Simple Calculations: A Canonical Example of the Mg2Si1–x–yGexSny Ternary Solid Solution

The dependence of the electronic band structure of Mg2Si0.3–xGexSn0.7 and Mg2Si0.3GeySn0.7–y (0 ≤ x, and y ≤ 0.05) ternary solid solutions on composition and temperature is explained by a simple linear model, and the lattice thermal conductivity of solid solutions with different Si/Ge/Sn ratios is predicted by the Adachi model. The experimental results show excellent consistency with the calculations, which suggests that the approach might be suitable for describing the electronic band structure and the lattice thermal conductivity of other solid solutions using these simple calculations. Beyond this, it is observed that the immiscible gap in the Mg2Si1–xSnx binary system is narrowed via the introduction of Mg2Ge. Moreover, for the Sb-doped solid solutions Mg2.16(Si0.3GeySn0.7–y)0.98Sb0.02 (0 ≤ y ≤ 0.05), the energy offset between the light conduction band and the heavy conduction band at higher temperatures (500–800 K) will decrease with an increase in Ge content, thus making a contribution to the conduc...

Spectroscopic ellipsometry study of Cu2ZnSnSe4 bulk crystals

Using spectroscopic ellipsometry we investigated and analyzed the pseudo-optical constants of Cu2ZnSnSe4 bulk crystals, grown by the Bridgman method, over 0.8–4.5 eV photon energy range. The structures found in the spectra of the complex pseudodielectric functions were associated to E0, E1A, and E1B interband transitions and were analyzed in frame of the Adachi's model. The interband transition parameters such as strength, threshold energy, and broadening were evaluated by using the simulated annealing algorithm. In addition, the pseudo-complex refractive index, extinction coefficient, absorption coefficient, and normal-incidence reflectivity were derived over 0.8–4.5 eV photon energy range.

Spectroscopic ellipsometry study of Cu2ZnGeSe4 and Cu2ZnSiSe4 poly-crystals

We report the room temperature spectroscopic ellipsometry study of Cu2ZnGeSe4 and Cu2ZnSiSe4 crystals, grown by modified Bridgman technique. Optical measurements were performed in the range 1.2–4.6 eV. The spectral dependence of the complex pseudodielectric functions as well as pseudo- complex refractive index, extinction coefficient, absorption coefficient, and normal-incidence reflectivity of Cu2ZnGeSe4 and Cu2ZnSiSe4 crystals were derived. The observed structures in the optical spectra were analyzed by Adachi's model and attributed to the band edge transitions and higher lying interband transitions. The parameters such as strength, threshold energy, and broadening, corresponding to the E0, E1A and E1B interband transitions, have been determined using the simulated annealing algorithm.

Temperature dependence of electronic band transition in Mn-doped SnO 2 nanocrystalline films determined by ultraviolet-near-infrared transmittance spectra

Mn-doped SnO 2 (SMO) nanocrystalline films with the composition from 2.5 to 12.5% have been prepared on quartz substrates by pulsed laser deposition. The temperature dependence of electronic structures and optical constants in the SMO films have been investigated by transmittance spectra from 5.3 to 300 K. Optical response functions have been extracted by fitting the transmittance spectra in the photon energy range of 0.5–6.5 eV with the Adachi's model. It was found that the absorption edge presents a red-shift trend with increasing Mn composition, and the optical band gap (OBG) is varied between 4.22 and 3.44 eV. Moreover, as the Mn composition increases, the temperature dependence of OBG becomes weaker. The band gap narrowing value [(5.3 K)–(300 K)] has been reduced from 98 to 3 meV and linearly decreases with the Mn composition. The phenomena could be attributed to the transition from low doping level SnO 2 band-like states to Mn-related localized states. Moreover, the Urbach energy shows the degree of the structural disorder, which could be explained by an empirical formulas in different temperature regimes.

Effects of crystalline structural transition on electronic-band structure of chromium-doped indium oxide

The electronic-band structure parameters of chromium-doped indium oxide films with crystalline structural transition were determined by fitting the dielectric functions of Drude and Adachi's model to near-normal reflectance spectra. The evaluated optical bandgap shows a maximum at an intermediate state and decreases respectively toward highly and less ideally crystallized states. We explain this phenomenon by combining the effect of intrinsic bandgap narrowing with the Burstein–Moss shift, both due to the crystalline structural transition. This work provides important insights into electronic-band structures of less ideally structured materials.

Structural, electronic, and optical properties of nanocrystalline As-doped ZnO films on quartz substrates determined by Raman scattering and infrared to ultraviolet spectra

Nanocrystalline As-doped ZnO films with different laser power energy (40 mJ and 55 mJ) and As doping concentrations (C As from 1% to 3%) have been grown on quartz substrates by pulsed laser deposition. The average grain size of the films was calculated from the (002) peak of x-ray diffraction patterns and is estimated to vary from 9 to 13 nm. Electronic transitions and optical properties of the films have been investigated by Raman scattering, far-infrared reflectance, and infrared-ultraviolet spectral transmittance technique. With increasing doping concentration, the A 1 longitudinal optical phonon mode shifts towards the lower energy side and can be described by (564-75C As) cm -1 owing to the increment of free carrier concentration. The E 1 transverse optical phonon frequency is located at about 415 cm -1 and does not show an obvious decreasing trend with the C As. The optical constants in the photon energy range of 0.5–6.5 eV have been extracted by fitting the experimental data with the Adachi's model. The refractive index dispersion in the transparent region can be well expressed by a Sellmeier's single oscillator function. Due to different doping concentration and hexagonal crystalline structure, the optical band gap of the films grown at 40 mJ linearly decreases with increasing As concentration. The phenomena agree well with the results from the theoretical calculations.

Germanium-based negative-index heterostructure for high speed modulation

We show that Ag/Ge/Ag ‘fishnet’-like metal-semiconductor metamaterials can be designed to exhibit negative index material (NIM) behavior around 1.5 µm in spite of interband absorption in Ge becoming substantial in this region. We use the FDTD method with the Adachi model for an accurate account of the dielectric response in Ge. The system is expected to perform faster than Si-based fishnet, which is corroborated by preliminary data.

Temperature dependent model dielectric function of highly disordered Ga0.52In0.48P

We report on the temperature dependence of the dielectric function of Ga0.52In0.48P from room temperature to 500°C, and for photon energies from 0.75eV to 5eV. The undoped, highly disordered Ga0.52In0.48P thin film was grown by metal-organic vapor phase epitaxy lattice matched onto a (001) GaAs substrate. The dielectric function of Ga0.52In0.48P was measured by in-situ spectroscopic ellipsometry, and analyzed using Adachi's composite critical point model. We provide a second-order temperature expansion parameter set for calculation of the Ga0.52In0.48P dielectric function and its temperature dependence, and which may become useful for in situ growth control or optoelectronic device performance evaluation at elevated temperatures. We discuss the temperature-induced shift of critical point transition energy parameters.

Comparative study of tetragonal Cu 2In 7Se 11.5 and trigonal CuIn 5Se 8 by spectroscopic ellipsometry

A comparative study of chalcopyrite-related Cu 2In 7Se 11.5 and CuIn 5Se 8 crystals by spectroscopic ellipsometry is presented. Their complex dielectric function ɛ( ω) = ɛ 1( ω) + iɛ 2( ω) has been determined in the 0.8–4.7 eV photon energy range. The spectral dependence of ɛ 1( ω) and ɛ 2( ω) as well as the complex refractive index, the absorption coefficient and the normal-incidence reflectivity for tetragonal Cu 2In 7Se 11.5 and trigonal CuIn 5Se 8 crystals have been modelled using the Adachi's model for interband transitions. The results are in a good agreement with the experimental data over the entire range of photon energies. The model parameters, including the energies corresponding to the lowest direct gap ( E 0) and higher critical points ( E 1 A , E 1 B ), have been determined using the simulated annealing algorithm. The results show that the tetragonal CuIn 3Se 5, the tetragonal Cu 2In 7Se 11.5 and the trigonal CuIn 5Se 8 crystals belong to different phases of the Cu 4In 2 n Se 2+3 n system, n = 6, n = 7 and n = 10, respectively.

Optical constants of Cu(In1−xGax)5Se8 crystals

Spectroscopic ellipsometry has been used to characterize the dielectric functions of bulk Cu(In1−xGax)5Se8 crystals. Spectra were measured at room temperature over the energy range 0.74–5.2 eV. The dielectric functions as well as the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity have been modeled using a modification of the Adachi model. The results are in a good agreement with the experimental data over the entire range of photon energies. The model parameters (strength, threshold energy, and broadening) have been determined using the simulated annealing algorithm. The transition energies E0 and E1A are found to shift linearly to higher energies as the gallium content increases.