Linear and nonlinear optical properties of multi-layered spherical nano-systems with donor impurity in the center

In this study, the linear, third-order nonlinear and total absorption coefficients (ACs) of multi-layered quantum dot (MLQD) and multi-layered quantum anti-dot (MLQAD) with a hydrogenic impurity are calculated. The analytical and numerical solutions of Schrödinger equation for both MLQD and MLQAD, within the effective mass approximation and dielectric continuum model, are obtained. As our numerical results indicate, an increase in the optical intensity changes the total AC considerably, but the intensity range that leads to these changes is different for MLQAD and MLQD. It is observed that by changing the incident photon energy, the AC curves corresponding to MLQAD and MLQD are of different shapes and behaviors. The peak heights of AC curves corresponding to MLQAD are strongly affected by changing the core antidot radius and the shell thickness values, however in these cases no considerable changes are observed in peak heights of MLQD. Furthermore, in contrast to MLQAD, the photon energies corresponding to total AC peaks of MLQD are more affected by changing the confining potentials (CPs).     

Effect of quantum dots on InAs/GaAs p-i-p quantum dots infrared photodetectors

ABSTRACT

In this paper, the InAs/GaAs p-i-p quantum dots infrared photodetectors (QDIPs) were successfully demonstrated by Apsys software. It consists of Al_0.3Ga_0.7As/GaAs structure to reduce dark current and InAs quantum dots (QDs) embedded in In_0.15Ga_0.85As as an active layer. The effect of structure parameters of InAs QDs on the dark current, photocurrent of the device and SNR (signal to noise) is discussed respectively, including different QDs density, the number of QD layer, GaAs thickness between QDs layers and Al_0.3Ga_0.7As, and GaAs thickness between two the QD layers.     

Effect of band parameters on interband optical absorption in quantum wire structure of low band gap III–V semiconductors

A generalized theory is presented to study the effect of band parameters on inter band optical absorption in quantum wire structure of III–V compound semiconductors considering the wave-vector ( $\vec{k}$ ) dependence of the optical transition matrix element (OME). The band structures of these low band gap semiconducting materials with sufficiently separated split-off valance band are frequently described by the three energy band model of Kane. This has been adopted to calculate the inter band optical absorption coefficient (IOAC) for a wide range of III–V compound semiconductors like, InAs, InSb, Hg_1?x Cd _x Te and In_1?x Ga _x As _y P_1?y lattice matched to InP, having varied split-off energy band compared to their energy band gap. It has been found that IOAC for quantum wires (QWRs) increases in oscillatory manner with increasing incident photon energy and the positions of peaks of oscillation of the coefficient are more closely spaced in the three band model of Kane than those with parabolic energy band approximations reflecting the direct the influence of band energy constants. This effect of band parameters is better revealed from the study of light polarization dependence of the absorption coefficient.     

Non-equilibrium quantum transport theory: current and gain in quantum cascade lasers

We have developed a self-consistent non-equilibrium Green’s function theory (NEGF) for charge transport and optical gain in THz quantum cascade lasers (QCL) and present quantitative results for the I-V characteristics, optical gain, as well as the temperature dependence of the current density for a concrete GaAs/Al_.15Ga_.85As QCL structure. Phonon scattering, impurity, Hartree electron-electron and interface roughness scattering within the self-consistent Born approximation are taken into account. We show that the characteristic QCL device properties can be successfully modeled by taking into account a single period of the structure, provided the system is consistently treated as open quantum system. In order to support this finding, we have developed two different numerically efficient contact models and compare single-period results with a quasi-periodic NEGF calculation. Both approaches show good agreement with experiment as well as with one another.     

High‐responsivity AlGaN–GaN multi‐quantum well UV photodetector

In this paper, we propose a set of AlGaN–GaN multi‐quantum well (MQW) photodetectors based on p‐i‐n heterostructures with 14 AlGaN–GaN MQW structures in i‐region, where GaN quantum well has 6 nm thickness and Al_xGa_1−xN barrier thickness is 3 nm. In this structure, the peak responsivity of 0.19 A/W at 246 nm is reported. In addition, we investigate effects of various parameters on responsivity, and we show that responsivity of MQW‐based photodetectors strongly depends on proper device design, that is, number of quantum wells, well thickness, barrier thickness, and mole fraction. We also show that increasing number of quantum wells, thickness of wells, and mole fraction as well as decreasing thickness of barriers, increase the responsivity. Using obtained results, we proposed optimal structure. Copyright © 2013 John Wiley & Sons, Ltd.     

Monte Carlo simulation of resonant phonon THz quantum cascade lasers

We report on Monte Carlo (MC) simulations aimed at the design and optimization of GaAs-based THz quantum cascade lasers. Results are presented for a GaAs/Al_0.15Ga_0.85As quantum cascade laser design based on LO phonon scattering depopulation, which operates at 2.8 THz. The obtained electron distribution functions in the subbands and the photoluminescence spectra are compared to experimental results. Also the dependence of the inversion and current density on the applied field is investigated, and the parasitic channels are identified based on the intersubband lifetimes.     

Self-constrained sintering of Al2O3/glass/Al2O3 ceramics by glass infiltration

Though need for precise alignment of interlayer patterning in LTCC application, there have been few reports about zero-shrinkage sintering techniques. In this study, ceramic substrate with minimal x–y shrinkage was prepared by glass infiltration method with ‘Al₂O₃/glass/Al₂O₃’ structure. Glass infiltration into alumina particle layer was observed with variation of both sintering temperature (700?≤?T _sint.?≤?900 °C) and alumina particle size distribution (0.5?≤?D ₅₀?≤?1.8 μm). Since glass had low viscosity enough to infiltrate at 700 °C, infiltration started at that temperature and infiltrated up to 20 μm or so with temperature increase, but infiltration depth did not increase noticeably above 750 °C. Based on these results, when sintered at 900 °C with controlled sheet thickness of both glass and alumina, the shrinkage in x–y direction was calculated as less than 0.2%, with 40% in z direction. Dielectric constant (? _r) measured 6.19 with quality factor (Q) of 552 at 1 GHz of frequency. From these results, it is thought that zero-shrinkage ceramic substrates would be obtained without de-lamination.     

NEGF simulation of the RTD bistability

The simulation of I-V characteristics of Al_0.3Ga_0.7As-GaAs and AlAs-GaAs resonant tunneling diodes (RTD) is presented. The nonequilibrium Green function (NEGF) based 1D quantum transport simulator Wingreen is used in our case. The plateau region on the IV characteristics usually present only by the Wigner function equation (WFE) based simulation appeared now by the NEGF simulation of our AlAs-GaAs RTD and its shape is comparable with our experimental measurements. Analysis of our results from point of view of the scattering and geometrical parameters of the RTD structure is presented.     

Synthesis and electrochemical properties of LiNi1−y Zn y O2

Zn doped LiNi_1?y Zn _y O₂ (0.00?≤?y?≤?0.100) composition was synthesized by an emulsion method. The emulsion-derived powder was calcined at the temperature range of 650?～?800 °C for 12?～?48 h. A single phase of LiNi_1?y Zn _y O₂ was obtained at 700 °C. The optimum condition for the synthesis of LiNi_1?y Zn _y O₂ was to be calcined at 750 °C for 36 h in oxygen stream. The composition of LiNi_0.995Zn_0.005O₂ showed the largest discharge capacity and improved cycle life. The initial and final discharge capacities were 163 and 154.5 mAh/g, respectively. The fading rate in discharge capacity after 20 cycles was only 5.2%.     

Phase transitions in Pb[(Ni1/3Sb2/3)xTiyZrz]O3 solid solution ceramics studied by piezoelectric measurements

The piezoelectric properties of Pb[(Ni_1/3Sb_2/3)_xTi_yZr_z]O₃ solid solution, where x+y+z?=?1, x?=?0.08 and y?=?0.44–0.49 have been investigated in a wide temperature region using a resonance technique. The physical properties of PZT near the morphotropic phase boundary modified with the relaxor Pb(Ni_1/3Sb_2/3)O₃ have been studied. The coefficients s ₁₁, k ₃₁ and d ₃₁ have been calculated basing on the damped harmonic oscillator model of piezoelectricly vibrating sample. Several anomalies in the temperature dependence of piezoelectric coefficients within the temperature range from 300 to 600 K have been found. For y?>?0.46 the coexistence of rhombohedral and tetragonal phases occurs. The observed low piezoelectric activity confirms the existence of polar regions above T _m in Pb[(Ni_1/3Sb_2/3)_xTi_yZr_z]O₃.     

Analytical modeling and analysis of Al m Ga1−m N/GaN HEMTs employing both field-plate and high-k dielectric stack for high-voltage operation

In this paper, a new high-voltage Al _m Ga_1?m N/GaN HEMT (High Electron Mobility Transistors) with Field-Plate and high-k dielectric stack, Graded two-dimensional electron gas (2DEG) Channel Field-Plate Stack dielectric (GCFPS) HEMTs structure has been reported. The proposed structure has shown enhancements of the performances of the GaN-based HEMTs taking into account the effects of spontaneous and piezoelectric polarization fields. In order to analyze this structure, a 2D analytical model has been developed where the expressions for 2D channel potential and electric field distribution have been derived. It was shown that the GCFPS design exhibits significantly reduction of the electric field peaks along the 2DEG channel. Therefore, the breakdown voltage (BV) is greatly improved in comparison with the standard AlGaN/GaN FP-HEMTs. The developed model is validated by the good agreement with the 2D simulated data.     

Optical properties of quantum dots versus quantum antidots: Effects of hydrostatic pressure and temperature

The effects of hydrostatic pressure and temperature on the linear, nonlinear and total absorption coefficients (ACs) of a hydrogenic impurity in the center of spherical quantum dot (QD) and quantum antidot (QAD) have been investigated. The comparative approach is used for presenting the results of both models. Our numerical results indicate that for QD nano-systems, by increasing the pressure, the resonance peak positions (RPPs) of ACs shift towards higher energies, while for QAD nano-systems, RPPs of ACs approximately remain unchanged. Furthermore, the larger pressure leads to the smaller height of resonance peak in both models. Also, our results show that the temperature increasing imposes the opposite effect on RPPs than the pressure increasing to the both models.     

Properties of pressure-induced phase transition in Nb-doped PZST ceramics

The structure of Nb-doped Pb_0.99Nb_0.02[(Zr_1?x Sn _x )_1?y Ti _y ]_0.98O₃ ceramics can be transformed from ferroelectric to antiferroelectric phase by using hydrostatic pressure. The dependences on both composition and temperature for the phase transition pressure (p _c) are studied. It is found that the p _c increases with increasing composition (y) of Titanium (Ti) if another composition (x) of Stannum (Sn) is fixed, while the p _c decreases with increasing of the composition (x) of Sn if the composition (y) of Ti is fixed. Regarding the influence of the temperature, it is found that the p _c increases with temperature as a linear function.     

Simulation study of an optimized current matching for In0.39Ga0.61N/In0.57Ga0.43N/In0.74Ga0.26N triple-junction solar cells

This paper deals with the design and optimization of a triple-junction (TJ) solar cell using indium gallium nitride (InGaN) material. Two tunnel diodes are used to ensure connection between the different subcells. A comprehensive study is performed by means of 2D numerical simulations to locate the best bandgap combination that leads to an optimized current matching. During the simulations, the doping concentration and the base thickness are considered as fitting parameters for the top and the middle subcells. The In_0.39Ga_0.61N/In_0.57Ga_0.43N/In_0.74Ga_0.26N bandgap combination is supposed to be 2.02 eV/1.52 eV/1.13 eV. A high short-circuit current density (13.313 mA/cm²) is achieved by assuming a base thickness of 1 µm for each subcell and a p/n doping ratio of 5?×?10¹⁸ cm^?3/5?×?10¹⁵ cm^?3 in the top cell, 1.5?×?10¹⁹ cm^?3/1.5?×?10¹⁶ cm^?3 in the middle cell, and 7.5?×?10¹⁸ cm^?3/7.5?×?10¹⁵ cm^?3 in the bottom cell. The optimized structure has an improved open-circuit voltage (2.877 V), fill factor (83%), and conversion efficiency (33.11%).

     

Characterization of Au/Pb(Zr0.96Ti0.04)O3/Al2O3/Si antiferroelectric field-effect transistors for memory application

Polarization-voltage (P-V) hysteresis loops and polarization retention were studied for Au/Pb(Zr_0.96Ti_0.04)O₃/Al₂O₃/Pt antiferroelectric capacitors with different Al₂O₃ layer thicknesses. The high-field ferroelectric phase after poling can be pertained to zero external field with the choice of an appropriate Al₂O₃ layer thickness. At the same time, a strong depolarization field across the Al₂O₃ layer is generated with the direction opposite to the field across the Pb(Zr_0.96Ti_0.04)O₃ layer. The depolarization-field direction can be reversed with the domain switching of the high-field ferroelectric phase, possessing the potential application of antiferroelectric memories. A large memory window of 10 V was observed for Au/Pb(Zr_0.96Ti_0.04)O₃ (50 nm)/Al₂O₃ (6.3 nm)/n-Si (100) field-effect transistors, as confirmed from the capacitance sweeping under voltages between ?19 and +19 V. The high/low capacitance ratio is over 8:1, and the ratio remains stable with time over 4 h after programming voltage of ±19 V at 80°C, in suggestion of the excellent retention of the memory.     

Optical properties of epitaxial plzt thin films

In the epitaxial (Pb_1?x , La _x )(Zr_1?y , Ti _y )_1?x/4O₃ [PLZT] films, the composition dependence of the refractive index and electric-optic (EO) coefficient near the morphotropic phase boundary (MPB) composition was investigated. A (100/001)-oriented PLZT 10/65/35 epitaxial film is found to have isotropic optical properties. Highly (100/001)-oriented epitaxial PLZT films with compositions near the MPB on Nb–SrTiO₃ substrates were fabricated using a sol–gel process. The value of birefringence from 4?×?10^?3 to 5?×?10^?4 in PLZT epitaxial film was smaller than that of lithium niobate single crystal. The refractive index decreases with increasing lanthanum content. The difference in the refractive index obtained depended upon the lanthanum content up to 2%. This value is adequate for fabrication of waveguide structures. The EO coefficient of PLZT 9/65/35 thin films was 45 pm/V, which is larger than that of lithium niobate single crystal. A very small polarization dependence of the EO coefficient was also observed.     

Electrical and memory window properties of Sr0.8-xBaxBi2.2Ta2-yZryO9 ferroelectric gate in metal-ferroelectric-insulator-semiconductor structure

Electrical characteristics of Sr_0.8-xBa_xBi_2.2Ta_2-yZr_yO₉ ferroelectric films grown on HfO₂/Si wafers by sol–gel spin coating technique were investigated from the viewpoint of application as ferroelectric gates in metal-ferroelectric-insulator-semiconductor (MFIS) stacks. It was observed that the leakage current density level was 10^-8 A/cm² under 14?V for moderate doping ratio. Determined memory windows from C-V characteristics of Sr_0.8Bi_2.2Ta₂O₉ (SBT) and Sr_0.8-xBa_xBi_2.2Ta_2-yZr_yO₉ (x?=?0.04, 0.08, 0.12 and y?=?0.1, 0.2, 0.3) are 0.59, 0.65, 0.75, and 0.86?V at gate sweeping bias of 5?V, respectively. Some part of electronic properties of Sr_0.8-xBa_xBi_2.2Ta_2-yZr_yO₉ with the objective to enhance memory window up to 45?% were discussed. It was interpreted that defects which are formed in Ba and Zr modified SBT affected the electronic processes like leakage current, memory window and charge trapping.     

Crystal Growth and High Temperature Piezoelectricity of La3Ta0.5Ga5.5 − x Al x O14 Crystals

For the application of high temperature piezoelectric devices, Al^{3 +} substituted La₃Ta_0.5Ga_5.5O₁₄ (LTG) crystals with chemical formula of La₃Ta_0.5Ga_{5.5 – x}Al_xO₁₄ (LTGA) were synthesized and grown by -PD (Micro-pulling-down) and Czochralski technique. LTGA compound has been shown to exhibit congruent melting in a wide compositional range without destabilization of the melt during growth, while distribution coefficient of the substituted Al^{3 +} ion was close to unity. LTGA crystals have shown preferable effects for temperature stability on piezoelectric properties in the range from R.T (room temperature) to 500_C.     

Analytical estimation of high-frequency properties of RF micro-inductors prepared by direct-write techniques

In this work, the high-frequency characteristics of micron size, square spiral-type RF thin film air-core inductors prepared by direct-write supersonic jet deposition of laser ablated nanoparticle were estimated by a software package. The important parameters that are determined by direct-write process, such as the materials, the cross-section shape, the thickness, and the conductivity of the coil, were varied systematically to estimate the trend of the inductors’ performances. The area dimensions of RF inductors utilized and the number of coil turns were 1.3 mm?×?1.3 mm and 3, respectively. The 96 wt% Al₂O₃, SiO₂-coated Al₂O₃, and SiO₂-coated Si wafers were used as the substrate material. The high-frequency characteristics of the inductance (L) and quality factor (Q) of the utilized inductors were simulated using a Maxwell three-dimensional field simulator (HFSS 8.5), which employs the finite element method. The used inductors, which have silver metal (Ag) coils, the trapezoid shaped cross-section of the coil, the coil thickness of 30 μm, and the coil conductivity of 70% of Ag bulk value, exhibit L of 8 to 9 nH. They also exhibit a maximum Q of 35.5 near the frequency of 750 MHz and a self-resonant frequency of 5.14 GHz. The simulated high-frequency data of the inductors were agreed well with those obtained from the equivalent circuit model of the utilized inductors.