Raman scattering from confined phonons in GaAs/AlGaAs quantum wires

We report on photoluminescence and Raman scattering performed at low temperature (T =  10 K) on GaAs/Al_0.3Ga_0.7As quantum-well wires with effective wire widths ofL =  100.0 and 10.9 nm prepared by molecular beam epitaxial growth followed by holographic patterning, reactive ion etching, and anodic thinning. We find evidence for the existence of longitudinal optical phonon modes confined to the GaAs quantum wire. The observed frequency at ο_L10 =  285.6 cm⁻¹forL =  11.0 nm is in good agreement with that calculated on the basis of the dispersive dielectric continuum theory of Enderleinas applied to the GaAs/Al_0.3Ga_0.7As system. Our results indicate the high crystalline quality of the quantum-well wires fabricated using these techniques.     

Effects of trench oxide and field plates on the breakdown voltage of SOI LDMOSFET

To improve the breakdown voltage, we propose a SOI-based LDMOSFET with a trench structure in the drift region. Due to the trench oxide and underneath boron implanted layer, the surface electric field in the drift region effectively reduced. These effects resulted in the increment of breakdown voltage for the trenched LDMOS more than 100 V compared with the conventional device. However, the specific on-resistance, which has a trade-off relationship, is slightly increased. In addition to the trench oxide on the device performance, we also investigated the influence of n− drift to n+ drain junction spacing on the off-state breakdown voltage. The measured breakdown voltages were varied more than 50 V with different n− to n+ design spaces and achieved a maximum value at L_DA = 2.0 μm. Moreover, the influence of field plate on the breakdown voltage of trench LDMOSFET was investigated. It is found that the optimum drain field plate over the field oxide is 8 μm.     

A CMOS bandgap reference voltage generator with reduced voltage variation and BJT area

This paper proposes a new bandgap reference (BGR) circuit which adopts a cascode current mirror biasing for reducing the reference voltage variation and a novel sizing method for reducing the PNP BJT area. The proposed BGR was designed and fabricated using 0.18 μm triple-well CMOS process which provides only normal V_TH transistors.The reference voltage variation of BGR was reduced from 0.5 mV (conventional) to 0.09 mV (proposed) using cascode current mirror biasing method. And the ratio of BJT emitter areas was reduced by a factor of 20 through the novel sizing method.     

Microfluidic acoustic trapping force and stiffness measurement using viscous drag effect

It has recently been demonstrated that it was possible to individually trap 70 μm droplets flowing within a 500 μm wide microfluidic channel by a 24 MHz single element piezo-composite focused transducer. In order to further develop this non-invasive approach as a microfluidic particle manipulation tool of high precision, the trapping force needs to be calibrated to a known force, i.e., viscous drag force arising from the fluid flow in the channel. However, few calibration studies based on fluid viscosity have been carried out with focused acoustic beams for moving objects in microfluidic environments.In this paper, the acoustic trapping force (F_trapping) and the trap stiffness (or compliance k) are experimentally determined for a streaming droplet in a microfluidic channel. F_trapping is calibrated to viscous drag force produced from syringe pumps. Chebyshev-windowed chirp coded excitation sequences sweeping the frequency range from 18 MHz to 30 MHz is utilized to drive the transducer, enabling the beam transmission through the channel/fluid interface for interrogating the droplets inside the channel. The minimum force (F_min,trapping) required for initially immobilizing drifting droplets is determined as a function of pulse repetition frequency (PRF), duty factor (DTF), and input voltage amplitude (V_in) to the transducer. At PRF = 0.1 kHz and DTF = 30%, F_min,trapping is increased from 2.2 nN for V_in = 22 V_pp to 3.8 nN for V_in = 54 V_pp. With a fixed V_in = 54 V_pp and DTF = 30%, F_min,trapping can be varied from 3.8 nN at PRF = 0.1 kHz to 6.7 nN at PRF = 0.5 kHz. These findings indicate that both higher driving voltage and more frequent beam transmission yield stronger traps for holding droplets in motion.The stiffness k can be estimated through linear regression by measuring the trapping force (F_trapping) corresponding to the displacement (x) of a droplet from the trap center. By plotting F_trapping – x curves for certain values of V_in (22/38/54 V_pp) at DTF = 10% and PRF = 0.1 kHz, k is measured to be 0.09, 0.14, and 0.20 nN/μm, respectively. With variable PRF from 0.1 to 0.5 kHz at V_in = 54 V_pp, k is increased from 0.20 to 0.42 nN/μm. It is shown that a higher PRF leads to a more compliant trap formation (or a stronger F_trapping) for a given displacement x. Hence the results suggest that this acoustic trapping method has the potential as a noninvasive manipulation tool for individual moving targets in microfluidics by adjusting the transducer’s excitation parameters.     

Recombination luminescence of CsI(Tl) under electron pulse irradiation

The luminescence kinetics of CsI(Tl) exposed to an electron pulse irradiation (E_e = 250 keV, t_1/2 = 10 ns, j = 2 ÷ 160 mJ/cm²) has been studied. It has been discovered that the slow emission rise is due to hole V_k–Tl⁰ recombination luminescence at temperature from 100 to 160 K and electron–V_kA recombination, where electrons released from single Tl⁰ at temperature from 180 to 300 K. The effect of Tl concentration on both processes has been investigated.     

Effect of material parameters on the open-circuit voltage in a GaInAsSb thermophotovoltaic cell

In this paper, a numerical simulation on the open-circuit voltage (V_OC) of the P-GaSb window/P-Ga_0.8In_0.2As_0.18Sb_0.82 emitter/N-Ga_0.8In_0.2As_0.18Sb_0.82 base/N-GaSb structure thermophotovoltaic (TPV) cell is performed and an analysis of the effects of device parameters on V_OC is presented. The simulations are carried out with the fixed spectral control filter and for the radiator temperature of T_rad = 950 °C, cell temperature of T_dio = 27 °C, the radiation photons are injected from the front P-region. The thick P-Ga_0.8In_0.2As_0.18Sb_0.82 emitter with the longer minority carrier diffusion length is the main optical absorption region. The simulated results are compared with the available experimental data, and a good agreement is obtained. The effects of the layer thickness, carrier concentration, injection level and main recombination mechanisms (e.g. the radiative, Auger, bulk Shockley–Read–Hall (SRH) and surface recombination) of the P-Ga_0.8In_0.2As_0.18Sb_0.82 emitter and N-Ga_0.8In_0.2As_0.18Sb_0.82 base on V_OC are analyzed. It indicates that the parameters of the emitter region have stronger effect than that of the base region on V_OC. Dependence of V_OC on the material parameters of P-GaSb window layer is also analyzed, both the carrier concentration and thickness of P-GaSb window layer have effect on V_OC. Moreover, adding a back surface reflector (BSR) to the TPV cell can increase V_OC.     

Nitrogen fixed into HNO3 by pulsed high voltage discharge

By applying pulsed high voltage discharge to a needle-mesh reactor that using seven acupuncture needles as discharge electrode and stainless steel wire mesh as ground electrode, nitrogen from bubbling gas could be fixed into NO₂^? and NO₃^? with equivalent mol H⁺ produced in the liquid phase and a small amount of NO and NO₂ yielded in the gas phase. The HNO₂ was originally formed and then converted into HNO₃. The ·OH and H₂O₂ stimulated the conversion reaction from HNO₂ to HNO₃, which caused HNO₂ concentration increased in the first 12 min and then decreased until lower than its detection limit. The concentration of HNO₃ still increased with discharge time. After 36 min, HNO₃ was the only and ultimate product in the liquid. The total yield of HNO₂ and HNO₃ could be affected by processing parameters such as electric factors of peak voltage and frequency, mesh size of ground electrode and content of nitrogen in N₂/O₂ bubbling. Increasing peak voltage or frequency, the total yield of HNO₂ and HNO₃ increased. Gas composition had a heavy impact on the fixation efficiency that obtained its maximum value at an oxygen content of 66.7% with bubbling O₂/N₂ gas. At the end of the 36 min discharge, the HNO₃ concentration with bubbling air was 2.215 mmol L^?1 at an applied voltage of 25 kV, pulse repetition frequency of 140 Hz and ground electrode mesh of 20 × 20. The energy yield was about 1.22 g (HNO₃)/kWh.     

The construction of a DC high voltage precision divider

A precise DC high-voltage divider was designed and constructed for use as a standard. The ratio of the divider for precise measurement must be known accurately independent of voltage and time. The changes in total resistance of a divider with voltage and time should be considered before design and construction. The divider consists of 100 wire-wound resistors with a total resistance of about 100 MΩ as the high-voltage arm and one wire-wound resistor with resistance of 100 kΩ as the low-voltage arm. The high voltage and ground electrodes were designed to prevent electric field concentration and corona formation at high voltages. A current measuring instrument with 6.5 digit resolution was developed to allow comparison of entering and existing currents for detection of current leakage. Considering the sources of error, a relative uncertainty of 66 ppm (parts per million) was obtained with coverage factor k = 2 for the constructed DC high-voltage divider.     

Electret properties of ethylene–propylene random co-polymer

Thermally stimulated current (TSC) spectra were examined for ethylene–propylene (EP) random co-polymer at different charging voltages V_p with positive and negative polarities. Observed TSC spectra showed two well-separated TSC bands, B_L and B_H, which respectively appeared in the temperature regions below and above 100 °C. Observed V_p dependence of B_L was quite different from that of typical polypropylene homo-polymer: As V_p increased, B_L band grew keeping its peak position same at 65 °C, and the band shape unchanged, as if the traps responsible for the B_L band are a single set of traps with the same trap depth and capture cross section. The trap depth of B_L was about 1.9 eV and 1.7 eV for positively charged EP and talc-containing EP samples, respectively. EP samples also showed unique TSC bands above 100 °C: one is a narrow TSC band peaked at 120 °C and the other is an unusual TSC band which was non-vanishing even at 165 °C just before destruction of samples by their melting. Consequently, the utmost stable charge density in EP co-polymer above 100 °C was found to be 3.5 × 10^?4 C/m² and 6.0 × 10 ^?4 C/m² for positively and negatively charged samples, respectively. These equivalent surface charge densities are much larger than those of usual polypropylene homo-polymer.     

Experimental and theoretical studies on bis(glycine) lithium nitrate (BGLiN): A physico-chemical approach

Good quality and bulk size single crystal (size: 20×13×8 mm³) of bis(glycine) lithium nitrate (BGLiN) was grown by a slow evaporation solution technique from the aqueous solutions at constant temperature i.e. 27 °C using synthesized materials. Crystal system and lattice parameters were determined by single crystals as well as powder X-ray diffraction analysis. The lattice parameters of the titled compound are a=10.0223 Å, b=5.0343 Å, c=17.0510 Å, and V=860.312 Å³ and it crystallized in an orthorhombic system with space group Pca2₁ obtained by single crystal XRD. Elemental composition was confirmed by energy dispersive X-ray spectroscopic analysis. Optical absorption spectrum was recorded and various optical parameters such as optical transmission (~60%), and optical band gap (4.998 eV) were calculated. Photoluminescence study shows that the grown crystal is free from major defects. Crystalline perfection of the grown crystal was assessed and found good. Ground state optimized geometry has been obtained by using DFT with 6-31G(d,p) basis set. HOMO and LUMO energy gap was found to be 6.01 eV and dipole moment was 1.65 D.     

Influence of rapid thermal annealing on electrical and structural properties of double metal structure Au/Ni/n-InP (1 1 1) diodes

The effect of rapid thermal annealing on the electrical and structural properties of Ni/Au Schottky contacts on n-InP have been investigated by current–voltage (I–V), capacitance–voltage (C–V), auger electron spectroscopy (AES) and X-ray diffraction (XRD) techniques. The Au/Ni/n-InP Schottky contacts are rapid thermally annealed in the temperature range of 200–500 °C for a duration of 1 min. The Schottky barrier height of as-deposited Ni/Au Schottky contact has been found to be 0.50 eV (I–V) and 0.86 eV (C–V), respectively. It has been found that the Schottky barrier height decreased with increasing annealing temperature as compared to as-deposited sample. The barrier height values obtained are 0.43 eV (I–V), 0.72 eV (C–V) for the samples annealed at 200 °C, 0.45 eV (I–V) and 0.73 eV (C–V) for those at 400 °C. Further increase in annealing temperature to 500 °C the barrier height slightly increased to 0.46 eV (I–V) and 0.78 eV (C–V) compared to the values obtained for the samples annealed at 200 °C and 400 °C. AES and XRD studies showed the formation of indium phases at the Ni/Au and InP interface and may be the reason for the increase in barrier height. The AFM results showed that there is no significant degradation in the surface morphology (rms roughness of 1.56 nm) of the contact even after annealing at 500 °C.     

Hump structure below Tc in the thermal conductivity of MgB2 superconductor

A reasonable cause of absence of hump structure in thermal conductivity of MgB₂ below the superconducting transition temperature (T_c) lies in the appearance of multigap structure. The gaps of lower magnitude can be suppressed by defects so that this system becomes effectively a single-gap superconductor. When such a situation is created, it is hoped that thermal conductivity (κ) will show hump below T_c. Proceeding along these lines, a sample of MgB₂ with a relatively higher residual resistivity ρ_o = 33.8 μΩ cm has been found to show a hump structure below T_c. The actual electronic thermal conductivity κ_el of this sample is less than that expected from the Wiedeman–Franz law by more than a factor of 2.6 in the considered temperature range.Modifying the Wiedeman–Franz law for the electronic contribution by replacing the Lorenz number L₀ = 2.45 × 10^?8 W Ω K^?2 by an effective Lorenz number L_eff (<L₀) we have obtained two sets of κ_el, namely those with L_eff = 0.1L₀ and 0.2L₀. Corresponding to these two sets of κ_el, two sets of the phonon thermal conductivity κ_ph are obtained. κ_ph has been analyzed in terms of an extended Bardeen–Rickayzen–Tewordt theory. The main result of this analysis is that the hump structure corresponds to a gap ratio of 3.5, and that large electron-point defect scattering is the main source of drastic reduction of the electronic thermal conductivity from that given by the usual Wiedeman–Franz law.     

Antiferromagnetic phase transition in garnet-type AgCa2Mn2V3O12 and NaPb2Mn2V3O12

Ferrimagnetism has been extensively studied in garnets, whereas it is rare to find the antiferromagnet. Present work will demonstrate antiferromagnetism in the two Mn–V-garnets. Antiferromagnetic phase transition in AgCa₂Mn₂V₃O₁₂ and NaPb₂Mn₂V₃O₁₂ has been found, where the magnetic Mn²⁺ ions locate only on octahedral A site. The heat capacity shows sharp peak due to antiferromagnetic order with the Néel temperature T_N=23.8 K for AgCa₂Mn₂V₃O₁₂ and T_N=14.2 K for NaPb₂Mn₂V₃O₁₂. The magnetic entropy change over a temperature range 0–50 K is 13.9 J K^?1 mol-Mn²⁺-ions^?1 for AgCa₂Mn₂V₃O₁₂ and 13.6 J K^?1 mol-Mn²⁺-ions^?1 for NaPb₂Mn₂V₃O₁₂, which are in good agreement with calculated value of Mn²⁺ ion with spin S=5/2. The magnetic susceptibility shows the Curie–Weiss behavior over the range 29–350 K. The effective magnetic moment μ_eff and the Weiss constant θ are μ_eff=6.20 μ_B Mn²⁺-ion^?1 and θ=?34.1 K (antiferromagnetic sign) for AgCa₂Mn₂V₃O₁₂ and μ_eff=6.02 μ_B Mn²⁺-ion^?1 and θ=?20.8 K for NaPb₂Mn₂V₃O₁₂.     

Low-bias,high-temperature operation of an InAs–InGaAs quantum-dot infrared photodetector with peak-detection wavelength of 11.7 μm

We report on a low-bias InAs–InGaAs quantum-dot (QD) infrared photodetector (QDIP) with operating temperature of 150 K. Longwave-infrared (LWIR) detection at the peak wavelength of 11.7 μm was achieved. Peak specific photodetectivity D¹ of 1.7 × 10⁹ and 9.0 × 10⁷ cm Hz^1/2/W were obtained at the operating temperature T of 78 K and 150 K, respectively. A large photoresponsivity of 8.3 A/W and high photoconductive gain of 1100 were demonstrated at a low-bias voltage of V = 0.5 V at T = 150 K. The low-bias and high-temperature performance demonstration based on InAs–GaAs material systems indicates that the QDIP technology is promising for LWIR sensing and imaging.     

A proposed organic Schottky barrier photodetector for application in the visible region

The fabrication and characterization of an organic photodetector (OPD) in the form of ITO coated glass/polycarbazole (PCz)/Al Schottky contact is reported. The device has been fabricated in our laboratory for the first time using the polymer synthesized by us. The device has been subsequently characterized in respect of electrical and optical properties in order to explore its potential for possible use as a detector in the visible region at 650 nm. It is observed that the detector exhibits a reasonably high value of peak detectivity (∼6 × 10⁶ cm Hz^1/2 W⁻¹) near zero bias voltage at V = 0.2 V.     

Investigation of a graded channel InGaAs/GaAs heterostructure transistor

An InGaAS/GaAs heterostructure transistor utilizing a gradedIn_xGa_1 − xAs channel grown by low-pressure metal-olorganic chemical vapor deposition has been demonstrated. A negative differential resistance (NDR) phenomenon is observed. Electron mobilities are significantly improved by using the graded InGaAs channel. For the In composition varying fromx =  0.25 (at the buffer–channel interface) to x =  0.1 (at the spacer–channel interface) structure, a peak extrinsic transconductance of 24.6 S mm ^− 1(atV_DS =  6.5 V,V_GSstep =  − 0.5 mV) and a saturation current density as high as 555 mA mm ^− 1for a gate length of 1.5 μ m are obtained.     

Effects of Ga contents on properties of CIGS thin films and solar cells fabricated by co-evaporation technique

This study examined the effects of Ga content in the CIGS absorber layer on the properties of the corresponding thin films and solar cells fabricated using a co-evaporation technique. The grain size of CIGS films decreased with increasing Ga content presumably because Ga diffusion during the 2nd stage of the co-evaporation process is more difficult than In diffusion. The main XRD peaks showed a noticeable shift to higher diffraction angles with increasing Ga content, which was attributed to Ga atoms substituting for In atoms in the chalcopyrite structure. Band gap energy and the net carrier concentration of CIGS films increased with Ga/(In + Ga) ratios. Regarding the solar cell parameters, the short circuit current density (J_SC) decreased linearly with Ga/(In + Ga) ratios due to the lack of absorption in the long-wavelength portion of the spectrum, while the open circuit voltage (V_OC) increase with those. However, V_OC values at high Ga/(In + Ga) regions (>0.35) was far below than those extrapolated from the low Ga contents regions, finally resulting in an optimum Ga/(In + Ga) ratio of 0.28 where the solar cell showed the highest efficiency of 15.56% with V_OC, J_SC and FF of 0.625 V, 35.03 mA cm⁻² and 0.71, respectively.     

Effect of series resistance on the performance of silicon Schottky diode in the presence of tin oxide layer

The current–voltage (I–V) characteristics of Al/SnO₂/p-Si (MIS) Schottky diodes prepared by means of spray deposition method have been measured at 80, 295 and 350 K. In order to interpret the experimentally observed non-ideal Al/SnO₂/p-Si Schottky diode parameters such as, the series resistance R_s, barrier height Φ_B and ideality factor n, a novel calculation method has been reported by taking into account the applied voltage drop across interfacial oxide layer V_i and ideality factor n in the current transport mechanism. The values obtained for V_i were subtracted from the applied voltage values V and then the values of R_s were recalculated. The parameters obtained by accounting for the voltage drop V_i have been compared with those obtained without considering the above voltage drop. It is shown that the values of R_s estimated from Cheung’s method were strongly temperature-dependent and decreased with increasing temperature. It is shown that the voltage drop across the interfacial layer will increase the ideality factor and the voltage dependence of the I–V characteristics. The interface state density N_ss of the diodes has an exponential growth with bias towards the top of the valance band for each temperature; for example, from 2.37 × 10¹³ eV⁻¹ cm⁻² in 0.70−E_v eV to 7.47 × 10¹³ eV⁻¹ cm⁻² in 0.62−E_v eV for 295 K. The mean N_ss estimated from the I–V measurements decreased with increasing the temperature from 8.29 × 10¹³ to 2.20 × 10¹³ eV⁻¹ cm⁻².     

Electromagnetically Induced Transparency and optical pumping processes formed in Cs sub-micron thin cell

The Electromagnetically Induced Transparency (EIT) effect in a Λ-system formed by Cs atoms (6S_1/2 ? 6P_3/2 ? 6S_1/2) confined in an extremely thin cell (ETC) (atomic column thickness L varies in the range of 800 nm –3 µm is studied both experimentally and theoretically. It is demonstrated that when the coupling laser frequency is in exact resonance with the corresponding atomic transition, the EIT resonance parameters weakly depend on L, which allows us to detect the effect at L = λ = 852 nm. EIT process reveals a striking peculiarity in case of the coupling laser detuned by Δ from the atomic transition, namely the width of the EIT resonance rapidly increases upon an increase in Δ (an opposite effect is observed in centimeter-scale cells). The strong broadening of the EIT resonance for large values of detunings Δ is caused by the influence of atom-wall collisions on dephasing rate of coherence. The influence of the coupling laser on the velocity selective optical pumping/saturation resonances formed in ETC has been also studied. The theoretical model well describes the observed results.     

Optical properties of GaAs/Ga 1-xAlxAs ridge quantum wire: Third-harmonic generation

In this paper, the third-harmonic generation (THG) in GaAs/Ga_1 ? xAl_xAs ridge quantum wires is studied in detail. An analytic expression for the THG is obtained using a compact density matrix approach and an iterative procedure. Numerical calculations are performed for the typical GaAs/Ga_1 ? xAl_xAs ridge quantum wire. The results show that the maximum THG over 10^? 9_? m²_?/V²_? can be obtained. Another important point is that the structural parameters have great influence on the THG in this system.