Numerical simulation of nonlinear properties of fullerene-containing one-dimensional photonic crystal

A model of interaction between a one-dimensional photonic crystal (PC) containing a defect (nonlinear optical layer of C₆₀ fullerene) and radiation with a wavelength ??₀= 1064 nm under steady-state conditions has been considered. This structure is a Fabry-Perot microcavity: a fullerene layer (with a thickness multiple of ??₀/2) is placed between the interference mirrors formed by alternating layers ??₀/4 thick. The PC under consideration (1) has a narrow transmission band in the vicinity of ??₀ against the background of a relatively wide 100%-reflection band (photonic band gap) in the linear mode and (2) provides multiple amplification of the radiation intensity in the intermediate layer with respect to the external radiation intensity. Since C₆₀ fullerene exhibits a significant optical Kerr nonlinearity, the optical thickness of the intermediate layer under irradiation deviates from the value multiple of ??₀/2; as a result, 100% transmission for ??₀ changes to almost 100% reflection at a certain radiation intensity. Thus, this structure behaves as a peculiar optical limiter.     

Determination of the hematocrit of human blood from the spectral values of the coefficients of extinction and small-angle scattering

An easily automated method for the determination of the hematocrit C _V of human blood from the extinction and small-angle scattering coefficients measured at radiation wavelengths of 0.37, 0.76, and 0.98 μm of semiconductor laser sources is proposed. The method does not require the solution of inverse problems and a priori information on the refractive index of erythrocytes. The effects that the measurement errors of the optical characteristics of blood, the dispersion of the refractive index, and the variations in the parameters of the distribution function of erythrocytes have on the determination accuracy of C _V are estimated. The possibility of determining C _V of whole blood is demonstrated.     

Investigation of p-type strained-layer InxGa1−xAs/AlyGa1−yAs quantum well infrared photodetectors for long wavelength infrared imaging arrays applications

Two p-type compressively strained-layer (PCSL) In_xGa_1−xAs/Al_yGa_1−yAs (where,x=0.4, 0.2;y=0, 0.15) quantum well infrared photodetectors (QWIPs) grown by MBE on (100) semi-insulating (SI) GaAs substrates have been investigated for 3-5 μm MWIR and 8-14 μm LWIR imaging arrays applications. The In_0.4Ga_0.6As/GaAs PCSL-QWIP utilizing the resonant transport mechanism between the heavy-hole type-I states and the light-hole type-II states shows a broadband double-peak response between 8 and 9 μm range. Using compressive-strain in the InGaAs quantum well, normal incident absorption was greatly enhanced by reducing the heavy-hole effective mass and increasing the density of states in the off-zone center. Maximum responsivities of 93 mA W⁻¹and 30 mA W⁻¹were obtained at peak wavelengths of λ_p1=8.9 μm and λ_p3=5.5 μm, respectively atV_b=1.6 V, and detectivity (D*) at λ_p1=8.9 μm was found to be 4.0×10⁹cm−√Hz/WatV_b=0.3 V andT=75 K for this QWIP. The In_0.2Ga_0.8As/Al_0.15Ga_0.85As PCSL-QWIP achieves two-color detection with peak wavelengths at 7.4 μm in the LWIR band and 5.5 μm in the MWIR band. This detector is under background limited performance (BLIP) atT=63 K for biases varying from −2.7 V to +3 V. A detectivity (D*) of 1.06×10¹⁰cm √Hz/Wwas obtained at λ_p7.4 μm,V_b1.0 V andT=81 K for this QWIP.     

Effect of thickness and porous structure of SiC layers on the spectral response of the Pd/SiC-pSi Schottky photodiodes

In this work, we study the effect of the thickness and porous structure of silicon carbide (PSC) layers on the electrical properties of Schottky photodiodes by using a palladium (Pd) layer deposited on non-porous silicon carbide (SiC) and porous-SiC (PSC) layers. The non-porous and porous-SiC layers were realized on a p-type silicon (Si(1 0 0)) substrate by pulsed laser deposition using a KrF laser (248 nm) and thermal deposition of a thin Pd layer. The porous structure of the SiC layer deposited was developed by an electrochemical (anodization) method. The electrical measurements were made at room temperature (295 K) in an air ambience. The effect of the porous surface structure and the thickness of the SiC layer were investigated by evaluating electrical parameters such as the ideality factor (n) and barrier height (?_Bp). The thickness of the porous layer significantly affects the electrical properties of the Schottky photodiodes. Analysis of current-voltage (I-V) characteristics showed that the forward current might be described by a classical thermal emission theory. The ideality factor determined by the I-V characteristics was found to be dependent on the SiC thickness a value For a thin SiC layer (0.16 μm) n was around 1.325 with a barrier height 0.798 eV, while for a thick layer (1.6 μm), n and ?_Bp were 1.026 and 0.890 eV, respectively for Pd/SiC-pSi. These results indicate Schottky photodiodes with high performance are obtained for thicker SiC layer and for thin layer of PSC. This effect showed the uniformity of the SiC layer. In the same case the ideality factor (n) decreases for Pd/PSC-pSi(1 0 0) for low SiC thickness by report of Pd/PSC-pSi(1 0 0) Schottky photodiodes, but for Pd/PSC-pSi(1 0 0) n increase for large SiC thickness layer. We notice that the barrier height (?_Bp) was reversely depend by report of ideality factor. A spectral response value of (SR) of 34 mA/W at λ = 400 nm was measured for Pd/0.16 μm SiC-pSi Schottky photodiode with low SiC thickness. On the other hand, a value of SR = 0.14 mA/W at λ = 900 nm was obtained when we used PSC layer (Pd/PSC-pSi(1 0 0)). A reverse behaviour occurs for thicker SiC layer. Finally, it was found that the thickness and surface porous structure have strong effect on sensitivity.     

Structure of the free exciton luminescence band of heteroepitaxial ZnSe/GaAs layers

The exciton reflectance and photoluminescence spectra of epitaxial ZnSe/GaAs layers with a thickness of 2–4 μm are investigated in the temperature range 10–120 K. It is shown that one of the causes of the formation of the doublet structure of the A _n=1 photoluminescence band is interference of the exciton radiation at the boundaries of the near-surface dead layer. Fiz. Tverd. Tela (St. Petersburg) 40, 881–883 (May 1998)     

Investigation of 4H-SiC epitaxial layers implanted by Al ions

4H-SiC epitaxial layers 26 μm thick with N _d ?N _a = 1 × 10¹⁵ cm^?3 grown by the CVD method on 4H-SiC commercial wafers were implanted by Al ions with energy of 100 keV and a dose of 5 × 10¹⁶ cm^?2. To produce the p ⁺?n junction, a rapid thermal annealing for 15 s at 1700°C was used. The obtained samples were studied by the local cathodoluminescence, X-ray diffractometry, and transmission electron microscopy. It was established that under specified conditions of implantation, the width of a region with a high content of radiation defects exceeded by two orders of magnitude, the depth of the projective range of Al ions and was equal to 40 μm. This result is explained by the combined contribution of the radiation enhanced defect diffusion and long-range action effect. A short-term high-temperature annealing resulted in the recrystallization of the specimen surface layer and enhancement of CVD layer structure.     

Giant magnetocaloric effect near the curie temperature in the Sm0.6Sr0.4MnO3 manganite

A method is proposed for the calculation of the magnetocaloric effect from simultaneous measurements of thermal expansion and magnetostriction made in different regimes (adiabatic and isothermal). The magnitude of the magnetocaloric effect for Sm_0.6Sr_0.4MnO₃ is estimated. It is found that near the Curie temperature T _C it passes through a maximum to reach a giant value ΔT=4.6 K for ΔB=0.84 T. In addition, in the neighborhood of T _C, we observed colossal magnetoresistance Δρ/ρ = [ρ(H) ? ρ(0)]/ρ(0) = 72% in a weak magnetic field of 0.84 T, a giant negative volume magnetostriction ω=?5×10^?4 in a field of the same strength, and a large change in the sample volume ΔV/V ≈ 0.1%.     

A new molecular C60 complex with bis(methylenedithio)tetrathiafulvalene (BMDT-TTF): Synthesis, crystal structure, and properties

A new molecular C₆₀ complex of the composition (BMDT-TTF) · C₆₀ · 2CS₂ (I) with the bis(methylenedithio)tetrathiafulvalene (BMDT-TTF) organic donor is synthesized. The molecular and crystal structures of this complex are determined by x-ray diffraction. The (BMDT-TTF) · C₆₀ · 2CS₂ (I) compound crystallizes in a monoclinic crystal system. The main crystal data are as follows: a=13.550(5) Å, b=9.964(7) Å, c=17.125(8) Å, β=99.52(4)°, V=2280(2) ų, M=1229.45, and space group P2₁/m. Crystals of I have a layered structure: layers consisting of C₆₀ molecules alternate with layers composed of BMDT-TTF and CS₂ molecules. It is found that, in complex I, the donor and C₆₀ molecules are linked through the shortest contacts, which leads to a change in the molecular geometry of BMDT-TTF. The donor molecules in a crystal layer are characterized by the shortest S...S contacts. The IR data indicate the electroneutrality of the fullerene molecule. The electrical conductivity of (BMDT-TTF) · C₆₀ · 2CS₂ single crystals is measured using the four-point probe method at room temperature: σ_RT=2×10^?5 Ω^?1 cm^?1.     

Electronic and interface state density distribution properties of Ag/p-Si Schottky diode

Electronic and interface state distribution properties of Ag/p-Si Schottky diode have been investigated. The diode indicates non-ideal current-voltage behavior with an ideality factor greater than unity. The capacitance-voltage (C-V) characteristic is linear in reverse bias indicating rectification behavior and charge density within depletion layer is uniform. From I-V and C-V characteristics, junction parameters such as diode ideality factor and barrier height were found as 1.66 and ?_B(I-V) = 0.84 eV (?_B(C-V) = 0.90 eV), respectively. The interface state density N_ss and relaxation time τ of the Schottky diode were determined by means of Schottky capacitance spectroscopy method. The results show the presence of thin interfacial layer between the metal and semiconductor.     

The fabrication and characterization of 4H SiC power UMOSFETs

The fabrication of 4H-SiC vertical trench-gate metal-oxide-semiconductor field-effect transistors(UMOSFETs) is reported in this paper.The device has a 15-μm thick drift layer with 3×1015 cm-3 N-type doping concentration and a 3.1μm channel length.The measured on-state source-drain current density is 65.4 A/cm2 at Vg = 40 V and VDS = 15 V.The measured threshold voltage(Vth) is 5.5 V by linear extrapolation from the transfer characteristics.A specific on-resistance(Rsp-on) is 181 mΩ·cm2 at Vg = 40 V and a blocking voltage(BV) is 880 V(IDS = 100 μA@880V) at Vg = 0 V.     

Electrical characteristics of polycrystalline Si layers embedded into high-k Al2O3 gate layers

The electrical characteristics of polycrystalline Si (poly Si) layers embedded into high-k Al₂O₃ (alumina) gate layers are investigated in this work. The capacitance versus voltage (C-V) curves obtained from the metal-alumina-polysilicon-alumina-silicon (MASAS) capacitors exhibit significant threshold voltage shifts, and the width of their hysteresis window is dependent on the range of the voltage sweep. The counterclockwise hysteresis observed in the C-V curves indicates that electrons originating from the p-type Si substrate in the inversion condition are trapped in the floating gate layer consisting of the poly Si layer present between the top and bottom Al₂O₃ layers in the MASAS capacitor. Also, current versus voltage (I-V) measurements are performed to examine the electrical characteristics of the fabricated capacitors. The I-V measurements reveal that our MASAS capacitors show a very low leakage current density, compared to the previously reported results.     

Optimization of the structure of an optical fiber polarizer with a metal film at a wavelength of 980 nm

The dependence of the extinction ratio of a 980-nm fiber polarizer with an Al film on the thickness of magnesium fluoride buffer layer has been experimentally and theoretically investigated. At the optimal thickness of the buffer layer, h _b = 0.078 (normalized to the wavelength), extinction ratios of 23 and 41 dB were obtained for polarizers having fiber shell residual layers of 2.01 and 1.607 μm, respectively. The optimal thin-film structure is determined, which will make it possible to design polarizers with extinction ratios up to 100 dB.     

A novel high-voltage light punch-through carrier stored trench bipolar transistor with buried p-layer

A novel high-voltage light punch-through(LPT) carrier stored trench bipolar transistor(CSTBT) with buried p-layer(BP) is proposed in this paper.Since the negative charges in the BP layer modulate the bulk electric field distribution,the electric field peaks both at the junction of the p base/n-type carrier stored(N-CS) layer and the corners of the trench gates are reduced,and new electric field peaks appear at the junction of the BP layer/N drift region.As a result,the overall electric field in the N drift region is enhanced and the proposed structure improves the breakdown voltage(BV) significantly compared with the LPT CSTBT.Furthermore,the proposed structure breaks the limitation of the doping concentration of the N-CS layer(NN CS) to the BV,and hence a higher NN CS can be used for the proposed LPT BP-CSTBT structure and a lower on-state voltage drop(Vce(sat)) can be obtained with almost constant BV.The results show that with a BP layer doping concentration of NBP = 7 × 1015 cm-3,a thickness of LBP = 2.5 μm,and a width of WBP = 5 μm,the BV of the proposed LPT BP-CSTBT increases from 1859 V to 1862 V,with NN CS increasing from 5 × 1015 cm-3 to 2.5 × 1016 cm-3.However,with the same N-drift region thickness of 150 μm and NN CS,the BV of the CSTBT decreases from 1598 V to 247 V.Meanwhile,the Vce(sat) of the proposed LPT BP-CSTBT structure decreases from 1.78 V to 1.45 V with NN CS increasing from 5 × 1015 cm-3 to 2.5 × 1016 cm-3.     

微晶硅薄膜晶体管稳定性研究

对有源区处于结构过渡区的微晶硅底栅薄膜晶体管，测试其偏压衰退特性时，观察到一种“自恢复”的衰退现象.当栅和源漏同时施加10 V的偏压时，测试其源-漏电流随时间的变化，发现源-漏电流先衰减、而后又开始恢复上升的反常现象.而当采用栅压为10 V、源-漏之间施加零偏压的模式时，源-漏电流随时间呈先是几乎指数式下降、随之是衰退速度减缓的正常衰退趋势.就此现象进行了初步探讨. 关键词： 过渡区硅材料 微晶硅薄膜晶体管 稳定性 自恢复衰退     

Efficiency of applying ammonium oxalate for protection of monumental limestone by poultice, immersion and brushing methods

Samples of cretaceous limestone have been treated with three application methods (poultice, immersion and brushing) using different concentrations of ammonium oxalate solution (AmOx) and varying treatment time in order to test the efficiency of surface and in-depth formation of a protective layer of calcium oxalate (CaOx). Synchrotron-based microanalytical techniques (SR-μXRD with 12.5 μm×7.5 μm (H×V) probe size, SR-μFTIR with 10 μm×10 μm and 8 μm×20 μm probe sizes) and laboratory μFTIR, XRD and SEM have been employed for analysis of the treated samples. Synchrotron-based techniques showed variations in the CaOx distribution along the surface on a micrometer scale. All treatments resulted in the development of a CaOx layer with a maximum thickness of approximately 40 μm. Application by the brushing method with 10 1-min applications with 5-min breaks during one hour showed a development of the calcium oxalate layer equivalent to the poultice treatment taking 10 h. This treatment could be preferred for large marble or limestone surfaces where poultice usage is economically not feasible.     

Theoretical modelling of XBn T2SLs InAs/InAsSb/B-AlAsSb mid-wave detector operating below thermoelectrical cooling

The paper reports on the barrier mid-wave infrared InAs/InAsSb (x_Sb = 0.4) type-II superlattice detector operating below thermoelectrical cooling. AlAsSb with Sb composition, x_Sb = 0.97; barrier doping, N_D < 2×10¹⁶ cm^?3 leading to valence band offset below 100 meV in relation to the active layer doping, N_D = 5×10¹⁵ cm^?3 was proved to be proper material not introducing extra barrier in valence band in the analyzed temperature range in XBn architectures. The detectivity of the simulated structure was assessed at the level of ～ 10¹¹ Jones at T ～ 100 K assuming absorber thickness, d = 3 μm. The detector’s architecture for high frequency response operation, τ_s = 420 ps (T ～ 77 K) was presented with a reduced active layer of d = 1 μm.     

The effect of phase difference of subpicosecond IR laser pulses propagating in a GaAs/ZnTe periodic structure on the efficiency of generation of the sum and difference frequencies radiations

The results of theoretical research and numerical simulation of the nonlinear interaction process of mutually orthogonal and linearly polarized IR subpicosecond laser pulses with the one-dimensional periodic structure of the GaAs/ZnTe are presented. In particular, the interaction of such pulses with the equal amplitude electric field of 70.71 MV/m, with the duration of 333 fs at the central wavelength λ₀ = 10 μm, with the periodic structure of the GaAs and the ZnTe with the thickness of layers equal to λ₀/2n_GaAs = 1.527 μm and λ0/4n_ZnTe = 0.931 μm, respectively, and with the number of periods equal to 100 is considered. It is shown that the efficiency of generation of sum and difference frequencies is proportional to the phase difference of the interacting IR pump pulses.     

Bias voltage dependence of inverted magnetoresistance on the annealing temperature in MgO-based magnetic tunnel junctions

MgO-based magnetic tunnel junctions (MTJs) with a layer sequence Ir₂₂Mn₇₈ or Fe₅₀Mn₅₀ (10 nm)/CoFe (2 nm)/Ru (0.85 nm)/CoFeB (0.5?t<2 nm)/MgO (2.5 nm)/CoFeB (3 nm) have been fabricated. The bias voltage dependence of tunneling magnetoresistance (TMR) is given as a function of the annealing temperature for these MTJs, which shows the TMR ratio changes its sign from inverted to normal at a critical bias voltage (V_C) when an unbalanced synthetic antiferromagnetic stack CoFe/Ru/CoFeB is used. V_Cs change with the thickness of the pinned CoFeB and annealing temperature, which implies one can achieve different V_Cs by artificial control. The asymmetric V_C values suggest that a strong density-of-states modification occurs at bottom oxide/ferromagnet interface.     

Coloring of silica glass with silver nanoparticles

The possibility of doping of surface layers of optical fused silica with silver nanogranules upon heating of a thin Ag film on the silica surface by a CO₂ laser beam (P ≈ 30 W, λ = 10.6 μm) is established. The effect of exposure time on the doped layer structure has been investigated. The absorption band of the colloidal solution of Ag in silica has been studied. It is shown that this band is homogeneously broadened and its peak (at 420 nm) corresponds to the small volume filling factor (q < 0.1) at an average granule radius of about 3 nm. Based on the measurements of the radial distributions of the reflectance and refractive index over the doped region at λ = 633 nm, it is revealed that the doped layer is an area-irregular asymmetric step waveguide.