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1.
We have investigated the effects of Bi doping on the crystal structure and high-temperature thermoelectric properties of the n-type layered oxide Ca2MnO4−γ . The electrical conductivity σ and the absolute value of the Seebeck coefficient S were, respectively, found to increase and decrease with Bi doping. The thermal conductivity κ of doped Ca2MnO4−γ is relatively low, 0.5 W/m K to 1.8 W/m K (27°C to 827°C). Consequently, the ZT value, ZT = σS 2 T/κ, increases with Bi doping. The maximum ZT is 0.023 for Ca1.6Bi0.18MnO4−γ at 877°C, which is ten times higher than that of the end member, Ca2MnO4−γ . The increase of ZT mainly results from the considerable increase of σ, which can be explained in terms of structural change. The␣Mn-O(1) and the Mn-O(2) distances in the c-direction and ab-plane, respectively, increase with increasing Bi concentration, indicating that the valence state of Mn ions decreases with the increase of electron carriers in the CaMnO3 layers. In addition, the Mn-O(2)-Mn bond angle increases linearly with Bi doping, leading to an improvement of the electron carrier mobility.  相似文献   

2.
Chemical–mechanical polishing of CdTe and Zn x Cd1−x Te single-crystal surfaces by bromine-evolving compositions based on aqueous solutions of H2O2(HNO3)–HBr–solvent has been investigated. The dependences of the chemical–mechanical polishing rate on the dilution of the base polishing etchant for various organic components have been determined. The surface condition after such polishing has been investigated using profilometry. The polishing etchant compositions for CdTe and Zn x Cd1−x Te single-crystal surfaces and the chemical polishing conditions have been optimized.  相似文献   

3.
Silicon wafers were steam oxidized at temperatures of 550–1000°C and pressures of 0.05–8 atm with H2 16O, H2 18O, and D2 16O. Deuterium (D) and18O profiles in 100–200 nm thick oxide films were measured with Cs+ beam secondary ion mass spectroscopy (SIMS). The use of D and18O isotopes enabled analysis of these elements without interference from the sputtering ambient. Peaks in the D profiles near the interface are due predominantly to abrupt changes in the ion yield and charging conditions as the interface is approached. Although the D concentration is nearly constant at ∼ 1 × 1020 cm−3 for 700–1000°C oxidations, it rises to a non-equilibrium value of 6 × 1020 cm−3 at 600°C. Analysis of steam indiffusions below 800°C indicates that the OD concentration is proportional to the (steam pressure)1/2, in agreement with earlier results on as-oxidized wet thermal oxides. The results of sequential isotope oxidations indicate that there is rapid exchange ofboth the hydrogen and oxygen species during transport of “water” to the SiO2. Si interface. We examine these results in terms of the concentrations of the interacting species in the oxide films, and our results are compared to similar reported studies.  相似文献   

4.
Recent work indicates that the alloy (Si2)x(GaAs)1−x can be formed within the GaAs quantum well of an AlxGa1−xAs-GaAs quantum well heterostructure (QWH) and results in a shift of laser operation to higher energy. In this paper we show, by SIMS and EDS measurements, that the Si concentration in the (Si2)x(GaAs)1−x layer far exceeds typical “doping” levels. The stability of these QWHs has been investigated with respect to thermal annealing and Zn impurity-induced layer disordering (Zn-IILD). Data are presented showing that the (Si2)x(GaAs)1−x alloy is stable against thermal annealing unless a rich source of Ga vacancies is provided, and that relatively low temperature Zn diffusion greatly enhances the disordering process of the alloy layer.  相似文献   

5.
The superconducting properties of (M x /YBa2Cu3O7−δy )N multilayer films were studied for varying layer thickness x. Different M phases were examined including green-phase Y2BaCuO5 (211), Y2O3, BaZrO3, CeO2, SmBa2Cu3O7−δ (Sm123), brown-phase La2BaCuO5 (La211), and MgO. Multilayer (M x /YBa2 Cu3O7−δy )N structures were grown by pulsed laser deposition onto SrTiO3 or LaAlO3 single-crystal substrates by alternate ablation of separate YBa2Cu3O7−δ (123) and M targets, at temperatures of 750°C to 790°C. The x layer thickness was varied from 0.1 nm to 4.5 nm, and the y 123 layer thickness was kept constant within a given range of 10 to 25 nm. Different M phase and x layer thicknesses caused large variations of the microstructural and superconducting properties, including superconducting transition (T c), critical current density as a function of applied magnetic field J c(H), self-field J c(77 K), and nanoparticle layer coverage. Strong flux-pinning enhancement up to 1 to 3x was observed to occur for M additions of 211 and BaZrO3 at 65 to 77 K, Y2O3 at 65 K, and CeO2 for H < 0.5 T. BaZrO3 had a noticeably different epitaxy forming smaller size nanoparticles ∼8 nm with 3 to 4x higher areal surface particle densities than other M phases, reaching 5 × 1011 nanoparticles cm−2. To optimize flux pinning and J c (65 to 77 K, H = 2 to 3 T), the M layer thickness had to be reduced below a critical value that correlated with a nanoparticle surface coverage <15% by area. Unusual effects were observed for poor pinning materials including Sm123 and La211, where properties such as self-field J c unexpectedly increased with increasing x layer thickness.  相似文献   

6.
The alloy composition of Hg1−xCdxTe should be controlled during growth, so that the desired band gap and the lattice-matched layer may be obtained. In-situ spectroscopic ellipsometry, now commercially available, enables one to acquire spectral data during growth. If one knows the optical dielectric function as a function of alloy composition and temperature, the technique can be fully used to monitor and control temperature, the thickness, and the alloy composition. For this purpose, we first obtained temperature dependent spectral data of Hg1−xCdxTe by spectroscopic ellipsometry (SE). The spectral data of Hg1−xCdxTe with x = 1,0.235, and 0.344 were obtained from room temperature to 800Kin the photon energy range from 1.3 to 6 eV. The spectral data revealed distinctive critical point structures at E0, E00, E1, E11, E2(X), and E2(Σ). Critical point energies decreased and linewidths increased monotonically as temperature increased. The model for the optical dielectric function enabled (i) the critical point parameters to be determined accurately, and (ii) the spectral data to be expressed as a function of temperature within and outside the experimental range.  相似文献   

7.
Interfacial reactions between the Ba2YCu3O6+x superconductor and the CeO2 buffer layers employed in coated conductors have been modeled experimentally by investigating the kinetics of the reaction between Ba2YCu3O6+x films and CeO2 substrates. At 810°C, the Ba2YCu3O6+x -CeO2 join within the BaO-Y2O3-CeO2-CuO x quaternary system is nonbinary, thereby establishing the phase diagram topology that governs the Ba2YCu3O6+x /CeO2 reaction. At a mole ratio of Ba2YCu3O6+x :CeO2 of 40:60, a phase boundary was found to separate two four-phase regions. On the Ba2YCu3O6+x -rich side of the join, the four-phase region consists of Ba2YCu3O6 +x , Ba(Ce1−z Y z )O3−x , BaY2CuO5, and CuO x ; on the CeO2 rich side, the four phases were determined to be Ba(Ce1−z Y z ) O3−x , BaY2CuO5, CuO x and CeO2. The Ba2YCu3O6+x /CeO2 reaction is limited by solid-state diffusion, and the reaction kinetics obey the parabolic rule, x = Kt 1/2, where x = thickness of the reaction layer, t = time, and K = a constant related to the rate constant; K was determined to be 1.6 × 10−3 μm/s1/2 at 790°C and 4.7 × 10−3 μm/s1/2 at 830°C. The activation energy for the reaction was determined to be E act = 2.67 × 105 J/mol using the Arrhenius equation.  相似文献   

8.
Cubic crystalline silicon-carbon nitride (Si1−x−yCxNy) films have been grown successfully using various carbon sources by rapid-thermal chemical-vapor deposition (RTCVD). The characteristics of the Si1−x−yCxNy films grown with SiH3CH3, C2H4, and C3H8 are examined and compared by x-ray photoelectron spectroscopy (XPS) spectra, scanning electron microscopy (SEM) images, and transmission electron microscopy (TEM) patterns. The XPS spectra show that the differences of chemical composition and chemical-bonding state are co-related to the C bonding type of the different carbon source. The SEM images and TEM analysis indicate that the better Si1−x−yCxNy film can be obtained using C3H8 gas as the carbon source. In addition, correlations between the growing stages to the microstructure of the cubic-crystalline Si1−x−yCxNy films have been illustrated in detail.  相似文献   

9.
The alloy compositions of GaXIn1−XAsyP1−y LPE layers lattice-matched to InP substrates have been determined by electron microprobe analysis. The composition data are well repre-sented by x = 0.40y + 0.067y2. The emission wavelengths of lattice-matched GaXIn1−XAsyP1−y/InP double-heterostructure diode lasers have been measured at 300 and 80 K. The photon energies for laser emission at 300 K are given by hΝ(eV) = 1.307 − 0. 60y + 0.03y2. The emission energies at 80 K are 57 meV higher. This work was sponsored by the Department of the Air Force.  相似文献   

10.
In this work, remote plasma-enhanced chemical vapor deposition (RPCVD) has been used to grow Ge x Si1−x /Si layers on Si(100) substrates at 450° C. The RPCVD technique, unlike conventional plasma CVD, uses an Ar (or He) plasma remote from the substrate to indirectly excite the reactant gases (SiH4 and GeH4) and drive the chemical deposition reactions. In situ reflection high energy electron diffraction, selected area diffraction, and plan-view and cross-sectional transmission electron microscopy (XTEM) were used to confirm the single crystallinity of these heterostructures, and secondary ion mass spectroscopy was used to verify abrupt transitions in the Ge profile. XTEM shows very uniform layer thicknesses in the quantum well structures, suggesting a Frank/ van der Merwe 2-D growth mechanism. The layers were found to be devoid of extended crystal defects such as misfit dislocations, dislocation loops, and stacking faults, within the TEM detection limits (∼105 dislocations/cm2). Ge x Si1−x /Si epitaxial films with various Ge mole fractions were grown, where the Ge contentx is linearly dependent on the GeH4 partial pressure in the gas phase for at leastx = 0 − 0.3. The incorporation rate of Ge from the gas phase was observed to be slightly higher than that of Si (1.3:1).  相似文献   

11.
Strained-layer SiGex-on-Si heteroepitaxy has been achieved by photolytic decomposition of disilane (Si2H6) and digermane (G e2H6) in an ultra high vacuum (UHV) chamber at substrate temperatures as low as 275°C. An ArF excimer laser (193 nm) shining parallel to the Si substrate was used as the UV light source to avoid surface damage and substrate heating. The partial pressures of the source gases in the reactor were chosen to vary the Ge mole fraction x from 0.06 to 0.5 in the alloy. The Si2H6 partial pressure was kept at 10 mTorr and the Ge2H6 partial pressure was varied from 0.13 to 2 mTorr with the laser intensity fixed at 2.75 × 1015 photons/cm2·pulse. To fit the Si1−xGex growth rate and Ge mole fraction data, the absorption cross section of Ge2H6 at 193 nm was set to 1 × 10−16 cm2, which is 30 times larger than that of Si2H6 (3.4 × 10−18 cm2). For Si1−xGex alloy growth, the deposition rate of Si increases with Ge mole fraction, resulting in increased Si1−xGex alloy growth rates for higher Ge content. The increase of the Si growth rate was attributed to the enhanced adsorption rate of Si2H6 pyrolytically in the presence of Ge, rather than due to photolytic decomposition reaction. The Ge mole fraction in Si1−xGex alloys can be predicted by a new model for Si and Ge pyrolytic and photolytic growth. The model describes the increased growth rate of Si1−xGex alloys due to a Ge2H6 catalytic effect during photo-enhanced chemical vapor deposition.  相似文献   

12.
Transparent semiconductor thin films of Zn1−x Mg x O (0 ≤ x ≤ 0.36) were prepared using a sol–gel process; the crystallinity levels, microstructures, and optical properties affected by Mg content were studied. The experimental results showed that addition of Mg species in ZnO films markedly decreased the surface roughness and improved transparency in the visible range. A Zn1−x Mg x O film with an x-value of 0.2 exhibited the best average transmittance, namely 93.7%, and a root-mean-square (RMS) roughness of 1.63 nm. Therefore, thin-film transistors (TFTs) with a Zn0.8Mg0.2O active channel layer were fabricated and found to have n-type enhancement mode. The Zn0.8Mg0.2O TFT had a field-effect mobility of 0.1 cm2/V s, threshold voltage of 6.0 V, and drain current on/off ratio of more than 107.  相似文献   

13.
Interfacial reactions and electrical properties of Hf/p-Si0.85Ge0.15 as a function of the annealing temperature were studied. Hf3(Si1−xGe)2 and Hf(Si1−xGe)2 were initially formed at 500°C and 600°C, respectively. At temperatures above 400°C, Ge segregation out of the reacted layers associated with strain relaxation of the unreacted Si0.85Ge0.15 films appeared. At 780°C, agglomeration occurred in the Hf(Si1−xGex)2 films. All the as-deposited and annealed Hf/p-Si0.85Ge0.15 samples showed the formation of an ohmic contact. The lowest specific contact resistance around 10−5 ω cm2 could be obtained for the Hf3 (Si1−xGex)2 contacts to p-Si0.85Ge0.15 formed at 500°C. Below 500°C, the decrease of specific contact resistance with the annealing temperature is mainly caused by the formation of Hf3(Si1−xGex)2 and an interfacial Ge-rich layer between the Hf3(Si1−xGex)2 and unreacted Si0.85Ge0.15 films, while above 600°C, the increase of specific contact resistance may be due to the formation of Hf(Si1−xGex)2 and SiC as well as the roughness of the Hf(Si1−xGex)2 films.  相似文献   

14.
In this paper we show that pseudomorphically strained heterostructures of InAs x P1−x /InP may be an alternative to lattice-matched heterostructures of In1−x Ga x As y P1−y /InP for optoelectronic applications. We first studied the group-V composition control in the gas-source molecular beam epitaxy (GSMBE) of the GaAs1-x P x /GaAs system. Then we studied GSMBE of strained InAs x P1−x /InP multiple quantum wells with the ternary well layer in the composition range 0.15 <x < 0.75. Structural and optical properties were characterized by high-resolution x-ray rocking curves, transmission electron microscopy, absorption and low-temperature photoluminescence measurements. High-quality multiple-quantum-well structures were obtained even for highly strained (up to 2.5%) samples. The achievement of sharp excitonic absorptions at 1.06, 1.3 and 1.55μm at room temperature from InAs x P1−x /InP quantum wells suggests the possibility of long-wavelength optoelectronic applications.  相似文献   

15.
The 1/f noise in photovoltaic (PV) molecular-beam epitaxy (MBE)-grown Hg1−xCdxTe double-layer planar heterostructure (DLPH) large-area detectors is a critical noise component with the potential to limit sensitivity of the cross-track infrared sounder (CrIS) instrument. Therefore, an understanding of the origins and mechanisms of noise currents in these PV detectors is of great importance. Excess low-frequency noise has been measured on a number of 1000-μm-diameter active-area detectors of varying “quality” (i.e., having a wide range of I-V characteristics at 78 K). The 1/f noise was measured as a function of cut-off wavelength under illuminated conditions. For short-wave infrared (SWIR) detectors at 98 K, minimal 1/f noise was measured when the total current was dominated by diffusion with white noise spectral density in the mid-10−15A/Hz1/2 range. For SWIR detectors dominated by other than diffusion current, the ratio, α, of the noise current in unit bandwidth in(f = 1 Hz, Vd = −60 mV, and Δf = 1 Hz) to dark current Id(Vd = −60 mV) was αSW-d = in/Id ∼ 1 × 10−3. The SWIR detectors measured at 0 mV under illuminated conditions had median αSW-P = in/Iph ∼ 7 × 10−6. For mid-wave infrared (MWIR) detectors, αMW-d = in/Id ∼ 2 × 10−4, due to tunneling current contributions to the 1/f noise. Measurements on forty-nine 1000-μm-diameter MWIR detectors under illuminated conditions at 98 K and −60 mV bias resulted in αMW-P = in/Iph = 4.16 ± 1.69 × 10−6. A significant point to note is that the photo-induced noise spectra are nearly identical at 0 mV and 100 mV reverse bias, with a noise-current-to-photocurrent ratio, αMW-P, in the mid 10−6 range. For long-wave infrared (LWIR) detectors measured at 78 K, the ratio, αLW-d = in/Id ∼ 6 × 10−6, for the best performers. The majority of the LWIR detectors exhibited αLW-d on the order of 2 × 10−5. The photo-induced 1/f noise had αLW-P = in/Iph ∼ 5 × 10−6. The value of the noise-current-to-dark-current ratio, α appears to increase with increasing bandgap. It is not clear if this is due to different current mechanisms impacting 1/f noise performance. Measurements on detectors of different bandgaps are needed at temperatures where diffusion current is the dominant current. Excess low-frequency noise measurements made as a function of detector reverse bias indicate 1/f noise may result primarily from the dominant current mechanism at each particular bias. The 1/f noise was not a direct function of the applied bias.  相似文献   

16.
Single crystals of Pb1−x Snx Te (0.06<x<0.08) have been grown by using an ingot-nucleation technique from a Te-rich source. The as-grown crystals have a p-type carrier concentration around 1019 cm−3 and dislocation density as low as 103 cm−2. Diode lasers fabricated from these crystals have contact resistances of 2×10−5 Ω-cm2 and a single-mode single-ended output power of 750 μW at heat sink temperatures around 15 K.  相似文献   

17.
In order to gain deep insight into the origin of the large thermoelectric power and metallic electrical conduction observed for La2−x Sr x CuO4 possessing a two-dimensional layered structure and the consequently obtained two-dimensional electronic structure, we investigate the energy–momentum dispersion and the lifetime of quasiparticles by using high-resolution angle-resolved photoemission spectroscopy. By calculating the thermoelectric power using the Bloch–Boltzmann theory and the experimentally obtained information about the electronic structure and electron scatterings, we found that the large thermoelectric power of La2−x Sr x CuO4 is mainly brought about by the anisotropic electron scattering caused by the strong electron correlation, and that the two-dimensional electronic structure makes almost no important contribution to the large S(T) except for rather minor effect of the van Hove singularity near the Fermi level.  相似文献   

18.
This paper presents central finite-dimensional H filters for linear systems with state or measurement delay that are suboptimal for a given threshold γ with respect to a modified Bolza–Meyer quadratic criterion including an attenuation control term with opposite sign. In contrast to the results previously obtained for linear time-delay systems, the paper reduces the original H filtering problems to H 2 (optimal mean-square) filtering problems, using the technique proposed in Doyle et al. (IEEE Trans. Automat. Contr. AC-34:831–847, 1989). The paper first presents a central suboptimal H filter for linear systems with state delay, based on the optimal H 2 filter from Basin et al. (IEEE Trans. Automat. Contr. AC-50:684–690, 2005), which contains a finite number of filtering equations for any fixed filtering horizon, but this number grows unboundedly as time goes to infinity. To overcome that difficulty, an alternative central suboptimal H filter is designed for linear systems with state delay, which is based on the alternative optimal H 2 filter from Basin et al. (Int. J. Adapt. Control Signal Process. 20(10):509–517, 2006). Then, the paper presents a central suboptimal H filter for linear systems with measurement delay, based on the optimal H 2 filter from Basin and Martinez-Zuniga (Int. J. Robust Nonlinear Control 14(8):685–696, 2004). Numerical simulations are conducted to verify the performance of the designed three central suboptimal filters for linear systems with state or measurement delay against the central suboptimal H filter available for linear systems without delays. The authors thank The London Royal Society (RS) and the Mexican National Science and Technology Council (CONACyT) for financial support under an RS International Incoming Short Visits 2006/R4 Grant and CONACyT Grants 55584 and 52953.  相似文献   

19.
In-rich In x Al1−x N films (0.47 ≤ x ≤ 1) were grown directly on nitridated (0001) sapphire substrates without employing a buffer layer by radiofrequency molecular-beam epitaxy. Both photoluminescence peak energy and optical absorption-edge energy of the In x Al1−x N films decreased monotonically with increasing In composition, which was consistent with the recently reported InN bandgap energies of ∼0.7 eV. The bowing parameter of this alloy was estimated to lie between 2.9 eV and 6.2 eV. Substrate nitridation with lower temperature and longer period conditions not only reduced misoriented In x Al1−x N phases remarkably but also produced narrower tilt distribution in the c-axis-oriented In x Al1−x N matrix.  相似文献   

20.
 By defining fuzzy valued simple functions and giving L1(μ) approximations of fuzzy valued integrably bounded functions by such simple functions, the paper analyses by L1(μ)-norm the approximation capability of four-layer feedforward regular fuzzy neural networks to the fuzzy valued integrably bounded function F : Rn → FcO(R). That is, if the transfer functionσ: R→R is non-polynomial and integrable function on each finite interval, F may be innorm approximated by fuzzy valued functions defined as to anydegree of accuracy. Finally some real examples demonstrate the conclusions.  相似文献   

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