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1.
High quality GaN epitaxial layers were obtained with Al xGa 1−xN buffer layers on 6H–SiC substrates. The low-pressure metalorganic chemical vapor deposition (LP-MOCVD) method was used. The 500 Å thick buffer layers of Al xGa 1−xN (0≤ x≤1) were deposited on SiC substrates at 1025°C. The FWHM of GaN (0004) X-ray curves are 2–3 arcmin, which vary with the Al content in Al xGa 1−xN buffer layers. An optimum Al content is found to be 0.18. The best GaN epitaxial film has the mobility and carrier concentration about 564 cm 2 V −1 s −1 and 1.6×10 17 cm −3 at 300 K. The splitting diffraction angle between GaN and Al xGa 1−xN were also analyzed from X-ray diffraction curves. 相似文献
2.
Selective wet chemical etching of the Al xGa 1−xAs/GaAs system has been applied to heterostructure characterization. Samples of LPE grown AlGaAs/GaAs laser double-heterostructures and separate confinement heterostructures as well as antiresonant reflecting optical waveguides heterostructures were treated with “I2 solution” (I 2:KI:H 2O) and hydrochloric acid. These compounds selectively etch the ternary Al xGa 1−xAs layers, but with different “threshold composition” xth values (the x value is that above which the etching rate of a given compound increases sharply). Selectively etched samples have been examined by SEM. The experimental dependence of etching rate on the x value for I2 solution has been derived. From this dependence, the x composition of any ternary layer can be estimated simply. Observations were made of the “microscopic” properties of the heterostructure, such as the smoothness of the interfaces and the uniformity of layers. All imperfections resulting from the growth process, such as interface perturbations or compositional nonuniformity of layers, are clearly seen. An additional advantage of this etching technique is its simplicity. It allows quick examination of grown heterostructure for the selection of wafers for further processing. 相似文献
3.
Polycrystalline samples with general formula Cd xCo 1−xFe 2−yCr yO 4 ( x=0, 0.25, 0.50, 0.75 and 1.00; y=0, 0.15 and 0.30) were prepared by standard ceramic techniques. The samples were characterized by XRD, IR, SEM and VSM techniques. The magnetic properties were studied at room temperature. The Neel's two-sublattice model is found to exist for lower concentration of Cd ( x<0.25). Thereafter ( x≥0.25) the canted spin model is found to exist for which Yafet–Kittel (Y–K) angles are calculated. The saturation magnetic moment is found to reduce with the substitution of Cr 3+, and is explained on the basis of cation distribution. No magnetic transformation is observed for the samples with higher cadmium concentrations (for x=0.75 and 1.00). 相似文献
4.
The E1 and E1+Δ 1 energy bands of metal–organic chemical vapor deposition grown Al xGa 1−xAs, with x in the range 0–0.55, have been determined using photoreflectance technique. The aluminum composition for each sample was determined using the energy of the room-temperature photoluminescence compensated peak value and a suitable fundamental band gap formula. The positions of the E1 and E1+Δ 1 peaks were determined from curve-fitting an appropriate theoretical model to our experimental data by a modified downhill simplex method. Using the results, we propose new E1 and E1+Δ 1 cubic expressions as functions of the aluminum composition, x, and compare them with the available reported expressions. 相似文献
5.
Solid solutions of Bi 3(Nb 1−xTa x)O 7 ( x = 0.0, 0.3, 0.7, 1) were synthesized using solid state reaction method and their microwave dielectric properties were first reported. Pure phase of fluorite-type could be obtained after calcined at 700 °C (2 h) −1 between 0 ≤ x ≤ 1 and Bi 3(Nb 1−xTa x)O 7 ceramics could be well densified below 990 °C. As x increased from 0.0 to 1.0, saturated density of Bi 3(Nb 1−xTa x)O 7 ceramics increased from 8.2 to 9.1 g cm −3, microwave permittivity decreased from 95 to 65 while Qf values increasing from 230 to 560 GHz. Substitution of Ta for Nb modified temperature coefficient of resonant frequency τf from −113 ppm °C −1 of Bi 3NbO 7 to −70 ppm °C −1 of Bi 3TaO 7. Microwave permittivity, Qf values and τf values were found to correlate strongly with the structure parameters of fluorite solid solutions and the correlation between them was discussed in detail. Considering the low densified temperature and good microwave dielectric proprieties, solid solutions of Bi 3(Nb 1−xTa x)O 7 ceramics could be a good candidate for low temperature co-fired ceramics application. 相似文献
6.
An electrode/electrolyte interface has been formed between an n-type CdSe 1−xTe x (0≤ x≤1) alloyed/mixed type semiconductor and a sulphide/polysulphide redox electrolyte. It has been investigated through the current–voltage, capacitance–voltage and spectrally selective properties. The dependence of the dark current through the junction and the junction capacitance on the voltage across the junction have been examined and analysed. It appeared that the current transport mechanism across the junction is strongly influenced by the recombination mechanism at the interface and series resistance effects. Upon illumination of the interface with a light of 20 mW cm −2, an open circuit voltage of the order of 0.35 V and a short circuit current of 212 μA cm −2 have been developed (for x=0.2), yielding an efficiency of energy conversion equal to 0.2% and a form factor of 45%. The action spectra in the 450–1000 nm wavelength range showed presence of the interface states at the electrode/electrolyte interface. The magnitudes of the barrier heights at the interfaces were also determined. It has been seen that a significant improvement in the electrochemical performance of a cell is noticed for the electrode composition with x=0.2. 相似文献
7.
Vickers microhardness indentations of 10 μm (001) oriented epilayers of Al xGa 1−xAs on GaAs substrates have been utilized to evaluate the hardness Hv, the internal stress, and the fracture toughness KIc of the layers as a function of their composition parameter x. The hardness Hv varies linearly according to: (6.9-2.2 x) GPa and KIc increases linearly with x according to: K1c = (0.44+1.30 x) MPa m 1/2. The influence of the substrate on these measurements was found to be negligible for the layer thickness (10 μm) and the indentation load (0.25 N) used, disregarding internal stresses. Internal film stresses were evaluated by the bimorph buckling method, and were found to depend on the composition parameter according to σ = 0.13x GPa. These stresses did not notably affect the Hv measurements, but for KIc corrections as large as 25% had to be made. The radial cracks observed were of the shallow Palmqvist type. In contradiction to previous reports on this type of cracking, it was found to initiate during unloading, not during loading, and a physical explanation for this deviation is given. No deep radial/median cracks were observed. It was found important to use expressions based on the correct crack geometry in the KIc evaluation. Also, a simple theory for the influence of internal stresses on the KIc results has been developed. 相似文献
8.
Lithium doped silver niobate (Ag 1−xLi xNbO 3, 0 < x < 0.1) is one of the candidate materials for lead-free piezoelectric materials. In this study, Ag 1−xLi xNbO 3 single crystals were successfully grown by a slow cooling method. Crystal structure was assigned to perovskite-type orthorhombic (monoclinic) phase. Dielectric properties were measured as a function of temperature. As a result, with increasing lithium contents, the phase transition at around 60 °C was shifted to lower temperature while the phase transition at around 400 °C was shifted to higher temperature. On the basis of these peak shifts, the lithium contents in Ag 1−xLi xNbO 3 single crystals were determined. Moreover, P– E hysteresis measurement revealed that pure silver niobate crystal was weak ferroelectrics with Pr of 0.095 μC/cm 2 while Ag 0.9Li 0.1NbO 3 (ALN10) crystal was normal ferroelectrics with Pr of 10.68 μC/cm 2. About this ALN10 crystal, polling treatment was performed and finally piezoelectric properties were measured. As a result, high electromechanical coupling coefficient k31 over 70% was observed. 相似文献
9.
We have developed Bi-2212 and 2223 tapes. For Bi-2212, two double stacked pancake type coils were fabricated using Bi-2212/Ag tapes prepared by a combination of the continuous dip-coating process and melt-solidification. A small coil (13 mm inner bore, 46.5 mm outer diameter) was inserted in a conventional superconducting magnet system. In a bias field of 20.9 T, the generated field of the coil was 0.9 T, at an Ic of 310 A (criterion 10 −13 Ωm) at 1.8 K. Thus, the superconducting magnet system achieved the generation of a field of 21.8 T in the full superconducting state. A large coil (20 mm inner bore, 94 mm outer diameter) generated a field of 2.6 T ( Ic = 385 A (10 −13 Ω m)) at 4.2 K and 1.53 T ( Ic = 225 A (10 −13Ω m)) at 20 K in self-field. For Bi-2223, tapes were prepared by the powder-in-tube technique using Ag-10% Cu- x%M ( x = 0–1.0, M = Ti, Zr, Hf or Au) alloy sheaths. The high Jc values of 5–7 × 10 4 A cm −2 at 4.2 K and 14 T were obtained for the tapes doped with x = 0.03–0.1 at.% Ti, 0.1 at.% Zr, 0.1 at.% Hf or 0.3% Au. These tapes have a modified Bi-2223 grain structure at the sheath/core interface and also a dense and more aligned microstructure, resulting in higher Jc values. 相似文献
10.
From the analysis of the variation of optical absorption coefficient with incident photon energy between 0.8 and 2.6 eV, obtained from ellipsometric data, the energy EG of the fundamental absorption edge and EG′ of the forbidden direct transition for CuIn xGa 1−xSe 2 alloys are estimated. The change in EG and the spin-orbit splitting Δ SO= EG′− EG with the composition x can be represented by parabolic expression of the form EG( x)= EG(0)+ ax+ bx2 and Δ SO( x)=Δ SO(0)+ a′ x+ b′ x2, respectively. b and b′ are called “bowing parameters”. Theoretical fit gives a=0.875 eV, b=0.198 eV, a′=0.341 eV and b′=−0.431 eV. The positive sign of b and negative sign of b′ are in agreement with the theoretical prediction of Wei and Zunger [Phys. Rev. B 39 (1989) 6279]. 相似文献
11.
The crystal structure, thermal expansion and electrical conductivity of the solid solution Nd 0.7Sr 0.3Fe 1−xCo xO 3 for 0≤ x≤0.8 were investigated. All compositions had the GdFeO 3-type orthorhombic perovskite structure. The lattice parameters were determined at room temperature by X-ray powder diffraction (XRPD). The pseudo-cubic lattice constant decreased continuously with x. The average linear thermal expansion coefficient (TEC) in the temperature range from 573 to 973 K was found to increase with x. The thermal expansion curves for all values of x displayed rapid increase in slope at high temperatures. The electrical conductivity increased with x for the entire temperature range of measurement. The calculated activation energy values indicate that electrical conduction takes place primarily by the small polaron hopping mechanism. The charge compensation for the divalent ion on the A-site is provided by the formation of Fe 4+ ions on the B-site (in preference to Co 4+ ions) and vacancies on the oxygen sublattice for low values of x. The large increase in the conductivity with x in the range from 0.6 to 0.8 is attributed to the substitution of Fe 4+ ions by Co 4+ ions. The Fe site has a lower small polaron site energy than Co and hence behaves like a carrier trap, thereby drastically reducing the conductivity. The non-linear behaviour in the dependence of log σT with reciprocal temperature can be attributed to the generation of additional charge carriers with increasing temperature by the charge disproportionation of Co 3+ ions. 相似文献
12.
The Ba xSr 1−xTiO 3 (BST)/Pb 1−xLa xTiO 3 (PLT) composite thick films (20 μm) with 12 mol% amount of xPbO–(1 − x)B 2O 3 glass additives ( x = 0.2, 0.35, 0.5, 0.65 and 0.8) have been prepared by screen-printing the paste onto the alumina substrates with silver bottom electrode. X-ray diffraction (XRD), scanning electron microscope (SEM) and an impedance analyzer and an electrometer were used to analyze the phase structures, morphologies and dielectric and pyroelectric properties of the composite thick films, respectively. The wetting and infiltration of the liquid phase on the particles results in the densification of the composite thick films sintered at 750 °C. Nice porous structure formed in the composite thick films with xPbO–(1 − x)B 2O 3 glass as the PbO content ( x) is 0.5 ≥ x ≥ 0.35, while dense structure formed in these thick films as the PbO content ( x) is 0.8 ≥ x ≥ 0.65. The volatilization of the PbO in PLT and the interdiffusion between the PLT and the glass lead to the reduction of the c-axis of the PLT phase. The operating temperature range of our composite thick films is 0–200 °C. At room temperature (20 °C), the BST/PLT composite thick films with 0.35PbO–0.65B 2O 3 glass additives provided low heat capacity and good pyroelectric figure-of-merit because of their porous structure. The pyroelectric coefficient and figure-of-merit FD are 364 μC/(m 2 K) and 14.3 μPa −1/2, respectively. These good pyroelectric properties as well as being able to produce low-cost devices make this kind of thick films a promising candidate for high-performance pyroelectric applications. 相似文献
13.
We present the results of ab initio calculations of the spin-polarized electronic structure of disordered bcc and fcc Fe 1 − xSi x alloys from dilute solid solutions of Si on iron up to FeSi intermetallic compound composition ( x=0.01..0.5) and ordered intermetallic phases FeSi in B20 structure and Fe 3Si in DO 3 structure. The self-consistent calculations were performed with the coherent potential approximation within the Korringa–Kohn–Rostoker method (KKR–CPA) for disordered case and by using the tight-binding linear muffin–tin orbital (TB LMTO) method for intermetallic compounds. In the last case the supercell approach has been utilized in order to take into account the structural defects for this ordered phase. In particular, we have performed a series of calculations in the BCC Fe 1 − xSi x alloys with x=0.01, 0.015, 0.1, 0.15, 0.25. 相似文献
14.
We have reported on the growth and magnetotransport properties of modulation p-doped Si 1−xGe x quantum wells on strained multilayers of 2.5 nm Si 1−xGe x/10 nm Si on vicinal (113) Si surfaces. Owing to the strong step-bunching properties of the (113) Si surface, both the Si 1−xGe x and the Si layers exhibited a regular pattern of large steps. Low-temperature magnetotransport measurements revealed a hole density (6–9×10 11 cm −2) independent of direction, whereas a pronounced mobility anisotropy was found. The mobility (1000–2000 cm 2/Vs) was approximately two times higher along the [33-2] direction compared to a perpendicular [−110] direction. This is attributed to anisotropic hole scattering caused by anisotropic shear strain which is always present in strained layers on (113) Si. No influence of the large regular steps, whose direction is given by the direction of the substrate miscut, on the mobility was found. 相似文献
15.
The crystalline quality of InSb substrates and their transparency in the 8–12 μm IR window compare favourably with those of CdTe. We report the first results obtained for Cd xHg 1−xTe (CMT) layers grown using molecular beam epitaxy over a buffer layer of CdTe on InSb. The (100)-oriented InSb wafer is chemically cleaned prior to thermal treatment which may be accompanied by ion beam cleaning; this process leads to a (2x4) reconstructed surface. Characterization of the surface using Auger spectroscopy, UV photoemission spectroscopy and X-ray photoelectron spectroscopy shows that it is free from oxygen and carbon. CdTe is grown at a substrate temperature of 523 K using a single CdTe cell. When 0.5 μm of CdTe has grown, the substrate temperature is lowered and the CMT is deposited epitaxially using three different cells for cadmium, mercury and tellurium. The layers exhibit X-ray rocking curves 6' wide and n-type conductivity with n = 3× 10 15 cm −3 and μ = 40 000 cm 2 V −1 s −1. Measurement of the strain in each layer shows that CMT is totally relaxed on the CdTe which in turn grows coherently on the InSb. 相似文献
16.
We report on the properties of (1− x)SrBi 2Ta 2O 9– xBi 3TaTiO 9 solid solution thin films for ferroelectric non-volatile memory applications. The solid solution thin films fabricated by modified metalorganic solution deposition technique showed much improved properties compared to SrBi 2Ta 2O 9. A pyrochlore free crystalline phase was obtained at a low annealing temperature of 600°C and grain size was found to be considerably increased for the solid solution compositions. The film properties were found to be strongly dependent on the composition and annealing temperatures. The measured dielectric constant of the solid solution thin films was in the range 180–225 for films with 10–50% of Bi 3TaTiO 9 content in the solid solution. Ferroelectric properties of (1− x)SrBi 2Ta 2O 9– xBi 3TaTiO 9 thin films were significantly improved compared to SrBi 2Ta 2O 9. For example, the observed remanent polarization (2 Pr) and coercive field ( Ec) values for films with 0.7SrBi 2Ta 2O 9–0.3Bi 3TaTiO 9 composition, annealed at 650°C, were 12.4 μC/cm 2 and 80 kV/cm, respectively. The solid solution thin films showed less than 5% decay of the polarization charge after 10 10 switching cycles and good memory retention characteristics after about 10 6 s of memory retention. The improved microstructural and ferroelectric properties of (1− x)SrBi 2Ta 2O 9– xBi 3TaTiO 9 thin films compared to SrBi 2Ta 2O 9, especially at lower annealing temperatures, suggest their suitability for high density FRAM applications. 相似文献
17.
Doping and electrical characteristics of in-situ heavily B-doped Si 1−x−yGe xC y (0.22< x<0.6, 0< y<0.02) films epitaxially grown on Si(100) were investigated. The epitaxial growth was carried out at 550°C in a SiH 4–GeH 4–CH 3SiH 3–B 2H 6–H 2 gas mixture using an ultraclean hot-wall low-pressure chemical vapor deposition (LPCVD) system. It was found that the deposition rate increased with increasing GeH 4 partial pressure, and only at high GeH 4 partial pressure did it decrease with increasing B 2H 6 as well as CH 3SiH 3 partial pressures. With the B 2H 6 addition, the Ge and C fractions scarcely changed and the B concentration ( CB) increased proportionally. The C fraction increased proportionally with increasing CH 3SiH 3 partial pressures. These results can be explained by the modified Langmuir-type adsorption and reaction scheme. In B-doped Si 1−x−yGe xC y with y=0.0054 or below, the carrier concentration was nearly equal to CB up to approximately 2×10 20 cm −3 and was saturated at approximately 5×10 20 cm −3, regardless of the Ge fraction. The B-doped Si 1−x−yGe xC y with high Ge and C fractions contained some electrically inactive B even at the lower CB region. Resistivity measurements show that the existence of C in the film enhances alloy scattering. The discrepancy between the observed lattice constant and the calculated value at the higher Ge and C fraction suggests that the B and C atoms exist at the interstitial site more preferentially. 相似文献
18.
From our measured differential cross section of Rayleigh scattering, the form factor for momentum transfers in the range 1 Å −1 ≤ x ≤ 50 Å −1 was deduced. Data for U, Pb, Pt, W, Sn, Cd, Ag, Mo and Cu at energies from 60 to 662 keV and scattering angles ranging from 5° to 140° were used. In addition, for the region 1 Å −1 ≤ x ≤ 50 Å −1 of momenta, experimental data from other authors at higher energies were analyzed. The experimental values obtained were compared with various form-factor theories and limits of validity are established. For the relativistic modified form factor G a good agreement always exists for θ ≤ 65°, independently of energy and atomic number. 相似文献
19.
Y xGd 1−xVO 4:Tm 3+ (5 mol%) phosphors were prepared by in situ co-precipitation technology with the different content ratio of Y/Gd ( x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, respectively). During the process, rare earth coordination polymers with o-hydroxylbenzoate were used as precursors, composing with polyethylene glycol (PEG) as dispersing media. After heat-treatment of the resulting multicomponent hybrid precursors at 900 °C, the samples were obtained. SEM indicated the particles present good crystalline state, whose crystalline grain sizes were about 0.2–2 μm. Under the excitation of 257 nm, all the materials show the characteristic emission of Tm 3+ which is the strong blue emission centered at 475 nm originating from 1G 4 → 3H 6 of Tm 3+. Besides this, concentration quenching appears in the system of YVO 4:Tm 3+ and GdVO 4:Tm 3+. And when x reaches 0.5, the system of Y xGd 1−xVO 4:Tm 3+ shows the strongest blue emission. 相似文献
20.
Quantum well (QW) structures of Al xGa 1–xAs/GaAs were characterized by photoluminescence technique as a function of the temperature between 10 and 300 K. The structures were grown on a 500 nm thick GaAs buffer layer with Molecular Beam Epitaxy technique. We have studied the properties of in-situ Cl 2-etched GaAs surfaces and overgrown QW structures as a function of the etching temperature (70 and 200 °C). Several models were used to fit the experimental points. Best fit to experimental points was obtained with the Pässler model. 相似文献
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