Bond-lengths of the atomic nearest-neighbor and next-nearest-neighbor in A1−xBxC1−yDy III–V solid solutions

On the basis of the FDUC model and the hypothesis of the constant covalent radii, the expressions of the atomic nearest-neighbor and the next-nearest-neighbor bond-lengths were derived for A_1−xB_xC_1−yD_y III–V quaternary solid solutions. This set of bond-length expressions predicts the averaged bond-lengths and bond angles at any concentration (x, y) for the III–V pseudobinary and quaternary solid solutions, which are only dependent on the lattice parameters and the concentrations of the pure end compounds. When x=0, 1 or y=0, 1, A_1−xB_xC_1−yD_y III–V quaternary solid solutions degenerate into the relative pseudobinary solid solutions, in which the nearest-neighbor and the next-nearest-neighbor bond-lengths agree well with the experimental results. Further discussion and comparison with other theoretical models are also given in this paper.     

Applications of variable angle spectroscopic ellipsometry to strained SiGe alloy heterostructures

Variable angle spectroscopic ellipsometry (VASE) has been used to characterize several Si_xGe_1−x/Ge heterostructures. First, Si_xGe_1−x/Ge superlattice (SL) structures were characterized in terms of the layer thicknesses, composition, x, of the Si_xGe_{1− x} layer, and oxide thickness. High-resolution X-ray diffraction results are also presented for the Si_xGe_1−x/Ge SL structures and are shown to be in close agreement with the VASE results once strain effects are taken into account. VASE has also been used to study thick, Ge-rich Si_xGe_1−x/Ge heterostructures that have been grown on Si substrates. A stepped buffer has been deposited first in order to minimize the strain in the Si_xGe_1−x/Ge layers. VASE can be used to give a qualitative determination of the residual strain along with the thickness of all layers within the optical penetration depth from the surface.     

Dielectric and piezoelectric properties of sol–gel derived Ca doped PbTiO3

Synthesis of Ca doped PbTiO₃ powder by a chemically derived sol–gel process is described. Crystallization characteristics of different compositions Pb_1−xCa_xTiO₃ (PCT) with varying calcium (Ca) content in the range x = 0–0.45 has been investigated by DTA/TGA, X-ray diffraction and scanning electron microscopy. The crystallization temperature is found to decrease with increasing calcium content. X-ray diffraction reveals a tetragonal structure for PCT compositions with x ≤ 0.35, and a cubic structure for x = 0.45. Dielectric properties on sintered ceramics prepared with fine sol–gel derived powders have been measured. The dielectric constant is found to increase with increasing Ca content, and the dielectric loss decreases continuously. Sol–gel derived Pb_1−xCa_xTiO₃ ceramics with x = 0.45 after poling exhibit infinite electromechanical anisotropy (k_t/k_p) with a high d₃₃ = 80 pC/N, ′ = 298 and low dielectric loss (tan δ = 0.0041).     

Determination of the E1 and E1+Δ1 energy bands by photoreflectance in AlxGa1−xAs in the region 0<x<0.55

The E₁ and E₁+Δ₁ 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+Δ₁ 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+Δ₁ cubic expressions as functions of the aluminum composition, x, and compare them with the available reported expressions.     

Investigation of MBE grown GaAs/AlGaAs/InGaAs heterostructures

This paper reports on the influence of the In mole fraction variation (0.1≤x≤0.25) of MBE grown pseudomorphic GaAs/Al_yGa_1−yAs/In_xGa_1−xAs heterostructures on the material quality and the performance of the fabricated devices. For x=0.1–0.15, the carrier mobility in the samples was 4500 cm² V⁻¹ s (at 300 K) and 37 000 cm²V⁻¹ s (at 77 K) and decreased significantly at low temperatures as x was increasing up to 0.25. Transistors with gate length of 0.8 m and In_0.1Ga_0.9As channels exhibited transconductances of 200–220 mSm mm⁻¹ and output conductances of 0.15–0.20 mSm mm⁻¹, while gate-source breakdown voltages were 27–28 V. Delay times of the designed and fabricated ICs frequency dividers by 2 were 130–140 ps.     

Optical and electrical properties of p-type AgInSnxS2−x (x = 0–0.04) thin films prepared by spray pyrolysis

AgInSn_xS_2−x (x = 0–0.2) polycrystalline thin films were prepared by the spray pyrolysis technique. The samples were deposited on glass substrates at temperatures of 375 and 400 °C from alcoholic solutions comprising silver acetate, indium chloride, thiourea and tin chloride. All deposited films crystallized in the chalcopyrite structure of AgInS₂. A p-type conductivity was detected in the Sn-doped samples deposited at 375 °C, otherwise they are n-type. The optical properties of AgInSn_xS_2−x (x < 0.2) resemble those of chalcopyrite AgInS₂. Low-temperature PL measurements revealed that Sn occupying an S-site could be the responsible defect for the p-type conductivity observed in AgInSn_xS_2−x (x < 2) thin films.     

Characterization of GaN epitaxial layers on SiC substrates with AlxGa1−xN buffer layers

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 cm² V⁻¹ s⁻¹ and 1.6×10¹⁷ cm⁻³ at 300 K. The splitting diffraction angle between GaN and Al_xGa_1−xN were also analyzed from X-ray diffraction curves.     

Optical properties of a-(Se70Te30)100−x(Se98 Bi2)x thin films

Measurements of optical constants (absorption coefficient, refractive index, extinction coefficient, real and imaginary part of the dielectric constant) have been made on a-(Se₇₀Te₃₀)_100−x (Se₉₈Bi₂)_x thin films (where x=0, 5, 10, 15 and 20) of thickness 2000 Å in the wavelength range 450–1000 nm. It is found that the optical bandgap decreases with the increase of Se₉₈Bi₂ concentration in the a-(Se₇₀Te₃₀)_100−x(Se₉₈Bi₂)_x system. The value of refractive index (n) decreases, while the extinction coefficient (k) increases with increasing photon energy. The results are interpreted in terms of concentration of localized states varying effective Fermi level.     

Sintering behavior and microwave dielectric properties of Bi3(Nb1−xTax)O7 solid solutions

Solid solutions of Bi₃(Nb_1−xTa_x)O₇ (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 Bi₃(Nb_1−xTa_x)O₇ ceramics could be well densified below 990 °C. As x increased from 0.0 to 1.0, saturated density of Bi₃(Nb_1−xTa_x)O₇ ceramics increased from 8.2 to 9.1 g cm⁻³, microwave permittivity decreased from 95 to 65 while Q_f values increasing from 230 to 560 GHz. Substitution of Ta for Nb modified temperature coefficient of resonant frequency τ_f from −113 ppm °C⁻¹ of Bi₃NbO₇ to −70 ppm °C⁻¹ of Bi₃TaO₇. Microwave permittivity, Q_f values and τ_f 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 Bi₃(Nb_1−xTa_x)O₇ ceramics could be a good candidate for low temperature co-fired ceramics application.     

The optical and magneto-optical properties of MnSbBi films

The optical and magneto-optical properties of Mn(Sb_1−xBi_x) films were investigated for incident photons within the energy range from 1.3 to 4.0 eV. The diagonal and off-diagonal optical conductivities were determined from the results of spectroscopic ellipsometry and the polar Kerr effect measurements. A large Kerr rotation angle of about 0.57° was observed at photon energy ω≈2.65 eV for the Mn₅₅Sb₃₆Bi₉ film with an average grain size of 30 nm. The changes of the exchange splitting and the spin–orbit interaction strength are responsible for the strong spectral dependence of the Kerr effect in Mn(Sb_1−xBi_x) nanocrystalline films.     

Synthesis and humidity sensitive properties of nanocrystalline Ba1−xSrxTiO3 thick films

Nanocrystalline Ba_1−xSr_xTiO₃ (x=0, 0.2, 0.4, 0.6, 0.8 and 1.0) precursors were synthesized using the stearic acid gel method. After the precursors had been calcined at 600–950°C for 0.5–1 h, nanocrystalline powders with the cubic perovskite structure were obtained and these were made into thick films. The powder samples were characterized by differential thermal analysis, X-ray diffraction and transmission electron microscopy, and the thick film samples were characterized by scanning electron microscopy and X-ray diffraction. The humidity-sensitive properties of the nanocrystalline Ba_1−xSr_xTiO₃ thick films were investigated. The results show that these nanocrystalline thick films possess higher humidity sensitivity and lower resistance than those of conventional materials.     

Pyroelectric properties of BST/PLT composite thick films with addition of xPbO–(1 − x)B2O3 glass

The Ba_xSr_1−xTiO₃ (BST)/Pb_1−xLa_xTiO₃ (PLT) composite thick films (20 μm) with 12 mol% amount of xPbO–(1 − x)B₂O₃ 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)B₂O₃ 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–0.65B₂O₃ glass additives provided low heat capacity and good pyroelectric figure-of-merit because of their porous structure. The pyroelectric coefficient and figure-of-merit F_D are 364 μC/(m² 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.     

Bulk Si1−xGex single- and poly-crystals: a new prospective material for electronics

Si_1−xGe_x is a prospective material for electronics. This is mostly because Si_1−xGe_x-based technology is close to silicon-based technology, which is advanced, widely applicable, and cheap. The majority of work on this material is devoted to Si_1−xGe_x-based heteroepitaxy, and in particular to the Si_1−xGe_x/Si system; few publications are devoted to bulk single-crystal. Here we focus on some interesting properties of bulk Si_1−xGe_x solid solutions. First, under heat treatment and alpha- and beta-irradiation the efficiency of defect introduction decreases with the increase of Ge composition of the Si_1−xGe_x single-crystal. This is because Ge atoms in a crystal lattice are annihilation centers for primary defects. Hence, this material is more resistant to temperature and radiation than silicon. Second, it is known that, since Z(Ge)Z(Si), the sensitivity of the material to irradiation should increase with the concentration of Ge. We show that Si_1−xGe_x nuclear detectors have efficiency three times higher than silicon detectors. Finally, we note that one of the major problems in materials based on solid solutions is the composition uniformity. Our investigations on the influence of composition fluctuations on material properties have shown that the material has a sufficient uniformity at x<0.1. Such an alloy is a prospective material for electronics.     

Spectroscopic ellipsometry analysis of GaAs1 − xNx layers grown by molecular beam epitaxy

In this work, we present the effect of nitrogen incorporation on the dielectric function of GaAsN samples, grown by molecular beam epitaxy (MBE) followed by a rapid thermal annealing (for 90 s at 680 °C). The GaAs_1 − xN_x samples with N content up to 1.5% (x = 0.0%, 0.1%, 0.5%, 1.5%), are investigated using room temperature spectroscopic ellipsometry (SE). The optical transitions in the spectral region around 3 eV are analyzed by fitting analytical critical point line shapes to the second derivative of the dielectric function. It was found that the features associated with E₁ and E₁ + Δ₁ transitions are blue-shifted and become less sharp with increasing nitrogen incorporation, in contrast to the case of E₀ transition energy in GaAs_1 − xN_x. An increase of the split-off Δ₁ energy with nitrogen content was also obtained, in agreement to results found with MOVPE GaAs_1 − xN_x grown samples.     

Structural characterization of ion-beam-induced epitaxially crystallized thin layers of III–V and IV–IV semiconductors

Structural properties of ion-beam-induced epitaxial crystallization (IBIEC) for amorphous layers of GaAs on GaAs(100), BP on BP(100) and Si_1−xGe_x and Si_1−x−yGe_xC_y on Si(100) have been investigated. Crystallization was induced by ion bombardment with 400 keV Ne, Ar or Kr at 150 °C for GaAs and at 350 °C for BP. Epitaxial crystallization up to the surface was observed both in GaAs and BP at temperatures much below those required for the solid phase epitaxial growth (SPEG). The growth rate per nuclear energy deposition density has shown a larger dependence on ion dose rate in cases of heavier ion bombardments both for GaAs and BP. Crystallization of a-GaAs with ions whose projected ranges are within the amorphous layer thickness was also observed at 150 °C. Epitaxial crystallization of Si_1−xGe_x and Si_1−x−yGe_xC_y layers (x = 0.13 and y = 0.014 at peak concentration) on Si(100) formed by high-dose implantation of 80 keV Ge and 17 keV C ions has been observed in the IBIEC process with 400 keV Ar ion bombardments at 300–400 °C. Crystalline growth by IBIEC has shown a larger growth rate in Si_1−x−yGe_xC_y/Si} than in Si_1−xGe_x/Si} with the same Ge concentration for all bombardments under investigation. X-ray rocking-curve measurements have shown a strain-compensated growth in Si_1−x−yGe_xC_y/Si}, whereas Si_1−xGe_x/Si} samples have shown a growth with strain accommodation.     

Composition and substrate temperature dependence of structural and optical studies on ZnSexCdS1−x alloy films

Thin (t−0.60 μm) films of ZnSe_xCdS_1−x were formed by vacuum evaporation on glass substrates held at 350 and 470 K. XRD studies showed that all the films were polycrystalline in nature. Films with x0.70 were hexagonal whereas films with x0.80 were cubic in structure. The structural transition was in the range 0.70<x<0.80. The lattice parameter was higher in films formed at a higher temperature. The lattice parameter followed Vegard's law. Grain size increased with substrate temperature. From the optical transmission spectra recorded in the wavelength range 300–2500 nm, the extinction coefficient, refractive index and band gaps were obtained. Band gap values showed a downward bowing with ‘x' with a bowing parameter of 0.40 eV.     

Photoelectrochemical investigations of n-CdSe1−xTex thin film electrodes

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.     

Effects of oxygen partial pressure on lead content of PLZT thin films produced by excimer laser deposition

Excimer laser ablation has been used to produce thin films of lanthanum-modified lead zirconate titanate (PLZT), or Pb_1−xLa_x(Zr_1−yTi_y)_1−x/4O₃. PLZT is an interesting class of materials since it has a wide range of compositionally dependent electro-optical properties and strong non-linear optical characteristics. PLZT thin films of 7/0/100, 28/0/100 and 0/0/100 compositions have been deposited onto crystalline Si100 and amorphous fused silica substrates. Effects of oxygen backfill pressure on the Pb:(Ti + La) ratios were investigated. The results indicate that controlling the oxygen backfill pressure during laser deposition strongly influences the stoichiometry and crystal structure of PLZT thin films.     

Probe microanalysis investigation and electroreflectance spectroscopy of Hg1−xCdxTe PLD films on silicon patterned substrates

Hg_1−xCd_xTe films were prepared on Si-patterned substrates by the pulse laser deposition technique from a Hg_1−xCd_xTe target (x≈0.2). The effects of different substrate temperatures, ranging from 293 to 543 K, different laser shots number in the range of 10–380, and the morphological type of the patterned substrate on the x-composition of films were studied by electron probe microanalysis (EPMA) and electroreflectance (ER) spectroscopy. The correlation between a film composition measured by EPMA and one determined from ER spectra data was observed.     

Growth of lithium doped silver niobate single crystals and their piezoelectric properties

Lithium doped silver niobate (Ag_1−xLi_xNbO₃, 0 < x < 0.1) is one of the candidate materials for lead-free piezoelectric materials. In this study, Ag_1−xLi_xNbO₃ 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_xNbO₃ single crystals were determined. Moreover, P–E hysteresis measurement revealed that pure silver niobate crystal was weak ferroelectrics with P_r of 0.095 μC/cm² while Ag_0.9Li_0.1NbO₃ (ALN10) crystal was normal ferroelectrics with P_r of 10.68 μC/cm². 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.