Molecular beam epitaxy synthesis of Si1−yCy and Si1−x−yGexCy alloys and Ge islands using an electron cyclotron resonance argon/methane plasma

We report the growth of Si_1−yC_y and Si_1−x−yGe_xC_y alloys on Si(001) by electron cyclotron resonance plasma-assisted Si molecular beam epitaxy using an argon/methane gas mixture. Various Si/Si_1−yC_y and Si/Si_1−x−yGe_xC_y multilayers have been grown and characterized principally by X-ray diffraction and Raman spectroscopy. The influence of growth parameters and electron cyclotron resonance plasma source operating conditions on the C substitutional incorporation was studied. Under optimum growth conditions the structures show good structural properties and sharp interfaces with carbon being essentially substitutionally incorporated up to concentrations of 1%. No significant carbon incorporation was measured in films grown under a high methane partial pressure without plasma excitation. Si_1−x−yGe_xC_y layers grown with this technique exhibit the strain compensation and enhanced thermal stability expected for these ternary alloys. Carbon pre-deposition of Si through surface exposure to the argon/methane plasma is shown to act as an antisurfactant on the growth of Ge islands by suppressing the formation of a Ge wetting layer on the surface.     

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.     

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.     

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.     

AlxGa−xAs/GaAs heterostructure characterization by wet chemical etching

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” (I₂:KI:H₂O) and hydrochloric acid. These compounds selectively etch the ternary Al_xGa_1−xAs layers, but with different “threshold composition” x_th 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.     

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.     

Magnetic behaviour of Cd and Cr substituted cobalt ferrites

Polycrystalline samples with general formula Cd_xCo_1−xFe_2−yCr_yO₄ (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 Cr³⁺, 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).     

Ag glasses based on Zn-phosphate

The partial substitution of Zn²⁺ for Ag⁺ in Ag₄P₂O₇ leads to the formation of a wide glassy domain of composition [Ag₄P₂O₇] _(1−y) [Zn₂P₂O₇] _(y) with 0.20y0.87. The introduction of AgI in these materials results in a new series of glasses of formula [(Ag₄P₂O₇)_(1−y) (Zn₂P₂O₇)_(y)] _(1−X) [AgI] _(x), which domain for the composition y = 0.25 corresponds to 0x 0.64. The structure as well as the thermal and electrical properties of these materials are compared with those of the [AgPO₃] _(1−X) [AgI] _(x) and [Ag₄P₂O₇] _(1−x) [AgI] _(x) glasses.     

Molecular beam epitaxy of CdTe and CdxHg1−xTe on InSb

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× 10¹⁵ cm⁻³ and μ = 40 000 cm² 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.     

Doping and electrical characteristics of in-situ heavily B-doped Si1−x−yGexCy films epitaxially grown using ultraclean LPCVD

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 SiH₄–GeH₄–CH₃SiH₃–B₂H₆–H₂ gas mixture using an ultraclean hot-wall low-pressure chemical vapor deposition (LPCVD) system. It was found that the deposition rate increased with increasing GeH₄ partial pressure, and only at high GeH₄ partial pressure did it decrease with increasing B₂H₆ as well as CH₃SiH₃ partial pressures. With the B₂H₆ addition, the Ge and C fractions scarcely changed and the B concentration (C_B) increased proportionally. The C fraction increased proportionally with increasing CH₃SiH₃ 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 C_B up to approximately 2×10²⁰ cm⁻³ and was saturated at approximately 5×10²⁰ 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 C_B 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.     

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.     

Contribution to the comprehension of dissipation phenomena in lead zirconate titanate: aliovalent doping effect

By the solid reaction method, undoped, potassium doped and niobium doped lead zirconate titanate (PZT) are elaborated. The mechanical losses measured in the range of the Hz as a function of temperature shows two peaks R₁ and R₂, and a ferroelectric transition peak P₁ between ferroelectric and para-electric states on the undoped PZT—Pb(Zr_0.54Ti_0.46)O₃—noted PZT54/46. Potassium doped PZT—Pb_1−xK_x(Zr_0.54Ti_0.46)O₃—shortly called PKZT 100x/54/46 shows an increase in the height of both the peaks at a doping content, x, less than 0.5 at.% but an opposite effect is observed above this value. Niobium doped PZT—Pb[(Zr_0.54Ti_0.46)_1−yNb_y]O₃—shortly called PNZT 100y/54/46, shows the vanish of the R₂ peak and the decrease of the height of the R₁ peak when the doping content increases.     

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.     

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.     

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.     

A high resolution silicon beam telescope

We describe the design and present the performances of a charged particle telescope used for high resolution silicon pixel developments. A telescope made of four x and four y single-sided silicon microstrip layers was built, providing an r.m.s. position resolution of 1 μm for high energy charged particles. A signal over noise ratio of 130 was achieved with minimum ionizing particles.     

Highly conductive microcrystalline silicon carbide films deposited by the hot wire cell method and its application to amorphous silicon solar cells

Microcrystalline silicon carbide (μc-Si_1−xC_x) films were successfully deposited by the hot wire cell method using a gas mixture of SiH₄, H₂ and C₂H₂. It was confirmed by Fourier transform infrared and X-ray diffraction analyses that the films consisted of μc-Si grains embedded in a-Si_1−xC_x tissue. The p-type μc-Si_1−xC_x films were deposited using B₂H₆ as a doping gas. A dark conductivity of 0.2 S/cm and an activation energy of 0.067 eV were obtained. The p-type μc-Si_1−xC_x was used as a window layer of a-Si solar cells, in which the intrinsic layer was deposited by photo-chemical vapor deposition, and an initial conversion efficiency of 10.2% was obtained.     

Physical properties of (La,Sr)Ti(O,N)3 thin films grown by pulsed laser deposition

Thin films of La_xSr_1−xTiO_3+x/2 (x = 0, 0.25, 0.5, 0.75, 1) were grown by laser ablation on two different kinds of substrates (SrTiO₃ (STO) and MgO) and were subsequently ammonolysed to yield the corresponding oxynitrides La_xSr_1−xTi(O,N)₃. For both substrates all films were found to grow epitaxially to the (1 0 0) direction of the cubic perovskite structure, except for x = 0.5 that grew parallel to the (1 1 0) direction. For some of the films TiN was detected as impurity phase. Scanning electron microscopy revealed that the films are dense and homogeneous with thicknesses around 350 nm. Atomic force microscopy showed that the surface roughness of the films varied between 4.2 and 14.1 nm. The employed substrate had a strong influence on the electrical properties. Films grown on STO exhibited a metallic behaviour, in contrast to the films grown on MgO, which were insulating.     

Growth and strain investigation of Cd0.96Zn0.04Te/GaAs by hot-wall epitaxy

A higher quality and a nearly stoichometric composition of Cd_1−yZn_yTe (y=0.04) epilayers have been successfully grown on a GaAs substrate by hot-wall epitaxy (HWE). The growth conditions regarding preheating treatment and Cd reservoir temperature were optimized. The relationship between quality and thickness was examined by four-crystal X-ray rocking curves and the best value of 120 arcsec for full width at half maximum (FWHM) was obtained. The dislocations on the interface, generated from the difference in lattice constants, were directly observed by high-resolution electron microscopy (HREM). The variation of strain with epilayer thickness shows that the density of extended defects in the epilayer decreases rapidly increasing the thickness up to 5 μm. When the epilayer thickness reaches 20 μm, the strain almost becomes zero. This result suggests that the high-quality epilayer, same as the bulk crystal, can be obtained by increasing the thickness. Photoluminescence (PL) spectra at 4.2 K show that bound-exciton (BE) emission is dominative. The strain relaxation by misfit dislocations were also explored by HREM.     

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.