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.     

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.     

Non-collinear magnetism and exchange couplings in FeCo/Mn superlattices

We present the main results of a systematic study of the magnetic properties of Fe_xCo_{1 − x}/Mn_n for various Fe concentrations x and Mn thicknesses n. We show that the magnetic order in the Mn spacer changes from collinear to non-collinear when the Fe concentration decreases. This behaviour is discussed in relation with a bulk ‘canted’ magnetic state nearly degenerate with the collinear AF order. The origin of the exchange of stability between these two magnetic states is ascribed to a stronger collinear character of the Fe/Mn interface than the Co/Mn one.     

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.     

The electronic structure of colossal magnetoresistive manganites

The ferromagnetic manganites A_{1 − x}B_xMnO₃ (A a trivalent element and B divalent) have been the subject of intensive study in the past few years. These manganites exhibit colossal magnetoresistance (CMR), i.e. their resistance can drop several orders of magnitude under influence of an external magnetic field. Electronic structure calculations show that these manganites are half-metallic: they are conducting for one spin direction exclusively. The possible relation between half-metallic magnetism and colossal magnetoresistance is discussed.     

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.     

Hardness, internal stress and fracture toughness of epitaxial AlxGa1−xAs films

Vickers microhardness indentations of 10 μm (001) oriented epilayers of Al_xGa_1−xAs on GaAs substrates have been utilized to evaluate the hardness H_v, the internal stress, and the fracture toughness K_Ic of the layers as a function of their composition parameter x. The hardness H_v varies linearly according to: (6.9-2.2x) GPa and K_Ic increases linearly with x according to: K_1c = (0.44+1.30x) 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 H_v measurements, but for K_Ic 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 K_Ic evaluation. Also, a simple theory for the influence of internal stresses on the K_Ic results has been developed.     

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.     

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.     

Computer simulation of martensitic textures

We consider a Ginzburg-Landau model free energy F(ε, e₁, e₂) for a (2D) martensitic transition, that provides a unified understanding of varied twin/tweed textures. Here F is a triple well potential in the rectangular strain (ε) order parameter and quadratic e₁², e₂² in the compressional and shear strains, respectively. Random compositional fluctuations η(r) (e.g. in an alloy) are gradient-coupled to ε, ˜ − ∑_rε(r)[(Δ_x² − Δ_y²)η(r)] in a “local-stress” model. We find that the compatibility condition (linking tensor components ε(r) and e₁(r), e₂(r)), together with local variations such as interfaces or η(r) fluctuations, can drive the formation of global elastic textures, through long-range and anisotropic effective ε-ε interactions. We have carried out extensive relaxational computer simulations using the time-dependent Ginzburg-Landau (TDGL) equation that supports our analytic work and shows the spontaneous formation of parallel twins, and chequer-board tweed. The observed microstructure in NiAl and Fe_xPd_{1 − x} alloys can be explained on the basis of our analysis and simulations.     

Structural and optical characterization of β-FeSi2 layers on Si formed by ion beam synthesis

Structural and optical properties have been investigated for surface β-FeSi₂ layers on Si(100) and Si(111) formed by ion beam synthesis using ⁵⁶Fe ion implantations with three different energies (140–50 keV) and subsequent two-step annealing at 600 °C and up to 915 °C. Rutherford backscattering spectrometry analyses have revealed Fe redistribution in the samples after the annealing procedure, which resulting in a Fe-deficient composition in the formed layers. X-ray diffraction experiments confirmed the existence of /gb-FeSi₂ by annealing up to 915 °C, whereas the phase transformation from the β to phase has been induced at 930 °C. In photoluminescence measurements at 2 K, both β-FeSi₂/Si(100) and β-FeSi₂/Si(111) samples, after annealing at 900–915 °C for 2 h, have shown two dominant emissions peaked around 0.836 eV and 0.80 eV, which nearly coincided with previously reported PL emissions from the sample prepared by electron beam deposition. Another β-FeSi₂/Si(100) sample has shown sharp emissions peaked at 0.873 eV and 0.807 eV. Optical absorption measurements at room temperature have revealed the allowed direct bandgap of 0.868–0.885 eV as well as an absorption coefficient of the order of 10⁴ cm⁻¹ near the absorption edge for all samples.     

Morphological changes induced by thermal anneals in NiFe/Ag multilayers; their relation to the resistive and magnetoresistive properties

(Ti, Al)N films have drawn much attention as alternatives for TiN coatings, which are oxidized easily in air above 500 °C. We have investigated the effect of Al content on the oxidation resistance of (Ti_{1 − x}Al_x)N films prepared by r.f. reactive sputtering.

(Ti_{1 − x}Al_xN films (O ≤ x ≤ 0.55) were deposited onto fused quartz substrates by r.f. reactive sputtering. Composite targets with five kinds of Al-to-Ti area ratio were used. The sputtering gas was Ar (purity, 5 N) and N₂ (5 N). The flow rate of Ar and N₂ gas was kept constant at 0.8 and 1.2 sccm, respectively, resulting in a sputtering pressure of 0.4 Pa. The r.f. power was 300 W for all experiments. Substrates were not intentionally heated during deposition. The deposited films (thickness, 300 nm) were annealed in air at 600 900 °C and then subjected to X-ray diffractometer and Auger depth profiling.

The as-deposited (Ti_{1 − x}Al_x)N films had the same crystal structure as TiN (NaCl type). Al atoms seemed to substitute for Ti in lattice sites. The preferential orientation of the films changed with the Al content of the film, x. Oxide layers of the films grew during annealing and became thicker as the annealing temperature increased. The thickness of the oxide layer grown on the film surface decreased with increasing Al content in the film. For high Al content films an Al-rich oxide layer was grown on the surface, which seemed to prevent further oxidation. All of the films, however, were oxidized by 900 °C annealing, even if the Al content was increased up to 0.55.     

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.     

Ferroelectric/superconductor heterostructures

This review covers the fabrication and characterization of ferroelectric/superconductor heterostructures such as Pb(Zr_xTi_1−x)O₃/YBa₂Cu₃O_7−δ (YBCO), BaTiO₃/YBCO and Ba_xSr_1−xTiO₃/YBCO etc. on various single crystal substrates. Pulsed laser deposition, laser molecular beam epitaxy, and magnetron-sputtering methods are compared. This report shows that pulsed laser deposition equipped with in situ reflection high-energy electron diffraction is a good method to control the growth mode of YBCO thin films. Furthermore, laser molecular beam epitaxy is a superb method for research of complex oxide films and their superlattices. Atomic force microscopy and transmission electron microscopy showed the ferroelectric films grown on the rough surface of the YBCO films produced high-density planar defects in the film and is detrimental to the ferroelectric/dielectric properties of the heterostructures. Therefore, for device usage, it is more advantageous to use SrRuO₃ than YBCO as the bottom electrode material. For growing atomically smooth surface films step-flow mode is highly recommended. Prospects of microwave device application of the ferroelectric/superconductor heterostructures are discussed, and proposed the BSTO films as the best candidate for passive microwave components.     

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.     

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.     

Epitaxial grown K1−xRbxTiOPO4 films with extremely flat surfaces for waveguiding

We report on epitaxial {1 0 0} K_1−xRb_xTiOPO₄ waveguide films for the visible spectral range grown on KTiOPO₄ substrates by liquid phase epitaxy. Using the m-line technique a refractive index increase of Δn_x≈0.007 and Δn_z≈0.004 for TM and TE polarisation has been determined for a K_0.78Rb_0.22TiOPO₄ film. Optical transmission and nearfield distribution are comparable to conventional ion-exchanged waveguides. Typical attenuation of about 1 dB/cm for both TM and TE polarisation was obtained at λ=532 and 1064 nm. Energy-dispersive X-ray spectrometry reveals solid-solution films with graded rubidium composition profiles. X-ray rocking curve analyses confirm the epitaxial growth process and indicate perfect and relaxed K_1−xRb_xTiOPO₄ films. Atomic force microscopy investigations reveal regular step structures with step heights Δh<1.3 nm resulting in rms-roughness values of ≈0.4 nm.     

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.     

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.     

Sb segregation in Si and SiGe: effect on the growth of self-organised Ge dots

The segregation and incorporation coefficients of antimony (Sb) in Si_1−xGe_x buried doped layers were investigated simultaneously using specific temperature sequences. We first showed an exponential kinetic evolution of Sb surface segregation in Si. In contrast such an evolution could not be observed in Si_1−xGe_x because of the Sb thermal desorption, at growth temperatures of 550°C. We also showed an increased surface segregation increasing with the partial Ge concentration in Si_1−xGe_x alloys, which was explained by a decrease of the kinetic barrier for Sb atoms mobility. It was, therefore, possible to determine the growth conditions to obtain a Si_1−xGe_x doped layer with a controlled incorporation level and a negligible surface segregation obtained by the thermal desorption of the Sb surface coverage. Finally, using Sb surfactant mediated growth, we found Ge dots with lateral sizes reduced by a factor of 2.8 and density multiplied by a factor of four as compared to dots directly deposited on Si(001).