Fabrication of a depletion mode GaAs MOSFET using Al2O3 as a gate insulator through the selective wet oxidation of AlAs

A 1 μm thick undoped GaAs buffer layer, a 1500 Å thick n-type GaAs layer, an undoped 500 Å thick AlAs layer and a 50 Å thick GaAs cap layer were consecutively grown by molecular beam epitaxy (MBE) on a [100] oriented semi-insulating GaAs substrate. The AlAs layer was oxidized in a N₂ bubbled H₂O vapor ambient at 400°C for 3 h and fully converted to Al₂O₃ for use as a gate insulator. The I–V characteristics, having a maximum drain current of 10.6 mA, a current cut-off voltage of −4.5 V and a maximum transconductance value of 11.25 mS/mm, indicate that the selective wet thermal oxidation of AlAs/GaAs was successful in producing a depletion mode GaAs MOSFET.     

Thin film TiO2 on nickel(110): an STM study

Titanium oxide films grown on the surface of a Ni(110) single crystal have been investigated using STM, LEED and AES for Ti coverages ranging from 1 to 10 ml [1 ml of Ti is defined here as equivalent to the number of top layer Ni atoms of Ni(110)]. The oxide overlayers were prepared by vapour phase deposition of Ti followed by oxidation in 1×10⁻⁷ mbar O₂ at 800 K. Oxidation of Ti coverages between 1 and 10 ML results in STM images indicating the presence of two terminations coexisting on the surface. One termination consists of islands of epitaxial rutile TiO₂(110), the second having cell parameters of 2.98±0.1×3.15±0.2 Å. The latter unit cell is consistent with TiO(001) (2.99×2.99 Ų). On oxidation of higher Ti coverages (10 ml), only epitaxial rutile TiO₂(110) islands are observed.     

Alkoxide route to La0.5Sr0.5CoO3 epitaxial thin films on SrTiO3

An all alkoxide based sol–gel route was investigated for preparation of epitaxial La_0.5Sr_0.5CoO₃ (LSCO) films on 100 SrTiO₃ (STO) substrates. Films with 20–30 to 80–100 nm thickness were prepared by spin-coating 0.2–0.6 M (metal) solutions on the STO substrates and heat treatment to 800 °C at 2 °C min^− 1, 30 min, in air. The films were epitaxial with a cube-on-cube alignment and the LSCO cell was strained to match the STO substrate of 3.905 Å closely; a and b = 3.894 Å and 3.897 Å for the 20–30 and 80–100 nm films, respectively. The c-axis was compressed to 3.789 Å and 3.782 Å for the 20–30 and 80–100 nm films, respectively, which resulted in an almost unchanged cell volume as compared to polycrystalline film and nano-phase powders prepared in the same way. The SEM study showed mainly very smooth, featureless surfaces, but also some defects. A film prepared in the same way on an -Al₂O₃ substrate was dense and polycrystalline with crystallite sizes in the range 10–50 nm and gave cubic cell dimensions of a_c = 3.825 Å. The conductivity of the ca 30–40 nm thick polycrystalline film was 1.7 mΩcm, while the epitaxial 80–100 nm film had a conductivity of around 1.9 mΩcm.     

Ni-Al2O3 selective cermet coatings for photothermal conversion up to 500°C

Spectrally selective Ni-Al₂O₃ composite films were prepared by r.f. planar magnetron sputtering using hot-pressed targets of two different compositions. The composition of the films were varied by co-sputtering the target with additional nickel pellets distributed uniformly on the target. The composition of the films were analysed by energy-dispersive X-ray analysis. Optical simulations were carried out with the experimentally measured n and k and the published n and k of the metallic substrate. R.f.-sputtered SiO₂ and Al₂O₃ were used as antireflection coatings. From the computer optimization studies we found that = 0.94 and (100°C) = 0.07 could be obtained with 650 Å of Ni-Al₂O₃ (f = 0.61) antireflected with 780 Å SiO₂ on a nickel-coated glass substrate. When molybdenum-coated nickel-plated stainless steel substrates were used, the films were found to be stable up to 500°C in air.     

HMTTEF.C60-A 2-D close-packed layered compound of C60

The analysis of high resolution synchrotron X-ray powder diffraction data of HMTTEF.C₆₀, (HMTTEF = hexamethylenetetratellurafulvalene) gave a triclinic unit cell with a 9.9297 Å, B = 9.9359 Å, C = 13.1472 Å, = 106.966 °, β = 95.887 ° and γ = 118.252 ° in the space group P . Steric considerations suggest that there is a nearly close-packed layer of C₆₀ molecules in the plane, and HMTTEF molecules are sandwiched between layers of C₆₀. The compound is insulating and weakly paramagnetic and the charge-transfer is small.     

Thermal stability and crystallization kinetics of sputtered amorphous Si3N4 films

The crystallization of thin silicon nitride (Si₃N₄) films deposited on polycrystalline SiC substrates was investigated by X-ray diffractometry as a function of annealing time. The amorphous Si₃N₄ films were produced by means of reactive r.f. magnetron sputtering. Annealing at temperatures between 1300 and 1700 °C led to the formation of crystalline films composed of -Si₃N₄ and β-Si₃N₄. The fraction of β-Si₃N₄ in the films reaches approximately 40% at temperatures above 1550 °C. Both polymorphic modifications were formed simultaneously during the crystallization process. A transformation of -Si₃N₄ to β-Si₃N₄ could not be observed in the time and temperature range investigated. The crystallization process of amorphous Si₃N₄ can be described according to the Johnson–Mehl–Avrami–Kolmogorov (JMAK) formalism, assuming a three-dimensional, interface controlled grain growth from pre-existing nuclei. The rate constants show an Arrhenius behaviour with an activation enthalpy of approximately 5.5 eV.     

Chemical-mechanical polishing of SiO2 thin films studied by X-ray reflectivity

Specular X-ray reflectivity from SiO₂ thin films prepared on silicon substrates by plasma-enhanced chemical vapor deposition showed the films to have a characteristic width of the decay in density at the free surface of 17 Å, to be about three-quarters the density of -quartz, and to have an interfacial layer at the silicon interface that was of the order of 100 Å wide and less dense than the bulk of the film. After chemical-mechanical polishing the characteristic width of the decay in density at the free surface was reduced to 10 Å; furthermore, the near-surface region to a depth of 30 Å had a greater density than the as-deposited film. Off-specular reflectivity confirmed that the decrease in characteristic width at the free surface was due to reduced roughness upon polishing and also revealed that the lateral correlation length in the limit of long wavelengths was the same for both polished and unpolished samples. The compression of the near-surface region during polishing is believed to enhance the dissolution of SiO₂ into the slurry which is necessary to achieve smooth surfaces.     

Direct liquid injection metal-organic chemical vapor deposition of HfO2 thin films using Hf(dimethylaminoethoxide)4

Hf(OCH₂CH₂NMe₂)₄, [Hf(dmae)₄] (dmae=dimethylaminoethoxide) was synthesized and used as a chemical vapor deposition precursor for depositing Hf oxide (HfO₂). Hf(dmae)₄ is a liquid at room temperature and has a moderate vapor pressure (4.5 Torr at 80 °C). It was found that HfO₂ film could be deposited as low as 150 °C with carbon level not detected by X-ray photoelectron spectroscopy. As deposited film was amorphous but when the deposition temperature was raised to 400 °C, X-ray diffraction pattern showed that the film was polycrystalline with weak peak of monoclinic (020). Scanning electron microscope analysis indicated that the grain size was not significantly changed with the increase of the annealing temperature. Capacitance–voltage measurement showed that with the increase of annealing temperature, the effective dielectric constant was increased, but above 900 °C, the effective dielectric constant was decreased due to the formation of interface oxide. For 500 Å thin film, the dielectric constant of HfO₂ film annealed at 800 °C was 20.1 and the current–voltage measurements showed that the leakage current density of the HfO₂ thin film annealed at 800 °C was 2.2×10⁻⁶ A/cm² at 5 V.     

Crystal structure and characterization of [2,5-(CH3O)2C6H3NH3]4P4O12·2H2O

The preparation, crystal structure, TG–DTA analysis and spectroscopy investigation are reported for the 2,5-dimethoxy phenyl ammonium cyclotetraphosphate dihydrate [2,5-(CH₃O)₂C₆H₃NH₃]₄P₄O₁₂·2H₂O. This new compound is triclinic P with unit cell dimensions: a = 7.438(5) Å, b = 11.841(7) Å, c = 12.354(4) Å,  = 96.61(4)°, β = 98.35(4)°, γ = 102.60(6)°, Z = 1 and V = 1038.0(1) Å³. Its crystal structure has been determined and refined to R = 0.049, with 5128 independant reflections. The structure can be described by rows of P₄O₁₂ ring anions along the a axis; between these rows are located the organic groups, connected to them by hydrogen bonds.     

Synthesis, structure refinement and characterization of tetrahydrated acid gadolinium diphosphate HGdP2O7·4H2O

Synthesis and single crystal structure are reported for a new gadolinium acid diphosphate tetrahydrate HGdP₂O₇·4H₂O. This salt crystallizes in the monoclinic system, space group P2₁/n, with the following unit-cell parameters: a = 6.6137(2) Å, b = 11.4954(4) Å, c = 11.377(4) Å, β = 87.53(2)° and Z = 4. Its crystal structure was refined to R = 0.0333 using 1783 reflections. The corresponding atomic arrangement can be described as an alternation of corrugated layers of monohydrogendiphosphate groups and GdO₈ polyhedra parallel to the () plane. The cohesion between the different diphosphoric groups is provided by strong hydrogen bonding involving the W4 water molecule.

IR and Raman spectra of HGdP₂O₇·4H₂O confirm the existence of the characteristic bands of diphosphate group in 980–700 cm⁻¹ area. The IR spectrum reveals also the characteristic bands of water molecules vibration (3600–3230 cm⁻¹) and acidic hydrogen ones (2340 cm⁻¹). TG and DTA investigations show that the dehydration of this salt occurs between 79 and 900 °C. It decomposes after dehydration into an amorphous phase. Gadolinium diphosphate Gd₄(P₂O₇)₃ was obtained by heating HGdP₂O₇·4H₂O in a static air furnace at 850 °C for 48 h.     

Graphite formation on Ni film by chemical vapor deposition

Thin film formation of graphite by chemical vapor deposition using 2-methyl-1,2′-naphthyl ketone as a starting material was carried out on Ni film substrates. On Ni films directly deposited on quartz glass, the graphite films were obtained when the Ni film thickness was above 1 000 Å and above 5 000 Å at 700 °C and 1 000 °C, respectively. Depositions on thinner Ni film substrates comprise amorphous carbon (a-C) or graphite tubes which was owing to the thermal coagulation of the Ni film into droplets. On the other hand, graphite film was obtained on the Ni film with thickness 10 Å when a-C was inserted between the Ni film and the quartz glass. The coagulation of the Ni film is considered to be avoided by inserting a-C layer.     

Growth kinetics of Mo3Si layer in the Mo5Si3/Mo diffusion couple

The diffusion processes and growth kinetics of the Mo₃Si silicide layer occurring at annealing the Mo₅Si₃/Mo diffusion couple between 1180 and 1800 °C were studied by electrothermography. The experimental results are supplemented with calculations on the behaviour of the initial Mo₅Si₃/Mo diffusion couple on the basis of the reaction diffusion model describing the transformation of the Mo₅Si₃ layer into a Mo₃Si one. The values of the parabolic growth constant for Mo₃Si layer were determined and the silicon diffusion coefficient in the Mo₃Si phase was calculated: D = 0.165 exp[(− 247 ± 10) / RT], cm²/s, where the activation energy is expressed in terms of kJ/mol.     

Physical model and numerical results of dissociation kinetics of hydrogen-passivated Si/SiO2 interface defects

A simple model of thermal dissociation and recovery of hydrogen-passivated silicon defects at the Si/SiO₂ interface, such as P_b - centers, during vacuum thermal annealing has been suggested. his model considers reactions of hydrogen with defect states at the Si/SiO₂ interface and diffusion of liberated atomic and molecular hydrogen in a silicon dioxide film. The rate constants were calculated in diffusion approximation. A good agreement was obtained between the experimental and numerical simulation results for oxides with different thickness (204–1024 Å), grown, both, (111) and (100) samples and annealed in the temperature range (480–700°C).     

Amorphous Ni---N---W film as a diffusion barrier between aluminum and silicon

The influence of nitrogen on the diffusion barrier properties of amorphous Ni---W films was studied. Nitrogen was introduced into the amorphous Ni---W film by co-sputtering nickel and tungsten in a premixed gas mixture of 90% Ar and 10% N₂, resulting in the formation of amorphous Ni₃₀N₂₁W₄₉ film. X-ray analysis indicates a detectable crystallization of the amorphous film after 30 min annealing in vacuum at 600°C, accompanied by the formation of W₂N, but backscattering spectrometry (BS) reveals a reaction with silicon only at about 725°C. The Schottky barrier height of this amorphous film on n-Si is stable for 30 min annealing up to at least 550°C. With an aluminum overlayer, BS indicates that an amorphous Ni₃₀N₂₁W₄₉ film effectively prevents the metallurgical interaction between aluminum and silicon for 30 min up to 600°C. The Schottky barrier height of that contact configuration is also stable up to at least 550°C, suggesting that amorphous Ni---N---W films have attractive features as diffusion barriers.     

Thickness-dependent properties of sprayed iridium oxide thin films

Iridium oxide thin films with variable thickness were deposited by spray pyrolysis technique (SPT), onto the amorphous glass substrates kept at 350 °C. The volume of iridium chloride solution was varied to obtain iridium oxide thin films with thickness ranging from 700 to 2250 Å. The effect of film thickness on structural and electrical properties was studied. The X-ray diffraction (XRD) studies revealed that the as-deposited samples were amorphous and those annealed at 600 °C for 3 h in milieu of air were polycrystalline IrO₂. The crystallinity of Ir-oxide films ameliorate with film thickness thereby preferred orientation along (1 1 0) remains unchanged. The infrared spectroscopic results show Ir–O and Ir–O₂ bands. The room temperature electrical resistivity (ρ_RT) of these films decreases with increase in film thickness. The p-type semiconductor to metallic transition was observed at 600 °C.     

The effect of hydrogen in the mechanism of aluminum-induced crystallization of sputtered amorphous silicon using scanning Auger microanalysis

The metal-induced crystallization (MIC) of hydrogenated sputtered amorphous silicon (a-Si:H) using aluminum has been investigated using X-ray diffraction (XRD) and scanning Auger microanalysis (SAM). Hydrogenated, as well as non-hydrogenated, amorphous silicon (a-Si) films were sputtered on glass substrates, then capped with a thin layer of Al. Following the depositions, the samples were annealed in the temperature range 200 °C to 400 °C for varying periods of time. Crystallization of the samples was confirmed by XRD. Non-hydrogenated films started to crystallize at 350 °C. On the other hand, crystallization of the samples with the highest hydrogen (H₂) content initiated at 225 °C. Thus, the crystallization temperature is affected by the H₂ content of the a-Si. Material structure following annealing was confirmed by SAM. In this paper, a comprehensive model for MIC of a-Si is developed based on these experimental results.     

Stress variation in epitaxial GaAs films on silicon under thermal treatment

High quality epitaxial GaAs films of 1.8 and 6.3 μm thickness on silicon substrates were examined for lattice distortion, misalignment and curvature by X-ray diffraction (Bond method) at 20–400 °C. These films were deposited by the metal-organic chemical vapour deposition method on the (001) plane of silicon using a buffer layer produced at T_b = 370 or 460 °C. A top layer was then grown at T_t = 560 or 650 °C. The GaAs films contract more strongly on cooling than the substrate, which causes a curvature and a tetragonal distortion below a critical temperature T_c. This temperature varies on thermal treatment at 200–400 °C and approaches T_b, the growth temperature of the buffer layer. The tetragonal distortion can be stabilized, so that T_c approximates T_b, if the GaAs films are annealed for several days at 400 °C.     

Preparation of highly c-axis-oriented Bi4Ti3O12 thin films and their crystallographic, dielectric and optical properties

Bismuth titanate (Bi₄Ti₃O₁₂) thin films with a high c-axis orientation up to 99% were prepared on (100)-oriented silicon wafers by r.f. planar magnetron sputtering using a Bi₂TiO₅ ceramic target at a substrate temperature of 600 °C. From the Auger electron spectroscopy depth profile of the film, there is no evidence of interdiffusion of a specific element between the film and the substrate. Relative dielectric constant of these films depends on film thickness. The behavior was explained assuming a low-dielectric-constant interface layer. Using this assumption, the relative dielectric constant of Bi₄Ti₃O₁₂ film was estimated to be approximately 140. This value is close to that along the c axis in a bulk form. The remanent polarization and the coercive field were 0.8 μC cm⁻² and 20 kV cm⁻¹, respectively.     

Epitaxy of atomically flat CaF2 films on Si(1 1 1) substrates

The growth of CaF₂ films with a thickness of approximately 3–4 nm on well-oriented Si(1 1 1) substrates by molecular beam epitaxy at temperatures between 410 and 560 °C were investigated by ex vacuo atomic force microscopy. Layer-by-layer growth producing atomically flat CaF₂ surfaces has been observed in a very narrow growth temperature window between approximately 430 and 470 °C. Perfect triangular shaped islands of one CaF₂ layer height are found on the surface with all corners aligned with the Si directions, indicating a pure B-stacking of the CaF₂ film. Surprisingly, also the substrate steps have been overgrown without visible defects. Below 410 °C, two different island orientations revealed a mixture of A- and B-stacking areas in the films. Above 520 °C non-wetting of the CaF interface layer leads to epitaxial films with a rough surface morphology.     

Comparison between measurements on Rayleigh scattering and form-factor theories for 1 Å ≤ x ≤ 50 Å

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.