High-temperature growth of very high germanium content SiGe virtual substrates

We have first of all studied (in reduced pressure–chemical vapour deposition) the high-temperature growth kinetics of SiGe in the 0–100% Ge concentration range. We have then grown very high Ge content (55–100%) SiGe virtual substrates at 850 °C. We have focused on the impact of the final Ge concentration on the SiGe virtual substrates’ structural properties. Polished Si_0.5Ge_0.5 virtual substrates were used as templates for the growth of the high Ge concentration part of such stacks, in order to minimize the severe surface roughening occurring when ramping up the Ge concentration. The macroscopic degree of strain relaxation increases from 99% up to values close to 104% as the Ge concentration of our SiGe virtual substrates increases from 50% up to 100% (discrepancies in-between the thermal expansion coefficients of Si and SiGe). The surface root mean square roughness increases when the Ge concentration increases, reaching values close to 20 nm for 100% of Ge. Finally, the field (the pile-up) threading dislocations density (TDD) decreases as the Ge concentration increases, from 4×10⁵ cm⁻² (1–2×10⁵ cm⁻²) for [Ge]=50% down to slightly more than 1×10⁵ cm⁻² (a few 10⁴ cm⁻²) for [Ge]=88%. For [Ge]=100%, the field TDD is of the order of 3×10⁶ cm⁻², however.     

Oxygen-pressure dependence of the crystallinity of MgO films grown on Si(1 0 0) by PLD

Effects of the oxygen partial pressure on pulsed-laser deposition of MgO buffer layers on silicon substrates were investigated. The overall growth process was monitored in situ by reflection high-energy electron diffraction (RHEED) method. It was found that the crystallinity and surface morphology of the MgO films were strongly affected by oxygen partial pressure in the deposition chamber. The oxygen-pressure dependence could be explained in terms of interactions of oxygen with species in the plume-like plasma. The MgO film obtained at an optimal oxygen-pressure range of 1×10⁻²–1 Pa exhibited an atomic-smooth and defect-free surface (the root-mean-square roughness being as low as 0.82 nm). For the metal–insulator–metal (MIM) structure of Au/MgO (150 nm)/TiN prepared at the optimal growth conditions achieved a very low leak current density of 10⁻⁷ A cm⁻² at an electric field of 8×10⁵ V cm⁻¹ and the permittivity (ε_r) of about 10.6, virtually the same as that of the bulk MgO single crystals.     

Diffusion of vanadium in GaAs

The diffusion of Vanadium has been studied in V-doped GaAs layers (GaAs:V) grown by Metal-Organic Chemical Vapour Deposition (MOCVD) using secondary ion mass spectroscopy (SIMS). The vanadium (V) concentration profiles of sandwiched structures made of alternatively undoped and V doped GaAs layers have shown a concentration independent diffusion coefficient (D_V) for varying V doping levels from 10¹⁸ to 10¹⁹ cm⁻³. Measurements of D_V at 550, 615 and 680 °C indicate that the temperature dependence of D_V can be represented by the Arrhenius equation:  cm² s⁻¹. It is suggested that V diffuses via interstitial sites.     

Multiple-step annealing for 50% enhanced p-conductivity of GaN

In this article, multiple-step rapid thermal annealing (RTA) processes for the activation of Mg doped GaN are compared with conventional single-step RTA processes. The investigated multiple-step processes consist of a low temperature annealing step at temperatures between 350°C and 700°C with dwell times up to 5 min and a short time high temperature step. With optimized process parameters, and multiple-step processes, we achieved p-type free carrier concentrations up to 1–2×10¹⁸ cm⁻³. The best achieved conductivity, so far, lies at 1.2 Ω⁻¹ cm⁻¹. This is a 50% improvement compared to conventional single-step process at 800°C, 10 min.     

Growth and properties of (Pb0.90La0.10)TiO3 thick films prepared by RF magnetron sputtering with a PbO buffer layer

The (Pb_0.90La_0.10)TiO₃ [PLT] thick films (3.0 μm) with a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering method. The PLT thick films comprise five periodicities, the layer thicknesses of (Pb_0.90La_0.10)TiO₃ and PbO in one periodicity are fixed. The PbO buffer layer improves the phase purity and electrical properties of the PLT thick films. The microstructure and electrical properties of the PLT thick films with a PbO buffer layer were studied. The PLT thick films with a PbO buffer layer possess good electrical properties with the remnant polarization (P_r=2.40 μC cm⁻²), coercive field (E_c=18.2 kV cm⁻¹), dielectric constant (ε_r=139) and dielectric loss (tan δ=0.0206) at 1 kHz, and pyroelectric coefficient (9.20×10⁻⁹ C cm⁻² K⁻¹). The result shows the PLT thick film with a PbO buffer layer is a good candidate for pyroelectric detector.     

Diffusion of Zn in stoichiometric LiTaO3

As stoichiometric LiTaO₃ (LT) draws a considerable attention for integrated optical waveguide devices, we have investigated Zn diffusion into this material by diffusing 70- nm-thick ZnO films deposited on y-cut LT substrates at 700–900 °C under various atmospheres. It was observed that the surface quality was very sensitive to pressure, but weakly affected by other diffusion conditions such as temperature and atmosphere. While the surface degraded, being covered with some residuals after heat treatment at the atmospheric pressure, it was very smooth and clear when the pressure was lowered below about 10 Torr. Another feature of Zn-diffused stoichiometric LT is that the crystal maintains its transparency even after diffusion at a pressure as low as 0.1 Torr, thus without a post-annealing step required. The diffusion coefficient varied from D=1.1×10^-2 to 5.5×10⁻¹ μm²/h in the given temperature range, with an activation energy of .     

A study of the degree of relaxation of AlGaN epilayers on GaN template

The strain state of 570 nm Al_xGa_1−xN layers grown on 600 nm GaN template by metal organic chemical vapor deposition was studied using Rutherford backscattering (RBS)/channeling and triple-axis X-ray diffraction measurements. The results showed that the degree of relaxation (R) of Al_xGa_1−xN layers increased almost linearly when x0.42 and reached to 70% when x=0.42. Above 0.42, the value of R varied slowly and Al_xGa_1−xN layers almost full relaxed when x=1 (AlN). In this work the underlying GaN layer was in compressive strain, which resulted in the reduction of lattice misfit between GaN and Al_xGa_1−xN, and a 570 nm Al_xGa_1−xN layer with the composition of about 0.16 might be grown on GaN coherently from the extrapolation. The different shape of (0 0 0 4) diffraction peak was discussed to be related to the relaxation.     

Growth and spectral properties of Nd:LaVO4 crystal

This paper reports the growth and spectral properties of 3.5 at% Nd³⁺:LaVO₄ crystal with diameter of 20×15 mm² which has been grown by the Czochralski method. The spectral parameters were calculated based on Judd–Ofelt theory. The intensity parameters Ω_λ are: Ω₂=2.102×10⁻²⁰ cm², Ω₄=3.871×10⁻²⁰ cm², Ω₆=3.235×10⁻²⁰ cm². The radiative lifetime τ_r is 209 μs and calculated fluorescence branch ratios are: β₁(0.88μm)=45.2, β₂(1.06μm)=46.7, β₃(1.34μm)=8.1. The measured fluorescence lifetime τ_f is 137 μm and the quantum efficiency η is 65.6%. The absorption band at 808 nm wavelength has an FWHM of 20 nm. The absorption and emission cross sections are 3×10⁻²⁰ and 6.13×10⁻²⁰ cm², respectively.     

Growth behavior and microstructure of Ge self-assembled islands on nanometer-scale patterned Si substrate

We investigate the growth behavior and microstructure of Ge self-assembled islands of nanometer dimension on Si (0 0 1) substrate patterned with hexagonally ordered holes of 25 nm depth, 30 nm diameter, and 7×10¹⁰ cm⁻² density. At 9 Å Ge coverage and 650 °C growth temperature, Ge islands preferentially nucleate inside the holes, starting at the bottom perimeter. Approximately 14% of the holes are filled by Ge islands. Moiré fringe analysis reveals partial strain relaxation of about 72% on average, which is not uniform even within a single island. Crystalline defects such as dislocation are observed from islands smaller than 30 nm. Increased Ge coverage to 70 Å forms larger aggregates of many interconnected islands with slightly increased filling factor of about 17% of the holes. Reducing the growth temperature to 280 °C results in much higher density of islands with a filling factor of about 80% and with some aggregates. The results described in this report represent a potential approach for fabricating semiconductor quantum dots via epitaxy with higher than 10¹⁰ cm⁻² density.     

Influence of cooling rate on the liquid-phase epitaxial growth of Zn3P2

We report the liquid-phase epitaxial growth of Zn₃P₂ on InP (1 0 0) substrates by conventional horizontal sliding boat system using 100% In solvent. Different cooling rates of 0.2–1.0 °C/min have been adopted and the influence of supercooling on the properties of the grown epilayers is analyzed. The crystal structure and quality of the grown epilayers have been studied by X-ray diffraction and high-resolution X-ray rocking measurements, which revealed a good lattice matching between the epilayers and the substrate. The supercooling-induced morphologies and composition of the epilayers were studied by scanning electron microscopy and energy dispersive X-ray analysis. The growth rate has been calculated and found that there exists a linear dependence between the growth rate and the cooling rate. Hall measurements showed that the grown layers are unintentionally doped p-type with a carrier mobility as high as 450 cm²/V s and a carrier concentration of 2.81×10¹⁸ cm⁻³ for the layers grown from 6 °C supercooled melt from the cooling rate of 0.4 °C/min.     

Textural properties of synthetic nano-calcite produced by hydrothermal carbonation of calcium hydroxide

The hydrothermal carbonation of calcium hydroxide (Ca(OH)₂) at high pressure of CO₂ (initial P_CO2=55 bar) and moderate to high temperature (30 and 90 °C) was used to synthesize fine particles of calcite. This method allows a high carbonation efficiency (about 95% of Ca(OH)₂–CaCO₃ conversion), a significant production rate (48 kg/m³ h) and high purity of product (about 96%). However, the various initial physicochemical conditions have a strong influence on the crystal size and surface area of the synthesized calcite crystals. The present study is focused on the estimation of the textural properties of synthesized calcite (morphology, specific surface area, average particle size, particle size distribution and particle size evolution with reaction time), using Rietveld refinements of X-ray diffraction (XRD) spectra, Brunauer–Emmett–Teller (BET) measurements, and scanning electron microscope (SEM) and transmission electron microscope (TEM) observations. This study demonstrate that the pressure, the temperature and the dissolved quantity of CO₂ have a significant effect on the average particle size, specific surface area, initial rate of precipitation, and on the morphology of calcium carbonate crystals. In contrast, these PTx conditions used herein have an insignificant effect on the carbonation efficiency of Ca(OH)₂.

Finally, the results presented here demonstrate that nano-calcite crystals with high specific surface area (S_BET=6–10 m²/g) can be produced, with a high potential for industrial applications such as adsorbents and/or filler in papermaking industry.     

Growth of single crystals of the new double perovskite Ba2HoRu1−xCuxO6 from high temperature solutions

Single crystals of Ba₂HoRu_1−xCu_xO₆ have been grown from high temperature solutions using PbO–PbF₂ as solvent in the temperature range 1150–1250 °C. Crystals with a six sided plate like morphology measuring up to 3 mm across and 0.5 mm thick and polyhedral habit measuring up to 2 and 1 mm in thick mass were obtained. Powder X-ray diffraction patterns obtained on the crystals were indexed to give a monoclinic space group P2₁/n with lattice parameters a=5.875(2), b=5.874(3), c=8.960(1) and β=89.995(2)°. The crystals with x=0 show a single anomaly at 6.5 K corresponding to an antiferromagnetic phase with . The crystals containing Cu show additional anomalies at 18 and 48 K. The SEM and EDS analysis reveals a 2116 phase.     

Compositional plane of a wide-gap semiconductor CaCdSeS lattice-matched to InP and GaAs

We have investigated compositional plane of a wide band gap solid solution semiconductor Ca_1−xCd_xSe_1−yS_y (x0.32) using powder synthesis under thermal equilibrium condition. The solubility limit at 1273 K varies with respect to the Se concentration y, taking a minimum Cd solubility limit of 0.12 at y=0.8 and a maximum limit of 0.32 at y=1.0. It is found that the system can be lattice-matched to GaAs and InP under covering the energy band gap of ultraviolet–visible region. These results allow to design optoelectronic devices adopting the Ca_1−xCd_xSe_1−yS_y system.     

Growth and spectral properties of Nd-doped Sr3Y(BO3)3 crystal

A new crystal of Nd³⁺:Sr₃Y(BO₃)₃ with dimension up to 25×35 mm² was grown by Czochralski method. Absorption and emission spectra of Nd³⁺: Sr₃Y(BO₃)₃ were investigated . The absorption band at 807 nm has a FWHM of 18 nm. The absorption and emission cross sections are 2.17×10⁻²⁰ cm² at 807 nm and 1.88×10⁻¹⁹ cm² at 1060 nm, respectively. The luminescence lifetime τ_f is 73 μs at room temperature     

Hydroxyapatite crystallization in the presence of acetaminophen

The effect of acetaminophen; a widely used analgesic and fever reducing medicine; in supersaturated solutions of calcium phosphate was investigated under plethostatic conditions, at 37 °C, 0.15 M NaCl, pH 7.40. The rates of crystal growth measured in the presence of acetaminophen 1.654×10⁻⁴ mol dm⁻³ to 6.616×10⁻⁴ mol dm⁻³ were reduced by 43% to 79%, respectively. The inhibition effect on the crystal growth rate may be explained through adsorption onto the active growth sites. Kinetic analysis suggested Langmuir-type adsorption of acetaminophen on the HAP surface with a affinity value of 2.4×10⁻⁴ dm³ mol⁻¹, for the substrate in the concentration range investigated. The electrophoretic mobility measurements showed that in the presence of acetaminophen the charge of the acetaminophen covered HAP particles was shifted to more negative values as compared to bare HAP. In the presence of acetaminophen no changes observed in the HAP overgrown morphology or in the apparent order of crystallization.     

SiC epitaxial layer growth in a novel multi-wafer vapor-phase epitaxial (VPE) reactor

Experimental results are presented for SiC epitaxial layer growths employing a unique planetary SiC-VPE reactor. The high-throughput, multi-wafer (7×2″) reactor, was designed for atmospheric and reduced pressure operation at temperatures up to and exceeding 1600°C. Specular epitaxial layers have been grown in the reactor at growth rates ranging from 3–5 μm/h. The thickest layer grown to date is 42 μm thick. The layers exhibit minimum unintentional n-type doping of 1×10¹⁵ cm⁻³, and room temperature mobilities of 1000 cm²/V s. Intentional n-type doping from 5×10¹⁵ cm⁻³ to >1×10¹⁹ cm⁻³ has been achieved. Intrawafer layer thickness and doping uniformities (standard deviation/mean at 1×10¹⁶ cm⁻³) are typically 4 and 7%, respectively, on 35 mm diameter substrates. Moderately doped, 4×10¹⁷ cm⁻³, layers, exhibit 3% doping uniformity. Recently, 3% thickness and 10% doping uniformity (at 1×10¹⁶ cm⁻³) has been demonstrated on 50 mm substrates. Within a run, wafer-to-wafer thickness deviation averages 9%. Doping variation, initially ranging as much as a factor of two from the highest to the lowest doped wafer, has been reduced to 13% at 1×10¹⁶ cm⁻³, by reducing susceptor temperature nonuniformity and eliminating exposed susceptor graphite. Ongoing developments intended to further improve layer uniformity and run-to-run reproducibility, are also presented.     

Mechanochemical–hydrothermal synthesis of hydroxyapatite from nonionic surfactant emulsion precursors

Nanocrystalline hydroxyapatite [HA, Ca₁₀(PO₄)₆(OH)₂] powders were synthesized by the mechanochemical–hydrothermal method using emulsion systems consisting of aqueous phase, petroleum ether (PE) as the oil phase and biodegradable Tomadol 23–6.5 as the nonionic surfactant. (NH₄)₂HPO₄ and Ca(NO₃)₂ or Ca(OH)₂ were used as the phosphorus and calcium sources, respectively. The calcium source and emulsion composition had significant effects on the stoichiometry, crystallinity, thermal stability, particle size and morphology of final products. Disperse HA crystals with a 160 nm length and aspect ratio of ca. 6 were formed in an emulsion system containing 10 wt% PE, 60 wt% water and 30 wt% surfactant. The HA particles had needle morphology with a specific surface area of . With this technique, HA nanopowders with specific surface areas in the range of 72– were produced.     

Growth of Na modified potassium lithium niobate crystal by the Czochralski method

Transparent Na modified potassium lithium niobate (Na_0.23K_2.60Li_1.82Nb_5.35O_15.70; NKLN) crystal was successively grown by the Czochralski method using RF induction heating from melt composition Na₂O:K₂O : Li₂O:Nb₂O₅=2:30:25:43 mol%. NKLN crystal showed a tetragonal tungsten bronze structure with lattice constants a=12.5446±0.0010 Å and c=4.0129±0.0005 Å at room temperature. The dielectric constant along the c-axis ε₃₃ showed a sharp maximum around 480 °C. Optical transmission edge was 370 nm and optical transmission spectra showed no absorption at wavelengths ranging from 380 to 800 nm. The structural and optical properties of NKLN were similar to those of the near stoichiometric KLN crystals. We believe that the growth of NKLN by the Czochralski method has an advantage for a large size and high-quality crystal.     

Growth and characterization of epitaxial layers on aluminum nitride substrates prepared from bulk, single crystals

A comparative study of epitaxy of AlN, GaN and their alloys, grown on c-axis and off-axis substrates of single-crystal aluminum nitride has been carried out. Growth on off-axis (>30°) substrates appears to result in rough surfaces and the absence of two-dimensional electron gas (2DEG). However, smooth morphologies were demonstrated for both homoepitaxial and heteroepitaxial growth on on-axis (<2°) substrates. On one of these oriented substrates a 2DEG, with a mobility of 1000 cm²/V s and a sheet density of 8.5×10¹² cm⁻² at room temperature, was also demonstrated for the first time.     

Calcite crystallization on oxadiazole-terpyridine copolymer

The nucleation and further crystallization of calcite on oxadiazole-terpyridine copolymer were investigated by the constant composition method. The apparent order of the crystallization process was found to be 2.3±0.2 indicative of a surface diffusion-controlled spiral growth mechanism. The number of ions forming the critical nucleus was found experimentally to be (n^*=) 3 in accordance with the simulation by the PM3 method included in the MOPAC program package. The surface energy of the growing phase was found to be 29.5 mJ m⁻².