Oxidative annealing of ZnSe/GaAs heterostructures

We annealed ZnSe/GaAs heterostructures in the oxygen atmosphere and investigated structural and optical properties of the forming films using X-ray diffraction and photoluminescence. While highly textured ZnO films were synthesized via low-temperature processing (~ 500 °C), high temperature processing (~ 800 °C) promoted reaction at the film/substrate interface and Zn loss from the film surface resulting in the polycrystalline ZnGa₂O₄ and ZnO₂.     

Modeling of In segregation, stress and strain in InGaAs/GaAs(100) quantum well heterostructures

An advanced model for simulation of In segregation phenomena, stress and strain distribution during metal-organic chemical vapor deposition of InGaAs/GaAs(100) quantum well (QW) heterostructures based on representation of boundary gas layer as “quasi-liquid” has been suggested. Elastic energy was taken into account by considering epitaxy as a sequence of growth acts each resulted in the formation of ultrathin imaginary layers. The assumption that elastic influence is not distributed throughout the whole thickness of the substrate but affects only its near-surface layer has been postulated. Results of calculations of In profiles and stress distribution for heterostructures with single and multiple QWs for varied epitaxy conditions are provided. Various options of exploring the developed model for other materials and the limitations of applicability are discussed.     

Laser annealing of Pb(Zr0.52Ti0.48)O3 thin films for the pyroelectric detectors

Lead zirconate titanate (Pb(Zr_0.52Ti_0.48)O₃, PZT) thin films fabricated by magnetron sputtering technique on the Pt/Ti/SiO₂/Si substrates at room temperature, were annealed by means of CO₂ laser with resulting average substrate temperature below 500 °C. The crystal structure, surface morphology and pyroelectric properties of the PZT films before and after annealing were investigated by X-ray diffraction, atomic force microscopy, and pyroelectric measurements. The results show that the annealed PZT thin film with a laser energy density of 490 W/cm² for 25 s has a typical perovskite phase, uniform crystalline particles with a size of about 90 nm, and a high pyroelectric coefficient with 1.15 × 10^− 8 Ccm^− 2 K^− 1.     

Evolution of composition distribution of Si-capped Ge islands on Si(001)

The evolution of the composition distribution and microstructures of Ge islands on Si(001) during the Si overgrowth was investigated by atomic force microscopy combined with selective wet etching procedures. With increasing Si coverage to 5.4 nm, the uncapped Ge islands were found to change their shapes dramatically from domes to truncated pyramids, nanorings and eventually to the fully buried islands. Different atomic composition profiles in SiGe islands were observed at different Si coverages. Especially, the nanorings were found to have a Ge-rich core with a Si-rich periphery. Based on the experimental results, the Ge redistribution in islands during Si capping is not only correlated with the intermixing between Si capping layer and Ge islands, but also a strain-driven process.     

ZnTe/Si heterostructures grown by isothermal closed space sublimation

ZnTe films with different thickness were grown onto (001) Si substrates using isothermal closed space sublimation by alternated sublimation of Zn and Te sources. As a result ZnTe polycrystalline films were obtained with a strong [111] orientation as revealed by X-ray diffraction patterns. The reason for this polycrystalline nature of the samples comes from incomplete removal of SiO₂ from the Si substrate surface. A preferential adsorption of Te in the first stages of the growth was concluded from Rutherford backscattering spectra analysis.     

Growth paths for the sulfurization of Cu-rich Cu/In thin films

In-situ energy-dispersive X-ray diffraction performed at the BESSY and HASY-LAB synchrotron facilities was used to observe sulfur pressure dependent growth paths for the formation of CuInS₂ thin films from Cu-rich metallic precursors. CuInS₂ can form directly from the intermetallic phases, through binary sulfides or via CuIn₅S₈. Particular attention is given to the latter reaction sequence, typical of rapid thermal processing (RTP), with complementary EDS and SEM analysis.     

Optical properties of SiO2/nanocrystalline Si multilayers studied using spectroscopic ellipsometry

Using variable-angle spectroscopic ellipsometry, we measure the pseudo-dielectric functions of as-deposited and annealed SiO₂/SiO_x multilayers. The SiO₂(2 nm)/SiO_x(2 nm) multilayers are prepared under various deposition temperatures by ion beam sputtering. Annealing at temperatures above 1100 °C leads to the formation of Si nanocrystals (nc-Si) in the SiO_x layer of multilayers. Transmission electron microscopy images clearly demonstrate the existence of nc-Si. We assume a Tauc-Lorentz lineshape for the dielectric function of nc-Si, and use an effective medium approximation for SiO₂/nc-Si multilayers as a mixture of nc-Si and SiO₂. We successfully estimate the dielectric function of nc-Si and its volume fraction. We find that the volume fraction of nc-Si decreases after annealing, with increasing x in as-deposited SiO_x layer. This result is compared to expected nc-Si volume fraction, which is estimated from the stoichiometry of SiO_x.     

Luminescence and microstructures of Eu-doped in triple phosphate Ca8MgR(PO4)7 (R = La, Gd, Y) with whitlockite structure

Eu³⁺-doped triple phosphate Ca₈MgR(PO₄)₇ (R = La, Gd, Y) was synthesized by the general high temperature solid-state reaction. This phosphor was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and emission spectra. XRD and FT-IR analysis indicated that Ca₈MgR(PO₄)₇ (R = La, Gd, Y) crystallized in single-phase component with whitlockite-like structure (space group R3c) of β-Ca₃(PO₄)₂. Under the excitation of UV light, the phosphors show bright red emission assigned to the transition (⁵D₀ → ⁷F₂) at 612 nm. The crystallographic sites of Eu³⁺ ions in Ca₈MgR(PO₄)₇ (R = La, Gd, Y) host were discussed on the base of site-selective excitation and emission spectra, luminescence decay and its host crystal structure.     

Fabrication of ferromagnetic Ni epitaxial thin film by way of hydrogen reduction of NiO

The ferromagnetic epitaxial Ni (111) thin film on the oxide substrate could be obtained by an epitaxy method, employing pulsed laser deposition (PLD) of epitaxial NiO (111) film on the sapphire (α-Al₂O₃) substrate and successive hydrogen reduction. The epitaxial NiO (111) film was deposited on the sapphire (0001) substrate at room temperature by PLD, and was reduced into the Ni epitaxial film by annealing (300 °C to 700 °C) in the hydrogen atmosphere, suggesting the possible formation of epitaxial [Ni metal/α-Al₂O₃] multilayer. The epitaxy of Ni film was proved by ex situ X-ray diffraction. The ferromagnetic anisotropy of the epitaxial Ni film was examined by superconducting quantum interference magnetometry.     

Optical and dielectric properties of CuAl2O4 films synthesized by solid-phase epitaxy

The synthesis and properties of CuAl₂O₄ thin films have been examined. The CuAl₂O₄ films were deposited via reactive direct current magnetron sputter using a CuAl₂ target. As-deposited films were amorphous. Post-deposition annealing at high temperature in oxygen yielded solid-phase epitaxy on MgO. The film orientation was cube-on-cube epitaxy on (001) MgO single-crystal substrates. The films were transparent to visible light. The band gap of crystalline CuAl₂O₄ was determined to be ∼ 4 eV using a Tauc plot from the optical transmission spectrum. The dielectric constant of the amorphous films was determined to be ∼ 20-23 at 1-100 kHz.     

Fabrication and structure properties of GaN nanowires by ammoniating Ga2O3 films

The GaN nanowires were successfully synthesized on Si(111) substrates by ammoniating the Ga₂O₃/ZnO films at 900 °C. The structure and morphology of the as-prepared GaN nanowires were studied by X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR), scanning electron microscopy (SEM) and field-emission transmission electron microscopy (FETEM). The results show that the single-crystal GaN nanowires have a hexagonal wurtzite structure with lengths of about several micrometers and diameters ranging from 30 nm to 120 nm, which are conducive to the application of nanodevices. Finally, the growth mechanism is also briefly discussed.     

CVD growth of cubic boron nitride: A theoretical/experimental approach

Similarities and dissimilarities in the growth of diamond vs. c-BN, in the present series of investigations, have been studied using quantum mechanical calculations. Hydrogen species have been observed to be very effective in stabilising both types of compounds. Very large similarities have also been observed when considering the adsorption of various growth species to these materials. However, it was found necessary to avoid mixtures of B- and N-containing species in the gas phase during c-BN growth, since they should most probably result in a mixture of these species also on the (111) and (110) surfaces. In addition, a careful gas phase design was found necessary in order to avoid a preferential initial growth of h-BN. These theoretical results can be used as guide lines in striving towards a thin film deposition of cubic boron nitride using gentle CVD methods like atomic layer deposition.     

Pb(Zr0.52Ti0.48)O3/TiNi multilayered heterostructures on Si substrates for smart systems

Ferroelectric/shape memory alloy thin film multilayered heterostructures possess both sensing and actuating functions and are considered to be smart. In this article, Pb(Zr_0.52Ti_0.48)O₃ (PZT) ferroelectric thin films and Ti-riched TiNi shape memory alloy thin films have been deposited on Si and SiO₂/Si substrates in the 400-600 °C temperature range by pulsed laser deposition technique. Deposition processing, microstructure and surface morphology of these films are described. The TiNi films deposited at 500 °C had an austenitic B2 structure with preferred (110) orientation. The surfaces of the films were very smooth with the root-mean-square roughness on a unit cell level. The structure of the TiNi films had a significant influence on that of the subsequently deposited PZT films. The single B2 austenite phase of the TiNi favored the growth of perovskite PZT films. The PZT/TiNi heterostructures with the PZT and TiNi films respectively deposited at 600 and 500 °C exhibited a polarization-electric field hysteresis behavior with a leakage current of about 2 × 10^− 6 A/cm².     

Residual carrier density in GaSb grown on Si substrates

The relationships between the densities of residual carriers and those of dislocation in GaSb films grown on Si substrates were investigated. Dislocation density was evaluated by cross-sectional transmission electron microscopy (TEM). The TEM images indicated that the dislocation density after a 5-μm-thick GaSb film was grown was below 1 × 10⁸/cm² although the density near the interface between the Si substrate and the GaSb film was about 3 × 10⁹/cm². Forming a dislocation loop by growing a thick GaSb layer may decrease the dislocation density. The density and mobility of the residual carrier were investigated by Hall measurement using the van der Pauw method. The residual carriers in GaSb grown on Si substrates were holes, and their densities decreased significantly from 4.2 × 10¹⁸ to 1.4 × 10¹⁷/cm³ as GaSb thickness was increased from 500 to 5500 nm.     

Superposed forward current-voltage characteristics in TbMnO3/n-Si and TbMnO3/p-Si heterostructures

TbMnO₃/n-Si (n-N) and TbMnO₃/p-Si (p-n) heterojunctions were fabricated under identical conditions. Good rectifying characteristics were found with almost the same forward current-voltage behavior in a temperature range from 150 to 300 K. Such intriguing superposed rectifying behaviours at the interfaces between TbMnO₃ and Si of two different doped types can be explained by a similar Schottky barrier diode behavior with its current-voltage dependence generally dominated by only one type of carrier. This work will favor both electronic transport analysis and future device applications.     

Fe3GaAs/GaAs(0 0 1): a stable and magnetic metal-semiconductor heterostructure

We show that in agreement with the ternary Fe-Ga-As phase diagram, the solid-state interdiffusions in epitaxial Fe/GaAs(0 0 1) heterostructures lead, at a temperature of approximately 500 °C, to the formation of thermodynamically stable Fe₃GaAs/GaAs(0 0 1) contacts quite similar to the well-known silicide/Si ones. The Fe₃GaAs films are made of grains epitaxial on GaAs with a well-defined interface. Their magnetic and electrical properties make Fe₃GaAs on GaAs an attractive metallization scheme for future magnetoelectronic devices. The results we report concern (25 or 80 nm Fe)/GaAs(0 0 1) heterostructures annealed at 480 and 500 °C for 10 min and characterized ex situ by He⁺ Rutherford backscattering and ion channeling, X-ray diffraction, transmission electron microscopy and alternating gradient field magnetometry.     

A model of preferential (100) crystal orientation of Si grains grown by aluminium-induced layer-exchange process

The aluminium-induced layer-exchange process allows to grow thin large-grained polycrystalline Si films on foreign substrates. A characteristic feature of these films is the preferential (100) orientation of Si grains, favourable for subsequent epitaxial thickening at low temperatures. In this work, a model based on the preferential nucleation is proposed, which elucidates a possible origin of the preferential (100) orientation and its sensitivity to the preparation and process conditions. The probability of Si nuclei to have respective orientation is attributed to the nucleation barrier, i.e. the critical value of the change of the Gibbs energy during nucleation. The preferential orientation is formed statistically by the nuclei having the lowest nucleation barriers.     

Cr1−xAlxN coatings deposited by lateral rotating cathode arc for high speed machining applications

In this work, a series of Cr_1−xAl_xN (0 ≤ x ≤ 0.7) coatings were deposited on high speed steel substrates by a vacuum arc reactive deposition process from two lateral rotating elemental chromium and aluminum cathodes in a flowing pure nitrogen atmosphere. The composition, structural, mechanical, and tribological properties of the as-deposited coatings were systematically characterized by energy dispersive analysis of X-rays, X-ray diffraction, nanoindentation, and ball-on-disc tribometer experiments. All of the as-deposited CrAlN coatings exhibited a higher hardness than CrN, showing a maximum hardness of about 40 GPa (at around X = 0.63) which is twice higher than that of the CrN. The wear performance under ambient conditions of the CrAlN coatings was found much better, with both lower friction coefficient and wear rate, than TiAlN coatings deposited by the same technique. The wear rate of the CrAlN coatings against alumina counterpart was about 2-3 orders in magnitude lower than that of the TiAlN coatings. Selected CrAlN coatings with the highest hardness were also deposited on some WC-based end-mills. An evident better performance of the CrAlN-coated end-mills was observed than the TiAlN-coated ones for cutting a hardened tool steel material under high speed machining conditions.     

Effect of ammoniating temperature on morphologic and optical properties of GaN nanostructured materials

GaN nanostructured materials have been obtained on Si(111) substrates by ammoniating the Ga₂O₃/ZnO films at different temperature in a quartz tube. X-ray diffraction (XRD), Scanning electron microscope (SEM), and photoluminescence (PL) are used to analyze the structure, morphology and optical properties of GaN nanostructured films. The results show that their properties are investigated particularly as a function of ammoniating temperature. The optimally ammoniating temperature of Ga₂O₃ layer is 950 °C for the growth of GaN nanorods. These nanorods are pure hexagonal GaN wurtzite structure with lengths of about several micrometers and diameters of about 200 nm, which is conducive to the application of nanodevices. Finally, the growth mechanism is also briefly discussed.