A Study of the Crystal Structure of Co<Subscript>40</Subscript>Fe<Subscript>40</Subscript>B<Subscript>20</Subscript> Epitaxial Films on a Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Topological Insulator

Laser molecular-beam epitaxy has been used to form Co₄₀Fe₄₀B₂₀ layers on Bi₂Te₃ topological insulator substrates, and their growth conditions have been studied. The possibility of growing epitaxial ferromagnetic layers on the surface of a topological insulator is demonstrated for the first time. The CoFeB layers have a body-centered cubic crystal structure with the (111) crystal plane parallel to the (0001) plane of Bi₂Te₃. 3D mapping in the reciprocal space of high-energy electron-diffraction patterns made it possible to determine the epitaxial relationships between the film and the substrate.     

Optical anisotropy and dielectric parameters of (Ba<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>)Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> films on a Pt(111)/Si(001) substrate

(Ba_0.5Sr_0.5)Nb₂O₆ films were synthesized on a Pt(111)/Si(001) substrate by RF gas-discharge sputtering in pure oxygen atmosphere. It was found that the films have a dominant crystallographic orientation in the [001] direction and natural unipolarity, which was revealed through analysis of dielectric and piezoelectric parameters. It was demonstrated that the optical parameters of film material in the Ba_0.5Sr_0.5, Nb₂O₆/Pt(111)/Si(001) heterostructure match those typical for a (Ba_0.5Sr_0.5)Nb₂O₆ single crystal.     

Growth and magnetic properties of spin coated Co<Subscript>0.6</Subscript>Zn<Subscript>0.4</Subscript>Mn<Subscript>0.3</Subscript>Fe<Subscript>1.7</Subscript>O<Subscript>4</Subscript> ultrathin films on silicon (100), (110) and (111) substrates

This paper reports growth of Co_0.6Zn_0.4Mn_0.3Fe_1.7O₄ (CZFMO) ultrathin films (thickness: 23–30 nm) by spin coating technique on silicon (100), (110) and (111) substrates. The deposited films were annealed at 700 °C for 1 h in the oxygen environment. All the films were found to be polycrystalline in nature. The CZFMO films were found to have minimal residual stress (13–50 MPa), which could be an encouraging feature for novel microwave miniaturized device applications. Room temperature magnetic measurements demonstrated completely saturated hysteresis loop with the highest squareness ratio (M _R /M _S )?~?60% for the film grown on Si (110) substrate. On the other hand CZFMO films on Si (100) and Si (111) substrates showed unsaturated hysteresis loops with M _R /M _S ~ 10 and 5%, respectively. The reason for the better magnetic properties of the ultrathin CZFMO film on Si (110) substrate seems to be its better crystalline quality and larger grain size compared to those of other films.     

Memory characteristics of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/LaAlO<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> multilayer structures with tunnel oxide thickness variation

Charge trap flash (CTF) memory devices are candidates to replace NAND flash devices. In this study, Pt/Al₂O₃/LaAlO₃/SiO₂/Si multilayer structures with lanthanum aluminate charge traps were fabricated for nonvolatile memory device applications. An aluminum oxide film was used as a blocking oxide for low power consumption in the program/erase operation and to minimize charge transport through the blocking oxide layer. The thickness of SiO₂ as tunnel oxide layer was varied from 30 to 50 Å. Thicknesses of oxide layers were confirmed by high resolution transmission electron microscopy (HRTEM) and all the samples showed amorphous structure. From the C–V measurement, a maximum memory window of 3.4 V was observed when tunnel oxide thickness was 40 Å. In the cycling test for reliability, the 30 Å tunnel oxide sample showed a relatively large memory window reduction by repeated program/erase operations due to the high electric field of ~10 MV/cm through tunnel oxide. The other samples showed less than 10% loss of memory window during 10⁴ cycles.     

Nanocoating of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> additive on ZrO<Subscript>2</Subscript> powder and its effect on the sintering behaviour in ZrO<Subscript>2</Subscript> ceramic

ZrO₂ powder was coated with Al₂O₃ precursor generated by a polymeric precursor method in aqueous solution. The system of nanocoated particles formed a core shell-like structure in which the particle is the core and the nanocoating (additive) is the shell. A new approach is reported in order to control the superficial mass transport and the exaggerated grain growth during the sintering of zirconia powder. Transmission electron microscopy (TEM) observations clearly showed the formation of an alumina layer on the surface of the zirconia particles. This layer modifies the sintering process and retards the maximum shrinkage temperature of the pure zirconia.     

Percolation effect in copper- and nickel-doped Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> crystals

The (0001) surface morphology of Bi₂Te₃〈M〉 (M = Cu, Ni) layered crystals has been studied using atomic force microscopy (AFM) and scanning electron microscopy. Two- and three-dimensional AFM images reveal charge transport paths through inhomogeneities created by nanostructured elements (5–20 nm) in the Te(1)-Te(1) interlayer spaces. The nanoparticle distribution in the (0001) plane is similar to the arrangement of model two-dimensional percolation clusters on a square lattice. The carrier mobility in Bi₂Te₃〈0.5 wt % Ni〉 crystals varies anomalously between 80 and 120 K.     

Orientation controlled deposition of Pb(Zr,Ti)O<Subscript>3</Subscript> films using a micron-size patterned SrRuO<Subscript>3</Subscript> buffer layer

Orientation controlled, micron-sized dot-patterned PZT films were grown by metal organic chemical vapor phase deposition (MOCVD), and their crystal structure was evaluated. A micron-size dot-patterned SrRuO₃ (SRO) buffer layer was initially prepared by MOCVD through a metal mask on the (111) Pt/Ti/SiO₂/Si substrate. Then, a PZT film was deposited over the entire substrate. Micron-beam X-ray diffraction and Raman spectroscopy indicated that (111)-orientated PZT was prepared on the SRO covered area, while the (100)/(001)-orientated one was directly grown on Pt-covered substrates. The PZT film grown on SRO was thinner than that on the Pt-covered substrate. The estimated ferroelectric property on the center of the dot pattern was larger than that at the circumference by Raman spectroscopy because the strain is accelerated at the center of the dot.     

Influence of C<Subscript>4</Subscript>F<Subscript>8</Subscript>/Ar/O<Subscript>2</Subscript> plasma etching on SiO<Subscript>2</Subscript> surface chemistry

The plasma assisted etching of SiO₂ in a commercial RF reactor with a variety of C₄F₈/Ar/O₂ chemistries has been studied by XPS, SIMS and FTIR. A simple model of surface reactions is proposed. In particular, the role of oxygen in the etch process has been investigated. According to our experiments, oxygen inhibits the formation of CFH based polymeric films on the surface. As the etching process is due to an exchange reaction between the oxygen in the SiO₂ and the gaseous fluorine species in the plasma, the presence of oxygen in the etch hinders this process by occupying adsorption sites on the surface. The results would confirm that argon does not participate in chemical reactions with the SiO₂ substrate but provides energy for reactions in which F, C and O are involved. The results also indicate that a thick fluorocarbon layer only forms on the surface in the absence of oxygen, regardless of the oxygen source. Consequently, only when the SiO₂ layer has been substantially removed does this film form.     

Synthesis,characterization and growth mechanism of ZnO nanowires on NiCl<Subscript>2</Subscript>-coated Si substrates

Well-crystallized ZnO nanowires have been successfully synthesized on NiCl₂-coated Si substrates via a carbon thermal reduction deposition process. The pre-deposited Ni nanoparticles by dipping the substrates into NiCl₂ solution can promote the formation of ZnO nuclei. The as-synthesized nanowires were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectrum. The results demonstrate that the as-fabricated nanowires with about 60 nm in diameter and several tens of micrometers in length are preferentially arranged along [0001] direction with (0002) as the dominate surface. Room temperature PL spectrum illustrates that the ZnO nanowires exist a UV emission peak and a green emission peak, and the peak centers locate at 387 and 510 nm. Finally, the growth mechanism of the nanowires is briefly discussed.     

Fe<Subscript>73.5</Subscript>Cu<Subscript>1</Subscript>Nb<Subscript>3</Subscript>Si<Subscript>15.5</Subscript>B<Subscript>7</Subscript> Thin Films Deposited by HiPIMS: Magnetic and Magnetostrictive Behavior

Results concerning the magnetic, magnetostrictive, structural, morphological, and topological properties of amorphous and nanocrystalline Fe _73.5Cu ₁Nb ₃Si _15.5 B ₇ thin films deposited using the high power impulse magnetron sputtering (HiPIMS) technique are reported. In as-deposited state, the samples are amorphous, the nanocrystalline state being achieved for samples isothermally annealed at adequate temperatures, in an electric furnace. For the optimum annealing temperature (475 ^°C), a decrease by about 70 % for the coercive magnetic field (50 A/m) and up to 1 order of magnitude for the saturation magnetostriction (~1×10^?6), compared to the as-deposited state, was obtained. The X-ray diffractometry (XRD) and scanning electron microscopy (SEM) results for samples thermally treated at 475 ^°C revealed a 53.6 % crystalline volume fraction of α-Fe(Si) nanograins with an average size of about 15 nm and a Si content of 10.78 %, uniformly dispersed in a residual amorphous matrix. Using the saturation magnetostriction values determined using the cantilever deflection method and the crystalline volume fraction of α-Fe(Si) nanograins, the contribution of crystalline phase to the saturation magnetostriction was also determined.     

Electric resistance and magnetoresistance of a two-layer epitaxial heterostructure (30 nm)La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>/(30 nm)La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

Two-layer epitaxial heterostructures (30 nm)La_0.67Ca_0.33MnO₃/(30 nm)La_0.67Ba_0.33MnO₃ (LCMO/LBMO) have been grown by laser deposition on single crystal (001)LaAlO₃ (LAO) substrates. In this system, the upper (LCMO) layer occurs under the action of tensile stresses in the substrate plane, whereas the lower (LBMO) layer exhibits biaxial compression. The formation of a 30-nm-thick LCMO film on the surface of the 30-nm-thick LBMO layer leads to an increase in the level of mechanical stresses in the latter layer. The maximum electric resistivity ρ of the (30 nm)LCMO/(30 nm)LBMO/LAO structure was observed at a temperature 25–30 K below that corresponding to the maximum of the ρ(T) curve for a single (30 nm)LBMO film on the same LAO substrate.     

Crystallographic relations between Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript> and TiC

Ti₃SiC₂ is a so-called not-so-brittle ceramic that combines the merits of both metals and ceramics. However, many previous works demonstrated that its bonding nature and properties were strongly related to TiC. In this paper the crystallographic relations between Ti₃SiC₂ and TiC were established and described based on the transmission electron microscopy investigation on the Ti₃SiC₂/TiC interface in Ti₃SiC₂ based material. At Ti₃SiC₂/TiC interface, the following crystallographic relationships were identified: (111)TiC//(001)Ti₃SiC₂, (002) TiC//(104)Ti₃SiC₂, and [11ˉ0]TiC//[110]Ti₃SiC₂. Based on the above interfacial relations an interfacial structure model was established. The structure of Ti₃SiC₂ could be considered as two-dimensional closed packed layers of Si periodically intercalated into the (111) twin boundary of TiC_0.67 (Ti₃C₂). The intercalation resulted in the transformation from cubic TiC_0.67 to hexagonal Ti₃SiC₂. In the opposite case, de-intercalation of Si from Ti₃SiC₂ caused the transformation from hexagonal Ti₃SiC₂ to cubic TiC_0.67. Understanding the crystallographic relations between Ti₃SiC₂ and TiC is of vital importance in both understanding the properties and optimizing the processing route for preparing pure Ti₃SiC₂. Received: 10 February 2000 / Reviewed and accepted: 17 March 2000     

Preparation and ferroelectric properties of (124)-oriented SrBi<Subscript>4</Subscript>Ti<Subscript>4</Subscript>O<Subscript>15</Subscript> ferroelectric thin film on (110)-oriented LaNiO<Subscript>3</Subscript> electrode

A (124)-oriented SrBi₄Ti₄O₁₅ (SBTi) ferroelectric thin film with high volume fraction of was obtained using a metal organic decomposition process on SiO₂/Si substrate coated by (110)-oriented LaNiO₃ (LNO) thin film. The remanent polarization (P _r) and coercive field (E _c) for (124)-oriented SBTi film are 12.1 μC/cm² and 74 kV/cm, respectively. No evident fatigue of (124)-oriented SBTi thin film can be observed after 1 × 10⁹ switching cycles. Besides, the (124)-oriented SBTi film can be uniformly polarized over large areas using a piezoelectric-mode atomic force microscope. Considering that the annealing temperature was 650 °C and the thickness of each deposited layer was merely 30 nm, a long-range epitaxial relationship between SBTi(124) and LNO(110) facets was proposed. The epitaxial relationship was demonstrated based on the crystal structures of SBTi and LNO.     

Growth of Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> films on Si with AlO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> and SiO<Subscript>2</Subscript> buffer layers by ion beam sputtering

Amorphous yttrium iron garnet films ranging in thickness from 100 to 600 nm have been produced on single-crystal silicon substrates by sputtering a polycrystalline target with the composition Y₃Fe₅O₁₂ (yttrium iron garnet) by a mixture of argon and oxygen ions. Before film growth, AlO _x or SiO₂ buffer layers up to 0.8 μm in thickness were grown on the Si surface. The heterostructures were crystallized by annealing in air at a temperature of 950°C for 30 min. The properties of the films were studied by magneto-optical techniques, using Kerr effect and ferromagnetic resonance measurements. The Gilbert damping parameter reached 2.8 × 10^–3 and the effective planar magnetic anisotropy field was independent of the nature of the buffer layer. This suggests that the thin-film heterostructures obtained in this study are potentially attractive for use in spin-wave semiconductor devices.     

Synthesis of In(OH)<Subscript>3</Subscript> and In<Subscript>2</Subscript>O<Subscript>3</Subscript> nanomaterials incorporating Au

In(OH)₃ and In₂O₃ nanocrystals of rectangular shape and incorporating Au were synthesized with a hydrothermal process and thermal decomposition. Powder X-ray diffraction, electron microscopy (SEM, TEM), and energy-dispersive spectroscopy studies reveal that elemental Au nanoparticles are dispersed on the surface of In(OH)₃ rectangular nanocrystals and incorporated into In₂O₃ nanoporous particles. UV–vis spectral measurements reveal a surface-enchanced plasma band near λ ~532 nm for both Au-incorporating nanomaterials. The BET surface areas of Au-incorporating In(OH)₃ and In₂O₃ are 26.2 and 35.5 m²/g, respectively. The incorporation of elemental Au in In(OH)₃ and In₂O₃ nanomaterials is attractive for sensor, catalyst and solar-cell applications.     

Formation of Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript> from Ti-Si-TiC powders by pulse discharge sintering (PDS) technique

Ti/Si/TiC powder mixture with molar ratios of 2:2:3 were sintered at various temperatures from 700–1300 °C for 15 min by PDS technique. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for the evaluation of phase composition in different samples for the understanding of the sintering mechanism for this system. Results showed that Ti₅Si₃ formed as the intermediate phase during sintering. The reaction between Ti₅Si₃ and TiC as well as Si induces the formation of Ti₃SiC₂, and TiSi₂ appears as the byproduct in this process. At temperature above 1000 °C, TiSi₂ reacts with TiC to form Ti₃SiC₂. High Ti₃SiC₂ phase content bulk material can be synthesized at 1300 °C for 15 min.     

Features of CoSi<Subscript>2</Subscript> phase formation by two-stage rapid thermal annealing of Ti/Co/Ti/Si(100) structures

A method of cobalt disilicide (CoSi₂) layer formation proceeding from a Ti(8 nm)/Co(10 nm)/Ti(5 nm)/Si(100) (substrate) structure prepared by magnetron sputtering is described. The initial structure was subjected to two-stage rapid thermal annealing (RTA) in nitrogen, and the samples after each stage were studied by the time-of-flight secondary-ion mass spectrometry, Auger electron spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The RTA-1 stage (550°C, 45 s) resulted in the formation of a sacrificial surface layer of TiN _x O _y , which gettered residual impurities (O, C, N) from inner interfaces of the initial structure. After the chemical removal of this TiN _x O _y layer, the enrichment with cobalt at the RTA-2 stage (830°C, 25 s) led to the formation of a low-resistance CoSi₂ phase.     

Responsivity to solar irradiation and the behavior of carrier transports for MoS<Subscript>2</Subscript>/Si and MoS<Subscript>2</Subscript>/Si nanowires/Si devices

The behavior of carrier transports and the responsivity to solar irradiation are studied for MoS₂/n-type Si (n-Si) and MoS₂/Si nanowires (SiNWs)/n-Si device. The MoS₂ thin films were prepared by the sol–gel method. The MoS₂/n-Si and MoS₂/SiNWs/n-Si devices exhibit reliable rectification behavior. The thermionic emission-diffusion model is the dominant process in these fabricated MoS₂/n-Si and MoS₂/SiNWs/n-Si devices. By applying the insertion of the SiNWs, the responsivity to solar irradiation can be effectively increased. Because of the low reflectance values for the MoS₂/SiNWs/n-Si sample, the increased photocurrent density for the MoS₂/SiNWs/n-Si device is due to high external light injection efficiency. MoS₂/SiNWs/n-Si devices benefit from the high surface to volume ratio for SiNWs leading to a high responsivity of the device. The photo-response results confirm that the decay in the photocurrent is due to the dominance of short-lifetime electron trapping in the MoS₂ thin film.     

Etching behavior of CdTe in aqueous H<Subscript>2</Subscript>O<Subscript>2</Subscript>-HI-C<Subscript>6</Subscript>H<Subscript>8</Subscript>O<Subscript>7</Subscript> solutions

This paper examines the dissolution behavior of the (111)A, (111)B, (110), and (100) surfaces of CdTe single crystals in aqueous H₂O₂-HI-C₆H₈O₇ (citric acid) solutions. We have determined the dissolution rate of the crystals as a function of temperature and solution concentration, located the composition regions of polishing and selective etchants, and studied the microstructure and roughness of surfaces polished with optimized etchants. The etching behavior of CdTe is shown to depend on its crystallographic orientation.     

Tunable SiO<Subscript>2</Subscript>/Si-based nanostructures

We model the formation of a nanoscale potential well with quantum wires on the semiconductor surface near the SiO₂/Si interface owing to a special charge distribution in the oxide. We consider an SiO₂/Si structure in the form of a cylindrical substrate covered with a coaxial oxide layer. The charge distribution in the oxide is taken to have the form of charged circular rings of finite thickness, coaxial with the cylindrical substrate. The parameters of the quantum wires are analyzed in relation to the charge distribution and density. Reducing the separation between two charged rings decreases the width of the quantum wires produced on the semiconductor surface and increases their depth.