Epitaxial growth of SrTiO3 films with different orientations on TiN buffered Si(001) by pulsed laser deposition

Epitaxial SrTiO₃ (STO) films have been grown on TiN buffered Si(001) by pulsed laser deposition. The TiN layer was in situ deposited at 540, 640 or 720°C whereas the STO film was grown at a fixed temperature of 640°C. We have studied the effect of the growth temperature of TiN on the epitaxial relationship of STO/TiN heterostructures. It is found that for TiN grown at 540 or 640°C the epitaxial relationship is 001STO 001TiN, and for TiN grown at 720°C it changes to (101)STO (001)TiN and [ 01]STO [1 0]TiN (or [ 01]STO [110]TiN). This change of relationship is accompanied by a sharp reduction in the out-of-plane lattice constant of the TiN layer. Fourier transform infrared spectra show that the longitudinal optic modes are active for all the STO films, but the absorption peak associated with the transverse optic mode is observed only in the (101) oriented STO films.     

Preparation and characterization of gold nanocrystals on NaCl(100) surfaces as labels for the dynamical x-ray tracking technique

We report on the fabrication of single crystalline gold nanocrystals of sizes between 10 nm and 25 nm. Gold was deposited under ultra high vacuum conditions on a freshly-cleaved NaCl(001) surface by means of the Molecular Beam Epitaxy technique. The preparation parameters substrate temperature and average thickness of the deposited gold were studied systematically. After the preparation, the crystals were characterized by means of different techniques, such as Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), X-Ray Diffractometry (XRD) and Laue Diffractometry.Measurements of the deposited cluster size, as measured by AFM, and the crystal size, as measured by XRD exhibited similar results at substrate temperatures between approx. 200 °C and 250 °C, indicating a high amount of single gold crystals. TEM images of gold crystals, made after the gold crystals had been dissolved in an aqueous 3-[(3-cholamidopropyl)dimethyl-ammonio]propane solution, support these results.Further, an effect of the substrate temperature during the deposition on the orientation of the gold crystals with respect to the NaCl(001) surface was observed. XRD measurements reveal a perpendicular texture of Au(111) parallel to NaCl(001) at approximately 310 °C. The second detected texture:Au(001)||NaCl(001) (in plane) and Au[100]||NaCl[100] (perpendicular) was observed over the complete studied temperature range (180 °C-330 °C).     

Thermal oxidation of chemical vapour deposited tungsten layers on silicon substrates for embedded non-volatile memory application

This research is targeted to enhance the functionality of bipolar complementary metal-oxide-semiconductor by innovative concepts of embedded resistive random access memory (RRAM) cells integration in the back-end-of-line (BEOL) region. The material of our interest is tungsten oxide as an insulator in RRAM cells and we focussed on the growth and characterisation of closed tungsten oxide layers. In this materials science study, we investigated the tungsten oxidation process under BEOL constraints (< 450 °C). Thin films of tungsten oxide (6-50 nm) were prepared by oxidising, under an atmosphere of one bar oxygen, the chemical vapour deposited tungsten layers on TiN covered silicon wafers. The X-ray photoelectron spectroscopy investigations indicate that the stoichiometric WO₃ grows after oxidation at 300 °C for an hour. The tungsten oxide layers prepared above 300 °C for longer than 15 min were non-stoichiometric. The X-ray diffraction investigations reveal the crystallisation of the WO₃ layers in monoclinic phase above 350 °C when oxidised for longer than 30 min; above 400 °C the (001) growth texture becomes dominant.     

Fully strained low-temperature epitaxy of TiN/MgO(001) layers using high-flux, low-energy ion irradiation during reactive magnetron sputter deposition

Epitaxial TiN layers, 0.3 μm thick, are grown on MgO(001) in the absence of applied substrate heating using very high flux, low-energy (below the lattice atom displacement threshold), ion irradiation during reactive magnetron sputter deposition in pure N₂ discharges. High-resolution x-ray diffraction, reciprocal lattice maps, and transmission electron microscopy analyses reveal that the TiN(001) films grow with an (001)_TiN||(001)_MgO and [100]_TiN||[100]_MgO orientation relationship to the substrate. The layers are fully coherent with no detectable misfit dislocations. For comparison, TiN/MgO(001) films grown at temperatures of 700-850 °C under similar conditions, but with no intentional ion irradiation, are fully relaxed with a high misfit dislocation density. Thus, the present results reveal that intense low-energy ion irradiation during film growth facilitates high adatom mobilities giving rise to low-temperature epitaxy, while the low growth temperature quenches strain-induced relaxation and suppresses misfit dislocation formation.     

Epitaxial growth of WOx nanorod array on W(001)

Nanorods of substoichiometric tungsten oxide (WO_x) were grown on W(001) substrates. Two methods for the growth of nanorods were used: oxidation of the substrate under appropriate conditions and the deposition of tungsten oxide from a tungsten foil heated in the presence of oxygen. The grown nanorods were observed using a scanning electron microscope and an atomic force microscope. The diameters of the nanorods were 5–20 nm. The nanorods were slightly inclined from the directions parallel or normal to the surface. The inclination of nanorods was explained in terms of the epitaxial relationship between WO₃ crystals and the W(001) substrate. The WO₃ crystals formed at the initial stage of growth act as the nuclei of WO_x nanorods. We observed selective enhancement of the growth in a certain epitaxial direction depending on the method of growth, and an array of WO_x nanorods was produced on the W(001) substrate.     

Microstructure and optical properties of cubic AlN/TiN bilayers deposited by laser molecular beam epitaxy

AlN/TiN bilayers were deposited on Si(100) substrates with varying laser pulse energy by laser molecular beam epitaxy (LMBE) technique, and their growth mode, crystal structure and optical properties were investigated. The results indicated that atomically flat TiN single films and AlN/TiN bilayers with layer-by-layer growth mode were successfully grown on Si(100) substrates at optimal laser pulse energy. Both TiN and AlN in the grown bilayers exhibited the NaCl-type cubic structure with the same (200) preferred orientation, showing an excellent epitaxial relationship. TiN single film was more reflective in the infrared range and presented a small transparent window centered at wavelength of 404 nm. Reflectance spectrum of AlN film on top of TiN indicated the sharp absorption at about 246 nm, yielding a bandgap energy of 5.04 eV comparable to the theoretical calculation of bulk cubic AlN, but scarcely reported by the experimental data.     

Epitaxial growth of WOx nanorod array on W(001)

Nanorods of substoichiometric tungsten oxide (WO_x) were grown on W(001) substrates. Two methods for the growth of nanorods were used: oxidation of the substrate under appropriate conditions and the deposition of tungsten oxide from a tungsten foil heated in the presence of oxygen. The grown nanorods were observed using a scanning electron microscope and an atomic force microscope. The diameters of the nanorods were 5–20 nm. The nanorods were slightly inclined from the directions parallel or normal to the surface. The inclination of nanorods was explained in terms of the epitaxial relationship between WO₃ crystals and the W(001) substrate. The WO₃ crystals formed at the initial stage of growth act as the nuclei of WO_x nanorods. We observed selective enhancement of the growth in a certain epitaxial direction depending on the method of growth, and an array of WO_x nanorods was produced on the W(001) substrate.     

Synthesis, growth, and characterization of nonlinear optical material l-arginine iodate crystal

Single crystals of nonlinear optical l-arginine iodate (l-Arg·2HIO₃) (C₆H₁₄N₄O₂·2HIO₃), were successfully grown for the first time by the temperature-lowering method and also by the slow evaporation method at a constant temperature (30 °C) from its aqueous solution at pH value of 6. Crystals were of average dimensions 3 × 2.5 × 1.0 cm³ by optimizing the growth parameters. Initially, solubility studies were carried out for four different solvents such as water, water-methanol, water-ethanol, and water-acetone. Among the four solvents, the solubility of l-Arg·2HIO₃ was found to be the highest in water and hence crystallization of l-Arg·2HIO₃ was carried out from aqueous solution. Morphology studies reveal that the crystals grow with reasonable growth rate along the [100], [001], [110¯] and [011] direction. As-grown crystals were characterized by density measurement, X-ray powder diffraction studies, infrared spectrophotometer, UV-Vis spectrometer, laser induced damage threshold studies and nonlinear optical study.     

Floating-zone crystal growth of WC

Single crystals of tungsten carbide were grown by the floating-zone technique by adding boron in the molten zone. It is confirmed that WC decomposes into cubic WC_1–x and carbon at an elevated temperature and cannot coexist with the liquid which consists of only tungsten and carbon. It was theoretically expected that addition of boron would stabilize the WC structure so that the WC phase could coexist with the liquid. This was proved by the float zoning through the WC rod which contained boron. The WC phase was deposited from the molten zone which consists of tungsten, carbon and boron. The growth of single crystals of WC was carried out under pressurized helium gas atmosphere of 1 M Pa. The molten zone, whose temperature was about 2900°C, was inductively heated and driven downwards. The compositions of the molten zone and the feed rod were [W]/[C]/[B] = 1/0.65/0.06 and 1/1.08/0.007 (atomic ratio), respectively. Single crystals with dimensions 9 mm diameter and 5 mm long can be cut out from the zone-passed rod. The boron content in the crystal could be lowered to the order of 100 p.p.m.     

HRTEM observation of the monoclinic-to-tetragonal m-t) phase transition in nanocrystalline ZrO2

The orientation relations m(100) || t(001), m[001] || t[110]; m(011) || t(100), m[100] || t[001]; m(100) || t(110), m[001] || t[001]; m(013) || t(116), m[001] || t[001] (indices for the primitive tetragonal cell) have been found between the tetragonal (t) and monoclinic (m) domains during the electron irradiation-induced m-t phase transition observed in-situ with HREM within isolated zirconia nanoparticles. Geometric models of the m-t interfaces are proposed.     

Investigation of thermal and hot-wire chemical vapor deposition copper thin films on TiN substrates using CupraSelect as precursor

Copper films were deposited on oxidized Si substrates covered with TiN using a novel chemical vapor deposition reactor in which reactions were assisted by a heated tungsten filament (hot-wire CVD, HWCVD). Liquid at room temperature hexafluoroacetylacetonate Cu(I) trimethylvinylsilane (CupraSelect) was directly injected into the reactor with the aid of a direct-liquid injection (DLI) system using N2 as carrier gas. The deposition rates of HWCVD Cu films obtained on TiN covered substrates were found to increase with filament temperature (65 and 170 degrees C were tested). The resistivities of HWCVD Cu films were found to be higher than for thermally grown films due to the possible presence of impurities into the Cu films from the incomplete dissociation of the precursor and W impurities caused by the presence of the filament. For HWCVD films grown at a filament temperature of 170 degrees C, smaller grains are formed than at 65 degrees C as shown from the taken SEM micrographs. XRD diffractograms taken on Cu films deposited on TiN could not reveal the presence of W compounds originating from the filament because the relative peak was masked by the TiN [112] peak.     

Edge dislocations with large burgers vector in sphalerite crystals

The X-ray diffraction topography technique based on the Borrmann effect was used to study the dislocated structure of Czochralski-grown indium-doped GaAs(001) crystals. Among the growth dislocations aligned in the [001] ingot growth direction, there are dislocations with large Burgers vectors (b=a〈100〉) and { 100} slip planes.     

Electromechanical coupling properties of [001], [011] and [111] poled Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 single crystals

The electric field induced “butterfly” curves and polarization loops, and the stress induced strain and polarization responses of [001], [011] and [111] oriented Pb(Mg_1/3Nb_2/3)O₃-0.32PbTiO₃ (PMN-0.32 PT) relaxor ferroelectric single crystals have been systematically investigated by experiment study. The focus is on the effect of constant compressive bias stress on the electromechanical coupling behavior along three crystallographic directions of PMN-0.32 PT single crystals. Dependence of the coercive field, remnant polarization, dielectric constant, and piezoelectric coefficient on the bias stress has been quantified for PMN-0.32 PT single crystals oriented in three different directions. Obtained results show that the large piezoelectric responses under zero compressive stress in [001] and [011] orientation are dominated by intrinsic crystal lattice while the engineered domain structure has a relatively minor effect. It is found that observed responses under stress cycle for [001] oriented crystals are due to polarization rotation and phase transformations. However, those for [011] and [111] oriented crystals are due to domain switching. The “butterfly” curves and polarization loops driven by electric field under different bias compression are described by two non-180° domain switching.     

Can micro-compression testing provide stress-strain data for thin films?: A comparative study using Cu, VN, TiN and W coatings

Micro-compression testing using an instrumented micro- or nanoindenter equipped with a flat punch is a promising approach to measure the stress-strain response of miniaturized materials and to complement hardness and modulus measurements gained by nanoindendation. Focussed ion beam milling is employed to fabricate micron-sized compression pillars from 1 µm thick single crystal Cu(001), TiN(001), and VN(001) films grown on MgO(001), and from a 6.7 µm thick polycrystalline W coating deposited on Si(001). In situ micro-compression tests in a scanning electron microscope reveal reproducible stress-strain curves for W, a considerable statistical scatter in the flow stress for Cu and VN, and failure of TiN pillars by cleavage. A linear work-hardening rate of 2.7 ± 1.2 GPa is determined for the polycrystalline W coating. The results are critically discussed taking into account material defects and the stiffness of the film-substrate system.     

Structural and magnetic properties of nickel catalyzed-tungsten oxide nanosheets synthesized using e-beam rapid thermal annealing

Nanosheets of nickel catalyzed tungsten oxide have been grown on Si (100) substrate by electron beam rapid thermal annealing (ERTA) process. The thin films of W and Ni were deposited in a conventional electron beam evaporation system under high vacuum conditions and then subjected to ERTA. Scanning electron microscopy and atomic force microscopy were used to study the systematic growth of the nanosheets. Nanosheets with a uniform thickness of 200 nm were obtained for the beam current of 9 mA with a voltage of 5 kV ERTA for 60 s. X-ray diffractograms indicate that the formation of multiple phases of nickel, nickel oxide and tungsten oxide with respect to the variation in the beam current. Vibrating sample magnetometer studies indicate that the magnetic properties of this system vary with the beam current.     

A simple hydrothermal route to GaOOH intersecting-rods with regular diamond-like cross section

GaOOH crystals were synthesized by a simple hydrothermal method at 90-200 °C for 18 h. The products were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscope (HRTEM). The rods synthesized at 110-150 °C exhibited intersecting morphology and regular diamond-like cross section with a diagonal length ratio of 2:1, and those synthesized at 180-200 °C exhibited nunchakus-like architecture. The formation of diamond-like cross section was due to homogeneous growth of lattice planes, namely {1 1 0}, along [4 1 0], [4 1¯ 0], [4¯ 1 0], and [4¯ 1¯ 0] directions of orthorhombic GaOOH. Moreover, the formation of the trunk was due to the growth along [0 0 1] direction.     

Surface morphology and preferential orientation growth of TaC crystals formed by chemical vapor deposition

TaC film was deposited on (002) graphite sheet by isothermal chemical vapor deposition using TaCl₅-Ar-C₃H₆ mixtures, with deposition temperature 1200 °C and pressure about 200 Pa. The influence of deposition position (or deposition rate) on preferential orientation and surface morphology of TaC crystals were investigated by X-ray diffraction and scanning electron microscopy methods. The deposits are TaC plus trace of C. The crystals are large individual columns with pyramidal-shape at deposition rate of 32.4-37.3 μm/h, complex columnar at 37.3-45.6 μm/h, lenticular-like at 45.6-54.6 μm/h and cauliflower-like at 54.6-77.3 μm/h, with <001>, near <001>, <110> and no clear preferential orientation, respectively. These results agree in part with the preditions of the Pangarov's model of the relationship between deposition rate and preferential growth orientation. The growth mechanism of TaC crystals in <001>, near <001>, <111> and no clear preferential orientation can be fairly explained by the growth parameter α with Van der Drift's model, deterioration model and Meakin model. Furthermore, a nucleation and coalescence model is also proposed to explain the formation mechanism of <110> lenticular-like crystals.     

Fabrication of low-resistivity and gold-colored TiN films by halide chemical vapor deposition with a low [NH3]/[TiCl4] flow ratio

This work describes the preparation of titanium nitride (TiN) films on Si (111) substrates by atmospheric pressure halide chemical vapor deposition (AP-HCVD). Various TiN films were obtained by exploiting TiCl₄ + NH₃ gas chemistry with flow ratios [NH₃]/[TiCl₄] from 0.2 to 1.4, and deposition temperatures (T_d) from 600 to 900 °C. When T_d = 800 °C gold-colored films with electrical resistivities of under 100 μΩ cm were formed at almost all of the investigated [NH₃]/[TiCl₄] flow ratios. In particular, a lowest resistivity of about 23.7 μΩ cm, which is quite close to that of bulk TiN, was achieved using an [NH₃]/[TiCl₄] flow ratio of 0.3. Atomic force microscopy indicated that the root mean square surface roughness of that film was only about 5.1 nm. Under the same [NH₃]/[TiCl₄] flow ratio as above, X-ray diffraction analyses revealed the presence of a cubic TiN phase with a preferred orientation of (200) for T_d ≤ 800 °C, while additional (111) and (220) orientations emerged when the film was deposited at 900 °C. In conclusion, a low resistivity (< 100 μΩ cm) TiN film can be formed by AP-HCVD with very low [NH₃]/[TiCl₄] flow ratios 0.3-1.4.     

Growth of epitaxial Cu on MgO(001) by magnetron sputter deposition

100-nm-thick Cu layers were grown on MgO(001) substrates by ultra-high vacuum magnetron sputter deposition at substrate temperatures T_s ranging from 40 to 300 °C. X-ray diffraction ω−2θ scans, ω-rocking curves, and pole figures show that layers grown at T_s = 40 and 100 °C are complete single crystals with a cube-on-cube epitaxial relationship with the substrate: (001)_Cu||(001)_MgO with [100]_Cu||[100]_MgO. In contrast, T_s ≥ 200 °C leads to polycrystalline Cu layers with 001, 203, and 1¯75-oriented grains. The transition from a single- to a polycrystalline microstructure with increasing T_s is attributed to temperature-induced mass transport that allows Cu nuclei to sample a larger orientational space and find lower energy (and/or lower lattice mismatch) configurations. The large Cu- to-MgO lattice mismatch of 14% is relieved by 7 × 7 Cu unit cells occupying 6 × 6 MgO cells. In addition, for T_s ≥ 200 °C, the 001-oriented grains relax by tilting by 4° or 15° about 〈110〉 or 〈100〉 axes, respectively, while the 203 and 1¯75-oriented grains exhibit complex epitaxial relationships with the substrate: (203)_Cu||(001)_MgO with [010]_Cu||[110]_MgO and [302¯]_Cu||[11¯0]_MgO; and (1¯75)_Cu||(001)_MgO with [211¯]_Cu||[100]_MgO and [43¯5]_Cu||[010]_MgO. The surface roughness, as determined by X-ray reflectivity, increases with growth temperature. The smoothest layers are grown at 40 °C and exhibit an rms surface and buried interface roughness of 0.7 and 1.4 nm, respectively.     

Single crystal growth in PMN-PT and PMN-PZT

Single crystal growth of lead-based piezoelectric ceramics Pb(Mg_1/3Nb_2/3)_0.68Ti_0.32O₃ (PMN-32PT) and Pb(Mg_1/3Nb_2/3)_0.42(Ti_0.638Zr_0.362)_0.58O₃ (PMN-37PT-21PZ) ceramics via templated grain growth (TGG) was investigated. (001)- and (111)-oriented BaTiO₃ (BT) single crystals and (001)-oriented SrTiO₃ (ST) single crystals (of approximately 2.5 × 2.5 × 1 mm) were utilized as seeds for the growth experiments. The piezoelectric single crystals were produced in a process that involves at first hot pressing of single crystal in cold isostatically pressed ceramics followed by subsequent sintering of the samples. Growth of (001)-oriented single crystals with BT seeds was observed in both PMN-32PT and PMN-37PT-21PZ matrices. The measured growth lengths were up to 140 and 65 μm, respectively. The grown (001)-oriented single crystals grown were rectangular. The measured growth lengths of the pyramidal-shaped (111) BT single crystals were up to 1 mm, which is much larger than the growth lengths of the (001) single crystals. Experiments on (001) ST-seeded single crystals were not successful. No single crystal growth was observed due to the dissolution of the ST single crystals in the PMN-PZT matrix. The differences were explained by defect-chemical considerations.