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.     

Structural study of thin MgO layers on Ag(001) prepared by either MBE or sputter deposition

Growth, morphology and structure of thin MgO films prepared on Ag(001) substrates by different preparation procedures have been investigated by AES, LEED and modulated electron emission. Oxide layers were prepared by Mg MBE in an oxygen atmosphere or sputter deposition from bulk MgO target. In both cases stoichiometric MgO forms, which initially grows in a rocksalt tetragonally distorted structure. For 3-ML films, 3.6% expansion of the interlayer spacing along the growth axis has been measured. The misfit strain progressively reduces as the film thickness increases, and the equilibrium spacing is completely recovered at 8–10 ML thickness. In spite of the similarity in structure and strain, different deposition procedures lead to different film morphology. Sputter deposited films only partially cover the substrate. Fractional coverage (50%) has been evaluated for the 3-ML thick film, and full coverage only occurs at approximately 10 ML thickness. The MBE procedure results in almost complete layers. Fourfold broadening of spot profiles has been observed in LEED patterns of both MBE and sputter deposited films, indicating the occurrence of a similar large-scale atomic arrangement and surface morphology.     

Growth and characterization of epitaxial anatase TiO2(001) on SrTiO3-buffered Si(001) using atomic layer deposition

Epitaxial anatase titanium dioxide (TiO₂) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer grown by molecular beam epitaxy (MBE) to serve as a surface template. The growth of TiO₂ was achieved using titanium isopropoxide and water as the co-reactants at a substrate temperature of 225-250 °C. To preserve the quality of the MBE-grown STO, the samples were transferred in-situ from the MBE chamber to the ALD chamber. After ALD growth, the samples were annealed in-situ at 600 °C in vacuum (10^− 7 Pa) for 1-2 h. Reflection high-energy electron diffraction was performed during the MBE growth of STO on Si(001), as well as after deposition of TiO₂ by ALD. The ALD films were shown to be highly ordered with the substrate. At least four unit cells of STO must be present to create a stable template on the Si(001) substrate for epitaxial anatase TiO₂ growth. X-ray diffraction revealed that the TiO₂ films were anatase with only the (004) reflection present at 2θ = 38.2°, indicating that the c-axis is slightly reduced from that of anatase powder (2θ = 37.9°). Anatase TiO₂ films up to 100 nm thick have been grown that remain highly ordered in the (001) direction on STO-buffered Si(001) substrates.     

Phase composition and microstructure of polycrystalline and epitaxial TaNx layers grown on oxidized Si(001) and MgO(001) by reactive magnetron sputter deposition

TaN_x is presently used in a variety of hard coating, wear-resistant, and diffusion barrier applications. However, the Ta–N system is inherently complex with more than 11 reported equilibrium and metastable phases and there has been little systematic study of the synthesis of these materials. Here, we report the results of an investigation of the ultrahigh vacuum reactive magnetron sputtering of Ta as a function of the N₂ fraction f_N2 in mixed Ar/N₂ discharges together with the phase composition and microstructure of TaN_x layers grown on MgO(001) and oxidized Si(001). Unlike the Ti–N system, for which TiN is the terminal phase, the abundance of N-rich phases in the Ta–N system results in the film deposition rate R and the N/Ta ratio of as-deposited layers varying continuously with f_N2, even for values >f_N2^*, where f_N2^* is the N₂ fraction corresponding to the maximum rate of N₂ uptake by deposited Ta. Phase composition results are summarized in a phase map plotted as a function of film growth temperature T_s (100–800°C) and f_N2. In pure Ar, the films are tetragonal β-Ta at T_s<150°C, bcc α-Ta at T_s>400°C, and a mixture of the two phases at intermediate temperatures. α-Ta layers grown on MgO(001) at T_s>500°C are epitaxial with a strain-driven 45° in-plane rotation with respect to the substrate: (001)_α-Ta∣∣(001)_MgO with [110]_α-Ta∣∣[100]_MgO. A series of lower nitrides — TaN_0.1, Ta₄N, and Ta₂N — are formed over narrow ranges of f_N2 between 0 and 0.10. This is followed by a wide single-phase field at T_s ≤ 650°C corresponding to the growth of metastable B1 NaCl-structure δ-TaN_x with x ranging from 0.94 (f_N2=0.10) to 1.37 (f_N2=0.275). The thermodynamically-stable hexagonal ε-TaN phase is formed at higher growth temperatures. δ-TaN_x layers grown on MgO(001) at T_s=550–650°C are epitaxial, exhibiting a cube-on-cube relationship: (001)_δ-TaN∣∣(001)_MgO with [100]_δ-TaN∣∣[100]_MgO. The relaxed lattice constant of δ-TaN_x(001) layers decreases linearly from 0.4350 nm with x=0.94 to 0.4324 nm with x=1.37. Finally, layers grown in pure N₂ are a two-phase mixture of δ-TaN_x and body-centered tetragonal TaN_x.     

Reactive sputter deposition of titanium dioxide

The microstructural, optical and electrical properties of reactively sputtered TiO_x films have been examined at points along the reactive deposition hysteresis curve. The deposition process utilizes feedback control loops of the current and oxygen/argon flowrates for operation in the internal region of the hysteresis curve. The variations in the optical properties are well correlated to the process conditions and the microstructure. For thicknesses of 60–100 nm, the films are polycrystalline for low deposition/high oxygen flow rates, and become amorphous for high deposition/low oxygen flow rates. Electron diffraction ring patterns of the polycrystalline phase are best fit to the d-spacings of the rutile structure. The index of refraction at 550 nm for the rutile phase polycrystalline films (10 nm grain size) is 2.6, this falls to 2.4 as the films become amorphous, and rises to 2.5 as the films become oxygen substoichiometric. We find the variation of the optical band gap of TiO_x to be less than 1%, and is less indicative of chemical and/or microstructural changes than the extinction coefficient at higher energies in the visible. The index of extinction at 350 nm shows a minimum at the same point as the index of refraction at 550 nm. We find the indirect fit of Tauc to the index of extinction gives the most linear fit. Variations in the complex index of refraction are consistent with the processing of r.f. sputtered films from titania targets.     

Epitaxial growth of CoSi2 on Si(001) by reactive deposition epitaxy: Island growth and coalescence

Epitaxial CoSi₂ layers, which are phase pure but contain {111} twins, are grown on Si(001) at 700 °C by reactive deposition epitaxy. Transmission electron microscopy analyses show that the initial formation of CoSi₂(001) follows the Volmer-Weber mode characterized by the independent nucleation and growth of three-dimensional islands whose evolution we follow as a function of deposited Co thickness t_Co in order to understand the origin of the observed twin density. We find that there are two families of island shapes: inverse pyramids and platelets. The rectangular-based pyramidal islands extend along orthogonal 〈110〉 directions, bounded by four {111} CoSi₂/Si interfaces, and grow with a cube-on-cube orientation with respect to the substrate: (001)_CoSi2||(001)_Si and [100]_CoSi2||[100]_Si. Platelet-shaped CoSi₂ islands are bounded across their long 〈110〉 directions by {111} twin planes (i.e. {111}(001)_CoSi2||{111}_Si) and their narrow 〈110〉 directions by {511}_CoSi2||{111}_Si interfaces. The top and bottom surfaces are {22¯1}, with {22¯1}_CoSi2||(001)_Si, and {1¯1¯1}, with {1¯1¯1}_CoSi2||{11¯1}_Si, respectively. The early stages of film growth (t_Co ≤ 13 Å) are dominated by the twinned platelets due to a combination of higher nucleation rates resulting from a larger number of favorable adsorption sites in the Si(001)2 × 1 surface unit cell and rapid elongation of the platelets along preferred 〈110〉 directions. However, at t_Co ≥ 13 Å island coalescence becomes significant as orthogonal platelets intersect and block elongation along fast growth directions. In this regime, where both twinned and untwinned island number densities have saturated, further island growth becomes dominated by the untwinned islands. A continuous epitaxial CoSi₂(001) layer, with a twin density of 2.8 × 10¹⁰ cm^− 2, is obtained at t_Co = 50 Å.     

Microstructure of thin film platinum electrodes on yttrium stabilized zirconia prepared by sputter deposition

(111) oriented thin film Pt electrodes were prepared on single crystals of yttrium-stabilized zirconia (YSZ) by sputter deposition of platinum. The electrodes were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDX), atomic force microscopy (AFM) and by profilometry. SEM images of the as-sputtered platinum film show a compact amorphous Pt film covering uniformly the substrate. Upon annealing at 1123 K, gaps and pores at the interface develop leading to a partial dewetting of the Pt film. Increasing the annealing temperature to 1373 K transforms the polycrystalline Pt film into single crystalline grains exhibiting a (111) orientation towards the substrate.     

Epitaxial growth of MgO/TiN multilayers on Cu

The growth of epitaxial MgO/TiN multilayer films on (001) Cu has been investigated. In particular, epitaxial structures were grown on (001) Cu layers that were epitaxial on (001) SrTiO₃. X-ray diffraction and reflection high-energy electron diffraction indicate that the multilayer structures are epitaxial on the (001) Cu surface. The motivation is the use of crystalline MgO/TiN multilayers as a diffusion barrier to both copper and oxygen. MgO/TiN multilayers are potentially useful as diffusion barriers for Cu interconnects on semiconductors as well as for superconducting wires based on the epitaxial growth of cuprate superconductors on biaxially textured copper.     

Growth and characterization of germanium epitaxial film on silicon (001) using reduced pressure chemical vapor deposition

High quality germanium (Ge) epitaxial film is grown directly on silicon (001) substrate using a “three-step growth” approach in a reduced pressure chemical vapor deposition system. The growth steps consist of sequential low temperature (LT) at 400 °C, intermediate temperature ramp (LT-HT) of ~ 6.5 °C/min and high temperature (HT) at 600 °C. This is followed by post-growth anneal in hydrogen at temperature ranging from 680 to 825 °C. Analytical characterizations have shown that the Ge epitaxial film of thickness ~ 1 μm experiences thermally induced tensile strain of 0.20% with a threading dislocation density of < 10⁷ cm^− 2 under optical microscope and root mean square roughness of ~ 0.9 nm. Further analysis has shown that the annealing time at high temperature has an impact on the surface morphology of the Ge epitaxial film. Further reduction in the RMS roughness can be achieved either through chemical mechanical polishing or to insert an annealing step between the LT-HT ramp and HT steps.     

Epitaxial growth of titanium oxide thin films on MgO(100) single-crystal substrates by reactive deposition methods

We synthesized titanium oxide thin films on MgO(100) single-crystal substrates by two reactive deposition methods and compared the structures of the thin films formed by these methods. In one method (pulsed-molecular-beam deposition method), molecular oxygen is supplied to the substrates by using a pulsed-molecular-oxygen beam source and deposition of one unit layer of titanium and subsequent supply of molecular oxygen are repeatedly performed. In the other method (radical beam deposition method), atomic oxygen is irradiated to the substrates by using an atomic oxygen beam generated by the radical beam source and irradiation of the atomic oxygen and deposition of titanium are simultaneously performed. In the case of the pulsed-molecular-beam deposition method, the crystal structure was changed by increasing the number of oxygen pulses supplied from the beam source. We found that the crystal structure of titanium oxide depended on the composition ratio of O:Ti in the film. The maximum ratio of O:Ti attainable by this method was 1.85, and at this ratio, (100)-oriented pseudorutile was formed. In the case of the radical beam deposition method, (100)-oriented anatase was formed below the titanium deposition rate of 0.10 nm/s and pseudorutile (TiO_2−δ) was formed above 0.15 nm/s. The pseudorutile structure synthesized on this experiment was very stable in air. We concluded that the crystal structure of the pseudorutile is a new crystal structure of titanium oxide.     

Gas pressure effects on thickness uniformity and circumvented deposition during sputter deposition process

In sputter deposition processes, the thickness distribution of the film is affected by experimental conditions such as gas pressure, target-substrate (T-S) distance and target elements. To study these effects, we have designed a sample holder with three quartz crystal microbalance (QCM) thickness monitors on its surface and measured the distribution of the depositing flux around the sample holder, including the circumvented deposition onto the back face. As for the gas pressure dependence for a T-S distance of about 50 mm, the relative deposition flux on the back face was found to be at its maximum at pressures of 1-2 Pa. Above this pressure, the uniformity of the flux on the front face became gradually worse. The observed characteristics could be reproduced by a Monte-Carlo simulation of the particle transport process. The reduction of the circumvented deposition and the uniformity degradation at high gas pressures were ascribed to the start of thermalization of sputtered particles and the shrinkage of its spatial profile toward the target.     

Two-dimensional interface structures of epitaxial (Ba,Sr)TiO3 film on miscut (001) MgO

We report on the effect of substrate miscut on the 2-dimensional interfacial structure and dielectric properties of the epitaxial Ba_0.6Sr_0.4TiO₃/MgO. Epitaxial Ba_0.6Sr_0.4TiO₃ films on vicinal (001) MgO grown by pulsed-laser ablation were studied using transmission electron microscopy (TEM). Plan-view TEM showed that the films grown on the substrate with miscut angles of 1.2°, 3.5°, and 5.3° have lattice mismatches of − 5.6%, − 6.0% and − 5.7% at the interface, larger than the values (− 5.4%, − 5.7% and − 5.5%, respectively) obtained using cross-section TEM. The films grown on 1.2° and 5.3° miscut substrates consist of commensurate domains with sizes about 30 to 40 nm at the interface, significantly larger than those of 10 to 20 nm obtained for the films grown on the 3.5° miscut substrate. The films with larger commensurate domains at the interface exhibit about 30% higher dielectric constant and dielectric tunability than those with smaller commensurate domains. Initial measurements show that their interfacial differences have a tremendous effect on the dielectric properties of the films.     

Microstructure of ZnO thin films produced by magnetron sputter oblique deposition

ZnO films deposited at different oblique angles of 40, 60 and 80°, under different Ar pressures 0.27, 0.67, 1.33 and 2.67 Pa, DC currents of 0.15 and 0.25 A, and distances of 10-15 cm from the target were studied. It was found that the film grains grow at an angle to the substrate when deposition angle is above 40°. It was shown that the grains consisted of a number of small crystals growing one on top of the other and shifted towards the target with the crystal orientation not along the grain growth but perpendicular to the substrate. Crystal size decreased with the deposition angle and internal stress disappeared when α = 80°. It was found that 1.33 Pa pressure provided the best balance between the deposition parameters. Growth rate reached maximum, samples had the biggest crystal size and high crystal density. However, crystal spatial alignment changed gradually with pressure and distance.     

Radiofrequency sputter deposition of germanium-selenide thin films for resistive switching

The superionic conducting properties of Ag-doped GeSe thin films make this material a promising candidate for future, resistively switching-based memories allowing for high integration densities and short switching times. This paper reports on the radiofrequency sputter deposition of GeSe thin films and on the properties of the deposited thin films with respect to non-volatile memory applications. As sputter deposition is a widely used deposition method for industrial applications, we focused on the influences of deposition parameters as power and pressure to examine the suitability of sputter deposition for fabricating random access memories using GeSe-based resistive memory cells. Multiple characterization methods were utilized to determine the quality of the deposited thin films. The results of our measurements showed that we obtained smooth, dense and amorphous layers, which reveal good switching properties after doping with Ag, suitable for the use in GeSe-based memories.     

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.     

Epitaxial growth of Cu(100) and Pt(100) thin films on perovskite substrates

Pulsed laser deposition has been used to grow epitaxially oriented thin films of Cu and Pt on (100)-oriented SrTiO₃ and LaAlO₃ substrates. X-ray diffraction results illustrated that purely epitaxial Cu(100) films could be obtained at temperatures as low as 100 °C on SrTiO₃ and 300 °C on LaAlO₃. In contrast, epitaxial (100)-oriented Pt films were attained on LaAlO₃(100) only when deposited at 600 °C. Atomic force microscopy images showed that films deposited at higher temperatures consisted of 3D islands and that flat, layered films were obtained at the lowest deposition temperatures. Importantly, Cu films deposited at 100 °C on SrTiO₃(100) were both purely (100)-oriented and morphologically flat. Pt and Cu films displaying both epitaxial growth and smooth surfaces could be obtained on LaAlO₃(100) only by using a three-step deposition process. High-resolution transmission electron microscopy demonstrated an atomically sharp Cu/SrTiO₃ interface. The crystalline and morphological features of Cu and Pt films are interpreted in terms of the thermodynamic and kinetic properties of these metals.     

Growth of SrTiO3 thin epitaxial films by aerosol MOCVD

SrTiO₃ thin films were prepared by aerosol metal-organic chemical vapour deposition on (001) MgO, R-plane Al₂O₃ and (001) Si single-crystal substrates. Strontium tetramethyl heptadionate and titanium n-butoxide dissolved in diethyleneglycol dimethyl ether were used as precursors. The structure of the films was investigated by X-ray diffraction and transmission electron microscopy. Epitaxial films with [001] and [111] orientation perpendicular to the substrate surface were obtained on MgO and Al₂O₃, respectively. The epitaxial films on the MgO substrate were found to be in a relaxed state with lattice parameters corresponding to the bulk values. SrTiO₃ films on the Si substrate were grown as highly textured in the [011]direction and randomly oriented in the plane parrallel to the substrate surface.     

Internal friction characteristics of Ti51.5Ni26.0Pd22.5 thin films formed by sputter deposition

The internal friction and the modulus have been measured on Ti_51.5Ni_26.0Pd_22.5 films under tension load, at three frequencies 0.5, 1.0, 5.0 Hz. in a wide range of temperatures covering the orthorhombic martensite and the reverse transformations. The effect of heat treatment and frequency on the internal friction has also been investigated. Only a transition around the orthorhombic transformation took place and a very sharp step-type internal friction was observed for all films with three kinds of heat treatments. The elastic modulus evolution reversed to internal friction change and a remarkable depression of the modulus corresponded to the internal friction peak. The internal friction of the films was enhanced by crystallization at 750 °C and the highest internal friction obtained at frequency 0.5 Hz. Ageing treatment at 450 °C after crystallizing at 750 °C promoted the peak of internal friction appearing and accelerated the microstructure variation. A high damping capacity of Q^− 1 = 0.05 or 0.06 was reached in the films crystallizing at 750 °C with or without ageing at 450 °C respectively and sustained from room temperature to 100 °C.     

Electronic and magnetic properties of ultra-thin epitaxial magnetite films on MgO(001)

The electronic and magnetic properties of epitaxial Fe₃O₄ (001) films on MgO(100) substrates were studied throughout the 2.5- to 30-nm thickness range using conversion electron Mössbauer spectroscopy. Despite the superparamagnetism that was observed for film thickness below 5 nm, the Verwey transition persisted even for the thinnest film. Temperature-dependent Mössbauer measurements between 80 K and 400 K revealed that the activation energy for the magnetic moment fluctuations in the 3-nm magnetite film is higher than the magnetic anisotropy energy by an order of magnitude.     

Preparation of thin epitaxial multilayer YBa2Cu3O7−x (Ag) films by aerosol deposition

Thin-textured YBa₂Cu₃O _y (Ag) films were prepared by a new CVD process consisting of aerosol deposition and low temperature annealing, both operations not exceeding 600°C. MgO and SrTiO₃ single crystals were used as substrates. The stoichiometry of the prepared films was not ideal 1–2–3, having the atomic quotient of copper always lower than 3. For an unoptimized preparation process, the onset temperatures were around 80 K and the zero resistanceT _c values around 70K. Properties of samples obtained by low-temperature annealing are compared with those obtained by high temperature annealing. “Cleaning” effect of vacuum-annealing upon removing the nitrate and hydroxide groups from the prepared films is supposed.