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.     

Transmission electron microscopy characterization of TiN/SiNx multilayered coatings plastically deformed by nanoindentation

Plastic deformation of TiN_5 nm/SiN_0.5 nm multilayers by nanoindentation was investigated by transmission electron microscopy in order to identify deformation mechanisms involved in film failure resulting from severe plastic deformation. The TiN layers exhibited a crystalline fcc structure with a [002] preferential orientation; further crystal growth was interrupted by the amorphous SiN_x layers. After severe plastic deformation collective vertical displacement of slabs of several TiN/SiN_x-bilayers, which resulted from shear sliding at TiN/TiN grain boundaries, was observed. They are, together with horizontal fractures along the SiN_x layers, vertical cracks under the indenter tip following the TiN grain boundaries and delamination from the substrate, the predominant failure mechanisms of these coatings. The deformation behaviour of these films provides an experimental support for the absence of dislocation activity in grains of 5 nm size.     

Growth, structural and electrical properties of polar ZnO thin films on MgO (100) substrates

ZnO films have been grown on (100) oriented MgO substrates by pulsed-electron beam deposition in the room temperature to 500 °C range. Highly (00·2) textured films are obtained for a growth temperature higher than 200 °C, and epitaxial films are formed at 500 °C with the following epitaxial relationships: (1-1·0)_ZnO // (110)_MgO and (11·0)_ZnO // (110)_MgO, despite the difference in symmetry between film and substrate. The low temperature resistivity curves evidenced a metal-semiconductor transition for the ZnO films grown in the 300 to 500 °C range which has been interpreted in the frame of the model of conductivity in disordered oxides.     

Effect of lattice matching on microstructure and electrical conductivity of epitaxial ARuO3 (A = Sr, Ca and Ba) thin films prepared on (001) LaAlO3 substrates by laser ablation

(001) SrRuO₃ (SRO), (001) CaRuO₃ (CRO) and (205) BaRuO₃ (BRO) thin films were epitaxially grown on (001) LaAlO₃ substrates by laser ablation, and the effect of lattice matching on the microstructure and electrical conductivity was investigated. (001) SRO and (001) CRO thin films had a terrace with orthogonal step structure, whereas (205) BRO thin film had an orthogonal structure with tetragonal grains. Epitaxial thin films showed metallic conduction, and the (001) CRO thin films exhibited the highest electrical conductivity, i.e. 1.5 × 10⁵ S m^− 1, among the (001) SRO, (001) CRO and (205) BRO thin films. The smaller misfit between thin film and substrate could be associated with the higher electrical conductivity.     

Effects of periodicity and oxygen partial pressure on the crystallinity and dielectric property of artificial SrTiO3/BaTiO3 superlattices integrated on Si substrates by pulsed laser deposition method

Epitaxial SrTiO₃(STO)/BaTiO₃(BTO) artificial superlattices have been grown on TiN buffered Si (001) substrates by pulsed laser deposition method and the effects of stacking periodicity and processing oxygen partial pressure on their crystallinity and dielectric properties were studied. The crystal orientation, epitaxy nature, and microstructure of STO/BTO superlattices were investigated using X-ray diffraction and transmission electron microscopy. The TiN buffer layer and superlattice thin films were grown with cube-on-cube epitaxial orientation relationship of [110](001)_films∣∣[110](001)_TiN∣∣[110](001)_Si. The c-axis lattice parameter of the STO/BTO superlattice decreased from 0.412 nm to 0.406 nm with increasing oxygen partial pressure and the dielectric constants, measured at the frequency of 100 kHz at room temperature, of the superlattices with 2 nm/2 nm periodicity increased from 312 at 1 × 10^− 5 Torr to 596 at 1 × 10^− 3 Torr. The dielectric constants of superlattices grown at oxygen partial pressure of 1 × 10^− 3 Torr increased from 264 to 678 with decreasing periodicity of the superlattices from 10 nm/10 nm to 1 nm/1 nm.     

Structure of epitaxial Mn-stabilized ZrO2 layers on yttria-stabilized zirconia single crystals prepared by sputtering

Epitaxial Mn-stabilized zirconia layers on yttria-stabilized ZrO₂ (YSZ) single crystals were prepared by sputtering a Mn_0.32Zr_0.68O₂ target. Substrates with (001), (110) and (111) orientation were used. Transmission electron microscopy cross sections showed a homogeneous thickness of the layers. Layers prepared at a substrate temperature of 600 °C grew epitaxially. X-ray diffraction showed that these layers are not exactly cubic. We found a slightly larger out-of-plane lattice parameter compared to the in-plane lattice parameter. This means the out-of-plane lattice misfit is lower than the in-plane lattice misfit, contrary to behaviour expected for epitaxially grown layers. This effect was largest for layers on YSZ(111) and small for layers on YSZ(001).     

Stress and texture in HIPIMS TiN thin films

Titanium nitride (TiN) films in the thickness range of 0.013 µm to 0.3 µm were grown by high power impulse magnetron sputtering (HIPIMS) on silicon substrates in two deposition modes: a) the substrate was grounded and b) − 125 V bias was applied to the substrate. On the films we performed microstructure-, film texture- and film stress-analysis. The films deposited under − 125 V bias experienced a more energetic ion bombardment than the films deposited on grounded substrates. This difference in ion bombardment energy is reflected in the different microstructure. In contrast to previous results for TiN films grown by conventional reactive magnetron sputtering, we observe no major film stress gradient for increasing film thicknesses. We explain this observation from the absence of a 200-to-111 texture crossover during film growth.A moderate ion bombardment leads to TiN films with (111) texture, while an intense ion bombardment leads to films with (001) texture (Greene et al.; Appl. Phys. Lett. 67 (20) 2928-2930 (1995)). At the same time (001) oriented grains are much more susceptible to compressive stress generation by ion bombardment than (111) oriented grains.     

Ferroelectric properties of epitaxial Bi3.15Nd0.85Ti3O12 films on SiO2/Si using biaxially oriented MgO as templates

High quality epitaxial Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) thin films with thicknesses from 30 to 80 nm have been integrated on SiO₂/Si substrates. MgO templates deposited by ion-beam-assisted deposition and SrRuO₃ (SRO) buffer layers processed by pulsed laser deposition have been used to initiate the epitaxial growth of BNT films on the amorphous SiO₂/Si substrates. The structural and ferroelectric properties were investigated. Microstructural studies by X-ray diffraction and transmission electron microscopy revealed high quality crystalline with an epitaxial relationship of (001)_BNT||(001)_SRO||(001)_MgO and [100]_BNT||[110]_SRO||[110]_MgO. A ferroelectric hysteresis loop with a remanent polarization of 3.1 μC/cm² has been observed for a 30 nm thick film. The polarization exhibits a fatigue-free characteristic up to 1.44 × 10¹⁰ switching cycles.     

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.     

Structural characterization of a Cu/MgO(001) interface using CS-corrected HRTEM

Epitaxial Cu(001) layers were deposited on MgO(001) substrates by magnetron sputtering and the atomic structure of the Cu-MgO interface was characterized by spherical aberration (C_S)-corrected high-resolution transmission electron microscopy (HRTEM). The interface structure and the misfit dislocation network were determined by imaging in both the <100> and <110> directions. The dislocation network was found to lie along the <100> directions with a Burgers vector of ½ a_Cu <100> deduced from HRTEM images and geometrical phase analysis. The dislocations do not fully accommodate the lattice mismatch, yielding residual stress at the interface and an elongation of the Cu lattice along the [001] direction.     

Influence of ion irradiation effects on the hydriding behavior of nanocrystalline Mg-Ni films

Mg-Ni films were grown on a silicon substrate using two magnetron sputter deposition sources and simultaneous Ar ion irradiation. X-ray diffraction microstructure and phase composition, EDX elemental composition and atomic force microscopy surface topography analysis showed that under low-energy Ar ion irradiation (bias voltages from 0 to −120 V), the Mg₂Ni phase was dominant and on the contrary with the increase of ion energy (bias voltages from −120 to −200 V), the MgNi₂ phase appeared. The Mg content changed from 63 at% down to 42 at% in films grown under bias voltages of 0 and −200 V, respectively. During hydrogenation at 8 bar, 270 °C for 3 h, films with a dominant phase of Mg₂Ni were transformed into Mg₂NiH₄. Hydrogen in MgNi₂ films was mainly in interstitials and tended to form bubbles.     

Effects of low temperature ZnO and MgO buffer thicknesses on properties of ZnO films grown on (0001) Al2O3 substrates by plasma-assisted molecular beam epitaxy

We investigated the structural properties of Zn-polar ZnO films with low temperature (LT) ZnO and MgO buffer layers grown by plasma-assisted molecular beam epitaxy on (0001) c-Al₂O₃ substrates using X-ray diffraction and transmission electron microscopy (TEM). The effects of MgO buffer layer thickness and LT ZnO buffer layer thickness were also examined. The optimum thicknesses for better crystal quality were 8 and 40 nm. One-pair and two-pair LT ZnO/MgO buffer layers were employed, and the changes in the structural properties of the high-temperature (HT) ZnO films using such buffer layers were studied. Contrary to the general tendency of c-ZnO films, the HT ZnO films on the LT ZnO/MgO buffer layers showed higher full width at half maximum (FWHM) values for X-ray rocking curves (XRCs) with (0002) reflection than those with (101?1) reflection. Compared with the one-pair LT ZnO/MgO buffer layers, the FWHM values of (0002) XRCs markedly decreased, whereas those of (101?1) XRCs slightly increased due to the insertion of one more pair of LT ZnO/MgO buffer layers into the previous film with one-pair LT ZnO/MgO buffer layers. The cross-sectional TEM observations with the two-beam condition confirmed that the screw dislocation was the dominant threading dislocation type—a finding that agreed well with the XRC results. On the basis of the plan-view TEM observations, the densities of the total threading dislocations for the HT ZnO films with the one- and two-pair LT ZnO/MgO buffer layers were determined as 2.3 × 10⁹ cm^− 2 and 1.6 × 10⁹ cm^− 2, respectively. The results imply that the crystal quality of Zn-polar ZnO films can be improved by two-pair LT ZnO/MgO buffer layers, and types of threading dislocations can be modified by adjusting the buffer system.     

Aberration-corrected HRTEM of defects in strained La2CuO4 thin films grown on SrTiO3

The structure of lattice defects in thin La₂CuO₄ films grown under tensile strain on SrTiO₃ (001) is investigated by the combination of state-of-the-art medium voltage aberration-corrected transmission electron microscopy together with numerical exit-plane wavefunction reconstruction. The interfacial reconstruction, the coordination in planar shear defects evolving from surface steps and misfit dislocations of the b = a[010] type are atomically resolved and analysed. Quantitative mapping and evaluation of peak data related to cation atom columns reveal the formation of a perovskite-like layer of lanthanum copper oxide analogous to the thermodynamically instable LaCuO_3−δ phase and a distortion in the octahedral coordination of copper at the interface to the substrate. The planar shear defects embody extra sites for cations and oxygen in a three-dimensional periodic arrangement which are partially filled and provide paths for vacancy hopping transport. The central structure of the misfit dislocation does not exhibit mirror symmetry around a plane containing the dislocation line owing to the asymmetric arrangement of cation columns.     

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.     

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.     

Line defects, planar defects and voids in SrTiO3 films grown on MgO by pulsed laser and pulsed laser interval deposition

Two and three dimensional growth of SrTiO₃ films on (001) MgO substrate was achieved by pulsed laser interval and pulsed laser deposition respectively. The growth mode was monitored by in-situ reflection high energy electron diffraction. Interval deposition forces layer-by-layer growth of materials even with such a large lattice misfit (~ 7.9%). A titanium dioxide buffer monolayer was deposited to allow the film to wet the substrate to encourage two dimensional growth of the strontium titanate. A variety of defects was investigated using transmission electron microscopy and high resolution scanning transmission electron microscopy. Misfit dislocations, steps at the interface, Ti-rich defects and regularly shaped nano-holes connected by anti-phase boundaries were found to be the dominant defects in these films grown layer by layer. The edges of the nano-holes were mainly along [010] and [100] for a [001] growth direction. The large strain between the two crystal systems with large lattice mismatch leads to in-plane tensile stress during the layer-by-layer growth. The stress is relieved in part by the holes. The films with a three dimensional growth mode possess a uniform surface with dislocations as the dominant defects. The individual densities of the various defects, including a Ti-rich phase and misfit and threading dislocations, are determined by the kinetics of the deposition method.     

Elastic strain and misfit dislocation density in Si0.92Ge0.08 films on silicon substrates

Thin (less than 1 μm) epitaxial Si_0.92Ge_0.08 films on (100) Si substrates were grown by an UHV evaporation technique at a substrate temperature of 750 °C. The film strain and misfit dislocation density were examined by means of X-ray diffraction and transmission electron microscopy, respectively. The films are shown to be in state of compression, and the misfit dislocation density depends strongly on film thickness. The critical film thickness below which pseudomorphic growth without misfit dislocations occurs is found to be about 0.1 μm. The extrapolation model of van der Merwe's misfit dislocation theory is modified assuming low lattice mismatch and a diamond structure. The misfit dislocation distances thus calculated are compared with the measured distances, and it is found that the former are always smaller than the latter.     

The effect of helium ion implantation on the relaxation of strained InGaAs thin films

Previous work on pseudomorphic SiGe on Si has shown that a significant reduction in the threading dislocation density can be achieved through appropriate ion beam processing. Helium ion implantation was used in an analogous study to induce strain relaxation within strained pseudomorphic InGaAs layers on GaAs through the intentional introduction of subsurface damage without the introduction of surface nucleated dislocations and their associated threading segments. Wafers of fully-strained 28 nm thick films of In_0.24Ga_0.76As were separately implanted with helium doses of 5 × 10¹⁴, 2 × 10¹⁵, and 1 × 10¹⁶ cm⁻² at 25 keV. These wafers became substrates for additional InGaAs film growth. The final InGaAs films always exhibited lower residual strain as compared to films grown directly on a control substrate of non-implanted GaAs. The broadening of the X-ray peaks indicates an increase in dislocation density within the InGaAs films and the strain relaxation was found to occur with a significant increase in surface roughness. This result stands in contrast to related work on SiGe films on Si where a reduction of the threading dislocation density within a SiGe film was observed. The reaction of the InGaAs/GaAs structure and materials to ion irradiation, with local disturbance to the stoichiometry, could preclude the use of ion beam techniques for realizing a reduction in threading dislocation density during strain relaxation.     

Ti2Al(O,N) formation by solid-state reaction between substoichiometric TiN thin films and Al2O3 (0001) substrates

Titanium nitride TiN_x (0.1 ≤ x ≤ 1) thin films were deposited onto Al₂O₃(0001) substrates using reactive magnetron sputtering at substrate temperatures (T_s) ranging from 800 to 1000 °C and N₂ partial pressures (pN₂) between 13.3 and 133 mPa. It is found that Al and O from the substrates diffuse into the substoichiometric TiN_x films during deposition. Solid-state reactions between the film and substrate result in the formation of Ti₂O and Ti₃Al domains at low N₂ partial pressures, while for increasing pN₂, the Ti₂AlN MAX phase nucleates and grows together with TiN_x. Depositions at increasingly stoichiometric conditions result in a decreasing incorporation of substrate species into the growing film. Eventually, a stoichiometric deposition gives a stable TiN(111) || Al₂O₃(0001) structure without the incorporation of substrate species. Growth at T_s 1000 °C yields Ti₂AlN(0001), leading to a reduced incorporation of substrate species compared to films grown at 900 °C, which contain also Ti₂AlN(101?3) grains. Finally, the Ti₂AlN domains incorporate O, likely on the N site, such that a MAX phase oxynitride Ti₂Al(O,N) is formed. The results were obtained by a combination of structural methods, including X-ray diffraction and (scanning) transmission electron microscopy, together with spectroscopy methods, which comprise elastic recoil detection analysis, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy.     

Epitaxy of In0.01Ga0.99As on Ge/offcut Si (001) virtual substrate

In_0.01Ga_0.99As thin films free of anti-phase domains were grown on 7° offcut Si (001) substrates using Ge as buffer layers. The Ge layers were grown by ultrahigh vacuum chemical vapor deposition using ‘low/high temperature’ two-step strategy, while the In_0.01Ga_0.99As layers were grown by metal-organic chemical vapor deposition. The etch-pit counting, cross-section and plane-view transmission electron microscopy, room temperature photoluminescence measurements are performed to study the dependence of In_0.01Ga_0.99As quality on the thickness of Ge buffer. The threading dislocation density of Ge layer was found to be inversely proportional to the square root of its thickness. The threading dislocation density of In_0.01Ga_0.99As on 300 nm thick Ge/offcut Si was about 4 × 10⁸ cm^− 2. Higher quality In_0.01Ga_0.99As can be obtained on thicker Ge/offcut Si virtual substrate. We found that the threading dislocations acted as non-radiative recombination centers and deteriorated the luminescence of In_0.01Ga_0.99As remarkably. Secondary ion mass spectrometry measurement indicated as low as 10¹⁶ cm^− 3 Ge unintended doping in In_0.01Ga_0.99As.