Epitaxial Orientation Relationship Research Articles

(Invited) Epitaxial Electrodeposition of Metals for Interconnects Beyond Cu

Cu has been the metal of choice for interconnects in silicon microprocessors since the mid-1990s. The continuous scaling of these interconnects in CMOS technology has not only produced a challenge with respect to power consumption and computing performance, but also with respect to reliability. As the critical dimensions of interconnects are scaled towards, and then below the electron mean free path (EMFP) of Cu (39 nm at room temperature), the resistivity is found to increase. This increase, termed the resistivity size-effect, results in resistance scaling beyond Ohm’s law dimensional scaling and leads to even larger power consumption and further limits improvements in computing performance. The resistivity rise with decreasing dimension is a result of electron scattering at grain boundaries and surfaces (vs. phonons). Given that electron scattering from grain boundaries is the major contributor to the resistivity rise in nanoscale interconnects, there has been increasing interest in producing and evaluating epitaxial, single-crystal conductors absent of grain boundaries. In this presentation, epitaxial electrodeposition of Co, Ru and Cu at room-temperature is reviewed. The choice of seed layer for electrodeposition, the role of symmetry of the seed layer and the epitaxial orientation relationship of the electrodeposited film and the seed layer are discussed. In addition, the impact of underpotential deposition on film nucleation and growth morphology is examined. The effects of electrolyte, ion concentration and pH are also addressed. Potential pathways for implementation of epitaxial electrodeposition in back-end-of-line processing will be discussed.

Unraveling solid–liquid phase transition and microstructural coarsening of semi-solid Al0.8Co0.5Cr1.5CuFeNi HEA with dual globules for thixoforming application

Multi-phase HEAs typically have potential of semi-solid processing due to wide semi-solid range. For thixoforming route of semi-solid processing (SSP), the semi-solid billet preparation by semi-solid isothermal heat treatment (SSIT) is one of the most important process steps. This work focuses on the microstructure evolution during SSIT of a semi-solid Al0.8Co0.5Cr1.5CuFeNi HEA with dual globules for thixoforming application. This semi-solid HEA contains BCC(B2), FCC globules and Cu-rich liquid film. Solidified liquid follows K-S, epitaxial orientation relationship withneighboringBCC(B2) and FCC globules. Solutionizing point for B2 phase is higher than roughly 1175 °C, and complete disordering of BCC(B2) globule occurs at 1225 °C. While, FCC globule persists stable disordered solid solution. Solid-liquid phase transition of globules provides liquid film proliferation. CALPHAD simulation on the “hypothetical” alloy system corresponding to component constitution of individual globule successfully forecasted solid–liquid phase transition. Coupling coarsening inhibition effects, including sluggish diffusion in globules, Cu-rich liquid film as inhibitory barrier, heterogeneous globule as obstacle, account for extremely sluggish microstructural coarsening (coarsening rate is only 2.44–11.25 μm3/s). Finally, through forming ofexemplar component, intriguing potential for SSP application in component manufacturing was verified.

Enhancing the mechanical and electrical properties of CNTs/Cu composites by synthesizing WC nanoparticles on the surface of CNTs

The trade-off between mechanical and electrical properties has been a long-standing challenge for carbon nanotubes (CNTs)/Cu composites. To alleviate such trade-off, a pre-decorated strategy, involving the introduction of WC nanoparticles on CNTs (WC@CNTs), is proposed to accomplish the prominent mechanical-electrical property synergy. It was found that WC was epitaxially grown on the CNTs, which renders the epitaxial orientation relationship (OR) as: (WC(001)//CNTs(0002)). Such epitaxial OR facilitates the robust WC-CNTs interface by reducing interfacial energy. Meanwhile, WC nanoparticles also enhance the bonding strength of CNTs-Cu interface by strong mechanical interlocking. The designed “CNTs-WC-Cu” interface not only guarantees accurate stress/strain transmission, but also enhances the interfacial conductivity by reducing interfacial scattering, which contributes to the considerable mechanical and electrical properties of WC@CNTs/Cu over CNTs/Cu. This research provides a feasible strategy for simultaneous improvement of the mechanical and electrical properties of composites.

Luminescence Properties of Epitaxial Cu2O Thin Films Electrodeposited on Metallic Substrates and Cu2O Single Crystals.

The luminescent properties of epitaxial Cu2O thin films were studied in 10-300 K temperature range and compared with the luminescent properties of Cu2O single crystals. Cu2O thin films were deposited epitaxially via the electrodeposition method on either Cu or Ag substrates at different processing parameters, which determined the epitaxial orientation relationships. Cu2O (100) and (111) single crystal samples were cut from a crystal rod grown using the floating zone method. Luminescence spectra of thin films contain the same emission bands as single crystals around 720, 810 and 910 nm, characterizing VO2+, VO+ and VCu defects, correspondingly. Additional emission bands, whose origin is under discussion, are observed around 650-680 nm, while the exciton features are negligibly small. The relative mutual contribution of the emission bands varies depending on the thin film sample. The existence of the domains of crystallites with different orientations determines the polarization of luminescence. The PL of both Cu2O thin films and single crystals is characterized by negative thermal quenching in the low-temperature region; the reason of this phenomenon is discussed.

Epitaxial electrodeposition of Cu2O on Ag substrates in sulfate baths

A series of Cu2O samples were electrodeposited on polycrystalline Ag substrates in an electrolyte containing 0.4 M cupric sulfate and 1.3 M lactic acid at different electrolyte temperatures, pH values, and current densities. The electron backscatter diffraction technique was used to characterize the epitaxial orientation relationship (OR) between the deposited Cu2O films and underlying Ag grains. Epitaxial growth was mostly prohibited for the films deposited at pH 9 and 25 °C, whereas epilayers consisting of crystals of a single orientation were deposited on most of the substrate grains at conditions of pH 12/25 °C, pH 9/60 °C, and pH 12/60 °C. In addition to the expected cube-on-cube OR, four more ORs were identified. The existence of multiple ORs was probably a result of coincidence site lattice matching. The adoption of the epitaxial OR for Cu2O films strongly depended on the orientations of the substrate grains. Finally, transmission electron microscopy analysis indicated that the epilayer deposited at 60 °C contained a relatively low density of threading dislocations on the order of 109 cm−2 and thus exhibited low electrical resistivity values of 1–3 × 103 Ω·cm.

(Digital Presentation) On Epitaxial Electrodeposition of Co, Ru and Cu for Interconnect Applications

As the critical dimensions of copper (Cu) interconnects approach dimensions near the mean free path of the metal (39.9 nm at room temperature), a rise in resistivity is observed. This phenomenon, termed resistivity size-effect, is the result of electron scattering at grain boundaries and surfaces. Given that electron scattering from grain boundaries is a major contributor to the resistivity rise in nanoscale interconnects, in this study we demonstrate epitaxial electrodeposition of single crystal and bicrystal films, using Co, Ru and Cu as three metals of choice. In detail, we will address the choice of seed layer, the role of symmetry of the seed layer, the sign and magnitude of the misfit strain (compressive, none, tensile), the epitaxial orientation relationship of the electrodeposited film and the seed layer, the impact of underpotential deposition on film nucleation, and the growth morphology. The role of electrolyte, ion concentration and pH will also be addressed.

Epitaxial Growth of Metals on (100) SrTiO3: The Influence of Lattice Mismatch and Reactivity

Abstract The model system Me/(100) SrTiO3 (Me: Pd, Pt, Cu, Ni, Cr, Mo, Nb, and Al) was used to show that there exists a simple correlation between the formation of particular epitaxial orientations, the lattice mismatch between metal and SrTiO3, and the oxygen affinity of the metal. The growth of the metal films on the (100) SrTiO3 surface was studied by reflection high-energy electron diffraction. Most metals (Pd, Pt, Ni, Nb and Al) grew with the following epitaxial orientation relationship on the SrTiO3: (100) SrTiO3 | | (100) Me, [001] SrTiO3 | | [001] Me. A second epitaxial orientation relationship was detected for Cr and Mo: (100) SrTiO3 | | (100) Cr, Mo, [001] SrTiO3 | | [011] Cr, Mo. For Mo, this orientation was detected only at very high growth temperatures. For each of the epitaxial orientation relationships, the substrate and film planes of four fold symmetry were parallel. Besides the second epitaxial orientation relationship, a third epitaxial orientation relationship was detected for Mo at growth temperatures below 900 K: (100) SrTiO3 | | (110) Mo, [001] SrTiO3 | | [001] Mo. In contrast to all other metals under investigation, Cu grew with a (111) fiber texture on the (100) SrTiO3 surface. The appearance of the first two epitaxial orientation relationships can be interpreted with a simple relationship between the metals’ oxygen affinities and the lattice mismatches with SrTiO3. This relationship can be used for other metals to predict epitaxy on the (100) SrTiO3 surface.

Asymmetric Interfaces in Epitaxial Off-Stoichiometric Fe3+xSi1−x/Ge/Fe3+xSi1−x Hybrid Structures: Effect on Magnetic and Electric Transport Properties

Three-layer iron-rich Fe3+xSi1−x/Ge/Fe3+xSi1−x (0.2 < x < 0.64) heterostructures on a Si(111) surface with Ge thicknesses of 4 nm and 7 nm were grown by molecular beam epitaxy. Systematic studies of the structural and morphological properties of the synthesized samples have shown that an increase in the Ge thickness causes a prolonged atomic diffusion through the interfaces, which significantly increases the lattice misfits in the Ge/Fe3+xSi1−x heterosystem due to the incorporation of Ge atoms into the Fe3+xSi1−x bottom layer. The resultant lowering of the total free energy caused by the development of the surface roughness results in a transition from an epitaxial to a polycrystalline growth of the upper Fe3+xSi1−x. The average lattice distortion and residual stress of the upper Fe3+xSi1−x were determined by electron diffraction and theoretical calculations to be equivalent to 0.2 GPa for the upper epitaxial layer with a volume misfit of −0.63% compared with a undistorted counterpart. The volume misfit follows the resultant interatomic misfit of |0.42|% with the bottom Ge layer, independently determined by atomic force microscopy. The variation in structural order and morphology significantly changes the magnetic properties of the upper Fe3+xSi1−x layer and leads to a subtle effect on the transport properties of the Ge layer. Both hysteresis loops and FMR spectra differ for the structures with 4 nm and 7 nm Ge layers. The FMR spectra exhibit two distinct absorption lines corresponding to two layers of ferromagnetic Fe3+xSi1−x films. At the same time, a third FMR line appears in the sample with the thicker Ge. The angular dependences of the resonance field of the FMR spectra measured in the plane of the film have a pronounced easy-axis type anisotropy, as well as an anisotropy corresponding to the cubic crystal symmetry of Fe3+xSi1−x, which implies the epitaxial orientation relationship of Fe3+xSi1−x (111)[0−11] || Ge(111)[1−10] || Fe3+xSi1−x (111)[0−11] || Si(111)[1−10]. Calculated from ferromagnetic resonance (FMR) data saturation magnetization exceeds 1000 kA/m. The temperature dependence of the electrical resistivity of a Ge layer with thicknesses of 4 nm and 7 nm is of semiconducting type, which is, however, determined by different transport mechanisms.

Effect of Electrolyte Concentration on Epitaxial Growth of ZnO on Cu Substrates through Electrochemical Deposition

The roles of zinc and nitrate ions in the electrochemical growth of ZnO were examined in this study. ZnO was deposited on a polycrystalline Cu substrate from electrolyte containing 0.0025–0.08 M zinc nitrate and 0.1–2.0 M potassium nitrate. The morphology, epitaxial orientation relationship, interfacial microstructure, and photoluminescence (PL) properties of the ZnO deposits were characterized. The ZnO morphology was found to be controlled mostly by the zinc ion concentration in the electrolyte. Here, 50–400 nm wide hexagonal pillars were deposited from electrolyte with potassium nitrate concentration as high as 0.4 M. All the deposited films had strong 〈0001〉//ND fibre texture. Most of the ZnO crystallites were grown epitaxially on Cu grains regardless of the electrolyte concentration. The major epitaxial orientation relationship was identified as (0001)ZnO//(103)Cu and [11 0]ZnO//[010]Cu, and three other epitaxial orientation relationships were also identified. Samples consisting of nanopillars or plates exhibited excellent PL characteristics with near bandgap–deep level emission intensity ratios of 2.4–4.3 and full width at half maximum of the NBE peak of 122–131 meV.

Oxidation behavior of interstitial free steel: The defining role of substrate crystallographic texture

The oxidation kinetics of interstitial free (IF) steel were shown to differ remarkably, by more than one order of magnitude, with the crystallographic texture of the metallic substrate. More specifically, γ-fiber or ND||<111> grains oxidized preferentially and provided epitaxial or pseudo-epitaxial growth of the corresponding oxide grains. Below 843K, the initial magnetite (Fe3O4) grains had a ∼45°<001> epitaxial orientation relationship with all the substrate ferrite grains. Specimens oxidized at temperature > 843K, on the other hand, had a pseudo-epitaxy: ND||<111> metallic-substrate grains generated ND||<001> pro-eutectoid magnetite. In both cases, the oxide grains growing from ND||<111> ferrite had more dislocations and higher tensile residual stresses. They also had more magnetite and less hematite. It was proposed that the oxide phase growing from ND||<111> ferrite grains, with higher dislocation content, has faster ionic diffusion. Faster diffusion in the magnetite also appeared to reduce the protective (lower ionic diffusion) outer hematite layer. A combination of these two generated thicker oxide layers on the ND||<111> ferrite grains.

Growth of narrow substrate temperature window on the crystalline quality of InN epilayers on AlN/Si(1 1 1) substrates using RF-MOMBE

Epitaxial InN films were grown on AlN/TMAl seed layer/Si(1 1 1) substrates in the range of temperature between 500 and 540 °C using pulsed-mode RF-MOMBE. The structural properties of the InN films were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy, and transmission electron microscopy (TEM). The electrical properties were characterized by Hall effect measurements. The results indicated that small changes in temperature significantly affect the quality of InN films. The XRD and TEM results indicated that the InN films grew oriented along the c-axis. Phi-scan XRD patterns indicated that the InN films grew epitaxially on the Si(1 1 1) substrates with the orientation relationship InN [1 1 −2 0]InN//[11 0]Si. When the substrate temperature at 530 °C, the TEM images indicated three-dimensional InN/AlN growth and growth rate was approximately 0.2 μm/h. The cross-sectional TEM images and SAD pattern showed a sharp InN/AlN interface and clear epitaxial orientation relationships of InN (−2 1 1 0)InN//(−2 1 1 0)AlN//(2 –2 0)Si, [0 0 0 1]InN//[0 0 0 1]AlN//[1 1 1]Si. The electron concentrations of the InN films were measured from 5.5 × 1019 to 1.3 × 1020 cm−3, and the mobility was measured from 116 to 360 cm2/V·s. The results indicate that controlling the substrate temperature is essential for engineering the growth of InN films on silicon wafers.

Structural study of epitaxial NdBa2Cu3O7-x films by laser chemical vapor deposition.

Epitaxial NdBa2Cu3O7−x films were prepared on (100) LaAlO3 single crystal substrates by laser chemical vapor deposition (laser CVD). The effect of deposition temperature on preferred orientation, crystallinity, microstructure, deposition rate of films was investigated. The preferred orientation of the NdBCO films changed from a, c-axis to c-axis, then back to a, c-axis, as the deposition temperature was increased from 993 to 1093 K. The highly c-axis-oriented epitaxial NdBa2Cu3O7−x film with critical transition temperature of 87 K was obtained at Tdep = 1033 K, with the in-plane epitaxial orientation relationship of NdBCO [100]∥LAO [010] and NdBCO [010]∥LAO [001]. The growth orientation varied from a-axis to c-axis as the film thickness increased in the case of the a, c-co-oriented NdBCO film. The contribution of thickness-dependent strain relaxation to preferred orientation of the NdBCO films was also discussed.

Epitaxial Fe Thin Films on {100} Y2Ti2O7: Model Interfaces for Nano-Oxide Dispersion Strengthened Steels

Nanostructured Ferritic Alloys, a variant of oxide dispersion strengthened steels, contain a high density of ≈2.5 nm Y-Ti-O nano-oxides (NOs) that provide remarkable irradiation tolerance, enhance recombination of vacancies and self-interstitial irradiation defects, and trap He in fine-scale bubbles at their interfaces. To complement studies of embedded NOs, mesoscopic-scale metal-oxide interfaces were fabricated by electron beam deposition of Fe films on {100} Y2Ti2O7 (YTO) bulk single-crystal substrates. We report, for the first time, the dominant epitaxial orientation relationship (OR) for the polycrystalline Fe film: {110}Fe\\{100}YTO and 〈111〉Fe\\〈110〉YTO. Further, one large grain region had an OR that is also found in embedded NOs: {100}Fe\\{100}YTO and 〈100〉Fe\\〈110〉YTO. HRTEM studies show clean, semicoherent interfaces with misfit dislocation spacings of 0.7 and 1.4 nm, respectively. These observations are important for the development of first principles models of metal-oxide interfaces, and the bilayers themselves will be used to observe the He partitioning between the Fe, YTO, and the corresponding interface.

Effect of precursors’ ratio on c -axis-oriented SmBCO film by MOCVD

Abstract Highly c -axis-oriented SmBa 2 Cu 3 O 7−x (SmBCO) epitaxial films were deposited on (100)-LaAlO 3 (LAO) single crystal substrates by metal-organic chemical vapor deposition using Sm(dpm) 3 , Ba(dmp) 2 /Ba(tmod) 2 and Cu(dpm) 2 as precursors. The effect of the precursors’ ratio on the crystalline phases, composition and microstructure of films were investigated. The highly c -axis-oriented SmBCO films with stoichiometry were obtained at the precursors’ molar ratio at Sm: Ba: Cu = 1: 5.12–5.54: 2.16–2.82, with the in-plane epitaxial orientation relationship of SmBCO[100] // LAO[010] and SmBCO[010] // LAO[001].

Studies on strain relaxation of La0.5Ba0.5MnO3 film by normal and grazing incidence X-ray diffraction

Perovskite manganite La0.5Ba0.5MnO3 (LBMO) films were deposited on (001)-oriented single-crystal SrTiO3 (STO) substrates by pulsed laser deposition. High-resolution X-ray diffraction and grazing incidence X-ray diffraction techniques were applied to characterize the crystal structure and lattice strain of LBMO films. The in-plane and out-of-plane growth orientations of LBMO films with respect to substrate surface have been studied. The epitaxial orientation relationship LBMO (001) [100] //STO (001) [100] exists at the LBMO/STO interface. The lattice strain of LBMO film begins to relax with the thickness of LBMO film up to 12 nm. When the thickness is further increased up to 43 nm, the film is in fully strain-relaxed state. Jahn–Teller strain plays an important role in LBMO/STO system. The mechanism for strain relaxation is in accordance with that of tetragonal distortion.

Oxygen-dependent epitaxial growth of Pt(001) thin films on MgO(001) by magnetron sputtering

The roles of oxygen gas in crystal orientation, surface morphology and electrical resistivity of Pt thin films grown on MgO(001) substrate by magnetron sputtering are studied. With a well-controlled oxygen ratio (15% oxygen) during sputtering deposition with Ar-O2 mixture ambient, (001) epitaxial growth of Pt film on MgO substrate is achieved with an epitaxial orientation relationship of (001)Pt//(001)MgO and [100]Pt//[100]MgO. Microstructural and electrical characterizations reveal that the (001) Pt thin films possess very smooth surface and good conductivity. The formation and subsequent decomposition of platinum oxides in the Pt films grown with more than 30% oxygen result in an increase of surface roughness and electrical resistivity. The high-quality Pt(001) film has large potential for integrated electronic device applications.

TEM characterization of a TiN-MgAl2O4 epitaxial interface

The interface between TiN and MgAl2O4 compounds was investigated using TEM analysis in order to understand the formation mechanism of TiN on the surface of MgAl2O4 compound through the orientation relationship of the TiN-MgAl2O4 interface. The epitaxial growth of TiN on the surface of MgAl2O4 compound is feasible due to the low planar lattice disregistry between materials as well as due to the same crystal system. TiN was confirmed to nucleate on the surface of MgAl2O4 compound due to an epitaxial orientation relationship between the TiN and MgAl2O4 phases.

Growth and characterization of epitaxial Zr(0001) thin films on Al2O3(0001)

The authors report the growth of epitaxial Zr(0 0 0 1) thin films on Al2O3(0 0 0 1) substrates at a temperature of 700 °C via dc magnetron sputtering in an ultrahigh vacuum deposition system equipped with facilities for chemical vapor deposition, low-energy electron diffraction, and Auger electron spectroscopy. Zr layers with a nominal thickness of ∼220 nm are deposited at a rate of ∼0.06 nm/s in 10 mTorr Ar atmosphere. In situ Auger electron spectra of the as-deposited film surface reveal the presence of a Zr peak at 145 eV and Hf peak at 172 eV, the latter due to the presence of Hf impurities in the Zr sputter target. In situ low-energy electron diffraction patterns acquired from the Zr sample show sixfold symmetric spots with an in-plane lattice spacing of 0.31 ± 0.02 nm, characteristic of Zr(0 0 0 1)–(1 × 1) surface. Cross-sectional transmission electron microscopy images reveal columnar growth and the formation of a crystalline, 22 ± 8 nm thick, interfacial layer. Energy dispersive x-ray spectra obtained from this region reveal the presence of both Zr and Al. The authors attribute the formation of this interfacial layer to plasma-induced substrate decomposition during sputtering followed by interdiffusion of Al and Zr at the film–substrate interface. ω-2θ x-ray diffraction data show that the Zr layers are single-phase with hexagonal close-packed structure. Using high-resolution symmetric as well as asymmetric reciprocal space maps, the authors determined that the film is fully relaxed with in-plane and out-of-plane orientation lattice parameters of 0.324 and 0.516 nm, respectively, and identified epitaxial orientation relationships as Zr(0 0 0 4) ‖ Al2O3(0 0 0 12) and Zr(101¯0) ‖ Al2O3(112¯0).

Atomic-Scale Structure and Local Chemistry of CoFeB-MgO Magnetic Tunnel Junctions.

Magnetic tunnel junctions (MTJs) constitute a promising building block for future nonvolatile memories and logic circuits. Despite their pivotal role, spatially resolving and chemically identifying each individual stacking layer remains challenging due to spatially localized features that complicate characterizations limiting understanding of the physics of MTJs. Here, we combine advanced electron microscopy, spectroscopy, and first-principles calculations to obtain a direct structural and chemical imaging of the atomically confined layers in a CoFeB-MgO MTJ, and clarify atom diffusion and interface structures in the MTJ following annealing. The combined techniques demonstrate that B diffuses out of CoFeB electrodes into Ta interstitial sites rather than MgO after annealing, and CoFe bonds atomically to MgO grains with an epitaxial orientation relationship by forming Fe(Co)-O bonds, yet without incorporation of CoFe in MgO. These findings afford a comprehensive perspective on structure and chemistry of MTJs, helping to develop high-performance spintronic devices by atomistic design.

Anti-phase boundary free two-dimensional epitaxial Fe3O4thin films: evidence of an unquenched orbital magnetic moment at room temperature

Two-dimensional (2-D) epitaxial ferrimagnetic Fe3O4 thin films are attractive choices for the next generation of spin devices due to their half-metallicity, high Curie temperature and high electrical conductivity. Despite having profound spin device compatibility, the use of Fe3O4 thin films has not been exploited to date due to the presence of a magnetic disorder known as anti-phase boundaries (APBs). Here we demonstrate the growth of 2D single crystalline APB free Fe3O4(100) thin films on TiN buffered Si(100). The epitaxial orientation relationship, Si(400)//TiN(200)//Fe3O4(400), was confirmed by reflection high-energy electron diffraction and polarized Raman analysis. The Fe3O4(100) thin films possess large in-plane magnetic domains in its remanent magnetization state and cubic in-plane magnetic anisotropy as confirmed using magnetic force microscopy and magneto-optic Kerr effect measurements, respectively. The orbital to spin angular moment ratio, ml/ms = 0.144, and the total magnetic moment extracted from X-ray magnetism circular dichroism (XMCD) measurements are close to the corresponding bulk value.