Vicinal Substrates Research Articles

Stability and magnetism on the atomic scale: Atom-wide wires on vicinal metal substrate

Density functional theory and kinetic Monte Carlo method are applied in our study to describe formation, stability and magnetic properties of atom-wide wires on vicinal metal substrate. We present temperature ranges of atom-wide wires’ formation for different systems. The lifetimes of nanowires were determined, and their stability was discussed. Our calculations for both ferromagnetic Fe and Co wires decorating Pt(775) step edge show that the easy magnetization is in the plane perpendicular to the wire, while for Cu(775) it is oriented along the wire. The wire properties obtained from our simulations will be useful both for creating a new generation of materials and for explaining the experimental data.

Comparison of Effective Carrier Mobility Between Wire on Well and Planar Superlattice Using Time-of-Flight Measurement

Wire on well (WoW) structure, which is formed on a vicinal substrate, has been under consideration for a bandgap adjustor of multijunction cells. The photocurrent of a single-junction cell containing WoW is higher than that of a cell containing planar superlattice (PSL). This work provides direct evidence for the superiority of WoW over PSL in terms of the transport of photogenerated carriers. The time of flight (ToF) of the carriers that were photoexcited near the top surface of a cell was measured with photoluminescence by embedding a probe well beneath the nanostructures (WoW or PSL). The effective mobility of carriers in the nanostructures was obtained from the relationship between the ToF and the electric field applied via external bias. It was revealed that the carrier mobility in WoW was higher than that of PSL. The merit of enhanced mobility for WoW is discussed with device simulation and the potential for higher conversion efficiency is evidenced.

Self-assembled line network in BiFeO3 thin films

In this work we report on the controlled fabrication of a self-assembled line network in highly epitaxial BiFeO3 thin films on top of LaAlO3 in the kinetically limited grown region by RF sputtering. As previously shown in the case of manganite thin films, the remarkable degree of ordering is achieved using vicinal substrates with well-defined step-terrace morphology. Nanostructured BiFeO3 thin films show mixed-phase morphology. Besides typical formation following (100) and (010) axes, some mixed phase nanodomains are detected also in-between the regular line network. These particular microstructures open a playground for future applications in multiferroic nanomaterials.

Cobalt nanowire arrays grown on vicinal sapphire templates by DC magnetron sputtering

A modified magnetron sputtering setup was used to fabricate planar nanowire (NW) arrays of Co. The structural characterization of these polycrystalline Co NW arrays confirms the coexistence of both hcp and fcc phases, in which their relative fractions depend on the Co deposition time as well as the presence (or absence) of a bcc nanowire seed layer, as shown for tungsten case. A model to explain how the Co nanowires grow is proposed: hcp Co phase is firstly stabilized on the vicinal sapphire substrate (or W seed layer), but as the NW thickness increases, a structural relaxation leads to the fcc Co phase. While different contributions to the magnetic anisotropies are considered, we find that the magnetic properties are dominated by shape effects with an easy axis oriented along the average direction of the wire axis. Thicker NW arrays also have a significant contribution from the magnetocrystalline anisotropy. From the behavior of the temperature dependent magnetization, we find that for thicker nanowires the magnetization reversal is governed by a curling reversal mode, whereas thinner nanowires switch their magnetization via coherent rotation.

Anisotropy of the transport properties of NdFeAs(O,F) thin films grown on vicinal substrates

NdFeAs(O,F) thin films with different fluorine contents were grown on 5° or 10° vicinal cut MgO and CaF2 single crystalline substrates by molecular beam epitaxy. Structural characterisations from reflection high-energy electron diffraction and x-ray diffraction confirmed the epitaxial growth of NdFeAs(O,F). The resistivities of the ab-plane and along the c-axis (ρab and ρc) were derived from the resistivity measurements in the longitudinal and transversal directions. The c-axis resistivity was always higher than ρab, resulting from the anisotropic electronic structure. The resistivity anisotropy at 300 K was almost constant in the range of 50 ≤ γρ ≤ 90 irrespective of the fluorine content. On the other hand, at 56 K showed a strong fluorine dependence: was over 200 for the films with optimum fluorine contents (superconducting transition temperature Tc around 50 K), whereas γρ was around 70 for the films in the under-doped regime (Tc between 35 and 45 K). The mass anisotropy ( and are the effective masses along the c-axis and on the ab-plane) close to Tc derived from the anisotropic Ginzburg-Landau approach using the angular-dependency of ρab was in the range from 2–5. On the assumption , those values are small compared to the normal state anisotropy.

A comparative study on GaSb epilayers grown on nominal and vicinal Si(100) substrates by molecular beam epitaxy

The direct growth of GaSb buffer layers on Si substrates is attracting considerable interest in the integration of group III-Sb based device structures on lower-cost Si substrates. Here, we present the effect of various growth steps on the defect types and defect density that are crucial for advancing high crystal quality GaSb buffer layer on nominal/vicinal Si substrate. As a growth step, the applied thermal annealing at an intermediate step provided a decrease in the threading dislocation (TD) density down to 1.72 × 108 cm−2, indicating a more effective method compared to post-growth annealing. Additionally, the importance of period number and position of GaSb/AlSb superlattice layers inserted in GaSb epilayers is demonstrated. In the case of the GaSb epilayers grown on vicinal substrates, the APB density as low as 0.06 µm−1 and TD density of 1.98 × 108 cm−2 were obtained for the sample grown on 4° miscut Si(100) substrate.

Strain-induced phase selection in epitaxial Ge2Sb2Te5 thin films

Controlling strain in hetero-epitaxial material systems can be used to optimize properties and functionalities for specific applications. For this purpose, a lattice mismatch between substrate and epitaxial layer is typically exploited. Beyond that, vicinal substrate surfaces can be used to enhance strain-induced effects. In both cases, a profound understanding of the impact of strain on the growth mode and the resulting structure and morphology of the epitaxial layer is of great importance in order to achieve the desired modifications. In this study, the impact of vicinal substrate surfaces on the phase, structure as well as on surface pattern formation in epitaxial phase-change material thin films is investigated. It is found that in the case of the prototypical phase-change material $\mathrm{G}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{5}$, the change from nominally flat Si(111) substrates to substrates with a miscut of 6\ifmmode^\circ\else\textdegree\fi{} and terrace widths lying between 2 and 5 nm enhances compressive strain and a transition from 2D island nucleation to step-flow growth. This results in a different $\mathrm{G}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{5}$ phase evolution (change from metastable to stable phase) and surface nanostructuring by regularly arranged terraces that resemble the surface topography of the substrates. This work therefore provides deeper insights into different regimes of growth of epitaxial $\mathrm{G}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{5}$ thin films and paves the way for strain-induced modification possibilities such as band-gap tuning, optimization of the switching energy or the figure of merit.

Thermodynamic model for metalorganic vapor-phase epitaxy of N-polar group-III nitrides in step-flow growth mode: Hydrogen, competitive adsorption, and configuration entropy

A thermodynamic model for metalorganic vapor-phase epitaxy (MOVPE) of the N-polar $(000\overline{1})$ binary group-III nitrides (AlN, GaN, and InN) in the step-flow growth mode is proposed based on the Burton, Cabrera, and Frank (BCF) theory. The coverages of the group-III adatoms are thermodynamically evaluated under competitive adsorption with hydrogen, which is used as a carrier gas or dissociated from the ${\mathrm{NH}}_{3}$ source gas during MOVPE. The chemical potentials of the group-III and H adatoms on N-polar group-III nitride surfaces are modeled using the respective bond energies with the surface N atoms of the nitride and the vibrational frequencies of the adatoms. The coverages of the coadsorbed group-III and H adatoms are calculated using the Langmuir adsorption isotherm with these chemical potentials. The configuration entropy of the group-III adatoms bridges the gap between the thermodynamic model and the BCF theory. The coverage of the group-III adatoms plays a role like partial pressure of the group-III gas in the thermodynamics. The equilibrium coverage of the group-III adatoms and the equilibrium pressure of the ${\mathrm{NH}}_{3}$ gas are evaluated from the conditions of Gibbs energy balance between the sources (group-III adatom and ${\mathrm{NH}}_{3}$ gas molecule) and products (group-III nitride and $3/2\phantom{\rule{0.28em}{0ex}}{\mathrm{H}}_{2}$ gas molecules) and of speed balance between group-III and N incorporation into step kinks. Fair agreements with the experimentally optimized growth conditions for MOVPE of N-polar GaN and InN are obtained by this method. Among the examined binary group-III nitrides, AlN growth is hardly affected by ${\mathrm{H}}_{2}$ gas pressure, GaN growth is controlled well by ${\mathrm{H}}_{2}$ gas pressure, and InN growth is strongly inhibited by ${\mathrm{H}}_{2}$ gas. A criterion for selecting the ${\mathrm{NH}}_{3}$/group-III flow ratio for maximum products/cost and minimum waste of the materials is demonstrated using the growth model and the estimated growth parameters. The offcut angle dependence of the growth rate on the vicinal substrates is also investigated.

Temperature Activated Dimensionality Crossover in the Nucleation of Quantum Dots by Droplet Epitaxy on GaAs(111)A Vicinal Substrates

A temperature activated crossover between two nucleation regimes is observed in the behavior of Ga droplet nucleation on vicinal GaAs(111)A substrates with a miscut of 2° towards (bar{1}bar{1}2). At low temperature (<400 °C) the droplet density dependence on temperature and flux is compatible with droplet nucleation by two-dimensional diffusion. Increasing the temperature, a different regime is observed, whose scaling behavior is compatible with a reduction of the dimensionality of the nucleation regime from two to one dimension. We attribute such behavior to a presence of finite width terraces and a sizeable Ehrlich-Schwöbel barrier at the terrace edge, which hinders adatom diffusion in the direction perpendicular to the steps.

Effects of vicinal substrates on the orientation of Bi2Sr2CaCu2O8+x thin films when the metal-organic decomposition method is used

We have prepared (010) (or (100)) oriented Bi2Sr2CaCu2O8+x (Bi2212) thin films by metal-organic decomposition (MOD) method which is one kind of solution method. The flat and vicinal (100) substrate (the miscut angle φ = 5° toward the direction [001]) of NdGaO3 (NGO) was used. When using vicinal substrates, the Bi2212 (020) (or (200)) peak appeared clearly in the XRD patterns. From the SEM image, it was found that elongated plate-like crystal grains of Bi2212 were grown. From the viewpoint of lattice matching, this elongated crystal grain is considered to be (010) (or (100)) oriented Bi2212 crystal grains.

Effects of Bi Irradiation on the Molecular Beam Epitaxy Growth of GaSb on Ge (111) Vicinal Substrates

The growth of GaSb on Ge (111) vicinal substrates is performed by molecular beam epitaxy with Bi irradiation previous to and during the growth. The effects of the Bi irradiation on the surface morphology and on the crystallinity are investigated using atomic force microscopy and X‐ray diffraction, respectively. It is shown that Bi works as a surfactant, which suppresses the generation of rotational twins in the GaSb layer.

In-plane orientation-dependent metal-insulator transition in vanadium dioxide induced by sublattice strain engineering

Selectively modulating the sublattices in 3D transition metal oxides via strains could tailor the electronic configurations with emerging anomalous properties, which provides new platforms for fundamental researches as well as designs of devices. Here, we report tailoring the oxygen octahedral sublattices in vanadium dioxide (VO2) thin films by anisotropic in-plane strains, and the observation of in-plane orientation-dependent metal–insulator transition. Through multimodal characterizations based on high-resolution X-ray diffraction, electrical transport measurements, and polarization-dependent X-ray absorption spectroscopy at different temperatures, we demonstrate that nonequal strains were successfully induced along A and B oxygen octahedral chains in VO2 films via a special design of epitaxial growth on vicinal substrates. The V 3d1 orbital configurations are modulated in the two oxygen octahedral chains, resulting in in-plane orientation-dependent metal–insulator transition behaviors such as reduced hysteresis width and anisotropic phase transition temperature. This work provides new fundamental insights on metal–insulator transitions, and more importantly, opens up new opportunities for material and device developments

Thick hydride vapor phase epitaxial growth of ZnSe on GaAs (1 0 0) vicinal and orientation patterned substrates

Zinc selenide is attractive for numerous optical applications in the mid and longwave infrared from solid state lasers to non-linear optical frequency conversion. Its use has been limited by the availability of single crystal and epitaxial materials. To meet these challenges we have developed a low-pressure hydride vapor phase epitaxy (HVPE) process for growth of high quality thick ZnSe layers on GaAs (1 0 0) vicinal and orientation-patterned GaAs (OP-GaAs) substrates. We show growth of >300 µm thick layers at growth rates >50 µm/hr. With only 0.26% lattice mismatch between ZnSe and GaAs, epitaxial layers with crystal quality comparable to bulk ZnSe substrates are achieved. X-ray reciprocal space maps show the ZnSe to be fully relaxed. Cross-sectional TEM imaging analysis shows stacking faults at the interface that propagate in to the growing film consistent with a full relaxed film. Surface morphology shows the presence of pits that have their long axis perpendicular to [1–10] direction. Low temperature photoluminescence reveals band to band emission that consisted of the 2.797 eV direct band gap along with the strong exciton emission at 2.802 eV. Growth on OP templates revealed low fidelity domains of thickness >50 µm. This work presents a process to produce high quality thick layers potentially suitable for laser and frequency conversion for defense and civilian applications.

Growth temperature dependence of cubic GaN step structures and cubic InN dot arrays grown on MgO (001) vicinal substrates

In this paper, we report on the detailed structural features of step-bunched surface formed on cubic (c-) GaN and self-assembled c-InN dot arrays grown on the c-GaN surface, particularly focusing on the growth temperature dependence. Samples were fabricated on MgO (001) vicinal substrates with off-cut angles of 2.0° and 3.5° toward [110] by RF-MBE. Multisteps and terraces with a certain periodicity in the vicinal direction were observed on c-GaN layers. The average terrace width narrowed with lowering the growth temperature and increasing the substrate off-cut angle. The formation of c-InN dots proceeded in the Stranski–Krastanov growth mode with critical thickness of 0.54–0.66 nm. The c-InN dot exhibited a variety of structural features depending on the growth conditions. Positional dot alignment along the multistep edges of the underlayer was observed under some conditions. The degree of alignment was found to be affected by the terrace width on the c-GaN underlayer.

Epitaxial Non c-Axis Twin-Free Bi2Sr2CaCu2O8+δ Thin Films for Future THz Devices

Thin films of (117) Bi2Sr2Ca2CuO8+δ (Bi-2212) were grown by Molecular Organic Chemical Vapor Deposition (MOCVD) on (110) SrTiO3 and (110) LaAlO3 substrates. Substrates were vicinal with off angles up to 20°. Films are 3D epitaxial and X-ray diffraction φ-ψ scans demonstrate that, while the films grown on a flat substrate are composed of twinned grains, the films on vicinal substrate are twin-free. A higher quality is obtained if growth is performed at two temperatures: Growth starts at 550–600 °C and continues at 700–750 °C. The twin-free film grown by the two-temperature method shows a zero-resistance critical temperature of 37 and 32 K when the measuring current is applied in-plane parallel and perpendicular to [001] direction of the substrate. Twin-free non c-axis thin films are promising for fabrication of novel planar THz devices.

Improvement of crystalline quality of N-polar green InGaN/GaN multiple quantum wells on vicinal substrate

The surface morphology, crystalline quality and optical characteristics of N-polar InGaN/GaN multiple quantum wells (MQWs) structures with emission in the green spectral range grown on planar and vicinal substrates were researched. The polarity of the samples was ascertained by convergent beam electron diffraction pattern. Scanning electron microscopy, high-resolution x-ray diffraction, cathodoluminescence, photoluminescence and Raman measurements demonstrated that MQWs structures on vicinal substrate had a smooth surface, better crystal quality, stronger luminescence intensity and less stress. Transmission electron microscopy illustrated that the structures on vicinal substrate could suppress the formation of inversion domains and the propagation of dislocations effectively.

How the Complementarity at Vicinal Steps Enables Growth of 2D Monocrystals.

Large 2D monocrystals are highly sought after yet hard to achieve; unlike graphene, most dichalcogenides and h-BN possess low symmetry, which allows for nucleation of mutually inverted pieces, merging into polycrystals replete with grain boundaries. On vicinal substrate surfaces such growing pieces were observed to orient alike, and very recently this effect apparently enabled the growth of large single crystal h-BN. Addressing the compelling questions of how such a growth process can operate and what the key mechanisms are is crucial in guiding the substrate selection for optimal synthesis of perhaps many materials. To this end, the basic crystallography and atomistic-modeling theory presented here reveal (i) how the undulations of the ever-wandering steps do not, surprisingly, disturb the orientations of the attached 2D-nuclei, whose direction remains robust owing to complementarity between the meandering step and h-BN counterpart if their kinks have similar size of negligible misfit, δ k < 0.1 Å. (ii) Stronger chemical affinity of metal to the N atoms at the zigzag edge of h-BN singles out its particular orientation, without evidence of any epitaxy, at the edge or to the surface. (iii) The monocrystal integrity requires unhindered growth spillover across the steps and the seamless healing of the residual fissures, caused by the very same steps necessary for co-orientation. Molecular dynamics simulations show this happening for the steps not taller than the BN bond, s < 1.44 Å. These criteria point to [-1 1 2] steps on the Cu (110) surface, in accord with experimental results (Wang et al. Towards the growth of single-crystal boron nitride monolayer on Cu. arXiv:1811.06688 Cond. Mat. Mtrl. Sci., 2018), while other possibilities can also be predicted.

Self-organization of SiGe planar nanowires via anisotropic elastic field

Strained epitaxial SiGe on vicinal Si(001) substrates develops a morphological instability perpendicular to the steps unlike the usual growth instabilities on vicinal substrates, eventually leading to planar nanowires. We assess both theoretically and experimentally the effect of strain anisotropy on the 1D elongation of the Asaro-Tiller-Grinfel'd (ATG) instability. The anisotropy of strain relaxation due to the presence of step edges is considered in a continuum model with two different effective strains in the surface plane. We show that the measured in-plane strain anisotropy and the theoretical model are consistent with the experimental morphologies. Nice network of ultrasmall aligned elongations are predicted resulting from a complex interplay of kinetic and energetic phenomena associated with strain anisotropy.

Tungsten self-organization nanowires prepared by DC magnetron sputtering

Tungsten nanowires were deposited by DC magnetron sputtering onto vicinal sapphire substrates with a specific geometry in which the entrance angle (solid angle) of the vapor beam relative to the substrate was modified. Through 2D auto-correlation function analysis, suitable conditions to obtain a nanoscale corrugated surface with a high-order correlation were determined concerning the annealing procedure (1300 °C for 5 h) of the sapphire substrates with miscut angle of 0.3°. Depending on the entrance angle of the tungsten vapor beam, different nanoparticles and/or nanowires were systematically produced under controlled conditions. AFM images, from the samples deposited with an entrance angle lower or equal to the miscut angle (0.3°) on the annealed sapphire substrates have shown homogeneous, uniform and isolated tungsten nanowires.

Sn-nanothreads in GaAs matrix and their sub- and terahertz applications

We report on theoretical and experimental studies of gated GaAs structures like the field-effect transistor with an array of parallel Sn nanothreads (Sn-NTs) embedded into GaAs matrix. Two device structures are proposed and considered: the pseudomorphic HEMT (PHEMT) with a doping profile of Sn-NTs and the terahertz hot-electron bolometer (THz HEB) both fabricated on vicinal GaAs substrates. PHEMT demonstrates the anisotropy of drain-source current I∥/I⊥ ~ 2.5 at 300 K when current flows along and across Sn-NTs which we relate to formation of quasi-one-dimensional conductivity channels in InGaAs quantum well. Maximum power gain cut-off frequency fmax ~ 0.15 THz is shown. The operation of THz HEB is associated with an increase in the density of the delocalized electrons due to their heating by the incoming THz radiation. The quantum and the classical device models are developed. Since the fraction of the delocalized electrons strongly depends on the average electron energy (effective temperature), the proposed THz HEB can exhibit an elevated responsivity compared with the HEBs based on more standard heterostructures. Due to a substantial anisotropy of the device structure, the THz HEB may demonstrate a noticeable polarization selectivity of the response to the in-plane polarized THz radiation.