Substrate Terraces Research Articles

Step-edge-guided nucleation and growth mode transition of α-Ga2O3 heteroepitaxy on vicinal sapphire

Abstract Controlling the epitaxial growth mode of semiconductor layers is crucial for optimizing material properties and device performance. In this work, the growth mode of α-Ga2O3 heteroepitaxial layers was modulated by tuning miscut angles (θ) from 0° to 7° off the (10-10) direction of sapphire (0002) substrate. On flat sapphire surfaces, the growth undergoes a typical three-dimensional (3D) growth mode due to the random nucleation on wide substrate terraces, as evidenced by the hillock morphology and high dislocation densities. As the miscut angle increases to θ=5°, the terrace width of sapphire substrate is comparable to the distance between neighboring nuclei, and consequently, the nucleation is guided by terrace edges, which energetically facilitates the growth mode transition into the desirable two-dimensional coherent growth. Consequently, the mean surface roughness decreases to only 0.62 nm, accompanied by a significant reduction in screw and edge dislocations to 0.16×107 cm-2 and 3.58×109 cm-2, respectively. However, the further increment of miscut angles to θ=7° shrink the terrace width less than nucleation distance, and the step-bunching growth mode is dominant. In this circumstance, the misfit strain is released in the initial growth stage, resulting in surface morphology degradation and increased dislocation densities.

Manipulation of BiFeO3 nanostructure by substrate terrace morphology

Topological domain structures, such as vortex and center domain have been found in BiFeO3 nanoislands fabricated by self-assemble approaches, which exhibit great application potential in nonvolatile storage. However, how to control the self-assembled nanostructure during preparation process is still an open question. Here, we successfully grew BiFeO3 films with nano-stripe, nano-maze and dense nanoisland structures on LaAlO3 (001) substrate by pulsed laser deposition. We found the formation of different nanostructures is related to the terrace morphology of the substrate. Nano-stripe structure with strip domain and high conduction tail-to-tail domain wall preferred to form on wide terrace, nano-maze structure with three-fold or four-fold center-type domain usually form on bunched terrace, while dense nanoisland structure with two/three-fold domain preferred to from on the narrow or rough terrace LaAlO3 substrate. These findings provide guidance for manipulating the ferroelectric topological domain structure during film preparation process.

Epitaxial growth of Si thin films with hexagonal close-packed structures on metal substrates.

We have studied the epitaxial growth of Si thin films on the Cd(0001) surface using low-temperature scanning tunneling microscopy. When deposited at low temperatures (100K), Si atoms form dendritic islands with triangular shapes, indicating the existence of anisotropic edge diffusion in the process of Si film growth. After annealing to elevated temperatures, the triangular dendritic Si islands become hexagonal compact islands. Moreover, the 2D Si islands located on two different substrate terraces exhibit different heights due to the influence of quantum-well states in Cd(0001) films. Based on high-resolution scanning tunneling microscopy images, it is observed that the first, second, and third Si layers show the pseudomorphic 1 × 1 structure. In particular, the first and second layer islands reveal the opposite triangles, indicating the hexagonal close-packed stacking of Si atoms. These results provide important information for the growth of pristine Si films on metal substrates and the understanding of Si-metal interaction.

Substrate Terrace Width Dependence of Direct Growth of GaN on ScAlMgO4 by Radio‐Frequency Molecular Beam Epitaxy

ScAlMgO4 (SAM) substrates have attracted considerable attention as platforms for GaN growth in recent years because GaN can be grown directly on SAM without any buffer layer. Herein, the effect of the terrace width of SAM substrates on direct GaN growth is investigated using radio‐frequency molecular beam epitaxy (RF‐MBE). A flat and single wurtzite (WZ)‐phase GaN film is grown on a SAM substrate with a large terrace width. In contrast, a rough GaN film with a mixture of WZ and zinc‐blende (ZB) phases is obtained on SAM with a small terrace width. The SAM step height of 0.8 nm corresponds to three GaN molecular layers. Therefore, ZB stacking faults may be generated during the coalescence of islands located on adjacent terraces, although only the WZ‐GaN islands grow in the initial growth stage. This indicates that SAM substrates with a larger terrace width, that is, a smaller off‐cut angle, are preferable for obtaining a flat WZ‐GaN film.

Deposition of Co islands on Cu(111) and Cu–Al(111): A comparative study from DFT calculations

The growth behaviors, stability and magnetic moments of Co islands on Cu(111) surface and Cu–Al(111) alloy surface were compared by means of Density Functional Theory calculations. Similar to the case of Pb-precovered Cu(111), it was found that the diffusivity of Co adatom on Cu–Al(111) was lower than that on Cu(111), which could suppress the agglomeration of Co atoms on Cu–Al(111). Such behaviors could be explained by the different electronic structures of surface atoms of Cu(111) and Cu–Al(111). Co adatoms deposited on one monolayer Co island descending a Co edge step to substrate terrace via push-out/exchange mechanism is more favored than direct hopping on both surfaces, and push-out/exchange descent is more favorable on Cu–Al(111) than Cu(111) with similar “Schwoebel-Ehrlich” barrier and larger exothermic reaction energy. Island formation energies calculated suggest layer-by-layer growth mode is more favored for Co islands deposited on Cu–Al(111) covered with Co films than that on Cu(111). In agreement with experiments, layer-by-layer growth is identified for Co islands on Cu–Al(111), suggesting perfect wetting of Cu–Al(111) by Co. Our calculations show Al impurities in Cu(111) enhance the stability and magnetic moments of Co islands, which may shed light on Co/Cu–Al(111) being potential applications in magnetic hyperthermia treatments.

Ultrafast Plasmon Thermalization in Epitaxial Graphene Probed by Time‐Resolved THz Spectroscopy

AbstractThe control of carrier transport by electrical, chemical, or optical Fermi level tuning is central to graphene electronics. Here, an optical pump—terahertz (THz) probe spectroscopy—is applied to investigate ultrafast sheet conductivity dynamics in various epitaxially grown graphene layers representing a large variety of carbon allotropes, including H2 intercalated films. The graphene layers display a prominent plasmonic response connected with induced THz transparency spectra on ultrashort timescale. It is generally believed that the plasmonic excitations appear due to wrinkles, and substrate terraces that bring about natural confinement potentials. It is shown that these potentials act within micrometer‐sized domains with essentially isotropic character. The measured ultrafast dynamics are entirely controlled by the quasi‐Fermi level of laser‐excited carriers through their temperature. The photocarriers undergo a disorder‐enabled super‐collision cooling process with an initial picosecond transfer of the optically deposited heat to the lattice followed by a sub‐nanosecond relaxation governed by the lattice cooling. The transient spectra is described by a two‐temperature Drude‐Lorentz model revealing the ultrafast evolution of the carrier temperature and chemical potential and providing crucial material parameters such as Fermi energy, carrier mobility, carrier confinement length, and disorder mean free path.

Parameter dependent fabrication of Chromium nano-structures on Au(111) surface

Circular and rectangular Cr islands of nanometre size and monoatomic height exhibit an impromptu long range order on locally stepped Au(111) surface with narrow terraces. The circular islands are detached from the {100} microfaceted Au(111) step edges and nucleate between a pair of discommensuration lines of the herringbone reconstruction on ≈ 9.5 nm wide terraces. In contrast, the rectangular islands are attached to the {111} microfaceted step edges and are formed on even narrower terraces of ≈ 6 nm width. Both types of islands prefer the hcp region of the substrate as the nucleation centre. Cr deposition at elevated temperatures such as 470 K results in formation of triangular islands and spiral dislocations on wide substrate terraces. Interestingly however, at higher coverages of 3–4 ML, huge elongated ridges that are 90–260 nm in length, 10–20 nm in width and up to almost 3 nm height are observed. These ridges occur primarily along <11¯0> direction, with the rest forming at an angle of 60 or 120° with respect to this direction.

Symmetry-Induced Structuring of Ultrathin FeO and Fe₃O₄ Films on Pt(111) and Ru(0001).

Iron oxide films epitaxially grown on close-packed metal single crystal substrates exhibit nearly-perfect structural order, high catalytic activity (FeO) and room-temperature magnetism (Fe3O4). However, the morphology of the films, especially in the ultrathin regime, can be significantly influenced by the crystalline structure of the used support. This work reports an ultra-high vacuum (UHV) low energy electron/synchrotron light-based X-ray photoemission electron microscopy (LEEM/XPEEM) and electron diffraction (µLEED) study of the growth of FeO and Fe3O4 on two closed-packed metal single crystal surfaces: Pt(111) and Ru(0001). The results reveal the influence of the mutual orientation of adjacent substrate terraces on the morphology of iron oxide films epitaxially grown on top of them. On fcc Pt(111), which has the same mutual orientation of adjacent monoatomic terraces, FeO(111) grows with the same in-plane orientation on all substrate terraces. For Fe3O4(111), one or two orientations are observed depending on the growth conditions. On hcp Ru(0001), the adjacent terraces of which are ‘rotated’ by 180° with respect to each other, the in-plane orientation of initial FeO(111) and Fe3O4(111) crystallites is determined by the orientation of the substrate terrace on which they nucleated. The adaptation of three-fold symmetric iron oxides to three-fold symmetric substrate terraces leads to natural structuring of iron oxide films, i.e., the formation of patch-like magnetite layers on Pt(111) and stripe-like FeO and Fe3O4 structures on Ru(0001).

Growth and characterization of sidewall graphene nanoribbons

We study the growth of epitaxial graphene nanoribbons on silicon carbide mesa sidewalls by means of scanning probe techniques, local transport, and Raman spectroscopy. The sidewall nanoribbons are demonstrated to consist of charge neutral monolayer graphene with a zig-zag type orientation. Two types of roughness, the step density of the substrate and the roughness of the sidewalls, were identified as being detrimental to the transport properties of these ribbons. By means of 4-point probe experiments, single channel ballistic transport was observed with a mean free path limited by the width of the underlying substrate terraces. Moreover, a transition from ballistic to one-dimensional diffusive transport can be obviously triggered by an increased roughness of the sidewall, e.g., by an enlarged depth of the mesa.

Nanoscale Morphology and Atomic Structure of Passive Films on Stainless Steel

Electrochemical Scanning Tunneling Microscopy (ECSTM) has been employed to investigate in situ the nanometer scale morphology and the atomic structure of a (100)-oriented Fe-l8Cr-l3Ni single crystal stainless steel surface passivated in 0.5 M H2SO4(aq). The results show that polarization in the pre-passive or passive domain does not destroy the typical as-prepared substrate topography but modifies the nanogranular morphology of the air-formed native oxide film initially covering the surface. In the pre-passive range (−0.06 V/SHE), dissolution of iron-rich oxide species has the main effect on the local changes, digging the surface preferentially on the substrate terraces where the native oxide film is thinner. In the passive domain, the 3D growth of oxide grains counteracts dissolution more rapidly than in the pre-passive range limiting the dissolution-induced modifications. The growth of the Cr(III)-enriched passive film is preferential along the substrate steps. Increasing the passivation potential (from 0.35 to 0.7 V/SHE) does not markedly influence the amplitude of the passivation-induced modifications of the morphology of the covering oxide film. However, it causes crystallization of the oxide. The atomic lattice measured in situ is consistent with the oxygen sublattice of the major oxide component (Cr2O3) of the Cr(III)-enriched passive film.

Layer-by-layer growth of sodium chloride overlayers on an Fe(001)-p(1 × 1)O surface

Ultra-thin NaCl films epitaxially grown on an Fe(001)-p(1 × 1)O surface have been investigated in ultra-high vacuum by non-contact atomic force microscopy and low energy electron diffraction. It has been found that at temperatures below 145 °C NaCl initially grows as monoatomic thick islands on substrate terraces, while at temperatures above 175 °C biatomic thick islands are also formed at substrate step edges. Both types of islands have the same Fe(001)–O[100] ∥ NaCl(001)[110] orientation, leading to a (4 × 4) superstructure, where the NaCl unit cell is oriented at 45° with respect to the substrate. Interestingly, no c(2 × 2) superstructure with the NaCl unit cell oriented at 0° has been observed. The oxygen on the iron surface promotes layer-by-layer growth, resulting in atomically flat films with 40–60 nm wide terraces at coverages ranging from 0.75 to 12 ML. Such NaCl films are of much higher quality than MgO films grown on Fe(001) and Fe(001)-p(1 × 1)O surfaces and represent a unique epitaxial system of an alkali halide on a pure metallic substrate. The reduced number of defects and the layer-by-layer mode of growth make this system very attractive for applications where an atomically defined tunnel barrier is required to control the properties of a device.

Intrinsic Terahertz Plasmons and Magnetoplasmons in Large Scale Monolayer Graphene

We show that in graphene epitaxially grown on SiC the Drude absorption is transformed into a strong terahertz plasmonic peak due to natural nanoscale inhomogeneities, such as substrate terraces and wrinkles. The excitation of the plasmon modifies dramatically the magneto-optical response and in particular the Faraday rotation. This makes graphene a unique playground for plasmon-controlled magneto-optical phenomena thanks to a cyclotron mass 2 orders of magnitude smaller than in conventional plasmonic materials such as noble metals.

Transport properties of high-quality epitaxial graphene on 6H-SiC(0001)

We have extensively studied the electronic properties of epitaxial graphene grown on the Si face of a 6H silicon carbide substrate by thermal decomposition in an argon atmosphere. Using e-beam lithography, large van der Pauw structures as well as Hall bars were patterned. Their size ranged from millimeters down to submicrometer-sized Hall bars, the latter entirely placed on atomically flat substrate terraces. We found reproducible electronic properties, independent of the sample size and orientation, over a broad temperature range. A comparison of the mobility values indicated no enhanced scattering at the macroscopic step edges of the SiC substrate and due to adsorbed molecules. However, the strong coupling to the substrate results in an elevated charge carrier density n and a reduced mobility μ compared to exfoliated graphene. If n is decreased the mobility rises substantially (up to 29 000 cm 2/V s at 25 K), and Shubnikov–de Haas oscillations and the graphene-like quantum Hall effect become visible. This leads to the conclusion that the electrons in epitaxial graphene have the same quasi-relativistic properties previously shown in exfoliated graphene and expected from theory.

Coverage Driven Formation of Homochiral Domains of an Achiral Molecule on Au(111)

Adsorption of achiral Ni-tetramethyl-tetraazaannulene on Au(111) leads to the assembly of chiral trimers, which serve as building blocks for larger chiral patterns. A racemic mixture is observed at low coverages. Beyond a critical coverage, extraordinarily large homochiral domains occur and cover entire substrate terraces. The chiral structures and the formation of homochiral domains are induced by steric interactions.

Formation of nanoscale clusters during the initial stages of CaF2 growth on miscut Si(111)

The initial stages of high temperature CaF2 growth by molecular beam epitaxy on Si(111) substrates with a 3° miscut were characterized using atomic force microscopy and low energy electron diffraction. At a growth temperature of 750 °C, electron diffraction measurements showed that the surface retained the (3×1) surface reconstruction up to a deposition thickness of at least 1.2 nm. The overall topography of the surface was defined by atomically flat terraces decorated with a large number of clusters. These clusters were confined to step edges and were typically 10–20 nm tall. The clusters appear to nucleate at the top of step edges and then grow in size until they extend across the step onto the neighboring terrace below. These results indicate that in this growth regime, the CaF2 molecules diffuse across terraces to aggregate into relatively large nanostructures after the formation of a thin wetting layer. The unusually rounded features and large heights seen in these clusters appear to arise from the topography of substrate terraces.

Quantum oscillations and quantum Hall effect in epitaxial graphene

We investigate the transport properties of high-quality single-layer graphene, epitaxially grown on a 6H-SiC(0001) substrate. We have measured transport properties, in particular charge carrier density, mobility, conductivity and magnetoconductance of large samples as well as submicrometer-sized Hall bars which are entirely lying on atomically flat substrate terraces. The results display high mobilities, independent of sample size and a Shubnikov-de Haas effect with a Landau level spectrum of single-layer graphene. When gated close to the Dirac point, the mobility increases substantially, and the graphene-like quantum Hall effect occurs. This proves that epitaxial graphene is ruled by the same pseudo-relativistic physics observed previously in exfoliated graphene.

Transport Properties of Single-Layer Epitaxial Graphene on 6H-SiC (0001)

We report on electrical measurements on epitaxial graphene on 6H-SiC (0001). The graphene layers were fabricated by thermal decomposition in Argon atmosphere. Large van der Pauw structures and Hall bars were patterned by e-beam lithography, the Hall bars ranged from rather large structures down to sub-micrometer sized Hall bars entirely placed on atomically °at substrate terraces. We present Hall measurements in a broad temperature range, Shubnikov de Haas oscillations and quantum Hall steps. The data lead to the conclusion that electrons in epitaxial graphene have the same quasi-relativistic properties previously shown in exfoliated graphene. A remarkable di®erence, however, is the stronger coupling to substrate phonons and the relatively high charging being an intrinsic property of this epitaxial system.

Growth of ultrathin layers of Au on LiNbO 3(0 0 0 1) measured with atomic force microscopy

Atomic force microscopy (AFM) has been used to characterize the growth of Au deposited via evaporation onto the positive face of single crystalline, lithium niobate, LiNbO 3(0 0 0 1) surface. In order to study the mechanisms for the ordering and aggregation of a noble metal on this ferroelectric surface, topographic and phase contrast imaging of the fractional surface coverage of Au were performed. Atomically flat, uniformly poled LiNbO 3 surfaces were prepared via an ambient high temperature anneal and served as a support for the thin gold films. These gold atomic layers were grown using electron bombardment evaporation sources under ultra-high vacuum (UHV) conditions and subsequently characterized under both vacuum and ambient environments. Using AFM it was found that gold preferentially nucleates at the top of LiNbO 3 substrate step edges. With increased coverage, island formation proceeds due to local aggregation of adsorbed gold on each substrate terrace. Based on local imaging of the growth morphology, the data is discussed in terms of thin film growth mechanisms as well as the influence of native surface features such as defects and charge distribution. Understanding growth mechanisms for gold layers on ferroelectric surfaces allows for a fuller appreciation of how atomic deposition of metal atoms on patterned poled LiNbO 3 surfaces would occur as well as yielding greater insight on the atomic characteristics of metals on ferroelectric interfaces.

Structure of ultrathin Pd films determined by low-energy electron microscopy and diffraction

Palladium (Pd) films have been grown and characterized in situ by low-energy electron diffraction (LEED) and microscopy in two different regimes: ultrathin films 2–6 monolayers (ML) thick on Ru(0001), and ∼20 ML thick films on both Ru(0001) and W(110). The thinner films are grown at elevated temperature (750 K) and are lattice matched to the Ru(0001) substrate. The thicker films, deposited at room temperature and annealed to 880 K, have a relaxed in-plane lattice spacing. All the films present an fcc stacking sequence as determined by LEED intensity versus energy analysis. In all the films, there is hardly any expansion in the surface-layer interlayer spacing. Two types of twin-related stacking sequences of the Pd layers are found on each substrate. On W(110) the two fcc twin types can occur on a single substrate terrace. On Ru(0001) each substrate terrace has a single twin type and the twin boundaries replicate the substrate steps.

Catalytic chemical vapor deposition of methane on graphite to produce graphene structures

Graphene structures, obtained by catalytic chemical vapor deposition of methane on highly oriented pyrolitic graphite (HOPG), were examined using scanning tunneling microscopy. Depending on the Fe catalyst coverage and localization on the substrate steps and terraces, different graphene structures were obtained: curved graphene sheets at the edges of topmost stacked graphene bilayers, laterally grown terraces at the edges of individual graphene layers parallel to the HOPG basal plane and planar graphene islands on the terraces. A growth mechanism is proposed that takes into account the specific features of the spatial distribution of Fe catalytic nanoparticles on the substrate surface, driven by metal film-substrate interaction. The present synthesis approach is promising for the controlled growth and modification of graphene layers, as well as for engineering the edge characteristics of graphene systems at the atomic scales.