Surface Step Structure Research Articles

Zig-zag surface step migration and structure modulation in metallic nanowires

Nanoscale metals frequently exhibit novel mechanical properties or physiochemical performances, due to the growing influence of surfaces. Here, we uncover a unique surface-mediated deformation behavior of Au nanowires by conducting in situ transmission electron microscope tensile testing. A zig-zag migration of surface steps via the transverse slips on alternate (111) and (001) planes is revealed in [112]-oriented Au nanowire, which contributes to unexpected uniform deformation with an elongation of 108.9%. Further, we discover that the unusual (001) slip can induce an anomalous stacking structure on the nanowire surface under uniaxial tension. These findings enrich our understanding of surface-mediated plasticity in metallic nanomaterials.

Magnetization reversal in Fe(001) films grown by magnetic field assisted molecular beam epitaxy

We studied the influence of a magnetic field (MF) on epitaxial growth and magnetic properties of Fe(001) films deposited on MgO(001). Thanks to modular sample holders and a specialized manipulator in our multi-chamber ultrahigh vacuum system, the films could be deposited and annealed in an in-plane MF of 100 mT. In situ scanning tunnelling microscopy showed that MF had a strong influence on the film morphology, and, in particular, on the structure of surface steps. The magnetic properties were studied ex situ using magneto-optic Kerr effect (MOKE) magnetometry and microscopy. We showed that the moderate in-plane magnetic field applied during growth has the visible impact on the magnetic properties. The observed angular dependence of the MOKE loops and domain structures were discussed based on a magnetization reversal model. In particular we found that magnetization reversal occurs via 90° domains and the reversal differs for the no-field and in-field grown samples, in correlation with the film morphology.

Width-dependent band gap of arm-chair graphene nanoribbons formed on vicinal SiC substrates by MBE

Formation and electronic states of graphene nanoribbons with arm-chair edges (AGNR) are studied on the SiC(0001) vicinal surfaces toward the [] direction. The surface step and terrace structures of both 4H and 6H-SiC substrates are used as the growth templates of one-dimensional arrays of AGNRs, which are prepared using the carbon molecular beam epitaxy followed by hydrogen intercalation. A band gap is observed above the π-band maximum by angle-resolved photoelectron spectroscopy (ARPES) for the both samples. The average widths of the AGNRs are 6 and 10 nm, and the estimated average band gaps are 0.40 and 0.28 eV for the 4H- and 6H- substrates, respectively. A simple and phenomenological inverse relation between the energy gap and AGNR width works in the analyses of the ARPES data.

Influence of the terrace width of pentacene underlayers on the crystallinity of C60 overlayers

Pentacene and fullerene (C60) form an efficient stacked heterostructure in organic solar cells, and C60 films exhibit high crystallinity on pentacene films. However, the mechanism of the crystallinity improvement in C60 overlayers remains to be fully elucidated. In this work, the surface (step and terrace structure) of pentacene films is investigated in terms of the terrace width using atomic force microscopy (AFM) to clarify the mechanism. We found that the terrace width becomes wider as the substrate temperature for pentacene film growth increases, and the terrace width becomes narrower as the thickness of pentacene films increases. In addition, we investigate the influence of the terrace width of pentacene underlayers on C60 overlayers using AFM and X-ray diffraction. We found that pentacene underlayers with wide terraces increase the grain size and improve the crystallinity of C60 overlayers. These findings indicate that growing pentacene underlayers with wider terraces allows for further enhancement in the crystallinity of C60 overlayers, leading to higher quality pentacene/C60 stacked heterostructures.

Effect of aluminum addition on the surface step morphology of 4H–SiC grown from Si–Cr–C solution

For the solution growth of 4H–SiC with Si1−xCrx solvents, the change in surface step structure by 4at% Al addition to the solvent was investigated. Without Al addition, step bunching resulted in the formation of giant macrosteps with height greater than several micrometers, and trench-like surface defects formed with solvent inclusion. The edge of the giant macrosteps composing trench-like defects was faceted into low-index facet planes, (11¯0m) (m=1–4). The formation of the trench-like surface defects is considered to originate from the self-pinning of macrosteps owing to the step-faceting phenomenon, which facilitates further development of macrosteps. On the other hand, the addition of Al to the solvents significantly improved the surface roughening, and suppressed the formation of the trench-like surface defects. In this case, smaller bunched steps with heights from several nanometers to about 10nm formed on the terraces of the macrosteps, while regularly arranged trains of unit-cell-size steps formed on the terrace of macrosteps in growth without Al addition. The decrease in step stiffness might be a possible cause for the formation of the smaller bunched steps on the terrace in growth with Al addition.

Structure and composition of bismuth telluride topological insulators grown by molecular beam epitaxy

The structure and composition of Bi2Te3−δtopological insulator layers grown by molecular beam epitaxy is studied as a function of beam flux composition. It is demonstrated that, depending on the Te/Bi2Te3flux ratio, different layer compositions are obtained corresponding to a Te deficit δ varying between 0 and 1. On the basis of X-ray diffraction analysis and a theoretical description using a random stacking model, it is shown that for δ ≥ 0 the structure of the epilayers is described well by a random stacking of Te–Bi–Te–Bi–Te quintuple layers and Bi–Bi bilayers sharing the same basic hexagonal lattice structure. The random stacking model accounts for the observed surface step structure of the layers and compares very well with the measured X-ray data, from which the lattice parametersaandcas a function of the chemical composition were deduced. In particular, the in-plane lattice parameterais found to continuously increase and the average distance of the (0001) hexagonal lattice planes is found to decrease from the Bi2Te3to the BiTe phase. Moreover, the lattice plane distances agree well with the linear interpolation between the Bi2Te3and BiTe values taking the strain in the epilayers into account. Thus, the chemical composition Bi2Te3−δcan be directly determined by X-ray diffraction. From analysis of the X-ray diffraction data, quantitative information on the randomness of the stacking sequence of the Bi and Te layers is obtained. According to these findings, the layers represent random one-dimensional alloys of Te–Bi–Te–Bi–Te quintuple and Bi–Bi bilayers rather than a homologous series of ordered compounds.

Surface Evolution of 4H-SiC(0001) during <i>In Situ </i>Surface Preparation and its Influence on Graphene Properties

The evolution of SiC surface morphology during graphene growth process has been studied through the comparison of substrate surface step structure after in-situ etching and graphene growth in vacuum. Influence of in-situ substrate surface preparation on the properties of graphene was studied through the comparison of graphene layers on etched and un-etched substrates grown under same conditions.

Improving p-type contact characteristics by Ni-assisted annealing and effects on surface morphologic evolution of InGaN LED films grown on Si (1 1 1)

Ni thin layer was deposited to assist to activate p-GaN and then was removed. The process was named Ni-assisted annealing (NA). We investigate the surface morphology and p-type contact behaviors of InGaN LED films grown on Si (1 1 1) substrates. Compared with conventional thermal annealing (TA), NA can improve the p-type contact characteristic at lower anneal temperature and a smaller specific contact resistivity ( ρ c = 6.1 × 10 −5 Ω cm 2) employing nonalloy Pt electrode was obtained. A wet etching method using acid–hydrogen peroxide was adopted to boil films surface after activation. We found that some nano-pits appeared on surfaces while original surface step structure was still clearly visible, which shows a defect-selective etching characteristic. Otherwise, we demonstrated that the surface morphology could be affected by NA while independent to TA. Some mechanisms for experimental phenomena were also discussed in the letter.

Relationship between surface structure and one-dimensional nanorod growth in ErBa2Cu307−δ films on vicinal SrTi03 substrates

Effects of vicinal growth in ErBa2Cu307−δ (ErBCO) with one-dimensional nanorods are discussed from their microstructures. BaNb2O6 doped ErBCO films were grown on vicinal substrates with tilted angles of 0°, 4° and 8° by a pulsed laser technique. Surface step structure appeared distinctly with increasing vicinal angles to 8° in images of scanning electron microscopy and atomic force microscopy. Then, Ba(Er0.5Nb0.5)O3 nanorods continuously grew from the step on vicinal substrate to film surface steps in transmission electron microscopy images of the film. The alignment of one-dimensional nanorods relate closely to film growth morphology.

A reactive force-field (ReaxFF) Monte Carlo study of surface enrichment and step structure on yttria-stabilized zirconia

To investigate surface segregation in yttria-stabilized zirconia (YSZ), DFT energies describing surface energy as a function of yttrium lattice position were used to parameterize a reactive-force field (ReaxFF). We used ReaxFF to perform Monte Carlo (MC) simulated annealing to sample structural configurations of flat YSZ (111) and vicinal YSZ (111) stepped surfaces. We evaluated yttrium surface segregation, oxygen vacancy position, and surface step composition for flat and stepped YSZ surfaces. It is thermodynamically favorable for yttrium atoms to segregate to the surface of YSZ, and specifically to step edge sites. Surface saturation of yttrium occurs at approximately 40% (40:60 Y:Zr ratio) while yttrium concentration at the step edge does not approach a saturation value, suggesting that steps on the YSZ surface are mainly yttria-terminated. We found that it is thermodynamically favorable for oxygen vacancies to occupy positions in the subsurface layer of YSZ, and a higher fraction of vacancies occupy positions NN to Y than NN to Zr. Yttrium segregation to step edges on the YSZ surface does not lower the surface formation energy of the stepped surface below that of the flat (111) termination, suggesting that the stability of YSZ surface steps observed experimentally is due to kinetic barriers for surface re-ordering.

Investigation of heavily nitrogen-doped n+ 4H–SiC crystals grown by physical vapor transport

Heavily nitrogen-doped n + 4H–SiC single crystals were grown by the physical vapor transport (PVT) method. The nitrogen incorporation kinetics in a heavily doped regime was studied in terms of growth temperature dependence, and it was revealed that the growth temperature substantially influenced the amount of nitrogen incorporated into the crystals and their surface step structures on the (0 0 0 1¯)C facet plane. The structural quality of heavily nitrogen-doped 4H–SiC crystals was examined by X-ray rocking curve measurements and defect selective etching by molten KOH at around 500 °C. The crystals contained an extremely low density of 3C–SiC inclusions and stacking faults and showed a comparable crystalline quality to conventionally doped 4H–SiC substrates. Furthermore the structural stability of the heavily nitrogen-doped 4H–SiC substrates during high-temperature treatments has been investigated. The substrates with a large {0 0 0 1} surface roughness showed a resistivity increase after annealing at 1100 °C for 2 h, which was confirmed to be caused by the formation and expansion of double Shockley-type basal plane stacking faults in the substrates. The occurrence of the stacking faults largely depended on the surface preparation conditions of substrates, which indicate that the primary nucleation sites of stacking faults exist in the near-surface regions of substrates.

OMVPE growth of highly strain-balanced GaInAs/AlInAs/InP for quantum cascade lasers

The growth and characterization of highly strain-balanced (SB) GaInAs/AlInAs/InP quantum-well heterostructures for mid-infrared quantum cascade lasers (QCLs) are reported. Growth conditions were established to provide a step-flow growth mode, which is believed to be important for precise thickness control of the ultrathin (∼1 nm) epilayers of the injector/active QCL regions. Heterostructures with GaInAs and AlInAs absolute layer strain ranging from 0.5% to as high as 1.5% were grown by organometallic vapor-phase epitaxy at a growth temperature of 580 °C and utilized tertiarybutyl arsine and tertiarybutyl phospine as group V sources and triethylgallium, trimethylindium, and trimethylaluminum as group III sources. Growth was optimized by characterization of SB GaInAs/AlInAs/InP multiple quantum-well structures using high-resolution X-ray diffraction (XRD) and atomic force microscopy. Similar surface step structure and XRD spectra were obtained over the 0.5–1.5% SB range. QCLs with nominal strain balance of 1% and based on a four-quantum-well double-phonon resonance-active region design were grown and processed as ridge lasers. These lasers operate at 4.8 μm in pulsed mode at room temperature with a threshold current density of 1.3 kA/cm 2, peak power of 1.74 W, and power-conversion efficiency of 9.7%. These results are comparable to state-of-the-art QCLs grown by molecular beam epitaxy.

In-situ surface preparation of nominally on-axis 4H-SiC substrates

A study of the in-situ surface preparation has been performed for both Si- and C-face 4H-SiC nominally on-axis samples. The surface was studied after etching under C-rich, Si-rich and under pure hydrogen ambient conditions at the same temperature, pressure and time interval using a hot-wall chemical vapor deposition reactor. The surfaces of all the samples were analyzed using optical microscopy with Normarski diffractional interference contrast and atomic force microscopy with tapping mode before and after in-situ etching. Polishing related damages were found to be removed under all etching conditions, also the surface step structure was uncovered and a few defect-selective etch pits were observed. For the Si-face sample, the best surface morphology was obtained after Si-rich etching conditions with more uniform and small macro-step height which resulted in the lowest surface roughness. For the C-face sample the surface morphology was comparable under all etching conditions and not much difference was found except for the etching rate. Si droplets were not observed under any etching conditions.

Organometallic vapor phase epitaxy of relaxed InPAs/InP as multiplication layers for avalanche photodiodes

Abstract InP 1− y As y epitaxial layers grown lattice-mismatched (LMM) on InP substrates were investigated as a new materials system for multiplication layers in Geiger-mode avalanche photodiodes (GM APDs) for detection of photons in the range 1.6–2.5 μm. LMM InP 1− y As y epilayers were grown on semi-insulating (1 0 0) InP substrates misoriented 0.2° and 2°→[1 1 0] by organometallic vapor phase epitaxy at a growth temperature of 580 °C. The growth scheme used for the InP 1− y As y buffer layer was optimized based on surface step structure and X-ray diffraction. It was found that step-flow growth is a minimum criterion for obtaining good material quality. A narrower XRD full-width at half-maximum values were measured for 2°-miscut substrates compared to 0.2°-miscut substrates. A high-quality buffer was obtained by step-grading the InP 1− y As y composition in increments of y =0.05 over a layer thickness of 0.5 μm to a final y =0.25. The device performance of LMM GM APDs was compared to that of measured more traditional lattice-matched GaSb-based devices. At 77 K, dark count rates of LMM devices are ∼50 kHz at 5 V overbias, and are comparable to GaSb-based p–i–n diodes operated in Geiger mode, while reset times of 0.02 ms are approximately 3 orders of magnitude lower than GaSb-based GM APDs.

Factors Affecting the Coverage Dependence of the Diffusivity of One Metal over the Surface of Another

Various factors that can affect the coverage dependence of diffusivity for the case of surface hetero-diffusion have been analyzed. Simple Monte Carlo modeling of the diffusion process shows that diffusivity can either increase or decrease with coverage, depending on whether the interaction between diffusing atoms is repulsive or attractive. The results of more detailed simulations show that these simple predictions are reasonable. If the diffusing species undergoes first-order phase transitions as a function of coverage, then important perturbations of the diffusion profiles will result. One final factor in coverage dependence of diffusivity is considered, namely, the possible effects of coverage on the surface step structure. An analysis of diffusion profiles obtained for the spreading of Bi over Cu(100) shows that the major effects of coverage on diffusivity come about from the presence of phase transitions, and from the inherent coverage dependence of diffusivity in systems with short-range repulsion.

STM study of Bi-on-Cu(1 0 0)

Scanning tunneling microscopy (STM) has been used to study the various possible structures of adsorbed Bi on the Cu(1 0 0) surface, after equilibration at a temperature of 520 K. All of the structures previously identified by X-ray diffraction (lattice gas, c(2 × 2), c(9√2 × √2) R45°, and p(10 × 10), in order of increasing Bi-coverage) were found to be present on a single sample produced by diffusing Bi onto the Cu(1 0 0) surface from a 3-d source. By investigating the possible coexistence of various pairs of phases, it was demonstrated that the c(2 × 2) phase transforms to the c(9√2 × √2) R45° phase by a first order transition, whereas the transition from c(9√2 × √2) R45° to p(10 × 10) is continuous. In addition, the structure of surface steps was studied as a function of Bi-coverage. The results showed that the presence of Bi changes the nature of the step–step interactions at the Cu(1 0 0) surface from repulsive to attractive. The attractive step–step interactions transform any small deviations from the nominal (1 0 0) orientation of the Cu substrate into (3 1 0) microfacets. When compared with the known equilibrium crystal shape (ECS) of Bi-saturated Cu, the observed microfaceting may imply that the ECS of Cu–Bi alloys is temperature dependent.

Origin of triangular island shape and double-step bunching during GaN growth by molecular-beam epitaxy under excess Ga conditions

GaN homoepitaxial growth by molecular-beam epitaxy under both excess gallium (Ga) and excess nitrogen (N) conditions is investigated. Based on two-dimensional island shape and surface step structures, we suggest the growth is kinetic-limited under the excess-Ga condition but diffusion-limited in the excess-N regime. The triangular GaN islands and double step bunching seen on surfaces prepared under excess-Ga are attributed to a difference in adatom attachment and/or site exchange rates between $A$ and $B$ steps, which is induced by surfactant Ga adlayers on GaN(0001).

Cubic inclusions in 4H-SiC studied with ballistic electron-emission microscopy

High-temperature-processing-induced “double-stacking fault” cubic inclusions in 4H-SiC were studied with ballistic electron emission microscopy (BEEM). Large BEEM current and a ∼0.53 eV local reduction in the Schottky barrier height (SBH) were observed where the inclusions intersect a Pt interface, confirming the quantum-well nature of the inclusions and providing nanometer scale information about local electronic behavior. Measured spatial variations in the BEEM current are related to the inclusion orientation and local surface step structure. An observation of an anomalously low SBH is discussed, suggesting the existence of a triple- or quadruple-stacking fault inclusion.

Atomic Force Microscope Observation of Growth and Defects on As-Grown (111) 3C-SiC Mesa Surfaces

AbstractThis paper presents experimental atomic force microscope (AFM) observations of the surface morphology of as-grown (111) silicon-face 3C-SiC mesa heterofilms. Wide variations in 3C surface step structure are observed as a function of film growth conditions and film defect content. The vast majority of as-grown 3C-SiC surfaces consisted of trains of single bilayer height (0.25 nm) steps. Macrostep formation (i.e., step-bunching) was rarely observed, and then only on mesa heterofilms with extended crystal defects. As supersaturation is lowered by decreasing precursor concentration, terrace nucleation on the top (111) surface becomes suppressed, sometimes enabling the formation of thin 3C-SiC film surfaces completely free of steps. For thicker films, propagation of steps inward from mesa edges is sometimes observed, suggesting that enlarging 3C mesa sidewall facets begin to play an increasingly important role in film growth. The AFM observation of stacking faults (SF's) and 0.25 nm Burgers vector screw component growth spirals on the as-grown surface of defective 3C films is reported.

Homoepitaxy of 6H-SiC on nearly on-axis (0 0 0 1) faces by chemical vapor deposition Part II: Evolution of surface steps

Homoepitaxial growth on 6H-SiC (0 0 0 1)-vicinal faces by horizontal cold-wall chemical vapor deposition operating at 1500°C has been re-investigated. It was found that the off-direction toward 〈0 1 1 ̄ 0〉 can provide smooth surfaces, even after long-time growth, as for 〈1 1 2 ̄ 0〉 -off, if epitaxial growth with a proper procedure is carried out on well-polished substrates. Based on observations of surface step structures by atomic force microscopy, the morphological tendency and evolution of surface steps by the growth on 〈0 1 1 ̄ 0〉 -off substrates are discussed.