High-index Surfaces Research Articles

Site-Specific Chemistry of Ethylene on Si(114)-(2 × 1)

Si(114)-(2 x 1) is a stable high-index surface with a single domain-like surface reconstruction composed of parallel rows of 3-fold coordinated rebonded atoms (resembling adatoms), dimers, and tetramers (dimers combined with non-dimerized atoms in diene-like structures). The distinct chemical properties expected for the oriented array of different structural motifs make this surface an interesting template for direct, comparative studies of structure-dependent reactions. We have studied the room-temperature adsorption of ethylene on rebonded atoms, dimers, and tetramers on Si(114)-(2 x 1) by atomic-resolution scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations of structure, energetics, and simulated STM images. Ethylene chemisorbs in four distinct geometries on Si(114), all of which are di-σ-bonded structures, with all adsorption sites occupied in a well-ordered monolayer at saturation coverage. In order of decreasing adsorption probability, the ethylene-Si reaction products were ethylene-bridged rebonded atoms, dimer [2 + 2] adducts, tetramer [4 + 2] adducts on one side of the tetramer, and (at high coverages only) tetramer [2 + 2] adducts on the dimer component of the tetramer. The calculations indicate that adsorption is kinetically limited at room temperature, with one energetically favorable reaction on the tetramer inhibited by an adsorption barrier. A particularly interesting aspect of the Si(114)-(2 x 1) surface demonstrated by this work is that it can act as both a dienophile (dimer sites) and a diene (tetramer sites) in cycloaddition adsorption reactions.

Red-shifting the surface plasmon resonance of silver nanoparticles for light trapping in solar cells

AbstractSurface plasmons in metallic nanoparticle arrays have been shown to increase the absorption of an underlying silicon substrate. This has wide ranging applications, not least in the photovoltaic industry. Incident light excites localised surface plasmons in the silver nanoparticles and is coupled into the silicon in trapped modes. The radiative behaviour of the nanoparticle film is changed by the proximity of a high refractive index surface, causing radiation to be directed into the silicon and providing a light-trapping layer. We investigate a simple and effective method of tuning the surface plasmon resonance frequency, and hence the spectral region at which the absorption enhancement is seen, by varying the underlying dielectric. The particle geometry and distribution are modified by the surface conditions provided by the dielectric layer, and both this and the change in refractive index alter the resonance position. Three common dielectrics used in the photovoltaic industry were investigated as surfaces on which to form arrays of self-assembled silver nanoparticles atmospheric pressure chemical vapour deposited titanium dioxide (APCVD TiO2), low pressure chemical vapour deposited silicon nitride (LPCVD Si3N4) and thermally grown silicon dioxide (SiO2). We show, by optical and electrical measurements, that the red-shifted resonances produced by nanoparticle films on APCVD TiO2, and LPCVD Si3N4 with relatively high refractive indices, correspond to an increase in optical absorption and external quantum efficiency in thin, crystalline solar cells at longer wavelengths.

Effects of Channel and Crystalline Orientations on the Electron Mobility in MOSFETs Fabricated on (114) and (5 5 12)-Silicon Substrates

This work reports the measurement of the electron mobility in the inversion layer of MOSFETs fabricated on Si(114) and Si(5 5 12), for medium and high effective transverse electric fields, Eeff. We studied the channel mobility in these surfaces as a function of the channel direction [1-4]. Results reveal the strong influence on electron mobility of the channel direction on high-index surfaces. It was noted that mobility on high index-surfaces is degraded when the channel direction goes far away from , but it is less affected when the channel runs parallel to this direction. Even though Si(0 0 1) has been the surface with the highest electron mobility reported, we found that mobility on Si(1 1 4) is higher than that on Si(0 0 1) when Eeff {greater than or equal to} 0.4 MV/cm.

Two distinct Sb-adsorption steps onSi(5512)−2×1: Indiffusion followed by preferential adsorption

Initial stages of antimony (Sb) adsorption on the $\mathrm{Si}(5\phantom{\rule{0.3em}{0ex}}5\phantom{\rule{0.3em}{0ex}}12)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}1$ surface have been studied by scanning tunneling microscopy in order to understand interfacial reaction between adsorbed Sb atoms and the Si template with one-dimensional (1D) symmetry. It has been found that there are two distinct steps, Sb indiffusion and preferential adsorption, at the initial Sb adsorption on $\mathrm{Si}(5\phantom{\rule{0.3em}{0ex}}5\phantom{\rule{0.3em}{0ex}}12)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}1$ held at $600\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Initially, deposited Sb atoms diffuse into the subsurface, cause indirect Si deposition, and result in surface reconstruction from a (5 5 12) terrace to (337) terraces with (113) steps. As soon as the subsurface Sb sites are saturated by indiffused Sb atoms, additionally deposited Sb atoms are preferentially adsorbed along the upper (113)-step edges and form 1D Sb wires with a spacing of about $10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, which corresponds to two periodic lengths of the original (5 5 12) surface. Once Sb-adsorption sites, (113) steps, are saturated, deposited Sb atoms cluster for themselves and do not contribute to nanowire fabrication. From the present studies, it has been found that both Sb indiffusion and preferential adsorption stabilize the high-index surface through relieving surface strain by way of either inserting or attaching Sb atoms, but once such surface strain is relieved, the 1D growth mode also terminates.

Sublimation behavior of SiO2 from low- and high-index silicon surfaces

The authors have used atomic force microscopy to investigate the sublimation behavior of 100-nm-thick oxide layers from the low-index Si(001) and Si(111) surfaces, as well as the stable, high-index Si(113) and Si(5 5 12) surfaces. Similar to previous thin-film (<50nm) sublimation studies, high vacuum annealing results in the formation of circular voids that grow laterally with annealing time (1–6min, 1150–1350°C). The depth of these voids is ∼200nm, or twice the thickness of the original oxide film, which is consistent with the thermal decomposition of SiO2 to form volatile SiO. There are subtle morphological differences, however, between the voids formed on the different surface orientations. Line profiles of the bottom Si surfaces inside the voids indicate flat terraces for the (111) and (113) orientations and sloped conic sections for the (001) and (5 5 12) orientations, indicating that the latter surfaces are less stable with respect to step formation during oxide sublimation. At the centers of voids, features such as deep pits (5μm diameter, 200–700nm depth) or vertical structures (1μm diameter, up to 700nm height) are present. Deep depressions are also observed at vertical sidewalls that occur during sequential anneals or on a patterned surface. After the thick oxide is completely desorbed, an undulating surface morphology occurs on all surface orientations due to the coalescence of the void sidewalls.

Adsorption and Vibration of O Atoms on Fe Low-index and Fe(211) High-index Surfaces

The 5-parameter Morse potential (5-MP) of the interaction between oxygen atoms and iron surfaces was constructed. The adsorption and diffusion of O atoms on Fe low-index and Fe(211) high-index surfaces were investigated by using 5-MP. All the critical characteristics of the system, such as adsorption site, adsorption geometry, binding energy, and eigenvalues for vibration, were calculated. The calculation results show that O atoms are located at the fourfold hollow site of the Fe(100) surface with an eigenvibration at 437 cm −1 . These results are in good agreement with the experimental and theoretical results obtained previously. With regard to the adsorption site of O—Fe(110) system, the authors of this study assume that the preferential adsorption state is the H 3 site and not the LB site, which is not in agreement with the experimental inferences obtained earlier. However, on the Fe(111) and Fe(211) surfaces, O atoms predominantly occupy the quasi-3-fold site.

A three-dimensional model for artificial atoms and molecules: influence of substrate orientation and magnetic field dependence

A full three-dimensional model for the calculation of the electronic structure of semiconductor quantum dots (QD) and molecules (QDM) grown on high index surfaces and/or in the presence of an external magnetic field is presented. The strain distribution of the dots is calculated using continuum elasticity and singe-particle states are extracted from the nonsymmetrized eight-band k·p theory. The model properly takes into account the effects of different substrate orientation by rotation of the coordinate system in such a way that one coordinate coincides with the growth direction, whereas the effects of a tilted external magnetic field are taken into account through the Zeeman effect and employing a gauge invariant scheme based on Wilson's formulation of lattice gauge theory. We point out the role of piezoelectricity for InAs/GaAs QDs grown on [11k], where k = 1,2,3,4,5,7,9 and for QDMs containing eight InAs/GaAs QDs grown on [11l], where l = 1,2,3. We predict the variation of the transition energies of the QDM as a function of substrate orientation and interdot distances in the molecule. We address the magnetic field direction dependent variation of the electronic properties of QD and QDM.

THE CALCULATION OF THE SURFACE ENERGY OF HIGH-INDEX SURFACES OF SILICON AT ZERO TEMPERATURE

We used the molecular dynamics simulation based on the Stillinger–Weber (SW) interatomic potential to calculate the high-index surface energies of surfaces containing any of the stereographic surfaces of silicon at zero temperature. An empirical formula based on the structural unit model was generalized for high-index surfaces. Our simulated results show that the generalized formula can give a good estimation of the surface energy and structural feature of the high-index surfaces not only on the edge of stereographic but also within it. Our simulation and empirical formula results reveal that the closest surface has the lowest energy, so the closest (101) surface has the lowest surface energy and the (101), (111) and (001) surfaces are the extremum on the curve of surface energy versus orientation angle. Both the theoretical simulation results and the empirical formula calculation results are consistent with the available first-principles theoretical data.

Energetics ofCOon stepped and kinked Cu surfaces: A comparative theoretical study

Our ab initio calculations of CO adsorption on several low and high miller index surfaces of Cu show that the adsorption energy increases as the coordination of the adsorption site decreases from 11 to 6, in qualitative agreement with experimental observations. On each surface the adsorption energy is also found to decrease with increase in coverage, although the decrement is not uniform. Calculated vibrational properties show an increase in the frequency of the metal-C mode with decrease in coordination whereas no such effect is found in the frequency of the CO stretch mode. Examination of the surface electronic structure shows a strong local effect of CO adsorption on the local density of state of the substrate atoms. We also provide some energetics of CO diffusion on Cu(111) and Cu(211).

Crystal growth of analcime studied by AFM and atomistic simulation

A combined atomic force microscopy (AFM) and atomistic simulation approach affords a high-resolution picture of the structure and surface termination of synthetic analcime and provides insight into the atomic-scale mechanism of crystal growth. AFM measurement reveals a repeated step height of ∼0.6 nm on the {2 1 1} crystal face corresponding to the interlayer separation for the {2 1 1} rather than higher index surfaces, as is observed in, e.g., zeolite A. Analysis of the possible terminating structures for the {2 1 1} face from surface energy evaluations indicates that a unique terminating structure is favoured implying a step height of ∼0.6 nm in accord with the experimental measurement. Analysis of intermediate stages of growth using classical simulation approaches suggests that the mechanism may be mediated purely by monomeric rather than oligomeric species, which contrasts with the presumed secondary building unit mechanism for many zeolitic materials.

Nanofaceting and alloy decomposition: From basic studies to advanced photonic devices

Most of the modern epitaxial structures for semiconductor lasers serving the needs of optical storage and fiber pumping are grown on misoriented GaAs(001) substrates. It has been found in metal-organic vapor-phase epitaxy that surface misorientation helps to achieve better epitaxial quality of the alloy layers. On the other hand, these misoriented or, in other definition, high-index surfaces are known to undergo phase transformations, depending on the misorientation angle, from nanofaceting (like (311)A, (311)B, (331), (211)B GaAs surfaces) to arrays of step bunches (like (775) GaAs, etc.). In the present paper, we consider growth-related effects during growth of both standard and advanced laser structures on GaAs (11¯8) substrates which are typically used for growth of 650nm GaAlInP devices. We show that the active region of the laser structures represents a corrugated superlattice with a ∼25nm in-plane periodicity, while the surrounding layers are natural superlattices with a ∼5nm vertical periodicity. Corrugated superlattice used as an active region manifests itself through a strong modification of optical properties. Strong in-plane polarization evidences the formation of arrays of quantum wires. Both standard and advanced red laser have been grown and processed. The advanced lasers have demonstrated a vertical beam divergence of only 7–8° full-width at half-maximum (FWHM). The advanced lasers with 10μm-wide stripes demonstrate continuous wave (CW) power up to ∼200mW and the lateral beam divergence of 4°, and those with 4μm-wide stripes show CW power up to ∼120mW and the lateral beam divergence of 6.5°. No facet passivation has been applied and the power is limited by the catastrophic optical mirror damage (COMD). 20W pulsed power has been achieved in 100μm-wide stripes for the advanced design and 6W has been obtained for the standard design. The advantage comes from the design optimization of the laser waveguide using the concept of the longitudinal photonic bandgap crystal. We believe that also the optimization of the active region with better utilization of the nanofaceting effects may enable, in addition, a dramatic extension of the emission wavelength towards bright red (620–630nm) and, probably, yellow (∼580nm) spectral ranges.

Atomic Structures of Benzene and Pyridine on Si(5 5 12)-2 × 1

The adsorption structures of benzene and pyridine on Si(5 5 12)-2 x 1 were studied at 80 K by using a low-temperature scanning tunneling microscope and density functional theory calculations. These structures are different from those observed on low-index Si surfaces: benzene molecules exclusively bind to two adatoms, that is, with di-sigma bonds between carbon atoms and silicon adatoms, leading to the loss of benzene aromaticity; in contrast, pyridine molecules interact with adatom(s) through either Si-N dative bonding or di-sigma bonds. Dative bonding configurations with pyridine aromaticity are the dominant adsorption features and are more stable than di-sigma bonding configurations. Thus the dative bonding of nitrogen-containing heteroaromatic molecules provides a strategy for the controlled attachment of aromatic molecules to high-index surfaces.

Observation of Ga droplet formation on (311)A and (511)A GaAs surfaces

Using (100) GaAs substrates as a reference, we present a study of the formation of Gadroplets on (311)A and (511)A GaAs substrates in which the effect of both the substratetemperature and the amount of Ga supplied on the droplet density and height for the threedifferent surfaces have been investigated. Droplets on (100) substrates show around shape; however, they appear as elongated balls with tails along the direction of the (311)A substrate and the direction of the (511)A substrate. It has been found that the Ga droplets on (511)Asurfaces have lower densities and higher heights than those on (100) substrates. In contrast,Ga droplets on (311)A surfaces have lower heights and much higher densitiescompared to those for both (100) and (511)A. We observed that the decrease in thedroplet density with increasing growth temperature for both (311)A and (511)A ismore than twice that for the (100)GaAs surface due to the larger drop in thenucleation rate. Based on these observations, we offer a physical explanationbased on the thermodynamics and the anisotropy of the high-index surfaces.

Study of the optical and structural properties of multi quantum well structures grown on high-index surfaces

The MOVPE overgrowth of high [01¯1]-oriented ridges confined at sides by facets related to {n11} crystallographic planes is reported. We studied the influence of the side tilt on the thickness of the AlGaAs and GaAs epitaxial layers grown under the condition of the kinetic growth mode. The multi quantum well (MQW) structures were prepared on the sides of ridges tilted at 54.7°, 45° and 30° to (100). The sidewall surface morphologies before and after epitaxial growth were evaluated and compared. We observed no tendency towards planarization towards a neighbouring high-index crystallographic plane, such as (211) and (311). We also showed that the quantum wells of the MQW structure make a smooth transition over the edge between the top surface and the facet as both AlGaAs and GaAs grew at similar rates on the surfaces.

Theoretical study of InAs/GaAs quantum dots grown on [11k] substrates in the presence of a magnetic field

Eight-band k.p theory including strain and piezoelectric effects are employed to calculate the strain distribution and electron and hole energy levels of InAs/GaAs quantum dots grown on [11k] substrates in the presence of an external magnetic field. Height of the dot determines how the increasing of k influences isotropic part of the strain tensor while biaxial part of the strain tensor is always reduced with increasing k. Because of the reduced symmetry of high index surfaces, influence of piezoelectric effect on the electronic structure becomes more dominant with increasing k. Electron energy levels are influenced by the isotropic part of the strain compared to the hole energy levels, where strong heavy hole–light-hole mixing is observed. For the dots grown on the [11k] surfaces magnetic field has smaller influence on the electron and hole energy levels as compared to the referent case.

Enhancing the sensitivity of a whispering-gallery mode microsphere sensor by a high-refractive-index surface layer

The use of whispering gallery mode (WGM) evanescent waves to detect adsorption of molecules to the surface of microsphere sensors and more particularly to the utilization of a high refractive index surface layer to increase the sensitivity thereof. The present invention examines the sensor capability of WGM in a dielectric sphere coated with a thin uniform dielectric layer of a high refractive index. Among the utilities of such a modified resonator for the sensing are to have an evanescent field of a different penetration depth without using a non-silica based microsphere or changing the laser wavelength, to further enhance the sensitivity by drawing the optical field of WGM into the coating layer, and to realize the same relative shifts for WGM of different radial modes, thus eliminating ambiguities in the measurement of a refractive index change in the surrounding medium.

The electron counting rule and passivation of compound semiconductor surfaces

A brief review is presented of the role of electron counting rule (ECR) in explaining structural stability and passivation of compound semiconductor surfaces. While III–V(1 1 0) and majority of III–V(0 0 1) and III–V(1 1 1) surfaces reconstruct in accordance with the ECR, there are a few low- and high-index surfaces which disobey the ECR but stabilize by sustaining significant elastic deformation in the surface region. We explain the latter scenario with the help of computational results for the geometric and electronic structure of GaSb(0 0 1)-( 1 × 3 ), GaSb(0 0 1)-c( 2 × 6 ), InP(1 1 1)A-( 3 × 3 ), and GaAs(1 1 1)B–Sb( 1 × 3 ). We also discuss hydrogen passivation of these surfaces. It is pointed out that the recently observed stable InP(1 1 1)A-( 3 × 3 ) surface can be both chemically and electronically passivated by exposing it to a hydrogen gas of one quarter of a monolayer coverage.

Cobalt Nanowire Fabrication on Activated Nitrogen-Templated Cu(210) Surface

We have investigated self-organized cobalt nanowires grown on activated nitrogen (AN)-templated Cu(210) high index surface. The morphology and magnetic properties of the grown Co nanowires were studied by scanning tunneling microscopy (STM) and magneto-optic Kerr effect (MOKE), respectively. The chemical state of the nanowires formed was characterized by in-situ X-ray photoelectron spectroscopy (XPS). It was found that Co grew preferentially along the narrow chains induced by the AN to form periodic Co nanowire arrays with diameter of 10 nm, which leads to a strong magnetic in-plane uniaxial anisotropy. Unlike the Co nanostructures formed on AN-modified low index Cu surfaces [Cu(100) and Cu(110)], these Co nanowires showed good thermal stability up to 470 °C, at which temperature the AN had desorbed from the surface.

Calculations on the adsorption of Au to MgO surfaces using SIESTA

The SIESTA suite of programs allows periodic density functional theory calculations to be carried out on system sizes of several hundred atoms. Here we make use of this to study the interface between Au and MgO surfaces. Atomic Au on MgO(100) shows preferential binding over surface anion sites, while the binding energy at a surface anion vacancy is significantly higher than on the stoichiometric surface. High index surfaces are used to generate kink site structures representative of extended defects and Au adsorption at these sites has a binding energy intermediate between MgO(100) and the anion vacancy. In contrast Au10 clusters are found to bind more strongly to kink sites than anion vacancies. Bader charge analysis demonstrates that electron transfer occurs from the surface to the Au species in all calculations and the distribution of this charge in the Au10 case is discussed.

Adsorption structures of benzene on a Si(5512)-2×1 surface: A combined scanning tunneling microscopy and theoretical study

In the first ever attempt to study the adsorption of organic molecules on high-index Si surfaces, we investigated the adsorption of benzene on Si(5 5 12)-(2x1) by using variable-low-temperature scanning tunneling microscopy and density-functional theory (DFT) calculations. Several distinct adsorption structures of the benzene molecule were found. In one structure, the benzene molecule binds to two adatoms between the dimers of D3 and D2 units in a tilted butterfly configuration. This structure is produced by the formation of di-sigma bonds with the substrate and of two C[Double Bond]C double bonds in the benzene molecule. In another structure, the molecule adsorbs on honeycomb chains with a low adsorption energy because of strain effects. Our DFT calculations predict that the adsorption energies of benzene are 1.03-1.20 eV on the adatoms and 0.22 eV on the honeycomb chains.