Session R40: Focus Session: Growth and Pattern Formation on Surfaces

Lateral Charge Transport in Atomically Clean, Ultra-thin Crystalline-Silicon Sheets

In very thin, atomically clean crystalline-Si sheets (``nanomembranes''), the electrical conductance is controlled by the surface. Conductance can occur either through surface transfer doping or directly in the clean-surface electronic bands. The thinner the sheet, the larger should be the contribution of the surface. We have earlier reported [1] conductance measurements on nanomembranes as thin as 77nm, and have shown that not only is the surface antibonding ($\pi^*$) band used to enhance ``bulk'' conduction in the membrane [2], but also the charges are additionally mobile in this band, providing a significant contribution to the overall conductance. We extend these measurements to thinner nanomembranes, between 64nm and 35nm thick, using a back-gated van der Pauw technique in ultra-high vacuum. The sheet conductance is measured after a high-temperature flash to obtain a high-quality Si(2$\times$1) reconstructed surface, and with H adsorbed on the surface. The maximum sheet resistance for a 64nm sample with H deposited in situ is higher than 24 G$\Omega$.\\[4pt] [1] W. Peng, et al., Nature Commun., under review.\\[0pt] [2] Zhang, P. P. et al., Nature 439, 703-706 (2006).   

Electronic properties of epitaxial silicene: a LT-STM/STS study

The astonishing properties of silicene, the Si-counterpart of graphene, together with pioneering experimental observations, triggered in the very recent years, an exponentially increasing interest for this atom-thick material, both at fundamental level and for applications in high-speed electronic devices. We demonstrated, that the spontaneous segregation of silicon on (0001) surface of zirconium diboride (ZrB$_2)$ thin films epitaxied on Si(111) wafers gives rise to a wide-scale uniform two-dimensional silicene sheet [1]. The silicene nature of the honeycomb structure imaged by scanning tunneling microscopy is evidenced by the observation of gap-opened $\pi $-electronic bands. The band gap opening is primarily due the specifically imprinted buckling. Here, we present the results of a low-temperature scanning tunneling spectroscopy investigation, which evidences the n-doped nature of silicene. The mapping of the local density of states, together with density functional theory give precious insights into the microscopic origin of the electronic bands of silicene. In particular, it shows the correlation between the degree of \textit{sp}$^{2}$ hybridization of different Si atoms in the internal structure and the character of the electronic bands. \\[4pt] [1] A. Fleurence \textit{et al.}, Phys. Rev. Lett. 108, 245501 (2012).   

A planar-like silicene on ZrB$_{\mathrm{2}}$(0001) surface revealed by a first-principles study

Given that a free standing planar silicene is unstable [1], it is of great interest to understand the mechanism of stability of any existing planar-like structure that would optimize the understanding of the intrinsic difference from its counterpart, graphene. Recently, silicene was epitaxially grown on the ZrB$_{\mathrm{2}}$(0001) surface and was demonstrated to have ($\sqrt 3 \times \sqrt 3 )$-reconstruction due to irregular buckling [2]. While the deviation from the regularly buckled structure is clearly made by experiment, two possible structures revealed by a first-principles calculation are still in the candidate list, neither one is completely ruled out from the possible groundstate structure. The energetically more favorable one possesses a planar-like structure, with all Si atoms residing in a plane except the one on top of a Zr atom becomes higher. However, this structure is less preferable from available experimental data. By studying the binding energy and electronic band structures of these two structures with and without the substrate, we will explain why such a planar-like structure can gain more energy than the regularly buckled-like phases via the interaction of the ZrB$_{\mathrm{2}}$(0001) surface and why the ground state advocated by density functional theory could become less preferable by experiment. [1] S. Cahangirov \textit{et al}., Phys. Rev. Lett. \textbf{102}, 236804 (2009). [2] A. Fleurence \textit{et al}., Phys. Rev. Lett. \textbf{108}, 245501 (2012).   

Growth and electronic properties of monolayer and multilayer silicene

Silicene, in which Si atoms replace C atoms in a two-dimensional honeycomb lattice in analogue with graphene, has been experimentally realized recently. In this work we report a systematic study of superstructures formed by sub- monolayer and multiple layer silicon grown on Ag(111), by scanning tunneling microscopy (STM) and spectroscopy (STS). We found that, depending on the substrate temperature and silicon coverage, several monolayer superstructures can form on Ag(111). At proper temperature and Si coverage, monolayer and multilayer silicene films were grown [1]. STS at 4K revealed quasiparticle interference (QPI) patterns suggesting intervalley and intravalley scattering of charge carriers, and a linear energy-momentum dispersion relation and a large Fermi velocity were derived [2]. These results unambiguously prove the existence of Dirac fermions in silicene, and provide a solid basis for further studies on the electronic property and device applications of silicene. [1] Nano Letters 12, 3507 (2012), [2]Physical Review Letters 109, 056804 (2012).   

Quasiparticle Interference in Silicene on Ag(111) Surface

Silicene, a single sheet of silicon atoms arranged in a honeycomb lattice analogous to graphene, has been successfully prepared on Ag(111) surface recently. The honeycomb atomic structure of silicene has been confirmed experimentally. However, more important details of the electronic structures of silicene, such as pseudospin or chirality of Dirac Fermions and the shapes of the Dirac cones, still remain illusive. Here we performed scanning tunneling microscopy and spectroscopy to investigate the electronic states of silicene on Ag(111) surface. From the quasiparticle interference (QPI) pattern observed in dI/dV maps, we derived linear energy-momentum dispersion and a large Fermi velocity, which prove the existence of Dirac fermion in silicene. Moreover, through mapping the QPI pattern in q space, we found the Dirac cones of silicene are not circular as in graphene, but significantly warped to hexagon. The theoretical calculations prove that the constant energy contours of Dirac cones of silicene are hexagonal warped due to the unique structure of silicene. Our results pave the way for exploiting anisotropic transport behavior and other exotic quantum effects in silicene.   

Nanostressor growth on Silicon Nanomembranes

Single-crystal semiconductor nanomembranes (NMs) have great potential for microelectronic materials heterointegration. In particular, they allow for the fabrication of custom-strained, dislocation-free growth interfaces. However, thin substrates are extremely compliant and it is, therefore, crucial to understand the added effects of residual processing strain and substrate bonding. We study the strain distributions on silicon NMs transferred to patterned Si substrates such that some NM regions are bonded and others freestanding. As the critical thickness for Stranski-Krastanow growth of quantum dots (QDs) is very strain dependent, we decorate the surface with Ge quantum dots (QDs) and use the resulting distribution as an easily visible indicator of strain. We see dramatic differences between QD distributions on the bound and freestanding regions, and also between the bound regions and the bulk Si substrate, suggesting that the buried interface may influence nanostressor growth.   

Investigation of Mn-Co surface alloy on Si(100)-2x1

Understanding of magnetic doping of Group-IV semiconductors is critical for the realization of spintronics devices. We present STM investigations of room temperature, sequential and co-deposition of Mn and Co on Si(100)-2x1. Monoatomic Mn-nanowires, which self-assemble on the Si surface, lose their continuity after deposition of 0.04-0.08 ML of Co. This loss in continuity is expressed in the wire length distributions, which are dominated by Mn dimers and ultrashort wires. Protrusions with a height of 0.5-0.8 {\AA} above the surface of the Mn wire are observed, which is evidence for adsorption of Co on wires. The Si defect structure is similar to exclusive Co deposition experiments on Si, and in agreement with the model of subsurface diffusion of Co atoms present in literature. Only 25{\%} of the deposited Co is observed on the surface and the rest are attached to the Mn structures. Wires form even during co-deposition of Mn and Co, indicating stronger Mn-Si and Mn-Mn interaction over Mn-Co interaction. The wire length distribution is dominated by ultrashort wires, similar to sequential deposition. A detailed discussion of the role of Co in breaking up the Mn wires will be presented. SQUID measurements are being performed to study the magnetic properties of Co-Mn-Si structures and will be discussed. DFT calculations for co-deposition of Mn and Co are presented and compared with experimental data.   

A Level-Set Approach to Simulate Mound Formation during Epitaxial Growth

We have developed an island dynamics model that uses the level-set approach to model epitaxial growth. In recent work we have implemented a numerical scheme to solve the diffusion equation for the adatom concentration with a (mixed) Robin boundary condition. Such a boundary condition properly describes multilayer growth when there is an additional step-edge barrier for atoms to diffuse over a step edge. We will discuss how variations of the boundary condition that correspond to variations of the step edge barrier affect the formation of mounds. We will furthermore discuss how the effect of downward funneling can be implemented within our approach and how it affects the slop of the mounds.   

How nucleation can cause stacking faults on the GaAs (111) B surface: A DFT study

GaAs grows along the [111] direction much faster than other crystal axes at high growth temperature. In this work, we leverage this anisotropy, using catalyst-free selective-area epitaxy, to grow high-aspect ratio nanopillars. However, we find that the resulting crystal structure exhibits a high density of stacking faults that can have detrimental effects on the electronic and optical properties of the material. Each stacking fault is equivalent to a monolayer of wurtzite embedded in an otherwise zinc-blende lattice. The origins of stacking faults are currently under debate, with both thermodynamic equilibrium arguments and nucleation arguments proposed to explain the segments of wurtzite that appear in the primarily zinc-blende crystal. Here we present a density-functional-theory study of nucleation and island growth on the (111)B surface of GaAs that demonstrates how the smallest stable nucleus can transition and stabilize in either a wurtzite or a zinc-blende orientation.   

First principles study of Bismuth growth on Nickel (111) surface

A recent experiment (Bollman, et al. 2011) suggests that Bismuth forms flat hexagonally close packed (hcp) films on the Nickel (111) surface, of heights 1, 3, 5 and 7 layers. A quantum size effect (QSE) based on free electrons was proposed in explanation. To test this idea, we calculated the total energy and QSE of Bismuth on Nickel (111) surface using density functional theory. We find the hcp films are destabilized by adding capping atoms which lead to puckering of the hcp layers and covalently bonded structures. Furthermore, we find the rhombohedral films based on the bulk Bi structure are energetically more favorable than the proposed hcp films. These structures also favor odd numbers of layers (a flat wetting layer followed by bulk-like rhombohedral bilayers), but owing to covalent chemical bonding rather than QSE.   

Absolute surface energies of polar and non-polar planes in GaN

Growth of high quality single crystals and epitaxial layers of GaN is very important for producing optoelectronic devices. First principles calculations can help in determining absolute surface energies, which are key quantities that control crystal-growth rates and fracture toughnesses. By means of hybrid functional calculations, we have determined absolute surface energies for the non-polar \{11-20\} and \{10-10\} and polar (0001) and (000-1) planes in wurtzite GaN. Low energy reconstructions of the bare and hydrogenated surfaces were considered under various conditions chosen to correspond to growth by molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). We find that the non-polar planes are close in energy, and lower in energy than the reconstructed (000-1) polar plane under all conditions considered. The reconstructions of the (0001) plane are lower in energy than the (000-1) plane over the whole range of conditions, and lower in energy than the non-polar reconstructions for high-pressure conditions. From these surface energies, lower bounds on the anisotropic fracture toughness of GaN are determined. Surface energies of polar planes for other III-nitrides will be compared to those of GaN.   

DFT studies of the early stages of growth of Nb on MgO(100)

Using DFT calculations of binding and adsorption energies for various sizes and shapes of Nb clusters on MgO(100) surfaces, we have examined the effects of cluster shape and a neutral O vacancy on the energies and stability of Nb[100] and [110] island structures. Similarly to other cases of metal adsorbates on MgO(100) surfaces, O-vacancy sites tend to act as nucleation sites for Nb adatoms, while the effect of a nearby O vacancy on the binding energy of a Nb cluster is much weaker. In particular, we find that the binding energy for a Nb monomer at an O site (O-vacancy site) is 1.52 eV (2.2 eV) while the energy barrier for Nb monomer diffusion is 0.58 eV. In addition, although both isolated 4-atom Nb [100] and 5-atom Nb [110] islands are isotropic with a slightly higher binding energy for the [100] island, for larger clusters an anisotropic Nb [110] structure is more stable than a square Nb [100] structure, which is in good agreement with a recent experimental result [1]. \\[4pt] [1] M. Krishnan et al., Phys. Rev. ST. Accel. Beams 15, 032001 (2012).   

Self Directed Growth of Nanopillar Arrays in Molten Polymer Films: Theory versus Experiment

It has been known for over a decade that molten polymer films exposed to a large uniform thermal gradient can develop spontaneous arrays of nanopillars. Theoretical predictions based on linear stability theory in the long wavelength approximation suggest that such formations arise from fluctuations either in the electrostatic attraction between the fluid and opposing substrate, acoustic phonon radiation pressure within the film, or thermocapillary forces along the air/polymer interface. Experimental confirmation of the mechanism responsible for such emergent structures requires measurements of the pattern formation process at very early times, a difficult task given that initial thickness fluctuations are of the order of a few nanometers. Here we report results of in-situ microscopy measurements of the dominant wavelength as a function of the applied thermal gradient and initial film thickness. Comparison to all three models indicates closest agreement with the thermocapillary mechanism. However, there remain discrepancies between theory and experiment with regard to the dominant wavelength observed and its corresponding growth rate. We discuss possible origins for the discrepancies, including non-stationary effects and simplified assumptions of the thermocapillary model.   

Guided Growth of Millimeter-Long Horizontal Nanowires with Controlled Orientations

The large-scale assembly of nanowires (NWs) on surfaces remains one critical challenge toward their integration into practical devices. We report the vapor-liquid-solid growth of perfectly aligned, millimeter-long horizontal GaN [1] and ZnO$^{\mathrm{\thinspace }}$[2] NWs with controlled orientations on different sapphire planes. The growth directions, crystallographic orientation and faceting of the NWs vary with each surface orientation, as determined by their epitaxial relation with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these NWs display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown NWs. Further control was recently achieved by patterning the catalyst nanoparticles to produce NWs with well-defined locations, orientation and length. This enables the massively parallel integration of NW circuits. The guided growth approach paves the way to highly controlled NW structures with potential applications not available by other means. [1] D. Tsivion, M. Schvartzman, R. Popovitz-Biro,P. von Huth and E. Joselevich, Science \textbf{333}, 1003 (2011). [2] D. Tsivion, M. Schvartzman, R. Popovitz-Biro and E. Joselevich, ACS Nano \textbf{6}, 6433 (2012).   

Low-energy Alkali Ion Scattering and X-ray Photoelectron Diffraction Studies of the Structure of Pt-Zn/Pt(111) Bimetallic Surfaces

Pt-Zn alloys have applications in heterogeneous catalysis, and studies on surfaces of well-defined, ordered Pt-Zn alloys, or intermetallic compounds, clarify the origins of changes that occur in catalysis by the alloy. Many stable intermetallic compounds of Pt and Zn occur in bulk materials, but no long-range ordered surface alloys were formed by depositing Zn on a Pt(111) single-crystal substrate in a search over a considerable range of conditions. These results can be contrasted to those from Pt-Sn, where ordered surface alloys were formed. Zn alloys with Pt upon heating, and XPD and ALISS were used to characterize the Pt-Zn alloy created by annealing one monolayer of Zn on Pt(111) to 650 K. This Pt-Zn/Pt(111) surface alloy had a diffuse (1x1) LEED pattern due to formation of a random, substitutional alloy between Pt and Zn with 0.05-monolayer Zn in the topmost layer. Zn atoms are substitutionally incorporated into Pt lattice positions and alloyed Zn atoms in the surface layer are located coplanar with the surface Pt atoms, without any buckling. TPD shows that both CO and NO chemisorb more weakly on the Pt-Zn alloy than on the clean Pt(111) surface, with NO more strongly affected.   

Positron states and annihilation characteristics of surface-trapped positrons at the oxidized Cu(110) surface

The process by which oxide layers are formed on metal surfaces is still not well understood. In this work we present the results of theoretical studies of positron states and annihilation characteristics of surface-trapped positrons at the oxidized Cu(110) surface. An ab-initio investigation of stability and associated electronic properties of different adsorption phases of oxygen on Cu(110) has been performed on the basis of density functional theory and using DMOl3 code. The changes in the positron work function and the surface dipole moment when oxygen atoms occupy on-surface and sub-surface sites have been attributed to charge redistribution within the first two layers, buckling effects within each layer and interlayer expansion. The computed positron binding energy, positron surface state wave function, and annihilation probabilities of surface trapped positrons with relevant core electrons demonstrate their sensitivity to oxygen coverage, elemental content, atomic structure of the topmost layers of surfaces, and charge transfer effects. Theoretical results are compared with experimental data obtained from studies of oxidized transition metal surfaces using positron annihilation induced Auger electron spectroscopy.