High-index Surfaces Research Articles

Microsize defects in InGaAs/GaAs [formula omitted]A/B multilayers quantum dot stacks

Surface morphology of microsize defects on the surface of various high-index GaAs substrates was investigated using an atomic force microscope (AFM). The surfaces investigated were the top layer of 1- and 17-period In 0.45 GaAs 0.55 / GaAs structures with quantum dots or buffer layer. These structures were characterized by the formation of oval defects on (1 0 0) surfaces, and microsize defects possessing the shape of multifaceted pits and hillocks on ( N 1 1 ) A / B ( N = 7 , 5 , 4 , 3 ) surfaces. The microsize defects were found to chaotically distribute on the surface and, as a rule, gathering in groups with some number of defects. Their density did not depend on the substrate orientation while the shape and orientation of the microsize defects were found to depend on the crystallographic orientation of the substrate. This dependence was determined to be the result of anisotropy of surface diffusion and surface elastic properties. The anisotropy of elastic properties of high-index surfaces was found to be the dominating factor in determining the microsize defect shape. We also report direct evidence of the fact that the effect of quantum dot lateral ordering observed on high-index ( N 1 1 ) B surfaces is determined by the anisotropy of surface elastic properties as well as elastic interaction between adjacent quantum dots.

半導体ナノ構造の構成要素としての表面構造 : 高指数表面の研究を通じて(最近の研究から)

半導体ナノ構造の構成要素としての表面構造 : 高指数表面の研究を通じて(最近の研究から)

InAs quantum dots onGaAs(2¯5¯11¯)B: STM and photoluminescence studies

The GaAss2511dB surface was prepared by molecular beam epitaxy and analyzed in situ by scanning tunneling microscopy sSTMd and low-energy electron diffraction. Atomically resolved STM images of GaAss2511dB revealed a 131 reconstruction, terminated by Ga dimers. The deposition of 1.5 ML of InAs onto GaAss2511dB resulted in the twoto three-dimensional transition with appearance of small InAs quantum dots sQD’sd with a very narrow size distribution and a high number density. Low-index s011d, s101d, and s111dB facets, a rounded vicinal s001d region for the main part, and a high-index s135dB surface for a flat base determine a shape of the QD’s that is totally unsymmetrical. Ex situ–performed photoluminescence measure-

Formation of antimony 1D-nanostructures on Si (5 5 12) surface

AbstractOf late, high index Si surfaces like, (5 5 12) are being explored for the formation of 1D nanostructures in the form of nanowires or chains. The surface topography of Si (5 5 12) presents an unique template for the growth of 1D nanostructures. In this paper we report the adsorption-desorption studies of Sb onto (2×1) reconstructed surface of Si (5 5 12). Motivated by our earlier studies of adsorption/desorption of Sb on low index surfaces (001) and (111), studying the interaction on high index surface like (5 5 12) is found to be interesting. The experiments have been performed in UHV with in-situ growth and probed by using AES, LEED and EELS techniques. The uptake curve shows initially a simultaneous multilayer growth of Sb up to 4 ML and then at 20 ML the entire Si surface is covered by Sb. The LEED patterns show a disorder during growth at RT above 1 ML. EELS studies at 250 eV, primary beam energy show that as the adsorption proceeds the features of bulk Sb start appearing above 4ML coverage. However, the bulk Sb features are not complete until a high enough coverage of Sb (˜20ML) which suggests the existence of the primary rows up to a higher coverage. Annealing studies have also been performed on the RT adsorbed system, to study the residual thermal desorption characteristics. Annealing the system to about 7900C results in Si (337) facets at a low coverage of 0.2 ML. The anisotropy in the LEED spots suggests an ordered Sb adsorption along (1 10) and a local disorder in the (665 ) direction. Thus we provide evidence for the formation of a zig-zag 1D-nanowire of Sb grown on the (5 5 12). The new phase is also accompanied by forming a dangling bond state at 3 eV in EELS.

Reconstruction of silicon surfaces: A stochastic optimization problem

Over the last two decades, scanning tunnelling microscopy (STM) has become one of the most important ways to investigate the structure of crystal surfaces. STM has helped achieve remarkable successes in surface science such as finding the atomic structure of Si(111) and Si(001). For high-index Si surfaces the information about the local density of states obtained by scanning does not translate directly into knowledge about the positions of atoms at the surface. A commonly accepted strategy for identifying the atomic structure is to propose several possible models and analyze their corresponding {\em simulated} STM images for a match with the experimental ones. However, the number of good candidates for the lowest-energy structure is very large for high-index surfaces, and heuristic approaches are not likely to cover all the relevant structural models. In this article, we take the view that finding the atomic structure of a surface is a problem of stochastic optimization, and we address it as such. We design a general technique for predicting the reconstruction of silicon surfaces with arbitrary orientation, which is based on parallel-tempering Monte Carlo simulations combined with an exponential cooling. The advantages of the method are illustrated using the Si(105) surface as example, with two main results: (a) the correct single-step rebonded structure [e.g., Fujikawa {\em et al.}, Phys. Rev. Lett. 88, 176101 (2002)] is obtained even when starting from the paired-dimer model [Mo {\em et al.}, Phys. Rev. Lett. 65, 1020 (1990)] that was assumed to be correct for many years, and (b) we have found several double-step reconstructions that have lower surface energies than any previously proposed double-step models.

Scanning tunneling microscopy studies of oxide growth and etching on Si(5 5 12)

We have used scanning tunneling microscopy to study how the Si(5 5 12) surface morphology evolves when exposed to oxygen at elevated temperatures, in particular when both oxide nucleation and etching occur simultaneously. This study includes results for sample temperatures of 650 to 750 °C at O2 pressures of ∼5×10−7 Torr and exposures of 50 to 600 L. It is already known that the Si(001) surface is significantly disrupted in this transition regime by etch pits and islands caused by etching around oxide-induced pinning sites. For the high-index Si(5 5 12) surface, no etch pits are found on the terraces, but pyramidal or linear islands are observed on terraces and along step edges. Both types of islands incorporate (113) facets, indicating an enhanced stability of this orientation against etching. The absence of etch pits and the presence of well-defined islands qualitatively distinguish the etching behavior of this high-index surface from its low-index counterparts.

Enhanced multilayer relaxation at high-index stepped Cu surfaces

Multilayer relaxation at high-index $\mathrm{Cu}(\mathrm{hkl})$ $(hkl=511,$ 320, and 410) stepped surfaces were determined by the first-principles all-electron full-potential linearized augmented plane-wave method within the framework of the local-density approximation and the generalized gradient approximation. The calculated relaxation of the interlayer distances, obtained by a geometry optimization procedure that minimizes the force on each atom, were compared with low-energy electron-diffraction (LEED) analysis of experimental data. In the case of Cu(511), the calculated results are in good agreement with the LEED analyses. On the other hand, for Cu(320) and Cu(410), there are large differences that may be understood from the fact that the LEED analyses of experiments consider up to only three or four layers from the surface, and that whereas even the fifth or sixth layers show large relaxation in our calculations, our results suggest a reanalysis of the LEED data with the inclusion of more layers.

Controlling (In, Ga)As quantum structures on high index GaAs surfaces

AbstractWe investigate the formation of (In, Ga)As self assembled quantum structures grown on different orientations of a GaAs substrate along one side of the stereographic triangle between (100) and (111)A surfaces. The samples were grown by Molecular Beam Epitaxy, monitored by Reflection High-Energy Electron Diffraction during the growth and characterized by in-situ Scanning Tunneling Microscopy and Atomic Force Microscopy. A systematic transition from zero dimensional (In, Ga)As quantum dots to one dimensional quantum wires was observed as the substrate was varied along the side of the triangle within 25° miscut from the (100) toward (111)A, which includes several high index surfaces. We propose an explanation for the role of the substrate in determining the type of the nanostructure that is formed.

Controlling (In,Ga)As quantum structures on high index GaAs surfaces

ABSTRACTWe investigate the formation of (In, Ga) As self assembled quantum structures grown on different orientations of a GaAs substrate along one side of the stereographic triangle between (100) and (111)A surfaces. The samples were grown by Molecular Beam Epitaxy, monitored by Reflection High-Energy Electron Diffraction during the growth and characterized by in-situ Scanning Tunneling Microscopy and Atomic Force Microscopy. A systematic transition from zero dimensional (In, Ga) As quantum dots to one dimensional quantum wires was observed as the substrate was varied along the side of the triangle within 25° miscut from the (100) toward (111)A, which includes several high index surfaces. We propose an explanation for the role of the substrate in determining the type of the nanostructure that is formed.

Controlling (In, Ga)As quantum structures on high index GaAs surfaces

ABSTRACTWe investigate the formation of (In, Ga)As self assembled quantum structures grown on different orientations of a GaAs substrate along one side of the stereographic triangle between (100) and (111)A surfaces. The samples were grown by Molecular Beam Epitaxy, monitored by Reflection High-Energy Electron Diffraction during the growth and characterized by in-situ Scanning Tunneling Microscopy and Atomic Force Microscopy. A systematic transition from zero dimensional (In, Ga)As quantum dots to one dimensional quantum wires was observed as the substrate was varied along the side of the triangle within 25° miscut from the (100) toward (111)A, which includes several high index surfaces. We propose an explanation for the role of the substrate in determining the type of the nanostructure that is formed.

The calculation of the surface energy of high-index surfaces in metals at zero temperature

We used the molecular dynamics simulation with interatomic potentials of the embedded atom method to calculate the high-index surface energies of the surfaces containing the 〈0 0 1〉 axis or 〈−1 1 0〉 axis in f.c.c. metal Al, Cu and Ni at zero temperature. We generalized an empirical formula based on structural unit model for high-index surfaces and present some new formulas that can be used to estimate the surface energy and structural feature of high-index surfaces very well. The results show that the closest surfaces have the lowest surface energy and the surface energies of the closest (1 1 1) surface and the next closest (1 1 0), (1 0 0) surfaces are the extremum on the curve of surface energy versus orientation angle. We also calculated the b.c.c. metal Fe and obtained a similar result. The difference is that in the b.c.c. metal the surface energies of the closest (1 1 0) surface and the next closest (1 0 0), (1 1 2) surfaces are the extremum on the curve of surface energy versus orientation angle. The results of theoretical simulation and the empirical formula consist well with the experiment data.

Analysis of a nonorthogonal pattern of misfit dislocation arrays in SiGe epitaxy on high-index Si substrates

We have investigated the formation of misfit dislocations resulting from the growth of partially strained Si0.7Ge0.3 epitaxial films on Si substrates with surface normals rotated off of the [001] axis toward [110] by 0°, 13°, and 25°. Transmission electron microscopy has shown that the dislocations form in a modified cross-hatch pattern for samples grown on the off-axis substrates. This modified cross hatch consists of three arrays along which the dislocations align. This is in contrast to the two orthogonal arrays found on the on-axis (001) substrates. These dislocations correspond well with the intersection of the (111) slip planes with the respective surfaces. We present a simple analysis of the amount of relaxation due to probable Burger’s vectors for these dislocations, which reveals the most likely directions for these vectors and shows that the density of dislocations only accounts for a fraction of the total film relaxation as measured by Raman peak shifts. These studies form the basis for the use of high index surfaces as components in modern devices, and provide pathways to possible templates for use in the growth of nanostructures.

Multilayer relaxation of Cu(210) studied by layer-doubling LEED analysis and pseudopotential density functional theory calculations

Multilayer relaxation of Cu(210) surface has been studied by layer-doubling low energy electron diffraction (LEED) analysis and pseudopotential density functional theory (DFT) calculations. An excellent agreement between the calculated and measured $I\ensuremath{-}V$ curves has been achieved as judged by direct inspection and a small Pendry R factor of 0.12. We suggest that the layer-doubling method is a suitable choice for quantitative LEED structural studies on high-index metal surfaces with interlayer spacings down to 0.8 \AA{}. Our pseudopotential DFT calculations have reproduced the relaxation sequence determined by the layer-doubling LEED analysis, i.e., $\ensuremath{-}$ $\ensuremath{-}$ $+$ $\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot},$ with the largest quantitative discrepancy of about 0.04 \AA{}. Comparison is made with LEED and DFT studies on other high-index Cu surfaces. Based on this comparison, a general trend for multilayer relaxations of open metal surfaces is proposed.

Surface electromigration of In-covered Si high-index surfaces

To clarify the role of facet in electromigration on the vicinal Si(001), we investigated in this study surface mass transport of In on Si high-index (103) and vicinal (001) surfaces, using scanning Auger microscope, low energy electron diffraction (LEED) and atomic force microscope (AFM). In parallel with (103), we also investigated similarly the most stable high-index (113) of Si for reference. Specimens with a large area of these surfaces were prepared from Si(001) wafer by cutting, grinding and polishing. No surface electromigration of In on Si(103) surface was observed at any temperature, while In overlayer exhibited a typical Stranski–Krastanov mode of growth. This fact strongly suggests that In adatoms move around over the intermediate layer, but there is no driving force of electromigration on them. The electromigration on the vicinal Si(001) depended upon the off-angle. It was most enhanced with the off-angle of around 4°. Based upon these results a model is proposed to explain the puzzling electromigration of In on vicinal Si(001). We found out that In on Si(113) exhibited a normal surface electromigration towards cathode.

Ga-induced nano-facet formation on Si(1 1 n) surfaces

Surface morphology of three kinds of Si(11n) surfaces during Ga deposition has been studied by scanning tunneling microscopy to investigate nano-facet formation processes on high-index silicon surfaces. It has been observed that Si(113) surfaces with vicinal angles of ±2° changed into nano-facet structures of (112) and (115) at 1ML Ga deposition as well as a flat Si(113). In contrast, no faceting occurred on Si(114). In all cases of nano-facet formation, (112) facet formed prior to (115) facet. Thus, it is considered that (112) facet formation is a driving force of nano-facet formation process. It is also considered that large angle difference between (112) and (114) prevents the facet formation on Si(114). Difference of vicinal direction appears in distribution of facet width. The facet width distribution strongly depends on the initial morphology on the surface tilted toward [332̄] direction, while on the surface tilted toward [3̄3̄2], initial morphology does not affect the final facet distribution.

First-Principles Calculation of the Work Functions for the High-Index Copper Surfaces

We calculated the work functions (WFs) for the high-index copper surfaces, (112), (321), and (413), as well as the low-index surfaces, (111), (100), and (110), using the full-potential linearized augmented plane wave (FLAPW) method within both the local density approximation (LDA) and the generalized gradient approximation (GGA). It is found that the GGA lowers rigidly the WFs about 0.46–0.49 eV from the values obtained by the LDA, for all the surfaces, and the experimental values for low-index surfaces are in between the calculated values by the LDA and GGA. It is also found that the WF does not decrease further, for the surfaces with higher indices than (112). This result is different from that of the recent measurements by the Kelvin probe technique [Appl. Phys. Lett.79, 4337 (2001)], which shows a linear dependence of WF on the atomic packing density. The result of our calculation is consistent with the WF anisotropy model, which was recently applied to tungsten surfaces.

Strain-driven facet formation on self-assembled InAs islands on GaAs (311)A

The shape of InAs three-dimensional islands grown on GaAs(311)A substrates by molecular-beam epitaxy was investigated by in situ scanning tunneling microscopy. The island is found to be laterally surrounded by (111)A and {110} facets together with a convex curved region close to the (100) facet. The top ridge of the islands is atomically resolved to be the most recently discovered high-index surface {11,5,2}. This observation points to the importance of the study of nanostructure growth on high-index surfaces and their characterization.

Investigation of stable high-index silicon surfacesby means of LEED pattern analysis

In the course of a systematic search for stable silicon surfaces, we have studied four stable high-index silicon surfaces, i.e., Si(1,1,11), (1 0 8), (2 1 2), and (15,1,17), by analyzing their low-energy electron diffraction (LEED) patterns. The results show not only that all these surfaces are stable but also that Si(15,1,17) is a major stable surface,whereas the rest three are minor stable surfaces, meaning that their unit cells consist of a nanometer facet of a major stable surface along with a double height step. In view of the periodic facet-step morphology of these minor stable surfaces, we suggest that these surfaces have the potential to be used as templates for growth of periodic nanowires of some suitable materials.

Ge nanostructures grown by self-assembly; influence of substrate orientation

The present paper discusses structural and optical properties of self-assembledrandom and non-random Ge dots on finite-area mesa substrates with facets withhigh-index orientation under conditions of strong surface diffusion. The islandformation on the (001) top surface of the mesas was found to be different if othercrystallographic planes are within the diffusion length. The growth on mesasis dominated initially by preferential nucleation of ordered islands on{hkl}facets and later by random nucleation of islands of more uniform sizes on the(001) top surface. The shape of ordered islands is influenced by the anisotropicproperties of high-index surfaces. A transition of ordered islands fromtrapezoid-based to elongated domes with high aspect ratio was identified, with amono-modal distribution in each stage of growth. We discuss these features interms of strong island–island interaction and fast surface diffusion. Therelevant change of the optical properties of ordered islands with respectto the random (001) case is the narrowing of photoluminescence peaks.

The phonon spectra and vibrational thermodynamical properties of Cu vicinal surfaces

We present a many-body empirical potential giving the total energy in Cu crystals, the transferability of which is proven by comparing with experiments the phonon band structure for the bulk and the three low index surfaces. After the determination of the multilayer relaxation of a set of vicinal surfaces, we investigate their vibrational spectra by means of their projected phonon band structure and the local spectral densities of states corresponding to a given value of k // which are discussed in relation with recent experiments. Finally, the root mean square displacement of specific atoms for low and high index surfaces are compared with the bulk one and the contribution of the vibrations to the free energy of steps is calculated as a function of temperature and of the terrace width for the p(1 0 0)×(1 1 1) and p(1 1 1)×(1 0 0) surfaces (2⩽ p⩽7). It is found that for these vicinal surfaces the step–step interactions mediated by phonons are attractive and may have the same order of magnitude as the other interactions.