High-index Si Research Articles

Formation of Sb submonolayer phases on high index Si(5 5 12) surface

This work is the first report on the submonolayer adsorption of antimony (Sb) on the high index Si(5 5 12) surface, studied in UHV by in situ surface sensitive probes such as Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), and low energy electron diffraction (LEED). The Auger growth curve which is a plot of the Sb(MNN)/Si(LVV) Auger peak intensity ratio with deposition time, provides a calibration of a deposition rate of 0.06 ML/min. A coverage of 0.2 ML is obtained by four different routes (a) adsorption at room temperature (RT), (b) adsorption at high substrate temperature (HT) of 680 °C, (c) annealing the RT adsorbed surface to 800 °C and (d) annealing the high substrate temperature (680 °C) adsorbed surface to 800 °C. The (2 2 5) facets are observed for the RT adsorbed system at coverage of 0.2 ML, while the pathway adopted to attain the 0.2 ML coverage by adsorption at elevated temperature shows two different atomic arrangements in the formation of the (3 3 7) facets. The HT adsorbed surface suggests the formation of nano-wire-like features while the 0.2 ML coverage obtained by annealing the HT as well as RT adsorbed surface suggests an anisotropic Sb atomic arrangement.

Influence of Si(001) substrate misorientation on morphological and optical properties of Ge quantum dots

We have investigated the correlation between morphological and optical properties of Ge dots deposited by molecular-beam epitaxy on a Si(001) surface and on a high-index Si(118) vicinal surface. Ge islands were confined on the top of an undulated Si0.5Ge0.5 template layer according to the Stranski-Krastanov growth mode. Atomic force microscopy measurements reveal that the main effect of the vicinal substrate is to transform hut islands on a nominal (001) substrate into wire-shaped islands on (118) substrates. We have observed a direct correlation between the elongated shape and polarization anisotropy of optical transitions in island. The island photoluminescence (PL) emission is partially (∼25%) polarized for dots deposited on a (118) substrate. PL spectroscopy investigations as a function of temperature and excitation power are reported. The results show that the PL of islands strongly depends on the pump excitation power: it broadens and is blueshifted by 28 and 14meV∕decade for structures grown on (001) and (118), respectively, as the excitation power density increases. The significant blueshift is interpreted in terms of band bending in type II recombination. Moreover, a detailed analysis of (118) island PL band shows the presence of two main peaks that could be attributed to the different island morphologies clearly identified by transmission electron and atomic force microscopies: wire islands and dome islands.

Kinetically controlled superstructural phases at the Sb/Si (5 5 12) interface

ABSTRACTThe adsorption of Sb on the high index Si (5 5 12) has been studied at higher substrate temperature (HT) (800°C), using in situ surface characterization techniques like Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED), and Electron Energy Loss Spectroscopy (EELS). The surface morphology of this high index Si (5 5 12) surface has row like trenches along (110) direction. We have performed the study of Sb adsorption and desorption on the Si (5 5 12) substrate held at different temperatures. The different pathways adopted during adsorption and desorption have suggested the dominant role of kinetics I forming various surface phases on the Si (5 5 12) substrate. The adsorption at room temperature resulted in the formation of (225) surface phase, while the adsorption at 680°C resulted in the (337) phase. The sequential thermal desorption of the room temperature and high temperature adsorbed surface resulted in the formation of (337) phases at 800°C, with anisotropic growth along one direction. While the adsorption at 800°C resulted in the formation of anisotropic (337) like phases, the further increase in the coverage formed a 2x Si (225) phase. Annealing this 2x Si (225) phase again resulted in the formation of anisotropic (337) phase. Thus formation of interface by controlling the growth kinetics can result in the formation of various tailored structures with desired properties.

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.

Ab initio pseudopotential calculations for the electronic and geometric structures of hydrogen covered Si(1 1 4)-(2×1)

Using a first principles pseudopotential method we have studied the adsorption of H on the high index Si(1 1 4)-(2×1) surface within the local density approximation. This stable surface is found to be both chemically and electronically passivated by two different coverages of hydrogen: 1.0 ML and 1.5 ML. For the 1.0 ML coverage all the A- and B-type dimer bond lengths have equalised to 2.40 Å and the rebonded lengths are slightly longer at 2.48 Å. Hydrogen passivation, for both coverages, leads the surface bands of the clean Si(1 1 4)-(2×1) surface to move into the bulk valence and conduction bands. Differences in the density of states for the clean, 1.0 ML and 1.5 ML coverages were observed and are discussed.

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.

Ab initio pseudopotential calculations for the geometry and electronic structure of Si(1 1 4)-c(2 × 2)

Ab initio pseudopotential calculations are performed to investigate the geometry, electronic structure and orbital characteristics of the high index Si(1 1 4)-c(2 × 2) surface. It is found that there are several states in and around the band gap of bulk silicon and that the surface is characterised by a small band gap of approximately 0.1 eV within the local density approximation.

Au-induced faceting of the Si(5512) surface

We have investigated Au-induced faceting of the high-index Si(5 5 12) surface for coverages up to ∼1 ML and temperatures between 600 and 800 °C using scanning tunneling microscopy. The surface predominantly forms sawtooth facets of the nearby (1 1 3) orientation, with an opposing facet plane that is determined by the Au coverage. These facet planes, which include (7 7 15), (3 3 7), (5 5 11), or (2 2 5), form one-domain, row-like surface structures with inter-row spacings that range from 2.2 to 3.5 nm. In contrast, the (1 1 3) plane forms either a clean 3 × 2 reconstruction with no long-range order or a disordered surface. Limited faceting to the low-index (1 1 1) plane is also observed at coverages above ∼0.2 ML.

New structural model of the high-index Si [formula omitted]2 × 1 surface

Combining first-principles total-energy calculation and scanning tunnelling microscopy (STM), we obtain the atomic structure of the high-index Si( 5 5 12 )2 × 1 surface, which is a promising template for growing one-dimensional metal nanowires. The equilibrium structure contains honeycomb chains, dimers, adatoms, and tetramers, which are building blocks of other reconstructed Si surfaces. The structure can account for STM images as well as other available experimental 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.

Hybrid deposition of sputtered and evaporated multilayer thin films

Si:H/CaF 2 and Si:H/HfF4 high reflectors with as many as 27 layers were deposited by a hybrid process that combined reactive magnetron sputtering and electron beam evaporation. The ultimate goal of this work was to deposit interference filters with low near-infrared (NIR) and LWIR absorption for use in thermophotovoltaic systems. The high index Si:H layers were deposited by reactive magnetron sputtering in mixtures of Ar+H2. Electron beam evaporation was the preferred deposition method for the CaF2 layers. The multilayer hybrid coatings were deposited in the same chamber by sequentially rotating the substrate over the Si sputtering target and the CaF2 evaporation source. The deposition rate of the evaporated CaF2 was 18.3 Å/s, compared to 0.42 Å/s for sputtered CaF2. The deposition rate of the Si:H layers was 1 Å/s. The refractive index of the SiH at NIR wavelengths was 3.45, with extinction coefficient <0.001. The refractive index of the CaF2 in the same wavelength range was 1.34, with an extinction coefficient <0.0001. The refractive index of HfF4 was 1.56 in the same wavelength range, with a comparable extinction coefficient. The resulting multilayer coatings had lower physisorbed water than those with sputtered CaF2 layers. The optical performance of the 9- and 27-layer all-sputtered and hybrid coatings was similar. The major advantage of the hybrid process was in deposition times. The deposition rate of the magnetron sputtered fluoride films was at least an order of magnitude lower than that of the e-beam evaporated films. The times required to deposit a quarter-wave (QW) optical thickness (3.2 μm) for hybrid (Si:H/HfF4) and (Si:H/CaF2) pairs were 34 min. The times to deposit comparable QW pairs by magnetron sputtering were 194 and 534 min, respectively. Advantages of this hybrid deposition method were reduced deposition times, optimum deposition process for each layer material, and improved optical and mechanical properties.

Oxygen etching of Si( [formula omitted]) and related surfaces

We have used scanning tunneling microscopy to study the oxygen etching behavior of the high index Si(5 5 12) surface, as well as nearby metal-induced facet planes formed by Cu and Au. The (5 5 12) surface is tilted 30.5° away from (0 0 1) towards (1 1 1) and forms a single-domain reconstruction composed of row-like structures. Oxygen etching has been observed for sample temperatures above 700 °C and O 2 exposures from 50 to 200 L, where pyramidal islands containing (1 1 3) facets oftentimes occur. As expected, the density of these islands increases at lower temperatures where oxide nucleation is favored. In addition to the clean surface, oxygen etching of metal-adsorbed surfaces that contain (1 1 3) planes has also been investigated. Both Cu and Au are known to cause faceting of the Si(5 5 12) surface: Cu forms sawtooth facets composed of (1 1 3) and (1 1 1) planes, whereas low coverages of Au produce (1 1 3)/(3 3 7) sawtooths. In both cases, oxygen etching at temperatures above 700 °C produces trapezoidal islands that again incorporate (1 1 3) planes. This prevalence of (1 1 3) facets during etching is explained via step pinning at oxide-induced pinning sites.

Electronic structure of a surface quantum-wire array

The Ga/Si(1 1 2) interface consists of a self-assembled, mesoscopic array of “atomic wires” on a high-index Si(1 1 2) surface. The structural uniformity of the atomic-wires or “quantum-wire array” appears far superior to those created by nano-lithography or STM atom manipulation, which presents an interesting opportunity to explore electronic transport in atomic wires of mesoscopic length. The structure and electronic structure of the atomic-wire array is investigated by means of first-principles total-energy calculations and photoemission experiments. The electronic conduction channels appear to be orthogonal to the crystallographic chains.

Au adsorption on Si([formula omitted]) surfaces and facet formation studied by high resolution in situ REM

Adsorption processes of Au on a high index Si(5512) surface were studied in situ by high resolution reflection electron microscopy with subunit cell resolution. Growth of new phases is quite anisotropic and rapid along the [11̄0] direction reflecting quite anisotropic substrate surface structure. At around 715°C, successive changes from (5512) to two-fold (337), (5511) and (225) facet were noted as an increase of Au deposit. (7717) surface units are locally formed before formation of the two-fold (337) surfaces. At higher temperature 780°C (above roughening temperature 750°C of (5511)-Au surface), a disordered surface is formed after formation of the two-fold (337) facet. The disordered surface is a higher temperature phase of the (5511) surface. Above changes are discussed in terms of characteristics of these high index surface structures.

High resolution REM studies of Si([formula omitted]) surfaces and their roughening phase transition

High resolution reflection electron microscopy has been applied to studies of a high index Si(5512) surface at a sub-unit cell resolution. Lattice fringes with different widths were found to be directly corresponding to the subunits of the (5512) surface. Defects on the surface were resolved as nano-facets with close orientation to (5512). (131331) and (7717) facets were identified based on the subunit arrangement. An incommensurate structure (q=a*bk/2.43) was revealed at a temperature just below the roughening transition. Formation of an incommensurate structure as well as defects was discussed in terms of surface steps.

1-D nanostructures grown on the Si(5 5 12) surface

We have used scanning tunneling microscopy (STM) to study the growth behavior of noble metals deposited on the high-index Si(5 5 12) surface. This unique surface is tilted 30.5° down from (0 0 1) and forms a single-domain reconstruction composed of row-like structures. When low coverages (<0.25 ML) of noble metals such as Ag and Au are deposited onto Si(5 5 12) and moderately annealed (∼450°C), they form overlayer “nanowires” with the periodicity of the Si surface (5.4 nm). It is the preferential reactivity of the underlying Si row structures that results in this well-ordered growth behavior. At higher coverages and temperatures, however, these metals can cause significant restructuring of the surface that leads to the creation of neighboring facet planes. Ag forms nanoscale sawtooth facets at coverages above 0.25 ML, but they are relatively narrow (5–10 nm wide) and do not cause the surface to undergo a dramatic restructuring. In contrast, Au deposition induces significant faceting of the surface, resulting in the formation of (1 1 3), (2 2 5), (3 3 7), (5 5 11), or (7 7 15) facet planes composed of periodic 1-D nanostructures.

STM studies of 1-D noble metal growth on silicon

Our scanning tunneling microscopy (STM) studies show that noble metals (Ag, Au) form a wide variety of 1-D structures on the high-index Si(5 5 12) surface. At coverages below 0.25 monolayer (ML), both metals grow as overlayer rows with an inter-row spacing of ∼5 nm. At higher coverages and annealing temperatures, the underlying Si reconstruction is removed, but periodic row structures persist. Au can also induce faceting to nearby planes, e.g. (7 7 15) and (2 2 5), at temperatures above 500°C. For all coverages and annealing temperatures studied here (0.02–1 ML, 450–800°C), the Si(5 5 12) template initiates 1-D growth of the deposited noble metals.

Au-Induced Faceting of Si(5 5 12)

The growth of Au on the stable, high-index Si(5 5 12) surface has been studied using scanning tunneling microscopy (STM). At very low coverages and moderate annealing temperatures (≤0.1 ML, 400–500°C), Au appears to decorate the underlying Si rows and form an array of rows that maintains the underlying (5 5 12) periodicity of 5.4 nm. For higher annealing temperatures and coverages, however, Au causes faceting to a number of nearby planes. The two primary facets formed at lower (∼0.15 ML) and higher (0.5–2 ML) coverages are the (337) and (225) planes, which are tilted 0.7° down [towards (111)] and 1.1° up from (5 5 12), respectively. Both orientations are in fact subunits of the (5 5 12) unit cell, so their presence is not surprising. In addition to these facets, two types of sawtooth morphologies composed of planes oriented further from (5 5 12) are found at very high annealing temperatures (800–900°C). These include (113)+(7 7 15) planes at very low coverage (0.05 ml) and (113)+(5 5 11) planes at higher coverage (∼1 ML), where (113) is tilted up by 5.3° and (7 7 15) and (5 5 11) are tilted down by 2.9° and 2.2°, respectively. Au adsorption on Si(5 5 12) therefore results in the formation of five possible facet planes: (113), (225), (337), (5 5 11), and (7 7 15).

Nanostructures obtained by self-organization of silicon surfaces

An overview of the formation of reconstructed facets on vicinal surfaces of Si(111), Si(100) and Si(110) is given, which have been observed by using scanning tunneling microscopy (STM). Most of the samples have been prepared by mechanical grinding, subsequent chemical etching leading to concave-shaped surfaces and by in situ treatment of high-index Si wafers. The orientation of the experimentally observed facets are represented in a stereographic projection, which allows the recognition of more general trends in facet formation. Moreover, it provides information on the availability of unit cells in a wide range of sizes and may therefore become important for the growth of self-organized surface structures.