High-index Si Research Articles

Graphene-based waveguide integrated dielectric-loaded plasmonic electro-absorption modulators

Theoretical investigations of graphene-based electro-optic plasmonic modulators with a dielectric ridge were analyzed and the results were presented. The effects of different ridge materials and different spacer dielectric functions were analyzed, showing that a 3 dB modulation with a 65 nm-long waveguide is possible with dielectric-loaded surface plasmon polariton waveguides (DLSPPWs), resulting in an energy per bit only 0.08 fJ/bit. The figure of merit, defined as the ratio between the extinction ratio and the insertion loss, was found to be about 5.2 with a low refractive index ridge and increased to over 17.3 for a high refractive index Si ridge, compared to 3.5 calculated and measured with photonic graphene-based waveguides. Additionally, it was shown that further improvement in terms of the figure of merit is possible with long-range dielectric-loaded surface plasmon polariton waveguides (LR-DLSPPWs), where it was calculated to exceed 72. For such waveguides, a 3 dB modulation was achieved with 10 μm-long waveguides with an energy per bit of 15.8 fJ/bit. The wavelength dependence of the graphene conductivity and, consequently, the attenuation of the waveguide were analyzed for different chemical potentials. It was shown that gate voltage applied across the graphene layers shifts the attenuation curve to shorter wavelengths, with the 3 dB modulation bandwidth exceeding 15 THz for a 12 μm-long DLSPP waveguide.

Nonpolar GaN films on high-index silicon: Lattice matching by design

We explore theoretically the possibility of growing GaN films in a nonpolar orientation on planar high-index Si($hhk$) substrates. Candidate substrates were identified by requiring that they are well lattice matched, on a length scale of several unit cells, to GaN in the nonpolar $m$-plane orientation. These candidate orientations were then used to construct atomistic models of the GaN/Si($hhk$) interface. Using density functional theory, we then computed the formation energies of these nonpolar interfaces and compared them to those of competing polar interfaces. We find that Si(112) and Si(113) offer potentially favorable substrates for the growth of nonpolar $m$-plane GaN.

Shape evolution of MBE grown Si1−xGex structures on high-index Si(5 5 12) surfaces: a temperature dependent study

The morphological evolution and the effect of growth temperature on size, orientation and composition of molecular beam epitaxy grown Ge–Si islands on Si(5 5 12) surfaces have been investigated in the temperature range from room temperature to 800 °C. Two modes of substrate heating, i.e. radiative heating (RH) and direct current heating (DH) have been used. The post-growth characterization was carried out ex situ by scanning electron microscopy, cross-sectional transmission electron microscopy and Rutherford backscattering spectrometry. In the RH case, we found spherical island structures at 600 °C with a bimodal distribution and upon increasing temperature, the structures got faceted at 700 °C. At 800 °C thick (∼122 nm) dome-like structures are formed bounded by facets. While in the case of dc heating, after the optimum critical temperature 600 °C, well aligned trapezoidal Si1−xGex structures with a graded composition starts forming along the step edges. Interestingly, these aligned structures have been found only around 600 °C, neither at low temperature nor at higher temperatures.

Universality in shape evolution of Si1−xGex structures on high-index silicon surfaces

The shape evolution of MBE grown Si1−xGex islands on ultraclean reconstructed high-index Si(5 5 12), Si(5 5 7) and Si(5 5 3) surfaces has been studied experimentally and explained using a phenomenological kinetic Monte Carlo (kMC) simulation. We show that a self-assembled growth at optimum thickness leads to interesting shape transformations, namely spherical islands to rectangular rods up to a critical size beyond which the symmetry of the structures is broken, resulting in a shape transition to elongated trapezoidal structures. We observe a universality in the growth dynamics in terms of aspect ratio and size exponent, for all three high-index surfaces, irrespective of the actual dimensions of Ge-Si structures. The shape evolution has been simulated using kMC by introducing a deviation parameter (ϵ) in the surface barrier term (ED) to take the effect of anisotropic diffusion as one of the plausible mechanisms.

Ga induced 2D superstructural phase diagram on trenched Si(5 5 12) surface

The high index Si(5 5 12) surface offers morphological trenches, which can be interesting for epitaxial growth. In this study, the evolution of Ga adsorption at a very low flux rate of 0.03ML/min on high index trenched Si(5 5 12)−2×1 reconstructed surface at various substrate temperatures ranging from room temperature (RT) to 600°C has been investigated using in-situ AES, LEED and EELS. The Auger uptake curves, which plot the Ga(LMM)/Si(LVV) Auger intensity ratio with Ga adsorption time, show that Ga grows in layer plus islands mode for substrate temperatures in the RT to 350°C range, while it grows in Volmer–Weber (3D islands) for higher substrate temperatures (>350°C). We also arrive at a complete 2D superstructural phase diagram for Ga/Si(5 5 12) interfacial system that shows the pathways to attain the different superstructural phases. The formation of Ga nanowires as (2 2 5), (3 3 7) phase and Ga 3D islands in the (1 1 2)−6×1, (1 1 2)−6×2 phases and other Ga induced superstructural phases like (7 7 17)+2x(1 1 3), (2 2 5)+(3 3 7), 1×1 has been carefully followed. The electronic structures of each of the observed phases have been probed by EELS and each of them is shown to have characteristic features.

Binding Structures of Pyrrole on Si(5 5 12)–2 × 1 Surfaces

In an effort to understand the reaction mechanisms involved in the adsorption of organic aromatic molecules on high-index Si surfaces, the reactions of pyrrole molecules adsorbed onto Si(5 5 12)–2 × 1 surfaces were studied using scanning tunneling microscopy and first principle calculations. The dissociation of one or two H atom(s) bonded to N (or N and C) from the pyrrole molecules was favored, and adsorption at adatom, tetramer, dimer, or honeycomb Si(5 5 12)–2 × 1 sites occurred to produce several distinct configurations. Pyrrole was most reactive toward the dimer site, yielding two dissociated hydrogen atoms and a vertical configuration. Pyrrole also adsorbed onto the tetramer and honeycomb sites, yielding two dissociated hydrogen atoms. On the adatom row, however, pyrrole bound to an adatom via a σ bond between the adatom and N to yield one dissociated H atom adsorbed onto a nearby adatom. No other hydrogen dissociation reactions were observed. In all configurations, the aromaticity of pyrrole was retained.

Ga-induced restructuring of Si(5 5 12) − 2 × 1 reconstructed surface at room temperature

Motivated by the need to form 1D-nanostructured dopants on silicon surfaces, we have attempted to grow Ga on the high index Si(5 5 12) surface which has a highly trenched (1D) morphology. The evolution of the interface with Ga adsorption in the monolayer regime has been probed by in situ AES, LEED and EELS. Controlling the kinetics by changing the Ga flux rates shows an interesting difference in the 1.0 to 1.5 ML region. The low flux rate (0.03 ML/minute) results in a Frank van der Merwe (layer by layer) growth mode up to 2 ML, while the higher flux rate (0.1 ML/minute) shows a transient island formation after the completion of 1 ML. The low rate shows the formation of 2 × (3 3 7) and (2 2 5) superstructures, while only the 2 × (3 3 7) is observed in a wide coverage range for the higher rate. The results demonstrate the ability to kinetically control the surface phases with different electronic properties of this technologically important interface.

Semipolar single component GaN on planar high index Si(11h) substrates

We present metal organic vapor phase epitaxy growth of polarization reduced, single component GaN on nonpatterned Si(112), Si(113), Si(114), Si(115), and Si(116) substrates. We find that the inclination angle of GaN c-axis with respect to the surface normal depends on the angle between Si(111) and above mentioned Si(11h)-surfaces. The growth of the GaN layer is essentially performed as c-axis oriented growth on the naturally occurring Si(111) facets of these Si(11h)-surfaces. The c-axis tilt-angle of GaN crystallites depends on the Si-surface direction and increases from Si(112) to Si(116) planes. GaN layers are investigated by x-ray analysis and scanning electron microscopy.

Clustering and layering of In adatoms on low and high index silicon surfaces: A comparative study

Using the morphological differences of low and high index surfaces as templates for metal growth, several low dimensional overlayer structures with novel structural and electronic properties can be formed. We present here a first report on submonolayer adsorption and residual thermal desorption studies of In adatoms on reconstructed high index Si (5 5 12)−2 × 1 surface and compare it with the observations on planar Si (111)−7 × 7 surface. The study is done by using in-situ Ultra High Vacuum surface sensitive probes like Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). These conventional wide area techniques provide an understanding of atomistic issues involved in the evolution of the interface. We have observed an anomalous growth mode during adsorption at room temperature (RT) above 2ML, which includes adatom layering and clustering on Si (111) surface. This is also manifested during the desorption experiments on both surfaces, and the subtle differences on the two surfaces are discussed. The observation of LEED pattern during the adsorption process shows formation of different superstructural phases on Si (111)−7 × 7 surface. On Si (5 5 12) 2 × 1 surface we observe the sequential 2× (225), 2× (337) and 2× (113) facet formation during adsorption/desorption, which include quasi 1D-nanowire/chain structures. A combination of lattice strain effects, presence of step-edge barrier and quantum size effects are employed to speculate the differences in adsorption and desorption.

Magnesium Induced Superstructural Changes in High Index Si (5 5 12) Surface: Formation of Quasi One Dimensional Structures

This is the first report of the initial formation of Mg/Mg2Si/Si interface at room temperature on high index Si (5 5 12) surface. The work describes the initial Mg2Si formation on the silicon surface, which forms a silicide layer after a critical coverage, and then metallic magnesium forms on top of the Mg2Si layers. The studies have been performed in situ in ultra high vacuum conditions (5 x 10(-11) Torr) and analyzed by surface sensitive techniques like Auger electron spectroscopy, electron energy loss spectroscopy, and low energy electron diffraction. The Auger uptake curve obtained by AES, shows the layered growth of Mg while the LEED and EELS shows the formation of surface phases, which are identified as the quasi 1D structure, with different electronic natures indicated by EELS studies. The growth process occurs in the following way that firstly the adsorption of Mg reconstructs the Si (5 5 12)-2 x 1 surface and forms two stable phases-Si (337) and Si (113) below the coverage of 1 ML, while there is Mg2Si formation at the coverage of 1 ML, further increase in the coverage of magnesium results in the metallic magnesium on top of the Mg2Si layer. Thus, with the Mg adsorption at RT not only the Si (5 5 12)-2 x 1 surface is reconstructed to give the stable phases but it also demonstrates the formation of Mg2Si at RT, which is the interfacial layer in between metallic Mg and Si.

Morphology of strained and relaxed SiGe layers grown on high-index Si substrates

We have investigated the surface morphology of strained and relaxed SiGe layers grown on Si substrates with surface normals rotated off of the [001] axis towards [111] by 0, 13, and 25°. Atomic force microscopy has revealed surface corrugations in thin layers prior to plastic relaxation on each of the surfaces due to the initial deposition of the strained films. Thicker partially relaxed layers have previously been shown to contain networks of misfit dislocations which create patterns that are unique to each substrate orientation. We find on these relaxed layers that the surface corrugations are well aligned with the dislocation networks forming a modified crosshatch pattern on the off-axis substrates. More strikingly, though, we find that these corrugations are comprised of smaller nanostructured features which are also unique to each surface. Topographs of the unrelaxed surfaces show no such organization indicating a correspondence between the misfit dislocations and the surface corrugations.

The preserved aromaticity of aniline molecules adsorbed on a Si(5 5 12)−2×1 surface

We present a scanning tunneling microscopy and first-principles calculations study of the adsorption structures of aniline on a Si(5 5 12)-2x1 surface. Dissociation from the aniline molecules of one or two H atom(s) bonded to N is favored, and then adsorption onto adatom, tetramer, and dimer rows of Si(5 5 12)-2x1 occurs in several distinct configurations. On the adatom row, aniline binds to an adatom in a tilted configuration, which is formed via a sigma bond between the adatom and N, with one dissociated H atom adsorbed on a nearby adatom. No further hydrogen dissociation occurs. On the tetramer and dimer rows, the structures with two dissociated hydrogens and upright configurations are the most stable. Aniline does not adsorb onto the honeycomb chains; this adsorption configuration has a low adsorption energy. In all the adsorption configurations of aniline on this surface, the molecule's aromaticity is preserved. Thus Si-N bonding of aromatic amine molecules provides a strategy for the homogeneous aromatic functionalization of high index Si surfaces.

Self-organization of In nanostructures on Si surfaces

Methods of forming various In nanostructures on Si surfaces are demonstrated. Using a high-index Si(311) surface, isolated nanotriangles and wires were grown by optimizing the deposition rate and substrate temperature. In contrast, nanodots were formed by the deposition of In on a Si(111)–In-31×31 surface at room temperature (RT) deposition. On a Si(111)–In-4×1/31×31 coexisting surface, nanowires were selectively grown in the Si(111)–In 4×1 area by RT deposition through the nucleation promoted by the boundary barrier produced by the surrounding 31×31 area. Details were studied using scanning tunneling microscopy.

Mechanical modulation method for ultrasensitive phase measurements in photonics biosensing

A novel polarimetry methodology for phase-sensitive measurements in single reflection geometry is proposed for applications in optical transduction-based biological sensing. The methodology uses altering step-like chopper-based mechanical phase modulation for orthogonal s- and p- polarizations of light reflected from the sensing interface and the extraction of phase information at different harmonics of the modulation. We show that even under a relatively simple experimental arrangement, the methodology provides the resolution of phase measurements as low as 0.007 deg. We also examine the proposed approach using Total Internal Reflection (TIR) and Surface Plasmon Resonance (SPR) geometries. For TIR geometry, the response appears to be strongly dependent on the prism material with the best values for high refractive index Si. The detection limit for Si-based TIR is estimated as 10(-5) in terms Refractive Index Units (RIU) change. SPR geometry offers much stronger phase response due to a much sharper phase characteristics. With the detection limit of 3.2*10(-7) RIU, the proposed methodology provides one of best sensitivities for phase-sensitive SPR devices. Advantages of the proposed method include high sensitivity, simplicity of experimental setup and noise immunity as a result of a high stability modulation.

High temperature superstructural phases of the Sb/Si (5 5 12) interface

The control of adsorption kinetics and post growth treatments yield several stable superstructural phases in heteroepitaxial growth, and are being exploited to form novel nanostructures with controlled shape, size and position assembly. In this paper, we report the study of the interface formation in the sub-monolayer Sb adsorption in UHV on the trenched template of the high index Si (5 5 12) − (2 × 1) reconstructed surface. The interface formation is monitored in-situ by Auger Electron Spectroscopy (AES), Electron Energy Loss Spectroscopy (EELS), and Low Energy Electron Diffraction (LEED). The trenches formed by the (337) and (225) facets separated by primary silicon rows are used to form 1D nanostructures. Performing adsorption, onto substrates held at various temperatures, has enabled us to observe several ordered superstructural phases. Evidence is provided to show that the pathways adopted in the formation of the interface determine the superstructural phases with characteristic electronic properties of this technologically and scientifically important interface.

Formation of n and p Regions in (114) and (5 5 12)-Silicon Substrates

In this work, the diffusion mechanisms of both boron and phosphorus dopants are analyzed when they are ion-implanted and thermally activated in high-index silicon substrates. Si(1 1 4) and Si(5 5 12) wafers are reported as highly promising substrates for the development of novel quantum structures specially for MOS devices. In order to obtain the final dopant distribution an electrochemical profiler was utilized. Because it is well known the lack of reported experimental data regarding the doping mechanism in these high-index silicon substrates, Si(1 0 0) and Si(1 1 1) substrates were utilized as reference. The experimental work was conducted varying the following ion implantation parameters: dose, energy, tilt angle, and silicon oxide masking film thickness; the thermal activation of the impurities was specially evaluated because their influence in the final distribution of dopants.

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.

Formation of 1D-Nanowires and 2D Nanophases in Heteroepitaxy of Sbon High Index Si(5 5 12) Surface

The surface topography of high index Si(5 5 12) presents a single-domain planar reconstruction that is composed of (225) and (337) regions with nanoscale widths and row like trenches and provides an unique template for the growth of nanostructures. In this study, Sb has been adsorbed on this Silicon surface to form various superstructural phases by steering the kinetic parameters and post growth annealing of the surface. Different pathways adopted have shown the formation of stabilized ordered superstructural phases corresponding to various coverages and substrate temperatures and are probed in-situ by complementary surface sensitive techniques such as Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES), and Electron Energy Loss Spectroscopy (EELS). The growth of Sb at 300 degrees C substrate temperature and annealing the formed system to different temperatures upto 820 degrees C leads to the formation of low dimensional phases having anisotropicity along the (110) direction like atomic wires. The results also demonstrate the pathways in tailoring 1 D and 2D nanostructure formation, on this technologically and scientifically important interface.

Adsorption induced faceting and superstructural phase diagram of the Sb/Si(5 5 12) interface

The planar high index Si(5 5 12) surface consists of trenches formed by the several proximal surface planes, that can be employed as templates for the adsorption of low dimensional nanostructures. This paper reports the results of an extensive UHV study of the adsorption of Sb, in the sub-monolayer coverage regime, onto the Si(5 5 12) surface. The evolution of the surface phases, surface morphology and electronic structure is monitored by Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED) and Electron Energy Loss Spectroscopy (EELS). A careful control of substrate temperatures and Sb coverages formed at a low flux rate of 0.06 ML/min enable us to extract a complete adsorption phase diagram of the important interface, for the first time. The phase diagram clearly demonstrates the conversion of the large Si(5 5 12) unit cell into facets of planes of smaller (2 2 5), (3 3 7) and (1 1 3) base units. The study also reveals the formation of various superstructural phases formed by steering the kinetic parameters.

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.