Oriented Si Surfaces Research Articles

Formation technology of flat surface with epitaxial growth on ion-implanted (100)-oriented Si surface of thin silicon-on-insulator

For the development of three-dimensional devices, selective epitaxial growth (SEG) technology has attracted much attention. SEG has been applied to fabricate many devices and it is expected to be used in future manufacturing processes. Therefore, its characteristics must be examined in detail to extend its application. For the fabrication of a three-dimensional device structure, the selectivity of epitaxial growth must be accurately controlled not only on Si and SiO2, but also on different impurity-type silicon surfaces. In this work, we investigated some characteristics of the SEG process, especially focusing on the surface roughness after SEG. Both vapor phase epitaxy (VPE) and solid phase epitaxy (SPE) were performed on ion-implanted silicon-on-insulator (SOI) thin wafers. It was often reported that epitaxial growth is very sensitive to the crystal condition of the substrate on which the films are deposited. However, we first revealed that the impurity type (p- or n-type) and its concentration at the substrate surface markedly changed the roughness and incubation times of the deposition. From our results, SPE with the oxide cap layer formation is effective for maintaining almost the same flatness as the original wafer surface. It is also effective to employ the low-temperature H2/Xe plasma treatment after the SEG to reduce roughness.

Combined Microstructural and Electrical Characterization of Metallization Layers in Industrial Solar Cells

Abstract Screen printed front side contacts of textured, mono-crystalline p-type silicon solar cells with n-type emitters were investigated. The different pastes (FSP1 and 2) and the different crystallographic orientations of the Si surfaces studied strongly affected the contact resistance. The microstructure of the contacts was analyzed in plan-view and cross-section by combined scanning and analytical transmission electron microscopy. A controlled grinding process rather than a chemical etching process was applied for the plan-view sample preparation. For textured cells processed with different pastes, pronounced differences were seen in the contact resistances (FSP1: efficiency 16.9% and contact resistance 20 mΩ cm 2 , FSP2: 17.8% and 2 ). A discontinuous glass layer was found for FSP1 but a continuous glass layer was found for FSP2, yielding a smaller contact resistance. Glass layers contained (Si 2 Pb)O x as a main constituent but different mole fractions of Zn, Ti, P, and B as minor constituents, varying for the different pastes. Glass layers were up to 500 nm thick and revealed inhomogeneously distributed spherical Ag colloids 5-200 nm in size. Planar cells were also studied and served as model systems: planar 〈111〉 oriented Si surfaces yielded specifically lower contact resistance as compared to planar 〈100〉 orientation. Pyramidal Ag crystallites were only observed for 〈100〉 oriented Si surfaces not for 〈111〉 surfaces. Therefore, it is concluded that pyramidal Ag crystallites are not necessary for contacts yielding low contact resistance. Instead, lens shaped Ag precipitates together with a high density of Ag colloids in the glass layer yield low contact resistance, as found for oriented Si surfaces. A percolative current path including charge transport via Ag colloids in the glass layer is proposed. For textured cells, in accordance with these results, pyramidal Ag crystallites were only observed at step edges of {111} faces or at the edges of the Si pyramids.

X‐ray pole figure analysis of ZnTe layers grown on lattice mismatched substrates

AbstractThe electro‐optical effect of ZnTe is recently highlighted, and various device structures utilizing ZnTe are explored. ZnTe substrates are recently commercially available, and high quality homoepitaxial layers can be grown. On the other hand, the cost of the substrate could be a concern for the practical device application. In this study, Si and Al2O3 substrates were used since they are widely used for many kinds of device applications. The lattice mismatch between those substrate materials and ZnTe is above 10% and those mismatch strain should be carefully considered. ZnTe layers were nucleated on chemically treated substrate surfaces. (111), (110), (211) oriented Si substrates and (0001), (11‐20), and (10‐10) planes of Al2O3 were used. By taking the pole figure data and studying the crystal symmetry of the domain, detail information associated with the formation of domains in the layer was discussed. The pole figure analysis revealed that several kinds of (110) oriented ZnTe domains rotated 60 degrees each other were formed on (110) oriented Si surface. On the other hand, preferentially oriented single domain of (111) was confirmed for the layer grown on the (0001) plane of Al2O3. ZnTe layers grown on (10‐10) plane Al2O3exhibited a unique domain formation. Pole figure data combined with the conventional theta‐2theta measurement indicated that (211) oriented plane of ZnTe was a dominant domain formed on the (10‐10) plane Al2O3. These crystallographic features would be related to the interface valency as well as the interfacial bond structure. X‐ray pole figure method was a useful and powerful method to study the domain formation of the severely lattice mismatched structures. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystallographic orientation engineering in silicon-on-insulator substrates

A bilayer silicon-on-insulator film was engineered to locally convert the crystallographic orientation from (100) to (110). The initial bilayer film is composed of a first 50 nm thick (110) oriented Si surface layer, above a second 20 nm thick (100) oriented Si underlayer. The bilayer film was elaborated using hydrophobic bonding to ensure an atomic contact between the two layers without any intermediate oxide. A local and deep-amorphization was developed by ion implantation to amorphize the (100) oriented Si underlayer, conserving also a partially crystalline (110) oriented surface layer. After such a deep amorphization, a solid phase epitaxy regrowth was performed at 900 °C. Transmission electron microscopy observations confirm that the partially crystalline surface layer acts as a seed for the epitaxial regrowth of the amorphized areas through the hydrophobic bonding interface. Thus, the orientation is locally converted from (100) to (110) in the underlayer, which could lead to the elaboration of hybrid films over a continuous insulating oxide that are mandatory for high performance electronic devices.

Nanopatterning Si(111) surfaces as a selective surface-chemistry route

Using wet-chemical self-assembly, we demonstrate that standard surface reactions can be markedly altered. Although HF etching of Si surfaces is known to produce H-terminated surfaces, we show that up to approximately 30% of a monolayer of stable Si-F bonds can be formed on atomically smooth Si(111) surfaces on HF reaction, when chemically isolated Si atoms are the target of the reaction. Similarly, approximately 30% Si-OH termination can be achieved by immersion of the partially covered F-Si(111) surface in water without oxidation of the underlying Si substrate. Such reactions are possible when H-terminated (111)-oriented Si surfaces are initially uniformly patterned with methoxy groups. These findings are contrary to the knowledge built over the past twenty years and highlight the importance of steric interactions at surfaces and the possibility to stabilize products at surfaces that cannot be obtained on chemically homogeneous surfaces.

Electronics and Photonics Integration on Non-Crystalline Substrates via Nanoheteroepitaxy

The ability to grow dissimilar materials with large lattice mismatches on a single substrate removes the integration related constraints of incorporating photonics with electronics. We review our recent nanoheteroepitaxy efforts on the epitaxial growth of bridged InP NWs between two vertical, (111)-oriented Si surfaces by forming connections during the NW growth facilitating their direct electrical probing via Si electrodes and a new method of synthesizing III-V NWs on a non-single crystalline surface that directly relaxes any lattice matching conditions. The current-voltage behavior with temperature and the resulting photo-enhanced thermal failure is also discussed.

Formation and some properties of chromium oxide nanolayers on semiconductors

Ultrathin chromium oxide layers (nanostructures) are synthesized by molecular laying (atomic layer deposition) on the (100)-oriented Si surface and the (100)- and (110)-oriented GaAs surfaces. The influence of technological conditions on the composition and basic regular trends in the formation of the layers is established. Some of the dielectric properties of the chromium oxide nanostructures and the quality of the semiconductor-insulator interface are assessed.

Void Formation in Differently Oriented Si in the Early Stage of SiC Growth

This study refers, through different microscopies, about the carbonization effects on differently oriented Si surfaces. A statistical study on the relationship between some process parameters (such as temperature, process time) and void dimensions and density, for three Si substrate orientations, is reported.

Carbonization Study of Different Silicon Orientations

3C-SiC/Si heteroepitaxy is hampered by large mismatches in lattice parameters (19.7%) and thermal expansion coefficient (8%) leading to 3C-SiC films containing high defects density. To reduce the presence of defects, a multi-step growth process in a CVD reactor is used. The aim of the work is to study the effect of carbonization on differently oriented Si surfaces, experiencing a 200°C-wide temperature range in a CVD reactor, to improve the crystalline quality. TEM analysis are carried out to evaluate thickness, crystal orientations and defects of carbonized layers with respect to the time-dependence of the process and to the different orientations of the Si substrate. It will be shown that process-related defects are strictly correlated to the substrate orientation either for size, density, occupied area, shape or thickness. Uniform, flat and crystalline thin SiC films are obtained with a low defect density.

Thin crystalline 3C-SiC layer growth through carbonization of differently oriented Si substrates

The growth of thin cubic silicon carbide (3C-SiC) buffer layers in an horizontal hot-wall chemical vapor deposition reactor, through the carbonization of differently oriented Si surfaces, is presented. A qualitative and quantitative study has been performed on statistical parameters related to voids due to the buffer layer growth on the different substrate orientations emphasizing shape, size, and density as a function of the substrate orientation. Variation in the void parameters can be attributed to the atomic packing density related to the substrate orientations, which were (100) Si, (111) Si, and (110) Si in this study. Scanning electron microscopy and transmission electron microscopy were performed to analyze the surface and the crystalline quality of the 3C-SiC films grown and, eventually, an empirical model for the carbonization of Si surfaces formulated. Large platens characterize the 3C-SiC films with shapes related to the orientations of the substrate. These platens derive from the two-dimensional growth of different SiC islands which enlarge during the process due to the continuous reaction between Si and C atoms. The interior part of platens was characterized by the presence of a pure crystalline material with the presence of small tilts affecting some grains in the 3C-SiC layer in order to relief the stress generated with the substrate.

Control of the Stability, Electron-Transfer Kinetics, and pH-Dependent Energetics of Si/H2O Interfaces through Methyl Termination of Si(111) Surfaces

Methyl-terminated, n-type, (111)-oriented Si surfaces were prepared via a two-step chlorination-alkylation method. This surface modification passivated the Si surface toward electrochemical oxidation and thereby allowed measurements of interfacial electron-transfer processes in contact with aqueous solutions. The resulting semiconductor/liquid junctions exhibited interfacial kinetics behavior in accord with the ideal model of a semiconductor/liquid junction. In contrast to the behavior of H-terminated Si(111) surfaces, current density vs. potential measurements of CH(3)-terminated Si(111) surfaces in contact with an electron acceptor having a pH-independent redox potential (methyl viologen(2+/+)) were used to verify that the band edges of the modified Si electrode were fixed with respect to changes in solution pH. The results provide strong evidence that the energetics of chemically modified Si interfaces can be fixed with respect to pH and show that the band-edge energies of Si can be tuned independently of pH-derived variations in the electrochemical potential of the solution redox species.

InP nanobridges epitaxially formed between two vertical Si surfaces by metal-catalyzed chemical vapor deposition

The heteroepitaxial growth of III-V compound semiconductors on Si would enable the integration of high-performance III-V materials with Si technology. We report epitaxial growth on (111)-oriented Si surfaces of highly aligned, single crystalline InP nanowires by chemical vapor deposition catalyzed by Au. We demonstrate laterally oriented InP nanowires bridging between vertical (111) Si surfaces formed by anisotropically etching a (110)-oriented Si substrate or the top Si layer of a silicon-on-insulator wafer. This method of connecting nanowires offers a facile way of integrating nanoscale III-V optoelectronic and photonic devices with Si.

Transmission Infrared Spectroscopy of Methyl- and Ethyl-Terminated Silicon(111) Surfaces

Transmission infrared spectroscopy (TIRS) has been used to investigate the surface-bound species formed in the two-step chlorination/alkylation reaction of crystalline (111)-oriented Si surfaces. Spectra were obtained after hydrogen termination, chlorine termination, and reaction of the Cl-Si(111) surface with CH(3)MgX or C(2)H(5)MgX (X = Cl, Br) to form methyl (CH(3))- or ethyl (C(2)H(5))-terminated Si(111) surfaces, respectively. Freshly etched H-terminated Si(111) surfaces that were subsequently chlorinated by immersion in a saturated solution of PCl(5) in chlorobenzene were characterized by complete loss of the Si-H stretching and bending modes at 2083 and 627 cm(-1)(,) respectively, and the appearance of Si-Cl modes at 583 and 528 cm(-1). TIRS of the CH(3)-terminated Si(111) surface exhibited a peak at 1257 cm(-1) polarized perpendicular to the surface assigned to the C-H symmetrical bending, or "umbrella" motion, of the methyl group. A peak observed at 757 cm(-1) polarized parallel to the surface was assigned to the C-H rocking motion. Alkyl C-H stretch modes on both the CH(3)- and C(2)H(5)-terminated surfaces were observed near 2900 cm(-1). The C(2)H(5)-terminated Si(111) surface additionally exhibited broad bands at 2068 and 2080 cm(-1), respectively, polarized perpendicular to the surface, as well as peaks at 620 and 627 cm(-1), respectively, polarized parallel to the surface. These modes were assigned to the Si-H stretching and bending motions, respectively, resulting from H-termination of surface atoms that did not form Si-C bonds during the ethylation reaction.

Anisotropy in the anodic dissolution of silicon elucidated by in situ infrared spectroscopy

The anodic dissolution of silicon in dilute HF electrolyte exhibits some anisotropy in the region of the first electropolishing plateau. This anisotropy can plausibly be assigned to an orientation-dependent surface chemistry. The silicon|dilute fluoride electrolyte interface has been investigated by in situ infrared vibrational spectroscopy in the differential mode, in the potential region where the surface turns from hydrogenated to oxidised. Silicon surfaces of different orientations exhibit closely similar behaviour. However, careful analysis of the ν OH region reveals a significant difference in the concentration of SiOH groups at (1 0 0)- and (1 1 1)-oriented Si surfaces. This observation points to the key role of the SiOH groups in determining the anisotropic behaviour of the electrochemical dissolution of silicon.

Infrared and X-ray Photoelectron Spectroscopic Studies of the Reactions of Hydrogen-Terminated Crystalline Si(111) and Si(100) Surfaces with Br2, I2, and Ferrocenium in Alcohol Solvents

The reaction chemistry of H-terminated crystalline Si(111) and Si(100) surfaces in CH3OH, CD3OD, CF3(CH2)3OH, C4H9OH, and C4D9OD solutions containing ferrocenium (Fc+)−BF4, I2, or Br2 was monitored using X-ray photoelectron (XP) spectroscopy and infrared (IR) spectroscopy. Addition of the one-electron oxidant Fc+, or addition of the oxidizing species I2 or Br2, produced diagnostic changes in the IR spectra that clearly indicated formation of surficial Si−OR groups. XPS data confirmed the conclusions of the IR studies. Under our reaction conditions, no detectable reaction occurred without the presence of the oxidant. The data are consistent with oxidative activation of the surficial Si−H bonds toward nucleophilic attack by the alcohols. The reaction chemistry was generally similar on (111)- and (100)-oriented Si surfaces, although some differences were observed in the ratio of reaction products on the two different surface orientations. Alkoxylated surfaces were also prepared by a two-step process in which...

Electrochemical and Electrical Behavior of (111)-Oriented Si Surfaces Alkoxylated through Oxidative Activation of Si−H Bonds

The reaction chemistry of H-terminated (111)-oriented Si surfaces has been explored in the presence of one-electron oxidants in alcohol solvents. Similar chemistry was observed for Si surfaces exposed to methanolic solutions of ferrocenium tetrafluoroborate or to methanolic solutions of I2, as well as for Si electrodes that were subjected to anodic electrochemical oxidation processes in methanol. Oxidation in alcohols activates the surficial Si−H bond toward nucleophilic attack, producing alkoxylated surfaces. This mechanism for Si−H activation is supported by infrared spectroscopy, temperature-programmed desorption spectroscopy, and by X-ray photoelectron spectroscopic studies of this system. These data explain a variety of observations regarding the surface chemistry and electrochemistry of Si electrodes and surfaces.

Effect of Fluorine on Surface Morphology in Selective Epitaxial TiSi2 Growth by Plasma‐Enhanced Chemical Vapor Deposition

We studied the effect of gas addition on the surface morphology of titanium silicide films grown by plasma‐enhanced chemical vapor deposition using a low‐density plasma of gases. We examined the use of , and as additional gases and found the F‐containing gases to be effective for forming continuous films. Deposition with SiCl4 and without additional gases results in films with slits among the silicide grains, showing that the effect on the surface morphology is due to fluorine. The surface morphology of the film indicated that the fluorine suppresses atom movement. Although gas addition caused etch pits in the substrate, it also resulted in a continuous epitaxial film with a C49‐structure being selectively deposited on a (001)‐oriented Si surface with a mask.

Modeling of the Surface Annihilation of Excess Self-Interstitials Generated by Gold Diffusion into Silicon

ABSTRACTBased on the kick-out mechanism, we have modeled the time evolution of the Si self-interstitial supersaturation during gold diffusion from evaporated surface layers into silicon substrates. The model comprises a limited annihilation velocity υI of self-interstitials at the Si surface, which accounts for the gradual increase of the Au boundary concentration observed during rapid thermal annealing at higher temperatures and furnace annealing at lower temperatures. Experimental data were fitted by computer simulation of the Au diffusion process. An approximate analytical expression describing the increase of the Au boundary concentration with time is also given. We obtain υI 1.35 × 1011 exp(−3.91 eV/kBT) ms-1 for gold-covered {100}-oriented Si surfaces between 845 °C and 1119 °C.

Optical second harmonic generation: A probe of atomic structure and bonding at Si–SiO2 interfaces, and other chemically modified Si surfaces

Optical second harmonic generation (SHG) is a highly surface-sensitive probe for studying crystalline Si surfaces because the inversion symmetry is broken and electric dipole optical SHG processes forbidden in the bulk are allowed. The polarized optical SHG from a perfectly oriented Si surface is inherently anisotropic, varying periodically as the in-surface projection of the polarization vector of the incident laser is rotated about a normal to the surface. The harmonic contributions to the angular anisotropy from the surface are characteristic of the surface bonding, and are modified by misorientation, chemical termination, as well as thermal treatments. This paper reviews the results of our previously reported optical SHG studies on Si(111) wafers with misorientations of 0°–5°±0.5° in the [112̄] direction for Si–H or Si–O terminated surfaces. Azimuthal anisotropy data are compared with an empirical model for the SHG intensity that is based on (i) the nonlinear response of anharmonic oscillators, and (ii) a phenomenological theory of azimuthal anisotropies expected for different surface orientations. This model is used as a framework for estimating ’’effective‘‘ resonance energies from single wavelength experiments, and in particular, for providing insights into the microscopic mechanisms that can contribute to the changes in these resonance energies with respect to different processing conditions. For example, important differences between thermally grown and plasma-oxidized interfaces are identified, and correlations between SHG and electrical performance of the Si–SiO2 interfaces are discussed.

Low-temperature ohmic Au/Sb contacts to n-type Si

Au/Sb films annealed at temperatures as low as 370 °C yield ohmic contacts to n-type Si. The contact formation is based on a liquid-phase epitaxy process of Sb-doped Si from Au solution. Measured contact resistivities range around 3×10−2 Ω cm2 and are at least one order of magnitude higher than what is expected from the solid solubility of Sb in Si. The discrepancy stems from local inhomogeneous etching and epitaxial regrowth of the (100)-oriented Si surface by and from the Au solution. Only a small fraction of the macroscopic contact area is doped by Sb and contributes to current transport across the Au/Si(100) interface.