Surface Silicon Atoms Research Articles

NONEMPIRICAL CLUSTER STUDY OF THE Au ADSORPTION ON THE Si(111) SURFACE

The nonempirical discrete-variational method of local density approximation was used to study the electronic structure and the most stable position of a single Au atom for the Si(111)–Au chemisorption system. Only the symmetric adsorption sites (H3, T4, "on-top") were considered. The three fold hollow site has been found to be energetically the most favorable position with the vertical distance between adsorbed Au atom and the topmost silicon atom plane of 1.59 Å, binding energy of 4.76 eV and vertical vibrational mode of 87 cm –1. The photoemission spectra were calculated and compared with the experimental spectra. Although the calculations suggest that Au atom can not penetrate under the silicon surface, the possibility has been shown for Au atom to replace the surface silicon atom upon the adsorption. The results of the calculations are discussed in connection with the existing models for initial stages of the Au growth on the Si(111) surface.

Interstitial precursor to silicide formation on Si(111)-(7 x 7).

We show that cobalt atoms deposited on Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) at room temperature occupy near-surface interstitial sites of the silicon lattice at very low coverages. These sites are visible in scanning tunneling microscopy images as slightly lowered groups of 2 or 3 adjacent Si adatoms in an otherwise intact Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) surface. At 150 \ifmmode^\circ\else\textdegree\fi{}C the interstitials are mobile and preferentially occupy sites directly under 3-coordinated silicon surface atoms (``rest atoms'') on the faulted side of the 7\ifmmode\times\else\texttimes\fi{}7 unit cell. An atom-displacing silicide reaction occurs only for higher coverages, when 7\ifmmode\times\else\texttimes\fi{}7 half-unit cells become multiply occupied.

Pathway of H2 desorption from dihydride Si(100).

In the present paper, the pathway of H 2 desorption from a dihydride species -SiH 2 on the Si(100) surface, -SiH 2 →Si+H 2 (-Si represents a surface Si atom site), is investigated. This pathway involves the recombination of two H atoms bonded to the same surface silicon atom. A three-layer cluster that consists of 12 silicon atoms and 20 hydrogen saturators is used to simulate the Si(100) surface and ab initio self-consistent-field and configuration-interaction theory is used to treat the cluster and desorption reactions. With the correction of zero-point vibrational energies, the H 2 desorption energy and activation barrier are computed to be 48 and 53 kcal/mol, respectively

Vacancy creation on the Si(111)−7 × 7 surface due to sulfur desorption studied by scanning tunneling microscopy

Ultra high vacuum scanning tunneling microscopy (STM) has been used to study the adsorption and subsequent thermal desorption of a sulfur overlayer deposited in situ, at room temperature, on the Si(111)-7 × 7 surface. Little ordering is visible in the STM images of this overlayer 1–2 monolayers thick, with the underlying silicon surface retaining the (7 × 7) reconstruction with weakened low energy electron diffraction (LEED) spot intensity. STM images of this surface following a thermal anneal at 375°C revealed the presence of a number of monolayer deep “holes” or voids in the (7 × 7) surface. The appearance of these voids is consistent with a coalescence of vacancy defects induced by the sulfur desorption process as also observed for oxygen induced etching of the silicon surface. In addition, we have observed small regions of the (√3 × √3)R30° and c(4 × 2) reconstructions within areas exhibiting a high degree of surface disorder, with the metastable (5 × 5) reconstruction also being found. Both reconstructions and disorder are attributed to vacancy diffusion to step edges causing a redistribution of surface silicon atoms.

A study of the morphology and microstructure of LPCVD polysilicon

The morphology and microstructure of polysilicon films deposited by low-pressure chemical vapour deposition (LPCVD) have been investigated as a function of deposition conditions. The deposition temperature was varied from 540–640‡C. As-deposited polysilicon films had a rough surface with (110) textured columnar grain structure, while as-deposited amorphous films had a smooth surface. The polysilicon film deposited at the amorphous to polycrystalline transition temperature had an extra-rough, rugged surface with (311) texture. At the transition temperature, the grain structure tended to shift from the polycrystalline to the amorphous state with increasing deposition pressure and film thickness. It was found that nucleation of amorphous film duringin situ annealing at the transition temperature without breaking the vacuum began to occur from surface silicon atom migration, in contrast to a heterogeneous nucleation during film deposition.

An X-ray diffraction study of the Si(111) (√3 × √3) R30°-indium reconstruction

The structure of the (√3 × √3)R30° reconstruction induced by the adsorption of 1 3 of a monolayer of In on the Si(111) surface has been determined using surface X-ray diffraction. The fractional order Patterson function obtained from structure factor intensities at zero perpendicular momentum transfer ( l = 0), indicates lateral displacement in the silicon surface atoms. Intensity profiles of fractional order rods and one integer order rod gives information concerning displacements normal to the surface of the silicon substrate. The indium adatoms are shown to occupy the 4-fold coordinated T 4 sites above the second layer silicon atoms. Keating elastic strain energy minimisation has been used to determine relaxations down to the sixth layer of the bulk.

An X-ray diffraction study of the Si(111)(√3 × √3)R30°-indium reconstruction

Abstract The structure of the (√3 × √3)R30° reconstruction induced by the adsorption of 1 3 of a monolayer of In on the Si(111) surface has been determined using surface X-ray diffraction. The fractional order Patterson function obtained from structure factor intensities at zero perpendicular momentum transfer (l = 0), indicates lateral displacement in the silicon surface atoms. Intensity profiles of fractional order rods and one integer order rod gives information concerning displacements normal to the surface of the silicon substrate. The indium adatoms are shown to occupy the 4-fold coordinated T4 sites above the second layer silicon atoms. Keating elastic strain energy minimisation has been used to determine relaxations down to the sixth layer of the bulk.

Thermally grown Si3N4 thin films on Si(100): Surface and interfacial composition.

Synchrotron photoemission measurements of the Si(2p) and N(1s) levels have been made on ${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ thin films grown in situ by high-temperature reaction of Si(100) with ${\mathrm{NH}}_{3}$. Surface sensitivity is enhanced in comparison to laboratory photoemission experiments by selecting photon energies that minimize the photoelectron mean free path in the nitride film. From the results, we are able to determine not only the types of chemical species present, but their approximate location within the film as well. Careful analysis of the Si(2p) photoemission spectra reveals the presence of a unique silicon species with a Si(2p) binding energy intermediate between elemental silicon and silicon in ${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$. Furthermore, the persistence of this species with increasing nitride-film thickness supports its assignment to a monolayer of silicon at the outermost surface layer, on top of the growing stoichiometric ${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ film. These surface silicon atoms can be distinguished from silicon atoms in intermediate oxidation states at the ${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$/Si interface. The spectroscopic evidence for chemically distinct surface, nitride, and interfacial silicon is discussed in terms of the silicon nitridation mechanism.

Investigation of microstructure and grain growth of polycrystalline silicon deposited using silane and disilane

The microstructures of polysilicon films deposited by low pressure chemical vapour deposition using silane (SiH 4) and disilane (Si 2H 6) as a silicon source have been investigated as a function of deposition conditions. The deposition temperature was varied from 485 °C to 640 °C. Disilane is more reactive and its amorphous-to-crystalline transition temperature is around 20 °C higher than that of silane. The film deposited at the transition temperature has an uneven surface with small (311) texture regardless of silicon source, its surface area is more than 1.5 times larger than the flat surface and it is a suitable substrate for capacitor dielectric films. It is found that nucleation during the annealing at the transition temperature without breaking vacuum starts from surface silicon atom migration, while grain growth starts from an heterogeneous nucleation site on the substrate during the deposition.

Adsorption site of alkali metal overlayers on Si(001) 2 × 1

The alkali metal semiconductor interfaces are currently being investigated by a variety of tools. Most studies to date at half a monolayer coverage have shown preference for either a quasi-hexagonal (H) site or a long-bridge (B) site. At this coverage one-dimensional chain structure for K on Si(001) 2 × 1 have now been confirmed by scanning tunneling microscopy (STM). The data, however, is consistent with either of the two sites. STM investigations at low coverages suggested that alkali metals like K and Cs occupy a novel site, Y, which is a bridge site between two Si atoms belonging to different dimers along the dimer row [110] direction. The total energy calculations for this new Y site, discovered by STM, have shown that it is indeed a site of (local) energy minimum. The ability of the surface silicon atoms, which are not adjacent to the alkali metal atom, to buckle makes the Y site a competitive adsorption site. We deduce the nature of bonding between alkali metals and Si using the STM data. It is concluded that the bond is substantially ionic in nature.

The chemisorption of NH 3 on the Si(100) surface

In this paper we report the results of calculations of the chemisorption of ammonia onto the Si(100)2×1 surface using the SLAB-MINDO molecular orbital method. Consistent with the latest experimental data, the ammonia molecules are assumed to dissociate into the species NH 2 and H which then attach themselves to the dangling bond orbitals of the surface silicon atoms of the 2×1 dimer reconstruction. The optimum topology appropriate to a uniform monolayer coverage has then determined by minimizing the total energy with respect to the coordinates of all the atoms lying above the fifth substrate layer. The Si-H bonds are found to be virtually identical to those determined previously for the Si(100)2×1 monohydride system. The Si-NH 2 configuration, on the other hand, is predicted to be almost planar in direct contrast to the trigonal pyramidal geometry proposed previously. Local density of states curves corresponding to the surface N, Si and H atoms are also presented and correlated with the experimental data.

Chemisorption of HCl, Cl 2 and F 2 on the Si(100) surface

In an earlier publication we have presented the results of some preliminary calculations on the chemisorption of HF and F 2 onto the Si(100)2 × 1 surface using the SLAB-MINDO molecular orbital method. In this paper we extend this work to the case of HCl and Cl 2, and report on some new results for F 2. All of the species are assumed to dissociate and adsorb onto the surface silicon atoms, and the optimum configuration appropriate to a uniform monolayer coverage determined by minimising the total energy with respect to variations of the atomic coordinates within the six topmost surface layers. The behaviour of HCl on the Si(100) surface is found to be very similar to that obtained previously for HF. The H and Cl atoms chemisorb onto the dangling bonds of the surface silicon atoms at close to the expected tetrahedral angles whilst retaining a 2 × 1 dimer topology for the silicon substrate as suggested by experiment. The adsorption of Cl 2 is also found to yield a 2 × 1 symmetric dimer topology with each Si-Cl bond being tilted away from the surface normal at an angle of 14.8°. The lowest energy configuration for Cl 2 on the Si(100) surface, however, is determined to be a 2 × 1 bridge site model in which the two chlorine atoms are sited 0.708 Å and 1.132 Å above the surface plane. An analogous bridge site model is also found for F 2 chemisorption with the fluorine atoms lying 0.390 Å and 0.015 Å above the silicon surface. While this topology is in complete contrast to the molecular type topology reported earlier for a monolayer coverage of F 2 on the Si(100)2 × 1 surface, both configurations are found to have equivalent energies and might thus be expected to coexist during the early stages of fluorine chemisorption.

Low-energy recoil ion spectroscopy studies of H on Si(111)-7 × 7

The adsorption of hydrogen on a Si(111)-7 × 7 surface has been studied by means of low-energy recoil ion spectroscopy (LERIS) and low-energy electron diffraction (LEED). From a series of experiments performed at low incident ion energies ranging from 400 to 700 eV, we find two peaks in the energy distributions of recoil hydrogen ions from hydrogen adsorbed on a Si(111) surface. One of the peaks can be satisfactorily explained by the binary elastic collision model, as being due to the direct-recoil process. The other peak, however, cannot be easily explained by the binary elastic collision model because it appears either on the high- or low-energy side of the direct-recoil ion peak, depending on the recoil angle. From the comparison of the experimental results with computer simulations, it is shown that this new peak can be interpreted by events in which the direct-recoil hydrogen atoms hit the surface silicon atoms and then exit as the ions, i.e., the surface-recoil process. Surface-recoil ion detection is very useful for obtaining information on the hydrogen locations at the Si(111) surface.

Comparison of Trichlorosilane and Trichlorogermane Decomposition on Silicon Surfaces Using FTIR Spectroscopy

ABSTRACTFourier transform infrared (FTIR) transmission spectroscopy was used to compare the decomposition of trichlorosilane (SiHCl3) and trichlorogermane (GeHCl3) on silicon surfaces. Chlorosilanes, such as SiHCl3 are employed in silicon chemical vapor deposition (CVD). Chlorosilanes and chlorogermanes are also possible molecular precursors for the controlled atomic layer growth of silicon and germanium. GeHCl3 may be useful for the deposition of germanium on silicon surfaces and the growth of Si1−xGex heterostructures. The FTIR studies were performed in-situ in an ultra-high vacuum chamber on high surface area, porous silicon samples. The FTIR spectra revealed that SiHCl3 dissociatively adsorbs at 200 K to form SiH, SiClx, ClSiH and Cl2SiH surface species. The presence of ClxSiH species is revealed by ClxSiH stretching (2196 cm−1) and bending (775, 744 cm−1) vibrations. The presence of these modes indicates that there is incomplete decomposition of SiHCl3 upon adsorption at 200 K. GeHCl3 also dissociatively adsorbs at 200 K to form SiH and SiClx species. An infrared absorption feature in the Ge-H stretching region (1970–1995 cm−1) was not detected in the FTIR spectrum. The absence of a Ge-H absorption feature argues that there is a complete transfer of hydrogen from germanium to surface silicon atoms at 200 K. The thermal stabilities of the surface species were studied with annealing experiments. The Clx SiH formed upon initial SiHCl3 exposures at 200 K were observed to decompose between 200–590 K and form additional surface SiH and SiCl species. For both GeHCl3 and SiHCl3 dissociative adsorption on porous silicon, the SiCL. (x = 2 or 3) surface species were converted to silicon monochloride surface species between 200–600 K. In addition, SiH surface species were lost upon annealing between 680–780 K as H2 desorbed from the surface. The adsorption kinetics of SiHCl3 and GeHCl3 were also monitored on porous silicon at various isothermal temperatures. These experiments provide insight into the surface chemistry of chlorosilanes and chlorogermanes during CVD and atomic layer controlled growth.

Ion-activated interface adsorption of oxygen during silver deposition on silicon

Abstract The process of ion-activated oxygen adsorption on silicon has been investigated using an experimental concept with simultaneous deposition of silver films. Auger electron spectroscopy in combination with sputter depth profiling is subsequently performed to determine the amount of oxygen adsorbed at the AgSi interface. Noble gas ions ( 4 He + , 20 Ne + and 40 Ar + )with energies between 50 and 175 keV were used, and it was found that for substrate temperatures of 300–700 K the oxygen adsorption depends strongly on ion mass, ion energy and ion flux density. For flux densities of 5 × 10 11 cm −2 s −1 or less, adsorption dominates and, in particular, for light-ion bombardment the majority of adsorbed oxygen atoms form chemical bonds with the silicon surface atoms (SiO). However, for heavy ions, physical sputtering starts to compete and limits the effective rate of adsorption. At sufficiently high ion fluxes the adsorption starts to decrease, and for all ions and energies used in this work it is found that, if the electronic energy deposition density exceeds a critical value of about 1.2 × 10 21 eV cm −2 s −1 , dissociation of the SiO bonds prevails with a corresponding loss in the adsorbed oxygen quantity.

Thermal stability of the methyl group adsorbed on Si( 100): CH 3I surface chemistry

The adsorption and thermal behavior of methyl iodide on Si(100)-(2 × 1) have been studied by kinetic uptake measurements, Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) mass spectroscopy. Methyl iodide adsorbs on Si(100) at room temperature with high efficiency (initial reaction probability of unity). From the kinetic uptake measurements and the AES measurements an average saturation coverage of (2.9 ± 0.3) × 10 14 methyl iodide molecules cm 2 is calculated, i.e., about one methyl iodide molecule per two silicon surface atoms. Experimental evidence is presented indicating that the molecule dissociates into a covalently bonded methyl group and an iodine atom upon adsorption. Heating causes the decomposition of the methyl group. The hydrogen atoms liberated from the methyl group mainly desorb as molecular hydrogen. The adsorbed iodine desorbs in the form of atomic iodine and possibly some small amount of hydrogen iodide as detectable by line-of-sight mass spectrometry. Less then 1% of the carbon desorbs in the form of C 2 hydrocarbon species. Neither the desorption of methane nor the desorption of methyl iodide is observed.

Desorption product yields following Cl 2 adsorption on Si(111) 7 × 7: coverage and temperature dependence

Abstract : Chlorine plays an essential role during the plasma etching of silicon surfaces. The interaction between chlorine and silicon is also important in the epitaxial growth of silicon during chemical vapor deposition with SiCl4 + H2 or SiCl2H2(4-6). An understanding of the structure and stability of chlorine on silicon surfaces is essential for a complete description of the surface reaction steps that define plasma etching and epitaxial growth processes. Previous work on chlorine adsorption on silicon surfaces has focused on the nature of the chloride species obtained after various stages of chlorine adsorption. X-ray photoemission spectroscopy (XPS) studies have monitored the silicon oxidation state after a saturation chlorine exposure Si(111)7x7 at 300k. These xps investigations measure oxidation states that were consistent with the bonding one, two and three chlorine atoms to individual silicon surface atoms. The photoemission studies also demonstrated that monochloride species were present at low chlorine coverages and di- and trichloride species were formed at higher chlorine coverages. Only monochloride species were observed to remain on the Si(111) 7x7 surface after annealing to 673 K (7).

FTIR studies of H 2O and D 2O decomposition on porous silicon surfaces

The decomposition of H 2O and D 2O on silicon surfaces was studied using transmission Fourier-transform infrared (FTIR) spectroscopy. These FTIR studies were performed in situ in an ultrahigh vacuum chamber using high surface area porous silicon samples. The FTIR spectra revealed that H 2O (D 2O) initially dissociates upon adsorption at 300 K to form SiH (SiD) and SiOH (SiOD) surface species, i.e., H 2O → SiH + SiOH. The decomposition of these surface species was then monitored using the SiH (SiD) stretch at 2090 cm −1 (1513 cm −1), SiOH (SiOD) stretch at 3680 cm −1 (2707 cm −1) and the SiOSi stretch at 900–1100 cm −1. As the silicon surface was annealed to 650 K, the FTIR spectra revealed that the SiOH surface species progressively decomposed to SiOSi species and additional SiH species, i.e., SiOH → SiH + SiOSi. Above 650 K, the SiH surface species decreased concurrently with the desorption of H −1 from the porous silicon surface. New blue-shifted infrared features in the SiH stretching region were observed at 2119, 2176 and 2268 cm −1 after annealing above 600 K. Additional infrared studies of partially hydrogen-covered porous silicon surfaces exposed to O 2 suggested that these blue-shifted SiH stretching vibrations were associated with silicon surface atoms backbonded to one, two or three oxygen atoms, respectively.

Anisotropic Etching of Crystalline Silicon in Alkaline Solutions: I . Orientation Dependence and Behavior of Passivation Layers

The anisotropic etching behavior of single‐crystal silicon and the behavior of and in an ethylenediaminebased solution as well as in aqueous , , and were studied. The crystal planes bounding the etch front and their etch rates were determined as a function of temperature, crystal orientation, and etchant composition. A correlation was found between the etch rates and their activation energies, with slowly etching crystal surfaces exhibiting higher activation energies and vice versa. For highly concentrated solutions, a decrease of the etch rate with the fourth power of the water concentration was observed. Based on these results, an electrochemical model is proposed, describing the anisotropic etching behavior of silicon in all alkaline solutions. In an oxidation step, four hydroxide ions react with one surface silicon atom, leading to the injection of four electrons into the conduction band. These electrons stay localized near the crystal surface due to the presence of a space charge layer. The reaction is accompanied by the breaking of the backbonds, which requires the thermal excitation of the respective surface state electrons into the conduction band. This step is considered to be rate limiting. In a reduction step, the injected electrons react with water molecules to form new hydroxide ions and hydrogen. It is assumed that these hydroxide ions generated at the silicon surface are consumed in the oxidation reaction rather than those from the bulk electrolyte, since the latter are kept away from the crystal by the repellent force of the negative surface charge. According to this model, monosilicic acid is formed as the primary dissolution product in all anisotropic silicon etchants. The anisotropic behavior is due to small differences of the energy levels of the backbond surface states as a function of the crystal orientation.

A critical assessment of the overcoordination model for the P b center at the 〈111〉 Si/SiO 2 interface

In this paper we will review our recent calculations on two models for the P b center at the 〈111〉 Si/SiO 2 interface. The first model is the Poindexter model, the trivalent silicon atom. The second and more recently proposed model is one in which an oxygen atom that is part of the SiO 2 network interacts with a surface silicon atom to form a three-fold coordinated oxygen atom (and a four-fold silicon atom). We compare our theoretical results to experimental 29Si, and 17O hyperfine results, electrical levels as obtained from DLTS and C V measurements, and to recent high pressure DLTS results. In all cases, the Poindexter model is in excellent agreement, while the overcoordination model is in striking disagreement. The most recent experiment is the pressure-dependent DLTS. This imposes severe constraints on the sign and magnitude of interaction between a network oxygen atom and an otherwise trivalent silicon atom.