SiClx Species Research Articles

Encapsulation of epitaxial silicene on ZrB2 with NaCl.

Silicene and other two-dimensional materials, such as germanene and stanene, have chemically reactive surfaces and are prone to oxidation in air, and thus require an encapsulation layer for ex situ studies or integration in an electronic device. In this work, we investigated NaCl as an encapsulation material for silicene. NaCl was deposited on the surface of epitaxial silicene on ZrB2(0001) thin films near room temperature and studied using synchrotron-based high-resolution photoelectron spectroscopy. The deposition of NaCl resulted in dissociative chemisorption, where the majority of epitaxial silicene reacted to form Si-Clx species.

Etching mechanisms of thin SiO2 exposed to Cl2 plasma

Plasma etching is the most standard patterning technology used in micro- and nano-technologies. Chlorine-based plasmas are often used for silicon etching. However, the behavior of thin silicon oxide exposed to such a plasma is still not fully understood. In this paper, we investigate how a thin silicon oxide layer on silicon behaves when it is exposed to a Cl2 plasma. The authors show that chlorine atoms diffuse and/or Cl+ ions are implanted through the thin (<2.5 nm) oxide, leading to the formation of a SiClx interface layer between the two layers of Si and SiO2. Chlorine accumulates at the interface until the SiO2 is thin enough to release volatile SiClx species and the silicon begins to be etched.

Molecular dynamics simulations of Cl+ etching on a Si(100) surface

Molecular dynamics simulations using improved Tersoff–Brenner potential parameters were performed to investigate Cl+ etching of a {2×1} reconstructed Si(100) surface. Steady-state Si etching accompanying the Cl coverage of the surface is observed. Furthermore, a steady-state chlorinated reaction layer is formed. The thickness of this reaction layer is found to increase with increasing energy. The stoichiometry of SiClx species in the reaction layer is found to be SiCl:SiCl2:SiCl3=1.0:0.14:0.008 at 50 eV. These results are in excellent agreement with available experimental data. While elemental Si products are created by physical sputtering, most SiClx (0<x<4) etch products are produced by chemical-enhanced physical sputtering.

Neutral species in inductively coupled SF6/SiCl4 plasmas

Inductively coupled SF6/SiCl4 plasmas interacting with a bulk silicon substrate and a SiO2-coated substrate have been investigated. Mass spectrometry and optical emission spectroscopy diagnostics were used to characterize the neutral population in the diffusion chamber. SiF4 molecules were detected as the dominant species, and their formation has been attributed to the high reactivity of F radicals with SiClx species. In a complementary experiment, a silicon chloride layer was deposited on the reactor walls during a SiCl4 plasma step and subsequently etched by a SF6 plasma. Time-resolved measurements of the neutral densities during the SF6 plasma step showed the importance of heterogeneous reactions between impinging F radicals and SiClx species deposited on the reactor walls. In SF6/SiCl4 plasmas, these reactions lead to a depletion in F radicals, which results in a decrease in the silicon substrate etch rate. Furthermore, this impacts on the concentration of SFx species and on the creation of new species, such as ClF, SF5Cl and S2Cl2.

Calculated energetics for adsorption and desorption steps during etching of Si(110) surface by Cl

Energetics and the configurational aspects related to the adsorption of Cl on the (110) index plane of Si and the subsequent desorption of SiClx species from the surface (leading to etching) were investigated. Calculations were conducted for varying surface Cl concentrations. First and second chlorination steps for surface Si atoms were analyzed and the role played by surface vacancies was investigated. On the Si(110) surface, steric effects coming from repulsive forces among the adsorbed Cl atoms, were found to be quite significant. Results indicate that the second chlorination step which leads to the formation of attached SiCl2 species, is very important in the overall eching process.

Energetics for bonding and detachment steps in etching of Si by Cl

Simulation calculations were carried out to investigate the energetics of adsorption and desorption processes taking place during etching of the Si(100) surface by Cl atoms. Clean and fully chlorinated surfaces with step edges were taken into consideration. Attachment energies of Cl atoms at step edge sites and subsequent detachment (etching) energies of SiClx species were calculated. Configurational characteristics of Cl bonded regions were also investigated. Results provide a systematic description of Cl adsorption and etching processes.

Mechanism of RF plasma induced fragmentation of SiCl4 and surface functionalization of polymeric substrates from SiClx species

The generation of extremely reactive species of silylium (silicenium) ions under plasma conditions opens up new ways for surface modification, even of the most inert polymeric surfaces. To control the plasma-induced chemistry and to tailor new molecular surface architecture, the mechanism of formation of the active species of the discharge must be understood. In this work the plasma-induced molecular fragmentation of SiCl4 was studied and the surface functionalization of cellulose paper (CP), polyester (PET), polypropylene (PP), and polytetrafluoroethylene (PTFE) substrates with silicon atom based active species was evaluated. Gas chromatograph mass spectroscopy (GC-MS) and low Energy Electron MS (LEEMS) measurements carried out both on SiCl4 and on the molecular mixture resulting from the recombination of plasma generated species indicate that the most predominant fragments are SiCl3+ cations accompanied by SiCl2+ and SiCl+ species. Survey and high-resolution electron spectroscopy for chemical analysis (ESCA) data collected from plasma functionalized surfaces show significant silicon and oxygen atom content regardless of the nature of the substrates. © 1996 John Wiley & Sons, Inc.

Mechanism of RF plasma induced fragmentation of SiCl4 and surface functionalization of polymeric substrates from SiClx species

The generation of extremely reactive species of silylium (silicenium) ions under plasma conditions opens up new ways for surface modification, even of the most inert polymeric surfaces. To control the plasma-induced chemistry and to tailor new molecular surface architecture, the mechanism of formation of the active species of the discharge must be understood. In this work the plasma-induced molecular fragmentation of SiCl4 was studied and the surface functionalization of cellulose paper (CP), polyester (PET), polypropylene (PP), and polytetrafluoroethylene (PTFE) substrates with silicon atom based active species was evaluated. Gas chromatograph mass spectroscopy (GC-MS) and low Energy Electron MS (LEEMS) measurements carried out both on SiCl4 and on the molecular mixture resulting from the recombination of plasma generated species indicate that the most predominant fragments are SiCl3+ cations accompanied by SiCl2+ and SiCl+ species. Survey and high-resolution electron spectroscopy for chemical analysis (ESCA) data collected from plasma functionalized surfaces show significant silicon and oxygen atom content regardless of the nature of the substrates. © 1996 John Wiley & Sons, Inc.

Semiempirical calculations of the chemisorption of chlorine on the Si(111)7*7 surface

The chemisorption of chlorine on the Si(111)7*7 surface has been studied using clusters and the well known AM1 semiempirical molecular orbital method. Both the equilibrium structures corresponding to n Cl atoms (1<or=n<or=7) chemisorbing onto the adatom and rest atom positions of the Si(111)7*7 reconstructed surface, and the etching energies for an adatom or rest atom bonded to x Cl atoms (0<or=x<or=4), have been obtained. We find that as successively more chlorine is chemisorbed, an adatom moves from its original threefold site (one chlorine) to an adjacent bridge site (two and three chlorines), and then to a neighbouring on top site (four or more chlorines). A rest atom bonded to up to four chlorine atoms, on the other hand, remains dose to its threefold clean surface position. Our results also predict, in agreement with experiment, that all of the SiClx species (1<or=x<or=4) will occur on an Si(111)7*7 surface exposed to atomic chlorine at low temperature. Increasing the temperature will first result in all of the SiCl3 and SiCl4, and most of the SiCl2, desorbing from the surface. This will be followed by the desorption of the remaining SiCl2, leaving only SiCl on the surface. The final stage of the desorption process involves the removal of the SiCl. Whilst the etching energies appropriate to a rest atom site are generally lower than the corresponding adatom values, the minimum etching energy is found to correspond to the desorption of an SiCl4 complex involving an adatom of the Si(111)7*7 reconstructed surface.

Adsorption and decomposition of trichlorosilane and trichlorogermane on porous silicon and Si(100)2×1 surfaces

The adsorption and decomposition of trichlorosilane (SiHCl3) and trichlorogermane (GeHCl3) on silicon surfaces were studied using Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption investigations. The FTIR transmission spectroscopy experiments were performed in situ in an ultrahigh vacuum chamber on high surface area porous silicon samples. The FTIR spectra revealed that SiHCl3 dissociatively adsorbed at 200 K and formed SiH, SiClx, ClSiH, and Cl2SiH surface species. The presence of ClxSiH species indicated incomplete decomposition of SiHCl3 upon adsorption at 200 K. GeHCl3 also dissociatively adsorbed at 200 K and formed SiH and SiClx species. The absence of an infrared absorption in the Ge–H stretching region suggests complete transfer of hydrogen from Ge to surface Si atoms at 200 K. The thermal stabilities of the surface species were then studied with annealing experiments. The ClxSiH species deposited by SiHCl3 exposure at 200 K decomposed between 200 and 590 K and formed additional SiH and SiCl species. The SiClx (x=2 or 3) species deposited by either GeHCl3 or SiHCl3 dissociative adsorption converted to silicon monochloride species between 200 and 600 K. The SiH surface species subsequently decreased at temperatures between 680 and 780 K as H2 desorbed from the silicon surface. FTIR spectroscopy was also utilized to monitor the adsorption of SiHCl3 and GeHCl3 on porous silicon at different surface temperatures. These adsorption studies were in agreement with the thermal annealing experiments. Temperature programmed desorption (TPD) studies were performed following saturation SiHCl3 and GeHCl3 exposures on Si(100)2×1. After SiHCl3 adsorption, the only desorption species were H2, HCl, and SiCl2, and they were observed at 810, 850, and 970 K, respectively. Following GeHCl3 adsorption, the TPD experiments monitored H2, HCl, SiCl2, and Ge at 810, 850, 950, and 1200 K, respectively. The adsorption kinetics of SiHCl3 and GeHCl3 were also measured on Si(100)2×1 and initial reactive sticking coefficients of S0≊.019 for SiHCl3 and S0≊1 for GeHCl3 were determined at 500 K. These experiments provide insight into the surface chemistry of chlorosilanes and chlorogermanes during Si, Ge, and Si1−xGex chemical vapor deposition on silicon surfaces.

Scanning tunneling microscopy study of the reaction of AlCl3 with the Si(111) surface

The initial stage of the reaction of aluminum trichloride (AlCl3) with the Si(111)-(7×7) surface and annealing effects of the adsorbed surface are investigated with a scanning tunneling microscope. Reacted and unreacted sites manifest in the contrast of adatom sites on the AlCl3-exposed surface. An AlCl3 molecule dissociatively adsorbs onto the Si(111)-(7×7) surface at room temperature, which yields chlorine atoms. Preferential adsorption site is found to be the center adatom site rather than the corner adatom site, and the chlorine atom adsorbs onto the top site of the center Si adatom. The migration of adsorbed molecules around the rest atom in the 7×7 unit cell is observed at room temperature. The Si–Clx species desorb from the surface by thermal annealing at 600 °C, leaving vacancies behind, after annealing at 1200 °C. Aluminum deposition occurs in a limited area and the surface shows the √7×√7 reconstruction.

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.

Mechanism of silicon film deposition in the RF plasma reduction of silicon tetrachloride

Plasma-chemical reduction of SiCl4 in mixtures with H2 and Ar has been studied by optical emission spectroscopy (OES) and laser interferometry techniques. It has been found that the Ar:H2 ratio strongly affects the plasma composition as well as the deposition (rD) and etch (rE) rates of Si: H, Cl films and that the electron impact dissociation is the most important channel for the production of SiClx species, which are the precursors of the film growth. Chemisorption of SiClx and the reactive surface reaction SiClx+H→−SiCl(x−1)0+HCl are important steps in the deposition process. The suggested deposition model givesrD ∞ [SiClx][H], in agreement with the experimental data. Etching of Si: H, Cl films occurs at high Ar: H2 ratio when Cl atoms in the gas phase become appreciable and increases with increasing Cl concentration. The etch rate is controlled by the Cl atom chemisorption step.