P.A. Taylor

Adsorption and thermal behavior of ethylene on Si(100)-(2 × 1)

The adsorption of ethylene on Si(100)-(2 × 1) has been studied in ultrahigh vacuum. Chemisorption was found to occur via a mobile precursor mechanism. The activation energy difference for desorption and chemisorption from the precursor (Ed − Er), was found to be 2.9 kcal mol−1. The saturation coverage of ethylene is 1 C2H4/Si2 dimer. Hydrogen-site blocking and thermochemical arguments suggest that C2H4 bonds as a di-σ surface complex to dimer sites; upon chemisorption of C2H4 the SiSi dimer is cleaved. Chemisorbed ethylene desorbs unimolecularlfrom Si(100) at ∼ 550 K, with approximately 2% of the monolayer undergoing dissociation. The activation energy of C2H4 desorption is 38 kcal mol−1, and for the di-σ C2H4Si2 complex, each SiC bond has a strength of ∼ 73 kcal mol−1. The low desorption activation energy allows C2H4 to desorb prior to signifi dissociation, preventing the formation of significant coverages of surfaces carbon and hydrogen.

The adsorption and decomposition of NH3 ON Si(100)-detection of the NH2(a) species

The dissociative adsorption of NH3 on Si(100)-(2 × 1) has been studied using accurate surface coverage measurements, temperature programmed desorption. Auger spectroscopy and digital ESDIAD/LEED methods. It has been found that NH2 surface species (amino species) are produced to a saturation coverage of 1 NH2/Si dimer at 120 K. This is accompanied by the production of a Si-H surface species. Digital ESDIAD measurements of the H+ angular distribution from NH2(a) species indicate that torsional oscillations about the Si-NH2 bond are responsible for the characteristic elliptical H+ pattern whose long axis is perpendicular to the Si-Si dimer bond direction. It has been shown that NH, dissociatively adsorbs with unity sticking probability at 120 K up to 86% of full coverage, indicative of a mobile precursor adsorption mechanism. The preadsorption of atomic H onto the Si dangling bond sites reduces the adsorptive capacity of the Si(100) surface, and 1 H/Si completely passivates the surface for NH3 chemisorption. The NH2(a) species, produced by adsorption at 120 K are stable up to about 600 K, where decomposition occurs to produce N(a) and H(a). A minor reaction channel involving NH2(a)+ H(a) to produce recombined NH3(g) is observed in the temperature range 600–700 K. Above 700 K, surface N(a), produced from NH2(a) decomposition, enters into the Si(100) lattice.

Direct determination of absolute monolayer coverages of chemisorbed C2H2 and C2H4 on Si(100)

A convenient kinetic uptake method has been employed to determine the absolute saturation monolayer coverage of C2H2 and C2H4 on a Si(100)(2×1) surface. Such measurements are important for postulating the structure of the chemisorbed hydrocarbon species on this surface. The saturation surface coverage for both chemisorbed molecules at 105 K is 2.5(±0.2)×1014 molecules/cm2 . This number is consistent with 1 hydrocarbon molecule per Si dimer site at monolayer coverage when the role of surface defects on Si(100) is considered. A di‐σ bonding model for both molecules is proposed at saturation coverage.

Hydrocarbon surface chemistry on Si(100)

Abstract The interaction of various hydrocarbon species with the Si(100) surface has been investigated using several surface science techniques. The efficiency of carbon deposition is related to the efficiency of SiC thin film formation. The hydrocarbon species studied include acetylene (C 2 H 2 ), ethylene (C 2 H 4 ), and the adsorbed methyl group (CH 3 (a)). In the case of the chemisorption of acetylene and ethylene, the π-bond ofthe olefinic molecules interacts with the dimer unit (Si 2 ) on the Si (100)−(2 × 1) surface. One monolayer of both acetylene and ethylene on Si(100) has been achieved by saturating the surface at 105 K, and a di-σ-bonding structure is proposed for one molecule per Si 2 dimer unit at monolayer coverage. Upon heating, the majority (> 95%) of the adsorbed acetylene undergoes dissociation to produce chemisorbed carbon and H 2 (g). In contrast, chemisorbed ethylene desorbs intact from Si(100) at ∼ 550 K, with approximately 2% of the monolayer undergoing dissociation. The low activation energy for desorption ( E ° d (C 2 H 4 )= 38 kcal mol -1 ) allows C 2 H 4 to desorb prior to significant decomposition. Investigations of the thermal behavior of CH 3 (a) on Si(100) show that the adsorbed methyl group is stable up to ∼600 K. At higher temperatures, CH 3 (a) decomposes to CH x (a) ( x 2 (g), leaving carbon on the surface. Less than 1% of the adsorbed carbon species (CH x , x ⩽ 3) desorbs in the form of C 2 hydrocarbon species upon heating. This indicates that the methyl group is an efficient source of surface carbon by thermal decomposition.

Chemical activity of the C=C double bond on silicon surfaces

Abstract The adsorption kinetics of propylene, propane and methane at 120 K on Si (100)−(2 × 1) are compared. Propane and methane have zero sticking probabilities at 120 K, while propylene reacts strongly with Si(100). The difference in sticking is shown in two ways. First, no kinetic uptake of propane and methane was observed during adsorption, while substantial uptake of propylene was observed. Second, measurements of the C(KLL)/Si(LVV) Auger peak-to-peak ratio before and after adsorption showed that no carbon was present on the surface after propane and methane exposures. It is shown therefore that the O=O double bond is an active molecular site for interaction with active sites on Si(100), whereas CH and CC single bonds are inactive at 120 K. This observation is of importance in models of chemical vapor deposition, plasma vapor deposition and reactive ion etching.

X‐ray photoelectron spectroscopy study of Si‐C film growth by chemical vapor deposition of ethylene on Si(100)

The growth of a thin film of SiC grown by chemical vapor deposition (CVD) of ethylene on Si(100) at 970 K was studied by x‐ray photoelectron spectroscopy (XPS). The growth of the film was observed through the behavior of the Si(2p) and C(1s) core levels and their plasmon losses. A 1.2‐eV (towards higher binding energy) shift is observed for the Si(2p) binding energy between silicon in Si(100) and silicon in SiC. The plasmon loss energies measured as a function of film thickness below the C(1s) emission indicate that the C/Si ratio of the Si‐C film throughout the CVD process is fairly constant.

Si-F bond directions on Si(100) — A study by ESDIAD

Abstract We report the first observation of ion angular distributions originating from electron stimulated desorption of an adsorbed atomic species on a semiconductor surface (ESDIAD). F+ is emitted from Si(100)-(2×1) along 4 azimuths corresponding to the principal crystal axes. The most probable F+ energy is 2.4 eV. The F+ emission angle, α ≈ 36° ± 5° to the surface normal, corresponds closely to the SiF surface bond direction. This F+ angular distribution is consistent with F bonding to Si dimers which are in two orthogonal reconstruction on Si(100)-(2×1). The threshold electron energy, VcT = 27.5 ± 1 eV for F+ production from the SiF surface species.

The dissociative adsorption of ammonia on Si(100)

Abstract The nature of the NH 3 adsorption process on Si(100) at 120 K was studied by isotopic mixing with adsorbed atomic deuterium and thermal desorption spectroscopy. NH 3 was found to dissociatively adsorb onto Si(100) dimer sites at 120 K as NH 2 (a) and H(a). The NH 2 (a) species persist up to about 700 K where two reaction channels become available. The major reaction channel leads to the decomposition of NH 2 (a) to N(a) and H(a). The minor channel is a recombination reaction that leads to the desorption of ammonia. This recombination reaction exhibits a deuterium kinetic isotope effect.

An ESDIAD study of chemisorbed hydrogen on clean and H-exposed Si(111)-(7 × 7)

Abstract : The chemisorption of H on Silicon(111)-(7x7) has been studied by digital ESDIAD and temperature programmed desorption methods. It has been found that residual Hydrogen in the bulk of the Si(111) can be transported to the surface upon annealing to temperatures above approx. 1000 K. The adsorption of atomic H on Si(111)-(7x7) results in a mixture of monohydride and polyhydride species as detected by H(+)ESDIAD. Thermal desorption from the H-saturated surface liberates (Beta 3(-), Beta 2(-) + B1 - H2) species as well as SiH4(g). Heating the H-saturated surface to 1040 K results in a significant disordering of the surface, leading to Si sites which produce highly tilted Si-h bond directions. The occupation of these sites with H produces surface species exhibiting high polar angles from the surface normal for H(+) desorption by an ESD process with a high ionic cross section compared to the cross section observed for normal mono- and polyhydride surface species. (JG)

Alkyl radical involvement in silicon surface chemistry

Abstract The reaction of Si(100)-(2 × 1) with C 3 H 6 has been observed to increase with the addition of atomic hydrogen to the C 3 H 6 overlayer. The enhancement in reactivity is postulated to originate from a free radical process involving the creation of the chemisorbed propyl species. The propyl species is bound to free valencies on the Si(100). The increase in reactivity is shown in two ways. First, the thermal desorption yield of C 3 H 6 decreases with increasing exposures of atomic hydrogen to the C 3 H 6 -covered Si(100). Second, measurements of the C(KLL) Si(LVV) Auger peak-to-peak ratio before and after thermal desorption show that more carbon remains on the surface after C 3 H 6 (ads) interaction with H. The ability to control the reaction of a hydrocarbon molecule with a semiconductor surface has several implications for processes of chemical vapor deposition (CVD), plasma vapor deposition (PVD), and reactive ion etching (RIE).