Room Temperature Adsorption Research Articles

Electron energy loss studies of room-temperature co adsorption on Cu(100) induced by electron or ion irradiation

A custom-built multi-technique portable spectrometer was used to study the vibrational (and electronic) excitations associated with the “anomalous” CO adsorption recently observed on clean and oxidized Cu(100) surfaces at room temperature. Results from the electron energy loss (and thermal desorption) experiments have provided strong evidence for in-situ CO production induced by low-energy electron or ion irradiation of CO or C 2H 4 on Cu(100). In particular, the C-O vibrational stretch frequency for the room-temperature CO adsorption was found to be red-shifted by 9 meV from its nominal position (259 meV) and could be observed even at temperature as high as 420 K. Several plausible mechanisms involving coadsorbate interactions with CO on metal surfaces have been discussed. A direct interaction model involving partial bonding between CO, adsorbed on an atop site, and a coadsorbed O atom in a four-fold hollow site was found to be adequate in explaining the observed red shift and the apparent stabilization of CO on Cu(100) at room temperature.

Adsorption and decomposition of diethylsilane and diethylgermane on Si(100): Surface reactions for an atomic layer epitaxial approach to column IV epitaxy

The room-temperature adsorption and desorption kinetics of diethylsilane (DES) and diethylgermane (DEG) on the Si(100)-(2×1) surface were investigated under ultrahigh vacuum using temperature programmed desorption, high resolution electron energy loss spectroscopy, and Auger electron spectroscopy. DES and DEG adsorb at room temperature in a self-limiting fashion, reaching saturation (0.4 and 0.3 monolayers, respectively), at exposures above 30 and 350 L, respectively. Temperature programmed desorption of the DES-saturated and DEG-saturated surfaces revealed only two species, hydrogen and ethylene, desorbing from either surface. In both systems, the hydrogen atoms desorbed primarily through the recombinative desorption of monohydride species, while the ethyl groups decomposed via β-hydride elimination and subsequently desorbed as ethylene. The hydrogen desorption peak temperature was 794 K for the DES-saturated surface and 788 K for the DEG-saturated surface. The desorption peak temperature for ethylene was significantly lower in the DEG/Si(100) system (700 K) than in the DES/Si(100) system (730 K) because of a lower activation energy and higher pre-exponential factor for β-hydride elimination from DEG-dosed Si(100). High resolution electron energy loss spectra of the DEG-saturated surface support an adsorption mechanism in which the ethyl groups remain bonded to the incoming germanium atom throughout the adsorption process.

Direct structural information from X-ray photoelectron diffraction: intermixing and on-surface adsorption of Na on Al surfaces

We have investigated the local structures of the room-temperature Na adsorption phases on Al(001) and (111) surfaces with X-ray photoelectron diffraction. Angular distributions of X-ray excited Na 1s photoelectrons and Na KVV Auger electrons emitted from c(2 × 2)-Na/Al(001), (√3 × √3)R30°-Na/Al(111) and p(2 × 2)-Na/Al(111) have been measured over the full solid angle above the Al surfaces. We show that angle-scanned photoelectron diffraction, in contrast to other structural techniques, gives very direct information on the degree of intermixing at the surface. Many structural models can be ruled out directly from the experimental results, and for the case of intermixing the interface structure can be determined by simple geometrical considerations.

Adsorption-state-specific C 1s core-level chemical shifts of thiophene on Si(111)7 x 7 by photoemission spectroscopy at 340 eV.

The room-temperature adsorption of thiophene on Si(111)7\ifmmode\times\else\texttimes\fi{}7 has been investigated using photoemission with monochromatic synchrotron radiation at 340-eV photon energy. The presence of two C 1s photopeaks at room temperature is consistent with the observation of two molecular-desorption states and with an earlier proposed adsorption model of thiophene on Si(111)7\ifmmode\times\else\texttimes\fi{}7 (that involves a surface-parallel \ensuremath{\pi}-bonded geometry and a \ensuremath{\sigma}-bonded geometry through the sulfur atom). The C 1s photoemission spectra have also been collected after annealing the sample to different temperatures. Although the observed core-level chemical shifts and the temperature dependence of the two photopeaks are consistent with the thermal-desorption data in the framework of a simple charge-transfer model, correlation between the photopeaks and thermal-desorption data cannot be used to unambiguously identify the nature of the proposed adsorption structures for the adsorption states. Some of the potential difficulties involved in this type of common practice will be discussed.

Atomically resolved scanning tunneling microscopy study of the adsorption and dissociation of methylchloride on Si(001)

The room-temperature adsorption and dissociation of methylchloride (CH3Cl) on Si(001) using scanning tunneling microscopy has been studied. Fragments are identified by the symmetry of their binding on the underlying Si(001) lattice. Methylchloride adsorbs dissociatively at room temperature, yielding a methyl group and a chlorine atom bound to the surface. Chlorine atoms bind in a variety of geometries, the most common being two chlorine atoms bonded to a single silicon dimer (silicon monochloride dimers). When the surface is heated to 150 °C, all Cl rearranges to this monochloride-type bonding configuration, and images reveal the beginnings of monochloride island formation. The Cl:CH3 ratio on the surface is found to be approximately 2:1, implying that not all methyl groups are bound to the surface in the reaction.

Toluene interactions with the 7 × 7 and sputtered Si(111) surfaces: evidence of methyl-induced and oxygen-enhanced dissociation pathways

The effects of methyl substitution on the room-temperature (RT) adsorption and reactivity of toluene and benzene on Si(111)7 × 7 have been investigated by thermal desorption spectrometry (TDS). In particular, the mass 100 (molecular) desorption profile for the low RT exposure of d 8-toluene to Si(111)7 × 7 consists of desorption peaks at 370 and 420 K, with the intensity of the peak at 420 K saturating at a lower exposure than the peak at 370 K. The similarities in the exposure dependence of the TDS profiles of toluene and those of benzene suggest a similar adsorption configuration, i.e. a π-bonded configuration with the ring (near-)parallel to the surface. In addition to the strong molecular desorption, the presence of a relatively weak mass 4 desorption peak at 800 K at higher exposure indicates a weak secondary surface process involving the deposition of D atoms on Si(111)7 × 7 as SiD. Furthermore, the observation of a similar mass 4 desorption peak at 800 K for the RT exposures of other selectively deuterated toluene isotopes CH 3C 6D 5 and CD 3C 6H 5 to Si(111)7 × 7 suggests that there is no exclusive D (H) abstraction at either the aromatic or aliphatic position of the adsorbed olefin molecule. The lack of a corresponding mass 4 desorption profile (at 800 K) for the RT exposure of d 6-benzene to Si(111)7 × 7 indicates that the deposition of D atoms on the Si surface originates from the complete dissociation of toluene into C and D atoms, which is likely induced by a methyl-to-surface interaction. Moreover, the overall increase in the mass 4 desorption for the RT exposure of d 8-toluene to a sputtered Si surface shows that the sputtered Si surface is more reactive than the 7 × 7 surface. The presence of a new broad mass 4 desorption peak at 540 K suggests an additional pathway by which D 2 molecules evolve directly from the dissociation of adsorbed toluene molecules on the sputtered Si surface. In addition, a RT O 2 post-exposure to d 8-toluene adsorbed on Si(111)7 × 7 is found to induce a large increase in the D 2 (mass 4) desorption at 800 K. The increase in the toluene dissociation can be attributed to either a surface-enhanced oxidation or an O 2-induced effect that stabilizes the adsorbed toluene molecules to a higher temperature (>; 350 K) where dehydrogenation and dissociation occur. It is, however, difficult to determine the mechanism for the enhanced dissociation from TDS alone.

Site selective SiH 4 adsorption on Si(111)(7 × 7) surfaces

Scanning tunneling microscopy (STM) was used to investigate room temperature adsorption and dissociation of SiH 4 on Si(111)(7 × 7) surfaces. The data show a pronounced site selectivity for this process. Initially the reaction involves exclusively the corner holes and the adjacent Si adatoms of the (7 × 7) reconstruction, with preferential adsorption of SiH 3 groups in the corner holes and of H atoms on one of the adjacent corner adatoms. For higher SiH 4 exposures the reactivity of the corner adatoms is significantly reduced, hydrogen adsorption occurs preferentially on the center adatoms. Deposited SiH x groups ( x = 2, 3) nucleate now in small clusters on the terraces. A higher density of these SiH x clusters on domain boundaries or at steps indicates a higher reactivity of these defect sites.

Thermal surface chemistry of l-, 3-, and 4-methyl-cyclohexene on Si( 111)7 × 7: dehydrogenation mechanism and the effects of methyl substitution

Recent studies of 6-membered cyclic olefins on Si(111)7 × 7 have revealed a surprisingly high level of reactivity as demonstrated by the catalytic dehydrogenation reaction of cyclohexene to benzene. The present work examines the room-temperature adsorption of 1-, 3-, and 4-methyl-cyclohexene on clean, sputtered and oxidized Si(111)7 × 7 surfaces by thermal desorption spectrometry. All three methyl-cyclohexene isomers are found to undergo dehydrogenation reactions to methyl-cyclohexadiene and toluene predominantly at 500–600 K. The dehydrogenated products appear to depend on the relative location of the methyl group with respect to the CC bond in the cyclic olefin. The reactivities of the three isomers are compared to those found for cyclohexene and 1-methyl-1,4-cyclohexadiene. Furthermore, the mass 78 desorption peak observed at ∼ 650 K is attributed to a methyl-elimination reaction that produces benzene and/or cyclohexadiene. A large post-exposure of H atoms prior to the thermal desorption of methyl-cyclohexene (cyclohexene) adsorbed on Si(111)7 × 7 at room temperature is found to inhibit toluene (benzene) evolution and concomitantly enhance molecular desorption. However, no significant reduction in methyl-cyclohexadiene (cyclohexadiene) evolution is found with the H post-exposure. Based on our present and previously reported results, we propose a primary dehydrogenation pathway that involves repositioning of the adsorbed olefin to sequentially interact with new Si (dangling bond and/or back bond) active sites by diffusion to other dangling-bond site and/or rotation about the adsorption site. The apparent ineffectiveness of the H post-exposure to inhibit methyl-cyclohexadiene (cyclohexadiene) evolution suggests a second dehydrogenation pathway that involves a stationary olefin interacting with two neighbouring Si back bonds simultaneously. Finally, the adsorption and dehydrogenation efficiency of methyl-cyclohexene are both found to be less than that of cyclohexene and appear to depend on a steric hindrance effect due to the methyl group and a methyl-to-surface interaction that reduces the mobility of the olefin on the Si surface, respectively.

Interactions between alkali metals and oxygen on a reconstructed surface: An STM study of oxygen adsorption on the alkali-metal-covered Cu(110) surface

Room-temperature adsorption of oxygen on potassium- and cesium-precovered Cu(110) surfaces was studied by scanning tunneling microscopy. Depending on the alkali-metal precoverage, two different scenarios exist for the structural evolution of the surfaces. For alkali-metal coverages ${\mathrm{\ensuremath{\theta}}}_{\mathrm{alk}}$\ensuremath{\le}0.13 ML [${\mathrm{\ensuremath{\theta}}}_{\mathrm{alk}}$=0.13 corresponds to the (1\ifmmode\times\else\texttimes\fi{}3) missing-row reconstructed Cu(110) surface], oxygen adsorption leads first to a transient contraction of the missing rows into islands of a (1\ifmmode\times\else\texttimes\fi{}2) structure. After longer exposures it causes the local removal of the alkali-metal-induced reconstruction, and the (2\ifmmode\times\else\texttimes\fi{}1) Cu-O ``added-row'' structure with ${\mathrm{\ensuremath{\theta}}}_{\mathrm{O}}$=0.5 is formed. In this structure the alkali-metal atoms are incorporated in the Cu-O chains. For higher alkali-metal precoverages, in the range of the (1\ifmmode\times\else\texttimes\fi{}2) reconstruction (${\mathrm{\ensuremath{\theta}}}_{\mathrm{alk}}$\ensuremath{\approxeq}0.2), more than one-half a monolayer of oxygen can be incorporated into the (1\ifmmode\times\else\texttimes\fi{}2) phase with only a minor structural effect before, at higher oxygen coverages, complex oxygen--alkali-metal--Cu structures with oxygen coverages well above 0.5 ML are formed. The saturation oxygen coverage is drastically enhanced beyond ${\mathrm{\ensuremath{\theta}}}_{\mathrm{O}}$=0.5, the quasisaturation value of the clean surface. Based on mass-transport arguments the substrate is reconstructed for all ratios of oxygen and alkali metal investigated here. Hence, adsorbate-substrate interactions are essential for these structures; they are not dominated by interactions between alkali metals and oxygen, i.e., by adsorbate-adsorbate interactions.

Room-temperature water adsorption on the Si(100) surface examined by UPS, XPS, and static SIMS

The adsorption of water on the reconstructed Si(100)-(2 x 1) surface at room temperature is studied using an integrated multi-technique approach. UPS, XPS, and static SIMS surface examination in a variable temperature mode of experimentation indicates dissociative adsorption of water to hydroxyl (OH) and hydride (H) groups saturating the single dangling bonds on adjacent dimer-pair Si atoms of the reconstructed surface. No evidence of molecular water is found. Upon heating the surface the hydroxyl decomposes to bridge-bonded oxygen (Si-O-Si) and additional silicon hydride groups. This is complete at ≈400°C. Above 400°C the surface loses hydrogen, presumably as H 2, and above 600°C the substoichiometric silicon oxide sublimes as SiO to return to the clean, reconstructed surface. The chemical state of oxygen on the surface is monitored as a function of temperature through the use of a Δ parameter (Δ = binding energy O(1s) - binding energy Si(2p)). These results are consistent with experimental evidence and surface chemistry first proposed by others. Assignment of the observed DOS features is made by comparison of the experimental DOS spectra to calculations for the system and for system analogs (SiOH radical, oxygen-saturated Si surface, hydrogen-saturated Si surface), and to another hydroxyl system (NaOH). Four major features of the saturated hydroxyl/hydride surface are found : 5 eV Si-H bond, Sisp 3 + H1s character; 6.4 eV O 2p lone pair, π symmetry nonbonding orbital; 7.6 eV O-H bond, O 2p + H1s character, σ symmetry; 11.9 eV Si-OH bond, sp 3 + O 2p character (binding energies relative to the Fermi level).

Local structure of c(2 x 2)-Na on Al(001): Experimental evidence for the coexistence of intermixing and on-surface adsorption.

Angular distributions of x-ray-excited Na 1s photoelectrons and Na KVV Auger electrons emitted from c(2\ifmmode\times\else\texttimes\fi{}2)-Na/Al(001) have been measured over the full 2\ensuremath{\pi} solid angle above the Al(001) surface. A detailed study involving single-scattering cluster calculations and an R-factor analysis has been performed to determine the room-temperature adsorption site of Na on Al(001). The best agreement with experiment is obtained for a configuration in which domains of Na atoms adsorbed in the hollow site and in the substitutional site coexist on the surface. This unusual and unexpected behavior has not been observed for an alkali-metal-on-metal adsorption system up to now, but it can be understood on the basis of very recent results from first-principles density-functional theory [C. Stampfl, J. Neugebauer, and M. Scheffler, Surf. Rev. Lett. 1, 222 (1994)].

The kinetics of O 2 dissociative chemisorption on Fe(110)

The dissociative chemisorption of O 2 on Fe(110) has been studied using molecular beam techniques. Dissociative chemisorption is facile, with an initial sticking coefficient of ~ 0.3 for a translational energy of 50 meV, incident at a room temperature sample. The sticking probability increases with translational energy and above 150 meV dissociation is a direct, activated process. Measurements of the angular and surface temperature dependence of dissociative chemisorption indicate that dissociation at lower energies is hindered by translational energy perpendicular to the surface, consistent with precursor trapping into a physisorption state prior to dissociative chemisorption. As the surface temperature is decreased the initial sticking probability increases slightly, with a complicated coverage dependence and a greatly increased dissociation probability at higher O coverages. Coincidentally, the saturation uptake of o 2 increases from just over a monolayer near room temperature to nearly 4 monolayers O at 160 K. For room temperature adsorption the sticking probability shows a minimum at 0.5 monolayer coverage of O, with a slight increase towards Θ = 1 monolayer, prior to saturation at high exposures. The sticking coefficient at Θ = 0.5 is sensitive to surface temperature and is associated with the formation of a relatively stable oxide layer. An abrupt change in the S(Θ) dependence is observed both at the completion of the ordered c(2 × 2) phase and at 0.5 monolayer for the activated dissociation channel.

Initial sticking coefficient of O2 on Ag(110)

We investigated the dynamics of the adsorption of O2 on Ag(110) with the molecular beam technique combined with EEL spectroscopy and with the method of King and Wells. The initial sticking coefficient S0 is reported for molecules impinging along both high symmetry azimuthal directions 〈001〉 and 〈11̄0〉 as a function of total energy and angle of incidence of the molecules and crystal temperature. The initial sticking coefficient is anisotropic for low as well as for room temperature adsorption. The dependence of the sticking coefficient on the temperature of the sample, Ts, indicates that adsorption takes place directly into the molecular well. Dissociation is eventually induced thermally for Ts≳150 K. The physisorbed state plays no role in the chemisorption process in the investigated impact energy range.

Adsorbate-induced surface stress: CO on Ni(100) and Ni(111)

We have determined the surface stress induced by the room-temperature adsorption of carbon monoxide on the (100) and (111) surfaces of nickel by measuring the bending of a thin nickel crystal. While CO adsorption induces exclusively a compressive stress on Ni(111), the stress is tensile on Ni(100) at CO coverages below 0.2 ML. The tensile stress passes through a maximum of + 0.96 N/m at θ = 0.09 to become compressive for θ > 0.25 with a value of −0.54 N/m at θ = 0.5. The latter value is small compared with the stress produced by other, more strongly chemisorbed adsorbates on the same surface. As CO molecules predominantly occupy terminal binding sites on Ni(100) at 300 K, we must exclude a site conversion process in order to explain the stress-coverage dependence. The sign reversal of surface stress might be attributed to a coverage dependent variation in the net charge transfer between the metal surface and the adsorbate, involving an enhanced splitting of the bonding and antibonding 2π ∗ CO orbitals of neighboring molecules at higher adsorbate coverages.

Vibrational and structural studies of oxygen adsorption on the Ag(110) surface

Adsorption of oxygen on the Ag(110)1×1 surface is studied by field ion-scanning tunneling microscopy (FI-STM) and high resolution electron energy loss spectroscopy (HREELS). Linear chain (added-row) formation in the [001] direction upon oxygen adsorption is observed. The step movement and step bunching are observed at the exposure lower than 60 L. Another type of chain formation are observed between the added rows with the local 3×1 configuration. The coverage and time-dependent HREELS data shows rather complicated dynamics in the case of room temperature adsorption. The HREELS data with the resolution of 4 meV (FWHM) shows an extra broad, weak peak at 15 meV, which has a maximum peak value at the exposure near 1800 L corresponding to the completion of the 3×1 phase.

Overlayer ordering induced by carbon monoxide adsorption on potassium pre-covered Ni(100)

Room temperature adsorption of CO on K pre-covered Ni(100) surfaces causes a strong ordering effect, as observed using low energy electron diffraction. This suggests formation of crystalline K/CO overlayers, even at very low K and CO coverages. The results are compared with recent models of the coadsorption process.

The comparative reactivity of various organic adsorbates with Cu(II) in CuH-SAPO-5 and CuH-SAPO-11 molecular sieves studied by electron spin resonance

The adsorption rates of various organic adsorbates on CuH-SAPO-5 and CuH-SAPO-11 molecular sieves have been studied from changes in the electron spin resonance (e.s.r.) spectra of Cu(II). The adsorption rate for hydrazine is much faster than for ethylene, and hydrazine coordinates to Cu(II) through its π-bond. The room-temperature adsorption of aromatic compounds (benzene; o-, m-, and p-xylene; toluene; ethylbenzene; and aniline), saturated nonpolar hydrocarbons ( n-hexane, cyclohexane, methylcyclohexane, 1,2-dimethylcyclohexane, and 1,3-dimethylcyclohexane), and polar cyclohexylamine on dehydrated CuH-SAPO-5 and CuH-SAPO-11, heat-treated at 65–200°C, is evaluated. With increased pretreatment temperature, the original Cu(II) e.s.r. intensity ( I A ) decreases and the intensity of a sharp singlet at g = 2.003 ( I B ), associated with adsorbate decomposition, increases. From the variation of the I A I B ratio as a function of pretreatment temperature, the relative reactivity of Cu(II) in CuH-SAPO-5 and CuH-SAPO-11 toward the various organic adsorbates is determined. It is of particular interest that the order of reactivity differs between the SAPO-5 material with a 12-ring channel and the SAPO-11 material with a 10-ring channel.

Contrasted behavior of Si(001) and Si(111) surfaces with respect to NH 3 adsorption and thermal nitridation: a N 1s and Si 2p core level study with synchrotron radiation

The role of surface structure in controlling NH 3 surface chemistry has been investigated by N 1s and Si 2p core level photoemission on Si(001)-2 × 1 and Si(111)-7 × 7, taking advantage of the surface sensitivity provided by synchrotron radiation tunability. Sequential treatments, i.e. 90 K adsorption/300 K annealing/1193 K nitridation, have been carried out. A common feature of low-temperature NH 3 adsorption on Si(001) and Si(111) is the growth of a solid NH 3 layer over a decomposed ammonia interlayer where, among other species, atomic nitrogen is present. Increasing the temperature causes this solid NH 3 layer to desorb. A 300 K annealing of the Si(001) surface eliminates also the adsorbed atomic nitrogen species to reach a situation in which only H atoms and NH 2 fragments decorate the silicon dimer broken bonds. In contrast to the Si(001) case, a 300 K annealing of the rougher Si(111) surface does not lead to a unique adsorption site/NH 3 species: in particular atomic N remains. The formation of higher subnitride states (already at 90 K) is also evidenced on Si(111) with respect to Si(001). The situation of greater complexity (due to the 7 × 7 reconstruction) of the Si(111) surface with respect to Si(001), when one considers low-and room-temperature adsorption processes, is strikingly reversed when one deals with thermal nitridation. The Si(111) subnitride distribution is compatible with an ideal abrupt interface, when the Si 3N 4 Si(001) interface appears as rougher. Using the clean and 300 K annealed surfaces as templates, the amounts (per unit area) of subnitrides at the Si 3N 4 Si interface are estimated.

Coverage-dependent adsorption sites for K/Cu(001) and Cs/Cu(001) determined by surface X-ray diffraction

Surface X-ray diffraction has been used to analyze in situ the room-temperature adsorption behaviour and the structure of K and Cs on Cu(100) at submonolayer coverages. Adsorption of K takes place in fourfold hollow sites up to coverages of about 0.25 monolayers (ML), where 1 ML corresponds to 1.53 × 10 15 atoms/cm 2. At higher coverages the formation of a quasi-hexagonal incommensurate adlayer is observed. In contrast, for Cs adsorption we observe from the very beginning the formation of the quasi-hexagonal structure up to the completion of the adlayer at about 0.30 ML. For K adsorption in the hollow sites we determine an adsorption height, d ⊥ = 2.25(15) A ̊ , corresponding to an effective K radius of r eff = 1.6(1) A ̊ close to the ionic radius of 1.33 Å. We do not observe a change in the effective radius as a function of coverage. For the quasi-hexagonal Cs structure we find an (average) adsorption height d ⊥ = 2.94 A ̊ corresponding to an effective radius of r eff = 2.18 and 1.93 A ̊ , for the limiting ca hollow- and bridge-site adsorption, respectively. The analysis of the superlattice reflections corresponding to the quasi-hexagonal incommensurate structures indicated that the K adlayer is strongly modulated. The first Fourier component of the substrate-induced modulation was determined to u 01 = 1.29(3) A ̊ . In contrast, for Cs/Cu(001) static modulation is much less important ( u 01 ⩽ 0.2 A ̊ ). Variation of the Cs adlayer density by changing the substrate temperature allows continuous expansion and contraction of the adsorbate unit cell. No commensurate-incommensurate transition has been observed.

Adsorption, thermal reaction, and desorption of disilane on Ge(111)-c(2 x 8).

Room-temperature adsorption of disilane (${\mathrm{Si}}_{2}$${\mathrm{H}}_{6}$) on Ge(111)-c(2\ifmmode\times\else\texttimes\fi{}8) and subsequent thermal reactions and desorption at elevated temperatures were studied using scanning tunneling microscopy and core-level photoemission. The initial adsorption results in the formation of various surface radicals, and the reacted areas on the surface grow laterally for increasing exposures. The sticking coefficient is rather low, and an exposure greater than about 30 000 langmuirs is needed to saturate the surface. The net amount of Si deposited for the saturated surface is about one-half of an atomic layer. Thermal annealing causes the hydrogen atoms to desorb and the Si atoms to move below the surface. For annealing temperatures beyond about 630 K, the desorption of hydrogen becomes complete, all of the Si atoms move below the surface, and the resulting surface resembles the starting clean Ge(111)-c(2\ifmmode\times\else\texttimes\fi{}8) surface except that the c(2\ifmmode\times\else\texttimes\fi{}8) long-range order is partially destroyed. Step flow and island coarsening, similar to growth by molecular-beam epitaxy, are observed.