B.G. Koehler

Desorption kinetics of hydrogen and deuterium from Si(111) 7×7 studied using laser‐induced thermal desorption

The desorption of hydrogen and deuterium from Si(111) 7×7 was studied using laser‐induced thermal desorption (LITD) and temperature programmed desorption (TPD) mass spectrometry. Isothermal LITD measurements enabled the surface coverage of hydrogen and deuterium to be monitored as a function of time. These isothermal results were used to obtain accurate desorption kinetics of hydrogen and deuterium from the high‐temperature β1 state on Si(111) 7×7. The desorption of hydrogen displayed second‐order kinetics with an activation barrier of 61±4 kcal/mol and a preexponential factor of 1.2×101±1.3 cm2/s. Likewise, the desorption kinetics of deuterium displayed second‐order kinetics with an activation barrier of 59±3 kcal/mol and a preexponential factor of 2.8×100±1.0 cm2/s. These desorption activation barriers yield upper limits of 82.6 and 81.6 kcal/mol for the Si–H and Si–D chemical bond energies, respectively, on Si(111) 7×7. TPD results obtained as a function of hydrogen coverage were consistent with second...

Adsorption and desorption kinetics for SiCl4 on Si(111)7×7

The adsorption and desorption kinetics for SiCl4 on Si(111)7×7 were studied using laser‐induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. TPD experiments monitored SiCl2 as the desorption product at approximately 950 K using a heating rate of β=9 K/s. SiCl2 was also observed as the desorption product in the LITD yield at all surface coverages. LITD measurements determined the initial reactive sticking coefficient (S0) of SiCl4 on Si(111)7×7 versus surface temperature. The sticking coefficient was observed to decrease with surface temperature from S0≊0.18 at 160 K to S0≊0.03 at 600 K. TPD experiments revealed that the chlorine surface coverage saturated after large SiCl4 exposures and the saturation coverage was independent of surface temperature. Isothermal LITD studies enabled the surface chlorine coverage to be monitored as a function of time during SiCl2 desorption. These studies revealed second‐order desorption kinetics for SiCl2 with a desorption activation bar...

Desorption product yields following Cl2 adsorption on Si(111)7 × 7: Coverage and temperature dependence

Desorption product yields obtained following adsorption of Cl2 on Si(111)7 × 7 were studied using temperature-programmed desorption (TPD) and laser-induced thermal desorption (LITD) techniques. At low chloride coverages of ΘΘs 0.6, a small SiCl4 TPD signal was also monitored at 950 K along with an additional SiCl2 TPD feature at 690 K. LITD experiments detected SiCl2 as the only desorption product in the LITD yield at low chloride coverages of ΘΘs 0.6. In t LITD studies, the SiCl2 LITD signals persisted until 950 K, whereas the SiCl3 LITD signals were only observed up to 700 K. The magnitude of the SiCl3 LITD signal following saturation chlorine exposures also decreased as a function of adsorption temperature. The SiCl2 desorption products were assigned to the recombinative desorption of SiCl + Cl → SiCl2. The SiCl3 LITD signals were attributed to either the direct desorption of SiCl3 surface species or the recombinative desorption of SiCl2 + Cl → SiCl3. Based on photoemission and scanning tunneling microscopy investigations, the SiCl2 and SiCl3 desorption yields were correlated with the existence of mono-, di- and trichloride species on the Si(111)7 × 7 surface. Many similarities were also observed between the chlorides and hydrides on the Si(111)7 × 7 surface.

Coverage dependence of the surface diffusion coefficient for hydrogen on Ru(001)

The coverage dependence of the surface diffusion coefficient for hydrogen on Ru(001) was studied using laser-induced thermal desorption(LITD) techniques. The LITD measurements were performed for a wide range of initial hydrogen surface coverages in the temperature range 230–270 K. At a given temperature, the surface diffusion coefficient was found to be constant as a function of coverage. The absence of coverage dependence in the surface diffusion coefficient indicates that adsorbate-adsorbate interactions between adsorbed hydrogen atoms on Ru(001) are negligible. This observation suggests that the coverage-dependent features observed in work function, high resolution electron energy loss spectroscopy and possibly temperature programmed desorption studies of hydrogen on Ru(001) are associated with two inequivalent hydrogen sites rather than repulsive lateral hydrogen interactions. Monte Carlo simulations reveal that the surface diffusion coefficient should be coverage independent on a surface with two inequivalent sites in the absence of adsorbate-adsorbate interactions.

Surface diffusion of hydrogen on carbon‐covered Ru(001) surfaces studied using laser‐induced thermal desorption

The effects of surface carbon on the surface diffusion of hydrogen on Ru(001) were studied using laser‐induced thermal desorption techniques. The surface mobility of hydrogen decreased by approximately a factor of 60 as a function of increasing surface carbon coverage from θC=0 to θC=0.42 monolayer at T=300 K. The observed reduction of hydrogen surface mobility vs surface carbon coverage was consistent with the trapping of hydrogen atoms by carbide species on the Ru(001) surface. A simple trapping model suggests that the potential energy wells of the carbon trap sites are ΔE>2.4 kcal/mol deeper than regular hydrogen adsorption sites. This estimate is also consistent with the results of Monte Carlo simulations.

Isotope effect in the surface diffusion of hydrogen and deuterium on Ru(001)

The surface diffusion of hydrogen and deuterium on Ru(001) at low coverage was studied using laser-induced thermal desorption techniques. In the temperature range from 260 to 300 K, the surface diffusion coefficients could be expressed in Arrhenius form as D H =6.9×10 −4 cm 2 /s exp(−3.6±0.5 kcal mol −1 / RT ) and D D =4.6×10 −4 cm 2 /s exp(−4.1±0.5 kcal mol −1 / RT ) for hydrogen and deuterium, respectively. The observed isotope effect was somewhat larger than predicted by simple transition state theory but was within the limits of experimental error. Quantum mechanical tunneling can be playing, at best, only a minor role in the surface migration of hydrogen on Ru(001) at these temperatures.

The decomposition of methanol on Ru(001) studied using laser induced thermal desorption

The decomposition reaction of methanol on Ru(001) was studied using laser induced thermal desorption (LITD). The LITD studies, combined with temperature programmed desorption and Auger electron spectroscopy measurements, allowed absolute product yields for the three competing surface pathways to be determined over the entire range of chemisorbed methanol coverages at a heating rate of β=2.6 K/s. At the lowest methanol coverages of θ≤0.07θs, where θs is the surface coverage of a saturated chemisorbed layer, all the methanol reacted between 220–280 K. This methanol decomposition reaction yielded desorption‐limited H2 and CO as reaction products. At higher coverages, molecular desorption and the second methanol decomposition reaction involving C–O bond breakage became increasingly important. At θ=θs, 50% of the initial methanol coverage desorbed, 24% produced H2 and CO and 26% left C on the surface. Isothermal LITD kinetic measurements were carried out at low methanol coverages of θ≤0.07θs at various tempera...

Adsorption and Desorption Kinetics for Chlorosilanes on Si(111) 7×7

The adsorption and desorption kinetics for SiCl 4 and SiCl 2 H 2 on Si(111) 7×7 were studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. Both LITD and TPD experiments monitored SiCl 2 as the main desorption product at 950 K at all coverages of SiCl 4 and SiCl 2 H 2 on Si(111) 7×7.HC1 desorption at 850 K and H 2 desorption at 810 K were also observed following SiCl 2 H 2 adsorption. Isothermal LITD measurements of SiCl 4 and SiCl 2 H 2 ) adsorption on Si(111) 7×7 revealed that the initial reactive sticking coefficient decreased with increasing surface temperature for both molecules. The temperature-dependent sticking coefficients were consistent with precursor-mediated adsorption kinetics. Isothermal LITD studies of SiC1 2 desorption revealed second-order SiCl 2 desorption kinetics. The desorption kinetics were characterizedby a desorption activation energy of E d = 67 kcal/mol and a preexponential of vd = 3.2 cm 2 /s. TPD studies observed that the HCI desorption yield decreased relative to H 2 and SiCl 2 desorption as a function of surface coverage following SiCl 2 H 2 exposure. These results indicate that when more hydrogen desorbs as H 2 at higher coverages, The remaining chlorine is forced to desorb as SiCl 2 .

Laser-induced thermal desorption of Silicon-containing surface reaction intermediates from Si(111)7 × 7

Abstract Laser-induced thermal desorption (LITD) was used to remove surface reaction intermediates from Si(111)7×7. Silicon-containing species were detected in the LITD yield from Si(111)7×7 surfaces covered with hydrogen, water or methanol. These silicon-containing LITD products include SiH n , Si(OH) n and Si(OCH 3 ) n where n = 1–3. None of these species were observed in the conventional temperature programmed desorption (TPD) spectra. TPD and temperature-programmed LITD studies were also performed and correlated for Si(111)7×7 surfaces as a function of hydrogen coverage. The SiH 2 LITD products were related directly to the low-temperature β 2 -peak in the H 2 TPD spectra. Previous infrared studies have demonstrated that this β 2 -peak corresponds to H 2 desorption from silicon dihydride species on the silicon surface. The ability of LITD to desorb silicon-containing surface reaction intermediates should be extremely useful in investigations of silicon surface reaction kinetics.