Adsorption, desorption, and decomposition of HCl and HBr on Ge(100): Competitive pairing and near-first-order desorption kinetics

Abstract

We have investigated the surface chemistry of coadsorbed hydrogen and halogen atoms on Ge(100), produced by dissociative chemisorption of HCl and HBr, by temperature-programmed desorption. The initial sticking probability S0 for HCl decreases from 0.6 at a substrate temperature of 270 K to 0.05 at 400 K, indicative of a precursor state to adsorption. For HBr S0 is constant at 0.7 over the same temperature range. A fraction f of adsorbed hydrogen atoms desorb associatively as H2 near 570 K, while the remaining (1−f) H atoms recombine with adsorbed halogen atoms and desorb as the hydrogen halide (HX) near 580–590 K. The activation energies for desorption of H2, HCl, and HBr are all approximately 40 kcal/mol. For both HCl and HBr f is 0.7 at low initial coverage and decreases slightly to 0.6 at saturation. The fraction f of adsorbed halogen atoms left on the surface following the competitive desorption of H2 and HX desorb as the dihalides GeCl2 and GeBr2 near 675 and 710 K, respectively. Desorption of H2, HCl, and HBr occurs with near-first-order kinetics, similar to the behavior of hydrogen adsorbed alone, which we attribute to preferential pairing induced by the π bond on unoccupied Ge dimers. We introduce and solve a generalized doubly occupied dimer model incorporating competitive pairing of H+H, H+X, and X+X on Ge dimers to explain the near-first-order kinetics. The model quantitatively accounts for both the desorption kinetics and the relative yields of H2 and HX with pairing energies of ≊3 kcal/mol. Implications of the present results for surface thermochemistry, chemical vapor deposition, and atomic layer epitaxy of Ge and Si(100)2×1 surfaces are discussed.

Bottom-Up Masking of Si/Ge Surfaces and Nanowire Heterostructures via Surface-Initiated Polymerization and Selective Etching

“…Upon completion of Ge segment growth, the GeH 4 supply is terminated, and the substrate temperature is increased from 270 °C to 430 °C with 10 sccm HBr. HBr passivates the Ge segment sidewalls, minimizing motion and diffusion of the gold seed particle during heating. ,, Upon reaching 430 °C, 30 sccm SiH 4 is delivered for 5 min at 2.8 Torr total pressure.…”

“…HBr passivates the Ge segment sidewalls, minimizing motion and diffusion of the gold seed particle during heating. 5,50,51 Upon reaching 430 °C, 30 sccm SiH 4 is delivered for 5 min at 2.8 Torr total pressure. Surface-Initiated Polymerization.…”

A metastable brominated nanodiamond surface enables room temperature and catalysis-free amine chemistry

“…13 For that reason, bromination of Si and Ge is more common due to the increase in crystal lattice spacing and decreased steric hindrance. [14][15][16] One previous bromination protocol was used with detonation NDs and was not well detailed, and the handling was performed largely under open-air conditions and in the presence of water for purification. 17 In this work, we show the following: alkyl bromide formation on high-pressure high-temperature (HPHT) NDs is possible using thionyl bromide (SOBr2), it displays enhanced chemical reactivity compared to that of brominated small molecule analogs and the reaction products must be carefully handled under inert conditions to retain alkyl bromide moieties.…”

Low-Temperature Growth of Axial Si/Ge Nanowire Heterostructures Enabled by Trisilane

“…[26][27] This approach, however, requires careful control of growth conditions to prevent surface roughening from halogen-induced surface atom removal (e.g., as SiX 2 or GeX 2 where X = Cl or Br). [28][29][30] A notable exception to the above challenges is vapor-solid-solid (VSS) growth with an alloyed catalyst (e.g., Au/Al, Au/Ag), which is able to produce sub-5 nm Ge domains along the length of Si nanowires. [31][32] Here, we demonstrate arrays of axial Si/Ge heterostructure nanowires with excellent sidewall morphologies and purely axial composition profiles.…”

“…To date, the large majority of axial Si/Ge heterostructure nanowire syntheses yield nonideal structural motifs, including diameter changes, , sidewall deposition, , kinking, ,, and/or crystallographic defects . Si/Ge heterostructure nanowires without these unwanted structural motifs are possible, but with growth rates usually too rapid to encode highly confined structures. , In many cases, co-flow of a second precursor, such as HCl, is also necessary to improve sidewall morphology. , This approach, however, requires careful control of growth conditions to prevent surface roughening from halogen-induced surface atom removal (e.g., as SiX 2 or GeX 2 where X = Cl or Br). − A notable exception to the challenges outlined above is vapor–solid–solid (VSS) growth with an alloyed catalyst (e.g., Au/Al or Au/Ag), which can produce sub-5 nm Ge domains along the length of Si nanowires. , …”