Si2H6 Dissociative Chemisorption and Dissociation on Si(100)-(2x1) and Ge(100)-(2x1)

Atomically controlled epitaxy of semiconductor-on-semiconductor is important for the construction of nanometer-scale devices such as quantum dots, qubits for quantum computing, nanoelectromechanical systems (NEMS) oscillators, and nanobiomedical devices. For instance, patterned atomic layer epitaxy (ALE) using a scanning tunneling microscopy tip to depassivate an area of a H-terminated Si(001) surface, creating a pattern for subsequent Si growth, should lead to a new generation of atomically precise structures. Vapor phase epitaxy is well-suited to achieve controlled deposition, as the precursors are not reactive with the H-terminated background, unlike Si atoms from solid-source evaporation. Disilane (Si2H6) is arguably the best precursor for Si ALE on Si or Ge surfaces at moderate temperatures; yet, its adsorption configuration and subsequent decomposition pathways are not well understood. Combining experimental data from in situ infrared absorption spectroscopy (IRAS), scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS) after saturation by disilane of both Si(100)-(2x1) and Ge(100)-(2 x 1) surfaces, with first-principles calculations of candidate surface structures, we show that Si2H6 chemisorbs through a beta-hydride elimination pathway as Si2H5 and H, instead of the previously proposed SiH3, and subsequently decomposes into an ad-monohydride dimer. The initial chemisorption process takes place on a single dimer and produces a monohydride ad-dimer oriented perpendicular to the substrate dimer rows. The ad-dimer can be located either in between two adjacent, initially clean dimers from the same dimer row, or in a bridging position over the trench between two adjacent dimer rows. These findings provide clear guidance for the formation of atomic size structures defined by local removal of hydrogen on H-terminated Si surfaces.