Adsorption and thermal reactions of disilane and the growth of Si films on Ge(100)-(2×1)

Scanning tunneling microscopy (STM), core-level photoemission spectroscopy using synchrotron radiation, and electron diffraction were employed to study the vapor-phase epitaxial growth of Si on Ge(100)-(2×1) using disilane (Si2H6). The dissociative chemisorption of a Si2H6 molecule on Ge(100)-(2×1) at room temperature results in two Si-trihydride (SiH3) radicals bonded onto two adjacent Ge dangling bonds. Some SiH2 and GeH species are also formed as a result of decomposition of SiH3. An initial sticking coefficient of ∼0.5 is deduced from STM images. An exposure of more than 2 langmuirs (1 langmuir=10-6 Torr s) of disilane at room temperature saturates the surface with SiH3, SiH2, and GeH species, and the resulting surface is disordered. The total amount of Si on the saturated surface is about 1/2 monolayer (ML). Successive annealing of the saturated surface to higher temperatures causes the conversion of SiH3 to SiH2, the conversion of SiH2 to SiH, and the desorption of H from GeH. These processes become complete at about 600 K, and the resulting surface is a clean Ge(100)-(2×1) interspersed with about 1/2 ML of Si-monohydride (SiH)-(2×1) islands. Desorption of hydrogen from these SiH islands occurs at even higher annealing temperatures, and is accompanied by indiffusion of Si into the Ge substrate. This process becomes complete at about 690 K, and the final system configuration is a clean Ge(100)-(2×1) with about 1/2 ML of Si buried in the subsurface region. Multilayer Si deposition was performed by atomic layer epitaxy, i.e., cyclic disilane adsorption at ∼340 K followed by thermal conversion at 820 K. For up to 18 cycles, the resulting surface consists of Ge only. The change in surface morphology is studied by STM. Differences between the clean Si(100)-(2×1) and Ge(100)-(2×1) surfaces as observed by STM are also reported.