Second-harmonic spectroscopy of Ge/Si(001) and Si1-xGex(001)/Si(001)

Ge0.1(001)/Si(001) films with SH photon energies 3.1<2hν<3.5 eV near the bulk E1 critical point of Si(001) or Si0.9Ge0.1(001). Ge was deposited on Si(001) by using atomic layer epitaxy cycles with GeH4 or Ge2H6 deposition at 410 K followed by hydrogen desorption. As Ge coverage increased from 0 to 2 monolayers the SH signal increased uniformly by a factor of seven with no detectable shift in the silicon E1 resonant peak position. SH signals from Si0.9Ge0.1(001)/Si(001) were also stronger than those from intrinsic Si(001). Hydrogen termination of the Si0.9Ge0.1(001) and Ge/Si(001) surfaces strongly quenched the SH signals, which is similar to the reported trend on H/Si(001). We attribute the stronger signals from Ge-containingsurfaces to the stronger SH polarizability of asymmetric Ge-Si and Ge-Ge dimers compared to Si-Si dimers. Hydrogen termination symmetrizes all dimers, thus quenching the SH polarizability of all of the surfaces investigated.