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

We report optical second-harmonic (SH) spectra for p-in/p-out polarization configuration of bare and hydrogen-terminated Ge/Si(001) and Si0.9Ge0.1 (00 1)/Si(001) films with SH photon energies 3.1 < 2hv < 3.5 eV near the bulk E l 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 410K followed by hydrogen desorption. As Ge coverage increased from 0 to 2 mono layers the SH signal increased uniformly by a factor of seven with no detec table shift in the silicon E-1 resonant peak position. SH signals from Si0. 9Ge0.1(001)/Si(001) were also stronger than those from intrinsic Si(001). H ydrogen 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(00 1). We attribute the stronger signals from Ge-containingsurfaces to the str onger 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.