Ellipsometry Analysis of Germanium-on-Insulator Wafers*

Germanium based photonic devices attract a lot of interest due to the fact that its band structure is easily influenced by strain and alloying with tin. Utilizing a germanium-on-insulator (GOI) substrate is a key feature for future silicon compatible germanium based devices, allowing for easier integration by the microelectronics industry.

Here, we analyzed the optical response of two GOI bonded wafers with different orientations and thicknesses via spectroscopic ellipsometry. The ellipsometric angles, psi and Delta, and the depolarization were acquired using a J.A. Woollam variable angle spectroscopic ellipsometer (VASE) and a Fourier transform infrared (FTIR) ellipsometer, allowing high precision measurements in the mid-IR to near-UV range.

The optical response of the GOI wafers was modeled with four layers (Si, buried oxide, Ge, native oxide). For Ge, we used the parametric semiconductor oscillator model and compared it to our previously determined optical constants of bulk germanium. We are particularly interested in differences near the direct band gap. We did not find any differences in the electronic structure of bulk Ge and the thin bonded Ge layers.