The newly developed technology EVG^® ComBond^® was used for the fabrication of Si-Ge heterostructures. In this technology, an ion beam bombardment is used to clean and to activate the Ge- and Si-surfaces at room temperature by removing the native oxides as well as organic contaminants without deteriorating chemical composition and microroughness of the surfaces.

After surface activation, the wafers were brought into contact at room temperature under high vacuum, which resulted in a wafer bond extending over the entire area.

The successful room temperature covalent bonding of Ge/Si with high bond strength (measured by the crack opening method), no micro-voids, and no thermal outgassing (inspection with scanning acoustic microscopy) was achieved. Cross-sectional transmission electron microscopy (TEM) of the bonded Ge/Si-interface revealed that the wafers bonded without damaging the crystal lattice except for a thin amorphous layer of <0.9 nm. This amorphous layer was completely removed by thermal annealing at 400°C for two hours. Current-voltage measurements on 1 cm² samples made from p-Si/n-Ge wafer bonds revealed diode behavior with a ratio of forward to reverse current of >4 and a threshold voltage of around 0.3V. The forward current yields an ideality factor of 1.1 after correcting for the series resistance which consists of a contact and interface resistance.

Figure : High-resolution TEM image of Ge/Si interface after thermal annealing at 400°C for 2 hours shows no obvious oxide layer and amorphous layer between the wafers.