| About this Abstract |
| Meeting |
2011 Electronic Materials Conference
|
| Symposium
|
2011 Electronic Materials Conference
|
| Presentation Title |
LL7, Electrical Conductivity of Directly Bonded Silicon/Germanium Hetero-Structures |
| Author(s) |
Isabelle Pauline Ferain, John Hayes, Ran Yu, Ki Yeol Byun, Farzan Gity, Brenda Long, Cindy Colinge |
| On-Site Speaker (Planned) |
Brenda Long |
| Abstract Scope |
Direct wafer bonding of Silicon (Si) to Germanium (Ge) has already been successfully demonstrated. However, little has been published about the relationship between electrical characteristics of Si/Ge hetero-structures and the quality of their bonded interface. In the present work, the impact of thin buried oxides (BOX) at the bonded interface on the electrical conductivity of bonded p-Si/p-Si/p-Ge stacks is investigated. p-Si/p-Si stacks were directly bonded to p-type Ge wafers. On the Si wafers, silicon dioxide films were used with a thickness range of a few Angstroms to 12nm. Prior to in-situ bonding, wafers were exposed for 10 minutes to free oxygen radicals generated by a remote plasma ring within the bonding chamber. After bonding, the overall thermal budget of the anneal was limited to 300 °C. Slow ramp-up rate ex-situ anneals helped to minimize the formation of thermally generated voids. Cross-sectional HR-TEM analysis of the bonded interface was used to characterise the BOX thickness. Limited discrepancy was observed between the oxide thicknesses prior to bonding and the subsequent thickness of the bonded interface. Ohmic contacts were fabricated on both sides of the bonded pair after formation of mesa structures. Due to the doping levels of the silicon and germanium, the total net current across the hetero-structure was expected to show a rectifying effect, with the total net current under positive bias (on the Si side) being significantly higher than its counterpart under negative bias. Such rectifying behaviour has been observed experimentally on our bonded Si/Ge samples. At low reverse bias, the electrical conductivity is shown to decrease exponentially as the BOX thickness increases linearly. As compared to an ideal oxide-free interface, bonded pairs with thin native oxides of silicon and germanium at the bonded interface present a dramatic (50 %) reduction of their conductivity in reverse bias. The rectifying behaviour measured on thin BOX samples was successfully matched to results of device simulations involving the same p-Si/p-Si/p-Ge hetero-structures and direct tunnelling across the bonded interface as main conduction mechanism. Electrical properties of SiO2, GeO and GeO2 were carefully selected and impact ionisation was neglected to a first approximation. These simulations helped to predict that the conductivity vs. BOX thickness trend remains valid at higher biases. They confirmed the substantial conductivity loss caused by native oxides. Our results stress the importance of an aggressive BOX scaling to enable the direct bonding approach for the fabrication of photodiodes for instance. This may pose some challenges in terms of bond strength, interface uniformity and yield. |
| Proceedings Inclusion? |
Undecided |