Hydrogen-terminated Silicon Surface Devices

Our research on H:Si devices is primarily motivated by the need to develop high quality (mobility) electron systems in Si that are amenable to fabrication techniques at or near the atomic scale.  The physics of electron systems in these devices is of interest in its own right: in particular the valley degeneracy of the electrons is dependent on surface orientation.  We are studying [111] oriented surfaces (in which each Si surface bond is pointing perpendicular to the surface and is terminated by one H atom) for two reasons: the valley degeneracy is predicted to be the maximum possible value of six and because chemical treatments of Si [111] yield atomically smooth surfaces.

H:Si[111] surface electrons in bonded FET structures  Because there is a very low density of surface states on H:Si surfaces, electrons can be induced on the surface by a strong electric field, and the induced electrons can be probed by transport measurements if ohmic contacts are present..  In our current devices contacts are created by ion implantation into a Si[111] chip, which is bonded to a silicon-on insulator (SOI) chip that contains a conducting gate and a vacuum cavity across which the electric field is applied.

All of the processes necessary for fabrication are standard except for the chip scale bonding step, which requires that the two surfaces be extremely flat and clean so that they adhere to one another by van der Waals forces.  Additionally, just prior to bonding, the Si[111] surface must be treated with NH4F in an oxygen-free environment. 

Once bonded together the chips are wired up using indium contacts and gold wire.  At low temperatures the electron mobility can exceed 300,000 cm2/Vsec, much higher that what is achievable in MOS (metal-oxide-silicon) FETs. Our devices can exhibit the fractional quantum Hall effect, and we have also recently fabricated devices in which a 2D hole system is created.