It is known that crystalline asymmetry on the microscopic scale can lead to the emergence of macroscopic stationary directed current when the crystal is irradiated by an external light source. The appearance of directed current induced by zero-mean force of radiation in absence of any external static voltage or current source has been named the photogalvanic effect or ratchet effect.

The term "ratchet effect" is now used in a wide range of fields from economics (referring to increasing market complexity) to science (referring to the directed motion of particles). In semiconductor material, when an electromagnetic wave interacts with electrons in the presence of an asymmetric potential, a directed current is generated and a charge motion arises.

In this current work, the asymmetric potential is created through a hexagonal lattice of semicircular antidots. These new devices configured as sensitive detectors and new micro-sized current and voltage generators can be considered for applications in innovative information technology. The units could be employed both in space and on the ground where microwave energies are found.

Fabrication steps

The antidot lattice is patterned at the Georgia Institute of Technology's Nanotechnology Research Center using a JEOL 9300FS Electron Beam Lithography tool. The equipment offers 50 kV and 100 kV accelerating voltage capabilities, a current range of 50 pA to 100 nA, and a beam diameter of 6–8 nm (at 2 nA). The hexagonal antidot lattice is fabricated with a lattice period of 600 nm and comprises semicircular antidots with a radius of 200 nm.

Large samples (measuring a few millimetres squared) were fabricated by standard photolithographic techniques and with ohmic electrical contacts at CNRS–LIMMS, a joint research centre based at the Institute of Industrial Science, University of Tokyo, Japan.

Magnetotransport measurements were performed from 1.4 to 77 K and at magnetic fields up to 15 T. Ratchet dc photovoltage measurements were carried out using a carcinotron tunable in the 33–50 GHz microwave frequency range. This work was performed at the High Magnetic Field Laboratory (CNRS-LNCMI) in Grenoble.

Based on the same physical principle, new devices made out of 8'' Si/SiGe substrates (provided by CEA-LETI France) are under development at LIMMS-CNRS/IIS-University of Tokyo in Japan.

Because a hydrophobic 2 nm silicon cap layer has to be preserved until the end of the process, all standard cleaning procedures from CMOS technology are prohibited. "To overcome this problem, the team has developed low-cost alternative technologies," explained Laurent Jalabert, a research engineer at LIMMS, in charge of the Si/SiGe sample fabrication in the MEMS cleanroom facilities of Hiroyuki Fujita at the Institute of Industrial Science of the University of Tokyo.