Silicon at the nanoscale using lithography control: Nanowires, nanopores and quantum dots

Nanostructures of silicon are becoming increasingly important as they may combine new opto-electronic features or devices with CMOS technology. This enables the integration of e.g. nanowire sensors with readout electronics or nanoscale LEDs grown directly on Si wafers. A problem here is that nanostructures are often fabricated using a bottom-up approach necessitating a second deposition step of random nature or involving a tedious alignment procedure. This is because current lithography techniques do not allow sub 10 nm precision for direct top-down fabrication. In this presentation I will show examples of fabricating some generic silicon structures such as nanowires, nanopores and quantum dots using optical or e-beam lithography where a second process step is used to shrink feature sizes in a controlled way. Thus, the lithography step for positioning of nanostructures and the size determining process step become decoupled, as shown by three examples from our work: (i) Silicon nanowires were fabricated using a top-down approach using optical lithography. Thinning of the device layer to ∼20 nm enabled picomolar sensitivity for detection of biomolecules, avoiding traditional CVD growth of nanowires. These “nanoribbons” allows multiple parallel detection and direct electrical readout. (ii) Nanopores are interesting for single molecule detection and DNA sequencing. We have fabricated silicon membranes with arrays of nanopores down to sub 10 nm diameter. Electro-chemical etching was used as the final size control. This allows multiple optical detection of DNA translocation. (iii) Finally, individual Si quantum dots have been fabricated using e-beam lithography and a final tuning of the core by oxidation in a “self-limiting” regime. This has allowed single-dot spectroscopy revealing quantum confined exciton recombination with ultra-sharp emission lines. Many other features such as blinking and spectral diffusion have been observed, similar to direct bandgap quantum dots. Recent data indicate that very high quantum yields are possible enabling new applications.