Description
Porous silicon is a unique nano-material exhibiting low optical losses in the infrared, high electrical resistivity (50 GΩ.cm) and low thermal conductivity (0.15 W/m.K). Our 100 µm thick 80% porosity porous silicon layer is formed through the annodisation of a 2” silicon wafer (p-type 0.1 Ω.cm), using a HF/Ethanol electrolyte followed by critical point drying. By laser writing onto the porous silicon material (λ=405 nm), extremely high film temperatures can be achieved. Selection of the precursor gas during laser writing, allows control of both the electrical and optical properties of the film – for example, using a hydrocarbon as the precursor, carbon displaces hydrogen at the silicon surface, and fills the nanometer sized pores with pyrolytic carbon. This leads to highly electrically conductive pathways being formed of nominally 50 mΩ.cm. Removal of the pyrolytic carbon using O2 plasma ashing leaves only the surface carbon, allowing optical waveguides to be created with low optical losses down to 1 dB/cm. However pyrolytic carbon formation significantly changes the thermal conductivity of the film and O2 plasma ashing removes any electrically conductive paths.
This work explores models to understand the exact surface temperature induced by the laser power and spot size, and in doing so, map the conditions to achieve low or high pyrolytic carbon formation. Experimental results indicate that low optical loss waveguides and high electrical conductivity pathways can be simultaneously formed on the films by correctly controlling the surface temperature, avoiding the need for subsequent O2 plasma ashing. The low index contrast in the waveguides fabricated results in large guided modes, comparable to mode field diameters from standard SMF-28 fibre, enabling low loss coupling into the guides and extremely simple alignment. The approach presented provides a unique pathway for achieving opto-electronic device fabrication on an all silicon platform.
| I am the presenting author | Yes |
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