Direct laser writing in semiconductors for photonics applications

Abstract

Research in silicon photonics is accelerated by the extensive understanding of silicon fabrication from the microelectronics industry, and silicon appears to be ideally placed to offer a solution for the integration of photonic layers onto an electronic chip. However, the manufacture of a crystalline silicon film requires high temperatures incompatible with back-end-of-line integration with the complementary metal-oxide semiconductor process flow. Significant interest therefore lies in research into low-temperature fabrication of alternatives to crystalline silicon. This thesis presents a direct laser crystallisation tech­nique suitable for use in a low-temperature fabrication process flow, and demonstrates its applications in a range of semiconductor materials. Laser writing is used to define waveguides in thin films of hydrogenated amorphous silicon, which has attracted attention for optical applications due to its low transmis­sion losses and high nonlinear refractive index. A lithography-free waveguide definition process is proposed, and the optical loss of the resulting waveguides is measured and compared to those in conventionally fabricated waveguides. The laser crystallisation process is then applied to etched wire structures in low tem­perature deposited amorphous silicon, maintaining a substrate temperature below the thermal budget for back-end-of-line integration. The material and optical properties of the resulting polycrystalline material are investigated by Raman spectroscopy, X-ray diffraction crystallography, cutback transmission measurements and nonlinear optical characterisation, and found to be comparable to those of single-crystal silicon. Most significantly, the lowest losses to date are recorded at 2.4 dB cm-1, and third-order non­linear characteristics in or near the range reported for crystalline silicon are measured. Finally, the application of the laser writing process to silicon-germanium is investigated. Silicon-germanium is a binary alloy with composition-dependent properties and applica­tions. The laser writing system is used to carry out localised tailoring of the germanium fraction. The change in local composition is investigated by Raman spectroscopy, elec­tron microscopy and finite-element simulation, and the effect on the photoconductivity is studied.

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Identifiers

ORCID for Stuart, James MacFarquhar: orcid.org/0000-0002-4378-7670

ORCID for Anna Peacock: orcid.org/0000-0002-1940-7172

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