Ion-implantation for defect engineering in silicon

Abstract

Over the years integrated circuits have been increasing in performance capabilities as a result of both an increasing transistor count according to the self-fulfilling Moore’s law, and a continuous reduction in feature size. At such small sizes, the time delay from metal interconnections far outweighs the delay across transistor gates, restricting the global chip operating speed, suffering high power loss and low bandwidth. Photonic interconnections are a promising solution to this problem, and benefit from wavelength division multiplexing capabilities to increase bandwidth. Silicon is a favourable platform since it builds upon mature CMOS fabrication technologies and processes to reduce the economic and technological costs. It’s transparency and high optical confinement, caused by the strong refractive index contrast against it’s natural oxide, is beneficial for signal routing but prevents generation and detection of light at communication wavelengths. This thesis focusses on a solution to the detection problem by intentionally introducing crystalline defects to extend silicon’s absorption capabilities beyond the intrinsic limit.

He+ ion implantation has been incorporated into a free-space photodetector to introduce vacancy-type defects and shift the absorption spectrum towards longer wavelengths. It was designed with the aid of a single-defect modelling approach to incorporate the enhanced photogeneration rate into a TCAD simulation model. This implantation has caused a 3.9x increase in responsivity to a wavelength of 1550 nm in experimental results, while reducing the intrinsic response to 633 nm by 29 %. B+ was also investigated, but it was found to compensate existing dopants and prevent ohmic contact if the metalsemiconductor interface is exposed during implantation. Then a waveguide integrated MSM photodetector is designed and fabricated, based upon a 340 nm SOI platform. The defect-mediated photoresponse is improved from the first design after a literary analysis of ion implantations. Ar+ is chosen since it introduces no doping, is a larger ion, and is implanted at an energy of 300 keV and dose of 1×1013 cm−2 to increase the concentration of damage produced. A TiN barrier-enhancement layer is also included to increase the Schottky barrier height up to 2.07x compared to an Al-only contact, and up to 1.37x compared to a Ti/Au enhancement layer which was also fabricated. The barrier height was further increased by 74 mV by the ion implantation, and dark current has been reduced further by substrate biasing up to -20 V.

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ORCID for Harold Chong: orcid.org/0000-0002-7110-5761

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