Preparation, Characterization and Applications of Nanocrystalline Porous Silicon
Michael J. Sailor
25 hours, 6 credits
July 5 - July 9, 2010
Dipartimento di Ingegneria dell'Informazione: Elettronica, Informatica, Telecomunicazioni, via Caruso, meeting room, ground floor
Contacts: Dr. Giuseppe Barillaro
Porous silicon is generated by etching crystalline silicon in aqueous or non-aqueous electrolytes containing hydrofluoric acid (HF). Initial interest focused on using the high surface area of porous silicon as a model of the crystalline silicon surface in spectroscopic studies, as a precursor to generate thick oxide layers on silicon, and as a dielectric layer in capacitance-based chemical sensors.
Interest in porous silicon, and in particular its nanostructure, exploded in the early 1990’s when Ulrich Goesele at Duke University identified quantum confinement effects in the absorption spectrum of porous silicon, and almost simultaneously Leigh Canham at the Defense Research Agency in England reported efficient, bright red-orange photoluminescence from the material. The quantum confinement effects arise when the pores become extensive enough to overlap with each other, generating nanometer-scale silicon filaments. As expected from the quantum confinement relationship, the red to green color of photoluminescence occurs at energies that are significantly larger than the bandgap energy of bulk silicon (1.1 eV, in the near infrared).
With the discovery of efficient visible light emission from porous silicon came a flood of work focused on creating silicon-based optoelectronic switches, displays, and lasers. Problems with the material’s chemical and mechanical stability and its disappointingly low electroluminescence efficiency led to a waning of interest by the mid 1990s. In the same time period, the unique features of the material - large surface area, controllable pore sizes, convenient surface chemistry, and compatibility with conventional silicon microfabrication technologies - inspired research into applications far outside of optoelectronics. Many of the fundamental chemical stability problems have been overcome as the chemistry of the material has matured, and various biomedical sensor, optics, and electronics applications have emerged.
This series of lessons describes basic electrochemical and chemical etching experiments that can be used to make the main types and structures of porous silicon. Beginning with fundamental properties and electrochemical mechanisms, experiments describing methods for characterization and key chemical modification reactions will be provided. A survey of the current applications, including chemical sensors, biosensors, drug delivery, and in-vivo diagnostics will be presented.