Material, Optical and Electro-optical Characterization of Si and Si-based Devices Under the Influence of High Energy Radiation
Radiation effects studies performed on electronics typically consist of electrical characterization of device performance to analyze the impact of radiation damage. Very few studies have focused on surface characterization of materials under high energy radiation, particularly those materials widely used in electronics such as silicon. This study addresses this gap in knowledge by first studying the influence of radiation on the surface of bare silicon, and then evaluating the high-energy radiation response of two silicon-based devices: a silicon photonic ring resonator and an amorphous silicon (a-Si:H) solar cell with a novel, metal covered, porous a-Si:H grating functioning as a back reflector. Irradiation of unpassivated bare silicon with 10-keV x-rays and 662-keV gamma rays, resulted in an enhanced rate of native oxide growth, which saturated at the typical native oxide thickness of ~ 2 nm. The oxidation rate was correlated to x-ray dose rate via an empirical thin oxide growth model. The transmission response of a silicon photonic ring resonator was then evaluated upon exposure to 10-keV x-rays and 662-keV gamma rays. Radiation exposure of the unpassivated ring resulted in a blue shift in resonance wavelength proportional to the incident total dose, while the transmission spectrum of a ring passivated with native oxide or a polymer coating remained immune to ionizing radiation. Next, an amorphous silicon n-i-p solar cell with a silver covered porous a-Si:H grating was fabricated and its performance was evaluated before and after irradiation with 4-MeV protons. The porous a-Si:H grating, fabricated by metal assisted chemical etching and a simple imprinting technique, led to a 30% increase in solar cell efficiency for thin active layers in comparison to identical solar cells with flat metal back reflectors. The enhancement was primarily attributed to increased photon absorption due to light scattering from the roughened metal grating. No degradation in solar cell performance was observed upon irradiation with 4-MeV protons for a fluence of 10^13 cm^(-2).