We report synthesis and photoluminescence optimization of luminescent Si nanocrystals in silicon rich oxide films using a CO<sub>2</sub> laser beam. Laser annealing allows for a very localized heat deposition. This results in appreciable temperature rise in an area that is equivalent to only a few spot sizes. This could be important in CMOS back-end compatible processing where high temperatures on the entire wafer scale might not be acceptable. Furthermore, temperature optimization studies in furnace annealing are time consuming because the furnaces have to be programmed to each individual temperature and the stabilization takes long times. In CO<sub>2</sub> laser annealing, the entire temperature range for nanocrystal formation is available along the radial and axial directions of the laser spot - thereby allowing temperature optimization in a single short experiment. Presence of crystalline nanoparticles is ascertained using structural analysis techniques like transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). We also report luminescence optimization with respect to laser power and annealing time. It is observed that laser annealing in an air ambient results in two peaks in the luminescence spectrum - on in the visible at 570 nm and one in the near infra read at 800 nm. Origin of luminescence in these two peaks is probed by hydrogen passivation and time resolved measurements.
In the second part of the paper, we focus on continuous wave characterization of photoluminescence from Si nanocrystals embedded in microdisk resonators. There have been numerous reports on observation of continuous-wave and transient gain in planar optical waveguides with Si nanocrystal active layer. However, there are relatively very few investigations focusing on photoluminescence emission from Si nanocrystals (quantum dots) embedded in on-chip optical microcavities. Microcavities spectrally filter the luminescence from the quantum dots and, depending on the (Q/V) ratio, can significantly alter the spontaneous photoemission from the quantum confined excitons. In our work, planar microcavities are patterned on the emitter layer by high resolution electron beam lithography and a combination of dry and wet chemical etching. Fabrication procedure is optimized to maximize the ratio of the quality factor and the mode volume. Continuous-wave photoluminescence measurements are performed by top-pumping the resonators with a 488 nm line of an argon ion laser. We study the photoemission from the microdisks for the polarization dependence, and quality factors. Contributions of various mechanisms leading to the observed loss are estimated. We believe that our studies will help gain further insight into photoemission physics of the group-IV nanostructures.
Demand for a silicon (Si) based optical modulator is becoming more pressing as optical interconnects are starting to be considered seriously as replacement for conventional copper wires in electronic chips. Difficulties in realizing this device in Si are well known as are the stringent requirements on its performance in terms of size (~μm), power (~μW-mW), speed (>1 GHz) and operating voltage (<5 V). Here we present a detailed numerical design and analysis of a compact, high-speed silicon-on-insulator (SOI) waveguide electro-optical modulator. The device operates by tuning the reflection resonance of a microring resonator by means of field-effect generated free carriers in metal-oxide-semiconductor accumulation layers. Electrical and optical analyses are carried out by solution of Poisson's, charge continuity, and Maxwell's equations by finite-element method. Our simulations predict a ~0.5 nm shift in the spectral response of the resonator around 1550 nm. With an appropriate pre-biasing, this leads to ~80% modulation depth switching with voltage swing of 2 V. Field-effect induced generation of free-carriers allows for operating bandwidth >5 GHz while consuming a total dynamic power of < 500 μW. Use of the field effect results in extremely thin charge layers of very high carrier concentration. We show that an appropriate placement of these layers in the modal field of strong-confinement SOI waveguides greatly enhances the charge-field interaction. This enables significant improvements in size and modulation depth and allows the device to operate at CMOS compatible power and voltage levels. Present work adds to the design space explored in the previous works and aims to advance the field-effect based micro-resonator modulator as an active photonic device to be used in future generations of opto-electronic circuits.