The basic challenge in developing a Si-based light source is overcoming the emission inefficiency of crystalline Si due to its indirect band structure. Numerous efforts have led to an array of Si-compatible materials from which efficient light emission was attained; these materials include Si nanocrystals (Si-ncs), Er doped SiO2 (Er:SiO2), and strained Ge on Si. Based on two of the most promising Si-compatible light emitting materials, Si-nc and Er:SiO2, we designed novel microcavities with the potential to be used in laser designs. We developed fabrication processes for both Si-nc and Er:SiO2 materials and performed extensive material characterization to attain the parameters governing their behavior in the ADE-FDTD model. The cavities designed, fabricated, and characterized in this work consisted of an in-plane corner-cut square microcavity, microdisks and microtoroids, and a concentric microdisk structure designed for a two-stage, CMOS-compatible Si laser.
The waveguide-coupled corner-cut square cavity was fabricated in-plane using E-beam lithography and selective dry etching. Both the lithography and etch processes were optimized to achieve smooth and vertical cavity sidewalls. We experimentally characterized this structure using lensed tapered fibers and saw excellent agreement with the simulated predictions. We identified an optimum corner-cut length which improved the Q-factor for a square cavity by as much as 2×.
We then focused on developing light emitting devices using the Si-nc and Er:SiO2 materials. While neither of these materials on their own satisfies all the requirements for an electrically pumped, CMOS-compatible laser at telecommunication wavelength, we proposed a concentric microdisk design which leverages the advantages of both materials. In the proposed structure, EL from an inner Si-nc microdisk acts as an optical pump for an Er:SiO 2 laser in the outer microdisk. Using our modeling tools, we confirmed the proposed device behavior and optimized the geometry.
To demonstrate the feasibility of this device, we fabricated a series of preliminary light emitting structures, including Si-nc microdisks, Er:SiO 2 microdisks and –toroids, and Si-nc/Er:SiO2 concentric microdisks. We developed two experimental characterization techniques to analyze the whispering-gallery modes (WGMs), one based on free-space collection from the edge of the microdisk and the other based on evanescent coupling to a tapered pulled fiber. The tapered fiber pulling process was refined to allow for in situ monitoring of the transmission and fiber diameter, which drastically improved the reliability and repeatability of this process. We compared these characterization setups and identified the regimes of operation in which each is appropriate. Using these characterization setups, we observed spectrometer limited Q-factors as high as 2×103 for Si-nc microdisks, comparable to the highest Q-factors reported in the literature, and Q-factors as high as 3×106 for Er:SiO2 microtoroids, which are high enough to achieve lasing given an optimized Er concentration. We then developed a fabrication process for the Si-nc/Er:SiO2 concentric microdisks in accordance with our two-stage laser design. Characterization of these concentric microdisks confirmed many of our predictions, including the existence of Si-nc based pump modes and Er:SiO2 based signal modes, the mitigation of free carrier absorption (FCA) loss from the signal modes, and indirect excitation of the Er-based film via Si-nc luminescence. The existence of active and passive modes at both Si-nc based (pump: ∼800 nm) and Er:SiO2 based (signal: ∼1530 nm) wavelengths were in good agreement with the simulated predictions. The FCA loss, which is the dominant loss mechanism in Er doped Si-nc compositions, is almost entirely mitigated in the concentric microdisk structure by spatially separating the pump and signal modes. Having the pump and signal modes spatially separated allowed us to use the Si-nc luminescence as a optical pump for the Er:SiO 2 film. This indirect excitation mechanism was the first demonstration of an integrated two-stage pumping scheme applied to these materials. Finally, we developed a semi-analytical model to predict lasing thresholds in this concentric microdisk structure. Based on this analysis, we identify the material and device optimizations required to achieve lasing in the concentric microdisk structure. (Abstract shortened by UMI.)