Development of a silicon-based on-chip light source could be facilitated by the incorporation of nanocrystalline silicon
(<i>nc</i>-Si) into a multislot waveguide structure, using erbium embedded in silicon oxide as a luminescence source. The
multislot waveguide confines TM polarized light in the oxide (low-index) layers, thus reducing the loss caused by
interaction with free carriers in the <i>nc</i>-Si layers. Here we demonstrate a lateral electrical injection scheme using a p-i-n
junction embedded into the multislot, allowing much more efficient charge injection than alternative vertical injection
approaches which have been limited by the highly insulating oxide layers. By exploiting the difference in the mode
profiles of TE and TM light, we were able to gauge the injection of free carriers as a function of applied voltage, by
measuring the polarization-dependent optical loss for light transmitted through the multislot waveguide. Experimental
measurements are well-predicted by numerical computations using both FDTD and the transfer matrix method.
Materials and devices.for compact optical amplification in Si photonics is reviewed. In particular, as the requirement for
high gain per length together with high refractive index renders traditional oxide-based approach problematic, Er-doping
of silicon-rich silicon nitride and erbium silicate nanocrystals are proposed and shown to be promising alternatives.
Using such new materials, microdisk resonators and slot waveguides that concentrate the light in a compact volume for
high functionality are fabricated and characterized.
Recent research progresses in slot waveguide have shown that it is possible to achieve photon confinement in low-refractive index region with nm thickness. To utilize this photon confinement, we propose a multilayer waveguide that could be the optimum design for future silicon light emission devices.
Our device consists of multiple alternating layers of Si and SiO<sub>2</sub> with nm thickness, which can be easily fabricated. Both transfer matrix method (TMM) and FDTD simulation are used to simulate the performance of this device. We calculated the propagation mode index, and photon confinement in SiO<sub>2</sub> layers. Birefringence as high as 0.8 is achieved with moderate design parameters, although a homogeneous slab waveguide also shows some birefringence, it cannot account for the high birefringence we have calculated. Thus it indirectly indicates that for TM polarization photons are actually confined in SiO<sub>2</sub> layers, where the refractive index is lower. Also our photon confinement simulation shows that, for a structure with multilayer region thickness of 0.52 μm, photon confinement in SiO<sub>2</sub> layers as high as 75% can be achieved with Si/SiO<sub>2</sub> layer thickness ratio close to 1.
We fabricated a few multilayer samples with different Si/SiO<sub>2</sub> thickness ratios and performed M-line measurement to measure the propagation mode index. The measurement results agrees well with our simulate results, indicates that for TM polarization photons can be strongly confined in SiO<sub>2</sub> layers in this multilayer structure. Thanks to this high confinement in SiO<sub>2</sub> layers, this structure could be an excellent choice for future silicon light emitting devices.
During the design of devices using Si nanostructures, it is often important to precisely know the dielectric function
ε, since it determines many of their electrical and optical properties. Several theoretical studies have predicted a
reduction in the dielectric constant ε as the nanostructure size decreases. Two competing physical mechanisms
have been proposed for the reduction: quantum confinement and surface effects (due to a breaking of polarizable
bonds on the surface).
There have been only a few experimental works on the size dependence of ε, in which ε was measured only
for one particular average size. In our work, we have measured the size-dependent ε of thin crystalline slabs at
different sizes using variable angle spectroscopic ellipsometry from 270 nm to 1700 nm at the incident angles of
65°, 70° and 75°. The thin crystalline slabs of different thicknesses (~ 15 nm to 2.5 nm) were fabricated by
repeatedly subjecting the top Si layer of SOI wafers to plasma oxidation and BOE etching. Ellipsometry and
surface profile measurements were performed between each etching step. At the wavelength of 1700 nm, for
which silicon is transparent and bulk ε is 11.7, we found thatε was reduced to 7.5 for a 2.5 nm thick Si slab. Our
results represent the first systematic measurement of the dielectric function of Si nanostructures as a function of
size and represent the first test of the theories.