Amorphous silicon waveguides and light modulators for integrated photonics realized by low-temperature plasma-enhanced chemical-vapor deposition

CMOS compatible athermal silicon photonic filters based on hydrogenated amorphous silicon

We report for the first time, wavelength filters with reduced thermal sensitivity, based on a combination of crystalline silicon and hydrogenated amorphous silicon (a-Si:H) waveguides, integrated on the same silicon on an insulator wafer through a Complementary Metal Oxide Semiconductor (CMOS) compatible process flow. To demonstrate the concept, we design and fabricate Mach Zehnder Interferometers (MZIs) and Arrayed Waveguide Gratings (AWGs) based on this approach, and we measure thermal drift <1[pm/°K] in MZIs and <10 [pm/°K] in AWGs at C band.

Widely tunable bandpass filter based on resonant optical tunneling

We describe a tunable bandpass filter and polarizer based on resonant tunneling through an air gap between two hemi-cylindrical prisms coated with 4-layer a-Si/SiO₂ matching stacks. Tuning is achieved by simultaneous variations in the incident angle and the air gap thickness, enabling the pass-band center wavelength to be continuously adjusted over a very wide range (potentially ~1000 - 1800 nm) with an approximately fixed fractional bandwidth (Δλ/λ ~1%). An analytical derivation of the conditions required to produce a flat-top TE pass-band at a desired wavelength is given. The filter provides excellent out-of-band rejection and strong suppression of the orthogonal TM polarization over the entire tuning range. For applications involving collimated light, it could be a useful alternative to existing widely tunable filters based on gratings or liquid crystals.

Athermal and wavelength-trimmable photonic filters based on TiO₂-cladded amorphous-SOI

Large-scale integrated silicon photonic circuits suffer from two inevitable issues that boost the overall power consumption. First, fabrication imperfections even on sub-nm scale result in spectral device non-uniformity that require fine-tuning during device operation. Second, the photonic devices need to be actively corrected to compensate thermal drifts. As a result significant amount of power is wasted if no athermal and wavelength-trimmable solutions are utilized. Consequently, in order to minimize the total power requirement of photonic circuits in a passive way, trimming methods are required to correct the device inhomogeneities from manufacturing and athermal solutions are essential to oppose temperature fluctuations of the passive/active components during run-time. We present an approach to fabricate CMOS backend-compatible and athermal passive photonic filters that can be corrected for fabrication inhomogeneities by UV-trimming based on low-loss amorphous-SOI waveguides with TiO2 cladding. The trimming of highly confined 10 μm ring resonators is proven over a free spectral range retaining athermal operation. The athermal functionality of 2nd-order 5 μm add/drop microrings is demonstrated over 40°C covering a broad wavelength interval of 60 nm.

Hydrogenated amorphous silicon photonic device trimming by UV-irradiation

A method to compensate for fabrication tolerances and to fine-tune individual photonic circuit components is inevitable for wafer-scale photonic systems even with most-advanced CMOS-fabrication tools. We report a cost-effective and highly accurate method for the permanent trimming of hydrogenated amorphous silicon photonic devices by UV-irradiation. Microring resonators and Mach-Zehnder-interferometers were utilized as photonic test devices. The MZIs were tuned forth and back over their complete free spectral range of 5.5 nm by locally trimming the two MZI-arms. The trimming range exceeds 8 nm for compact ring resonators with trimming accuracies of 20 pm. Trimming speeds of ≥ 10 GHz/s were achieved. The components did not show any substantial device degradation.

Low-temperature deposition of high-quality silicon oxynitride films for CMOS-integrated optics

The growth of silicon oxynitride thin films applying remote inductively coupled, plasma-enhanced chemical vapor deposition is optimized toward high optical quality at a deposition temperature as low as 150°C. Propagation losses of 0.5±0.05 dB/cm, 1.6±0.2 dB/cm, and 0.6±0.06 dB/cm are measured on as-deposited waveguides for wavelengths of 1300, 1550, and 1600 nm, respectively. Films were deposited onto a 0.25 μm technology mixed-signal CMOS chip to show the application perspective for three-dimensional integrated optoelectronic chips.

Broadband all-optical modulation in hydrogenated-amorphous silicon waveguides

We demonstrate broadband all-optical modulation in low loss hydrogenated-amorphous silicon (a-Si:H) waveguides. Significant modulation (approximately 3 dB) occurs with a device of only 15 microm without the need for cavity interference effects in stark contrast to an identical crystalline silicon waveguide. We attribute the enhanced modulation to the significantly larger free-carrier absorption effect of a-Si:H, estimated here to be alpha = 1.6310(-16)N cm(-1). In addition, we measured the modulation time to be only tau(c) approximately 400 ps, which is comparable to the recombination rate measured in sub-micron crystalline silicon waveguides, illustrating the strong dominance of surface recombination in similar sized (460 nm x 250 nm) a-Si:H waveguides. Consequently, a-Si:H could serve as a high performance platform for backend integrated CMOS photonics.

Electro-optically induced absorption in alpha-Si:H/alpha-SiCN waveguiding multistacks

Electro optical absorption in hydrogenated amorphous silicon (proportional-Si:H)--morphous silicon carbonitride (proportional-SiCxNy) multilayers have been studied in two different planar multistacks waveguides. The waveguides were realized by plasma enhanced chemical vapour deposition (PECVD), a technology compatible with the standard microelectronic processes. Light absorption is induced at lambda = 1.55 microm through the application of an electric field which induces free carrier accumulation across the multiple insulator/semiconductor device structure. The experimental performances have been compared to those obtained through calculations using combined two-dimensional (2-D) optical and electrical simulations.