Group IV functionalization of low index waveguides

Low fabrication error sensitivity, integration density, channel scalability, low switching energy and low insertion loss are the major prerequisites for future on-chip WDM systems and interfacing with optical fibres. A number of device geometries have already been demonstrated that fulfil these criteria, at least in part, but combining all of the requirements is still a difficult challenge.Two contenders that could fulfil these criteria are the low loss nitride waveguiding platform and the high index group IV compounds for active photonic devices. Silicon Oxynitride (SiON) and Silicon Nitride (SiN) based waveguides are extremely powerful and central to today’s optical communications networks. The intermediate refractive index provides low footprint devices but eases the fabrication demands that can result in phase errors and repeatability problems in the all silicon approach. This enables multiplexers and demultiplexers with very low crosstalk and insertion loss and extremely low loss long range waveguides, making them very attractive for the optical backplanes and rack to rack links inside supercomputers and data centers. Group IV Photonics GeSi has a number of attractive optical characteristics for modulation, absorption and detection in a small volume area enabling low power and high density integration.Here, we propose and demonstrate a novel architecture consisting of the interfacing of a range of deposition method using low temperature PECVD and HWCVD nitride waveguides, Photonic crystal modulators [1] but also detectors [2] connected by a silicon nitride bus waveguide. The architecture features very high scalability due to the small size of the devices (~100 micrometre square) and the modulators operate with an AC energy consumption of less than 1fJ/bit

Low temperature silicon nitride waveguides for multilayer platforms

Several 3D multilayer silicon photonics platforms have been proposed to provide densely integrated structures for complex integrated circuits. Amongst these platforms, great interest has been given to the inclusion of silicon nitride layers to achieve low propagation losses due to their capacity of providing tight optical confinement with low scattering losses in a wide spectral range. However, none of the proposed platforms have demonstrated the integration of active devices. The problem is that typically low loss silicon nitride layers have been fabricated with LPCVD which involves high processing temperatures (<1000 ºC) that affect metallisation and doping processes that are sensitive to temperatures above 400ºC. As a result, we have investigated ammonia-free PECVD and HWCVD processes to obtain high quality silicon nitride films with reduced hydrogen content at low temperatures. Several deposition recipes were defined through a design of experiments methodology in which different combinations of deposition parameters were tested to optimise the quality and the losses of the deposited layers. The physical, chemical and optical properties of the deposited materials were characterised using different techniques including ellipsometry, SEM, FTIR, AFM and the waveguide loss cut-back method. Silicon nitride layers with hydrogen content between 10-20%, losses below 10dB/cm and high material quality were obtained with the ammonia-free recipe. Similarly, it was demonstrated that HWCVD has the potential to fabricate waveguides with low losses due to its capacity of yielding hydrogen contents <10% and roughness <1.5nm

Photonic crystal waveguides on silicon rich nitride platform

We demonstrate design, fabrication, and characterization of two-dimensional photonic crystal (PhC) waveguides on a suspended silicon rich nitride (SRN) platform for applications at telecom wavelengths. Simulation results suggest that a 210 nm photonic band gap can be achieved in such PhC structures. We also developed a fabrication process to realize suspended PhC waveguides with a transmission bandwidth of 20 nm for a W1 PhC waveguide and over 70 nm for a W0.7 PhC waveguide. Using the Fabry–Pérot oscillations of the transmission spectrum we estimated a group index of over 110 for W1 PhC waveguides. For a W1 waveguide we estimated a propagation loss of 53 dB/cm for a group index of 37 and for a W0.7 waveguide the lowest propagation was 4.6 dB/cm

Hot-wire chemical vapour deposition for silicon nitride waveguides

In this work, we demonstrate the use of HWCVD as an alternative technique to grow SiN layers for photonic waveguides at temperatures <400ºC. In particular, the effect of the ammonia flow and the filament temperature on the material structure, optical properties and propagation losses of the deposited films was investigated. SiN layers with good thickness uniformity, roughness as low as 0.61nm and H concentration as low as 10.4×1021 atoms/cm3 were obtained. Waveguides fabricated on the studied materials exhibited losses as low as 7.1 and 12.3 dB/cm at 1310 and 1550nm respectively

Si-rich silicon nitride for nonlinear signal processing applications

Nonlinear silicon photonic devices have attracted considerable attention thanks to their ability to show large third-order nonlinear effects at moderate power levels allowing for all-optical signal processing functionalities in miniaturized components. Although significant efforts have been made and many nonlinear optical functions have already been demonstrated in this platform, the performance of nonlinear silicon photonic devices remains fundamentally limited at the telecom wavelength region due to the two photon absorption (TPA) and related effects. In this work, we propose an alternative CMOS-compatible platform, based on silicon-rich silicon nitride that can overcome this limitation. By carefully selecting the material deposition parameters, we show that both of the device linear and nonlinear properties can be tuned in order to exhibit the desired behaviour at the selected wavelength region. A rigorous and systematic fabrication and characterization campaign of different material compositions is presented, enabling us to demonstrate TPA-free CMOS-compatible waveguides with low linear loss (~1.5dB/cm) and enhanced Kerr nonlinear response (Re{γ} = 16 Wm-1). Thanks to these properties, our nonlinear waveguides are able to produce a pi nonlinear phase shift, paving the way for the development of practical devices for future optical communication applications

Silicon nitride for integrated photonic applications

Due to its flexible optical properties silicon nitride is an attractive material for integrated photonic circuits. In this paper, we review the results we have obtained on near-infrared photonic devices including low loss waveguides based on SiN layers deposited with low temperature PECVD using an ammonia-free chemistry. In particular, we discuss the fabrication of subwavelength suspended structures to extend the use of SiN to mid-infrared photonic devices

NH₃-free PECVD silicon nitride for photonic applications

Silicon Photonics has open the possibility of developing multilayer platforms based on complementary metal-oxide semiconductors compatible materials that have the potential to provide the density of integration required to fabricate complex photonic circuits. Amongst these materials, silicon nitride (SiN) has drawn attention due to its fabrication flexibility and advantageous intrinsic properties that can be tailored to fulfil the requirements of different linear and non-linear photonic applications covering the ultra-violet to mid-infrared wavelengths. Yet, the fabrication techniques typically used to grow SiN layers rely on processing temperatures > 400 C to obtain low propagation losses, which deem them inappropriate for multilayer integration. This thesis presents a systematic investigation that provided a comprehensive knowledge of a deposition method based on an NH3-free plasma enhanced chemical vapour deposition recipe that allows the fabrication of low-loss silicon nitride layers at temperatures < 400 C. The results of this study showed that the properties of the studied SiN layers depend mostly on their N/Si ratio, which is in fact one of the only properties that can be directly tuned with the deposition parameters. These observations provided a framework to optimise the propagation losses and optical properties of the layers in order to develop three platforms intended for specific photonic applications. The first one comprises 300nm stoichiometric SiN layers with refractive index (n) of 2 that enable the fabrication of photonic devices with propagation losses < 1 dB/cm at l = 1310nm and < 1:5 dB/cm at l = 1550 nm, which are good for applications that require efficient routing of optical signals. The second one consists on 600nm N-rich layers (n = 1.92) that allow fabricating both devices with propagation losses < 1 dB/cm at l = 1310 nm, apt for polarisation independent operation and coarse wavelength division multiplexing devices with cross-talk < 20 dB and low insertion losses. Finally, the last platform consisted of suspended Si-rich layers (n = 2.54) that permits the demonstration of photonic crystal cavities with Q factors as high as 122 000 and photonic crystal waveguides capable of operating in the slow-light regime. Hopefully, the demonstration of these platforms will stimulate the development of more complex SiN devices for multilayer routing, wavelength division multiplexing applications and non-linear integrated photonics in the future

Athermal silicon nitride angled MMI wavelength division (de)multiplexers for the near-infrared

WDM components fabricated on the silicon-on-insulator platform have transmission characteristics that are sensitive to dimensional errors and temperature variations due to the high refractive index and thermo-optic coefficient of Si, respectively. We propose the use of NH3-free SiNx layers to fabricate athermal (de)multiplexers based on angled multimode interferometers (AMMI) in order to achieve good spectral responses with high tolerance to dimensional errors. With this approach we have shown that stoichiometric and N-rich SiNx layers can be used to fabricate AMMIs with cross-talk<30dB, insertion loss <2.5dB, sensitivity to dimensional errors <120pm/nm, and wavelength shift <10pm/°C

Towards a fully functional integrated photonic-electronic platform via a single SiGe growth step

Silicon-germanium (Si1-xGex) has become a material of great interest to the photonics and electronics industries due to its numerous interesting properties including higher carrier mobilities than Si, a tuneable lattice constant, and a tuneable bandgap. In previous work, we have demonstrated the ability to form localised areas of single crystal, uniform composition SiGe-on-insulator. Here we present a method of simultaneously growing several areas of SiGe-on-insulator on a single wafer, with the ability to tune the composition of each localised SiGe area, whilst retaining a uniform composition in that area. We use a rapid melt growth technique that comprises of only a single Ge growth step and a single anneal step. This innovative method is key in working towards a fully integrated photonic-electronic platform, enabling the simultaneous growth of multiple compositions of device grade SiGe for electro-absorption optical modulators operating at a range of wavelengths, photodetectors, and bipolar transistors, on the same wafer. This is achieved by modifying the structural design of the SiGe strips, without the need to modify the growth conditions, and by using low cost, low thermal-budget methods

Fully Integrated SiN/SOI (De)Multiplexer for the O-band

We present the experimental demonstration of a silicon nitride (de)multiplexer fully integrated with a thick silicon-on-insulator platform for coarse wavelength division multiplexing applications in the O-band