Integrated photonic platforms - The case for SiC
The potential of SiC integrated photonics is starting to be unleashed, thanks to the unique photonic properties of this material and the development of high-quality epitaxial growth.
BY HAIYAN OU FROM THE TECHNICAL UNIVERSITY OF DENMARK
The most mature and applied semiconductor ever, silicon, is the pillar for modern microelectronics. For the last 50 years or more it has been revolutionising human life, with progress in computation prowess spurred on by adherence to Moore Law, leveraging planar processing.
However, the shrinking of device dimensions cannot continue forever. Line widths are now approaching their physical limit, leading those at the forefront of this technology to search for a way forward to address the looming bottleneck. One promising option involves turning to integrated photonics.
Electrons versus photons
A crucial difference between integrated electronics and integrated photonics is the carrier. Evaluated in this regard, the latter has much appeal. The merits of the photon include: an absence of charge, preventing interference from electromagnetic fields; and an absence of mass, enabling devices to operate at far faster speeds than their electronic counterparts (see Table 1). However, there are challenges in the photonic domain, including the manipulation of this carrier.
In integrated electronics, silicon dominates. Meanwhile, in integrated photonics multiple material platforms co-exist, including silicon, Si3N4, GaAs, InP, GaP, AlN, LiNbO3 and SiC. This high level of diversity reflects the fact that no single material provides the six basic building blocks for an integrated photonic chip: a light source, a waveguide, a modulator, detection, low-cost assembly and intelligence.
Silicon is playing a very active role in photonics, thanks to a well-developed material and its mature processing, refined over many decades of manufacture of integrated electronics. However, silicon has two significant drawbacks. The first is that it is an indirect bandgap semiconductor, so is inherently inefficient for light emission; and the second is that due to a bandgap of 1.12 eV, strong two-photon absorption occurs at telecommunication wavelengths, screening other nonlinearities, such as four-wave mixing, which provides wavelength conversion.
These two restraints create opportunities for other materials, such as III-Vs and wide bandgap semiconductors. For example, direct bandgap III-V semiconductors are ideal for integrated lasers solutions, while wide bandgap semiconductors have negligible two-photon-absorption within the telecommunication wavelength range.
Table 1. Comparison of photons and electrons
SiC: Pros and cons
Within this family of compound semiconductors, SiC is emerging as a very promising candidate for integrated photonics. This wide bandgap material has many strengths, including being CMOS compatible, bio-compatible, abundant, non-toxic and thus sustainable; and it has unique photonic properties, such as both high second-order and third-order nonlinearities, a high refractive index, a wide bandgap, and a low intrinsic material loss. Additional strengths include having more than 250 polytypes with variable properties, a thermal conductivity that’s more than three times that of silicon, and stable mechanical, physical and chemical characteristics. Furthermore, the processing of SiC is advanced – there is a mature growth technology for producing high crystal-quality SiC, engineers know how to dope this material, and mature material growth and device fabrication has already been established for SiC power electronics, laying the foundation for efforts to scale SiC in integrated photonics.
The wide adoption of SiC power devices, particular in electric vehicles, has laid a solid foundation for the deployment of this material system in SiC photonics. However, despite these advances in materials and fabrication, three substantial challenges have to be addressed: the formation of SiC-on-insulator (SiCOI) stacks, in order to confine the light in SiC; the development of nanofabrication technology, for low loss and dispersion control; and the introduction of an efficient coupling scheme for the coupling of light in and out of a chip.
In photonic circuits, the path of the photon is controlled by the waveguide, using total internal reflection to steer light through the structure. To ensure that photons are confined within a SiC layer, this material is embedded in low refractive index material, such as SiO2. Commercially available SiC tends to be in the form of wafers, typically 500 mm-thick, while the SiC layer in a SiCOI stack is normally less than 1 µm.
The first big challenge to address is to transfer SiC from a wafer to a thin layer. Our team at the Technical University of Denmark initially took on this challenge with an ion-cut method, also known as smart cut, well established for the production of SOI wafers. Unfortunately, this approach led to a high optical loss of around 6 dB/cm, resulting from ion implantation.
This loss could not be reduced dramatically by thermal annealing, due to limitations associated with the lower melting temperature of the silicon substrate. One possible solution might be to turn to laser annealing, rather than furnace annealing, as this source can be focused to a small spot, allowing just the SiC layer to experience a high temperature for defect recovery, while maintaining the silicon substrate below its melting point.
Figure 1. The microcomb development roadmap.
Our current approach differs from this. We avoid ion implantation altogether, using a bonding and grinding method to form our SiCOI structures. However, the yield is still very low.
Another concern is waveguide loss, resulting from imperfections in fabrication. If there is any roughness at the surface of the SiC waveguide, this increases its loss. To minimise this we have optimised our SiC waveguide fabrication process, which consists of electron-beam lithography, dry etching and top-cladding SiO2 deposition. This approach enables us to form SiC waveguides with a well-defined geometry and a smooth surface.
A typical cross-sectional geometry of our SiC waveguides is around 500 nm by 500 nm. While this is much bigger than the feature size of a FET, the transistor used to control electron transport in ICs, it is still much smaller than a spot size from a standard single mode fibre – that’s around 10 mm. Due to this significant difference in size, we have devoted much effort to ensuring efficient light coupling between a standard single-mode fibre and a SiC waveguide, using mode conversion on the chip. We also aim to achieve extremely low loss within the waveguide, so that we don’t need an on-chip amplifier, a device that doesn’t exist yet.
To test our SiC optical chips, we use a high magnification microscope to aid the alignment of SiC waveguides and optical fibres (see opening image). This task is supported by mounting the optical chip and the fibre ends on three-dimensional adjustable stages.