Dr. Evil and On-Chip "LASERS" for Silicon Photonics

In the 1999 comedy, The Spy Who Shagged Me, Dr. Evil laments about why he can’t have sharks with “laser beams” attached to their heads. I get the feeling that silicon photonic designers sometimes feel the same way about why they don’t yet have integrated on-chip laser light sources. While off-chip light sources have good light-emitting efficiency and thermal stability they suffer from relatively large coupling losses between the laser and the photonic IC (PIC) and higher packaging costs. At ITC 2015 W.R. Bottoms presented on ensuring reliability in the era of heterogeneous integration (see paper here). In his presentation he stated that 2015 was the year in which the number of mobile-connected devices first exceeded the number of people on the earth. He went on to project that broadband speeds need to more than double by the year 2019 and that in order to do this our concept of network architectures will need to change. Key to that change will be the movement of network photonics closer to the chip level.

On-chip light sources have the potential for moving photonics onto the chip itself promising higher integration density, compact size and better energy efficiency. Unfortunately, silicon(Si) is an indirect band gap semiconductor and is very inefficient at light generation. This has caused the on-chip light source to be one of the last lagging components of a truly integrated photonics solution. Companies like Luxtera have been successful using off-chip light sources in telecom markets but lack of progress on an on-chip light source is currently limiting the progress of chip level optical interconnect technology. According to an article published in Light Sciences and Applications, an ideal on-chip light source should be able to emit light at 1310 or 1550nm wavelengths to connect directly to the external fiber optical networks, lase under electrical pumping for compact size & high integration density, display high power efficiency for sufficient output power and low energy cost-per-bit transmission and be able to integrate on Si with CMOS compatible fabrication techniques for large scale manufacturing. The paper reviews three most likely solutions for an on-chip light source, those being Erbium (Er) related light sources, Germanium-on-Si lasers and III-V-based hybrid Si lasers. Table 1 from the article lists these light source candidates with their advantages and disadvantages.

While good progress has been made on ER-doped fiber amplifiers and lasers (EDFAs/EDFLs) they have yet to make the jump to electrically pumped lasers, one of the key criteria for an on-chip light source so for now Er is not on the short term horizon.

Germanium (Ge) is an interesting candidate for on-chip lasing in that it is the material most closely matched to Si, to the point that it too is an indirect band gap material. However, Ge is different from Si in that it exhibits a pseudo-direct band gap behavior that enables it to emit light of approximately the magical 1550nm wavelength. Much research exists around what is known as band gap engineering with the idea to modify the band structure of Ge enough to effectively turn it into a direct band gap material. Good progress has been made to this end using strategies such as enhanced n-type doping to fill-up the valence band with electrons used for lasing, and using tensile strain and alloys of Ge and Tin (Sn) to shrink the band gap to enable efficient lasing.

One of the main challenges is to establish a trade-off between these strategies in terms of optimizing the performance of a Ge laser while also avoiding operating wavelength redshift, an artifact of narrowing the bandgap. An additional challenge yet to be overcome is the relatively high threshold current density required for Ge lasers. Challenges notwithstanding, Ge’s large gain spectrum and ability to work at high temperatures makes it very attractive in wavelength division multiplexing (WDM) systems and high-density optical-electrical ICs. Additionally, Ge is also widely used for modulation and detection and therefore could simultaneously address all of these areas in a monolithic integrated SiGe-based photonic platform while maintaining compatibility within a CMOS process flow needed to reduce process complexity and cost.

In the meantime, III-V-based hybrid Si lasers using various bonding techniques currently represent the most practical on-chip silicon photonic light sources. These lasers however suffer from poor heat dissipation due to the high thermal resistance of the bonding layers. Given this, these types of lasers may not be suitable for large-scale dense monolithic integration in terms of yield and cost over the long term. The alternative with growing momentum is high-quality quantum dot (QD) materials that have been successfully grown on Si using direct hetero-epitaxial growth (III-V QDs). These III-V lasers have been demonstrated to maintain lasing operation at up to 120 °C with low threshold current densities of 62.5 A/cm[SUP]2[/SUP]. Monolithically grown on Si, they could be more promising as on-chip lasers, and may satisfy the requirements for low-cost, high-yield, temperature-insensitive, and large-scale high-density monolithic integration.

So what would Dr. Evil have done with an integrated on-chip “laser”? Strapped it onto the head of “Mini-Me” of course!