How to Grow a Laser on Silicon

Today, silicon photonic circuits connect server racks and play key roles in chemical sensors, biosensors, and lidar for self-driving cars. But making these devices work requires an external light source, or multistep manufacturing processes to bring lasers onto silicon chips. You can’t make a quality laser from silicon itself, and the best laser materials, which are compound semiconductors like gallium arsenide, don’t play well with silicon.

On 28 January at the SPIE Photonics West conference in San Francisco, researchers from the Belgian electronics research organization Imec described a technique that seems to sidestep these problems, making it possible to grow gallium arsenide lasers directly on standard, 300-millimeter silicon wafers using existing processes and materials.

Silicon photonics devices typically use lasers located off chip, or require various workarounds to bring a laser on board. Engineers can bond gallium arsenide dies or small wafers to silicon waveguides, then build the lasers on top, a process that requires specialized equipment and results in material waste. Or, they can transfer finished lasers to a silicon chip. Transfer processes require careful alignment, and it takes great care and time to get it right. [See “4 Ways to Put Lasers on Silicon,” IEEE Spectrum, 8 April 2023.]

Growing lasers directly on silicon as part of the manufacturing process is an appealing prospect—it should make it possible to create these devices at lower cost and at greater scale and speed. But the necessary light-emitting materials, compound semiconductors made of elements pulled from the third and fifth column of the periodic table, don’t work with silicon. The crystal structures of these III-V semiconductors don’t align with those of silicon. So when they are grown on silicon, they strain and warp, generating performance-killing flaws.

Nanoridge Lasers

The Imec team has found a way to work with this material mismatch by accepting and confining such flaws. Instead of growing their lasers directly on top of the silicon wafer, they start by covering the wafer in silicon dioxide, then cutting arrowhead-shaped trenches through this top material, all along the surface of the wafer, like a field plowed prior to planting. Next, they deposit gallium arsenide inside the trenches so that it touches only silicon at the bottom of the trench. Thanks to this uncomfortable crystalline contact, defects build up inside the trench, but they are in effect buried there, Imec product engineer Didit Yudistira told Photonics West attendees. The defects don’t spread into the nanoscale ridges of III-V material grown on top of the trenches to make the active part of the nanoridge laser.

Imec researchers managed to produce nanoridge lasers across an entire 300-millimeter wafer. The inverted-arrowhead shape at the bottom of the nanoridges traps crystal defect that would otherwise prevent lasing.Imec

To finish the lasers, the Imec group grows more gallium arsenide along the ridges, embedding it with layers of indium gallium arsenide that help control the wavelength of light. The devices are finished with electrical contacts and mirrors. The Imec team has made hundreds of these compound semiconductor lasers, as well as photodetectors, on 300-mm silicon wafers, the size used in advanced fabs.

Imec researchers described this result earlier in January in Nature. In a commentary accompanying the scientific paper, University College Cork photonics researcher Brian Corbett writes that this approach “could offer a scalable, low-cost way to produce photonic circuits.”

The nanoridge lasers work at a wavelength of about 1,020 nanometers, which is shorter than the wavelengths typically used for telecommunications. At Photonics West, Yudistra said the group is now working to make devices that lase at slightly longer wavelengths. The nanoridge lasers also show some signs of wear and tear, because the semiconductor material develops some defects where it abuts the device’s electrical contacts. Yudistra said they are working on a different design to prevent this problem.