Lasers made of quantum dots could work with silicon

In an effort to break the ‘silicon bottleneck’ that occurs when the signal from fibre-optic cables arrives at data centres, researchers are working to integrate photonics into silicon devices.

Computers run on electrons moving through silicon-based chips, which are less efficient than photonics. With this in mind, scientists have been developing lasers — a crucial component of photonic circuits — that work seamlessly on silicon. Now, researchers from the University of California, Santa Barbara have proposed that the future of silicon-based lasers may be in tiny, atom-like structures called quantum dots.

According to Justin Norman, a graduate student at UC Santa Barbara, replacing the electronic components that connect devices with photonic components could cut energy use by 20 to 75%. “It’s a substantial cut to global energy consumption just by having a way to integrate lasers and photonic circuits with silicon,” he said.

Unfortunately, silicon does not have the right properties for lasers, so the UC Santa Barbara researchers turned to a class of materials from Groups III and V of the periodic table because these materials can be integrated with silicon. The team initially struggled to find a functional integration method, but ultimately ended up using quantum dots because they can be grown directly on silicon.

Quantum dots are semiconductor particles only a few nanometres wide — small enough that they behave like individual atoms. When driven with electrical current, electrons and positively charged holes become confined in the dots and recombine to emit light — a property that can be exploited to make lasers.

The researchers made their III–V quantum-dot lasers using a technique called molecular beam epitaxy. They deposited the III–V material onto the silicon substrate and its atoms self-assembled into a crystalline structure.

But the crystal structure of silicon differs from III–V materials, leading to defects that allow electrons and holes to escape, degrading performance. Fortunately, because quantum dots are packed together at high densities — more than 50 billion dots/cm² — they capture electrons and holes before the particles are lost.

The team’s lasers also have many other advantages, Norman said. For example, quantum dots are more stable in photonic circuits because they have localised, atom-like energy states. They can also run on less power because they don’t need as much electric current. Moreover, they can operate at higher temperatures and be scaled down to smaller sizes.

Writing in the journal APL Photonics, the researchers have revealed that their lasers can operate at 35°C without much degradation. They also report that the lasers’ lifetime could be up to 10 million hours.

The team are now testing lasers that can operate at 60–80°C, which is the more typical temperature range of a data centre or supercomputer. They are also working on designing epitaxial waveguides and other photonic components, Norman said.

Image caption: One type of laser that’s particularly suited for quantum dots is a mode-locked laser, which passively generates ultrashort pulses less than one picosecond in duration. Image credit: Peter Allen.