First Electrically-Injected Laser Bumps Speed and Efficiency in Optoelectronics

August 11, 2020 by Antonio Anzaldua Jr.

When used as a semiconductor, the newfound diode laser made of germanium and tin significantly improves on-chip photonics and nano-electronics.

When we think of lasers, we may remember scenes straight out of science fiction movies. In reality, lasers serve many purposes—for instance, in ranging devices, bar code scanners, and optical communications.

In 1962, the first laser made of gallium arsenic (GaAs) sparked interest in lasers that could push the limits of current semiconductor processes.

Now, materials science researchers at the University of Arkansas (UARK) have used germanium (Ge) and tin (Sn) to create the first electrically-injected laser, which could improve the speed of micro-processing and efficiency. When tested, the GeSn laser "operated in pulsed conditions up to 100 kelvins, or 279 degrees below zero Fahrenheit."


Optically-Pumped vs. Electrically-Injected Lasers

Manufacturers may use different structures of semiconductors depending on lasing wavelength, threshold, temperature operation, power consumption, and the size of end devices. There are two types of lasers used for semiconductors: optically-pumped and electrically-injected laser devices.

For optically-pumped lasers, the optical field can be well confined since there is nothing but air above the capping layer of the structure.

However, for electrically-injected laser devices, which have metal contact above the cap layer, the thickness of the material’s cap layer needs to be carefully optimized to minimize the optical loss via the metal thin film. 


An Advance in Si-Based Group IV Lasers

Silicon, germanium, and their alloys have been key materials for the electronics industry to progress, allowing developers to improve the semiconductor fabrication processes. Si-based group IV (Si) GeSn alloys offer a tunable bandgap from indirect to direct, making them useful candidates for on-chip photonics and nano-electronics.

Material science researchers at the University of Arkansas (UARK), led by electrical engineering professor Shui-Qing “Fisher” Yu, have demonstrated the first electrically-injected laser made with germanium tin. Used as a semiconducting material for circuits on electronic devices, the diode laser could improve micro-processing speed and efficiency at much lower costs, according to the researchers. 


The power output and spectrum characteristics of an electrically-injected GeSn laser

The power output and spectrum characteristics of an electrically-injected GeSn laser. Image used courtesy of Professor Yu and his research team at UARK


Professor Yu expressed his excitement about his team's discovery: “Our results are a major advance for group-IV-based lasers. They could serve as the promising route for laser integration on silicon and a major step toward significantly improving circuits for electronic devices.”

The material could lead to the development of low-cost, lightweight, compact, and low power-consuming electronic components that use light for information transmission and sensing.

With Sn content over 8%, GeSn turns into a direct bandgap material, which is essential for efficient light emission. Furthermore, the GeSn epitaxy is monolithic on Si and fully compatible with CMOS processes.


The Effect of Electrically-Injected GeSn Lasers on Electronics

This recent study on GeSn lasers is appealing to circuit designers because it suggests that the ability to integrate a large variety of devices (such as amplifiers, ultra-low power switches, and optoelectronic devices) on the same Si substrate may be around the corner.

In a previous article, AAC contributor Luke James discussed how silicon may not be a suitable candidate for emitting light during chip fabrication. In this respect, group IV photonics promises to reduce the power consumption of electric circuits through the addition of optical components.

In order to transfer electrically-stored information to an optical circuit, an on-chip laser is highly desirable.


The structure of the laser device along with researchers’ calculations of type II alignment between GeSn active and SiGeSn cap layers

The structure of the laser device along with researchers’ calculations of type II alignment between GeSn active and SiGeSn cap layers. Image used courtesy of Professor Yu and his research team at UARK


The study completed at UARK demonstrated that adding a SiGeSn buffer can enhance the hole confinement and improve material quality. Additionally, this fabrication technique can minimize interface defects and surface roughness so internal optical losses can be reduced.


What's Next for GeSn Lasers?

To further improve the device performance, investigations of new structure designs are underway. These experiments include increasing the Sn content to boost the bandgap directness so that injection efficiency can be increased. 

Electrically-injected lasers such as the GeSn laser are more complicated to achieve due to the need for doping and building structures with solar cells. Many GeSn structures have been investigated for LED applications.

Researchers aim to find direct bandgap GeSn layers with enough doping concentration for n- and p-type doping. However, professor Yu’s research of optically-pumped GeSn for bulk and micro-disk devices may lead to a fully-operable, electrically-injected GeSn laser.


Featured image used courtesy of UARK