February 10, 2011 — Nanolasers grown directly on a silicon surface could be a starting point for better microprocessors, biochemical sensors, and other optoelectronic products. UC Berkeley researchers grew nanopillars made of indium gallium arsenide, a III-V material, onto a silicon surface at 400°C.

The researchers point out that marrying III-V with silicon to create a single optoelectronic chip has been problematic. The atomic structures of the two materials are mismatched.

"Growing III-V semiconductor films on silicon is like forcing two incongruent puzzle pieces together," said study lead author Roger Chen, a UC Berkeley graduate student in electrical engineering and computer sciences. "It can be done, but the material gets damaged in the process."

Figure 1. Shown is a schematic (left) and various scanning electron microscope images of nanolasers grown directly on a silicon surface. The achievement could lead to a new class of optoelectronic chips. (Courtesy Connie Chang-Hasnain Group)

Moreover, the manufacturing industry is set up for the production of silicon-based materials, so for practical reasons, the goal has been to integrate the fabrication of III-V devices into the existing infrastructure, the researchers said.

"Today’s massive silicon electronics infrastructure is extremely difficult to change for both economic and technological reasons, so compatibility with silicon fabrication is critical," said the study’s principal investigator, Connie Chang-Hasnain, UC Berkeley professor of electrical engineering and computer sciences. "One problem is that growth of III-V semiconductors has traditionally involved high temperatures — 700°C or more — that would destroy the electronics. Meanwhile, other integration approaches have not been scalable."

The UC Berkeley researchers overcame this limitation by finding a way to grow nanopillars made of indium gallium arsenide, a III-V material, onto a silicon surface at the relatively cool temperature of 400°C.

“Working at nanoscale levels has enabled us to grow high quality III-V materials at low temperatures such that silicon electronics can retain their functionality,” said Chen.

The researchers used metal-organic chemical vapor deposition (MOCVD) to grow the nanopillars on the silicon. "This technique is potentially mass manufacturable, since such a system is already used commercially to make thin film solar cells and LEDs," said Chang-Hasnain.

"This is the first bottom-up integration of III-V nanolasers onto silicon chips using a growth process compatible with the CMOS technology now used to make integrated circuits," said Chang-Hasnain.

Figure 2. The unique structure of the nanopillars grown by UC Berkeley researchers strongly confines light in a tiny volume to enable subwavelength nanolasers. Images on the left and top right show simulated electric field intensities that describe how light circulates helically inside the nanopillars. On the bottom right is an experimental camera image of laser light from a single nanolaser. (Courtesy Connie Chang-Hasnain Group)

Once the nanopillar was made, the researchers showed that it could generate near infrared laser light (about 950nm wavelength) at room temperature. The hexagonal geometry dictated by the crystal structure of the nanopillars creates a new, efficient, light-trapping optical cavity. Light circulates up and down the structure in a helical fashion and amplifies via this optical feedback mechanism.

The researchers describe their work in a paper published in the journal Nature Photonics. Access it here: http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2010.315.html

The unique approach of growing nanolasers directly onto silicon could lead to highly efficient silicon photonics, the researchers said. They noted that the miniscule dimensions of the nanopillars — smaller than one wavelength on each side, in some cases — make it possible to pack them into small spaces with the added benefit of consuming very little energy.

"Our results impact a broad spectrum of scientific fields, including materials science, transistor technology, laser science, optoelectronics and optical physics," said Chang-Hasnain. "Ultimately, this technique may provide a powerful and new avenue for engineering on-chip nanophotonic devices such as lasers, photodetectors, modulators and solar cells," said Chen.

In the future, the researchers expect to improve the characteristics of these lasers and control them electronically.

The Defense Advanced Research Projects Agency and a Department of Defense National Security Science and Engineering Faculty Fellowship helped support this research.

Courtesy of Sarah Yang, Media Relations, UC Berkeley