A joint research team from the University of Houston, Applied Optoelectronics Inc. (AOI) and Cornell University has won an intense race to develop a better way to build lasers and other optoelectronic devices.
Just as the development of structural steel revolutionized the construction industry, this new technique dramatically expands the field of epitaxy, the process used to manufacture high performance semiconductor devices such as lasers, optical detectors, and microwave devices and circuits. Dubbed the "compliant universal substrate," Dr. Chau-Hong Kuo of the Space Vacuum Epitaxy Center (SVEC), the NASA Commercial Space Center at UH, unveiled this new technique for creating epitaxial thin film devices last week, at the North American Molecular Beam Epitaxy Conference in Pennsylvania.
"This technique will allow us to create lasers and optoelectronic devices with better performance and lower costs by relieving a lot of the materials constraints," said Steven Pei, associate director for research at SVEC.
The compliant universal substrate concept was first proposed by scientists at Cornell University in 1996. Since then, research groups around the world have raced to produce the first device on the compliant substrate. "AOI initiated this research under a grant from the National Science Foundation's Small Business Innovation Research program," said Dr. Thompson Lin, president of AOI, a spin-off company from SVEC.
Epitaxy is a technique for growing single crystal materials on a base or substrate with atomic precision. A combination of layers might produce a laser, while another combination produces a high efficiency solar cell. However, traditional epitaxy faces one substantial hurdle: the substrate's crystalline structure must match the material being placed on top. It's like trying to align the grids on two pieces of graph paper--the grids must match. Currently, only a few substrate materials are available, fewer still are affordable. Thus, materials requiring substrates with different "grid sizes" can not be used, greatly limiting researchers' options.
The "compliant universal substrate" is like a grid printed on a piece of rubber loosely bonded to a conventional substrate. It expands or contracts to match the grid of the epitaxy thin film grown on top of it. By eliminating concerns about matching the grids on the underlying conventional substrate, the universally compliant substrate dramatically increases the choices of epitaxy thin films / substrate combinations for optoelectronic applications and may even lead to less expensive base / substrate materials.
Researchers from SVEC and AOI demonstrated the viability of the technique by building a mid-infrared laser on the new compliant universal substrate bonded to an otherwise incompatible substrate. The new structure significantly improved the laser's cooling, allowing it to produce more power. The NSF, Air Force and Ballistic Missile Defense Organization are currently funding the research at SVEC and AOI to develop semiconductor mid-infrared lasers for environmental monitoring and jamming of heat-seeking missiles.
Although applicable to all epitaxy thin films, researchers expect this new technique will be most useful for developing lasers and other optoelectronic devices. For example, the development of blue and ultraviolet lasers has been hindered by the lack of an appropriate substrate. With the compliant universal substrate, researchers are one step closer to producing blue lasers for color display and high-density optical storage applications.