A new type of traffic signal began appearing in U.S. cities a few years ago. Brighter, longer lasting, and more energy efficient than standard incandescent traffic lights, these new signals relied on light emitting diodes (LEDs), simple devices made of semiconducting materials with the ability to produce light when a current is applied.
This new lighting technology has the potential to replace most types of incandescent and fluorescent lighting, with an energy savings estimated at more than $90 billion annually in the U.S. alone. But before LEDs can replace existing lighting, more breakthroughs are necessary to lower materials cost and raise efficiency.
Group III Nitride Thin Films
At Penn State, materials scientist Joan Redwing is pursuing an understanding of the Group III Nitrides, and gallium nitride thin films in particular, that are used for LEDs and many electronic applications. These materials are important because of their large band gap energy, says Redwing, professor of materials science and engineering. “The band gap sets the wavelength for the light the material emits, so for example, if you make a structure like a light emitting diode and pass a current through it, it gives off light, and the color of the light is roughly equivalent to the band gap energy.”
“The real interest in the nitride materials developed about ten years ago when there was a lot of interest in coming up with red, blue, and green LEDs for full-color displays, like the giant screens in football stadiums,” Redwing says. “The mixing of green, blue, and red light gives you a whole variety of different colors. At that time there was a good red emitting diode, but a less good green, and no blue diode. So there was a flurry to find a good blue emitting material.”
Solid State Lighting
Now interest has shifted to the development of solid state lighting based on light emitting diodes, with the hope that they can become efficient enough in the next few years to replace incandescent and fluorescent lighting. Much of the research on LEDs is being funded through the Department of Energy.
Military Uses For Gallium Nitride Semiconductors
The military also has interest in nitrides, Redwing says. “If you can make the bandgap energy larger and larger, the material can withstand greater voltages. You can operate the devices at higher power without failure occurring compared to silicon, which is the standard semiconductor.” The military has an interest in gallium nitride semiconductors for radar systems and also for any kind of microwave-based communications system.
In addition, Redwing says, the military has an interest in blue or UV lasers. These could have applications in high data storage military systems and chemical biological reagent sensing, as well as covert communications. She is working with the Penn State Electro-Optics Center on these potential uses of nitride materials.
Understanding stress in nitride materials
Superconducting Thin Films and Semiconductor Nanowires
While much of their work is in non-optical materials like superconducting thin films and semiconductor nanowires for electronic devices, her group is also studying gallium nitride growth on silicon substrates for optical applications. Currently, light emitting diodes are deposited on sapphire substrates, and sapphire is a relatively expensive material, says Redwing. If they could overcome problems associated with putting nitride materials on a silicon substrate, then industry would be able to create lower cost LEDs and begin to integrate nitride devices with the logic functions that can be fabricated on silicon substrates.
“There is significant interest in being able to integrate nitride materials onto a silicon platform,” Redwing says. “The problem with using silicon as the substrate is that there is a big difference in the thermal expansion coefficients of silicon and gallium nitride. When you heat the substrate up to 1000°C and deposit this material and then cool it down to room temperature, the silicon contracts at a different rate than the gallium nitride. This causes the gallium nitride films to crack. We've been studying the stress that's induced in the film and coming up with methods to minimize it and prevent the cracking.”
Metal Organic Chemical Vapor Deposition (MOCVD)
Redwing recently received funding from the National Science Foundation for her gallium nitride/silicon research. Her group uses a technique called Metal Organic Chemical Vapor Deposition (MOCVD) to synthesize nitride thin film layers. This involves heating a substrate in a reactor and then introducing gases: trimethyl gallium, nitrogen, and ammonia. These gases pass into the reactor and decompose as they become heated at the substrate surface, forming a thin film on the substrate. Her group utilizes a specialized optical system to measure the thin film deposition rate and changes in the curvature of the substrate while the film is depositing. The changes in substrate curvature provide information on the stress in the film.
Film Layer Cracking
The cracking of the film layer comes about because the gallium nitride is under tensile stress and is being pulled apart by the silicon substrate. To cancel the tensile stress, her group is trying to build in a compressive stress. They do this by using a transition layer between the silicon and the gallium nitride, in this case by alloying the film with aluminum. They start with a high amount of aluminum mixed with gallium nitride, and gradually decrease the aluminum until the film is just gallium nitride. The intermediate aluminum gallium nitride layer is about one micron in thickness, as is the pure gallium nitride layer. The silicon substrate is about 500 microns thick.
“We’ve studied this structure using the optical wafer curvature measurement system. What happens is that the film starts out with a high density of defects, which is typical because of the difference in lattice constant between the film and the substrate. By using this graded intermediate layer which introduces a compressive stress, we can get the dislocations to bend and annihilate each other so the gallium layer on top has a lower density of dislocations. Also, the compressive stress that is introduced by the graded layer offsets the tensile stress from thermal expansion and significantly reduces film cracking.”
Thin Film Stresses
Understanding what causes stress in thin film materials is important for making any device, Redwing says. “The type of research we do is to understand on a fundamental level the origin and evolution of stress in these films,” she concludes. “The application for our work is in improving LEDs and transistors, and integrating gallium nitride on silicon for new optical and electronic devices.”