Scientists at the Department of Energy's Sandia National Laboratories in Albuquerque, N.M., are researching ways to use indium gallium arsenide nitride (InGaAsN), a semiconductor alloy, as a photovoltaic power source for communications satellites and fiber-optic lasers. The researchers have discovered that the addition of 1% or 2% nitrogen to GaAs dramatically alters the alloy's optical and electrical properties and causes, in their own words, "crazy physics." These reactions provide the alloy with characteristics that suit it for satellite photovoltaics and laser applications.
Nitrogen, a small atom with high electronegativity, has a large effect on gallium arsenide's bandgap structure (the minimum energy necessary for an electron to transfer from the valence band into the conduction band and create current). The addition of nitrogen reduces the material's bandgap energy by nearly one-third.
"In the semiconductor world, this is unheard of," says Eric Jones, a Sandia physicist who has been working with InGaAsN for three years. "The new material allows designers to tailor properties for maximum current production with different bandgaps. This is what makes the material unique."
InGaAsN has captured the interest of the satellite communications industry, which sees it as a potential power source for satellites and other space systems. The new material, which may be used as part of an electricity-generating solar cell, has a potential 40% efficiency rate when put into a state-of-the-art multilayer cell. That's nearly twice the efficiency rate of a standard silicon solar cell.
Sandia's scientists produce InGaAsN through a metal-organic chemical-vapor-deposition (MOCVD) process. A GaAs wafer is heated to between 500°C and 800°C in an MOCVD reactor manufactured by EMCORE Corp. in Albuquerque. Various gases containing indium, gallium, arsenic, and nitrogen flow together into the chamber. The heat causes the source chemicals containing the elements to decompose and the elements themselves to form a crystal on the wafer, resulting in the InGaAsN alloy.
InGaAsN was developed in Japan about 10 years ago. Sandia got involved with it in the mid-1990s when Hong Hou, now chief technology officer of EMCORE's Albuquerque operations, joined the facility from Bell Labs. His Ph.D. dissertation at the University of California, San Diego, investigated the material. It was about this time that the U.S. Department of Energy (DOE) Center of Excellence for the Synthesis and Processing of Advanced Materials, headed by George Samara at Sandia, selected InGaAsN as the focus of a new line of research in photovoltaic materials.
Jones explains that an InGaAsN solar cell that could provide power to a satellite will ultimately have four layers. The top layer would consist of indium gallium phosphide (InGaP). The second would be made out of GaAs. The third would include 2% nitrogen with indium in GaAs. The fourth would be germanium.
Each layer absorbs light at different wavelengths of the solar spectrum. The first layer absorbs yellow and green light, while the second absorbs between green and deep red. The AsN layer absorbs between deep red and infrared light, and the germanium layer absorbs infrared and far infrared light. The absorbed light creates electron hole pairs. Electrons are drawn to one terminal as the holes are drawn to the other, producing electrical current.
Existing satellite systems use either silicon for solar cells or a two-layered solar panel made up of the InGaP layer and the GaAs layer. Silicon space solar cells have a maximum theoretical efficiency around 23%, while the dual-layer InGaP/GaAs solar cells' efficiency is around 30%. The layered solar cell containing InGaAsN is expected to achieve 40% efficiency. (Each percentage figure is the maximum efficiency rate possible in perfect conditions.)
The trick will be to realize these theoretical gains in practice. Commercial application becomes interesting if the addition of the InGaAsN junction can add 4% or 5% to overall cell efficiency, compared to the best commercial devices available today.
The bandgap and crystal structure (i.e., lattice constant) of InGaAsN makes it an ideal material for solar cells in space power systems (see the figure). It results in reduced satellite mass and launch cost, along with increased payload and satellite mission.
"You get two times the power from the new material compared to silicon," Jones says. "With InGaAsN, the size of the solar collecting package can be smaller, meaning the satellite will weigh less, come in a smaller package, and be cheaper to launch."
But before InGaAsN can realistically be used in a photovoltaic system, researchers must better understand the material. A higher-quality alloy also has to be developed. "We are doing a lot of tweaking to try to make the material viable," says Sandia researcher Andy Allerman. "This includes changing some things in the growth process, like temperature, and then measuring its effects after the InGaAsN is grown. We're trying to understand the optical and electrical properties."
Possible Laser Source
The same properties that make InGaAsN a prospect for photovoltaics systems for space satellites also make it a likely source for lasers designed for fiber optics. Scientists in Sandia's Semiconductor Materials and Processes department consider InGaAsN a candidate laser material that will produce the 1.3-µm bandgap needed for short-distance fiber optics systems, like those used to wire an office building.
Peter Esherick, another researcher at Sandia, notes that current fiber-optics systems use a semiconductor alloy with a base of InP as a laser source because it can grow crystals in the right bandgap range. "However," he notes, "for a lot of reasons—including because it is cheaper—researchers would prefer to use GaAs as the base substrate."
Without the addition of nitrogen, GaAs' bandgap is too high to serve as a laser source. But with the nitrogen, the bandgap falls within the usable range of 0.7 to 1.4 µm. By changing the amount of nitrogen doped into the GaAs, researchers can alter the laser's bandgap.
There are two types of semiconductor lasers: edge emitters and vertical-cavity surface-emitting lasers (VCSELs). In the edge emitter, which currently dominates the semiconductor laser market, photons shoot out of one edge of the semiconductor wafer after rebounding off mirrors that have been cleaved out of the crystalline substrate. In the VCSEL laser, photons bounce vertically between mirrors grown into the structure and then shoot straight up from the wafer surface.
The differences seem simple, but their potential consequences for manufacturing efficiency and new applications are tremendous. Most critically, laser devices that emit light from their upper surface can be fabricated side by side on a wafer in vast numbers. Sandia's researchers have successfully built an edge emitter out of the new material. This is the first step toward incorporating it into a VCSEL structure.
More information is available on the web at www.sandia.gov/media/NewsRel/NR2000/InGaAsN.htm. Eric Jones can be reached at (505) 844-8752 or [email protected] Andy Allerman is available at (505) 845-3697 or [email protected] Also, Peter Esherick can be contacted at (505) 844-5857 or [email protected]