Addition of nitrogen to gallium arsenide dramatically alters optical and electrical properties
There’s "crazy physics going on."
That’s what Sandia researchers say about a new semiconductor alloy, indium gallium arsenide nitride (InGaAsN), they are developing for possible use as a photovoltaic power source for space communications satellites.
The addition of one or two percent nitrogen in gallium arsenide, a standard semiconductor material, dramatically alters the alloy’s optical and electrical properties.
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. In fact, the addition of the nitrogen reduces the material’s bandgap energy by nearly one-third.
"In the semiconductor world, this is unheard of," says Eric Jones (1113), a physicist who has been working with the material 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 that sees it as a potential power source for satellites and other space systems. The new material, which may be used as an electricity-generating solar cell, has a potential 40 percent efficiency rate when put into a multi-layer cell. That is nearly twice the efficiency rate of a standard silicon solar cell.
Sandia scientists make InGaAsN using a metal-organic chemical vapor deposition (MOCVD) process. A gallium arsenide wafer is heated to between 500 and 800 degrees C in an MOCVD reactor manufactured by EMCORE Corp. Various gases containing indium, gallium, arsenic, and nitrogen are flowed 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, creating the InGaAsN alloy.
The nitrogen-carrying source used in this process is dimethylhydrazine (DMHy) — which is creating some of the roadblocks in the research.
"The nitrogen source we are getting from suppliers is not very pure. And purity is something needed to grow high quality crystals," says Andy Allerman (1711), one of the researchers who grows and studies the material. "Once we get a pure source, we will be able to determine how effective InGaAsN alloys will be in devices."
InGaAsN was first 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 Corp. Albuquerque Operations, joined the Labs from AT&T Bell Labs. His PhD dissertation at the University of California, San Diego, was on the material.
It was about this time that the DOE Center of Excellence for the Synthesis and Processing of Advanced Materials, headed by George Samara (1152), selected High Efficiency Photovoltaics as one of the center’s technical thrusts. This brought together research funded by DOE’s Basic Energy Sciences (BES) program and the photovoltaic division of DOE’s Energy Efficiency and Renewable Energy Program. The photovoltaics research was to have two directions — a search for a way to build better and less expensive silicon solar cells and development of the next-generation thin film for advanced photovoltaics.
InGaAsN’s potential as a photovoltaic material was so intriguing it was selected to be the focus of the new line of research.
Competition is keen to develop the InGaAsN into a viable material that will actually work as a laser or photovoltaic system for space systems. But Sandia appears to be ahead of the pack.
"We’re in a worldwide race to come up with a high quality indium gallium arsenide nitride material," says Andy. "Many people can make lasers and solar cells, including us. We have the distinct advantage of having state-of-the-art growth capabilities, material characterization techniques, and advanced computing that allow us to understand these materials and to make them into useful devices."
Team members working on material development are Andy, Eric, Steve Kurtz (1713), Rob Sieg (1711), and Normand Modine (1113). Providing their expertise in the photovotaic area are James Gee and Dan Aiken, both 6219.
Eric heads up a DOE BES committee made up of scientists from several national laboratories called the Thin Films for Advanced Photovoltaics Group, which collaborates on InGaAsN research. Funding for Sandia’s photovoltaic efforts comes from DOE, Lockheed Martin, and the Air Force Research Laboratory.
Eric says an InGaAsN solar cell that could provide power to a satellite will ultimately have four layers. The top layer would consist of the alloy indium gallium phosphide; the second of gallium arsenide; the third of two percent nitrogen with indium in gallium arsenide; and the fourth, germanium.
Each layer absorbs light at different wavelengths of the solar spectrum. The first layer, for example, absorbs yellow and green light, while the second absorbs between green and deep red. The arsenide nitride layer absorbs between deep red and infrared, and the germanium absorbs infrared and far infrared. The absorbed light creates electron hole pairs. Electrons are drawn to one terminal and the holes 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 indium gallium phosphide layer and the gallium arsenide layer. Silicon space solar cells have a maximum efficiency around 23 percent, while that of the dual-layer indium gallium phosphide/gallium arsenide solar cell is around 30 percent. That compares to the 40 percent efficiency rate predicted for the layered solar cell containing InGaAsN. (Each percentage figure is the maximum efficiency rate possible in perfect conditions.)
The right bandgap and crystal structure (i.e., lattice constant) of InGaAsN is what makes it an ideal material for solar cells in space power systems. It results in reduced satellite mass and launch cost and increased payload and satellite mission.
Two times the power
"You get two times the power from the new material as from silicon," Eric says. "With InGaAsN, the size of the solar collecting package can be smaller, meaning that 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, a better understanding of the material needs to be achieved and a higher quality alloy must be developed. That is what Sandia scientists are striving to accomplish.
"We are doing a lot of tweaking to try to make the material viable," Andy says. "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."
To make sure they have grown a "good crystal" the researchers observe the reactor on computer monitors during the deposition process and do material characterization, X-ray diffraction, and electrical measurements. They also take it apart and test it physically.
Eric says many questions about the new material remain to be answered and much research is left to do.
"Our goal now is to determine how to improve the growth process, obtain a better understanding of why the material does what it does, and figure out what the material is capable of," he says. "What we learn from studying this unusual system will allow us to better understand other laser and solar cell materials."
Last modified: February 29, 2000