New Alloy May Benefit Satellite Power Systems

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New Alloy May Benefit Satellite Power Systems

A new semiconductor alloy, indium gallium arsenide nitride (InGaAsN), being researched at the Department of Energy's Sandia National Laboratories may prove to be a photovoltaic power source for space telecommunication satellites and for lasers in fiber optics (InGaAsN was first developed in Japan about 10 years ago). The addition of one or two percent nitrogen in gallium arsenide dramatically alters the alloy's optical and electrical properties giving it characteristics suitable 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 the nitrogen reduces the material's bandgap energy by nearly one-third. The new material, which may be used as part of an electricity-generating solar cell, has a potential 40 percent efficiency rate when put into a state-of-the-art multi-layer cell, nearly twice the efficiency rate of a standard silicon solar cell. InGaAsN is made 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 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, creating the InGaAsN alloy. 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 theoretical efficiency around 23 percent, while the dual-layer indium gallium phosphide/gallium arsenide solar cell is around 30 percent. An InGaAsN solar cell that could provide power to a satellite would 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. The bandgap and crystal structure (i.e., lattice constant) of InGaAsN makes it an ideal material for solar cells in space power systems. Its use would result in reduced satellite mass and launch cost and increased payload and satellite mission performance.

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March 20, 2000

Andrews Space & Technology