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New semiconductor alloy's crazy physics makes it a possible photovoltaic power source for satellites

DOE/Sandia National Laboratories : 30 December, 2006  (New Product)
Scientists at the Department of Energy's Sandia National Laboratories are researching ways to use a new semiconductor alloy, indium gallium arsenide nitride, as a photovoltaic power source for space communications satellites and for lasers in fiber optics.
The addition of one or two percent nitrogen in gallium arsenide, a standard semiconductor material, dramatically alters the alloy's optical and electrical properties and causes 'crazy physics' to occur, giving it characteristics suitable for satellite photovoltaics and laser applications, says Eric Jones, a Sandia physicist who has been working with the material for three years.

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,' Jones says. 'The new material allows designers to tailor properties for maximum current production with different bandgaps. This is what makes the material unique.'

High efficiency rate
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 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. That is nearly twice the efficiency rate of a standard silicon solar cell.

Sandia scientists make InGaAsN using a metal-organic chemical vapor deposition process. They heat a gallium arsenide wafer 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.

It was about this time that the 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 material.

Four layers
Jones 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 theoretical efficiency around 23 percent, while 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.

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 percent of 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. It results in reduced satellite mass and launch cost and increased payload and satellite mission.

'You get two times the power from the new material as from 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, and a higher quality alloy must be developed.

'We are doing a lot of tweaking to try to make the material viable,' Sandia researcher Andy Allerman 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.'

Possible laser source
The same properties that make InGaAsN a prospect for photovoltaics systems for space satellites cause Sandia scientists to eye it closely as a possible source for lasers used in fiber optics.

Scientists in Sandia's Semiconductor Materials and Processes Department view InGaAsN as a candidate laser material that will produce the 1.3 micron bandgap needed for short-distance fiber optics systems, like those used to wire an office building. A laser produces an intense, coherent, directional beam of light from a semiconductor material that has been excited (light stimulating protons and protons stimulating more protons). The light impulse, which is encoded information then flows through fiber optic lines at the speed of light.

Sandia researcher Peter Esherick says current fiber optics systems use a semiconductor alloy with a base of indium phosphide as a laser source because it can grow crystals in the right bandgap range.

However, Esherick notes, 'for a lot of reasons', including because it is cheaper, researchers would prefer to use gallium arsenide as the base substrate.

Without the addition of nitrogen, gallium arsenide's 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 microns. By changing the amount of nitrogen doped into the gallium arsenide, researchers can alter the laser's bandgap.

Two types of semiconductor lasers exist, edge emitter and vertical cavity surface emitting laser. In the edge emitter, which currently dominates the semiconductor laser market, photons shoot out 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 researchers have successfully built an edge emitter out of the new material, which is the first step toward incorporating the new material into a VCSEL structure.
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