New semiconductor alloy’s ‘crazy physics’ makes it a possible photovoltaic power source for satellites. Scientists at the Department of Energy’s Sandia National Laboratories are researching ways to use a new semiconductor alloy, indium gallium arsenide nitride (InGaAsN), 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 (MOCVD) 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.
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 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 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.
