Taking III-V Growth Into The Next Dimension
Engineers at the University of California, Berkeley, have developed an inexpensive new way to grow thin films of InP. This achievement could bring high-end solar cells within reach of consumer pocketbooks.
The work, led by Ali Javey, UC Berkeley associate professor of electrical engineering and computer sciences, is described in a paper published in Scientific Reports, Nature’s peer-reviewed open access journal.
“Performance is everything in the solar cell industry, but performance at a reasonable cost is key," says Javey, who is also a faculty scientist at the Lawrence Berkeley National Laboratory. “The techniques we are reporting here should be a game-changer for III-V solar cells, as well as for LEDs."
The most efficient photovoltaics are made from III-V compounds. However, the complex manufacturing requirements for III-V materials make them up to ten times more expensive than silicon. This limits their use in military applications and NASA satellites.
UC Berkeley engineers could help make high-end solar cells, currently used in satellites and other space and military applications, affordable for consumer markets. (iStockPhoto)
The conventional growth of III-Vs requires expensive epitaxial growth substrates, low precursor utilisation rates, long growth times, and large equipment investments.
Addressing this issue, UC Berkeley researchers decided to explore cheaper ways to grow the III-V material indium phosphide (InP). They demonstrated that InP can be grown on thin sheets of metal foil in a process that is faster and cheaper than traditional methods, yet still comparable in optoelectronic characteristics.
The researchers used a process they call Vapour Liquid Solid (VLS) growth.
In this research, the scientists deposited indium films onto electropolished molybdenum foils by either electron-beam (e-beam) evaporation or electroplating, followed by e-beam evaporation of a 50 nm silicon oxide (SiOx) cap. The Mo/In/SiOx stack was then heated in hydrogen to a growth temperature above the melting point of indium (~157°C).
After temperature stabilisation, phosphorous vapour was introduced into the chamber. The diffusion of phosphorous vapour through the capping layer and dissolution in the liquid indium resulted in the precipitation of solid InP crystals.
Scanning electron micrograph of the InP (Credit: Ali Javey, Rehan Kapadia and Zhibin Yu)
Using this technique they demonstrated high quality 1 - 3 μm thick InP thin-films on molybdenum foils with ultra-large grain size up to 100 μm. The researcher say this is about 100 times larger than those obtained by conventional growth processes such as MOCVD.
The films exhibited electron mobilities as high as 500 cm2/V-s and minority carrier lifetimes as long as 2.5 ns. What's more, under 1-sun equivalent illumination, photoluminescence efficiency measurements indicated that an open circuit voltage of up to 930 mV can be achieved, only 40 mV lower than measured on a single crystal reference wafer.
This work is described in detail in the paper, "A direct thin-film path towards low-cost large-area III-V photovoltaics," by Kapadia et al in Scientific Reports,3, Article number 2275. doi:10.1038/srep02275
The full paper can be accessed via the link http://www.nature.com/srep/2013/130724/srep02275/full/srep02275.html
The U.S. Department of Energy helped fund this research.