Solar-Tectic on course for sapphire success
As cheap sapphire glass production gathers pace, solar cell start-up, Solar-Tectic, has set its sights on smartphones and more. Compound Semiconductor reports.
Set to scale cheap sapphire glass production, Solar-Tectic will target solar cell, smartphone and LED applications.
If Solar-Tectic's latest developments are anything to go by, 2015 looks set to be a big year for the US-based solar cell start-up.
Plans to produce sapphire glass, consisting of a thin film of sapphire on MgO-buffered soda-lime glass, are well underway with smartphone covers clearly being the number one application.
What's more, company researchers have also deposited highly textured silicon films on the MgO/soda-lime glass substrate, opening the door to cheap fabrication of photovoltaics and more. And with the technology also being honed for LEDs, the future is looking bright.
Solar-Tectic was set up in 2010, to commercialise the thin film technology developed by the late Dr Praveen Chaudhari, IBM materials physicist, Brookhaven National Laboratory director and 1995 US National Medal of Innovation and Technology winner.
As his son, Dr Ashok Chaudhari, explains: "His idea for solar cells at the time was to deposit high quality but inexpensive thin films onto inexpensive substrates."
"He wanted to deposit silicon onto a cheap substrate, with the trick being to deposit a thin film of sapphire onto glass," adds Chaudhari. "If you could do this, then you have your crystalline substrate on which you can deposit silicon or another semiconductor film."
Five years on and the 'trick' is being put into practice. Late last year, and working alongside colleagues from thin film deposition system manufacturer, Blue Wave Semiconductors, US, the company received a US patent for its growth of single crystal semiconductor films, on inexpensive substrates, including glass.
"Soda lime glass is very cheap and that makes a big difference," says Chaudhari.
"Other researchers hadn't really thought about using this glass as it has a very low melting point," he adds. "But we can deposit thin films onto this glass at much low temperatures than other deposition processes."
Traditional science
Chaudhari's success lies in the materials phenomenon, 'eutectic melting'. When two elements, such as silicon and a metal, combine to form a superlattice, known as a eutectic system, the melting point of each is lowered.
At the so-called eutectic point, both elements within the superlattice will melt at the lowest possible melting temperature. However, fix the relative compositions of each as required, and your element of choice will still melt from the superlattice at a lower temperature than if in its pure form.
Chaudhari has harnessed this effect to develop his proprietary eutectic deposition method.
Here, silicon films can be deposited from a metal-semiconductor system, such as an Al-Si eutectic melt, onto glass via an electron beam evaporation system between temperatures of 300 to 600°C. These growth temperatures are higher than the materials systems' eutectic point, but don't melt the glass substrate.
As Chaudhari explains: "When depositing silicon from the eutectic melt, we keep the temperature constant, above the eutectic temperature, and increase the amount of silicon until heterogeneous nucleation takes place. This allows for large grained thin film deposition."
So far the results are very promising. Initial X-ray diffraction of sapphire films, deposited via the electron beam evaporation system onto the Mg-buffered soda lime glass, reveals continuous highly textured Al2O3, crucial for smartphone cover applications.
At the same time, similar analyses of silicon films, deposited from an Al-Si eutectic melt, reveal a highly textured silicon film, ready for GaN deposition and, of course, LEDs.
TEM diffraction pattern of sapphire/MgO interface showing crystallinity of each. UV visible spectroscopy has also shown near perfect transparency of sapphire glass. [Chaudhari]
Chaudhari soon hopes to scale the process. Right now he and colleagues are working with small sample sizes, but in his words: "We don't foresee any issues for scalability here."
And crucially, the process is without a doubt relatively low-cost. As sapphire furnace maker and supplier of sapphire material for Apple's smartphones, GT Advanced Technologies, files for bankruptcy, Chaudhari claims his deposition process is 'many orders of magnitude cheaper'.
"It also consumes much less energy than the Apple-GTAT furnaces that heated sapphire to 2000°C. We only heat to 550°C and for a much shorter time," he says.
"Our process is very traditional materials science, and why neither Apple nor GTAT took this route puzzles me," he adds. "Apple is very cunning, but employees are primarily designers, not materials scientists, so maybe they just missed this traditional materials view."
So where next for the start-up? According to Chaudhari, a next step is to deposit GaN on his highly textured silicon layers. In-house work starts soon, and companies are already contacting him to collaborate and develop the technology further.
"We're a start-up and a family-run business with a limited budget," he says. "We've got great results with silicon so need a company to work with that can really take this forward."
