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Components | June 22, 2012

Breakthrough in cubic silicon carbide materials growth

Swedish researchers have developed a method for a material which can be used for highly efficient solar cells. The material, cubic silicon carbide, may allow collecting more than twice the energy from the sun compared with silicon based solar cells.
The group from Linköping University has developed a growth method with a growth rate that is even higher than in comparable methods for growth of commercial hexagonal silicon carbide.

‐ The trick is to decrease the growth temperature, which allows formation of the cubic silicon carbide, while maintaining the growth rate by adjusting other parameters, says Mikael Syväjärvi who heads the research group. A high growth rate is important for the cost consideration, while a large cost is usually given by the packaging. The group now wants to explore the solar cell properties of the material, and potentially create a start‐up company since there is no other competitive method.

Michael Syväjärvi is researching new materials, especially semiconductors for energy and environment. His research focuses on materials for white LEDs for general lighting and photovoltaics, as well as the Nobel Prize material graphene. He is one of the founders of Graphensic AB.
© Mikael Syväjärvi
Cubic silicon carbide is considered very suitable for making highly efficient solar cells. But the quality has been poor. Now researchers in Sweden have developed a method for growth of cubic silicon carbide in world leading quality similar to the commercial hexagonal silicon carbide.

The cubic silicon carbide is a perfect material for impurity (intermediate bandgap) solar cell. For boron doped cubic SiC, the dopant band of B in the bandgap of 3C‐SiC leads to a efficient use of sun light so that an efficiency up to 48‐60% could be achieved depending on the theoretical model.

But cubic silicon carbide has shown to be the black sheep of the silicon carbide family. The hexagonal silicon carbide types have been commercialized since many years, while the cubic silicon carbide has faced challenges. It is metastable, meaning that it does not really want to form. One has to decrease the growth temperature to make it form, but at the same time the growth rate is decreased. The common approach is to use silicon as substrate, but the lattice and thermal mismatch causes defects and stress.

The researchers at Linköping University have applied a bulk growth approach like used in production of hexagonal silicon carbide. The trick is to lower the growth temperature while adjusting other parameters to maintain a high growth rate. The group applies hexagonal silicon carbide as substrate, and the material is transformed to the cubic structure during initial stage of growth. The advantage of using hexagonal substrate is the perfect matching.

"The trick is to decrease the growth temperature, which allows formation of the cubic silicon carbide, while maintaining the growth rate by adjusting other parameters."
The growth rate is 1 mm/hr, and structural measurements show a similar quality like in hexagonal commercial material. The key parameter in showing off the quality is the carrier lifetime. Previously this had a lifetime of about 0.1 μs, while the new record value is 8.2 μs in as‐grown material, an increase of almost two orders of magnitude. In comparison, this is even slightly better than that in as‐grown hexagonal silicon carbide.

Todays silicon solar cells have an efficiency of 20%. In order to increase the efficiency of solar cells, multijunction (thin film) solar cells with different bandgaps is one of the most promising. The best efficiency of such solar cells demonstrated on the research scale is 43.5%. However, the challenges in fabrication of multijunction solar cells lie in the growth of multi‐stacked material and balance of junction currents. Cubic silicon carbide in a single material which is doped during growth, and having a high growth rate like 1 mm/h, could pave the way for more efficient solar cell concepts.

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