US scientists grow GaAs films on low cost substrates
III-V semiconductors such as GaAs make highly efficient solar cells but the GaAs or Ge substrates used for their epitaxial growth account for more than half of the cost, giving limited use in terrestrial photovoltaic applications.
While efforts have been made to grow high quality single-crystalline GaAs and III-V materials on low-cost substrates such as metal foils, they have not been very successful. Either the cost-reduction has been insufficient for making solar cells for terrestrial use or the opto-electronic properties have become degraded so the power conversion efficiency is low.
Now a research team from the University of Houston, South Dakota School of Mines and Technology, and NASA Johnson Space Center, has reported the heteroepitaxial growth of high mobility, single-crystalline-like n and p-type doped GaAs thin films on inexpensive, flexible metal foils using an MOCVD process. The GaAs films exhibited hole and electron mobilities as high as 66 and 300cm2/V-s, respectively.
The picture above shows: a) epitaxial GaAs thin film on flexible metal foil, and (b) schematic of the multilayer architecture developed in this work to grow epitaxial GaAs thin films on IBAD templates on metal foils.
The epitaxy enabling substrate for GaAs growth was single-crystalline-like Ge thin film on biaxially textured templates made by ion beam assisted deposition on metal foil.
According to the team, the GaAs films on metal foils showed single-crystalline-like nature, strong biaxial texture, low grain misorientation (less than 2degrees) and strong photoluminescence at room temperature. Precise control of doping and carrier concentration was achieved, resulting in GaAs films with high carrier mobility and electrical conductivity.
The researchers believe the work can pave the path for roll-to-roll manufacturing of flexible III-V solar cells for the mainstream photovoltaics market.
'High mobility single-crystalline-like GaAs thin films on inexpensive flexible metal substrates by metal-organic chemical vapor deposition' by P. Dutta et al, appeared in Appl. Phys. Lett. 105, 092104 (2014); http://dx.doi.org/10.1063/1.4895388
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