Indium Gallium Arsenide
Indium gallium arsenide (InGaAs) is also a compound III-V semiconductor. It can be applied in two ways for use in TPVs. When lattice-matched to an InP substrate, InGaAs has a bandgap of 0.74 eV, which is not an improvement on traditional GaSb. Devices of this configuration have been produced with a fill factor of 69% and an efficiency of 15%. However, to absorb higher wavelength photons, the bandgap may be engineered by changing the ratio of In to Ga. The range of bandgaps for this system is from about 0.4 to 1.4 eV. However, these different structures cause strain with the InP substrate. This can be controlled with graded layers of InGaAs with different compositions. This was done to develop of device with a quantum efficiency of 68% and a fill factor of 68% grown by molecular beam epitaxy. This device also had a bandgap of 0.55 eV achieved in the compound In0.68Ga0.33As. InGaAs has the advantage of being a well-developed material. InGaAs can also be made to lattice match perfectly with Ge resulting in very low defect densities. Being able to use Ge as a substrate is a significant advantage over more expensive or harder to produce substrates.
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