Bandgap of indium nitride measured as 0.7 eV
Using MBE-grown samples from Cornell University and Ritsumeikan University, Japan, the LBNL team measured the optical properties of a wide range of InN and InGaN films. It was found that the bandgap of InGaN decreased with increasing In content, from 3.4 eV for pure GaN to 0.7 eV for InN.
The discovery came about after the LBNL researchers attempted to study light emission from InN samples at around 2 eV (equivalent to a wavelength of around 0.62 microns). "Even though the bandgap of InN was reported to be 2 eV, nobody could get light out of it at 2 eV," said LBNL s Wladek Walukiewicz. "But when we looked at a longer wavelength, all of a sudden there was lots of light."
The bandgap of 0.7 eV for InN is equivalent to a wavelength of around 1.77 microns. The value of 2 eV is thought to have been derived from InN samples fabricated by sputtering; the possible introduction of impurities such as oxygen could displace the bandgap of the material.
LBNL is interested in the applications of InGaN as a solar cell material. The bandgap range of InGaN from 0.7 to 3.4 eV provides a very close match to the solar spectrum. The most efficient solar cells combine several materials with different bandgaps to utilize the different portions of the solar spectrum. Each cell absorbs light with an energy just above the material s bandgap, while light below the bandgap energy passes through to the next cell. Triple-junction solar cells manufactured by Spectrolab combine GaInP (bandgap = 1.85 eV), GaAs (1.43 eV) and Ge (0.66 eV), and have an efficiency of around 28%.
Walukiewicz says that two layers of InGaN, one tuned to a bandgap of 1.7 eV (around 52% In) and the other to 1.1 eV (around 80% In), could attain the maximum theoretical efficiency for a double-junction cell of 50%. Lattice mismatches will cause problems when attempting to grow monolithic structures; however it has proved possible to get efficient emission from highly defective material to make InGaN-based LEDs. Other issues, such as growing p-type material, will also need to be addressed.
