Electrical and optical characterization of group III-nitride alloys for solar energy conversion

The group III-nitride alloys In1- xAlxN and In1- xGaxN are promising materials for high efficiency solar cells as well as photoelectrochemical cells for hydrogen production. By varying x, the band gaps of the alloys are tunable across the solar spectrum.;Native defects can dominate the electrical and optical properties of these materials. The effects of the native defects in In-rich films are studied using high energy irradiation with 2 MeV He+ ions to introduce controlled quantities of native point defects. Typically, the defects are triple donors, creating n-type films even when there is no intentional doping. The absorption edge and the photoluminescence peak show a blue shift with increasing defect concentration that is attributed to the corresponding increase in the electron concentration. Further, the high concentration of defects creates a metallically conductive layer on the surfaces of In-rich films that complicates device operation.;It is important to consider the effects of the heavy n-type doping and the narrow band gaps in these alloys when analyzing the fundamental material properties. As an example, the compositional dependence of the band gap in InAlN is found by accounting for the high electron concentrations as well as the nonparabolicity of the conduction band.;Due to their triple charge state, there is a strong Coulomb repulsion between the donor defects. In InN with high defect concentrations, it is energetically favorable for the defects to form an ordered configuration, which reduces their scattering efficiency. Rapid thermal annealing of irradiated InN films provides sufficient energy for the defects to diffuse and correlate their positions. As a result, the films have high electron mobilities with high electron concentrations.;A study of n-type InGaN photoelectrodes finds that the incorporation of a tandem solar cell will be required for efficient production of hydrogen from sunlight. The n-type films corrode under operation, further necessitating modification of the surface with a catalyst and/or a protective oxide coating. P-type films may provide better corrosion resistance.