Highly Mismatched GaN1-xAsx Alloys Across the Entire Composition Range
Typically only dilute (up to 10%) highly mismatched alloys (HMAs) can be grown due to the large differences in atomic size and electronegativity of the host and the alloying elements. Recently, we overcame the miscibility gap of the GaN1-xAsx system using low temperature molecular beam epitaxy (LT-MBE) and successfully synthesized alloys over a wide composition range. In the intermediate composition range (0.10 < x < 0.75) the resulting alloys are amorphous. Including the amorphous alloys, the band gap energy of the GaN1-xAsx alloy spans from ~0.8 eV – 3.4 eV opening a wide range of possible applications, especially multi-junction solar cells with a single system. The ability to understand and control the properties of these alloys is important for the band gap engineering of energy-related semiconductor devices.
The amorphous GaN1-xAsx alloys have been grown on sapphire, silicon, and inexpensive glass substrates. Transmission electron microscopy (TEM) selected area diffraction (SAD) patterns and high resolution micrographs show that the alloys in the intermediate composition range are homogenously amorphous. Optical absorption spectra exhibit a systematic shift of the absorption edge with composition and are sharp, indicating strong local ordering of the amorphous alloy. The band gap can also be determined from photomodulated reflectance, which probes the critical points of the density of states.