Optical and electrical characterization of ternary and quaternary gallium nitride based alloys

Progress in epitaxial growth techniques in the last 20 years has led to the availability of high quality ternary (InGaN) and quaternary (AlInGaN) alloys for optical devices [1], high efficient solar cells, high electron mobility transistors, microwave power applications [2] and, recently, photo-electrochemical (PEC) devices for water splitting cells [3]. Their most interesting feature is the bandgap tunable with composition, covering the whole visible spectrum. Despite extensive research on In-based alloy systems, the dependence of the bandgap, polarization properties, role of dislocations and strain relaxation mechanism on the In-content is still debated, and the effect of crystal defects on optical and electrical properties of heterostructures based on InxGa1-xN and AlxInyGa1-x-yN alloys is not yet clear. This contribution presents the results of electrical and optical characterization of InxGa1-xN/GaN and AlxInyGa1-x-yN/GaN structures by Surface Photovoltage (SPV) Spectroscopy and deep level transient spectroscopy (DLTS) in correlation with Transmission electron microscopy analysis (TEM) and light-assisted Kelvin probe force microscopy (KPFM). The influence of In content and Si-doping concentration on the misfit dislocations and electronic transitions in InxGa1-xN layers has been investigated has been discussed. Furthermore, the two-dimensional electron gas at the AlxInyGa1-x-yN/GaN heterointerfacehas been characterized by SPV and the main recombination centers in quaternary AlxInyGa1-x-yN/GaN alloys have been determined in terms of its activation enthalpy and capture cross-section. Furthermore, light-assisted KPFM reveals direct visualization of sites associated with recombination centers/traps, which includes threading dislocations, misfit dislocations and coalescence boundaries. The present analysis has allowed us to clarify the role of In content and misfit dislocations on alloy disorder and to characterize the defects acting as strong recombination center in ternary and quaternary GaN based alloys. [1] S. Nakamura et al., Proc. IEEE 101, 2211 (2013). [2] N. M. Johnson et al., Phys Today 53, 31 (2000). [3] J. Jia et al., Nat. Commun. 7, 13237 (2016)