LAUSR

Since the early 1960s, alloys are commonly grouped into two classes, featuring bound states in the bandgap (I) or additional, non-discrete band states (II). Microscopic material parameters for class I alloys can directly be extracted from photoluminescence (PL) spectra, whereas any conclusions drawn for class II alloys usually remain indirect and limited to macroscopic assertions. Nonetheless, here, we present a spectroscopic study on exciton localization in a so-called mixed crystal alloy (class II) that allows us to access microscopic alloy parameters. We study bulk In$_x$Ga$_{1-x}$N epilayers at the onset of alloy formation (0 $\leq$ $x$ $\leq$ 2.4%) in order to understand the material's particular robustness to defects. Based on an in-depth PL analysis it is demonstrated how different excitonic complexes (free, bound, and complex bound excitons) can serve as a probe to monitor the dilute limit of class II alloys. From an $x$-dependent linewidth analysis we extract the length scales at which excitons become increasingly localized, meaning that they convert from a free to a bound particle upon alloy formation. Already at x = 2.4% the average exciton diffusion length is reduced to 5.7 $\pm$ 1.3 nm at a temperature of 12 K, thus, detrimental exciton transfer mechanisms towards non-radiative defects are suppressed. In addition, the associated low temperature PL data suggests that a single indium atom does not suffice in order to permanently capture an exciton. Micro-PL spectra even give access to a forthright probing of silicon bound excitons embedded in a particular environment of indium atoms, thanks to the emergence of a hierarchy of individual, energetically sharp emission lines (full width at half maximum $\approx$ 300 $\mu$eV). Consequently, the present study allows us to extract first microscopic alloy properties formerly only accessible for class I alloys.