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Since the early 1960s, alloys are commonly grouped into two classes,featuring bound states in the bandgap (I) or additional, non-discrete bandstates (II). Microscopic material parameters for class I alloys can directly beextracted from photoluminescence (PL) spectra, whereas any conclusions drawnfor class II alloys usually remain indirect and limited to macroscopicassertions. Nonetheless, here, we present a spectroscopic study on excitonlocalization in a so-called mixed crystal alloy (class II) that allows us toaccess microscopic alloy parameters. We study bulk In$_x$Ga$_{1-x}$N epilayersat the onset of alloy formation (0 $\leq$ $x$ $\leq$ 2.4%) in order tounderstand the material's particular robustness to defects. Based on anin-depth PL analysis it is demonstrated how different excitonic complexes(free, bound, and complex bound excitons) can serve as a probe to monitor thedilute limit of class II alloys. From an $x$-dependent linewidth analysis weextract 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 transfermechanisms towards non-radiative defects are suppressed. In addition, theassociated low temperature PL data suggests that a single indium atom does notsuffice in order to permanently capture an exciton. Micro-PL spectra even giveaccess to a forthright probing of silicon bound excitons embedded in aparticular environment of indium atoms, thanks to the emergence of a hierarchyof individual, energetically sharp emission lines (full width at half maximum$\approx$ 300 $\mu$eV). Consequently, the present study allows us to extractfirst microscopic alloy properties formerly only accessible for class I alloys.

Probing alloy formation using different excitonic species: The particular case of InGaN