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

Since the 1960s, alloys are grouped into two classes, featuring bound states in the bandgap (I - GaP:N, ZnTe:O, etc.) or additional, non-discrete, band states (II - SiGe, GaAsP, InGaAs, etc.). Therefore, one can observe either a rich and informative zoo of excitons bound to isoelectronic impurities (I), or the typical bandedge emission of a semiconductor that shifts and broadens with rising isoelectronic doping (II). Because no such strongly localized excitons are found in the photoluminescence (PL) spectra of the investigated bulk In(x)Ga(1-x)N epilayers ( 0 ≤ x ≤ 2.4%, 100 nm thick, growth on bulk GaN substrates), we suggest to utilize shallow impurities as a tool to study the distribution of isoelectronic centers. By micro-PL, we directly observe an entire hierarchy of bound excitons related to silicon-indium complexes as individual, energetically sharp emission lines appear (full width at half-maximum ≈ 300μeV). In order to exemplify our approach, we determine nanoscopic parameters of the InGaN alloy like the exciton diffusion length. We expect that our spectroscopic analysis represents a general pathway for studying mixed crystal alloys associated to class II, which approaches the high level of spectroscopic sophistication evidenced in literature for class I alloys.