A key aspect of the physics of recombination in InGaN quantum wells (QWs) grown on the polar c-plane of GaN is charge carrier separation across the QW resulting from internal electric fields arising from spontaneous and piezoelectric polarisation.  A consequence of these fields is that holes in polar QWs probe the lower quantum well interface (where InGaN was grown on GaN) and electrons probe the upper interface.  Holes tend to be localised at randomly-occurring regions of increased indium content, whereas electrons may be localised at well width fluctuations[1].  These differences in localisation spatially separate carriers within the plane of the QW, in addition to the field-related separation across the well.  Signatures of this carrier separation in photoluminescence measurements include long recombination lifetimes and variation of these lifetimes across the QW emission band. This spectral dependence of the timescale of the luminescence decays results from changes in strain and hence macroscopic built-in electric field associated with the randomly-occurring variations in local indium content responsible for hole localisation. Non-polar QWs, in contrast, exhibit much shorter recombination lifetimes, with little variation in lifetime across the QW emission band.  Here, in the absence of internal fields, the electron and hole are co-localised and strong wavefunction overlap leads to fast decays for all wavelengths[2].

This understanding of the impact of internal electric fields on the recombination dynamics of InGaN QWs can be used to interpret time-resolved cathodoluminescence data from a range of InGaN nano-objects, thus providing new nanoscale insights not achieveable by photoluminescence.  For InGaN/GaN core-shell nanorods which have various polar, non-polar and semi-polar facets, the polar facets exhibit longer recombination lifetimes and a marked variation in lifetime across the lineshape, which is reduced for the non- and semi-polar planes. For trench defects in InGaN/GaN polar QWs, strain relaxation within a region surrounded by a trench reduces the internal fields, which is again observed as a reduced wavelength dependence of the carrier lifetime.  Overall, the measurement of the spectral variations in recombination lifetimes at the nanoscale provides a valuable probe of the impact of nanoscale structure on carrier recombination mechanisms in nitride-based materials.  

[1] Dawson et al. The nature of carrier localisation in polar and nonpolar InGaN/GaN quantum wells J. Applied Physics 119, 181505(2016)

[2] Schulz et al. Structural, electronic, and optical properties of m-plane InGaN/GaN quantum wells: Insights from experiment and atomistic theory Phys. Rev. B 92, 235419(2015)