A study of the optical and polarisation properties of InGaN/GaN multiple quantum wells grown on a-plane and m-plane GaN substrates

摘要

In this paper we report on a comparative study of the low temperatureemission and polarisation properties of InGaN/GaN quantum wells (QWs) grown onnonpolar a-plane and m-plane free-standing bulk GaN substrates where the Incontent varied from 0.14 to 0.28 in the m-plane series and 0.08 to 0.21 for thea-plane series. The low temperature photoluminescence spectra from both sets ofsamples are very broad with full width at half-maximum height increasing from81 to 330 meV as the In fraction increases. Comparative photoluminescenceexcitation spectroscopy indicates that the recombination mainly involvesstrongly localised carriers. At a temperature of 10 K the degree of linearpolarisation of the a-plane samples is much smaller than of the m-planecounterparts and also varies across the spectrum. From polarisation-resolvedphotoluminescence excitation spectroscopy we measured the energy splittingbetween the lowest valence sub-band states to lie in the range of 23-54 meV forboth a-and m-plane samples in which we could observe distinct exciton featuresin the polarised photoluminescence excitation spectroscopy. Thus, the thermaloccupation of a higher valence subband cannot be responsible for the reductionof the degree of linear polarisation. Time-resolved spectroscopy indicates thatin a-plane samples there is an extra emission component which at least partlyresponsible for the reduction in the degree of linear polarisation.