Dynamical study of the radiative recombination processes in GaN/AlGaN QWs

Abstract

The effects of the Si doping level on the recombination dynamics and carrier (exciton) localization in modulation-doped GaN/Al_0.07Ga_0.93N multiple-quantum-well (MQW) structures were studied by means of photoluminescence (PL) and time-resolved PL measurements. All samples with different doping levels show a QW emission which is blue shifted with respect to the 3.48 eV PL peak from the GaN buffer layer. The decay time at the peak position remains nearly constant in the range of 320–420 ps at 2 K for all doping levels. For the undoped and low doped samples (3 × 10¹⁸ cm⁻³), which have less free electrons in the QWs, a non-exponential PL decay behaviour at 2 K is attributed to localized exciton recombination. The more highly doped samples (5 × 10¹⁸ cm⁻³ to 10²⁰ cm⁻³) show almost exponential decay curves at 2 K, suggesting the recombination of free electrons and localized holes. The internal polarization-induced fields of the medium and highly-doped samples are partly screened by the electrons originating from the doping in the barriers. The emission peaks in time delayed PL spectra of these samples exhibit almost no shift as time evolves. Only the PL peak of the undoped and low-doped samples shows a redshift with time delay, related to the photogenerated carriers [1]. The decay time for the undoped sample shows non-exponential behaviour typical for localized excitons in III-N QWs. The same behaviour of decay time as a function of emission energy has been reported for InGaN QWs [2].