Photomodulated Reflectivity Measurement of Free-Carrier Dynamics in InGaN/GaN Quantum Wells

We describe a novel technique for measuring carrier dynamics in solid-state optical materials based on photomodulated reflectivity (PMR) and, as an example, apply it to a study of an InGaN/GaN multi-quantum-well (QW) structure grown on a c-plane sapphire substrate. The technique is a form of frequency modulation spectroscopy and relies on probing changes in refractive index induced by fluctuations in free-carrier density during optical excitation. We show that it is possible to accurately determine both carrier density and lifetime, independent of any photoluminescence (PL) measurement and with no knowledge of the incident, or fraction of absorbed, laser power, quantities that can give rise to considerable uncertainties in PL studies. We demonstrate that such uncertainties can lead to an order of magnitude underestimation of the total photogenerated carrier density and compromised accuracy in determining carrier lifetime. We determine, by a comparison of the two techniques, PMR and PL, the nonradiative Shockley–Reed–Hall (SRH), radiative (excitonic), and nonradiative Auger-related coefficients (from the standard ABC model). We find marked differences in the carrier-density-dependent lifetime, determined from PMR, translate to significant differences in the SRH and excitonic coefficients, which we believe relate to the more accurate determination of carrier densities in PMR than in PL. We also find evidence from the PMR for a change in effective mass of the photoexcited carriers with excitation intensity, which points to a complex localization/delocalization mechanism, likely facilitated by random fluctuations in indium content and QW width, consistent with previous findings by independent methods.