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

Abstract

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.

Bimolecular and Auger Recombination in Phase-Stable Perovskite Thin Films from Cryogenic to Room Temperature and Their Effect on the Amplified Spontaneous Emission Threshold

“…We use transient reflection (TR) spectroscopy to study the dynamics of the perovskite thin film on a Si substrate and make use of the strong signal in the near-infrared (NIR) region to monitor the charge-carrier density as a function of time. This signal reveals the change in refractive index, which is proportional to the concentration of free carriers in the material. − Exemplary spectra as a function of time are given in Figure S2.…”

Efficiency droop in zincblende InGaN/GaN quantum wells

“…Full details of the technique, as well as the process to extract carrier density from the experimental data can be found in ref. 30.…”

“…Combined PL and photoreflection (PR) measurements were conducted at room temperature, in a similar manner to ref. 30, to measure the carrier density corresponding to the onset of droop in the zb-QW samples. Firstly, the variation of emis- sion efficiency with incident power density was determined using a CW 405 nm laser to resonantly excite the QWs and so avoid inhomogeneous carrier distribution effects (see section IIIA2), with ND filters employed to alter the excitation power density.…”

Self-Cleaning and Self-Powered UV Sensors for Highly Reliable Outdoor UV Detection

“…Theoretically, UV light can excite photogenerated carriers in BaTiO 3 , which will increase the conductance of BaTiO 3 and lead to a reduced impedance of the PENG. However, the density of the photogenerated carriers n is related to the carrier lifetime τ where, τ is the carrier lifetime, and G is the carrier generation rate, which is equal to the volume density of the absorbed photons . The lifetime of photogenerated carriers in BaTiO 3 is too short (in <60 ps) to generate appreciable carrier density changes in BaTiO 3 .…”

“…where, τ is the carrier lifetime, and G is the carrier generation rate, which is equal to the volume density of the absorbed photons. 34 The lifetime of photogenerated carriers in BaTiO 3 is too short (in <60 ps) 35 to generate appreciable carrier density changes in BaTiO 3 . So, in practice, the influence of UV on the PENG's impedance is negligible.…”