Challenges for reliable internal quantum efficiency determination in AlGaN-based multi-quantum-well structures posed by carrier transport effects and morphology issues

Abstract

We report on a systematic study of the determination of the internal quantum efficiency (IQE) in AlGaN-based multiple-quantum-well (MQW) structures using different optical evaluation methodologies and experimental conditions, in order to derive a standard set of measurement conditions for reliable IQE determination. Several potential sources of error that may distort the IQE obtained by optical measurements are discussed, such as carrier transport effects, excitation conditions failing to fulfill ideal resonance conditions, and morphology issues. A series of nominally identical AlGaN-based MQW structures is grown on an AlGaN layer separated by an AlN interlayer of varying thickness. The MQW structures are studied both by resonant and quasiresonant photoluminescence spectroscopy, and IQEs are determined via different commonly employed methods. The obtained values are shown to be significantly affected by the employed excitation conditions, as well as the evaluation techniques. In addition, growth morphology issues and carrier transport effects need to be considered in the interpretation of the measured data, with the latter being investigated in greater detail. The results emphasize the need for an appropriate choice of both experimental conditions and evaluation methodology in order to extract reliable and comparable IQE values.

Carrier Dynamics in Al‐Rich AlGaN/AlN Quantum Well Structures Governed by Carrier Localization

“…AlN barriers were used for reasons of homogeneous QW growth and to ensure comparability with the SQW structures. [ 21 ] The Al content of the MQW samples is slightly larger (≈53%) compared with the SQW samples, whereas the thicknesses are identical. On top of all structures, a 50 nm‐thick AlN cap layer was deposited to avoid any surface effects, such as band bending due to Fermi‐level pinning.…”

“…In this power region, spanning over more than two orders of magnitude and commonly labeled “plateau region,” the IQE was assumed to be close to unity due to negligible nonradiative recombination and quasi‐resonant excitation of carriers into the QWs. [ 21 ] Increasing the carrier density further up to 2 × 10 20 cm −3 , a drastic decrease in PL efficiency by ≈45% is observed, representing the well‐known efficiency droop. [ 10–18 ] Simultaneously, the FWHM of the PL spectrum was found to increase, closely correlating with the droop in PL efficiency.…”

Assessment of AlGaN/AlN superlattices on GaN nanowires as active region of electron-pumped ultraviolet sources

“…In this pumping regime, the photogenerated carrier densities are higher than the doping level of the original structure (high injection) and nonradiative recombination paths are partially saturated. Therefore the obtained IQE is significantly higher than the low-injection value and depends on the excitation power density [49][50][51][52]55,59]. Maximum values of IQE are obtained for an excitation power density around 10-100 kW/cm 2 [52,55].…”

Influence of the Growth Substrate on the Internal Quantum Efficiency of AlGaN/AlN Multiple Quantum Wells Governed by Carrier Localization

“…believed to work adequately when the excitation is resonant and a "plateau region" of the PL efficiency as a function of excitation power density is observed at low temperatures. 4,6 Time-resolved and timeintegrated CL measurements were performed in an FEI Inspect F50 SEM. A Horiba Jobin Yvon ihR550 spectrometer equipped with a Peltier-cooled charge coupled device detector and a 600 grooves/mm diffraction grating was used for spectral analysis.…”

“…9 injected into the structures is directly proportional to the applied laser power. According to the ABC model, 4,6 the maximum of the PL efficiency curve is obtained at n=√(A/C), where n is the carrier density,…”