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

Christian Frankerl,Marc Patrick Hoffmann,Matthew John Davies,Axel Hoffmann,Hans-Jürgen Lugauer,Roland Zeisel,Felix Nippert,Christian Brandl

doi:10.1002/pssb.202000242

Abstract

The carrier dynamics of Al‐rich AlGaN/AlN quantum well (QW) structures in the presence of strong carrier localization is reported. Excitation density‐dependent photoluminescence (PL) measurements at low temperatures reveal a clear correlation between the onset of efficiency droop and the broadening of the time‐integrated PL spectra. While the droop onset is heavily impacted by the localization strength, the PL emission broadening is observed almost exclusively on the high energy side of the emission spectrum. Spectrally resolved PL decay transient measurements reveal a strong dependency of the carrier lifetimes on the emission photon energy across the spectrum, consistent with a distribution of localized states, as well as on the temperature, depending on the localization strength of the investigated structure. The characteristic “S”‐shaped temperature dependence of the PL emission energy is shown to be directly correlated to the thermal redistribution of carriers between localized states. Based on these findings, the role of carrier localization in the recombination processes in AlGaN QW structures is underlined and its implications for efficiency droop are discussed.

Highlights

Blue light-emitting diodes (LEDs) based on the InGaN material. Despite their continuing inferiority to blue LEDs, research efforts on AlGaN-based devices emitting in the deep ultraviolet (UV) spectrum have determined IQEs as high as %65%,[18] signifisystem have found widespread application into a multitude of cantly exceeding the maximum values expected by simple estimacommercial optoelectronic components.[1,2]
In a recent research efforts, the efficiencies of such devices could be raised study, we have shown that the degree of carrier localization in to very high standards.[3]
We found that one universal decay shape accurately described the PL time decay of all photon energies except for 4.769 and 4.805 eV. This finding strongly suggested that the vast majority of the PL emission originated from carriers localized among potential minima of varying depth, induced by local fluctuations of alloy composition and quantum wells (QWs) thickness, with the nonexponential decay shape reflecting the microscopic structure of the potential “landscape.”[26] The decay transients corresponding to photon energies of 4.769 and 4.805 eV follow a slightly different dependency, deviating in shape from the rest of the decay transients

Summary

Introduction

Despite their continuing inferiority to blue LEDs, research efforts on AlGaN-based devices emitting in the deep ultraviolet (UV) spectrum have determined IQEs as high as %65%,[18] signifisystem have found widespread application into a multitude of cantly exceeding the maximum values expected by simple estimacommercial optoelectronic components.[1,2] Due to extensive tions based solely on the influence of the TDDs.[19]. AlGaN/AlN QW structures emitting in the UV-C spectral range. A. Hoffmann Institut für Festkörperphysik Technische Universität Berlin Hardenbergstr. 36, Berlin 10623, Germany ments enable us to directly trace the carrier density-induced and thermal redistribution of carriers between localized states and reveal its consequences for the recombination dynamics in the system, including efficiency droop, altogether highlighting the importance of carrier localization in AlGaN-based. The copyright line was corrected on 21 August 2020, after initial publication online

Sample Structure and Measurement Methods

Experimental Section

Conclusions

Findings

Conflict of Interest