Temperature-dependent efficiency droop in AlGaN epitaxial layers and quantum wells

Abstract

Luminescence efficiency droop has been studied in AlGaN epitaxial layers and multiple quantum wells (MQWs) with different strength of carrier localization in a wide range of temperatures. It is shown that the dominant mechanism leading to droop, i.e., the efficiency reduction at high carrier densities, is determined by the carrier thermalization conditions and the ratio between carrier thermal energy and localization depth. The droop mechanisms, such as the occupation-enhanced redistribution of nonthermalized carriers, the enhancement of nonradiative recombination due to carrier delocalization, and excitation-enhanced carrier transport to extended defects or stimulated emission, are discussed.

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

“…[ 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. At carrier densities up to 2 × 10 18 cm −3 , corresponding to IQE ≈ 100%, the FWHM does not change significantly with carrier density.…”

“…[ 20 ] We confirm this by temperature‐dependent PL measurements, which reveals the characteristic “S”‐shaped dependence of the PL emission energy, as exemplarily shown in Figure 3 a for sample M25. The data were normalized to the PL peak emission energy at 15 K, and the redshift region of the ‘S’‐shape was fit by the commonly used equation [ 6,7,13,17 ] …”

“…Recently, a similar effect was reported and studied in AlGaN‐based epilayers and quantum wells (QWs), which generally suffer from a similar magnitude of TDDs compared with InGaN‐based structures. [ 11–17 ] 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 ] significantly exceeding the maximum values expected by simple estimations based solely on the influence of the TDDs. [ 19 ] In a recent study, we have shown that the degree of carrier localization in AlGaN/AlN QWs is substantially larger than commonly reported in equivalent InGaN‐based structures and directly affects the IQE and efficiency droop.…”

Kinetic Monte Carlo simulations of the dynamics of a coupled system of free and localized carriers in AlGaN

“…The PL efficiency dependences on excitation power density generally exhibit efficiency droop, and are used to evaluate the contribution of Auger recombination [31,33], which is commonly accepted as the main origin of the efficiency droop in InGaN materials [42,43], and has been suggested to be important also in AlGaN [33]. Nevertheless, the Auger processes have also been shown to be affected by carrier localization [5,6,34], while recently, the importance of carrier localization to efficiency droop has reemerged [30,44,45]. This situation motivated us for an attempt to simulate the carrier dynamics in AlGaN by properly describing the influence of carrier localization and without the involvement of the Auger processes.…”

Extremely high internal quantum efficiency of AlGaN-based quantum wells on face-to-face annealed sputter-deposited AlN templates

“…Thus, the efficiency droop was not taken into consideration in the exciton rate equation model used in this analysis, and the fitting was performed only for the lower intensity side of the efficiency curves. Although there are several possible origins of the efficiency droop phenomena, such as the Auger recombination, 34) the delocalization of carriers or excitons, [35][36][37] the saturation of radiative recombination processes, 38) we consider that the effects of the efficiency droop phenomenon on our analysis were negligible.…”