Strongly localized carriers in Al-rich AlGaN/AlN single quantum wells grown on sapphire substrates

Abstract

Carrier dynamics in AlGaN-based single quantum well (QW) structures grown on sapphire are studied by means of time-integrated and time-resolved photoluminescence spectroscopy (PL) in a wide temperature range from 5 K to 350 K. The samples cover a broad compositional range, with aluminum contents ranging between 42% and 60% and QW widths between 1.5 nm and 2.5 nm. All samples reveal the characteristic “S”-shape temperature dependence of the PL emission energy as frequently reported in InGaN-based systems, albeit with significantly larger localization strengths of up to 60 meV. It is shown that in the compositional range investigated, carrier localization is determined primarily by the QW width and, in contrast, exhibits a much weaker dependence on aluminum concentration. By the combination of time-integrated and time-resolved PL measurements, the localization of carriers is demonstrated to have a significant impact on the recombination dynamics of AlGaN/AlN QWs grown on sapphire, heavily affecting the internal quantum efficiency and efficiency droop even in standard LED operation conditions.

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

“…In contrast, as we previously determined, the QW width seems to be the main parameter defining σ . [ 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 ] …”

“…[ 20 ] Four samples are investigated in this article, two single‐QW (SQW) and two multi‐QW (MQW) structures. The SQW samples were investigated recently in another work, [ 20 ] whereas the MQW samples were grown specifically for this study. The SQW samples are of similar Al content (≈50%) but varying QW width, and are labeled S25 (2.5 nm‐thick QW) and S15 (1.5 nm‐thick QW).…”

“…[ 15,20 ] To determine whether this finding also holds for low temperatures, Figure 2 a shows the repetition of the carrier density‐dependent PL efficiency measurements for sample S15, which was found to exhibit a significantly stronger localization of σ = 58 meV. [ 20 ] Sample S15 already entered droop regime at a carrier density of 3 × 10 17 cm −3 , about an order of magnitude lower than the carrier density required to induce droop in sample S25, suggesting that droop onset was determined by localization strength at arbitrary temperatures. We emphasized that despite the drastically different carrier density required to cause droop in comparison with sample S25, the correlation between droop onset and asymmetric FWHM broadening is still evident in sample S15, as shown in Figure 2a,b.…”

“…Additional growth details can be found in previous study. [ 20 ] Four samples are investigated in this article, two single‐QW (SQW) and two multi‐QW (MQW) structures. The SQW samples were investigated recently in another work, [ 20 ] whereas the MQW samples were grown specifically for this study.…”

“…[ 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. [ 20 ]…”