Enhancement of Auger recombination induced by carrier localization in InGaN/GaN quantum wells

The origin of efficiency droop in state-of-the-art quality InGaN/GaN and GaN/AlGaN quantum wells (QWs) grown on various crystal planes is studied by means of time-resolved photoluminescence spectroscopy associated with a precise determination of the QW carrier density. In a first set of experiments, it is shown that a polar InGaN/GaN QW under nonresonant high optical excitation shows clear signatures of Auger loss mechanism and thus behaves quite differently compared to its binary based GaN/AlGaN QW counterpart, where no Auger signature is observed. In order to get rid of the impact of the built-in polarization field and illustrate the dominant role of carrier localization, similar experiments have been conducted on two $m$-plane InGaN/GaN QWs with similar In composition but a different degree of disorder. We demonstrate that carrier localization strongly enhances the Auger recombination process in nonpolar InGaN/GaN QWs. We also show that this effect may be further amplified by the presence of polarization fields on polar QWs. The relaxation of the $k$-selection rule during the Auger recombination process, resulting from QW potential disorder, can account for the enhancement of the efficiency droop in InGaN/GaN QWs.

Figure 1

HRTEM images of the $m$-plane QWs, in (a) the $m$-QW1 sample and (b) the $m$-QW2 sample. Contrast line scans along the growth direction (c) in the $m$-QW1 sample, and (d) the $m$-QW2 sample.

Figure 2

Temperature dependence of the emission energy of the four QW samples and their corresponding FWHM. (a) $c$-plane GaN/AlGaN SQW, (b) $c$-plane InGaN/GaN SQW, (c) $m$-QW1, and (d) $m$-QW2.

Figure 3

Comparison of the PL characteristics of the two $m$-plane InGaN/GaN QW samples probed under low injection condition ($\sim 100\mathrm{nJ}/\mathrm{c}{\mathrm{m}}^2$ per pulse). Measured QW effective lifetime (black circles), as well as theoretical radiative (red solid line) and computed effective lifetime (black solid line) for samples (a) $m$-QW1 and (b) $m$-QW2. Densities of free and localized excitons, deduced from our modeling, are shown for (c) $m$-QW1 and (d) $m$-QW2. The arrows mark the temperature at which the free and localized carrier densities become equal, i.e., the delocalization temperature $T_{\mathrm{d}}$.

Figure 4

Comparison of the $c$-plane GaN/AlGaN and the $c$-plane InGaN/GaN SQW systems probed under high injection ($300\mu \mathrm{J}/\mathrm{c}{\mathrm{m}}^2$) at $T=4\mathrm{K}$. Streak image of (a) the $c$-plane GaN (3 nm)/$\mathrm{A}{\mathrm{l}}_{0.09}\mathrm{G}{\mathrm{a}}_{0.91}\mathrm{N}$ and (b) the $c$-plane $\mathrm{I}{\mathrm{n}}_{0.09}\mathrm{G}{\mathrm{a}}_{0.91}\mathrm{N}$ (2 nm)/GaN SQW recorded at $T=4\mathrm{K}$. (c) tr spectra (from 0 to 20 ps) obtained at 4 K and under high injection conditions for the GaN/AlGaN QW (blue) and the InGaN/GaN QW (green) and the corresponding e-h plasma modeling (red lines). For the sake of comparison, the spectral axis has been shifted to match the position of the two QW ground states ($E_{\mathrm{QW}}$) in the low density regime. (d) PL intensity decays integrated over the whole QW emission energy range [GaN/AlGaN QW (blue line) and InGaN/GaN QW (green line)].

Figure 5

Power-dependent tr-PL for the $c$-plane InGaN/GaN SQW measured at $T=4\mathrm{K}$. The PL intensity has been integrated between 3.1 and 3.3 eV.

Figure 6

Comparison of $m$-QW1 and $m$-QW2 probed under high injection ($300\mu \mathrm{J}/\mathrm{c}{\mathrm{m}}^2$) at $T\approx 4\mathrm{K}$. (a) and (b) tr spectra obtained at 4 K and under high injection conditions for samples $m$-QW1 and $m$-QW2, respectively. The PL spectra are extracted every 50 ps after the excitation up to a time delay of 250 ps and then every 100 ps. The red solid line shows a typical fit of the spectra with our e-h plasma model. The integrated PL intensity at early delays is displayed on top of each series of spectra for both samples. $I_0$ is the integrated PL intensity at early delays for $m$-plane QW1 and is taken as a reference. (c) PL decays, integrated over the whole QW emission energy range, for the two $m$-plane InGaN/GaN QWs. The dashed lines show the initial decay time. The black solid lines illustrate the fitting of the decays of the investigated samples.

Figure 7

Comparison of $m$-QW1 and $m$-QW2 probed at low temperature ($T=6\mathrm{K}$) as a function of laser fluence. Power-dependent tr-PL for both $m$-plane samples [(a) $m$-QW1 and (b) $m$-QW2]. The PL intensity has been spectrally integrated over the entire QW emission range. (c) and (d) Laser fluence dependence of the QW PL lifetime at early delays for $m$-QW1 and $m$-QW2.