Reducing the efficiency droop by lateral carrier confinement in InGaN/GaN quantum-well nanorods

Efficiency droop is a major obstacle facing high-power application of InGaN/GaN quantum-well (QW) light-emitting diodes. In this letter, we report the suppression of efficiency droop induced by density-activated defect recombination in nanorod structure of a-plane InGaN/GaN QWs. In the high carrier density regime, the retained emission efficiency in a dry-etched nanorod sample is observed to be over two times higher than that in its parent QW sample. We further argue that the improvement is a combined effect of the amendment contributed by lateral carrier confinement and the deterioration made by surface trapping.

InGaN/GaN quantum wells (QWs) are perfectly suitable for demonstrating light-emitting diodes (LEDs) in the short-wavelength region. 1,2 However, their high-power applications have been hindered by an enduring issue of efficiency droop-the decrease in quantum efficiency of light emission with increasing carrier density. [3][4][5][6][7] To solve this problem, it is essential to avoid the leaky processes that reduce the emission efficiency at high carrier density. Some important progresses have been made on suppressing the efficiency droop in the past few years. [6][7][8][9][10][11][12][13][14][15][16] These advances have basically been achieved by meliorating the issues of current leakage [8][9][10][11][12][13][14][15] and Auger recombination. 16,17 Recently, another process of the density-activated defect recombination (DADR) has been identified to be also responsible for the efficiency droop in InGaN/GaN QWs. [18][19][20] Nevertheless, the way to avoid such efficiency droop has not been really investigated yet. Here, we propose that the effect of lateral carrier confinement in QW nanostructures can be employed to reduce this undesired DADR-induced efficiency droop.
The process of DADR decreases the emission efficiency with excess defect recombination at high carrier density as schematically shown in Figure 1a. [18][19][20] Upon increasing carrier density, the enhanced carrier scattering drives carriers to overcome the energy barriers and to populate the defect states. [18][19][20] In principle, such process can be suppressed if carrier motion can be confined in lateral directions by proper material/structure designs. To test this idea, we present a systematic optical study on a sample of InGaN/GaN nanorods in comparison with its parent of a-plane QWs. We have observed reduced efficiency droop in the nanorod sample and confirmed it as a result of lateral carrier confinement. We also argue that the extent of droop amendment made by lateral carrier confinement is harmed by the negative effect of surface trapping in the QW nanorods.
The nonpolar a-plane QW samples were grown on r-plane sapphire substrates consisting of a GaN buffer layer, an n-GaN layer, a 15 nm thick InGaN single-QW layer, and a p-GaN capping layer. The InGaN/GaN nanorods were fabricated with a dry-etching procedure as described in an earlier publication. 21 Second harmonic generation at 400 nm of ultrafast pulses from Ti:sapphire femtosecond laser was employed as excitation source for photoluminescence (PL) measurement. The emission was collected at the direction normal to the substrate and analyzed by a spectrograph (Sp 2500i, Princeton Instruments) equipped with a charge-coupled device cooled by liquid nitrogen. The excitation residual was eliminated by an ultrasteep long-pass filter (BLP01-405R-25, Semrock). The time-resolved PL (TRPL) spectrum was measured with the technique of time-correlated single-photon counting at a temporal resolution of ~ 50 ps provided by a fast single-photon avalanche diode (PDM, Picoquant) as described previously. 22 The PL lifetime was then extracted by fitting the decay component in the temporal window of first 10 ns with an exponential or biexponential decay function.
The scenario of lateral carrier confinement in a nanorod sample is depicted in DADR is a type of carrier delocalization process which becomes dominant with increasing carrier density due to enhanced carrier scattering. 18,19 The carrier scattering drives the escape of carriers from localized states (Figure 1a There are rapidly growing interests on optimizing InGaN LEDs with nanoarchitectured designs in the past few years, benefiting from some unique merits of nanostructures including strain relaxation and enhanced light extraction. 17,[27][28][29][30] In this work, we carefully design the nanorod size to make sure that the procedure of nanofabrication mainly affects the process of DADR. We employ a parent sample of single InGaN/GaN QW grown on a nonpolar substrate, in which the DADR-induced efficiency droop has been identified very recently. 20 The average radius of QW nanorods (~ 130 nm, Figure 1b) is set to be in the same length scale as the carrier diffusion length in InGaN samples (60-500 nm ). [31][32][33] This size is much larger than the Bohr radius (~ 3 nm) of excitons in InGaN samples, 34 so that the size effect on Auger recombination can be neglected. Here, we focus our study on the process of DADR by monitoring the correlation between efficiency droop and defect recombination. work. 20 The saturation effect of defect states can be safely excluded here as the defect emission becomes much stronger with shorter-wavelength excitation. 20 We compare the experimental data recorded from the nanorod sample and its parent sample. Upon  (Figure 1a). More importantly, this effect also blocks the channels of carrier escape from localized states to defect states, suppressing the process of DADR.
The above discussion has affirmed the potential to suppress the DADR-induced efficiency droop by incorporating nanorod structures in LEDs. However, a long-standing issue exists in such technology, i.e. surface states can be hardly avoided during nanofabrication. 29,39,40 The lifetime of PL decay in the nanorod sample is ~ 0.56 ns, which is very close to the value of ~ 0.58 ns in the parent sample ( Figure 4).
Such tiny difference suggests that the effect of surface states on emission dynamics is less important here than the cases of nanorods fabricated from c-plane QWs. 29,39,40 This result can be explained by the unique emission dynamics in a-plane samples benefiting from the absence of piezoelectric polarization. 41,42 The polarization field in c-plane samples causes carrier separation (quantum-confined Stark effect) that reduces the recombination rate. 34,43 Without polarization field, the electrons and holes in a-plane samples are better overlapped than that in c-plane samples. 41,42 In consequence, the carrier recombination are much faster in a-plane samples, 44