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Efficiency droop is a major obstacle facing high-power application of InGaN/GaN quantum-well (QW) light-emitting diodes (LEDs). In this paper, we report the suppression of efficiency droop induced by the process of density-activated defect recombination in nanorod structures 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 such improvement is a net effect that the lateral carrier confinement overcomes the increased surface trapping introduced during fabrication.
© 2014 Optical Society of America
1. Introduction
InGaN/GaN quantum wells (QWs) are perfectly suitable for light-emitting diode (LED) applications 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 [3–7]. To solve this problem, it is essential to avoid the carrier leakages that reduce emission efficiency at the high carrier density. Some important progresses have been made on suppressing the efficiency droop in the past few years [6–16]. These advances have basically been achieved by meliorating the issues of current leakage [8–15] and Auger recombination [16,17]. Recently, another process, i.e. the density-activated defect recombination (DADR), has been identified to be also responsible for the efficiency droop in InGaN/GaN QWs [18–20]. 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 DADR process decreases the emission efficiency with excess defect recombination at high carrier density as schematically shown in Fig. 1(a) [18–20]. Upon increasing carrier density, the enhanced carrier scattering drives carriers to overcome the energy barriers and to populate the defect states [18–20]. 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 systematic optical studies on samples of InGaN/GaN nanorods in comparison with the parent a-plane QWs. We have observed reduced efficiency droop under high density excitation in these nanorod samples. We further argue that the efficiency improvement is a net effect that the lateral carrier confinement overcomes the increased surface trapping introduced during fabrication.
Fig. 1 Lateral carrier confinement in QW nanorods. (a) Schematic sketch of the impact of lateral carrier confinement on the process of DADR (not in scale). (b) A SEM image of the nanorod sample. The average radius is ~130 nm.
2. Experiments
In this study, the InxGa1-xN (x ~0.15) QW samples were prepared with the method of metal-organic chemical vapor deposition. 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 control samples of multiple QWs were grown on a c-plane sapphire substrate consisting of a GaN buffer, an undoped GaN layer, an n-type GaN layer, five pairs of 2.5 nm-thick InGaN QWs sandwiched between 13 nm GaN barriers and a p-type capping layer. The InGaN/GaN nanorods were fabricated from the QW LED structure by inductively-coupled-plasma etching using a novel etch mask of self-assembled indium tin oxide (ITO)-based nanodots [21]. By controlling the size of the ITO-based nanodots and the ICP etching conditions, nanorod samples with radius of ~130 nm and ~50% filling factor were fabricated successfully. Surface passivation on surface of the control sample of multiple QW nanorods was performed with a 10 nm thick Al2O3 layer by using atomic layer deposition.
For PL measurements, we employed the second harmonic field (400 nm) generated from a femtosecond Ti:sapphire regenerative amplifier (pulse duration ~90 fs, Libra, Coherent). The PL emission was collected at the direction normal to the substrate. The residual excitation light was eliminated by an ultra-steep long-pass filter (BLP01-405R-25, Semrock). The emission spectra were collected and analyzed by a spectrograph (Sp 2500i, Princeton Instruments) equipped with a charge-coupled device cooled by liquid nitrogen. The integrated intensity of QW emission at the blue band (IB) and defect emission at the yellow band (ID) are separated by using a fitting procedure with Gaussian functions as previously described [20]. The TRPL spectra at the center wavelength of the emission band were measured with the time-correlated single-photon counting technique [22]. Second-harmonic generation at 400 nm of a femtosecond oscillator (Vitara, Coherent) was used as the excitation source. The temporal resolution for the detection is ~50 ps with a fast single-photon avalanche photodiode (PDM, Picoquant). The amplitude of delayed component was taken from the kink point of the rising edge in the TRPL spectrum. We analyzed the emission decay behavior in the temporal window of the first 10 ns. The PL lifetime was then extracted by fitting the decay component with an exponential or biexponential decay function.
3. Results and discussion
The scenario of lateral carrier confinement in a nanorod sample is depicted in Fig. 1(a). Localized states with potential minima decrease the possibility of defect recombination in InGaN QWs [23–25]. This effect of carrier localization induced by indium fluctuations ensures high quantum efficiency for QW emission [24,25]. The DADR is a process of carrier delocalization due to enhanced carrier scattering with increasing carrier density [18,19]. The carrier scattering enables the escape of carriers from localized states [Fig. 1(a)], which recombine through the defect states or other excess nonradiative centers, leading to an efficiency decrease of QW emission. Assuming the defect states to be evenly distributed in space, the boundaries [Fig. 1(a), dashed green lines] of nanorods can physically block the channels linked between localized states inside the nanorods and defect states outside [21,26], which potentially amends the DADR-induced efficiency droop.
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 [17,27–30]. In this work, we have carefully designed the nanorod size to make sure that the procedure of nanofabrication mainly affects the process of DADR. We employ the parent sample of a single InGaN/GaN QW grown on a nonpolar substrate, in which the DADR-induced efficiency droop has been identified y recently [20]. The average radius of the QW nanorods [~130 nm, Fig. 1(b)] is set to be in the same length scale as the carrier diffusion length in such InGaN samples (60-500 nm) [31–33]. This size is much larger than the exciton Bohr radius (~3 nm) in the InGaN sample [34], so that the size effect on Auger recombination can be neglected. Here, we focus our attention on the DADR process by monitoring the correlation between the efficiency droop and the defect recombination.
We evaluate the efficiency droop by monitoring the integrated intensity of QW emission per unit excitation (IEm/IEx) as a function of the excitation fluence. The value of IEm/IEx is proportional to the internal quantum efficiency of light emission from InGaN QWs as previously established in literature [35,36]. As a signature of efficiency droop, the dependence of IEm/IEx on the fluence evolves from a “plateau” regime to a “decreasing” regime upon increasing the excitation power [Fig. 2(a)] [36]. The efficiency droop is tightly associated with the defect recombination (i.e. yellow luminescence) where the intensity ratio between the defect emission and QW emission (ID/IB) increases abruptly [Figs. 2(a) and 2(b)]. The peak of QW emission slightly shifts to the blue side due to the effect of state filling [Fig. 2(c)]. These results confirm the presence of DADR-induced efficiency droop in the sample as discussed previously [20]. The saturation effect of defect states can be safely excluded here since the defect emission becomes much stronger when excited with a shorter wavelength [20]. Next, let’s compare the experimental data recorded from the nanorod sample and its parent QW sample. Upon raising the excitation fluence (>40 μJ/cm2), the retained efficiency is much higher in the nanorod sample, which means that the efficiency droop is partially reduced after nanofabrication [Fig. 2(a)]. The smaller value of ID/IB in the nanorod sample indicates that such efficiency retention is realized with suppression of defect recombination.
Fig. 2 Reduced efficiency droop in the nanorod sample. (a) The normalized QW emission intensity per unit excitation power (logarithm scale) and (b) intensity ratio between the defect emission and QW emission are plotted versus excitation fluence. The data from the nanorod sample and the parent QW sample are compared. (c) PL emission spectra from the nanorod sample recorded under different excitation fluences. The dashed line indicates the fluence-dependent shift of the emission peak.
To further identify the role played by lateral carrier confinement, we comparatively study the steady-state and transient PL emissions in these two samples. PL spectra recorded from the two samples are shown in Fig. 3 under the same excitation fluence. The emission spectra from both samples exhibit two bands, one for the blue QW emission and another for the yellow defect emission. For the nanorod sample, the light extraction is significantly enhanced with promoted QW emission [Fig. 3]. In spite of this, the defect emission from the nanorod sample becomes weaker than that from the parent QW sample. This result can be well explained by the suppression of defect recombination with a lateral carrier confinement in the nanorod sample, which is also evidenced by a blue-shift of the QW emission peak [Fig. 3]. The lateral carrier confinement caused by the nanorod boundaries restrains carrier diffusion between localized states [37,38]. In this case, the possibility of carrier recombination through strongly localized states (with low potential minima) decreases, leading to the QW emission with a higher photon energy.
Fig. 3 Time-integrated PL spectra from the nanorod sample and the parent QW sample recorded under the same excitation fluence at ~15 μJ/cm2.
The emission dynamics can provide more direct information about the carrier diffusion. Figures 4(a) and 4(b) show the normalized TRPL spectra from the nanorod sample and its parent QW sample under the weak excitation condition (~10 nJ/cm2) as compared to the profile of instrumental response function (IRF). The onsets of TRPL spectra for both samples exhibit two build-up components, i.e., an abrupt rise followed by a delayed slow rise, representing different stages of the carrier dynamics. Upon pulse excitation, the photo-excited carriers relax to the bottom states in the conduction band through thermalization process, leading to the abrupt rise in the TRPL spectra. This process occurs in a time scale of picoseconds, which cannot be resolved with the temporal resolution of ~50 ps used in this study. This is why the abrupt rise is close to the IRF profile for both samples. Due to the spatial inhomogeneity, some localized states are strongly localized and others are weakly localized [Fig. 1(a)]. The process of thermally-activated diffusion drives the carriers from the weakly-localized states to the strongly-localized states in a time scale of hundreds of picoseconds. Such process induces accumulation of carriers at the strongly-localized states, which leads to the delayed-rise component in the TRPL spectra [21, 26, 37, 38]. The highlighted amplitude of the delayed-rise component [Figs. 4(a) and 4(b)] in the nanorod sample is about half of that in their parent QW sample, implying that the lateral carrier diffusion is strongly suppressed in the nanorod sample. More interestingly, this effect also blocks the channels of carrier escape from localized states to defect states, therefore suppressing the DADR process.
Fig. 4 Transient optical evidence of lateral carrier confinement in nanorods. Normalized TRPL spectra recorded at the center wavelength of QW emission from (a) the nanorod sample and (b) the QW sample are shown in comparison with the profile of instrumental response function (IRF). Following the initial abrupt rise, the curves recorded from both samples exhibit a delayed-rise component. The amplitude (D) of delayed-rise component is marked from the kink point of the rising edge in the TRPL spectra. The red dashed lines are the curves fitting to single-exponential decay functions.
The above discussion has affirmed that the DADR-induced efficiency droop can be reduced by manipulating the lateral carrier diffusion with 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 QW sample [Fig. 4]. Such tiny difference suggests that the effect of surface states on emission dynamics is insignificant here [29,39,40]. We roughly evaluated the internal quantum efficiency of PL in these samples by measuring the intensity ratio between the QW emissions at room temperature and 5K, respectively. The internal PL quantum efficiency in the nanorod sample (~35 ± 6%) is slightly higher than that in the parent QW sample (~28 ± 7%), which also indicates that the surface states do not play an important role. These results can be well explained by the unique emission dynamics in the a-plane samples benefiting from the absence of piezoelectric polarization [41,42]. The polarization field in c-plane samples causes carrier separation within the QW (via the quantum-confined Stark effect) that reduces the recombination rate [34,43]. Without a polarization field, the wave functions of electrons and holes in the a-plane samples have better overlap than that in the c-plane samples [41,42]. As a consequence, the carrier recombination is much faster in the a-plane samples [44,45], so that the trapping effect of surface states is less distinct in the TRPL spectra recorded from a-plane QW nanorods. Despite of the negligible effect on emission dynamics, the surface states provide additional channels for carrier recombination [Fig. 1(a)] which may limit the efficiency retention contributed by lateral carrier confinement. In other words, the observed efficiency improvement at the high density regime is a net effect that the amendment of the lateral carrier confinement overcomes the deterioration made by the increased surface trapping effect.
To particularly see the negative effect caused by the surface trapping effect, we also performed control experiments on specially fabricated c-plane QW samples. The PL decay lifetime is generally much longer in the c-plane samples than in the a-plane samples [44,45], so that the surface effect can be more explicitly seen in the TRPL spectra. The control samples are fabricated from parent multiple QWs grown on c-plane substrates. In comparison to the a-plane samples studied above, the density of defect states is much lower with a weaker defect emission in the c-plane QW sample [Fig. 5(a), inset]. The emission in the control QW sample decays much slower [~5.28 ns, Fig. 5(a)]. In the nanorods (c-NR) fabricated from the c-plane QWs, the TRPL spectrum consists of two decay components. The lifetime parameters (amplitude ratios) of these two components are ~0.7 ns (~17%) and ~4.85 ns (~83%), respectively. The appearance of the faster component is an evidence of pronounced surface trapping effect [Fig. 5(a)]. The recombination of surface states is likely to be non-radiative since no significant defect emission is observed from the nanorod sample [Fig. 5(a), inset]. The dependence of IEm/IEx on fluence indicates that, rather than being suppressed, the efficiency droop is deteriorated in the c-plane nanorod sample [Fig. 5(b)]. To check whether this result is a combined effect of lateral carrier confinement and surface trapping, we have further investigated another nanorod sample with surface passivation (c-NR-S). The surface passivation is realized by depositing a layer of Al2O3 on surface of the nanorod with the technique of atomic layer deposition. In the TRPL spectrum, the surface trapping component is not distinctly observable any more in the surface-passivated nanorod sample. The TRPL spectrum can be reproduced by an exponential decay function with a lifetime of ~4.90 ns, suggesting that the density of surface states diminishes in this surface-passivated nanorod sample. The efficiency droop in the surface-passivated sample is significantly reduced as compared to that in the as-etched nanorod sample [Fig. 5(b)], which confirms the negative role played by the surface states. From the above presented experimental evidences and discussions, we can safely conclude that the degree of suppressing the efficiency droop achieved by the lateral carrier confinement is harmed by the surface trapping effect.
Fig. 5 Control experiments on the surface trapping effect. Time-resolved (a) and time-integrated (a, inset) PL spectra of three control samples (the c-plane QWs (c-MQW), as-etched QW nanorods (c-NR), and surface-passivated QW nanorods (c-NR-S) recorded under the same conditions are compared. The dashed line highlights the ultrafast decay component. (b) The normalized QW emission intensities per unit excitation power from the three samples are compared as a function of the excitation fluence.
4. Summary
In summary, we have found that the DADR-induced efficiency droop can be partially amended by lateral carrier confinement in QW nanorod structures. The full potential of this method may be approached by further reducing nanorod radius. However, the effect of surface trapping influences the efficiency retention helped by lateral carrier confinement in nanorods. This drawback can be potentially removed by surface engineering with certain post-fabrication technologies or more practically by employing epitaxial grown nano-LEDs with minimal density of surface states [14,17,27,28,46]. The technology studied here, together with the methods proposed in literature on inhibiting other efficiency droop mechanisms [6–16], can be integrated for further development towards droop-free LEDs, which will be particularly desirable for high-power applications owing to the enhanced light extraction in nanorod systems.
Acknowledgments
This work is supported by the Program of International S&T Cooperation (2011DFA01400, MOST), the National Basic Research Program of China (2013CB932903 and 2012CB921801, MOST), the National Science Foundation of China (91233103, 61108001, 11227406 and 11321063), and NRF of Korea (K2011-0017325). C.Z. acknowledges financial support from the New Century Excellent Talents program (NCET-09-0467), Fundamental Research Funds for the Central Universities, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). J.S.K acknowledges financial support from BK21 PLUS program in SCNU.
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