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Excitation power and temperature dependences of the photoluminescence (PL) spectra are studied in InGaN/GaN multiple quantum wells (MQWs). The excitation power dependences of the PL peak energy and linewidth indicate that the emission process of the MQWs is dominated first by the Coulomb screening effect and then by the localized states filling at low temperature, and that the nonradiative centers are thermally activated in low excitation range at room temperature. The anomalous temperature dependences of the peak energy and linewidth are well explained by the localized carrier hopping and thermalization process, and by the exponentially increased density of states with energy in the band tail. Moreover, it is also found that internal quantum efficiency is related to the mechanism conversion from nonradiative to radiative mechanism, and up to the carriers escaping from localized states.
©2012 Optical Society of America
1. Introduction
InGaN alloys have been attracting much attention as potential materials for the fabrication of high brightness light emitting diodes (LEDs) and continuous wave (cw) blue laser diodes because of the advantage of tuning ability of the alloy bandgap. Understanding the emission mechanism in InGaN multiple quantum well (MQW) structures is a key issue for further developing such optoelectronics devices [1–3]. The widely accepted viewpoint is that the inhomogeneous distribution of indium facilitates high quantum efficiency of nitride-based LEDs in spite of the tremendous density of dislocations of InGaN/GaN MQWs grown on the lattice-mismatched substrates [4]. The localized excitons within indium-rich regions resulting from partial phase segregation in InGaN alloys are considered to prevent them from reaching nonradiative recombination sites and play an important role for spontaneous emission. However, to our knowledge, the character of carrier motion, and the relevant process of establishing their distribution over the localized states in InGaN remain to be further explored [5, 6].
In this paper, in order to clarify the underlying physics of light emission from InGaN/GaN MQWs, we measured the excitation power (P) and temperature (T) dependences of the photoluminescence (PL) spectra, and revealed the physical mechanism behind by analyzing the emission energy, linewidth, intensity, and internal quantum efficiency (IQE).
2. Experiments
The InGaN/GaN MQWs were grown on a (0001)-oriented sapphire using metalorganic chemical vapor deposition (MOCVD). The precursors of Ga, In, N, and Si were trimethylgallium (TMGa), trimethylindium (TMIn), ammonia (NH3), and silane (SiH4), respectively. The QWs were grown under N2 ambient after a 1.5-μm-thick undoped GaN buffer layer and a 2.5-μm-thick Si-doped GaN layer. The active region consisted of 8 MQWs with 3-nm-thick InGaN wells and 14-nm-thick GaN barriers. The indium content of the active region is about 15%.
For excitation power and temperature dependent PL measurement, the sample was mounted in a closed-cycle He cryostate and the temperature was controlled from 6 to 300 K. A 405 nm cw semiconductor laser was used as an excitation light source with the spot size of ~170 μm, and the excitation power changed from 0.001 to 50 mW. The PL signal from the sample was dispersed by a Jobin-Yven iHR320 monochromator and detected by a thermoelectrical cooled Synapse CCD detector.
3. Results and discussions
Figure 1 shows the peak energy and linewidth of the spectra as a function of the excitation power at 6 and 300 K. At 6 K, as shown in Fig. 1(a), the peak energy monotonically increases with increasing excitation power. While the linewidth first decreases in the low excitation range of P < 15 mW, and then increases as the excitation power further increased. The behavior of the spectra can be explained as follows. The first is Coulomb screening of quantum-conðned Stark effect (QCSE) resulting from the internal electric field, since increasing photogenerated carrier density weakens the QCSE [7], and that results in the increasing of the peak energy with increasing excitation power at P < 15 mW. As the screening effect dominates the emission process, it accompanied a reduction in linewidth [8]. With further increasing excitation power (P > 15 mW), the band-filling effect of high-energetic localized centers resulting from the inhomogeneous distribution of the indium starts interfering and becomes dominant, that also induces a blueshift of the emission energy. But, unlike the effect of QCSE, the effect accompanies the broadening of linewidth.
Fig. 1 Emission peak energy and full width at half maximum (FWHM) as a function of excitation power for the InGaN/GaN MQWs at 6 K (a) and 300 K (b).
At 300 K, however, different phenomena were observed. In contrast to the case at 6 K, the peak energy shows a redshift obviously, accompanied by the broadening of the linewidth with increasing excitation power (P < 0.1 mW), as shown in Fig. 1(b). It is because the nonradiative centers are thermally activated at high temperature under lowest excitation power, and the defect-related nonradiative recombination dominates the recombination process. As a consequence, the carrier lifetime is shortened and that prompts excited carriers to recombine at higher energy extended states before reaching into lower energy localized states [9]. Thus as compared to the recombination in low-energy localized state, the transition of these higher energy extended states would emit higher photon energy. Since there may exist many of higher energy extended states, this kind of recombination would accompany by the broadening of linewidth. With increasing excitation power in low excitation range (P < 0.1 mW), the carrier lifetime increases, thus the excited carriers can transfer from higher extended states to lower localized states. As a result, the redshift of the peak energy and the broadening of the linewidth are observed. Moreover, with further increasing excitation power above 0.1 mW, the nonradiative centers become approximately saturated and the radiative recombination starts to dominate the recombination process, therefore, similar phenomenon as we discussed in Fig. 1(a) was observed in Fig. 1(b). The reason for the origin of the nonradiative centers is not clear now. It might be due to the presence of the threading dislocation and/or the shallow level impurity in the InGaN/GaN MQWs. In addition, we also notice that the emission peak energy shows a slight redshift with increasing excitation power in the very low excitation range below 0.003 mW at 6 K, as shown in Fig. 1(a). But, unlike the case at 300 K which nonradiative recombination dominates the recombination process in the excitation range below 0.1 mW, this behavior may be resulting from that the localized carriers have a higher recombination efficiency than the free carriers due to its stronger localization effect, this results in that the former has a faster increase in emission intensity than the later with increasing excitation power, thus leading to a slight redshift of the emission peak energy of the MQWs in the very low excitation range below 0.003 mW at 6 K.
Figure 2(a) plots the temperature dependence of the peak energy over a broader excitation power range. The anomalous temperature behavior is clearly observed in the curves measured at P = 0.05 mW. First, it redshifts until a temperature (noted as Tmin) of ~60 K corresponding to a maximum of the localization energy, then, it blueshifts up to the full-delocalization temperature (noted as Tmax) of ~170 K, where it starts redshifting again. The anomalous temperature behavior of the peak energy is S-shaped (decrease-increase-decrease) [9–14]. The peculiarity manifests itself also in the temperature dependence of the PL linewidth [Fig. 2(b)]. It decreases slowly until a temperature close to the Tmin. Then it increases significantly up to a temperature slightly higher than the Tmin. Further, it decreases up to a temperature close to the Tmax. Finally, the linewidth increases steadily up to 300 K again. The change of the linewidth shows a W-shaped (decrease-increase-decrease-slowly increase-increase) temperature dependence.
Fig. 2 Temperature dependence of PL peak energy (a) and FWHM (b) measured at different excitation powers. The temperature Tmin and Tmax, corresponding to the minimum and maximum of the peak energy at different excitation powers, respectively, are shown by the arrow as a guide to the eye. The solid curve is calculated using the band-tail model [Eq. (1)]. The box marks the initial decreasing of the temperature dependent linewidth in the temperature range of T < 40 K.
To explain in detail above mentioned the S-shaped (W-shaped) temperature dependent behavior of the peak energy (linewidth) observed at P = 0.05 mW in our present study (Fig. 2), the schematic diagrams indicating the possible mechanism of carriers transferring in the MQWs structure at different temperatures with P = 0.05 mW is shown in Fig. 3 . At low temperature of 6 K, carriers are randomly distributed among the potential minima [Fig. 3(a)]. As the temperature increases from 6 K up to Tmin ≈60 K, weakly localized carriers are thermally activated and relax down into other strongly localized states via hopping [15–17] and reach a saturated redistribution [Fig. 3(b)], which results in the initial redshift of the peak energy as large as 18 meV [Fig. 2(a)]. It is consistent with the initial decreasing of the PL linewidth in the temperature range of T ≈6–40 K [Fig. 2(b)]. After 60 K, increasing temperature enable carriers to achieve the thermal equilibrium with the lattice and to occupy higher-energy levels of the localized states [Fig. 3(c)], thus results in the blueshift of the peak energy as large as 33 meV toward the free-exciton ground state up to Tmax ≈170 K [Fig. 2(a)]. Accordingly, the rapid increase of the linewidth in T ≈40–70 K represents a crossover from nonthermalized to thermalized energy distribution of localized excitons [Fig. 2(b)] [18, 19]. A quick decrease of the linewidth in T ≈70–110 K as shown in Fig. 2(b), is explained as that when further increasing temperature above 70 K, even the most localized carriers become progressively mobile. Therefore the carrier distribution narrows [Fig. 3(d)], and the linewidth decreases. After T ≈110 K, the role of the regular thermalization of carriers starts to become more and more important, which results in the linewidth increase at a lower rate up to the full-delocalization temperature of Tmax ≈170 K [Fig. 2(b)]. It is consistent with the slow increase of the peak energy in the temperature range [Fig. 2(a)]. Finally, the peak energy decreases and the linewidth increases markedly up to 300 K, in agreement with a regular thermalization of the carriers.
Fig. 3 Schematic diagrams indicating the possible mechanism of carriers transferring in the MQWs structure at different T at P = 0.05 mW. (a)–(d) represent respectively the case of the carriers distribution at lowest T (such as 6 K), Tmin (60 K), a T slightly higher than Tmin, and a T close to Tmax.
The temperature-indueced blueshit of the peak energy can be described by the band-tail model [20, 21]:
where E(T) is the emission energy at T, Eg(0) the energy gap at 0 K, and α and β are Varshni coefficients. The third term comes from the localization effect, in which σ indicates the degree of localization effect, i.e., the large value of σ means a strong localization effect, and kB is the Boltzmann constant. One of the fitting curves made by Eq. (1) is shown in Fig. 2(a) with the following parameters: Eg(0) = 2.72 eV, α = 0.77 meV/K, β = 1000 K, σ = 23.34 meV, and it fits well with our experimental data in the temperature range of 80–300 K. Through fitting [Fig. 2(a)], the values of σ obtained at various excitation powers, as a function of the excitation power, are plotted in Fig. 4(a) . The decreasing trend of the parameter σ as a function of the excitation power implies the reduced localized effect. As a result, the temperature dependent redshift or blueshift of the peak energy become less prominent with increasing excitation power, correspondingly, the temperature behavior of the peak energy gradually evolves from the S-shaped temperature dependence into a weak S-shaped, and until into an inverted V-shaped temperature dependence [Fig. 2(a)]. While the temperature dependence of the linewidth, as a function of the excitation power, also gradually evolves from the W-shaped temperature dependence into a weak W-shaped, and until into a monotonically increased temperature dependence [Fig. 2(b)]. Although beyond the scope of our study in present work, it can be expected that when the excitation power is large enough, the localization effect will completely disappear, and the temperature behavior of the peak energy will closely follow Varshni law. Moreover, as seen from Fig. 2, as the excitation power increases the band energy blueshifts, corresponding to this process, the linewidth first decreases (P < 15 mW) and then increases (P > 15 mW) at low temperatures, which is attributed to the coulomb screening of the QCSE and the band-filling of the localized states as discussed in Fig. 1. At the same time, both the temperatures Tmin and the Tmax decrease, reflecting the reduced localization effect. The values of the Tmin and Tmax, as a function of the excitation power, are shown in Fig. 4(b).Fig. 4 The values of σ (a), Tmin and Tmax (b) obtained at various excitation powers [see Fig. 2(a)], as a function of the excitation power.
Furthermore, as shown in Fig. 2, the initial redshifting of the temperature dependent peak position in the temperature range of T < 60 K, becomes less and less significant with increasing excitation power [Fig. 2(a)]. It is in agreement with the reduced localization effect as discussed above. However, surprisingly, the initial narrowing of the temperature dependent linewidth in the temperature range of T < 40 K [see dashed box in Fig. 2(b)], becomes more and more significant with increasing excitation power (P < 5 mW), which seems to be in contradiction with the expected experimental result that increasing excitation power should make the initial narrowing effect of the temperature dependent linewidth weaken due to the reduced localization effect. In fact, to explain the strange phenomenon, density of states in the band tail [22], which exponentially increases with energy up to the free-exciton energy, must be taken into account. The density of state is expressed by
where E is the energy of a trap with respect to the mobility edge, and NL is the total number of localized states. The temperature T0 characterizes the width of the exponential. At low excitation power, as the temperature slightly increases, weakly localized carriers are thermally activated and redistributed into lower-energy localized states via hopping, and this process is expected to result in a pronounced narrowing of the linewidth. However, the increasing temperature simultaneously also make the carriers undergo a markedly band-filling process in the lower levels of localized states due to the smaller density of states, and results in an observably broadening of the linewidth. Therefore, the emission linewidth of the MQWs indicates a slowly decreasing with the temperature at low excitation power since the broadening partially canceled the narrowing. When the excitation power is elevated (P < 5 mW), as the low levels of the localized states will be fully occupied and the empty higher levels have a larger density of states, the emission linewidth of the MQWs, as a function of the temperature, indicates a pronounced decrease due to the reduced broadening effect induced by the band-filling. However, when the excitation power is increased further in the range of 5 to 8 mW, the localized states are further filled, accordingly the initial decreasing of the temperature dependent linewidth gradually weakens and untill disappears since the temperature dependent relaxation effect of the carriers down into the low-energy localized states reduces, while the temperature dependent thermal broadening effect of the carriers in localized states relatively enhances. After the critical excitation power of 8 mW, the linewidth will show a continuous increase with the temperature since the thermally broadening of the carriers in localized states start to dominate the emission process of the MQWs with increasing temperature (T < 40 K). Here, it must notice that the explanation using the exponential density of states in the band tail is in agreement with the previous discussion on the S- and W-shaped temperature dependences of the emission energy and linewidth, and further reinforces the discussion.Figure 5(a) shows the IQE of the InGaN/GaN MQWs as a function of the excitation power. Here, the IQE is defined as the ratio of the integrated PL intensity at 300 K and 6 K. One can clearly see that the IQE increases markedly in the excitation range of about P < 15 mW, then increases at a lower rate, and finally shows an approximatively saturated tendencywith further increasing excitation power (P > 15 mW). In other words, the increase rate of the IQE gradually decreases with excitation power in our measured range. Now we discuss the mechanism responsible for the dependence of the IQE on the excitation power.
Fig. 5 Internal quantum efficiency (a) and integrated PL intensity at 6 and 300 K (b) of MQWs as a function of the excitation power.
In general, the collected PL intensity is proportional to the injected carrier density with a power index [23, 24]. Here, we assume that excitation power P is approximately proportional to the injected carrier density for a fixed spot size of pumping laser. Therefore, the PL intensity could be expressed as
where I is integrated PL intensity, and parameter F physically reflects the various recombination processes. If F equals to 1, it indicates the radiative recombination dominates. On the other hand, if F > 1, the Shockley–Read–Hall recombination occurs, relating to the presence of nonradiative centers that provide a shunt path to the carriers. Figure 5(b) summarized the relationship between the excitation power and the integrated PL intensity for the MQWs. At 6 K, the intensity is linearly varied with increasing excitation power (F = 1), which indicates that the radiative recombination dominates the recombination process and the nonradietive centers are quenched at low temperature. However, under low excitation power at 300 K, the superlinear dependence of I on P is observed, showing that the defect-related nonradiative recombination dominates in the low excitation range. But as excitation power continuously increases up to an excitation power close to P ≈15 mW, the linear dependence of the emission intensity on the excitation power is exhibited. That is, at 300 K, the value of F decreases gradually from 1.22 to 1 with increasing excitation power, meaning that the nonradiative centers are saturated and lead to the gradual suppression of the nonradiative recombination, in agreement with the discussion done in Fig. 1(b). At P > 15 mW, the radiative recombination almost completely dominate the recombination process [Fig. 5(b)]. Based on the discussion, the mechanism responsible for the dependence of the IQE on the excitation power [Fig. 5(a)], can be explained as follows. Since the radiative recombination dominates the emission process in whole excitation range at 6 K and, in contrast, the emission process at 300 K undergoes a mechanism converting from the nonradiative recombination to the radiative recombination with increasing excitation power, the IQE pronouncedly increases with increasing excitation power up to P ≈15 mW, as shown in Fig. 5(a). On the other hand, after P ≈15 mW, as the band-filling effect of localized states starts interfering and becomes dominated as mentioned above, which prompts the injected carriers to escape more easily from localized states at 300 K than that at 6 K, the increase of the IQE is gradually suppressed with increasing excitation power in our measurement range (P < 50 mW).Above mentioned results indicate that though carrier localization is an important aspect in understanding the radiative recombination and improving the IQE in the InGaN QW, the existence of the internal electric fields (i.e., polarization fields) leading to charge separation issues and nonradiative centers are also important limitation in achieving large spontaneous emission rate and high IQE QW active region. Therefore, to achieve a high IQE for InGaN QWs based LEDs, various methods are being pursued by many researchers [25–31], such as, the use of the staggered InGaN QW and InGaN-delta-InN QW both with improved electron-hole wavefuctions overlap design for the enhancement of its radiative recombination rate [28, 30], and the application of patterned sapphire substrates (PSS) to reduce the threading dislocation density and overcome the nonradiative recombination [31].
4. Conclusions
In summary, we have investigated the carriers transferring and recombining mechanism of the MOCVD-grown InGaN/GaN MQWs over the excitation power range of 0.001 to 50 mW and the temperature range of 6 to 300 K. The S- and W-shaped temperature dependences of the emission energy and linewidth reflect the conversion of the carrier transferring mechanisms from nonthermalized to thermalized distribution of localized carriers, and finally to the regular thermalization of the carriers. The disappearance of the S- and W-shaped temperature dependences with increasing excitation power, is attributed to the reduced localization effect. The initial decreasing in the W-shaped temperature dependent linewidth strengthens first and then weakens with increasing excitation power, which we attribute to the exponentially increased density of states with energy in the band tail. The excitation power dependence of the emission intensity, together with that of the emission energy and linewidth, shows that the emission process of the MQWs is dominated by the radiative recombination at low temperature, and by nonradiative recombination at room temperature within low excitation range. The conclusion is also manifested in the excitation power dependence of the IQE. It is improved due to the pronounced enhancement of the radiative recombination mechanism at room temperature with increasing excitation power, and then tends to a constant with further increasing excitation power, this is because when band-filling effect dominates, the injected carriers escape more easily from localized states, especially at room temperature. Accordingly, we can conclude that to achieve a high-quantum efficiency of InGaN-based LED, it would be essential to overcome the nonradiative recombination, weaken the internal electric field in the QW, and increase the depth of localized states to suppress carriers escaping to extended states. The experimental results will provide a useful guidance to fabricate a high-performance LED with high-quantum efficiency.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 10874101), by the Science Foundation of Shandong province, China (Grant No. Y2008A10), and by National Basic Research Program of China (973 Program) through Grant No. 2009CB930503.
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