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GaN multiple quantum wells
Superlattices and Microstructures 97 (2016) 186e192
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Influence of InGaN growth rate on the localization states and optical properties of InGaN/GaN multiple quantum wells X. Li a, D.G. Zhao a, *, J. Yang a, D.S. Jiang a, Z.S. Liu a, P. Chen a, J.J. Zhu a, W. Liu a, X.G. He a, X.J. Li a, F. Liang a, L.Q. Zhang b, J.P. Liu b, H. Yang b, Y.T. Zhang c, G.T. Du c a
State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing, 100083, People’s Republic of China Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou, 215123, People’s Republic of China c State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun, 130023, People’s Republic of China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2016 Received in revised form 11 June 2016 Accepted 13 June 2016 Available online 16 June 2016
The localization effect is studied in blue-violet light emitting InGaN/GaN multiple quantum wells (MQWs) with varying InGaN growth rate. The temperature-dependent photoluminescence (PL) measurement shows that for higher-growth-rate samples two emission peaks appear in their PL spectra. Further analysis reveals that two different localization luminescence states (i.e., deep and shallow localization states) exist in the InGaN QWs with higher QW growth rate, and the competition of radiative recombination between the two localization states determines the relative intensity of the two emission peaks. It is also found that, as InGaN growth rate reduces, the deep localization state depth is almost unchanged while the shallow localization state weakens. When the QW growth rate reduces to a certain value, the shallow localization state disappears and only a single main peak induced by deep localization state appears in the PL spectra. Finally, it is noted that an intermediate InGaN growth rate results in a better light emission efficiency of the MQW. © 2016 Elsevier Ltd. All rights reserved.
Keywords: InGaN/GaN multiple quantum well Localization effect Growth rate
1. Introduction InGaN/GaN MQW structures have been widely used as active layers in InGaN-based optoelectronic devices [1e6], such as photodetectors, light-emitting diodes (LEDs) and laser diodes (LDs), due to their tunable optical band gap from ultraviolet to infrared spectral range. In spite of the high density of defects and strong quantum confined Stark effect (QCSE), the luminescence efficiency of light emission in InGaN/GaN MQWs can be surprisingly high. A prevailing consensus is that the dominant emission from InGaN/GaN MQW structures involves the carrier recombination of strongly localized centers, which can localize carriers to hinder them moving to nonradiative recombination centers and hence reduce the effect of nonradiative recombination at dislocations [7e9]. The dominant mechanisms for the formation of such localization centers are still a matter of debate, and different mechanisms have been proposed. The possible mechanisms of carrier localization that have been widely cited are QW width fluctuations [10,11], random alloy fluctuations [12], including In-N-In chains [13], and
* Corresponding author. E-mail address: [email protected] (D.G. Zhao). http://dx.doi.org/10.1016/j.spmi.2016.06.023 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
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indium-rich clustering [14]. These mechanisms can be affected by the growth parameters of InGaN/GaN MQWs, such as growth temperature, growth rate and source flux. As a key growth parameter, the growth rate of InGaN QWs can influence localization effect and luminescence properties by affecting indium incorporation and QW layer thickness during the QW growth [15e17]. Therefore, it is rather important to get a deep insight into the influences of InGaN growth rate on localization effect for improving the luminescence efficiency of light emission. In this paper, by varying TEGa flux of InGaN QWs, three InGaN/GaN MQW samples with varying InGaN growth rate are grown. The carrier localization effect in InGaN/GaN MQWs is investigated by means of temperature-dependent PL measurement. The experimental results show that for higher-growth-rate InGaN/GaN MQW samples, two emission peaks (lowenergy peak and high-energy peak) appear in their PL spectra, while the emission peak becomes a single one when slowing down the growth rate of InGaN QW. Further analysis reveals that there are two different localization states in the InGaN QWs with higher QW growth rate, acting as two radiative recombination centers, and the PL intensity variation is attributed to the transitions of radiative recombination centers between the two localization states. The effects of these two localization states on light emission are also analyzed. 2. Experiment The three samples (labeled as S15, S25 and S35, respectively) studied in this work were grown on c-plane sapphire substrates using AIXTRON close-coupled showerhead 3 2 in. vertical metaleorganic chemical vapor deposition (MOCVD) reactor, spinning at 100 rpm. The used precursors of In, N, and Si were trimethylindium (TMIn), ammonia (NH3), and silane (SiH4), respectively. The precursor of Ga was trimethylgallium (TMGa) anywhere, except in InGaN/GaN MQW region growth triethylgallium (TEGa) is used. After thermal cleaning of the substrates in a H2 ambient for 10 min at 1080 C, a 25 nm LT-GaN buffer layer was first grown at 550 C and then the temperature was increased to 1080 C for the growth of a 2-mm-thick Sidoped GaN bulk layer. These layers formed the template on which three periods of InGaN/GaN MQWs with well and barrier growth temperature of 730 C and 830 C were grown, followed by a 150 nm Mg-doped p-type GaN cladding layer to fabricate MQW structures. The barrier growth temperature is increased with respect to that of the QW layers in order to get closer to the optimum GaN deposition conditions for improving material quality [18]. The growth conditions of these three samples were nearly the same except the InGaN QW growth rate which is modulated by varying the TEGa source flow rate and the InGaN QW growth time. These 3 structures were denoted as Samples S15, S25, and S35 for the TEGa flux of 15, 25, and 35 sccm. The InGaN growth rate is almost linearly increased with increasing TEGa flow rate [16], but the growth time of InGaN QWs for these samples has to be adjusted to maintain a constant QW width, leaving the growth rate as a single variable. The method of growth rate control will be discussed in detail below in the experiment part of this article. The localization effect had been investigated using temperature-dependent PL measurements, carrying out with a 325-nm line of HeCd laser at an excitation density of 0.8 W/cm2, and the temperature was controlled to change from 10 to 300 K using a closed-cycle refrigerator of CTI Cryogenics. The detected PL spectral lines were fitted by Gaussian functions to eliminate the influence induced by the Fabry-Perot interference fringes for analyzing the accurate peak energy, peak intensity and full width at half maximum (FWHM). The surface morphology of samples was characterized by atomic force microscopy (AFM). 3. Results and discussion The indium mole fraction and InGaN QW width cannot be determined by high resolution X-ray diffraction (HR-XRD) for these three samples. Because the number of QWs is small and there is an additional thick layer on the top of active region, the intensity of MQW layer peak signal is too low to measure [18]. To obtain the InGaN growth rate of these three samples, three additional thick InGaN layers of about 37.5 nm-thick, were separately grown on GaN template using the same growth conditions with the InGaN QW of Samples S15, S25, and S35, to take as reference samples. Their thickness is controlled by the in situ monitoring system, respectively (data not shown here). Accordingly, the growth time of 3 samples was recorded as 4260, 2840, and 2060 s. The InGaN growth rate of Samples S15, S25, and S35 was calculated to be 0.0088, 0.0132, and 0.0182 nm/s, respectively. Furthermore, the growth time of each individual InGaN QW for Samples S15, S25, and S35 was adjusted to be 270,180, and 130 s, respectively and thus the QW width for the three samples was able to keep constant (~2.4 nm). Fig. 1 shows the PL spectra of the InGaN/GaN MQW Samples S15, S25, and S35 recorded at temperature of 30 K. At first, we observed InGaN related main emission peak at 3.02, 3.04, and 3.05 eV for Samples S15, S25, and S35, respectively. In fact, the three MQW samples have the same well and barrier layer thicknesses, the only difference is the growth rate of InGaN layer and the crystalline quality of InGaN layer. The difference in their PL properties can be mainly attributed to the difference in InGaN layers. The InGaN band gap energy depends on the indium composition and can be calculated by Eg(InxGa1xN) ¼ Eg(GaN) (1x) þ Eg(InN)x bx(1x) (eV), where b is the bowing parameter [19]. It is noted that the peak energies of InGaN MQWs have no any remarkable difference. The minor indium composition difference can be mainly attributed to the enhanced indium incorporation efficiency with increasing TEGa flow in the process of MQW growth [15]. As comprehensive results of increasing indium incorporation efficiency and TEGa flow, the indium composition of S25 is largest, accompanied with smallest peak emission energy of InGaN/GaN MQWs. However, it is found that the peak intensity of these samples is quite different. It is not monotonously changing with increasing growth rate from S15 to S25 to S35, which may be partly related to varying material quality. To compare the defect-related material quality of the InGaN QW as far as possible, the surfaces of three individual InGaN single layers, separately grown using the same growth conditions as the InGaN QW
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Fig. 1. The PL spectra of the InGaN/GaN MQW Samples S15 (black), S25 (red), and S35 (blue) recorded at temperature 30 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
samples, are measured by AFM and the results are depicted in Fig. 2. As shown in Fig. 2(c), when the InGaN growth rate increases to 0.0182 nm/s (corresponding to Sample S35), a large number of pits and black spots appear on the surface, caused by reduced migration time of adatoms on the growth surface with a larger InGaN growth rate [16]. The rough surface may be accompanied with a degraded material quality related to defects, which can form a large number of non-radiative recombination centers. As a result, the increased defect-related non-radiative recombination centers in Sample S35 may capture more carriers, leading to much deteriorated PL intensity. The relatively high indium composition in MQWs always leads to large strains, accompanied easily with defects in MQW structure and thus poor material quality. However, for S25 with the largest indium composition, as shown in Fig. 2(b), the surface of InGaN layer is much smoother and no obvious surface pits, and corresponds to a better material quality due to the slow growth rate of 0.0132 nm/s. Therefore, it shows the migration time of adatoms on the growth surface subject to growth rate is more influential than the relatively large indium composition induced defects in MQW structure on material quality, causing a better PL quality. When the InGaN growth rate is reduced to 0.0088 nm/s (corresponding to sample S15) and thus the adatoms migration time on the growth surface increases, it is found that, the surface is unexpectedly rough and many surface pits also exist as shown in Fig. 2(a). The possible reason for the degraded material quality is that the longer growth time gives impurity atoms in the reaction chamber more opportunity to be introduced into epitaxial layers. However, it is also well known that the PL intensity of InGaN is affected not only by the non-radiative centers dependent on the material quality [15,16], but also by the localization states which form the radiative recombination centers [7e9]. We will investigate this aspect below. In order to study the PL emission properties in detail, the PL spectra of these three samples at various temperatures are plotted in Fig. 3(a)e(c), where the peak intensity of PL spectra is normalized for each sample to clearly recognize the variation of the spectral line shape with increasing temperature. It is noticed that the InGaN-related PL spectral shape of Sample S15 seems almost symmetric and there is only a single main peak in spite of the variation of temperature. For Samples S25 and S35, the PL spectral line shape is asymmetric, a main emission peak (low energy peak PL) and an additional emission peak (high energy peak PH) located in the right shoulder with different intensity can be observed. The intensity of PH varies with increasing temperature and even exceeds the intensity of PL when the temperature increases to 270 K for Sample S35. Therefore, it is reasonable to use single Gaussian peak fitting for S15 and to analyze separately PL and PH with Gaussian fitting for S25 and S35. The PL peak energy as a function of temperature is demonstrated in Fig. 4 for all samples. The solid lines are fitting curves based on Equation (1). Obviously, an anomalous S-shape behavior of PL peak energy is observed for Sample S15. For S25 and S35, both PL and PH undergo an S-shape shift with increasing temperature. The S-shape behavior can act as an evidence of the existence of localization effect and has been explained very well by the change in carrier dynamics with increasing temperature, and attributed to the existence of band-tail states in the density of states [20e22]. So it can be considered that only a single class of localization center dominates the radiative recombination for S15, while two classes of localized states exist in Samples S25 and S35, which are responsible for the two emission peaks (PL and PH). The degree of localization effect in nitride-based devices could be obtained according to the band-tail states model. The temperature dependence of Eg is described as:
Eg ðTÞ ¼ Eð0Þ
aT 2 s2 ; T þ b KB T
(1)
where E(0) is the band-gap energy at 0 K; T is the temperature in Kelvin; and a and b are Varshni fitting parameters. The third term describes the broadening effect and the shape of the band tail below the nominal band edge, in which s indicates the
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Fig. 2. AFM images of the InGaN layer surfaces: (a) TEGa ¼ 15 sccm, (b) TEGa ¼ 25 sccm, (c) TEGa ¼ 35 sccm and their growth rate is 0.0088, 0.0132, and 0.0182 nm/s, respectively.
Fig. 3. The PL spectra of Samples (a) S15, (b) S25, and (c) S35 at various temperatures. The PL intensity is individually normalized at each temperature to clearly observe the variation of the PL spectral shape with increased temperature.
Fig. 4. Temperature dependent peak energy position for Sample (a) S15 and PL (black squares) and PH (red circles) from Samples (b) S25 and (c) S35, respectively. The solid lines are fitting curves based on Equation (1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
degree of the localization effect. The fitted values of s are 18.1 and 10.9 meV for the PL and PH of S25, while for the PL and PH of S35, the value of s is 18.3 and 15.2 meV, respectively. The fitted values of E(0), a and b are listed in Fig. 4. It is noted that the s value of PL is relatively large and almost equivalent (~18 meV) for Samples S25 and S35, indicating strong and deep localization states (DLS). The In-rich regions, where composition fluctuation easily happens, always form In clusters and quantumdots-like structures, which can form deep and strong traps acting as localized radiative recombination centers. Therefore, the PL peak of S25 and S35 as well as the single main peak of S15, corresponding to DLS, are most likely ascribed to indium compositional fluctuation. In addition, the references [17,23e25] also attributed DLS to compositional fluctuation, further supporting our point. It is also noted that the localization states for PH are weaker than those for PL and the localization states
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for PH correspond to the shallow localization states (SLS). Since the indium content outside the In-rich regions is low and thus the carrier confinement there is too weak, the localized carriers escape easily from there into nonradiative recombination centers, leading to a weakened radiative recombination. Therefore, the existence of radiative recombination centers outside the In-rich regions seems difficult to be understood. The mechanism forming weak carrier localization in these areas can be ascribed to well width fluctuations. First, the SLS is weakened with the decreasing growth rate, which accords with the reducing well width fluctuation due to the reducing growth rate. Second, references [23,25] have demonstrated similar guess about SLS, which can further support our viewpoints. When the QW growth rate is reduced to 0.0088 nm/s, the localization effect is less significant and thus only a single PL peak induced by DLS appears for S15. In addition, when the Equation (1) is used to fit PL and PH individually, there are two different band gaps with the respective band tail states for PL and PH because of the different average indium content. Actually, it has been also demonstrated by De et al. [26,27] that two distinct classes of carrier localization centers exist in InGaN/GaN MQWs. The deep traps and the shallow traps originate from local compositional fluctuations of indium content and thickness variation of the active layers, respectively. To further confirm the intensity variation of PL and PH, the temperature-dependent integrated PL intensity for the PL and PH of Samples S25 and S35 is shown in Fig. 5, respectively. It is shown that the integrated PL intensity of PL for Samples S25 and S35 increases slightly at first, and then drops monotonically with increasing temperature. Actually, at very low temperature, the photo-generated carriers are randomly distributed in QWs, and the recombination process occurs across the whole QW region. As temperature rises, the thermal mobility of the carriers increases, the thermally activated carriers move to the lowest localized states via hopping, i.e., more electrons and holes recombine radiatively within the potential minima where the PL is originated, causing an increase in the integrated intensity of PL. Along with continually rising temperature, the carriers can further obtain enough thermal energy to escape from potential minima, while meantime the non-radiative recombination starts to more and more dominate over the radiative recombination process, resulting in a rapid reduction of the integrated intensity of PL with increasing temperature. For the PH of Samples S25 and S35, the integrated intensity reduces monotonically with rising temperature. The reasons can be ascribed two aspects. On the one hand, the carriers tend to escape from the SLS with low average indium content to non-radiative recombination centers. On the other hand, combining Figs. 4 and 5, it is found that, in the low temperature range the emission energy red-shifts of PH accompanied with a decrease of the integrated PL intensity for S25 and S35. In fact, the redshift of the emission energy at low temperature is attributed to the redistribution of carriers from shallower localization states to the SLS. It suggests that some defects related to nonradiative recombination centers may exist in SLS in S25 and S35. Moreover, it is found that the integrated intensity of PH in Sample S25 (S35) is much smaller than that of PL for T < 210 K(170 K), and starts to approach the integrated intensity of PL in the temperature range 210 K(170 K) < T < 290 K(270 K). When the temperature continues to increase, the integrated intensity of PH begins to reach and even exceed the intensity of PL, especially for Sample S35. At very high temperature, as InGaN growth rate increases, the integrated PL intensity of PH relative to that of PL is enhanced, which is consistent with the increased localization degree for PH with increasing InGaN growth rate. The temperature dependent integrated PL intensity for the PL and PH proves that the DLS dominates the radiative recombination at low temperature, and the radiative recombination centers begin to transform from DLS to SLS at middle temperature range, and at very high temperature the radiative recombination is mainly led by the SLS. To explain the above mentioned temperature dependent peak energy and intensity variation for all samples in detail, a schematic energy band diagram describing the possible mechanism of carrier transferring in the MQW structure at different
Fig. 5. The temperature dependent integrated PL intensity for the PL (black) and PH (red) of Samples (a) S25 and (b) S35. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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temperatures is demonstrated in Fig. 6. At low temperature of 10 K, carriers do not have sufficient thermal energy, and can not relax down to the lower energy level states, so that they are randomly distributed in the potential minima (Fig. 6(a)). As the temperature increases from 10 to 70 K, weakly localized carriers are thermally activated and relax down into lower localized states via hopping (Fig. 7(b)) [28,29], which causes an initial red shift of PL peak energy. The subsequent blue shift is often related to the Boltzmann occupation of localized states for T ¼ 70e150 K [30]. A further increase of temperature enables carriers to achieve the thermal equilibrium with the lattice and to occupy higher-energy levels of the localized states (Fig. 6(c)). In fact, the temperature induced band-gap shrinkage also occurs as soon as the temperature rises. The temperature induced band-gap shrinkage starts to dominate for each PL peak including PL and PH when temperature increases from 150 K to 300 K, giving rise to a redshift process as predicted by the well known Varshni equation [31]. This is the possible mechanism of carriers transferring induced S-shaped behavior in the MQWs structure at different temperatures. As for the intensity variation of PL and PH for S25 and S35, it may be due to the transform of radiative recombination centers from DLS to SLS when further increasing temperature. The increased temperature enables carriers in DLS to be further excited and occupy higher-energy localized states, i.e., transfer to SLS. As temperature continues to increase, the number of carriers in SLS increases and it may be even larger than that of carriers in DLS when the temperature exceeds a certain value (270 K and 210 K for S25 and S35, respectively) (Fig. 6(d)). Meanwhile, the radiative recombination centers are transferred from DLS to SLS and the intensity of PH exceeds the intensity of PL. The SLSs with stronger localization can contain more carriers, which can lead to a rather stronger PH. As a result, the intensity of PH for Sample S35 becomes stronger and even exceeds that of PL at very high temperatures (as seen in Figs. 5(b) and 3(c)). Based on the discussion about localization states, the PL intensity of all samples such as shown in Figs. 1 and 3 should be further analyzed. For Sample S35 with the highest InGaN growth rate, the majority of carriers are transferred to or trapped in SLS, and thus the PL emission intensity is dominated by PH, especially at higher temperature (as shown in Fig. 5(b)). Since the SLS may originate from low average indium regions with weak carrier confinement, the carriers are likely to escape easily from SLS to nonradiative recombination centers, causing a weakened PL integral intensity. Additionally, the material quality deteriorates due to the reduced adatoms migration time on the growth surface, leading to a large number of nonradiative recombination centers. As discussed before, the nonradiative recombination centers may appear near the localization state regions, which has been also mentioned in Ref. [20]. Thereby the photo-generated carriers are easily captured by nonradiative recombination centers, further deteriorating the PL intensity. Sample S25 with medial InGaN growth rate displays the best PL emission intensity, which is attributed to two aspects. On the one hand, the nonradiative recombination centers can be largely diminished due to the improved material quality (as seen in Fig. 2(b)). On the other hand, the carriers transferred into SLS are reduced, further decreasing the opportunity to be captured by nonradiative recombination centers. As for Sample S15, it would be difficult to understand that the PL intensity is weak although only DLS dominates the radiative recombination. Thus the relatively low PL intensity should be attributed to the poor material quality (as seen in Fig. 2(a)), and thus massive nonradiative recombination centers may consume a lot of photo-generated carriers.
4. Conclusion In conclusion, three blue-violet light emitting InGaN/GaN MQW samples with varying InGaN growth rate are grown. The temperature dependent PL is measured to investigate the localization effect in InGaN QWs. Two peaks located in the lowenergy side and high-energy side are observed in InGaN/GaN MQW samples with higher InGaN growth rate, and only one
Fig. 6. Schematic energy band diagrams indicating the possible mechanism of carriers transferring in the MQW structures for Samples S15, S25 and S35 at different temperature.
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single main peak appear in the PL spectra when slowing down the InGaN growth rate. It is found that these two peaks correspond to the transitions from two classes of localization states, i.e., DLS which may be induced by In-rich clusters and SLS which may be induced by well width fluctuation in the InGaN QWs, and the competition of radiative recombination between DLS to SLS leads to the relative change of PL and PH in integrated PL intensity. As InGaN growth rate increases, the SLS is enhanced due to increased well width fluctuations, and thus the intensity of high energy peak is increased owing to the increased carriers captured by SLS. It is noted that in the InGaN/GaN MQW with an intermediate InGaN growth rate, the majority of carriers are still trapped in the DLS at higher temperature and meantime the nonradiative recombination centers is reduced due to the improved material quality, which results in a better light emission efficiency of this sample. Acknowledgments The authors acknowledge the support from the National Natural Science Foundation of China (Grant Nos. 61474110, 61377020, 61376089, 61223005 and 61321063), One Hundred Person Project of the Chinese Academy of Sciences, and Basic Research Project of Jiangsu Province (Grant No. BK20130362). References [1] Xiaojing Li, Degang Zhao, Desheng Jiang, Zongshun Liu, Ping Chen, Lingcong Le, Jing Yang, Xiaoguang He, Shuming Zhang, Jianjun Zhu, Hui Wang, Baoshun Zhang, Jianping Liu, Hui Yang, J. Vac. Sci. Technol. B 32 (2014) 031204. [2] Fu Binglei, Liu Naixin, Liu Zhe, Li Jinmin, Wang Junxi, J. Semicond. 35 (2014) 114007. [3] Wei Liu, De Gang Zhao, De Sheng Jiang, Ping Chen, Zong Shun Liu, Jian Jun Zhu, Xiang Li, Ming Shi, Dan Mei Zhao, Jian Ping Liu, Shu Ming Zhang, Hui Wang, Hui Yang, J. Vac. Sci. Technol. A 33 (2015) 061502. [4] X. Li, Z.S. Liu, D.G. Zhao, D.S. Jiang, P. Chen, J.J. Zhu, J. Yang, L.C. Le, W. Liu, X.G. He, X.J. Li, F. Liang, L.Q. Zhang, J.P. Liu, H. Yang, Appl. 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Influence of InGaN growth rate on the localization states and optical properties of InGaN/GaN multiple quantum wells X. Li a, D.G. Zhao a, *, J. Yang a, D.S. Jiang a, Z.S. Liu a, P. Chen a, J.J. Zhu a, W. Liu a, X.G. He a, X.J. Li a, F. Liang a, L.Q. Zhang b, J.P. Liu b, H. Yang b, Y.T. Zhang c, G.T. Du c a
State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing, 100083, People’s Republic of China Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou, 215123, People’s Republic of China c State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun, 130023, People’s Republic of China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2016 Received in revised form 11 June 2016 Accepted 13 June 2016 Available online 16 June 2016
The localization effect is studied in blue-violet light emitting InGaN/GaN multiple quantum wells (MQWs) with varying InGaN growth rate. The temperature-dependent photoluminescence (PL) measurement shows that for higher-growth-rate samples two emission peaks appear in their PL spectra. Further analysis reveals that two different localization luminescence states (i.e., deep and shallow localization states) exist in the InGaN QWs with higher QW growth rate, and the competition of radiative recombination between the two localization states determines the relative intensity of the two emission peaks. It is also found that, as InGaN growth rate reduces, the deep localization state depth is almost unchanged while the shallow localization state weakens. When the QW growth rate reduces to a certain value, the shallow localization state disappears and only a single main peak induced by deep localization state appears in the PL spectra. Finally, it is noted that an intermediate InGaN growth rate results in a better light emission efficiency of the MQW. © 2016 Elsevier Ltd. All rights reserved.
Keywords: InGaN/GaN multiple quantum well Localization effect Growth rate
1. Introduction InGaN/GaN MQW structures have been widely used as active layers in InGaN-based optoelectronic devices [1e6], such as photodetectors, light-emitting diodes (LEDs) and laser diodes (LDs), due to their tunable optical band gap from ultraviolet to infrared spectral range. In spite of the high density of defects and strong quantum confined Stark effect (QCSE), the luminescence efficiency of light emission in InGaN/GaN MQWs can be surprisingly high. A prevailing consensus is that the dominant emission from InGaN/GaN MQW structures involves the carrier recombination of strongly localized centers, which can localize carriers to hinder them moving to nonradiative recombination centers and hence reduce the effect of nonradiative recombination at dislocations [7e9]. The dominant mechanisms for the formation of such localization centers are still a matter of debate, and different mechanisms have been proposed. The possible mechanisms of carrier localization that have been widely cited are QW width fluctuations [10,11], random alloy fluctuations [12], including In-N-In chains [13], and
* Corresponding author. E-mail address: [email protected] (D.G. Zhao). http://dx.doi.org/10.1016/j.spmi.2016.06.023 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
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indium-rich clustering [14]. These mechanisms can be affected by the growth parameters of InGaN/GaN MQWs, such as growth temperature, growth rate and source flux. As a key growth parameter, the growth rate of InGaN QWs can influence localization effect and luminescence properties by affecting indium incorporation and QW layer thickness during the QW growth [15e17]. Therefore, it is rather important to get a deep insight into the influences of InGaN growth rate on localization effect for improving the luminescence efficiency of light emission. In this paper, by varying TEGa flux of InGaN QWs, three InGaN/GaN MQW samples with varying InGaN growth rate are grown. The carrier localization effect in InGaN/GaN MQWs is investigated by means of temperature-dependent PL measurement. The experimental results show that for higher-growth-rate InGaN/GaN MQW samples, two emission peaks (lowenergy peak and high-energy peak) appear in their PL spectra, while the emission peak becomes a single one when slowing down the growth rate of InGaN QW. Further analysis reveals that there are two different localization states in the InGaN QWs with higher QW growth rate, acting as two radiative recombination centers, and the PL intensity variation is attributed to the transitions of radiative recombination centers between the two localization states. The effects of these two localization states on light emission are also analyzed. 2. Experiment The three samples (labeled as S15, S25 and S35, respectively) studied in this work were grown on c-plane sapphire substrates using AIXTRON close-coupled showerhead 3 2 in. vertical metaleorganic chemical vapor deposition (MOCVD) reactor, spinning at 100 rpm. The used precursors of In, N, and Si were trimethylindium (TMIn), ammonia (NH3), and silane (SiH4), respectively. The precursor of Ga was trimethylgallium (TMGa) anywhere, except in InGaN/GaN MQW region growth triethylgallium (TEGa) is used. After thermal cleaning of the substrates in a H2 ambient for 10 min at 1080 C, a 25 nm LT-GaN buffer layer was first grown at 550 C and then the temperature was increased to 1080 C for the growth of a 2-mm-thick Sidoped GaN bulk layer. These layers formed the template on which three periods of InGaN/GaN MQWs with well and barrier growth temperature of 730 C and 830 C were grown, followed by a 150 nm Mg-doped p-type GaN cladding layer to fabricate MQW structures. The barrier growth temperature is increased with respect to that of the QW layers in order to get closer to the optimum GaN deposition conditions for improving material quality [18]. The growth conditions of these three samples were nearly the same except the InGaN QW growth rate which is modulated by varying the TEGa source flow rate and the InGaN QW growth time. These 3 structures were denoted as Samples S15, S25, and S35 for the TEGa flux of 15, 25, and 35 sccm. The InGaN growth rate is almost linearly increased with increasing TEGa flow rate [16], but the growth time of InGaN QWs for these samples has to be adjusted to maintain a constant QW width, leaving the growth rate as a single variable. The method of growth rate control will be discussed in detail below in the experiment part of this article. The localization effect had been investigated using temperature-dependent PL measurements, carrying out with a 325-nm line of HeCd laser at an excitation density of 0.8 W/cm2, and the temperature was controlled to change from 10 to 300 K using a closed-cycle refrigerator of CTI Cryogenics. The detected PL spectral lines were fitted by Gaussian functions to eliminate the influence induced by the Fabry-Perot interference fringes for analyzing the accurate peak energy, peak intensity and full width at half maximum (FWHM). The surface morphology of samples was characterized by atomic force microscopy (AFM). 3. Results and discussion The indium mole fraction and InGaN QW width cannot be determined by high resolution X-ray diffraction (HR-XRD) for these three samples. Because the number of QWs is small and there is an additional thick layer on the top of active region, the intensity of MQW layer peak signal is too low to measure [18]. To obtain the InGaN growth rate of these three samples, three additional thick InGaN layers of about 37.5 nm-thick, were separately grown on GaN template using the same growth conditions with the InGaN QW of Samples S15, S25, and S35, to take as reference samples. Their thickness is controlled by the in situ monitoring system, respectively (data not shown here). Accordingly, the growth time of 3 samples was recorded as 4260, 2840, and 2060 s. The InGaN growth rate of Samples S15, S25, and S35 was calculated to be 0.0088, 0.0132, and 0.0182 nm/s, respectively. Furthermore, the growth time of each individual InGaN QW for Samples S15, S25, and S35 was adjusted to be 270,180, and 130 s, respectively and thus the QW width for the three samples was able to keep constant (~2.4 nm). Fig. 1 shows the PL spectra of the InGaN/GaN MQW Samples S15, S25, and S35 recorded at temperature of 30 K. At first, we observed InGaN related main emission peak at 3.02, 3.04, and 3.05 eV for Samples S15, S25, and S35, respectively. In fact, the three MQW samples have the same well and barrier layer thicknesses, the only difference is the growth rate of InGaN layer and the crystalline quality of InGaN layer. The difference in their PL properties can be mainly attributed to the difference in InGaN layers. The InGaN band gap energy depends on the indium composition and can be calculated by Eg(InxGa1xN) ¼ Eg(GaN) (1x) þ Eg(InN)x bx(1x) (eV), where b is the bowing parameter [19]. It is noted that the peak energies of InGaN MQWs have no any remarkable difference. The minor indium composition difference can be mainly attributed to the enhanced indium incorporation efficiency with increasing TEGa flow in the process of MQW growth [15]. As comprehensive results of increasing indium incorporation efficiency and TEGa flow, the indium composition of S25 is largest, accompanied with smallest peak emission energy of InGaN/GaN MQWs. However, it is found that the peak intensity of these samples is quite different. It is not monotonously changing with increasing growth rate from S15 to S25 to S35, which may be partly related to varying material quality. To compare the defect-related material quality of the InGaN QW as far as possible, the surfaces of three individual InGaN single layers, separately grown using the same growth conditions as the InGaN QW
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Fig. 1. The PL spectra of the InGaN/GaN MQW Samples S15 (black), S25 (red), and S35 (blue) recorded at temperature 30 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
samples, are measured by AFM and the results are depicted in Fig. 2. As shown in Fig. 2(c), when the InGaN growth rate increases to 0.0182 nm/s (corresponding to Sample S35), a large number of pits and black spots appear on the surface, caused by reduced migration time of adatoms on the growth surface with a larger InGaN growth rate [16]. The rough surface may be accompanied with a degraded material quality related to defects, which can form a large number of non-radiative recombination centers. As a result, the increased defect-related non-radiative recombination centers in Sample S35 may capture more carriers, leading to much deteriorated PL intensity. The relatively high indium composition in MQWs always leads to large strains, accompanied easily with defects in MQW structure and thus poor material quality. However, for S25 with the largest indium composition, as shown in Fig. 2(b), the surface of InGaN layer is much smoother and no obvious surface pits, and corresponds to a better material quality due to the slow growth rate of 0.0132 nm/s. Therefore, it shows the migration time of adatoms on the growth surface subject to growth rate is more influential than the relatively large indium composition induced defects in MQW structure on material quality, causing a better PL quality. When the InGaN growth rate is reduced to 0.0088 nm/s (corresponding to sample S15) and thus the adatoms migration time on the growth surface increases, it is found that, the surface is unexpectedly rough and many surface pits also exist as shown in Fig. 2(a). The possible reason for the degraded material quality is that the longer growth time gives impurity atoms in the reaction chamber more opportunity to be introduced into epitaxial layers. However, it is also well known that the PL intensity of InGaN is affected not only by the non-radiative centers dependent on the material quality [15,16], but also by the localization states which form the radiative recombination centers [7e9]. We will investigate this aspect below. In order to study the PL emission properties in detail, the PL spectra of these three samples at various temperatures are plotted in Fig. 3(a)e(c), where the peak intensity of PL spectra is normalized for each sample to clearly recognize the variation of the spectral line shape with increasing temperature. It is noticed that the InGaN-related PL spectral shape of Sample S15 seems almost symmetric and there is only a single main peak in spite of the variation of temperature. For Samples S25 and S35, the PL spectral line shape is asymmetric, a main emission peak (low energy peak PL) and an additional emission peak (high energy peak PH) located in the right shoulder with different intensity can be observed. The intensity of PH varies with increasing temperature and even exceeds the intensity of PL when the temperature increases to 270 K for Sample S35. Therefore, it is reasonable to use single Gaussian peak fitting for S15 and to analyze separately PL and PH with Gaussian fitting for S25 and S35. The PL peak energy as a function of temperature is demonstrated in Fig. 4 for all samples. The solid lines are fitting curves based on Equation (1). Obviously, an anomalous S-shape behavior of PL peak energy is observed for Sample S15. For S25 and S35, both PL and PH undergo an S-shape shift with increasing temperature. The S-shape behavior can act as an evidence of the existence of localization effect and has been explained very well by the change in carrier dynamics with increasing temperature, and attributed to the existence of band-tail states in the density of states [20e22]. So it can be considered that only a single class of localization center dominates the radiative recombination for S15, while two classes of localized states exist in Samples S25 and S35, which are responsible for the two emission peaks (PL and PH). The degree of localization effect in nitride-based devices could be obtained according to the band-tail states model. The temperature dependence of Eg is described as:
Eg ðTÞ ¼ Eð0Þ
aT 2 s2 ; T þ b KB T
(1)
where E(0) is the band-gap energy at 0 K; T is the temperature in Kelvin; and a and b are Varshni fitting parameters. The third term describes the broadening effect and the shape of the band tail below the nominal band edge, in which s indicates the
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Fig. 2. AFM images of the InGaN layer surfaces: (a) TEGa ¼ 15 sccm, (b) TEGa ¼ 25 sccm, (c) TEGa ¼ 35 sccm and their growth rate is 0.0088, 0.0132, and 0.0182 nm/s, respectively.
Fig. 3. The PL spectra of Samples (a) S15, (b) S25, and (c) S35 at various temperatures. The PL intensity is individually normalized at each temperature to clearly observe the variation of the PL spectral shape with increased temperature.
Fig. 4. Temperature dependent peak energy position for Sample (a) S15 and PL (black squares) and PH (red circles) from Samples (b) S25 and (c) S35, respectively. The solid lines are fitting curves based on Equation (1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
degree of the localization effect. The fitted values of s are 18.1 and 10.9 meV for the PL and PH of S25, while for the PL and PH of S35, the value of s is 18.3 and 15.2 meV, respectively. The fitted values of E(0), a and b are listed in Fig. 4. It is noted that the s value of PL is relatively large and almost equivalent (~18 meV) for Samples S25 and S35, indicating strong and deep localization states (DLS). The In-rich regions, where composition fluctuation easily happens, always form In clusters and quantumdots-like structures, which can form deep and strong traps acting as localized radiative recombination centers. Therefore, the PL peak of S25 and S35 as well as the single main peak of S15, corresponding to DLS, are most likely ascribed to indium compositional fluctuation. In addition, the references [17,23e25] also attributed DLS to compositional fluctuation, further supporting our point. It is also noted that the localization states for PH are weaker than those for PL and the localization states
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for PH correspond to the shallow localization states (SLS). Since the indium content outside the In-rich regions is low and thus the carrier confinement there is too weak, the localized carriers escape easily from there into nonradiative recombination centers, leading to a weakened radiative recombination. Therefore, the existence of radiative recombination centers outside the In-rich regions seems difficult to be understood. The mechanism forming weak carrier localization in these areas can be ascribed to well width fluctuations. First, the SLS is weakened with the decreasing growth rate, which accords with the reducing well width fluctuation due to the reducing growth rate. Second, references [23,25] have demonstrated similar guess about SLS, which can further support our viewpoints. When the QW growth rate is reduced to 0.0088 nm/s, the localization effect is less significant and thus only a single PL peak induced by DLS appears for S15. In addition, when the Equation (1) is used to fit PL and PH individually, there are two different band gaps with the respective band tail states for PL and PH because of the different average indium content. Actually, it has been also demonstrated by De et al. [26,27] that two distinct classes of carrier localization centers exist in InGaN/GaN MQWs. The deep traps and the shallow traps originate from local compositional fluctuations of indium content and thickness variation of the active layers, respectively. To further confirm the intensity variation of PL and PH, the temperature-dependent integrated PL intensity for the PL and PH of Samples S25 and S35 is shown in Fig. 5, respectively. It is shown that the integrated PL intensity of PL for Samples S25 and S35 increases slightly at first, and then drops monotonically with increasing temperature. Actually, at very low temperature, the photo-generated carriers are randomly distributed in QWs, and the recombination process occurs across the whole QW region. As temperature rises, the thermal mobility of the carriers increases, the thermally activated carriers move to the lowest localized states via hopping, i.e., more electrons and holes recombine radiatively within the potential minima where the PL is originated, causing an increase in the integrated intensity of PL. Along with continually rising temperature, the carriers can further obtain enough thermal energy to escape from potential minima, while meantime the non-radiative recombination starts to more and more dominate over the radiative recombination process, resulting in a rapid reduction of the integrated intensity of PL with increasing temperature. For the PH of Samples S25 and S35, the integrated intensity reduces monotonically with rising temperature. The reasons can be ascribed two aspects. On the one hand, the carriers tend to escape from the SLS with low average indium content to non-radiative recombination centers. On the other hand, combining Figs. 4 and 5, it is found that, in the low temperature range the emission energy red-shifts of PH accompanied with a decrease of the integrated PL intensity for S25 and S35. In fact, the redshift of the emission energy at low temperature is attributed to the redistribution of carriers from shallower localization states to the SLS. It suggests that some defects related to nonradiative recombination centers may exist in SLS in S25 and S35. Moreover, it is found that the integrated intensity of PH in Sample S25 (S35) is much smaller than that of PL for T < 210 K(170 K), and starts to approach the integrated intensity of PL in the temperature range 210 K(170 K) < T < 290 K(270 K). When the temperature continues to increase, the integrated intensity of PH begins to reach and even exceed the intensity of PL, especially for Sample S35. At very high temperature, as InGaN growth rate increases, the integrated PL intensity of PH relative to that of PL is enhanced, which is consistent with the increased localization degree for PH with increasing InGaN growth rate. The temperature dependent integrated PL intensity for the PL and PH proves that the DLS dominates the radiative recombination at low temperature, and the radiative recombination centers begin to transform from DLS to SLS at middle temperature range, and at very high temperature the radiative recombination is mainly led by the SLS. To explain the above mentioned temperature dependent peak energy and intensity variation for all samples in detail, a schematic energy band diagram describing the possible mechanism of carrier transferring in the MQW structure at different
Fig. 5. The temperature dependent integrated PL intensity for the PL (black) and PH (red) of Samples (a) S25 and (b) S35. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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temperatures is demonstrated in Fig. 6. At low temperature of 10 K, carriers do not have sufficient thermal energy, and can not relax down to the lower energy level states, so that they are randomly distributed in the potential minima (Fig. 6(a)). As the temperature increases from 10 to 70 K, weakly localized carriers are thermally activated and relax down into lower localized states via hopping (Fig. 7(b)) [28,29], which causes an initial red shift of PL peak energy. The subsequent blue shift is often related to the Boltzmann occupation of localized states for T ¼ 70e150 K [30]. A further increase of temperature enables carriers to achieve the thermal equilibrium with the lattice and to occupy higher-energy levels of the localized states (Fig. 6(c)). In fact, the temperature induced band-gap shrinkage also occurs as soon as the temperature rises. The temperature induced band-gap shrinkage starts to dominate for each PL peak including PL and PH when temperature increases from 150 K to 300 K, giving rise to a redshift process as predicted by the well known Varshni equation [31]. This is the possible mechanism of carriers transferring induced S-shaped behavior in the MQWs structure at different temperatures. As for the intensity variation of PL and PH for S25 and S35, it may be due to the transform of radiative recombination centers from DLS to SLS when further increasing temperature. The increased temperature enables carriers in DLS to be further excited and occupy higher-energy localized states, i.e., transfer to SLS. As temperature continues to increase, the number of carriers in SLS increases and it may be even larger than that of carriers in DLS when the temperature exceeds a certain value (270 K and 210 K for S25 and S35, respectively) (Fig. 6(d)). Meanwhile, the radiative recombination centers are transferred from DLS to SLS and the intensity of PH exceeds the intensity of PL. The SLSs with stronger localization can contain more carriers, which can lead to a rather stronger PH. As a result, the intensity of PH for Sample S35 becomes stronger and even exceeds that of PL at very high temperatures (as seen in Figs. 5(b) and 3(c)). Based on the discussion about localization states, the PL intensity of all samples such as shown in Figs. 1 and 3 should be further analyzed. For Sample S35 with the highest InGaN growth rate, the majority of carriers are transferred to or trapped in SLS, and thus the PL emission intensity is dominated by PH, especially at higher temperature (as shown in Fig. 5(b)). Since the SLS may originate from low average indium regions with weak carrier confinement, the carriers are likely to escape easily from SLS to nonradiative recombination centers, causing a weakened PL integral intensity. Additionally, the material quality deteriorates due to the reduced adatoms migration time on the growth surface, leading to a large number of nonradiative recombination centers. As discussed before, the nonradiative recombination centers may appear near the localization state regions, which has been also mentioned in Ref. [20]. Thereby the photo-generated carriers are easily captured by nonradiative recombination centers, further deteriorating the PL intensity. Sample S25 with medial InGaN growth rate displays the best PL emission intensity, which is attributed to two aspects. On the one hand, the nonradiative recombination centers can be largely diminished due to the improved material quality (as seen in Fig. 2(b)). On the other hand, the carriers transferred into SLS are reduced, further decreasing the opportunity to be captured by nonradiative recombination centers. As for Sample S15, it would be difficult to understand that the PL intensity is weak although only DLS dominates the radiative recombination. Thus the relatively low PL intensity should be attributed to the poor material quality (as seen in Fig. 2(a)), and thus massive nonradiative recombination centers may consume a lot of photo-generated carriers.
4. Conclusion In conclusion, three blue-violet light emitting InGaN/GaN MQW samples with varying InGaN growth rate are grown. The temperature dependent PL is measured to investigate the localization effect in InGaN QWs. Two peaks located in the lowenergy side and high-energy side are observed in InGaN/GaN MQW samples with higher InGaN growth rate, and only one
Fig. 6. Schematic energy band diagrams indicating the possible mechanism of carriers transferring in the MQW structures for Samples S15, S25 and S35 at different temperature.
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single main peak appear in the PL spectra when slowing down the InGaN growth rate. It is found that these two peaks correspond to the transitions from two classes of localization states, i.e., DLS which may be induced by In-rich clusters and SLS which may be induced by well width fluctuation in the InGaN QWs, and the competition of radiative recombination between DLS to SLS leads to the relative change of PL and PH in integrated PL intensity. As InGaN growth rate increases, the SLS is enhanced due to increased well width fluctuations, and thus the intensity of high energy peak is increased owing to the increased carriers captured by SLS. It is noted that in the InGaN/GaN MQW with an intermediate InGaN growth rate, the majority of carriers are still trapped in the DLS at higher temperature and meantime the nonradiative recombination centers is reduced due to the improved material quality, which results in a better light emission efficiency of this sample. Acknowledgments The authors acknowledge the support from the National Natural Science Foundation of China (Grant Nos. 61474110, 61377020, 61376089, 61223005 and 61321063), One Hundred Person Project of the Chinese Academy of Sciences, and Basic Research Project of Jiangsu Province (Grant No. BK20130362). References [1] Xiaojing Li, Degang Zhao, Desheng Jiang, Zongshun Liu, Ping Chen, Lingcong Le, Jing Yang, Xiaoguang He, Shuming Zhang, Jianjun Zhu, Hui Wang, Baoshun Zhang, Jianping Liu, Hui Yang, J. Vac. Sci. Technol. B 32 (2014) 031204. [2] Fu Binglei, Liu Naixin, Liu Zhe, Li Jinmin, Wang Junxi, J. Semicond. 35 (2014) 114007. [3] Wei Liu, De Gang Zhao, De Sheng Jiang, Ping Chen, Zong Shun Liu, Jian Jun Zhu, Xiang Li, Ming Shi, Dan Mei Zhao, Jian Ping Liu, Shu Ming Zhang, Hui Wang, Hui Yang, J. Vac. Sci. Technol. A 33 (2015) 061502. [4] X. Li, Z.S. Liu, D.G. Zhao, D.S. Jiang, P. Chen, J.J. Zhu, J. Yang, L.C. Le, W. Liu, X.G. He, X.J. Li, F. Liang, L.Q. Zhang, J.P. Liu, H. Yang, Appl. 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