- Email: [email protected]
Study of photoluminescence and absorption in phase-separation InGaN films
ARTICLE IN PRESS
Physica B 344 (2004) 292–296
Study of photoluminescence and absorption in phase-separation InGaN films Z.Z. Chen*, Z.X. Qin, X.D. Hu, T.J. Yu, Z.J. Yang, Y.Z. Tong, X.M. Ding, G.Y. Zhang State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China Received 2 May 2003; accepted 19 October 2003
Abstract InGaN/GaN layers have been grown under low pressure by metal-organic vapor phase epitaxy on sapphire substrate. X-ray diffraction (XRD), photoluminescence, and optical absorption measurements have been performed to study the radiative recombination mechanisms in the samples. The In composition was determined by XRD measurement using Vegard’s law. With increasing In composition, a red shift of absorption edge and a broad Urbach tail in absorption spectra were observed. The InN inclusions in InGaN played a key role in long-wavelength absorption. There existed large Stokes shifts in all thick InGaN layers with different In compositions. The intensities of low-energy emissions were enhanced compared to those of high-energy ones with increasing In composition. We attributed low-energy emissions to deep levels and high-energy emissions to In-rich quantum dots. r 2003 Elsevier B.V. All rights reserved. PACS: 77.65Ly; 78.40Fy; 78.55Cr Keywords: InGaN; Phase separation; Photoluminescence excitation; Quantum dots
1. Introduction Recently, InGaN the ternary alloy which furnishes the active layers in UV-green light emitting diodes [1] and laser diodes [2] has been the focus of scientific efforts for understanding its emission mechanism [3–11] and structures [6,12– 15]. It is well known that chemical demixing will *Corresponding author. State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China. E-mail address: [email protected] (Z.Z. Chen).
occur in the ternary alloy due to the high miscibility gap between InN and GaN [16]. In contrast to a conventional alloy, the high miscibility gap leads to an InGaN layer composed of In-rich quantum dots (QDs) buried in an Indeficient matrix, since the solubility limit of Ga in InN is less than 5% [12]. Many authors consider the In-rich QDs as the origin of the high brightness emission in the InGaN-active region [4–7]. Several groups report that the piezoelectric effect plays an important role in the emitting process [9,10]. Under high injection conditions, the extended defects may be involved in radiative recombination
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.10.008
ARTICLE IN PRESS Z.Z. Chen et al. / Physica B 344 (2004) 292–296 100000 GaN 10000
Intensity(CPS)
[8,11]. However, more clear evidences should be obtained to confirm the origin of the InGaN emission. In this article, we performed photoluminescence (PL), PL excitation (PLE) and optical absorption measurements on a set of InGaN samples with different In composition. According to our results, the origin of absorption and emission in InGaN were obtained.
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InGaN
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InN
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2. Experiment
1 28
The InGaN layers were grown on 0.5 mmthick GaN layers on the sapphire substrates by metal-organic chemical vapor deposition (MOCVD). Trimethylindium (TMI), trimethylgallium (TMG), and ammonia (NH3) have been used as sources of In, Ga, and N, respectively. H2 and N2 were used as carrier gases. A GaN buffer layer was deposited at 550 C. Then a GaN epilayer was grown at 1060 C. The InGaN layer was grown at 800 C on the GaN layer. The typical thicknesses of GaN and InGaN layers are 0.5 and 1.0 mm, respectively. The composition of In in InGaN layer was controlled by carrier gases flow rate, flow rate ratio of TMG to TMI, growth temperature, and so on. The In compositions in InGaN layers were determined by the measurement of X-ray diffraction (XRD) using Philips X’pert MRD diffractometer. Optical transmission spectra were obtained by U3410 UV-visible-infrared spectrometer. PL and PLE spectra were measured by EG&G Fluoro MAX-2 Spectrometer at room temperature, and the light source was xenon lamp.
3. Results and discussion The 2y scan pattern of a typical InxGa1 xN layer, named as sample A, is shown in Fig. 1. There are three peaks in Fig. 1, corresponding to GaN (0 0 0 2), InGaN (0 0 0 2) and InN (0 0 0 2) diffraction peaks. On the lower-angle side of InGaN (0 0 0 2), two arrow-indicated shoulders are visible. These results provide direct evidence of phase separation, which is attributed to spinodal decomposition [12]. According to the Bragg’s
30
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2 θ (deg)
Fig. 1. XRD curve of sample A. On the lower-angle side of InGaN (0 0 0 2), two arrow-indicated shoulders are visible.
angle of InGaN (0 0 0 2), the In mole composition, x, in InGaN layers can be obtained as 0.11 using Vegard’s law. Similarly, the values of x of samples B, and C are obtained as 0.156 and 0.177, respectively. The inclusions of InN, Z, in InGaN layer of samples A–C are determined as 1.485%, 0.975% and 1.069% by XRD rocking curve data. The details of the calculation theory are discussed in Ref. [17]. Optical transmission spectra are shown in Fig. 2. The interference effects on the transmission spectra are modified by the envelope curve method [18]. There are two obvious steps in each curve, corresponding to absorption edge of GaN and InGaN layers, respectively. With increasing x, the InGaN absorption edge is red-shifted, the Urbach tail is broadened and the optical absorption is strengthened. Short-wavelength absorption of sample B is more than that of sample A, but weaker for long-wavelength absorption. With increase of In composition in InGaN layer, the band gap will become narrower, and the lattice will be deteriorated because of lattice misfit between GaN and InGaN layers. Moreover, the phase separation of InGaN layer may be composed of InN segregation, InGaN composition modulation, and even In droplet when x is large. The random distribution of the In-rich region may influence the long-wavelength absorption of the whole InGaN layer, absorption and scattering by the In-rich region, which reduces the transmission
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294 80 70
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60 50 40 30 x=0.110, η=1.485% x=0.156, η=0.975% x=0.177, η=1.069%
20 10 0 300
400
500
600
700
Wavelength (nm) Fig. 2. Optical transmission spectra of InGaN samples with different x. The interference effects on the spectra are modified by the envelop curve method.
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Fig. 3. PL spectra of the InGaN samples with different x, where the excitation energy is 3.1 eV. The short dash lines indicate the results of Gaussian fitting for experiment data.
of the photons. The red-shift of optical absorption edge is dependent on x, while the phase separation of InGaN layer will determine the long-wavelength absorption (for samples A and B). Fig. 3 shows the PL spectra of the InGaN samples where the excitation energy is 3.1 eV. The
short dash lines are the results of Gaussian fitting for experiment data. Considering the lower energy excitation, the PL can be attributed to the emission from InGaN layer, while the emission from GaN layer can be neglected. There exists a large Stokeslike shift (the shift from optical absorption edge to
ARTICLE IN PRESS Z.Z. Chen et al. / Physica B 344 (2004) 292–296
emission peak in PL spectrum) of 300 meV more in all samples. For the samples with x=0.110, 0.156, 0.177, the ratios of intensities at the peaks of 2.4 eV to those of 2.2 eV are 5.1, 1.9, and 1.25, respectively. This causes the dominant peaks redshift slightly. The total PL intensities also decrease. The very large Stokes-like shift implies a strong localized effect in InGaN layer. So it can be concluded that PL emissions originate from: (1) In-rich region rather than from bulky InGaN layer, (2) deep levels in InGaN layer. In the thick InGaN film (typical thickness=1.0 mm), self-assembly QDs may be due to the compositional modulation in InGaN rather than the interface fluctuation [16,19]. With increasing x, the average size of In-rich QDs could be larger, which leads to the decrease of the radiative recombination efficiency due to the poorer quantum confinement, and the red-shift of PL emission. On the other hand, much more deep levels will be generated with increasing x due to the formation of lattice defects. So the emissions at 2.2 and 2.4 eV may be attributed to In-rich QDs and deep levels, respectively. The mechanisms of the two emissions will be studied further by PLE spectra in the following. Fig. 4 shows the PLE spectra of sample A for emission peaks at 510 and 550 nm in PL spectra. There are two peaks in each PLE spectrum, which are corresponding to GaN exciton peak and a broad InGaN peak, respectively. The intensity of GaN exciton peak is larger than that of InGaN 50000
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Fig. 4. PLE spectra of sample A for emission peaks at 510 and 550 nm in PL spectra.
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peaks for emission at 550 nm. It is reversed for emission at 510 nm. It means that the emission at 550 nm comes from the In-deficient matrix excited by high-energy photons and the phaseseparated InGaN regions excited by photons with an energy lower than 3.4 eV. The photongenerated carriers are recombined at deep levels where light is emitted at 550 nm. But the emission at 510 nm comes mainly from the phase-separated InGaN regions with lower-energy excitation. It seems that there exists a barrier for photon-generated carrier in matrix to transfer to QDs. It is a rather easy way for them to recombine in deep levels. When the excited carriers are trapped by In-rich QDs, they have no choice but to radiatively recombine. So the InGaN peak is dominant in the PLE spectrum for emission at 510 nm. Fig. 5 shows the PLE spectra of the InGaN samples, with the emissions at (a) 510 nm and (b) 550 nm. As In composition increases, the InGaN peaks are strengthened and the GaN peaks are weakened. At the same time, the intensities of emissions at 510 nm are decreased rapidly, obviously when x is more than a certain value, while the ones at 550 nm are decreased slowly and homogenously. With increasing x, InGaN peaks are broadened and multiple for both emissions at 510 and 550 nm. With increasing x, the emission from phase-separated InGaN layers will be strengthened comparatively. Moreover, the one from In-deficient matrix will be weakened because the excitation light to the matrix will be absorbed and scattered by In-rich region. The absolute decreases for both emissions with the increase in x are due to the lattice defects like non-radiative levels and weak confinement of In-rich regions for trapped carriers. The increases of x will cause severe InGaN phase separation, which will broaden the InGaN peaks. For the emissions related to QDs, the larger size of In-rich QDs could cause them to decrease rapidly due to the poorer quantum confinement. However, as to the emissions related to deep levels, they are changed smoothly because the efficiency of carrier recombination is dependent on the density of lattice defects, which could not be changed abruptly.
ARTICLE IN PRESS Z.Z. Chen et al. / Physica B 344 (2004) 292–296
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Fig. 5. PLE spectra of the InGaN samples, with the emissions at (a) 510 nm and (b) 550 nm.
4. Conclusions In summary, we have performed XRD, PL, PLE, and optical absorption spectra measurements on the InGaN samples with different x. The absorption edge of InGaN is red-shift and has a broad Urbach tail with increasing x. The intensities of low-energy emissions increase compared to those of high-energy ones. The large Stokesshift implies a strong localized effect in InGaN layer. According to the results of PL and PLE spectra, we conclude that the low-energy emissions are attributed to deep levels and the high ones are attributed to In-rich QDs in thick InGaN layers.
Acknowledgements This work was supported by National Science Foundation of China (NSFC) No.69876002.
References [1] T. Mukai, M. Yamada, S. Nakamura, Jpn. J. Appl. Phys. 28 (Part 1) (1999) 3976. [2] S. Nakamura, M. Senoh, S.-I. Nagahama, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, T. Mukai, Jpn. J. Appl. Phys. 38 (Part 2) (1999) L226. [3] R. Singh, D. Doppalapudi, T.D. Moustakas, L.T. Romano, Appl. Phys. Lett. 70 (9) (1997) 1089. [4] K.P. O’Donnell, R.W. Martin, G. Middelton, Phys. Rev. Lett. 82 (1) (1999) 237.
[5] A. Tabata, L.K. Teles, L.M.R. Scolfaro, J.R. Leite, A. Kharchenko, T. Frey, D.J. As, D. Schikora, K. Lischka, J. Furthm.uller, F. Bechstedt, Appl. Phys. Lett. 80 (5) (2002) 769. [6] G. Pozina, J.P. Bergman, B. Monemar, T. Takeuchi, H. Amano, I. Akasaki, J. Appl. Phys. 88 (5) (2000) 2677. [7] Yong-Tae Moon, Dong-Joon Kim, Jin-Sub Park, Jeong-Tak Oh, Ji-Myon Lee, Young-Woo Ok, Hyunsoo Kim, Seong-Ju Park, Appl. Phys. Lett. 79 (5) (2001) 599. [8] S. Srinivasan, F. Bertram, A. Bell, F.A. Ponce, S. Tanaka, H. Omiya, Y. Nakagawa, Appl. Phys. Lett. 80 (4) (2002) 550. [9] E. Kuokstis, R. Gaska, M.S. Shur, J.W. Yang, G. Simin, M. Asif Khan, Appl. Phys. Lett. 80 (6) (2002) 977. [10] A. Vertikov, I. Ozden, A.V. Nurmikko, J. Appl. Phys. 86 (1999) 4697. [11] H. Kollmer, J.S. Im, S. Heppel, J. Off, F. Scholzand, A. Hangleiter, Appl. Phys. Lett. 74 (1999) 82. [12] Y.S. Lin, K.J. Ma, C. Hsu, S.W. Feng, Y.C. Cheng, C.C. Liao, C.C. Yang, C.C. Chou, C.M. Lee, J.I. Chyi, Appl. Phys. Lett. 77 (19) (2000) 2988. [13] K. Watanabe, Jer-Ren Yang, N. Nakanishi, K. Inoke, M. Shiojiri, Appl. Phys. Lett. 80 (5) (2002) 761. [14] A.N. Westmeyer, S. Mahajan, Appl. Phys. Lett. 79 (17) (2001) 2710. [15] S.Y. Karpov, MRS Internet J. Nitride Semicond. Res. 3 (2000) 16. [16] I-hsiu Ho, G.B. Stringfellow, Appl. Phys. Lett. 69 (1996) 2701. [17] Z. Qin, Z. Chen, Y. Tong, S. Lu, G. Zhang, Appl. Phys. A. 74 (2002) 655. [18] K. Yang, R. Zhang, Y.D. Zheng, L.H. Qin, B. Shen, H.T. Shi, Z.C. Huang, J.C. Chen, Mat. Res. Soc. Symp. Proc. 423 (1996) 747. [19] L. Nistor, H. Bender, A. Vantomme, M.F. Wu, J. Van Landuyt, K.P. O’Donnell, R. Martin, K. Jacobs, I. Moerman, Appl. Phys. Lett. 77 (4) (2000) 507.
Physica B 344 (2004) 292–296
Study of photoluminescence and absorption in phase-separation InGaN films Z.Z. Chen*, Z.X. Qin, X.D. Hu, T.J. Yu, Z.J. Yang, Y.Z. Tong, X.M. Ding, G.Y. Zhang State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China Received 2 May 2003; accepted 19 October 2003
Abstract InGaN/GaN layers have been grown under low pressure by metal-organic vapor phase epitaxy on sapphire substrate. X-ray diffraction (XRD), photoluminescence, and optical absorption measurements have been performed to study the radiative recombination mechanisms in the samples. The In composition was determined by XRD measurement using Vegard’s law. With increasing In composition, a red shift of absorption edge and a broad Urbach tail in absorption spectra were observed. The InN inclusions in InGaN played a key role in long-wavelength absorption. There existed large Stokes shifts in all thick InGaN layers with different In compositions. The intensities of low-energy emissions were enhanced compared to those of high-energy ones with increasing In composition. We attributed low-energy emissions to deep levels and high-energy emissions to In-rich quantum dots. r 2003 Elsevier B.V. All rights reserved. PACS: 77.65Ly; 78.40Fy; 78.55Cr Keywords: InGaN; Phase separation; Photoluminescence excitation; Quantum dots
1. Introduction Recently, InGaN the ternary alloy which furnishes the active layers in UV-green light emitting diodes [1] and laser diodes [2] has been the focus of scientific efforts for understanding its emission mechanism [3–11] and structures [6,12– 15]. It is well known that chemical demixing will *Corresponding author. State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China. E-mail address: [email protected] (Z.Z. Chen).
occur in the ternary alloy due to the high miscibility gap between InN and GaN [16]. In contrast to a conventional alloy, the high miscibility gap leads to an InGaN layer composed of In-rich quantum dots (QDs) buried in an Indeficient matrix, since the solubility limit of Ga in InN is less than 5% [12]. Many authors consider the In-rich QDs as the origin of the high brightness emission in the InGaN-active region [4–7]. Several groups report that the piezoelectric effect plays an important role in the emitting process [9,10]. Under high injection conditions, the extended defects may be involved in radiative recombination
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.10.008
ARTICLE IN PRESS Z.Z. Chen et al. / Physica B 344 (2004) 292–296 100000 GaN 10000
Intensity(CPS)
[8,11]. However, more clear evidences should be obtained to confirm the origin of the InGaN emission. In this article, we performed photoluminescence (PL), PL excitation (PLE) and optical absorption measurements on a set of InGaN samples with different In composition. According to our results, the origin of absorption and emission in InGaN were obtained.
293
InGaN
1000
100
InN
10
2. Experiment
1 28
The InGaN layers were grown on 0.5 mmthick GaN layers on the sapphire substrates by metal-organic chemical vapor deposition (MOCVD). Trimethylindium (TMI), trimethylgallium (TMG), and ammonia (NH3) have been used as sources of In, Ga, and N, respectively. H2 and N2 were used as carrier gases. A GaN buffer layer was deposited at 550 C. Then a GaN epilayer was grown at 1060 C. The InGaN layer was grown at 800 C on the GaN layer. The typical thicknesses of GaN and InGaN layers are 0.5 and 1.0 mm, respectively. The composition of In in InGaN layer was controlled by carrier gases flow rate, flow rate ratio of TMG to TMI, growth temperature, and so on. The In compositions in InGaN layers were determined by the measurement of X-ray diffraction (XRD) using Philips X’pert MRD diffractometer. Optical transmission spectra were obtained by U3410 UV-visible-infrared spectrometer. PL and PLE spectra were measured by EG&G Fluoro MAX-2 Spectrometer at room temperature, and the light source was xenon lamp.
3. Results and discussion The 2y scan pattern of a typical InxGa1 xN layer, named as sample A, is shown in Fig. 1. There are three peaks in Fig. 1, corresponding to GaN (0 0 0 2), InGaN (0 0 0 2) and InN (0 0 0 2) diffraction peaks. On the lower-angle side of InGaN (0 0 0 2), two arrow-indicated shoulders are visible. These results provide direct evidence of phase separation, which is attributed to spinodal decomposition [12]. According to the Bragg’s
30
32
34
36
38
2 θ (deg)
Fig. 1. XRD curve of sample A. On the lower-angle side of InGaN (0 0 0 2), two arrow-indicated shoulders are visible.
angle of InGaN (0 0 0 2), the In mole composition, x, in InGaN layers can be obtained as 0.11 using Vegard’s law. Similarly, the values of x of samples B, and C are obtained as 0.156 and 0.177, respectively. The inclusions of InN, Z, in InGaN layer of samples A–C are determined as 1.485%, 0.975% and 1.069% by XRD rocking curve data. The details of the calculation theory are discussed in Ref. [17]. Optical transmission spectra are shown in Fig. 2. The interference effects on the transmission spectra are modified by the envelope curve method [18]. There are two obvious steps in each curve, corresponding to absorption edge of GaN and InGaN layers, respectively. With increasing x, the InGaN absorption edge is red-shifted, the Urbach tail is broadened and the optical absorption is strengthened. Short-wavelength absorption of sample B is more than that of sample A, but weaker for long-wavelength absorption. With increase of In composition in InGaN layer, the band gap will become narrower, and the lattice will be deteriorated because of lattice misfit between GaN and InGaN layers. Moreover, the phase separation of InGaN layer may be composed of InN segregation, InGaN composition modulation, and even In droplet when x is large. The random distribution of the In-rich region may influence the long-wavelength absorption of the whole InGaN layer, absorption and scattering by the In-rich region, which reduces the transmission
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60 50 40 30 x=0.110, η=1.485% x=0.156, η=0.975% x=0.177, η=1.069%
20 10 0 300
400
500
600
700
Wavelength (nm) Fig. 2. Optical transmission spectra of InGaN samples with different x. The interference effects on the spectra are modified by the envelop curve method.
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Fig. 3. PL spectra of the InGaN samples with different x, where the excitation energy is 3.1 eV. The short dash lines indicate the results of Gaussian fitting for experiment data.
of the photons. The red-shift of optical absorption edge is dependent on x, while the phase separation of InGaN layer will determine the long-wavelength absorption (for samples A and B). Fig. 3 shows the PL spectra of the InGaN samples where the excitation energy is 3.1 eV. The
short dash lines are the results of Gaussian fitting for experiment data. Considering the lower energy excitation, the PL can be attributed to the emission from InGaN layer, while the emission from GaN layer can be neglected. There exists a large Stokeslike shift (the shift from optical absorption edge to
ARTICLE IN PRESS Z.Z. Chen et al. / Physica B 344 (2004) 292–296
emission peak in PL spectrum) of 300 meV more in all samples. For the samples with x=0.110, 0.156, 0.177, the ratios of intensities at the peaks of 2.4 eV to those of 2.2 eV are 5.1, 1.9, and 1.25, respectively. This causes the dominant peaks redshift slightly. The total PL intensities also decrease. The very large Stokes-like shift implies a strong localized effect in InGaN layer. So it can be concluded that PL emissions originate from: (1) In-rich region rather than from bulky InGaN layer, (2) deep levels in InGaN layer. In the thick InGaN film (typical thickness=1.0 mm), self-assembly QDs may be due to the compositional modulation in InGaN rather than the interface fluctuation [16,19]. With increasing x, the average size of In-rich QDs could be larger, which leads to the decrease of the radiative recombination efficiency due to the poorer quantum confinement, and the red-shift of PL emission. On the other hand, much more deep levels will be generated with increasing x due to the formation of lattice defects. So the emissions at 2.2 and 2.4 eV may be attributed to In-rich QDs and deep levels, respectively. The mechanisms of the two emissions will be studied further by PLE spectra in the following. Fig. 4 shows the PLE spectra of sample A for emission peaks at 510 and 550 nm in PL spectra. There are two peaks in each PLE spectrum, which are corresponding to GaN exciton peak and a broad InGaN peak, respectively. The intensity of GaN exciton peak is larger than that of InGaN 50000
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Fig. 4. PLE spectra of sample A for emission peaks at 510 and 550 nm in PL spectra.
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peaks for emission at 550 nm. It is reversed for emission at 510 nm. It means that the emission at 550 nm comes from the In-deficient matrix excited by high-energy photons and the phaseseparated InGaN regions excited by photons with an energy lower than 3.4 eV. The photongenerated carriers are recombined at deep levels where light is emitted at 550 nm. But the emission at 510 nm comes mainly from the phase-separated InGaN regions with lower-energy excitation. It seems that there exists a barrier for photon-generated carrier in matrix to transfer to QDs. It is a rather easy way for them to recombine in deep levels. When the excited carriers are trapped by In-rich QDs, they have no choice but to radiatively recombine. So the InGaN peak is dominant in the PLE spectrum for emission at 510 nm. Fig. 5 shows the PLE spectra of the InGaN samples, with the emissions at (a) 510 nm and (b) 550 nm. As In composition increases, the InGaN peaks are strengthened and the GaN peaks are weakened. At the same time, the intensities of emissions at 510 nm are decreased rapidly, obviously when x is more than a certain value, while the ones at 550 nm are decreased slowly and homogenously. With increasing x, InGaN peaks are broadened and multiple for both emissions at 510 and 550 nm. With increasing x, the emission from phase-separated InGaN layers will be strengthened comparatively. Moreover, the one from In-deficient matrix will be weakened because the excitation light to the matrix will be absorbed and scattered by In-rich region. The absolute decreases for both emissions with the increase in x are due to the lattice defects like non-radiative levels and weak confinement of In-rich regions for trapped carriers. The increases of x will cause severe InGaN phase separation, which will broaden the InGaN peaks. For the emissions related to QDs, the larger size of In-rich QDs could cause them to decrease rapidly due to the poorer quantum confinement. However, as to the emissions related to deep levels, they are changed smoothly because the efficiency of carrier recombination is dependent on the density of lattice defects, which could not be changed abruptly.
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Excitation Photon Energy (eV)
Fig. 5. PLE spectra of the InGaN samples, with the emissions at (a) 510 nm and (b) 550 nm.
4. Conclusions In summary, we have performed XRD, PL, PLE, and optical absorption spectra measurements on the InGaN samples with different x. The absorption edge of InGaN is red-shift and has a broad Urbach tail with increasing x. The intensities of low-energy emissions increase compared to those of high-energy ones. The large Stokesshift implies a strong localized effect in InGaN layer. According to the results of PL and PLE spectra, we conclude that the low-energy emissions are attributed to deep levels and the high ones are attributed to In-rich QDs in thick InGaN layers.
Acknowledgements This work was supported by National Science Foundation of China (NSFC) No.69876002.
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