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GaN SQD nanocolumn and InGaN nanocolumn
Physics Procedia 2 (2009) 327–333 www.elsevier.com/locate/procedia
Title: Optical properties of InGaN/GaN SQD nanocolumn and InGaN nanocolumn Author’s name: Naoki Suzuki [a], Kazuaki Kouyama [a],[c], Yuta Insose [a], Hideyuki Kunugita [a],[c], Kazuhiro Ema [a],[c], Hiroto Sekiguchi [b],[c]
, Akihiko Kikuchi [b],[c], and Katumi Kisino [b],[c]
Address: a
Department of Physics, Sophia University, 7-1 Chiyoda-Ku, Tokyo, 102-8554, Japan
b
Department of Electrical and Electronics Engineering, Sophia University, Tokyo, Japan
c
Core Research for Evolutional Science and Technology (CREST), JST
Abstract: We have studied the emission mechanism of InGaN/GaN nanocolumns. We extracted only the effect of localized states and investigated the difference between InGaN/GaN single-quantum-disc (SQD) and InGaN nanocolumns by means of photoluminescence and photoluminescence excitation measurements. The difference between the localized states was interpreted by the band tail model. We conclude that the InGaN nanocolumn has widely distributed localized states, while the InGaN/GaN SQD has a single localized state.. Keywords: Nano crystal Corresponding author: Tel: +81-3-3238-3339, Fax: +81-3-3238-3341, E-mail: [email protected] Main Text: 1. INTRODUCTION InGaN is a promising material for light-emitting devices because its direct band gap energy can be made to vary from 0.7 eV (InN) to 3.5 eV (GaN) with change in the alloy composition[1]. However, the luminescent properties of InGaN rapidly deteriorate if the luminescence wavelength is lengthened from blue to green because of increases in the threading dislocation and the quantum confined Stark effect (QCSE)
[2]
caused by the internal piezo field. Another
problem in InGaN is the existence of spatial potential fluctuations due to indium compositional fluctuation leading to exciton localization [3], which has a significant effect on the performance of InGaN-based optical devices. Self-organized nanocolumn is a crystal with a columnar shape, whose diameter is about 100 nm and height about 1ȝm [4]
. Such columnar crystals can be synthesized in III-nitrides, i.e. GaN, AlN, InN and also InGaN. Moreover InGaN/GaN
heterostructure can also be synthesized, in which InGaN quantum discs (QDCs) are formed in the GaN nanocolumns. They have attracted much attention in the field of light-emitting devices as they show quite strong photoluminescence (PL) [4]. This attractive feature is considered to mainly originate from the fact that the nanocolumns are dislocation-free crystals. In addition, for the GaN nanocolumns, unique excitonic properties due to the columnar morphology have been investigated and it has been confirmed that the emissions of the exciton polariton from the side surface greatly contribute to the strong PL
[5]
. However the optical properties of localized excitons in InGaN and InGaN/GaN
nanocolumns have not been well understood as yet. In this study, we measured the PL, photoluminescence excitation (PLE) spectra and mobility edge [6] for InGaN nanocolumn and InGaN/GaN single-quantum-disk (SQD) nanocolumn (see Figure 1). For SQD nanocolumns, it has been shown that the internal electric field is suppressed [7]. Therefore we can extract only the effect of localized states and investigate the relationship between localized carrier dynamics and dimensionality. doi:10.1016/j.phpro.2009.07.015
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2. SAMPLE AND EXPERIMENTAL PROCEDURER We measured two kinds of samples; InGaN/GaN Single Quantum Disk (SQD) and InGaN nanocolumns. In Figure 1 the structure of SQD and InGaN nanocolumns is shown. In InGaN, the formation of In-rich regions causes the localization of excitons as quantum dots, which results in widely distributed localized states. PLE measurement and the excitation energy dependence on PL spectra are powerful tools for understanding the degree of localization. We performed PL and PLE measurements at 7K. Since the measurements did not use micro spectroscopy, the obtained spectra were from the nanocolumn ensemble containing hundreds of thousands of nanocolumns. The light source was quasi-monochromatic light dispersed by a monochromator from a xenon lamp. 3. RESULTS AND DISCUSSION We measured PL spectra by changing the excitation photon energy just above the PL peak energies. Figures 2 and 3 show the PL spectra of both samples. For InGaN nanocolumn, the PL peak position depends on the excitation energy as is shown in the inset of Figure 2; the PL peak shows a drastic redshift below 2.7 eV with decrease of excitation photon energy. This result shows that the InGaN nanocolumn has a mobility edge (the energy separating extended from localized states) at 2.7 eV. This feature is the same as reported in InGaN films [8]. On the other hand, the PL peak of InGaN/GaN SQD remains constant with the decrease of excitation photon energy (see Figure 3).This result shows that there is no mobility edge for InGaN/GaN SQD nanocolumn. Figure 4 and 5 show the PL and PLE spectra of both samples. For InGaN nanocolumn, the PLE measurement is obtained by monitoring at three different energy positions; the emission peak (~2.3 eV), low energy side (~2.1 eV) and high energy side (2.5 eV). The PLE signals for all monitoring points increase monotonically with increasing excitation energy, reach a maximum at ~2.8 eV and remain almost constant at higher energies. The three PLE spectral shapes are different with the monitoring wavelength. This feature of the PLE is also the same as in InGaN films and SQW [9] [10]. These results from Figures 2 and 4 show that InGaN nanocolumn has many localized states which form an exponential tail below the band edge. On the other hand, for InGaN/GaN SQD nanocolumn, the three PLE signal shapes do not change with the monitoring wavelength (2.25 eV, 2.06 eV, 1.89 eV). From the results shown in Figure 3 and Figure 5 we can suppose that InGaN/GaN SQD nanocolumn has a single localized state or a few states without a wide distribution and carriers cannot move from one localized state to another localized state. That is to say, all InGaN/GaN SQD nanocolumns have common continual density of state (DOS) and a few localized states separated from the band edge. Figure 6 shows the estimated localized states of these samples. For the InGaN nanocolumns, we can explain the experimental results by the fact that the PL is the emission from localized states that are distributed as an exponential tail below the band edge (Figure 6(a)).This model reveals the cause of the PL peak position depending on the excitation photon energy. When the InGaN nanocolumn is excited above the mobility edge, the excited carriers are trapped in the In-rich region and form the localized excitons. Therefore the PL is from the continuous tail of the localized state below
N. Suzuki et al. / Physics Procedia 2 (2009) 327–333
the band edge. When the excitation energy is below the mobility edge, the excited carriers cannot move to above the excitation energy and the PL is from the tail of the localized state below the excitation energy. For the InGaN/GaN SQD nanocolumns, the origin of the PL emission is considered to be from only one localized state on each disk (Figure 6(b)).The model shows that the PL peak is independent of the excitation photon energy if all the SQD nanocolumns have a common band-edge. The broad PL spectral width is caused by the fact that each SQD has a different localized energy and the spectrum is from the SQD ensemble.
4. CONCLUSIONS We have investigated the differences in the localized excitons between the InGaN/GaN SQDnanocolumn and the InGaN nanocolumn.InGaN nanocolumn has a mobility edge (2.7 eV) similar to the InGaN films, while the InGaN/GaN SQD nanocolumn does not. Based on the band tail model [11], we can suppose that for InGaN nanocolumn, the PL is from the continuous tail of the localized state below the band edge, in the same way as for InGaN films, while for InGaN/GaN SQD nanocolumn, it has a single localized state separated from the continuous level.
References: [1] S.Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, Y. Sugimoto, and H. Kiyoku, Appl. Phys. Lett. 70, 1417 (1997) [2] H. S.Kim, J. Y. Lin, H. X.Jiang, W. W. Chow, A. Botcharev, and H. Morkc, Appl. Phys. Lett. 73, 3426 (1998) [3] Y. Narukawa, Y. kawakami, M. Funato, S. Fujita, and S. Nakamura, Appl. Phys. Lett. 70, 981 (1996) [4] A. Kikuchi, K. Yamano, M. Tada, and K. Kishino, Phys. Stat. Sol. (b), 241, 2754 (2004) [5] K. Kouyama, M. Inoue, N. Suzuki, H. Sekiguchi, H. Kunugita, K. Ema, A. Kikuchi and K. Kishino, J. Lumi, 128, 969 (2008) [6] Semiconductor Optics, edited by C.Klingshirn (Springer-Verlag, 2001), Sect.9.6. [7] Y. Kawakami, S. Suzuki, A. Kaneta, M. Funato, A. Kikuchi and K. Kishino, Appl. Phys. Lett. 89, 163124 (2006) [8] A. Satake, Y. Masumoto, T. Miyajima, T. Asatsuma, F. Nakamura, and M. Ikeda, Phys. Rev. B 57, R2041 (1997) [9] S.Chichibu, T.Azuhata, T.Sota, and S. Nakamura, Appl. Phys. Lett. 70, 2822 (1997) [10] Yong-Hoon Cho, J. J. Song, S. Keller, M.S. Minsky, E. Hu, U. K. Mishra, and S. P. DenBaars, Appl. Phys. Lett. 73, 1128 (1998) [11] S.Chichibu, T.Azuhata, T.Sota, and S. Nakamura, Appl. Phys. Lett. 69, 4188 (1996)
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Figure captions: List of figure captions Figure 1 Structures of (a) InGaN/GaN SQD, and (b) InGaN nanocolumn sample Figure 2 PL spectra of InGaN nanocolumn. Arrows show the excitation photon energy. The inset shows the PL peak energy as a function of the excitation photon energy. Figure 3 PL spectra of InGaN/GaN SQD nanocolumn Arrows show the excitation photon energy. Figure 4 PL and PLE spectra in InGaN nanocolumn Figure 5 PL and PLE spectra in InGaN SQD nanocolumn Figure 6 Localized states in InGaN nanocolumn (a) and InGaN/GaN SQD nanocolumn (b)
N. Suzuki et al. / Physics Procedia 2 (2009) 327–333
Figures
Figure 1
Figure 2
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N. Suzuki et al. / Physics Procedia 2 (2009) 327–333
Figure 3 600
PL Intensity (arb. units)
700
W a v e le n g th (n m ) 500 400
P L E (2 .5 2 e V ) P L E (2 .3 8 e V ) P L E (2 .1 5 e V ) PL
1 .8 2 .0 2 .2 2 .4 2 .6 2 .8 3 .0 3 .2 3 .4 3 .6 P h o to n E n e r g y (e V )
Figure 4
PL Intensity (arb. units)
W avelength (nm ) 800 700 600 500 400 PLE(2.25eV) PLE(2.06eV) PLE(1.89eV) PL
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Photon Energy (eV)
Figure 5
N. Suzuki et al. / Physics Procedia 2 (2009) 327–333
Energy
Energy
(b)
㩷
㩷㩷
(a)
Mobility edge
Localized state Density of state
Figure 6
Localized state Density of state
333
Title: Optical properties of InGaN/GaN SQD nanocolumn and InGaN nanocolumn Author’s name: Naoki Suzuki [a], Kazuaki Kouyama [a],[c], Yuta Insose [a], Hideyuki Kunugita [a],[c], Kazuhiro Ema [a],[c], Hiroto Sekiguchi [b],[c]
, Akihiko Kikuchi [b],[c], and Katumi Kisino [b],[c]
Address: a
Department of Physics, Sophia University, 7-1 Chiyoda-Ku, Tokyo, 102-8554, Japan
b
Department of Electrical and Electronics Engineering, Sophia University, Tokyo, Japan
c
Core Research for Evolutional Science and Technology (CREST), JST
Abstract: We have studied the emission mechanism of InGaN/GaN nanocolumns. We extracted only the effect of localized states and investigated the difference between InGaN/GaN single-quantum-disc (SQD) and InGaN nanocolumns by means of photoluminescence and photoluminescence excitation measurements. The difference between the localized states was interpreted by the band tail model. We conclude that the InGaN nanocolumn has widely distributed localized states, while the InGaN/GaN SQD has a single localized state.. Keywords: Nano crystal Corresponding author: Tel: +81-3-3238-3339, Fax: +81-3-3238-3341, E-mail: [email protected] Main Text: 1. INTRODUCTION InGaN is a promising material for light-emitting devices because its direct band gap energy can be made to vary from 0.7 eV (InN) to 3.5 eV (GaN) with change in the alloy composition[1]. However, the luminescent properties of InGaN rapidly deteriorate if the luminescence wavelength is lengthened from blue to green because of increases in the threading dislocation and the quantum confined Stark effect (QCSE)
[2]
caused by the internal piezo field. Another
problem in InGaN is the existence of spatial potential fluctuations due to indium compositional fluctuation leading to exciton localization [3], which has a significant effect on the performance of InGaN-based optical devices. Self-organized nanocolumn is a crystal with a columnar shape, whose diameter is about 100 nm and height about 1ȝm [4]
. Such columnar crystals can be synthesized in III-nitrides, i.e. GaN, AlN, InN and also InGaN. Moreover InGaN/GaN
heterostructure can also be synthesized, in which InGaN quantum discs (QDCs) are formed in the GaN nanocolumns. They have attracted much attention in the field of light-emitting devices as they show quite strong photoluminescence (PL) [4]. This attractive feature is considered to mainly originate from the fact that the nanocolumns are dislocation-free crystals. In addition, for the GaN nanocolumns, unique excitonic properties due to the columnar morphology have been investigated and it has been confirmed that the emissions of the exciton polariton from the side surface greatly contribute to the strong PL
[5]
. However the optical properties of localized excitons in InGaN and InGaN/GaN
nanocolumns have not been well understood as yet. In this study, we measured the PL, photoluminescence excitation (PLE) spectra and mobility edge [6] for InGaN nanocolumn and InGaN/GaN single-quantum-disk (SQD) nanocolumn (see Figure 1). For SQD nanocolumns, it has been shown that the internal electric field is suppressed [7]. Therefore we can extract only the effect of localized states and investigate the relationship between localized carrier dynamics and dimensionality. doi:10.1016/j.phpro.2009.07.015
328
N. Suzuki et al. / Physics Procedia 2 (2009) 327–333
2. SAMPLE AND EXPERIMENTAL PROCEDURER We measured two kinds of samples; InGaN/GaN Single Quantum Disk (SQD) and InGaN nanocolumns. In Figure 1 the structure of SQD and InGaN nanocolumns is shown. In InGaN, the formation of In-rich regions causes the localization of excitons as quantum dots, which results in widely distributed localized states. PLE measurement and the excitation energy dependence on PL spectra are powerful tools for understanding the degree of localization. We performed PL and PLE measurements at 7K. Since the measurements did not use micro spectroscopy, the obtained spectra were from the nanocolumn ensemble containing hundreds of thousands of nanocolumns. The light source was quasi-monochromatic light dispersed by a monochromator from a xenon lamp. 3. RESULTS AND DISCUSSION We measured PL spectra by changing the excitation photon energy just above the PL peak energies. Figures 2 and 3 show the PL spectra of both samples. For InGaN nanocolumn, the PL peak position depends on the excitation energy as is shown in the inset of Figure 2; the PL peak shows a drastic redshift below 2.7 eV with decrease of excitation photon energy. This result shows that the InGaN nanocolumn has a mobility edge (the energy separating extended from localized states) at 2.7 eV. This feature is the same as reported in InGaN films [8]. On the other hand, the PL peak of InGaN/GaN SQD remains constant with the decrease of excitation photon energy (see Figure 3).This result shows that there is no mobility edge for InGaN/GaN SQD nanocolumn. Figure 4 and 5 show the PL and PLE spectra of both samples. For InGaN nanocolumn, the PLE measurement is obtained by monitoring at three different energy positions; the emission peak (~2.3 eV), low energy side (~2.1 eV) and high energy side (2.5 eV). The PLE signals for all monitoring points increase monotonically with increasing excitation energy, reach a maximum at ~2.8 eV and remain almost constant at higher energies. The three PLE spectral shapes are different with the monitoring wavelength. This feature of the PLE is also the same as in InGaN films and SQW [9] [10]. These results from Figures 2 and 4 show that InGaN nanocolumn has many localized states which form an exponential tail below the band edge. On the other hand, for InGaN/GaN SQD nanocolumn, the three PLE signal shapes do not change with the monitoring wavelength (2.25 eV, 2.06 eV, 1.89 eV). From the results shown in Figure 3 and Figure 5 we can suppose that InGaN/GaN SQD nanocolumn has a single localized state or a few states without a wide distribution and carriers cannot move from one localized state to another localized state. That is to say, all InGaN/GaN SQD nanocolumns have common continual density of state (DOS) and a few localized states separated from the band edge. Figure 6 shows the estimated localized states of these samples. For the InGaN nanocolumns, we can explain the experimental results by the fact that the PL is the emission from localized states that are distributed as an exponential tail below the band edge (Figure 6(a)).This model reveals the cause of the PL peak position depending on the excitation photon energy. When the InGaN nanocolumn is excited above the mobility edge, the excited carriers are trapped in the In-rich region and form the localized excitons. Therefore the PL is from the continuous tail of the localized state below
N. Suzuki et al. / Physics Procedia 2 (2009) 327–333
the band edge. When the excitation energy is below the mobility edge, the excited carriers cannot move to above the excitation energy and the PL is from the tail of the localized state below the excitation energy. For the InGaN/GaN SQD nanocolumns, the origin of the PL emission is considered to be from only one localized state on each disk (Figure 6(b)).The model shows that the PL peak is independent of the excitation photon energy if all the SQD nanocolumns have a common band-edge. The broad PL spectral width is caused by the fact that each SQD has a different localized energy and the spectrum is from the SQD ensemble.
4. CONCLUSIONS We have investigated the differences in the localized excitons between the InGaN/GaN SQDnanocolumn and the InGaN nanocolumn.InGaN nanocolumn has a mobility edge (2.7 eV) similar to the InGaN films, while the InGaN/GaN SQD nanocolumn does not. Based on the band tail model [11], we can suppose that for InGaN nanocolumn, the PL is from the continuous tail of the localized state below the band edge, in the same way as for InGaN films, while for InGaN/GaN SQD nanocolumn, it has a single localized state separated from the continuous level.
References: [1] S.Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, Y. Sugimoto, and H. Kiyoku, Appl. Phys. Lett. 70, 1417 (1997) [2] H. S.Kim, J. Y. Lin, H. X.Jiang, W. W. Chow, A. Botcharev, and H. Morkc, Appl. Phys. Lett. 73, 3426 (1998) [3] Y. Narukawa, Y. kawakami, M. Funato, S. Fujita, and S. Nakamura, Appl. Phys. Lett. 70, 981 (1996) [4] A. Kikuchi, K. Yamano, M. Tada, and K. Kishino, Phys. Stat. Sol. (b), 241, 2754 (2004) [5] K. Kouyama, M. Inoue, N. Suzuki, H. Sekiguchi, H. Kunugita, K. Ema, A. Kikuchi and K. Kishino, J. Lumi, 128, 969 (2008) [6] Semiconductor Optics, edited by C.Klingshirn (Springer-Verlag, 2001), Sect.9.6. [7] Y. Kawakami, S. Suzuki, A. Kaneta, M. Funato, A. Kikuchi and K. Kishino, Appl. Phys. Lett. 89, 163124 (2006) [8] A. Satake, Y. Masumoto, T. Miyajima, T. Asatsuma, F. Nakamura, and M. Ikeda, Phys. Rev. B 57, R2041 (1997) [9] S.Chichibu, T.Azuhata, T.Sota, and S. Nakamura, Appl. Phys. Lett. 70, 2822 (1997) [10] Yong-Hoon Cho, J. J. Song, S. Keller, M.S. Minsky, E. Hu, U. K. Mishra, and S. P. DenBaars, Appl. Phys. Lett. 73, 1128 (1998) [11] S.Chichibu, T.Azuhata, T.Sota, and S. Nakamura, Appl. Phys. Lett. 69, 4188 (1996)
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Figure captions: List of figure captions Figure 1 Structures of (a) InGaN/GaN SQD, and (b) InGaN nanocolumn sample Figure 2 PL spectra of InGaN nanocolumn. Arrows show the excitation photon energy. The inset shows the PL peak energy as a function of the excitation photon energy. Figure 3 PL spectra of InGaN/GaN SQD nanocolumn Arrows show the excitation photon energy. Figure 4 PL and PLE spectra in InGaN nanocolumn Figure 5 PL and PLE spectra in InGaN SQD nanocolumn Figure 6 Localized states in InGaN nanocolumn (a) and InGaN/GaN SQD nanocolumn (b)
N. Suzuki et al. / Physics Procedia 2 (2009) 327–333
Figures
Figure 1
Figure 2
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N. Suzuki et al. / Physics Procedia 2 (2009) 327–333
Figure 3 600
PL Intensity (arb. units)
700
W a v e le n g th (n m ) 500 400
P L E (2 .5 2 e V ) P L E (2 .3 8 e V ) P L E (2 .1 5 e V ) PL
1 .8 2 .0 2 .2 2 .4 2 .6 2 .8 3 .0 3 .2 3 .4 3 .6 P h o to n E n e r g y (e V )
Figure 4
PL Intensity (arb. units)
W avelength (nm ) 800 700 600 500 400 PLE(2.25eV) PLE(2.06eV) PLE(1.89eV) PL
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Photon Energy (eV)
Figure 5
N. Suzuki et al. / Physics Procedia 2 (2009) 327–333
Energy
Energy
(b)
㩷
㩷㩷
(a)
Mobility edge
Localized state Density of state
Figure 6
Localized state Density of state
333