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Dynamics of electron–hole plasmas and localized excitons in highly excited GaN-based ternary alloys
ARTICLE IN PRESS
Journal of Luminescence 128 (2008) 712–714 www.elsevier.com/locate/jlumin
Dynamics of electron–hole plasmas and localized excitons in highly excited GaN-based ternary alloys D. Hirano, Y. Kanemitsu Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Available online 4 November 2007
Abstract We have studied photoluminescence (PL) spectra of GaN crystals and InGaN ternary alloys at low temperatures as a function of the femtosecond laser excitation intensity. With an increase of the intensity, the broad PL due to electron–hole plasmas (EHP) appears below the biexciton PL in the GaN sample. On the other hand, the broad EHP PL appears above the localized exciton PL in the InGaN sample. The intensity dependence of PL properties of InGaN crystals is completely different from that of GaN crystals. The effect of alloy disorder on PL processes in ternary alloys is discussed. r 2007 Elsevier B.V. All rights reserved. Keywords: Electron–hole plasma; Biexciton; Localized exciton
Over the past decade, there have been extensive studies on optical and electronic properties of wide-gap nitride semiconductors [1]. The GaN-based ternary alloys, InGaN and AlGaN crystals, are the heart material for optical devices in the blue spectral region. Today, wide-gap GaNbased semiconductors have also been recognized as key materials for fundamental research of exciton many-body effects, because of their large exciton-binding energies. For example, we can clearly observe the photoluminescence (PL) spectra of high-density exciton states, such as biexcitons (M-lines) and P-lines [2], because the excitonbinding energy of GaN is rather large (24.8 meV) [3]. Furthermore, in GaN-based semiconductors, the electron–hole (e–h) pair correlation or excitonic effects play an essential role in dynamics of e–h plasmas (EHP) and band-gap renormalization [4,5]. In ternary alloys, potential fluctuations due to alloy disorders cause the localization of excitons and biexcitons [6–10]. It is believed that the formation of the localized states strongly affects the dynamics of the excitons and e–h systems and optical gain spectra under strong excitation [11,12]. However, the dynamical behaviors of highly dense Corresponding author. Fax: +81 774 38 4511.
E-mail address: [email protected] (D. Hirano). 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.10.013
excitons and e–h systems in GaN-based ternary alloys are not well understood. In this work, we studied PL and optical gain spectra of GaN and InGaN crystals as a function of excitation laser intensity at low temperatures, and discuss the effects of localized states on optical processes of EHP. The samples used in this work were GaN and InxGa1xN (x=0.05) epitaxial films [13,14]. The excitation photon energies were set to 17575 meV above the Aexciton energy in each sample. The excitation pulse duration and the repetition rate were 150 fs and 1 kHz, respectively. Optical gain spectra were measured by a variable stripe length method [15]. The excitation laser was focused on crystal surfaces by a cylindrical lens and the stripe length was varied with a mask. The edge emissions propagating along the sample surface were detected. The laser spot sizes focused on sample surfaces were carefully measured by a knife-edge method. Fig. 1 shows time-integrated PL spectra of GaN crystals and InGaN ternary alloys at low temperatures as a function of the femtosecond laser excitation intensity. In GaN, under very weak excitation, free and bound exciton PL are observed near the exciton energy, where the free exciton PL energy is indicated by Ex. With an increase of the excitation intensity, the M-line due to biexcitons appears, as indicated by Exx in Fig. 1(a), where the
ARTICLE IN PRESS D. Hirano, Y. Kanemitsu / Journal of Luminescence 128 (2008) 712–714
In1-xGaxN(x=0.05) EHP 3.2 mJ/cm2
800 µJ/cm2
580 µJ/cm2
Exx
123 µJ/cm2
5.0
120 µJ/cm2 22 µJ/cm2
µJ/cm2
Ex
Ex
Absorption (arb.units)
PL Intensity (arb.units)
EHP 10 mJ/cm2
3.1 µJ/cm2
0.48 µJ/cm2
3.40 3.45 3.50 3.55 Photon Energy (eV)
3.20 3.25 3.30 3.35 Photon Energy (eV)
Fig. 1. Time-integrated PL spectra of (a) GaN and (b) InxGa1xN (x ¼ 0.05) samples as a function of the excitation density at 7 K. The dashed line shows absorption spectrum of the (b) InxGa1xN (x ¼ 0.05) sample.
PL Intensity (arb.units)
Gain (cm-1)
50 40 30 20 10
600
PL Intensity (arb.units)
GaN
density is below the critical density of the Mott transition. Under extremely strong excitation, a broad PL band appears above the absorption peak. This PL band is due to EHP near the band edge. The excitation intensity dependence of the InGaN ternary crystal is completely different from that of the GaN binary crystal. In order to determine the origin of the narrowing of the localized exciton PL and the broad EHP PL above the absorption edge, we measured optical gain spectra using a variable stripe method. Fig. 2 shows the optical gain and time-integrated PL spectra at different excitation densities. The PL spectra were measured in lateral configuration (edge-emission). There are two different origins of the optical gain in the InGaN sample. At 0.34 mJ/cm2 excitation, a narrow PL band appears at lower energy below the absorption edge, as shown in Fig. 2(a). The narrow PL spectrum coincides well with the optical gain spectrum. The narrow PL is due to the stimulated emission. Similar PL is observed in ZnSe-based and InGaN-based quantum-well structure [11,12,16]. The population inversion of localized excitons occurs in the tail state below the band edge. Under intense excitation, two bands are observed in the optical gain spectrum in Fig. 2(b). The main peak of the gain spectrum is above the Ex peak and the large gain is caused by EHP. In addition, the observed time-resolved PL spectrum can be fitted by the PL spectrum calculation for EHP [17]. Then, the optical gain and PL are due to the highly dense EHP. The small peak is below the Ex peak, similar to the case of the weak excitation experiment in Fig. 2(a). This is caused by the localized excitons in the tail state. In Fig. 1(b), with an increase of the excitation intensity, the small redshift of the narrow PL band is also observed. This redshift can be explained by thermal distribution of excitons and the increase in homogeneous broadening due to exciton–LO phonon scattering [12,18]. The tail state in the ternary compounds strongly affects the PL and optical gain spectra. In conclusion, we studied PL and optical gain spectra of InGaN ternary alloys as a function of the femtosecond laser excitation intensity. There are two different origins of
Gain (cm-1)
intensity Exx is proportional to the square of the laser excitation intensity. Under extremely intense excitation, a broad PL band appears at the low-energy region. When the exciton density exceeds the critical density of the exciton Mott transition, EHP become stable. The band-gap renormalization occurs in highly excited semiconductors, because of the many-body effects of EHP. These spectral changes in GaN crystals are very simple and similar excitation dependence is usually observed in wide-gap semiconductors: the GaN crystal is a textbook sample. In the InGaN sample, on the other hand, the intensity dependence of the PL spectrum is complicated. Under weak excitation, a broad and strong PL band appears at about 50 meV below the exciton absorption peak Ex, as shown in Fig. 1(b). The efficient PL is due to localized excitons. With an increase of the excitation intensity, the PL peak energy is blueshifted and the spectral width becomes narrower. The narrowing of the PL band implies the stimulated emission from localized states. In these experimental conditions (e.g., 800 mJ/cm2), the carrier
713
500 400 300 200 100 0
3.20 3.25 3.30 3.35 Photon Energy (eV)
3.20 3.25 3.30 3.35 Photon Energy (eV)
Fig. 2. Time-integrated PL (dashed line) and optical gain (solid line) spectra of the InGa1xN (x ¼ 0.05) sample in lateral configuration under (a) 0.34 and (b) 2.4 mJ/cm2 excitation at 7 K.
ARTICLE IN PRESS 714
D. Hirano, Y. Kanemitsu / Journal of Luminescence 128 (2008) 712–714
the stimulated emission in InGaN ternary compounds: EHP and localized excitons. The intensity dependence of PL properties of InGaN crystals is completely different from that of the GaN crystal. The localized tail states due to alloy disorder affect the PL process even in highly excited ternary compounds. Part of this work is supported by the Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 18340089). One of the authors (D.H.) would like to thank The Marubun Research Promotion Foundation for financial support. References [1] S. Nakamura, S. Pearton, G. Fasol, The Blue Laser Diode, Springer, Berlin, 2000. [2] T. Nagai, A. Yamamoto, Y. Kanemitsu, Phys. Rev. B 71 (2005) 121201(R). [3] K. Kornitzer, T. Ebner, K. Thonke, R. Sauer, C. Kirchner, V. Schwegler, M. Kamp, M. Leszczynski, I. Grzegory, S. Porowski, Phys. Rev. B 60 (1999) 1471. [4] T. Nagai, T.J. Inagaki, Y. Kanemitsu, Appl. Phys. Lett. 84 (2004) 1284.
[5] W.W. Chow, A. Knorr, S.W. Koch, Appl. Phys. Lett. 67 (1995) 754. [6] E. Cohen, M.D. Sturge, Phys. Rev. B 25 (1982) 3828. [7] S. Permogonov, A. Renznitskii, S. Verbin, G.O. Muller, P. Flogel, M. Nikiforova, Phys. Status Solidi (B) 113 (1982) 589. [8] Y. Yamada, Y. Ueki, K. Nakamura, T. Taguchi, Y. Kawaguchi, A. Ishibashi, T. Yokogawa, Appl. Phys. Lett. 84 (2004) 2082. [9] Y. Kanemitsu, K. Tomita, D. Hirano, H. Inouye, Appl. Phys. Lett. 88 (2006) 121113. [10] Y. Kanemitsu, K. Tomita, H. Inouye, Appl. Phys. Lett. 87 (2005) 151120. [11] J. Ding, H. Jeon, T. Ishihara, M. Hagerott, A.V. Nurmikko, H. Luo, N. Samarth, J. Furdyna, Phys. Rev. Lett. 69 (1992) 1707. [12] J. Ding, M. Hagerott, T. Ishihara, H. Jeon, A.V. Nurmikko, Phys. Rev. B 47 (1993) 10528. [13] C. Sasaki, H. Naito, M. Iwata, H. Kudo, Y. Yamada, T. Taguchi, T. Jyouichi, H. Okagawa, K. Tadatomo, H. Tanaka, J. Appl. Phys. 93 (2003) 1642. [14] K. Tadatomo, H. Okagawa, Y. Ohuchi, T. Tsunekawa, Y. Imada, M. Kato, T. Taguchi, Jpn. J. Appl. Phys. 40 (2001) L583. [15] K. Shaklee, R.E. Nahory, R.F. Leheny, J. Lumin. 7 (1973) 284. [16] A. Satake, Y. Masumoto, T. Miyajima, T. Asatsuma, M. Ikeda, Phys. Rev. B 60 (1999) 16660. [17] D. Hirano, T. Tayagaki, Y. Kanemitsu, unpublished. [18] Y. Yamada, in: T. Ogawa, Y. Kanemitsu (Eds.), Optical Properties of Low-Dimensional Materials, World Scientific, Singapore, 1995, p. 202.
Journal of Luminescence 128 (2008) 712–714 www.elsevier.com/locate/jlumin
Dynamics of electron–hole plasmas and localized excitons in highly excited GaN-based ternary alloys D. Hirano, Y. Kanemitsu Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Available online 4 November 2007
Abstract We have studied photoluminescence (PL) spectra of GaN crystals and InGaN ternary alloys at low temperatures as a function of the femtosecond laser excitation intensity. With an increase of the intensity, the broad PL due to electron–hole plasmas (EHP) appears below the biexciton PL in the GaN sample. On the other hand, the broad EHP PL appears above the localized exciton PL in the InGaN sample. The intensity dependence of PL properties of InGaN crystals is completely different from that of GaN crystals. The effect of alloy disorder on PL processes in ternary alloys is discussed. r 2007 Elsevier B.V. All rights reserved. Keywords: Electron–hole plasma; Biexciton; Localized exciton
Over the past decade, there have been extensive studies on optical and electronic properties of wide-gap nitride semiconductors [1]. The GaN-based ternary alloys, InGaN and AlGaN crystals, are the heart material for optical devices in the blue spectral region. Today, wide-gap GaNbased semiconductors have also been recognized as key materials for fundamental research of exciton many-body effects, because of their large exciton-binding energies. For example, we can clearly observe the photoluminescence (PL) spectra of high-density exciton states, such as biexcitons (M-lines) and P-lines [2], because the excitonbinding energy of GaN is rather large (24.8 meV) [3]. Furthermore, in GaN-based semiconductors, the electron–hole (e–h) pair correlation or excitonic effects play an essential role in dynamics of e–h plasmas (EHP) and band-gap renormalization [4,5]. In ternary alloys, potential fluctuations due to alloy disorders cause the localization of excitons and biexcitons [6–10]. It is believed that the formation of the localized states strongly affects the dynamics of the excitons and e–h systems and optical gain spectra under strong excitation [11,12]. However, the dynamical behaviors of highly dense Corresponding author. Fax: +81 774 38 4511.
E-mail address: [email protected] (D. Hirano). 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.10.013
excitons and e–h systems in GaN-based ternary alloys are not well understood. In this work, we studied PL and optical gain spectra of GaN and InGaN crystals as a function of excitation laser intensity at low temperatures, and discuss the effects of localized states on optical processes of EHP. The samples used in this work were GaN and InxGa1xN (x=0.05) epitaxial films [13,14]. The excitation photon energies were set to 17575 meV above the Aexciton energy in each sample. The excitation pulse duration and the repetition rate were 150 fs and 1 kHz, respectively. Optical gain spectra were measured by a variable stripe length method [15]. The excitation laser was focused on crystal surfaces by a cylindrical lens and the stripe length was varied with a mask. The edge emissions propagating along the sample surface were detected. The laser spot sizes focused on sample surfaces were carefully measured by a knife-edge method. Fig. 1 shows time-integrated PL spectra of GaN crystals and InGaN ternary alloys at low temperatures as a function of the femtosecond laser excitation intensity. In GaN, under very weak excitation, free and bound exciton PL are observed near the exciton energy, where the free exciton PL energy is indicated by Ex. With an increase of the excitation intensity, the M-line due to biexcitons appears, as indicated by Exx in Fig. 1(a), where the
ARTICLE IN PRESS D. Hirano, Y. Kanemitsu / Journal of Luminescence 128 (2008) 712–714
In1-xGaxN(x=0.05) EHP 3.2 mJ/cm2
800 µJ/cm2
580 µJ/cm2
Exx
123 µJ/cm2
5.0
120 µJ/cm2 22 µJ/cm2
µJ/cm2
Ex
Ex
Absorption (arb.units)
PL Intensity (arb.units)
EHP 10 mJ/cm2
3.1 µJ/cm2
0.48 µJ/cm2
3.40 3.45 3.50 3.55 Photon Energy (eV)
3.20 3.25 3.30 3.35 Photon Energy (eV)
Fig. 1. Time-integrated PL spectra of (a) GaN and (b) InxGa1xN (x ¼ 0.05) samples as a function of the excitation density at 7 K. The dashed line shows absorption spectrum of the (b) InxGa1xN (x ¼ 0.05) sample.
PL Intensity (arb.units)
Gain (cm-1)
50 40 30 20 10
600
PL Intensity (arb.units)
GaN
density is below the critical density of the Mott transition. Under extremely strong excitation, a broad PL band appears above the absorption peak. This PL band is due to EHP near the band edge. The excitation intensity dependence of the InGaN ternary crystal is completely different from that of the GaN binary crystal. In order to determine the origin of the narrowing of the localized exciton PL and the broad EHP PL above the absorption edge, we measured optical gain spectra using a variable stripe method. Fig. 2 shows the optical gain and time-integrated PL spectra at different excitation densities. The PL spectra were measured in lateral configuration (edge-emission). There are two different origins of the optical gain in the InGaN sample. At 0.34 mJ/cm2 excitation, a narrow PL band appears at lower energy below the absorption edge, as shown in Fig. 2(a). The narrow PL spectrum coincides well with the optical gain spectrum. The narrow PL is due to the stimulated emission. Similar PL is observed in ZnSe-based and InGaN-based quantum-well structure [11,12,16]. The population inversion of localized excitons occurs in the tail state below the band edge. Under intense excitation, two bands are observed in the optical gain spectrum in Fig. 2(b). The main peak of the gain spectrum is above the Ex peak and the large gain is caused by EHP. In addition, the observed time-resolved PL spectrum can be fitted by the PL spectrum calculation for EHP [17]. Then, the optical gain and PL are due to the highly dense EHP. The small peak is below the Ex peak, similar to the case of the weak excitation experiment in Fig. 2(a). This is caused by the localized excitons in the tail state. In Fig. 1(b), with an increase of the excitation intensity, the small redshift of the narrow PL band is also observed. This redshift can be explained by thermal distribution of excitons and the increase in homogeneous broadening due to exciton–LO phonon scattering [12,18]. The tail state in the ternary compounds strongly affects the PL and optical gain spectra. In conclusion, we studied PL and optical gain spectra of InGaN ternary alloys as a function of the femtosecond laser excitation intensity. There are two different origins of
Gain (cm-1)
intensity Exx is proportional to the square of the laser excitation intensity. Under extremely intense excitation, a broad PL band appears at the low-energy region. When the exciton density exceeds the critical density of the exciton Mott transition, EHP become stable. The band-gap renormalization occurs in highly excited semiconductors, because of the many-body effects of EHP. These spectral changes in GaN crystals are very simple and similar excitation dependence is usually observed in wide-gap semiconductors: the GaN crystal is a textbook sample. In the InGaN sample, on the other hand, the intensity dependence of the PL spectrum is complicated. Under weak excitation, a broad and strong PL band appears at about 50 meV below the exciton absorption peak Ex, as shown in Fig. 1(b). The efficient PL is due to localized excitons. With an increase of the excitation intensity, the PL peak energy is blueshifted and the spectral width becomes narrower. The narrowing of the PL band implies the stimulated emission from localized states. In these experimental conditions (e.g., 800 mJ/cm2), the carrier
713
500 400 300 200 100 0
3.20 3.25 3.30 3.35 Photon Energy (eV)
3.20 3.25 3.30 3.35 Photon Energy (eV)
Fig. 2. Time-integrated PL (dashed line) and optical gain (solid line) spectra of the InGa1xN (x ¼ 0.05) sample in lateral configuration under (a) 0.34 and (b) 2.4 mJ/cm2 excitation at 7 K.
ARTICLE IN PRESS 714
D. Hirano, Y. Kanemitsu / Journal of Luminescence 128 (2008) 712–714
the stimulated emission in InGaN ternary compounds: EHP and localized excitons. The intensity dependence of PL properties of InGaN crystals is completely different from that of the GaN crystal. The localized tail states due to alloy disorder affect the PL process even in highly excited ternary compounds. Part of this work is supported by the Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 18340089). One of the authors (D.H.) would like to thank The Marubun Research Promotion Foundation for financial support. References [1] S. Nakamura, S. Pearton, G. Fasol, The Blue Laser Diode, Springer, Berlin, 2000. [2] T. Nagai, A. Yamamoto, Y. Kanemitsu, Phys. Rev. B 71 (2005) 121201(R). [3] K. Kornitzer, T. Ebner, K. Thonke, R. Sauer, C. Kirchner, V. Schwegler, M. Kamp, M. Leszczynski, I. Grzegory, S. Porowski, Phys. Rev. B 60 (1999) 1471. [4] T. Nagai, T.J. Inagaki, Y. Kanemitsu, Appl. Phys. Lett. 84 (2004) 1284.
[5] W.W. Chow, A. Knorr, S.W. Koch, Appl. Phys. Lett. 67 (1995) 754. [6] E. Cohen, M.D. Sturge, Phys. Rev. B 25 (1982) 3828. [7] S. Permogonov, A. Renznitskii, S. Verbin, G.O. Muller, P. Flogel, M. Nikiforova, Phys. Status Solidi (B) 113 (1982) 589. [8] Y. Yamada, Y. Ueki, K. Nakamura, T. Taguchi, Y. Kawaguchi, A. Ishibashi, T. Yokogawa, Appl. Phys. Lett. 84 (2004) 2082. [9] Y. Kanemitsu, K. Tomita, D. Hirano, H. Inouye, Appl. Phys. Lett. 88 (2006) 121113. [10] Y. Kanemitsu, K. Tomita, H. Inouye, Appl. Phys. Lett. 87 (2005) 151120. [11] J. Ding, H. Jeon, T. Ishihara, M. Hagerott, A.V. Nurmikko, H. Luo, N. Samarth, J. Furdyna, Phys. Rev. Lett. 69 (1992) 1707. [12] J. Ding, M. Hagerott, T. Ishihara, H. Jeon, A.V. Nurmikko, Phys. Rev. B 47 (1993) 10528. [13] C. Sasaki, H. Naito, M. Iwata, H. Kudo, Y. Yamada, T. Taguchi, T. Jyouichi, H. Okagawa, K. Tadatomo, H. Tanaka, J. Appl. Phys. 93 (2003) 1642. [14] K. Tadatomo, H. Okagawa, Y. Ohuchi, T. Tsunekawa, Y. Imada, M. Kato, T. Taguchi, Jpn. J. Appl. Phys. 40 (2001) L583. [15] K. Shaklee, R.E. Nahory, R.F. Leheny, J. Lumin. 7 (1973) 284. [16] A. Satake, Y. Masumoto, T. Miyajima, T. Asatsuma, M. Ikeda, Phys. Rev. B 60 (1999) 16660. [17] D. Hirano, T. Tayagaki, Y. Kanemitsu, unpublished. [18] Y. Yamada, in: T. Ogawa, Y. Kanemitsu (Eds.), Optical Properties of Low-Dimensional Materials, World Scientific, Singapore, 1995, p. 202.