- Email: [email protected]
AlN multiple quantum disk nanocolumns
ARTICLE IN PRESS
Journal of Luminescence 128 (2008) 1084–1086 www.elsevier.com/locate/jlumin
Ultrafast intersubband relaxation dynamics at 1.55 mm in GaN/AlN multiple quantum disk nanocolumns Kaiichi Tanakaa,, Keita Ikunoa, Yohei Kasaia, Kazuya Fukunagaa, Hideyuki Kunugitaa,c, Kazuhiro Emaa,c, Akihiko Kikuchib,c, Katsumi Kishinob,c a Department of Physics, Sophia University, 7-1 Kioi-cho, Tokyo 102-8554, Japan Department of Electrical and Electronics Engineering, Sophia University, 7-1 Kioi-cho, Tokyo 102-8554, Japan c Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Japan
b
Available online 19 November 2007
Abstract We observed intersubband transition in GaN/AlN multiple quantum disk (MQD) in nanocolumns by absorption spectra. We also investigated ultrafast relaxation dynamics at 1.55 mm by time-resolved saturable absorption. We have found that the temporal response of nanocolumns is almost the same as the multiple-quantum-well (MQW) sample. This indicates that the delay caused by multiple scatterings in MQD nanocolumns is negligible. The saturation intensity for the Brewster configuration in MQD was also estimated. r 2007 Elsevier B.V. All rights reserved. Keywords: Nitride; GaN; Nanocolumn; Intersubband; Quantum well; Optical communication wavelength
1. Introduction The electronic relaxation process of intersubband transition (ISBT) in semiconductor quantum wells with a large conduction band offset is extremely fast due to strong electron–phonon interactions [1]. ISBT has attracted a lot of attention as a good candidate for all-optical switching devices, because of its speed and tunability to the optical communication wavelengths of 1.2–1.6 mm (0.8–1.0 eV) [2–8]. Ultrafast ISBT relaxation in several multiple quantum wells (MQWs) such as InGaAs/AlAsSb [2,3], (CdS/ZnSe)/BeTe [4], GaN/Al(Ga)N [5–8], has been demonstrated. Among these, the GaN-based ISBT has the important properties of faster relaxation time and lower toxicity than any other ISBT materials. However, owing to lattice mismatch between GaN(AlN) and the sapphire substrate, the GaN/AlN epitaxial layer exhibits high-density threading dislocations. Because threading dislocations cause undesirable absorption in the optical communication wavelength range with the same polariza-
Corresponding author. Tel.: +81 3 3238 3339; fax: +81 3 3238 3341.
E-mail address: [email protected] (K. Tanaka). 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.11.062
tion dependence as ISBT, they limit its suitability for switching devices [8]. Self-organized GaN nanocolumns [9] are essentially free of threading dislocations [10], and a GaN/AlN multiple quantum disk (MQD) can be inserted into GaN nanocolumns as shown in Fig. 1. Therefore, the nanocolumn-ISBT would be expected to improve the switching performance. However, since the nanocolumns stand densely at random positions, there is concern that multiple scatterings and/or optical localization among the nanocolumns may negatively affect this ultra-high-speed characteristic. In this study, we investigated the ultrafast relaxation dynamics of ISBT at 1.55 mm in GaN/AlN MQD nanocolumns using time-resolved saturable absorption measurements and confirmed the high-speed performance of the ISBT was preserved. We also estimated the saturation intensity of the nanocolumn ISBT. 2. Experimental procedure The sample of GaN/AlN MQD nanocolumns investigated was fabricated on a (0 0 0 1) c-plane Al2O3 substrate using rf plasma-assisted molecular beam epitaxy (RFMBE) [11]. The nanocolumns were grown at high density
ARTICLE IN PRESS K. Tanaka et al. / Journal of Luminescence 128 (2008) 1084–1086
GaN/AlN: 300pair GaN 300nm AlN Nucleation (0001)Sapphire Substrate
Absorption (arb. units)
Fig. 1. Schematic diagram of the GaN/AlN MQD nanocolumns.
500
600
700
800 900 1000 1100 1200 Energy (meV)
Fig. 2. Intersubband absorption spectrum obtained by subtracting Spolarized spectrum from P-polarized spectrum.
(1010 cm2), perpendicular to the substrate surfaces. Approximately 300 periods of GaN/AlN multi-layer structure were fabricated on GaN nanocolumns. The thicknesses of the GaN well and AlN barrier layers were estimated to be 10 A˚ (4 ML) and 27 A˚ (11 ML), respectively. Silicon was selectively doped into the AlN barriers. Electrons emitted from the donor levels in the AlN barriers accumulate in the GaN wells, with a concentration evaluated to be 2 1019 cm3. The diameter of the MQD nanocolumns ranged from 80 to 400 nm. Fig. 2 shows the room-temperature ISBT absorption spectrum obtained by subtracting the absorption spectrum for P-polarized light from that for S-polarized light. Each absorption spectrum was measured by single transmission with the beam incident at the Brewster’s angle. The fullwidth at half-maximum (FWHM) is broadened about 230 meV, mainly due to inhomogeneous components of several nanocolumns with about 300 pair wells. We investigated the ISBT ultrafast relaxation dynamics using a degenerative time-resolved pump–probe technique. The pump and probe pulses were produced by an optical parametric amplifier (OPA) excited by an amplified modelocked Ti:Al2O3 laser at a repetition rate of 100 kHz. The temporal width of the optical pulses was 100 fs. The peak wavelength was set to 1.55 mm. The beam was divided into pump and probe pulses by a half-mirror. In order to ensure sufficient interaction with the ISBT dipole moment, and also to suppress multiple reflections within the sample, the incident angle of the pump pulse was set equal to the Brewster angle for the sample. The pump beam was
chopped at a frequency of 1.270 kHz for lock-in detection. The probe pulse was incident at about 101 to the pump pulse. 3. Results and discussion 3.1. Relaxation process In Fig. 3, the solid line shows the temporal trace of the change in the transmittance of the P-polarized probe pulse induced by the P-polarized pump pulse in nanocolumn structures, and the dashed line shows that induced by the S-polarized pump and probe pulses. The effective pump beam intensity was about 1.4 pJ/mm2 per pulse. The ultrafast change appeared only for P-polarization, indicating that it is due to absorption saturation signals of ISBT. The FWHM are about 230 fs which is almost the same as in the MQW sample (Fig. 3 inset). This indicates that there is no delay in the response time due to the multiple scattering and localization effect in this sample. This result confirms the preservation of the high-speed performance of the ISBT in the nanocolumns and enhances the suitability of MQD nanocolumns in switching devices. 3.2. Saturation intensity It is appropriate to estimate absorption saturation intensity Is as evaluating the figure of merit for a switching device of this sample. Is is the intensity at which the absorption is halved due to saturation. Fig. 4 shows the pump-intensity dependence of the change in transmittance at zero delay. The linear relationship between these two quantities indicates the third-order nonlinear susceptibility w(3) region. From the gradient of the line (m ¼ 3.4 103 cm2/GW) in Fig. 4, the interaction length to the Brewster angle for the sample (L ¼ 3.5 105 cm)
MQD nanocolumns Intensity (arb.units)
GaN 600nm
1085
-500
MQW structure P-porarization
FWHM:230fs
-500
0
500
1000
FWHM: 230fs P-polarization S-polarization
0
500 1000 Delaytime (fs)
1500
Fig. 3. Experimental results of pump probe measurement for Ppolarization (solid line) and S- polarization (dotted line). The signal only appears for P-polarization, which indicates the ISBT relaxation process. The inset is a result for P-polarization in the MQW structure.
ARTICLE IN PRESS K. Tanaka et al. / Journal of Luminescence 128 (2008) 1084–1086
1086 5
ΔT/T (x10-3)
4 3 2 1 0 0.6
0.8
1.0
1.2
If these factors are improved, the figure of merit will be much better. In conclusion, we investigated self-organized GaN/AlN MQD nanocolumn-based ISBT. The ISBT in GaN/AlN MQD nanocolumns was confirmed for the first time by absorption spectrum, and the ultrafast relaxation dynamics was observed using the degenerate pump probe technique at 1.55 mm. Our results show that the delay caused by multiple scatterings in MQD nanocolumns is negligible and the localization effect is not observable around communication wavelengths.
Pump Intensity (GW/cm2)
Fig. 4. The dependence of the change in transmittance on the pump intensity. The dotted line is a fitted linear relationship.
and the absorption coefficient in a weak intensity limit (a0 ¼ 6.5 103 cm1), we can estimate the absorption saturation intensity Is to be 67 GW/cm2, which corresponds to 67 pJ/mm2 for a 100 fs pulse. This value is not notably different from that of the MQW sample [7], although a great improvement in Is compared to the MQW sample was expected. We can consider several reasons for this: (1) Stray light due to scattering of the pump pulses from the randomly arranged nanocolumns hinders ISBT signals. As shown in the previous section, the effect of multiple scattering on the response time is negligible, but the scattering of the pump pulses influences the estimation of Is. (2) The dope concentration, the number of wells, and the homogeneity of a column are not optimized.
References [1] B.K. Ridley, Phys. Rev. B 39 (1989) 5282. [2] T. Akiyama, G. Georgive, T. Mozume, H. Yoshida, A. Venu Gopal, O. Wada, Electron. Lett. 37 (2001) 129. [3] A.V. Gopal, H. Yoshida, T. Shimoyama, N. Georgieve, T. Mozume, H. Ishikawa, Appl. Phys. Lett. 80 (2002) 4696. [4] R. Akimoto, B.S. Li, K. Akita, T. Hasama, Appl. Phys. Lett. 87 (2005) 181104. [5] N. Iizuka, K. Kaneko, N. Suzuki, T. Asano, S. Noda, O. Wada, Appl. Phys. Lett. 77 (2000) 648. [6] C. Gmachl, H.M. Ng, G. Chu, A.Y. Cho, Appl. Phys. Lett. 77 (2000) 3722. [7] J. Hamazaki, S. Matsui, H. Kunugita, K. Ema, H. Kanazawa, T. Tachibana, A. Kikuchi, K. Kishino, Appl. Phys. Lett. 84 (2004) 1102. [8] N. Iizuka, K. Kaneko, N. Suzuki, J. Appl. Phys. 99 (2006) 093107. [9] M. Yoshizawa, A. Kikuchi, M. Mori, N. Fujita, K. Kishino, Jpn. J. Appl. Phys. 36 (1997) L459. [10] A. Kikuchi, K. Yamano, M. Tada, K. Kishino, Phys. Stat. Sol. (b) 241 (2004) 2754. [11] K. Kishino, A. Kikuchi, H. Kanazawa, T. Tachibana, Appl. Phys. Lett. 81 (2002) 1234.
Journal of Luminescence 128 (2008) 1084–1086 www.elsevier.com/locate/jlumin
Ultrafast intersubband relaxation dynamics at 1.55 mm in GaN/AlN multiple quantum disk nanocolumns Kaiichi Tanakaa,, Keita Ikunoa, Yohei Kasaia, Kazuya Fukunagaa, Hideyuki Kunugitaa,c, Kazuhiro Emaa,c, Akihiko Kikuchib,c, Katsumi Kishinob,c a Department of Physics, Sophia University, 7-1 Kioi-cho, Tokyo 102-8554, Japan Department of Electrical and Electronics Engineering, Sophia University, 7-1 Kioi-cho, Tokyo 102-8554, Japan c Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Japan
b
Available online 19 November 2007
Abstract We observed intersubband transition in GaN/AlN multiple quantum disk (MQD) in nanocolumns by absorption spectra. We also investigated ultrafast relaxation dynamics at 1.55 mm by time-resolved saturable absorption. We have found that the temporal response of nanocolumns is almost the same as the multiple-quantum-well (MQW) sample. This indicates that the delay caused by multiple scatterings in MQD nanocolumns is negligible. The saturation intensity for the Brewster configuration in MQD was also estimated. r 2007 Elsevier B.V. All rights reserved. Keywords: Nitride; GaN; Nanocolumn; Intersubband; Quantum well; Optical communication wavelength
1. Introduction The electronic relaxation process of intersubband transition (ISBT) in semiconductor quantum wells with a large conduction band offset is extremely fast due to strong electron–phonon interactions [1]. ISBT has attracted a lot of attention as a good candidate for all-optical switching devices, because of its speed and tunability to the optical communication wavelengths of 1.2–1.6 mm (0.8–1.0 eV) [2–8]. Ultrafast ISBT relaxation in several multiple quantum wells (MQWs) such as InGaAs/AlAsSb [2,3], (CdS/ZnSe)/BeTe [4], GaN/Al(Ga)N [5–8], has been demonstrated. Among these, the GaN-based ISBT has the important properties of faster relaxation time and lower toxicity than any other ISBT materials. However, owing to lattice mismatch between GaN(AlN) and the sapphire substrate, the GaN/AlN epitaxial layer exhibits high-density threading dislocations. Because threading dislocations cause undesirable absorption in the optical communication wavelength range with the same polariza-
Corresponding author. Tel.: +81 3 3238 3339; fax: +81 3 3238 3341.
E-mail address: [email protected] (K. Tanaka). 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.11.062
tion dependence as ISBT, they limit its suitability for switching devices [8]. Self-organized GaN nanocolumns [9] are essentially free of threading dislocations [10], and a GaN/AlN multiple quantum disk (MQD) can be inserted into GaN nanocolumns as shown in Fig. 1. Therefore, the nanocolumn-ISBT would be expected to improve the switching performance. However, since the nanocolumns stand densely at random positions, there is concern that multiple scatterings and/or optical localization among the nanocolumns may negatively affect this ultra-high-speed characteristic. In this study, we investigated the ultrafast relaxation dynamics of ISBT at 1.55 mm in GaN/AlN MQD nanocolumns using time-resolved saturable absorption measurements and confirmed the high-speed performance of the ISBT was preserved. We also estimated the saturation intensity of the nanocolumn ISBT. 2. Experimental procedure The sample of GaN/AlN MQD nanocolumns investigated was fabricated on a (0 0 0 1) c-plane Al2O3 substrate using rf plasma-assisted molecular beam epitaxy (RFMBE) [11]. The nanocolumns were grown at high density
ARTICLE IN PRESS K. Tanaka et al. / Journal of Luminescence 128 (2008) 1084–1086
GaN/AlN: 300pair GaN 300nm AlN Nucleation (0001)Sapphire Substrate
Absorption (arb. units)
Fig. 1. Schematic diagram of the GaN/AlN MQD nanocolumns.
500
600
700
800 900 1000 1100 1200 Energy (meV)
Fig. 2. Intersubband absorption spectrum obtained by subtracting Spolarized spectrum from P-polarized spectrum.
(1010 cm2), perpendicular to the substrate surfaces. Approximately 300 periods of GaN/AlN multi-layer structure were fabricated on GaN nanocolumns. The thicknesses of the GaN well and AlN barrier layers were estimated to be 10 A˚ (4 ML) and 27 A˚ (11 ML), respectively. Silicon was selectively doped into the AlN barriers. Electrons emitted from the donor levels in the AlN barriers accumulate in the GaN wells, with a concentration evaluated to be 2 1019 cm3. The diameter of the MQD nanocolumns ranged from 80 to 400 nm. Fig. 2 shows the room-temperature ISBT absorption spectrum obtained by subtracting the absorption spectrum for P-polarized light from that for S-polarized light. Each absorption spectrum was measured by single transmission with the beam incident at the Brewster’s angle. The fullwidth at half-maximum (FWHM) is broadened about 230 meV, mainly due to inhomogeneous components of several nanocolumns with about 300 pair wells. We investigated the ISBT ultrafast relaxation dynamics using a degenerative time-resolved pump–probe technique. The pump and probe pulses were produced by an optical parametric amplifier (OPA) excited by an amplified modelocked Ti:Al2O3 laser at a repetition rate of 100 kHz. The temporal width of the optical pulses was 100 fs. The peak wavelength was set to 1.55 mm. The beam was divided into pump and probe pulses by a half-mirror. In order to ensure sufficient interaction with the ISBT dipole moment, and also to suppress multiple reflections within the sample, the incident angle of the pump pulse was set equal to the Brewster angle for the sample. The pump beam was
chopped at a frequency of 1.270 kHz for lock-in detection. The probe pulse was incident at about 101 to the pump pulse. 3. Results and discussion 3.1. Relaxation process In Fig. 3, the solid line shows the temporal trace of the change in the transmittance of the P-polarized probe pulse induced by the P-polarized pump pulse in nanocolumn structures, and the dashed line shows that induced by the S-polarized pump and probe pulses. The effective pump beam intensity was about 1.4 pJ/mm2 per pulse. The ultrafast change appeared only for P-polarization, indicating that it is due to absorption saturation signals of ISBT. The FWHM are about 230 fs which is almost the same as in the MQW sample (Fig. 3 inset). This indicates that there is no delay in the response time due to the multiple scattering and localization effect in this sample. This result confirms the preservation of the high-speed performance of the ISBT in the nanocolumns and enhances the suitability of MQD nanocolumns in switching devices. 3.2. Saturation intensity It is appropriate to estimate absorption saturation intensity Is as evaluating the figure of merit for a switching device of this sample. Is is the intensity at which the absorption is halved due to saturation. Fig. 4 shows the pump-intensity dependence of the change in transmittance at zero delay. The linear relationship between these two quantities indicates the third-order nonlinear susceptibility w(3) region. From the gradient of the line (m ¼ 3.4 103 cm2/GW) in Fig. 4, the interaction length to the Brewster angle for the sample (L ¼ 3.5 105 cm)
MQD nanocolumns Intensity (arb.units)
GaN 600nm
1085
-500
MQW structure P-porarization
FWHM:230fs
-500
0
500
1000
FWHM: 230fs P-polarization S-polarization
0
500 1000 Delaytime (fs)
1500
Fig. 3. Experimental results of pump probe measurement for Ppolarization (solid line) and S- polarization (dotted line). The signal only appears for P-polarization, which indicates the ISBT relaxation process. The inset is a result for P-polarization in the MQW structure.
ARTICLE IN PRESS K. Tanaka et al. / Journal of Luminescence 128 (2008) 1084–1086
1086 5
ΔT/T (x10-3)
4 3 2 1 0 0.6
0.8
1.0
1.2
If these factors are improved, the figure of merit will be much better. In conclusion, we investigated self-organized GaN/AlN MQD nanocolumn-based ISBT. The ISBT in GaN/AlN MQD nanocolumns was confirmed for the first time by absorption spectrum, and the ultrafast relaxation dynamics was observed using the degenerate pump probe technique at 1.55 mm. Our results show that the delay caused by multiple scatterings in MQD nanocolumns is negligible and the localization effect is not observable around communication wavelengths.
Pump Intensity (GW/cm2)
Fig. 4. The dependence of the change in transmittance on the pump intensity. The dotted line is a fitted linear relationship.
and the absorption coefficient in a weak intensity limit (a0 ¼ 6.5 103 cm1), we can estimate the absorption saturation intensity Is to be 67 GW/cm2, which corresponds to 67 pJ/mm2 for a 100 fs pulse. This value is not notably different from that of the MQW sample [7], although a great improvement in Is compared to the MQW sample was expected. We can consider several reasons for this: (1) Stray light due to scattering of the pump pulses from the randomly arranged nanocolumns hinders ISBT signals. As shown in the previous section, the effect of multiple scattering on the response time is negligible, but the scattering of the pump pulses influences the estimation of Is. (2) The dope concentration, the number of wells, and the homogeneity of a column are not optimized.
References [1] B.K. Ridley, Phys. Rev. B 39 (1989) 5282. [2] T. Akiyama, G. Georgive, T. Mozume, H. Yoshida, A. Venu Gopal, O. Wada, Electron. Lett. 37 (2001) 129. [3] A.V. Gopal, H. Yoshida, T. Shimoyama, N. Georgieve, T. Mozume, H. Ishikawa, Appl. Phys. Lett. 80 (2002) 4696. [4] R. Akimoto, B.S. Li, K. Akita, T. Hasama, Appl. Phys. Lett. 87 (2005) 181104. [5] N. Iizuka, K. Kaneko, N. Suzuki, T. Asano, S. Noda, O. Wada, Appl. Phys. Lett. 77 (2000) 648. [6] C. Gmachl, H.M. Ng, G. Chu, A.Y. Cho, Appl. Phys. Lett. 77 (2000) 3722. [7] J. Hamazaki, S. Matsui, H. Kunugita, K. Ema, H. Kanazawa, T. Tachibana, A. Kikuchi, K. Kishino, Appl. Phys. Lett. 84 (2004) 1102. [8] N. Iizuka, K. Kaneko, N. Suzuki, J. Appl. Phys. 99 (2006) 093107. [9] M. Yoshizawa, A. Kikuchi, M. Mori, N. Fujita, K. Kishino, Jpn. J. Appl. Phys. 36 (1997) L459. [10] A. Kikuchi, K. Yamano, M. Tada, K. Kishino, Phys. Stat. Sol. (b) 241 (2004) 2754. [11] K. Kishino, A. Kikuchi, H. Kanazawa, T. Tachibana, Appl. Phys. Lett. 81 (2002) 1234.