- Email: [email protected]
InP Bragg-spaced quantum wells grown by MOCVD
Optics Communications 285 (2012) 4759–4762
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Temperature tuning of the Bragg resonance of InAsP/InP Bragg-spaced quantum wells grown by MOCVD Wei Yan, Xiao-Ming Li, Tao Wang n Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, Hubei, PR China
a r t i c l e i n f o
abstract
Article history: Received 19 October 2011 Received in revised form 13 June 2012 Accepted 9 July 2012 Available online 24 July 2012
In this article, we presented a study of InAs0.04P0.96/InP Bragg-spaced quantum wells (BSQWs), which were grown by metal organic chemical vapor deposition (MOCVD). The quantum wells were characterized by photoluminescence (PL), double-crystal x-ray diffraction (DC-XRD), and reflection spectra. We found that the BSQWs structure grown at 580 1C appears to be extremely abrupt, uniform, free of misfit dislocations, and of narrow PL line width. From the reflection spectra at different temperatures, we presented a theoretical analysis of the changes in band structure for resonant and near-resonant wells, and proposed a new scheme of using the temperature to tune the Bragg resonance of Bragg spaced quantum wells. & 2012 Elsevier B.V. All rights reserved.
Keywords: InAsP/InP Bragg-spaced quantum wells Bragg resonance Metal organic chemical vapor deposition
1. Introduction In the last two decades, semiconductor BSQWs and superlattice structures have become very promising in the realization of optoelectronic devices employed in modern optical fiber communication systems [1–5], due to their unique optical properties demonstrated by those quantum-confined structures. The primary advantage of InP-based materials for such tasks is a wider range of available materials as compared to GaAs-based BSQWs systems. In particular, intentionally strained InAsP/InP heterostructures have become an interesting alternative system for the realization of optical devices operating in the 900–1600 nm range [6]. Currently, molecular beam epitaxy (MBE) and MOCVD are the two most widely used technologies for the growth of BSQWs. For example, Farley et al. reported a 25 period InAsP/InP BSQW grown by solid source MBE for the modulation of light at 1550 nm [7]. Yildirim et al. reported nearroom-temperature all-optical polarization switch based on 40 periods GaAs/AlGaAs BSQWs grown by MBE [5]. On the other hand, there are many researchers who grew InAsP/InP BSQWs by MOCVD, including Lee et al., who reported 1300 nm InAsP/InP BSQW laser diodes [1,13,14]. But, there are rarely letters reporting more than 20 periodic BSQWs grown by MOCVD, especially the InAsP/InP. In this paper we shall report 10, 20, 30 and 60 periods InAsP/InP BSQWs grown by MOCVD at the same conditions, and obtain the photoluminescence and reflection spectra at different temperatures.
n
Corresponding author. E-mail address: [email protected] (T. Wang).
0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.07.037
Many papers [4,8–11] investigated in semiconductor BSQWs, when the Bragg condition is satisfied d ¼ l/2 (Bragg wavelength¼exciton wavelength), the formation of Bragg resonant effect (here the multiple reflections from all N quantum wells add constructively to give the strong reflectivity and superradiant broadening of N coupled oscillators), where d is the period of the BSQWs, l is the wavelength of the 1s electron–heavy-hole exciton (E ¼hc/l), and N is the number of BSQWs. But, if d a l/2 (a little shift, near-Bragg resonance) there will be a transmission window in the reflectivity stop band. Hence, we propose a new method of using the temperature to control the Bragg resonance, inducing da l/2. 2. Experimental procedure The InAsP/InP BSQWs structures were grown on undoped (001) InP substrate at 580 1C in a horizontal cold-wall quartz reactor by the Thomas Swan low-pressure MOCVD at a pressure of 150 mbar. Arsine (AsH3), phosphine (PH3) and trimethylindium (TMIn) were used as source materials. The V/III ratio for InAsP growth was 240; the InAsP and InP growth rates were 0.19 nm/s and 0.31 nm/s, respectively for 10, 20, 30 and 60 periods InAsP/ InP BSQWs at the same growth conditions, the difference being the growth time. DC-XRD was used to characterize crystal structure of the BSQWs. The PL spectra measurements were performed with an Arþ laser (l ¼514.5 nm) as the excitation source. The signal was dispersed by a 1 m spectrometer and detected by a liquid-nitrogen-cooled Ge p–i–n photodiode using conventional lock-in techniques. The sample temperature was
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W. Yan et al. / Optics Communications 285 (2012) 4759–4762
maintained in a continuous flow helium cryostat operating in the 10–300 K range and the reflection spectra also was obtained.
Structural properties of InAsP/InP BSQWs were investigated using DC-XRD. The pattern of 20 periods InAs0.04P0.96/InP BSQWs grown at 580 1C together with a simulation based on software of X’pert Epitaxy is shown in Fig. 1. From the figure we can know that the simulation highly agrees with the experiment, and the wells width of InAs0.04P0.96 was 7 nm and the barriers of InP 126.9 nm. The angular separation between the zeroth-order satellite peak and the InP peak provides a measure of the average strain of the BSQWs epitaxial layers. The intensity of the satellite peak is a good indicator of the quality of BSQWs, that is, of the degree of the crystallinity and of the uniformity of the layer thickness. If misfit dislocations are present or the layer thickness do not repeat well, then the x-ray peaks will be broadened and have reduced intensity. From Bragg’s law, the BSQWs period L can be obtained by L ¼ l cos(y)Dy/2, where l is the x-ray wavelength, y is the Bragg angle corresponding to InP, and Dy is the angular separation between the satellite peaks. However, sharp and well-defined satellite peaks are detected in this study. This implies that the BSQWs structure maintains its structural integrity throughout the deposition sequence with smooth and abrupt interfaces, and well-defined periodicity. Fig. 2 shows the PL spectra of the InAsP/InP BSQWs structure at various temperatures between 10 and 300 K. The dominant PL peak positions shift to lower energies and the linewidths broaden with temperature. The maximum of PL spectra shown in Fig. 2 is due to the E1H transition between the n ¼1 electron subband and the n¼1 heavy-hole subband. At 300 K the light-hole exciton peak E1L can be seen and its intensity decreases with temperature. The above temperature-dependent behavior of the luminescence peak energy has been ascribed to free exciton recombination, while recombination is via excitons trapped to material defects, such as disorder defects and misfit dislocation. Fig. 3 shows the variations of PL peak energy and FWHM as a function of temperature for 20 periods InAsP/InP BSQWs structure. The temperature dependence of the PL peak energy can be expressed as the Varshni Equation [15]: Eg ðTÞ ¼ Eg ð0ÞaT 2 =ðT þ bÞ
ð1Þ
where Eg(0) is the energy gap at 0 K, a and b are material constants. The fitted a and b are 8.48 10 4 eV/K and 1600 K,
Fig. 1. DC-XRD pattern of 20 periods InAs0.04P0.96/InP MSQWs at 580 1C, the upper and the lower are experiment and simulation, respectively.
PL Intensity (a.u.)
3. Results and discussion
0.2
300k 200K 150K 90K 10K
0.15
0.1
0.05
0 1.28
1.3
1.32
1.34 1.36 1.38 Energy (eV)
1.4
1.42
Fig. 2. PL spectra of 20 periods InAsP/InP BSQWs at various temperatures between 10 and 300 K.
Fig. 3. Variations of the PL peak energy and FWHM as a function of the temperature for 20 periods InAsP/InP BSQWs.
respectively. The Eg(0) is 1.382 eV. The FWHM is found to increase with temperature, because of scattering by longitudinal optical phonons [16]. In general, the luminescence line shape is a convolution of an inhomogeneous part and a temperature-dependent homogeneous part. The inhomogeneous linewidth in the strained quantum well structure is mainly due to the interface roughness and random alloy disorder. Nevertheless, the minimum PL linewidths for BSQWs structures may be limited by the presence of atomic-scale clustering at the hetero-interface, as observed in MBE grown structures [17]. The nanoscale compositions within the InAsP alloy layers lead to an asymmetry in interface quality. Because of the symmetrical PL line shape and no impurity transition involved in the low-energy side of the lowtemperature luminescence spectra, the inhomogeneous broadening is not taken into account. As the temperature increases, scattering by longitudinal optical phonons becomes dominant due to the increasing phonon population, which gives rise to the homogeneous part of the linewidth broadening. However, the broadening in PL spectra significantly reduced for InAs0.04P0.96/InP BSQWs structure, due to the suppression of roughness in the interface. The FWHM is 15 and 51 meV at 10 K and 300 K, compared with the 24.6 and 50.5 meV at the 10 K and 296 K of five period InAs0.5P0.5/InP, respectively [13]; we got that a high crystalline quality 20 periods InAs0.04P0.96/InP BSQWs structures successfully grown by MOCVD which agrees with Fig. 1.
W. Yan et al. / Optics Communications 285 (2012) 4759–4762
Reflection
0.15
0.10
0.05
0.00 890
895
900 905 910 Wavelength (nm)
5 4.5 4 3.5 3 2.5 2 1.5 1
HWHM Total Linewidth γ
10 20 30 60
915
4761
experiment simulation 0
10
20 30 40 50 60 Number of quantum wells
70
Fig. 4. (a) Experimental reflection spectra of BSQWs, 10, 20, 30 and 60 periods at 10 K and (b) square dot represents experimental HWHM line width for samples and the solid line is generated from simulation for the BSQWs samples.
x 10-7
0.05
9.05 W avelength (m )
Reflection
0.10
20k 77k 92k 136k 200k
9.04 9.03 9.02 9.01 9
890 895 900 905 910 915 920 Wavelength(nm)
20
40 60 80 100 120 140 Temperature (k)
Fig. 5. (a) Reflection of 20-period BSQWs at various temperatures and (b) the red solid line represents Bragg wavelength and the dashed line represents exciton wavelength. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
From Fig. 4 we can get the principal experimental results that the reflectivity and linewidth increase with N at Bragg resonance (d¼ l/2), the so-called superradiant response. Fig. 4(b) conclusively shows that experimentally the linewidth in reflection for the collective system does indeed increase linearly with N; in accordance with the linear dispersion theory [10], the total damping rate
g ¼ G þN G0
ð2Þ
where G is the nonradiative exciton damping rate of a single quantum well, and G0 is the radiative damping rate of a single quantum well. The total damping rate is N times enhanced compared to radiative damping rate of a single quantum well. Thus Eq. (2) allows a convenient way to extract the radiative linewidth from the slope of linewidth versus N and the remaining contributions to the linewidth from the intercept. The fitted G and G0 are 0.053 meV and 1.054 meV, respectively. G0 can also be determined by precision reflection measurements from a single quantum well. The variation in the quantum wells of BSQWs sample is expected to have very little effect on G0, since it depends mostly on the well material and thickness. The reflection spectra of 20 period InAsP/InP BSQWs structure at various temperatures from 10 K to 200 K are shown in Fig. 5(a). We can see that all the peaks positions shift to longer wavelength and FWHM broadens with temperature. We can also see that if the temperature drifts from 10 K (near-Bragg resonance), the reflectivity peak will begin to collapse, and the higher the temperature, the more serious the collapse. Furthermore, the reflectivity peak almost vanishes at 200 K. The above temperature-dependent behaviour of the reflection spectra has been ascribed to free exciton recombination and the scattering by
longitudinal optical phonons. When the temperature is above 200 K, the exciton binding energy is destructed, so the reflection spectra vanish. The above data suggest that if the Bragg resonance condition is destructed, the reflection spectra will split. So we can control the Bragg resonance by temperature. And then, we will present a theoretical analysis of temperature-dependence of band structure in BSQWs, for Bragg resonance and near-Bragg resonance. Fig. 5(b) shows the temperature versus Bragg wavelength and exciton wavelength. From the figure we can know that the Bragg wavelength equals the exciton wavelength at 10 K. Some letters [4,8–11] reported that in the case when Bragg wavelength equals the heavy hole exciton wavelength, the photonic band structure forms a forbidden gap (and an associated reflection stopgap) with both resonance frequencies located within the gap. Fig. 6 demonstrates the photonic band structure of the model BMQWs at 10 K with the plane waves expansion method [18]; from Fig. 6(b) we can see that there is a stopgap at almost 900 nm, which agrees with the experiment of reflection spectra at 10 K. When the temperature increases to 77 K, from Fig. 5(b) we can know that the Bragg wavelength is longer than the exciton wavelength (near-Bragg resonance). Yang et al. [12] reported that if oBrg a ox, there will be a transmission window in the reflectivity stop band. The presence of two characteristic frequencies breaks the photonic band structure into three bands, as illustrated in Fig. 7; it agrees with Fig. 5 at 77 K. The intermediate transmission band has been shown to have the analytic form cosðKaÞ ¼ cosðqaÞ þ
G
o
ðo þ igÞox ox
sinðqaÞ
ð3Þ
W. Yan et al. / Optics Communications 285 (2012) 4759–4762
2.5
1.44
2
1.42 Energy (ev)
Energy (ev)
4762
1.5 1 0.5
1.40 1.38 1.36 1.34
-3
.0 4
.0 2 -3
8
.0 6
.0
-3
-3
Re (k)*a
-3
3
.1
2
-3
1
-3
0
.1
-1
.1 4
-2
-3
-3
2
0
Re (k)*a
Fig. 6. (a) Photonic band structure of the model BMQWs at 10 K and (b) expanded scale of left red ring. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1.44 1.42
2
Energy (ev)
Energy (ev)
2.5
1.5 1 0.5
1.4 1.38 1.36 1.34
-3
-3
.0
2
4
6
.0 -3
.0
8
-3
.0 -3
.1 -3
3
2
2
.1
0 1 Re (k)*a
-3
-1
.1
-2
-3
-3
4
0
Re (k)*a Fig. 7. (a) Photonic band structure of the model BMQWs at 77 K, where an intermediate band is formed and (b) the intermediate band is shown in (a) on an expanded scale.
where o(k) is the polariton angular frequency (wave vector), ox is the 1s-heavy-hole quantum well excitonic resonance frequency, c the speed of light in vacuum, q¼nbo/c (nb is the background index), a is the periodic spacing of the quantum wells, and G(g) the radiative decay (dephasing) rate. According to the above conclusion we propose a new scheme of using the temperature to control the transmission window opening and closing. Based on this fact, we devise the following procedure. (1) First set the temperature off 10 K and d a l/2, opening a transmission window for the incident light to propagate into the BMQWs. (2) We then tune the temperature back to 10 K to close the transmission window and stop the light inside the BMQWs at the same time. (3) The temperature is then reset to its original value, reopening the transmission window and releasing the light. The scheme can be used not only in slow light devices, but also in optical switch devices.
4. Conclusion We have demonstrated a growth condition to grow high quality, long-period InAs0.04P0.96/InP Bragg-spaced quantum wells by MOCVD, and studied the influence of temperature on the PL and reflection spectra. We found that the peaks of PL shift to longer wavelength and the FWHM broaden with temperature, and at Bragg resonance, the intensity and linewidth of reflection increase with N. By researching the reflection spectra at different temperatures, we presented a theoretical analysis of the changes in band structure for resonant and near-resonant cases, and proposed a new scheme of using the temperature to control the Bragg resonance.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant no. 60877040), and National Basic Research program of China (Grant. 2010CB923204). References [1] C.Y. Lee, M.C. Wu, H.P. Shiao, T.T. Shi, W.J. Ho, Solid-State Electronics 43 (1999) 2141. [2] F. Carreno, M.A. Anton, Jornal of Physics B: Atomic Molecular and Optical Physics 42 (2009) 21550801. [3] T. Wang, G. Li, Z. Chen, Optics Express 16 (2008) 127. [4] T. Wang, Q. Li, D.S. Gao, Chinese Science Bulletin 54 (2009) 3663. [5] M. Yildirim, J.P. Prineas, Journal of Applied Physics 98 (2005) 0635061. [6] C. Monier, I. Serdiukova, L. Aguilar, F. Newman, M.F. Vilela, A. Frenudlich, Journal of Vacuum Science and Technology B 17 (1999) 1158. [7] R.J. Farley, S.K. Haywood, V.A. Hewer, J.H.C. Hogg, Proceedings of the 10th Inernational Conference on Indium Phosphide and Related Materials, vol. 13, 1998 p. 564. [8] L.I. Deych, A.A. Lisyansky, Physical Review B 62 (2000) 4242. [9] M. Hubner, J.P. Prineas, C. Ell, P. Brick, E.S. Lee, G. Khitrova, H.M. Gibbs, S.W. Koch, Physical Review Letters 83 (1999) 2841. [10] J.P. Prineas, C. Ell, E.S. Lee, G. Khitrova, H. Gibbs, S.W. Koch, Physical Review B 61 (2000) 13863. [11] J.P. Prneas, C. Cao, M. Yildirim, W. Johnston, M. Reddy, Journal of Applied Physics 100 (2006) 0631011. [12] Z.S. Yang, N.H. Kwong, R. Binder, Journal of the Optical Society of America B; Optical Physics 22 (2005) 2145. [13] C.Y. Lee, M.C. Wu, H.P. Shiao, W.J. Ho, Journal of Crystal Growth 208 (2000) 137. [14] Y.G. Zhao, Y.H. Zou, X.L. Huang, J.J. Wang, Y.D. Qin, Journal of Applied Physics 8 (1998) 4430. [15] Y.P. Varshni, Physica 34 (1967) 149–152. [16] W.Z. Shen, Y. Chang, S.C. Shen, W.G. Tang, Y. Zhao, A.Z. Li, Journal of Applied Physics 79 (1996) 2139. [17] S.L. Zuo, W.G. Bi, C.W. Tu, E.T. Yu, Applied Physics Letters 72 (1998) 2135. [18] W.T. Rhodes, Photonic Crystals. Atlanta, 2009.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Temperature tuning of the Bragg resonance of InAsP/InP Bragg-spaced quantum wells grown by MOCVD Wei Yan, Xiao-Ming Li, Tao Wang n Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, Hubei, PR China
a r t i c l e i n f o
abstract
Article history: Received 19 October 2011 Received in revised form 13 June 2012 Accepted 9 July 2012 Available online 24 July 2012
In this article, we presented a study of InAs0.04P0.96/InP Bragg-spaced quantum wells (BSQWs), which were grown by metal organic chemical vapor deposition (MOCVD). The quantum wells were characterized by photoluminescence (PL), double-crystal x-ray diffraction (DC-XRD), and reflection spectra. We found that the BSQWs structure grown at 580 1C appears to be extremely abrupt, uniform, free of misfit dislocations, and of narrow PL line width. From the reflection spectra at different temperatures, we presented a theoretical analysis of the changes in band structure for resonant and near-resonant wells, and proposed a new scheme of using the temperature to tune the Bragg resonance of Bragg spaced quantum wells. & 2012 Elsevier B.V. All rights reserved.
Keywords: InAsP/InP Bragg-spaced quantum wells Bragg resonance Metal organic chemical vapor deposition
1. Introduction In the last two decades, semiconductor BSQWs and superlattice structures have become very promising in the realization of optoelectronic devices employed in modern optical fiber communication systems [1–5], due to their unique optical properties demonstrated by those quantum-confined structures. The primary advantage of InP-based materials for such tasks is a wider range of available materials as compared to GaAs-based BSQWs systems. In particular, intentionally strained InAsP/InP heterostructures have become an interesting alternative system for the realization of optical devices operating in the 900–1600 nm range [6]. Currently, molecular beam epitaxy (MBE) and MOCVD are the two most widely used technologies for the growth of BSQWs. For example, Farley et al. reported a 25 period InAsP/InP BSQW grown by solid source MBE for the modulation of light at 1550 nm [7]. Yildirim et al. reported nearroom-temperature all-optical polarization switch based on 40 periods GaAs/AlGaAs BSQWs grown by MBE [5]. On the other hand, there are many researchers who grew InAsP/InP BSQWs by MOCVD, including Lee et al., who reported 1300 nm InAsP/InP BSQW laser diodes [1,13,14]. But, there are rarely letters reporting more than 20 periodic BSQWs grown by MOCVD, especially the InAsP/InP. In this paper we shall report 10, 20, 30 and 60 periods InAsP/InP BSQWs grown by MOCVD at the same conditions, and obtain the photoluminescence and reflection spectra at different temperatures.
n
Corresponding author. E-mail address: [email protected] (T. Wang).
0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.07.037
Many papers [4,8–11] investigated in semiconductor BSQWs, when the Bragg condition is satisfied d ¼ l/2 (Bragg wavelength¼exciton wavelength), the formation of Bragg resonant effect (here the multiple reflections from all N quantum wells add constructively to give the strong reflectivity and superradiant broadening of N coupled oscillators), where d is the period of the BSQWs, l is the wavelength of the 1s electron–heavy-hole exciton (E ¼hc/l), and N is the number of BSQWs. But, if d a l/2 (a little shift, near-Bragg resonance) there will be a transmission window in the reflectivity stop band. Hence, we propose a new method of using the temperature to control the Bragg resonance, inducing da l/2. 2. Experimental procedure The InAsP/InP BSQWs structures were grown on undoped (001) InP substrate at 580 1C in a horizontal cold-wall quartz reactor by the Thomas Swan low-pressure MOCVD at a pressure of 150 mbar. Arsine (AsH3), phosphine (PH3) and trimethylindium (TMIn) were used as source materials. The V/III ratio for InAsP growth was 240; the InAsP and InP growth rates were 0.19 nm/s and 0.31 nm/s, respectively for 10, 20, 30 and 60 periods InAsP/ InP BSQWs at the same growth conditions, the difference being the growth time. DC-XRD was used to characterize crystal structure of the BSQWs. The PL spectra measurements were performed with an Arþ laser (l ¼514.5 nm) as the excitation source. The signal was dispersed by a 1 m spectrometer and detected by a liquid-nitrogen-cooled Ge p–i–n photodiode using conventional lock-in techniques. The sample temperature was
4760
W. Yan et al. / Optics Communications 285 (2012) 4759–4762
maintained in a continuous flow helium cryostat operating in the 10–300 K range and the reflection spectra also was obtained.
Structural properties of InAsP/InP BSQWs were investigated using DC-XRD. The pattern of 20 periods InAs0.04P0.96/InP BSQWs grown at 580 1C together with a simulation based on software of X’pert Epitaxy is shown in Fig. 1. From the figure we can know that the simulation highly agrees with the experiment, and the wells width of InAs0.04P0.96 was 7 nm and the barriers of InP 126.9 nm. The angular separation between the zeroth-order satellite peak and the InP peak provides a measure of the average strain of the BSQWs epitaxial layers. The intensity of the satellite peak is a good indicator of the quality of BSQWs, that is, of the degree of the crystallinity and of the uniformity of the layer thickness. If misfit dislocations are present or the layer thickness do not repeat well, then the x-ray peaks will be broadened and have reduced intensity. From Bragg’s law, the BSQWs period L can be obtained by L ¼ l cos(y)Dy/2, where l is the x-ray wavelength, y is the Bragg angle corresponding to InP, and Dy is the angular separation between the satellite peaks. However, sharp and well-defined satellite peaks are detected in this study. This implies that the BSQWs structure maintains its structural integrity throughout the deposition sequence with smooth and abrupt interfaces, and well-defined periodicity. Fig. 2 shows the PL spectra of the InAsP/InP BSQWs structure at various temperatures between 10 and 300 K. The dominant PL peak positions shift to lower energies and the linewidths broaden with temperature. The maximum of PL spectra shown in Fig. 2 is due to the E1H transition between the n ¼1 electron subband and the n¼1 heavy-hole subband. At 300 K the light-hole exciton peak E1L can be seen and its intensity decreases with temperature. The above temperature-dependent behavior of the luminescence peak energy has been ascribed to free exciton recombination, while recombination is via excitons trapped to material defects, such as disorder defects and misfit dislocation. Fig. 3 shows the variations of PL peak energy and FWHM as a function of temperature for 20 periods InAsP/InP BSQWs structure. The temperature dependence of the PL peak energy can be expressed as the Varshni Equation [15]: Eg ðTÞ ¼ Eg ð0ÞaT 2 =ðT þ bÞ
ð1Þ
where Eg(0) is the energy gap at 0 K, a and b are material constants. The fitted a and b are 8.48 10 4 eV/K and 1600 K,
Fig. 1. DC-XRD pattern of 20 periods InAs0.04P0.96/InP MSQWs at 580 1C, the upper and the lower are experiment and simulation, respectively.
PL Intensity (a.u.)
3. Results and discussion
0.2
300k 200K 150K 90K 10K
0.15
0.1
0.05
0 1.28
1.3
1.32
1.34 1.36 1.38 Energy (eV)
1.4
1.42
Fig. 2. PL spectra of 20 periods InAsP/InP BSQWs at various temperatures between 10 and 300 K.
Fig. 3. Variations of the PL peak energy and FWHM as a function of the temperature for 20 periods InAsP/InP BSQWs.
respectively. The Eg(0) is 1.382 eV. The FWHM is found to increase with temperature, because of scattering by longitudinal optical phonons [16]. In general, the luminescence line shape is a convolution of an inhomogeneous part and a temperature-dependent homogeneous part. The inhomogeneous linewidth in the strained quantum well structure is mainly due to the interface roughness and random alloy disorder. Nevertheless, the minimum PL linewidths for BSQWs structures may be limited by the presence of atomic-scale clustering at the hetero-interface, as observed in MBE grown structures [17]. The nanoscale compositions within the InAsP alloy layers lead to an asymmetry in interface quality. Because of the symmetrical PL line shape and no impurity transition involved in the low-energy side of the lowtemperature luminescence spectra, the inhomogeneous broadening is not taken into account. As the temperature increases, scattering by longitudinal optical phonons becomes dominant due to the increasing phonon population, which gives rise to the homogeneous part of the linewidth broadening. However, the broadening in PL spectra significantly reduced for InAs0.04P0.96/InP BSQWs structure, due to the suppression of roughness in the interface. The FWHM is 15 and 51 meV at 10 K and 300 K, compared with the 24.6 and 50.5 meV at the 10 K and 296 K of five period InAs0.5P0.5/InP, respectively [13]; we got that a high crystalline quality 20 periods InAs0.04P0.96/InP BSQWs structures successfully grown by MOCVD which agrees with Fig. 1.
W. Yan et al. / Optics Communications 285 (2012) 4759–4762
Reflection
0.15
0.10
0.05
0.00 890
895
900 905 910 Wavelength (nm)
5 4.5 4 3.5 3 2.5 2 1.5 1
HWHM Total Linewidth γ
10 20 30 60
915
4761
experiment simulation 0
10
20 30 40 50 60 Number of quantum wells
70
Fig. 4. (a) Experimental reflection spectra of BSQWs, 10, 20, 30 and 60 periods at 10 K and (b) square dot represents experimental HWHM line width for samples and the solid line is generated from simulation for the BSQWs samples.
x 10-7
0.05
9.05 W avelength (m )
Reflection
0.10
20k 77k 92k 136k 200k
9.04 9.03 9.02 9.01 9
890 895 900 905 910 915 920 Wavelength(nm)
20
40 60 80 100 120 140 Temperature (k)
Fig. 5. (a) Reflection of 20-period BSQWs at various temperatures and (b) the red solid line represents Bragg wavelength and the dashed line represents exciton wavelength. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
From Fig. 4 we can get the principal experimental results that the reflectivity and linewidth increase with N at Bragg resonance (d¼ l/2), the so-called superradiant response. Fig. 4(b) conclusively shows that experimentally the linewidth in reflection for the collective system does indeed increase linearly with N; in accordance with the linear dispersion theory [10], the total damping rate
g ¼ G þN G0
ð2Þ
where G is the nonradiative exciton damping rate of a single quantum well, and G0 is the radiative damping rate of a single quantum well. The total damping rate is N times enhanced compared to radiative damping rate of a single quantum well. Thus Eq. (2) allows a convenient way to extract the radiative linewidth from the slope of linewidth versus N and the remaining contributions to the linewidth from the intercept. The fitted G and G0 are 0.053 meV and 1.054 meV, respectively. G0 can also be determined by precision reflection measurements from a single quantum well. The variation in the quantum wells of BSQWs sample is expected to have very little effect on G0, since it depends mostly on the well material and thickness. The reflection spectra of 20 period InAsP/InP BSQWs structure at various temperatures from 10 K to 200 K are shown in Fig. 5(a). We can see that all the peaks positions shift to longer wavelength and FWHM broadens with temperature. We can also see that if the temperature drifts from 10 K (near-Bragg resonance), the reflectivity peak will begin to collapse, and the higher the temperature, the more serious the collapse. Furthermore, the reflectivity peak almost vanishes at 200 K. The above temperature-dependent behaviour of the reflection spectra has been ascribed to free exciton recombination and the scattering by
longitudinal optical phonons. When the temperature is above 200 K, the exciton binding energy is destructed, so the reflection spectra vanish. The above data suggest that if the Bragg resonance condition is destructed, the reflection spectra will split. So we can control the Bragg resonance by temperature. And then, we will present a theoretical analysis of temperature-dependence of band structure in BSQWs, for Bragg resonance and near-Bragg resonance. Fig. 5(b) shows the temperature versus Bragg wavelength and exciton wavelength. From the figure we can know that the Bragg wavelength equals the exciton wavelength at 10 K. Some letters [4,8–11] reported that in the case when Bragg wavelength equals the heavy hole exciton wavelength, the photonic band structure forms a forbidden gap (and an associated reflection stopgap) with both resonance frequencies located within the gap. Fig. 6 demonstrates the photonic band structure of the model BMQWs at 10 K with the plane waves expansion method [18]; from Fig. 6(b) we can see that there is a stopgap at almost 900 nm, which agrees with the experiment of reflection spectra at 10 K. When the temperature increases to 77 K, from Fig. 5(b) we can know that the Bragg wavelength is longer than the exciton wavelength (near-Bragg resonance). Yang et al. [12] reported that if oBrg a ox, there will be a transmission window in the reflectivity stop band. The presence of two characteristic frequencies breaks the photonic band structure into three bands, as illustrated in Fig. 7; it agrees with Fig. 5 at 77 K. The intermediate transmission band has been shown to have the analytic form cosðKaÞ ¼ cosðqaÞ þ
G
o
ðo þ igÞox ox
sinðqaÞ
ð3Þ
W. Yan et al. / Optics Communications 285 (2012) 4759–4762
2.5
1.44
2
1.42 Energy (ev)
Energy (ev)
4762
1.5 1 0.5
1.40 1.38 1.36 1.34
-3
.0 4
.0 2 -3
8
.0 6
.0
-3
-3
Re (k)*a
-3
3
.1
2
-3
1
-3
0
.1
-1
.1 4
-2
-3
-3
2
0
Re (k)*a
Fig. 6. (a) Photonic band structure of the model BMQWs at 10 K and (b) expanded scale of left red ring. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1.44 1.42
2
Energy (ev)
Energy (ev)
2.5
1.5 1 0.5
1.4 1.38 1.36 1.34
-3
-3
.0
2
4
6
.0 -3
.0
8
-3
.0 -3
.1 -3
3
2
2
.1
0 1 Re (k)*a
-3
-1
.1
-2
-3
-3
4
0
Re (k)*a Fig. 7. (a) Photonic band structure of the model BMQWs at 77 K, where an intermediate band is formed and (b) the intermediate band is shown in (a) on an expanded scale.
where o(k) is the polariton angular frequency (wave vector), ox is the 1s-heavy-hole quantum well excitonic resonance frequency, c the speed of light in vacuum, q¼nbo/c (nb is the background index), a is the periodic spacing of the quantum wells, and G(g) the radiative decay (dephasing) rate. According to the above conclusion we propose a new scheme of using the temperature to control the transmission window opening and closing. Based on this fact, we devise the following procedure. (1) First set the temperature off 10 K and d a l/2, opening a transmission window for the incident light to propagate into the BMQWs. (2) We then tune the temperature back to 10 K to close the transmission window and stop the light inside the BMQWs at the same time. (3) The temperature is then reset to its original value, reopening the transmission window and releasing the light. The scheme can be used not only in slow light devices, but also in optical switch devices.
4. Conclusion We have demonstrated a growth condition to grow high quality, long-period InAs0.04P0.96/InP Bragg-spaced quantum wells by MOCVD, and studied the influence of temperature on the PL and reflection spectra. We found that the peaks of PL shift to longer wavelength and the FWHM broaden with temperature, and at Bragg resonance, the intensity and linewidth of reflection increase with N. By researching the reflection spectra at different temperatures, we presented a theoretical analysis of the changes in band structure for resonant and near-resonant cases, and proposed a new scheme of using the temperature to control the Bragg resonance.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant no. 60877040), and National Basic Research program of China (Grant. 2010CB923204). References [1] C.Y. Lee, M.C. Wu, H.P. Shiao, T.T. Shi, W.J. Ho, Solid-State Electronics 43 (1999) 2141. [2] F. Carreno, M.A. Anton, Jornal of Physics B: Atomic Molecular and Optical Physics 42 (2009) 21550801. [3] T. Wang, G. Li, Z. Chen, Optics Express 16 (2008) 127. [4] T. Wang, Q. Li, D.S. Gao, Chinese Science Bulletin 54 (2009) 3663. [5] M. Yildirim, J.P. Prineas, Journal of Applied Physics 98 (2005) 0635061. [6] C. Monier, I. Serdiukova, L. Aguilar, F. Newman, M.F. Vilela, A. Frenudlich, Journal of Vacuum Science and Technology B 17 (1999) 1158. [7] R.J. Farley, S.K. Haywood, V.A. Hewer, J.H.C. Hogg, Proceedings of the 10th Inernational Conference on Indium Phosphide and Related Materials, vol. 13, 1998 p. 564. [8] L.I. Deych, A.A. Lisyansky, Physical Review B 62 (2000) 4242. [9] M. Hubner, J.P. Prineas, C. Ell, P. Brick, E.S. Lee, G. Khitrova, H.M. Gibbs, S.W. Koch, Physical Review Letters 83 (1999) 2841. [10] J.P. Prineas, C. Ell, E.S. Lee, G. Khitrova, H. Gibbs, S.W. Koch, Physical Review B 61 (2000) 13863. [11] J.P. Prneas, C. Cao, M. Yildirim, W. Johnston, M. Reddy, Journal of Applied Physics 100 (2006) 0631011. [12] Z.S. Yang, N.H. Kwong, R. Binder, Journal of the Optical Society of America B; Optical Physics 22 (2005) 2145. [13] C.Y. Lee, M.C. Wu, H.P. Shiao, W.J. Ho, Journal of Crystal Growth 208 (2000) 137. [14] Y.G. Zhao, Y.H. Zou, X.L. Huang, J.J. Wang, Y.D. Qin, Journal of Applied Physics 8 (1998) 4430. [15] Y.P. Varshni, Physica 34 (1967) 149–152. [16] W.Z. Shen, Y. Chang, S.C. Shen, W.G. Tang, Y. Zhao, A.Z. Li, Journal of Applied Physics 79 (1996) 2139. [17] S.L. Zuo, W.G. Bi, C.W. Tu, E.T. Yu, Applied Physics Letters 72 (1998) 2135. [18] W.T. Rhodes, Photonic Crystals. Atlanta, 2009.