InAlGaAs quantum dots

Thin Solid Films 541 (2013) 68–71

Contents lists available at SciVerse ScienceDirect

Thin Solid Films journal homepage: www.elsevier.com/locate/tsf

Bimodal luminescence behavior of spatially-ordered seven-stacked InAs/InAlGaAs quantum dots Jae Won Oh a, Mee-Yi Ryu a,⁎, Byounggu Jo b, Jin Soo Kim b, T.R. Harris c, Yung Kee Yeo c a b c

Department of Physics, Kangwon National University, Chuncheon 200-701, Republic of Korea Division of Advanced Materials Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea Department of Engineering Physics, Air Force Institute of Technology, Wright-Patterson AFB, OH 45433, USA

a r t i c l e

i n f o

Available online 17 September 2012 Keywords: Quantum dot Optical property Time-resolved photoluminescence

a b s t r a c t Seven-stacked InAs/InAlGaAs quantum dots (QDs) grown on InP substrates have been studied using photoluminescence (PL) and time-resolved PL techniques, and the results show a bimodal luminescence behavior. The seven-stacked QD sample shows a redshifted PL emission energy, a broad PL linewidth, and a strong temperature dependent PL compared with the single layer QD sample. The decrease of the PL decay time with increasing emission wavelength has been observed in the seven-stacked sample, but not in the single layer sample. All of these results are attributed to the bimodal size distribution of QDs in the seven-stacked sample, which originates from different sizes of spatially ordered QDs formed on different layers. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor quantum dot (QD) structures have been extensively studied for the potential applications in lasers, solar cells, single photon emitters, photodetectors, etc. Numerous studies have been performed for producing QD arrays with high uniformity, ordering and positioning [1–3]. In order to accomplish vertically ordered as well as laterally ordered QD arrays, many researchers have focused on stacked structures of self-assembled QDs [4–10]. QDs grown on an upper layer, which is strain-coupled to a bottom layer of dots, align with the dots at the bottom layer, and typically show a narrower QD size distribution. Heitz et al. [4] reported the improved shape uniformity for stacked InAs/GaAs QDs, which is attributed to the contribution of the buried QDs to the surface strain, altering the QD formation kinetics and energetics. It has been reported [5] that the fundamental transition energy for the vertically ordered QDs in multilayer InAs/GaAs structures is dependent on both the interlayer separation and the number of QD layers. Colocci et al. [6] showed that the radiative lifetime of selfassembled QDs can be tuned by controlling the electronic coupling among electrons and holes through the appropriate choice of the number of stacked layers and the thickness of the space layer. However, the optical properties of multilayer InAs/InAlGaAs QDs on InP have not been systematically investigated yet. In this paper, the luminescence properties and recombination dynamics of single layer and seven-stacked layers of self-assembled InAs/InAlGaAs QDs grown on InP substrates have been investigated as functions of temperature and emission wavelength using

⁎ Corresponding author. Tel.: +82 250 8474; fax: +82 257 9689. E-mail address: [email protected] (M.-Y. Ryu). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.09.023

photoluminescence (PL) and time-resolved PL (TRPL) measurements, and the results are reported. 2. Experimental details The InAs/InAlGaAs QD samples were grown on n-type InP (001) substrates by a V80 molecular beam epitaxy system. Single layer (QD1) and seven-stacked layers (QD2) of InAs/InAlGaAs QDs were grown at 480 °C by the conventional Stranski–Krastanov growth mode after depositing an InAlGaAs buffer layer at 510 °C. Each layer for the seven stacks of InAs/InAlGaAs QDs was separated by 29.5-nm-thick InAlGaAs spacers grown at 480 °C. For the PL and TRPL measurements, an InAlGaAs capping layer was deposited on the InAs QDs at 480 °C. PL and TRPL measurements were carried out at temperatures ranging from 10 to 300 K. The PL measurements were made using an 830 nm radiation from a Ti:Sapphire laser which was pumped by an Ar-ion laser at 514 nm. The PL signals were dispersed with 0.50-m spectrometer and collected with an extended InGaAs detector. TRPL measurements were performed using an Edinburgh Instruments FLS 920 spectrometer. The InAs QD samples for TRPL were excited with a picosecond pulsed diode laser (λ = 635 nm, pulse width = 93 ps), and the signal was collected with a near-infrared photomultiplier tube (NIR-PMT). The luminescence decays were measured using a timecorrelated signal photon counting system. 3. Results and discussion Fig. 1 shows the PL spectra of the InAs/InAlGaAs QD samples measured at 10 and 300 K. The PL spectra of the single layer InAs QDs (QD1) measured at 10 and 300 K show one dominant peak at 1352

J.W. Oh et al. / Thin Solid Films 541 (2013) 68–71

PL Intensity (arb. units)

(a) QD1

1400 10 K

1600

1800

2000

300 K

x 70

(b) QD2

10 K

300 K

A

x 65

B 1200

1400

1600

1800

2000

Wavelength (nm) Fig. 1. PL spectra of (a) sample QD1 and (b) sample QD2 measured at 10 and 300 K. The Gaussian fitted peaks are also displayed in (b).

and 1500 nm with a full width at half maximum (FWHM) of 85 and 111 meV, respectively, at each temperature and a weak shoulder peak on the low-energy side. For the seven-stacked layers of InAs QDs (QD2), the PL spectrum measured at 10 K exhibits a strong asymmetric shape peak and can be fitted with two Gaussian peaks (peak A and peak B) as shown in Fig. 1(b). The peak positions of the deconvoluted Gaussian peaks A and B are 1365 nm and 1548 nm, and their FWHM is 110 and 86 meV, respectively. As the temperature increases to 300 K, peak B becomes dominant and redshifts to 1694 nm

PL Intensity (arb. units)

(a)

QD1

10 K

having a FWHM of 77 meV as shown clearly in Fig. 4. The main PL peaks of QD2 are observed at longer wavelengths (lower energies) compared to those of QD1 at both 10 and 300 K. This result of redshift can be explained by the larger QD size variation of sample QD2 as compared with sample QD1. The formation of InAs/InAlGaAs QDs was investigated by crosssectional transmission electron microscopy (TEM) (not shown here) [10]. The lateral sizes (heights) of QDs in sample QD2 for the first, second, and third layers were measured to be 17.7 ±4.0, 20.8± 5.2, and 25.8 ±3.7 nm (2.8 ±0.4, 3.9± 0.4, and 4.7 ± 0.6 nm), respectively. Although the sizes of the QDs increased rapidly from the first to the third layer, the rate of increase in QD sizes significantly reduced in the upper layers beyond the third layer. For example, the lateral sizes (heights) of QDs in the seventh layer increased only slightly to 29.2± 3.7 (5.9 ±0.4) from 26.1± 2.13 nm (5.5 ± 0.7 nm) for the fourth layer. The lateral sizes and heights of QDs in sample QD1 were similar to those of the first layer of sample QD2 because the growth conditions for QD1 and QD2 were exactly the same except the added number of QD layers in sample QD2. Initially, the average QD size for the subsequent upper layers up to several QD layers increased because of the strain accumulation due to the influence of the lower underlying QD layers. However, the average QD size for the subsequent upper layers was nearly saturated with increasing number of stacked QD layers as explained by the intrinsic phase separation of InAlGaAs barrier [10–12]. Without this effect, the average QD size for the subsequent upper layers would continuously increase due to continued strain accumulation as the number of QD layers increases. Considering the TEM results and the relatively strong PL intensity of peak A (high-energy side peak in Fig. 1(b)), it is believed that the asymmetric shape of the PL spectrum of sample QD2 is mainly caused by inhomogeneous broadening due to QDs with two different sizes. In order to verify the origin of the double peak features in the PL spectrum for sample QD2, laser excitation power-dependent PL measurements were also carried out. As shown in Fig. 2, the shapes of PL spectra of QD1 and QD2 are not significantly changed with increasing laser excitation power from 5 to 300 mW, and overall PL intensities are enhanced almost linearly for both samples. The intensity of the high-energy peak A remains stronger than that of the low-energy peak B even at low excitation power, and their relative intensity is nearly the same over the entire excitation power range from 5 to 300 mW.

(b)

10 K

QD2

300 mW

300 mW

250 mW

250 mW 200 mW

200 mW 150 mW 100 mW 75 mW 50 mW 30 mW 20 mW 10 mW 5 mW

x 1.5 x2 x3 x6 x 15 1200

1400

1600

Wavelength (nm)

1800

x2 x3 x4 x6

150 mW 100 mW 75 mW 50 mW 30 mW 20 mW

x 10 x 10

10 mW 5 mW

x 1.5

1200

1400

1600

PL Intensity (arb. units)

1200

69

1800

Wavelength (nm)

Fig. 2. Excitation power-dependent PL spectra of (a) sample QD1 and (b) sample QD2 measured at 10 K. All spectra are shifted vertically for clarity.

J.W. Oh et al. / Thin Solid Films 541 (2013) 68–71

QD2, peak A

0.92

0.88

0.84

0.80

0.76

0.72 0

50

100

150

200

250

300

Temperature (K) Fig. 3. Temperature dependence of the PL peak energies of sample QD2. The solid curves are the calculated transition energies using Varshni's equation with the parameters of bulk InAs.

Seven-stacked InAs QDs

10 K

τ1 at 10 K

125 K

τ1 at 125 K

3.0 2.5 2.0 1.5

300 K 0.5 0.0 1100

1250

1400

1550

225 K

x5

2.4 150 K

80 K A

(b) QD2

10 K

1700

1850

τ at 10 K 1

125 K

τ at 125 K 1

2.0 1.6 1.2 0.8

Normalized PL Intensity

PL Intensity (arb. units)

(a) QD1

3.5

1.0

B

A

x 50

4.0

Decay Time τ1(ns)

These results strongly indicate that the two peaks of QD2 do not stem from the excited and ground states of QDs, but are instead attributed to the bimodal size distribution of QDs in the seven-stacked layers. The temperature dependence of the PL peak energies of sample QD2 is shown in Fig. 3. The energies of the high-energy peak (peak A) show a sigmoidal temperature dependence. The solid lines in Fig. 3 are calculated transition energies using Varshni's equation [13] with the parameters for bulk InAs, and are vertically shifted to match the peak energies at 10 and 300 K. The energies of peak A follow well the calculated energy curves for temperatures lower than 50 K and higher than 200 K, while at intermediate temperatures (50 − 200 K), the peak energies decrease much faster with increasing temperature than

Decay Time τ1 (ns)

PL Peak Energy (eV)

QD2, peak B

expected. The low-energy peak (peak B) of QD2, however, shows different temperature dependences as shown in Fig. 3. The energy of peak B is blueshifted from 0.801 to 0.832 eV as temperature is increased from 10 to 125 K, and then the peak energy is constantly redshifted to 0.732 eV with increasing temperature up to 300 K. It is believed that the blueshift of the peak B with temperature up to 125 K is due to a significant thermal activation of carriers localized at relatively small QDs and simultaneously recapture of carriers by relatively large QDs located in upper layers, resulting in the increased PL intensity of peak B as described below. Fig. 4 shows the temperature-dependent PL spectra of sample QD2 for selected temperatures. Each spectrum shows an asymmetric shape, which can be deconvoluted into two Gaussian peaks A and B. The peaks A and B are assigned to the emission from the smaller QDs and the larger QDs, respectively. The PL intensity of the high-energy peak A is dominant up to 125 K, but as temperature increases further, the PL intensity of the low-energy peak B becomes gradually dominant up to 300 K as shown in Fig. 4. This phenomenon can be interpreted by the redistribution of carriers between different size dots as described above. With increasing temperature carriers are thermally excited from the small dots outside into the wetting layer and/or barrier and then preferentially relax into large dots [14]. Fig. 5 shows the PL decay times and the PL spectra of samples QD1 and QD2 measured as a function of emission wavelength at 10 and 125 K. The PL decay time of QD1 at 10 K (125 K) increases from 0.99 ns (0.90 ns) to 1.50 ns (3.36 ns) as emission wavelength increases from 1216 nm (1300 nm) to 1306 nm (1476 nm), and then starts to decrease to 0.95 ns (2.20 ns) as the emission wavelength increases further up to 1526 nm (1600 nm). As shown in Fig. 5(a), the decay

Normalized PL Intensity

70

0.4 10 K

B 1200

1400

0.0 1100

1600

1800

2000

1250

1400

1550

1700

1850

Wavelength (nm)

Wavelength (nm) Fig. 4. Temperature-dependent PL spectra of sample QD2. Each of them is deconvoluted into two Gaussian peaks.

Fig. 5. PL decay times (circles and triangles) of (a) sample QD1 and (b) sample QD2 as a function of emission wavelength taken at 10 and 300 K. The PL spectra measured at 10 and 300 K are also displayed.

J.W. Oh et al. / Thin Solid Films 541 (2013) 68–71

time is the longest around the PL peak, which can be interpreted as being due to the increased carrier migration from higher energy states (smaller dots) and/or wetting layer. At 125 K, the decay time of QD1 exhibits a similar dependence on the emission wavelength as for the 10 K, but it shows much longer decay times for longer wavelengths greater than 1360 nm as shown in Fig. 5(a). The increase of the decay time at 125 K compared with 10 K is attributed to the increased radiative lifetime, which typically increases with temperature [15–18]. The decay times of both samples taken at the PL peaks increase with temperature from 10 to 140 K, and then start to decrease as the temperature increases further up to 300 K (not shown here), as usual, due to increased thermal activation of nonradiative processes. Although the wavelength dependence of the decay time of QD2 at 125 K as shown in Fig. 5(b) shows a similar trend as that of QD1 as shown in Fig. 5(a), the decay time of QD2 measured at 10 K shows an entirely different decay behavior. The decay time of QD2 at 10 K decreases steadily from 1.83 to 1.22 ns as the emission wavelength increases from 1130 to 1600 nm. This behavior can be explained by the improved carrier confinement and enhanced wave function overlap due to increased QD size and improved aspect ratio with increasing number of stacked quantum dot layers. 4. Conclusion The luminescence properties of single layer (QD1) and sevenstacked layers (QD2) of InAs/InAlGaAs QDs grown on InP substrates have been investigated using PL and TRPL measurements. The PL spectrum of sample QD2 presents an asymmetric shape and has been deconvoluted into two Gaussian peaks. It is believed that the two peaks of QD2 are due to the bimodal size distribution of QDs. The PL peak of QD2 is observed at longer wavelength with a broader linewidth compared to that of QD1, which could be due to a larger size variation in sample QD2 with increasing stacked QD layers. QD1 exhibits the longest decay time around the PL peak due to the increased carrier migration from higher energy states (smaller dots) and/or wetting layer. The decay time of QD2 measured at 10 K decreases steadily with increasing emission wavelength, which might

71

be due to the improved carrier confinement and the enhanced wave function overlap due to increased QD size and improved QD shape with increasing number of QD layers.

Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0073699 and 2010-0021555). Time-resolved photoluminescence measurements were performed at the Central Lab of Kangwon National University.

References [1] In: D. Bimberg, M. Grundmann, N.N. Ledentsov (Eds.), Quantum Dot Heterostructures, John Wiley & Sons, Inc., 1999. [2] In: O. Schmidt (Ed.), Lateral Alignment of Epitaxial Quantum Dots, Springer, Berlin, 2008. [3] H. Lan, Y. Ding, Nano Today 7 (2012) 94. [4] R. Heitz, A. Kalburge, Q. Xie, M. Grundmann, P. Chen, A. Hoffmann, A. Madhukar, D. Bimberg, Phys. Rev. B 57 (1998) 9050. [5] S. Taddei, M. Colocci, A. Vinattieri, F. Bogani, S. Franchi, P. Frigeri, L. Lazzarini, G. Salviati, Phys. Rev. B 62 (2000) 10220. [6] M. Colocci, A. Vinattieri, L. Lippi, F. Bogani, M. Rosa-Clot, S. Taddei, A. Bosacchi, S. Franchi, P. Frigeri, Appl. Phys. Lett. 74 (1999) 564. [7] S.Y. Shah, N. Halder, S. Sengupta, S. Chakrabarti, Mater. Res. Bull. 47 (2012) 130. [8] Z. Mi, P. Bhattacharya, J. Appl. Phys. 98 (2005) 023510. [9] G.S. Solomon, J.A. Trezza, A.F. Marshall, J.S. Harris Jr., Phys. Rev. Lett. 76 (1996) 952. [10] K.J. Lee, B. Jo, C.-R. Lee, I.-H. Lee, J.S. Kim, D.K. Oh, J.S. Kim, S.J. Lee, S.K. Noh, J.-Y. Leem, M.-Y. Ryu, J. Appl. Phys. 109 (2011) 113505. [11] J.S. Kim, C.-R. Lee, B.S. Choi, H.-S. Kwack, C.W. Lee, E.D. Sim, D.K. Oh, Appl. Phys. Lett. 90 (2007) 153111. [12] J.S. Kim, J.H. Lee, S.U. Hong, W.S. Han, H.-S. Kwack, C.W. Lee, D.K. Oh, IEEE Photon. Technol. Lett. 16 (2004) 1607. [13] Y.P. Varshni, Physica 34 (1967) 149. [14] Y.C. Zhang, C.J. Huang, F.Q. Liu, B. Xu, J. Wu, Y.H. Chen, D. Ding, W.H. Jiang, X.L. Ye, Z.G. Wang, J. Appl. Phys. 90 (2001) 1973. [15] W. Yang, R.R. Lowe-Webb, H. Lee, P.C. Sercel, Phys. Rev. B 56 (1997) 13314. [16] H. Gotoh, H. Ando, T. Takagahara, J. Appl. Phys. 81 (1997) 1785. [17] H.J. Lee, M.-Y. Ryu, J.S. Kim, J. Appl. Phys. 108 (2010) 093521. [18] H.Y. Kim, M.-Y. Ryu, J.S. Kim, J. Lumin. 132 (2012) 1759.