ZnTe quantum dots

ZnTe quantum dots

Journal of Alloys and Compounds 735 (2018) 2119e2122 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: htt...

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Journal of Alloys and Compounds 735 (2018) 2119e2122

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Thermal escape and carrier dynamics in multilayer CdTe/ZnTe quantum dots Sung Hwan Jin a, Su Hwan Kim a, Minh Tan Man b, Jin Chul Choi a, Hong Seok Lee b, * a b

Department of Physics, Yonsei University, Wonju, 26493, Republic of Korea Department of Physics, Research Institute Physics and Chemistry, Chonbuk National University, Jeonju, 54896, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 September 2017 Received in revised form 21 November 2017 Accepted 29 November 2017 Available online 1 December 2017

We present an optical spectroscopy study of multilayer CdTe/ZnTe quantum dots by photoluminescence (PL) and time-resolved PL. We demonstrate that the PL blueshift and enhanced PL intensity were due to an increase in the material intermixing and the density of the emitting states. An integrated PL as a function of temperature revealed that the separated energy of two different states was 4.5e6.2 meV and was nearly independent of the stack period. The main thermal escape was assisted by multi-longitudinal optical phonons at high temperature with activation energies in the range 48e54 meV. © 2017 Elsevier B.V. All rights reserved.

Keywords: Multilayer quantum dots Cadmium telluride Thermal escape Carriers dynamics

1. Introduction Semiconductor quantum dots (QDs) have been intensively studied for both fundamental physics and optoelectronic device applications [1e5]. In particular, QDs have been applied to laser application fields because they have a lower threshold current density, a higher differential gain, and a higher characteristic temperature than quantum well (QW) lasers [6e8]. However, technical problems, such as the enhancement of thermal stability, carrier collection ability, and the QDs size uniformity control, must be solved in order to apply QDs to a laser device. To overcome these problems, various structures have been demonstrated, such as coupled QD and QW structures, vertically stacked QD structures, and QD-in-QW structures [9e11]. In particular, multilayer QD structures allow the enhancement of the QD density in the laser active region and a reduction of the dot size distributions [12]. Furthermore, the multilayer QD structures facilitate improved tunneling of carriers into the QDs through a thin separation barrier [13]. Carrier dynamics of QDs are very important, not only for the understanding of the fundamental physics of zero-dimensional structures, but also for improving the photoluminescence (PL) efficiency and performance of optoelectronic devices [14,15]. II-VI

* Corresponding author. E-mail address: [email protected] (H.S. Lee). https://doi.org/10.1016/j.jallcom.2017.11.374 0925-8388/© 2017 Elsevier B.V. All rights reserved.

QDs have significant advantages due to their large band gaps and high exciton-binding energy [16]. Among these II-VI QDs, CdTe QDs are of great interest in optoelectronic devices operating in the green spectral range because of their optical properties, such as a large energy band gap and direct excitonic transitions [17]. In this work, we investigated the optical properties of multilayer CdTe/ZnTe QDs on GaAs substrates with different periods of stacks grown by molecular beam epitaxy (MBE) and atomic layer epitaxy (ALE). The PL measurements were carried out to study the characteristics of interband transitions in the multilayer CdTe/ZnTe QDs with various periods of stacks. The temperature-dependent and time-resolved PL measurements were performed to study the thermal escape and carrier dynamics of the multilayer CdTe/ZnTe QDs as a function of stack period. 2. Experimental details Several multilayer CdTe/ZnTe QDs with different periods of stacks were grown on GaAs (100) by using MBE and ALE procedures (Fig. 1). The GaAs substrates were degreased in warm trichloroethylene, cleaned in acetone and then in a Br-methanol solution, and thoroughly rinsed in de-ionized water. After chemical etching, the substrates were blown by nitrogen. The substrates were mounted on a molybdenum susceptor and thermally cleaned at 580  C for 5 min. First, a 900 nm ZnTe buffer layer was grown on the GaAs substrate using MBE. Then, a 4.5 monolayer (ML) CdTe with

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monochromator using a multichannel plate photomultiplier tube. A 405-nm wavelength, picosecond pulsing laser diode with an 80 MHz repetition rate was used as an excitation source. The temperature-dependent PL spectra were measured using a helium closed-cycle Displex refrigerator with a temperature range of 25e110 K. The time-resolved PL decay curves were measured using a time-correlated single photon counting (TCSPC) method. A commercially available TCSPC module (PicoHarp, PicoQuant GmbH, Berlin, Germany) was used to obtain the PL decay curves. The full width at half maximum of the total instrument response function (IRF) was less than 130 ps.

3. Results and discussion

Fig. 1. Schematic diagram of multilayer CdTe/ZnTe QDs on GaAs substrates with different periods of stacks.

different periods of stacks (N ¼ 1, 3, and 7) were grown using ALE. The CdTe QDs were separated by a 15 nm ZnTe spacer layer grown using MBE. The CdTe QDs were then capped with a 100 nm ZnTe layer grown using MBE. The ZnTe and CdTe layers were deposited at a substrate temperature of 310  C. The Zn and Te source temperatures used for the deposition of the ZnTe layer were 280 and 300  C, respectively; the Cd and Te source temperatures used for the deposition of the CdTe layer were 195 and 300  C, respectively. For the optimized ALE, a Cd effusion cell was opened for 8 s and interrupted for 1 s. Thereafter, a Te effusion cell was opened for 8 s and interrupted for 5 s. These interruptions stabilized the positive and negative ions on the surface, improving the thin film quality. The PL measurements were performed using a 150 mm

Fig. 2. (a) PL spectra at 25 K for the multilayer CdTe/ZnTe QDs with 1, 3, and 7 periods. (b) Peak position and intensity of the PL spectra for multilayer CdTe/ZnTe QDs as a function of the stack period.

Fig. 2(a) shows the PL spectra at 25 K for the multilayer CdTe/ ZnTe QDs on GaAs substrates with different periods of stacks. The main peaks corresponded to the exciton transition from the ground-state electronic subband to the ground-state heavy-hole band (E1-HH1) in the multilayer CdTe/ZnTe QDs with various periods of stacks. Fig. 2(b) shows the peak position and intensity of the E1-HH1 transition for the multilayer CdTe/ZnTe QDs as a function of the stack period. The peak position corresponding to the E1HH1 transition in the multilayer CdTe/ZnTe QDs shifted to a higher energy as the number of stacks was increased. This blueshift in the PL peak position of the multilayer CdTe/ZnTe QDs was due to an

Fig. 3. PL spectra at several temperatures for multilayer CdTe/ZnTe QDs with (a) 1, (b) 3, and (c) 7 periods.

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attributable to a decrease in the size of QDs [19]. The intensity of the E1-HH1 transition for multilayer CdTe/ZnTe QDs increased with increasing number of stacks. This behavior was attributed to an increase in the density of the emitting states with increasing number of stacks [20]. To explain the activated thermal energy that redistributes the carriers into the localization potentials, and the thermal escape of carriers assisted by multiple phonons at high temperature, Fig. 3 presents the temperature dependence of the PL spectra for the multilayer CdTe/ZnTe QDs with different periods of stacks on the GaAs substrates. All samples showed an anomaly in the emission shift and the quenching of the PL spectra with temperature, which was caused by the inhomogeneous distribution of the localized carriers to escape and diffuse to the trap. Commonly, tracking the integrated PL intensities as a function of temperature of the confined carriers in the stacked QDs reveals that the main process is related to the thermal escape process, which can be explained by the redistribution of carriers in the localization potentials [21]. As shown in Fig. 4(a), the thermal-induced integrated PL intensities can be properly modelled according to following equation [21],

IPL ðTÞ ¼

Fig. 4. (a) Integrated PL intensities for the multilayer CdTe/ZnTe QDs with 1, 3, and 7 periods as a function of inverse temperature. (b) Thermal activation energy and thermal escape energy of multilayer CdTe/ZnTe QDs as a function of the stack period.

increase in the material intermixing between the QDs and separation layers with an increasing number of stacks [18]. Furthermore, the PL peak position of the multilayer CdTe/ZnTe QDs shifts to higher energy with an increasing number of stacks might be

Ið0Þ 1þ

2aeDE=kB T

þ

3be2DE=kB T

þ c eELO =kB T  1

m

(1)

where a, b, and c are constants relating to the relative energy density of states. I(0) is the integrated PL intensity at 0 K, DE is the thermal activation energy of the low temperature quenching processes; m is the number of LO phonons involved in the thermal escape of carriers, and ELO is extracted from Eq. (1). The fitting parameters describing the low and high temperature quenching processes can all be reproduced by our model, and the experimental data were fitted to Eq. (1) by fixing the m values from ~2.3 to ~2.5 for the multilayer CdTe/ZnTe QDs with various periods of stacks. As shown in Fig. 4(b), we obtained the DE values of 4.5 ± 0.3, 5.5 ± 0.2, and 6.2 ± 0.2 meV for the multilayer CdTe/ZnTe QDs with 1, 3, and 7 periods, respectively, which were the energy separations

Fig. 5. Time-resolved PL spectra at 25 K for multilayer CdTe/ZnTe QDs with (a) 1, (b) 3, and (c) 7 periods. (d) Decay times of multilayer CdTe/ZnTe QDs as a function of the stack period.

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between the difference states at low temperature. This decrease appeared to be due to the increased energy splitting occurring in the smaller stacks and potential fluctuations on the surface of the multilayer CdTe/ZnTe QDs. The main non-radiative process at high temperatures was the thermal escape of carriers assisted by multiphonons with average LO energies, ELO, of 19.2, 20.8, and 21.6 meV for the multilayer CdTe/ZnTe QDs with 1, 3, and 7 periods, respectively. As shown in Fig. 4(b), the thermal escape energy, Eescape (¼m*ELO), increased from 48 ± 0.5 to 54 ± 0.7 meV as the stack period increased, which led to an increase in the energy that a carrier had to absorb to jump from one state to the following one. This result is in qualitative agreement with both the theoretical predictions and experimental studies performed on the CdTe QDs [21]. We performed time-resolved PL measurements to understand the carrier dynamics of the multilayer CdTe/ZnTe QDs on GaAs substrates with various periods of stacks. Fig. 5(a)-(c) shows the time-resolved PL spectra at 25 K for the multilayer CdTe/ZnTe QDs with 1, 3, and 7 periods, respectively. The PL decay curves display different decay behaviors depending on the number of stacked QDs layers. To determine the PL decay time, we carried out reconvolution fitting with the IRF for the time-resolved PL results, which fitted well with a biexponential function. The shortest decay time can be determined with reconvolution fitting is approximately a tenth of the of the IRF width [22]. As reported in the literature, the fast component can be attributed to the decay of bright excitons in the QDs and the slow component is related to a recombination of dark excitons [23]. Hereafter, we focus only on the fast decay rate. The decay times of the multilayer CdTe/ZnTe QDs as a function of stack period are shown in Fig. 5(d). The decay times of fast components, from the fitting results, are 303 ± 6, 326 ± 6, and 353 ± 6 ps for 1, 3, and 7 layers, respectively. The PL decay time of the multilayer QDs increased with increasing number of stacked dot layers. This behavior is attributed to the strong coupling effects among electrons and holes induced by strain in multilayer QD structures [24]. 4. Conclusions A detailed knowledge of the larger valence band offset, and the intermixing effects between QDs and separation layers in multilayer CdTe/ZnTe QDs on GaAs substrates is crucial for understanding carrier dynamics. The multilayer CdTe/ZnTe QDs with various periods of stacks were fabricated. We demonstrated that an increase in the material intermixing and the density of the emitting states caused a PL blueshift and enhanced PL intensity. As a result, the separated energy of two different states (4.5e6.2 meV), and thermal escape energy (48e54 meV) increased as the stack period increased. The experiment results help to improve the understanding of carrier dynamics and activation energies in multilayer CdTe/ZnTe QDs with different periods of stacks. Acknowledgment This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2015R1D1A3A01015595).

References [1] A.P. Alivisatos, Semiconductor clusters, nanocrystals and quantum dots, Science 271 (1996) 933e937. [2] L. Sun, J.J. Choi, D. Stachnik, A.C. Bartnik, B.R. Hyun, G.G. Malliaras, T. Hanrath, F.W. Wise, Bright infrared quantum-dot light-emitting diodes through interdot spacing control, Nat. Nanotechnol. 7 (2012) 369e373. [3] Y.C. Lien, J.M. Shieh, W.H. Huang, C.H. Tu, C. Wang, C.H. Shen, B.T. Dai, C.L. Pan, C. Hu, F.L. Yang, Fast programming metal-gate Si quantum dot nonvolatile memory using green nanosecond laser spike annealing, Appl. Phys. Lett. 100 (2012), 143501. [4] Y.-F. Lao, S. Wolde, A.G.U. Perera, Y.H. Zhang, T.M. Wang, H.C. Liu, J.O. Kim, T. Schuler-Sandy, Z.-B. Tian, S.S. Krishna, InAs/GaAs p-type quantum dot infrared photodetector with higher efficiency, Appl. Phys. Lett. 103 (2013), 241115. [5] C.-H.M. Chuang, P.R. Brown, V. Bulovíc, M.G. Bawendi, Improved performance and stability in quantum dot solar cells through band alignment engineering, Nat. Mater. 13 (2014) 796e801. [6] Y. Arakawa, H. Sakaki, Multidimensional quantum well laser and temperature dependence of its threshold current, Appl. Phys. Lett. 40 (1982) 939e941. [7] A. Moritz, R. Wirth, A. Hangleiter, A. Kurtenbach, K. Eberl, Optical gain and lasing in self-assembled InP/GaInP quantum dots, Appl. Phys. Lett. 69 (1996) 212e214. [8] S.S. Mikhrin, A.R. Kovsh, I.L. Krestnikov, A.V. Kozhukhov, D.A. Livshits, N.N. Ledentsov, Yu.M. Shernyakov, I.I. Novikov, M.V. Maxi- mov, V.M. Ustinov, Zh.I. Alferov, High power temperature-insensitive 1.3 mm InAs/InGaAs/GaAs quantum dot lasers, Semicond. Sci. Technol. 20 (2005) 340e342. [9] H.S. Lee, K.H. Lee, J.C. Choi, H.L. Park, T.W. Kim, D.C. Choo, Enhancement of the activation energy in coupled CdTe/ZnTe quantum dots and quantum-well structures with a ZnTe thin separation barrier, Appl. Phys. Lett. 81 (2002) 3750e3752. [10] J.S. Wang, G. Lin, R.S. Hsiao, C.S. Yang, C.M. Lai, C.Y. Liang, H.Y. Liu, T.T. Chen, Y.F. Chen, J.Y. Chi, J.F. Chen, Continuous-wave high-power (320 mW) single mode operation of electronic vertically coupled InAs/GaAs quantum dot narrow-ridge-waveguide lasers, Appl. Phys. B 81 (2005) 1097e1100. [11] W.I. Han, J.H. Lee, J.S. Ju, J.C. Choi, H.S. Lee, Carrier dynamics and activation energy of CdTe quantum dots in a CdxZn1-xTe quantum well, Appl. Phys. Lett. 99 (2011), 231908. [12] J. Tersoff, C. Teichert, M. Lagally, Self-organization in growth of quantum dot superlattices, Phys. Rev. Lett. 76 (1996) 1675e1678. [13] G.S. Solomon, J.A. Trezza, A.F. Marshall, J.S. Harris Jr., Vertically aligned and electronically coupled growth induced InAs Islands in GaAs, Phys. Rev. Lett. 76 (1996) 952e955.  , G. Visimberga, A. Salhi, M. De Vittorio, A. Passaseo, R. Cingolani, [14] G. Raino M. De Giorgi, Simultaneous filling of InAs quantum dot states from the GaAs barrier under non resonant excitation, Appl. Phys. Lett. 90 (2007), 111907. [15] M.T. Man, H.S. Lee, Carrier transfer and thermal escape in CdTe/ZnTe quantum dots, Opt. Express 22 (2014) 4115e4122. [16] N. Pelekanos, J. Ding, A.V. Nurmikko, H. Luo, N. Samarth, J.K. Furdyna, Quasitwo-dimensional excitons in (Zn,Cd)Se/ZnSe quantum wells: reduced excitoneLO-phonon coupling due to confinement effects, Phys. Rev. B 45 (1992) 6037e6042. [17] H.S. Lee, H.L. Park, T.W. Kim, Optical properties of CdTe/ZnTe quantum dots sandwiched between two quantum wells with ZnTe separation barriers, Appl. Phys. Lett. 89 (2006), 181929. [18] M.O. Lipinski, H. Schuler, O.G. Schmidt, K. Eberl, N.Y. Jin-Phillipp, Straininduced material intermixing of InAs quantum dots in GaAs, Appl. Phys. Lett. 77 (2000) 1789e1791. [19] C.Y. Lee, J.D. Song, J.M. Kim, K.S. Chang, Y.T. Lee, T.W. Kim, Dependence of the optical properties on the GaAs spacer thickness for vertically stacked InAs/ GaAs quantum dots, Mater. Res. Bull. 39 (2004) 135e139. [20] S. Sanguinetti, M. Padovani, M. Guirioli, E. Grilli, M. Guzzi, A. Vinattieri, M. Colocci, P.F.S. Frigeri, Carrier transfer and photoluminescence quenching in InAs/GaAs multilayer quantum dots, Appl. Phys. Lett. 77 (2000) 1307e1309. [21] M.T. Man, H.S. Lee, Discrete states and carrier-phonon scattering in quantum dot population dynamics, Sci. Rep. 5 (2015) 8267. [22] D.J.S. Birch, R.E. Imhof, Kinetic interpretation of fluorescence decays, Anal. Instrum. 14 (1985) 293e329. [23] I. Favero, G. Cassabois, C. Voisin, C. Delalande, Ph. Roussignol, R. Ferreira, rard, Fast exciton spin relaxation in single C. Couteau, J.P. Poizat, J.M. Ge quantum dots, Phys. Rev. B 71 (2005), 233304. [24] M. Colocci, A. Vinattieri, L. Lippi, F. Bogani, M. Rosa-Clot, S. Taddei, A. Bosacchi, S. Franchi, P. Frigeri, Controlled tuning of the radiative lifetime in InAs selfassembled quantum dots through vertical ordering, Appl. Phys. Lett. 74 (1999) 564e566.