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ARTICLE IN PRESS Physica E 26 (2005) 422–426 www.elsevier.com/locate/physe Carrier capture and relaxation of self-assembled ZnTe/ZnSe quantum dots prepared under Volmer–Weber and Stranski–Krastanow growth modes M.-E. Leea, Y.-C. Yehb, Y.-H. Chungb, C.-L. Wub, C.-S. Yangc, W.-C. Chouc, C.-T. Kuob, D.-J. Jangb, a Department of Physics, National Kaoshiung Normal University, Kaoshiung, 80264 Taiwan, ROC Department of Physics, National Sun Yat-sen University, 70 Lienhai Road, Kaoshiung, 80441 Taiwan, ROC c Department of Electrophysics, National Chiao-Tung University, Hsinchu, 30056 Taiwan, ROC b Available online 24 November 2004 Abstract The carrier capture and relaxation of type II ZnTe/ZnSe quantum dots have been investigated with ultrafast timeresolved photoluminescence upconversion. The carrier capture times were 7 and 38 ps for the Volmer–Weber mode and Stranski–Krastanow mode, respectively. We found that the carrier relaxation of QDs exhibits faster decay under the Volmer–Weber growth mode than under the Stranski–Krastanow growth mode. We attribute the difference of carrier relaxation to the wetting layer formed in the Stranski–Krastanow growth mode. r 2004 Elsevier B.V. All rights reserved. PACS: 78.47.+p; 73.63. b; 78.67. n Keywords: Type II quantum dots; Time-resolved photoluminescence; Carrier capture; Carrier relaxation; Auger process Semiconductor quantum dots (QDs) have been extensively studied for potential applications in optoelectronic devices and for their novel electronic and optical properties in 3-D confined nanostructures [1,2]. Carrier capture and relaxation are among the important properties that affect the device performance such as threshold current and Corresponding author. Fax: 886-75253709. E-mail address: [email protected] (D.-J. Jang). temperature stability. Recently, the impact of the wetting layer on the carrier capture and relaxation in QDs has been investigated. Several reports indicated that the wetting layer is important for carriers to relax excess energy easily through a continuum tail of wetting layer defect states [3]. The carrier dynamics in the wetting layer in relation to the capture into the QDs have also been investigated [4]. However, recently, a report found that the two-dimensional character of the 1386-9477/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2004.08.092 ARTICLE IN PRESS M.-E. Lee et al. / Physica E 26 (2005) 422–426 wetting layer is not relevant in determining the QD capture and relaxation [5]. While the details of the mechanism of carrier capture and carrier relaxation remain controversial and most of the studies were focused on type I QDs of III–V materials, the present work demonstrate that the wetting layer is significant to the carrier dynamics in type II QDs of II–VI materials. In order to identity the impact of wetting layers on the carrier capture and relaxation, we investigate the carrier capture and carrier relaxation of self-assembled ZnTe quantum dots grown in a ZnSe matrix by molecular beam epitaxy with two growth modes. One of the growth methods is the Stranski–Krastanow (SK) mode that a layer was grown first in two dimensions then followed by three-dimensional islands after a critical thickness. The other growth mode is the Volmer–Weber (VW) mode in which no two-dimensional layer was formed before three dimension islands were grown. The type-II QD structure, with electrons and holes confined in different spatial regions, also exhibits interesting physical properties, such as slower radiative lifetime, tunability of emission energy, and reduction of electron–hole interaction. While holes are confined in the ZnTe layer, electrons are localized in ZnSe barriers. We study the carrier capture and relaxation with ultrafast time-resolved photoluminescence (PL) that provides a temporal resolution of better than 300 fs. The high temporal resolution is essential to study the fast carriers captured by QDs and the PL decay at the first 20 ps after photoexcitation. The carrier capture times were determined from the time-resolved photoluminescence to be 7 and 38 ps for the VW mode and SK mode, respectively. We found that carrier relaxation of QDs in the VW growth mode exhibits faster decay than that of QDs in the SK growth mode due to the wetting layer in the SK mode providing a pathway for carriers to diffuse and migrate from large (small) to small (larger) QDs. Self-assembled ZnTe QDs were grown in a ZnSe matrix on the GaAs substrate by using a Riber 32P molecular beam epitaxy system with two growth modes of VW and SK. The VW mode of QDs with 2.9 MLs coverage were capped with a ZnSe thickness of 5 nm and the SK mode of QDs with 423 3.0 MLs coverage were capped with a ZnSe thickness of 50 nm. Atomic force microscopy (AFM) showed that two families with different sizes of diameters, large and small, of QDs were grown for both samples. The AFM also indicated that mainly small size QDs were grown for small ZnTe deposition (thickness of 1.3 MLs). The dependence of the sizes of QDs on ZnTe deposition thickness were also reported for InAs/GaAs QDs [6]. Large QDs with dot densities of 3.6 108 and 1.5 109 cm 2 for the VW mode and SK mode, respectively, were estimated by atomic force microscopy as well as the small QDs with dot densities of 4.0 109 and 1.0 1010 cm 2 for the VW mode and SK mode, respectively. The details and growth parameters of these two modes are given elsewhere [7,8]. To study the carrier capture and relaxation of two different growth mode of QDs, the all optical PL up-conversion spectroscopy [9,10] was used to measure the time-resolved photoluminescence of these samples. This upconversion technique, in contrast to a pump probe, covers a very wide spectral range of PL without the need of a tunable laser source. A beam with a 150 fs pulsewidth from a Kerr-lens modelocked Ti:sapphire laser at a repetition rate of 76 MHz was split into two beams. One of the two beams was frequency-doubled with a 4 betabarium-borate (BBO) nonlinear crystal and was used to illuminate the samples. For the present work, 100 mW of average power was focused on a 50 mm spot on the surface of the sample mounted in a low-vibration closed cycle cryostat kept at temperature of 35 K for the present study. The PL was collected and focused by a pair of off-axial parabolic mirrors onto another BBO nonlinear crystal, where the PL was mixed with the fundamental beam that was sent through an optical delay. The signal of the sum frequency was generated by angle-tuning the BBO crystal to the spectrum of interest according to the phasematching condition. The up-converted signal was filtered with a band-pass filter to remove any contribution that might come from either the fundamental or frequency-doubled beams. The signal was then dispersed by a monochromator of 30 cm focal length to enhance the spectral resolution before collection by a thermal-electrically ARTICLE IN PRESS 424 M.-E. Lee et al. / Physica E 26 (2005) 422–426 cooled GaAs photomultiplier. Eventually, we used a standard photon-counting instrument to analyze the signal. The time-integrated PL of the QDs photoexcited nonresonantly with the frequency-doubled light from the fundamental of the Ti:sapphire laser shows that the peaks of the PL for the VW mode and SK mode QDs are 2.33 and 2.22 eV, respectively. The difference of the peak energies of these two similar height QDs is the result of the wetting layer and larger QDs formed, whose energies are lower, in the SK mode. The width of 40 nm for the time-integrated PL of the VW mode and 30 nm for the SK mode are due to the size distribution of QDs. The time-resolved PL of both samples for energies at the barriers and the peaks of the timeintegrated PL are shown in Fig. 1. The PL intensity of the barrier for the VW mode QDs (Fig. 1(a)) increases rapidly with a time scale of about 0.5 ps and is followed by a decay with a time scale of 7 ps. The PL of the barrier for the SK mode QDs reveals a slower rise time of 3 ps and a decay time of 38 ps. The differences of the PL rise times for these two different growth modes may be due to the different thicknesses of the capping layers. The ratio of the rise time to the decay time is consistent for both samples, which may indicate Fig. 1. The time-resolved PL of ZnTe/ZnSe QDs grown by VW mode ( triangle down) and SK mode (circle) at energy of barrier (a) and at the peak of the time-integrated PL (b). The inset shows the time-resolved PL in the first 100 ps. The sample’s temperature is 35 K. The lines are for visual guidance and the vertical scale is linear. that only the thickness of the capping layer affects the increase in rise and decay times in the SK mode. Fig. 1(b) shows the time-resolved PL at peak energies of the time-integrated PL. For the VW QDs, we observe that the PL intensity increases with a risetime of 2 ps and the decay of the PL can be characterized bi-exponentially with two time constants: a fast decay time of 20 ps and a slower decay time of 1.6 ns. The figure also shows a much slower PL risetime of 15 ps and a PL lifetime of more than 4 ns for the SK growth mode. The times of carrier capture by QDs, determined from the lifetimes of the barriers, are 7 and 38 ps for the VW mode and SK mode, respectively. We attribute the fast PL decay of the VW mode within the first 20 ps after photoexcitation to the Auger processes due to the higher carrier density generated inside the dots [11]. As the carrier density decreases with delay time, the Auger process is less effective. Because of the nature of the type II QDs structure of ZnTe/ZnSe, holes were confined in the ZnTe dots and electrons are localized in the ZnSe layer around the dots. The spatial separation of the wave functions of holes and electrons accounts for the slow PL decay time of 1.6 ns for the VW mode. In comparison, we found that for the SK mode the PL decay time is more than 4 ns, which is about 3 times slower than that for the VW mode. We attribute this slow decay, in addition to the spatial separation of wave function of holes and electrons, to the wetting layer formed in this structure. The carriers generated from the capping layer by photoexcitation are funneled into the wetting layer before they are captured by QDs. Once the carriers reach the wetting layer, either from barriers or QDs, they will not escape easily to the barrier as a result of the confinement by the barrier, whose band gap energy is larger than that of the wetting layers. This could also partially explain the slow PL rise time of the SK mode compared to that of the VW mode in Fig. 1(b). In contrast, the carriers in the QDs of the VW mode are not confined before they are captured by the QDs and, thus, the probability of escaping from the QDs increases. Therefore, the wetting layer in the SK mode plays an important role in carrier capture and relaxation. The slow decay lifetime for both samples are ARTICLE IN PRESS M.-E. Lee et al. / Physica E 26 (2005) 422–426 slower/comparable to that of type I CdSe/ZnSe QDs [12,13]. In order to understand the role of wetting layer in carrier capture and relaxation, we measured the decay of PL as a function of energy for both samples, as shown in Fig. 2. The PL decay of the SK mode at energies above the peak of the timeintegrated PL exhibits bi-exponential decay. For comparison, we determined the PL decay at all energies with two time constants, one refers to the slow decay and the other refers to the fast decay of the PL although the PL shown in Fig. 1(b) can easily be explained by a single exponential. As shown in Fig. 2, the slow decay time of the SK mode decreases with energy. We attribute the decrease of slow decay times also to the wetting layer because the carriers in the small QDs, whose energy states are larger, can be excited to the wetting layer either by the Auger process or by thermal emission and will later relax to the large QDs. Carriers in the confined wetting layer migrate from small QDs to the large QDs by diffusion. On the contrary, the slow decay times of the VW mode level around 1.6 ns indicates that the probabilities of carrier capture into the QDs are equal and thus the carriers excited by the Auger process and thermal emission will relax to large 425 and small QDs with the same probabilities since no path is provided for carriers to diffuse and migrate from large (small) to small (larger) QDs. The same probabilities of carrier capture in the VW mode is confirmed by studying the PL rise times of the QDs and we found that they were all about 2 ps, regardless the detected energy. The above argument could also explain why the slow PL decay times for the SK mode are smaller than the VW mode at energies above 2.3 eV. The carriers escape away from small QDs in the SK mode and diffuse to the large QDs through the wetting layer. Therefore, fewer carriers are relaxed to the bandgap of the small QDs than to the large QDs and the slow decay times are thus smaller than those of the VW mode. In conclusion, we studied the carrier capture and relaxation of ZnTe/ZnSe QDs grown in the VW and SK modes with ultrafast PL upconversion. We found that the carrier capture times were 7 and 38 ps for the VW mode and SK mode, respectively. We attribute the fast PL decay of the VW mode within the first 20 ps after photoexcitation to the Auger processes. 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