Carrier tunneling and thermal escape in asymmetric double quantum dots

Carrier tunneling and thermal escape in asymmetric double quantum dots

Accepted Manuscript Full Length Article Carrier tunneling and thermal escape in asymmetric double quantum dots Kee Hong Lim, Minh Tan Man, Chang-Lyoul...

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Accepted Manuscript Full Length Article Carrier tunneling and thermal escape in asymmetric double quantum dots Kee Hong Lim, Minh Tan Man, Chang-Lyoul Lee, Sang-Youp Yim, Jin Chul Choi, Hong Seok Lee PII: DOI: Reference:

S0169-4332(18)31764-1 https://doi.org/10.1016/j.apsusc.2018.06.197 APSUSC 39705

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

16 August 2017 21 March 2018 22 June 2018

Please cite this article as: K. Hong Lim, M. Tan Man, C-L. Lee, S-Y. Yim, J. Chul Choi, H. Seok Lee, Carrier tunneling and thermal escape in asymmetric double quantum dots, Applied Surface Science (2018), doi: https:// doi.org/10.1016/j.apsusc.2018.06.197

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Carrier tunneling and thermal escape in asymmetric double quantum dots Kee Hong Lima, Minh Tan Manb, Chang-Lyoul Leec, Sang-Youp Yimc, Jin Chul Choia, Hong Seok Leea,* a

Department of Physics, Yonsei University, Wonju 26493, South Korea

b

Department of Physics, Research Institute of Physics and Chemistry, Chonbuk National

University, Jeonju 54896, South Korea c

Advanced Photonics Research Institute, Gwangju Institute of Science and Technology,

Gwangju 61005, South Korea *Corresponding author. E-mail address: [email protected] (H.S. Lee).

Abstract We investigated the effects of ZnTe separation layer thickness on the optical properties of asymmetric CdTe/ZnTe double quantum dots (QDs) deposited by molecular beam epitaxy and atomic layer epitaxy on Si substrates. For asymmetric CdTe/ZnTe double QDs, the exitonic peaks of the large QDs (LQDs) were blue shifted with decreasing separation layer thickness because of intermixing caused by strain from the small QDs (SQDs). The relative peak intensity of the LQDs with respect to that of the SQDs increased with decreasing separation layer thickness due to carrier tunneling from the SQDs to the LQDs. We propose that the separation layer plays a key role in the thermal escape process, and can therefore be used to modulate carrier capture in optoelectronic devices.

Keywords: Multilayer quantum dot; Carrier dynamics; Carrier tunneling; Thermal escape

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1. Introduction Quantum dots (QDs) continue to attract keen interest for applications in highperformance optoelectronic devices because of their unique properties, which include tunable emission wavelengths, low threshold currents, and high saturation gain [1-5]. For optoelectronic device applications, an understanding of the carrier dynamics in QDs as well as the ability to control the QD size, shape, and size distribution are very important for improving device performance. A promising advance in this field has been the development of asymmetric double QDs, which exhibit improved exciton oscillator strength and photoluminescence (PL) spectral tuning due to inhomogeneous broadening and lower groundstate saturation gain [6-9]. Among the asymmetric double QDs reported to date, wide-gap CdTe/ZnTe QD structures, which have the advantage of higher excitonic binding energies, are of great interest due to their potential applications in optoelectronic devices operating in the green spectral range [10]. To control the quantum confinement and optical properties, CdTe/ZnTe QDs have been studied on GaAs substrates due to the relatively small lattice mismatch (~8%) between the ZnTe layer and the GaAs substrate, as opposed to the larger lattice mismatch (~12%) between the ZnTe layer and a Si substrate [11, 12]. However, devices with larger areas and higher quality can be fabricated on Si substrates than on GaAs substrates [13]. Furthermore, a detailed experimental investigation of the recombination and relaxation processes from the barrier energy states into the discrete energy states of asymmetric CdTe/ZnTe double QDs is required [14, 15]. In this work, we investigated the effects of the ZnTe separation layer thickness on the optical properties of asymmetric CdTe/ZnTe double QDs on Si substrates. Atomic force microscopy (AFM) and PL measurements were performed to characterize the structural properties and interband transitions, respectively, in the asymmetric CdTe/ZnTe double QDs. Temperature-dependent PL measurements were carried out to investigate the variation in the thermal escape process of the double QDs with varying separation layer thickness. 2

2. Experimental details Asymmetric double CdTe/ZnTe QDs were grown on Si(100) using molecular beam epitaxy (MBE) and atomic layer epitaxy (ALE). The Si substrates were etched in a mixture of NH4F and HF (V/V=7:1) for 10 min and cleaned with deionized water. After chemical etching, the substrates were blown with nitrogen. The substrates were mounted on a molybdenum susceptor and thermally cleaned at 580 °C for 5 min. Following vapor deposition, a 900 nm ZnTe buffer layer was grown on the Si substrate by MBE. Smaller quantum dots (SQDs), specifically 3.0 monolayer (ML) CdTe QDs, were grown by ALE. Next, Zn and Te source cells were opened simultaneously and ZnTe separation layers of 45, 30, 15, or 8 nm were deposited on the SQDs for each sample. Larger quantum dots (LQDs), specifically 4.0 ML CdTe QDs, were then grown. Finally, the samples were capped with a 100 nm ZnTe layer grown using MBE. The ZnTe and CdTe layers were deposited at a substrate temperature of 300 °C. The temperatures of the Cd, Zn, and Te sources were set at 195, 300, and 280 °C, respectively. For the optimized ALE, a Cd effusion cell was opened for 8 s and interrupted for 1 s. Then, 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. AFM measurements were performed using a Park NX10 instrument operated in non-contact mode. For the PL measurements, the PL was collected by a 150 mm 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. Temperature-dependent PL spectra were measured using a helium closed-cycle Displex refrigerator over a temperature range of 20 to 110 K.

3. Results and discussion Fig. 1 shows the PL spectra for the SQDs, LQDs, and asymmetric CdTe/ZnTe double 3

QDs with ZnTe separation layer thicknesses of 45, 30, 15, and 8 nm. Two peaks were observed in the spectra of the asymmetric double QDs with ZnTe separation layer thicknesses of 45, 30, and 15 nm, whereas a single peak was observed for the system with an 8-nm separation layer thickness. The peaks at 2.218 eV for SQDs and 2.101 eV for LQDs are attributed to the exciton transition from the ground state electronic sub-band to the groundstate heavy-hole band, which demonstrated a narrow full width at half maximum (FWHM) of 66 and 123 meV for the SQDs and LQDs, respectively. Fig. 1(b) and (c) show AFM images of uncapped SQDs and LQDs, respectively. The diameters of the SQDs were between 50 and 65 nm, and their average height and density were 9 nm and 1 × 10 10 cm−2, respectively. The diameters of the LQDs were between 75 and 95 nm, and their average height and density were 12 nm and 6 × 109 cm−2, respectively. The height and diameter of the CdTe QDs increased with increasing CdTe layer thickness. Fig. 2(a) plots the PL peak position and the FWHM for the asymmetric CdTe/ZnTe double QDs as a function of the ZnTe separation layer thickness. The PL peaks were blue shifted with decreasing ZnTe separation layer thickness due to intermixing caused by strain induced in the SQDs [16]. In addition, reducing the thickness of the separation layer narrowed the FWHM of the LQDs in the asymmetric double QDs owing to improved uniformity of the LQDs resulting from the strain in the SQDs [17]. The relative peak intensity of the LQDs with respect to the intensity of the SQDs with ZnTe separation layer thicknesses of 45, 30, and 15 nm is shown in Fig. 2(b). The peak intensity of the LQDs with respect to the SQDs increases with decreasing separation layer thickness. This behavior is attributed to carrier tunneling from the SQDs to the LQDs [18]. Although the carriers initially relax into the SQDs, most subsequently travel to the LQDs through tunneling, leading to recombination. As a result, the PL of the asymmetric double QDs exhibited a single peak at 2.117 eV for the ZnTe separation layer thickness of 8 nm. To confine the relaxation of the electrons in the asymmetric double QDs on the Si 4

substrate, we performed temperature-dependent PL measurements. Fig. 3 shows the temperature-dependent PL spectra of LQDs and the asymmetric CdTe/ZnTe double QDs with a separation layer thickness of 8 nm. As the temperature increased, the peak positions in the spectra of both systems shifted to low energy due to the decreasing band gap between the electrons and holes. At high temperature, a decrease in the PL intensity was observed. Examination of the integrated PL intensities of the confined carriers as a function of temperature revealed that the main process is related to thermal escape, as shown in Fig. 4(a). The temperature-dependent integrated PL intensity can be determined using the following Arrhenius equation [19]:

I PL (T ) 

I0 1  a exp( E / k BT )  b exp( 2E / k BT )  c[exp( E / k BT )  1) m

(1)

where I0 is the integrated PL intensity at 0 K, a, b, and c are constants related to the energy density of states, E is the thermal activation energy of the low-temperature quenching processes, and ELO and m are the phonon energy and the number of LO phonons, respectively. The best fit values for the thermally activated transition between two different states separated by E were found to be about 5.4 and 6.8 meV for the LQDs and asymmetric double QDs with a separation layer thickness of 8 nm, respectively. This transition may correspond to the transition between intrinsic and defect states that affects the temperature dependence of the PL intensity at low temperature. The thermal escape energy is defined by Eescape=m×ELO, which assisted by scattering with the number m of LO phonons at high temperature [19]. The calculated thermal escape energy (Eescape) was 42.5 meV for LQDs. The greatest thermal escape energy, 59.1 meV, was observed for the asymmetric double QDs with a separation layer thickness of 8 nm, and the thermal escape energy reduced with increasing separation layer thickness (Fig. 4 (b)). It can be clarified evidence of the occurred escape processes

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induced by interactions of SQDs with LQDs and the separation layer [19-21], as well as into the carrier tunneling rates [18]. The fluctuations of the separation layer thickness induce different charge densities and configurations of the carriers surrounding the QDs, which play an essential role in carrier tunneling and the carrier redistribution process. Furthermore, the increase in the relative PL intensity for the asymmetric CdTe/ZnTe double QDs as a function of the ZnTe separation layer thickness (Fig. 2 (b)) provides further evidence for a pronounced increase in carrier tunneling rate, paving the way to tune the thermal escape process in optoelectronic devices.

4. Conclusions We investigated the carrier tunneling and thermal escape processes in CdTe/ZnTe asymmetric double QDs grown on Si substrates by MBE and ALE. PL measurements showed that the PL peaks of the LQDs in the asymmetric double QDs were blue shifted with decreasing separation layer thickness because of intermixing caused by strain from the SQDs. The FWHM of the PL peak corresponding to the LQDs in the asymmetric double QDs decreased as the separation layer thickness was reduced due to improved uniformity of the LQDs resulting from the strain in the SQDs. Moreover, the observed increases in the relative peak intensity and thermal escape energy with decreasing separation layer thickness are consistent with a pronounced increase in carrier tunneling rate and carrier redistribution. We conclude that the separation layer thickness controls carrier capture in optoelectronic devices by modulating the thermal escape process.

Acknowledgment This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2018R1A2B6001019). 6

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Figure Captions Fig. 1. (a) PL spectra for the 3.0 ML CdTe/ZnTe QDs (SQDs), 4.0 ML CdTe/ZnTe QDs (LQDs), and asymmetric CdTe/ZnTe double QDs at 20 K with ZnTe separation layers of thickness 45, 30, 15, and 8 nm. AFM images of the (b) SQDs and (c) LQDs. Fig. 2. (a) Peak position and FWHM of the PL spectra for asymmetric CdTe/ZnTe double QDs as a function of ZnTe separation layer thickness. (b) PL intensity of 4.0 ML CdTe/ZnTe QDs (LQDs) with respect to the intensity of 3.0 ML CdTe/ZnTe QDs (SQDs) as a function of ZnTe separation layer thickness. Fig. 3. PL spectra at varying temperatures for (a) the 4.0 ML CdTe/ZnTe QDs (LQDs) and (b) the asymmetric CdTe/ZnTe double QDs with a ZnTe separation layer of 8 nm. Fig. 4. (a) Integrated PL intensities as a function of reciprocal temperature for the 4.0 ML CdTe/ZnTe QDs (LQDs) and the asymmetric CdTe/ZnTe double QDs with a ZnTe separation layer of 8 nm. (b) Thermal escape energy of the asymmetric CdTe/ZnTe double QDs as a function of ZnTe separation layer thickness.

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Graphical abstract

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Highlights 1. Carrier tunneling and thermal escape in asymmetric double quantum dots were investigated. 2. Band gap of large quantum dots shifts to higher energy with decreasing ZnTe separation layer thickness. 3. Thermal escape energy of large quantum dots decreases with wider ZnTe separation layer thickness. 4. Thermal transfer of carriers between double quantum dots increases with narrower separation layers.

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