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We report on the carrier transfer and thermal escape in CdTe/ZnTe quantum dots (QDs) grown on a GaAs substrate. The significant emission-energy-dependent decay time at high excitation intensity (35 W/cm2) is attributed to the lateral transfer of carriers in the QDs. At low temperature (< 35 K) and low emission energy (< 2.168 eV), a thermally activated transition occurs between two different states separated by approximately 9 meV, while the main contribution to nonradiative processes is the thermal escape from QDs that is assisted by carrier scattering via the emission of longitudinal phonons through the excited QD states at high temperature, with energies of approximately 19 meV.
© 2014 Optical Society of America
1. Introduction
Quantum dots (QDs) are particularly interesting owing to their unique physical properties and their promising applications in high-performance optoelectronic devices such as single-electron transistors [1], lasers [2], light-emitting diodes [3], infrared photodetectors [4], and solar cells [5]. For these applications, the study of carrier dynamics and time-resolved photoluminescence (PL) in QDs as well as the precise control of shape and size distribution of QDs is very important for improving the performance of optoelectronic devices [6]. The study of carrier dynamics not only offers a convenient way to clarify their structures but also provides useful information for extending their applications. Furthermore, time-resolved PL in QDs have emerged as an important tool that helps understand the process of recombination, relaxation, and the interaction between carriers. In particular, wide band-gap CdTe/ZnTe QDs are characterized by high excitonic binding energies and are currently of interest owing to their potential application in optoelectronic devices operating at short wavelengths [7, 8]. A detailed experimental investigation of the recombination and relaxation processes from the barrier states into the discrete energy states involved in optoelectronic devices has thus become necessary. Cascade processes have been generally found to involve radiative relaxation, energy transfer between dots of different dimensions [9], Auger recombination scattering [10], thermal escape from the dot [11], or trapping in surface and/or defect states [12, 13]. Moreover, theoretical study of the nonstationary secondary emission from the lowest-energy states of the QDs has been reported [14].
In this paper, we investigate the carrier dynamics in CdTe/ZnTe QDs using time-resolved PL measurements. The analysis of the PL decay time at 20 K as a function of the emission energies clearly indicates that the carrier dynamics can be attributed to the lateral transfer of carriers in the QDs. In addition, we determined localization energy (Elocal), the mobility edge (Eme), and the nonradiative activation energy assisted by the thermal escape process. Using time-resolved PL measurements at various excitation intensities, we also demonstrate that the decreasing decay time as a function of excitation power corresponds to a full filling process of the ground state of each QD, resulting in an increase in the electron-hole wave function overlap with increasing carrier concentration in the dots.
2. Experimental details
The studied sample was grown on a GaAs (100) substrate via molecular beam epitaxy (MBE) and atomic layer epitaxy (ALE). The GaAs substrate was degreased in warm trichloroethylene, cleaned in acetone, cleaned in methanol, and thoroughly rinsed in deionized water. Immediately after the chemical cleaning process, the GaAs substrate was mounted on a molybdenum susceptor. After the GaAs substrate was thermally cleaned at 600 °C for 5 min, a 900 nm ZnTe buffer layer was first grown on the GaAs substrate at 320 °C using MBE, followed by the deposition of 3.5 monolayer (ML) CdTe at 320 °C using ALE, resulting in the formation of QDs. The CdTe QDs were then capped with a 100 nm thick ZnTe layer grown at 320 °C using MBE. The Zn and Te source temperatures for the ZnTe layer were 280 and 300 °C, respectively, while the Cd and Te source temperatures for the CdTe layer were 195 and 300 °C, respectively. One cycle of ALE growth was carried out using an optimum growth process in which the Cd effusion cell was opened for 8 s and growth was interrupted for 1 s. Thereafter, the Te effusion cell was opened for 8 s and growth was interrupted for 5 s. Note that an interrupted process was introduced to improve the film quality by stabilizing positive and negative ions on the surface. Atomic force microscopy (AFM) measurements were performed using a multimode atomic force microscope from Digital Instruments, operating in the tapping mode. Time-resolved PL decay curves were acquired using a time-correlated single photon counting (TCSPC) method. We used 400 nm frequency-doubled femtosecond pulses from a 76 MHz mode-locked Ti:sapphire laser system as an excitation source, and the sample temperature was kept between 20 and 110 K using a He closed-cycle refrigerator displex system. The PL was dispersed using a 15-cm monochromator and detected using a multichannel plate photomultiplier tube. A commercially available TCSPC module (PicoHarp, PicoQuant GmbH) was used to obtain PL decay curves. The full width at half maximum (FWHM) of the total instrument response function (IRF) was less than 130 ps.
3. Results and discussion
Figure 1 shows the PL spectrum at 20 K for the 3.5 ML CdTe/ZnTe QDs with an excitation power of 1 mW. The dominant peak at 2.175 eV corresponds to the exciton transition from the ground-state electronic subband to the ground-state heavy-hole band (E1-HH1) in the CdTe/ZnTe QDs. The FWHM of the E1-HH1 peak for the CdTe/ZnTe QDs is approximately 32 meV. The inset of Fig. 1 shows an AFM image of the uncapped surface for the 3.5 ML CdTe/ZnTe QDs. The AFM image reveals that the CdTe QDs are embedded in an undoped ZnTe matrix, and uniform CdTe QDs are formed. The average heights of the formed CdTe QDs are between approximately 7 and 10 nm, and their diameters are between approximately 40 and 50 nm. The density of the CdTe QDs is approximately 5 × 1010 cm−2.
Fig. 1 PL spectrum at 20 K for the 3.5 ML CdTe/ZnTe QDs with an excitation power of 1 mW. The inset shows the AFM image of the 3.5 ML CdTe QDs grown on a ZnTe buffer layer.
Let us turn our attention to time-resolved PL decay time. For the experiments described in this section, we used an excitation density (I0) of approximately 3.5 mW with an excitation photon energy of 3.098 eV and a laser repetition rate equal to 76 MHz, corresponding to a photon fluence <jp> of approximately 8 × 1011 photons cm−2 per laser pulse. With a density of the CdTe QDs of approximately 5 × 1010 cm−2 and a spot diameter of approximately 100 μm, the average excitation per dot is given by <N0> = jpσ0, where σ0 is the absorption cross section of the dot [15, 16]. The calculated <N0> is approximately 1.2. Figures 2(a)–(d) show the PL decay time obtained from experimental observations at four emission energies in the PL spectra at 20 K. To extract PL decay time, we performed reconvolution fitting with the IRF for time-resolved PL results. In general, the decay curves fit well with the bi-exponential function reported in the literature, which consists of fast and slow components attributable to “bright” and “dark” excitons [17]; hereafter, we focus only on the fast component. The decay times of the CdTe/ZnTe QDs as a function of emission energy are shown in Fig. 2(e). The decay time depends strongly on the emission energy. The maximum decay time occurs around 2.175 eV, which interestingly coincides with the emission attributed to CdTe QDs in the green band. At the lower-energy region around 2.175 eV, the PL decay time slightly increases, while the decay time on the higher-energy side decreases. In general, two thermally activated processes of the dynamic luminescence have been observed: (1) carrier transfer between dots, possibility due to giant oscillator-strength effect and the quantum confined Stark effect in large QDs, which mainly depend on electron-hole wave function overlap, and (2) the possibility of quenching due to the escape of carriers from QDs [18, 19]. The electron-hole superposition is affected by the inhomogeneous sizes and shapes of QDs. Such maxima could indicate a crossover among two different effects. In our case, the increase in the PL decay time with increasing emission energy is attributed to the different ground state oscillator strength related to the QDs height but influenced by the electron-hole (e-h) short-range exchange interaction along the vertical direction due to the enhanced electron-hole pair polarizability. In this case, the exchange of heavy holes at localized states is still insufficient to affect the resizing of dot height, and the electron-hole overlap is influenced by the electron wave function [20]. Above the maximum of the peak, we observe a dramatic increase in the radiative lifetime with increasing energy due to the decrease in the exciton coherence volume in large QDs. This behavior is attributed to the reduced giant oscillator strength and quantum confined Stark effect due to a strong built-in electric field in large QDs [21]. This reduces the exciton oscillator strength by separating the wave functions of electrons and holes and loosening the Coulomb attraction that controls the extension of their in-plane relative motion [22]. At the same time, the coupling of heavy holes begins to become necessary, and the electron-hole overlap occurs on the QD volume. Similar results, i.e., the observation of PL decay time strongly increasing with decreasing emission energy, have been reported for self-assembled InGaAs [20] and GaN-AlGaN QDs [22]. In addition to the usual behavior, the observed PL decay time can perhaps also be attributed to the important contribution of the in-plane tunneling between nanostructures and the large barrier penetration of the excited electron wave function. Therefore, we conclude that the carrier dynamics can be attributed to the lateral transfer of carriers in the QDs.
Fig. 2 (a)-(d) Time-resolved PL spectra at 20 K for the 3.5 ML CdTe/ZnTe QDs at different emission energies. (e) Decay time of the 3.5 ML CdTe/ZnTe QDs as a function of emission energy. The solid line represents PL spectrum at 20 K for the 3.5 ML CdTe/ZnTe with an excitation power of 3.5 mW. The dashed line indicates the fitting curve for the radiative lifetime of transfer energies.
To determine the radiative lifetime of transfer energies triggered at high energies as well as their average localization energy and mobility characteristics, Gourdon and Lavallard [9] proposed an expression for the lateral energy transfer in high-density QDs in the form
where τrad is the radiative lifetime, Eme is the energy for which the radiative lifetime equals the lateral transfer time, and Elocal is a characteristic energy for the density of localized states of electrons or holes. The latter parameter is a measure of the average localization energy of the QDs, and Eme could be interpreted as the mobility edge for excitons. We found the best fit value for the radiative lifetime of τ rad to be 480 ps, the best fit for an average localization energy of Elocal to be 9.2 meV, and that for the mobility edge of Eme to be 2.15 eV at 20 K. Interestingly, the strong redshift of Eme is comparable to that of the PL maximum (2.175 eV). It is exciting that the value of Elocal corresponds to the low-energy band. This low energy quenching was assumed to be due to the ionization of shallow donor levels, and the two quenching stages probably point toward a donor-acceptor recombination [23] or a thermally activated transition from intrinsic states to higher-energy localized surface states [24], or transitions between intrinsic and defect states [25].For a more adequate description of the carrier dynamics, PL decay times at various temperatures were also considered. Figures 3(a)–(e) show the time-resolved PL spectra for peak energies of the 3.5 ML CdTe/ZnTe QDs at several temperatures. The decay times of the 3.5 ML CdTe/ZnTe QDs as a function of temperature are shown in Fig. 3(f). The results show a significant difference between the coupled and separated QDs, revealing that the transfer processes between different localized states primarily determine the increase of the decay time. Three different processes explain the decay of localized excitons: radiative recombination, transfer processes, and nonradiative recombination. The PL decay time slightly increases for increasing temperatures up to approximately 35 K, and then it decreases with increasing temperature. The increase in the decay times at low temperature is due to heavy-hole coupling, as predicted by the thermal activation of coupled dot, for the carrier wavefunction. The electron-hole overlap is influenced by the electron wave function, while the hole wave function is strongly affected because the coupling of heavy holes is still insufficient to affect the resizing of the dot height. This delocalization is also reported by Colocci et al. [26] in their InAs/GaAs coupled QDs. It is known that at low temperature, the exciton transfer is limited to tunneling processes, whereas the thermally induced coupling between optically active localized excitons and mobile exciton states is enhanced with increasing temperature. In the high temperature region (above 35 K), the PL decay time decreases with increasing temperatures.
Fig. 3 (a)-(e) Time-resolved PL spectra for peak energies of the 3.5 ML CdTe/ZnTe QDs at several temperatures. (f) Decay time of the 3.5 ML CdTe/ZnTe QDs as a function of temperature. The solid line indicates the fitting curve obtained using the thermal escape model.
In a single approach, the recombination rate of the QD ground state is given by [27]
where τdecay is the measured PL decay time, and ΓR(T) and ΓNR(T) are the radiative and non-radiative recombination rates, respectively. With the use of these models, it has been reported [28, 29] that the thermal population of the higher exciton states, which have lower oscillator strengths, accounts for the prolonged net radiative recombination lifetime in the higher temperature region. The variation of the radiative rate due to the presence of a localized state is given by [29]where ΔE is the thermally activated energy difference between the ground state and the localized states, g is the ratio of the degeneracy of the localized state to that of the ground state, and τ0 is the radiative time at 0 K. The nonradiative recombination suggests that the main contribution to the nonradiative processes is the thermal escape out of QDs, with the nonradiative recombination rate given by [30]where Eesp is the average energy for thermal carrier escape, which corresponds to only one confined level. Γ0 is the escape attempt frequency. For Eq. (2), the above expression becomes:The experimental decay time values are well-reproduced by the best-fit curve to Eq. (5) for ΔE = 9.4 meV, Eesp = 19.3 meV, and τ0 = 484 ps. We observed that the best-fit values of ΔE and τ0 are very similar to Elocal and τrad extracted from the energy-dependent PL decay time analysis, respectively. This suggests that the low-temperature quenching is due to the same thermally activated transition between two different states energetically separated by ΔE. This transition could be due to the transition between intrinsic and defect states that affect the PL decay time temperature dependence at high excitation intensity. For the value of Eesp, it is interesting to note that the carrier escape energy, Eesp, is consistent with the values of average phonon energies obtained on colloidal CdTe QDs [31], with a magnitude lower than the theoretical value estimated by Rubin et al. (approximately 24.5 meV for bulk CdTe) [32]. We expect that thermal carrier escape, which could be due to carrier scattering via emission of longitudinal phonons through the excited QD states, is the main nonradiative process for CdTe/ZnTe QDs at high temperature.Figure 4(a) shows the PL decay time obtained from experimental observations at different excitation intensities at 20 K. We compare two PL decay time traces taken with high and low excitation intensities at an emission energy of 2.175 eV [see Fig. 4(b)], at which a change in decay lifetime as a function of excitation intensity at 20 K takes place. This quantity exhibits two distinct regimes. The PL decay time is constant at low excitation power with the average number of excited photons in each dot <N0> << 1, which is sufficiently small to neglect Auger scattering [15]. At approximately 1 mW, the decay time decreases sublinearly. The decreasing decay time as a function of excitation power corresponds to a full filling process of the ground state of each QD, resulting in an increasing electron-hole wave function overlap with increasing carrier concentration in the dot (primarily heavy-hole). The filling process shows a similar behavior as that reported for InAs/GaAs QDs [33, 34]. The observed lifetime shortening at high excitation power with <N0> > 1 is attributed to Auger recombination effects [10]. The efficiency of Auger processes are mediated by strong Coulomb electron-electron interactions in semiconductor QDs [35]. The excess energy of the e-h pair is not efficiently released as a photon, but is instead transferred to a third particle (an electron, a hole, or an exciton) that is re-excited to higher energy states.
Fig. 4 (a) Time-resolved PL spectra at several excitation intensities for the 3.5 ML CdTe/ZnTe QDs. (b) Decay time of the 3.5 ML CdTe/ZnTe QDs as a function of excitation intensity.
4. Conclusion
In conclusion, we have investigated the factors leading to the deterioration of the optical performance in terms of quantum efficiency for CdTe/ZnTe QDs using time-resolved PL measurements at high excitation intensity (35 W/cm2). The significant emission energy-dependent decay time at high excitation intensity is attributed to the lateral transfer of carriers in the QDs. At low temperature and low emission energy, a thermally activated transition occurs between two different states separated by approximately 9 meV, while the main contribution to the nonradiative processes is the thermal escape out of QDs assisted by carrier scattering via emission of longitudinal phonons through the excited QD states at high temperature, with average energies of approximately 19 meV.
Acknowledgments
This work 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 (2010-0021189).
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