V. Talalaev, P. Werner, and N.D. Zakharov
Exciton luminescence from the InAs/GaAs quantum dot molecules
The interaction of quantum dots (QDs) in self-organized arrays provides a potential basis for manufacturing new optoelectronic devices. In the vertical direction correlation of QDs in layers the conditions for the quantum dot molecules (QDMs) are established. At the transition from single QDs to QDMs the energy spectrum of array is transformed. The appearance of molecule terms may ensure the intra-subbands radiative recombination, which spans the IR- and THz- ranges. However, the realization of such emission requires a certain kinetics of excitation relaxation in QDM structures. This aspect of the problem has not been properly studied.
The current work deals with the study of exciton luminescence kinetics in the InAs/GaAs structures having two InAs QD layers. Obviously the spacing between the QD planes in such array is an important parameter defining the interaction between confined excitons. For larger spacings the carrier transfer between the QD planes is hindered, whereas for smaller spacings the tunnel coupling between neighboring QDs becomes strong enough to obtain the QDMs. We present the results of a transmission electron microscopy (TEM) and time resolved photoluminescence (TRPL) study of two sample types with InAs QDs grown by molecular beam epitaxy. In our case the sample with 5 nm GaAs spacer between QD planes (M-type) exhibits a QDM behavior compared to the reference sample with a 10 nm spacer (S-type) showing a behavior of the single QDs.
Figure 1(a) shows a dark field TEM-image of the M-type structure. Figure 1(b) presents the indium distribution across one pair of QDs. Both QDs appear very similar in size and composition. The maximum of the In content in QDs is the same (60%). The distance between the maxima is about 7 nm. Therefore, there is a clear evidence for the existence of QDMs in the M-type structure.
Fig. 1. (a) Cross-section dark-field TEM image of a QDM sample with 5 nm GaAs spacer; (b) indium concentration obtained by analysis of the high resolution TEM image of a individual QDM after Fourier-filtering along the growth direction; (c) cw PL data at T = 10 K for QDM array versus excitation power density: 0.2, 2, 10, 13, 25, 50, 100, 130, 200 and 300 Wcm-2, from the bottom to the top.
The steady state PL spectrum of the M-type sample consists of at least 3 well-separated optical transitions assigned to molecular energy terms (Fig. 1(c)). The + sign stands for the bonding (symmetric) terms. For low excitation density we find an energy gap of 45 meV between the s+ and p+ PL lines and 24 meV between p+ and d+. Plots of the line intensities versus excitation power show an almost linear dependence with saturation thresholds at 50 and 130 Wcm-2, for the s+ and p+ lines, respectively. The ratio of 1:3 for their integrated intensities at the threshold points reflects the relative occupation probability of s+ and p+ states. For d+ line the saturation is not reached within the steady state experiment. Arrhenius plots of the cw PL temperature dependent yield a thermal activation energy of 124, 82 and 60 meV for the s+, p+ and d+ lines, respectively.
For the QDM array the 45 meV is assigned to the energy gap between s+ and p+ states. This value matches the difference of thermal activation energies between s+ and p+ states. The observed p+–d+ splitting of 24 meV and 22 meV obtained from cw PL spectra and Arrhenius analysis, respectively, indicates that s+, p+ and d+ transitions involve the same hole level, being energetically separated from the GaAs barrier by about 190 meV. This value corresponds well to theoretical predictions of a strong hole localization in QDMs.
Fig. 2(a) presents the transient PL behavior for the M-type array at the s+, p+, and d+ line positions. The recombination from s+ and p+ states is characterized by a mono-exponential PL decay with time constants of 1.3 and 1.0 ns, respectively. The d+ band exhibits a bi-exponential PL decay, namely a fast (0.8 ns) and a slow (1.7 ns) transient.
Fig. 2. (a) TRPL of the M-type sample at the s+, p+, and d+ line positions; (b) TRPL of the S-type sample taken for both doublet components; (c) scheme of the energy states and optical transition in QDM. The excitation density – 2.4×1011 photons/(pulse×cm2), measurement temperature – 10 K.
Summary: a consistent interpretation of steady state PL and TRPL data for QDMs is presented. The TRPL spectra indicate the increase of the exciton lifetime in the QDM compared with a single QD. A detailed analysis of the transient luminescence behavior shows an efficient d+ → p+ transition that potentially could be used for the creation of mid-IR light sources.