Self-assembled (In,Ga)As/GaAs quantum dots (QDs) demonstrate many favorable physical properties that makes them suitable for numerous device applications 1234. They possess high radiative efficiency and easily controllable areal densities; allow engineering of the energy levels permitting efficient carrier injection from a semiconductor matrix. Typically, (In,Ga)As/GaAs QDs have ~2–5 nm height and ~20–50 nm base width. Due to small QD sizes and high confinement potential, the electron and hole states are fully quantized and their wavefunctions are strictly localized within the QD area with a δ-like density of states. Therefore, the self-assembled QDs can be treated to some extent as nearly ideal zero-dimensional (0D) systems behaving themselves like a “frozen ideal gas”.

Each individual QD offers several advantages as a source for single photons. It has high oscillator strength and narrow spectral linewidth (~0.01 meV) and does not suffer from photobleaching or shelving. However, these idealizations have to be substantially modified in case of QD densities lying in the range of ~10¹⁰–10¹¹ cm⁻². These densities result in mean dot separation of the order of tens to hundreds of nanometers which is comparable with the sizes of the individual QDs. Unavoidable size fluctuations within the QD ensemble lead to an additional inhomogeneous broadening of the optical spectra, frequently making the observation of the intrinsic 0D behavior difficult. Another characteristic of QD ensemble the interdot coupling is usually grouped into two categories with regard to whether it occurs via overlapping wave functions (electronic coupling) of the spatially separated QDs or via long-range electromagnetic interactions.

Depending on the strength of electronic coupling between neighboring QDs, their individual electronic states and the relaxation of the photo-excited carriers through those states can be significantly altered. In the case of strong coupling, they can form QD molecules 5, resulting in new physics and a number of applications such as an excitonic qubit system with potential scalability 6. In the case of intermediate or weak coupling, the energy and carrier transfer between QDs occurs through quantum–mechanical tunneling that substantially affects recombination, carrier injection, and lasing in the QD system 7891011.

The long-range radiative coupling between QDs in an ensemble can be interpreted in terms of successive emission and reabsorption of photons resulting in collective modes of several QDs. In this case, the exciton state of a single QD cannot be treated as a stationary state since its excitation in an individual QD will be transferred to other QDs 12. In addition, energy transfer between dots can be in the form of an electrostatic dipole–dipole interaction, frequently cited as Förster energy transfer 13. In the case of InP QDs with interdot distance of 7 nm, the Förster time for the excitation transfer between two QDs has been estimated 14 to be in the range of 10²–10³ ps. This Förster transfer rate is proportional to the fourth power of the dipole moment matrix element and decays as the sixth power of the interdot distance 14. Complementary to the Förster energy transfer, the polariton coupling is also expected to have a long spatial range, of the order of a few photon wavelengths. This mechanism results in the transfer of electron–hole excitation between distant QDs mediated by the emission and reabsorption of the transverse electromagnetic field 12. Typical transfer rates attributed to polariton coupling are in the range of 10⁻³–10⁻⁴ ps⁻¹. It is proportional to the square of the dipole moment matrix element and decays as the inverse of the interdot distance, if the transfer is mediated by a propagating field in two dimensions. Thus, in presence of radiative interaction, in fact, even very remote QDs cannot be considered as isolated systems, and the QD ensemble is expected to develop signatures of cooperative radiation or superradiation 15. In order to strengthen the QD coupling through their radiation field, the use of semiconductor microcavities has been proposed and strong interaction between single QDs and the cavity mode has been already demonstrated 16.

In the case of QD ensembles with well-separated QDs, the fingerprint of cooperative effects in radiation is the change of the photoluminescence (PL) decay time with a change in the number of interacting QDs and their respective separation 1217. The number and interdot separation of the dots can be varied using postprocessing of as-grown samples. For example, the dots could be covered by a mask containing small apertures through which the optical excitation as well as the collection of the signal is done. A lateral patterning of the sample surface offers another possibility. In this case, small mesa structures are fabricated which contain only a single QD or a few QDs. In case of CdSe/ZnSe QDs studied in Ref. 17, mesas of the same size were arranged in a grid-like pattern over an area larger than the size of the laser spot on the sample. Mesas of sizes down to a few tens of nanometers were etched into the QD sample removing the QDs from between the mesas. It has been shown 17 that the radiative interaction between QDs disappears with decreasing mesa size.

Recently, the new laterally ordered self-assembled (In,Ga)As/GaAs QD stack has been grown by molecular-beam epitaxy (MBE) 18. QDs align in long rows forming QD chains. It has been found that the dot chains can be substantially increased in length by the introduction of growth interruptions during the initial stages of growth of the GaAs spacer layer. The growth procedure has been also used to create a template to arrange InAs dots into chains with a predictable dot density. The resulting dot chains offer the possibility to engineer interdot coupling for novel physical phenomena and potential devices. Here, the distance between neighboring QD chains can be made sufficiently large as to prevent the immediate electron–hole wavefunction tunneling as well as the Förster energy transfer between QDs belonging to different chains. This looks like as a natural mesa between chains. The objective of our research is the study of the peculiarities of interdot coupling in these new laterally ordered self-assembled (In,Ga)As/GaAs QD structures. In this case, we can expect the superradiance effect from the dot-chain ensemble. It is important that in-chain interdot coupling is easily changeable also thus permitting to study the cooperative effects caused by immediate interdot carrier tunneling.

We use the multilayer (In,Ga)As/GaAs dot-chain structure to reveal various cooperative effects arising both through overlapping wave functions of the spatially separated QDs and through long-range radiative interactions between QDs. “Experiment” includes the details of sample growth together with results of AFM analysis of the grown structure. The details of the PL are given. “Temperature cw PL measurements” includes the results of the temperature investigation of the cw PL. This results in information about the energy scheme of the system and provides evidence of interdot coupling and carrier transfer in QD chains. “Time-resolved PL measurements and mechanisms of interdot coupling” represents the results of the time-resolved PL measurements and contains the theoretical model used for the interpretations of the time-resolved PL data. Last section contains concluding remarks.

The multilayer (In,Ga)As/GaAs dot-chain structures were prepared in order to study the actual mechanisms of interdot coupling which lead in part to the appearance of cooperative effects. In_xGa_1−xAs QDs were grown by molecular beam epitaxy (MBE) on semi-insulating GaAs (100) substrates such that dot chains form 18. Following a 0.3-μm GaAs buffer layer grown at 580°C, the growth of In_0.4Ga_0.6As QDs capped by 60 MLs of GaAs, both at 540°C, was repeated for a 15 layer superlattice. The In_0.4Ga_0.6As QD layers were grown by the deposition of 8.5 ML of material. A final QD layer was grown and left uncapped for morphology analysis. The atomic force microscopy (AFM) image of the final, uncapped QD layer, Fig. 1a, demonstrates that these InGaAs QDs are formed in chains along the [0-11] direction. Figure 1b shows the Fourier transformed AFM image of an area with sizes 4 × 4 μm². The arrows pointed out the maxima along the directions of the QD alignment, [0-11] and [011]. Figure 1c presents a histogram of the QD aspect ratio. It is seen that the mean aspect ratio is ~0.11. Based on these results, the average spacing between chains is estimated to be 90 nm, while the separation between QDs in the chains is around 45 nm, close to the lateral size of QDs. As a result, it is expected that any observed lateral electronic coupling will be predominantly between the QDs in a single chain and negligible between interchain QDs. However, analysis of the PL reveals a much more complicated system of interaction and coupling between the dots. Note that in our multilayer structures the GaAs spacer between adjacent QD layers was thick enough (approximately 60 MLs thick) to prevent any vertical QD ordering. In this case, adjacent QD layers can be considered as independent QD systems (see e.g. Ref. 11) and many layers serve predominantly to enhance the total PL gain.

The PL was performed using the 532-nm line from a frequency doubled Nd:YAG (neodymium-doped yttrium aluminum garnet) laser for continuous-wave PL excitation and the 2-ps pulses at λ_exc = 750 nm from a mode-locked Ti:sapphire laser for transient PL measurements. The optical pulse train at 76 MHz and an excitation density that was varied between 10⁹ and 2 × 10¹⁴ photons/(pulse × cm²) was used. The steady-state PL signal from the sample was dispersed by a 0.5-m single-grating monochromator and detected by a LN-cooled OMA V: InGaAs photodiode detector array. The PL transients were detected with a Hamamatsu synchroscan streak camera C5680 with an infrared enhanced S1 cathode. The overall time resolution of the system used in the time-resolved PL measurements was ~15 ps.

In order to get the information about both the energy scheme of the InGaAs/GaAs dot chain sample and the channels of the interdot carrier transfer, the temperature-dependent PL spectra are measured (Fig. 2). The low temperature (T = 10 K) and low excitation intensity (I_exc = 0.4 W/cm²) cw PL measurements reveal a single peak for the ground state exciton emission with a maximum at E_max = 1.247 eV and full width at half maximum (FWHM) of Γ ~ 48 meV (Fig. 2). Increasing the temperature from 10 K up to 300 K shifts the PL band to the red by ~67 meV and broadens almost twice, up to ~87 meV. At the temperatures as high as 150 K, the PL is found to broaden asymmetrically, favoring the higher energy. The results of fit with two Gaussians, Γ₁ = 49 meV and Γ₂ = 60 meV, are shown in the PL spectrum measured for T = 150 K by dashed lines (Fig. 2). Therefore, the high temperature asymmetry is partly a result of thermal population of the higher energy confined states in the dots. More precise behavior of the PL peak maximum and the FWHM versus temperature is shown in Fig. 3. The temperature dependence of the FWHM (Fig. 3a) demonstrates a non-monotonic behavior. It reaches a minimum at ~80 K and then grows significantly with increasing temperature. The PL redshift (~32 meV, see Fig. 3b) is greater than the redshift of InAs (~13 meV, curve 3 in Fig. 3b) or GaAs (~17 meV, not shown here) upon increasing the temperature from 10 K to 100 K.

The initial PL spectrum narrowing in the temperature range between 10 and 80 K is ascribed to thermally induced carrier transfer between the QDs within each chain. According to a simple thermionic model 1920, at low T, carriers are “frozen” randomly into the QD states (after initial partial redistribution) due to the tunneling between QDs within each chain. With increasing T, carriers are expected to be thermally activated outside the dots into the 2D InGaAs wetting layer (WL) or GaAs barrier and/or hopping between dots will favor a drift of carriers toward the dots having lower ground state energies. This results in a narrowing of the PL spectrum and a redshift for the PL peak that is larger than expected from the thermal shrinkage of the InAs band gap under elevating T. In order to describe such a process, one can use either the rate equation model 19 or a simplified quasi-thermodynamic model 20 that introduces a temperature dependent function β(T) allowing to parameterize the degree of thermalization of carriers. This function continuously interpolates between the well-defined low and high-temperature limits of the carrier distribution function and describes the observed peculiarities in the PL spectra with just two fitting parameters. In fact, such a description is equivalent to the assumption that partially thermalized carriers are still described by equilibrium statistics, but with a higher effective temperature T_th. Following to Ref. 20, the QD PL spectra (I(E)) can be treated using a relation like

with I₀(T) being the temperature-dependent scale factor 21 and E_g representing the temperature-dependent energy gap for InAs described, say, by the Varshni formula 22. Parameter A enters the relationship between the transition energies and the dot sizes 23 and is taken to be equal A = 2.37 eV nm^1/2 for our QDs. The QD ensemble is characterized by a Gaussian size distribution with a variance o²/2 and mean basal length a₀ corresponding to the ground state transition energy E₀. The thermalization function β(T) reflects all the carrier dynamics through the multiple pathways and can be approximated by expression 20

with T_th and Δ being the only two fit parameters of the model. T_th is the temperature when β = 0.5 and Δ corresponds to the temperature interval over which the transition from a “glassy” to a quasi-equilibrium state occurs.

Using Eqs (1) and (2) with the set of parameters E₀ = 1.247 eV and Δ = 48 meV taken at the lowest temperature (T = 10 K) for a Gaussian describing the low temperature PL spectrum (Fig. 2), we find the best fit of the FWHM (T) (Fig. 3a) and E_max(T) (Fig. 3b) dependences with the parameters Δ = 15 K and T_th = 60 K in the temperature range from 10 to 125 K. While the theoretical analysis carried out earlier is applied only for the ground state QD transitions our fit is valuable in the temperature range, where the contribution of the excited states is negligible. It follows from Fig. 2 that this range spreads from ~10 K up to ~150 K. It is important to note that the narrowing of PL the band width with an increase in temperature from ~10 to 100 K (see Fig. 3a) is not intuitive. One might expect the contrary, i.e., the FWHM increasing due to thermal broadening. Therefore, a good fit of the FWHM (T) dependence in this temperature range supports well the description of data in terms of the quasi-thermodynamic model presented earlier. In turn, the description of the E_max(T) dependence (see Fig. 3b) in the low-temperature range requires more elaborate analysis of the physical processes that determine the band-gap behavior in the QD system with increased temperature, e.g., electron–phonon coupling. The absence of such analysis can be in part an origin for the absence of a perfect fit for the E_max(T) data shown in Fig. 3b performed using the Varshni formula 22 for the temperature dependence of the energy gap in InAs QDs. For higher temperatures, the contribution of the excited states in the FWHM formation becomes significant leading to a significant deviation of calculated dependences from measured dependences observed in Fig. 3 in the temperature range above 150 K. Significantly small T_th value in comparison with that T_th = 127 K found earlier for the InAs/GaAs QDs 1920 gives evidence of the existence of non-thermal relaxation mechanism favoring the thermalization of the QD system even at the lowest temperatures. Such a mechanism is the interdot tunneling caused by the electronic coupling between the dots of the same chain. In this case, carriers escape from the smaller QDs with higher ground state energy to the larger QDs with lower ground state energy. Such escape results in the higher population of the lower energy QD states in comparison with the population of the energy states of smaller QDs under the steady-state PL excitation and equal rate of the carrier trap on the small and large QDs. As a result, the PL band of the QD ensemble, which possesses a symmetric QD size distribution described by a Gaussian, has to be asymmetric with a more abrupt high-energy side. Introducing the asymmetry measure as the difference δ = E − E_max, one can expect the positive asymmetry in case of low temperatures. Such determined asymmetry of the PL band is shown in Fig. 4. It occurs positive at the T = 10 K evidencing the non-thermal carrier distribution in the QD ensemble at low temperatures. Further temperature increase leads to the subsequent temperature population of the higher energy levels of smaller QDs thus decreasing the PL band asymmetry and finally switches the sign of asymmetry to the negative one. Such a behavior of the PL band asymmetry under the temperature increasing is demonstrated in Fig. 4. As one can see from Fig. 4, the well-pronounced PL band develops at the high-energy side of the dominant PL band. This band in the difference spectrum is shifted by ~38 meV from the PL peak position toward higher energy that quite well correlates with the energy difference found from the PL line shape analysis carried out in Fig. 2 for T = 150 K. Our calculations of the QD energy spectrum show that in the In_0.4Ga_0.6As/GaAs QD with the aspect ratio ~0.1 only one level e₁ is generated in the conduction band and at least two levels of heavy hole states (hh₁ and hh₂) arise in the valence band. Thus, we ascribe the main PL peak to the e₁ → hh₁ intradot transition, whereas complementary high-energy peak arising at high temperatures is assigned to the e₁ → hh₂ transition developing due to the thermal population of the hh₂ hole state. This additional transition significantly modifies the PL line shape at higher temperatures (Fig. 2) and introduces additional PL band broadening resulting in a significant deviation of the calculated temperature dependences from measured temperature dependences of FWHM and PL E_max within the temperature range 150–300 K (Fig. 3a, 3b). It is clear that the asymmetry of the PL band, measured directly (Fig. 4), gives information about both the intra- and interdot-chain carrier relaxation. Thus, it provides more information than one can obtain, say, from a Gaussian fit of the PL spectra (Fig. 2).

Analysis of the temperature dependence of the integrated PL for the multilayer In_0.4Ga_0.6As/GaAs QD chain sample gives further verification of the contribution from the e₁ → hh₂ QD transition. Figure 5 shows this dependence. At least three Arrhenius-type activation energies can be extracted from the plot in Fig. 5: , , and quite satisfactorily agrees with the energy 38 meV extracted from the high temperature difference PL spectra (Fig. 4) for the e₁ → hh₂ transition. The energy we ascribe to the transition from the QD electron ground state (e₁) to the 2D conduction band () of WL, . Indeed this predicts the 2D WL in our system to be ~1.468 eV which is very close to the energy 1.445 eV that has been observed for the 2D In_0.36Ga_0.64As WL in similar QD samples under high optical pumping 24. We were not able to saturate the QD states to see the WL in our samples due to the large density of dots. The energy we ascribe to the transition from the QD ground states to a 1D WL. Its energy is of about 1.391 eV. The existence of such WL is specific for the dot chain sample and has been proved recently by means of scanning electron, transmission electron, and atomic force microscopies 25 as well as by optical spectroscopy 26. It has been shown that in addition to the conventional 2D WL involved in the Stranski–Krastanov growth a 1D-postwetting layer along the [01 − 1] direction arises that connects the QDs in each chain. Whereas the 2D WL exists before the QD chains form, the 1D-postwetting layer develops during the GaAs capping of already existing dot chains. It arises due to anisotropic surface diffusion of In atoms that accompanies the change in strain profile during capping and therefore produces the steady-state material distribution that includes a 1D-postwetting layer as a result.

Thus, the existence of various discrete and continuous states in the multilayer In_0.4Ga_0.6As/GaAs QD chain sample substantially complicates the real picture of relaxation and coupling of the QDs. Based on the temperature dependence of the PL spectra, we can distinguish the low temperature effects caused by the in-chain interdot coupling and high temperature effects related to the QD coupling through the WL and GaAs barrier states. To go further, we focus on the low-temperature interdot coupling. We have demonstrated the cooperative effects caused by in-chain, interdot electronic coupling that becomes transformed into a change of the low temperature PL band and its line-shape. The features related to the interdot-chain, long-range radiative coupling are explored by means of time-resolved PL spectroscopy.

The low temperature PL transients for the multilayer In_0.4Ga_0.6As/GaAs(001) QD chain sample are shown in Fig. 6. The excitation density is chosen to be comparatively low in order to provide the mono-exponential PL decay following the law under excitation with different wavelengths. Here, the time t₀ corresponds to the beginning of declining part of the I_PL(t) dependence and is set to be 0 in further analysis. The rate of decay is characterized with time τ_d. The characteristic rise time for all PL transients is found to not exceed ~20 ps indicating that the carriers are trapped quickly by all QDs. Figure 6a shows the transients measured with the excitation wavelength λ_exc = 750 nm and the detection wavelengths λ_det = 930 nm, 950 nm, 995 nm, and 1030 nm covering the region of the QD ground state emission. For comparison the laser pulse is also shown indicating that the observed PL decay is related to the QD system response only. Figure 6b shows the transients measured at the peak of the ground state emission, λ_det = 995 nm, with pulsed laser excitation varied between λ_exc = 720 nm (above the GaAs bandgap) and λ_exc = 870 nm (below the GaAs bandgap and the InGaAs WL as shown in “Temperature cw PL measurements”). The observed exponential PL decay is well characterized by a single τ_d value depending nevertheless both on the λ_exc and λ_det. Figure 7 summarizes the τ_d(E_det) data measured for various excitation energies E_exc. The dependences are plotted versus the difference. Qualitative analysis of all dependences allows four items to be explained: i) rapid increasing of with decreasing E_det throughout the QD PL spectrum; ii) reaching the maximal when E_det reaches the PL maximum; iii) decreasing the maximal values with decreasing the excitation energy E_exc; and iv) appearance of the dip on the dependences for the detection energies lower than E₀. In order to address these four points we involve two different mechanisms of interdot coupling and the mechanisms of QD coupling with the host material and inherent defects finally affecting the τ_d value.

Finally, peculiar dependences of the PL decay time on the excitation and detection energies are revealed in the multilayer In_0.4Ga_0.6As/GaAs QD chain sample and ascribed to the peculiarities of the carrier and energy relaxation caused by both immediate electronic interdot coupling and long-range coupling through the radiation field. Summarizing, we have established: i) appearance of low temperature asymmetry of the PL band caused by the non-thermal carrier distribution through in-chain interdot wave function coupling. The energy scheme of the In_0.4Ga_0.6As/GaAs QD chain sample is specified and 1D WL, 2D InGaAs WL, and the heavy hole excited states are identified from the temperature cw PL measurements: ii) the in-chain interdot carrier transfer forms the swing of the decay time τ_d versus detection energy dependence and can lead to the state saturation effect shifting the maximal τ_d value close to QD PL maximum; iii) superradiant mode of the emission field couples the QDs of different dot chains and leads to a significant change of the decay time value just in the vicinity of the QD PL maximum. This radiant interdot coupling defines the dependence of the τ_d value on the excitation wavelength. Coexistence of various mechanisms of interdot coupling is determined here by special design of the QD samples providing effective interdot in-chain electronic coupling and effective radiative interchain coupling due to superradiance. This latter type of coupling could play a significant role in ultrafast applications.

E. Marega Jr: On leave from Departamento de Física e Ciência dos Materiais, Instituto de Física de São Carlos-USP São Carlos SP 13560-970, Brazil.