On the importance of antimony for temporal evolution of emission from self-assembled (InGa) (AsSb)/GaAs quantum dots on GaP(001)

PAPER • OPEN ACCESS On the importance of antimony for temporal evolution of emission from self-assembled (InGa) (AsSb)/GaAs quantum dots on GaP(001) To cite this article: Petr Steindl et al 2021 New J. Phys. 23 103029 View the article online for updates and enhancements. You may also like Interfacing quantum emitters with propagating surface acoustic waves Matthias Weiß and Hubert J Krenner - Charge carrier relaxation in InGaAs-GaAs quantum wire modulation-doped heterostructures S V Kondratenko, S A Iliash, Yu I Mazur et al. - Quantitative description of carrier dynamics in GaSb/GaAs quantum-ringwith-dot structures Maetee Kunrugsa - This content was downloaded from IP address 161.111.10.230 on 30/03/2022 at 12:15 New J. Phys. 23 (2021) 103029 https://doi.org/10.1088/1367-2630/ac2bd6 OPEN ACCESS RECEIVED 19 July 2021 REVISED 22 September 2021 ACCEPTED FOR PUBLICATION 30 September 2021 PUBLISHED 21 October 2021 Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. PAPER On the importance of antimony for temporal evolution of emission from self-assembled (InGa) (AsSb)/GaAs quantum dots on GaP(001) Petr Steindl1,2, Elisa Maddalena Sala3,4,BenitoAl ´ en5, Dieter Bimberg3,6and Petr Klenovsk´ y1,7,∗ 1Department of Condensed Matter Physics, Faculty of Science, Masaryk University, Kotlᡠrská 267/2, 61137 Brno, Czech Republic 2Huygens-Kamerlingh Onnes Laboratory, Leiden University, PO Box 9504, 2300 RA Leiden, The Netherlands 3Center for Nanophotonics, Institute for Solid State Physics, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany 4EPSRC National Epitaxy Facility, The University of Sheffield, North Campus, Broad Lane, S3 7HQ Sheffield, United Kingdom 5Instituto de Micro y Nanotecnología, IMN-CNM, CSIC (CEI UAM+CSIC) Isaac Newton, 8, E-28760, Tres Cantos, Madrid, Spain 6‘Bimberg Chinese-German Center for Green Photonics’ of the Chinese Academy of Sciences at CIOMP, 13033 Changchun, People’s Republic of China 7Czech Metrology Institute, Okružní 31, 63800 Brno, Czech Republic ∗Author to whom any correspondence should be addressed. E-mail: kleno[email protected] Keywords: III–V semiconductors, quantum dots, photoluminescence, carrier dynamics, lifetimes Supplementary material for this article is available online Abstract Understanding the carrier dynamics of nanostructures is the key for development and optimization of novel semiconductor nano-devices. Here, we study the optical properties and carrier dynamics of (InGa)(AsSb)/GaAs/GaP quantum dots (QDs) by means of non-resonant energy and time-resolved photoluminescence depending on temperature. Studying this material system is fundamental in view of the ongoing implementation of such QDs for nano memory devices. The structures studied in this work include a single QD layer, QDs overgrown by a GaSb capping layer, and solely a GaAs quantum well, respectively. Theoretical analytical models allow to discern the common spectral features around the emission energy of 1.8 eV related to the GaAs quantum well and the GaP substrate. We observe type-I emission from QDs with recombination times between 2 ns and 10 ns, increasing towards lower energies. Moreover, based on the considerable tunability of the QDs depending on Sb incorporation, we suggest their utilization as quantum photonic sources embedded in complementary metal-oxide-semiconductor platforms, due to the feasibility of a nearly defect-free growth of GaP on Si. Finally, our analysis confirms the nature of the pumping power blue-shift of emission originating from the charged-background induced changes of the wavefunction spatial distribution. 1. Introduction In the last few decades, semiconductor nano-structures as self-assembled III–V quantum dots (QDs) have been investigated due to their wide range of novel physical properties. Such QDs can be employed in a number of different applications, such as active media in semiconductor lasers [1–3], as building blocks for quantum information devices, particularly for quantum repeaters [4–6], as efficient single and entangled photon sources [7–16], including highly-entangled states for quantum computing [17–20], or as nanomemories [21–25]. Among III–V QDs, particularly type-I indirect (InGa) (AsSb)/GaAs QDs embedded in a GaP(001) matrix [26,27] have recently attracted attention due to their promising use as storage units for the QD-flash nanomemory cells [26,27], as potentially effective entangled photon sources [28], owing to their smaller fine-structure splitting (FSS) of the ground state exciton compared to well-known type-I systems such as (InGa)As/GaAs [13,14], and as quantum gates [28–31]. The concept of © 2021 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft New J. Phys. 23 (2021) 103029 PSteindlet al hole storage QD-flash was initially suggested by Bimberg and co-workers [21–25,32] following first pioneering studies [32] regarding the mechanisms of electron escape from InAs/GaAs QDs, by using the deep level transient spectroscopy. The key feature of the QD-flash is to combine the fast access times of dynamic random access memories with the non-volatility of the flash, which leads to a universal memory type, potentially simplifying future computer architectures. Recently, type-I indirect (InGa) (AsSb)/GaAs/GaP QDs showed an improvement of one order of magnitude in the storage time compared to pure In0.5Ga0.5As/GaAs/GaP QDs [33,34], reaching ∼1 h at room temperature [26,27]. This result represents to date the experimental record for metal-organic vapor phase epitaxy (MOVPE)-grown QDs, thus opening up the possibility to use this technique to fabricate memory devices based on high-quality III–V semiconductor QDs. The storage time can be increased by further systematic growth parameter optimization of the Sb/P-based type II system, up to the non-volatile regime with storage times exceeding 10 years [35]. Additionally, in reference [28] the authors theoretically discussed the physical properties of such material system—particularly the quantum confinement type—depending on the relative In/Ga and As/Sb contents in the QDs. It was found that these QDs showed concurrently both direct and indirect optical transitions for increasing Sb content, finally leading to type-II band alignment [28]. That made such QDs be excellent candidates for quantum information technologies. Increasing the Sb content in the QDs has been previously made possible by overgrowing (InGa) (AsSb)/GaAs/GaP QDs with a GaSb capping layer, which has effectively modified the QD composition [36]. Moreover, through detailed investigations of their optical properties, it was found that such procedure led to an energy swapping of the Γand L states, thereby increasing the wavefunction leakage outside the QDs [28,36]. This property is indeed very appealing for further improvement of storage times since an increased Sb incorporation into the QDs leads to increased hole localization energy [24,25,28]. Finally, fabricating QDs on GaP substrates is advantageous in terms of integration on silicon platforms, since the lattice mismatch between GaP and Si amounts to just 0.4%, thus making defect-free MOVPE growth of GaP on Si possible [37]. In this work, we study the carrier dynamics of (InGa) (AsSb)/GaAs/GaP QDs by means of time-resolved-photoluminescence (TRPL) for varying detection energy and sample temperature. This allows us to better separate the spectrally overlapping optical transitions previously observed in our recent work [36]. First, we provide a brief overview of our sample structures. Afterwards, we discuss the experimental results on carrier lifetimes for varying measurement conditions. Analytical models, describing the observed physical phenomena are provided, leading us to discern the different types of optical transitions involved. We would like to point out that, to date, there is no such detailed optical investigation on this material system. Finally, we discuss the material structure in the context of quantum technology applications and comment on the current experimental limitations. 2. Sample structures The samples were grown by MOVPE in Stranski–Krastanov (SK) mode on GaP(001) substrates at the TU Berlin [26,27]. The structures of the samples studied in the present work are schematically depicted in all figures as insets. All samples include 5 Ml-thick GaAs interlayer (IL), a crucial ingredient for the subsequent QD formation, as pointed out by Sala et al [26,38]. The sample having the IL only is referred to as Sw/o,that labeled Swith (Scap) contains (InGa) (AsSb) QDs, without (with) ∼1 Ml GaSb capping. The QDs of density approximately 1 ×1011 cm−2are of truncated pyramid shape, with basis diameter of ∼15 nm and height of 2.5±0.4nm[26,28,36,39] and were grown with 1 s Sb-flush triggering QDs surface passivation by As–Sb exchange, leading to a decreased tendency of defect formation during their formation [38]. For detailed information about the growth procedure, see references [26,27,36]. Additional details on their structure, particularly on size, shape, and composition, can be found in very recent work on XSTM and atom probe tomography investigations on such QD samples [39]. The sample photoluminescence (PL) is found at ∼1.8 eV and shows several not well spectrally separated bands, representing a combination of momentum direct and indirect type-I transitions from QDs [36], visualized in band-scheme diagrams in figure 1calculated by eight-band k·pmethod using NEXTNANO++ simulation suite [40,41]. For more information about calculations, we refer to detail theoretical study of electronic states of this material system provided in reference [28]. 3. Experimental setup for TRPL measurements To populate the whole structure with carriers and thus study their dynamics, TRPL experiments were carried out with a pulsed laser with the wavelength of 405 nm, focussed on 0.06 mm2area with a 60 ps 2 New J. Phys. 23 (2021) 103029 PSteindlet al Figure 1. Cross-section along growth direction obtained by NEXTNANO++ [40] simulation of band schemes including eigenenergies indicated by dotted lines for samples (a) Sw/o,(b)Swith,and(c)Scap. The insets show the experimentally observed recombination times τi(icorresponds to recombination origin, see text), transition (taken from fits of PL in the time domain, solid lines) and escape (dashed line) energies derived later in the text. Light (lh) and heavy hole (hh) splitting of valance band are indicated by different line styles (lh: dashed, hh: solid). pulse-width. The emitted PL spectrum was dispersed by 1200 grooves/mm ruled grating and detected around 1.8 eV by a Si avalanche photodiode. First, we cooled the samples to 15 K, and detected in 200 ns temporal window the energy-resolved TRPL signal for each wavelength. Then, within temperature-resolved TRPL, the sample temperature Twas varied in the range 15–130 K. Here, the temporal window was modified to maximize the resolution from 200 ns for lower T,to25nsforhigherT. Changing the temporal window is connected with changes in repetition rate, which was varied between 5 MHz (for the temporal window 200 ns; used also for energy-resolved TRPL) and 80 MHz (for 25 ns). 4. Spectral line-shape model For the description of macro-PL in the time domain (TDPL), we take advantage of the similarity in the grown structures, leading to expected shared spectral features across samples associated with carriers confined in the GaAs IL, i.e. zero-phonon (ZPL) and phonon-replica (rep-ZPL) transitions of electrons from Xxy conduction minima to Γvalence band maximum [36,43]. Through analysis of the line-shape in the Sw/osample, we conclude that the convolution of two asymmetrical bands with maximum emission energy Emax concurrently showing a small high-energy and a prominent low-energy band-tail produce better results than the purely Gaussian spectral deconvolution used in reference [36]. The low energy tail shall be related to carrier localization into long-range IL potential fluctuations [44]. Meanwhile, high energy tails shall be related to phonon-assisted thermal population of delocalized states, especially at large excitation powers/temperatures or during the initial stages of the relaxation process. We follow the work of Almosni et al to describe the low energy tail long-range fluctuations through the following equation [44] I∝exp(/Elong) Elong exp(−exp(/Elong)), (1) where a single parameter Elong characterizes the long-range potential disorder energy. Meanwhile, hot carrier population is taken into account through an nphonon-assisted thermalization process by line-shape [45] In∝5/2−nexp − kBTca (2) with carrier thermalization energy of kBTca;=E−Emax. We limit our description of IIL (convolution of equations (1)and(2)) to one-phonon process (n=1) only. 3 New J. Phys. 23 (2021) 103029 PSteindlet al Figure 2. Excitation power dependence of emission energies of samples (a) Sw/o,(b)Swith,and(c)Scap. Symbols represent the emission energies fitted from steady-state PL spectra. A typical (normalized) spectrum of each sample measured with D=3.3Wcm −2together with colored band-reconstruction over spectral range of 1650–1900 meV is shown in insets (semi-logarithmic scale). The emission energies evolve in agreement with diffuse interface model for spatial type-I transitions [36,42] (solid lines). Low-power emission energies of IL transitions in Swith (Scap) are red-shifted by Ew(Ec)inrespecttothatin Sw/o. 5. Excitation power resolved steady-state PL Before moving to time-resolved analysis, we show in figure 2the validity of the fitting model by applying it to the steady-state PL vs continuous-wave excitation power dependence Dmeasured at 15 K and published in our previous study [36]. As it can be seen in figure 2, two replicas of the above lineshape model account for most of the PL emission in these samples, yet not completely. To describe the full PL spectrum, two additional Gaussian profiles are necessary, consistent with continuous-wave macro-PL experiment resolved by excitation power, polarization and sample temperature [36]. One of them describes a rather broad band (FWHM larger than 35 meV), clearly observable only at very low excitation powers, likely originating in the donor-acceptor pair (DAP) transitions in GaP [46,47] or other defect induced during GaAs IL and QDs formation (the latter in the case of samples with QDs). We attribute the second Gaussian band to the recombination from QDs, being due to non-optimized excitation wavelength, and thus very weak and observable mainly for high excitation powers. Similarly as there, the fitted peak energies are used to analyse the emission blue-shift with increasing D, in order to determine the type of carrier spatial confinement. Although elsewhere in the literature [48–52] the presence of blue-shift is automatically assigned to indirect spatial alignment, the so-called type-II, we examine here the blue-shift by E=E0+Uln(D)+βD1/3[36,42] allowing us to disentangle type-II band-bending, due to state squeezing represented by the parameter β,fromthespatialalignment independent blue-shift caused by crystalline defects described by the Urbach energy tail U.Havingβ negligible, the analysis in figure 2suggests that the emission bands of our heterostructures are of type-I, i.e. spatially direct, as also previously reported based on Gaussian fits [36] and in agreement with k·p simulations [28]. Moreover, we observe that ZPL and rep-ZPL transitions of samples Swith and Scap are red-shifted in respect to their energies observed from PL of Sw/oby Ew=52 meV and Ec=82 meV, respectively. This shift partially reflects the strain-relaxation initialized by constituent segregation from QD-layer [39] and, thus, partially induced change in band confinement. The former is connected also with the natural spectral broadening when additional localized defect states are created in the heterostructure. These additional states then form an effective background potential increasing with excitation power, leading to the energy blue-shift of bands of samples with QDs, characterized by the Urbach energy. However,thebandsofthesamplewithonlyGaAsILdonot manifest blueshift themselves. A similar shift can be also observed in the time domain after the non-resonant pulse-excitation when the carriers first thermalize into the trap states and form the initial background potential. As those recombine, Elong decreases, the potential weakens and, thus, the emission energy is gradually red-shifted, as we will discuss later in more detail. This potential weakening is connected also with the spreading of the state wavefunctions, effectively observable as an increase in recombination times in the excitation resolved TRPL, see supplemental information (https://stacks.iop.org/NJP/23/103029/mmedia)[53]. Although we attribute the QD band in the emission of samples with dots, we expect, in the studied spectral range, even richer spectral response related to momentum-indirect transitions of QDs [28]and their compositional variations [39]. These are most likely shadowed by much stronger GaAs IL emission whichincomparisonwithQDshaving3Dquantumconfinement provides much more states for the 4 New J. Phys. 23 (2021) 103029 PSteindlet al population. To reveal those in future studies, micro-PL experiments with pre-optimized excitation wavelength will be necessary. 6. Emission energy dependent TRPL In this section, we study the energy-resolved carrier dynamics in our heterostructures by TRPL. To assign the recombination times to the characteristic bands, we first fit the signal (see raw experimental data in figure 3) in individual time bins by the spectral shape model discussed in the previous part, and we refer to this analysis as time-domain PL (TDPL). For the best-fit results presented in figure 4,weusethe parameters obtained from steady-state excitation power dependency. Later, we analyse the signal for each wavelength also by the double mono-exponential model (2ME) I(t)=A1exp(−t/τ1)+A2exp(−t/τ2), (3) characterized by amplitude A1(A2)anddecaytimeτ1(τ2) for the slow (fast) decay process. In the case of samples with QDs, we added to the analysis also the third exponential decay component (τ3), representing the electron–hole recombination in QDs. Typical TRPL deconvolution to individual decay channels for each sample taken at the PL maximum is shown in figures 3(d)–(f); extracted time constants qualitatively agree with TDPL analysis. Finally, we analyze the spectral distribution of the time decay constants τ1–τ3by an analytical model developed by Gourdon and Lavallard [54]: τ=τr 1+exp[(E−Eme)/U0](4) which is widely used in the literature [55,56], even though in equation (4) the hopping processes [54]or temperature dependence [57] are not included. The meaning of the parameters in equation (4) is as follows: τris the exciton radiative lifetime, Eme the characteristic energy for which the radiative time equals the transfer one, analogously to a mobility edge [56,58], and U0is the measured energy of localized states, similar to Urbach energy tail, responsible for the observed energy blue-shift [42]. Note, that τ1process decays rather slowly and does not completely disappear in one temporal window, therefore we take into account its repumping from previous pulses in TRPL fits, as discussed in the appendix. This issue is overcome in TDPL by disentangling individual transitions by line-shape model fitting, where the slowest decay is assigned to (mainly non-radiative) pair recombination of DAP in GaP [46,47]. Moreover, in spectral dependence for the evaluation of τ1we need to extend the model (4) by an additional contribution, likely connected with other defects created during the epitaxial growth process. Note, that we do not assume effects such as dark excitons of QDs and dark states in general as well, which are typically much weaker than bright-state QD emission, particularly in macro-PL of samples with a non-optimized excitation wavelength and a large spectral overlap of transitions. These effects can become observable and more information about the structure dynamics can be gained if only few QDs are isolated, either by near-field optical methods or in micro-PL experiments, ideally with more efficient optical addressing of the QD area by excitation laser closer to the energy of the transitions (∼1.8eV),yetbelow deep level resonances in GaP (approx. 2.2 eV) [47] to minimize absorption in substrate, or by optical pumping of the wetting layer [59,60]. 6.1. Sample without QDs Sw/o We start our discussion with the sample Sw/o. TDPL deconvolution allows us to study not only the relaxation-time constants of the considered decay process but also the energy changes of the state in the time domain. Specifically, the emptying of the impurity states entails an exponential-like decrease of the emission energies of the total energy ΔEfor both ZPL and rep-ZPL bands, also recently observed for relaxed GaAs/GaP QDs with type-I band-alignment [61]: E(t)=E0+ΔEexp(−t/τE), (5) where E0+ΔEis the energy of the observed state after laser excitation, which exponentially decays proportionally to the time constant τE(an effective time when impurities and defects affect the electron state) to electron energy E0. That can be equally well understood as due to defects at the interfaces between segments of the heterostructure, which create a local electric field (non-equilibrium carriers) leading to red-shift ΔEof the electron state with energy E0. The carriers then recombine for τEupon which the eigenvalue of electron state returns to its value without the presence of the local field E0. Note, that the shift ΔEcannot be caused by inter-valley scattering, which is three orders of magnitude faster than the observed τE[62,63], nor by the thermalization of higher excited states (since τE>radiative recombination times) or 5 New J. Phys. 23 (2021) 103029 PSteindlet al Figure 3. False-color plots of PL intensity as a function of time and emission energy for samples (a) Sw/o,(b)Swith,and(c)Scap. The color scale is identical for all samples. Deconvolution of TRPL signal to individual channels taken at PL maximum of the sample: (d) Sw/oat 1.84 eV, (e) Swith at 1.80 eV, and (f) Scap at 1.77 eV. The fit (solid black line) of experimental data (gray symbols) includes Gaussian pulse detector response (purple), background of 50 counts (not shown), and 2 or 3 exponential decays distinguished by color (QD: orange dash–dotted; ZPL: green dashed; DAP: red dotted) including repumping. thermalization of free-carrier created after excitation which is of one order of magnitude faster, see Tca in supplemental information [53]. Even though both bands are shifted by few units of meV, similarly to the total blue-shift observed in steady-state experiments, the integral TDPL spectrum taken at different times of measurement does not show any significant shift and decays equally in time proportionally to the decay around 10–15 ns, see inset of figure 4(a) and table 1. Note, that since for the studied samples the energy level separations of IL, DAP, and QDs are not clearly distinguishable, we use double mono-exponential decay function (with time constants τTDPL 1and τTDPL 2) to deconvolute the emission intensity, where the origin of the second time constant is assigned according to the following: DAP and other non-radiative defects decay slowly (τTDPL 2>40 ns), whereas QD transition is fast (τTDPL 2<10 ns). The standard TRPL deconvolution at each wavelength in figure 5(a) shows two contributions. The faster, being in good agreement with ZPL and rep-ZPL TDPL band decays, with time constants around 13 ns contributes more or less constantly by 20% to the total intensity (panel (b)). The slower process, related to DAP and crystalline defects, increases the time-constant up to ∼200 ns towards lower energies where none transition from GaAs IL is expected [28,43] and is saturated below 1.79 eV as expected from the similarity with the two other samples. Note, that similar behaviour with extremely slow (up to few μs) low-energy transition were independently reported for (In, Ga)As/GaP [64,65], Ga(As,P)/GaP [66], and GaSb/GaP [67] as momentum-indirect transitions from QDs. Because we observe such transition not only for our QDs with completely different stoichiometry but also for GaAs/GaP sample clearly without any QDs, we tend to assign the slow transition to defects in GaP substrate [68,69], common for all reported structures. Furthermore, we note in figure 5(a) a good agreement between TDPL and TRPL time constants, allowing us to deduce, in power and temperature resolved experiments, the character of relaxation based on the results of TRPL measurements only. 6.2. Sample with QDs Swith The whole spectrum of Swith (figure 3(b)), including ZPL and rep-ZPL bands, is also red-shifted in TDPL in respect to that of Sw/o, approximately by Ew,seefigure4(b) and table 2. That is close to the energy shift of 6 New J. Phys. 23 (2021) 103029 PSteindlet al Figure 4. Fitted TDPL emission energies (symbols) which exhibit exponential-like energy red-shift with temporal evolution (fit, black solid lines). While for Sw/oin (a), the shift is timid, for samples Swith (b) and Scap (c) it exceeds 10 meV and leads to an observable spectral-shape variation within temporal evolution (see panels (a)–(c) of figure 3and emission energy shift of the spectra in insets with color-coded fitted emission bands across the spectral range of 1.65–1.9 eV at t=0 and after 135 ns). The broken grey vertical lines indicate the moment of the laser pulse excitation (t=0 in insets). Table 1. Summary of the best-fit parameters of the spectral shape model applied to the excitation power resolved PL and TDPL of all studied samples. Symbol ∗(∗∗) refers to a discrepancy of +10 meV (−5meV)inE0from TDPL in respect to the extracted value from the excitation power-dependent PL. For QDs, we give FWHM as Elong. Sample Transition Elong (meV) E0(meV) U(meV) ΔE(meV) τE(ns) τTDPL 1(ns) τTDPL 2(ns) Sw/o ZPL 10 1858 ±0.40.5±0.21.3±0.450±40 10.7±0.252±1 Rep-ZPL 14 1826 ±0.40.8±0.15.9±0.431±511±387.6±0.7 Swith ZPL 19 1796∗±13.9±0.4 13.8±0.541±46.8±0.147±1 Rep-ZPL 20 1765∗±12.8±0.411±146±6 12.9±0.547±1 QDs 19 1777∗±23.6±0.6 14.3±0.535±4 10.4±0.1 Scap ZPL 20 1764 ±0.44.4±0.117±144±7 14.9±0.12.0±0.1 Rep-ZPL 23 1733 ±0.43.1±0.25.4±0.719±468±4 QDs 8 1796∗∗ ±0.60.7±0.210±14.1±0.47.7±2 Eme(Sw/o)−Eme(Swith)=47 meV for ZPL (55 meV for rep-ZPL) and together with similar time constants τTDPL 1, pointing to similar physics behind the IIL transitions. The best fit emission energies of ZPL and rep-ZPL after excitation show non-equilibrium carrier background potential, initially squeezing the electron wavefunction [48,70]. Later, as the potential weakens, the wavefunction spatially spreads, leading to the gradual red-shift ΔEof 14 meV and 11 meV for ZPL and rep-ZPL bands, respectively, to their steady-state energies. This time, in agreement with large blue-shift in excitation power-dependent PL, the shifts are more prominent due to significantly increased number of defects created within QD layer formation and later due to additional atom segregation [39]. In addition to the sample Sw/o,weobservealsoΔEof 14 meV for the TDPL QD band with time constant of ∼10 ns, suggesting impurity induced dynamics connected with the GaAs layer. The TRPL signal, deconvoluted by equation (3) by three mono-exponential decay contributions, shows two patterns: one similar to that observed for Sw/o, and also a much faster one, which we attribute to the emission from QDs. These processes, depicted in panels (c) and (d) of figure 5, have different weight [wi=Aiτi/(3 jAjτj)] across the measured spectral range. While for energies below 1.75 eV the DAP dynamical processes dominate in weight, they lose importance for larger energies in favor to the processes involving the GaAs IL. The QD contributions are almost negligible in weight in the whole spectral range, except for an increase of w3, corresponding to QDs, centered around 1.80 eV and 1.83 eV, where w3is larger than 10%. The mean values of τ3in these spectral ranges are 9.0±1.0nsand6.0±1.0ns, respectively. Note that we cannot find any conclusion about the QD transition brightness purely from its weight, since its value is affected by the recombination times of transitions which QDs are compared with—in our case decay times of up to two orders magnitude higher than QDs. On the other hand, the TRPL deconvolution in figure 3and also fit of TDPL data in inset of figure 4fully reveal the contribution to 7 New J. Phys. 23 (2021) 103029 PSteindlet al Figure 5. The energy dispersion of (a) time constants and (b) corresponding weights w[wi=Aiτi/(jmax jAjτj), jmax =2(3) for 2ME (3ME) fitting model] for sample Sw/oobtained by fitting the TRPL signal by the double mono-exponential model using equation (3) (symbols) and fitted by the Gourdon–Lavallard’s model (4) (solid lines) [54]. That for samples Swith and Scap obtained from fitting of the TRPL signal by triple mono-exponential model using equation (3) is shown in panels ((c) and (d)) and ((e) and (f)), respectively. The deconvoluted time constants show good agreement with TDPL intensity decays (full symbols with arrows representing time-domain ΔEshift; transitions are assigned by color in agreement with figure 4). Shaded areas of 1–10 ns, 10–40 ns, and >100 ns correspond to different recombination channels. Table 2. Parameters obtained from Gourdon and Lavallard model, equation (4). Units of the variables are: τi r is in ns, Ei me and Ui 0are in meV. GaAs IL GaAs IL, phonon rep. Sample τZPL rEZPL me UZPL 0τrep rErep me Urep 0 Sw/o13.0±1.0 1882 ±34±2 14.4±2.4 1856 ±24.3±1.4 Swith 31.5±0.7 1835 ±18.0±0.6 30.7±0.3 1801 ±22.7±1.1 Scap 18.4±0.5 1792 ±111±1 18.8±0.3 1743 ±12.5±0.9 Growth defects DAP in GaP Sample τd rEd me Ud 0τDAP rEDAP me UDAP 0 Sw/o90 ±1 1877 ±15.3±0.2 260 ±30 1776 ±315.6±0.5 Swith 284 ±2 1810 ±1 14.9±0.1 561 ±1 1781 ±117.0±0.1 Scap 1156 ±1 1737 ±117.6±0.2 the signal, which is prominent few nanoseconds after excitation when it clearly exceeds concurrent DAP recombination, even though w3≈10%. For the spectral characteristic of the transitions, the Gourdon and Lavallard model [54]wasusedby means of one contribution for the process τ2,andtwocontributionsfortheprocessτ1.Thebest-fitvalues (see table 2) show the mobility edge of the ZPL transition in IL shifted with respect to that of Sw/oby 47 meV, which is in the agreement with the shift of the whole spectrum discussed previously. On the other hand, the mobility edge of DAP in GaP remains not affected by the heterostructure. The radiative time of the ZPL (rep-ZPL) band is 31.5±0.7ns(30.7±0.3 ns), which is more than two times larger than that of the sample without QDs. That increase can be understood in terms of different material distribution, as an effect of strain relaxation discussed in [36] due to the GaAs IL overgrowth with QDs, leading to the change of the confinement potentials. On the other hand, disorder energies U0originating from material redistribution—in our case mainly due to the strain relaxation—are higher than for Sw/o, indicating increased disorder of GaAs IL interface. 6.3. Sample with GaSb-capped QDs Scap As previously shown in [36] by comparison of experimental and k·psimulated emission energies, overgrowing the QDs with a thin (∼1 Ml) GaSb cap leads to an effective increase of the Sb content in QDs. Through the TDPL analysis of sample Scap using the line-shape model with emission energies and FWHM 8 New J. Phys. 23 (2021) 103029 PSteindlet al [34] Stracke G, Sala E M, Selve S, Niermann T, Schliwa A, Strittmatter A and Bimberg D 2014 Indirect and direct optical transitions in In0.5Ga0.5As/GaP quantum dots Appl. Phys. Lett. 104 123107 [35] Nowozin T, Bimberg D, Daqrouq K, Ajour M N and Awedh M 2013 Materials for future quantum dot-based memories J. Nanomater. 1–6 215613 [36] Steindl P, Sala E M, Al´ en B, Marr´ onDF,BimbergDandKlenovsk ´ y P 2019 Optical response of (InGa)(AsSb)/GaAs quantum dots embedded in a gap matrix Phys. Rev. B100 195407 [37] Grassman T J, Carlin J A, Galiana B, Yang L-M, Yang F, Mills M J and Ringel S A 2013 Nucleation-related defect-free GaP/Si(100) heteroepitaxy via metal-organic chemical vapor deposition Appl. Phys. Lett. 102 142102 [38] Sala E M et al 2016 Growth and structure of In0.5Ga0.5Sb quantum dots on GaP(001) Appl. Phys. Lett. 109 102102 [39] Gajjela R S R et al 2021 Structural and compositional analysis of (InGa)(AsSb)/GaAs/GaP Stranski–Krastanov quantum dots Light Sci. Appl. 10 125 [40] Birner S, Zibold T, Andlauer T, Kubis T, Sabathil M, Trellakis A and Vogl P 2007 IEEE Trans. Electron Devices 54 2137 [41] Zibold T 2007 PhD Thesis Technische Universität München [42] Abramkin D S, Gutakovskii A K and Shamirzaev T S 2018 Heterostructures with diffused interfaces: luminescent technique for ascertainment of band alignment type J. Appl. Phys. 123 115701 [43] Prieto J A, Armelles G, Pistol M-E, Castrillo P, Silveira J P and Briones F 1997 Optical studies of GaAs quantum wells strained to GaP Appl. Phys. Lett. 70 3449–51 [44] Almosni S et al 2016 Correlations between electrical and optical properties in lattice-matched GaAsPN/GaP solar cells Sol. Energy Mater. Sol. Cells 147 53–60 [45] Amtout A 1995 Calculation of the line shape of one-phonon replicas in polar semiconductors having direct forbidden band gaps Phys. Rev. B52 13955–64 [46] Dean P J, Henry C H and Frosch C J 1968 Infrared donor–acceptor pair spectra involving the deep oxygen donor in gallium phosphide Phys. Rev. 168 812–6 [47] Dean P J 1970 Recombination processes associated with ‘deep states’ in gallium phosphide J. Lumin. 1–2 398–419 [48] Klenovsk´ y P, Steindl P and Geffroy D 2017 Excitonic structure and pumping power dependent emission blue-shift of type-II quantum dots Sci. Rep. 745568 [49] Jo M, Sato M, Miyamura S, Sasakura H, Kumano H and Suemune I 2012 Origin of the blueshift of photoluminescence in a type-II heterostructure Nanoscale Res. Lett. 7654 [50] Ledentsov N N et al 1995 Radiative states in type-II GaSb/GaAs quantum wells Phys. Rev. B52 14058–66 [51] Jin C Y, Liu H Y, Zhang S Y, Jiang Q, Liew S L, Hopkinson M, Badcock T J, Nabavi E and Mowbray D J 2007 Optical transitions in type-II InAs/GaAs quantum dots covered by a GaAsSb strain-reducing layer Appl. Phys. Lett. 91 021102 [52] Gradkowski K, Ochalski T J, Williams D P, Healy S B, Tatebayashi J, Balakrishnan G, O’Reilly E P, Huyet G and Huffaker D L 2009 Coulomb effects in type-II Ga(As)Sb quantum dots Phys. Status Solidi B246 752–5 [53] Supplemental information (URL to be inserted by publisher). [54] Gourdon C and Lavallard P 1989 Exciton transfer between localized states in CdS1−xSexalloys Phys. Status Solidi B153 641–52 [55] Rubel O, Stolz W and Baranovskii S D 2007 Spectral dependence of the photoluminescence decay in disordered semiconductors Appl. Phys. Lett. 91 021903 [56] Sugisaki M, Ren H-W, Nishi K, Sugou S and Masumoto Y 2000 Excitons at a single localized center induced by a natural composition modulation in bulk Ga0.5In0.5PPhys. Rev. B61 16040–4 [57] Su Z and Xu S 2017 A generalized model for time-resolved luminescence of localized carriers and applications: dispersive thermodynamics of localized carriers Sci. Rep. 713 [58] Oueslati M, Benoit C and Zouaghi M 1988 Resonant Raman scattering on localized states due to disorder in GaAs1−xPxalloys Phys. Rev. B37 3037–41 [59] Johansen J, Stobbe S, Nikolaev I S, Lund-Hansen T, Kristensen P T, Hvam J, Vos W L and Lodahl P 2008 Size dependence of the wavefunction of self-assembled InAs quantum dots from time-resolved optical measurements Phys. Rev. B77 1–4 [60] Johansen J, Julsgaard B, Stobbe S, Hvam J M and Lodahl P 2010 Probing long-lived dark excitons in self-assembled quantum dots Phys. Rev. B81 1–4 [61] Shamirzaev T S, Abramkin D S, Gutakovskii A K and Putyato M A 2010 High quality relaxed GaAs quantum dots in gap matrix Appl. Phys. Lett. 97 023108 [62] Zollner S, Gopalan S and Cardona M 1989 Intervalley deformation potentials and scattering rates in zinc blende semiconductors Appl. Phys. Lett. 54 614–6 [63] Sosnowski T S, Norris T B, Jiang H, Singh J, Kamath K and Bhattacharya P 1998 Rapid carrier relaxation in In0.4Ga0.6As/GaAs quantum dots characterized by differential transmission spectroscopy Phys. Rev. B57 R9423–6 [64] Robert C et al 2012 Electronic, optical, and structural properties of (In, Ga)As/GaP quantum dots Phys. Rev. B86 205316 [65] Robert C et al 2016 Electronic wave functions and optical transitions in (In, Ga)As/GaP quantum dots Phys. Rev. B94 075445 [66] AbramkinDS,PutyatoMA,BudennyySA,GutakovskiiAK,SemyaginBR,PreobrazhenskiiVV,KolomysOF,StrelchukVV and Shamirzaev T S 2012 Atomic structure and energy spectrum of Ga(As, P)/GaP heterostructures J. Appl. Phys. 112 083713 [67] Abramkin D S, Putyato M A, Gutakovskii A K, Semyagin B R, Preobrazhenskii V V and Shamirzaev T S 2012 New system of self-assembled GaSb/GaP quantum dots Semiconductors 46 1534–8 [68] Mannik L, Jedral L and Ruda H E 1992 Decay of green photoluminescence in GaP: Zn, O Phys. Rev. B46 3826–9 [69] Moser K, Baumgartner R and Prettl W 1984 Decay of the red Cu-S pair luminescence in GaP Solid State Commun. 49 761–4 [70] Llorens J M et al 2019 Topology driven g-factor tuning in type-II quantum dots Phys.Rev.Appl.11 044011 [71] Rautert J, Shamirzaev T S, Nekrasov S V, Yakovlev D R, Klenovsk´ y P, Kusrayev Yu G and Bayer M 2019 Optical orientation and alignment of excitons in direct and indirect band gap (In, Al)As/AlAs quantum dots with type-I band alignment Phys. Rev. B99 195411 [72] Schimpf C et al 2019 Resolving the temporal evolution of line broadening in single quantum emitters Opt. Express 27 35290–307 [73] Manna U, Zhang Q, Dhomkar S, Salakhutdinov I F, Tamargo M C, Noyan I C, Neumark G F and Kuskovsky I L 2012 Radiative transitions in stacked type-II ZnMgTe quantum dots embedded in ZnSe J. Appl. Phys. 112 063521 [74] Alonso-´ Alvarez D 2011 Strain balanced epitaxial stacks of quantum dots and posts PhD Thesis, Dissertation Universidad Aut´ onoma de Madrid [75] Gurioli M, Vinattieri A, Colocci M, Deparis C, Massies J, Neu G, Bosacchi A and Franchi S 1991 Phys. Rev. B44 3115–24 [76] Daly E M, Glynn T J, Lambkin J D, Considine L and Walsh S 1995 Behavior of In0.48Ga0.52P quantum-well luminescence as a function of temperature Phys. Rev. B52 4696–9 15 New J. Phys. 23 (2021) 103029 PSteindlet al [77] Oberli D Y, Dupertuis M-A, Reinhardt F and Kapon E 1999 Effect of disorder on the temperature dependence of radiative lifetimes in V-groove quantum wires Phys. Rev. B59 2910 [78] Citrin D S 1993 Radiative lifetimes of excitons in quantum wells: localization and phase-coherence effects Phys. Rev. B47 3832–41 [79] Klenovsk´ yP,K ˇ rápek V, Munzar D and Humlíˇ cek J 2010 Modelling of electronic states in InAs/GaAs quantum dots with GaAsSb strain reducing overlayer J. Phys.: Conf. Ser. 245 012086 [80] Klenovsk´ yP,K ˇ rápek V, Munzar D and Humlíˇ cek J 2010 Electronic structure of InAs quantum dots with GaAsSb strain reducing layer: localization of holes and its effect on the optical properties Appl. Phys. Lett. 97 203107 [81] Klenovsk´ yP,HemzalD,SteindlP,ZíkováM,K ˇ rápek V and Humlíˇ cek J 2015 Polarization anisotropy of the emission from type-II quantum dots Phys. Rev. B92 241302(R) [82] Dadgostar S 2016 (Al, Ga)(As, P) structures in the GaP matrix: growth and characterization Doctoral Thesis Humboldt-Universität Berlin, Berlin [83] Skazochkin A V, Krutogolov Yu K and Kunakin Yu I 1995 Nature of some electron traps in GaP Semicond. Sci. Technol. 10 634 [84] Ioffe Institute NSM archive—physical properties of semiconductors www.ioffe.rssi.ru (accessed 7 October 2021) [85] Abramkin D S, Petrushkov M O, Putyato M A, Semyagin B R, Emelyanov E A, Preobrazhenskii V V, Gutakovskii A K and Shamirzaev T S 2019 GaAs/GaP quantum-well heterostructures grown on Si substrates Semiconductors 53 1143–7 [86] Desplanque L, Coinon C, Troadec D, Ruterana P, Patriarche G, Bonato L, Bimberg D and Wallart X 2017 Morphology and valence band offset of GaSb quantum dots grown on GaP(001) and their evolution upon capping Nanotechnology 28 225601 [87] Nowozin T 2014 Self-Organized quantum dots for memories, electronic properties and carrier dynamics Doctoral Thesis Technische Universität Berlin, Berlin [88] Sala E M, Godsland M, Trapalis A and Heffernan J 2021 Effect of cap thickness on InAs/InP quantum dots grown by droplet epitaxy in metal-organic vapor phase epitaxy Phys. Status Solidi RRL 15 2100283 [89] Emberger V, Hatami F, Ted Masselink W and Peters S 2013 AlP/GaP distributed Bragg reflectors Appl. Phys. Lett. 103 1–5 [90] Rauter P, Spindlberger L, Schäffler F, Fromherz T, Freund J and Brehm M 2018 Room-temperature group-IV led based on defect-enhanced ge quantum dots ACS Photonics 5431–8 [91] Murphy-Armando F, Brehm M, Steindl P, Lusk M T, Fromherz T, Schwarz K and Blaha P 2021 Light emission from direct band gap germanium containing split-interstitial defects Phys. Rev. B103 85310 16