# Investigation of Non-Radiative Processes in InAs/(Ga,In)(N,As) Quantum Dots.

Maxime Hugues, Mirja Richter, Jean-Michel Chauveau, Benjamin Damilano, Jean-Yves Duboz, Jean Massies, Thierry Taliercio, Pierre Lefebvre, Thierry Guillet, Pierre Valvin, et al.

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## To cite this version:

Maxime Hugues, Mirja Richter, Jean-Michel Chauveau, Benjamin Damilano, Jean-Yves Duboz, et al. Investigation of Non-Radiative Processes in InAs/(Ga,In)(N,As) Quantum Dots. Japanese Journal of Applied Physics, 2007, 46 (4L), pp.L317. 10.1143/JJAP.46.L317. hal-01304760

HAL Id: hal-01304760

https://hal.science/hal-01304760v1

Submitted on 2 Dec 2024

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# Investigation of Non-Radiative Processes in InAs/(Ga,In)(N,As)
## Quantum Dots

Maxime HUGUES, Mirja RICHTER*, Jean-Michel CHAUVEAU, Benjamin

DAMILANO†, Jean-Yves DUBOZ, Jean MASSIES, Thierry TALIERCIO¹, Pierre

LEFEBVRE¹, Thierry GUILLET¹, Pierre VALVIN¹, Thierry BRETAGNON¹, Bernard

GIL¹, Andreas D. WIECK²

Centre National de la Recherche Scientifique - Centre de Recherche sur l'Hétéro-Epitaxie
et ses Applications, Rue B. Gregory, 06560 Valbonne, France.

¹Groupe d'Etude des Semiconducteurs, Université Montpellier II, 34095 Montpellier
Cedex 5, France.

²Lehrstuhl für Angewandte Festkörperphysik, Ruhr-Universität Bochum,
Universitätsstraße 150, D-44780 Bochum, Germany.

The photoluminescence properties of InAs/(Ga,In)(N,As) quantum dots (QDs) were analyzed and compared to those of InAs/(Ga,In)As QDs. While the photoluminescence (PL) intensity and the PL decay times of these samples are similar at low temperature, their decrease when the temperature increases is stronger in the case of InAs/(Ga,In)(N,As) QDs. This is attributed to a weaker confinement of the carriers and to

<footnote>* Also at affiliation 2.</footnote>

<footnote>† E-mail address : bd@crhea.cnrs.fr</footnote>
the presence of structural defects as revealed by the analysis of plane-view and cross-
section transmission electronic microscopy images.

KEYWORDS: Quantum Dots, InAs, (Ga,In)(N,As), Photoluminescence, Decay Times, TEM.
It has been recently demonstrated that InAs quantum dots (QDs) grown on GaAs substrate can emit in the 1.5 µm range if they are encapsulated by a thin (Ga,In)(N,As) (GINA) layer¹,²,³). In the perspective of using these nanostructures for device applications, such as laser diodes for telecommunication, it is crucial to optimize their radiative efficiency. Indeed, the radiative efficiency of InAs/GINA QDs can be affected by defects due both to the partial relaxation of the elastic strain accumulated during the growth and to specific GINA related defects. The formation of InAs QDs results from the elastic strain relaxation when the InAs layer thickness exceeds a value of ~1.7 monolayers. However, above this value, dislocations may form. Regarding the GINA layer itself, point defects can appear during growth due to the low growth temperature and also the use of a nitrogen plasma source for which the emission of highly energetic ions cannot be entirely eliminated.⁴) The quality of the GINA layer is important because the carriers generated in the GaAs matrix are captured by the asymmetric quantum well formed by the InAs wetting layer and the GINA cap layer before they are transferred into the QDs. If the GINA layer is affected by many defects, a part of the carriers recombines non-radiatively there, prior to their capture by the QDs. Therefore the presence of such defects reduces the photoluminescence (PL) efficiency of InAs/GINA QDs. In previous papers, the radiative efficiency of InAs/GINA QDs has been evaluated by comparing their room temperature PL to that of GINA/GaAs quantum wells²) or of InAs/(Ga,In)As QDs³) emitting at 1.3 µm. To the best of our knowledge, no temperature-dependent and time-resolved (TR) PL measurements were published to date, although these experiments are known to provide direct and quantitative information on the QD luminescence efficiency.⁵)
In this letter we investigate the recombination processes in InAs/GINA QDs through their temperature dependence and TRPL characteristics. These properties are compared to those of InAs/(Ga,In)As QDs. A correlation between the optical and structural properties is made.

The samples were grown by molecular beam epitaxy in a Riber 32P reactor on GaAs (001) substrates using Ga, In and As solid sources and an Addon radio-frequency N plasma source. InAs QDs were grown at 520°C on a 1 µm-thick undoped GaAs buffer layer. The nominal InAs thickness is 2.8 monolayers. The QDs were encapsulated either by 5 nm of Ga<sub>0.85</sub>In<sub>0.15</sub>As (sample A) or Ga<sub>0.85</sub>In<sub>0.15</sub>As<sub>0.988</sub>N<sub>0.012</sub> (sample B). Then, a GaAs barrier layer (100 nm) was grown. Finally, the QDs with the same parameters as the buried ones were deposited on the surface. In order to decrease the density of point defects in the GINA layer, Sample B was annealed *ex situ* during 30 sec at 640°C. These conditions were those leading to the optimum room temperature PL intensity. More details regarding the sample growth can be found in Ref. 3).

TRPL and time-integrated PL were carried out with a picosecond Ti-sapphire laser, tuned to 790 nm (photon energy above the band gap of GaAs), with a repetition rate of 4 MHz. The decay times were measured by using the method of time-correlated single photon counting. The temporal response of the setup is typically lower than 400 ps, limited by the Hamamatsu infrared photomultiplier, cooled to 190 K. The deconvolution procedure allows to measure decay times as small as 190 ps. To adjust the temperature between 10 K and 300 K, a helium closed cycle cryostat was used.

Transmission electron microscopy (TEM) was carried out in a JEOL 2010F field emission microscope operating at 200kV. Both cross-sectional and plane-view samples
were prepared by mechanical thinning, followed by ion milling to electron transparency.

In order to investigate the buried quantum dots, the surface was further etched with ion
milling.

Figure 1 shows the 10 K PL spectra of samples A and B, obtained by using the 514.5 line of an ionized argon laser with a low power density (0.3 W/cm²). Such a low excitation density ensures that defect-related levels are not saturated. The PL maximum for sample A is measured at 1.014 eV, while the presence of 1.2 % nitrogen in the GINA layer red-shifts the peak to 0.931 eV, for sample B. The PL intensity of sample B is only 1.5 times lower than sample A.This shows that the introduction of N results in a strong decrease of the PL peak energy without strongly affecting the low temperature PL efficiency providing that an optimum post-growth annealing is performed. However, this is no more the case for higher temperatures as it is shown by the analysis of the temperature dependence of the integrated PL intensity for samples A (squares) and B (triangles), with an excitation density of 0.3 W/cm² (Fig. 2). In the low temperature range, the PL intensity remains nearly constant for both samples. However, beyond critical temperatures of T<sub>A</sub> = 130 K and T<sub>B</sub> = 70 K (inset of Fig. 2) for samples A and B, respectively, further increasing the temperature decreases the intensity for both samples.

The decline in intensity is much more pronounced for sample B than for sample A. The
nitrogen incorporation in the capping layer seems to be the origin of the stronger radiative
efficiency decrease with the temperature in sample B. This is confirmed by TRPL
measurements. Figure 3 shows the temperature dependence of the PL decay times for
samples A (squares) and B (triangles). The minimum decay time measurable with our
experimental setup is shown by the horizontal line at 190 ps corresponding to the limit
response function (LRF). The low temperature (8K) decay times of the two samples are almost the same, close to 600 ps. These decay times remain constant in the low temperature range, up to 150 K and 70 K for samples A and B, respectively. It is worth pointing out that these temperatures are very close to T$_{A}$ and T$_{B}$ defined above. Beyond these temperatures, we observe a decrease of the decay time for both samples, but once again much more pronounced for sample B than for sample A.

The decay time reaches the LRF at a temperature of 290 K and 230 K for samples A and B, respectively. In the low temperature range, considering the stability of the intensity and the decay time we can assume that the internal quantum efficiency (η), of the QDs is 1 due to the localization of electron-hole pairs in QDs. In other words, the recombination mechanisms are dominated by radiative processes. The decrease of the decay times corresponds to the progressive increase of nonradiative processes, resulting in a decrease of η. We can conclude that the incorporation of nitrogen impacts negatively on the radiative efficiency at room temperature. Several physical phenomena can be put forward to explain this: (i) N incorporation reduces the barrier height that confines the carriers in the QDs (ii) the specific growth conditions may introduce defects in the vicinity of the quantum dots, or even (iii) in the QDs. The last point can be eliminated at first sight because in the growth procedure used, the InAs dots are formed without any nitrogen supply³. Hence, there is no difference in the structures of samples A and B until the encapsulation of the QDs. Furthermore, TEM analysis showed that the QDs were free of any defect.

The activation energy deduced from the PL intensity variation as a function of the temperature is generally attributed to the thermal escape of the carriers from the QDs to
the QW constituted by the wetting layer and the (Ga,In)As or GINA layer (in the
following this will be referred to "the QW"). Indeed, the incorporation of nitrogen in the
(Ga,In)As encapsulation layer induces the desired redshift of the QD PL emission, but
also a reduction of the carrier confinement in the QD (on the order of 60 meV).⁶) This
trend is verified regarding the activation energies since a decrease is found for samples A
and B, from 260 down to 150 meV, respectively (Fig. 2). Therefore it can be concluded
that part of the PL intensity decrease arises from a weaker carrier confinement in the case
of InAs/GINA QDs compared to InAs/(Ga,In)As QDs. TEM experiments were used to
investigate the crystalline quality of the GINA and (Ga,In)As layers. Different kind of
structural defects can be found in the GINA encapsulation layer. Figure 4a displays a
(004) dark-field image taken from a plane-view of sample B. This image evidences the
presence of the surface QDs (indicated by white arrows) but also of a stacking fault (SF)
(marked by the black arrows in Fig. 4a). Cross-sectional images have shown that the SFs
were nucleated in the GINA layer. The SF density is very low (<10⁴ cm⁻¹) with a highly
non-homogeneous distribution. In addition very few, misfit dislocations have been
observed but with a density lower than 10⁶ cm⁻². While extended defects are nucleated in
the GINA encapsulation layer of sample B, their very low density cannot fully explain
the differences measured in PL of sample A and B. The investigation of buried QDs in
sample B, have revealed dislocation loops directly above the QDs in the GINA layer (see
black arrows in Fig. 4b). Such defects, whose density is very high, have not been detected
in sample A while they have already been observed by Herrera *et al*. for GINA QWs⁷). In
addition to the weaker carrier confinement, the strong PL intensity decrease for sample B
is thus also related to the presence of these defects. Point defects in GINA may also play
a role in the radiative efficiency decrease with the temperature as it has been widely discussed in the literature for GINA QW.<sup>8,9,10,11)</sup>

In conclusion, we have analyzed the temperature dependence of the PL and the TRPL of InAs/GINA and InAs/(Ga,In)As QDs. We have shown that, although their low temperature characteristics are comparable, the quenching of the PL intensity with temperature is faster in the case of InAs/GINA QDs. We assign this phenomenon both to a decrease of the carrier confinement compared to InAs / (Ga,In)As QDs and to the nucleation of defects directly above the QDs in the GINA layer. With the aim of realizing efficient devices with InAs/GINA QDs, the structures have to be optimized (in particular the GINA thickness) in order to minimize the carrier thermal escape out of the dots.

Moreover, the GINA growth conditions have to be improved to avoid, as far as possible, the formation of extended defects.

This work is partially supported by the French Ministry of Research “AC nanosciences BAND” project. One of the authors (M.R.) thanks the “Evangelisches Studienwerk Haus Villigst”.
1) A. Y. Egorov, D. Bedarev, D. Bernklau, G. Dumitras, and H. Riechert: Phys. Status Solidi B **224** (2001)839.

2) M. Ustinov, A.Y. Egorov, V.A. Odnoblyudov, N.V. Kryzhanovskaya, Y.G. Musikhin, A.F. Tsatsul'nikov, and Z.I. Alferov: J. Cryst. Growth **251** (2003) 388.

3) M. Richter, B. Damilano, J.-Y. Duboz, and J. Massies, A. D. Wieck: Appl. Phys. Lett. **88** (2006) 231902.

4) J. Miguel-Sánchez, A. Guzmán, and E. Muñoz, Appl. Phys. Lett. **85** (2004) 1940.

5) A. Markus, A. Fiore, J. D. Ganière, U. Oesterle, J. X. Chen, B. Deveaud, M. Ilegems, H. Riechert: Appl. Phys. Lett. **80** (2002) 911.

6) The carrier confinement is defined as the energy difference between the QD ground state emission and the QW (wetting layer plus (Ga,In)As or GINA layer) fundamental energy level.

7) M. Herrera, D. González, J. G. Lozano, R. García, M. Hopkinson, H. Y. Liu, M. Gutierrez, and P. Navaretti: J. Appl. Phys. **98** (2005) 023521.

8) S.G. Spruytte, C.W. Coldren, J.S. Harris, W. Wampler, P. Krispin, K. Ploog, M.C. Larson: Appl. Phys. Lett. **89** (2001) 4401.

9) E. Tournié, M.-A. Pinault, and A. Guzman: Appl. Phys. Lett. **80** (2002) 4148.

10) R. Kudrawiec, G. Sek, J. Misiewicz, D. Gollub, and A. Forchel: Appl. Phys. Lett. **83**, (2003) 2772.

11) W. Li, M. Pessa, T. Ahlgren, and J. Decker: Appl. Phys. Lett. **79** (2001) 1094.
Fig.1. 10 K PL spectra of samples A and B.

Fig.2. Temperature dependence of the integrated PL intensity of the samples A (square) and B (triangle).

Fig.3. Temperature dependence of the decay times for the samples A (square) and B (triangle).

Fig.4. (004) dark-field images taken from sample B (plane-view). a) Stacking Fault (SF) and surface QDs (marked by white arrows). b) Detail of burried QDs exhibiting dislocation loops (arrows).
<figure><img src="image_3.png" /><figcaption>Fig. 1</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 2</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 4</figcaption></figure>