Materials Science and Engineering B88 (2002) 234– 237
Optical properties of InAs/Aly Ga1 − y As/GaAs quantum dot structures P. Altieri a, S. Sanguinetti a, M. Gurioli a,*, E. Grilli a, M. Guzzi a, P. Frigeri b, S. Franchi b, G. Trevisi b a
I.N.F.M. and Dipartimento di Scienza dei Materiali, Uni6ersita` di Milano Bicocca, Via Cozzi 53, 20125 Milan, Italy b CNR-MASPEC, Parco delle Scienze 37A, 43010 Fontanini, Parma, Italy
Abstract We present a detailed study, by means of photoluminescence measurements, of the optical properties of self-assembled InAs/Aly Ga1 − y As/GaAs quantum dot (QD) structures, grown by Atomic Layer Molecular Beam Epitaxy. We found a blue shift of the fundamental QD energy transition when increasing the Al content in the barrier. The comparison of the experimental data with previous findings and with a simple effective mass model suggests that the emission blue shift cannot be completely attributed to the increase of the confining barriers band gap. At the same time we show that the use of ALMBE allows a better control of the QD size distribution with respect to standard MBE growth. The increase of the barrier height enhances the QD radiative efficiency at high temperature, allowing to observe the QD emission up to T= 430 K. Important pieces of information on the thermal activation of non radiative channels are obtained by comparing the QD PL temperature dependence with non-resonant and resonant excitation of the QD levels. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Quantum dots; InAs; Photoluminescence
1. Introduction In the last decade, the interest in zero-dimensional semiconductor heterostructures (known as quantum dots (QDs)) embedded in larger gap materials (barriers) has increased due to their potential technological applications . The three-dimensional quantum confinement leads to an atomic-like electronic density of states that is expected to result in a number of advantageous properties for optoelectronic devices. So far, the most successful QD heterostructures are fabricated using the Stranski –Krastanov growth mode, correlated to self-assembling phenomena in strongly lattice mismatched systems, that allows the nucleation of defectfree semiconductor 3D islands. There has been a number of recent reports  on the attractive properties of self-organized QD lasers even if the device performances are far from theoretical predictions, mainly because of a still unsatisfactory luminescence efficiency
at room temperature and of the inhomogeneous broadening of the electronic density of states associated with the QD size fluctuations. Moreover, the self-assembled growth permits only a limited control of the QDs structural properties and then of the optical transition tuning. Recently increasing attention has been devoted to the use of ternary alloys for both QD and barrier materials in order to increase the degrees of freedom in the QD band gap engineering [3–5]. Our study concerns the Al mole fraction dependence of the optical properties of InAs QDs embedded in Aly Ga1 − y As barriers. The QD optical properties have been investigated by photoluminescence measurements under both non-resonant excitation (PL), above the AlGaAs barrier, and in conditions of near-resonant excitation (RPL) below the GaAs barrier.
2. Sample growth and experimental detail * Corresponding author. Tel.: + 39-02-6448-5158; fax: + 39-026448-5400. E-mail address: [email protected]
InAs QDs embedded in Aly Ga1 − y As cladding layers were grown by Atomic Layer Molecular Beam Epitaxy
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 8 6 3 - 7
P. Altieri et al. / Materials Science and Engineering B88 (2002) 234–237 Table 1 Structural parameters of the samples
d1 (nm) d2 (nm) yAl
3. Results and discussion
0 0 0.00
4 6 0.15
10 10 0.15
4 6 0.30
d1, d2 are the Aly Ga1−y As layer thicknesses and yAl is the Al mole fraction.
(ALMBE) . The features of the four studied samples are reported in Table 1. A 100-nm-thick GaAs buffer layer and a d1-thick Aly Ga1 − y As layer were grown by Molecular Beam Epitaxy (MBE) at 600 °C on a (100) GaAs substrate. Then the substrate temperature was lowered to 460 °C and a 3ML-thick InAs layer was deposited by ALMBE. Finally a d2-thick Aly Ga1 − y As layer and a 10-nm-thick GaAs cap layer were grown by ALMBE at a low temperature (360 °C) in order to reduce In segregation . The growth was interrupted for 210 s before and after the InAs deposition to stabilize the growth temperature of both Aly Ga1 − y As layer and QDs. In the QD0 reference sample (InAs QDs in a GaAs matrix), the Aly Ga1 − y As layers are absent. The Al mole fraction (y) is chosen to be 0.15 in the QD15A and QD15B samples and 0.30 in the QD30 sample. In order to check the growth reproducibility we studied four different QD0 samples. The PL spectra were measured using a grating monochromator with a Peltier cooled InGaAs photodiode. The excitation source was a multiline Ar+ laser. RPL spectra were resolved by a double grating monochromator and detected by a cooled Ge photodiode. The excitation source, in the 780– 930 nm range, was a Ti– Sa laser, pumped by a multiline Ar+ laser. PL measurements were performed between 13 and 430 K, using a coldfinger closed cycle He cryostat, whereas RPL measurements were performed between 80 and 300 K, using a bath-type cryostat.
Fig. 1(a) shows non-resonant PL spectra of QDs, at T= 13 K and power density of 20 W cm − 2. The spectra are normalized to the peak intensity value. All the samples show a weak but extended high-energy shoulder, which can be mainly attributed to ground state PL of smaller QDs. Indeed, the comparison between the PL spectra obtained with different excitation power densities, shown in Fig. 1(b), reveals that the contribution of excited states, in the high-energy part of the PL spectrum, is negligible. The presence of a second family of QDs is also shown by Atomic Force Microscopy (AFM) of a single uncapped layer of InAs/ GaAs QDs . It is interesting to note that the use of Aly Ga1 − y As confining layers enables the tuning of the emission energy towards the blue side. The dependence of the blue shift of the PL band on the Al mole fraction is reported in Fig. 2(a). In the case of QD0 we include an error bar, defined as the spread of the data of a series of nominally identical structures. We also report in Fig. 2(a) data taken from Refs. [3,4]. It is worth noting that the data in Ref. [3,4] refer to MBE QDs with PL emission energy at 1.25 and 1.17 eV, respectively. The blue shift obtained in our data, as the Al content increases, is comparable with the results of Polimeni et al. , but it is small if compared to that observed in Ref. . This experimental discrepancy suggests that the increase of the PL emission energy cannot be completely ascribed to the effects of the barrier gap increase and therefore variation of size and/or composition of the QDs may occur for different Al content. On one side, the lattice mismatch and the strain field depend very little on the Al content of the barriers and therefore we do not expect strain-driven changes in the QDs shape and size in our samples. On the other hand, the presence of Al during the Stranski– Krastanov growth of InAlGaAs QDs dot has been shown to sharply reduce the dot size , possibly suggesting different growth kinetics on Al-containing substrates. In order to give an estimation of the in-
Fig. 1. (a) Normalized PL spectra (T= 13 K) of InAs QDs in Aly Ga1 − y As; (b) PL spectra of QD30 sample are shown, for the excitation power densities of 20 and 2 W cm − 2.
P. Altieri et al. / Materials Science and Engineering B88 (2002) 234–237
Fig. 2. Blue shift (a) and (b) FWHM of the PL band as the Al content is increased. Different symbols refer to our data (circles), data reported in Ref.  (triangles) and data reported in Ref.  (squares). Lines in panel (a) refer to the model predictions with different Q factors.
Fig. 3. Temperature dependence of the integrated (a) PL and (b) RPL intensity. In (a) the excitation is above the AlGaAs absorption edge; the data are normalized to the 13 K value. In (b) the excitation, below GaAs exciton, follows the temperature dependence of the GaAs band gap; the data are normalized to the 80 K value.
crease of carrier confinement energy related to the Al content in the Aly Ga1 − y As barriers, we have developed a very simple effective mass model. The QD shape is approximated by a parallelepiped with an aspect ratio Q (Q =height/base size). In the calculations we kept Q constant for all Al mole fractions. The bands are assumed parabolic using the parameters given in Ref. . The QD height is then chosen to fit the QD0 transition energy and then kept constant while increasing the Al content in the barrier. The model results are reported in Fig. 2(a) for different values of Q around the experimental value of Q = 0.3, as given by AFM measurements . The model underestimates the observed emission blue shifts. Thus, other effects, related to the change in the QD aspect ratio and/or in the In segregation when increasing the Al content in the barriers, are expected to play a role in the QD emission blue shift. Clearly a more detailed model is needed to obtain quantitative information on this aspect. The trends of the broadening of the PL band when increasing the Al content in the barriers are reported in Fig. 2(b). Again our data are compared with previous findings in the literature [3,4]. In our case the PL full width at half maximum (FWHM) is quite small (50 meV) and almost constant for different Al content in the barriers. In contrast, larger values of the FWHM and a strong
increase of it with Al mole fraction are found in Refs. [3,4] on MBE QDs. This result demonstrates that the ALMBE allows a better control of the size homogeneity of the QD ensemble even in the case of AlGaAs barriers. Other interesting pieces of information can be deduced from the temperature dependence of the PL integrated intensity shown in Fig. 3: panel (a) refers to PL with excitation above the AlGaAs absorption edge and panel (b) refers to RPL with excitation below the GaAs absorption edge. In the RPL case, the excitation energy is varied when increasing T, following the temperature dependence of the GaAs band gap. According to previous studies [3,4] we find that the increasing of the confining barrier produces a reduction of the thermal quenching of the PL. In particular, for the case 0.3 Al mole fraction we were able to follow the QD emission up to 430 K. This is very likely to be ascribed to the larger confinement of the carriers in the QDs and, as a consequence, to the less efficient carrier escape out of the QDs, when deepening the confinement potential of the barriers. Nevertheless we also found a strong reduction of the PL thermal quenching when using RPL. We suggest that the quenching curve obtained with non-resonant excitation, above AlGaAs barriers, is the result of two mechanisms: the carrier thermal escape described above and a temperature-dependent en-
P. Altieri et al. / Materials Science and Engineering B88 (2002) 234–237
ergy relaxation and capture processes involving the AlGaAs barriers as well as the GaAs buffer. Our results therefore show that, in the case of non-resonant excitation, an important contribution of the PL quenching arises from the carrier losses during the energy relaxation processes. Similar conclusions were drawn in Refs. [10,11], by comparing data of PLE and time-resolved PL measurements. We therefore believe that care has to be exercised when extracting information on the radiative efficiency of QD structures, which is a crucial parameter for the optimization of QD lasers, from non-resonant PL measurements.
relevance of carrier losses during the energy relaxation processes and the utility of the use of resonant excitation in order to obtain direct insights on the QD radiative efficiency.
We have shown that the use of Aly Ga1 − y As barriers allows to blue-shift the fundamental optical transition of InAs QDs and to increase the high temperature PL efficiency. The comparison of the experimental data with previous findings and with a simple effective mass model allows us to conclude that the emission blue shift cannot be completely attributed to the increase of the confining barriers band gap. Other effects, related to the exact characteristics of the growth kinetics of QD on AlGaAs substrates, seem to play a role in determining the QD emission behaviour. At the same time we demonstrate that the use of ALMBE allows a better control of the QD size distribution with respect to standard MBE growth. In addition, the increase of the barrier height enhances the QD radiative efficiency at high temperature, allowing to observe the QD emission up to T=430 K. By comparing the QD PL temperature dependence with non-resonant and resonant excitation of the QD levels, we finally point out the
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Acknowledgements This work has been supported by the PRA ‘Nanotecnologie’. P.A. acknowledges the INFM support by the ‘PIE’ project.