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GaAs self assembled quantum dots
Materials Science and Engineering B88 (2002) 252– 254
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Resonant quenching of photoluminescence in Inx Ga1 − x As/Aly Ga1 − y As/GaAs self assembled quantum dots P. Altieri a, S. Lozzia a, S. Sanguinetti a, M. Gurioli a,*, E. Grilli a, M. Guzzi a, P. Frigeri b, S. Franchi b a
Department di Scienza dei Materiali, Ist. Nazion di Fisica della Materia, Uni6ersita´ di Milano, Bicocca, Via Cozzi 53, 20125 Milan, Italy b CNR-MASPEC, Parco delle Scienze 37A, 43010 Fontanini, Parma, Italy
Abstract We investigated a large series of self-assembled InAs and Inx Ga1 − x As quantum dot (QD) structures by means of photoluminescence (PL) and PL excitation (PLE) techniques. A pronounced dip of the QD PLE spectra just below the GaAs absorption edge has been observed in all the investigated samples, denoting a resonant quenching of the QD PL intensity. PL spectra with excitation in such spectral region show, beside the QD PL band, a new extrinsic band at 1.356 eV with strong GaAs– LO phonon replicas. The PLE spectrum of this extrinsic band is almost specular to the QD PLE, denoting a competition in the carrier capture between the QDs and the defects associated with the 1.356 eV emission. The chemical etching of the QD and of the surrounding barrier layers does not eliminate the extrinsic PL band, which is, therefore, attributed to a native defect of the GaAs buffer. The spectral position and shape of the 1.356 eV band allows us to associate it to a complex of Ga vacancy with deep acceptor states in the GaAs layers. We conclude that the resonant PL quenching of QDs arises from the competition in the carrier capture between the QDs and deep defects in the GaAs layers. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Quantum dots; InGaAs; Photoluminescence
1. Introduction Semiconductor quantum dots (QDs) have recently gained increasing attention due to their unique feature of being solid-state atomic-like systems, opening new prospects for both interesting physics [1] and relevant technological applications [2]. The state of the art in the synthesis of QDs for opto-electronic devices is nowadays the Stranski – Krastanov epitaxial growth. Such a growth mode, based on self-organization phenomena in strong lattice mismatched systems, is known to lead to almost defect free semiconductor QDs, which show high radiative efficiency. Nevertheless, the characterization of non-radiative channels, extended defects, and photoluminescence thermal quenching is one of the
* Corresponding author. Tel.: + 39-02-644-85158; fax: + 39-02644-85400. E-mail address: [email protected] (M. Gurioli).
most relevant topics for the optimization of the material quality in view of the device [3]. Our study concerns a detailed characterization of QD photoluminescence (PL) quenching channels resonant with the wetting layer (WL) states, in a large set of QD structures. The analysis has been performed through variable wavelength excitation measurements of the QD PL. We have identified the presence of an efficient recombination channel competitive with the WL-to-QD thermalization process. The fingerprint of this channel is a strong reduction of the QD PL yield in a narrow spectral region just below the GaAs absorption edge. The reduction of the QD PL efficiency has been systematically observed in a large series of InAs and In0.5Ga0.5As QDs grown on GaAs substrates and embedded in either GaAs or very thin AlGaAs barriers. Moreover, this effect is clearly recognizable in the QD PLE spectra reported in the literature [4,5]. We attribute the resonant PL quenching to a competition in the carrier capture between the QDs and deep defects in the GaAs layers.
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 8 6 - 8
P. Altieri et al. / Materials Science and Engineering B88 (2002) 252–254
253
2. Sample growth and experimental details
3. Results and discussion
We investigated 15 different samples of InAs and In0.5Ga0.5As QDs embedded in GaAs and/or Aly Ga1 − yAs (y =0.15, 0.3) cladding layers. The structures were grown by Atomic Layer Molecular Beam Epitaxy (ALMBE) [6]; they consist of 100 nm-thick GaAs buffer layers grown by Molecular Beam Epitaxy (MBE) at 600 °C on (100) GaAs substrates. Then the substrate temperature was lowered to 460 °C and an InAs or In0.5Ga0.5As layer of suitable thickness was deposited by ALMBE. Finally, a GaAs cap layer was grown by ALMBE at a temperature (360 °C) lower than the InAs QDs growth temperature in order to reduce local In segregation [6]. The growth was interrupted for 210 s before and after the InAs deposition to stabilize the growth temperature of different layers. In few cases the QDs were embedded in thin (5 nm) Aly Ga1 − y As layers. Either singles QD layer or stacked QD layers have been investigated. The PLE and PL 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. The measurements were performed between 2 and 300 K using a bath type cryostat.
The phenomenology presented is extremely similar in all the investigated samples. Therefore, we will avoid to specify details of the structures to which the following figures refer. Fig. 1(a) shows typical PL spectra of InAs QD structure at T= 80 K in a semilogarithmic scale for two different excitation energies. For the same structure and temperature we report in Fig. 1(b) the PLE spectra for two different emission energies. The solid line in Fig. 1(a) refers to an excitation energy above the GaAs absorption edge and shows the PL from the QDs. The corresponding PLE spectrum at the maximum of the PL band of the QDs is reported in Fig. 1(b) as solid line. Besides the GaAs excitonic peak at 1.505 eV, the PLE spectrum of the QDs shows a pronounced dip just below the GaAs absorption edge, denoting a strong reduction of the PL from the QDs for this resonant excitation. It is also worth noting that the presence of a dip, just below the GaAs absorption edge, in the QD PLE spectra [4,5] and even in InGaAs quantum wells [7], is a rather common feature in the literature. The PL spectrum for an excitation energy of 1.493 eV, that is the center of the PLE dip at T= 80 K, is reported as dotted line in Fig. 1(a). A clear reduction of the QD PL emission is indeed observed, without any major variation in the lineshape. In addition, a strong PL band appears at 1.356 eV, which is not present for excitation above the GaAs band gap. The PLE spectrum of this new band is reported in Fig. 1(b) as dotted line. It presents a strong resonance in the same spectral position of the dip in the PLE of the QDs. As a matter of fact, the PLE spectra of the 1.356 eV PL band is almost specular to the QD PLE, denoting a competition in the carrier capture between the QDs and this new radiative center. In order to investigate the nature of the PL emission at 1.356 eV we have performed a detailed analysis of its temperature dependence. The PL spectra at T= 2 K with resonant excitation at 1.495 eV is reported in Fig. 2(a). We clearly resolve up to four replicas spaced by the GaAs LO phonon energy of 36 meV. The PL lineshape of each replica is strongly asymmetric, and a multi Gaussian analysis does not fit. In order to deconvolute the PL intensity of the phonon replica we fit their low energy side with an exponential tail (dashed lines in Fig. 2(a)). The solid bar in Fig. 2(a) represents the intensity of each replica, which follows a nearly exact Poisson distribution. This agrees with the Hopfield predictions for the intensity of the phonon replica, expressed in Eq. (1), based on a configuration coordinate model [8]. The estimated:
Fig. 1. (a) Typical PL spectra at T= 80 K for excitation energy of 1.53 eV (solid line) and of 1.493 eV (dotted line). (b) PLE spectra at T =80 K for detection energy at the QD-PL (solid line) and at the 1.356 eV emission (dotted line).
Fig. 2. (a) PL spectra of the 1.356 eV band at T= 2 K (solid line). The dashed lines are the exponential fits of the low energy tails of each replica; the bars show the Poisson distribution with S= 0.5. (b). Temperature dependence of the PL band at 1.356 eV.
Sn n!
In = A exp{− S}
(1)
Huang–Rhys factor is S=0.5 and indicates that the center responsible for the emission at 1.356 eV is a deep
254
P. Altieri et al. / Materials Science and Engineering B88 (2002) 252–254
Fig. 3. (a) Temperature dependence of the PL integrated intensity, the full line is the fit with activation energy of 120 9 10 meV. The inset reports the shift of the 1.356 eV with increasing temperature (dots). (b) Comparison of the PL from the virgin and etched sample.
defect associated with strong lattice distortion. Fig. 2(b) reports the evolution of the PL spectra of the 1.356 eV band for increasing temperature. A large broadening of the replica is already observed above T =50 K and for T \100 K the phonon replica cannot be anymore resolved. In addition a strong quenching of the PL band is found. A summary of the temperature dependence of the PL band is reported in Fig. 3(a). The PL integrated intensity shows an activation energy of 1209 10 meV, which corresponds nicely with the separation between the PL band and the relative PLE resonance. It is well known that the PL thermal quenching of the deep centers in GaAs is due to the thermal activation of carriers from the ground to the excited state of the defects [9]. We therefore attribute the PLE resonance to the excited state transition of the deep defects, which also acts as an efficient quenching channel of the PL emission in the case of high temperature. The inset in Fig. 3(a) shows the temperature dependence of the emission energy (dots) in comparison with the prediction of the Varshni law for the GaAs, scaled at 1.356 eV for low T [9]. Clearly the shift of the extrinsic band does not follow the GaAs band gap variation, as expected for deep level transitions [9]. Finally we have performed a chemical etching of the QD and of the surrounding barrier layer in order to check if the presence of this defect would be related to the nucleation of three dimensional islands. The comparison within the PL spectrum at T= 2 K of the virgin sample and the etched sample is reported in Fig. 3(b). The PL band at 1.356 eV is still present in the etched sample and it shows a comparable PL intensity in the two structures. We then con-
clude that this extrinsic band is related to a native defect in the GaAs buffer layer. A comparison with previous findings in the literature allows us to assign this GaAs native defect to a complex of a Ga vacancy with a deep acceptor. Indeed the PL band of the well-known complex CuVGa has almost the same features of the band here studied [10]. Note also that in the previous studies the 1.356 eV band was easily observed using non resonant excitation with Ar+ laser for a copper concentration lower than 1015 cm − 3 [10]. In our QD structures the PL band at 1.356 eV cannot be resolved for excitation above the GaAs absorption edge. This fact suggests a small density of defects, in our structures, and indeed the extrinsic PL band can be resolved only by using resonant excitation on the excited level of the defect. In addition this excited level turns out to be resonant with the electronic states of the wetting layer on which the QDs nucleate. We believe that this resonant coupling is indeed responsible for the competition in the carrier capture between the deep defect in the GaAs and the QDs.
Acknowledgements This work has been supported by the Consiglio Nazionale delle Ricerche (CNR), MADESS project. P.A. acknowledges the INFM support by the ‘PIE’ project.
References [1] A.D. Yoffe, Adv. Phys. 50 (2001) 1. [2] M. Grundmann, Physica E 5 (2000) 167. [3] E.C. Le Ru, P.D. Siverns, R. Murray, Appl. Phys. Lett. 77 (2000) 2446. [4] M.J. Steer, D.J. Mowbray, W.R. Tribe, M.S. Skolnick, M.D. Sturge, M. Hopkinson, A.G. Cullis, C.R. Whitehouse, R. Murray, Phys. Rev. B 54 (1996) 17738. [5] R. Heitz, O. Stier, I. Mukhametzhanov, A. Madhukar, D. Bimberg, Phys. Rev. B 62 (2000) 11017. [6] A. Bosacchi, P. Frigeri, S. Franchi, P. Allegri, V. Avanzini, J. Cryst. Growth 175 – 176 (1997) 771. [7] P. Borri, M. Gurioli, M. Colocci, F. Martelli, M. Capizzi, A. Patane´ , A. Polimeni, J. Appl. Phys. 80 (1996) 3011. [8] J.J. Hopfield, J. Phys. Chem. Solids 10 (1959) 110. [9] L. Pavesi, M. Guzzi, J. Appl. Phys. 75 (1994) 4779. [10] J. Van de Ven, W.J.A. Hartmann, L.J. Giling, J. Appl. Phys. 60 (1986) 3735.
www.elsevier.com/locate/mseb
Resonant quenching of photoluminescence in Inx Ga1 − x As/Aly Ga1 − y As/GaAs self assembled quantum dots P. Altieri a, S. Lozzia a, S. Sanguinetti a, M. Gurioli a,*, E. Grilli a, M. Guzzi a, P. Frigeri b, S. Franchi b a
Department di Scienza dei Materiali, Ist. Nazion di Fisica della Materia, Uni6ersita´ di Milano, Bicocca, Via Cozzi 53, 20125 Milan, Italy b CNR-MASPEC, Parco delle Scienze 37A, 43010 Fontanini, Parma, Italy
Abstract We investigated a large series of self-assembled InAs and Inx Ga1 − x As quantum dot (QD) structures by means of photoluminescence (PL) and PL excitation (PLE) techniques. A pronounced dip of the QD PLE spectra just below the GaAs absorption edge has been observed in all the investigated samples, denoting a resonant quenching of the QD PL intensity. PL spectra with excitation in such spectral region show, beside the QD PL band, a new extrinsic band at 1.356 eV with strong GaAs– LO phonon replicas. The PLE spectrum of this extrinsic band is almost specular to the QD PLE, denoting a competition in the carrier capture between the QDs and the defects associated with the 1.356 eV emission. The chemical etching of the QD and of the surrounding barrier layers does not eliminate the extrinsic PL band, which is, therefore, attributed to a native defect of the GaAs buffer. The spectral position and shape of the 1.356 eV band allows us to associate it to a complex of Ga vacancy with deep acceptor states in the GaAs layers. We conclude that the resonant PL quenching of QDs arises from the competition in the carrier capture between the QDs and deep defects in the GaAs layers. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Quantum dots; InGaAs; Photoluminescence
1. Introduction Semiconductor quantum dots (QDs) have recently gained increasing attention due to their unique feature of being solid-state atomic-like systems, opening new prospects for both interesting physics [1] and relevant technological applications [2]. The state of the art in the synthesis of QDs for opto-electronic devices is nowadays the Stranski – Krastanov epitaxial growth. Such a growth mode, based on self-organization phenomena in strong lattice mismatched systems, is known to lead to almost defect free semiconductor QDs, which show high radiative efficiency. Nevertheless, the characterization of non-radiative channels, extended defects, and photoluminescence thermal quenching is one of the
* Corresponding author. Tel.: + 39-02-644-85158; fax: + 39-02644-85400. E-mail address: [email protected] (M. Gurioli).
most relevant topics for the optimization of the material quality in view of the device [3]. Our study concerns a detailed characterization of QD photoluminescence (PL) quenching channels resonant with the wetting layer (WL) states, in a large set of QD structures. The analysis has been performed through variable wavelength excitation measurements of the QD PL. We have identified the presence of an efficient recombination channel competitive with the WL-to-QD thermalization process. The fingerprint of this channel is a strong reduction of the QD PL yield in a narrow spectral region just below the GaAs absorption edge. The reduction of the QD PL efficiency has been systematically observed in a large series of InAs and In0.5Ga0.5As QDs grown on GaAs substrates and embedded in either GaAs or very thin AlGaAs barriers. Moreover, this effect is clearly recognizable in the QD PLE spectra reported in the literature [4,5]. We attribute the resonant PL quenching to a competition in the carrier capture between the QDs and deep defects in the GaAs layers.
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 8 6 - 8
P. Altieri et al. / Materials Science and Engineering B88 (2002) 252–254
253
2. Sample growth and experimental details
3. Results and discussion
We investigated 15 different samples of InAs and In0.5Ga0.5As QDs embedded in GaAs and/or Aly Ga1 − yAs (y =0.15, 0.3) cladding layers. The structures were grown by Atomic Layer Molecular Beam Epitaxy (ALMBE) [6]; they consist of 100 nm-thick GaAs buffer layers grown by Molecular Beam Epitaxy (MBE) at 600 °C on (100) GaAs substrates. Then the substrate temperature was lowered to 460 °C and an InAs or In0.5Ga0.5As layer of suitable thickness was deposited by ALMBE. Finally, a GaAs cap layer was grown by ALMBE at a temperature (360 °C) lower than the InAs QDs growth temperature in order to reduce local In segregation [6]. The growth was interrupted for 210 s before and after the InAs deposition to stabilize the growth temperature of different layers. In few cases the QDs were embedded in thin (5 nm) Aly Ga1 − y As layers. Either singles QD layer or stacked QD layers have been investigated. The PLE and PL 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. The measurements were performed between 2 and 300 K using a bath type cryostat.
The phenomenology presented is extremely similar in all the investigated samples. Therefore, we will avoid to specify details of the structures to which the following figures refer. Fig. 1(a) shows typical PL spectra of InAs QD structure at T= 80 K in a semilogarithmic scale for two different excitation energies. For the same structure and temperature we report in Fig. 1(b) the PLE spectra for two different emission energies. The solid line in Fig. 1(a) refers to an excitation energy above the GaAs absorption edge and shows the PL from the QDs. The corresponding PLE spectrum at the maximum of the PL band of the QDs is reported in Fig. 1(b) as solid line. Besides the GaAs excitonic peak at 1.505 eV, the PLE spectrum of the QDs shows a pronounced dip just below the GaAs absorption edge, denoting a strong reduction of the PL from the QDs for this resonant excitation. It is also worth noting that the presence of a dip, just below the GaAs absorption edge, in the QD PLE spectra [4,5] and even in InGaAs quantum wells [7], is a rather common feature in the literature. The PL spectrum for an excitation energy of 1.493 eV, that is the center of the PLE dip at T= 80 K, is reported as dotted line in Fig. 1(a). A clear reduction of the QD PL emission is indeed observed, without any major variation in the lineshape. In addition, a strong PL band appears at 1.356 eV, which is not present for excitation above the GaAs band gap. The PLE spectrum of this new band is reported in Fig. 1(b) as dotted line. It presents a strong resonance in the same spectral position of the dip in the PLE of the QDs. As a matter of fact, the PLE spectra of the 1.356 eV PL band is almost specular to the QD PLE, denoting a competition in the carrier capture between the QDs and this new radiative center. In order to investigate the nature of the PL emission at 1.356 eV we have performed a detailed analysis of its temperature dependence. The PL spectra at T= 2 K with resonant excitation at 1.495 eV is reported in Fig. 2(a). We clearly resolve up to four replicas spaced by the GaAs LO phonon energy of 36 meV. The PL lineshape of each replica is strongly asymmetric, and a multi Gaussian analysis does not fit. In order to deconvolute the PL intensity of the phonon replica we fit their low energy side with an exponential tail (dashed lines in Fig. 2(a)). The solid bar in Fig. 2(a) represents the intensity of each replica, which follows a nearly exact Poisson distribution. This agrees with the Hopfield predictions for the intensity of the phonon replica, expressed in Eq. (1), based on a configuration coordinate model [8]. The estimated:
Fig. 1. (a) Typical PL spectra at T= 80 K for excitation energy of 1.53 eV (solid line) and of 1.493 eV (dotted line). (b) PLE spectra at T =80 K for detection energy at the QD-PL (solid line) and at the 1.356 eV emission (dotted line).
Fig. 2. (a) PL spectra of the 1.356 eV band at T= 2 K (solid line). The dashed lines are the exponential fits of the low energy tails of each replica; the bars show the Poisson distribution with S= 0.5. (b). Temperature dependence of the PL band at 1.356 eV.
Sn n!
In = A exp{− S}
(1)
Huang–Rhys factor is S=0.5 and indicates that the center responsible for the emission at 1.356 eV is a deep
254
P. Altieri et al. / Materials Science and Engineering B88 (2002) 252–254
Fig. 3. (a) Temperature dependence of the PL integrated intensity, the full line is the fit with activation energy of 120 9 10 meV. The inset reports the shift of the 1.356 eV with increasing temperature (dots). (b) Comparison of the PL from the virgin and etched sample.
defect associated with strong lattice distortion. Fig. 2(b) reports the evolution of the PL spectra of the 1.356 eV band for increasing temperature. A large broadening of the replica is already observed above T =50 K and for T \100 K the phonon replica cannot be anymore resolved. In addition a strong quenching of the PL band is found. A summary of the temperature dependence of the PL band is reported in Fig. 3(a). The PL integrated intensity shows an activation energy of 1209 10 meV, which corresponds nicely with the separation between the PL band and the relative PLE resonance. It is well known that the PL thermal quenching of the deep centers in GaAs is due to the thermal activation of carriers from the ground to the excited state of the defects [9]. We therefore attribute the PLE resonance to the excited state transition of the deep defects, which also acts as an efficient quenching channel of the PL emission in the case of high temperature. The inset in Fig. 3(a) shows the temperature dependence of the emission energy (dots) in comparison with the prediction of the Varshni law for the GaAs, scaled at 1.356 eV for low T [9]. Clearly the shift of the extrinsic band does not follow the GaAs band gap variation, as expected for deep level transitions [9]. Finally we have performed a chemical etching of the QD and of the surrounding barrier layer in order to check if the presence of this defect would be related to the nucleation of three dimensional islands. The comparison within the PL spectrum at T= 2 K of the virgin sample and the etched sample is reported in Fig. 3(b). The PL band at 1.356 eV is still present in the etched sample and it shows a comparable PL intensity in the two structures. We then con-
clude that this extrinsic band is related to a native defect in the GaAs buffer layer. A comparison with previous findings in the literature allows us to assign this GaAs native defect to a complex of a Ga vacancy with a deep acceptor. Indeed the PL band of the well-known complex CuVGa has almost the same features of the band here studied [10]. Note also that in the previous studies the 1.356 eV band was easily observed using non resonant excitation with Ar+ laser for a copper concentration lower than 1015 cm − 3 [10]. In our QD structures the PL band at 1.356 eV cannot be resolved for excitation above the GaAs absorption edge. This fact suggests a small density of defects, in our structures, and indeed the extrinsic PL band can be resolved only by using resonant excitation on the excited level of the defect. In addition this excited level turns out to be resonant with the electronic states of the wetting layer on which the QDs nucleate. We believe that this resonant coupling is indeed responsible for the competition in the carrier capture between the deep defect in the GaAs and the QDs.
Acknowledgements This work has been supported by the Consiglio Nazionale delle Ricerche (CNR), MADESS project. P.A. acknowledges the INFM support by the ‘PIE’ project.
References [1] A.D. Yoffe, Adv. Phys. 50 (2001) 1. [2] M. Grundmann, Physica E 5 (2000) 167. [3] E.C. Le Ru, P.D. Siverns, R. Murray, Appl. Phys. Lett. 77 (2000) 2446. [4] M.J. Steer, D.J. Mowbray, W.R. Tribe, M.S. Skolnick, M.D. Sturge, M. Hopkinson, A.G. Cullis, C.R. Whitehouse, R. Murray, Phys. Rev. B 54 (1996) 17738. [5] R. Heitz, O. Stier, I. Mukhametzhanov, A. Madhukar, D. Bimberg, Phys. Rev. B 62 (2000) 11017. [6] A. Bosacchi, P. Frigeri, S. Franchi, P. Allegri, V. Avanzini, J. Cryst. Growth 175 – 176 (1997) 771. [7] P. Borri, M. Gurioli, M. Colocci, F. Martelli, M. Capizzi, A. Patane´ , A. Polimeni, J. Appl. Phys. 80 (1996) 3011. [8] J.J. Hopfield, J. Phys. Chem. Solids 10 (1959) 110. [9] L. Pavesi, M. Guzzi, J. Appl. Phys. 75 (1994) 4779. [10] J. Van de Ven, W.J.A. Hartmann, L.J. Giling, J. Appl. Phys. 60 (1986) 3735.