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InAlAs quantum wires grown on InP (001)
Materials Science and Engineering B101 (2003) 259 /261 www.elsevier.com/locate/mseb
Effect of spacer layer thickness on the optical properties of stacked InAs/InAlAs quantum wires grown on InP (001) B. Salem a,*, G. Bre´mond a, M. Hjiri b, F. Hassen b, H. Maaref b, O. Marty c, J. Brault d, M. Gendry d a
LPM, UMR CNRS 5511, INSA de Lyon, 69621 Villeurbanne, France b LPSCE, Faculte´ des Sciences de Monastir, Monastir, Tunisia c LENAC, UCB Lyon1, 69622 Villeurbanne, France d LEOM, UMR CNRS 5512, Ecole Centrale de Lyon, 69134 Ecully, France
Abstract Stacked structures containing InAs quantum wires (QWRs) have been successfully grown on In0.52Al0.48As/InP(001) by solidsource molecular beam epitaxy (MBE). The influence of the In0.52Al0.48As spacer layer thickness (SLT) on the structural and optical properties has been studied by means of transmission electron microscopy (TEM), atomic force microscopy (AFM) and photoluminescence (PL). A large full width at half maximum of the PL peak (FWHM /120 meV) has been observed on a structure with a single layer of wires. In contrast, a stacked structure with a SLT of 10 nm presents a PL peak FWHM reduced to 92 meV, which shows that the stacking process improves the wire size homogeneity. Polarized photoluminescence (PPL) experiments carried out on the stacked structures show a strong polarization anisotropy (35%) when the SLT is in the 10 /15 nm range. On the contrary, for SLT equal to 5 nm, a weak degree of polarization is obtained (P /6%) combined with a red shift of the PL line. This behaviour is attributed to vertical electronic coupling between the InAs QWRs when the SLT is lowered to within the 5 nm range. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Quantum wires; Photoluminescence; Stacked structures; Polarization
1. Introduction In recent years, extensive work has been devoted to the fabrication and the characterisation of self-assembled quantum nanostructures obtained by the Stranski/Krastanov (S/K) growth mode. The interest is due to potential applications to optoelectronic devices such as lasers or photodetectors using semiconductor quantum dots (QDs) as the active region of heterostructures [1 /6]. The stacking of quantum dots in multilayer structures has been proposed as an efficient mechanism to improve dot size uniformity [7,8]. This effect has been demonstrated in the case of InAs/GaAs self-organized quantum dots, where the deposition of multiple layers of InAs separated by GaAs spacers
* Corresponding author. Tel.: /33-4-7243-8066; fax: /33-4-72438531. E-mail address: [email protected] (B. Salem).
results in vertically correlated quantum dots of better structural and optical properties [7,9 /11]. The self-assembled formation of InAs quantum wires (QWRs) in single and stacked InAs/In0.52Al0.48As structures grown on (001) InP substrates by molecular beam epitaxy (MBE) has previously been reported [12 /16]. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements have shown a high-density of QWRs elongated in the [1 /10] direction. Furthermore, in the stacked structures, crosssection TEM images have revealed a specific anticorrelated vertical arrangement of these InAs QWRs. In this paper, we study the influence of the In0.52Al0.48As spacer layer thickness (SLT) on the structural and optical properties of the stacked InAs QWRs. The effect of vertical electronic coupling on photoluminescence (PL) is reported. These results are correlated to the optical anisotropy measured by polarized photoluminescence (PPL).
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00691-8
260
B. Salem et al. / Materials Science and Engineering B101 (2003) 259 /261
2. Experimental details The samples studied in this work were grown at 525 8C on a semi insulating InP substrate using solid source molecular beam epitaxy in a Riber 2300 reactor. The stacked structures made of 10 InAs QWR layers separated by In0.52Al0.48As spacer layers lattice matched to InP with various SLTs of 5, 10 and 15 nm. The InAs deposit was fixed at a 3 monolayer (ML) equivalent thickness, just above the critical threshold for the 2D/3D growth mode transition (2.5 ML) as measured by reflection high-energy electron diffraction (RHEED). The surface morphologies and structural properties of the samples were characterized by AFM and TEM measurements. The PL was excited using the 514.5 nm line of an Ar -ion laser. The emission was dispersed by a spectrometer and detected by a 77 K cooled Ge photodetector using a conventional lock-in technique. The polarization measurements were performed on the PL emission normal to the surface via a Glan-Thompson near infrared polarizer at the entrance slit of the monochromator, followed by a quarter wave plate, in order to obtain a PL signal independent of any light polarization effect of the optical system response.
3. Results and discussion Fig. 1(a) and (b) show typical cross-section (g /(200) dark-field) TEM images of stacked InAs QWR samples with SLTs of 5 and 15 nm, respectively. No sharp contrast revealing dislocations is observed on the samples: they are dislocation-free on the field of view scale. For the two samples, we see a remarkable anticorrelated vertical arrangement of the stacked InAs QWRs. In fact, the origin of this arrangement is the phase separation taking place in the InAlAs spacer layer [15]. The AFM image in Fig. 1(c) shows the tendency for the wire-like shaped InAs islands (elongated along the [1 /10] direction) observed for all stacked structures when the SLT is lower than 50 nm. Fig. 2 shows the PL spectra at 8 K of a reference sample with a single layer of QWRs, and of stacked structures with SLTs equal to 5, 10 and 15 nm. The linewidth reduction, from 120 to 92 meV, observed in the stacked sample with a SLT /10 nm, indicates the QWR homogeneity improvement induced by the stacking process. On the contrary, for SLT /5 nm, a low energy shoulder is apparent in the spectrum leading to an increase of the spectral linewidth (FWHM /155 meV). Vertical electronic coupling between the InAs QWRs can be responsible for this red shift [17] through the formation of a low energy mini-band. In Fig. 3, we have plotted the PPL spectra and the degree of linear polarization of the stacked structures with different SLT. For the stacked samples with
Fig. 1. (a) and (b) Cross sectional (g /(200) dark field) TEM images showing the anti-correlated vertical arrangement in InAs/InAlAs stacked structures with SLTs equal to 5 and 15 nm, respectively. (c) AFM image of the InAs QWRs in stacked structures.
Fig. 2. Low temperature PL spectra of an InAs QWR single-layer sample and of stacked samples.
SLTs /10 and 15 nm, the degree of linear polarization P [18] which is around 35%, indicates a quantum wire
B. Salem et al. / Materials Science and Engineering B101 (2003) 259 /261
261
Fig. 3. 8 K polarized PL spectra of stacked samples with SLTs equal to 5, 10 and 15 nm. The corresponding polarization degrees are also plotted.
signature in good agreement with the AFM measurements. On the contrary, in the structure with SLT /5 nm, a weak degree of linear polarization is obtained (P /6%). This can strengthen the presumption of vertical electronic coupling inducing an increase of the symmetry of the wave functions in the growth planes. Work is in progress to verify this point.
4. Conclusion We have investigated, using PL and PPL spectroscopies, the optical properties of stacked InAs/InAlAs quantum wires grown on InP(001) as a function of the InAlAs spacer layer thickness. For stacked structures with SLTs /10 and 15 nm, we have measured a strong linear polarization degree related to the wire shape of the InAs islands. The PL peak FWHM decreases from 120 meV for a single-layer sample to 92 meV for the stacked sample with optimal SLT /10 nm. When SLT /5 nm, a red shift of the PL line and a linewidth increase are observed. Based on PPL measurements, we attribute these behaviours to the effect of vertical electronic coupling between the InAs QWR layers.
References [1] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars, P.M. Petroff, Appl. Phys. Lett. 63 (1993) 3203.
[2] R. Leon, P.M. Petroff, D. Leonard, S. Fafard, Science 267 (1966) 1995. [3] V.M. Ustinov, A.E. Zhukov, A.Yu. Egorov, A.R. Kovsh, S.V. Zaitsev, N.Yu. Gordeev, Electron. Lett. 34 (1998) 670. [4] N. Kirsta¨dter, N.N. Ledentsov, M. Grundmann, D. Bimberg, V.M. Ustinov, S.S. Ruvimov, M.V. Maximov, P.S. Kop’ev, Zh.I. Alferov, Electron. Lett. 30 (1994) 1416. [5] F. Klopf, J.P. Reithmaier, A. Forchel, Appl. Phys. Lett. 77 (2000) 1419. [6] L. Chu, M. Arzberger, A. Zrenner, G. Bohm, G. Abstreiter, Appl. Phys. Lett. 75 (1999) 2247. [7] Q. Xie, A. Madhukar, P. Chen, N.P. Kobayashi, Phys. Rev. Lett. 75 (1995) 2542. [8] J. Tersoff, C. Teichert, M.G. Lagally, Phys. Rev. Lett. 76 (1996) 1675. [9] R. Heitz, A. Kalburge, Q. Xie, M. Grundmann, P. Chen, A. Hoffmann, A. Madhukar, D. Bimberg, Phys. Rev. B 57 (1998) 9050. [10] P. Frigeri, A. Bosacchi, S. Franchi, P. Allegri, V. Avanzini, J. Cryst. Growth 201/202 (1999) 1136. [11] E. Petitprez, N.T. Moshegov, J.E. Marega, A. Mazel, D. Dorignac, R. Fourmeaux, J. Vac. Sci. Technol. B 18 (2000) 1493. [12] H. Li, J. Wu, Z. Wang, T. Daniels-Race, Appl. Phys. Lett. 75 (1999) 1173. [13] J. Wu, B. Xu, H.X. Li, J. Cryst. Growth 197 (1999) 95. [14] H. Li, T. Daniels-Race, M.-A. Hasan, J. Vac. Sci. Technol. B 19 (2001) 1471. [15] J. Brault, M. Gendry, O. Marty, M. Pitaval, J. Olivares, G. Grenet, G. Hollinger, Appl. Surf. Sci. 162 /163 (2000) 584. [16] J. Brault, M. Gendry, G. Grenet, G. Hollinger, J. Olivares, B. Salem, T. Benyattou, G. Bremond, J. Appl. Phys. 92 (2002) 506. [17] G.S. Solomon, J.A. Trezza, A.F. Marshall, J.S. Harris, Jr, J. Vac. Sci. Technol. B 14 (1996) 2208. [18] The degree of linear polarization P of the emitted light is classically defined by the following equation: P (I[110] ¯ ¯ and I [110] are the PL intensities I[110] )=(I[110] ¯ I[110] ) where I/[110] ¯ and [110]. along the two orthogonal directions [110]
Effect of spacer layer thickness on the optical properties of stacked InAs/InAlAs quantum wires grown on InP (001) B. Salem a,*, G. Bre´mond a, M. Hjiri b, F. Hassen b, H. Maaref b, O. Marty c, J. Brault d, M. Gendry d a
LPM, UMR CNRS 5511, INSA de Lyon, 69621 Villeurbanne, France b LPSCE, Faculte´ des Sciences de Monastir, Monastir, Tunisia c LENAC, UCB Lyon1, 69622 Villeurbanne, France d LEOM, UMR CNRS 5512, Ecole Centrale de Lyon, 69134 Ecully, France
Abstract Stacked structures containing InAs quantum wires (QWRs) have been successfully grown on In0.52Al0.48As/InP(001) by solidsource molecular beam epitaxy (MBE). The influence of the In0.52Al0.48As spacer layer thickness (SLT) on the structural and optical properties has been studied by means of transmission electron microscopy (TEM), atomic force microscopy (AFM) and photoluminescence (PL). A large full width at half maximum of the PL peak (FWHM /120 meV) has been observed on a structure with a single layer of wires. In contrast, a stacked structure with a SLT of 10 nm presents a PL peak FWHM reduced to 92 meV, which shows that the stacking process improves the wire size homogeneity. Polarized photoluminescence (PPL) experiments carried out on the stacked structures show a strong polarization anisotropy (35%) when the SLT is in the 10 /15 nm range. On the contrary, for SLT equal to 5 nm, a weak degree of polarization is obtained (P /6%) combined with a red shift of the PL line. This behaviour is attributed to vertical electronic coupling between the InAs QWRs when the SLT is lowered to within the 5 nm range. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Quantum wires; Photoluminescence; Stacked structures; Polarization
1. Introduction In recent years, extensive work has been devoted to the fabrication and the characterisation of self-assembled quantum nanostructures obtained by the Stranski/Krastanov (S/K) growth mode. The interest is due to potential applications to optoelectronic devices such as lasers or photodetectors using semiconductor quantum dots (QDs) as the active region of heterostructures [1 /6]. The stacking of quantum dots in multilayer structures has been proposed as an efficient mechanism to improve dot size uniformity [7,8]. This effect has been demonstrated in the case of InAs/GaAs self-organized quantum dots, where the deposition of multiple layers of InAs separated by GaAs spacers
* Corresponding author. Tel.: /33-4-7243-8066; fax: /33-4-72438531. E-mail address: [email protected] (B. Salem).
results in vertically correlated quantum dots of better structural and optical properties [7,9 /11]. The self-assembled formation of InAs quantum wires (QWRs) in single and stacked InAs/In0.52Al0.48As structures grown on (001) InP substrates by molecular beam epitaxy (MBE) has previously been reported [12 /16]. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements have shown a high-density of QWRs elongated in the [1 /10] direction. Furthermore, in the stacked structures, crosssection TEM images have revealed a specific anticorrelated vertical arrangement of these InAs QWRs. In this paper, we study the influence of the In0.52Al0.48As spacer layer thickness (SLT) on the structural and optical properties of the stacked InAs QWRs. The effect of vertical electronic coupling on photoluminescence (PL) is reported. These results are correlated to the optical anisotropy measured by polarized photoluminescence (PPL).
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00691-8
260
B. Salem et al. / Materials Science and Engineering B101 (2003) 259 /261
2. Experimental details The samples studied in this work were grown at 525 8C on a semi insulating InP substrate using solid source molecular beam epitaxy in a Riber 2300 reactor. The stacked structures made of 10 InAs QWR layers separated by In0.52Al0.48As spacer layers lattice matched to InP with various SLTs of 5, 10 and 15 nm. The InAs deposit was fixed at a 3 monolayer (ML) equivalent thickness, just above the critical threshold for the 2D/3D growth mode transition (2.5 ML) as measured by reflection high-energy electron diffraction (RHEED). The surface morphologies and structural properties of the samples were characterized by AFM and TEM measurements. The PL was excited using the 514.5 nm line of an Ar -ion laser. The emission was dispersed by a spectrometer and detected by a 77 K cooled Ge photodetector using a conventional lock-in technique. The polarization measurements were performed on the PL emission normal to the surface via a Glan-Thompson near infrared polarizer at the entrance slit of the monochromator, followed by a quarter wave plate, in order to obtain a PL signal independent of any light polarization effect of the optical system response.
3. Results and discussion Fig. 1(a) and (b) show typical cross-section (g /(200) dark-field) TEM images of stacked InAs QWR samples with SLTs of 5 and 15 nm, respectively. No sharp contrast revealing dislocations is observed on the samples: they are dislocation-free on the field of view scale. For the two samples, we see a remarkable anticorrelated vertical arrangement of the stacked InAs QWRs. In fact, the origin of this arrangement is the phase separation taking place in the InAlAs spacer layer [15]. The AFM image in Fig. 1(c) shows the tendency for the wire-like shaped InAs islands (elongated along the [1 /10] direction) observed for all stacked structures when the SLT is lower than 50 nm. Fig. 2 shows the PL spectra at 8 K of a reference sample with a single layer of QWRs, and of stacked structures with SLTs equal to 5, 10 and 15 nm. The linewidth reduction, from 120 to 92 meV, observed in the stacked sample with a SLT /10 nm, indicates the QWR homogeneity improvement induced by the stacking process. On the contrary, for SLT /5 nm, a low energy shoulder is apparent in the spectrum leading to an increase of the spectral linewidth (FWHM /155 meV). Vertical electronic coupling between the InAs QWRs can be responsible for this red shift [17] through the formation of a low energy mini-band. In Fig. 3, we have plotted the PPL spectra and the degree of linear polarization of the stacked structures with different SLT. For the stacked samples with
Fig. 1. (a) and (b) Cross sectional (g /(200) dark field) TEM images showing the anti-correlated vertical arrangement in InAs/InAlAs stacked structures with SLTs equal to 5 and 15 nm, respectively. (c) AFM image of the InAs QWRs in stacked structures.
Fig. 2. Low temperature PL spectra of an InAs QWR single-layer sample and of stacked samples.
SLTs /10 and 15 nm, the degree of linear polarization P [18] which is around 35%, indicates a quantum wire
B. Salem et al. / Materials Science and Engineering B101 (2003) 259 /261
261
Fig. 3. 8 K polarized PL spectra of stacked samples with SLTs equal to 5, 10 and 15 nm. The corresponding polarization degrees are also plotted.
signature in good agreement with the AFM measurements. On the contrary, in the structure with SLT /5 nm, a weak degree of linear polarization is obtained (P /6%). This can strengthen the presumption of vertical electronic coupling inducing an increase of the symmetry of the wave functions in the growth planes. Work is in progress to verify this point.
4. Conclusion We have investigated, using PL and PPL spectroscopies, the optical properties of stacked InAs/InAlAs quantum wires grown on InP(001) as a function of the InAlAs spacer layer thickness. For stacked structures with SLTs /10 and 15 nm, we have measured a strong linear polarization degree related to the wire shape of the InAs islands. The PL peak FWHM decreases from 120 meV for a single-layer sample to 92 meV for the stacked sample with optimal SLT /10 nm. When SLT /5 nm, a red shift of the PL line and a linewidth increase are observed. Based on PPL measurements, we attribute these behaviours to the effect of vertical electronic coupling between the InAs QWR layers.
References [1] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars, P.M. Petroff, Appl. Phys. Lett. 63 (1993) 3203.
[2] R. Leon, P.M. Petroff, D. Leonard, S. Fafard, Science 267 (1966) 1995. [3] V.M. Ustinov, A.E. Zhukov, A.Yu. Egorov, A.R. Kovsh, S.V. Zaitsev, N.Yu. Gordeev, Electron. Lett. 34 (1998) 670. [4] N. Kirsta¨dter, N.N. Ledentsov, M. Grundmann, D. Bimberg, V.M. Ustinov, S.S. Ruvimov, M.V. Maximov, P.S. Kop’ev, Zh.I. Alferov, Electron. Lett. 30 (1994) 1416. [5] F. Klopf, J.P. Reithmaier, A. Forchel, Appl. Phys. Lett. 77 (2000) 1419. [6] L. Chu, M. Arzberger, A. Zrenner, G. Bohm, G. Abstreiter, Appl. Phys. Lett. 75 (1999) 2247. [7] Q. Xie, A. Madhukar, P. Chen, N.P. Kobayashi, Phys. Rev. Lett. 75 (1995) 2542. [8] J. Tersoff, C. Teichert, M.G. Lagally, Phys. Rev. Lett. 76 (1996) 1675. [9] R. Heitz, A. Kalburge, Q. Xie, M. Grundmann, P. Chen, A. Hoffmann, A. Madhukar, D. Bimberg, Phys. Rev. B 57 (1998) 9050. [10] P. Frigeri, A. Bosacchi, S. Franchi, P. Allegri, V. Avanzini, J. Cryst. Growth 201/202 (1999) 1136. [11] E. Petitprez, N.T. Moshegov, J.E. Marega, A. Mazel, D. Dorignac, R. Fourmeaux, J. Vac. Sci. Technol. B 18 (2000) 1493. [12] H. Li, J. Wu, Z. Wang, T. Daniels-Race, Appl. Phys. Lett. 75 (1999) 1173. [13] J. Wu, B. Xu, H.X. Li, J. Cryst. Growth 197 (1999) 95. [14] H. Li, T. Daniels-Race, M.-A. Hasan, J. Vac. Sci. Technol. B 19 (2001) 1471. [15] J. Brault, M. Gendry, O. Marty, M. Pitaval, J. Olivares, G. Grenet, G. Hollinger, Appl. Surf. Sci. 162 /163 (2000) 584. [16] J. Brault, M. Gendry, G. Grenet, G. Hollinger, J. Olivares, B. Salem, T. Benyattou, G. Bremond, J. Appl. Phys. 92 (2002) 506. [17] G.S. Solomon, J.A. Trezza, A.F. Marshall, J.S. Harris, Jr, J. Vac. Sci. Technol. B 14 (1996) 2208. [18] The degree of linear polarization P of the emitted light is classically defined by the following equation: P (I[110] ¯ ¯ and I [110] are the PL intensities I[110] )=(I[110] ¯ I[110] ) where I/[110] ¯ and [110]. along the two orthogonal directions [110]