- Email: [email protected]
InP core-multishell nanowires
ARTICLE IN PRESS Journal of Luminescence 129 (2009) 1941–1944
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Spectral diffusion of type-II excitons in wurtzite InP/InAs/InP core-multishell nanowires Bipul Pal a,1, Ken Goto a, Michio Ikezawa a, Yasuaki Masumoto a,, Premila Mohan b, Junichi Motohisa b, Takashi Fukui b a b
Institute of Physics, University of Tsukuba, Tsukuba 305-8571, Japan Research Center for Integrated Quantum Electronics, Hokkaido University, Sapporo 060-8628, Japan
a r t i c l e in fo
abstract
Available online 7 May 2009
We study the optical properties of a periodic array of InP/InAs/InP core-multishell nanowires by means of time- and spectrally resolved PL and PL excitation spectroscopy. Inhomogeneous broadening is present in our sample due to short range monolayer fluctuation and As–P intermixing at the interface, resulting in exciton localization. Acoustic phonon assisted migration of exciton to lower energy localized states leads to an energy-dependent PL decay time. A time-dependent redshift of the PL spectra (spectral diffusion) resulted from this is observed and a localization energy of 4 meV is estimated. & 2009 Elsevier B.V. All rights reserved.
PACS: 73.21.b 78.47.+p 78.55.m 78.67.n 81.07.b Keywords: Core-multishell nanowires Wurtzite InP/InAs heterostructures Time-resolved PL spectra Spectral diffusion
Semiconductor nanowires (NWs) can act as waveguide for light as well as for charge carriers. Thus, they have the potential to work as both the active device element and the interconnects for the integrated electronic and photonic circuits [1]. The possibility of layer-by-layer assembly of NW building blocks is promising for three-dimensional integrated electronics [2]. Several nanoelectronic and photonic devices, including lasers [3], polarization sensitive photodetectors [4], and field-effect transistors [5] have been fabricated by using NWs. Incorporation of heterostructures into a NW can enhance its capability for multifunctional devices. Axial heterostructured NWs have been used to fabricate single photon emitters [6], single electron transistors [7], etc. Introduction of radial heterostructures by forming core-shell and coremultishell structures has additional benefits. It passivates the surface states and helps to improve the optical cavity properties. Possible strain effects between lattice mismatched core and shell offer flexibility in band structure engineering. Very recently the fabrication of core-shell and core-multishell nanowires (CMNs) and regular array of such structures has become successful [8,9]. However, the optical properties of these novel structures remained virtually unexplored, so far.
Corresponding author.
E-mail address: [email protected] (Y. Masumoto). Present address: Indian Institute of Science Education and Research, Kolkata, Mohanpur, Nadia 741252, India. 1
0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.04.044
In this paper we investigate optical properties of InP/InAs/InP CMNs by using time-resolved (TR) and spectrally resolved (SR) PL and PL excitation (PLE) spectroscopy. Multi-peak CW-PL spectra is observed due to monolayer (ML) scale variation in the InAs layer thickness. Inhomogeneous broadening arises in this sample due to short range interface nanoroughness and As–P intermixing at the interface. Disorder induced potential fluctuation causes exciton localization. Decay time of PL becomes energy dependent due to acoustic phonon assisted relaxation of excitons to the lower energy localized states within the inhomogeneous broadening. This results in a time-dependent redshift of PL peak (spectral diffusion). A localization energy of 4 meV is estimated. Our sample is a periodic array of vertically oriented, highly uniform InP/InAs/InP CMNs, grown by using selective area metalorganic vapor phase epitaxy [9]. Transmission electron microscopy study showed that InP and InAs have a wurtzite crystal structure in CMNs [10,11]. Each CMN has a hexagonal cross-section and a translational symmetry along the axis. Schematics of the vertical and horizontal cross-section of a CMN is shown in Fig. 1(a). Details of the sample structure, growth procedure, and scanning electron microscopy images may be found in Ref. [9]. Standard setup with a tunable CW Ti:sapphire laser and a double monochromator (spectral resolution 0:02 meV) is used for SR-PL and PLE measurements. A modelocked Ti:sapphire laser (repetition rate 82 MHz, pulse width 2 ps) and a streak camera (time resolution 30 ps) are used in TR-PL measurements. The focused laser spot on the sample is
ARTICLE IN PRESS 1942
B. Pal et al. / Journal of Luminescence 129 (2009) 1941–1944
150 mm in diameter covering 105 CMNs. We estimate the steady state carrier density for 1 mW CW excitation to be less than three electron–hole pairs per CMN, on an average. All measurements are performed at 2 K for excitation at 1.38 eV, below the bandgap of 1.5 eV of wurtzite InP [12] (see also Refs. [11,13]). We plot in Fig. 1(b) a PL spectrum of the sample. It shows multiple peaks. One ML fluctuation of the layer thickness is
SiO2
2 ML
PLE
PL (x3)
2 1 0 1.1
1.2
1.3 1.4 Energy (eV)
1.5
1.6
Fig. 1. (Color online) (a) Schematics of the horizontal (i) and vertical (ii) crosssection of a CMN. (b) PL (solid line) and PLE spectra for detection at 1-ML (filled circles) and 2-ML (open circles) PL peaks.
1.8 IPL (a. u.)
1.5
2.24E6 1.68E6
0.9
1.13E6
0.6
5.68E5 1.00E4
0.3 0.0
1.16
1.18 1.20 Energy (eV)
1.22
Fig. 2. Gray-scale-coded PL intensity on time–energy surface.
6
1.181 Spectral Weight (eV)
Time (ns)
1.2
0.05 ns 1.75 ns
1.179
4
1.177 2
1.175 1.173 0.0
0.6 1.2 Time (ns)
1.8 1.14
1.17 1.20 Energy (eV)
IPL (arb. units)
(ii)
3
1 ML
InAs
T = 2 K, P = 10 mW
3 ML
InP
Intensity (arb. units)
4
(i)
common in the epitaxial growth of semiconductor heterostructures [14]. When the lateral dimension of the ML-islands is larger than the exciton in plane diameter, carrier recombination in such ML-islands results in separate peaks in the PL spectra, as seen in Fig. 1(b). A previous study [9] on an identical sample assigned the PL peaks from higher to lower energy to the regions of nominal thickness 1, 2, and 3 MLs, respectively, based on a calculation of the ground-state transition energy in a strained InAs/InP quantum well (QW). This assignment is in approximate agreement with other experimental and theoretical results available in the literature on ultrathin InAs/InP QWs [15,16]. We continue to designate the PL peaks from higher to lower energy in Fig. 1(b) as 1-, 2-, and 3-ML peak, respectively. When the lateral dimension of the ML-islands is smaller than the exciton in plane diameter, it causes inhomogeneous broadening and exciton localization [14]. Broad PL peaks with a full width of half maximum of 30 meV indicate inhomogeneous broadening in our sample. Inhomogeneous broadening may also be caused by the random intermixing of As- and P-atoms at the interface during sample growth [16]. We also show in Fig. 1(b) the PLE spectra detected at 1- and 2-ML peaks. Rise of the PLE signal at about 1.5 eV is related to absorption in the wurtzite InP layer [12]. The shoulder-like structures at 1.25 and 1.39 eV may be assigned to the absorption for 2-ML and 1-ML regions, respectively. We note that the PLE spectrum measured at 2-ML PL peak shows a structure at 1.39 eV, where the absorption for 1-ML peak appears. This indicates that carrier relaxation from thinner to thicker QW regions (lateral migration) takes place within a NW, possibly through the interaction with phonons. We observe a large Stokes shift of 70 meV between PL and PLE spectra. It is known that for QWs the Stokes shift for the inhomogeneously broadened exciton PL band almost universally becomes 0.6 times the FWHM of the excitonic absorption band. This universality is based on a large number of experimental observations and is also supported by theoretical calculations [17]. In our case the Stokes shift is more than twice the PL linewidth of 30 meV. Relaxation in an inhomogeneously broadened band cannot explain such alarge Stokes shift. The large Stokes shift possibly arises here due to type-II transition [18], because the absorption takes place at a higher energy than the PL in a type-II system. The type-II band alignment in this sample was concluded from a previous study by the observation of a blue shift of PL peaks with a cube-root dependence on excitation energy and a long PL decay time of 16 ns [19]. We study the dynamics of type-II excitons in this inhomogeneously broadened sample. For this we measure PL kinetics at different energies within the 2-ML peak. The result is presented in Fig. 2 by a plot of gray-scale-coded PL intensity on the timeenergy surface. A spectral diffusion (time dependent shift in the spectral feature) is clearly seen. To illustrate it further, we define
0 1.23
Fig. 3. (Color online) (a) Spectral weight as function of time. (b) Normalized TR-PL spectra at 0.05 and 1.75 ns.
ARTICLE IN PRESS B. Pal et al. / Journal of Luminescence 129 (2009) 1941–1944
3
4 4
3 2
2 1 2 3
0.0
0.5 1.0 Time (ns)
1.5
2.0
0
1.15 1.20 Energy (eV)
(ns)
2 102
5
6
1
IPL (arb. units)
IPL (arb. units)
103
1943
1 0
Fig. 4. (Color online) (a) PL decay measured at energy E ¼ 1:174, 1.185, and 1.191 eV. (b) Energy dependence of PL decay time t is shown on the CW PL spectrum around 2ML peak. Detection energies for the PL transients of (a) are indicated by arrows.
R R the spectral weight as hEðtÞi ¼ ½ EIPL ðE; tÞ dE=½ IPL ðE; tÞ dE, where IPL ðE; tÞ is the PL intensity measured at energy E at time t. The spectral weight computed from the data of Fig. 2 is shown as function of time in a semilogarithmic plot in Fig. 3(a). Spectral weight shifts towards lower energy at longer times with a nearly constant energy loss rate [20]. We plot in Fig. 3(b) the TR-PL spectra at 0.05 and 1.75 ns, which shows a spectral shift of about 4 meV. This suggests that the PL decay time (t) is energy dependent with shorter t at higher energy. This is indeed seen from the TR-PL data of Fig. 4(a). PL decay time t obtained from exponential fits to such decay curves is plotted as a function of energy in Fig. 4(b), along with a PL spectrum. A short t of 1:6 ns is seen in the high-energy tail of the PL spectrum. With decreasing energy, t increases. It again decreases at the low-energy tail of the PL spectrum, possibly due to rapid carrier capture by the nonradiative defect centers [21]. On the high-energy sidet decreases because the excitons migrate to nearby low energy localized states during their radiative lifetime. At low temperatures, the spectral relaxation of excitons within the inhomogeneous linewidth occurs through interaction with acoustic phonons, and it is predominantly towards lower energy [22]. Moreover, the localized excitons have a longer radiative lifetime due to the shrinkage of exciton wavefunction and reduced coherence volume [23]. The energy dependence of PL decay time seen at the high energy side of PL peak in Fig. 4(b) is similar to that observed in Fig. 2 of Ref. [24]. The data was explained by a model based on the acoustic phonon assisted spectral relaxation of excitons. The corner regions of the CMNs are likely to have lower exciton energy compared to that in the side ultrathin QWs. Excitons from the side regions may finally migrate to the corner regions and get trapped. A simple calculation suggests a difference in the confinement energy between the side and corner regions to be 3 meV [25], comparable to the observed spectral diffusion of 4 meV. Observed spectral diffusion of 4 meV is very small compared to the Stokes shift of 70 meV seen in Fig. 1(b). This can be explained by a type-II model for excitons. We propose that the photoabsorption takes place in the InAs layer by a direct transition between a quantum confined hole state and an unstable scattering state of electron (above the InP conduction band edge). The electron then relax from the unstable scattering state in a picosecond time scale to a stable state in the InP layer (confinement arises due to transient band bending effect) and form a type-II exciton with the confined hole in the InAs QW. Radiative recombination of the type-II excitons gives rise to PL signal. The large Stokes shift takes place during the formation of
type-II exciton in a picosecond time scale [26], and the spectral diffusion is caused by the localization of the type-II exciton over a nanosecond time scale. In conclusion, we present a detailed optical study on a periodic array of InP/InAs/InP core-multishell nanowires by using timeand spectrally resolved PL and PLE measurements. Short range ML fluctuation and interface disorder arising from possible As–P intermixing give rise to inhomogeneous broadening and exciton localization. Localization and spectral diffusion of the type-II exciton is observed in TR-PL measurements. These results will be important for further progress in the fabrication of the coremultishell nanowires and for the development of nanodevices based on such structures.
References [1] D.J. Sirbuly, M. Law, H. Yan, P. Yang, J. Phys. Chem. B 109 (2005) 15190. [2] A. Javey, S. Nam, R.S. Friedman, H. Yan, C.M. Lieber, Nano Lett. 7 (2007) 773. [3] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [4] J. Wang, M.S. Gudiksen, X. Duan, Y. Cui, C.M. Lieber, Science 293 (2001) 1455. [5] B.K. Kim, J.J. Kim, J.O. Lee, K.J. Kong, H.J. Seo, C.J. Lee, Phys. Rev. B 71 (2005) 153313. [6] M.T. Borgstro¨m, D. Bonacchi, L. Zobbi, F.M. Piras, E.A. Speets, A. Caneschi, A. Cornia, A. Magnani, B.J. Ravoo, D.N. Reinhoudt, R. Sessoli, D. Gatteschi, Nano Lett. 5 (2005) 1439. [7] C. Thelander, T. Ma˚rtensson, M.T. Bjo¨rk, B.J. Ohlsson, M.W. Larsson, L.R. Wallenberg, L. Samuelson, Appl. Phys. Lett. 83 (2003) 2052. [8] L.J. Lauhon, M.S. Gudiksen, D. Wang, C.M. Lieber, Nature 420 (2002) 57; M. Liu, X. Li, H. Imrane, Y. Chen, T. Goodrich, Z. Cai, K.S. Ziemer, J.Y. Huang, Appl. Phys. Lett. 90 (2007) 152501. [9] P. Mohan, J. Motohisa, T. Fukui, Appl. Phys. Lett. 88 (2006) 133105. [10] P. Mohan, J. Motohisa, T. Fukui, Appl. Phys. Lett. 88 (2006) 013110. [11] Y. Kobayashi, M. Fukui, J. Motohisa, T. Fukui, Physica E 40 (2008) 2204. [12] For wurtzite-type InP, the bandgap is larger by 84 meV compared to that of zincblende-type InP. [13] M. Murayama, T. Nakayama, Phys. Rev. B 49 (1994) 4710. [14] C. Weisbuch, R. Dingle, A.C. Gossard, W. Wiegmann, Solid State Comm. 38 (1981) 709; J. Hegarty, M.D. Sturge, J. Opt. Soc. Amer. B 2 (1985) 1143. [15] R. Houdre´, J.F. Carlin, A. Rudra, J. Ling, M. Ilegems, Superlattices Microstruct. 13 (1993) 67; A.-M. Vasson, A. Vasson, J. Leymarie, P. Disseix, P. Boring, B. Gil, Semicond. Sci. Technol. 8 (1993) 303; R. Leonelli, C.A. Tran, J.L. Brebner, J.T. Graham, R. Tabti, R.A. Masut, S. Charbonneau, Phys. Rev. B 48 (1993) 11135; P. Paki, R. Leonelli, L. Isnard, R.A. Masut, Appl. Phys. Lett. 74 (1999) 1445; V. Albe, L.J. Lewis, Physica B 301 (2000) 233. [16] N. Shtinkov, P. Desjardins, R.A. Masut, Phys. Rev. B 66 (2002) 195303. [17] F. Yang, M. Wilkinson, E.J. Austin, K.P. O’Donnell, Phys. Rev. Lett. 70 (1993) 323. [18] T.T. Chen, C.H. Chen, W.Z. Cheng, W.S. Su, M.H. Ya, Y.F. Chen, P.W. Liu, H.H. Lin, J. Appl. Phys. 93 (2003) 9655.
ARTICLE IN PRESS 1944
B. Pal et al. / Journal of Luminescence 129 (2009) 1941–1944
[19] B. Pal, K. Goto, M. Ikezawa, Y. Masumoto, P. Mohan, J. Motohisa, T. Fukui, Appl. Phys. Lett. 93 (2008) 073105. [20] Y. Masumoto, S. Shionoya, H. Kawaguchi, Phys. Rev. B 29 (1984) 2324. [21] Yu.I. Mazur, B.L. Liang, Zh.M. Wang, G.G. Tarasov, D. Guzun, G.J. Salamo, T.D. Mishima, M.B. Johnson, J. Appl. Phys. 100 (2006) 054313. [22] J.F. Donegan, R.P. Stanley, J.P. Doran, J. Hegarty, J. Lumin. 58 (1994) 216.
[23] D.S. Citrin, Phys. Rev. B 47 (1993) 3832. [24] C. Gourdon, P. Lavallard, Phys. Status Solidi (b) 153 (1989) 641. [25] K. Goto, M. Ikezawa, S. Tomimoto, B. Pal, Y. Masumoto, P. Mohan, J. Motohisa, T. Fukui, Jpn. J. Appl. Phys. 48 (2009) 04C203. [26] Observation of the formation dynamics of type-II excitons and PL rise time is limited by the 30 ps time resolution of our TR-PL measurements.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Spectral diffusion of type-II excitons in wurtzite InP/InAs/InP core-multishell nanowires Bipul Pal a,1, Ken Goto a, Michio Ikezawa a, Yasuaki Masumoto a,, Premila Mohan b, Junichi Motohisa b, Takashi Fukui b a b
Institute of Physics, University of Tsukuba, Tsukuba 305-8571, Japan Research Center for Integrated Quantum Electronics, Hokkaido University, Sapporo 060-8628, Japan
a r t i c l e in fo
abstract
Available online 7 May 2009
We study the optical properties of a periodic array of InP/InAs/InP core-multishell nanowires by means of time- and spectrally resolved PL and PL excitation spectroscopy. Inhomogeneous broadening is present in our sample due to short range monolayer fluctuation and As–P intermixing at the interface, resulting in exciton localization. Acoustic phonon assisted migration of exciton to lower energy localized states leads to an energy-dependent PL decay time. A time-dependent redshift of the PL spectra (spectral diffusion) resulted from this is observed and a localization energy of 4 meV is estimated. & 2009 Elsevier B.V. All rights reserved.
PACS: 73.21.b 78.47.+p 78.55.m 78.67.n 81.07.b Keywords: Core-multishell nanowires Wurtzite InP/InAs heterostructures Time-resolved PL spectra Spectral diffusion
Semiconductor nanowires (NWs) can act as waveguide for light as well as for charge carriers. Thus, they have the potential to work as both the active device element and the interconnects for the integrated electronic and photonic circuits [1]. The possibility of layer-by-layer assembly of NW building blocks is promising for three-dimensional integrated electronics [2]. Several nanoelectronic and photonic devices, including lasers [3], polarization sensitive photodetectors [4], and field-effect transistors [5] have been fabricated by using NWs. Incorporation of heterostructures into a NW can enhance its capability for multifunctional devices. Axial heterostructured NWs have been used to fabricate single photon emitters [6], single electron transistors [7], etc. Introduction of radial heterostructures by forming core-shell and coremultishell structures has additional benefits. It passivates the surface states and helps to improve the optical cavity properties. Possible strain effects between lattice mismatched core and shell offer flexibility in band structure engineering. Very recently the fabrication of core-shell and core-multishell nanowires (CMNs) and regular array of such structures has become successful [8,9]. However, the optical properties of these novel structures remained virtually unexplored, so far.
Corresponding author.
E-mail address: [email protected] (Y. Masumoto). Present address: Indian Institute of Science Education and Research, Kolkata, Mohanpur, Nadia 741252, India. 1
0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.04.044
In this paper we investigate optical properties of InP/InAs/InP CMNs by using time-resolved (TR) and spectrally resolved (SR) PL and PL excitation (PLE) spectroscopy. Multi-peak CW-PL spectra is observed due to monolayer (ML) scale variation in the InAs layer thickness. Inhomogeneous broadening arises in this sample due to short range interface nanoroughness and As–P intermixing at the interface. Disorder induced potential fluctuation causes exciton localization. Decay time of PL becomes energy dependent due to acoustic phonon assisted relaxation of excitons to the lower energy localized states within the inhomogeneous broadening. This results in a time-dependent redshift of PL peak (spectral diffusion). A localization energy of 4 meV is estimated. Our sample is a periodic array of vertically oriented, highly uniform InP/InAs/InP CMNs, grown by using selective area metalorganic vapor phase epitaxy [9]. Transmission electron microscopy study showed that InP and InAs have a wurtzite crystal structure in CMNs [10,11]. Each CMN has a hexagonal cross-section and a translational symmetry along the axis. Schematics of the vertical and horizontal cross-section of a CMN is shown in Fig. 1(a). Details of the sample structure, growth procedure, and scanning electron microscopy images may be found in Ref. [9]. Standard setup with a tunable CW Ti:sapphire laser and a double monochromator (spectral resolution 0:02 meV) is used for SR-PL and PLE measurements. A modelocked Ti:sapphire laser (repetition rate 82 MHz, pulse width 2 ps) and a streak camera (time resolution 30 ps) are used in TR-PL measurements. The focused laser spot on the sample is
ARTICLE IN PRESS 1942
B. Pal et al. / Journal of Luminescence 129 (2009) 1941–1944
150 mm in diameter covering 105 CMNs. We estimate the steady state carrier density for 1 mW CW excitation to be less than three electron–hole pairs per CMN, on an average. All measurements are performed at 2 K for excitation at 1.38 eV, below the bandgap of 1.5 eV of wurtzite InP [12] (see also Refs. [11,13]). We plot in Fig. 1(b) a PL spectrum of the sample. It shows multiple peaks. One ML fluctuation of the layer thickness is
SiO2
2 ML
PLE
PL (x3)
2 1 0 1.1
1.2
1.3 1.4 Energy (eV)
1.5
1.6
Fig. 1. (Color online) (a) Schematics of the horizontal (i) and vertical (ii) crosssection of a CMN. (b) PL (solid line) and PLE spectra for detection at 1-ML (filled circles) and 2-ML (open circles) PL peaks.
1.8 IPL (a. u.)
1.5
2.24E6 1.68E6
0.9
1.13E6
0.6
5.68E5 1.00E4
0.3 0.0
1.16
1.18 1.20 Energy (eV)
1.22
Fig. 2. Gray-scale-coded PL intensity on time–energy surface.
6
1.181 Spectral Weight (eV)
Time (ns)
1.2
0.05 ns 1.75 ns
1.179
4
1.177 2
1.175 1.173 0.0
0.6 1.2 Time (ns)
1.8 1.14
1.17 1.20 Energy (eV)
IPL (arb. units)
(ii)
3
1 ML
InAs
T = 2 K, P = 10 mW
3 ML
InP
Intensity (arb. units)
4
(i)
common in the epitaxial growth of semiconductor heterostructures [14]. When the lateral dimension of the ML-islands is larger than the exciton in plane diameter, carrier recombination in such ML-islands results in separate peaks in the PL spectra, as seen in Fig. 1(b). A previous study [9] on an identical sample assigned the PL peaks from higher to lower energy to the regions of nominal thickness 1, 2, and 3 MLs, respectively, based on a calculation of the ground-state transition energy in a strained InAs/InP quantum well (QW). This assignment is in approximate agreement with other experimental and theoretical results available in the literature on ultrathin InAs/InP QWs [15,16]. We continue to designate the PL peaks from higher to lower energy in Fig. 1(b) as 1-, 2-, and 3-ML peak, respectively. When the lateral dimension of the ML-islands is smaller than the exciton in plane diameter, it causes inhomogeneous broadening and exciton localization [14]. Broad PL peaks with a full width of half maximum of 30 meV indicate inhomogeneous broadening in our sample. Inhomogeneous broadening may also be caused by the random intermixing of As- and P-atoms at the interface during sample growth [16]. We also show in Fig. 1(b) the PLE spectra detected at 1- and 2-ML peaks. Rise of the PLE signal at about 1.5 eV is related to absorption in the wurtzite InP layer [12]. The shoulder-like structures at 1.25 and 1.39 eV may be assigned to the absorption for 2-ML and 1-ML regions, respectively. We note that the PLE spectrum measured at 2-ML PL peak shows a structure at 1.39 eV, where the absorption for 1-ML peak appears. This indicates that carrier relaxation from thinner to thicker QW regions (lateral migration) takes place within a NW, possibly through the interaction with phonons. We observe a large Stokes shift of 70 meV between PL and PLE spectra. It is known that for QWs the Stokes shift for the inhomogeneously broadened exciton PL band almost universally becomes 0.6 times the FWHM of the excitonic absorption band. This universality is based on a large number of experimental observations and is also supported by theoretical calculations [17]. In our case the Stokes shift is more than twice the PL linewidth of 30 meV. Relaxation in an inhomogeneously broadened band cannot explain such alarge Stokes shift. The large Stokes shift possibly arises here due to type-II transition [18], because the absorption takes place at a higher energy than the PL in a type-II system. The type-II band alignment in this sample was concluded from a previous study by the observation of a blue shift of PL peaks with a cube-root dependence on excitation energy and a long PL decay time of 16 ns [19]. We study the dynamics of type-II excitons in this inhomogeneously broadened sample. For this we measure PL kinetics at different energies within the 2-ML peak. The result is presented in Fig. 2 by a plot of gray-scale-coded PL intensity on the timeenergy surface. A spectral diffusion (time dependent shift in the spectral feature) is clearly seen. To illustrate it further, we define
0 1.23
Fig. 3. (Color online) (a) Spectral weight as function of time. (b) Normalized TR-PL spectra at 0.05 and 1.75 ns.
ARTICLE IN PRESS B. Pal et al. / Journal of Luminescence 129 (2009) 1941–1944
3
4 4
3 2
2 1 2 3
0.0
0.5 1.0 Time (ns)
1.5
2.0
0
1.15 1.20 Energy (eV)
(ns)
2 102
5
6
1
IPL (arb. units)
IPL (arb. units)
103
1943
1 0
Fig. 4. (Color online) (a) PL decay measured at energy E ¼ 1:174, 1.185, and 1.191 eV. (b) Energy dependence of PL decay time t is shown on the CW PL spectrum around 2ML peak. Detection energies for the PL transients of (a) are indicated by arrows.
R R the spectral weight as hEðtÞi ¼ ½ EIPL ðE; tÞ dE=½ IPL ðE; tÞ dE, where IPL ðE; tÞ is the PL intensity measured at energy E at time t. The spectral weight computed from the data of Fig. 2 is shown as function of time in a semilogarithmic plot in Fig. 3(a). Spectral weight shifts towards lower energy at longer times with a nearly constant energy loss rate [20]. We plot in Fig. 3(b) the TR-PL spectra at 0.05 and 1.75 ns, which shows a spectral shift of about 4 meV. This suggests that the PL decay time (t) is energy dependent with shorter t at higher energy. This is indeed seen from the TR-PL data of Fig. 4(a). PL decay time t obtained from exponential fits to such decay curves is plotted as a function of energy in Fig. 4(b), along with a PL spectrum. A short t of 1:6 ns is seen in the high-energy tail of the PL spectrum. With decreasing energy, t increases. It again decreases at the low-energy tail of the PL spectrum, possibly due to rapid carrier capture by the nonradiative defect centers [21]. On the high-energy sidet decreases because the excitons migrate to nearby low energy localized states during their radiative lifetime. At low temperatures, the spectral relaxation of excitons within the inhomogeneous linewidth occurs through interaction with acoustic phonons, and it is predominantly towards lower energy [22]. Moreover, the localized excitons have a longer radiative lifetime due to the shrinkage of exciton wavefunction and reduced coherence volume [23]. The energy dependence of PL decay time seen at the high energy side of PL peak in Fig. 4(b) is similar to that observed in Fig. 2 of Ref. [24]. The data was explained by a model based on the acoustic phonon assisted spectral relaxation of excitons. The corner regions of the CMNs are likely to have lower exciton energy compared to that in the side ultrathin QWs. Excitons from the side regions may finally migrate to the corner regions and get trapped. A simple calculation suggests a difference in the confinement energy between the side and corner regions to be 3 meV [25], comparable to the observed spectral diffusion of 4 meV. Observed spectral diffusion of 4 meV is very small compared to the Stokes shift of 70 meV seen in Fig. 1(b). This can be explained by a type-II model for excitons. We propose that the photoabsorption takes place in the InAs layer by a direct transition between a quantum confined hole state and an unstable scattering state of electron (above the InP conduction band edge). The electron then relax from the unstable scattering state in a picosecond time scale to a stable state in the InP layer (confinement arises due to transient band bending effect) and form a type-II exciton with the confined hole in the InAs QW. Radiative recombination of the type-II excitons gives rise to PL signal. The large Stokes shift takes place during the formation of
type-II exciton in a picosecond time scale [26], and the spectral diffusion is caused by the localization of the type-II exciton over a nanosecond time scale. In conclusion, we present a detailed optical study on a periodic array of InP/InAs/InP core-multishell nanowires by using timeand spectrally resolved PL and PLE measurements. Short range ML fluctuation and interface disorder arising from possible As–P intermixing give rise to inhomogeneous broadening and exciton localization. Localization and spectral diffusion of the type-II exciton is observed in TR-PL measurements. These results will be important for further progress in the fabrication of the coremultishell nanowires and for the development of nanodevices based on such structures.
References [1] D.J. Sirbuly, M. Law, H. Yan, P. Yang, J. Phys. Chem. B 109 (2005) 15190. [2] A. Javey, S. Nam, R.S. Friedman, H. Yan, C.M. Lieber, Nano Lett. 7 (2007) 773. [3] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [4] J. Wang, M.S. Gudiksen, X. Duan, Y. Cui, C.M. Lieber, Science 293 (2001) 1455. [5] B.K. Kim, J.J. Kim, J.O. Lee, K.J. Kong, H.J. Seo, C.J. Lee, Phys. Rev. B 71 (2005) 153313. [6] M.T. Borgstro¨m, D. Bonacchi, L. Zobbi, F.M. Piras, E.A. Speets, A. Caneschi, A. Cornia, A. Magnani, B.J. Ravoo, D.N. Reinhoudt, R. Sessoli, D. Gatteschi, Nano Lett. 5 (2005) 1439. [7] C. Thelander, T. Ma˚rtensson, M.T. Bjo¨rk, B.J. Ohlsson, M.W. Larsson, L.R. Wallenberg, L. Samuelson, Appl. Phys. Lett. 83 (2003) 2052. [8] L.J. Lauhon, M.S. Gudiksen, D. Wang, C.M. Lieber, Nature 420 (2002) 57; M. Liu, X. Li, H. Imrane, Y. Chen, T. Goodrich, Z. Cai, K.S. Ziemer, J.Y. Huang, Appl. Phys. Lett. 90 (2007) 152501. [9] P. Mohan, J. Motohisa, T. Fukui, Appl. Phys. Lett. 88 (2006) 133105. [10] P. Mohan, J. Motohisa, T. Fukui, Appl. Phys. Lett. 88 (2006) 013110. [11] Y. Kobayashi, M. Fukui, J. Motohisa, T. Fukui, Physica E 40 (2008) 2204. [12] For wurtzite-type InP, the bandgap is larger by 84 meV compared to that of zincblende-type InP. [13] M. Murayama, T. Nakayama, Phys. Rev. B 49 (1994) 4710. [14] C. Weisbuch, R. Dingle, A.C. Gossard, W. Wiegmann, Solid State Comm. 38 (1981) 709; J. Hegarty, M.D. Sturge, J. Opt. Soc. Amer. B 2 (1985) 1143. [15] R. Houdre´, J.F. Carlin, A. Rudra, J. Ling, M. Ilegems, Superlattices Microstruct. 13 (1993) 67; A.-M. Vasson, A. Vasson, J. Leymarie, P. Disseix, P. Boring, B. Gil, Semicond. Sci. Technol. 8 (1993) 303; R. Leonelli, C.A. Tran, J.L. Brebner, J.T. Graham, R. Tabti, R.A. Masut, S. Charbonneau, Phys. Rev. B 48 (1993) 11135; P. Paki, R. Leonelli, L. Isnard, R.A. Masut, Appl. Phys. Lett. 74 (1999) 1445; V. Albe, L.J. Lewis, Physica B 301 (2000) 233. [16] N. Shtinkov, P. Desjardins, R.A. Masut, Phys. Rev. B 66 (2002) 195303. [17] F. Yang, M. Wilkinson, E.J. Austin, K.P. O’Donnell, Phys. Rev. Lett. 70 (1993) 323. [18] T.T. Chen, C.H. Chen, W.Z. Cheng, W.S. Su, M.H. Ya, Y.F. Chen, P.W. Liu, H.H. Lin, J. Appl. Phys. 93 (2003) 9655.
ARTICLE IN PRESS 1944
B. Pal et al. / Journal of Luminescence 129 (2009) 1941–1944
[19] B. Pal, K. Goto, M. Ikezawa, Y. Masumoto, P. Mohan, J. Motohisa, T. Fukui, Appl. Phys. Lett. 93 (2008) 073105. [20] Y. Masumoto, S. Shionoya, H. Kawaguchi, Phys. Rev. B 29 (1984) 2324. [21] Yu.I. Mazur, B.L. Liang, Zh.M. Wang, G.G. Tarasov, D. Guzun, G.J. Salamo, T.D. Mishima, M.B. Johnson, J. Appl. Phys. 100 (2006) 054313. [22] J.F. Donegan, R.P. Stanley, J.P. Doran, J. Hegarty, J. Lumin. 58 (1994) 216.
[23] D.S. Citrin, Phys. Rev. B 47 (1993) 3832. [24] C. Gourdon, P. Lavallard, Phys. Status Solidi (b) 153 (1989) 641. [25] K. Goto, M. Ikezawa, S. Tomimoto, B. Pal, Y. Masumoto, P. Mohan, J. Motohisa, T. Fukui, Jpn. J. Appl. Phys. 48 (2009) 04C203. [26] Observation of the formation dynamics of type-II excitons and PL rise time is limited by the 30 ps time resolution of our TR-PL measurements.