- Email: [email protected]
(Mg,Zn)O quantum wells
Available online at www.sciencedirect.com
Physica E 21 (2004) 671 – 675 www.elsevier.com/locate/physe
Internal electric eld e"ect on luminescence properties of ZnO/(Mg,Zn)O quantum wells T. Makinoa;∗ , A. Ohtomob , C.H. Chiaa;1 , Y. Segawaa;1 , H. Koinumac; d , M. Kawasakib; d a Photodynamics
Research Center, RIKEN (Institute of Physical and Chemical Research), Sendai 980-0845, Japan b Institute for Materials Research, Tohoku University, Sendai 980-0857, Japan c Frontier Collaborative Research Center, Tokyo Institute of Technology, Yokohama 226-8503, Japan d Combinatorial Materials Exploration and Technology, Tsukuba 305-0044, Japan
Abstract Spectroscopic studies have been conducted for wurtzite ZnO=Mg0:27 Zn0:73 O multiple quantum wells. Internal electric elds at interfaces are found to have an in6uence on the photoluminescence (PL) properties for well width (Lw ) greater than 4:2 nm. These experimentally observed features are characteristic of the quantum-con ned Stark e"ects; the magnitude of the electric eld due to spontaneous and piezoelectric polarizations and the depth of the triangle-shaped potential wells are the monotonically increasing functions of Mg concentration and the Lw , respectively. Results of the time-resolved PL study are also presented. ? 2003 Elsevier B.V. All rights reserved. PACS: 78.55.Et; 81.15.Fg; 71.35.Cc; 72.15.−v Keywords: Quantum wells; II–VI semiconductors; Quantum-con ned Stark e"ect; Phonon–exciton interactions
1. Introduction During the last few years, developments in the eld of ZnO has been progressing steadily toward the applications for short wavelength light-emitting devices. Epitaxial ZnO lms having suBciently high quality can now be grown by various epitaxial methods. ZnO has several analogous features with the other famous semiconductor, GaN, e.g., band gap energy and the crystal structure. In strained nitride heterolayers ∗
Corresponding author. E-mail address: [email protected] (T. Makino). URL: http://www.riken.jp/lab-www/photophys/top.html 1 Also at Department of Physics, Tohoku University, Sendai, Japan.
grown in the wurtzite structure with the c-axis parallel to the growth direction, piezoelectric and spontaneous polarization e"ects are present [1–6]. These polarization e"ects have been extensively investigated in GaN-related heterostructures. As pointed out in our previous study [7], the piezoelectric elds for ZnO on MgZnO are expected to be much smaller than those for GaN-related systems because of the small di"erence in the lattice constants between ZnO and MgZnO. On the other hand, there may exist a sizeable spontaneous eld across the ZnO layers. This is more likely if the large spontaneous polarization coeBcient of ZnO, comparable with that of GaN, is considered. Nevertheless, the observation of the radiative recombination of the electron–hole pairs that are spatially separated due to such quantum-con ned Stark (QCS) e"ects
1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.110
672
T. Makino et al. / Physica E 21 (2004) 671 – 675
has not been reported so far in ZnO-based quantum wells (QWs). In this short communication, a sequel of [8], we describe the possibility of the presence of spontaneous polarization mismatches at interfaces of ZnO=Mg0:27 Zn0:73 O heterostructures (higher Mg concentration studied here). This spontaneous polarization e"ect leads to the observation of the radiative recombination from the electron–hole pairs in6uenced by the QCS e"ects. As a result, a Stokes-shifted photoluminiscience (PL) band located 40 meV below the localized exciton (LE) emission is observed in the PL spectra taken at a temperature of 5 K. To investigate the interplay between the distance and the oscillator strength, spectral distribution of the PL decay times is also discussed. 2. Experimental procedures The ZnO/ZnMgO MQWs were grown on a ScAlMgO4 (0 0 0 1) substrate by laser–molecular-beam epitaxy method. The detailed preparation process has been described elsewhere [9]. The total structures consist of 10 periods of alternating ZnO well layers and 5-nm-thick (Zn,Mg)O barrier layers. The Mg content of x = 0:12 is slightly lower than the solubility limit (x =0:15) of this alloy lm [9], while the content of x = 0:27 is far above it, corresponding to a barrier height of about 0:5 eV. The thicknesses indicated below are the averaged ones of 10 well layers. All the layer thicknesses were set as integer of molecular layers by the prescribed deposition time. The SCAM is transparent to the spectral region of interest. The samples were kept in a cryostat. The PL was excited by a He–Cd laser (325 nm in wavelength) and was monitored using a monochromator with a charge-coupled device. Absorption measurements were carried out using a xenon lamp. 3. Results and discussion A band diagram of wurtzitic QWs under the polarization elds is schematically shown in Fig. 1. Because the band diagram of the well regions is not 6at, electrons and holes tend to be relaxed. In other words, the internal electric eld pushes the electron and hole toward opposite sides of the well. In such a situation, the
cond.
QCS
LE
valence
Fig. 1. Schematic diagram of the conduction and the valence bands of wurtzitic QWs under the spontaneous and piezoelectric polarization eld. Electrons and holes distributed in the well regions (closed circles) are spatially separated due to the QCS e"ects.
PL transition energy is likely to be smaller than that of absorption. The energy di"erence becomes more signi cant with an increase in the well width. We show in Fig. 2 low-temperature PL and absorption spectra corresponding to four ZnO=Mg0:27 Zn0:73 O QW samples with di"erent well widths (Lw ). There are distinct structures in these absorption spectra, which have been already assigned in Ref. [10]: the free excitonic transitions. A sharp peak structure has no longer been observed for the samples with Lw greater than 4:2 nm. In other words, it is not possible to conrm here the excitonic peak (e.g., Gaussian) although this is possible for some thinner QWs. This is unusual because the spatial 6uctuations on the relevant heterostructure size have less in6uence on the width of absorption bands in these thicker QWs. It is well known that the QCS e"ects somewhat reduce the excitonic oscillator strength as shown in Fig. 2. The well width dependence of the absorption peak energies is shown in Fig. 3. Because sometimes the sharp structure was not observed in the experimental data, we used the calculation absorption energy obtained by Coli and Bajaj [11]. We hereafter discuss whether the PL properties can be consistently interpreted from a viewpoint of the
T. Makino et al. / Physica E 21 (2004) 671 – 675
673
3.65 ABS PL
3.60
QCSE
Peak Energy (eV)
3.55 3.50 3.45 3.40 3.35
LE QCSE
3.30 1
2
3
4
Well Width (nm) Fig. 3. Peak energies of PL (open circles) and absorption (open squares) are plotted against well width in ZnO=Mg0:27 Zn0:73 O MQWs. The QCS peak energies are also shown. Fig. 2. PL spectra of ZnO QW samples measured at 5 K. The well widths of QWs are shown in the gure. Nomenclatures of LE and QCS, respectively, mean the localized exciton and the quantum-con ned Stark e"ect. The LO-phonon replicas are indicated as arrows. The excitonic absorption spectra (dotted lines) are shown for comparison.
internal electric eld. The zero-phonon peaks of PL labeled with “LE” in Fig. 2 (located at energies of 3.38, 3.50, and 3:56 eV) are attributed to the radiative recombination of the localized excitons [10], which were based on their peak energy plot [12]. When the PL peak energies were plotted as a function of Lw , the amount of Stokes shift is a monotonically decreasing function of Lw . In other words, these energy positions smoothly connect with respect to each other (cf. open squares in Fig. 3). This behavior is typically seen in the case of radiative recombination from the excitons localized at the potentials induced by spatial 6uctuations on the relevant heterostructure size, because such 6uctuation has a more sensitive e"ect for a very thin well. As long as the data for the Lw smaller than or equal to 3:7 nm are discussed, it seems to be unnecessary to take the polarization e"ects into account. The
discussion of barrier height 6uctuation e"ect has been given elsewhere [10]. On the other hand, there are two prominent zero-phonon PL peaks in the 4.7- and 4.2-nm-thick QWs. The higher-energy PL bands are found to have the same origin with the main zero-phonon peaks observed in the samples with Lw smaller than or equal to 3:7 nm. Lower-energy side “QCS” bands have evidently larger Stokes shift than that of LE bands. These PL bands are located 40 meV in energy below the emission band of the LE and 60 meV below the absorption energy of the free-exciton transition. It should be noted, although the corresponding spectra are not shown in gures, that such kind of emission is not observed in any QW sample with the smaller Mg concentration (x = 0:12), namely, only the LE bands could be observed in ZnO=Mg0:12 Zn0:88 O QWs. The magnitude of the electric eld in the case of x = 0:27 is larger than that of x =0:12, because of the larger lattice mismatch between ZnO and ZnO=Mg0:27 Zn0:73 O (cf. Fig. 3 of Ref. [9]). This internal eld, present along the growth axis of the system, is caused by piezoelectric and spontaneous polarizations. It is
T. Makino et al. / Physica E 21 (2004) 671 – 675
2000 T=5 K
QCS
LW
1500
=4.23nm 1000
LE
500
Decay Time (ps)
considered easier to observe the QCS bands in the sample with higher barrier height. The Stokes-type shift of PL is due to the QCS e"ects induced by this strong internal electric eld. We cannot conclude here whether the amount of spontaneous polarization mismatch is a monotonically increasing function of well width. It is beyond our scope to consider the e"ect of the spontaneous polarization because its coeBcient has not been reported for MgO. We can safely mention that only the in-plane (lateral) band gap inhomogeneity determined the overall Stokes-type shift of the PL for the ZnO=Mg0:27 Zn0:73 O QWs with Lw of 0.7–3:75 nm, whereas the QCS e"ect also contributes to the Stokes shift for QW with Lw greater than 4:2 nm. As can be understood from the band diagram shown in Fig. 1, the radiative transition from spatially separated carriers (closed circles) resides on the lower side of energy than that of the LE. In the case of small Lw , however, the energy di"erence can be neglected because the depth of triangle-shaped potential well is smaller than the case of wider well. Both the electron and hole wave functions are con ned in the wells even when the electric eld is present. We thus observe the single PL peak (LE band) in this case. On the other hand, in the opposite case (wider wells), the well depth becomes larger. Carrier wave functions drops at opposite sides of the well layer with respect to each other. Thus, the energy di"erence between QCS and LE becomes no longer negligible. We think that the well-width-dependent screening of the built-in eld is responsible for the coexistence of two kinds of zero-phonon PL peaks in 4.2- and 4.7-nm-thick QWs. As addressed in [6] and called a mesoscopic capacitor e"ect, the photocreated electron–hole pairs are separated by the polarization elds and accumulated in the well layers. The e"ect of such a screening has been found to be more signi cant when the nonradiative recombination time is relatively long compared with the radiative recombination time as a consequence of the depletion-induced recovery of the elds. This gives rise to a variation in PL energies in the case of thicker QWs (cf. Fig. 3 of Ref. [6]) dependent on the nonradiative recombination time. Probably our QW samples have two nonradiative recombination channels whose characteristic times are di"erent from each other, leading to the coexistence of the localized exciton and such Stokes-like shifted luminescences observed in thicker QWs.
T-I PL Intensity (arb. units)
674
0 3.30
3.32
3.34
3.36
3.38
Photon Energy (eV) Fig. 4. Time-integrated PL in a ZnO=Mg0:27 Zn0:73 O MQW taken at a temperature of 5 K. PL decay times are shown as a function of monitored photon energy.
Fig. 4 shows a time-integrated PL spectrum and PL decay times as a function of monitored photon energy at 5 K in our MQW. The line shape of the PL spectrum is somewhat di"erent from that in Fig. 1, presumably resulting from the optical excitation power. For the decay time measurement, an intense ultrafast laser was used. There is a large di"erence in the decay time between QCS and LE bands, which is consistent with the reduced oscillator strength of the separated carriers. The intensity distribution of longitudinal-optical (LO) phonon replicas is estimated as a function of the well width. Luminescence band denoted by QCS-LO is the radiative recombination of the carriers with simultaneous creation of LO phonon. The energy di"erence between the QCS and the QCS-LO bands is equal to the energy of the LO phonon of ZnO (72 meV). The 1LO and 2LO phonon replicas of the QCS bands (e.g., QCS-LO) are clearly seen in the upper two traces of Fig. 2. This is not the case for the LE emission. The intensities of the LO phonon replicas relative to the intensities of the zero-phonon peaks for 4.2- and 4.7-nm-thick QWs are 0.046 and 0.068, respectively. On the other hand, the corresponding ratio is too small to be deduced in the case of Lw = 0:9 nm. It is well known that emission intensities of phonon replicas is related to the coupling strength with the LO phonons. The coupling strength between the electron–hole pairs separated due to QCS e"ects and the LO phonons is signi cantly larger than that between the LEs and the phonons.
T. Makino et al. / Physica E 21 (2004) 671 – 675
The phonon coupling strength (PCS) generally depends strongly on the spatial distributions of electron and hole charge densities and sometimes deviates from the bulk value [13]. This is more characteristic in cases where the wurtzitic heterostructures are in6uenced by the strong internal electric eld caused by piezoelectric and spontaneous polarizations [14,15]. It is thus easy to infer that the reduced overlap of these electron and hole charge densities must be responsible for the observed increase of the PCS. This results in the di"erence in the PCS between QCS and LE bands. Conversely, this enhancement supports our spectral assignment concerning the QCS band. 4. Summary We observed the photoluminescence of electron– hole pairs which are spatially separated due to the QCS e"ects in 4.2- and 4.7-nm-thick wurtzite ZnO=Mg0:27 Zn0:73 O quantum wells at 5 K. This PL band is located 40 meV in energy below the emission band of the localized excitons and 60 meV below the absorption energy of the free-exciton transition. As a result of its dependence on the Mg concentration (x = 0:12 and 0.27) and the well width, we reached such a spectral assignment. It is considered that the strong electric eld present along the growth axis of the system increases the distance between electron and hole charge distributions, decreases the overlap between electrons and holes, and leads to the suppression of excitonic oscillator strength and to the enhancement in their phonon interaction.
675
References [1] F. Bernardini, V. Fiorentini, D. Vanderbilt, Phys. Rev. Lett. 79 (1997) 3958. [2] S.H. Park, S.-L. Chuang, J. Appl. Phys. 87 (2000) 353. [3] R. Cingolani, G. Larocca, H. Kalt, K. Ploog, J. Mann, Phys. Rev. B 43 (1991) 9662. [4] R. Langer, J. Simon, V. Ortiz, N.T. Pelekanos, A. Barski, R. AndrRe, M. Godlewski, Appl. Phys. Lett. 74 (1999) 3827. [5] M. Leroux, N. Grandjean, M. Laugt, J. Massies, B. Gil, P. Lefebvre, P. Bigenwald, Phys. Rev. B 58 (1998) R13371. [6] A. Di Carlo, A. Reale, P. Lugli, G. Traetta, M. Lomascolo, A. Passaseo, R. Cingolani, A. Bon glio, M. Berti, E. Naopoliani, M. Natali, S. Sinha, A. Drigo, P. Vinattieri, M. Collocci, Phys. Rev. B 63 (2001) 235305. [7] T. Makino, Y. Segawa, A. Ohtomo, K. Tamura, T. Yasuda, M. Kawasaki, H. Koinuma, Appl. Phys. Lett. 79 (2001) 1282. [8] T. Makino, K. Tamura, C.H. Chia, Y. Segawa, M. Kawasaki, A. Ohtomo, H. Koinuma, Appl. Phys. Lett. 81 (13) (2002) 2355. [9] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1237. [10] T. Makino, N.T. Tuan, H.D. Sun, C.H. Chia, Y. Segawa, M. Kawasaki, A. Ohtomo, K. Tamura, H. Koinuma, Appl. Phys. Lett. 77 (2000) 975. [11] G. Coli, K.K. Bajaj, Appl. Phys. Lett. 78 (2001) 2861. [12] T. Makino, C.H. Chia, N.T. Tuan, Y. Segawa, M. Kawasaki, A. Ohtomo, K. Tamura, H. Koinuma, Appl. Phys. Lett. 76 (2000) 3549. [13] S. Kalliakos, X.B. Zhang, T. Taliercio, P. Lefebvre, B. Gil, N. Grandjean, B. Damilano, J. Massies, Appl. Phys. Lett. 80 (2002) 428. [14] Y. Matsumoto, M. Murakami, Z.W. Jin, A. Ohtomo, M. Lippmaa, M. Kawasaki, H. Koinuma, Jpn. J. Appl. Phys. Part 2 38 (1999) L603. [15] T. Makino, N.T. Tuan, H.D. Sun, C.H. Chia, Y. Segawa, M. Kawasaki, A. Ohtomo, K. Tamura, T. Suemoto, H. Akiyama, M. Baba, S. Saito, T. Tomita, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1979.
Physica E 21 (2004) 671 – 675 www.elsevier.com/locate/physe
Internal electric eld e"ect on luminescence properties of ZnO/(Mg,Zn)O quantum wells T. Makinoa;∗ , A. Ohtomob , C.H. Chiaa;1 , Y. Segawaa;1 , H. Koinumac; d , M. Kawasakib; d a Photodynamics
Research Center, RIKEN (Institute of Physical and Chemical Research), Sendai 980-0845, Japan b Institute for Materials Research, Tohoku University, Sendai 980-0857, Japan c Frontier Collaborative Research Center, Tokyo Institute of Technology, Yokohama 226-8503, Japan d Combinatorial Materials Exploration and Technology, Tsukuba 305-0044, Japan
Abstract Spectroscopic studies have been conducted for wurtzite ZnO=Mg0:27 Zn0:73 O multiple quantum wells. Internal electric elds at interfaces are found to have an in6uence on the photoluminescence (PL) properties for well width (Lw ) greater than 4:2 nm. These experimentally observed features are characteristic of the quantum-con ned Stark e"ects; the magnitude of the electric eld due to spontaneous and piezoelectric polarizations and the depth of the triangle-shaped potential wells are the monotonically increasing functions of Mg concentration and the Lw , respectively. Results of the time-resolved PL study are also presented. ? 2003 Elsevier B.V. All rights reserved. PACS: 78.55.Et; 81.15.Fg; 71.35.Cc; 72.15.−v Keywords: Quantum wells; II–VI semiconductors; Quantum-con ned Stark e"ect; Phonon–exciton interactions
1. Introduction During the last few years, developments in the eld of ZnO has been progressing steadily toward the applications for short wavelength light-emitting devices. Epitaxial ZnO lms having suBciently high quality can now be grown by various epitaxial methods. ZnO has several analogous features with the other famous semiconductor, GaN, e.g., band gap energy and the crystal structure. In strained nitride heterolayers ∗
Corresponding author. E-mail address: [email protected] (T. Makino). URL: http://www.riken.jp/lab-www/photophys/top.html 1 Also at Department of Physics, Tohoku University, Sendai, Japan.
grown in the wurtzite structure with the c-axis parallel to the growth direction, piezoelectric and spontaneous polarization e"ects are present [1–6]. These polarization e"ects have been extensively investigated in GaN-related heterostructures. As pointed out in our previous study [7], the piezoelectric elds for ZnO on MgZnO are expected to be much smaller than those for GaN-related systems because of the small di"erence in the lattice constants between ZnO and MgZnO. On the other hand, there may exist a sizeable spontaneous eld across the ZnO layers. This is more likely if the large spontaneous polarization coeBcient of ZnO, comparable with that of GaN, is considered. Nevertheless, the observation of the radiative recombination of the electron–hole pairs that are spatially separated due to such quantum-con ned Stark (QCS) e"ects
1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.110
672
T. Makino et al. / Physica E 21 (2004) 671 – 675
has not been reported so far in ZnO-based quantum wells (QWs). In this short communication, a sequel of [8], we describe the possibility of the presence of spontaneous polarization mismatches at interfaces of ZnO=Mg0:27 Zn0:73 O heterostructures (higher Mg concentration studied here). This spontaneous polarization e"ect leads to the observation of the radiative recombination from the electron–hole pairs in6uenced by the QCS e"ects. As a result, a Stokes-shifted photoluminiscience (PL) band located 40 meV below the localized exciton (LE) emission is observed in the PL spectra taken at a temperature of 5 K. To investigate the interplay between the distance and the oscillator strength, spectral distribution of the PL decay times is also discussed. 2. Experimental procedures The ZnO/ZnMgO MQWs were grown on a ScAlMgO4 (0 0 0 1) substrate by laser–molecular-beam epitaxy method. The detailed preparation process has been described elsewhere [9]. The total structures consist of 10 periods of alternating ZnO well layers and 5-nm-thick (Zn,Mg)O barrier layers. The Mg content of x = 0:12 is slightly lower than the solubility limit (x =0:15) of this alloy lm [9], while the content of x = 0:27 is far above it, corresponding to a barrier height of about 0:5 eV. The thicknesses indicated below are the averaged ones of 10 well layers. All the layer thicknesses were set as integer of molecular layers by the prescribed deposition time. The SCAM is transparent to the spectral region of interest. The samples were kept in a cryostat. The PL was excited by a He–Cd laser (325 nm in wavelength) and was monitored using a monochromator with a charge-coupled device. Absorption measurements were carried out using a xenon lamp. 3. Results and discussion A band diagram of wurtzitic QWs under the polarization elds is schematically shown in Fig. 1. Because the band diagram of the well regions is not 6at, electrons and holes tend to be relaxed. In other words, the internal electric eld pushes the electron and hole toward opposite sides of the well. In such a situation, the
cond.
QCS
LE
valence
Fig. 1. Schematic diagram of the conduction and the valence bands of wurtzitic QWs under the spontaneous and piezoelectric polarization eld. Electrons and holes distributed in the well regions (closed circles) are spatially separated due to the QCS e"ects.
PL transition energy is likely to be smaller than that of absorption. The energy di"erence becomes more signi cant with an increase in the well width. We show in Fig. 2 low-temperature PL and absorption spectra corresponding to four ZnO=Mg0:27 Zn0:73 O QW samples with di"erent well widths (Lw ). There are distinct structures in these absorption spectra, which have been already assigned in Ref. [10]: the free excitonic transitions. A sharp peak structure has no longer been observed for the samples with Lw greater than 4:2 nm. In other words, it is not possible to conrm here the excitonic peak (e.g., Gaussian) although this is possible for some thinner QWs. This is unusual because the spatial 6uctuations on the relevant heterostructure size have less in6uence on the width of absorption bands in these thicker QWs. It is well known that the QCS e"ects somewhat reduce the excitonic oscillator strength as shown in Fig. 2. The well width dependence of the absorption peak energies is shown in Fig. 3. Because sometimes the sharp structure was not observed in the experimental data, we used the calculation absorption energy obtained by Coli and Bajaj [11]. We hereafter discuss whether the PL properties can be consistently interpreted from a viewpoint of the
T. Makino et al. / Physica E 21 (2004) 671 – 675
673
3.65 ABS PL
3.60
QCSE
Peak Energy (eV)
3.55 3.50 3.45 3.40 3.35
LE QCSE
3.30 1
2
3
4
Well Width (nm) Fig. 3. Peak energies of PL (open circles) and absorption (open squares) are plotted against well width in ZnO=Mg0:27 Zn0:73 O MQWs. The QCS peak energies are also shown. Fig. 2. PL spectra of ZnO QW samples measured at 5 K. The well widths of QWs are shown in the gure. Nomenclatures of LE and QCS, respectively, mean the localized exciton and the quantum-con ned Stark e"ect. The LO-phonon replicas are indicated as arrows. The excitonic absorption spectra (dotted lines) are shown for comparison.
internal electric eld. The zero-phonon peaks of PL labeled with “LE” in Fig. 2 (located at energies of 3.38, 3.50, and 3:56 eV) are attributed to the radiative recombination of the localized excitons [10], which were based on their peak energy plot [12]. When the PL peak energies were plotted as a function of Lw , the amount of Stokes shift is a monotonically decreasing function of Lw . In other words, these energy positions smoothly connect with respect to each other (cf. open squares in Fig. 3). This behavior is typically seen in the case of radiative recombination from the excitons localized at the potentials induced by spatial 6uctuations on the relevant heterostructure size, because such 6uctuation has a more sensitive e"ect for a very thin well. As long as the data for the Lw smaller than or equal to 3:7 nm are discussed, it seems to be unnecessary to take the polarization e"ects into account. The
discussion of barrier height 6uctuation e"ect has been given elsewhere [10]. On the other hand, there are two prominent zero-phonon PL peaks in the 4.7- and 4.2-nm-thick QWs. The higher-energy PL bands are found to have the same origin with the main zero-phonon peaks observed in the samples with Lw smaller than or equal to 3:7 nm. Lower-energy side “QCS” bands have evidently larger Stokes shift than that of LE bands. These PL bands are located 40 meV in energy below the emission band of the LE and 60 meV below the absorption energy of the free-exciton transition. It should be noted, although the corresponding spectra are not shown in gures, that such kind of emission is not observed in any QW sample with the smaller Mg concentration (x = 0:12), namely, only the LE bands could be observed in ZnO=Mg0:12 Zn0:88 O QWs. The magnitude of the electric eld in the case of x = 0:27 is larger than that of x =0:12, because of the larger lattice mismatch between ZnO and ZnO=Mg0:27 Zn0:73 O (cf. Fig. 3 of Ref. [9]). This internal eld, present along the growth axis of the system, is caused by piezoelectric and spontaneous polarizations. It is
T. Makino et al. / Physica E 21 (2004) 671 – 675
2000 T=5 K
QCS
LW
1500
=4.23nm 1000
LE
500
Decay Time (ps)
considered easier to observe the QCS bands in the sample with higher barrier height. The Stokes-type shift of PL is due to the QCS e"ects induced by this strong internal electric eld. We cannot conclude here whether the amount of spontaneous polarization mismatch is a monotonically increasing function of well width. It is beyond our scope to consider the e"ect of the spontaneous polarization because its coeBcient has not been reported for MgO. We can safely mention that only the in-plane (lateral) band gap inhomogeneity determined the overall Stokes-type shift of the PL for the ZnO=Mg0:27 Zn0:73 O QWs with Lw of 0.7–3:75 nm, whereas the QCS e"ect also contributes to the Stokes shift for QW with Lw greater than 4:2 nm. As can be understood from the band diagram shown in Fig. 1, the radiative transition from spatially separated carriers (closed circles) resides on the lower side of energy than that of the LE. In the case of small Lw , however, the energy di"erence can be neglected because the depth of triangle-shaped potential well is smaller than the case of wider well. Both the electron and hole wave functions are con ned in the wells even when the electric eld is present. We thus observe the single PL peak (LE band) in this case. On the other hand, in the opposite case (wider wells), the well depth becomes larger. Carrier wave functions drops at opposite sides of the well layer with respect to each other. Thus, the energy di"erence between QCS and LE becomes no longer negligible. We think that the well-width-dependent screening of the built-in eld is responsible for the coexistence of two kinds of zero-phonon PL peaks in 4.2- and 4.7-nm-thick QWs. As addressed in [6] and called a mesoscopic capacitor e"ect, the photocreated electron–hole pairs are separated by the polarization elds and accumulated in the well layers. The e"ect of such a screening has been found to be more signi cant when the nonradiative recombination time is relatively long compared with the radiative recombination time as a consequence of the depletion-induced recovery of the elds. This gives rise to a variation in PL energies in the case of thicker QWs (cf. Fig. 3 of Ref. [6]) dependent on the nonradiative recombination time. Probably our QW samples have two nonradiative recombination channels whose characteristic times are di"erent from each other, leading to the coexistence of the localized exciton and such Stokes-like shifted luminescences observed in thicker QWs.
T-I PL Intensity (arb. units)
674
0 3.30
3.32
3.34
3.36
3.38
Photon Energy (eV) Fig. 4. Time-integrated PL in a ZnO=Mg0:27 Zn0:73 O MQW taken at a temperature of 5 K. PL decay times are shown as a function of monitored photon energy.
Fig. 4 shows a time-integrated PL spectrum and PL decay times as a function of monitored photon energy at 5 K in our MQW. The line shape of the PL spectrum is somewhat di"erent from that in Fig. 1, presumably resulting from the optical excitation power. For the decay time measurement, an intense ultrafast laser was used. There is a large di"erence in the decay time between QCS and LE bands, which is consistent with the reduced oscillator strength of the separated carriers. The intensity distribution of longitudinal-optical (LO) phonon replicas is estimated as a function of the well width. Luminescence band denoted by QCS-LO is the radiative recombination of the carriers with simultaneous creation of LO phonon. The energy di"erence between the QCS and the QCS-LO bands is equal to the energy of the LO phonon of ZnO (72 meV). The 1LO and 2LO phonon replicas of the QCS bands (e.g., QCS-LO) are clearly seen in the upper two traces of Fig. 2. This is not the case for the LE emission. The intensities of the LO phonon replicas relative to the intensities of the zero-phonon peaks for 4.2- and 4.7-nm-thick QWs are 0.046 and 0.068, respectively. On the other hand, the corresponding ratio is too small to be deduced in the case of Lw = 0:9 nm. It is well known that emission intensities of phonon replicas is related to the coupling strength with the LO phonons. The coupling strength between the electron–hole pairs separated due to QCS e"ects and the LO phonons is signi cantly larger than that between the LEs and the phonons.
T. Makino et al. / Physica E 21 (2004) 671 – 675
The phonon coupling strength (PCS) generally depends strongly on the spatial distributions of electron and hole charge densities and sometimes deviates from the bulk value [13]. This is more characteristic in cases where the wurtzitic heterostructures are in6uenced by the strong internal electric eld caused by piezoelectric and spontaneous polarizations [14,15]. It is thus easy to infer that the reduced overlap of these electron and hole charge densities must be responsible for the observed increase of the PCS. This results in the di"erence in the PCS between QCS and LE bands. Conversely, this enhancement supports our spectral assignment concerning the QCS band. 4. Summary We observed the photoluminescence of electron– hole pairs which are spatially separated due to the QCS e"ects in 4.2- and 4.7-nm-thick wurtzite ZnO=Mg0:27 Zn0:73 O quantum wells at 5 K. This PL band is located 40 meV in energy below the emission band of the localized excitons and 60 meV below the absorption energy of the free-exciton transition. As a result of its dependence on the Mg concentration (x = 0:12 and 0.27) and the well width, we reached such a spectral assignment. It is considered that the strong electric eld present along the growth axis of the system increases the distance between electron and hole charge distributions, decreases the overlap between electrons and holes, and leads to the suppression of excitonic oscillator strength and to the enhancement in their phonon interaction.
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