- Email: [email protected]
Growth and optical characterization of dense arrays of site-controlled quantum dots grown in inverted pyramids
Available online at www.sciencedirect.com
Physica E 21 (2004) 193 – 198 www.elsevier.com/locate/physe
Growth and optical characterization of dense arrays of site-controlled quantum dots grown in inverted pyramids S. Watanabe∗ , E. Pelucchi, B. Dwir, M. Baier, K. Leifer, E. Kapon Laboratory of Physics of Nanostructures, Institute of Quantum Photonics and Electronics, Swiss Federal Institute of Technology Lausanne (EPFL), 1015 Lausanne, Switzerland
Abstract We report on the growth and optical properties of dense arrays of single GaAs/AlGaAs quantum dot (QD) heterostructures with pitches as small as 300 nm. The samples were grown by organometallic chemical vapor deposition in dense inverted pyramids on {1 1 1}B GaAs substrate pre-patterned using electron beam lithography and wet chemical etching. The growth conditions such as deoxidation and growth temperatures, growth rates, and V/III ratio, had to be chosen quite di6erently from those employed with micron-size pyramids. Low-temperature micro-photoluminescence and cathodoluminescence spectra of the samples show distinct luminescence from the QDs with a linewidth of less than 1 meV and uniform emission energy for an ensemble of 900 QDs. The possibility of incorporating such QD arrays inside optical microcavity structures is also discussed. ? 2003 Elsevier B.V. All rights reserved. PACS: 85.35.Be; 78.55.Cr; 73.21.La; 78.60.Hk; 78.66.Fd Keywords: Quantum dot; Site-controlled; Micro-photoluminescence; Cathodoluminescence
1. Introduction Semiconductor quantum dot (QD) structures have been attracting great interest because of their fundamental physical properties as well as their potential applications in high performance devices. For both aspects of QD studies, it is quite important to produce arrays of QDs with well-controlled positions and with identical structure and composition. This would be desirable for realizing high performance lasers [1] or photonic crystal devices [2], as well as for inducing eAcient coupling between QDs and cavity modes [3]. Most of the studies of QDs have been devoted to ∗ Corresponding author. Tel.: +41-21-693-58-62; fax: +41-21693-54-80. E-mail address: [email protected] (S. Watanabe).
self-organized Stranski–Krastanow (S–K) QDs. Much e6ort has been invested to improve the uniformity of the S–K QDs [4] or to arrange them in ordered arrays [5,6]. It is quite diAcult, however, to control these properties of S–K QDs due to their self-organized formation mechanism. Pyramidal QDs [7,8] have a potential advantage to overcome the alignment problems of QDs, since each QD position is precisely controlled by the substrate recess pattern deFned before growth. The QD size is also well controlled by the growth conditions, making it possible to achieve nearly identical QDs. Since there is also the possibility to stack identical QDs in the growth direction [8], the pyramidal structure is a good candidate for achieving also 3D arrays of identical QDs. In this paper, we report on the growth and optical properties of single-QD arrays grown in dense
1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.005
194
S. Watanabe et al. / Physica E 21 (2004) 193 – 198
inverted pyramids with pitches of 500 and 300 nm. The samples were grown by organometallic chemical vapor deposition (OMCVD) in dense inverted pyramids on {1 1 1}B GaAs substrate prepared by electron beam lithography (EBL) and wet chemical etching. We observed distinct luminescence from the QD structures using micro-photoluminescence (PL) and cathodoluminescence (CL) measurements. The spectral line width of each QD is less than 1 meV, and CL image shows uniform emission energy for an ensemble containing nearly 1000 QDs. 2. Sample preparation details The fabrication process starts with the preparation of a substrate consisting of triangular arrays of inverted pyramid patterns with sub-micron pitch. We used undoped {1 1 1}B substrate (misoriented by 2◦ toward [0 1 −1]), coated with a 20-nm thick SiO2 mask layer. Circular holes with pitches of 500 and 300 nm were made in the SiO2 layer using EBL and wet chemical etching with bu6ered hydroEuoric acid. Total area of the arrays was 300 m × 300 m. The inverted tetrahedral pyramids were etched by Br 2 – methanol through the holes in the SiO2 mask, which resulted in straight {1 1 1}A pyramidal sidewalls with sharply deFned corners. Top-view scanning electron microscope (SEM) images of the substrate are shown in Fig. 1 for (a) 500 nm and (b) 300 nm pitch pyramids. In order to avoid excessive material transport during the growth from non-patterned regions, these arrays were surrounded by 30 m wide stripes of 5 m pitch pyramids. After deoxidation, OMCVD growth of a GaAs/ AlGaAs heterostructure was performed on top of the patterned substrate. The top-view images after OMCVD growth for each pitch of pyramids are shown in Fig. 1(c) and (d). Note that precise dot positioning is obtained via the substrate patterning. The growth conditions such as deoxidation and growth temperatures, growth rates, and V/III ratio, had to be chosen quite di6erently from those employed with micron-size pyramids [8,9], as we further discuss below. First, the temperature has to be low enough to avoid mass transport during deoxidation. Fig. 2 shows top-view SEM images of the substrate before/after deoxidation under AsH3 Eux at di6erent tempera-
Fig. 1. SEM micrographs of (a), (c) 500 nm pitch and (b), (d) 300 nm pitch pyramidal array before [(a), (b)] and after [(c), (d)] OMCVD growth. The magniFcations are the same for all the images.
Fig. 2. SEM images of the patterned substrate (a) before and (b) – (d) after deoxidation. Deoxidation temperature is (b) ∼730◦ C, (c) ∼620◦ C, and (d) ∼530◦ C, respectively. The pitch of the pyramid arrays is 300 nm.
tures. In spite of the sharp pyramidal pits in the substrate before deoxidation seen in Fig. 2(a), the same sample after annealing at around 730◦ C (Fig. 2(b))
S. Watanabe et al. / Physica E 21 (2004) 193 – 198
shows that the substrate is completely planarized due to surface mass transport. These deoxidation conditions are routinely used in our laboratory for growth on V-groove gratings with comparable pitch etched into (0 0 1) substrates, where no such major e6ect can be observed. The planarization of the substrate is obviously induced by a large di6usion length of Ga adatoms on the {1 1 1}B surface. Fig. 2(c) and (d) show a substrate deoxidized at ∼620◦ C and at ∼530◦ C, respectively. When the deoxidation temperature is decreased, only a modest rounding at the bottom of the pyramidal structure can be observed. Quite clearly, only in the last case mass transport did not signiFcantly perturb the original pyramidal patterns. We thus Fxed the deoxidation temperature at around 530◦ C. Second, the actual growth rate inside the pyramids is much higher than that for larger, micron-size pyramids. We estimated from cross-sectional atomic force microscopy (AFM) images (not shown) that it is 10 –15 times higher than the nominal growth rate determined by growth on a planar (0 0 1) GaAs wafer. The reason for this high growth rate is the large surface di6usion length relative to the size of the patterned region (300 m × 300 m) combined with the absence of growth on large {1 1 1}B non-patterned surfaces. We note that the volume of each pyramidal recess pattern is quite small, hence the total thickness of deposition is restricted to a few hundreds of nm, which corresponds to 20 –30 nm in nominal thickness. At the moment, it is diAcult to precisely control the absolute thickness of each layer due to the short deposition time (typically 8 s for the QD layer). Attempts to reduce the growth rate so as to achieve better control on the QD size are currently underway. Following to the above-mentioned results, we prepared samples incorporating a single QD per pyramid using the following procedure. After the deoxidation process at ∼530◦ C, temperature was ramped up to ∼660◦ C during nominally 1.0-nm-thick-GaAs bu6er deposition maintaining low growth rate (0:006 nm=s nominal) and very high V/III ratio (¿ 5000). Following a 2-nm-thick (nominal)-Al0:55 Ga0:45 As bottom cladding layer, an Al0:3 Ga0:7 As/GaAs/Al0:3 Ga0:7 As heterostructure was then grown with nominal thickness of 0:35 nm for GaAs and 5 nm for Al0:3 Ga0:7 As, which realize a single QD at the bottom of each pyramidal pattern. An upper Al0:55 Ga0:45 As cladding layer
195
is added before a GaAs cap layer. After OMCVD growth, the GaAs substrate was removed from the samples and upright-standing pyramids were obtained to increase QD photoluminescence eAciency as was described in Ref. [9]. 3. Micro-PL characterizations We performed micro-PL characterization of the sample, which was maintained at 10 K in a He-Eow cryostat, using continuous wave Ar+ ion laser ( = 514 nm) focused into a 1 m diameter spot. In this conFguration, we excited about 4 to 10 pyramids simultaneously for the 500- and 300-nm pitch pyramids, respectively. In the spectrum of the 500-nm pitch pyramids shown in Fig. 3(a) (top), PL from the GaAs bulk, the QDs, and the surrounding barrier quantum structures are observed. The PL energy of the QDs is 1:61 eV. The barrier quantum structures consist of GaAs and AlGaAs quantum wells (QWs) and quantum wires (QWRs), as identiFed at these energies in spectrally and spatially resolved CL spectra of larger pyramidal QD structures [9,10]. The sharp peaks observed in the spectra of the barrier quantum structures are indicative of exciton localizations in these very thin structures. The PL of the 300-nm pitch pyramids, on the other hand, shows a QD emission lie at the same energy as for the 500-nm pitch sample, but lacks the spectral features related to the barrier structures. This can be explained by an eAcient carrier capture from the surrounding barriers into the QDs in the 300 nm pitch, smaller pyramids. In contrast, some of the excited carriers (generated throughout the structure due to the above-barrier excitation energy) in the 500 nm pitch arrays are captured in the barriers and recombine there. Hence, the carrier capture distance in these structures is smaller than ∼250 nm but larger than ∼100 nm. It is also interesting to note that the QD emission energy is not di6erent for the 300 nm pitch and the 500 nm pitch arrays. This indicates that the surface di6usion of the growth species that gives rise to the dot formation takes place on a length scale much smaller than 500 nm, as expected from studies of growth of V-groove QWRs [11]. Fig. 3(b) shows the excitation power dependence of a micro-PL spectrum of the 300-nm pitch pyramidal QDs. At low excitation power, one can see a
196
S. Watanabe et al. / Physica E 21 (2004) 193 – 198
Fig. 3. (a) Micro-PL spectra of 500- and 300-nm pitch pyramidal QD structures at T = 10 K, excited at a power density of 2.3 and 1:1 kW=cm2 , respectively. PL intensity is shown with a reduced scale by factor of 10 and 5 below 1:56 eV and above 1:65 eV for the 500-nm pitch pyramids. (b) Excitation power dependence of the micro-PL spectra of the 300-nm pitch pyramidal QDs.
prominent peak at 1:612 eV, with a full width at half maximum of less than 1 meV. The exact structure of such low-excitation micro-PL spectra depends sensitively on the position of the pump spot; often, we observe several such sharp peaks within a range of ∼10 meV. The peaks in these low excitation spectra are assigned to emission from several, nearby dots that have slightly di6erent sizes, corresponding to a di6erence in emission energy of 1–2 meV. When the excitation power is increased, the spectral peaks broaden and peak of the envelope shifts to lower energy. This evolution is consistent with the formation of multi-exciton states in each dot, which broadens and red-shifts the spectrum of each dot [12].
Fig. 4. (a) CL spectra of 500-nm pitch pyramidal QDs at T = 7 K with electron beam voltage of 5 keV and current of 1 nA, when the excitation region contains 1, 10, and 100 pyramidal structure(s) respectively. (b) A wavelength-dispersive CL image at T = 7 K taken at 1:55 meV. Total area of the image region is 12:5×16:5 m2 where ∼900 QDs are contained. Spectral window of the CL image is 7 meV.
4. CL characterizations We performed large-scale CL measurements on a di6erent sample with 500-nm pitch pyramidal arrays to investigate the uniformity of the QDs. The grown structure consists of Al0:3 Ga0:7 As=GaAs=Al0:3 Ga0:7 As heterostructure with nominal thickness of 0:25 nm for GaAs and 2:5 nm for AlGaAs. While the nominal size of the GaAs QD layer is smaller than for the Frst sample, the actual size of the GaAs QDs is larger, as inferred from the lowest emission energy. 1 Fig. 4(a) 1 The di6erent ratio of nominal/actual size of the QD is caused by the di6erent ratio of patterned/non-patterned region which a6ects the mass transport e6ect during the growth.
S. Watanabe et al. / Physica E 21 (2004) 193 – 198
shows CL spectra taken at T = 7 K with an acceleration voltage of V = 5 keV and a beam current of I = 1 nA. In these measurements, an excitation region containing 1, 10, and 100 pyramidal structure(s), respectively, was scanned during the same integration time. We observed Fve peaks in each spectrum, which correspond to ground (1:553 eV) and excited states of the QDs. The energy di6erence between the ground and Frst excited state transition energies is 26 meV. Note that the spectral peak energies and linewidths do not change when the excitation area is increased up to 5 × 5 m2 where 100 QDs are contained. Fig. 4(b) shows a wavelength-dispersive CL image taken at 1:55 eV emission energy. The total area of the image region (12:5 × 16:5 m2 ) contains 900 QDs and the collection slit width corresponds to an energy window of 7 meV. The triangular pattern pitch is 500 nm, whereas the spatial resolution is around 200 nm, enough to clearly distinguish the CL signal of each individual quantum dot. Bright luminescence was observed at the center of more than 99% of the pyramidal structures scanned in this image, indicating good uniformity of the QDs and a small number of defective dots. Some of the dots show a less intense signal, partly because of the di6erent emission energy, which causes only the tail of their emission to fall within the 7 meV measurement window. 5. Planarization and regrowth One interesting application of such dense QDs is their incorporation into optical waveguides or optical microcavities in order to control the photon emission or to realize eAcient QD lasers. For example, one useful conFguration would employ such dense QD arrays conFned between Bragg mirrors. For such applications, planarization of the grown surface is necessary. This can be achieved at quite high growth temperatures (∼810◦ C) which results in two-dimensional (2D) growth on the {1 1 1}B surface. Fig. 5 shows a cross-sectional AFM image of a GaAs=Al0:3 Ga0:7 As multilayer sequence grown inside the 500-nm pitch pyramidal array. Following the deposition of a 3-nm Al0:3 Ga0:7 As layer, the GaAs=Al0:3 Ga0:7 As sequence was grown with a nominal period chosen to be quite short (GaAs 2 nm=Al0:3 Ga0:7 As 6 nm nominal) at the beginning of the growth and then increased to
197
Fig. 5. Cross-sectional AFM image of GaAs=Al0:3 Ga0:7 As layer sequences grown on 500-nm pitch pyramid arrays. GaAs(dark region)/AlGaAs layers could be observed via the height di6erence caused by the di6erent surface oxidation time on the cleaved surface.
GaAs 10 nm=Al0:3 Ga0:7 As 25 nm nominal once planarization was achieved. We could observe the height di6erence between GaAs and Al0:3 Ga0:7 As layers in a cross-sectional AFM picture owing to the di6erent surface oxidation time on the cleaved, scanned surface [13]. Clear planarization and 2D growth are obtained on the {1 1 1}B surface, which opens the way for epitaxy of the di6erent planar structures mentioned above. The growth rate inside the pyramid is much higher than that on the {1 1 1}B surface because of the mass transport and capillarity e6ects. A signiFcant thickening of the GaAs layer at the bottom of the pyramid is evident, an expected feature of the nonplanar growth giving rise to the pyramidal QDs [8,11]. 6. Conclusions In conclusion, we reported on the growth and optical characteristics of dense, pyramidal QD arrays with pitch of 500 and 300 nm. We clearly observed the QD luminescence features from both types of arrays.
198
S. Watanabe et al. / Physica E 21 (2004) 193 – 198
The luminescence linewidth of each dot is less than 1 meV and CL measurements demonstrate the high uniformity of these dense QD arrays across m-scale areas. The high uniformity and the accurate position control o6ered by such dense QD arrays should make them useful in applications such as highly eAcient QD lasers and active photonic crystals.
Acknowledgements This work was supported by the Fonds National Suisse pour la recherchNe scientiFque. S.W. would like to thank the Japan Society for the Promotion of Science for partial Fnancial support.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Y. Arakawa, H. Sakaki, Appl. Phys. Lett. 40 (1982) 939. T. Yoshie, et al., Appl. Phys. Lett. 79 (2001) 4289. J. VuQcoviNc, et al., Phys. Rev. A 66 (2002) 023808. S. Fafard, C.N. Allen, Appl. Phys. Lett. 75 (1999) 2374. T. Yang, et al., J. Appl. Phys. 93 (2003) 1190. S. Kohmoto, et al., Appl. Phys. Lett. 75 (1999) 3488. Y. Sugiyama, et al., Appl. Phys. Lett. 67 (1995) 256. A. Hartmann, et al., Appl. Phys. Lett. 71 (1997) 1314. A. Hartmann, et al., J. Phys.: Condens. Matter 11 (1999) 5901. K. Leifer, et al., Appl. Phys. Lett. 77 (2000) 3923. G. Biasiol, E. Kapon, Phys. Rev. Lett. 81 (1998) 2962; G. Biasiol, et al., Phys. Rev. B 65 (2002) 205306. A. Hartmann, et al., Phys. Rev. Lett. 84 (2000) 5648. F. Reinhardt, et al., Appl. Phys. Lett. 68 (1996) 3168; B. Dwir, et al., Mater. Sci. Eng. B 37 (1996) 83.
Physica E 21 (2004) 193 – 198 www.elsevier.com/locate/physe
Growth and optical characterization of dense arrays of site-controlled quantum dots grown in inverted pyramids S. Watanabe∗ , E. Pelucchi, B. Dwir, M. Baier, K. Leifer, E. Kapon Laboratory of Physics of Nanostructures, Institute of Quantum Photonics and Electronics, Swiss Federal Institute of Technology Lausanne (EPFL), 1015 Lausanne, Switzerland
Abstract We report on the growth and optical properties of dense arrays of single GaAs/AlGaAs quantum dot (QD) heterostructures with pitches as small as 300 nm. The samples were grown by organometallic chemical vapor deposition in dense inverted pyramids on {1 1 1}B GaAs substrate pre-patterned using electron beam lithography and wet chemical etching. The growth conditions such as deoxidation and growth temperatures, growth rates, and V/III ratio, had to be chosen quite di6erently from those employed with micron-size pyramids. Low-temperature micro-photoluminescence and cathodoluminescence spectra of the samples show distinct luminescence from the QDs with a linewidth of less than 1 meV and uniform emission energy for an ensemble of 900 QDs. The possibility of incorporating such QD arrays inside optical microcavity structures is also discussed. ? 2003 Elsevier B.V. All rights reserved. PACS: 85.35.Be; 78.55.Cr; 73.21.La; 78.60.Hk; 78.66.Fd Keywords: Quantum dot; Site-controlled; Micro-photoluminescence; Cathodoluminescence
1. Introduction Semiconductor quantum dot (QD) structures have been attracting great interest because of their fundamental physical properties as well as their potential applications in high performance devices. For both aspects of QD studies, it is quite important to produce arrays of QDs with well-controlled positions and with identical structure and composition. This would be desirable for realizing high performance lasers [1] or photonic crystal devices [2], as well as for inducing eAcient coupling between QDs and cavity modes [3]. Most of the studies of QDs have been devoted to ∗ Corresponding author. Tel.: +41-21-693-58-62; fax: +41-21693-54-80. E-mail address: [email protected] (S. Watanabe).
self-organized Stranski–Krastanow (S–K) QDs. Much e6ort has been invested to improve the uniformity of the S–K QDs [4] or to arrange them in ordered arrays [5,6]. It is quite diAcult, however, to control these properties of S–K QDs due to their self-organized formation mechanism. Pyramidal QDs [7,8] have a potential advantage to overcome the alignment problems of QDs, since each QD position is precisely controlled by the substrate recess pattern deFned before growth. The QD size is also well controlled by the growth conditions, making it possible to achieve nearly identical QDs. Since there is also the possibility to stack identical QDs in the growth direction [8], the pyramidal structure is a good candidate for achieving also 3D arrays of identical QDs. In this paper, we report on the growth and optical properties of single-QD arrays grown in dense
1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.005
194
S. Watanabe et al. / Physica E 21 (2004) 193 – 198
inverted pyramids with pitches of 500 and 300 nm. The samples were grown by organometallic chemical vapor deposition (OMCVD) in dense inverted pyramids on {1 1 1}B GaAs substrate prepared by electron beam lithography (EBL) and wet chemical etching. We observed distinct luminescence from the QD structures using micro-photoluminescence (PL) and cathodoluminescence (CL) measurements. The spectral line width of each QD is less than 1 meV, and CL image shows uniform emission energy for an ensemble containing nearly 1000 QDs. 2. Sample preparation details The fabrication process starts with the preparation of a substrate consisting of triangular arrays of inverted pyramid patterns with sub-micron pitch. We used undoped {1 1 1}B substrate (misoriented by 2◦ toward [0 1 −1]), coated with a 20-nm thick SiO2 mask layer. Circular holes with pitches of 500 and 300 nm were made in the SiO2 layer using EBL and wet chemical etching with bu6ered hydroEuoric acid. Total area of the arrays was 300 m × 300 m. The inverted tetrahedral pyramids were etched by Br 2 – methanol through the holes in the SiO2 mask, which resulted in straight {1 1 1}A pyramidal sidewalls with sharply deFned corners. Top-view scanning electron microscope (SEM) images of the substrate are shown in Fig. 1 for (a) 500 nm and (b) 300 nm pitch pyramids. In order to avoid excessive material transport during the growth from non-patterned regions, these arrays were surrounded by 30 m wide stripes of 5 m pitch pyramids. After deoxidation, OMCVD growth of a GaAs/ AlGaAs heterostructure was performed on top of the patterned substrate. The top-view images after OMCVD growth for each pitch of pyramids are shown in Fig. 1(c) and (d). Note that precise dot positioning is obtained via the substrate patterning. The growth conditions such as deoxidation and growth temperatures, growth rates, and V/III ratio, had to be chosen quite di6erently from those employed with micron-size pyramids [8,9], as we further discuss below. First, the temperature has to be low enough to avoid mass transport during deoxidation. Fig. 2 shows top-view SEM images of the substrate before/after deoxidation under AsH3 Eux at di6erent tempera-
Fig. 1. SEM micrographs of (a), (c) 500 nm pitch and (b), (d) 300 nm pitch pyramidal array before [(a), (b)] and after [(c), (d)] OMCVD growth. The magniFcations are the same for all the images.
Fig. 2. SEM images of the patterned substrate (a) before and (b) – (d) after deoxidation. Deoxidation temperature is (b) ∼730◦ C, (c) ∼620◦ C, and (d) ∼530◦ C, respectively. The pitch of the pyramid arrays is 300 nm.
tures. In spite of the sharp pyramidal pits in the substrate before deoxidation seen in Fig. 2(a), the same sample after annealing at around 730◦ C (Fig. 2(b))
S. Watanabe et al. / Physica E 21 (2004) 193 – 198
shows that the substrate is completely planarized due to surface mass transport. These deoxidation conditions are routinely used in our laboratory for growth on V-groove gratings with comparable pitch etched into (0 0 1) substrates, where no such major e6ect can be observed. The planarization of the substrate is obviously induced by a large di6usion length of Ga adatoms on the {1 1 1}B surface. Fig. 2(c) and (d) show a substrate deoxidized at ∼620◦ C and at ∼530◦ C, respectively. When the deoxidation temperature is decreased, only a modest rounding at the bottom of the pyramidal structure can be observed. Quite clearly, only in the last case mass transport did not signiFcantly perturb the original pyramidal patterns. We thus Fxed the deoxidation temperature at around 530◦ C. Second, the actual growth rate inside the pyramids is much higher than that for larger, micron-size pyramids. We estimated from cross-sectional atomic force microscopy (AFM) images (not shown) that it is 10 –15 times higher than the nominal growth rate determined by growth on a planar (0 0 1) GaAs wafer. The reason for this high growth rate is the large surface di6usion length relative to the size of the patterned region (300 m × 300 m) combined with the absence of growth on large {1 1 1}B non-patterned surfaces. We note that the volume of each pyramidal recess pattern is quite small, hence the total thickness of deposition is restricted to a few hundreds of nm, which corresponds to 20 –30 nm in nominal thickness. At the moment, it is diAcult to precisely control the absolute thickness of each layer due to the short deposition time (typically 8 s for the QD layer). Attempts to reduce the growth rate so as to achieve better control on the QD size are currently underway. Following to the above-mentioned results, we prepared samples incorporating a single QD per pyramid using the following procedure. After the deoxidation process at ∼530◦ C, temperature was ramped up to ∼660◦ C during nominally 1.0-nm-thick-GaAs bu6er deposition maintaining low growth rate (0:006 nm=s nominal) and very high V/III ratio (¿ 5000). Following a 2-nm-thick (nominal)-Al0:55 Ga0:45 As bottom cladding layer, an Al0:3 Ga0:7 As/GaAs/Al0:3 Ga0:7 As heterostructure was then grown with nominal thickness of 0:35 nm for GaAs and 5 nm for Al0:3 Ga0:7 As, which realize a single QD at the bottom of each pyramidal pattern. An upper Al0:55 Ga0:45 As cladding layer
195
is added before a GaAs cap layer. After OMCVD growth, the GaAs substrate was removed from the samples and upright-standing pyramids were obtained to increase QD photoluminescence eAciency as was described in Ref. [9]. 3. Micro-PL characterizations We performed micro-PL characterization of the sample, which was maintained at 10 K in a He-Eow cryostat, using continuous wave Ar+ ion laser ( = 514 nm) focused into a 1 m diameter spot. In this conFguration, we excited about 4 to 10 pyramids simultaneously for the 500- and 300-nm pitch pyramids, respectively. In the spectrum of the 500-nm pitch pyramids shown in Fig. 3(a) (top), PL from the GaAs bulk, the QDs, and the surrounding barrier quantum structures are observed. The PL energy of the QDs is 1:61 eV. The barrier quantum structures consist of GaAs and AlGaAs quantum wells (QWs) and quantum wires (QWRs), as identiFed at these energies in spectrally and spatially resolved CL spectra of larger pyramidal QD structures [9,10]. The sharp peaks observed in the spectra of the barrier quantum structures are indicative of exciton localizations in these very thin structures. The PL of the 300-nm pitch pyramids, on the other hand, shows a QD emission lie at the same energy as for the 500-nm pitch sample, but lacks the spectral features related to the barrier structures. This can be explained by an eAcient carrier capture from the surrounding barriers into the QDs in the 300 nm pitch, smaller pyramids. In contrast, some of the excited carriers (generated throughout the structure due to the above-barrier excitation energy) in the 500 nm pitch arrays are captured in the barriers and recombine there. Hence, the carrier capture distance in these structures is smaller than ∼250 nm but larger than ∼100 nm. It is also interesting to note that the QD emission energy is not di6erent for the 300 nm pitch and the 500 nm pitch arrays. This indicates that the surface di6usion of the growth species that gives rise to the dot formation takes place on a length scale much smaller than 500 nm, as expected from studies of growth of V-groove QWRs [11]. Fig. 3(b) shows the excitation power dependence of a micro-PL spectrum of the 300-nm pitch pyramidal QDs. At low excitation power, one can see a
196
S. Watanabe et al. / Physica E 21 (2004) 193 – 198
Fig. 3. (a) Micro-PL spectra of 500- and 300-nm pitch pyramidal QD structures at T = 10 K, excited at a power density of 2.3 and 1:1 kW=cm2 , respectively. PL intensity is shown with a reduced scale by factor of 10 and 5 below 1:56 eV and above 1:65 eV for the 500-nm pitch pyramids. (b) Excitation power dependence of the micro-PL spectra of the 300-nm pitch pyramidal QDs.
prominent peak at 1:612 eV, with a full width at half maximum of less than 1 meV. The exact structure of such low-excitation micro-PL spectra depends sensitively on the position of the pump spot; often, we observe several such sharp peaks within a range of ∼10 meV. The peaks in these low excitation spectra are assigned to emission from several, nearby dots that have slightly di6erent sizes, corresponding to a di6erence in emission energy of 1–2 meV. When the excitation power is increased, the spectral peaks broaden and peak of the envelope shifts to lower energy. This evolution is consistent with the formation of multi-exciton states in each dot, which broadens and red-shifts the spectrum of each dot [12].
Fig. 4. (a) CL spectra of 500-nm pitch pyramidal QDs at T = 7 K with electron beam voltage of 5 keV and current of 1 nA, when the excitation region contains 1, 10, and 100 pyramidal structure(s) respectively. (b) A wavelength-dispersive CL image at T = 7 K taken at 1:55 meV. Total area of the image region is 12:5×16:5 m2 where ∼900 QDs are contained. Spectral window of the CL image is 7 meV.
4. CL characterizations We performed large-scale CL measurements on a di6erent sample with 500-nm pitch pyramidal arrays to investigate the uniformity of the QDs. The grown structure consists of Al0:3 Ga0:7 As=GaAs=Al0:3 Ga0:7 As heterostructure with nominal thickness of 0:25 nm for GaAs and 2:5 nm for AlGaAs. While the nominal size of the GaAs QD layer is smaller than for the Frst sample, the actual size of the GaAs QDs is larger, as inferred from the lowest emission energy. 1 Fig. 4(a) 1 The di6erent ratio of nominal/actual size of the QD is caused by the di6erent ratio of patterned/non-patterned region which a6ects the mass transport e6ect during the growth.
S. Watanabe et al. / Physica E 21 (2004) 193 – 198
shows CL spectra taken at T = 7 K with an acceleration voltage of V = 5 keV and a beam current of I = 1 nA. In these measurements, an excitation region containing 1, 10, and 100 pyramidal structure(s), respectively, was scanned during the same integration time. We observed Fve peaks in each spectrum, which correspond to ground (1:553 eV) and excited states of the QDs. The energy di6erence between the ground and Frst excited state transition energies is 26 meV. Note that the spectral peak energies and linewidths do not change when the excitation area is increased up to 5 × 5 m2 where 100 QDs are contained. Fig. 4(b) shows a wavelength-dispersive CL image taken at 1:55 eV emission energy. The total area of the image region (12:5 × 16:5 m2 ) contains 900 QDs and the collection slit width corresponds to an energy window of 7 meV. The triangular pattern pitch is 500 nm, whereas the spatial resolution is around 200 nm, enough to clearly distinguish the CL signal of each individual quantum dot. Bright luminescence was observed at the center of more than 99% of the pyramidal structures scanned in this image, indicating good uniformity of the QDs and a small number of defective dots. Some of the dots show a less intense signal, partly because of the di6erent emission energy, which causes only the tail of their emission to fall within the 7 meV measurement window. 5. Planarization and regrowth One interesting application of such dense QDs is their incorporation into optical waveguides or optical microcavities in order to control the photon emission or to realize eAcient QD lasers. For example, one useful conFguration would employ such dense QD arrays conFned between Bragg mirrors. For such applications, planarization of the grown surface is necessary. This can be achieved at quite high growth temperatures (∼810◦ C) which results in two-dimensional (2D) growth on the {1 1 1}B surface. Fig. 5 shows a cross-sectional AFM image of a GaAs=Al0:3 Ga0:7 As multilayer sequence grown inside the 500-nm pitch pyramidal array. Following the deposition of a 3-nm Al0:3 Ga0:7 As layer, the GaAs=Al0:3 Ga0:7 As sequence was grown with a nominal period chosen to be quite short (GaAs 2 nm=Al0:3 Ga0:7 As 6 nm nominal) at the beginning of the growth and then increased to
197
Fig. 5. Cross-sectional AFM image of GaAs=Al0:3 Ga0:7 As layer sequences grown on 500-nm pitch pyramid arrays. GaAs(dark region)/AlGaAs layers could be observed via the height di6erence caused by the di6erent surface oxidation time on the cleaved surface.
GaAs 10 nm=Al0:3 Ga0:7 As 25 nm nominal once planarization was achieved. We could observe the height di6erence between GaAs and Al0:3 Ga0:7 As layers in a cross-sectional AFM picture owing to the di6erent surface oxidation time on the cleaved, scanned surface [13]. Clear planarization and 2D growth are obtained on the {1 1 1}B surface, which opens the way for epitaxy of the di6erent planar structures mentioned above. The growth rate inside the pyramid is much higher than that on the {1 1 1}B surface because of the mass transport and capillarity e6ects. A signiFcant thickening of the GaAs layer at the bottom of the pyramid is evident, an expected feature of the nonplanar growth giving rise to the pyramidal QDs [8,11]. 6. Conclusions In conclusion, we reported on the growth and optical characteristics of dense, pyramidal QD arrays with pitch of 500 and 300 nm. We clearly observed the QD luminescence features from both types of arrays.
198
S. Watanabe et al. / Physica E 21 (2004) 193 – 198
The luminescence linewidth of each dot is less than 1 meV and CL measurements demonstrate the high uniformity of these dense QD arrays across m-scale areas. The high uniformity and the accurate position control o6ered by such dense QD arrays should make them useful in applications such as highly eAcient QD lasers and active photonic crystals.
Acknowledgements This work was supported by the Fonds National Suisse pour la recherchNe scientiFque. S.W. would like to thank the Japan Society for the Promotion of Science for partial Fnancial support.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Y. Arakawa, H. Sakaki, Appl. Phys. Lett. 40 (1982) 939. T. Yoshie, et al., Appl. Phys. Lett. 79 (2001) 4289. J. VuQcoviNc, et al., Phys. Rev. A 66 (2002) 023808. S. Fafard, C.N. Allen, Appl. Phys. Lett. 75 (1999) 2374. T. Yang, et al., J. Appl. Phys. 93 (2003) 1190. S. Kohmoto, et al., Appl. Phys. Lett. 75 (1999) 3488. Y. Sugiyama, et al., Appl. Phys. Lett. 67 (1995) 256. A. Hartmann, et al., Appl. Phys. Lett. 71 (1997) 1314. A. Hartmann, et al., J. Phys.: Condens. Matter 11 (1999) 5901. K. Leifer, et al., Appl. Phys. Lett. 77 (2000) 3923. G. Biasiol, E. Kapon, Phys. Rev. Lett. 81 (1998) 2962; G. Biasiol, et al., Phys. Rev. B 65 (2002) 205306. A. Hartmann, et al., Phys. Rev. Lett. 84 (2000) 5648. F. Reinhardt, et al., Appl. Phys. Lett. 68 (1996) 3168; B. Dwir, et al., Mater. Sci. Eng. B 37 (1996) 83.