GaAs quantum wire superlattices

Materials Science and Engineering B51 (1998) 233 – 237

Absence of ground state PLE peak in crescent-shaped AlGaAs/GaAs quantum wire superlattices Xue-Lun Wang *, Mutsuo Ogura, Hirofumi Matsuhata Electrotechnical Laboratory, 1 -1 -4 Umezono, Tsukuba 305, Japan

Abstract ˚ )/GaAs (45 A ˚ ) quantum wire superlattice (QWR-SL) grown on V-grooved substrate The optical properties of an AlGaAs (135 A by flow rate modulation epitaxy are investigated using photoluminescence (PL) and PL excitation (PLE) spectroscopy. The QWR-SL showed step-like PLE spectra with the first step having almost no detectable PLE intensity which are completely ˚ thick single QWR sample. The unusual optical properties of the different from the sawtooth-shaped PLE spectra of a 45 A QWR-SL are probably due to the interaction of the one dimensional QWR electronic state with optical field since the QWR electronic states are electronically isolated by the thick AlGaAs barrier layer in this sample. © 1998 Elsevier Science S.A. All rights reserved. Keywords: AlGaAs/GaAs; Quantum wire superlattice; V-grooved substrate; Photoluminescence excitation spectroscopy; Ground state; Excited state

1. Introduction Low dimensional semiconductor quantum structures, such as the one dimensional quantum wires (QWRs) and the zero dimensional quantum dots (QDs) are the objects of intensive current investigation due to the predicted various new quantum effects. On the other hand, quantum wire and quantum dot superlattice (QWR-SL and QD-SL) structures which represent an intermediate electronic system between the two dimensional quantum well (QWL) and the one dimensional QWR and that between the one dimensional QWR and the zero dimensional QD are expected to exhibit novel quantum effects which could not be achieved with single quantum structure due to quantum state interaction between isolated structures [1,2]. Recently, we have succeeded in the fabrication of highly uniform AlGaAs/ GaAs quantum wire superlattices on V-grooved substrates by flow rate modulation epitaxy (FME) [3]. Very unique optical characteristics different from those appeared in single QWR (SQWR) were observed from a ˚ )/GaAs (45 A ˚ ) QWR-SL [4]. 20 period AlGaAs (90 A (1) Several side peaks which are most likely due to excited state related emissions were observed in low * Corresponding author. 0921-5107/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 9 2 1 - 5 1 0 7 ( 9 7 ) 0 0 2 6 7 - 5

temperature photoluminescence (PL) spectra even at an excitation power density four orders of magnitude lower than that required for the observation of excited ˚ thick SQWR. (2) The state emissions from a 45 A ˚ )/ ground state emission peak of the AlGaAs (90 A ˚ ) QWR-SL showed a radiative lifetime GaAs (45 A ( 2.1 ns) almost four times longer than that of the SQWR sample. These unique optical properties were attributed to the results of electronic coupling between different QWRs since superlattice minibands of several meV width are expected to be formed in that case. In this paper, we show that the unique optical behaviors can be observed even in electronically isolated AlGaAs/ GaAs QWR-SL.

2. Experimental The samples used here are grown on 4.8 mm pitch V-grooved GaAs substrates by metalorganic vapor phase epitaxy (MOVPE). The layer structure of the grown sample consists of a 330 nm thick GaAs buffer layer, a 990 nm thick AlGaAs barrier layer, a 20 period AlGaAs/GaAs QWR-SL layer and a 160 nm thick AlGaAs barrier layer, where the layer thickness values are those at the V-groove bottom. The Al composition

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of the AlGaAs barrier layer is different for different crystalline facets which is about 0.33 and 0.39 for the (001) mesa top flat and the (111)A side wall facets, and about 0.29 for the vertical quantum well (VQWL) formed at the V-groove center. The GaAs wire layer was grown by FME, a modified MOVPE growth technique which was found to have superior low temperature growth selective and layer thickness controllability compared with the conventional MOVPE selectivity growth [5,6]. The growth temperature was about 630°C. At this temperature, the AlGaAs barrier layer growth profile does not change with growth thickness and hence we can ignore the influence of the change in AlGaAs growth profile on the size of QWR [3]. It was also found experimentally that an AlGaAs barrier layer thickness about twice that of the GaAs wire layer is necessary for the complete recovery of the distorted V-groove bottom curvature [3]. For example, Fig. 1 shows the cross-sectional transmission electron microscopy (TEM) image of a 20 period AlGaAs (90 ˚ )/GaAs (45 A ˚ ) QWR-SL. Except for the first two to A three periods, very uniform superlattice structure was obtained. Moreover, the AlGaAs/GaAs heterointerfaces of the FME-grown superlattice samples were found to be atomically uniform by a high resolution

˚ )/GaAs (45 A ˚ ) QWR-SL and Fig. 2. PL spectra of the AlGaAs (135 A ˚ thick SQWR measured at 12 K. a 45 A

TEM observation and a microscopic optical characterization [7,8]. The optical properties of the grown samples were characterized by PL and PL excitation (PLE) measurements at low temperatures. Before the measurements, the (001) flat and part of the (111)A side-wall QWL regions were selectively removed to create a wide optical window between the QWR and the (111)A side-wall peaks using the processes given in Fig. 4(a–c) [9,10]. This is very convenient for the investigation of excited state luminescence by PLE. An Ar + laser was used as the excitation source with an incident angle of 30° with respect to the surface normal in PL measurements, while for PLE measurements a Ti:sapphire laser was used in back scattering geometry. In PLE measurements, the luminescence was detected with a JobinYvon triple monochromator (T6400) and a cooled charge coupled device (CCD) camera.

3. Results

Fig. 1. Cross-sectional TEM image of a 20 period AlGaAs (90 ˚ )/GaAs (45 A ˚ ) QWR-SL. A

Fig. 2 shows the PL spectra measured at 12 K of the ˚ )/GaAs (45 A ˚ ) QWR-SL and a 45 A ˚ AlGaAs (135 A thick SQWR reference sample. Luminescence from QWR regions at about 1.64 eV dominates the spectra due to the enhanced carrier capture efficiency of the QWR regions by the removal of the (001) flat and the (111)A side-wall regions [9,10]. Only a single peak corresponding to the ground state electron to heavyhole recombination (1e-1hh) can be observed in the spectrum of the SQWR sample at such a low excitation

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power density ( 150 mW cm − 2). However, in the case of the QWR-SL sample, a small peak indicated by s1 appeared at the high energy side of the 1e-1hh main peak with an energy separation of about 28.9 meV from the 1e-1hh peak. Similar peaks were also observed in ˚ )/GaAs (45 A ˚) the PL spectra of the AlGaAs (90 A QWR-SL sample in our previous works [4] and they were attributed to excited state related emissions from their energy positions and the excitation power dependence of their peak intensities. The origin of the s1 peak was further investigated by PLE measurements. Fig. 3 shows the 20 K PLE spectra of the QWR-SL and the SQWR samples, where PL spectra measured using Ti:sapphire laser (wavelength= 705 nm) were also given for reference. In the PLE spectra of the SQWR sample, strong PLE peak resulted from the 1e-1hh recombination, together with two excited heavy-hole state related PLE peaks were observed in the investigated energy range when the excitation laser beam is polarized parallel to the wire axis. This is characteristic of the sawtooth-shaped density of states (DOS) of QWR structures and is also a good indication of the high quality of the FME-grown QWRs. A more detailed description of these spectra can be found else˚ )/GaAs where [7]. To the contrary, the AlGaAs (135 A ˚ (45 A) QWR-SL sample exhibited PLE spectra completely different from those of the SQWR sample as shown in Fig. 3(b). Several striking features can be seen from these spectra. (1) Instead of the sharp sawtoothshaped PLE spectra of the SQWR, step-like PLE structures were observed in the superlattice case, reminiscent of the DOS of two dimensional QWL. (2) Similar high-energy PLE structures were observed when the detection energy is fixed at the peak energy of the s1 side peak. This is considered as a direct evidence that the s1 side peak is due to the excited state emission of the 1e-1hh main peak but not due to the QWR size fluctuation as we supposed previously. (3) There is almost no PLE peak around the energy position of the 1e-1hh transition. The ground state PL emission seems to mainly result from optical absorption by higher energy states and the subsequent carrier relaxation to the ground state. (4) The energy separation between the ground and the first excited state PLE peaks (32 meV) is much smaller than that of the SQWR sample (46 meV). The surface roughness (grating) can sometimes affect the optical properties of QWRs or QDs grown on patterned substrate [11]. As illustrated in Fig. 4(c), the surface of the sample used for optical measurements contains quite large roughness. In the following, we investigated the effect of the surface grating by flattening the substrate surface using the processes given in Fig. 4(e–f). After the removal of photoresist, we regrew a thick AlGaAs layer (2 mm) with the same nominal Al composition as that of the first grown

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AlGaAs barrier layer (Fig. 4(d)). Then, the sample surface was completely flattened by mechanical lapping (Fig. 4(e)). Fig. 4(f) shows a scanning electron microscopy (SEM) image of the cross section of the sample after mechanical lapping. PL and PLE measurements were then carried out on the flattened sample and the spectra are given in Fig. 5. Almost the same PL and PLE spectra were obtained as those of the sample

Fig. 3. PLE spectra of (a) the 45 A, thick SQWR and (b) the AlGaAs ˚ )/GaAs (45 A ˚ ) QWR-SL. PLE spectra for both the parallel and (135 A perpendicular incident laser beam polarization are given for the SQWR sample, while only the spectra for parallel polarization are given for the superlattice sample. The bold vertical arrows indicate the detection energy position.

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tures. The step-like PLE spectra imply the formation of considerably wide electronic subband. However, almost no direct electronic coupling between QWRs can be expected in the present superlattice sample because the ˚ thick AlGaAs barrier layer isolates the QWRs 135 A completely. This is also confirmed by theoretical calculation using finite element method. As a possibility, we are considering the interaction of QWR electronic state with optical field or interwire coupling via optical field [1]. It is well known that the emission and absorption properties of semiconductor quantum structures can be significantly modified through the interaction with optical field in semiconductor microcavities [12,13]. In that case, the optical field can be effectively confined into the cavities by careful design of the cavity structure. However, no optical confinement structures were intentionally incorporated into the QWR-SL sample investigated here. Therefore, the optical properties observed here probably represent a new kind of electron-photon interaction different from that appeared in the conventional microcavity structures. However, it is apparent that further experimental and theoretical investigations are necessary for the clarification of the physical processes occurred in such structures.

5. Conclusions ˚ )/GaAs (45 A ˚ ) QWR-SL with An AlGaAs (135 A high size uniformity was grown on V-grooved substrate by FME and its optical properties were investigated using PL and PLE measurements. The AlGaAs (135 ˚ )/GaAs (45 A ˚ ) QWR-SL exhibited very unique optical A

Fig. 4. Processes used for the preparation of PL and PLE samples (a– c) and those used for the achievement of a flat sample surface (d, e). The cross-sectional SEM image of the sample obtained after mechanical lapping (f).

with surface grating. Therefore, we can safely eliminate the effect of the surface grating as the origin of the abnormal PLE spectra observed in the superlattice structures.

4. Discussion ˚ )/GaAs (45 A ˚ ) QWR-SL showed The AlGaAs (135 A very unusual optical properties compared with those of SQWR. These optical properties can not be attributed to extrinsic origins, for examples, impurities, size fluctuations or surface grating effects, but rather seem to be intrinsic properties characteristic of QWR-SL struc-

Fig. 5. PLE spectra of the flattened sample. The bold vertical arrows indicate the detection energy.

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properties which are completely different from those of the SQWR sample, i.e. (1) the appearance of excited state emission in low temperature PL spectra even with very low excitation power density; and (2) the observation of step-like PLE spectra. These unique optical properties can not be explained by the direct electronic coupling due to the very thick Al˚ ) barrier layer. The interaction of the GaAs (135 A one dimensional QWR electronic state with optical field is considered as a possible reason for these unique optical behaviors.

Acknowledgements The authors would like to thank Dr Tsunenori Sakamoto and Dr Keizou Shimizu for their encouragement on this work. They would also like to thank Dr Ryosaku Kazi for his help in theoretical calculation and Dr Kazuhiro Komori for helpful discussion.

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