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aPhys. Status Solidi C 10, No. 11, 1509–1512 (2013) / DOI 10.1002/pssc.201300228

Fabrication of highly stacked quantum dots on vicinal (001) InP substrates using strain-compensation technique Kouichi Akahane* and Naokatsu Yamamoto

National Institute of Information and Communications Technology, 4-2-1, Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan

Received 19 June 2013, revised 12 July 2013, accepted 26 August 2013 Published online 23 September 2013

Keywords quantum dash, quantum dot, strain compensation, vicinal surface * Corresponding author: e-mail akahane@nict.go.jp, Phone: +81 42 327 6804, Fax: +81 42 327 7938

Highly stacked InAs quantum dots (QDs) were grown on vicinal (001) InP substrates by the strain-compensation technique. The vicinal surface and As2 flux played im-portant roles in changing quantum dashes into QDs on

the (001) surface. Strain compensation was employed to stack 30 layers of QDs without degrading the quality of the crystal. In addition, the QDs exhibited strong photo-luminescence emission at 1.55 μm and room temperature.

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Research on quantum dots (QDs) and quantum dashes (QDHs) has attracted significant attention because of their applications to high-performance optical devices such as semiconductor lasers and semiconductor optical amplifiers [1-4]. High-density QDs and QDHs are necessary for fabricating such devices. Usually, a self-assembling technique is used to obtain high-density QDs or QDHs and a stacked structure is used to dramatically in-crease their density. However, their average size gradually increases from one layer to the next in the stacked structure [5-8]. Moreover, having too many layers degrades their quality because internal strain accumulates as the number of layers increases, causing defects and dislocations [9, 10]. To address these issues, we developed a strain-compensation technique that is resistant to defects and dis-locations, thus enabling the successful growth of InAs QDs and QDHs. Our method increases the number of layers of InAs QDs and QDHs that can be stacked on top of InP substrates [11-14], and it can be used to obtain QD struc-tures on (311)B surfaces and QDH structures on (001) sur-faces. Although the anisotropic shape of QDH structures can be used to control the polarization of light during the development of planar-type optical devices, QD structures are better suited for fabricating waveguide devices because QD structures strongly confine carriers. Because (001) sur-faces are commonly used in industry, they are inexpensive,

and thus QD structures can profitably be fabricated on InP(001) substrates. In this study, we used vicinal (001) substrates and As2 flux to develop a method of growing QD structures on (001) InP substrates. Highly stacked QD structures were successfully fabricated on vicinal (001) InP substrates using the proposed strain-compensation tech-nique.

2 Experimental All the samples were fabricated us-

ing molecular beam epitaxy (MBE) on vicinal (001) InP substrates. The substrates were heated to 520 °C in order to thermally clean them. A 300-nm-thick lattice-matched In0.52Al0.48As buffer layer was grown on the cleaned sub-strates, and then 4-monolayer (ML) InAs QDs and 20-nm-thick In1-x-yGaxAlyAs spacer layers were subsequently grown at 470 °C on top of the buffer layer, producing a stacked structure. The InAs and spacer layers satisfied strain-compensation conditions that prevent the degrada-tion of QD quality. We defined the strain-compensation conditions as

dInAs εInAs = −dsp εsp, (1) εInAs = (aInAs – asub)/asub , (2) εsp = (asp – asub)/asub. (3)

1510 K. Akahane and N. Yamamoto: Fabrication of highly stacked quantum dots on vicinal (001) InP substrates

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In these equations, dInAs and dsp are the thicknesses of the deposited InAs and spacer layers, respectively, and aInAs, asub, and asp are the respective lattice constants of the InAs, InP, and InGaAlAs spacer layers. This definition is based on the simple approximation that the total strain energy of an InAs layer/spacer layer pair is zero. The post-growth surface morphology was observed using an atomic force microscope (AFM) under normal atmospheric conditions for samples without capping layers. A yttrium vanadate (YVO4) laser operated at 532 nm, a 250-mm monochro-mator, and an electrically cooled PbS detector were used to measure the photoluminescence (PL) of the samples at room temperature.

3 Results and discussion We first fabricated

two-layer stacked samples of InAs QDH structures without strain compensation to evaluate their structural and optical properties. The InAs QDH structures were grown on In0.52Ga0.24Al0.24As layer. Figure 1 shows the AFM images of the InAs QDH structures grown on the InP(001) substra-tes with As4 flux. The QDHs are elongated along the

[1-10] direction because of the difference in the surface diffusion length of In adatoms during the growth of the InAs QDHs [15-17]. The typical width (along the [110] di-rection) and height of the QDHs were approximately 24.4 and 1.4 nm, respectively. Figures 1 (b) and (c) show AFM images of the InAs QDH structures grown on (001) InP 2° off and 4° off (along the [1-10] direction) the substrate, respectively, with As4 flux. The InAs nanostructure chan-ged slightly from QDHs into QDs. However, the QDH structure was dominant in these samples. The typical width and height of the structures were (b) 24.8 and 2.1 nm and (c) 27.0 and 2.7 nm, respectively. Therefore, the suppressi-on of the surface diffusion of In adatoms along the [1-10] direction with As4 flux is not enough under these growth conditions.

Figure 2 shows AFM images of the InAs QD struc- tures grown on (001) InP 0o, 2o and 4o off (along the [1-10] direction) the substrate with As2 flux. The InAs nanostruc-ture obviously changed from QDHs into QDs. The (001) vicinal surface increased the atomic step density along the [1-10] direction, which decreased the surface diffusion of

(a) (b) (c)

(a) (b) (c)

Figure 1 AFM images of the InAs QDH structures grown on (001) InP substrates with As4: (a) 0° off, (b) 2° off, and (c) 4° off.

Figure 2 AFM images of the InAs QD structures grown on (001) InP substrates with As2: (a) 0° off, (b) 2° off, and (c) 4° off.

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In adatoms during the growth of the InAs QDs. However, the As2 flux also played an important role in forming the QD structure. Indeed, when either the vicinal surface or the As2 flux was used separately, an InAs QDH structure was formed and it was shorter along the [1-10] direction. The As2 flux was more active than the As4 flux, so the As2 flux also decreased the surface diffusion of In adatoms during the growth of the InAs QDs. The lateral size (along the [110] direction) and height of the QDs were (a) 29.9 and 2.9 nm, (b) 32.0 and 2.7 nm, and (c) 32.9 and 3.5 nm, respectively.

Figure 3 shows PL spectra for the QDH and QD structures shown in Fig. 1 and Fig. 2 measured at room temperature. The spectra labeled (a)–(e) correspond to the structures shown in Figs. 1 (a)–(c) and Figs. 2 (b) and (c), respectively. All the samples show PL in the region around 1.55 µm. The PL peak wavelengths are located at ~1600 nm in the spectra for the QD structures, as indicated by (d) and (e), and at 1450 nm in the spectra for the QDH struc-ture, as indicated by (a). The difference in emission wave-length is attributed to the difference in the QDH and QD structures; that is, the height of QDH structure is smaller than that of QD structure, shorter emission wavelength ap-peared in the spectrum for the QDH structure. The emis-sion wavelength for the QD structure is almost the same as that for layers of InAs QDs grown to the same thickness on InP(311)B substrates [11, 12]. The spectrum of Fig. 3(b) showed wider emission than the other samples, which me-ans that QDH and QD structures were mixed in this sam-ple.

We then fabricated highly stacked structures of InAs QDs on (001) InP 4° off the substrates. Figure 4 shows an AFM image of 30th layer of the InAs QDs. The shape of the QDs was maintained after 30 layers were stacked on top of each other. In addition, the surface morphology of the InAs QD structure did not degrade. Figure 5 shows the PL spectrum for the 30-layer stacked sample. The spec-trum of a single-layer emission is also shown as a refer-

Figure 3 PL spectra of one-layer InAs QDHs and QDs grown on (001) InP substrates measured at 300 K: (a) 0o off with As4, (b) 2o off with As4, (c) 4o off with As4, (d) 2o off with As2, and (e) 4o off with As2.

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Figure 4 AFM images of 30-layer stack of InAs QDs grown on InP 4° off substrate with As2.

Figure 5 PL spectrum of 30-layer stack of InAs QDs (solid line) and single-layer InAs QDs (dotted line) mea-sured at room temperature.

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ence. Strong PL was observed at 1532 nm at room tempe-rature, indicating that the crystal quality was maintained after 30 layers of InAs QDs were stacked. To satisfy the strain-compensation condition as shown in (1)-(3), the Al composition was increased from the condition mentioned in Fig. 1 and Fig. 2. Therefore, the shift of the peak wave-length of the 30-layer sample indicates the effect of the strain-compensation layer, which has slightly larger Al composition and compressive strain for the InAs QDs. Ob-viously, the PL intensity increased significantly with sta-cking. Therefore, this method of fabricating InAs QDs on vicinal (001) InP substrates is useful to fabricating high-density QDs on other commonly used substrates.

4 Summary Highly stacked InAs QDs were grown on

vicinal (001) InP substrates by the strain-compensation technique. The vicinal surface and As2 flux played impor-tant roles in changing the quantum dash structure into the QD structure on the (001) surface. Strain compensation was employed to fabricate a 30-layer stack of QDs without degrading the qualities of the structure or the crystal. In addition, the QDs exhibited strong PL emission at 1.55 μm and room temperature.

Acknowledgements We would like to acknowledge the

staff of the Photonic Device Laboratory at National Institute of Information and Communications Technology for their technical support.

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