Controlling polarization anisotropy of site-controlled InAs/InP (100) quantum dots

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Controlling polarization anisotropy of site-controlled InAs/InP

(100) quantum dots

Citation for published version (APA):

Yuan, J., Wang, H., Veldhoven, van, P. J., Wang, J., Vries, de, T., Smalbrugge, E., Jin, C., Nouwens, P. A. M., Geluk, E. J., Silov, A. Y., & Nötzel, R. (2011). Controlling polarization anisotropy of site-controlled InAs/InP (100) quantum dots. Applied Physics Letters, 98(20), 1-3. [201904]. https://doi.org/10.1063/1.3591155

DOI:

10.1063/1.3591155

Document status and date: Published: 01/01/2011

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Controlling polarization anisotropy of site-controlled InAs/InP (100)

quantum dots

Jiayue Yuan, Hao Wang, René P. J. van Veldhoven, Jia Wang, Tjibbe de Vries et al.

Citation: Appl. Phys. Lett. 98, 201904 (2011); doi: 10.1063/1.3591155 View online: http://dx.doi.org/10.1063/1.3591155

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Controlling polarization anisotropy of site-controlled InAs/InP

_„100…

quantum dots

Jiayue Yuan,a兲Hao Wang, René P. J. van Veldhoven, Jia Wang, Tjibbe de Vries, Barry Smalbrugge, C. Y. Jin, Peter Nouwens, Erik Jan Geluk, Andrei Yu. Silov, and Richard Nötzelb兲

Department of Applied Physics, COBRA Research Institute, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands

共Received 3 January 2011; accepted 22 April 2011; published online 17 May 2011兲

We report on the shape and polarization control of site-controlled multiple and single InAs quantum dots 共QDs兲 on InP pyramids grown by selective-area metal-organic vapor phase epitaxy. With increasing growth temperature the QDs elongate causing strong linear polarization of the photoluminescence. With reduced pyramid base/pyramid top area/QD number, the degree of polarization decreases, attributed to the symmetric pyramid top, reaching zero for single QDs grown at lower temperature. This control of linear polarization is important for entangled photon sources operating in the 1.55 ␮m wavelength region. © 2011 American Institute of Physics.

关doi:10.1063/1.3591155兴

Semiconductor quantum dots 共QDs兲 provide the basis for novel devices in photonics and quantum information.1–3 The envisioned devices such as cavity enhanced single and entangled photon sources4,5require precise site control of the QDs. This has been achieved by growth on truncated pyramids,6–8 in V-grooves,4,9 and nanoholes10,11 for InAs/ GaAs and InAs/InP QDs with emission in the important 1.55 ␮m telecom wavelength region.12,13 In addition, con-trol of the QD shape and consequently linear polarization of the emission is crucial for entangled photon sources. They require symmetric QDs with zero degree of polarization 共DOP兲 and, hence, vanishing fine structure splitting 共FSS兲. This makes InAs/InP QDs particularly promising exhibiting an about ten times smaller FSS for a certain shape than InAs/ GaAs QDs.14Various approaches to control the polarization of QDs have been demonstrated employing 共111兲 oriented substrates,9 thermal annealing,15 and in-plane magnetic fields.16

Here, we report a different approach to control the shape and polarization of site-controlled 1.55 ␮m region multiple and single InAs QDs by growth on InP 共100兲 pyramids formed by selective-area metal-organic vapor phase epitaxy 共MOVPE兲. The size of the QDs increases with increasing growth temperature and the QDs elongate causing strong lin-ear polarization of the photoluminescence共PL兲. Importantly, the DOP decreases with reduced pyramid base, i.e., pyramid top area, i.e., QD number induced by the symmetric pyramid top, reaching zero for single QDs grown at lower tempera-ture.

The samples were grown by MOVPE on InP共100兲 sub-strates misorientated 2° toward 共110兲. Trimethyl-indium, trimethyl-gallium, phosphine, and tertiarybutyl-arsine were used as source materials. The substrates were masked by 100 nm SiNx with circular openings of 0.7– 1.5 ␮m diameter and 10 ␮m pitch. Truncated InP

pyra-mids were selectively grown at 610 ° C. On the pyramid top, three monolayers InAs QDs were grown at 490 and 515 ° C which were capped by 70 nm InP. The uncapped InAs QDs were characterized by atomic force microscopy共AFM兲 in air. Micro-PL spectroscopy of the capped QDs was performed by exciting the samples, mounted in a He-flow cryostat, with a continuous-wave He–Ne laser operating at 635 nm. Excita-tion and detecExcita-tion of the PL were through a microscope ob-jective with a spatial resolution of ⬃2 ␮m. The PL was dispersed by a 0.25 m single monochromator and detected by an InGaAs photodiode array. For the polarization analysis, a linear polarizer followed by a quarter-wave plate was in-serted in front of the monochromator. The polarization-dependent PL from 34 multiple QD and 26 single QD samples was measured for statistical analysis.

Figures1共a兲–1共f兲show the AFM images of the multiple and single InAs QDs on top of the InP pyramids grown at 关共a兲–共c兲兴 490 °C and 关共d兲–共f兲兴 515 °C. In general, for the larger pyramid top areas 共pyramid bases兲, the QDs nucleate on the high-index兵103其 and 兵115其 facets surrounding a 共100兲 center facet.7,13For shrinking pyramid top area the QD

num-a兲_{Electronic mail: j.yuan@tue.nl.}

b兲_{Present address: ISOM Institute for Systems Based on Optoelectronics and}

Microtechnology, Technical University of Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain. (b) Diameter: 1.42 m Diameter: 0.75 m Diameter: 1.39 m [01-1] 2 m (a) (c) (d) (f) Diameter: 0.78 m [0-1-1] 515 °C 515 °C 490 °C Diameter: 1.07 m Diameter: 0.96 m (b) (e) 515 °C 490 °C 490 °C 515 °C

FIG. 1.共Color online兲 AFM images 共2⫻2 ␮m2_{兲 of the multiple and single}

InAs QDs grown at关共a兲–共c兲兴 490 °C and 关共d兲–共f兲兴 515 °C on the InP pyra-mids with base diameters varying between 1.5 and 0.7 ␮m.

APPLIED PHYSICS LETTERS 98, 201904共2011兲

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ber decreases and close to pinch-off single QDs form on the symmetric diamond-shaped 共100兲 top facet. For higher growth temperature the QDs strongly elongate along 关011¯兴 and number and size increase.

The average QD width along关011¯兴关Fig.2共a兲兴 increases

from 50 nm for the QDs grown at 490 ° C to 75 nm for the QDs grown at 515 ° C with a broader distribution while the average width along关011兴关Fig. 2共b兲兴 of 50 nm remains al-most unchanged. The increase in the QD number and size for higher growth temperature is attributed to the enhanced As/P exchange during QD growth, leading to more incorporation of InAs,17which also accounts for the broader size distribu-tion. The pronounced QD elongation along 关011¯兴 at higher growth temperature is attributed to the increased adatom sur-face migration length which is generally larger along 关011¯兴 and the increased QD size leading to a shape transition from round to elongated.18,19

Micro-PL spectra taken at 5 K of the capped multiple and single QDs grown at 490 ° C are shown in Fig. 3. The PL spectrum of the multiple QDs on the pyramid with the largest base is centered at 1465 nm with a full width at half maximum共FWHM兲 of 34 nm while the PL line of the single QD is at 1471.4 nm with the FWHM of 0.6 nm limited by the spectrometer resolution. For the higher QD growth tem-perature of 515 ° C, the PL spectrum of the multiple QDs is centered at 1495 nm with a broader FWHM of 59 nm while the PL line of the single QD is at 1483 nm. This PL redshift and broader FWHM are in agreement with the larger size and less uniform size distribution of the QDs grown at higher temperature. The excitation power dependent integrated PL intensity of a typical single QD grown at 490 ° C, plotted in the inset of Fig. 3, increases linearly with a slope of 1.08 indicating high optical quality of the single QD.20–23

The shape anisotropy of the QDs results in distinct linear polarization of the PL. Figures4共a兲and4共b兲show the polar plot of the PL peak intensity共I兲 of the multiple 共on the pyra-mids with the largest base兲 and single InAs QDs grown at

490 and 515 ° C. 0° designates the PL polarized along关011¯兴 and 90° the PL polarized along 关011兴. The angular depen-dence of the PL intensity is fitted by I = I_关011兴sin2_␪

+ I_关011¯兴cos2_␪_{, where}_␪ _{is the polarization angle. Most}

obvi-ous for the QDs grown at 515 ° C, the PL is polarized along 关011¯兴 which is the direction of QD elongation.24,25

Most im-portant, the DOP=共I_关011¯兴− I_关011兴兲/共I_关011¯兴+ I_关011兴兲⫻100% at the PL peak position reduces with reduced pyramid base, i.e., reduced pyramid top area and QD number, plotted in Fig.

4共c兲. The DOP for the multiple QDs on the pyramid with the largest base area of 1.58 ␮m2 _关Fig. _1共d兲_{兴 is 38%. This is}

comparable with polarization measurements of InAs QD en-sembles on GaAs truncated pyramids.6 The DOP decreases monotonically down to 17% for the single QD on the pyra-mid with the smallest base area of 0.48 ␮m2关Fig.1共f兲兴. This is attributed to an increasing influence of the shape of the shrinking pyramid top area, approaching the symmetric 共100兲 top facet with diamondlike boundary, on the QD shape rendering it more symmetric. For the QDs grown at 490 ° C, which are generally more symmetric, the slight DOP for the multiple QDs is completely eliminated for the single QD.

It is difficult to quantitatively relate the DOP to the FSS which is the figure of merit for realizing entangled photon sources and within the spectral resolution the FSS cannot be resolved directly. In general, for InAs/InP QDs the FSS mainly depends on the QD shape.14The FSS from the intrin-sic Dresselhaus term26 due to the bulk inversion asymmetry is a minor factor because of the small lattice mismatch共3%兲 and the strong confinement of holes. Moreover it has been reported that the FSS is directly related to the DOP.9,15,16 Hence, zero DOP implies the sought for zero FSS for our single symmetric InAs QDs grown at reduced temperature on the InP pyramids close to pinch-off.

In conclusion, we have studied the shape and polariza-tion control of site-controlled 1.55 ␮m region multiple and single InAs QDs on InP共100兲 pyramids grown by selective-area MOVPE. The QD size increases with elevated growth temperature and the QDs strongly elongate causing pro-nounced polarization of the PL. Most important the DOP reduces with reduced pyramid base/pyramid top area/QD number due to increasing influence of the symmetric pyra-mid top on the QD shape, reaching zero for single QDs

20 40 60 80 100 120 140 0 5 10 15 20 25 Co un ts Width along [01-1] (nm) 490qC 515qC 20 40 60 80 100 120 140 0 5 10 15 20 25 490qC 515qC Count s Width along [011] (nm)

(a)

(b)

FIG. 2. 共Color online兲 Histograms of 共a兲 width along 关011¯兴 and 共b兲 width along关011兴 of the QDs grown at 490 and 515 °C on the pyramids with the largest base area.

1420 1440 1460 1480 1500 1520 1540 1560 PL Intensity (a.u. ) Wavelength (nm) 0.1 1 10 100 101 102 103 Int. PL Intensity (a. u.) Excitation Power (PW) X 6 Slope: 1.08 Single QD Peak at 1471.4 nm FWHM = 0.6 nm Tg: 490 °C Multiple QDs Peak at 1465 nm FWHM = 34 nm 5 K

FIG. 3. 共Color online兲 Micro-PL spectra taken at 5 K of the multiple and single InAs QDs grown at 490 ° C. The inset shows the integrated PL in-tensity at 5 K of a single InAs QD grown at 490 ° C as a function of the excitation power.

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grown at lower temperature. This is important for the real-ization of single and entangled photon sources operating in the 1.55 ␮m telecom wavelength region.

The authors gratefully acknowledge the support of the Smart Mix Programme of The Netherlands Ministry of Eco-nomic Affairs and The Netherlands Ministry of Education, Culture and Science.

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26_{G. Dresselhaus,}_{Phys. Rev.} _{100, 580}_共1955兲. 0.0 0.5 1.0 0 30 60 90 120 150 180 210 240 270 300 330 0.0 0.5 1.0 490°C 515°C Nor m. PL Intens ity (a.u.) 0.0 0.5 1.0 0 30 60 90 120 150 180 210 240 270 300 330 0.0 0.5 1.0 N orm. PL Int ens ity (a. u. ) 490°C 515°C 0.4 0.8 1.2 1.6 0 10 20 30 40 50 DO P ( % )

Area of pyramid base (μm2) 0.8 1.0 1.2 1.4 Diameter of pyramid base (μm)

[01-1] [011]

(a) Multiple

(b) Single

[01-1] [011]

(c)

490 °C

515 °C

100% [01-1] [011] [01-1] [011] I I DOP= I I − × +

Single

QD

Multiple

QDs

FIG. 4. 共Color online兲 Polar plot of the polarized PL peak intensity for共a兲 multiple and 共b兲 single InAs QDs grown at 490 and 515 ° C. The solid lines represent fits to the experimental data by I = I_关011兴sin2_␪_{+ I}

关011¯兴cos2␪.

共c兲 DOP of the QD PL peak intensity versus the area of pyramid base for the QDs grown at 490 and 515 ° C. The red and blue dashed lines are guide to the eyes.

Controlling polarization anisotropy of site-controlled InAs/InP (100) quantum dots

Controlling polarization anisotropy of site-controlled InAs/InP

(100) quantum dots

Controlling polarization anisotropy of site-controlled InAs/InP (100)

quantum dots

Related Articles

Additional information on Appl. Phys. Lett.

Controlling polarization anisotropy of site-controlled InAs/InP

„100…

quantum dots

(a)

(b)

(a) Multiple

(b) Single

(c)

490 °C

515 °C

Single

QD

Multiple

QDs

_„100…