Physica E 42 (2010) 2753–2756

Contents lists available at ScienceDirect

Physica E

1386-94

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physe

Suppression of indefinite peaks in InAs/GaAs quantum dot spectrum by lowtemperature capping in the indium-flush method

N. Kumagai a,n, S. Ohkouchi a,b, S. Nakagawa a,d, M. Nomura a, Y. Ota a,d, M. Shirane a,b, Y. Igarashi a,b,S. Yorozu a,b, S. Iwamoto a,c,d, Y. Arakawa a,c,d

a Institute for Nano Quantum Information Electronics (NanoQuine), The University of Tokyo, 4-6-1 Komaba, Meguroku, Tokyo 153-8505, Japanb Nano Electronics Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japanc Institute of Industrial Science (IIS), The University of Tokyo, 4-6-1 Komaba, Meguroku, Tokyo 153-8505, Japand Research centre of advanced science and technology (RCAST), The University of Tokyo, 4-6-1 Komaba, Meguroku, Tokyo 153-8505, Japan

a r t i c l e i n f o

Article history:

Received 15 September 2009

Received in revised form

3 December 2009

Accepted 20 December 2009Available online 15 January 2010

Keywords:

Self-assembled quantum dot

Molecular beam epitaxy

Indium arsenide

Photoluminescence

Single photon emitters

77/$ - see front matter & 2009 Elsevier B.V. A

016/j.physe.2009.12.042

esponding author. Tel./fax: +81 3 5452 6807

ail address: nkumagai@iis.u-tokyo.ac.jp (N. Ku

a b s t r a c t

We have investigated effects of growth temperature of thin GaAs capping layer in the initial stage of

indium-flush process using atomic force microscopy and microscopic photoluminescence (m-PL)

methods. The shape of capped InAs quantum dot (QD) and its m-PL properties are sensitive to the

growth temperature of thin GaAs capping layer. In the case of the high temperature cap, the QD shape

in initial capping stage is elongated along the [1 1 �0] direction, and m-PL spectrum shows several

peaks accompanied with indefinite peaks. On the other hand, the low temperature case, the QD shape is

kept in isotropic and m-PL spectrum shows distinctive emissions from excitonic states of the QD with

suppressed indefinite peaks. These results indicate that the low temperature capping is effective to keep

an isotropic shape of QD and suppress indefinite peaks.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

InAs/GaAs quantum dot (QD) has been attractive for source ofsingle photon emitter and generation of entangled photon pair inquantum information technology [1]. Beside, study of quantumelectrodynamics by combination between InAs QDs and nano-cavity system has been also carried out actively [2]. For theseapplications, following features are required in InAs QD sample:(1) low density for optical addressing and spatial separating fromthe others; (2) distinct emission from excitonic state with lowbackground emission (high signal to noise ratio). In Addition,shortening of emission-wavelength to less than 1 mm is alsorequired in order to use sensitive silicon based photo-detector. Tosatisfy these requirements simultaneously, tailoring of micro-scopic optical property of InAs QD has become more crucial [3],since it is directly linked to control of a lot of parameters such asshape, size, and composition.

Recently, initial capping process has become known as one ofthe important keys to control microscopic structural properties ofInAs QDs and also the process is complex and sensitive to manygrowth parameters [3–7]. However, tailoring of microscopicoptical property of InAs QD by growth parameters of initial

ll rights reserved.

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magai).

capping process has not been so investigated systematically [7].Therefore we have focused attention on the growth temperatureof thin GaAs capping layer in indium-flush (In-flush) process. In-flush method (sometimes called by double capping method orpartial cap and anneal method) is a certain overgrowth techniqueto shorten emission-wavelength by reduction of QD height [6–9].That is, QD is covered with thin GaAs cap partially and thenannealed [10,11].

In this study, we have systematically investigated the effectof growth temperature of thin GaAs capping layer in In-flushprocess on microscopic property of InAs QD by atomic forcemicroscopy (AFM) and microscopic photoluminescence (m-PL)measurement.

2. Experimental

All InAs QD samples were grown on semi-insulating (0 0 1)oriented GaAs substrate by molecular beam epitaxy (MBE).The growth temperature of InAs QDs and the equivalent beampressure of As4 during growth of QDs were set at 485 1Cand 5�10�6 Torr. To obtain adequate low density QDs withreproducibility, we employed various low growth rates of InAsQDs from 0.0042 to 0.015 ML/s as a pilot experiment, andexamined relation between the nominal coverage of InAs andQD density by AFM.

N. Kumagai et al. / Physica E 42 (2010) 2753–27562754

Then, we fabricated QD samples with the optimized growthrate for both AFM and m-PL measurements. For observation ofinitial shape of InAs QD in capping process, 1 nm-thick GaAscapping layer was grown on InAs QDs at various temperaturesfrom 435 to 485 1C. QD height and shape were examined by AFMin air at room temperature.

2.0 2.1 2.2 2.3 2.4 2.5 2.6107

108

109

1010

Den

sity

[cm

-2]

Nominal coverage [ML]

0.0125ML/s0.015ML/s0.0042ML/s

Fig. 1. (a) Plots of QD density as function of InAs nominal coverage for three growth rate

growth rate of 0.0042 ML/s with nominal coverage of 2.2 ML. QD density is around 5�

[110]

[110]

4

3

22

1

Hei

ght [

nm]

1

042

0

a b

dc

435 deg.

485 deg.

Fig. 2. (a)–(c) AFM images of InAs QDs with 1 nm-thick GaAs cap for 1 mm2. (a) Thin

height and anisotropy of QD shape as a function of growth temperature of 1 nm-thick Ga

and [1 1 0] direction.

For m-PL measurements, In-flushed InAs QDs were embeddedin GaAs matrix. In the In-flush process, the partial capping layersof GaAs were grown at 435 and 485 1C, corresponding to thelowest and highest growth temperature of AFM samples,respectively. The thickness of partial cap was determined to1.6 nm for shortening the emission-wavelength to less than 1 mm

2.7

1µm2

s (0.0125, 0.015 and 0.0042 ML/s). (b) Typical AFM image of InAs QDs grown by the

108 cm�2.

6

55

4

3

2

3

2

1 Height

0 Anisotropy

0 440 460 480 5000

Capping temperature [deg.]

Ani

sotro

py ([

110]

/ [1

10])

460 deg.

cap was grown at 435 1C, (b) 460 1C and (c) 485 1C, respectively. (d) Averaged QD

As cap. Here, anisotropy is defined as a ratio of diameters of QD between the [1 1 0]

N. Kumagai et al. / Physica E 42 (2010) 2753–2756 2755

at low temperature. In-flushing (annealing) was done at the samegrowth temperature of partial GaAs cap. Following In-flushprocess, InAs QD samples were embedded with 80 nm-thick GaAslayer. First 20 nm of 80 nm-thick GaAs was grown at 450 1C withthe growth rate of 0.083 nm/s. And then, the remaining 60 nm-thick GaAs was grown with the rate of 0.222 nm/s. During thegrowth of 60 nm-thick GaAs, the growth temperature wasincreased from 450 to 500 1C with the rate of 0.2 1C/s. Thesesamples were processed to mesa structures, and m-PL measure-ments were performed at 10 K with Ti:Al2O3 laser. The spot size ofexcitation and wavelength were around 3 mm and 780 nm.

3. Results and discussion

At first, the result of the pilot experiment on optimization oflow growth rate was performed. Fig. 1(a) shows the relationbetween nominal coverage of InAs and QD density for threegrowth rates. In the case of the lowest growth rate of 0.0042 ML/s,QD density was proportional to nominal coverage of InAs in lowdensity region from 108 to 109 cm�2, indicating that this growthrate is suitable to control the QD density in this region withreproducibility. While other higher rates are difficult to controllow density because of the fast fall-off in this region. Fig. 1(b) isthe typical AFM image of InAs/GaAs QDs with density of�5�108 cm�2 in 1 mm2 area. Averaged QD height was around

950 955 960 965 970

PL

inte

nsity

[a.u

.]

Wavelength [nm]

880 885 890 895 900

PL

inte

nsity

[a.u

.]

Wavelength [nm]

435 °C capping

485 °C capping

Fig. 3. Representative m-PL spectra at 10 K from the mesa structures with around 1 mm

((c) and (d)), respectively.

13 nm, and its shape was isotropic. The low density QDs havebeen well fabricated using the optimized growth rate. We appliedthis growth condition to grow AFM and m-PL samples.

Fig. 2(a)–(c) shows AFM images of InAs QDs with 1 nm-thickGaAs capping layer for various capping temperatures. When thecapping layer was grown at 435 1C, isotropic QD shape has beenkept as shown in Fig. 2(a). On the other hand, QD shapes for 460and 485 1C were elongated to [1 1 0] direction by hightemperature capping as shown in Fig. 2(b)–(d) is the plot ofaveraged QD height with 1 nm-thick of GaAs cap and anisotropyof the QD shape. Here, the anisotropy is defined as the ratio ofdiameter between [1 1 0] and [1 1 0] direction. Averaged QDheight after the growth of 1 nm-thick GaAs was reduced toaround 2 nm. This drastic reduction of QD height is probably dueto rapid diffusion of In atoms from top of InAs QDs to Ga richregion around QD base [3]. Anisotropy of QD shape has becomelarge with increasing of capping temperature. Since migration ofIn and Ga atoms and inrtermixing of these atoms are enhanced athigh temperatures, formation of an anisotropic InGaAs tail aroundthe bottom of QD may be promoted [3–5,10,11].

Representative m-PL spectra of In-flushed InAs QDs whosepartial caps were grown at 435 1C are shown in Fig. 3(a) and (b),and those grown at 485 1C are shown in Fig. 3(c) and (d),respectively. m-PL peaks are attributed to only 1 or 2 QDs, since afabricated mesa structure has included 1 or 2 QDs, taking accountof the QD density and the diameter of the mesa (�1 mm). When

950 955 960 965 970Wavelength [nm]

895 900 905 910 915Wavelength [nm]

435 °C capping

485 °C capping

diameter. The partial capping layers were grown at 435 1C ((a) and (b)), and 485 1C

896 898 900 902 904 906

PL

Inte

nsity

[a.u

.]

Wavelength [nm]

XX

X+

X

100 101 102 103101

102

103

104

105

linear

linear

X+ XX X

Inte

nsity

[a.u

.]

Pump power [a.u.]

square

Fig. 4. Representative m-PL spectra with peak assignment and excitation power

dependence of PL peak intensity (inset). X+, X and XX is charged exciton, neutral

exciton and neutral biexciton, respectively.

N. Kumagai et al. / Physica E 42 (2010) 2753–27562756

the partial caps were grown at 435 1C, distinctive peaks wereobserved without background emission and/or broad emission(Fig. 3(a) and (b)). On the other hand, in the case of 485 1Ccapping, m-PL spectra have shown several peaks accompaniedwith indefinite broad peaks, and the broad peaks tend to tail tolonger wavelength as shown in Fig. 3(c) and (d). Microscopicstructures of anisotropic QDs with large In–Ga intermixing regionand resulting m�PL spectra are more complex than those ofisotropic QDs formed by low temperature growth of thin GaAscap. Although the relation between the anisotropic shape of QDsand indefinite emissions still remains to be elucidated, our resultsindicate that isotropic embedding of QD by low temperaturecapping leads suppression of indefinite emission.

Fig. 4 shows another representative m-PL spectrum from InAsQD whose partial cap was grown at 435 1C. The inset is the powerdependence of PL peak intensities. In our case, the most dominantpeak was assigned as charged exciton (X+), and the others werealso assigned as exciton (X) and biexciton (XX), respectively.Details of assignments for these distinct peaks and confirmationof single photon emission by photon correlation measurementswere described elsewhere [12]. We have succeeded to grow high

quality InAs QDs that is suitable for single dot spectroscopy bylow temperature capping in In-flush process.

4. Conclusion

The effects of growth temperature of thin GaAs capping layerin In-flush process on initial shape and optical property of InAsQDs has been studied by AFM and m-PL methods. Low growthtemperature of thin GaAs capping layer suppresses anisotropicdeformation of QD shape and indefinite emission. Using this lowgrowth temperature condition, high quality InAs QD for single dotspectroscopy is achieved.

Acknowledgement

This work was accomplished by the Special CoordinationFunds for Promoting Science and Technology. We thankDr. Watanabe (NanoQuine, The University of Tokyo) for hissupport on MBE operation.

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