Journal of Crystal Growth 255 (2003) 57–62

Influence of InGaAs overgrowth layer on structural andoptical properties of InAs quantum dots

Jin Soo Kim*, Jin Hong Lee, Sung Ui Hong, Won Seok Han,Ho-Sang Kwack, Dae Kon Oh

Basic Research Laboratory, Electronics and Telecommunications Research Institute (ETRI), 161 Gajeong-dong Yuseong-gu,

Daejeon, South Korea

Received 5 March 2003; accepted 24 March 2003

Communicated by M. Schieber

Abstract

Self-assembled InAs quantum dots (QDs) covered by In0.15Ga0.85As layer with different thickness were grown by

molecular beam epitaxy and their structural and optical properties were investigated by transmission electron

microscopy (TEM) and photoluminescence (PL) spectroscopy. Cross-sectional TEM images showed that the shape of

InAs QD, particularly the height, was controlled by changing the thickness of InGaAs overgrowth layer, thus changing

the optical properties. The emission peak position of InAs QDs covered by 8.5 nm In0.15Ga0.85As layer was 1.30 mm at

room temperature with the energy-level spacing between the ground states and the first excited states of 80meV, which

is about 1.5 times larger than that of InAs QDs capped by GaAs layer. The longer emission wavelength with relatively

larger energy-level spacing was related to the QD aspect ratio (height/width) that was confirmed by TEM images.

r 2003 Elsevier Science B.V. All rights reserved.

PACS: 78.66.Fd; 81.05.Ea; 81.15.Hi

Keywords: A1. Energy-level spacing; A1. Quantum dot aspect ratio; A3. Quantum dots

1. Introduction

Self-assembled quantum dots (QDs) in largelylattice-mismatched systems grown by the Stranski-Krastanov mode have attracted much interest inthe past several years, because of the possibility ofrealizing the improved device performance such aspromising high thermal stability, low threshold

current density and high differential gain for alaser diode [1,2]. However, their overall perfor-mance in devices has remained inferior to that ofthe quantum wells mainly because of the size,shape, and compositional fluctuations in QDs [3].So, the formation of small QD at nanometer scalecan be systematically controlled in both size andshape to have much closer ideal QD properties andto improve the device performance.The emission wavelengths for the convention-

ally grown InAs QDs in GaAs matrix were around1.15 mm at room temperature and was not appro-

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*Corresponding author. Tel.: +82-42-860-6021; fax: +82-

42-860-6248.

E-mail address: kjinsoo@etri.re.kr (J.S. Kim).

0022-0248/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0022-0248(03)01210-7

priate for fiber optic communication system [4,5].Therefore, it has been a strongly interesting subjectto extend the emission wavelength of QDs to1.30 mm because of the transparency window ofoptical fiber [6,7]. There have been several reportson the energy-level control to 1.3 mm by using theso-called strain-reducing layer [8,9], however, theeffects of this strain-reducing layer on the struc-tural and optical properties, such as the energy-level spacing, of InAs QDs have not been fullydiscussed yet.In the present work, the influence of the

In0.15Ga0.85As overgrowth layer with differentthickness on the structural and optical propertiesof self-assembled InAs QDs was investigated bytransmission electron microscopy (TEM) androom temperature photoluminescence (PL) spec-troscopy. Cross-sectional TEM images indicatedthat changing the thickness of the InGaAs over-growth layer could control the shape of InAs QD,thus changing the optical properties. The emissionpeak position of InAs QDs capped by 8.5 nmIn0.15Ga0.85As layer was 1.30 mm at room tem-perature with the relatively larger energy-levelspacing between the ground states and the firstexcited states compared to that of InAs QDscapped by GaAs layer. The results obtained fromTEM images and PL spectra indicated that thelonger emission wavelength with the larger energy-level spacing could be attributed to the QD aspectratio (height/width).

2. Experimental details

The samples used in the present work weregrown by a V80 molecular beam epitaxy onsilicon-doped GaAs substrates. Two differentstructures were prepared to investigate the effectsof In0.15Ga0.85As overgrowth layer on the struc-tural and optical properties of InAs QDs and atypical InAs QD structure embedded in a GaAsmatrix was also grown as a reference (QD1). Theschematic diagrams for the reference QD1 sampleand the QD samples with the In0.15Ga0.85Asovergrowth layer of 6 nm (QD2) and 8.5 nm(QD3) thickness are shown in Fig. 1.

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n-GaAs Sub.

InAs QDs

GaAs Buffer 150 nm

InGaAs Spacer 6 (8.5) nm

InGaAs 6 (8.5) nm

3 Periods

GaAs Spacer 24 (21.5) nm

GaAs Capping 50 nm

InAs QDs

n-GaAs Sub.

InAs QDs

GaAs Buffer 150 nm

5 Periods

GaAs Spacer 30 nm

GaAs Capping 50 nm

InAs QDs

QD1 QD2 and QD3 (a) (b)

Fig. 1. Schematics of the QD samples: (a) the reference QD1

sample, (b) the QD samples with the In0.15Ga0.85As overgrowth

layer (6 and 8.5 nm).

(c)

(b)

(a)

Fig. 2. (a) Plan-view TEM image of the bright field from the

QD1 sample, and cross-sectional TEM images of (b) the QD1

sample, and (c) the QD3 sample.

J.S. Kim et al. / Journal of Crystal Growth 255 (2003) 57–6258

Before depositing the InAs QD layer, thesubstrate temperature was set to 540�C for thegrowth of GaAs buffer layer and then, thesubstrate temperature was lowered to 480�C forthe InAs QD layer and the In0.15Ga0.85As layer.An undoped 50 nm GaAs cap layer was grownafter the deposition of the InAs QD layer followedby 10 s growth interruption time under As-richcondition and the In0.15Ga0.85As layer.TEM specimens were prepared by standard ion

milling and dimpling, and were investigated in aPhilips EM 20 operating at 200 keV. In PLmeasurement, an Argon ion laser was used as anexcitation source to generate electron–hole pairs.The luminescence light from the sample wasfocused with collection lenses, dispersed by a1.2m SPEX single grating monochromator anddetected by a liquid nitrogen cooled Ge detector.

3. Results and discussions

Fig. 2 shows (a) the plan-view TEM image ofthe bright field from the QD1 sample, and cross-sectional TEM images of (b) the QD1 sample, and(c) the QD3 sample, which show the formation ofQDs clearly. No indication of the dislocations dueto the large strain is observed, which can be easilyseen in a large QD. From Fig. 2(a) and (b), thelateral size (height) and density of QDs for theQD1 sample are approximately 20.571 nm(5.570.5 nm) and 7� 1010 cm�2, respectively. InFig. 2(c) for the QD3 sample, the lateral size(height) of QDs is 2271 nm (8.570.5 nm), wherethe QD aspect ratio is increased compared to thereference QD1 sample, resulting in the modifica-tion of the optical properties of the QDs that willbe discussed later.

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InGaAs

InGaAsInGaAs

n-type GaAs Sub. (d)

GaAs Spacer

(a) (b)

(c)

Fig. 3. HRTEM images for: (a) the QD1 sample, (b) the QD2 sample, and (c) the QD3 sample, in which the size of the QD (the aspect

ratio) is related to the thickness of In0.15Ga0.85As layer. (d) Schematic illustration for the formation of the shape-controlled QD

covered by InGaAs layer.

J.S. Kim et al. / Journal of Crystal Growth 255 (2003) 57–62 59

The high-resolution TEM (HRTEM) images for(a) the QD1 sample, (b) the QD2 sample, and (c)the QD3 sample are shown in Fig. 3, in which thesize of InAs QD (the aspect ratio) is related to thethickness of the In0.15Ga0.85As overgrowth layer.The difference in QD size, particularly the height,can be attributed to the local directional migrationof Ga and In adatoms around the InAs QD due tothe surface chemical potential and the suppressionof In segregation reducing the compositionalmixing, because In already exists in the InGaAslayer, when depositing In0.15Ga0.85As layer onInAs QD layer [10–12]. A possible model thatexplains the formation for the shape-engineeredInAs QD by using InGaAs layer is schematicallyillustrated in Fig. 3(d). From these results, theeffective shape of InAs QD can be tailored bychanging the thickness of InGaAs overgrowthlayer.Fig. 4(a) shows the PL spectra of the QD2

sample with different excitation intensity measuredat room temperature. The PL signals are verystrong even at low excitation intensity indicatingto the efficient capture of carriers into the QDs,even though no intended confining layers forcarriers were introduced. Signals from the wettinglayer and the GaAs layer are not clearly shown inthe PL spectra. At low excitation intensity onlyone peak is observed at 1.26 mm due to the ground-state emission and the additional peaks areobserved with an increase in the excitationintensity implying that the peaks result from theexcited-state emission, which is often observed inthe previous reports [13,14]. This is a distinctiveproperty of QDs and is due to the nature of thecarrier capture, recombination and state fillingeffect. By using two Gaussian curves (dotted lines)to fit the measured spectrum at an excitationintensity of 250mW as shown in Fig. 4(b), twooverlapping spectra are separated, and the emis-sion peak position and the energy-level spacingbetween the ground states and the first excitedstates can be determined accurately. The emissionpeak position for the ground states and the energy-level spacing of the QD2 sample are 0.984 eV(1.26 mm) and 67meV, respectively.Fig. 5(a) shows the PL spectra from InAs QDs

measured at room temperature, where the excita-

tion intensity is the same as for the case of PLspectrum shown in Fig. 4(b). With an increase inthe thickness of the In0.15Ga0.85As layer, theemission peak position for the ground states ismore red-shifted from that of the reference QD1sample. The emission peak position for the groundstates of the QD3 sample is 1.30 mm, which is quiteimportant region for fiber optic communications,with linewidth broadening of 36meV. The red-shift in the PL peak position could be due to anincrease in QD size, especially an increase in theQD aspect ratio, and the low potential barrier ofthe InGaAs layer [4,15]. The emission peakposition and the energy-level spacing between the

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0.9 1.0 1.1 1.2 1.3 1.4

(a)

x 5

10 mW

50 mW1 mW

100 mW

250 mW

300 K

PL

Int

ensi

ty (

arb.

uni

ts)

Wavelength (µm)

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

∆E=67 meV

(b)

300 K

PL

Int

ensi

ty (

arb.

uni

ts)

Photon Energy (eV)

Fig. 4. (a) PL spectra from the QD2 sample measured at room

temperature with different excitation intensity. (b) Gaussian

fitting curves (dotted lines) to the measured spectrum at an

excitation intensity of 250mW.

J.S. Kim et al. / Journal of Crystal Growth 255 (2003) 57–6260

ground states and the first excited states aresummarized in Fig. 5(b), which were obtained bya same fitting procedure as in Fig. 4(b), and thedashed lines are only guides for the eyes. Bycomparing the PL results with the HRTEMimages as shown in Fig. 3, the emission wavelengthfor the ground states is longer as expected due tothe size quantization effect with an increase in thesize of a QD. However, the energy-level spacingbetween the ground states and the first excitedstates also becomes larger. If we consider only thesize quantization effect, the energy-level spacingwould be smaller with an increase in QD size. Inthe previous work, the numerical calculation forthe energy-level spacing with various heights at afixed diameter was done, where the wave functionof QD is more confined in the QD with the highaspect ratio resulting in stronger quantum effects

[16]. From TEM and PL results, the red-shift ofthe emission peak position with the relativelylarger energy-level spacing for this work can beexplained by the increase in the QD aspect ratio.

4. Conclusions

The InAs QDs covered by the In0.15Ga0.85Aslayer with two different thicknesses were grownand their structural and optical properties wereinvestigated by TEM and room temperature PL.The emission peak position for the InAs QDscovered by 8.5 nm In0.15Ga0.85As layer showed1.30 mm at room temperature. The red-shift in thePL of the QDs covered by the In0.15Ga0.85As layerfrom that of the QDs capped by the GaAs layercan be attributed to an increase in the size of theQD and a low potential barrier. While theemission peak position was red-shifted with anincrease in the thickness of the InGaAs over-growth layer, the energy-level spacing between theground states and the first excited states of theQDs was increased. The enlarging in the energy-level spacing can be attributed to the fact thatwave function of QD can be more confined in theQD with the high aspect ratio resulting in strongerquantum effects.

Acknowledgements

This work was supported by Ministry ofInformation and Communication (Grant No.03MB2510).

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