effects of the thickness of gaas spacer layers on the structure of multi layer

8/14/2019 Effects of the Thickness of GaAs Spacer Layers on the Structure of Multi Layer

http://slidepdf.com/reader/full/effects-of-the-thickness-of-gaas-spacer-layers-on-the-structure-of-multi-layer 1/5

Effects of the thickness of GaAs spacer layers on the structure of multilayer

stacked InAs quantum dots

Hyung Seok Kim a, Ju Hyung Suh a, Chan Gyung Park a,Ã, Sang Jun Lee b, Sam Kyu Noh b, Jin Dong Song c,Yong Ju Park c, Won Jun Choi c, Jung Il Lee c

a Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Kyungbuk 790-784, Republic of Koreab Quantum Dot Technology Laboratory, Korea Research Institute of Standards and Science (KRISS), Daejeon 305-600, Republic of Koreac Nano Device Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea

a r t i c l e i n f o

Article history:

Received 13 September 2005

Received in revised form

13 October 2008

Accepted 20 October 2008

Communicated by K.H. PloogAvailable online 1 November 2008

PACS:

68.37.Lp

68.43.Hn

68.55.Jk

68.65.Hb

Keywords:

A1. Microstructure characterizationA1. Quantum dots

A1. Transmission electron microscopy

(TEM)

B1. Multilayer InAs/GaAs

a b s t r a c t

The effects of the thickness of GaAs spacer layers on the structure of multilayer stacked InAs quantum

dots (QDs) grown by molecular-beam epitaxy were studied using transmission electron microscopy. To

investigate QD structure depending on spacer layer growth, first uncapped free-standing QDs were

grown and their structure compared with that of multilayer stacked QDs. In addition, vertically

nonaligned and aligned stacked QDs were grown by adjusting the thickness of GaAs spacer layers. The

uncapped QDs were found to form a lens-shaped structure with side facets. Upon capping with a GaAs

spacer, the apex of nonaligned QDs flattened by In diffusion. However, the aligned QDs maintained their

lens-shaped structure with round apex after capping. It is believed that their apex did not flatten

because the chemical potential gradient of In was relatively low due to the adjacent InAs QD layers. The

results demonstrate the possibility of controlling QD structure by adjusting the thickness of spacer

layers.

& 2008 Published by Elsevier B.V.

1. Introduction

Recently self-assembled heteroepitaxial quantum dots (QDs)

have been grown in Stranski-Krastanow mode [1] and consider-

able effort has been devoted to fabricating laser devices [2],

photodetectors [3] and advanced memory [4] by using self-

assembled QDs. To understand and control the optoelectronic

properties of QD devices, the shape and size of QDs have to bemeasured exactly because their optoelectronic properties are

significantly dependent on their structural properties such as

shape, size, uniformity and density. In addition, the study of the

detailed structure of QDs provides new insight into understanding

the growth characteristics of QDs and controlling their growth

parameters.

There are many important growth parameters such as growth

temperature, deposition rate, growth interruption and the growth

condition of cap layers, which affect the QD structures and their

optoelectronic properties. In particular, the growth procedure of a

cap layer plays a crucial role in determining QD structure and

various QD shapes, depending on cap layer growth have been

reported including truncation [5], ride-valley transition [6] and

dissolution of QDs [7]. Typical analysis of the effect of cap layer

growth on the QD structure was performed using atomic force

microscopy (AFM) in uncapped or partially capped QDs andshowed an important material redistribution during cap layer

growth [8,9]. However, the AFM investigation is not capable of

resolving QD structure owing to the well-known tip convolution

effect, and it is impossible to investigate the structure of fully

capped QDs. Transmission electron microscopy (TEM) is a unique

analysis technique for investigating QD structures capped with an

overlayer and atomic scale analyses are also possible by high-

resolution electron microscopy (HREM).

In the present study, multilayer stacked InAs QDs were grown

on GaAs by molecular-beam epitaxy (MBE) and their structural

properties were investigated by field emission gun-TEM (FEG-TEM)

and high-voltage electron microscopy (HVEM) depending on the

thickness of spacer cap layers. The uncapped QDs were found to

ARTICLE IN PRESS

Contents lists available at ScienceDirect

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

Journal of Crystal Growth

0022-0248/$- see front matter & 2008 Published by Elsevier B.V.doi:10.1016/j.jcrysgro.2008.10.054

ÃCorresponding author. Tel.: +8254 2792139; fax: +8254 2792399.

E-mail addresses: [email protected] (H.S. Kim).

[email protected] (C.G. Park).

Journal of Crystal Growth 311 (2009) 258–262

http://-/?-

http://www.elsevier.com/locate/jcrysgro

http://dx.doi.org/10.1016/j.jcrysgro.2008.10.054

http://dx.doi.org/10.1016/j.jcrysgro.2008.10.054

http://www.elsevier.com/locate/jcrysgro

http://-/?-



form a lens-shaped structure with side facets. In addition,

vertically nonaligned and aligned stacked QDs were grown by

adjusting the thickness of GaAs spacer layers. Upon capping by

GaAs, the apex of nonaligned QDs flattened by In diffusion

although the aligned QDs maintained their lens-shaped structure

with round apex.

2. Experimental procedure

The InAs QDs were grown on semi-insulating GaAs(0 0 1)

substrates using the MBE system (RIBER32P). In order to

investigate the effects of GaAs spacer growth on the QD structure,

uncapped QDs were grown and their structures were investigated

by AFM and HREM. In addition, two different samples, one

vertically aligned and the other nonaligned along the growth

direction, were grown by varying the thickness of GaAs spacer

layers. Fig. 1 shows the schematic diagram of five-period stacked

InAs/GaAs QDs heterostructures. The GaAs buffer layer with a

thickness of 250 nm was deposited on the substrate at 5601C. The

InAs QDs with an equivalent thickness of 2.5 monolayers (ML)

were formed and GaAs overlayers were deposited at 4801C. Forthe growth of vertically nonaligned and aligned QDs, 49 and 9 nm

thick GaAs spacers were deposited, respectively. The growth of

InAs QDs and GaAs spacers was repeated five times with growth

rates of 1.4 and 12.4 nm/min, respectively.

The structural properties of QDs were studied using AFM

(Dimension 3100, Digital Instruments) in tapping mode and

200 kV HR-TEM (JEM-2010F, JEOL). In addition, scanning-TEM

(STEM; JEM2100F, JEOL) with energy dispersive X-ray spectro-

scopy (EDS) and 1.25 MV HVEM (JEM-ARM1300S, JEOL) located

at the Korea Basic Science Institute (KBSI) were used for the

observation of QD structures on an atomic-length scale. TEM

investigations were performed by conventional bright field (BF)

TEM and HREM techniques. TEM images were recorded on an

image plate and their resolution was improved enough to measure

QD size using digital intensity profiling through fast Fourier

transformation (FFT) and the inverse FFT processes.

3. Results and discussion

Fig. 2(a) shows an AFM height image of uncapped InAs QDs

measured in tapping mode. The QDs were lens-shaped and their

average height and diameter were 3.5 and 35 nm, respectively.

The QDs were distributed randomly and had a tendency to grow

together in twos or threes. The QDs density was 7.7Â1010cmÀ2.

Fig. 2(b) is an AFM height profile of single QD indicated in the

Fig. 2(a) by the dotted line A–B. The section profile shows that the

QD has symmetrical dimensions and QD height and diameter

were measured as 3.75 and 36 nm, hence the aspect ratio was

about 0.1.

However, the cross-sectional HREM of uncapped QDs revealedthat the lateral dimension measured by AFM was enlarged by

about two times although the QDs height by AFM nearly coincided

with the result by TEM. Fig. 2(c) shows the HREM image of

uncapped QD on [110] zone. The QD height and the diameter

of base were measured as $4.2 and $18.5nm, respectively, and

hence the aspect ratio was 0.23 which is more than twice as large

as the result by AFM. The resolution of AFM is limited by the

sharpness and shape of the tip whose normal radius of curvature

is 20–60nm [10]. In particular, lateral resolution is much more

dependent on the dimension of the tip than vertical resolution in

nanometer-sized samples because of the measuring geometry

between the tip and sample [11]. Considering the QD structure by

TEM, it is believed that the lateral dimension of QDs measured by

AFM is not reliable although their vertical dimension by AFM isreasonable. The HREM also shows that the QDs are coherent

islands without any defects such as dislocations or stacking faults.

The uncapped QDs were lens-shaped with side facets whose

wetting angles were about 261. The phenomenon of equilibrium

faceting plays a crucial role in determining the QD shape [12]. The

facets with high Miller indices such as (136), (137), (125) and

(2511) were reported in many studies using scanning tunneling

microscopy, AFM and reflection high-energy electron diffraction.

The facet angle was measured as 23–281 and the main facet angle

was 261 which coincides with the (137) facet [12].

We have investigated the structure of uncapped InAs QDs

before GaAs spacer growth. However, the QDs have to be covered

with a cap layer which is needed for the passivation of the QDs

for device application. In addition, multilayer stacked QDs are

required to increase the QD density and the optoelectronic

efficiency of QD devices [13]. In the present study, therefore,

five-period stacked QD structures were grown with varying the

thickness of spacer layers, after which the effects of GaAs spacer

overlayer on the QD structures were investigated.

Fig. 3 shows a cross-sectional TEM BF image of five-period

stacked InAs QDs with 49 nm thick GaAs spacer layers and the

[110] zone HREM image (b). The five-period stacked QDs were

successively grown and randomly distributed along the growth

direction. A lens-shaped very dark contrast was observed on and

beneath the wetting layers under the g 004 two beam condition.

The InAs/GaAs heteroepitaxy has a 7.2% lattice misfit and the

misfit strain induces a dark contrast in TEM observations using

phase contrast as well as diffraction contrast. The QD height and

diameter were measured as 3.5–4.5 and 15–20 nm, respectively.In particular, the BF and HREM images show that QD apexes

flattened after capping with a spacer layer. The effects of elastic

energy and surface energy can explain the QD apex flattening

[6,14]. The increase of the elastic energy and surface energy of

InAs QDs by depositing a GaAs overlayer induces the diffusion of

In from the QDs [14]. Therefore, QD apex flattening is possible due

to the In diffusion induced by the compressive strain from GaAs to

QDs and the chemical potential gradient of In during overlayer

growth at high temperature. The QD height and base diameter

were measured as $4.6 and $23.2nm in the HREM. However,

exact determination of the shape and size of capped QDs was

difficult because of the indistinct boundary between InAs and

GaAs as shown in Fig. 3(b). In the BF image under dynamical two

beam and the HREM on-zone axis multi beam conditions, thediffraction contrast image of QDs can be observed largely by strain

ARTICLE IN PRESS

Fig. 1. Schematic diagram of five-period stacked InAs QDs grown on GaAs.

H.S. Kim et al. / Journal of Crystal Growth 311 (2009) 258–262 259



ARTICLE IN PRESS

Fig. 2. AFM height image of a free-standing uncapped InAs QDs in tapping mode (a), section height profile indicated by dotted line A–B in (a), and a [110] zone cross-

sectional HREM image of a single QD showing lens-shaped structure.

Fig. 3. Cross-sectional BF image of five-period stacked InAs QDs with 49 nm thick GaAs spacers in g 004 two beam condition (a) and the [110] HREM image of the QD

showing flat QD apex (b).

Fig. 4. Annular dark field (ADF) STEM image of capped QD with FFT pattern (a) and EDS line-scan profile for In along the denoted line in the ADF image (b).

H.S. Kim et al. / Journal of Crystal Growth 311 (2009) 258–262260



field around the QDs, rather than their original shape and size

[15]. In addition, the diffusion of In during overlayer growth may

also increase the QD size in TEM observations.

To investigate more exact QD shape and size, the annular dark

field (ADF) observation of capped QDs and the EDS line scan for

In were performed using a STEM-EDS system. Fig. 4 shows the

ADF-STEM image of capped QD with FFT pattern (a) and EDS line-

scan profile for In along the denoted line in the ADF image (b). Themaximum spatial resolution of ADF-STEM is defined as the size of

electron beam probe and the STEM imaging was performed using

electron beam probe with 0.2 nm diameter in the present study.

The QDs and wetting layers were bright in the ADF image and an

interdiffusion of Ga and In producing an indistinct boundary

between In QDs and Ga matrix was observed. This transition

region ranged from 0.5 to 1 nm. The QD has flat apex and the

height and base diameter were $4.3 and $20 nm, respectively.

Although QD shape and size were observed more clearly

compared with the previous observations such as BF and HREMimages, it was very difficult to observe the distinct QD shape and

size on an atomic-length scale. We believe that one possible

reason for the difficulty of the distinct observation of QD structure

is the nature of the indistinct boundary between InAs and GaAs

due to the interdiffusion of In and Ga.

The QDs have flat apexes and their height and base diameter

were $4.3 and $20 nm, respectively. Although QD shape and size

were observed more clearly compared with the previous observa-

tions such as BF and HREM images, it was very difficult to observe

the distinct QD shape and size on an atomic-length scale.

We believe that one possible reason for the difficulty of the

distinct observation of QD structure is the nature of the indistinct

boundary between InAs and GaAs due to the interdiffusion of

In and Ga.The QDs with 9 nm thick spacer layers were grown vertically

aligned. Fig. 5 shows the BF image of aligned QDs (a) and the

HREM image of QDs from the first to the fourth period (b). It was

reported that the driving force of vertically aligned growth is the

interacting strain fields induced by the under period QDs which

give rise to a preferred migration of In adatoms [16]. Therefore,

the QDs are grown vertically aligned due to the strain fields

induced by under period QDs when the thickness of spacer layers

is within the range of the strain fields. The QD height and

diameter were measured as $5 and $20 nm, respectively. The QD

height and diameter increased slightly than those of uncapped

QDs. Considering the previous observations of the uncapped and

capped QDs, we confirm that the size of capped QDs is about

10–20% greater than that of uncapped QDs in TEM observations

due to strain fields and In diffusion.

The BF TEM and HREM also revealed that aligned QDs have a

round apex which differs from the flattened apex of nonaligned

QDs. It is believed that the round apex of uncapped QDs was

maintained because of the relatively low chemical potential

gradient of In compared with that of nonaligned QDs. Fig. 6

shows the schematic illustrations of the structures of nonaligned

and aligned QDs. The nonaligned QDs have flat apexes due to In

diffusion. After QD formation, compressive strain is induced to the

capped QDs from GaAs overlayers and their surface energy

increases by depositing GaAs [6]. Therefore, the In atoms in QD

apexes become unstable and their diffusion results in the apex

flattening as shown in Fig. 6(a). However, the aligned QDs are

ARTICLE IN PRESS

Fig. 5. Cross-sectional BF image of vertically aligned InAs QDs with 9 nm thick

GaAs spacers in g 004 two beam condition (a) and the [110] HREM image of aligned

QDs from the first to the fourth period (b).

Fig. 6. Schematic illustrations of the structures of vertically nonaligned (a) and aligned QDs (b).

H.S. Kim et al. / Journal of Crystal Growth 311 (2009) 258–262 261



more stable than the nonaligned QDs because the strain fields

caused by under period QDs reduce the misfit strain between InAs

and GaAs. It has been demonstrated that the compressive strain

in aligned QDs is significantly lower than that in single-layer or

nonaligned QDs [17]. In addition, it is confirmed that the chemical

potential gradient of In is relatively low due to the adjacent InAs

layers. Therefore, the nonaligned QDs flattened but the aligned

QDs did not flatten after spacer layer growth.

4. Conclusion

Vertically nonaligned and aligned InAs QDs were grown by

adjusting the thickness of GaAs spacer layers. The apexes of

nonaligned QDs were flattened by In diffusion after capping with a

spacer layer, although the uncapped QDs were lens-shaped

structures with round apexes. However, the aligned QDs main-

tained their lens-shaped structure after capping. Efficient adjust-

ing of the thickness of spacer layers may be a crucial factor for

controlling the structure of multilayer stacked QDs.

Acknowledgements

This work was supported in part by the Ministry of Science and

Technology through the National Research Laboratory on Quan-

tum-dot Technology at the Korea Research Institute of Standards

and Science (M1-0104-00-0127). The work in KIST was supported

in part by Nano R&D project by MOCIE ROK and QC project by

MOST ROK. One of authors (S.K. Noh) acknowledges the partial

support provided by the Korea Science and Engineering Founda-

tion through the Quantum-functional Semiconductor Research

Center at Dongguk University.

References

[1] D.J. Eaglesham, Phys. Rev. Lett. 64 (1990) 1943.

[2] I.N. Kaiander, R.L. Sellin, T. Kettler, N.N. Ledentsov, D. Bimberg, N.D. Zakharov,P. Werner, Appl. Phys. Lett. 84 (2004) 2992.[3] J. Phillips, K. Kamath, P. Bhattacharya, Appl. Phys. Lett. 72 (1998) 2020.[4] K. Yano, T. Ishii, T. Hashimoto, T. Kobayashi, F. Murai, K. Seki, IEEE Trans.

Electron Devices 41 (1994) 1628.[5] V. Tokranov, M. Yakimov, A. Katsnelson, M. Lamberti, S. Oktyabrsky, Appl.

Phys. Lett. 83 (2003) 833.[6] R. Songmuang, S. Kiravittaya, O.G. Schmidt, J. Crystal Growth 249 (2003) 416.[7] A. Lenz, H. Eisele, R. Timm, S.K. Becker, R.L. Sellin, U.W. Pohl, D. Bimberg,

M. Dahne, Appl. Phys. Lett. 84 (2004) 2992.[8] N.P. Kobayashi, T.R. Ramachandran, P. Chen, A. Madhukar, Appl. Phys. Lett. 68

(1996) 3299.[9] B. Basnar, H. Hiener, E. Gornic, G. Strasser, J. Crystal Growth 264 (2004) 26.

[10] Q. Gong, J.B. Liang, B. Xu, D. Ding, H.X. Li, C. Jiang, W. Zhou, F.Q. Liu, Z.G. Wang,X.H. Qiu, G.Y. Shang, C.L. Bai, J. Crystal Growth 192 (1998) 376.

[11] Shen-De Chen, Chiou-Yun Tsai, Si-Chen Lee, J. Nanopart. Res. 6 (2004) 407.[12] K. Jacobi, Prog. Surf. Sci. 71 (2003) 185.[13] H. Heidemeyer, S. Kiravittaya, C. Muller, N.Y. Jin-Phillipp, Appl. Phys. Lett. 80

(2002) 1544.

[14] N.N. Ledentsov, V.A. Shchukin, M. Grundmann, N. Kirstaedter, J. Bohrer,O. Schmidt, D. Bimberg, V.M. Ustinov, A.Y. Egorov, A.E. Zhukov, P.S. Kop’ev,S.V. Zaitsev, N.Y. Gordeev, Z.I. Alferov, A.I. Borovkov, A.O. Kosogov,S.S. Ruvimov, P. Werner, U. Gosele, J. Heydenreich, Phys. Rev. B 54 (1996)8743.

[15] X.G. Liao, J. Zou, X.F. Duan, D.J.H. Cockayne, R. Leon, C. Lobo, Phys. Rev. B 58(1998) R4235.

[16] Quianghua Xie, Anupam Madhukar, Ping Chen, Nobuhiko P. Kobayashi, Phys.Rev. Lett. 75 (1995) 2542.

[17] N.Y. Jin-Phillipp, F. Phillipp, J. Appl. Phys. 88 (2000) 710.

ARTICLE IN PRESS

H.S. Kim et al. / Journal of Crystal Growth 311 (2009) 258–262262

effects of the thickness of gaas spacer layers on the structure of multi layer

Documents