Journal of Crystal Growth 234 (2002) 105–109

Growth of Si-doped InAs quantum dots and annealing effectson size distribution

Jin Soo Kima, Phil Won Yua, Jae-Young Leemb,*, Joo In Leec, Sam Kyu Nohc,Jong Su Kimc, Gu Hyun Kimc, Se-Kyung Kangc, Seung Il Banc, Song Gang Kimd,

Yu Dong Jange, Uk Hyun Leee, Jung Soon Yime, Donghan Leee

aDepartment of Information and Communications, Kwangju Institute of Science and Technology (K-JIST),

Kwangju 500-712, South KoreabDepartment of Optical Engineering, Inje University, Kimhae 621-749, South Korea

cMaterials Evaluation Center, Korea Research Institute of Standards and Science, Taejon 305-340, South KoreadDepartment of Information and Telecommunication, Joongbu University, Gumsan-Gun 132-940, South Korea

eDepartment of Physics, Chungnam National University, Taejon 305-764, South Korea

Received 1 August 2001; accepted 14 August 2001

Communicated by M. Schieber

Abstract

We investigated the Si-doped InAs quantum dots (QDs) grown by molecular beam epitaxy and the annealing effectson the QD size distribution through photoluminescence (PL) spectroscopy. A double-peak feature in PL was observedfrom as-grown InAs QDs with Si-doping, and excitation intensity dependence of PL indicated that the double-peak

feature is related to the ground-state emission from InAs QDs with bimodal size distribution. The PL spectrum fromSi-doped InAs QDs subjected to annealing treatment at 8001C in nitrogen ambient showed three additional PL peaksand blue-shift of the double-peak feature observed from as-grown sample. The excitation-intensity-dependent PL

and consideration of thermal stability of carriers through temperature-dependent PL measurement demonstratedthat three additional peaks come from the InAs QDs with three new branches of QD occurring during annealingprocess. r 2002 Elsevier Science B.V. All rights reserved.

PACS: 78.66.Fd; 78.67.Hc; 81.05.Ea; 81.15.Hi

Keywords: A1. Optical microscopy; A1. Quantum dots; A3. Molecular beam epitaxy; B2. Semiconducting III–V materials

1. Introduction

In the past several years, self-assembled quan-tum dots (QDs) formed by the Stranski–Krasta-nov growth mode have been reported in highlystrained systems such as In(Ga)As/GaAs [1–3],

*Corresponding author. Materials Evaluation Center, Korea

Research Institute of Standards and Science (KRISS), 1

Toryong-dong, Yusong-gu, Taejon 305-340, South Korea.

Tel.: +82-55-320-3716; fax: +82-55-320-3631.

E-mail address: jyleem@ijnc.inje.ac.kr (J.-Y. Leem).

0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 6 6 5 - 7

AlInAs/AlGaAs [4,5], and GaSb/GaAs [6]. Theprevious investigations predicted that these quasizero-dimensional quantum structures have pro-vided the explanation of unique and interestingphysical properties [7,8]. It is also expected torealize the improved device performance such aspromising low-threshold current density, highdifferential gain and high-temperature stabilitydue to the d-function-like character of the jointdensity of state in QDs [9,10]. However, theiroverall performance in devices has remainedinferior to that of the quantum wells (QWs),mainly because of the size fluctuations in QDs[11–13]. In order to control the formation of QDs,lots of research efforts have been made by varyingthe growth parameters such as growth tempera-ture, III/V ratio, and growth rate [14,15]. Therehave also been a few reports on the doping effectson the formation of QDs, that is, bimodaldistribution in QD size [16–18]. However, theeffects of post-growth annealing on the QD sizedistribution for intentionally impurity-doped QDshave not been discussed yet.

In the present work, we studied Si-doped InAsQDs grown by molecular beam epitaxy (MBE),and the effects of post-growth annealing treatmenton the QD size distribution through excitationintensity and temperature-dependent photo-luminescence (PL) spectroscopy. A double-peakfeature in PL from as-grown InAs QDs withSi-doping was observed and excitation-intensity-dependent PL measurement indicated that thesetwo peaks were related to the ground-stateemission from two different QD branches. Afterannealing treatment on the Si-doped QD sample,the double-peak feature in PL of the as-grownsample was blue-shifted and three additional peakswere also observed. In order to identify theseadditional peaks from InAs QDs subjected toannealing treatment, the excitation intensity andtemperature-dependent PL measurement werecarried out.

2. Experimental procedure

The samples used in the present work weregrown by a Riber 32P MBE on (1 0 0) semi-

insulating GaAs substrates. The substrate tem-perature was set to 5801C for the growth of GaAsbuffer layer and then, the substrate temperaturewas lowered to 4401C for the deposition of theInAs QD layer. The substrate temperature wasmeasured by an optical pyrometer, which wascalibrated using the substrate surface oxide deso-rption temperature. Si is continuously suppliedduring the growth of InAs QDs layer with aconcentration of 2� 1017 cm�3. InAs QD layerwas grown at a rate of 0.07ML s�1 and theformation of InAs QD was verified by theobservation of the 2D–3D transition by in situreflection high-energy electron diffraction(RHEED) pattern, after 1.7ML deposition ofInAs. An undoped 25 nm GaAs cap layer wasgrown after InAs QD deposition followed by 30 sgrowth interruption time under as-rich condition.The undoped InAs QDs sample was also grown asa reference.

In PL measurement, an Argon ion laser with awavelength of 514.5 nm was used, as an excitationsource to generate electron–hole pairs. The PLsignal was dispersed by a 1m monochromator anddetected with a liquid-nitrogen-cooled Ge detector.

The rapid thermal annealing (RTA) process wasperformed in nitrogen ambient at a temperature of8001C for 30 s. Before RTA treatment, the samplecut from the central region of the wafer wascapped with bulk GaAs for As not to remove fromthe surface.

3. Results and discussion

Fig. 1 shows PL spectra from (a) the referencesample and (b) Si-doped InAs QD samplemeasured at 10K. The dip at 1.093 eV (1133 nm)in PL spectra is due to the water absorption. ThePL signals for both samples are very strong even atlow excitation intensity indicating the efficientcapture of carriers into QDs, even though nointended confining layers for carrier were intro-duced. Any signal from the wetting layers andGaAs layer is not clearly shown in the PL spectra.In Fig. 1(a) for the reference sample at lowexcitation intensity, only one peak is observed at1.108 eV due to ground-state emission and the

J.S. Kim et al. / Journal of Crystal Growth 234 (2002) 105–109106

additional peaks are observed with increase in theexcitation intensity implying that the peaks resultfrom the excited-state emission, which is oftenobserved in the previous reports [7,12]. This is adistinctive property of QDs and is due to thenature of the carrier capture, recombination andstate-filling effect. In Fig. 1(b), a double-peakfeature from Si-doped InAs QDs is observed.The PL intensities of two peaks linearly increasedepending on the excitation intensity and theirrelative intensities are kept similar but independentof each other at different powers of excitation. The

double-peak feature can be also seen distinctivelyeven at very low excitation intensity. From theseresults, the double-peak feature in PL correspondsto the ground-state emission from InAs QDs withtwo different branches. If we consider the confine-ment effects, the high-energy peak and low-energyone are associated with the small QD branch andlarge QD branch, respectively. By using twoGaussian curves (solid lines in the inset ofFig. 1(b)) to fit the measured spectrum at anexcitation intensity of 20mW, two overlappingspectra are separated and the peak position andfull-width at half-maximum (FWHM) can bedetermined accurately. The energy positions(FWHM) for peaks A and B in the inset ofFig. 1(b) are 1.063 eV (55meV) and 1.119 eV(116meV), respectively.

Fig. 2 shows the PL spectrum from Si-dopedInAs QDs subjected to annealing treatment at8001C at an excitation intensity of 20mW, and thedouble-peak feature in PL of as-grown InAs QDsis blue-shifted mainly due to the inter-diffusion ofIn and Ga atoms at the interface between the InAsQDs and GaAs resulting in a change in the sizeand the composition of the QDs during annealing[19,20]. The peak separation between peak A0 andpeak B0 is 74meV, which is larger than that forthe as-grown sample of 56meV. This indirectlydemonstrates that the double-peak feature in PL

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

1 mW (x 2)

100 mW

70 mW

30 mW

20 mW

10 mW

PL

Int

ensi

ty (a

rb. u

nits

)

Photon Energy (eV)

Peak B

Peak A

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

100 mW

50 mW

1 mW (x 7)

10 mW (x 2)20 mW

PL

Int

ensi

ty (

arb.

uni

ts)

Photon Energy (eV)(a)

(b)

Fig. 1. PL spectra from (a) the reference sample and (b) Si-

doped InAs QDs measured at 10K with different excitation

intensities. The inset of (b) shows the Gaussian fitting curves

(solid lines) to the measured spectrum (filled circles) at an

excitation intensity of 20mW.

0.9 1.0 1.1 1.2 1.3 1.4 1.5

WL

Peak A’

Peak B’

PL

Int

ensi

ty (

arb.

uni

ts)

Photon Energy (eV)

Exp. Data Fit. Curves

Fig. 2. PL spectrum from Si-doped InAs QDs subjected to

annealing treatment at 8001C. Three additional peaks between

a double-peak feature and WL signal are observed.

J.S. Kim et al. / Journal of Crystal Growth 234 (2002) 105–109 107

of the as-grown sample comes from two differentQD branches. When the QD size is different, thechange in its shape and size caused by inter-diffusion of In and Ga could be different. TheFWHM for peaks A0 and B0 from annealed InAsQDs sample are 43 and 90meV, respectively,indicating that the uniformity of each dot branchis increased by annealing treatment.

Three additional peaks between the double-peakfeature and wetting layer (WL) signal are observedin Fig. 2, whose intensities are not strong but wellresolved. The energy positions of these three peaksare 1.307, 1.363, and 1.417 eV, respectively. Inorder to identify the origin of these peaks, theexcitation intensity and temperature dependenceof PL spectra were taken into account.

Fig. 3 shows the PL intensities from annealedsample with different excitation intensities andintensities of peaks A0 and B0 (not shown), andthree additional peaks are enhanced with increasein the excitation intensity. The intensity of eachpeak is linearly and independently increased, justas in the case of the as-grown sample. Three weakPL signals are also observed even at very lowexcitation intensity. When the dot size is smallenough for only ground states of electrons to exist,

the transition between the ground states ofelectrons and holes become a dominant recombi-nation process. The emission peaks with transitionbetween the ground states of electrons and theexcited states of holes can be also observed at highexcitation intensity because of non-orthogonalitydue to anisotropy of dot shape, however, theirintensities are fairly weak compared with that ofthe ground-state emission. In this work, theintensity of the additional peaks is almost thesame and their relative intensities with increase inexcitation intensity are also similar indicating thatthese peaks are not associated with the transitionsbetween ground states of electrons and excitedstates of holes.

Fig. 4 shows the temperature-dependent PLspectra at an excitation intensity of 100mW. Withincrease in the temperature, the peaks at high-energy side successively disappear. This can beascribed to the thermal stability of carriers relatedto the confinement caused by the QD size. Thecarrier at higher energy due to the higher confine-ment effects can be thermally activated easily thanthat at lower energy requiring high activationenergy.

From these results of excitation intensity andtemperature dependence of PL, we can make aconclusion that three additional peaks are strongly

1.25 1.30 1.35 1.40 1.45

WL

1 mW

10 mW

30 mW

50 mW

10 K

100 mW

PL

Int

ensi

ty (

arb.

uni

ts)

Photon Energy (eV)

Fig. 3. Excitation-intensity-dependent PL spectra from

Si-doped InAs QDs subjected to the RTA process. The intensity

of three additional peaks is enhanced with increase in the

excitation intensity. The intensity of each peak is linearly and

independently increased just as in the case of the as-grown

sample.

1.30 1.35 1.40 1.45

20 K

25 K

35 K

30 K

10 K

PL

Int

ensi

ty (

arb.

uni

ts)

Photon Energy (eV)

Fig. 4. Temperature-dependent PL spectra at an excitation

intensity of 100mW. With increase in the temperature, the

peaks from high-energy side successively disappear.

J.S. Kim et al. / Journal of Crystal Growth 234 (2002) 105–109108

correlated to the InAs QDs with three new QDbranches occurring during annealing. That is, theQDs originally included in the main branchesleading to broad line-width of PL before RTAprocess could be changed in dot shape and sizeduring annealing resulting in three new QDbranches.

4. Conclusions

Si-doped and undoped InAs QD samples weregrown and their optical properties were investi-gated by PL spectroscopy. A double-peak featurewas observed from Si-doped InAs QD sample atvery low excitation intensity, while there was onlyone emission peak from the reference InAs QDsample. The excitation intensity dependence of PLspectra showed that the double-peak feature fromSi-doped InAs QDs corresponds to the ground-state emission from InAs QDs with two differentsize branches. After RTA process, the double-peakfeature was blue-shifted and three additional peakswere newly observed. From the excitation intensityand temperature dependence of PL spectra, it canbe demonstrated that these peaks come from theInAs QDs with three different branches of QDoccurring during annealing process.

References

[1] J.S. Kim, P.W. Yu, J.Y. Leem, J.I. Lee, S.K. Noh, Jong Su

Kim, S.M. Kim, J.S. Son, U.H. Lee, J.S. Yim, D. Lee,

Appl. Phys. Lett. 21 (2001) 3247.

[2] R. Leon, Y. Kim, C. Jagadish, M. Gal, J. Zou, D.J.H.

Cockayne, Appl. Phys. Lett. 69 (1996) 1888.

[3] J.M. Garcia, T. Mankad, P.O. Holtz, P.J. Wellman,

P.M. Petroff, Appl. Phys. Lett. 72 (1998) 3172.

[4] B.L. Liu, Z.Y. Xu, H.Y. Liu, Z.G. Wang, J. Crystal

Growth 220 (2000) 51.

[5] J. Phillips, P. Bhattacharya, U. Venkateswaran, Appl.

Phys. Lett. 74 (1999) 1549.

[6] F. Hatami, N.N. Ledenstov, M. Grundmann, J. Bohrer,

Z.I. Alferov, Appl. Phys. Lett. 67 (1995) 656.

[7] J.I. Lee, H.G. Lee, E.J. Shin, S. Yu, D. Kim, G. Ihm,

Appl. Phys. Lett. 70 (1997) 2885.

[8] S. Farad, K. Hinzer, S. Raymond, M. Dion, J. McCaffrey,

Y. Feng, S. Charbonneau, Science 274 (1996) 1350.

[9] Y. Arakawa, H. Sakaki, Appl. Phys. Lett. 40 (1982) 939.

[10] K. Nishi, H. Saito, S. Sugou, J.-S. Lee, Appl. Phys. Lett.

74 (1999) 1111.

[11] F. Heinrichsdorff, M.H. Mao, N. Kirstaedter, A. Krost,

D. Bimberg, A.O. Kosogov, P. Werner, Appl. Phys. Lett.

71 (1997) 22.

[12] M. Grundmann, N.N. Ledenstov, O. Stier, D. Bimberg,

V.M. Ustinov, P.S. Kop’ez, Zh. Alferov, Appl. Phys. Lett.

68 (1996) 979.

[13] U.H. Lee, D. Lee, H.G. Lee, S.K. Noh, J.Y. Leem, H.J.

Lee, Appl. Phys. Lett. 74 (1999) 1597.

[14] H. Kissel, U. M .uller, C. Walther, W.T. Masselink, Yu.I.

Mazur, G.G. Tarasov, M.P. Lisitsa, Phys. Rev. B 62 (2000)

7213.

[15] L. Chu, M. Arzberger, G. B .ohm, G. Abstreiter, J. Appl.

Phys. 85 (1999) 2355.

[16] H.L. Wang, D. Ning, S.L. Feng, J. Crystal Growth 209

(2000) 630.

[17] Y.C. Zhang, C.J. Huang, F.Q. Liu, B. Xu, D. Ding,

W.H. Jiang, Y.F. Li, X.L. Ye, J. Wu, Y.H. Chen, Z.G.

Wang, J. Crystal Growth 219 (2000) 199.

[18] Z. Qian, F. Songlin, N. Dong, Z. Haijun, W. Zhiming, D.

Yuanming, J. Crystal Growth 200 (1999) 603.

[19] S.J. Xu, X.C. Wang, S.J. Chua, C.H. Wang, W.J. Fan,

J. Jiang, X.G. Xie, Appl. Phys. Lett. 72 (1998) 3335.

[20] S. Malik, C. Roberts, R. Murray, M. Pate, Appl. Phys.

Lett. 71 (1997) 1987.

J.S. Kim et al. / Journal of Crystal Growth 234 (2002) 105–109 109