Modified growth of Ge quantum dots using C2H4 mediation by ultra-high vacuum chemical vapor deposition

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Modiﬁed growth of Ge quantum dots using C

2

H

4

mediation by

ultra-high vacuum chemical vapor deposition

S.W. Lee

a,

*

, P.S. Chen

b

, S.L. Cheng

a,c

, M.H. Lee

d

, H.T. Chang

a

, C.-H. Lee

e

, C.W. Liu

e

a

Institute of Materials Science and Engineering, National Central University, Jhong-Li 32001, Taiwan, ROC

b

Department of Materials Science and Engineering, Minghsin University of Science and Technology, Hsinchu 30401, Taiwan, ROC

c

Department of Chemical and Materials Engineering, National Central University, Jhong-Li 32001, Taiwan, ROC

d

Institute of Electro-optical Science and Technology, National Taiwan Normal University, Taipei 11677, Taiwan, ROC

e

Department of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC

1. Introduction

Recently, the growth of self-assembled Ge quantum dots (QDs) on Si substrates has been intensively investigated for the next generation of Si-based optoelectronic devices[1–3]. Deposition of Ge onto Si(0 0 1) leads to a strain-induced spontaneous formation of the three-dimensional islands as soon as the Ge epilayer exceeds a critical thickness of 3–4 monolayers, which is the well-known Stranski–Krastanow (S–K) growth mode[4,5]. The self-formation of Ge QDs is considered to be energetically favorable since the gain of strain energy due to the partial strain relaxation overcompen-sates the increased surface energy. However, thermodynamics and kinetics impose certain restrictions on the growth and geometrical dimensions of those Ge QDs. For example, at low growth temperatures, low-adatom mobilities kinetically limit the QDs formation and then result in a thick wetting layer or a broader dot-size distribution within the Ge dot ensemble[6,7]. To circumvent these restrictions, several approaches, such as selective epitaxial

growth [8] and thermal annealing [9], have been reported.

Recently, Schmidt et al. demonstrated that a very small amount of pre-deposited C onto the Si substrate induces very small Ge dots with enhanced light-output below the critical thickness [10]. However, the size uniformity of those C-induced Ge dots remains a critical issue [11]. It is well known that growth kinetics of the epilayer is controlled by hydrogen desorption in the epitaxial growth front under ultra-high vacuum chemical vapor deposition (UHV/CVD) condition[12]. In the present study, we use ethylene

(C2H4) mediation to modify the growth mode of Ge dots at

relatively low growth temperature in an UHV/CVD system. The results demonstrate that the elongated Ge hut clusters can be transformed to highly uniform Ge domes with a high Ge composition by a modiﬁed dot-formation process.

2. Experimental procedures

10–25

V

cm, 100 mm diameter p-type (0 0 1)-oriented Si

wafers were used in the present study. All the Ge QDs investigated in this work were grown at 550 8C in a commercially available multi-wafer UHV/CVD system. Pure SiH4, 2% C2H4, and 5% GeH4

diluted in He were used as precursors. The Si wafers were dipped in a 10% HF solution to achieve the hydrogen-passivated surface, and then transferred into an UHV/CVD system. A 60-nm-thick Si buffer layer was ﬁrst grown to cover the wafer surface. After depositing

Applied Surface Science 254 (2008) 6261–6264

A R T I C L E I N F O Article history:

Available online 22 March 2008 PACS: 68.65Hb, 68.65.Ac, 68.37.Ps, 61.72.Ff Keywords: Ge Self-assembled Quantum dot UHV/CVD A B S T R A C T

C2H4mediations were used to modify the Stranski–Krastanow growth mode of Ge dots on Si(0 0 1) at

550 8C by ultra-high vacuum chemical vapor deposition. With appropriate C2H4-mediation to modify the

Si surface, the elongated Ge hut clusters can be transformed to highly uniform Ge domes with a high Ge composition at the core. These C2H4-mediated Ge dots, almost bounded by {1 1 3} facets, have an average

diameter and height of 55 and 9 nm, respectively. We propose two major mechanisms to depict the formation of these C2H4-mediated Ge dots: (i) an almost hydrogen-passivated Si surface to limit the

nucleation sites for dot formation, and (ii) the incorporation of Ge atoms, repelled by the C-rich areas, into the existing Ge dots. This work provides a useful scheme to tune the topography of Ge dots in an UHV/ CVD condition for possible optoelectronic applications.

* Corresponding author at: Institute of Materials Science and Engineering, National Central University, No. 300, Jhongda Road, Jhong-li City, Taoyuan County 32001, Taiwan, ROC. Tel.: +886 3 4227151x34905; fax: +886 3 2805034.

E-mail address:swlee@ncu.edu.tw(S.W. Lee).

C o n t e n t s l i s t s a v a i l a b l e a tS c i e n c e D i r e c t

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the Si buffer, Si surface was then pre-mediated by 10-sccm C2H4

with different durations prior to Ge dot growth. Subsequently, a 6.8 eq-MLs Ge layer was deposited to form the self-assembled Ge QDs on those C2H4-mediated Si surfaces. Note that the amount of

Ge deposition is expressed in the unit of equivalent-monolayers (eq-MLs, 1 eq-ML = 6.27 1014_{Ge atom/cm}2₎_[13]_.

The Ge dot shape and size distribution were studied ex situ by atomic force microscopy (AFM) in tapping mode. Both plan view and cross-section transmission electron microscopy (XTEM) images were used to provide detailed information about facets and microstructures of the Ge dots. TEM in conjunction with an energy dispersion spectrometer (EDS) was utilized to determine the composition of Ge dots. For the EDS analysis the electron beam can be converged to a size as small as 1.5 nm.

3. Results and discussion

3.1. Topography by atomic force microscopy

Fig. 1illustrates the topographic evolution of 6.8 eq-MLs Ge dots grown at 550 8C with different C2H4-mediated duration. As

shown inFig. 1(a), the Ge dots are predominantly elongated huts accompanying a few pyramid-like islands, which are typically observed at lower growth temperatures. These huts elongated along the two orthogonal h1 1 0i directions were found to have the base width of 10–95 nm with an average height of 2.8 nm. Although with a density up to 1 1011cm2_{, these hut islands}

exhibit the large size ﬂuctuations, which limit their practical applications. However, with C2H4mediation for 15 s, the resulting

Ge QDs were found to change their shapes drastically from huts to multi-faceted domes accompanying some superdomes, indicated by arrowheads in theFig. 1(b). These superdomes may be caused by the agglomerations of neighbor Ge islands, which nucleated simultaneously in a larger C-free region. However, with increasing duration of C2H4-mediation to 1 min [Fig. 1(c)], superdomes have

vanished completely and multi-faceted domes with a narrow size distribution remained. In addition, the density of these C2H4

-mediated Ge dots was found to be about 5.4 109_cm2_{, higher}

than that corresponding toFig. 1(b). Those larger C-free regions in

Fig. 1(b), with increasing duration of C2H4-mediation, are likely to

be separated into parts by more C-containing Si surfaces. Consequently, more and smaller C-free regions remain. Therefore, the agglomerations of neighbor Ge islands into superdomes are expected to be suppressed. On the other hand, more nucleation sites for Ge dot formation are provided by those larger C-free regions, resulting in an increase of Ge dot density. However, further C2H4-mediation for 10 min [Fig. 1(d)], these C2H4-mediated Ge

QDs shrunk and island uniformity degraded. In Fig. 2, we

summarize the dependence on C2H4-mediated duration of both

the Ge dot density and size obtained from AFM images. It indicates that, with appropriate C2H4-mediation (1 min in this case) to

modify the Si surface, the growth of highly uniform Ge QDs with an acceptable dot density can be achieved.

3.2. Microstructures of C2H4-mediated Ge QDs

Fig. 3shows the XTEM images of two typical C2H4-mediated Ge

dots, which formed in the condition corresponding to that of

Fig. 1. AFM images (1mm 1mm) of 6.8 eq-MLs Ge QDs on Si(0 0 1) grown at 550 8C (a) without and with the 10-sccm C2H4mediation for (b) 15 s, (c) 1 min, (d) 10 min prior

to Ge growth.

S.W. Lee et al. / Applied Surface Science 254 (2008) 6261–6264 6262

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Fig. 1(b) and (c), respectively. As shown inFig. 3(a), some threading dislocations were found to form in the 15-s-C2H4-mediation Ge

superdome. It is possibly caused by the simultaneous nucleations and subsequent agglomeration of neighbor Ge islands conﬁned at a limited region. With increasing duration of C2H4-mediation to

1 min, no defects such as threading dislocations were observed inside the Ge islands. These defect-free C2H4-mediated dots have

an average diameter and height of 55 and 9 nm, respectively, yielding an aspect ratio about 3.2 times to that of Ge hut islands. Unlike the larger multi-faceted Ge domes observed at 600 8C in a previous study [14], these C2H4-mediated Ge dots are almost

bounded by {1 1 3} facets with an inclination of 25.28,

accom-panying small {1 0 5} facets with an inclination of 11.38 at the apex. In addition, EDS measurement data indicate that the average Ge composition at the core of C2H4-mediated dots is about 83%, much

higher than 63% of that of Ge domes grown at 600 8C. These XTEM and AFM observations imply that the original S–K growth mode of Ge islands on Si(0 0 1) has been modiﬁed by C2H4mediations prior

to Ge growth.

3.3. Mechanism for C2H4-mediated dot formation

In this case, due to the stronger C–H bonds than Si–H ones, C2H4-mediation prior to Ge growth produces a C-containing Si

surface with almost full monohydride coverage [15]. This

C-containing Si surface, having a lower rate of hydrogen desorption, leads to a reduction of sticking coefﬁcient of GeH4on Si surfaces

and then impedes the subsequent deposition of Ge atoms. Therefore, the nucleation sites for the Ge QDs were mostly conﬁned at Si-rich or C-free regions. As can be expected, with the appropriate conditions of pretreatment of Si surface with C2H4, the

growth of highly uniform Ge QDs can be achieved, as shown in

Fig. 1(c).

Considering the much larger volumes of C2H4-mediated Ge dots

than those of Ge huts, there should be another dot-formation mechanism to supply the increased amount of Ge atoms. It has been reported that the formation of Ge–C bond in the SiGeC material system would involve severe strain because of a difference in bond length as high as 37% [16]. Therefore, the difﬁculty to form Ge–C bonds drives the Ge atoms to migrate from the C-enriched surface regions to the existing Ge islands, where the lattice mismatch is the smallest. Consequently, the 3D growth of C2H4-mediated islands continues with the incorporation of Ge

atoms repelled by the C-rich areas. This assumption is also consistent with our TEM observations that C2H4-mediated dots

have an extremely high Ge content. Therefore, as illustrated in

Fig. 4, two mechanisms can be concluded to depict the formation of C2H4-mediated Ge dots: (1) an almost hydrogen-passivated Si

surface to limit the nucleation sites for dot formation; (2) the incorporation of Ge atoms, repelled by the C-rich areas, to enlarge the existing Ge QDs.

It is worthwhile to note that the submonolayer C coverage on Si surfaces in the present study plays a different role in the dot

formation compared to Schmidt’s work [10], in which the

pre-deposited C atoms act as nucleation centers for the Ge dot formation. Over the past years, extensive works have been carried out to investigate the surfactant-meditated growth of Ge QDs. For

Fig. 2. Dot density and average size of the Ge dots vs the duration of C2H4

-mediation. Both are obtained from the AFM measurements.

Fig. 3. XTEM images of two typical C2H4-mediated Ge dots, which correspond to Fig. 1(b) and (c), respectively. The D marked inside the Ge dot refers to a threading dislocation.

Fig. 4. A schematic view illustrating two mechanisms for the formation of C2H4

-mediated Ge dots: (i) an almost hydrogen-passivated Si surface and (ii) the repulsive interaction of C and Ge atoms.

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comparison, the methodologies and their dot-formation mechan-isms are summarized inTable 1The present study, in particular, proposed the mechanism for C2H4-mediated Ge dot formation in

the UHV/CVD system.

4. Summary and conclusions

In summary, modiﬁed growth of self-forming Ge dots on Si(0 0 1) by C2H4-mediation at 550 8C was investigated and it was

found that growth of self-assembled Ge domes with a narrow size distribution can be achieved with appropriate C2H4-mediation to

modify the Si surface. Two major mechanisms are proposed to

depict this unique growth mode for the formation of C2H4

-mediated Ge dots: (i) an almost hydrogen-passivated Si surface to limit the nucleation sites for dot formation, and (ii) the

incorporation of Ge atoms, repelled by the C-rich areas, into the existing Ge dots. This work provides a useful pathway to tune the topography of the Ge QDs in an UHV/CVD condition for potential optoelectronic applications.

Acknowledgements

The research was supported by the National Science Council through a grant No. NSC 97-2218-E-008-003 and the National Nano Device Laboratories.

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Comparison of dot-formation mechanisms for the different surfactant-meditated growth of Ge QDs

Authors Surfactant System Dot-formation mechanism Schmidt et al.a _C _SSMBEd _{C atoms act as nucleation}

center for dot formation Portavoce et al.b

Sb SSMBE Sb atoms suppress the

surface diffusion and reduce the dot size Chen et al.c

B UHVCVD B atoms weaken H-dangling

bonds and provide more nucleation sites Present study C UHVCVD C atoms limit the nucleation

sites for dot formation

a_Reference_[10]_. b Reference[17]. c Reference[18]. d

Solid-source molecular beam epitaxy.

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