Controllable Growth of Ni Nanocrystals Embedded in BaTiO<sub>3</sub>/SrTiO<sub>3</sub> Superlattices

doi:10.4236/mnsms.2013.31B002

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Modeling and Numerical Simulation of Material Science, 2013, 3, 4-8

Published Online January 2013 (http://www.SciRP.org/journal/mnsms)

Controllable Growth of Ni Nanocrystals Embedded in

BaTiO3/SrTiO3 Superlattices

Zhengwei Xiong, Weidong Wu

Science and Technology on Plasma Physics Laboratory Research Center of Laser Fusion, China Academy of Engineering Physics

Mianyan, China

Email: [email protected]

Received 2012

ABSTRACT

BaT i O3/SrTiO3 superlattices with embedded Ni nanocrystals (NCs) have been grown on SrTiO3 (001) substrate using

laser molecular beam epitaxy (L-MBE). In situ reﬂection high-energy electron diffraction (RHEED) was employed to

investigate the process of lattice strain in the self-organization of Ni NCs and the epitaxial growth of BaTiO3/SrTiO3

superlattices. The results indicated that the strain from large lattice mismatch drove the self-organization of Ni NCs.

Als o , the layer-by-layer growth of BaTiO3 /SrTiO3 superlattices and the island growth of Ni NCs were controllable ac-

curately. The fine alternation of the two processes would provide a possible route to engineer controllably the nano-

composite microstructure.

Keywords: Nanocrystal; Superlattices; Self-organization

1. Introduction

Oxide artificial superlattices, especially (001) oriented

BaT i O3/SrTiO3 superlattices (BTO/STO SLs), have been

attracted more attention because their dielectricand fer-

roelectric properties can be enhanced by controlling the

lattice strain along the polarized direction [1-3]. The di-

electric matrix with the embedded metal NCs has widely

potential application in nonlinear optical and electronic

device [4, 5]. Therefore, several methods have been con-

sidered to fabricate the BTO and STO-based material,

including sol-gel [6], reactive evaporation [7],

rf-sputtering [8] and laser ablation methods [9]. However,

the controllable fabrication of nanostructure remains the

daunting challenge for many deposition methods. There

is an excellent method referred to as self-or ga nized

growt h, in which the strain would drive the

three -dimensional (3D) island to form in the lattice mis-

matched growth process [10, 11]. Especially for the fa-

brication of quantum dot (QD) structures, the

self-or ga nized g rowth is greatly succe ss ful in semicon-

ductor devices, such as InGaAs on GaAs [10], AlN on

GaN [12]. Wu et al. have successfully fabricated

Co:BTO and Ni:BTO composite film using

self-organized method [13-15].

In this paper, the laser molecular beam epitaxy was

used to embed the Ni NCs in the epitaxial films of Ba-

TiO3/SrTiO3 superlattices (Ni NCs:BTO/STO SLs). The

fabrication of the nanocomposite system is interesting

and significant for both fundamental and application as-

pects. Such composite films offer the combination of

ferroelectric and ferromagnetic characteristics. This

magnetoelectric interaction will be particularly strong in

systems which simultaneously exhibit both ferromagnet-

ism and ferroelectricity, i.e., in magnetoelectric multi-

ferroics [16-18].

2. Experimental

The Ni NCs:BTO/STO SLs films were grown on STO

(001) substrate with STO buffer layer by L-MBE. The

acceleration voltage of RHEED electron gun was 25kev

and the grazing incidence angle was about 1-3°. The

growth process was performed in an ultrahigh -vac uu m

(UHV) system and the background vacuum was ～2

×10-6 Pa. The schematic diagram of Ni NCs:BTO/STO

SLs film was shown in Fig. 1. The experimental parame-

ters were listed in detail in Table 1. Before the deposition,

the substrate STO (001) wa s in situ annealed about 2

hours in order to remove surface contamination. At 10 Pa

oxygen pressure, the sample was annealed about 20 mi-

nutes after accomplishing the BTO layer. The samples

wer e characterized by XRD with Cu ka radiation and

high -resolution transmission electron microscopy

(HRTEM).

3. Results and discussion

Fig.2 sho ws the oscillation of the RHEED intensity and

Z. XIONG, W. WU

Figure 1. A schematic diagram of Ni NCs:BTO/STO SLs

film. One deposition cycle involves a number of pulses on

the Ni target in ultra-high vacuum, followed by the epitaxial

growth of BTO/STO SLs.

Table 1. Experimental parameters for Ni NCs:BTO/STO

SLs films fabrication

Background vacuum ~2×10-6 Pa

Working vacuum ~2×10-5 Pa

Target

BaTiO3 purity >99.99%

SrTiO3 purity >99.99%

Ni purity >99.99%

Substrate SrTiO3(001)

Laser pulse

frequency

1Hz for SrTiO3 dep os i tion

1Hz for BaTiO3 dep os ition

2Hz for Ni deposition

The distance between

the target and

substrate

5 cm

Annealing condition 650 ℃ 20 min

10Pa O2 press u re

diffraction patterns of BTO deposited on STO surface.

The diffraction sharp streaks indicate a smooth surface

and good crystalline quality [19]. Compared with the

unannealed STO substrate [Fig. 2(a)], the diffraction

streaks become more obvious after in situ anneal at

650℃ [Fig. 2(b)], indicated that the crystallization of

STO surface is improved. During the growth processes,

the characteristic streaks of BTO became more striking

shown in Fig.1(c) and Fig.1 (d). The prefect epitaxial

layers of BTO were formed on the STO s urface. In this

case, the intensity oscillations are obviously observed. It

is concluded that the deposited BTO layers are grown

with la yer -by-layer growth mode.

Figure 2. Variation of RHEED intensity oscillation and

patterns along the [100] azimuth of BTO on STO layer: (a)

unannealed STO substrate; (b) annealed STO surface; (c)

deposited 60s BTO; (d) deposited 120s BTO

Figure 3. Transition of RHEED patterns during the self-organized

embe dded Ni process: (a) BTO(001) surface; (b) 100 pulses Ni; (c)

500 pulses Ni; (d) 1000 pulses Ni

As indicated in Fig. 3, the self-o rga nize d Ni NC s along

the [100] azimuth were deposited the new surface of

BTO (001). It is obvious that the streaky patterns of BTO

disappeared gradual ly wit h the increasing Ni pulses,

while the spots of Ni NCs become more striking. Addi-

tionally, due to the large lattice mismatch between BTO

and Ni (>10%), that results in an overall decrease in

RHEED intensity. Thus, this case is considered as the

formation of 3D islands. The surface-lattice parameter,

which is proportional to the inverse of the distance be-

tween the different diffraction spots or streaks, can be

directly measured from the RHEED patterns [19]. By

measuring the vertical spacing and horizontal spacing for

the dominant Ni diffraction spots, the lattice constant of

Ni in cubic structure was c onf ir med to 0.354nm in real

space. This result approaches to the lattice constant of

bulk metal Ni (0.352 nm) in cubic structure [20]. There-

for e, it is demonstrated that the deposited Ni maintains

metallic property.

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Figure 4. Variation of RHEED intensity oscillation during STO

layer deposited on the Ni NCs. The insets are the RHEED patterns

recorded at 10s, 40s and 120s, respective ly.

After the completion of Ni NCs, the next layer is STO.

As shown in Fig. 4, at the initial stage of STO deposition,

the RHEED intensity decreases instantaneously because

of the increasing random distribution of the incoming

STO atoms on the surface. Then, the deposited STO

atoms and the formed clusters are rearranged on the sur-

face. That induces the increasing RHEED oscillated in-

tensity. For the insets Fig. 4(a)-(d), the dominant Ni spots

change to the characteristic streaks of STO with the in-

creasing STO pulses. Meanwhile, the RHEED intensity

oscillations are observed again. Those results demon-

strate that the STO layer covers the geometrically irregu-

lar surface provided self-orga nize d Ni NCs, and is gro w n

wit h layer-by-layer growth mode. The accurate thickness

of every individual layer is controlled by RHEED inten-

sity oscillation which is extremely sensitive to the pres-

ence of surface atoms in the top crystalline layer [21, 22].

After the laser ended for some time, the oscillated inten-

sity increased slowly, the lattice relaxation tended to

smooth for the surface roughness. Therefore, the Ni NCs

are embedded successfully in BTO/STO SLs by the al-

ternation of 3D island Ni NCs and perfect epitaxial

BTO/STO SLs. Importantly, the embedded Ni NCs can

not disturb the epitaxial growth of BTO/STO SLs.

In order to explain the formation mecha nis m of 3D

island Ni NCs, the changes in the (aNi-aBTO)/aBTO as a

function of the pulses, using the spot spacing for Ni NCs

and the streak spacing for BTO, were calculated (Fig. 5).

Initially, the Ni NCs retain an in-plane lattice parameter

identical to that of the BTO layer, and then recover to the

value of bulk Ni rapidly. Owing to the large lattice mis-

match between Ni and BTO (>10%), Ni NCs were sub-

jected to considerable in-plane tensile strain. Simulta-

neously, this in-plane straining would shorten the

out-of-plane lattice parameter of Ni NCs, leading to the

structural transition from cubic to tetragonal. With the

deposition being continued, the increasing strain energy

was relieved by the transformation from 2D to 3D islands

as the thickness exceeded the critical thickness hc, in

which the lattice was relaxed laterally to the unstrained

value, and the critical thickness is in inverse proportional

to the lattice mismatch, according to the Mat-

the ws -Blakeslee formula. The strain from lattice mis-

match acts as a source of driving force for the

self-organization of Ni NCs. Therefore such metal NCs

growth route is feasible especially in the large lattice

mismatch system.

Figure 5. The in-plane lattice relaxation during the

self-organization of Ni NCs. Inset an expanded view of the first 90

pulses

Figure 6. The cross-sectional HRTEM image of Ni NCs:BTO/STO

SLs nanocomposition film

Fig. 6 is the cross-section HRTEM image of Ni

NCs:BTO/STO SLs composition films. The irregular

interface of strained Ni NC layers is alternated with 7

times. We estimate the sizes of Ni NCs about 5nm. As

shown in Fig. 6, the perfect laye r-by-laye r epitaxia

growth of STO and BTO layers is performed. The sepa-

rate BTO layer and STO layer have the uniform thick-

ness of 8 nm and 10 nm accordingly. The thickness of

Z. XIONG, W. WU

every separate layer is accurately controlled by RHEED

intensity oscillation which is extremely sensitive to the

presence of surface atoms in the top crystalline layer [3].

The individual BTO layer and STO layer have the uni-

form thickness of 14nm and 21nm, respectively. Ac-

cording to the results of RHEED monitored, the growth

rate of STO and BTO are estimated to be 0.037 ML/s and

0.043 ML/s, where 1 monolayer (ML) corresponds to a

layer thickness of 0.781 nm (STO) and 0.806 nm (BTO),

the double value of the c-axis lattice constant of bulk

STO (0.3905 nm) and BTO (0.4029 nm) [5], respectively.

So it is concluded that one period of RHEED intensity

oscillation corresponds to the growth of two STO unit

cells and two BTO unit cells, respectively. We achieve

atomic-level control for the growth of the nanocomposite

fil m.

Figure 7. XRD θ-2θ scans of the composite films on STO (001)

substrate; the inset is an expand view around the BTO (002) peak

Fig. 7 shows the X-ray diffraction θ-2θ scans of the

composite films grown on STO (001) substrate. The

characteristic peaks of BTO and STO show that the epi-

taxial growth is satisfying along basically c-axis with the

lower diffraction angles. Using STO single-crystal sub-

strate, we have obtained very high quality epitaxial films.

Magnifying the BTO (002) peak for the inset, a weak

peak is observed beside the leading peak. We suppose

that the weak peak is Ni (111).The crystallographic

orientation with Ni (111) takes priority of the fcc metal

structure. The appearance of the Ni (111) is supposed to

utmost compressed plane in order to get minimum sur-

face energy and remain metallic property, so as to give

the flat surface to the vicinal separation layer. This way

reduces extremely the effect of the strained distortion by

the large lattice mismatch between the different interfac-

es. The results of the XRD characterization are in good

agree with RHEED analysis.

4. Conclusion

In summary, Ni NCs:BaTiO3/SrTiO3 superlattices were

fabricated by L-MBE and controlled accurately by in situ

monitoring RHEED. With the self-or ga nized method, the

alternate growth of Ni NCs was embedded successfully

in BaTiO3/SrTiO3 superlattices and could not disturb the

epitaxial growth of BaTiO3/SrTiO3 superlattices, in good

agree with the anal ysi s of XRD and HRTEM. The results

wer e concluded that the layer-by-layer growth of Ba-

TiO3/SrTiO3 superlattices and the t hree -dimensional isl-

and growth of Ni NCs. The strain in the composite films

from lattice mismatch acts as a source of driving force

for the self-organization of Ni NCs. Therefore such

self-or ga nized metal NCs growth route is propitious to

fabricate three -di mensio nal quantum dot devices.

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