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264 In situ characterization of thin ﬁlm growth

there are fewer holes remaining and interlayer transfer approaches zero. With

completion of the base layer interlayer transport can occur only into the top

layer and the value turns positive and increases with subsequent shots until

coalescence is reached. after coalescence, interlayer transport into the layer

slows down as the number of holes into which interlayer transport can occur

decreases rapidly when approaching full coverage.

The second plot Fig. 9.13(c) shows that the growth-of-a-layer Dq

as a

function of the number of laser shots mirrors nicely the fast interlayer transport

plot. Dq

reaches a maximum when the base layer q

is near lling up and

the top layer coverage is ~0.4. Because effectively there is only one layer

open here, there can be little or no interlayer transport and all material must

go into Dq

. however, because of the two layer nature of growth a laser shot

going 100% in a single layer is never observed. Even at the maximum (near

0.4 coverage) less than 90% of the laser shot contributes to the growth of

a single layer. Past the maximum, Dq

develops a downward trend because

the increasing layer coverage q

effectively blocks the surface and decreases

the exposed coverage (q

– q

) and with that the fraction f

of the plume that

is directly contributed to Dq

The behavior of the slow thermally driven interlayer transport component

shown in Fig. 9.13(d) is the easiest to understand. A distinctive feature of

this plot is that it clearly depends on the dwell time. The amount of material

transferred is determined from the data in Fig. 9.8 to increase with increasing

dwell time from about 5% of a single shot for 0.2 s dwell time to about

20% of a single shot for 50 s dwell time. This behavior represents clear

conrmation of the thermal nature of this interlayer transport component.

The thermal interlayer transport component corresponds to transport of

already crystallized STO unit cells. The plot in Fig. 9.13(d) shows that the

transport out of the layer below 0.5 coverage is balanced by the transport

into the layer above 0.5 coverage.

9.3.6 Diffuse scattering

Another advantage of using synchrotron X-rays is that they provide sufcient

intensity to perform time-resolved measurements of the very weak diffuse

scattering component that originates from incoherent uctuations in local

order and lateral organization (Fuoss et al., 1992; Robinson et al., 1996).

Diffuse scattering measurements are performed by using an area detector,

such as a CCD camera to capture the entire diffraction prole including both

the sharp central peak and the broad diffuse scattering. Presented in the form

of two-dimensional diffraction intensity maps shown in Fig. 9.14, diffuse

scattering data provide information on the size of islands as illustrated by the

change of the q (nm

–1

) distribution of intensity as a function of deposition

given in terms of the number of laser shots. The variations in the grayscale



265Real-time studies of epitaxial ﬁlm growth using SXRD

shades are used to aid visualization of the intensity changes. These two-

dimensional diffraction maps contain the complete information about the

formation and the evolution of the surface structure during STO lm growth.

The vertical line that cuts through the diffraction maps at q = 0 describes the

growth intensity oscillations of the sharp central peak discussed previously.

The larger elliptical shape features appearing at q ≠ 0 on both sides of the

central peak correspond to diffuse scattering.

The diffuse scattering describes the spatial distribution of the islands

on the growing surface (Fuoss et al., 1992; Alvarez et al., 1998; Ferguson

et al., 2009). The presence of the diffuse side lobes indicates that a well-

dened correlation exists between the islands. The q dependence of these

features given by Dq = 2p/L describes a characteristic length scale in terms

of L, the correlation length that corresponds to the island separation l in

the pre-coalescence stage of the growth. Therefore, the shifting of q of the

diffuse side lobes toward q = 0 correspond to increasing island sizes. From

–1.0 0.0 1.0

q (nm

–1

)

Laser shot

120

100

9.14 The complete diffraction intensity map from growth of 17

u.c. of STO obtained by measuring the (0 0 ½) diffraction proﬁle

using a CCD for growth at 10 s dwell time at 680 °C. The spine of

the plot corresponds to the growth intensity oscillations of the

specular beam. The side lobes at q π 0 on both sides give the diffuse

scattering that is indicative of a well-developed correlation between

the islands.



266 In situ characterization of thin ﬁlm growth

time-dependent measurements it is estimated that these changes are slow

and are in the timescale of seconds.

The relationship between the central peak and the diffuse side lobes in

layer lling during growth of the rst three layers is illustrated in Fig. 9.15.

As Fig. 9.14 shows the diffuse scattering signal is weak and distributed over

a wide q range when the islands are very small at the onset of growth. less

noisy data are obtained by using d-scans with a point detector. as expected,

the plot shows that the intense central peak and the diffuse scattering are

oscillating out of phase. The rst appearance of the weak diffuse scattering

component is observed at q @ 0.15 after the rst laser shot. The island size

estimated from Dq = 2p/r is r = 5 nm. An increasing island density after the

second laser shot would be manifested as an increase in q. Simple layer lling

at a xed island density with increasing island sizes would result only in

the change of the intensity of the diffuse side lobes located at a xed q. at

a full layer the diffuse scattering should vanish and nucleation of the next

layer would occur for a q equal or close to the value for the rst shot. Instead

the plots in Figs 9.14 and 9.15 show that q decreases continuously with

subsequent laser shots. This behavior is unexpected for simple layer lling

9.15 Single shot data obtained using d-scans with a point detector

showing the relative intensity of the specular and diffuse intensity

peaks during growth of the ﬁrst 3 u.c. of STO. The diffuse scattering

oscillates out of phase with the specular intensity during layer ﬁlling.

Laser shot

Normalized intensity (l/l

)

q (Å

–1

)

–0.15

–0.10

–0.05

0.00



267Real-time studies of epitaxial ﬁlm growth using SXRD

and indicates that the island size is increasing during growth. This trend is

characteristic of thermally driven processes such as island ripening. it was

conrmed that q shifts to smaller values quicker for longer dwell times at a

xed temperature, and for increasing temperature at a xed dwell time.

The shifting of q toward smaller values even when deposition of a new

layer – referenced to the maximum intensity of the specular peak – is started,

indicates the absence of independent renucleation on a at growth surface.

The dependence of the q for a newly growing layer on the q of the lling

layer suggests that some type of communication exists between these two

layers. This memory effect between the growth of two successive layers is

a conrmation of the simultaneous growth of two layers in the S2L growth

mode as concluded from the time-dependent coverage evolution. The most

likely explanation for the existence of the memory effect is that interlayer

transport makes the growth of two successive layers interdependent on each

other. a careful examination of the scattering maps reveals that after the

rst few layers the diffuse scattering between two successive layers never

fully vanishes in agreement with the conclusion that a at growing surface

is never again restored.

9.4 Future trends

The sXrD studies reveal new details of the PlD growth mechanism, leading

to the conclusion that the potential of PlD is currently underutilized. The

most important of these ndings is that the actual lm growth of STO in

PLD – where growth is equivalent to initial crystallization conrmed by the

rst detectable presence of the crystalline STO phase – occurs on a much

faster timescale than previously thought. The crystallization time in PlD as

the upper limit for combination of surface transport needed for the arrival

of the growth species at the proper crystallographic sites was determined to

occur faster than it can be measured with ms scale time resolution. using

diffuse scattering measurements it was shown that a second much slower step

that corresponds to continuous island ripening occurs throughout the growth

process. The island ripening process occurs by thermally driven conversion

of already crystalline smaller sTo islands into larger ones. The formation

of larger islands (by long dwell times) is not only unnecessary for PLD,

it is actually detrimental to lBl growth because it reduces the nucleation

density and favors premature nucleation of the second layer and eventually

acceleration of the interface broadening and 3D growth. longer dwell times

and higher substrate temperatures increase the impact of thermal processes

and diminish the non-equilibrium effects of PlD growth.

The S2L growth mode will persist indenitely during high quality growth

and represents the highest perfection growth mode physically possible by any

growth method. Post-growth AFM characterizations show that STO lms



268 In situ characterization of thin ﬁlm growth

grown in the s2l growth mode reveal no interface broadening even after the

growth of more than 100 u.c. thick STO. In terms of a kinetic picture it is

now clear why PlD is so similar to lBl growth. although the presence of

the second layer certainly interferes and slows down fast interlayer transport

into the base layer, it appears that interlayer transport is sufciently effective

to enable a ‘stretched out’ lBl growth mode to complete one full layer over

two periods.

it is appealing to think about a ‘pure’ PlD growth process in which

the thermal component is suppressed and the surface smoothing effects of

energetic deposition (non-equilibrium growth) from the plume are fully

preserved. The rst step toward realizing such a process is to reduce the

dwell time. An added benet of eliminating or reducing the dwell times is an

increase in the average growth rate that is presently used with PlD growth

conditions not much higher than typical MBE growth rates of about 1 ML/

min. it is concluded that an effective growth manipulation scheme for PlD

lm growth (Koster et al., 1999) must be based on maximizing the role of

the fast interlayer transport step and suppressing the contribution from the

sluggish thermal interlayer transport component during the dwell time, by

reducing the substrate temperature, or the dwell time, or a combination of

the two.

The growth kinetics picture of sTo PlD that emerges from sXrD

data is part of a general framework that is also applicable in homo- and

heteroepitaxial growth of other perovskites. The present studies of kinetics

from single laser shot deposition show that departure from lBl growth in

homoepitaxy happens as a consequence of impeded fast interlayer transport.

This is the factor that is most likely to adversely affect lBl growth in

heteroepitaxy as well. But the mechanisms through which the interactions

between the dissimilar atoms of the substrate and the lm can alter the rate

of interlayer transport in layer lling at the onset of growth are not known.

The macroscopic manifestation of these interactions includes 3D growth,

strain, chemical inhomogeneity, and phase change that fundamentally alter

the properties of the near-interface region. Manipulating the growth kinetics

becomes an indispensible tool in heteroepitaxial growth because it provides

a way to open lm growth pathways that are closed by thermodynamics in

near-equilibrium growth methods. The special importance that the growth

of the rst layer plays in homoepitaxial kinetics becomes magnied in

heteroepitaxy by the fact that it is both kinetically and structurally different

from the rest of the layers that make up the lm. Direct determination of

interlayer transport rates from the time-dependent coverages, island size

distributions, and island size evolution from diffuse scattering can be used for

understanding how interface sharpness will be affected by island nucleation

and island growth governed by the additional factors that are present in

heteroepitaxial growth.



269Real-time studies of epitaxial ﬁlm growth using SXRD

9.5 Acknowledgment

research sponsored by the Materials sciences and Engineering Division, Basic

Energy Sciences (BES), US Department of Energy (DOE), and performed

in part at the advanced Photon source, a DoE-BEs user facility.

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