Island Distance in One–Dimensional Epitaxial Growth
Harald Kallabis1,2, Paul L. Krapivsky2 and Dietrich E. Wolf1,2
arXiv:cond-mat/9806140v3 [cond-mat.stat-mech] 17 Sep 1998
1
FB 10, Theoretische Physik, Gerhard–Mercator–Universität Duisburg, 47048 Duisburg, Germany
2
Center for Polymer Studies, Boston University, Boston, 02215 MA, USA
January 9, 2014
The typical island distance ℓ in submonlayer epitaxial growth depends on the growth conditions via
an exponent γ. This exponent is known to depend on the substrate dimensionality, the dimension
of the islands, and the size i∗ of the critical nucleus for island formation. In this paper we study
the dependence of γ on i∗ in one–dimensional epitaxial growth. We derive that γ = i∗ /(2i∗ + 3) for
i∗ ≥ 2 and confirm this result by computer simulations.
PACS numbers: 81.15-z, 81.10.Bk, 05.50.+q
I. INTRODUCTION
γ=
Molecular beam epitaxy (MBE) is of outstanding importance for the fabrication of microelectronic devices.
The highly controllable experimental conditions make
this technique an ideal tool for crystal growth. The thickness of the deposited layers can be controlled down to the
atomic scale while growth proceeds layerwise.
Typically the growth of multiple layers is of primary
interest in most experimental and theoretical studies of
MBE. However, deeper insight into the physics of multilayer growth requires a comprehensive understanding of
the submonolayer regime (see Ref. [1], for instance). The
most important length scale in submonolayer growth is
the island distance or diffusion length ℓ. The classical result [2] for this length is that it depends on the diffusion
constant D of adatoms and the deposition rate F like
ℓ ∼ (D/F )γ .
i∗
.
2i∗ + d + df
(3)
Our motivation for studying the one-dimensional case
is the following. Much theoretical work is dedicated to
the study of basic mechanisms of epitaxial growth. For
computational reasons, the numerical work in this area
is often restricted to models in surface dimension d = 1.
As mentioned above, such studies rely on a good understanding of the physics in the submonolayer regime.
Also, quasi one–dimensional MBE may be realized
experimentally on surfaces with extremely anisotropic
diffusion constant, like the 2x1–dimer–reconstructed
Si(001) surface (see, e.g., Ref. [8]). There, the estimated ratio of the diffusion constants along rows and
perpendicular to them is of the order of 1000:1 in typical experiments [9]. A computer simulation study of this
growth process leads to the estimate that for a deposition rate of F = 1/600 ML/s and at temperatures below
T = 450 K, this anisotropy should be pronounced enough
to lead to quasi one–dimensional MBE [10]. However,
there is a controversy concerning the relative importance
of anisotropic diffusion and anisotropic bonding [11,12].
Another experimental example is the reconstructed surface of Au(100) [13].
This paper is organized as follows. In section II the
expression (3) will be derived. The subsequent sections
are devoted to the numerical confirmation of this result. In section III we employ the coarse–grained MBE
(CGMBE) model [3], which allows for a simple algorithmic implementation of a variable critical nucleus size i∗ .
To ensure that the results are independent of the chosen
algorithm, we reproduce them with a model that is not
coarse grained (MBE model) in section IV. It is shown
that the results of the MBE and the CGMBE models
compare favourably with each other and with the theoretical prediction. The final section V contains a general
discussion of the results.
(1)
The analysis of rate equations yields that the exponent
γ depends on the substrate dimension d, on the size i∗ of
the critical nucleus and the fractal dimension df of the
islands [2–4]:
γ=
i∗
.
2i∗ + 3
(2)
The critical nucleus is defined such that all islands consisting of i∗ or less atoms can shrink by emitting adatoms,
while an island of size i∗ + 1 is stable. Eq. (2) applies
if island diffusion and adatom desorption are not important.
Eq. (2) has been demonstrated to hold in d = 2 for
i∗ = 1, 2, 3, 4, see Refs. [5,6]. In this paper we study the
dependence (1) in surface dimension d = 1 for different
values of the critical nucleus i∗ . In this case, df = 1. So
far, only the case i∗ = 1 [3,7] has been studied in one
dimension, to our knowledge, and it is in agreement with
(2). Here we show that (2) is no longer valid for i∗ ≥ 2,
where
1
Setting t ∝ F −1 gives the result (3).
For i∗ = 2 there is a logarithmic correction to (6) [15],
II. ANALYTICAL CONSIDERATIONS
Most treatments of the monolayer growth processes
are based on a mean-field rate equation approach. This
is appropriate in the most important two–dimensional
case since the basic kinetic mechanism – the two-particle
diffusion-controlled aggregation – has an upper critical
dimension dc = 2 [14]. In one dimension, however, classic rate equations do not apply and we shall employ a
modified rate equation approach [7].
We denote by A and I densities of adatoms and stable
immobile islands, respectively. We start with adatoms
and write
dA
A
=F − .
dt
τ
DA3
dI
=
,
dt
| ln I|
whereas (5) remains unchanged. Repeating the same
analysis gives
ℓ∼
D
F
2/7
ln
D
F
1/7
.
(9)
III. NUMERICAL RESULTS
The idea of the coarse grained MBE (CGMBE) model,
introduced in Ref. [3], successfully applied in Refs. [5,6],
and studied in detail in Ref. [16], is to avoid the timeconsuming simulation of the surface diffusion on the
atomic scale by resolving it only on a much coarser scale
∆x. As long as ∆x is smaller than the island distance ℓ,
one still gets correct information about the surface morphology.
Specificially the model is implemented as follows: The
substrate is divided into cells of linear extension ∆x, so
that a monolayer corresponds to ∆x atoms per cell. The
partial filling of a cell at site x is given by a counter
m(x) = 0, 1, . . . , ∆x − 1, which means the number of
atoms on top of completed monolayers in cell x. The
height h(x) denotes the number of complete monolayers.
For the complete description of the state of a cell, an edge
indicator is(x) is introduced. If is(x) = 1, atoms cannot
leave cell x because they are irreversibly incorporated
into the island edge present in the cell. This allows for
a simple implementation of the concept of the critical
nucleus i∗ : if upon diffusion or deposition of an atom
into cell x the counter reaches the value m(x) = i∗ , is(x)
is set to one. Consequently, m(x) can only increase from
then on, until m(x) = ∆x, in which case h(x) is increased
by one and m(x) and is(x) are set to zero. As long as
is(x) = 0, m(x) can also decrease, when atoms diffuse
into neighbouring cells.
The dynamics has to take into account that a single
adatom move from one cell to a neighbouring one represents the diffusion over a distance of ∆x lattice sites. This
happens with frequency νD = D/(∆x)2 for all adatoms
in the system. The deposition of one atom happens with
frequency νF = F L∆x. Hence the probability for deposition during one time step ∆t = (νF + νD )−1 of the
computer simulation is
(5)
This differs from the two–dimensional situation where the
number of different sites visited by an adatom grows linearly with time and the adatom density evolves according
to dA/dt = F − DIA. Actually, Eq. (5) contains an additional loss term due to the nucleation of new islands.
However, this loss term is asymptotically subdominant,
since most adatoms are captured by the existing islands.
The rate equation for the island density is derived by
noting that islands are formed upon colliding of i∗ + 1
adatoms. Hence
∗
dI
= DAi +1 .
dt
(4)
Here τ is a lifetime, i.e., the time between the deposition of an adatom and its incorporation into a stable
√ island. During the time interval τ , the adatom visits Dτ
different
sites, so the adatom lifetime is determined by
√
I Dτ ≈ 1, and Eq. (4) becomes
dA
= F − DI 2 A.
dt
(8)
(6)
The many-particle diffusion-controlled reaction process
(i∗ + 1)A → I has an upper critical dimension dc = 2/i∗
(see e.g. [15] and references therein). The fact that the
upper critical dimension gets smaller when i∗ increases
is simple to appreciate. Indeed, many-particle collisions
occur rarely, so the system is well mixed and the meanfield approach should become correct earlier. For i∗ > 2
we have dc < 1, so Eq. (6) applies; the case i∗ = 2 is
marginal, so we anticipate logarithmic corrections to the
mean-field results. For i∗ = 1 correlations dominate the
diffusion process in one dimension, so that (6) has to be
modified [14] and leads to γ = 1/4 [7].
Eq.(3) is obtained from (6) and (5) in the following way: From (5) it follows that asymptotically A ≃
(F/D)I −2 . Inserting this into (6) and intergrating we
find
i∗ /(2i∗ +3)
∗
F
(F t)1/(2i +3) .
(7)
I∼
D
p = νF /(νF + νD ).
(10)
1 − p is the probability with which each of the adatoms is
allowed to diffuse into a neighboring cell. If no adatoms
2
are present, νD = 0. This algorithm speeds the simulation up by a factor of about (∆x)2 .
In the simulations presented in the following, the
coarse graining length had the constant value ∆x = 20.
The system size was chosen to be L∆x = 2 · 105 and
averages were taken over 100 independent runs.
model was simulated in systems of size L = 65536.
The island distance was measured here as follows. During deposition, the number of islands was calculated for
times 0 < F t < 1. An island was defined as a connected
region where h(x) ≥ a⊥ , where a⊥ is the vertical lattice
constant. For early times, the number of islands increases
due to nucleation. At roughly F t ≃ 0.5 the number of
islands decreases again since the islands coalesce. This
means that the island distance has a minimum which was
used for evaluation.
4
10
*
i =1
*
i =2
*
i =3
*
i =4
4
10
10
0.4
0.3
γeff
3
10
island distance
island distance
3
2
10
0.2
0.1
0.0
0
2
2
10
4
6
log(D/F)
8
10
1
10
4
10
6
8
10
10
10
10
D/F
1
10
FIG. 1.
The island distance as a function of D/F
for i∗ = 1, 2, 3, 4.
The solid lines represent fits to
the last three decades of data and have the slopes
γ = 0.248, 0.293, 0.329, 0.355, respectively.
10
0
2
10
4
6
10
10
8
10
10
10
D/F
FIG. 2.
The island distance as a function of D/F for
i∗ = 1 (circles) and i∗ = 2 (squares). The solid lines with
slopes γ = 0.246 and γ = 0.288 represent fits to the last
three decades of data. The inset shows how the local slopes
approach the theoretically predicted exponents (dotted lines)
for large D/F .
In the CGMBE model, nucleation events can be identified easily. Thus the island distance can be determined
by dividing the system size by the total number of nucleation events in the first layer. Measurements of this
quantity as a function of D/F for different i∗ are shown
in Fig. 1. Note that the asymptotic algebraic behaviour
ℓ ∼ (D/F )γ is reached with ℓ as function of D/F having
a weak negative curvature.
In Fig. 2, the measurements of the island distance as a
function of D/F for the MBE model with i∗ = 1 and i∗ =
2 are shown. From the effective exponent γeff ≡ log(10 ·
D/F ) − log(D/F ) shown in the inset, it becomes clear
that the asymptotic behaviour ℓ ∼ (D/F )γ is reached
only for large values of D/F . The dotted lines indicate
the theoretical values from Eq. (2), γ(i∗ = 1) = 1/4, and
Eq. (3), γ(i∗ = 2) = 2/7, respectively. Note also that the
asymptotic exponent is approached from below.
IV. RESULTS WITHOUT COARSE GRAINING
In the (non–coarse grained) MBE model atoms are deposited onto the surface of a simple cubic crystal (i.e.
a square lattice in d = 1) with a rate of F atoms per
unit time and area. Atoms with no lateral neighbors are
allowed to diffuse with diffusion constant D. In the simplest variant of the model atoms with lateral neighbors
are assumed to be immobile so that, e.g., dimers are immobile and stable. Therefore, i∗ = 1. Extension of the
rules from i∗ = 1 to i∗ = 2 is straightforward: Dimers
are allowed to split into two adatoms, while trimers are
stable. In either case, growth commences with a flat
substrate, h(x, 0) = 0 for all sites x. On deposition at x,
h(x, t) is increased by one. We neglect barriers to interlayer transport (Ehrlich–Schwoebel barriers [17]). Thus
the only parameter of the model is the ratio D/F . This
V. DISCUSSION
In Fig. 3, the comparison of the simulation results of
the CGMBE model (Fig. 1) and the MBE model (Fig. 2)
with various analytical formulas is shown. The small difference between γ measured in the CGMBE and the MBE
model is not only due to statistical noise, but also due to
the asymptotic behaviour of ℓ(D/F ) being reached with
negative and positive curvature, respectively. Therefore, even better agreement between the results of the
CGMBE and the MBE models for larger D/F can be
3
expected. As a consequence, the values for higher i∗ ,
obtained with the CGMBE model, can be regarded as
reliable.
length in one dimension for critical island size i∗ ≥ 2 and
have confirmed the result by computer simulations.
VI. ACKNOWLEDGEMENTS
0.40
D. E. W. acknowledges support by DFG within SFB
166. H. K. acknowledges support by DAAD within
the Hochschulsonderprogramm III. P. L. K. acknowledges
NSF grant DMR-9632059 and ARO grant DAAH04-961-0114 for financial support.
γ
0.35
0.30
VII. NOTE ADDED IN PROOF
0.25
0.20
1
2
3
Following the suggestion of one of the referees we note
that our rate equation approach applies to the general
case of surface dimension d and island dimension df . The
fractal island dimension can be smaller or larger than the
surface dimension which is integer in physically relevant
cases. In this general case we use F tℓd = ℓdf with ℓ =
I −1/d to obtain
4
*
i
FIG. 3. The submonolayer exponent γ as a function of i∗
in d = 1. Squares and diamonds are the results of the simulations of the CGMBE and the MBE models, respectively. The
solid curve is the theoretical prediction for i∗ ≥ 2, Eq. (3).
The dashed curve corresponds to formula (2) and the dotted
line are the exponents expected, if islands up to size i∗ can
diffuse, Eq. (11).
γ=
where ν = 2 for d = 1 and ν = 1 for d ≥ 2. This
does not apply to the exceptional case d = i∗ = 1 where
d < dc = 2/i∗ . There equation (6) should read I˙ = DA2 I
and one finds γ = 1/(3 + df ).
The agreement of the numerical results with the theoretical prediction (3) is very good. For comparison, we
also show Eq. (2), which in one dimension applies only
for i∗ = 1. It is interesting to notice that the exponent
for i∗ = 2 coincides with the one expected, if islands up
to size i∗ can diffuse without decaying [3,18]
γ=
i∗
i∗
= ∗
.
∗
(d + 2)i + df
3i + 1
i∗
,
dν(i∗ + 1) + df
(11)
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The specific way we implemented the decay of a cluster
in our computer simulations makes this tendency very
clear: Whenever a dimer has formed it must break up
in the next diffusion step, which means that each of the
two atoms moves to one of the neighboring cells. With
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further adatom was met there, this amounts simply to a
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down, however, for i∗ > 2.
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epitaxial growth in a one dimensional model where islands of size up to i∗ are unstable while larger islands are
stable and immobile. We have derived the exponent γ
which determines the D/F –dependence of the diffusion
4
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5