2.3
. Formation of Ge nano
-islands on Si
surface by ion
-
stimulated method
Interest is due to unique physical features of quantum dots associated with
an atomic-like energy spectrum and due to a possibility of producing
optoelectronic devices of a new generation on their basis. From the point of view
of devices, in the case when the distance between the levels is much greater
than
thermal
energy an atomic-
like electronic spectrum of charges at quantum dots
makes it possible to remove the main problem of present-day micro- and
optoelectronics, namely “smearing” of charge carriers in an energy window of an
order of
kT
, which
leads to degradation of device features for higher temperature.
Surface irradiation with low
-energy ions during the growth is a perspective
method of controlling the density of formed islands and the island distribution in
size. However, the mechanisms of ion
effect on the processes of nucleation and
growth remains unclear in many respects.
The use of kinetic rate equations allows at
an atomic level describing
nucleation of nano-
dimensional islands in the early (submonolayer) stage of the
epitaxial growth under conditions far from equilibrium. In this dissertation, this
method is used to study the effect of low-energy ions present in a flow of adsorbed
atoms on concentration of formed islands.
It was accepted that ion collisions with surface leads to defect formation –
surface vacancies or vacancy clusters –
being points of preferable nucleation. Two
mechanisms of facilitated formation of nucleus on defects
were considered:
a
)
a nucleus is formed from the surface atoms
knocked-out by an ion with
the probability
close to 1;
b
)
knocked-out atoms play no special role in nucleus formation but the
nucleus formation rate at a defect is higher than that at a defect-
free area (owing to
less size or/and higher energy of dissociation of critical nucleus).
Numerical calcula
tions of generalized kinetic equations for the values of
growth temperature
T
, adsorbed atom flow
F
and activation energy of surface
diffusion typical for the Ge/Si structure were performed.
The integration results
118
showed that irrespective of a mechanism of nucleus formation the island
concentration increases with the raise in a share of ions in the adsorption flow
Λ
and decreases with the decrease in
T
or with the raise in
F
. For rather low
T
or high
F
the island c
oncentration corresponds to the growth when there are no ions. That
is associated with the fact that under indicated conditions an adsorbed atom has no
time to reach defect owing to surface migration until it meets another adsorbed
atom (the similar effect
for the initially defected surface was
found by
P
. Jensen
и
et al, Surf
.
Sci. 412/413, 1998,
p
. 458)
.
The expressions connecting a characteristic value of the ion share for which
the ion component plays a vivid role in island formation with crystallization
parameters were obtained. In the case of nucleus formation according to the
mechanism (a)
1
3
/
1
)
/
(
~
−
θ
Λ
D
F
(2.1
4
)
where
D
is the coefficient of surface diffusion,
Ft
=
θ
is the degree of surface
coating for time
t
. For the nucleus formation under mechanism (b)
}
)
2
/(
]
)
2
(
)
2
exp{[(
)
/
(
~
)
2
/(
)
2
2
(
)
2
/(
)
(
2
T
k
i
E
i
E
j
D
F
B
j
i
i
j
i
i
j
i
+
+
−
+
+
−
+
−
+
−
θ
Λ
(2.1
5
)
where
i
and
j
are the size of critical nucleus on the defect-free surface and on the
defect, respectively;
i
E
and
j
E
are the energy of nucleus
dissociation into separate
adatoms.
It is known that the maximal density of Ge islands with linear size of about
10 nm is to be an order of 10
12
cm
-2
(for higher density the islands are collected in
a continu
ous layer). Further increase in the nano
-island density is possible only
when their linear sizes decrease. The effects observed under ion irradiation such as
transformation of the function of nucleus distribution in size and change in island
concentration on surface give hopes that under ion action it will be possible to
obtain denser blocks of quantum islands by decreasing a size of the critical
nucleus. The facilitated nucleus formation at charged defect centers (generated
under ion irradiation) is a resu
lt of decreasing an activation barrier of nucleus
formation in electrostatic interaction of charge with nucleus atoms.
A mechanism describing a decrease in activation barrier of nucleus
formation in electron capture by a metastable fluctuation of adatoms was also
considered. The estimations points to a significant role of free carriers of charges
generated under ion irradiation in the process of growth
from partially ionized
flows; the sizes of critical nucleus are ¼
of the critical size of neutral nucleus. Also
by analytical and numerical methods the regularities of nucleus formation under
low-energy ion irradiation during the process of condensation of Ge molecular
flow to Si were obtained. As a result, it was confirmed that under ion irradiation
the islan
d density increases and becomes more homogeneous, i.e. the function
island distribution in size is narrowed.
The
formation processes of nano
-
dimensional Ge islands in Ge/Si
heterosystems obtained by molecular-
beam epitaxy have been experimentally
studied. A partially ionized flow of Ge was used to vary the features of a block of
119
Ge nano-
islands of the silicon surface. Positively charged ions of Ge were obtained
in the process of flow formation of a material evaporated from electron-beam
evaporator (EBE). Io
n bombardment of growing film surface modifies a near-
surface layer of crystal by defect formation, charged complex creation, penetration
of potentially accelerated particle into crystal.
Structure and morphology of Ge islands were studied by scanning tunn
eling
microscopy (STM). The STM data allowed the evolution of sizes and density of
Ge islands to be observed depending on ion component energy
(fig.2.8 and 2.9)
.
The estimation of the limiting factors influencing the density and sizes of islands
and their places of preferable nucleation was made.
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