Article Type : Research Article
Authors : Zavodinsky V* and Gorkusha O
Keywords : Density functional theory; Modelling; Density of electronic states; Band gap; Total energy
Within the framework of the density functional theory and the pseudopotential method, the electronic structure calculations of the systems "metal-Si (100)" systems with Li, Be or Al coatings of one to three monolayers thickness, were carried out. Band gaps in the densities of states were observed in the one monolayer Li-Si (100) and Be-Si (100) systems (1.02 eV and 0.36 eV, respectively). In the Li-Si (100) system, the gap disappears with increasing thickness of the metal coating to two monolayers, and in the Be-Si (100) system it increases to 0.40 eV for two monolayer and disappearing when three monolayers are deposited. No gap was found in the Al-Si (100) system. This behavior of the band gaps can be explained by the passivation of the substrate surface states and the peculiarities of the electronic structure of the deposited metals.
Metal layers on silicon have long attracted
attention of researchers. However, this mainly refers to layers of refractory
3d transition metals that form stable silicides. Layers of low-melting metals
on silicon have not been studied enough. The most indicative include works,
where the atomic and electronic structure of the ultra-thin layers on Si (100)
and Si (111) surfaces were studied. In the work of Kotllyar et al [1]. The
high-temperature interaction of Al with Si (100) surface at low Al coverages
was studied using low-energy electron diffraction, Auger electron spectroscopy
and STM. At low Al coverages (
Is of particular interest to us, since it
describes in detail first-principles calculations of the electronic structure
of the Li-Si (100) system, which is one of the objects of our work. They find
that Li adatoms interact mainly with the dangling-bond orbitals of Si dimers.
The analysis of charge densities demonstrates a large charge transfer from the
Li adatom to a dangling-bond orbital of a Si dimer, which is responsible for a
large decrease of work function at submonolayer coverages. We will present
their results in more detail below in comparison with ours. The work of
Morikawa et al [5]. Is devoted to the
theoretical support to the double-layer model for potassium adsorption on the
Si (001) surface. Haye et al [6]. Nave studied sodium-doped dimer rows on Si
(001). The stability and electronic structure of a nanowire were studied by
first-principles calculations. The wire consisted of a single depassivated
silicon dimer row on the hydrogen passivated Si (001)-2×1 surface. In the work
of Jeong and Kang [7]. The atomic and electronic structure of the Na/Si (111) –
(3×1) surface was studied using the pseudopotential
density-functional total-energy method. Authors predicted that sodium atoms
evaporated onto this surface stick preferentially at the depassivated row and
partially fill the empty one-dimensional states of this row. This leads to a
thin metallic wire of atomic size dimensions. At room temperature the sodium
atoms are mobile along the depassivated row; they become immobile at
temperatures below 120ºK. Based on the calculated electronic structures,
authors proposed that a saturation Na coverage is 1/3 monolayer. This model can
explain the experimental results: the semiconducting ground state, double-row
scanning-tunneling-microscopy image, chemical passivation for oxidation.
Rysbaev et Al [8]. Studied the formation of silicide films of metals (Li, Cs,
Rb, and Ba) during ion implantation in Si and subsequent thermal annealing. Hite,
Tang and Sprunger [9]. Used scanning tunneling microscopy and photoelectron
spectroscopy to investigate the nucleation, growth, and structure of beryllium
on Si (111)-(7×7). They indicated that a chemical reaction occurs at
temperatures as low as 120ºK, resulting in a nano-clustered morphology,
presumed to be composed of a beryllium silicide compound. Upon annealing to
higher temperatures, their data indicate that beryllium diffuses into the
selvage region. Saranin et al [10]. Studied ordered arrays of Be-encapsulated
Si nanotubes on Si (111) surface using scanning tunneling microscopy. Gordeev,
Kolotova and Starikov [11]. Investigated the formation of metastable aluminum
silicide as intermediate stage of Al-Si alloy crystallization. They noted that
the mechanism of Al-Si alloy crystallization from an amorphous state is still
unclear. Despite the absence of equilibrium compounds for this binary system,
there are several experimental evidences confirming the formation of metastable
silicide at annealing of amorphous Si mixed with Al. Authors considered the
properties of aluminum silicide Al2Si structure, which is a probable candidate
for the role of the observed metastable compound. Their investigation was based
on the atomistic simulations with the interatomic potential and density
functional theory approach. Terekhov et al [12].
Have studied a possibility of the metastable Al3Si
phase formation in composite Al-Si films obtained by ion-beam and magnetron
sputtering. Endo et al [13]. Made an
elementary analysis of each metal atoms on the Si (001) surface by scanning
tunneling microscopy and spectroscopy; the result was evaluated with the first
principles calculations of quantum mechanics. As metallic contaminations,
sub-monolayer of aluminum is evaporated on Si (001)2×1. The local density of
states on the Al/Si (001)-2×1 was measured by scanning tunneling spectroscopy
at room temperature. Experimental results are in good agreement with that
obtained from the calculations. Our work is devoted to computer modeling of
sub-nano (of one to three monolayers thickness) layers of Al, Be and Li on the
Si (100) surface using the Kohn-Sham approach within the framework of the
density functional theory [14-15] and the pseudopotential method [16].
All calculations were performed using the FHI96md
package [17]. Pseudopotentials were found using the FHI98pp package. To
calculate the exchange-correlation energy, the local electron density
approximation was used [18-19]. In all cases, the energy cutoff of a set of
plane waves was taken to be 10 Ry; calculations were carried out with the five
k-points: (0.5; 0.5; 0.0), according to the
One metal monolayer
Our calculations showed that for studied metals, the
arrangement of their atoms on the Si (100) surface turned out to be very
different. Schemes of the optimized arrangement of atoms in the “metal
monolayer + Si (100)” systems are presented in (Figure 1). It can be seen from
this figure that on the clean Si (100) surface, asymmetric (skewed in different
directions) dimers with a length of 2.32 Å and a bevel angle of 13.5° are
formed and p(2×2) symmetry is established, which is consistent with the
conclusions of works [23-24]. When applying a monolayer of Li, silicon dimers
lose their asymmetry and increase their length to 2.78 Å, and in the case of
beryllium and aluminum, silicon dimers as such disappear, their length is
compared with the distance between the underlying silicon atoms (2.90 Å). Note
also that beryllium atoms are immersed in the silicon substrate and are
installed between the first and second layers.
Figure 1: Arrangement of Li, Be and Al atoms on the Si (100)
surface in the case of deposition of 1 ML. The gray circles are Si atoms, the
small black circles are hydrogen atoms, and the big black circles are metal
atoms. The clean structure on the Si (100) surface is also shown.
Figure 2:
Density of electronic states formed when the first monolayer of Li, Be and Al
is deposited on the Si (100) surface in comparison with the density of states
for the clean Si (100)- (2×1) surface. The Fermi level corresponds to
zero energy.
Figure 3: Arrangement of Li and be atoms on the Si (100) at the 2 ML
coatings.
Figure 4: DOS calculated for 2 Li
and 2 be nonolayers on the Si (100) surface.
Figure 5: Arrangement of be atoms on the Si (100) surface at 3 ML coating.
Figure 6: DOS for three beryllium ML on the Si (100) surface.
In (Figure 2) we plot the densities of states
calculated for the first ML of lithium, beryllium and aluminum. The DOS for the
clean Si (100) surface is also shown there for comparison. From this figure it
is clear that in the Li-Si (100) and Be-Si (100) systems a forbidden gap Egap
presents. Namely, for Li Egap = 1.02 eV, for B-Si (100) Egap = 0.36 eV. In the
Al-Si (100) case there is no band gap. For the clean Si (100) surface we
obtained Egap = 0.54 eV. We were able to make a comparison with literature data
only for lithium and pure silicon. In theoretical work [4], with an arrangement
of lithium atoms close to ours, but with eight layers of silicon, an energy gap
width of 1.35 eV was obtained. In the work, using electron energy loss
spectroscopy Egap = 0.4 eV was determined. The authors of have found Egap = 0.5
eV using scanning tunneling spectroscopy. Those results are close to ours, so we
have reason to believe that our results obtained for me and Al monolayers on Si
(100) are also quite reliable.
Due to the fact that we were interested in the
formation of a band gap in electronic states, in this section we studied only
the Li-Si (100) and Be-Si (100) systems, since in the Al-Si (100) system the
band gap did not appear already at deposition of one ML of metal. The
energetically favorable arrangement of Li and be atoms on the Si (100) surface
with a two-layer coating is shown in (Figures 3-4). Shows the DOS for these two
systems. From this figure it follows that in the Li-Si (100) system the ordering
of the arrangement of metal atoms does not disappear, but becomes different
compared to a monolayer case. Li atoms penetrate deep into the silicon
substrate and occupy positions between the first and second silicon layers. In
the Be-Si (100) system a half of metal atoms are situated practically at the
same lever with the first silicon layer. From this figure it can be seen that
in the electronic structure of the 2ML Li-Si (100) system there is no energy
gap, but in the 2ML Be-Si (100) system there is a gap with a width of 0.40 eV,
a little larger than in the 1 ML case.
We studied this case only for the Be-Si (100) system.
The optimized structure of this system is shown in (Figure 5). From which it
can be seen that the arrangement of metal atoms has become practically
disordered. Shows the DOS for this system. It is easy to see that with a
coating 3 ML thick, some mixing of be and Si atoms is occur, and the disordered
arrangement of metal atoms becomes obvious. The DOS for this system is shown in
(Figure 6). We see that when three beryllium ML are deposited on the Si (100)
surface, the energy gap in the density of states disappears.
In the case of systems with layer-by-layer atomic
structure (and composition), it is very important to understand how the
electronic structure changes from layer to layer. This also applies to the case
of pure silicon, with the surface that undergoes structural restructuring. To
clarify this issue, we performed calculations, layer-by-layer, local densities
of electronic states (LDOS). In order not to clutter the paper with many
similar results, we limited ourselves to pure silicon with the Si (100)-p (2×2)
surface and systems of 1 and 2 ML Li on Si (100). The first case is interesting
due to the presence of dimers on the silicon surface, which leads to a band gap
significantly smaller than in bulk silicon; the second case demonstrates a
sharp increase in the Egap width and its sharp disappearance. Local densities
of states for pure silicon with the Si (100)-p (2×2) surface are shown in
(Figure 7).
This figure shows that in the LDOS corresponding to
the top layer, the band gap has a small width (0.54 eV) and is limited on both
sides by high peaks of occupied and unoccupied states. Next, we see that
gradually, from layer to layer, the relative intensities of the nearby free
states, marked by arrows, decrease and go to zero towards the last layer,
thereby opening an approximately twice wider gap (?1.0 eV), satisfactorily
corresponding to the value of Egap in bulk silicon (1.12 eV) (Figure 8). Shows
the LDOS for systems of 1 and 2 monolayers of Li on Si (100). This figure shows
that in the case of 1 ML Li coating (A panels) densities of states near the
Fermi level are very similar in the top, middle and bottom of the silicon slab,
and the band gap remains practically the same everywhere. On contrary, 2 ML Li
coating (B panels) leads to disappearing of the band gap, and metal-induced
unoccupied states present at the Fermi level up to the bottom of the silicon
slab.
Figure 7:
Local densities of states for pure silicon with the Si (100)-p (2× 2) surface.
The numbers show the numbers of layers, starting from the surface where the
dimers are formed.
Our calculations showed that in the Li-Si (100) system
there is a noticeable mixing of the metal and silicon layers. In the cases of
beryllium and aluminum the ordered and disordered structures are formed without
signs of significant mixing with silicon. As for the electronic structure, in
the 1 MC Li-Si (100) and 1 MC Be-Si (100) systems, the energy gap is observed,
while in the Al-Si (100) system there is no gap. When deposited the second
monolayer, the gap in the Li-Si (100) system disappears, but in the Be-Si (100)
system the gap remains. When three monolayers of beryllium are deposited, the
gap disappears. Calculations of local densities of states have shown that when
computer studying the electronic structure of near-surface regions of silicon,
one need to be very careful because the metal-induced states penetrate even at
the twelve-atomic layer deep.