Chemistry Reference
In-Depth Information
3.2 Novel Low-Temperature Kinetic Pathways
for the QSE-Driven Uniform Height Islands
After this short introduction to expectations from classical diffusion experiments,
some basic results obtained during metal growth at low temperatures controlled by
quantum size effects (QSEs) and in particular for the Pb/Si system will be reviewed.
It has been a surprise that electron confinement can play a key role in the self-
organization of metallic nanostructures [
7
]. Confinement becomes important when
the structure dimensions are comparable to the Fermi wavelength
λ
F
, i.e., for metal-
lic structures when the thickness is a few layers. Traditionally, spectroscopic tech-
niques were used to map out the position of the energy levels of confined electrons
as they shift with the change of the film thickness, high-quality spectra indicating
the growth of good films [
8
,
9
]. However film or island thickness was expected to
change monotonically as the deposited amount was varied. In addition for typical
confining potentials (at most a few tens of electron volts defined by the position
of the valence band with respect to the vacuum) the difference in the energy level
spacing between confining wells of adjacent thickness was considered too small
(
tens of millielectron volts) to have an effect in the grown film morphology.
However, in several metallic systems Ag/GaAs(100) [
10
], Ag/Si(111) [
11
],
Pb/Si(111) [
12
], In/Si(111) [
13
], Ag/NiAl(100) [
14
], Ag/quasicrystals [
15
],
Pb/Cu(111) [
16
], and Ag/Fe(100) [
17
,
18
] uniform height selection was found.
Different experimental techniques have been applied (STM, STS [
19
,
20
], high-
resolution electron diffraction [
12
], surface X-ray scattering [
21
], ARPES [
22
], in
situ conductivity [
23
], etc.) and theoretical calculations using different approxima-
tion [
24
,
25
] have been used to confirm that QSE is the reason for the observed
height selection. Possibly Pb/Si(111) is the most widely studied system among
these that exhibits QSE-induced morphology because of the ease in height selec-
tion (immediately after deposition) and the unusual sharpness of the height dis-
tribution. As reviewed in Chapter 4 by Jia et al. extensive work has been carried
out both experimentally and theoretically to understand the role of QSE in film
growth. Different theoretical techniques have been applied on different models (free
electron model and first principles calculations) to calculate the observed stable
heights in Pb/Si(111). In addition the chapter discusses the control of other film
properties with height. It was found that several properties (lattice relaxation, work
function, thermal expansion, superconductivity, and chemical reactivity) oscillate
with height, as a result of how the electronic structure depends on changes; and
in particular the variation of the density of states of the confined electrons with
height. However, besides understanding how island stability relates to electron con-
finement, it is also essential to know how mass transport operates in the system
that directs the randomly deposited atoms to ideal positions to build the defect-free
islands.
QSE-driven height selection results from the dependence of the electronic energy
on island height. The island height can increase only by multiples of the step height
d
while the electron wavefunction periodicity is defined by the Fermi wavelength
λ
F
. At the preferred heights, the leakage of the wavefunction outside the confining
∼
