The silicon (100) surface with a silicon dioxide layer forms the
interface of most relevance to modern microelectronics technology,
as it lies at the heart of modern MOSFETs.
The semiconductor/oxide interface is being studied by
density functional theory (DFT) calculations on model systems.
Silicon oxide layers formed between silicon substrates and oxides
with large dielectric constants (high k) play an important role in
determining electrical properties for microelectronic applications
such as gates stacks and capacitors.
In particular, interfacial layers set limits on the equivalent
oxide thickness (EOT) for high k oxides formed on silicon.
For sub-nanometer interfacial layers,
an amorphous, non-stoichiometric oxide forms; an increase in the
dielectric constant relative to SiO2 in this layer has been inferred
from experiment and theoretically predicted.
Formation of oxygen vacancies within interfacial layers
further reduces the oxygen content and acts to increase the
dielectric constant.
We have investigated oxygen vacancy formation at silicon/oxide interfaces
and find that their formation is energetically more favorable in
non-stoichiometric SiOx than in SiO2,
implying a higher density of oxygen vacancies within interfacial layers.
A representative interface structure
is shown here.
For oxide layers occurring at high k/silicon interfaces (shown on right),
the reduced stoichiometry relative to bulk silica
implies that oxygen removal has taken place.
Coupled to the observation that oxygen vacancy formation in
silica is more favorable than in HfO2,
it is inferred that oxygen is kinetically driven away from the
interface towards the hafnia layer.
This effect contributes to reducing the stoichiometry in the SiOx
region, with an accompanying increase in interfacial dielectric constant.
Hafnium pseudo-binary oxides,
(HfO2)x(SiO2)1-x,
are being considered as high-k gate dielectrics in field effect transistors,
as a compromise between high permittivity (k) and
thermal stability during processing.
Using model oxide mixtures based on hafnon,
we have examined the stability before and after annealing
using static density functional theory and ab initio molecular dynamics.
We find that hafnium silicates are thermodynamically unstable with
respect to HfO2 and SiO2.
SiO2-rich mixture (blue=Hf, yellow=Si, red=O).
Mechanisms of segregation on the atomic scale are elucidated,
showing why SiO2-rich mixtures undergo spinodal decomposition
to give 4-coordinate Si (left).
By contrast, HfO2-rich phases are metastable,
but also decompose during annealing below typical process temperatures
to give 6-coordinate Hf (right).
