A thin film is a layer of material ranging from fractions of a
nanometer (
monolayer) to several
micrometer Micrometer can mean:
* Micrometer (device), used for accurate measurements by means of a calibrated screw
* American spelling of micrometre
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...
s in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many applications. A familiar example is the household
mirror
A mirror or looking glass is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the im ...
, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of
silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as
sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as
magnetic recording media,
electronic semiconductor devices,
integrated passive devices,
LEDs,
optical coatings (such as
antireflective
An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses, other optical elements, and photovoltaic cells to reduce reflection. In typical imaging systems, this improves the effic ...
coatings), hard coatings on cutting tools, and for both energy generation (e.g.
thin-film solar cells) and storage (
thin-film batteries). It is also being applied to pharmaceuticals, via
thin-film drug delivery. A stack of thin films is called a
multilayer.
In addition to their applied interest, thin films play an important role in the development and study of materials with new and unique properties. Examples include
multiferroic materials, and
superlattices that allow the study of quantum phenomena.
Nucleation
Nucleation is an important step in growth that helps determine the final structure of a thin film. Many growth methods rely on nucleation control such as atomic-layer epitaxy (atomic layer deposition). Nucleation can be modeled by characterizing surface process of
adsorption
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
,
desorption
Desorption is the physical process where a previously adsorbed substance is released from a surface. This happens when a molecule gains enough energy to overcome the activation barrier of the bounding energy that keeps it in the surface.
There ...
, and
surface diffusion
Surface diffusion is a general process involving the motion of adatoms, molecules, and atomic clusters ( adparticles) at solid material surfaces.Oura, Lifshits, Saranin, Zotov, and Katayama 2003, p. 325 The process can generally be thought of in ...
.
Adsorption and desorption
Adsorption
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
is the interaction of a vapor atom or molecule with a substrate surface. The interaction is characterized the
sticking coefficient, the fraction of incoming species thermally equilibrated with the surface.
Desorption
Desorption is the physical process where a previously adsorbed substance is released from a surface. This happens when a molecule gains enough energy to overcome the activation barrier of the bounding energy that keeps it in the surface.
There ...
reverses adsorption where a previously adsorbed molecule overcomes the bounding energy and leaves the substrate surface.
The two types of adsorptions,
physisorption
Physisorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption.
Overview
The fundamental interacting force of physisorption is Van der Waals force. Even ...
and
chemisorption
Chemisorption is a kind of adsorption which involves a chemical reaction between the surface and the adsorbate. New chemical bonds are generated at the adsorbent surface. Examples include macroscopic phenomena that can be very obvious, like cor ...
, are distinguished by the strength of atomic interactions. Physisorption describes the
Van der Waals bonding between a stretched or bent molecule and the surface characterized by adsorption energy
. Evaporated molecules rapidly lose kinetic energy and reduces its free energy by bonding with surface atoms. Chemisorption describes the strong electron transfer (ionic or covalent bond) of molecule with substrate atoms characterized by adsorption energy
. The process of physi- and chemisorption can be visualized by the potential energy as a function of distance. The equilibrium distance for physisorption is further from the surface than chemisorption. The transition from physisorbed to chemisorbed states are governed by the effective energy barrier
.
Crystal surfaces have specific bonding sites with larger
values that would preferentially be populated by vapor molecules to reduce the overall free energy. These stable sites are often found on step edges, vacancies and screw dislocations. After the most stable sites become filled, the adatom-adatom (vapor molecule) interaction becomes important.
Nucleation models
Nucleation kinetics can be modeled considering only adsorption and desorption. First consider case where there are no mutual
adatom interactions, no clustering or interaction with step edges.
The rate of change of adatom surface density
, where
is the net flux,
is the mean surface lifetime prior to desorption and
is the sticking coefficient: