64
3 Modification of the Work Function
Higher or longer heaang
a)
b)
c)
Sub.
film
Sub.
film
Sub.
film
Sub.
alloy, intermetallics
Grain boundary diffusion
is dominant
Fig. 3.30 Schematic illustrations of nonequilibrium surface segregation, where diffusion stops
at (a) and no further interface reaction occurs. With annealing at higher temperature or much
longer time, interface reaction starts (b) and finally film element is consumed to form an alloy or
intermetallic (c). (see text for explanation)
(ZrO 2 ) coating instead of a thoria coating has been developed for the same application
to avoid the use of radioactive thorium. The adsorption of alkali metals on various
materials is also a popular method of decreasing the work function. To generate
heavy ions for implantation, a Cs coating is used to promote the ionization of heavy
ions. Hydrogen termination on semiconductors, such as silicon, diamond, and GaAs,
is a frequently used technique for modifying surface band bending and sometimes
results in the formation of a surface with a negative electron affinity (NEA), which
is utilized in photodetectors.
3.4 Deposition
To form a surface with adsorbate atoms, as mentioned in Sect. 3.3, adsorbate atoms
are deposited on the mother surface. If the amount of deposited atoms increases
so that the coverage of the deposited atoms exceeds one layer, we call this process
“deposition” instead of “adsorption”. Here the surface is completely covered by the
deposited atoms, whereas the surface is partially covered in the case of adsorption.
The work function decreases with adsorption for most elements, as shown in Fig. 3.29.
When the thickness of the adsorbed layer exceeds one atomic layer, we can see that
the work function gradually approaches that of the deposited element and reaches
that of the deposited element at around 3–4 atomic layers. This situation is realized
when the deposited layer grows in a layer-by-layer mode, as schematically illustrated
in Fig. 3.32. If a deposited layer does not grow in a layer-by-layer mode, islands are
formed, and the work function reaches that of the layer of metal with a rough surface
(lower than that with a flat surface, as discussed in Sect. 2.3) when more than 3–4
layers of the surface are completely covered by the overlayer metal. An exception
to the above case of layer-by-layer growth occurs in mismatched lattice epitaxy,
where a film with a crystal structure not in the bulk phase diagram grows to a certain
thickness and has a different work function from that of a film with a crystal structure
in the bulk diagram.
3 Modification of the Work Function
Higher or longer heaang
a)
b)
c)
Sub.
film
Sub.
film
Sub.
film
Sub.
alloy, intermetallics
Grain boundary diffusion
is dominant
Fig. 3.30 Schematic illustrations of nonequilibrium surface segregation, where diffusion stops
at (a) and no further interface reaction occurs. With annealing at higher temperature or much
longer time, interface reaction starts (b) and finally film element is consumed to form an alloy or
intermetallic (c). (see text for explanation)
(ZrO 2 ) coating instead of a thoria coating has been developed for the same application
to avoid the use of radioactive thorium. The adsorption of alkali metals on various
materials is also a popular method of decreasing the work function. To generate
heavy ions for implantation, a Cs coating is used to promote the ionization of heavy
ions. Hydrogen termination on semiconductors, such as silicon, diamond, and GaAs,
is a frequently used technique for modifying surface band bending and sometimes
results in the formation of a surface with a negative electron affinity (NEA), which
is utilized in photodetectors.
3.4 Deposition
To form a surface with adsorbate atoms, as mentioned in Sect. 3.3, adsorbate atoms
are deposited on the mother surface. If the amount of deposited atoms increases
so that the coverage of the deposited atoms exceeds one layer, we call this process
“deposition” instead of “adsorption”. Here the surface is completely covered by the
deposited atoms, whereas the surface is partially covered in the case of adsorption.
The work function decreases with adsorption for most elements, as shown in Fig. 3.29.
When the thickness of the adsorbed layer exceeds one atomic layer, we can see that
the work function gradually approaches that of the deposited element and reaches
that of the deposited element at around 3–4 atomic layers. This situation is realized
when the deposited layer grows in a layer-by-layer mode, as schematically illustrated
in Fig. 3.32. If a deposited layer does not grow in a layer-by-layer mode, islands are
formed, and the work function reaches that of the layer of metal with a rough surface
(lower than that with a flat surface, as discussed in Sect. 2.3) when more than 3–4
layers of the surface are completely covered by the overlayer metal. An exception
to the above case of layer-by-layer growth occurs in mismatched lattice epitaxy,
where a film with a crystal structure not in the bulk phase diagram grows to a certain
thickness and has a different work function from that of a film with a crystal structure
in the bulk diagram.