20
2 What is the Work Function?: Definition and Factors …
Table 2.2 Influence of crystal orientation on work function values for some metals
Crystal orientation
Metal
Crystal lattice
(110)
(100)
(111)
Cu
fcc
4.48
4.59
4.94
Ag
fcc
4.52
4.64
4.74
Ni
fcc
5.04
5.22
5.35
Ir
fcc
5.42
5.67
5.76
Au
fcc
5.37
5.47
5.31
Al
fcc
4.06
4.41
4.24
W
bcc
5.25
4.63
4.47
Mo
bcc
4.95
4.53
4.55
Ta
bcc
4.80
4.15
4.00
Nb
bcc
4.87
4.02
4.36
Level out of spreading
electrons
Fig. 2.14 Schematic of cross section of a stepped surface (see text for explanation)
whose flatness is guaranteed. The reason is that after ion sputtering to remove the
contaminant on the surface, which makes the surface rough, the heating to recover
the surface smoothness by atomic diffusion is sometimes insufficient, especially for
metals with high melting temperatures, resulting in a clean but rough surface. There
are many steps on a rough surface, resulting in a lower work function than that on a
flat surface. Some examples for Pt and Rh [13] are listed in Table 2.3.
Concerning surface roughness, one more interesting phenomenon has been
proposed. When a film is grown homoepitaxially, the surface roughness changes in a
manner dependent on the growth mode. The possibility of distinguishing the growth
mode from the change in the work function between the instantaneous nucleation
mode and the progressive nucleation mode has been proposed [14]. The calculated
changes in the work function during the growth for the two modes are shown in the
left side of Fig. 2.16.