78
4 Measurement of Work Function
-100
0
100 200 300 400 500 600
-5
-4
-3
-2
-1
0
1
2
3
4
5
ln(J/T 2
)
(hν-e )/k B T
Fig. 4.8 Example of a plot represented by Eq. (4.9) when photoelectron emission current density
is measured as a function of temperature (closed circles), the solid line represents Eq. (4.9) with φ
= 0, and the broken line represents Eq. (4.9) shifted to fit the closed circles, giving φ = 2.6 eV
the energy of emitted electrons having zero kinetic energy with respect to the Fermi
level should give the work function. Work function measurement by photoelectron
emission yield spectroscopy (PYS), where the photoemission current is measured
as a function of the excited photon energy, is based on the former principle. To use
the latter principle, the measurement of the electron energy is essential, where the
electrons can be excited in many ways. Because the threshold energy of emitted
electrons having zero kinetic energy is called the secondary electron cutoff, this type
of method is called “secondary electron cutoff spectroscopy” in this chapter. Usually,
different names are given for different methods of electron excitation, such as UPS
(ultraviolet photoelectron spectroscopy), XPS (X-ray photoelectron spectroscopy),
and AES (Auger electron spectroscopy), where the primary aim of these techniques
is not the measurement of the work function. In order to obtain the absolute value
of the work function, the energy of emitted electrons with respect to the Fermi level
must be known. In Fig. 4.9, the principles of the above two types of electron emission
spectroscopy are schematically illustrated.
4.2.1 Photoelectron Emission Yield Spectroscopy (PYS)
The principle of photoelectron emission yield spectroscopy is schematically illustrated in Fig. 4.10. In this method, the photoelectron emission yield is measured as a
function of the photon energy. Let us assume a specimen having the DOS of an electron as shown in Fig. 4.10. Electrons having energy above E V AC − hν are excited by
photons, making the photoelectron yield proportional to the area of the DOS above
E V AC − hν. Because no electrons are excited by photons having energy below φ,
4 Measurement of Work Function
-100
0
100 200 300 400 500 600
-5
-4
-3
-2
-1
0
1
2
3
4
5
ln(J/T 2
)
(hν-e )/k B T
Fig. 4.8 Example of a plot represented by Eq. (4.9) when photoelectron emission current density
is measured as a function of temperature (closed circles), the solid line represents Eq. (4.9) with φ
= 0, and the broken line represents Eq. (4.9) shifted to fit the closed circles, giving φ = 2.6 eV
the energy of emitted electrons having zero kinetic energy with respect to the Fermi
level should give the work function. Work function measurement by photoelectron
emission yield spectroscopy (PYS), where the photoemission current is measured
as a function of the excited photon energy, is based on the former principle. To use
the latter principle, the measurement of the electron energy is essential, where the
electrons can be excited in many ways. Because the threshold energy of emitted
electrons having zero kinetic energy is called the secondary electron cutoff, this type
of method is called “secondary electron cutoff spectroscopy” in this chapter. Usually,
different names are given for different methods of electron excitation, such as UPS
(ultraviolet photoelectron spectroscopy), XPS (X-ray photoelectron spectroscopy),
and AES (Auger electron spectroscopy), where the primary aim of these techniques
is not the measurement of the work function. In order to obtain the absolute value
of the work function, the energy of emitted electrons with respect to the Fermi level
must be known. In Fig. 4.9, the principles of the above two types of electron emission
spectroscopy are schematically illustrated.
4.2.1 Photoelectron Emission Yield Spectroscopy (PYS)
The principle of photoelectron emission yield spectroscopy is schematically illustrated in Fig. 4.10. In this method, the photoelectron emission yield is measured as a
function of the photon energy. Let us assume a specimen having the DOS of an electron as shown in Fig. 4.10. Electrons having energy above E V AC − hν are excited by
photons, making the photoelectron yield proportional to the area of the DOS above
E V AC − hν. Because no electrons are excited by photons having energy below φ,