4.3 Utilizing Contact Potential Difference Measurement
91
E VAC
E C
E V
E F
IP
Δ
BP
Nonmetallic
specimen
Nonmetallic
specimen
Reference
probe
(metal)
Reference
probe
(metal)
Fig. 4.24 Schematic illustration of contact potential difference measurement for a nonmetallic
specimen (see text for explanation)
is not easy to maintain the same distance when the specimen must be removed for
treatment in another chamber or when the specimen is exchanged. From Eq. (4.15),
it can be seen that when the amplitude of the induced current is plotted as a function
of BP, the gradient of the plot is inversely proportional to x
2
0 . Therefore, by adjusting
the distance so that the gradient in Fig. 4.23 is kept constant throughout the whole
set of measurements, the effect of the distance on CPD measurements is minimized.
In this method, no direct current flows between the reference and the specimen; therefore, the CPD value of semiconducting or insulating specimens can be
measured. In Fig. 4.24, a potential diagram of the capacitor, where the specimen is
either a semiconductor or insulator (nonmetallic), is shown. Because CPD values are
determined by the position of the Fermi level regardless of the electron density, the
principle of the measurement is exactly the same as that described above for metal
specimens.
4.3.2 Diode Method
This method is less commonly used these days since many sophisticated methods
have been developed. However, this method used to be popular owing to its concise
experimental setup [11, 12]. Figure 4.25 illustrates the measurement principle. One
must prepare a cathode that emits electrons. E is the electric field applied to induce
electron emission from the cathode. When the work function of the cathode (φ B ) is
lower than that of the specimen (φ A ), the emitted electrons cannot reach the specimen
owing to the potential barrier under zero BP. Therefore, no current flows between
the cathode and the specimen. While applying a BP to the specimen, a current (i)
starts flowing when the potential barrier disappears. The BP at this time is equal
to the work function difference between the cathode and the specimen. A plot of
current versus BP is schematically illustrated in Fig. 4.26. In practice, the current–
BP curve for the reference specimen is first measured and then the BP(V 0 ) required
91
E VAC
E C
E V
E F
IP
Δ
BP
Nonmetallic
specimen
Nonmetallic
specimen
Reference
probe
(metal)
Reference
probe
(metal)
Fig. 4.24 Schematic illustration of contact potential difference measurement for a nonmetallic
specimen (see text for explanation)
is not easy to maintain the same distance when the specimen must be removed for
treatment in another chamber or when the specimen is exchanged. From Eq. (4.15),
it can be seen that when the amplitude of the induced current is plotted as a function
of BP, the gradient of the plot is inversely proportional to x
2
0 . Therefore, by adjusting
the distance so that the gradient in Fig. 4.23 is kept constant throughout the whole
set of measurements, the effect of the distance on CPD measurements is minimized.
In this method, no direct current flows between the reference and the specimen; therefore, the CPD value of semiconducting or insulating specimens can be
measured. In Fig. 4.24, a potential diagram of the capacitor, where the specimen is
either a semiconductor or insulator (nonmetallic), is shown. Because CPD values are
determined by the position of the Fermi level regardless of the electron density, the
principle of the measurement is exactly the same as that described above for metal
specimens.
4.3.2 Diode Method
This method is less commonly used these days since many sophisticated methods
have been developed. However, this method used to be popular owing to its concise
experimental setup [11, 12]. Figure 4.25 illustrates the measurement principle. One
must prepare a cathode that emits electrons. E is the electric field applied to induce
electron emission from the cathode. When the work function of the cathode (φ B ) is
lower than that of the specimen (φ A ), the emitted electrons cannot reach the specimen
owing to the potential barrier under zero BP. Therefore, no current flows between
the cathode and the specimen. While applying a BP to the specimen, a current (i)
starts flowing when the potential barrier disappears. The BP at this time is equal
to the work function difference between the cathode and the specimen. A plot of
current versus BP is schematically illustrated in Fig. 4.26. In practice, the current–
BP curve for the reference specimen is first measured and then the BP(V 0 ) required