8.2. INFRARED FREQUENCY RANGE
203
8.2.3. Raman Spectroscopy
The general principles of Raman spectroscopy were explained in Section 3.4.1.
Raman scattering measures the frequency shift AW = Wphonon = I w, , - wScatI
between the incident uinc and scattered uscat light frequencies when Wphonon is an
optical phonon mode vibration. When Wphonon is an acoustic phonon, the process
is called Brillouin scattering, discussed in the next section. Figure 2.10 makes it
clear that the optical mode corresponds to high-frequency lattice vibrations,
and the acoustic mode conforms to lattice vibrations at much lower frequencies:
w , ~ ~ , ~
<< coopt. The scattered frequency has the value uscat = wine f Wphonon where the
negative sign in the preceding expression for Am corresponds to a Stokes line, and the
positive sign denotes an anti-Stokes line, as explained in Section 3.4.1. These two
types of scattering that entail a change in frequency of the emitted photon are called
inelastic. When there is no frequency shift (Le., Am = 0), then the scattering is the
elastic Rayleigh type that takes place in X-ray diffraction. Here and in the next section
we describe some Raman and Brillouin spectra from Milani and Bottani (2000).
The Raman spectrum of bulk crystalline germanium exhibits a narrow absorption
line, E 3 cm-' wide, arising from the r25+ optical phonon mode at the frequency of
300cm-', as indicated in the inset to Fig. 8.10. When Ge is deposited on a silica
(Si02) film, the Raman spectrum is featureless except for a broad shoulder near
270cm-', as shown in the lowest spectrum of this figure. Annealing causes the
shoulder to disappear, and the 300-cm-' crystalline silica peak to appear. Figure 8.1 1
illustrates how this peak broadens and shifts to lower frequencies with decreasing
particle size. Its width decreases with an increase in temperature and annealing time,
as shown in Fig. 8.12, and these data provide an estimated particle size between 6
250
300
350
frequency shift (cm-1)
Figure 8.10. Raman spectra of Ge films on Si02 substrates as deposited and after oxidation at
800 and 1000°C. [From P. Milani and C. E. Bottani, in Nalwa (2000), Vol. 2, Chapter 4, p. 243.1
203
8.2.3. Raman Spectroscopy
The general principles of Raman spectroscopy were explained in Section 3.4.1.
Raman scattering measures the frequency shift AW = Wphonon = I w, , - wScatI
between the incident uinc and scattered uscat light frequencies when Wphonon is an
optical phonon mode vibration. When Wphonon is an acoustic phonon, the process
is called Brillouin scattering, discussed in the next section. Figure 2.10 makes it
clear that the optical mode corresponds to high-frequency lattice vibrations,
and the acoustic mode conforms to lattice vibrations at much lower frequencies:
w , ~ ~ , ~
<< coopt. The scattered frequency has the value uscat = wine f Wphonon where the
negative sign in the preceding expression for Am corresponds to a Stokes line, and the
positive sign denotes an anti-Stokes line, as explained in Section 3.4.1. These two
types of scattering that entail a change in frequency of the emitted photon are called
inelastic. When there is no frequency shift (Le., Am = 0), then the scattering is the
elastic Rayleigh type that takes place in X-ray diffraction. Here and in the next section
we describe some Raman and Brillouin spectra from Milani and Bottani (2000).
The Raman spectrum of bulk crystalline germanium exhibits a narrow absorption
line, E 3 cm-' wide, arising from the r25+ optical phonon mode at the frequency of
300cm-', as indicated in the inset to Fig. 8.10. When Ge is deposited on a silica
(Si02) film, the Raman spectrum is featureless except for a broad shoulder near
270cm-', as shown in the lowest spectrum of this figure. Annealing causes the
shoulder to disappear, and the 300-cm-' crystalline silica peak to appear. Figure 8.1 1
illustrates how this peak broadens and shifts to lower frequencies with decreasing
particle size. Its width decreases with an increase in temperature and annealing time,
as shown in Fig. 8.12, and these data provide an estimated particle size between 6
250
300
350
frequency shift (cm-1)
Figure 8.10. Raman spectra of Ge films on Si02 substrates as deposited and after oxidation at
800 and 1000°C. [From P. Milani and C. E. Bottani, in Nalwa (2000), Vol. 2, Chapter 4, p. 243.1