3.2. Characterization Methods
3.2.9. Raman Spectroscopy
Raman spectroscopy relies on the interaction of light with matter. When light is scat- tered by a material two fundamental modes can occur, elastic and inelastic scattering. The most dominant one is elastic scattering (Rayleigh scattering) where the incident and final wavelengths are the same. However, a certain portion of the incident light is scattered inelastically, meaning the final differs from the incident wavelength. This inelastic scattering is also referred to as Raman scattering. It is a result of the interac- tion of radiation with the vibrational or rotational modes of molecules or phonons in the case of solids. The scattering event consists of the excitation of an electron from a ground state with a photon at the laser frequency νL to a virtual intermediate state.
This excited state simultaneously decays to emit a scattered photon of frequency νS.
The electron returns to a ground state, that can be in a different vibrational mode. The scattered photon will hence take up a different frequency. If νS < νL this process
In terms of Raman scattering by convention wavenumbers ¯ν i.e. inverse wavelengths are used: ¯ν = 1 λ = ν c. ¯ν is given in units of cm −1
, therefore a factor 107 has to be
included to calculate the wavenumber from wavelengths in nm. For Raman spectra the Raman shift ∆¯ν rather than absolute wavenumbers are used. ∆¯ν = ¯νRaman− ¯νLaser.
A Raman shift of ∆¯ν = 0 represents the excitation frequency of the laser [177, p. 20-
21]. Because the Raman shift is calculated relative to the wavenumber of the laser, it is independent of the actual laser frequency. Different excitation frequencies can be applied, while the peaks are observed at the same position in the Raman spectra. The used Raman spectrometer is a LabRam Aramis spectrometer from HORIBA Jobin
Figure 3.12.: Raman spectrometer with schematic light path. The laser light (473 nm or 633 nm) is directed on the sample after passage through a pinhole a neutral density filter for light attenuation as well as a microscope objective. The sample is placed in confocal configuration under the objective, that collects the light from the sample. The light passes a Notch filter for blocking the central laser wavelength and is then spectrally analyzed by a grating and a CCD detector, from [178].
Yvon. The schematic configuration including the optical path is illustrated in Fig. 3.12. Two different lasers can be used as excitation source, a 473 nm diode laser and a 633 nm Helium Neon laser. The laser light first passes an adjustable pinhole and a neutral density filter. With the filter the light intensity is adjusted. This is often necessary when adjusting the sample. However too high intensities would alter the sample properties by recrystallization etc. The light is then focused on the sample with a microscope objective. The scattered and reflected light is collected by the same objective, since the sample is mounted in a confocal configuration. The collected light passes a Notch filter for blocking the Rayleigh scattered light at the central laser wavelength. Finally it is spectrally analyzed by a grating and a CCD detector.
3.2. CHARACTERIZATION METHODS
As discussed above the choice of the laser wavelength does not change the results that are obtained. Due to the different penetration depth of blue and red light into the material (a-Si:H for example), the use of both lasers gives information concerning bulk and surface properties. The collection depth dcol for the Raman signal is effectively
the half of the penetration depth (dcol= 2α1 ), because the scattered light has to leave
the sample in reflection geometry [177, p. 32-34]. Compared to Fig. 5.19 (b) where the penetration depth of an a-Si:H sample is shown, blue light penetrates approximately 50 nm deep into the material. The red laser reaches several hundreds of nanometers deep. In the first case information about near surface regions are extracted, while the red laser gives information about the bulk properties of a film. The combination of both measurements contains valuable information on the growth behavior of materials with crystallinity variation. For example in µc-Si:H where the crystal growth starts from a seeding region in a column-like growth [179]. Fig. 3.13 (a) illustrates the Raman
Figure 3.13.:(a) Raman spectra of amorphous and highly microcrystalline material measured with 633nm excitation wavelength. (b) Fitting of a Raman spectrum with three Gaussian peaks at I480 corresponding to a-Si:H, I510 and I520 corresponding to the crystalline
phase.
spectra of an a-Si:H and a µc-Si:H sample. The a-Si:H sample shows a characteristic peak at 480 cm−1
. The µc-Si:H sample exhibits a dominant peak near 520 cm−1
similar to crystalline silicon but also with a contribution originating from the amorphous phase near 480 cm−1
. This is understandable because an amorphous phase is still evident in the µc-Si:H material. The detailed analysis of this spectrum is shown in (b). It is fitted by means of three individual Gaussian peaks at 480 cm−1
corresponding to the amorphous phase, a peak at 520 cm−1
related to crystalline silicon and an additional peak in between at 510 cm−1
. The latter is correlated to a defective crystalline phase [179]. From the areas under the Gaussian peaks I480, I510 and I520 a crystallinity factor
ΦC can be extracted:
ΦC =
I510+ I520
I480+ I510+ I520
, (3.17)
that gives the ratio of Raman intensities according to the crystalline phase (I510+ I520)
in relation to the total intensity. This crystallinity factor is not equal to the actual crystalline volume fraction in device grade material, but can be considered as a lower limit for it [180]. To estimate the real volume fraction of crystalline material a correction factor that accounts for different cross sections of Raman scattering for a-Si:H and c-Si material has to be taken into account [181]. In this work the absolute value for the determination of the crystalline fraction is not of major importance, only the trend of crystallinity for different samples is compared (see section 5.5.5). Therefore, a correction for the different sensitivity of silicon phases was not considered.