Raman Spectroscopy Study of Pulsed Laser Induced Structural Transformations in Amorphous Ge Films
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Rayleigh criterion, d = 1.22 X / NA (where X is the laser wavelength and NA is the Numerical Aperture of the microscope objective). This results in laser beam diameters at the focal plane smaller than 1 gm, which is the spatial resolution of the Raman microprobe (typically X = 514.5 nm and NA= 0.95). In our case this resolution was poorer due to the low thermal conductivity of our samples, either amorphous or polycrystalline, which resulted in laser induced overheating, producing spectral broadening and local annealing. In order to avoid these experimental drawbacks, the excitation laser beam was defocused for all the measurements presented herein, that were thus free of laser induced artifacts. The size of the laser beam was a few tms. The detection of the Raman scattered light was made with an intensified silicon photodiode array. The microRaman signal was very weak for a-Ge, requiring thus long integration times and tedious spectra acquisition. The samples of the present study are 50 nm thick amorphous Germanium (a-Ge) films grown at room temperature by DC sputtering on either glass or (100) silicon substrates covered by its native oxide layer. TEM (Transmission Electron Microscopy) and Spectroscopic Ellipsometry showed that the films were amorphous with a low density of voids [9]. These samples were irradiated in air with 10 ps laser pulses (X,=583 nm) provided by synchronously pumped dye (Rhodamine 66), which the output is amplified by pulsed dye laser amplifier (Kitton Red-620). The laser beam (elliptical) is focused onto the sample surface to a size in the order of 700 pm. The laser fluences at the maximum of the intensity distribution ranged typically from 0 to 200 mJ/cm 2 . A network of ablations regularly spaced were made prior to the irradiations in order to easily localize the irradiation spots, which were surrounded by the ablations, fig. 1.
Figure 1.- Network of laser ablations, the irradiation area is indicated. Raman spectroscopy of itc-Ge and a-Ge Raman spectra of a-Ge and c-Ge are shown in fig.2. The first Raman spectrum of crystalline Ge presents an optical mode at 302 cm-1 with a FWHM (Full Width at Half Maximum) of 3cm- 1 . The momentum conservation rule allows only scattering by zone center (q--0) phonons. When this rule is relaxed, due to translational symmetry breakdown, other phonons can participate in the Raman scattering resulting in asymmetrical broadening and frequency downshift of the first order Raman spectrum. Assuming a Gaussian localization factor, W(q) -exp ( -q2 L 2 /4 ), where L is the phonon correlation length (average distance between defects or crystal size) [3, 5] this size can be estimated experimentally by fitting the Raman intensity to the following relationship [3, 5]:
436
TO
Fo= 4 cm-1
-
a o
0
20
•0
400
/2
100
60
. ...... IL ......
200
Wavenumber (cm-I)
300
Wavenumber (cm-1)
Figure 2.- Typical Raman spectrum of crystalline Ge (left) and amorphous Ge (right) 1(w)
0
2 2 exp(q L /4) d~q 2 2 [w(q) - Wo] + (F0/2)
where q is expressed in Brillouin unit
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