Confocal Micro-Raman Characterization of SiC Epilayers
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233 Mat. Res. Soc. Symp. Proc. Vol. 588 ©2000 Materials Research Society
large zone phonon dispersions of the 3C SiC and has been proven by successful mapping of the [111 ] phonon dispersion of 3C SiC using the phonon frequencies of various polytypes [2,5]. In a polar semiconductor such as SiC, the LO phonon interact with the free-carrier plasmon modes, resulting in coupled modes at
are the bare LO and TO phonon frequencies, and where wo 1 and ow,
ie-
tP
(2)
is the free-carrier plasmon frequency. For visible Raman scattering in wide-gap semiconductors, the resonance effect can be neglected due to smaller photon energy. In this case, the light scattering by the coupled modes through the deformation potential (u) and electro-optic (E) mechanisms dominate over that through the charge density fluctuation mechanism. The scattering cross section can be written as [3] d2S
dd2
with
-
A =(w)2 -
F(nm,+ 1)o 2 2
2oF0(w1o)- )2)2
/A
(3)
2 2 ) ((o2 - o)) +o) 212F((o2 -(
22
)
(3a)
and
(3b) z = 0o 2(l + C), where F is a scaling factor, no, is the Bose-Einstein factor, F the plasmon damping constant, and C is the Faust-Henry coefficient determining the ratio of scattering by LO and TO phonons in undoped crystals. Equation (3) can be used to determine the free-carrier concentration and damping constant of the plasmon mode. The phonons and plasmons in most of the SiC polytypes show anisotropic behaviors. In case of the (u, E) scattering mechanism, the plasmon excitations involving the anisotropic effective mass and dielectric constant behave as single component plasmons. This work only deals with the coupled mode of AI symmetry where both the electronic and lattice displacements are along the [0001] axis. Therefore, the axial effective mass m//* and dielectric constant eP,are used in the data analysis. EXPERIMENT The bulk and epitaxial SiC samples used in this work were commercially obtained from Cree Research and ATMI/Epitronics. All 4H and 6H substrates were miscut 8' and 3.5' off the {0001 } plane, respectively. The samples were characterized using atomic force microscopy (AFM), Rutherford backscattering (RBS), spectroscopic ellipsomitry and high-resolution transmission electron microscopy (HRTEM). Raman measurements were performed using a Dilor XY800 spectrometer. The laser beam at 4880 A and 5145 A from an Ar+-ion laser was focused down to about 0.8 jim in diameter on to the {0001} surface of the samples by a 100x (N.A.=0.95) objective. The scattered light was collected through the same objective and directed to an adjustable pinhole placed in the image plane of the microscope. The optically conjugated confocal arrangement ensures that only the
234
light coming from the focal plane reaches the spectrometer. By adjusting the size of the confocal pinhole depth resolution smaller than 2 jim can be achieved. Three 1800 grooves/mm gratings were used in the spectrometer with the first two in the fore-monochromator working subtractively and serving as a filter stage. The spectral resolution is about 3 cml1 in thi
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