Spectroscopy of localized and collective vibrational modes in compound semiconductors, their alloys and epilayers
Abstract
The longitudinal and transverse zone-center optical phonons in thin films of polar semiconductors, as well as, the local vibrational modes (LVMs) and gap modes of impurities in zincblende and wurtzite semiconductors are addressed with Fourier transform infrared spectroscopy. The observation of the zone-center transverse optic (TO) and longitudinal optic (LO) phonons of a polar semiconductor crystal as transmission minima in its infrared spectrum, when measured with radiation obliquely incident on a thin film of the crystal, i.e., the Berreman effect, is applied to a diverse group of II-VI and III-V epilayers and superlattices grown by molecular beam epitaxy, as well as epilayers of SiC on a Si substrate and free-standing CdS films grown by chemical vapor deposition. Whereas the TO phonon appears irrespective of the polarization of the incident radiation for both normal and oblique incidence, the LC phonon can be detected only in the latter with light polarized in the plane of incidence. Localized vibrations of substitutional $\rm Mg\sp{2+},\ Ca\sp{2+}$ and S$\sp{2-}$ in CdTe, ZnTe and CdSe are observed as sharp absorption lines in the infrared and display as many lines as there are isotopes of the impurity, their intensities being proportional to their natural abundances. The lowering of the site symmetry from T$\sb{d}$ in the zincblende CdTe and ZnTe to C$\sb{3v}$ in the wurtzite CdSe causes the triply degenerate $\Gamma\sb5$ mode in the former to split into a singlet $\Gamma\sb1$ and doublet $\Gamma\sb3$ in the latter, the singlet and the doublet appearing only for the electric vector along the optic axis and normal to it, respectively. The frequency of a LVM of a light impurity substituting for either the group II of the group VI host atom in a II-VI semiconductor exhibits two types of isotope shifts: in one, the shift is caused by the specific isotope of the impurity whereas in the other a shift occurs in association with the isotopes of the nearest neighbor (NN) atoms of the impurity. The shifts associated with the four tetrahedrally bonded NNs can be accounted for by modeling the impurity and its NNs as a XY$\sb4$ molecule. The model also satisfactorily describes the anisotorpic effects for an LVM in a wurtzite host.
Degree
Ph.D.
Advisors
Ramdas, Purdue University.
Subject Area
Condensation|Materials science
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