Infrared 45° reflectometry of very thin films

Abstract

It is shown that the spectra given by the difference between the reflectivity for p-polarized light and the squared reflectivity for s-polarized light (Rp−Rs2) with an angle of incidence of 45° can be used for directly determining the frequencies of zone-center transverse optical (ωTO) and longitudinal optical (ωLO) phonons in very thin polar-semiconductor films on substrates. These spectra are also useful for measuring resonance frequencies of both transverse and longitudinal optical confined phonons in ultrathin layers of superlattices.

Infrared 45� Reflectometry of Anisotropic Ultrathin Films and Heterostructures

“…In practice, that condition can be satisfied only by superlattices with layers, where the optical phonons are quantized [12 to 14]. Since these modes give rise to poles and zeros of the dielectric functions for the individual layers [6,33,34], the infrared 45 reflectometry turns out to be also appropriate for determining the frequencies of confined optical modes in superlattices [18]. Now, let us consider a superlattice formed by alternating isotropic layers, which has a total thickness (d SL ) much smaller than the wavelength of the incident light (d SL ( l).…”

“…Recently, another possibility to use infrared spectroscopy for determining the frequencies of optical phonons in thin films was proposed by the authors [18]. The new method, named infrared 45 reflectometry, is based on the measurement of the difference…”

“…In our previous paper [18], we developed a theory for interpreting infrared D 45 spectra of very thin films on substrate. These spectra allow to determine clearly the frequen-cies of both transverse and longitudinal optical phonons in the film.…”

“…These spectra allow to determine clearly the frequen-cies of both transverse and longitudinal optical phonons in the film. The theory [18] is valid only for optically isotropic films and, therefore, its application is limited. In the present paper, we shall revise the theory of infrared 45 reflectometry and generalize it to the case of anisotropic thin films (Section 2).…”

“…It should be noted that for the case of an isotropic thin film (e x e y e z e f ) the expressions (7) and (8) go over into previously-published linear formulae for R s and R p (see, for example [3, 15, 29 to 31]), and D 45 (10) acquires the form [18,24]…”