Infrared observation of transverse and longitudinal polar optical modes of semiconductor films: Normal and oblique incidence

Structural and chemical analysis of annealed plasma-enhanced atomic layer deposition aluminum nitride films

“…43 Both of the N 2 :H 2 flow ratio annealing series exhibited similar FTIR spectra. In the low energy end of the spectra, the typical IR phonon modes of AlN are present and in good agreement with the previous research: [44][45][46][47] The valleys at 669 and 615 cm À1 can be assigned to the E1(TO) and A1(TO) modes, respectively. Moreover, the broad valley at 885 cm À1 can be associated with the A1(LO) mode.…”

Far-infrared transmission in GaN, AlN, and AlGaN thin films grown by molecular beam epitaxy

“…In contrast, both the TO and LO modes can be excited by radiation that impinges on the film at an angle i with the electric field polarized parallel to the plane of incidence ͓i.e., a p wave, also known as transverse-magnetic ͑TM͒ wave͔. 19,20 In such case, the electric field has a component in the direction of the particle motion for both the TO and LO phonons.…”

“…20 and references therein for details͒. In what follows, we focus on TM waves since only this radiation allows the observation of transmission minima at the frequency of LO modes.…”

“…3 As first demonstrated by Berreman, 19 the frequency of both zone-center TO and LO phonons of polar materials can be directly measured in far-IR transmission experiments for radiation at oblique incidence and with the electric vector polarized parallel to the plane of incidence ͑TM waves͒. 19,20 The observation of transmission minima at the frequency of LO modes ͑i.e., the Berreman effect͒ and TO modes is limited to samples that are significantly thinner than the wavelength of the electromagnetic radiation corresponding to the reststrahlen band of the material. This condition, easily fulfilled for epilayers and heterostructures produced by molecular beam epitaxy ͑MBE͒ or chemical vapor deposition, provides an alternative means to determine the frequency of both TO and LO phonons in polar compounds and structures.…”

“…This condition, easily fulfilled for epilayers and heterostructures produced by molecular beam epitaxy ͑MBE͒ or chemical vapor deposition, provides an alternative means to determine the frequency of both TO and LO phonons in polar compounds and structures. 20,21 The transmission measurements at oblique incidence also allow one to directly measure the frequency of LO-plasmon coupled modes, from which the free-carrier concentration of the samples can be evaluated. 22 The observation of transmission minima corresponding to TO or LO phonons in group-III nitrides is however limited to samples grown on substrates that are not highly light absorbing in the spectral region of interest.…”