Infrared reflectance and transmission spectra in II-VI alloys and superlattices

Abstract

Room temperature measurements of the far-infrared (FIR) reflectance spectra are reported for the polar optical phonons in a series of bulk Cd x Zn 1−x Te (0 x 1) and CdSe x Te 1−x (0 < x 0.35) crystals grown by Bridgman technique. The composition-dependent spectra exhibited many fundamental aspects of lattice vibrations, while the results of long-wavelength optical modes for the end member binary compounds displayed phonon values in good agreement with the existing inelastic neutron scattering and/or Raman spectroscopy data. Using a standard methodology of multilayer optics with effective-medium dielectric tensors, we simulated the FIR transmission and reflectivity spectra at oblique incidence (Berreman's effect) in many II-VI free-standing thin films, epilayers and superlattices. Berreman's approach provided us with a strong basis for analyzing the infrared experimental data and offered an effective means of estimating the zone-center transverse and longitudinal optical (ω TO , ω LO) phonon frequencies in polar-semiconductor thin films and heterostructure materials of increasing technological importance.

Polarization Dependent Reflectivity and Transmission for Cd1-Xznxte/GaAs(001) Epifilms in the Far-Infrared and Near-Infrared to Ultraviolet Region

“…The Cd x Zn 1-x Te (0 ≤ x ≤ 1) samples used in the FIR (5 meV ≤ E ≤ 100 meV) reflectivity measurements [9] were grown at the Massachusetts Institute of Technology using the Bridgman technique. The CZT ternary alloys were prepared by reacting the 99.9999% pure elemental constituents in the evacuated sealed quartz tubes at ~ 1150°C.…”

“…The II-VI based electronics demand [7][8][9][10] high-quality crystalline materials with fewer defects. In the as grown CZT alloys, the constraints of phase diagram necessitating higher growth temperatures usually instigate intrinsic defects.…”

“…In the electronic industry, as the veracity of using semiconductor materials intensifies -so does the compulsion of employing reliable and reproducible methods for appraising their distinctive qualities. In assessing the nature of defects and degree of crystallinity, many experimental techniques have been employed in the past, such as the Fourier transform infrared (FTIR) reflectivity and transmission [9,10], Raman scattering (RS) [8], photoluminescence (PL) [11], synchrotron X-ray diffraction (S-XRD) topography [1][2][3], and deep level transient spectroscopy (DLTS) [12], etc. In addition, the spectroscopic ellipsometry (SE) is perceived as an equally valuable tool for appraising the optical constants of semiconductor materials and evaluating the epifilm thickness [13].…”

“…Again, no SE measurements are available for assessing the Cd 1-x Zn x Te/GaAs epifilm thickness. Earlier, the vibrational properties of Cd 1-x Zn x Te were acquired by using FTIR [9,10] and RS [8] spectroscopy with a limited alloy composition, x. The results when analyzed by a classical method envisioned a "two-phonon-mode" behavior.…”

“…Accurate model dielectric functions are established appositely to extort the optical constants of all the involved semiconductor materials. The simulated dielectric functions ( ) ε ω  and refractive indices ( ) n ω  are meticulously appraised in the FIR → NIR → UV energy range by comparing them against the existing spectroscopic FTIR [9] and ellipsometry [20] data. A traditional approach of multilayer optics is used to simulate polarization dependent reflectivity [R(λ)] and transmission [T(λ)] spectra for ultrathin Cd 1-x Zn x Te/GaAs (001) epifilms of thickness ranging between 22 nm ≤ d ≤ 1 µm.…”