Infrared-active phonon modes and static dielectric constants in α-(AlxGa1−x)2O3 (0.18 ≤ x ≤ 0.54) alloys

Abstract

We determine the composition dependence of the transverse and longitudinal optical infrared-active phonon modes in rhombohedral α-(Al xGa1− x)2O3 alloys by far-infrared and infrared generalized spectroscopic ellipsometry. Single-crystalline high quality undoped thin-films grown on m-plane oriented α-Al2O3 substrates with x = 0.18, 0.37, and 0.54 were investigated. A single mode behavior is observed for all phonon modes, i.e., their frequencies shift gradually between the equivalent phonon modes of the isostructural binary parent compounds. We also provide physical model line shape functions for the anisotropic dielectric functions. We use the anisotropic high-frequency dielectric constants for polarizations parallel and perpendicular to the lattice c axis measured recently by Hilfiker et al. [Appl. Phys. Lett. 119, 092103 (2021)], and we determine the anisotropic static dielectric constants using the Lyddane–Sachs–Teller relation. The static dielectric constants can be approximated by linear relationships between those of α-Ga2O3 and α-Al2O3. The optical phonon modes and static dielectric constants will become useful for device design and free charge carrier characterization using optical techniques.

Determination of anisotropic optical properties of MOCVD grown m-plane α-(Al x Ga1−x )2O3 alloys

“…The IR DF is dominated by the IR active phonons. In contrast to the work by Stokey et al 52) the Al-rich side (with x = 0.76) is also discussed in the IR part of this study. The UV DF yields the dielectric limit ε ∞ and the Γ-point transition energies.…”

“…The phonon modes follow approximately a linear trend except for the lower wavenumber phonons A 2u (1) and E u (2). For comparison the values of Stokey et al 52,66) on m-plane α-Ga Table II. Phonon wavenumbers (ω 0 ) of the IR active optical phonon modes E u (E⊥c) and A 2u (E || c) by IR spectroscopic ellipsometry for all investigated α-(Al x Ga 1−x ) 2 O 3 samples and a m-plane sapphire wafer (x = 1), along with literature data on c-plane Ga 2 O 3 , 16,18) m-plane Ga 2 O 3 , 65,66) and c,a-, and m-plane Al 2 O 3 .…”

“…The missing phonon E u (1) is expected to appear below the experimentally accessible spectral range starting at 250 cm −1 for all samples till x ≈ 0.26. 52) The fact that it is not found for higher Al concentrations either is probably related to its Table I. Al contents (x), lattice parameters (a, m and c) and FWHM by GIID and high resolution XRD (HRXRD), together with thicknesses (d) by UV spectroscopic ellipsometry (UVSE) for all investigated α-(Al x Ga 1−x ) 2 O 3 samples are listed, along with literature data on Ga 2 O 3 powders, 54) c-plane Ga 2 O 3 , 18) m-plane Ga 2 O 3 , 65) and c-plane Al 2 O 3 .…”

“…Hilfiker et al investigated the optical absorption onset 34) and dielectric limits ε ∞ 51) by spectroscopic ellipsometry. Additionally Stokey et al 52) determined IR-active phonon modes and static dielectric constants with the same technique in the IR.…”

The Role of Optical Phonon Confinement in the Infrared Dielectric Response of III–V Superlattices

“…[4,5] However, PhPs suffer from limited spectral tunability as they can only be supported within the relatively narrow, nominally fixed spectral window bound by the transverse (TO) and longitudinal optical (LO) phonons of the host crystal. [6] Therefore, there is significant interest in exploring methods for tuning the optical phonon response, such as through alloying, [7,8] ion intercalation, [9] or manipulating the isotopic content, [10][11][12] carrier concentration, [13,14] or strain [15][16][17][18][19][20] in the material.…”