Investigation of optically active E1 transversal optic phonon modes in AlxGa1−xN layers deposited on 6H–SiC substrates using infrared reflectance

Abstract

We report an investigation, performed in the full composition range x=0–1, of the change in infrared reflectivity spectra of AlxGa1−xN layers deposited on 6H–SiC substrates. We have found two different transverse E1(TO) phonon frequencies that can be assigned to AlN-like and GaN-like modes. The composition dependences of these frequencies can be well approximated by linear functions and the oscillator strengths scale like the corresponding Al and Ga mole fractions, respectively. On a purely experimental basis, this establishes evidence of a two-mode behavior for this controversial alloy system. The frequencies of the impurity mode of Ga in AlN (622 cm−1) and of the impurity mode of Al in GaN (643 cm−1) were determined.

The Low-Field Electron Mobility in Bulk AlGaN

“…The main concern here is with the Fro È hlich interaction involving the LO modes in an alloy. This has been discussed by Swierkowski et al [4] and Nash et al [5], and the approach of the latter work is followed to describe LO modes in wurtzite Al x Ga 1± ±x N. It is found that, in fact, the alloy exhibits two-mode behaviour but that the Fro È hlich coupling associated with the lower frequency mode is extremely weak, making the alloy virtually a one-mode alloy as far as LO polar coupling is concerned, in agreement with Wisniewski et al [12].…”

Surface phonon polariton mode of wurtzite structure AlxGa1−xN (⩽x⩽1) thin films

“…[2][3][4][5][6][7] In contrast, the knowledge of the SPP mode of the Al x Ga 1−x N alloy remains unclear or unexplored. For its constituents, the study of the SPP mode is also rarely reported and has not received sufficient attention.…”

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

“…Numerous studies have employed Raman scattering and far-infrared ͑IR͒ spectroscopies to investigate several physical properties of GaN, AlN, and the ternary alloy AlGaN such as their phonon modes, the crystal quality and the orientation of the as-grown material, free-carrier effects, and the piezoelectric response. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] It is now clear that the longitudinal optic ͑LO͒ phonons ͓A 1 ͑LO͒ and E 1 ͑LO͔͒ of hexagonal AlGaN display a one-mode behavior, whereas the rest of the Raman or IR active phonon modes ͓E 2 , A 1 ͑TO͒ and E 1 ͑TO͔͒ exhibit a two-mode behavior. 13,17 Similarly, it has been shown that the LO and transverse optic ͑TO͒ phonons of cubic AlGaN exhibit one-mode and two-mode behaviors, respectively.…”

“…1,[4][5][6][7][8][9][12][13][14][15] In contrast, only a few works reported IR transmission measurements, 3,9,12,18 most of which were performed in normal-incidence configuration. Reports dealing with oblique-incidence IR transmission to study the phonons of group-III nitrides are scarce.…”

“…In spite of the uniaxial symmetry of wurtzite compounds, the isotropic approximation has been widely used in the literature to analyze the IR optical response of group-III nitrides. 2,4,6,8 The bandgap of AlGaN is significantly higher than the energy corresponding to visible laser excitations. As a consequence, the use of visible Raman scattering to investigate structures containing very thin films of AlGaN ͑i.e., films with very small scattering volumes͒ may be hampered by the low Raman efficiency exhibited by this ternary alloy, in particular for large Al contents.…”