We present a comprehensive study of the phonon mode behavior and the optical interband transitions of cubic AlxGa1-xN films (0≤x≤0.20) using spectroscopic ellipsometry from the midinfrared to the vacuumultraviolet spectral range (0.05-8.5 eV). The ≈0.5-μm-thick AlxGa1-xN layers were grown by radiofrequency plasma-assisted molecular-beam epitaxy and possess free-electron concentrations in the range of 6 × 1019-1 × 1020 cm-3. A two-mode behavior for the transverse-optical phonon of AlxGa1-xN is observed, which is consistent with theoretical predictions. Due to the high free-electron concentration, the observed AlxGa1-xN fundamental band-gap energy E0 is subject to a strong Burstein-Moss shift and band-gap renormalization. We quantify the amount of both band-gap shifting mechanisms, and provide an estimate for the composition dependence of the AlxGa1-xN band-gap energy E0, which depends approximately linearly on the alloy composition with E0 (eV) = 3.19 + 2.77x at room temperature and for carrier-depleted material. For cubic GaN, the L-point interband transition E1 shifts to lower energies with increasing free-electron concentration, whereas the interband transition E2 remains unshifted. Increasing A1 content induces a blueshift (redshift) of the E1 (E2) transition of AlxGa1-xN for x≤0.12. Due to strong broadening effects, both transitions cannot be differentiated anymore for x≥0.16.
|Original language||English (US)|
|Number of pages||16587193|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - May 1 2002|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics