Infrared study on graded lattice quality in thin GaN crystals grown on sapphire

Transverse-optical (TO) and longitudinal-optical (LO) phonons in Si-doped and undoped n-GaN films have been studied by infrared reflection spectroscopy. The former film has been grown by metalorganic chemical vapor deposition (MOCVD) to a thickness of 2.3 μm, while the latter by hydride vapor phase epitaxy (HVPE) to a thickness of 35 μm. Both films are deposited on the c-surface of sapphire substrates with the surface of the c-plane of the wurtzite structure. Oblique-incidence infrared reflection spectroscopy has permitted us to observe the A₁ mode, as well as the E₁ mode, of polar optical phonons. The reflection spectra observed for s- and p-polarizations can be described well in terms of Gervais and Piriou's four-parameter semi-quantum oscillator model taking account of the LO-phonon-plasmon coupling. It is found that in the MOCVD-grown film, the damping energies of the phonons are markedly graded along the growth direction of the film. It is suggested that the lattice strain is quite large in the interface zone but is reduced rapidly upon going beyond about 1 μm from the interface. Accordingly, the HVPE film has a good lattice quality throughout the film.