Optical characterization of AlN films grown by plasma source molecular beam epitaxy

Thin films of AlN were grown on Si(111), sapphire (1 (1) over bar 02), and sapphire (0001) substrates by plasma source molecular beam epitaxy (PSMBE). Optical transmission spectra of PSMBE grown AlN/sapphire show neither significant band edge distortions nor deep level absorption in the spectral range 0.2-6.5 mu m. Furthermore, there is no evidence of deep levels in the spectra of AlN/Si in the range 2.5-25 mu m. The optical phonon modes of AlN/Si films were directly observed in infrared transmission and 80 degrees angle of incidence reflectance spectra. Longitudinal optical (900 cm(-1)) and transverse optical (680 cm(-1)) modes are shifted toward each other comparable to previously reported bulk values. Optical spectra of thick textured film samples with low degree of orientation have phonon energies close to bulk values (probably due to strain relaxation). (C) 1998 American Institute of Physics. [S0021-8979(98)06416-0].