Preparation And Optical Properties Of AlN Thin Films Deposited On Al₂O₃（0001） Substrates

The Ⅲ-Ⅴ group AlN thin film semiconductors possess broad applications foreground in optoelectronic and microelectronic devices due to their excellent physical and chemical properties. For the epitaxial growth of AlN films on A12O3substrates, the crystalline and surface qualities of AlN films are seriously affected by the large lattice mismatch between AlN and A12O3and the different surface polarity of AlN films. In this paper, AlN films were deposited on Al2O3(0001) substrates by the laser molecular beam epitaxy technique, and AlN buffer layers were pre-deposited under ultra-high vacuum to improve the film qualities. Effects of processing parameters and pre-deposition time on the film qualities were analyzed in detail, and influences of the crystal structure and external temperature on the optical properties of AlN films were discussed. The main results are as follows:1. The crystalline and surface qualities of AlN films can be greatly improved by the AlN buffer layer pre-deposited under ultra-high vacuum. The orthogonal experiments indicate that the suitable substrate temperature and laser energy for depositing AlN films are750℃and150mJ/p respectively.2. AlN films exhibit the single (0002) preferred orientation and the Al-polarity when the pre-deposition time is5min and10min, and the crystallinity and surface morphology improve with the increasing of the pre-deposition time. The best crystallinity is obtained at the pre-deposition time of20min, but the polarity inversion from the Al-polarity to the N-polarity occurs. At the pre-deposition time of30min, AlN can not crystallize due to the unbalance of the Al and N atomic ratio during the deposition.3. Based on the Swanepoe’s theoretical model, the analysis for the optical transmittance spectra of AlN films indicate that the bandgap and refractive index of AlN films decrease as the increasing of pre-deposition time, which may be resulted from the decreasing of the axial ratio c/a.4. In the temperature range of77-450K, the absorption curves of AlN films moves towards longer wavelength as the temperature increases, indicating that the bandgap of AlN films decreases with increasing temperature. The temperature dependence of the bandgap could be well described by the Passler model, in which the Debye temperature is estimated to be1067K. Effective phonon energy is obtained as68.6meV from the temperature dependence of the Urbach’s energy. This relatively small value is associated with the electron-longitudinal optical phonon and acoustic phonon and/or nonpolar E2(low) phonon interactions.