Paper EM-TuP7
Formation of AlN Thin Films by Direct Nitridation of Aluminum Thin Films and Their Visible Photoluminescence Property

Tuesday, November 11, 2014, 6:30 pm, Room Hall D

AlN has the widest band-gap of 6.3 eV among the direct band-gap semiconductors and is therefore suitable for shorter wavelength light emitting devices and power devices. AlN thin films have been prepared by MOCVD and reactive sputtering with the deposition rate of about 0.1-10 nm/min. Meanwhile, AlN powders and AlN bulks have been prepared by reducing nitridation and direct nitridation methods. In this study, AlN thin films were formed by direct nitridation (DN) of Al thin films and their properties were compared with those formed by reactive sputtering (RS). For photoluminescence (PL) measurements, doping of Eu into AlN was performed to observe visible PL from Eu ions in AlN films.

Al thin films with a thickness of 150 nm were deposited on c-axis sapphire substrates by radio frequency (RF) magnetron sputtering using an Al target (φ4 inch, Koujundokagaku, Japan) in constant Ar (8.0 sccm) flow. Co-sputtering of Eu₂O₃ and Al was performed by placing 2 pieces of Eu₂O₃ tablets on the Al target for PL measurements. The films were subsequently annealed at 900°C for 15 sec in N₂ or NH₃ gas using a conventional electric quartz tube furnace, which can form AlN films with a theoretical thickness of 300 nm. On the other hand, AlN thin films with a thickness of 500-700 nm were deposited on c-axis sapphire substrates by RS using an Al target in constant N₂ (5.6 sccm) and Ar (2.4 sccm) flow. Co-doping of Eu₂O₃ and Si during the formation of AlN films by RS was performed by placing several pieces of Eu₂O₃ and Si tablets on the Al target. Doping of Si was to observe the blue luminescence from Eu²⁺ in AlN by replacing Al atoms by Si atoms in AlN, which can prevent the formation of Eu³⁺. RF sputtering power was set at 200 W through this study. The films were subsequently annealed at 900°C for 30 min in N₂ with rapid thermal annealing system. Growth time of AlN films with a thickness of 300 nm by RS was approximately 30 times longer than that by DN. These samples were analyzed by X-ray diffraction (XRD), PL, optical transmittance and energy dispersive X-ray spectrometry.

The wurtzite AlN (002) XRD peak appeared from the samples formed by DN. PL spectra of Eu₂O₃ doped AlN films formed by DN exhibited broad emissions at 400 nm and 530 nm, which were assigned to oxygen defect and Eu²⁺, respectively. The optical direct band-gap of AlN films doped with Eu₂O₃ was calculated to be 5.85 eV from their optical transmission spectra. These results were also obtained from the samples formed by RS. It is interesting to note that the PL spectral features of Eu₂O₃ doped AlN films formed by DN resembles those of Eu₂O₃ and Si co-doped AlN films formed by RS.