Tunable Energy Band Research Articles

High quality optoelectronic grade epitaxial AlN films on α-Al 2O 3, Si and 6H-SiC by pulsed laser deposition

AlN is one of the most important optoelectronic materials in the wide band gap III–V semiconductors because of its wide and tunable energy band gap in conjunction with other nitrides, high thermal conductivity, doping capabilities, and high hardness. The proposed optoelectronic devices require high quality epitaxial films on various substrates. Here, we present our recent work on the fabrication of high quality epitaxial AlN films on Al 2O 3(0001), Si(111) and 6H-SiC(0001) by pulsed laser deposition (PLD). The PLD is a nonequilibrium technique where thin film growth temperature can be reduced by more than 250–350°C and epitaxial films comparable in quality to MOCVD (equilibrium technique) obtained. The laser fluence and the substrate temperature were found to be crucial processing parameters for the formation of high quality monocrystalline AlN films. The AlN films deposited above 750°C and laser energy densities of 2–3 J cm −2 were found to be epitaxial with c-axis normal to substrate plane. The X-ray rocking curve of epitaxial films on sapphire and SiC yielded full-width-at-half-maximum of ∼0.06–0.07°. The transmission electron microscopy also revealed that the films were epitaxial with an orientational relationship of AlN[0001]‖Al 2O 3[0001], AlN[0001]‖Si[111], AlN[0001]‖SiC[0001] and in-plane alignment of AlN[1̄21̄0]‖ Al 2O 3[01̄10], AlN[101̄0]‖Al 2O 3[2̄110], AlN[21̄1̄0]‖Si[011̄] and AlN[01̄10]‖SiC[01̄10]. The optical absorption edge measured by UV-visible spectroscopy for the epitaxial AlN film on sapphire was sharp and the band gap was found to be 6.1 eV. The electrical resistivity of the films was about 5–6×10 13 Ω cm −1 with a breakdown field of 5×10 6 V cm −1.