Influences of repetition rate of laser pulses on growth of crystalline AlN films on sapphire(0001) substrates by pulsed laser deposition Kazushi Sumitani a, ⁎, Ryota Ohtani a , Tomohiro Yoshida b , You Nakagawa b , Satoshi Mohri b , Tsuyoshi Yoshitake b a Saga Light Source, 8-7 Yayoigaoka, Tosu, Saga 841-0005, Japan b Department of Applied Science for Electronics and Materials, Kyushu University, 6-1 Kasuga, Fukuoka 816-8580, Japan abstract article info Available online 11 December 2009 Keywords: AlN Pulsed laser deposition Repetition rate We investigated influences of a repetition frequency of laser pulses on growth of AlN crystalline films by pulsed laser deposition. The structural and morphological properties of the films were studied by X-ray diffraction and scanning electron microscopy. Employment of high frequency laser pulses not only enhanced the growth of AlN crystallites, but also afforded the crystal growth at higher nitrogen pressures. Growth of α-AlN was dramatically enhanced with an increase in the laser pulse frequency, while β-AlN was grown at the high frequency of laser pulses and high nitrogen pressures. © 2009 Elsevier B.V. All rights reserved. 1. Introduction AlN is widely investigated as a promising material for short wavelength light emitting devices, microelectronic devices, and acoustic devices with several outstanding properties [1,2]. Hexagonal α-AlN has a significantly wide bandgap (6.2 eV), high thermal and chemical stability, and acoustic properties [3–6], while metasta- ble β-AlN, which has a cubic structure, is also known as a structural phase which possesses higher thermal conductivity, ballistic velocity, and acoustic velocity due to its better symmetry than α-AlN [7]. We have tried the growth of β-AlN films on sapphire(0001) substrates by pulsed laser deposition (PLD) [8,9]. PLD growth is a quite simple and effective method for fabrication of compound films. Especially, a non-equilibrium condition in PLD process is beneficial on the growth of films with metastable phases. It is not easy to fabricate crystalline films because many parameters such as a target, substrate, ambient gas, laser, etc. affect the growth condition. Recently, we have found that the growth condition of the crystalline AlN films can be affected by the repetition frequency of the laser pulses. In this study, we investigated the influences of the repetition frequency on the crystallinity of the AlN films. 2. Experimental methods AlN thin films were deposited on ultrasmooth step sapphire(0001) substrates [10] in ambient nitrogen at a substrate temperature of 700 °C by PLD using a sintered AlN target that was composed of α-AlN. A chamber for the film preparation was evacuated under 10 -6 Pa by a turbo molecular pomp. The pressure during the deposition was changed from 50 mTorr to 40 Torr by adjusting a gate valve under a fixed nitrogen inflow of 15 sccm. The distance between the substrate and the target was 15 mm. An ArF excimer laser beam [λ = 193 nm, full width at half maximum (FWHM) = 20 ns] with an energy of 100 mJ was focused through a spherical lens onto a rotating target at an incidence angle of 45°. The irradiation area on the target was approximately 2 mm 2 . The fluence and irradiance were 10 J/cm 2 and 1.5 × 10 8 W/cm 2 , respectively. The repetition rates of the pulses were selected as 10 Hz and 50 Hz. Structural and morphological properties of the films were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). 3. Results and discussions The XRD results from the films grown with the repetition frequency of 10 Hz and 50 Hz at different nitrogen pressures are shown in Fig. 1. The measurements were performed in θ–2θ arrangement. At both pressures, the diffraction intensities of α-AlN were enhanced with an increase in the frequency from 10 to 50 Hz. In particular at the nitrogen pressure of 2 Torr, their enhancement was marked. Deposition at a high frequency of laser pulses makes possible enhancement of the crystal growth and furthermore allows the growth under higher nitrogen pressures. We also performed 2θ scans with a fixed incident angle of 4°. The diffraction peaks from AlN were also observed at the same scattering angle as in the θ–2θ measurements. This indicates that the films were composed of polycrystalline AlN. Fig. 2(a) and (b) shows SEM images of the films deposited at the nitrogen pressure of 2 Torr and the repetition frequencies of 10 and Diamond & Related Materials 19 (2010) 618–620 ⁎ Corresponding author. Tel.: +81 942 83 5017; fax: +81 942 83 5196. E-mail address: sumitani@saga-ls.jp (K. Sumitani). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.12.004 Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond