Structural and optical properties of c-axis oriented aluminum nitride thin films prepared at low temperature by reactive radio-frequency magnetron sputtering Aurelian C. Galca a, ⁎, George E. Stan b, ⁎, Liliana M. Trinca a , C. Catalin Negrila b , Leona C. Nistor c, ⁎ a Laboratory of Multifunctional Materials and Structures, National Institute of Materials Physics, RO-077125, Măgurele-Bucharest, Romania b Laboratory of Nanoscale Condensed Matter Physics, National Institute of Materials Physics, RO-077125, Măgurele-Bucharest, Romania c Laboratory of Atomic Structures and Defects in Advanced Materials, National Institute of Materials Physics, RO-077125, Măgurele-Bucharest, Romania abstract article info Article history: Received 25 January 2012 Received in revised form 14 August 2012 Accepted 11 October 2012 Available online 16 October 2012 Keywords: Aluminum nitride Radio-frequency magnetron sputtering Transmission electron microscopy X-ray diffraction Ellipsometry Spectroscopic ellipsometry, X-ray diffraction and transmission electron microscopy experiments are employed to characterize aluminum nitride (AlN) thin films obtained by radio-frequency magnetron sputtering at low tem- perature (≈50 °C). To understand the growth mechanism and to get in depth information of such films by using ex situ characterization techniques, the AlN thin film sample series were prepared for different sputtering times, while keeping constant all the other deposition conditions. The diffraction studies reveal a [002] oriented growth of the AlN thin films. The misorientation of this crystallographic axis to the normal to the surface reduces progressively with film growth. A nonmonotonic behavior of the AlN pseudo-refractive index versus deposition time indicates a complex depth profile of the AlN thin films optical properties. The difference in orientation dis- persion of the [002] crystallite axis, the variation of defects concentration and each constituent atom density in- fluence the refractive index evolution. Our interpretation validity was verified by producing and characterizing samples obtained at intermediate deposition time. The AlN thin films show also very good pull-out adherence values. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Among the wide band gap materials, the aluminum nitride (AlN) has the largest piezoelectric coefficient and the higher corrosion re- sistance, and has attracted a great technological interest due to excep- tional properties such as energy band gap (6.2 eV), high breakdown voltage (≈3 MV/cm), high electrical resistivity (≈10 15 Ω cm), high hardness (11–15 GPa), good thermal stability (≈1000 °C in air), high thermal conductivity (≈170 W/m⋅ K) and high surface acoustic velocity (up to 6000 m/s) [1–6]. Highly c-axis oriented AlN thin films are especially attractive for micro-electromechanical (MEMS) and nano-electromechanical (NEMS) devices [7,8]. The MEMS and NEMS resonators are regarded as promising technologies for many hi-tech applications (electrometry, chemical and biological sensing, and scanning probe techniques) [7]. Other applications such as ultraviolet light-emitting diodes and laser diodes, gate dielectrics, high power devices, and insulating layers are also targeted. The acoustic response and piezoelectric coefficients of AlN structures depend mainly on their crystallinity, crystal orientation, reduced surface roughness and polarity distribution [9–12]. However, extensive research is still necessary to find out the influence of the crystallographic structure and of the crystallinity degree of AlN films on their functionality as MEMS/NEMS devices. In order to develop better opto- and micro-electronical devices, a wide range of deposition methods have been employed with the aim to grow high quality oriented AlN thin films, as chemical vapor deposition [13], molecular beam epitaxy [14], metal organic chemical vapor deposition [15–18], hybrid vapor phase epitaxy [19], pulsed laser deposition [20,21] and direct current (DC) and radio-frequency (RF) magnetron sputtering [22–26]. The compatibility with the cur- rent microelectronic processes requires a low temperature deposition process. However, the synthesis of AlN films with a definite crystal- line structure at low temperature is still challenging. Reactive RF magnetron sputtering (RF-MS) presents the advantage of low deposi- tion temperatures (usually below 200 °C), allowing the synthesis of AlN films with the preferred crystal orientations and reduced surface roughness at low pressure [24,27]. Moreover, RF-MS possesses many other advantages such as reproducibility, efficiency, versatility and the ability to grow high quality adherent thin films [28]. In this work AlN layers of different thicknesses were grown onto (100) silicon substrates in identical experimental conditions [29] by reac- tive RF-MS. We investigated the films growth architecture, their structur- al and optical qualities versus thickness and their behavior during prolonged exposure to ambient media. The presented results could be important for the further development of self-sustainable opto- and micro-electronical devices designs. The study attempts to offer a compre- hensive and insightful view of such c-axis oriented AlN layers grown on Thin Solid Films 524 (2012) 328–333 ⁎ Corresponding authors. E-mail addresses: ac_galca@infim.ro (A.C. Galca), george_stan@infim.ro (G.E. Stan), leonis@infim.ro (L.C. Nistor). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.10.015 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf