Effect of thickness on optical and microwave dielectric properties of Hydroxyapatite films deposited by RF magnetron sputtering Apurba Das a , Anil Kumar Chikkala a , Gyan Prakash Bharti a , Rasmi Ranjan Behera b , Ravi Sankar Mamilla b , Alika Khare a , Pamu Dobbidi a, * a Department of Physics, Indian Institute of Technology Guwahati, Guwahati, 781039, India b Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati, 781039, India article info Article history: Received 22 August 2017 Received in revised form 18 December 2017 Accepted 24 December 2017 Available online 29 December 2017 Keywords: HAp films RF sputtering XRD Nonlinear optical properties Dielectric properties abstract This study present findings on the structural, optical and dielectric properties of polycrystalline Hydroxyapatite [HAp, Ca 10 (PO 4 ) 6 (OH) 2 ] films, deposited using radio-frequency (RF) magnetron sputter- ing. The X ray diffraction (XRD) studies revealed that the unit cell volume and crystallite size of the films deposited on quartz substrates enhanced with an increase in film thickness. The Young's modulus (E hkl ) and the Poisson ratio (n hkl ) of the thin films along different crystallographic directions have been calculated using the X-ray elastic constants. The Young's modulus of the films exhibited crystallographic direction dependence which suggests that the sputtered films are anisotropic. The dielectric constant ε r and the loss tangent tand of the sputtered films were in the range 29e85 and 0.0028e0.0014 respectively at a frequency of 1 MHz. These values are by far the best for films deposited under a pure argon envi- ronment. The improvement in crystallinity and uniform grain size of the films are used as parameters to understand the variation in dielectric properties of the sputtered films. The best microwave dielectric properties were in the range ε r ¼ 75 - 65 and tand ¼ 0.014e0.032 when measured at 5 and 10 GHz, employing split post dielectric resonator (SPDR) technique. The obtained results suggest that HAp sputtered films can be promising for optical limiting and applications in tunable microwave devices. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Hydroxyapatite (HAp) is a widely known bioactive ceramic that mimics the chemical composition of bones and teeth [1e5]. Bone is composed of organic and inorganic constituents. The inorganic component of natural bone is constituted of biological apatites along with trace elements like sodium, magnesium, strontium etc. which are known to supply strength to the skeleton. These biological apatites shows structural similarity with mineral apatites such as HAp [Ca 10 (PO 4 ) 6 (OH) 2 ] and Brushite [CaHPO 4 $2H 2 O]. Crystallographic studies on HAp have revealed that it has a complex crystal structure with 44 atoms arranged in a hexagonal primitive cell and crystallizing in a space group of P6 3 /m [6e8]. The OH groups are found to be aligned in columns parallel to the c-axis, along with Ca 2þ and (PO 4 3 ) ions [9e12]. Owing to the excellent bioactivity and biocompatibility both in vivo and in vitro, Hydroxyapatite has been widely used as a feasible bone substitute material [13e16]. Initial research on HAp focussed mainly on developing the biocompatibility and mechan- ical properties of HAp for load bearing applications [14e19]. The current research trend in HAp has been in studying the electrical properties which has proven to be phenomenal in supplementing the bioactivity studies [19e26]. Commercially, the most utilized method to deposit HAp is plasma spraying. This technique has a very high deposition rate and the films deposited are reported to show good bioactivity [27]. But despite a lot of clinical success, because of certain issues like poor long time adherence of the coating to substrates, tendency to resorb and non-uniformity of the deposited layer, researchers have focussed on developing other novel techniques to obtain HAp coatings which will eliminate the shortcomings of the method previously described. It is not hard to appreciate the role of sputtering in the growth of uniform thin films over all types of substrates be it metallic, ceramic or polymer. Sputtering usually generates films which are either amorphous or polycrystalline with precise control over deposition * Corresponding author. E-mail address: pamu@iitg.ernet.in (P. Dobbidi). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2017.12.293 0925-8388/© 2017 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 739 (2018) 729e736