coatings Article Fabrication and Characterization of Aluminum Nitride Nanoparticles by RF Magnetron Sputtering and Inert Gas Condensation Technique Ishaq Musa 1,* , Naser Qamhieh 2,* , Khadija Said 2, Saleh T. Mahmoud 2 and Hussain Alawadhi 3 1 Department of Physics, Palestine Technical University-Kadoorie, P. O. Box 7, Tulkarem 300, Palestine 2 Department of Physics, UAE University, Al-Ain 15551, UAE; [email protected] (K.S.); [email protected] (S.T.M.) 3 Center for Advanced Materials Research, University of Sharjah, Sharjah 27272, UAE; [email protected] * Correspondence: [email protected] (I.M.); [email protected] (N.Q.) Received: 3 March 2020; Accepted: 30 March 2020; Published: 21 April 2020 Abstract: Aluminum nitride nanoparticles (AlN-NPs) were fabricated by a RF magnetron sputtering and inert gas condensation technique. By keeping the source parameters and sputtering time of 4 h fixed, it was possible to produce AlN-NPs with a size in the range of 2–3 nm. Atomic force microscopy (AFM), Raman spectroscopy, X-ray diffraction (XRD), and UV-visible absorption were used to characterize the obtained AlN-NPs. AFM topography images showed quazi-sphere nanoparticles with a size ranging from 2 to 3 nm. The XRD measurements confirmed the hexagonal wurtzite structure of AlN nanoparticles. Furthermore, the optical band gap was determined by the UV-visible absorption spectroscopy. The Raman spectroscopy results showed vibration transverse-optical modes A1(TO), E1(TO), as well as longitudinal-optical modes E1(LO), A1(LO). Keywords: AlN nanoparticles; RF magnetron sputtering; atomic force microscopy; Raman spectroscopy; UV-visible absorption 1. Introduction Aluminum nitride (AlN) is a large and direct band-gap semiconductor material (Eg = 6.2 eV). It has a hexagonal wurtzite structure similar to zinc oxide (ZnO) and lattice constants of a = 0.311 nm and c = 0.498 nm. It is characterized by high thermal conductivity and chemical stability, a high melting point, low coefficient of thermal expansion, high electrical resistivity,low dielectric loss, high mechanical stiffness, and high acoustic wave velocity [1–3]. Hence, it has attracted considerable attention of researchers due to its unique property applications in surface acoustic wave (SAW) devices, sensors, thin-film resonators, metal-oxide-semiconductors (MOS) [4], and microelectronic devices [5]. Other versatile applications are used in optoelectronic devices, for example, deep-ultraviolet light-emitting diodes and laser diodes [6], which can be used for a living environment [7]. Many regions of the world are suffering from the pollution of water; therefore, it is necessary to use sterilization systems to clean the water. According to the World Health Organization (WHO), every hour more than 100 children die from water-borne bacteria [8]. Currently, deep ultraviolet light sources such as mercury lamps or excimer lasers are capable of killing these bacteria. Nonetheless, these UV-light sources are not reliable due to their large size, low efficiency, and their toxic substances that cause serious environmental problems [9]. Among wide bandgap semiconductor materials, such as GaN, AlN, and AlGaN, only aluminum nitride-based UV-light-emitting diodes (LEDs) have potential applications for killing water-borne bacteria. In the past decade, various techniques were employed to produce aluminum nitride (AlN) Coatings 2020, 10, 411; doi:10.3390/coatings10040411 www.mdpi.com/journal/coatings Coatings 2020, 10, x FOR PEER REVIEW 2 of 8 Coatings 2020, 10, 411 2 of 8 laser deposition [13], molecular beam epitaxy [14], and the common deposition process DC reactive thinmagnetron films and sputtering nanostructures [15]. [10–12], such as pulsed laser deposition [13], molecular beam epitaxy [14], and theNanomaterials common deposition offer different process advantages, DC reactive such magnetron as flexible sputtering space [for15]. simplicity reconstruction, enhancedNanomaterials mechanical off stability,er different large advantages, surface area, such and as flexiblesuitable spacecoating, for which simplicity may reconstruction,lead to unique enhancedapplications mechanical in different stability, nanoelectronic-scale large surface area,devices and [16]. suitable The selection coating, whichin fabrication may lead techniques to unique of applicationsnanostructures in diprovidesfferent nanoelectronic-scale the freedom to modify devices the [16physical]. The selectionproperties in of fabrication materials. techniques Synthesis ofof nanostructuresnanoparticles by provides magnetron the freedomsputtering to modifyand inert-gas the physical condensation properties may of open materials. the door Synthesis for basic of nanoparticlesresearch to study by magnetronthese properties sputtering such as and in inert-gashybrid structures condensation composites may openby depositing the door or for coating basic researchAlN nanoparticles to study these on propertiesmulti-wall such carbon as in nanotubes hybrid structures (MWCNTs). composites The transfer by depositing of the or energetic coating AlNelectrons nanoparticles excited by on surface multi-wall plasmon carbon from nanotubes metal (M (MWCNTs).WCNTs) to The the transfer conduction of the band energetic of the electrons emitting excitedmaterial by is surface expected plasmon to enhance from metalthe UV (MWCNTs) emission. toPrevious the conduction studies, bandsuch as of theZnO-coated emitting materialMWCNTs, is expectedshow that to the enhance UV bandgap the UV emission.emission greatly Previous enhanc studies,ed while such as the ZnO-coated green emission MWCNTs, arising show from that defects the UVwas bandgap reduced emission [17]. greatly enhanced while the green emission arising from defects was reduced [17]. InIn thisthis research,research, aluminumaluminum nitridenitride (AlN)(AlN) nanoparticlesnanoparticles areare producedproduced forfor thethe firstfirst timetime usingusing thethe RFRF magnetronmagnetron sputteringsputtering andand inert inert gas gas condensation condensation technique. technique. 2.2. MaterialsMaterials andand MethodsMethods AluminumAluminum nitridenitride nanoparticlesnanoparticles werewere fabricatedfabricated byby RFRF magnetronmagnetron sputteringsputtering withwith inertinert gasgas condensationcondensation inin thethe suitablesuitable system.system. WeWe purchasedpurchased thethe nanoparticlesnanoparticles sourcesource fromfrom MantisMantis DepositionDeposition Ltd.Ltd. (Oxfordshire,(Oxfordshire, UK).UK). AA schematicschematic diagramdiagram ofof thethe experimentalexperimental researchresearch set-upset-up inin ourour lablab forfor thethe fabricationfabrication ofof AlN nanoparticles is is illustrated illustrated in in Figure Figure 1.1 .The The main main parts parts of ofthe the system system shown shown in this in thisfigure figure consist consist of a ofnanoparticle a nanoparticle source source that includ that includeses an RF an magnetron RF magnetron sputtering sputtering unit, a unit, turbo a turbopump pump(TP), a (TP), quadrupolar a quadrupolar mass mass filter filter (QMF), (QMF), and and the the deposition deposition chamber. chamber. The The two two turbo turbo pumps werewere 8−8 utilizedutilized to to evacuate evacuate the the main main and and source source chambers chambers to to a a base base pressure pressure of of 10 10− mbar.mbar. TheThe RFRF magnetronmagnetron typetype dischargedischarge waswas usedused toto generategenerate nanoparticlesnanoparticles fromfrom thethe AlNAlN targettarget purchasedpurchased fromfrom thethe KurtKurt J.J. LeskerLesker CompanyCompany (Je(Jeffersonfferson Hills, PA, PA, USA) USA) with with a a pu purityrity of of 99.8%. 99.8%. Argon Argon ga gass was was used used to create to create the theplasma, plasma, sputter sputter material material from from its its target, target, establis establishh nanoparticle nanoparticle condensati condensation,on, and create pressurepressure gradientgradient betweenbetween thethe sourcesource andand depositiondeposition chamberschambersthat thatallowed allowednanoparticles nanoparticlesto topass pass throughthroughthe the massmass filter.filter. InIn thisthis research,research, thethe massmass filterfilter waswas usedused isis aa quadrupolequadrupole massmass filterfilter locatedlocated betweenbetween thethe nanoparticle’snanoparticle's sourcesource andand thethe depositiondeposition chamber.chamber. Figure 1. Schematic diagram of the deposition system, including the nanoparticle source and deposition Figure 1. Schematic diagram of the deposition system, including the nanoparticle source and chamber [18]. deposition chamber [18]. The principle of QMF is based on applying alternating current AC and direct current DC voltages [ (U +TheVcos principle!t)] to fourof QMF straight is based metal rods.on applying The two alternating rods were connectedcurrent AC to aand positive direct voltage current while DC ± thevoltages other two[±(U rods + V tocos the ωt negative)] to four voltage. straight Here, metalU isrods. the DCThe voltage, two rodsV is were the amplitude connected of to AC a voltage,positive !voltageis frequency, while the and othert is two time. rods Herein, to the a negative grid was voltage. placed Here, at the U outer is the part DC ofvoltage, the mass V is filter the amplitude that was of AC voltage, ω is frequency, and t is time. Herein, a grid was placed at the outer part of the mass Coatings 2020, 10, 411 3 of 8 usedCoatings to measure 2020, 10, thex FOR ion PEER flux REVIEW of the selected mass/size, and the resulting current