Understanding the growth of micro and nano-crystalline AlN by thermal plasma process Nilesh S. Kanhe a , Ashok B. Nawale a , Rupesh L. Gawade b , Vedavati G. Puranik b , Sudha V. Bhoraskar a , Asoka K. Das c , Vikas L. Mathe a,n a Department of Physics, University of Pune, Pune 411007, India b Center for Materials Characterizations, National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune 411008, India c Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India article info Article history: Received 9 August 2011 Received in revised form 21 October 2011 Accepted 6 November 2011 Communicated by K. Nakajima Available online 1 December 2011 Keywords: A1. Crystal morphology A2. Growth from high temperature solutions B1. Nitride B2. Semiconducting aluminum compound abstract We report the studies related to the growth of crystalline AlN in a DC thermal plasma reactor, operated by a transferred arc plasma torch. The reactor is capable of producing the nanoparticles of Al and AlN depending on the composition of the reacting gas. Al and AlN micro crystals are formed at the anode placed on the graphite and nano crystalline Al and AlN gets deposited on the inner surface of the plasma reactor. X-ray diffraction, Raman spectroscopy analysis, single crystal X-ray diffraction and TGA-DTA techniques are used to infer the purity of post process crystals as a hexagonal AlN. The average particle size using SEM was found to be around 30 mm. The morphology of nanoparticles of Al and AlN, nucleated by gas phase condensation in a homogeneous medium were studied by transmis- sion electron microscopy analysis. The particle ranged in size between 15 and 80 nm in diameter. The possible growth mechanism of crystalline AlN at the anode has been explained on the basis of non- equilibrium processes in the core of the plasma and steep temperature gradient near its periphery. The gas phase species of AlN and various constituent were computed using Murphy code based on minimization of free energy. The process provides 50% yield of microcrystalline AlN and remaining of Al at anode and that of nanocrystalline h-AlN and c-Al collected from the walls of the chamber is about 33% and 67%, respectively. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Aluminum nitride (AlN) is an attractive ceramic material, which possess high thermal conductivity and high electrical resistivity ( 410 14 O cm) similar to alumina. Theoretical predic- tions indicate that AlN ceramic should have a room-temperature thermal conductivity of 3.2 W/cm K, which is comparable to that of copper, which is 4 W/cm K. However, measurement indicates that residual impurities in commercial AlN, particularly oxygen, are responsible for lowering its thermal conductivity. Thus formation of AlN in its pure form, both in the bulk crystalline form, as well as in the nano crystalline form is technologically attractive. Its low coefficient of expansion, like that of silicon, provides high thermal shock resistance. It is a hard ceramic having good mechanical strength and excellent corrosion resis- tance. It is also a potential material in the optoelectronic and high power microwave devices [1–4]. It is versatile semiconductor ceramic, which has high technological potentials. Crystalline AlN can also serve as a good catalytic material, however on account of its high temperature of reaction its growth under the normal temperature and pressure is difficult. Here we report the synthesis of crystalline AlN using a high temperature route involving thermal plasma as the source of heat. In the present method highly oriented ultra pure AlN crystals were grown in the plasma plume at the anode; whereas nano- particles of aluminum and AlN were also grown simultaneously by gas phase reaction by the in-flight homogenous nucleation process and were deposited on the walls of the reactor, depending on the nature of the ambient gas and pressure chosen. The oriented growth of bulk crystals is explained on the basis of high core temperature along the axis in the transfered torch operated plasma device while attempting to evaporate Al block for producing nano-particles of aluminum by vapor phase con- densation. The technique is thus versatile and is capable of producing high purity nano aluminum as well as crystals depend- ing upon the kind of ambient gas chosen. Thermal plasma method is a novel route to synthesize metal nanoparticles of metal oxides and metal nitrides as has been reported in our earlier commu- nications [5–8]. This method provides high plasma power density, clean and controlled environment for material synthesis, single Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.11.011 n Corresponding author. Tel.: þ91 20 25692678; fax: þ91 20 25691684. E-mail address: vlmathe@physics.unipune.ac.in (V.L. Mathe). Journal of Crystal Growth 339 (2012) 36–45