Titanium Nitride Epitaxy on Tungsten (100) by Sublimation Crystal Growth
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Titanium Nitride Epitaxy on Tungsten (100) by Sublimation Crystal Growth Lisa Mercurio1, James H. Edgar1, Li Du1, and E. A. Kenik2 1 Chemical Engineering, Kansas State University, Durland Hall, Manhattan, KS, 66506-5102 2 Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, TN, 37831-6064 Abstract Titanium nitride crystals were grown by the sublimation-recondensation technique from titanium nitride powder on tungsten substrates. The bright golden TiN crystals displayed a variety of shapes including cubes, truncated tetrahedrons, truncated octahedrons, and tetrahedrons bounded by (111) and (100) crystal planes. The TiN crystals formed regular, repeated patterns within individual W grains that suggested epitaxy. X-ray diffraction and electron backscattering diffraction revealed that the tungsten foil was highly textured in the (100) orientation and confirmed epitaxial TiN deposition with the orientational relationship TiN(100)║W(100) and TiN[100]║W[110], that is, the TiN crystals were rotated 45∞ in-plane with respect to the tungsten. Because of its larger coefficient of thermal expansion compared to W, upon cooling from the growth temperature, the TiN crystals were under in-plane tensile strain, causing many cracks within the TiN crystals. Introduction Sublimation-recondensation has proven to be a reliable technique for producing single crystals of aluminum nitride and silicon carbide [1, 2], yet other than these semiconductors, this method remains relatively undeveloped. Gu et al. [3] extended the range of materials treated by this technique by growing ScN crystals from sintered ScN in a tungsten crucible. Growth of ScN took place on tungsten foil at the top of the crucible in a resistively-heated tungsten furnace at a rate as high as 79.3 mg/h. These crystals were of good quality with etch pit densities of 104 cm-2 to 106 cm-2 [4]. The present study extends the technique further by demonstrating the feasibility of the sublimation-recondensation method for producing TiN single crystals. Titanium nitride is a refractory group IVB transition metal nitride with a striking golden color and a variety of unique and useful characteristics, making its single crystal growth desirable. TiN has excellent mechanical properties including high stability (both chemical and metallurgical), high resistance to wear and corrosion, and high hardness at 21-24 GPa [5]. The stability and strength of TiN results from the covalent nature of the Ti-N bonds and its rock salt crystal structure [6], which has a lattice constant of 4.24 ≈ [7]. Cubic TiNx is stable in the composition range of TiN0.6 to TiN1.2, with vacancy concentrations up to 50 at% [8]. In addition, TiN possesses such qualities as a high melting point of 2930 ∞C, a coefficient of thermal expansion of 9.35x10-6 K-1 at room temperature, and good thermal conductivity (0.192 W/cm∑K) [7]. It has a low electrical resistivity of 20±10 µΩ∑cm at room temperature [5] and adheres well to SiO2 and Al. These qualities make TiN thin films suitable for use as protective
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