Microstructural and Compositional Tunability in Sputter-Deposited Transition-Metal Carbide Thin Films using Ultra-Low Reactive Gas Pressures and 2D Layers
Tantalum carbides (Ta2C, Ta3C2, Ta4C3, Ta6C5, and TaC) are refractory compounds with a mixture of covalent, ionic, and metallic bonding. Interest in these compounds stems from the fact that their structure and properties vary with their microstructure, composition, and orientation. In this dissertation, I investigated growth-related aspects of tantalum carbide (Ta-C) thin films grown on single-crystalline MgO(001) and Al2O3(0001) substrates via ultra-high vacuum direct current magnetron sputtering of a TaC compound target using pure Ar and Ar/C2H4 gas mixtures at substrate temperatures Ts between 1073 K and 1373 K. In particular, I focused on the effects of ultra-low pressures (pc = 0.0025% ~ 2.5% of the total pressure) of C2H4 and the role of two-dimensional (2D) layers on compositional and microstructural evolution of Ta-C thin films.
I find that the Ta-C layers sputter-deposited on MgO(001) in pure Ar (i.e., pc = 0) and Ar/C2H4 gas mixtures with pc = 5 � 10^-7 Torr are polycrystalline trigonal-structured α-Ta2C (a = 0.310 nm and c = 0.492 nm). At pc = 5 � 10^-6 Torr, I obtain cubic rock salt (B1), 002-textured TaC0.76 (a = 0.442 nm) porous thin film with facetted surface. Interestingly, I also observed 111-oriented twins due to four crystals rotated with respect to each other. Film grown at higher pc = 5 � 10^-4 Torr is relatively dense with smoother surface and composed of a two-phase mixture of nanocrystalline TaC and amorphous carbon.
Similar experiments carried out with pc = 0 as a function of Ts resulted in 0001-oriented α-Ta2C thin films on Al2O3(0001) at all Ts between 1073 K and 1373 K. With increasing Ts, I obtain smoother and thinner layers with enhanced out-of-plane coherency and decreasing unit cell volume. Interestingly, the Ta2C 0001 texture improves with increasing Ts up to 1273 K above which the layers are relatively more polycrystalline. At Ts = 1373 K, during early stages of deposition, the Ta2C layers grow heteroepitaxially on Al2O3(0001) with (0001)Ta2C || (0001)Al2O3 and [10-10]Ta2C || [11-20]Al2O3. With increasing deposition time t, I observed the formation of anti-phase domains and misoriented grains resulting in polycrystalline layers. I attribute the observed enhancement in 0001 texture to increased surface adatom mobilities and the development of polycrystallinity to reduced incorporation of C in the lattice with increasing Ts.
With the introduction of small amounts of ethylene (pc = 0.1% to 1% of the total pressure), I obtain Ta-C layers with pc-dependent composition and morphology. I find that the layers grown using lower pc exhibit strongly facetted surfaces with columnar grains while those grown using higher pc are rough with irregular features. Films deposited using higher pc show primarily B1-TaC 111 reflections. At the lower Ts (= 1123 K) and lower pc (= 5.0 � 10^-6 Torr), I obtain a two-phase mixture of rhombohedral-Ta3C2(0001) and B1-TaC(111) oriented with respect to the Al2O3(0001) substrate as: (111)TaC || (0001)Al2O3,[-211]TaC || [11-20]Al2O3 and (111)TaC || (0001)Al2O3), [-1-12]TaC || [11-20]Al2O3 and (0001)Ta3C2 || (0001)Al2O3, [10-10]Ta3C2 || [11-20]Al2O3.
Finally, I investigated the effect of 2D hexagonal boron nitride (hBN) layer on the crystallinity of sputter-deposited Ta2C/Al2O3(0001) thin films. In these experiments, hBN is deposited via pyrolytic cracking of borazine at pressures pborazine up to 2.0 � 10^-4 Torr, Ts = 1373 K for t = 10 min. I discovered that the Ta2C film sputter-deposited on hBN-covered Al2O3(0001) surface exhibits significantly higher crystallinity than the samples grown on bare Al2O3(0001) substrates. Furthermore, I find that the crystallinity of thicker Ta2C layers can be improved by inserting hBN layers at regular intervals.
My studies demonstrate compositional and microstructural tunability during sputter-deposition of transition-metal carbide thin films using small amounts of the reactive gas and 2D layered materials as buffer layers. I expect that these results open up the exciting possibility of growth of highly oriented cubic-TaC and/or layered-Tan+1Cn phases, with n = 1, 2, 3, etc. with the appropriate choice of deposition parameters.