ICMCTF2009 Session TS1: Computational & Experimental Studies of Thin Film Growth: An Atomistic View
Tuesday, April 28, 2009 1:30 PM in Room Sunset
TS1-1 In Situ, Real-Time Observation of Thin Film Deposition: Roughening, Zeno, Grain Boundary Corssoin Barrier, and Steering
M.J. Rost (Leiden University, Netherlands)
Thin polycrystalline metal films are becoming increasingly important, as is reflected in the multitude of applications in nanotechnology, nanooptics, microelectronics, vacuum coating, catalysis, medical science, sensor elements, wear protection layers, decorative coatings, and the synthesis of new materials. @paragraph@ As thin film properties are intrinsically linked to the precise film structure, one would like to control the overall film morphology down to the nanometer scale. This clearly demands fundamental research that links well-known atomic processes, such as diffusion and nucleation, with the mesoscopic film evolution during film growth.@paragraph@ Applying video-rate Scanning Tunneling Microscopy (STM)@super 1@, we succeeded in visualizing film growth with atomic-scale resolution in real-time@super 2@. We evaporated several tens of monolayers of gold on top of a well-annealed polycrystalline gold film, while continuously observing the evolving surface with the microscope. These measurements directly visualize atomic processes that take place during film growth.@paragraph@ Analyzing the evolving film structure, we observe a significant increase in the film roughness, which we explain by considering both “well-known”, single crystalline growth modes in combination with additional polycrystalline effects@super 2,3@. The grain boundaries play a crucial role in the evolution, as they initiate mound formation, thereby significantly increasing the total film roughness. A possible additional roughness contribution comes from atom steering, which also can delay the film closure in the early stages during film growth. @paragraph@@super 1@M.J. Rost et al.; Rev. Sci. Instr. 76, 053710-1 (2005) @paragraph@@super 2@M.J. Rost; Phys. Rev. Lett. 99, 266101 (2007) @paragraph@@super 3@M.J. Rost et al.; Phys. Rev. Lett. 91, 026101 (2003)}
TS1-3 Evolution of Residual Stress During Thin Film Growth: Effect of Competing Kinetic Processes
E. Chason (Brown University)
During deposition, thin films go through a range of stress states, changing from compressive to tensile and back again. This evolution can be understood as a kinetic competition between different mechanisms of stress generation and relaxation. The balance between them shifts as the microstructure evolves from isolated islands, through coalescence and finally into a steady state uniform film. Real-time measurements of stress using wafer curvature enable us to monitor these changes while the film is growing. We will review some experiments of stress evolution in thin films grown by different processes (sputtering, evaporation, electrodeposition). A kinetic model will be described that balances the different stress generation processes and show how it can explain the effect of changing the growth conditions on the resulting stress.
TS1-6 Atomistic Simulations of Plasma Sprayed Thin Film Growth
E. Webb (Sandia National Laboratories)
Atomistic simulations of metallic droplet impingement onto a metal substrate have been performed to model thin film growth via plasma spray processing. The plasma spray process involves injection of solid material into a heat source, or plasma, melting of the constituent particles, and acceleration of the molten particles towards a substrate. Upon impact with the substrate, molten particles deform and flatten onto the substrate and then solidify as they cool. Due to the typically high kinetic energy of impacting particles, their final solidified shape is highly asymmetric as they form disks or splats on the substrate. The shape of individual solidified particles manifests itself in the microstructure of deposited films as a lamellar morphology often emerges. This morphology is associated with many of the properties inherent in plasma sprayed coatings including beneficial thermal, chemical, and mechanical resistance so it is of use to understand how droplet properties ma nifest in final splat morphology. While conventional plasma spray processes incorporate feedstock materials (e.g., powders) with particle sizes on the order of tens to even hundreds of microns, recent work@super 1@ has demonstrated that nanoscale particles can be sprayed and that the nanoscale feature size is retained in the solidified coating. Studying the impact and solidification behavior of nanoscale particles is well suited to molecular dynamics simulations where the fundamental entities are atoms of the constituent materials. Furthermore, the Weber number, or ratio of inertial to interfacial energy, of nanoscale particles is significantly smaller than that for conventional plasma sprayed particles. As such, it is uncertain how well existing engineering relations between the droplet and final splat diameters apply to nanoscale particles. Using MD simulations, we explore the flattening of nanoscale droplets as a function of their impingement velocity and size and rev e al that recently proposed relations@super 2@ between flattening and droplet properties do not readily extend to nanoscale plasma sprayed particles. @paragraph@@super 1@J. Gang, et al.; Scripta Mater., v.48, pp. 1599-1604 (2003).@paragraph@@super 2@C-J. Li, et al.; Surf. & Coat. Technol., v.191, pp. 375-383 (2005).
TS1-7 Tunable Molecular Beams: A New Frontier in Vacuum Deposition of Organic Semiconductors
A. Amassian (Cornell University, Ithaca)
Organic electronics are widely believed to be the most viable platform to manufacture pervasive and disposable electronics on flexible substrates cheaply and with a lesser environmental impact than conventional electronics. The performance of organic electronic devices is closely tied to the packing structure, morphology and interfaces in organic semiconductor thin films, which in turn are intricately linked to molecular processes operant during their assembly. Typically, vacuum sublimation/evaporation is used to fabricate ordered molecular films (e.g., pentacene). While the simplicity of thermal deposition processes makes them attractive, they provide few knobs in way of process control. Supersonic molecular beams have emerged as a way to tailor the assembly of complex molecular building blocks by manipulating the state of incident molecules (e.g., kinetic energy, vibro-rotational states, molecular clustering). Using a powerful combination of in situ real-time synchrotron X-ray scattering, scanning probe microscopy, rate equation modeling and molecular dynamics simulations, we investigate molecular-scale processes of adsorption, assembly, and molecular crystallite formation during supersonic molecular beam deposition of small-molecule thin films of pentacene and diindenoperylene. Our research shows that tunable supersonic molecular beams can tailor the mode and kinetics of growth, resulting in a control of the morphology and packing structure of organic small-molecule semiconductors in unprecedented ways. These changes can influence the field effect mobility of organic semiconductors and offer a pathway to control the performance of organic electronic materials and devices thereof. Our findings indicate that molecular-scale control of interfaces and thin films is achievable; it is contingent upon the development of adequate processing strategies.
TS1-9 A First-Principles Investigation of the Phase Stability of Ti@sub 2@AlC Upon Oxygen Incorporation
M. Dahlqvist, B. Alling, I.A. Abrikosov, J. Rosén (Linköping University, Sweden)
The phase stability of Ti@sub 2@AlC upon oxygen incorporation has been studied by means of first-principles calculations. Experimental observations of this so called MAX-phase (M = early transition metal, A = A-group element, X = C or N) show that the characteristic nanolaminated structure is retained upon oxygen incorporation, with strong indications of O substituting for C. Therefore, a solid solution of C and O on the carbon sublattice has been simulated by the so called Special Quasi-random Structure (SQS) method. The energy of formation of Ti@sub 2@Al(C@sub 1-x@,O@sub x@) has been compared to those of all known competing binary and ternary phases, and has been found favourable for all C to O ratios at the composition of the MAX-phase. A negative isostructural formation enthalpy have also been predicted for Ti@sub 2@Al(C@sub 1-x@,O@sub x@). Moreover, the energy of the alloy was lower than what has been calculated for a corresponding mixture of different Ti(C,O)-ternaries and TiAl. Altogether this indicates that a mixture of C and O in the MAX-phase is thermodynamically stable for a wide range of oxygen content, x, which is consistent with experimental observations. Furthermore, the effect of oxygen incorporation on electrical and mechanical properties is discussed, and compared to what is observed experimentally. These results are of importance for an increased fundamental understanding of phase formation and material properties tuned by the incorporation of oxygen.
TS1-10 A Formula for Increased Hardness and/or Ductility in TiN-Based Thin Films
D.G. Sangiovanni, V. Chirita, L. Hultman (Linköping University, Sweden)
TiN-based thin films, such as Ti@sub 1-x@Al@sub x@N and their alloys, are known to have excellent mechanical and thermal properties. In this paper we report the initial results of our ab-initio investigations of four novel ternary compounds, Ti@sub 1-x@M@sub x@N, obtained by alloying TiN with Nb, V, Mo and W, in concentrations of up to 50%. The elastic constants as well as bulk, shear and Young’s moduli of these compounds were evaluated using density functional theory calculations within the generalized gradient approximation, and compared with the corresponding properties of TiN and Ti@sub 1-x@Al@sub x@N. Significantly, we find that the addition of all these elements, but primarily Mo and W, results in a substantial increase in bulk modulus values compared to TiN (up to 15%) and Ti@sub 1-x@Al@sub x@N (up to 30%). At the same time, we observe a dramatic decrease (up to 70%) in the values of C@sub 44@, and a reversal in the Cauchy pressure, C@sub 12@-C@sub 44@, from negative to positive values, results which are indicative of significantly increased ductility in all of these compounds. These trends are in total contrast to what is known for Ti@sub 1-x@Al@sub x@N, which exhibits increased brittleness and lower bulk modulus values as the Al content is increased. We analyze the electronic structure of these compounds and observe a significant shift in the ratio of ionic-to-covalent bonding, as compared to TiN and/or Ti@sub 1-x@Al@sub x@N. This information can be used to understand the mechanisms which promote the transition from strong directional to more metallic bonding, and as it will be shown, is essential in designing compounds with mechanical properties tailored to a variety of applications.
TS1-11 DFT Studies of Graphene Thin Films
C.V. Ciobanu (Colorado School of Mines)
We present a theory of Moire patterns that are generated by superposing graphene layers on substrates with triangular or rectangular symmetries.@paragraph@ While these quasi-periodic patterns are determined solely by the lattice constants of graphene and substrate and their relative orientation, the structure, interfacial stability, and electronic properties are determined by the atomic-scale interactions. The interactions include the rehybridization of the carbon atoms into sp3 states, which can induce a bandgap in graphene and thus result in distinct signatures in the STM measurements. We will exemplify with the case of graphene on noble metals and transition-metal oxides, and explore the possibilities and the limitations of engineering the bandgap in graphene by using various substrates.