Liquid-Like Mass Transport, Without Melting, In Solid Metallic Films via Ultrafast Laser Enhancement of Point Defect Diffusion

Ultrashort laser pulses (150 fs) at fluences below that needed to melt a material excite electrons into unoccupied states for brief moments of time significantly weakening bonding for possibly as long as 10 nanoseconds. The ions drift with their room-temperature velocity and create transient lattice disorder. Approximately 1-2% of the atoms in the high velocity tail of the Boltzmann distribution will have sufficient energy to create a Frenkel pair. Repeated irradiation leads to an accumulation of point defects and vastly increases (12 to 20 orders of magnitude) mass transport via an ultrafast laser enhanced diffusion mechanism.<br/>Our previous work with semiconductors has demonstrated that repeated exposures of a gallium arsenide (GaAS) surface to ultrashort laser pulses leads to the generation followed by enhanced diffusion of point defects. The defects diffuse to the surface and their accumulation leads to morphological changes that are observed as Laser Induced Periodic Surface Structures (LIPSS). We have shown that high spatial frequency LIPSS (HSFL) are formed while irradiating GaAs with 1000 pulses. The diffusion process is electronically driven and occurs on timescales shorter than 10 ns, the time it takes for a majority of the excited carriers to equilibrate after irradiation. This suggests that the ultrafast laser enhanced diffusion coefficient is increased by up to 20 orders of magnitude over the room temperature value. This is the same order of magnitude value as diffusion in a melt. Hence we observe liquid-like mass transport while remaining in the solid phase.<br/>This talk will show that a similar process can also work in metals, leading to atomic scale mixing from defect- and disorder-driven departures from a perfect lattice. We set out to study this mixing with ultrashort laser irradiation using fluences below that needed to melt a material in a single exposure. We chose a well studied system (Ni/W) that offers the promise of thermally stable nanostructured materials previously grown with electrochemical methods [1].<br/>Alternating film stacks of Ni and W were deposited onto a glass substrate by magnetron sputter deposition. A working gas pressure of 2 mTorr and 3 mTorr was used for growing the Ni and W, respectively. A 100 nm Ni film was first deposited on the glass substrate as a heat sink. 12 alternating layers of Ni and W were then deposited on the heat sink such that a layer of W was present on the surface. The Ni layers were 2.5 nm and the W layers were 1.5 nm thick so that the layers are an average of 27 at% W. Multiple pulses from a Ti:Sapphire laser with λ = 780 nm and pulse duration t = 150 fs were used to irradiate the surface of the stack, on the W layer.<br/>We will present scanning electron micrographs showing that irradiating the alternating film stack with 1,000 pulses leads to the formation of HSFL on the surface of the films. Using transmission electron microscopy, we will show that the HSFL is composed of the top 6 layers, meaning that the laser affected the material to a depth of only ~12 nm below the surface. Furthermore, we will show with x-ray dispersive spectroscopy that the layers are mixed. This solid-state mass transport enhancement process will be discussed in the context of ultrashort lattice disordering, bond softening, diffusion and defect generation mechanisms.<br/>[1] A. J. Detor, M. K. Miller, and C. A. Schuh, “Solute distribution in nanocrystalline Ni–W alloys examined through atom probe tomography,” <i>Philos. Mag.</i>, vol. 86, no. 28, pp. 4459–4475, Oct. 2006, doi: 10.1080/14786430600726749.