Pulsed Laser Deposition (PLD) Anne Reilly College of William and Mary Department of Physics

Outline 1. Thin film deposition 2. Pulsed Laser Deposition a) Compared to other growth techniques b) Experimental Setup c) Advantages and Disadvantages 3. Basic Theory of PLD 4. Opportunities

Thin Film Deposition Transfer atoms from a target to a vapor (or plasma) to a substrate

Thin Film Deposition Transfer atoms from a target to a vapor (or plasma) to a substrate After an atom is on surface, it diffuses according to: D=D o exp(-  D /kT)  D is the activation energy for diffusion ~ 2-3 eV kT is energy of atomic species. Want sufficient diffusion for atoms to find best sites. Either use energetic atoms, or heat the substrate.

target substrate Evaporation (Molecular beam epitaxy-MBE) Ways to deposit thin films target substrate Chemical vapor deposition- CVD Ar + substrate gas Sputtering

Low energy deposition (MBE): ~0.1 eV may get islanding unless you pick right substrate or heat substrate to high temperatures High energy deposition (Sputtering ~ 1 eV) smoother films at lower substrate temperatures, but may get intermixing

Low energy deposition (MBE): ~0.1 eV may get islanding unless you pick right substrate or heat substrate to high temperatures High energy deposition (Sputtering ~ 1 eV) smoother films at lower substrate temperatures, but may get intermixing

CCD /PMT spectrometer Target Substrates or Faraday cup laser beam Pulsed Laser Deposition

CCD /PMT spectrometer Target Substrates or Faraday cup laser beam Pulsed Laser Deposition Target: Just about anything! (metals, semiconductors…) Laser: Typically excimer (UV, 10 nanosecond pulses) Vacuum: Atmospheres to ultrahigh vacuum

Advantages of PLD  Flexible, easy to implement  Growth in any environment  Exact transfer of complicated materials (YBCO)  Variable growth rate  Epitaxy at low temperature  Resonant interactions possible (i.e., plasmons in metals, absorption peaks in dielectrics and semiconductors)  Atoms arrive in bunches, allowing for much more controlled deposition  Greater control of growth (e.g., by varying laser parameters)

Disadvantages of PLD Uneven coverage High defect or particulate concentration Not well suited for large-scale film growth Mechanisms and dependence on parameters not well understood

Processes in PLD Laser pulse

Processes in PLD e- Electronic excitation

Processes in PLD e- Energy relaxation to lattice (~1 ps) lattice

Processes in PLD Heat diffusion (over microseconds) lattice

Processes in PLD Melting (tens of ns), Evaporation, Plasma Formation (microseconds), Resolidification lattice

Processes in PLD lattice If laser pulse is long (ns) or repetition rate is high, laser may continue interactions

Processes in Pulsed Laser Deposition 1. Absorption of laser pulse in material Q ab =(1-R)I o e -  L (metals, absorption depths ~ 10 nm, depends on ) 2. Relaxation of energy (~ 1 ps) (electron-phonon interaction) 3. Heat transfer, Melting and Evaporation when electrons and lattice at thermal equilibrium (long pulses) use heat conduction equation: (or heat diffusion model)

Processes in Pulsed Laser Deposition 4. Plasma creation threshold intensity: goverened by Saha equation: 5. Absorption of light by plasma, ionization (inverse Bremsstrahlung) 6. Interaction of target and ablated species with plasma 7. Cooling between pulses (Resolidification between pulses)

Incredibly Non-Equilibrium!!! At peak of laser pulse, temperatures on target can reach >10 5 K (> 40 eV!) Electric Fields > 10 5 V/cm, also high magnetic fields Plasma Temperatures K Ablated Species with energies 1 –100 eV

PLD with Ultrafast Pulses (< 1 picosecond) see Stuart et al., Phys. Rev. B, (1996) A new research area! If the pulse width < electron lattice-relaxation time, heat diffusion, melting significantly reduced! Means cleaner holes and cleaner ablation Direct conversion of solid to vapor, less plasma formation Reactive chemistry: energetic ions, ionized nitrogen, high charge states Leads to less target damage (cleaner holes), and smoother films (less particulates)

PLD with Ultrafast Pulses (< 1 picosecond) see Stuart et al., Phys. Rev. B, (1996) A new research area! If the pulse width < electron lattice-relaxation time, heat diffusion, melting significantly reduced! Means cleaner holes and cleaner ablation Direct conversion of solid to vapor, less plasma formation Reactive chemistry: energetic ions, ionized nitrogen, high charge states Leads to less target damage (cleaner holes), and smoother films (less particulates)  > 50 ps Conventional melting, boiling and fracture Threshold fluence for ablation scales as  1/2  < 10 ps Electrons photoionized, collisional and multiphoton ionization Plasma formation with no melting Deviation from  1/2 scaling

TARGET FILM (deposited on silicon) 20 ns EXCIMER versus 1 ps TJNAF-FEL Cobalt ~20 mJ/pulse, 20 ns, 308 nm, 25 Hz, 1 x Torr Steel, ~20  J/pulse, 18 MHz, 3.1 micron 1 x Torr, 60 Hz pulsed, rastered beam Less melting! Few particulates! for Nb: < 1 per cm -2 SEMs by B. Robertson, T. Wang, TJNAF

Opportunities Ultrahigh quality films Circuit writing Isotope Enrichment New Materials Nanoparticle production

Magnetic Moment of fcc Fe(111) Ultrathin Films by Ultrafast Deposition on Cu(111) J. Shen et al., Phys. Rev. Lett., 80, pp MBEPLD Higher quality films, better magnetic properties

MICE Direct writing of electronic components- in air! Rapid process refinement No masks, preforms, or long cycle times True 3-D structure fabrication possible Single laser does surface pretreatment, spatially selective material deposition, surface annealing,component trimming, ablative micromachining, dicing and via-drilling

Isotope Enrichment in Laser-Ablation Plumes and Commensurately Deposited Thin Films P. P. Pronko, et al. Phys Rev. Lett., 83, pp Over twice the natural enrichment of B 10 /B 11, Ga 69 /Ga 71 in BN and GaN films Plasma centrifuge by toroidal and axial magnetic fields of 0.6MG!

Transient States of Matter during Short Pulse Laser Ablation K. Sokolowski-Tinten et al., Phys. Rev. Lett., 81, pp Fluid material state of high index of refraction, optically flat surface

New Materials and Nanoparticles D.B. Geohegan-ORNL Carbon/carbon collisons- buckyballs Fast carbon ions- diamond films Study of plasma plume and deposition of carbon materials

References “Pulsed Laser Vaporization and Deposition”, Wilmott and Huber, Reviews of Modern Physics, Vol. 72, 315 (2000) “Pulsed Laser Deposition of Thin Films”, Chrisey and Hubler (Wiley, New York, 1994) “Laser Ablation and Desorption”, Miller and Haglund (Academic Press, San Diego, 1998)