Pulsed Laser Deposition (PLD)

• Pulsed Laser Deposition (PLD) is a physical vapor deposition technique that uses high-power pulsed lasers to ablate targets and deposit thin films with precise stoichiometry.
• It leverages controlled laser ablation, plasma plume dynamics, and real-time diagnostics like RHEED and spectroscopic ellipsometry for atomic-level film growth.
• PLD is widely applied to synthesize complex oxides, nanostructures, metal films, and hybrid materials by fine-tuning process parameters and substrate conditions.

Pulsed Laser Deposition (PLD) is a laser-based, physical vapor deposition technique that enables stoichiometric, high-purity, and atomically controlled thin-film and nanostructure growth across a broad range of materials classes. A high-power pulsed laser ablates a condensed-phase target in vacuum or low-pressure gas, generating a transient plasma plume which delivers material to a substrate, where growth can be monitored and manipulated at the atomic or molecular level through careful adjustment of process parameters. In advanced research, PLD is employed for systems as diverse as complex oxides, chalcogenides, metal halides, sp-carbon nanostructures, and metallic films, with applications spanning quantum materials, photonics, catalysis, and functional devices. The technique’s unique attribute is precise control over kinetic energy, surface supersaturation, stoichiometry transfer, and real-time feedback, enabling the synthesis of films unattainable by conventional deposition methods (Harris et al., 2023, Casari et al., 2016, Kashir et al., 2019, Ulbrandt et al., 2019, Solomon et al., 10 Mar 2025).

1. Fundamental Principles of Laser Ablation and Plasma Plume Dynamics

PLD centers on pulsed laser ablation, typically using a nanosecond-scale excimer laser (e.g., KrF, λ=248 nm, τ≈20–30 ns) operated at fluences in the 0.3–15 J/cm² range, and repetition rates from 0.5 to 10 Hz (Harris et al., 2023, Kashir et al., 2019, Zhang et al., 2018). Each pulse delivers energy $E=F\cdot A$ (where $F$ is fluence, $A$ spot area), vaporizing a thin surface layer. The resultant high-temperature plasma plume consists of atoms, ions, molecules, radicals, clusters, and particulates; its kinetic energy spectrum ($\sim$0.1–100 eV/atom) is governed by target properties, spot geometry, laser fluence, and background gas (Harris et al., 2020).

The plume’s expansion depends critically on collision frequency with ambient gas: in vacuum or low-pressure regimes (<10⁻⁵ Torr), species retain high kinetic energy, supporting rapid surface coverage and ballistic migration; at elevated background pressure (0.1–500 Pa), plume–gas collisions confine and thermalize species, enhancing shock fronts and enabling cluster aggregation (Casari et al., 2016, Mateus et al., 2018). The evolution can be described by blast-wave (Sedov–Taylor) scaling:

$$R_s(t) = \xi \left(\frac{E_{\text{pl}}}{\rho_{\text{gas}}}\right)^{1/5} t^{2/5}$$

where $R_s$ is shock front radius, $E_{\text{pl}}$ is plume energy, $\rho_{\text{gas}}$ is gas density.

Plume kinetic regimes may be distinguished by plasma diagnostics (e.g., Langmuir probe): at photon-to-absorber ratios $\Phi/N_{\text{abs}} < 1$ (thermal regime), electron density $n_e \sim 10^{17}$–$10^{18}$ m⁻³, electron temperature $T_e \lesssim 0.5$ eV; above threshold ($\Phi/N_{\text{abs}} \gg 1$), $n_e \sim 10^{19}$–$10^{20}$ m⁻³, $T_e \sim 1$–5 eV, and high-energy, strongly ionized plumes are realized (Harris et al., 2020).

2. Film Growth Modes, Substrate Effects, and Kinetic Control

Film nucleation and morphology in PLD are determined by the interplay between plume species’ kinetic energy, background gas conditions, substrate temperature, and target–substrate distance. Layer-by-layer, Stranski–Krastanov, island (Volmer–Weber), and porous growth modes can be tuned by selecting optimal conditions for supersaturation, ad-atom mobility, and impingement rate (Ulbrandt et al., 2019, Mateus et al., 2018).

For crystalline and epitaxial films (e.g., oxides on perovskites or halides on single crystals), substrate temperature ($T_s$), lattice mismatch ($\delta$), and the local flux determine registry, strain, and defect density (Kashir et al., 2019, Hanzawa et al., 2019). In complex oxides, misfit dislocations and strain relaxation obey Matthews–Blakeslee scaling,

$$t_c \approx \frac{b}{2\pi|f|}\left[1 + \ln\left(\frac{t_c}{b}\right)\right],$$

where $t_c$ is critical thickness, $b$ the Burgers vector, $f$ the lattice mismatch (Kashir et al., 2019).

Deposition rate $R$ is often described by threshold-dominated kinetics:

$R \propto (F-F_{\text{th}})^m,$

where $F_{\text{th}}$ is material- and wavelength-dependent threshold fluence, $m \simeq 1$–2 (Kashir et al., 2019, Solomon et al., 10 Mar 2025, Kodu et al., 2018).

Substrate selection directly affects nucleation density, epitaxial quality, and functional response. For example, ultrathin SrTiO₃ buffer layers grown by PLD enable coherent growth of perovskite oxides on substrates otherwise plagued by valence mismatch and interface charge transfer, as shown for SrNbO₃ on KTaO₃ and GdScO₃ (Palakkal et al., 1 Oct 2024).

3. Process Monitoring: In Situ Diagnostics and Feedback

Real-time diagnostics are central to process control and optimization in PLD. Layer-by-layer epitaxy and interface abruptness are routinely tracked via reflection high-energy electron diffraction (RHEED), with specular spot intensity oscillations mapping monolayer completion:

$$I(t) = I_0 + \Delta I \cos\left(\frac{2 \pi t}{\tau_{\text{mono}}}\right)$$

where $\tau_{\text{mono}}$ is the monolayer deposition time (Gruenewald et al., 2013, Orvis et al., 2019). Simultaneous in situ spectroscopic ellipsometry yields the dielectric function $\epsilon(\omega, t)$ and allows extraction of film thickness and electronic structure evolution in real time.

Atomic force microscopy (AFM) integrated into PLD systems enables direct, unit-cell-level surface topography imaging at growth temperatures up to 700 °C and background pressures to 1 mbar. Such systems resolve atomic steps, nucleation/island density, and dynamic roughness evolution on timescales down to $\sim$1 s after a pulse (Wessels et al., 2017). In situ Auger electron spectroscopy (AES) provides monolayer depth resolution of surface composition during heterostructure growth (Orvis et al., 2019).

New machine-learning approaches fuse in situ plume imaging (ICCD/EMCCD-based movies) with deep convolutional neural networks to regress both process (pressure, laser energy) and film-growth kinetic parameters from spatiotemporal plume features with $r^2 > 0.9$ for process variables and $r^2 \sim 0.82$ for kinetic coefficients (sticking probabilities, flux). These frameworks enable real-time anomaly detection, pre-screening of growth conditions, and autonomous process optimization (Harris et al., 2023).

4. Materials Systems: Complex Oxides, Nanostructures, Halides, and Metals

PLD has been proven in the synthesis of diverse materials:

• Complex/doped oxides: Enables epitaxial films (e.g., SrTiO₃, NiO, MnO, EuO, SrNbO₃, LaMnO₃+δ, BiFeO₃), with applications in spintronics, ferroelectrics, and oxide electronics (Kashir et al., 2019, Gruenewald et al., 2013, Palakkal et al., 1 Oct 2024). For rocksalt oxides, fine control over $T_G$, $F$, $p_{\text{O}_2}$, and lattice matching is required for single-phase, strained growth (Kashir et al., 2019).
• Chalcogenides and Fe-based superconductors: Utilized for growth of epitaxial multilayers and metamagnetic/quantum phases, e.g., FeSe films on CaF₂ or SrTiO₃ (Shen et al., 2017, Harris et al., 2020, Hanzawa et al., 2019). Film properties (nematic transitions, $T_c$, carrier density) correlate with subtle PLD parameter modulations.
• Carbon nanostructures: Enables synthesis of 1D sp-carbon atomic wires embedded in sp² amorphous matrices. Critical parameters include operating in high Ar pressure ($\sim$500 Pa), $L > 1$ plume geometry, moderated fluence ($\sim$2–3 J/cm²), and room-temperature substrates for "soft landing" and prevention of re-hybridization (Casari et al., 2016).
• 2D and hybrid perovskite halides: Room-temperature PLD produces oriented $(\mathrm{PEA})_{2}\mathrm{PbI}_4$ Ruddlesden–Popper films with thickness control via pulse count and phase-pure $n=1$ orientation monitored by in situ PL (Solomon et al., 10 Mar 2025).
• Metals and functional layers: Compact TiO₂ electron-transport layers for perovskite solar cells with optimized thickness (32 nm, $\sim$4 000 pulses at 5 Hz, $F=100$ mJ/pulse) yield >13% PCE and outperform spin-coated layers in compactness, pinhole suppression, and contact resistance (Zhang et al., 2018). NiFe metallic films are possible at room temperature with controlled fluence ($F_L\geq3$ J/cm²), producing $M_s>500$ emu/cc (Yan et al., 2023).
• Surface functionalization: Sub-monolayer deposition on 2D materials, e.g., graphene, enables surface modification by Ag or ZrO₂ with per-pulse areal atomic densities $\sim7\times10^{12}$ cm⁻² (Kodu et al., 2018).

5. Process Modulation, Library Creation, and Interface Engineering

PLD lends itself to combinatorial and interface-engineering strategies owing to the directionality of the ablation plume and precise, pulse-resolved control. Combinatorial PLD (CPLD) uses synchronized masks and multiple targets to fabricate continuous in-plane composition spreads and interface libraries (Wolfman et al., 2020). For example, modulation of the buffer layer stoichiometry (e.g., LSMO$_x$) in tunnel junctions allows fine-tuning of interfacial magnetism and Schottky barriers by $\sim$100–400 meV within a single wafer.

For phase-selective growth of layered Ruddlesden–Popper and related oxides, plume cross-section (set by laser spot area $A$), angular spread, and deposition rate must be minimized (e.g., $A\sim0.25$ mm², $R\lesssim0.03$ nm/s) to suppress thermodynamically competing phases (Seo et al., 2016). Real-time spectroscopic ellipsometry or optical conductivity monitoring provides fingerprinting of phase purity, complementing standard RHEED.

6. Process Optimization, Anomaly Detection, and Autonomous Control

Closed-loop control frameworks are increasingly being deployed utilizing real-time diagnostics (RHEED, SE, AFM, ICCD plumes) and data-driven modeling. ML-based regression of plume images achieves $r^2>0.9$ for physical process parameters (pressure $P$, energies $E_1, E_2$), and $r^2\sim0.85$ for kinetic model parameters ($J, s_0, s_1$), allowing for automated optimization and early anomaly detection. Anomalies (e.g., deviations beyond $3\sigma$ of target setpoints) are automatically identified and flagged, with typical root causes including optical window fouling, drift in gas flows, and misalignment or degradation of targets (Harris et al., 2023).

In situ, high-speed X-ray scattering methods similarly resolve ultrafast surface transport and allow the researcher to quantify island-breakup, diffusion, and interlayer migration directly after each pulse, informing feedback for process tuning (Ulbrandt et al., 2019).

7. Challenges and Best Practices in PLD

Despite its versatility, PLD presents several technical challenges. Achieving uniformity and stoichiometry across large areas requires homogenization of the laser beam, precise rastering, and routine calibration of deposition rates (pulses/monolayer via RHEED/SE) due to target erosion and spot reshaping. Cation off-stoichiometry can arise if the fluence drifts or ablation profile is not uniform, necessitating ex situ RBS/WDS checks (Wolfman et al., 2020).

Film roughness, defect density, and interfacial structure are highly sensitive to substrate preparation (RMS roughness $<0.3$ nm preferred for ultrathin metal films), ambient pressure, plume geometry, and substrate temperature. Maintenance of ultra-high vacuum is critical for non-oxidic and metallic films to avoid contamination.

Pulse parameter regimes must be carefully selected: for instance, threshold fluences must be exceeded for full ablation, but excessive $F$ can cause particulates and plasma shielding. The synergy of stoichiometric transfer, monolayer control via in situ RHEED or SE, and plume engineering enables the creation of tailored heterostructures with functional properties unattainable by other physical vapor deposition techniques (Gruenewald et al., 2013, Yan et al., 2023).

---

References

• (Harris et al., 2023) Deep learning with plasma plume image sequences for anomaly detection and prediction of growth kinetics during pulsed laser deposition
• (Casari et al., 2016) Carbon-atom wires produced by nanosecond pulsed laser deposition in a background gas
• (Kashir et al., 2019) Pulsed Laser Deposition of Rocksalt Magnetic Binary Oxides
• (Ulbrandt et al., 2019) Homoepitaxial growth of SrTiO$_3$ by Pulsed Laser Deposition: energetic vs thermal growth
• (Solomon et al., 10 Mar 2025) Oriented 2D Ruddlesden-Popper Metal Halides by Pulsed Laser Deposition
• (Shen et al., 2017) Electronic structure and nematic phase transition in superconducting multi-layer FeSe films grown by pulsed laser deposition method
• (Palakkal et al., 1 Oct 2024) Growth Engineering of SrNbO3 Perovskite Oxide by Pulsed Laser Deposition and Molecular Beam Epitaxy
• (Seo et al., 2016) Selective growth of epitaxial Sr2IrO4 by controlling plume dimensions in pulsed laser deposition
• (Wessels et al., 2017) Imaging Pulsed Laser Deposition oxide growth by in-situ Atomic Force Microscopy
• (Orvis et al., 2019) In situ Auger electron spectroscopy of complex oxide surfaces grown by pulsed laser deposition
• (Gruenewald et al., 2013) Pulsed laser deposition with simultaneous in situ real-time monitoring of optical spectroscopic ellipsometry and reflection high-energy electron diffraction
• (Wolfman et al., 2020) Interface combinatorial pulsed laser deposition to enhance heterostructures functional properties
• (Yan et al., 2023) High-quality NiFe thin films on oxide/non-oxide platforms via pulsed laser deposition at room temperature
• (Zhang et al., 2018) Application of compact TiO$_2$ layer fabricated by pulsed laser deposition in organometal trihalide perovskite solar cells
• (Harris et al., 2020) Geometrical and energy scaling in the pulsed laser deposition plasma during epitaxial growth of FeSe thin films
• (Kodu et al., 2018) Highly sensitive NO2 sensors by pulsed laser deposition on graphene
• (Hanzawa et al., 2019) Stabilization and heteroepitaxial growth of metastable tetragonal FeS thin films by pulsed laser deposition
• (Mateus et al., 2018) Helium load on W-O coatings grown by pulsed laser deposition