SiO₂ Encapsulation via PVD

• SiO₂ encapsulation via PVD deposits uniform, amorphous thin films using electron-beam evaporation under ultra-high vacuum, ensuring complete coverage for air-sensitive materials like NbOI₂.
• Comprehensive characterizations, including AFM, Raman spectroscopy, and SHG mapping, confirm that the process preserves crystal structure, optical transparency, and nonlinear properties vital for photonic applications.
• Optimized deposition parameters such as controlled rate, substrate temperature, and film thickness yield enhanced mechanical robustness and long-term environmental stability suitable for integrated electronic and photonic devices.

Silicon dioxide (SiO₂) encapsulation via physical vapor deposition (PVD) is a critical process for protecting sensitive materials—particularly air-unstable van der Waals layered compounds such as niobium oxide diiodide (NbOI₂)—from environmental degradation while preserving their structural, chemical, and optical functionalities. PVD-grown amorphous SiO₂ barriers are central to platforms involving photonics, electronics, and quantum technologies, offering a pathway for integration with foundry-compatible processes and long-term device stability (Ngo et al., 18 Jan 2026, Jambur et al., 2022).

1. Physical Vapor Deposition of SiO₂: Workflow and Parameters

SiO₂ encapsulation by PVD is typically realized using electron-beam evaporation of high-purity SiO₂ “chunks” or powders in high-vacuum environments. NbOI₂ encapsulation exemplifies the stringent requirements for uniform film formation and chemical inertness.

Key process parameters are summarized below:

Parameter	Typical Value (NbOI₂ study (Ngo et al., 18 Jan 2026))	Notes
Source material	SiO₂, 99.99% purity	Electron-beam evaporated
Base pressure	~10⁻⁶–10⁻⁷ mbar	Ultra-high vacuum
Deposition rate	0.4 nm/s (24 nm/min)	Monitored by quartz crystal sensor
Target thickness	75 nm	Sufficient for full encapsulation
Substrate temperature	20–25 °C (ambient)	No heating for fragile 2D materials
Substrate preparation	Ionized air blow, no plasma/ion assist	Minimizes substrate/material damage

The deposited thickness $d$ follows the relation $d = R \cdot t$, where $R$ is the rate and $t$ the deposition time. For a 75 nm film at 0.4 nm/s: $t \approx 188$ s (Ngo et al., 18 Jan 2026).

SiO₂ films for mechanical testing have also been grown at higher substrate temperatures, with reported values of 60–900 °C yielding thicknesses up to 1 μm (Jambur et al., 2022).

2. Structural and Chemical Characterization of PVD-Grown SiO₂ Films

Comprehensive structural analysis of encapsulated heterostructures and reference SiO₂ films reveals several essential properties:

• Morphology: AFM and SEM confirm continuous, pinhole-free films with sub-nanometer RMS roughness on encapsulated flakes. Islanding and roughness are visible only on unprotected, oxidized NbOI₂.
• Interface quality: No delamination or void formation detected by AFM.
• Density and Porosity: For films grown on Si at various $T_s$, densities range from 2.10 g/cm³ (60 °C) to 2.17 g/cm³ (900 °C), with bulk melt-quenched a-SiO₂ at 2.20 g/cm³. Porosity (relative pore volume $\phi_{\mathrm{rel}}$) grows almost linearly with $T_s$ but is offset by enhanced network densification (smaller Si–O rings) at high $T_s$ (Jambur et al., 2022).
• Phase and Stoichiometry: XRD on as-grown NbOI₂ confirms preservation of the C2 ferroelectric phase after encapsulation. EDS and XPS analyses reveal characteristic Nb:I:O signatures and chemical state separation from degradation products such as Nb₂O₅ (Ngo et al., 18 Jan 2026).
• Crystal Structure: Raman spectroscopy shows no mode shift or broadening—Raman-active vibrations at $P_1$ = 104 cm⁻¹ through $P_5$ = 612 cm⁻¹—demonstrating lattice retention and absence of strain transfer from the encapsulation (Ngo et al., 18 Jan 2026).

3. Optical and Nonlinear Properties Preservation

One of the principal challenges in van der Waals heterostructure engineering is the retention of optical functionalities post-encapsulation. The SiO₂ cap layer must exhibit high optical transparency and low-loss characteristics across device-relevant spectral windows.

• Refractive index: Standard PVD or thermal SiO₂ has $n \approx 1.45$ for visible and near-infrared (VIS-NIR) regions; intrinsic propagation losses are $\ll 1$ dB/cm. A 75 nm cap contributes $<1$\% attenuation at both 1550 nm and 775 nm (Ngo et al., 18 Jan 2026).
• Second Harmonic Generation (SHG): Spatially resolved SHG mapping under 1550 nm fs excitation reveals total suppression ($>$90% loss in thin regions by day 10, zero by day 31) for unprotected NbOI₂, versus full retention (variation $<$5%) for SiO₂-encapsulated samples (40–360 nm) over 31 days. Power dependence obeys $I_{2\omega} \propto P^n$ with $n = 2.03$, confirming a second-order process and indicating preservation of $\chi^{(2)}$ (Ngo et al., 18 Jan 2026).
• Other nonlinear processes: No direct SPDC metrics were reported, but general scaling for SHG efficiency $\eta_\mathrm{SHG} \propto |\chi^{(2)}|^2 L^2 I_0$ holds, with encapsulation retaining full nonlinear response.

4. Environmental Stability and Degradation Kinetics

A central motivation for encapsulation is the mitigation of moisture- and oxygen-driven degradation, especially for materials such as NbOI₂, which amorphizes and loses its functional properties in ambient air.

• Kinetics: For unencapsulated NbOI₂, kinetic modeling follows $I_{2\omega}(t) = I_0 \exp(-t/\tau)$ with $\tau \lesssim 10$ days for thin flakes; SHG vanishes and in-plane area contracts by $>$10\% within one month. SiO₂ encapsulation extends $\tau \gg 31$ days—no observable loss over the measurement period (Ngo et al., 18 Jan 2026).
• Comparison: Raman and SHG response are fully preserved for encapsulated samples, while unprotected flakes exhibit Nb₂O₅ formation and complete optical signal loss.

5. Mechanical and Barrier Properties of PVD SiO₂

The ability of SiO₂ films to serve as encapsulation layers is contingent on a combination of network density, mechanical robustness, and low permeability. The elastic modulus ($E$) and hardness ($H$) scale with deposition temperature:

$T_s$ (°C)	$\rho$ (g/cm³)	$E$ (GPa)	$H$ (GPa)	$\phi_{\mathrm{rel}}$
60	2.10	50	5.0	0.76
600	2.15	55	6.0	0.79
800	2.16	60	7.0	-
900	2.17	-	-	1.00

Mechanical modulus increases with both growth and measurement temperature (anomalous $dE/dT_m>0$, uncommon in glasses), with a 60 °C film stiffening by 3.0 GPa upon heating to 400 °C. Densification through smaller network rings (noted in Raman and MD simulations) is responsible for the increase in $E$ despite higher total porosity at elevated $T_s$ (Jambur et al., 2022). For encapsulation, $T_s \sim 600$ °C offers an optimal compromise between density and manageable increases in porosity.

6. Implications for Photonic and Electronic Integration

Electron-beam PVD SiO₂ encapsulation is compatible with back-end processes in silicon photonics foundries. Key recommendations for encapsulating fragile van der Waals materials include:

• Deposition conditions: Avoid ion/plasma assistance and maintain moderate deposition rates (0.2–0.5 nm/s) to minimize mechanical and radiation damage.
• Thickness targeting: Adjust SiO₂ thickness to just exceed the tallest region of the underlying flake (typically 70–100 nm) for complete coverage without perturbing photonic or nonlinear optical modes.
• Alternative barriers: When even higher environmental tolerance is required, 75 nm Al₂O₃ layers can be deposited by the same approach, providing similar chemical protection and more uniform optical interference patterns (Ngo et al., 18 Jan 2026).
• Mechanical optimization: Films grown at $T_s \geq 600$ °C exhibit high elastic modulus ($E > 55$ GPa), high density ($\rho > 2.15$ g/cm³), and favorable microstructure for long-term integrity (Jambur et al., 2022).

A plausible implication is that careful selection of growth parameters allows tuning of encapsulation performance for environments ranging from ambient laboratory conditions to high-temperature device operation.

7. Summary and Outlook

PVD SiO₂ encapsulation effectively transforms air-sensitive, nonlinear-active 2D materials such as NbOI₂ into environmentally robust, foundry-compatible components for integrated photonic and quantum devices. The process preserves crystal structure, chemical state, and nonlinear optics over extended ambient exposure (at least 31 days), contingent on the selection of appropriate vacuum, deposition rate, film thickness, and—where barriers demand superior robustness—substrate temperature. Future developments may focus on integrating such encapsulation protocols with advanced lithographic steps and multilayer photonic device architectures (Ngo et al., 18 Jan 2026, Jambur et al., 2022).