metal-oxide-interfaces

name	metal-oxide-interfaces
description	Domain knowledge for metal oxide interface analysis in reflectometry. Covers native oxide formation on metals (Cu→CuO, Ti→TiO₂), oxide thickness ranges, and rules for when to add or avoid oxide layers. Always use when the sample involves metals, copper, titanium, native oxides, or SiO₂. The user may not have specified the oxide layer, but if the metal is exposed to ambient conditions, it is likely present and should be considered in the model.
metadata	{"author":"aure","version":"1.0"}

Native Metal Oxide Formation

When a metal layer is directly in contact with the ambient medium (air, solvent, etc.) and no oxide, SEI, or other surface layer is already present, a thin native oxide layer typically forms.

When to Add an Oxide Layer

• Metal is the outermost layer (in contact with ambient)
• No existing oxide, SEI, or surface layer between the metal and ambient
• The fit shows systematic residuals suggesting a missing interface layer

When NOT to Add an Oxide Layer

• An oxide or surface layer already exists between the metal and ambient
• The metal is a buried layer (e.g., Ti adhesion layer beneath Cu)
• The metal is already covered by an SEI or other surface layer
• Do NOT split an existing oxide into sublayers (e.g., CuO + Cu₂O) — keep it simple

Common Metal Oxide Properties

Oxide	SLD (×10⁻⁶ Å⁻²)	Typical Thickness
CuO	5.0	10–50 Å
Cu₂O	4.0	10–30 Å
TiO₂	2.6	10–50 Å
Fe₂O₃	7.2	10–30 Å
Al₂O₃	5.7	20–50 Å
NiO	6.9	10–30 Å
SiO₂	3.47	10–20 Å (native)

Oxide Layer Fitting Guidelines

• Thickness bounds: 5–200 Å (for initial oxide layers)
• SLD bounds: use ±2.0 around nominal oxide SLD
• Oxide roughness is typically 3–15 Å
• When a metal in contact with a reactive ambient has no oxide in the model, adding one is normally the top-ranked structural hypothesis. Do NOT discard it on the grounds of model simplicity — let BIC decide after it has been tried once.

Adhesion Layers

Adhesion layers (e.g., Ti or Cr beneath Au or Cu) are internal layers and should NOT have oxide layers added on them. Their SLD bounds should be wider (±3.0) to account for intermixing with adjacent layers:

• Titanium adhesion: SLD range -5.0 to 1.0
• Chromium adhesion: SLD range 1.0 to 5.0

Refinement Strategy — Metal Oxide Interfaces

When refining a model with metal layers, apply these rules in order:

Adhesion Layer Parameter Drift

Thin adhesion layers (Ti, Cr) are prone to parameter trade-offs during fitting:

• Thickness inflation: If fitted Ti thickness is >2× nominal (e.g., 100 Å when 50 Å is expected), tighten thickness bounds close to the nominal value (e.g., 30–80 Å for a nominal 50 Å layer). The fitter uses the adhesion layer as a contrast sink when given too much freedom.
• SLD drift: If fitted Ti SLD deviates >50% from nominal (-1.95), check whether the SLD bounds are too wide. Narrowing to ±2.0 around nominal may help, but keep bounds wide enough for realistic intermixing (minimum ±1.5).
• Roughness pinning: If adhesion layer roughness is pinned at its lower bound, the bound may be too restrictive. Ensure roughness_min ≥ 5 Å and roughness_max is at least 20 Å.

Metal Surface Oxide

If an outermost metal layer is in contact with air, D₂O/H₂O, or another reactive ambient and there is no oxide in the model, a native oxide is the single most likely structural gap — add it as a high-ranked hypothesis in the structural-hypothesis-ranking list at intake. For Cu in aqueous ambient: CuO, 10–50 Å, SLD 4.5–5.5. For Ti in aqueous ambient: TiO₂, 10–50 Å, SLD 2.0–3.2. Roughness typically 3–15 Å.

During refinement, if parameter-only tweaks have not reached the acceptance threshold and this hypothesis is pending, try it before further bound fiddling. Do not gate oxide addition on arbitrary χ² thresholds such as "only if χ² > 10" — a structural gap produces systematic residuals even at moderate χ², and the BIC guardrail will automatically revert the change if the added complexity is not justified.

Do NOT add an oxide layer to a metal that is buried under another layer or a solvent unless the sample description explicitly asks for it.

Multi-Layer Metal Stacks (e.g., Cu on Ti on Si)

• The Cu/Ti interface roughness should typically be 5–15 Å. If it grows larger than 20 Å, consider whether intermixing is being used to compensate for a missing interface layer.
• Keep the Cu SLD bounds within ±2.0 of the nominal 6.55 value.
• For Ti under Cu, the Ti SLD can deviate more due to intermixing — use bounds of -5.0 to 1.0 but flag deviations beyond -4.0 or above 0.0 as potential unphysical behavior.