Optical Thin Film Composition Insights
Introduction to Optical Thin Films
Optical filters rely on thin films formed by depositing materials onto substrates through phase transitions (solid-liquid-gas-solid or solid-gas-solid). These films differ from bulk materials in microstructure, crystal structure, and composition, impacting their performance in applications like spectroscopy and imaging.
Evaporation of Elements
Most metals evaporate as single atoms, but semiconductors and semimetals often form clusters. For example, antimony (Sb) films primarily contain Sb4 clusters, with some Sb2 and trace Sb. Arsenic (As) forms As4 and As2, while bismuth (Bi) and tellurium (Te) include Bi2 and Te2. Similar clusters appear in carbon (C), cerium (Ce), selenium (Se), and silicon (Si).
Compound Thin Films
Compounds behave complexly during deposition. Sulfides like ZnS decompose (e.g., 2ZnS → 2Zn + S2) but recombine on the substrate, yielding films close to bulk stoichiometry. Cadmium telluride (CdTe) breaks into Cd and Te2. Fluorides experience defluorination, increasing moisture absorption. Using large-capacity evaporation sources minimizes this issue.
Fluoride Film Defluorination
The table below shows fluorine loss in fluoride films:
| Film | Atoms | Concentration Ratio |
|---|---|---|
| Na3AlF6 | F/Na, F/Al | 1.3, 3.6 |
| MgF2 | F/Mg | 1.3 |
| SrF2 | F/Sr | 1.4 |
| BaF2 | F/Ba | 1.5 |
| NaF | F/Na | 0.6 |
Oxide Films
Oxides lose oxygen during evaporation. Even stable SiO2 contains SiO and O2. Severe oxygen loss occurs in TiO2, Ta2O5, and NiO, while MgO, Al2O3, BeO, and CoO closely match bulk compositions.
Sputtering and Clusters
Sputtering produces composite atoms. For copper (Cu) at 100 eV with Ar+, only 5% is single Cu atoms; most are Cu2 or Cu+n (n=1–11) for single-crystal targets. Aluminum (Al) sputtering yields Aln clusters (n=1–7 or 1–18). For GaAs, 99% of sputtered material is neutral Ga and As atoms.
Composition Analysis
Surface analysis techniques, using photons, electrons, or ions, identify thin film composition by analyzing secondary particle emissions from surface interactions.
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