Diasemi —Diamond (111) Homoepitaxy via Hot-Filament CVD: Growth Mode, Transition-Metal Incorporation, and Twin Formation
October31, 2025
Diamond (111) Homoepitaxy via Hot-Filament CVD: Growth Mode, Transition-Metal Incorporation, and Twin Formation
Methodology
Substrate: HPHT Ib diamond (111), 2–4° off-axis.
Filament: Ta, Ø 0.25 mm × 95 mm, ≈ 3000 °C.
Process: 4 kPa H₂ + CH₄ (0.35–1.4 %), substrate ≈ 1000 °C.
Tools: AFM (surface morphology) and SIMS (Ta quantification).
Mesa method: atomically flat (111) mesas via NiC etching + MPCVD planarization for true homoepitaxy.
Key Observations
| CH₄ conc. | Growth Mode | Growth Rate (μm h⁻¹) | Ta Conc. (atoms cm⁻³) | Morphology |
|---|---|---|---|---|
| 0.35 % | 2D island (bilayer steps ≈ 0.21 nm) | 0.5 | 1 × 10²⁰ | Smooth triangular islands + many rotational twins |
| 0.7 % | Mixed 2D→3D | 1.6 | 5 × 10¹⁹ | Larger islands (~2 nm steps) |
| 1.4 % | 3D rough | 3.3 | 1 × 10¹⁹ | Particle-like surface, no twins |
→ Increasing CH₄ drives 2D→3D transition, while Ta incorporation drops by one order.
Flux and Sticking Analysis
Product of [Ta conc × growth rate] ≈ constant → Ta incorporation flux independent of growth mode.
Ta sticking coefficient αTa ≈ 2 × 10⁻³, nearly constant.
Incorporation governed by gas-phase Ta/C ratio, not by surface morphology.
Effective TaC evaporation rate under HFCVD ≈ 0.19 µg cm⁻² s⁻¹ (two orders below vacuum).
Large atomic-radius metals (Ta 146 pm vs C 77 pm) have low incorporation efficiency due to lattice-strain barriers; only ~10⁻³ of arriving atoms are trapped.
Twin Formation Mechanism
Twin density ≈ 2 × 10⁸ cm⁻², while Ta areal density ≈ 2 × 10¹² cm⁻².
→ Twins ≪ incorporated Ta → single Ta atoms do not directly trigger twins.Twins arise mainly under 2D island growth, specific to HFCVD.
Proposed mechanism:
→ Local Ta atoms or Ta–vacancy complexes reduce {111} stacking-fault energy (~300 mJ m⁻²), facilitating 60° rotational twinning.
→ Energy barrier lowering is statistical, hence low twin density despite high Ta content.
Conclusions
Growth mode transition (2D→3D) with increasing CH₄ mirrors MPCVD behavior.
Ta incorporation decreases linearly with CH₄ due to dilution, not morphology.
αTa ≈ constant ⇒ incorporation controlled by gas-phase Ta/C ratio.
Twin formation is not a direct result of Ta atoms but of complex Ta-defect interactions.
HFCVD can potentially yield large-area epitaxial diamond with controlled defect engineering via transition-metal interaction.
