Wide-bandgap photonic + piezoelectric platform with UV transparency
Physics / mechanism
AlN has a wide bandgap (~6.2 eV — wider than SiC), making it transparent through UV. Refractive index ~2.1 at 1550 nm. AlN is non-centrosymmetric, so it exhibits second-order nonlinearity (χ²), enabling efficient electro-optic modulation and frequency conversion. It is also strongly piezoelectric, making it the dominant material for film bulk acoustic resonators (FBARs) in RF filters — the same property opens electro-acoustic modulators in photonics.
CMOS-fab compatibility is decent: AlN deposits via sputtering at moderate temperatures and is already used in MEMS lines. The challenge is high-quality crystalline AlN (vs polycrystalline) — single-crystal AlN-on-sapphire is the lab path; commercial AlN-on-Si is polycrystalline and lossy.
Competitive landscape
AlN photonic waveguides have been demonstrated since 2012 (Yale group, IBM Zurich). Best loss ~0.5–1 dB/cm. Commercial photonic platforms: none yet. RF/MEMS commercial: Soitec (POI substrates), Skyworks, Qorvo, Broadcom for FBARs.
ScAlN (scandium-doped AlN) has emerged as the higher-performance variant — adding Sc increases piezoelectric and electro-optic coefficients 5–10× over pure AlN. ScAlN also exhibits ferroelectricity at certain compositions, opening additional functionality. See Scandium Aluminum Nitride (ScAlN).
Frontier (open questions)
See frontmatter frontier: block.