Gan Growth Relies on Sapphire Substrates for New Applications

Imagine modern LED lighting without a material that combines strength, transparency, and stability under extreme temperatures. Such technology might not exist at all. Synthetic sapphire, a single-crystal form of aluminum oxide (Al₂O₃), plays this pivotal role—not only as an ideal substrate for III-nitride epitaxial growth but also as a material with broad applications in semiconductors, electronics, and optics.

Unlike polycrystalline aluminum oxide, sapphire's single-crystal structure grants it exceptional physical and chemical properties that make it ideal for specialized applications:

• Outstanding chemical stability: Resistant to various chemical agents, even at high temperatures.
• Excellent electrical properties: Extremely high resistivity (typically >10¹¹ Ω·cm at ~300K), though its relatively low thermal conductivity (<30 W/(m·K) near room temperature) can be limiting for LED applications.
• Superior dielectric properties: High dielectric strength with constants of 11.5 (parallel to c-axis) and 9.3 (perpendicular to c-axis) at 298K across 10³–10⁹ Hz frequencies.
• Remarkable mechanical strength: High compressive strength (~2 GPa or ~3×10⁵ psi) though with lower tensile strength (275–400 MPa). Flexural strength measures ~1.03 GPa (parallel to c-axis) and ~758 MPa (perpendicular).
• Exceptional hardness: Knoop hardness of 1900 kg/mm² (parallel) and 2200 kg/mm² (perpendicular).
• High refractoriness: Maintains properties under extreme heat.

Synthetic sapphire for electronics consists of ultra-pure single-crystal Al₂O₃ without pores or grain boundaries—distinct from gem-grade sapphires containing trace elements that create characteristic colors. This pure crystalline form is also called α-alumina or corundum, representing the most thermodynamically stable phase among alumina's many polymorphs.

Sapphire's dominance as the substrate of choice for GaN heteroepitaxy stems not only from its hexagonal crystal structure's similarity to GaN's wurtzite form but also from its exceptional chemical and thermal stability. With a melting point of 2323K (2030°C) and boiling point of 3253K (2980°C), sapphire remains stable even during high-temperature GaN buffer layer epitaxy above 1000°C.

During typical MOCVD GaN growth processes where hydrogen serves as both carrier gas and byproduct of hydride cracking, sapphire maintains stability where other materials would decompose. Even so, minor surface decomposition occurs—oxygen release from heated sapphire surfaces that later incorporates into initial GaN growth layers, creating thin oxygen-doped regions near the interface.

The complex crystallography of (0001) sapphire surfaces requires careful preparation. Standard procedures involve annealing in flowing H₂ at 1000–1100°C to restructure surface chemistry before chemical exposure. Atomic force microscopy reveals how annealing times between 2–40 minutes develop step-terrace microstructures with ~0.2nm step heights (one monolayer).

Direct growth on polished c-plane sapphire produces poor GaN quality due to significant lattice mismatch (14%) and thermal expansion differences. This leads to wide XRD peaks (15–30 arc-min FWHM), high residual electron concentrations (≥10¹⁸ cm⁻³), and rough 3D microstructures. The solution came through buffer layer technology—though it reduces rather than eliminates these fundamental mismatches.

Nitridation has become a crucial pretreatment step, where sapphire surfaces exposed to flowing NH₃ at ≥800°C form thin AlN layers that improve subsequent III-nitride growth. This process modifies surface energy and reduces lattice mismatch while affecting film microstructure, polarity, defect density, and electronic properties. Optimal nitridation times under 3 minutes produce smoother surfaces, while longer durations increase roughness through stress-induced features.

Despite sapphire's advantages, researchers continue exploring alternatives to address lattice and thermal expansion mismatches:

• Silicon Carbide (SiC): The second-most popular III-nitride substrate, particularly for blue/green/white LEDs and HEMTs. 4H- and 6H-SiC offer hexagonal structures with better lattice matching (~3.5% mismatch vs. GaN) than sapphire.
• Silicon (Si): Economically attractive due to mature manufacturing of large-diameter wafers (>12"), though GaN quality on Si(111) still trails sapphire-based growth.
• Zinc Oxide (ZnO): Promising with only ~1.9% lattice mismatch to GaN, but suffers from decomposition at typical growth temperatures and impurity diffusion challenges.
• Bulk GaN substrates: The ideal but costly solution, produced through ammonothermal growth or HVPE techniques. While offering low dislocation densities (~10⁵ cm⁻²), current pricing and wafer size limitations hinder widespread LED adoption.

Beyond III-nitride epitaxy, sapphire shows promise in advanced material synthesis:

• Graphene growth: Serves as a lower-cost alternative to SiC for MBE graphene synthesis, benefiting from hexagonal surface symmetry.
• Carbon nanotube alignment: Atomic steps on miscut c-plane sapphire (0.2nm height) can template highly aligned single-wall nanotube growth through van der Waals interactions.

Flip-chip (FC) LED designs address two critical limitations of conventional nitride LEDs: poor light extraction and sapphire's low thermal conductivity. By placing contacts on the bottom and using the sapphire as the light exit window, FCLEDs achieve:

• Better heat dissipation through direct metal bonding
• Improved light extraction via thicker window layers and reduced refractive index contrast (n_sapphire=1.76 vs. n_air=1.0)
• Metal contacts that double as reflective mirrors

Further enhancements come from combining conductive omnidirectional reflectors (ODRs) with micro-pillar array (MPA) texturing on sapphire surfaces—creating structures that simultaneously improve electrical contact and photon escape probability.

Studies demonstrate how modified sapphire geometries boost LED efficiency:

• Truncated inverted pyramid structures improve light extraction
• Undercut sidewalls enhance output through multiple photon escape opportunities
• Wave-like textured sidewalls increase power output by ~10%
• 22° undercut sidewalls significantly improve light emission

These approaches share a common principle—increasing photon opportunities to find escape cones within critical angles. Recent developments in sapphire shaping (SS) technology, particularly inclined sidewall fabrication, show particular promise for high-brightness applications.