MOCVD

Metalorganic chemical vapour deposition (MOCVD) is a method for depositing thin crystalline films onto a substrate by introducing metalorganic precursor chemicals into a heated reaction chamber. The precursors decompose or react in the gas phase, and their products condense onto the hot substrate surface in a controlled, layer-by-layer manner. It is the dominant technique for manufacturing compound semiconductors like gallium nitride (GaN), gallium arsenide (GaAs), and indium phosphide (InP), which power LEDs, laser diodes, and high-frequency transistors.

The process works by flowing carrier gases (often hydrogen or nitrogen) that transport vaporized metalorganic precursors into a heated reaction chamber, typically maintained at 500 to 1100 degrees Celsius depending on the materials being grown. Common precursors include trimethylgallium (TMG) for gallium, trimethylindium (TMI) for indium, and arsine (AsH3) or phosphine (PH3) for group V elements. The precursor molecules break down near the substrate surface, and atoms recombine to form the desired crystalline compound. Dopants for electrical doping (silicon for n-type, magnesium for p-type) are introduced as separate gaseous precursors in controlled concentrations.

Reactor Types and Process Variants

MOCVD systems vary widely in scale and design. Atmospheric pressure reactors operate at around 1 atm and are simpler and lower cost; low-pressure reactors (10 to 100 torr) offer better uniformity and thickness control across larger wafers. Horizontal tube reactors position the wafer horizontally with gas flowing over it; vertical reactors (such as barrel or close-coupled showerhead designs) direct gas downward onto the wafer and are preferred for production because they handle larger wafer sizes and multiple wafers per run. Production systems can load six to ten 2-inch (50 mm) wafers or full 6-inch (150 mm) or 8-inch (200 mm) diameter wafers.

The quality of films depends critically on temperature uniformity, precursor flow rates, gas-phase chemistry, and precursor purity. Temperature errors of just a few degrees Celsius cause significant variations in growth rate and dopant incorporation across the wafer. Precursor flow is regulated by bubbling carrier gas through heated liquid sources or using pressure-controlled evaporators for solid precursors. Unwanted reactions can occur if precursor mixtures react prematurely in the gas phase rather than only on the substrate, a phenomenon called gas-phase nucleation that produces powder and degrades film quality. Metalorganic precursors are expensive, hazardous (many are pyrophoric or toxic), and their cost and safety handling burden are the main drawbacks of the process compared to simpler epitaxy methods.

MOCVD dominates the III-V semiconductor industry because it allows precise control of alloy composition and dopant levels in single-crystal layers only tens of nanometres thick, and because it can coat complex three-dimensional structures (such as the sidewalls of etched features). The term metalorganic refers to the organic ligands (methyl groups, typically) bonded to the metal atoms in the precursor molecules; the organic part provides volatility so the precursor can be delivered as a vapour. MOCVD competes with liquid-phase epitaxy (LPE) for older compound semiconductor work and with molecular beam epitaxy (MBE) for research and niche applications requiring ultra-high purity or ultra-thin control, but MOCVD remains the workhorse of LED and optoelectronic device manufacturing.