MBE vs MOCVD vs HVPE: Epitaxy Methods, Growth Rate and Uses
MBE, MOCVD and HVPE all grow compound semiconductor crystals that follow the substrate crystal (epitaxial growth), but they deliver the sources differently and grow at very different speeds. MBE aims beams of evaporated source material at the substrate in ultra-high vacuum. It is slow, typically about 1 µm per hour, but controls thickness down to single atomic layers. MOCVD sends metal-organic and hydride gases to a heated substrate in a flowing gas at reduced to atmospheric pressure and grows at typically 1 to a few µm per hour. It can process many wafers at once and is the workhorse for LED and laser production. HVPE uses gallium chloride, made from gallium and hydrogen chloride, together with ammonia to grow thick GaN at tens to hundreds of µm per hour, and is used to make free-standing substrates.
| Aspect | MBE | MOCVD | HVPE |
|---|---|---|---|
| Growth ambient and pressure | Ultra-high vacuum (base pressure on the order of 10⁻⁸ Pa); source atoms and molecules fly to the substrate with almost no gas collisions | A flowing hydrogen or nitrogen carrier gas, typically from reduced pressure (a few kPa) up to atmospheric | Typically near atmospheric pressure; often a hot-wall reactor whose walls are also heated |
| Sources | Solid elements (Ga, Al, In, As, etc.) evaporated in heated effusion cells; nitrides use plasma-activated nitrogen | Group III metal-organics such as trimethylgallium; group V hydrides such as ammonia, arsine and phosphine | Gallium chloride (GaCl), formed by passing HCl over hot gallium metal, plus ammonia |
| Typical growth rate | Typically about 1 µm per hour (roughly one atomic layer per second for GaAs) | Typically 1 to a few µm per hour | Typically tens to hundreds of µm per hour, tens of times faster than MOCVD |
| Thickness and interface control | The most precise: shutters switch sources instantly, giving abrupt interfaces at the atomic-layer level | Switching gases changes composition on a scale of a few nanometers, enough for quantum wells and other thin multilayers | Poor: suited to growing thick films fast rather than thin layers or abrupt interfaces |
| In-situ monitoring | RHEED (reflection high-energy electron diffraction) shows the surface state and counts grown layers | Pressure is too high for RHEED; optical reflectance, wafer temperature and wafer bow are measured | Limited |
| Throughput | Low: few wafers per run; mainly research and production of specific devices | High: some reactors process dozens of wafers per run | Grows thick films fast, but not many wafers at once |
| Main materials | III-V compounds such as GaAs, AlGaAs and InGaAs, plus nitrides, oxides and others | III-V compounds such as GaN, InGaN, AlGaN, GaAs and InP | GaN (thick films and substrates), Ga2O3 and others |
| Handling and issues | Ultra-high vacuum must be maintained and the system serviced; solid sources such as arsenic are toxic | Pyrophoric metal-organics and highly toxic hydrides; carbon from the metal-organics can be incorporated, and hydrogen passivates Mg in Mg-doped GaN | Corrosive hydrogen chloride; thick growth makes bowing and cracking from thermal-expansion mismatch with the substrate a concern |
| Typical uses | RF devices such as HEMTs, research on quantum wells and superlattices, quantum devices | LEDs, laser diodes, GaN HEMTs and power devices, multi-junction solar cells | Free-standing GaN substrates, thick buffer layers, Ga2O3 epitaxial layers |
Figures are typical orders of magnitude and vary by process generation and product.