What is the manufacturing process of gallium nitride epitaxial wafers?

The manufacturing process of gallium nitride (GaN) epitaxial wafers is a core link in the industry chain, with extremely high technical barriers. It directly determines the quality of the epitaxial wafers and the performance of the final devices.

Currently, the mainstream commercial production method isMetal-Organic Chemical Vapor Deposition (MOCVD)。

The following is a detailed analysis of the GaN epitaxial wafer manufacturing process based on the MOCVD method:

Core equipment: MOCVD reactor

MOCVD equipment is a large, precise vacuum deposition system, the core component of which is the reaction chamber. Its working principle is:

• Metal Organic Source (MO Source)： As a source of Group III elements (such as Ga), for example, trimethylgallium (TMGa).

• Nitrogen Source: As a source of Group V elements, ammonia (NH₃) is usually used.

• Carrier Gas: High-purity hydrogen (H₂) or nitrogen (N₂) is used to carry the metal-organic source vapor to the reaction chamber.

These reaction precursors are precisely controlled in flow and injected into a high-temperature heated reaction chamber, where they undergo chemical reactions on the heated substrate surface and deposit to form gallium nitride crystal films.

Detailed explanation of the manufacturing process

Step 1: Substrate Preparation

• Substrate selection： Choose different substrates according to the application, such asSapphire, silicon (Si), silicon carbide (SiC)etc. Silicon substrates have become the mainstream choice for power electronics due to their cost and large size advantages.

• Cleaning： Before being placed in the reaction chamber, the substrate must undergo a rigorous cleaning process to remove contaminants such as particles, organic matter, and metal ions to ensure that the starting point of epitaxial growth is absolutely clean.

Step 2: Substrate loading and preheating

• Place the cleaned substrate on the graphite susceptor of the MOCVD equipment.

• The reaction chamber is closed and evacuated to remove oxygen and moisture.

• The carrier gas is introduced and the substrate is heated to a certain preheating temperature in preparation for epitaxial growth.

Step 3: Nucleation Layer Growth

• This is one of the most critical and technically challenging steps, especially when growing on foreign substrates such as silicon.

• challenge： The lattice constant and thermal expansion coefficient of GaN and silicon are very different. Direct growth will lead to high-density defects and even cracks.

• Solution： First, a thin layer ofAluminum nitride (AlN) or aluminum-rich aluminum gallium nitride (AlGaN) nucleation layerThis layer of material acts as a buffer transition, effectively preventing the reaction between the silicon substrate and GaN, and providing nucleation points for the subsequent growth of GaN.

Step 4: Stress Management & Buffer Layer Growth

• Purpose: Release the huge stress caused by thermal mismatch and lattice mismatch, prevent the epitaxial wafer from warping or cracking, and reduce the threading dislocation density.

• Craftsmanship： On top of the nucleation layer, a layer of several microns thick is grownHigh-resistance GaN or AlGaN buffer layerThis is usually a complexsuperlattice structure(Such as alternating growth of AlN/GaN or AlGaN layers with different Al compositions), the dislocation lines are bent and terminated through a multilayer structure, thereby significantly reducing the defect density extending to the active region.

• For silicon substrates, this step is the core technology for achieving high-performance, crack-free epitaxial wafers.

Step 5: High-quality GaN channel layer growth

• On top of the buffer layer, high-qualityIntrinsic non-doped GaN (i-GaN) layer。

• This layer is the area with the highest crystal quality and will become the channel for electron transport in the device.

Step 6: Barrier Layer Growth

• A thin layer of GaN is grown on the channel layer.AlGaN layer(Usually the Al component is 20%-30%).

• Energy Band Engineering： Due to the different band gaps of GaN and AlGaN and the presence of piezoelectric polarization effect, a strong piezoelectric polarization field is formed at the interface between the two, thereby inducing aHigh-concentration, high-mobility two-dimensional electron gas (2DEG)。

• This 2DEG region is the channel for electron flow in HEMT devices, which can achieve extremely high electron concentration and mobility without doping. This is the physical basis for the high performance of GaN devices.

Step 7: Cap Layer Growth (Optional)

• In order to further optimize the device performance, a very thin layer is sometimes grown on the top layer.n-type doped GaN (n-GaN)Cap layer.

• effect： Protect AlGaN surface, reduce ohmic contact resistance, improve device linearity, etc.

Step 8: Cooling & Unloading

• After growth is complete, the material is cooled at a strictly controlled rate to avoid material damage or performance degradation due to thermal stress.

• After cooling to a safe temperature, inert gas is filled into the reaction chamber and the grown epitaxial wafer is taken out.

Step 9: In-situ Monitoring & QC

• Throughout the growth process, advanced MOCVD equipment will be integratedIn-situ monitoring system(such as laser interferometer, optical pyrometer, etc.) to monitor the growth rate, thickness, temperature and curvature changes in real time to ensure the accuracy and repeatability of the process.

Step 10: Ex-situ Testing

• After the epitaxial wafer is removed, a comprehensive inspection is required, including:

  • X-ray diffraction (XRD): Detect crystal quality, stress, composition and thickness.

  • Atomic force microscopy (AFM): Detect surface roughness.

  • Hall Measurement: Measure the carrier concentration, mobility and sheet resistance of 2DEG.

  • CV test： Analyze the carrier concentration distribution.

  • Photoluminescence (PL): Evaluate optical quality (especially for optoelectronic devices).

  • Raman spectroscopy: Measure stress.

Technical Challenges and Key Points

1. Crack control： Especially on silicon substrates, how to release stress through complex buffer layer structure design is the core technology.

2. Defect Control： How to minimize dislocation density is the key to improving device breakdown voltage and reliability.

3. Uniformity and consistency： Ensuring high consistency in performance between the same wafer and wafers from different batches is the lifeline of mass production.

4. Doping control： Achieving p-type doping (usually using magnesium Mg) and precise n-type doping (usually using silicon Si) is the basis for manufacturing PN junctions and enhancement-mode devices.

5. Carbon impurity controlDuring MOCVD growth, reaction byproducts introduce carbon impurities, which compensate for doping and cause leakage in the buffer layer. Precisely controlling growth conditions (such as V/III ratio, pressure, and temperature) to manage carbon concentration is another major challenge in manufacturing high-performance power devices.

In short, the manufacture of GaN epitaxial wafers is an extremely delicate "cooking" process that requires a deep understanding of materials science, fluid mechanics, thermodynamics, and chemistry, and countless process experiments to optimize each parameter. Only then can high-quality GaN crystal films that can be used to manufacture high-performance devices be "grown" on non-natural substrates.