Comparison between MBE (Molecular Beam Epitaxy) and MOCVD (Metal-Organic Chemical Vapor Deposition)

Common Features of MBE and MOCVD

Working Environment:

Both MBE and MOCVD operate in cleanroom environments.

Application Range:

In certain material systems, such as arsenides, both techniques can produce similar epitaxial effects.

Differences between MBE and MOCVD

MBE (Molecular Beam Epitaxy) Working Principle:

MBE uses high-purity elemental precursors, which are heated in an evaporator to form molecular beams for deposition. It typically operates under ultra-high vacuum (UHV) conditions to prevent contamination by air molecules.

Equipment Structure:

MBE consists of a sample transfer chamber and a growth chamber. The growth chamber is usually sealed and only opened during maintenance. The substrate is mounted on a heated fixture, surrounded by a liquid nitrogen-cooled cold screen to capture impurities and atoms that are not caught on the substrate surface.

Monitoring Tools:

MBE employs in-situ monitoring tools such as Reflection High-Energy Electron Diffraction (RHEED) to monitor the growth surface, laser reflection, thermography, and chemical analysis (mass spectrometry, Auger spectroscopy) to analyze the evaporated material's composition. Other sensors measure temperature, pressure, and growth rate to adjust process parameters in real-time.

Growth Rate:

Typically, the growth rate is about one-third of a monolayer per second (0.1 nm, 1 Å). It is influenced by the flux rate (the number of atoms arriving at the substrate surface, controlled by the source temperature) and the substrate temperature (which affects the diffusion and desorption characteristics of atoms on the substrate). Growth rates and material supply are controlled by mechanical shutter systems, enabling the reliable and repeatable growth of ternary and quaternary alloys and multilayer structures.

Material Characteristics:

• Silicon: High temperatures (>1000°C) are required for growth on silicon substrates to ensure the desorption of oxides. This requires special heaters and substrate fixtures. The mismatch in lattice constants and thermal expansion coefficients makes the growth of III-V materials on silicon an active research topic.

• Antimony: For III-Sb semiconductors, low substrate temperatures are required to prevent desorption from the surface. High temperatures may cause “inconsistency,” where one atomic species is preferentially evaporated, leaving the material with a non-stoichiometric ratio.

• Phosphorus: For III-P alloys, phosphorus may deposit inside the chamber, requiring a lengthy cleaning process, which could make short production runs infeasible.

• Strained Layers: Typically, lower substrate temperatures are needed to reduce atomic diffusion on the surface, thereby reducing the likelihood of layer relaxation. This can lead to defects, as reduced atomic mobility causes voids in the epitaxial layer, which may be encapsulated and cause failure.

MOCVD (Metal-Organic Chemical Vapor Deposition) Working Principle:

MOCVD is a chemical vapor process that uses ultra-pure gaseous sources for deposition, requiring the handling of toxic gases and their treatment. Metal-organic precursors (such as trimethylgallium for Group III elements and hydrides like arsine and phosphine for Group V elements) are used for epitaxial layer deposition.

Equipment Structure:

MOCVD features a high-temperature, water-cooled reaction chamber where substrates are placed on graphite bases heated by RF, resistive, or infrared heating. Reaction gases are vertically injected into the process chamber above the substrate.

Monitoring Tools:

MOCVD uses thermography with emissivity correction for in-situ temperature measurement of the substrate surface; reflectivity is used to analyze surface roughness and epitaxial growth rate. Laser reflection is used to measure substrate bending, and ultrasonic gas monitoring helps track the concentration of the organic metal precursors to improve growth process accuracy and repeatability.

Growth Conditions:

The growth temperature is primarily determined by the thermal decomposition requirements of the precursors and then optimized for surface migration. The growth rate is governed by the vapor pressure of the III-V metal-organic sources in the gas phase. For aluminum-containing alloys, higher temperatures (>650°C) are typically required for growth, while phosphorus-based layers grow at lower temperatures (<650°C), although AlInP may be an exception.

Material Characteristics:

• High Strain Layers: Due to the ability to conventionally use arsenides and phosphides, strain balancing and compensation are achievable, such as with GaAsP barriers and InGaAs quantum wells (QWs).

• Antimonides: MOCVD growth of antimonide materials is limited because appropriate precursor sources are lacking, leading to the inadvertent (and usually undesirable) incorporation of carbon in AlSb, which limits alloy choices and hinders the use of MOCVD for antimonide growth.

Summary

Monitoring Options:

MBE typically offers more in-situ monitoring options than MOCVD, with the epitaxial growth being adjusted by flux rates and substrate temperatures. These parameters are controlled separately, and correlated in-situ monitoring provides a clearer, more direct understanding of the growth process.

Material Applicability:

MOCVD is a highly versatile technique. By varying the precursor chemistry, a wide range of materials can be deposited, including compound semiconductors, nitrides, and oxides. The cleaning time in MOCVD chambers is faster than in MBE.

Application Advantages:

MBE is the preferred method for Sb material growth, while MOCVD is typically preferred for P materials. For arsenide-based materials, both techniques have similar capabilities. For more advanced structures like quantum dots and quantum cascade lasers, MBE is usually the preferred method for base epitaxy. MOCVD is often preferred for subsequent epitaxial regrowth due to its flexibility in etching and masking.

Special Applications:

MOCVD is well-suited for distributed feedback (DFB) lasers, buried heterostructure devices, and regrowth of coupled waveguides, which may include in-situ etching of semiconductors. MOCVD is also used for single-chip InP integration. While GaAs single-chip integration is still in its early stages, MOCVD can achieve selective-area growth, aiding in the separation of emission/absorption wavelengths. MBE, on the other hand, has challenges in this area, as polycrystalline deposition tends to form on dielectric masks.

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