6.4 Epitaxial growth techniques

Epitaxial growth techniques are crucial for creating high-quality crystalline layers in nanoelectronics. These methods, including liquid phase epitaxy, vapor phase epitaxy, and molecular beam epitaxy, allow precise control over material composition and structure at the atomic level.

Understanding epitaxial growth is essential for fabricating advanced semiconductor devices. This section explores different techniques, their advantages, and applications, highlighting how they enable the creation of complex nanostructures and improve device performance.

Epitaxial Growth Types

Homoepitaxy and Heteroepitaxy

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• Homoepitaxy involves growing a crystalline layer on a substrate of the same material (GaAs on GaAs)
• Produces high-quality films with minimal defects due to perfect lattice matching
• Heteroepitaxy grows a crystalline layer on a substrate of a different material (GaN on sapphire)
• Challenges in heteroepitaxy include lattice mismatch and thermal expansion coefficient differences
• Lattice mismatch can lead to strain and defects in the grown layer
• Thermal expansion mismatch may cause film cracking or delamination during cooling

Strain Engineering and Lattice Matching

• Strain engineering manipulates lattice mismatch to modify material properties
• Intentionally introduces strain to alter band structure and carrier mobility
• Compressive strain decreases in-plane lattice constant and increases out-of-plane constant
• Tensile strain increases in-plane lattice constant and decreases out-of-plane constant
• Lattice matching minimizes strain between substrate and epitaxial layer
• Achieved by selecting materials with similar lattice constants or using buffer layers
• Buffer layers gradually transition from substrate to epitaxial layer lattice constant
• Graded buffer layers reduce dislocation density in the active layer

Liquid and Vapor Epitaxy

Liquid Phase Epitaxy (LPE)

• Growth technique where the epitaxial layer crystallizes from a supersaturated solution
• Substrate immersed in a melt containing desired growth elements
• Cooling the melt causes supersaturation and crystal growth on the substrate
• Advantages include simplicity, low cost, and high growth rates (up to 1 μm/min)
• Limitations include difficulty in controlling layer thickness and composition
• Primarily used for III-V compound semiconductors (GaAs, InP)
• Applications in LED and solar cell fabrication

Vapor Phase Epitaxy (VPE) and MOCVD

• VPE grows epitaxial layers from vapor phase precursors
• Precursors react or decompose on the heated substrate surface
• Hydride VPE uses hydride gases (AsH3, PH3) as group V precursors
• Chloride VPE employs metal chlorides (GaCl, AlCl) as group III precursors
• Metal-Organic Chemical Vapor Deposition (MOCVD) uses metalorganic precursors
• MOCVD precursors include trimethylgallium (TMGa) and triethylgallium (TEGa)
• Advantages of MOCVD include precise control of composition and doping
• MOCVD enables growth of complex heterostructures and quantum wells
• Used in production of high-performance optoelectronic devices (lasers, LEDs)

Molecular Beam Epitaxy

MBE Process and Equipment

• Molecular Beam Epitaxy (MBE) grows epitaxial layers in ultra-high vacuum conditions
• Utilizes atomic or molecular beams of elements directed at a heated substrate
• Ultra-high vacuum (< 10^-10 Torr) ensures minimal impurity incorporation
• Source materials heated in effusion cells (Knudsen cells) to produce molecular beams
• Substrate rotation ensures uniform deposition across the wafer
• In-situ monitoring techniques include Reflection High-Energy Electron Diffraction (RHEED)
• RHEED provides real-time information on surface structure and growth rate
• Growth rates typically low (< 1 μm/hour) allowing precise control of layer thickness

Doping and Advanced Structures

• MBE enables precise control of doping profiles and concentrations
• Dopant atoms introduced from separate effusion cells
• Allows for abrupt doping transitions and delta-doping
• Delta-doping creates ultra-thin, highly doped layers (few atomic layers thick)
• MBE facilitates growth of complex heterostructures and superlattices
• Quantum wells and quantum dots can be fabricated with atomic-layer precision
• Enables band-gap engineering for advanced electronic and optoelectronic devices
• Applications include high-electron-mobility transistors (HEMTs) and quantum cascade lasers