Surface Science

Atomic layer deposition (ALD) and molecular beam epitaxy (MBE) are advanced thin film deposition techniques. ALD uses sequential gas pulses for precise atomic-scale control, while MBE employs molecular beams in ultra-high vacuum for epitaxial growth.

Both methods offer unique advantages in thin film fabrication. ALD excels in conformal coatings and low-temperature deposition, while MBE enables precise control over composition and crystal structure, making them vital in semiconductor and nanotechnology industries.

Atomic layer deposition vs molecular beam epitaxy

Working principles of ALD

  • Thin film deposition technique based on sequential, self-limiting surface reactions between gaseous precursors and a substrate surface
    • Enables precise control over film thickness and composition at the atomic scale
  • Growth proceeds in a cyclic manner, with each cycle consisting of alternating pulses of precursor gases separated by purge steps
    • Self-limiting nature of the surface reactions ensures uniform and conformal coating of complex 3D structures (high aspect ratio trenches, porous materials)
  • Can deposit a wide range of materials at relatively low temperatures (typically <400°C)
    • Oxides (Al2O3, HfO2), nitrides (AlN, TiN), sulfides (ZnS, SnS), and metals (Pt, Ru)

Working principles of MBE

  • Ultra-high vacuum (UHV) technique for epitaxial growth of thin films
    • Allows precise control over the composition, doping, and thickness of the deposited layers
  • Elemental or molecular beams are generated by heating high-purity source materials in effusion cells
    • Beams are directed towards a heated substrate, where they react and form a crystalline film
  • UHV environment (typically <10^-10 Torr) minimizes contamination
    • Enables the use of in-situ characterization techniques, such as reflection high-energy electron diffraction (RHEED), to monitor the growth process in real-time
  • Particularly suitable for the growth of compound semiconductors
    • III-V materials (GaAs, InP), II-VI materials (CdTe, ZnSe)
    • Precise control over interface abruptness and doping profiles

Advantages of ALD and MBE for thin films

Advantages of ALD over other deposition techniques

  • Atomic-level control over film thickness
    • Ability to deposit uniform and conformal films on high-aspect-ratio structures (deep trenches, nanopores) and porous materials (aerogels, zeolites)
  • Excellent compositional control
    • Self-limiting surface reactions ensure a constant growth rate per cycle, independent of precursor flux
  • Low deposition temperatures
    • Enables coating of temperature-sensitive substrates (polymers, biomaterials)
    • Facilitates integration with other processes in device fabrication (back-end-of-line processing)

Advantages of MBE over other epitaxial growth techniques

  • Precise control over the growth rate, typically in the range of 1 μm/h
    • Allows formation of abrupt interfaces and quantum well structures with atomic layer precision
  • Low growth temperatures (typically <800°C)
    • Reduces the risk of interdiffusion and enables the growth of metastable materials (GaInNAs, GaMnAs)
  • In-situ monitoring capabilities, such as RHEED
    • Enables real-time analysis of the growth process and surface morphology
  • Compatibility with a wide range of dopants
    • Ability to achieve high doping concentrations (>10^20 cm^-3), essential for device applications (laser diodes, solar cells)

Surface chemistry in thin film growth

Role of surface chemistry in ALD

  • Choice of precursors and substrates affects the surface reactions, growth rate, and film composition
    • Precursors must exhibit sufficient reactivity, thermal stability, and volatility to ensure efficient and self-limiting surface reactions
  • Surface termination and the presence of functional groups influence the nucleation and growth behavior
    • Affects film morphology and conformality (island growth vs layer-by-layer growth)
  • Kinetics of surface reactions determine the saturation behavior and the required exposure and purge times for each ALD cycle
    • Adsorption, desorption, and surface diffusion processes

Role of surface kinetics in MBE

  • Substrate temperature and flux of the molecular beams affect the adsorption and desorption rates of the growth species
    • Influences the growth rate and the incorporation of dopants
  • Surface diffusion of adatoms plays a crucial role in determining the growth mode and surface morphology of the epitaxial films
    • Layer-by-layer growth (Frank-van der Merwe), step-flow growth, or 3D island growth (Volmer-Weber)
  • Presence of surface reconstructions and the formation of surface phases can affect the incorporation of dopants and the electronic properties of the grown films
    • GaAs (2x4) reconstruction, Si (7x7) reconstruction
  • Strain induced by lattice mismatch between the film and the substrate can influence the surface kinetics and the growth mode
    • Leads to the formation of defects (misfit dislocations) or self-assembled nanostructures (quantum dots)

Applications of ALD and MBE in materials science

Applications of ALD

  • High-k dielectrics for metal-oxide-semiconductor field-effect transistors (MOSFETs) and dynamic random-access memory (DRAM) capacitors
    • HfO2, ZrO2, Al2O3
  • Conformal coatings for complex nanostructures
    • Nanoporous materials, nanoparticles, and nanowires
    • Enables the synthesis of functional core-shell structures and heterogeneous catalysts
  • Protective coatings for various applications
    • Moisture barriers for organic electronics (OLEDs)
    • Corrosion-resistant layers for metal surfaces (Al2O3 on steel)
  • Synthesis of 2D materials by precise control over the layer thickness and composition
    • Transition metal dichalcogenides (MoS2, WS2)

Applications of MBE

  • Growth of high-mobility semiconductor heterostructures for high-frequency and high-power applications
    • Modulation-doped field-effect transistors (MODFETs)
    • High electron mobility transistors (HEMTs)
  • Fabrication of quantum well and superlattice structures for optoelectronic devices
    • Lasers (quantum cascade lasers), photodetectors (quantum well infrared photodetectors), and solar cells (multi-junction cells)
    • Enables the realization of novel device concepts and improved performance
  • Synthesis of low-dimensional structures for quantum technologies and nanoscale sensors
    • Quantum dots (single-photon sources, qubits)
    • Nanowires (field-effect transistors, gas sensors)
  • Growth of topological insulators and other exotic materials with unique electronic and magnetic properties
    • Bi2Se3, Bi2Te3
    • Opens up new avenues for fundamental research and potential spintronic applications
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.