Epitaxial Deposition Abu Syed Md. Jannatul Islam Lecturer, Dept. of EEE, KUET, BD 1 Department of Electrical and Electronic Engineering Khulna University of Engineering & Technology Khulna-9203

2 Epitaxy  The term Epitaxy comes from the Greek word meaning ‘ordered upon’.  Epitaxy means the growth of a single crystal film on top of a crystalline substrate.  For most thin film applications (hard and soft coatings, optical coatings, protective coatings) it is of little importance.  However, for semiconductor thin film technology it is crucial.

3 Homoepitaxy:  The film and the substrate are the same material.  Often used in Si on Si growth (A on A)  Epitaxially grown layers are purer than the substrate and can be doped independently of it. Types of Epitaxy Heteroepitaxy:  Film and substrate are different materials.(Growth of AlAs on Si or GaAs on Si).  Trying to grow a layer of a different material on top of a substrate leads to unmatched lattice parameters.  This will cause strained or relaxed growth and can lead to interfacial defects.  Such deviations from normal would lead to changes in the electronic, optic, thermal and mechanical properties of the films.  Allows for optoelectronic structures and band gap engineered devices.

4 Ordered, crystalline growth Epitaxial growth Epitaxial Growth NOT epitaxial  High Quality Film (1μm or less thickness) deposited on a high quality substrate.  To ensure high crystalline quality, the lattice parameters of the thin layer should match with that of the substrate (to minimize strain).

5  While Si is not the ideal material from an electronic and optical point of view, its abundance, ease of processing and availability of a good native oxide have made it the backbone of semiconductor industry.  Combining Si substrates with compound semiconductor films would enable higher optoelectronic functionality and higher speeds. However, there are severe lattice mismatch and chemical compatibility issues between Si and most III-V alloys that preclude direct growth.  Metal-Semiconductor Hetero-epitaxy: Metal-semiconductor structures are used for contact applications. While not essential, epitaxial growth allows increased electron mobility through a junction.  Epitaxial growth is useful for applications that place stringent demands on a deposited layer:  High purity, Low defect density, Abrupt interfaces, Controlled doping profiles  High repeatability and uniformity, Safe, efficient operation  Can create clean, fresh surface for device fabrication Why Epitaxial Growth

6  Engineered wafers  Clean, flat layer on top of less ideal Si substrate  On top of SOI structures  Ex.: Silicon on sapphire  Higher purity layer on lower quality substrate (SiC)  In CMOS structures  Layers of different doping  Ex. p - layer on top of p + substrate to avoid latch-up Why Epitaxial Growth  To make layer which is not available in nature  Very important in III-V semiconductor production  Bipolar Transistor (Needed to produce buried layer)  III-V Devices (Interface quality key, Hetero-junction Bipolar Transistor, LED, Laser).

7 Steps:  Absorption of ad atoms  Surface diffusion  Crystal growth  Evaporation of adatoms Parameters:  Growth temperature  Growth pressure  Flow amount of reactants  Substrate and treatment Epitaxial Growth Steps & Parameters

8 Scheme of Epitaxial Deposition

9 Epitaxial Deposition Techniques Epitaxial growth can be performed at temperatures considerably below the melting point of the substrate crystal. A variety of methods are used to provide the appropriate atoms to the surface of the growing layer.  Vapor Phase Epitaxy/Chemical vapor deposition (grown from Vapor)  Liquid phase epitaxy (grown from a Melt)  Molecular beam epitaxy (an evaporation of the elements in a Vacuum) With this wide range of epitaxial growth techniques, it is possible to grow a variety of crystals for device applications, having properties specifically designed for the electronic and optoelectronic device being made.

10 Liquid Phase Epitaxy  Reactants are dissolved in a molten solvent at high temperature  Substrate dipped into solution while the temperature is held constant  Example: SiGe on Si  Bismuth used as solvent  Temperature held at 800°C  High quality layer Molecular Beam Epitaxy  Very promising technique  Beams created by evaporating solid source in UHV  Not ideal for large area layers or abrupt interfaces  Thermodynamic driving force relatively very low Epitaxial Deposition Techniques

11  MOMBE---means when Metel Organic Source used for MBE  Sputtering---the layer quality is very poor. Thus it is used for making contact with the help of metal related source.  HVPE---Hydride Vapor Phase Epitaxy  Pulse laser Deposition (PLD)  Reactive Evaporation  Electron Beam Plasma Technique  Solvo thermal Method Epitaxial Deposition Techniques *Advantages, Disadvantages, and Applications of all these techniques are very much important. Please collect all the information……………….

12 TechniquesStrengthsWeaknesses LPE (liguid phase epitaxy)Simple, High purityScale economies Inflexible, Non-uniformity HVPE( hydride vapor phase epitaxy) Well developed Large scaleNo Al alloys Complex process/reactor control difficult, Hazardous sources MBESimple process, Uniform, Abrupt interface In-situ monitoring As/P alloy difficult, Expensive, Low throughput MOCVD/OMVPE/OMCVD MOVPE Most flexible, Large scale production Abrupt interface Simple reactor, High purity, selective in situ monitoring Expensive sources Most parameters to control Accurately Hazardous precursors Overview of Epitaxy Techniques