Epitaxy growth

An important phenomenon found in semicoductors,its types and applications,liquid,vapour and molecular beam epitaxy

1. PRESENTED BY: RUCHI SHARMA:14105A0007 RUTUJA SOLKAR:14105A0008 EPITAXY GROWTH

2. Epitaxy refers to the method of depositing a mono-crystalline film on a mono-crystalline substrate. The deposited film is denoted as epitaxial film or epitaxial layer. The term epitaxy comes from the Greek roots, Epi means “above” and taxis means “deposition in ordered manner”. Epitaxial films may be grown from gaseous or l iquid precursors. The substrate acts as a seed crystal , the deposited film takes on a lattice structure a nd orientation identical to those of the substrate.

3. Types of Epitaxial films Epitaxial films can be classified into two broad categories: Homoepitaxy - The film and the substrate are the same material. - Epitaxially grown layers are purer than the substrate and can be doped independently o f it. Pseudo- homoepitaxy : Epi film and substrate are of same material but doping in epi layer can be different from that of substrate (doped Si/ undoped Si) Heteroepitaxy - Film and substrate are of different materials Pseudo heteroepitaxy : chemical commonality between film and substrate

4. Applications of Epitaxial Growth Nanotechnology semiconductor fabrication high quality crystal growth(silicon –germanium, gallium-nitride) to grow layers of pre-doped silicon (in pacemakers, vending machine) To deposit organic molecules onto crystalline substrate.

5. Liquid Phase Epitaxy Liquid phase epitaxy (LPE) is a method to grow semiconductor crystal layers from the melt on solid substrates This happens at temperatures well below the melting point of the deposited semiconductor The semiconductor is dissolved in the melt of another material. At conditions that are close to the equilibrium between dissolution and deposition the deposition of the semiconductor crystal on the substrate is slowly and uniform The equilibrium conditions depend very much on the temperature and on the concentration of the dissolved semiconductor in the melt

6. ADVANTAGES High growth rates. These are typically . 1–10μm / h, i.e . faster than in VPE or MBE Favorable segregation of impurities into the liquid phase Ability to produce very flat surfaces and excellent structural perfection Wide selection of dopants G rowth can be made to occur over a wide range of temperatures Absence of highly toxic precursors or byproducts. LIMITATIONS T o control of layer thickness, alloy compositions,doping,interface smoothness and difficulties in growing certain combinations of interest for hetero structure devices. Poor reproducibility, problems with scaling up in size or throughput

7. Atomic layer deposition  ( ALD ) is a  thin film deposition technique that is based on the sequential use of a gas phase chemical process ALD is considered a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals, typically called  precursors These precursors react with the surface of a material one at a time in a sequential, self-limiting, manner Through the repeated exposure to separate precursors, a thin film is slowly deposited Vapour Phase Epitaxy

8. Advantages VPE provides a very controlled method to produce a film to an atomically specified thickness. Also, the growth of different multilayer structures is straightforward. Due to the sensitivity and precision of the equipment, it is very beneficial to those in the field of microelectronics and nanotechnology in producing small, but efficient semiconductors Disadvantages High purity of the substrates is very important, and as such, high costs will ensue Once the layer has been made and the process is complete, there may be a requirement of needing to remove excess precursors from the final product

9. To make an interesting new crystal using MBE, you start off with a base material called a  substrate , which could be a familiar semiconductor material such as silicon, germanium, or gallium arsenide First , you heat the substrate, typically to some hundreds of degrees (for example, 500–600°C or about 900–1100°F in the case of gallium arsenide) Then you fire relatively precise beams of atoms or molecules (heated up so they're in gas form) at the substrate from "guns" called  effusion cells The molecules land on the surface of the substrate, condense, and build up very slowly and systematically in ultra-thin layers, so the complex, single crystal you're after grows one atomic layer at a time That's why MBE is an example of what's called  thin-film deposition Molecular Beam Epitaxy

10. Basic elements of MBE system : Heated substrate Effusion cells and shutter Reflection High Energy Electron Diffraction (RHEED system- RHEED gun & screen) Ultra High Vacuum ( UHV) Liquid Nitrogen cryopanelling

11. The solid source material sublimates They provide angular distribution of atoms or molecules in beam The substrate is heated to the necessary temperature The gaseous elements then condense on the wafer where they may react with each other to form a layer Atoms on clean surface are free to move until finding correct position in the crystal lattice to bond Process

12. Advantages: It's particularly good for making high-quality (low-defect, highly uniform) semiconductor crystals from compounds or from a number of different elements, instead of from a single element It also allows extremely thin films to be fabricated in a very precise, carefully controlled way Disadvantages: It's a slow and laborious method (crystal growth rate is typically a few microns per hour), which means it's more suited for scientific research laboratories than high-volume production, and the equipment involved is complex and very expensive (partly because of the difficulty of achieving such clean, high-vacuum conditions ) Advantages and Disadvantages of MBE

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