Ch.1 Introduction
• Optoelectronic devices:
- devices deal with interaction of electronic and optical processes
•Solid-state physics:
- study of solids, through methods such as quantum mechanics, crystallography,
electromagnetism and metallurgy
• Elemental semiconductors:
- Si, Ge, ..etc.
- indirect bandgap, low electric-optics conversion efficiency
• Compound semiconductors
- III-V (e.g. GaN, GaAs), II-VI
- direct bandgap, high electric-optics conversion efficiency
• GaAs, InP
- higher mobility than Si, Ge,
- energy band gap, Eg: 1.43 (GaAs), 1.35 (InP)
- most common substrate, used to grow up compound semiconductors
Periodic Table
Band structure
• Band structure:
- results of crystal potential that originates from equilibrium arrangement of atoms
in lattice
- directed from potential model and electron wave equation (Schrodinger equation)
time-dependent Schrodinger equation
E: electron energy, φ:wave equation, m: electron mass, ħ: Plank constant
Electron energy band diagram v.s. wave number
Energy bandgap v.s. lattice constant
Bonding in solids
• Van der Waals bonding:
formation of dipoles between atoms and their electrons
e.g.: inert gas, like Ar
• Ionic bonding:
electron exchange between atoms produces positive and negative ions
which attract each other by Coulomb-type interactions
e.g. NaCl, KCl
• covalent bonding
sharing of electrons between neighboring atoms
e.g.: elemental and compound semiconductors
• Metallic bonding:
valence electrons are shared by many atoms (bonding not directional, electron
free or nearly free contributed to conductivity)
e.g.: Zn
Body-Centered Cubic (BCC) structure
•
http://stokes.byu.edu/bcc.htm
e.g. iron, chromium, tungsten, niobium
Face-Centered Cubic (FCC) structure
e.g.: aluminum, copper, gold, silver
•
http://stokes.byu.edu/fcc.htm
Diamond Cubic (FCC) structure
•
http://zh.wikipedia.org/zh-tw/File:Diamond_Cubic-F_lattice_animation.gif
Zincblende structure
•
Diamond structure,
e.g.: Si, Ge
Zincblende structure
e.g.: aluminum, GaAs
Atomic arrangement in different solids
Dislocation & strain
• Dislocation occurs if
- epitaxial layer thickness > hc (critical thickness), or
- epitaxial layer thickness < hc, but with large mismatch
• Strain occurs if
- epitaxial layer thickness < hc, and with small mismatch
Strain semiconductor
• a) lattice match
b) compressive strain
c) tensile strain
• Strain offer flexibility for restriction of lattice mismatch
Crystal Growth
• Bulk growth:
- furnace growth
- pulling technique
• Epitaxial growth:
- Liquid Phase Epitaxy (LPE)
- Vapor Phase Epitaxy (VPE), or termed Chemical Vapor Deposition (CVD)
- Molecular Beam Epitaxy (MBE)
Epitaxy
• epi means “above”
taxis means “in order manner”
epitaxy can be translated to “to arrange upon”
• with controlled thickness and doping
• subtract acts as a seed crystal, deposited film takes on a lattice structure and
orientation identical to the subtract
• different from thin film deposition that deposit polycrystalline or amorphous film
• - homoepitaxy: epi and subtract are with the same material
epi layer more pure than subtract and have different doping level
- hetroepitaxy:
• used for
- Si-based process for BJT and CMOS, or
- compound semiconductors, such as GaAs
Epitaxy Material Growth Methods
• Liquid Phase Epitaxy
• Vapor Phase Epitaxy (VPE), or termed Chemical Vapor Deposition (CVD)
- formation of condensed phase from gas of different chemical composition
- distinct from physical vapor deposition (PVD) such as sputtering, e-beam
deposition, MBE (condensation occurs without chemical change)
- gas stream through a reactor and interact on a heated subtract to grow
epi layer
• Molecular Beam Epitaxy
Doping of Semiconductors
• Intrinsic materials: undoped
- Undoped materials by epitaxy technology have more carriers than in intrinsic
material. e.g. GaAs: 1013 /cm3 (instrinsic carrier concentration: 1.8x106 /cm3)
- impurity comes from source materials, carrier gases, process equipment, or
subtract handle
• Extrinsic materials:
- n-type: III sub-lattice of III-V compound is substituted by V elements:
impurity terms “donor”
- p-type: V sub-lattice of III-V compound is substituted by III elements:
impurity terms “acceptor”
http://www.siliconfareast.com/sigegaas.htm
Optical fiber
- lowest loss at 1.55 um
- lowest dispersion” 1.3 um
Energy band theory