Heterostructures
Most semiconductor optoelectronic devices, other than silicon photodetectors, are made from heterostructures. Heterostructures - structures made using more than one material These are important because (1) use of different materials allows us to control where the electrons and holes go in the devices (critical for lasers) (2) different materials have different refractive indices, which allow us to make waveguides and mirror structures (3) structures can be made in which only certain parts absorb or emit light at a desired wavelength (other parts being transparent) (4) in advanced optoelectronic devices, the different materials allow us to quantum-confine the electrons and holes in very thin layers, enabling quantum-mechanically engineered devices.
Band offsets and line-ups
To design devices, we need to know how the conduction and valence bands line up at the interface between materials with different bandgaps. Band "line- up” determines, for example, in which material the electrons and holes will be found (i.e., in which material they have the lower energy, hence the extent to which electrons and holes are confined in a particular material layer. Terminology for the heterostructure interface. ΔEc - conduction band offset (or discontinuity) ΔEv - valence band offset (or discontinuity) ΔEc/ΔEv - offset ratio
Band offsets and line-ups
Heterojunction Band Diagramming
Epitaxial growth of heterostructures
Need to grow layers, often very thin, with different materials, alloy compositions and doping Three main techniques 1. Liquid phase epitaxy (LPE) 2. Metal organic vapor phase epitaxy (MO-VPE) 3. Molecular beam epitaxy (MBE) Basic epitaxial concept Start with substrate of bulk semiconductor (e.g., GaAs or InP) polished to a flat surface (a wafer), with a particular crystal orientation (e.g., (100) x direction) Then grow thin layers epitaxially on the substrate (i.e., with a high-quality crystalline structure based on that of the substrate template).
Lattice mismatch & critical layer thickness
We can grow very thin layers even when there is substantial lattice mismatch Strained layers are useful for making surprisingly reliable high-performance lasers, despite the very large strains in the lattice in the thin "quantum well” layer because in spite of the strain, the epitaxial layer is in its lowest energy state.
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Growth regions of III-V compounds
Liquid-solid phase diagram for GaAs
Liquid phase epitaxy
Basic concept of LPE--Thermodynamic equilibrium growth Pass a saturated melt of compound (As in Ga) to be grown over surface of substrate and reduce the temperature, which reduces the solubility of As and results in deposition of GaAs horizontal growth technique substrate is pulled in sequence under several different melts to grow a multiple layer structure
Lattice mismatch, strain & dislocations
The degree of strain and introduction of dislocations depends upon epitaxial layer lattice mismatch and thickness--if the strain energy is less than required to create a dislocation, the layer remains strained, if not, it is relaxed with dislocations.
LPE can successfully and inexpensively grow heterostructures, but precise control over thickness, surface morphology and formation of very abrupt interfaces between materials are difficult.
What makes a good heterojunction system
Growth compatibility
Iso-electronic pairs (non-doping) Lattice match or commensurate match between layers Predictable and known values of ΔEc and ΔEv Known and shallow ionization energy P and N dopants Potential for compositional grading Available substrates
Lattice constants and band-gap energies
To successfully grow one crystalline material on another, lattice constants need to be very closely matched, otherwise the epitaxial layer has a very large number of crystalline defects This tends to significantly degrade device performance and can result in catastrophic failure (e.g., in light-emitting devices, defects shorten the lifetime, making them practically useless).
Lattice matched materials systems
Most optoelectronic devices (with the specific exception of strained "quantum well" lasers) use lattice matched materials AlxGa1-xAs AlAs and GaAs have very similar lattice constants and allow arbitrary sequences and thicknesses of layers and alloys extensively used for devices (e.g., compact disc lasers) In0.53Ga0.47As and InP Critical for devices operating near 1.55µm wavelength "ternary" alloy - three constituent materials one d. of f. (In/ Ga ratio) - control lattice constant to match to InP GaxIn1- xAsyP1-y & In1-x-yAlxGayAs - quaternary (four constituent) alloys with two degrees of freedom (x and y) Control over both bandgap energy and lattice parameter to match, for example, to InP. Both alloys are useful for band gaps in 1.3 - 1.5µm wavelength region. Important for long distance optical fiber systems.