Molecular Beam Epitaxy
09/26/2008
MSE 576: Thin Films
Deepak Rajput
Graduate Research Assistant
Center for Laser Applications
Materials Science & Engineering
University of Tennessee Space Institute
Tullahoma, Tennessee 37388-9700
Email: drajput@utsi.edu
Web: http://drajput.com

Outline
* Epitaxy

* Molecular Beam Epitaxy

* Molecular Beam

* Problems and Diagnostics

Epitaxy

* Method of depositing a monocrystalline film.

* Greek root: epi means ?above? and taxis means ?ordered?.

* Grown from: gaseous or liquid precursors.

* Substrate acts as a seed crystal: film follows that !

* Two kinds: Homoepitaxy (same composition) and Heteroepitaxy (different composition).
Epitaxy
* Homoepitaxy:

# To grow more purified films than the substrate.
# To fabricate layers with different doping levels

* Heteroepitaxy:

# To grow films of materials of which single crystals cannot be grown.
# To fabricate integrated crystalline layers of different materials
Epitaxy
* Vapor Phase Epitaxy (VPE)
SiCl4(g) + 2H2(g) ? Si(s) + 4HCl(g) (at 12000C)
# VPE growth rate: proportion of the two source gases

* Liquid Phase Epitaxy (LPE)
Czochralski method (Si, Ge, GaAs)
# Growing crystals from melt on solid substrates
# Compound semiconductors (ternary and quaternary III-V compounds on GaAs substrates)

* Molecular Beam Epitaxy (MBE)
# Evaporated beam of particles
# Very high vacuum (10-8 Pa); condense on the substrate
Molecular Beam Epitaxy
Molecular Beam Epitaxy: Idea !
* Objective: To deposit single crystal thin films !

* Inventors: J.R. Arthur and Alfred Y. Chuo (Bell Labs, 1960)

* Very/Ultra high vacuum (10-8 Pa)

* Important aspect: slow deposition rate (1 micron/hour)

* Slow deposition rates require proportionally better vacuum.
Molecular Beam Epitaxy: Process
* Ultra-pure elements are heated in separate quasi-knudson effusion cells (e.g., Ga and As) until they begin to slowly sublimate.

* Gaseous elements then condense on the wafer, where they may react with each other (e.g., GaAs).

* The term ?beam? means the evaporated atoms do not interact with each other or with other vacuum chamber gases until they reach the wafer.

Molecular Beam
* A collection of gas molecules moving in the same direction.

* Simplest way to generate: Effusion cell or Knudsen cell

Molecular beam
* Oven contains the material to make the beam.

* Oven is connected to a vacuum system through a hole.

* The substrate is located with a line-of-sight to the oven aperture.

* From kinetic theory, the flow through the aperture is simply the molecular impingement rate on the area of the orifice.
Molecular Beam
* Impingement rate is:

* The total flux through the hole will thus be:

* The spatial distribution of molecules from the orifice of a knudsen cell is normally a cosine distribution:

Molecular Beam
* The intensity drops off as the square of the distance from the orifice.

* High velocity, greater probability; the appropriate distribution:

Molecular Beam
* Integrating the equation gives:

as the mean translational energy of the molecules

MBE: In-situ process diagnostics
* RHEED (Reflection High Energy Electron Diffraction) is used to monitor the growth of the crystal layers.

* Computer controlled shutters of each furnace allows precise control of the thickness of each layer, down to a single layer of atoms.

* Intricate structures of layers of different materials can be fabricated this way e.g., semiconductor lasers, LEDs.

* Systems requiring substrates to be cooled: Cryopumps and Cryopanels are used using liquid nitrogen.
ATG Instability
* Ataro-Tiller-Grinfeld (ATG) Instability: Often encountered during MBE.

* If there is a lattice mismatch between the substrate and the growing film, elastic energy is accumulated in the growing film.

* At some critical film thickness, the film may break/crack to lower the free energy of the film.

* The critical film thickness depends on the Young?s moduli, mismatch size, and surface tensions.

Assignment
* Solve the equation to find the mean translational energy (Etr) of the molecules:

* What fraction of the molecules in a molecular beam of N2 formed by effusion of N2 gas initially at 300 K from an orifice at a large Knudsen number will have kinetic energies greater than 8kcal/mol?