Aaron Vallett EE 518 April 5 th , 2007 Principles and Applications of Molecular Beam Epitaxy Instructor: Dr. J. Ruzyllo
Dec 24, 2015
Aaron VallettEE 518April 5th, 2007
Principles and Applications of Molecular Beam Epitaxy
Instructor: Dr. J. Ruzyllo
Outline Introduction
Review of epitaxial growth
MBE Process Chamber construction Beam sources Characterization
MBE Applications Devices R&D/Commercial
Summary
Introduction
Invented in late 1960s at Bell Labs by J. R. Arthur and A. Y. Cho
An epitaxial growth process involving one or more molecular beams of atoms or molecules physically arranging themselves on a crystalline surface under ultrahigh-vacuum conditions
Growth is tightly controlled – layer compositions and thickness can be adjusted at an atomic scale
Epitaxy Review
Growth of thin, high quality, single-crystal layers on a similar-type crystal substrate
Molecules are adsorbed on the surface Diffuse across the surface until finding a suitable crystal
site
Image from http://www.phys.ubbcluj.ro/~rote/Zahn/Introduction.pdf
MBE Process Overview
Beam impinges on heated substrate (600°C) Incident molecules diffuse around the surface to the proper
crystal sites and form crystalline layers Characterization tools allow growth to be monitored in-situ
Image modified from http://projects.ece.utexas.edu/ece/mrc/groups/street_mbe/mbechapter.html
Very similar to thermal evaporation with one big difference - UHV (10-8 - 10-11 torr)
Solid source materials are heated to melting point in effusion cells
UHV gives source molecules a large mean free path, forming a straight beam
MBE Chamber
Stainless steel chamber and seals reduce leaks After servicing, chamber must be baked and outgassed
at ~200°C for 2-5 days UHV achieved through use of cryo, Ti-sublimation and
ion pumps – no oil Cryo-shroud promotes condensation of contaminants
and stray particles
Image from http://ocw.mit.edu/NR/rdonlyres/Electrical-Engineering-and-Computer-Science/6-772Spring2003/B5D923F5-9B4C-4436-A1F1-0343B35E1928/0/lect8_part1.pdf
Sample Preparation and Loading
Starting substrate must be ultra clean and flat
Wafer usually comes “epi-ready” with a protective oxide
Substrate loaded in load-lock and heated for outgassing for several hours
Substrate may then move to a buffer chamber and be outgassed again
Growth substrate then loaded onto holder in growth chamber
Protective oxide desorbed by heating substrate on the chuck in UHV
Goal is to keep the chamber and sample as pure as possible
Image from http://www.uwo.ca/isw/images/Mbeiiism.gif
Effusion Sources and the Molecular Beam
Effusion: the process where individual molecules flow through a hole without collisions
Source material is heated to vapor phase Ultra-low pressure in UHV leads to molecules with mean free paths
of hundreds of meters Opening in effusion cell is small – molecules travel straight out of it
with no collisions, forming a beam
Images from http://www.mbe-kompo.de/products/effusion/effusioncell_ome.html
and http://zumbuhllab.unibas.ch/060929GufeiMBE.pdf
Effusion Cell Construction A typical MBE system may feature 8 effusion cells Crucible is constructed of pyrolytic boron nitride (PBN) to withstand
temperatures up to 1400°C Thermocouple must accurately measure crucible temperature
Change in T of .5°C changes flux by 1% During the day flux variations of <1%, day-to-day <5% T must be controlled within a half-degree at 1000°C
Images from http://www.riber.com/en/public/solidcells.htm
and http://www.hlphys.jku.at/fkphys/epitaxy/mbe.html
Sources seated in a cooling shroud to maintain flux and eliminate thermal crosstalk between cells
Mechanical shutters in front of sources control the beam
In-situ Characterization Deposition in UHV allows unique in-situ measurements to be taken RHEED – reflection-high-energy-electron-diffraction
Electrons from a gun strike the growing surface at a shallow angle The crystal reflects electrons into a diffraction pattern Diffraction pattern and intensity can provide information on the state of the
surface Mass spectrometry
Used to measure surface and chamber composition
Ionization gage Used to measure chamber pressure or molecular beam flux
Images from http://www.elettra.trieste.it/experiments/beamlines/lilit/htdocs/people/luca/tesihtml/node25.html
and http://www.phys.ubbcluj.ro/~rote/Zahn/Introduction.pdf
MBE Abilities
Deposition rate is ~ 1 μm/hr or 1 monolayer/sec
Computer controlled shutter can be opened or closed in 100 mS
Defect free, super abrupt, single-atom layers can be grown – only MBE allows this precision
Multiple beams can impinge the surface at once to create III-V materials or dope a layer during growth
Images from http://www.phys.ubbcluj.ro/~rote/Zahn/Introduction.pdf and http://research.yale.edu/boulder/Boulder-2005/Lectures/Willett/boulder1.pdf
15 monolayers
AlGaAs/GaAs alternating layers
Device Applications
Traditionally used for very specific, commonly compound-semiconductor, applications
HBTs, MESFETs and HEMTs Quantum wells Semiconductor lasers Silicon-on-sapphire growth
Images from http://www.micro.uiuc.edu/mbe/laserd.htm
and Thompson et. al. IEEE Trans. On Semicon. Manufacturing, Vol. 18, No.1, February 2005
Also being considered for use in commercial production of SiGe MOSFETs
MBE in Industry By nature MBE has a very low throughput If it is needed for future CMOS processing, manufacturers will
install clustered MBE chambers to increase throughput
Images from http://users.ece.gatech.edu/~alan/ECE6450/Lectures/ECE6450L13and14-CVD%20and%20Epitaxy.pdf
Summary
MBE creates near-perfect crystalline layers
MBE is a purely physical process, so blocking the beam can stop layer growth
Slow growth time allows atomically thin and super abrupt layers to be grown
Mixing of beams permits growth of compound semiconductor and doped layers
MBE is a costly and time consuming technique, but its high level of precision may drive it into the commercial CMOS world
ReferencesParker, E. “Technology and Physics of Molecular Beam Epitaxy” 1985
Chang, L. and K. Ploog “Molecular Beam Epitaxy and Heterostructures” 1985
Liu, W. “Fundamentals of III-V Devices” 1999
http://www.phys.ubbcluj.ro/~rote/Zahn/Introduction.pdf
http://projects.ece.utexas.edu/ece/mrc/groups/street_mbe/mbechapter.html
http://www.uwo.ca/isw/images/Mbeiiism.gif
http://www.mbe-kompo.de/products/effusion/effusioncell_ome.html
http://zumbuhllab.unibas.ch/060929GufeiMBE.pdf
http://www.riber.com/en/public/solidcells.htm
http://www.hlphys.jku.at/fkphys/epitaxy/mbe.html
http://www.elettra.trieste.it/experiments/beamlines/lilit/htdocs/people/luca/tesihtml/node25.html
http://www.phys.ubbcluj.ro/~rote/Zahn/Introduction.pdf
http://research.yale.edu/boulder/Boulder-2005/Lectures/Willett/boulder1.pdf
http://www.micro.uiuc.edu/mbe/laserd.htm
R M Sidek et al 2000 Semicond. Sci. Technol. 15 135-138
Thompson et. al. IEEE Trans. On Semicon. Manufacturing, Vol. 18, No.1, February 2005
http://users.ece.gatech.edu/~alan/ECE6450/Lectures/ECE6450L13and14-CVD%20and%20Epitaxy.pdf