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