Slide 1OutlineIntroductionEpitaxy ReviewMBE Process OverviewMBE ChamberSample Preparation and LoadingEffusion Sources and the Molecular BeamEffusion Cell ConstructionIn-situ CharacterizationMBE AbilitiesDevice ApplicationsMBE in IndustrySummaryReferencesAaron VallettEE 518April 5th, 2007Principles and Applications of Molecular Beam EpitaxyInstructor: Dr. J. RuzylloOutlineIntroductionReview of epitaxial growthMBE ProcessChamber constructionBeam sourcesCharacterizationMBE ApplicationsDevicesR&D/CommercialSummaryIntroduction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 scaleEpitaxy 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 siteImage from http://www.phys.ubbcluj.ro/~rote/Zahn/Introduction.pdfMBE 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-situImage 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 beamMBE 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 particlesImage from http://ocw.mit.edu/NR/rdonlyres/Electrical-Engineering-and-Computer-Science/6-772Spring2003/B5D923F5-9B4C-4436-A1F1-0343B35E1928/0/lect8_part1.pdfSample 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 possibleImage from http://www.uwo.ca/isw/images/Mbeiiism.gifEffusion 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 beamImages from http://www.mbe-kompo.de/products/effusion/effusioncell_ome.htmland http://zumbuhllab.unibas.ch/060929GufeiMBE.pdfEffusion 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°CImages 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 beamIn-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 fluxImages from http://www.elettra.trieste.it/experiments/beamlines/lilit/htdocs/people/luca/tesihtml/node25.htmland http://www.phys.ubbcluj.ro/~rote/Zahn/Introduction.pdfMBE 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 growthImages from http://www.phys.ubbcluj.ro/~rote/Zahn/Introduction.pdf and http://research.yale.edu/boulder/Boulder-2005/Lectures/Willett/boulder1.pdf15 monolayersAlGaAs/GaAs alternating layersDevice ApplicationsTraditionally used for very specific, commonly compound-semiconductor, applicationsHBTs, MESFETs and HEMTsQuantum wellsSemiconductor lasersSilicon-on-sapphire growthImages from http://www.micro.uiuc.edu/mbe/laserd.htmand Thompson et. al. IEEE Trans. On Semicon. Manufacturing, Vol. 18, No.1, February 2005Also being considered for use in commercial production of SiGe MOSFETsMBE 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 throughputImages from http://users.ece.gatech.edu/~alan/ECE6450/Lectures/ECE6450L13and14-CVD%20and%20Epitaxy.pdfSummary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 worldReferencesParker, E. “Technology and Physics of Molecular Beam Epitaxy” 1985Chang, L. and K. Ploog “Molecular Beam Epitaxy and Heterostructures” 1985Liu, W. “Fundamentals of III-V Devices”