Epitaxial DepositionOutlineEpitaxial GrowthMotivationGeneral Epitaxial Deposition RequirementsGeneral SchemeThermodynamicsKineticsKinetics ExampleVapor Phase EpitaxyPrecursors for VPEVarieties of VPEOther MethodsDoping of Epitaxial LayersProperties of Epitaxial LayerApplicationsMore applicationsSummaryReferencesEpitaxial DepositionDaniel LentzEE 518Penn State UniversityMarch 29, 2007Instructor: Dr. J. RuzylloOutlineIntroductionMechanism of epitaxial growthMethods of epitaxial depositionProperties of epitaxial layersApplications of epitaxial layersEpitaxial GrowthDeposition of a layer on a substrate which matches the crystalline order of the substrateHomoepitaxyGrowth of a layer of the same material as the substrateSi on SiHeteroepitaxyGrowth of a layer of a different material than the substrateGaAs on SiOrdered, crystalline growth; NOT epitaxialEpitaxial growth:MotivationEpitaxial growth is useful for applications that place stringent demands on a deposited layer:High purityLow defect densityAbrupt interfacesControlled doping profilesHigh repeatability and uniformitySafe, efficient operationCan create clean, fresh surface for device fabricationGeneral Epitaxial Deposition RequirementsSurface preparationClean surface neededDefects of surface duplicated in epitaxial layerHydrogen passivation of surface with water/HFSurface mobilityHigh temperature required heated substrateEpitaxial temperature exists, above which deposition is ordered Species need to be able to move into correct crystallographic locationRelatively slow growth rates resultEx. ~0.4 to 4 nm/min., SiGe on SiGeneral SchemeModified from http://www.acsu.buffalo.edu/~tjm/MOVPE-GaN-schematic.jpgThermodynamicsSpecific thermodynamics varies by processChemical potentialsDriving forceHigh temperature process is mass transport controlled, not very sensitive to temperature changesSteady stateClose enough to equilibrium that chemical forces that drive growth are minimized to avoid creation of defects and allow for correct orderingSufficient energy and time for adsorbed species to reach their lowest energy state, duplicating the crystal lattice structureThermodynamic calculations allow the determination of solid composition based on growth temperature and source compositionKineticsGrowth rate controlled by kinetic considerationsMass transport of reactants to surfaceReactions in liquid or gasReactions at surfacePhysical processes on surfaceNature and motion of step growthControlling factor in orderingSpecific reactions depend greatly on method employedKinetics ExampleAtoms can bond to flat surface, steps, or kinks.On surface requires some critical radiusEasier at stepsEasiest at kinksAs-rich GaAs surfaceAs only forms two bonds to underlying GaVery high energyReconstructs by forming As dimersLowers energyCauses kinks and steps on surfaceResults in motion of steps on surfaceIf start with flat surface, create step once first group has bondedGrowth continues in same wayhttp://www.bnl.gov/nsls2/sciOps/chemSci/growth.aspVapor Phase EpitaxySpecific form of chemical vapor deposition (CVD)Reactants introduced as gasesMaterial to be deposited bound to ligandsLigands dissociate, allowing desired chemistry to reach surfaceSome desorption, but most adsorbed atoms find proper crystallographic positionExample: Deposition of siliconSiCl4 introduced with hydrogenForms silicon and HCl gasAlternatively, SiHCl3, SiH2Cl2SiH4 breaks via thermal decompositionPrecursors for VPEMust be sufficiently volatile to allow acceptable growth ratesHeating to desired T must result in pyrolysisLess hazardous chemicals preferableArsine highly toxic; use t-butyl arsine insteadVPE techniques distinguished by precursors usedVarieties of VPEChloride VPEChlorides of group III and V elementsHydride VPEChlorides of group III elementGroup III hydrides desirable, but too unstableHydrides of group V elementOrganometallic VPEOrganometallic group III compoundHydride or organometallic of group V elementNot quite that simpleCombinations of ligands in order to optimize deposition or improve compound stabilityEx. trimethylaminealane gives less carbon contamination than trimethylalluminumhttp://upload.wikimedia.org/wikipedia/en/thumb/e/e5/Trimethylaluminum.png/100px-Trimethylaluminum.png, http://pubs.acs.org/cgi-bin/abstract.cgi/jpchax/1995/99/i01/f-pdf/f_j100001a033.pdf?sessid=6006l3Other MethodsLiquid Phase EpitaxyReactants are dissolved in a molten solvent at high temperatureSubstrate dipped into solution while the temperature is held constantExample: SiGe on SiBismuth used as solventTemperature held at 800°CHigh quality layerFast, inexpensiveNot ideal for large area layers or abrupt interfacesThermodynamic driving force relatively very lowMolecular Beam EpitaxyVery promising techniqueElemental vapor phase methodBeams created by evaporating solid source in UHVDoping of Epitaxial LayersIncorporate dopants during depositionTheoretically abrupt dopant distributionAdd impurities to gas during depositionArsine, phosphine, and diborane commonLow thermal budget resultsHigh T treatment results in diffusion of dopant into substrateReason abrupt distribution not perfectProperties of Epitaxial LayerCrystallographic structure of film reproduces that of substrateSubstrate defects reproduced in epi layerElectrical parameters of epi layer independent of substrateDopant concentration of substrate cannot be reducedEpitaxial layer with less dopant can be depositedEpitaxial layer can be chemically purer than substrateAbrupt interfaces with appropriate methodsApplicationsEngineered wafersClean, flat layer on top of less ideal Si substrateOn top of SOI structuresEx.: Silicon on sapphireHigher purity layer on lower quality substrate (SiC)In CMOS structuresLayers of different dopingEx. p- layer on top of p+ substrate to avoid latch-upMore applicationsBipolar TransistorNeeded to produce buried layerIII-V DevicesInterface quality keyHeterojunction Bipolar TransistorLEDLaserhttp://www.veeco.com/library/elements/images/hbt.jpghttp://www.search.com/reference/Bipolar_junction_transistorSummaryDeposition continues crystal