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Epitaxial Deposition M.H.Nemati Sabanci University

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  • Epitaxial DepositionM.H.Nemati

    Sabanci University

  • OutlineIntroductionMechanism of epitaxial growthMethods of epitaxial depositionApplications of epitaxial layers

  • Epitaxial GrowthDeposition of a layer on a substrate which matches the crystalline order of the substrateHomoepitaxyGrowth of a layer of the same material as the substrateSi on SiHeteroepitaxyGrowth of a layer of a different material than the substrateGaAs on SiOrdered, crystalline growth; NOT epitaxialEpitaxial growth:

  • MotivationEpitaxial growth is useful for applications that place stringent demands on a deposited layer:High purityLow defect densityAbrupt interfacesControlled doping profilesHigh repeatability and uniformitySafe, efficient operationCan create clean, fresh surface for device fabrication

  • General Epitaxial Deposition RequirementsSurface preparationClean surface neededDefects of surface duplicated in epitaxial layerHydrogen passivation of surface with water/HFSurface mobilityHigh temperature required heated substrateEpitaxial temperature exists, above which deposition is ordered Species need to be able to move into correct crystallographic locationRelatively slow growth rates resultEx. ~0.4 to 4 nm/min., SiGe on Si

  • General Scheme

  • ThermodynamicsSpecific thermodynamics varies by processChemical potentialsDriving forceProcess involves High temperature process is mass transport controlled, not very sensitive to temperature changesClose enough to equilibrium that chemical forces that drive growth are minimized to avoid creation of defects and allow for correct orderingSufficient energy and time for adsorbed species to reach their lowest energy state, duplicating the crystal lattice structureThermodynamic calculations allow the determination of solid composition based on growth temperature and source composition

  • KineticsGrowth rate controlled by kinetic considerationsMass transport of reactants to surfaceReactions in liquid or gasReactions at surfacePhysical processes on surfaceNature and motion of step growthControlling factor in orderingSpecific reactions depend greatly on method employed

  • Methods of epitaxial depositionVapor Phase EpitaxyLiquid Phase EpitaxyMolecular Beam Epitaxy

  • Vapor Phase EpitaxySpecific form of chemical vapor deposition (CVD)Reactants introduced as gasesMaterial to be deposited bound to ligandsLigands dissociate, allowing desired chemistry to reach surfaceSome desorption, but most adsorbed atoms find proper crystallographic positionExample: Deposition of siliconSiCl4(g) + 2H2(g) Si(s) + 4HCl(g),SiCl4 introduced with hydrogenForms silicon and HCl gasSiH4 breaks via thermal decompositionReversible and possible to do negative (etching)

  • Precursors for VPEMust be sufficiently volatile to allow acceptable growth ratesHeating to desired T must result in pyrolysisLess hazardous chemicals preferableArsine highly toxic; use t-butyl arsine insteadVPE techniques distinguished by precursors used

  • Liquid Phase Epitaxy

    Reactants are dissolved in a molten solvent at high temperatureSubstrate dipped into solution while the temperature is held constantExample: SiGe on SiBismuth used as solventTemperature held at 800CHigh quality layerFast, inexpensiveNot ideal for large area layers or abrupt interfacesThermodynamic driving force relatively very low

  • Molecular Beam EpitaxyVery promising techniqueBeams created by evaporating solid source in UHVEvaporated beam of particle travel through very high vaccum and then condense to shape the layerDoping is possible to by adding impurity to source gas by(e.g arsine and phosphors)Deposition rate is the most important aspect of MBEThickness of each layer can be controlled to that of a single atomdevelopment of structures where the electrons can be confined in space, giving quantum wells or even quantum dotsSuch layers are now a critical part of many modern semiconductor devices, including semiconductor lasers and light-emitting diodes.

  • Doping of Epitaxial LayersIncorporate dopants during deposition(advantages)Theoretically abrupt dopant distributionAdd impurities to gas during depositionArsine, phosphine, and diborane commonLow thermal budget results(disadvantages)High T treatment results in diffusion of dopant into substrateCant independently control dopant profile and dopant concentration

  • ApplicationsEngineered wafersClean, flat layer on top of less ideal Si substrateOn top of SOI structuresEx.: Silicon on sapphireHigher purity layer on lower quality substrate (SiC)In CMOS structuresLayers of different dopingEx. p- layer on top of p+ substrate to avoid latch-up

  • More applicationsBipolar TransistorNeeded to produce buried layerIII-V DevicesInterface quality keyHeterojunction Bipolar TransistorLEDLaser

    http://www.veeco.com/library/elements/images/hbt.jpghttp://www.search.com/reference/Bipolar_junction_transistor

  • SummaryDeposition continues crystal structureCreates clean, abrupt interfaces and high quality surfacesHigh temperature, clean surface requiredVapor phase epitaxy a major method of depositionEpitaxial layers used in highest quality wafersVery important in III-V semiconductor production

  • ReferencesP. O. Hansson, J. H. Werner, L. Tapfer, L. P. Tilly, and E. Bauser, Journal of Applied Physics, 68 (5), 2158-2163 (1990).G. B. Stringfellow, Journal of Crystal Growth, 115, 1-11 (1991).S. M. Gates, Journal of Physical Chemistry, 96, 10439-10443 (1992).C. Chatillon and J. Emery, Journal of Crystal Growth, 129, 312-320 (1993).M. A. Herman, Thin Solid Films, 267, 1-14 (1995).D. L. Harame et al, IEEE Transactions on Electron Devices, 42 (3), 455-468 (1995).G. H. Gilmer, H. Huang, and C. Roland, Computational Materials Science, 12, 354-380 (1998).B. Ferrand, B. Chambaz, and M. Couchaud, Optical Materials, 11, 101-114 (1999).R. C. Cammarata, K. Sieradzki, and F. Spaepen, Journal of Applied Physics, 87 (3), 1227-1234 (2000).R. C. Jaeger, Introduction to Microelectronic Fabrication, 141-148 (2002).R. C. Cammarata and K. Sieradzki, Journal of Applied Mechanics, 69, 415-418 (2002).A. N. Larsen, Materials Science in Semiconductor Processing, 9, 454-459 (2006).

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