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1、TransportinSilicon-GermaniumHeterostructuresDanielChrastina3GeneralTheoryofTransportinSilicon-GermaniumAlloysIgnoringdopingforthemoment,therearetwowaysinwhichanalloylayermaybeincorporatedwithinaheterostructure.Inapseudomorphicstructure(suchasthatdescribedinFigure3.1)asilicon-germaniumalloyisg
2、rowndirectlyonpuresilicon.(Itisnormalpracticenottogrowthealloydirectlyontothesiliconsubstratebuttodepositafewhundrednanometresofpuresiliconfirst.)Thealloylayeristhengenerallycappedbymorepuresilicon(thereareissuesregardingtheoxidizationofa1silicon-germaniumalloy).Thealloylayerwillmatchitslatti
3、ceconstanttothatofthepuresilicon,provideditisnottoothick,thegermaniumconcentrationisnottoohigh,orthegrowthtemperatureisnottoohigh.Forthisreason,whilstpseudomorphicstructuresaregenerallysimpletogrow,theyaregenerallylimitedtolowgermaniumconcentrationsandthinalloylayers.Alternatively,toallowforg
4、reaterflexibilityregardingthickeralloyswithhighergermaniumconcentrationsorpuresiliconundertensilestrain(forelectronchannels,seeChapter6)avirtualsubstratemaybegrown.Abufferlayerofalloyisgrownandallowedtorelax,andthentheactivechannelsaregrownonthis.Thegermaniumconcentrationwithinthebuffermaybeg
5、radedtoincreaseupwards,butisgenerallyconstantforthefewhundrednanometresbelowtheactivechannel.Itisimportanttoensurethatthebufferlayerisfullyrelaxed(bygrowingitatahighenough2temperature)andthatthedefectsarenotmigratingupintotheactivechannel.3.1PrincipleAdvantagesInUsingASilicon-GermaniumHeteros
6、tructureTherearethreeprincipleadvantagestousingasilicon-germaniumheterostructureasopposedtoaplainsilicon-silicondioxide(Si/SiO2)system,arisingfromthelowereffectivemass,thepossibilityofband-structureengineering,andtheenergybandsplittinginstrainedlayers.7TransportinSilicon-GermaniumHeterostruct
7、uresDanielChrastinaFigure3.1Aschematicbandprofileofaninverted,pseudomorphicstructurewhichshowshowa2-dimensionalcarriergas(inthiscase,ofholes)isformed.Whenbroughttogether,dopantatomsionizeandchargeaccumulatesatthenearestheterointerface.8Transp
