Aztec pyramid

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Materials Today, 19, 5, pp. 292-293, 2016-05-04

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Aztec pyramid

SpringThorpe, Tony; Caballero, Juan; Barrios, Pedro; Wasilewski, Zbigniew

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MaterialsTodayVolume19,Number5June2016 _UNCOVERED

Uncovered

Aztec

pyramid

Electrochemical

etching

of

Te-doped

gallium

arsenide

structures

Tony

SpringThorpe

,

Juan

Caballero

,

Pedro

Barrios

*

and

Zbigniew

Wasilewski

1_{NationalResearchCouncilofCanada,Canada}

2_{UniversityofWaterloo,Canada}

Thosewithvividimaginationsmightthinkthattheimageon thisissue’scoverisanaerialviewoftheSpanishconquistadors,on horseback, preparing to assault the palace of the Aztec leader Montezuma-IIin1520CE.Inrealityitistheresultofthe electro-chemical etching (ECE) [1] of a tellurium (Te) doped gallium arsenide (GaAs) molecular beam epitaxy (MBE) structure. Te is ann-typedopantinGaAs,andisincorporatedontheAssitesinthe crystal lattice.This dopantcanbeusedcontrollably toproduce

electricalcarrierconcentrationsintherange1E16to>1E19/cm3. Thecompletestructurewasaseriesoffive350nmthickTe-GaAs layers,inwhichtheTeconcentrationwasdecreasedstepwisefrom 1E19/cm3 nearthe initial substrateto 1E17/cm3at the final growthsurface.Thiswasachievedbyreducingthetemperatureof thesourceoftheTeatomsforeachlayer.Bysubsequentanalysis thetemperatureoftheTesourcecanthenberelatedtotheamount ofTeincorporatedineachlayer.Oncethiscalibrationis complet-ed complex structures can then be grown with the accurately controlledcarrierconcentrationsthatarenecessaryfor ‘state-of-the-art’deviceperformance.

ECEwasused,togetherwithcapacitancevs.voltage(CV) anal-ysis,tomeasurethecarrierconcentrationvariationofthe struc-ture. This was carried out from the surface of the layer, as a functionofdepthin 10nmincrements,to the underlying sub-strate.TheetchingwasperformedusingaWEPCVP21ECVprofiler

[2],witha3mmdiametersealingringtodefinetheetchedarea. Theelectrolyteusedwasthetraditionalammonium tartrate/am-moniasolution[3].Opticalilluminationwasusedduringetching togenerateelectron/hole pairs,andthe dissolutioncurrentwas controlledat1mA/cm2(70.7mA).UsingFaraday’slawsof elec-trolysis, the time to remove 10nm of GaAs was automatically calculated,andtheetchprocesswasthenstoppedtoenableaCV measurementtobecarried out,anda calculationofthe carrier concentrationtobemade. Thiswasrepeatedone hundredand seventyfivetimes,untiltheetchdepthwas1.75mm,togenerate agraphicaldepthprofileoftheTe-dopingvariation.

Normally,ifthefinalepitaxialsurfaceisdefectfree,theetching proceedsinaplanarmanner.However,thisparticularsamplehad a large number of optically visible defects (700/cm2_{), which}

servedasthenucleationsitesforthefeaturesthatwereobserved atthetermination oftheECVmeasurementsequence.The fea-turesareetchpits,withoveralldimensions500mm500mm. Thattheyarepitscanbeseenincleavedsection,althoughtheycan appeartotheeyetoberaisedpyramid-likestructures.Sincethe GaAssubstratehada[100]crystalsurface,thesidesofthepyramid aretheslowetching{111}planesoftheGaAscubiccrystallattice, alignedtothetwomutuallyperpendicular(110)directions.

The formation ofpits duringetching isdue to the presence of crystalline defects, such as dislocations and/or stacking

*Correspondingauthor:Barrios,P.(Pedro.Barrios@nrc-cnrc.gc.ca)

292

1369-7021/CrownCopyrightß2016PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).http://dx.doi.org/10.1016/

faults. Strain fields associated with the defective crystal lattice acceleratethe etchprocessrelativetothe initial planarsurface. InthecaseofECE,thestrainfieldsactaslowresistancecurrent paths,whichresultinenhancedmaterialremoval.Howeveritis notclearastowhytheseparticularpitsformedwiththewire-like featuresregularly arranged aroundthe centralinverted stepped pyramid.

Profiles at other locations on the wafer produced the same features.Soitisa reproducible,butinexplicable,phenomenon. However,etchingusinganalternativeelectrolytewasmoreplanar, butdidnotproducesimilarfeatures.Soitwouldappearthatthese pitsareapeculiarityoftheammoniumtartrate/ammonia electro-lyteECEprocess.

Furtherreading

[1]T.Ambridge,M.M.Faktor,J.Appl.Electrochem.5(1975)319–328.

[2]http://wepcontrol.com/cv-profiler/index.htm.

[3]PN4200Polaron’sSemiconductorprofiler,Instructionmanual,Bio-Rad,

Rich-mond,CA.

MaterialsTodayVolume19,Number5June2016 UNCOVERED

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