Carrier Compensation Induced by Thermal Annealing in Al-Doped ZnO Films

18 Pages·2017·7.1 MB·English

materials
Article
Carrier Compensation Induced by Thermal
Annealing in Al-Doped ZnO Films
TakashiKoida1,*,TetsuyaKaneko1,2andHajimeShibata1
1 ResearchCenterforPhotovoltaics,NationalInstituteofAdvancedIndustrialScienceandTechnology(AIST),
Central2,Umezono1-1-1,Tsukuba,Ibaraki305-8568,Japan;[email protected](T.K.);
[email protected](H.S.)
2 SchoolofEngineering,TokaiUniversity,4-1-1,Kitakaname,Hiratsuka,Kanagawa259-1292,Japan
* Correspondence:[email protected];Tel.:[phone]
AcademicEditors:AndreaLiBassiandCarloS.Casari
Received:26December2016;Accepted:3February2017;Published:8February2017
Abstract: Thisstudyinvestigatedcarriercompensationinducedbythermalannealinginsputtered
ZnO:Al(Al O :0.25,0.5,1.0,and2.0wt%)films. Thefilmswerepost-annealedinaN atmosphereat
2 3 2
low(1×10−23atm)andhigh(1×10−4atm)oxygenpartialpressures(P ).InZnO:Alfilmswithlow
O2
Alcontents(i.e.,0.25wt%),thecarrierdensity(n)begantodecreaseatannealingtemperatures(T )
a
of600◦CatlowP . AthigherP and/orAlcontents,nvaluesbegantodecreasesignificantlyat
O2 O2
lowerT (ca.400◦C).Inaddition,Znbecamedesorbedfromthefilmsduringheatinginahighvacuum
a
(i.e., <1 × 10−7 Pa). Theseresultssuggestthefollowing: (i)ZninterstitialsandZnvacanciesare
createdintheZnOlatticeduringpost-annealingtreatments,therebyleadingtocarriercompensation
byacceptor-typeZnvacancies;(ii)ThecompensationbehaviorissignificantlyenhancedforZnO:Al
filmswithhighAlcontents.
Keywords: transparentconductingoxides;ZnO;doping;carriercompensation;mobility
1. Introduction
Transparent conducting oxide (TCO) films have been widely used as window electrodes in
optoelectronic devices such as flat panel displays and solar cells. Most TCO films are based on
In O , SnO , or ZnO, of which ZnO is the most important due to its advantages of low cost and
2 3 2
nontoxicitycomparedtoIn O . Additionally,ZnOfilmsexhibithigherconductivitythanSnO films
2 3 2
at relatively low growth temperatures. Recently, ZnO films have been used in thin-film solar cell
applications[1],however,theytypicallyexhibitlowconductivitycomparedtoIn O -basedTCO[2].
2 3
ThemostimportantpropertiesofTCOfilmsforuseaswindowelectrodesaretheirconductivityand
absorptioncoefficient.Bothoftheseparametersarerelatedtothecarrierdensity(n)andmobility(µ)[3].
Thecarrierdensity(n=N −N )isdeterminedbythedonor(N )andacceptor(N )densities,andthese
d a d a
twovaluesinfluenceµthroughCoulombinteractionsthatcausefreeelectronscatteringbycharged
donorandacceptordefects. InthecaseofAl-dopedZnO(ZnO:Al)films,forexample,thedominant
donoristhesinglychargeddopantAl •, whereasthecompensatingacceptorisconsideredtobe
Zn
adoublychargedzincvacancy(v ”)[4–7]. Hence,suppressingv ”isimportantforachievingboth
Zn Zn
highnandµ.
Materials2017,10,141;doi:10.3390/ma10020141 www.mdpi.com/journal/materials
Materials2017,10,141 2of18
The changes in concentration of intrinsic defects such as v ” are typically described using
Zn
Brouwerdiagrams[8].Lanyetal.calculatedintrinsicdefectdensitiesandnforpureZnOandZnO:Al
as a function of growth temperature and oxygen partial pressure (P ) based on the formation
O2
energiesofthedefectsandthethermodynamicsoftherelevantdefectreactions[6]. InZn Al O,
0.99 0.01
thedonordensityremainsuncompensatedatlowP ,whereasndecreaseswithincreasingP as
O2 O2
aresultofcompensationbythev ”acceptor. Zakutayevetal. havealsoinvestigatedtheconnection
Zn
betweendefecttheoryandthin-filmgrowthofZnO:Gabycomparingtheoreticallyandexperimentally
obtainednvalues[9].Theydemonstratedthatthehighconductivityofthefilmswasduetothehighly
non-equilibrium, metastable state that resulted from growth by pulsed laser deposition (PLD) at
relativelylowtemperaturesandlowP . Inaddition,Looketal. determinedN andN densities
O2 d a
fromµandnvaluesforZnO:GathinfilmsgrownbyPLDbasedonamobilityanalysisinwhichµwas
mainlydeterminedbythescatteringfromionizeddonorsandacceptors[10].
Thepurposeofthepresentworkistoinvestigatethecarriercompensationbehaviorinsputtered
ZnO:Alfilmsduringpost-annealingtreatmentatlow(1×10−23 atm)andhighP (1×10−4 atm).
O2
Becausethesputteringprocessisknowntobeanon-equilibriummethod,theas-depositedZnO:Al
filmsarefarfromanequilibriumstate.Duringpost-annealingathightemperaturesunderconstantP ,
O2
thefilmequilibrateswithO throughinteractionwiththesurroundinggasphase. Consequently,the
2
density of v ” defects should approach a certain value determined by the P and annealing
Zn O2
temperature.Therefore,monitoringthevariationofnvaluescanbeusedtoevaluatecarriercompensation
inducedbythethermalannealing.
WecharacterizedZnO:AlfilmswithvariousAlcontents.Theoretically,thedensityofcompensating
acceptor-typev ”increaseswiththeconcentrationofdonor-typeAl •becausetheformationenergy
Zn Zn
ofv ”decreaseswithincreasesintheFermienergy[4–7]. Atthesametime,theAlcontentofthefilms
Zn
typicallyinfluencestheoptimumfilmgrowthtemperature[11,12],andtheextentofnon-equilibrium
intheas-depositedfilmsincreaseswithdecreasinggrowthtemperatures(T ). Inthispaper,wefirst
g
describe the effects of T on the structural and electrical properties of ZnO:Al films with different
g
Alcontents. AfteroptimizingT foreachZnO:Alfilm,wedescribethevariationsinnandµvalues
g
duringthepost-annealingtreatmentatlowandhighP anddiscusstheoriginofthevariations.
O2
2. ResultsandDiscussion
2.1. PropertiesofAs-DepositedZnO:AlFilms
Figure1shows(a)θ–2θscan(out-of-plane)with002ωscanand(b)2θ –ϕscan(in-plane)X-ray
x
diffraction(XRD)patternsofa~240-nm-thickZnO:Al(2.0wt%)filmgrownat250◦Casanexample.
Only the diffraction peaks due to (002) and (004) planes were observed in the θ–2θ scan, whereas
the diffraction peaks due to (100), (110), (200), (210), and (300) planes were observed. All of the
preparedfilmswithvariousAlcontentsandT exhibitedpreferredorientationofthec-axisnormal
g
to the substrate plane and did not contain any additional phases (e.g., ZnAl O or Al O phases).
2 4 2 3
Because the solid solubility limit of Al in Al O -doped ZnO sintered body prepared at 1400 ◦C
2 3
was reported to be ~0.3 at % [13], the ZnO:Al films fabricated in this study are metastable from
athermodynamicpointofview,andasupersaturatedsolidsolutionoccursasaresultofthehighly
non-equilibriumgrowthprocess. Figure2showsacross-sectionaltransmissionelectronmicroscopy
(TEM)imageoftheZnO:Al(2.0wt%)film. Abundantsmallcolumnargrainswerepresentatthe
interfacebetweenthefilmandsubstrate,whichhadslightlytiltedlong-axisorientationsnormalto
thesubstratesurface. Forfilmthicknessesabove50nm,thesmallgrainsbecameconnected,andthe
deviation of each columnar grain decreased. Consequently, relatively large columnar grains with
widthsof20–40nmappearedwhenthefilmthicknessexceeded100nm.
Materials2017,10,141 3of18
Materials 2017, 10, 141 3 of 17
Materials 2017, 10, 141 3 of 17
(a) (b)
(a) (b)
FFaFii iggZguunurrOreee : 11A1... l (( (a(aa2))) . 0θθθ ––w–222tθθθ % s sscc)ca aafnninl m( (o(oo.uu utt-t-o-ooff-f-p-ppllalaannneee))) w wwiititthhh 0 00000222 ω ωω s sscccaaannn;;; a aannnddd ( (b(bb))) 2 22θθθxx–x––ϕϕϕ s sscccaaannn ( (i(ininn---pppllalaannneee))) X XXRRRDDD pppaaatttttteeerrrnnnsss ooofff
aZnO:Al(2.0wt%)film.
a ZnO:Al (2.0 wt %) film.

Figure 2. A cross-sectional TEM image of an as-deposited ZnO:Al (2.0 wt %) film.
Figure 2. A cross-sectional TEM image of an as-deposited ZnO:Al (2.0 wt %) film.
Figure2.Across-sectionalTEMimageofanas-depositedZnO:Al(2.0wt%)film.
Figure 3 displays plots of the c-axis length, the full width at half maximum (FWHM) of the 002
Figure 3 displays plots of the c-axis length, the full width at half maximum (FWHM) of the 002
diffraFcigtiuorne 3pedaiskp l(aFyWspHlMots002o)f, tahnedc -athxies lFeWngHthM,t hoef furlolcwkiindgth cautrhvaelsf mfoarx itmheu m00(2F WdHiffMra)ctoifonth ep0e0a2k
diffraction peak (FWHM002), and the FWHM of rocking curves for the 002 diffraction peak
d(FifWfraHctMioωn0p02e)a aks( FaW fuHnMction), oanf dTtgh feoFr WthHe M~2o4f0r-oncmki-nthgiccukr vZensOfo:Artlh feil0m02s.d Hifferraec,t iFoWnpHeMak00(2F WrefHleMctωs the)
(FWHMω002) as a functi0o02n of Tg for the ~240-nm-thick ZnO:Al films. Here, FWHM002 reflects t0h02e
out-of-plane crystallite size and the differences in c-axis length along the growth direction due to
as a function of T for the ~240-nm-thick ZnO:Al films. Here, FWHM reflects the out-of-plane
out-of-plane crystagllite size and the differences in c-axis length along t0h0e2 growth direction due to
cnroynst-aulnliitfeorsmiz emaincrdo-tshteredssifefse,r ewnhceerseians cF-WaxHisMleωn0g02t hreaflleocntgs tthhee cg-raoxwis tthiltd. iFreocrt ieoanchd ZuenOto:Anlo fnil-mun wifoitrhm a
non-uniform micro-stresses, whereas FWHMω002 reflects the c-axis tilt. For each ZnO:Al film with a
mgiivceron- sAtrle cssoenst,ewnth, eFreWasHFMW0H02 Manωd FWreHflMectωs0t0h2 edce-carxeiassteildt. gFroardeuaachllyZ nwOit:hA linficlrmeawsiinthg aTgg iuvepn tAo lac coenrtteanint,
given Al content, FWHM002 and 0F02WHMω002 decreased gradually with increasing Tg up to a certain
FoWptHimMal teamnpdeFraWtuHreM (ωTopt),d wechrielea stehdesger avdaulualelsy iwncitrheainsecdre aasbionvgeT Touptp. Ttohea vcaerlutaeisn oofp Ttiompt awlteerme 2p5e0ra, t3u0r0e,
optimal0 0t2emperature (To0p0t)2, while these values increased above Tgopt. The values of Topt were 250, 300,
(3T50, )a,nwdh 3i7le0 t°hCe sfeorv aZlnuOes:Ainl cfrilemasse fdabarbiocavteedT us.inTgh ZenvOa lcueersamoficT targwetesr eco2n5t0a,in30in0g, 325.00,, 1a.n0d, 03.57,0 a◦nCd
35o0p, tand 370 °C for ZnO:Al films fabricated uopsting ZnO ceramic toaprtgets containing 2.0, 1.0, 0.5, and
f0o.r25Z wnOt %:A lAfli2lOm3s, rfeasbprieccattivedelyu.s Tinogpt ZdencOrecaesreasm wicithta irngcertesacsoinngta Ainli ncogn2te.0n,t1. .T0h,0e .5v,aarinadtio0n.2 5ofw tth%e FAWlHOM,
0.25 wt % Al2O3, respectively. Topt decreases with increasing Al content. The variation of the FWH2 M3
values can be explained as follows: The diffusion length of adatoms and precursors at the growth
respectively. T decreases with increasing Al content. The variation of the FWHM values can be
values can be oepxtplained as follows: The diffusion length of adatoms and precursors at the growth
surface increase with Tg up to Topt, thereby leading to greater crystallinity in the films. Conversely,
surface increase with Tg up to Topt, thereby leading to greater crystallinity in the films. Conversely,
further increases in Tg would significantly promote diffusion of the adatoms and precursors, leading
further increases in Tg would significantly promote diffusion of the adatoms and precursors, leading
Materials2017,10,141 4of18
explainedasfollows: Thediffusionlengthofadatomsandprecursorsatthegrowthsurfaceincrease
withT uptoT ,therebyleadingtogreatercrystallinityinthefilms. Conversely,furtherincreases
g opt
Materials 2017, 10, 141 4 of 17
in T would significantly promote diffusion of the adatoms and precursors, leading to migration
g
ofionsinthebulkfilms,anddesorptionofZnatthegrowthsurface. Consequently,excessAlmay
to migration of ions in the bulk films, and desorption of Zn at the growth surface. Consequently,
degrade the film crystallinity by forming local atomic structures similar to a homologous phase
excess Al may degrade the film crystallinity by forming local atomic structures similar to a
((ZnO) Al O )[14,15]. TheeffectofinsertinghomologousphasesinaZnOfilmwouldbetoincrease
homolomgou2s p3hase ((ZnO)mAl2O3) [14,15]. The effect of inserting homologous phases in a ZnO film
theout-of-planedimensionofthefilm. Indeed, weobservedanincreaseinthec-axislengthatT
would be to increase the out-of-plane dimension of the film. Indeed, we observed an increase in theg
aboveT (Figure3).
c-axis lenopgtth at Tg above Topt (Figure 3).

Figure 3. Plots of c-axis length, the full width at half maximum (FWHM) of the 002 diffraction peak
Figure3.Plotsofc-axislength,thefullwidthathalfmaximum(FWHM)ofthe002diffractionpeak
(FWHM002), and the FWHM of rocking curves for the 002 diffraction peak (FWHMω002) as a function
(FWHM ),andtheFWHMofrockingcurvesforthe002diffractionpeak(FWHMω )asafunction
002 002
of growth temperature for ZnO:Al (0.25, 0.5, 1.0, and 2.0 wt %) films.
ofgrowthtemperatureforZnO:Al(0.25,0.5,1.0,and2.0wt%)films.
Figure 4 displays the resistivity (ρ), n, and μ of the films plotted as a function of Tg. With
decreFaisgiunrge A4ld cisopnltaeynst,t hne mreosnisotitvointyic(aρl)ly,n d,eacnrdeaµsoedft haleofinlgm wspitlho tat egdraadsuaaflu inncctrioenasoef iTng μ..W Ciothndseeqcrueeanstilnyg,
Aρ lgcroandtueanltl,yn inmcorenaosteodn iwcailtlhy ddeeccrreeaassiendg aAloln cgonwtietnht.a Mgroardeu iamlpinocrrteaanstelyi,n inµ .eCacohn sZenqOue:Antll y(0,.ρ25g,r 0a.d5u, a1l.l0y,
ainncdr e2a.s0e wdtw %it)h fidlmec,r ebaostihn gn Aanldc oμn teexnhti.bMiteodre mimaxpimorutamn tvlya,liunees aacth thZen TOo:pAt vla(0lu.2e5s, i0n.5d,ic1a.0te,dan idn 2F.i0guwrte% 3.)
Tfihlme ,rebsoutlhtsn suanggdeµst etxhhaitb iimtepdermfeacxtiimonusm of vcarylusetasl aatreth teheT coaptuvsea loufe csairnrideirc tartaepdpiinngF iagnudr esc3a.ttTehriengre. sTuhltiss
psuhgegneosmttehnaotnim isp aelrsfoe cotibosnesrvoefdc riyns ZtanlOar:eAtlh feilcmasu spereopfacraerdri eart tdraifpfeprienngt afnildms tchaitctekrninesgs.eTsh. Fisigpuhreen 5o msheonwons
cis-aaxlisso loebnsgetrhv,e dFWinHZMnO002:A, lanfidlm FsWprHepMaωre0d02 aatsd iaff efruenncttifiolnm othf icfiklmne stsheisc.kFniegsusr efo5r sheoacwhs Zc-naxOi:sAlel nfgiltmh,
dFWepHosMite0d02 ,aat nTdoptF. WReHgMardωl0e0s2s aosf aAflu cnocntitoennto, fbofitlhm FtWhicHkMne0s02s afnodr eFaWchHZMnωO0:0A2 ldfieclmreadseepdo rsaitpeiddlayt wToitpht.
iRnecgreaardsilnegss ofiflmA ltchoinctkennets,sb outhp FtWo H1M000–0220a0n dnmFW, HanMdω 0d0e2cdreeacsreedas egdraradpuiadlllyy woivther in1c0r0ea–s2i0n0g finlmm.
Cthoicrrkensepsosnudpintog 1to00 t–h2e0 0chnamng,ae nidn dcreycsrteaalsleindeg qruadaluitayll,y boovthe rn1 0a0n–d2 0μ0 innmcre.aCsoerdr ersappoidndlyi nwgittoh tihnecrcehaasningge
finilmcr ythstiaclklnineessq uuapl ittoy, 1b0o0t–h2n00a nndmµ, ainncdr eianscerdearsaepdi dglryawduitahlliyn corveears i1n0g0fi–2lm00t hnimck fnoers seaucpht oZn1O00:–A2l0 (00n.2m5,,
0a.n5d, 1in.0c,r aeansde d2.g0r wadtu %al)l,y aosv sehro1w00n– i2n0 0Fingmurfeo r6ae.a cFhigZunreO 6:Ab lp(0lo.2ts5 ,a0 .r5e,la1t.0io,nasnhdip2 .b0ewtwte%e)n, ans asnhdow μn. Iinn
FFiigguurree 66ab., Ftihgiuckre s6oblipdl olitnseas raerlaet icoonnsnheipctbedet wtoe ethnen failnmdsµ h.aInviFnigg usrime6ilba,rt hfiilcmk stohliicdklninesesseasr. eInco gnenneecrteadl,
mtootbhielitfiyl mofs dheagveinngersaitmedil asremfilimconthdiuckctnoersss edse.crIenasgeesn ewraitlh, minocbreilaitsye oinf dceagrreineer radteendsisteym, sicinocned udcotnoorsr
idmecprueraistieess wwitohrkin acsre sacsaettienricnagrr iceerndteernss iotfy ,fsreinec ecadrroinerosr. iImndpeuerdit,i emsewaosurkreads μsc afottre rtihnigckc efniltmerss wofitfhr eae
tchaircrkienress.sI nodf e~e8d0,0m nemas duerecrdeaµsfeodr wthiitchk afinl minscrweaitshe aint hAiclk cnoenstsenotf o~r8 0n0. Hnmowdeevcerer,a fsoerd twhiinth fialmnsin wcriethas ae
tihniAcklnceosns toefn lteosrs nth.aHn o1w00e vnemr,, fmorotbhiliintyfi ilnmcrsewasiethd awtihthic ckanrerisesro dfelnessistyth. aTnhe1 0b0ehnamvi,omr ocabnil ibtye einxpcrleaainseedd
woni ththcea rbraiesirsd oefn ssictayt.teTrhinegb eaht atvhieo rgcraaninb beoeuxnpdlaairnye d(GoBn) thraethbears isthoafns ciaotntiezreindg imatpthueritgireasi n[1b6o,1u7n].d aGrBy
s(GcaBt)terraitnhge rist hdaensciroibneizde dbyi mthpeu Sreittioe sm[o16d,e1l7, ]i.nG wBhsiccaht ttehrei nmgoibsidlietsyc irsi bdeodmbiynathteedS beyto thmeordmeilo,ninicw ehmicishsitohne
amcorobsilsi tgyriasind obmarirniaertesd, wbyhetrhee rimmipounriictieems iosrs iootnhearc rdoesfsegcrtsa iinndbaurcreie erlse,cwtrhoenr etriampps uarti tGieBsso [r1o6t]h. eFrodr effielmctss
with very high n values, the depletion width formed at a GB is narrow, enabling tunneling through
the barriers by free electrons. These results shown in Figures 3–6 clearly indicate that imperfections
in the crystal lattice and concentration of GB are the cause of carrier trapping and scattering. It
should be noted that the values of n, μ, and Topt obtained for each ZnO:Al are comparable to those
previously reported for ZnO:Al films [11,12,18].
Materials2017,10,141 5of18
induceelectrontrapsatGBs[16]. Forfilmswithveryhighnvalues,thedepletionwidthformedat
a GB is narrow, enabling tunneling through the barriers by free electrons. These results shown in
Figures3–6clearlyindicatethatimperfectionsinthecrystallatticeandconcentrationofGBarethe
causeofcarriertrappingandscattering. Itshouldbenotedthatthevaluesofn,µ,andT obtained
opt
foreachZnO:AlarecomparabletothosepreviouslyreportedforZnO:Alfilms[11,12,18].
Materials 2017, 10, 141 5 of 17
Materials 2017, 10, 141 5 of 17

Figure 4. Plots of resistivity, carrier density, and mobility in ZnO:Al (0.25, 0.5, 1.0, and 2.0 wt %) films
Figure4.Plotsofresistivity,carrierdensity,andmobilityinZnO:Al(0.25,0.5,1.0,and2.0wt%)films
Figure 4. Plots of resistivity, carrier density, and mobility in ZnO:Al (0.25, 0.5, 1.0, and 2.0 wt %) films
as a function of growth temperature.
asafunctionofgrowthtemperature.
as a function of growth temperature.

Figure 5. Plots of c-axis length, the FWHM of the 002 diffraction peak (FWHM002), and the FWHM of
FFrioiggcukurirneeg 55 .c. PuPlrloovttesss o offfo crc- -atahxxeiiss 0 ll0ee2nn ggdttihhff,,r ttahhceeti FoFWnW pHHeMMak o o(ffF ttWhheeH 00M0022ω dd00ii2ff)ff rraaascc tatii oofunnn ppceetaiaokkn (( FFoWWf tHHhiMMck00n002e2)s), ,sa anfnoddr tZthhnee OFFW:AWHlH (M0M.2 o5of,f
rr0oo.5cck,k 1iinn.0gg, cacunurdrvv 2ee.ss0 ffwoorrt t%thh)ee f 00il00m22s dd. iiffffrraaccttiioonn ppeeaakk ((FFWWHHMMωω002) )aass aa ffuunncctitoionn oof ftthhicickknneessss foforr ZZnnOO:A:Al l(0(0.2.255, ,
002
0.5, 1.0, and 2.0 wt %) films.
0.5,1.0,and2.0wt%)films.
Materials2017,10,141 6of18
MMataetreirailasl s2 2001177, ,1 100, ,1 14411 66 o of f1 71 7

Figure 6. (a) Plots of resistivity, carrier density, and mobility in ZnO:Al (0.25, 0.5, 1.0, and 2.0 wt %)
Figure 6. (a) Plots of resistivity, carrier density, and mobility in ZnO:Al (0.25, 0.5, 1.0, and 2.0 wt %)
Ffiiglmurse a6s. a(a f)uPnlcottisono forfe fsiilsmti vtihtiyc,kcnaersrsi;e r(bd)e Rnseiltayt,ioannsdhimp obbeitlwityeeinn cZanrrOie:rA dle(0n.s2i5ty, 0a.5n,d1 .m0,oabnildity2 .f0owr tth%e )
films as a function of film thickness; (b) Relationship between carrier density and mobility for the
fiflimlmssa.s afunctionoffilmthickness;(b)Relationshipbetweencarrierdensityandmobilityforthefilms.
films.
FFigiguurere7 7s hshoowwss( a(a))t rtraannssmmiittttaannccee aanndd rreeflfleeccttaannccee;; aanndd ((bb)) aabbssoorrppttiioonn ccooeeffffiicciieenntt ssppeeccttrraa oof fththee
Figure 7 shows (a) transmittance and reflectance; and (b) absorption coefficient spectra of the
~8~08000-n-nmm-t-hthicickkfi flimlmss ggrroowwnn aatt TToopptt.. AAlll lththee fifilmlms swweerer etrtarnanspsparaernetn itni nvivsiibsilbe lwewavaevleenlegnthg trhegreiognio annadn ad
~800-nm-thick films grown at Topt. All the films were transparent in visible wavelength region and a
addeeccrereaasese inin ttrraannssmmitittatanncec einin nneeaarr-i-ninfrfarareredd wwaavveelelennggthth rereggioionn wwaas soobbsseervrveedd dduuee ttoo ffrreeee--ccaarrrrieierr
decrease in transmittance in near-infrared wavelength region was observed due to free-carrier
ababsosorprptitoionn..I nInt htheeu ultlrtarvavioioleltetw wavaveleelnenggththr ergeigoinon,a, nano nosnestewt wavaevleelnegntght,ha, tawt whihchicha basbosroprtpiotinonb ebgeagnanto
absorption. In the ultraviolet wavelength region, an onset wavelength, at which absorption began
intocr ienacsree,asshei,f stehdiftteods thoo srhteorrwtera vwealevnegletnhgstwhsi twhiitnhc irnecarseinasginAgl Acol nctoennttedntu deutoe Btou Brsutresitne-iMn-oMssosssh sifht.ift.
to increase, shifted to shorter wavelengths with increasing Al content due to Burstein-Moss shift.

Figure 7. (a) Transmittance and reflectance; and (b) absorption coefficient spectra of the
Figure7.(a)Transmittanceandreflectance;and(b)absorptioncoefficientspectraofthe~800-nm-thick
F~ig8u00r-en m7-. th(iac)k ZTnraOn:Asml (i0tt.a2n5,c e0 .5a, n1.d0 , arnedfl e2c.t0a wncte %; ) afnildm s.( b) absorption coefficient spectra of the
ZnO:Al(0.25,0.5,1.0,and2.0wt%)films.
~800-nm-thick ZnO:Al (0.25, 0.5, 1.0, and 2.0 wt %) films.
2.2. Properties of ZnO:Al Films Post-Annealed at Low and High Oxygen Partial Pressures
2.2. PropertiesofZnO:AlFilmsPost-AnnealedatLowandHighOxygenPartialPressures
2.2. Properties of ZnO:Al Films Post-Annealed at Low and High Oxygen Partial Pressures
We investigated the effects of post-annealing treatments at low (1 × 10−23 atm) and high (1 × 10−4
We investigated the effects of post-annealing treatments at low (1 × 10−23 atm) and high
(1at×mW)1 P0eO− 2i4 ninva teZmsnt)iOgP:aAtel d(0i nt.h25eZ, ne0fO.f5e:,Ac 1tl.s0 (o, 02f. .2p05o w,st0t- .%a5n,)n 1fei.l0am,lis2n .gg0r towrwetant% ma)te Tfnioltpmst. asFti ggloruowrwe (8n1 s×ah t1o0Tw−2s3 ta.htemF c)ihg aaunnrdeg eh8si gisnhh no( 1wa n×s d1t h0μ−e 4
avtmal)u PesO 2f oinr ZZnnOO::AAOll 2((20..02 5w, 0t .%5,) 1 (.F0i,g 2u.0re w 8ta %) a)n fdil mZns Ogr:Aowl (n0 .a2t5 T woptt .% F)i g(Fuirgeu 8r es h8obw)o fpsit ltmhes acht aa nPgOe2 so ifn 1 n × a 1n0d−2 3μ
changes in n and µ values for ZnO:Al (2.0 wt %) (Figure 8a) and ZnO:Al (0.25 wt %) (Figure 8b)
vaatlmue sa sf oar fZunnOct:iAonl (o2f.0 s wpet c%im) e(Fni gteumrep 8ear)a taunrde Z(TnsO) :dAulr (in0.g2 5p wostt -%an) n(Feaigliunrge u8bsi)n fgil ma sH aatl la mPOe2a osuf r1e ×m 1e0n−t2 3
filmsataP of1×10−23atmasafunctionofspecimentemperature(T )duringpost-annealingusing
astmys taesm a. fTuOhn2ec timone aosfu rsepmeceinmt ensy tsetmemp erisa tudrees c(rTibs)e ddu rinin gS epcotsiot-na n3n.e aAlisns gs huosiwnng ai nH aFlilg mureea s8uar,e mfivene t
aHallmeasurementsystem.ThemeasurementsystemisdescribedinSection3.AsshowninFigure8a,
sypsotsetm-a.n nTehaeli nmg teraesautmreemnetsn wt esryes pteemrf orims edde: s(ci)r i5b0e–d3 00in– 50S°eCc;t i(oiin) 503–. 40A0s– 5s0h °oCw; (nii i)i n5 0–F5ig00u–r5e0 8°Ca,; (fiivv)e
fivepost-annealingtreatmentswereperformed: (i)50–300–50◦C;(ii)50–400–50◦C;(iii)50–500–50◦C;
p5o0s–t-6a0n0n–e5a0l °inCg; atrneda t(mv)e 5n0t–s6 w50e–r5e0 p °eCrf. oCrlmoseedd: ((io)p 5e0n–)3 s0y0m–5b0o°lCs ;r (eipi)r e5s0e–n4t0 d0–a5ta0 m°Ce;a (siuiir)e 5d0 d–5u0r0in–g5 0h °eCat;i n(igv )
5(0c–o6o0l0in–g5)0 i°nC e; aacnhd p (ovs)t -5a0n–n6e5a0l–in5g0 °trCe.a Ctmloesnetd. N(oop leanr)g sey cmhabnoglse sr einp rne saenndt μd awtae rme eoabssuerrevde dd uupri ntog 4h0e0a °tiCn,g
(c ooling) in each post-annealing treatment. No large changes in n and μ were observed up to 400 °C,
Materials2017,10,141 7of18
M(ivat)er5ia0ls– 2600107–, 1500, 1◦4C1 ; and (v) 50–650–50 ◦C. Closed (open) symbols represent data measured du7 roifn 1g7
heating(cooling)ineachpost-annealingtreatment. Nolargechangesinnandµwereobservedup
wtoh4e0r0ea◦sC a, wlahrgeer edaescarelaasreg eind nec wreaass eobinsenrvwedas woibthse irnvcerdeawsiinthg iTnsc arbeaosvien g40T0s°Cab. oTvhee 4v0a0lu◦eCs. oTfh ne dvuarluinegs
coofonlidnugr iinn geaccoho lainngneinaleinagch traenantmeaelnintg wtererae tnmeeanrltyw idereenntiecaarl ltyoi dtheen tni cvaalltuoet hate tnhev amluaexiamttuhme mananxeimaluinmg
taenmnpeaelriantgurteem (Tpae)r. aFtuurrteh(eTram).oFrue,r tthheer msaomrep,leths easlsaom epxlhesibailtseod esxihmibiliatre dn svimaliulaers nduvarilnuge shdeuartiinngg hine aetainchg
sinubesaecqhuseunbts eaqnuneenatlianngn etraeliantgmterneat tmupe nttou tpemtopteermaptuerreast ulreesss lethssanth athnet hmeamxaimxiummum TaT ainin ththee pprreevviioouuss
aannnneeaalliinngg ttrreeaattmmeenntt..T Thheesseer erseusultlstsc lcelaeralyrldy edmeomnosntrsattreatthe atthtahte tehqeu eilqiburilaitbiorantitoimn etirmapei dralypiidnclyre ianscersewasieths
wdeitchre adsiencgreTass.inTgh erTesf. orTeh,tehreefnorvea, lutehsem ne asvuarleudesd umrineagstuhreedco odliunrginpgr octheess rceofolelicntgq upasrio-ceeqsusi lirberfiluemct
qstuaatesis-eaqnudilfirborziuenmd setfaetcets daennds iftrioeszeant tdheefemcta xdiemnusimtieTs aato fththe emaanxniemaulimng Ttra eoaft mtheen atn. nSiemaliilnagr cthreaantgmeesnitn.
Snimvailluare schaasnagfeusn icnt ino nvaolfuTess wase rae foubnscetriovned off oTrs twheerZen oOb:sAelrv(0e.d2 5fowr tth%e) ZfinlmO,:Aasl (s0h.o25w nwti n%F)i gfiulmre, 8abs.
sHhoowwenv einr, Fthigeucrhe a8nbg. eHsoinwnevvearl,u tehse wcheraengveesr yins mn avllacluoems pwareerde vtoertyh ossmeaolfl tchoemZpnaOre:dA lto( 2thwotse% o)ff itlhme.
ZFingOu:rAel9 (s2u wmtm %a)r ifzilems.t hFeigcuhraen 9g essuminmρ,anri,zaens dthµe vcahlaunegsefso rint hρe, Zn,n aOn:dA lμ( 0v.a2l5u,e0s. 5f,o1r .t0h,e2 .Z0nwOt:A%l) (f0il.m25s,
0m.5e,a 1s.u0r,e 2d.0a wtrto %om) fitlemmsp mereaatusurereadft eart rthoeompo tsetm-anpnereaatluinrge tarfetaetrm theen tpsoastt-aaPnneaolfin1g× tr1e0a−tm23eanttms .atT ah ePOd2a otaf
O2
1a r×e p10lo−2t3t eadtmas. Tahfue ndcattioa naroef tphleotmteadx aims ua mfuTnactiinone aocfh tahne nmeaalxinimgutrmea Ttma einn te.aIcnht earnesnteinalginlyg, tthreeaotmveernatl.l
Ibnethearvesiotirnognlyh, etahtei nogvweraaslls btreohnagvliyord oepne hnedaetnintgo nwtahse sAtrlocnognlyte dnetpinenthdeenfitl mons. tFhoer Athl ecoZnntOen:At lin( 0t.h2e5 fwiltm%s.)
Ffiolmr t,hµe inZcnrOea:Aseld (0fr.2o5m w4t2 %to) 5fi4lmw,h μe rienacsrneadsiedd nforotmch a4n2 gteo s5ig4 nwifihcearnetalsy nw idthidi nncorte achsiannggTe s.iIgnncifoicnatnratlsyt,
a
wfoirthth iencZrneOas:iAngl (T2.a0. Iwn tco%n)trfialsmt,, fnord tehcere ZanseOd:Aral p(2id.0l ywatt %T)a failbmo,v ne d40e0cr◦eCas,ewdh rearpeiadslyµ adti dTan aobtovvaer 4y0a0t °aCll,,
washaelrseoassh μo wdind innoFt ivgaurrye a8ta ,abl.l, Fausr athlseor mshoorwe,nn ivna Fluigeusrbee 8gaa,nb.t oFudretchreeramseoraet, lno wvaelruTes vbaelguaens tfoo rdZecnrOea:Asel
a
afitl mloswwerit hTah vigahlueersA folcr oZnnteOn:tAs,l rfeilsmulst iwngithin hnigvhaelur eAsla cfotenrtethnetsh, irgehsu-Tltainagn nine anli nvgaltureesa tamfteenr ttshteh hatigwhe-Trea
ainndneepaelinndge tnrteaotfmtheentAs lthcoant twenert:e 5in.0d–e5p.3en×d1e0n2t0 ocfm th−e3 Afol rcZonntOen:At:l 5(.20.–05a.3n d× 110.020w cmt%−3 )fofirlm ZsnO(T:A=l (520.00 a◦nCd),
a
12..08 –w2.t9 %×) 1fi0l2m0sc m(T−a 3= f5o0r0Z °nCO),: A2.l8(–22.0.9, 1×. 01,0a20n cdm0−.35 fwort Z%n)Ofil:Amls ((2T.0a,= 1.600, 0an◦Cd) 0,.a5n wdt2 %.0–) 2f.i4lm×s 1(T02a0 =c 6m0−0 3°Cfo)r,
aZnndO 2:A.0l–(22.4.0 ×, 11.002,00 c.5m,−a3n fdor0 Z.2n5Ow:At%l ()2.fi0l,m 1.s0(, T0a.5=, a6n5d0 0◦C.2)5. wt %) films (Ta = 650 °C).

Figure 8. Changes in carrier density and mobility in (a) ZnO:Al (2.0 wt %) and (b) ZnO:Al (0.25 wt %)
Figure8.Changesincarrierdensityandmobilityin(a)ZnO:Al(2.0wt%)and(b)ZnO:Al(0.25wt%)
ffiillmmss aass aa ffuunnccttiioonn ooff ssppeecciimmeenn tteemmppeerraattuurree dduurriinnggp poosstt--aannnneeaalliinnggt trreeaattmmeenntstsa atta aP PO2 ooff 11 ×× 1100−−232 3atamtm. .
O2
TThhee oobbsseerrvveedd cchhaannggeess iinn nn aanndd µμ wweerree ddeeppeennddeenntt oonn bbootthh TTa aanndd PPO2.. FFiigguurree 1100 sshhoowwss rroooomm
a O2
tteemmppeerraattuurree vvaalluueess ooff ρρ,, nn,, aanndd µμ ffoorr ZZnnOO::AAll ((00..2255 aanndd 22..00 wwtt %%)) fifillmmss ppllootttteedd aass aa ffuunnccttiioonn ooff TTa aatt
a
PPO2 vvaalluueess ooff 11 ×× 1010−2−3 2a3nadn 1d ×1 1×0–41 0a–t4ma (tFmig(uFrigeu 1r0ea1) 0aan)da nthdet ZhenOZn:AOl: A(0l.5( 0w.5t w%t) %fil)mfislm psospto-astn-naneanleeadl eadt
O2
6a0t06 0°C0 ◦aCt vaatrivoaursio PuOs2 Pvaluevsa l(uFiegsu(rFei g10ubre). 1F0igbu).reF 1ig0ua riend1i0caatiensd tihcaatt ethset hμa vtathlueeµs fvoar ltuhees ZfonrOt:hAel (Z0n.2O5 :wAtl
O2
%(0.)2 f5ilwmt i%nc)rfielamseidn cornelays eadftoenr lpyoasftt-earnpnoesatl-ianngn aeta llionwg aPtOl2o. wInP add.iItnioand, dthiteio nn ,vtahleuensv faolru ethsef oZrnthOe:AZln (O0:.A25l
O2
and 2.0 wt %) films decreased significantly after annealing at a higher PO2. Figure 10b shows the
results for two films that were post-annealed at 600 °C at different PO2 values. One film was annealed
at a PO2 of 1 × 10−23 atm (dotted lines), while the other film was annealed at a PO2 of 1 × 10−4 atm,
followed by an annealing treatment at a PO2 of 1 × 10−23 atm and a subsequent annealing treatment at
Materials2017,10,141 8of18
(0.25and2.0wt%)filmsdecreasedsignificantlyafterannealingatahigherP . Figure10bshowsthe
O2
resultsfortwofilmsthatwerepost-annealedat600◦CatdifferentP values. Onefilmwasannealed
O2
Materials 2017, 10, 141 8 of 17
ataP of1×10−23atm(dottedlines),whiletheotherfilmwasannealedataP of1×10−4atm,
O2 O2
foa lPloOMw2 aoteefdr i1abl s× y2 01a107n−, 41a 0an,t 1nm4e1 a (slionlgidt rleinaetms).e Bnottaht afilPmOs2 eoxfh1ib×ite1d0− s2im3ailtamr tarnadnsapsourtb sperqoupeenrttieasn nafetaelri nthget raena8nt omefa 1el7in ntga t
atrqtrePueaOaatta2strm imePo-aeeOfetq2nm1n uotts×efisl n1i abt 1a×tsr0t it1 u−ah0tm4th−e 4ae tassh tttameasmm ate (ms(essas.omeo Pl l ieiOdPd 2O Pll2ii(O nn12(e e1ss×( )1).. × B1 B×o0 1ot−10th0h−2 2−3f32ifi 3l omlomorrsr s1 e11xe × xh××hi b 1i1ib10t00ei−t−−de444 dsaaaitmsttmmmimi)l)),a, i, rlta tththhrreaeetrnrrereaesbbpnbyyos yp rrterro feeprlffletrlocepetpccirntetoigirnpnt gieget rhsttt eihahierfeest ieirtarrh f tettthehhrmeree rtaromhmndenoyoeanddanaylymninnneaigcaam lmiinci gc
quasi-equilibriumstates.
quasi-equilibrium states.

FigFuirgeu 9r.e R 9o. Romoo-mte-mtepmepreartautruer ere rseissitsitvivitiyty, ,c caarrrriieerr ddeennssiittyy,, aanndd mmoobbiliiltiyty i nin Z ZnnOO:A:Al (l0 (.205.2, 50,. 50,. 15.,0 1, .a0n, dan 2d.0 2.0
Figure9.Room-temperatureresistivity,carrierdensity,andmobilityinZnO:Al(0.25,0.5,1.0,and2.0wt%)
fiwlmt %swat) s%fial)m ffuislnm acsst iaaos nf auo nffucantincotnnioe noa lfoi naf ngannteenamelaiplniengrg at ettuemmrepp.eePrraoatstuut-rraeen.. PnPeooasstlt-i-naangnnnweeaaalsliinnpgge wr wfaoasr smp peeredfrofarotmramedPe dat aaot fPa1O P2 ×Oo2f 1o10 f×− 1 12 ×30 −a123t0 m−23.
atma.t m. O2

Figure 10. Room-temperature resistivity, carrier density, and mobility in (a) ZnO:Al (0.25 and 2.0 wt
%) films plotted as a function of annealing temperature at PO2 values of 1 × 10−23 and 1 × 10−4 atm and
Figure 10. Room-temperature resistivity, carrier density, and mobility in (a) ZnO:Al (0.25 and 2.0 wt
Figu(bre) t1w0.o RZonoOm:A-tle (m0.5p ewrta t%u)r efilrmessi sptoivsti-tayn,cnaerarlieedr adte 6n0s0i t°yC, aunnddemr ovbariliiotuysi nPO(2a v)aZluneOs.: Al(0.25and2.0wt%)
fi%lm) fsilpmlost pteldotatesda afsu na cftuionnctoiofna nonf eaanlnineaglitnemg tpeemrapteurraetuarteP at PvOa2 lvuaelsuoefs 1of× 1 1×0 1−02−323a anndd1 1× × 1100−−4 4atamtm anandd
O2
(b(b))t Twtwoo oeZ lZunncOOid:A:Aaltle( ( 00t.h.55e w wottr %i%g)i)n fif ilolmmf stsh ppeoo scstht--aaannnngneeeasa llieendd naa tta 66n00d00 μ◦°CC, w uuenn ddceherra vrvaaacrritioeouruiszs ePPdO2 tvhvaela usluetrseu.s c.tural properties of
O2
the films using TEM and XRD, and the chemical properties using X-ray photoelectron spectroscopy
To elucidate the origin of the changes in n and μ, we characterized the structural properties of
(XPS) and thermal desorption spectroscopy (TDS). Figure 11 displays plan-view TEM images of the
the afsil-mdesp uossiitnegd TanEdM p aonstd-a XnRneDa,l eadn Zd ntOhe:A clh (e0m.25ic aanl dp r2o.0p ewrtt i%es) ufislimnsg aXt -6r5a0y °pCh. oMtooeslte gctrraoinn bsopuenctdraorsiceos py
(XPS) and thermal desorption spectroscopy (TDS). Figure 11 displays plan-view TEM images of the

as-deposited and post-annealed ZnO:Al (0.25 and 2.0 wt %) films at 650 °C. Most grain boundaries
Materials2017,10,141 9of18
Toelucidatetheoriginofthechangesinnandµ,wecharacterizedthestructuralpropertiesof
Mattehriealsfi 2lm017s, u10s,i n14g1 TEMandXRD,andthechemicalpropertiesusingX-rayphotoelectronspectrosco9p yof 17
(XPS)andthermaldesorptionspectroscopy(TDS).Figure11displaysplan-viewTEMimagesofthe
obsaesr-vdeedp oinsi ttehde afinlmdps owste-raen [n0e0a1le] dtilZt-ntOyp:Ae lb(o0u.2n5daanrdie2s. 0wwitht %a )[0fi0lm1]s raotta6t5i0on◦C a.nMgloes. tTghriasi nisb boeucnaduasrei ebsoth
filmosb seexrhviebditiendt hc-eaxfiilsm psrwefeerrere[d00 o1r]ietinltt-atytipoensb,o ausn idnadriiecsatwedit hbya t[h0e0 1F]WroHtaMtioωn a nogfl ec.a.T 2h°i–s3i°s (bFeicgauursee 3).
002
Figbuorteh 1fi1lam,bs ecxlehairbliyte dshco-awxsis tphraetf ebrorethd oorfi etnhtea tiaosn-ds,epasosiintdedic aftielmd sb ycothnetaFinWedH Mstωra0i0n2eodf rceag.io2n◦s– 3t◦hat
pro(pFiagguarteed3) .toFwigaurrde 1th1ae, bgrcaleinar ilnytsehrioowr sfrtohmat bthoeth GoBfsth. eThase- dsterpaoinsietded rfieglmiosncso anrtea iansesdusmtreadin teod hreagvieo nbseen
thatpropagatedtowardthegraininteriorfromtheGBs. Thestrainedregionsareassumedtohavebeen
produced by unavoidable orientation mismatches between adjacent columns during crystal growth.
producedbyunavoidableorientationmismatchesbetweenadjacentcolumnsduringcrystalgrowth.
After the post-annealing treatment, many of the boundaries appeared flat and located parallel to the
Afterthepost-annealingtreatment,manyoftheboundariesappearedflatandlocatedparalleltothe
c-axis at least within a thickness of the TEM foil for both films (Figure 11c,d). This feature may be
c-axisatleastwithinathicknessoftheTEMfoilforbothfilms(Figure11c,d). Thisfeaturemaybe
attributed to grain growth that reduces the grain boundary area to achieving energetically favorable
attributedtograingrowththatreducesthegrainboundaryareatoachievingenergeticallyfavorable
boundaries. Indeed, the FWHM slightly decreased with T . Figure 12 shows the a- and c-axis
100 a
boundaries.Indeed,theFWHM slightlydecreasedwithT .Figure12showsthea-andc-axislengths
100 a
lengths and cell volume (Figure 12a) along with FWHMω , FWHM , and FWHM (Figure 12b)
002 002 100
andcellvolume(Figure12a)alongwithFWHMω ,FWHM ,andFWHM (Figure12b)forthe
002 002 100
forp tohset -paonsnte-aanlendefiallemds failtmPs atv PalOu2 evsaolufe1s× of1 10 −×2 010a−n20d a1nd× 11 0×− 140a−4t matmas aasf au nfuctniocntioonf Tof .TTa.h Tesheesvea lvuaelsues
O2 a
were strongly dependent on the Al content as a result of the strong influence of T and thickness on
were strongly dependent on the Al content as a result of the strong influence of Tg and thickness
g
the structural properties, as discussed in Section 2.1. Figure 12b shows that the FWHM and
onthestructuralproperties, asdiscussedinSection2.1. Figure12bshowsthattheFWHM 0a02nd
002
FWFHWMH1M00 decdreeacrseeads ewditwhi Ttha aTboavbeo v50e05 0°C0,◦ wC,itwhi tthhet hsaemsaem cehachnagnegs eosbosbersverevde dat abtobtoht hlolwow anadn dhihgihg hPO2.
100 a
FurPthe.rFmuorrthee, rnmoo rseig,nnoifisciagnnitfi dcaifnfterdeinffceeresn icne stihnet hoethoethr emrmeaesausruerded vvaalluueess wweerree ddeetetectcetdedb ebtewteweenetnh ethe
O2
filmfislm psopsto-satn-annenaeleadle data ltolwow anandd hhigighh PPOO2.2 .InIn ccoonnttrraasstt,, tthhee μµ aanndd nn vvaalluueesss hshoowwnnin inF iFgiugruer1e0 1s0t rsotnrognlygly
depdeenpdenedde odno PnOP2.O T2.hTehreefroerfoer, et,hteh ededcerceraesaese inin ththee ssttrraaiinneedd rreeggiioonn aarroouunnddt htheeG GBsBsa nadnda sal isglhigthint cirnecarseease
in tihnet hine-ipnl-apnlaen cerycrsytasltlailtlei tseizsiez einidndiciactaetded inin FFigiguurreess 1111 aanndd 1122 wweerree nnoottr reessppoonnsisbilbelefo frotrh tehien icnrecareseasien iµn. μ.
AdAdditdioitnioanllayl,l yt,hteh deidffifefreernent tnn vvaalulueess pprroodduucceedd bbyy ppoosstt--aannnneeaalilningga tadt idffieffreernetnPtO P2Ov2 avlauleuseasr earneo ntodtu deutoe to
chacnhgaensg eins itnheth setrsutrcutuctruarla plrporoppeertriteiess oobbsseerrvveedd bbyy TTEEMM aannddX XRRDDa annalaylsyissi.s.

FigFuirgeu 1re1.1 P1.laPnla-vni-evwie wTETMEM imimagaegse soof fththee aass--ddeeppoossiitteedd fifillmmss ooff( (aa))Z ZnnOO:A:All( 0(.02.525w wt%t %)a)n adn(db )(bZ)n ZOn:AOl:Al
(2.0( 2w.0t w%t)%; a)n;adn pdopsot-sat-nannenaelaelded fifilmlms soof f((cc)) ZZnnOO::AAll ((00..2255 wwtt %%)) aanndd( d(d))Z ZnnOO:A:Al(l2 (.20.w0 wt%t )%fi)l mfilsmast 6a5t 065◦C0 °C
at aa tPaO2P oOf2 1o f× 11×0−213 0a−tm23.a tm.
Materials2017,10,141 10of18
Materials 2017, 10, 141 10 of 17

Figure 12. Plots of (a) a- and c-axis lengths and cell volumes as a function of annealing temperature,
Figure12.Plotsof(a)a-andc-axislengthsandcellvolumesasafunctionofannealingtemperature,
anadn d(b()b F)WFWHHMM oof frorocckkiningg ccuurrvveess ffoorr 000022 ddiiffffrraaccttiioonn ppeeaakkss ((FFWWHHMMωω002)) aanndd FFWWHHMM ooff 000022a anndd1 01000
002
didffirffarcatcitoinon ppeeaakkss( F(WFWHMHM0,02F, WFHWMHM)1a00s) afausn cati onfuonfcatnionne aloinf gatenmnpeaelriantgu retfeomrpfielmrastupores t-afonrn eafillemds
002 100
poastta-aPnneaolfe1d× at1 a0 −P2O02 (osfo 1li d× 1li0n−e20s ()saonldido lfin1e×s)1 a0n−d4 (odf o1t t×e d10l−i4n (edso)tattemd .lines) atm.
O2
Conversely, high-temperature annealing at low PO2 is likely to change the chemical properties
Conversely,high-temperatureannealingatlowP islikelytochangethechemicalpropertiesof
of the films. Figure 13 shows XPS spectra for the sOu2rface regions (Figure 13a) and bulk regions
thefilms.Figure13showsXPSspectraforthesurfaceregions(Figure13a)andbulkregions(Figure13b)
(Figure 13b) of the as-deposited and post-annealed ZnO:Al films. Figure 13a indicates that the Al
oftheas-depositedandpost-annealedZnO:Alfilms. Figure13aindicatesthattheAlcontentsinthe
contents in the surface regions were significantly higher than in the bulk films for both
surfaceregionsweresignificantlyhigherthaninthebulkfilmsforbothpost-annealedZnO:Al(0.25and
po2s.0t-wantn%ea)lfieldm Zs.nTOh:eAcl o(n0.c2e5n tarnadti o2n.0s woftA %l,) Zfinlm,asn. dTOhe( Cconc(eant%tra),tiCons (oaft A%l),, Zannd, aCnd( Oat (%C)A)l e(satti m%a),t eCdZn
Al Zn O
(aftr %om), tahnedX CPOS (sapt e%ct)r)a easrteimsuamtedm farroizmed thine TXaPbSle sp1.eTcthrea raerseu sltusmsumgagreisztetdh aint mTainboler d1.e Tcohme preossuitlitosn soufgtgheest
that minor decomposition of the ZnO layer occurs in the surface region during high-temperature
ZnOlayeroccursinthesurfaceregionduringhigh-temperatureannealingatlowP . Thisisalso
O2
ansnuepaplionrgte datb lyotwh ePTOD2. STshpies citsr aaflosor tshuepZpnoOrt:eAdl fiblym tsh.eF iTgDurSe s1p4eschtroaw fsotrh tehTe DZSnsOp:eAclt rfailomfsd. eFsoigruprtieo n14
shows the TDS spectra of desorption species from the ZnO:Al (0.25 wt %) and ZnO:Al (2.0 wt %)
speciesfromtheZnO:Al(0.25wt%)andZnO:Al(2.0wt%)filmsgrownatT . Wemonitoredthe
opt
filmmass sgr(omw/nz) aotf T(aop)t.6 W4;e(b m)3o2n;iatnodre(dc )ththee mraatsios (bmet/wz)e eonf (tah)e 6t4w; o(.bH) 3e2re; ,aannd m(c/) zthvea lruaetioof b6e4twinecleund tehseo tnwlyo.
HZerne+, ,awnh mer/eza svaalnume o/fz 6o4f 3in2cilnucdluesd eosnblyo tZhnZ+,n w2+haenrdeaOs a+n. Tmh/ez boefh 3a2v iionrcolufdthees mbo/thz =Z3n22+ saignnda Ols2w+. eTrhee
2
beshimaviliaorr tooft hthoes emo/fzt h=e 3m2 /sizg=na6l4s swigenrea lssiwmiitlhara tcoo nthsotasnet orfa ttihoe, emsp/ze c=ia 6ll4y sfiogrnTalsb weloitwh a5 3c0o◦nCst,atnhte rreabtiyo,
s
esipnedciicaaltliyn gfotrh aTts mboeslotwof 5th3e0 m°C/,z t=he3r2esbiyg nianldsibcealtoinwg 5t3h0a◦t Cmroesflte cotf tthheed mes/ozr p=t i3o2n soifgZnanlsra btheelorwth a5n30O °C.
2
reIfnleccot ntthrea sdt,eastotrepmtipoenr aotfu rZens arbaothveer5 t3h0a◦nC ,Oth2.e Imn /cozn=tr3a2sst,i ganta tlesmshpaerrpaltyuirnecsr eaabsoedvew 5i3th0 T°sCa, ntdhee xmce/zed =e d32
sigthneallesv sehlaorfptlhye imnc/rzea=se6d4 swigintha lTs,s tahnerde beyxcieneddiceadti nthget hleavteOl 2owf tahsea lmso/zd =es 6o4rb seidg.nHalesn, cthe,edreebcoym inpdoisciatitoinng
thoaft tOhe2 fiwlmass ablesgoa dneastoTrbsevda.l uHeesnacbeo, vdee5c0o0m◦pCoisnitiaohni gohf tvhaec ufiulmms( ib.ee.g,a<n1 ×at 1T0s− v7aPluae).s above 500 °C in a
high vacuum (i.e., <1 × 10−7 Pa).