UNIVERSITÀ’+DEGLI+STUDI+DI+TRIESTE+
+++++ XXV +++ CICLO+DEL+DOTTORATO+DI+RICERCA+IN+
+ + +
NANOTECNOLOGIE+
+
+
+ +
MBE+growth+of+self?assisted+GaAs+nanowires+
and+
their+characterization+
+ + + +
Settore+scientifico?disciplinare:+ 02/B1+
++
+ +
DOTTORANDO+/+A+
+STEFANO+F.+AMBROSINI+
+
+ +
COORDINATORE+
+PROF.+MAURIZIO+FERMEGLIA+
+
+ +
SUPERVISORE+DI+TESI+
+DR.+SILVIA+RUBINI+
+
+
+
ANNO+ACCADEMICO+2011+/+2012+
+ +
+ + + + + + + + + + + +
Veni, Vidi, Vici +
Caius+Iulius+
De+bello+Gallico+ +
Abstract((ita)(
La+ricerca+nel+campo+dei+nanofili+appartiene+ancora+al+laboratorio+e+non+all’industria.+
Cionondimeno,+i+nanofili+sono+oggetto+di+grande+attrazione+nella+comunità+scientifica+
e+ nanotecnologica,+ sia+ per+ il+ loro+ elevato+ “aspect+ ratio”,+ che+ per+ le+ loro+ capacità+
semiconduttrici.+Il+primo+ne+fa+buoni+candidati+per+usi+di+sensoristica+e+interazione+
con+ la+ luce,+ le+ seconde+ potrebbero+ invece+ sfruttate+ per+ la+ miniaturizzazione+ di+
componenti+(opto)elettronici.+
Il+ GaAs+ è+ un+ materiale+ semiconduttore+ III?V+ con+ un+ band?gap+ diretto+ di+ 1,43+ eV+ a+
temperatura+ ambiente.+ Il+ suo+ band+ gap+ ne+ fa+ un+ materiale+ più+ pregiato+ del+ silicio,+
rispetto+ al+ quale+ ha+ una+ minore+ sensibilità+ al+ calore+ ed+ una+ maggiore+ mobilità+
elettronica.+ Tali+ pregi+ sono+ accompagnati+ da+ un+ prezzo+ maggiore,+ che+ oggigiorno+
limita+ l’uso+ del+ GaAs+ nei+ componenti+ optoelettronici+ a+ più+ alta+ qualità,+ come+ per+
esempio+ negli+ usi+ satellitari.+ La+ sintesi+ di+ nanofili+ di+ GaAs+ di+ alta+ qualità+ è+ una+
possibiità+intrigante+per+combinare+le+qualità+superiori+del+GaAs+con+il+minor+costo+
del+silicio+e+della+sua+tecnologia.+
L’epitassia+ da+ fascio+ molecolare+ è+ una+ tecnica+ per+ sintetizzare+ materiali+ con+ la+
massima+ purezza.+ Operando+ a+ pressioni+ minori+ di+ 1E
?10+ Torr,+ essa+ è+ ideale+ per+ la+
ricerca+ scientifica+ e+ per+ la+ sintesi+ di+ prodotti+ di+ altissima+ qualità.+ I+ prezzi+ di+ tali+
caratteristiche+sono+un’estrema+delicatezza+dell’apparato+ed+un+controllo+maniacale+
delle+condizioni+di+crescita+per+assicurare+riproducibilità+dei+risultati.++
All’inizio+ del+ 2009,+ la+ comunità+ di+ crescitori+ di+ nanofili+ era+ alle+ prese+ con+ alcune+
problematiche+riguardanti+la+scelta+di+un+materiale+diverso+dall’oro+per+assistere+la+
crescita+ e+ conseguentemente+ di+ un+ substrato+ acconcio;+ ulteriori+ complicazioni+ alla+
sintesi+ di+ nanofili+ di+ alta+ qualità+ è(ra)+ il+ politipismo+ cristallino+ dei+ nanofili+ di+ GaAs.+
L’uso+di+oro+nella+sintesi+di+nanofili+comporta+inquinamento+chimico+responsabile+di+
difetti+nel+materiale+e+la+presenza+di+nanoparticelle+di+oro+all’estremità+dei+nanofili+li+
nanofili+di+GaAs+su+GaAs+e+i+difetti+cristallini+erano+una+piaga+presso+che+impossibile+
da+eliminare.+
La+mia+tesi+di+dottorato+è+stata+votata+allo+studio+di+queste+due+problematiche.+
Abbiamo+ studiato+ un+ protocollo+ semplice+ per+ la+ realizzazione+ di+ substrati+ di+ GaAs+
capaci+di+ospitare+la+crescita+di+nanofili+di+GaAs+in+assenza+di+Au,+depositando+uno+
strato+ subnanometrico+ di+ silicio+ su+ substrati+ deossidati+ di+ GaAs+ ed+ espondendoli+
quindi+ all’aria.+ Più+ semplice+ ancora,+ la+ deposizione+ di+ Si+ direttamente+ su+ substrati+
GaAs+epiready+si+è+dimostrata+altrettanto+efficace.+
Abbiamo+ affrontato+ il+ problema+ del+ politipismo+ cristallino+ sia+ durante+ la+ crescita,+
investigando+le+relazioni+tra+parametri+di+crescita+e+struttura+cristallina+dei+nanofili,+
sia+dopo+la+crescita,+cercando+di+alterarla+tramite+riscaldamento.+
Abbiamo+così+scoperto+che+condizioni+ricche+in+As+durante+la+crescita+assistita+da+Ga+
favoriscono+ la+ struttura+ cristallina+ wurtzite,+ al+ contrario+ della+ più+ stabile+ struttura+
zincoblenda,+propria+del+GaAs+di+bulk.+Il+protrarsi+di+condizioni+ricche+in+As+portano+
quindi+alla+estinzione+della+nanoparticella+di+Ga.+
In+collaborazione+con+dr.+Jakob+B.+Wagner+(DTU+CEN,+Copenaghen,+Danimarca),+dott.+
mag.+ Damiano+ Cassese+ ed+ il+ gruppo+ di+ ricerca+ di+ dr.+ Marco+ Lazzarino+ (IOM+ CNR,+
Trieste,+ Italia)+ abbiamo+ scoperto+ che+ un+ periodo+ di+ riscaldamento+ di+ cento+ ore+ in+
flusso+ di+ As+ è+ capace+ di+ alterare+ la+ struttura+ wurtzite+ di+ nanofili+ assistiti+ Au+ in+
zincoblenda.+Tale+scoperta+è+stata+in+principio+fatta+tramite+microscopia+elettronica+
in+trasmissione+e+quindi+confermata+per+mezzo+di+spettroscopia+Raman.+
Abbiamo+ studiato+ la+ crescita+ di+ nanofili+ di+ GaAs+ in+ assenza+ di+ nanoparticella.+ In+
collaborazione+ con+ dott.+ mag.+ Giacomo+ Priante+ (IOM+ CNR,+ Trieste,+ Italia)+ e+ prof.+
Vladimir+G.+Dubrovskii+(SPBAU,+San+Pietroburgo,+Russia)+abbiamo+trovato+che+anche+
dopo+la+totale+consunzione+della+nanoparticella+di+Ga,+i+nanofili+possono+riprendere+
la+crescita+in+modo+assistito+da+Ga.++
Altri+studi+di+minore+entità+sono+stati+condotti,+come+la+crescita+di+nanofili+di+Si+su+
GaAs+senza+catalizzatore,+lo+studio+di+nanotubi+in+carbonio+contenenti+nanoparticelle+
di+ catalizzatore,+ lo+ studio+ di+ microscopia+ elettronica+ in+ trasmissione+ della+ crescita+
laterale+ dei+ nanofili,+ l’effetto+ di+ “optical+ limiting”+ esercitato+ da+ sistemi+ ibridi.+ I+
risultati+di+questi+studi,+stonando+nella+logica+della+tesi,+sono+presentati+in+appendice.+
Altresì,+in+appendice+è+raccolta+una+sistematica+della+crescita+di+nanofili+di+GaAs.+
Durante+i+beam+times+ad+ELETTRA,+il+Sincrotrone+di+Basovizza,+Trieste,+il+mio+lavoro+è+
stato+principalmente+votato+alla+sintesi+di+nanofili+di+GaAs+drogati+Si+sui+substrati+di+
cui+ sopra.+ I+ nanofili+ sono+ stati+ studiati+ con+ spettroscopia+ a+ raggi+ X+ risolta+
spazialmente+sulla+linea+di+luce+ESCAMICROSCOPY,+in+collaborazione+con+dr.+Mattia+
Fanetti+e+lo+staff+della+linea+di+luce.+
La+prima+sessione+di+esperimenti+ha+mostrato+che+i+nanofili+assisiti+da+Ga+sono+meno+
conduttivi+degli+assistiti+da+Au+per+lo+stesso+drogaggio+nominale.+La+seconda+tranche+
di+ esperimenti+ ha+ permesso+ la+ comprensione+ del+ meccanismo+ di+ incorporazione+
delle+impurezze+di+Si+durante+la+crescita+mediante+lo+studio+della+posizione+del+livello+
di+ Fermi+ nella+ band+ gap+ dei+ nanofili+ di+ GaAs.+ Abbiamo+ così+ capito+ che+ alterando+ il+
rapporto+ As/Ga+ e+ senza+ cambiare+ la+ temperatura+ di+ crescita+ (andando+ cioè+ a+
crescere+in+assenza+di+nanoparticella)+è+possibile+realizzare+eterogiunzioni+radiali+p?n+
di+alta+qualità,+con+la+stessa+quantità+di+drogante.+
Questa+ tesi+ di+ dottorato+ è+ organizzata+ come+ segue:+ ad+ ogni+ argomento+ principale+
(substrato,+politipismo,+ripresa+della+crescita,+spettroscopia+con+luce+di+sincrotrone)+è+
dedicata+un+capitolo.+In+ogni+capitolo,+l’introduzione+spiega+l’interesse+del+particolare+
argomento,+ lo+ stato+ dell’arte+ e+ la+ nostra+ soluzione+ ai+ problemi+ aperti.+ Seguono+ i+
nostri+risultati+con+discussione+ed+infine+le+conclusioni.+
Buona+lettura.+
+
( (
Abstract((eng)(
Although+still+belonging+to+the+laboratory+world,+nanowires+are+the+object+of+a+big+
attraction+in+scientific+community+for+their+exploitation+in+technological+devices.+Two+
of+the+most+alluring+aspects+of+nanowires+are+their+high+aspect+ratio,+which+makes+
them+ candidates+ for+ sensing+ uses+ as+ well+ as+ light+ harvesting,+ and+ their+
semiconducting+ nature,+ that+ puts+ forward+ their+ employment+ for+ nanoscale+
transistors+and+logic+units.++
GaAs+is+a+well?known+III?V+semiconductor+material+with+a+direct+band?gap+of+1.43+eV+
at+room+temperature.+It+has+better+properties+than+Si,+such+as+a+large+direct+band?
gap,+which+also+provides+lower+sensibility+to+heating,+and+higher+electron+mobility.+
Despite+its+higher+cost,+GaAs+is+nowadays+commonly+used+for+highest+performance+
logic+ units+ and+ optoelectronics.+ The+ synthesis+ of+ high+ quality+ GaAs+ nanowires+ and+
their+implementation+on+Si+technology+encourages+the+idea+of+putting+together+the+
superior+GaAs+with+the+cheaper+Si.+
Molecular+beam+epitaxy+is+a+growth+technique+that+offers+the+highest+possible+purity+
by+ means+ of+ ultra+ high+ vacuum+ growth+ regime+ (base+ pressure+ <+ 1E
?10+ Torr).+ It+ is,+
therefore,+ ideal+ for+ both+ research+ and+ high+ quality+ products.+ Prices+ of+ such+
advantages+are+an+extreme+delicateness+of+the+setup+and+a+maniacal+control+of+the+
growth+conditions,+to+ensure+reproducibility+in+scientific+results.++
At+the+beginning+of+2009,+the+nanowire+grower+community+was+facing+some+issues+
such+as+the+choice+of+a+material+different+from+gold+assisting+the+nanowire+growth,+
the+ growth+ substrate+ and+ the+ crystal+ polytypism.+ The+ use+ of+ Au+ as+ the+ assistant+
nanowire+ growth+ material+ had+ two+ main+ drawbacks.+ Firstly,+ the+ verified+ pollution+
with+ Au+ of+ the+ growing+ nanowires+ would+ create+ defects+ in+ the+ material;+ secondly,+
the+presence+of+Au+nanoparticles+at+the+nanowire+free+end+would+make+them+non?
compatible+ with+ Si?based+ technology.+ Focusing+ on+ GaAs,+ there+ was+ no+ simple+
protocol+for+the+substrate+treatment+in+order+to+trigger+the+Au?free+one?dimensional+
nanowire+growth+on+GaAs.+Nanostructure+crystal+defects+were+almost+impossible+to+
avoid.++
My+Ph.D.+work+has+been+devoted+to+the+study+of+these+problems.++
We+studied+a+simple+protocol+to+obtain+GaAs+nanowires+on+GaAs+by+MBE.+We+proved+
that+a+sub?nanometric+Si+layer+epitaxially+deposited+on+the+deoxidized+GaAs+surface+
followed+ by+ atmospheric+ oxidation,+ or+ direct+ Si+ deposition+ onto+ epiready+ GaAs+
wafers+is+able+to+trigger+Ga+assisted+GaAs+nanowire+growth.+
We+ tackled+ the+ nanowire+ polytypism+ issue+ both+ in+ a+ “during+ growth”+ action,+ by+
studying+the+effects+of+the+growth+parameters+on+the+nanowire+crystal+structure;+and+
with+ an+ “after+ growth”+ action,+ by+ attempting+ to+ change+ the+ nanowire+ crystal+
structure+by+thermal+annealing+after+the+growth+itself.+
In+ particular+ we+ found+ that+ As+ rich+ conditions+ during+ Ga+ assisted+ GaAs+ nanowire+
growth+leads+to+a+favored+wurtzite+structure+as+compared+to+the+default+zincblende+
lattice+of+the+bulk+GaAs+material.+Eventually,+As+rich+conditions+lead+to+the+total+Ga+
nanoparticle+consumption.+
In+ collaboration+ with+ dr+ Jakob+ B.+ Wagner+ (DTU+ CEN,+ Copenhagen,+ Denmark),+ dott.+
mag.+ Damiano+ Cassese+ and+ the+ research+ group+ of+ dr.+ Marco+ Lazzarino+ (IOM+ CNR,+
Trieste,+Italy),+we+discovered+that+a+100+hours+long+nanowire+annealing+step+under+
As+ flux+ is+ capable+ of+ transforming+ the+ wurtzite+ structure+ of+ Au+ assisted+ GaAs+
nanowires+ in+ zincblende.+ This+ was+ carried+ out+ by+ means+ of+ transmission+ electron+
microscopy+and+was+double+checked+by+means+of+Raman+spectroscopy.++
We+studied+the+growth+of+GaAs+nanowires+in+absence+of+growth+assisting+droplet.+In+
collaboration+with+dott.+Giacomo+Priante+(IOM+CNR,+Trieste,+Italy)+and+prof.+Vladimir+
G.+ Dubrovskii+ (SPBAU,+ Saint+ Petersburg,+ Russia),+ we+ found+ that+ after+ the+ total+
consumption+ of+ the+ Ga+ nanoparticle,+ GaAs+ nanowires+ can+ resume+ the+ Ga+ assisted+
growth.+
Other+ minor+ projects+ such+ as,+ the+ MBE+ growth+ of+ self+ catalyzed+ Si+ nanowires+ on+
When+being+assigned+beam+time+at+the+ELETTRA+Synchrotron+facility+in+Trieste,+my+
work+ has+ mainly+ been+ the+ synthesis+ of+ Si?doped+ GaAs+ NWs+ on+ our+ innovative+
substrates.+Nanowires+were+then+studied+in+spatially+resolved+X?ray+photoemission+
spectroscopy+ experiments+ on+ the+ ESCAMICROSCOPY+ beamline,+ performed+ in+
collaboration+with+dr.+Mattia+Fanetti+and+the+ESCAMICROSCOPY+staff.++
The+ first+ measurement+ set+ showed+ that+ Ga+ assisted+ nanowires+ exhibit+ a+ much+
weaker+ conductivity+ than+ their+ Au+ catalyzed+ counterparts,+ for+ the+ same+ two?
dimensional+nominal+doping.+The+second+set+of+experiments+gave+details+about+the+
incorporation+ of+ Si+ in+ nanowires+ during+ the+ growth,+ by+ the+ detection+ of+ the+ Fermi+
level+in+the+energy+gap.+We+realised+that+by+tuning+the+As/Ga+flux+ratio+without+need+
of+changing+the+growth+temperature++(i.e.,+shifting+from+vapor?liquid?solid+to+vapor?
solid+ regime)+ and+ for+ the+ same+ impurity+ flux,+ core?shell+ p?n+ junctions+ could+ be+
synthesized,+with+a+high+uniformity+grade+along+the+whole+nanowire+length.+
This+ Ph.D.+ thesis+ is+ organized+ as+ follows:+ to+ each+ main+ topic+ (substrates,+ nanowire+
polytypism,+ nanowire+ growth+ resumption,+ synchrotron+ radiation+ experiments)+ a+
thesis+chapter+is+dedicated.+In+each+chapter,+an+introduction+explains+the+motivation+
and+interests+of+the+given+topic,+illustrates+the+state+of+the+art+and+demonstrates+our+
strategy+to+the+solution+of+the+open+problems.+Prior+to+the+conclusion,+our+results+are+
presented+and+discussed.+
Have+a+good+reading.+
+
+ +
+ +
(
1( ( ( (
(
Introduction( pag( 1(
(
1.1( ( (
(
Nanowire(generalities( pag( 2(
(
1.2( ( (
(
Nanowire(synthesis( pag( 4(
(
1.3( ( (
(
Nanowire(characterization( pag( 4(
2( ( ( (
(
Si@treated(substrates( pag( 5(
(
2.1( ( (
(
Motivation(and(interests( pag( 5(
(
2.2( ( (
(
State( pag( 6(
(
2.3( ( (
(
Our(strategy( pag( 7(
( (
2.3.1(
( (
Epitaxial(substrates( pag( 8(
( (
2.3.2(
( (
Epiready,substrates, pag( 9(
(
2.4( ( (
(
Experimental(results( pag( 10(
( (
2.4.1(
( (
Si
2p(region( pag( 11(
( (
2.4.2(
( (
As3d(region( pag( 13(
( (
2.4.3(
( (
Ga3d(region( pag( 15(
(
2.5( ( (
(
Discussion( pag( 17(
( (
2.5.1(
( (
Epitaxial(substrates, pag( 17(
( (
2.5.2(
( (
Epiready,substrates, pag( 17(
( (
2.5.3(
( (
Si(thickness( pag( 18(
(
2.6( ( (
(
Effectiveness(of(Si@treated(substrates( pag( 19(
(
2.7( ( (
(
Conclusions( pag( 20(
3( ( ( (
(
GaAs(nanowire(polytypism( pag( 21(
( (
3.0.1(
( (
Introduction( pag( 21(
( (
3.0.2(
( (
Motivation(and(interests( pag( 22(
( (
3.0.3(
( (
Our(strategy( pag( 23(
(
3.1( ( (
(
Influence(of(the(growth(parameters( pag( 24(
( (
3.1.1(
( (
Substrate(material( pag( 24(
( (
3.1.2(
( (
Catalyst(material( pag( 25(
( ( ( ( 3.1.2.1( Vapor(liquid(solid(growth( pag( 26(
( ( ( ( 3.1.2.2( Au@assisted(growth( pag( 28(
( ( ( ( 3.1.2.3( Ga@assisted(growth( pag( 29(
( ( ( ( 3.1.2.4( Summary( pag( 31(
( (
3.1.3(
( (
Role(of(the(material(fluxes( pag( 32(
( ( ( ( 3.1.3.1( Our(strategy( pag( 33(
( (
3.1.4(
( (
Substrate(orientation( pag( 32(
( ( ( ( 3.1.4.1( Droplet@terminated(nanowires( pag( 35(
( ( ( ( 3.1.4.2( Pyramid@terminated(nanowires( pag( 35(
Summary'
State(of(the(Art(
( ( ( ( 3.1.4.3( Local(fluctuations(of(the(growth(parameters( pag( 35(
( ( ( ( 3.1.4.4( Consequences(on(the(crystal(structure( pag( 37(
( ( ( ( (3.1.4.5( Summary( pag( 39(
( (
3.1.5(
( (
Role(of(the(growth(temperature( pag( 40(
(
3.2( ( (
(
Post(growth(processes( pag( 41(
( (
3.2.1(
( (
Motivation(and(interests( pag( 41(
( (
3.2.2(
( (
State(of(the(art( pag( 42(
( (
3.2.3(
( (
Our(strategy( pag( 43(
( ( ( ( 3.2.3.1( Why(TEM?( pag( 44(
( ( ( ( 3.2.3.2( Why(600(°C?( pag( 44(
( (
3.2.4(
( (
TEM(results( pag( 45(
( ( ( ( 3.2.4.1( First(experiments( pag( 45(
( ( ( ( 3.2.4.2( Similar(ndipendent(experiments( pag( 47(
( ( ( ( 3.2.4.2( Further(experiments( pag( 48(
( (
3.2.5(
( (
Raman( pag( 49(
( ( ( ( 3.2.5.1( Why(Raman?( pag( 49(
( ( ( ( 3.2.5.2( Our(strategy( pag( 50(
( ( ( ( 3.2.5.3( Raman(results( pag( 50(
( ( ( ( 3.2.5.4( Single(nanowire(issues( pag( 51(
( (
3.2.6(
( (
Conclusions( pag( 52(
4( ( ( (
(
GaAs(nanowire(growth(resumption( pag( 53(
(
4.1( ( (
(
Ga(nanoparticle(consumption( pag( 53(
(
4.2( ( (
(
Non@assisted((vapour(solid)(GaAs(nanowire(growth( pag( 56(
( (
4.2.1(
( (
Motivation(and(interests( pag( 56(
( (
4.2.2(
( (
State(of(the(art( pag( 57(
( (
4.2.3(
( (
Our(strategy( pag( 60(
( (
4.2.4(
( (
Discussion( pag( 63(
( (
4.2.5(
( (
Conclusions( pag( 64(
(
4.3( ( (
(
Ga@assisted(GaAs(nanowire(growth(resumption( pag( 65(
( (
4.3.1(
( (
Experimental(results( pag( 65(
( (
4.3.2(
( (
Morphology(result( pag( 66(
( (
4.3.3(
( (
Radial(and(Axial(rowth(rate( pag( 68(
4.3.4( Crystal(structure( pag( 70(
5( ( ( ( (
Scanning(photoemission(spectroscopy( pag( 81(
(
5.1( ( (
(
Introduction( pag( 81(
(
5.2( ( (
(
Motivation(and(interests( pag( 81(
(
5.3( ( (
(
What(is(SPEM?( pag( 84(
(
5.4( ( (
(
How(is(SPEM(performed?( pag( 85(
(
5.5( ( (
(
Why(SPEM?( pag( 86(
( 5.6( ( (
(
pag( 87(
( ( ( pag(
( ( 5.6.1( ( ( Previous(experiments(on(Au@assistes(GaAs(nanowires( pag( ( 87(
( ( 5.6.2( ( ( Our(Strategy( pag( 88(
( (
5.6.3(
( (
Ga@assisted(nanowires( pag( 88(
( (
5.6.4(
( (
Ga@assisted(versus(Au@assisted(nanowires( pag( 90(
( (
4.6.5(
( (
Effect(of(Si@doping( pag( 92(
(
5.7( ( (
(
Fermi(energy(within(GaAs(band@gap(in(nanowires( pag( 93(
( (
5.7.1(
( (
Axial(junctions( pag( 96(
( (
5.7.2(
( (
Doping(and(growth(mode( pag( 98(
(
5.8( ( (
(
Discussion( pag( 102(
(
5.9( ( (
(
Conclusions( pag( 103(
6( ( ( (
(
Conclusions(and(open(points( pag( 104(
(
6.1( ( (
(
Summary(of(the(results( pag( 104(
(
6.2( ( (
(
Open(points(and(future(research( pag( 105(
Appendix(A( Systematics(of(GaAs(nanowire(growth( pag( 106(
(
A.0( ( (
(
Preface( pag( 106(
(
A.1( ( (
(
Nucleation(stage( pag( 107(
( ( ( A.1.0.1(
(
Motivation(and(interests( pag( 107(
( ( ( A.1.0.2(
(
Si(surfaces( pag( 107(
( ( ( A.1.0.3(
(
GaAs(surfaces( pag( 109(
( ( ( A.1.0.4(
(
Our(strategy( pag( 109(
( (
A.1.1(
( (
Experimental(results( pag( 110(
( ( ( A.1.1.1(
(
XPS( pag( 110(
( ( ( A.1.1.2(
(
GaAs(deposition( pag( 113(
( (
A.1.2(
( (
Conclusions( pag( 114(
(
A.2( ( (
(
Nanowire(systematics(on(Si@treated((100)(substrates( pag( 115(
( (
A.2.0(
( (
Introduction(and(experimental(details( pag( 115(
( (
A.2.1(
( (
Nanowire(growth( pag( 115(
( (
A.2.2(
( (
Growth(rate( pag( 118(
( (
A.2.3(
( (
Epiready,substrates, pag( 120(
((Contactless(conductivity(measurements((
((on(GaAs(nanowires(
( (
A.2.4(
( (
Comparison(of(epiready(and(epitaxial,substrates( pag( 120(
( (
A.2.5(
( (
Temperature(range( pag( 121(
( (
A.2.6(
( (
Structural(properties( pag( 122(
( (
A.2.7(
( (
Optical(properties( pag( 124(
( (
A.2.8(
( (
Conclusions( pag( 126(
(
A.3( ( (
(
Nanowire(systematics(on(Si@treated((111)B(substrates( pag( 127(
( (
A.3.1(
( (
Introduction( pag( 127(
( (
A.3.2(
( (
Experimental(details( pag( 127(
( (
A.3.3(
( (
Experimental(results( pag( 129(
( ( ( A.3.3.1(
(
Axial(growth(eate( pag( 129(
( ( ( A.3.3.2(
(
Radial(growth(rate( pag( 130(
( ( ( A.3.3.3(
(
Aspect(ratio( pag( 131(
( ( ( A.3.3.4(
(
Structural(properties( pag( 133(
( ( ( A.3.3.5(
(
Optical(properties( pag( 133(
( (
A.3.4(
( (
Conclusions( pag( 133(
Appendix(B( Catalyst(free(Si(nanowires(on(( pag( 134(
( (
B.1( ( (
(
Introduction( pag(
(
( ( (
(
epiready(GaAs((111)B(substrates( pag( 134(
(
B.2( ( (
(
Motivation(and(interests( pag( 134(
(
B.3( ( (
(
State(of(the(art( pag( 135(
(
B.4( ( (
(
Our(strategy( pag( 137(
(
B.5( ( (
(
Experimental(details( pag( 137(
(
B.6( ( (
(
Experimental(results( pag( 137(
( (
B.6.1(
( (
Growth(rate( pag( 138(
( (
B.6.2(
( (
Effect(of(As(flux(on(nanowire(morphology( pag( 139(
(
B.7( ( (
(
Discussion( pag( 140(
(
B.8( ( (
(
Open(points( pag( 141(
(
B.9( ( (
(
Conclusions( pag( 141(
Appendix(C(
( (
Linear(with(time(GaAs(shell(( pag( 142(
(
C.1( ( (
(
Introduction( pag( 142(
(
C.2( ( (
(
Motivation(and(interests( pag( 142(
(
C.3( ( (
(
Our(strategy( pag( 143(
C.4( Experimental(results( pag( 144(
Appendix(D(
Optical(limiting(as(result(a(of(photoinduced(electron(
transfer(in(( pag( 147(
hybrid(systems(with(CdSe/ZnS(quantum(dots,(C60,(and(
Perylene( pag(
( (
D.1( ( (
(
Introduction( pag( 147(
(
D.2( ( (
(
Motivation(and(interests( pag( 147(
(
D.3( ( (
(
State(of(the(Art( pag( 148(
(
D.4( ( (
(
Our(strategy( pag( 149(
(
D.5( ( (
(
Experimental(details( pag( 150(
(
D.6( ( (
(
Results(and(discussion( pag( 151(
(
D.7( ( (
(
Conclusions( pag( 155(
Appendix(E( Catalyst@embedded(multi@walled(carbon(nanotubes( pag( 156(
(
E.1( ( (
(
Introduction( pag( 156(
(
E.2( ( (
(
Motivation(and(interests( pag( 156(
(
E.3( ( (
(
Experimental(details( pag( 158(
(
E.4( ( (
(
Results( pag( 158(
( (
E.4.1(
( (
Theory(simulation( pag( 159(
( (
E.4.2(
( (
Overview( pag( 161(
( (
E.4.3(
( (
Morphology( pag( 162(
( (
E.4.4(
( (
Chemical(analysis( pag( 163(
( (
E.4.5(
( (
Tilt(series( pag( 164(
( (
E.4.6(
( (
Dark(field(imaging( pag( 165(
(
E.5( ( (
(
Conclusions( pag( 166(
Appendix(F( Experimental(instruments( pag( 167(
(
F.1( ( (
(
Molecular(beam(epitaxy( pag( 167(
( (
F.1.1(
( (
Riber(32P( pag( 168(
( (
F.1.2(
( (
Soviet(MBE(system( pag( 170(
(
F.2( ( (
(
Transmission(electron(microscope( pag( 172(
( (
F.2.1(
( (
Tecnai(G200( pag( 175(
( (
F.2.2(
( (
Libra(200( pag( 175(
( (
F.2.3(
( (
Jeof(2100F( pag( 176(
( ( ( ( ( ( ( (
Bibliography( ( ( ( ( ( ( ( ( (
( (
pag$ $$177$
((
+
1.#Introduction#
This! document! has! been! compiled! as! a! Ph.D.! thesis! for! the! Graduate! School! in!
Nanotechnology! located! in! Trieste,! Italy,! under! the! supervision! of! Dr.! Silvia!
Rubini.! The! objective! of! this! research! was! the! growth! and! characterization! of!
nanostructures,!mainly!nanowires.!The!main!interest!was!for!the!Au!free!growth!
of!GaAs!nanowires!by!means!of!molecular!beam!epitaxy.!
At!TASC!IOM!CNR,!in!Basovizza!(TS),!Italy,!the!core!of!the!growth!contribute!to!
my!Ph.D.!took!place!in!collaboration!with!my!supervisor,!dr.!Silvia!Rubini.!
I! spent! four! months! at! Danske! Technische! Universitet! (DTU)! –! Center! for!
Electron!Nanoscopy!(CEN)!and!DTU!Nanotech,!!located!in!Copenhagen,!Denmark.!
At! DTU,! I! worked! with! the! assistance! of! Dr.! Jakob! B.! Wagner,! under! the!
supervision! of! Prof.! Peter! Boggild.! The! objective! of! this! stay! was! the!
familiarization!with!transmission!electron!characterization!of!nanostructures,!in!
particular!GaAs!nanowires.!This!stage!proved!the!Ph.D.!research!to!be!worth!the!
title!of!Doctor!Aeuropeus.!!
A! final! eight! months! research! took! place! at! Saint! Petersburg! (SPB)! Accademic!
University!(AU)!SPB!State!University!(GU),!located!in!Saint!Petersburg,!Russia.!At!
SPBAU! I! focused! on! growing! GaAs! based! nanowires,! both! in! AuQ,! MnQ! and! GaQ!
assisted!experiments!under!the!supervision!of!Prof.!George!E.!Cirlin.!At!SPBGU!I!
mainly! performed! transmission! electron! microscopy! characterization! of! GaAs!
based! nanowires! in! collaboration! with! dr.! Evgenii! A.! Ubyivovk! under! the!
supervision!of!Prof.!Oleg!F.!Vyvenko.!
This! document! is! divided! in! chapters! plus! appendixes.! The! present! chapter!
introduces!my!work!by!explaining!the!physics!involved!in!GaAs!nanowires,!the!
motivation!and!interest!of!my!research!and!the!goals!we!defined!in!the!beginning.!
The!second!chapter!reports!in!detail!on!the!research!of!a!effective!substrate!to!
grow!GaAs!nanowires!in!absence!of!Au.!!!!!!!
The! third! chapter! is! dedicated! to! the! study! of! crystal! polytypism! of! GaAs!
nanowires.! It! has! been! divided! into! the! study! of! the! impact! of! the! growth!
parameters!on!the!nanowire!crystal!structure,!and!a!second!section!reporting!the!
research!performed!in!Copenhagen,!further!continued!in!Trieste!in!collaboration!
with! dott.! mag.! Damiano! Cassese! and! Dr.! Marco! Lazzarino’s! group! about! the!
possibility!to!modify!the!crystal!structure!after!the!growth.!
The!first!part!of!the!fourth!chapter!has!been!dedicated!to!the!study!of!the!growth!
of! GaAs! nanowires! in! AuQfree! and! GaQfree! mode,! which! is! called! vapourQsolid!
nanowire!growth.!Next,!!the!study!of!the!growth!resumption!in!GaQassisted!mode!
of! GaAs! nanowires! after! the! consumption! of! the! GaQassistant! nanoparticle! is!
analysed.! This! study! has! been! conducted! in! collaboration! with! dott.! mag.!
Giacomo!Priante.!
The! fifth! chapter! comprises! the! results! of! the! experiments! performed! on! GaAs!
nanowires! by! means! of! scanning! photoemission! spectroscopy! at! the! ESCA!
beamline!in!ELETTRA!synchrotron!facility!located!in!Trieste,!Italy.!This!chapter!
is!divided!into!three!sections.!First,!the!properties!of!GaQassisted!GaAs!nanowires!
are! compared! with! those! of! AuQassisted! GaAs! nanowires.! Then,! the!
characterisation! of! the! doping! concentration! by! means! of! the! measurement! of!
the! Fermi! energy! level! position! within! the! band! gap! of! GaAs! in! nanowires! is!
shown.! Finally,! the! consequences! of! the! growth! mode! (GaQassisted! and! non!
assisted)!on!the!dopant!incorporation!are!addressed.!
In! the! final! chapter! a! summary! of! the! results! is! presented.! In! addition,! the!
conclusions! of! the! research! along! with! the! consideration! of! open! areas! for!
further!research!are!underlined.!!
Results!that!could!not!fit!in!the!main!thesis!line!are!put!in!the!appendixes.!Such!
results!concern!growth!systematics,!the!study!of!other!kinds!of!nanostructures,!
as!carbon!nanotubes!or!molecular!systems.!
!
diameters!can!range!down!to!few!nanometers,!while!nanowire!length!can!easily!
range! to! several! tens! of! microns.! At! such! diameter! values,! quantum! effect!
becomes! important.! ! Therefore,! quantum! confinement! of! the! charge! carriers!
along!the!radial!direction!constitutes!one!of!the!two!most!interesting!properties!
of! nanowires.! Another! attractive! feature! of! nanowires! is! their! high! surfaceQtoQ volume!ratio,!that!allows!a!very!effective!interaction!with!light!and!make!them!
interesting!for!sensing!application.!
Nanowires! can! be! comprised! of! metals! and! semiconductors.! Semiconducting!
nanowires!in!particular!may!allow!the!miniaturization!of!electronic!devices,!and!
in!case!of!IIIQV!semiconductor!nanowires,!of!optoelectronic!devices.!
A!variety!of!different!IIIQV!nanowire!based!optoelectronic!device!prototypes!such!
as!photodetectors,!chemical!and!gas!sensors,!waveguides,!light!emitting!diodes,!
microcavity! lasers,! solar! cells,! and! nonlinear! optical! converters! have! been!
demonstrated.!
[1]! Moreover,! a! particular! interest! has! been! dedicated! to! the!
growth! of! Au! free! nanowires.! Taking! the! optoelectronic! integration! into!
consideration,! Au! free! GaAs! nanowires! showed! a! greater! internal! quantum!
efficiency!compare!to!Au!assisted!nanowires!of!the!same!material.!
[2]!
Effective!exploitation!of!IIIQV!based!nanowires!for!optoelectronics!would!occur!
for! structures! free! of! crystal! structure! defects,! which! were! proven! to! be!
detrimental!from!the!technological!point!of!view!
[3].!
In!conclusion,!the!synthesis!of!defect!free!and!Au!free!GaAs!nanowires!have!been!
the!main!objectives!of!this!Ph.D.!research.!
!
! !
1.2#Nanowire#synthesis#
Generally! speaking,! the! methods! to! obtain! nanowires! can! be! divided! into! two!
categories.! First,! topQdown! methods,! where! nanowires! are! fabricated! by!
reducing!the!size!of!pieces!of!material!down!to!the!nanoscale.!Such!processes!are!
brought! about! by! means! of! lithographic! techniques! or! electrophoresis.! Second,!
bottomQup! approaches,! that! exploit! the! selfQaggregation! of! atoms! to! produce!
nanowires.!Such!processes!took!place!by!means!of!suspension,!electrochemical!
deposition,!laser!ablation,!vapour!deposition!and!epitaxy.!
Among!the!other,!molecular!beam!epitaxy!was!the!technique!used!to!synthesize!
nanowires! for! the! present! thesis.! This! technique! will! further! be! discussed! in!!
Appendix!X.!!
1.3#Nanowire#characterization#
Characterization!of!nanostructures!can!be!structural,!morphological,!optical!and!
electrical.! This! thesis! covers! all! these! types! of! characterization.! Structural!
characterization! can! be! performed! by! means! of! XQray! diffraction,! Raman!
Spectroscopy,! transmission! electron! microscopy.! The! latter,! in! particular,! was!
used! during! this! thesis! work.! The! most! efficient! way! to! obtain! morphological!
information! on! the! nanowires! has! been! scanning! electron! microscopy.! Optical!
characterization!was!carried!out!by!means!of!photoluminescence!spectroscopy,!
while! some! electrical! characterization! was! performed! by! means! of! scanning!
photoemission!spectroscopy.!!
! !
2.#Si:treated#GaAs#substrates#
At!the!beginning!of!this!chapter,!motivations!that!drive!Au!free!nanowire!growth!
research! are! presented;! a! brief! literary! review! of! the! recent! years! interesting!
papers!is!then!given.!In!the!following,!I!will!point!out!the!prerequisites!to!grow!
GaAs! nanowires! on! GaAs! and! then! explain! our! strategy.! The! results! section!
begins! with! the! substrate! characterization! and! finally! their! effectiveness! in!
hosting!Au!free!GaAs!nanowires.!
!
2.1.#Motivation#and#interest#
Although!being!the!most!used!metal!to!assist!MBE!nanowire!growth!
[4],!Au!has!
some! drawbacks.! First,! it! was! shown! that! it! can! diffuse! in! nanowires! during!
growth!
[5,! 6].! Second,! the! wellQknown! incompatibility! of! Au! with! Si! has! to! be!
considered,!because!of!the!deep!traps!it!creates!in!the!Si!indirect!band!gap!
[7,8].!
Third! and! concerning! GaAs,! Au! assisted! nanowires! grow! to! a! temperature! at!
which!the!Ga!diffusion!length!becomes!rapidly!smaller!than!the!nanowire!length.!
This!might!be!a!source!of!unwanted!tapering!
[9,4].!
Fourth,!Au!assisted!GaAs!nanowires!present!a!usually!defected!crystal!structure,!
unless!special!attention!is!paid!to!the!growth!protocol!
[10].!
There! are! alternatives! to! Au.! Concerning! GaAs,! such! alternatives! employ! Ga! as!
metal! trigger! to! drive! the! nanowire! growth.! In! this! case,! nanowires! present!
strong! pros! in! all! the! quoted! cons! of! Au.! We! reported! a! good! example! of! Ga!
assisted!MBE!nanowires!properties!
[11].
!GaQassisted!GaAs!nanowires!by!MBE!are!almost!untapered,!have!a!close!to!phaseQ purity! crystal! structure,! grow! in! a! wide! temperature! range! and! have! excellent!
optical!properties.!
The! biggest! drawback! of! this! approach,! addressed! in! this! thesis! but! almost!
neglected! in! literature,! are! the! difficulty! to! control! nanowire! diameter! and!
density.!
2.2.#State#of#the#art#
Successful!examples!of!Au!free!MBE!GaAs!nanowire!growth!were!reported!on!Si!
[12Q16]!
and! on! GaAs!
[17Q19].! In! most! cases!
[12Q15]!authors! reported! the! presence! of!
native!oxides!on!the!silicon!substrate!surface!prior!to!nanowire!growth.!The!Si!
oxide! alters! the! adatom! diffusivity! on! the! surface! as! well! as! the! nucleation!
density.!Clean!Si!surfaces!hosted,!for!the!same!temperature,!a!greater!density!of!
NWs!as!compared!to!the!oxidized!Si!
[16].!In!the!same!paper!it!was!shown!that!the!
growth!temperature!range!for!the!best!NW!yield!was!also!different!on!the!two!
surfaces.!Clean!Si!surface!did!not!host!nanowires!at!660!°C!or!above.!
No!direct!GaAs!nanowire!growth!on!GaAs!has!been!reported!due!to!the!lack!of!
nucleation!sites!for!selective,!anisothropic!crystal!growth!as!opposed!to!layerQbyQ layer!twoQdimensional!growth!
[20].!Such!a!selectivity!has!been!shown!to!be!given!
for!instance!by!the!presence!of!a!thin!Si!oxide!layer!on!the!surface,!which!has!a!
lower! sticking! coefficient! than! the! underlying! GaAs!
[17].! .In! principle,! other! SiQ based! compound! masks! could! be! as! effective.! Possible! substitutes! to! SiO
x! are!
Si
3N
x! and! SiH
x.! The! X! chemical! pedices! indicates! the! possible! presence! of! Si!
suboxides!different!from!Si
4+![11].!!
In!the!years!immediately!following!the!present!research!work!though,!nanowire!
growth!on!clean!Si!surface!was!demonstrated,!so!the!interest!in!changing!SiO
x!as!
mask! material! lost! importance! and! was! then! abandoned.! Furthermore,! no!
detailed!study!of!the!Ga!sticking!coefficient!on!Si!compounds!was!ever!available!
in!literature.!
GaAs!nanowire!growth!was!reported!on!GaAs!substrates!coated!with!a!sputtered!
SiO
2! layer! 6! to! 30! nm! in! thickness! and! dipped! in! HF! aqueous! solution! before!
being!loaded!in!the!MBE!system.!
[17Q19].
!It!was!shown!how!a!too!thick!Si!oxide!layer!prevented!the!establishment!of!an!
clear.!It!was!proposed!that!SiO
x!become!mobile!above!530!°C!
[14,!19,!21].!!
It!was!observed!that!such!a!high!temperature!led!to!the!opening!of!small!craters!
in!the!SiO
x!adlayer!on!InAs!
[22],! where!the!indium!adatoms!would!coalesce!and!
give! rise! to! the! Au! free! NW! growth,! in! a! In! assisted! vapour! liquidQsolidQ mechanism.! The! actual! coalescence! of! In! nanoparticles! and! their! catalyst! role!
was! though! debated! because! of! the! absence! of! In! droplets! at! the! Au! free! InAs!
nanowires!at!the!end!of!the!growth
[23Q24].!
!It! will! be! shown! in! the! following! that! for! GaAs,! the! NW! density! is! almost!
insensible! to! any! annealing! procedure! prior! to! the! NW! growth.! The! nanowire!
density! increases! with! increasing! growth! temperature,! up! to! the! GaAs!
evaporation! temperature,! above! which! the! density! abruptly! falls! to! zero.!
Although! we! cannot! rule! out! a! temperature! dependent! nucleation! step,! we!
proved! that! temperature! annealing! by! itself! is! not! able! to! enhance! the! GaAs!
nanowire!density!on!GaAs.!
!!
2.3.#Our#strategy#
We! wanted! to! minimize! the! substrate! surface! treatment! to! achieve! Au! free!
nanowire!growth.!The!goal!was!to!obtain!a!surfaceQenergy!modulated!substrate,!
where! the! vaporQtoQsolid! reaction! activation! energy! would! be! lowered! by! the!
coalescence!of!Ga!nanoparticles.!
Our! road! to! the! growth! of! GaAs! nanowires! on! GaAs! did! not! employ! any!
sputtering! nor! chemical! etching,! nor! other! mechanical! or! chemical! processes.!
The! necessary! few! monolayer! thick! oxidized! silicon! layer! was! obtained! in! two!
methods:! epitaxial! deposition! of! Si! on! thermally! deoxidized! GaAs! wafers,!
followed!by!atmospheric!oxidation!(epitaxial!substrates),!or!epitaxial!deposition!
of!Si!on!epiready!(oxidized)!GaAs!wafers!(epiready!substrates).!The!last!method!
in!particular!showed!the!possibility!to!achieve!GaAs!nanowire!growth!totally!in, situ,,directly!on!commercially!available!GaAs!wafers,!and!therefore!proved!to!be!
the!simplest!way!ever,!to!grow!GaAs!nanowires!on!GaAs.!
The! key! points! of! our! preparation! method! were! two:! the! thickness! of! the!
deposited! Si! layer! and! the! oxidation! time! (not! available! into! account! in! the!
second! approach! of! ours).! The! Si! oxide! thickness! was! shown! to! be! crucial! to!
achieve!epitaxial!nanowire!growth!
[17Q18],!because!a!too!thick!adlayer!resulted!in!
loss!of!epitaxial!relation!between!nanowires!and!substrate.!On!the!contrary!GaAs!
deposition!on!clean!Si!(SiO
x!thickness!=!0)!led!to!epitaxial!2D!growth!as!opposed!
to! 1D! anisothropic! nanowire! growth!
[25].! Unfortunately! neither! of! these! two!
points! have! been! yet! completely! rationalized! and! only! empirical!
phenomenological!indications!can!be!given.!!
We!found!that!the!oxidation!time!seemed!to!be!more!or!less!non!influent!up!to!
one!week!after!the!substrate!preparation,!i.e.!growth!experiments!performed!on!
substrates! up! to! one! week! old! led! to! comparable! results! in! terms! of! nanowire!
density!and!length.!On!purpose!of!the!quantity!of!Si!deposited!on!the!wafers,!we!
found!a!small!range!of!thickness!values,!for!which!the!growth!experiments!led!to!
a! satisfactory! nanowire! yield.! Outside! such! extremes,! the! nanowire! density!
dropped,!very!likely!due!to!a!surface!chemical!environment!not!suitable!for!the!
nanowire!growth!process!to!take!place.!!
XQray! photoemission! spectroscopy! was! widely! used,! to! understand! what! were!
the! surfaces! that! led! to! dense! nanowire! population! and! what! were! not.!
Investigated!core!levels!were!(Ga
3p!,!Si
2p,!,!Ga
3d!as!well!as!!As
3d)!
!
! !
2.3.1.#Epitaxial(substrates#
Epitaxial,substrates!were!prepared!as!following.!nQtype!GaAs!(001)!wafers!(Si!
doped,!N
D=!1x10
16!cm
Q3)!were!outgassed!at!300!°C!in!a!buffer!modulus!before!
their!introduction!in!the!MBE!growth!chamber.!Substrates!were!then!thermally!
deoxidized!at!580!°C.!A!0,3QµmQthickQbuffer!GaAs!layer!was!then!grown!at!600!°C!
at!a!growth!rate!of!1!µm/h,!with!a!Ga!to!As!equivalent!beam!pressure!ratio!of!10!
to!12,!as!determined!by!means!of!an!ion!gauge!positioned!at!the!sample!location.!
After!the!GaAs!buffer!growth,!the!sample!temperature!was!lowered!to!500°C.!4!
monolayers!Si!were!then!deposited!(1!Si!monolayer!=!6.26!x!10
14!cm
Q2)!under!As!
flux!at!a!rate!of!0.02!ML/s.!The!Si!growth!rate!was!calibrated!in!the!previous!(not!
shown).!These!growth!conditions!are!consistent!with!those!used!to!obtain!high!
quality!Si/GaAs!superlattices!
[25].!!
The! substrates! were! then! exposed! to! air! for! 15! min! at! 300! °C! on! a! hot! plate!
located! in! the! clenroom,! before! being! introduced! again! in! the! UHV! system! and!
degassed!at!300°C!for!30!minutes.!
,
2.3.2#Epiready(substrates#
Epiready, substrates! were! prepared! as! follows.! Epiready! GaAs! substrates! as!
received!by!the!supplier!
[26]!were!just!degassed!in!UHV!at!300!°C!for!30!min!prior!
to!the!deposition!of!a!Si!layer!in!the!same!conditions!reported!above!for!epitaxial!
substrates.!XPS!analysis!of!the!substrates!after!different!Si!deposition!times!(not!
shown)!revealed!that!the!Si!equivalent!thickness!obtained!on!the!epiready!GaAs!
susbtrates! was! reduced! by! a! factor! of! 2,! as! compared! to! the! epitaxial! case! for!
similar! deposition! times.! In! other! words,! the! same! deposition! time! gave! on!
epiready! substrates! half! the! thickness! they! would! give! on! epitaxial, substrates.!
We!interpreted!this!result!invoking!a!lower!sticking!coefficient!of!Si!adatoms!on!
oxidized! GaAs! as! compared! to! the! epitaxial! surface.! In! the! following,! we! quote!
the!Si!equivalent!thickness!(in!MLs)!as!estimated!from!XPS.! !
2.4#Experimental#Results#
What! follows! is! to! be! considered! representative! of! all,measured! substrates.! In!!
table!1,!the!relevant!binding!energy!values,!referred!to!the!spectrometer!Fermi!
level,!are!summarized.!
!
!
!
Core!
Electron!
Peak!
Binding!Energy! SpinQorbit!
splitting! Fitting!Procedure!
As
3d! ≈!41!eV! 0.71!eV! 2!Voigt!curves!
Ga
3p! 104Q108!eV! 2.45!eV! 2!Voigt!curves!
Ga
3d! ≈!20!eV! 0.43!eV! 1!Voigt!curve!
Si
2p! ≈!100!eV! 0.6!eV! 1!Voigt!curve!
#
Table#2.1:#Core#electron#peak#data#that#appear#in#the#upcoming#analysis#
!
!
! !
2.4.1.#Si
2p#region#
In!fig.!2.1,!the!binding!energy!region!from!99!to!114!eV,!as!recorded!on!our!SiQ treated!substrates,!is!shown.!
!
!
Figure#2.1:#Ga#3p#and#Si#2p#core#level#photoemission#recorded#on#different#substrates.#(a)#Epitaxial#substrates#
with#4#MLs#of#Si#as#grown#and#(b)#after#expo:#sure#to#air.#(c)#Epiready#GaAs#(001)#wafers#as#received#from#the#
supplier#and#(d)#after#deposition#of#4#MLs#of#Si.#
In!fig.!2.1,!the!binding!energy!region!from!99!to!114!eV,!as!recorded!on!our!SiQ treated! substrates,! is! shown.! The! first! two! rows! (curves! aQb)! refer! to! epitaxial, substrates,! where! 4! Si! monolayers! were! deposited.! In! curve! (a)! the! Ga
3d! spin!
orbit!doublet!and!the!Si
2p!core!electron!peak!are!clearly!separated,!identified!and!
fitted.!Curve!(b)!is!taken!after!exposure!to!the!air.!We!see!that!the!lowest!energy!
peak,!correspondent!to!the!Si
2p!electron!emission!shifts!towards!higher!binding!
energies! and! merges! with! the! Ga
3d3/2! peak.! This! is! a! distinctive! sign! of! the!
atmoshperic!oxidation!process.!
Similarly,!curve!(c)!shows!the!surface!of!epiready!GaAs!(100)!wafers!as!received!
from! the! supplier.! Electron! peaks! are! slightly! larger! than! their! thermally!
deoxidized!correspondent!in!(a).!Curve!(d)!shows!the!same!surface!after!the!Si!
deposition:!again,!a!new!peak!appear,!merged!with!Ga
3d3/2!but!the!peak!of!native!
Si!does!not!completely!disappear,!as!in!the!case!of!epitaxial,substrates.!
#
!
! !
2.4.2.#As
3d#region#
In! fig.! 2.2,! the! binding! energy! region! from! 38! to! 47! eV,! as! recorded! on! our! SiQ treated!substrates,!is!shown.!
!
Figure#2.2:#As#3d#core#level#photoemission#recorded#on#different#substrates.#(a)#Epitaxial#substrate#with#4#MLs#of#
Si# as# grown# and# (b)# after# exposure# to# air.# (c)# Epiready# GaAs# as# received# from# the# supplier# and# (d)# epiready#
substrate#after#deposition#of#4#MLs#of#Si.#
Fig.!2.2!shows!the!correspondent!changes!in!the!As
3d!core!electron!peaks,!upon!
oxidation! of! the! surface.! in! (a)Q(b)! the! epitaxial, surface! is! shown:! no! change!
occurs!in!the!As
3d!lineshape!(except!a!hardly!detectable!small!contribution!4.2!eV!
higher! in! binding! energy! than! the! As
3d! spin! orbit! doublet,! compatible! with! the!
As
5+!oxidation!state.!In!(c)Q(d)!the!epiready!surface!is!shown:!(c)!shows!both!the!
As
3+!and!As
5+!oxidation!states,!while!in!(d)!the!reduction!by!Si!is!total!and!only!
the!native!As!3d!spin!orbit!doublet!is!visible.!
! !
2.4.3.#Ga
3d#region#
In! fig.! 2.3,! the! binding! energy! region! from! 17! to! 25! eV,! as! recorded! on! our! SiQ treated!substrates,!is!shown.!
!
Figure#2.3:#Ga3d#core#level#photoemission#recorded#on#different#substrates.#(a)#Epitaxial#substrate#with#4#MLs#of#
Si#as#grown#and#(b)#after#exposure#to#air.#(c)#Epiready#GaAs#as#received#from#the#supplier#and#(d)#after#deposition#
of#4#MLs#of#Si.#
! !
Fig.!2.3!shows!the!same!four!cases!for!the!Ga!3d!core!electron!peak.!As!for!the!
previous!case,!curve!(a)!shows!a!pristine!surface!while!in!curve!(b)!a!small!oxide!
contribution!is!visible!(Ga
1+!oxidation!state);!curve!(c)!shows!on!the!contrary!a!
fully!oxidized!surface!(both!Ga
1+!and
3+!oxide!levels!are!visible!on!the!left!side!of!
the!native!Ga
3d!electron!peak)!while!curve!(d)!shows!a!much!reduced!lineshape,!
with!less!Ga
1+!and!almost!no!Ga
3+!states.!
! !
2.5.#Discussion#
The!effect!of!the!deposition!of!4!ML!Si!on!GaAs!substrates!is!shown!in!fig.s!2.1Q2.!
A!comparative!discussion!is!hereby!presented.!
!
2.5.1.(Epitaxial(substrates(
On!thermally!deoxidized!GaAs,!Si!deposition!resulted!in!a!clean!a!Si!layer!until!it!
got!oxidized!by!air!exposure.!In!this!case,!Si!oxide!is!found!on!the!surface!while!
the!underlying!GaAs!remains!pristine.!The!small!oxide!contribution!visible!in!in!
fig.!2.2(b)!can!be!explained!simply!by!the!atmospheric!oxidation!of!few!As!atoms!
segregated!on!the!Si!surface!during!its!deposition!in!As!flux.!!
By! analyzing! the! chemical! shift! of! the! oxidized! silicon! peak! in! 1(b)! from! 1(a),!
corresponding! to! 2.9! eV,! we! evaluated! the! oxidized! surface! to! host! the! Si
3+! suboxide!state!
[27].!
,
2.5.2.#Epiready#substrates#
On! epiready! GaAs! substrates,! Si! deposition! resulted! in! a! chemical! reduction! of!
the! GaAs! surface.! Concomitantly,! the! Si! oxidizeed;! after! the! reaction,! the! GaAs!
surface!was!only!slightly!oxidized,!while!a!part!of!the!deposited!4!Si!monolayers!
did!not!react.!GaAs!reduction!by!means!of!Si!at!room!temperature!was!already!
shown!to!be!effective!
[28].
! !The!presence!of!both!the!oxidized!and!unreacted!Si!allowed!the!direct!evaluation!
of!the!chemical!shift,!which!is!found!to!be!2.27!eV,!close!to!the!Si
4+!oxidation!state.!
[27]
!
Although!the!oxidation!reaction!is!not!quantitative!nor!presents!a!single!product,!
such! a! procedure! was! shown! not! to! have! any! consequences! on! the! GaAs!
nanowire!yield.!
[11]!
! !
2.5.3.#Si#thickness#
Outside! the! investigated! values,! i.e.! for! θ
Si! <! 3! ML! or! θ
Si! >! 8! ML,! the! density!
showed!a!clear,!but!not!easyQtoQunderstand!behavior.!In!particular,!in!case!of!too!
thin!Si!layer,!the!oxidation!step!resulted!in!the!oxidation!of!the!underlying!GaAs!
(not! shown).! In! this! case,! the! nanowire! density! dropped! abruptly! to! 0.! On! the!
contrary,!increasing!the!Si!thickness!reduced!the!NW!density,!up!to!a!nonQsharp!
value! close! to! 8! ML! Si.! Unfortunately,! given! the! empirical! approach! offered! by!
this!method,!no!quantitative!information!can!be!given!(both!the!epitaxial,and!the!
epiready!substrates!employed!a!non!quantitative!controlled!oxidation!agent)!and!
only! qualitative! estimation! together! with! trialQandQerror! method! proved! to! be!
useful.! Reduced! nanowire! density! was! for! instance! required! in! case! of! SPEM!
experiments,! or! to! investigate! the! nanowire! morphology! in! detail! by! scanning!
electron!microscopy.!
!
! !
2.6.#Effectiveness#of#Si:treated#substrates#
In! figure! tempi_corti_confronto! is! visible! that! a! 3! minutes! long! material!
deposition!results!in!nanowire!growth.!The!substrate!was!SiQtreated!substrates!
with! 4! monolayers! Si.! The! substrate! was! prepared! in! the! epiready! approach.!
Equivalent!results!were!found!by!means!of!the!epitaxial!approach!(see!appendix).!
I! particularly! show! epiready! samples! because! of! their! greater! simplicity! of!
preparation.!
!
Figure# 2.4:# an# ensembe# of# SEM# micrographs# of# a# nanowire# growth# experiment# performed# on# (100)# Si:treated#
GaAs#substrates#with#4#equivalent#monolayers#Si#in#panels#(a:c)#and#on#(111)B#Si:treated#substrates#in#panels#(b:
d).#
