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Chemical Spray Pyrolysis Deposition of Zinc Sulfide Thin Films and Zinc Oxide Nanostructured Layers.

by Dedova, Tatjana, PhD


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Raman Spectroscopy

Raman
intensity,
a.u.

100

10

E (low)

2

2nd order

b)

a)

E (high)

2

E (TO)

1

A (TO)

1

100 200 300 400 500 600

Raman shift, cm
-1

Figure 3.24 Sprayed ZnO nanorod layers Raman spectra from samples composed of
a) nearly vertically standing ZnO rods (SEM micrographs are presented in Figures
3.12b); b) tripods (SEM micrograph is presented in Figure 3.12c)

Figure 3.24 shows the Raman spectra of ZnO layers composed of well c-axis
orientated ZnO rods (curve a) and of tripods (curve b). Strong Raman bands at 99
and 438 cm-1 were observed for both samples. These bands correspond to ZnO
nonpolar optical phonon E2 (low) and E2 (high) modes of hexagonal wurtzite phase
respectively and are characteristic of high crystal quality wurtzite [137, 138]. The
FWHM of the E2 (high) mode at 438 cm
-1 of sprayed ZnO nanorods is around 6
cm-1 and comparable to 8 cm-1 of ZnO nanowires [139]. The small difference in the
spectra is the presence of vanishingly weak peak of E1(TO) band at 410 cm
-1 for
the tripod-like ZnO layers (Figure 3.24b). The E1(TO) mode is sensitive to the
orientation of nanostructure. This mode is detected from the tilted nanowires and
randomly oriented nanobelts and cannot be usually observed from the vertically
aligned nanowires [138, 140].

In summary, according to PL and Raman measurements, the ZnO nanorods
prepared from ZnCl2 solution by spray pyrolysis possess excellent optical quality
and high purity. The SAED studies demonstrated that individual nanorod is singlecrystalline.
The sample prepared form solution with addition of thiocarbamide is
not so pure but probably contaminated by impurities originated from thiourea.

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CONCLUSIONS

ZnS thin films

The phase composition, crystallinity, morphology and optical properties of ZnS
thin films prepared by chemical spray pyrolysis technique using aqueous solutions
of zinc chloride and thiocarbamide are controlled by both growth temperature and
the precursor molar ratio in spraying solution.

1. The films deposited at temperatures of 240-380 °C are amorphous and
contain high amount of residues originated from the precursors. Using
growth temperatures above 400 ºC, major part of impurities are expelled
and the films become crystalline.

2. Highly (002) oriented ZnS films with the wurtzite structure and closely
stoichiometric composition could be grown at 500-600 °C in air using the
1:2 molar ratio of precursors (ZnCl2 and SC(NH2)2) in the spraying
solution. ZnS films from the 1:1 solution have dominantly sphalerite
structure, Zn-rich composition and contain ZnO as secondary phase at
growth temperatures close to 600 °C. Films obtained from Zn-rich solution
(Zn:S=2:1) have nonuniform surface and composition consisting of
mixture of ZnS and ZnO phases.

3. The band gap and refractive indices of the ZnS films prepared by spray of
1:2 solution are higher (Eg of 3.67 eV and n=1.82 (at 632.8 nm)) compared
to those obtained from the 1:1 solution (Eg of 3.59 eV and n=1.48 (at 632.8
nm)). Sprayed ZnS films exhibit optical transparency in the order of 70-80
% in the visible spectrum region and demonstrate antireflective coating
properties reducing the reflectivity of Si wafers from 36% to 10%.

ZnO nanostructured layers

1. ZnO nanostructured layers comprising nanorods can be grown by a simple
and cost-effective chemical spray pyrolysis technique using zinc chloride
as precursor. The morphology of ZnO layers, dimensions and orientation
of nanorods, substrate coverage and quality of the crystals are controlled
by growth temperature, precursor concentration, substrate type and
additives in the solution.

2. Compact ZnO films develops to the structured layers when the growth
temperatures of 450ºC and higher are used. Well-shaped highly crystalline
hexagonal ZnO rods with preferred c-axis orientation on a substrate could
be grown at temperatures around 550 °C using ZnCl2 concentrations of

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0.05-0.1 mol/l in the spray solution. Higher concentrations of ZnCl2 (c=0.2
mol/l) lead to the compact layer formation in the case of deposition on
TCO substrates and ZnO tripods-like crystals development on glass
substrates. The diameter of the rods could be decreased and aspect ratio
increased using alcohol-based solvents or small addition of thiourea into
the ZnCl2 solution (Zn:S =1:0.25).

3. ZnO nanorods prepared by chemical spray of zinc chloride solutions are of
high optical and crystalline quality single crystals as confirmed by Raman
and PL spectroscopic studies and selective area electron diffraction
measurements.

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ACKNOWLEDGEMENTS

This thesis is based on the experimental work carried out during 2003-2007 in
the Laboratory of Semiconductor Materials at the Materials Science Department,
Tallinn University of Technology.

Above all, I would like to express my deepest gratitude to my supervisor –
Research Professor Malle Krunks for her excellent guidance, support and
encouragement during this work.

I would like to express my special thanks to the unique person who
unfortunately is not anymore among us –Prof. Peter Enn Kukk for showing me the
entrance door to the Laboratory of Semiconductors Materials and the Head of the
Department of Materials Science, Professor Enn Mellikov for opening this door
and giving me the possibility to carry out the present study.

I owe my gratitude to the co-authors and colleagues at the Department of
Materials Science. Special thanks are due to Dr. Arvo Mere for teaching me the
optical and electrical measurements, being always ready to help and to answer any
questions; Atanas Katerski for keeping the instruments working and always helping
should a need arise, Olga Volobujeva and Dr. Valdek Mikli for SEM, EDX and
SAED studies, Maarja Grossberg for PL measurements, and others for their
friendship and support.

I am also deeply thankful to my former teacher and advisor Dr. Mare Altosaar,
who is greatly contributing to my knowledges on semiconductor materials both in
theory and in practice.

I am very grateful to the opponents - Prof. Lauri Niinistö and Dr. Thierry
Pauporté - for their valuable time and comments.

Most importantly, my heartfelt thanks belong to my family, especially, to my
husband Andrei for his invaluable support and encouragement during this work.

And finally, financial support from the Estonian Science Foundation grants No.
5612 and 6954, Estonian Ministery of Education and Science target financing
project 0142515s03SF and Estonian Doctoral School of Materials Science and
Materials Technology (MMTDK) are gratefully acknowledged. This support
enabled attendance in International Scientific Conferences.

Tallinn, December 2007

Tatjana Dedova

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ABSTRACT

Chemical Spray Pyrolysis Deposition of Zinc Sulfide Thin Films and Zinc
Oxide Nanostructured Layers

This thesis is focused on the studies on chemical spray pyrolysis deposition of ZnS
thin films and ZnO nanostructured layers consisting of nanorods.

The properties of ZnS thin films were studied, dependent on the molar ratio of
precursors (ZnCl2 : tu) in solution and growth temperature (TS =230–600 °C) by
means of various techniques including XRD, SEM, EDX, FTIR, UV-VIS and
ellipsometry. It was found that the phase composition, crystallinity, structure,
morphology and optical properties of sprayed films are controlled by both the
substrate temperature and molar ratio of precursors in the solution.

The films deposited at temperatures below 400 ºC were amorphous and
contaminated by the residues originating from the precursors. Films became
crystalline if deposited at temperatures around 400 ºC or above. Slightly Zn-rich
(Zn/S=1.11) films with the sphalerite structure were grown in the temperature
interval of 490-530ºC using the precursor molar ratio of 1:1, whereas the films
from 1:2 solution at TS=530ºC were highly oriented wurtzite having nearly
stoichiometric composition (Zn/S=1.01). Additional ZnO phase was detected on
the diffractogram of 1:1 film deposited at TS=600ºC while this phase was not
detected in the XRD patterns of the films deposited from the 1:2 solution at this
temperature. Films utilizing the 2:1 solution were Zn-rich (Zn/S=up to 10) with
ZnO phase incorporation even at low deposition temperatures (TS=400-540ºC).

Refractive indices and band gap values were higher for the ZnS films deposited
at TS=530ºC from the 1:2 solution (n=1.8 at 632.8 nm and Eg=3.67 eV compared
to those obtained from the 1:1 solution (n=1.5 at 632.8 nm and Eg=3.59 eV). All
films independent of the molar ratio demonstrated high optical transmittance (70-
80 %) in the visible and infrared spectral region and reduced the reflectivity of Si
wafers from 36% to 10% thus showing their applicability as antireflection coatings
in devices.

The second part of the thesis was focused on the deposition of ZnO nanorod
layers by chemical spray pyrolysis technique. The ZnO layer formation,
composition, structure, morphology, dimensionality and orientation of the ZnO
crystals as well as optical properties were investigated with respect to growth
temperature, solution concentration, substrate type (glass, ITO, SnO2, ZnO seed
layer) and presence of additives (solvents, thiocarbamide) in the spraying solution.
SEM, XRD, SAED, PL and Raman spectroscopic measurements were carried out
to characterize the ZnO layers.

Dense ZnO thin films formed at temperatures around 400 ºC were transformed
to highly structured layers at temperatures around 450 ºC or above. Well-shaped
highly crystalline hexagonal ZnO rods with preferred c-axis orientation on glass or
on transparent conductive oxide covered substrates could be grown at temperatures

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around 550 °C using ZnCl2 concentrations of 0.05-0.1 mol/l in the spraying
solution. The concentration of 0.2 mol/l results in branched and tilted crystals,
mainly tripods on glass substrates while compact ZnO layer formation was
observed on conductive electrode substrates. Generally, the nanorods deposited
onto TCO substrates had more regular and uniform sizes, smaller diameters, better
orientation perpendicular to the substrate and higher substrate coverage compared
to those grown on bare glass. The addition of organic solvent (ethanol or
isopropanol) into the spraying solution supported the development of significantly
smaller rods (d=70-80 nm, L=600 nm) in comparison to the rods obtained from
purely aqueous solutions (d=200-300 nm, L=900 nm). Furthermore, in this study it
was found that small additions of thiocarbamide into spraying solution of ZnCl2
(ZnCl2 :tu =1:0.25) promoted the development of significantly thinner ZnO
nanorods with a higher aspect ratio.

According to the PL and Raman studies, ZnO nanorods, deposited from the
ZnCl2 solutions are of high optical and crystalline quality. According to SAED
each separate nanorod is a single crystal. The results obtained, together with
simplicity and low cost of the spray procedure, make the technology promising for
preparation of ZnO nanorods which are applicable in nanodevices.

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KOKKUVÕTE

Tsinksulfiidi õhukesed kiled ning tsinkoksiidi nanostruktuursed kihid
keemilise pihustuspürolüüsi meetodil

Käesolev doktoritöö on pühendatud tsinksulfiidi õhukeste kilede ja tsinkoksiidi
nanostruktureeritud, nanovarrastest koosnevate kihtide kasvatamisele keemilise
pihustamise meetodil.

Uuriti pihustatud ZnS kilede omadusi sõltuvalt kasvatamise temperatuurist
vahemikus 230–600 °C ja lähteainete (ZnCl2 ja tiokarbamiid) kontsentratsioonide
molaarsuhtest lahuses. Kilede uurimiseks kasutatati XRD, SEM, EDX, FTIR, UV-
VIS ja ellipsomeetria mõõtmismeetodeid. Kasvutemperatuuridel alla 400 ºC
sadestatud kiled sisaldavad olulisel määral tsinktiokarbamiidkloriidi laguprodukte
ja on halva kristallilisusega. Kristallilised kiled saadi kasvatamisel 400 ºC juures ja
sellest kõrgematel temperatuuridel. Kiled, mis kasvatati temperatuuridel üle 500
ºC, ei sisaldanud enam lähteainetest tulenevaid lisandeid. Kasutades ekvimolaarset
(1:1) pihustuslahust ja sadestustemperatuure 490-530 ºC, saadakse kuubilise
struktuuriga Zn-i rikkad kiled (Zn/S=1.11), mis kõrgemal kasvutemperatuuril (600
ºC) sisaldavad kristallilist ZnO faasi. Kilede sadestamisel 500-600 ºC juures
tsinkkloriidi ja tiokarbamiidi molaarsuhtel 1:2 saadi heksagonaalse struktuuriga
ligikaudu stöhhiomeetrilise koostisega (Zn/S=1.01) ZnS kiled, mis on c-telje
suunas tekstureeritud. Kiled, mis saadi lähtelahuste molaarsuhtel 2:1, on tsingi
rikkad ja sisaldavad lisaks ZnS-le ka ZnO faasi.

ZnS kiled, mis sadestati 1:2 lahusest 530 ºC juures, omavad suuremat
keelutsooni laiust ja murdumisnäitajat kui 1:1 lahusest valmistatud kiled. Näiteks
1:2 ja 1:1 lahustest kasvatatud kilede murdumisnäitajad on vastavalt 1.8 ja 1.5
(632.8 nm juures) ning keelutsooni laiused vastavalt 3.67 ja 3.59 eV. Pihustatud
ZnS kiled omavad 70-80 %-list optilist läbilaskvust nähtavas ja infrapunases
spektri osas. Leiti, et pihustatud ZnS kiled vähendavad oluliselt valguse
peegeldumist räni pinnalt ja seega võivad leida kasutamist ka odavate
antireflektoorsete katetena.

Teine osa doktoritööst käsitleb ZnO nanovarrastest koosnevate kihtide
kasvatamist keemilise pihustamise meetodil. Kirjanduse andmetel ei ole ZnO
nanovardaid varem keemilise pihustamise meetodil saadud. Tsinkoksiidi
nanostruktureeritud kihtide moodustumist ZnCl2 lahuste pihustamisel, sealhulgas
kihtide faasikoostist, struktuuri, morfoloogiat, kristallide mõõtmeid ning
orienteeritust kihis uuriti sõltuvalt kasvatamise temperatuurist, lähtesoola
kontsentratsioonist pihustuslahuses, aluse tüübist (klaas; ITO, SnO2 ja ZnO-ga
kaetud klaas) ja lisanditest pihustuslahuses. Kihte uuriti SEM, XRD, SAED,
Raman ja PL spektroskoopia meetoditega. Ilmnes, et sadestustemperatuuri
tõstmisel üle 450 C muutub sile, kompaktne ZnO kile struktureeritud kihiks, mis
koosneb eraldi seisvatest heksagonaalsetest kristallidest. Hästi väljakujunenud
heksagonaalse struktuuriga tsinkoksiidi nanovardad moodustuvad kasvatamisel 550

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°C juures kasutades ZnCl2 kontentratsioone pihustuslahuses 0.05-0.1 mol/l.
Kõrgema kontsentratsiooniga lahuste (0.2 mol/l) pihustamisel moodustuvad
klaasalustele tripoodide-kujulised kristallide kogumikud, kusjuures juhtiva
oksiidiga kaetud aluste (TCO) puhul kujunes välja kompaktne ZnO kiht. Üldiselt,
TCO peale sadestatud nanovardad on ühtlasema mõõduga, väiksema diameetriga,
parema c-teljelise orienteeritusega ja annavad aluse parema kattetiheduse kui
klaasile kasvatatud nanovardad. Kasutades lahustina alkohole moodustuvad
tunduvalt väiksemad ZnO nanovardad (d=70-80 nm, L=600 nm) kui samadel
tingimustel vesilahustest pihustatud kristallid (d=200-300 nm, L=900 nm). Väikese
koguse tiokarbamiidi lisamine pihustuslahusesse (ZnCl2 :tu =1:0.25) viis tunduvalt
peenemate ja suurema kuvasuhtega nanovarraste moodustumisele. PL ja Raman
spektroskoopia ning SAED analüüsi tulemused näitavad, et keemiliselt pihustatud
ZnO nanovardad on puhtad, kõrge kvaliteediga monokristallid. Saadud tulemused
koos keemilise pihustuspürolüüsi lihtsuse ja odavusega näitavad, et see tehnoloogia
on paljulubav nanoseadiste jaoks vajalike omadustega ZnO nanovarraste
valmistamiseks.

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