Probing oxide heterostructure interfaces through angle resolved and resonant electron ...

205 Pages·2015·22.14 MB·English

UNIVERSITA´ DEGLI STUDI DI MILANO &
UNIVERSITA´ CATTOLICA DEL SACRO CUORE
SCUOLA DI DOTTORATO IN
FISICA, ASTROFISICA E FISICA APPLICATA
DIPARTIMENTO
MATEMATICA E FISICA
DOTTORATO DI RICERCA IN
FISICA, ASTROFISICA E FISICA APPLICATA
Ciclo XXVII
Probing oxide heterostructure interfaces
through angle resolved and resonant electron
spectroscopies
Settore scientifico disciplinare FIS/03
Coordinatore: Prof. Marco Bersanelli
Tutore: Prof. Luigi Sangaletti
Tesi di Dottorato di:
Gabriele Salvinelli
Anno Accademico 2013-2014
Dedicated to my wife.
“La scienza conduce a un punto oltre il quale non ci pu`o piu` guidare;
e l’uomo, giunto a questo punto, `e necessitato a cercare un’altra guida,
che non trova se non sostituendo alla conoscenza scientifica la fede religiosa.
Scienza e Religione non sono in contrasto, ma hanno bisogno l’una dell’altra
per completarsi nella mente di un uomo che pensa seriamente.” (Max Plank)
“Benedetta sii tu, Materia universale.
Durata senza limiti, Etere senza sponde
- triplice abisso di stelle, di atomi e di generazioni -
tu che superando e dissolvendo le nostre anguste misure
ci riveli le dimensioni di Dio.”
(Teilhard de Chardin)
4
Contents
Introduction 9
1 Experimental and theoretical details 13
1.1 X-ray Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . 13
1.1.1 Three-step model . . . . . . . . . . . . . . . . . . . . . . . 18
1.2 X-ray Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . 20
1.3 Resonant Photoemission Spectroscopy . . . . . . . . . . . . . . . 22
1.3.1 Fundamental aspect of ResPES . . . . . . . . . . . . . . . 24
1.3.2 The case of Ti L and La M edge . . . . . . . . . . . 28
2,3 4,5
1.4 X-ray Photoemission Electron Microscopy . . . . . . . . . . . . . 30
1.5 Mirror Electron Microscopy . . . . . . . . . . . . . . . . . . . . . 30
1.6 Modeling for XPS: the BriXias code . . . . . . . . . . . . . . . . 32
1.6.1 Effective XPS core-level intensity modeling . . . . . . . . 33
2 Cd Sn O /TiO heterojunctions for solar cell applications 35
x y z 2
2.1 Dye-sensitized solar cells and recombination processes . . . . . . 36
2.2 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.2.1 Transparent conductive oxide . . . . . . . . . . . . . . . . 40
2.2.2 DSSC fabrication . . . . . . . . . . . . . . . . . . . . . . . 41
2.3 XPS results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.4 AR-XPS results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.4.1 CTO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.4.2 The abrupt model for the CTO-TOs . . . . . . . . . . . . 45
2.4.3 The intermix model for CTO-TOs . . . . . . . . . . . . . 47
6 CONTENTS
2.5 Interface Band Diagram . . . . . . . . . . . . . . . . . . . . . . . 50
2.6 Comparison with the literature . . . . . . . . . . . . . . . . . . . 56
2.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3 The case of LaAlO /SrTiO heterointerface 61
3 3
3.1 Perovskite oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.1.1 LaAlO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3
3.1.2 SrTiO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3
3.1.3 LaAlO /SrTiO interface . . . . . . . . . . . . . . . . . . 68
3 3
3.2 Materials and Methods. . . . . . . . . . . . . . . . . . . . . . . . 71
3.2.1 Experimental details . . . . . . . . . . . . . . . . . . . . . 72
3.2.2 Cleaning Method . . . . . . . . . . . . . . . . . . . . . . . 73
3.3 XPS characterization . . . . . . . . . . . . . . . . . . . . . . . . . 74
3.4 Band offset analysis . . . . . . . . . . . . . . . . . . . . . . . . . 82
3.4.1 Valence band offset by spectra decomposition . . . . . . . 89
3.5 Band Profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
3.5.1 The LAO band: flat or bent? . . . . . . . . . . . . . . . . 98
3.5.2 Resonant photoemission spectroscopy data . . . . . . . . 99
3.5.3 Band alignment at the interface. . . . . . . . . . . . . . . 107
3.6 About the origin of the interface states . . . . . . . . . . . . . . . 112
3.7 High photon flux photoemission electron microscopy and micro-
spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
3.7.1 Photoemission electron microscopy data . . . . . . . . . . 118
3.7.2 µ-XPS data as a function of the x-ray exposure time . . . 121
3.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
4 AR-XPS study on LaAlO /SrTiO 135
3 3
4.1 Method and Model . . . . . . . . . . . . . . . . . . . . . . . . . . 138
4.2 Abrupt interface model: stoichiometric case . . . . . . . . . . . . 141
4.2.1 Terrace roughness model. . . . . . . . . . . . . . . . . . . 141
4.2.2 Surface cation vacancies . . . . . . . . . . . . . . . . . . . 145
4.3 Abrupt interface model: La/Al stoichiometry correction . . . . . 146
4.3.1 Terrace roughness model with La/Al correction . . . . . . 151
4.3.2 Remarks about the abrupt interface model . . . . . . . . 151
4.4 Interdiffusion model . . . . . . . . . . . . . . . . . . . . . . . . . 154
4.5 Abrupt interface model: La/Al and Ti/Sr stoichiometry correction157
CONTENTS 7
4.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
Appendix A 163
A.1 Determination of the uncertainty in the band offset analysis . . . 163
A.2 Reliability of the band offset analysis . . . . . . . . . . . . . . . . 165
A.3 Remark on the bare CTO energy gap. . . . . . . . . . . . . . . . 167
A.4 Evaluation of the bare CTO energy gap . . . . . . . . . . . . . . 168
Bibliography 171
Acknowledgements 199
Publications and Activities 201

Introduction
In the recent years, devices have become smaller and faster, more power-
ful and efficient, as well as cheaper. However, the size of the building blocks
of the current electronics - i.e. the transistors - is quickly approaching the
limitsofmanufactureandtheneedofanewgenerationofdevicesbasedonnovel
mechanisms appears to be essential [1]. In this scenario, well-defined interfaces
between oxide materials have produced novel electronic systems displaying a
spectacularvarietyofpropertieswithpromisingpotentialitiesforfuturedevices,
such as colossal magnetoresistance, high-temperature superconductivity, mag-
netismattheinterfacebetweennonmagneticoxides,aswellastwo-dimensional
electron gas (2DEG) between two oxide insulators [2]. Because of their chem-
ical complexity, the quality of oxide materials was believed to be lower with
respect to the conventional semiconductors, but the recent advances in growth
techniques based on pulsed laser deposition (PLD) or molecular beam epitaxy
(MBE) have retracted this viewpoint, opening to the fabrication of oxide in-
terfaces with the same atomic precision as semiconductors [3,4]. At the same
time, however, therichelectronicphasesexhibitedbyheterojunctionsinvolving
transitionmetaloxides,whicharedominatedbythehighlyinteractingelectrons
in d-orbitals, may be the result of a subtle balance of minute perturbations [3].
Even though the thin film growth technology has been suitably tailored for
oxide compounds, a central issue remains the sample quality, where the control
of defects may play a key role towards a future step of technology [4].
In this context, interfaces between perovskite oxides can display properties
at the nanometer scale that are qualitatively different from their single build-
ing blocks, allowing one to engineer novel functionalities through the growth
of epitaxial heterostructures. Indeed, the discovery of a 2DEG at the interface
10 CONTENTS
between lanthanum aluminate, LaAlO , and strontium titanate, SrTiO , [5,6]
3 3
hasdisclosedunexpectedperspectivesinthephysicsofoxide-basedjunctions. So
far,alargenumberofstudieshaveattemptedtodeterminethephysicalproper-
tiesofthe2DEGandtodevelopnoveldevicesbasedonthecapabilitytocontrol
the interface properties through chemical interactions at the surface, suitable
materials combination, or strain engineering [7]. The study of the electronic
properties of the LaAlO /SrTiO heterojunction can thus provide fundamen-
3 3
tal information to understand the origin of the relevant physical phenomena
observed in these systems, while the profiling of energy-level alignment across
the interface would disclose the basic information for the design, characteriza-
tion, and tuning of devices based on the intrinsic electric fields arising at the
interface, such as photovoltaics [8,9] and field-effect transistors [10], as well as
devices based on tunable conductivity and charge writing [11,12].
In non-oxide semiconductors, the relaxation of polar discontinuity in
heterointerfaces is usually achieved by an atomic reconstruction process [5],
where the interface stoichiometry is altered by interdiffusion, point defect, and
dislocation and in general by a structural roughening. In oxides, the possibil-
ity of multiple valence ions allows even an electronic reconstruction that, in
the LaAlO /SrTiO case, should move electrons from the surface to the empty
3 3
Ti d-levels. Nevertheless, both atomic and electronic reconstructions could be
present in oxide interfaces and the possibility to probe non-destructively the
cation depth profiles across the junction would provide further details useful to
understand the oxide physics. Indeed, the latter is also valid for less-ordered
systems, i.e. amorphous thin films, in which the interfacial electronic proper-
ties can be driven by complex substitutional effects across the interface, such
as stoichiometry gradients, vacancies and cation interdiffusion. This is the case
of many ultrathin interfaces that are playing a key role in the development of
new architectures to be tested in photovoltaic (PV) applications. Also in this
field, noveldevicescanbeengineeredprovidedthatthecapabilitytocontrolin-
terfaces at the nanoscale is demonstrated. Among the most popular solar cells,
electrochemicalPVcellsdisplayastackofinterfaceswithdifferentthicknessand
chemicalproperties,fromthefrontcontacttothebackelectrode. Thefrontcon-
tact constitutes itself an interface where a transparent conducting oxide (TCO)
layer is usually matched with a nanosized blocking layer (BL). The quality of
this interface may have relevant consequences on the overall cell performances.