Research Article
Received: 28 August 2009
Revised: 29 January 2010
Accepted: 5 February 2010
Published online in Wiley Interscience: 29 March 2010
(www.interscience.wiley.com) DOI 10.1002/sia.3355
Characterization of thin and ultrathin
transparent conducting oxide (TCO) films
and TCO-Si interfaces with XPS, TEM
and ab initio modeling†
S. Diplas,a∗ O. M. Løvvik,a,c H. Nordmark,b D. M Kepaptsoglou,c
J. Moe Graff,a C. Ladam,c F. Tyholdt,a J. C. Walmsley,b A. E. Gunnaes,c
R. Fagerbergb and A. Ulyashina
Interfaces play an important role in solar cell heterostructures, especially when film thicknesses decrease. In this work, we
use XPS, transmission electron microscopy (TEM) and density functional theory (DFT) to study both the film and the film–Si
interface of electron beam deposited indium tin oxides (ITO) and pulsed laser deposition (PLD) deposited ZnO on p-Si (100).
Vacuum evaporation of ITO resulted in a film that contained elemental Sn and In which oxidized after annealing at 300 ◦ C for
30 min. Ar etching of the HF-treated Si substrate in the PLD deposition chamber caused an increase of the interfacial oxide
thickness independently of the deposition temperature as a result of Ar etching-induced dangling bonds. Deposition as well as
annealing at elevated temperature also increases the interfacial oxide thickness for both ITO-Si and ZnO-Si systems. Copyright
c 2010 John Wiley & Sons, Ltd.
Keywords: ITO; ZnO; thin films; interfaces; XPS; TEM; DFT
Introduction
874
Transparent conducting oxides (TCO) like Indium Tin Oxides (ITO)
and ZnO are widely used for different types of solar cells, both as
antireflection coatings and transparent conducting electrodes,
due to their attractive combination of electrical conductivity
and transparency to visible light. In particular, ZnO (doped
with Al) is a reliable and cost effective substitute for SiN
as an antireflection coating material for Si-based solar cells.
Interfaces may limit the performance of solar cells and other
electronic devices. In heterojunction photovoltaics, a significant
and increasing proportion of charge carrier loss is due to interfaces
as film thickness decreases. Interfaces are also strongly influenced
by processing conditions. For many applications, high-quality
epitaxial films or fully amorphous films forming an abrupt interface
with the substrate are desired. However, realizing such perfectly
sharp interfaces may be very difficult. An important reason for this
is the great complexity of the interfaces. It has been shown that
TCO layers deposited on various Si-based substrates at different
temperatures can exhibit formation of a SiOx thin layer on the
TCO/Si interfaces.[1,2] Moreover, after high temperature anneals of
TCO/Si interfaces formation of a specific interface layer occurs due
to interdiffusion between the TCO material and Si.[3]
The TCO/Si interface structure should be therefore known at the
atomic level in order to understand the mechanisms governing its
formation and function. In this work, we used XPS, high resolution
electron microscopy (HREM) and density functional theory (DFT)
modeling to study the morphology and electronic structure of
thin ITO and ZnO films, as well as their interfaces with the Si
substrate focusing on the effect of deposition and heat treatment
Surf. Interface Anal. 2010, 42, 874–877
on the composition of ITO and the ITO-Si interface and the effect
of substrate treatment on the ZnO-Si interface.
Experimental
ITO films were deposited on p-Si (100) 1–10 ohm sq with e-beam
deposition in a CVC SC-5000 chamber having a Temescal STIH-2701 e-beam gun. The substrates were treated in 1% HF solution to
remove native oxide before they were loaded into the chamber and
pumped down to 5 × 10−6 Torr. However, the pressure increased
significantly during evaporation, up to 10−5 Torr, suggesting that
oxygen was removed from the ITO during the process. This was
confirmed by the resulting dark and nontransparent samples.
Annealing in air at 300 ◦ C for 30 min followed the deposition in
order to reoxidize the samples. The In : Sn ratios (measured with
XPS) in the target and the deposited films were 5 : 1 and 4 : 1,
respectively.
The ZnO films were grown on p-Si (100) substrates by pulsed
laser deposition (PLD) using a ZnO target (American Elements,
∗
Correspondence to: S. Diplas, SINTEF Materials and Chemistry, Forskningsvn 1,
O-0314, Oslo, Norway. E-mail: spyros.diplas@sintef.no
† Paper published as part of the ECASIA 2009 special issue.
a SINTEF Materials and Chemistry, Forskningsvn 1, O-0314, Oslo, Norway
b SINTEFMaterialsandChemistry,Høgskoleringen 5,O-7465,Trondheim,Norway
c Department of Physics, University of Oslo, Blindern, N-0316, Oslo, Norway
c 2010 John Wiley & Sons, Ltd.
Copyright Thin and ultrathin conducting oxide films
Figure 1. TEM images of ZnO deposited at 500 ◦ C in O2 without (a) and after (b) Ar etching of the Si substrate; c) EDS line scan across the ZnO/Si interface
in Fig. 1 b) and d) Si 2p HRXPS spectra of ultra thin films (3 nm) deposited at RT and at 500 ◦ C with and without Ar etching of the Si substrate.
99.999%). Prior to the PLD, the substrates were etched in 5% HF
for 30 sec. On some of the substrates, before the ZnO deposition,
a gentle argon ion milling was performed for 2 min inside the PLD
chamber to further remove any remaining oxide. The films were
grown using a KrF excimer laser at 248 nm wavelength (Lambda
Physik LPX Pro 210i). The base pressure of the vacuum chamber
before deposition was less than 1 × 10−6 Torr. Depositions were
carried out at ambient temperature, or at 500 ◦ C, in the oxygen
remaining base pressure or in 10−3 Torr oxygen atmosphere.
Two film thicknesses were produced, 3–5 and 80 nm. The
thinnest films allowed for direct investigation of the TCO-Si
interface by XPS standard (zero angle of emission) and angle
resolved (AR) mode. XPS was performed using a KRATOS AXIS
ULTRA with monochromatic Al Ka radiation (hν = 1486.6 eV) at
15 kV and 10 mA. TEM was performed on a field emission gun TEM
Jeol 2010F equipped with an EDS detector. DFT calculations were
performed using the Vienna ab initio Simulation Package (VASP).
Results and Discussion
ZnO/p-type Si (100) interface
Surf. Interface Anal. 2010, 42, 874–877
c 2010 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/sia
875
Figures 1 a) and b) show HREM images of ZnO deposited at
500 ◦ C with (Fig. 1b)) and without (Fig. 1a)) Ar sputtering of the
Si substrate, respectively, prior to deposition. High-resolution Si
2p XPS (HRXPS) spectra of ultrathin ZnO films treated at various
conditions, in Fig. 1 d) show that the interfacial oxide is much
thicker where the Si substrate was Ar-etched in the deposition
chamber prior to deposition.
The interfacial oxide is shown to be mixed Si-Zn oxide(s) by the
broad peak at ∼102 eV in the Si 2p XPS spectra which corresponds
to Si-Zn mixed oxide(s).[4,5] This is supported by the intermixing of
the Si and Zn profiles in the line scan across the ZnO-Si interface
shown in Fig. 1 c). It appears that there is a compositional gradient
with a Zn-rich and a Si-rich side on the ZnO and Si-substrate
side, respectively. This, combined with a contrast change in the
interfacial oxide in Fig. 1 b) indicates that the interfacial oxide
consists of two layers; a Zn-rich mixed oxide on the ZnO side
of the interface, and a Si-rich mixed oxide on the Si side. The
thickness of the interfacial oxide approximately defined by the
distance between the two vertical broken lines in Fig. 1 c), is about
7–8 nm in agreement with the HREM image in Fig. 1 b). It should
be mentioned that experimental conditions during EDS line scans
like specimen drift (although drift compensation was applied)
and carbon contamination can affect the sharpness of the profile.
Owing to this we performed point analysis which confirmed mixing
of Zn and Si in the oxide. Moreover, the XPS evidence (Fig. 1 d))
for existence of a Zn-Si oxide along the interface of the ultrathin
films supports the presence of Si and Zn gradients in the interfacial
oxide.
In the absence of Ar etching (Fig. 1 a)) of the substrate prior to
deposition, the oxide was much thinner (∼3 nm) for deposition
in O2 , both at RT and at 500 ◦ C (comparison not shown here).
EDS analysis (both point and line scans) on the interfacial oxide
formed during RT deposition, look similar to that formed during
deposition at 500 ◦ C.
Considering the XPS results in Fig. 1 d), and with respect
to the interfacial oxide formation between the thin films and
the substrates, we can suggest that in the initial stages the
oxide(s) formed between the Ar-etched substrate and the film
is of mixed nature (Zn-Si oxide). Deposition at RT on non-Aretched substrate results in a negligible initial interfacial oxide
S. Diplas et al.
Figure 2. a) and b) HREM images of ITO e-beam deposited on p-Si(100), as-deposited and annealed at 300 ◦ C for 30 mins, respectively; c) EDS line scan
across the interface in Fig. 2 b), showing chemical composition variation across the interface; and d) high-resolution Si 2p XPS of 3 nm ITO on p-type Si
(001), as-deposited and after heat treatment, showing negligible Si-oxide at the interface and an increase in the interfacial oxide, respectively.
in Si create vacancy-related complexes.[8,9] However, it is difficult
to distinguish whether an amorphous Si-oxide forms before ZnO
deposition, followed by a formation of a mixed amorphous Zn-Si
oxide which is covered finally by ZnO, or an amorphous ZnO or
Zn-Si mixed oxide forms from the first moments of deposition.
Amorphization of the Si substrate due to Ar etching cannot be
excluded.[10]
ITO/p-type Si (001) interface
Figure 3. Detail from an atomistic model of a ITO/p-Si interface before
(left) and after (right) FP molecular dynamics for 1300 fs at 300 K.
876
formation with average oxidation state probably lower than 3+.
Annealing promotes an oxidation state closer to 4+, as shown by
the shift of the oxide to higher binding energy, whilst deposition
at 500 ◦ C seems to promote a thicker (as shown by the XPS
intensity) interfacial oxide close to SiO2 . It seems that the interfacial
oxide thickness depends on the ZnO thickness, and for small
ZnO thicknesses it depends on the deposition temperature. This
indirectly supports the mixed nature of the interfacial oxide. The
increase in the interfacial oxide thickness in the samples with
Ar sputtering occurring prior to deposition is attributed to an
increase of the surface roughness (more effective area subjected
to oxidation)[6] and/or to the activation of dangling bonds induced
by the Ar sputtering.[7] It has been also shown that noble gases
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Figures 2 a) and b) show HREM images of the ITO/p-Si (100)
interface, in the as-deposited condition and after air annealing
at 300 ◦ C for 30 min, respectively. Figure 2c) shows EDS line scan
profiles across the interface in Fig. 2b). Figure 2d) shows Si 2p
HRXPS spectra of ultrathin of the as-deposited and annealed
ITO films. The as-deposited film was amorphous and became
fully crystalline after annealing at 300 ◦ C for 30 minutes. During
e-beam deposition in vacuum, a thin (∼2 nm) amorphous layer of
Si containing oxide (not stoichiometric SiO2 as shown by XPS in
Fig. 2d)) is formed at the Si–ITO interface, as a result of a redox
reaction between In, Sn and O,[11] which is typical of ITO deposition
with other methods.[1] Air annealing leads to the crystallization of
the ITO layer, while the thickness of the interfacial amorphous layer
increases and becomes less uniform than that of the as-deposited,
possible due to further oxidation of Si during annealing.
Figure 3 illustrates the mechanism of the oxidation as revealed
by first principle molecular dynamics for 1300 fs at 300 K. Several Si-
c 2010 John Wiley & Sons, Ltd.
Copyright Surf. Interface Anal. 2010, 42, 874–877
Thin and ultrathin conducting oxide films
Figure 4. a, b) XPS Sn 3d and O 1s spectra 3 nm from ITO on p-type Si (001) e-beam deposited, respectively and c) XPS VB spectra 3 nm ITO on p-type Si
(001) e-beam deposited) showing presence of increase intensity (DOS) in the near EF region.
O bonds are formed during the simulation, and one oxygen atom
(marked with arrow) has started to move into the Si matrix. This
may be regarded as the first step of oxidation of Si, leaving metallic
Sn and In close to the interface. Since annealing was performed in
air the oxygen vacancy is expected to be replaced during diffusion.
A metal-rich interfacial layer has also been detected by SIMS in
ITO/Si structure.[12]
Effect of deposition on ITO oxidation state
Figures 4a), b) and c) show Sn 3d, O 1s HRXPS and valence band
XPS spectra of ultrathin ITO (3–5 nm) deposited on p-Si (100) (also
representative for the 80 nm samples). The e-beam deposition
in vacuum resulted in the presence of nonoxidized Sn and In
(not shown) in the film. After air annealing, the films appear
reoxidized and the XPS core level spectra (e.g. In Fig. 3 d)) obtain
their normal asymmetry which is characteristic of the ITO.[13,14]
ARXPS (not shown) showed that the presence of elemental Sn and
In was more enhanced in the bulk of the film, due to expected
reoxidation in the surface and near-surface areas. The presence of
elemental Sn and In is attributed to partial reduction of ITO during
evaporation in vacuum.
The O 1s spectrum in Fig. 4 b) is also indicative of the influence
of deposition and annealing on the oxidation state of the film. In
ITO films the low binding energy O1s component (labeled as A) at
∼530 eV is attributed to oxygen in the oxide crystal, while the high
binding energy one(s) (labeled as B) at ∼532 eV has been assigned
to: (a) oxygen ions in oxygen-deficient regions, (b) oxygen atoms
in an amorphous phase, (c) oxygen bound to Sn, (d) adsorbed
oxygen species (Omx-) (e) hydroxyl groups associated to either
adsorbed OH or In(OH)x, e.g.[15,16]
Angle-resolved XPS data (not shown) suggested that adsorbed
species did not have significant contribution to the presence
of the high-binding energy shoulder. We suggest that the 532 eV
component is attributed to the oxygen deficiency and the presence
of the as-deposited amorphous ITO phase (Fig. 2 a) above). VB
spectra in Fig. 4 d) show increased intensity in the near-EF region,
presumably due to enhanced density of states (DOS) owing to
presence of metallic Sn and/or In.
Conclusion
ZnO films deposited with PLD on p-Si (100) showed that Ar etching
of the Si substrate prior to deposition dramatically increases the
interfacial oxide thickness. The interfacial oxide, which seems to
consist of a mixture of Si and Zn oxides, also increases in thickness
for deposition at elevated temperatures compared to deposition at
ambient conditions. Electron beam deposition of ITO on p-Si (001)
produces an oxygen-deficient (Sn, In-rich) film which reoxidizes
upon air annealing. Air annealing also causes a small increase in
the interfacial ITO-Si oxide.
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Surf. Interface Anal. 2010, 42, 874–877
c 2010 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/sia