Electronic Properties of Post-transition Metal Oxide Semiconductor Surfaces

Abstract

Metal oxides such as ZnO, Ga2O3, CdO, In2O3, and SnO2 exhibit high degree of transparency to visible light while supporting high levels electrical conductivity. The causes of
the conductivity and the role played by the surface are current topics of research. This chapter presents a systematic study
of the electronic structure and electrical properties of these post-transition metal oxides (PTMO) using a combination of
X-ray photoelectron spectroscopy, angle-resolved photoelectron spectroscopy, Hall effect, infrared reflectivity, and optical
absorption spectroscopy measurements. Evidence of surface electron accumulation in these PTMO is presented. It is found that
for CdO and In2O3, electron accumulation is observed even in the absence of extremely high doping levels. The results also indicate that despite
the strong tendency to exhibit surface electron accumulation, these materials can also exhibit an electron depletion layer
under the appropriate surface stoichiometry conditions or when certain anions are adsorbed. The proclivity towards surface
electron accumulation shown by the PTMOs is discussed in terms of bulk band structure, surface states, and the position of
their band edges in an absolute energy scale. The electronic properties of thin films and bulk crystals of the PTMO surfaces
also provide information vital for the interpretation of conductivity measurements of PTMO nanostructures, which are often
dominated by surface effects.

Full text

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Chapter 6
Electronic Properties of Post-transition Metal
Oxide Semiconductor Surfaces
T.D. Veal, P.D.C. King, and C.F. McConville
6.1Introduction
The post-transition metal oxides (PTMOs) have traditionally been classified as
transparent conductors. The normally contradictory properties of transparency to
visible light and electrical conductivity have made materials such as Sn-doped
In2O3and Al-doped ZnO suitable for use as transparent electrodes in devices such
as solar cells and flat panel displays. However, the PTMOs are increasingly being
considered as semiconductors for potential use as the active layer in a wide range of
transparent (opto)electronic devices.
The electronic properties of the surfaces of semiconductors are important for
many device applications. Due to different atomic coordination and broken trans-
lational symmetry at the surface, the electronic properties can differ markedly in
this region compared to in the bulk of a semiconductor. The presence of charged
surface states can induce a macroscopic redistribution of free carriers close to the
surface, in order to screen the electric fields associated with such a surface charge,
and therefore minimize energy. Such surface space-charge regions are associated
with a bending of the electronic bands relative to the Fermi level, over distances
defined by the electronic screening length of the material. Typical length scales for
such a band bending in semiconductors are on the order of 5–500 nm. Therefore,
while such surface electronic properties are important for manydevice applications,
they become even more important in nanostructured semiconductors, where the
surface space-charge region can account for a large proportion of the volume of the
material. Indeed, transport properties of semiconductor nanowires are known to
depend on the nature of the surface space charge of the particular material. For
example, in InN nanowires the conductivity increases as the diameter is reduced
due to the presence of surface electron accumulation, while in GaN nanowires
T.D. Veal (*) • P.D.C. King • C.F. McConville
Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
e-mail: Timothy.Veal@warwick.ac.uk
J. Wu et al. (eds.), Functional Metal Oxide Nanostructures, Springer Series
in Materials Science 149, DOI 10.1007/978-1-4419-9931-3_6,
# Springer Science+Business Media, LLC 2012
127

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reducing the diameter decreases the conductivity as a result of the surface electron
depletion layer [1, 2]. This follows the surface space-charge behaviour of thin films
of InN and GaN [3, 4]. The surface space-charge region is widely recognized as
important to the wide variety of applications of metal oxide nanostructures, but
frequently the only type of intrinsic space-charge region considered for n-type
semiconductors is an electron depletion layer [5]. However, as will be discussed
below, this is generally an incorrect assumption for PTMOs. The electronic
properties of metal oxide nanostructures provide the motivation for this chapter
on the electronic properties of the surfaces of PTMO thin films. Until recently,
many of the fundamental surface properties of these materials were unknown or
misunderstood. A knowledge and understanding of the fundamental surface space-
charge properties of this class of materials will be of interest to those researching
both the application of these materials in transparent (opto)electronics and the
transport properties of PTMO nanostructures.
The post-transition metallic elements and oxygen are highlighted in Fig. 6.1 on a
portion of the periodic table that depicts the electronegativity and atomic radius of
the elements. This chapter is limited to the surface electronic properties of the
period 4 (ZnO and Ga2O3) and period 5 (CdO, In2O3, and SnO2) PTMO
semiconductors. The period 6 PTMOs (HgO, Tl2O3, PbO, PbO2, and Bi2O3) have
not yet had their surface electronic properties investigated. In Sect. 6.2, the surface
space-charge properties will be presented for each of the PTMOs of periods 4 and 5.
This will be followed in Sect. 6.3 by an explanation of the overriding cause of these
and related properties in terms of the nature of the bulk band structure of the
materials and the location of the band edges with respect to the charge neutrality
level (CNL).
Fig. 6.1 The post-transition
metal elements and oxygen
highlighted ona portion of the
periodic table. The shading
corresponds to the
electronegativity of the
elements. The atomic number
(top left), atomic radius in pm
(top right) and the
electronegativity in Pauling
units (bottom) are also shown
for each element
128T.D. Veal et al.

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6.2Surface Space-Charge Properties
The surface space-charge properties of an n-type semiconductor can be determined,
to a large extent, by measuring the surface and bulk Fermi levels, with the
difference between them being accounted for by the band bending. In practice,
the surface Fermi level with respect to the valence-band maximum (VBM) can
be determined using X-ray photoemission spectroscopy (XPS) and knowledge of
the band gap of the semiconductor enables the Fermi level with respect to the
conduction-band minimum (CBM) to be obtained.1The bulk Fermi level can be
estimated by measuring either the free-electron density using the Hall effect, the
conduction electron plasma frequency using infrared reflectivity, or, for a degener-
ately doped material, by using optical absorption spectroscopy. The determined
surface and bulk Fermi levels can then be used as boundary conditions to solve the
Poisson equation within the modified Thomas–Fermi approximation [6]. The solu-
tion of the Poisson equation gives the band bending and charge profiles as a
function of depth below the semiconductor surface and also the surface state
density. Here, the surface space-charge properties of all the period 4 and 5
PTMOs investigated using these methods are presented within the context of the
previous understanding of their surface electronic properties.
6.2.1ZnO
The first member of the period 4 PTMOs, ZnO, has the wurtzite structure and a
room temperature band gap of 3.35 eV. The electronic properties of its surfaces
have been studied for over 40 years [7]. Some of the earliest work showed that it is
possible to modify the surface space charge from electron depletion to strong
electron accumulation by varying the surface treatment. Exposure of the ZnO
surface to oxygen induces an electron depletion layer, whereas atomic hydrogen
treatment produces surface electron accumulation [7]. However, the “intrinsic”
state of the space charge at ZnO surfaces is still debated [8, 9] and the role of
surface conductivity in ZnO nanowires is far from being resolved [10].
Figure 6.2 shows valence-band XPS of bulk ZnO crystals grown by both the
hydrothermal (HT) and pressurized-melt (PM) techniques. Spectra were collected
fromboth the Zn- andO-polar facesof the same c-axis crystals grown byeachmethod
and also from non-polar crystals (m-plane and a-plane). The room temperature bulk
electron densities were found, from multiple field Hall effect measurements, to be
2–3 ? 1014cm?3for the HT ZnO, 5 ? 1016cm?3for the a-plane PM ZnO, and
1While this picture may be complicated somewhat by many-body effects within the surface space-
charge region, as will be discussed later in this chapter, it still provides a correct qualitative and
indeed semi-quantitative picture of the surface space-charge characteristics of a given material.
6Electronic Properties of Post-transition Metal Oxide Semiconductor Surfaces129

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1.5 ? 1017cm?3for the c-plane PM ZnO [9]. The separation,z, between the VBM
and the Fermi level at the surface is determined by extrapolating a linear fit to the
lower binding energy edge of the valence band photoemission to a line fitted through
the background data. The Fermi level is calibrated to be thezero ofthe binding energy
scale by using the Fermi edge of a silver reference sample.
98765
Binding Energy (eV)
Intensity (cps)
Intensity (cps)
4
HT ZnO
Zn-polar face
O-polar face
a
b
HT ZnO
m-plane face
PM ZnO
a-plane face
ζ = 3.60 eV
ζ = 3.54 eV
ζ = 3.71 eV
ζ = 3.52 eV
ζ = 3.61 eV
ζ = 3.50 eV
PM ZnO
Zn-polar face
O-polar face
3
9 8 7 6 5
Binding Energy (eV)
Intensity (arb.units)
Intensity (cps)
4 3 2 1 0−1
9 8 7 6 5
Binding Energy (eV)
4 3 2 1 0−1
210
−1
98765
Binding Energy (eV)
43210
−1
Fig. 6.2 Valence-band XPS spectra from the Zn-polar and O-polar faces of (a) the same bulk
c-axis HT ZnO wafer and (b) the same bulk c-axis PM ZnO wafer, showing the extraction of x, the
valence-band maximum to Fermi level separation, for each face. The inset of (a) shows the
spectrum from the m-plane face of a separate HT ZnO wafer. The inset of (b) shows the spectrum
from the a-plane face of a separate PM ZnO wafer. Figure reproduced with permission from [9].
Copyright (2010) by the American Physical Society
130T.D. Veal et al.

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From the XPS data, the surface Fermi level was found to be located between
3.50 and 3.71 eV above the VBM, with the highest values being for the Zn-polar
surface, the lowest values for the O-polar surface, and the non-polar surfaces in
between [9]. Since the measured bulk carrier densities correspond to bulk Fermi
levels ranging from 3.10 to 3.27 eV above the VBM, the surface Fermi level is
always higher than the bulk Fermi level, indicating the presence of surface electron
accumulation for all samples investigated. A greater difference between the surface
Fermi levels of the Zn- and O-polar surfaces was found for the HT ZnO than for the
much higher bulk carrier density PM ZnO. The greater electron accumulation at the
Zn-polar face, particularly for the HT ZnO, is attributed to the spontaneous polari-
zation [11] associated with the wurtzite structure. The effect is reduced for the PM
ZnO due to partial screening by the higher bulk electron density of these samples.
The surface band bending and carrier concentration profiles for the various ZnO
samples were determined by numerical solution of Poisson’s equation within the
modified Thomas–Fermi approximation [6] and are shown in Fig. 6.3.
From the measured amount of band bending, solving the Poisson equation
enabled the surface sheet densities to be determined as 7 ? 1012cm?2
(3 ? 1012
5 ? 1012cm?2(2.5 ? 1012cm?2) for the Zn-polar (O-polar) face of the PM
ZnO. These surface electrons have been found to have a significant influence on
the electrical properties of high resistivity HT ZnO at temperatures below 200 K
and are a major cause of the anomalously low maximum electron mobility
measured using single magnetic field Hall effect measurements. For PM ZnO, the
surface layer has little effect onits electrical properties above 50 K due to the higher
concentration of uncompensated shallow donors in this material [9]. Surface elec-
tron accumulation appears to be the natural state of both polar and non-polar ZnO
surfaces and can significantly influence electrical measurements made on heavily
compensated ZnO material. Consequently it must be taken into account when
investigating the electrical properties of potentially p-type material. Additionally,
it can be seen from Fig. 6.3 that, for the HT ZnO, the free-electron density 100 nm
away from the surface is still significantly higher than the bulk value, illustrating
how important surface properties can be for nanoscale structures.
cm?2) for the Zn-polar (O-polar) face of the HT ZnO and
6.2.2Ga2O3
The second member of the period 4 PTMOs, Ga2O3, exhibits five different
polymorphs [12], with the most stable being b-Ga2O3with a monoclinic structure
and a room temperature band gap in the range of 4.4–4.9 eV [13, 14]. In spite of the
surface electronic properties of Ga2O3not receiving much attention, thin films of
this material have been successfully used as gas sensors. Rather than exhibiting
sensitivity to anion-like chemisorption, such as O2, they have been found to be good
sensors of reducing gases, such as H2and CO, when chemisorption of donor-like
species occurs [15]. This behaviour was explained in terms of an electron
6Electronic Properties of Post-transition Metal Oxide Semiconductor Surfaces 131

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Fig. 6.3 Poisson-MTFA calculations of the band banding and the carrier concentration profiles in
the electron accumulation layer at the Zn-polar (a) and (c) and O-polar faces (b) and (d) of HT
ZnO; and at the Zn-polar face (e) and (g) and O-polar face (f) and (h) of PM ZnO. Figure
reproduced with permission from [9]. Copyright (2010) by the American Physical Society
132T.D. Veal et al.

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accumulation layer model of the Ga2O3surface [16]. The presence of an electron
accumulation is also suggested by recent photoemission data, where the surface
Fermi level can be seen to be approximately 4.5 eV above VBM [17]. Such a
surface Fermi level, very close to or above the CBM, is above typical Ga2O3bulk
Fermi levels, indicating downward band bending and surface electron accumula-
tion. Further investigations of Ga2O3are, however, required to verify the nature of
its surface electronic properties.
6.2.3CdO
The first member of the period 5 PTMOs, CdO, has the rock salt structure and a
room temperature direct band gap of 2.2 eV [18, 19], with two indirect band gaps of
less than 1.1 eV [19–21]; the VBM is not at the G-point due to repulsion between
the Cd 4d and O 2p states being symmetry forbidden at G for the rock salt structure
[22]. Until recently, CdO had long been assumed to exhibit surface electron
depletion [23]. However, several recent studies have contradicted this, finding
evidence of surface electron accumulation [24–26].
Specifically, a combination of valence-band XPS, Hall effect, and optical absorp-
tion and reflectivity has been used to determine the surface and bulk Fermi levels of
CdO(001) films grown by metal organic vapour phase epitaxy on sapphire substrates.
The valence-band XPS measurements, shown in Fig. 6.4a, give the L-point (indirect)
VBM to surface Fermi level separation as x ¼ 1.29 ? 0.05 eV. Taking the separa-
tion of the G-point and L-point maxima of the valence band as 1.2 eV from
calculations [24], the surface Fermi level is found to be 2.49 eV above the G-point
VBM. Meanwhile, from Hall effect measurements (n ¼ 1.6 ? 1019cm?3, m ¼ 106
cm2V?1s?1)andtheopticalabsorptionandIRreflectivitydatashowninFig.6.4b,c,
the G-point VBM to bulk Fermi level separation was determined as ? ¼ 2.23
? 0.05 eV. Therefore, the Fermi level lies higher relative to the band extrema at
the surface than in the bulk, implyinga downwardbending ofthe bands atthe surface
of 0.26 eV. The calculated band bending is shown in Fig. 6.4d along with the large
accumulation of electrons in the near surface region shown in Fig. 6.4e.
The surface electron accumulation has additionally been found to be quantized.
Angle-resolved photoemission spectroscopy (ARPES) has revealed that the con-
duction electrons at the surface occupy two-dimensional subbands created by the
confining potential well associated with the downward band bending [24, 26]. The
photoemission maps for a CdO surface-quantized two-dimensional electron gas
(Q2DEG) are shown in Fig. 6.5. Similar Q2DEG states are expected at the surface
of PTMO nanowires. However, if the nanowire size is sufficiently small, the
wavefunction tails from the Q2DEG states of one surface may overlap with those
from the surface on the opposite side of the nanowire. In this case, these states could
interact, potentially giving rise to novel physics between two spatially separated,
but nonetheless coupled, Q2DEGs.
6Electronic Properties of Post-transition Metal Oxide Semiconductor Surfaces133

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Intriguingly, such detailed spectroscopic measurements of the electronic states
within a surface electron accumulation layer reveal a discrepancy with the simple
band-bending picture discussed so far. While the valence-band bending determined
by XPS (Fig. 6.4a), as well as core-level and other valence-band photoemission
measurements, (Fig. 6.5e) indicates a downward band bending of only 0.26 eV, a
deep enough quantum well to give the measured conduction-band subband as low
as 0.5 eV below the Fermi level requires the conduction band to bend downward by
1.1 eV. This discrepancy between the amount of valence- and conduction-band
bending can be resolved by considering many-body effects within the electron
accumulation layer [26]. Instead of the conduction and valence bands undergoing
the same amount of band bending as in the conventional one-electron picture of
Fig. 6.4 (a) Valence-band XPS, (b) squared optical-absorption coefficient, and (c) measured
(points) and simulated (line) IR reflectivity spectra for an undoped CdO sample following
annealing at 600?C in UHV for 2 h. (d) Band bending [CBM, indirect (L-point) and direct
(G-point) VBMs] and (e) carrier concentration as a function of depth below the surface in the
electron accumulation layer. Figure reproduced with permission from [25]. Copyright (2009) by
the American Physical Society
134 T.D. Veal et al.

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surface space charge, band gap renormalization occurs in the accumulation layer
where the electron density is the highest, shrinking the band gap close to the surface,
as illustrated in Fig. 6.6. This band gap shrinkage has a large influence on the surface
sheet density; within the one-electron picture, the valence-band bending implies a
sheet density of 1 ? 1013cm?2, while the actual sheet density, determined from the
Luttinger area, N2D¼ k2
ARPES data, is 4.4 ? 1013cm?2. Such many-body effects probably increase the
surface sheet electron densities of other materials discussed elsewhere in this chapter
compared with the reported values based on valence-band photoemission data.
F=2p, with the Fermi wave vector, kF, taken directly from the
6.2.4 In2O3
The second member of the period 5 PTMOs, In2O3, has the body-centred-cubic
(bcc) bixbyite structure or the less stable rhombohedral (rh) structure. The direct
band gap of cubic In2O3was long thought to be 3.7 eV, based on the onset of optical
absorption, with an indirect gap of about 2.6 eV [27, 28]. However, recent theoreti-
cal studies have found no evidence for any significant difference between the direct
and indirect band gaps [29, 30]. Experimental studies using photoemission, X-ray
emission and absorption, and optical absorption are consistent with a band gap in
Fig. 6.5 (a) Quantized conduction-band subbands of a surface Q2DEG. (b) E vs. k|| and
(c) constant energy cuts through the subband dispersions measured by ARPES, shown here for a
CdO(001) surface recorded with a photon energy of hn ¼ 30 eV at room temperature. The
constant energy cuts are integrated over ?3 meV about (c1) the Fermi level and (c2) 0.15, (c3)
0.30, and (c4) 0.45 eV below the Fermi level, respectively. (d) Schematic of downward bending of
the conduction band and (inset) corresponding increase in free-carrier density within the semicon-
ductor surface electron accumulation layer (Q2DEG). (e) Angle-integrated PES measurements of
the valence bands and Cd 4d core levels in CdO, recorded at a photon energy of hn ¼ 120 eV.
Figure reproduced with permission from [26]. Copyright (2010) by the American Physical Society
6 Electronic Properties of Post-transition Metal Oxide Semiconductor Surfaces135

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the range 2.6–2.95 eV [29, 31]. The theoretical calculations indicate that the weak
nature of optical absorption around this energy can be attributed to transitions
between the highest valence-band states and states at the CBM being dipole
forbidden or having only minimal dipole intensity [29, 30].
Thisrevisionofthebandgaphasbroughtaboutare-evaluationofthesurfacespace-
chargepropertiesofIn2O3.Before2008,photoemissionspectroscopyhadbeenusedto
locatethesurfaceFermilevelatabout3eVabovetheVBM[32,33].Withtheaccepted
band gap at that time being 3.7 eV, this, along with a bulk Fermi level close to the
CBM, implied the presence of upward band bending and a surface electron depletion
layer. However, withthe recently established band gap of 2.9 eV (3.0 eV) for bcc (rh)
In2O3,thisandmorerecentphotoemissiondata,suchasthatshowninFig.6.7forboth
bcc- and rh-In2O3, indicate that the surface Fermi level is above the bulk Fermi level,
corresponding to downward band bending and electron accumulation [31, 34].
For In2O3, with the shallow dispersion of its topmost valence bands [30], the
VBM position with respect to the Fermi level cannot be accurately determined by
extrapolating the leading edge of the valence-band photoemission; this method
leads to an underestimation of the VBM to surface Fermi level separation due to the
effects of instrumental and lifetime broadening. Instead, as shown in Fig. 6.7, the
experimental data are compared with the broadened calculated valence-band den-
sity of states (VB-DOS). A 3.4 eV (3.5 eV) shift is required to align the calculated
VB-DOS with the XPS data from undoped bcc (rh) In2O3, indicating that the
surface Fermi level is 0.5 eV above the CBM for both bcc and rh In2O3. Along
with the bulk Fermi levels for the different samples, determined from Hall effect
and infrared reflectivity measurements, the downward band bending was found to
be in the range 0.4–0.5 eV [31]. When heavily Sn-doped, the bulk Fermi level is
much closer to the surface Fermi level, resulting in approximately flat bands. The
band bending and carrier concentration profiles as a function of depth below
the surface, calculated by solving the Poisson equation, are shown in Fig. 6.8
[31]. The surface sheet density can be estimated from these calculations to be
1.3 ? 1013cm?2for all of the undoped In2O3samples.
Fig. 6.6 One-electron (solid
line) and schematic
renormalized (dashed line)
band bending at the surface of
CdO, and corresponding
calculated one-electron
(solid) and measured
renormalized (dashed)
subband energies. Figure
reproduced with permission
from [26]. Copyright (2010)
by the American Physical
Society
136T.D. Veal et al.

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Fig. 6.7 Background-
subtracted valence-band
photoemission spectra from
(a) undoped and Sn-doped
bcc-In2O3(001) and undoped
(111) (grown by plasma-
assisted molecular-beam
epitaxy and metal organic
vapour phase epitaxy) and
(b) undoped rh-In2O3(0001).
The conduction-band
emission is magnified 25
times. The density functional
theory calculated valence-
band density of states is
shaded in grey and is also
shown after lifetime and
instrumental broadening.
Figure is adapted from [31]
Fig. 6.8 Poisson-MTFA
calculations of band bending
(a) and (c) and carrier
concentration profiles (b) and
(d) in the electron
accumulation layer at the
surface of bcc-In2O3(a) and
(b) and rh-In2O3(c) and (d).
Figure is adapted from [31]
6Electronic Properties of Post-transition Metal Oxide Semiconductor Surfaces 137

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The presence of surface electron accumulation is further corroborated by a
comparison of Al KaXPS (hn ¼ 1486.6 eV) and synchrotron radiation high energy
XPS (hn ¼ 6,000 eV) [35].AscanbeseeninFig.6.7, anadditional peakis present in
the photoemission spectra justbelow the Fermi level. This corresponds to photoemis-
sion from the occupied conduction-band states which have predominantly In 5s
character. After correcting for matrix element effects, Zhang et al. found a larger
contribution from filled conduction-band states in the more surface sensitive
1486.6 eV XPS than for the 6,000 eV XPS [35]. This indicates that there is an
increasedconcentrationofconductionelectronsclosetothesurfacethandeeperinthe
bulk, confirming the picture of a surface electron accumulation layer in In2O3.
6.2.5SnO2
The third and final member of the period 5 PTMOs, SnO2, has the rutile structure
and a band gap of 3.59 eV (3.50 eV at room temperature) [36, 37]. The surface
properties of SnO2 have been comprehensively reviewed quite recently [38].
The surface of SnO2 can exhibit surface electron depletion or accumulation,
depending on the stoichiometry of the surface and the bulk Fermi level. Similar
to ZnO, adsorption of anion species leads to electron depletion, while cation
adsorption leads to accumulation, making SnO2a good material for gas sensing
via corresponding changes in conductivity. Indeed, nanoscale structures of SnO2
are particularly suited to this task [39], where the larger surface to bulk ratio
provides a relatively higher density of surface adsorption sites compared to thin
films, enabling the adsorbed species to have an even greater influence on the total
measured conductivity. The presence of an electron accumulation layer at the
surface of SnO2under certain conditions is long established [40], but whether it
is the “intrinsic state” of the surface is not yet well established.
Valence-band XPS from an undoped bulk SnO2(100) crystal after cleaning by 2 h
of vacuum annealing at 400?C is shown in Fig. 6.9. The VBM to surface Fermi level
separationis3.70eV.Witha roomtemperaturebandgapof3.50eV,thiscorresponds
to the surface Fermi level being 0.20 eV above the CBM. This is well above the non-
degenerate bulk Fermi level of undoped SnO2, indicating the presence of downward
band bending and electron accumulation. Before annealing a VBM to surface Fermi
level separation of 3.60 eV was observed, again indicating electron accumulation but
to a lesser extent than after the surface was reduced by annealing.
These results, parallels with the other PTMOs, some recent studies [37, 41], and
the discussion in the next section, indicate that SnO2surfaces do have an inherent
tendency to exhibit electron accumulation in contrast to the surface depletion layer
of the majority of n-type semiconductors. However, the presence of anion species
when used as a gas detector or exposure to oxygen plasma treatment can result in
surface electron depletion [38, 41].
138T.D. Veal et al.

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6.3Bulk Band Structure Origin of Electron
Accumulation Propensity
From the preceding sections, it is clear that the PTMOs exhibit a tendency towards
surface electron accumulation, rather than the charge depletion common to almost
all conventional semiconductors. This raises the question whether some universal
characteristic ofthePTMOsdrivesthis apparently unconventionalsurface behaviour.
Inadditiontotheir propensityforstrongly n-type surfaces, thesematerials alsohavea
proclivity towards n-type bulk properties, with many defect centres that are tradition-
ally considered to counteract the prevailing conductivity, including hydrogen
impurities [42–45] and several native defects, exhibiting an extreme propensity for
donor behaviour in PTMOs. Any model which explains the surface properties of
these materials must also be consistent with these striking bulk properties.
In fact, both the surface and bulk characteristics of PTMOs can naturally be
explained from gross features of their bulk band structure. A typical PTMO bulk
band structure, for In2O3, is shown in Fig. 6.10. While multiple and quite complex
valence bands are observed, the conduction band is characterized by a single low-
lying band at the zone centre. Throughout the rest of the Brillouin zone, the
conduction band lies much higher. The character of the so-called deep, that is
localized, defect, is determined from the band structure across the Brillouin zone
(that is, they have a very extended k-space nature). Indeed, for broken symmetry
defect states such as surface states, native defects, and hydrogen impurities, it is
possible to define an energy level at the mid-point of the average band gap across the
Brillouin zone which determines the charge transition from positively to negatively
charged defect centres [46–49]. If this so-called CNL lies within the fundamental
band gap, it is generally favourable for compensating defect centres and surface
charge depletion, as shown in Fig. 6.11. However, for a bulk band structure of the
form depicted in Fig. 6.10, the low-lying CBM can actually occur below the mid-gap
Fig. 6.9 Background-
subtracted valence-band
photoemission spectrum from
bulk-grown SnO2(100)
6 Electronic Properties of Post-transition Metal Oxide Semiconductor Surfaces 139

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Fig. 6.11 Schematic formation energies for dominant native defects and interstitial hydrogen
impurities and schematic band bending in conventional semiconductors (left) and PTMOs (right).
The influence of the CNL, within the band gap in conventional semiconductors but above the CBM
in PTMOs, can clearly be seen. Figure adapted from [25]
Fig. 6.10 Calculated band structure of In2O3. Figure adapted from [30]
140T.D. Veal et al.

Page 15

position averaged across the entire Brillouin zone. In this case, even for bulk Fermi
levels above the bottom of this CBM, it remains favourable for donor-like surface
states to donate their electrons into the conduction band, forming a surface electron
accumulation.Similarly,native defectsandhydrogenimpuritiescanbothgainenergy
by donating free-carriers into the conduction band (see Fig. 6.11).
Consequently,thegeneralpropensityfordonor-typesurfacestatesaswellasdonor
bulkdefectsandhydrogeninPTMOscanallbeunderstoodtoresultfromthebottomof
theconductionbandlyingbelowtheCNLinthesematerials.Indeed,fromanumberof
direct measurements [25, 34], relative valence-band alignments from valence-band
offset measurements [50–52], and calculations [53, 54], the CNL lying above the
bottom of the conduction band can indeed be seen as a universal property of the
PTMOs, as shown in Fig. 6.12. In all of these cases, the low-lying CBM gives rise to
thisbandalignment:suchfeaturesofthebulkbandstructureareapparentinelectronic
structurecalculationsformanyPTMOs[22,37,55].Thisitselfcanbeunderstoodasa
consequence ofthe large sizeandelectronegativity mismatchbetweentheconstituent
post-transitionmetalelementandoxygen[56](seeFig.6.1).Asimilarsituationoccurs
inthesemiconductorInN,withtheCBMalsolocatedbelowtheCNL[56],asshownin
Fig. 6.12. Indeed, the properties of InN exhibit many similarities with the PTMOs
[56–58]. While we stress that this is not a microscopic model, such similarities, and
their differences from conventional semiconductors such as Si and GaAs, are well
explained by the CNL concept (see Fig. 6.12).
Fig. 6.12 Band line-up of a number of PTMOs and other semiconductors, relative to the charge
neutrality level. The PTMOs and InN are highlighted and compared with the conventional
semiconductors, Si and GaAs. Ga2O3is omitted due to the lack of quantitative information
about the location of its band edges with respect to the CNL. The positions of the bands relative
to the CNL are derived from the measurements and calculations in [25, 31, 50–53, 56]
6Electronic Properties of Post-transition Metal Oxide Semiconductor Surfaces141