APPLIED PHYSICS LETTERS 86, 042104 共2005兲
Unraveling the conduction mechanism of Al-doped ZnO films
by valence band soft x-ray photoemission spectroscopy
Mercedes Gabása兲
Departamento de Física Aplicada I, Universidad de Málaga, 29071 Málaga, Spain
Susana Gota
CEA-DSM/DRECAM-SPCSI, CEA-Saclay, 91191 Gif-sur-Yvette, France
José Ramón Ramos-Barrado and Miguel Sánchez
Departamento de Física Aplicada I, Universidad de Málaga, 29071 Málaga, Spain
Nicholas T. Barrett
CEA-DSM/DRECAM-SPCSI, CEA-Saclay, 91191 Gif-sur-Yvette, France
José Avila and Maurizio Sacchi
LURE, Centre Universitaire Paris Sud, B.P. 34, 91898 Orsay Cedex, France
共Received 6 April 2004; accepted 13 December 2004; published online 18 January 2005兲
We report on the correlation between the electrical behavior and valence band spectra of undoped
and Al-doped ZnO films, obtained by using x-ray photoelectron spectroscopy. Although Al-doping
can induce a conductivity increase of two orders of magnitude, we show that the gap persists and
there is no semiconductor–metal transition upon doping. For the 3% Al-doped ZnO film, we
measure a reduction in the band gap of ⬃150 meV with respect to the undoped and the 1% doped
films. Our results suggest that the band conduction mechanism proposed for undoped ZnO at room
temperature still dominates the conduction process in doped films. © 2005 American Institute of
Physics. 关DOI: 10.1063/1.1856141兴
Transparent conductive oxide 共TCO兲 films have been extensively used in optoelectronic devices because of their high
visible transmittance and low dc resistivity. Recent investigations point out that ZnO, combining semiconducting behavior with optical transparency in the visible range, will be
one of the most competitive semiconductors in the near
future.1 The addition to ZnO of small amounts of Al results
in a marked increase in the electrical conductivity,2 the optical properties remaining excellent for devices.3 Similar effects were observed with other IIIa elements as dopants, such
as indium and gallium.4 From the technological point of
view, spray pyrolysis is an interesting alternative to other
methods to obtain such antireflective TCO coatings.5 Several
reports exist on the dc conductivity of ZnO thin films and
crystals at room temperature, where a band conduction
mechanism has been proposed.6 Variable-range hopping conduction has been invoked in H2-annealed films below
250 K.7 The influence of Al-doping on the electronic structure of ZnO has been addressed by Ohashi et al.8 using bandedge emission to probe shallow donor states in the band gap.
Some details of the doping effect on the band structure,
though, remain unexplained. An open question is whether
increased conductivity in doped films is accompanied by
changes in the conduction mechanism compared to undoped
ones. For elucidating this point, we have performed photoelectron spectroscopy 共PES兲 in the valence band 共VB兲 region
as a function of the Al-doping in ZnO films.
Pure ZnO and Al-doped films about 100 nm thick were
prepared onto quartz substrates by spray pyrolysis. The
deposition parameters were chosen to ensure a good optical
transparency. The structure and chemical analysis confirmed
a兲
Electronic mail: mgabas@uma.es
the formation of ZnO and the absence of other phases or
compounds. More details about preparation and characterization of undoped and Al-doped ZnO films have been published elsewhere.9 The resistivity of the films was determined
using Van der Pauw’s method. It decreases by two orders of
magnitude 共from 816 to 15.35 ⍀ cm兲 with respect to the undoped sample for an atomic ratio 关Al兴 / 关Zn兴 of 3%. Higher
Al-doping leads to increased resistivity.10
PES spectra were recorded using synchrotron radiation
at the SA73 beamline of the SuperACO storage ring 共LURE,
Orsay兲. Before spectra acquisition, samples were cleaned by
Ar+ etching 共energy 500 eV兲. High resolution valence band
PES spectra were recorded at room temperature, using photon energies of 35 and 70 eV. In both cases, the overall resolution 共convolution of the beamline and the electron analyser
contributions兲 was ⬃60 meV. The photon energy h␯
= 35 eV corresponds to the best compromise between the
maxima of the beamline photon flux and of the VB cross
section. The other energy of 70 eV was chosen because of its
closeness to the Al-2p core level binding energy, avoiding
second-order effects. The binding energies are referred to the
chemical potential measured on a clean Cu sample 共␮Cu兲 in
electrical and thermal contact with the samples.
Figure 1 shows the spectra measured at h␯ = 35 eV: we
observe three broad peaks at binding energies 共Eb兲 of ⬃11,
⬃8, and ⬃6 eV. The peak at the deepest Eb 共⬃11 eV兲 is
attributed to the Zn-3d band. In the VB region, the peak at
Eb ⬃ 8 eV involves the O-2p orbitals hybridized with Zn-4s
and Zn-4p ones, while the lowest Eb feature 共⬃6 eV兲 is attributed mainly to the O-2p orbitals.11,12 In Fig. 2 共h␯
= 70 eV兲, we observe again the same three broad bands, now
centered at Eb ⬃ 11, 7.9, and 5.2 eV. After referring the binding energies to ␮Cu, the spectra were aligned to the leading
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86, 042104-1
© 2005 American Institute of Physics
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042104-2
Gabás et al.
FIG. 1. PES spectra of the VB region recorded with a photon energy of
h␯ = 35 eV for the undoped ZnO and 1% and 3% Al doped samples. The
chemical potential 共␮兲 measured in a clean Cu sample in electrical contact
with the samples is plotted together. Inset: zoom of the low Eb part of the
VB region.
edge of the Zn-3d peak. The spectra intensity was interpolated by fitting the background at binding energies well
above 共⬍2 eV兲 and below 共⬎14 eV兲 the VB. The results
presented in Figs. 1 and 2 are fully reproducible using a
second series of samples. For a given photon energy, we
observe that the overall spectra are almost independent of
doping. These data match the VB PES results previously
published for undoped single crystals11–13 and powdered14
ZnO. In addition, our results provide a comparison between
the VB PES spectra of undoped and Al-doped ZnO thin
films. An important result is the evidence that none of the
samples 共undoped ZnO, and 1% and 3% doped films兲 presents any density of states in the forbidden band gap region
for both series of spectra 共h␯ = 35 eV and h␯ = 70 eV兲.
Appl. Phys. Lett. 86, 042104 共2005兲
The small differences between the spectra with doping
are localized at the uppermost edge of the PES VB spectra.
The insets of Figs. 1 and 2 are an enlargement of this region.
In the case of the 3% Al doped sample, we measure a shift of
⬃150 meV of the uppermost edge of the VB to lower binding energies with respect to the other samples 共1% Al-doped
and undoped ZnO兲. The same result is obtained at h␯ = 35
and 70 eV, supporting its reliability.
We interpret this shift as a narrowing of the band gap. It
is known 共see, e.g., Ref. 8, and references therein兲 that the
hybridization between states of the ZnO matrix and of the Al
dopant yields new donor electronic states located just below
the lowermost edge of the conduction band 共CB兲. As a consequence, the chemical potential of the 3% Al-doped ZnO
films shifts ⬃150 meV upwards, approaching the bottom of
the CB. This fact can be pictured as a narrowing of the
effective band gap, and gives rise to an increase of the conductivity. Conclusions about the occupation of the conduction band as a function of doping cannot be drawn directly
from our PES experiments.
Recently, Imai et al. have published electronic band calculations of Ga-doped and undoped ZnO and ZnS, using
density functional theory.15 Previous experimental reports indicate that similar effects are observed upon Al and Ga
doping.16 Assuming Ga in a substitutional unrelaxed site,
Imai et al. obtain a similar density of states for undoped and
Ga-doped ZnO. They also find that the chemical potential of
undoped ZnO 共␮ZnO兲 is well below the bottom of the CB.
Our PES data agree with the former result, but are at variance with the latter. In both sets of PES spectra 共h␯ = 35 eV
and h␯ = 70 eV兲, we observe a separation of ⬃3 eV between
the edge of the VB and ␮Cu. Therefore, the chemical potential of both the undoped and the Al-doped ZnO films is located very close to the lowermost conduction band edge,
assuming that the gap of undoped ZnO is 3.4 eV. This position of the chemical potential is consistent with an n-type
semiconductor. Although Imai et al. also obtain a displacement of ␮ in the case of doped ZnO toward the conduction
band edge, the shift does not agree quantitatively with our
experimental results. They suggest that ␮ shifts into the CB,
i.e., the electrons from the dopant Ga atoms occupy the CB,
resulting in metallic behavior of the Ga-doped ZnO. Our
experimental results, for Al doping, disagree with this finding. The energy distance between the uppermost edge of the
VB and ␮Cu is about 3.05 and 2.9 eV for the undoped and
the 3% doped sample, respectively.17 This means that the
chemical potential of the 3% Al-doped sample shifts toward
the lowermost CB edge by ⬃150 meV with respect to the
other samples, but the semiconductor character remains in
spite of the doping. This shift of ⬃150 meV is very small but
measurable, thanks to the good energy resolution obtained by
combining synchrotron radiation and a high performance
electron analyzer.
The consequence of the experimental results presented
in this letter is that the band conduction mechanism generally
admitted for ZnO6 should also be dominant for the conduction process in doped films. Therefore, a semiconductor–
metal transition accompanying the conductivity jump should
be excluded. A slight decrease of the optical band gap
共⬃90 meV兲 has also been observed in transmittance measurements when comparing undoped and 3% Al-doped ZnO
FIG. 2. PES spectra of the VB region recorded with a photon energy of
h␯ = 70 eV for the 1% and 3% Al doped samples. The chemical potential 共␮兲
measured in a clean Cu sample in electrical contact with the samples is
plotted together. Inset: zoom of the low Eb part of the VB region.
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042104-3
Appl. Phys. Lett. 86, 042104 共2005兲
Gabás et al.
films prepared by spray pyrolysis.10 The absorption edge
shifts toward higher values as the aluminum concentration
increases up to a ratio of 关Al兴 / 关Zn兴 = 3%, the ratio at which
the resistivity minimum occurs. For higher values, absorption edges overlap. These results compare well, qualitatively
and quantitatively, with our above-reported findings. Results
similar to what we found in ZnO upon doping are observed
in Fe3O4 across the Verwey transition.18 Magnetite exhibits a
first-order phase transition at TV ⬵ 120 K, with the dc conductivity abruptly increasing by two orders of magnitude
when heating through TV. PES spectra taken above 共130 K兲
and below 共110 K兲 TV are identical, except in the region very
near the valence band threshold. Neither spectra show a
Fermi step. The only detectable change between them is that
the high temperature spectrum is shifted by ⬃50 meV to
lower binding energy. The band gap does not collapse, but is
merely reduced by ⬃50 meV, with no sign of a real
insulator–metal transition. Although the parameter controlling the transition in magnetite is the temperature, the persistence of the gap and its reduction are very similar to the
results we found for ZnO upon doping. The Fe3O4 and ZnO
band gap behavior across the conductivity transition is quantitatively similar, with a shift of several tens of millielectron
volts.
In summary, the hybridization between the orbitals of the
Al dopant and of the ZnO matrix should lead to the appearance of electronic states located just below the lowermost
edge of the conduction band. This is reflected in the upwards
shift of the chemical potential, i.e., in the narrowing of the
effective band gap. No semiconductor–metal transition accompanied the conductivity jump in our 3% Al-doped films.
As a consequence, the band conduction mechanism should
also be the principal contribution to the conduction process
in doped films, as is the case in the undoped ZnO.
This work was funded by the Acción Integrada HF20010145 D.G.I. and by MAT2000-1505 MCyT 共Spain兲, and by
the Programme d’Actions Intégrées PICASSO n. 04344SH,
Ministère des Affaires étrangères et Ministère de la Recherche 共France兲. The authors are grateful to the S.C.A.I. of
Málaga University and V. Pérez-Dieste for their technical
collaboration.
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