Unraveling the conduction mechanism of Al-doped ZnO ﬁlms by valence band soft x-ray photoemission spectroscopy

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Unraveling the conduction mechanism of Al-doped ZnO films by valence band soft x-ray photoemission spectroscopy

Mercedes Gabás^a^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 ex-

tensively used in optoelectronic devices because of their high visible transmittance and low dc resistivity. Recent investi- gations 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.¹The addition to ZnO of small amounts of Al results in a marked increase in the electrical conductivity,²the 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.⁵Several reports exist on the dc conductivity of ZnO thin films and crystals at room temperature, where a band conduction mechanism has been proposed.⁶Variable-range hopping conduction has been invoked in H₂-annealed films below 250 K.⁷The influence of Al-doping on the electronic struc- ture of ZnO has been addressed by Ohashi et al.⁸using band- edge 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

the formation of ZnO and the absence of other phases or compounds. More details about preparation and characteriza- tion of undoped and Al-doped ZnO films have been published elsewhere.⁹The 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 un-

doped 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E

b

兲 of ⬃11,

⬃8, and ⬃6 eV. The peak at the deepest E

b

共⬃11 eV兲 is

attributed to the Zn-3d band. In the VB region, the peak at E_b

⬃8 eV involves the O-2p orbitals hybridized with Zn-4s

and Zn-4p ones, while the lowest E_b feature

共⬃6 eV兲 is at-

tributed 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 E_b

⬃11, 7.9, and 5.2 eV. After referring the bind-

ing energies to␮Cu, the spectra were aligned to the leading

a兲Electronic mail: mgabas@uma.es

APPLIED PHYSICS LETTERS 86, 042104

共2005兲

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/042104/3/$22.50 86, 042104-1 © 2005 American Institute of Physics Downloaded 13 Apr 2007 to 150.214.40.140. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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edge of the Zn-3d peak. The spectra intensity was interpo- lated 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 crystals^11–13 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 pre-

sents any density of states in the forbidden band gap region for both series of spectra

共h

␯= 35 eV and h␯^{= 70 eV}

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 bind-

ing 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 con-

sequence, 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 cal- culations of Ga-doped and undoped ZnO and ZnS, using density functional theory.¹⁵Previous experimental reports in- dicate 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 vari- ance 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 posi- tion of the chemical potential is consistent with an n-type semiconductor. Although Imai et al. also obtain a displace- ment 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 find- ing. 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 mea-

surements when comparing undoped and 3% Al-doped ZnO

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 Ebpart of the VB region.

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 Ebpart of the VB region.

042104-2 Gabáset al. Appl. Phys. Lett. 86, 042104

共

2005

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Downloaded 13 Apr 2007 to 150.214.40.140. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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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 Fe₃O₄across the Verwey transition.¹⁸Magnetite exhibits a first-order phase transition at T_V

⬵120 K, with the dc con-

ductivity abruptly increasing by two orders of magnitude when heating through T_V. PES spectra taken above

共130 K兲

and below

共110 K兲 T

Vare 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 control- ling the transition in magnetite is the temperature, the persis- tence of the gap and its reduction are very similar to the results we found for ZnO upon doping. The Fe₃O₄and 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 appear- ance 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 HF2001- 0145 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 Recher- che

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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