Elsevier

Thin Solid Films

Volume 519, Issue 7, 31 January 2011, Pages 2103-2110
Thin Solid Films

Measurement method for carrier concentration in TiO2 via the Mott–Schottky approach

https://doi.org/10.1016/j.tsf.2010.10.071Get rights and content

Abstract

In direct contrast to the way in which silicon is precisely doped for integrated circuit applications in order to optimize device performance, there is little nuanced understanding of the correlation between TiO2 doping level, charge carrier concentration, and the operation of TiO2-based photocatalysts, dye-sensitized solar cells, and sensors. The present work outlines a rigorous methodology for the determination of free carrier concentration for doped metal oxide semiconductors such as TiO2 that are not amenable to standard metrology methods. Undoped, Cr-, Mn-, and Nb-doped polycrystalline anatase TiO2 are synthesized via atomic layer deposition (ALD) using Ti(OCH(CH3)2)4, H2O, Cr(C5H7O2)3, Mn(DPM)3 (DPM = 2,2,6,6-tetramethyl-3, 5-heptanedionato), and Nb(OCH2CH3)5 as the source materials for Ti, O, Cr, Mn, and Nb, respectively. Chemical composition and crystallinity are investigated and a thorough “device-like” characterization of TiO2 Schottky diodes is carried out to justify the subsequent extraction of carrier concentration values from capacitance–voltage (C–V) measurements using the Mott–Schottky approach. The influence of factors such as substrate type, contact metal type, and surface and interface preparation are examined. Measurements of donor carrier concentration are obtained for undoped, Cr-, Mn-, and Nb-doped TiO2 synthesized by ALD. Possible causes for the obtained carrier concentrations are discussed.

Introduction

Many metal oxide semiconductors with wide band gaps (> 3 eV) possess electronic properties that, in combination with chemical stability and modest cost, suit them well for important applications such as photocatalysis [1], [2], dye-sensitized solar cells [3], and gas sensing of environmental pollutants [4], combustion products [5], and explosives [6]. Such applications would benefit greatly from the same precision in controlling charge carrier concentration via doping that is presently possible for conventional microelectronic devices based on silicon and III–V semiconductors. In photocatalysts and dye-sensitized solar cells, for example, such control would permit manipulation of near-surface or near-interface electric fields to optimize the flow of photogenerated charge carriers [7]. In gas sensors based on electrical conductivity such control would improve the sensitivity, which depends upon background charge carrier concentration [8].

Unfortunately, doping of metal oxides to achieve the requisite control is notoriously difficult due to the lack of readily available dopants providing shallow donor and acceptor levels [9]. Similarly troublesome is the accuracy of the available metrology tools for measuring carrier concentration. Commonly used methods such as four-point-probe or Hall effect measurements require ohmic contacts to the oxides, yet real contacts to wide-bandgap semiconductors usually create Schottky barriers that act like diodes. Both problems probably contribute to the wide variance of n-type carrier concentrations found in the literature in the case of anatase TiO2, ranging from approximately 1 × 1016 cm−3 to 1 × 1020 cm−3 [10], [11]. Such variations complicate attempts to perform defect engineering in TiO2 [12].

This study focuses on metrology — in particular, the use of “Schottky diode” test structures for determining the carrier concentrations of anatase TiO2 thin films synthesized via atomic layer deposition (ALD) based upon capacitance–voltage (C–V) measurements [13]. Related studies have been reported elsewhere for anatase TiO2 deposited using alternative methods, as well as other metal oxides such as ZnO [14], but vital electrical characteristics of the diode structures are not always described or characterized adequately. In some instances, necessary current–voltage (I–V) data are not shown [15] or even collected [11], [16]. Sometimes both I–V and C–V data are missing [17], which is problematic for such nontrivial measurements. Similar problems afflict literature for single crystal rutile TiO2 [18], [19].

The evaluation of metal oxide carrier concentration in this manner is an attractive alternative to conventional four-point-probe or Hall effect measurements for several reasons. Given the 3.9 eV work function of TiO2 [20], a characterization method that precludes forming an ohmic contact to the TiO2 surface is preferable. Additionally, ohmic contact recipes that entail an annealing step (i.e. Al to n-type Si wherein the Al–Si eutectic region formed under the Al contact is critical) are out of the question, since high-temperature processing irreversibly alters the stoichiometry and thus carrier concentration of metal oxides. Lastly, the C–V method circumvents the need to form multiple contacts to the semiconductor surface. The cloverleaf, square, or rectangular four contact geometries necessary for Hall effect measurements may be restrictive with respect to sample size or preparation method. Since these geometries assume a laterally homogeneous metal oxide film, it is also not possible to probe spatial variations in carrier concentration as a result of gradients in thickness or doping level.

The present work outlines a sound metrological method for the evaluation of TiO2 charge carrier concentration using I–V and C–V measurements. In particular, detailed electrical characterization of TiO2 Schottky diodes is performed. The influence of transition metal doping on TiO2 donor carrier concentration is investigated using the Mott–Schottky approach.

Section snippets

Preparation and characterization of polycrystalline TiO2 films

Thin film polycrystalline anatase TiO2 was synthesized via atomic layer deposition (ALD) and in some cases doped to ~ 2 at.% with transition metals such as Cr, Mn, and Nb. Deposition took place within an approximately 3 L stainless steel vacuum chamber evacuated with a mechanical pump to a base pressure of about 6.7 Pa. The substrates were mounted on a resistively heated chuck, whose temperature was maintained at 200 °C during deposition and monitored with a chromel–alumel (type K) thermocouple.

Structural and chemical characterization

XPS confirmed introduction of the metal dopants into the films and provided an estimate of surface elemental composition. Fig. 2 shows the XPS spectra for the Cr 2p, Mn 2p, and Nb 3d core-levels. The binding energy scale has been referenced to the C 1s peak at 285 eV. Quantitative analysis indicates that Cr, Mn, and Nb doping levels of approximately 2 at.% were obtained. Ti 2p3/2 and Ti 2p1/2 peaks are situated at 458.6 and 464.3 eV for undoped TiO2 and shift by ~ 0.3 eV in all doped films. The

Conclusion

The doping science of metal oxide semiconductors such as TiO2 is hardly straightforward. As a consequence, TiO2 “device” performance is typically correlated to atomic percentage of dopant in the bulk [40], [41]. A wide variety of dopants have been investigated, although nuances such as electrical activation and solubility have yet to be fully fleshed out. Indeed, there is still much to be learned about the energy levels of impurity atoms [9] and the exact way in which these species alter the

Acknowledgements

This work was carried out in part at the Center for Microanalysis of Materials at the Frederick Seitz Materials Research Laboratory, University of Illinois, which is partially supported by the U.S. Department of Energy (DE-FG02-07ER46453 and DE-FG02-07ER46471). We acknowledge funding from the National Science Foundation through a Graduate Research Fellowship (M.C.K.S.) and Grant DMR 07-04354. We are grateful for the assistance of Mauro Sardela, Rick Haasch, Dane Sievers, and Edmond Chow.

References (48)

  • A. Vaseashta et al.

    Sci. Technol. Adv. Mater.

    (2007)
  • N.O. Savage et al.

    Sensor Actuat. B - Chem.

    (2001)
  • M. Takahashi et al.

    Thin Solid Films

    (2001)
  • J.T. Mayer et al.

    J. Electron. Spectrosc. Relat. Phenom.

    (1995)
  • J. Zhu et al.

    Appl. Catal., B

    (2006)
  • M. Ni et al.

    Sol. Energy Mater. Sol. Cells

    (2006)
  • A.P. Singh et al.

    Int. J. Hydrogen Energy

    (2008)
  • T. Ioannides et al.

    J. Catal.

    (1995)
  • J.A. von Windheim et al.

    Diamond Relat. Mater.

    (1993)
  • K. Hashimoto et al.

    Jpn. J. Appl. Phys.

    (2005)
  • A.L. Linsebigler et al.

    Chem. Rev.

    (1995)
  • B. O'Regan et al.

    Nature

    (1991)
  • S. Banerjee et al.

    Nanotechnology

    (2009)
  • D. Morris et al.

    J. Mater. Chem.

    (2001)
  • E.G. Seebauer et al.

    Charged Semiconductor Defects: Structure, Thermodynamics, and Diffusion

    (2009)
  • C.S. Enache et al.

    J. Electroceram.

    (2004)
  • E.-J. Lee et al.

    J. Appl. Electrochem.

    (1992)
  • M.K. Nowotny et al.

    J. Phys. Chem. C

    (2008)
  • D.K. Schroder

    Semiconductor Material and Device Characterization

    (1998)
  • C.F. Windisch et al.

    J. Vac. Sci. Technol. A

    (2000)
  • R. O'Hayre et al.

    J. Phys. Chem. C

    (2007)
  • J. Yan et al.

    J. Vac. Sci. Technol. B

    (1996)
  • T. Miyagi et al.

    Jpn. J. Appl. Phys.

    (2004)
  • K. Kobayashi et al.

    J. Appl. Phys.

    (1986)
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