Article pubs.acs.org/JPCC
Impact of Nitrogen Doping on Electrical Conduction in Anatase TiO2 Thin Films Y. P. Yu,†,‡ W. Liu,§ S. X. Wu,† and S. W. Li†,* †
State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics & Engineering, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China ‡ Department of Physics, Shantou University, Shantou 515063, People’s Republic of China § Institute of Optoelectronics, Shenzhen University, Shenzhen 518060, People’s Republic of China S Supporting Information *
ABSTRACT: Nitrogen doping-induced changes in the electrical conduction of anatase TiO1‑xNx (x ≤ 0.12) thin films were investigated by combining electrical measurement with structural characterization. The Hall effect data indicate that when the doping level reaches 4 at.%, the O substitution with N results in a p-type conduction, in spite of the self-compensation effect, and the Hall mobility of holes is more than 20 cm2/(V s). On the basis of the experimental results from X-ray diffraction, X-ray photoemission spectroscopy, and temperature-resistance relationship, a mechanism involving hopping conduction and band conduction is proposed to interpret the transport behavior of carriers. In addition, the origin of the p-type based on the structural character of the film will also be discussed.
1. INTRODUCTION Anatase titanium dioxide (TiO2) is a wide-band gap (∼3.2 eV) semiconductor material and has been widely used in many fields, such as photocatalysis, photovoltaics, and transparent electronics.1−5 To meet the technological demands from diverse applications, it is necessary to modify the structural and electrical properties of TiO2 through doping with various dopants, including metal and nonmetal ions. Among these, the approach of Nitrogen doping (N-doping), which has the competitive virtues of efficient visible-light sensitivity and less deep-level trap, has attracted much attention for the past decade. However, most studies on N-doped anatase TiO2 are focused on microstructure, electronic structure, optical property, and photocatalyst activity, as well as magnetic origin.6−14 As to the electrical transport properties, K. Pomoni et al.15,16 have investigated two N-induced experimental phenomena, i.e., decrease in conduction activation energy and enhancement of electron trapping. Nevertheless, the detailed study on the transport mechanism in this doped material, to the best of our knowledge, is still limited so far, especially for holeconduction. However, several models, including band conduction,17−19 variable range hopping conduction,20 grainboundary limitation,21−25 small-polaron hopping conduction,26,27 and multitrap filling,28,29 have been suggested to interpret the different performance of electron-conduction in crystalline and nanocrystalline TiO2-based films. However, it is noteworthy that these models are closely related to the structure character of sample. The electrical transport-processes in samples with different crystallinity or doping level would always be different and hence be described by different models. © 2012 American Chemical Society
Thus, two questions are raised, one is which mechanism is dominant for N-doped anatase TiO2, and the other is to what extent is the difference in conduction between N-doped and stoichiometric anatase TiO2. In the present work, pure anatase N-doped TiO2 films are prepared by PAMBE (plasma-assisted molecular beam epitaxy) technology. The N-derived variations in the electrical properties have been investigated by combining temperature-dependent resistance and Hall measurement with structural characterization.
2. EXPERIMENTAL SECTION The sample films were grown on LaAlO3 (001) substrate in OMICRON PAMBE system using the gas mixture of N2 and O2. The high-purity of the raw materials (>99.999%) and the high-vacuum in the growth chamber ensure remarkable suppression for unintended impurity in the film. The substrate temperature was held at 460 °C. The gas flux ratio of N2 to O2 was adjusted to obtain different N concentration in film. The film thicknesses are all about 100 nm estimated by deposition time and growth rate. The high-resolution X-ray photoemission spectroscopy (XPS) was measured using an ESCALAB 250 system (Thermo-VG Scientific) with a monochromatic Al Kα X-ray source at 15 kV and 150 W. The crystal structure was characterized by a high-resolution advance powder X-ray diffractometer using Cu Kα radiation (Bruker-D8). The Halleffect measurement was carried out in an MMR K-2500 system Received: January 2, 2012 Revised: August 7, 2012 Published: August 7, 2012 19625
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at 300 K. The area of the square-shaped sample is 5 × 5 mm2, and four soldered-Indium-contacts, forming van der Pauw configuration, are placed at four edges of the square. Such contact placement ensures the correct carrier-type identification corresponding to the sign of Hall coefficient (RH) even if the investigated film is inhomogeneous in carrier density or mobility.30 The applied magnetic field is perpendicular to the film surface. A vacuum cryostat (Advance Instruments) and a Keithley 617 electrometer were used to measure the temperature dependence of dark resistance (Rd) of film. During the experiment process, a DC bias voltage of 80 V, within the linear interval of current−voltage curve, was continuously applied to the coplanar Indium-contacts separated by a distance of 4.0 mm.
3. RESULTS AND DISCUSSION Prior to the electrical measurement, the characterizations for composition and crystallography of the films were carried out. Figure.1a shows the measured XPS survey spectrum of TiO1.88N0.12 film, and the other two samples have the similar spectrum (not shown) as this. Obviously, only four elements, Ti, N, O, and C, are found in our samples. The Carbon signal is mainly introduced by the surface adventitious carbon during the XPS measurement or the reserve process. As to the Nincorporation in the lattice, the binding energy of 395.8 eV, shown in Figure 1b, indicates that the N3‑ ions primarily substitute for the O2‑ ions in TiO2 host lattice.31,32 The small peaks at 399.8 and 401.8 eV can be ascribed to the titanium oxynitrides in surface.33 On the basis of the integral areas and sensitivity factors of N 1s and O 1s (Figure 1c) signals in the core level spectra, the estimated substitutional N percentage in anion sublattice is 0 at. %, 1.5 at.%, and 6.0 at.%, respectively.31,33 In the X-ray diffraction patterns displayed in Figure 2, only TiO2 anatase (004) peak at 37.6° is observed except the peaks referred to the LaAlO3 substrate, indicating that the crystallites in the films are pure anatase-phase, and that the orientation is preferred. These structure characteristics result from the Oxygen-rich epitaxial growth condition and from the good in-plane lattice matching between LaAlO3 (001) plane and anatase TiO2 (001) plane. Moreover, because the crystallographic quality is lowered by Ndoping, the shape of the (004) peak varies with N concentration, as shown in the inset in Figure 2. From the fwhm’s of the peaks, the average grain-sizes of our samples can be estimated to be more than 19 nm according to the Sherrer’s formula. In brief, the investigated films consist of the orientation-preferred columnar anatase-pure grains. This finding agrees with the TEM results reported by Cheung et al.,34 in which the highly crystalline and phase-pure N-doped TiO2 films were prepared by the same PAMBE technology as this work. In general, dark resistance of TiO2 film is temperature dependent, and some conduction features of charge carriers could be recognized by this relationship. Figure 3 shows the plots of ln Rd versus 103/T in the temperature range 130−290 K for the films. Because the measurement was performed under the nearly same conditions of film thickness and contact configuration, these plots also manifest the relative variation in film resistivity. In Figure 3, two linear curves in each plot imply that there are two contributions to the conduction mechanism of the samples, depending on different domains of temperature. The least-squares fitting to the data of the lines gives the thermal activation energy (Ea) displayed in the figure.
Figure 1. (a) XPS survey spectrum of TiO1.88N0.12 film. (b), (c) N 1s and O 1s core level spectra of TiO2, TiO1.97N0.03, and TiO1.88N0.12 films. The Ns3− denotes the substitutional N3− for lattice O2−.
It is well-known that the electrical conduction of semiconductor material is determined by carrier concentration and its transport feature. As to the anatase TiO2, Oxygen vacancy (Vo) has been known as the most common native defect, which introduces a donor-like local level below conduction band minimum CBM. If temperature is increased, electrons will be activated from the Vo levels to the conduction band to become mobile. The transport process of these electrons is free of small-polaron effect because the weak electron-multiphonon couplings cannot compensate the energy required for the strong local lattice distortion in the anatase.35 Thus, in the higher temperature domain, the linear decrease in Rd of the undoped film can be attributed to a band-conduction of the thermally activated electrons, which is quite different from the small-polaron hopping process in rutile TiO2.26,36 Moreover, because the density (n) of the electron activated into 19626
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of the reduced Rd or Ea by N-doping is attributed to a higher n arising from the more Vo’s.15,16 However, such reason cannot account for the results of the TiO1.97N0.03 film. First, the rises of n and Vo density will enhance the electron hopping-conduction in principle, leading to a lower Rd than that of the undoped TiO2 at low temperature. Obviously, this expectation is contrary to the shown result that the Rd of TiO1.97N0.03 film is about 1 order of magnitude higher than that of the TiO2 in T < 170 K range. Second, the n rise means that the Fermi level shifts toward CBM, producing a lowered Ea, but the measured values of Ea (0.25 vs 0.24 eV) do not support this point. The above analysis, thereby, suggests that the conduction mechanism in the doped films should be different from that in anatase TiO2, and that a reasonable explanation should not rely on the electron-conduction. Actually, many experiments and DFT (density functional theory) calculations have revealed that substitutional N dopants introduce localized N 2p states just above valence band maximum.10,41,42 Because N-doping also leads to both lattice and crystal-field to be somewhat disordered, these states, at sufficiently high doping level, may mix with O 2p states at the top of valence band to form localized band-tail in terms of Anderson localization. When the N dopants are not completely compensated by Vo’s (i.e., the N density is more than two times the Vo density), there are a few unoccupied N or O 2p states acting as acceptors. In such a case, hole-hopping between the neighboring localized centers (localized states in band-tail) is responsible for the electrical conduction at low temperature. When temperature increases, the holes are thermally activated to the extended states above the mobility edge of valence band, so that band-conduction becomes dominant. This mechanism can be verified by the experimental results of the doped films, shown in Figure 3. As temperature increases, the conduction is changed from the hole hopping-conduction to the hole bandconduction. The critical temperature is about 170 K. An increment in N concentration causes not only the lower resistance, but also the changed Ea. For the hopping process, Ea stands for the average energy-difference between neighboring localized centers.43 A slight increase in Ea from 0.03 to 0.05 eV can be ascribed as an enhancement of wave function overlap between these centers due to the N concentration increase. For the band-conduction in the range of 290−170 K, Ea, referring to the distance between the Fermi level and the mobility edge, will decrease with increasing acceptor density (through increasing N concentration), as displayed in Figure 3. Accordingly, a width of the band-tail in N-doped films is estimated from the Ea value to be about 0.2 eV, which matches the observed data in the experiments on optical absorption and XPS.32,33,44 To further find experimental evidence, we conducted the Hall measurement on our samples at room temperature. As the resistances of the TiO2 and the TiO1.97N0.03 films exceed the measurement high-limit, only the Hall data of the TiO1.88N0.12 film can be obtained. The experimental results measured with an applied field B sweep in the range from −11 000 to 11 000 G are plotted in Figure 4a. It can be found that the RH is a monotonically decreasing function of field strength (B < 8000 G). This feature, being a typical anomalous behavior also found in other semiconductors,45 has been attributed to the deficient field strength and the various scattering processes.45−48 The scattering centers in the film mainly include charged Vo+, Vo2+, and N3−. For the same sakes, the variation in the resistivity is under the measurement limit, and the corresponding magneto-
Figure 2. XRD patters of TiO2, TiO1.97N0.03, TiO1.88N0.12, films and LaAlO3 substrate. The inset shows the variation in anatase (004) peak.
Figure 3. Dark resistance Rd as a function of 103/T, measured on TiO2, TiO1.97N0.03, and TiO1.88N0.12 films. Ea is the thermal activation energy.
conduction band is proportional to exp[−(Ec − EVo)/2kT], (Ec and EVo are the energy of the CBM and the Vo level, respectively. Variable k is the Boltzmann’s constant), it can be deduced from the Ea value of 0.24 eV in activation domain that the EVo locates around 0.5 eV below the CBM. This is in accord with the conclusions in refs 37 and 38, where the calculations reveal that the Vo induces a shallow donor-level in TiO2. In the range of 130−210 K (in Figure 3), the Rd of the undoped anatase TiO2 film is almost temperature-independent (Ea ≈ 0), indicating that most electrons are not activated in the low temperature environment. Accordingly, the transport of these localized electrons is via hopping between adjacent Vo centers. This hopping conduction is essentially a quantum tunneling process, and its probability is always insensitive to temperature, and will decrease with increasing the mean distance between Vo’s. Once a few O2− ions are replaced with N3− ions, the conduction property will be influenced. The studies on Ndoped TiO2 have revealed that the substitution of N to O brings about three effects: the rise in Vo density, the compensation to Vo’s and the enhancement of electrontrapping.15,16,39,40 The latter two will lower the density and the mobility of the mobile electron respectively, giving a rise in Rd. Thus, in the case of electron band-conduction, the observation 19627
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density, which indicates that the boundary barrier effect is minor for the carrier transport. It is well-known that the doping effect is closely related to the microstructure. Both anatase and rutile structures can be viewed as a network of coordinated TiO6 octahedra. The coordination of octahedra is less dense in anatase than in rutile, leaving anatase in a more open and flexible structure.51 Thus, the finding of p-type conduction in N-doped rutile TiO2, reported by Bao et al. and Matsushita et al. recently,52,53 convinces us that for N-doped anatase, substitutional N-doping is also a feasible approach to realize hole-conduction. Moreover, N-doping for the perfect lattice will facilitate the formation of Vo’s to release the local stress and to keep the electrical neutrality. As a result, the microstructure containing Vo’s becomes slightly disordered and more flexible, leading to an increase in the solubility of N dopant. This should be a predominant cause for the appearance of p-type conduction. In addition, the doping efficiency is also influenced by the defects at grain-boundaries, especially trap centers. At a high defectdensity, the Fermi level will be pinned by these defects, so that the conduction type is difficult to adjust well.24 For our investigated films, however, this unintended effect could be efficiently suppressed by the structure characteristics of preferred grain-orientation and pure phase. In other words, owing to the orientation-preferred growth and the nice epitaxy technology, less defects exist at grain-surfaces or boundaries compared to those multiphase or multiorientation films.
4. CONCLUSIONS For the anatase TiO2 thin film with preferred orientation, the substitution of O with N leads to a degradation of crystallinity, but the electrical conduction can be changed from n to p-type. As temperature increases from 130 to 290 K, two thermally activated processes with exponential characteristic appear in the lower (T < 170 K) and the higher temperature regions, respectively. Accordingly, the hole-transport mechanism in the N-doped film is changed from the hopping-conduction between localized states in band-tail to the band-conduction above the mobility edge in valence band. When the N concentration reaches 4 at.%, the estimated hole concentration in the film is lower than 1 × 1017 cm−3, and the Hall mobility is higher than 20 cm2/(V s).
Figure 4. (a) Plot of the Hall coefficient RH and the resistivity ρ as a function of magnetic field B, measured on TiO1.88N0.12 film. The inset shows the van der Pauw contact-placement in the Hall measurement. (b) Plot of RH−1 versus B2 corresponding to part a. A straight-line appears in weak field region.
resistance could be negligible. Most importantly, the signs of the RH’s are positive, indicating clearly that the major carriers are holes. To our knowledge, it should be the first observation for N-doped anatase TiO2. In the quasi-classical approach, the field dependence of RH in weak field region (ωτ ≪ 1) can be expressed as RH = −(pe/rH + pe3⟨τ2⟩B2/m*2)−1,49 where ω is the cyclotron frequency; τ is the momentum relaxation time; p, e, m*, and rH are the hole density, electron charge, effective mass, and Hall factor, respectively. A straight-line dependence corresponding to small fields is revealed in the plot of RH−1 versus B2 presented in Figure 4b, and a linear extrapolation to zero gives a value of pe/rH to be 0.008 Coul/cm3. Considering that the rH has a theoretical value between 1 and 2,45,49 the values of hole density and Hall mobility in the TiO1.88N0.12 film are determined to be within the range of 5 × 1016−1 × 1017 cm−3 and 20−40 cm2/(V s), respectively. The latter is consistent with the reported data in ref 50. Comparing the hole density with the N concentration, we find that N dopants are almost compensated by the Vo’s induced by dopant-selfbecause of the self-compensation effect. From the value of p, the Debye length LD = (εεokT/pe2)1/2 (ε is the low frequency dielectric constant; εo is the dielectric constant of vacuum) is evaluated to be over 20 nm,43,51 which means that the depletion width is larger than the average grain-size. Therefore, the band bending around boundary is insignificant due to the low carrier
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ASSOCIATED CONTENT
S Supporting Information *
The dependence of the dark resistance on temperature, and the achievement of the activation energy. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The authors would like to acknowledge financial support from the National Natural Science Foundation of China (Grant No. 60977021) and Natural Science Foundation of Guangdong Province (Grant No. S2011020001190). 19628
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