Interrelation between Chemical, Electronic, and Charge Transport

Article pubs.acs.org/JPCC

Interrelation between Chemical, Electronic, and Charge Transport Properties of Solution-Processed Indium−Zinc Oxide Semiconductor Thin Films Marc Ham ̈ ing,*,†,‡,∥ Alexander Issanin,‡,§ Daniel Walker,‡,§,● Heinz von Seggern,‡,§ Wolfram Jag̈ ermann,‡,§ and Klaus Bonrad†,‡ †

Merck KGaA, D-64293 Darmstadt, Germany Merck − TU Darmstadt Laboratories, D-64287 Darmstadt, Germany § Department of Materials Science, Technical University of Darmstadt, D-64287 Darmstadt, Germany ‡

S Supporting Information *

ABSTRACT: Solution-processed metal oxide semiconductors are of high interest for the preparation of high-mobility transparent metal oxide (TMO) semiconductor thin films and thin film transistors (TFTs). It has been shown that the charge transport properties of indium−zinc oxide (IZO) thin films from molecular precursor solutions depend strongly on the preparation conditions, in particular on the precursor conversion temperature Tpc and, to some surprise, also on the concentration of the precursor solution. Therefore, the chemical and the electronic structure of solution-processed IZO thin films have been studied in detail with Xray photoelectron spectroscopy (XPS) under systematic variation of Tpc and the concentration of the precursor solution. A distinct spectral feature is observed in the valence band spectra close to the Fermi level at EB = 0.45 eV binding energy which correlates with the trends in the sheet resistivity, the field effect mobility μFE, and the optical gap Egopt from four-pointprobe (4PP), TFT, and UV−vis measurements, respectively. A comprehensive model of the interrelation between the conditions during solution-processing, the chemical and electronic structure, and the charge transport properties is developed. and short energy payback time,10 compatibility with flexible substrates, and the perspective to easily customize products by digital printing techniques.11 Previous research activities were predominantly focused at organic materials and devices.12−16 Recently, very promising printing and coating approaches have been developed for producing high-mobility TMO semiconductor thin films and TFTs from molecular precursor solutions in a two-step process in air: First, a solution of precursor molecules is casted onto a substrate, and in a second step, the precursor film is decomposed and converted into a TMO thin film by either heating or irradiation.17−21 It has been demonstrated that IZO and IGZO TFTs of high quality can be fabricated from molecular precursor solutions which meet several important criteria for future applications, e.g. fabrication and operation in air, operation voltages of a few V, large Ion − Ioff ratio, small hysteresis, and several cm2/(V s) high field-effect (FE) mobility.17−22 Therefore, solution-processed high-mobility TMO semiconductor thin films and TFTs in general and solution-processed IZO and IGZO thin films and TFTs in particular23−25 have become of high interest in addition to semiconductors and TFTs based on graphene,26 carbon

1. INTRODUCTION Transparent metal oxide (TMO) semiconductors are of high technological interest as they are key components for the fabrication of transparent electrical contacts, transparent electronic circuits, and corresponding electronic devices such as flexible displays, solar cells, and electrochromic smart windows.1,2 Several methods have been developed for fabricating TMO thin films of high quality based on sputtering and chemical or physical vapor deposition (CVD/PVD), respectively.2 The basic material’s properties of TMO thin films in general and ZnO and indium−tin oxide (ITO) thin films in particular have been studied intensively during the last decades.2−8 More recently, amorphous TMO semiconductors, in particular indium−zinc oxide (IZO) and indium−gallium−zinc oxide (IGZO) semiconductor films, moved into the focus of current research as these semiconductors provide opportunities for fabricating high-end transparent thin film transistors (TFTs) and electronics for novel displays with large area deposition methods.2,9 Their high charge carrier mobility of up to 40 cm2/(V s) and the high on-to-off ratio of corresponding TFTs, for example, allow for new developments (e.g., portable electronic devices with high-resolution OLED displays and very low power consumption).1,2,9 Furthermore, printed electronics is expected to become a key technology due to attractive opportunities of solution-based printing and coating methods, including low fabrication cost © 2014 American Chemical Society

Received: February 25, 2014 Revised: May 23, 2014 Published: June 5, 2014 12826

dx.doi.org/10.1021/jp501956z | J. Phys. Chem. C 2014, 118, 12826−12836

The Journal of Physical Chemistry C

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structure of thin films and surfaces. During the measurements, the sample is irradiated with X-rays of a given photon energy (photoionized), and the corresponding photoelectron spectrum, the energy distribution of emitted photoelectrons, is recorded with high surface sensitivity. Each material has a characteristic photoelectron spectrum. The intensity and the signature of spectral features provide information on the material’s composition as well as on the chemical and the electronic structure (e.g., valency, electronic properties, impurities, and charge transport states).38−44 A photoelectron spectroscopy investigation of the upper valence band region is of particular interest for a direct analysis of the charge transport states.44−46 Note that a direct study of electrically active impurity and dopant states in a semiconductor requires a careful and detailed XPS analysis because the concentration of impurities (and the corresponding spectroscopy signal) is typically very small.35−37 For this reason, the chemical and the electronic properties of a series of solution-processed IZO semiconductor thin films has been investigated comprehensively and in detail with XPS under systematic variation of the precursor conversion temperature Tpc and the concentration of the precursor solution. The XPS signal from the upper valence band and the electronic gap is carefully analyzed with respect to changes of the charge transport states and electrically active impurity states. A consistent picture of the interrelation between the chemical structure, the electronic structure, and the charge transport properties is developed by correlating the findings from the XPS analysis with complementary data from fourpoint-probe (4PP), TFT, and UV−vis absorption measurements.

nanotubes,16,27 nano-object−electrolyte compounds,28,29 polymers, and small molecules.12,13,30 It has been studied intensively how the properties of sputtered TMO semiconductors can be tailored and improved with respect to different applications.2,31−33 The approach to print TMO semiconductor thin films from molecular precursor solutions in air is comparatively new and less understood. Therefore, it is crucial to obtain a fundamental and detailed understanding of the general interrelation between the growth of solution-processed TMO semiconductors, their chemical and electronic structure, and their charge transport properties. It has been shown that the precursor conversion temperature Tpc and the concentration of the precursor solution are very critical parameters for controlling the properties of solution-processed IZO semiconductor thin films and TFTs.22,25 Particularly, the FE mobility increases with increasing precursor conversion temperature Tpc and decreasing concentration of the precursor solution, respectively. IZO TFTs with FE mobility μFE up to 20 cm2/(V s) have been fabricated from metal oximate precursor solutions by optimizing the preparation process accordingly.22 Additionally, it has been found that solution-processing of IZO thin films induces a distinct nanoporous film structure not observed for TMO thin films prepared by sputtering33 and atomic layer deposition.34 Furthermore, optimization of the preparation process for high FE mobility μFE leads to an increase of the material density according to X-ray reflectivity measurements.22 However, even nanoporous thin films with roughly half the material density of an ideal IZO film yield several cm2/(V s) high charge carrier mobility,22 which is to some surprise only a little lower than that of sputtered IZO thin films9 despite of significant structural imperfections. Therefore, solution-processed IZO semiconductor thin films are of great interest for manufacturing electronic devices by large-scale coating and printing processes. However, their charge transport properties are not fully understood, and hence, a detailed analysis of their electronic structure is absolutely necessary with particular focus at electronic states mediating charge transport. In general, the charge transport properties of semiconductors are determined by electronic states of energy close to the chemical potential.35−37 The energy position of the charge transport states, in particular the energy position of the valence band maximum (VBM) and the conduction band minimum (CBM) with respect to the chemical potential has a strong influence on the charge transport properties. Moreover, already a very low concentration of electrically active impurities contributing to electronic (dopant) states within the electronic gap can have a strong impact on the charge transport properties.9,35−37 In return, tailoring the charge transport properties of semiconductors requires a detailed understanding and precise control of impurities and electronic (dopant) states. Consequently, a detailed and direct analysis of the charge transport states and impurity states in relation to the chemical structure is essential for a fundamental understanding of how to control and tailor the charge transport properties of solutionprocessed TMO semiconductor thin films. However, this very important aspect has not been investigated sufficiently.17−25 Therefore, the charge transport states of different solutionprocessed IZO semiconductor thin films are analyzed in the following in relation to the chemical structure and the charge transport properties. Photoelectron spectroscopy in general and X-ray photoelectron spectroscopy (XPS) in particular are very valuable techniques for studying the chemical and the electronic

2. EXPERIMENTAL SECTION 2.1. Thin Film Formation. Different IZO thin films were deposited on two different types of substrates, namely, quartz glass for UV−vis absorption spectroscopy and 4PP measurements, and on bottom-gate SiO2/Si TFT substrates for FE transport and XPS measurements. All IZO thin films were produced by several cycles of, first, spin coating of a metal oximate precursor solution and, second, conversion of the precursor film into a metal oxide thin film by a 4 min long annealing step on a hot plate in air as illustrated in Figure S1. The precursor solution was prepared from In- and Zn-oximate powder dissolved in 2-methoxyethanol with the molar ratio cIn/ cZn = 1.5:1. Each sample series was prepared at the same spincasting speed, either at 2500 or 1500 rpm, respectively. The discussed trends have been found to be similar between different sample series. For the investigation of the influence of the precursor conversion temperature Tpc on the thin film properties, several sample series were prepared under systematic variation of Tpc (255 ± 15 °C, 305 ± 20 °C, 340 ± 20 °C, and 425 ± 25 °C as determined via a thermocouple (K-type) and an infrared thermometer Infratherm IE7). These sample series were prepared from a precursor solution of either 30 mg/g precursor concentration (30 mg solid precursor per 1.00 g of precursor solution) or 100 mg/g precursor concentration. To study the influence of a variation of the precursor concentration, different types of IZO thin films of comparable thickness were prepared on quartz and on TFT substrates by spin coating 2, 4, 7, and 13 layers, respectively, using precursor solutions of 100, 50, 30, and 15 mg/g concentration, respectively. The respective samples are referred to as S2 × 100, S4 × 50, S7 × 30, and 12827



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Figure 1. XPS core level spectra of a systematic series of IZO multilayer thin films from a 30 mg/g precursor solution prepared at different precursor conversion temperatures Tpc. Note that the spectra in (b) and (d) are plotted at the same intensity scale.

S13 × 15 in the following. According to SEM measurements, as shown exemplarily in Figure S5a, the corresponding thin films are homogeneous and smooth with a significant porosity in the nanometer range for Tpc = 425 °C. 2.2. XPS Measurements. The XPS data was recorded from IZO thin films on TFT substrates after the TFT measurements in order to prevent any modification of the charge transport properties from X-ray irradiation.45,46 The XPS measurements were carried out using a monochromatic Al Kα X-ray source and a VG ESCALab 250 spectrometer with an overall energy resolution of ΔE = 400 meV and a pressure of 1 × 10−9 mbar. The spectrometer was carefully calibrated to the Fermi edge and the Au 4f peaks of a clean Au foil, and no evidence for charging of the samples was observed, which allows a detailed evaluation of the energy position of the XPS signals. The core level spectra were normalized to the background for a detailed comparison of relative intensities and spectral signatures. For each sample the first XPS measurement was repeated at the end of the recording cycle. A comparison of the XPS spectra of the pristine sample and the irradiated sample revealed no significant difference, and hence, it can be concluded that radiation damage is not significant for the discussed XPS data. Note that a very small but significant C 1s signal from carbon contaminants is observed for all investigated IZO thin films, as shown exemplarily in Figure S2 of the Supporting Information, which is not surprising for air-exposed samples. Additionally, a small chlorine signal is observed for samples prepared at low Tpc due to residues from the precursor synthesis.21 2.3. Electrical Characterization. The 4PP measurements were carried out at room temperature at in-line geometry with equidistant probe spacing (3 mm distance between the four probes) using an Agilent B1500A electronics analyzer. For this purpose, different IZO thin films were prepared on quartz glass substrates of 25 × 25 mm2 size according to the procedure described above. The value of the sheet resistivity R□ from a single measurement is an average of four values corresponding to four different currents being applied to the outer probes. Each sample was probed several times with the sample being rotated by 90° in between. No significant dependence of R□ on

the measurement direction was observed, as is expected for homogeneous and amorphous thin films. IZO−semiconductor−TFTs were prepared in bottom-gate bottom-contact geometry using well-defined commercially available SiO2/Si substrates with Au/ITO source and drain contacts, as illustrated in the inset of Figure S6. The TFT channel width and length was chosen to be W = 10 mm and L = 20 μm, respectively. The TFT transfer curves have a very small hysteresis (Figure S6) and a steep increase of the sourcedrain current ID at the turn-on point, which is an indication for high quality of the investigated TFTs. Accordingly, the FE mobility μEF was calculated from the saturation region of the TFT transfer curves (see Figure S6 for example) using the relation μFE

2Ld ⎛ Δ ID ⎜⎜ = Wε0εSiO2 ⎝ ΔVG

⎞2 ⎟⎟ ⎠

(1)

with d = 90 nm thickness of the SiO2 gate dielectrics and the dielectric constant of εSiO2 = 3.9.37 2.4. UV−vis Spectroscopy. The UV−vis absorption coefficient α of different IZO thin films on a quartz substrate was determined from transmission and specular reflectivity measurements with an Agilent Cary 5000 spectrometer with Δλ = 2 nm energy resolution in a V−W configuration for measurement of the transmittance T (V-configuration) and the specular reflectance R (W-configuration). The absorption coefficient α was then calculated according to ⎛ I − IR ⎞ α(hν) = ln⎜ 0 ⎟/d ⎝ IT ⎠

(2)

The thickness d of these IZO thin films was estimated with a Zygo NewView 6k white light interferometer (WLI) at several positions for each sample. For this purpose the following steps were carried out: (i) etching of a hole into the IZO films using an aqueous H2SO4 solution of 10% molar concentration, (ii) rinsing with water, (iii) sputtering a 40 nm thick platinum film on top, and (iv) measuring the step height of the film edges with the WLI. This procedure turned out to be accurate and 12828



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reproducible. For example, the IZO film height of a sample series prepared with 2500 rpm spin speed, (S2 × 100, S4 × 50, S7 × 30, and S13 × 15, respectively) was determined to be 43.5 ± 2.2 nm, 41.5 ± 2.9 nm, 35.4 ± 3.5 nm, and 28.6 ± 3.0 nm, respectively.

signals to lower binding energy is observed with increasing Tpc in combination with a continuous broadening of the photoemission lines and a change of their signature. In particular, the In 3d5/2 peak shifts 0.51 eV to lower binding energy from EB = 445.43 eV (Tpc = 255 °C) to 444.92 eV (Tpc = 425 °C), and the peak width increases significantly from fwhm = 1.29 to 1.69 eV. Additionally, the signature of the In 3d5/2 peak of the sample prepared at Tpc = 425 °C is significantly asymmetric as a steep leading edge and a broad trailing edge is observed, which suggests the presence of a distribution of chemically different In species in the sample. Moreover, a significant increase of the signal-to-background ratio is observed with increasing Tpc. This effect can be primarily understood in relation to a change of the chemical structure of the IZO film with increasing Tpc as a change from hydroxidic to oxidic character corresponds to a decrease of the molar concentration of oxygen species and consequently to an increase of the molar concentration of In species. The Zn 2p3/2 signal shifts 0.42 eV to lower binding energy with increasing Tpc, from EB = 1022.47 eV (Tpc = 255 °C) to EB = 1022.05 eV (Tpc = 340 °C), accompanied by a slight increase of the peak width from fwhm = 1.47 to 1.61 eV. Moreover, the maximum of the Zn 2p3/2 signal from the sample prepared at Tpc = 425 °C is located at EB = 1022.29 eV, which is 0.24 eV higher than in case of Tpc = 340 °C (EB = 1022.05 eV). Additionally, the peak is significantly broadened (fwhm = 1.89 eV) and asymmetric, namely, a broad leading edge, which resembles the signature of the Tpc = 340 °C sample, and a steep trailing edge is observed. This again suggests the presence of a distribution of chemically different Zn species. The general shift of the In 3d5/2 and the Zn 2p3/2 signals to lower binding energy is in agreement with the trend observed for the O 1s signal. It can be understood with respect to a downshift of the Fermi level relative to the VB maximum or a decrease of the oxidation state of the zinc and indium species with decreasing hydroxidic character of the film in agreement with a previous XPS study of solution-processed Zn(O, OH) thin films, which showed analogous shifts of the Zn 3d and O 1s signal to lower binding energy upon elimination of hydroxide species.48 Note that the described change of the chemical structure of the IZO film with increasing precursor decomposition temperature Tpc is accompanied by a change of the film morphology, as illustrated by the SEM images in Figure S5. Accordingly, a porous film structure is formed for Tpc = 425 °C with a few nm large pores, which are not observed for the film prepared at Tpc = 255 °C. Moreover, the asymmetry of the In 3d5/2 and the Zn 2p3/2 peaks of the sample prepared at Tpc = 425 °C indicates a gain in intensity at the high energy side of the peaks in combination with a high-EB-shift of signal “B” in the corresponding O 1s spectrum in Figure 1a. The finding that the increase in intensity of the high-EB-edge is larger for the Zn 2p3/2 signal than for the In 3d5/2 can be explained by a lower photoelectron mean free path and higher surface sensitivity of the Zn 2p3/2 signal than for the In 3d5/2 signal.51 In this context, the signal at the high-EB-edge of the In 3d5/2 and the Zn 2p3/2 signal as well as the high-EB-shift of signal “B” in the O 1s spectrum of the sample prepared at Tpc = 425 °C can be attributed to the formation of a few Å thin surface component, which contributes at higher binding energy than the bulk component. Then the overall signal (sum of the surface and the bulk component) can result in asymmetric core level peaks. Hence, this observation may be a hint either for the formation of a surface component with a stronger hydroxidic character

3. RESULTS AND DISCUSSION 3.1. Influence of the Precursor Conversion Temperature Tpc. 3.1.1. XPS Investigation of the Chemical and the Electronic Structure. Figure 1 illustrates the XPS signals of the O 1s, N 1s, Zn 2p3/2, and In 3d5/2 core levels of a systematic series of IZO thin films which were prepared at different precursor conversion temperatures (Tpc). The O 1s spectrum in Figure 1a for the 255 °C films consists of one main peak “B” located at EB = 532.16 eV with 1.35 eV full width at halfmaximum (fwhm), and a distinct shoulder “A” is located at ca. 1.5 eV lower binding energy, namely, at EB = 530.65 eV. Signal “B” can be attributed to oxygen species in a chemical environment of nonideal metal oxide due to the formation of hydroxides, peroxides and surface species during the precursor decomposition, whereas signal “A” in Figure 1a can be assigned to oxygen species in a chemical environment, which is typical for stoichiometric IZO.5,42,47,48 Moreover, the O 1s spectrum of the 255 °C sample differs significantly from previously reported data of sputtered IZO and IGZO thin films,49,50 which is a strong indication for a significant difference in chemical structure. Note that spectral contributions from molecular precursor fragments due to incomplete decomposition of the nitrogen containing oximate precursors are very unlikely, because no evidence for the presence of any nitrogen species is found in the N 1s region in Figure 1b. Moreover, Figure 1a shows significant and continuous changes of the O 1s spectrum with increasing Tpc. The intensity ratio between the signals “B” and “A” decreases strongly with increasing Tpc. For Tpc = 255 °C, for example, the ratio of the peak maxima is 4:1, whereas in the case of Tpc = 425 °C it is 1:2. This is accompanied by a change of the peak position. Signal A shifts to lower binding energy by 250 meV from EB = 530.65 eV (Tpc = 255 °C) to 530.40 eV (Tpc = 425 °C). Peak B shifts in parallel to peak A for 255 °C ≤ Tpc ≤ 340 °C, namely, from EB = 532.16 eV (Tpc = 255 °C) to 531.90 eV (Tpc = 340 °C). However, for Tpc = 425 °C, peak “B” is shifted to higher binding energy (EB = 532.10 eV), which corresponds to a differential chemical shift of 200 meV between peak “A” and “B”. Altogether, these findings indicate that for low Tpc, a thin film with predominantly hydroxidic character is formed with a small metal oxide contribution, and with increasing Tpc the oxidic character of the film increases and the hydroxidic character decreases. We assign feature “B” to hydroxide species according to detailed studies of ZnO surfaces.5 The hydroxide− oxide stoichiometry at the surface can be derived to first-order from the intensity ratio of the signals “B” and “A”. According to the estimate illustrated in Figure S3 in the Supporting Information, the hydroxide-to-oxide ratio of the surface decreases from 5.2:1 (Tpc = 255 °C) to 1.9:1 (Tpc = 305 °C), 0.5:1 (Tpc = 340 °C), and 0.5:1 (Tpc = 425 °C). Furthermore, the observation of a differential chemical shift between the signals “A” and “B” is a significant and strong indication for a difference in the chemical structure between the films prepared at Tpc = 340 and 425 °C despite similar intensity ratio. The change of the chemical structure is also reflected in the In 3d5/2 and in the Zn 2p3/2 signal in Figure 1c,d. A shift of both 12829



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illustrated in the Supporting Information Figure S4. The error bars correspond to differences in slope of the subtracted background. From Figure 2b, it becomes evident that the area of feature “C” increases significantly and continuously with increasing precursor conversion temperature Tpc. If the concentration of oxygen atoms, Nox, in the film is known, the electron density, nC, related to the spectral feature “C” can be estimated from the ratio of the intensity, IC, of the spectral feature “C” and the intensity of the O 2p derived signal, IO 2p, (integration of the XPS signals from 2.5 eV < EB < 7.0 eV) after accounting for the different photoionization cross sections of the metal s electrons (σs = 0.5 × 0.10 × 10−2 Mb) and the fully occupied O 2p (6e−) states (σO 2p = 1.5 × 0.24 × 10−3 Mb)55 according to

than the bulk component in analogy to previous investigations of ZnO crystals5 or for surface band bending.47 In Figure 2a, a significant and continuous change of the VB spectrum can be observed with increasing Tpc. For Tpc = 255 °C

IC nCσs = IO2p NoxσO2p

(3)

The concentration of oxygen atoms can be estimated to be Nox = 3.4 × 1022 1/cm3 under the assumption of a stoichiometric IZO film with the composition of 3In2O3 4ZnO, a material density of 5 g/cm3,22 and the molar weights of 16u (O), 65u (Zn), and 113u (In). Accordingly, the estimated density of electrons nC ranges between (1.3 ± 0.4) 1019 1/cm3 (S3 × 100, Tpc = 340 °C) and (5.0 ± 1.7) 1019 1/ cm3 (S7 × 30, Tpc = 425 °C) in excellent agreement with a previous hard-XPS study of sputtered IGZO thin films.47 The comparatively large error accounts mainly for the uncertainty in determining the absolute intensity of the O 2p signal from the XPS spectra due to hybridization with Zn- and In-derived states and due to deviations from the assumed stoichiometry because of a significant hydroxydic character of the thin films as discussed above. 3.1.2. Charge Transport Properties. In Figure 3 (top), the sheet resistivity R□ of a representative series of solutionprocessed IZO thin films on quartz glass is plotted for different Tpc as determined by 4PP measurements at more than five

Figure 2. (a) VB spectra of the systematic series of IZO thin films from Figure 1 and (b) peak area of the feature “C” in the VB spectra of different sample series.

the VB signal has a peak-like signature with its intensity maximum at EB = 5.34 eV, and the VB onset is located at EB = 3.92 ± 0.13 eV, with a significant tail to lower binding energy. This signal can predominantly be assigned to O 2p derived VB states with minor Zn 3d and In 4d character.9,47,52 With increasing Tpc and decreasing hydroxidic character of the thin film, the VB signal broadens, and the onset, which corresponds to the VBM, shifts to lower binding energy, namely, to EB = 3.01 ± 0.10 eV (Tpc = 425 °C) in analogy to the trend observed in a previous investigation of solution-processed Zn(O, OH) thin films.48 Note that the signature of the VB onset is not ideally sharp, but a significant tail is observed for all spectra, in particular for the sample prepared at Tpc = 255 °C. Additionally, a very small but significant signal labeled “C” can be observed in Figure 2a at EB = 0.45 eV for Tpc = 340 and 425 °C, respectively, with significantly higher intensity in case of Tpc = 425 °C. Because in general donor states of an n-type semiconductor are located directly below the chemical potential,35−37 the close vicinity of feature “C” to the chemical potential (indicated by “EF”) raises the question whether the corresponding electronic states are crucial for the charge transport properties of the IZO thin films. A similar spectral feature has been discussed previously for V2O5 and TiO2 thin films (EB = 1.3 eV)53,54 and sputtered IGZO thin films (EB = 0.2 eV)47 in relation to defect states due to oxygen vacancies and in relation to near conduction band minimum (near-CBM) states, respectively. Note that the binding energy position of feature “C” (EB = 0.45 eV) differs from the typical binding energy of In-derived (EB < 0.1 eV) and oxygen-vacancy-derived (EB ∼ 0.3 eV) donor states in sputtered ZnO films.2 The dependence of the intensity of “C” on Tpc has been studied for several sample series and plotted in Figure 2b. The intensity of “C” is determined by integrating the PE signal from EF to 1.25 eV and careful subtraction of a linear background as

Figure 3. (top) Sheet resistivity R□ of a series of IZO thin films on quartz glass, whereas R□ was found to be above the measurement limit (8 × 109 Ω/□) for (*) S7 × 30, Tpc = 255 °C and (**) S3 × 100, Tpc = 340 °C, respectively. (bottom) The FE mobility μFE of different IZO thin films on SiO2/Si bottom-gate bottom-contact TFTs. 12830



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255 °C) to 3.91 ± 0.12 eV (Tpc = 340 °C), and to hν = 3.09 ± 0.10 eV (Tpc = 425 °C). This trend is similar to a previous study of ZnO films grown by chemical vapor deposition where a decrease of the optical gap from Egopt = 4.06 eV to 3.13 eV was reported with increasing growth temperature from 200 to 500 °C.58 A comparison of Figure 4 and Figure 1 indicates that Egopt decreases with decreasing hydroxidic character of the thin film, in good agreement with previous reports of a larger optical gap for In(OH)3 and Zn(OH)2 (Egopt > 5 eV) than for In2O3 and ZnO (Egopt ∼ 3.7 eV and ∼3.3 eV).7,59,60 Moreover, a decrease of Egopt of solution-processed Zn(O,OH) films with decreasing hydroxidic character has also been found.48 Note the small but significant tail of the absorption edge in case of Tpc = 340 and 255 °C, which extends from the absorption onset to lower photon energies. This tail can be explained by the presence of VB and CB tail states in agreement with the XPS VB spectra in Figure 2a where a comparatively large and broad tail is observed at the VBM of the Tpc = 255 °C sample and a significantly smaller tail in case of the Tpc = 425 °C sample. 3.1.4. Correlation of Chemical, Electronic, and Charge Transport Properties. The significant change of the charge transport properties with increasing Tpc can be well understood with respect to a change of the energy position of the conduction states. The latter can be estimated from a comparison of the energy position of the VBM in Figure 2a and the optical gap Egopt from Figure 4, which can be considered as lower limit for the electronic gap. Note that for stoichiometric ZnO films, the exciton binding energy is of the order of 60 meV7 and hence comparatively small. The corresponding trend is illustrated in Figure 5. In case of Tpc

different positions per sample. For the S7 × 30 series, the sheet resistivity of the Tpc = 255 °C sample is observed to be higher than the upper limit of the measurement setup (>8 × 109 Ω/ □). This is indicated in Figure 3 by an arrow. With increasing Tpc, the sheet resistivity decreases to (1.69 ± 0.80) × 109 Ω/□ (Tpc = 340 °C) and (4.22 ± 0.50) × 105 Ω/□ (Tpc = 425 °C). Note that the lower value is only 2 to 3 orders of magnitude higher than the sheet resistivity of corresponding IZO and ITO thin films from sputter processes, which are highly ndoped.49,56,57 The sheet resistivity of an analogously prepared S3 × 100 series is observed to be (6.50 ± 1.23) × 108 Ω/□ in the case of Tpc = 425 °C, and for Tpc = 340 °C it is above the upper limit of the 4PP measurement range. Consequently, the relative trend within this series is similar to that of the S7 × 30 series. In both cases, the sheet resistivity decreases drastically with increasing Tpc. The finding of a generally higher sheet resistivity for the S3 × 100 series than for the S7 × 30 series is discussed with respect to Figure 7. Figure 3 (bottom) shows the FE mobility μFE of in total 20 bottom-gate bottom-contact TFTs with an IZO semiconductor thin film according to the description in section 2.3. It becomes evident that μFE increases continuously with increasing Tpc for both the S7 × 30 series and the S3 × 100 series. In case of the S7 × 30 series, for example, the FE mobility μFE increases from (3.25 ± 1.08) × 10−4 cm2/(V s) (Tpc = 255 °C) to 6.74 ± 0.99 cm2/(V s) (Tpc = 425 °C), indicating that the investigated solution-processed IZO TFTs are of high quality. The finding of generally lower μFE for the S2 × 100 series than for the S7 × 30 series will be discussed below. The comparison of Figure 3 with Figure 2b suggests that the decrease of the sheet resistivity R□ and the increase of the FE mobility μFE is related to the increase of the intensity of the spectral feature “C”. 3.1.3. UV−vis Absorption Spectroscopy. In Figure 4, the UV−vis absorption coefficient α(hν) is plotted over the photon

Figure 5. (left) Schematic energy diagram of occupied (dark gray) and unoccupied (light gray) states of solution-processed IZO thin films. (right) Illustration of charge percolation paths due to occupation of the near-CBM density of states. Figure 4. UV−vis absorption coefficient α of S3 × 100 IZO thin films on quartz glass for different precursor conversion temperatures Tpc. A linear extrapolation of the absorption edge is indicated for estimating the optical gap.

= 255 °C the VBM in Figure 2a is located at EF − EVBM = 3.92 ± 0.13 eV, and the optical gap is Egopt = 4.28 ± 0.15 eV according to Figure 4. This suggests that for low precursor conversion temperature Tpc, the CBM is located significantly above the Fermi level in agreement with the finding of a high sheet resistivity R□. For Tpc = 425 °C, the VBM is located at EF − EVBM = 3.01 ± 0.10 eV, and the optical gap is Egopt = 3.09 ± 0.10 eV. Consequently, the CBM can assumed to be close to the Fermi level and hence near-CBM tail states become occupied leading to low sheet resistivity R□, as observed in Figure 3. In this context, the decrease of the tail below the optical absorption edge in Figure 4 with increasing Tpc can at least partially be attributed to the occupation of near-CBM states in analogy to the Burstein−Moss effect.61,62

energy. In the case of Tpc = 425 °C, the absorption coefficient is close to zero for low photon energy hν and increases drastically for high photon energy. The energy position of the absorption edge corresponds to the optical gap Egopt. For the following discussion Egopt is estimated from the α(hν) curves by linear extrapolation of the onset without further assumptions concerning the character of the optical transitions (e.g., direct or indirect, respectively). Figure 4 shows a clear decrease of the optical gap of the IZO thin films with increasing Tpc from hν = 4.28 ± 0.15 eV (Tpc = 12831



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Figure 6. (a−c) XPS core level spectra and (d) valence band spectra of a series of IZO thin films prepared at Tpc = 425 °C using precursor solutions with different precursor concentration, namely, with 100, 50, 30, and 15 mg/g.

IZO thin films in Figure 6a are relatively similar. However, a detailed analysis of the peak position and the spectral signature reveals small but significant differences. In case of S2 × 100, peak “A” is located at EB = 530.40 eV and the shoulder “B” at 531.91 eV, whereas for S13 × 15 peak “A” is shifted by 0.09 eV to lower binding energy (EB = 530.31 eV), and the shoulder “B” is shifted 0.29 eV to higher binding energy (EB = 532.20 eV). Additionally, a continuous increase of the signal of the trailing edge at EB = 533.8 eV and a significant decrease of the signalto-background ratio is found. The observed modifications of the O 1s spectra, in particular the differential peak shifts, in this series from S2 × 100 to S13 × 15 are strong indications for a small but significant variation of the chemical structure (in the surface region). Moreover, the trend from S2 × 100 to S13 × 15 is less distinct but similar to the modification of the O 1s spectrum in Figure 1a due to an increase of Tpc from 340 to 425 °C. Moreover, the maximum of the Zn 2p3/2 signal in Figure 6b shifts 0.20 eV to higher binding energy from EB = 1022.07 eV (S2 × 100) to 1022.27 eV (S13 × 15) and increases in width from fwhm = 1.78 to 2.06 eV in analogy to the trend observed in Figure 1c when increasing Tpc from 340 to 425 °C. Additionally, the maximum of the In 3d5/2 signal in Figure 6b shifts 0.12 eV to lower binding energy from EB = 444.92 eV (S2 × 100) to 444.80 eV (S13 × 15) with similar peak width (fwhm = 1.53 and 1.54 eV, respectively). This change is accompanied by a significant modification of the spectral signature of the In 3d5/2 peak from symmetric (S2 × 100) to clearly asymmetric (S13 × 15), in particular by the formation of a steep leading edge and a broad trailing edge, suggesting the presence of a distribution of chemically slightly different In species in analogy to the trend observed in Figure 1d with increasing Tpc from 340 to 425 °C. Again, for the surface sensitive Zn 2p3/2 signal, more spectral weight is shifted to higher binding energy than for the less surface sensitive O 1s and In 3d5/2 signal. Moreover, all VB spectra in Figure 6d show a significant spectral feature “C” close to the Fermi level at EB = 0.45 eV. After a careful evaluation of the intensity of “C” according to the procedure described above the area of the feature “C” is plotted in Figure 7a for two different sample series. It can clearly be observed that the area

Furthermore, the observation of feature “C” in the VB spectra in Figure 2a only for Tpc = 340 and 425 °C, with higher intensity in the case of Tpc = 425 °C, can also be understood with respect to the lowering of the energy position of the CBM with increasing Tpc. Accordingly, the spectral feature “C” can be attributed to occupied near-CBM states (in the surface region). Consequently, the concentration of free charge carriers increases with increasing intensity of the spectral feature “C”, leading to a decrease of the sheet resistivity R□. Moreover, the scenario discussed above can also explain the observed increase in the FE mobility μFE with increasing precursor conversion temperature Tpc. In relation to earlier investigations of amorphous ionic TMO the CBM states can be assumed to have predominantly metal s-type character, and hence, they can considered to be diffuse and delocalized.8,9,63 This implies that the occupation of delocalized near-CBM states (associated with spectral feature “C”) leads to the formation of charge percolation paths, as illustrated schematically in Figure 5. Because an increase of the electron mean free path translates directly into an increase of the charge carrier mobility, the experimental finding with respect to Figure 3, namely, the increase of the FE mobility μFE with increasing Tpc is straightforward. This interpretation is in line with previous Hall measurements of IGZO and ITO thin films prepared by pulsed laser deposition, which showed an increase of the Hall mobility with increasing carrier concentration.9,64 3.2. Influence of the Precursor Concentration. It has been reported previously, that the FE mobility of solutionprocessed IZO-TFTs increases not only with increasing precursor decomposition temperature Tpc but also with decreasing concentration of the molecular precursor solution.22 Consequently, the question arises whether a modification of the chemical structure and the charge transport states can be observed upon variation of the concentration of the precursor solution in agreement with the explanation developed above. 3.2.1. XPS Investigation of the Chemical and the Electronic Structure. Figure 6 shows the XPS core level and VB spectra of a series of IZO thin films which were prepared at T pc = 425 °C using precursor solutions of different concentration. At first glance, the O 1s spectra of the different 12832



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3.2.3. UV−vis Absorption Spectroscopy. The change of the electronic structure is also reflected in the UV−vis absorption coefficient α in Figure 8. The onset of the α curve shifts 0.28 eV

Figure 8. UV−vis absorption coefficient α of IZO thin films on quartz glass prepared at Tpc = 425 °C using precursor solutions with different precursor concentration, namely, with 100, 50, 30, and 15 mg/g.

to lower photon energy with decreasing precursor concentration from hν = 3.19 ± 0.10 eV (S2 × 100) to 2.91 ± 0.10 eV (S13 × 15). A comparison of these values with the energy position of the VBM derived in Figure 6d, namely, EF − EVBM = 2.99 ± 0.14 eV (S2 × 100) and 2.88 ± 0.12 eV (S13 × 15), suggests that the energy position of the CBM is slightly lowered toward the Fermi level with decreasing concentration of the precursor solution. Consequently, the increase of the intensity of the spectral feature “C” in Figure 6d and Figure 7 (top), as well as the decrease of the sheet resistivity and the increase of the FE mobility μFE, can be explained along the same line of arguments as discussed with respect to the influence of the precursor conversion temperature Tpc, namely, by an increase of the occupation of near-CBM states. This is an important aspect in addition to the previously discussed contribution of the film structure to the charge carrier mean free path.22

Figure 7. (a) Intensity of the valence band feature “C”, (b) sheet resistivity of IZO thin films on quartz glass, and (c) average FE mobility μFE of in total 34 IZO TFTs. All films were prepared at Tpc = 425 °C precursor conversion temperature.

of “C” increases significantly and continuously with decreasing precursor concentration. Altogether, the XPS data indicates a small but significant modification of the chemical and electronic structure with decreasing concentration of the precursor solution along the same trend as observed for increasing Tpc from 340 to 425 °C. 3.2.2. Charge Transport Properties. Figure 7b indicates a strong and monotonic decrease of the sheet resistivity R□ with decreasing precursor concentration from (6.21 ± 1.74) × 108 Ω/□ (S2 × 100) to (5.83 ± 1.17) × 104 Ω/□ (S13 × 15) and a significant increase of the FE mobility μFE (Figure 7c) from 1.19 ± 0.17 cm2/(V s) (S2 × 100) to 10.89 ± 0.86 cm2/(V s) (S13 × 15). This finding is in agreement with previous work which demonstrated that the FE mobility can be increased up to 20 cm2/(V s) by further decrease of the concentration of the precursor solution due to densification of the film and an increase of the charge carrier mean free path, respectively.22 Moreover, the interrelation between the trends of the area of the spectral feature “C”, the sheet resistivity R□, and the FE mobility μFE is identical to what has been discussed above with respect to an increase of the precursor conversion temperature Tpc. This finding corroborates further the given interpretation that the change of the charge transport properties is due to a modification of the electronic structure with decreasing concentration of the precursor solution which follows the same trend as what has been observed for an increase of Tpc. Note that the increase of μFE is accompanied by a decrease of the IZO film thickness from 43.5 ± 2.2 nm (S2 × 100) to 28.6 ± 3.0 nm (S13 × 15) according to WLI measurements. This interrelation is reverse to the previously observed trend for TFTs with few nm thick organic semiconductor thin films where the channel height is limited by the semiconductor film thickness.14

4. SUMMARY AND CONCLUSION The chemical and the electronic structure of IZO semiconductor thin films from molecular precursor solutions has been investigated systematically upon variation of two important preparation process parameters, in particular, the precursor decomposition temperature Tpc and the concentration of the precursor solution. A comprehensive and very detailed XPS analysis indicates complete precursor decomposition for all precursor conversion temperatures and reveals significant modifications of the stoichiometry, the chemical, and the electronic structure of the IZO thin films with increasing Tpc, namely, from predominantly hydroxidic to predominantly oxidic. An important finding is the observation of a very small spectral feature “C” in the XPS valence band spectra in the case of high precursor conversion temperatures Tpc, which is located in the electronic gap at EB = 0.45 eV. The presented data shows a direct correlation between the intensity of the spectral feature “C” and the charge transport properties, namely, the sheet resistivity R□ and the FE mobility μFE. A comparison of the XPS valence band and the UV−vis absorption measurements indicates that the energy position of the spectral feature “C” is close to the CBM and that its intensity increases when the CBM approaches EF. Therefore, the spectral feature “C” is assigned to a density of occupied near-CBM states, and the 12833



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Present Addresses

decrease of the sheet resistivity R□ and the increase of the FE mobility μFE with increasing intensity of “C” can be attributed to an increase of the concentration of free charge carriers and an increase of the electron mean free path. The discussed changes in the XPS core level data of the different sample series (variation of the energy position, intensity ratio, and spectral signature of different signals) indicate a significant and complex modification of the chemical structure including but not limited to a change of the hydroxide−oxide stoichiometry (in the surface region). Consequently, the occupation of near-CBM states due to a significant modification of the chemical structure can be considered as “chemical doping” in contrast to substitutional doping in a classical semiconductor. In summary, the following points have been demonstrated: 1 IZO semiconductor thin films from molecular precursor solutions are very promising for applications in printed electronics due to compatibility with a simple preparation process in air, optical transparency, high conductivity, and high charge carrier mobility in corresponding TFTs. 2 Optimizing the IZO thin film preparation process for low sheet resistivity R □ and high FE mobility μ FE corresponds to an increase of n-type doping due to lowering of the CBM energy position toward the chemical potential and occupation of a high density of near-CBM states (∼1019 1/cm3). 3 The change of the charge transport states is accompanied by a significant and complex modification of the chemical structure, including a change of the hydroxide−oxide stoichiometry (in the surface region), which can be considered as chemical doping. 4 A direct and systematic spectroscopy analysis of the electronic structure of solution-processed IZO semiconductor thin films is crucial for the development of a detailed and fundamental understanding of their charge transport properties. Altogether, this study shows not only a systematic and detailed XPS analysis of solution-processed IZO semiconductor thin films in combination with complementary electrical and optical measurements, but additionally, a fundamental and detailed understanding of the electronic structure and the charge transport properties has been developed in relation to the conditions during the IZO thin film preparation, which can expected to be of general relevance for solution-processed TMO semiconductor thin films.

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∥

Karlsruhe Institute of Technology (KIT), Institute for Photon Science and Synchrotron Radiation (IPS), D-76344 Eggenstein-Leopoldshafen, Germany. ● Merck KGaA, University Parkway, Southampton Science Park, Hampshire SO16 7QD, United Kingdom. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS TMO, transparent metal oxide; IZO, indium−zinc-oxide; XPS, X-ray photoelectron spectroscopy; TFT, thin film transistor; 4PP, four-point-probe; FE mobility, field effect mobility; VBM, valence band maximum; CBM, conduction band minimum; fwhm, full width half-maximum

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Schematic illustration of the IZO preparation process, XPS survey spectra, XPS O 1s spectra, XPS VB spectrum of a S13 × 15 IZO thin film with indication of the area of the spectral feature “C”, SEM images of two IZO-TFT, and exemplary TFT transfer curves with an illustration of the TFT layout. This material is available free of charge via the Internet at http:// pubs.acs.org.

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ACKNOWLEDGMENTS

We thank P. Pacak, A. Klyszcz, and K. Schönauer (all Merck KGaA) for support during the thin film preparation and electrical and optical characterization. P. Mundt, J. J. Schneider, R. Hoffmann, K. Hofman, and E. Dörsam (all TU Darmstadt) contributed to interesting discussions. Finally, we thank TU Darmstadt and Merck KGaA for financial support.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +49 721 60828269. 12834



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Interrelation between Chemical, Electronic, and Charge Transport

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