Restoring the Properties of Transparent Al-Doped ZnO Thin Film

Article pubs.acs.org/JPCC

Restoring the Properties of Transparent Al-Doped ZnO Thin Film Electrodes Exposed to Ambient Air Martin Mickan,†,‡ Mathieu Stoffel,† Hervé Rinnert,† Ulf Helmersson,‡ and David Horwat*,† †

Université de Lorraine, UMR CNRS 7198, Institut Jean Lamour, Nancy F-54011, France Plasma & Coatings Physics Division, IFM-Material Physics, Linköping University, SE-581 83 Linköping, Sweden

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‡

ABSTRACT: The properties of Al-doped ZnO (AZO) films are known to degrade with exposure to humidity. Different AZO films deposited using reactive direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS) have been aged in ambient laboratory conditions and annealed at temperatures between 160 and 180 °C in a N2 atmosphere. Their electrical and optical properties, which have been investigated both ex situ and in situ during the annealing, are improved. The results of the in situ measurements are interpreted in terms of a diffusion process, where hydroxyl groups are decomposed and water is diffusing out of the films. As hydroxyl groups are known to act as a trap for charge carriers in ZnO, their removal from the film can explain the improvement of the electrical properties by the annealing.

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INTRODUCTION Al-doped ZnO (AZO) is a transparent conducting oxide. It is often used as a transparent electrode in solar cells1 or in flat panel displays.2 Other applications of AZO films are antistatic films,3 low emissivity coatings,4 or transparent heaters in defrosting windows.5 In order to be used as a transparent electrode, AZO films need to have a resistivity in the order of 10−4 Ω·cm, while being transparent in the visible range. AZO films can be produced by various methods, such as magnetron sputtering,6 spray pyrolysis,7 or chemical vapor deposition.8 Recently, the possibility of depositing highly conductive AZO films close to room temperature (about 45 °C) has been shown using high power impulse magnetron sputtering (HiPIMS).9 Such low deposition temperatures are interesting for transparent electronics on flexible substrates.10 However, the electrical properties of AZO films are known to degrade in humid environments.11 There have been several studies of this phenomenon12−14 that relate the degradation of the electrical properties to the absorption of water and the formation of Zn(OH)2. These hydroxyl groups at the grain boundaries can act as trap states which decrease the charge carrier concentration.14 The increase in the trap states at the grain boundaries also induces an increase in the potential barrier at the grain boundaries, thus decreasing the mobility of charge carriers.14,15 Tohsophon et al.12 showed that annealing the ZnO films in vacuum at a temperature above 150 °C can partially restore the electrical properties after a damp heat treatment. This suggests that compounds formed after reaction with water may decompose and absorbed water can diffuse out of the film during a low temperature annealing. Hüpkes et al.13 showed that a high temperature annealing at 650 °C can cause the ZnO films to become stable toward damp heat conditions. They © 2017 American Chemical Society

attributed this stability to the reconstruction of the grain boundaries, which serve as diffusion paths for water molecules. Hence, grain boundaries may act as diffusion shortcuts for water in ZnO. While annealing heat-sensitive multilayer devices is not feasible from a practical point of view, an annealing procedure to restore the properties of the material in applications of single layer AZO, such as antistatic films, low emissivity coatings, or transparent heaters, can be of great interest. The diffusion of water in oxides has been studied by Doremus.16 He proposes a model where the diffusion of water is coupled with the formation of the hydroxides to give an effective diffusion coefficient. The diffusion of water is rather difficult to measure directly. However, its detrimental influence on the electrical and optical properties may be used to evidence it indirectly by measuring these physical properties upon water uptake or outflow. In the present work AZO films prepared by various sputtering processes have been aged under ambient laboratory conditions and then annealed under N2 at temperatures between 160 and 180 °C. The evolution of the electrical and optical properties of the films has been characterized in situ during the annealing. Additionally, information about the chemical composition and the bonding state of the samples has been obtained before and after the annealing. The results of the in situ analyses are interpreted in terms of a diffusion process out of the AZO films of absorbed water-related species, resulting from the decomposition of hydroxide species. Received: March 30, 2017 Revised: June 9, 2017 Published: June 19, 2017 14426

DOI: 10.1021/acs.jpcc.7b03020 J. Phys. Chem. C 2017, 121, 14426−14433

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The Journal of Physical Chemistry C

Figure 1. Resistivity and optical band gap (a) and charge carrier concentration and mobility (b) of AZO films before (full symbols) and after an annealing at 180 °C (open symbols) as a function of distance from the target axis.

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EXPERIMENTAL SECTION Thin films of AZO have been deposited using an alloyed Zn0.97Al0.03 target using reactive direct current magnetron sputtering (DCMS) and reactive HiPIMS onto unheated glass substrates. For the DCMS-deposited films three different total pressures, 0.3, 1, and 1.5 Pa, were used. The O2 flow rate was optimized to obtain conductive films as reported by Jullien et al.,17 and a sputtering current of 0.1 A was applied. Additionally, AZO films have been deposited by HiPIMS in a different deposition system at a pressure of 1 Pa. Pulses with a length of 100 μs with a frequency of 1000 Hz were supplied by a Melec Spik 2000A. An Advanced Energy Pinnacle Plus DC power supply was used to charge the HiPIMS unit at a constant voltage of 570 V. In this system the films have been placed on a rotating substrate holder, with the magnetron placed excentric to the substrate holder rotation axis. This setup allows to study the film properties as a function of the position of the substrate with respect to the magnetron. The setup is described in more detail elsewhere.9 In order to study the behavior of the films after aging, the films have been stored in ambient laboratory environment (average relative humidity around 35%) for at least 6 months. This allowed the films to react with the ambient moisture. Then the films have been annealed at temperatures between 160 and 180 °C under N2 flow between 1.5 and 3 h. The annealing took place in sealed temperature controlled stages which allowed in situ characterization during the annealing. A Linkam FTIR 600 stage and a Linkam THMS 600 stage were used for in situ optical and electrical characterization, respectively. Transmittance measurements were done with a Cary 5000 UV− vis−NIR spectrophotometer in the ultraviolet, visible, and nearinfrared ranges and with a Thermo Scientific Nicolet 6700 Fourier transform infrared (FTIR) spectrometer in the nearinfrared range. For the electrical characterization a four-point probe setup was used both in situ during the annealing and ex situ before and after the annealing. Additional characterization of selected films was performed ex situ before and after the annealing in N2. The charge carrier concentration and the mobility were measured using a HMS 5000 Hall effect measurement setup. The composition profile of the films was measured using a CAMECA IMS 7f secondary ion mass spectrometer (SIMS). Room temperature photoluminescence (PL) spectra have also been recorded using a Horiba iHR320 spectrometer equipped with a Horiba Syncerity multichannel CCD detector. The excitation source was a 266 nm CryLas FQCW266-50 laser. Two different gratings have been used that are optimized to record the spectra in the UV and visible ranges, respectively.

X-ray photoelectron spectra (XPS) were measured using nonmonochromatized Al Kα radiation at 1486.6 eV. The photoelectrons were collected by an Omicrometer EA 125 hemispherical analyzer. In order to analyze the chemical composition inside the film, the samples were etched by several cycles of Ar+ bombardment (3 × 10−6 mbar) with an energy of 5 keV and a current of 38 mA. The Ar+ bombardment was performed prior to and after annealing at 175 °C for 90 min in vacuum.

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RESULTS Figure 1 shows the evolution of the resistivity (Figure 1a), as well as the charge carrier concentration and mobility (Figure 1b) measured using the Hall effect setup, as a function of the lateral position of the sample relative to the center of the target. We have considered here an AZO film deposited using HiPIMS at a discharge voltage of 570 V. The film was annealed at 180 °C for 2 h. The deposition parameters were chosen to achieve low resistivity values to keep the variation of the properties across the sample surface relatively small9 and to improve the activation of dopants.18 Moreover, the band gap was calculated from the transmittance spectra using the Tauc method.19 The resistivity decreases by a factor of 2−8 while the band gap increases after annealing. The charge carrier concentration and mobility also increase for most of the sample positions despite the low annealing temperature and the inert atmosphere. Under these annealing conditions changes to the intrinsic defects or the crystalline structure are not expected. This suggests the influence of an extrinsic factor such as absorbed water. In Table 1 the resistivity ρ, the charge carrier concentration n, the Table 1. Electrical and Optical Properties of an AZO Film Deposited Using HiPIMS at a Distance of 3 cm from the Target Axis aged ρ (10−4 Ω·cm) n (1020 cm−3) μ (cm2/(V s)) Egap (eV) Tavg (%)

as-deposited

before annealing

after annealing

4.2 ± 0.4

22.8 ± 2.3 5.5 ± 0.3 4.9 ± 0.3 3.52 ± 0.03 78.0 ± 8.5

7.2 ± 0.7 8.2 ± 0.3 10.5 ± 0.3 3.60 ± 0.04 81.5 ± 6.7

mobility μ, the band gap Egap, and the average transmittance in the visible range Tavg are shown for the AZO film deposited at a distance of 3 cm from the target axis together with the initial resistivity of the film after the deposition. The position of 3 cm from the target axis was selected as it shows the lowest resistivity. The electrical properties are improved, and the 14427


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The Journal of Physical Chemistry C average transmittance in the visible range is increased after the annealing. The resistivity after the annealing approaches the initial resistivity of the film after the deposition but is still higher. The transmittance of the HiPIMS-deposited films has been measured in situ during the annealing at 180 °C in the spectrophotometer. The resulting spectra close to the band gap are shown in Figure 2 for the sample deposited at a distance of

Figure 2. Transmittance of an AZO film deposited using HiPIMS at a distance of 3 cm from the target axis. The transmittance was measured during annealing at 180 °C under N2 as a function of time.

Figure 3. Absolute (a) and relative (b) change of the transmittance at 2500 nm as a function of the annealing time for two different AZO films. The error of the measurement is smaller than the size of the symbols.

3 cm from the target axis. It can be seen that the band gap increases with time during the annealing. This increase can be related to an increase in the charge carrier density due to the Burstein−Moss effect.20 Another indirect way to obtain information about the charge carrier density is the bulk plasmon resonance. The bulk plasmon resonance causes a decrease of the transmittance in the near-infrared range. An increase in the charge carrier concentration leads to a shift of the plasmon resonance, and therefore the onset of the decrease in transmittance shifts toward lower wavelengths.21 This means that the evaluation of the transmittance at a certain wavelength (e.g., 2500 nm) can be used as an indirect indicator of the charge carrier concentration. The evolution of the transmittance in the near-infrared during the annealing is shown in Figure 3a for two different samples. One of the samples is the same HiPIMSdeposited sample as in Figure 2, which was annealed at 180 °C while measuring its transmittance in the spectrophotometer. The other sample is an AZO film deposited using DCMS at a pressure of 1.5 Pa. The DCMS-deposited AZO film was annealed at 160 °C while measuring its transmittance using the FTIR spectrometer. As the electrical properties of the DCMSdeposited film are not as good as for the HiPIMS-deposited film, the near-infrared transmittance is higher for the DCMSdeposited film due to its lower charge carrier concentration (see also Figure 4). The difference in the electrical properties of the films can be explained by the more pronounced substoichiometry9 and the higher activation of dopants18 which leads to an improvement in the homogeneity of the properties in the case of HiPIMS-deposited AZO films even for depositions close to room temperature. For both samples the transmittance in the near-infrared decreases with the annealing time, indicating an increase in the charge carrier concentration. Despite the large difference in the electrical and optical properties of the two samples, the evolution of the charge carrier concentration during the annealing is very similar. This can be seen in the relative evolution T/T0 of the near-infrared transmittance with respect to the initial transmittance T0 that is shown in Figure 3b.

Figure 4. Absolute (a) and relative (b) change of the sheet resistance with the annealing time for AZO films deposited using DCMS at different pressures. The annealing temperature is fixed at 170 °C. The error in the measurement in smaller than the size of the symbols. The solid lines in (b) correspond to a fit of the function using eq 4.

Similarly to the optical measurements, in situ resistance measurements have been carried out on AZO films deposited by DCMS at different pressures. The evolution of the sheet resistance is shown as a function of the annealing time in Figure 4a. Even though the initial sheet resistance of the three samples varies over 2 orders of magnitude, the evolution with the annealing time is very similar. The sheet resistance decreases with the annealing time. Figure 4b shows the relative change R/ R0 of the sheet resistance with respect to the initial resistance R0 of each sample. The solid lines correspond to a fit of the function using eq 4 given in the Discussion section. 14428


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deposited on Si substrates using DCMS at 0.5 Pa. The XPS spectra have been collected from the surface and after several successive Ar+ bombardment steps. The O 1s core level spectra which are shown in Figure 7 were fitted using the software CasaXPS. The background was treated by the Shirley method.27 The O 1s peak can be fitted by four contributions, which are a mixture of Gaussian and Lorentzian functions. The contributions are labeled A, B, C, and D. Contribution A is centered around 529.9 ± 0.05 eV and can be attributed to the Zn−O bonds in the ZnO lattice.28,29 The second contribution, B (centered around 531.1 ± 0.05 eV), can be attributed to Zn− O bonds in O-deficient regions in the ZnO lattice.28,29 Contribution C (centered around 532.4 ± 0.06 eV) can be attributed to the contamination with for example −CO3, water, and hydroxyl groups.28,29 Contribution D (centered at 533.7 ± 0.23 eV) was found by Kunat et al.30 and confirmed by Kotsis and Staemmler31 to be characteristic for hydroxide species adsorbed on the polar (0001) surface of ZnO. This contribution disappears after the Ar+ bombardment of the sample. From the areas of the peaks corresponding to the contributions A to D, their respective concentration can be calculated. The error in the concentration can be estimated using a Monte Carlo algorithm within the CasaXPS software. The content of O belonging to hydroxyl groups in the films can be estimated by the ratio (C + D)/(A + B + C + D). This O content is probably overestimated, more particularly at the film surface, because contribution C is not just attributed to hydroxyl groups but also to other contaminants. The content of hydroxyl groups is shown in Figure 8a as a function of the Ar+ bombardment time. The content of hydroxyl groups within the films decreases from around 20% before the annealing to around 17% after annealing. In Figure 8b, the ratio of A/(A + B) is shown as a function of the bombardment time. This ratio shows the relative amount of O atoms present in a stoichiometric ZnO environment with respect of the total amount of O atoms involved in Zn−O bonds. After the annealing, an increasing fraction of O atoms are participating in a stoichiometric ZnO environment.

For the HiPIMS-deposited sample grown at a distance of 1 cm from the target axis, the concentration profile for the elements Zn, Al, O, and H was measured with SIMS before and after annealing at 180 °C for 2 h. The profile of the different elements did not experience significant changes except for H, which is shown in Figure 5. It can be seen that the H

Figure 5. H concentration profile measured in an AZO film, deposited using HiPIMS at a distance of 1 cm from the target axis, prior to and after annealing at 180 °C for 2 h.

concentration decreases by 65% after the annealing to a low level throughout the entire film thickness, showing that Hrelated species have diffused out of the film. Room temperature photoluminescence spectra of an AZO film deposited on a Si substrate using HiPIMS at a pressure of 0.6 Pa have been recorded before and after an annealing at 180 °C for 2 h. The resulting spectra are shown in Figure 6. Two

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DISCUSSION The results of the in situ characterization during the annealing in Figure 4 show that the electrical properties are improved by the annealing. Both the charge carrier density and the mobility are increased after the annealing, as can be seen from the transmittance data in Figure 3 and from the Hall effect measurements in Figure 1. This improvement of the electrical properties could be explained by the removal of hydroxyl groups. Zn(OH)2 is believed to play a role in the degradation of the electrical properties of ZnO films due to humidity,11 as it is a resistive wide band gap semiconductor with a band gap that was calculated to be 5.645 eV.32 Zn(OH)2 is known to decompose at a temperature of 150 °C.33 Kim et al. have shown that the increase in resistance during a damp heat treatment of AZO can be explained by the chemisorbed OH− at grain boundaries14 that forms trap states for charge carriers and increases the potential barrier at grain boundaries. The resistance of the film can be modeled as the sum of the resistance within the grains and the resistance due to the grain boundaries. The OH− groups at the grain boundaries can trap charge carriers and therefore increase the grain boundary resistance.

Figure 6. Room temperature PL spectra of a HiPIMS-deposited AZO film before and after annealing at 180 °C under N2.

peaks can be identified in the spectral range 300−900 nm. The first one centered at 350 nm corresponds to the near band edge photoluminescence.22 Its intensity increases with the annealing. Usually, the near band edge photoluminescence is found at higher wavelengths around 380 nm.23 The shift to lower wavelength can be attributed to the high charge carrier concentration. High charge carrier concentrations are known to cause a broadening and a shift of the emission peak.24 In degenerate semiconductors impurity assisted recombination processes can lead to the emission of higher energy photons.25 The shoulder that can be seen around 400 nm might be related to Zn vacancies.26 The second peak in the visible range is much broader and can be attributed to point defects.22 Its intensity decreases slightly after annealing as expected. It should be noted that the intensity of the defect emission is much smaller than the intensity of the near band edge emission. XPS spectra of AZO films have been measured before and after annealing in vacuum at 175 °C on two samples placed next to each other during the deposition. The films were 14429


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Figure 7. O 1s core-level spectra measured before annealing prior to (a) and after 15 min Ar+ bombardment (b). The same spectra were then measured after annealing at 175 °C for 90 min in vacuum prior to (c) and after 15 min Ar+ bombardment (d).

of water is assumed to be constant within the film. At the film surface the concentration of water is fixed at 0. This approximation can be justified as there is a constant flow of N2 over the film, so any water molecules that desorb from the surface are transported away with the gas flow. With these assumptions, the concentration profile of water C(x,t) in the film is given by C(x , t ) 4 = π C0

∞

∑ n=0

⎛ −D(2n + 1)2 π 2t ⎞ ( −1)n exp⎜ ⎟ 2n + 1 4l 2 ⎝ ⎠

⎛ (2n + 1)πx ⎞ × cos⎜ ⎟ ⎝ ⎠ 2l

(1)

with C0 being the initial concentration of water and D the diffusion coefficient.34 There is no direct experimental data available on the actual amount of the hydroxyl groups present in the film, but it is possible to interpret the resistance as an indirect measurement of C(x,t)/C0. The film can be divided into N thin slices, each with a resistance of Ri(x,t). As the current is flowing parallel to the film surface and therefore perpendicular to the diffusion direction x, the resistances Ri(x,t) are connected in parallel. The total resistance of the film is then given by

Figure 8. Ratio of the fitting contributions of the O 1s XPS signal. (a) Contribution of O belonging to hydroxyl groups. (b) Contribution of O within the stoichiometric ZnO lattice.

1 = R (t )

It can be assumed that Zn(OH)2 or hydroxyl groups are decomposed during annealing, producing molecular water that diffuses out of the grain boundaries.16 The change of the resistance with the annealing time in Figure 4 can therefore be interpreted as a consequence of the out-diffusion process of the molecular water. In order to model this diffusion process, we can consider the film as a planar sheet of thickness l where diffusion only occurs in the direction perpendicular to the film surface, which is denoted x. The interface between the film and substrate can be viewed as impermeable. The initial distribution

N

∑ i=1

1 R i(x , t )

(2)

For N a value of 200 was chosen, which gives a slice width of 0.5 nm for a film of 100 nm thickness. Each resistance Ri(x,t) can be related to the hydroxyl group concentration C(x,t)/C0 at the position x. For this, it is necessary to normalize Ri(x) in order to obtain a quantity that is proportional to the fraction C(x,t)/C0. At time t = 0, the total film resistance is equal to the initial value Rmax, and therefore 14430


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The Journal of Physical Chemistry C Ri(x,0) needs to be NRmax. At t → ∞ all the water is removed from the film, and the total film resistance reaches its final value Rmin, which means that limt→∞ Ri(x,t) is equal to NRmin. The normalization can therefore be described by ⎤ ⎡ C(x , t ) (R max − R min) + R min ⎥ R i(x , t ) = N ⎢ ⎦ ⎣ C0

Figure 6 is also in agreement with the hypothesis of the decomposition of the hydroxyl groups. It has been shown that the presence of Zn(OH)2 decreases the intensity of the excitonic emission.40 The fitting results of the O 1s core level XPS peak in Figure 8a also show that the concentration of hydroxyl groups is decreasing after the annealing. However, the concentration of hydroxyl groups is not decreasing to zero, meaning that not all of the hydroxide related species are removed. This could suggest that there are different environments of hydroxyl groups that might be more or less difficult to remove. The stability of the adsorbed hydroxyl groups depends on the adsorbing surface. Hydroxyl groups are most stable on the polar (0001) surfaces,41 which could explain why contribution D is not removed after the annealing.42 On other surfaces such as the nonpolar (101̅0) surface or (112̅0) surface the adsorption energy of hydroxyl groups is lower.41,43 Another observation is the increase in the concentration of O in a stoichiometric ZnO lattice as can be seen in Figure 8b. This could indicate that when hydroxyl groups are removed from the film, the environment around the remaining O atoms changes to resemble more the environment of stoichiometric ZnO. Water can then diffuse out of the film along the grain boundaries, as discussed above.

(3)

with Rmax the initial (maximum) value of the resistance and Rmin the final (minimum) value of the resistance R(t). It is possible to rewrite eq 2 in order to fit it to the measured relative change of the resistance R(t)/Rmax. N ⎞−1 R (t ) ⎛ 1 ⎟⎟ = ⎜⎜∑ R max ⎝ i = 1 R i(x , t )/R max ⎠

(4)

Inserting eqs 1 and 3 into eq 4, there is only one parameter for the fit, the diffusion coefficient D. The resulting curves are shown in Figure 4b. The results for the diffusion coefficient D are summarized in Table 2. Table 2. Effective Diffusion Coefficient Obtained from Fitting Eq 4 to the Measured Resistance pressure (Pa)

D (10−13 cm2/s)

0.3 1.0 1.5

1.19 ± 0.06 0.48 ± 0.02 2.10 ± 0.08

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CONCLUSION Various AZO films have been annealed in a N2 atmosphere at temperatures between 160 and 180 °C after aging in ambient air. The evolution of their electrical and optical properties has been followed in situ as a function of the annealing time. The electrical and optical properties improve after the annealing, independent of the initial properties. The improvement of the electrical properties can be modeled considering out-diffusion of water, with the fastest diffusion found for films deposited at the highest pressure. SIMS measurements have shown that H is removed from the films, while the O 1s core level spectra show that the content of hydroxyl groups decreases. This shows that hydroxyl groups are decomposed and water is removed from the films during the annealing.

According to the diffusion-reaction model by Doremus, the diffusion of water in oxides can be separated in the diffusion of molecular water and the formation of hydroxyl groups.16 This leads to an effective diffusion coefficient that can be several orders of magnitude lower than the actual diffusion coefficient of water molecules in oxides. For the diffusion of water in silica, effective diffusion coefficients in the order of 10−15 cm2/s35 to the order of 10−13 cm2/s36 have been found for annealing temperatures of 200−300 °C. From this data Doremus calculated diffusion coefficients for molecular water in the order of 10−10 cm2/s.37 The effective diffusion coefficients determined in this work are in the order of 10−13 cm2/s. Considering the lower annealing temperature in this work, our values for the effective diffusion coefficient are higher than the effective diffusion coefficients given above for the diffusion of water in silica. This can be understood, considering that our material is not amorphous but a nanocrystalline material with a large density of grain boundaries, that offers easier diffusion pathways. This hypothesis is supported by the fact that the fitted diffusion coefficient has the highest value for the sample deposited with the highest pressure. A high deposition pressure leads to a lower kinetic energy of the atoms impinging on the growing film,38 which leads to a more fine-grained microstructure according to the structure zone model by Thornton.39 A fine-grained microstructure means a larger density of grain boundaries which allows for faster diffusion. This suggests that the hydroxyl groups in the grain boundaries might decompose and water might be diffusing out of the material using the grain boundaries as diffusion paths. The SIMS results in Figure 5 confirm that H is removed from the films during the annealing. This is in agreement with the model of the decomposition of hydroxyl groups. The increase in the PL intensity of the excitonic peak as seen in

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.H.). ORCID

David Horwat: 0000-0001-7938-7647 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.M. thanks the European Commission for the “Erasmus Mundus” scholarship within the DocMASE project. The authors are grateful to Denis Mangin for help with SIMS measurements.

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Restoring the Properties of Transparent Al-Doped ZnO Thin Film

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