Unraveling the Oxygen Effect on the Properties of Sputtered BaSnO3

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Unraveling the oxygen effect on the property of sputtered BaSnO3 thin films Bingcheng Luo, and JunBiao Hu ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00007 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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ACS Applied Electronic Materials

Unraveling the oxygen effect on the property of sputtered BaSnO3 thin films Bingcheng Luo*, Junbiao Hu Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710072 China * Corresponding author. E-mail address: [email protected] Abstract: The perovskite oxide BaSnO3 is a highly topical material for next-generation transparent conducting semiconductors. The foreign chemical doping strategy is largely used to realize the high conductivity in BaSnO3. Removal of oxygen in BaSnO3 is another route to get high conductivity but the oxygen vacancy-property relationships have not been realized completely, which is of fundamental importance for the future optoelectronic applications. Here we report how the oxygen vacancies in sputtered BaSnO3 films can be accounted for the structural and chemical environment, the transport phenomena, and the optoelectronic properties. Oxygen vacancies in BaSnO3 films cause the expansion of the unit cell along the out-of-plane direction, enhance the conductivity, transform the electronic transport mechanism from the thermal activation model to the variable-range hopping (VRH) model, and spawn the sub-band level-assisted photoconduction. The present work further strengthens our understanding of the oxygen effect on the physical properties in BaSnO3-based systems. Keywords: BaSnO3, transparent conductive oxide, thin film, transport property, photo response, oxygen vacancy

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Introduction As the member of perovskite family, alkaline-earth stannates (RSnO3; R=Ba, Sr, and Ca) have been intensively investigated for various applications.1-5 Among RSnO3, BaSnO3 (BSO) with an ideal cubic perovskite structure and large optical band gap (>3 eV) is particularly promising.6, 7 La-doped BSO single crystals possess extremely high room temperature mobility (~320 cm2 V-1 s-1) and excellent thermal stability,8 reflecting great prospects as a candidate transparent conducting oxide to replace the industry standard Sn doped In2O3 (ITO). Especially, the power conversion efficiency and the stability of organic-inorganic perovskite solar cells were improved by replacing mesoporous TiO2 with La-doped BSO.9 When using La-doped BSO as the channel layer, the large on-off ratio (>107) was realized in all-perovskite field-effect transistors.10 Additionally, theoretical prediction indicated that hybrid improper ferroelectricity could be achieved in stannate-based superlattices11 and two dimensional electron gases at interfaces between CaSnO3 or ZnSnO3 and KTaO3 or KNbO3 could be controlled by electric field.12 These unique characteristics make one believe that stannate-based oxides have the potential to be as impactful to future oxide electronic devices as silicon in modern devices.6 Previously, doping with trivalent rare-earth ions at Ba2+ site or pentavalent ions at Sn4+ site is commonly used to transform BSO from an insulator to a conductor. So far, most of the research efforts have focused on the La-doped BSO, since La doping could produce the highest electron mobility and lowest electrical resistivity in both single crystal and epitaxial film. Nevertheless, other chemical substitutions are also 2


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investigated. For example, strontium substitution could widen the band gap of BSO thin films,13 and the large magnetic moment could be realized in Gd/Nd/Pr-doped BSO.14,

15

In addition to the A/B-site substitutional dopants, removal of oxygen in

BSO is also attractive since oxygen vacancies are closely related to the physical properties

(e.g.,

transport,

magnetic,

resistive

switching,

and

persistent

photoconductivity properties). Previously, the theoretical results suggested that oxygen vacancies in BSO were unfavorable for high levels of n-type conductivity due to high formation energies.16 Fortunately, by lowering oxygen pressure during the film growth17 or vacuum annealing at high temperatures18,

19,

the oxygen-deficient

BSO films could exhibit semiconductor and even metal charge transport phenomena. Such behavior has been attributed to the presence of shallow states near the conduction band. The existence of defect states within the band gap of BSO can apparently affect the conductivity,17-22 but the importance and activity of such sub-band levels to the photoconductivity has been largely neglected, which is rather important for optoelectronic devices. Therefore, it is highly expected to study the optoelectronic properties of BSO films with oxygen vacancies. On the other hand, compared to other physical methods such as pulsed laser deposition (PLD) and molecular beam deposition (MBE), sputtering techniques are more easily scaled for applications. Very recently, we demonstrated that the BSO films grown by radio-frequency (RF) magnetron sputtering technique under a reduced oxygen atmosphere could modify the transport and magnetic photovoltaic properties of La0.7Sr0.3MnO3/BaSnO3junctions.5 These previous results indicated that defects if 3



controlled properly could offer an alternative opportunity to engineer the physical properties of BSO films, because they may act as donors/acceptors, compensating centers, charge traps, or recombination centers. Herein, we compared the electrical transport and photo-response properties of sputtered BSO films which were deposited under different oxygen atmospheres. This work would reveal the role of oxygen vacancy on the property evolution of BSO films and in turn, provide important design guideline for desired functionalities. Experimental The epitaxial BaSnO3 (BSO) thin films of thickness ~100 nm were deposited on TiO2-terminated SrTiO3 (STO) (001) substrates by radio-frequency (RF) magnetron sputtering technique with BSO ceramic target.23 The deposition conditions are as follows: the deposition pressure of 20 mTorr, RF power intensity of 2.5 W/cm2, and the substrate temperature of 800℃. When depositing the BSO films, different oxygen partial pressures (i.e.,O2/(Ar + O2)) from 50% to 0%, were chose for controlling different oxygen vacancy concentrations. After deposition, the films were kept in-situ for 30 min at the corresponding deposition condition and then cooled to room temperature. The phase structures of the films were checked by X-ray diffraction (XRD) (Bruker D8) with Cu Kα radiation in the usual θ-2θ geometry. The valence states were examined using X-ray photoelectron spectroscopy (XPS) (ESCALAB 250) with Al Kα radiation. For minimizing the surface effect of the films, the XPS signal was collected after a thin top layer (~15 nm) was removed away by Ar+ iron beam. 4


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Temperature-dependent resistivity was measured along the in-plane direction by a four-probe method, and the room-temperature carrier mobility and concentration were determined through Hall measurement. The surface morphology and local current mapping were characterized at room temperature by scanning probe microscope (SPM) (MFP-3D, Asylum Research) operated in conductive atomic force microscopy (CAFM) mode, as described concretely in prior work.24 For measuring the spectral response, two silver electrodes with the distance of 1 mm were vacuum-evaporated onto the film, through which the 6487 Keithley electrometer was connected by a probe station. Prior to illumination, BSO films were heated up to 120℃ for 10 min to exclude the effect of the absorbed molecules, and subsequently stored in the dark under vacuum environment for one day to eliminate persisting effects of previous light exposure. The wavelength scanning regime is between 300 and 700 nm. More measurement details can be found in prior work.25 Results and discussion Figure 1 (a) presents XRD patterns of BSO films deposited at various oxygen partial pressures. Only the (00l) peaks corresponding to the substrate can be seen, confirming that all the films are well (001)-oriented along the normal to the substrate and have a perovskite structure without any impurity phase. The positions of the (00l) peaks are shifted towards lower angle with decreasing the oxygen partial pressure, reflecting the expansion of the out-of-plane lattice parameter. Similar phenomena were observed in oxygen-deficient BSO films produced by PLD.17 The observed lattice expansion is an indication of the increased concentration of oxygen vacancies because the formation 5



of oxygen vacancies would enhance the electrostatic repulsion between the cations, and a resultant expansion of the lattice. To better understand the cause of this lattice expansion, XPS measurements were used to investigate the oxidation states and chemical compositions of BSO films. Figure 1 (b) shows the high-resolution O 1s XPS spectra and the corresponding de-convolution. From the best peak de-convolution, one can see two spectral components, centered at around 529.4±0.2 eV and 531.0±0.1 eV, respectively. The first spectral component, located at the low binding energy (~529.4 eV), is ascribed to the lattice oxygen (OL) in BSO-based compounds, while the second one is related to the oxygen vacancies (OV).25, 26 The ratio of peak area (OV/OL), which is positively correlated with the relative concentration of oxygen vacancies,25 is presented in Figure 1 (d), confirming that the number of oxygen vacancies increases with decreasing the oxygen partial pressure. To meet the requirement of charge neutrality, it is reasonably believed that the Sn2+ ions should exist in oxygen-deficient BSO films.27 Thus, the high-resolution Sn 3d5/2 XPS spectra could be de-convoluted into two peaks, as shown in Figure 1 (c). The first peak, appeared at the high binding energy (~494.5±0.2 eV), results from the Sn with high valence (Sn4+), and the second one (~493.7±0.3 eV) could be assigned to Sn2+. Note that the chemical shift between Sn4+ and Sn2+ is about 0.8±0.1 eV, corresponding with the previously reported values (~0.7±0.05 eV) in tin oxides.28 Similarly, the evolution of the relative concentration of Sn2+ ions in BSO films is presented in Figure 1 (d), implying the enhanced concentration of Sn2+ ions with decreasing the oxygen partial pressure. 6


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Having established that the growth oxygen atmospheres could be used to modify the concentration of Sn2+ ions and oxygen vacancies, we proceed to study the influence of such defects on the electronic and optoelectronic properties. The conductivity behaviors involving the oxygen effect are first compared. Figure 2 (a) shows the room-temperature (298 K) resistivity (ρ), carrier density (n) and mobility (μ) of BSO films as a function of the oxygen partial pressure. The resistivity decreases systematically with decrease of the oxygen partial pressure, while the carrier density and mobility are boosted. The resistivity of BSO film deposited in oxygen-poor (0%) atmosphere is about 29 mΩ cm, which is smaller than that of BSO film deposited in oxygen-rich (50%) atmosphere by approximately four orders of magnitude. Similar results have also been observed in BSO-based thin films.17, 19 As reported, the energy level of the Sn2+ ions in BSO system is situated at ~1.4 eV above the valence-band edge,29 forming the deep acceptor level. Thus, the electrical behavior is primary determined by oxygen vacancies. Ionized oxygen vacancies could provide the additional mobile carriers for conduction, leading to the increased carrier density. Meanwhile, oxygen vacancies can neutralize the negative charges at threading dislocations, which suppresses the contribution of dislocation scattering.21 As a result, the mobility is boosted. Mobility of 10.7 cm2 V-1 s-1 at 1.9×1019 cm-3 was obtained in BSO film deposited in oxygen-poor (0%) atmosphere. The electrical behaviors of BSO film is closely related to many variables including the fabrication method, the substrate choice, the growth parameters, the annealing conditions, and the thickness, however, these values of our film are comparable to those of high-temperature 7



vacuum annealed BSO films deposited on MgO substrates (e.g., ~0.8 cm2 V-1 s-1 at 2×1017 cm-3 when annealed at 600℃; ~22 cm2 V-1 s-1 at 5×1019 cm-3 when annealed at 900℃).19 To understand the conduction mechanism, the temperature-dependent resistivity of BSO films was also measured, as illustrated in Figure 2 (b). Although vacuum annealed BSO films at higher temperature (775 ℃ ) could exhibit the metallic behavior,19 all the BSO films in our case exhibit the semi-insulating behaviors. This discrepancy may be due to the different fabrication conditions. According to the study on BSO-based compounds,19, 23 the thermal activation model and the variable-range hopping (VRH) model were commonly utilized to analyze the conduction mechanism. Both the models can be described with general relation ρ∝exp(T0/T)p, where T0 is the characteristic parameter, T is the temperature and n is the exponential term. The value of p can be used to distinguish different conduction mechanism, such as the thermal activation model (p=1), Mott-VRH model (p=1/4)30 and Efros-Shklovskii (ES) VRH model (p=1/2).31 Therefore, the exponent p should be determined first in a self-consistent way, i.e., Zabrodskii analysis.32 The logarithmic derivative w=− dln(ρ)/dln(T)=p×(T0/T)p is given. By plotting ln(w) vs. ln(T), the value of p can be obtained from the slope, as shown in Figure 2 (b). The p values are close to 1 for highly oxygenated BSO films (50% and 25%), whereas they are close to 0.25 for oxygen-deficient BSO films (10% and 0%), implying a crossover from the thermal activation model to the Mott-VRH model. As expected, the oxygen-deficient BSO films are a disordered system with localized electron carriers due to the inevitable 8


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oxygen vacancies introduced during the thin-film deposition. For comparison, all the resistivity data were fitted according to the thermal activation model or the Mott-VRH model, as given in Figure 2 (b) (solid lines). From the fitted results, we obtain the values of the characteristic parameter, T0, which are 1062.8 K (50%), 656.1 K (25%), 363862.0 K (10%), and 2621.2 K (0%) for BSO films deposited at different oxygen partial pressures. Therefore, the activation energy can be estimated using the relation Ea= kBT0 for the thermal activation model, or Ea=0.25kBT (T0/T)0.25 for Mott-VRH model (kB is the Boltzmann constant). As presented in Figure 2 (d). the activation energy values at 300 K are given, which decreases systematically from 90.7 meV to 11.0 meV when the O2/(Ar + O2) ratio decreases from 50% to 0%. Oxygen vacancies could form defect states within the BSO band gap and shift the Fermi level upward.33 As a result, the activation energy is decreased. Beyond the above-mentioned macroscopic electrical behaviors, understanding the local transport phenomena is also attractive due to the miniaturization of next-generation electronic devices. To this end, conductive atomic force microscopy (CAFM), which could simultaneously give surface morphology and local conductivity map, is employed to gather information on the spatial distribution of the electrical conductance with nanoscale resolution. Figure 3 shows the CAFM results of BSO films under a constant sample bias of 1.0 V. As visible from Figure 3 (a), the surface topographies of BSO films exhibit a uniform crack-free microstructure, suggesting that the topography will have a minimal impact on CAFM current for our samples. The root-mean-square (RMS) surface roughness was analyzed through Igor 9



Pro software. The RMS values are 0.32 nm (50%), 0.75 nm (25%), 0.92 nm (10%), and 1.21 nm (0%) for BSO films deposited at different oxygen partial pressures. The oxygen-deficient BSO film possesses larger RMS value, which is likely associated with the oxygen vacancies. Oxygen vacancies could accelerate oxygen ions motion, leading to the increment of the grain growth rate and thus the increased surface roughness.34 As seen from Figure 3 (b), these corresponding current images exhibit spatially inhomogeneous current magnitudes. Also, the corresponding features are obviously absent in the cross-section profiles between topographic image and current image, as shown in Figure 3 (c), indicating that the spatial distribution of the local current is not related to the topographical features. Additionally, the mean current magnitudes are 0.12 nA (50%), 0.33 nA (25%), 1.58 nA (10%), and 14.23 nA (0%) for BSO films deposited at different oxygen partial pressures, clearly reflecting the growth in the carrier-transporting capability with decreasing the oxygen partial pressure, consistent with the above-discussed macroscopic transport behaviors. Oxygen vacancies can significantly modify the electronic properties of BSO films. Also, the oxygen effect can be reflected in the photo-response spectra of BSO films, which is important for photo-detector applications. Thereby, the spectral responses of BSO films were measured, as shown in Figure 4 (a). The sketch of the measurement geometry is given in Figure 4 (b). Generally, the photo-response arises from electronic transitions which occur when illumination photons with sufficient energy (E) are absorbed. We thus divide the response spectra into three regimes, depending on the absorption edge of BSO ~3.0 eV,7 and obtain a number of comparisons as 10


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follows. First, in the super-band gap regime (E>3.2eV), all the responses decrease with increasing the photon energy. At this regime, the photons are primarily absorbed at or near the surface of the film where the carriers intensely recombine due to the high surface recombination. As a result, the detected response is suppressed. Note that the initiation of the fall is at ~3.26 and 3.13 eV for the highly oxygenated BSO film (50%) and the oxygen-deficient BSO film (0%), respectively. This red shift with decreasing the oxygen partial pressure may be closely related to the band tailing effect. As demonstrated above, the oxygen-deficient BSO films are a disordered system, and disorder can induce the Urbach tail in the conduction band edge.23 Second, the responses rise quickly near the band gap regime (E~3.0±0.2eV). When the photon energy is equal to the band gap, the carriers from the valence band (EV) are excited to the conduction band (EC), resulting in the free electrons in the conduction band and the free holes in the valence band. Both contribute to the current, and thus the response is increased rapidly. Nevertheless, compared to the highly oxygenated BSO film (50%), the other responses become slowly less with decreasing the photon energy due to the Urbach absorption. Therefore, the experimental data can be fitted according to the exponential law (∝exp(E/Eu), Eu is the Urbach energy), as shown in Figure 4 (a) (solid line). The Eu values extracted from the fitting results are about 151, 348, and 452 meV for BSO films deposited under 25%, 10% and 0% oxygen atmosphere, respectively. Third, in the sub-band gap regime (E

Unraveling the Oxygen Effect on the Properties of Sputtered BaSnO3

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