Sensing and impedance characteristics of ybtao4 sensing membranes

ABSTRACT In this study we developed ytterbium tantalum oxide (YbTaO4) sensing membranes for use in electrolyte–insulator–semiconductor (EIS) pH sensors. The influence of rapid thermal

annealing (RTA) treatment on the sensing and impedance properties of the YbTaO4 sensing membranes deposited through reactive co-sputtering onto Si substrates was explored. X-ray diffraction,

atomic force microscopy, and X-ray photoelectron spectroscopy revealed the structural, morphological, and chemical features, respectively, of these YbTaO4 films annealed at 700, 800 and 900

 °C. The YbTaO4 EIS device annealed at the 800 °C exhibited a super-Nernstian response of 71.17 mV/pH within the pH range of 2–12. It also showed the lowest hysteresis voltage ( < 1 mV)

and the lowest drift rate (0.22 mV/h) among the tested systems. Presumably, the optimal annealing temperature improved the stoichiometry of YbTaO4 film and increased its (−131)-oriented

nanograin size. Moreover, the impedance properties of YbTaO4 EIS sensors were investigated by using the capacitance–voltage method. The resistance and capacitance of YbTaO4 sensing films

annealed at three various temperatures were evaluated by using different frequency ranges in accumulation, depletion, and inversion regions. The semicircle diameter of the YbTaO4 EIS sensor

became smaller, due to a gradual decrease in the bulk resistance of the EIS device, as the RTA temperature was increased. SIMILAR CONTENT BEING VIEWED BY OTHERS RESISTIVE SWITCHING AND

BATTERY-LIKE CHARACTERISTICS IN HIGHLY TRANSPARENT TA2O5/ITO THIN-FILMS Article Open access 31 August 2023 DIRECT MEASUREMENT OF THE THERMOELECTRIC PROPERTIES OF ELECTROCHEMICALLY DEPOSITED

BI2TE3 THIN FILMS Article Open access 21 October 2020 LI IONTRONICS IN SINGLE-CRYSTALLINE _T_-NB2O5 THIN FILMS WITH VERTICAL IONIC TRANSPORT CHANNELS Article Open access 27 July 2023

INTRODUCTION An ion-sensitive field-effect-transistor (ISFET) device was first developed by Bergveld in 1970 as a replacement for a fragile glass electrode1. Over the past four decades,

research on various kinds of ISFET-based biosensors, for example glucose, urea, protein, DNA hybridization, and DNA methylation detection2,3,4,5,6,7, has been conducted because they have the

advantages of fast response, small size, and low cost. In general, the gate electrode of a conventional metal-oxide-semiconductor field-effect transistor (MOSFET) is replaced by a

chemically sensitive oxide layer, an electrolyte, and a reference electrode to become an ISFET device. An electrolyte-insulator-semiconductor (EIS) capacitor has a simple structure and easy

fabrication with respect to an ISFET device. The surface potential of such an ISFET is modulated by the change in the charges at the interface between the oxide layer and electrolyte, thus

leading to the shift of the threshold voltage and the variation of the drain-source current. As a result, quality and chemical stability of the sensing film play a key role in the

applications of an ISFET or EIS device. The most widely used sensitive oxide film is SiO2 in ISFETs, but its application is severely limited by its poor sensitivity8. Therefore, Si3N4,

Al2O3, Ta2O5, TiO2, ZrO2, and HfO2 films were investigated as alternative sensitive oxides in ISFETs or EISs to improve their sensitivities9,10,11,12. However, the pH sensitivity of the

traditional ISFET or EIS sensors could not exceed the Nernstian limit (59.18 mV/pH at 25 °C) because of intrinsic properties of these metal oxide films. In addition, there are some materials

related problems with silicide at the interface of oxide film/Si substrate after thermal treatment and dangling bonds on the sensing film during the thin-film deposition to impact their

sensing performance13,14. In search of the low defect density and high thermal stability in high dielectric constant (κ) metal oxide films, rare-earth (RE) oxide thin films have been studied

for use as a replacement MOSFET gate dielectric due to their high κ values, large bandgap energies, good thermodynamic properties, high resistivities, and high conduction-band offsets15,16.

Of these RE oxides, ytterbium oxide (Yb2O3) thin film turns out to be a potential gate oxide because of its novel properties, including excellent thermal and chemical stability, large

bandgap (~5 eV), and high κ (~15), in which the κ values depend on deposition processing17,18. Nevertheless, due to the highly hygroscopic nature, there is a need to develop the fabrication

of stable RE oxide film, suppressing the formation of a hydroxide layer on the film because of the presence of oxygen vacancies19. The formation of oxygen vacancies can contribute to the

occurrence of a sufficient atomic reorganization in the film structure. In order to solve the aforementioned shortcoming, in an effort to remove the oxygen vacancies in the RE oxide film,

the addition of Ti or TiO2 into the film could be reduced the moisture absorption of RE oxides, thereby improving the structural and electrical properties20,21. Recently, our group

previously demonstrated the structural and sensing characteristics of Yb2Ti2O7 sensing films in an EIS sensor22, in contrast, the resistance to the level of moisture absorption for the

incorporation of Ta into the Yb2O3 film after post-annealing treatment to eliminate the oxygen vacancies is still unclear. The aim of this paper is to explore the effect of post-annealing

treatment on the structural, sensing, and impedance characteristics of ytterbium tantalum oxide (YbTaO4) sensing films deposited on Si substrates through reactive rf co-sputtering. X-ray

diffraction (XRD), atomic force microscope (AFM), and X-ray photoelectron spectroscopy (XPS) were employed to examine the film structures, surface morphologies, and chemical compositions of

YbTaO4 films annealed at three different temperatures (700, 800 and 900 °C), respectively. In addition, structural characteristics of the YbTaO4 films are correlated to sensing and impedance

properties of the EIS sensors after annealing at three temperatures. In this study, the YbTiO4 membrane after RTA at 800 °C showed a higher pH sensitivity (71.17 mV/pH), a smaller

hysteresis voltage (<1 mV) and a lower drift rate (0.22 mV/h), compared with other RTA temperatures. METHODS FABRICATION Prior to the deposition of sensing film, the Si substrate was

cleaned and hydrogen-terminated by using diluted HF. The YbTaxOy thin films were grown on p-Si (100) substrates with a resistivity of 5–10 Ω-cm by rf co-sputtering using from metal Yb and Ta

as target materials in a mixture of Ar/O2 (5 sccm/20 sccm). The plasma power of Yb and Ta targets was 100 W. The chamber pressure was 1 × 10−3 Torr during the growth process and the

substrate temperature was 27 °C. The growth rate and physical thickness of the YbTaxOy film were ~2 nm/min and ~60 nm, respectively. Subsequently, the samples were annealed at three various

temperatures (700, 800 and 900 °C) by rapid thermal annealing (RTA) in oxygen (O2) ambient for 30 s to form an YbTaO4 compound. Next, the back-side oxide of Si wafer was etched by buffer

oxide etchant (BOE) solution and Al (400 nm thick) as a back-side electrode was deposited by thermal evaporation to achieve good electrical contact. The sensing area of 3.14 mm2 was defined

by a robotic dispensing system using an adhesive silicone gel (S181) as an isolating layer. The EIS device was mounted on the Cu-coated printed circuit board (PCB) with an Al back-side

contact through Ag paste. Finally, the non-sensing region of the EIS sensor was covered with epoxy. CHARACTERIZATION The orientation and phase of these films were investigated by XRD (Bruker

D8 discover diffractometer) in a scan range of 2θ = 20°–60° using a step time of 1 s and a step size of 0.05°. The Cu Kα radiation (λ = 1.5406 Å) was run under a voltage of 40 kV and a

current of 20 mA. A tapping mode AFM (NT-MDT Solver P47) was employed to explore the surface topography and determine the surface roughness (root-mean-square, Rrms) of the YbTaO4 films

annealed at three temperatures. The Rrms roughness of these samples was estimated in scanning areas of 3 × 3 μm2. The film composition of YbTaO4 films was analyzed using XPS (Thermo VG

Scientific Microlab 350 system) with a monochromatic Al Kα source (1486.7 eV). The binding energy scale of each element was calibrated by setting the main hydrocarbon peak at a binding

energy of 285 eV (C 1 s). MEASUREMENT The pH sensitivity, hysteresis voltage, and drift rate of the YbTaO4 EIS devices were evaluated by capacitance–voltage (C–V) curves. The C–V

measurements for different pH buffer solutions (Merck Inc.) were performed using inductance–capacitance–resistance (LCR) meter (Agilent 4284 A), operated at 500 Hz with an applied ac voltage

of 10 mV. The Ag/AgCl electrode (commercial liquid-junction electrode) electrode as a reference electrode was used. The impedance measurement of EIS devices was performed by a combination

of Agilent 4284 A and 4285 A LCR meters. To achieve the steady results, all samples were kept in reverse osmosis (RO) water for 24 h before measurement. The sensing membrane was washed with

deionized water before transferring to subsequent pH solution. In order to prevent light and noise interference, all the measurements were performed in a dark box. Three devices on each

sample were repeated at least three times to measure sensing performance of the YbTaO4 EIS sensors. RESULTS AND DISCUSSION STRUCTURAL PROPERTIES OF YBTAO4 SENSING FILMS The XRD patterns of

all YbTaO4 films deposited on a Si substrate after RTA at different temperatures (700–900 °C) in O2 ambient were presented in Fig. 1. The YbTaO4 films after RTA at three temperatures have a

monoclinic structure. The YbTaO4 diffraction peaks were recorded at (310) and (−131) peaks for the film annealed at 700 °C. Apart from the increment in the intensity of the existing YbTaO4

peaks, an additional YbTaO4 (011), (−111), (111), (020), (200), (120), (022), and (202) peaks (JCPDS: 00-024-1416) were detected when the annealing temperature was raised up to 800 °C,

suggesting a polycrystalline structure. These peaks became more noticeable as the annealing temperature increased. By further increasing the annealing temperature to 900 °C, the increment in

the intensity of YbTaO4 (−111), (111), (020), (200), and (120) peaks was observed. Overall, as the RTA temperature increased from 700 °C to 800 °C, the intensity of the YbTaO4 (−131) peak

increased. This condition may indicate the reduction of oxygen vacancies present in the film due to the highest stability offered by (−131)-oriented YbTaO4. The crystallite size (or grain

size) of the YbTaO4 film after three RTA temperatures is determined by using the Scherrer’s equation for the main diffraction peak23. The average grain size was calculated to be 0.12, 0.21,

and 0.24 nm for the sample annealed at 700, 800 and 900 °C, respectively. Moreover, the crystallite size of the (−131) peak for the YbTaO4 film annealed at 700, 800 and 900 °C was 0.12, 0.35

and 0. 19 nm, respectively. The surface morphology of YbTaO4 thin films deposited on a Si substrate was evaluated by AFM to examine the effect of RTA temperatures. Figure 2 depicts AFM 3-D

surface topographies and Rrms roughnesses of the YbTaO4 films annealed at various temperatures. In general, the sample is uniform and smooth without defects, such as a crack or void. It can

be observed in Fig. 2 that bubblelike grains were developed for all samples. When the film was annealed at 900 °C, more bubblelike grains were formed in contrast to the film annealed at 800

and 700 °C. The Rrms roughness of the YbTaO4 films annealed at 700, 800 and 900 °C was 0.51, 0.69 and 0.83 nm, respectively. The Rrms roughness of the sample is in the increasing trend as

the RTA temperature increases. The reason of increase of surface roughness might be attributed to the growth of grain size due to the formation of grain agglomerations in the film after

annealing at high temperature in oxygen gas. This result indicates that the grain size increases with increasing RTA temperature, as expected, leading to the enhanced crystallinity and the

sharpened diffraction peaks. Therefore, the obtained Rrms roughness from AFM matches with the XRD results. Hence, more YbTaO4 was formed, and a noticeable change in the surface morphology

was observed for the sample annealed at 900 °C. The valence states of Yb, Ta, and O elements in a YbTaO4 layer were analyzed by XPS with Ar+ ion etching. The 30 s etching process was

conducted to remove the surface contamination. The obtained Yb 3d, Ta 4 f and O 1 s spectra of the YbTaO4 films after RTA at three temperatures were shown in Fig. 3. After the subtraction of

Shirley-type backgrounds, the raw data were all fitted by Gaussian-Lorentzian line shapes. To determine the valence state of Yb in the films annealed at various temperatures, we

investigated the XPS spectra of the Yb 4d level in the YbTaO4. The ytterbium compounds have two valence states. The filled 4 f shell of divalent ytterbium (Yb2+) in the 4d spectrum is a

doublet with a peak area ratio of 3:2, while partially filled 4 f shell of trivalent ytterbium (Yb3+) is a multiplet24. The electronic structure of the Yb 4d multiplet peaks in these films

is in good agreement with multiplet splitting in the Yb 4d spectrum of Yb2O3, which is different from the 4d doublet of Yb2+. There are two main Yb 4d3/2 and 4d5/2 peaks centered at 199.5 ± 

0.1 eV and 185.3 ± 0.1 eV, respectively, which are consistent with the binding energy of Yb 4d for Yb2O3 ref.25. The Yb 4d5/2 peak of the film annealed at 800 and 900 °C was slightly shifted

toward a higher binding energy, suggesting the incorporation of more Ta atom into the Yb2O3 film forming a YbTaO4 compound. Figure 3(b) shows the double peak features in the Ta 4f XPS

spectra for YbTaO4 films annealed at three temperatures. The Ta 4f5/2 and 4f7/2 peaks located at binding energies of 28.1 and 26.2 eV, respectively, are assigned to pure Ta2O5 film26. The

spin-orbit splitting energy of Ta2O5 film was 1.9 eV from Ta 4f5/2 to Ta 4f7/2 peak. The deconvolution results of these YbTaO4 films demonstrated a perfect fit for Ta 4f5/2 and 4f7/2 peaks

located at two 28 ± 0.1 and 26.1 ± 0.1 eV and splitting energy of ~1.9 eV, which are in good agreement with the reported values for Ta2O5 ref.26 indicating that the Ta exhibits the highest

oxidation state. These sensing films consist of tantalum suboxides (Ta2O5) with no TaOx content. The splitting energy of about 1.9 eV can consider strong binding interaction between the

tantalum and the oxygen atoms. The reaction of Ta2O5 with Yb2O3 can form a YbTaxOy compound. The O 1 s XPS spectra of the YbTaO4 films after RTA at different temperatures were deconvoluted

with Gaussian-Lorentzian curve fitting after the Shirley background, as shown in Fig. 3(c). The fitting results depicted three peaks located at 531.6, ~530.5, and 529.8 eV. The high

binding-energy peak of 531.6 eV corresponds to the surface contamination is caused by adsorption of atmospheric carbon-containing species. The median binding-energy peak of 530.5 eV can be

related to the O2− species occurring in Ta2O526. The low binding-energy peak of 529.8 eV can be attributed to the Yb-O-Ta bonding. These low peak intensities are assigned to O-C (or O=C)

bonds resulting from various species present in the organic carbon overlayer on top of the YbTaO4 films. The peak intensities of YbTaO4 and Ta2O5 components for the film annealed at 800 °C

exhibited higher and lower, respectively, in the O 1 s signal compared with other temperatures. In addition, the O 1 s peak corresponding to YbTaO4 for the sample annealed at 900 °C was a

lower intensity than that at 800 °C. During high-temperature annealing, the more Ta or/and Yb atoms diffuse readily from the YbTaO4 film to form a thicker silicate layer at the YbTaO4–Si

interface14,15. SENSING AND IMPEDANCE CHARACTERISTICS OF YBTAO4 EIS SENSORS After the film material analyses, the sensing performance (pH sensitivity, hysteresis voltage and drift rate) of

YbTaO4 EIS sensors after RTA at three temperatures was investigated. The flatband voltage (_V__FB_) of an ISFET or EIS device can be expressed as:27 $${{V}}_{{FB}}={{E}}_{{ref}}-{{\psi

}}_{{\rm{0}}}+{{\chi }}^{{sol}}-\frac{{{\rm{\Phi }}}_{{Semi}}}{{q}}-\frac{{{Q}}_{{eff}}}{{{C}}_{{ox}}}$$ (1) where _E__ref_ is the reference electrode potential, _ψ_0 is the surface

potential, _χ__sol_ is the surface dipole potential of the solution, _Φ__Semi_ is the semiconductor work function, _q_ is the elementary charge, _Q__eff_ is the effective charge produced at

the oxide-semiconductor interface and in the oxide by the different types of charges (e.g., interface trapped charge, fixed oxide charge, oxide trapped charge), and _C__ox_ is the gate oxide

capacitance. Apart from _ψ_0, all these terms are fixed values. This term causes an ISFET or EIS device responsive to the tested solution as a result of the polarization and formation of

the potential barrier, which is related to the H+ concentration. Therefore, an ISFET or EIS device can be sensitive to the solution pH. The potential of electrolyte-insulator interface

corresponding to pH can be explained by using a combination of the site-binding model and the Gouy–Chapman–Stern theory28. The pH sensitivity of an ISFET device is derived from Bergveld28 as

follows: $$\frac{\delta {\psi }_{0}}{\delta {p}{{H}}_{{s}}}=2.303\alpha \frac{{{k}}_{{B}}{T}}{{q}},\,{with}\,{\alpha }=\frac{1}{1+\frac{2.303{{k}}_{{B}}{T}{{C}}_{{dif}}}{{{q}}^{2}{{\beta

}}_{{int}}}}$$ (2) where _α_ represents a dimensionless sensitivity parameter which varies between 0 and 1, _C__diff_ indicates the differential capacitance of the electric double-layer, and

_β__int_ stands for the intrinsic buffer capacity of the oxide surface. The above equation gives information that the _β__int_ and _C__diff_ are effectively influenced the pH sensitivity of

an ISFET device. The _C__diff_ is obtained from the Gouy-Chapman-Stern model. In addition, the _β__int_ is related to the density of binding sites on the gate oxide. The higher the

intrinsic buffer capacity _β__int_ is, the higher the pH sensitivity of the sensing film will achieve. The intrinsic buffer capacity is associated with the surface roughness and the material

quality of the film. The change in the pH value of the electrolyte solution could give rise to a shift of the flatband voltage in the C-V curves. In order to evaluate pH sensitivity of the

YbTaO4 EIS sensors after RTA at different temperatures, a set of C-V curves measured in a wide range of pH 2–12 was tested. Figure 4(a–c) show the normalized C-V curves of the YbTaO4 EIS

devices annealed at 700, 800 and 900 °C, respectively. For a p-type Si substrate, three zones were visible, namely the accumulation, inversion and depletion regions. The accumulation region

is caused by a hole channel on the Si surface when a high positive voltage is applied to substrate electrode. In contrast, when a high negative voltage is applied to substrate electrode, an

inversion layer of electrons is formed in the inversion region. The C-V measurements around the depletion region were conducted for electrolyte solutions with pH ranging from 2 to 12. It is

evidence that the kinks of the EIS sensors annealed at 800 and 900 °C were found in the depletion region of the C-V curves, possibly indicating the presence of interface state at the

oxide-substrate29. In an ISFET operation, an adequate number of surface hydroxyl (OH) groups is essential on the gate oxide material for pH measurement at the Nernstian limit. The OH groups

can protonate (positively charged, OH2+) or deprotonate (negatively charged, O−)27, which relies on the solution pH value. Therefore, the surface potential of the gate oxide changes, which

can be measured via a capacitance variation of an EIS device. A negative shift of the reference voltage (VREF) with increasing pH values of the investigated electrolyte solution reflects a

more negatively charged gate oxide surface. The inset of Fig. 4(a–c) depicts the sensitivity and linearity of the YbTaO4 EIS devices after RTA at 700, 800 and 900 °C, respectively. The

0.5Cmax was set as the reference to extract the VREF versus the change of the pH value. The pH sensitivity and linearity of the C-V curves were achieved by linear regression. The YbTaO4 EIS

device annealed at 800 °C exhibited the highest sensitivity of 71.17 ± 2.36 mV/pH among these RTA temperatures (52.37 ± 3.25 mV/pH for 700 °C and 61.13 ± 2.26 mV/pH for 900 °C). This result

is mainly attributed to the film with a stoichiometric YbTaO4 structure and a larger grain size of (−131) plane enhancing the pH sensitivity. Furthermore, the optimal RTA temperature in the

oxygen ambient might improve the surface and interfacial material quality and increase the intrinsic buffer capacity (_β__int_). Furthermore, the pH sensitivity of the YbTaO4 EIS sensor is

higher than the theoretical Nernstian value of 59.4 mV/pH at 27 °C. This super-Nernstian value of our EIS sensor could be related to the mechanism of one transferred electron per 1.5 H+

ion30. The decrease in pH sensitivity after RTA at 900 °C is the decrease of the surface state density of the OH groups31. Furthermore, the hysteresis curves of the YbTaO4 EIS sensors with

three RTA temperatures were shown in Fig. 5(a). The EIS sensors were submerged in the solutions of the pH loop of 7 → 4 → 7 → 10 → 7. The hysteresis phenomenon might be due to the defects

(e.g., oxygen vacancies, dangling bonds) of the film, these defects could react with the hydroxyl groups, thereby leading to hysteresis effect32. The YbTaO4 EIS device after RTA at 800 °C

had the lowest hysteresis voltage of 1 ± 0.2 mV among these annealing temperatures (63 ± 8.9 mV for 700 °C and 11 ± 2.8 mV for 900 °C), suggesting that the optimal RTA temperature could

effectively reduce the oxygen vacancies and dangling bonds. On the contrary, the EIS sensor annealed at 700 °C showed a relatively larger hysteresis voltage of 63 mV compared to other

temperatures. This result could be attributed to a high number of defects in the YbTaO4 film because these defects cannot be removed by the low annealing temperature. Figure 5(b) depicts the

drift characteristics of the YbTaO4 sensing films annealed at three RTA temperatures. Each of the EIS sensors was measured in a pH 7 buffer solution for a period of time. The change in the

VREF can be expressed as ΔVREF = VREF(t)–VREF(0). The slope of the drift characteristics indicates the drift rate of an EIS device. The drift effect can be interpreted by the hopping and/or

trap-limited transport of water-related species33, which the localized defects could interact with the tested solution, thus resulting in the gate voltage shift. Figure 5(b) demonstrates

that the YbTaO4 EIS sensor with the 800 °C had the lowest drift rate of 0.22 ± 0.03 mV/h, whereas the EIS device with the 700 °C featured the highest drift rate of 0.36 ± 0.08 mV/h. The

lower drift rate may be due to the fact that the crystal defects could be eliminated by optimal RTA temperature in O2 ambient, hence causing a lower capacitance of hydrated layer. In

contrast, the higher drift rate could be contributed to the higher capacitance value of hydrated layer. In Table 1, the sensing performance of the YbTiO4 membrane is compared with commonly

used materials for EIS or ISFET-based sensors such as Ta2O59, Al2O310, ZrO211, HfO211, TiO234, SnO235, and Yb2Ti2O722. It is found that our YbTiO4 membrane demonstrated a higher pH

sensitivity (71.17 mV/pH), a smaller hysteresis voltage (<1 mV) and a lower drift rate (0.22 mV/h), relative to those of these materials. Figure 6 shows the reference voltage of the

YbTiO4 EIS sensor annealed at 800 °C for various H+, K+, Na+, Ca2+, and Mg2+ ion concentrations in pH 7. The response curves were measured in these ion concentrations ranging from 10−5 to

0.05 M. It is clear that the YbTiO4 sensing membrane had high selectivity to H+ ions and less selectivity to other ions, e.g. K+, Na+, Ca2+, Mg2+. Electrochemical impedance spectroscopy is

well established as a powerful tool for investigating the mechanisms of electrochemical reactions and measuring the dielectric and transport properties of materials. In addition, it can be

successfully applied for the characterization of biosensing surfaces and/or in estimation of bioanalytical signals produced by biosensors. A small sinusoidal perturbation of potential

measured is usually applied to an electrochemical device at different frequencies, thus monitoring the variation of electric current or voltage. Figure 7(a) depicts an equivalent circuit of

the oxide surface/electrolyte solution interface in an EIS device. The Nyquist plots of the YbTaO4 EIS devices annealed at three different RTA temperatures and tested at different solution

pH values were presented in Figs 8–10. Each of the plots in all the YbTaO4 EIS devices demonstrated a depressed semicircle portion at high frequency region and a slanted straight portion at

low frequency region. The results indicate that the semicircle portion is related to charge transfer-limited process and the linear portion is associated with diffusion limited process or

mass transfer process at oxide–electrolyte interface. At higher frequency end the intercept corresponds to the Si substrate impedance (ZS) and at lower frequency corresponds to the sum of ZS

and charge transfer resistance (Rct). The value of Rct is a measure of electron transfer across the exposed area of the oxide surface. The conductance method, indicating the loss mechanism

because of interface trap capture and emission of carriers, is generally employed to evaluate the density of interface trap states (Dit) in the depletion region for a MOSFET device29. If the

capacitance has small losses in the oxide, a simplified equivalent circuit of an EIS sensor for conductance method was shown in Fig. 7(b). The measured conductance originates from the

contribution of the interfacial trap states. According to the equivalent circuit of Fig. 7(b), two semicircle features may be observed in Nyquist plots: one is related to the sensing

membrane response at higher frequencies, and the other is associated with the oxide–solution interface response at lower frequencies. Figures 8–10 demonstrate that single semicircles were

observed in the frequency range 0.2 to 8 MHz for the YbTaO4 EIS sensors with different RTA temperatures from 700 to 900 °C. At pH 4, a semicircle diameter of the YbTaO4 EIS sensors annealed

at 700 °C exhibited larger compared with other temperatures, as shown in Fig. 8(a–c), suggesting that the EIS devices after RTA at higher temperature produce low bulk resistances. On the

other hand, the diameter of the semicircle was almost the same value as the accumulation, depletion, and inversion regions. Figure 9(a–c) depict that the Nyquist plots of the YbTaO4 EIS

sensors after RTA at three temperatures were performed under three regions and then tested at pH 7. Here, a lower real impedance value (small semicircle) occurred at higher frequency,

whereas a higher real impedance value (large semicircle) appeared at lower frequency. The impedance spectra (Zim vs. Zre) of the YbTaO4 EIS sensors annealed at the three temperatures and

then tested at pH 10 under the three regions were shown in Fig. 10(a–c). The radius of the semicircles for these EIS sensors gradually decreased from ~3–3.6 to ~1.8–2.1 kΩ as the RTA

temperature increased. Moreover, the radius of the semicircle in accumulation region was higher than those of depletion and inversion regions. Figures 8 and 10 demonstrate that the radius of

the semicircles for the EIS sensors tested in the alkaline (pH 10) solution were higher than those in the acid (pH 4) solution. The size of H3O+ ions is larger than that of the HO− ions,

thus leading to a low diffusion rate. Therefore, the bulk resistance of the EIS devices gradually increased from ~0.4 to ~1 kΩ as the pH value increased. CONCLUSIONS The effect of RTA

treatment on the structural, sensing and impedance characteristics of YbTaO4 sensing films on Si substrates by means of reactive rf cosputtering was explored in the paper. Material analyses

indicate that the YbTaO4 sensing film after RTA at 800 °C could form a stoichiometric YbTaO4 structure and increase the grain size of (−131) plane. The YbTaO4 sensing film annealed at 800 °C

exhibited a higher pH sensitivity of 71.17 mV/pH, a smaller hysteresis voltage of 1 mV and a lower drift rate of 0.22 mV/h, in comparison with other RTA temperatures. These results are

attributed this RTA temperature to the reduction in the defects and the improvement in the material quality of the film and the interface of YbTaO4-Si substrate. The semicircle radius of the

Nyquist plot for YbTaO4 EIS device performed under the accumulation region was larger than those under the depletion and inversion regions. Furthermore, the bulk resistance gradually

decreased with increasing the RTA temperature, while it clearly increased with increasing the pH value. The YbTaO4-based EIS sensor annealed at 800 °C is suitable for use in future medical

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authors would like to thank the Ministry of Science and Technology of Taiwan for financially supporting this research under contract of MOST-106-2221-E-182-063. AUTHOR INFORMATION AUTHORS

AND AFFILIATIONS * Department of Electronics Engineering, Chang Gung University, Taoyuan, 33302, Taiwan Tung-Ming Pan & Yu-Shu Huang * Division of Urology, Chang Gung Memorial Hospital,

Taoyuan, 33305, Taiwan Tung-Ming Pan * Division of Natural Science, Center for General Education, Chang Gung University, Taoyuan, 33302, Taiwan Jim-Long Her Authors * Tung-Ming Pan View

author publications You can also search for this author inPubMed Google Scholar * Yu-Shu Huang View author publications You can also search for this author inPubMed Google Scholar * Jim-Long

Her View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS T.-M.P. and J.-L.H. conceived experiments and data analysis and wrote manuscript.

Y.S.H. assisted with experiment design and performed experiments and analyzed data. All authors discussed and commented on the manuscript. CORRESPONDING AUTHOR Correspondence to Tung-Ming

Pan. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing interests. ADDITIONAL INFORMATION PUBLISHER'S NOTE: Springer Nature remains neutral with regard to

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http://creativecommons.org/licenses/by/4.0/. Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Pan, TM., Huang, YS. & Her, JL. Sensing and Impedance Characteristics of YbTaO4

Sensing Membranes. _Sci Rep_ 8, 12902 (2018). https://doi.org/10.1038/s41598-018-30993-7 Download citation * Received: 15 January 2018 * Accepted: 09 August 2018 * Published: 27 August 2018

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