Influence of NH3 plasma and Ti doping on pH-sensitive CeO2 electrolyte-insulator-semiconductor biosensors

In this study, CeO2 pH-sensitive sensing membranes in electrolyte-insulator-semiconductor structures on silicon substrate were fabricated. To enhance sensing performance, the membrane underwent Ti doping and NH3 plasma treatment on the surface. To examine the effects of Ti doping and plasma treatment, multiple material properties evaluations were conducted using field-emission scanning electron microscopy, X-ray diffraction, atomic force microscopy, and secondary ion mass spectroscopy. Results indicate that Ti doping and plasma treatment can remove defects and enhance crystallization, thereby achieving improved pH-sensing performance of the membrane with high sensitivity, high linearity, low hysteresis voltage and low drift voltage. CeO2-based EIS membranes with Ti doping and NH3 plasma treatment show promise for future portable pH-sensitive biosensors.

Within this decade, pH-sensing technologies have been intensively studied for biochemical applications. This is due in large part to the fact that pH value is key to the health of living organisms, influencing function, development and growth of living systems. In the early 20th century, researchers have used colorimetric and electrometric methods to examine the pH values in different solutions by observing their colors and measuring their voltage variations 1 . Since then, light detection and voltage detection have become two distinct methods by which to carry out pH sensing 2 . For pH sensing using the light detection approach, optical fibers were utilized to monitor color changes of the dye in solutions in the 1980s 3 . Recently, Li et al. used detection of fluorescent light absorption related to energy level variations to evaluate pH values 4 . Alternately, detection of voltage variation from a semiconductor device began from 1970, when Bergveld invented the first ion-sensitive field effect transistor (ISFET) 5 . An ISFET is derived from a metal oxide semiconductor field effect transistor with an ion-sensing gate structure, in contact with a buffer solution. In the 1990s an electrolyte-insulator-semiconductor (EIS) structure was invented using a sandwiched insulating sensing membrane in contact with an electrolyte on top and a semiconductor on the bottom 6 . EIS biosensing devices have been attracting intensive attention because of their rapid response, robustness, compact size, and possible integration with an on-chip circuit 7,8 . Over the past decade, various types of metal oxides such as Nb 2 O 5 9 , HfO 2 10 , and TiO 2 11 have been used as the sensing insulator in an EIS structure. Recently, some rare earth oxides, with advantages including wide band gaps, large band offsets on Si, and high dielectric constants have been demonstrated as good sensing insulators for EIS biosensing devices 12,13 . Among these rare earth oxides, CeO 2 , with a wide bandgap of 3.19 eV and a high dielectric constant, has been used as the sensing material for EIS biosensing devices 14,15 . In addition, Kao et al. have proposed the positive effects of annealing on the CeO 2 membrane in 2014 16 , and the influence of CF 4 plasma treatment on the CeO 2 sensing insulator in 2015 17 . However, to further improve the material properties of the sensing membrane and hence boost the sensing capability, alternative processes or distinctive treatments are worth exploration and investigation. In addition to the conventional thermal annealing treatment, incorporation of atoms during membrane layering by co-sputtering 18 or the addition of different atoms after membrane deposition with plasma treatment 19 have been proposed to reinforce the membrane in order to reducing defects. Recently, the incorporation of Ti atoms 20 and NH 3 plasma treatment 21 have been utilized to improve the sensing membrane performance. Based on the previous report 20 , Ti doping in the insulator layer can fix defects in the membrane 22 . Furthermore, addition of N atoms through the NH 3 plasma treatment on the surface of the sensing insulator can mitigate dangling bonds on the membrane surface and hence ameliorate the solution/insulator interface during the sensing operation 23,24 . In this paper, we combined Ti doping and NH 3 plasma treatment to optimize the sensing performance of the membrane.
In this research, CeO 2 membrane-based EIS biosensors were fabricated with Ti doping into the membrane by cosputtering and N-atom incorporation by NH 3 plasma treatment on the surface of the membrane. Moreover, multiple material analyses, including secondary ion mass spectroscopy (SIMS), field-emission scanning electron  microscopy (FESEM), X-ray diffraction (XRD), and atomic force microscopy (AFM) were performed to study the improvements of material properties caused by Ti doping and NH 3 plasma treatment. SIMS results show that the piling up of N atoms may fix interfacial dangling bonds. Moreover, FESEM images clearly indicate that both Ti doping and NH 3 plasma treatment can enhance granization. Consistent with the FESEM images and XRD analysis, AFM images reveal that incorporation of Ti atoms can reinforce the crystallization of the CeO 2 insulator. In addition, the pH-sensing sensing capabilities were measured 25,26 . In line with the material analysis, results indicate that incorporation of Ti doping and NH 3 plasma treatments could effectively improve pH sensing behavior and sensing capability. The Ti-doped NH 3 plasma treated EIS biosensors have the potential to develop future portable biochemical sensors at an industrial level.

Results and Discussion
The detailed EIS structure is illustrated in Fig. 1. Material analyses were performed on the membrane film and sensing measurements were conducted on the EIS sensors. To characterize the influence of Ti addition and NH 3 plasma treatment on the CeO 2 membrane, multiple material characterizations including FESEM, XRD, AFM, and SIMS were performed on CeO 2 and Ce 2 Ti 2 O 7 films with and without NH 3 plasma treatment. First, we used FESEM to view the surface morphologies of CeO 2 and Ce 2 Ti 2 O 7 films as shown in Fig. 2(a,b,c and d). An FESEM  image of the as-deposited CeO 2 film is shown in Fig. 2(a). Compared with the as-deposited CeO 2 film, the FESEM image of the Ti-doped CeO 2 film without NH 3 plasma treatment as shown in Fig. 2(b) reveals that clearer nanograins could be observed, indicating that Ti addition might reinforce crystallization. Similarly, the CeO 2 film treated with NH 3 plasma as shown in Fig. 2(c) exhibited clearer grain images than the CeO 2 as-deposited film, showing that NH 3 plasma treatment could enhance crystallization as well. Furthermore, CeO 2 film incorporating Ti atoms with NH 3 plasma treatment as shown in Fig. 2(d) exhibited the strongest crystallization among all the samples. In addition, slight cracks around the grains might further enhance the contact area in the membrane/ electrolyte surface and boost the sensing performance.
Additionally, we used XRD to examine the CeO 2 films and the Ce 2 Ti 2 O 7 films incorporated with Ti atoms in various NH 3 plasma treatment conditions, as shown in Fig. 3(a and b). Consistent with the FESEM images as  shown in Fig. 2(a,b,c and d), XRD patterns reveal that NH 3 plasma treatment for 3 min could drastically enhance the CeO 2 (400) peak and the CeO 2 (200) peak intensity indicative of crystallization during NH 3 plasma treatment. Furthermore, Ti addition could cause CeO 2 crystals to form Ce 2 Ti 2 O 7 crystals as shown in Fig. 3(b). Furthermore, NH 3 plasma treatment for 3 min could enhance crystallization as 3 min NH 3 plasma treatment increased the Ce 2 Ti 2 O 7 (2,2,1) peak and the Ce 2 Ti 2 O 7 (2,1,2) peak drastically. The results show that Ti addition and NH 3 plasma treatment could effectively strengthen crystallization and hence improve the sensing performance of the CeO 2 membrane.
In addition, AFM images as shown in Fig. 4 reveal that the surface roughness of the CeO 2 and Ce 2 Ti 2 O 7 films treated in various NH 3 plasma conditions. Compared with the CeO 2 film and Ce 2 Ti 2 O 7 as shown in Fig. 4, Ti doping could effectively increase the surface roughness and cause the grain on the surface to become more noticeable. Similarly, NH 3 plasma treatment could increase roughness and cause grain growth as well. Moreover, incorporating both Ti doping and NH 3 plasma treatment for 3 min could drastically increase the roughness from 0.424 nm to 0.815 nm and clearly enhance the grains, as shown in the AFM images.
Furthermore, SIMS analysis as shown in Fig. 5 reveals the distribution of various atoms inside the CeO 2 /Si film and the Ce 2 Ti 2 O 7 /Si films. Figure 5 shows stronger N atom concentrations in both the CeO 2 film and the Ce 2 Ti 2 O 7 film after NH 3 plasma treatment. Moreover, noticeable accumulation could be observed in the membrane/Si interface. Since NH 3 plasma treatment could infuse N atoms into the CeO 2 and Ce 2 Ti 2 O 7 films, stronger N-Ce and N-Si bonds might be formed and dangling bonds or defects around the interface might be fixed 19,23 .
Strengthening the material quality in the bulk and in the interface might increase the sensitivity and linearity of pH sensing of the films. Furthermore, a high concentration of Ti atoms in the SIMS profile for the Ce 2 Ti 2 O 7 films could be observed. Since Ti addition could also fix traps and dangling bonds, a high concentration of Ti atoms in the Ti-doped CeO 2 film could cause the Ti atoms to reduce the defects 18 . In addition, a slight increase of concentration of Ti atoms might indicate the accumulation of the Ti atoms around the interface and fix the dangling bonds near the interface.
Incorporating a CeO 2 membrane as the electroactive gate film in the pH-based EIS structure allows the incorporated sensor to detect fine pH value variations. The influence of the plasma treatment and Ti doping can be realized by studying the site binding model 27 . The reference voltage is closely related to the surface potential, which is dependent on the pH value of the electrolyte and the membrane material. The ϕ value can be calculated from the above equation (1), where k is the Boltzmann's constant, T is the temperature, and β is a parameter in terms of the chemical sensitivity of the membrane [28][29][30] . The value of β is proportional to the density of surface hydroxyl groups and is determined by the following equation (2).
N s is the number of surface sites per unit area. K a is the equilibrium constant of the acid point and K b is the equilibrium constant of the base point, respectively. C DL is the bilayer capacitance calculated from the Gouy-Chapman-Stern model 31 .
According to the above theories, addition of Ti atoms and incorporation of NH 3 plasma treatment could enhance crystallization and reduce the dangling bonds. Therefore, the number of the surface sites could be increased and the sensing performance could be boosted. To characterize the sensing performance of the CeO 2 and Ce 2 TiO 7 membranes with and without NH 3 plasma treatment, the C-V curves of these various membranes are shown in Fig. 6(a,b,c and d). The linearity and sensitivity of the CeO 2 and Ce 2 TiO 7 membranes with and without NH 3 plasma treatment extracted from normalized C-V curve are shown in Fig. 7(a,b,c and d). As shown in Fig. 6(a) and 7(a), the as-deposited CeO 2 membrane exhibited a low sensitivity of 34.37 mV/pH. Moreover, fluctuated C-V curves could be observed indicating multi-capacitance effects were present, signifying that defects in the bulk or in the interface might be present. As the membrane underwent the NH 3 plasma treatment for 3 min, the sensitivity was boosted to 48.62 mV/pH and the C-V curves became smoother as shown in Figs 6(b) and 7(b). Similarly, as the as-deposited CeO 2 membrane and the as-deposited Ce 2 TiO 7 membrane were compared as shown in Fig. 7(a and c), the sensitivity was improved when the Ti atoms were incorporated into the CeO 2 membrane. Furthermore, as the Ce 2 TiO 7 membrane went through NH 3 plasma treatment for 3 min, the sensitivity was greatly improved to 54.43 mV/pH. Furthermore, much smoother C-V curves could be observed, indicating single capacitance with high material quality film could be formed. Combining Ti doping and NH 3 plasma treatment could enhance the CeO 2 film material quality and sensing capability.
Furthermore, to study the hysteresis effects of the CeO 2 and Ce 2 TiO 7 films which underwent through various plasma treatment conditions, hysteresis voltages were measured for the CeO 2 film and the Ce 2 Ti 2 O 7 film with different plasma treatment conditions, as shown in Fig. 8(a and b). The as-deposited CeO 2 film with a hysteresis voltage of 27.7 mV and the as-deposited Ce 2 Ti 2 O 7 film with a hysteresis voltage of 20.8 mV could be observed, indicating that Ti-doping might reduce the dangling bonds and traps to lower the hysteresis voltage. Furthermore, the CeO 2 with NH 3 plasma treatment had a low hysteresis voltage of 7.8 mV, and the Ce 2 Ti 2 O 7 film with NH 3 plasma voltage had the lowest hysteresis voltage of 5.6 mV. The results indicate that NH 3 plasma treatment could passivate defects and enhance sensing performance, consistent with the material analysis.
Finally, the CeO 2 membrane and the Ce 2 TiO 7 membrane prepared in different plasma treatment conditions were tested for gate drift voltage, as shown in Fig. 9(a and b). Each of the samples were dipped in a pH7 buffer solution. In line with all the previous analyses, results reveal that the drift voltage shift could be reduced either by Ti doping or NH 3 plasma treatment for 3 min. Moreover, the Ce 2 Ti 2 O 7 membrane with NH 3 plasma treatment had the least drift voltage of 0.21 mV/hr, signifying that Ti doping combined with NH 3 plasma treatment could effectively enhance sensing capability. Moreover, to compare EIS membranes composed of various materials and treated with different treatment, the EIS pH biosensing devices incorporating ZnO, Gd 2 O 3 , Gd 2 TiO 7 and Nb 2 O 5 membranes are compared with CeO 2 and Ce 2 TiO 7 membranes as shown in Table 1

Conclusions
In this study, CeO 2 EIS biosensors incorporated Ti doping and NH 3 plasma treatment were fabricated. Multiple material analyses suggest that the addition of Ti atoms to form Ce 2 Ti 2 O 7 film could enhance grain growth and suppress dangling bonds. Furthermore, inclusion of N atoms by NH 3 plasma treatment could reinforce crystallization and remove defects. Therefore, the sensitivity and linearity of the CeO 2 EIS biosensor might be boosted and the hysteresis and the drift voltage could be reduced. CeO 2 -based EIS membranes with Ti doping and NH 3 plasma treatment show promise for future industrial biosensing applications.

Methods
Electrolyte-insulator-semiconductor (EIS) structures incorporating CeO 2 and Ce 2 Ti 2 O 7 sensing membranes were fabricated on 4 inch n-type (100) silicon wafers with a resistivity of 5-10 Ω-cm. After standard RCA cleaning, the samples were dipped into 1% hydrofluoric acid to etch native oxide from the surface. For the first type of samples, a 50 nm CeO 2 film was deposited on the Si substrate by reactive radio frequency (rf) sputtering from a cerium target in diluted O 2 ambient (Ar/O 2 = 25 sccm/0 sccm). For the second group of samples, a 50 nm Ce 2 Ti 2 O 7 sensing film was deposited by reactive radio frequency (rf) co-sputtering on an n-type silicon wafer, sputtered from a cerium target and a titanium target in diluted O 2 ambient (Ar/O 2 = 20 sccm/5 sccm). The rf power and chamber pressure were 100 W and 20 mTorr, respectively. After deposition, CeO 2 and Ce 2 Ti 2 O 7 were subjected to a post-NH 3 plasma treatment in a plasma-enhanced chemical vapor deposition (PECVD) system with rf power of 30 W and a processing pressure of 500 mTorr for 1 min, 3 min and 6 min. Next, two types of samples were subsequently treated with rapid thermal annealing (RTA) using a conventional thermal annealing system under ambient N 2 condition for 30 sec at a temperature of 600 °C. After that, the back-side contact of the Si wafer was deposited by Al film with a thickness of 300 nm. The sensing membrane size was defined through photolithographic processing under a photosensitive epoxy (SU8-2005, Micro-Chem). EIS structures were then fabricated on the copper lines of a printed circuit board (PCB) by using a silver gel to form conductive lines. Epoxy was utilized to separate the EIS structure and the copper line. The detailed EIS structure is illustrated in Fig. 1