Structural and Sensing Characteristics of Dy2O3 ... - ACS Publications

Sep 17, 2010 - In this paper, we describe an electrolyte−insulator−semiconductor device for biomedical engineering applications prepared from Dy2O3 an...
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J. Phys. Chem. C 2010, 114, 17914–17919

Structural and Sensing Characteristics of Dy2O3 and Dy2TiO5 ElectrolyteInsulator-Semiconductor pH Sensors Tung-Ming Pan*,†,‡ and Chao-Wen Lin† Department of Electronics Engineering, Chang Gung UniVersity, Taoyuan 333, Taiwan, R.O.C., and Bio-Sensor Group, Bio-Medical Engineering Research Center, Chang Gung UniVersity, Taoyuan 333, Taiwan, R.O.C. ReceiVed: August 16, 2010; ReVised Manuscript ReceiVed: September 1, 2010

In this paper, we describe an electrolyte-insulator-semiconductor device for biomedical engineering applications prepared from Dy2O3 and Dy2TiO5 sensing membranes deposited on Si substrates by means of reactive radio frequency sputtering. The structural and morphological features of these films with annealing at various temperatures were studied by X-ray diffraction, atomic force microscopy, and X-ray photoelectron spectroscopy. Compared with the Dy2O3 film, electrolyte-insulator-semiconductor devices incorporating a Dy2TiO5 sensing film annealed at 800 °C exhibited a higher sensitivity (57.59 mV/pH in the solutions from pH 2 to 12), a smaller hysteresis voltage (0.2 mV in the pH loop 7 f 4 f 7 f 10 f 7), and a lower drift rate (0.362 mV/h in the pH 7 buffer solution), presumably because of its thinner low-k interfacial layer at the oxide/Si interface and its higher surface roughness. 1. Introduction Ion-sensitive field-effect transistors (ISFETs) first demonstrated by Bergveld in 1970, are solid-state electronic devices for chemical sensing of ion concentrations in solution (for example, hydrogen ions for pH measurement).1 ISFET devices are being studied extensively because of their small size, rapid response, ease of processing, and high compatibility for integration with complementary metal-oxide-semiconductors (CMOSs). Metal-oxidesemiconductor field-effect-transistor (MOSFET) devices consist mainly of the metal, the oxide, the source/drain, and the semiconductor substrate; the difference between an ISFET and a MOSFET device is that no metal gate electrode is employed in the former. An electrolyte-insulator-semiconductor (EIS) device is currently considered to be one of the basic structural elements of semiconductor-based biosensors due to its simple structure and, therefore, uncomplicated manufacture.2 This type of sensor utilizes the variations in capacitance of its gate insulator/semiconductor structure because of the chemically induced surface potential. For example, a SiO2 film had been employed as a pH sensing membrane for ISFET applications.3 Due to their higher pH response, several other insulator materials, including Al2O3, Si3N4, Ta2O5, and WO3,4-7 have been investigated as pH-sensitive membranes. These materials have some disadvantages, however, such as hysteresis and drift effects, which lower the accuracy of their resulting sensors.8,9 To extend the application of pH sensors, it remains necessary to develop insulator materials that possess high pH sensitivity, small hysteresis, high stability, and low drift. High-dielectricconstant (high-k) materials, such as Gd2O3, ZrO2, HfO2, Y2O3, and Pr2O3,10-14 have recently been proposed as pH-sensitive membranes because of their good sensing performance in EIS devices. To improve the quality of the interface between the high-k material and the silicon substrate, growing a thin SiO2 * To whom correspondence should be addressed. Tel: 886-3-211-8800, ext. 3349. Fax: 886-3-211-8507. E-mail: [email protected]. † Department of Electronics Engineering. ‡ Bio-Sensor Group, Bio-Medical Engineering Research Center.

film on the Si substrate imparts the pH sensing membrane with a smaller density of the interfacial state, lower stress, and good adhesion.15 Recently, rare-earth metal oxides have been investigated for advanced CMOS devices due to high dielectric constants, high resistivities, and large energy band-gap characteristics.16,17 Thin dysprosium oxide (Dy2O3) film has been considered as a potential gate insulator to achieve a less than 1.0 nm equivalent oxide thickness. Several characteristics of Dy2O3 have been reported, including the large band gap (>4.9 eV), conduction band offset, relative dielectric constant (14-18), interfacial properties, moisture absorption, and the Gibbs free energy (112.4 kcal/mol) of Dy2O3 in contact with Si.17-19 It has been reported that a Dy2O3 thin film deposited on the Si substrate exhibited excellent electrical characteristics, such as a high electron mobility, low gate leakage current, and small equivalent oxide thickness.20,21 Moreover, the incorporation of TiO2 or Ti into the lanthanide oxide dielectrics has attracted much attention as a method to achieve a gate insulator material with excellent physical and electrical properties for CMOS device applications.22,23 In this paper, we describe the structural and sensing characteristics of Dy2O3 and Dy2TiO5 sensing films deposited on Si substrates. We applied X-ray diffraction (XRD) to determine the growth directions and crystallinity of the films, X-ray photoelectron spectroscopy (XPS) to monitor the chemical structure of the Dy2O3 and Dy2TiO5 films, and atomic force microscopy (AFM) to measure the surface morphology of the sensing films after annealing at different temperatures. We found that the postdeposition annealing temperature played an important role in the formation of the low-k interfacial layer at the oxide-Si interface. Furthermore, we determined the effect of postdeposition annealing treatment on the sensing characteristics (pH sensitivity, hysteresis, and drift) of the Dy2O3 and Dy2TiO5 films. 2. Experimental Section EIS devices incorporating Dy2O3 and Dy2TiO5 sensing membranes were fabricated on 4 in. p-type Si(100) wafers. After

10.1021/jp107733u  2010 American Chemical Society Published on Web 09/17/2010

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Figure 1. XRD patterns of (a) Dy2O3 and (b) Dy2TiO5 films annealed at different temperatures.

standard RCA (Radio Corporation of America) cleaning, the Si substrate was dipped in 1% HF solution for 10 s to remove the native oxide. An ∼20 nm Dy2O3 film was deposited on the Si substrate by reactive radio frequency (rf) sputtering from a dysprosium target in diluted O2 ambient (Ar/O2 ) 5 sccm/2 sccm) at a substrate temperature of 27 °C, whereas an ∼20 nm Dy2TiO5 film was deposited by reactive rf cosputtering from a dysprosium target and titanium target. All samples were performed at various annealing temperatures (700, 800, and 900 °C) by rapid thermal annealing (RTA) in O2 ambient for 30 s. The backside contact (a 400 nm thick Al film) of the Si wafer was deposited using a thermal coater. The sensing membrane size was defined through photolithographic processing under a photosensitive epoxy (SU8-2005, MicroChem Inc.) that behaves as an antiacid polymer. EIS devices were then fabricated on the copper lines of a printed circuit board by using a silver gel to form conductive lines. A handmade epoxy package was used to encapsulate the EIS structure and the copper line. The film structures of the Dy2O3 and Dy2TiO5 sensing films after RTA at various temperatures were studied by XRD. XRD analyses were carried out using a Siemens D5000 with a Cu KR (λ ) 1.542 Å) radiation. The composition and chemical bonding in Dy2O3 and Dy2TiO5 films were investigated using a Physical Electronics Quantum 2000 XPS instrument. The surface morphologies of the films were observed using an NTMDT Solver P47 (AFM). The AFM was operated in the tapping mode for imaging; the scan area for measurement of the roughness was 1 × 1 µm. The pH sensitivities of the Dy2O3 and Dy2TiO5 sensing membranes were determined by measuring capacitance-voltage (C-V) curves of the EIS devices. The C-V curves were measured using buffer solutions of various values of pH (Merck Inc.), a Ag/AgCl reference electrode, and a Hewlett-Packard 4284A LCR meter operated at an ac signal frequency of 100 Hz. All setups were performed in a dark box to avoid interference from light and noise. 3. Results and Discussion 3.1. Structural Properties. A detailed study of the crystalline structure of the Dy2O3 and Dy2TiO5 before and after RTA treatment was realized by X-ray diffraction measurements, as shown in Figure 1a,b. A strong (400) peak and two weak (026) and (622) peaks are observed for the as-deposited sample with the Dy2O3 film. In contrast, for the Dy2TiO5 sample, a strong (400) peak and two weak (620) and (533) peaks were found in the 2θ diagram. At annealing temperatures of 700 and 800 °C, the Dy2O3 (222) reflection peak is found in the XRD patterns, as shown in Figure 1a. Additionally, (440), (026), and (622) reflection peaks are observed, but with less intensity than for

Figure 2. XPS spectra of (a) Dy 4d and (b) O 1s for Dy2O3 films after annealing at different temperatures.

the (222) peak. It was found that the (400) peak becomes stronger at a higher annealing temperature of 900 °C, indicative of a preferential orientation of the crystallites with the (400) planes of Dy2O3 parallel to the substrate. However, this annealing temperature will easily generate the formation of an amorphous SiO2 and a nonstoichiometric silicate layer at the oxide/Si interface. In contrast, the film annealed at 700 and 800 °C exhibited one strong (222) peak and five weak (400), (440), (531), (620), and (533) peaks for the Dy2TiO5 sample, as shown in Figure 1b. The intensity of the Dy2TiO5 (222) peak is stronger than that of the other peaks in the 800 °C spectrum, indicative of a better Dy2TiO5 structure. From these measurements, it can be seen that the sample annealed at 800 °C is uniaxially textured along the (111) direction of the faced-centered cubic phase (space group: F4j3m) of Dy2TiO5 because the XRD patterns reveal mainly the (222) diffraction line. Figure 2a shows the Dy 4d XPS spectra for the Dy2O3/Si interface after RTA treatment. The Dy 4d3/2 and 4d5/2 peaks of the reference Dy2O3 are located at 158.8 and 155.8 eV,24 respectively. For the as-deposited films, the Dy 4d peak is shifted lower in binding energy by about 1.2 eV as compared with the Dy2O3 reference position, indicative of a poor Dy2O3 structure. It is interesting to find that the shift of the Dy 4d peak to a higher binding energy increases upon increasing the RTA temperature. The chemical shift of the Dy 4d doublet peaks moved toward higher binding energy for the film annealed 900 °C (4d3/2 and 4d5/2 peaks at 159 and 156 eV, respectively) as compared with the sample annealed 800 °C. This shift is due to the high-Dy-content metal oxide film. The O 1s spectra for the Dy2O3 films annealed at various temperatures are shown in Figure 2b with their appropriate peak curve-fitting lines corre-

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Figure 3. XPS spectra of (a) Dy 4d, (b) Ti 2p, and (c) O 1s for Dy2TiO5 films after annealing at different temperatures.

sponding to chemical states. Each fitting peak followed the general shape of the Lorentzian-Gaussian function. The low energy state at 529.4 eV is attributed to O in Dy2O3.24 The intermediate energy state at 531.6 eV is attributed to interfacial O atoms in nonstoichiometric silicate (DySixOy). The high energy state at 533 eV is attributed to O in SiO2.25 The intermediate energy feature at 531.6 eV is distinctly different from both O in SiO2 and O in Dy2O3. It is reasonable to attribute this chemical state to a mixture of Dy2O3 and SiO2, that is, a DySixOy. The as-grown sample appears to be composed of Dy2O3, DySixOy, and SiO2. The intensity of the O 1s peak corresponding to Dy2O3 gradually increases but suddenly changed in the peak position at 900 °C. Furthermore, the intensity of the O 1s peak corresponding to SiO2 was rather constant up to 800 °C but suddenly increased at 900 °C. This result suggests that the high-temperature annealing increases the formation of amorphous silica and decreases the presence of the Dy silicate layer at the Dy2O3/Si interface. Figure 3 shows the Dy 4d, Ti 2p, and O 1s core level XPS spectra for the Dy2TiO5 film annealed at various temperatures. For the sample without RTA treatment, the Dy 4d5/2 and 4d3/2 peaks at 152.6 and 155.6 eV, respectively, suggest a poor DyOH structure due to the hydroxide formation after exposure to an air ambient.18 The Dy 4d peak position of the film after RTA at 700 °C shifts to a higher binding energy by about 1.1 eV relative to the DyOH position, suggesting a lesser amount of Dy reacting with OH, leading to a thinner DyOH. The chemical shift moving toward higher binding energy can be thought of as the decrease in the thickness of the hydroxide and the change of the chemical nature of the hydroxide from DyOH to Dy(OH)x, respectively. This can be explained by the presence of less water reactive TiO2 in Dy2TiO5 after RTA treatment. For annealing performed at 800 °C, the chemical shift of the Dy 4d core level moved toward higher binding energy for the case of the Dy2TiO5 film (4d5/2 and 4d3/2 peaks at 155.5 and 158.5 eV, respectively) as compared with the Dy2O3 film (4d5/2 and 4d3/2 peaks at 155.8 and 158.8 eV, respectively). The chemical shift to higher binding energy for the Dy 4d peak indicates the difference between the Dy-O-Ti bonding in Dy2O3 and that in Dy2TiO5. The Dy 4d5/2 and 4d3/2 peaks at 156.2 and 159.2 eV corresponding to Dy-O-Si bonds appear in the film annealed at 900 °C. This finding indicates that the formation of the silicate interfacial layer occurred as a result of oxygen atoms escaping from the Dy2TiO5 film and moving to the Dy2TiO5/Si interface. The peaks for the Ti 2p1/2 and 2p3/2 energy levels appeared at 464.2 and 458.6 eV, respectively, for the as-deposited Dy2TiO5 film (Figure 3b), suggesting a poor TiO2 structure incorporating Ti in the

form of TiO2 or TiOH, which probably formed in the surface region of the sample during its exposure to air.17 For the film annealed at 800 °C, the Ti 2p doublet (Ti 2p1/2 and Ti 2p3/2 at 465.6 and 460 eV, respectively) is shifted to higher binding energy compared with the TiO2 reference position (Ti 2p1/2 and Ti 2p3/2 at 464.3 and 458.7 eV, respectively).22 This shift was attributed to Ti in the Dy2TiO5 compound. Figure 3c displays the O 1s spectraswith appropriate curve-fitting of peakssof the Dy2TiO5 films before and after RTA treatment. In the four sets of spectra, the O 1s peaks at 534.8, 533, 532, and 530.5 eV represent the Dy-O-H, Si-O, Dy-O-Si, and Dy-O-Ti bonds,25 respectively. The intensity of the as-deposited film exhibited one large O 1s peak at 534.8 eV, representing DyOH groups, revealing that the reaction between the Dy and water caused hydroxide units to form in the Dy2TiO5 film. For the annealed film, no O 1s signal corresponding to DyOH was found for the Dy2TiO5 film, suggesting that the addition of Ti to the Dy2O3 film suppresses the thickness of the hydroxide. The O 1s peak intensity corresponding to Dy2TiO5 increased upon increasing the RTA temperature but suddenly changed in the peak position at 900 °C (O 1s peak at 531.2 eV). We attribute this behavior to the reactions of oxygen with the Dy and Ti atoms to form a Dy2TiO5 structure. Moreover, the area and intensity of the O 1s peak at 533 eV corresponding to SiO2 remain almost unaltered with high-temperature annealing (e800 °C), indicating a well-crystallized Dy2TiO5 structure, resulting in a higher thermal stability and a lower diffusivity of oxygen. For the film annealed at 900 °C, the O 1s peak intensity corresponding to silicate and SiO2 increased. This suggests the formation of a low-k interfacial layer at the dielectric/Si interface during thermal annealing. The surface roughness of the Dy2O3 and Dy2TiO5 sensing films after RTA at various temperatures is shown in Figure 4. The as-deposited Dy2O3 exhibited a larger surface roughness than the annealed film. We believe that the sensing film is condensed during this annealing temperature as the density increased. The availability of defect sites due to lower density can reduce self-diffusion of dysprosium and oxygen, resulting in a lower grain-boundary velocity during annealing and, therefore, small grains. For the annealed sample, the Dy2TiO5 sample shows a higher surface roughness compared with the Dy2O3 sample. This is attributed to that the Ti incorporating into the Dy2O3 film increased the growth of the grain size.26 Furthermore, the surface roughness of the Dy2TiO5 film increases upon increasing the RTA temperature but suddenly decreases at 900 °C. We suggest that this behavior is due to an increased self-diffusion of dysprosium, titanium, and oxygen

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Figure 4. Surface roughness of Dy2O3 and Dy2TiO5 films as a function of RTA temperature.

Figure 5. Reference voltage of (a) Dy2O3 and (b) Dy2TiO5 EIS devices annealed at 800 °C as a function of pH at room temperature. The inset shows the C-V curve response of (a) Dy2O3 and (b) Dy2TiO5 sensing films annealed at 800 °C when inserted into solutions with pH values from 2 to 12.

during high-temperature annealing, leading to the enhancement of the clustering of grains, thus increasing the surface roughness of the Dy2TiO5 film.17 3.2. Sensing Characterization. The effect of the postdeposition annealing temperature on the pH sensitivity of the Dy2O3 and Dy2TiO5 sensing films can be understood by using the sitebinding model to describe the ionic absorption processes at the electrolyte/oxide interface.27 The reference voltage is related to the surface potential (Ψ), which relies on the acido-basic characteristics of the sensitive material and on the electrolyte pH. The value of Ψ can be determined from eq 1

ψ ) 2.303

kT β (pHpzc - pH) qβ+1

(1)

where k is Boltzmann’s constant, T is the temperature of the system, q is the elementary charge, pHpzc is the pH at the point of zero charge, and β is a parameter that indicates the chemical sensitivity of the gate oxidesit is dependent on the density of surface hydroxyl groups.28 The pHpzc of pH-ISFET is given by eq 2

pHpzc ) -log10

() Ka Kb

0.5

(2)

where Ka and Kb represent the equilibrium constants at acid and base point, respectively. In addition, the β value is related to the chemical sensitivity of the gate oxide and is determined by the density of surface hydroxyl groups, which is given as eq 3

β)

2q2Ns√KaKb kTCDL

(3)

where Ns is the total number of surface site per unit area and CDL is the double-layer capacitance derived from the GouyChapman-Stern model.27 On the basis of the above equations, it can be inferred that the higher Ns (or the higher β), accordingly, contributes to a higher sensitivity and also the more linear response of pH detection.29 Variations of Ns could be connected with the structure and the surface roughness of the Dy2O3 and Dy2TiO5 films. These values are in good agreement with the literature reported in ref 14. In pH-EIS sensing, the change in the pH of the solution caused a shift in the flatband voltage of the C-V curves. This

Figure 6. pH sensitivity of EIS devices with Pr2O3 and Dy2TiO5 sensing membranes as a function of RTA temperatures.

result is mainly due to the ionization of the surface hydroxyl groups by either hydrogen ions or hydroxyl ions. Figure 5 depicts the pH dependence of the reference voltage (the voltage required to achieve a normalized capacitance of 0.5) of Dy2O3 and Dy2TiO5 sensing membranes that had been subjected to RTA at 800 °C. During a cycle from pH 2 to 12, a Dy2TiO5 EIS device shows a high pH sensitivity of 57.59 mV/pH and a good linearity of 99.66%. The insets in Figure 5 show the pH dependence of one group of C-V curves for the EIS structure prepared using Dy2O3 and Dy2TiO5 films annealed at 800 °C. These normalized C-V curves were shifted as a result of H+ ions modifying the surface potential through dipole formation on the sensing membrane. The EIS device using the Dy2O3 sensing film exhibits smooth C-V curves in the solutions from pH 2 to 12, whereas the Dy2TiO5 film has distorted C-V curves. This may be attributed to the presence of oxide traps in the Dy2TiO5. To evaluate the sensing performance of the Dy2O3 and Dy2TiO5 EIS devices annealed at different temperatures, we recorded a set of C-V curves at the pH ranging from pH 2 to 12. The pH sensitivity of Dy2O3 and Dy2TiO5 sensing membranes as a function of RTA temperatures is shown in Figure 6. EIS devices using a Dy2TiO5 film show a larger pH sensitivity than that of the Dy2O3 film. This result could be attributed to the higher surface roughness of the Dy2TiO5 sensing film as observed in the previous AFM analysis. With the higher surface roughness, the Ns will increase accordingly, which, therefore, contributes to a higher detection sensitivity, as previously discussed in the site-binding model. We find that the Dy2TiO5 sensing membrane annealed at 800 °C has a higher pH sensitivity of 57.59 mV/pH as compared to other RTA tem-

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Figure 7. Hysteresis voltage of (a) Dy2O3 and (b) Dy2TiO5 EIS devices annealed at various temperatures during the pH loop of 7 f 4 f 7 f 10 f 7.

Figure 8. Drift characteristics of (a) Dy2O3 and (b) Dy2TiO5 EIS devices annealed at various temperatures as a function of time in the pH ) 7 solution.

peratures. This is due to a larger surface roughness and the formation of a thinner low-k interfacial layer during this RTA temperature, thus a high density of surface hydroxyl groups. In contrast, the Dy2TiO5 EIS device annealed at 900 °C had a lower pH sensitivity than the film annealed at 800 °C. This outcome is due to the formation of an amorphous silica layer film during this RTA temperature, thus possibly leading to a low density of binding sites. The hysteresis characteristic can be due to the intrinsic defects of a dielectric film, causing the formation of porous structures. The interior sites of these porous defects could react with the ions existing in the tested solution and thus lead to a hysteresis response. Another possible cause is the interaction of ions in the solution with the responding sites along the boundaries of grains on an oxide film.30 It is thought that the addition of Ti or TiO2 into the Dy2O3 can improve the hysteresis characteristic due to the reduction of the defect chemistry (oxygen vacancies) and the formation of a barrier of grain boundaries that suppresses electromigration of the defect chemistry.31 Figure 7 shows the hysteresis phenomenon (gate voltage variation) of Dy2O3 and Dy2TiO5 sensing membranes subjected to RTA at different temperatures during the pH loop of 7 f 4 f 7 f 10 f 7 over a period of 1500 s. The hysteresis voltage here is defined as the gate voltage difference between the initial and terminal voltages measured in the above cycle. From the experimental results, the EIS device with a Dy2TiO5 sensing film exhibits a lower gate voltage compared with the Dy2O3 film. This result is attributed to the fast response of the EIS device with a Dy2TiO5 sensing film due to a better Dy2TiO5 structure, a lower defect chemistry, and a thinner interfacial layer. The Dy2TiO5 EIS capacitor annealed at 800 °C had a smaller hysteresis

voltage of 0.2 mV than the other annealing temperatures. This is because of a low number of crystal defects. In contrast, the 700 °C-annealed sample shows a large hysteresis voltage, indicative of a high density of crystal defects, which creates interior sites. These would be sites that can respond to changes of the chemical composition of the tested solution, resulting in large variations in the gate voltage. The drift behavior is typically characterized by a relatively slow, monotonic, temporal change in the threshold voltage of the ISFET device, which is not caused by variations in the electrolyte composition. The drift phenomenon is explained by electric field enhanced ion migration within the gate insulator and electrochemical nonequilibrium conditions at the insulatorsolution interface,32,33 injection of electrons from the electrolyte at strong anodic polarizations, creating negative space charge inside the insulator films,34 and proton tunneling.35 Figure 8 depicts the drift characteristics of EIS capacitors with Dy2O3 and Dy2TiO5 films annealed at various temperatures as a function of time, measured in the pH 7 solutions for 12 h. The change in the gate voltage can be written as ∆Vg ) Vg(t) Vg(0). The measured temporal variation of the reciprocal of the insulator capacitance follows a time dependence of the form {1 - exp[-(t/τ)R]},35 where τ is the time constant associated with structural relaxation and R is the dispersion parameter characterizing dispersive transport. For the sample annealed at 800 °C, the EIS device prepared at a Dy2TiO5 sensing film exhibited a better stability of 0.362 mV/h than that of the Dy2O3 film. The improvement of the drift effect was accomplished by the formation of an interfacial layer at the oxide/Si interface by this annealing temperature. Consequently, extrinsic ions can neutralize and compensate these defects, giving rise to the

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TABLE 1: Comparison of Sensing Parameters for EIS Devices Fabricated with a Si3N4, Al2O3, WO3, HfO2, Y2O3, Pr2O3, PrTiO3, Dy2O3, and Dy2TiO5 sensing membrane

pH sensitivity (mV/pH)

hysteresis voltage (mV)

drift rate (mV/h)

Si3N4 Al2O3 WO3 HfO2 Y 2 O3 Pr2O3 PrTiO3 Dy2O3 Dy2TiO5

53-55 54-56 45-56 45-58 56.09 52.9 56.8 48.6 57.59

3 0.8 15.7 5-30 13.6 17.5 2.84 27.5 0.2

0.8 0.3 7.2-26 2.5-125 1.24 2.15 5.4 1.77 0.362

decrease in the defects. However, a higher drift rate of the Dy2TiO5 sample after RTA at 700 °C was considered to be caused by a higher density of crystal defects. The measured as well as extracted sensing parameters are summarized in Table 1, where the data from EIS devices incorporating a Si3N4,36,37 Al2O3,36,37 WO3,38 HfO2,29 Y2O3,12 Pr2O3,13 and PrTiO339 are shown for comparison. Although a Dy2TiO5 sensing membrane has a slightly lower sensitivity and larger drift, the nitrogen incorporated into the Dy2TiO5 film can reduce the low-k silica layer at the oxide/Si interface and hence improve the sensing characteristics.40 4. Conclusion In this paper, we studied the structural and sensing properties of Dy2O3 and Dy2TiO5 sensing films deposited on a Si(100) substrate by means of reactive sputtering. We used XRD, XPS, and AFM analyses to confirm the presence of Dy2O3 and Dy2TiO5 structures in the EIS devices. The Dy2TiO5 EIS device that had been subjected to RTA at 800 °C exhibited a high sensitivity (57.59 mV/pH), a small hysteresis voltage (0.2 mV), and a low drift rate (0.362 mV/h), features that we attribute to (i) the well-crystallized Dy2TiO5 structure, (ii) suppression of the formation of an interfacial layer at the oxide/Si interface, and (iii) its higher surface roughness. EIS devices incorporating Dy2TiO5 sensing membranes appear to be very promising systems for use in biomedical engineering applications. Acknowledgment. This work was supported by the National Science Council, Taiwan, Republic of China, under Contract No. NSC-98-2111-E-182-056-MY3. References and Notes (1) Bergveld, P. IEEE Trans. Biomed. Eng. 1970, 17, 70–71. (2) Kramer, M.; Pita, M.; Zhou, J.; Ornatska, M.; Poghossian, A.; Schoning, M. J.; Katz, E. J. Phys. Chem. C 2009, 113, 2573–2579. (3) Chin, Y. L.; Chou, J. C.; Sun, T. P.; Liao, H. K.; Chung, W. Y.; Hsiung, S. K. Sens. Actuators, B 2001, 75, 36–42. (4) Chou, J. C.; Weng, C. Y.; Tsai, H. M. Sens. Actuators, B 2002, 81, 152–157.

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