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GaN HEMT based pH sensors by controlling the threshold voltage
Sensors & Actuators: B. Chemical 306 (2020) 127609
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Enhancing the sensitivity of the reference electrode free AlGaN/GaN HEMT based pH sensors by controlling the threshold voltage
T
Dongyang Xuea, Heqiu Zhanga,*, Aqrab ul Ahmada, Hongwei Lianga,*, Jun Liua, Xiaochuan Xiaa, Wenping Guob, Huishi Huangc, Nanfa Xub a
School of Microelectronics, Dalian University of Technology, Dalian 116024, Liaoning, China Shandong NovoShine Optoelectronics Co., Ltd., Weifang 261061, Shandong, China c Jiangsu Xinguanglian Technology Co., Ltd., Wuxi 214192, Jiangsu, China b
ARTICLE INFO
ABSTRACT
Keywords: AlGaN/GaN HEMT pH sensor Photoelectrochemical oxidation method Threshold voltage Transconductance
The threshold voltage (VT) of the AlGaN/GaN HEMT based pH sensor was adjusted by the method of the photoelectrochemical (PEC) oxidation on the GaN cap layer surface. After the PEC oxidation treatments, the VT of the device shifted from -3.46 V to -1.15 V and the gate voltage (VG) corresponding to the maximum transconductance (gmMAX) position (VG|gmMAX) of the device shifted from -2.6 V to -0.1 V. The drain current (ID) variation per pH of the AlGaN/GaN HEMT based pH sensor without reference electrode increased from 0.7 μA to 14 μA when the drain voltage (VD) was 0.5 V. The sensitivity of the reference electrode free AlGaN/GaN HEMT based pH sensor can be significantly increased by regulating the VT to make VG|gmMAX approached the equivalent VG when liquid droplet on the sensing window surface (VG-EQU), which is beneficial to the miniaturization and integration of the AlGaN/GaN HEMT based sensors in the future.
1. Introduction With the development of internet of things technology, semiconductor-based sensors have attracted a lot of attention due to their potential of miniaturization and integration [1–7]. Recently in the domains of medical detection and environmental monitoring [8–11], the AlGaN/GaN HEMT based sensors are becoming the focus of many scholars gradually due to the high-temperature resistance, strong radiation resistance, stable chemical properties and remarkable biological compatibility of Ⅲ-Ⅴ nitride materials [4,12–14]. The AlGaN/GaN HEMT based pH sensors are devices that converting the information about H+ concentration of a solution (pH) into electrical signals. Owing to piezoelectric polarization and spontaneous polarization of Ⅲ-Ⅴ nitride materials, a high concentration of two-dimensional electron gas (2DEG) is formed into the interface between AlGaN and GaN [15], which is very sensitive to the change of surface charges. According to the site-binding model [16], the GaN natural/artificial oxide layer or other metal oxides will adsorb hydroxyl groups to form amphoteric oxides in the solution. When the pH value of the solution changes, the surface charges will change accordingly, which will affect the 2DEG to change the ID of the sensor. Two main factors are affecting the sensitivity of the sensor. One is the sensitivity of the sensing area surface to
⁎
H+, and the Nernst limit is 59 mV/pH at room temperature. The other is the control ability of gate to 2DEG, i.e. transconductance (gm). The sensitivity of the sensors surface to the H+ in solutions could be improved by some surface treatment methods that have been reported, such as oxidation with H2O2 [17], oxygen plasma treatment and thermal oxidation treatment [18,19], or artificial growth of an oxide layer such as Al2O3 [20]. The main reason for the increased sensitivity of the device is that the treated surfaces by these methods are easier to form hydroxyl groups in solution, providing binding sites for H+ [20]. On the other hand, the methods that have been studied to increase the gm of the sensors were to optimize the structure of devices, such as increasing the width-length ratio of the sensing window [21–23] and reducing the thickness of barrier layer [24–26]. For the AlGaN/GaN HEMT based pH sensors without the reference electrode, the threshold voltage (VT) of the devices is also a key factor because it determines the position of the gate voltage (VG) corresponding to the maximum transconductance (gmMAX) position (VG|gmMAX), which is not a problem for the devices with the reference electrodes because the operating VG of devices can be biased at any value by the reference electrodes. However, traditional reference electrodes are poor mechanical properties, difficulty in miniaturization and incompatibility with semiconductor fabrication technology, which seriously restrict the
Corresponding authors. E-mail addresses: [email protected] (H. Zhang), [email protected] (H. Liang).
https://doi.org/10.1016/j.snb.2019.127609 Received 11 September 2019; Received in revised form 16 December 2019; Accepted 18 December 2019 Available online 19 December 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Sensors & Actuators: B. Chemical 306 (2020) 127609
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possibility of miniaturization, integration and harsh biochemical environment application of AlGaN/GaN HEMT based pH sensors. The pseudo-reference electrode such as Au [27], Pt, etc., or some solid electrode modified with organic membrane [28], fabricated on the sensor chip may be a way to solve the problem of traditional reference electrodes. Another way is to study the AlGaN/GaN HEMT based pH sensor without the reference electrode. Abidin et al. studied the AlGaN/ GaN HEMT based pH sensor without the reference electrode, but the nonlinearity response signals of the drain current (ID) to pH changes were obtained [29]. Giacinta Parish et al. reported that the GaN cap layer was necessary for the AlGaN/GaN HEMT based pH sensor to obtain a linear response signal of ID to pH changes, which is beneficial to the design of the reference electrode-free AlGaN/GaN HEMT based pH sensors [30]. And the sensitivity of the AlGaN/GaN HEMT based pH sensors without the reference electrodes could be improved by controlling the composition of Al and the thickness of the barrier layer of AlGaN during the epitaxy growth of AlGaN/GaN HEMT to adjust the VT, which was demonstrated with simulation by Anna Podolska et al. [31]. It could also change the VT and gm of devices by photoelectrochemical (PEC) method which have been reported that the positive bias voltage was applied to AlGaN barrier layer of device immersed in phosphoric acid [25] or NaOH solutions [32] under the irradiation of 325 nm He-Cr laser to realize the anodic oxidation of AlGaN. It is noteworthy that the GaN cap layer is necessary to ensure the linear signal of the reference electrode free AlGaN/GaN HEMT based pH sensor, but in the above method, phosphoric acid may etch GaN and AlGaN, which need to be avoided. Moreover, it should be avoided that the VT of the devices shifting to a positive value because the normally-off devices are disabled for measuring the pH of solutions without the reference electrodes. In this paper, the AlGaN/GaN HEMT based pH sensor with a GaN cap layer was used for obtaining a linear response signal of the device without the reference electrode. The PEC oxidation method was adapted to regulate the VT of the device to make VG|gmMAX approached 0 V that close to equivalent VG when liquid droplet on the sensing window surface (VG-EQU). And the sensitivity of the reference electrode free AlGaN/GaN HEMT based pH sensor was improved significantly.
steps. The first step was etching on the wafer by inductively coupled plasma (ICP) using BCl3/Cl2 as an etchant. The etching depth was 650 nm. The second step was to deposit the ohmic electrodes (Ti/Al/Ti/ Au/Pt with a thickness of 30/300/30/50/30 nm) by electron beam evaporation (EBM). Then the devices were annealed in a nitrogen atmosphere at 780℃ for 2 min to form ohmic contacts. The third step was depositing a 1.5 μm SiO2 passivation layer by plasma chemical vapor deposition (PECVD). Then the window of sensing area and the windows of the testing area were etched by the buffered oxide etch (BOE). The micrograph of the device is shown in Fig. 1(b). The width of the sensing window is 400 μm and the length of the sensing window is 40 μm. In the treatment of the PEC oxidation, the GaN cap layer connected with the ohmic contact electrodes, deionized water droplet, and the external Pt electrode together with the Keithley 4200 semiconductor characterization system constituted an electrolytic cell system. Under the irradiation of ultraviolet light, the voltage was applied between the ohmic contact electrodes and the Pt electrode. To avoid the reaction of the AlGaN barrier layer, a 365 nm ultraviolet LED lamp was used to guarantee that ultraviolet light was absorbed only by GaN but not by AlGaN. And the illuminance of the ultraviolet lamp was 70 mW/cm2. Deionized water was used as a reaction solution to reduce the reaction rate of the GaN cap layer to prevent the VT of the device from moving too fast to a positive value. The X-ray photoelectron spectrometer (XPS) system (K-Alpha+) was used to characterize the elements on the surface of samples before and after the treatment of the PEC oxidation. During testing the performance of the sensors, the pH buffer solutions were used with 4, 7, and 10 pH values. The ID was measured with the drain voltage (VD) scanned from 0 V to 1 V. When analyzing the VT and gm of the device, the sensing window was contacted with deionized water to form the electric double layer (EDL) gate at the interface, and the VG was controlled by pseudo-reference electrode (Pt) connected the deionized water on the sensing window to sweep from -10 V to 6 V. Then the ID-VG curve was analyzed to obtain the VT and gm of the device. To assess the time-drift of the device, the drain current-time curves (ID-t) of the device added varied pH solution was measured when VD was biased at 0.5 V. 3. Results and discussion
2. Experimental
Before the PEC oxidation, the measurement signals of the device at different pH and the analyzation of the VT and gm of the device are shown in Fig. 2. Fig. 2(a) shows the ID-VG curve and gm-VG curve of the device in deionized water with pseudo-reference electrode when VD is 0.5 V. The physical mechanism of the Z-shape curve (ID-VG curve) formation is similar to the MOSFET or gated diode [33,34]. And the VT of the device can be obtained from the ID-VG curve, which is -3.46 V. The maximum point of slope of ID-VG curve, which is -2.6 V, is the VG|gmMAX (peak position of gm-VG curve). Fig. 2(b) shows the ID-VD curves of the
As shown in Fig. 1(a), the wafer was grown by metal organic chemical vapor deposition (MOCVD) on the sapphire substrate, which consisted of 2 μm GaN buffer layer, 0.5 nm AlN spacer layer, 22 nm unintentionally doped Al0.26Ga0.74N barrier layer, and 2 nm GaN cap layer. The carrier density and mobility of 2DEG were 1.12 × 1013 cm−2 and 1520 cm2/(V∙s), respectively, which was measured with the Hall effect test system at room temperature. The fabrication process of the devices was mainly divided into three
Fig. 1. (a) The schematic diagram of the device section. (b) The micrograph of the device. 2
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Fig. 2. The measurement signals at different pH and the analyzation of the VT and gm of the device before the PEC oxidation. (a) The ID-VD curves of the device without a reference electrode at pH 4, pH 7 and pH 10, respectively. (b) The transfer characteristic curve (ID-VG) and gm-VG curve of the device in deionized water when VD was 0.5 V.
device without reference electrode at pH 4, pH 7 and pH 10, respectively. As shown in Fig. 2(b), the variation of ID per pH is only about 0.7 μA when VD is 0.5. The gm of the device is very small when the VG is nearby 0 V, which shown in Fig. 2(a), and that is why the current signal response of the device without reference electrode to the change of pH is weak. The ID-VG curves, gm-VG curves and IG-VG curves of the device measured in the dark before the PEC oxidation and after each time treatment are shown in Fig. 3. As Fig. 3(a) shows, the VT of the device before and after each time treatment is -3.46 V, -2.1 V, -1.54 V and -1.15 V, respectively. Fig. 3 shows that the VG|gmMAX of the device before and after each time treatment are -2.6 V, -1.2 V, -0.5 V and -0.1 V, respectively. Under the irradiation of 365 nm ultraviolet light, the GaN cap layer could generate photo-generated carriers to promote the anodic oxidation reaction on the GaN cap layer surface localized the sensing window area, resulting in Ga2O3 formation on the surface, which shown as the reaction formula (1). A similar anodic oxidation reaction has been reported by Ching-Ting Lee et al. [25]. At the same time, due to the catalytic action of Pt, the cathodic reduction reaction occurs on the surface of the Pt electrode which shown as the reaction formula (2). During the reaction, bubbles could be observed near the sensing window area and the Pt electrode. Therefore, the reason for the right shift of VT is the formation of Ga2O3 on the sensing window
surface, which resulted in the increase of the solid-liquid interface barrier due to the bandgap of Ga2O3 surface is larger than GaN cap layer and Al0.26Ga0.74N barrier layer. It is also found that the range of right shift of VT was smaller and smaller with the increase times of the treatment due to the thicker Ga2O3 surface would be more resistant to the surface oxidation reaction. And the gate leakage current (IG) decreases with the increase of the surface oxidation degree, which shown in Fig. 3(c).
2GaN + 6h+ + 6OH
6H+ + 6e
Pt
3H2
356nmUV
Ga2 O 3+3H2 O + N2
(1) (2)
In Fig. 4(a), the Ga 3d core level spectrum of the untreated sample is divided into three peaks. The main peak at 19.3 eV arises from Ga which combined with N (Ga-N) [35], the peak of Ga that combined with O (Ga-O) is at 20.4 eV and the peak at 17.6 eV arises from N 2 s [32,36]. It can be calculated the ratio of the Ga-N/Ga-O is 0.06. The rare oxygen may come from the natural oxide layer on the GaN cap layer surface. Fig. 4(b) shows that the Ga 3d core level spectrum of the sample which has been treated three times by the PEC oxidation. In the figure, the peak position of N 2 s, Ga-N, and Ga-O is 17.6 eV, 19.8 eV, and 20.4 eV, respectively [32,36]. The ratio of Ga-N/Ga-O is 1.03. After the PEC oxidation treatment, the atomic percentage of oxygen increases Fig. 3. The ID-VG curves, gm-VG curves and IGVG curves of the device measured in the dark before the PEC oxidation and after each time treatment. (a) The ID-VG curves of the device before the PEC oxidation and after each time treatment. (b) The gm-VG curves of the device before the PEC oxidation and after each time treatment. (c) The IG-VG curves of the device before the PEC oxidation and after each time treatment.
3
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Fig. 4. (a) The Ga 3d core level spectra of the untreated sample. (b) The Ga 3d core level spectra of the sample which has been treated three times by the PEC oxidation.
Fig. 5. (a) The energy band diagram of the sensor untreated by PEC. (b) The energy band diagram of the sensor treated by PEC. Fig. 6. The measurement signals at different pH and the analyzation of the VT and gm of the device after the once PEC oxidation. (a) The IDVD curves of the device without a reference electrode at pH 4, pH 7 and pH 10, respectively. (b) The transfer characteristic curve (IDVG) and gm-VG curve of the device in deionized water with Pt as a quasi-reference electrode when VD was 0.5 V.
Fig. 7. The measurement signals at different pH and the analyzation of the VT and gm of the device after the thrice PEC oxidation. (a) The ID-VD curves of the device without a reference electrode at pH 4, pH 7 and pH 10, respectively. (b) The transfer characteristic curve (IDVG) and gm-VG curve of the device in deionized water with Pt as pseudo -reference electrode when VD was 0.5 V.
obviously, so it is speculated that Ga2O3 is formed by surface oxidation. The mechanism for the formation of Ga2O3 on the sensing window surface making the right shift of VT is further illuminated with the energy band theory model as shown in Fig. 5(a) and (b). The interface of GaN/AlN and the surface of the sensing area are equivalent to two sides of the capacitor, which is the series capacitance of the oxide capacitance and the channel-layer capacitance [33]. Eq. (3) could be obtained due to the Gauss’ law. The ns is concentration of 2DEG, σpol is the surface density of polarized charge, ε is approximately equal to
dielectric constant of AlGaN, d is the distance from 2DEG to surface, ΔΦSS-TL and ΔΦREF-TL represent the surface potential barrier of sensing area surface and reference electrode with test liquid respectively, Δ means the part of the conduction band (EC) that under the Fermi level and ΔEC is the conduction band step between AlGaN and GaN. The 2DEG will be depleted approximately and Δ could be considered as 0 when VG = VT. Then combining with Eq. (3), Eq. (4) could be obtained [37]. Factors affecting VT include ΔΦSS-TL, ΔΦREF-TL, and other parameters that are intrinsic to the device itself. Ga2O3 would form a higher 4
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changes caused by variation of pH can be derived. It can be calculated that the surface of the sensing window response to H+ (dΔΦSS-TL /pH) increases to 53.3 mV/pH due to the increase of the surface oxidation degree. For the AlGaN/GaN HEMT based pH sensor without reference electrode, the main reason for the improvement of the sensitivity of the device treated by the PEC oxidation is the change of VT caused the VG|gmMAX approach VG-EQU of the sensors. Fig. 8 shows the ID-t curves of the device with the varied pH solution on the sensing area. The VD was biased at 0.5 V, and the ID of the device in acid solution (pH = 4), neutral solution (pH = 7) and alkaline solution (pH = 10) was measured, respectively. Each solution was measured for 100 s and a cycle was 300 s, with a total of five cycles. In the five cycles, the difference value between the maximum ID and the minimum ID is 7 μA, 9 μA, and 10 μA when the device added the acid solution, neutral solution, and alkaline solution, respectively. The signal sensitivity of the device is 14 μA/pH. Therefore, due to the timedrift during the measurement, the resolution of the device is between 0.5–1 pH. Further optimization of the sensor is still needed to reduce the time-drift and improve its resolution.
Fig. 8. The ID-t curves of the device with the varied pH solution on the sensing area.
4. Conclusions
barrier with the solution due to its wider bandgap [38,39] and more hydroxyl binding sites. Therefore, the VT shifts right after PEC treatment because the ΔΦSS-TL of Ga2O3 is larger than ΔΦSS-TL of GaN. When the test liquid changes, the ΔΦREF-TL will almost keep constant if the reference electrode good enough, and the variation of the ΔΦSS-TL is determined by the sensitivity of the surface of the sensor.
ens =
VT =
pol
pol d 0
0
d
(
+(
SS TL
SS TL
REF TL
REF TL )
+
EC e
e
EC e
VG )
In this report, the PEC oxidation method was applied to the GaN cap layer surface localized the sensing window area of the AlGaN/GaN HEMT based pH sensor. The VT of the device shifted to the right with the increase of the PEC oxidation treatment times. The sensitivity of the reference electrode free AlGaN/GaN HEMT based pH sensor was increased to 20 times by regulating the VT of the device to make VG|gmMAX approaching VG-EQU of the sensors. According to the results of ID-t curves, the resolution of the device is between 0.5–1 pH, which needs to be further optimized. The work may be beneficial to the miniaturization and integration of the AlGaN/GaN HEMT based pH sensors in the future.
(3)
(4)
After the first PEC oxidation treatment, the measurement signals at different pH and the analyzation of the VT and gm of the device are shown in Fig. 6. The ID-VD curves of the device without reference electrodes in different pH solutions are measured, which shown in Fig. 6(a). The change of the current signal of the device at different pH values, which is 3 μA/pH (VD = 0 V), is more obvious than that of the untreated device. According to the current value of the device when VD is 0.5 V in the ID-VD curves and the ID-VG curve of the device, the approximate range of VG-EQU corresponding to the device in different pH solutions could be deduced, which is shown in Fig. 6(b). However, the gm corresponding to the VG-EQU was still not optimal. The sensitivity of the sensing window surface response to H+ could be calculated to be 50.0 mV/pH from the insert of the Fig.6(b). The measurement signals at different pH and the analyzation of the VT and gm of the device which after the thrice PEC oxidation are shown in Fig. 7. As Fig. 7(a) shows the ID-VD curves of the device in different pH solutions are measured without reference electrodes. Variation of ID per pH is about 14 μA when VD is 0.5 V, which is 20 times as much as that of the untreated device and about 5 times as much as the device after once treatment. It should be noted that the device tends to be saturated when VD at about 1 V. The VG-EQU is near 0 V and the VT is -1.15 V, so the saturation drain voltage (VD-Sat = VG-VT) is near 1 V. When VD is greater than or equal to VD-Sat, the drain current will reach saturation. When the sensing window is dipped in the solution with higher pH, the VG-EQU will be smaller, the saturation voltage will be closer to 1 V, and the current saturation trend will be more obvious. As shown in Fig. 7(b), according to the current value of the device when VD is 0.5 V in the ID-VD curves and the ID-VG curve of the device, the approximate range of VG-EQU corresponding to the device in different pH solutions could be deduced, which is close to the VG|gmMAX. The corresponding VG of the ID values (VD = 0.5 V) in different pH solutions can be found from the transfer characteristic curve, which shown in the insert of the Fig.7(b). According to this, the surface potential
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 61574026, 11675198, 11875097, 61774072), the Dalian Science and Technology Innovation Foundation (No. 2018J12GX060). References [1] T.C. Yeow, M. Haskard, D. Mulcahy, H. Seo, D.J.S. Kwon, A.B. Chemical, A very large integrated pH-ISFET sensor array chip compatible with standard CMOS processes, Sens. Actuators B Chem. 44 (1997) 434–440. [2] M. Castellarnau, N. Zine, J. Bausells, C. Madrid, A. Juárez, J. Samitier, et al., Integrated cell positioning and cell-based ISFET biosensors, Sens. Actuators B Chem. 120 (2007) 615–620. [3] M.J. Milgrew, D.R.S. Cumming, Matching the transconductance characteristics of CMOS ISFET arrays by removing trapped charge, IEEE Trans. Electron Devices 55 (2008) 1074–1079. [4] N. Chaniotakis, N. Sofikiti, Novel semiconductor materials for the development of chemical sensors and biosensors: a review, Anal. Chim. Acta 615 (2008) 1–9. [5] E.S.Y.J. Gang, U.S. Paten (Ed.), Method and Apparatus for Magnetically Guided Catheter for Renal Denervation Employing MOSFET Sensor Array, 2016 United States. [6] K. Xu, L. Huang, Z. Zhang, J. Zhao, Z. Zhang, L.W. Snyman, et al., Light emission from a poly-silicon device with carrier injection engineering, Mater. Sci. Eng. B 231 (2018) 28–31. [7] K. Xu, Y. Chen, T.A. Okhai, L.W. Snyman, Micro optical sensors based on avalanching silicon light-emitting devices monolithically integrated on chips, Opt. Mater. Express 9 (2019). [8] S.J. Pearton, F. Ren, Y.-L. Wang, B.H. Chu, K.H. Chen, C.Y. Chang, et al., Recent advances in wide bandgap semiconductor biological and gas sensors, Prog. Mater.
5
Sensors & Actuators: B. Chemical 306 (2020) 127609
D. Xue, et al.
[30] G. Parish, F.L.M. Khir, N.R. Krishnan, J. Wang, J.S. Krisjanto, H. Li, et al., Role of GaN cap layer for reference electrode free AlGaN/GaN-based pH sensors, Sens. Actuators B Chem. 287 (2019) 250–257. [31] A. Podolska, D. Broxtermann, J. Malindretos, G.A. Umana-Membreno, S. Keller, U.K. Mishra, et al., Method to predict and optimize charge sensitivity of ungated AlGaN/GaN HEMT-Based ion sensor without use of reference electrode, IEEE Sens. J. 15 (2015) 5320–5326. [32] L. Li, X. Li, T. Pu, Y. Liu, J.-P. Ao, Normally off AlGaN/GaN ion-sensitive field effect transistors realized by photoelectrochemical method for pH sensor application, Superlattices Microstruct. 128 (2019) 99–104. [33] K. Xu, Silicon MOS optoelectronic micro‐nano structure based on reverse‐biased PN junction, Phys. Status Solidi 216 (2019). [34] C.D. Young, A. Neugroschel, K. Matthews, C. Smith, H. Dawei, P. Hokyung, et al., Gated diode investigation of bias temperature instability in High- $\kappa$ FinFETs, Ieee Electron Device Lett. 31 (2010) 653–655. [35] Y. Zhong, Y. Zhou, H. Gao, S. Dai, J. He, M. Feng, et al., Self-terminated etching of GaN with a high selectivity over AlGaN under inductively coupled Cl 2 /N 2 /O 2 plasma with a low-energy ion bombardment, Appl. Surf. Sci. 420 (2017) 817–824. [36] M. Mishra, S. Krishna Tc, P. Rastogi, N. Aggarwal, A.K.S. Chauhan, L. Goswami, et al., New approach to clean GaN surfaces, Mater. Focus. 3 (2014) 218–223. [37] Y. Chen, D. Xu, K. Xu, N. Zhang, S. Liu, J. Zhao, et al., Optoelectronic properties analysis of silicon light-emitting diode monolithically integrated in standard CMOS IC, Chin. Phys. B 28 (2019) 107801. [38] W. Wei, Z. Qin, S. Fan, Z. Li, K. Shi, Q. Zhu, et al., Valence band offset of betaGa2O3/wurtzite GaN heterostructure measured by X-ray photoelectron spectroscopy, Nanoscale Res. Lett. 7 (2012) 562. [39] M. Grodzicki, P. Mazur, S. Zuber, J. Brona, A. Ciszewski, Oxidation of GaN(0001) by low-energy ion bombardment, Appl. Surf. Sci. 304 (2014) 20–23.
Sci. 55 (2010) 1–59. [9] F. Ren, S.J. Pearton, Sensors using AlGaN/GaN based high electron mobility transistor for environmental and bio‐applications, Physica status solidi c 9 (2012) 393–398. [10] V. Cimalla, Label-free biosensors based on III-Nitride semiconductors, Label-Free Biosensing (2017) 59–102. [11] V.K.J.Fi.S. Khanna, Robust HEMT microsensors as prospective successors of MOSFET/ISFET detectors in harsh environments HEMT microsensors, Front. Sens. 1 (2013) 38–48. [12] S. Strite, H. Morkoç, GaN, AlN, and InN: a review, J. Vac. Sci. Technol. B 10 (1992) 1237–1266. [13] A. Ould-Abbas, O. Zeggai, M. Bouchaour, H. Zeggai, N. Sahouane, M. Madani, et al., Study on functionalizing the surface of AlGaN/GaN high electron mobility transistor based sensors, J. Optoelectron. Adv. Mater. 15 (2013) 1323–1327. [14] M. Rais-Zadeh, V.J. Gokhale, A. Ansari, M. Faucher, D. Theron, Y. Cordier, et al., Gallium nitride as an electromechanical material, J. Microelectromechanical Syst. 23 (2014) 1252–1271. [15] C. Wood, D. Jena, Polarization Effects in Semiconductors: From Ab Initio Theory to Device Applications: Springer Science & Business Media, (2007). [16] D.E. Yates, S. Levine, T.W. Healy, Site-binding model of the electrical double layer at the oxide/water interface, J. Chem. Soc. Faraday Trans. 1: Physical Chem. in Condensed Phases 70 (1974) 1807–1818. [17] C.-C. Chen, H.-I. Chen, H.-Y. Liu, P.-C. Chou, J.-K. Liou, W.-C. Liu, On a GaN-based ion sensitive field-effect transistor (ISFET) with a hydrogen peroxide surface treatment, Sens. Actuators B Chem. 209 (2015) 658–663. [18] L. Wang, Y. Bu, J.-P. Ao, Effect of oxygen plasma treatment on the performance of AlGaN/GaN ion-sensitive field-effect transistors, Diam. Relat. Mater. 73 (2017) 1–6. [19] L. Wang, Y. Bu, L. Li, J.-P. Ao, Effect of thermal oxidation treatment on pH sensitivity of AlGaN/GaN heterostructure ion-sensitive field-effect transistors, Appl. Surf. Sci. 411 (2017) 144–148. [20] L. Wang, L. Li, T. Zhang, X. Liu, J.-P. Ao, Enhanced pH sensitivity of AlGaN/GaN ion-sensitive field effect transistor with Al2O3 synthesized by atomic layer deposition, Appl. Surf. Sci. 427 (2018) 1199–1202. [21] M.S. Abidin, A.M. Hashim, M.E. Sharifabad, S.F. Rahman, T. Sadoh, Open-gated pH sensor fabricated on an undoped-AlGaN/GaN HEMT structure, Sens.(Basel) 11 (2011) 3067–3077. [22] Y. Dong, D.-h. Son, Q. Dai, J.-H. Lee, C.-H. Won, J.-G. Kim, et al., AlGaN/GaN heterostructure pH sensor with multi-sensing segments, Sens. Actuators B Chem. 260 (2018) 134–139. [23] H. Zhang, J. Tu, S. Yang, K. Sheng, P. Wang, Optimization of gate geometry towards high-sensitivity AlGaN/GaN pH sensor, Talanta 205 (2019). [24] T. Brazzini, A. Bengoechea-Encabo, M.A. Sánchez-García, F. Calle, Investigation of AlInN barrier ISFET structures with GaN capping for pH detection, Sens. Actuators B Chem. 176 (2013) 704–707. [25] C.-T. Lee, Y.-S. Chiu, Gate-recessed AlGaN/GaN ISFET urea biosensor fabricated by photoelectrochemical method, IEEE Sens. J. 16 (2016) 1518–1523. [26] Y. Dong, D.-H. Son, Q. Dai, J.-H. Lee, C.-H. Won, J.-G. Kim, et al., High sensitive pH sensor based on AlInN/GaN heterostructure transistor, Sensors 18 (2018) 1314. [27] J. Xing, D. Huang, Y. Dai, Y. Liu, Y. Ren, X. Han, et al., Influence of an integrated quasi-reference electrode on the stability of all-solid-state AlGaN/GaN based pH sensors, J. Appl. Phys. 124 (2018). [28] P. Sjöberg, A. Määttänen, U. Vanamo, M. Novell, P. Ihalainen, F.J. Andrade, et al., Paper-based potentiometric ion sensors constructed on ink-jet printed gold electrodes, Sens. Actuators B Chem. 224 (2016) 325–332. [29] M.S.Z. Abidin, H.A. Shahjahan, A.M. Hashim, Surface reaction of undoped AlGaN/ GaN HEMT based two terminal device in H+ and OH-ion-contained aqueous solution, Sains Malays. 2 (2013) 197–203.
Dongyang Xue received his B.E. degree from Dalian University of Technology in 2015. He is now a PhD candidate at Dalian University of Technology. His research interests are interdisciplinary and mainly include chemical sensors and biosensors based on group Ⅲ-N materials. Heqiu Zhang received her B.S. degree in 1997, and M.S. degree in 2000, respectively, from Dalian University of Technology. And she obtained PhD from Peking University in 2003. She is now an associate professor at the School of Microelectronics, Dalian University of Technology. Current research work mainly focuses on ZnO ultraviolet detectors and HEMT based sensors. Aqrab ul Ahmad had received his Master’s degree in Bio-Physics from Govt. College University, Faisalabad, Pakistan in 2015. Currently he is doctoral student in School of Microelectronics Dalian University of Technology, China under Chinese Government Scholarship. His research interests are synthesis of 2D materials (Boron nitride nanostructures) and their usage in multidisciplinary fields, biomedical, nano-electronics and energy related. Hongwei Liang received his B.S degree from Jilin University in 2000 and PhD from Academy of Chinese academy of sciences in 2005. He is now a professor at the School of Microelectronics, Dalian University of Technology. The major research directions are the third generation semiconductor materials and light emitting devices (Ga2O3 materials, novel BN semiconductor materials, GaN-based ultraviolet, blue, green and yellow LEDs), GaN-based high electron mobility power electronic devices (HEMT devices and sensors) and high temperature and radiation resistant detector devices, etc.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Enhancing the sensitivity of the reference electrode free AlGaN/GaN HEMT based pH sensors by controlling the threshold voltage
T
Dongyang Xuea, Heqiu Zhanga,*, Aqrab ul Ahmada, Hongwei Lianga,*, Jun Liua, Xiaochuan Xiaa, Wenping Guob, Huishi Huangc, Nanfa Xub a
School of Microelectronics, Dalian University of Technology, Dalian 116024, Liaoning, China Shandong NovoShine Optoelectronics Co., Ltd., Weifang 261061, Shandong, China c Jiangsu Xinguanglian Technology Co., Ltd., Wuxi 214192, Jiangsu, China b
ARTICLE INFO
ABSTRACT
Keywords: AlGaN/GaN HEMT pH sensor Photoelectrochemical oxidation method Threshold voltage Transconductance
The threshold voltage (VT) of the AlGaN/GaN HEMT based pH sensor was adjusted by the method of the photoelectrochemical (PEC) oxidation on the GaN cap layer surface. After the PEC oxidation treatments, the VT of the device shifted from -3.46 V to -1.15 V and the gate voltage (VG) corresponding to the maximum transconductance (gmMAX) position (VG|gmMAX) of the device shifted from -2.6 V to -0.1 V. The drain current (ID) variation per pH of the AlGaN/GaN HEMT based pH sensor without reference electrode increased from 0.7 μA to 14 μA when the drain voltage (VD) was 0.5 V. The sensitivity of the reference electrode free AlGaN/GaN HEMT based pH sensor can be significantly increased by regulating the VT to make VG|gmMAX approached the equivalent VG when liquid droplet on the sensing window surface (VG-EQU), which is beneficial to the miniaturization and integration of the AlGaN/GaN HEMT based sensors in the future.
1. Introduction With the development of internet of things technology, semiconductor-based sensors have attracted a lot of attention due to their potential of miniaturization and integration [1–7]. Recently in the domains of medical detection and environmental monitoring [8–11], the AlGaN/GaN HEMT based sensors are becoming the focus of many scholars gradually due to the high-temperature resistance, strong radiation resistance, stable chemical properties and remarkable biological compatibility of Ⅲ-Ⅴ nitride materials [4,12–14]. The AlGaN/GaN HEMT based pH sensors are devices that converting the information about H+ concentration of a solution (pH) into electrical signals. Owing to piezoelectric polarization and spontaneous polarization of Ⅲ-Ⅴ nitride materials, a high concentration of two-dimensional electron gas (2DEG) is formed into the interface between AlGaN and GaN [15], which is very sensitive to the change of surface charges. According to the site-binding model [16], the GaN natural/artificial oxide layer or other metal oxides will adsorb hydroxyl groups to form amphoteric oxides in the solution. When the pH value of the solution changes, the surface charges will change accordingly, which will affect the 2DEG to change the ID of the sensor. Two main factors are affecting the sensitivity of the sensor. One is the sensitivity of the sensing area surface to
⁎
H+, and the Nernst limit is 59 mV/pH at room temperature. The other is the control ability of gate to 2DEG, i.e. transconductance (gm). The sensitivity of the sensors surface to the H+ in solutions could be improved by some surface treatment methods that have been reported, such as oxidation with H2O2 [17], oxygen plasma treatment and thermal oxidation treatment [18,19], or artificial growth of an oxide layer such as Al2O3 [20]. The main reason for the increased sensitivity of the device is that the treated surfaces by these methods are easier to form hydroxyl groups in solution, providing binding sites for H+ [20]. On the other hand, the methods that have been studied to increase the gm of the sensors were to optimize the structure of devices, such as increasing the width-length ratio of the sensing window [21–23] and reducing the thickness of barrier layer [24–26]. For the AlGaN/GaN HEMT based pH sensors without the reference electrode, the threshold voltage (VT) of the devices is also a key factor because it determines the position of the gate voltage (VG) corresponding to the maximum transconductance (gmMAX) position (VG|gmMAX), which is not a problem for the devices with the reference electrodes because the operating VG of devices can be biased at any value by the reference electrodes. However, traditional reference electrodes are poor mechanical properties, difficulty in miniaturization and incompatibility with semiconductor fabrication technology, which seriously restrict the
Corresponding authors. E-mail addresses: [email protected] (H. Zhang), [email protected] (H. Liang).
https://doi.org/10.1016/j.snb.2019.127609 Received 11 September 2019; Received in revised form 16 December 2019; Accepted 18 December 2019 Available online 19 December 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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possibility of miniaturization, integration and harsh biochemical environment application of AlGaN/GaN HEMT based pH sensors. The pseudo-reference electrode such as Au [27], Pt, etc., or some solid electrode modified with organic membrane [28], fabricated on the sensor chip may be a way to solve the problem of traditional reference electrodes. Another way is to study the AlGaN/GaN HEMT based pH sensor without the reference electrode. Abidin et al. studied the AlGaN/ GaN HEMT based pH sensor without the reference electrode, but the nonlinearity response signals of the drain current (ID) to pH changes were obtained [29]. Giacinta Parish et al. reported that the GaN cap layer was necessary for the AlGaN/GaN HEMT based pH sensor to obtain a linear response signal of ID to pH changes, which is beneficial to the design of the reference electrode-free AlGaN/GaN HEMT based pH sensors [30]. And the sensitivity of the AlGaN/GaN HEMT based pH sensors without the reference electrodes could be improved by controlling the composition of Al and the thickness of the barrier layer of AlGaN during the epitaxy growth of AlGaN/GaN HEMT to adjust the VT, which was demonstrated with simulation by Anna Podolska et al. [31]. It could also change the VT and gm of devices by photoelectrochemical (PEC) method which have been reported that the positive bias voltage was applied to AlGaN barrier layer of device immersed in phosphoric acid [25] or NaOH solutions [32] under the irradiation of 325 nm He-Cr laser to realize the anodic oxidation of AlGaN. It is noteworthy that the GaN cap layer is necessary to ensure the linear signal of the reference electrode free AlGaN/GaN HEMT based pH sensor, but in the above method, phosphoric acid may etch GaN and AlGaN, which need to be avoided. Moreover, it should be avoided that the VT of the devices shifting to a positive value because the normally-off devices are disabled for measuring the pH of solutions without the reference electrodes. In this paper, the AlGaN/GaN HEMT based pH sensor with a GaN cap layer was used for obtaining a linear response signal of the device without the reference electrode. The PEC oxidation method was adapted to regulate the VT of the device to make VG|gmMAX approached 0 V that close to equivalent VG when liquid droplet on the sensing window surface (VG-EQU). And the sensitivity of the reference electrode free AlGaN/GaN HEMT based pH sensor was improved significantly.
steps. The first step was etching on the wafer by inductively coupled plasma (ICP) using BCl3/Cl2 as an etchant. The etching depth was 650 nm. The second step was to deposit the ohmic electrodes (Ti/Al/Ti/ Au/Pt with a thickness of 30/300/30/50/30 nm) by electron beam evaporation (EBM). Then the devices were annealed in a nitrogen atmosphere at 780℃ for 2 min to form ohmic contacts. The third step was depositing a 1.5 μm SiO2 passivation layer by plasma chemical vapor deposition (PECVD). Then the window of sensing area and the windows of the testing area were etched by the buffered oxide etch (BOE). The micrograph of the device is shown in Fig. 1(b). The width of the sensing window is 400 μm and the length of the sensing window is 40 μm. In the treatment of the PEC oxidation, the GaN cap layer connected with the ohmic contact electrodes, deionized water droplet, and the external Pt electrode together with the Keithley 4200 semiconductor characterization system constituted an electrolytic cell system. Under the irradiation of ultraviolet light, the voltage was applied between the ohmic contact electrodes and the Pt electrode. To avoid the reaction of the AlGaN barrier layer, a 365 nm ultraviolet LED lamp was used to guarantee that ultraviolet light was absorbed only by GaN but not by AlGaN. And the illuminance of the ultraviolet lamp was 70 mW/cm2. Deionized water was used as a reaction solution to reduce the reaction rate of the GaN cap layer to prevent the VT of the device from moving too fast to a positive value. The X-ray photoelectron spectrometer (XPS) system (K-Alpha+) was used to characterize the elements on the surface of samples before and after the treatment of the PEC oxidation. During testing the performance of the sensors, the pH buffer solutions were used with 4, 7, and 10 pH values. The ID was measured with the drain voltage (VD) scanned from 0 V to 1 V. When analyzing the VT and gm of the device, the sensing window was contacted with deionized water to form the electric double layer (EDL) gate at the interface, and the VG was controlled by pseudo-reference electrode (Pt) connected the deionized water on the sensing window to sweep from -10 V to 6 V. Then the ID-VG curve was analyzed to obtain the VT and gm of the device. To assess the time-drift of the device, the drain current-time curves (ID-t) of the device added varied pH solution was measured when VD was biased at 0.5 V. 3. Results and discussion
2. Experimental
Before the PEC oxidation, the measurement signals of the device at different pH and the analyzation of the VT and gm of the device are shown in Fig. 2. Fig. 2(a) shows the ID-VG curve and gm-VG curve of the device in deionized water with pseudo-reference electrode when VD is 0.5 V. The physical mechanism of the Z-shape curve (ID-VG curve) formation is similar to the MOSFET or gated diode [33,34]. And the VT of the device can be obtained from the ID-VG curve, which is -3.46 V. The maximum point of slope of ID-VG curve, which is -2.6 V, is the VG|gmMAX (peak position of gm-VG curve). Fig. 2(b) shows the ID-VD curves of the
As shown in Fig. 1(a), the wafer was grown by metal organic chemical vapor deposition (MOCVD) on the sapphire substrate, which consisted of 2 μm GaN buffer layer, 0.5 nm AlN spacer layer, 22 nm unintentionally doped Al0.26Ga0.74N barrier layer, and 2 nm GaN cap layer. The carrier density and mobility of 2DEG were 1.12 × 1013 cm−2 and 1520 cm2/(V∙s), respectively, which was measured with the Hall effect test system at room temperature. The fabrication process of the devices was mainly divided into three
Fig. 1. (a) The schematic diagram of the device section. (b) The micrograph of the device. 2
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Fig. 2. The measurement signals at different pH and the analyzation of the VT and gm of the device before the PEC oxidation. (a) The ID-VD curves of the device without a reference electrode at pH 4, pH 7 and pH 10, respectively. (b) The transfer characteristic curve (ID-VG) and gm-VG curve of the device in deionized water when VD was 0.5 V.
device without reference electrode at pH 4, pH 7 and pH 10, respectively. As shown in Fig. 2(b), the variation of ID per pH is only about 0.7 μA when VD is 0.5. The gm of the device is very small when the VG is nearby 0 V, which shown in Fig. 2(a), and that is why the current signal response of the device without reference electrode to the change of pH is weak. The ID-VG curves, gm-VG curves and IG-VG curves of the device measured in the dark before the PEC oxidation and after each time treatment are shown in Fig. 3. As Fig. 3(a) shows, the VT of the device before and after each time treatment is -3.46 V, -2.1 V, -1.54 V and -1.15 V, respectively. Fig. 3 shows that the VG|gmMAX of the device before and after each time treatment are -2.6 V, -1.2 V, -0.5 V and -0.1 V, respectively. Under the irradiation of 365 nm ultraviolet light, the GaN cap layer could generate photo-generated carriers to promote the anodic oxidation reaction on the GaN cap layer surface localized the sensing window area, resulting in Ga2O3 formation on the surface, which shown as the reaction formula (1). A similar anodic oxidation reaction has been reported by Ching-Ting Lee et al. [25]. At the same time, due to the catalytic action of Pt, the cathodic reduction reaction occurs on the surface of the Pt electrode which shown as the reaction formula (2). During the reaction, bubbles could be observed near the sensing window area and the Pt electrode. Therefore, the reason for the right shift of VT is the formation of Ga2O3 on the sensing window
surface, which resulted in the increase of the solid-liquid interface barrier due to the bandgap of Ga2O3 surface is larger than GaN cap layer and Al0.26Ga0.74N barrier layer. It is also found that the range of right shift of VT was smaller and smaller with the increase times of the treatment due to the thicker Ga2O3 surface would be more resistant to the surface oxidation reaction. And the gate leakage current (IG) decreases with the increase of the surface oxidation degree, which shown in Fig. 3(c).
2GaN + 6h+ + 6OH
6H+ + 6e
Pt
3H2
356nmUV
Ga2 O 3+3H2 O + N2
(1) (2)
In Fig. 4(a), the Ga 3d core level spectrum of the untreated sample is divided into three peaks. The main peak at 19.3 eV arises from Ga which combined with N (Ga-N) [35], the peak of Ga that combined with O (Ga-O) is at 20.4 eV and the peak at 17.6 eV arises from N 2 s [32,36]. It can be calculated the ratio of the Ga-N/Ga-O is 0.06. The rare oxygen may come from the natural oxide layer on the GaN cap layer surface. Fig. 4(b) shows that the Ga 3d core level spectrum of the sample which has been treated three times by the PEC oxidation. In the figure, the peak position of N 2 s, Ga-N, and Ga-O is 17.6 eV, 19.8 eV, and 20.4 eV, respectively [32,36]. The ratio of Ga-N/Ga-O is 1.03. After the PEC oxidation treatment, the atomic percentage of oxygen increases Fig. 3. The ID-VG curves, gm-VG curves and IGVG curves of the device measured in the dark before the PEC oxidation and after each time treatment. (a) The ID-VG curves of the device before the PEC oxidation and after each time treatment. (b) The gm-VG curves of the device before the PEC oxidation and after each time treatment. (c) The IG-VG curves of the device before the PEC oxidation and after each time treatment.
3
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Fig. 4. (a) The Ga 3d core level spectra of the untreated sample. (b) The Ga 3d core level spectra of the sample which has been treated three times by the PEC oxidation.
Fig. 5. (a) The energy band diagram of the sensor untreated by PEC. (b) The energy band diagram of the sensor treated by PEC. Fig. 6. The measurement signals at different pH and the analyzation of the VT and gm of the device after the once PEC oxidation. (a) The IDVD curves of the device without a reference electrode at pH 4, pH 7 and pH 10, respectively. (b) The transfer characteristic curve (IDVG) and gm-VG curve of the device in deionized water with Pt as a quasi-reference electrode when VD was 0.5 V.
Fig. 7. The measurement signals at different pH and the analyzation of the VT and gm of the device after the thrice PEC oxidation. (a) The ID-VD curves of the device without a reference electrode at pH 4, pH 7 and pH 10, respectively. (b) The transfer characteristic curve (IDVG) and gm-VG curve of the device in deionized water with Pt as pseudo -reference electrode when VD was 0.5 V.
obviously, so it is speculated that Ga2O3 is formed by surface oxidation. The mechanism for the formation of Ga2O3 on the sensing window surface making the right shift of VT is further illuminated with the energy band theory model as shown in Fig. 5(a) and (b). The interface of GaN/AlN and the surface of the sensing area are equivalent to two sides of the capacitor, which is the series capacitance of the oxide capacitance and the channel-layer capacitance [33]. Eq. (3) could be obtained due to the Gauss’ law. The ns is concentration of 2DEG, σpol is the surface density of polarized charge, ε is approximately equal to
dielectric constant of AlGaN, d is the distance from 2DEG to surface, ΔΦSS-TL and ΔΦREF-TL represent the surface potential barrier of sensing area surface and reference electrode with test liquid respectively, Δ means the part of the conduction band (EC) that under the Fermi level and ΔEC is the conduction band step between AlGaN and GaN. The 2DEG will be depleted approximately and Δ could be considered as 0 when VG = VT. Then combining with Eq. (3), Eq. (4) could be obtained [37]. Factors affecting VT include ΔΦSS-TL, ΔΦREF-TL, and other parameters that are intrinsic to the device itself. Ga2O3 would form a higher 4
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changes caused by variation of pH can be derived. It can be calculated that the surface of the sensing window response to H+ (dΔΦSS-TL /pH) increases to 53.3 mV/pH due to the increase of the surface oxidation degree. For the AlGaN/GaN HEMT based pH sensor without reference electrode, the main reason for the improvement of the sensitivity of the device treated by the PEC oxidation is the change of VT caused the VG|gmMAX approach VG-EQU of the sensors. Fig. 8 shows the ID-t curves of the device with the varied pH solution on the sensing area. The VD was biased at 0.5 V, and the ID of the device in acid solution (pH = 4), neutral solution (pH = 7) and alkaline solution (pH = 10) was measured, respectively. Each solution was measured for 100 s and a cycle was 300 s, with a total of five cycles. In the five cycles, the difference value between the maximum ID and the minimum ID is 7 μA, 9 μA, and 10 μA when the device added the acid solution, neutral solution, and alkaline solution, respectively. The signal sensitivity of the device is 14 μA/pH. Therefore, due to the timedrift during the measurement, the resolution of the device is between 0.5–1 pH. Further optimization of the sensor is still needed to reduce the time-drift and improve its resolution.
Fig. 8. The ID-t curves of the device with the varied pH solution on the sensing area.
4. Conclusions
barrier with the solution due to its wider bandgap [38,39] and more hydroxyl binding sites. Therefore, the VT shifts right after PEC treatment because the ΔΦSS-TL of Ga2O3 is larger than ΔΦSS-TL of GaN. When the test liquid changes, the ΔΦREF-TL will almost keep constant if the reference electrode good enough, and the variation of the ΔΦSS-TL is determined by the sensitivity of the surface of the sensor.
ens =
VT =
pol
pol d 0
0
d
(
+(
SS TL
SS TL
REF TL
REF TL )
+
EC e
e
EC e
VG )
In this report, the PEC oxidation method was applied to the GaN cap layer surface localized the sensing window area of the AlGaN/GaN HEMT based pH sensor. The VT of the device shifted to the right with the increase of the PEC oxidation treatment times. The sensitivity of the reference electrode free AlGaN/GaN HEMT based pH sensor was increased to 20 times by regulating the VT of the device to make VG|gmMAX approaching VG-EQU of the sensors. According to the results of ID-t curves, the resolution of the device is between 0.5–1 pH, which needs to be further optimized. The work may be beneficial to the miniaturization and integration of the AlGaN/GaN HEMT based pH sensors in the future.
(3)
(4)
After the first PEC oxidation treatment, the measurement signals at different pH and the analyzation of the VT and gm of the device are shown in Fig. 6. The ID-VD curves of the device without reference electrodes in different pH solutions are measured, which shown in Fig. 6(a). The change of the current signal of the device at different pH values, which is 3 μA/pH (VD = 0 V), is more obvious than that of the untreated device. According to the current value of the device when VD is 0.5 V in the ID-VD curves and the ID-VG curve of the device, the approximate range of VG-EQU corresponding to the device in different pH solutions could be deduced, which is shown in Fig. 6(b). However, the gm corresponding to the VG-EQU was still not optimal. The sensitivity of the sensing window surface response to H+ could be calculated to be 50.0 mV/pH from the insert of the Fig.6(b). The measurement signals at different pH and the analyzation of the VT and gm of the device which after the thrice PEC oxidation are shown in Fig. 7. As Fig. 7(a) shows the ID-VD curves of the device in different pH solutions are measured without reference electrodes. Variation of ID per pH is about 14 μA when VD is 0.5 V, which is 20 times as much as that of the untreated device and about 5 times as much as the device after once treatment. It should be noted that the device tends to be saturated when VD at about 1 V. The VG-EQU is near 0 V and the VT is -1.15 V, so the saturation drain voltage (VD-Sat = VG-VT) is near 1 V. When VD is greater than or equal to VD-Sat, the drain current will reach saturation. When the sensing window is dipped in the solution with higher pH, the VG-EQU will be smaller, the saturation voltage will be closer to 1 V, and the current saturation trend will be more obvious. As shown in Fig. 7(b), according to the current value of the device when VD is 0.5 V in the ID-VD curves and the ID-VG curve of the device, the approximate range of VG-EQU corresponding to the device in different pH solutions could be deduced, which is close to the VG|gmMAX. The corresponding VG of the ID values (VD = 0.5 V) in different pH solutions can be found from the transfer characteristic curve, which shown in the insert of the Fig.7(b). According to this, the surface potential
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 61574026, 11675198, 11875097, 61774072), the Dalian Science and Technology Innovation Foundation (No. 2018J12GX060). References [1] T.C. Yeow, M. Haskard, D. Mulcahy, H. Seo, D.J.S. Kwon, A.B. Chemical, A very large integrated pH-ISFET sensor array chip compatible with standard CMOS processes, Sens. Actuators B Chem. 44 (1997) 434–440. [2] M. Castellarnau, N. Zine, J. Bausells, C. Madrid, A. Juárez, J. Samitier, et al., Integrated cell positioning and cell-based ISFET biosensors, Sens. Actuators B Chem. 120 (2007) 615–620. [3] M.J. Milgrew, D.R.S. Cumming, Matching the transconductance characteristics of CMOS ISFET arrays by removing trapped charge, IEEE Trans. Electron Devices 55 (2008) 1074–1079. [4] N. Chaniotakis, N. Sofikiti, Novel semiconductor materials for the development of chemical sensors and biosensors: a review, Anal. Chim. Acta 615 (2008) 1–9. [5] E.S.Y.J. Gang, U.S. Paten (Ed.), Method and Apparatus for Magnetically Guided Catheter for Renal Denervation Employing MOSFET Sensor Array, 2016 United States. [6] K. Xu, L. Huang, Z. Zhang, J. Zhao, Z. Zhang, L.W. Snyman, et al., Light emission from a poly-silicon device with carrier injection engineering, Mater. Sci. Eng. B 231 (2018) 28–31. [7] K. Xu, Y. Chen, T.A. Okhai, L.W. Snyman, Micro optical sensors based on avalanching silicon light-emitting devices monolithically integrated on chips, Opt. Mater. Express 9 (2019). [8] S.J. Pearton, F. Ren, Y.-L. Wang, B.H. Chu, K.H. Chen, C.Y. Chang, et al., Recent advances in wide bandgap semiconductor biological and gas sensors, Prog. Mater.
5
Sensors & Actuators: B. Chemical 306 (2020) 127609
D. Xue, et al.
[30] G. Parish, F.L.M. Khir, N.R. Krishnan, J. Wang, J.S. Krisjanto, H. Li, et al., Role of GaN cap layer for reference electrode free AlGaN/GaN-based pH sensors, Sens. Actuators B Chem. 287 (2019) 250–257. [31] A. Podolska, D. Broxtermann, J. Malindretos, G.A. Umana-Membreno, S. Keller, U.K. Mishra, et al., Method to predict and optimize charge sensitivity of ungated AlGaN/GaN HEMT-Based ion sensor without use of reference electrode, IEEE Sens. J. 15 (2015) 5320–5326. [32] L. Li, X. Li, T. Pu, Y. Liu, J.-P. Ao, Normally off AlGaN/GaN ion-sensitive field effect transistors realized by photoelectrochemical method for pH sensor application, Superlattices Microstruct. 128 (2019) 99–104. [33] K. Xu, Silicon MOS optoelectronic micro‐nano structure based on reverse‐biased PN junction, Phys. Status Solidi 216 (2019). [34] C.D. Young, A. Neugroschel, K. Matthews, C. Smith, H. Dawei, P. Hokyung, et al., Gated diode investigation of bias temperature instability in High- $\kappa$ FinFETs, Ieee Electron Device Lett. 31 (2010) 653–655. [35] Y. Zhong, Y. Zhou, H. Gao, S. Dai, J. He, M. Feng, et al., Self-terminated etching of GaN with a high selectivity over AlGaN under inductively coupled Cl 2 /N 2 /O 2 plasma with a low-energy ion bombardment, Appl. Surf. Sci. 420 (2017) 817–824. [36] M. Mishra, S. Krishna Tc, P. Rastogi, N. Aggarwal, A.K.S. Chauhan, L. Goswami, et al., New approach to clean GaN surfaces, Mater. Focus. 3 (2014) 218–223. [37] Y. Chen, D. Xu, K. Xu, N. Zhang, S. Liu, J. Zhao, et al., Optoelectronic properties analysis of silicon light-emitting diode monolithically integrated in standard CMOS IC, Chin. Phys. B 28 (2019) 107801. [38] W. Wei, Z. Qin, S. Fan, Z. Li, K. Shi, Q. Zhu, et al., Valence band offset of betaGa2O3/wurtzite GaN heterostructure measured by X-ray photoelectron spectroscopy, Nanoscale Res. Lett. 7 (2012) 562. [39] M. Grodzicki, P. Mazur, S. Zuber, J. Brona, A. Ciszewski, Oxidation of GaN(0001) by low-energy ion bombardment, Appl. Surf. Sci. 304 (2014) 20–23.
Sci. 55 (2010) 1–59. [9] F. Ren, S.J. Pearton, Sensors using AlGaN/GaN based high electron mobility transistor for environmental and bio‐applications, Physica status solidi c 9 (2012) 393–398. [10] V. Cimalla, Label-free biosensors based on III-Nitride semiconductors, Label-Free Biosensing (2017) 59–102. [11] V.K.J.Fi.S. Khanna, Robust HEMT microsensors as prospective successors of MOSFET/ISFET detectors in harsh environments HEMT microsensors, Front. Sens. 1 (2013) 38–48. [12] S. Strite, H. Morkoç, GaN, AlN, and InN: a review, J. Vac. Sci. Technol. B 10 (1992) 1237–1266. [13] A. Ould-Abbas, O. Zeggai, M. Bouchaour, H. Zeggai, N. Sahouane, M. Madani, et al., Study on functionalizing the surface of AlGaN/GaN high electron mobility transistor based sensors, J. Optoelectron. Adv. Mater. 15 (2013) 1323–1327. [14] M. Rais-Zadeh, V.J. Gokhale, A. Ansari, M. Faucher, D. Theron, Y. Cordier, et al., Gallium nitride as an electromechanical material, J. Microelectromechanical Syst. 23 (2014) 1252–1271. [15] C. Wood, D. Jena, Polarization Effects in Semiconductors: From Ab Initio Theory to Device Applications: Springer Science & Business Media, (2007). [16] D.E. Yates, S. Levine, T.W. Healy, Site-binding model of the electrical double layer at the oxide/water interface, J. Chem. Soc. Faraday Trans. 1: Physical Chem. in Condensed Phases 70 (1974) 1807–1818. [17] C.-C. Chen, H.-I. Chen, H.-Y. Liu, P.-C. Chou, J.-K. Liou, W.-C. Liu, On a GaN-based ion sensitive field-effect transistor (ISFET) with a hydrogen peroxide surface treatment, Sens. Actuators B Chem. 209 (2015) 658–663. [18] L. Wang, Y. Bu, J.-P. Ao, Effect of oxygen plasma treatment on the performance of AlGaN/GaN ion-sensitive field-effect transistors, Diam. Relat. Mater. 73 (2017) 1–6. [19] L. Wang, Y. Bu, L. Li, J.-P. Ao, Effect of thermal oxidation treatment on pH sensitivity of AlGaN/GaN heterostructure ion-sensitive field-effect transistors, Appl. Surf. Sci. 411 (2017) 144–148. [20] L. Wang, L. Li, T. Zhang, X. Liu, J.-P. Ao, Enhanced pH sensitivity of AlGaN/GaN ion-sensitive field effect transistor with Al2O3 synthesized by atomic layer deposition, Appl. Surf. Sci. 427 (2018) 1199–1202. [21] M.S. Abidin, A.M. Hashim, M.E. Sharifabad, S.F. Rahman, T. Sadoh, Open-gated pH sensor fabricated on an undoped-AlGaN/GaN HEMT structure, Sens.(Basel) 11 (2011) 3067–3077. [22] Y. Dong, D.-h. Son, Q. Dai, J.-H. Lee, C.-H. Won, J.-G. Kim, et al., AlGaN/GaN heterostructure pH sensor with multi-sensing segments, Sens. Actuators B Chem. 260 (2018) 134–139. [23] H. Zhang, J. Tu, S. Yang, K. Sheng, P. Wang, Optimization of gate geometry towards high-sensitivity AlGaN/GaN pH sensor, Talanta 205 (2019). [24] T. Brazzini, A. Bengoechea-Encabo, M.A. Sánchez-García, F. Calle, Investigation of AlInN barrier ISFET structures with GaN capping for pH detection, Sens. Actuators B Chem. 176 (2013) 704–707. [25] C.-T. Lee, Y.-S. Chiu, Gate-recessed AlGaN/GaN ISFET urea biosensor fabricated by photoelectrochemical method, IEEE Sens. J. 16 (2016) 1518–1523. [26] Y. Dong, D.-H. Son, Q. Dai, J.-H. Lee, C.-H. Won, J.-G. Kim, et al., High sensitive pH sensor based on AlInN/GaN heterostructure transistor, Sensors 18 (2018) 1314. [27] J. Xing, D. Huang, Y. Dai, Y. Liu, Y. Ren, X. Han, et al., Influence of an integrated quasi-reference electrode on the stability of all-solid-state AlGaN/GaN based pH sensors, J. Appl. Phys. 124 (2018). [28] P. Sjöberg, A. Määttänen, U. Vanamo, M. Novell, P. Ihalainen, F.J. Andrade, et al., Paper-based potentiometric ion sensors constructed on ink-jet printed gold electrodes, Sens. Actuators B Chem. 224 (2016) 325–332. [29] M.S.Z. Abidin, H.A. Shahjahan, A.M. Hashim, Surface reaction of undoped AlGaN/ GaN HEMT based two terminal device in H+ and OH-ion-contained aqueous solution, Sains Malays. 2 (2013) 197–203.
Dongyang Xue received his B.E. degree from Dalian University of Technology in 2015. He is now a PhD candidate at Dalian University of Technology. His research interests are interdisciplinary and mainly include chemical sensors and biosensors based on group Ⅲ-N materials. Heqiu Zhang received her B.S. degree in 1997, and M.S. degree in 2000, respectively, from Dalian University of Technology. And she obtained PhD from Peking University in 2003. She is now an associate professor at the School of Microelectronics, Dalian University of Technology. Current research work mainly focuses on ZnO ultraviolet detectors and HEMT based sensors. Aqrab ul Ahmad had received his Master’s degree in Bio-Physics from Govt. College University, Faisalabad, Pakistan in 2015. Currently he is doctoral student in School of Microelectronics Dalian University of Technology, China under Chinese Government Scholarship. His research interests are synthesis of 2D materials (Boron nitride nanostructures) and their usage in multidisciplinary fields, biomedical, nano-electronics and energy related. Hongwei Liang received his B.S degree from Jilin University in 2000 and PhD from Academy of Chinese academy of sciences in 2005. He is now a professor at the School of Microelectronics, Dalian University of Technology. The major research directions are the third generation semiconductor materials and light emitting devices (Ga2O3 materials, novel BN semiconductor materials, GaN-based ultraviolet, blue, green and yellow LEDs), GaN-based high electron mobility power electronic devices (HEMT devices and sensors) and high temperature and radiation resistant detector devices, etc.
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