Enhanced pH sensitivity over the Nernst limit of electrolyte gated a-IGZO thin film transistor using branched polyethylenimine

Abstract

This paper reports the use of branched polyethylenimine (BPEI) coated amorphous indium gallium zinc oxide (a-IGZO) based electrolyte gated thin film transistors (EGTFTs) to obtain a pH sensitivity of ∼110 mV pH⁻¹ which is above the Nernst limit. The a-IGZO, used both as the active layer and pH sensitive layer, obviates the need of a separate high-K dielectric thereby simplifying the process of fabrication of TFT based sensors along with their operation at a low voltage range (−2 V to 2 V). Two EGTFT devices, one with bare a-IGZO and the other with BPEI coated a-IGZO were utilized in this work. The electrical double layer (EDL) formation at the a-IGZO/electrolyte interface was studied with the capacitance–voltage analysis which showed well defined accumulation and depletion regimes at a small voltage range (−1 V to 1 V) with the EDL capacitance of ∼2.0 μF cm⁻². The EGTFT with bare a-IGZO film showed a good I_ON/I_OFF ratio of the order of ∼10³, a sub-threshold swing of 238 mV dec⁻¹, and saturation mobility of 3.0 cm² V⁻¹ s⁻¹ but a pH sensitivity of 24 mV pH⁻¹. The utilization of BPEI with this structure, reported for the first time, resulted in ∼4.6 fold enhancement in the sensitivity. The BPEI layer enhances the pH sensitivity and could also facilitate the use of such devices for biosensing as the amine groups of the BPEI could be used as linkers for immobilizing biomolecules for capturing target analytes. The device structure used in the work is compatible for fabrication on flexible substrates which enable lower cost and applications needing form factor.

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1. Introduction

Thin film transistor (TFT) based pH sensors have drawn attention due to their ease of fabrication, low cost, and compatibility of fabrication on flexible substrates,^1–3 and these could replace the conventional bulk silicon based ion sensitive field effect transistors (ISFETs) for use as chemical and bio-sensors. The liquid electrolyte gated thin film transistors (EGTFTs) are easy to fabricate requiring the formation of source/drain electrodes and a thin film of the desired semiconductor, and obviate the need of separate gate dielectrics. Many organic EGTFTs have been studied in the last decade due to the high electric double layer (EDL) capacitance at the electrolyte/semiconductor interface which allows operation at low voltages (<2 V).^4–9 Few organic EGTFTs have also been employed for pH sensing; Buth et al.¹⁰ obtained a low sensitivity of 9 mV pH⁻¹ while Kofler et al.¹¹ obtained a sensitivity of 52 mV pH⁻¹ using an additional polymeric ion-selective membrane integrated with the EGTFT. However, relatively faster degradation of organic EGTFTs in comparison with the inorganic EGTFTs limits their utilization in sensing applications. The TFT characteristics of some inorganic EGFETs based on ZnO and MoS₂ have been studied; and a graphene based device has been studied for pH detection which suffered from instability and low pH sensitivity (∼26 mV pH⁻¹).^12–15 The sensitivity of the semiconductor surface to the ions is crucial to justify the replacement of the high-K dielectrics in these field effect based sensors. The sensitivity enhancement aspect of EGTFTs for their utilization in chemical and bio-sensing have received scant attention and hence is the interest of the present work. Amorphous indium gallium zinc oxide (a-IGZO) based TFTs have been reported to be more stable with controlled carrier concentrations and desired mobilities.^16,17 Whereas, TFTs based on other ZnO based compounds such as IZO based TFTs while providing high carrier mobilities suffered from high carrier concentrations leading to an always “ON” state,¹⁸ and GZO based TFTs while showing improved stabilities had low mobilities.¹⁹ Recently, few a-IGZO based EGTFTs have been studied to understand the effect of different electrolytes (solid and liquid) on the TFT characteristics.^20–24 In addition to improving the device performance (mobility, threshold voltage, ON/OFF current ratio) of a-IGZO based EGTFTs, the pH sensitivities of these structures is also critical which have not been studied so far. The a-IGZO film as the pH sensitive layer was studied in our previous work using a silicon based electrolyte–insulator–semiconductor (EIS) structure which showed a maximum pH sensitivity of 53.3 mV pH⁻¹.²⁵ This study provided a scope to utilize a-IGZO as the pH sensitive layer which can be used as the active layer in TFT based sensors.

In this work, we report the effect of liquid electrolyte with varying pH values on the characteristics of an a-IGZO based EGTFT which could be utilized as pH sensors as well as bio-sensors. The formation of an EDL at the electrolyte/a-IGZO interface which acts as a highly capacitive thin insulator film was studied by capacitance–voltage (C–V) analysis. To further enhance the pH sensitivity, the a-IGZO surface was coated with a thin film of the branched polyethylenimine (BPEI) and the effect of BPEI coating on the C–V and transfer characteristics of the EGTFT were studied. BPEI is a water soluble polymer which has been utilized for surface modification of silicon and platinum based potentiometric pH sensors for sensitivity enhancement.^26,27 In addition to pH sensitivity enhancement, the BPEI could also be used as linker molecules for immobilization of biomolecules.²⁸ Therefore, the utilization of BPEI for pH sensitivity enhancement also obviates the need of chemical functionalization of the surface to create amine groups for immobilization of biomolecules for use of such structures in bio-sensing applications. In addition, to the best of our knowledge, we are not aware of any report on the use of the BPEI in EGTFTs to enhance their sensitivities.

2. Experimental

2.1. Materials

n⁺-Si wafers with 150 nm of thermally grown oxide were obtained from Silicon Materials, USA. The sputtering target (4-inch diameter) of composition InGaZnO₄ was purchased from Able Target Limited, China. The branched polyethylenimine with a number average molecular weight (M_n) of 60 000 was obtained from Sigma Aldrich (Fluka Analytical), Germany. The standard pH buffer solutions made from potassium hydrogen phthalate (pH 4.0), potassium dihydrogen phosphate/disodium hydrogen phosphate (pH 7.0), and boric acid/potassium chloride/sodium hydroxide (pH 9.0) were purchased from Merck Millipore, India. The pH values of these solutions were verified by commercial pH meter. Epoxy (SU-8 2005) was obtained from Microchem, USA.

2.2. Device fabrication and characterization

First, a 30 nm thick film of a-IGZO was deposited over a piece of oxidized n⁺-Si wafer at room temperature (between 22–30 °C) by RF magnetron sputtering at 70 W at 8 × 10⁻³ mbar in Ar plasma atmosphere with a deposition rate of 2 nm min⁻¹. A copper shadow mask of opening 10 mm (L) × 1 mm (W) was used. The deposited IGZO film was annealed in a tubular furnace at 400 °C with an oxygen flow rate of 100 sccm. The film characterizations were done with a stylus profilometer (Model: Dektek XTL) for thickness measurement, an electron probe micro analyzer (Model: JXA-8230; JEOL) for compositional analysis, an X-ray diffractometer (Model: Panalytical XPert) for phase analysis and atomic force microscopy (Model: MFP-3D; Asylum Research) for surface topography. The source (S) and drain (D) were fabricated by thermal evaporation of aluminum of thickness 100 nm followed by annealing in N₂ ambience in a box furnace at 250 °C for 30 minutes. The channel dimension of the fabricated device was 600 μm (W) × 200 μm (L) with the S and D overlapping of 400 μm. Then a 100 nm thick film of a-IGZO was deposited and annealed at 400 °C in oxygen ambience. Thus the sandwiched Al source/drain contacts between two a-IGZO layers overcome the problem of higher contact resistance associated with the structure having bottom source/drain contacts²⁹ and the critical passivation of top source/drain contacts to protect from exposure to the electrolyte. The reservoir of the size 1.0 × 1.0 mm² was fabricated over the top of the a-IGZO layer by photolithography using an epoxy to hold the electrolyte solution. A schematic of the fabricated EGTFT is shown in Fig. 1a. To optimize the BPEI thin film, 2% BPEI solution in DI water was spin coated at 5000 rpm for 1 min and heated on a hot plate in open air for 30 minutes at 100, 150, and 200 °C. The film annealed at 200 °C provided uniform thickness of 23 nm measured with stylus profilometer while the penetration of the stylus probe into the film annealed till 150 °C caused an error in thickness measurement attributed to the remaining water content in the film. The film was not annealed above 200 °C to avoid the decomposition of BPEI.³⁰ To fabricate the MOS device, the optimized BPEI layer was spin coated on a cleaned p-type bare silicon substrate, and 100 nm thick Al was deposited using a shadow mask of 3 mm opening to define the area of the capacitor. Capacitance–voltage (C–V) and current–voltage (I–V) characterization of all the devices were conducted using a semiconductor parametric analyzer (Agilent B1500A). Tungsten (W) probes which are used to make connections with source/drain electrodes were also used as pseudo-reference electrodes to make contact with the electrolyte filled in the reservoir.

Fig. 1 (a) Schematic of the fabricated EGTFT structure (b) illustration of the electric double layer formation in the metal/electrolyte/semiconductor structure.

3. Results and discussion

3.1. Thin film characterization

The proportion of In : Ga : Zn in the IGZO film measured with EPMA was found to be 0.52 : 1 : 0.61 which indicates a Ga rich film. The XRD pattern showed amorphous character as no crystalline peak was observed. The surface topographies of the sputtered a-IGZO and the spin coated BPEI films as obtained from AFM are shown in Fig. 2. The a-IGZO film showed a granular structure with an RMS roughness of 0.51 nm which is comparable to the reported value of 0.53 nm.³¹ The BPEI film annealed at 200 °C in air showed an RMS roughness of 0.21 nm without any clear grain formation. The dielectric properties of the BPEI film were studied from the C–V characteristics of a MOS (Al–BPEI–p-Si) structure at 1 kHz. The dielectric constant of the BPEI layer was measured to be ∼2.0.

Fig. 2 AFM images of (a) bare a-IGZO film annealed at 400 °C (b) BPEI film on a-IGZO annealed at 200 °C.

3.2. C–V analysis of bare and BPEI coated a-IGZO/electrolyte interface

The formation of the EDL at the electrolyte/semiconductor interface (shown in Fig. 1b) was studied by the C–V measurement of the devices with both bare a-IGZO and the BPEI coated a-IGZO film where the bias voltage was applied at the liquid gate with the source grounded. The C–V characteristics of both the devices measured at 1 kHz frequency with the solution (electrolyte) of pH 4 are shown in Fig. 3. Similar C–V curves were obtained at pH 7 and 9 (not shown here). In the device with the bare a-IGZO film, the capacitance starts increasing at −0.4 V confirming the accumulation of negative charge carriers in the a-IGZO which sharply increases near the applied voltage of 0 V and saturates at 0.9 V. Further increase in the voltage reduces the capacitance which can be attributed to the induced charge transport through the electric double layer. The value of the maximum capacitance (C_max) is ∼4 times higher than the minimum capacitance (C_min) and this validates the strong capacitive nature of the EDL (C_EDL ∼ 2.0 μF cm⁻²) in a small voltage range (∼1 V). On the other hand, in the device with BPEI coated a-IGZO film, the negative shift in the C–V curve predicts an early accumulation of the charge carriers which could be attributed to the positively charged amine groups (–NH₃⁺, –NH₂⁺) in the BPEI film. BPEI has primary (–NH₂), secondary (–NH), and tertiary (–N) amine groups with the probability of occurrence of the tertiary group being much lower. The primary and secondary amines get protonated at acidic pH to form –NH₃⁺ and –NH₂⁺groups. Similar negative threshold voltage shift was observed in our previous work upon surface (SiO₂) modification with self assembled monolayers of 3-aminopropyl triethoxysilane.³² The decreased accumulation capacitance of the device with BPEI coated a-IGZO indicated the low capacitive nature of the BPEI film with respect to the EDL at the electrolyte/a-IGZO interface. The sharp decrease in the accumulation capacitance above 0.8 V could be due to the conductive nature of the BPEI film which leads to large leakage current above 1 V. A hump in the C–V curve near −0.7 V could be attributed to the positively charged trap states present at the BPEI/a-IGZO interface which has also been observed at the Al₂O₃/InGaAs interface of an InGaAs based MOS structure.³³

Fig. 3 C–V characteristics of W/electrolyte/a-IGZO/Al structure with bare and BPEI coated a-IGZO measured at pH 4.

3.3. Characteristics of the EGTFTs with bare and BPEI coated a-IGZO

The transfer characteristics of a-IGZO based liquid electrolyte gated TFT at pH 4, 7, and 9 are shown in Fig. 4. The TFT was operated with the gate voltage varying between −2 V to 2 V at a constant drain voltage of 2 V. The operating voltage range of the EGTFT is in good agreement with the reported literature values.^21,22 The device showed an I_ON/I_OFF ratio of the order of ∼10³, a sub-threshold swing of 238 mV dec⁻¹, a saturation mobility of 3.0 cm² V⁻¹ s⁻¹, and the threshold voltage (V_TH) of −70 mV measured at pH 4. The low mobility and higher sub-threshold swing of the EGTFT compared to reported literature²¹ could be attributed to the Ga rich IGZO film.¹⁹ The Ga rich IGZO film has been reported to improve the stability of the device due to its strong oxygen binding nature which facilitates the utilization of these structures for sensing applications with minimized drift and hysteresis.³⁴ Negative V_TH is attributed to the positively charged a-IGZO surface at acidic pH which induces the accumulation of negatively charged carriers in the channel. The decrease in the OFF current and the increased V_TH i.e. 0 mV at pH 7, could be due to the neutralized a-IGZO surface with zero net surface charge. Further decrease in the OFF current and the further increased V_TH, i.e. 50 mV at a pH value of 9, confirmed the negatively charged a-IGZO surface. The gate leakage currents at all pH values are in the range of ∼10 nA till 1 V and 100 nA between 1 to 2 V which is nearly 10² orders lower than the ON current of the TFT. The pH sensitivity of the a-IGZO surface is 24 mV pH⁻¹ which is lower than the value of 53 mV pH⁻¹ reported by us using an electrolyte-insulator-semiconductor (EIS) structure with a-IGZO as a pH sensitive layer with SiO₂ as the dielectric and crystalline silicon as the semiconductor.²⁵ While, in the case of the EGTFT, the active layer (a-IGZO) itself was employed as pH sensitive surface and this could have reduced its sensing characteristics. A comparable value of pH sensitivity of ∼26 mV pH⁻¹ was also obtained by Ohno et al.¹² using a graphene based EGTFT which showed a voltage shift from −0.06 V to 0.05 V with pH values varying between 4 to 8.2.

Fig. 4 Transfer characteristics of the EGTFT with bare a-IGZO with varying values of pH.

To enhance the pH sensitivity of the a-IGZO based EGTFT, the a-IGZO surface was coated with ∼23 nm of polyethylenimine (BPEI) layer. The transfer characteristics of the BPEI coated a-IGZO based EGTFT are shown in Fig. 5a. The device showed slightly diminished TFT performance parameters with the I_ON/I_OFF ratio of the order of 3.5 × 10², a sub-threshold swing of 282 mV dec⁻¹, saturation mobility of 2.2 cm² V⁻¹ s⁻¹, and the threshold voltage (V_TH) of −388 mV measured at pH 4. These reduced parameters could be attributed to the reduced capacitive nature of EDL upon the coating of BPEI film. A more negative V_TH was observed in the BPEI coated a-IGZO when compared to the bare a-IGZO based EGTFT. The primary and secondary amine groups in the BPEI film show strong protonation at acidic pH by converting to –NH₃⁺ and –NH₂⁺ resulting in a strong accumulation of the negative charge carriers in the channel which leads to a negative shift in the V_TH. The positive shift in the V_TH and the reduced OFF current with increased pH values showed similar trends as observed for the EGTFT without BPEI. The gate leakage currents at all pH values were higher compared to those obtained in the device with the bare a-IGZO surface and this could be attributed to the low dielectric constant and conductive nature of the BPEI film which is also confirmed by the C–V characteristics in Fig. 2. The a-IGZO coated with BPEI showed a pH sensitivity of 110 mV pH⁻¹ which is ∼4.6 fold higher than that obtained with the bare a-IGZO surface. The reversibility test was also performed by measuring the transfer characteristics with pH cycle 4–7–9–7–4. As shown in Fig. 5b, no significant change in total voltage shift was observed in the reverse direction i.e. from pH 9 to pH 4 except that the voltage difference between pH 9 and pH 7 was reduced. Also a slight increase in ON current was observed. To study the time dependent degradation, the device was tested after being kept in the open air for two months and showed stable transfer characteristics without any significant change in voltage shift between pH 4 to 7. However, a significant decrease in the voltage shift between pH 7 to 9 was observed. Both the devices showed unstable transfer characteristics when tested at extreme values of pH (1.86 and 12.45). In addition to use as the pH sensors, these EGTFTs with BPEI coating over a-IGZO are also potential candidates for utilization as enzymatic bio-sensors where the enzyme–substrate reaction leads to change in the pH. The amine groups are used as linkers for immobilizing sensing biomolecules. Thus such a structure, i.e. EGTFT based on a-IGZO coated with BPEI, could obviate the need of chemical functionalization and dielectrics such as Si₃N₄, Al₂O₃, and Ta₂O₅ in the fabrication of the enzymatic bio-sensors.^35–37

Fig. 5 Transfer characteristics of BPEI coated a-IGZO EGTFT (a) with varying values of pH (b) reversibility test with pH cycle 4–7–9–7–4.

4. Conclusion

In summary, two EGTFT devices, one with bare a-IGZO and the other with BPEI coated a-IGZO were employed in this work. The formation of the EDL at the electrolyte/a-IGZO interface was studied using the C–V analysis along with the effect of thin film coating of BPEI layer over the a-IGZO surface. Both the devices showed good C–V characteristics with well defined accumulation and depletion regimes at a low voltage range i.e. −1 V to 1 V with the EDL capacitance of ∼2.0 μF cm⁻². The EGTFT with bare a-IGZO film showed a good I_ON/I_OFF ratio of the order of ∼10³, a sub-threshold swing of 238 mV dec⁻¹, and saturation mobility of 3.0 cm² V⁻¹ s⁻¹. However, the a-IGZO in the EGTFT showed low pH sensitivity (24 mV pH⁻¹) which could be attributed to the utilization of a-IGZO as both the active layer and the pH sensitive surface. The BPEI coated a-IGZO showed ∼4.6 fold enhancement in the pH sensitivity (∼110 mV pH⁻¹) due to strong protonation efficiency of the amine (–NH₂ and –NH) groups present in the BPEI film. However, the TFT performance parameters were slightly diminished which showed the I_ON/I_OFF ratio of the order of 3.5 × 10², a sub-threshold swing of 282 mV dec⁻¹, and saturation mobility of 2.2 cm² V⁻¹ s⁻¹. In addition to the pH sensitivity enhancement, the BPEI could also be utilized for immobilization of the biomolecules. Thus, the EGTFTs based on the BPEI coated a-IGZO will obviate the need of a dielectric and chemical functionalization in the fabrication of the enzymatic bio-sensors. Such structures could also be fabricated on flexible substrates where low temperature ∼200 °C solution processed a-IGZO and BPEI annealed at ∼200 °C will be utilized.

Acknowledgements

Authors acknowledge the help of Mr Abhi Mukherjee for some useful technical discussions. The financial support of the DST Science and Engineering Research Board, India (Grant number SB/S3/CE/055/2013) is acknowledged.

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