sSOI quantum-well for high voltage sensitivity

    Ion-sensitive field-effect transistor with sSi/Si 0.5 Ge0.5 /sSOI quantum-well for high voltage sensitivity Jiao Wen, Qiang Liu, Chang Liu, Yize Wang, Bo Zhang, Zhongying Xue, Zengfeng Di, Jiahua Min, Wenjie Yu, Xinke Liu, Xi Wang, Qing-Tai Zhao PII: DOI: Reference:

S0167-9317(16)30349-5 doi: 10.1016/j.mee.2016.06.017 MEE 10311

To appear in: Received date: Revised date: Accepted date:

16 February 2016 25 April 2016 22 June 2016

Please cite this article as: Jiao Wen, Qiang Liu, Chang Liu, Yize Wang, Bo Zhang, Zhongying Xue, Zengfeng Di, Jiahua Min, Wenjie Yu, Xinke Liu, Xi Wang, Qing-Tai Zhao, Ion-sensitive field-effect transistor with sSi/Si0.5 Ge0.5 /sSOI quantum-well for high voltage sensitivity, (2016), doi: 10.1016/j.mee.2016.06.017

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Ion-Sensitive Field-Effect Transistor with

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sSi/Si0.5Ge0.5/sSOI Quantum-Well for High

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Voltage Sensitivity

Jiao Wen1,2, Qiang Liu1,2, Chang Liu1, Yize Wang1, Bo Zhang1, Zhongying Xue1,

State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of

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Zengfeng Di1, Jiahua Min2, Wenjie Yu1,*, Xinke Liu3,*, Xi Wang1, Qing-Tai Zhao4

Microsystem and Information Technology, CAS, Shanghai, 200050, China School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China

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College of Materials Science and Engineering, Shenzhen Key Laboratory of Special Functional

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Materials, Shenzhen University, Shenzhen, 518060, China Peter Grünberg Institute 9, JARA-FIT, Forschungszentrum Jülich, Jülich, 52425, Germany

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E-mail: [email protected] (Wenjie Yu); [email protected] (Xinke Liu)

Abstract: An ion-sensitive field-effect transistor (ISFET) with improved sensing current and voltage sensitivity is realized by using a sSi/Si0.5Ge0.5/sSOI quantum-well (QW) heterostructure and an Al2O3 dielectric layer as sensing membrane coupling with the back-gate. The voltage sensitivity is much higher than the reference SOI ISFET, typically at low drain current due to the high hole mobility confined in the SiGe QW. A high voltage sensitivity 360 mV/pH with a linearity of 99.89% was achieved for QW ISFET. The results of the planar QW ISFET show great potential for low cost, real-time monitoring of bio-chemicals due to its simplified process. Key words: SiGe, quantum-well, ion-sensitive field-effect transistor, voltage sensitivity

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ACCEPTED MANUSCRIPT 1. Introduction

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Since ion-sensitive field effect transistor (ISFET) was first proposed by Bergveld in 1970s [1], it has been attracted great interests due to its wide applications in

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environmental monitoring, biomedicine, life sciences and food security [2-6]. ISFET is

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similar to a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET), but with a liquid gate replacing the metal gate, which is compatible with existing sophisticated

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complementary MOS (CMOS) technology. The planar Si based ISFETs which based on

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standard Si CMOS technology provides even lower cost due to the simple process. Meanwhile, electrochemical biosensor based on ISFET demonstrates a lot of advantages

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such as on-chip integration, small size, fast response time, label-free detection [7, 8].

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Investigations of the sensitive membrane materials, device structures and the

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methods of measurement have been underway for decades. Similar to a conventional MOSFET, the sensing current of ISFET (ISENGSING) is proportional to the gate dielectric capacitance COX and the carrier mobility µ, ISENGSING ∝ µCOX, where the gate refers to the liquid gate with the detecting bio-chemicals. Thus, a large COX and high µ can improve the sensing current of ISFET sensors and so as to boost the signal-to-noise margin. Several high-κ dielectric membranes like Al2O3 [9], ZnO [10], TiO2 [11], ZrO2 [12], SnO2 [13], Ta2O5 and WO3 [14] were applied for pH-ISFET. Moreover, nanostructures have been used to increase the contact area of the active sensor area. [15, 16] However, the complicated process and large noise are the main challenges of nanowire ISFET sensors [17, 18]. Recently, dual-gate FETs and extended-gate FETs were also proposed to increase the voltage sensitivity [19, 20].

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ACCEPTED MANUSCRIPT The main challenge for ISFET sensors for a small signal is the signal-to-noise ratio (SNR). SiGe and Si/SiGe showed much lower noise than Si devices [21, 22]. Quantum

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well (QW) heterostructures with tensely strained Si (sSi) on compressively strained

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Si0.5Ge0.5 grown on strained silicon on insulator (sSOI) substrate provide intrinsically high hole mobility in the SiGe layer with quantum confinement effects [23]. High sensing

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current and SNR are expected in such QW sSi/SiGe ISFET sensors.

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In this work we present a sSi/Si0.5Ge0.5/sSOI (QW) ISFET for the first time and experimentally investigated the performance of QW p-ISFET with Al2O3 sensing

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membrane by applying back-gate biases and floating electrolyte. High sensitivity of QW

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2. Device fabrication

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ISFET was achieved by comparison with conventional SOI ISFETs.

The starting wafer is a biaxially strained SOI (sSOI) (100) substrate with a tensile

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strain of ε=0.8%, which contains a 12-nm-thick top sSi layer on a 145-nm-thick buried oxide layer (BOX). A 25-nm-thick Si0.5Ge0.5 layer was pseudomorphically grown on the lightly p-doped sSOI substrate with reduced pressure chemical vapor deposition (RPCVD). As the thickness of Si0.5Ge0.5 layer is below the critical thickness for strain relaxation, the SiGe channel is under a -2.1% biaxial compressive strain. A 5-nm-thick sSi cap layer was grown on the Si0.5Ge0.5 layer and the strain of this sSi cap layer corresponds to that of the sSOI. The QW ISFET device with sSi/Si0.5Ge0.5/sSOI structure is schematically shown in Fig. 1. After mesa definition, an implantation of BF2+ (10 keV/21015 cm-2) was carried out to define the source/drain (S/D) regions followed by an activation anneal at 600 °C for 1 min. The low temperature anneal was chosen in order to fully conserve the strain in

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ACCEPTED MANUSCRIPT both the SiGe channel and S/D regions, and avoid the inter-diffusion of Ge into the sSi layers [23]. Then, an Al2O3 gate dielectric layer, as a sensing membrane, was deposited

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by atomic layer deposition (ALD) with a thickness of 10 nm shortly after a standard RCA

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pre-high-κ cleaning process. After the source and drain contact window patterning by lithography, the Al2O3 layers at S/D were etched by 1% HF followed by 300nm thick Al

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deposition and a lift-off process to form the S/D contacts. Finally, the back gate metal

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contact with a 300-nm-thick Al layer was formed by e-beam evaporation after an HF dip to remove the oxides on the backside while the up-side of sensor chip was covered with

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photoresist for protection.

All the devices were annealed at 400 °C for 1 min in the forming gas (mixture of 5%

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H2 in N2) in order to improve the contacts. To eliminate the interference of the metal contacts during sensor tests, the S/D metal contacts of the ISFET were isolated by

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subsequent deposition of 150-nm-thick Si3N4 coating. The length and width of the active sensor area are 14 μm and 50 μm, respectively. As a reference, SOI ISFETs with a 30 nm top Si layer and 120 nm BOX were also fabricated with same process.

Fig. 1: Schematic of the ISFET device with sSi/SiGe QW.

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ACCEPTED MANUSCRIPT 3. Results and discussion

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SOI ISFET VDS=-0.5V

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SOI ISFET VDS=-0.1V

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Fig. 2: Transfer characteristics of a QW ISFET with a back gate. For comparison the

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transfer characteristics of a SOI ISFET is also displayed, showing higher currents and steeper subthreshold slope of the QW ISFET.

Transconductance (S)

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maximum Gm for the QW device compared to the SOI ISFET.

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VBG = -5V

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Energy Band Alligiment (eV)

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Fig.4: Simulated hole density in the QW ISFET under different applied back gate voltages (VBG). The inset shows the band energies from the top to the sSOI/BOX interface at VBG=-5V. The channel geometries are 50 μm (W) and 10 μm (L).

Fig. 2 presents the transfer characteristics (ID-VBG) curves of a QW ISFET and a fully depleted SOI substrate with 30 nm top Si ISFET by sweeping the back gate voltage VBG without a top gate. Thus both devices were characterized as conventional MOSFETs with a back gate. The sensing current of SiGe QW transistor is significantly higher than the SOI device, which is consistent with results obtained in previous paper [24]. The subthreshold slope SS for the QW ISFET is also smaller than the SOI device, corresponding to SS=0.72V/dec for SOI device and 0.45V/dec for the QW ISFET.

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ACCEPTED MANUSCRIPT However, the off-current of QW ISFET at higher positive gate voltage, which is the gate induced drain leakage (GIDL), increases exponentially with increasing VBG. GIDL occurs

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in the overlap region between the drain and the gate and is attributed to band-to-band

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tunneling. This GIDL effect is more pronounced in SiGe channel devices because of the smaller band gap of the strained SiGe. [25]. Optimized process can reduce the GIDL.

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Fig.3 shows the transconductance Gm extracted from Fig.2 at VDS=-0.5V. The maximum

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transconductance of the QW ISFET is about one order of magnitude higher than the SOI ISFET, which is caused mainly due to the high mobility of holes in the strained SiGe

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layer and the big drain-source resistance for SOI ISFET. The simulation results conducted by Sentaurus TCAD show that holes are mainly confined in the Si0.5Ge0.5 QW

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at lower back gate voltage (|VBG|<5V ) owing to the valence band offset between Si and Si0.5Ge0.5 layers, which indicates a buried channel of QW ISFET [26]. In addition, the Si

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layer between BOX and SiGe layer reduces the carrier scattering at the oxide/semiconductor interface, leading to a decreased low-frequency noise compared to the accumulation-mode SOI devices [27]. Consequently, the higher sensing currents and transconductance of the QW ISFET provide an improved SNR and a higher driving power for signal amplifier when the ISFET is integrated in an electronic circuitry.

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pH9 pH8 pH7 pH6 pH5 VDS= -0.1V

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Drain Current ID (A)

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ID=1

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Fig.6: VBG shift as a function of pH at different ID for (a) QW ISFET and (b) SOI ISFET. Both devices show very good pH sensing linearity of >99.5%.

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sSi/SiGe/sSOI SOI

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Sensitivity (mV/pH)

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Fig.7 Summary of the voltage sensitivity of QW and SOI ISFETs at different ID, showing

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higher voltage sensitivity of the QW ISFET.

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Time (s) Fig. 8: Real-time ID response of the QW ISFET sensor in a pH loop of 7-6-5 at VBG=-1V For fully depleted devices, the capacitive coupling effect between the top gate dielectric and bottom BOX layer amplifies the magnitude of Vth-shift, which helps conventional ISFET to surpass the Nernst-limit [28]. In this work, ISFETs were 10

ACCEPTED MANUSCRIPT characterized by applying the back-gate and floating electrolyte with standard pH phosphate

buffer

solutions

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The

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reservoir

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polydimethylsiloxane (PDMS) was formed around the exposed sensing area to hold pH

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buffer solutions. All the measurements were performed at room temperature. Fig. 5 shows the transfer curves at different pH values for the QW ISFET (a) and SOI ISFET

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(b), respectively. Consistent with p-type MOSFET behavior, the surface potential of the

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Al2O3 gate dielectric and was changed by an additional voltage resulting from the acidity and basicity of the pH solutions, resulting in modulation of the potential in the

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channel. At lower pH values, the hydroxyl (-OH) groups of Al2O3 surface were protonated. Accordingly, the additional negative charges on the surface increases the

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hole concentration in the channel. As the pH increases, more surface hydroxyl groups are deprotonated, the surface is gradually covered by negative charges like an additional

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top gate voltage, which in turn increases the conductance of the device. Therefore, the subthreshold voltage (Vth) of the ISFETs is modulated by varying the pH values of the detecting solutions. Fig.6 shows the VBG changes at different pH for both QW ISFET and SOI ISFET. Both devices show very good linearity of >99.5% at ID from 1µA to 4µA. The pH voltage sensitivity of the devices are summarized in Fig.7. It is clear that the QW ISFET shows higher sensitivity than the SOI ISFET, typically at small ID. At ID=1µA, a sensitivity of 231mV/pH for the QW ISFET was achieved, almost 2 times of the SOI device (122 mV/pH), indicating advantages of the QW ISFET for small signals. The sensitivity increases with the drain current. A high pH sensitivity of 360 mV/pH was obtained at ID = 4 μA with a good linearity of 99.89% for the QW ISFET, while the SOI ISFET shows a sensitivity of 329.7mV/pH.

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For a small ID value a low VBG is

ACCEPTED MANUSCRIPT needed. In this case the holes are mainly confined in the QW SiGe layer with high mobility. As the current increases with a larger VBG holes are created in both the

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strained Si and the SiGe layers as shown in Fig.3, leading to decreased hole mobility

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and thus resulting smaller sensitivity difference from the reference SOI device.

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Fig.8 presents the transient characteristics of the QW ISFET for pH values from 7 to 5 and the mole concentration of the solution is 1 mM. The ID step varies significantly

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with the pH solution applied to the QW ISFET, indicating great potential for the real-time bio-sensing applications.

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4. Conclusions

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A QW ISFET with sSi/Si0.5Ge0.5/sSOI has been experimentally fabricated and

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characterized. The QW ISFET sensor showed a much higher sensing current and transconductance compared to the SOI device due to the high hole mobility. A high

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voltage sensitivity 360mV/pH with very good linearity has been achieved for QW ISFETs with Al2O3 sensing membrane by applying back-gate biases and floating electrolyte. The voltage sensitivity for the QW ISFET is much higher than the SOI devices at smaller drain currents, showing advantages of sensing small signals by using QW ISFETs.

Acknowledgments This work was supported by National Natural Science Foundation of China (project number: 61306126, 61306127, 61106015), CAS International Collaboration and Innovation Program on High Mobility Materials Engineering.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights

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1. sSi/Si0.5Ge0.5/sSOI (QW) ISFETs were fabricated and experimentally investigated for the first time. 2. A high voltage sensitivity 360 mV/pH with very good linearity has been achieved with QW ISFET. 3. Compared with SOI devices, QW ISFETs show higher voltage sensitivity of small signals at smaller drain currents.

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