Gallium Nitride-Based Chemical Sensors

Transcription

1 Gallium Nitride-Based Chemical Sensors N.Kornilios 1, N.Chaniotakis 2, G.Konstandinidis 3 1 Electr.Engin. Dept, T.E.I. of CRETE, Heraklion Crete Greece kornil@stef.teiher.gr 2 Analytical Chemistry, Dept. Of Chem., Univ. of Crete, Heraklion-Crete, Greece 3 Microelectronics Research Group IESL, FORTH, P. O. Box 1527, Heraklin-Crete, Greece Abstract The rapid progress in semiconductor materials and devices has promoted the development of the sensors industry. Large band gap semiconductors are ideal candidates for a variety of sensor applications. In particular, gallium nitride (GaN) is used as the sensing element for the development of chemical sensors. The recognition mechanism is based on the selective interaction of anions in solution with the epitaxial Ga-face polarity GaN wurtzite crystal film, grown on sapphire. The native GaN crystal is used for the development of ion sensors. Also AlGaN/GaN high electron mobility transistors can be used as sensing elements by monitoring the drain source current. The chemical resistivity of the GaN crystal favours sensors with excellent lifetime, signal stability, and reproducibility. Keywords GaN, Sensors 1. Introduction GaN based material are very promising candidates for high power and high temperature device applications but also for green and blue light emitters, due to their large band gap energy, high breakdown electric field, high thermo chemical stability and high electron saturation velocity. Large bandgap semiconductors ensure minimal problems due to unwanted optical or thermal generation of charge carriers. The strong chemical bonding between the constituent atoms gives rise to a quite favourable (and sometimes even exceptional) mechanical, thermal, and chemical stability of this class of materials. The optimised sensors require certain maturity in the deposition, processing, and customisation of the base materials, as all competitive electronic devices. The remarkable progress made in the last decade particularly in the field of wide-gap semiconductors such as, GaN, or the II VIs has opened up new possibilities not only in optoelectronics and high power/high frequency devices, but also for novel sensors and complex integrated systems featuring embedded sensor functions. For most of the materials mentioned above, advanced epitaxy or heteroepitaxy methods are now available on an industrial scale, and the control of defects and dopants has been achieved at least to a degree where serious device applications can be considered. GaN sensors elements may operate in harsh environmental conditions such as high temperature and chemically corrosive ambient. Their unique physical and chemical properties allow for their application in a variety of industrial environments. These materials are being used for the development of high power FETs and optoelectronic devices. Since these materials are used in the semiconductor components industry mainly for their bulk

2 semiconductor properties, surface studies are very limited. The interaction of the surface of the wurtzite type crystal of GaN (0001) with the different ligands is a relatively new field. Even though there are some reports indicating that this kind of materials can be used for monitoring of certain gases such as hydrogen (H 2 ), methane (CH 4 ), carbon monoxide (CO), acetylene (C 2 H 2 ) and nitrous oxide (NO 2 ) at relatively high temperatures [1], very little is known of the specific chemical interaction. In this work, we presents some results concerning the use of GaN semiconductor as an all solid-state potentiometric sensor for measuring anions in solutions as well as the transducer in cation sensitive potentiometric sensors. Moreover except the bulk material of GaN high electron mobility transistors AlGaN/GaN can be also used as sensors by observing the changes occurring in the drain source current due to different ions concentrations. The main advantage of the GaN crystal is its extremely high stability. The crystal structure cannot under any wet chemical conditions be etched or destroyed. Etching of the GaN surface can be performed to a limited degree only under photo electrochemical activation and in very corrosive environments. Based on this fact, it is expected that the potential of the GaN sensor will be very stable over time. Indeed, the signal stability of the sensor over time is excellent, since there was no significant potential drift when the sensor was immersed in electrolyte solution up to 5 days. 2. Experimental The crystals used for the sensor development consisted of heteroepitaxial Ga-polarity GaN (0001) films grown on Al 2 O 3 (0001) substrates, with a total thickness of 2 3 mm. The Ga-face GaN (0001) films were grown on Al 2 O 3 (0001) either by molecular beam epitaxy (MBE) using a radio- frequency (RF) nitrogen plasma source (RFMBE), or by a combination of RFMBE and metalorganic vapour phase epitaxy (MOVPE) [2]. In the latter case, a 0.5 mm film was grown on commercially available MOVPE GaN/Al 2 O 3 (0001) templates, using optimized growth conditions for step-flow growth. The optimized tuning of the RFMBE growth allowed for the development of a smooth GaN surface characteristic of a step-flow growth mechanism, as shown by atomic force microscopy. Pieces of the wafer, approximately 5 by 5 mm, were used for the construction of the chemical sensor. A 0.1 mm diameter copper wire was bonded with indium at the edge of the GaN. The bonding pad, the sides and the backside were electrically insulated leaving exposed to the solution only the GaN surface (Figure 1). Fig. 1: Schematic representation for the GaN-based sensor.

3 The signal is obtained via a copper wire bonded onto GaN using indium metal. The wire and indium are electrically insulated from solution. The potassium selective GaN-based sensor was constructed by casting a PVC-based liquid polymeric membrane. The potassium selective membrane constructed contained 1 wt.% valinomycin as the Kþ carrier and 0.3 wt.% potassium tetrakis (4-chlorophenyl) borate as the lipophilic anionic sites, and was used as such without any further optimization. The plasticizer used was bis (2-ethylhexyl) sebacate (DOS) while the polymer matrix was high molecular weight poly (vinyl chloride) (PVC) in 2:1 weight ratio. One hundred mg of this mixture was dissolved into 400 ul of freshly distilled THF. The evaluation of the K-selective GaN sensor was performed in the same way as above. The zero current electrochemical measurements were performed using the Keithley 6340 source meter, versus a double-junction Ag/AgCl reference electrode (90-02, Thermo Orion, USA). The impedance spectra were obtained using an Autolab PGSTAT 30 potentiostat/galvanostat equipped with a frequency response analyser (Eco Chemie, Netherlands). The ph response experiments were performed using a ph glass electrode (Model BN, Thermo Orion, USA) in 0.1 M tris (hydroxymethyl) amino-methane (TRIS) solution, adding small aliquots of HCl. [3] In alls the above cases the native GaN crystal surface selectively reacts with Lewis basic species, including thiols and anions. The sensor concepts discussed in the following are based on the modulation of the carrier density in a surface-near two-dimensional electron gas (2DEG), which is spontaneously created at the interface of AlGaN /GaN heterostructures. III-nitride semiconductors exhibit strong spontaneous and piezoelectric polarization along the 0001 axis. The polarization discontinuity at an AlGaN/GaN or InAlN/GaN 0001 heterojunction results in a net positive polarization charge at the interface. This phenomenon is compensated for by the formation of a mobile two-dimensional electron gas 2DEG at the GaN side, thus forming the conductive channel of high-electronmobility transistor HEMT devices. The 2DEG density is modulated by the potential at the surface of the channel gate of the HEMTs and thus, devices without gate metallization could directly sense charges adsorbed into the exposed gate area. The unique chemical characteristics of IIInitrides make them ideal matrices for the development of solid-state chemical and biochemical sensors. In addition, the precise control of surface potential is a very important factor for the development of stable microwave HEMT devices. These devices suffer from current instabilities a problem that can be overcome by improving the device passivation. Recent work has demonstrated the potential of AlGaN/GaN HEMT structures for the fabrication of chemical and biochemical sensors. Neuberger [4] reported on the response of electrolyte-gate HEMT [EGHEMT] devices to liquids of different polarities, such as water, isopropanol, acetone, and methanol. The drain-source current I DS versus drain-source voltage V DS characteristics exhibited a reduction in the saturation current region when the gate was exposed to the polar liquid and the maximum reduction was observed for acetone. In this work, we found its selective response to anions study the response of AlGaN/GaN EGHEMT sensors to different potassium salt concentrations in water, which confirmed the preferential anionic binding on the GaN surface. In addition, it is shown that the size of the exposed gate areas is also an important factor that drastically influences the chemical sensitivity of the sensor. These results clearly demonstrate the capability of the AlGaN/GaN HEMTs to be used as matrices for the development of novel solidstate sensors of anions in an aqueous solution, as well as that serious consideration must be placed on the optimisation of passivation processes for microwave transistors. EGHEMT sensors, as well as reference conventional metal gate HEMT devices, were fabricated from the same AlGaN/GaN HEMT structure grown by plasma-assisted molecular-beam epitaxy. A 2nm GaN cap layer covered the AlGaN barrier to secure the electrochemical characteristics of the GaN surface for the

4 EGHEMT devices. The 2DEG density was 1.2x10 13 cm 2 with a mobility of approximately 900 cm 2 V 1 s 1 at 300 K and 3000 cm 2 V 1 s 1 at 77 K, according to Hall Effect measurements on Van der Pauw specimens with alloyed In contacts. EGHEMT and reference HEMT devices with a gate width gate length Lg in the range of μm were processed on the same chip. Cl 2 -based reactive ion etching formed device mesas with a depth of 0.4μm. The source and drain ohmic contacts of the device consisted of a Ti/Al/Ni/Au multilayer annealed at 800 C. Schottky contacts of reference HEMTs were formed by Ni/Au metallization. At the end, the EGHEMT metallic contacts were encapsulated by polyimide. For the gate sensing areas, windows were opened in the polyamide film using a wet chemical solution. After dicing, chips with Lg=40μm and 80μm EGHEMT devices were bonded on a specially fabricated glass carrier with long electrical interconnection stripes. The interconnection stripes were finally encapsulated by epoxy and the so packaged EGHEMT devices could be immersed in various solutions for electrochemical response measurements.[5] The response to aqueous solutions with ph in the range of and four potassium salts KCl, KBr, KNO3, and KSCN with concentration between 10 4 and 1 mol/lt M was systematically investigated. The response to ph was measured in a 2-N-morpholinoethanesulfonic acid 0.1 M buffered solution by gradually adding amounts of 0.1 M KOH solution. The ph of the solution was monitored using a glass ph electrode and a corresponding ph meter Thermo Orion. A double-junction Ag/AgCl reference electrode Orion was always used to define the electrolytes reference potential. The EGHEMT source contact was used as the contact point for the GaN bulk. Potentiometric measurements were performed using the Keithley 6340 source meter. The drain-source current versus I DS - V DS characteristics of the HEMTs were recorded using a curve tracer Sony Tektronix. 3. Results and Discussion From the data of ph and K measurements using a GaN bulk crystal, the results have indicated that the GaN is a stable and reproducible sensing element for electrochemical sensor applications. The fast response time of the GaN crystal together with its stable potential over a prolonged period of time further supports the postulation that the observed potentiometric response is a purely surface phenomenon. However, it is important the surface to remain clean down to atomic level in order to achieve the optimum sensitivity and detection limit to the anions. The sensitivity observed is shown to be due to the direct interaction of the Lewis basic anions with the Lewis acidic surface gallium atoms, which act as the fixed sites for the reversible anion coordination. In the second case we use GaN HEMT. After each measurement, the device was rinsed with deionized DI water and then was dried by blowing nitrogen. The measurements were repeated for the same type of aqueous solution and the results were practically unchanged. Figure 2 shows the I DS -V DS characteristics of a reference conventional HEMT device with a metal gate of Lg=76μm and Wg =100μm.The optimized design, growth, and processing of the AlGaN/GaN HEMT structure has resulted in obtaining devices with unique performance for the given gate length. The transistor is characterized by low drain-source saturation voltage V DS sat of 2.5 V at V GS =0 V, and I DS sat=45ma/mm and high transconductance g m in the current saturation region g m sat of approximately 30 ms/mm. A similar V DS sat but even higher g m sat of approximately 55mS/mm were measured for the Lg=36μm HEMT devices. Low values of V DS sat are very important for EGHEMT sensor applications since they will allow EGHEMTs to operate near or within the high transconductance regime I DS saturation region at low V DS. This condition ensures high sensitivity operation with minimized power consumption and generated heat as well as elimination of any electrochemical side reactions. Initially, the response of EGHEMTs to ph changes was examined.

5 Figure 3 shows the I DS -V DS characteristics of an EGHEMT with Lg =80μm for three ph values of 3.45, 7.0, and The response of I DS to ph at V DS = 3.0 V was ma/ph. Fig. 2. I DS -V DS characteristics of a reference HEMT transistor with Lg=76μm and Wg=100μm, fabricated from the same AlGaN / GaN structure used for EGHEMTs. The V GS step is 1 V. It can be concluded that since I DS decreases with increasing ph, negatively charged species OH are being adsorbed into the active GaN 0001 surface. Alternatively, this may be explained as the outcome of an increase of the barrier height, i.e., the energy difference between the conduction band E C and the Fermi level E F, at the GaN 0001 surface. The next step was to study the effect of different concentrations C of KCl, KBr, KNO3, or KSCN salt in water. Fig. 3. I DS -V DS characteristics of an EGHEMT with Lg=80μm and Wg=100μm, measured in air black solid line and within aqueous solutions with ph=3.35. dotted line, ph=6.84. dashed-dotted line and ph=12.45 dashed line Figure 4 gives the I DS -V DS characteristics for an EGHEMT sensor with Lg=80μm immersed in different KSCN solutions. As shown in Figure 4, pure DI water has also a reduction effect on I DS and this is attributed to the selective coordination of OH ions on the GaN 0001 surface, as it was discussed previously. Also the dependence of I DS was measured at V DS =2.5 V versus the

6 logarithm of concentration log C of KCl, KBr, KNO3, or KSCN. A similar behaviour is observed for all the investigated salts. In particular, the I DS decreases monotonically with log C, exhibiting a rather linear dependence on log C. Significant deviations from the linear I DS versus log C dependence were found only for EGHEMTs with the narrowest gate of Lg=40μm. Fig. 4. Color online I DS -V DS characteristics of an EGHEMT with Lg=80μm and Wg =100μm, measured in air solid line and within pure DI water dashed line and water with diluted KSCN concentration 10 4 M... short dotted line, 10 3 M dashed-dotted line, 10 1 M dashed-dotted-dotted line and 1 M... black dotted line. The average response of EGHEMTs with Lg =80μm, in the concentration range between 10 4 M and 1 M was 159A/decade for KCl, 104A/decade for KBr, 82A/decade for KNO3, and 118A/decade for KSCN at V DS = 2.5V. In all cases, the decrease of I DS indicates the Br, or SCN on selective adsorption of the anions Cl, NO 3 the GaN surface. The increased negative charge on the surface partly depletes the channels 2DEG density and reduces I DS. A similar trend was also observed for the measured potential difference between the Ag/AgCl reference and the GaN-EGHEMT electrode, contacted through its source contact, in agreement with previous results. This potential difference will generally drop both across a space-charge region in the electrolyte and a space-charge region in the semiconductor. Thus, the measured variations of the voltage between Ag/AgCl electrode and GaN bulk versus logc cannot be directly associated with the potential differences between the EGHEMT s gate GaN surface and channel GaN bulk. These measurements can be used, however, for a qualitative understanding of the response of the AlGaN/GaN EGHEMTs to anions. The EGHEMT sensors exhibited significantly higher changes of I DS versus ph compared to the changes of I DS versus logc of potassium salts. A complete explanation of this is not clear at this moment and more work is needed to understand the influence of different electrolyte solutions on the transistors current. A possible explanation is that the barrier height at the GaN surface is determined by the ph, and thus the electric field is actually induced within the semiconductor. Therefore, the addition of salts may only modify the charge on the surface and slightly change the potential drop within the semiconductor. 4. Conclusions Gallium nitride crystal, grown on sapphire substrate, was demonstrated as an efficient sensing element for the development of anion-sensitive potentiometer sensors. It was shown that the

7 gallium atoms of the outer surface of the GaN (0001) crystal coordinate selectively and reversibly with anions in solution. The observed ph response is based on the interaction of the electron deficient Ga, while the increased response upon oxidative treatment indicates that there is a mixed response of the surface to both hydroxide and hydrogen ions. Based on the potentiometric response of the GaN surface to anions with drastically different lipophilicities, it is concluded that the induced potential generated is due to the specific interaction of the basic anionic ligands with the acidic gallium atoms of the GaN surface. The fast response time and the stability and reproducibility of the signal suggest that the sensor elements act according to the fixed site model that governs the solid-state membranes. The semiconducting characteristic of the GaN allows for the use of this sensor element to be used also as a transducer for the formation of a composite liquid membrane sensor, with sensitivity that depends on the composition of the ion sensitive membrane placed onto the GaN transducer. The novel characteristics of the GaN sensor element can find a wide range of application for the development of nanosensors, CHEMFETs and BIOFETs. Finally the presented results reveal a clear response of bulk GaN and AlGaN/GaN EGHEMT devices to anions in an aqueous solution. HEMTs of the above material show a strong dependence of drain-source current upon exposing the gate region to various chemicals solutions. 5. Acknowledgment This work has been supported by Hellenic Ministry of Education YPEPTH. "The Project is cofunded by the European Social Fund and National Resources- EPEAEK II-ARXIMIDIS" 6. References [1] J. Schalwig, G. Muller, M. Eickhoff, O. Ambacher, M. Stutzmann, Sens.Actuators B 2002, 87, 425. [2] A.Georgakilas, Ph. Komninou in Nitride Semiconductors, Handbook on Material and Devices Wiley-VCH, Berlin 2003, ch.3 p.107. [3] N.A.Chaniotakis, Y.Alifragis, G.Konstantinidis, A.Georgakilas, Anal.Chem.Vol.76, No.18, Sept 2004 [4] R. Neuberger, G.Müller, O.Ambacher, and M.Stutzmann, Phys. Status Solidi A 185, [5] Y.Alifragis, A.Georgakilas, G. Konstantinidis, E. Iliopoulos, A. Kostopoulos, N. Chaniotakis Ap. Physics Letters,