Diamond & Related Materials 18 (2009) 884–889

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Diamond & Related Materials

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Combining diamond electrodes with GaN heterostructures for harshenvironment ISFETs

M. Dipalo a,⁎, Z. Gao a, J. Scharpf a, C. Pietzka a, M. Alomari a, F. Medjdoub a, J.-F. Carlin b, N. Grandjean b,S. Delage c, E. Kohn a

a Institute of Electron Devices and Circuits, University of Ulm, 89081 Ulm, Germanyb EPFL, CH 1015, Lausanne, Switzerlandc Alcatel-Thales III–V Laboratory, Route de Nozay, F-91460 Marcoussis, France

⁎ Corresponding author. Tel.: +49 731 5026187; fax:E-mail address: michele.dipalo@uni-ulm.de (M. Dip

0925-9635/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.diamond.2009.01.011

a b s t r a c t

a r t i c l e i n f o

Available online 21 January 2009

Keywords:

Electrochemistry and biochinertness and stability, senproposed to date, however

ElectrochemistryBoron doped NCDGaN heterostructure

further improvements are still needed to obtain functional devices that can beoperated efficiently beyond the reach of the well established silicon ISFET technology [P. Bergveld, Sensor andActuators B, Chem. 88 (2003), pp. 1].

emistry have always been ideal applications for diamond due to its chemicalsitivity and biocompatibility. Several diamond ChemFET concepts have been

In this paper we describe a novel ISFET structure in which a boron doped diamond electrochemical gateelectrode is combined and monolithically integrated with an InAlN/GaN HEMT structure. The new devicemerges the high chemical stability of diamond with the high transconductance and low pinch-off voltage ofInAlN/GaN heterostructure FETs, resulting in a highly stable ISFET with high sensitivity. First devices havebeen fabricated and electrochemically characterized, expressing high current levels, a pH sensitivity of about50 mV/pH, complete current modulationwhen operated within the electrochemical window of the electrodein the range of pH 1 to pH 13 and high stability upon pH cycling and the application of high anodicoverpotentials.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Research on diamond for electrochemical and bio-sensing applica-tions has been very active in the past years, especially towards thedevelopment of new chemically stable and biocompatible ISFETstructures, such devices being thought to be the ideal alternative toSi-based and GaN-based ISFETs for harsh environment applicationsand in vivo monitoring.

Several new ISFET concepts have already been introduced takingadvantage of the outstanding diamond properties, the most repre-sentative ones being the hydrogen terminated diamond Solution GateFET (SGFET) [1,2] and the boron δ-doped channel ISFET with oxygenterminated surface [3,4]. Solution gate FETs based on the hydrogeninduced surface conductivity have shown high pH sensitivity,however strongly dependent on the presence of other elementspartially replacing hydrogen in the surface termination [5–7].Secondly, the carbon–hydrogen groups at the diamond surface arereplaced by carbon–oxygen groups under high anodic overpotentialsor when in contact with strongly oxidizing media, shifting thethreshold voltage toward negative potentials and thus reducing thedevice performance within the diamond potential window. The

+49 731 5026155.alo).

ll rights reserved.

oxygen termination can only be replaced by hydrogen again by ahydrogen plasma treatment at high temperature; the recovery of thecharacteristics of these devices is therefore not a practical solution.

ISFETs based on oxygen terminated boron δ-doped diamond havebeen demonstrated on both single crystal [3] and nanocrystallinediamond (NCD) [4]. In this case the gate is represented by a standardoxygen terminated diamond electrochemical electrode and the pHsensitivity is given by the acid–base equilibrium at the diamond/electrolyte interface [8]. In contrast to hydrogen terminated diamondISFETs, these devices have shown high chemical stability in harshenvironment, including aggressive media and strong anodic over-potentials. However, the delta channel technology is still highlyexperimental and the single crystal boron δ-doped channel ISFETsrealized up to now show poor current modulation (and thus a lowtransconductance), in consequence resulting in poor chemicalsensitivity. The boron δ-doped ISFET with nano-crystalline diamondchannel showed higher current modulation but low current densities,the growth of suitable δ-doped layers on large area NCD buffers stillbeing a major challenge.

Therefore, the combination of a diamond gate electrode with ahigh performance FET structure on a foreign substrate in a singledevice would be an attractive solution. However, nano-crystallinediamond with high electrochemical quality needs to be grown underconditions not compatible with C MOS [9] or AlGaN/GaN [10]

Fig. 1. Micrograph of the finished ISFET device. The exposed NCD area is 200×300 µm.

885M. Dipalo et al. / Diamond & Related Materials 18 (2009) 884–889

technologies. Only ultrananocrystalline diamond (UNCD) has beendeposited successful onto CMOS circuits at temperatures as low as350 °C without degrading the MOSFET characteristics. However, largegrain boundary content and extended grain boundary network ofUNCD has resulted in severe electrochemical degradation especiallywhen operated at high anodic overpotentials (by etching of grainboundary defects).

On the other hand, InAlN/GaN HEMTs have shown the highestthermal stability of all FET structures realized to date, which has beenillustrated by operation at 1000 °C (in vacuum) [11]. Thus, the InAlN/GaN heterostructure may be the most suitable candidate for such aheterostructure ISFET device presently. Indeed, recently we havereported on the high temperature growth (up to 750 °C) of highquality NCD films onto lattice-matched InAlN/GaN [12,13], showingthat such an approach is in fact feasible.

In this work we have used these initial overgrowth experiments tocombine the inertness and robustness of the diamond electrode withthe high gain of the GaN-based FET leading to high stability andsensitivity; the resulting structure resembles that of an EGFET(Extended Gate FET [14,15]) where the diamond electrode acts asgate extension of the InAlN/GaN HEMT.

As in the case of diamond delta channel ISFETs, the InAlN/GaN FETgate input will tolerate a certain level of leakage current, which issufficient to operate the gate electrode in the amperometric regime.Therefore, both potentiometric as well as amperometric modes ofoperation are possible with this new device, allowing to monitorsustained redox reactions. Furthermore, if realized on sapphire theentire materials stack is transparent and therefore compatible withmost fluorescence techniques used in biochemical applications.

2. Device technology

The starting samples have been InAlN/GaN lattice-matchedheterostructures grown on 2 in. sapphire wafers by MOCVD. Thesewafers have been cut in 1×1 cm2 pieces for device fabrication. Theheterostructure comprised of a 2 µm GaN buffer, a 1 nm AlN spacerlayer and a 7 nm InAlN barrier, with a resulting polarization inducedsheet charge density in the range of 2×1013 cm−2 at the InAlN/GaNinterface, the thickness of the barrier layer determining the FET pinch-off voltage [16]. With this thickness, taking into account the influenceof all processing steps as discussed below, an FET pinch off voltage ofapprox. VP=−1 V (vs. SCE) is obtained.

Before diamond growth, the InAlN/GaN heterostructure substratehas been patterned by mesa etching and ungated FET structuresrealized by ohmic contact deposition. Mesa etching was performed byreactive ion etching (RIE) in Ar plasma. The ohmic contact metalliza-tion has been Ti/Al/Ni/Ta alloyed by Rapid Thermal Annealing (RTA)at 900 °C. The FET geometry was rather relaxed with a source to drainspacing of 10 µm, a gate length of 2 µm and a gate width of 50 µm. Allpatterns were realized by optical lithography.

The first step of diamond growthwas the deposition of a silicon basedinterlayer for diamond nucleation by PECVD, which was then performedby BEN in aHot Filament CVD reactor at a temperature of 680 ° C [17]. Theinterlayer thickness was approx. 150 nm and the resulting nucleationdensity approx. 2×1010 cm−2. Subsequently, a 150 nm thick intrinsic NCDlayer has been grown at 700 °C byHot Filament CVD in CH4/H2 chemistry,used for electrical back insulation of the electrode. The last diamondgrowth stepwas the growth of a highly boron dopedNCD layer by plasmaCVDwith anASTeXmicrowave plasma CVD. This layerwas also outgrownat 700 ° C in CH4/H2 chemistry with a thickness of about 400 nm. Borondoping in the range of 1020 cm−3 was obtained, using a solid boron rodinserted in the plasma [18]. The boron doped NCD surface was finallyoxidized in oxygen plasma for 2 min and in sulphuric acid with hydrogenperoxide (H2SO4:H2O, 2:1) for 10 min.

The entire NCD layer stack and the interlayer have then beenetched by a RIE plasma processing step in Ar and O2, only leaving a

300×500 µm electrode area in the proximity of the FET structure.Subsequently, a 2 µmNi/Au Schottky gate has been deposited onto theFET surface between source and drain, with the gate pad overlappingpartially onto the NCD electrode to connect the electrode surface tothe FET Schottky gate.

In a last step the ISFET device has been passivated with 5 µm thickSU8 epoxy, which is patterned with three openings; two openings forthe ohmic contacts of the FET and one for the NCD electrode. Theopening of the NCD electrode has a size of 200×300 µm and definesthe diamond area, which is in direct contact with the electrolyte. Thefinal structure is shown in Fig. 1.

3. Results and discussion

3.1. NCD characterization

Fig. 2 shows the surface of the substrate after biased enhancednucleation. As can be seen, the nucleation has been highly uniformalthough the base material of the substrate has been sapphire andthus insulating. The SEM pictures in Fig. 3A and B illustrate the NCDelectrode surface morphology after outgrowth and the layer stackcross section. Randomly oriented diamond grains are clearly obser-vable in Fig. 3A; the grain size being in the range of 200 nm. In Fig. 3Bthe bright contrast of the thin InAlN barrier layer (7 nm) can still beobserved between the GaN buffer and the silicon based interlayer,showing that there has been no decomposition of GaN during the CVDprocess at high temperature.

The NCD quality has also been investigated by UV Ramanspectroscopy. The Raman spectrum of the NCD electrode is shown inFig. 4. In the spectrum the characteristic diamond (sp3 bond) peak at1332 cm−1 and the G-band peak at 1580 cm−1 can be identified, theratio between the diamond and G-band peak amplitudes being typicalfor the grain size in question.

3.2. NCD electrochemical characterization

The diamond electrodes have been characterized in their electrodeconfiguration by short circuiting the source and drain contacts, in 0.1MH2SO4 (pH 1) and 0.1 M KOH (pH 13), using an electrochemical celldescribed in [3]; the measurement setup is sketched in Fig. 5.Voltammogramshave been obtainedmeasuring the I–V characteristicsbetween the short circuited source/drain FET contacts and theplatinum counter electrode (VSPt) with reference to a standard calomelelectrode (VE vs. SCE) (see Fig. 5). In thismode the diamond electrode/Pt counter electrode arrangement is in series to the FET gate Schottky

Fig. 4. UV Raman spectrum of boron doped NCD grown on InAlN/GaN.Fig. 2. Photograph of the nucleated InAlN/GaN heterostructure.

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diode and the potential drop across the (rectifying) Schottky diode hasalso to be taken into account for the interpretation of the results. InFig. 6 the voltammograms in pH 1 and pH 13 are shown in the linearscale for evaluation of the potential window; the graphs represent thesame potential range as usually applied in the analysis of boron dopedNCD electrodes grown on silicon [4]. The curve obtained in pH 13presents current suppression under high anodic potential. This occursdue to pinch-off of the FETchannel under these bias conditions due to apinch-off voltage of approximately VP=−1 V vs. SCE as alreadymentioned above and aswill be discussed in the next paragraph on theISFET characteristics. Thus, this phenomenon will not be observed in

Fig. 3. A: SEM picture of the surface of boron doped NCD grownon InAlN/GaN. Diamondgrains have size in the range of 200 nm. B: SEM cross section of the entire structure.

FETs with higher pinch-off voltage, where the channel is not yetpinched off under these conditions.

The voltammogram in pH 1 is depicted again in semi-logarithmicscale (Tafel plot) in Fig. 7. The background current density in thepotential window is shown in Fig. 7A; it is equal to approximately1 nA/mm2, which is also obtained for boron doped NCD grown onsilicon [4]. In Fig. 7B the voltammogram in pH 1 is depicted usingabsolute current values and is compared with the I–V characteristicsof the Ni/Au Schottky diode. As can be seen, the Schottky diodeleakage current is several orders of magnitude above the electrodecurrent even when driven into hydrolysis with hydrogen and oxygenevolution under both anodic and cathodic conditions and even takinginto account the different surface areas of the electrode and Schottkycontact gate. At +2 V (vs. SCE) anodic polarization the electrodecurrent (in electrolyte) begins to saturate at approximately 100 nA/mm2, when it starts to be limited by the conductivity of the electrolyte[3,4]. Thus, the reaction limiting effect of the gate Schottky contact iswidely negligible even in its reverse direction and the amperometricmode of operation is hence not prevented.

3.3. ISFET characteristics

When discussing the ISFET characteristics it is important to recallthe convention used for the polarity of the applied bias conditions. Thegate bias is usually measured against source as VGS, which is oppositeto the electrode potential measured against the Pt counter electrodeVSPt (VSPt=−VGS).VSPt is in fact the potential of the source contact (viathe electrode surface potential) vs. the Pt counter electrode, whereasVGS is the potential of the Pt counter electrode measured vs. the sourcepotential (via the electrode surface potential). The same relationship isvalid for the potential of the source contact vs. the standard calomelelectrode (SCE), namely VG (vs. SCE)=−VE (vs. SCE).

The devices have been characterized in their ISFET mode byapplying a drain source voltage VDS in addition to VGS, which isactually applied between the platinum electrode and the sourcecontact as sketched in Fig. 5. The output characteristics of such anISFET device are shown in Fig. 8A in H2SO4 (pH 1) electrolyte. In thiscase the pinch-off voltage is approx. VP=−1.2 V (vs. SCE) and themaximum open channel current IDSmax=40 mA/mm at VGmax=+0.8 V (vs. SCE). This current level may be compared to the level ofdiamond ISFETs with oxygen terminated electrode realized so far inthe µA/mm range [3,4]. The maximum transconductance is 72 mS/mm at the open channel current level.

From the output characteristics it can be observed that the total gatebias range is limited to 2 V. For pH 1 this is from pinch-off at VP=−1.2 Vvs. SCE to open channel at VGmax=+0.8 V (vs. SCE); while VGmax=+0.8 V vs. SCE corresponds to the onset of hydrogen evolution (VE=−0.8 V vs. SCE as shown in the voltammogram in Fig. 6), the pinch-off at

Fig. 5. Electrochemical cell setup.

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VP=−1.2 V vs. SCE occurs approximately 0.8 V before the onset ofoxygen evolution. Therefore this specific device does not show highsensitivity over the entire 3 V range of the diamond potential window inwater because of its low pinch-off voltage, which is mainly correlated tothe thin InAlN barrier thickness of 7 nm as alreadymentioned above. Thepinch-off voltage can be however be varied by choosing a different InAlNbarrier thickness. For example, a barrier of approx.10 nmwould increasethe pinch-off voltage to about −2 V vs. SCE, allowing thus ranging overthe entire diamond potential window in water.

The ISFET mode of operation may be better analyzed by thetransfer characteristics in the semi-logarithmic scale as shown inFig. 8B for pH 1 at VDS=1.5 V, which is already in the saturationregime of the FET output characteristics. Here it can be seen thatchannel currentmodulation can be observed in the usual linear chargecontrol regime as well as in the exponential sub-threshold regimedown to VG=−1.7 V (vs. SCE), where the buffer leakage level isreached. As can be seen the on/off current ratio is 105. This channelcurrent modulation by the gate even in the sub-threshold regime isenabled by the very low electron transfer current level of the electrodein the electrolyte as already indicated with Fig. 7B. The currentmodulation and thus sensitivity range stretches therefore in factacross approximately 5 orders of magnitude covering the entirediamond potential window in water.

At the forward gate bias of VGmax=+0.8 V (vs. SCE), the hydrogenevolution starts to appear at the electrode as mentioned above.

Fig. 6. Cyclic voltammetry in pH 1 and pH 13 with 20 mV/s scan rate in electrodeconfiguration and in linear scale.

Operated with the Schottky gate alone the input can be biased up toapprox. VGS=+2 V and thus the gate diode will not be shuntedthrough even during hydrogen evolution.

As a result of the current modulation in the sub-threshold regimedown to the oxygen evolution and in the linear regime up to thehydrogen evolution, the device proves to be sensitive in the cathodicas well as in the anodic regime, where Solution Gate FETs with H-terminated diamond or AlGaN electrodes cannot operate because ofoxidation or ion drift and corrosion.

Fig. 7. A: Cyclic voltammetry in pH 1 in semi-logarithmic scale (Tafel plot) with 20mV/sscan rate. B: Cyclic voltammetry in pH 1 in semi-logarithmic scale (Tafel plot) with20mV/s scan rate and comparedwith the I–V characteristics of the gate Schottky diode.

Fig. 8. A: Output characteristic in pH 1. The ISFET device shows IDMAX=40 mA/mm atVG=0.8 V vs. SCE and pinch off at VG=−1 V vs. SCE. B: Transfer characteristics in pH 1in semi-logarithmic scale.

Fig. 10. pH cycling between pH 1 and pH 13 at VDS=1.5 V and VG=0.6 V vs. SCE. Themeasurements are taken after anodic treatment at VE=1.5 V vs. SCE for 2 min in pH 13.

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In addition, in this configuration the FET gate input is metallic andthus at a constant potential, which is in contrast to the Solution GateFET with the FET gate electrode in direct contact with a solution ofmuch higher resistivity than a metal, also adding to the electrode biasresolution and thus sensitivity to redox energy levels.

Fig. 9. Transfer characteristic in pH 1 and pH 13 at VDS=1.5 V. The two curves show ashift in the pinch off voltage of approximately 600 mV.

Shown in Fig. 9 are the transfer characteristics in pH 1 and in pH13 in the linear scale at VDS=1.5 V, the curve shift (and thusthreshold or pinch-off voltage shift) between pH 1 and pH 13 beingapproximately 600 mV. The average pH input voltage sensitivity isthus ΔVP=50 mV/pH, the output current sensitivity is 3 mAmm−1/pH at a gate length of LG=2 µm. This may be compared to thecurrent sensitivity of nanocrystalline ISFETs with oxygen terminatedsurface of approximately 4 nAmm−1/pH at LG=200 µm [4] or ofInAlN/GaN ISFETs with InAlN electrode surface of approximately16 µAmm−1/pH at LG=500 µm [19]. Even in KOH electrolyte withpH 13 the anodic regime can be fully scanned. The Nernst limit of59.2 mV/pH is not yet reached in these first experimental structures,which is believed to be mainly due to parasitic effects.

The stability of the fully packaged device has been testedrepeatedly in pH 1 and pH 13 after strong anodic oxidation in pH13 at VE=+1.5 V (vs. SCE) for 2 min (total charge transfer N1016

electrons/cm2). Fig. 10 shows repeated pH cycles with the devicebiased to VDS=1.5 V and VGS=+0.6 V (vs. SCE). Each data pointwas extracted from a voltammogram and associated transfercharacteristic, the FET being exposed to the electrolyte for 10 min.As can be seen, the current levels are essentially constant and fit thetransfer characteristic in Fig. 9, thus no degradation is observed. Itmay be worth noting that under similar anodic oxidation (totalcharge transfer ~1×1016 electrons/cm2) 90% of the output currentwas lost in the case of InAlN/GaN FETs with the InAlN gate area indirect contact with the solution [19].

4. Conclusions

In this study we have developed a novel hybrid diamond/InAlNISFET structure on GaN, combining the electrochemical characteristicsof a diamond electrode with the electronic properties of a GaN-basedFET structure. In this new approach of an extended gate ISFET a highquality boron doped NCD electrode structure was grown at hightemperature directly onto a lattice-matched InAlN/GaN HEMT devicestructure. Electrochemical characterization has confirmed that thedevice could be operated reliably in the cathodic as well as anodicregimewith the gate electrode operating in the potentiometric as wellas in the amperometric mode. Thus, this is the first time that a GaN-based FET device structure could be successfully overgrown withnanocrystalline diamond of high electrochemical quality, leading tofully preserved FET characteristics.

The InAlN/GaN heterostructure allowed obtaining high channelcurrent densities in the order of 40 mA/mm and thus hightransconductance and high outputting current pH sensitivity. Theanalyzed device showed a low pinch-off voltage of approx. −1 V vs.SCE, thus the high current sensitivity of the device could not be

889M. Dipalo et al. / Diamond & Related Materials 18 (2009) 884–889

exploited in the anodic regime. The pinch-off voltage is however aparameter which can be adjusted by the InAlN barrier thickness andsurface treatment during processing; the ISFET device can be thereforeoptimized for operation in the entire diamond potential window inwater including hydrogen and oxygen evolution. Moreover thetransfer characteristic showed pH sensitivity of approximately50 mV/pH, which is comparable to conventional oxygen terminatednanocrystalline boron doped electrodes and delta-channel NCD ISFETswith oxygen terminated electrode surface [4]. The oxygen terminationhas indeed resulted in high chemical stability when operating thedevice under high anodic overpotentials.

These first structures are still highly experimental, also concerningtheir passivation and encapsulation and have not been optimized forspecific applications. However, reducing the Schottky gate length to0.25 µm, which is the standard gate length in microwave GaN basedFET structures, an open channel current up to 2.5 A/mm can beexpected and thus a further improvement in output current pHsensitivity of 2 orders of magnitude [11].

An additional important advantage of the new ISFETconcept is thatGaN-based heterostructures are amainstreammaterials technology inoptoelectronics and electronics. Therefore, it can be expected thatlarge size substrates beyond the presently used 2 in. diameter will beavailable in the future. This will enable to realize integrated structuresof high complexity comparable to those of CMOS based lab-on-chipconcepts or bio-Chips which can operate under highly toxic andcorrosive conditions like in blood.

Furthermore, the ISFET structure will generally be fabricated on asapphire substrate, since this is one of the standard substrates forGaN-based devices; the devices and arrays will hence be transparent,allowing the combination with fluorescence techniques used for invivo cell monitoring.

Acknowledgments

The authors would like to acknowledge the European DRIVE(512224) and UltraGaN (6903) projects for financial support. The first

author would like to thank Simeon Renz, Ulm University for histechnical support and help in the work.

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