Role of GaN cap layer for reference electrode free AlGaN/GaN-based pH sensors

https://doi.org/10.1016/j.snb.2019.02.039Get rights and content

Highlights

  • We demonstrate reference electrode free AlGaN/GaN transistor-based pH sensors.

  • We show that such sensors require a GaN cap or else anion selectivity is observed.

  • Mitigation of coulombic interaction effects accounts for influence of the GaN cap.

  • These findings are critical for practical applications of AlGaN/GaN pH sensors.

Abstract

AlGaN/GaN-based pH sensors offer unique advantages of compact size, high sensitivity, and compatibility with lab-on-a-chip technologies. However, under reference electrode-free operation, researchers have reported two types of pH sensor response: linear (related to pH selectivity) and U-shaped (related to anion selectivity). To date, this distinct difference in behaviour has not been well explained and appropriate control experiments have not been conducted to elucidate the cause. In this paper, we compare the pH response of a reference electrode-free AlGaN/GaN device with and without a GaN capping layer. The results show that in the absence of a reference electrode, a linear response towards pH requires a GaN cap layer. This behaviour can be explained by the mitigation, when a GaN cap is used, of Coulombic interaction effects that occur between the electrolyte and surface states. Such interaction effects are only dominant when there is no reference electrode controlling the surface potential. There may also be secondary effects due to differences in the chemical surface chemistry of the oxides of aluminium and gallium. In the design of a reference electrode-free GaN transistor-based sensor technology, these factors must be taken into account.

Introduction

Monitoring aqueous pH is essential for a wide variety of industrial, agricultural and environmental processes and applications. The glass pH selective electrode was successfully developed in the 1960s and is still widely used [[1], [2], [3], [4], [5]]. Unfortunately, glass electrodes are fragile and not suitable for operation in harsh environments (such as high pressure, temperature and/or extreme pH). In the 1970s, Bergveld pioneered a Si-based solid-state pH sensor using a modified approach to the traditional metal-oxide-semiconductor field effect transistor (MOSFET) concept [6]. The Si-based pH sensor exhibited improved mechanical robustness compared to the glass electrode; however, chemically it is less stable and exhibits significant response drift. Furthermore, it requires a reference electrode to turn on the device (i.e. populate the transistor conduction channel with electrons) and sufficiently tune sensitivity. Although on-chip reference electrodes are used in commercially available Si-based sensors these only work for individual sensors and cannot be used for arrays, precluding any multifunctional sensor platform. Zirconia electrodes were developed later and shown to operate at elevated temperatures, pressures, and pH values [[7], [8], [9]]. However, these electrodes also require the use of an accompanying reference electrode, which ultimately limits their stability and lifetime.

More recently there has been significant investigation into the use of GaN-based materials, which exhibit excellent chemical and mechanical robustness [10], for chemical sensing [11,12]. Usually these have utilised the AlGaN/GaN heterostructure, in which spontaneous polarisation at the interface results in a high electron mobility, two-dimensional electron gas (2DEG) layer which forms the transistor conduction channel for AlGaN/GaN-based transistors. To date, AlGaN/GaN-based pH sensors with and without a reference electrode have been demonstrated by numerous research groups [[13], [14], [15], [16], [17], [18]]. Recent investigations have included practical considerations such as temperature dependence [19] and drift compensation [20]. Across these studies AlGaN/GaN transistor-like structures with and without caps have been used, with one study also investigating the role of doping of the GaN cap [19]. The role of thermally grown or other oxides such as Al2O3 or Sc2O3 has also been investigated by multiple groups and found to influence sensitivity, most likely through changes to surface state influence and to surface chemistry [14,18,[21], [22], [23]]. In most reports a reference electrode was utilised in measurements, and in those cases a linear pH response was always observed [[13], [14], [15],17,22,23]. However Abidin et al. [24] and Podolska et al. [25] obtained a U-shape response for their AlGaN/GaN sensors operated without a reference electrode, indicating anion selectivity rather than pH selectivity. The work of Podolska et al. went further and studied the device response to pH-neutral NaCl solutions to confirm anion sensitivity of the unfunctionalised devices. Anion selectivity in pH-neutral solutions had also been observed by Chaniotakis, Alifragis et al. when using a reference electrode [26,27]. The selectivity towards anions in solution even when pH was varied was suggested by Podolska et al. to be related to not using a reference electrode. However, Encabo et al. reported a linear response in their observations of a reference electrode-free GaN/AlN/GaN-based pH sensor, as did Brazzini et al. for a reference electrode-free GaN/AlInN/AlN/GaN-based pH sensor [16,28]. A schematic summarising these comparisons of the differences in device design and measurement protocol versus reported behaviour as a function of pH changes is given in Fig. 1.

With reference-electrode free operation being critical for multi-function array-based sensing platforms or for practical in situ application of individual sensors, this paper seeks to elucidate the causes for these differences in behaviour. In this study reference electrode-free GaN-capped (GaN/AlGaN/GaN) and uncapped (AlGaN/GaN) pH sensors have been fabricated and the pH response monitored carefully and critically compared. We also compared the results using ion buffered solutions (where the ionic strength is kept constant) versus non-ionic strength buffered solutions at various pH values.

Section snippets

Materials and methods

Three different (GaN)/AlGaN/GaN heterostructure samples were used in this study, all grown on a sapphire substrate by metal organic chemical vapour deposition (MOCVD). The structures grown with and without GaN cap had slightly different AlGaN layer properties in order to maintain comparable 2DEG properties [29]. All structures were grown with reduced 2DEG density (compared to high-current AlGaN/GaN transistor structures) designed to optimise the sensitivity of the source-drain current to the

AFM studies

AFM was used to assess the effects of exposure to pH 3.5–11 aqueous solution on the surface morphology of uncapped AlGaN/GaN devices and GaN-capped AlGaN/GaN devices, as shown in Fig. 3 (GaN-capped) and Fig. 4 (uncapped). Some slight differences in morphology are seen for AlGaN as opposed to GaN surface layer, as is expected [35], however roughness is not significantly different. Importantly for this study, by comparing the solution exposure images to the images in air, no discernible

Conclusions

We have demonstrated that AlGaN/GaN-based sensors operated without a reference electrode will exhibit a linear response towards pH variations when a GaN-capped heterostructure is used. Without such a cap and with no ionic strength buffering used as part of the measurement configuration, the response in acidic solutions is reversed, resulting in the U-shaped pH response curve that has been reported elsewhere. The difference in the response at low pH can be attributed to the difference in

Acknowledgements

This work was partially funded by the Australian Research Council [DP40100827]; the Office of Naval Research [N00014-08-1-0655]; and the National Science Foundation [DMR 1121053]. This work was performed in part at the Western Australian Node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and micro-fabrication facilities for Australia's researchers. FLMK was funded by a PhD scholarship

Giacinta Parish received the B.S. degree in Chemistry in 1995, and the B.E. and M.Eng.Sc degrees in electronic engineering in 1995 and 1997, respectively, all from The University of Western Australia, Perth, and the Ph.D. degree in electrical engineering in 2001, from the University of California, Santa Barbara. She joined The University of Western Australia as an Australian Postdoctoral Fellow in 2001, and is now an Associate Professor at the same institution. Her main research interests are

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    Giacinta Parish received the B.S. degree in Chemistry in 1995, and the B.E. and M.Eng.Sc degrees in electronic engineering in 1995 and 1997, respectively, all from The University of Western Australia, Perth, and the Ph.D. degree in electrical engineering in 2001, from the University of California, Santa Barbara. She joined The University of Western Australia as an Australian Postdoctoral Fellow in 2001, and is now an Associate Professor at the same institution. Her main research interests are III–V nitride materials and devices, and more recently porous silicon. Specific interests within these areas currently include development of processing technology, transport studies, and development of novel chem- and bio-sensors.

    F.L.M. Khir received the B.S. degree in Physics in 2004, and the M.Sc. degrees in Solid State Physics in 2007 respectively, all from Universiti Sains Malaysia, Malaysia, She is currently a graduate student at The University of Western Australia, Perth. Her main research interests are III–V nitride materials and devices particularly in chemical sensors.

    Radha Krishnan Nachimuthu received his Ph.D in Electrical and Electronic Engineering from the University of Western Australia (UWA) in 2016 and worked as a Research Associate in the Microelectronics Research Group (MRG). His research interests include III-V nitride materials, Magneto-optics and micro & nano- optoelectronic device characterisation, processing and fabrication.

    Jianan Wang obtained his B.Sc. in Chemistry from Nanjing University, China in 2017. He is now a PhD candidate at The University of Western Australia. His research interests are interdisciplinary and mainly include developing novel chemical sensors based on group III-N materials and studying the properties of solid-liquid interfaces experimentally.

    Haoran Li received the Bachelor degree in optical engineering from Zhejiang University, Hangzhou, China, and the Master's degree in electrical engineering from University of California, Santa Barbara, CA. She is currently a PhD candidate of electrical and computer engineering with University of California, Santa Barbara (UCSB), working under the supervision of Professor Umesh Mishra. Her research interest includes group-III nitrides epitaxy by Metal-Organic Chemical Vapor Deposition (MOCVD), as well as (Al, Ga, In)N based electronic devices.

    Gilberto A. Umana-Membreno received his Ph.D. in Electrical and Electronic Engineering from the University of Western Australia (UWA) in 2007, where he currently holds a senior research appointment with the Microelectronics Research Group (MRG). His research interests are in the general area of semiconductor materials, micro- and nano-electronic/optoelectronic device characterisation, design and optimisation, and processing and fabrication. At the MRG, he is currently leading research efforts in the development and optimisation of practical devices in the following semiconductor material technologies: HgCdTe and Type-II superlattice for infrared photodetection; AlGaN/GaN, GaNSiC for high frequency/power electronics and chemical sensors; and high-k dielectrics on graphene and SiC.

    Stacia Keller received the Diploma and Ph.D. degree in chemistry from the University of Leipzig, Germany, in 1983 and 1986, respectively. In 1994, she joined the Electrical and Computer Engineering Department, University of California at Santa Barbara. Her research interests include crystal growth and characterization of group-III nitrides, as well as nitride based electronic and opto-electronic devices. She has authored or co-authored over 500 technical publications and holds 27 US patents.

    Umesh K. Mishra received the Bachelor of Technology degree from the Indian Institute of Technology, Kanpur, India, the Master's degree from Lehigh University, Bethlehem, PA, and the Ph.D. degree from Cornell University, Ithaca, N.Y., all in electrical engineering. He is currently a Professor of electrical and computer engineering with the University of California at Santa Barbara (UCSB). A recognized leader in the area of high-speed field-effect transistors, he has made major contributions at every laboratory and academic institution for which he has worked, including North Carolina State University, Raleigh, Hughes Research Laboratories, Malibu, CA, The University of Michigan at Ann Arbor, and General Electric, Syracuse, NY. He cofounded Wi-Tech (later Nitres), which was acquired by Cree Inc., to commercialize both GaN-based laser-emitting diodes (LEDs) and transistors in 1996, and co-founded Transphorm in 2007, to commercialize GaN-based power systems. Transphorm was selected as a World Economic Forum 2013 Technology Pioneer. He has authored or coauthored around 1000 papers and holds over 80 patents in the area of compound semiconductors. Prof. Mishra is the recipient of numerous awards, including the 2007 IEEE David Sarnoff Technical Field Award and 2012 Welker Medal. In 2009, he was elected to the National Academy of Engineering in recognition of his contributions to the development of GaN electronics and other high-speed high-power semiconductor electronic devices.

    Murray Baker obtained his Ph.D. in organic chemistry with Prof. Les Field at the University of Sydney in 1988, for work on Csingle bondH activation by iron phosphine complexes. After a year in the Biologicals Group at ICI Australia's research group at Ascot Vale, Melbourne, and two years learning surface and materials chemistry with Prof. George M. Whitesides at Harvard University, he joined The University of Western Australia, where he is Professor of Chemistry. He has research interests in organometallic chemistry (especially applications of organometallic compounds in medicine and catalysis), biomaterials, and surface chemistry.

    Brett D. Nener received his B.E. degree in 1977 and Ph.D. degree in 1987 from The University of Western Australia, and the M.E.Sc. (1980) from the University of Tokyo. He is Professor and Head of Electrical, Electronic and Computer Engineering at The University of Western Australia. His current areas of research interest are Biosensors; III-N devices; Modelling of atmospheric effects such as refraction, scintillation and aerosol scattering; Electro-optic systems; Infrared and UV photodetector devices and models; and the measurement of semiconductor deep-level traps. He is a senior member of IEEE.

    Matthew Myers obtained his B.S. in Chemical Engineering from the California Institute of Technology in 2003 and his Ph.D. in organic chemistry and materials science with Prof. Colin Nuckolls at the Columbia University (New York) in 2008, for work on curved polycyclic aromatic hydrocarbons and their applications in organic electronics. He is currently interested in nanomaterials, polymers and chemical sensors.

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