Boron delta-doped FET on diamond for chemical sensing
Final Report Abstract
This project was aimed to investigate the role of surface functional groups and surface processing methods of boron-doped diamond on its electrochemical and electronic barrier characteristics in contact with inorganic electrolytes. Here we developed a method of surface analysis which uses a boron delta-doped ion-sensitive FET on diamond as the analytical tool, operated both as FET and as electrochemical electrode. In the previous research project the concept of boron delta-doped ISFET on diamond was proposed for pH sensing in harsh environments. This study has focused on diamond surface terminated by fluorine-, nitrogen- and oxygenrelated functional groups by novel processing techniques. A correlation has been established between the nature of chemical bonds and the induced electronic surface barrier. This observation confirms that surface bonding of different nature generate electronic states at specific energy levels in the diamond bandgap. In all cases studied here, the density of the induced surface states has been sufficient to pin the surface Fermi level, thus forming electronic barrier (surface depletion layer at the surface), and which is specific for each termination case. The case of fluorine-termination could be very promising for ISFET applications, as it demonstrated high chemical stability upon anodic polarization (the range of gate biases for channel modulation), and possibly low electronic barrier at the surface. A novel method of plasma-assisted termination was proposed, which includes in-vacuo preheating and shielding in plasma, thus avoiding contamination and plasma-induced damages of the surface. Another part of this project was targeted to answer a question of how the termination-induced electronic barrier at the surface of diamond electrodes may affect the kinetics of redox reaction in aqueous electrolytes. Based on the results of electrochemical impedance spectroscopy measurements, we proposed a model of electron tunneling across the surface depletion layer of highly boron-doped diamond. This barrier is in series to the electronic barrier formed by ligand charges of certain redox species on the electrode surface. Such double-barrier structure explains the high selectivity of diamond electrodes with oxygen-, fluorine- and nitrogen-terminated surfaces, and is consistent with the data on H-terminated diamond, described in the literature. Such surface barrier might be a fundamental limitation of semiconducting diamond electrodes. Based on the obtained results it was proposed to modify the crystalline structure of a surface layer in diamond towards a metal-like phase, where the surface functional groups will not be as critical as in the case of semiconducting material. The preliminary experiments demonstrated that high-temperature annealing may lead to the formation of an amorphous carbon (disordered diamond?) phase, which is chemically stable and shows catalytic activity comparable to that of Pt metal. A “technology-relevant” method to convert diamond surface into such amorphous phase could be solid-state reactions with carbide-forming metals, like Ti, W or Mo metals or alloys, followed by chemical etching the metallic and graphitic phase from the surface. What is left behind is a chemically stable carbon phase with the desired electrochemical characteristics. The proposed technique may open new possibilities for diamond-based sensors. The technical part of the project included explorative studies on (i) CVD diamond growth, targeting sub-nanometer flat surfaces, (ii) evaluation of atomic boron profiles in the deltadoped channels of FET, and (iii) evaluation of electronic defects /traps in the grown diamond buffer. In future research all these tools could be helpful to improve the electronic characteristics of boron delta-doped channels, needed for both ion sensitive FETs and for microwave power devices. Regarding the electrochemical aspects of this project, one could foresee two independent research projects which are based on the achieved results: One project might be devoted to further optimization of delta-doped ISFETs with respect to transport phenomena in the doped channel and surface barrier characteristics by fluorine termination, both targeting pH sensing in harsh environments. Another project might be aimed to (i) the detailed investigation of the conducting amorphous carbon phase on diamond found by the annealing experiments, (iii) to exploring novel methods of surface conversion, like solid state reactions with carbide-forming metals and others, and (iii) to novel electrode structures for harsh environment and biosensing applications and which explore the sensing areas by the induced amorphous carbon phase. Successful completion of these projects in the future might be a step towards the practical implementation of diamond for chemical sensing, and which is still an actual issue in spite of many efforts in the research community.
Publications
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“Characteristics of boron delta-doped diamond for electronic applications”. Diamond Related Mater. 17 (2008) 409
H. El-Hajj, A. Denisenko, A. Bergmaier, G. Dollinger, M. Kubovic, E. Kohn
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“The electronic surface barrier of boron-doped diamond by anodic oxidation”. J. Appl. Phys. 103 (2008) 014904
A. Denisenko, C. Pietzka, A. Romanyuk, H. El-Hajj, E. Kohn
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“Doped diamond electron devices”. In: “CVD Diamond for Electronic Devices and Sensors” by R.S. Sussmann (ed.), Wiley Publ. NY (2009) 571 pp
E. Kohn, A. Denisenko
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“Surface modification of single-crystal boron-doped diamond electrodes for low background current”. Diamond and Related Materials, 18 (2009) 816
C. Pietzka, A. Denisenko, L.A. Kibler, J. Scharpf, Y. Men, E. Kohn
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“Effect of surface disorder by RF- oxygen plasma on the electrical properties of thin boron-doped diamond layers”. In: Proc. of Hasselt Diamond Workshop 2010 SBDDXV Feb. 22-24, 2010 Hasselt, Belgium, Paper 5.36
J. Scharpf, A. Denisenko, C. Pietzka, E. Kohn
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“Electronic surface barrier properties of boron-doped diamond oxidized by plasma treatment”. Diamond and Related Materials, 19 (2010) 213
C. Pietzka , A. Denisenko, A. Romanyuk, P.J. Schäfer, L.A. Kibler, J. Scharpf , E. Kohn
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“Surface damages in diamond by Ar/O2 plasma and their effect on the electrical and electrochemical characteristics of borondoped layers”. J. Appl. Phys. 108 (2010) 074901
A. Denisenko, A. Romanyuk, C. Pietzka, J. Scharpf and E. Kohn
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“Surface structure and surface barrier characteristics of boron-doped diamond in electrolytes after CF4 plasma treatment in RF-barrel reactor”. Diamond and Related Materials, 19 (2010) 423
A. Denisenko, A. Romanyuk, C. Pietzka, J. Scharpf, E. Kohn
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„Lateral depletion of contact to metallic nano-particles on boron-doped diamond electrodes”. J. Electrochem. Soc. 157 (2010) H343
A. Denisenko, C. Pietzka, L. A. Kiebler, E. Kohn
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“Catalytic activity of platinum nanoparticles on highly boron-doped and 100-oriented epitaxial diamond towards HER and HOR”. Physical Chemistry Chemical Physics (2011)
T. Brülle, A. Denisenko, H. Sternschulte, U. Stimming
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“Electrochemical Response of Biomolecules on Carbon Substrates: Comparison between Oxidized HOPG and O- Terminated Boron-Doped CVD Diamond”. In: Nanotechnological Basis for Advanced Sensors (NATO Science for Peace and Security Series B) by J. Reithmaier, P. Paunovic, W. Kulisch, C. Popov (Eds.), Springer Netherlands (2011), 545 pp
C. Baier, H. Sternschulte, A. Denisenko, A. Schlichtiger, U. Stimming
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“Electronic surface barrier properties of fluorine-terminated boron-doped diamond in electrolytes”. Surf. Sci., 605 (2011) 632
A. Denisenko, A. Romanyuk, J. Scharpf, E. Kohn
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“Surface characteristics and electronic characteristics of boron-doped diamond exposed to N2 r.f.-plasma”. J. Electroanalytical Chem. 657 (2011) 164
A. Denisenko, A. Romanyuk, L.A. Kibler, E. Kohn
