LetterStabilizing diamond surface conductivity by phenol-formaldehyde and acrylate resins
Introduction
Diamond exhibits unique properties that can be exploited in electrochemical and electronic applications including bioelectronics [1], [2], [3], [4]. The best conductivity can be achieved in monocrystalline diamonds [5]. Yet nanocrystalline diamond (NCD) films may have good enough quality for many applications and the films can be deposited on diverse substrates and on large areas. On both types of diamond, hydrogen-terminated surface can exhibit a two-dimensional surface conductivity [6] which is practical for device fabrication as it can be easily patterned by oxidation through masks. However, unlike bulk doping, the surface conductivity is strongly affected by molecular gas species presented in ambient air and is changing significantly over time [6], [7], [8]. Therefore, stabilization of the surface conductivity is crucial for device applications. Diverse organic and inorganic compounds were tested to generate and/or passivate surface conductivity, such as fullerenes [9] or calcium fluoride [10].
Here we report on achieving stabilization of NCD surface conductivity by phenol-formaldehyde and acrylate resin coatings. These resins are commonly available, inexpensive, can be applied by spin-coating, and can be easily micro-patterned by UV or e-beam lithography. These features make them suitable and compatible with common device fabrication technologies. We also show that the resin coating can increase the surface conductivity compared to the bare surfaces in ambient air.
Section snippets
Experimental details
Diamond films were deposited in nanocrystalline form on fused silica substrates (10 × 10 mm). The substrates were mechanically seeded in ultrasonic bath with a diamond powder (nominal size 5 nm) for 40 min [11].
The depositions were performed by microwave plasma chemical vapor deposition process with the following parameters: hydrogen gas flow 300 sccm, methane gas flow 3 sccm, total vacuum pressure 30 mbar, microwave power 800–1400 W. The deposition temperature was in the range 420–800 °C. The
Results and discussion
Fig. 1 illustrates that the surface conductivity of H-terminated NCD films deposited at 600 °C is generally preserved after rinsing as well as after application of resin coatings. The resin coatings also exhibit an additional effect of slight decrease or enhancement of the surface conductivity. By repeating the experiments on four different samples and on five different microscopic channels on each sample we obtained the statistical information about these effects. The MA15 decreases the
Conclusions
To conclude, the resins provide stabilization of the surface conductivity which is initially generated by exposure of H-terminated diamond surfaces to ambient air. In spite of rinsing, drying, and other processing steps, the surface conductivity of undoped H-terminated diamond is generally preserved under phenol-formaldehyde and acrylate resin coatings. The decrease or increase is within 60% of the original value in air. We propose that the change is most likely due to the field-effect of
Acknowledgments
We would like to acknowledge a kind assistance of Vlastimil Jurka with lithographic processing, Zdena Poláčková with chemical treatments, and Antonín Fejfar with software development for the data acquisition. We also gratefully acknowledge financial support by the projects KAN400100701 (GAAV), AV0Z 10100521, LC06040 (MŠMT), LC510 (MŠMT), and the Fellowship J.E. Purkyně (GAAV).
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