Accelerated publicationExamining epitaxial graphene surface conductivity and quantum Hall device stability with Parylene passivation
Graphical abstract
Introduction
Over the last few years, graphene has become a beacon to many scientific communities, partially because of its useful electrical properties [[1], [2], [3]]. Graphene can be grown on silicon carbide (SiC) substrate by silicon sublimation, and it has been demonstrated that this form of graphene exhibits properties that make it appropriate for use in metrological applications like developing a quantized Hall resistance (QHR) standard [[4], [5], [6], [7], [8], [9], [10]]. Developing this standard would require the properties of millimeter-scale EG devices, such as the surface conductivity, carrier density, and mobility to be relatively stable over time and unaffected by changes in the environment. Although millimeter-scale EG devices have already been realized for metrology, passivation efforts for devices of this size and application have not been heavily explored [11,12]. Similar work has been reported for much smaller EG devices, but quantum Hall measurements were not monitored [13].
If left exposed to air at ambient atmospheric conditions, the carrier density and surface conductivity of the unprotected EG varies over time [[14], [15], [16], [17]]. Because electrical stability is vital to the mass production of EG for other large-scale electrical applications, finding a suitable passivation material is a high priority. This holds especially true in the field of electrical metrology, where desired passivation entails electrical properties to be stable within unit percentages over several weeks in air. Efforts have been made to understand multiple forms of epitaxial graphene passivation, with some forms involving the use of poly-methyl methacrylate (PMMA) [18], various dielectric materials available with atomic layer deposition tools [[19], [20], [21], [22]], and amorphous boron nitride [13,23]. One of the main issues with atomic layer deposition is the imparted carrier density by the various films, which span from 5 × 1012 cm−2 to 9 × 1012 cm−2. For resistance metrology, densities on order of 1012 or lower are required for epitaxial graphene devices to outperform the current standards based on gallium arsenide.
One possibility for passivation popular with organic field effect transistors and similar devices is Parylene, which shows excellent promise for stabilizing mass-producible electronics [[24], [25], [26], [27], [28], [29]]. In this work, we report on the use of Parylene C and Parylene N as encapsulating agents for EG. Their effectiveness to protect both EG quantum Hall devices and large areas of unprocessed monolayer EG is evaluated by remeasuring the electrical properties of samples after they have been exposed to various environmental conditions, such as a high temperature or high humidity. We report that Parylene C and N can passivate the surface conductivity to within 20% of its previously measured value for EG areas on millimeter scales, and these results, despite not meeting the criterion for electrical resistance metrology, may have a more fruitful impact on general device engineering that does not rely on measuring resistances to one part in 10 [8]. A general, added advantage to using Parylene is that devices can be fully packaged and wire-bonded before deposition, enabling testing before process steps.
Section snippets
High temperature EG growth
EG is formed by sublimating Si atoms from the surface of the SiC substrate's Si face during an annealing process. Samples used for this study were grown both on square and rectangular SiC chips diced from 76 mm 4H-SiC(0001) semi-insulating wafers (CREE[see notes]) which has a miscut of about 0.10°. SiC chips were then submerged in a 5:1 diluted solution of hydrofluoric acid and deionized water, making an effective concentration of <10%. Following a deionized water bath, chips were placed on top
Device transport measurements
Transport measurements of our EG devices were performed at 1.6 K in a 9 T superconducting magnet cryostat. Once the devices were fabricated, they were characterized by atomic force microscopy and Raman spectroscopy [32], and were mounted onto sample holders for magneto-transport measurements. The measurements were collected using a Labview[see notes] script to acquire I–V curves, magnetic field sweeps, and temperature data. The three main device characteristics of interest were longitudinal
Parylene C and N effects on EG devices
To evaluate the effectiveness of Parylene passivation, two sets of tests are performed. The first test involves the passivation of fabricated quantum Hall devices. Before analyzing the results, it is important to recall the relation between the mobility and carrier density of the EG devices, which is shown in Reference [32]. The general trend of the mobility is that it asymptotically reaches zero as ne goes to infinity, and the changes in mobility become subtler for higher ne. One can tune ne
Conclusion
In this work, the effectiveness of Parylene C and Parylene N as passivation layers for EG has been evaluated by two different testing methods, electronic transport of EG quantum Hall devices and the non-contact microwave perturbation measurements. The reported results showing EG electrical property passivation are significant for the mass production of millimeter-scale graphene devices with stable electrical properties.
Notes
Commercial equipment, instruments, and materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology or the United States government, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose. The authors declare no competing financial interest.
Funding sources
Work done by Y.Y. was supported by federal grant #70NANB12H185.
Acknowledgment
The work of C.-I.L and B.Y.W. at NIST was made possible by arrangement with Prof. C.-T. Liang of National Taiwan University. A.F.R and H.M.H would like to thank the National Research Council's Research Associateship Program for the opportunity.
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These authors contributed equally to this work. C.-I.L. designed the experiment. All authors have given approval to the final version of the manuscript.