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Automated Non-Destructive Imaging and Characterization of the Graphene/hBN Moiré Pattern with Non-Contact Mode AFM


Ardavan Zandiatashbar ,* Byong Kim , Young-kook Yoo , and Keibock Lee Park Systems Inc. , 3040 Olcott St. , Santa Clara , CA 95054


* ardavan@parkafm.com Introduction


Graphene has attracted researchers’ attention because of its unique band gap structure, which allows it to be used in high-mobility semiconductor devices. However, realization of such a graphene-based high-performance device has been challenging due to lack of a suitable substrate. T is challenge has been recently addressed by the development of epitaxial growth of graphene on hexagonal boron nitride (hBN) [ 1 , 2 ]. T is material is a suitable substrate for graphene because of its similar hexagonal structure. A moiré pattern is generated as the result of a ~2% mismatch between graphene and hBN lattices and the van der Waals interaction between the two layers [ 3 ]. T e moiré superlattice has periodicity values larger than each of the two materials’ lattice constants by two orders of magnitude [ 4 ]. T e step height of moiré pattern is below one angstrom. Scanning probe microscopy (SPM) is a key technique for characterization of moiré pattern. In this characterization method, an SPM probe scans the surface of top mono-layer graphene and a three-dimensional surface topography image is generated to detect sub-angstrom surface variations due to the formation of moiré pattern. Optical and electron microscopes can provide images that have high lateral resolution but are limited in providing the required vertical resolution to detect sub-angstrom surface topography. SPM has been known to provide the highest vertical resolution (sub-angstrom) in addition to high lateral resolution [ 5 ]. T erefore it is useful for verifi cation of successful fabrication of graphene/hBN devices by epitaxial growth methods. However, SPM has been challenging for two major reasons: there is a steep learning curve for optimization of operating parameters, and the cost is high for the specialized tips required for imaging at the highest resolution. In addition, frictional mode SPM involves mechanical tip-sample engagement, making it a destructive method for characterizing the graphene/hBN devices. Currently most research performed on characterization of the moiré pattern uses destructive SPM modes [ 1 , 2 , 4 ].


Non-contact mode atomic force microscopy (AFM) is a non-destructive SPM technique available since the late 1980s [ 6 ]. In order to perform non-contact mode imaging, the tip-sample separation must be controlled accurately. T is was one of the limitations at the introduction of this technique. However, thanks to research and development, the technique has reached maturity over the past decade and is now provided as the standard AFM imaging mode by Park Systems. In this article, the need for easy, non-destructive SPM characterization of moiré patterns has been addressed by an automated non-contact mode AFM imaging technique


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developed by Park Systems. T e commercial name of the product is SmartScan™ Auto Mode. Until today, SPM charac- terization of the moiré pattern has been challenging, expensive, destructive, and time-consuming. Now with this new solution researchers can easily and reliably perform quality control of fabricated graphene-based devices.


Materials and Methods Graphene epitaxy on hBN . For this study, hBN samples were prepared by mechanical cleavage of BN crystals on a silicon substrate with a 300 nm SiO 2 epilayer. Graphene was grown by CVD epitaxial growth. Detailed information on sample preparation can be found in the reference by Gallagher et al. [ 7 ] Non-contact mode AFM imaging . Non-contact mode imaging is performed by modulating the cantilever’s oscillation amplitude at a frequency slightly higher than the resonance frequency of the cantilever in air (far from the sample surface). As the tip approaches the sample surface, tip-sample interaction enters the attractive state, where non-contact mode imaging is performed. By further decreasing tip-sample separation, the interaction switches to the repulsive state (dynamic or tapping mode). T e cantilever oscillation phase, which is representing a phase diff erence between driving signal and output signal, is negative in the attractive state and positive in the repulsive state. Note that this assignment is for AFM systems manufactured by Park Systems. T e value and sign of the phase varies depending on the AFM manufacturer.


Variation of amplitude as a function of tip-sample separation


is shown in Figure 1 and is referred to as the “A-d curve” hereaſt er [ 8 ]. As shown in Figure 1 , when the oscillating cantilever approaches the sample, oscillation amplitude decreases due to tip-sample interaction forces. However, as the tip descends, cantilever amplitude has a small sudden increase in value. T is slight jump in amplitude is associated with the changing sign of the phase from negative to positive. T is indicates that tip-sample interaction switches from the attractive to the repulsive state. In other words, the cantilever switches from non-contact mode to tapping mode. As the tip continues approaching the surface, the amplitude value approaches zero. If the cantilever is liſt ed and tip-sample separation increases, the jump from the repulsive to the attractive state occurs again, but at a greater distance from the sample surface. If imaging is performed with cantilever oscillation amplitudes larger (smaller) than the second (fi rst) jump point, imaging is performed in non-contact (tapping) mode, and tip-sample interaction is maintained in the attractive (repulsive) state. If imaging is performed with amplitude values between the fi rst and second jump, the tip-sample interaction will be unstable and switch between attractive and repulsive states, which


doi: 10.1017/S1551929515000942 www.microscopy-today.com • 2015 November


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