Chemical and structural analysis of sub-20 nm graphene patterns generated by scanning probe lithography
Graphical abstract
Introduction
Scanning probe lithography (SPL) is being used to fabricate different nanoelectronic devices by modifying and/or manipulating 2D electronic materials [1], [2], [3]. Among SPL methods, oxidation scanning probe lithography (o-SPL) enables the direct and resist-less nanopatterning of a large variety of materials [4], [5], [6], from silicon to 2D electronic materials; from self-assembled monolayers to biomolecules. The direct and resist-less lithography offered by o-SPL has generated a variety of nanopatterns on layered materials such as graphene and transition metal dichalcogenides [7], [8], [9], [10], [11], [12]. The patterning capability has also been exploited to fabricate several electronic devices including quantum point contacts and field-effect transistors on layered materials [13], [14], [15], [16], [17], [18], [19], [20].
Oxidation SPL operates by confining laterally the local oxidation of a surface. A water bridge between the tip and the sample produces the confinement. This water bridge is induced by the application of an external voltage (Fig. 1(a)). The water bridge also provides oxyanions that drive the oxidation process in the presence of an applied voltage. The dielectric properties, the size and the geometry of the o-SPL patterns are the key features to confine the electron flow on the active sections of the graphene layer (Fig. 1(b)).
Advances in the performance of those devices require a full understanding of the structural and chemical properties of those dielectric barriers. Current knowledge on the chemical and dielectric properties of o-SPL patterns is based on experiments performed on silicon [21], [22]. Recently, micro-Raman and X-ray spectroscopy have been applied to characterize submicrometer o-SPL patterns on graphene [23], [24]. However, there are no studies on the chemical and the structural properties of those nanopatterns that have the spatial resolution relevant for the development of high-performance devices.
Here we report a combined force microscopy, high-resolution transmission electron microscopy (HRTEM), focused ion beam nanolithography and high spatial resolution electronic spectroscopy to characterize the structure and chemical composition of nanopatterns fabricated by o-SPL on epitaxial graphene. The nanopatterns characterized in this report have a lateral width in the sub-20 nm range while their height is below 4 nm.
Epitaxial graphene is being actively studied for its potential towards wafer-scale fabrication of graphene-based devices [25], [26], [27]. The patterning of graphene by optical or electron-beam lithographies requires the use of resists. Those resists might deteriorate the structural and/or electrical properties of the patterns and devices [28]. This observation supports the use of a resist-free direct lithography technique such as o-SPL to pattern epitaxial graphene. Previous results of o-SPL on epitaxial graphene have shown the potential of this technique for the patterning of graphene on SiC [29], [30], [31], although high-resolution nanopatterns have not been produced so far for this type of graphene. Moreover, a deep characterization of the processes involved in o-SPL of graphene with nanometer resolution is missing. In our work, the spectroscopy analysis shows that the patterns contain C and O which confirms the existence of a graphene oxide. The o-SPL patterns grow 1–3 nm above and below the graphene baseline. This also implies that the patterning affects the SiC underneath by the incorporation of Si into the graphene oxide. However, by tuning the oxidation parameters involved in the o-SPL process, it is possible to limit the oxidation process to the graphene layer.
The patterns have a trapezoidal shape dominated by the width of the base. We have shown that for the smallest and thinnest oxides (total thickness of about 2 nm), the shape is almost rectangular. This is important in order to define the real distance between the dielectric barriers in a quantum dot device.
Section snippets
Experimental
The o-SPL experiments were performed by operating the atomic force microscope (dimension V, Bruker, USA) in the amplitude modulation mode with a free amplitude in the 5–10 nm range and a set point amplitude/free amplitude ratio of about 0.9 [5]. The local anodic oxidation experiments were carried out by using n+-doped silicon cantilevers (NCH-W, NanoWorld, Germany) with a force constant of about 40 N/m and a resonant frequency of about 300 kHz. During the nanolithography process, the tip and
Results and discussion
Fig. 1(a) shows a scheme of the o-SPL patterning on graphene monolayer grown on a SiC substrate. Fig. 1(b) shows a pattern on graphene that confines the flow of the electrons through an 18 nm wide constriction. This type of patterns has been fabricated to study the electronic properties of graphene quantum dots [16].
To perform the structural and spectroscopic characterization of the nanopatterns we have patterned arrays of lines on epitaxial graphene (Fig. 1(c–d)). Those patterns have been
Conclusions
In short, we have shown here that using o-SPL it is possible to produce high-resolution nanopatterns on epitaxial graphene. We have performed a thorough characterization of the structural and chemical properties of sub-4 nm in height and sub-20 nm in width graphene oxide patterns created by o-SPL on epitaxial graphene.
HRTEM analysis reveals that, due to the appropriate tuning of the oxidation parameters involved in the nanopatterning, o-SPL process alters the graphene layer and 1–2 nm of the
Acknowledgements
Financial support by several projects are acknowledged: MAT2014-51982-C2-1-R, MAT2014-51982-C2-2-R and MAT2015-69725-REDT, MAT2016-76507-R from MINECO (including FEDER funding) and Aragón Regional Government. This work was also funded by the European Union FP7/2007-2013 under Grant Agreement No. 318804 (SNM).
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