Detachment of epitaxial graphene from SiC substrate by XUV laser radiation

Abstract

The thermal decomposition on silicon carbide (SiC) is one of the most used growth techniques for fabrication of epitaxial graphene. However, it significantly diminishes graphene’s otherwise exceptional carrier mobility. Reduction of the substrate influence is therefore essential for keeping conductivity at high levels. Here we present a novel technique where a sample with epitaxial graphene grown on SiC was exposed to intense 21.2 nm radiation. A sub-nanosecond pulse at low fluence in an interval 0.4–0.7 J/cm² was used to break covalent sp³ bonds between the SiC substrate and buffer (the first graphene layer) which remains, except for release of its intrinsic strain, almost unaffected. A detailed analysis of the irradiated area examined by several microscopic and spectroscopic methods such as white-light interferometry and micro-Raman spectroscopy shows a clear evidence of a graphene layer detached from the substrate. Higher fluences induce damage to SiC substrate which expands due to the amorphization process. Damage thresholds were obtained by an advanced method of ablative imprints and compared with those calculated by the hybrid code XTANT.

Introduction

Graphene, a two-dimensional planar sheet of sp² bonded carbon atoms organized into a benzene-ring structure [1], is a promising candidate for a new generation of electronic devices due to its exceptional parameters such as high thermal conductivity [2], excellent carrier mobility [3,4] and possible scalability [5,6]. Unfortunately, one of the most used and reliable techniques for growing large-scale grains, thermal decomposition on silicon carbide (SiC) [7], significantly reduces the carrier mobility. It is affected partially due to crystal imperfections and partially due to scattering of charge carriers by substrate phonons [8,9]. Elimination of the substrate is therefore essential to suppress the influence of substrate phonons and keep the carrier mobility at a very high level. Production of the so-called quasi-free-standing graphene can be done by reducing number of strong sp³ bonds between the silicon face of SiC and the first semiconducting graphene layer called buffer [10,11]. This has been recently attained in a hydrogen atmosphere [12,13], by oxygen intercalation during an annealing in air [14] or by a rapid-cooling technique, which utilizes different thermal expansion coefficients of the graphene and the SiC substrate [15]. Here we introduce an alternative method which employs an intense laser beam at 21.2 nm focused on epitaxial graphene (EG) grown on the silicon side of the SiC. Whereas J. Hicks et al. [16] observed only a partial peeling of the top graphene layers at random places after exposing the sample to a 10 keV x-ray source, our method provides a uniform decoupling of the buffer from the substrate with a high spatial resolution which is given by the size of the laser focus. This technique has therefore a great potential to be used in micro-electronics for building highly conductive paths.

An impact of intense short-wavelength radiation on carbon structures has been studied during last several years at various intensities, photon energies, and for various pulse durations. Recently, damage experiment at Free-electron laser in Hamburg (FLASH) showed that exposure of amorphous carbon to intense femtosecond pulses results in thermally activated graphitization [17]. Similar conditions were used for irradiation of diamond which also graphitized [18,19], but this anomalous solid-to-solid phase transition was non-thermal and occurred at the femtosecond time-scale. These experimental results have been verified with advanced simulations [20,21]. A similar effect has been observed in fullerene (C₆₀) crystals exhibiting very low damage threshold due to efficient material removal caused by Coulomb explosion [22]. Despite the described results, exposure of graphene to high-intensity visible laser irradiation resulted in pure ablation instead of graphitization [23]. Potential graphitization of graphene induced by short-wavelength radiation has not been described yet.

Irradiation of graphene at fluences below the damage threshold demonstrably induces breaking of the $s{p}^2$ bonds and formation of defects which can be clearly observed via Raman spectroscopy as a strong increase of the D-peak intensity [16,[23], [24], [25], [26]]. This defect formation partially originates from EUV-induced oxidation which might be enhanced by photo-ionized electrons from the valence band breaking the $s{p}^2$ bonds [27].

Here we present results of exposure of the EG on SiC to sub-nanosecond XUV pulses. At low fluences we observe detachment of the graphene layer including buffer from the substrate while keeping both of these unharmed. This is feasible due to high damage thresholds of SiC and graphene compared to relatively weak binding energy between the buffer and the substrate.