2014 Theses Doctoral

Large-Area Graphene Synthesized by Chemical Vapor Deposition for High-Performance, Flexible Electronics

Petrone, Nicholas Walker

Graphene is an ideal candidate for use in flexible field-effect transistors (FETs) which require both high flexibility and high operating frequencies, because it offers exceptional electronic properties (room temperature mobility in excess of 10,000 cm² Vⁱ s⁻¹ and high saturation velocity of 3-7x10⁷ cm s⁻¹) as well as outstanding mechanical performance (strain limits up to 25%). Indeed, graphene FETs (GFETs) fabricated on rigid substrates from single crystals of mechanically exfoliated graphene have demonstrated unity power gain cut-off frequencies, fmax, up to 34 GHz, even at modestly scaled channel lengths of 600 nm. However, in order to realize commercial production of graphene-based technologies, it is essential to integrate large-area graphene produced by scalable synthesis methods into device fabrication.
Chemical vapor deposition (CVD) offers a promising method to produce low-cost, large-area films of graphene, crucial for the commercial realization of graphene-based technologies. However, the electronic performance of CVD-grown graphene has remained problematic. Compared to exfoliated graphene, CVD graphene exhibits lower mobility, greater impurity doping, and higher asymmetry between electron and hole conduction, indicative of disorder and scattering processes that are not present in exfoliated samples. In order to achieve commercial scalability of high-performance graphene-based technologies, it is prerequisite to minimize disorder present in CVD graphene and achieve equivalent electronic properties to exfoliated graphene.
In this work, I present a detailed study of the electronic transport behavior of CVD graphene in which the predominant sources of intrinsic disorder, grain-boundary scattering, is eliminated and extrinsic disorder, transfer induced contamination and substrate-induced scattering, are minimized. Grain boundaries within fabricated devices are eliminated by varying the CVD synthesis conditions to yield CVD graphene with large grain sizes, up to 250 μm in dimension. Process-related contamination is minimized by employing a novel dry-transfer technique that greatly reduces the extrinsic doping in CVD graphene devices, and samples are transferred onto hexagonal boron nitride (hBN), a dielectric which minimizes substrate-induced scattering and permits for the most precise assessment of the intrinsic performance of graphene. By minimizing the presence of these three predominant sources of disorder in CVD graphene, measurements presented in this work are the first demonstration that large-area graphene can not only be synthesized but also transferred onto arbitrary substrates while reproducibly achieving electrical performance comparable to that of high-quality exfoliated graphene. Related research demonstrates that the CVD graphene synthesized in this work additionally demonstrates equivalent mechanical properties to exfoliated graphene.
After demonstrating that CVD graphene films can achieve both exceptional electronic and mechanical properties, the synthesis and transfer methods developed are subsequently applied to the fabrication of high-performance, flexible, radio-frequency FETs (RF-FETs), an application demanding both high-frequency operation and high mechanical flexibility. Methods to fabricate RF-FETs on flexible substrates using CVD graphene as the active channel material are presented. Devices fabricated with channel lengths of 500 nm show extrinsic values of unity current gain cut-off frequency, fT, and unity power gain cut-off frequency, fmax, up to 10.7 GHz and 3.7 GHz, respectively, and strain limits of 1.75%. By reducing the channel length to 260 nm, extrinsic values of fT and fmax increase to 23.6 GHz and 6.5 GHz, respectively, with intrinsic fmax = 28.2 GHz and strain limits of 2% attainable. Flexible graphene RF-FETs fabricated with channel lengths of 260 nm not only represent the highest values of fmax achieved in any flexible technology to date, but they also show an order of magnitude improvement in strain limit over flexible technologies demonstrating the next highest reported value of fmax.
The structure of flexible GFETs is further improved by encapsulating the graphene channel in hBN dielectric layers and by implementing a self aligned fabrication scheme. RF-FETs fabricated with channel lengths of 375 nm demonstrate extrinsic cut-off frequencies fT and fmax of 12.0 GHz and 10.6 GHz, respectively, and intrinsic fT and fmax of 29.7 GHz and 15.7 GHz, respectively. The improved extrinsic cut-off frequencies indicate that using both a self-aligned fabrication scheme and hBN encapsulation are paramount to improving RF performance in flexible GFETs.
Collectively, this work demonstrates that CVD graphene can achieve both outstanding electronic and mechanical performance and establishes CVD graphene as a competitive semiconductor technology for use in flexible RF-FETs. As such, it reveals the potential of CVD graphene as a material to enable a wide-range of flexible technologies requiring both high frequency operation and high mechanical flexibility.