A study on nanofabricated fully suspended graphene microribbons and their photophysics.

摘要

Graphene exhibits extraordinary electrical, mechanical and optical properties which have attracted tremendous attention for applications in nanoelectronics, nanophotonics and novel sensor technologies. Properties such as wavelength-independent optical absorption and high carrier motilities are of particular interest for photodetection applications. While photodetectors made from mechanically exfoliated graphene are well reported in literature, a scalable approach, such as photodetectors made from chemical vapor deposition (CVD)-grown graphene, is highly desired from a practical standpoint. However, the photophysics of CVD-graphene involves complex mechanisms arising from inherent grain boundaries and defect levels, which are not well understood. Furthermore, the fabrication and characterization of suspended CVD-graphene structures are challenging, since they require the incorporation of several unique methodologies to create high performance photodetectors.;This dissertation presents a study of CVD- graphene microribbons suspended between the metal contacts in photodetector applications. Several fabrication techniques, including larger-area CVD growth and polymer free transfer of monolayer graphene, full suspension of graphene microribbons and laser-current annealing, are utilized to obtain high-quality suspended graphene microribbons. In this study, Full suspension of CVD-graphene microribbons is found to enable four-fold improvement in photoresponse over substrate-supported microribbons, which is a significant step towards enhancing responsivity of future generation photodetectors. The photophysics of fully suspended graphene microribbons is analyzed using light-current input/output (L-I) analysis, which describes incident power dependent characteristics of photoelectric and/or photo-thermoelectric effects. From the analysis, it is found that the photoelectric effect dominates the photocurrent generation mechanism in fully suspended graphene, in contrast to the photo-thermoelectric effect dominating in substrate-supported graphene. This finding implies that, in the future, one can optimize graphene photodetectors by tailoring their design and fabrication processes. These characteristics are also promising for wafer-scale fabrication of graphene photodetectors approaching THz cut-off frequency. This dissertation addresses several fundamental questions regarding the mechanical and electro-optical properties of CVD-graphene microribbons exposed to varying electrical field, incident light intensity and temperature. Furthermore, this work presents a new understanding of individual contributions from photoelectric and photo-thermoelectric effects, as well as the built-in electric field, which is of paramount importance to future design of efficient graphene-based photodetectors.