Geometrical magnetoresistance effect and mobility in graphene field-effect transistors

Further development of the graphene field-effect transistors (GFETs) for high-frequency electronics requires accurate evaluation and study of the mobility of charge carriers in a specific device. Here, we demonstrate that the mobility in the GFETs can be directly characterized and studied using the geometrical magnetoresistance (gMR) effect. The method is free from the limitations of other approaches since it does not require an assumption of the constant mobility and the knowledge of the gate capacitance. Studies of a few sets of GFETs in the wide range of transverse magnetic fields indicate that the gMR effect dominates up to approximately 0.55 T. In higher fields, the physical magnetoresistance effect starts to contribute. The advantages of the gMR approach allowed us to interpret the measured dependencies of mobility on the gate voltage, i.e., carrier concentration, and identify the corresponding scattering mechanisms. In particular, the range of the fairly constant mobility is associated with the dominating Coulomb scattering. The decrease in mobility at higher carrier concentrations is associated with the contribution of the phonon scattering. Analysis shows that the gMR mobility is typically 2-3 times higher than that found via the commonly used drain resistance model. The latter underestimates the mobility since it does not take the interfacial capacitance into account.Comment: The following article has been submitted to Applied Physics Letters. After it is published, the DOI will be found her

Photoresponse of Graphene-Gated Graphene-GaSe Heterojunction Devices

Altres ajuts: This project has received funding from the European Union's Horinon 2020 research and innovation programme. This work was also partially funded by the Ministerio de Economía y Competitividad. The authors also acknowledge the funding from the Academy of Finland (Grants 276376, 284548, 295777, 304666, 312294, 312297, 312551, and 314810), TEKES-the Finnish Funding Agency for Technology and Innovation. The authors also thank Dr. Stephan Suckow in AMO GmbH for fruitful discussions about photonic device behavior.Because of their extraordinary physical properties, low-dimensional materials including graphene and gallium selenide (GaSe) are promising for future electronic and optoelectronic applications, particularly in transparent-flexible photodetectors. Currently, the photodetectors working at the near-infrared spectral range are highly indispensable in optical communications. However, the current photodetector architectures are typically complex, and it is normally difficult to control the architecture parameters. Here, we report graphene-GaSe heterojunction-based field-effect transistors with broadband photodetection from 730-1550 nm. Chemical-vapor-deposited graphene was employed as transparent gate and contact electrodes with tunable resistance, which enables effective photocurrent generation in the heterojunctions. The photoresponsivity was shown from 10 to 0.05 mA/W in the near-infrared region under the gate control. To understand behavior of the transistor, we analyzed the results via simulation performed using a model for the gate-tunable graphene-semiconductor heterojunction where possible Fermi level pinning effect is considered

MoS₂ Based High-Performance FET Devices for Memory Circuits on Flexible Platform

Two-dimensional transition metal dichalcogenides (TMDs) are fertile ground for fundamental material science and emergent applications in high-performance electronics. The mechanical flexibility of 2D materials allows their incorporation in variable form factor designs such as smart wearables and foldable electronics, where their electronic and optical properties outperform conventional flexible materials such as organic polymers. In the current work, we present high-performance ALD grown MoS2 based FET devices transferred on polyimide substrates. The growth method employs ALD to grow an MoO3 layer on a 6 inch wafer which is subsequently annealed in an H2S atmosphere to convert to MoS2. The ALD allows for excellent uniformity and because the layer acts as a template the process is scalable to larger sizes. The resulting MoS2 film is 2-3 layers thick. The film was transferred on the patterned target substrate using polystyrene assisted transfer of MoS2 layer. More than 90% device yield, excellent current ON to OFF ratio of 106 and mobility values up to 32 cm2 V-1 s-1 have been measured from micron scale devices with L/W in the range of two. We will present results on the yield and homogeneity on large area memory networks aiming for low power neuromorphic circuits