"PhD Defense: Electronic and spintronic devices using two-dimensional materials"

Who: CFM Auditorium

Place: nanoGUNE seminar room, Tolosa Hiribidea 76, Donostia - San Sebastian

Date: Tuesday, 30 May 2017, 11:00

Ever since in 2004 atomically-thin two-dimensional van der Waals materials became available to the scientific community, at the reach of manual microexfoliation techniques, their implementation in novel device structures and concepts promised disruptive new applications and motivated research in a vast range of fields. Confined to the thinnest possible thickness, electrons in these materials exhibit a plethora of electronic properties, from semiconducting MoS2, to superconductor NbSe2, dielectric BN, and, jack-of-all-trades, graphene. 

In this thesis, we explore fundamental and applied aspects of chemical vapor deposition (CVD) graphene, MoS2, and WSe2 using electronic device structures that use them as transporting channel, namely field-effect transistors (FETs), Hall bars, and diodes. 

MoS2 is a n-type semiconducting 2D vdW that complements one of the weak aspects of graphene-based transistors, which is the small ratio between the maximum current output and of the minimum current output of the transistors (ON/OFF ratio). Using MoS2 we identify an electron doping constraint for performing stable magnetotransport measurements, and we investigate the origins of the strong current fluctuations of the FETs. We study the low-frequency noise (LFN) of the current output of devices made with different layer thicknesses, and use the strong light-matter interactions of MoS2 to employ photodoping techniques together with the electrostatic gating to dope the channel. By converging all these conditions, we are able to discern a mechanism behind the different types of LFN noise reported in literature for MoS2, while at the same time identifying a LFN crossover driven by photodoping from carriernumber fluctuations to Hooge-mobility fluctuations. 

With p-type semiconducting WSe2 we optimize the electron and hole transport properties of ambipolar FETs by considering BN as a top and bottom interface substrate and encapsulation layer, respectively. By doing so, we are able to address to some extent the strong hysteretic effects that adversely affect the operation of WSe2 FETs on oxide substrates, and improve the overall device performance. The versatility of CVD graphene allows us to do both applied and fundamental studies, both related to spintronics and electronics. 

The unique properties of graphene make it a core material in the search of fullelectrical approaches to generate, transport, and detect spin currents without the use of magnetic elements. Using a Hall bar shaped sample, non-local signals in graphene have been demonstrated to be associated with spin transport. In our case, we use the large area availability of CVD graphene to study non-local effects in an unlikely scenario for the transport of spins. We study the non-local signals of millimeter-sized Hall bars of CVD graphene, and by doing a systematic study as a function of device scale, from macro-to-microscale we identify a mechanism that cannot be connected with spin diffusion that also leads to large signals. By evaluating the microscopic details of the samples, and the different effects observed, we propose the counterpropagating edge states shunted by grain boundaries to drive such effects. 

In a more applied manner, we use CVD graphene for two other types of devices. First, we study the use of graphene as an electrode material for lateral and vertical fieldeffect transistors that operate using organic channels, and determine that the low density of states of graphene allows for unscreened electric fields to reach the organic layer and enable the transistor operation in the vertical geometry. 

The second applied study is the large-scale fabrication of diodes using CVD graphene. Benefiting from the ultra-thin cross-section of graphene, and using a lateral geometry, we demonstrate the reliable fabrication of lateral metal/insulator/graphene diodes using Ti and TiO2 as metal and insulating layers, respectively. The time constants determined from the direct-current analysis place the operation of the fabricated devices in the THz range. Additionally, the material combination considered enabled large current densities based on field emission processes.

Luis Hueso and Felix Casanova

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