416. van der Waals integrated devices based on nanomembranes of 3D materials

Fig. 1 Schematics of the fabrication flow. (a) Fabrication of Pt electrodes and Si strips. (b) Picking up process of Pt electrodes and Si strips. (c) Schematics of the back-gate β-Ga2O3 transistor. (d) Transfer process of the Pt electrodes and Si strips onto the β-Ga2O3 channel.

Fig. 2 Schottky barrier diode based on β-Ga2O3 and transferred Pt electrodes. (a) Schematics of the MESFET. (b) False-color SEM image of the MESFET. (c) Cross-sectional TEM image of the Pt/β-Ga2O3 interface. (d) Ids-Vds curves of the sameβ-Ga2O3 SB diode. (e) Richardson’s plot of ln(I0/T2) vs 1000/T. (inset) The corresponding energy alignment diagram.

Fig. 3 Characterizations of MESFET. (a) Transfer characteristics of the MESFET. The channel length and width of the transistor are 6.6 and 7 μm, respectively. The gate length is 3.5 μm. (b) Output characteristics of the MESFET.

Yang Liu, et al, Nano Lett. 20, 1410 (2020)
https://doi.org/10.1021/acs.nanolett.9b05027

(1) A physical assembly approach that explores the van der Waals (vdW) force to integrate distinct materials without direct chemical bonds offers an alternative integration strategy without lattice and processing limitations.
(2) The single-crystalline β-Ga2O3 with a monoclinic structure could be cleaved into ultrathin flakes along the (100) plane direction, similar to those exfoliation processes in layered 2D materials.
(3) The back-gate β-Ga2O3 transistor could be tuned into normally-on state via a rapid thermal annealing (RTA) process with typical on-off ratio larger than 1×109 and linear output behavior.
(4) The temperature-dependent forward I-V characteristics can be analyzed by the Richardson’s plot, that is, the (ln(I0/T2)-1/T) plot.
(5) The Si/β-Ga2O3 heterostructure exhibits typical rectification behavior with a rectification ratio of ∼107.
(6) The traditional epitaxial heterostructures usually involve strong chemical bonds (200-1000 kJmol-1) and often aggressive processing conditions, which could generate substantial interfacial disorder, and thus seriously limit the materials that can be combined. In contrast, the vdW interaction (∼10 kJmol-1) does not involve direct chemical bond or aggressive chemical processes and thus could allow flexible integration of highly disparate materials with preserved chemical and electronic properties for producing heterostructures with nearly ideal interfaces.
(7) The reason of the Ohmic contact behavior was attributed that annealing in argon at 450 °C for 1 min, led to the formation of oxygen vacancies at the Ti/β-Ga2O3 interface, thus effectively reducing the Schottky barriers at the interface and providing reliable Ohmic contacts.