Ultimate limit in size and performance of WSe2 vertical diodes

Abstract

Precise doping-profile engineering in van der Waals heterostructures is a key element to promote optimal device performance in various electrical and optical applications with two-dimensional layered materials. Here, we report tungsten diselenide- (WSe2) based pure vertical diodes with atomically defined p-, i- and n-channel regions. Externally modulated p- and n-doped layers are respectively formed on the bottom and the top facets of WSe2 single crystals by direct evaporations of high and low work-function metals platinum and gadolinium, thus forming atomically sharp p–i–n heterojunctions in the homogeneous WSe2 layers. As the number of layers increases, charge transport through the vertical WSe2 p–i–n heterojunctions is characterized by a series of quantum tunneling events; direct tunneling, Fowler–Nordheim tunneling, and Schottky emission tunneling. With optimally selected WSe2 thickness, our vertical heterojunctions show superb diode characteristics of an unprecedentedly high current density and low turn-on voltages while maintaining good current rectification.

Ultrafast 27 GHz cutoff frequency in vertical WSe2 Schottky diodes with extremely low contact resistance

“…The f cutoff equation below genuinely explains the above thickness-dependent results, but we also calculated f cutoff following Eq. (1) and confirmed that calculation results are almost the same as the measured value in each thickness 22,49 :…”

“…The detailed characteristics of Schottky diodes using different WSe 2 thicknesses are shown in Supplementary Fig. 5 22 . Figure 1f shows the junction capacitance-voltage (C j -V or 1/C j 2 -V) characteristics for our Schottky diode.…”

“…3d is an equivalent small signal circuit model of high frequency RF measurements on device under test (DUT) structure. Since Al 2 O 3 is deposited on WSe 2 flake rather than Pt electrode, oxideinduced capacitance (C ox ) is parallel to junction resistance (R j ) and junction capacitance (C j ) 22 . At a sufficiently high frequency, the C j dominates the R j .…”

“…The vertical transistors and diodes could be alternative device designs for 2D electronic applications. Vertically stacked layered crystals have been used in different device designs and have shown great advantages over lateral devices, such as tunneling or ultrafast charge transfer-mediated device operation 8,[17][18][19][20][21][22] . The band alignment through a combination of 2D crystals and contact engineering in these vertical devices is very crucial to achieve highperformance electron devices [23][24][25] .…”

“…Some other part of the top surface is electrically isolated by thin aluminum oxide (Al 2 O 3 ), which becomes a self-aligned parallel capacitor suitable for amplitude modulation (AM) demodulation circuits on glass substrate. More importantly, by modulating the thick 2D layered crystals in the vertical diodes with capacitive dimension, very low series resistance and capacitance (0.145 pF) are simultaneously achieved, allowing an ultrahigh cutoff frequency up to record high 27.1 GHz 22,31 . Equipped with practically facile fabrication process using thick and large-area van der Waals TMD crystals, our vertical TMD Schottky diodes exhibit high device stability and reproducibility.…”