Intrinsic transport in 2D heterostructures mediated by hexagonal boron nitride tunneling contacts

【introduction】

Recently, transition metal dichalcogenides have entered the forefront of materials research due to their unique properties when they are spatially constrained. When these van der Waals materials are mechanically vapor deposited to achieve a single layer limit, they are mechanically stripped from the blocks, and they exhibit A phenomenon that is substantially different from its large counterpart. These include indirect to direct bandgap conversion. In addition, two-dimensional materials can be stacked or stitched together to form a van der Waals heterostructure, creating innovative devices for next-generation optoelectronics.

[Introduction]

Recently, Vinayak P. from Northwestern University The team of Dravid (corresponding author) is at Nano Lett. An article entitled Intrinsic Transport in 2D Heterostructures Mediated through h-BN Tunneling Contacts was published, using a hexagonal boron nitride (h-BN) tunneling scheme to detect the characteristics of lateral TMD heterojunctions grown by chemical vapor deposition. The electronic properties across the junction are first measured by scanning a photocurrent microscope, and then the photoelectron generation mechanism is elucidated. This work is the first to apply this packaging solution to lateral heterostructures and as a reference for future electronic material measurements. It is also a framework for more accurate evaluation of the electronic transmission characteristics of 2D heterostructures and better information for future device architectures.

[Graphic introduction]

Figure 1: MoS2 / WS2 lateral heterostructure

a: a schematic diagram of a fixed Fermi level resulting in a Schottky barrier at the interface;

b: a schematic diagram of an ohmic contact enabled by a hexagonal boron nitride tunnel layer;

c: atomic model of the lateral heterostructure of MoS2/WS2;

d: schematic diagram of a chemical vapor deposition apparatus;

e: Schematic of a packaged heterojunction device.

Figure 2: Characterization of the lateral heterostructure of MoS2 / WS2

a: Raman diagram of the lateral heterostructure;

b: Raman spectrum from a single material;

c: photoluminescence map;

d: photoluminescence spectrum of a single material

e: atomic force microscopy of the transverse heterostructure;

f: an atomic force microscope height profile on a white dotted line in part e;

g: Secondary ion mass spectrometry (SIMS) pattern of transverse heterostructure.

Figure 3: Packaging method

a: outputting the curves of WS2(a) and MoS2(d) by varying gate biases;

b: output curve on packaged WS2(b) and MoS2(e) devices with varying gate biases;

c: transmission curve across non-packaged WS2 (red curve), package WS2 (black curve), (f) across non-packaged MoS2 (red curve), package MoS2 (black curve);

Figure 4: Properties of the encapsulated node

a: depicting the effects of various biasing schemes on the energy band structure of MoS2/WS2;

b: semi-logarithmic output curve at the junction of the package;

c: Source-drain scan with and without illumination.

Figure 5: Scanning photocurrent mapping analysis

a: photocurrent generation diagram under forward bias;

b: a line scan across the horizontal line in the forward bias state;

c: photocurrent generation diagram under reverse bias;

d: photocurrent generation map without bias;

e: Line scan of horizontal dashed lines in reverse bias and unbiased state.

【summary】

The team used hexagonal boron nitride encapsulation and tunneling methods to examine the properties of the MoS2/WS2 lateral heterojunction grown by CVD. There is a strong rectification behavior using conventional metal contact geometries, but the researchers noticed that the rectification ratio of the package structure has dropped by nearly two orders of magnitude, which is more representative of 2D lateral heterostructure properties and can be used as a reference for future electronic measurements. At the same time, the photoelectric measurement of the heterostructure is realized by the optical transparency of the h-BN tunnel barrier. By scanning the photocurrent microscopy results, it is possible to examine the mechanism of photocurrent generation that causes no external variables in the lateral heterostructure. Through this work, they emphasized that the electron transport of two-dimensional materials is very susceptible to the external environment, but the photoelectric properties of the single-layer TMD and its heterostructures can be secretly extracted by means of tunnel contact.