Intrinsic Electronic Transport Properties and Carrier Densities in PtS₂ and SnSe₂: Exploration of n⁺‐Source for 2D Tunnel FETs

Abstract

Boltzmann tails can be cut off in band-to-band tunneling (BTBT). However, despite intensive research on conventional semiconductor heterojunction TFETs, carrier generation at the p-n heterointerface degrades the SS due to the difficulty in reducing the active defect levels at the heterointerface. [4][5][6][7] Here, 2D materials provide an ideal platform for TFETs because the dangling-bond-free van der Waals (vdW) heterointerface minimizes carrier generation and the short tunnel distance at the vdW interface increases the on current. [8,9] To date, n-type 2D-TFETs have been mainly investigated because degenerately doped p + -2D materials, such as Nb-doped p + -MoS 2 , [10,11] intrinsic p + -black phosphorus (BP), [12][13][14][15] and externally doped p + -WSe 2 , [16,17] are available as source materials. Degenerately doped source materials are indispensable for TFETs because their high carrier densities afford sharp SS values as well as high on current by modulating the band alignment only in the channel region, not in the source region. However, contrary to n-type 2D-TFETs, degenerately doped n-type source materials in p-type 2D-TFETs have been very limited. Indeed, only intrinsic n + -SnSe 2 is utilized for the source. [18][19][20][21][22][23] Although there are some reports on p-type 2D-TFETs with nondegenerately doped source materials, [24][25][26][27] electrostatic doping by an additional local gate electrode has been applied to the sources, which causes other problems, such as a complicated device structure and an increase in the parasitic capacitance. Recently, the substitutional doping technique during 2D growth has been intensively investigated. [28][29][30][31][32] However, controlling the dopant concentration and crystallinity is still a major challenge.Other air-stable n + -2D crystals except n + -SnSe 2 are in high demand to extend our knowledge of p-type 2D-TFETs. In general, the source material with a narrow bandgap (E G ) is favorable to TFETs in combination with the wide E G channel material because it reduces the barrier height for BTBT. [2] From this perspective, PtS 2 , one of the noble metal dichalcogenides, is a reasonable candidate because of the narrow E G for bulk PtS 2 that has been reported (0.25 eV from experiments and 0.48 eV from density functional theory (DFT) calculations). [33]

Quantitative Determination of Contradictory Bandgap Values of Bulk PdSe2 from Electrical Transport Properties

“…To gain insight into the transport mechanism, Hall measurements were conducted under a magnetic field (B) ranging from 0 to 0.9 T perpendicular to the cleavage plane (z-direction in Figure 2a). [42,53] With a drain voltage (V D ) applied in the x-direction under the magnetic field in the z-direction, a Hall voltage (V H ) was generated in the y-direction. The B-dependent component of the transverse resistance (R xy ) was obtained; this component is proportional to V H , as shown in Figure S6 (Supporting Information).…”

“…Finally, E G is quantitatively evaluated based on the W Dm -N A (N D ) diagram for various E G values, which we constructed in our previous paper. [42] N D is the donor density. The W Dm for a p-type semiconductor can be expressed as follows [57]…”

“…Finally, E G is quantitatively evaluated based on the W Dm ‐ N A ( N D ) diagram for various E G values, which we constructed in our previous paper. [ 42 ] N D is the donor density. The W Dm for a p ‐type semiconductor can be expressed as follows [ 57 ] where ε and n i are the dielectric constant of the semiconductor material and intrinsic carrier density, respectively.…”

“…In this study, apart from optical absorption measurements, various electrical transport measurements, such as Hall, four probe and dual‐gate FET measurements, were conducted for PdSe 2 to solve the contradiction in E G values for bulk PdSe 2 . Based on the maximum depletion width ( W Dm ) – acceptor density ( N A ) diagram for various E G values constructed recently, [ 42 ] the bandgap energy for bulk PdSe 2 is quantitatively estimated to be ≈0.3 eV. Although this is not a desirable result from the viewpoint of far infrared material, it helps us to correctly understand the mechanism of the photoresponse of PdSe 2 .…”

A wafer-scale two-dimensional platinum monosulfide ultrathin film via metal sulfurization for high performance photoelectronics

“…Indeed, our calculation shows validity in interpreting the formation of either PtS or PtS 2 under different growth conditions. First, the chemical vapor transport technique leads to formation of bulk crystals of PtS 2 , 35,37–39 in which the confined space of the sealed ampoule facilitates a high partial pressure of S vapor. In contrast, the open-space sintering of mixed Pt/S powders results in the synthesis of PtS crystals 40–44 in the early stage or the sulfurization of Pt metal, 20,45,46 which provided PtS powders for the XRD investigation to obtain the standard diffraction pattern data.…”