Self-Limiting Oxides on WSe₂ as Controlled Surface Acceptors and Low-Resistance Hole Contacts

Abstract

Transition metal oxides show much promise as effective p-type contacts and dopants in electronics based on transition metal dichalcogenides. Here we report that atomically thin films of under-stoichiometric tungsten oxides (WOx with x < 3) grown on tungsten diselenide (WSe2) can be used as both controlled charge transfer dopants and low-barrier contacts for p-type WSe2 transistors. Exposure of atomically thin WSe2 transistors to ozone (O3) at 100 °C results in self-limiting oxidation of the WSe2 surfaces to conducting WOx films. WOx-covered WSe2 is highly hole-doped due to surface electron transfer from the underlying WSe2 to the high electron affinity WOx. The dopant concentration can be reduced by suppressing the electron affinity of WOx by air exposure, but exposure to O3 at room temperature leads to the recovery of the electron affinity. Hence, surface transfer doping with WOx is virtually controllable. Transistors based on WSe2 covered with WOx show only p-type conductions with orders of magnitude better on-current, on/off current ratio, and carrier mobility than without WOx, suggesting that the surface WOx serves as a p-type contact with a low hole Schottky barrier. Our findings point to a simple and effective strategy for creating p-type devices based on two-dimensional transition metal dichalcogenides with controlled dopant concentrations.

WSe 2 -contact metal interface chemistry and band alignment under high vacuum and ultra high vacuum deposition conditions

“…However, generation of oxygen defects to form a-WO x (x < 3) results in a FL shift 0.20 eV towards the conduction band and corresponding appearance of additional chemical states at lower binding energy from the W 6+ oxidation state in the W 4f core level spectrum (W 5+ , W 4+ ) and higher binding energy from the intense oxide state in the O 1s core level spectrum. The high vacuum work functions of both a-WO 3 and a-WO x we report here are in agreement with previous reports of WO x serving as a p-type interlayer between channel and contact metal in WSe 2 -based devices [18]. In addition, the high vacuum work functions of a-WO 3 and a-WO x indicate that both have potential to serve as Ohmic hole contacts in FETs employing a number of different TMDs as the channel material.…”

“…Analogous doping schemes involving surface charge transfer [15,16] or substitution [17] have been developed for TMDs, although doping effects degrade in air for many of these strategies. Alternative methods have been employed recently in TMD-based devices for reduced contact resistance and WSe 2 -contact metal interface chemistry and band alignment under high vacuum and ultra high vacuum deposition conditions 2 C M Smyth et al include but are not limited to a WO x hole injection layer [18], metallic edge contacts [12,19], vacuum annealing [20], and metallic 2D top contacts [11,21]. Contact resistances from 0.2 to 2 kΩ μm have been achieved in WSe 2 -based FETs employing the strategies listed above.…”

2D Tunnel Field Effect Transistors (FETs) with a Stable Charge‐Transfer‐Type p+‐WSe2 Source

“…[26] First, based on the previous experiment, [23] the O 3 annealing condition to form WO x on the multilayer WSe 2 prepared from the exfoliation of bulk WSe 2 crystals was optimized as follows: temperature, 200 °C; annealing time, 1 h; and O 3 concentration, 2800 ppm. [24] Therefore, it has been considered that the electron affinity change of WO x by the air exposure causes the recovery from p + -WSe 2 to p-WSe 2 . The detailed experiment is provided in Figure S1 of the Supporting Information.…”

“…Both BP and SnSe 2 used as the p + -source are notably unstable in air, which deteriorates the interface. [23,24] If this charge-transfer-type p + -WSe 2 is used as the source in the 2D-2D TFET, the atomically sharp band alignment at the p + -2D/n-2D interface will be formed because the charge transfer occurs at the p + -2D/n-2D interface to cancel the energy difference in Fermi level (E F ), and the screening length determines the sharpness of the band alignment. [20] This unfavorable circumstance for source selection in 2D-2D heterostructures results from the challenging situation that substitutional doping by the conventional ion implantation technique is not suitable for 2D materials because of defect formation.…”

“…[18,19] Therefore, the significantly low SS of 31.1 mV dec −1 for four orders has only been obtained from a conventional highly doped germanium source with a MoS 2 channel by using a liquid ion gate with extremely high capacitance. [23,24] If this charge-transfer-type p + -WSe 2 is used as the source in the 2D-2D TFET, the atomically sharp band alignment at the p + -2D/n-2D interface will be formed because the charge transfer occurs at the p + -2D/n-2D interface to cancel the energy difference in Fermi level (E F ), and the screening length determines the sharpness of the band alignment. [21,22] Recently, a strong p + -WSe 2 has been demonstrated by charge-transfer doping from the self-limiting WO x surface oxide layer, which is formed by an ozone treatment.…”