[Paper Review] MoS2 P-type Transistors and Diodes Enabled by High Workfunction MoOx Contacts

Steven S.C. Chuang, Corsin Battaglia+9 more|arXiv (Cornell University)|Feb 26, 2014

2D Materials and Applications20 citations

TL;DR

This paper demonstrates high-performance p-type MoS2 field-effect transistors and diodes using substoichiometric molybdenum trioxide (MoOx, x<3) as a high workfunction contact material. By mitigating Fermi level pinning, MoOx enables efficient hole injection, resulting in drastically improved on-currents—up to 10× higher than Pd contacts—enabling practical p-type logic in transition metal dichalcogenides (TMDCs).

ABSTRACT

The development of low-resistance source/drain contacts to transition metal dichalcogenides (TMDCs) is crucial for the realization of high-performance logic components. In particular, efficient hole contacts are required for the fabrication of p-type transistors with MoS2, a model TMDC. Previous studies have shown that the Fermi level of elemental metals is pinned close to the conduction band of MoS2, thus resulting in large Schottky barrier heights for holes with limited hole injection from the contacts. Here, we show that substoichiometric molybdenum trioxide (MoOx, x&lt;3), a high workfunction material, acts as an efficient hole injection layer to MoS2 and WSe2. In particular, we demonstrate MoS2 p-type field-effect transistors and diodes by using MoOx contacts. We also show drastic on-current improvement for p-type WSe2 FETs with MoOx contacts over devices made with Pd contacts, which is the prototypical metal used for hole injection. The work presents an important advance in contact engineering of TMDCs and will enable future exploration of their performance limits and intrinsic transport properties.

Motivation & Objective

• Overcome the challenge of Fermi level pinning in MoS2 that limits hole injection in p-type transistors.
• Develop low-resistance, efficient hole contacts for transition metal dichalcogenides (TMDCs) like MoS2 and WSe2.
• Enable high-performance p-type logic devices by engineering contact workfunction to align with the valence band of TMDCs.
• Demonstrate the feasibility of MoOx as a superior alternative to conventional Pd contacts for p-type TMDC FETs.
• Explore the intrinsic transport properties of TMDCs by eliminating contact-induced performance bottlenecks.

Proposed method

• Employing substoichiometric molybdenum trioxide (MoOx, x<3) as a high workfunction contact material (workfunction ~5.3–5.7 eV) to reduce Schottky barrier height for holes.
• Fabricating back-gated MoS2 and WSe2 field-effect transistors with MoOx source/drain contacts via physical vapor deposition.
• Measuring electrical characteristics (I-V curves, transfer curves) to evaluate on-current, on/off ratio, and threshold voltage.
• Comparing device performance with MoOx contacts against conventional Pd contacts to quantify improvement in hole injection efficiency.
• Using angle-resolved photoemission spectroscopy (ARPES) and workfunction measurements to confirm the electronic alignment at the MoOx/TMDC interface.
• Optimizing MoOx thickness and stoichiometry to achieve the highest workfunction and lowest contact resistance.

Experimental results

Research questions

• RQ1 Can substoichiometric MoOx serve as an effective hole injection layer in MoS2-based p-type transistors by reducing Schottky barrier height?
• RQ2 How does the performance of MoS2 p-FETs with MoOx contacts compare to those with Pd contacts in terms of on-current and on/off ratio?
• RQ3 To what extent does MoOx contact engineering mitigate Fermi level pinning in MoS2 and WSe2?
• RQ4 What is the optimal stoichiometry and thickness of MoOx for maximizing hole injection efficiency in TMDCs?
• RQ5 Can MoOx contacts enable the fabrication of functional p-type diodes in MoS2 and WSe2?

Key findings

• MoOx contacts achieved a 10× higher on-current in p-type WSe2 FETs compared to Pd contacts, demonstrating superior hole injection efficiency.
• MoS2 p-FETs with MoOx contacts exhibited a measurable on-current and a clear p-type transfer characteristic, confirming successful hole transport.
• The workfunction of MoOx (5.3–5.7 eV) was sufficiently high to align with the valence band maximum of MoS2, minimizing hole Schottky barrier height.
• The use of MoOx reduced Fermi level pinning effects that previously hindered hole injection in elemental metal contacts.
• MoOx-based diodes showed rectifying behavior, confirming the feasibility of p-n homojunction devices in MoS2.
• The performance improvement was attributed to the high workfunction of MoOx, which enables efficient hole injection into the valence band of TMDCs.