Black Phosphorus Transistors with Near Band Edge Contact Schottky Barrier.

Zhi-Peng Ling,K Gopinadhan,Kah-Wee Ang,Soumya Sakar,Jun-Tao Zhu,Sinu Mathew,T Venkatesan

doi:10.1038/srep18000

Abstract

Black phosphorus (BP) is a new class of 2D material which holds promise for next generation transistor applications owing to its intrinsically superior carrier mobility properties. Among other issues, achieving good ohmic contacts with low source-drain parasitic resistance in BP field-effect transistors (FET) remains a challenge. For the first time, we report a new contact technology that employs the use of high work function nickel (Ni) and thermal anneal to produce a metal alloy that effectively reduces the contact Schottky barrier height (ΦB) in a BP FET. When annealed at 300 °C, the Ni electrode was found to react with the underlying BP crystal and resulted in the formation of nickel-phosphide (Ni2P) alloy. This serves to de-pin the metal Fermi level close to the valence band edge and realizes a record low hole ΦB of merely ~12 meV. The ΦB at the valence band has also been shown to be thickness-dependent, wherein increasing BP multi-layers results in a smaller ΦB due to bandgap energy shrinkage. The integration of hafnium-dioxide high-k gate dielectric additionally enables a significantly improved subthreshold swing (SS ~ 200 mV/dec), surpassing previously reported BP FETs with conventional SiO2 gate dielectric (SS > 1 V/dec).

Highlights

Progressing towards the international technology roadmap for semiconductors[1] demands devices with ever shrinking dimensions
Schottky barrier heights as low as ~30 meV was determined through temperature-dependent studies using a suitably low work function metal such as scandium
We present the realization of high performance black phosphorus (BP) field-effect transistors with a back-gate configuration on a ~15 nm thick BP channel

Summary

Introduction

Progressing towards the international technology roadmap for semiconductors[1] demands devices with ever shrinking dimensions. Combining the thickness-dependent tunable direct bandgap characteristics with experimentally demonstrated record high field-effect hole mobility[22] approaching 1000 cm[2] V−1 s−1 , black phosphorus is a promising candidate for both nanoelectronics and optoelectronics applications in the future For these atomically thin 2D materials to be successfully integrated into high-performance devices, the major performance-limiting factors must be identified and optimized. Unlike MoS2, the metal/BP interface is not strongly affected by the Fermi level pinning effect[31] and the choice of the contact metal can effectively tune the Schottky barrier heights at the interface This combined with the ability to switch between p-type and n-type conduction through the control of the gate and drain bias allows ambipolar operations and the potential for CMOS integrations. As part of the CMOS integration process, a thermal annealing process is typically required to improve the contact quality, but which is seldom investigated in previously reported BP FETs till date

Methods

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Conclusion