Contact morphology and revisited photocurrent dynamics in monolayer MoS2

Eric Parzinger,Martin Hetzl,Alexander W. Holleitner,Ursula Wurstbauer

doi:10.1038/s41699-017-0042-2

Abstract

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have emerged as promising materials for electronic, optoelectronic, and valleytronic applications. Recent work suggests drastic changes of the band gap and exciton binding energies of photo-excited TMDs with ultrafast non-radiative relaxation processes effectively heating the crystal lattice. Such phenomena have not been considered in the context of optoelectronic devices yet. We resolve corresponding ultrafast photoconductance dynamics within monolayer MoS2. The data suggest that a bolometric contribution as well as a defect-related conductance dominate the overall photoconductance. We further reveal that a focused laser illumination, as is used in many standard optoelectronic measurements of MoS2, can modify and anneal the morphology of metal contacts. We show that a junction evolves with lateral built-in electric fields, although Raman spectra and photoluminescence spectra indicate no significant changes, such as a crystal phase transition. We highlight how such optimized devices can drive ultrafast electromagnetic signals in on-chip high-frequency and THz circuits.

Highlights

A lot of progress has been made in the fabrication of metal contacts with reduced contact resistance,[3,6,7,10,25,26] the corresponding impact on optoelectronic phenomena in transition metal dichalcogenides (TMDs) is not yet fully understood.[4,6]
Low resistant contacts remain still the key bottleneck for the realization of a high device performance, and they are especially interesting for optoelectronic devices to avoid depletion regions at the contacts
Our study is based on several tens of different monolayer MoS2flakes, which are micromechanically exfoliated from bulk crystals and transferred by an all-dry viscoelastic stamping technique.[36]

Summary

Introduction

Future applications of transition metal dichalcogenides (TMDs) rely on the fabrication of good and well-defined contacts.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24] a lot of progress has been made in the fabrication of metal contacts with reduced contact resistance,[3,6,7,10,25,26] the corresponding impact on optoelectronic phenomena in TMDs is not yet fully understood.[4,6] low resistant contacts remain still the key bottleneck for the realization of a high device performance, and they are especially interesting for optoelectronic devices to avoid depletion regions at the contacts. Substrate junction annealed contact junction annealed contact between jD and jlifetime, and we interpret the observed timescale of ~3 ps with the non-radiative relaxation and recombination times[33,46] and fast capture mechanisms by traps via Auger processes[47] or surface defects.[48] Consistent with this interpretation, we observe that the FWHM of I1 increases with a lower laser fluence (data not shown).[33] we can neglect a photothermoelectric current for this first contribution I1, as we do not detect it for a laser excitation below the band gap of MoS2 (cf Supplementary Fig. S4).

Results

Conclusion