Direct Atomic Layer Deposition of Ultrathin Aluminum Oxide on Monolayer MoS2 Exfoliated on Gold: The Role of the Substrate

Emanuela Schilirò,Fabrizio Roccaforte,Marco Cannas,Raffaella Lo Nigro,Salvatore E Panasci,Filippo Giannazzo,Simonpietro Agnello,Franco M Gelardi

doi:10.1002/admi.202101117

Abstract

AbstractIn this paper, the authors demonstrate the atomic layer deposition (ALD) of highly homogeneous and ultrathin (≈3.6 nm) Al2O3 films with very good insulating properties (breakdown field of ≈10–12 MV cm−1) directly onto monolayer (1L) MoS2 exfoliated on gold. Differently than in the case of 1L MoS2 supported by a common insulating substrate (Al2O3/Si), a better nucleation process of the high‐k film is observed on the 1L MoS2/Au system since the ALD early stages. Atomic force microscopy analyses show a ≈50% Al2O3 surface coverage just after 10 ALD cycles, its increase to >90% (after 40 cycles), and a uniform ≈3.6 nm film (after 80 cycles). The Al2O3 density on bilayer MoS2 is found to be significantly reduced with respect to 1L MoS2/Au, suggesting a role of screened interface charges with the metal substrate on the adsorption of ALD precursors. Finally, Raman and photoluminescence spectroscopy show a p‐type doping and tensile strain of 1L MoS2 induced by the Au substrate, providing an insight on the evolution of vibrational and optical properties after the Al2O3 deposition. The direct ALD growth of Al2O3 on large‐area 1L MoS2 induced by the Au underlayer can be of wide interest for electronic applications.

Highlights

In this paper, the authors demonstrate the atomic layer deposition (ALD)of highly homogeneous and ultrathin (≈3.6 nm) Al2O3 films with very good insulating properties directly onto monolayer (1L) MoS2 exfoliated on gold
Two higher resolution morphological images of 1L MoS2/Au and 1L MoS2/Al2O3 are reported in Figures 1e and 1f, respectively, from which similar RMS values (0.17 and 0.18 nm) to those measured on the bare substrates were obtained
After demonstrating the formation of a compact ≈3.6 nm Al2O3 insulating film on top of 1L MoS2/Au by 80 ALD cycles at 250 °C, we investigated the film nucleation and growth stages by atomic force micro scopy (AFM) analyses performed after a reduced number of ALD cycles at the same temperature

Summary

Introduction

Of highly homogeneous and ultrathin (≈3.6 nm) Al2O3 films with very good insulating properties (breakdown field of ≈10–12 MV cm−1) directly onto monolayer (1L) MoS2 exfoliated on gold. This short overview about ALD of high-k dielectrics on TMDs indicates that the seeding layers and prefunctionalization approaches explored so far present some disadvantages, while direct thermal ALD of ultrathin films would be highly desirable In this respect, the interaction of atomically thin MoS2 layers with the underlying substrate is expected to play an important www.advmatinterfaces.de role in the ALD nucleation stage, to what observed for 1L graphene residing on some specific substrates.[26,27] As an example, Dlubak et al.[26] reported an enhanced Al2O3 nucleation on CVD-grown 1L graphene residing on the native metal substrates (Cu, Ni), that was ascribed to an improved ALD-precursor adsorption due the electrostatic effect of polar traps located at graphene/metal interface.[28,29] More recently, the uniform growth of ultrathin (≈2.4 nm) Al2O3 films by direct thermal ALD (at 250 °C) on 1L epitaxial graphene on 4H-SiC(0001) has been ascribed to the beneficial effect of the carbon buffer layer at the interface with the substrate.[30] To the best of our knowledge, analogous substrate effects on the ALD nucleation onto 1L TMDs have not been reported so far. Further analysis by micro-Raman and micro-photoluminescence (PL) spectroscopy provided an insight on the substrate-related doping and strain of MoS2/Au and MoS2/Al2O3 samples, as well as on the effect of the ALD process on the vibrational and optical emission properties

Results and Discussion

Conclusion

Experimental Section

Data Availability Statement