Nucleation and growth mechanisms of Al2O3 atomic layer deposition on synthetic polycrystalline MoS2

Abstract

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are of great interest for applications in nano-electronic devices. Their incorporation requires the deposition of nm-thin and continuous high-k dielectric layers on the 2D TMDs. Atomic layer deposition (ALD) of high-k dielectric layers is well established on Si surfaces: the importance of a high nucleation density for rapid layer closure is well known and the nucleation mechanisms have been thoroughly investigated. In contrast, the nucleation of ALD on 2D TMD surfaces is less well understood and a quantitative analysis of the deposition process is lacking. Therefore, in this work, we investigate the growth of AlO (using Al(CH)/HO ALD) on MoS whereby we attempt to provide a complete insight into the use of several complementary characterization techniques, including X-ray photo-electron spectroscopy, elastic recoil detection analysis, scanning electron microscopy, and time-of-flight secondary ion mass spectrometry. To reveal the inherent reactivity of MoS, we exclude the impact of surface contamination from a transfer process by direct AlO deposition on synthetic MoS layers obtained by a high temperature sulfurization process. It is shown that AlO ALD on the MoS surface is strongly inhibited at temperatures between 125°C and 300°C, with no growth occurring on MoS crystal basal planes and selective nucleation only at line defects or grain boundaries at MoS top surface. During further deposition, the as-formed AlO nano-ribbons grow in both vertical and lateral directions. Eventually, a continuous AlO film is obtained by lateral growth over the MoS crystal basal plane, with the point of layer closure determined by the grain size at the MoS top surface and the lateral growth rate. The created AlO/MoS interface consists mainly of van der Waals interactions. The nucleation is improved by contributions of reversible adsorption on the MoS basal planes by using low deposition temperature in combination with short purge times. While this results in a more two-dimensional growth, additional H and C impurities are incorporated in the AlO layers. To conclude, our growth study reveals that the inherent reactivity of the MoS basal plane for ALD is extremely low, and this confirms the need for functionalization methods of the TMD surface to enable ALD nucleation.

Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor

“…as well as nanometer scalability (to reduce their effective oxide thickness "EOT" for enhanced gate control over the MoS 2 channel). The interested reader is directed to various literature reports for further reading on these topics [317][318][319][320][321][322][323][324][325][326][327][328][329][330][331][332][333]. The focus of the following discussion is on the influence of dielectric engineering on important device parameters, such as doping, carrier mobility, ON-currents and contact resistance, and how dielectric engineering can be used to help enhance the performance of MoS 2 -based devices.…”

Controlled Layer-by-Layer Oxidation of MoTe2 via O3 Exposure

“…The hole-doping is likely caused by local oxidation of underlying single layer MoTe2 and/or hole injection from top high working function oxide film (MoOx, with x<3) [3][4][5] . However, we rule out the possibility of local oxidation by transport measurement of oxidized monolayer MoTe2.…”

“…Our findings point to a simple and effective strategy for growing homogenous surface oxide film on MoTe2, which is promising for several purposes in metal-oxidesemiconductor transistor, ranging from surface passivation to dielectric layers. 3 Thin oxide layer is critical for the development of modern semiconductor manufacturing technology, which has been widely adopted for insulating in active devices, for physical masking in patterning process, for passivation to protect function materials from contamination 1 . SiO2 or atomic-layer-deposited (ALD) Al2O3 has been widely adopted as dielectric or insulating layers in transitional metal dichalcogenides (TMDs)-based complementary metal oxide semiconductor (CMOS) devices 2,3 .…”

Nucleation mechanism during WS2 plasma enhanced atomic layer deposition on amorphous Al2O3 and sapphire substrates

“…Thus, the top surface consists of the WS2 crystal basal planes and grain boundaries. It has been observed that the crystal basal planes of 2D materials typically have a low reactivity for ALD [36][37][38] . Instead, the chemisorption of ALD precursors on 2D materials can occur at line defects and grain boundaries 38 .…”