Dr. Toma Stoica
Dr. Ciurea Lidia Magdalena
Dr. Ionel Stavarache
Dr. Adrian Slav
Dr. Ana-Maria Lepadatu
Dr. Elena Matei
Dr. Catalin Palade
Dr. Ioana Maria Avram Dascalescu
PhD Student Ovidiu Cojocaru

SUMMARY OF STAGE 1

The goal of the 2DOPTOSENS project is the manufacture of multifunctional optoelectric sensors obtained by nucleation and selective growth of 2D-MoS2 thin layers on structured SiO2/Si substrates by Physical Vapor Deposition (PVD) method.

Stage 1/2022 has as a main objective the preparation of the conditions for carrying out selective depositions of 2D-MoS₂ and for the manufacture of optoelectronic devices in the following stages, through studies on the operating parameters of the 2D-MoS₂ reactor and obtaining test samples of depositions on structured supports, as well as photolithographic mask design. This objective was achieved through the activities proposed for Stage 1 of the Implementation Plan: ● Calibration of the operating parameters of the 2D-MoS₂ reactor; ● PVD growth of 2D-MoS₂ on test substrates; ● Statistical characterization of 2D-MoS₂ growth; ● Morphological and structural characterization; ● Creation of the project web page and permanent updating.

In this Stage, experiments were carried out to find out the operating parameters of the equipment, the performances of the depositions on structured and unstructured substrates that led to the in-depth understanding of the processes involved in these growths of 2D-MoS₂ layers and the establishment of measures to improve the depositions and their characterization. First of all, the parameters and operating conditions of the installation and the deposition process were investigated and improvements were made, reducing the leakage rate by over an order of magnitude. The thermogravimetry measurements showed the stability of the MoS2 powder up to 1100^oC in the inert atmosphere, but also its oxidation at over 500^oC in the atmosphere containing oxygen, thus demonstrating the importance of reducing traces of oxygen in the working atmosphere, the exchange reaction of replacing S by O being stronger the higher the oxygen partial pressure and the higher the temperature. The temperature gradient in the oven inside the tube was calibrated in order to evaluate the temperature of the samples during deposition. The composition and crystalline quality of the MoS2 source was tested by Raman scattering measurements. A number of 10 test depositions of 2D-MoS2 were carried out, varying various parameters such as the temperatures set in the three zones of the furnace (the temperature of the MoS₂ powder set between 970^oC and 1050^oC), the duration of the deposition (30 – 720 min), the flow of Ar (36 – 144 sccm) and working pressure (10 – 27 mbar). 1x1 cm2 SiO2/Si substrates with ~300nm thick SiO₂ layer were used, either structured with SiO₂ windows obtained by chemical etching in HF, or unstructured.

The statistics on a number of 12-18 samples/deposition of the selective growths by nucleation in the SiO₂ windows, as well as at the edge of the samples, was investigated by optical microscopy. The temperature gradient achieved in the three-zone furnace ensures lower deposition temperatures the more the samples were located at greater distances from the hottest zone where the MoS₂ powder was placed. Consistent selective deposits are observed from 10 cm to 25 cm away from the source of MoS2 at temperatures below 800^oC and above 630^oC. The zone of maximum efficiency of the deposition expands and moves slightly when the flow of Ar increases and the working pressure decreases. Raman scattering measurements showed that traces of oxygen in the working atmosphere have the effect of oxidizing the MoS₂ deposits and transforming them into monoclinic MoO₂. By reducing the vacuum leaks, selective depositions of non-oxidized crystalline MoS₂ were obtained. In this stage, deposition tests were also carried out using supports structured by the deposition of metallic Mo strips, which showed the accumulation of MoS₂ deposited at the edge of these strips. In order to increase the efficiency and quality of the 2D-MoS₂ deposits, the temperature gradient and the limit vacuum will be increased in the future. For the next stages, photolithographic masks were designed for structuring the supports before the depositions and the electrodes of the devices for opoelectronic measurements.

In conclusion, the objectives and activities proposed for Stage 1/2022 were fully achieved.

SUMMARY STAGE 2

The project goal of 2DOPTOSENS project is to fabricate multifunctional optoelectronic sensors through selective growth of thin 2D-MoS₂ layers on structured Mo/SiO₂/Si substrates. The main objective of Stage 2/2023 is selective growth of 2D-MoS₂ and the fabrication of optoelectronic samples. To achieve the goals of Phase 2, the following activities were carried out as provided in the Implementation Plan: ● Fabrication of structured substrates; ● Selective growth of 2D-MoS2 on structured substrates; ● Investigations of morphology and crystalline structure of the selective 2D-MoS₂ growths; ● Fabrication and optimization of optoelectronic test samples – Part I; ● Dissemination of results and updating the project's website.

The activities in this Phase 2/2023 focused on achieving high-quality selective depositions on SiO₂/Si substrates with a structured Mo layer. The first activity involved making improvements to the deposition installation: better isolation between furnace zones, a better degassing vacuum using a turbo pump, and improved sealing. Another practical activity for achieving the phase objectives was manufacturing photolithographic masks and samples with a structured Mo layer using photolithography. Selective depositions were obtained by the PVD of 2D-MoS₂ close to Mo pads, extending up to 160 µm from Mo, with decreasing thickness down to 1-2 MLs. A critical analysis of the PVD deposition mechanism using MoS₂ powder as a precursor showed that the basic mechanism is not well established in the literature, with the PVD process possibly involving CVT (chemical vapor transport), as well as Mo-CVD, a deposition using oxidized Mo and sulfur vapors as precursors. Mo-CVD resulted in selective depositions restricted to the immediate vicinity and in contact with Mo pads. Good results were obtained using substrates structured with naturally oxidized Mo. It was found that the native oxide is sufficient to lead to selective 2D MoS₂ depositions filling the gap between Mo electrodes from 5 µm to 20 µm wide. SEM and AFM measurements showed these depositions consist of MoS₂ plates (flakes) with thicknesses of 4 nm (6 MLs) in the 5 µm gap, and only 1 – 2 MLs in gaps of 15 µm and 20 µm. The selective depositions thus obtained using substrates with Mo "finger contacts" geometry structures could be used to fabricate field-effect devices directly from the deposited samples, by contacting the large pads of the structures, serving as source-drain electrodes, and the doped Si substrate as the gate electrode. Multiple characterization tests/measurements were conducted on these devices. In these devices, the dark current dramatically reduces by nearly 5 orders of magnitude when changing the gate voltage from +7V to -7V, demonstrating the n-type semiconductor nature of the 2D MoS₂ layers. Relative photosensitivity of the order of 10⁵(%) was achieved by monochromatic illumination of 4.5x10^-4 mW/cm² at 650 nm. The spectral responsivity of 15 – 25 A/W at 600 nm wavelength showed a photosensitivity threshold at 1.72 – 1.77 eV corresponding to 2 MLs MoS₂ thickness.

The web page of the 2DOPTOSENS project has been updated: https://infim.ro/en/project/multifunctional-optoelectrical-sensor-based-on-two-dimensional-mos2-atomically-thin-layers-grown-by-selective-nucleation/.

In conclusion, the objectives and activities proposed for Stage 2/2023 were fully achieved.

Articles published in ISI-quoted journals

• "Atomically thin MoS₂ layers selectively grown on Mo patterned substrates for field effect controlled photosensors", I. Stavarache, C. Palade, A. Slav, I. Dascalescu, A.-M. Lepadatu, L. Trupina, E. Matei, M. L. Ciurea, T. Stoica, submitted to ACS Applied Nano Materials

International Conference presentations 

• Oral presentation: "Vapor transport growth of localized atomically thin 2D MoS2 layers",
  I. Stavarache, C. Palade, A. Slav, I. Dascalescu, A.-M. Lepadatu, E. Matei, M. L. Ciurea, T. Stoica
  International Semiconductor Conference, 11–13 October 2023, Sinaia, Romania

Project Contact Person:

Project Director:  Dr.Toma Stoica

National Institute of Materials Physics, Atomistilor 405A., 077125 Magurele - Bucharest, ROMANIA

Tel:+40--(0)21-2418-171

Email: toma.stoica@infim .ro