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.

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