Annotation of the results obtained in 2021
Within the framework of the third year of the project execution, the minimal achievable thickness of WS2 films obtained via ALD-grown oxide sulfurization was determined. In this case, it was assumed that a necessary condition for the continuity of the WS2 films is the continuity of the initial WO3 film. In this regard, the possibility of obtaining ultrathin WO3 films by the ALD method was preliminary studied, namely, the moment of the onset of continuity on various substrates (sapphire, silicon oxide) was determined. Since a necessary condition for the reproducibility of the ALD process is operation under saturation conditions, these conditions were first determined. It has been shown that the thermal ALD process WH2 (Cp) 2 + O3, carried out at a temperature of 300 degrees, has a clearly pronounced saturation with a duration of WH2 (Cp) 2 supply of at least 2.0 seconds and exposure to O3 — at least 8 seconds. The GPC measured under these conditions was approximately 1.1 A / cycle. Then, using XPS with the angular resolution, the initial stages of growth were studied, where the number of ALD cycles varied in the range of 5-50. It turned out that the continuity of the films (f = 0.97) occurs at 20 and 30 ALD cycles on sapphire and silicon oxide, which in both cases corresponds to a thickness of about 1.5 nm, and upon reaching this moment, the thickness begins to increase linearly with the number of cycles. Presumably, at the moment there is a transition from the Wolmer-Weber mechanism (island growth) to layer-by-layer growth. The resulting ultrathin (up to 4-5 nm) films had a low (RMS less than 0.3 nm) roughness value, which is a consequence of their amorphous structure. Indeed, it was confirmed by X-ray diffraction that crystallization of these films into a stable monoclinic phase begins to be observed from a thickness of about 9 nm. The influence of the thickness of the initial WO3 film on the structure and electronic properties of the resulting WS2 has also been investigated. Sulfidization of WO3 films was carried out in a tubular furnace at a temperature of 900 degrees for 30 minutes in an atmosphere of 5% H2 + Ar. Immediately before sulfurization, the in-situ reduction of the WO3 film to the metallic state was carried out. Using XPS, it was found that the WS2 film obtained by sulfurization of a continuous WO3 film with a thickness of 1.5 nm is also continuous (f≥0.96), and its thickness is approximately 2 nm, which corresponds to 3 monolayers. AFM studies have shown that the thickness of the resulting WS2 is linearly correlated with the thickness of the initial WO3, and the largest grains (up to 50 nm) are observed in the thinnest film. Additional TEM studies showed that these grains show signs of the presence of texture both in the plane of the film and outside it, which indicates the possibility of the orienting action of the substrate and epitaxial growth. However, a significant amount of amorphous phase was also found between the grains, the proportion of which decreased with increasing thickness. The average crystal grain size in thicker films was approximately 15–20 nm. The study of the obtained films by Raman spectroscopy revealed that WS2 films of thinner thickness contain a higher concentration of defects. XPS analysis showed that they also contain a significant proportion of the oxysulfide component, which correlates with the presence of an amorphous phase. The thicker films were found to be entirely composed of tungsten sulfide. However, in all cases, the calculated value of the ratio of atomic concentrations [S] / [W] turned out to be more than 2. Electrophysical measurements carried out by the TLM method showed that the measured value of the resistivity of the films practically does not change in the thickness range of 2.8-4.7 nm, but increases significantly with a decrease in thickness to 1.9 nm. Since a decrease in the crystallite size should lead to an increase in the scattering of charge carriers at grain boundaries and, consequently, to an increase in resistance, thicker films should have a higher resistivity. Since this does not agree with the experimental data obtained, it was assumed that the presence of an amorphous phase containing oxysulfide compounds at the boundaries between neighboring crystallites, rather than their size as such, has the strongest effect on the electrical resistance of the films. Thus, the features of ultrathin WS2 films with a thickness of about 2 nm are the relatively large size of crystal domains and epitaxial growth on sapphire substrates. However, for the complete phase transformation of the amorphous WO3 phase into the 2H-WS2 crystalline phase, a more careful selection of sulfurization conditions is required. Since one of the most important tasks of the project being implemented is to obtain homogeneous MoS2 / WS2 films on technologically significant (wafer scale) areas, experimental testing of the scalability of the implemented approach was carried out. The resulting WS2 film was shown to be uniform over an area of about 30 sq. cm. and contains 4-5 monolayers, and on an area of 40 sq. cm. a deviation in the number of monolayers does not exceed 2. The task of creating electronic devices, in particular, transistors, based on synthesized 2D TMD layers inevitably implies the need to obtain ultrathin high-quality high-k dielectrics on them by the ALD method. In this regard, the initial stages of HfO2 growth on the synthesized WS2 layer were studied using XPS with angular resolution. It turned out that the use of ozone as an oxidizing agent leads to the growth of the film starting from the first deposition cycles, and its continuity is achieved in about 30-40 cycles, which corresponds to a thickness of about 3 nm. In this case, however, it was found that the surface oxidation of the WS2 layer occurs at the initial stage. On the contrary, the use of water avoids this negative effect and is, therefore, more preferable. Apparently, the morphological features of the synthesized WS2, namely, the relatively small size of the crystal grain and a sufficient number of surface defects in the form of their boundaries, contribute to efficient nucleation in the ALD process and, as a consequence, lead to early continuity of the grown film. The transistor structures were also created on the basis of synthesized MoS2 and WS2 layers with a thickness of no more than 3-4 monolayers. The WS2-based structure demonstrated ambipolar modulation of conductivity with a maximum modulation index of about 10, which indicates the true semiconductor nature of the material obtained. However, the absence of the channel break effect, as well as the observed asymmetry of the modulation towards positive voltages, made it possible to draw a conclusion about its weak n-type doping. In contrast, the MoS2 structure exhibited clearer n-type conductivity and the ability to break the conductive channel by applying a negative gate voltage. The maximum achieved modulation value was approximately 20. The presented data allow us to conclude that the developed approaches to sulfurization of ultrathin oxide films obtained by the ALD method make it possible to obtain high-quality two-dimensional homogeneous layers of MoS2 and WS2 on large substrates.

 

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Annotation of the results obtained in 2019
The main purpose of this project is a synthesis of high-quality 2D MoS2/WS2 layers by the high-temperature sulfurization of preliminary ALD-grown molybdenum and tungsten oxide thin films on the wafer-scale substrates. The metal oxide growth is a classical ALD task which was strongly extended over the last two decades. Really, ALD is based on the self-saturated surface chemical reactions, which in practice allows obtaining ultrathin uniform films with subnanometer thickness accuracy and high conformality even on the large area substrates with complicated surface morphology. On the other hand, despite a wide range of the reported ALD processes for different metal oxides, there are only a few papers on MoO3 and WO3 growth. Moreover, taking into account the project specifics, especially ultrathin (up to 5 nm) and continuous films are mainly of interest since the final molybdenum or tungsten disulfide nanosheets possess unique properties beeing only a few monolayers thick. However, in the mentioned works the possibility of ultrathin films growth was not even discussed. Moreover, despite some existing data on ALD saturations, the detailed film analysis (chemical composition, structure, thickness uniformity) is still lacking. Therefore, the main project task at the current stage was the ALD processes development both for molybdenum and tungsten oxides on wafer-scale substrates. First, during the project execution, the ALD process of molybdenum oxide from Mo(CO)6 and oxygen plasma on 100 mm wafers was developed. It was found that under saturated conditions this process offers a high growth rate of 0.7 A/cycle and a good thickness uniformity (1σ≈3.5%). In contrast to the reported previously plasma-assisted ALD process which, apparently, resulted in the ion bombardment of growing film and, consequently, poor stoichiometry, the developed process allowed to obtain a completely oxidized film (Mo 6+) while the O/Mo ratio was found to be near 3. Next, 50 ALD cycle-grown film was analyzed by angle-resolved XPS. It is found that this film is continuous with an average thickness of about 3 nm. Therefore, the developed process allows for obtaining ultrathin films appropriate for further sulfurization. In particular, the sulfurization test of the preliminary grown 1.5 nm-thick Mo-oxide film on the sapphire stripe (1.5x9.5 cm) resulted in the molybdenum sulfide formation under sulfur vapor at 900C. Next, within the project framework, the obtainment possibility of the two-dimensional crystallographic alpha-modification of MoO3 was investigated since it is also a van-der-Waals material and, therefore, has a chance to be obtained in this form. In this regard, the initially amorphous molybdenum oxide films were annealed in oxygen atmosphere. It was found that the (0k0)-oriented alpha-MoO3 crystals may be obtained on the sapphire substrate at 450C. Their average size was more than 100 mkm2 which is significantly larger compared to the mechanically exfoliated crystals (~1 mkm2). An ALD-process for tungsten oxide using metal-oxide compound WH2(Cp)2 and oxygen plasma was also developed. As in the case of the molybdenum oxide process, the same saturation studies were also carried out. Apparently, a high precursor activity resulted in the visible incubation absence (less than 50 cycles) that leads to linear correlation between the film thickness and reaction cycles number even within low thickness range (about 3.5 nm). The obtained film thickness uniformity on 100 nm wafers was found to be better than 4%. The XPS-analysis showed the presence only W6+ oxidation state for the metal atoms and stoichiometric ratio O/W near 3. To summarize, the developed ALD-processes allow to synthesize ultrathin (2-3 nm) continuous molybdenum and tungsten oxide films on wafer-scale (>50 cm2) substrates, which can be subsequently utilized for the synthesis of two-dimensional transition metal dichalcogenides (2D-TMDs), exhibiting unique properties only at the thickness of few monolayers.

 

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Annotation of the results obtained in 2020
Within the framework of the research project, the influence of active oxygen radicals dose on the properties of ALD-grown WOx films was investigated. In-situ XPS analysis revealed that the low radicals dose resulted in incomplete ligand removal and high film carbon contamination. Nevertheless, in this case, a significant amount of tungsten atoms showed a 4+ oxidation state. The increase of oxygen radicals dose resulted in a visible increase of 5+ and 6+ tungsten fraction as well as a suppression of existing oxygen vacancies, which concentration did not exceed 1 at. %. Also, it was shown that the oxygen vacancy concentration can be tuned by the deposition temperature within an existing ALD window, but the achievable range was as low as 0.35 at. %. Therefore, the simple oxygen radicals dose variation within the ALD process does not allow to obtain chemically clean WOx films which may contain a significant amount of W4+ state, making an additional reduction step inevitable. The role of the additional stoichiometry parameter, namely active hydrogen radicals treatment, was also investigated in two different cases: treatment of the completely grown film and an appropriate ALD cycle modification. In the first case, only a visible film reduction to the 5+ state was found, and the achieved oxygen deficiency was as high as 30 at. %. However, such an approach has a serious disadvantage due to the limited hydrogen penetration depth, which may result in an oxygen concentration gradient across the film. Therefore, the ALD cycle modification by the addition of hydrogen radicals treatment step seems to be a more relevant approach. In this case, the increase of the hydrogen treatment time resulted in a monotonic W5+ fraction increase with a clear saturation tendency. The achievable oxygen vacancy level was about 5 at. % which was confirmed by the direct ab-initio DFT simulations. Also, the obtained WOx films were found to be conductive: the increase of the oxygen vacancy concentration from 2,5 at. % to 4,5 at. % resulted in a resistivity decrease from 10^6 down to 1 Ohm×cm. Therefore, the low thermal budget allows only to effectively tune the oxygen deficiency level, but the lower tungsten oxidation states potentially can not be obtained. Additional experiments on tungsten oxide film reduction by the molecular hydrogen were carried out in a tube furnace. It was found that 500 deg. C reduction resulted in the formation of oxidation states continuum from 6+ to metallic W, while their distribution across the thickness was found to be uniform. The reduction temperature of 600 deg. C allowed obtaining a pure metallic W film, which was accompanied by a thickness decrease from 5-6 nm down to 2 nm without the continuity violation, which was confirmed by angle-resolved XPS analysis and X-ray reflectivity. It’s worth noting that the initial oxide film was amorphous while the reduced one already showed a weak crystallinity even at such low thickness. Therefore, molecular hydrogen reduction in a furnace tube allows controlling the tungsten chemical state in WOx films in a significantly wider range compared to the modified ALD process. The influence of the sulfurization temperature of ALD-grown MoO3 films on the structure of the obtained MoS2 films within the temperature range of 700-1000 deg. C was also examined. The performed XPS analysis established that the successful molybdenum disulfide formation took place over the entire temperature range since no signs of residual oxide were found. However, it turned out that the sulfurization temperature had a strong effect on the surface morphology of the obtained MoS2 films: the relatively smooth surface of the films obtained at lower temperatures (500-700 deg. C) turned into the granular one at temperatures of 900-1000 deg. C, which was accompanied by an increase in roughness from 0.3 nm up to 0.7 nm. In addition, the TEM study clearly indicated an accompanying crystalline structure improvement. In particular, the films consisted of large crystalline grains up to 50-200 nm in size, predominantly oriented by the basal planes parallel to the substrate, which was shown by X-ray diffraction measurements, were obtained at temperatures of 900-1000 deg. C. Thus, the direct sulfurization of ALD-grown MoO3 films, performed at temperatures of 900-1000 deg. C, allows obtaining continuous films with a 2D structure and large grains, potentially suitable for use in electronics. In the case of tungsten oxide sulfurization, the preliminary seed oxide film reduction was found to be a critical factor directly affecting the continuity of the resulting WS2 layer: for example, direct sulfurization in an Ar flow led to the formation of separate isolated islands. The utilization of a hydrogen-containing carrier gas allowed obtaining the continuous film; however, a significant difference was found between the films obtained with and without preliminary reduction of the seed oxide layer to the metallic state. Despite the fact that XPS did not show any difference between these cases since both showed a successful WS2 formation, its surface morphology and the crystalline structure were found to depend strongly on the presence of preliminary oxide reduction. The surface investigation carried out by AFM and SEM clearly showed that the films obtained without seed oxide reduction were characterized by a visible grain size inhomogeneity: large grains up to 200 nm in size stood out against the background of the underlying homogeneous layer consisting of smaller grains (20-50 nm in size). The presence of large grains caused significant (up to 10-12 nm) height variations, resulting in a high RMS value of 2.0 nm. In contrast, the WS2 film obtained with preliminary oxide reduction was characterized by a more uniform distribution of grains with an average size of about 50 nm, and a significantly smaller height difference (2-3 nm) influenced the RMS value, which in this case was found to be 1.1 nm. X-ray diffraction measurements showed that in both cases films have a layered structure typical for WS2, but the quality of grain orientation turned out to be noticeably better in the case of preliminary reduction. TEM analysis revealed that the large grains being present in the films obtained without the reduction step are rather thick (6-7 nm) single crystals, while the observed grains in the opposite film consist of even smaller grains. The electrical properties of the films under investigation were also found to be drastically different: the preliminary reduction step led to a decrease in sheet resistance by more than an order of magnitude. The carrier mobility estimation using the space-charge-limited electron transport mechanism also revealed a significant advantage of previously reduced films. The maximum achieved value of mobility was approximately 0.9 cm2/Vs. Therefore, the preliminary reduction step is highly desirable for obtaining high-quality continuous 2D WS2 layers. Finally, the vertical-structure (electrode/MoS2(WS2)/electrode-type) experimental photodetectors based on the synthesized MoS2 and WS2 films were fabricated and their main parameters were determined. Thus, the short-circuit current of the graphene/MoS2/FTO structure with a diameter of 100 μm was about 1.03 μA, and for a similar structure with WS2 of the same diameter it was only 0.1 μA. In the case of MoS2 utilization, a photosensitivity value of up to 0.7 A/W with an external quantum efficiency of 0.03 was achieved at a voltage of 2 V. The similar WS2-based structure demonstrated both significantly lower photosensitivity (at 1 V) - about 15 mA/W and efficiency (0.0027). For the structures under investigation, the electronic band alignment was also determined by XPS. Thus, both MoS2 and WS2 films obtained by the sulfurization of corresponding ALD-grown films can be successfully used as functional layers of optoelectronic devices.

 

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