Mikhail Chubarov

Diffusion-Controlled Epitaxy of Large Area Coalesced WSe2 Monolayers on Sapphire

A multistep diffusion-mediated process was developed to control the nucleation density, size, and lateral growth rate of WSe2 domains on c-plane sapphire for the epitaxial growth of large area monolayer films by gas source chemical vapor deposition (CVD). The process consists of an initial nucleation step followed by an annealing period in H2Se to promote surface diffusion of tungsten-containing species to form oriented WSe2 islands with uniform size and controlled density. The growth conditions were then adjusted to suppress further nucleation and laterally grow the WSe2 islands to form a fully coalesced monolayer film in less than 1 h. Postgrowth structural characterization demonstrates that the WSe2 monolayers are single crystal and epitaxially oriented with respect to the sapphire and contain antiphase grain boundaries due to coalescence of 0° and 60° oriented WSe2 domains. The process also provides fundamental insights into the two-dimensional (2D) growth mechanism. For example, the evolution of domain size and cluster density with annealing time follows a 2D ripening process, enabling an estimate of the tungsten-species surface diffusivity. The lateral growth rate of domains was found to be relatively independent of substrate temperature over the range of 700-900 °C suggesting a mass transport limited process, however, the domain shape (triangular versus truncated triangular) varied with temperature over this same range due to local variations in the Se/W adatom ratio. The results provide an important step toward atomic level control of the epitaxial growth of WSe2 monolayers in a scalable process that is suitable for large area device fabrication.

Chemical vapour deposition of epitaxial rhombohedral BN thin films on SiC substrates

Epitaxial growth of rhombohedral boron nitride (r-BN) on different polytypes of silicon carbide (SiC) is demonstrated using thermally activated hot-wall chemical vapour deposition and triethyl boron and ammonia as precursors. With respect to the crystalline quality of the r-BN films, we investigate the influence of the deposition temperature, the precursor ratio (N/B) and the addition of a minute amount of silicon to the gas mixture. From X-ray diffraction and transmission electron microscopy, we find that the optimal growth temperature for epitaxial r-BN on the Si-face of the SiC substrates is 1500 °C at a N/B ratio of 642 and silicon needs to be present not only in the gas mixture during deposition but also on the substrate surface. Such conditions result in the growth of films with a c-axis identical to that of the bulk material and a thickness of 200 nm, which is promising for the development of BN films for electronic applications.

Early stages of growth and crystal structure evolution of boron nitride thin films

A study of the nucleation and crystal structure evolution at the early stages of the growth of sp2-BN thin films on 6H-SiC and α-Al2O3 substrates is presented. The growth is performed at low pressure and high temperature in a hot wall CVD reactor, using ammonia and triethylboron as precursors, and H2 as carrier gas. From high-resolution transmission electron microscopy and X-ray thin film diffraction measurements we observe that polytype pure rhombohedral BN (r-BN) is obtained on 6H-SiC substrates. On α-Al2O3 an AlN buffer obtained by nitridation is needed to promote the growth of hexagonal BN (h-BN) to a thickness of around 4 nm followed by a transition to r-BN growth. In addition, when r-BN is obtained, triangular features show up in plan-view scanning electron microscopy which are not seen on thin h-BN layers. The formation of BN after already one minute of growth is confirmed by X-ray photoelectron spectroscopy.

In-plane x-ray diffraction for characterization of monolayer and few-layer transition metal dichalcogenide films

There is significant interest in the growth of single crystal monolayer and few-layer films of transition metal dichalcogenides (TMD) and other 2D materials for scientific exploration and potential applications in optics, electronics, sensing, catalysis and others. The characterization of these materials is crucial in determining the properties and hence the applications. The ultra-thin nature of 2D layers presents a challenge to the use of x-ray diffraction (XRD) analysis with conventional Bragg-Brentano geometry in analyzing the crystallinity and epitaxial orientation of 2D films. To circumvent this problem, we demonstrate the use of in-plane XRD employing lab scale equipment which uses a standard Cu x-ray tube for the analysis of the crystallinity of TMD monolayer and few-layer films. The applicability of this technique is demonstrated in several examples for WSe2 and WS2 films grown by chemical vapor deposition on single crystal substrates. In-plane XRD was used to determine the epitaxial relation of WSe2 grown on c-plane sapphire and on SiC with an epitaxial graphene interlayer. The evolution of the crystal structure orientation of WS2 films on sapphire as a function of growth temperature was also examined. Finally, the epitaxial relation of a WS2/WSe2 vertical heterostructure deposited on sapphire substrate was determined. We observed that WSe2 grows epitaxially on both substrates employed in this work under all conditions studied while WS2 exhibits various preferred orientations on sapphire substrate which are temperature dependent. In contrast to the sapphire substrate, WS2 deposited on WSe2 exhibits only one preferred orientation which may provide a route to better control the orientation and crystal quality of WS2. In the case of epitaxial graphene on SiC, no graphene-related peaks were observed in in-plane XRD while its presence was confirmed using Raman spectroscopy. This demonstrates the limitation of the in-plane XRD technique for characterizing low electron density materials.

Review Article: Challenge in determining the crystal structure of epitaxial 0001 oriented sp2-BN films

Boron nitride (BN) as a thin film is promising for many future electronic applications. On 0001 α-Al2O3 and 0001 4H/6H-SiC substrates, chemical vapor deposition yields epitaxial sp2-hybridized BN (sp2-BN) films oriented around the c-axis. Here, the authors seek to point out that sp2-BN can form two different polytypes; hexagonal BN (h-BN) and rhombohedral BN (r-BN), only differing in the stacking of the basal planes but with the identical distance between the basal planes and in-plane lattice parameters. This makes structural identification challenging in c-axis oriented films. The authors suggest the use of a combination of high-resolution electron microscopy with careful sample preparation and thin film x-ray diffraction techniques like pole figure measurements and glancing incidence (in-plane) diffraction to fully distinguish h-BN from r-BN.

Initial stages of growth and the influence of temperature during chemical vapor deposition of sp2-BN films

Knowledge of the structural evolution of thin films, starting by the initial stages of growth, is important to control the quality and properties of the film. The authors present a study on the initial stages of growth and the temperature influence on the structural evolution of sp2 hybridized boron nitride (BN) thin films during chemical vapor deposition (CVD) with triethyl boron and ammonia as precursors. Nucleation of hexagonal BN (h-BN) occurs at 1200 °C on α-Al2O3 with an AlN buffer layer (AlN/α-Al2O3). At 1500 °C, h-BN grows with a layer-by-layer growth mode on AlN/α-Al2O3 up to ∼4 nm after which the film structure changes to rhombohedral BN (r-BN). Then, r-BN growth proceeds with a mixed layer-by-layer and island growth mode. h-BN does not grow on 6H-SiC substrates; instead, r-BN nucleates and grows directly with a mixed layer-by-layer and island growth mode. These differences may be caused by differences in substrate surface temperature due to different thermal conductivities of the substrate mate...

Characterization of Boron Nitride thin films

Rhombohedral Boron Nitride layers were grown on sapphire substrate in a hot-wall CVD reactor. The characterization of those layers is reported and the results are discussed in correlation with the various growth parameters used.

Chemical Vapor Deposition of Boron Nitride Thin Films on SiC

We give here an overview of our recent work on growth of rhombohedral boron nitride (r-BN) thin films on SiC substrates by chemical vapor deposition (CVD). We demonstrate the growth of twinned r-BN on various SiC polytypes at 1500 °C, using H2 as carrier gas and triethyl boron and ammonia as precursors with an N/B ratio of ~ 640. The epitaxial relation with various substrates is determined from XRD and TEM. Adding Si to the gas phase stabilizes the r-BN phase but does not alter the electric properties of the material which remains electrically insulating.

PATTERNED LASER CRYSTALLIZATION OF a-Si

PATTERNED LASER CRYSTALLIZATION OF a-Si Thin films of amorphous Si on glass were crystallized by pulsed nano- and picosecond lasers. Two methods for creating the desired patterns of crystallized regions were used. In the former, the pattern is produced by a focused laser beam, and in the latter it is made using a prefabricated mask. The electric conductivity of crystallized films increases by more than 4 orders of magnitude in comparison with untreated amorphous films. AMORFA SILĪCIJA (a-Si) STRUKTURĒTA LĀZERKRISTALIZĒŠANA Amorfa silīcija plānās kārtiņas uz stikla tika kristalizētas ar impulsu nanosekunžu un piko-sekunžu lāzeriem. Izmantotas divas metodes polikristāliska silīcija raksta iegūšanai. Pirmajā metodē raksts iegūts, pārvietojot fokusētu lāzera staru, bet otrajā metodē rakstu iegūst, paraugu apstarojot caur iepriekš izgatavotu masku. Iegūtā polikristāliskā silīcija vadāmība pieaug vairāk kā par 4 kārtām, salīdzinot ar neapstrādātu paraugu.

NANOMECHANICAL PROPERTIES AND POSSIBLE APPLICATIONS OF MECHANOACTIVATED ZNO COATINGS

NANOMECHANICAL PROPERTIES AND POSSIBLE APPLICATIONS OF MECHANOACTIVATED ZNO COATINGS A simple and cost-effective mechanoactivated oxidation method (MAOM) has been developed for producing ZnO coatings on glass. To achieve the transparency of initially opaque mechanoactivated ZnO coatings they were annealed at the temperatures modifying the surface morphology of nanostructured coatings. Investigation of nanomechanical properties has revealed that MAOM coatings in as-obtained state have higher hardness (up to 3 GPa) as compared with bulk metallic zinc (160 MPa). In the annealed state the coatings show stabily high adhesion with glass, with the ability to form a grained structure (hardness 8-10 GPa) or a whiskers nanostructure (hardness 18-20 GPa). The Young modulus of coatings ranged from 80 to 120 GPa. The nanomechanical and optical properties were compared with those of bulk ZnO single crystal and commercial ZnO films. The optical properties of ZnO films with a whiskers structure are typical of a transparent high-quality ZnO single crystal. The coatings obtained have high electrical resistance, which decreases in response to the UV exposure. The photocurrent increases by two orders of magnitude, which is easily detectable and makes it possible to use such coatings as UV emission detectors. MEHANOAKTIVĒTO ZNO PĀRKLĀJUMU NANOMEHĀNISKĀS ĪPAÂĪBAS UN IESPĒJAMIE PIELIETOJUMI Šajā darbā ir izpētītas ar mehanoaktivētās oksidēšanas metodi iegūto ZnO pārklājumu uz stikla optiskās, elektriskās un nanomehāniskās īpašības. Veikts salīdzinājums ar ZnO monokristālu un ar magnetronu putinātajiem ZnO pārklājumiem. Rezultāti liecina, ka ar mehanoaktivētās oksidēšanas metodi iegūtajiem pārklājumiem ir lieliska adhēzija ar pamatni, var veidoties graudaina struktūra ar cietību 8-10 GPa, kā arī var veidoties ar viskersiem pārklātas virsmas morfologija ar cietību līdz 18 GPa. Iegūto pārklājumu Junga modulis ir robežās no 80-120 GPa, kas saskan ar rūpnieciski iegūto magnetronu putinātajiem ZnO pārklājumiem. Iegūto pārklājumu ar viskersu struktūru optiskās īpašības (absorbcija un luminiscence) atbilst tipiskiem augstas kvalitātes caurspīdīgiem ZnO pārklājumiem. Darbā paradīta iespēja izmantot iegūtos pārklājumus UV detektoros.