ALD/ALE 2022 Session EM-MoP: Emerging Materials Poster Session

Ternary oxides can show a wide range of very interesting physical properties and several have already been successfully deposited with ALD¹. One that, to our knowledge, hasn’t yet been reported with ALD is CaVO₃, ALD-processes for calcium and vanadium oxides have been reported already in the early 1990s and 2000respectively^2,3.

CaVO₃ is a correlated metal. These materials with strongly correlated charge carriers hold promise for a new type of transparent conductor (as opposed to highly doped wide bandgap materials like indium tin oxide)⁴. VO₂ and CaVO₃ also show a metal-insulator transition depending on film thickness which could make it an interesting phase change material^5–7.

An ALD process of CaVO₃ could therefore be a great step towards the utilization and further research of this material because ALD is scalable and has great control over the thickness and composition of the deposited films. Here we want to present the status of our work with the ALD of CaO, V_xO_y and the mixed Ca_xV_yO_z Oxides.

References

¹ A.J.M. Mackus, J.R. Schneider, C. MacIsaac, J.G. Baker, and S.F. Bent, Chem. Mater. 31, 1142 (2019).

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⁵ S. Beck, G. Sclauzero, U. Chopra, and C. Ederer, Phys. Rev. B 97, 075107 (2018).

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In recent years, liquid atomic layer deposition (LALD)¹ has emerged as a much simpler and versatile strategy to overcome some of the current constraints of its gas phase homolog for the deposition of metal-organic frameworks (MOF) thin films (e.g. thermal decomposition of precursors at high temperatures, poor control over the crystallinity).

This work describes the automated deposition of Cu₂ (bdc)₂ (dabco) thin films on silicon and glass substrates using a 3D-printed microfluidic chip. Films with preferred (001) and (100) orientations were obtained by changing the temperature of the reaction, the concentration of the reactants, and the dimensions of the microfluidic reactor as demonstrated by GIXRD measurements. In addition, the area of the thin film was successfully controlled by changing the flow rates of the precursors in a continuous flow mode.

1O. Graniel, J. Puigmartí-Luis and D. Muñoz-Rojas, Dalt. Trans., 2021, 50, 6373–6381.

Layered two-dimensional (2D) materials exhibit many exotic physical, chemical, and electronic properties,¹ which allows them to create exciting new opportunities as a test-bed for many fundamental theories of materials science,^2,3 as well as pave the path for a wide variety of applications, such as optoelectronic and nanoelectronic devices,^4–6 clean energy harvesting,⁷ catalysis materials,⁸ bioengineering,⁹ and others. As an additional paradigm, a precise layering of quasi-2D building blocks of different materials in well-controlled sequences can provide an additional degree of complexity in terms of materials design and harnessing novel nano- and quantum-scale phenomena.

This work will discuss the current progress regarding the ALD synthesis of sulfides and selenides of tin, molybdenum, and tungsten on bare silicon and oxide-capped silicon (001) substrates. A comparative study, in terms of the structure, the morphology, and the growth rate of the films, for chloride and dimethylamido-based metallic precursors, will be presented. Some of the initial results of the ALD synthesis of 2D multi-layered films will be discussed. The work will also highlight the alteration to the crystal structure, morphology, and orientation of the as-grown films, brought about by post-growth annealing treatments.

Reference:

¹ S.Z. Butler, S.M. Hollen, L. Cao, Y. Cui, J.A. Gupta, H.R. Gutiérrez, T.F. Heinz, S.S. Hong, J. Huang, A.F. Ismach, E. Johnston-Halperin, M. Kuno, V. V. Plashnitsa, R.D. Robinson, R.S. Ruoff, S. Salahuddin, J. Shan, L. Shi, M.G. Spencer, M. Terrones, W. Windl, and J.E. Goldberger, ACS Nano 7, 2898 (2013).

² X.X. Zhang, Y. You, S.Y.F. Zhao, and T.F. Heinz, Phys. Rev. Lett. 115, 257403 (2015).

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Scandium nitride is a III-V semiconductor from group III and group XV, with properties distinct from those of more common III-Vs from group XIII and group XV.⁽¹⁾Among the most important applications of ScN, however, is when it is alloyed with AlN to form Al_(1-x)Sc_xN.Thin films of Al_(1-x)Sc_xN have shown enhanced piezoelectricity⁽²⁾, and recently ferroelectricity⁽³⁾, enabling novel electronic devices, such as FE-FETs⁽⁴⁾.While ScN has been deposited by a variety of techniques including hybrid vapor-phase epitaxy (HVPE), magnetron sputtering, MBE, and MOCVD, an ALD technique would have advantages for CMOS integration --- including low temperature, wide-area uniformity, 3D conformality, precise thickness control, etc.While Sc₂O₃ ALD has been reported⁽⁵⁾, to the best of our knowledge, this represents the first example of ScN by an ALD technique.

Scandium nitride was deposited at 250 °C from tris(N,N’-di-isopropylformamidinato)scandium(III) (Sc(amd)₃) and N₂/Ar plasma in a Kurt J. Lesker ALD150LX plasma-enhanced ALD reactor.The Sc(amd)₃ was delivered from a source held at 160 °C.XPS results demonstrate 1:1 Sc to N composition, with a small amount of carbon (3.8%) and very low oxygen in the bulk of the film (~1%).In nitride PEALD, ultra-high purity (UHP) process conditions have been shown to be necessary to obtain low oxygen content in readily oxidizable thin films, such as TiN⁽⁶⁾, which we believe is critical to low-impurity ScN PEALD.Grazing incidence X-ray diffraction (GIXRD) shows evidence of polycrystalline ScN at this growth temperature from a film grown on Si (with native oxide), with peaks corresponding to the (200), (220), (311), and (222) peaks of reference cubic ScN.A UHP process for ScN with compatible temperature window for PEALD of AlN, such as this, is anticipated to enable ultra-thin ALD-grown Al_(1-x)Sc_xN for applications in 3D piezoelectric MEMS devices and/or ferroelectric memory which would be difficult or impossible to achieve via existing non-ALD deposition techniques.

¹Biswas, B.; and Sava, B. Physical Review Materials3, 020301 (2019).

²Akiyama, M.; et al.Adv. Mater.21, 593 (2009).

³Ficktner, S.; Wolff, N.; Lofink, F,; Kienle, L.; and Wagner, B.J. Appl. Phys.125, 114103 (2019).

⁴Liu, X.; et al. Nano. Lett.21, 3753-3761 (2021).

⁵Wang, X.; et al. Appl. Phys. Lett.101, 232109 (2012).

⁶Rayner, Jr., G.B.; O’Toole, N.; Shallenberger, J.; Johs, B. J.Vac. Sci. Technol. A38, 062408 (2020).

There is a desire to produce ALD coatings of yttrium-fluoride materials, such as YOF and YF₃, but the implementation of such coatings is challenging and requires special considerations in the choice of precursor chemicals and reactants as well as ALD tool designs due to the corrosive nature of these processes.We will be presenting an approach to produce thin films of ALD-based YOF and YF₃ by depositing ALD Y₂O₃ and then converting the oxide film into YOF and/or YF₃ with a post-coat chemical vapor (non-plasma) conversion process. Utilizing this method YF₃ and YOF layers with thicknesses of more than 100nm have been produced and applied to high aspect ratio structures.

Film structure, composition, chemical bonding arrangement and morphology have been studied using techniques such as XPS, XRD, EDAX, and FIB SEM. It is believed that the fluoride is formed by an O-> F exchange reaction, converting the Y₂O₃ into YF₃ or YOF.

Furthermore, we will discuss how blends of YF₃ and YOF, with a gradual transition of the composition from YF₃ to YOF and Y₂O₃ can be obtained and how that may be advantageous for the implementation of such fluoride coatings.