ALD/ALE 2022 Session EM1-WeA: Emerging Materials

Wednesday, June 29, 2022 1:30 PM in Room Auditorium
Wednesday Afternoon

Session Abstract Book
(346KB, May 7, 2022)
Time Period WeA Sessions | Abstract Timeline | Topic EM Sessions | Time Periods | Topics | ALD/ALE 2022 Schedule

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1:45 PM EM1-WeA-2 ALD of In1-XGaXN
Henrik Pedersen, Polla Rouf, Chih-Wei Hsu (Linköping University, IFM)

Alloying group 13-nitrides to ternary phases allows tuning of the bandgap from 6.2 eV for pure AlN down to 0.7 eV for pure InN. The bandgap of In1-xGaxN can theoretically span from UV to IR (3.4–0.7 eV), including the whole visible light range by varying x, making it promising material for optoelectronic applications. However, the ability to vary the composition of In1-xGaxN is limited by the theoretically predicted metastability of In1-xGaxN for 0.05 < x < 0.95, which leads to phase separation into their binary materials. The deposition of In1-xGaxN is also hindered by the low thermal stability of InN, which decomposes into In metal and N2 at around 500 °C, making traditional CVD approaches ill-suited. We have recently shown that ALD is a promising technique to deposit InN thin films with excellent structural quality,1 ALD is therefore a promising alternative to CVD for In1-xGaxN with x close to 0.5. In our efforts to deposit In0.5Ga0.5N we have explored two ALD approaches:

By using a short period superlattice, with alternating monolayers of GaN and InN, In0.5Ga0.5N deposition was attempted from repeated n InN and m GaN monolayers (n=m= 1, 2, 3…) using triethyl gallium (TEG), trimethyl indium (TMI) and ammonia plasma at 320 °C. This approach afforded single-crystalline In1-xGaxN with tunable x between 0.3 and 0.7 by varying the ratio between n and m. The crystalline quality of In1-xGaxN prepared by this multilayer approach ALD is remarkably better than that prepared by conventional continuous CVD and earlier reported ALD work using a multilayer approach with thicker layers of InN and GaN in the multilayer.

By mixing solid Ga(III) and In(III) triazenides in the same evaporator and co-subliming the two metal precursors In1-xGaxN was deposited using a single, mixed metal pulse and NH3 plasma at 350 °C.2 In1-xGaxN was successfully deposited using this approach and the value of x could be tuned by changing the sublimation- and deposition temperatures, and the ratio of the two metal precursors. An In1-xGaxN film with x = 0.5 was deposited and found to have a band gap of 1.94 eV. The In1-xGaxN film grew epitaxially on 4H–SiC(0001) without need for a buffer layer and without phase segregation or decomposition of the In1-xGaxN into the binary materials or In droplets.

Our results reveal a promising potential of ALD over conventional growth techniques to prepare ternary group 13-nitrides with tunable composition at low temperature, which provides the possibility to grow heterostructures with metastable alloys for device application.

Refs.

  1. Hsu et al. Appl. Phys. Lett. 2020, 117, 093101.
  2. Rouf et al. J. Mater. Chem. C2021, 9, 13077.
2:00 PM EM1-WeA-3 Silicon-Based Polymer-Derived Ceramic Coatings by Post-Processing of Pre-Ceramic MLD Thin Films
Kristina Ashurbekova, Mato Knez (CIC nanoGUNE)
Si-based polymer-derived ceramics (PDCs) belong to an emerging class of advanced materials that provide high strength, hardness, corrosion protection and heat dissipation, even upon use in extreme environments like high temperatures or chemically reactive plasma conditions. For example, wet-chemically synthesized aluminum doped SiOC PDCs retained their mechanical properties up to 1900°C in addition to an increased creep and corrosion resistance [1].
In the present work, MLD-deposited siloxane-alumina (SiAlCHO) thin films have been used as pre-ceramic polymers for a polymer-derived amorphous silicoaluminum oxycarbide (SiAlCO) synthesis by high-temperature post-processing. Pre-ceramic SiAlCHO films were grown by applying sequential surface reactions between 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (V4D4) and trimethylaluminum [2]. To increase the mass yield during the polymer-to-ceramic transformation, cross-linking of the growing chains is desired. For this purpose, we introduced di-tert-butyl peroxide into the MLD process to cross-link the chains through their vinyl groups. The resulting film exhibited improved properties, such as 12% higher film density and enhanced thermal stability, if compared to the non-cross-linked film [3].
The fabrication of the final SiAlCO PDCs coatings was carried out by pyrolyzing the SiAlCHO MLD films in an Ar atmosphere and in vacuum at 900oC. The Raman spectra showed D and G peaks at 1350 cm–1 and 1590 cm–1, respectively, thereby indicating the formation of free sp2-hybridized carbon in the resulting PDCs film. The in situ sp2-carbon, formed by decomposition of Me and Vi groups in the SiO2MeVi moieties within the SiAlCO PDC film was identified by X-ray photoelectron spectroscopy (XPS). The spectra showed presence of C=C sp2 bonds and C-H bonds at the interface of free carbon nanoclusters. The elimination of a part of the organic groups is confirmed with the XPS survey scan data, where the Si:C ratio in the film after pyrolysis was reduced from 1:3 to 1.5:1. Transmission electron microscopy confirmed that the PDC film remained amorphous and defect-free after pyrolysis. Interestingly, annealing a 5 nm thick SiAlCO PDC film in vacuum at 900°C showed the formation of a conformal graphene shell on the surface of the amorphous SiAlCO PDC (Supplementary Fig. 1). This MLD-derived conformal SiAlCO PDC thin film showed exceptional uniformity, linear shrinkage, and thermal stability up to 1100°C.
[1] Wen Q., et al., Prog. in Mater. Science, 2020, 109, 100623.
[2] Ashurbekova Kr., et al., Chem. Mater., 2021, 33, 3, 1022–1030.
[3] Ashurbekova Kr., et al., Chem. Commun., 2021,57, 2160-2163.
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2:15 PM EM1-WeA-4 Closing in on Room-Temperature Metal-Insulator-Transitions for Next Generation Electronics by Epitaxial Nickelate ALD
Linn Rykkje, Henrik Sønsteby, Ola Nilsen (University of Oslo)

Complex oxides exhibiting metal-insulator transitions (MITs) are exemplar materials systems with strong correlation and emergent functional phenomena. Particularly the rare-earth nickelates (RENiO3, with trivalent rare-earth RE = La, Pr, Nd, …, Lu) are of interest as their MITs occur concomitantly with a structural transition. Underlying their rich phase diagram and the MIT’s physical origin is a complex interplay of interactions; though it remains an unsolved puzzle in fundamental research, the exotic properties rooted in it have great potential for electronics applications.

Among the RENiO3s, the MIT temperature of NdNiO3 (TMI = 200 K) is the closest to room temperature. Tuning the TMI can be carried out using strain or by partial substitution of Nd with larger RE cations (see phase diagram). A more significant challenge, however, has been to develop a synthesis route that stabilizes Ni3+ and provides sufficient control under industrially relevant conditions. For instance, high temperatures and ultrahigh vacuum (UHV) typically facilitate epitaxy, but are incompatible with monolithic device integration.

In this talk we show that with ALD – since long embraced by the electronics industry – we can grow high-quality epitaxial NdNiO3 thin films with excellent control of thickness, uniformity, and chemical composition. This is achieved at low temperatures (225 oC) without constraints to the substrate geometry or need for UHV. Thin films of stoichiometric composition show low resistivities at room temperature and a sharp MIT, which are desired properties of a functional electronic switch in future neuromorphic architectures. Quaternary oxide thin films of the form (RE,Nd)NiO3 have been successfully deposited using ALD with the aim of tuning the TMI close to 273 K. Further chemical and electrical characterizations are needed, however, to establish and control the effect of partial RE substitution on the TMI.

Although much of the fundamental behavior of the RENiO3s remains contested, their potential for applications is undisputed; in fact, many members are already found in various device concepts. The success in using low-temperature ALD to grow high-quality NdNiO3 (stoichiometric and cation substituted) thin films with a sharp MIT could promote the implementation of such switching-materials in next-generation electronics. A complex oxide field-effect transistor may thus be more within reach than previously anticipated, offering a viable alternative and/or complement to Si-based circuitry. Based on fundamentally different mechanisms, this could pave the path for a greener and more sustainable integrated circuit technology in the future.

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2:30 PM EM1-WeA-5 Plasma-Enhanced Atomic Layer Deposition of Spinel Ferrite CoFe2O4 and NiFe2O4 Thin Films
Mari Napari (University of Southampton); Mikko Heikkila (University of Helsinki); Sami Kinnunen, Jaakko Julin (University of Jyvaskyla); Themis Prodromakis (University of Southampton)

Thin films of insulating ferro- and ferrimagnetic complex oxides with high Curie temperatures, such as spinel ferrites, are essential for many emerging applications utilising room temperature spin-polarisation and magneto-optical effects, e. g. spintronics and sensors [1]. There is a need for a synthesis method for high quality magnetic oxides with large scale processing compatibility. Here, we have developed PEALD processes for two spinel ferrite materials, CoFe2O4 (CFO) and NiFe2.5O4 (NFO) using ferrocene and cobalt(III)- or nickel(II) acetylacetonate as precursors in direct plasma PEALD at 250°C. The CFO films were deposited with 1:2 Co:Fe ratio, while the NFO films were grown iron-rich to ensure that the ferrimagnetic property is not hampered by a parasitic antiferromagnetic nickel oxide component [2]. Stoichiometry of the grown ternary oxide films was confirmed with time-of-flight elastic recoil detection analysis measurements, which also showed that the low light element impurity content of the films (H < 2.0 at. -%, C < 0.3 at. -%,) originates mainly from the acetylacetonate sources. According to the X-ray diffraction measurements of 40 nm thick films, the PEALD CFO and NFO have the desired (inverse) spinel structure, and the films grown on sapphire substrates are strongly (111) oriented already as-deposited. Helium ion microscopy and atomic force microscopy both showed that the films are continuous and free of aggregations. The oriented CFO films on sapphire have a very smooth surface (rrms < 0.3 nm) but the NFO with a same thickness has a higher surface roughness (rrms > 1.5 nm), which is in accordance with the previous observations of the ALD-grown iron-rich NFO [3]. In addition to the growth and structural characteristics we will present the results of the magnetic property measurements of the films.

[1] Hirohata et al. IEEE Trans. Magnetics 5 (2015) 0800511

[2] Napari et al. InfoMat 2 (2020) 769

[3] Bratvold et al. J. Vac. Sci. Technol. A 37 (2019) 021502

2:45 PM EM1-WeA-6 Engineering Maxwell-Wagner Polarization in Al2O3/TiO2/Al2O3 Nanolaminates Grown by Atomic Layer Deposition
Partha Sarathi Padhi (Raja Ramanna Centre for Advanced Technology); R.S. Ajimsha, S. K. Rai, Pankaj Misra (Raja Ramanna Centre for Advanced Technology)

Recently multilayered nanolaminates (NLs) of two dielectrics with conductivity contrast exhibiting giant dielectric constant owing to interface induced Maxwell–Wagner (M-W) relaxation have emerged as potential candidate for high density storage capacitors. The M-W polarization can be engineered precisely by controlling the thicknesses of sublayers and number of interfaces. We report growth of Al2O3/TiO2 (ATA) NLs on Si and Au/Si substrates using atomic layer deposition, wherein M-W relaxation induced high dielectric constant was realized and engineered by tuning sublayer thicknesses. Trimethylaluminum (Al (CH3)3) and Titanium tetrachloride (TiCl4) were used as source for Al and Ti respectively, while deionized water (H2O) was used as source for oxygen. Depositions were carried out at 200 °C and the average growth per cycle for TiO2 and Al2O3 was ~ 0.4 and 1.6 Å respectively. The thickness of Al2O3 and TiO2 layers were kept same in a given NL and was reduced from ~ 2.4 to 0.17 nm in different NLs keeping the total stack thickness fixed at ~ 60 nm. X-ray reflectivity curves from these NLs with intense Bragg peaks and clean Kiessig fringes, as shown in Fig. 1, confirmed the multilayer structures with uniform thickness along with distinct interfaces. The dielectric properties of ATA NLs were studied in Au/ATA/Au device configuration using impedance spectroscopy in frequency range of 10–106 Hz. The dielectric constant of ATA NLs at 10 Hz was found to increase from ~ 23 to 290 with decreasing sublayer thicknesses from ~ 2.4 to 0.17 nm (Fig. 2(a)), while the dielectric loss was initially found to reduce from ~ 0.8 to 0.06 with reduction in sublayer thicknesses down to ~ 0.48 nm and then increased up to ~ 0.24 with further reduction in sublayer thicknesses down to ~ 0.17 nm (Fig. 2(b)). The dielectric constant of ~ 290 obtained for the ATA NL with ~ 0.17 nm sublayer thickness is significantly larger than that of both Al2O3 (K ~10) and TiO2 (K ~ 20) and is proposed to be due to M-W type dielectric relaxation caused by space charge polarization across the interfaces of Al2O3/TiO2. Temperature dependent dispersion in dielectric constant and loss of ~ 0.48 nm ATA NL clearly revealed two sets of thermally activated relaxations, confirming existence of interfacial M-W relaxation (Fig. 3). The ATA NLs of sublayer thickness ~ 0.17 nm showed high capacitance density of ~ 43.1 fF/μm2, low loss of ~ 0.24 at 10 Hz, low EOT of ~ 0.8 nm, high breakdown field of ~ 0.265 MV/cm, low leakage current density of ~ 8.5 x 10 -4 A/cm2 at 1V and cut-off frequency of ~ 12KHz which are promising for development of next generation high density storage capacitors.

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3:30 PM Break
Session Abstract Book
(346KB, May 7, 2022)
Time Period WeA Sessions | Abstract Timeline | Topic EM Sessions | Time Periods | Topics | ALD/ALE 2022 Schedule