NAMBE 2022 Session NM-TuM1: Novel Materials

Molecular‐beam epitaxy (MBE) has come to the forefront for the thin film synthesis of materials with an exceptionally high structural quality, and with the best figures of merits whether it is related to their electrical or optical properties. From its beginnings as a successful method for III–V semiconductor growth to today for the growth of many contenders for next‐generation electronics and spintronics devices, several new synthesis challenges have however emerged. For instance, it has been notoriously difficult to grow metal oxides, in an atomically precise manner, of metals having ultra-low vapor pressures and difficulty of oxidation.

In this talk, we will review these issues and will present our group’s effort to address these challenges using a novel solid-source metal-organic MBE approach. We show, for the first time, controlled synthesis of metal and metal oxides of these “stubborn” elements with the same ease and control as afforded by III-V MBE. We will present detailed growth study utilizing chemistry of source materials as a controlling knob to navigate synthesis. With the goal to understand and control electronic ground states in defect-managed complex oxide films and nano-membranes, we will discuss how chemistry of source materials can be used to navigate synthesis on-demand.

The kagome antiferromagnet Mn3Sn has become fascinating in the current times because it’s one of the rare antiferromagnets that exhibits large anomalous Hall and Nernst effects. This opens a new area of research using functional antiferromagnets¹, but for future device applications, it requires fabricating high-quality thin films. There are reports of the controlled growth of Mn₃Sn on substrates like m-plane sapphire,² Pt/Al2O3 (0001)³ and others using sputtering growth, but this often can result in polycrystalline films. In this work, the goal is to grow a crystalline high quality Mn₃Sn film using molecular beam epitaxy. Effusion cells are used for Mn and Sn sources which are calibrated using a quartz crystal thickness monitor. The growth is monitored in-situ using reflection high energy electron diffraction (RHEED) and measured ex-situ using X-ray diffraction, Rutherford backscattering and cross-sectional STEM. The samples are grown at 500 ± 9°C and 416 ± 9 °C with Mn: Sn atomic flux ratio of 3.2: 1 on c-plane sapphire for 60 mins. We observed that, for both temperatures, the RHEED patterns are streaky; however, the resulting orientations of the films are different. Additional results pertaining to surface morphologies, film orientations, chemical compositions, as well as empirical models will be discussed in detail.

Acknowledgement:

The authors acknowledge support from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317. The authors would like to thank Dr. Stinaff and his students for back coating the sapphire substrates, as well as Ashok Shrestha for useful discussions and help.

¹ S. S. Zhang et al., "Many body resonance in a correlated topological Kagome Antiferromagnet", Physical Review Letters 125, 046401 (2020).

² S. Oh, T. Morita, T. Ikeda, M. Tsunoda, M. Oogane, and Y. Ando, "Controlled growth and magnetic property of a-plane-oriented Mn3Sn thin film", AIP Advances 9,035109 (2019).

³ Y. Cheng, S. Yu, M. Zhu, J. Hwang, and F. Yang, "Tunable topological Hall effects in noncollinear antiferromagnets Mn3Sn/Pt bilayers", APL Materials 9, 051121 (2021).

Ferromagnetic shape memory alloys such as Ni₂MnGa have great potential as microelectronic actuators due to their ability to reversibly transform between multiple crystalline phases. However, epitaxial growth of shape memory alloys can prevent their ability to undergo structural phase transformations through substrate clamping effects, especially in nanometer scale films below their relaxation thickness. Here, we demonstrate interfacial decoupling between the ferromagnetic shape memory alloy Ni₂MnGa and MgO via the introduction of a graphene interlayer. X-ray diffraction experiments of Ni₂MnGa films grown by molecular beam epitaxy on bare and graphene terminated MgO(001) show that the constraints of epitaxy are relaxed on graphene terminated surfaces, allowing for multiple epitaxial relationships between substrate and film. This decoupling from graphene allows the Ni₂MnGa to be exfoliated, providing an etch-free method to produce free-standing membranes. Finally, we will discuss the similarities and differences between the three sample types based on magnetization, transport, and structural measurements.

Ferroelectric and multiferroic materials are of great importance for functional devices including low-power non-volatile memories and logic. Proper ferroelectrics, in which spontaneous polarization drives the ferroelectric phase transition, have attracted broad interest in the past few decades. Both theoretical and experimental studies indicated that critical thicknesses exist for proper ferroelectrics. For example, a synchrotron X-ray study revealed that, at room temperature, PbTiO₃ films can be structurally ferroelectric with thicknesses down to 3-unit cells [1], which agrees well with theoretical predictions [2]. Unfortunately, far less is known about improper ferroelectrics, where an ordering other than polarization (e.g., structural, charge or spin ordering) drives a phase transition that results in ferroelectricity.

To explore the intrinsic properties of improper ferroelectrics, we have investigated a model system, hexagonal LuFeO₃. Phase-pure films of this improper ferroelectric are grown on epitaxial iridium bottom electrodes on YSZ (111) substrates by MBE. Atomic force microscopy (AFM) shows that h-LuFeO₃ films grow smoothly on the iridium bottom electrode in a layer-by-layer fashion with an RMS roughness of 0.55 nm. X-ray ω-rocking curves are measured to assess the structural quality. The full width at half maximum (FWHM) of 111 Ir peak is 29 arc sec (0.008°) and of the 002 h-LuFeO₃ peak is 0.12°, narrower than any prior reports [3,4] by factors of about 60 and 2, respectively. Our results show that h-LuFeO₃ films as thin as 1-unit cell are still ferroelectric at ~900 °C.

[1] Fong, D. D. et al. Ferroelectricity in ultrathin perovskite films. Science 304, 1650-1653 (2004).

[2] Ghosez, Ph. et al. Microscopic model of ferroelectricity in stress-free PbTiO₃ ultrathin films. Applied Physics Letters 76, 2767-2769 (2000).

[3] Pandey, A. D. et al. Single orientation graphene synthesized on iridium thin films grown by molecular beam epitaxy. Applied Physics Letters 98, 181903 (2011).

[4] Sinha, K. K. Growth and characterization of hexagonal rare-earth ferrites (h-RFeO₃; R = Sc, Lu, Yb). University of Nebraska, PhD dissertation (2018).

YbRh₂Si₂ is a heavy fermion material [1, 2] crystallizing in the tetragonal ThCr₂Si₂-type structure (space group I4/mmm). It has a well-defined quantum critical point [2, 3], a linear-in-temperature strange metal behavior resulting from a dynamical electron localization-delocalization transition [1] and shows superconductivity below 10 mK [4]. YbRh₂Si₂ crystal thin films have been recognized as promising for novel applications, such as THz transmission spectroscopy and shot noise detection.

In this work, we study the growth of YbRh₂Si₂ on Ge(001) substrates by molecular beam epitaxy (MBE). The substrate chosen is Ge (fcc, a=5.658 Å [5]), due to the low lattice mismatch between its nearest neighbor distance a'=(a√2)/2= 4.001 Å and the a lattice parameter of YbRh₂Si₂ (a= 4.007 Å, c= 9.860 Å [6]). The thickness of the deposited films is around 60 nm.

Complementary characterization techniques, in situ reflection high-energy electron diffraction (RHEED), ex situ atomic force microscopy (AFM), x-ray diffraction (XRD), and energy-dispersive x-ray (EDX) spectroscopy, demonstrate that epitaxial crystalline films are obtained.

The crystal structure analysis highlights that small changes in the out-of-plane lattice parameter correlate with the Rh content. In particular, larger values are directly related to higher Rh concentration. XRD ϕ-scans show that the films have a four-fold symmetry and are rotated by 45° with respect to the substrate. Resistivity measurements show that the 60 nm thin film YbRh₂Si₂ exhibit similar characteristics to bulk single crystals.

[1] L. Prochaska, et al., Science 367, 285 (2020).
[2] O. Trovarelli, et al., Phys. Rev. Lett. 85, 626(2000).
[3] P. Gegenwart, et al., Phys. Rev. Lett. 89, 056402 (2002).
[4] D. H. Nguyen, et al., Nature Communications, 12, 4341 (2021).
[5] L. E. Vorobyev, in Handbook Series on Semiconductor Parameters, M. Levinshtein, Ed. (World Scientific, 1996), pp. 33-57.
[6] S. Wirth, et al., J. Phys.: Condens. Matter 24, 294203 (2012).