Hexagonal $AB$O$_3$ oxides ($A$, $B$ = cation) are a rich materials class for realizing novel quantum phenomena. Their hexagonal symmetry, oxygen trigonal bipyramid coordination and quasi-two dimensional layering give rise to properties distinct from those of the cubic $AB$O$_3$ perovskites. As bulk materials, most of the focus in this materials class has been on the rare earth manganites, $R$MnO$_3$ ($R$ = rare earth); these materials display coupled ferroelectricity and antiferromagnetic order. In this review, we focus on the thin film manifestations of the hexagonal $AB$O$_3$ oxides. We cover the stability of the hexagonal oxides and substrates which can be used to template the hexagonal structure. We show how the thin film geometry not only allows for further tuning of the bulk-stable manganites but also the realization of metastable hexagonal oxides such as the $R$FeO$_3$ that combine ferroelectricity with weak ferromagnetic order. The thin film geometry is a promising platform to stabilize additional metastable hexagonal oxides to search for predicted high-temperature superconductivity and topological phases in this materials class.
Hexagonal group IV materials like silicon and germanium are expected to display remarkable optoelectronic properties for future development of photonic technologies. However, the fabrication of hexagonal group IV semiconductors within the vapour-liquid-solid method has been obtained using gold as a catalyst thus far. In this letter, we show the synthesis of hexagonal Ge on self-assisted GaAs nanowires using molecular beam epitaxy. With an accurate tuning of the Ga and As molecular beam flux we selected the crystal phase, cubic or hexagonal, of the GaAs NWs during the growth. A 500 nm-long hexa…
RF plasma assisted MBE growth of Scandium Nitride (ScN) thin films on GaN (0001)/SiC, AlN (0001)/Al2O3 and on 6H-SiC (0001) hexagonal substrates is found to lead to a face centered cubic (rock-salt) crystal structure with (111) out-of-plane orientation instead of hexagonal orientation. For the first time, cubic (111) twinned patterns in ScN are observed by in-situ electron diffraction during epitaxy, and the twin domains in ScN are detected by electron backscattered diffraction, and further corroborated with X-ray diffraction. The epitaxial ScN films display very smooth, sub nanometer surface …
Epitaxial films which contains more than one crystallographic orientation are of interest due to the possibility of altered magnetic, electrical, and optical properties. Thin films of FeSe have been grown on single-crystal MgO substrates under conditions that produces the simultaneous, epitaxial growth of tetragonal and hexagonal phases. We show that this double epitaxy is characterized by phase domains with a well-defined epitaxial relationship to each other and that the relative phase fraction can be controlled. For growth temperatures of 350-450 degrees C, the (001)-oriented tetragonal phas…
Hexagonal boron nitride (h-BN) is a layered two-dimensional material with properties that make it promising as a dielectric in various applications. We report the growth of h-BN films on Ni foils from elemental B and N using molecular beam epitaxy. The presence of crystalline h-BN over the entire substrate is confirmed by Raman spectroscopy. Atomic force microscopy is used to examine the morphology and continuity of the synthesized films. A scanning electron microscopy study of films obtained using shorter depositions offers insight into the nucleation and growth behavior of h-BN on the Ni sub…
Hexagonal boron nitride (hBN) has been grown on sapphire substrates by ultra-high temperature molecular beam epitaxy (MBE). A wide range of substrate temperatures and boron fluxes have been explored, revealing that high crystalline quality hBN layers are grown at high substrate temperatures, $>$1600$^\circ$C, and low boron fluxes, $\sim1\times10^{-8}$ Torr beam equivalent pressure. \emph{In-situ} reflection high energy electron diffraction (RHEED) revealed the growth of hBN layers with $60^\circ$ rotational symmetry and the $[11\bar20]$ axis of hBN parallel to the $[1\bar100]$ axis of the sapp…
$WSe_2$ Monolayers Grown by Molecular Beam Epitaxy on hBN
Institute of Experimental Physics, Faculty of Physics, University of Warsaw（华沙大学物理学院实验物理研究所）；Institute of Physics, Polish Academy of Sciences（波兰科学院物理研究所）；Research Center for Materials Nanoarchitectonics, National Institute for Materials Science（国家材料科学研究所材料纳米构造研究中心）；Research Center for Electronic and Optical Materials, National Institute for Materials Science（国家材料科学研究所电子与光学材料研究中心）
GaN-based resonators supporting whispering gallery modes are suggested as a promising approach for fabrication of efficient nanophotonic applications. A modification of excitonic emission due to coupling with cavity modes is observed experimentally in the GaN planar hexagonal microcavities grown by a selective area metal-organic vapor phase epitaxy. Low temperature cathodoluminescence spectra measured in-situ scanning electron microscope show two or three emission peaks split by 100 meV or 160 meV for the resonators formed by double- and single-wall micro-nuts, respectively. The results are co…
We report the growth of self-assembled Bi2Se3 quantum dots (QDs) by molecular beam epitaxy on GaAs substrates using the droplet epitaxy technique. The QD formation occurs after anneal of Bismuth droplets under Selenium flux. Characterization by atomic force microscopy, scanning electron microscopy, X-ray diffraction, high-resolution transmission electron microscopy and X-ray reflectance spectroscopy is presented. The quantum dots are crystalline, with hexagonal shape, and have average dimensions of 12 nm height (12 quintuple layers) and 46 nm width, and a density of $8.5 \cdot 10^9 cm^{-2}$. T…
We demonstrate the growth of thin films of molybdenum ditelluride and molybdenum diselenide on sapphire substrates by molecular beam epitaxy. In-situ structural and chemical analyses reveal stoichiometric layered film growth with atomically smooth surface morphologies. Film growth along the (001) direction is confirmed by X-ray diffraction, and the crystalline nature of growth in the 2H phase is evident from Raman spectroscopy. Transmission electron microscopy is used to confirm the layered film structure and hexagonal arrangement of surface atoms. Temperature dependent electrical measurements…
Magnetic tunnel junctions (MTJ) have been grown by using molecular beam epitaxy on c-plane Al2O3 substrates. The MTJ stacks consist of two ferromagnetic hcp-Co layers separated by a thin insulating h-BN barrier. The samples have been grown in a single run revealing single crystalline epitaxial structures with sharp interfaces as observed by applying transmission electron microscopy. The in-plane magnetization experiments have revealed separate magnetization switching of a thin top Co (soft) layer and a thick bottom Co (hard) layer. At zero magnetic field the two Co layers are found in an antip…
Bi$_2$Te$_3$ is a material with high efficiency of thermoelectric energy conversion. Recently, it was also recognized as a topological insulator, and is often used as the basis for creation of other types of topological matter. Pulsed laser deposition (PLD) is widely considered as a simple method for growing of multicomponent films, but not as a tool for van der Waals epitaxy. We demonstrate here that the van der Waals epitaxy of Bi$_2$Te$_3$ is indeed impossible in vacuum PLD, but is possible in the presence of a background gas, which is confirmed by the results of scanning tunneling microsco…
Hexagonal perovskites are an attractive group of materials due to their various polymorph phases and rich structure-property relationships. BaRuO3 (BRO) is a prototypical hexagonal perovskite, in which the electromagnetic properties are significantly modified depending on its atomic structure. Whereas thin-film epitaxy would vastly expand the application of hexagonal perovskites by epitaxially stabilizing various metastable polymorphs, the atomic structure of epitaxial hexagonal perovskites, especially at the initial growth stage, has rarely been investigated. In this study, we show that an in…
$Al_xGa_{1-x}N$ is a critical ultra-wide bandgap material for optoelectronics, but the deposition of thick, high quality epitaxial layers has been hindered by a lack of lattice-matched substrates. Here we identify the (111) face of transition metal carbides as a suitable class of materials for substrates lattice matched to (0001) $Al_xGa_{1-x}N$ and demonstrate the growth of thin film TaC which has an effective hexagonal lattice constant matched to $Al_{0.45}Ga_{0.55}N$. We explore growth conditions for sputtered TaC on sapphire substrates and investigate the effects of sputter power, layer th…
Reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM), vibrating sample magnetometry and other physical property measurements are used to investigate the structure, morphology, magnetic and magneto-transport properties of (001)-oriented Cr$_2$Te$_3$ thin films grown on Al$_2$O$_3$(0001) and Si(111)-(7$\times$7) surfaces by molecular beam epitaxy (MBE). Streaky RHEED patterns indicate flat smooth film growth on both substrates. STM studies show the hexagonal arrangements of surface atoms. Determination of the lattice parameter from atomically resolved STM imag…
Chromium selenide thin films were grown epitaxially on Al${_2}$O${_3}$(0001) and Si(111)-(7${\times}$7) substrates using molecular beam epitaxy (MBE). Sharp streaks in reflection high-energy electron diffraction and triangular structures in scanning tunneling microscopy indicate a flat smooth film growth along the c-axis, and is very similar to that from a hexagonal surface. X-ray diffraction pattern confirms the growth along the c-axis with c-axis lattice constant of 17.39 Å. The grown film is semiconducting, having a small band gap of about 0.034 eV, as calculated from the temperature depend…
In their recent publication, Zhao et al. [J. Appl. Phys. 110, 033523 (2011)] claim to have found a new three-dimensional relationship for niobium-on-sapphire epitaxy. However, two critical errors were made in the analysis of x-ray diffraction measurements. The crystal structure of sapphire (alpha-Al2O3) was erroneously cited as hexagonal close-packed, and crystallographic orientations of sapphire were misidentified. Correcting these errors, one finds their claim unjustified.
We report on luminescent centers contained in a few-layer-thick hexagonal boron nitride (h-BN) film grown on Ni by molecular beam epitaxy. After transfer to a SiO$_2$/Si substrate, sharp lines are observed in photo- and cathodoluminescence spectra in both the ultraviolet and the visible range. Spatially resolved measurements reveal that the luminescent centers responsible for these lines are localized within microscopic multi-layer islands that form at the nucleation centers of the h-BN film. The comparison of their energy, polarization and phonon replica emission with previous theoretical pre…
Thin crystalline layers of TaAs Weyl semimetal (9 and 18 nm thick) are grown by molecular beam epitaxy on GaAs(001) substrates. The (001) planes of the tetragonal TaAs lattice are parallel to the GaAs(001) substrate, but the corresponding in-plane crystallographic directions of the substrate and the layer are rotated by 45°. In spite of a substantial lattice mismatch (about 19%) between GaAs(001) substrate and TaAs epilayer no misfit dislocations are observed at the GaAs(001)/TaAs(001) interface. Only stacking fault defects in TaAs are detected with transmission electron microscopy. Thorough X…
Anti-ThCr$_2$Si$_2$ type $RE_2$O$_2$Sb ($RE$ = rare earth) with Sb square net has shown insulating conduction so far. Here we report the synthesis of La$_2$O$_2$Sb epitaxial thin films for the first time by multilayer solid-phase epitaxy. The valence state of Sb was about -2 evaluated from X-ray photoemission spectroscopy measurement, and the indirect band gap of 0.17 eV was observed. The La$_2$O$_2$Sb epitaxial thin film showed unexpectedly high electrical conduction as a narrow gap semiconductor, whose resistivity at room temperature was approximately ten-thousand-fold lower than that of La$…
The effect of film morphology on its surface chemistry and band structure has been analyzed for gallium nitride epitaxial films grown by molecular beam epitaxy. The film morphology has been studied using scanning electron microscopy and atomic force microscopy, and the bandstructure, defect and emission properties have been studied by X ray photoelectron spectroscopy and cathodoluminescence spectroscopy. It was found that the highly porous GaN nanowall network shows the highest relative conductivity and does not have defect related luminescence. The flatter films were more resistive and showed…
Direct observations using scanning transmission electron microscopy unveil an intriguing interfacial bi-layer that enables epitaxial growth of a strain-free, monoclinic, bronze-phase VO2(B) thin film on a perovskite SrTiO3 (STO) substrate. We observe an ultrathin (2-3 unit cells) interlayer best described as highly strained VO2(B) nanodomains combined with an extra (Ti,V)O2 layer on the TiO2 terminated STO (001) surface. By forming a fully coherent interface with the STO substrate and a semi-coherent interface with the strain-free epitaxial VO2(B) film above, the interfacial bi-layer enables t…
We describe a theoretical study of the role of adsorbate interactions in island nucleation and growth, using Ag/Pt(111) heteroepitaxy as an example. From density-functional theory, we obtain the substrate-mediated Ag adatom pair interaction and we find that, past the short range, a repulsive ring is formed about the adatoms. The magnitude of the repulsion is comparable to the diffusion barrier. In kinetic Monte Carlo simulations, we find that the repulsive interactions lead to island densities over an order of magnitude larger than those predicted by nucleation theory and thus identify a sever…
Pritzker School of Molecular Engineering, University of Chicago（芝加哥大学普里茨克分子工程学院）；Department of Chemistry, James Franck Institute, University of Chicago（芝加哥大学化学系、詹姆斯·弗兰克研究所）；Department of Materials Science and Engineering, Rensselaer Polytechnic Institute（伦斯勒理工学院材料科学与工程系）；X-ray Science Division, Advanced Photon Source, Argonne National Laboratory（阿贡国家实验室先进光子源部X射线科学分部）；Center for Nanoscale Materials, Argonne National Laboratory（阿贡国家实验室纳米材料中心）；Electron Microscopy Core, University of Illinois Chicago（伊利诺伊大学芝加哥分校电子显微镜核心设施）
MnBi2Te4 has recently been predicted and shown to be a magnetic topological insulator with intrinsic antiferromagnetic order. However, it remains a challenge to grow stoichiometric MnBi2Te4 films by molecular beam epitaxy (MBE) and to observe pure antiferromagnetic order by magnetometry. We report on a detailed study of MnBi2Te4 films grown on Si(111) by MBE with elemental sources. Films of about 100 nm thickness are analyzed in stoichiometric, structural, magnetic and magnetotransport properties with high accuracy. High-quality MnBi2Te4 films with nearly perfect septuple-layer structure are r…
In recent years, two-dimensional van der Waals materials have emerged as an important platform for the observation of long-range ferromagnetic order in atomically thin layers. Although heterostructures of such materials can be conceived to harness and couple a wide range of magneto-optical and magneto-electrical properties, technologically relevant applications require Curie temperatures at or above room-temperature and the ability to grow films over large areas. Here we demonstrate the large-area growth of single-crystal ultrathin films of stoichiometric Fe5GeTe2 on an insulating substrate us…
The search for topological superconductivity has motivated investigations into materials that combine topological and superconducting properties. The half-Heusler compound YPtBi appears to be such a material, however experiments have thus far been limited to bulk single crystals, drastically limiting the scope of available experiments. This has made it impossible to investigate the potential topological nature of the superconductivity in this material. Experiments to access details about the superconducting state require sophisticated lithographic structures, typically based on thin films. Her…
N-polar (In,Ga)N/GaN quantum wells prepared on freestanding GaN substrates by plasma-assisted molecular beam epitaxy at conventional growth temperatures of about 650 °C do not exhibit any detectable luminescence even at 10 K. In the present work, we investigate (In,Ga)N/GaN quantum wells grown on Ga- and N-polar GaN substrates at a constant temperature of 730 °C. This exceptionally high temperature results in a vanishing In incorporation for the Ga-polar sample. In contrast, quantum wells with an In content of 20% and abrupt interfaces are formed on N-polar GaN. Moreover, these quantum wells e…
The Pd, and Pt based ABO2 delafossites are a unique class of layered, triangular oxides with 2D electronic structure and a large conductivity that rivals the noble metals. Here, we report successful growth of the metallic delafossite PdCoO2 by molecular beam epitaxy (MBE). The key challenge is controlling the oxidation of Pd in the MBE environment where phase-segregation is driven by the reduction of PdCoO2 to cobalt oxide and metallic palladium. This is overcome by combining low temperature (300 °C) atomic layer-by-layer MBE growth in the presence of reactive atomic oxygen with a post-growth …
The growth and microstructural properties of ternary monolayers of two-dimensional hexagonal materials are examined, including both individual two-dimensional crystalline grains and in-plane heterostructures, multijunctions, or superlattices. The study is conducted through the development of a ternary phase field crystal model incorporating sublattice ordering and the coupling among the three atomic components. The results demonstrate that a transition of compositional pattern or modulation in this type of two-dimensional ternary crystals, from phase separation to geometrically frustrated latt…
The magnetic order associated with the degree of freedom of spin in two-dimensional (2D) materials is subjected to intense investigation because of its potential application in 2D spintronics and valley-related magnetic phenomena. We report here a bottom-up strategy using molecular beam epitaxy to grow and dope large-area (cm$^2$) few-layer MoSe$_2$ with Mn as a magnetic dopant. High-quality Mn-doped MoSe$_2$ layers are obtained for Mn content of less than 5 % (atomic). When increasing the Mn content above 5 % we observe a clear transition from layer-by-layer to cluster growth. Magnetic measur…
Superconducting thin films of magnesium diboride (MgB$_2$) were prepared on MgO (001) substrate by a molecular beam epitaxy (MBE) method with the co-evaporation conditions of low deposition rate in ultra-high vacuum. The structural and physical properties of the films were studied by RHEED, XRD, XPS, resistivity and magnetization measurements.All films demonstrated superconductivity without use of any post-annealing process.The highest {\it T}$_{c,onset}$ determined by resistivity measurement was about 33K in the present samples.Anisotropic superconducting properties were evaluated by the resi…
One of the challenges of oxide molecular beam epitaxy (MBE) is the synthesis of oxides containing metals with high electronegativity (metals that are hard to oxidize). The use of reactive organometallic precursors can potentially address this issue. To investigate the formation of radicals in MBE, we explored three carefully chosen metal-organic precursors of tin for SnO2 and BaSnO3 growth: tetramethyltin (TMT), tetraethyltin (TET), and hexamethylditin (HMDT). All three precursors produced single-crystalline, atomically smooth, and epitaxial SnO2 (101) films on r-Al2O3 in the presence of an ox…
As-grown superconducting MgB_2 thin films have been grown on SrTiO_3(001), MgO(001), and Al_2O_3(0001) substrates by a molecular beam epitaxy (MBE) method with novel co-evaporation conditions of low deposition rate in ultra-high vacuum. The structural and physical properties of the films were studied by RHEED, XRD, electrical resistivity measurements, and SQUID magnetometer. The RHEED patterns indicate three-dimensional growth for MgB_2. The highest T_c determined by resistivity measurement was about 36K in these samples. And a clear Meissner effect below T_c was observed using magnetic suscep…
Mn5Ge3 thin films have been demonstrated as a promising spin-injector material for germanium-based spintronic devices. So far, Mn5Ge3 has been grown epitaxially only on Ge (111) substrates. In this letter we present the growth of epitaxial Mn5Ge3 films on Ge (100) substrates. The Mn5Ge3 film is synthetized via sub-second solid-state reaction between Mn and Ge upon flash lamp annealing for 20 ms at the ambient pressure. The single crystalline Mn5Ge3 is ferromagnetic with a Curie temperature of 283 K. Both the c-axis of hexagonal Mn5Ge3 and the magnetic easy axis are parallel to the Ge (100) sur…
We report growth of superconducting Sr2RuO4 films by oxide molecular beam epitaxy (MBE). Careful tuning of the Ru flux with an electron beam evaporator enables us to optimize growth conditions including the Ru/Sr flux ratio and also to investigate stoichiometry effects on the structural and transport properties. The highest onset transition temperature of about 1.1 K is observed for films grown in a slightly Ru-rich flux condition in order to suppress Ru deficiency. The realization of superconducting Sr2RuO4 films via oxide MBE opens up a new route to study the unconventional superconductivity…
Gray tin, also known as α-Sn, has been attracting research interest recent years due to its topological nontrivial properties predicted theoretically. The Dirac linear band dispersion has been proved experimentally by angle resolved photoemission spectroscopy. We have grown a series of α-Sn thin film samples in two types with different substrates and thicknesses by molecular beam epitaxy. To explore the possible exotic physical properties related to the topological band structures, we have measured the electrical transport properties of our α-Sn thin film samples and observed multiple supercon…
Mn concentration depth profiles in Mn-doped GaAs thin films grown at substrate temperatures of 580 and 250 °C using various Mn cell temperatures have been investigated with dynamic secondary ion mass spectrometry and Auger electron spectroscopy. When the samples are grown at a low substrate temperature of 250 °C, the Mn distributes uniformly. For the samples grown at a high substrate temperature of 580 °C, the concentration depth profiles are easily fitted with a temperature-dependent Fermi function only if the Mn concentration is above the solubility limit. However, when the Mn concentration …
We report on the systematic investigation of the optical properties of a selectively grown quantum dot gain material assisted by block-copolymer lithography for potential applications in active optical devices operating in the wavelength range around 1.55 um and above. We investigated a new type of diblock copolymer PS-b-PDMS (polystyrene-block-polydimethylsiloxane) for the fabrication of silicon oxycarbide hard mask for selective area epitaxy of InAs/InP quantum dots. An array of InAs/InP quantum dots was selectively grown via droplet epitaxy. Our detailed investigation of the quantum dot car…
The structural transition at about 1000 °C, from the hexagonal to the orthorhombic phase of LuFeO3, has been investigated in thin films of LuFeO3. Separation of the two structural phases of LuFeO3 occurs on a length scale of micrometer, as visualized in real space using X-ray photoemission electron microscopy (X-PEEM). The results are consistent with X-ray diffraction and atomic force microscopy obtained from LuFeO3 thin films undergoing the irreversible structural transition from the hexagonal to the orthorhombic phase of LuFeO3, at elevated temperatures. The sharp phase boundaries between th…
We present a systematic study of the morphology of homoepitaxial InP films grown by metalorganic vapor-phase epitaxy which are imaged with ex situ atomic force microscopy. These films show a dramatic range of different surface morphologies as a function of the growth conditions and substrate (growth temperature, V/III ratio, and miscut angle < 0.6deg and orientation toward A or B sites), ranging from stable step flow to previously unreported strong step bunching, over 10 nm in height. These observations suggest a window of growth parameters for optimal quality epitaxial layers. We also present…
The stoichiometric "111" iron-based superconductor, LiFeAs, has attacted great research interest in recent years. For the first time, we have successfully grown LiFeAs thin film by molecular beam epitaxy (MBE) on SrTiO3(001) substrate, and studied the interfacial growth behavior by reflection high energy electron diffraction (RHEED) and low-temperature scanning tunneling microscope (LT-STM). The effects of substrate temperature and Li/Fe flux ratio were investigated. Uniform LiFeAs film as thin as 3 quintuple-layer (QL) is formed. Superconducting gap appears in LiFeAs films thicker than 4 QL a…
The heteroepitaxial growth of Re (0001) films on Nb (110) surfaces has been investigated. Nb/Re bilayers were grown on A-plane sapphire -- alpha-Al2O3 (11-20) -- by molecular beam epitaxy. While Re grew with a (0001) surface, the in-plane epitaxial relationship with the underlying Nb could be best described as a combination of Kurdjumov-Sachs and Nishiyama-Wassermann orientations. This relationship was true regardless of Re film thickness. However, an evolution of the surface morphology with increasing Re thickness was observed, indicative of a Stranski-Krastanov growth mode. Re (0001) layers …
