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Silicene, the silicon analogue of graphene, consists of an atomically buckled honeycomb lattice of silicon atoms. Theory predicts exceptional electronic properties, including Dirac fermions and a topological spin Hall insulator phase. An important obstacle impeding exploration of such properties in electronic devices is the chemical sensitivity of silicene, hampering its incorporation in layer stacks. Here we show experimentally that epitaxial silicene and hexagonal boron nitride (h-BN) can be stacked without perturbing the electronic properties of silicene. Intercalated silicene underneath epitaxial h-BN on ZrB 2 (0 0 0 1) substrate films is obtained by depositing Si atoms at room temperature. Using (angle resolved) photoelectron spectroscopy (ARPES, PES) and scanning tunneling microscopy (STM) we find that the intercalated silicene exhibits the same electronic properties as epitaxial silicene on ZrB 2 , while it resists oxidation in air up to several hours. This is an essential step towards the development of layer stacks that allow for fabrication of devices.

The realization of silicene-free ZrB_2(0001) thin films grown on Si(111) by Ar^+ ion bombardment allowed for studying the spontaneous formation of silicene on their surfaces. Imaging the bare ZrB_2(0001) surface by STM revealed the structures of Zr-terminated and B-terminated ZrB_2(0001) created by the bombardment. The spontaneous formation of a continuous silicene sheet on asputtering-induced disordered ZrB_2 surface demonstrates that silicene does not require an atomically-flat crystalline template to be stabilized. This opens the way to the fabrication of large scale single-crystal sheets and points out the potential of silicene to be used in the next generation silicon-based technologies., identifier:https://dspace.jaist.ac.jp/dspace/handle/10119/14720

We present a method for the formation of an epitaxial surface layer involving B, N, and Si atoms on a ZrB2(0001) thin film on Si(111). It has the potential to be an insulating growth template for 2D semiconductors. The chemical reaction of NH3 molecules with the silicene-terminated ZrB2 surface was characterized by synchrotron-based, high-resolution core-level photoelectron spectroscopy and low-energy electron diffraction. In particular, the dissociative chemisorption of NH3 at 400 °C leads to surface nitridation, and subsequent annealing up to 830 °C results in a solid phase reaction with the ZrB2 subsurface layers. In this way, a new nitride-based epitaxial surface layer is formed with hexagonal symmetry and a single in-plane crystal orientation., Electrical Engineering, Mathematics and Computer Science

Since epitaxial silicene is not chemically inert under ambient conditions, its application in devices and the ex-situ characterization outside of ultrahigh vacuum environments require the use of an insulating capping layer. Here, we report on a study of the feasibility of encapsulating epitaxial silicene on ZrB_2(0001) thin films grown on Si(111) substrates by aluminum nitride (AlN) deposited using trimethylaluminum (TMA) and ammonia (NH_3) precursors. By in-situ high-resolution core-level photoelectron spectroscopy, the chemical modifications of the surface due to subsequent exposure to TMA and NH_3 molecules, at temperatures of 300 °C and 400 °C, respectively, have been investigated. While an AlN-related layer can indeed be grown, silicene reacts strongly with both precursor molecules resulting in the formation of Si–C and Si–N bonds such that the use of these precursors does not allow for the protective AlN encapsulation that leaves the electronic properties of silicene intact., identifier:https://dspace.jaist.ac.jp/dspace/handle/10119/12911

We investigated the electronic properties of epitaxial silicene on ZrB_2(0001) thin film grown on Si(111) by means of low-temperature scanning tunneling spectroscopy and density functional theory calculations. The position of silicon atoms and thus, the localization of the valence and conduction states were deducted from the comparison of the spectra and the computed local density of states. We point out the strong contribution of p_z orbitals of specific atoms to those states which indicates the π character of the conduction and valence bands. A clear correlation between hybridization of the orbitals of the Si atoms and the buckling was evidenced.

As silicene is not chemically inert, the study and exploitation of its electronic properties outside of ultrahigh vacuum environments require the use of insulating capping layers. In order to understand if aluminum oxide might be a suitable encapsulation material, we used high-resolution synchrotron photoelectron spectroscopy to study the interactions of Al atoms and O_2 molecules, as well as the combination of both, with epitaxial silicene on thin ZrB_2(0001) films grown on Si(111). The deposition of Al atoms onto silicene, up to the coverage of about 0.4 Al per Si atoms, has little effect on the chemical state of the Si atoms. The silicene-terminated surface is also hardly affected by exposure to O_2 gas, up to a dose of 4500 L. In contrast, when Al-covered silicene is exposed to the same dose, a large fraction of the Si atoms becomes oxidized. This is attributed to dissociative chemisorption of O_2 molecules by Al atoms at the surface, producing reactive atomic oxygen species that cause the oxidation. It is concluded that aluminum oxide overlayers prepared in this fashion are not suitable for encapsulation since they do not prevent but actually enhance the degradation of silicene., identifier:https://dspace.jaist.ac.jp/dspace/handle/10119/12321

The evolution of the electronic structure and the structural stability of epitaxial silicene on ZrB_2(0001) thin films exposed to K atoms has been studied by angle-resolved photoelectron spectroscopy and low-energy electron diffraction. Potassium adsorption leads to charge donation to the silicene lattice, which is accompanied by the partial filling of a formerly unoccupied π＾* band and by the increasing hybridization between the diboride surface state and the lower branch of the back-folded π band. The results allow an identification of silicene-derived π electronic states and confirm that before K adsorption, the interactions at the silicene-substrate interface are rather weak.

We report on the crystal and electronic structures of single-crystalline anthracene multilayer films grown onto Cu(110) surfaces at 140 K. Electron diffraction patterns and angle-resolved ultraviolet photoelectron spectra are consistent with a thin film phase characterized by a “standing-up” molecular orientation and a squarelike unit cell within the plane of herringbone-stacked molecular layers. A single orientation of the film is achieved in an epitaxial relation with the lattice of the substrate surface. A clear dispersion of intermolecular bands relates anisotropic charge-carrier transport to particular crystallographic directions., identifier:https://dspace.jaist.ac.jp/dspace/handle/10119/9522

Integration of metals and semiconductors having three- or sixfold symmetry on device-oriented [i.e., (001)] silicon wafers, which have fourfold symmetry, has been a long-standing challenge. The authors demonstrate that, by using symmetry-converted (111) silicon on insulator, wurtzite-structure gallium nitride, which has threefold symmetry, can be integrated with Si(001). The stability of the symmetry-converted Si(111) layer makes this technique appealing to the commercial integration of wide-ranging important materials onto Si(001) base wafers. c 2007 American Institute of Physics., identifier:https://dspace.jaist.ac.jp/dspace/handle/10119/4970

The effect of nitridation on the epitaxial growth of GaN on lattice-matched ZrB_2(0001) films prepared ex situ and in situ was studied using an ultrahigh-vacuum molecular-beam epitaxy (MBE)-scanning probe microscopy system. The growth of GaN was carried out by rf-plasma-assisted MBE, and epitaxy of wurtzite GaN was observed on both ex situ and in situ prepared ZrB_2 samples. The polarity was found to be consistently N-polar regardless of the samples, based on the observation of a series of N-polar Ga-rich reconstructions: (3×3), (6×6), and c(6×12). The nitridation of ZrB_2 film was conducted by exposing it to active nitrogen and well-ordered hexagonal-BN (h-BN) formation was observed when the annealing temperature was above 900 °C. The partially formed BN layer affected neither the epitaxy nor the polarity of GaN, but when the surface was fully covered with well-ordered h-BN, GaN growth did not occur., identifier:https://dspace.jaist.ac.jp/dspace/handle/10119/4516

GaN is directly grown on Si(111) by radio-frequency plasma-assisted molecular-beam epitaxy, and the surface is studied using in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). By optimizing the growth condition, well-defined surface reconstructions are observed in atomically-resolved STM images after the additional Ga deposition, indicating the uniform N-polarity of the grown film. We show that N-rich condition in the initial GaN growth and slightly Ga-rich condition in the subsequent growth are critical in order to achieve monopolar uniform GaN films., identifier:https://dspace.jaist.ac.jp/dspace/handle/10119/4527

Low hydrogen diffusivity and solubility of aluminum oxide make a vapor deposited aluminum oxide (Al_2O_3) film an anticipated hydrogen permeation barrier coating. In this paper, amorphous Al_2O_3 film was deposited using filtered vacuum arc method. As a substrate, vapor deposited amorphous tungsten oxide (WO_3) film was used in order to characterize hydrogen permeation barrier performance of the Al_2O_3 film utilizing coloration of WO_3 when it forms H_xWO_3. The samples were exposed to the flux and angular quantified atomic hydrogen beam, and the degree of coloration was characterized by visible to near infrared range transmission spectroscopy. Using this method, a half-micron thick amorphous Al_2O_3 film reducing atomic hydrogen reaching the underlying WO_3 film to 3 ×10^<-4> was measured. Furthermore, Al_2O_3 film as thin as 20 nm showed atomic hydrogen reduction of 4 × 10^<-3>. The results indicate effectiveness of thin vapor deposited Al_2O_3 film as a permeation barrier against atomic hydrogen., identifier:https://dspace.jaist.ac.jp/dspace/handle/10119/4966