Hexagonal boron nitride

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Mechanical decoupling of quantum emitters in hexagonal boron nitride from low-energy phonon modes

Mechanical decoupling of quantum emitters in hexagonal boron nitride from low-energy phonon modes

Quantum emitters in hexagonal boron nitride were recently reported to hold unusual narrow homogeneous linewidths of tens of megahertz within the Fourier transform limit at room temperature. This unique observation was traced back to decoupling from in-plane phonon modes. Here, we investigate the origins for the mechanical decoupling. New sample preparation improved spectral diffusion, which allowed us to reveal a gap in the electron-phonon spectral density for low phonon frequencies. This sign for mechanical decoupling persists up to room temperature and explains the observed narrow lines at 300 kelvin. We investigate the dipole emission directionality and reveal preferred photon emission through channels between the layers supporting the claim for out-of-plane distorted defect centers. Our work provides insights into the underlying physics for the persistence of Fourier transform limit lines up to room temperature and gives a guide to the community on how to identify the exotic emitters.
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Raman Spectroscopy on Graphene Encapsulated in Hexagonal Boron Nitride

Raman Spectroscopy on Graphene Encapsulated in Hexagonal Boron Nitride

As a consequence, Raman spectroscopy is a highly suitable tool for sample characterization in graphene research, since such short-ranged strain variations have been shown to be a limitin[r]

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Ultra Thin Silicon Nitride Interface Engineering

Ultra Thin Silicon Nitride Interface Engineering

Having a broader perspective regarding the impact of the silicon nitride applications the interface engineering approach becomes quite appealing for superconducting sil- icon based Josephson junctions where the Schottky barriers at the silicon surface require high doping concentrations at degenerate state canceling any gate modu- lation of the silicon channel. Here, a lower doping concentration in combination with the silicon nitride provides a sophisticated concept. Moreover, the ohmic con- tacts are promising if the random dopant distribution becomes a limiting factor. Cryogenic MOSFETs try to bypass the integration of dopants, which results into Schottky-barrier MOSFET transfer characteristics [ 257 ]. A second field of interest is the incorporation of the silicon nitride for Tunnel-FETs. The huge advantage of a highly doped substrate in combination with a Schottky-Mott junction (see Fig. 8.1 ) does not require any implantation or electrostatic side gates. Silicon nitride is not the only promising interface engineering material. Keeping in mind that germanium will play a greater role in the semiconductor industry, first steps on germanium ni- tride as a Fermi level depinning tool have been already performed [ 271 ]. Whereas, plasma nitridation assisted by thermal annealing of germanium is more intense stud- ied, a thermal nitridation in ammonia atmosphere was not investigated yet [ 272 , 273 ]. Hexagonal boron nitride with an excellent crystalline nature and passivation skills could be the key of encapsulating 2D materials, especially for emerging research materials [ 274 ].
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Properties of nanocrystalline cubic boron nitride films

Properties of nanocrystalline cubic boron nitride films

Recent reviews on various preparation methods and their parameter spaces, given in [Kulisch 1999], indicate that in most preparational approaches various mixtures of nitrogen and argon ions are used for the bombardment with energies of typically some hundred eV. As a consequence, one expects an incorporation of Ar atoms into c-BN films with the total amount especially depending on the applied energy of the Ar + ions as well as the deposition temperature, both parameters influencing the sticking probability of argon. Furthermore, with this probability being fixed, the concentration of incorporated Ar is anticipated to scale with the ratio of the argon and the boron fluxes impinging onto the substrate. With all these parameters kept constant during the film deposition, a rather homogeneous Ar depth profile is expected [Deyneka 2003]. There is, however, an additional subtle detail related to the growth of c-BN films at least on top of the silicon substrate. When analysing the growing layers in more detail, e.g. by High Resolution Transmission Electron Microscopy (HRTEM) (see Sec. 3.2.7) starting from the substrate surface, one observes, after a strongly disordered and intermixed Si/BN interface layer, a nanocrystalline highly textured hexagonal boron nitride (h-BN) layer, which has its basal planes perpendicular to the substrate [Boyen 2002]. In the context of the present work such a layer sequence opens the possibility that argon is preferentially incorporated in one of the above phases, e.g. in the less dense h-BN layer resulting in an inhomogeneous Ar depth profile. In that case, the question arises whether the c-BN layer being on top of the h-BN acts as a diffusion barrier impeding the argon from diffusing out even at elevated temperatures. Such a behaviour might be expected from the structural similarity of c-BN and diamond, for which considerable thermal diffusion is known only for a few elements like hydrogen/deuterium, nitrogen, oxygen and boron [Popovici 1995; Chrenko 1977; Narducci 1990; Ahlgren 1998].
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Quantum Transport and Shot Noise in Graphene-Boron Nitride Heterostructures

Quantum Transport and Shot Noise in Graphene-Boron Nitride Heterostructures

Soon after the first successful isolation of graphene monolayers in 2004, Novoselov et al. [ Nov05b ] started applying the “Scotch-Tape” technique to other layered materials, already including hBN. At that time, silicon wafers with 300 nm thermally grown SiO 2 were the substrate of choice, since interference in the oxide layer increases the optical contrast of graphene, allowing the identifi- cation of monolayers among thicker graphite pieces after exfoliation. With the development of transfer techniques for graphene, starting in 2008 [ Rei08 ], new possibilities to investigate the influence of different substrates on graphene were opened up. A tremendous improvement of sample quality was reached by Dean et al. [ Dea10 ] by combining graphene and hexagonal boron nitride, an atomically flat, insulating isomorph of graphite. Since it can only be synthesized as powder of millimeter size crystals [ Tan07 ][ Wat04 ], direct exfoliation onto hBN would not have been practically possible. With this breakthrough, the possibilities and the importance of the ability to transfer and manipulate 2d crystals became obvious, eventually giving rise to a new research field of “van-der-Waals heterostructures” [ Gei13 ]. Mayorov et al. encapsulated graphene in hBN by transferring first graphene onto a hBN crystal, and then another hBN crystal on top of graphene. While the deposition of material onto graphene generally deteriorates its properties, hBN ecapsulation was shown to not only protect the graphene from the environment, but to even further improve the mobility [ May11 ]. The latest advancement was introduced by Wang et al., who found a way to encapsulate graphene in hBN without introducing any impurities in the process. This technique, which was employed for the sample fabrication in this thesis, works by starting the stack from the top hBN and using it to pick up the graphene and bottom hBN. Using this technique, a mean free path of up to 15 μm was found[ Wan13 ].
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Hyperbolic phonon-polaritons in boron nitride for near-field optical imaging and focusing

Hyperbolic phonon-polaritons in boron nitride for near-field optical imaging and focusing

During the search for better plasmonic materials 14,15 , polar dielectrics capable of supporting phonon-polaritons such as silicon carbide 16–20 and hexagonal boron nitride (hBN) 10,11,21 have been demonstrated as superior alternatives to metals at mid-infrared to THz frequencies. Interestingly, many phonon- resonant materials such as quartz 22 , zinc oxide 23 , calcite 24 and hBN 10,11,21 are natural hyperbolic materials 25–27 . These natural hyperbolic materials support hyperbolic phonon-polariton modes within homogeneous crystals with atomic-scale unit cells, thus the upper limit on the highest propagating wavevectors k associated with artificial metal-dielectric HMMs is no longer an issue. Instead, photonic confinement within tiny volumes in the few nanometre range becomes possible. This was recently demonstrated by Dai et al. 11 where surface phonon polariton propagation within a three monolayer (o1 nm) thin flake of hBN was reported. It is the propagation of such high-k fields that are scattered off or launched from deeply sub-diffractional objects that is at the heart of super-resolution imaging. These benefits are also coupled with a drastic reduction in the optical losses compared with HMMs, which results in improved performance, that is, higher field confinement 10,11 and improved image resolution.
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Excellent electronic transport in heterostructures of graphene and monoisotopic boron-nitride grown at atmospheric pressure

Excellent electronic transport in heterostructures of graphene and monoisotopic boron-nitride grown at atmospheric pressure

1. Introduction In recent years, a large number of two-dimensional (2D) materials have been discovered [ 1 – 6 ], invest- igated and used in first prototype devices. These materials cover almost all types of different material classes, including metals, semimetals, semiconduct- ors, insulators, superconductors [ 7 , 8 ], and even fer- romagnets [ 9 , 10 ]. However, the number of insulat- ors is very limited since up to now only hexagonal boron nitride (BN) is available as a true 2D layered insulator. This gives hexagonal BN a special signi- ficance in particular since most properties of func- tional 2D materials - which consist only of surface atoms - are strongly influenced by the direct envir- onment, making substrate materials and capping lay- ers highly crucial for the effective material quality and device performance. Indeed, encapsulating 2D mater- ials in hexagonal BN opened the way for improv- ing device performance [ 1 – 3 , 11 – 13 ] and to thor- oughly studying the rich electronic, mechanical and optical properties of 2D materials. For example, phe- nomena observed in 2D materials thanks to the BN
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Ultra Thin Silicon Nitride Interface Engineering

Ultra Thin Silicon Nitride Interface Engineering

Having a broader perspective regarding the impact of the silicon nitride applications the interface engineering approach becomes quite appealing for superconducting sil- icon based Josephson junctions where the Schottky barriers at the silicon surface require high doping concentrations at degenerate state canceling any gate modu- lation of the silicon channel. Here, a lower doping concentration in combination with the silicon nitride provides a sophisticated concept. Moreover, the ohmic con- tacts are promising if the random dopant distribution becomes a limiting factor. Cryogenic MOSFETs try to bypass the integration of dopants, which results into Schottky-barrier MOSFET transfer characteristics [ 257 ]. A second field of interest is the incorporation of the silicon nitride for Tunnel-FETs. The huge advantage of a highly doped substrate in combination with a Schottky-Mott junction (see Fig. 8.1 ) does not require any implantation or electrostatic side gates. Silicon nitride is not the only promising interface engineering material. Keeping in mind that germanium will play a greater role in the semiconductor industry, first steps on germanium ni- tride as a Fermi level depinning tool have been already performed [ 271 ]. Whereas, plasma nitridation assisted by thermal annealing of germanium is more intense stud- ied, a thermal nitridation in ammonia atmosphere was not investigated yet [ 272 , 273 ]. Hexagonal boron nitride with an excellent crystalline nature and passivation skills could be the key of encapsulating 2D materials, especially for emerging research materials [ 274 ].
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Indium gallium nitride nanostructures for optoelectronic applications

Indium gallium nitride nanostructures for optoelectronic applications

time for InN NWs. The crystal structure of the V-shaped NWs was investigated in detail by TEM studies. They were found to crystallise in wurtzite crystal structure. A report on V-shaped NWs for indium arsenide (InAs) material was published by Conesa-Boj et al. [139]. They found a pyramidal-shaped nucleus at the NW bottom enabling the V-shape of their NWs. Here, no such nucleus at the bottom of the InN NWs could be found. In this work, the V-shaped profile of the NW top is caused by a twin-plane reaching through the entire vertical NW axis. This defect separates two opposite crystal planes. A similar V-shaped profile was reported by Gamalski et al. [135] where a twin-plane re-entrant mechanism was found for Ge NWs. The twin- plane is suggested to be a preferential nucleation site, promoting the growth along the [¯110¯1] direction, due to a reduced nucleation barrier at the twin-plane re-entrant groove by the presence of a line energy. Originally this mechanism was proposed by Wagner, Hamilton and Seidensticker [140, 141] based on preferential nucleation at the re-entrant-groove of the surfaces at the twin boundary bounded by a low energy habit plane and the self-perpetuating two-dimensional growth of the nuclei. So far, this model has been applied to the anisotropic growth of various materials with repeated twinned structures, e.g. semiconductor dendrites, [140–142] or face centred cubic met- als [143, 144]. Furthermore, Gamalski et al. [135] noted that the triple phase line (point where vapour, solid and liquid phase are in equilibrium), which is known to be the preferential nucleation site in standard VLS growth [77], is not the preferred nucleation site in the twin-plan re-entrant growth mode. This growth mode was not specifically reported for III-nitride NWs. Nevertheless, their main statements were found to be consistent with the observations in this work presented the first time for InN NWs.
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The unirationality of Hurwitz spaces of hexagonal curves of small genus

The unirationality of Hurwitz spaces of hexagonal curves of small genus

We consider two applications of this result for k = 6. First we study hexago- nal curves with a view towards the theory of Gorenstein ideals of codimen- sion 4. In the cases covered by the unirationality construction we consider the general hexagonal canonical curve as subvariety of the rational normal scroll of dimension 5 which is spanned by the special linear series and show that it has the expected syzygies.

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An equation of state of plutonium nitride fuel

An equation of state of plutonium nitride fuel

Note that with the data chosen, the U pressure is lower than the nitrogen pressure over the temperature range of interest, so that the model does not predict congruent [r]

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Electronic Structure of Nitride-based Quantum Dots

Electronic Structure of Nitride-based Quantum Dots

To understand the electronic properties of nitride-based QDs it is important to have a detailed understanding of the valence-band (VB) structure of the involved materials. In this work QDs with wurtzite crystal structure are considered. Therefore, the VB structure of the wurtzite phases of the group-III nitrides under different forms of strain shall be analyzed here in detail. GaN will serve as an example where quantitative results are presented, but the considerations can be fully transfered to wurtzite InN or InGaN. AlN is an exception, because the crystal-field splitting (see below) is negative in this material. First, the VB structure under symmetric biaxial strain in the basal plane, typical for quantum dots (QDs) or quantum wells (QWs) grown on the c- plane, will be discussed in Sec. 5.1 . Thereafter, an additional strain anisotropy in the basal plane will be considered in Sec. 5.2 . The later analysis is of particular importance for the electronic structure of QDs with low symmetry of the confinement potential in the basal plane due to, e.g., an elongated shape, an inhomogeneous composition profile, or externally induced strain anisotropy. The results of this analysis will be used in Secs. 6.5 and 7.4 to interpret the results of the QD electronic structure calculations.
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Normally-off transistor topologies in gallium nitride technology

Normally-off transistor topologies in gallium nitride technology

6.2. Selective Etching of p-GaN Completely removing the p-GaN without attacking the underlying AlGaN barrier re- quires highly selective etch process, i.e. a large etch rate for p-GaN is desired, whereas the etch rate of AlGaN should ideally be zero. Achieving different etch rates for mate- rials is primarily based on chemical reactions in contrast to physical etching by sputter- ing. Therefore, a dry-etch process in an inductively coupled plasma reactive ion etching (ICP-RIE) tool is preferred, in which the plasma density can be independently con- trolled through the ICP power. The dry-etching agent of choice for gallium nitride is chlorine, which shows a high etch rate for GaN and, slightly weaker, for AlGaN [120]. Oxygen acts as a retarding agent in the dry-etching process with higher effectiveness for AlGaN compared to GaN [121] and can be used to adjust the selectivity. Another possi- bility is the use of a fluorine chemistry as retarding agent [122]. For both, a competing process between the passivation of the surface by an oxide/fluoride due to chemical reactions, and etching by sputtering this passivation through ion bombardment and at- tacking the underlying material is established. These two processes need to be balanced to achieve high selectivity. This type of process was first introduced by Lee et al. in 2000 [123] with a Cl 2 /O 2 /Ar chemistry leading to a selectivity of 24 and improved by Han et al. [124] with a Cl 2 /O 2 /N 2 chemistry and a selectivity of 60. Particularly the process from Han et al. has seen growing interest recently and shows very good re- sults [117, 125–127].
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Large Nonlinear Optical Response in Carbon Nitride Nanotube 

Large Nonlinear Optical Response in Carbon Nitride Nanotube 

Theoretical and experimental studies have shown that conjugated /r-electron organic systems, fullerenes and carbon nanotubes, are potentially important in photonics owing to their lar[r]

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Excitation of solitons in hexagonal lattices and ways of controlling electron transport

Excitation of solitons in hexagonal lattices and ways of controlling electron transport

where index n identifies a particle among all N par- ticles of the ensemble. Further Znk = Zn − Zk and znk = (Zn − Zk)/|Zn − Zk | is the unit vector defining the direction of the interaction force Fnk, corresponding to the Morse potential, between the n-th and the k-th atoms in the lattice. As mentioned above, we consider dimensionless spatial coordinates rescaled with σ as unit length. Time is normalized to the inverse frequency of linear oscillations near the minimum of the Morse potential well, ω 0 −1 , whereas energy is scaled with 2D. In view of the above only those lattice units with coordinates Z k , satisfying the condition |Z n − Zk | < 1.5, are taken into account in the sum in Eq. (13). In computer simulations the interaction of lattice units is considered to take place inside a rectangular cell Lx · Ly with periodic boundary conditions and depending on the symmetry of an initial distribution of units and their number N . The initial condition is defined by the regular hexagonal configuration. Using data about trajectories of particles Zn(t) and their velocities we calculate the lattice atom distribution described by the core electron probability density ρ(Z, t).
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Ultramicroporous silicon nitride ceramics for CO2 capture

Ultramicroporous silicon nitride ceramics for CO2 capture

Tour: Asphalt-derived high surface area activated porous carbons for carbon dioxide capture. Seifollahi Bazarjani, H-J[r]

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Polarity control and doping in aluminum gallium nitride

Polarity control and doping in aluminum gallium nitride

The two challenges that will be solved are: (1) incorporation of compensating charged point defects in p- and n-type AlGaN alloys which limit the free carrier concentrations [r]

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Defunctionalisation catalysed by boron Lewis acids

Defunctionalisation catalysed by boron Lewis acids

6 For reviews of boron Lewis acids/frustrated Lewis pair- catalysed H–H and Si–H bond activation, see: (a) D. Weber and M. R. Gagn´ e, in Organosilicon Chemistry: Novel Approaches and Reactions, ed. T. Hiyama and M. Oestreich, Wiley-VCH, Weinheim, 2019, pp. 33–85; (b) T. Hackel and N. A. McGrath, Molecules, 2019, 24, 432–461; (c) M. Oestreich, J. Hermeke and J. Mohr, Chem. Soc. Rev., 2015, 44, 2202–2220; (d) D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2015, 54, 6400–6441; (e) D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 46–76.

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Laser Spectroscopy of the Boron Isotopic Chain

Laser Spectroscopy of the Boron Isotopic Chain

These developments are particularly interesting for the light elements at least up to carbon, which offer a suitable transition for laser spectroscopy from the ortho- helium like state. This state can be prepared in an electron-beam ion source, allowing to measure absolute charge radii in the stable isotopes of all light elements with Z ≤ 6. This includes measurements on 10 B 3+ and 11 B 3+ that in combination with the foreseeable results from the online experiment provide an accurate absolute charge radius of 8 B. Furthermore, a more precise value of the 7 Be charge radius, which could be measured in the respective Be 2 + state, would help to determine the supposed proton-halo character of 8 B, since it defines the size of the halo core. This would also be the first measurement to provide a reliable δ〈r c 2 〉 value between two isotopes of neighboring elements. The combina- tion of all-optical nuclear charge radii with the results from existing and proposed isotope shift measurements would significantly increase our knowledge not only of the boron isotopic chain but across the “light” part of the nuclear chart.
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OPUS 4 | Experimental investigation of the flow over macroscopic hexagonal structured surfaces

OPUS 4 | Experimental investigation of the flow over macroscopic hexagonal structured surfaces

Experiments were performed to investigate the flow over structured surfaces (structured cylinders and plates) taking into consideration the aerodynamic quantities like pressure drag, momentum thickness, shear stress and moments in the wake regions. Investigations of the flow over patterned or structured cylinders with patterns of k/D = 1.98 x 10 -2 were conducted for Reynolds numbers ranging from 3.14 x 10 4 to 2.77 x 10 5 . The drag coefficient of the cylinder with patterns pressed outwards at 90° to the flow was found to be the lowest of all, i.e. 0.65 times of S and remains nearly unchanged for a wider range of Reynolds numbers, whereas the drag coefficient of the configuration I0 has a value very close to the one of smooth cylinder. It was shown with the help of velocity profiles in the wake region and flow visualization techniques that the boundary layer separation is much more delayed for patterned cylinder with the lowest drag coefficient and occurs between 110° and 115°. A hexagonal bump (outwardly curved structure) disturbs the incoming flow in such a way that an early partial separation takes place. This partial separation initiates instability along the separating shear layer and increase the turbulent intensity of the flow near the surface. The flow reattaches to the surface with higher momentum and leaves a separation bubble behind it. The reattached flow is able to overcome the adverse pressure gradient delaying the main separation significantly. On the other hand, the configurations with patterns pressed inwards possess a drag coefficient closer to the one of smooth cylinder. The higher drag coefficient suggests that the separation is not that much delayed as in case of the outwardly curved structures. Oil flow patterns and velocity profiles above the surface confirmed the earlier separation of flow. Hence, it is evident that inwardly curved structures do not behave exactly in a way as the dimples do, although they resemble dimples much more in form and shape than the above mentioned hexagonal bumps. Furthermore, it was revealed that the separation of flow over patterned cylinders cannot be specified by a single line at a particular angle, but a wave with its crests and troughs indicating the points of main separations along the length of the cylinder. On the basis of these results, it can confidently be established that the delayed boundary layer separation induced by a separation bubble is mainly responsible for the drag reduction of patterned cylinders.
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