Crystal Channels Research Articles

Electron mobility enhancement using ultrathin pure Ge on Si substrate

We demonstrate enhancement of electron mobility in nMOSFET using an ultrathin pure Ge crystal channel layer directly grown on a bulk Si wafer. A thin Si crystal layer is also grown on top of a Ge crystal channel layer as a capping layer. Using the Si/Ge/Si structure, a maximum 2.2X enhancement in electron mobility is achieved while good gate dielectric properties and junction qualities of bulk Si devices are maintained.

Analytic model for ion channeling in successive implantations in crystalline silicon

Successive ion implantations are frequently used in the fabrication of silicon devices. Each ion implantation increases the crystal damage in the region near the surface. For successive implantations, the ion channeling in perfect crystal channels is reduced and the channeling tail is lowered. In the process simulation, the impurity distribution after ion implantation is often calculated as the sum of two Pearson functions, of which the second covers the channeling ions. In this work, we present a general model for the reduction of channeling in successive implantations. The crystal damage from the implantations is monitored and used for the calculation of a differential channel dose. The model allows a fast analytic calculation of impurity profiles in 1D, 2D, and 3D process simulators. It provides a similar accuracy as Monte Carlo simulations and gives excellent agreement with SIMS data of successive ion implantations.

Ethylenediamine assisted growth of single crystal tellurium channels

Single crystal tellurium channels have been synthesized through the ethylenediamine assisted hydrothermal treatment method. The products were characterized by SEM, TEM, SAED, HRTEM, EDS and XRD measurements. The results show that the as-prepared tellurium channels have the 1D channel like nanostructures with the width about 600 nm in diameter. It is also shown that the tellurium channels with the hexagonal phase crystallinity grow preferentially along the [001] direction.

Channeling technique to make nanoscale ion beams

Particle channeling in a bent crystal lattice has led to an efficient instrument for beam steering at accelerators [Biryukov et al., Crystal Channeling and its Application at High Energy Accelerators, Springer, Berlin, 1997], demonstrated from MeV to TeV energies. In particular, crystal focusing of high-energy protons to micron size has been demonstrated at IHEP with the results well in match with Lindhard (critical angle) prediction. Channeling in crystal microstructures has been proposed as a unique source of a microbeam of high-energy particles [Bellucci et al., Phys. Rev. ST Accel. Beams 6 (2003) 033502]. Channeling in nanostructures (single-wall and multi-wall nanotubes) offers the opportunities to produce ion beams on nanoscale. Particles channeled in a nanotube (with typical diameter of about 1nm) are trapped in two dimensions and can be steered (deflected, focused) with the efficiency similar to that of crystal channeling or better. This technique has been a subject of computer simulations, with experimental efforts under way in several high-energy labs, including IHEP. We present the theoretical outlook for making channeling-based nanoscale ion beams and report the experience with crystal-focused microscale proton beams.

Mathematical theories of classical particle channeling in perfect crystals

We present an overview of our work on rigorous mathematical theories of channeling for highly energetic positive particles moving in classical perfect crystal potentials. Developed over the last two decades, these theories include: (i) a comprehensive, highly mathematical theory based on Nekhoroshev’s theorem which embraces both axial and planar channeling as well as certain non-channeling particle motions, (ii) a theory of axial channeling for relativistic particles based on a single-phase averaging method for ordinary differential equations and (iii) a theory of planar channeling for relativistic particles based on a two-phase averaging method for ordinary differential equations. Here we touch briefly on (i) and (ii), then focus on (iii). Together these theories place Lindhard’s continuum model approximations on a firm mathematical foundation, and should serve as the starting point for more refined mathematical treatments of channeling.

The schemes of proton extraction from IHEP accelerator using bent crystals

IHEP Protvino has pioneered the wide practical use of bent crystals as optical elements in high energy beams for beam extraction and deflection on permanent basis since 1989. In the course of IHEP experiments, crystal channeling has been developed into efficient instrument for particle steering at accelerators, working in predictable, reliable manner with beams of very high intensity over years. Crystal systems extract 70GeV protons from IHEP main ring with efficiency of 85% at intensity of 1012, basing on multi-pass mechanism of channeling proposed theoretically and realized experimentally at IHEP. Today, six locations on the IHEP 70GeV main ring of the accelerator facility are equipped by crystal extraction systems, serving mostly for routine applications rather than for research and allowing a simultaneous run of several particle physics experiments, thus significantly enriching the IHEP physics program. The long successful history of large-scale crystal exploitation at IHEP should help to incorporate channeling crystals into accelerator systems worldwide in order to create unique systems for beam delivery. We report recent results from the research and exploitation of crystal extraction systems at IHEP.

Suppression of ferroelectricity in ultrathin barium titanate films grown within mica-4 interlayer space

The critical thickness for ferroelectricity has assumed importance because of the integration of ferroelectric perovskite films into microelectronic devices. Recent calculations reported by Junquera and Ghosez (J. Junquera and Ph. Ghosez, Nature, 2003, 422, 506, ref. ) have shown the critical thickness in the case of BaTiO3 thin films to be ca. 2.4 nm. We have grown BaTiO3 films of thickness ca. 1.2 nm within the crystal channels of sodium fluorophlogopite mica of chemical composition Na4Mg6Al4Si4O20F4·H2O. Dielectric permittivity measurements on the composites do not show any transition at 393 K. Several possible causes for the suppression of this dielectric anomaly have been examined e.g., reduction of BaTiO3, sodium ion introduction into BaTiO3 and generation of thermal stress on these films. All of these have been ruled out on the basis of available experimental data. We believe our results are consistent with the recently reported theoretical predictions.

Studying and applying channeling at extremely high bunch charges

The potentially high plasma densities possible in solids might produce extremely high acceleration gradients. However, solid-state plasmas could pose daunting challenges. Crystal channeling has been suggested as a mechanism to ameliorate these problems. A high-density plasma in a crystal lattice could quench the channeling process. There is no experimental or theoretical guidance on channeling for intense charged particle beams. An experiment has been carried out at the Fermilab A0 photoinjector to observe electron channeling radiation at high bunch charges. An electron beam with up to 8nC per electron bunch was used to investigate the electron–crystal interaction. No evidence was found of quenching of channeling at charge densities two orders of magnitude larger than in earlier experiments. Possible new channeling experiments are discussed for the much higher bunch charge densities and shorter times required to probe channeling breakdown and plasma behavior.

Nanostructures versus crystals in particle channeling

Crystal channeling is a well-established technique at particle accelerators. Nanotechnology offers regular structures of nanochannels, with unique characteristics and with possibilities to vary them over a broad range. Does channeling in nanostructured material offer unique capabilities as compared to channeling in crystal lattices? We address this question with Monte Carlo simulations of channeling in nanotubes of various geometry (SWNT and MWNT) and in crystal lattices. We compare channeling and dechanneling in nanostructures and in crystal lattices.

Crystal undulator experiment at IHEP

Accelerator-based sources of hard-photon radiation are a rapidly developing field. Coherent radiation of a particle beam in an undulator is a popular choice. The crystalline undulators with periodically deformed crystallographic planes offer electromagnetic fields of the order of 1000T and could provide a period L in the sub-millimeter range. In this way, a hundred-fold gain in the energy of emitted photons would be reached, as compared to a usual undulator. The first real construction of a crystal channeling undulator was recently proposed and realised by the present collaboration. After the sine-like deformation of crystal was proven in X-ray tests, four undulators were tested for channeling in a beam of 70-GeV protons. It was observed in the experiment that the crystal cross-section is efficiently channeling high energy particles, similarly to usual crystal deflectors. The experimentally established transparence for channeling of high energy particles allows one to start a direct experiment on photon production from a positron beam in the crystalline undulator. Details of commissioning of the experimental setup in the first run with 3GeV positron beam are presented.

Dynamics of ferrocene in AlPO4-5 single crystals probed by nuclear forward scattering

We have investigated the dynamics of ferrocene in the channels of single crystals of AlPO(4)-5, using nuclear forward scattering. Around 150 K, rotational dynamics sets in, which is strongly anisotropic up to 235 K. Assuming planar rotation of the ferrocene molecules within the channels we obtained an activation energy of (8.2+/-1.0) kJ mol(-1). Between 235 K and room temperature, the rotation becomes isotropic.

Xe NMR lineshapes in channels of peptide molecular crystals.

To further an understanding of the nature of information available from Xe chemical shifts in cavities in biological systems, it would be advantageous to start with Xe in regular nanochannels that have well known ordered structures built from amino acid units. In this paper, we report the experimental observation of Xe NMR lineshapes in peptide channels, specifically the self-assembled nanochannels of the dipeptide L-Val-L-Ala and its retroanalog L-Ala-L-Val in the crystalline state. We carry out grand canonical Monte Carlo simulations of Xe in these channels to provide a physical understanding of the observed Xe lineshapes in these two systems.

Exciton emission in PTCDA films andPTCDA∕Alq3multilayers

We investigate the exciton emission in 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) thin films and PTCDA/aluminium-tris-hydroxyqinoline $(\mathrm{Al}{\mathrm{q}}_{3})$ multilayers by photoluminescence spectroscopy in the temperature range from $10\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The films are grown by organic molecular beam deposition on Si(001) and Pyrex™ substrates at high vacuum. The obtained temperature dependence of the different recombination channels arising from indirect Frenkel excitons, charge transfer excitons, excimers, and relaxed excited monomers is compared to the recombination channels in single PTCDA crystals. A strong recombination band is observed at $1.63\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in $\mathrm{PTCDA}∕\mathrm{Al}{\mathrm{q}}_{3}$ multilayers. This emission band is attributed to charge transfer transitions between stacked PTCDA molecules that are compressed by strain fields within the PTCDA layers. This assignment is supported by strain-dependent photoluminescence measurements on pure PTCDA films.

Ultrarelativistic nuclei in a crystal channel: Coulomb scattering, coherence, and absorption

We incorporate the effect of lattice thermal vibrations into the Glauber-theory description of particle and nucleus-crystal Coulomb interactions at high energy. We show that taking the lattice thermal vibrations into account produces a strong absorption effect: the phase shift function of the multiple-diffraction scattering on a chain of N identical atoms acquires a large imaginary part, and the radius of the absorption region in the impact parameter plane grows logarithmically with N. Consequences of this observation for the elastic and quasi-elastic Coulomb scattering are discussed. The practically interesting example of the coherent Coulomb excitation of ultrarelativistic particles and nuclei passing through a crystal is considered in detail.

Tunable photonic crystal coupled-cavity laser

We report the development of a tunable photonic crystal coupled-cavity laser diode based on InP. The laser consists of two photonic crystal channel waveguides that are coupled through a photonic crystal mirror segment in /spl Gamma/-M direction. The cavity lengths define the respective mode spacings in the cavities. The tuning characteristics of the device are compared with results obtained from a transfer matrix model. Quasi-continuous tuning is achieved in a 29.6-nm window with 36 wavelength division multiplexing channels spaced 0.8 nm apart (ITU grid). The simplicity of fabrication and promising output characteristics should make this tunable laser design an interesting source for monolithic integration into highly integrated photonic circuits.

Ion channels in crystals of gramicidin D complex with RbCl. Atomic resolution low-temperature synchrotron X-ray data

Ion channels in crystals of gramicidin D complex with RbCl. Atomic resolution low-temperature synchrotron X-ray data

Mode anti-crossing and carrier transport effects in tunable photonic crystal coupled-cavity lasers

We report the tuning characteristics of a widely tunable photonic crystal coupled-cavity laser and find good agreement with simulations based on a transfer matrix algorithm. The device consists of two photonic crystal channel waveguides that are longitudinally coupled through a photonic crystal section. The cavity modes exhibit slightly different mode spacing and allow a wavelength switching by utilizing the Vernier effect. The supermodes of the integrated structure exhibit an anti-crossing behavior as a result of the contra-directional coupling between the two resonant cavities.

Optimization of transmission properties of two-dimensional photonic crystal channel waveguide bends through local lattice deformation

We present an extended study of different topologies for lattice-deformed two-dimensional single line-defect (W1) photonic crystal channel waveguide bends and their effect on the optimization of the bend transmission properties. An enhancement of the spectral response by a factor of six was obtained, with the optimal design providing transmission greater than 95% over a 5.2% relative bandwidth. Experimental results for devices realized in III–V semiconductor epitaxial structure show a transmission efficiency of 95%.

Raman characterization of 0.4 nm single-wall carbon nanotubes using the full-symmetry line group

Raman spectra of single-wall carbon nanotubes produced in the channels of zeolite AFI single crystals have been analyzed in the light of the full symmetry group, the line group. The phonon dispersion curves of the tubes (5,0), (3,3), and (4,2) are calculated based on the lattice dynamical model and the phonon branches are assigned to their quantum numbers (irreducible representations). The structures of Raman spectra of different samples are reproduced well by the density of states of relevant Raman-active phonons. The result is useful to evaluate the contents of these tubes in real crystals where the contents are not well defined before.

Temperature Derivative Fluorescence Spectroscopy as a Tool to Study Dynamical Changes in Protein Crystals

Motions through the energy landscape of proteins lead to biological function. At temperatures below a dynamical transition (150–250 K), some of these motions are arrested and the activity of some proteins ceases. Here, we introduce the technique of temperature-derivative fluorescence microspectrophotometry to investigate the dynamical behavior of single protein crystals. The observation of glass transitions in thin films of water/glycerol mixtures allowed us to demonstrate the potential of the technique. Then, protein crystals were investigated, after soaking the samples in a small amount of fluorescein. If the fluorophore resides within the crystal channels, temperature-dependent changes in solvent dynamics can be monitored. Alternatively, if the fluorophore binds to the protein, local dynamical transitions within the biomolecule can be probed directly. A clear dynamical transition was observed at 175 K in the active site of crystalline human butyrylcholinesterase. The results suggest that the dynamics of crystalline proteins is strongly dependent on solvent composition and confinement in the crystal channels. Beyond applications in the field of kinetic crystallography, the highly sensitive temperature-derivative fluorescence microspectrophotometry technique opens the way to many studies on the dynamics of biological nanosamples.