The layered dichalcogenides of Nb and Ta easily form intercalation compounds with metal hydroxide... more The layered dichalcogenides of Nb and Ta easily form intercalation compounds with metal hydroxides (alkali, alkaline earth, ammonium hydroxide, and their derivatives) in aqueous solution. The increase per unit layer in the c lattice parameter produced upon intercalation varies from 2 to 6 A and is dependent on the host dichalcogenide, the metal hydroxide employed, and its solvation number. A formula of the type T~S~(MOH)O,Z_O.~(HZO)~.~-~.~ was obtained from chemical analyses data. The formation of different intercalation stages, structural aspects, and superconductivity of these compounds are described and discussed.
We have investigated the recrystallization of ion-implanted amorphous GaAs using a frequency-doub... more We have investigated the recrystallization of ion-implanted amorphous GaAs using a frequency-doubled, 10-8 s pulsed Nd:YAG laser. The best results were obtained by spatially overlapping laser pulses at 0.2 J/cm2. The recorder GaAs samples were examined by He-ion backscattering and Raman scattering. At power densities above 20 MW/cm2 (0.2 J/cm2), not only does the GaAs surface begin to sow uneven
Anomalous, symmetry-forbidden first-order Raman scattering has been observed in the cubic magneti... more Anomalous, symmetry-forbidden first-order Raman scattering has been observed in the cubic magnetic semiconductors EuO, EuS, EuSe, and EuTe. The scattering observed for T & Tc in the europium chalcogenides is characterized by (1) the occurrence of a peak in the scattering intensity at co, where (kpp (c00(cubi p (2) a broad linewidth corresponding to the frequency range of the LO phonons throughout the Brillouin zone, (3) a lack of polarization sensitivity of the spectrum, (4) a lack of sensitivity to whether the sample is electrically conducting or insulating, (5) a quenching of the scattered intensity when the temperature is lowered below Tc, (6) a quenching of the scattered intensity by application of an external magnetic field when the temperature is close to but greater than Tc, and (7) the observation of harmonic structures at multiples of boa in EuS and EuSe. The properties of this scattering cannot be explained by conventional models for symmetry-forbidden Raman scattering. A new light-scattering mechanism is proposed, in which the spin disorder lifts the translational and inversion symmetries of the electronic wave functions in these materials. With this spin-disorder mechanism it is possible to explain qualitatively many of the observed features of this broad-line Raman scattering. It is also possible to account for the relative intensities of the higher harmonics of the broad-line scattering observed in EuS and EuSe for T p Tc by considering a model proposed by Williams and Smit.
Electroluminescence in long-channel, ambipolar carbon nanotubes is highly localized in a small tu... more Electroluminescence in long-channel, ambipolar carbon nanotubes is highly localized in a small tube region where electron-and hole currents overlap. This region, reminiscent of a pn junction in a light-emitting diode, but produced without static dopants, can be moved along the entire carbon nanotube simply by adjusting the voltage at a uniform backgate. The movement of the spot, its size, the spectrum of the emitted light, and the efficiency of infrared generation allows a much better understanding of the electronics in ambipolar carbon nanotube field-effect transistors.
30th European Solid-State Device Research Conference, 2000
A non-invasive characterization of a fast CMOS ring oscillator is reported. The optoelectronic te... more A non-invasive characterization of a fast CMOS ring oscillator is reported. The optoelectronic technique exploits infrared emission from hot-carriers in high-field regions of switching transistors. By means of a fast solid-state single-photon detector (Single Photon Avalanche Detector SPAD), high time resolution and sensitivity are obtained. Experimental data with 50ps resolution enable to measure systematic variations of the period and jitter of switching transitions due to phase noise in a ring oscillator. The luminescence of pchannel MOSFET's, previously reported to be barely detectable, is also measured.
1999 IEEE International Solid-State Circuits Conference. Digest of Technical Papers. ISSCC. First Edition (Cat. No.99CH36278)
Picosecond imaging circuit analysis (PICA) is shown to be a valuable means for measuring internal... more Picosecond imaging circuit analysis (PICA) is shown to be a valuable means for measuring internal timing of microprocessors operating at speed. In this paper, PICA is applied to the analysis of delays and skew of the IBM POWER3 64b microprocessor clock distribution
This letter reports the first observation of Roman scattering in the ternary sulphide ZnIn2S 4. W... more This letter reports the first observation of Roman scattering in the ternary sulphide ZnIn2S 4. While most of the sulphides of the type MEIn2Sd, where Me ~ Mg, Ca, Zn, Cd, Hg, Mn, Co, Ni and Fe crystallize either in the normal or in the inverse spinel structure (1), Znin2S 4 is an exception in that it possesses a layer-type structure. It was not until 1962 that LAPPE et al. (2) determined its crystal structure to be rhombohedral with the space group of C35~(R3m), using X-ray diffraction from single crystals. A study of its optical properties such as absorption and photoluminescence by SHIONOrA and coworkers (2.4) shows that it has a large band gap of ~-~ 2.8 eV and the onset of absorption occurs in the visible region around 5000/~. This makes the argon-ion laser a suitable source of radiation for Raman scattering. The crystal structure of ZnIn2S 4 can be visualized more easily in terms of the hexagonal unit cell which has the dimensions a = 3.85/~ and e = 37.1 ~, so that it has an unusually large value of ~ 10 for the c/a ratio. This arises from the fact that this hexagonal unit cell, which contains 21 atoms corresponding to three units of the chemical formula, can be considered as built of 3 chemical blocks stacked along the c-axis, and rotated by 120 ~ with respect to each other. Each block consists of a sequence of 3 cations: tetrahedral In, octahedral In and tetrahedral Zn bonded to sulphur atoms. The intrabloek bonding is essentially ionic, whereas the interblock bonding is due to the weak S-S van der Waals interaction. This accounts for the layer structure with easy cleavage in the c-plane.
Raman scattering has been observed in EuO, EuS, EuSe and EuTe. The Raman frequencies are lower th... more Raman scattering has been observed in EuO, EuS, EuSe and EuTe. The Raman frequencies are lower than those reported for the infrared L.O. modes and the lines are extremely broad. Observations have been made for T > T, and T < Tc in EuS and EuSe.
The superconducting properties of the transition-metal dichalcogenides TaS"TaSe"NbS"and NbSe, hav... more The superconducting properties of the transition-metal dichalcogenides TaS"TaSe"NbS"and NbSe, have been studied as a function of structural disorder induced by heavy-ion bombardment and by deintercalation at temperatures between 300 and 800 K. Changes in T, similar to those seen on intercalation by metal ions and organic molecules were observed but there was no increase in the anisotropy of the critical magnetic fields. The changes in T, are attributed to changes in the electronic structure at the respective Fermi surfaces beyond previously considered changes in the density of states at the Fermi surface on intercalation.
ABSTRACT We present a comprehensive picture of the phonon populations in an electrically-driven c... more ABSTRACT We present a comprehensive picture of the phonon populations in an electrically-driven carbon nanotube transistor, including the Raman-active G and radial breathing modes (RBM), and also the Raman-inactive zone boundary mode (K), and intermediate-frequency mode (IFP), populated by anharmonic decay. The effective temperature of the RBM is considerably lower than the intermediate- and high-frequency mode temperatures, which we explain by a phonon-decay bottleneck. We include substrate polar phonon scattering to fully account for the device electronic characteristics.
ABSTRACTMetal-catalyzed graphitization from vapor phase sources of carbon is now an established t... more ABSTRACTMetal-catalyzed graphitization from vapor phase sources of carbon is now an established technique for producing few-layer graphene, a candidate material of interest for post-silicon electronics. Here we describe two alternative metal-catalyzed graphene formation processes utilizing solid phase sources of carbon. In the first, carbon is introduced as part of a cosputtered Ni-C alloy; in the second, carbon is introduced as one of the layers in an amorphous carbon (a-C)/Ni bilayer stack. We examine the quality and characteristics of the resulting graphene as a function of starting film thicknesses, Ni-C alloy composition or a-C deposition method (physical or chemical vapor deposition), and annealing conditions. We then discuss some of the competing processes playing a role in graphitic carbon formation and review recent evidence showing that the graphitic carbon in the a-C/Ni system initially forms by a metal-induced crystallization mechanism (analogous to what is seen with Al-...
The layered dichalcogenides of Nb and Ta easily form intercalation compounds with metal hydroxide... more The layered dichalcogenides of Nb and Ta easily form intercalation compounds with metal hydroxides (alkali, alkaline earth, ammonium hydroxide, and their derivatives) in aqueous solution. The increase per unit layer in the c lattice parameter produced upon intercalation varies from 2 to 6 A and is dependent on the host dichalcogenide, the metal hydroxide employed, and its solvation number. A formula of the type T~S~(MOH)O,Z_O.~(HZO)~.~-~.~ was obtained from chemical analyses data. The formation of different intercalation stages, structural aspects, and superconductivity of these compounds are described and discussed.
We have investigated the recrystallization of ion-implanted amorphous GaAs using a frequency-doub... more We have investigated the recrystallization of ion-implanted amorphous GaAs using a frequency-doubled, 10-8 s pulsed Nd:YAG laser. The best results were obtained by spatially overlapping laser pulses at 0.2 J/cm2. The recorder GaAs samples were examined by He-ion backscattering and Raman scattering. At power densities above 20 MW/cm2 (0.2 J/cm2), not only does the GaAs surface begin to sow uneven
Anomalous, symmetry-forbidden first-order Raman scattering has been observed in the cubic magneti... more Anomalous, symmetry-forbidden first-order Raman scattering has been observed in the cubic magnetic semiconductors EuO, EuS, EuSe, and EuTe. The scattering observed for T & Tc in the europium chalcogenides is characterized by (1) the occurrence of a peak in the scattering intensity at co, where (kpp (c00(cubi p (2) a broad linewidth corresponding to the frequency range of the LO phonons throughout the Brillouin zone, (3) a lack of polarization sensitivity of the spectrum, (4) a lack of sensitivity to whether the sample is electrically conducting or insulating, (5) a quenching of the scattered intensity when the temperature is lowered below Tc, (6) a quenching of the scattered intensity by application of an external magnetic field when the temperature is close to but greater than Tc, and (7) the observation of harmonic structures at multiples of boa in EuS and EuSe. The properties of this scattering cannot be explained by conventional models for symmetry-forbidden Raman scattering. A new light-scattering mechanism is proposed, in which the spin disorder lifts the translational and inversion symmetries of the electronic wave functions in these materials. With this spin-disorder mechanism it is possible to explain qualitatively many of the observed features of this broad-line Raman scattering. It is also possible to account for the relative intensities of the higher harmonics of the broad-line scattering observed in EuS and EuSe for T p Tc by considering a model proposed by Williams and Smit.
Electroluminescence in long-channel, ambipolar carbon nanotubes is highly localized in a small tu... more Electroluminescence in long-channel, ambipolar carbon nanotubes is highly localized in a small tube region where electron-and hole currents overlap. This region, reminiscent of a pn junction in a light-emitting diode, but produced without static dopants, can be moved along the entire carbon nanotube simply by adjusting the voltage at a uniform backgate. The movement of the spot, its size, the spectrum of the emitted light, and the efficiency of infrared generation allows a much better understanding of the electronics in ambipolar carbon nanotube field-effect transistors.
30th European Solid-State Device Research Conference, 2000
A non-invasive characterization of a fast CMOS ring oscillator is reported. The optoelectronic te... more A non-invasive characterization of a fast CMOS ring oscillator is reported. The optoelectronic technique exploits infrared emission from hot-carriers in high-field regions of switching transistors. By means of a fast solid-state single-photon detector (Single Photon Avalanche Detector SPAD), high time resolution and sensitivity are obtained. Experimental data with 50ps resolution enable to measure systematic variations of the period and jitter of switching transitions due to phase noise in a ring oscillator. The luminescence of pchannel MOSFET's, previously reported to be barely detectable, is also measured.
1999 IEEE International Solid-State Circuits Conference. Digest of Technical Papers. ISSCC. First Edition (Cat. No.99CH36278)
Picosecond imaging circuit analysis (PICA) is shown to be a valuable means for measuring internal... more Picosecond imaging circuit analysis (PICA) is shown to be a valuable means for measuring internal timing of microprocessors operating at speed. In this paper, PICA is applied to the analysis of delays and skew of the IBM POWER3 64b microprocessor clock distribution
This letter reports the first observation of Roman scattering in the ternary sulphide ZnIn2S 4. W... more This letter reports the first observation of Roman scattering in the ternary sulphide ZnIn2S 4. While most of the sulphides of the type MEIn2Sd, where Me ~ Mg, Ca, Zn, Cd, Hg, Mn, Co, Ni and Fe crystallize either in the normal or in the inverse spinel structure (1), Znin2S 4 is an exception in that it possesses a layer-type structure. It was not until 1962 that LAPPE et al. (2) determined its crystal structure to be rhombohedral with the space group of C35~(R3m), using X-ray diffraction from single crystals. A study of its optical properties such as absorption and photoluminescence by SHIONOrA and coworkers (2.4) shows that it has a large band gap of ~-~ 2.8 eV and the onset of absorption occurs in the visible region around 5000/~. This makes the argon-ion laser a suitable source of radiation for Raman scattering. The crystal structure of ZnIn2S 4 can be visualized more easily in terms of the hexagonal unit cell which has the dimensions a = 3.85/~ and e = 37.1 ~, so that it has an unusually large value of ~ 10 for the c/a ratio. This arises from the fact that this hexagonal unit cell, which contains 21 atoms corresponding to three units of the chemical formula, can be considered as built of 3 chemical blocks stacked along the c-axis, and rotated by 120 ~ with respect to each other. Each block consists of a sequence of 3 cations: tetrahedral In, octahedral In and tetrahedral Zn bonded to sulphur atoms. The intrabloek bonding is essentially ionic, whereas the interblock bonding is due to the weak S-S van der Waals interaction. This accounts for the layer structure with easy cleavage in the c-plane.
Raman scattering has been observed in EuO, EuS, EuSe and EuTe. The Raman frequencies are lower th... more Raman scattering has been observed in EuO, EuS, EuSe and EuTe. The Raman frequencies are lower than those reported for the infrared L.O. modes and the lines are extremely broad. Observations have been made for T > T, and T < Tc in EuS and EuSe.
The superconducting properties of the transition-metal dichalcogenides TaS"TaSe"NbS"and NbSe, hav... more The superconducting properties of the transition-metal dichalcogenides TaS"TaSe"NbS"and NbSe, have been studied as a function of structural disorder induced by heavy-ion bombardment and by deintercalation at temperatures between 300 and 800 K. Changes in T, similar to those seen on intercalation by metal ions and organic molecules were observed but there was no increase in the anisotropy of the critical magnetic fields. The changes in T, are attributed to changes in the electronic structure at the respective Fermi surfaces beyond previously considered changes in the density of states at the Fermi surface on intercalation.
ABSTRACT We present a comprehensive picture of the phonon populations in an electrically-driven c... more ABSTRACT We present a comprehensive picture of the phonon populations in an electrically-driven carbon nanotube transistor, including the Raman-active G and radial breathing modes (RBM), and also the Raman-inactive zone boundary mode (K), and intermediate-frequency mode (IFP), populated by anharmonic decay. The effective temperature of the RBM is considerably lower than the intermediate- and high-frequency mode temperatures, which we explain by a phonon-decay bottleneck. We include substrate polar phonon scattering to fully account for the device electronic characteristics.
ABSTRACTMetal-catalyzed graphitization from vapor phase sources of carbon is now an established t... more ABSTRACTMetal-catalyzed graphitization from vapor phase sources of carbon is now an established technique for producing few-layer graphene, a candidate material of interest for post-silicon electronics. Here we describe two alternative metal-catalyzed graphene formation processes utilizing solid phase sources of carbon. In the first, carbon is introduced as part of a cosputtered Ni-C alloy; in the second, carbon is introduced as one of the layers in an amorphous carbon (a-C)/Ni bilayer stack. We examine the quality and characteristics of the resulting graphene as a function of starting film thicknesses, Ni-C alloy composition or a-C deposition method (physical or chemical vapor deposition), and annealing conditions. We then discuss some of the competing processes playing a role in graphitic carbon formation and review recent evidence showing that the graphitic carbon in the a-C/Ni system initially forms by a metal-induced crystallization mechanism (analogous to what is seen with Al-...
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Papers by James Tsang