ALBERT

Description: We present an impedance engineered Josephson parametric amplifier capable of providing bandwidth beyond the traditional gain-bandwidth product. We achieve this by introducing a positive linear slope in the imaginary component of the input impedance seen by the Josephson oscillator using a λ / 2 transformer. Our theoretical model predicts an extremely flat gain profile with a bandwidth enhancement proportional to the square root of amplitude gain. We experimentally demonstrate a nearly flat 20 dB gain over a 640 MHz band, along with a mean 1-dB compression point of −110 dBm and near quantum-limited noise. The results are in a good agreement with our theoretical model.

Description: The physical mechanism of low-thermal-budget Au-free ohmic contacts to AlGaN/GaN heterostructures is systematically investigated with current-voltage, high-resolution transmission electron microscopy, and temperature-dependent contact resistivity characterizations. With a low annealing temperature of 600 °C, pre-ohmic recess etching of the AlGaN barrier down to several nanometers is demonstrated to be an effective method to reduce the contact resistance between Ti/Al/Ti/W ohmic metals and AlGaN/GaN heterostructures. However, further over recess of the AlGaN barrier leads to only sidewall contact to 2D electron gas channel and thus degraded contact performance. It is verified by temperature-dependent contact resistivity measurements that field emission (tunneling) dominates the current transport mechanism in Au-free ohmic contacts with AlGaN barrier partially and over recessed, while both field emission and thermionic emission contribute to traditional Ti/Al/Ni/Au ohmic contacts to AlGaN/GaN heterostructures that annealed at high temperature (850 °C).

Description: In this paper, a bistable optical-driven silicon-nanowire memory is demonstrated, which employs ring resonator to generate optical gradient force over a doubly clamped silicon-nanowire. Two stable deformation positions of a doubly clamped silicon-nanowire represent two memory states (“0” and “1”) and can be set/reset by modulating the light intensity (

Description: In order to improve the operation speed of phase change memory (PCM), superlattice-like Ge 2 Sb 2 Te 5 /Sb (SLL GST/Sb) thin films were prepared in a sputtering method to explore the suitability as an active material for PCM application. Compared with GST, SLL GST/Sb thin film has a lower crystallization temperature, crystallization activation energy, thermal conductivity, and smaller crystalline grain size. A faster SET/RESET switching speed (10 ns) and a lower operation power consumption (the energy for RESET operation 9.1 × 10 −13 J) are obtained. In addition, GST/Sb shows a good endurance of 8.3 × 10 4 cycles.

Description: Negative differential resistance (NDR) and bipolar resistive switching (RS) phenomena were observed in Au/Ga 2 O 3−x /Nb:SrTiO 3 /Au heterostructures fabricated by growing amorphous gallium oxide thin films on 0.7%Nb-doped SrTiO 3 substrates using pulsed laser deposition technique. The RS behavior is reproducible and stable without the forming process. The NDR phenomenon happened during the course of RS from low resistance state to high resistance state and was dependent much on the applied forward bias. The bias dependent charge releasing from oxygen vacancies was considered to contribute to the NDR behavior. The results show that there is a very close relationship between NDR and RS.

Description: We report the manipulation of a liquid plug inside a rectangular microchannel, when one of the channel walls is a deformable membrane, which adjoins another parallel microchannel. Elastocapillary flow of a driving liquid (DL) through the adjoining microchannel, when approaches the plug, tries to pull the membrane near the plug, which is initially deflected into the plug, towards the DL. The plug is transported due to a differential pressure that develops across the plug owing to the increase in the radius of curvature of the trailing meniscus of the plug. A theoretical model is proposed to predict the plug velocity, which depends on a parameter J and plug length L ̃ . The predictions of the theoretical model show good agreement with experimental data. The dynamic behaviour of the plug and DL is presented and discussed.

Description: Metalorganic vapor phase epitaxy of III-V compounds commonly involves arsenic. We study the formation of atomically well-ordered, As-modified Si(100) surfaces and subsequent growth of GaP/Si(100) quasisubstrates in situ with reflection anisotropy spectroscopy. Surface symmetry and chemical composition are measured by low energy electron diffraction and X-ray photoelectron spectroscopy, respectively. A two-step annealing procedure of initially monohydride-terminated, (1 × 2) reconstructed Si(100) in As leads to a predominantly (1 × 2) reconstructed surface. GaP nucleation succeeds analogously to As-free systems and epilayers free of antiphase disorder may be grown subsequently. The GaP sublattice orientation, however, is inverted with respect to GaP growth on monohydride-terminated Si(100).

Description: The energy levels created in supersaturated n-type silicon substrates with titanium implantation in the attempt to create an intermediate band in their band-gap are studied in detail. Two titanium ion implantation doses (10 13  cm -2 and 10 14  cm -2 ) are studied in this work by conductance transient technique and admittance spectroscopy. Conductance transients have been measured at temperatures of around 100 K. The particular shape of these transients is due to the formation of energy barriers in the conduction band, as a consequence of the band-gap narrowing induced by the high titanium concentration. Moreover, stationary admittance spectroscopy results suggest the existence of different energy level configuration, depending on the local titanium concentration. A continuum energy level band is formed when titanium concentration is over the Mott limit. On the other hand, when titanium concentration is lower than the Mott limit, but much higher than the donor impurity density, a quasi-continuum energy level distribution appears. Finally, a single deep center appears for low titanium concentration. At the n-type substrate, the experimental results obtained by means of thermal admittance spectroscopy at high reverse bias reveal the presence of single levels located at around E c -425 and E c -275 meV for implantation doses of 10 13  cm −2 and 10 14  cm −2 , respectively. At low reverse bias voltage, quasi-continuously distributed energy levels between the minimum of the conduction bands, E c and E c -450 meV, are obtained for both doses. Conductance transients detected at low temperatures reveal that the high impurity concentration induces a band gap narrowing which leads to the formation of a barrier in the conduction band. Besides, the relationship between the activation energy and the capture cross section values of all the energy levels fits very well to the Meyer-Neldel rule. As it is known, the Meyer-Neldel rule typically appears in processes involving multiple excitations, like carrier capture and emission in deep levels, and it is generally observed in disordered systems. The obtained Meyer-Neldel energy value, 15.19 meV, is very close to the value obtained in multicrystalline silicon samples contaminated with iron (13.65 meV), meaning that this energy value could be associated to the phonons energy in this kind of substrates.

Description: The dynamics of point-like Brownian particles in a periodic confined channel with oscillating boundaries has been studied. Directional transport (DT) behavior, characterized by net displacement along the horizontal direction, is observed even without external force which is necessary for the conventional DT where the boundaries are static. For typical parameter values, the average velocity V t of DT reaches a maximum with the variation of the noise intensity D , being alike to the phenomenon of stochastic resonance. Interestingly, we find that V t shows nontrivial dependences on the particle gravity G depending on the noise level. When the noise is large, V t increases monotonically with G indicating that heavier particle moves faster, while for small noise, V t shows a bell-shape dependence on G , suggesting that a particle with an intermediate weight may move the fastest. Such results were not observed for DT in a channel with static boundaries. To understand these findings, we have adopted an effective one-dimensional coarsening description, which facilitates us to introduce an effective entropic force along the horizontal direction. The average force is apparently nonzero due to the oscillatory boundary, hence leading to the net transport, and it shows similar dependences as V t on the noise intensity D and particle gravity G . The dependences of the DT behavior on other parameters describing the oscillatory channel have also been investigated, showing that DT is more pronounced for larger oscillation amplitude and frequency, and asymmetric geometry within a channel period and phase difference between neighboring periods are both necessary for the occurrence of DT.

Description: A new nine-dimensional potential energy surface (PES) and dipole moment surface (DMS) for silane have been generated using high-level ab initio theory. The PES, CBS-F12 HL , reproduces all four fundamental term values for 28 SiH 4 with sub-wavenumber accuracy, resulting in an overall root-mean-square error of 0.63 cm −1 . The PES is based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set limit, and incorporates a range of higher-level additive energy corrections to account for core-valence electron correlation, higher-order coupled cluster terms, and scalar relativistic effects. Systematic errors in computed intra-band rotational energy levels are reduced by empirically refining the equilibrium geometry. The resultant Si–H bond length is in excellent agreement with previous experimental and theoretical values. Vibrational transition moments, absolute line intensities of the ν 3 band, and the infrared spectrum for 28 SiH 4 including states up to J = 20 and vibrational band origins up to 5000 cm −1 are calculated and compared with available experimental results. The DMS tends to marginally overestimate the strength of line intensities. Despite this, band shape and structure across the spectrum are well reproduced and show good agreement with experiment. We thus recommend the PES and DMS for future use.