Study of Ti/Au,Ti/Al/Au,and Ti/Al/Ni/Au ohmic contacts to <Emphasis Type="Italic">n</Emphasis>-GaN

The contacts of Ti/Au, Ti/Al/Au, and Ti/Al/Ni/Au films deposited on n-GaN were studied by current–voltage (I–V) and transmission-line-method measurements. The effect of annealing temperature on specific contact resistivity has been investigated by changing the annealing temperature from 400 to 900 °C. Ti/Al/Au and Ti/Al/Ni/Au films were superior to the bilayer (Ti/Au) in ohmic contact characteristics and thermal stability. The Ti/Al/Ni/Au composite showed the best thermal stability due to the fact that Ni plays a more important role than the alloy of Ti/Al in preventing the interdiffusion of Ti, Al, and Au. The lowest contact resistivity (10^-7cm²) to n-GaN was obtained for the Ti/Al/Ni/Au sample by short-time/high-temperature annealing. The formation mechanism of ohmic contacts to n-GaN is also discussed. PACS 73.40.Cg; 73.61.Ey     

Leakage current and pyroelectric properties of compositionally graded (Pb,Ca)TiO<Subscript>3</Subscript> films

Highly (100)-oriented, compositionally graded (Pb,Ca)TiO₃ (PCT) thin films with a Ca content from 0 to 24 mol% on Pt/Ti/SiO₂/Si substrates were prepared by a sol-gel process. The graded structure of the Au/PCT/Pt film capacitor showed a well-saturated hysteresis loop at an applied field of 500 kV/cm with remanent polarization (P_r), and coercive electric field (E_c) values of 9.35 C/cm² and 130 kV/cm, respectively. At 100 kHz, the dielectric constant and dielectric loss of the film were 129 and 0.024, respectively. The leakage current density of the graded PCT film was less than 1.0×10^-7 A/cm² over a voltage range from 0 to 4 V. The conduction current depended on the voltage polarity. At low electric field (110 and 180 kV/cm, respectively, for Pt and Au electrodes biased negatively), the Au/PCT and PCT/Pt interfaces form a Schottky barrier. At high electric field (>110 kV/cm), the Au electrode biased negatively shows space-charge-limited current (SCLC) behavior. The temperature dependencies of the pyroelectric coefficients of the graded PCT film were measured by a dynamic technique. From 20 to 82 °C, the pyroelectric coefficients of graded PCT film remain steady in the range 106 to 118 C/m²K. The detectivity figure of merit (F_D) of the graded PCT film was 6.7×10^-6 Pa^-0.5. PACS 77.80.-s; 77.70.+a; 77.22.-d; 51.50.+v; 68.37.-d     

A theoretical study of the Y$\mathsf{_{3}}$O clusters

Hybrid density functional calculations are performed to study the structural and electronic properties of neutral, anionic and cationic Y₃O clusters. The most stable structures of these clusters are found to be triply bridging oxygen atom structures with C_S symmetry. The ground states of Y₃O, Y₃O^- and Y₃O ⁺ are doublet (²A), triplet (³A) and singlet (¹A), respectively. The calculated electron affinities and ionization potentials are in good agreement with the available experimental data. Time-dependent density functional theory is used to calculate the low-lying excited states. A theoretical assignment for the features in the experimental photoelectron spectra is given.Received: 5 November 2003, Published online: 20 January 2004PACS: 36.40.Mr Spectroscopy and geometrical structure of clusters - 31.15.Ew Density-functional theory - 34.50.Gb Electronic excitation and ionization of molecules; intermediate molecular states (including lifetimes, state mixing, etc.)     

Imidazolium ion-terminated self-assembled monolayers on Au: effects of counteranions on surface wettability

Self-assembled monolayers presenting imidazolium ions at the tail ends (SAMIMs) having different counteranions have been prepared on Au, and the measurement of water contact angles of the surfaces proved to be an extremely valuable simple technique for quantifying the effects of counteranions on hydrophilicity and hydrophobicity of SAMIMs, which will be extrapolated to the water miscibility of the related ionic liquids.     

Measurement of nonrandom event-by-event fluctuations of average transverse momentum in square root of (sNN)=200 GeV Au+Au and p+p collisions

Event-by-event fluctuations of the average transverse momentum of produced particles near midrapidity have been measured by the PHENIX Collaboration in square root of (sNN)=200 GeV Au+Au, and p+p collisions at the Relativistic Heavy Ion Collider. The fluctuations are observed to be in excess of the expectation for statistically independent particle emission for all centralities. The excess fluctuations exhibit a dependence on both the centrality of the collision and on the pT range over which the average is calculated. Both the centrality and pT dependence can be well reproduced by a simulation of random particle production with the addition of contributions from hard-scattering processes.     

Infimum Properties Differ in the Weak Truth-table Degrees and the Turing Degrees

Abstract We prove that there are non-recursive r.e. sets A and C with A < _T C such that for every set . Both authors are supported by “863” and the National Science Foundation of China     

Wideband tunable fiber short-pass filter based on side-polished fiber with dispersive polymer overlay

It is known that dispersive fiber has an LP01 mode cutoff wavelength that acts as a wavelength filter. With a dispersive polysiloxane polymer overlay on a side-polished single-mode fiber, an in-line tunable fiber short-pass filter with a tuning range of approximately 400 nm (1250-1650 nm) and a temperature variation of 15 degrees C was demonstrated. The rejection efficiency was greater than 50 dB, whereas the insertion and polarization-dependent losses were below 0.26 and 0.09 dB, respectively.