Yabin Jin

Gradient Index Devices for the Full Control of Elastic Waves in Plates

In this work, we present a method for the design of gradient index devices for elastic waves in plates. The method allows the design of devices to control the three fundamental modes, despite the fact that their dispersion relation is managed by different elastic constants. It is shown that by means of complex graded phononic crystals and thickness variations it is possible to independently design the three refractive indexes of these waves, allowing therefore their simultaneous control. The effective medium theory required for this purpose is presented, and the method is applied to the design of the Luneburg and Maxwell lenses as well as to the design of a flat gradient index lens. Finally, numerical simulations are used to demonstrate the performance of the method in a broadband frequency region.

An aluminum-gate metal-oxide-silicon capacitor with a tunnel-thin oxide under the bidirectional electric stress

Bidirectional electric stresses are performed in metal—oxide—silicon capacitors with ultrathin (2.5 nm) oxides and with highly doped substrates. The observed effects are compared to those occurring under the conventional unidirectional constant-current overload. The effects observed after stress of ultrathin oxides bring into play defects of comparable nature to those noted for thick oxides, namely, neutral traps. Furthermore, soft breakdown events are scrutinized based on defect creation throughout the oxide and modulation of charge state trapping at the interfaces.

Electric transport in a AlGaAs∕GaAs structure from 300Kto4.2K

Some AlGaAs∕GaAs heterostructures with different high electron mobilities were electrically characterized as a function of temperature. Hall measurements were made at 4.2K and current-voltage characteristics were measured from 300Kto4.2K. Experimental results have revealed a saturation of the current at electric fields as low as 5V∕cm at low temperature (<100K). The saturation was linked to the very high mobility combined with the presence of alloy diffusion in the AlGaAs∕GaAs channel. The pinch-off effect was excluded because of the absence of the control gate. Drift velocities were carried out at 4.2K in these experiments.