Gap reduction

Abstract: Ab initio calculations show that the band gap of boron nitride (BN) nanotubes can be greatly reduced by a transverse electric field. This gap reduction arises from a mixing of states within the highest occupied molecular orbital and lowest unoccupied molecular orbital complexes and leads to a spatial separation of electrons and holes across the tube diameter. The gap modulation increases with tube diameter and is nearly independent of chirality. For BN nanotubes of diameters of 5 nm or more, a sizable gap reduction should be achievable with laboratory fields. This effect provides a possible way to tune the band gap of BN tubes for various applications.

Abstract: This paper presents a robust fabrication technique for manufacturing ultrasensitive micromechanical capacitive accelerometers in thick silicon-on-insulator substrates. The inertial mass of the sensor is significantly increased by keeping the full thickness of the handle layer attached to the top layer proof mass. High-aspect-ratio capacitive sense gaps are fabricated by depositing a layer of polysilicon on the sidewalls of low aspect- ratio trenches etched in silicon. Using this method, requirements on trench etching are relaxed, whereas the performance is preserved through the gap reduction technique. Therefore, this process flow can potentially enable accelerometers with capacitive gap aspect-ratio values of greater than 40:1, not easily realizable using conventional dry etching equipment. Also, no wet-etching step is involved in this process which in turn facilitates the fabrication of very sensitive motion sensors that utilize very compliant mechanical structures. Sub-micro-gravity in-plane accelerometers are fabricated and tested with measured sensitivity of 35 pF/g, bias instability of 8 mug, and footprint of <0.5 cm2.

Abstract: The authors review briefly the existing theoretical treatments of the various effects that contribute to the reduction of the energy gap in heavily doped Si, namely electron-electron and electron-impurity interactions and the effect of disorder in the impurity distribution. They then turn to the longstanding question why energy-gap reductions extracted from three different types of experiments have persistently produced values with substantial discrepancies, making it impossible to compare with theoretical values. First, they demonstrate that a meaningful comparison between theory and experiment can indeed be made if theoretical calculations are carried out for actual quantities that experiments measure, e.g. luminescence spectra, as recently done by Selloni and Pantelides. Then, they demonstrate that, independent of any theoretical calculations, the optical absorption spectra are fully consistent with the luminescence spectra and that the discrepancies in the energy-gap reductions extracted from the two sets of spectra are caused entirely by the curve-fitting procedures used in analyzing optical-absorption data. Finally, they show explicitly that, as already believed by many authors, energy-gap reductions extracted from electrical measurements on transistors do not correspond to true gap reductions. They identify two corrections that must be added to the values extracted from the electrical data in order to arrive at the true gap reductions and show that the resulting values are in good overall agreement with luminescence and absorption data. They, therefore, demonstrate that the observed reduction in emitter injection efficiency in bipolar transistors is not strictly due to a gap reduction, as generally believed, but to three very different effects.

Abstract: A detailed theoretical investigation to explore the effects of external pressure (0.0GPa, 2.5GPa, 5.0GPa, 7.5GPa, 10GPa, 12.5GPa, 15GPa, 17.5GPa, 20GPa and 25GPa) on structural stability, electronic structure, band gap engineering and its impact on optical properties in LiNbO3 by using CASTEP code, based on ab-initio density functional theory, with ultra-soft pseudopotential and exchange correlation functional GGA-PBE, is presented. From band structure, it is observed that the top of the valence band is typically controlled by O-2p and Nb-4d orbital electrons and the band gap exhibits nearly linear decreasing trend (3.542eV–2.905eV) with the increasing external pressure. Decreasing trend of the electronic band gap with increase of external pressure is explained with the help of total density of states and elemental partial density of states. Moreover, with the increase of external pressure beyond 25 GPa stability of the structure degrades significantly. In addition, to study the impact of band gap reduction on the optical properties, we have also calculated the refractive index, the reflectivity, absorption function and the energy loss function of the LiNbO3 by the complex dielectric function under various external pressures. The optical results reveal that the static refractive index increases with the increase of pressure and the variation ranges of the absorption function and the reflectivity rate are also broadened. Absorption spectra, under increase of pressure, show the red shift and the major plasmon peaks display blue shift, which is the thumb proof of electronic band gap reduction. Both the electronic structures and optical properties of LiNbO3 can be constructively tempered with pressure, which contributes an effective theoretical basis for the further application of the LiNbO3 under pressure.