Search results for " band gap"
showing 10 items of 78 documents
Modulation of the electronic properties of GaN films by surface acoustic waves
2003
We report on the interaction between photogenerated electron-hole pairs and surface acoustic waves (SAW) in GaN films grown on sapphire substrates. The spatial separation of photogenerated carriers by the piezoelectric field of the SAW is evidenced by the quenching of the photoluminescence (PL) intensity. The quenching levels in GaN are significantly smaller than those measured in GaAs under similar conditions. The latter is attributed to the lower exciton ionization efficiency and carrier separation probabilities mediated by the piezoelectric effect. The PL spectra also evidence energy shifts and broadenings of the electronic transitions, which are attributed to the band gap modulation by …
Pressure dependence of the exciton absorption and the electronic subband structure of aGa0.47In0.53As/Al0.48In0.52As multiple-quantum-well system
1992
We have measured the optical absorption of a ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/${\mathrm{Al}}_{0.48}$${\mathrm{In}}_{0.52}$As multiple quantum well at 10 K for pressures up to 7 GPa. The energies of optical transitions between heavy- and light-hole subbands and electron levels of the wells show a blueshift with pressure similar to the bulk lowest direct band gap. We observe a decrease with pressure of the energy splitting between heavy- and light-hole subbands with the same quantum number n. From the analysis of the absorption line shape, we have obtained the pressure dependences of exciton binding energies, oscillator strengths, and linewidths. These results are interpreted in…
Topological Insulators in Ternary Compounds with a Honeycomb Lattice
2010
One of the most exciting subjects in solid state physics is a single layer of graphite which exhibits a variety of unconventional novel properties. The key feature of its electronic structure are linear dispersive bands which cross in a single point at the Fermi energy. This so-called Dirac cone is closely related to the surface states of the recently discovered topological insulators. The ternary compounds, such as LiAuSe and KHgSb with a honeycomb structure of their Au-Se and Hg-Sb layers feature band inversion very similar to HgTe which is a strong precondition for existence of the topological surface states. In contrast to graphene with two Dirac cones at K and K' points, these material…
Role of dispersion on zero-average-index bandgaps
2009
We consider periodic multilayers combining ordinary positive index materials and dispersive metamaterials with negative index in some frequency ranges. These structures can exhibit photonic bandgaps which, in contrast with the usual Bragg gaps, are not based on interference mechanisms. Changing the dispersion models for the constituent metamaterial, we investigate its role in the production of zero-average-index bandgaps. In particular, we show the effect of each constitutive parameter on both bandgap edges. Finally, we give some approximated analytical expressions in terms of average parameters for the determination of the upper and lower limits of the zero-average refractive-index bandgap…
Crystal symmetry and pressure effects on the valence band structure ofγ-InSe andε-GaSe: Transport measurements and electronic structure calculations
2005
This paper reports on Hall effect and resistivity measurements under high pressure up to 3--4 GPa in $p$-type $\ensuremath{\gamma}$-indium selenide (InSe) (doped with As, Cd, or Zn) and $\ensuremath{\epsilon}$-gallium selenide (GaSe) (doped with N or Sn). The pressure behavior of the hole concentration and mobility exhibits dramatic differences between the two layered compounds. While the hole concentration and mobility increase moderately and monotonously in $\ensuremath{\epsilon}$-GaSe, a large increase of the hole concentration near 0.8 GPa and a large continuous increase of the hole mobility, which doubled its ambient pressure value by 3.2 GPa, is observed in $\ensuremath{\gamma}$-InSe.…
Effects of pressure and temperature on the dielectric constant of GaS, GaSe, and InSe: Role of the electronic contribution
1999
In this work we report on direct measurements of the temperature and pressure dependences of the low-frequency dielectric constant along c axis $({\ensuremath{\varepsilon}}_{\ensuremath{\parallel}})$ of GaS, GaSe, and InSe. The temperature dependence of both the ordinary and extraordinary refractive indexes is also presented. A large increase of ${\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}$ under pressure has been observed. In the framework of a rigid ion model, the lattice contribution to ${\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}$ is shown to increase slightly under pressure, due to the change of the angle between the anion-cation bond and the layer plane. Consequently, …
THE LIMITING EFFICIENCY OF FOUR-BAND CELLS REVISITED
2014
The limiting theoretical efficiency of four-band solar cells is revisited. In previous work, researchers have looked at the theoretical efficiency of four band cells where the smallest of the three sub-band gaps is closest to the valence band and the largest closest to the conduction band. In this work, limits are calculated also for other possible band configurations. In multi-band cells, photon selectivity can be assured by adjusting the band widths. The present work shows that previous authors have put too rigid constraints on the band structure to achieve spectral selectivity. Relieving these constraints gives a considerably higher limiting efficiency for cells with band width restricti…
Failure analysis of normally-off GaN HEMTs under avalanche conditions
2020
Gallium nitride (GaN) high electron-mobility transistors (HEMTs) are promising devices in the power electronics field owing to their wide bandgap (WBG). However, all the potential advantages provided by their WBG require reliability improvement. In industrial applications, robustness is one of the main factors considered by circuit designers. This study focuses on the observation of the degradation behavior of the main waveforms of unclamped inductive-switching (UIS) test circuits of two different commercial GaN HEMT structures. The relevance of this study lies in the potential applications of these devices to high-voltage applications and automotive systems where they are subjected to many…
Azido bridged binuclear copper(ii) Schiff base compound: synthesis, structure and electrical properties
2019
An azido bridged dinuclear complex [Cu(L−)(μ1,1N3)]2 (1) was synthesized by a 1 : 1 condensation of N-cyclohexyl-1,3-propanediamine and 5-bromosalicylaldehyde (HL). The complex was subsequently characterized based on elemental analyses, IR, single-crystal X-ray diffraction, a Hirshfeld study, FESEM, ESI-MS, powder XRD and also DFT studies successfully. The X-ray crystal structure of complex 1 revealed that the Cu(II) ion exhibited a definite five-coordinate square pyramidal coordination arrangement with a centro-symmetric μ 1,1-azido bridging in the end-on mode and formed a dimeric structure. The optical direct band gap of complex 1 was estimated to be 2.84 eV. The current–voltage character…