We report on p-n junction light-emitting diodes fabricated from MgZnO / ZnO / AlGaN / GaN triple heterostructures. Energy band diagrams of the light-emitting diode structure incorporating piezoelectric and spontaneous polarization fields were simulated, revealing a strong hole confinement near the n-ZnO / p-AlGaN interface with a hole sheet density as large as 1.82 ϫ 10 13 cm −2 for strained structures. The measured current-voltage ͑IV͒ characteristics of the triple heterostructure p-n junctions have rectifying characteristics with a turn-on voltage of ϳ3.2 V. Electron-beam-induced current measurements confirmed the presence of a p-n junction located at the n-ZnO / p-AlGaN interface. Strong optical emission was observed at ϳ390 nm as expected for excitonic optical transitions in these structures. Experimental spectral dependence of the photocurrent confirmed the excitonic origin of the optical transition at 390 nm. Light emission was measured up to 650 K, providing additional confirmation of the excitonic nature of the optical transitions in the devices.
We report on ensemble Monte Carlo transport simulations for semiconducting, single-wall, zigzag carbon nanotubes. The basis for the Monte Carlo simulations is provided by electronic structure calculations within the framework of a simple tight-binding model that takes the effect of the tube curvature on the band structure into account. The principal scattering mechanisms considered are due to the electron–phonon interactions involving longitudinal acoustic and optical phonons. Using ensemble Monte Carlo simulations, the steady-state and transient characteristics are explored. The steady-state velocity saturates due to optical-phonon scattering, and negative differential mobility is obtained for large electric fields. The results also show interesting transient phenomena that are caused by the limited phase space of these dynamically one-dimensional structures.
We report ensemble Monte Carlo transport simulation results for single-wall semiconducting zigzag carbon nanotubes. The effects of electron scattering by radial breathing mode phonons are investigated. The basis for the Monte Carlo simulations is provided by electronic structure calculations in the framework of the tight-binding model. Scattering mechanisms considered are due to electron-phonon interactions involving longitudinal acoustic, longitudinal optical, and radial breathing mode phonons. The steady-state velocity is lower for low and moderate electric fields when radial breathing mode phonons are taken into account. Electron scattering by radial breathing mode phonons does not appear to affect strongly the steady-state electron transport within a carbon nanotube at high electric fields. Oscillations in the transient velocity show increased damping.
We report on p-type AlGaN∕GaN superlattice designs with significantly improved vertical and lateral electrical conductivities (σV and σL). Composition-graded p-AlGaN layers produce a polarization charge distribution, which together with an appropriate Mg doping in the structure leads to more than an eightfold reduction of barrier height and a ∼50% increase in the sheet hole density in the p-GaN wells compared to typical modulation-doped superlattice structures. Using the optimized structure, more than 13 orders of magnitude and 35 times improvement is shown for σV, compared to typical superlattice and σL, compared to bulk p-GaN, respectively. Both σV and σL are found to improve significantly at higher temperatures in the optimized structure.
We report measurements of the Schottky barrier heights of Ni/Au contacts on Ga-polarity and N-polarity n-GaN under hydrostatic pressure and applied in-plane uniaxial stress. Under hydrostatic pressure the two different polarities of GaN yield significantly different rates of Schottky barrier height increase with increasing pressure. Uniaxial stress parallel to the surface affects the Schottky barrier height only minimally. The observed changes in barrier height under stress are attributed to a combination of band structure and piezoelectric effects.
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