Controlled and reproducible growth of GaN nanowires is demonstrated by pulsed low-pressure metalorganic chemical vapor deposition. Using self-assembled Ni nanodots as nucleation sites on (0001) sapphire substrates we obtain nanowires of wurtzite-phase GaN with hexagonal cross sections, diameters of about 100nm, and well-controlled length. The nanowires are highly oriented and perpendicular to the growth surface. The wires have excellent structural and optical properties, as determined by x-ray diffraction, cathodoluminescence, and Raman scattering. The x-ray measurements show that the nanowires are under a complex strain state consistent with a superposition of hydrostatic and biaxial components.

We suggest a system of two wells connected with Charge Asymmetric Resonance Tunneling (CART) as a basic element of light emitting diode (LED) structure for semiconductors with different masses of electrons and holes. The system consists of an emitter of electrons, an emitter of holes and an active layer. The hole emitter is coupled with the active in such a way that holes can be freely supplied into the active layer without a barrier. The electron emitter is coupled to the active layer via a barrier. The barrier design uses the charge asymmetric resonance tunneling phenomenon which allows to make the barrier transparent for electrons and blocking for holes. Advantages of this design are: the increased capture efficiency of the electrons into the active layer due to direct resonance tunneling of the electrons from the electron emitter on bound electron level in the active quantum well, the suppression of electron leakage into the hole emitter, the elimination of the parasitic light generated outside the active layer, and the electron emitter acts also as a good current spreading layer. First results of experimental investigation and theoretical modeling of the CART LED devices are reported.Introduction Commonly single or multiple quantum wells are used as the active layers in LEDs [1]. For the fabrication of a highly efficient device the number of carriers recombining inside the active layer should be maximized and the number of carriers recombining outside should be minimized. This requires the optimation of capture rates for electrons and holes into the active layer. In polar III±V and II±VI semiconductors the most effective channel of the carrier capture in quantum well is via emission of polar optical phonons. The corresponding carrier capture rate can be estimated roughly as the quantum well width divided by the product of the carrier thermal velocity over polar phonon emission time [2,3]. Thus, the capture rate depends on the quantum well parameters and the carrier masses. As a rule in III±V and II±VI semiconductors the electron effective masses are much lighter and the corresponding thermal velocities are higher than those for holes. For this reason, in the narrow quantum well, which provides optimal carrier confinement and maximal optical matrix element, a part of the electrons are not captured in the active layer and recombine outside of it. This reduced the efficiency of LED devices [4].To solve this problem we suggest a LED structure based on a system of two wells with Charge Asymmetric Resonance Tunneling which allows to enhance the number of the electrons captured into the active layer with the quantum well. The phenomenon of