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

The composition dependence of emission energy of pseudomorphically strained InGaN layers with In content up to 0.2 is obtained. It is found that the main reason of “scatter” in published values of the InGaN bowing parameter is the uncertainty of the Poisson's ratio determination. It is shown that after recalculation to the same Poisson's ratio, most published data yield essentially the same results as compared to experimental uncertainty.

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The effect of high-temperature annealing on stress in AlxGa1−xN in different ambients and at different temperatures was studied using ultraviolet micro-Raman spectroscopy. Low (x=0.08) and high (x=0.31 and x=0.34) composition AlGaN, grown by metalorganic chemical vapor deposition (MOCVD) and molecular-beam epitaxy (MBE), were compared. Compositional and morphological changes were monitored using Auger electron spectroscopy (AES) and atomic force microscopy (AFM), respectively. The Raman results demonstrate that all samples exhibit maximum stress changes in the compressive direction when annealed in an air ambient. AES confirms this to be due to higher oxygen incorporation after annealing in the air ambient, and shows higher oxygen incorporation in the vicinity of cracks and defects. MOCVD and MBE samples of a similar composition were found to reach the same biaxial stress, despite differences in initial stress and growth temperature. Relaxation of a parabolic intercrack stress profile to homogeneous stress was observed with annealing in all ambients for cracked samples. AFM results on cracked samples show an increase in width of the primary cracks along the 〈21̄1̄0〉 directions, and the formation of secondary cracks along the 〈11̄00〉 directions.

The absorption and magnetoabsorption of a set of (In, Ga)As/GaAs quantum wells has been studied. The oscillatory structure of magnetoabsorption allows one to reconstruct the energy positions of the Landau levels of the HHlE1 and LHlE1 excitonic states, taking into account the exciton binding energies calculated variationally. We have found that the potential profile for light holes is nearly flat with small deviations towards the type II quantum well. Calculation taking account of a combined potential for light holes shows that the excitonic transitions associated with the light hole states remain spatially direct in the system under study due to the 'Coulomb well' effect (additional hole confinement in the Coulombic potential created by an electron). The oscillator strength of the transitions has been shown to be quite high, which is suppotted by the data available. Because of a rather high oscillator strength, one can expect the appearance of a doublet spectral StNCtUre of light-hole exciton absorption, which was formed by the second light-hole 'oscillatory' states in the combined quantum well potential.

The annealing of n-type thin GaN films grown by metal-organic chemical vapour deposition in vacuum has been studied by beam-based positron annihilation spectroscopy. The results are consistent with a model in which Ga vacancies (VGa) exist alongside dislocations and are stable up to 900 °C. It is suggested that dislocations are shallow positron traps. Upon annealing at ⩽500 °C the decrease of dislocation density increases the effective positron diffusion length (L+eff) and the probability of trapping at VGa. While L+eff continues to change, the trapping of positrons at VGa is saturated upon annealing above 500 °C. The formation of N vacancies near the surface at high temperatures is considered to introduce a potential that retards positron back-diffusion. At 900 °C dissociation of GaN at a rate of ~5 nm s-1 is observed. Oxygen clusters, stable up to 900 °C, appear to exist near the interface between the GaN film and the sapphire substrate.

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