No abstract

No abstract

No abstract

Relaxation of tensile strain in AlGaN heterostructures grown on GaN template can lead to the formation of cracks. These extended defects locally degrade the crystal quality resulting in a local increase of non-radiative recombinations. The effect of such cracks on the optical and structural properties of core-shell AlGaN/AlGaN multiple quantum wells grown on GaN microwires are comprehensively characterized by means of spectrally and time-correlated cathodoluminescence (CL). We observe that the CL blueshifts near a crack. By performing 6x6 k.p simulations in combination with transmission electron microscopy analysis, we ascribe this shift to the strain relaxation by the free surface near cracks. By simultaneously recording the variations of both the CL lifetime and the CL intensity across the crack, we directly assess the carrier dynamics around the defect at T = 5 K. We observe that the CL lifetime is reduced typically from 500 ps to less than 300 ps and the CL intensity increases by about 40% near the crack. The effect of the crack on the optical properties is therefore of two natures. First, the presence of this defect locally increases non-radiative recombinations while at the same time, it locally improves the extraction efficiency. These findings emphasize the need for time-resolved experiments to avoid experimental artifacts related to local changes of light collection.

Thermal annealing of top−down fabricated GaN nanocolumns (NCs) was investigated over a wide range of temperatures for ammonia-rich atmospheres of both nitrogen and hydrogen. It was found that in contrast to the annealing of planar GaN layers, where surface morphology change is governed purely by material decomposition, reshaping of GaN NCs is strongly affected by competition between different crystallographic facets, which in turn depends on ambient atmosphere and temperature. A qualitative mechanism explaining the observed behavior has been proposed. On the basis of the analysis of these annealing results, growth conditions suitable for either predominantly lateral expansion of the NCs turning their sidewalls into six well-defined vertical m-plane facets, or, vice versa, their infilling from the base regions between the NCs were determined. GaN NC arrays of increased filling factors as compared to the as top−down fabricated ones have been demonstrated using these optimized growth conditions.

No abstract

Fully functional InAlN-based ultraviolet LEDs emitting at 340–350 nm were demonstrated for the first time; detailed electrical and optical characterization is presented and discussed. Results from the measurements at pulsed conditions are in agreement with the attribution of the dominant electroluminescence peak to near-band-edge emission. The composition of the AlGaN barriers was chosen to give the same internal polarization field as that of the InAlN wells. A simulation study of this polarization-matched heterostructure shows a significant increase in the electron-hole overlap integral if compared with a standard AlGaN/AlGaN active region having the same level of carrier confinement. Limitations and problems of these preliminary devices are also presented, and possible future work aimed at increasing their efficiency is discussed.

Silicon-doped n-type (0 0 0 1) AlGaN materials with 60% and 85% AlN content were studied close to the doping condition that gives the lowest resistivity (Si/III ratios in the ranges 2.8-34 × 10 −5 and 1.3-6.6 × 10 −5 , respectively). Temperature-dependent conductivity and Hall-effect measurements showed that, apart from the diffusion-like transport in the conduction band, a significant amount of the conductivity was due to phonon-assisted hopping among localized states in the impurity band, which became almost completely degenerate in the most doped sample of the Al 0.6 Ga 0.4 N series. In the doping range explored, impurityband transport was not only dominant at low temperature, but also significant at roomtemperature, with contributions to the total conductivity up to 46% for the most conductive sample. We show that, as a consequence of this fact, the measurements of Hall carrier concentration and Hall mobility using the usual single-channel approach are not reliable, even at high temperatures. We propose a simple method to separate the contributions of the two channels. Our model, although only approximate, can be used to gain insight into the doping mechanism: particularly it shows that the room-temperature free-electron concentration in the conduction band of the Al 0.6 Ga 0.4 N material reaches its maximum at about 1.6 × 10 18 cm −3 , well below the value that would have been obtained with the standard single-channel analysis of the data. This maximum is already achieved at dopant concentrations lower than the one that gives the best conductivity. However, further increase of the doping levels are required to enhance the impurity-band channel, with concentrations of the carriers participating in this type of transport that increase from 2.1 × 10 18 cm −3 up to 4.3 × 10 18 cm −3 . For the Al 0.85 Ga 0.15 N, even though it was not possible to estimate the actual carrier concentrations, our measurements suggest that a significant impurity-band channel is present also in this material.