The Auger recombination coefficient in quasi-bulk InxGa1−xN (x∼9%–15%) layers grown on GaN (0001) is measured by a photoluminescence technique. The samples vary in InN composition, thickness, and threading dislocation density. Throughout this sample set, the measured Auger coefficient ranges from 1.4×10−30to2.0×10−30cm6s−1. The authors argue that an Auger coefficient of this magnitude, combined with the high carrier densities reached in blue and green InGaN∕GaN (0001) quantum well light-emitting diodes (LEDs), is the reason why the maximum external quantum efficiency in these devices is observed at very low current densities. Thus, Auger recombination is the primary nonradiative path for carriers at typical LED operating currents and is the reason behind the drop in efficiency with increasing current even under room-temperature (short-pulsed, low-duty-factor) injection conditions.

We report the observation of periodic 180 degrees stripe domains below the ferroelectric transition in thin films. Epitaxial PbTiO3 films of thickness d=1.6 to 42 nm on SrTiO3 substrates were studied using x-ray scattering. Upon cooling below T(C), satellites appeared around Bragg peaks indicating the presence of 180 degrees stripe domains of period Lambda=3.7 to 24 nm. The dependence of Lambda on d agrees well with theory including epitaxial strain effects, while the suppression of T(C) for thinner films is significantly larger than that expected solely from stripe domains.

The determination of the structure of transient molecules, such as photoexcited states, in disordered media (such as in solution) usually requires methods with high temporal resolution. The transient molecular structure of a reaction intermediate produced by photoexcitation of NiTPP-L2 (NiTPP, nickeltetraphenylporphyrin; L, piperidine) in solution was determined by x-ray absorption fine structure (XAFS) data obtained on a 14-nanosecond time scale from a third-generation synchrotron source. The XAFS measurements confirm that photoexcitation leads to the rapid removal of both axial ligands to produce a transient square-planar intermediate, NiTPP, with a lifetime of 28 nanoseconds. The transient structure of the photodissociated intermediate is nearly identical to that of the ground state NiTPP, suggesting that the intermediate adopts the same structure as the ground state in a noncoordinating solvent before it recombines with two ligands to form the more stable octahedrally coordinated NiTPP-L2.

The performance of III‐nitride based high‐power light emitting diodes (LEDs) is reviewed. Direct color high‐power LEDs with 1 × 1 mm2 chip size in commercial LUXEON® Rebel packages are shown to exhibit external quantum efficiencies at a drive current of 350 mA ranging from ∼60% at a peak wavelength of ∼420 nm to ∼27% at ∼525 nm. The short wavelength blue LED emits ∼615 mW at 350 mA and >2 W at 1.5 A. The green LED emits ∼110 lm at 350 mA and ∼270 lm at 1.5 A. Phosphor‐conversion white LEDs (1 × 1 mm2 chip size) are demonstrated that emit ∼126 lm of white light when driven at 350 mA and 381 lm when driven at 1.5 A (Correlated Color Temperature, CCT ∼ 4700 K). In a similar LED that employs a double heterostructure (DH) insign instead of a multi‐quantum well (MQW) active region, the luminous flux increases to 435 lm (CCT ∼ 5000 K) at 1.5 A drive current. Also discussed are experimental techniques that enable the separation of internal quantum efficiency and extraction efficiency. One technique derives the internal quantum efficiency from temperature and excitation‐dependent photoluminescence measurements. A second technique relies on variable‐temperature electroluminescence measurements and enables the estimation of the extraction efficiency. Both techniques are shown to yield consistent results and indicate that the internal quantum efficiency of short wavelength blue (λ ∼ 420 nm) high‐power LEDs is as high as 71% even at a drive current of 350 mA. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We present in situ x-ray scattering measurements of the surface structures of PbTiO(3) (001) in equilibrium with PbO vapor. At 875 to 1025 K, a reconstruction having c(2x2) symmetry is present under most conditions, while a 1 x 6 reconstruction occurs under PbO-poor conditions. The atomic structure of the c(2x2) phase is found to consist of a single layer of an antiferrodistortive structure with oxygen cages counter-rotated by 10 degrees about the titanium ions.