Ultra-fast photoluminescence in fused silica surface flaws susceptible to laser damage

ABSTRACTUsing high-sensitivity confocal time-resolved photoluminescence (PL) techniques, we found an ultrafast PL (40 ps-5 ns) from impurity-free surface flaws on fused silica. This PL is excited by the single-photon absorption of sub-band gap light. Regions which exhibit this PL are strongly absorptive well below the band gap, as evidenced by a propensity to damage with 3.5 eV nanosecond-scale laser pulses. Very high defect densities are needed to explain the damage thresholds observed. For such high defect densities, significant interactions between defects may strongly affect the temporal characteristics of the emission of electronic excitations. We propose that the distribution in lifetimes observed is not simply due to a large variety of defect states, but due to a variety of energy transfer interactions between defect states.

THE REASON FOR THE HIGH EMISSION EFFICIENCY OF GaInN/GaN BASED LEDS

We explain the mechanism of defect screening in GaInN/GaN quantum wells, which are used as active layers in white and blue light emitting diodes (LEDs). Despite the fact that these devices have now been commercially available for some time, the reason for the high luminescence efficiency had not been really understood. The high defect densities in these devices commonly would not allow the use as an optical emitter. We present the mechanism turning an actually poor-quality material into a powerful optical emitter.

Structural Characterization of Bulk AlN Single Crystals Grown from Self-Seeding and Seeding by SiC Substrates

Using a combination of synchrotron white beam x-ray topography (SWBXT) and high resolution x-ray diffraction (HRXRD), the structural quality of AlN crystals grown by various sublimation-based techniques have been non-destructively analyzed. Spontaneously nucleated AlN crystals are characterized by very low defect densities but their size is small. Self-seeding results in nucleation of multiple grains of different orientations, a few of which are of good quality while most are highly strained. Using readily available commercial 4H and 6H-SiC substrates, several growth runs have been carried out using different growth conditions to obtain thick AlN layers, either attached to the seed or free-standing. While attached layers are typically cracked and highly strained, crack-free free-standing layers can be obtained by delamination or SiC decomposition. X-ray characterization reveals these crystals have good purity but moderately high defect densities.

Dislocation Behavior During Deformation- Combining Experiments, Simulation and Modeling.

In situ straining in the transmission electron microscope has been combined with molecular dynamics computer simulations to investigate the nature of the interaction of glissile dislocations with radiation-produced defects (loops, stacking-fault tetrahedra, and He bubbles), and to determine the mechanisms by which the dislocation loops and stacking-fault tetrahedra are annihilated and defect-free channels are created. The defect pinning strength depends on the defect and on the interaction geometry. The experiments and simulations show that a single interaction is not always sufficient to annihilate a dislocation loop or a stacking-fault tetrahedra and that the nature of the defect may be changed because of the interaction. The edge/screw character of the dislocation is also important as they have different efficiencies for annihilating a defect. The dislocations responsible for creating the defect-free channels are not the preexisting dislocations but originate from grain boundaries and other stress concentrators. Cross-slip of dislocations within the channels is important for clearing and widening the channel and can create new channels. Based on these observations a dispersed-barrier hardening model in which the influence of the radiation defects and dislocation density are combined. The resulting model predicts the observed behavior, including the apparent yield drop at high defect densities.

Stress Evolution During Growth of Sputtered Ni/Cu Multilayers

Results from in-situ measurements of stress during growth of (111)-textured Ni/Cu multilayers with small and large bilayer periods are presented. In multilayers with small bilayer periods, Ni is in uniform tension and Cu in uniform compression. This behavior is modeled as the growth of a coherent multilayer with alloying in the layers. In multilayers with large bilayer periods, stress relaxation is observed but the measured stresses are much lower than expected based on a Mathews-Blakeslee relaxation process. An alternative stress relaxation mechanism based on high defect densities is presented and discussed.