physica status solidi (c)
Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Editor: Stefan Hildebrandt (Editor-in-Chief)
Online ISSN: 1610-1642
Cover Picture: Phys. Status Solidi C 3–4/2014
A hexagonal closed-packed array of nano-pillars patterned by nanosphere litho-graphy is embedded within an InGaN light-emitting diode by means of epitaxial lateral overgrowth. Li and coworkers describe on pp. 742–745 that the regrowth of a continuous p-doped contact layer across the nanostructures enables bridging of tens of thousands of nano-emitters, whilst fulfilling the task of electrical isolation between the p- and n-doped layers concurrently, representing a pio-neering technique for realizing nano-LED practically. As the nano-pillars are embedded within the LED beneath the regrown layer, the air gaps between pillars, as well as their light extraction enhancing capabilities, are fully retained even after device encapsulation. The nano-pillar structures also contribute to strain relaxation of the quantum wells, evident from the mild spectral shifts with increasing injection currents compared to conventional LEDs.
Inside Front Cover: Phys. Status Solidi C 3–4/2014
In typical designs of planar nitride green light emitting diodes (LEDs), the active layers suffer from a large strain between the InGaN quantum wells (QWs) and the underlying n-GaN layer. Stranski–Krastanov quantum dots (SK QDs), character-ized by their small sizes and potential for efficient strain accommodation, may allow the growth of defect-free LED active layers at green wavelengths. However, SK growth, being ultimately conditioned by strain and relative surface energies of the system, is sensitive to the substrate defects. In contrast to planar GaN substrates which comprise high amounts of dislocations, GaN nanowires (NWs) can be grown dislocation-free and therefore act as ideal templates for the growth of nitride SK QDs. In their work, Bi and coworkers (see pp. 421–424) used metal-organic vapor phase epitaxy to grow GaN NWs selectively through nanometer-scale circular openings in a SiN mask deposited on GaN/Si substrates. InN QDs were grown subsequently on such GaN NWs. The InN QDs show a high uni-formity in size, shape and density. The side facet edges of the GaN NWs act as energetically favorable nucleation sites, but QD growth can also be realized on the side facets by changing the vapor phase supersaturation and the growth temperature. Periodic formation of I1-type stacking faults is observed along the c-axis of the InN QDs, attributed to a large lattice mismatch (about 10%) between InN and GaN.
Back Cover: Phys. Status Solidi C 3–4/2014
As the efficiency and output power of III-nitride blue laser diodes (LDs) improve, they potentially become interesting for solid-state lighting (SSL). In fact, state-of-the-art blue LDs have higher efficiencies than blue light-emitting diodes (LEDs) when driven at high input powers. This is because the processes that cause the drop in efficiency at high powers in LEDs are clamped in LDs operated under stimulated emission. In their paper on pp. 674–677, Wierer and coworkers show that blue LDs have the potential to be nearly as efficient as blue LEDs, and also with similar white lumen output when pumping phosphors. Therefore, a white source consisting of phosphors pumped by a blue LD could someday become a competitive and viable SSL source. A simple demonstration is shown in the cover photo where a collimated blue LD beam (right) is directed and absorbed from the edge in a traditional phosphor plate used for LEDs (left) to produce brilliant white light. (Photograph by Randy Montoya of Sandia National Laboratories. Phosphors in photograph courtesy of Intematix.)