No abstract

Vertically aligned GaN nanorods have recently obtained substantial interest as a new approach to solid state lighting. In comparison to conventional planar LEDs, 3D NanoLEDs are expected to offer substantial advantageous: very low defect density, quasi free‐standing, no strain due to mismatch of thermal expansion coefficients, no substrate bending even when grown on large area silicon. Core‐shell strategies are another very interesting aspect. The active LED surface per wafer could be increased by more than one order of magnitude. However, most of these advantages have not yet been proven in real devices, which would include a quantitative comparison of light emission. Related to the 3D character, there are also technological risks. In the following we will discuss the main developments which have paved the way up to this point, including a detailed discussion of possible benefits and risks connected with the NanoLED approach (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

During the metal organic vapor phase epitaxy of GaN microcolumns, both nitrogen-and gallium-polar GaN in the same structures could be detected on patterned SiO x /sapphire templates. To clarify its origin, the spatial distribution of surface polarity has been analyzed by both Kelvin probe force microscopy and selective etching techniques. A new "truncated pyramid + column" growth method was developed to effectively avoid the formation of mixed polarity and realize selective area growth of single N-polar GaN columns. The strong yellow luminescence in mixed polarity structures can substantially be eliminated in single N-polar core−shell lightemitting diodes.

The behavior of GaN surfaces during photoassisted Kelvin probe force microscopy is demonstrated to be strongly dependant on surface polarity. The surface photovoltage of GaN surfaces illuminated with above-band gap light is analyzed as a function of time and light intensity. Distinct differences between Ga-polar and N-polar surfaces could be identified, attributed to photoinduced chemisorption of oxygen during illumination. These differences can be used for a contactless, nondestructive, and easy-performable analysis of the polarity of GaN surfaces.

Surface magnetic structure of epitaxial magnetite thin films grown on MgO (001) A long-range ordered magnetic domain structure was found in magnetitelike ͑Fe 3−␦ O 4 , ␦ Ϸ 0.03͒ thin films prepared by molecular beam epitaxy on MgO ͑100͒ substrates. The stripelike magnetic domain structure arising after suitable postprocessing differs significantly from earlier observations. The field-dependent domain structure was investigated by magnetic force microscopy in external magnetic fields. The magnetic domain structure results from a moderate perpendicular anisotropy. The domain structure is pinned, and the pinning centers arise from the magnetite antiphase domain structure.