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Dortmund, Germany

Sintonen S.,Aalto University | Ali M.,Aalto University | Torma P.T.,Optogan GmbH | Suihkonen S.,Aalto University | And 6 more authors.
Physica Status Solidi (C) Current Topics in Solid State Physics

Owing to its great potential in optoelectronic devices, a lot of effort has recently been put into improving the crystal quality of heteroepitaxial GaN. In this study, GaN layers grown on patterned GaN/sapphire substrates are non-destructively investigated by X-ray diffraction techniques. The effect of the patterning is characterized by determining the strain and lattice tilt and twist in the overgrown GaN layers. High resolution X-ray diffraction (HRXRD) scans and reciprocal space maps (RSMs) were utilized for this purpose. Synchrotron radiation X-ray topography (SR-XRT) was used to examine the microstructure of the samples. The patterning was found to significantly improve the crystal quality of the GaN layers, especially by reducing the amount of edge dislocations. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Suihkonen S.,Aalto University | Ali M.,Aalto University | Svensk O.,Aalto University | Sintonen S.,Aalto University | And 5 more authors.
Physica Status Solidi (C) Current Topics in Solid State Physics

We present the results of sapphire/GaN substrate patterning on GaN film quality and on LED performance. MOVPE was used to grow GaN films on c-plane sapphire. The films were then patterned with hexagonal holes with various diameters. When regrown with GaN, voids were formed in the sapphire/GaN interface. The void shape could be controlled from nearly vertical to inclined by changing the pattern dimensions. Patterning resulted also in reduced dislocation density of the overgrown GaN film. X-ray diffraction and transmission electron microscopy results showed that the reduction is due to bending of edge-type dislocations during the regrowth of the patterned substrate. Standard LEDs were grown on reference and patterned substrates and processed into chips. When grown on a patterned substrate LED chips showed approximately 10% increase in light output when compared to ones grown on a reference sample. The increase was attributed to improved light extraction. More improvement can be expected by optimizing the pattern geometry and the void shape. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Torma P.T.,AaltoUniversity | Ali M.,AaltoUniversity | Svensk O.,AaltoUniversity | Suihkonen S.,AaltoUniversity | And 5 more authors.

A study of GaN films and nitride based light emitting diodes (LEDs) grown on low density pillar structure (LDPS) and high density pillar structure (honeycomb like) sapphires patterned by chemical wet etching is described. Both types of patterned sapphire substrate (PSS) offered reduced defect density and improved performance of near-ultraviolet LED. In the case of LDPS patterned sapphire the correct choice of the pillar depth was found to be crucial for high quality crystal growth. A reduction of threading dislocation (TD) density from the level of 108 cm-2 down to the level of 2 × 109 cm-2 was observed. It was found that mostly enhanced light extraction rather than improved material quality caused the improvement of the LED performance. © 2010 The Royal Society of Chemistry. Source

Ali M.,Aalto University | Svensk O.,Aalto University | Riuttanen L.,Aalto University | Kruse M.,Optogan GmbH | And 7 more authors.
Semiconductor Science and Technology

We present near-UV GaN light-emitting diodes (LEDs) grown on patterned GaN/sapphire templates with improved material quality and light extraction efficiency. Enhancement of light extraction efficiency is attributed to voids generated at the GaN/sapphire interface. The sidewall inclination angle of the voids can be controlled from nearly vertical ( 85°) to fully inclined ( 60°) by changing the initial patterning dimensions. Light extraction efficiency and material quality improve with a decreasing void sidewall angle. A 20% increase in the light output is observed at 20mA of input current for LED structures with 60° inclined sidewall voids. © 2012 IOP Publishing Ltd. Source

Ali M.,Aalto University | Romanov A.E.,Optogan GmbH | Suihkonen S.,Aalto University | Svensk O.,Aalto University | And 5 more authors.
Journal of Crystal Growth

We present the results of GaN re-growth on hexagonally patterned GaN templates. Sapphire was used as the original substrate and the samples were grown by metalorganic vapor phase epitaxy (MOVPE). The re-growth on the patterned templates results in the formation of voids at the GaN/sapphire interface. Our extensive scanning electron microscopy (SEM)-based experimental investigations show that the void shape can be controlled from nearly vertical to fully inclined configurations. It was found that the initial hexagon hole diameter plays a key role in determining the final profile of the void sidewalls. X-ray diffraction analysis of the GaN layers indicates that the layers with inclined sidewall voids have an improved crystalline quality. Knowledge of the void configurations in the GaN layers and a possibility to control their shape can help in enhancing light extraction from the light emitting structures. © 2010 Elsevier B.V. Source

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