The tensile stress in light-emitting diode (LED)-on-Si(1 1 1) multilayer structures must be reduced so that it does not compromise the multiple quantum well emission wavelength uniformity and structural stability. In this paper it is shown for non-optimized LED structures grown on Si(1 1 1) substrates that both emission wavelength uniformity and structural stability can be achieved within the same growth process. In order to gain a deeper understanding of the stress distribution within such a structure, cross-sectional Raman and photo-luminescence spectroscopy techniques were developed. It is observed that for a Si:GaN layer grown on a low-temperature (LT) AlN intermediate layer there is a decrease in compressive stress with increasing Si:GaN layer thickness during MOCVD growth which leads to a high level of tensile stress in the upper part of the layer. This may lead to the development of cracks during cooling to room temperature. Such a phenomenon may be associated with annihilation of defects such as dislocations. Therefore, a reduction of dislocation intensity should take place at the early stage of GaN growth on an AlN or AlGaN layer in order to reduce a build up of tensile stress with thickness. Furthermore, it is also shown that a prolonged three dimensional GaN island growth on a LT AlN interlayer for the reduction of dislocations may result in a reduction in the compressive stress in the resulting GaN layer.
The GaN-based light emitting diodes (LEDs) were grown on the cone-shaped patterned sapphire substrates (PSSs) by metal-organic chemical vapor deposition. To enhance the epilayer quality and optoelectronic performance of LEDs, the PSSs were further wet etched to form the modified top-tip cone shapes. After the wet etching process, some dry-etched induced damage on sapphire surface generated in the PSS fabrication process could be removed, leading to a higher crystal quality in epilayer and better device performance. As the wet etching time was increased to 3 min, the LED grown on the cone-shaped PSS had 53% and 8% improvement in light output (@ 20 mA) compared to that on conventional sapphire substrate (CSS) and the cone-shaped PSS without wet etching, respectively. It indicates that the technique of the modification in PSS shape has a high potential in LEDs application.
Source:IOPscience
Cu-Co alloy substrate was formed on the Au-seeded p-GaN side of sapphire wafer specimens to function as a secondary substrate of vertical structure GaN-based light emitting diode (LED), using electrodeposition method at constant potentials in an acidic citrate electrolyte. Potentiostatic deposition process was available to change the Co content of Cu-Co alloy substrates with varying cathodic deposition potential. Rotation speed of wafer specimens during electrodeposition also affected seriously the microstructure as well as Co content of the substrates. Cu-Co alloy layers deposited on non-rotating specimens showed coarse, particulate microstructures, whereas Cu-Co alloy substrates formed with applying specimen rotation had dense, grained microstructures. The chemical composition and microstructure of electrodeposited Cu-Co alloy substrates were closely related with their electrical and mechanical properties, which should be essentially considered for a appropriate secondary substrate of vertical structure LEDs. Residual stress, electrical resistivity, and Vickers hardness of the substrates were measured, respectively, according to cathodic deposition potential and rotation speed. In conclusion, mechanically strengthened Cu-Co alloy substrate in comparison with ductile Cu substrate was fabricated having very low residual stress and acceptable electrical resistivity by alloying Cu with low Co content of less than 3.15 at% in potentiostatic deposition process.
Source:IOPscience
Enhancement of light output intensity for GaN-based vertical light-emitting diodes (LEDs), combining wafer bonding and the laser lift-off (LLO) process, employing an omnidirectional extraction surface with synthesized single-crystal ZnO nanorod arrays in aqueous solution at room temperature is presented. The light output intensity and wall-plug efficiency of the GaN-based LLO vertical LED with the omnidirectional extraction surface by ZnO nanorod arrays shows 38.9 and 
increases, respectively, at 200 mA current injections compared to that of a vertical LED without ZnO nanorod arrays. The ZnO nanorod arrays not only support a current spreading layer but enhance the probability of photon escape through the omnidirectional extraction surface.
Source:IOPscience
Based on an InGaN/GaN light emitting diode (LED) structure, monolithically integrated vertical driving metal-oxide-semiconductor field-effect transistors (MOSFETs) were designed and experimentally implemented using a selective area growth (SAG) method. A simple p-GaN/n-GaN stack was selectively regrown on top of the LED wafer to realize an n/p/n structure for the vertical MOSFET fabrication. The integrated vertical MOSFET, which can effectively modulate the injection current through the serially connected LED, exhibited high performance such as an enhancement-mode (E-mode) operation with a relatively high output current density. However, on-resistance (R ON) degradation was observed in the fabricated vertical MOSFET at a low drain bias level (V DS < 2 V). Through a 2D TCAD simulation, the origin of the high R ON was revealed to be an electron barrier induced by the LED's p-AlGaN electron blocking layer (EBL). The simulation results also demonstrated that it can be improved by band engineering of the EBL.
Source:IOPscience
High quality and crack-free AlN films were obtained by using nano-patterned sapphire substrates (NPSS) grown at AMEC Prismo HiT3TM MOCVD platform. It is believed that the introduced epitaxial lateral overgrowth can annihilate most of dislocations and the grain boundary induced tensile stress can be significantly suppressed by NPSS. For a 5 μm thick AlN film, FWHMs of 173 arcsec and 335 arcsec were observed from AlN (002) and (102) X-ray rocking curves, respectively, indicating the high crystalline quality. The surface of AlN films grown on NPSS shows a typical step-bunching morphology with atomic steps on the bunched terrace. In addition, we also obtained excellent thickness uniformity for AlN films grown on NPSS with within-wafer and wafer-to-wafer thickness uniformity of 0.69% and 0.92%, respectively.
Source:IOPscience
The heteroeptixial integration of the III-V semiconductor compound, which was limited by lattice mismatch, can now be accomplished by using the wafer bonding process. In this study, a line-patterned 
layer was successfully transferred to the GaP wafer through the wafer bonding process for the successive growth of high-brightness light-emitting diode (LED) structures. © 2004 The Electrochemical Society. All rights reserved.
Source:IOPscience
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