Micro and nano-structured green gallium indium nitride/gallium nitride light-emitting diodes

Light-emitting diodes (LEDs) are commonly designed and studied based on bulk material properties. In this thesis different approaches based on patterns in the nano and micrometer length scale range are used to tackle low efficiency in the green spectral region, which is known as "green gap".;Since light generation and extraction are governed by microscopic processes, it is instructive to study LEDs with lateral mesa sizes scaled to the nanometer range. Besides the well-known case of the quantum size effect along the growth direction, a continuous lateral scaling could reveal the mechanisms behind the purported absence of a green gap in nanowire LEDs and the role of their extraction enhancement. Furthermore the possibility to modulate strain and piezoelectric polarization by post growth patterning is of practical interest, because the internal electric fields in conventional wurtzite GaN LEDs cause performance problems. A possible alternative is cubic phase GaN, which is free of built-in polarization fields. LEDs on cubic GaN could show the link between strong polarization fields and efficiency roll-off at high current densities, also known as droop. An additional problem for all nitride-based LEDs is efficient light extraction. For a planar GaN LED only roughly 8% of the generated light can be extracted. Novel lightextraction structures with extraction-favoring geometry can yield significant increase in light output power.;To investigate the effect of scaling the mesa dimension, micro and nano-sized LED arrays of variable structure size were fabricated. The nano-LEDs were patterned by electron beam lithography and dry etching. They contained up to 100 parallel nano-stripe LEDs connected to one common contact area. The mesa width was varied over 1 mum, 200 nm, and 50 nm. These LEDs were characterized electrically and optically, and the peak emission wavelength was found to depend on the lateral structure size. An electroluminescence (EL) wavelength shift of 3 nm towards smaller values was observed when the stripe width was reduced from 1 mum to 50 nm. At the same time a strong fourfold enhancement of the light emission from the patterned region over the unpatterned area was observed. Micro-patterned LEDs showed non-linear scaling of the light output power, and an enhancement of 39 % was achieved for structures with an area fill ratio of 0.5 over an LED with square mesa.;Growth of cubic GaN and cubic GaInN/GaN LEDs was shown by M-OVPE in Vshaped grooves formed by the {111} planes of etched silicon. SEM images of the GaN layer in small (~0.5 mum) regions show a contrast change where the phase boundary between cubic and wurtzite GaN is expected to occur. The growth parameter space is explored for optimal conditions while minimizing the alloying problem for GaN growth on Si. The cubic GaN phase is confirmed by electron back-scatter diffraction (EBSD) in the V-groove center, whereas wurtzite GaN is found near the groove edges. Luminescence of undoped GaN and GaInN/GaN multi-quantum well structures was studied by cathodoluminescence (CL). The undoped cubic GaN structure showed strong band-edge luminescence at 385 nm (3.22 eV) at 78 K, whereas for the MQW device strong emission at 498 nm is observed, even at room temperature. Full cubic LED structures were grown, and wavelength-stable electroluminescence at 489 nm was demonstrated.;LEDs with integrated light extraction structures are grown on free-standing GaN substrates with different off-cut angles. The devices with different off-cut show pronounced features at the top surface that also penetrate the active region. For a 2.24° off-cut, these features resemble fish scales, where the feature sizes are in the mum-range. The 2.24° off-cut LED shows a 3.6-fold increased light output power compared to a LED on virtually on-axis substrate with 0.06° off-cut. The enhancement found in the fish scale LEDs is attributed to increased light scattering, effectively reducing the fraction of trapped light.;These results show the potential of structures on the micro and nanometer scale for LED device performance and the progress on cubic GaN could open alternative ways to understand the droop problem.