Dukkyu Bae

Experimental determination of current spill-over and its effect on the efficiency droop in InGaN/GaN blue-light-emitting-diodes

We report the experimental determination of current spill-over in InGaN/GaN blue light emitting diodes by measuring the change in the forward current generated by a resonant excitation. To quantify accurately, the absorption of the laser as a function of the forward current was also determined. Two samples that have clearly different behavior of efficiency droop were compared to clarify the relationship between the current spill-over and the efficiency droop. We conclude that the carrier spill-over does occur and can be a significant cause for the efficiency droop but cannot single-handedly account for the efficiency droop.

Microstructured Air Cavities as High-Index Contrast Substrates with Strong Diffraction for Light-Emitting Diodes

Two-dimensional high-index-contrast dielectric gratings exhibit unconventional transmission and reflection due to their morphologies. For light-emitting devices, these characteristics help guided modes defeat total internal reflections, thereby enhancing the outcoupling efficiency into an ambient medium. However, the outcoupling ability is typically impeded by the limited index contrast given by pattern media. Here, we report strong-diffraction, high-index-contrast cavity engineered substrates (CESs) in which hexagonally arranged hemispherical air cavities are covered with a 80 nm thick crystallized alumina shell. Wavelength-resolved diffraction measurements and Fourier analysis on GaN-grown CESs reveal that the high-index-contrast air/alumina core/shell patterns lead to dramatic excitation of the low-order diffraction modes. Large-area (1075 × 750 μm(2)) blue-emitting InGaN/GaN light-emitting diodes (LEDs) fabricated on a 3 μm pitch CES exhibit ∼39% enhancement in the optical power compared to state-of-the-art, patterned-sapphire-substrate LEDs, while preserving all of the electrical metrics that are relevant to LED devices. Full-vectorial simulations quantitatively demonstrate the enhanced optical power of CES LEDs and show a progressive increase in the extraction efficiency as the air cavity volume is expanded. This trend in light extraction is observed for both lateral- and flip-chip-geometry LEDs. Measurements of far-field profiles indicate a substantial beaming effect for CES LEDs, despite their few-micron-pitch pattern. Near-to-far-field transformation simulations and polarization analysis demonstrate that the improved extraction efficiency of CES LEDs is ascribed to the increase in emissions via the top escape route and to the extraction of transverse-magnetic polarized light.

Formation of flat, relaxed Si1−xGex alloys on Si(001) without buffer layers

Atomically flat, fully strained Si1−xGex layers with thicknesses ranging from 40 to 240nm were grown on Si(001) at 450°C by ultrahigh-vacuum chemical vapor deposition and subjected to annealing at 1000°C for 20min to induce relaxation. In order to minimize surface diffusion during annealing and thereby inhibit strain-induced roughening in favor of misfit dislocation formation, SiO2 capping layers are deposited prior to annealing. The overall process results in smooth, relaxed alloy layers without the necessity of using several-μm-thick compositionally graded buffer layers.