Published Version

This letter explores the impact of quantum well placement and photonic crystal (PhC) etch depth on the emission directionality of thin-film InGaN PhC light-emitting diodes (LEDs). The far-field pattern of 800-nm-thick PhC LEDs is tuned by varying only the etch depth of a surface-patterned hexagonal PhC from 90 to 440 nm. This dependence on etch depth is shown to arise from the preferential excitation of a subset of the allowed guided modes. Selective excitation of the TE0 and TE1 modes is utilized to achieve a vertically directional emission pattern comprised of only these two modes.

The light extraction efficiency of photonic-crystal ͑PhC͒ light-emitting diodes ͑LEDs͒ relies on the competition between the PhC extraction and dissipation mechanisms of the guided light within the LED. This work presents the experimental determination of the PhC extraction length of each guided mode and the absorption coefficient of the active region ͑AR͒ and quantum wells ͑QWs͒ from the observation of the LED far-field emission using a high-resolution angle-spectrum-resolved measurement. The angular and spectral linewidths of the extracted guided modes reveal, depending on the spectral range, the modal extraction length of the PhCs, the AR absorption length, or a combination of both. Modes with a high confinement with the QWs presented a shorter absorption length compared with their extraction length by a shallow surface PhC ͑95-nm-deep͒, meaning that the AR absorption was a more efficient mechanism than the PhC extraction. The measured modal extraction length of the shallow surface PhC varied in the range of 55-120 m, which determines the minimum dimensions of the device and the maximum acceptable dissipation length for an efficient extraction of the guided light by the PhCs. This paper presents also a discussion on the PhC designs that yield PhC extraction lengths shorter than other dissipation lengths, a fundamental requirement for high-efficiency PhC LEDs. The same technique was also applied to estimate the absorption coefficient of the InGaN-based QWs, and can be extended to experimentally determine losses by metallic layers from electrical contacts or other dissipation mechanisms, which are parameters of interest to a broader class of optoelectronic devices, not only PhC LEDs.

This work presents the experimental measurement of the extraction lengths of individual guided modes of embedded photonic-crystal (PhC) lightemitting diodes (LEDs), which corroborates its superior guided light extraction compared to surface PhC LEDs. While in surface PhC LEDs a number of low order modes were not observed experimentally, in embedded PhC LEDs, the stronger interaction of all guided modes with the PhCs resulted in the diffraction of all low order modes with short extraction lengths, ranging from 60 to 80 m.

Thin-film InGaN photonic crystal (PhC) light-emitting diodes (LEDs) with a total semiconductor thickness of either 800 nm or 3.45 μm were fabricated and characterized. Increased directional radiance relative to Lambertian emission was observed for both cases. The 800-nm-thick PhC LEDs yielded only a slight improvement in total light output over the 3.45-μm-thick PhC LEDs. Simulations indicate that, except for ultrathin devices well below 800 nm, the balance between PhC extraction and metal absorption at the backside mirror results in modal extraction efficiencies that are almost independent of device thickness, but highly dependent on mirror reflectivity.