This paper presents an overview of integrated optics routing and coupling devices based on multimode interference. The underlying self-imaging principle in multimode waveguides is described using a guided mode propagation analysis. Special issues concerning the design and operation of multimode interference devices are discussed, followed by a survey of reported applications. It is shown that multimode interference couplers offer superior performance, excellent tolerance to polarization and wavelength variations, and relaxed fabrication requirements when compared to alternatives such as directional couplers, adiabatic X- or Y-junctions, and diffractive star couplers. >

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Optical multi-mode interference devices based on self-imaging: principles and applications

Status and future outlook of III-V compound semiconductor visible-spectrum light-emitting diodes (LEDs) are presented. Light extraction techniques are reviewed and extraction efficiencies are quantified in the 60%+ (AlGaInP) and ~80% (InGaN) regimes for state-of-the-art devices. The phosphor-based white LED concept is reviewed and recent performance discussed, showing that high-power white LEDs now approach the 100-lm/W regime. Devices employing multiple phosphors for "warm" white color temperatures (~3000-4000 K) and high color rendering (CRI>80), which provide properties critical for many illumination applications, are discussed. Recent developments in chip design, packaging, and high current performance lead to very high luminance devices (~50 Mcd/m2 white at 1 A forward current in 1times1 mm2 chip) that are suitable for application to automotive forward lighting. A prognosis for future LED performance levels is considered given further improvements in internal quantum efficiency, which to date lag achievements in light extraction efficiency for InGaN LEDs

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Status and Future of High-Power Light-Emitting Diodes for Solid-State Lighting

Blue light-emitting diodes with a light extraction efficiency of 73% are reported. The InGaN–GaN devices use a photonic-crystal structure for superior optical mode control; their performance has been characterized experimentally and modelled theoretically.

III -nitride photonic-crystal light-emitting diodes with high extraction efficiency

Al2O3 is a versatile high-κ dielectric that has excellent surface passivation properties on crystalline Si (c-Si), which are of vital importance for devices such as light emitting diodes and high-efficiency solar cells. We demonstrate both experimentally and by simulations that the surface passivation can be related to a satisfactory low interface defect density in combination with a strong field-effect passivation induced by a negative fixed charge density Qf of up to 1013 cm−2 present in the Al2O3 film at the interface with the underlying Si substrate. The negative polarity of Qf in Al2O3 is especially beneficial for the passivation of p-type c-Si as the bulk minority carriers are shielded from the c-Si surface. As the level of field-effect passivation is shown to scale with Qf2, the high Qf in Al2O3 tolerates a higher interface defect density on c-Si compared to alternative surface passivation schemes.

/pdf/on-the-c-si-surface-passivation-mechanism-by-the-negative-4z2pi0oo3h.pdf

On the c-Si surface passivation mechanism by the negative-charge-dielectric Al2O3

Electromagnetic band structure can produce either an enhancement or a suppression of spontaneous emission from two-dimensional (2-D) photonic crystal thin films. We believe that such effects might be important for light emitting diodes. Our experiments were based on thin-film InGaAs-InP 2-D photonic crystals at ambient temperature, but the concepts would apply equally to InGaN thin films, for example. We show that the magnitude of Purcell enhancement factor, F/sub p//spl sim/2, for spatially extended band modes, is similar to that for a tiny mode in a three dimensional (3-D) nanocavity. Nonetheless, light extraction enhancement that arises from Zone folding or Bragg scattering of the photonic bands is probably the more important effect, and an external quantum efficiency >50% is possible. Angle resolved photoluminescence from inside the photonic crystal gives a direct spectral readout of the internal 2-D photonic band dispersion. The tradeoffs for employing various photonic crystal structures in high efficiency light-emitting diodes are analyzed.

/pdf/spontaneous-emission-extraction-and-purcell-enhancement-from-4v6yj9wnms.pdf

Spontaneous emission extraction and Purcell enhancement from thin-film 2-D photonic crystals

Surface recombination is an important characteristic of an optoelectronic material. Although surface recombination is a limiting factor for very small devices it has not been studied intensively. We have investigated surface recombination velocity on the exposed surfaces of the AlGaN, InGaAs, and InGaAlP material systems by using absolute photoluminescence quantum efficiency measurements. Two of these three material systems have low enough surface recombination velocity to be usable in nanoscale photonic crystal light-emitting diodes.

/pdf/surface-recombination-measurements-on-iii-v-candidate-2l4899i4rj.pdf

Surface Recombination Measurements on III-V Candidate Materials for Nanostructure Light-Emitting Diodes

The control of quantum well shapes in GaAs/AlGaAs material after growth has been investigated both theoretically and experimentally. Double quantum well samples capped either by SiO/sub 2/ or fluorides of the group IIA elements were annealed, and energy gap shifts were measured by photoluminescence. These experimental energy shifts were compared to a theoretical model to obtain the diffusion coefficient of aluminum into the quantum wells. Fluorides were found to inhibit the intermixing process almost completely, whereas SiO/sub 2/ is known to enhance it. The aluminum diffusion coefficients for samples annealed at 920/spl deg/C for 30 s are 4.0/spl times/10/sup -17/ cm/sup 2//s and 2.1/spl times/10/sup -15/ cm/sup 2//s for SrF/sub 2/ and SiO/sub 2/ caps, respectively. The activation energies found were 4.09 and 6.40 eV for the same two caps. >

Postgrowth control of GaAs/AlGaAs quantum well shapes by impurity-free vacancy diffusion

Strip‐loaded semiconductor ring lasers were successfully fabricated in the GaAs/AlGaAs material system. The radius was 400 μm and a 2×2 3 dB multimode interference coupler 320 μm long was used to extract the output signal. Devices operated continuous wave at 140 mA threshold current with a differential quantum efficiency of 7% per output, which we believe is the highest value reported to date for lasers of this type.

Strip‐loaded semiconductor ring lasers employing multimode interference output couplers

Very low loss extended cavity lasers have been fabricated using the impurity-free vacancy diffusion technique. The average loss, obtained from the slope of measured loss as a function of the extended cavity length, was 10 dB/cn for extended cavities annealed at 900 degrees C for 30 s. The lowest loss of 3.6 dB/cm was obtained from a device annealed at 950 degrees C for 30 s.

Very low loss extended cavity GaAs/AlGaAs lasers made by impurity-free vacancy diffusion

Bandgap tuning by impurity-free vacancy disordering was investigated on carbon and zinc p-doped laser structures. Zinc diffused during annealing and consequently only carbon-doped material lased. After annealing, threshold current densities rose from 225 to 255 A cm-2 due to an increase in the transparency current; the gain constant of the quantum wells remained constant.

https://iopscience.iop.org/article/10.1088/0268-1242/9/11/020/pdf

I. Gontijo

Papers

Surface Recombination Measurements on III-V Candidate Materials for Nanostructure Light-Emitting Diodes

Postgrowth control of GaAs/AlGaAs quantum well shapes by impurity-free vacancy diffusion

Strip‐loaded semiconductor ring lasers employing multimode interference output couplers

Very low loss extended cavity GaAs/AlGaAs lasers made by impurity-free vacancy diffusion

A comparison of carbon and zinc doping in GaAs/AlGaAs lasers bandgap-tuned by impurity-free vacancy disordering