Broadband Light-emitting Diodes Research Articles

Solution-Processed Infrared Optoelectronics: Photovoltaics, Sensors, and Sources

The fabrication of optoelectronic devices via spin-coating onto an arbitrary substrate offers ease of integration, low cost, and physical flexibility. Reports of active optoelectronic devices operating in the IR, and made via solution-processing, emerged in the early 2000s. Here, we review progress in IR solar cells, image sensors, and optical sources based on solution-processed materials. The latest solution-processed photovoltaics (PV) now provide 4.2% power conversion efficiencies in the IR, placing them a factor of 3 away from enabling a doubling in overall solar power conversion efficiency of today's best visible-wavelength solution-processed PV. The best solution-processed photodetectors now provide sensitivities in the 1013 Jones D* range, exceeding the sensitivity of the best epitaxially grown short-wavelength infrared photodetectors. Infrared optical sources, both broadband light-emitting diodes and, more recently, lasers, have now also been reported at 1.5 mum. We review the progress and future prospects of this rapidly advancing field.

A Multi-Source Portable Light Emitting Diode Spectrofluorometer

A portable luminescence spectrofluorometer weighing only 1.5 kg that uses multiple light emitting diodes (LEDs) as excitation sources was developed and evaluated. Excitation using a sequence of seven individual broad-band LED emission sources enabled the generation of excitation-emission spectra using a light weight (<1.5 kg) spectrometer. Limits of detection for rhodamine 6G, rhodamine B, and fluorescein were 2.9, 3.2, and 11.0 nM, respectively. Generation of excitation-emission matrices (EEMs) enabled the analysis of samples containing mixtures of rhodamine B and fluorescein. Buffered saline plant and animal feed extracts were also analyzed using this instrument. These samples included the woody plants Pistacia lentiscus (Evergreen pistache or Mastic) and Philyria latifolia, and the herbaceous species Medicago sativa (alfalfa), Trifolium spp. (clover), and a feed concentrate. Application of multi-way principal component analysis (MPCA) to the resulting three-dimensional data sets enabled discernment among these various diet constituents.

Barrier-width dependence of emission in triple-quantum-well broadband light-emitting diodes

Broadband (1.3-1.9 /spl mu/m) emission has been demonstrated using triple-quantum-well light-emitting diodes fabricated from InGaAs/InGaAlAs digital alloys. Obtaining comparable emission intensities from the three wells requires careful choice of the barrier-layer thicknesses.

High slope efficiency, “cascaded” midinfrared lasers with type I InAsSb quantum wells

Lasers and light-emitting diodes with multistage, type I InAsSb/InAsP quantum well active regions are reported. These ten stage, cascaded devices were grown by metalorganic chemical vapor deposition. The broadband light-emitting diodes produced high average powers, &amp;gt;2 mW (∼80 K, 3.7 μm) and &amp;gt;0.1 mW (∼300 K, 4.3 μm). A 3.8–3.9 μm laser structure operated up to T=180 K. At 80 K, peak power &amp;gt;100 mW and a slope efficiency of 48% (4.8% per stage) were observed in our gain guided lasers. The slope efficiency was strongly dependent on cavity length, and analysis of efficiency data suggests an internal differential quantum efficiency &amp;gt;1 and a loss coefficient ⩾100 cm−1.

The growth of InAsSb/InAsP strained-layer superlattices for use in infrared emitters

We describe the metalorganic chemical vapor deposition of InAsSb/InAsP strained-layer superlattice (SLS) active regions for use in mid-infrared emitters. These SLSs were grown at 500°C, and 200 Torr in a horizontal quartz reactor using TMIn, TESb, AsH3, and PH3. By changing the layer thickness and composition, we have prepared structures with low temperature (≤20K) photoluminescence wavelengths ranging from 3.2 to 4.4 µm. Excellent performance was observed for a SLS light emitting diode (LED) and both optically pumped and electrically injected SLS lasers. An optically pumped, double heterostructure laser emitted at 3.86 µm with a maximum operating temperature of 240K and a characteristic temperature of 33K. We have also made electrically injected lasers and LEDs utilizing a GaAsSb/InAs semi-metal injection scheme. The semi-metal injected, broadband LED emitted at 4 µm with 80 µW of power at 300K and 200 mA average current. The InAsSb/InAsP SLS injection laser emitted at 3.6 µm at 120K.

Midinfrared lasers and light-emitting diodes with InAsSb/InAsP strained-layer superlattice active regions

The properties of InAsSb/InAsP strained-layer superlattice (SLS), midwave infrared materials, and devices are reported. SLSs were grown by metal-organic chemical vapor deposition and characterized by magnetophotoluminescence and x-ray diffraction. Excellent performance was observed for an SLS light emitting diode (LED) and an optically pumped SLS laser. The semimetal injected, broadband LED emitted at 4 μm with 80 μW of power at 300 K and 200 mA average current. The laser displayed 3.86 μm emission at 240 K, the maximum operating temperature of the laser, and a characteristic temperature of 33 K.

Progress in InGaAs-GaAs selective-area MOCVD toward photonic integrated circuits

The progress toward integrated photonic devices by selective-area metalorganic chemical vapor deposition (MOCVD) is reviewed. Processing steps involved with fabricating buried heterostructures (BHs) by a three-step technique are outlined, and a computational model is presented that predicts the enhancement behavior of selective-area MOCVD. Results are reviewed for several discrete and integrated photonic devices. These include low-threshold BH lasers, laser diodes integrated with either intracavity or external cavity modulators, dual-channel emitters integrated with both modulators and passive y-junction waveguides, and broad-band light-emitting diodes (LEDs).

A low-cost, high-speed, near-infrared hygrometer

We describe a simple near-infrared hygrometer based on a broadband light-emitting diode (LED) centered at 1320 nm. Light from the LED is collimated and passes through a rotating optical-filter assembly and a sample cell. The water vapor concentration is measured using a synchronous detection scheme. This compact and robust instrument has a response time of 0.15 s and can reliably detect short-term absolute humidity changes as small as 0.6 mg/L (which equate to 0.02% absorption); the long-term stability of the absolute humidity measurement over a period of 4 days is approximately 1 mg/L. These performance characteristics are particularly well suited to measurements associated with human respiration, and we present results in this humidity regime.

Broad-band light-emitting diode for 1.4-2.0 μm using variable-composition InGaAs quantum wells

We describe a novel broad-band light-emitting diode employing InGaAs quantum wells with different bandgaps. The device structures were grown by molecular-beam epitaxy on InP substrates and consist of three In/sub x/Ga/sub 1-x/As quantum wells (x=0.4, 0.53, and 0.66) imbedded in the p-doped region of an InAlAs p-n junction diode. Electrons injected into the p-region are captured into the wells and recombine with holes to produce radiation at the three bandgap wavelengths. Broad-band emission over a large wavelength range (1.4-2.0 μm) is easily achieved with this approach. These novel solid-state sources have a variety of potential applications, for example, in miniature spectrometer and sensor systems."

5.5-MeV proton irradiation of a strained quantum-well laser diode and a multiple quantum-well broadband LED

The degradation under 5.5-MeV proton irradiation of two classes of quantum-well-based fiber-optic light sources was evaluated for satellite applications. The first was an InGaAs/GaAs strained-layer quantum-well (QW) laser, and the second was a broadband light-emitting diode (LED) based on dual asymmetric quantum wells in the InGaAs/GaAs/AlGaAs system. The QW LEDs were more tolerant of proton irradiation (-3 dB power at approximately 3*10/sup 13/ protons/cm/sup 2/) than the QW lasers (-3 dB power at approximately 3*10/sup 12/ protons/cm/sup 2/). The LEDs were operated far into gain saturation with a high-loss cavity structure, while the lasers were operated in a region where gain was more sensitive to current density. Therefore, atomic displacement-related recombination sites had a greater detrimental effect upon the lasers than the LEDs. The lasers held constant slope efficiency, and current thresholds increased linearly with proton fluence, while both LED power and slope efficiency decreased with proton fluence. The determined damage factors for both devices were found to fall within values predicted from a universal damage relation previously reported by G.P. Summers et al. (1988).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Broad-band emission from a multiple asymmetric quantum-well light-emitting diode

The authors demonstrate broadband light-emitting diode (LED) emission, with a full-width-at-half-maximum (FWHM) values >100 nm, based on concurrent multiple-state transitions in a single active layer containing two asymmetric quantum wells in the GaAs/AlGaAs material system. This spectral width is much broader (by a factor of 2.5) than that for commercial edge-emitting LEDs in the GaAs/AlGaAs system. The LED device is well suited for broadband source applications in wavelength-multiplexing-based, fibre-optic sensor network systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fiber-optic angular sensor with interleaved channel spectra

The angular position of a polarizer determines the power ratio of two light beams with interleaved channel spectra, filtered from a broadband light-emitting diode. The beams are transmitted simultaneously through a 750-m-long multimode fiber and then analyzed. An angular resolution of 0.5 degrees over a 70 degrees range is obtained, independently of the attenuation of the fiber link.