We have developed a range of un-cooled mid-IR LEDs and photodiodes for IR gas sensing applications. Varying the composition of MBE grown Indium Aluminium Antimonide (In(1-x)AlxSb) epi-layers on GaAs allows us to engineer the emission/detection wavelength for a particular gas up to λmax≈6μm. The relatively high series resistance, LED drive requirements, and the non-optimised impedance matching of the un-biased photodiodes restricts the market for these components. Sub-dividing single element devices into N smaller devices connected in series enable the LED current and voltage requirements to be tailored to match the source, and improves the photodiode impedance matching.
We report the development of the necessary growth and photolithography technologies for series-connecting InAlSb diodes on GaAs substrates. We include results from multi-element Co2 (Al(x)=4.5%) and CH4 (Al(x)=8.5%) sensing LEDs and photodiodes. These impedance matched LEDs represent a 9-fold improvement in the wall-plug efficiency compared with single element LEDs with the same light output. The impedance of the multi-element photodiodes is increased significantly with respect to the series resistance, which gives up to a 5-fold improvement in sensitivity since the noise contributions from the external amplifier and series resistance are minimised. These advances have greatly improved the suitability of these components for gas sensing, and further improvements in the performance are expected through optimisation of the epi-layer design and the device geometry.
The high gains in performance predicted for optical immersion are difficult to achieve in practice due to total internal reflection at the lens/detector interface. By reducing the air gap at this interface optical tunneling becomes possible and the predicted gains can be realized in practical devices. Using this technique we have demonstrated large performance gains by optically immersing mid-infrared heterostructure InA1Sb LEDs and photodiodes using hypershperical germanium lenses. The development of an effective method of optical immersion that gives excellent optical coupling has produced a photodiode with a peak room temperature detectivity (D*) of 5.3 x 109 cmHz½W-1 at λpeak=5.4μm and a 40° field of view. A hyperspherically immersed LED showed a f-fold improvement in the external efficiency, and a 3-fold improvement in the directionality compared with a conventional planar LED for f/2 optical systems. The incorporation of these uncooled devices in a White cell produced a NO2 gas sensing system with 2 part-per-million sensitivity, with an LED drive current of <5mA. These results represent a significant advance in the use of solid state devices for portable gas sensing systems.
Although mid infrared research into sources and detectors has made considerable progress in recent years, requirements for gas sensing purposes for source power and the detectivity of diode detectors - particularly in combination - remain to be convincingly demonstrated in an industrial context. Published results are often confusing in that they apply to a variety of pulse lengths and duty cycles. We suggest a standardized approach in terms of an averaged cw power output. Parameters such as radiance, drive current and electrical power are also important. We discuss the relative merits of lasers and LEDs, i.e. use of line or band absorption on gas sensing. We report recent advances in the use of immersion optics leading to detectors with D*~5x109 cmHz1/2W-1 at 5.4micrometers , LEDs with outputs improved by a factor of 5 and an LED/Diode/White cell gas sensing demonstration giving 2ppm sensitivity for NO2 with an electrical power requirement of only 0.25mW. Further consideration includes sensitivity of gas sensing, path length and volume, time constant and temperature stability. Latest results are assessed on the basis of the above and combined with some market indications.
We report on electroluminescence and photoluminescence studies of arsenic rich InAs1-xSbx heterostructure LED's for the MIR region. Single-quantum- well LED's have demonstrated 300 K of approximately 24 (mu) W and approximately 50 (mu) W and approximately 8 micrometers , respectively, with corresponding internal quantum efficiencies of 0.8% and 1.6%. We also demonstrate 4.2 micrometers , 300 K emission from strained-layer superlattice (SLS) LED's with AlSb electron confining barriers with output powers > 0.1 mW. In reverse bias, these SLS devices exhibit negative luminescence efficiencies of approximately 14% at 310 K.
InAs/In(As,Sb) heterostructure LEDs are studied in forward (FB) and reverse (RB) bias where the phenomenon of 'negative luminescence' is seen for the first time in this materials system. Pseudomorphic 300K SQW LEDs, lattice matched to InAs and emitting at λ-5 micrometers and λ-8 micrometers , have internal conversion efficiencies of > 1.3 percent and > 0.83 percent respectively and maximum outputs in excess of 50 μW, in spite of an extremely low overall epilayer Sb content. Strain-relaxed InAs/In(As,Sb) SLS LEDs with AlSb barriers for electron confinement give 300K outputs in excess of 0.1mW at λ-4.2μm, approximately 3.5 times greater than control devices without the AlSb barrier. In RB the same SLS diodes exhibited efficient negative luminescence with output powers which increase with increasing device temperature to within 0.8 of the FB figures at 320 K.
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