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The presence of surface states, which pins the Fermi level within the bandgap, contributes to the degradation in the performance of light-emitting diodes (LEDs) and p-i-n photodiodes due to an increase in the non-radiative recombination rate. Chemical modification on the facet or device surface can greatly enhance the output power of LED's and photocurrent of p-i-n photodiodes. Adsorbing molecules can change either the density or energy distribution of surface states. This effect leads to changes in surface recombination which result in systematic variations in light output, and thus the effect can be used for detection of analytes. This mechanism was used to realize a compact chemical sensor based on III/V LED/Detector structures. Initially, we fabricated InGaAlP/InGaP/InGaAlP double heterostructure (DH) LEDs (400 X 1000 micrometer2) with three different active region thicknesses: 50, 250 and 500 nm. In constant current mode, the DH LED exhibits electroluminescence (EL) at approximately 670 nm for an InGaP active region. The EL intensity changes of the LED in various gaseous ambients (NH3, NH2(CH3), NH(CH3)2, N(CH3)3, and SO2) are measured. The data show reproducible trends: DH LED structures with thicker active regions result in larger emission intensity changes due to analyte adsorption. Our findings are consistent with active-layer surface area dependence. Thicker active layer devices have larger carrier losses due to nonradiative surface recombination, and thus show a stronger sensitivity to the surface chemistry. Furthermore, we used this DH LED design to build a highly versatile compact sensor. The MOCVD-grown LED wafer is patterned and chemically etched to fabricate integrated GaAs/AlGaAs edge-emitting LEDs and p-i-n photodiode units. The light emitted from the edge-emitting LEDs is absorbed at the sidewall of the adjacent photodiode, and the resulting photocurrent is measured. The device design concept is based on increasing the ratio of analyte-accessible facet area to the volume of the active region. This integrated LED- photodiode device can serve as an on-line chemical sensor for a variety of analytes.
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