Semiconductor devices1 make good chemical sensors, being responsive, selective and compact. Some gas analytes, for example, can be photometrically detected by the electroluminescence they absorb from semiconductor light-emitting diodes23. Here we show that the intensity of this electroluminescence can be modulated in another way entirely, namely as a result of the adsorption of analyte onto the surface of the light-emitting diode, which affects its radiative efficiency by altering its surface-state structure and causes rapid, stable and reversible changes in electroluminescent intensity. Such devices are complementary to existing metal oxide-based resistance sensors, offering the advantages of low-power, room-temperature operation and ready integration into array-based devices.

Light-emitting diodes (LEDs) designed for high optical efficiency employ thin quantum-well active regions45. However, the LED-based chemical sensors we describe here are designed to optimize the active layer's surface interactions so that adsorption effects on carrier recombination processes are accentuated. Such chemical adsorption can alter the nature and energy distribution of surface states that affect surface recombination velocity and radiative efficiency67.

We used a double-heterostructure LED (Fig. 1a), grown by low-pressure metal–organic chemical vapour deposition. This has a conventional p-type/n-type junction arrangement, consisting of an active layer of undoped In0.49Ga0.51P, and p-type and n-type cladding layers of (Al0.5Ga0.5)0.51In0.49P, grown to be lattice-matched to GaAs substrates; a highly doped p-type GaAs contact layer completes the structure. Each cladding layer is 0.5 μm thick and the active-layer thickness was varied from 50 to 500 nm. We processed the LED structures into broad-stripe (500×1,000 μm2) devices with cleaved (110) output facets.

Figure 1: Diode structure and the response of its electroluminescence to sulphur dioxide.
figure 1

a, Double heterostructure p–n junction diode. Electrical contact is made to the top and bottom surfaces with metal films (layer dimensions not drawn to scale). b, Changes in electroluminescence intensity at 670 nm of a double-heterostructure diode upon exposure to SO2 gas, relative to a vacuum ambient. The partial pressure of SO2 is indicated; the sample is returned to the vacuum ambient between each exposure.

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The experimental set-up is adapted from semiconductor photoluminescent studies8. When forward-biased, the LEDs give red electroluminescence with band maxima at about 670 nm. Packaged devices were operated continuously at low and constant current levels (around 20 mA) to minimize heating and superluminescence. Electroluminescence experiments were carried out in the presence of five optically transparent analyte gases: ammonia (NH3), methylamine (NH2CH3), dimethylamine (NH(CH3)2), trimethylamine (N(CH3)3) and sulphur dioxide (SO2). A vacuum (10−3 torr) served as the reference ambient.

Figure 1b shows a typical trace of electroluminescence intensity against time for a LED exposed to SO2. Enhancement increases with active-layer thickness, reaching several hundred per cent for the double heterostructures with the thickest active layer. The reversible responses (Fig. 1b) demonstrate the potential for online sensing with these structures. All of the analyte–diode structure combinations displayed adsorption and desorption times of less than a few minutes at room temperature. For all analytes, enhancements are seen at pressures as low as about 0.01 torr, with the response saturating at around 0.1–1 torr. The electroluminescent response is reproducible even after weeks of storage in air.

This adsorbate-induced modulation of electroluminescent intensity from simple double-heterostructure diodes illustrates their application as transducers that can couple semiconductor surface chemistry to an optical signal. The remarkable features of these structures (robust, inexpensive, small, low power consumption, compositionally tunable emission spectra) could allow arrays to be used to identify a variety of analytes. Highly sensitive, ultrasmall LEDs with large surface-to-volume ratios should then be easily integrated with diodes that serve as photodetectors, forming monolithic chemical sensors. From photoluminescent studies of semiconductors6 9 10 and from electroluminescence of molecular diodes11, it should be possible to customize the selectivity, sensitivity and speed of the electroluminescent response by surface modification.