The detection of infrared radiation at wavelengths 0.75 to 20 micrometres is important for industrial process control, scientific imaging, thermography and radiometry, and surveillance; infrared cameras can be built from arrays of Schottky barrier detectors. The response of infrared detectors, based on Schottky diodes or heterostructures, to incident radiation is limited by the height of the internal potential barrier, and the cut-off wavelength depends on their construction. The longest cut-off wavelength, of 12 micrometres, has recently been achieved by iridium and platinum based Schottky detectors and by a SiGe/Si heterostructure. The cutoff wavelength can be changed only by lowering the Schottky barrier height, and the detectors can be tuned to only a few tens of meV.

However, if a detector had an asymmetrical metal-semiconductor-metal heterostructure, there should be modulation of several hundred meV, and an improved photoresponse. Now, researchers from France Telecom-CNET, have fabricated such a detector, which utilises an iridium electrode, and is tunable. The cut-off wavelength has moved from 2 to over 6 micrometres (I. Sagnes, Y. Campidelli and P. A. Badoz, J. Electron. Mater., 1994, 23, (6), 497–501).

The tunable infrared photoemission sensor, TIPS, of iridium/silicon/erbium silicide/silicon, is effectively two back-to-back Schottky diodes separated by silicon, creating an asymmetric potential barrier between the iridium and erbium silicide films. An iridium diode was made from evaporated iridium film; iridium and erbium silicide contacts were attached to the diode and an iridium dot, respectively.

With an external bias applied between the iridium and erbium silicide electrodes, the variation in the effective barrier height is over ten times larger than that of a standard Schottky barrier, and the cut-off wavelength can be modulated over a large range. On infrared illumi-nation photocurrents are created, two, specific to TIPS, have tunable thresholds, of size depending on the incident photon energy, and on the applied bias. High detection levels are expected at 2 micrometres wavelength and at 125 K, and if it is combined with large focal plane arrays and with developing microelectronics technology novel detectors can be produced.