Quantum Well Infrared Photodetectors: Device Physics and Light Coupling

Abstract

It is customary to make infrared (IR) detectors in the long wavelength range (8 – 20 urn) by utilizing the interband transition which promotes an electron across the bandgap (Eg) from the valence band to the conduction. These photo-electrons can be collected efficiently, thereby producing a photocurrent in the external circuit. Since the incoming photon has to promote an electron from the valence band to the conduction band, the energy of the photon (hv) must be higher than the Eg of the photosensitive material. Therefore, the spectral response of the detectors can be controlled by manipulating Eg of the photosensitive material. Detection of very long wavelength IR radiation up to 20 μm requires small bandgaps down to 62 meV. Examples of such materials meeting these requirements are Hg_1-xCd_xTe and Pb_1-xSn_xTe in which the energy gap can be controlled by varying x. It is well known that these low bandgap materials are more difficult to grow and process than large bandgap semiconductors such as GaAs. These difficulties motivate the exploration of utilizing the intersubband transitions in multi quantum well (MQW) structures made of large bandgap semiconductors.