Positive and Negative Luminescent Infrared Sources and Their Applications
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The growth of these structures is described by Johnson in an associated paper at this meeting The long wavelength IR devices are made from mercury cadmium telluride (MCT), and comprise double heterostructures. Techniques to fabricate large area InSb devices, up to lcm2, which include the use of degenerately doped substrates to provide transparency and the integration of optical concentrators in the substrate material to improve optical efficiency are described in section 3. Emission data from both material systems are presented in section 4. The application of positive and negative luminescent devices to gas sensing, improved thermal imagers and imager testing is discussed in section 5. 2. DEVICE STRUCTURES The structures, for each material system, used to make the luminescent devices have been described previously 2-4 , and are shown in figure 1,together with the associated equilibrium band diagrams. Briefly, the devices have an 'active', narrow-gap region between n- and p-type contact regions which have a very high doping level and, ideally, a wider energy-gap. For optical devices, the active region is doped p-type, normally to a low level such that it is intrinsic at room temperature and so is referred to as n-type. The structure, therefore, contains one pn junction and one isotype junction. The diffusion lengths in the contact regions are sufficiently short that ohmic metal contacts do not influence the carrier densities at the junctions to the active region. The products of generation/recombination rates and diffusion lengths are also low in the contact 153 Mat. Res. Soc. Symp. Proc. Vol. 484 ©1998 Materials Research Society
regions, so the thermally mediated carrier flows to and from the active region are small and the majority of the device current arises only from processes in the active region. This has benefit for conventional detectors, which are operated at zero bias, by constraining the volume of material which generates thermal noise to be no more than is necessary to obtain the required quantum efficiency. Under reverse bias, minority carrier extraction occurs at the pn junction and exclusion takes place at the isotype junction. At ambient temperature, this leads to a reduction in the minority carrier density in the active region by several orders of magnitude. In order to maintain approximately zero space charge, there is also a large reduction of the majority carrier density by, typically, two orders of magnitude down to the background doping level. The consequence is that the Auger carrier generation processes, in particular Auger-1 and Auger-7, and their associated noise, are suppressed leading to an improvement in performance of uncooled detectors and minimisation of the current necessary to drive negative luminescent devices. Under forward bias, the extracting and excluding contacts become injecting and accumulating, respectively, and serve to confine the electrons and holes in the active region, with little leakage into the contact regions, so maximising the radiative recombination rate and
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