| Popis: |
This paper describes long wavelength (LW) infra-red detectors made from HgCdTe grown by Metal Organic Vapour Phase Epitaxy (MOVPE) and the performance in a low photon flux background compatible with a multispectral requirement. The detectors are staring, focal plane arrays consisting of HgCdTe mesa-diode arrays bump bonded to silicon read-out circuits. The HgCdTe structure is grown on GaAs and consists of an absorber layer sandwiched between wider band-gap cladding layers. Device processing is wafer-scale. Wet etching is used to define the mesas and the mesa sidewalls are passivated with inter-diffused CdTe. The GaAs substrate is removed after bump bonding to minimise the thermal stress on cooling. The technology is sufficiently advanced to enable production not only of LWIR detectors but also dual band MWIR/LWIR detectors, as reported last year. Cameras for both types have been developed. There is now increasing interest in using the technology for LWIR multispectral imaging. Due to the requirement for narrow bandwidths, resulting in low radiant flux, the diode quality, in terms of dark current and resistance, must be exceptionally good. This requirement has been difficult to achieve in many technologies, however MOVPE grown MCT has consistently provided LWIR arrays with the necessary low dark current and high resistance. Performance from arrays of size 640x512 with 24 μm pixels and having a cut-off of 10 μm will be described. These achieve diode impedances of several GΩ's with less than 1 nA dark current at 90K. |
