J.G.A. Wehner

Mercury cadmium telluride resonant-cavity enhanced photoconductive infrared detectors

Resonant-cavity-enhanced Hg1−xCdxTe photoconductive detectors for midwave infrared wavelengths are investigated for use in multi- and hyper-spectral sensor systems. Resonant-cavity-enhanced performance is modeled, and compared with measured performance of fabricated devices. The responsivity of fabricated devices shows resonant cavity enhancement, with performance limited by surface recombination.

Towards MEMS-based infrared tunable microspectrometers

Silicon nitride membrane based MEMS technologies are used in numerous optical micro-systems for many practical applications. In recent years significant effort has been invested into miniaturization of present spectrometers leading to development of compact systems based on linear detector arrays. However, further miniaturization requires the application of MEMS technology. MEMS based micro-Fabry-Perot cavity structures with a flexible mirrors operating in the visible and near infrared range have already been demonstrated. Extension of the technology to mid- (MWIR-3μm to 5μm) and long-wave infrared (LWIR - 8μm to 12μm) seems to be a natural development. However, this requires much larger deflection of the movable mirror of the Fabry-Perot cavity: ~1μm for 3μm to 5μm and ~2μm for 8μm to 12μm wavelengths range. Consequently, precise control of the intrinsic stress in the silicon nitride support film is needed. Our experiments show that suitable silicon nitride properties can be obtained by carefully controlling the process parameters during plasma enhanced CVD growth. In addition to the material requirements, the mechanical structure of the flexible mirror is also important. In order to optimize the mirror structure/shape, finite element analysis was undertaken. The results show that for certain silicon nitride support shapes and thicknesses, mirror deflections needed for both the MWIR and LWIR wavelength regions are possible.

Annealing and Shunting in RCE HgCdTe Photoconductors

Resonant cavity enhanced HgCdTe structures have been grown by molecular beam epitaxy, and photoconductors have been modelled and fabricated based on these structures. Ex-situ annealing for 20 hours under a Hg atmosphere did not markedly degrade cavity performance, and resulted in much more uniform material. Responsivity measured on the ex-situ annealed material is low, and shows evidence of shunting of absorber layer by mirror layers.

Resonant-cavity-enhanced HgCdTe photodetectors

Resonant cavity enhanced HgCdTe structures have been grown by molecular beam epitaxy, and photoconductors have been modelled and fabricated based on these structures. Responsivity has been measured and shows a peak responsivity of 8 x 104 V/W for a 50 X 50 μm2 photoconductor at a temperature of 200K. The measured responsivity shows good agreement with the modelled responsivity across the mid-wave infrared window (3-5μm). The measured responsivity is limited by surface recombination, which limits the effective lifetime to ~15ns. The optical cut-off of the detector varies with temperature as modelled from 5.1 um at 80K to 4.4 um at 250K. There is strong agreement between modelled and measured peak responsivity as a function of temperature from 80-300K.

Mercury Cadmium Telluride/Cadmium Telluride Distributed Bragg Reflectors

A HgCdTe/CdTe system is investigated for use in distributed Bragg reflectors. The modelled performance is described, and compared to an as-grown structure. As-grown 15 layer structures with arithmetically varying layer thickness are also annealed at 250°C and performance re-evaluated at 2, 7 and 24 hours anneal times. There is some shifting of the absorption edge after annealing , and some degradation in reflectivity, but the mirror is still functioning

Refractive index engineering for a distributed Bragg reflector for a resonant-cavity-enhanced detector

Current infra-red detectors are limited to detect broad windows in transmission. By adding Fabry-Perot filtering to these detectors multi- and hyper-spectral detectors can be fabricated. However, filtering will reduce the signal available to the detector. In order to decrease the noise (thereby increasing the signal to noise ratio), the detector can be moved into the resonant cavity of the filter. The design of the mirrors is changed by placing the detector with the resonant cavity. Materials for the design of a resonant cavity enhanced mercury cadmium telluride detector are investigated in this paper.

Optical and membrane modelling in a MEMS hyperspectral imaging system

Hyperspectral imaging using micro-electro-mechanical systems (MEMS) is investigated. A Fabry-Perot cavity is created using a silicon nitride membrane suspended over a variable length air gap, to support a distributed Bragg reflector. Models of membrane behaviour and reflector optical performance are given.