Mid-infrared Optoelectronic materials & devices

Abstract

The use of Pyroelectic array detectors in Infrared cameras in the 8 to 14 µm region necessitates a means for area-modulation, or 'chopping' of the infrared scene, thus generating an element of change for measurement by the detector. Currently this is achieved by mechanically 'chopping' the image using a rotating blade, with drawbacks such high electric motor power, and 'blurring' as camera motion changes the rotational speed. Adequate modulation using a novel method has been achieved in the 8 to 14 µm region by introducing moderate levels of excess carriers to suitably prepared Germanium. These were introduced via excitation from a diode laser source. The process utilised is greatly superior to existing attempts to produce a broadband area-modulation device. The process described uses inter valence band transitions from the light-hole to heavy-hole band, requiring power densities in the order of Watts cm^-2. Modulation of from 44% to 6% transmission at 10 µm using a power density of 28.6 W cm^-2 from an AlGaAs 809nm laser is easily achievable, and an illumination intensity which is constantly being improved upon. Low carrier density n-Germanium has been identified as the most suitable material for the modulator, and much of the work concentrates on this material. Development of the modulator has revealed the importance of long carrier lifetimes, necessitating high bulk purity material with suitably-treated surfaces to reduce and maintain a low surface recombination velocity. We will report on the relative importance of these effects, in particular maintaining low recombination at different surfaces using various etching methods, the effects of optical excitation at different wavelengths, and another application for the modulator in gas concentration sensing. The results of the modelling of the carrier diffusion after optical excitation will also be presented, showing the relative importance of the optical and electrical parameters of the device.

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