Detector Array Predevelopments for FIRST
FIRSA and FIRGA are two ESA sponsored photoconductor array development programs which are in progress. The goal of the FIRSA study (Far-InfraRed Stressed Ge:Ga Array) is to build and test two fully functional demonstrator arrays of 1x16 stressed Ge:Ga detector elements each with associated cryogenic readout electronics (CRE). The stressed Ge:Ga technology is chosen as the detector baseline for the FIRST-PHOC instrument covering the wavelength range 80-220 Ám in spectrometric and photometric imaging modes. The camera requires a 25x16 pixel array. Each of the 25 linear array modules consists of a frame with a stressing mechanism, 16 detector elements in integrating cavities with a light cone fore-optics. The module carries on a ceramic substrate the multiplexed 1x16 channel CRE chip. Two complete demonstrator arrays were manufactured. They are currently under test in a 1.6 K environment. The feasibility of the chosen concept could be demonstrated in several functional tests meanwhile. Valuable experience is gained to guide development and manufacture of the next generation of CREs and the final version of the array modules.
A gallium arsenide (GaAs) photoconductor array which is sensitive at wavelengths from about 60 to 300 Ám might offer the advantage of extending considerably the long wavelength cut-off of presently available photodetectors. The aim of the FIRGA study (Far-InfraRed GaAs Array) is the development of a 1x32 or 4x32 pixel demonstrator array using Te-doped GaAs. The long wavelength peak in the spectrum of GaAs:Te shown below corresponds to the 1s-2p transition of the hydrogenic donors in the GaAs. The impurities lie about 6 meV below the conduction band, excitation of charge carriers is produced by a photo-thermal ionization event. We have prepared sample detectors and investigated their electrical and infrared characteristics. The monolithic 4x32 element demonstrator (see sketch below) is considered a pacesetter for a large 50x32 pixel array for far infrared astronomy. Our current activities are concentrating on the optimization of the photo-response of bulk detector materials as well as on the preparation of ultra high purity GaAs layers, of those having impurity band conduction properties and of highly doped epitaxial contact layers.
Katterloher, R., G. Jakob, E. Bauser, S.Zehender, E.E. Haller, J. Beeman, Th. Henning and G. Pilbratt : The GaAs Photoconductor - Characteristics of LPE grown Sample Detectors, Submillimetre and Far-infrared Space Instrumentation. In: Proc. 30th ESLAB Symposium, Ed. E.J. Wolfe, ESA SP-388, 33-36 (1996)