Far-infrared photoconductors for Herschel and SOFIA

Abstract

We are presently developing large format photoconductor arrays for the Herschel Space Observatory and for the Stratospheric Observatory For Infrared Astronomy (SOFIA). These arrays are based on individual Ge:Ga detectors contained in integrating cavities which are fed by an array of light cones to provide for area-filling light collection in the focal plane of an instrument. In order to detect light at wavelengths > 120 μm, uniaxial stress has to be applied to each detector crystal. We have developed a method to efficiently stress an entire stack of detector elements which allows us to form two-dimensional arrays from an arbitrary number of linear detector modules. Each linear module is read out by a cryogenic readout electronics circuit which operates at 4 K and is mechanically integrated into the module. We have measured effective quantum efficiencies of the light cone / detector /read-out chain of > 30% under realistic background conditions. GaAs photoconductive detectors could extend the spectral response cut-off up to > 300 μm. In the past, a continuous progress in material research has led to the production of pure, lightly and heavily doped n-type GaAs layers using the liquid phase epitaxy technique (LPE). Sample detectors demonstrated the expected infrared characteristics of bulk type devices. Modeling of BIB detector types predicts an improved IR sensitivity due to the attainable higher doping of the infrared sensitive layer. However, the modeling gives also an estimate of the severe material requirements for the n-type blocking layer. With a new centrifugal technique for the LPE material growth we intend to achieve this goal. Technical details of this unique equipment, first results of the achieved material quality in the initial growth runs and future steps to optimize operational parameters are reported. If successful, this detector technology will be first implemented in our spectrometer FIFI LS for SOFIA.