Development of far-infrared Ge:Ga photoconductor 2D array for 3 THz imaging

Gallium-doped germanium (Ge:Ga) extrinsic semiconductor has been used as sensitive far-infrared detectors with a cutoff wavelength of 110 micron (2.7 THz), especially in the fields of astronomy, the spectroscopy of molecules and solids, and plasma diagnostics. Developing Ge:Ga photoconductor arrays to take two-dimensional (2D) THz images is now an important target for research fields such as the space astronomy. We present the basic idea of a 20X3 Ge:Ga far-infrared photoconductor array directly hybridized to a Si p-MOS readout integrated circuit (IC) using indium bump technology, which is an advanced structure for far-infrared detector arrays. The main issues to achieve in creating a 2D array are summarized as follows: the fabricatinon of a monolithic Ge:Cra 2D array with longitudinal configuration, development of the cryogenic readout electronics, and the development of technology for connecting the detector with the electronics. We report that the detector was cooled to 2.1 K, the best responsivity obtained was 16.2 A/W and the best NEP was 2.6 X 10(-17) W/Hz(1/2) at a bias field of 1.2 V/cm. We demonstrated that p-MOS FETs can be used as the cryogenic readout electronics. And we succeeded in the far-infrared detection by using direct hybrid structure photoconductors. The results of vibration and cooling tests were conducted to confirm the strength of the direct hybrid structure. Our Ge:Ga photoconductor direct hybrid 2D array will be an excellent device for taking 3 THz images.