Position sensitive photon detectors using epitaxial InGaAs/InAlAs quantum wells

This work deals with the investigation of novel position-sensitive devices based on InGaAs/InAlAs quantum wells, which could be applied to several applications of either synchrotron or conventional light sources. Such devices may be used as fast and efficient detectors due to the direct, low-energy band gap and high electron mobility at room temperature. Metamorphic In0.75Ga0.25As/In0.75Al0.25As quantum wells containing a two-dimensional electron gas were grown by molecular beam epitaxy. Two devices with size of 5 × 5 mm2 were prepared by using optical lithography. In the first, the active layers were segmented into four electrically insulated quadrants. Indium ohmic contacts were realized on the corner of each quadrant (for readout) and on the back surface (for bias). In the second, the quantum well was left unsegmented and covered by 400 nm of Al providing a single bias electrode, while four readout electrodes were fabricated on the back side by depositing and segmenting a Ni/Ge/Au layer. Photo-generated carriers can be collected at the readout electrodes by biasing from either the QW side or the back side of the devices during beam exposure. Individual currents obtained from each electrode allow monitoring of both the position and the intensity of the impinging beam for photon energies ranging from visible to hard X-ray. Such detector prototypes were tested with synchrotron radiation. Moreover, the position of the beam can be estimated with a precision of 800 nm in the segmented QW. A lower precision of 10 μm was recorded in the unsegmented QW due to the charge diffusion through the 500-μm-thick wafer, with however a lower electronic noise due to the better uniformity of the contacts.