Development of photo-diodes for Pohang-Accelerator-Laboratory X-ray free-electron laser

The Pohang-Accelerator-Laboratory X-ray free-electron laser (PAL-XFEL) provides intense ultra-short X-ray pulses via a self-amplified spontaneous emission (SASE) process. Since fluctuations are induced by the intrinsic characteristics of the SASE process, we designed and fabricated photo-diodes (PDs) to be used in quadrant beam position and pop-in monitoring systems for measuring intensity and position, and beam size, respectively. Six 500 mu m-thick high-resistivity silicon wafers were used for different fabrication processes to vary the thicknesses of bias electrode and anti-reflective coating layers optimized for the quantum efficiency (QE) at typical wavelengths of 266 and 400 nm used for pump-probe experiments at the PAL-XFEL. In this paper, we present the electrical characteristics, QE, signal-to-noise ratio, energy resolution of the fabricated PDs, and compared them with those of commercial PD.