Performance Measurements of Photodiodes for X-Ray Detection

The X-ray free-electron laser (XFEL) at the Pohang Accelerator Laboratory (PAL), Pohang, South Korea, provides X-ray energies of up to 15 keV depending on the experimental purpose. Silicon-based devices have been used as diagnostic devices and detectors for X-rays in experimental stations. Considering recent domestic and international circumstances, developing silicon-based detectors in-house is necessary. We developed a silicon p-intrinsic-n (PIN) photodiode (PD) for X-ray detection, and its performance was investigated and compared with that of a commercial PD used in the PAL-XFEL. The PD was designed to be 1 cm $\times1$ cm in size, and it used the junction side for signal readout and the ohmic side for X-ray entrance. Considering the absorption length of 12-keV X-rays suitable for crystallography, a 500- $ {\mu }\text{m}$ -thick silicon wafer with a high resistivity was used for PD fabrication. It has a guard ring, an n+ edge field shaper, and an antireflection coating (ARC) and is available in four different types based on the metal structure of the junction and ohmic sides. We present here the electrical characteristics of the PDs. The depletion voltage at which the bulk of the PD is fully depleted was determined to be 119.7 ± 8.2 V from bulk capacitance measurements, and the operation voltage of the PD was set to 200 V. PDs with leakage currents less than 30 nA/cm ² at the operation voltage were chosen for performance measurements. The signal-to-noise ratio (SNR) with a Sr-90 radioactive source corresponding to the minimum ionizing particle (MIP) and the quantum efficiency (QE) for wavelengths of 300–1000 nm were measured. Beam tests were conducted at the PAL-XFEL using 600-, 900-, and 1200-eV X-rays where the responses to ultrahigh brightness and ultrashort X-ray pulses were checked. The energy resolutions were measured using gamma-ray radioactive sources, Am-241 (59.5 keV), and Ba-133 (31.0 and 81.0 keV). The differences in performance among the PD types are presented herein in terms of the electrical characteristics, SNR, QE, response to X-ray pulses, and energy resolutions. The results of the fabricated PDs were also compared with those of a commercial PD.