Characterization of charge multiplication and charge collection in semiconductor detectors using advanced Transient Current Technique methods

In the proposed project we will measure the properties of impact ionization in low gain avalanche detectors (LGAD) and charge collection in silicon carbide (SiC) particle detectors. For this purpose, we will develop new measurement methods in laser based transient current technique (TCT). LGAD based systems for measuring particle arrival times with a resolution of a few 10 ps are becoming an important component of experiments with high collision densities at hadron colliders. First such systems will be ATLAS HGTD and CMS HGCAL, which will be installed in 2028 during the High Luminosity Large Hadron Collider (HL-LHC) upgrade. Other fields using radiation hard timing detectors are monitoring of fusion power reactors with temperature resistant silicon carbide detectors, and monitoring and imaging applications in medical ion beams. For a high signal-to-noise ratio, which is essential for achieving the required time resolution, LGADs employ a thin multiplication layer to generate internal gain via built-in charge multiplication by impact ionization. The gain factor depends on the radiation damage, caused by hadrons during detector operation, and on the density of free charge carriers, which reduce gain by screening of the electric field. In the proposed project we will quantify these effects in LGADs using new TCT methods. We will also develop a TCT system with UV light for one of the first TCT measurements of charge collection in SiC detectors. Measurements will be performed on neutron irradiated samples with fluences characteristic for the HL-LHC (2.5×1015 neq/cm2) and up to extreme fluences (1017 neq/cm2). The option of proton irradiation will also be considered. Measurements of the dependence of impact ionization on fluence will be performed with Two Photon Absorption TCT (TPA-TCT), with the beam directed through the edge of the LGAD sample (Edge-TPA). The gain factor due to impact ionization in the multiplication layer will be determined from the ratio of the signal amplitudes in the LGAD sample and a reference sample without internal gain. TPA-TCT uses tight beam focusing – 1 μm laterally, 30 μm along the beam direction – which allows charge injection at a well-defined distance to the collection electrode. This will allow to separate between the gain contribution in multiplication layer from the contribution in the silicon bulk, which presents a background for this measurement. This will be the first ever Edge-TPA measurement with LGAD samples. For measurements of the gain dependence on effects related to charge carrier concentration, a TCT system for combining multiple laser beams into a single source will be developed (Multi-beam TCT). The source laser beam will be split into a train of two pulses, where the first will be used for generation of desired charge concentration in the sample, and the second will be used to inject probe signals. For precise adjustment of the relative time delay between the two pulses, a module with a tunable optical path length in one branch will be developed. From the amplitude of the signal produced by the second pulse, we will determine the gain loss due to screening, and using variable delays, we will determine the time dependence of screening and gain restoration time. In addition, the system will include a laser beam with a constant (DC) illumination for generating a constant current for filling radiation induced charge traps. This will allow to study charge collection efficiency as a function of the fraction of occupied traps. We will also develop one of the first systems for TCT measurements in SiC detectors, which due to a larger band gap require excitation with UV light. For this purpose, an existing scanning TCT setup will be upgraded with an UV optical system. The system will be used to measure uniformity of charge collection, depletion depth and charge carrier velocity in SiC detectors as a function of fluence.