Investigation of the insulator layers for segmented silicon sensors before and after X-ray irradiation

For the proper simulation and understanding of segmented silicon sensors the surface boundary conditions and the charge density distribution in the SiO ₂ layer (and other insulator layers if present), as well as at the Si-SiO ₂ interface have to be known. It has been observed previously, that the boundary conditions on the sensor surface change with relative humidity, RH. We therefore have measured the surface conductivity of SiO ₂ -Si ₃ N ₄ at room temperature for RH values between 30 and 46 % using a Gate Controlled Diode fabricated on n-type high-ohmic Si, and for RH = 50 % using a MOSFET. For determining the effective oxide-charge density, N _ox ^eff , which is required for sensor simulations, as function of ionizing dose and biasing conditions, capacitance-voltage-frequency (C-V-f ) measurements on MOS capacitors (MOS-C) irradiated up to SiO ₂ doses of 1 GGy by ~10 keV X-rays were performed previously. Large hysteresis effects were observed when the voltage was ramped from accumulation to strong inversion and back. We interpreted these shifts as evidence for field-enhanced injection of charges from the Si into the SiO ₂ . Here we present C- V-f measurements on MOS-Cs fabricated on <;100> and <;111> high-ohmic Si, without irradiation and after X-ray irradiation to 1 GGy. In order to determine the time- and field-dependence of the injection of positive charges from the Si into the SiO ₂ , the MOS-Cs have been biased at different voltages in inversion for different time intervals.