Mid Sweden University

In this work we present a numerical study of charge sharing in photon counting X-ray imaging detectors. The study is based on charge transport simulations combined with a system level Monte Carlo simulation code to calculate the energy resolution of different pixel detector configurations. Our simulations show that the charge sharing is very sensitive to the electric field distribution in the device, and that the higher doping levels used in GaAs detectors reduce the effect of charge sharing significantly. Our study concludes that one of advantage's in using very heavy semiconductor materials in X-ray imaging detectors is the possibility to suppress charge sharing utilizing structures with much higher electric field. A 100 mum thick epitaxial GaAs detector absorbs 52% of the photons, while a 300 pin thick Silicon detector absorbs only 8% of the photons (30keV source). In addition to the superior stopping power, the GaAs detector has 5 times lower charge diffusion, resulting in superior spatial and energy resolution.

A Monte Carlo study of the transient response of single photon absorption in X-ray pixel detectors is presented. The simulation results have been combined with Monte Carlo simulation of the X-ray photon transport and absorption, and used to estimate the image properties of a detector system, including the pixel array and readout electronics. The study includes several different simulation challenges, such as full band Monte Carlo simulation of charge transport in large devices (300 mu m * 100 mu m), modelling of three-dimensional electrostatic effects using cylindrical coordinates, Monte Carlo simulation of photon transport and absorption, and a system level Monte Carlo simulation of the entire pixel detector and readout

The charge transport and X-ray photon absorption in three-dimensional (3D) X-ray pixel detectors have been studied using numerical simulations. The charge transport has been modelled using the drift-diffusion simulator MEDICI, while photon absorption has been studied using MCNP. The response of the entire pixel detector system in terms of charge sharing, line spread function and modulation transfer function, has been simulated using a system level Monte Carlo simulation approach. A major part of the study is devoted to the effect of charge sharing on the energy resolution in 3D-pixel detectors. The 3D configuration was found to suppress charge sharing much better than conventional planar detectors.

Numerical simulations of microscopic and macroscopic device properties of field effect transistors in 4H- and 6H-SiC are presented. The microscopic properties have been simulated using a full band (ab initio method) Monte Carlo simulation model and the macroscopic properties have been simulated using a drift-diffusion model with transport parameters obtained from the Monte Carlo simulations. Different models for the SiC/SiO2 interface in SiC MOSFETs have been evaluated and compared with experimental data. Finally, we present a comparison of simulated device performance for MOSFETs and MESFETs in 4- and 6H-SiC technologies. Both vertical (SIT) and lateral MESFET structures have been considered

A fully stochastic model for the imaging properties of X-ray silicon pixel detectors is presented. Both integrating and photon counting configurations have been considered, as well as scintillator-coated structures. The model is based on three levels of Monte Carlo simulations; photon transport and absorption using MCNP, full band Monte Carlo simulation of charge transport and system level Monte Carlo simulation of the imaging performance of the detector system. In the case of scintillator-coated detectors, the light scattering in the detector layers has been simulated using a Monte Carlo method. The image resolution was found to be much lower in scintillator-coated systems due to large light spread in thick scintillator layers. A comparison between integrating and photon counting readout methods shows that the image resolution can be slightly enhanced using a photon-counting readout. In addition, the proposed model has been used to study charge-sharing effects on the energy resolution in photon counting detectors. The simulation shows that charge-sharing effects are pronounced in pixel detectors with a pixel size below 170 * 170 mu m2. A pixel size of 50 * 50 mu m2 gives a highly distorted energy spectrum due to charge sharing. This negative effect can only be resolved by introducing advanced counting schemes, where neighbouring pixels communicate in order to resolve the charge sharing.

In this paper, we present numerical studies of the high frequency performance of a submicron MOSFET in 2H-, 4H- and 6H-SiC. The studies are based on simulations where commercial two-dimensional drift-diffusion and hydrodynamic carrier transport models have been used. The results have been compared with those obtained from full band Monte Carlo simulations. The Monte Carlo carrier transport model is based on data from a full potential band structure calculation using the Local Density Approximation to the Density Functional Theory. In 6H-SiC the bulk transport properties in the direction perpendicular to the c-axis, are slightly lower than in 2H- and 4H-SiC. However, in the direction parallel to the c-axis the transport properties are considerably less favourable than in the other two polytypes. The effects of these differences, on surface mobility device performance and carrier energy, have been studied.

The 4H-SiC static induction transistor (SIT) is a very competitive device for high frequency and high power applications (3-6 GHz range). The large breakdown voltage and the high thermal conductivity of 4H-SiC allow transistors with extremely high current density at high voltages. The SIT transistor shows better output power capabilities but the unity current-gain frequency is lower compared to a MESFET device. In this work we show, using a very accurate numerical model, that a compromise between the features given by the SIT structure and the ordinary MESFET structure can be obtained using the vertical MESFET structure. The device dimension has been selected very aggressively to demonstrate the performance of an optimized technology. We also present results from drift-diffusion simulations of devices, using transport parameters obtained from the Monte Carlo simulation. The simulations indicate that 2H-SiC is superior to both 4H and 6H-SiC for vertical devices. For lateral devices, 2H-SiC is slightly faster compared to an identical 4H-SiC device

Silicon Carbide is a very interesting semiconductor material for high temperature, high frequency, and high power applications. The main reasons are its high saturation velocity, large thermal conductivity, high Schottky barriers, and high breakdown voltages. High quality 4H-SiC and 6H-SiC polytype substrates and epitaxial layers are commercially available today. An additional advantage of SiC is the native oxide that allows fabrication of MOS devices. A large effort has been devoted towards the development of high performance devices in SiC. The largest success has been for unipolar devices like Schottky diodes and different kinds of MESFETs. MOSFETs have also been fabricated in both 4H- and 6H-SiC. Unfortunately, the MOSFET performance was found to be much worse than expected, due to a very low surface mobility. Nevertheless, the technology developed is very interesting and includes possible large scale integration of digital circuits operating at very high temperatures. In this work we present numerical simulations of the device performance of different Field Effect Transistors (FETs). Both full band Monte Carlo simulations and macroscopic modeling using the drift-diffusion approach have been utilized in this work. The Monte Carlo simulations have been used to extract transport parameters and to evaluate the macroscopic models in a device configuration

UV-enhanced photodetectors of both n⁺-p and p⁺-n type have been processed in silicon. Photodetectors of the p⁺-n type display a responsivity close to the theoretical limit with an antireflective coating of either thermally grown dry silicon dioxide or deposited oxide (TEOS), followed by a short wet oxidizing step. This holds, irrespective of whether the detector window is doped by boron through ion implantation or diffusion from a solid source. However, for p⁺-n photodiodes with a TEOS-oxide in the as-deposited state the responsivity decreases substantially for wavelenghts below 500 nm compared to the theoretical predictions. This is attributed to a high recombination velocity at the silicon dioxide/silicon interface, as supported by computer simulations of the detector performance. In contrast, n⁺-p photodiodes are found to be rather insensitive with respect to the properties of the silicon dioxide/silicon interface. These results provide the first experimental demonstration that high built in electric fields, caused by abrupt dopant profiles, can suppress the influence of a high interface carrier recombination velocity.