Monte Carlo Simulations of Homogeneous and Inhomogeneous Transport in Silicon Carbide

University dissertation from Kista : Mikroelektronik och informationsteknik

Abstract: The importance of simulation is increasing in the researchon semiconductor devices and materials. Simulations are used toexplore the characteristics of novel devices as well asproperties of the semiconductor materials that are underinvestigation, i.e. generally materials where the knowledge isinsufficient. A wide range of simulation methods exists, andthe method used in each case is selected according to therequirements of the work performed. For simulations of newsemiconductor materials, extremely small devices, or deviceswhere non-equilibrium transport is important, the Monte Carlo(MC) method is advantageous, since it can directly exploit themodels of the important physical processes in the device.One of the semiconductors that have attracted a lot ofattraction during the last decade is silicon carbide (SiC),which exists in a large number of polytypes, among which3C-SiC, 4H-SiC and 6H-SiC are most important. Although SiC hasbeen known for a very long time, it may be considered as a newmaterial due to the relatively small knowledge of the materialproperties. This dissertation is based on a number of MCstudies of both the intrinsic properties of different SiCpolytypes and the qualities of devices fabricated by thesepolytypes. In order to perform these studies a new full-bandensemble device MC simulator, the General Monte CarloSemiconductor (GEMS) simulator was developed. Algorithmsimplemented in the GEMS simulator, necessary when allmaterial-dependent data are numerical, and for the efficientsimulation of a large number of charge carriers in high-dopedareas, are also presented. In addition to the purely MC-relatedstudies, a comparison is made between the MC, drift-diffusion,and energy-balance methods for simulation of verticalMESFETs.The bulk transport properties of electrons in 2H-, 3C-, 4H-and 6H-SiC are studied. For high electric fields the driftvelocity, and carrier mean energy are presented as functions ofthe field. For 4H-SiC impact-ionization coefficients,calculated with a detailed quantum-mechanical model ofband-to-band tunneling, are presented. Additionally, a study oflow-field mobility in 4H-SiC is presented, where the importanceof considering the neutral impurity scattering, also at roomtemperature, is pointed out.The properties of 4H- and 6H-SiC when used in short-channelMOSFETs, assuming a high quality semiconductor-insulatorinterface, are investigated using a simple model for scatteringin the semiconductor-insulator interface. Furthermore, theeffect is studied on the low and high-field surface mobility,of the steps formed by the common off-axis-normal cutting ofthe 4H- and 6H-SiC crystals. In this study an extension of theprevious-mentioned simple model is used.