Degree Granting Department
Andrew Hoff, Ph.D.
Oxidation, Remote plasma, Non-contact, Corona-voltage, Mass spectrometry
Oxide thin films were grown on 4H-SiC at low pressure and reduced temperatures using a remote plasma afterglow thermal oxidation method, achieving significantly faster growth rates than standard atmospheric furnace processes. The resulting SiO2/SiC structures were characterized by a non-contact corona-voltage metrology technique in order to extract capacitance-voltage information, to facilitate further analysis of the afterglow oxidation growth mechanism, and to determine the electrical behavior of defects. In addition, mass spectrometry experiments revealed the concentration of nitric oxide species in the afterglow reactor gas exhaust produced by the cracking of N2O molecules in the microwave plasma discharge. Oxidations were performed on n- and p-doped epitaxial 4H-SiC wafers at growth temperatures between 700Â°C and 1100Â°C. The afterglow oxidation process was determined to be primarily in the parabolic growth regime, and thus rate-limited by diffusion processes.
Analysis of the parabolic growth rate temperature dependence revealed a break in activation energy between 0.46 eV and 1.51 eV at lower and higher temperature ranges, indicating a change in the dominating oxidation mechanism. In the proposed transport-limited mechanism, afterglow oxidation was suggested to be rate-limited by parallel diffusion of atomic oxygen radicals and excited singlet oxygen molecules to the SiO2/SiC interface. An alternative stress-relief mechanism suggested that viscous flow of SiO2 could relieve compressive stress in the oxide above 960Â°C. In this case, growth would be stress-limited at low temperatures and diffusion-limited at higher temperatures. Regardless of the exact mechanism or temperature range, the data developed in this work suggest that afterglow oxidation rates of 4H-SiC are faster than atmospheric growth rates mainly because significant quantities of atomic and excited oxygen are generated in the microwave discharge independent of temperature.
Using flatband voltages and accumulation capacitance values extracted from C-V measurements, worst-case charge densities associated with the oxide-semiconductor interfacial region were estimated. The charged defects were found to exist in the 1012/cm2 range regardless of growth temperature or oxide thickness. The charged defects were attributed to interface traps which capture majority carriers while the SiC is electrically stressed into accumulation during measurement. It was suggested that the traps failed to emit their charges within the time of measurement, even when the semiconductor was swept into depletion, and thus caused a shift in the observed flatband voltage. Mass spectrometry analysis showed that no thermal cracking of gas species occurs in the furnace at the detection level of the measurement, but rather significant quantities of nitric oxide are produced by the cracking of N2O molecules in the microwave plasma discharge independent of furnace temperature.
Scholar Commons Citation
Short, Eugene L., "Growth of oxide thin films on 4H- silicon carbide in an afterglow reactor" (2006). Graduate Theses and Dissertations.