Das's Disertation Defense
Posted: Thursday, November 5th, 2009
DISSERTATION TITLE: CHARACTERIZATION OF DEFECTS AND EVALUATION OF MATERIAL QUALITY OF LOW TEMPERATURE EPITAXIAL GROWTH
DATE AND TIME: November 19, 2009 (Thursday), 2:30pm-4:30pm
WHERE: Simrall conference room (Room 228, Second floor)
CANDIDATE: Hrishikesh Das
COMMITTEE:
Dr. Yaroslav Koshka
Associate Professor of Electrical and Computer Engineering
(Major Advisor and Dissertation Director)
Dr. Michael Mazzola
Professor of Electrical and Computer Engineering
(Committee Member)
Dr. Raymond S. Winton
Professor of Electrical and Computer Engineering
(Committee Member)
Dr. Seong-Gon Kim
Associate Professor of Physics and Astronomy
(Committee Member)
ABSTRACT:
A novel process for low-temperature epitaxial growth of silicon carbide (SiC) was recently developed at Mississippi State University. The method is based on using halo-carbon growth precursor replacing the traditional hydrocarbon precursor propane. To facilitate adoption of the new method by the SiC semiconductor device industry, additional progress needs to be achieved in minimizing defects in blanket epitaxial growth, improving quality of selective epitaxial growth, achieving highly doped epitaxial layers, etc. However, only limited information was available about defects and impurity incorporation in epitaxial layers produced by this new method. This dissertation aims at filling the gap in our knowledge about defect and doping in the low-temperature epitaxial growth method by conducting a comprehensive characterization of different types of epitaxial layers produced by this technique.
A method for delineating lattice defects in SiC epitaxial layers and separating contribution from the substrate and the epitaxial layer was developed. The method is based on a combination of molten potassium hydroxide (KOH) etching and mechanical polishing. The technique was applied to establish concentration of dislocations and distinguish between the dislocation types. Various epitaxial layers grown at different growth conditions were characterized.
Growth with HCl addition was aimed at increasing the growth rate and improving the epilayer morphology. It was found that such growth conditions enhance the conversion efficiency of basal plane dislocations (propagating from the substrate) into threading dislocations in the epitaxial layers. Under optimum growth conditions, concentration of defects in the epitaxial layers were found to be less than or equal to that in the substrate, which established the good quality of the low temperature growth process. However, the low temperature growth process with HCL addition and high growth rate was found to introduce a new major problem in that a significant concentration of triangular defects formed in the epitaxial layers. Similar defects are often observed in more traditional (higher temperature) growth processes. It is often debated if the substrate quality is responsible for the triangular defects in the epitaxial layers. Answering this question in each particular case is very important when trying to eliminate the generation of the triangular defects.
The triangular defects in the low-temperature epitaxial layers studied in this work were traced down to the substrate by a combination of repeated polishing and molten KOH etching steps. The triangular defects were found not to originate from any substrate defects, which is different from some of the earlier reports of similar defects observed after epitaxial growth of SiC at regular (higher) temperatures. The structure and composition of the triangular defects was investigated by Scanning Electron Microscopy (SEM) and defect oxidation technique and were found to include single or multiple stacking faults bound by partial dislocations, and, in some cases, inclusions of other SiC polytypes. Evidences were obtained to assign the origin of these triangular defects to the formation of small polycrystalline islands on the growth surface. The polycrystalline islands often stay at the epilayer surface for a limited time and subsequently get evaporated, which had made it impossible to detect them and suspect their influence on the triangular defect generation prior to this work.
Various lattice defects were characterized in samples produced by novel low temperature selective epitaxial growth (SEG) method. SEG epitaxial layers were found to have a slightly higher number of dislocations compared to that in the substrate. Defects at the sidewall edges were observed at concentrations strongly depending on the crystallographic orientation of the mesa sidewall.
Heavily aluminum doped p+ layers (that are required for good ohmic contacts) produced by the low-temperature epitaxial growth method were analyzed in order to establish the dependence of the dislocation density on the Al dopant incorporation. Gradual degradation of the epitaxial morphology was found with increase in the level of doping, followed by much steeper degradation when approaching the solubility limit of Al in 4H-SiC. Precipitates were observed beyond Al doping concentrations of 3.5x10^20 cm^-3 and were found to be the dominating defect at the highest levels of doping.
A model for describing dislocation generation in heavily doped epitaxial layers has been developed taking the stress in the lattice caused by the Al doping into effect. The model provided good quantitative fit for the experimental data. Several material parameters like the multiplication factor, the strain hardening factor and other material constants of the SiC epitaxial layers have been extracted from this model.
