Nonuniformities of electrical resistivity in undoped 6H-SiC wafers

Abstract

Electrical and microstructural properties of highly boron-implantation doped 6H-SiCChemical elemental analysis, temperature-dependent Hall measurements, deep-level transient spectroscopy, and contactless resistivity mapping were performed on undoped semi-insulating ͑SI͒ and lightly nitrogen-doped conducting 6H-SiC crystals grown by physical vapor transport ͑PVT͒. Resistivity maps of commercial semi-insulating SiC wafers revealed resistivity variations across the wafers between one and two orders of magnitude. Two major types of variations were identified. First is the U-shape distribution with low resistivity in the center and high in the periphery of the wafer. The second type had an inverted U-shape distribution. Secondary-ion-mass spectrometry measurements of the distribution of nitrogen concentration along the growth axis and across the wafers sliced from different locations of lightly nitrogen-doped 6H-SiC boules were conducted. The measured nitrogen concentration gradually decreased along the growth direction and from the center to the periphery of the wafers. This change gives rise to the U-like distribution of resistivity in wafers of undoped SI-SiC. The concentrations of deep electron traps exhibited similar dependence. Compensation of nitrogen donors by these traps can result in the inverted U-like distribution of resistivity. Possible reasons for the observed nonuniformities include formation of a ͑0001͒ facet in PVT growth coupled with orientation-dependent nitrogen incorporation, systematic changes of the gas phase composition, and increase of the deposition temperature during boule growth.

Effects of hydrogen on the properties of SiC crystals grown by physical vapor transport: Thermodynamic considerations and experimental results

“…6. It can be seen that the concentrations of all of the deep traps related to Si-rich growth conditions [6,7] are strongly reduced with only the 0.4 eV trap remaining in any significant concentration.…”

“…In SiC crystals grown by physical vapor transport, the absolute nitrogen concentration and the amount of its variation along the growth axis is determined by the changes in the effective C/Si ratio via the wellknown site competition effect [5]. As growth progresses, the ambient becomes more C-rich, which suppresses nitrogen incorporation into the C sites [6]. Concomitantly, the most prominent electron traps strongly decrease in concentration as the effective C/Si ratio increases suggesting that they are defects related to the Si-rich growth conditions [7].…”

Electrical properties and deep levels spectra of bulk SiC crystals grown by hybrid physical–chemical vapor transport method

“…Therefore, the yield of semi-insulating crystals depends on one's ability to reduce the concentration of shallow centers. In earlier publication, Li et al [4] have shown that nitrogen distribution along the growth axis in 6H-SiC boules is to a large extent determined by the effective C/Si ratio in the gas phase during growth. At the beginning of the PVT growth process, the growth ambient is typically highly Si-rich and changes to less Si-rich toward the end [5].…”

“…Since efficiency of nitrogen incorporation decreases with the C/Si ratio, its concentration is higher in the seed portion of the crystal compared to the tail portion. Concomitantly with change in nitrogen level, the concentrations of major deep electron traps decrease along the crystal growth axis [4,6]. These non-uniformities are inherent in PVT growth method and result in change of electrical properties both along the growth axis and across the boule diameter.…”

Growth of nitrogen-doped SiC boules by halide chemical vapor deposition

“…This process relies on sublimation and re-condensation of solid SiC, which experiences a significant drift in the C/Si ratio as the source material is consumed [1,2]. The changing stoichiometry leads to significant changes in material properties and doping concentrations both radially and axially in the crystal [3].…”