A Study of Nitrogen Incorporation in PVT Growth of n+ 4H SiC

Abstract

A detailed understanding of the incorporation of N2 gas during PVT growth of SiC is required to achieve high performance, low resistivity n+ SiC substrates necessary for power device applications. In this report, nitrogen incorporation is investigated for growth of 4H SiC crystals from 2” to 3” diameter in conditions ranging from unintentionally doped to low resistivity (0.015 - cm). For a wafer in a particular boule a resistivity uniformity of ± 5% is typical although the uniformity decreases when the wafer orientation is cut off axis from the bulk growth direction. Within a boule growth, the nitrogen incorporation is found to be a function of growth time. As growth continues, the resistivity of wafers cut further from the seed increases. A typical 3” on axis sliced wafer has a within wafer resistivity uniformity of 5% compared with an average seed to tail variation of 10%. Due to the axial resistivity gradient the within wafer resistivity uniformity of off axis sliced wafers is 8%. These axial and radial gradients are thought to be a function of the changing C/Si ratio during growth. Nitrogen incorporation as a function of PVT geometry, N2 partial pressure, and growth temperature are investigated and discussed. In particular, nitrogen incorporation is found to depend on the crucible size and nitrogen partial pressure, but is not strongly dependent on the absolute growth temperature, for growth temperature ranging over 150°C. Modeling of PVT growth shows the axial resistivity gradient can be linked with a change in the C/Si ratio versus time. Trends and N2 gas incorporation behavior will be discussed using resistivity mapping, SIMS, and Hall effect data.