Photothermal ionization spectroscopy of shallow nitrogen donor states in 4H-SiC

Photothermal ionization spectroscopy (PTIS) measurements were carried out on a free-standing, high purity and high quality 4H-SiC epitaxial layer at various temperatures. The two step photothermal ionization process is clearly reflected in the temperature dependence of the photoconductivity. The PTI spectrum at a temperature of 25.6 K exhibits one order of magnitude higher energy resolution than the infrared absorption spectra of 4H-SiC bulk material. It reveals five strong, well resolved electronic transition lines associated with the shallow nitrogen donor. The ionization energy of the shallow nitrogen donor is deduced to be 60.2 +/- 0.5 meV based on experimental results. Furthermore, PTI magnetospectroscopy measurements were performed to investigate the symmetry properties of these transitions in Faraday configuration. No linear Zeeman splitting is observed, however, these lines show a diamagnetic shift. It indicates that the excited states of the shallow nitrogen donor are nondegenerate at zero magnetic field, which is consistent with the fact that the effective mass tensor of 4H-SiC has three different diagonal components. (C) 2000 American Institute of Physics. [S0021-8979(00)00208-5].