Photothermal ionization spectroscopy (PTIS) measurements were carried out o
n a free-standing, high purity and high quality 4H-SiC epitaxial layer at v
arious temperatures. The two step photothermal ionization process is clearl
y 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 mate
rial. It reveals five strong, well resolved electronic transition lines ass
ociated with the shallow nitrogen donor. The ionization energy of the shall
ow nitrogen donor is deduced to be 60.2 +/- 0.5 meV based on experimental r
esults. Furthermore, PTI magnetospectroscopy measurements were performed to
investigate the symmetry properties of these transitions in Faraday config
uration. No linear Zeeman splitting is observed, however, these lines show
a diamagnetic shift. It indicates that the excited states of the shallow ni
trogen 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].