  We investigate properties of doping-induced metal-insulator transition in
GaN:Si by means of electron spin resonance and Hall effect. While increasing
the doping concentration, Si-related bands are formed below the bottom of the
GaN conduction band. The D0 band of single-occupied Si donor sites is centered
27 meV below the bottom of the GaN conduction band, the D- band of
double-occupied Si states at 2.7 meV below the bottom of the GaN conduction
band. Strong damping of the magnetic moment occurs due to filling of the D-
states at Si concentrations approaching the metal-insulator transition.
Simultaneously, shortening of electron spin relaxation time due to limited
electron lifetime in the single-occupied D0 band is observed. The
metal-insulator transition occurs at the critical concentration of
uncompensated donors equal to about 1.6 * 10^18 cm^-3. Electronic states in
metallic samples beyond the metal-insulator transition demonstrate non-magnetic
character of double-occupied states.
