While the physics of electrical doping of semiconductors has been well understood for decades, magnetic doping and the interactions between the carriers and the magnetic moments in semiconductors are still under active investigation for various applications, such as spintronics and quantum computing. Our systematic studies on transition-metal-doped (TM-doped) and rare-earth-doped (RE-doped) amorphous group IV elemental semiconductors provide unique insight into the rich physics of this type of materials.

Our model system is the e-beam coevaporated a-Gd_xSi_1-x films. Magnetron cosputtered a-Gd_xSi _1-x films, despite having very different film morphology at the 10-nm scale from the e-beam coevaporated films, are demonstrated to possess almost the same physical properties. Cosputtered a-Gd_xC_1-x (:H_y) and Gd ion-implanted ta-C (ta-C_1-x:Gd _x) films are studied for Gd in different a-C matrices with different sp²/sp ³ ratio. All doped a-C films are on the insulating side of the metal-insulator transition. Very similar to a-Gd _xSi_1-x films, Gd possesses a large magnetic moment in a-C. The moment-moment and moment-carrier interactions lead to a spin-glass ground state and large negative magnetoresistance (MR) below a crossover temperature T' in both a-Gd _xC_1-x<(:Hy) and ta-C_1-x:Gd_x films. A small positive MR is found above T'.

Transition metal Mn has always been believed to possess a large local moment in Si or Ge. However, e-beam coevaporated a-Mn_xSi_1-x films are found to show a quenched local moment for Mn concentration as low as x=0.005 and up to x=0.175. All films are purely paramagnetic and have very small saturation moments. Unlike Gd, which provides both carriers and local moment, Mn only provides electrical carriers in a-Si. These results suggest an itinerant non-magnetic Mn states in a-Si; the insulating behavior is a result of the strong structural disorder. This quenching of the local Mn moment has not been predicted by any existing theory. Consistent with the small Mn moment, a-Mn_xSi_{1- x} films show a very small positive MR, in contrast to Gd-doped films.