UC Berkeley

While remarkable progress has been made towards understanding the properties of Mn-doped GaAs, the fundamental nature of carrier-mediated ferromagnetism in Mn-doped III-V semiconductors remains unclear. The research described in this dissertation focuses on the synthesis of novel ferromagnetic semiconductor alloys using ion implantation and pulsed-laser melting to investigate how changing the host from GaAs to another semiconductor affects ferromagnetism and transport. Using the Ga_1-xMn_xAs system as a reference, the chemistry of the anion sublattice is manipulated by performing isovalent anion substitution in which either the entire anion sublattice is changed from As to another Group V element (e.g. P) or the As sublattice is dilutely alloyed with isovalent P or N. By choosing isovalent elements of shorter atomic radius the interplay of carrier localization (determined by the Mn acceptor level in the host semiconductor) and exchange strength (determined by the energetic alignment of the Mn 3d and anion p states) can be explored.

It will be shown that changing the host semiconductor from GaAs to GaP leads to significant localization of ferromagnetism-mediating holes. Nonetheless, robust carrier-mediated ferromagnetism is observed in Ga_1-xMn_xP as determined by combined of ion-channeling, SQUID magnetometry, magnetotransport, X-ray magnetic circular dichroism, and magnetic anisotropy experiments. This finding indicates that hole localization does not destroy the carrier-mediated ferromagnetic phase, though T_C is generally lower in localized systems. Ternary semiconductor hosts are also explored with particular attention paid to the Ga_1-xMn_xAs_1-yP_y system that has attracted considerable theoretical attention as a system in which it is predicted that carrier delocalization and exchange strength are simultaneously maximized. However, this research indicates that T_C is not enhanced by dilute P alloying into Ga_1-xMn_xAs, which is attributed to the scattering of ferromagnetism-mediating holes by the alloy disorder introduced onto the anion sublattice. Finally, the magnetic anisotropy of Ga_1-xMn_xP is explored in detail and found to be substantially similar to that observed in other III_1-xMn_xV materials. Collectively this work demonstrates the importance of considering effects of hole localization for predictions of ferromagnetism and transport in III_1-xMn_xV materials.