Ferromagnetism and localization in Ga$_{1-x}$Mn$_{x}$As, Ga$_{1-x}$Mn$_{x}$P, and in between

Because of their potential as both injectors and filters of spin-polarized carriers, ferromagnetic semiconductors may play an important role in spin-based electronics, or \textit{spintronics}. Ferromagnetic semiconductors are formed by the substitution of a relatively small fraction of host atoms with a magnetic species. Ga$_{1-x}$Mn$_{x}$As has been the most thoroughly studied material among these, and ferromagnetism in it arises from hole-mediated inter-Mn exchange. The Curie temperature T$_{C}$ in Ga$_{1-x}$Mn$_{x}$As has been shown to increase with increasing concentration of substitutional Mn acceptors. However, room temperature ferromagnetism in this canonical system has been elusive due to challenges in materials synthesis---namely, raising $x$ while avoiding the formation of second phases or compensating defects. Increasing $p-d$ exchange by modifying the host semiconductor via anion substitution (e.g., replacing As with P) is a significantly less explored route by which T$_{C}$ may be raised. We are investigating the effect of anion substitution in ferromagnetic Ga$_{1-x}$Mn$_{x}$As$_{1-y}$P$_{y}$ formed by ion implantation followed by pulsed-laser melting. In the endpoint compound Ga$_{1-x}$Mn$_{x}$P T$_{C}$ is found to vary linearly with $x$, and non-metallic transport is observed for $x$ up to $\sim $4.2{\%}, corresponding to a T$_{C}$ of $\sim $62 K compared to $\sim $112 K for Ga$_{1-x}$Mn$_{x}$As with a similar $x$. Dilution of the endpoint compound Ga$_{1-x}$Mn$_{x}$As with P results in a precipitous decrease in T$_{C}$ to below 60 K for y=2.8{\%}. Remarkably, Ga$_{1-x}$Mn$_{x}$As$_{1-y}$P$_{y}$ films undergo a metal-insulator transition between $y$=1.5{\%} and 2.3{\%} even as $x$ is held approximately constant indicating that alloy disorder in the anion sublattice induces hole localization, which in turn may be responsible for a strong suppression of T$_{C}$. Thus, while anion substitution may enhance $p-d$ exchange, localization effects must be considered when developing a suitable picture for ferromagnetism in these materials.