Lattice location of Mn in GaAs and GaN

The field of dilute magnetic semiconductors (DMS) has seen a lot of development in the past decades, both from a fundamental interest in the link between magnetic and conducting properties and with an eye to potential applications in computer technology. While the presence of semiconducting properties and magnetism in a given material is not out of the ordinary, DMS materials stand out because the charge carriers actually mediate between magnetic moments in the lattice, causing the ferromagnetic ordering. These magnetic moments and charge carriers are supplied by transition-metal (TM) dopants in a classic semiconductor. The location where these dopants are incorporated will determine if they will act as either an acceptor or donor and how they will couple to other magnetic moments. Hence, in order to achieve a better understanding of DMS, accurate knowledge of the lattice location the TM takes up in the crystal is vital. In this thesis the lattice location of Mn in GaAs and GaN is studied, two model materials from respectively the narrow-gap and wide-gap DMS families. For Mn implanted GaAs, written more compactly as (Ga,Mn)As, the ferromagnetic behavior is relatively well understood by the charge-carrier mediated mechanism described above, which aligns the substitutional Mn magnetic moments. Aside from substitutional Mn (Mn$_{Ga}$) also interstitial Mn (Mn$_{int}$) can be present which is known to be a donor, compensating the hole charge carriers introduced by the substitutional Mn acceptors. Moreover it couples anti-ferromagnetically to MnGa reducing the ferromagnetism. Using the technique of emission channeling (EC) the lattice location of Mn$_{int}$ in a (Ga,Mn)As thin film at 4% impurity concentration prepared by ion implantation and pulsed laser melting (II-PLM) was determined to be the T$_{As}$ site. The thermal stability was studied as well by considering the fractions of Mn present after different annealing steps. For diffusion of substitutional Mn an activation energy (E$_{a}$) of 2.1 eV was found. Previous findings on (Ga,Mn)As thin films prepared by molecular beam epitaxy (MBE) of 1% and 5% impurity concentration found an activation energy respectively higher and lower than for the II-PLM sample. We suggest that the diffusion of substitutional Mn is an effect dependent mainly on the concentration and is best interpreted in terms of vacancy-assisted diffusion in a percolation cluster of Mn atoms. For interstitial MnE$_{a}$ was determined to be 0.9-1.2eV. This value is much lower than the activation energy found in the aforementioned MBE samples. That Mn_int has a significantly lower thermal stability in the II-PLM film compared to the MBE films is interpreted as a consequence of the presence of an internal electric field, enhancing the diffusivity of Mn$_{int}$. The electric field is assumed to be generated by a non-uniform charge carrier distribution, a result of the depth profile of electrically active Mn$_{sub}$ in the II-PLM film. Although in GaN cation substitution by Mn is accepted there have also been reports of minority anion substitution. Since this anion fraction (MnN) may affect the electrical and magnetic properties by acting as a compensating defect (similar to Mn$_{int}$ in (Ga,Mn)As), it is important to determine whether or not it is present, and if so, in which amount. On the basis of earlier EC experiments also a selection mechanism for anion substitution to take place, depending on the location of the Fermi-level in the band-gap, had been proposed. To test this hypothesis the lattice location of Mn implanted (Ga,Mn)N, p-type GaN and n-type GaN was determined with EC. For all three samples the Mn$_{Ga}$ and Mn$_{Ga}$ were found to be displaced towards the AB$_{Ga}$ site. This displaced fraction is attributed to the formation of a defect complex with nitrogen vacancies created during implantation. No qualitative difference in displacement is found between the doped GaN samples, contrary to what is expected on basis of the location of the Fermi-level. We interpret this as being due to implantation damage causing the Fermi-level to be pinned in the middle of the bandgap, locally counteracting the effect of the dopants.