Local Structure and Magnetism of (Ga,Mn)As

Abstract

Throughout the years, dilute magnetic semiconductors (DMS) have emerged as promising materials for semiconductor-based spintronics. In particular, (Ga,Mn)As has become the model system in which to explore the physics of carrier-mediated ferromagnetism in semiconductors and the associated spintronic phenomena, with a number of interesting functionalities and demonstrated proof-of-concept devices. It constitutes the perfect example of how the magnetic behavior of DMS materials is strongly influenced by local structure. In this thesis, we address key aspects of the interplay between local structure and ferromagnetism of (Ga,Mn)As. We unambiguously identify the lattice site occupied by interstitial Mn as the tetrahedral interstitial site with As nearest neighbors T(As). We show, furthermore, that the T(As) is the most energetically favorable site regardless of the interstitial atom forming or not complexes with substitutional Mn. We also evaluate the thermal stability of both interstitial and substitutional Mn sites occupied by Mn for two representative Mn concentrations (1% and 5%), and its influence on the material’s structure and magnetism. We show that compared to the substitutional Mn, interstitial Mn becomes mobile at lower temperatures, for both low (1%) and high (5%) Mn concentration. Moreover, the diffusion temperatures are lower for the high concentration than for the low concentration case. These diffusion temperatures are concentration dependent, with aggregation of impurities occurring for the high concentration at lower temperatures than for the low concentration. These findings translate into two key conclusions: at typical growth temperatures (200-300°C) the interstitial Mn is mobile for high concentration (5%) but not for low concentration (1%); and substitutional Mn impurities become mobile in a temperature regime that is well below what has been previously reported. We also observe a decrease in the activation energy for the diffusion of both substitutional and interstitial impurities with increasing Mn concentration. This decrease has a different origin for each case: for substitutional diffusion, with activation energies of 2.3-2.6 eV at 1% Mn to 1.9-2.0 eV at 5% Mn, a vacancy-assisted mechanism occurs that is favored with increasing impurity concentration; for interstitial diffusion, with activation energies of 1.5-2.1 eV at 1% Mn to 1.3-1.8 eV at 5% Mn, as charge screening effects become stronger with increasing Mn (and consequently carrier) concentration, interstitial Mn defects are effectively “neutralized”, and therefore experience lower migration barriers. Additionally, we conducted a comprehensive study of the local structure and magnetism in the different diffusion regimes. We show that annealing at 200°C promotes the passivation of the interstitial Mn for 5% Mn (Ga,Mn)As, as an increase in TC and magnetization is observed. No improvement is observed in the 1% Mn case, which can be understood from the absence of interstitial impurities incorporated during growth for this concentration regime. Annealing at 300°C induces the precipitation of Mn into Mn-rich regions for both concentrations studied, with no signs of ferromagnetism. Finally, annealing at 600°C led to the formation of well-defined secondary-phases in both concentrations, consistent essentially of superparamagnetic MnAs nanoclusters of two types: zincblende and hexagonal NiAs-type. The results presented in this thesis are direct evidence for the complex interplay between the local structure and the carrier-mediated ferromagnetism present in this DMS system. It constitutes an important step in the understanding of the fundamental physics behind (Ga,Mn)As, motivating a wider investigation of other dilute magnetic semiconductors within the III-Mn-V family