Ferromagnetismus bei Raumtemperatur in mehrphasigen (Ga,Mn)N Schichten und Heterostrukturen

Abstract

(Ga,Mn)N is a GaN-based dilute magnetic semiconductor (DMS) and thus a promising system for semiconductor spintronic applications. Within the scope of this thesis thin layers of Mn doped GaN were grown by radio-frequency plasma-assisted molecular beam epitaxy (MBE) under various growth conditions and characterized with respect to their structural and magnetic properties. Structural characterizations exhibit a mediocre crystal quality which leads - in certain cases - to the formation of frustrated antiferromagnetic coupling and thus spin-glass behavior among the Mn atoms. Ferromagnetism at room temperature could not be achieved as Mn impurities could not be incorporated in such quantity to establish the Mn-Mn interaction (double-exchange) without precipitation.Although the first results could elucidate the parameter space of Mn incorporation it was not possible to establish room temperature ferromagnetism in (Ga,Mn)N thin layers. Due to the fact that increasing the Mn incorporation leads to formation of precipitates it is essential to make other approaches. By replacing the Si(111) substrate with GaN template the crystal quality of the MBE grown layers increases noticeably. (Ga,Mn)N thin layers with 3% Mn concentration grown on templates exhibit ferromagnetic behavior at room temperature. Measured magnetization of about 10-3 &muB per Mn atom leads to the assumption that only a fractional amount of incorporated Mn are involved in ferromagnetic coupling. Further investigations shall clarify the influence of (Al,Ga)N/(Ga,Mn)N heterojunctions on Mn incorporation and magnetization. The grown heterostructures exhibit not only a strong increase in magnetization but also an increase of the coercive field by a factor of 20. Mechanical strain and band bending effects at the heterojunctions however does not seem to have a direct influence on Mn incorporation and thus the Mn-Mn ferromagnetic coupling. The magnetism appears to be stabilized locally and does not depend on the layer thickness. Further analysis by TEM, EDX and analysis of the SQUID data reveal several magnetic phases in the (Ga,Mn)N layer: a superparamagnetic phase with a uniaxial anisotropy and a blocking behavior at TB = 8 K composed of MnGa nanoclusters with an average diameter of 1.25 nm and a magnetic moment of 210 &muB/cluster, a room-temperature ferromagnetic phase of high coercivity consisting of large accumulations of MnGa clusters (100 nm diameter) and a ferro/paramagnetic phase of dilute Mn in GaN with TC = 12 K and a magnetic moment of 2 &muB/Mn. The latter corresponds to current theoretical calculations which predict no room-temperature ferromagnetism for (Ga,Mn)N with Mn concentrations below 20%.Through this investigation a better understanding of the growth, Mn incorporation process and magnetic behaviour of (Ga,Mn)N layers and heterostructures could be achieved. However the results also indicate that (Ga,Mn)N DMS does not seem to be praticable for spintronic applications, but MnGa as layers or precipitations in GaN still have a certain potential as spin-injectors.