S. Nau

Anisotropy of the magnetotransport in (Ga,Mn)As/MnAs paramagnetic-ferromagnetic hybrid structures

We investigated the temperature-dependent magnetoresistance of granular (Ga,Mn)As/MnAs hybrids grown on (100) GaAs in different transport geometries. The observed magnetoresistance effects are much bigger than for a corresponding (Ga,Mn)As reference sample without MnAs nanoclusters. We find that the magnetoresistance effects depend strongly on the chosen transport geometry. When the external field is perpendicular to the sample plane the effects are largest. The smallest effects occur when the external field is in the sample plane and parallel to the current. Furthermore, we have established by ferromagnetic resonance studies that the magnetic properties of the ensemble of ferromagnetic MnAs nanoclusters is similar for the magnetic field orientations studied. Therefore, the observed anisotropy of the magnetoresistance mainly reflects the difference in current path through the sample which leads to a variation of the degree of interaction between the free carriers in the matrix and nanoclusters.

Overgrowth experiments of ferromagnetic (MnGa)As-cluster layers by MOVPE

We present the results of the epitaxial overgrowth of magnetic (MnGa)As-cluster structures with GaAs, (AlGa)As and AlAs using metal organic vapor phase epitaxy (MOVPE). The structural differences in the overgrowth are investigated by means of atomic force microscopy (AFM) combined with transmission electron microscopy (TEM), in particular, to proof the successful overgrowth of the cluster layers with AlAs. Out of these experiments a first model for the overgrowth is developed. Measurements using a SQUID-magnetometer confirm the existence of ferromagnetism above room temperature in the cluster layers after overgrowth; however, other magnetic properties as the coercitive field are influenced by the overgrowth process.

Experimental investigation of structures of interior interfaces in GaAs

A method for the structural investigation of interior inverted GaAs/AlAs interfaces is presented which combines highly selective etching and subsequent atomic force microscopy. It provides three-dimensional mappings of interior GaAs interfaces on a lateral scale on the order of micrometers with angstrom z resolution. The perfection of this method is demonstrated, which allows the observation of the real interface monolayer island and terrace structure. Potential aluminum residues on the uncovered interfaces are below the detection limit of Auger electron spectroscopy, which is estimated to 16% of aluminum in a single monolayer on a GaAs crystal. The structure of an interior interface can differ significantly from that of a corresponding surface layer after cooling down from the growth temperature. The substantial restructuring of the interface morphology caused by growth interruptions is investigated in detail for metalorganic vapor phase epitaxy.

Correlation of the physical properties and the interface morphology of AlGaAs/GaAs heterostructures

We have investigated the influence of interface roughness in GaAs/AlGaAs heterostructures on both the optical and the electronic properties by systematically varying the two growth parameters substrate temperature and growth interruption. We prove that the optimization of samples for optics and transport, respectively, requires different growth parameters. Whereas the optical properties are exclusively determined by the roughness of the two quantum well interfaces, the transport properties are additionally influenced by the ionized impurity scattering. The number of impurities increases during growth interruption and, consequently, in contrast to the optical samples, an optimization of the growth parameters is not as straightforward since it depends on the background impurities originating from the ultrahigh vacuum system. A direct correlation with information obtained from atomic force microscopy images is therefore only possible for the optical properties. At growth temperatures higher than 620 °C in ad...

100 fs Carrier Dynamics in GaAs under 100 nm Diameter Apertures

We study the decay of an optically excited cloud of carriers as a function of its initial diameter d. Values as low as d = 100 nm are possible via metallic apertures on bulk GaAs fabricated by electron beam lithography. We find that the decay time τ versus d scales as τ ∝ d2 for large d, as expected for diffusive transport, and as τ ∝ d for submicron values of d, indicating strong deviations from diffusive transport. The dependence on parameters such as initial carrier kinetic energy, carrier density and sample temperature is discussed.

Mesoscopic island structure at GaAs/(AlGa)As interfaces grown by MBE

The study of interface morphology is of key importance for the optical as well as the transport properties of low-dimensional structures and can also lead to detailed understanding of growth processes. In this paper we report the results of systematic studies of the influence of the growth temperature on the morphology of GaAs/(AlGa)As quantum well (QW) interfaces. A combination of highly selective etching and subsequent atomic force microscopy (AFM) was used to determine the interface morphology. This technique enables three-dimensional mapping of the interfaces with an atomic height resolution on lateral scales from 10 nm up to 50 /spl mu/m.