L. Däweritz

Origin of high-temperature ferromagnetism in (Ga,Mn)N layers grown on 4H–SiC(0001) by reactive molecular-beam epitaxy

We report on the growth, structural as well as magnetic characterization of (Ga,Mn)N epitaxial layers grown directly on 4H–SiC(0001) by reactive molecular-beam epitaxy. We focus on two layers grown under identical conditions except for the Mn/Ga flux ratio. Structural characterization reveals that the sample with the lower Mn content is a uniform alloy, while in the layer with the higher Mn content, Mn-rich clusters are found to be embedded in the (Ga,Mn)N alloy matrix. Although the magnetic behavior of both the samples is similar at low temperatures, showing antiferromagnetic characteristics with a spin-glass transition, the sample with higher Mn content additionally exhibits ferromagnetic properties at and above room temperature. This ferromagnetism most likely originates from the Mn-rich clusters in this sample.

Periodic elastic domains of coexisting phases in epitaxial MnAs films on GaAs

The surface topography of epitaxial MnAs films on GaAs(001) is studied by scanning probe microscopy. We provide direct experimental evidence for temperature-dependent elastic domains of the coexisting ferromagnetic αMnAs and paramagnetic βMnAs phases. The results agree well with a theoretical model for the elastic equilibrium of periodic domains.

Uniform quantum-dot arrays formed by natural self-faceting on patterned substrates

Of the approaches currently under investigation for the fabrication of functional III–V semiconductor nanostructures, self-organized growth mechanisms and directed growth on patterned substrates have yielded quantum wires and dots with the best structural and electronic properties. In patterned growth, high densities of structures are difficult to obtain; self-organization, on the other hand, can provide densely packed structures with good crystal quality, but generally offers limited control over nanostructure uniformity and spatial position. In the case of quantum dots, non-uniformity of size and shape is clearly undesirable, as the resulting structures will exhibit a broad range of electronic and optical properties, effectively smearing out the sought-for zero-dimensional behaviour of the dot ensemble. Here we demonstrate a method for improving size uniformity, while maintaining a high density of quantum dots, that combines elements of both self-organization and patterning. The photoluminescence spectrum of the resulting ordered arrays of quantum dots is dominated by a single sharp line, rather than the series of sharp lines that would indicate transitions in quantum dots of different sizes.

Kinetics of MnAs growth on GaAs(001) and interface structure

On different As-rich GaAs(001) templates, well characterized by reflectance difference spectroscopy, nucleation and growth of NiAs-type MnAs is investigated in real time by reflection high-energy electron diffraction. Using very high As4/Mn flux ratios and low growth rates, one of the two occurring azimuthal alignments of the (1100) orientation can be nearly suppressed even in the nucleation stage, and it vanishes completely with further growth. Annealing is found to be very effective in surface smoothing. In dependence on the As/Mn ratio the MnAs(1100) surface develops different reconstructions. This finding is important for further investigations in the growth of double heterostructures. High-resolution transmission electron microscopy of as-grown MnAs/GaAs samples reveals an abrupt interface. The lattice mismatch accommodation is anisotropic with regularly arranged misfit dislocations along the [110] direction and less localized coherency strain in the [110] direction, consistent with a near-coincid...

Magnetic structure of epitaxially grown MnAs on GaAs(001)

We investigate in detail the occurrence of magnetic domains in epitaxially grown MnAs films on GaAs(001) by magnetic force microscopy (MFM). MnAs layers exhibit in their demagnetized state a very complex magnetic domain structure. High resolution MFM images reveal detailed information on the domain wall. Additionally, we imaged magnetic domains in the dependence on the applied magnetic field. This detailed investigation gives new insight into the correlation between film topography and magnetic domain structures. Systematic magnetization measurements in-plane and out-of-plane have shown high anisotropy in our films. The out-of-plane magnetization determined as a function of the applied field reveals that the direction of the magnetic moments in the domain walls are out-of-plane, thus the domain walls are determined as 180° Bloch type.

Combined molecular beam epitaxy and diffractometer system for in situ x-ray studies of crystal growth

A combination of a molecular beam epitaxy (MBE) machine and a six circle diffractometer has been constructed at a dedicated wiggler beamline at the storage ring BESSY II for in situ investigations of III–V compound crystal growth. The growth conditions in our system reach a high MBE standard with a noncooled base pressure of 2×10−10 mbar. A fast entry load lock is available for sample exchange. Large-area Be windows in the ultrahigh vacuum chamber allow us to measure reflections at entrance and exit angles up to 45°, i.e., large perpendicular momentum transfers are possible. In situ reflection high energy electron diffraction and x-ray fluorescence measurements can be performed simultaneously with x-ray scattering. A GaAs(001) surface prepared and examined in our system reveals terrace widths of 450 nm and β(2×4) reconstruction domain sizes of 210 nm. The possibility of time-resolved x-ray diffraction studies is demonstrated by observation of intensity oscillations during layer-by-layer homoepitaxial grow...

Controlled n-type doping of AlN:Si films grown on 6H-SiC(0001) by plasma-assisted molecular beam epitaxy

We study the properties of Si-doped AlN films grown on 6H-SiC(0001) by plasma-assisted molecular beam epitaxy. Whereas nominally undoped AlN films are invariably insulating in nature, Si-doped films are found to be semiconducting with an electron concentration up to 7.4×1017cm−3, and a resistivity approaching 1 Ω cm at room temperature. Even heavy Si-doping (1×1020cm−3) does not degrade the structural properties of the AlN films. The morphology of these films is characterized by Si-induced step-bunching, but remains smooth with a rms roughness of about 1 nm.

Magnetic anisotropy and switching process in diluted Ga1−xMnxAs magnetic semiconductor films

Magnetic anisotropy and magnetization reversal phenomena are studied in Ga1−xMnxAs films grown on GaAs(001). From the azimuthal dependence of the magneto-optical Kerr rotation and magnetic linear dichroism, the in-plane 〈100〉 directions are unambiguously assigned as axes of easy magnetization. The values of the cubic and uniaxial anisotropies are determined. Magnetization reversal proceeds by noncoherent spin rotation and irreversible jumps by formation of 90° domains.

Phase-transition-induced residual strain in ferromagnetic MnAs films epitaxially grown on GaAs(001)

We investigate the atomic interface structure and the residual strain state of ferromagnetic α (hexagonal) MnAs layers on cubic GaAs(001) by means of high-resolution transmission electron microscopy and electron diffraction. Despite the different symmetries of the adjacent planes at the heterointerface and the large and orientation-dependent lattice mismatch, the hexagonal MnAs grows epitaxially on GaAs(001) with the (11.0) prism plane parallel to the cubic substrate. The atomic arrangement at the interface, which is defined by the accommodation of the large lattice mismatch, explains this extreme case of heteroepitaxial alignment. The anisotropic residual strain distribution is discussed with respect to the particular process of lattice misfit relaxation in the presence of the ferromagnetic phase transition.

Atomic scale morphology of self-organized periodic elastic domains in epitaxial ferromagnetic MnAs films

The surface of epitaxial MnAs layers grown on GaAs(001) substrates by molecular beam epitaxy is studied by atomic force microscopy and scanning tunneling microscopy (STM). A periodic array of alternating ridges and grooves is observed. The periodicity ranges from 200 to 900 nm and increases with increasing layer thickness. The terrace-step morphology and the surface reconstruction on the ridges and in the grooves are imaged by STM. It is found that both are independent from the ridge-groove structure, supporting the idea that the formation of the ridge-groove structure is due to elastic distortion of the film during cooling after growth.