Hans-Peter Schönherr

Epitaxial growth of Fe3Si/GaAs(001) hybrid structures

We have established an optimized growth temperature range, namely, 150 °C<TG<250 °C, where ferromagnetic Fe3Si/GaAs(001) hybrid structures with high crystalline and interfacial quality can be fabricated by molecular-beam epitaxy. The composition of the Fe3Si layers, which can be regarded as a Heusler alloy, was tuned within the stable Fe3Si phase. The layers show high magnetic moments with a value of 1050 emu/cm3, which is close to that of bulk Fe3Si.

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.

Selectivity of growth on patterned GaAs (311)A substrates

We report on the selectivity of growth on patterned GaAs (311)A substrates by solid‐source molecular beam epitaxy. For mesa stripes oriented along the [01‐1] direction, the selectivity of growth is qualitatively different from that on patterned GaAs (100) substrates with a higher growth rate on one of the side facets of the stripes. This growth mode develops a convex curved surface profile enclosing thicker wirelike regions of GaAs due to preferential migration of Ga atoms from both sides toward the sidewall leaving behind thinner regions on the adjacent mesa top and bottom areas. A mechanism for the formation of the surface profile is proposed.

Evolution of the surface morphology of Fe grown on GaAs (100), (311)A, and (331)A substrates by molecular beam epitaxy

We study the growth of Fe by molecular beam epitaxy on GaAs (100), (311)A, and (331)A substrates in dependence on the termination (reconstruction) of the GaAs surface and Fe growth temperature. Crystal quality and surface morphology of the 20- and 160-nm-thick Fe layers are characterized by double-crystal x-ray diffraction and atomic force microscopy. On GaAs (100) substrates we obtain very smooth Fe layers for As-rich surface reconstructions at a growth temperature of 50 °C. Less As-rich surface reconstructions produce macroscopic defects whose density increases on more Ga-rich surface reconstructions. On GaAs (311)A and (331)A substrates smooth layers with good crystal quality are obtained at 0 °C. The high density of macroscopic defects in these Fe layers is again eliminated on As-saturated surfaces. The evolution of the Fe surface morphology on the micron-length scale and the successful elimination of macroscopic defects on As-saturated GaAs substrates is highly relevant for application of these layer...

Patterned growth on high‐index GaAs (n11) substrates: Application to sidewall quantum wires

We have recently found a new phenomenon in the selectivity of growth by molecular‐beam epitaxy on patterned GaAs (311)A substrates to form a fast growing sidewall on one side of mesa stripes oriented along the [01−1] direction. Preferential migration of Ga atoms from the mesa top and bottom toward the sidewall forms a smooth convex curved surface profile without facets. Comparison of patterned growth on other high‐index (n11)A&B surfaces shows this growth mode to be unique for GaAs (311)A substrates. Lateral quantum wires are realized for step heights in the quantum‐size regime. Quantum confinement of excitons in the wires is demonstrated by the transition from two‐dimensional to magnetic confinement with increasing magnetic field. For device applications it is important that the wires can be vertically stacked in the growth direction without increase in interface roughness and wire size fluctuations.

Self-organized quantum wires formed by elongated dislocation-free islands in (In,Ga)As/GaAs(100

Long and fairly uniform quantum wire arrays have been fabricated by the growth of (In,Ga)As/GaAs multilayer structures. The structural properties of the quantum wires are characterized by atomic force microscopy, x-ray diffractometry, and transmission electron microscopy. The lateral carrier confinement in the quantum wires is confirmed by linear polarization dependent photoluminescence (PL) and magneto-PL measurements.

Uniform multiatomic step arrays formed by atomic hydrogen assisted molecular beam epitaxy on GaAs (331) substrates

Coherently aligned multi-atomic step arrays are naturally formed during molecular beam epitaxy (MBE) of GaAs/(AlGa)As heterostructures on GaAs (331) substrates. The step height systematically increases with the substrate temperature while the lateral periodicity remains almost unchanged. With atomic hydrogen irradiation the step height is larger by more than a factor of 2 compared to that in conventional MBE which is attributed to a larger surface migration length of adatoms. The higher uniformity in atomic hydrogen assisted MBE allows the formation of step arrays, 12–13 nm high with a lateral periodicity around 250 nm, and straight step edges over 10 μm length. The step arrays reveal a strong influence on the electron transport of Si-modulation-doped GaAs/(AlGa)As heterostructures with the conductivity parallel to the step edges at cryogenic temperatures more than one order of magnitude larger than that perpendicular to the steps.

Shape transition of coherent three-dimensional (In, Ga)As islands on GaAs(100)

The shape transition of coherent three-dimensional (3D) islands is observed experimentally in the (In,Ga)As/GaAs(100) material system. In the molecular-beam epitaxy of a 1.8-nm-thick In0.35Ga0.65As single layer, we find that the shape of the coherent 3D islands transforms from round to elongated when increasing the growth temperature. A quantitative agreement of our experimental data with the theoretical work of Tersoff and Tromp is achieved.

Micro-Photoluminescence Study at Room Temperature of Sidewall Quantum Wires Formed on Patterned GaAs (311)A Substrates by Molecular Beam Epitaxy

Lateral quantum wires are grown by molecular beam epitaxy of GaAs/(AlGa)As multilayer structures on patterned GaAs (311)A substrates along the sidewall of 15–20 nm heigh mesa stripes oriented along [01–1]. The wire formation relies on the preferential migration of Ga atoms from the mesa top and bottom toward the sidewall. The quantum wires having a lateral width of ~50 nm are characterized by micro-photoluminescence spectroscopy between 8 K and room temperature. In the whole temperature regime the quantum wire exhibits a clear luminescence peak, well separated from the quantum well peak at the mesa top and bottom forming the lateral barriers. Micro-photoluminescence linescans reveal the strong spatial confinement of the photogenerated carriers even at room temperature.

Device quality submicron arrays of stacked sidewall quantum wires on patterned GaAs (311)A substrates

Three-dimensional arrays of vertically stacked sidewall quantum wires are fabricated by molecular beam epitaxy on GaAs (311)A substrates patterned with 500-nm-pitch gratings. The cathodoluminescence spectra at low temperature are dominated by the emission from the quantum wires with narrow linewidth accompanied by a very weak emission from the connecting thin quantum wells due to localization of excitons at random interface fluctuations. When the carriers in the quantum well become delocalized at elevated temperature, only the strong emission from the quantum-wire array is observed revealing perfect carrier capture into the quantum wires without detectable thermal repopulation of the quantum well up to room temperature. Thus, unpreceded device quality of this quantum-wire structure is demonstrated.