G. Springholz

Direct formation of self-assembled quantum dots under tensile strain by heteroepitaxy of PbSe on PbTe (111)

Direct growth of tensile-strained PbSe quantum dots by molecular beam epitaxy on 5.5% lattice mismatched PbTe (111) is investigated by atomic force microscopy and in situ reflection high energy electron diffraction. After wetting layer formation, two types of PbSe islands are formed distinguishable in size and ordering behavior. All islands exhibit a well defined pyramidal shape with triangular base and steep (100) side facets. In addition, the dots exhibit a remarkably narrow size distribution with a relative variation of height and width as low as ±7%.

Negligible Surface Reactivity of Topological Insulators Bi2Se3 and Bi2Te3 towards Oxygen and Water

The long-term stability of functional properties of topological insulator materials is crucial for the operation of future topological insulator based devices. Water and oxygen have been reported to be the main sources of surface deterioration by chemical reactions. In the present work, we investigate the behavior of the topological surface states on Bi2X3 (X = Se, Te) by valence-band and core level photoemission in a wide range of water and oxygen pressures both in situ (from 10(-8) to 0.1 mbar) and ex situ (at 1 bar). We find that no chemical reactions occur in pure oxygen and in pure water. Water itself does not chemically react with both Bi2Se3 and Bi2Te3 surfaces and only leads to slight p-doping. In dry air, the oxidation of the Bi2Te3 surface occurs on the time scale of months, in the case of Bi2Se3 surface of cleaved crystal, not even on the time scale of years. The presence of water, however, promotes the oxidation in air, and we suggest the underlying reactions supported by density functional calculations. All in all, the surface reactivity is found to be negligible, which allows expanding the acceptable ranges of conditions for preparation, handling and operation of future Bi2X3-based devices.

Vertical and lateral ordering in self-organized quantum dot superlattices

Abstract The formation of vertically and laterally ordered dot superstructures in self-organized quantum dot superlattices is described. The ordering is based on the long-range elastic interactions between the strained self-assembled quantum dots, providing a driving force for a spatially correlated dot nucleation. For various materials systems, different types of ordered structures have been observed, ranging from vertically aligned dot superlattices for Si/Ge or III–V semiconductors to a fcc-like ABCABC… stacking in IV–VI materials. It is shown that the elastic anisotropy of the spacer material plays a crucial role in this self-organization process. In particular, for materials with very high elastic anisotropy and growth orientations parallel to an elastically soft direction, layer-to-layer dot correlations inclined to the growth direction can be formed. This is shown to be particularly effective for inducing a lateral ordering of the dots within the growth plane, which can lead to a significant narrowing of the dot size dispersion. These conclusions are also supported by Monte Carlo growth simulations.

Midinfrared surface-emitting PbSe/PbEuTe quantum-dot lasers

Midinfrared laser emission from self-organized PbSe quantum dots in a high-finesse vertical-cavity surface-emitting laser structure is reported. The structure was grown by molecular-beam epitaxy and consists of high reflectivity epitaxial EuTe/PbEuTe Bragg mirrors with a PbSe/Pb1−xEuxTe quantum-dot superlattice as the active region. Narrow laser emission at 4.2–3.9 μm induced by optical pumping is achieved at temperatures up to 90 K. The observation of simultaneous two-mode emission indicates a width of the inhomogeneously broadened PbSe dot gain spectrum of about 18 meV.

Above-room-temperature mid-infrared lasing from vertical-cavity surface-emitting PbTe quantum-well lasers

Above-room-temperature operation of vertical-cavity surface-emitting lasers emitting in the mid-infrared is reported. The stimulated emission is generated in PbTe quantum wells embedded in two-wavelength microcavities by optically pumping with fs laser pulses. The spectrum of the laser modes is broadened and blue-shifted due to dynamic band filling. The intensity of the mid-infrared emission and the laser threshold depends on the energy of the microcavity resonance. At a wavelength of 3.1 μm, laser operation is obtained up to a temperature of 65 °C, limited by nonradiative recombination processes.

Spiral growth and threading dislocations for molecular beam epitaxy of PbTe on BaF2 (111) studied by scanning tunneling microscopy

Molecular beam epitaxy of PbTe on BaF2 (111) is studied using UHV–scanning tunneling microscopy and atomic force microscopy. It is shown that PbTe growth is totally dominated by growth spirals formed around threading dislocations (TD) that originate from the growth on the 4.2% lattice‐mismatched substrate. Due to dislocation annihilation, the TD density rapidly decreases with layer thickness, which results in a dramatic increase of the electron mobilities in the layers.

Epitaxial Bragg mirrors for the mid-infrared and their applications

Bragg interference mirrors consisting of stacks of dielectric layers with an optical thickness of a quarter wavelength are of great importance for optoelectronic device applications. For the mid-infrared spectral range mirrors with high reflectivity stop bands are fabricated from combinations of Pb1� xEuxTe/EuTe materials by molecular beam epitaxy on BaF2 substrates. These mirrors designed by the transfer matrix method exhibit reflectivities in excess of 99% by only 3 Bragg mirror layer pairs and very wide stop band regions, reaching a width of up to 60% of the target wavelength. Based on these very efficient mirrors, planar microcavities are demonstrated with an ultra-high effective finesse of up to 1700. Stimulated emission between 3 and 6mm is obtained by optically pumping a vertical-cavity surface-emitting laser containing PbTe quantum wells with Pb1� xEuxTe barriers as active medium embedded between two dielectric Bragg mirrors. Depending on the design of the resonator, pulsed laser operation is observed up to 651C. The enhancement of light absorption in the cavity is used to study the absorption of superlattices containing correlated self-organized PbSe quantum dots. r 2002 Elsevier Science Ltd. All rights reserved.

Size control and midinfrared emission of epitaxial PbTe∕CdTe quantum dot precipitates grown by molecular beam epitaxy

Epitaxial quantum dots with symmetric and highly facetted shapes are fabricated by thermal annealing of two-dimensional (2D) PbTe epilayers embedded in a CdTe matrix. By varying the thickness of the initial 2D layers, the dot size can be effectively controlled between 5 and 25nm, and areal densities as high as 3×1011cm−2 can be achieved. The size control allows the tuning of the quantum dot luminescence over a wide spectral range between 2.2 and 3.7μm. As a result, ultrabroadband emission from a multilayered quantum dot stack is demonstrated, which is a precondition for the development of superluminescent diodes operating in the near infrared and midinfrared.

Molecular beam epitaxy of strained PbTe/EuTe superlattices

Using reflection high energy electron diffraction (RHEED), the phase diagram of strained layer heteroepitaxy of EuTe by molecular beam epitaxy on PbTe(111) surfaces was determined. The EuTe(111) surface exhibits different surface reconstructions corresponding to a Te‐stabilized or an Eu‐stabilized surface state. For perfect two‐dimensional layer‐by‐layer heteroepitaxial growth, only a narrow process window exists which can be determined by RHEED. Using such established growth conditions, we have fabricated strained PbTe/EuTe superlattices with superior structural perfection as shown by high resolution x‐ray diffraction.

4.8 μm vertical emitting PbTe quantum-well lasers based on high-finesse EuTe/Pb1−xEuxTe microcavities

Vertical laser emission at 4.8 μm from PbTe quantum wells in high-finesse Pb0.95Eu0.05Te/EuTe microcavity structures at temperatures between 35 and 85 K is reported. The vertical-cavity laser structure was grown by molecular-beam epitaxy on BaF2(111) substrates, and consisted of a 2λ cavity with four 20 nm quantum wells at the cavity antinodes. Laser emission was excited by optical pumping with a pulsed Nd:YVO4 laser. The comparison of the cavity mode positions with envelope function calculations of the quantum-well energy levels indicates that, in this temperature range, lasing is due to transitions between the ground level of the oblique valleys in the conduction and valence bands.