U.W. Pohl

Size-dependent fine-structure splitting in self-organized InAs/GaAs quantum dots

A systematic variation of the exciton fine-structure splitting with quantum dot size in single quantum dots grown by metal-organic chemical vapor deposition is observed. The splitting increases from to as much as with quantum dot size. A change of sign is reported for small quantum dots. Model calculations within the framework of eight-band theory and the configuration interaction method were performed. Different sources for the fine-structure splitting are discussed, and piezoelectricity is pinpointed as the only effect reproducing the observed trend.

Gain studies of (Cd, Zn)Se quantum islands in a ZnSe matrix

By inserting stacked sheets of nominally 0.7 monolayer CdSe into a ZnSe matrix we create a region with strong resonant excitonic absorption. This leads to an enhancement of the refractive index on the low-energy side of the absorption peak. Efficient waveguiding can thus be achieved without increasing the average refractive index of the active layer with respect to the cladding. Processed high-resolution transmission electron microscopy images show that the CdSe insertions form Cd-rich two-dimensional (Cd, Zn)Se islands with lateral sizes of about 5 nm. The islands act as quantum dots with a three-dimensional confinement for excitons. Zero-phonon gain is observed in the spectral range of excitonic and biexcitonic waveguiding. At high excitation densities excitonic gain is suppressed due to the population of the quantum dots with biexcitons.

Quantum dots: promises and accomplishments

Exploration of the Stranski-Krastanow growth of strained semiconductor hetero structures marked the major breakthrough for easy fabrication of defect-free quantum dots (QDs). For the first time, single QDs are facilitating the development of electrically operated emitters of single polarized or entangled photons on demand: an essential component for quantum communication systems. QDs inserted in quantum wells, stacked in planes upon each other, have led to semiconductor lasers that can operate at wavelengths that were previously impossible, or at least difficult to reach, for a given semiconductor family. These lasers show excellent temperature stability, large radiation resistance, and excellent temporal and spatial stability. In this review we discuss recent progress in the field of quantum dot devices.

Reversed truncated cone composition distribution of In0.8Ga0.2As quantum dots overgrown by an In0.1Ga0.9As layer in a GaAs matrix

We present cross-sectional scanning tunneling microscopy results of self-organized In0.8Ga0.2As quantum dots covered by an In0.1Ga0.9As film inside a GaAs matrix prepared by metalorganic chemical vapor deposition. From images of quantum dots with atomic resolution, we determine a spatial distribution of the In composition within the dots with a shape of a reversed truncated cone. The wetting layer and the overgrown In0.1Ga0.9As layer show vertical intermixing.

In situ monitoring of GaN metal-organic vapor phase epitaxy by spectroscopic ellipsometry

Epitaxy of high-quality GaN on sapphire requires a rather sophisticated substrate preparation prior to the GaN epilayer growth, namely nitridation of the substrate’s surface, growth of a GaN nucleation layer at a relative low temperature, and reduction of the defect density of this layer by a subsequent annealing step. For studying both, the detailed mechanisms of this complex procedure and its growth parameter dependencies, we attached an in situ spectroscopic ellipsometer to a nitride metal-organic vapor phase epitaxy reactor. First, the high-temperature dielectric function of GaN was measured using samples from different suppliers. Based on these data, the effect of growth parameter variations on the crystal quality of GaN epilayers could be monitored in situ. In particular, we determined the threshold temperature and the duration of the substrate nitridation under ammonia as well as the thermal threshold and duration of the nucleation layer transformation. Additionally, based on the in situ measuremen...

High-power semiconductor disk laser based on InAs∕GaAs submonolayer quantum dots

An optically pumped semiconductor disk laser using submonolayer quantum dots (SML QDs) as gain medium is demonstrated. High-power operation is achieved with stacked InAs∕GaAs SML QDs grown by metal-organic vapor-phase epitaxy. Each SML-QD layer is formed from tenfold alternate depositions of nominally 0.5 ML InAs and 2.3 ML GaAs. Resonant periodic gain from a 13-fold nonuniform stack design of SML QDs allows to produce 1.4W cw at 1034nm. The disk laser demonstrates the promising potential of SML-QD structures combining properties of QD and quantum-well gain media for high-power applications.

Alternative-precursor metalorganic chemical vapor deposition of self-organized InGaAs/GaAs quantum dots and quantum-dot lasers

Metalorganic chemical vapor deposition of laser diodes based on triple stacks of self-organized InxGa1−xAs/GaAs quantum dots (QDs) as active medium using the alternative precursor tertiarybutylarsine (TBAs) is reported. Epitaxy of monodispersed QDs using TBAs is demonstrated. Due to the high cracking efficiency of TBAs at low temperatures, the crucial growth parameters V/III ratio and temperature can be tuned almost independently. Ridge-waveguide QD lasers show a transparency current of 29.7 A/cm2—equivalent to 9.9 A/cm2 per QD layer—an internal quantum efficiency of 91.4%, and an internal optical loss of 2.2 cm−1.

Optical study of octahedrally and tetrahedrally coordinated MnSe

Abstract Octahedrally and tetrahedrally coordinated MnSe MBE-layers and polycrystalline thin films and octahedrally coordinated (Mn, Cd)Se (xMn ⩾ 0.8) bulk crystals are investigated by luminescence and excitation spectroscopy. The results are discussed in the framework of a ligand-field-model. Below the Neel temperature (TN) a significant shift of the Mn2+ d-d excitation bands to higher energies is found for the polycrystalline and bulk samples, which is caused by a spin-ordering- induced energy relaxation of the various Mn2+ states. No indication for an antiferromagnetic phase transition is found in the case of the quasi-2D-MBE-layers of MnSe.

Electrically driven single photon source based on a site-controlled quantum dot with self-aligned current injection

Electrical operation of single photon emitting devices employing site-controlled quantum dot (QD) growth is demonstrated. An oxide aperture acting as a buried stressor structure is forcing site-controlled QD growth, leading to both QD self-alignment with respect to the current path in vertical injection pin-diodes and narrow, jitter-free emission lines. Emissions from a neutral exciton, a neutral bi-exciton, and a charged exciton are unambiguously identified. Polarization-dependent measurements yield an exciton fine-structure splitting of (84 ± 2) μeV at photon energies of 1.28–1.29 eV. Single-photon emission is proven by Hanbury Brown and Twiss experiments yielding an anti-bunching value of g(2)(0) = 0.05 under direct current injection.

Vertical-cavity surface-emitting quantum-dot laser with low threshold current grown by metal-organic vapor phase epitaxy

Ground state lasing of electrically driven vertical-cavity surface-emitting lasers with a quantum-dot (QD) gain medium grown using metal-organic vapor phase epitaxy was realized. The devices use stacked InGaAs QD layers, placed in the field intensity antinodes of the cavity formed by selectively oxidized distributed Bragg reflectors. Devices with 3×3 QD layers demonstrate at 20°C a cw output power of 1.45mW at 1.1μm emission wavelength. The peak external efficiency was 45%, limited by lateral carrier spreading within the 4λ cavity and a reduction of the internal efficiency above 60°C. A minimum threshold current of 85μA was obtained from a device with a 1μm aperture.