F. Henneberger

Visible band-gap ZnCdO heterostructures grown by molecular beam epitaxy

Single-phase ZnCdO alloys with a band gap extending from the violet to yellow spectral range are fabricated by molecular beam epitaxy using extremely low growth temperatures in conjunction with O-rich growth conditions. The Cd concentration can be systematically adjusted via the Cd∕Zn beam pressure ratio. Despite growth temperatures as low as 150°C, layer-by-layer growth is accomplished allowing for the preparation of ZnCdO∕ZnO quantum well structures. Both epilayers and quantum wells exhibit strong band-gap-related emission at room temperature in the whole composition range.

Growth of high-quality ZnMgO epilayers and ZnO∕ZnMgO quantum well structures by radical-source molecular-beam epitaxy on sapphire

We report on a specific growth procedure combining low-temperature growth of ZnMgO and postgrowth annealing at intermediate temperatures. Despite the large lattice misfit induced by the sapphire substrate, layer-by-layer growth is accomplished up to the phase-separation limit found at a c-lattice constant of 0.5136nm and Mg mole fraction of 0.40. The procedure allows us to grow quantum wells with atomically smooth interfaces in a wide range of structural designs exhibiting prominent emission features up to room temperature.

Self-assembled CdSe quantum dots Formation by thermally activated surface reorganization

Abstract The formation of self-assembled CdSe quantum dots on ZnSe by a reorganization process occurring during growth interruption and thermal activation is demonstrated. Optical data yield a dot size in the 3–5 nm range and a density of 10 10 –10 11 cm −2 . Our measurements reveal a complex coupling with lattice vibrations.

Transmission electron microscopy investigation of structural properties of self-assembled CdSe/ZnSe quantum dots

CdSe quantum dots on ZnSe, grown by molecular beam epitaxy and formed during reorganization of an initially uniform film by thermal activation, are microstructurally elucidated in cross section and plan view, using transmission electron microscopy. In diffraction contrast, an almost uniform wetting layer is clearly visible. Dark contrast features with a distinctly larger extension into growth direction mark the location of quantum dots. Individual quantum dots can be identified in high-resolution imaging both by lattice expansion and contrasts arising from their strain fields. Plan-view images show the coexistence of two classes of quantum dots with an average lateral size of ⩽10 nm (area density 100 μm−2) and 10–50 nm (20 μm−2), respectively. The shape of the larger entities is pyramidlike.

In-plane polarization anisotropy of the spontaneous emission of M-plane GaN/(Al,Ga)N quantum wells

We study the in-plane polarization of wurtzite GaN/(Al, Ga)N multiple quantum wells. Identical M-plane (1100) and C-plane (0001) structures are grown by plasma-assisted molecular-beam epitaxy on γ-LiAlO2(100) and 6H–SiC(0001), respectively. While the emission from the conventional [0001] oriented wells is isotropic within the growth plane, we observe a strong polarization anisotropy of over 90% for the M-plane sample. The luminescence is polarized normal to [0001] and shows no spectral shift with polarization angle, i.e., it originates solely from A excitons (px and py valence band states). The deviation of the polarization degree from unity is attributed to the mixing with pz valence band states due to quantum confinement.

Electronic excitations and longitudinal optical phonon modes of self-assembled CdSe quantum dots revealed by microprobe studies

The electronic states of self-assembled CdSe quantum dots and the longitudinal optical phonon mediated energy relaxation among these states are investigated. Measurements with microprobe resolution demonstrate that each quantum dot develops specific phonon modes. There is no evidence for a well-defined phonon bottleneck in the intra-quantum-dot relaxation. We observe single quantum-dot bi-exciton emission and bi-exciton binding energies exceeding 20 meV.

Luminescence related processes in semiconductor nanocrystals —The strong confinement regime

Abstract This paper summarizes systematic studies on the luminescence from CdSe nanocrystals in the situation of strong zero-dimensional confinement. We discuss the role of surface and defect states, the origin of the photo-darkening and demonstrate by calorimetric adsorption measurements that the internal quantum efficiency is in the range of some 10%. The lifetimes of the intrinsic excitations are strongly size-dependent. While the radiative recombination (≈ 1 ns) is dominant for larger nanocrystals, we observe a dramatic increase of the non-radiative rate below 4 nm as a result of the reduced volume to surface ratio. The combination of ultra-fast and size-selective excitation allows us to observe a progression of sharp LO-phonon lines signifying the zero-dimensional character of the nanocrystals and providing information on the strength of the phonon coupling and the homogeneous width of the quantum-confined ground state.

Binding energy of charged excitons in ZnSe-based quantum wells

Excitons and charged excitons (trions) are investigated in ZnSe-based quantum well structures with (Zn,Be,Mg)Se and (Zn,Mg)(S,Se) barriers by means of magneto-optical spectroscopy. Binding energies of negatively

Fine structure of the trion triplet state in a single self-assembled semiconductor quantum dot

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Efficient light emission from inorganic and organic semiconductor hybrid structures by energy-level tuning

and positively