V. G. Talalaev

Pseudomorphic GeSn/Ge(001) quantum wells: Examining indirect band gap bowing

A study of the bandgap character of compressively strained GeSn0.060-0.091/Ge(001) quantum wells grown by molecular beam epitaxy is reported. The built-in strain in GeSn wells leads to an increased separation between L and Γ conduction band minima. The prevalent indirect interband transitions in GeSn were probed by photoluminescence spectroscopy. As a result we could simulate the L-valley bowing parameter in GeSn alloys, bL = 0.80 ± 0.06 eV at 10 K. From this we conclude that even compressively strained GeSn/Ge(001) alloys could become direct band gap semiconductors at the Sn-fraction higher than 17.0 at. %.

Miniband-related 1.4–1.8 μm luminescence of Ge/Si quantum dot superlattices

The luminescence properties of highly strained, Sb-doped Ge/Si multi-layer heterostructures with incorporated Ge quantum dots (QDs) are studied. Calculations of the electronic band structure and luminescence measurements prove the existence of an electron miniband within the columns of the QDs. Miniband formation results in a conversion of the indirect to a quasi-direct excitons takes place. The optical transitions between electron states within the miniband and hole states within QDs are responsible for an intense luminescence in the 1.4–1.8 µm range, which is maintained up to room temperature. At 300 K, a light emitting diode based on such Ge/Si QD superlattices demonstrates an external quantum efficiency of 0.04% at a wavelength of 1.55 µm.

Transient carrier transfer in tunnel injection structures

InGaAs tunnel injection nanostructures consisting of a single quantum well as injector and a quantum dot layer as emitter are studied by time-resolved photoluminescence spectroscopy. The quantum dot photoluminescence undergoes substantial changes when proceeding from direct quantum dot excitation to quantum well excitation, which causes an indirect population of the dot ground states. This results in a lowered effective carrier temperature within the dots. Results on the carrier transfer versus barrier thickness are discussed within the Wentzel–Kramers–Brillouin approximation. Deviations for barrier thicknesses <5nm are assigned to the formation of nanobridges that are actually detected by transmission electron microscopy.

Surface recombination and facet heating in high-power diode lasers

Surface recombination velocities and surface temperatures at front facets of standard broad-area lasers emitting at 808nm were investigated by time-resolved two-color photoluminescence and micro-Raman spectroscopy. Surface recombination velocities in the range between <105 and 106cm∕s are determined for devices with tailored surface properties. The results clearly show that increased surface recombination velocities are accompanied by increased facet temperatures. Reabsorption of light generated in the diode lasers leads to an additional enhancement of facet heating for surfaces of minor structural quality. The methodological approach presented here paves the way for improved analytical access to diode laser facet properties.

Optimized annealing conditions identified by analysis of radiative recombination in dilute Ga(As,N)

Rapid thermal annealing parameters for Ga(As,N) with 0.5% N are experimentally analyzed. The criteria taken into account are the relative intensity of the observed deep level photoluminescence, the intensity of edge luminescence contributions, and the luminescence decay time. For a 60 s treatment, we find an optimum annealing temperature of 850 °C with a clear tendency of lower optimum annealing temperatures for increasing N contents. Luminescence decay times of up to 400 ps for recombination of delocalized carrier pairs match well the lifetime region specific for direct III–V semiconductors and provide clear evidence of the type-I band alignment in the Ga(As,N) samples investigated.

Transient spectroscopy of InAs quantum dot molecules

Coupled pairs of InAs quantum dots are grown by molecular-beam epitaxy. Structural and optical characterization is done by means of transmission electron microscopy and photoluminescence, respectively. Photoluminescence spectra consist at least of three well-separated optical transitions that are assigned to molecular energy terms and a substantial exciton lifetime increase is observed. Detailed spectral analysis of the transient luminescence behavior indicates “intraterm” transitions that could be favorably used for the creation of midinfrared light sources.

Optically pumped semiconductor disk laser with graded and step indices

Summary form only given. This paper reports continuous-wave (cw) and passive mode-locked laser operation of diode-pumped semiconductor disk lasers based on gain sections with step and graded index designs, each containing 6 InGaAs quantum wells. Mode-locked laser operation is achieved applying both structures, using a V-shaped cavity including a fast saturable semiconductor absorber mirror. Results show that the graded index structure exhibits clear advantages over the step index structure in cw-laser operation with respect to the saturation of the absorption. However, for mode-locked operation, shorter pulses are generated using the step index structure.

Si/Ge Nanostructures for Optoelectronics Applications

The optical and structural properties of multilayer Si/Ge structures with precritical, as well as close-to-critical, germanium inclusions in a silicon matrix, for which the transition from the two-dimensional to island growth occurs, were studied. The possibility of obtaining intense photoluminescence at room temperature in both cases under optimally chosen growth parameters is demonstrated. The proposed approaches to producing an active region appear promising for applications in silicon-based optoelectronics.

Composite system based on CdSe/ZnS quantum dots and GaAs nanowires

The possibility of fabricating a composite system based on colloidal CdSe/ZnS quantum dots and GaAs nanowires is demonstrated and the structural and emission properties of this system are investigated by electron microscopy and photoluminescence spectroscopy techniques. The good wettability and developed surface of the nanowire array lead to an increase in the surface density of quantum dots and, as a consequence, in the luminosity of the system in the 600-nm wavelength region. The photoluminescence spectrum of the quantum dots exhibits good temperature stability in the entire range 10–295 K. The impact of surface states on energy relaxation and the role of exciton states in radiative recombination in the quantum dots are discussed.

Recombination emission from InAs quantum dots grown on vicinal GaAs surfaces

Photoluminescence (PL) spectra of InAs/GaAs heteroepitaxial structures with quantum dots (QDs) have been studied. The structures were grown by submonolayer migration-enhanced epitaxy on vicinal substrates with the amount of deposited InAs close to the critical value of 1.8 monolayer (ML). The origin and evolution of the structure of PL spectra were studied in relation to the direction and angle of misorientation, temperature, and power density and spectrum of the exciting radiation. A blue shift and narrowing of the PL band with increasing misorientation angle was established experimentally. The fact that QDs become smaller and more uniform in size is explained in terms of a lateral confinement of QDs on terraces with account taken of the step bunching effect. The temperature dependences of the positions and full widths at half-maximum (FWHM) of PL bands are fundamentally different for isolated and associated QDs. The exciton ground states contribute to all low-temperature spectral components. The excited exciton state contributes to the recombination emission from QDs, as evidenced by the temperature dependence of the integrated intensity of the PL bands. A quantitative estimate is given of the electronic structure of different families of InAs QDs grown on GaAs substrates misoriented by 7° in the [001] direction.