Raman scattering from confined phonons in GaAs/AlGaAs quantum wires

We report on photoluminescence and Raman scattering performed at low temperature (T = 10 K) on GaAs/Al 0.3Ga 0.7As quantum-well wires with effective wire widths of L = 100.0 and 10.9 nm prepared by molecular beam epitaxial growth followed by holographic patterning, reactive ion etching, and anodic thinning. We find evidence for the existence of longitudinal optical phonon modes confined to the GaAs quantum wire. The observed frequency at ω L10 = 285.6 cm -1 for L = 11.0 nm is in good agreement with that calculated on the basis of the dispersive dielectric continuum theory of Enderlein† as applied to the GaAs/Al 0.3Ga 0.7As system. Our results indicate the high crystalline quality of the quantum-well wires fabricated using these techniques. © 1998 Academic Press

High-efficient up-conversion of photoluminescence in CdSe quantum dots grown in ZnSe matrix

The intensive up-conversion photoluminescence at low temperatures in CdSe/ZnSe structures with single CdSe inserts of the nominal thickness 1.5 monolayer was observed. Excitation power dependensies show a nearly quadratic character up-converted photoluminescence signal from CdSe quantum dots. Up-conversion photoluminescence mechanism was interpreted on the basis of a non-linear process of two-step two-photon absorption through deep defect states including cation vacancies localized at the interface of quantum dots. This observation of photoluminescence up-conversion demonstrates the influence of quantum dot environment on its properties

Polarized micro-photoluminescence spectroscopy of GaN nanocolumns

International audienceWe propose inversion domains (IDs) to be the origin of the 3.42 eV photoluminescence (PL) band in GaN epilayers and nanocolumns. A shift of the band relatively to the near‐edge PL band is induced presumably by different strain in the IDs. Micro‐PL studies of nanocolumns enriched by IDs reveal anti‐correlated intensity variation as well as a similarity between temperature and power dependences of both bands. A change of dominant polarization takes place across the spectra, being likely related to variation of exciton level ordering at tensile strain. Discrete narrow lines observed in the spectra are considered as manifestation of strain‐induced one‐dimensional carrier confinement in the ID

Dynamics of doubly resonant Raman scattering and resonant luminiscence in ultrathin InAs/GaAs quantum wells

We have studied the transient behavior of a sharp emission line, which emerges close to the heavy-hole (hh) exciton photoluminescence line in a 1.6-monolayer-thick InAs layer embedded in a GaAs matrix. The sharp emission line appears under resonant excitation of the light-hole (Ih) exciton transition, whose energy separation from the hh exciton transition amounts to one GaAs LO phonon. We have measured the decay time of the sharp emission line in addition to the lifetime of the hh and the Ih excitons, using time-correlated single-photon counting and two-wavelength picosecond time-resolved pump–probe transmission techniques. We find that the decay time of the sharp emission line is equal to the lh exciton lifetime for excitation on resonance but that it strongly decreases for excitation off resonance. We conclude that, whereas the sharp line originates from doubly resonant Raman scattering for off-resonant excitation, it is predominantly due to resonant luminescence for resonant excitation. We derive a lh exciton dephasing time of 40 ps, which suggests that the length scale of potential fluctuations from disorder is larger than the lh exciton Bohr radius

High resolution x-ray diffraction and reflectivity studies of vertical and lateral ordering in multiple self-organized InGaAs quantum dots

We have investigated multiple layers of self-assembled InGaAs quantum dots by high-resolution X-ray diffraction reciprocal space mapping and reflectivity. An anisotropy in the dot spacing was found, which proves an ordering of the islands in a disordered two-dimensional square lattice with main axes parallel to the direction and a lateral lattice parameter of 55 nm. Vertical correlations in the dot multilayers were investigated nondestructively in the regime of total external reflection of X-rays

Structural characterization of self-assembled quantum dot structures by x-ray diffraction techniques

We have investigated, by means of X-ray diffraction reciprocal space mapping and X-ray reflectivity, multilayers of self-organized InGaAs quantum dots grown on GaAs by MBE. An anisotropy of the average inter-dot spacings in the [100]and [110]directions was found, consistent with an ordering of the dots in a two-dimensional square lattice with main axes along the -directions and a lattice parameter of 55 nm. The nearly perfect vertical alignment (stacking) of the dots was deduced consistently from the diffraction peak shape and from measurements of the resonant diffuse scattering in the X-ray reflection regime

Spectral and dynamic properties of InAs-GaAs self-organized quantum-dot lasers

The spectral and dynamic properties of InAs-GaAs MOCVD-grown vertically stacked self-organized quantum-dot lasers are studied experimentally. A strong mode grouping effect (quasi-periodic modulation of the lasing spectrum) is observed and interpreted as a result of wavelength-dependent losses in the laser waveguide associated with substrate leakage and reflection. Some samples also display a broader spectral modulation which may be attributed to lasing from different groups of dots, or energy levels. Experimental observations are in agreement with a theoretical explanation involving increased optical nonlinearities due to the localized nature of carriers. In relaxation oscillation pulse trains, a substructure is observed which we believe to be a dynamic manifestation of the same carrier localization effects; a preliminary rate-equation simulation supports this interpretation