R. Engelhardt

Room-temperature lasing of strain-compensated CdSe/ZnSSe quantum island laser structures

Efficient resonant excitonic waveguiding is achieved in laser structures, grown by metallorganic chemical vapor deposition, with stacked CdSe quantum islands which were separated by ternary ZnSSe barriers. Plastic relaxation within the stack is shown to be suppressed by adjusting the sulfur content in the barriers to compensate the strain. Excitonic lasing with low threshold intensities is demonstrated well above room temperature with Ith77 K=0.8 kW/cm2 and Ith300 K=55 kW/cm2.

MOCVD of vertically stacked CdSe/ZnSSe quantum islands

Stacks of nominally one monolayer thick CdSe sheets, separated by ZnSSe barriers, were grown by metallorganic chemical vapor deposition. Cadmium interdiffusion and interface roughening was minimized at a VI/II ratio close to stoichiometry. CdSe quantum islands formed in the stacked sheets show a strong electronic coupling for barrier thicknesses below 50 A. The excitonic luminescence of coupled islands is red-shifted with respect to the emission of uncoupled islands. Evidence for rather weak vertical, structural correlation of island coupling is found. Plastic relaxation of larger stacks can be suppressed by strain-compensating barriers. Such stacked CdSe/ZnSSe structures are particularly interesting for lateral excitonic waveguide structures.

Line broadening and localization mechanisms in CdSe/ZnSe quantum dots

Abstract We analyze the line-broadening mechanisms of single CdSe quantum dot (QD) emission lines. A jitter in the emission energy of individual CdSe QDs is reported for the first time. The jitter is caused by the quantum-confined Stark effect induced by the randomly fluctuating charges of defects in the vicinity of the QDs. These random processes lead to a broadening of the emission lines and usually inhibit the determination of a true homogeneous line width. On the other hand, identical jitter allows the unambiguous assignment of groups of emission lines to single QDs. A strong thermal broadening of the QD emission lines is observed. From our observations, parameters of the phase relaxation due to acoustic and LO phonon scattering, which is the main line broadening mechanism, are derived.

Quantum dots formed by ultrathin insertions in wide-gap matrices

We report on experimental and theoretical studies on a new type of quantum-dot (QD) structures obtained using ultrathin, i.e. below the critical thickness for 2D‐3D transition, strained narrow gap insertions in wide bandgap matrices. We concentrate on submonolayer (SML) or slightly above 1 ML CdSe insertions in a wide-gap II‐VI matrices and give the first results on ultrathin InGaN insertions in a GaN matrix. A discussion on detailed comparison of our original results with the results of other authors is presented. The formation of dense arrays (up to 10 12 cm 22 ) of nanoscale two-dimensional (2D) islands is revealed in processed high-resolution transmission electron microscopy images. In the case of stacked sheets of SML insertions, the islands in the neighboring sheets are formed predominantly in correlated or anticorrelated way for thinner and thicker spacer layers, respectively. Different polarization of photoluminescence (PL) emission recorded in edge geometry for vertically-uncoupled and coupled QDs confirms the QD nature of excitons. By monitoring of sharp lines due to single QDs using cathodoluminescence the 3D confinement is manifested. We demonstrate significant squeezing of the QD exciton wavefunction in the lateral direction using magneto-optical experiments. We point to complete suppression of lateral motion of excitons bound to islands in case of wide-gap (ZnMgSSe) matrices, as follows from PL excitation studies. A resonant (0-phonon) lasing is observed in ultrathin CdSe insertions and proves the lifting of the k-selection rule for QD excitons. We show that lack of exciton screening in QDs up to high excitation densities enables strong resonant modulation of the refractive index in stacked ultrathin insertions and allows realization of resonant (excitonic) waveguiding and lasing. This enables the realization of a new type of heterostructure laser operating without external optical confinement by layers having lower average refractive indices. Ultrahigh QD excitonic gain in dense arrays of stacked QDs allows a new type of surface-emitting laser. q 2000 Elsevier Science S.A. All rights reserved.

Three-dimensionally confined excitons in MOCVD-grown ultrathin CdSe depositions in ZnSSe matrix

Up to 3 monolayer (ML) thick highly strained CdSe insertions in lattice matched ZnSSe/GaAs were grown by metalorganic chemical vapor deposition (MOCVD). The samples show a bright photoluminescence band near the barrier band edge exhibiting a strong red shift with increasing CdSe deposition. The assignment to recombinations of three-dimensionally confined excitons is confirmed by cathodoluminescence. An additional-low energy band appearing for thicker CdSe depositions is attributed to defects.

High quantum efficiency InP mesas grown by hybrid epitaxy on Si substrates

Selective area liquid phase epitaxy of InP mesas has been performed on masked Si(001) substrates previously overgrown with InP by metalorganic chemical vapour deposition (MOCVD). The resulting mesas are bordered by very smooth crystal facets which are free of defects over typically 300 μm and exhibit an etch pit density of ≤1 × 104 cm−2. The X-ray rocking curve half width of the mesas amounts to 144 arcsec and the quantum efficiency is increased by two orders of magnitude as compared to the underlying InP buffer layer. Thermally induced strain is observed by triple X-ray analysis to relax anisotropically in perfect agreement with finite element analysis. These results are encouraging to use the selectively grown mesas as buffer layers for device structures to be grown in a subsequent MOCVD process step.

Negative exciton mass in ZnCdSe/ZnSSe superlattices observed by excitonic polariton Raman scattering

Resonant Raman scattering of excitonic polaritons at LO phonons in ZnCdSe/ZnSSe superlattices at room temperature reveals strong spatial dispersion effects near the transverse excitonic resonance frequency . For incident photon energies below , the LO phonon anti-Stokes frequency shifts exceed the Stokes shifts, while the inverse relation holds above . All experimental results can be well described in the excitonic polariton formalism in terms of exciton branches with positive and negative excitonic masses above and below , respectively, and an additional damping parameter.

Efficient lateral index guiding of II–VI laser structures by implantation-induced disordering

Abstract Implantation-induced disordering in ZnCdSe-based quantum wells and superlattices is investigated and used to generate lateral index-guided laser structures. The structures are implanted with Nitrogen at doses of 1 × 10 15 4 × 10 15 cm −2 at energies ranging between 110 and 230 keV and subsequently annealed at temperatures up to 460°C in Zn-atmosphere. Distinct intermixing is identified by secondary-ion-mass-spectroscopy. Lateral selective implantation is performed by using stripe masks 5–30 μm wide. Lateral selectivity is proven by cathodoluminescence. Waveguiding of selectively implanted superlattices and laser structures is demonstrated by recording the near-field pattern. The implanted structures exhibit index guiding with and without subsequent annealing, demonstrating that index guiding is already achieved by the weak intermixing induced by implantation.