V. Türck

Low-pressure metal organic chemical vapor deposition of GaN on silicon(111) substrates using an AlAs nucleation layer

We have investigated the growth of GaN on silicon by low-pressure metal organic chemical vapor deposition. Good quality GaN layers are grown on silicon(111) using an AlAs nucleation layer. AlAs is thermally stable even at 1050 °C and, unlike GaN and AlN buffer layers, the formation of SiNx on the Si surface is prevented. Single crystalline GaN films are obtained by introducing a thin low-temperature GaN buffer layer grown on the AlAs nucleation layer. The GaN layers are characterized by x-ray diffraction, atomic force microscopy, secondary ion mass spectroscopy, photoluminescence, and cathodoluminescence.

InGaAs quantum wires grown by low pressure metalorganic chemical vapor deposition on InP V‐grooves

Single InGaAs quantum wires were fabricated by low pressuremetal organic chemical vapor deposition on V‐grooved InP substrates. For substrate patterning a new wet chemical etching process that leads to high quality V‐grooves with {111}A facets was used. The growth parameters of the InP buffer layer have a strong impact on the quantum wire formation. Scanning electron microscopy,photoluminescence, and spatially resolved cathodoluminescence experiments have been performed to characterize the structures. The crescent shaped InGaAs quantum wires have dimensions of about 13 nm height and 100 nm width. The wire luminescence is found to be at λ=1575 nm (FWHM=17 meV).

High-gain excitonic lasing from a single InAs monolayer in bulk GaAs

We report the observation of highly efficient laser emission from a single InAs layer with an effective thickness of 1.5 monolayers (ML) embedded in bulklike GaAs. Lasing action is obtained at the wavelength of the InAs thin-layer luminescence (870 nm) by cw optical pumping with a threshold power density of 0.9(3) kW/cm2 at 10 K. Gain measurements yield a very high material gain of 1.0(5)×104 cm−1 for the InAs layer when pumped with ∼10 kW/cm2 at low temperatures. The 0 dimensional character of the emission as determined from cathodoluminescence and the absence of band-gap renormalization with increasing pump level speak for an excitonic mechanism of population inversion.

LATERALLY STRUCTURED ZNCDSE/ZNSE SUPERLATTICES BY DIFFUSION INDUCED DISORDERING

Thermally induced disordering of ZnCdSe/ZnSe superlattices was studied by secondary‐ion‐mass‐spectroscopy and cathodoluminescence. The structures were grown by molecular beam epitaxy and annealed in Zn, H2, or Se atmosphere. In Zn atmosphere the superlattice was found to be stable up to 500 °C, whereas a heat treatment in H2 and Se atmosphere leads to a complete intermixing at temperatures of 450 and 430 °C, respectively. A Si3N4 stripe was used to protect the superlattice locally from the annealing atmosphere and to achieve a laterally structured intermixing. The disordering area is controlled to better than 600 nm.

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.

LP-MOCVD growth of GaN on silicon substrates: Comparison between AlAs and ZnO nucleation layers

Abstract The impact of AlAs and ZnO nucleation layers on the low-pressure MOCVD growth of GaN on silicon substrates is the subject of this study. In both cases, prior to the high-temperature deposition of the GaN main layer a thin low-temperature GaN buffer layer has been found to improve the crystal quality. Especially in the case of a ZnO nucleation layer, no single crystalline growth of GaN was obtained without a low-temperature GaN buffer layer. Furthermore, the crystallographic quality of the GaN layer is strongly dependent on the strength of the (0001) texture of the ZnO grains. The ZnO (0001)-texture is independent of the Si(111) or Si(001) orientation and allows for single crystalline growth of GaN on both types of Si surfaces. TEM investigations show, that the ZnO nucleation layer has completely vanished after the epitaxial process. In contrast, AlAs nucleation layers remain stable as proven by X-ray diffraction and TEM measurements.

Few-Particle Effects in Self-Organized Quantum Dots

Few-particle effects are investigated both experimentally and theoretically for self-organized quantum dots (QDs). The actual confinement potential, reflecting the structure-dependent low-symmetry and inhomogeneous strain, is demonstrated to have a strong impact on the few-particle states. Eight-band k·p calculations for the InAs/GaAs model system indicate both binding and anti-binding biexciton states depending on the separation of the electron and hole wave functions. Single-dot experiments on CdSe/ZnSSe QDs enable the identification of the neutral and charged exciton and biexciton states and demonstrate a large variation of the corresponding binding energies attributed to the variation of the structural properties of the probed QDs. First eight-band k·p model calculations for epitaxial CdSe QDs demonstrate a pronounced influence of the Cd-concentration. Finally, lateral energy transfer processes originating at weakly localizing Cd-fluctuations are identified in time-resolved experiments. The unique properties of three-dimensionally confined excitons in semiconductor quantum dots (QDs) have been studied intensively in recent years, see e.g. [1] and references therein. The last three years saw an increasing interest in the physics of multi-exciton and charged-exciton states in selforganized QDs [2,3,4,5,6,7]. Such multi-particle states are of interest both for basic physics and device applications. Understanding the properties of manyparticle states improves insight into the complex interplay of the Coulomb interaction and the external confinement. At the same time the detailed knowledge of many-particle effects is essential for the development of semiconductor devices operating at high excitation densities like, e. g., lasers. Single-QD spectroscopy has proven to be a powerful tool in the study of few-particle effects, see e.g. [8] and references therein. It allows to investigate emission spectra without the obstructing influence of inhomogeneous broadening resulting from statistical variations of the structural properties. In spite of the enormous progress in the single-QD spectroscopy in recent years, which has been stimulated by defect-free self-organized QDs allowing to investigate the intrinsic optical properties, the interpretation of experimental data as well as theoretical predictions of few-particle states generally neglect the intricate structural properties of real mesoscopic QD structures. In particular interband transitions, involving both electron and hole states, are sensitive to the structural properties of self-organized QDs leading to asymmetric electron and hole wave functions [9]. The related effects of the B. Kramer (Ed.): Adv. in Solid State Phys. 41, 39–50 (2001) c

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.

Correlation of InGaAs/GaAs quantum dot and wetting layer formation

Abstract The formation and correlation of the wetting layer (WL) and quantum dots (QD) is investigated for the Stranski-Krastanow growth of InGaAs on GaAs in metalorganic chemical vapour deposition. For nominally 1.8 nm In 0.4 Ga 0.6 As deposition QDs coexist with a complex structured WL. Temperature dependent photoluminescence (PL) and PL excitation (PLE) results show the WL thickness to vary locally between 5 and 9 monolayers, with thermal activation being necessary for carrier transfer from the thicker WL regions into the QDs. We find an (anti) correlation between QD size and WL thickness from which the most efficient capture into the QDs results, i.e. larger QDs capture preferentially from thinner WL regions. The existence of WL growth islands which did not transform into QDs and the anti-correlation of QD size and WL thickness hint at an incomplete InGaAs QD formation for the investigated deposition despite a growth interruption of three minutes after deposition of the InGaAs layer.

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.