M. Straßburg

Growth of Ru doped semi-insulating InP by low pressure metalorganic chemical vapor deposition

Abstract Ruthenium doping has been successfully applied for the growth of semi-insulating (s.i.) InP. In contrast to the 3d-transition metal (TM) Fe, the isovalent 4d-TM Ru exhibits a four orders of magnitude lower diffusion coefficient and shows no interdiffusion with p-type dopants. Most important, Ru compensates electrons as well as holes, a prerequisite for complete compensation under double injection conditions. The growth of Ru doped InP layers has been investigated using bis(η5-2,4-dimethylpentadienyl)ruthenium(II) for Ru doping, different P-precursors (PH3, TBP, DTBP) and H2 or N2 carrier gas.

Diffusion of Cd, Mg and S in ZnSe-based quantum well structures

Diffusion of cadmium, magnesium and sulfur in ZnSe-based quantum well structures is studied by thermal treatment under different ambient atmospheres and depth selective ion implantation. Secondary ion mass spectroscopy analysis and X-ray diffraction demonstrate that metal components diffuse via group II lattice sites already at temperatures below 500°C. Both, p-type doping and the generation of group II vacancies induce a strong enhancement of diffusion. The more stable group VI element sulfur diffuses by a kick-out mechanism. Diffusion coefficients for different annealing and doping conditions are determined and the consequences for device design are discussed.

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.

Raman study of nitrogen-doped ZnSSe/GaAs epilayers

b ¨¨ ¨ Abstract N-doped ZnSSeyGaAs epilayers were grown by metalorganicvapor phase epitaxy, using monomer (tBu) Se or dimer Me Se 22 2 precursors. Structural characteristics of the epilayers were obtained by analyzing the X-ray diffraction patterns and the Raman frequencies of the ZnSe- and ZnS-like, LO and TO modes. Thereby, inferior structural properties were found for layers grown using the dimer precursor. Raman coupled plasmon-LO-phonon modes were observed from the GaAs interface area due to photo- excited carriers and were analyzed in terms of band bending effects. 2002 Elsevier Science B.V. All rights reserved. PACS: R 403; C 68; H 187; S 429

Optical identification of quantum dot types in CdSe/ZnSe structures

Two different types of quantum islands, formed either by strain-modified island growth or in the Stranski-Krastanow (SK) mode, were identified by optical and structural investigations. Pronounced differences in the size, shape and Cd content of these islands lead to different localisation energies of three-dimensionally confined excitons allowing a clear distinction of both island types. The classification was confirmed by plan-view and cross-sectional transmission electron microscopy.

Interplay of surface charges and excitons localized in CdSe/ZnSe quantum dots

Abstract Spectral diffusion is an effect often observed in epitaxially grown II–VI quantum dots. It is shown that in such structures spectral diffusion is caused by electric fields generated from charges on the sample surface. The interaction mechanism is a linear quantum-confined Stark effect revealing an electric dipole moment of the zero-dimensional multi-exciton states in the quantum dot. The dipole length of all the observed multi-exciton states is almost the same suggesting a negligible correlation of the electron and hole wave functions caused by a strong vertical confinement.

Doping-dependent Mg diffusion in ZnMgSSe/ZnSSe-structures

Diffusion of magnesium in ZnMgSSe/ZnSSe-structures is investigated. The superlattices structures is partly implanted and annealed. Annealing atmosphere, time and temperature are varied. Secondary-ion mass spectroscopy investigations demonstrate that magnesium diffuses via group II lattice sites. Diffusion enhancement results for p-type doping as well as additional group II vacancy generation. Diffusion coefficients for the different annealing and doping conditions are determined.

Epitaxy and structural characterization of ZnSeTe layers grown on InP substrates

ZnSeTe epilayers were grown on InP substrates with and without ZnCdSe buffer layers. Structural X-ray and Raman characterization results indicate a large range of Se composition in the epilayers. The introduction of a buffer layer and the control of the growth conditions improve the structure by limiting compositional variations close to the stoichiometric value where lattice matching conditions are met. Effects of plastic strain relaxation are thereby minimized.

CdSe/ZnSSe quantum islands grown by MOVPE on homoepitaxial GaAs buffers

Abstract Single and stacked sheets of CdSe quantum islands, embedded in a ZnSSe matrix, were grown by metalorganic vapor-phase epitaxy on GaAs homoepitaxial buffers to induce a narrow size distribution of the quantum island ensemble. The insertion of a GaAs buffer leads to a distinctly smoother II–VI/III–V interface. The full-width at half-maximum of the CdSe-related localized excitonic emission is found to be reduced for structures grown on a GaAs buffer proving the success of this approach. GaAs buffers grown in the same reactor prior to II–VI epitaxy lead to slightly better results than buffers grown in a separate reactor.

Gain to absorption conversion by increasing excitation density in excitonic waveguides

Exciton waveguiding was proposed to improve optical confinement of II—VI lasers. Strong resonant exciton absorption induces a refractive index enhancement (*n) on the low-energy side of the absorption peak. By stacking sheets of submonolayer (SML) CdSe insertions into a ZnSe matrix, we fabricated a region in ZnSe with strong resonant exciton absorption. In this region, the k-selection rule is lifted due to exciton localization by the SML islands. Therefore, a zero-phonon lasing mechanism resonant to the exciton waveguiding region could be realized. Using the variablestripe-length method, we study the intensity-dependent optical gain of the ZnSe/CdSe submonolayer superlattices in a ZnS 0.06 Se 0.94 matrix. ( 1998 Elsevier Science B.V. All rights reserved.