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The influence of the growth conditions during capping of CdSe/ZnSe quantum structures grown on GaAs(001) by molecular-beam epitaxy (MBE) were systematically investigated by high-resolution x-ray diffraction, transmission electron microscopy, and temperature dependent, partly time-resolved photoluminescence spectroscopy. The results clearly indicate formation of quantum wells with potential fluctuations if conventional MBE is used for capping the CdSe by ZnSe. In contrast, quantum dot formation occurs using migration enhanced epitaxy for this growth step. In the latter case, quantum dots can be obtained without formation of stacking faults.

The growth and vertical organization of CdSe quantum dots in three-layer stacks consisting of CdSe with a nominal thickness of 2.5 monolayers (ML) and ZnSe spacers with thicknesses between 10 and 20 ML was investigated by reflection high energy electron diffraction during the growth and different transmission electron microscopy techniques. The samples were grown by molecular beam epitaxy at 400 °C. It was found that up to 10 ML spacer thickness all three CdSe layers and ZnSe spacers form one broad (Cd, Zn)Se alloy layer with a small Cd concentration containing Cd-rich islands with a size of ∼15 nm. For spacers with a larger thickness (12–20 ML) three separated ternary (Cd, Zn)Se layers are observed which contain Cd-rich inclusions (small islands) with a size of less than 10 nm. A preferential vertical correlation of the small islands occurs for the 12 ML spacer thickness. With increasing spacer thickness, the number of the correlated small islands is reduced displaying a tendency to uncorrelated growth.

The origin of the “coffee-bean” strain contrast is studied, that is observed in the plan-view transmission electron microscopy (TEM) images of CdSe/ZnSe quantum dot structures. The samples were grown by two different methods: standard molecular-beam epitaxy at 350 °C and atomic layer epitaxy at 230 °C with annealing at 340 °C after the CdSe deposition. The nominal CdSe thickness was above 3 ML. In situ reflection high energy electron diffraction during the growth or during the annealing shows the transition from the two- (2D) into the three-dimensional (3D) surface morphology for both samples. The coffee-bean contrast is usually assigned to three-dimensional islands which are generated after the morphological 2D/3D transition. It is found that the coffee-bean contrast in plan-view TEM images is alternatively associated with pairs of stacking faults on {111} lattice planes which are inclined against each other. The stacking faults, which are bound by Shockley partial dislocations, are preferably generated ...

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.

GaN nanotubes with coaxial InGaN quantum wells were analyzed by scanning transmission electron microscopy in order to determine their structural properties as well as the indium distribution across the InGaN quantum wells. For the latter, two process steps are necessary. First, a technique to prepare cross-sectional slices out of the nanotubes has been developed. Second, an existing scanning transmission electron microscopy analysis technique has been extended with respect to the special crystallographic orientation of this type of specimen. In particular, the shape of the nanotubes, their defect structure, and the incorporation of indium on different facets were investigated. The quantum wells preferentially grow on m-planes of the dodecagonally shaped nanotubes and on semipolar top facets while no significant indium signal was found on a-planes. An averaged indium concentration of 6% to 7% was found by scanning transmission electron microscopy analysis and could be confirmed by cathodoluminescence measu...

The modified atomic scattering amplitudes (MASAs) of mixed Ga1-xInxAs, GaAs1-xNx, and InAs1-xNx are calculated using the density functional theory approach and the results are compared with those of the binary counterparts. The MASAs of N, Ga, As, and In for various scattering vectors in various chemical environments and in the zinc-blende structure are compared with the frequently used Doyle and Turner values. Deviation from the Doyle and Turner results is found for small scattering vectors (s-1) and for these scattering vectors the MASAs are found to be sensitive to the orientation of the scattering vector and on the chemical environment. The chemical environment sensitive MASAs are used within zero pressure classical Metropolis Monte Carlo, finite temperature calculations to investigate the effect of well size on the electron 002 and 220 structure factors (SFs). The implications of the use of the 002 (200) spot for the quantification of nanostructured Ga1-xInxAs systems are examined while the 220 SF across the well is evaluated and is found to be very sensitive to the in-plane static displacements.

The near-surface reduction of the Sb mole fraction during the growth of GaAsSb nanowires (NWs) and GaAs NWs with GaAsSb inserts has been studied using quantitative high-angle annular dark field scanning transmission electron microscopy (STEM). A model for diffusion of Sb in the hexagonal NWs was developed and employed in combination with the quantitative STEM analysis. GaAsSb NWs grown by Ga-assisted molecular beam epitaxy (MBE) and GaAs/GaAsSb NWs grown by Ga- and Au-assisted MBE were investigated. At the high temperatures employed in the NW growth, As-Sb exchange at and outward diffusion of Sb towards the surface take place, resulting in reduction of the Sb concentration at and near the surface in the GaAsSb NWs and the GaAsSb inserts. In GaAsSb NWs, an increasing near-surface depletion of Sb was observed towards the bottom of the NW due to longer exposure to the As beam flux. In GaAsSb inserts, an increasing change in the Sb concentration profile was observed with increasing post-insert axial GaAs grow...

Surface structuring with ultrashort laser pulses is of high interest as a scalable doping technique as well as for surface nanostructuring applications. By depositing a layer of antimony before the irradiation of ZnO, we were able to incorporate a large quantity of Sb atoms into the single crystalline region of the laser modified surface for potential p-type doping. We have studied the incorporation of antimony and the material properties of laser-induced periodic surface structures (LIPSS) on c-plane ZnO upon femtosecond laser processing at two different peak fluences. We observe high spatial frequency LIPSS with structure periods from 200–370 nm and low spatial frequency LIPSS with periods of 600–700 nm. At a fluence of 0.8 J/cm2, close the ablation threshold of ZnO, the LIPSS are single crystalline except for a few nanometers of amorphous material. At a peak laser fluence of 3.1 J/cm2, they consist of polycrystalline and single crystalline ZnO areas. However, the polycrystalline part dominates with a thickness of about 500 nm.

Al1−xInxN layers with an indium content between x=10.5% and x=24% were grown by metal-organic vapor-phase epitaxy and characterized concerning their optical, structural and morphological properties with regard to the realization of optoelectronic devices. The indium content and the strain of these layers were measured by high resolution x-ray diffraction. Ellipsometric measurements were used to determine the optical constants [refractive index n(λ) and extinction coefficient κ(λ)] in dependence of wavelength and indium content. The values determined for the electronic bandgaps are in good agreement with theoretical predictions and previous publications on this topic but are more focused on AlInN layers which are pseudomorphically grown on GaN. A bowing parameter of b=10.3±0.1 was determined for fully strained layers with an indium content between 13% and 24%. In order to investigate the suitability of these layers for use in distributed Bragg reflectors, the surface morphology is characterized with respec...

III/V semiconductors containing dilute amounts of nitrogen are metastable and need to be thermally treated after growth to optimize optoelectronic properties. The influence of thermal annealing on the nitrogen depth profile in metal organic vapor phase epitaxygrown GaNAs/GaAs as well as GaInNAs/GaAs heterostructures is examined on a nanometer scale by combining several high resolution transmission electron microscopy techniques, also with Rutherford backscattering spectrometry. Annealing conditions, which are optimized for quaternary alloys with respect to photoluminescence intensity, do not result in element redistribution for the In containing material. Contrary to the quaternary material, the result of annealing the ternary GaNAs is a pronounced pileup of the nitrogen profile without any out diffusion of nitrogen. These findings have important influence on device structures, which often contain GaNAs barriers for strain-compensation purposes together with GaInNAs active regions. In the light of metastability considerations for the ternary and quaternary alloy, one can conclude that the In contained in the quaternary material stabilizes the material and suppresses phase separation. Consequently GaInNAs is more stable than its ternary counterpart GaNAs .© 2007 American Institute of Physics. DOI: 10.1063/1.2794739

We compared the optical properties and material nanostructures between several InGaN∕GaN multiple quantum-well (QW) samples of different interfacial layers. In some of the samples, InN interfacial layers were inserted between the wells and barriers to improve the QW quality and hence the light-emission efficiency. Compared with a widely used barrier-doped QW structure, the insertions of the InN interfacial layers (silicon doped or undoped) do enhance the photon emission efficiencies. Of the two samples with InN interfacial layers, the one with intrinsic InN interfacial layers had the higher photoluminescence (PL) and electroluminescence (EL) efficiencies. Cluster structures are clearly observed in this sample, resulting in strong carrier localization. In this sample, we also observed a temperature-dependent S-shape variation in the PL spectral peak, a strong photoluminescence excitation (PLE) intensity, and a steep PL decay time variation beyond its peak as a function of temperature. On the other hand, bo...