Transmission electron microscopy investigation of segregation and critical floating-layer content of indium for island formation in InGaAs

We have investigated InGaAs layers grown by molecular-beam epitaxy on GaAs(001) by transmission electron microscopy (TEM) and photoluminescence spectroscopy. InGaAs layers with In-concentrations of 16, 25 and 28 % and respective thicknesses of 20, 22 and 23 monolayers were deposited at 535 C. The parameters were chosen to grow layers slightly above and below the transition between the two- and three-dimensional growth mode. In-concentration profiles were obtained from high-resolution TEM images by composition evaluation by lattice fringe analysis. The measured profiles can be well described applying the segregation model of Muraki et al. [Appl. Phys. Lett. 61 (1992) 557]. Calculated photoluminescence peak positions on the basis of the measured concentration profiles are in good agreement with the experimental ones. Evaluating experimental In-concentration profiles it is found that the transition from the two-dimensional to the three-dimensional growth mode occurs if the indium content in the In-floating layer exceeds 1.1+/-0.2 monolayers. The measured exponential decrease of the In-concentration within the cap layer on top of the islands reveals that the In-floating layer is not consumed during island formation. The segregation efficiency above the islands is increased compared to the quantum wells which is explained tentatively by strain-dependent lattice-site selection of In. In addition, In0.25Ga0.75As quantum wells were grown at different temperatures between 500 oC and 550 oC. The evaluation of concentration profiles shows that the segregation efficiency increases from R=0.65 to R=0.83.Comment: 16 pages, 6 figures, 1 table, sbmitted in Phys. Rev.

Growth of Oriented Au Nanostructures: Role of Oxide at the Interface

We report on the formation of oriented gold nano structures on Si(100) substrate by annealing procedures in low vacuum (\approx10-2 mbar) and at high temperature (\approx 975^{\circ} C). Various thicknesses of gold films have been deposited with SiOx (using high vacuum thermal evaporation) and without SiOx (using molecular beam epitaxy) at the interface on Si(100). Electron microscopy measurements were performed to determine the morphology, orientation of the structures and the nature of oxide layer. Interfacial oxide layer, low vacuum and high temperature annealing conditions are found to be necessary to grow oriented gold structures. These gold structures can be transferred by simple scratching method.Comment: 13 pages, 3 figures, Accepted in J. Appl. Phy

Increase of the mean inner Coulomb potential in Au clusters induced by surface tension and its implication for electron scattering

Electron holography in a transmission electron microscope was applied to measure the phase shift induced by Au clusters as a function of the cluster size. Large phase shifts Df observed for small Au clusters cannot be described by the well-known equation Df=C_E V_0 t (C_E: interaction constant, V_0: mean inner Coulomb potential (MIP) of bulk gold, t: cluster thickness). The rapid increase of the Au MIP with decreasing cluster size derived from Df, can be explained by the compressive strain of surface atoms in the cluster

Interrelation of structural and electronic properties of InGaN/GaN quantum dots using an eight-band k.p model

We present an eight-band k.p model for the calculation of the electronic structure of wurtzite semiconductor quantum dots (QDs) and its application to indium gallium nitride (InGaN) QDs formed by composition fluctuations in InGaN layers. The eight-band k.p model accounts for strain effects, piezoelectric and pyroelectricity, spin-orbit and crystal field splitting. Exciton binding energies are calculated using the self-consistent Hartree method. Using this model, we studied the electronic properties of InGaN QDs and their dependence on structural properties, i.e., their chemical composition, height, and lateral diameter. We found a dominant influence of the built-in piezoelectric and pyroelectric fields, causing a spatial separation of the bound electron and hole states and a redshift of the exciton transition energies. The single-particle energies as well as the exciton energies depend heavily on the composition and geometry of the QDs

Suppression of the ferromagnetic state in LaCoO3 films by rhombohedral distortion

Epitaxially strained LaCoO3 (LCO) thin films were grown with different film thickness, t, on (001) oriented (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates. After initial pseudomorphic growth the films start to relieve their strain partly by the formation of periodic nano-twins with twin planes predominantly along the direction. Nano-twinning occurs already at the initial stage of growth, albeit in a more moderate way. Pseudomorphic grains, on the other hand, still grow up to a thickness of at least several tenths of nanometers. The twinning is attributed to the symmetry lowering of the epitaxially strained pseudo-tetragonal structure towards the relaxed rhombohedral structure of bulk LCO. However, the unit-cell volume of the pseudo-tetragonal structure is found to be nearly constant over a very large range of t. Only films with t > 130 nm show a significant relaxation of the lattice parameters towards values comparable to those of bulk LCO.Comment: 31 pages, 10 figure

Novel microstructural characteristics and properties of spray formed Al-RE-TM based alloys

Recent studies on the synthesis of bulk Al-RE (Rare Earth)-TM (Transition Metal) based alloys, from melt spun ribbons and gas atomized powders, have shown that a partially amorphous or nano-crystalline structures lead to a high specific strength. In the present study, therefore, spray atomization and deposition process has been used to produce plates of Al85Y8Ni5Co2 (deposit D1) and Al83Y5La5Ni5Co2 (deposit D2) based alloys so as to synthesize bulk deposit of nano-crystalline and/or partial amorphous matrix composite in a single step. The rapid solidification and high undercooling of droplets during atomization and a chilling effect on undercooled liquid upon deposition are expected to give rise to the above microstructural features. The microstructural features of deposits as well as overspray powders were studied using optical, scanning and transmission electron microscope. The alloys invariably showed a large fraction of nano-crystalline and amorphous structures, characterized by featureless regions at optical resolution, along with distribution of primary equilibrium phases. The differential scanning calorimetric (DSC) analysis of the deposits showed all the crystallization peaks as is observed during crystallization of fully amorphous melt spun ribbons of respective compositions. A glass transition phenomenon is observed in Al-Y-Ni-Co based deposit. The transmission electron microscopy of deposit D1 showed the presence of 50-100 nm size fcc-Al precipitates in an amorphous matrix decorated with 5-20 nm fcc-Al crystallites. The annealing treatment of deposits at different temperatures, determined from the crystallization peaks of the deposit, showed precipitation of nanoscale fcc-Al and intermetallic phases giving rise to a remarkable increase in hardness. The bulk hardness of the deposits D1 and D2 was 391 and 427 HV, respectively. Whereas, the heat treated deposits showed a bulk hardness value of 476 HV for deposit D1 at 298 oC and 582 HV for deposit D2 at 380 oC. An attempt has been made to bring out the possible mechanism of microstructural evolution during spray deposition of these alloys, and the effect of microstructural features on the mechanical properties has been discussed

Nano scale phase separation in Au-Ge system on ultra clean Si(100) surfaces

We report on the formation of lobe-lobe (bi-lobed) Au-Ge nanostructures under ultra high vacuum (UHV) conditions (\approx 3\times 10^{-10} mbar) on clean Si(100) surfaces. For this study, \approx 2.0 nm thick Au samples were grown on the substrate surface by molecular beam epitaxy (MBE). Thermal annealing was carried out inside the UHV chamber at temperature \apprx 500{\deg}C and following this, nearly square shaped Au_{x}Si_{1-x} nano structures of average length \approx 48 nm were formed. A \approx 2 nm Ge film was further deposited on the above surface while the substrate was kept at a temperature of \approx 500{\deg}C. Well ordered Au-Ge nanostructures where Au and Ge residing side by side (lobe-lobe structures) were formed. In our systematic studies, we show that, gold-silicide nanoalloy formation at the substrate (Si) surface is necessary for forming phase separated Au-Ge bilobed nanostructures. Electron microscopy (TEM, STEM-EDS, SEM) studies were carried out to determine the structure of Au - Ge nano systems. Rutherford backscattering Spectrometry measurements show gold inter-diffusion into substrate while it is absent for Ge.Comment: 23 pages, 6 Figures, 1 Tabl