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We present a comprehensive study relating the magnetic properties to structural properties of Mn+-implanted Si1-xGex films as a function of Ge content (x=0–0.5). Ferromagnetic ordering with three critical temperatures, TB∼10–16 K, TC1∼650–780 K, and TC2∼825–860 K, are reported in this material system. Element specific x-ray absorption fine structure results show that the majority of the Mn ions are nonsubstitutional in all samples. The transmission-electron microscopy coupled with z contrast and chemical analysis reveals the presence of Mn-rich nanosized clusters including Mn4Si7 in Si-rich samples and Mn7Ge3 phases in Ge-rich samples. A composition transition occurred at x∼0.2–0.3, where we observe a change in bond lengths and defect structures. Additionally, an enhancement in magnetizations with an increase in both TB and TC1 as well as a conversion from n-type to p-type conduction are also detected. [ABSTRACT FROM AUTHOR]

We report the observation of large scale self-assembly of long horizontal nanowires into orthogonally oriented bundles, during in situ annealing of a few monolayers of Ge on Si(001). Results are interpreted in terms of a collective wave-propagation mechanism, previously suggested for interpreting ripple faceting on Ge/Si(1 1 10) surfaces. Quantitative agreement between experiments and theory is found. The onset of the mechanism, the number of wires in the bundles, and their total density can be controlled by carefully tuning the growth parameters. [ABSTRACT FROM AUTHOR]

The three-dimensional composition profiles of individual SiGe/Si(001) islands grown on planar and pit-patterned substrates are determined by atomic force microscopy (AFM)-based nanotomography. The observed differences in lateral and vertical composition gradients are correlated with the island morphology. This approach allowed us to employ AFM to simultaneously gather information on the composition and strain of SiGe islands. Our quantitative analysis demonstrates that for islands with a fixed aspect ratio, a modified geometry of the substrate provides an enhancement of the relaxation, finally leading to a reduced intermixing. [ABSTRACT FROM AUTHOR]

The morphological evolution of both pits and SiGe islands on patterned Si(001) substrates is investigated. With increasing Si buffer layer thickness the patterned holes transform into multifaceted pits before evolving into inverted truncated pyramids. SiGe island formation and evolution are studied by systematically varying the Ge coverage and pit spacing and quantitative data on the influence of the pattern periodicity on the SiGe island volume are presented. The presence of pits allows the fabrication of uniform island arrays with any of their equilibrium shapes. [ABSTRACT FROM AUTHOR]

The model systems for self-organized quantum dots formed from elemental and compound semiconductors, namely Ge grown on Si(001) and InAs on GaAs(001), are comparatively studied by scanning tunneling microscopy. It is shown that in both material combinations only two well-defined families of faceted and defect-free nanocrystals exist (and coexist). These three-dimensional islands, pyramids, and domes show common morphological characteristics, independent of the specific material system. A universal behavior is further demonstrated in the capping-passivation process that turns the nanocrystals in true quantum dots. [ABSTRACT FROM AUTHOR]

We show that a reduced critical thickness observed in stacked layers of self-assembled Ge hut-cluster islands can be quantitatively related to the stress field originating from buried islands. The experimental data for stacks with up to five island layers and Si spacer layers between 3 und 9 nm thick are compared with a simple model for stress-induced reduction of critical thickness. While it is not possible to fit the experimental data using the inverse cubic stress-field scaling of a spherical island, a more accurate fit may be determined by using an inverse linear dependence derived for a pyramid. The best fit was obtained using numerical results calculated for the stress from a truncated pyramid. Si intermixing in stacked layers of hut clusters is experimentally observed and included in the model. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on the photoluminescence (PL) properties of Ge hut cluster islands on Si(001) that were overgrown at temperatures as low as 250 °C. We find that the island-related photoluminescence systematically redshifts as the overgrowth temperature is reduced from 500 to 360 °C, which is attributed to a reduced Ge segregation. For even lower overgrowth temperatures, the emission energy saturates at 0.63 eV or 1.96 μm, more than 110 meV smaller than the band gap of unstrained bulk Ge. We report a PL peak centered at 2.01 μm at low excitation power, in good agreement with the estimated transition energy for a spatially indirect transition between holes confined in the strained Ge island and electrons confined in the surrounding Si matrix. PL is observed up to a temperature of 185 K and an activation energy of 40 meV is deduced from fitting the temperature-dependent peak intensity. Annealing experiments reveal a systematic blueshift of the hut cluster-related PL, thus verifying unambiguously, that the PL signal originates from the hut clusters and not from defects. [ABSTRACT FROM AUTHOR]

Self-assembled Ge islands grown by solid-source molecular-beam epitaxy were investigated by resonant Raman scattering under hydrostatic pressure at room temperature. We utilize the effect of pressure to tune the electronic transition through laser excitation energies in the Ge islands. The pressure coefficient of this resonating electronic transition thus obtained is ∼2.7±0.5 meV/kbar, which is significantly smaller than the pressure shift of the E[sub l] transition in bulk Ge. This is attributed to the fact that the Ge islands are strongly constrained by the surrounding Si lattice, leading to a smaller deformation as compared to the bulk Ge, when subjected to the same pressure. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Intense photoluminescence (PL) originating from single layers of germanium hut clusters grown on silicon (001) is investigated using PL spectroscopy. We propose that the luminescence originates from phononless recombination within a spatially indirect, type-II neighboring confinement structure. Enhanced no-phonon (NP) luminescence is attributed to exciton localization at the Ge/Si interfaces. The PL intensity is sensitive to the growth temperature during interface formation, as well as to post-growth thermal annealing, illustrating the influence of atomic-level Si–Ge intermixing on exciton localization and NP enhancement. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Self-assembled growth in combination with prepatterning yields ordered lines of Ge islands on a planar Si (001) surface. The self-assembled Ge nanostructures are grown on top of a 15-period Si/SiGe superlattice, which is deposited on a prepatterned Si substrate. The pattern consists of 10 nm deep trenches with a period of 250 nm. The superlattice translates the surface modulation of the substrate into a strain-field modulation at the growth front of the superlattice. This strain field modulation provides the template for the ordered nucleation of self-assembled Ge islands. Our method gives rise to the long-range ordering of perfectly passivated nanostructures and can in principle be applied to any other strained material system. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have investigated the vertical correlation properties of SiGe islands in a series of Si/SiGe multilayers using grazing incidence x-ray diffraction. The degree of island correlation is found to strongly depend on the thickness of the Si spacer layer separating subsequent SiGe layers. A comparison with results obtained from transmission electron microscopy demonstrates the feasibility of the x-ray diffraction method for the investigation of sample series, and an improved statistical accuracy of the obtained parameters: with x-ray diffraction, the statistical average of typically 10[sup 6]-10[sup 7] island columns is obtained, compared to only few in the case of transmission electron microscopy. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ge/Si islands grown with molecular-beam epitaxy at 630 °C are overgrown with Si at different temperatures T[sub cap], and their photoluminescene spectra are recorded. Both the island-related and wetting-layer-related energy transitions redshift with lowered T[sub cap], which is explained by reduced material intermixing. The mandatory growth interruption, which is introduced during the temperature drop, causes island ripening and shifts the island (wetting layer) photoluminescence peaks slightly to lower (higher) energies. The growth interruption quenches the quantum efficiency of the wetting layer by more than an order of magnitude, whereas the island-related photoluminescence intensity even slightly increases. The island's superior resistance against growth interruptions, and hence interface contamination, is explained by effective carrier localization in the Ge nanostructures. Room-temperature photoluminescence is reported for Ge islands overgrown at 460 °C. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The growth of 2.4 monolayers Ge on 0.2 monolayers of predeposited C on Si results in the formation of 15 nm size Ge islands. Fifty stacked layers of these C-induced Ge dots are grown on Si (001) at 460 °C. Different pieces of the wafer are annealed at temperatures between 460 and 950 °C and for times ranging from 1 to 20 min at 850 °C. For temperatures higher than 550 °C, a pronounced energy blueshift and an evolution from one broad photoluminescence peak to two well-resolved lines reflect a gradual transition from quantum dot states to quantum well-like states. As transmission electron microscopy images illustrate, diffusion processes completely smear out the sharp interfaces between the dots and the surrounding Si. An activation energy of only 1.6 eV and temperature-dependent diffusion coefficients are derived from simple model calculations. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Self-assembled Ge wires with a height of only 3 unit cells and a length of up to 2 micrometers were grown on Si(001) by means of a catalyst-free method based on molecular beam epitaxy. The wires grow horizontally along either the [100] or the [010] direction. On atomically flat surfaces, they exhibit a highly uniform, triangular cross section. A simple thermodynamic model accounts for the existence of a preferential base width for longitudinal expansion, in quantitative agreement with the experimental findings. Despite the absence of intentional doping, the first transistor-type devices made from single wires show low-resistive electrical contacts and single-hole transport at sub-Kelvin temperatures. In view of their exceptionally small and self-defined cross section, these Ge wires hold promise for the realization of hole systems with exotic properties and provide a new development route for silicon-based nanoelectronics. [ABSTRACT FROM AUTHOR]