SiGe nanostructures: new insights into growth processes

International audienceDuring the last decade, Si/Si 1−x Ge x heterostructures have emerged as a viable system for use in CMOS technology with the recent industrial production of heterojunction bipolar transistor-based integrated circuits. However, many key problems have to be solved to further expand the capabilities of this system to other more attractive devices. This paper gives a comprehensive review of the progress achieved during the last few years in the understanding of some fundamental growth mechanisms. The discrepancies between classical theories (in the framework of continuum elasticity) and experimental results are also specially addressed. In particular, the major role played by kinetics in the morphological evolution of layers is particularly emphasized. Starting from the unexpected differences in Si 1−x Ge x morphological evolution when deposited on (001) and on (111), our review then focuses on: (1) the strain control and adjustment (from fully strained to fully relaxed 2D and 3D nanostructures)-in particular, some original examples of local CBED stress measurements are presented; (2) the nucleation, growth, and self-assembly processes, using self-patterned template layers and surfactant-mediated growth; (3) the doping processes (using B for type p and Sb for type n) and the limitations induced by dopant redistribution during and after growth due to diffusion, segregation, and desorption. The final section will briefly address some relevant optical properties of Si 1−x Ge x strained layers using special growth processes

Formation and stability of a two-dimensional nickel silicide on Ni (111) an Auger, LEED, STM, and high-resolution photoemission Study

Using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), scanning tunnelling microscopy (STM) and high resolution photo-electron spectroscopy (HR-PES) techniques we have studied the annealing effect of one silicon monolayer deposited at room temperature onto a Ni (111) substrate. The variations of the Si surface concentration, recorded by AES at 300{\deg}C and 400{\deg}C, show at the beginning a rapid Si decreasing followed by a slowing down up to a plateau equivalent to about 1/3 silicon monolayer. STM images and LEED patterns, both recorded at room temperature just after annealing, reveal the formation of an ordered hexagonal superstructure(rot3xrot3)R30{\deg}-type. From these observations and from a quantitative analysis of HR-PES data, recorded before and after annealing, we propose that the (rot3 x rot3)R30{\deg}superstructure corresponds to a two dimensional (2D) Ni2Si surface silicide.Comment: Journal Physical Review B (2012

Sb-surfactant mediated growth of Ge nanostructures

International audienceThis paper reports the AFM and HREM study of the Sb surfactant mediated growth of Ge on Si(0 0 1). We show that very dense self-organised Ge dots of small lateral dimensions can be grown by using a sub-monolayer coverage of Sb on Si(0 0 1) in the transient growth regime between 2D nucleation and step flow. The dramatic Ge growth change induced by Sb is attributed to both kinetic and thermodynamic effects. Indeed, HREM observations evidence mainly two phenomena: the close-packing of ultra-small Ge islands indicating a lower surface diffusion in presence of Sb and a mono-modal island shape and size that strongly differs from the bimodal islands 'huts' and 'domes' commonly observed without Sb. Morphological and microstructural features of Ge islands formed with and without Sb are studied and the differences between facets and aspect ratio are exhibited. Moreover, at lower growth temperature (in the 2D nucleation regime, T g 0 350°C), a delay to 3D island nucleation is observed and defect free 2D flat layers can be grown up to thicknesses of 18 A ,. At higher growth temperature, (in pure step flow at T g 750°C) large, well separated 'dome' islands partially relaxed by dislocation nucleation on their edges are obtained. Such islands are very similar to those obtained without Sb coverage. The complete desorption of Sb on Ge rich surface at T\ 720°C explains this result. This study which improves the understanding on the formation of ultra-small dense islands is very promising for the fabrication of quantum devices that require highly homogeneous islands of small lateral sizes and of MOSFET heterostructures with strained SiGe n-channel which require flat Ge rich layers with abrupt interfaces

Nanometric size effect on Ge diffusion in polycrystalline Si

The nanosize effect on Ge diffusion (850 \u3c = T \u3c = 1000 degrees C) in polycrystalline Si layers is investigated. The Ge diffusion coefficients in microcrystalline and nanocrystalline Si layers made of 30 mu m and 40 nm wide grains, respectively, are measured and compared. In the microcrystalline Si layer, the Ge diffusion coefficient in micrograin boundaries is measured using a conventional analytical solution of Fick\u27s equations corresponding to the Fisher model. In the nanocrystalline Si layer, the Ge diffusion coefficients in nanograins and in nanograin boundaries are measured via a method based on two-dimensional simulations using the Fisher model geometry. The diffusivities in nanograins and nanograin boundaries are one order of magnitude higher than in micrograins and micrograin boundaries, respectively. However, the nanosize effect appears to be different in grains and grain boundaries; despite that the activation energy for diffusion in 40 nm wide grains is at least 1 eV lower than in Si bulk. The activation energy in nanograin boundaries is about the same as in micrograin boundaries

VACANCY-MEDIATED ATOMIC TRANSPORT IN NANO-CRYSTALS

International audienceAtomic transport in nano-crystals is still poorly studied experimentally. However, the knowledge of atomic transport kinetic and of the mechanisms allowing atoms to move in a volume exhibiting nano-scale dimensions (< 100 nm) is important for i) improving our fundamental knowledge concerning point defects' formation and migration energies, and atom-point defect interactions in nano-structures, as well as for ii) predicting mass transport in nano-structures, allowing the design of nano-structure fabrication processes to be developed at lower cost. In this article, atom probe tomography measurements were used to investigate the Ge distribution in 40 nm-wide Si nano-crystals in which the Ge flux was found to be ten times faster than in the bulk of a Si mono-crystal. The Ge atoms were found to be randomly distributed in the nano-crystals. No extended defect was found being able to explain an increase of Ge transport kinetic in the nano-crystals. Consequently, a scenario based on a higher equilibrium vacancy concentration at the nano-crystal surface (or interface) is proposed in order to explain the faster atomic kinetic measured in Si nano-crystals

Lattice diffusion and surface segregation of B during growth of SiGe heterostructures by molecular beam epitaxy: effect of Ge concentration and biaxial stress

Si1-xGex/Si1-yGey/Si(100) heterostructures grown by Molecular Beam Epitaxy (MBE) were used in order to study B surface segregation during growth and B lattice diffusion. Ge concentration and stress effects were separated. Analysis of B segregation during growth shows that: i) for layers in epitaxy on (100)Si), B segregation decreases with increasing Ge concentration, i.e. with increased compressive stress, ii) for unstressed layers, B segregation increases with Ge concentration, iii) at constant Ge concentration, B segregation increases for layers in tension and decreases for layers in compression. The contrasting behaviors observed as a function of Ge concentration in compressively stressed and unstressed layers can be explained by an increase of the equilibrium segregation driving force induced by Ge additions and an increase of near-surface diffusion in compressively stressed layers. Analysis of lattice diffusion shows that: i) in unstressed layers, B lattice diffusion coefficient decreases with increasing Ge concentration, ii) at constant Ge concentration, the diffusion coefficient of B decreases with compressive biaxial stress and increases with tensile biaxial stress, iii) the volume of activation of B diffusion () is positive for biaxial stress while it is negative in the case of hydrostatic pressure. This confirms that under a biaxial stress the activation volume is reduced to the relaxation volume

Triple-junction contribution to diffusion in nanocrystalline Si

The influence of triple-junctions on experimental Ge diffusion profiles (850-1000 degrees C) in nanocrystalline Si is investigated using three-dimensional finite element simulations. We found that triple-junction diffusion is not negligible in nanocrystalline Si made of 40 nm wide grains. Ge triple-junction diffusion coefficient follows the Arrhenius law 5.72x10(4) exp(-3.24 eV/kT)cm(2) s(-1). It is approximately 4.7x10(2) times higher than grain boundary diffusion coefficient, even though diffusion in triple-junction and in grain boundary exhibits similar activation energy

B diffusion in implanted Ni2Si and NiSi layers

B diffusion in implanted Ni2Si and NiSi layers has been studied using secondary ion mass spectrometry, and compared to B redistribution profiles obtained after the reaction of a Ni layer on a B-implanted Si(001) substrate, in same annealing conditions (400-550 degrees C). B diffusion appears faster in Ni2Si than in NiSi. The B solubility limit is larger than 10(21) atom cm(-3) in Ni2Si, while it is similar to 3x10(19) atom cm(-3) in NiSi. The solubility limit found in NiSi is in agreement with the plateau observed in B profiles measured in NiSi after the reaction of Ni on B-implanted Si

Realization of vertically aligned, ultra-high aspect ratio InAsSb nanowires on graphite

The monolithic integration of InAs1–xSbx semiconductor nanowires on graphitic substrates holds enormous promise for cost-effective, high-performance, and flexible devices in optoelectronics and high-speed electronics. However, the growth of InAs1–xSbx nanowires with high aspect ratio essential for device applications is extremely challenging due to Sb-induced suppression of axial growth and enhancement in radial growth. We report the realization of high quality, vertically aligned, nontapered and ultrahigh aspect ratio InAs1–xSbx nanowires with Sb composition (xSb(%)) up to ∼12% grown by indium-droplet assisted molecular beam epitaxy on graphite substrate. Low temperature photoluminescence measurements show that the InAs1–xSbx nanowires exhibit bright band-to-band related emission with a distinct redshift as a function of Sb composition providing further confirmation of successful Sb incorporation in as-grown nanowires. This study reveals that the graphite substrate is a more favorable platform for InAs1–xSbx nanowires that could lead to hybrid heterostructures possessing potential device applications in optoelectronics