Didier Dentel

Continuous germanene layer on Al(111).

Germanene, a 2D honeycomb structure similar to silicene, has been fabricated on Al(111). The 2D germanene layer covers uniformly the substrate with a large coherence over the Al(111) surface atomic plane. It is characterized by a (3 × 3) superstructure with respect to the substrate lattice, shown by low energy electron diffraction and scanning tunnelling microscopy. First-principles calculations indicate that the Ge atoms accommodate in a very regular atomic configuration with a buckled conformation.

Si adatom surface migration biasing by elastic strain gradients during capping of Ge or Si1−xGex hut islands

Hut cluster formation during Ge or Si1−xGex solid source molecular beam epitaxial growth on Si(001) is a well-known kinetic pathway for partial strain relief. It results in undulated morphologies with {105} facets allowing a∥ lattice parameter relaxation on the island apexes. Here, we show how subsequent Si coverages, grown at 500 °C, avoid being tensile strained and impede further increase of stored elastic strain energy. Dominant inhomogeneous Si surface diffusions take place as proven by a Ge marker technique able to provide transmission electron microscopy or high-resolution transmission electron microscopy images of the initial Si morphology stages and by reflection high-energy electron diffraction examinations. This mechanism prevails for high enough Si growth rates, able to quench lateral Ge diffusion and limit chemical strain relief. Mediated by stress variations on the noncapped island curvatures, Si is depleted from the top of the islands and accumulates in the troughs of the ripples where it ac...

Surface smoothing induced by epitaxial Si capping of rough and strained Ge or Si1−xGex morphologies: a RHEED and TEM study

The same Si/Ge/Si or Si/Si1−xGex/Si structures grown at 400°C on Si(0 0 1) are compared, either in real time by reflection high-energy electron diffraction (RHEED) or on the final product by transmission electron microscopy (TEM). This allows us to follow interface morphology variations during Si re-growth upon Ge containing layers of various Ge thicknesses or alloy x fractions. As shown by the passage from spotty to streaky RHEED patterns and by specular beam intensity oscillation evolutions, the surfaces roughen systematically during strained Ge or Si1−xGex (SiGe) growth and smooth rapidly during subsequent growth of 4 to 6 Si monolayers, at least in the elastic hut-clustering islanding range with {1 0 5} facets. With the help of TEM examinations, a coherent picture may be proposed for these surface smoothing observations: (i) A dominant mechanism in form of a quick Si surface diffusion occurring initially on the Ge-strained surfaces. It ensures a heteregeneous Si accumulation towards the places of minimized misfit, i.e., in the troughs of the Ge or SiGe morphologies. (ii) A slower Ge diffusion (as occuring on Si) depleting the emerging island crests and contributing to an overall Ge surface termination (Ge surface segregation) and to a complementary island smoothing. The latter mechanism, only important at low growth kinetics, favours the formation of alloyed interfaces as a by-product of the island smoothing and lateral intermixing. At high Si growth kinetics the former mechanism prevails leading to better preserved island morphologies and final interfaces appearing chemically more abrupt but less flat.

Influence of a pre-deposited carbon submonolayer on the Ge island nucleation on Si(001)

Si surfaces manipulated by a carbon (C) pre-deposition have been used to modify the growth morphology of Ge islands. In situ reflection high-energy electron diffraction and x-ray photoelectron diffraction and ex situ atomic force microscopy studies have been conducted for constant C seeding and varying growth temperatures and Ge coverages, with the aim of deepening the understanding of the relevant Ge quantum dot formation. With temperatures ranging from 400 to 600 °C, well structured Ge islands grow in a Volmer–Weber mode as soon as 0.4 ML of C and 1 ML of Ge are deposited. Strongly modified behaviors are nevertheless observed by changing the Ge growth temperature from 500 to 600 °C. By increasing the Ge coverage from 1 to 6 ML at 500 °C, the island height increases at constant density, whereas, at 600 °C, a strong reduction of the density is observed, with a three-dimensional-two-dimensional transition probably due to a partial Ge intermixing in the Si matrix. These different nucleation schemes are conn...

AFM and RHEED study of Ge/Si(001) quantum dot modification by Si capping

Abstract Atomic force microscopy (AFM) images of 6 Ge equivalent monolayers (EML) deposited on Si(001) at 500°C by solid source molecular beam epitaxy reveal the formation of ∼2 nm high islands with a height/base dimension ratio close to 1/10. Clear differences are observed, by both reflection high-energy electron diffraction and AFM, according to the Si capping conditions of such hut clusters. At room temperature the impinging Si adatoms homogeneously cover the Ge clusters due to very low Si and Ge surface diffusions nearly preserving the corrugation of uncovered Ge clusters. In contrast, at 500°C and a low Si deposition rate (∼2 EML min −1 ), a surface smoothing occurs after deposition of a thickness as low as 5 Si EML. A Si adatom diffusion from the apex towards the bottom of the islands concomitant with a Ge lateral segregation lead to a rapid collapse of the buried islands. This scenario is supported by the observation of a (2×9) superstructure, characteristic of the formation of SiGe alloy.

Two dimensional Si layer epitaxied on LaAlO3(111) substrate: RHEED and XPS investigations

We report the epitaxial growth of one Silicon monolayer on the LaAlO3(111) substrate, a high-κ crystalline oxide. Structural and chemical properties were investigated in-situ using reflection high energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS). The deposition was achieved by molecular beam epitaxy in the temperature range RT-500°C. A two-dimensional epitaxial growth mode is observed for a deposition at temperature between 300°C and 500°C. The deposited single layer is formed by two dimensional (2D) structures of Si.

Reflection high-energy electron diffraction intensity oscillations during Si growth on Ge(001) by solid source/molecular beam epitaxy

Abstract RHEED intensity oscillations of the specular beam are used to follow the growth of Si on Ge(001) by solid source molecular beam epitaxy (SSMBE) in a large range of substrate temperatures and Si deposition rates. From room temperature to 300°C, a kinetic governed layer-by-layer growth mode is observed, the periods of the oscillations corresponding to a single atomic layer. The slow damping of the oscillations in this regime is related to a progressive appearance of surface defects, the RHEED patterns remaining two-dimensional but becoming more diffuse after complete damping of the oscillations. The damping is faster at either higher temperature or lower deposition rates, both effects favouring the increase of the surface diffusion length. Thus, at 400°C, only two oscillations with a doubled period are observed before damping. At 500°C, no RHEED oscillations are obtained in line with the setting up of the thermodynamically expected Volmer–Weber growth mode.

Tip-Induced Switch of Germanene Atomic Structure

A new germanene crystallographic structure is investigated by scanning tunnelling microscopy and density functional theory calculations. We found that germanene can crystallize in two stable but different structures when grown on Al(111) at the same temperature. These structures are evidenced in scanning tunnelling images by a honeycomb contrast and by a hexagonal contrast. These contrasts are relevant of a Ge network with one (hexagonal) or two (honeycomb) Ge atoms per unit cell shifted upward with respect to the other Ge atoms. These structures appear alternatively and can be turned on and off by a tip-induced process.

HRTEM study of strained Si/Ge multilayers

Deformation in Si/Ge strained multilayers has been characterised by HRTEM. The HRTEM strain profile determined by image treatment has been used to compare two growth techniques: Hot wire assisted gas source and ultra high vacuum molecular beam epitaxy. We have shown that for the first technique, germanium layers are highly strained. This is probably due to the incorporation of atomic hydrogen which would prevent relaxation by stacking faults formation.

Original Ge-induced phenomena on various SiC(0 0 0 1) reconstructions

Using complementary surface analysis techniques, we study the Ge growth on distinct SiC(0 0 0 1) reconstructions and elucidate complex mechanisms occurring by thermal activation. Two Si-rich reconstructions, (3 × 3) and , and one C-rich, , are concerned, on which Ge is found to grow in Stranski–Krastanov and Volmer–Weber modes, respectively. The best Ge-wetting layer is favoured on the (less Si-rich) because closest to a perfect truncated SiC(0 0 0 1) termination. At sufficiently high temperature, the Ge-wetting layer is organized in the form of a (4 × 4)Ge reconstruction for which we propose a first atomic model that is based on the 3 × 3 structure. Annealing Ge on the (3 × 3) and surfaces provokes spectacular successive 2D/3D and unusual 3D/2D transitions not only of Ge but also of Si and C, respectively, coming from the surface initial richness. In both cases, a phase separation is observed either in the 2D or 3D structures, which is unexpected for the Ge/Si binary system and somewhat usual for the Ge/C one. In the case of Ge on , a special 2D heterostructure graphite/Ge/SiC is achieved at the atomic level. This acts as a Schottky barrier and then can be promising for future possible applications.