J.-M. Baribeau

Misfit strain relaxation in GexSi1−x/Si heterostructures: The structural stability of buried strained layers and strained‐layer superlattices

The critical thickness‐strain relationships for buried strained layers and strained‐layer superlattices (SLSs) are derived using an energy balance model. Relaxation of the entire heterostructure and individual strained layers by both 60° type a/2〈011〉 and pure edge dislocations is considered. GexSi1−x/Si heterostructures designed to investigate the stability regimes predicted by the model were grown by molecular‐beam epitaxy. The extent of relaxation and the detailed dislocation structure were assessed in annealed structures by x‐ray rocking curve analysis, transmission electron microscopy, and Nomarski microscopy of etched samples. Comparison of metastable as‐grown and post‐growth annealed microstructures revealed the evolution of misfit dislocation structure as equilibrium was approached on annealing in the temperature range 600–900 °C. The predominant relaxation mechanism for most GexSi1−x/Si heterostructures was by 60° a/2 〈011〉 misfit dislocations at the first strained‐layer/substrate interface. Howe...

Growth and characterization of Si1−xGex and Ge epilayers on (100) Si

Two approaches to the growth of high‐quality epitaxial Ge epilayers on (100) Si have been investigated. The first consisted of compositional‐grading Si1−xGex layers and the use of strained‐layer superlattices as dislocation filters. In general, this method produced unsatisfactory results, due to the difficulty in achieving good epitaxial growth in the Ge concentration interval 30%−70%. The second approach consisted of simply depositing pure Ge directly on (100) Si. Excellent epitaxial films with dislocation densities of less than 107 cm−2 and smooth morphology were obtained after optimization of the growth parameters. The initial growth temperature and post‐growth annealing were found to be critical in obtaining good epitaxial material.

Ge dots and nanostructures grown epitaxially on Si

We review recent progress in the growth and characterization of Si1−xGex islands and Ge dots on (001) Si. We discuss the evolution of the island morphology with Si1−xGex coverage, and the effect of growth parameters or post-growth annealing on the shape of islands and dots. We outline some of the structural, vibrational, and optical properties of Si1−xGex islands and review recent advances in the determination of their composition and strain distribution. In particular, we present an analytical electron transmission microscopy study of the Ge spatial distribution in Ge dots and Si /Si1−xGex island superlattices grown by molecular beam epitaxy and ultra-high vacuum chemical vapour deposition. We describe the use of undulated Si1−xGex island superlattices for infrared detection at telecommunication wavelengths. Finally, we discuss various approaches currently being investigated to engineer Si1−xGex quantum dots and, in particular, control their size, density, and spatial distribution. As examples, we show how C pre-deposition on Si(001) can influence nucleation and growth of Ge islands and how low temperature Si homo-epitaxy can lead to a particular surface cusp morphology that may promote dot nucleation.

Solid phase epitaxial regrowth of Si1−xGex/Si strained‐layer structures amorphized by ion implantation

Strained‐layer structures consisting of ∼30–35 nm Si1−xGex (x=0.16–0.29) and 33 nm Si deposited by molecular beam epitaxy on a (100)Si substrate have been amorphized by ion implantation at 40 K with 120 keV As+. Rutherford backscattering/channeling measurements using 2 MeV He+ ions have been used to measure the thickness of the amorphous layer and monitor the subsequent epitaxial regrowth occurring as a result of annealing at 560–600 °C. Angular scans across the {110} planar channels indicate that the initial crystallinity and strain was recovered for x=0.16; however, for x=0.29 crystal quality was greatly reduced and the coherency at the substrate‐alloy layer interface was destroyed.

Heterogeneous nucleation sources in molecular beam epitaxy-grown GexSi1−x/Si strained layer superlattices

Abstract In the past few years, a number of studies have indicated the importance of heterogeneous sources in understanding defect nucleation mechanisms. Nonetheless, the majority of critical thickness research is still based on homogeneous misfit dislocation nucleation theory to predict the onset of plastic relaxation at strained layer interfaces. It will be shown that heterogeneous sources are plentiful even in molecular beam epitaxy (MBE)-grown material wherein the generation of defects depends on the oxygen/carbon contamination levels. Several metastable Ge x Si 1− x /Si strained layer superlattices have been grown under slightly different conditions in order to vary the concentration of oxygen/carbon at the substrate-buffer interface. Subsequent annealing treatments were carried out in order to accentuate any characteristic differences in the plastic relaxation behaviour of these metastable structures. Chemical defect etching, plan-view and cross-sectional transmission electron microscopy have been used extensively to characterize misfit dislocation nucleation behaviour at the strained layer interfaces based on differences in the number and type of heterogeneous sources.

Interdiffusion and strain relaxation in (SimGen)p superlattices

We report an x‐ray diffraction study of interdiffusion and strain relaxation in (SimGen)p short‐period superlattices. An interdiffusion coefficient Dλ was determined by monitoring, as a function of time, the decay upon annealing of the first order 000 satellite peak arising from the compositional modulation of the superlattice. Strain relaxation was obtained from the shift of the 400 superlattice peak on annealing. In the early stage of annealing the low angle satellite exhibited a rapid nonexponential decay after which a slower exponential decay was observed indicating a larger initial interdiffusion coefficient. This enhancement was correlated with the presence of strain and it disappeared upon relaxation. Diffusion was faster in structures alternating thin Si and thick Ge layers suggesting that migration of Si into Ge is the dominant diffusion process.

An annealing study of strain relaxation and dislocation generation in Si1−xGex/Si heteroepitaxy

The generation of interface misfit dislocations, and the accompanying strain relaxation, in a molecular‐beam epitaxy grown 0.17 μm thick metastable Si0.82Ge0.18/Si(100) strained epilayer have been studied in detail as a function of rapid thermal annealing treatments over the 500–850 °C temperature range. Charge collection and transmission electron microscopy were used to determine the onset of relaxation by directly imaging misfit dislocations and to investigate the variation in dislocation density with increasing anneal temperature. The strain variation in the epilayer was carefully monitored using double‐crystal x‐ray diffraction and Raman spectroscopy, and the annealing induced changes in strain related to the electron microscopy observed density of interface misfit dislocations. The relative merit of each experimental technique is discussed in the light of these results. The generation of strain relieving dislocations was found to be an activated process, with an activation energy on the order of 1.5 ...

Visible light emitting SiSiO2 superlattices

Abstract Six-period superlattices of Si SiO 2 have been grown at room temperature using molecular beam epitaxy. With this mature technology, the ultra-thin (1–3 nm) Si layers were grown to atomic layer precision. These layers were separated by ∼1 nm thick SiO2 layers whose thickness was also well controlled by using a rate-limited oxidation process. The chemical and physical structures of the multilayers were characterized by cross-sectional TEM, X-ray diffraction, Raman spectroscopy, Auger sputter-profile, and X-ray photoelectron spectroscopy. The analysis showed that the Si layer is free of impurities and is amorphous, and that the SiO 2 Si interface is sharp (∼0.5 nm). Photoluminescence (PL) measurements were made at room temperature using 457.9 nm excitation. The PL peak occurred at wavelengths across the visible range for these multilayers. The peak energy position E was found to be related to the Si layer thickness d by E (eV) = 1.60+0.72d−2 in accordance with a quantum confinement mechanism and the bulk amorphous-Si band gap.

Selective amorphization of ion‐bombarded SiGe strained‐layer superlattices

Si/SixGe1−x multilayers were implanted with Si ions of 540 keV at doses between 1.0×1014 and 2.5×1015 ions/cm2. Channeling spectra were taken using 3 MeV B++ ions. These measurements showed a rapid increase of the Ge minimum yield with implantation dose. The increases were paralleled by a growth of disorder peaks in those parts of the Si backscattering spectrum corresponding to the SixGe1−x layers. After 1.2×1015 Si ions/cm2 the SiGe layers were completely amorphized. Cross‐sectional transmission electron microscope pictures confirmed the selective amorphization of the SiGe layers. Annealing of an irradiated sample resulted in recrystallization of all the amorphous layers in the 450–550 °C temperature range.

Growth and characterization of SiGe atomic layer superlattices

Abstract The MBE growth of ultrathin (Si m Ge n ) p atomic layer superlattices (ALSs) consisting of p periods of alternating m and n monolayers of silicon and germanium is reported. Various structures were prepared on (100) Si, (100) Ge and on a 20-nm Si 0.5 Ge 0.5 buffer on (100) Si. The physical properties of the ALSs were investigated by Raman scattering (RS), double-crystal X-ray diffraction, transmission electron microscopy and photoluminescence (PL). The samples were found to have accurate stoichiometry and thicknesses. Substrate-dependent variations in the growth morphology were observed. The microstructures showed some waviness on silicon or germanium substrates and were heavily defected when an alloy buffer was introduced. Phonon peaks due to folding of acoustic modes were seen in the frequency range 20–200 cm −1 . The optical modes in RS were calculated using a linear chain model and were compared with the experimental spectra. Good fits were obtained after introducing interdiffusion in the model by adjusting the mass of the interfacial atoms. The PL investigation revealed features related to defects in silicon. However, no strong luminescent features that could be ascribed to a direct band-gap transition were observed.