Strain In Epitaxial Layers Research Articles

Analysis of Mechanical Strain in AlGaN/GaN HFETs

Herein, the influence of mechanical strain induced by passivation layers on the electrical performance of AlGaN/GaN heterostructure field‐effect transistor is investigated. We studied the physical mechanism of a threshold voltage (Vth) shift for the monolithically fabricated on/off devices reported earlier by our group. For that, theoretical calculations, simulation‐based analysis, and nano‐beam electron diffraction (NBED) measurements based on STEM are used. Strain distribution in the gate vicinity of transistors is compared for a SiNx passivation layer with intrinsic stress from ≈0.5 to −1 GPa for normally on and normally off devices, respectively. The strain in epitaxial layers transferred by intrinsic stress of SiNx is quantitatively evaluated using NEBD method. Strain dissimilarity Δε = 0.23% is detected between normally on and normally off devices. Using this method, quantitative correlation between 1.13 V of Vth shift and microscopic strain difference in the epitaxial layers caused by 1.5 GPa intrinsic stress variation in passivation layer is provided. It is showed in this correlation that about half of the reported threshold voltage shift is induced by strain, i.e., by the piezoelectric effect. The rest of Vth shift is caused by the fabrication process. Therefore, various components/mechanisms contributing to the measured Vth shift are distinguished.

Quantum dots in strained layers—preventing relaxation through the precipitate hardening effect

The internal strain in epitaxial layers due to lattice misfit has long been recognized as a limiting factor in the design of semiconductor structures. In strained layer structures above a critical thickness hc, this strain is relaxed by the introduction of misfit dislocations. Here, we show that the interaction between the strain fields of a self-assembled quantum dot and a dislocation can lead to a threading dislocation being trapped, or pinned, by the quantum dot. The strength of this interaction is always larger than the force exerted on the dislocation by a surrounding layer with lower misfit strain. This gives a significant increase in the critical thickness for relaxation hc(QD). In layers between hc and hc(QD), threading dislocations can at best move only small distances, effectively preventing relaxation. Furthermore it is not possible to destabilize such a layer by the deposition of strained layers above it. The classical critical thickness condition thus does not apply to these structures, and they can be produced with essentially no limits to thickness before relaxation occurs. This may be expected to have significant technological consequences and allow a much wider range of structures to be produced than is possible using current strained layer design rules.

Surface Morphology of SiGe Epitaxial Layers Grown on Uniquely Oriented Si Substrates

AbstractThe 4% lattice mismatch between Si and Ge creates strain in epitaxial layers of SiGe alloys on Si, and this strain can manifest itself in the morphological structure of the surface of the epitaxial layer. This study explores the relationship of the evolution of the surface morphology of SiGe layers grown on a range of Si surface orientations. We have grown thin, strained and thick, relaxed layers of Si0.7Ge0.3 by solid source molecular beam epitaxy on substrates with surface normals rotated from [001] towards [111] by angles of θ = (0, 2, 4, 10, 22) degrees. The surface morphology was investigated by atomic force microscopy, which showed considerable ordering of surface features on relaxed samples. These features evolve from hut-like structures at 0 degrees to large mesa-like structures separated by pits and crevices at 22 degrees. The organization of these features is also shown to vary with the substrate orientation. Each surface has characteristic directions along which features are aligned, and these directions vary continuously with the angle of rotation of the substrate. Transmission electron microscopy confirmed that misfit dislocations had formed along those same directions. The state of relaxation of each layer is quantified by Raman spectroscopy in order to make a direct correlation between residual strain and surface morphology.

Diffusion Mechanisms in Sige Alloys

ABSTRACTInterest in diffusion processes in SiGe alloys arises from their potential in HBT's, HFET's, and optoelectronics devices, where migration over distances as small as a few nanometres can be significant. Successful modelling of these processes requires a much improved understanding of the mechanisms of self- and dopant diffusion in the alloy, although recent progress has been made. It is the purpose of this review to set this in the context of diffusion processes in elemental silicon and germanium, and to identify how this can help to elucidate behaviour in the alloy. Firstly, self diffusion processes are reviewed, from general agreement that self-diffusion in germanium is dominated by neutral and acceptor vacancies, to the position in silicon which is still uncertain. Germanium diffusion in silicon, however, appears to be via both vacancy and interstitial processes, and in the bulk alloy there is evidence for a change in dominant mechanism at around 35 percent germanium. Next, a review of dopant diffusion begins with Sb, which appears to diffuse in germanium by a mechanism similar to self-diffusion, and in silicon via monovacancies also, from marker layer evidence. In SiGe, the effects of composition and strain in epitaxial layers on Si substrates are also consistent with diffusion via vacancies, but questions still remain on the role of charged defects. The use of Sb to monitor vacancy effects such as grown-in defects by low temperature MBE, are discussed. Lastly, progress in assessing the role of vacancies and interstitials in the diffusion of boron is reviewed, which is dominated by interstitials in silicon-rich alloys, but appears to change to domination by vacancies at around 40 percent germanium, although studies in pure germanium are greatly needed.

Measurement of laser annealed SiCGe by nuclear microprobe analysis

Incorporation of C into SiGe offers a way of improving the lattice match and reducing strain in epitaxial layers on Si substrates. A novel method for the synthesis of SiCGe films involves the deposition of SiGe films on Si single crystal substrates, C ion implantation of the SiGe film, followed by millimetre-sized laser-spot annealing of the film to promote epitaxial regrowth. We have used a nuclear microprobe to obtain channeling contrast images of the annealed regions of films with a composition of Si 1− x−y Ge x C y ( x ∼ 8% and y ∼ 1%). We find that the χ min of the annealed film is a strong function of laser fluence between 400 and 600 mJ/cm 2 (the onset of melting). Also, the regrowth within a 1 × 2 mm 2 laser spot is non-uniform with variations in crystal quality of up to 20%.

Temperature-dependent measurements on ZnSe heterostructures by high-resolution X-ray diffraction

Thermally induced strain in ZnSe epitaxial layers on GaAs occurs due to the different expansion coefficients of the single materials. This strain is directly investigated by high-resolution X-ray diffraction in the temperature range from 10 to 600 K. The strain is measured for a thick relaxed and a thin pseudomorphic layer. Furthermore, the lattice constants of ZnSe and GaAs bulk materials are determined at variable temperatures. It was observed that the thermal expansion is quite linear for GaAs and ZnSe only in the temperature range from 250 to 600 K. The measured temperature-dependent lattice constants and the derived expansion coefficients are input parameters for a model to calculate the thermally induced strain in the epitaxial layers. Good agreement could be obtained between the experimental data and the model.

Determination of elastic strains in epitaxial layers by HREM

A new technique is presented to directly measure strains in epitaxial systems from high-resolution electron microscope (HREM) images. This method involves the calculation of the cumulative sum (CUSUM) of deviations from an average lattice fringe spacing value. Even when the variation in individual lattice fringe spacings is large compared to the difference in fringe spacing due to strain, the CUSUM method is capable of providing reliable strain determinations. The CUSUM approach was applied to three samples of semiconductor In 1−xAl xSb/InSb (0≤x≤0.5) bilayer and superlattice systems in [11¯0] zone-axis projection. It was found that the epitaxial strains obtained from the HREM CUSUM method agreed with the bulk X-ray diffraction values when the sample thicknesses were on the order of 80–100 nm. Thinner specimens, on the order of 10–20 nm, displayed significant surface relaxation effects.

Modulated reflectance study of strain in epitaxial GaAs on silicon

The electrolyte electroreflectance technique was used to study the nature of the strain in epitaxial layers of GaAs grown by laser-assisted metalorganic chemical vapor deposition on (100) Si substrate. The GaAs epilayer, about 140 nm thick, was chemically etched in steps of about 20 nm and spectra of the differential reflectivity (ΔR/R) versus energy were recorded in the 2.4–4.0 eV region immediately following each etch procedure. The E1 and E1+Δ1 transitions of GaAs and their broadening parameters were calculated using the low-field analysis of the electrolyte electroreflectance line shapes. The uniformity of carriers and the effects of the etching procedure were studied with three-dimensional amplitude maps. The variations of the spectral characteristics are interpreted in terms of strains and electronic changes as one approaches the GaAs-Si interface.