Relaxation Of Layers Research Articles

Investigation of the Thermal Stability of Strained Ge Layers Grown at Low Temperature by Reduced-pressure Chemical Vapour Deposition on Si0.2Ge0.8 Relaxed Buffers

AbstractHigh quality strained Ge (s-Ge) epitaxial layers are a promising candidate to achieve high mobility channel MOSFETs suitable for the 22 nm technology node and beyond, due to the intrinsically higher mobility of Ge compared to Si, and the additional performance enhancements from strain [1]. In order to achieve an s-Ge channel more than a few monolayers thick it is necessary to engineer a relaxed Si1-xGex buffer with a high Ge content (x > 0.5). We have recently reported high quality s-Ge layers grown by RP-CVD at low temperature (T ≤ 450 °C), on a fully relaxed Si0.2Ge0.8 buffer [2]. By using a reverse-grading approach, we achieved a high Ge composition in the buffer, with a smooth surface (rms surface roughness of ~2 nm), low threading dislocations density (~ 4 x 106 cm-2) and much thinner (~ 2.1 μm) than can be achieved with conventional linear grading [3].In this work, the thermal stability of s-Ge epilayers (up to 80 nm thick) grown on relaxed Si0.2Ge0.8 buffers has been investigated by in-situ annealing in H2 ambient at temperatures up to 650 °C. These temperatures are similar to those currently used during fabrication of advanced CMOS devices. All s-Ge layers were grown at 400 °C using GeH4 gaseous precursor. The relaxation of the annealed layers has been studied using high-resolution XRD reciprocal space maps (RSMs), and was found to depend strongly on both annealing temperature and thickness of the Ge epilayer. Strained Ge layers up to 50 nm thick remained fully strained after annealing at 450 °C, whereas after annealing at 550 °C s-Ge layers thicker than 20 nm were on the onset of relaxation; after annealing at 650 °C all s-Ge layers showed significant relaxation with defects clearly visible at the Si0.2Ge0.8/Ge interface. All annealed s-Ge layers exhibited higher surface roughness than s-Ge control samples without annealing (rms ~ 2 nm). Annealing at 450 °C resulted in only a slight increase in surface roughness (rms ~ 3 nm), almost independent of s-Ge thickness. However, annealing at 550 °C and 650 °C resulted in significant surface roughening (with maximum rms values of 5 nm and 35 nm, respectively) due to the formation of Ge islands, which were observed by AFM. At these higher temperatures, the surface roughness of the s-Ge layers was found to be thickness dependent, with a Ge smoothing effect observed for layers greater than 50 nm.These results are particularly important for the fabrication of s-Ge MOSFETs, for which the surface passivation prior to gate stack formation is critical to the performance of the device. Based on the results presented here, the thermal budget should be kept below 550 °C to avoid relaxation and roughening of the s-Ge epilayer, which could degrade the device performance.

Growth of Metamorphic InGaP for Wide-Bandgap Photovoltaic Junction by MBE

AbstractMetamorphic triple-junction solar cells can currently attain efficiencies as high as 41.1%. Using additional junctions could lead to efficiencies above 50%, but require the development of a wide bandgap (2.0-2.2eV) material to act as the top layer. In this work we demonstrate wide bandgap InyGa1-yP grown on GaAsxP1-x via solid source molecular beam epitaxy. Unoptimized tensile GaAsxP1-x buffers grown on GaAs exhibit asymmetric strain relaxation, along with formation of faceted trenches 100-300 nm deep in the [01-1] direction. Smaller grading step size and higher substrate temperatures minimizes the facet trench density and results in symmetric strain relaxation. In comparison, compressively-strained graded GaAsxP1-x buffers on GaP show nearly-complete strain relaxation of the top layers and no evidence of trenches. We subsequently grew InyGa1-yP layers on the GaAsxP1-x buffers. Photoluminescence and transmission electron microscopy measurements show no indication of phase separation or CuPt ordering. Taken in combination with the low threading dislocation densities obtained, MBE-grown InyGa1-yP layers are promising candidates for future use as the top junction of a multi-junction solar cell.

Layer and orientation resolved bond relaxation and quantum entrapment of charge and energy at Be surfaces

The chemistry and physics of under-coordination at a surface, which determines the process of catalytic reactions and growth nucleation, is indeed fascinating. However, extracting quantitative information regarding the coordination-resolved surface relaxation, binding energy, and the energetic behavior of electrons localized in the surface skin from photoelectron emission has long been a great challenge, although the surface-induced core level shifts of materials have been intensively investigated. Here we show that a combination of the theories of tight binding and bond order-length-strength (BOLS) correlation [C. Q. Sun, Prog. Solid State Chem., 2007, 35, 1-159], and X-ray photoelectron spectroscopy (XPS) has enabled us to derive quantitative information, by analyzing the Be 1s energy shift of Be(0001), (1010), and (1120) surfaces, for demonstration, regarding: (i) the 1s energy level of an isolated Be atom (106.416 ± 0.004 eV) and its bulk shift (4.694 eV); (ii) the layer- and orientation-resolved effective atomic coordination (3.5, 3.1, 2.98 for the first layer of the three respective orientations), local bond strain (up to 19%), charge density (133%), quantum trap depth (110%), binding energy density (230%), and atomic cohesive energy (70%) of Be surface skins up to four atomic layers in depth. It is affirmed that the shorter and stronger bonds between under-coordinated atoms perturb the Hamiltonian and hence the fascinating localization and densification of surface electrons. The developed approach can be applied to other low-dimensional systems containing a high fraction of under-coordinated atoms such as adatoms, atomic defects, terrace edges, and nanostructures to gain quantitative information and deeper insight into their properties and processes due to the effect of coordination imperfection.

Structure and water dynamics of aqueous peptide solutions in the presence of co-solvents

We perform neutron diffraction and quasi-elastic neutron scattering (QENS) to probe hydration water structure, and dynamics down to supercooled temperatures, of a concentrated amphiphilic peptide system with the co-solvents glycerol and dimethyl sulfoxide. We find that the kosmotropic co-solvent glycerol preserves the hydration structure near the peptide that is observed in the water solvent alone, that in turn preserves the dynamical temperature trends of two water relaxation processes--one corresponding to a localized relaxation process of the peptide bound surface water and a second relaxation process of the outer hydration layers. By contrast the chaotropic co-solvent, by disrupting the hydration layer near the peptide surface, eliminates the inner hydration layer relaxation process induced by the peptide, to show a single time scale for translational water dynamics.

Resonant MEMS based on cubic GaN layers

AbstractIn this work, we report the fabrication of GaN microelectromechanical structures (MEMS) based on cubic GaN grown by molecular beam epitaxy on 3C‐SiC(100)/Si(100) pseudosubstrates. Free‐standing beam resonators with a width of 5 μm and lengths from 250 to 1000 μm were patterned and characterized. Magnetomotively and electrostatically actuated resonators were analyzed under ambient and vacuum conditions resulting in quality factors of up to 250 under ambient conditions up to 40.000 under vacuum (1.9x10‐5 mbar) and resonant frequencies from 97 to 406 kHz for the fundamental resonant mode. These results allow for the determination of the Young's modulus and the residual strain of the free standing structures. The measured GaN resonators exhibited Young's moduli from 235 to 260 GPa and high residual axial strains from 1.0x10‐3 to 1.25x10‐3. For the cubic GaN layers from which the resonators were fabricated, residual strains between 3.3x10‐3 and 5.8x10‐3 have been extracted from XRD measurements. These values are on the same order of magnitude but a definite factor higher compared to the measurement results obtained for the fabricated GaN‐beams, which indicates a partial layer relaxation caused by the influences of the MEMS fabrication processes. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural perfection of InGaN layers and its relation to photoluminescence

AbstractThe relation between structural perfection and optical properties of InGaN with 10% In are discussed. Transmission Electron Microscopy, X‐ray diffraction and Rutherford backscattering spectrometry measurements show that only strained layers with a thickness not exceeding 100 nm are defect free and In concentration is lower than the nominal value. Extension of layer thickness leads to layer sequestration into sublayers with different In contents and the formation of planar defects as a result of layer relaxation. In concentration in such sublayers reach and in some cases exceed the nominal concentration. A single band edge photoluminescence peak is observed only for the thinnest layer. Samples with larger film thickness showed multiple PL peaks corresponding to layers with different In content. Much higher In content would be required to explain the presence of some PL peaks, suggesting that some PL peaks originate from the defective areas of the film. This was confirmed by cathodoluminescence studies performed on the same samples used earlier for TEM studies. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Efficient relaxation of strained-SiGe layers induced by thermal oxidation

The new route to fabricate compositionally graded and highly-relaxed Si 1-xGe x layers using thermal oxidation at high temperatures is investigated. Ge atoms behavior during thermal oxidation of Si 1-xGe x layers is strongly dependent on the oxidation temperature. For low temperature oxidation processes Ge is incorporated as GeO 2 in the grown oxide layer, while for higher temperatures it accumulates below the grown oxide into a layer with a higher concentration than the initial Si 1-xGe x. However, Si 1-xGe x layers oxidized at 1000 °C did not show such an accumulation layer because Ge diffusion efficiently occurred, resulting in the formation of a compositionally graded and relaxed Si 1-xGe x layer. Such layers could be used as virtual substrates for the strained-Si and relaxed-SiGe applications.

Ion energy and dose dependence of strain relaxation for thin SiGe buffer layers using Si + implantation

We systematically investigated ion energy and dose dependencies of strain relaxation for SiGe buffer layers using Si + ion implantation. It was found that for the SiGe grown on Si substrates implanted at the energy of 25 keV and with a dose of 5 × 10 14 cm − 2, relaxation ratio was highly enhanced with very smooth surface. The obtained ratio was 65% higher than that of unimplanted samples. However, strain relaxation ratios were seen to reduce as ion energy increased or ion dose increased. This result indicated that the End-of-Range (EOR) defects generated by ion implantation and post annealing were responsible for strain relaxation and that the depth of the EOR defects from the SiGe/Si heterointerface was very crucial for strain relaxation of the SiGe layers.

Dislocation-interaction-based model of strained-layer relaxation

A strained layer relaxes plastically when dislocations propagate within the layer, leaving behind an array of misfit dislocations at the layer interface. We develop an analytical model of this process, based on the idea that relaxation is frustrated when propagating dislocations are trapped or annihilated by encounters with previously created misfit dislocations or other propagating dislocations. The theory characterizes the evolving density of the misfit array and the density of propagating dislocations in terms of a pair of coupled rate equations. The two trapping functions which appear in these equations are evaluated quantitatively by numerically investigating all possible dislocation-dislocation encounters. Fluctuations in the local stress field driving the individual dislocations are explicitly taken into account when evaluating the trapping functions. Analysis of the rate equations shows that there are two regimes in the strain-relaxation dynamics. Initially, the strain decreases rapidly following a universal dependence on time scaled with the initial dislocation density n0. At a (rescaled) crossover time that increases with n0, the strain levels off from the universal relaxation curve and saturates to an asymptotic residual strain level, which decreases with n0. Microscopically, our model reveals that the initial fast strain-relaxation regime is dominated by collisions between propagating dislocations, while the slow saturation regime is dominated by the trapping of propagating dislocations by the misfits. In the end, the self-trapping of the propagating dislocations by the misfit array they themselves have generated leaves the layer in a frustrated state with residual strain higher than the critical strain. The predictions of the theory are found to be in good agreement with experimental measurements and with large-scale numerical simulations of layer relaxation.

Relaxation of strained pseudomorphic SixGe1−x layers on He-implanted Si/δ-Si:C/Si(100) substrates

In this letter we present a method to increase the efficiency of SiGe layer relaxation by He+ ion implantation and annealing. Preferential nucleation of He platelets along a δ-impurity layer grown in the Si substrate below the SiGe layer results in planar localization and homogenization of dislocation loop sources inducing a more uniform distribution of misfit dislocations. We demonstrate this for a thin Si:C layer grown by reduced pressure chemical vapor deposition. The optimization of the conditions for efficient relaxation and layer quality is studied with respect to the position of the Si:C layer and the process parameters. Relaxation degrees up to 85% are obtained for Si0.77Ge0.23 layers.

Substrate-induced antiferromagnetism of a Fe monolayer on the Ir(001) surface

We present detailed ab initio study of structural and magnetic stability of a Fe-monolayer on the fcc(001) surface of iridium. The Fe-monolayer has a strong tendency to order antiferromagnetically for the true relaxed geometry. On the contrary an unrelaxed Fe/Ir(001) sample has a ferromagnetic ground state. The antiferromagnetism is thus stabilized by the decreased Fe-Ir layer spacing in striking contrast to the recently experimentally observed antiferromagnetism of the Fe/W(001) system which exists also for an ideal bulk-truncated, unrelaxed geometry. The calculated layer relaxations for Fe/Ir(001) agree reasonably well with recent experimental LEED data. The present study centers around the evaluation of pair exchange interactions between Fe-atoms in the Fe-overlayer as a function of the Fe/Ir interlayer distance which allows for a detailed understanding of the antiferromagnetism of a Fe/Ir(001) overlayer. Furthermore, our calculations indicate that the nature of the true ground state could be more complex and display a spin spiral-like rather than a c(2x2)-antiferromagnetic order. Finally, the magnetic stability of the Fe monolayer on the Ir(001) surface is compared to the closely related Fe/Rh(001) system.

Vacancy-driven stress relaxation in layers

The role of vacancies together with their generally non-ideal sources and sinks has been investigated previously in the context of multi-component diffusion. This concept is applied to a thin layer on a substrate subjected to an initial eigenstress state. The surface of the layer is assumed to act as an ideal source and sink for vacancies. The interface may act either as an ideal or as no source and sink for vacancies. Non-ideal sources and sinks are supposed in the bulk. The formation of a vacancy site fraction profile across the layer due to active sources and sinks and due to diffusion is studied, provoking a relaxation of the residual eigenstress state. A set of two nonlinear partial differential equations for both the vacancy site fraction and the eigenstress distributions is developed. The problem is reformulated in dimension-free quantities. The relation between the diffusivity and the activity of sources and sinks for vacancies in the bulk is characterized by a vacancy intensity parameter k. Numerical solutions are presented, and the influence of nonlinear terms in the evolution equations is evaluated. The simulations provide the evolution of the vacancy site fraction and of the hydrostatic stress profiles in the layer. The development of a film of newly deposited or removed atoms at the layer surface and of the total thickness of the layer is also calculated. The results of simulations are discussed.

Strain relaxation of SiGe in a Si/SiGe/Si heterostructure under proton irradiation

We have studied the mechanisms underlying strained layer relaxation by means of point defect interaction. During high temperature (300 °C) proton irradiation, vacancies generated in the vicinity of SiGe layer migrate and accumulate within the compressively strained SiGe layer. The accumulating vacancies are stabilized by hydrogen, which diffuses from the implanted region, thus allowing the nucleation and growth of hydrogen-vacancy (V-H) complexes. The formation of V-H complexes is accompanied by gradual strain relief in SiGe layer. Since the diffusion of both vacancies and hydrogen is limited by the irradiation temperature, strain relaxation of the SiGe layer is not realized during room temperature (20 °C) proton irradiation. The study supports the idea that the compressive stress in the SiGe layer induces the indiffusion of vacancies and H, and reveals the important role of point defects in the strain relaxation of the strained SiGe layer.

Heterogeneous Onset of Myocardial Relaxation in Subendocardial and Subepicardial Layers Assessed With Tissue Strain Imaging: Comparison of Normal and Hypertrophied Myocardium

We sought to investigate the existence of a time difference in myocardial relaxation between the subendocardium and subepicardium in patients with and without myocardial hypertrophy. Regional differences in mechanical and electrical properties between the subendocardium and subepicardium have been described for the left ventricle in animals. However, this difference has not been well evaluated in clinical conditions. Time-to-peak radial strain with reference to the QRS complex was measured at the subendocardium and subepicardium at the mid-posterior wall of the left ventricle in 12 normal subjects, 14 patients with hypertensive heart disease, and 27 patients with aortic stenosis (16 with and 11 without strain electrocardiogram [ECG] pattern) using tissue Doppler-based strain imaging. Time-to-peak radial strain in the subepicardium (381 +/- 60 ms) was shorter than that in the subendocardium (463 +/- 29 ms; p < 0.001) in normal subjects, suggesting that the subepicardial relaxation precedes subendocardial relaxation. No significant difference was found between normal subjects and patients with hypertensive heart disease (388 +/- 67 ms for the subepicardium; 455 +/- 36 ms for the subendocardium in hypertensive heart disease). In cases with hypertrophied myocardium due to aortic stenosis, time-to-peak radial strain in the subendocardium was shortened and that in the subepicardium was prolonged. In 10 (63%) of 16 patients with aortic stenosis and strain ECG pattern, the timing of peak strain in the subendocardium (417 +/- 63 ms) preceded that in the subepicardium (452 +/- 62 ms). There is heterogeneous onset of myocardial relaxation in the subendocardial and subepicardial layers at the mid-posterior wall of the left ventricle. Subepicardial myocardial relaxation precedes subendocardial relaxation in normal subjects. In contrast, there is inversion of the transmural sequence of myocardial relaxation between the subendocardium and subepicardium in some patients with aortic stenosis and strain ECG pattern.

Dielectric behaviour and relaxation processes of montmorillonite clay nano‐platelet colloidal suspensions in poly(vinyl pyrrolidone)–ethylene glycol oligomer blends

AbstractBACKGROUND: Intercalated and exfoliated montmorillonite (MMT) clay structures in polymer matrices improve the thermal, mechanical, electrical and pharmaceutical properties of organic–inorganic materials. Poly(vinyl pyrrolidone) (PVP)–ethylene glycol oligomer (EGO) blends are biocompatible and non‐toxic materials. The dielectric characterization of MMT clay nano‐platelet colloidal suspensions in PVP–EGO blends is important in understanding the ionic conduction behaviour in many complex phenomena occurring in biological systems, and in selective membranes and their use in controlled drug release systems and in liquid electrolytes.RESULTS: An investigation using dielectric spectroscopy in the 20 Hz to 1 MHz frequency range of MMT clay nano‐platelet colloidal suspensions in PVP–EGO blends confirmed that the PVP segmental motion, ionic conduction relaxation time, electric double layer relaxation time and direct current electrical conductivity are significantly influenced by the clay concentration and EGO chain length. In these materials, ionic motion and PVP segmental dynamics are strongly coupled. Intercalation of EGO structures in clay galleries and exfoliation of clay platelets by adsorption of PVP–EGO structures on clay surfaces are governed by hydrogen bonding interactions between the carbonyl groups of PVP monomer units, the hydroxyl groups of EGOs and the hydroxylated aluminate surfaces of the MMT clay.CONCLUSION: The dielectric behaviour of intercalated and exfoliated structures of MMT clay nano‐platelet colloidal suspensions in PVP–EGO blends provides a convenient way to obtain liquid organic‐inorganic polymeric nanocomposite electrolytes with tailored ionic conduction properties. Copyright © 2009 Society of Chemical Industry

Heterogeneous nucleation and adatom detachment at one-dimensional growth of In onSi(100)−2×1

Growth of atomic indium chains---one-dimensional islands---on the $\text{Si}(100)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}1$ surface was observed by scanning tunneling microscopy (STM) at room temperature and simulated by means of a kinetic Monte Carlo method. Density of indium islands and island-size distribution was obtained for various deposition rates and coverage. STM observation of growth during the deposition of indium provided information on growth kinetics and relaxation of grown layers. Important role of $C$-type defects at adsorption of metal atoms was observed. Measured growth characteristics were simulated using a microscopic model with anisotropic surface diffusion and forbidden zones along the metal chains. An analysis of experimental and simulation data shows that the detachment of indium adatoms from the chains substantially influences a growth scenario and results in monotonously decreasing chain length distribution function at low coverage. Diffusion barriers determined from the simulations correspond to almost isotropic diffusion of indium adatoms on the surface. The results are discussed with respect to data reported in earlier papers for other metals.

Control of Sn Precipitation and Strain Relaxation in Compositionally Step-Graded Ge1-xSnx Buffer Layers for Tensile-Strained Ge Layers

We investigated the relationship between Sn precipitation and strain relaxation in Ge1-xSnx buffer layers grown by the compositionally step-graded (CSG) method on a virtual Ge substrate. We found that the strain in the upper Ge1-xSnx layers is reduced by Sn precipitation rather than the lateral propagation of misfit dislocations at the interfaces of upper Ge1-xSnx layers in the CSG method. The critical misfit strain was increased to 5.8 ×10-3 compared with that in our previous work by lowering the temperature of the postdeposition annealing, and a Sn content of 6.3% in the Ge1-xSnx buffer layer was achieved with a large degree of strain relaxation using only two stacked layers of the CSG structure. An in-plane tensile strain of 0.62% in a 30-nm-thick Ge layer fabricated on these Ge1-xSnx buffer layers was achieved.

Role of Substrate Misorientation in Relaxation of 3C-SiC Layers on Silicon

We analyze in detail the evolution of curvature of 3C-SiC layers grown on vicinal silicon substrates. A common feature of (100) and (111) oriented layers is a strongly asymmetric wafer bending that may suggest an anisotropic stress relaxation within the layer. A comparative study of the curvature, lattice parameter, surface morphology and structural defects for the off-angles ranging from 0.5° to 6° is performed in order to confirm or disprove this hypothesis. We find a homogeneous, tensile in-plane lattice deformation. We also show the correlation of the orientation of the high and low curvature axes with the morphology and defect pattern of the layer.

Surface relaxation and stress for 5d transition metals

Using the density functional theory, we present a systematic theoretical study ofthe layer relaxation and surface stress of 5d transition metals. Our calculationspredict layer contractions for all surfaces, except for the (111) surface of facecentered cubic Pt and Au, where slight expansions are obtained similarly to the caseof the 4d series. We also find that the relaxations of the close packed surfacesdecrease with increasing occupation number through the 5d series. The surfacestress for the relaxed, most closely packed surfaces shows similar atomic numberdependence as the surface energy. Using Cammarata’s model and our calculatedsurface stress and surface energy values, we examine the possibility of surfacereconstructions, which is in reasonable agreement with the experimental observations.

Magnetoresistance calculation in current-perpendicular-to-plane giant magnetoresistance spin valves with current-confined paths

We compared the magnetoresistance (MR) ratios of current-confined-path (CCP) current-perpendicular-to-plane (CPP) giant MR spin valves (SVs) calculated using the two-current-model. The simplified two-current model, which places the parasitic resistance external to the two-current network, has been inaccurate in determining the overall resistance of a SV in parallel magnetization configuration. The characteristics of the error in MR depend on the parasitic resistance value, the spin asymmetry coefficient, and the spacer layer resistivity. In comparison to the complete two-current model, we found that although the simplified model could generally predict the trend of various MR behaviors, it also overestimates MR for different CCP densities. A more comprehensive calculation on the CCP SVs was then performed using the spin-drift-diffusion equations based on the Valet–Fert model. It was found that the MR behavior upon considering spin relaxation in the ferromagnetic layers has a closer match with results predicted by the simplified model due to the strong reduction in spin accumulation in the parasitic layers. Spin relaxation in the spacer layer, however, shows a competing effect that tends to reduce the MR ratios. Our results in this paper are useful for accurate evaluation of MR performance in a CPP SV sensor with current-confined paths.