Relaxation Of Layers Research Articles

X-ray Study of Strain Relaxation in Heteroepitaxial AlGaAs Layers Annealed under High Hydrostatic Pressure

The effect of treatment at up to 1270 K under hydrostatic argon pressure, up to 1.2 GPa, on strain relaxation of AlGaAs layers was investigated by X-ray diffraction and related methods. The 1.5 μm thick AlGaAs layers weregrown by molecular beam epitaxy method on 001 oriented semi-insulating GaAs substrate at 950 K. An increase in intensity of X-ray diffuse scattering, originating from hydrostatic pressure-induced misfit dislocations, was observed for all treated samples. For the samples treated at 920 K during 1 h under 0.6 GPa, the diffuse scattering was confined to the [110] crystallographic direction perpendicular to the direction of dislocations. For the samples treated at 1.2 GPa at the same temperature and time conditions as for 0.6G Pa, a different behaviour is observed, namely the diffuse scattering extends along all azimuthal directions, indicating that dislocations are created in both [110] and [110] directions. The change of strain after the treatment was most pronounced for the samples treated at 1.2 GPa for 1 h at 920 K.

ANGLE-SCANNED PHOTOELECTRON DIFFRACTION: FROM CLEAN SURFACES TO COMPLEX ADSORPTION SYSTEMS

We exploit the capabilities of photoelectron diffraction (PED) to provide quantitative information on the local structure of the first layers of clean and adsorbate-covered surface systems. Selected studies of low-energy PED are presented, highlighting the advantages of the angle-scanning approach. In the first experiment, we evaluate the clean Rh(110) surface layer relaxation by employing the PED of the Rh 3d5/2 surface component. The resulting relaxation is in good accord with previous low energy electron diffraction data. In the second experiment, a system lacking long-range order is examined, namely the saturation layer formed by nitrogen monoxide on Rh(100) at 123 K. Preliminary results confirm the bridge adsorption geometry model. The last example is a chemical shift PED study of the c(4 × 2) phase of carbon monoxide on Pt(111). In this system, CO molecules are coadsorbed at two different adsorption sites, the energy separation of the respective C 1s components being 0.7 eV. Structural determination has been achieved by an independent analysis of the diffraction yield originated by the two chemically shifted C 1s components. The structure of Pt (111) + c(4 × 2)-2CO has been refined with an automated search of the best parameters using a modified version of MSCD (multiple scattering calculation of diffraction) code.

Thermal expansion behaviour of CdHgTe epitaxial layers on CdZnTe substrates

Abstract In order to clarify critical thickness issues for epitaxial layers of Cd x Hg 1− x Te on Cd 1− y Zn y Te substrates, it is desirable to have accurate data for the difference in thermal expansion between these materials. Based on molecular beam epitaxial samples, we have measured thermal expansion of the two materials simultaneously, over the temperature range 25–110°C, using high-resolution X-ray diffraction. Measured CdHgTe values are corrected for effects of layer strain. Data cover the composition ranges x =0–0.7 and y =0–0.05. Thermal expansion of narrow gap CdHgTe is found to exceed that of lattice matched CdZnTe substrates by approximately 0.3 ppm/K. This can quantitatively account for a previously reported asymmetry in layer relaxation.

Hydrogen adsorption on an open metal surface:H2/Pd(210)

We present self-consistent density-functional calculations of the adsorption of atomic and molecular hydrogen on the (210) surface of palladium using a plane-wave basis set with optimized ultrasoft pseudopotentials. The layer relaxations of the (210) surface and the preferred adsorption sites for atomic hydrogen adsorption are determined. Furthermore, we show that on the rather open Pd(210) surface a molecular ${\mathrm{H}}_{2}$ adsorption state becomes stabilized by the presence of atomic hydrogen on the surface. This provides a consistent explanation of recent experiments. An analysis of the bonding situation in terms of the local density of states is also presented.

Fourier-transform infrared investigations of Si 1− yC y structures for hetero bipolar transistor applications

With the rapid proliferation of commercial SiGe hetero bipolar transistor (HBT) devices, incompatibilities with mainstream integration technologies become an important issue. A common problem in device processing is transient enhanced diffusion of boron out of the base region due to poly-emitter implantation and annealing. It has been shown that co-doping of the base with C can suppress the diffusion of B, but it is accompanied by strong C diffusion. By a combination of Fourier Transform Infrared spectroscopy (FTIR), X-ray diffraction and SIMS studies, we investigated in which form C is present in MBE grown Si 1− y C y layers and in SiGe:C:B HBTs with implanted poly-Si emitter. With the FTIR technique we are sensitive to substitutional C, coherent and incoherent SiC precipitates and some C-containing complexes. With these techniques we studied the time-scale on which the metastable C-containing layer relax by SiC formation.

Observed Critical Thickness in Selectively and Non-Selectively Grown Si1-xGex Layers on Patterned Substrates

Relaxation of SiGe layers grown selectively or non-selectively on oxide-patterned substrates using reduced pressure chemical vapor deposition was investigated. The influences of the buffer layer, the polycrystalline layer on the oxide and the opening size on the critical thickness for relaxation of SiGe layers have been studied in detail. High resolution reciprocal lattice mapping, atomic force microscopy and Normanski optical microscope have been used as the main characterization tools.

Relaxed InAsP layers grown on step graded InAsP buffers by solid source MBE

AbstractSi-doped InAsxP1-x layers with As mole fractions ranging from 0.05 to 0.50 were grown on InAsxP1-x step-graded buffer layers on InP substrates by solid source molecular beam epitaxy. The growth parameters consisted of a P:In flux ratio of 7:1, a growth temperature of ∼ 485°C, a growth rate of 2.2 Å/s, and an As:In flux ratio of 0.37-2.36 for varying As mole fractions. The As mole fraction and the layer relaxation were determined using triple axis x-ray diffraction measurements. Near complete relaxation (>93%) was achieved for all Si-doped InAsxP1-x epilayers. The structural morphology indicated that the InAsxP1-x graded buffer layers were effective in relieving the lattice mismatch strain as evidenced by a well-developed crosshatch morphology and low rms surface roughness. The electron concentration, mobility, and Si donor activation energy for each InAsxP1-x composition were determined using temperature dependent Hall measurements. At a constant electron carrier concentration of %3.5×1016 cm-3, the 300 K carrier mobility increased from 2700 to 4732 cm2/V-sec with increasing As mole fraction from 0.05 to 0.50.

Strain relaxation of strained-Si layers on SiGe-on-insulator (SGOI) structures after mesa isolation

ABSTRACTStrained-Si-On-Insulator (Strained-SOI) MOSFETs are one of the most promising device structures for high speed and/or low power CMOS. In realizing strained-Si MOS LSI, fabrication of strained-Si MOSFETs with small sizes are indispensable and thus, the investigation of the strain relaxation is an important issue. Therefore, the strain relaxation of strained-SOI mesa islands with small active area was investigated in this study. Thin strained-Si films were grown on thin relaxed SiGe-on-insulator (SGOI) structures (x=0.28). The isolation process was carried out by using chemical-dry-etching (CDE) to fabricate samples with small active areas. Using Raman spectroscopy with resolution of > 1 micron meter, strained-Si islands on SGOI substrates with the size of 5 micron meter square were investigated. Rapid-thermal-annealing (RTA) in N2 atmosphere was performed to study the strain relaxation during heating processes. As a result, it was confirmed that the strained-Si layers grown on relaxed SiGe (x=0.28) before and after mesa isolation, down to 5 micron meter in size, had almost no relaxation after the RTA process at 1000°C. Furthermore, it was confirmed that the nano-beam electron diffraction (NBD) measurement showed similar tendency regarding the strain relaxation.

Lattice Relaxation of AlN Buffer on Surface-Treated SiC in Molecular-Beam Epitaxy for Growth of High-Quality GaN

ABSTRACTThe effects of SiC surface treatment on the lattice relaxation of AlN buffer layers and the crystalline quality of GaN layers grown on the buffer layers were studied. AlN buffer layers and GaN main layers were grown by plasma-assisted molecular-beam epitaxy on on-axis 6H-SiC (0001)Si substrates. High-temperature HCl-gas etching resulted in an atomically flat SiC surface with (√3×√3)R30° surface reconstruction, while HCl-gas etching followed by HF chemical treatment resulted in an atomically flat surface with (1×1) structure. The AlN layer grown on the (1×1) surface showed slower lattice relaxation. GaN grown on the AlN buffer layer exhibited a (0002) X-ray rocking curve of 70 arcsec and 107 cm−2 of screw-type dislocation density, which was superior than that of GaN grown on (√3×√3)R30° surface.

Discrete Mosaic “Spread” at the Surface of ZnSe Layers on GaAs(001)

Thin pseudomorphic ZnSe films can be grown by molecular beam epitaxy on GaAs(001) up to a layer thickness of about 100 nm. Above this critical thickness the topmost layers relax by forming large mosaic-domains which are tilted by 0.2° with respect to the macroscopic surface. This is evidenced by satellites arising in discrete directions close to the fundamental diffraction spots in high resolution low energy electron diffraction (SPA-LEED). Energy dependent SPA-LEED measurements were performed to characterize the satellites and identify their origin. High resolution X-ray diffraction (HRXRD) experiments indicate that this mosaicity is restricted to a near-surface region and does not penetrate deeply into the bulk of the ZnSe film.

Epitaxial growth of InGaAs on misoriented GaAs(1 0 0) substrate by metal-organic vapor phase epitaxy

In 0.35Ga 0.65As layers were grown on GaAs(1 0 0) substrates by metal-organic vapor phase epitaxy using graded buffer layers. GaAs(1 0 0) substrates misoriented toward (1 1 1)A and (1 1 1)B and exactly oriented GaAs(1 0 0) substrates were used. Surface morphology of InGaAs layers strongly depended on the substrate misorientation direction. The lowest root mean square of surface roughness was obtained for the layer on the substrate misoriented toward (1 1 1)A under optimized conditions. Strain relaxation in InGaAs layers was influenced by substrate misorientation. Threading dislocation densities were 6.4×10 6, 3.6×10 6, and 2.0×10 6 cm −2 in InGaAs layers on the exactly oriented substrate and substrates misoriented toward (1 1 1)A and (1 1 1)B, respectively.

Development of giant low-field magnetostriction in a-TerfecoHan-based single layer, multilayer and sandwich films

Giant magnetostriction in low magnetic field has been achieved with different approaches, among which single, multilayer and sandwich films with rare-earth base are considered as most promising ones. Enhancement of the 3d(Fe,Co)-magnetic moment with respect to those of 3d(Fe) and 3d(Co) strengthens the Tb–FeCo exchange energies in amorphous Tb(Fe 0.55Co 0.45) 1.5 (named as a-TerfecoHan) films. This is thought to cause diminishing of the Tb-sperimagnetic cone-angle and, thus, leading to enhancement of the magnetostriction. Indeed, a saturation magnetostriction of λ γ,2 ∼10 −3 has been obtained. After annealing, a parallel magnetostrictive susceptibility of χ λ∥ =1.8×10 −2 T −1 has been achieved at μ o H=10 mT. TbFeCo/YFe multilayers combining exchange-coupled giant magnetostrictive TerfecoHan layers and large-magnetisation Y 0.2Fe 0.8 ones exhibit an excellent magnetic and magnetostrictive softness. Subsequent annealing at 350°C leads to the relaxation of the a-TbFeCo layers and to a nanocrystallisation of the YFe layers. In this state, a magnetic coercive field μ o H C=0.3 mT and a huge magnetostrictive susceptibility, χ λ∥ =13×10 −2 T −1 have been obtained. Mössbauer studies indicate that the magnetic softness as observed is associated to the evolution of the nanocrystalline Fe-particles. Magnetostriction is also presented for Fe/TerfecoHan/Fe sandwiches. In this case, giant magnetostriction is already developed at low fields in as-deposited films. Technologically, this seems to be one of the simplest ways to prepare magnetostrictive films for microsystem applications.

Micro-Raman Study of Wurtzite AlN Layers Grown on Si(111)

Strain relaxation in cracked wurtzite AIN layers grown on Si(111) substrates was investigated by micro-Raman spectroscopy, under excitations at 2.54 and 3.81 eV. Crack-free areas of the layers exhibit a residual tensile stress, which is evidenced by the redshift of the frequency of the E 2 phonon of AIN. A cartography of the strains in a 0.4 μm thick AIN layer was derived from measurements of the spatial variation of the E 2 phonon frequency. We deduced that the cracking of the AIN layer leads to a relaxation of residual strain of about 60%.

Spectroscopic and structural characterisation of a VO x ( x≈1) ultrathin epitaxial film on Pt (111)

VO x ultrathin epitaxial films (0.8≤ x≤1.3), grown on Pt(111) by evaporating vanadium in a controlled water background (1×10 −7 Pa), have been chemically characterised by X-ray photoelectron spectroscopy (XPS) and X-ray-excited Auger electron spectroscopy (AES), which confirm the presence of V(II). The VO film shows a NaCl-type structure exposing the (111) plane, as proven by XPD. Multiple scattering calculations are compatible with an O-terminated surface and a surface relaxation of the outermost atomic layers, which leads to a VO bond length contraction amounting to 7%.

Brown Fat UCP1 Is Specifically Expressed in Uterine Longitudinal Smooth Muscle Cells

Until now, uncoupling protein 1 (UCP1) was considered as unique to brown adipocytes. It supports a highly regulated uncoupling of oxidative phosphorylation that is associated with diet as well as with non-shivering thermogenesis. Here we report that UCP1 is not specific to brown adipocytes and can be expressed in longitudinal smooth muscle layers. In the uterus, this conclusion was drawn from different convergent data. A specific antibody against mouse UCP1 revealed, in mitochondrial fractions, a protein with the same molecular weight as brown fat UCP1. Sensitive and specific reverse transcriptase-polymerase chain reaction detected a mRNA whose sequence was totally homologous to that of brown fat UCP1 mRNA. Antibody against UCP1 as well as a UCP1 antisense probe specifically stained uterine longitudinal smooth muscles. UCP1 was also expressed in longitudinal smooth muscle of digestive and male reproductive tracts but was never expressed in other types of smooth muscle, including those of arterial vessels. In uterine tract, UCP1 content was increased after cold exposure or beta-adrenergic agonist treatment. It was also up-regulated during the postovulatory period after sexual cycle synchronization. Its content transiently increased during gestation and decreased markedly after birth. These regulations strongly argue about a role for UCP1 in thermogenesis as well as in relaxation of longitudinal smooth muscle layers.

Micro-Raman Study of Wurtzite AlN Layers Grown on Si(111)

Strain relaxation in cracked wurtzite AIN layers grown on Si(111) substrates was investigated by micro-Raman spectroscopy, under excitations at 2.54 and 3.81 eV. Crack-free areas of the layers exhibit a residual tensile stress, which is evidenced by the redshift of the frequency of the E 2 phonon of AIN. A cartography of the strains in a 0.4 μm thick AIN layer was derived from measurements of the spatial variation of the E 2 phonon frequency. We deduced that the cracking of the AIN layer leads to a relaxation of residual strain of about 60%.

Structural characterization of InGaAs/InP heterostructures grown under compressive and tensile stress

A systematic study of strain relaxation mechanisms by TEM, XRD, RBS-channelling, SEM-CL and AFM in In x Ga 1− x As/InP heterostructures grown by MOCVD under tensile and compressive initial misfit is reported. It is found that the layers under compression (0.61< x<0.74) relax nearly symmetrically along the two 〈1 1 0〉 directions. The residual strain vs. the layer thickness follows the same law experimentally determined for compressive MBE-grown In x Ga 1−x As/GaAs (x<0.2) specimens showing no composition, growth technique or temperature effect. The tensile layers (0.2< x<0.36) relax asymmetrically with a critical thickness larger than for the compressive case. The relaxation is faster along the [1 1 0]-direction with the asymmetry that increases by increasing the tensile misfit. Cracks, formed after the growth, are found to present a different density along [1 1 0] and [1 −1 0] depending on the residual strain. Grooves also develop along the two 〈1 1 0〉 directions as soon as a measurable strain relaxation appears. A possible correlation between grooves, planar defects and the mechanism of strain release is discussed. Strain-balanced In x Ga 1− x As/In y Ga 1− y As multi-quantum wells (MQWs) grown on a compositionally graded In x Ga 1− x As/GaAs buffer to extend the absorption edge of photovoltaic devices to 1 eV are finally investigated. The MD network results in a marked cross-hatched morphology that affects the lateral strain distribution in the whole structure. Lateral composition fluctuations in the MQW are discussed in terms of step bunching mechanism occurring at the valleys of the cross-hatched surface.

Active displacement field in the Suez–Sinai area: the role of postseismic deformation

By means of a spherical viscoelastic model of co- and postseismic deformation we compute the postseismic displacement associated with the November, 1995, M w=7.2, Aqaba (Jordan) earthquake. We compare our results with the observational data obtained by a GPS campaign performed in a wide portion of Sinai between November 1997 and May 1998. Though the original purpose of the campaign was to explain the tectonic features of the region, the displacement values deduced from the GPS observations are hardly reconcilable with any of the proposed tectonic models of Sinai. Our hypothesis is that the detected deformation field could be due more to the postseismic relaxation following the Aqaba earthquake rather than to tectonic movements. Our results show that both GPS observational data and the postseismic simulations predict a compressional regime in the Gulf of Suez though the contraction detected by GPS is generally larger. The best agreement with the data is obtained for an asthenospheric and lower crust viscosity of 10 18 Pa s and an absence of aseismic afterslip on the fault plane. We conclude that the contribution coming from the postseismic viscoelastic relaxation of the ductile shallow layers following the 1995 Aqaba earthquake is likely to play an important role in determining the present deformation field in Sinai. The transient postseismic relaxation seems to play a more important role than the secular tectonic deformation.

Electronic properties of the Fe/GaAs(0 0 1) interface

The electronic properties of Fe/GaAs(1 0 0) depend on the energetics of the GaAs(1 0 0)-(2×4) reconstruction as well as on the relaxation of Fe layers. Employing a pseudopotential ab initio method, we show that the GaAs(0 0 1)-(2×4) As-terminated surface is composed of two Ga and two As dimers with dimer axes perpendicular to each other in agreement with experiment. The influence of relaxing all atoms on the Schottky barrier has been calculated for different super-cell sizes showing a trend towards larger barrier heights for larger volumes of the Fe layers.

Electrical Properties of 3C-SiC Layers Grown on Silicon Substrates with a Novel Stress Relaxation Structure

Silicon (Si) substrates having cavities just beneath the surface layer (multicavity Si substrates) were examined as the stress relaxation structure in 3C-SiC heteroepitaxial growth on Si substrates. Single-crystalline 3C-SiC layers were grown on both the multicavity Si substrates and conventional ones by means of low pressure chemical vapor deposition. The layers’ quality was characterized by cross-sectional transmission electron microscope (TEM) observations, micro-Raman spectroscopy, current-voltage characteristics, and capacitance-voltage (C-V) characteristics. The TEM results showed that this structure reduced the defect density in the 3C-SiC layers. The averaged full width at half-maximum of TO Raman band in the 3C-SiC layers on the multicavity Si substrates was narrower than that on the conventional ones. Furthermore, the characteristics of the Schottky barrier structures showed that the reverse leakage current of the diodes using the multicavity Si substrates was lower than that using the conventional ones. The depth profile of the carrier concentration in the 3C-SiC layers determined by the C-V characteristics implied the improvement of the crystal quality of the 3C-SiC layers grown on the multicavity substrates. These results indicate that the multicavity Si substrates are effective for stress relaxation in the 3C-SiC layers. © 2001 The Electrochemical Society. All rights reserved.