Significant Strain Relaxation Research Articles

Nanoair-bridged lateral overgrowth of GaN on ordered nanoporous GaN template

We report the growth of high-quality GaN epilayers on an ordered nanoporous GaN template by metalorganic chemical vapor deposition. The nanopores in GaN template were created by inductively coupled plasma etching using anodic aluminum oxide film as an etch mask. The average pore diameter and interpore distance is about 65 and 110nm, respectively. Subsequent overgrowth of GaN first begins at the GaN crystallite surface between the pores, and then air-bridge-mediated lateral overgrowth leads to the formation of the continuous layer. Microphotoluminescence and micro-Raman measurements show improved optical properties and significant strain relaxation in the overgrown layer when compared to GaN layer of same thickness simultaneously grown on sapphire without any template. Similar to conventional epitaxial lateral overgrown GaN, such overgrown GaN on a nanopatterned surface would also serve as a template for the growth of ultraviolet-visible light-emitting III-nitride devices.

In Situ Synchrotron Measurements of Oxide Growth Strains

Synchrotron x-rays are used for in situ determination of oxide strain, during oxide formation on a Kanthal A1 FeCrAlZr substrate at 1160°C. The measurements rely on use of high-energy (~80keV) x-rays and transmission geometry, and the methodology of the strain measurements is presented. Oxide growth strains at elevated temperature, relative to pure alumina, were seen to be small, while temperature excursions induced significant strains. Furthermore, significant strain relaxation was observed during isothermal holds, suggesting oxide creep as a major relaxation mechanism. Upon cooling to room temperature, significant residual strains developed, with a corresponding in-plane residual stress of -3.7 GPa.

Strain relaxation of epitaxial CoSi2 and SiGe layers in cap-Si/Si0.83Ge0.17/Si(0 0 1) and epi-CoSi2/Si0.83Ge0.17/Si(0 0 1) structures

Strain relaxation behaviors of the epitaxial CoSi2 (epi-CoSi2) and Si0.83Ge0.17 layers in epi-CoSi2/Si0.83Ge0.17/Si(001) and cap-Si/Si0.83Ge0.17/Si(001) structures were investigated by high-resolution X-ray diffraction (HR-XRD) analyses. Samples were treated at the temperature, TA = 650–900°C by rapid thermal annealing. Comparative measurements showed a different strain relaxation behavior in the SiGe layers with and without CoSi2 layer. And Ge content and lattice mismatch in the SiGe film of the epi-CoSi2/Si0.83Ge0.17/Si(001) are smaller than those in the SiGe layer in cap-Si/SiGe/Si(001) possibly due to the diffusion of Ge into the tensile-stressed epi-CoSi2 layer to reduce the compressive stress in the SiGe layer at elevated temperature. The analyses of high-resolution ω–2θ scan spectra and reciprocal space mapping showed that epi-CoSi2 layer is under tensile residual stress and a significant strain relaxation starts at TA = 900°C indicating of thermal stability up to TA = 850°C.

Air-bridged lateral growth of crack-free Al0.24Ga0.76N on highly relaxed porous GaN

We report on the strain reduction in AlGaN layers grown on porous GaN (P-GaN) by metalorganic chemical vapor deposition (MOCVD). The P-GaN was obtained by ultraviolet radiation-enhanced electroless wet chemical etching of thick hydride vapor phase epitaxial grown GaN (HVPE-GaN) templates over (001) sapphire substrates. By optimizing the growth conditions, lateral growth of AlGaN was enhanced resulting in air-bridge formation between the P-GaN and the AlGaN layers. X-ray diffraction studies showed significant strain relaxation in AlGaN layers primarily due to the strain sharing between P-GaN and the AlGaN layers. This allowed us to grow crack-free good optical quality layers with thickness exceeding the critical limits for AlGaN deposition on the conventional MOCVD GaN or HVPE-GaN. The obtained results demonstrate the potential of this approach for the development of efficient ultraviolet light emitters.

Structure stability of short-period InAs/AlSb superlattices

The structure stability of short-period InAs/AlSb superlattices (SLs) grown by molecular beam epitaxy was examined by X-ray diffraction, Raman scattering and far-infrared reflectance spectroscopy. The combination of these techniques allowed us to identify modification of the structure properties of InAs/AlSb SLs annealed at various temperatures. Pronounced structure degradation was detected upon short annealing at temperatures as low as 490°C. Significant strain relaxation was also observed. These experimental results are understood on the basis of As/Sb exchange during annealing.

Impact of MOCVD-GaN ‘templates’ on the spatial non-uniformities of strain and doping distribution in hydride vapour phase epitaxial GaN

Thick HVPE-GaN layers are grown on Si-doped and undoped MOCVD-GaN ‘template’ layers as well as directly on sapphire, with the aim to investigate the effect of the MOCVD template on the strain relaxation and spatial distribution of free carriers in the overgrown HVPE films. Spatially resolved cross-sectional micro-Raman measurements, cathodoluminescence and transmission electron microscopy show improved crystalline quality resulting in elimination of the non-uniformities of electron distribution, a low free carrier concentration (<10 17 cm −3) as well as a significant strain relaxation effect.

Strain relaxation and dopant distribution in the rapid thermal annealing of Co with Si/Si 1− xGe x/Si heterostructure

The reaction of cobalt with the Si-sacrificial cap in the strained Si/Si 1− x Ge x /Si MBE grown heterostructure was studied. The Si-cap is added to prevent the relaxation of the SiGe and to guarantee uniform and reliable silicidation reaction. The Si 1− x Ge x epilayer, with Ge content between 18 and 28 at%, was highly B doped, while the Si-cap was undoped or B doped either during growth or by ion implantation. Cobalt evaporation was followed by rapid thermal annealing at 450–700°C for 30 sec in N 2 or Ar+10%H 2. When the silicide penetrated the Si-cap/Si 1− x Ge x interface, noticeable out-diffusion of Ge and B to the surface was observed. In spite of the presence of the Si-cap significant strain relaxation was observed in three cases: (1) in the implanted samples, although the implantation was confined to the Si-cap, (2) when the Co layer was too thick, such that the silicide penetrated the SiGe layer and (3) when the Ge content in the SiGe layer was relatively high (27.5%).

Strain conservation in implantation-doped GeSi layers on Si(100)

Metastable pseudomorphic GeSi layers grown by chemical vapor deposition or by molecular beam epitaxy on Si(100) substrates were implanted at room temperature. The implantations were performed with 90 keV As ions to a dose of 1 x 1013 cm-2 for Ge0.08Si0.92 layers and 70 keV BF2 ions to a dose of 3x 1013 cm-2 for Ge0.06Si0.94 layers. The samples were subsequently annealed for short 10–40 s durations in a lamp furnace with a nitrogen ambient, or for a long 30 min period in a vacuum tube furnace. For Ge0.08Si0.92 samples annealed for a 30 min-long duration at 700°C, the dopant activation can only reach 50% without introducing significant strain relaxation, whereas samples annealed for short 40 s periods (at 850°C) can achieve more than 90% activation without a loss of strain. For Ge0.06Si0.94 samples annealed for either 40 s or 30 min at 800°C, full electrical activation of the boron is exhibited in the GeSi epilayer without losing their strain. However, when annealed at 900°C, the strain in both implanted and unimplanted layers is partly relaxed after 30 min, whereas it is not visibly relaxed after 40 s.

Advantage of short over long annealing to activate As implanted in metastable pseudomorphic Ge0.08Si0.92 layers on Si(100)

Metastable pseudomorphic Ge0.08Si0.92 layers grown by chemical-vapor deposition on Si(100) substrate were implanted at room temperature with 90 keV As ions to a dose of 1×1013 cm−2. The samples were subsequently annealed for short 40 s durations in a lamp furnace with a nitrogen ambient, or for long 30 min periods in a vacuum tube furnace. For samples annealed for a 30-min-long duration at 700 °C, the dopant activation can only reach 50% without introducing significant strain relaxation, whereas samples annealed for short 40 s periods (at 850 °C) can achieve more than 90% activation without a loss of strain. We conclude that it is advantageous to anneal a low-dose As-implanted pseudomorphic and metastable GeSi epilayers briefly at an elevated temperature, rather than to anneal it for a 30-min-long period at a lesser temperature, when high activation without a strain loss is desired.

An X-ray diffraction study of the lattice strain relaxation in MOVPE GaAs/Ge heterostructures

GaAs/Ge heterostructures were grown by metal-organic vapour phase epitaxy with different V/III flow ratios; the layer thickness was always much larger than the critical threshold for the elastic strain relaxation. The structural properties of the specimens were investigated by both X-ray topography and high-resolution X-ray diffractometry. It has been found that, at high V/III ratios, the layers are affected by a high density of stacking faults, whereas misfit dislocations are completely absent; the fault density decreases on decreasing the V/III value. A small, but significant strain relaxation has also been observed, thus demonstrating the effectiveness of the partial dislocations bordering the faults in relaxing the strain. At lower V/III ratios the dominant defects are the misfit dislocations to which the strain relaxation can be attributed.

Advantage of rapid thermal annealing over furnace annealing for P-implanted metastable Si/Ge0.12Si0.88

Metastable pseudomorphic Ge0.12Si0.88 films were grown by molecular beam epitaxy on Si(100) substrates and then implanted with 100 keV 31P at room temperature for a dose of 5×1013/cm2. Samples were subsequently annealed by rapid thermal annealing (RTA) in nitrogen and by steady-state furnace annealing in vacuum. Both damage and strain introduced by implantation can be completely removed, within instrumental sensitivity, by RTA at 700 °C for 10–40 s. Vacuum annealing for 30 min at 500–550 °C removes most of the damage and strain induced by the implantation but the activation of the P is poor. At 700 °C, the activation is nearly 100%, but the crystallinity worsens and the pseudomorphic strain begins to relax. We conclude that for a lightly implanted metastable and pseudomorphic GeSi epilayer on Si, steady-state vacuum annealing cannot achieve good dopant activation without introducing significant strain relaxation to the heterostructure, while RTA can.

Film stress of sputtered W/C multilayers and strain relaxation upon annealing

Sensitive curvature measurements were performed on 40 Å period W/C multilayer structures on Si substrates using a two beam laser reflection technique. A compressive stress of approximately 1530 MPa was measured in these sputtered multilayer films. Thermal annealing to 500 °C resulted in very little strain relaxation in the multilayers but x-ray diffraction data show a slight increase of the multilayer period. Significant strain relaxation, though, was observed when a 400 Å W buffer layer was included. Thermal annealing of these samples to 400–500 °C resulted in large strain relaxation due to the growth of α-W crystals in the buffer layer. Large oxide formation on air annealed samples did not significantly change the strain state of the films.

Steady-State Versus Rapid Thermal Annealing of Phosphorusimplanted Pseudomorphic Si(100)/Ge0.12Si0.88

ABSTRACTPseudomorphic Ge0.12Si0.88 films 265 nm thick grown by molecular beam epitaxy on p- Si(100) substrates were implanted with 100 keV 31P at room temperature for a dose of 5 x 1013/cm2. The projected range of the implanted P is about half the epilayer thickness. The implanted layers, together with non-implanted virgin samples, were subsequently annealed by both rapid thermal annealing in nitrogen and by steady-state furnace annealing in vacuum. The damage and strain of the annealed layers were studied by 4He channeling and x-ray doublecrystal diffraction. For a dose of 5 x 1013 P /cm2, both the damage and strain introduced by implantation can be completely removed, within instrumental sensitivity, by rapid thermal annealing at 700 °C for 10 - 40 s. Furnace annealing at 550 °C for 30 min for this sample removes most of the damage and strain induced by implantation. Furnace annealing at 700 °C or higher worsens the crystallinity of the layer and the strain relaxes. Hall measurements were performed on the same samples. Furnace annealing cannot achieve good dopant activation without introducing significant strain relaxation to the heterostructure, while rapid thermal annealing can.

Strain Relaxation and Oxide Formation on Annealed W/C Multilayers

ABSTRACTTungsten-carbon (W/C) Multilayer structures are used as X-ray mirrors and other optical elements. The optical properties of such elements are highly sensitive to changes in strain due to thermal processing. Sensitive curvature measurements were performed on 40A period W/C Multilayer structures on Si substrates using a two beam laser reflection technique. A compressive stress of approximately 1530 MPa was measured in these sputtered multilayer films. Thermal annealing to 500 C in air and under vacuum resulted in very little strain relaxation in the multilayers but X-ray diffraction data show a slight increase of the multilayer period. Significant strain relaxation, though, was observed when a 400Å W buffer layer was included. Thermal annealing of these samples to 400–500°C resulted in large strain relaxation due to the formation of a-W crystals in the buffer layer. Moderate oxide formation on air annealed samples as measured by SIMS was shown not to be a dominant mechanism of strain relaxation.

Depth non-uniformities in thin CdTe layers grown by MBE on InSb substrates

Measurements have been made of the carrier concentration as a function of depth through CdTe layers grown by MBE on InSb (001) substrates. Both In-doped and undoped layers are n-type but the free carrier concentration diminishes in the vicinity of the CdTeInSb interface, possibly due to the presence of an increased density of extended defects in this region resulting from lattice mismatch at the interface. In order to provide more detailed information concerning the origin of these depth non-uniformities, double crystal X-ray diffraction (DCXRD) techniques have been employed to provide information about the crystal quality and lattice strain within the layers. By studying a variety of samples with different structure and quality, it is clearly demonstrated that regions of reduced carrier concentration correspond to those regions where significant strain relaxation has occured.

Strain relaxation kinetics in Si1−xGex/Si heterostructures

Strain relaxation in Si1−xGex/Si superlattices and alloy films is studied as a function of ex situ anneal treatment with the use of x-ray diffraction and Raman spectroscopy. Samples are grown by molecular-beam epitaxy at an unusually low temperature (≊365 °C). This results in metastably strained alloy and superlattice films significantly in excess of critical thicknesses previously reported for such structures. Significant strain relaxation is observed upon anneal at temperatures as low as 390 °C. After a 700 °C, 2 h anneal, superlattices are observed to relax less fully (∼43% of coherent strain) than corresponding alloys (∼84% of coherent strain). Also, the strain relaxation kinetics of a Si1−xGex alloy layer is studied quantitatively. Alloy strain relaxation is approximately described by a single, thermally activated, first order kinetic process having activation energy Ea=2.0 eV. The relevance of our results to the microscopic mechanisms responsible for strain relaxation in lattice-mismatched semiconductor heterostructures is discussed.

Thermal stability of Pb-alloy Josephson junction electrode materials. IV. Effects of crystal structure of Pb-Bi counter electrodes

Previous investigations have shown that the stability of Pb-alloy Josephson junction devices during repeated thermal cycling between 298 and 4.2 K can be improved by using ε-phase Pb-Bi films as the counter electrode. The improvement was attributed to the hcp structure that has more resistance to strain relaxation by dislocation glide than fcc materials. In the present paper the strain behavior and microstructure changes of α(fcc) and ε(hcp)-phase Pb-Bi films, which were prepared by co-deposition of Pb and Bi onto Si substrates from a single source at 273 K and then cooled to as low as 4.2 K or heated to 350 K, have been studied by using x-ray diffraction and transmission electron microscopy (TEM). Most of the strain introduced into the films upon cooling to 4.2 K due to the thermal expansion coefficient mismatch between the films and the substrate was found to be supported elastically by ε-phase Pb-Bi films thinner than 0.8 μm, and no change in microstructure was observed by TEM upon cooling 0.2-μm-thick films to 100 K. However, significant strain relaxation was observed in α-phase Pb-Bi films as thin as 0.2 μm. The dominant strain relaxation mechanism in the α-phase film at low temperatures was very similar to that observed in Pb films: Dislocations glided across the grains to relax the strain, and the glide motion was impeded by the grain boundaries, the native oxide on the film surface, the substrate and twins. Upon warming the film to room temperature, a large number of dislocations remained, and did not slip out as in the case of the Pb films. In addition to strong resistance to strain relaxation of the ε-phase films due to the hcp structure, the fiber structure of the films was found to be unfavorable for dislocation glide. Grain growth in the α-phase films commenced upon heating to 340 K, while in the ε-phase films grain growth was first observed at the higher temperature of 350 K, despite the fact that the ε-phase film has a lower melting temperature. From the present results it is clear that the thermal stability of the ε-phase Pb-Bi films is superior to that of the α-phase films.

Residual strains of Pb thin films deposited onto Si substrates

An investigation has been carried out of the strain and strain-relaxation in (111) oriented Pb films deposited onto oxidized Si substrates at 300 K and then cooled down to temperatures as low as 4.2 K. The strain in the Pb caused by differences in thermal expansion of the Pb and Si was measured using X-ray diffraction techniques. For film thicknesses of ~1400 Å, the magnitude of the observed strain at 4.2 K was found to approximate the strain value calculated from the difference in thermal expansion coefficients using the biaxial strain model. For greater film thicknesses, the value of the strain measured immediately after cooling was much lower, indicating that significant strain-relaxation occurred during the cooling process. It is probable that most of the strain was relaxed by a dislocation-slip mechanism. During isothermal annealing above 50 K a second, slow relaxation process was observed. Nonuniformity of the strain normal to the film-surface was found by analyzing the asymmetric X-ray line broadening that occurred during cooling. A strong dependence of strain on grain orientation was observed, and interpreted in terms of the biaxial strain model.