Array Of Misfit Dislocations Research Articles

Growth of strained Si on high-quality relaxed Si1−xGex with an intermediate Si1−yCy layer

An intermediate Si1−yCy layer in the Si1−xGex film, replacing the conventionally graded buffer layer, was used to form the high-quality relaxed SiGe substrate. With the 700 nm thick Si0.8Ge0.2 overlayer, such a Si1−xGex∕Si1−yCy∕Si1−xGex (x=0.2, y−0.014) heterostructure has a threading dislocation density of 5.5×105cm−2 and a residual strain of only 2%. The surface roughness was measured to be about 4.2 nm. The long-range misfit dislocation array was formed mainly at the interface of top Si1−xGex and Si1−yCy. Strained-Si n-channel metal-oxide-semiconductor transistors with various buffer layers were fabricated and examined. Effective electron mobility for the strained-Si device with this substrate technology was found to be 95% higher than that of Si control device. The scheme for the formation of the relaxed Si1−xGex film serving as a virtual substrate shall be applicable to high-speed strained-Si devices.

Strain-assisted nanoscale patterning of Co thin films

The effect of a nanometer-scale oscillating surface strain field, due to a buried misfit dislocation array, on the growth of Co on Pt(001) is investigated by examining the structure and magnetic properties of the deposited Co film. $1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick Co films are sputter-deposited on epitaxial Pt underlayers in which the strain field is generated by a square array of interfacial misfit dislocations due to the lattice mismatch of the Pt(001) film with the underlying MgO(001) substrate. The strength of the surface strain field is controlled by varying the thickness of the Pt underlayer from $2\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Remarkable changes in the magnetic remanence behavior, measured using the $\ensuremath{\Delta}m$ technique, are observed as a function of the thickness of the Pt underlayer. These changes can be attributed to changes in the Co nucleation density and growth morphology associated with the different strength of the surface strain field for different Pt underlayer thicknesses. This indirect approach suggests that nanopatterned, magnetically isolated Co films are produced over a very narrow range of Pt underlayer thickness.

In situ grazing incidence x-ray diffraction study of strain evolution during growth and postgrowth annealing of MnAs on GaAs(113)A

We study the strain evolution during molecular beam epitaxy growth of MnAs on GaAs(113)A, its change during postgrowth annealing and the interfacial structure by in situ grazing incidence x-ray diffraction. A MnAs peak is detected at a coverage as low as 0.5 monolayer (ML). The film is compressively strained at the onset of growth and relaxes as the thickness increases. The in-plane grain size increases in two distinct stages: a fast increase up to 4 ML nominal thickness is followed by a slow increase. Postgrowth annealing of the layer improves the crystal quality and leads to a further relaxation of the layer. An ordered array of misfit dislocations is found at the interface.

Electrical, Structural, and Chemical Analysis of Defects in Epitaxial SiGe -Based Heterostructures

The electrical and structural properties of defects in epitaxial with content from 5 to 41% are investigated using electron-beam-induced current contrast measurements and bevel polishing with preferential defect etching. At room temperature, no electrical activity of misfit and threading dislocations is evident for wafers with 14, 21, and 41% , while at a crossgrid array of misfit dislocations is observed on all these wafers and shallow level traps from threading dislocation defects are seen on the 41% wafer. Thermal treatments reduce the electrical activity of the defects. Bevel polishing and preferential chemical etching with the Wright and Secco etches are utilized to analyze the dislocation density and distribution throughout the heterostructure. The etch chemistry difference between and is characterized as a function of content to correctly interpret results. The etch rate increases with increasing concentration up to 15% , then saturates at a rate of for higher values, which is about 3 times that for an unstrained substrate. The etch rate is strongly dependent on the defect concentration, stress, and defect density in the layers.

Misfit dislocation formation in the AlGaN∕GaN heterointerface

Heteroepitaxial growth of AlxGa1−xN alloy films on GaN results in large tensile strain due to the lattice mismatch. During growth, this strain is partially relieved both by crack formation and by the coupled introduction of dense misfit dislocation arrays. Extensive transmission electron microscopy measurements show that the misfit dislocations enter the film by pyramidal glide of half loops on the 1∕3⟨11̱23⟩∕{112̱2} slip system, which is a well-known secondary slip system in hcp metals. Unlike the hcp case, however, where shuffle-type dislocations must be invoked for this slip plane, we show that glide-type dislocations are also possible. Comparisons of measured and theoretical critical thicknesses show that fully strained films can be grown into the metastable regime, which we attribute to limitations on defect nucleation. At advanced stages of relaxation, interfacial multiplication of dislocations dominates the strain relaxation process. This work demonstrates that misfit dislocations are important mechanisms for relaxation of strained III-nitride heterostructures that can contribute significantly to the overall defect density.

HRTEM observation of interfacial dislocations at faceted Al–Pb interfaces

Al–Pb ribbons containing 1 at.% Pb have been produced by melt-spinning and subsequently investigated by high-resolution transmission electron microscopy. It is shown that the lattice mismatch of about 22% between the nanometre-sized Pb inclusions and the surrounding Al matrix is accommodated by a periodic array of misfit dislocations at the Al–Pb interface. The closing failures of Burgers circuits drawn around misfit dislocations on {111} and {100} facets identify the corresponding Burgers vectors as (a 0/4)⟨ 211 ⟩ and (a 0/2)⟨ 110⟩, respectively. The Burgers vector of (a 0/4)⟨ 211⟩ corresponds to the projected edge part of a 60° (a 0/2)⟨ 110⟩ dislocation. The Pb inclusions themselves appear to be free of defects.

Spatial variation of luminescence from AlGaN grown by facet controlled epitaxial lateral overgrowth

Interesting phenomena have been observed in the epitaxial lateral overgrowth of AlxGa1−xN alloys using facet control on serrated GaN templates. A complex microstructure is observed that involves misfit dislocation arrays that are closely related to regions with significantly large variations in composition. The dislocations are on inclined planar boundaries and result from basal-plane slip, which is allowed in this inclined facet geometry. The spatial variation of the aluminum composition in the overgrowth region is determined by cathodoluminescence spectroscopy and ranges from x=0.06to0.27, for constant growth conditions that after planarization result in a uniform composition at x=0.16. These results indicate that aluminum incorporation depends significantly on the growth direction with marked preference for facets parallel to the basal plane.

Microstructural assessment of InN-on-GaN films grown by plasma-assisted MBE

The structural properties of InN thin films, grown by rf plasma-assisted molecular beam epitaxy on Ga-face GaN/Al 2O 3(0001) substrates, were investigated by means of conventional and high resolution electron microscopy. Our observations showed that a uniform InN film of total thickness up to 1 μm could be readily grown on GaN without any indication of columnar growth. A clear epitaxial orientation relationship of [ 0001 ] GaN ∥ [ 0001 ] InN , ( 10 1 ̄ 0 ) GaN ∥ ( 10 1 ̄ 0 ) InN was determined. The quality of the InN film was rather good, having threading dislocations as the dominant structural defect with a density in the range of 10 9–10 10 cm −2. The crystal lattice parameters of wurtzite InN were estimated by electron diffraction analysis to be a = 0.354 nm and c = 0.569 nm, using Al 2O 3 as the reference crystal. Heteroepitaxial growth of InN on GaN was accomplished by the introduction of a network of three regularly spaced misfit dislocation arrays at the atomically flat interface plane. The experimentally measured distance of misfit dislocations was 2.72 nm. This is in good agreement with the theoretical value derived from the in-plane lattice mismatch of InN and GaN, which indicated that nearly full relaxation of the interfacial strain between the two crystal lattices was achieved.

In situ investigation of MnAs/GaAs(001) growth and interface structure using synchrotron x-ray diffraction

Using grazing-incidence synchrotron x-ray diffraction, we have quantitatively studied the strain evolution of epitaxial MnAs films in situ during growth by molecular-beam epitaxy. We find that the MnAs layer grows in four distinct stages, being compressively strained at the beginning of growth and relaxing as the thickness increases. The evolution of the in-plane grain size is determined as a function of layer thickness. We find a one-dimensional ordered array of periodic misfit dislocations at the interface, having a periodicity of 4.9±0.05 nm along GaAs[110]. An annealing of the film increases the grain size as well as improves the ordering of dislocation array.

Modeling crosshatch surface morphology in growing mismatched layers. Part II: Periodic boundary conditions and dislocation groups

We present further developments and understanding of the commonly observed crosshatch surface morphology in strain-relaxed heteroepitaxial films. We have previously proposed that the crosshatch morphology is directly related with strain relaxation via threading dislocation glide which results in both surface step and misfit dislocation (MD) formation [see Andrews et al., J. Appl. Phys. 91, 1933 (2002)—now referred to as Part I]. In this article, we have used solutions for the stress fields and displacement fields for periodic MD arrays which include the effects of the free surface. These solutions avoid truncation errors associated with finite dislocation arrays that were used in Part I. We have calculated the surface height profile for relaxed films where the misfit dislocations were introduced randomly or the misfit dislocations were placed in groups with alternating sign of the normal component of their Burgers vector. We have calculated the surface height profiles where the slip step remains at the surface [“slip step only” (SSO)] and where the slip step is eliminated [“slip step eliminated” (SSE)] due to annihilation of opposite sense steps, such as could happen during growth or lateral mass transport. For relaxed films, we find that the surface height undulations, characteristic of crosshatch, increase with increasing film thickness for the SSO case, whereas the surface becomes flatter for the SSE case. Experiments on relaxed In0.25Ga0.75As films on (001) GaAs show that the surface height undulations in the [110] direction increase with increasing film thickness. Thus, we conclude that with increasing film thickness the crosshatch in the slow diffusion [110] direction is best described by the SSO case.

Microstructure investigation of NiAl–Cr(Mo) interface in a directionally solidified NiAl–Cr(Mo) eutectic alloyed with refractory metal

Abstract The microstructure of a directionally solidified NiAl–Cr(Mo) eutectic alloyed with refractory metal in as-processed and heat-treated states has been studied by means of scanning electron microscopy and high resolution electron microscopy (HREM). The microstructure of the NiAl–Cr(Mo) eutectic was characterized by lamellar Cr(Mo) phases embedded within NiAl matrix with common growth direction of 〈1 1 1〉. The interface between NiAl and lamellar Cr(Mo) did not have any transition layers. Misfit dislocations were observed at the NiAl–Cr(Mo) interface. In addition to lamellar Cr(Mo) phases, coherent Cr(Mo, Ni, Al) precipitates and NiAl precipitates were also observed in the NiAl matrix and lamellar Cr(Mo) phases, respectively. After hot isostatic pressing and heat treatment, the NiAl–Cr(Mo) interfaces became smooth and straight. Square array of misfit dislocations was directly observed at the (0 0 1) interface between NiAl and Cr(Mo, Ni, Al) precipitate. The configuration of misfit dislocation network showed a generally good agreement with prediction based on the geometric O-lattice model.

Structures of BaF2-CaF2 heterolayers and their influences on ionic conductivity.

Recently, artificial ion conductors have been prepared by growing epitaxial heterolayers consisting of BaF2-CaF2 using molecular beam epitaxy. The ionic conductivity of these heterolayers shows a strong dependence on the layer thickness [N. Sata, S. Eberman, K. Eberl, and J. Maier, Nature 408, 996 (2000)]. In this paper three such heterolayers with different spacings (sample A: 80 nm, sample B: 10 nm, sample C: 1 nm) are investigated by conventional transmission electron microscopy and high-resolution transmission electron microscopy. The spacings are chosen such that they fall into the three conductivity regimes observed in N. Sata et al. (l > 50 nm; 8 < l < 50 nm; l < 8 nm). In accordance with conductivity studies, the samples with spacings of 10 nm or greater (A,B) are epitaxial and continuous, whereas in the case of extremely small spacing (C) the continuity of the layers is destroyed by formation of a column-like structure. Analytical electron microscopy reveals that, instead of forming multilayers, Ca and Ba separate in different columns in sample C. The structure properties of sample A (large l) are quite ideal: Planar interfaces with regular arrays of misfit dislocations with their Burgers vectors on the interface are observed. In the case of sample B (medium l) the lattice misfit is accommodated, in addition, by wavy interfaces associated with dislocations characterized by a Burgers vector that makes a large angle to the interfaces. The (111) lattice spacing very close to the interfaces is markedly changed due to this novel relaxation mechanism in the multilayer. The influences of the crystallographic defects on the ionic conductivity are also discussed.

The Strain-Assisted Nucleation and Growth Behavior of Co Islands: An Indirect Study of Nanopatterning Using the Remanent Magnetization Measurement (Δm)

AbstractNanopatterning through strain-assisted nucleation of metal adatom islands is experimentally studied using a magnetic remanence or Δm measurement. The strain field is generated by a square array of interfacial misfit dislocations with a spacing of 4.1 nm due to the lattice mismatch of the epitaxial Pt(002) film on MgO(002) substrates. The intensity of strain is controlled by changing the thickness of the Pt underlayer ranging from 2.0 to 10.0 nm. The amount of exchange coupling among the Co grains grown on Pt template is measured using the μm technique as an indirect indicator of magnetic grain distribution. From the measured μm values, the morphology of adatom islands is indirectly deduced. The Co islands on a 5.0 nm thick Pt underlayer show the overall negative behavior of Δm curve, i.e., the strongest nanopatterning effect. Strain driven nucleation is found to be achievable through a narrow selection of experimental growth conditions.

Abrupt changes and hysteretic behavior of90°domains in epitaxial ferroelectric thin films with misfit dislocations

The influence of misfit dislocations on the statics and dynamics of 90° domains (twins) in epitaxial thin films is studied theoretically. The misfit dislocations create additional potential barriers, which hinder the movements of domain walls along the film/substrate interface. For ferroelastic 90° walls, these barriers are considerable because an elastic interaction exists between 90° domains and misfit dislocations owing to the coupling of the dislocation stresses to the spontaneous strains generated at the proper or improper ferroelastic phase transition. The energy of elastic interaction is evaluated for a single 90° domain embedded into a tetragonal film containing a periodic array of edge misfit dislocations. On this basis. the equilibrium width of a single embedded 90° domain is calculated as a function of the misfit strain in the film/substrate systems. It is found that, at some threshold values of the misfit strain, the gradual variation of the domain width is interrupted by steplike changes. The evolution of the 90° domain pattern during the polarization switching in ferroelectric films with misfit dislocations is analyzed in detail. The calculations show that the electric-field-induced growth of an embedded 90° domain involves three different stages. Remarkably, at some critical value of the applied electric field, one of the 90° domain walls jumps over the neighboring misfit dislocation, which is accompanied by abrupt changes of the domain width and position in the film. The steplike movements of 90° domain walls over the misfit dislocations are proposed as a possible microscopic mechanism of the Barkhausen effect occurring in ferroelectric thin films during the polarization reversal. The hysteretic behavior of 90° domains at the cyclic variation of an applied electric field is also demonstrated.

Slip systems and misfit dislocations in InGaN epilayers

We have studied the microstructure of InGaN layers grown on two different GaN substrates: a standard GaN film on sapphire and an epitaxial lateral overgrown GaN (ELOG) structure. These two materials exhibit two distinct mechanisms of strain relaxation. InGaN epilayers on GaN are typically pseudomorphic and undergo elastic relaxation by the opening of threading dislocations into pyramidal pits. A different behavior occurs in the case of epitaxy on ELOG where, in the absence of threading dislocations, slip occurs with the formation of periodic arrays of misfit dislocations. Potential slip systems responsible for this behavior have been analyzed using the Matthews-Blakeslee model and taking into account the Peierls forces. This letter presents a comprehensive analysis of slip systems in the wurtzite structure and considers the role of threading dislocations in strain relaxation in InGaN alloys.

Analysis of a nonorthogonal pattern of misfit dislocation arrays in SiGe epitaxy on high-index Si substrates

We have investigated the formation of misfit dislocations resulting from the growth of partially strained Si0.7Ge0.3 epitaxial films on Si substrates with surface normals rotated off of the [001] axis toward [110] by 0°, 13°, and 25°. Transmission electron microscopy has shown that the dislocations form in a modified cross-hatch pattern for samples grown on the off-axis substrates. This modified cross hatch consists of three arrays along which the dislocations align. This is in contrast to the two orthogonal arrays found on the on-axis (001) substrates. These dislocations correspond well with the intersection of the (111) slip planes with the respective surfaces. We present a simple analysis of the amount of relaxation due to probable Burger’s vectors for these dislocations, which reveals the most likely directions for these vectors and shows that the density of dislocations only accounts for a fraction of the total film relaxation as measured by Raman peak shifts. These studies form the basis for the use of high index surfaces as components in modern devices, and provide pathways to possible templates for use in the growth of nanostructures.

Atomic arrangement at the AlN/Si (111) interface

High-quality GaN epilayers have been grown on Si (111) substrates by metalorganic vapor phase epitaxy using a low-temperature AlN nucleation layer. The atomic arrangement at the epilayer/substrate interface has been investigated by high-resolution electron microscopy. A crystallographically abrupt interface is observed along most of the epilayer, indicating that the AlN/Si interface is thermodynamically stable and of high crystalline quality. Lattice images at the interface show a periodic array of misfit dislocations. The lattice images have been analyzed in detail in order to obtain information about the atomic arrangement, and interface bonding models are proposed.

Threading and misfit-dislocation motion in molecular-beam epitaxy-grown HgCdTe epilayers

Lattice mismatch between the substrate and the absorber layer in single-color HgCdTe infrared (IR) detectors and between band 1 and band 2 in two-color detectors results in the formation of crosshatch lines on the surface and an array of misfit dislocations at the epi-interfaces. Threading dislocations originating in the substrate can also bend into the interface plane and result in misfit dislocations because of the lattice mismatch. The existence of dislocations threading through the junction region of HgCdTe IR-photovoltaic detectors can greatly affect device performance. High-quality CdZnTe substrates and controlled molecular-beam epitaxy (MBE) growth of HgCdTe can result in very low threading-dislocation densities as measured by the etch-pit density (EPD ∼ 104cm−2). However, dislocation gettering to regions of high stress (such as etched holes, voids, and implanted-junction regions) at elevated-processing temperatures can result in a high density of dislocations in the junction region that can greatly reduce detector performance. We have performed experiments to determine if the dislocations that getter to these regions of high stress are misfit dislocations at the substrate/absorber interface that have a threading component extending to the upper surface of the epilayer, or if the dislocations originate at the cap/absorber interface as misfit dislocations. The preceding mechanisms for dislocation motion are discussed in detail, and the possible diode-performance consequences are explored.

Effects of misfit dislocation array on phase separation during codeposition of binary thin films

Effects of misfit dislocation array on the surface phase separation of binary films codeposited on a lattice mis-matched substrate is studied by using computer simulations of a dynamical continuum model. The model is described in terms of compositional field of the binary film and involves a coupling effect between the compositional field and the dislocation induced stress field. Several two-dimensional simulation results demonstrate that such a coupling might provide a novel process to control the steady state compositional patterns on the surface of phase separating thin film.

Strain Relaxation by Dislocation Arrays in Thin Films

AbstractAn analytical model for strain relaxation by misfit dislocation arrays in thin films is presented that takes into account all components of the strain tensor, including shear strains. The model is developed for (001) films and applied to strain relaxation in (011) oriented FCC metal films. Our results show that shear strains strongly influence the total strain energy of the film. Since both the critical strain for dislocation formation, and the equilibrium spacing of dislocations in arrays depend on the minimum energy values, these quantities are found to be different from those predicted by previous models. This model is useful for understanding both critical strain data and strain relaxation in films.