Introduction Of Misfit Dislocations Research Articles

Experimental and Theoretical Analysis of Strain Relaxation in GexSi1-x/Si(100) Heteroepitaxy

ABSTRACTBy analyzing in-situ strain relaxation measurements of GexSi1-x/Si(100) epitaxy in a Transmission Electron Microscope, we are able to quantify the fundamental parameters which describe strain energy relaxation via misfit dislocation introduction. Quantitative descriptions of misfit dislocation nucleation, propagation and interaction processes are derived. The numerical parameters obtained from these experiments are then incorporated into a predictive theoretical model of strain relaxation whichrelies only upon experimentally measured quantities. Good agreement between experiment and theory is obtained over a wide range of data.

Modified Dislocation Structures in GexSi1-x Double Epilayers on (001)Si

AbstractDouble epilayers of different compositions of GexSi1-x on (001)Si are observed to have dislocation contents which differ markedly from similar single epilayers. An initial epilayer, grown below its critical thickness, underwent substantial misfit dislocation introduction, while a second epilayer, grown at a composition where edge-type misfit dislocations are normally observed to dominate the morphology, contained mostly 60° type dislocations. It is suggested that dislocation entry into the upper, high mismatch epilayer allows many dislocations to enter the buried, low mismatch epilayer, and that this in turn affects the dislocation morphology in the upper layer through strain relief.

Influence of lattice misfit on heterojunction bipolar transistors with lattice-mismatched InGaAs bases

AlGaAs/InGaAs/GaAs heterojunction bipolar transistors (HBTs) with lattice-mismatched InGaAs bases have been studied for determining the effect of misfit defects on device performance. Defect structures were investigated using scanning cathodoluminescence and transmission electron microscopy. Misfit dislocations were found to form at the bottom interface between n-InGaAs and n-GaAs in the collector. Electrical properties of lattice-misfitted n-GaAs/n-InGaAs heterojunctions were studied using p+-GaAs/n-GaAs/n-InGaAs diode structures. Deep-level transient spectroscopy measurements revealed electron traps (Et=0.49–0.55 eV) and hole traps associated with the lattice-misfitted interface. Devices showed high dc current gains in spite of the presence of misfit dislocations. Poorer saturation of collector current was observed with increasing defect density. HBTS with lattice-mismatched InGaAs bases are considered to be relatively insensitive to the introduction of misfit dislocations in terms of dc characteristics.

An exactly solvable model for calculating critical misfit and thickness in epitaxial superlattices: Layers of equal elastic constants and thicknesses

A parabolic interaction potential has been used to develop a model for calculating the misfit dislocation (MD) energy in the case of a superlattice of alternating layers of materials with equal elastic constants and thicknesses. The model, which is believed to be a good one for small misfits and to have some merit for covalent bonded materials, is exactly solvable for the critical thickness above which it is energetically favorable to lose coherency by the introduction of MDs into the interfaces. It was found, for a given misfit f, that the critical thickness for epitaxial superlattices free from their substrate is somewhat more than 4 times that for a single epilayer on a thick substrate. Furthermore, the critical thickness varies almost inversely with misfit to the power 1.22 when Poisson’s ratio is 1/3. It was also shown that the critical misfit fc obtained by equating maximal misfit strain and MD energies is a significant overestimate of fc. The results for a superlattice are compared with those of a thin layer on a thick substrate.

Morphological transformations of thin heteroepitaxial films

The features of two-and three-dimensional growth of thin heteroepitaxial films are analysed with allowance for the energy of misfit dislocation (MD) networks. Criteria for distinguishing between Frank-van der Merwe, Stranski-Krastanov and Volmer-Weber growth modes are suggested. For the Stanski-Krastanov mode the transition from layer to island growth is considered to be associated with an increase in the interface energy owing to MD formation. The critical thickness of the pseudomorphic film in the case of the formation of dislocated islands is much less than that in the case of the introduction of MDs into the continuous film and depends on an average migration length of the adatoms. Some theoretical results are compared with experimental results for germanium films obtained by molecular beam epitaxy on Si(001) and Si(111) substrates.

Molecular Dynamics Studies of Semiconductor Thin Films and Interfaces

ABSTRACTThe structure and stability of thin epitaxial films have been investigated using molecular dynamics computer simulations. One issue of interest is the stability of a smooth film relative to 3-dimensional clusters. The simulation results show that the uniform film is never the lowest energy state for a system with finite misfit. However, the uniform film, produced in a layer-by-layer growth mode, can persist in a metastable state at substantial misfits, e.g. 10% at 1/2 the melting point. This is a result of the large nucleation barrier to the formation of clusters.The second issue is the quality of the interface between the film and the substrate. At equilibrium, the critical thickness for the introduction of misfit dislocations is larger for films on the diamond cubic (100) substrate than on the (111), and differs from predictions based on continuum mechanics. We find that coherent films remain in metastable equilibrium far beyond the critical misfit calculated for full equilibrium, because a large free energy barrier inhibits the introduction of misfit dislocations.

Some 15 young scientists win student awards

Winners of the Materials Research Society's Graduate Student Awards for 1984 will address nine of the technical symposia. A record number of award winners were selected from an unprecedented avalanche of applications-more than three times the number submitted last year.“A significantly greater number of graduate students will participate in this year's Fall Meeting than ever have before,” says Kathleen C. Taylor, Treasurer, who initiated the awards program for students. “Applications were received from a number of academic institutions which hadn't participated in previous competitions, indicating that the program is achieving one of its principal goals, which is to encourage the broadest possible participation in our meetings.”The awards are made to outstanding students recommended by their major professor who will present papers judged superior by the chairmen of the MRS symposia. Thus, explains Kathy, “the students selected have demonstrated the intellectual excellence, technical competence, and hard work required to achieve distinguished research results.”The award winners and the titles of the papers they will present are:Thomas C. Banwell, applied physics, California Institute of Technology; “Ion Induced Mixing in Ni—SiO2 Bilayers.”Bobbi Roop, physical chemistry, Massachusetts Institute of Technology; “Reactivity of XeF2, F2, and CF3 on Si(111)7×7 and Si (100) 2×1.”Gary B. Bronner, electrical engineering, Stanford University; “Physical Modeling of Backside Gettering.”Ali S.M. Salih, materials engineering, North Carolina State University, Raleigh; “Extrinsic Gettering via the Controlled Introduction of Misfit Dislocations.”James Kakalios, physics, University of Chicago; “Amorphous Semiconductor Doping Superlattices.”Evert Jan van Loenen, physics, FOM Institute for Atomic and Molecular Physics, Amsterdam; “The Orientation of Ultrathin NiSi2 Films: Correlations with Film Morphology.”

The effect of substrate relaxation on the introduction of misfit dislocations in misfitting epitaxial layers

The effects of substrate relaxation and curvature on the stress levels and the introduction of misfit dislocations in epitaxial layers are considered. The stress distribution in single- and multi-layer epitaxial structures are calculated, employing elastic energy minimization. The results are found to differ significantly from those previously reported, the stress levels being up to 60% greater. These results are combined in a calculation of substrate curvatures and critical thicknesses for misfit dislocation introduction taking into account the substrate relaxation. It is shown that, under certain conditions, Substrate relaxation and misfit dislocation interaction with the epilayer stress both markedly impede the initial introduction of misfit dislocations into the interface. Die Einflusse von Substratrelaxation und -Krummung auf das Spannungsniveau und die Ein fuhrung von Misfit-Versetzungen in Epitaxieschichten werden betrachtet. Die Spannungsvertei-lung in Ein- und Mehrschicht-Epitaxiestrukturen werden mittels Minimierung der elastischen Energie berechnet. Es wird gefunden, das die Ergebnisse sich betrachtlich von fruher veroffent-lichten unterscheiden, das Spannungsniveau ist bis zu 60% groser. Diese Ergebnisse werden bei einer Berechnung der Substratkrummung und der kritischen Dicke fur die Einfuhrung von Misfit-versetzungen kombiniert, wobei Substratrelaxation berucksichtigt wird. Es wird gezeigt, das unter gewissen Bedingungen sowohl Substratrelaxation als auch Misfit-Versetzungswechselwirkung mit der Epischichtspannung merklich die anfungliche Einfuhrung von Misfitversetzungen in die Interface beeinflussen.

Enhanced elastic moduli in Cu-Ni films with compositional modulation

Films with modulated composition 8-Å Cu/8-Å Ni show an increase in the Young’s and torsional moduli of a factor of 2, relative to bulk or longer-wavelength samples. The film structure is found to be highly textured with coherent lamella for the short-wavelength-enhanced moduli samples, but becomes more random with the probable introduction of misfit dislocations at longer wavelengths. The moduli results support the previous findings of Tsakalakos and Hilliard.

The epitaxial growth of iron on (111) gold platelets at substrate temperatures of about 500°C

Iron was vacuum deposited on (111) gold platelets at substrate temperatures of about 500°C. Defects were observed in the FeAu layers with increasing deposit thickness. Layers with iron deposit thicknesses of less than 20 Å revealed short dislocation segments with dislocation lines aligned roughly along the 〈110〉 and 〈112〉 Au directions. Burgers vector determinations indicated that segments along the 〈112〉 Au directions were edge dislocations having Burgers vectors of the type ± 1 2 〈110〉 lying in the interfacial plane while defects along the 〈110〉 Au directions were most likely total dislocations with Burgers vectors of the type ± 1 2 〈110〉 inclined to the interfacial plane. Dislocation networks were observed to form when iron was deposited to thicknesses exceeding 20 Å. Burgers vector determinations confirmed that these dislocations (aligned along the 〈112〉 Au directions) were edge-type dislocations with Burgers vectors in the plane of the interface. An analysis of dislocation spacing allowed the misfit to be calculated from which it is concluded that a saturated FeAu solid solution forms at about 500°C. Possible mechanisms for the generation and introduction of misfit dislocations are discussed.

Misfit dislocations in PbTe-PbSnTe heterojunction

Misfit dislocations at the heterojunction of PbTe-PbSnTe layers grown by liquid phase epitaxy have been investigated by etch pit studies. The linear density of misfit dislocations along the 〈100〉 direction parallel to the heterojunction was in good agreement with the predicted value. This agreement indicates that the strain due to lattice mismatch between PbTe and PbSnTe is completely relieved by the introduction of misfit dislocations. It has been shown from the distribution of etch pits, electron probe analysis, and calculations, that the misfit dislocations spread around the heterojunction in the region determined by Sn self-diffusion during epitaxial growth.

A one-dimensional model of the mobility of small epitaxial islands I: Parabolic substrate potential

Abstract A one-dimensional model of small epitaxial islands assuming a parabolic representation for the substrate potential is considered. General explicit solutions for the atom displacements and the potential energies of the chains as functions of the chain lengths and the number and location of the misfit dislocations are obtained. It is shown that the equilibrium chain configurations are stable. The activated states for the translation of the dislocations across one lattice parameter of the substrate are atoms-on-peaks configurations. The activation energies for reversible movement of the chains as a whole are calculated assuming the misfit dislocation slip mechanism proposed by Frenkel and Kontorova. It is shown that an observable mobility of small epitaxial islands is expected if the natural misfit is sufficiently greater than the critical misfit, when misfit dislocations can be introduced spontaneously at the free ends of the chains. Mobility is also expected for very small clusters whose sizes are comparable with the size at which the first misfit dislocation becomes energetically favoured. If the natural misfit is slightly greater than the critical misfit an occasional mobility of separate crystallites is expected. An azimuthal misorientation enhances the cluster mobility. An increase in the misfit leads to a reduction in the activation energies for surface diffusion of sufficiently long chains. For short chains the reverse dependence is observed. The shape of the barriers for surface diffusion is not symmetric if the chains are longer than the critical length, when a spontaneous introduction of misfit dislocations at the free ends becomes possible.

Growth mechanisms of AuCu II plates

The growth mechanisms of AuCu II plates are considered upon the basis of the measurements of their lengthening and thickening rates as reported by Pedraza and Kittl ( Acta Met. 24, 835, 1976). These rates are 3–4 orders of magnitude smaller than those estimated for incoherent AuCu II: disordered matrix (interphase) boundaries. During growth, the interphase boundaries are deduced to be coherent. At the edges of plates, coherency is maintained because the succession of ledges is too rapid to give the misfit dislocation structure time to develop. At the broad faces of plates, the closely spaced twins (initially, ca. 50 Å apart) in the AuCu II greatly diminish the driving force for introduction of misfit dislocations until substantial coarsening of the twinned structure has occurred. Ledge growth at the broad faces is inhibited by the requirement that such growth occur parallel to the twins; this effect appears responsible for the order of magnitude larger spacing between ledges at the broad faces than at the edges of the plates and for the growth of AuCu II crystals as plates rather than as idiomorphs. The principal features of AuCu II growth were shown to be consistent with a massive rather than a martensitic mode of transformation.

A calculation of the energy of misfit dislocations and the critical thickness in graded epitaxial layers

Abstract The energy of a mixed dislocation in a compositionally graded heterojunction is calculated, and from this result the critical thickness, at which the introduction of misfit dislocations is energetically favourable, may be obtained. Explicit expressions for the energy and critical thickness were determined for the particular case of the 60° dislocation in III–V compounds. The graded epilayer thickness at which all substrate-inherited threading dislocations are eliminated is also calculated.

Energy of a dislocation near an epitaxial interface

The energy of a straight mixed dislocation in a coherent epitaxial layer at a uniform distance t from the interface is calculated. It is shown that, in addition to terms associated with coherency strain and the dislocation energy, there is a term associated with the interaction of the coherency and dislocation strain fields which makes the total energy decrease as the dislocation approaches the interface, but this term is negligible when the dislocation is nearer the epilayer surface than the interface. The result is used to modify a calculation of Jesser and Matthews of the critical thickness of an epitaxial layer at which the introduction of misfit dislocations becomes energetically favourable. The present work reduces the calculated critical thickness by 30–50%.

The Identification, Annihilation, and Suppression of Nucleation Sites Responsible for Silicon Epitaxial Stacking Faults

A study of the formation of epitaxial stacking faults in 2 in. diameter, dislocation‐free (111) silicon wafers used in the fabrication of standard buried collector transistors has been made. The nucleation sites for the epitaxial faults are introduced during the initial oxidation of the wafer and are correlated with the presence of a high density of shallow, flat‐bottomed, saucer‐shaped etch pits. The saucer pits are selectively annihilated in the diffused or implanted regions during the fabrication of the Sb‐doped buried collectors. For the ion‐implanted process the annihilation of saucer pits extends laterally from 50 to 100 μm beyond the boundaries of the collectors. Following epitaxial growth, epitaxial stacking faults, at a density of 104 cm−2, are only found in those nonburied layer regions which have a saucer pit density of 106–107 cm−2 before epitaxy. Therefore, epi stacking faults are not found in ion‐implanted material with a separation between buried collectors of 100 μm or less. Most 3 in. diameter, and the central regions of 2 in. diameter wafers do not have saucer pits or epi stacking faults. This is attributed to anin situ gettering of nucleation sites by precipitates, which are known to form in wafers, or regions of wafers, with a high initial oxygen concentration. Additional procedures for deliberately suppressing or gettering the nucleation sites are presented. These include deliberate abrasion, deposition of strained layers, introduction of misfit dislocations, and the use of an Sb ion implant, which are performed on the back side of wafers before the initial masking oxidation.

The Elimination of Stacking Faults by Preoxidation Gettering of Silicon Wafers: III . Defect Etch Pit Correlation with p‐n Junction Leakage

Recently described procedures for the elimination of oxidation‐induced stacking faults have been incorporated into a device processing line to yield data on the leakage in p‐n junction tester diodes. The gettering processes, which take place on the back side of a wafer before the first oxidation, include the deliberate introduction of misfit dislocations and/or the deposition of a highly strained layer of . The results show that leakage currents can be reduced by several orders of magnitude and diode yields increased from about 10% to greater than 90%. By doing “post mortem” chemical etch pit studies on previously electrically mapped arrays of diodes the relative effectiveness of the two gettering schemes are compared. Factors such as the type of stacking fault, whether it be related to a native or process‐induced source, or whether it is clean or decorated are discussed as well as the influence of slip‐type dislocations.

Abstract: Properties of interfacial defects in III–V compound semiconductors

A most common technique used in the fabrication of optoelectronic devices involves the growth of one or several epitaxial layers of a given AIII BV semiconductor compound over a single crystal substrate. The choice of the epitaxial compound composition (binary, ternary, or even quaternary compounds are used) is usually guided by the desire to achieve a given band gap. As a result, a lattice mismatch is generally introduced between the epitaxial layers and substrate. The lattice mismatch parameter f , which is a function of the film composition, and the temperature of the layer‐substrate structure, is an important parameter for achieving control over the film quality and the device performances. The misfit‐induced defect generation and the role of these defects on the device performances are analyzed. The results presented which cover experiments on Ga1−xAlxAs1−yPy–GaAs double heterojunction structures and GaP homojunction structures are general enough to be applied to other types of mismatched epitaxial structures.The buildup in misfit strain energy Ef during the growth of a mismatched epitaxial layer is released by elastic and/or plastic deformation of the lattice. The introduction of misfit dislocations, one form of the film plastic deformation, takes place in several stages which are functions of the magnitutde of Ef and the structure temperature.1,2 Substrate defects such as dislocations, dislocation loops, and precipitates are playing a major role in the first stage of the misfit dislocation formation since they originate from these substrate defects. During the first stage of formation an interesting asymmetrical distribution of misfit dislocations along a 〈110〉 direction is also observed for {100} Ga1−xAlxAs1−yPy–GaAs structures.1,2 Use of this asymmetrical behavior is made to obtain defect‐free double heterostructures. The second stage of formation corresponds to the generation of ’’60° type’’ dislocation half loops to accomodate the misfit strain in the other 〈110〉 direction. The third stage results in an increase in the density of interfacial dislocations with Burger’s vectors in the interface plane.In the case of GaP epitaxial layers3 with small misfit values, several defect‐formation stages are observed during the film growth. The first one corresponds to a large increase (10–100 times) in the epitaxial‐layer dislocation density over that of the substrate. These new dislocations which originate from substrate dislocation loops near the interface are not efficient in relieving the misfit strain. The other stages are analogous to those described above for the formation of misfit dislocations in Ga1−xAlxAs1−yPy–GaAs structures. The role of dislocations on the device efficiency and degradation behavior1,4 is analyzed. Evidence for a stimulated dislocation motion and multiplication process upon minority carrier injection conditions is presented. This dislocation motion is attributed to the recombination‐enhanced mobility of point defects in these materials and to the efficient point‐defect sink behavior of the dislocations.5

Dislocations in vapor-grown compositionally graded (In,Ga)P

Dislocation morphologies in compositionally graded vapor-phase epitaxial (VPE) layers of (In,Ga)P deposited on (100) GaP substrates have been determined via transmission electron microscopy using (011) cross-sectional samples. Evidence for the introduction of misfit dislocations by slip and from substrate dislocations has been found. These processes lead to the familiar crossed array of dislocations at an interfacial misfit plane. Evidence for Frank-Read sources, which are pinned dislocation segments in the crossed array, and dislocation annihilation is also presented. Asymmetric arrays of dislocations at compositional steps have been observed.

The growth and structure of thin copper films on (001) surfaces of nickel

Abstract The epitaxial growth of thin copper films on (001) surfaces of nickel in ultra-high vacuum has been investigated using Auger electron spectroscopy and electron microscopy. At room temperature, deposits tend to grow in layer-like fashion and are strained compressively to be coherent with the substrate up to a thickness of about 8 A. Above this thickness the elastic strain decreases with the introduction of misfit dislocations, but more rapidly than theories predict. At 300°C, copper grows by a Stranski–Krastenov mechanism.