Dislocations In Si Research Articles

Optical bandsrelated to dislocations in Si

First-principles calculations are used to investigate theinteraction of self-interstitial aggregates with the 90°partial dislocation in Si. We find that I4 is bound to the linewith an energy of around 3 eV. The defect causes deep levels toappear in the band gap and optical transitions between these levelsmay account for the luminescent bands relating to plasticallydeformed Si.

Photoluminescence characterization of defects in Si and SiGestructures

Low-temperature photoluminescence (PL) spectroscopy is a very sensitive toolto investigate the presence of dislocations in Si. The maindislocation-related bands (D1-D4) have been attributed to a wide range ofcauses, either intrinsic properties of the dislocation or impurity related. PLis a competitive recombination process and the non-radiative processes need tobe measured to understand the overall effect of impurities. PL spectroscopysamples a large volume in comparison to the dislocation itself and thereforegives an average effect. High-resolution room-temperature PL mapping (SiPHER)has been used to detect defects in both Si and SiGe wafers. Whole-wafer PLmaps reveal the presence of slip on 300 mm Si wafers. Comparison studies withdefect etching show that there is a one-to-one correlation between defectsdetected in the PL micro-scans and those revealed by defect etching.Whole-wafer mapping has revealed a number of different defect types in SiGeepilayers. The ability to record whole-wafer PL maps facilitates the rapididentification of inhomogeneities and defects. High-resolution PL micro-mapsshowed the defect area to contain a high density of misfit dislocations, andthe nucleation site has strong non-radiative recombination. One common defecttype was analysed using plan view transmission electron microscopy (TEM) andoptical microscopy; these results revealed the presence of a high density ofdefect loops and stacking faults consistent with the high recombination rateat the defect site.

Modelling of HREM and nanodiffraction for dislocation kinks and corereconstruction

The ability to directly image individual dislocation kinks (Kolar H et al 1996Phys. Rev. Lett. 77 4031) opens up many possibilities for the study of kink dynamicsby in situ TEM. Unfortunately, this technique is limited by surfaceroughness. For ceramics, however, high temperature annealing has been found toproduce inert and atomically smooth surfaces that even survive ambientpressures (Susnitzky D 1992 J. Am. Ceram. Soc. 75 2463). We haveprepared such samples of 3C-SiC, in order to image kinks by the forbiddenreflections method. Using multislice simulations for 30 and 90° partialdislocations in Si we show that not only the number of kinks along thedislocation core can be determined, but also their structure assuming anaberration corrected TEM.We also show how the recently proposed double period reconstruction along the90° partial dislocation in Si can easily be verified experimentallyusing convergent beam electron diffraction.

Intrinsic Defects and the D1 to D4 Optical Bands Detected in Plastically Deformed Si

The properties of multi-vacancy and multi-interstitial defects that possess luminescent bands around 1 eV are reviewed. Prominent among these are the hexavacancy and tri- and tetra-self-interstitial defects. It is suggested that the formation of these defects on dislocation cores could lead to the D1 to D4 photoluminescent bands linked to dislocations in Si and SiGe.

Vacancy interaction with dislocations in silicon: the shuffle-glide competition

Competition between the two alternative positions (shuffle and glide 111 plane subsets) for the core of a 30 degrees partial dislocation in Si is examined. Using a combination of ab initio total energy calculations with finite temperature free-energy calculations based on an interatomic potential, we obtained free energies for the relevant vacancy-type core defects. Generally, the free energy of vacancy formation in the core of a 30 degrees glide partial dislocation is considerably lower (by more than 1 eV) than in the bulk. However, even at high temperatures, the predicted thermal concentration of the shuffle segments comprised of a row of vacancies in the core is low, placing the 30 degrees partial dislocation in the glide subset position.

Do we really understand dislocations in semiconductors?

The properties of dislocations in Si, GaAs and GaN are reviewed. Although, theoretical investigations favour a reconstruction eliminating, or reducing, the electrical activity in each case, with a consequent increase in the barrier for dislocation motion, there are problems in reconciling these results with experiment. It may be that the influence of impurities or point defects cannot be overlooked.

Hydrogen interaction with dislocations in Si

An H plasma has a remarkable effect on dislocation mobility in silicon, reducing its activation energy to 1.2 eV. Applying density functional theory to the interactions of H and H2 with the core of the 90 degrees partial dislocation in Si, we have identified a path for motion involving kink formation and migration at hydrogenated core bonds which conforms exactly to the experimentally measured activation energy.

First-principles study of impurity segregation in edge dislocations in Si

Using ab initio calculations, the segregation of As, Ga, and Ge atoms in the core regions of perfect edge dislocations in Si is examined. When all nearest neighbors of an impurity are Si atoms, As favors the core site at maximum compression and has a segregation energy of 0.25 eV/atom. Ga and Ge impurities favor sites under maximum tension and have segregation energies of 0.44 and 0.19 eV per atom, respectively. For As impurities, however, a pairing mechanism yields an even larger segregation energy of 0.64 eV/atom. (c) 2000 The American Physical Society.

Carrier Recombination at Screw Dislocations in n-Type AlGaN Layers

We analyse the recombination and scattering at dislocations in Si doped AlGaN layers by transmission electron microscopy (TEM), electron beam induced current (EBIC) and cathodoluminescence. We show that c-type screw dislocations are nonradiative recombination centres. From temperature dependent measurements of the EBIC contrast we find a shallow acceptor state 20 meV above the valence band. The level can be understood iri terms of one-dimensional dislocation bands split off by the dislocation deformation field. In addition piezoelectric potentials play a role.

Deformation behavior of silicon

Dynamic processes of individual dislocations in Si, such as generation, motion, and multiplication, are reviewed on the basis of direct observations by means of X-ray topography and transmission electron microscopy. The characteristics in the reaction of oxygen impurities with dislocations are clarified by real time observations with these techniques. Immobilization of dislocations due to the reaction is demonstrated. Stress-strain behavior of Si in the yield deformation is described in detail. It is sensitive to the deformation temperature, strain rate, and also to the density of dislocations initially contained in the crystal. Though the yield behavior of Si crystals free from dislocations is not influenced by oxygen impurities contained, that of dislocated crystals is affected significantly by these impurities. All of these characteristics in the yield deformation of Si are shown to be successfully described with a model that takes into account the dynamic processes of individual dislocations revealed by the direct observations.

The core reconstruction of the 90degree partial dislocation in silicon

The free energy of a recently proposed new structure for the core of the 90o partial dislocation in Si, displaying double the crystal periodicity along the dislocation line, is found to differ from that of the previously accepted singlecrystal period core structure by an insignificant amount at the temperatures at which dislocations become mobile in Si. It is proposed, therefore, that both core structures exist. We have found that, although one should, in principle, be able to distinguish between these core structures with modern 400kV high-resolution electron microscopes, the published experimental micrographs are unable to distinguish between these competing core structures.

Extended Point Defect Structures at Intersections of Screw Dislocations in Si: A Molecular Dynamics Study

Molecular dynamics computer simulations have been employed with the Tersoff interatomic potential to examine the atomic structure of (a/2) 〈110〉 screw dislocations forming regular two-dimensional arrays in silicon. The main attention is focused on the atomic configurations of dislocation intersections. The dislocations are assumed to be undissociated, following HREM observations on the low-angle (001) twist boundaries produced by silicon wafer bonding in ultrahigh vacuum. It is shown that cores of the dislocation intersections are formed by closed characteristic groups of atoms (extended point defects). The symmetry of these defects strongly depends on the fact whether the screw dislocation arrays generate a twist or a shear boundary.

Strained SiGe Materials for High Quantum Efficiency Photodiodes at µ = 1.3 to 1.5µm

AbstractSilicon-gemanium is a promising materials system for delivering near-IR photodetection on the ubiquitous silicon substrate. The 4% lattice mismatch leads to biaxial strain for SiGe films grown coherently on (100) Si substrates, which both extends the long wavelength detection limit by reducing the band gap and defines the critical thickness for the SiGe film. We have studied the use of strained-layer superlattices grown on high quality relaxed SiGe buffers. Graded buffers are ideal virtual substrates for achieving a complete range of compositions and strains for GeSi alloys. A physical model has been created to map out the absorption coefficient as a function of Ge fraction and (100) biaxial strain. Based on calculated absorption spectra as a function of composition and strain, we have designed and grown a Ge/Ge0.5Si0.5 strain-balanced superlattice on a high quality Ge0.75Si0.25 relaxed buffer using UHV-CVD. The strain-balanced superlattice has an effective absorption coefficient of 5000 cm−1. The relaxed buffer serves as a virtual substrate with relatively low threading dislocation density (106 cm-−2). The effect of the threading dislocations on the leakage current of P-I-N junctions is also investigated. Electron beam induced current (EBIC) is used to determine the dislocation density of SiGe relaxed buffer junctions grown with different grading rates. Mesa isolated devices are used to determine bulk leakage current densities for the SiGe junctions. The bulk leakage current density scales directly with the dislocation density. Bulk leakage currents density per dislocation length (2×10−5 A cml) agrees with deep level transient spectroscopy (DLTS) determined literature values for SiGe capture cross-section and defect density per dislocation in Si.

Structural transformation in the90°partial dislocation in Si due to Ga impurities

Ga impurities in the 90{degree} Shockley partial dislocation in silicon have been investigated using first-principles total-energy pseudopotential calculations. The results indicate that Ga segregates to the core of the dislocation and destabilizes the asymmetric fourfold coordinated structure, which is known to be the low-energy configuration in pure Si. The segregation energy for Ga in the symmetric core configuration is 0.53 eV/atom. The atomic mechanism for this spontaneous transformation to the symmetric structure is the passivation of quasifive-fold sites in the symmetric core by Ga. {copyright} {ital 1998} {ital The American Physical Society}

Dynamics of Dissociated Dislocations in SI: A Micro-Meso Simulation Methodology

AbstractThe theory of dislocation motion in materials with high Peierls stress relates dislocation mobility to the underlying kink mechanisms. While one has been able to describe certain qualitative features of dislocation behavior, important details of the atomic core mechanisms are lacking. We present a hybrid micro-meso approach to modeling the mobility of a single dislocation in Si in which the energetics of defect cores and kink mechanisms are treated by atomistic simulation, while dislocation motion under applied stress and at finite temperature is described through kinetic Monte Carlo. Three important aspects pertaining to treating the details of local structure and dynamics of kinks are incorporated in our approach: (1) Realistic complexity of (multiple) kink mechanisms in the dislocation core. (2) Full Peach-Koehler formalism for treatment of curved dislocation. (3) Detailed investigation of interaction between partials. This simulation methodology is used to calculate micron-scale dislocation mobility, with no adjustable parameters; specifically we obtain temperature and stress dependent velocity results that can be compared with experimental measurements.

Dislocation and Grain Boundary Energies in Si and Ge from an Anharmonic Bond Charge Model

The paper presents calculated line energy values for the reconstructed 60° and 90° glide-set partial dislocations in Si and Ge, formation and migration energy for the reconstructed kink on the reconstructed 90° partials, and energy data for the symmetric Σ = 9 and for two variants of the symmetric Σ = 11 tilt grain boundaries. Criteria are formulated to identify interatomic force field models which are able to provide reliable energy estimates. The anharmonic bond charge (a.bc) model is introduced as an example that approximately fulfills the basic criteria, i.e., describes well the second and third order elastic constants and the phonon dispersion curves. Deviations between energy estimates from the a.bc model and less reliable approaches are discussed. It is shown that in case of the = 11 tilt grain boundary the a.bc model gives different energetical ranking for the so-called Σ = 11A and the Σ = 11B variants in Si and Ge, in agreement with the experimental observations.

Optical and electrical non-uniformity around dislocations in silicon doped GaAs

Highly spatially resolved mappings of photoluminescence (PL) and capacitance-voltage (C-V) measurements have been used to characterise the optical and electrical non-uniformity around dislocations in Si doped GaAs crystals. A broad emission band with a peak at 0.95 eV commonly appears in Si doped GaAs wafers with carrier concentrations ranging from 3 × 1017 to 4 × 1018 cm-3. The average intensity of the 0.95 eV band is lower in the dislocated area than in the dislocation free area. The microscopic intensity pattern of the band agrees with the dislocation pattern. The intensity increases in the core region of dislocations and decreases in the surrounding area. The band edge emission shows a complementary intensity pattern. The C-V mapping of a wafer indicates that the carrier concentration is higher in the dislocated area, correlating with the PL intensity pattern. These results suggest that the microdefects responsible for the 0.95 eV band act as acceptors compensating Si donors and that they are gettered by dislocations.

Period-Doubled Structure for the90°Partial Dislocation in Silicon

We suggest that the commonly-accepted core structure of the 90-degree partial dislocation in Si may not be correct, and propose instead a period-doubled structure. We present LDA, tight-binding, and classical Keating-model calculations, all of which indicate that the period-doubled structure is lower in energy. The new structure displays a broken mirror symmetry in addition to the period doubling, leading to a wide variety of possible soliton-like defects and kinks.

Core effects in dislocation intersection

In summary, we have demonstrated the feasibility of atomistic simulations for the study of dislocation intersections. Through the specific examples of partial dislocations in Si, we have examined the mechanisms of intersection in full atomistic detail. We have also shown that formation of the intersection defects can be viewed as a convolution of two dislocation densities distributed in two non-parallel glide planes, in agreement with the quasi-continuum Peierls concept. On the other hand, energetics of the intersections is strongly dependent on the details of atomistic structure in the core, such as core reconstruction, which can be addressed only within a fully atomistic approach.

Ordering of As impurities in a Si dislocation core

We demonstrate by ab initio calculations that segregation of As in a dislocation core in Si occurs in the form of an ordered chain of As atoms running along the dislocation pipe. All As atoms in the chain achieve threefold coordination and the segregation energy is close to 1 eV per As atom.