Dislocations In Silicon Research Articles

Structures responsible for radiative and non‐radiative recombination activity of dislocations in silicon

AbstractRegular dislocation networks (DN) formed by direct bonding of Si wafers actively participate in recombination of minority carriers. Besides nonradiative recombination (NRR), dislocation‐related luminescence (DRL), i.e. radiative recombination could be observed. We show that the type of recombination is defined by the type of the dislocations and the structure of the network. Screw dislocations are mostly DRL active, while NRR is mostly related to edge dislocations. Moreover, the DRL intensity strongly depends on the misalignment angle between the crystal orientations of both bonded wafers. There exists an optimal alignment between bonded Si wafers, for which the DRL intensity has a maximum. Such alignment could be used for dislocation based all‐Si light emitting devices in the future on‐chip optical communication components. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mechanism behind the stress‐induced change in boron and arsenic diffusion in silicon devices and its experimental verification

AbstractThere are contradictory reports regarding the effects of uniaxial compressive stress on boron diffusion in silicon. Some reported on the stress‐induced retardation of boron diffusion, whereas some reported on the stress‐induced enhancement of boron diffusion. This work serves to reconcile the conflicting reports on the effects of shallow trench isolation‐induced compressive stress on boron diffusion in silicon. We propose that there are two opposing mechanisms that affect the strain‐induced change in boron diffusion. Mechanism 1, which is related to the strain‐induced reduction in the silicon lattice constant, will lead to a retardation of the boron diffusion of the S/D extension implants. Mechanism 2, which is related to the formation of transient dislocations in silicon during the ramping up period of the S/D annealing, will lead to an enhancement of boron diffusion of the S/D extension implants. The interactions between the two opposing mechanisms would affect the on current versus S/D overhang characteristics as well as the off current versus S/D overhang characteristics. Similar theory can be extended to arsenic diffusion.

An optical microscopy study of dislocations in multicrystalline silicon grown by directional solidification method

With the growing market shares for directionally solidified multicrystalline silicon (mc-Si) based solar cells in recent years, it is of practical interest to investigate crystal defects present in the mc-Si materials. Dislocation is the primary crystal defect in mc-Si, and it plays an important role in influencing the photovoltaic properties of mc-Si solar cells. In this work, we employed optical microscopy to investigate dislocations in mc-Si grown by the industrial directional solidification method. It was found that the distribution of dislocations in mc-Si is highly inhomogeneous from one grain to another. High inhomogeneity in dislocation distribution was also observed in individual grains. A large number of slip dislocations were generally observed in mc-Si. The origin of dislocations, the distribution inhomogeneity of dislocations, and their effects on the photovoltaic properties of mc-Si solar cells were discussed.

Study of SiC and Si 3N 4 inclusions in industrial multicrystalline silicon ingots grown by directional solidification method

In directional solidification of multicrystalline silicon ingots for solar cells, the concentration of C and N impurities in the silicon melts increases with progression of solidification due to their relatively low segregation coefficients. In the case of supersaturation of C and N in silicon melts, SiC and Si 3N 4 inclusions are formed. In this work, a piece of multicrystalline silicon was selected from a central block, which was cut out from an industrial multicrystalline silicon ingot grown by directional solidification method. The distribution of SiC and Si 3N 4 inclusions from the top to bottom regions was systematically studied. It was found that majority of SiC and Si 3N 4 inclusions are present in the top region and the amount of inclusions decreases exponentially from the top surface down to the bulk of the ingot. Morphologies and characteristics of the SiC and Si 3N 4 inclusions were investigated. The presence of SiC and Si 3N 4 inclusions generates high density of dislocations in multicrystalline silicon, and sometimes can also introduce pores into multicrystalline silicon. The results of this work will be of practical interest to the photovoltaic industry.

Dislocation dynamics and slip band formation in silicon: In-situ study by X-ray diffraction imaging

White beam X-ray diffraction imaging (topography) with an optimised CCD-detector system is used to monitor in-situ and in real time the nucleation, growth and movement of dislocations in silicon at high temperatures. It can be shown, that damage like microcracks and the surrounding strain fields in a wafer act as sources for dislocation loops, which end in slip bands far away from the source. The dislocations are arranged in channels of parallel {111} glide planes, which become visible as bands of parallel surface steps when the dislocations slip out on the back or front sides of the wafer. The width of such a channel or band depend on the dimensions of the damaged volume where the dislocations nucleate. This can be explained with a simple geometrical model.

Light scattering from dislocations in silicon

Nondecorated glide dislocations in Czochralski grown silicon have been studied by laser scattering tomography technique. Dependence of intensity of scattered light on polarization of the incident light has been measured for different orientations of the dislocation line and Burgers vector. Detailed theory of light scattering by dislocation in silicon crystals is presented. It is shown that by combination of polarization and tomography measurements it is possible to determine slip system of nondecorated mixed dislocation in Si.

3D numerical analysis of the influence of material property of a crucible on stress and dislocation in multicrystalline silicon for solar cells

We carried out calculations to investigate the influence of thermal conductivity of the wall of a crucible on thermal stress and dislocations in a silicon ingot during a solidification process using a three-dimensional global analysis. It was found that the m–c interface shape and the temperature gradient in a silicon ingot have significant influence on thermal stress and dislocations due to different thermal conductivity of the wall of a crucible. Therefore, we should control not only the m–c interface shape, but also temperature gradient in a silicon ingot in order to reduce thermal stress and dislocations in a silicon ingot during a solidification process.

Theoretical study of hydrogen stability and aggregation in dislocation cores in silicon

The interaction between hydrogen and a dislocation in silicon has been investigated using first-principles calculation. We consider $30\ifmmode^\circ\else\textdegree\fi{}$ and $90\ifmmode^\circ\else\textdegree\fi{}$ partial dislocations with both single and double periodic structures and nondissociated screw dislocation starting from the case of one single H to a fully H-filled dislocation line. In the case of a single H atom, H is preferentially located in a bond-centered-like site after a possible breaking of a Si--Si bond. In case of two H atoms, the molecular ${\text{H}}_{2}$ can be stable but is never the lowest energy configuration. If initially located in a bond-centered site, ${\text{H}}_{2}$ usually spontaneously dissociates into two H atoms and breaks the Si--Si bond followed by the passivation of resulting dangling bonds by H atoms. When additional H atoms are inserted into partial dislocation cores, they first induce the breaking of the largely strained Si--Si bonds in the dislocation core, then passivate the created dangling bonds. Next the insertion of stable ${\text{H}}_{2}$ near the dislocation core becomes favorable. A maximum H density is determined as 6 H atoms per length of Burgers vector and the largest energy gain in energy is obtained for a $90\ifmmode^\circ\else\textdegree\fi{}$ single periodic partial dislocation. Our calculations also suggest that the presence of few hydrogens could have a non-negligible influence on the dislocation structures, inducing core reconstructions. The mobility of H along the dislocation line is briefly addressed in the case of the $90\ifmmode^\circ\else\textdegree\fi{}$ single periodic partial dislocation core.

On the dissociation of dislocations in semiconductor crystals

An analysis of moving dissociated dislocations subjected to both a quasi-static viscous friction force and a periodic lattice resistance is performed: Metastable configurations are expected, the range of the dissociation distances being asymmetrically distributed with respect to the equilibrium dissociation distance. The data of earlier investigations on the influence of high external stress on the dissociation of dislocations in silicon and germanium, as determined by transmission electron microscopy, are re-examined. It is shown that in most cases the range of observed dissociation widths is consistent with the predictions of the model

Lifetime limiting recombination pathway in thin-film polycrystalline silicon on glass solar cells

The minority carrier lifetimes of a variety of polycrystalline silicon solar cells are estimated from temperature-dependent quantum efficiency data. In most cases the lifetimes have Arrhenius temperature dependences with activation energies of 0.17–0.21 eV near room temperature. There is also a rough inverse relationship between lifetime and the base dopant concentration. Judging by this inverse law, the activation energies of the lifetimes, and the absence of plateau behavior in the lifetimes of the higher doped cells at low temperatures, it is inferred that the dominant recombination pathway involves the electronic transition between shallow states which are 0.05–0.07 eV below the conduction band and 0.06–0.09 eV above the valence band, respectively, consistent with the shallow bands in silicon dislocations. The modeled recombination behavior implies that deep levels do not significantly affect the lifetimes for most of the cells at and below room temperature.

Molecular dynamics studies of the dissociated screw dislocation in silicon

Characterizing the motion of dislocations through covalent, high Peierls barriermaterials is a key problem in materials science, while despite the progress inexperimental studies the actual observation of the atomistic behaviour involved incore migration remains limited. We have applied a hybrid embedding schemeto investigate the dissociated screw dislocation in silicon, consisting of two30° partials separated by a stacking fault ribbon, under the influence of a constant externalstrain. Our ‘learn on the fly’ hybrid technique allows us to calculate the forces on atoms inthe vicinity of the core region using the tight binding Kwon potential, whilst the remainderof the bulk matrix is treated within a classical approximation. Applying a 5% strain to thedissociated screw dislocation, for a simulation time of 100 ps at a temperature of 600 K, weobserve movement of the partials through two different mechanisms: double kink formationand square ring diffusion at the core. Our results suggest that in these conditions, therole of solitons or anti-phase defects in seeding kink formation and subsequentmigration is an important one, which should be taken into account in future studies.

Evaluation of the Strain around an Isolated Shallow Trench and the Impact of Stress on LSI Device Performance

We propose a stress-controlled fabrication process for shallow trench isolation (STI) that can reduce stress-originated leakage current. In this paper, the relationships between the electrical characteristics of a transistor and the STI fabrication process parameters were obtained by measurement. Direct measurements of mechanical strain around an STI structure were executed by convergent beam electron diffraction (CBED) analysis, and mechanical strain was simulated by the finite element method (FEM). Transmission electron microscopy (TEM) analysis was also performed. It was revealed that the undesirable leakage current in a transistor was caused by a dislocation in crystal silicon, which was induced by tensile strain perpendicular to the silicon surface. We also found that the mechanical strain is controllable by optimizing the amount of recess of the gap-fill oxide in the STI structure after chemical mechanical polishing (CMP).

MNM-3A-3 単結晶シリコンの疲労とシャフルセット転位(セッション 3A 単結晶・多結晶シリコンの疲労寿命評価とメカニズムの解明)

MNM-3A-3 単結晶シリコンの疲労とシャフルセット転位(セッション 3A 単結晶・多結晶シリコンの疲労寿命評価とメカニズムの解明)

Computing transition rates of thermally activated events in dislocation dynamics

We present a numerical method to compute the transition rates of thermally activated events in dislocation dynamics on the atomistic scale, based on the formula from the transition state theory. The method is applied to the migration of kinks in 30° partial dislocations in silicon. To our knowledge, this is the first time that the contribution of entropy to the transition rate of such events has been calculated using reliable atomistic models.

A generalized Peierls–Nabarro model for kinked dislocations

We present a generalized Peierls–Nabarro model for curved dislocations incorporating directly the Peierls energies for both straight dislocations and dislocation kinks. In our model, the anisotropic elastic energy is calculated efficiently using the discrete Fourier transform on the discrete lattice sites of the slip plane, and the discreteness in both the elastic energy and the misfit energy is included. We have used our model to calculate the kink migration and nucleation energies of the 30° dislocations in silicon. The results agree well with those obtained using atomistic potentials and first principles calculations, and the experimental results.

TEM observations in Si: An attempt to link deformation microstructures and electrical activity

The deformation microstructures of Si single crystals have been studied by transmission electron microscopy (TEM) in the temperature range of deformation where “dislocation trails” are evidenced. The main features of the deformation microstructures are characterized by dislocations dipoles and their debris. These microscopic observations are discussed in relation with the mesoscopic properties of dislocation trails as well as the possible transformation between two different core structures of dislocations in silicon.

LM-STEM Study of Dislocations in Thick Silicon

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Effects of tool edge radius on ductile machining of silicon: an investigation by FEM

The submicron-level orthogonal cutting process of silicon has been investigated by the finite element approach, and the effects of tool edge radius on cutting force, cutting stress, temperature and chip formation were investigated. The results indicate that increasing the tool edge radius causes a significant increase in thrust force and a decrease in chip thickness. A hydrostatic pressure (∼15 GPa) is generated in the cutting region, which is sufficiently high to cause phase transformations in silicon. The volume of the material under high pressure increases with the edge radius. Temperature rise occurs intensively near the tool–chip interface while the highest cutting temperature (∼300 °C) is far lower than the necessary temperature for activating dislocations in silicon. As the edge radius is beyond a critical value (∼200 nm), the primary high-temperature zone shifts from the rake face side to the flank face side, causing a transition in the tool wear pattern from crater wear to flank wear. The simulation results from the present study could successfully explain existing experimental phenomena, and are helpful for optimizing tool geometry design in silicon machining.

Molecular dynamics simulations of the interaction between 60° dislocation and self-interstitial cluster in silicon

Abstract Molecular dynamics simulations are performed to investigate the interaction between 60° shuffle dislocation and tetrainterstitial ( I 4 ) cluster in silicon, using Stillinger–Weber (SW) potential to calculate the interatomic forces. Based on Parrinello–Rahman method, shear stress is exerted on the model to move the dislocation. Simulation results show that the I 4 cluster can bend the dislocation line and delay the dislocation movement. During the course of intersection the dislocation line sections relatively far away from the I 4 cluster accelerate first, and then decelerate. The critical shear stress unpinning the 60° dislocation from the I 4 cluster decreases as the temperature increases in the models.

Nitrogen diffusion and interaction with dislocations in single-crystal silicon

The results of dislocation unlocking experiments are reported. The stress required to unpin a dislocation from nitrogen impurities in nitrogen-doped float-zone silicon (NFZ-Si) and from oxygen impurities in Czochralski silicon (Cz-Si) is measured, as a function of the unlocking duration. It is found that unlocking stress drops with increasing unlocking time in all materials tested. Analysis of these results indicates that dislocation locking by nitrogen in NFZ-Si is by an atomic species, with a similar locking strength per atom to that previously deduced for oxygen atoms in Cz-Si. Other experiments measure dislocation unlocking stress at 550 °C in NFZ-Si annealed at 500–1050 °C. The results allow an effective diffusivity of nitrogen in silicon at 500–750 °C to be inferred, with an activation energy of 3.24 eV and a diffusivity prefactor of approximately 200 000 cm2 s−1. This effective diffusivity is consistent with previous measurements made at higher temperatures using secondary ion mass spectrometry. When the results are analyzed in terms of a monomer-dimer dissociative mechanism, a nitrogen monomer diffusivity with an activation energy in the range of 1.1–1.4 eV is inferred. The data also show that the saturation dislocation unlocking stress measured at 550 °C in NFZ-Si is dependent on the anneal temperature, peaking at 600–700 °C and falling toward zero at 1000 °C.