Boron-interstitial Clusters Research Articles

Structure and diffusion of interstitial boron pairs in silicon

Using the plane wave pseudopotential approach, the structure and mechanism of migration of an interstitial boron pair are proposed. The energy of the proposed configuration is lower by at least 0.2 eV in comparison to other interstitial boron pairs. The proposed model, which is equivalent to a ${\mathrm{B}}_{2}{I}_{2}$ pair (using the notation where ${\mathrm{B}}_{m}{I}_{n}$ refers to $m$ boron atoms and $n$ silicon atoms occupying $m$ regular sites), migrates in the 〈110〉 channel with energy barriers between intermediate sites not greater than 0.1 eV and a total energy barrier of 0.6 eV between stable sites. Conventional continuum and kinetic Monte Carlo models of formation of boron interstitial clusters (BIC's) do not consider the mobility of the proposed configuration in the growth process. Its stability against dissociation could have considerable implications for the modeling of transient enhanced diffusion of boron in silicon.

Concentration Dependence of Boron-Interstitial Cluster (BIC) Formation in Silicon-on-Insulator (SOI)

ABSTRACTThe effect of boron concentration on boron-interstitial cluster (BIC) dissolution was studied in separation by implantation of oxygen (SIMOX) and SOITEC silicon-on- insulator (SOI) materials. SOI substrates having surface silicon thickness of 750 Å and 1450 Å were ion implanted with 11B+ at a energy of 15 keV. The dose of the implant was varied from 3×1014 cm−2 to 1×1015 cm−2 to provide different boron peak concentrations. Anneals were performed at 825 °C in a nitrogen ambient using RTA for short times and furnace anneals for longer times. The retained dose of boron in the surface silicon layer was estimated using simulations from UT-Marlowe 5.0. Hall Effect was used to measure the sheet resistance, mobility, sheet number, and fraction of active boron. At lower concentrations, SOI exhibited significantly lower sheet numbers. As the boron concentration was increased SOI began to approach bulk Si. However, degraded mobility and sheet resistance was observed at all concentrations. Truncating the boron profile over the surface Si/buried oxide interface enhanced the fraction of active boron in the 750 Å SOI. This also lead to a higher fractional activation in 750 Å SOI than bulk Si as the concentration increased due to a loss of interstitials, effectively reducing the BIC population.

BIC Formation and Boron Diffusion in Relaxed Si0.8Ge0.2

ABSTRACTThe relationships between Boron Interstitial Cluster (BIC) evolution and boron diffusion in relaxed Si0.8Ge0.2 have been investigated. Structures were grown by Molecular Beam Epitaxy (MBE) with surface boron wells of variant composition extending 0.25 [.proportional]m into the substrate, as well as boron marker layers positioned 0.50 [.proportional]m below the surface. The boron well concentrations are as follows: 0, 7.5×1018, 1.5×1019, and 5.0×1019 atoms/cm3. The boron marker layers are approximately 3 nm wide and have a peak concentration of 5×1018 atoms/cm3. Samples were ion implanted with 60 keV Si+ at a dose of 1×1014 atoms/cm2 and subsequently annealed at 675°C and 750°C for various times. Plan-view Transmission Electron Microscopy (PTEM) was used to monitor the agglomeration of injected silicon interstitials and the evolution of extended defects in the near surface region. Secondary Ion Mass Spectroscopy (SIMS) concentration profiles facilitated the characterization of boron diffusion behaviors during annealing. Interstitial supersaturation conditions and the resultant defect structures of ion implanted relaxed Si0.8Ge0.2 in both the presence and absence of boron have been characterized.

Boron-Interstitial Cluster Kinetics: Extraction of Binding Energies from Dedicated Experiments

ABSTRACTA description of the transient diffusion and activation of boron during post-implantation an- nealing steps is one of the most challenging tasks. In industrially relevant situation, it needs to address diffusion at extrinsic concentrations, the agglomeration of self-interstitials, and the formation of boron-interstitial clusters. This article describes the experimental work performed or used to calibrate model parameters as independently as possible. In particular, the experiments used to extract information about the energetics of boron-interstitial clusters are described.

Lattice strain and composition of Boron-Interstitial Clusters in Crystalline Silicon

ABSTRACTIn this work we have investigated the average composition of Boron Interstitials Clusters (BICs) and the strain induced in the Si crystal by BICs. We have formed BICs by Si implantation and subsequent annealing of two Si samples, grown by molecular beam epitaxy, containing thin buried layers doped with different B concentrations (1019 and 1020at/cm3). By B chemical profiles diffusion analysis, we have extracted the doses of Si self-interstitials (I) and Boron atoms trapped at the BICs. The B/I stoichiometric ratio is about 1 for the low B concentration and about 3.5 for the high B concentration sample. High-resolution x-ray diffraction analyses provided an estimate of strain profile. While in the low B concentration sample no appreciable strain was detected after BIC formation, at the higher B concentration we found that the tensile strain present in the as grown B doped layer changes to a strong compressive strain as a consequence of BICs formation. For this kind of clusters, the mean volume expansion with respect to the Si matrix is of (29 ± 6) Å3 for each B atom trapped at the BICs.

Modeling B clustering in Si and SiGe

ABSTRACTOur experiments show boron-interstitial cluster dissolution is reduced during oxidation, a behavior not predicted by the current models. Both our experiments and others on the time dependence indicate the reactivation is coming from more than one cluster. Since oxidation induced interstitial injection reduces reactivation, one of the significant clusters has to release an interstitial release during dissolution. Recent ab-initio calculations provide a qualitatively different B clustering model, with two significant cluster species. Based on these energetics, we have developed a new physically based model that for the first time accounts for the experimentally observed cluster dissolution time and ambient dependence.Anomalous B diffusion behavior is also observed in an investigation of Ge influence on B diffusion. Silicon wafers were subjected to a Si preamorphization implant (PAI), followed by Ge and B implants contained within the existing amorphous layer. The control sample received only Si PAI and B implant. Upon annealing, the peak of the B profile shifts towards the surface and increases in magnitude, exhibiting uphill diffusion. The control samples subjected to the same thermal processing exhibit TED, but no uphill diffusion behavior. This is consistent with the model of B diffusion in Si1−xGex, accounting for trapping of B at Ge sites through formation of GeB complex.

Enhancement of Boron Activation in Shallow Junctions by Hydrogen

ABSTRACTThe ability to activate greater amounts of dopants at lower temperatures is a persistent contingency in the continual drive for device scaling in Si microelectronics. We report on the effect of incorporating atomic hydrogen on the activation of implanted boron in shallow junctions. Hydrogen incorporation into the sample was carried out by exposure to an electron cyclotron resonance (ECR) hydrogen plasma. Enhanced activation was observed in hydrogenated samples for post-implantation annealing temperatures of 450°C and below, as measured by spreading resistance profilometry, and confirmed by identical boron atomic profile in hydrogenated and unhydrogenated samples. The enhancement in boron activation at lower temperature is attributed to the creation of vacancies in the boron-implanted region, the lattice-relaxation effect by the presence of atomic hydrogen, and the effect of atomic hydrogen on boron-interstitial cluster formation.

Ion beam induced defects in crystalline silicon

A short review of the current understanding and modelling of the formation of ion beam induced defects in crystalline silicon is given in the first part of this article. Some recent experiments on the evolution of {1 1 3} defects formed after Si + implants in CVD grown wafers and on the formation of small clusters in ultra-low energy high-dose B implanted Si are then presented. It is found that, independently of the experimental conditions, {1 1 3} defects evolve in all cases following a non-conservative Ostwald Ripening mechanism. In some cases, {1 1 3} defects have been found to transform into dislocation loops, while in other cases they completely dissolve during annealing. After RTA annealing of ultra-low energy high-dose B + implanted Si, small two-dimensional {1 0 0} loops are formed. Both Si and B atoms are contained in the defects. These results indicate that boron interstitial clusters can be imaged by TEM and thus can be much bigger than generally assumed.

Optical and electrical activity of boron interstitial defects in Si

Density functional theory is used to investigate boron interstitial clusters and defectsformed with carbon and oxygen. Using data from experimental techniques such as deeplevel transient spectroscopy, and electron paramagnetic resonance, photoluminescenceand infrared studies, we are able to assign structures to many observed centresand begin to develop a series of reaction paths for the evolution of boron withannealing temperature depending on the relative concentrations of impurities.Among other results we demonstrate that a metastable defect composed of two boroninterstitials and a self-interstitial has symmetry, vibrational modes and an electronicstructure consistent with the I2 photoluminescence centre, also known as the Y centre.

A Tight-binding Molecular Dynamics Study of the Dissociation of Boron Clusters in c-Si

Boron clusters with silicon self-interstitials have been implicated in the phenomenon of transient enhanced diffusion (TED) following ion implantation of boron and subsequent annealing steps. This paper explores possible dissolution mechanisms for boron-interstitial clusters during the simulation of a typical annealing process. Using tight-binding molecular dynamics (TBMD) and employing a Goodwin-Skinner-Pettifor sp-based TB model, we have been able to observe the complete dissolution of a B 4 I 4 cluster into the surrounding crystalline silicon matrix. Many unsuccessful attempts to observe dissolution are also presented, highlighting the effect of cluster stability, temperature and the role of vacancies in cluster dissolution. Though we can make no unambiguous statements on the definitive dissolution mechanism of boron-defect clusters based on one successful dissolution event, we can hint at key events that appear to be important, such as the diffusion of self-interstitials (presciently predicted by Pelaz et al. ), the "stranding" of boron atoms in their wake, and the importance of mobile boron-self-interstitial (B-I) pairs. The intrinsic diffusivity of boron in a c-Si lattice and its retardation of the diffusivity of Si self-interstitials is also discussed.

Dissolution kinetics of boron-interstitial clusters in silicon

In this work, we have investigated the stoichiometry of boron-interstitial clusters (BICs) produced in a molecular-beam-epitaxy-grown B box by Si implantation and annealing, and their dissolution during further prolonged annealing cycles. Low-concentration B delta doping was used to quantitatively monitor the interstitial (I) flux. A stoichiometric ratio of about 1.2 between I and B was found for the BICs formed at 815 °C. The BIC dissolution kinetics was investigated by analyzing the concentration profiles at different times and temperatures (in the range 815–950 °C) with a simulation code able to deconvolve the processes of B diffusion and B release from clusters. We found that the main mechanism for cluster dissolution is the release of interstitial boron atoms, with a thermal activation energy of 3.2±0.4 eV. These data are discussed and compared with existing literature data.

Studies of boron–interstitial clusters in Si

The large number of self-interstitials created during implantation mediate the fasttransient diffusion of implanted boron, leading to clustering. Sophisticatedannealing strategies based on knowledge of the formation energy of the clustersare required to achieve full activation of the implant. In recent years attemptshave been made to determine these data a priori from theoretical calculations.However, energy calculations alone are not sufficient to establish the key playersin the clustering process of boron. The present paper describes a systematicfirst-principles quantum mechanical study of the characteristic vibrationfrequencies of a large number of boron–interstitial clusters (including possibleconfigurational isomers). Comparison with the first Raman spectra obtained onB-implanted samples after high temperature annealing is presented.

Identification of boron clusters and boron-interstitial clusters in silicon

The identity of boron clusters and boron interstitial clusters in Si, produced by radiation or implantation, has been studied theoretically. Local vibration modes of the single-boron interstitial and the diboron split interstitial $({\mathrm{B}}_{2}\mathrm{I})$ and its metastable isomer as well as substitutional dimers are found to be in good agreement with observations. The modes of a diboron defect that has trapped three interstitials ${\mathrm{B}}_{2}{\mathrm{I}}_{3}$ are close to those observed for the boron-related $I2$ luminescent center. The annealing of these centers around $400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ coincides with the main recovery of the electrical activity of boron, but the formation of defects which are metastable casts doubt on previous modeling strategies.

Transient-diffusion effects

Transient effects on diffusion and activation during post-implantation anneals are a major obstacle for the further miniaturization of ultra-large-scale integrated semiconductor devices. The article reviews recent developments in the simulation of such phenomena with particular emphasis on models for the kinetics of self-interstitial agglomerates and boron–interstitial clusters.

Kinetics of boron reactivation in doped silicon from Hall effect and spreading resistance techniques

In this work, a series of 13 boron implants were performed into Czochralski silicon substrates with doses of 2×1014–1.6×1015 cm−2 at energies of 10–80 keV. The boron was deliberately clustered with a 750 °C anneal of 10 or 30 min and the electrical activation of the boron implants was determined following a second anneal at 750 or 850 °C with a Hall effect system with certain samples also being analyzed with a spreading resistance technique. Analysis of the reactivation rates allows for the determination of the net energy to boron reactivation to be approximately 3.0 eV assuming the reactivation process is mediated by release of a boron interstitial with a migrational energy of 0.3 eV. This results in a critical binding energy of approximately 2.7 eV from the process limiting the dissolution of the most stable boron-interstitial cluster.

Current Understanding and Modeling of Boron-Interstitial Clusters

AbstractScaling of devices requires not only shallow junctions but also high levels of dopant activation. For boron as the main p-type dopant, the latter requirement is especially problematic since small clusters of boron atoms and self-interstitials, known also as boron-interstitial clusters (BICs), were found to deactivate and immobilize large fractions of the implanted atoms during post-implantation annealing. In this article, the properties of BICs are reviewed and their influence on semiconductor processes are highlighted.

Electrical activation of B in the presence of boron-interstitials clusters

Boron marker-layer structures have been used to analyze the evolution of boron-interstitial clusters (BICs) formed during transient enhanced diffusion. Our approach is based on the measure of B activation by spreading resistance profiling after annealing of Si implantation damage. We investigated a wide range of implant conditions in terms of defect densities below and above the amorphization threshold of Si. We found a common behavior of BICs in terms of trapping and release processes of B atoms. The BICs density as a function of time for different concentration ratios of I and B has been determined.

Issues on boron electrical activation in silicon: Experiments on boron clusters and shallow junctions formation

A comprehensive understanding of dopant activation mechanisms in crystalline Si is required in order to form shallow junctions. In this paper, we will review several experimental assessments on boron clustering and novel methods to form shallow junctions. Boron marker-layer structures have been used to investigate the fundamental aspects of formation and ripening boron-interstitial clusters (BICs) and their influence on the associated transient enhanced diffusion (TED). The samples were damaged by Si implants at different doses in the sub-amorphizing range and annealed at high temperatures. We found that BICs act as a sink for interstitials at supersaturations values S( t)>10 4. This implies that silicon self-interstitial defects are the primary source of interstitials driving TED, and that BICs act as a secondary “buffer” for the interstitial supersaturation. These clusters are less sensitive to the ripening process than pure interstitial clusters do, so that their size remain below 2 nm regardless of the annealing time. Moreover, we found that BICs formed in the early stage of the annealing reduce considerably the active amount of B. An extensive characterization of the electrical activation of ultra-low energy implanted boron in silicon is also reported. The spreading resistance profiling (SRP) technique has been used, in a suitable configuration, for measuring doped layers shallower than 100 nm, in order to extract the carrier concentration profiles. High ramp rates are certainly useful in order to form shallow and highly active layers. Laser annealing represents the most promising approach to match the requirements for the needs of fabrication of the future devices. The feasibility of laser annealed ultra-shallow junctions, with depths below 100 nm and high electrical activation, is demonstrated.

A kinetic lattice Monte-Carlo approach to the evolution of boron in silicon

A kinetic 3D lattice Monte-Carlo (MC) model is introduced, which allows to describe the evolution of boron-rich silicon, especially the formation of boron-interstitial clusters of various structures and compositions. Using a refined bcc lattice with lattice constant a bcc= a Si/4 a large variety of defect configurations can be mapped essentially without geometrical distortions. The energetics of defects can be specified by means of a set of energy parameters. Boron diffusion is implemented via an interstitial-assisted mechanism. First results of interstitial-mediated boron clustering are presented. The future capabilities to study the kinetics of B n I m cluster formation are outlined.

Effects of boron-interstitial silicon clusters on interstitial supersaturation during postimplantation annealing

Boron marker-layer structures have been used to investigate the effects of B doping on the evolution of the implantation damage and of the associated transient enhanced diffusion. The samples were damaged by Si implants at different doses in the range 2×1013–1×1014 cm−2 and annealed at 740 °C for times between 2 s and 4 h. The values of interstitial supersaturation, from the beginning of the annealing up to the complete damage recovery, have been determined for the different Si doses for a given B doping level. Damage removal has been followed by double crystal x-ray diffraction. Our results confirm that the formation of boron-interstitial silicon clusters traps a relevant fraction of the interstitials produced by the implantation. This trapping action gives rise to a strong reduction of the interstitial supersaturation, prevents the interstitial clusters from being transformed in {113} defects and modifies the time evolution of the transient enhanced diffusion. X-ray analyses indicate also that the size of the boron-interstitial silicon clusters remains below 2 nm.