Si Self-interstitials Research Articles

Determination of Silicon self-interstitial diffusivity using isotopically pure 30Silicon multi-layer

Si self-diffusion under oxygen ambient has been studied using multi-layer structures consisting of alternative layers with isotopically enriched 30Si and natural Si in order to determine the diffusivities of Si self-interstitials. Spreading of 30Si spikes of each layer due to the diffusion of Si self-interstitials generated at the surface was measured with SIMS analysis. The diffusivity of Si self-interstitials, DI, is obtained from the fitting with experimental results. In the temperature range between 820 and 920 °C, DI and thermal equilibrium concentration of Si self-interstitials, CIi, are described by the Arrhenius equations, D I = 3.48 × 10 4 exp ( − 3.82 eV / K T ) (cm 2/s) and C Ii = 9.62 × 10 18 exp ( − 0.475 eV / K T ) (cm −3), respectively.

Boron-interstitial clusters in crystalline silicon: stoichiometry and strain

In this work, we have investigated the strain induced in a Si crystal by Boron-interstitial clusters (BICs). BICs were formed by Si implantation and subsequent annealing of two Si samples, grown by molecular beam epitaxy (MBE), containing thin buried layers doped with different B concentrations (10 19 and 10 20 at./cm 3). By B chemical profile diffusion analysis the doses of Si self-interstitials (Is) and Boron atoms trapped in the BICs were extracted. 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 (HRXRD) analyses provided a measure of the strain profile. While in the low B concentration sample, no appreciable strain was detected after BICs 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 BIC formation. For this kind of clusters, the mean volume expansion with respect to the Si matrix is 29 ± 6 Å 3 for each B atom trapped in the BICs.

Formation of p+ shallow junctions using SiGe barriers

Ultrashallow p+ junctions are required for next generation electronics. We present a technique for the formation of ultrashallow p+ junctions that increases the thermal stability of the junctions formed by either epitaxy or ion implantation. By using a 10nm Si1−xGex barrier layer, the diffusion of B is inhibited during high temperature processes. Alloys having a composition from x=0–0.4 were investigated and it is shown that the most effective barrier had the maximum Ge fraction. The junction depth decreased to 36.7nm for a 5×1015∕cm2 1kV BF3 plasma implant spike annealed at 1050°C, compared to a junction depth of 48nm for a Si control sample having the identical implant and anneal. It is hypothesized that the inhibition of B diffusion in the alloy layer is caused by a reduction of the Si self-interstitials in the alloy.

Investigation of two-dimensional diffusion of the self-interstitials in crystalline silicon at 800 °C and at room temperature

The two-dimensional (2D) diffusion of self-interstitials (I) in crystalline Si, both at room temperature and at 800 °C, has been studied by quantitative scanning capacitance microscopy measurements. The 2D I emission from an I source laterally confined down to submicrometer dimensions, obtained by low-energy implantation through a patterned oxide mask, has been observed. At room temperature, I diffusion was monitored by measuring the electrical deactivation of B corresponding to the diffusing interstitial tail and it was demonstrated that this deactivation is due to compensating levels introduced by defects in the Si band gap. At 800 °C I diffusion was monitored by measuring the transient enhanced diffusion of B spikes due to interstitial supersaturation produced during the annealing. In both cases, a dependence of the I depth-penetration on the original source size has been shown.

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.

The Use of SiGe Barriers During the Formation of p+ Shallow Junctions by Ion Implantation

ABSTRACTUltra-shallow p+ junctions are required for next generation electronics. We present a technique for the formation of ultra-shallow p+ junctions that increases the thermal stability of the junctions formed by ion implantation. By using a 10 nm Si1−xGex barrier layer, the diffusion of B is inhibited during high temperature processes. Alloys having a composition from x = 0 to 0.4 were investigated and it is shown that the most effective barrier had the maximum Ge fraction. The junction depth decreased to 36.7 nm for a 5×1015/cm2 1kV BF3 plasma implant spike annealed at 1050°C, compared to a junction depth of 48 nm for a Si control sample having the identical implant and anneal. It is hypothesized that the inhibition of B diffusion in the alloy layer is caused by a reduction of the Si self-interstitials in the alloy.

Coupling of atom-by-atom calculations of extended defects with B kick-out equations: application to the simulation of boron ted

We present simulations of B TED based on a complete calculation of the extended defect growth/shrinkage during annealing. The Si self-interstitial supersaturation calculated at the extended defect depth is coupled to the set of equations for the B kick-out diffusion through a generation/recombination term in the diffusion equation of the Si self-interstitials. The simulations are compared to the measurements performed on a Si wafer containing several B marker layers, where the amount of TED varies from one peak to the other. The good agreement obtained on this experiment is very promising for the application of these calculations to the case of ultra-shallow B + implants.

First-principles study of the atomic structure of B-related defects in crystalline Si predoped with phosphorus

We investigate the effect of phosphorus doping on the atomic structure of B-related defects in crystalline Si through first-principles pseudopotential calculations. We find that a B–P complex is the most stable defect, with a binding energy of about 0.3eV, compared with isolated B and P ions. When Si self-interstitials (Is) are generated by ion implantation, B and P dopants form an Is–B–P complex. The stability of the Is–B–P complex is greatly enhanced as the Fermi level increases, compared with the Is–B complex. The formation of the B–P and Is–B–P complexes is suggested to be responsible for the suppression of B diffusion in Si predoped with donor impurities.

Diffusion of phosphorus in relaxed Si1−xGex films and strained Si/Si1−xGex heterostructures

Phosphorus diffusion has been studied in relaxed Si1−xGex samples (x=0.11 and 0.19) and strained Si/Si1−xGex/Si heterostructures (x=0.08, 0.13, and 0.18). The diffusivity of P is found to increase with increasing Ge content, while the influence of compressive strain results in a decrease in diffusivity as compared to that in relaxed material. The effect of strain is found to be equivalent to an apparent activation energy of −13 eV per unit strain, where the negative sign indicates that the P diffusion is mediated by interstitials in Si1−xGex (x<0.20). This conclusion is also supported by an experiment utilizing injection of Si self-interstitials, which results in an enhanced P diffusion in strained Si1−xGex. Further, P is found to segregate into Si across Si/Si1−xGex interfaces and the segregation coefficient increases with increasing Ge concentration.

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.

Phosphorus and boron diffusion in silicon under equilibrium conditions

The intrinsic diffusion of phosphorus and boron in high-purity epitaxial silicon films has been studied. Phosphorus diffusion in a wide temperature range (810 to 1100 °C) revealed diffusion coefficients with an Arrhenius behavior exhibiting an activation energy of 2.74±0.07 eV and a pre-exponential factor of (8±5)×10−4 cm2/s. In the temperature range of 810 to 1050 °C, boron was found to diffuse with an activation energy of 3.12±0.04 eV and a pre-exponential factor of 0.06±0.02 cm2/s. These results differ from those of many previous studies, but this deviation may to a large extent be attributed to slow transients before equilibrium concentrations of point defects are established at temperatures below ∼1000 °C. Despite a similar diffusion mechanism mediated by Si self-interstitials, P exhibits a lower activation energy than B because of stronger bonding to the Si self-interstitial.

Direct observation of two-dimensional diffusion of the self-interstitials in crystalline Si

The two-dimensional (2D) diffusion of Si self-interstitials (I) from a submicron laterally confined source has been investigated in detail. High-resolution scanning capacitance microscopy was used for quantitative measurements of the B transient enhanced diffusion induced by the I flux generated by a low-energy Si implantation through a submicron dimension oxide mask. We show that the I depth penetration strongly depends on the original lateral size of the source. The 2D I diffusion has been well described by a 2D rate equations model, using the same physical parameters valid for one-dimensional diffusion.

Two dimensional boron diffusion determination by scanning capacitance microscopy

Two-dimensional boron diffusion has been investigated on sub-micron patterned samples implanted with 1 keV B ions and diffused at 1100 °C for different times. Carrier profiles were measured by scanning capacitance microscopy (SCM) whose depth and lateral resolution was enhanced by a double bevelling sample preparation. Implants were performed at two different doses (1×10 14 and 1×10 15 cm −2) into patterned wafers with several stripe widths ranging from 0.5 to 5 μm. B transient enhanced diffusion is strongly reduced with decreasing feature size, depending also on the implanted dose. The phenomenon has been related to the influence of Si self-interstitials on the B transient diffusion.

Influences of Point and Extended Defects on As Diffusion in Si

The influences of point and extended defects introduced by ion implantation on the transient enhanced diffusion (TED) of As have been examined. The implantation of a subamorphizing dose of Si into Si wafers with nearly uniform As background doping results in the slight segregation of As into {311} defects after annealing at 670°C, whereas an amorphizing dose of Si is found to induce significant segregation of As into end-of-range (EOR) dislocation loops after annealing at 820°C, regardless of the background As concentration. In the second experiment, As ions were implanted into Si wafers at 30 keV to low and medium doses. Some of the wafers were preannealed in order to recrystallize the amorphous layer and subsequently implanted with subamorphizing doses of Si at 50 keV. The resulting profiles reveal that the TED of As increases with Si implantation dose, indicating that transient As diffusion occurs via Si self-interstitials in the same way as for B and P. A suppression of the TED of As in the early stages of annealing at higher As and Si implantation doses is also observed. This is considered to originate from the formation of immobile arsenic-vacancy clusters.

Diffusion of iridium in silicon: Changeover from a foreign-atom-limited to a native-defect-controlled transport mode

Diffusion of Ir into Si was investigated by annealing of front-side deposited floating-zone Si wafers at temperatures between 1000 and 1200 \ifmmode^\circ\else\textdegree\fi{}C. Concentrations-depth profiles were measured by means of neutron activation analysis in conjunction with mechanical sectioning. The observed deep penetrations and anomalous profile shapes point to a diffusion process involving both highly mobile interstitial ${\mathrm{Ir}}_{\mathrm{i}}$ as diffusion vehicle and immobile substitutional ${\mathrm{Ir}}_{\mathrm{s}}$ as dominating species. The experimental findings are best reproduced by making allowance for ${\mathrm{Ir}}_{\mathrm{i}}\ensuremath{-}{\mathrm{Ir}}_{\mathrm{s}}$ exchanges that simultaneously take place via Si self-interstitials (kickout mechanism) and Si vacancies (dissociative mechanism). As a unique property it is found that the Si:Ir system is subject to a gradual transition from a ${\mathrm{Ir}}_{i}$-controlled diffusion mode at higher temperatures to a self-interstitial-controlled diffusion mode at lower temperatures.

Modeling the suppression of boron transient enhanced diffusion in silicon by substitutional carbon incorporation

Recent work has indicated that the suppression of boron transient enhanced diffusion (TED) in carbon-rich Si is caused by nonequilibrium Si point defect concentrations, specifically the undersaturation of Si self-interstitials, that result from the coupled out-diffusion of carbon interstitials via the kick-out and Frank–Turnbull reactions. This study of boron TED reduction in Si1−x−yGexCy during 750 °C inert anneals has revealed that the use of an additional reaction that further reduces the Si self-interstitial concentration is necessary to describe accurately the time evolved diffusion behavior of boron. In this article, we present a comprehensive model which includes {311} defects, boron-interstitial clusters, a carbon kick-out reaction, a carbon Frank–Turnbull reaction, and a carbon interstitial-carbon substitutional (CiCs) pairing reaction that successfully simulates carbon suppression of boron TED at 750 °C for anneal times ranging from 10 s to 60 min.

Ab initio modeling study of boron diffusion in silicon

We present investigations of boron diffusion in crystalline silicon using ab initio calculations (W. Windl et al., Phys. Rev. Lett. 83 (1999) 4345). Based on these results, a new mechanism for B diffusion mediated by Si self-interstitials was proposed. Rather than kick-out of B into a mobile channel-interstitial, one- or two-step diffusion mechanisms have been found for the different charge states. The predicted activation energy of 3.5–3.8 eV, migration barrier of 0.4–0.7 eV, and diffusion-length exponent of −0.6 to −0.2 eV are in excellent agreement with experiment. We also present results of ab initio calculations for the structure and energetics of boron-interstitial clusters in Si. We show how these first-principles results can be used to create a physical B diffusion model within a continuum simulator which has strongly enhanced predictive power in comparison to traditional diffusion models.

Stability of defects in crystalline silicon and their role in amorphization

Using molecular-dynamics simulation techniques, we have investigated the role that point defects and interstitial-vacancy complexes have on the silicon amorphization process. We have observed that accumulation of interstitial-vacancy complexes in concentrations of 25% and above lead to homogeneous amorphization. However, we have determined the basic properties of the interstitial-vacancy complex, and showed that it is not as stable at room temperature as previously reported by other authors. From our simulations we have identified more stable defect structures, consisting of the combination of the complex and Si self-interstitials. These defects form when there is an excess of interstitials or by incomplete interstitial-vacancy recombination in a highly damaged lattice. Unlike the interstitial-vacancy complex, these defects could survive long enough at room temperature to act as embryos for the formation of extended amorphous zones and/or point defect clusters.

Formation ofradiation defects in silicon structures under low-intensityelectron irradiation

Formation of A-centres and divacancies in siliconp+-n-n+ structures was investigated for 4 MeVelectron irradiation in the low-intensity range of 1011-5×1012 cm-2 s-1. It is shown that theintroduction rates of both A-centres and divacancies increasewith intensity in this range and then saturate at intensitiesabove 1012 cm-2 s-1. Using the data fromthe literature the introduction rates of these defects arediscussed in a wide range of intensities of electronirradiation of 1011-1015 cm-2 s-1,indicating a consistent role of carbon interstitial atoms andelectron-enhanced migration of Si self-interstitials in theobserved behaviour of introduction rates.

Influence of boron concentration on the enhanced diffusion observed after irradiation of boron delta-doped silicon at 570°C

A sequence of three boron spikes, grown in silicon by molecular beam epitaxy (MBE) and characterized by secondary-ion mass spectrometry, has been employed to investigate the mechanisms of boron diffusion during irradiation with 2.5 MeV protons at an elevated sample temperature of 570°C. The energy of the proton beam has been chosen so that the generation rate of point defects can be considered as uniform throughout the boron delta-doped silicon. The influence of the B concentration (ranging from 5×10 15 to 3×10 18 cm −3 in different samples) on the enhanced diffusion of boron have been studied in detail. For each concentration the B diffusion coefficient is increased by a factor greater than 5×10 4 under irradiation as compared to the B diffusion coefficient at 570°C in unirradiated areas. In the first approximation this enhancement is independent on the depth position of the boron spike but depends on the boron concentration. The diffusion of boron under irradiation exhibits an exponential behavior. This kind of diffusion has already been reported in the literature for boron diffusion in silicon during oxidation experiments at 600°C. The direct comparison with oxidation experiments where Si self-interstitials are injected in a non-uniform way provides further information on the basics of the boron diffusion mechanisms in silicon.