J. M. Poate

PHYSICAL MECHANISMS OF TRANSIENT ENHANCED DOPANT DIFFUSION IN ION-IMPLANTED SILICON

Implanted B and P dopants in Si exhibit transient enhanced diffusion (TED) during annealing which arises from the excess interstitials generated by the implant. In order to study the mechanisms of TED, transmission electron microscopy measurements of implantation damage were combined with B diffusion experiments using doping marker structures grown by molecular-beam epitaxy (MBE). Damage from nonamorphizing Si implants at doses ranging from 5×1012 to 1×1014/cm2 evolves into a distribution of {311} interstitial agglomerates during the initial annealing stages at 670–815u2009°C. The excess interstitial concentration contained in these defects roughly equals the implanted ion dose, an observation that is corroborated by atomistic Monte Carlo simulations of implantation and annealing processes. The injection of interstitials from the damage region involves the dissolution of {311} defects during Ostwald ripening with an activation energy of 3.8±0.2 eV. The excess interstitials drive substitutional B into electric...

B diffusion and clustering in ion implanted Si: The role of B cluster precursors

A comprehensive model for B implantation, diffusion and clustering is presented. The model, implemented in a Monte Carlo atomistic simulator, successfully explains and predicts the behavior of B under a wide variety of implantation and annealing conditions by invoking the formation of immobile precursors of B clusters, prior to the onset of transient enhanced diffusion. The model also includes the usual mechanisms of Si self-interstitial diffusion and B kick-out. The immobile B cluster precursors, such as BI2 (a B atom with two Si self-interstitials) form during implantation or in the very early stages of the annealing, when the Si interstitial supersaturation is very high. They then act as nucleation centers for the formation of B-rich clusters during annealing. The B-rich clusters constitute the electrically inactive B component, so that the clustering process greatly affects both junction depth and doping level in high-dose implants.

Evolution from point to extended defects in ion implanted silicon

We present a quantitative study of the evolution of point defects into clusters and extended defects in ion-implanted Si. Deep level transient spectroscopy (DLTS) measurements are used to identify and count the electrically active defects in the damaged region produced by Si ion implantation at energies of 145 keV–2 MeV, and fluences from 1×108 to 5×1013u2009Si/cm2. Analyses of silicon annealed in the temperature range 100–680u2009°C allow us to monitor the transition from simple point defects to defect clusters and extended defects that occur upon increasing the ion fluence and the annealing temperature. At low doses, <1010u2009Si/cm2, only about 2% of the Frenkel pairs generated by the ion beam escape recombination and are stored into an equal number of interstitial and vacancy-type point defects. Thermal treatments produce a concomitant annealing of interstitial and vacancy-type defects until, at temperatures above 350u2009°C, only two to three interstitial-type defects per ion are left, and the DLTS spectra contain s...

Solid‐phase epitaxy of implanted silicon by cw Ar ion laser irradiation

Arsenic‐ and antimony‐implanted silicon wafers have been annealed by a cw Ar ion laser. Glancing‐angle Rutherford backscattering and transmission electron and optical microscopy measurements indicate that the mechanism for recrystallization is one of thermal solid‐phase regrowth from the underlying crystalline‐amorphous interface. No implant redistribution is observed.

Interactions of ion‐implantation‐induced interstitials with boron at high concentrations in silicon

Ion implantation of Si (60 keV, 1×1014/cm2) has been used to introduce excess interstitials into silicon predoped with high background concentrations of B, which were varied between 1×1018 and 1×1019/cm3. Following post‐implantation annealing at 740u2009°C for 15 min to allow agglomeration of the available interstitials into elongated {311} defects, the density of the agglomerated interstitials was determined by plan‐view transmission electron microscopy observation of the defects. We report a significant reduction in the fraction of excess interstitials trapped in {311} defects as a function of boron concentration, up to nearly complete disappearance of the {311} defects at boron concentrations of 1×1019/cm3. The reduction of the excess interstitial concentration is interpreted in terms of boron‐interstitial clustering, and implications for transient‐enhanced diffusion of B at high concentrations are discussed.

The interstitial fraction of diffusivity of common dopants in Si

The relative contributions of interstitials and vacancies to diffusion of a dopant A in silicon are specified by the interstitial fraction of diffusivity, fA. Accurate knowledge of fA is required for predictive simulations of Si processing during which the point defect population is perturbed, such as transient enhanced diffusion. While experimental determination of fA is traditionally based on an underdetermined system of equations, we show here that it is actually possible to derive expressions that give meaningful bounds on fA without any further assumptions but that of local equilibrium. By employing a pair of dopants under the same point-defect perturbance, and by utilizing perturbances very far from equilibrium, we obtain experimentally fSb⩽0.012 and fB⩾0.98 at temperatures of ∼800u2009°C, which are the strictest bounds reported to date. Our results are in agreement with a theoretical expectation that a substitutional dopant in Si should either be a pure vacancy, or a pure interstitial(cy) diffuser.

Interstitial defects in silicon from 1–5 keV Si+ ion implantation

Extended defects from 5-, 2-, and 1-keV Si+ ion implantation are investigated by transmission electron microscopy using implantation doses of 1 and 3×1014u2009cm−2 and annealing temperatures from 750 to 900u2009°C. Despite the proximity of the surface, {311}-type defects are observed even for 1 keV. Samples with a peak concentration of excess interstitials exceeding ∼1% of the atomic density also contain some {311} defects which are corrugated across their width. These so-called zig-zag {311} defects are more stable than the ordinary {311} defects, having a dissolution rate at 750u2009°C which is ten times smaller. Due to their enhanced stability, the zig-zag {311} defects grow to lengths that are many times longer than their distance from the surface. It is proposed that zig-zag {311} defects form during the early stages of annealing by coalescence the high volume density of {311} defects confined within a very narrow implanted layer. These findings indicate that defect formation and dissolution will continue to con...

Growth interface breakdown during laser recrystallization from the melt

Morphological instability occurring during high‐velocity Si crystal growth from an impurity containing melt is examined in detail. The experimental conditions are achieved by annealing an ion‐implanted Si layer with pulsed laser radiation. Computer modeling is employed to understand the heat flow and impurity diffusion behavior that occurs. The stability and size of impurity segregation cells observed to occur under particular conditions are related to the predictions of morphological stability theory.

Iron gettering mechanisms in silicon

Boron implantation into silicon offers a unique system for studying the gettering mechanisms of Fe. Using deep level transient spectroscopy to monitor the remaining Fe in the gettered region and secondary‐ion‐mass spectroscopy to measure the concentration of Fe redistributed to the B region, we show that the gettering mechanisms can be quantitatively described. A combination of Fermi‐level‐induced Fe+ charge‐state stabilization and Fe+–B− pairing acts to lower the free energy of Fe in p+ regions. This can lead to Fe partition coefficients as high as 106 at a p+/p interface at temperatures below ≊400u2009°C. The dynamic response of the system is diffusion limited during the cooling cycle. B gettering is more effective than gettering produced by Si implantation damage and more effective than trapping by a neutral impurity such as C. These mechanisms also make a large contribution to the effective gettering of Fe by p/p+ epitaxial silicon wafers. The Fermi‐level/pairing gettering mechanism is also expected to op...

TRANSIENT ENHANCED DIFFUSION OF SB AND B DUE TO MEV SILICON IMPLANTS

We measure the transient enhanced diffusion of shallow molecular-beam-epitaxy grown marker layers of Sb and B due to deep MeV Si+ ion implants at very high doses (≈1016u2009cm−2). We expect the near-surface region of these implants to be vacancy rich, and we observe transient enhanced diffusion of Sb (the classic vacancy diffuser). The large enhancements imply a significant vacancy supersaturation (≈700 at 740u2009°C). Double implantation of the high-dose MeV Si followed by a shallow (40 keV) Si implant and annealing produces a greatly reduced number of {311} defects compared to a 40 keV implant into virgin Si, again consistent with a vacancy-rich region in the near-surface region of an MeV implant. However, the shallow B marker layers also show transient enhanced diffusion for the same MeV implant under similar annealing conditions, implying that an interstitial supersaturation is present at the same time. We discuss possible mechanisms for a simultaneous supersaturation of both types of point defects.We measure the transient enhanced diffusion of shallow molecular-beam-epitaxy grown marker layers of Sb and B due to deep MeV Si+ ion implants at very high doses (≈1016u2009cm−2). We expect the near-surface region of these implants to be vacancy rich, and we observe transient enhanced diffusion of Sb (the classic vacancy diffuser). The large enhancements imply a significant vacancy supersaturation (≈700 at 740u2009°C). Double implantation of the high-dose MeV Si followed by a shallow (40 keV) Si implant and annealing produces a greatly reduced number of {311} defects compared to a 40 keV implant into virgin Si, again consistent with a vacancy-rich region in the near-surface region of an MeV implant. However, the shallow B marker layers also show transient enhanced diffusion for the same MeV implant under similar annealing conditions, implying that an interstitial supersaturation is present at the same time. We discuss possible mechanisms for a simultaneous supersaturation of both types of point defects.