Silicon Boride Research Articles

Boride-enhanced diffusion in silicon: Bulk and surface layers

Epitaxial silicon boride layers, located at the surface or within the bulk of single-crystal silicon, give rise to enhanced diffusion of B during annealing. A submonolayer buried boride layer releases ≈0.4 interstitials per B atom in the layer, generating a transient diffusion enhancement in the range of 10–100 for several minutes at 900 °C. The resulting profile broadening is comparable to that caused by ion implantation damage. At the same temperature, surface boride layers generate a diffusion enhancement of ∼6, part of which arises from the B diffusion flux and part from the chemical influence of the boride layer.

Boron-enhanced diffusion of boron from ultralow-energy ion implantation

We have investigated the diffusion enhancement mechanism of boron-enhanced diffusion (BED), wherein boron diffusivity is enhanced four to five times over the equilibrium diffusivity at 1050 °C in the proximity of a silicon layer containing a high boron concentration. It is demonstrated that BED is driven by excess interstitials injected from the high boron concentration layer during annealing. For evaporated layers, BED is observed above a threshold boron concentration between 1% and 10%, though it appears to be closer to 1% for B-implanted layers. For sub-keV B implants above the threshold, BED dominates over the contribution from transient-enhanced diffusion to junction depth. For 0.5 keV B, this threshold implantation dose lies between 3×1014 and 1×1015 cm−2. It is proposed that the excess interstitials responsible for BED are produced during the formation of a silicon boride phase in the high B concentration layers.

Boron-enhanced diffusion of boron: Physical mechanisms

Silicon layers containing B in excess of a few atomic percent create a supersaturation of Si self-interstitials in the underlying Si, resulting in enhanced diffusion of B in the substrate [boron-enhanced diffusion (BED)]. The temperature and time dependence of BED is investigated here. Evaporated boron as well as ultralow energy 0.5 keV B-implanted layers were annealed at temperatures from 1100 to 800 °C for times ranging from 3 to 3000 s. Isochronal 10 s anneals reveal that the BED effect increases with increasing temperature up to 1050 °C and then decreases. In contrast, simulations based on interstitial generation via the kick-out mechanism predict a decreasing dependence leading to the conclusion that the kick-out mechanism is not the dominant source of excess interstitials responsible for BED. The diffusivity enhancements from the combined effects of BED and transient-enhanced diffusion, measured in 2×1015 cm−2, 0.5 keV B-implanted samples, show a similar temperature dependence as seen for evaporated B, except that the maximum enhancement occurs at 1000 °C. The temperature-dependent behavior of BED supports the hypothesis that the source of excess interstitials is the formation of a silicon boride phase in the high-boron-concentration silicon layer.

High-Temperature Thermoelectric Properties of Silicon Boride Ceramics as a Smart Material

AbstractSeveral silicon boride phases such as SiB4, SiB6, SiB6-x, SiB6+x, and Si11B31, were previously reported. Among them, SiB6has proved to be a potentially useful material because of its excellent electrical conductivity, high degree of hardness, moderate melting point, and low specific gravity. The sintering conditions and thermoelectric properties of silicon boride (SiB6) ceramics produced by hot pressing were investigated in order to determine the suitability of this material for high-temperature thermoelectric applications as a smart material. The relative density increased with increasing sintering temperature. With a sintering temperature of 1923 K, a sintered body having a relative density of more than 99% was obtained. X-ray diffraction analysis showed no crystalline phase other than SiB6 in the sintered body. The specimens were prepared for measurement of the electrical conductivity and Seebeck coefficient by the D.C. four-terminal method. The thermal conductivity of SiB6 was obtained by calculation from the thermal diffusivity and specific heat capacity of the specimen. The electrical conductivity of SiB6 increased with increasing temperature. The electrical conductivity of the polycrystalline SiB6 (99% dense) was 0.5 to 1.1 × 103 S/m at 298 to 1273 K. The thermal conductivity decreased with increasing temperature in the range of room temperature to 1273 K. The thermal conductivity was 9.1 to 2.5 W/mK in the range of room temperature to 1273 K. The Seebeck coefficient of SiB6 increased with increasing temperature. The Seebeck coefficient of SiB6 was 140 × 10−6 V/K at 1273 K. The figure of merit Z of SiB6 increased with increasing temperature. The Z of SiB6 reached 8.1 × 10−6/K at 1273 K. The ZT value is useful to evaluate the ability of thermoelectric materials. The ZT value reached 0.01 at 1273 K. Based on the results, SiB6 showed very good thermoelectric material characteristics at high temperature.

Temperature- and Time-Dependence of Boron-Enhanced Diffusion From Evaporated- and Ultra-Low Energy Ion-Implanted Layers

ABSTRACTSilicon layers containing B in excess of a few atomic percent create a supersaturation of Si self-interstitials in the underlying Si, resulting in enhanced diffusion of B in the substrate (boron-enhanced diffusion, BED). The temperature- and time-dependence of BED is investigated here. Evaporated-boron as well as ultra-low energy 0.5-keV B-implanted layers were annealed at temperatures from 1100 to 800°C for times ranging from 3 to 3000s. Isochronal 10s anneals reveal that the BED effect increases with increasing temperature up to 1050°C and then decreases. In contrast, a simulation of interstitial generation via the kick-out mechanism predicts a decreasing dependence on increasing temperature leading to the conclusion that the kick-out mechanism is not the dominant source of excess interstitials responsible for BED. The diffusivity enhancements from the combined effects of BED and TED (transient-enhanced diffusion), measured in 2×105cm−2, 0.5-keV B-implanted samples, show a similar temperature dependence as seen for evaporated B, except that the maximum enhancement occurs at 1000°C. The temperature-dependent behavior of BED supports the hypothesis that the source of excess interstitials is the formation of a silicon boride phase in the high B concentration silicon layer.

Critical issues in ion implantation of silicon below 5 keV: Defects and diffusion

Continued use of ion implantation for doping of silicon integrated circuits will soon require implantation energies below 5 keV in order to form electrical junctions < 50 nm deep. At such low energies, dopant diffusion and formation of extended defects may be modified by both the proximity of the surface and by the large volume concentrations of point defects and dopant atoms that will arise from reduced range straggling. This brief review summarizes our recent experiments measuring defect formation and evolution, as well as transient enhanced diffusion (TED), in silicon implanted with Si + and B + ions < 10 nm deep. The results have demonstrated that {311}-type extended defects are generated from Si implants even within 3 nm of the surface. However, when these defects eventually dissolve, the surface acts as a perfect sink to efficiently annihilate the released interstitials. As a result, the amount of TED measured at epitaxially-grown B markers decreases approximately linearly with decreasing ion energy (at least, for Si + implantation). For low energy B + implants typical doses required for source-drain doping lead to the formation of a fine-grain polycrystalline silicon boride phase during activation annealing.

The coexistence of silicon borides with boron-saturated silicon: Metastability of SiB3

The silicon-rich end of the Si-B phase diagram, defining the silicon boride(s) that coexist in equilibrium with boron-saturated silicon, is poorly known. Understanding this equilibrium has implications for the processing of p+ silicon wafers, whose boron concentrations are near the solubility limit. Additionally, silicon boride precipitates produced by boron-ion-implantation and annealing of crystalline silicon have recently been shown to be efficient internal getters of transition metal ions. The experiments described in this paper probe the stability of these silicon borides. A phase with a boron-carbide-like structure, SiB3, grows from boron-saturated silicon in both the solid and the liquid state. However, SiB3 is not found to be stable in either circumstance. Rather, SiB3 is a metastable phase whose formation is driven by the relative ease of its nucleation and growth. The silicon boride that exists in stable equilibrium with boron-saturated silicon is SiB6. A qualitative understanding of the metastability of SiB3 comes from recognizing the conflict between the bonding requirements of icosahedral borides such as SiB3 and the size mismatch between silicon and boron atoms.

Damage, defects and diffusion from ultra-low energy (0–5 keV) ion implantation of silicon

Continued use of ion implantation for doping of silicon integrated circuits will soon require implantation energies below 5 keV to form electrical junctions less than 50 nm deep. At such low energies, dopant diffusion and formation of extended defects is modified by both the proximity of the surface and by the large volume concentrations of point defects and dopant atoms that arise from reduced range straggling. This brief review summarizes our recent experiments which measured defect formation and evolution, as well as enhanced diffusion, in silicon implanted with Si + and B + ions at energies as low as 0.5 keV. The results have demonstrated that {311}-type extended defects are generated from Si + implants even within 3 nm of the surface. However, when these defects eventually dissolve, the surface acts as a perfect sink to efficiently annihilate the released interstitials. As a result, the amount of TED from Si + implantation measured by epitaxially-grown B markers decreases approximately linearly with decreasing ion energy. For sub-keV B + implants typical doses currently used for source-drain doping lead to a boron diffusion enhancement of 3–4× despite the proximity of the surface. Enhanced diffusion is also observed from molecular beam-deposited silicon layers containing a high boron concentration. This newly emerged diffusion enhancement mechanism, boron-enhanced-diffusion (BED), is associated with the formation of a fine-grain polycrystalline silicon boride phase in the implanted layer during activation annealing. These investigations of ultra-low energy (ULE) implantation have thus reinforced and validated our understanding of the role of implantation damage in enhancing dopant diffusion in silicon, while simultaneously revealing some important new materials issues which will impact semiconductor processing in coming device generations.

Association of broad icosahedral Raman bands with substitutional disorder inSiB3and boron carbide

The structure of silicon boride, ${\mathrm{SiB}}_{3},$ is based on 12-atom, boron-rich icosahedra in which silicon atoms substitute for some boron atoms. Raman bands associated with vibrations of icosahedral atoms in ${\mathrm{SiB}}_{3}$ are quite broad, reflecting this substitutional disorder. Comparing the Raman spectra of other icosahedral borides with ${\mathrm{SiB}}_{3},$ only boron carbides have similarly broad icosahedral Raman bands. The direct correlation of broad icosahedral Raman bands with substitutional disorder supports the proposition that carbon atoms replace icosahedral boron atoms in boron carbides of all compositions.

Oxidation Resistance of Silicon Tetra Boride Powder

The oxidation of silicon boride (SiB4) powder at high temperatures was investigated in order to determine the possibility of its high-temperature utility. SiB4 powder with an average particle size of about 10μm was used in this experiment. The sample oxidized at 200 to 1100°C for 5min to 50h in air; the weight changes were measured to estimate the oxidation resistance. The oxidation of sample, at the short oxidation time of 5min, started at 500 to 600°C, and weight gain changes with increasing oxidation temperature. The sample at the oxidation time of 0.5 to 1h, exhibited weight gain change with increasing oxidation temperature, and then the oxidation saturated at 900°C. On the other hand, at the oxidation time of 10h, it had the maximum value of the weight gain at 600°C, after that the weight gain monotonically decreased with increasing oxidation temperature. SiB4 is oxidized to vitreous SiO2 and B2O3 at about 700°C. The B2O3 of oxidation product vaporized above at 900°C. The vitreous SiO2 transformed into the cristobalite.

Chemical Vapor Deposition of Multiphase Boron-Carbon-Silicon Ceramics

AbstractSpecific compositions of boron-carbon-silicon ceramics exhibit improved abrasive wear and good thermal shock resistance, but require bulk sintering at temperatures in excess of 2100K. The formation of such phases by chemical vapor deposition was investigated in the temperature range of 1073K–1573K. Methyltrichlorosilane (CH3SiCl3), boron trichloride, and methane were chosen as reactant gases, with hydrogen as a carrier gas and diluent. The coatings were deposited in a computer-controlled, hot-wall reactor at a pressure of 33 MPa (200 Torr).Below 1473K the coatings were amorphous. At higher temperatures non-equilibrium reactions controlled the deposition process. The most common coating consisted of a silicon carbide matrix and a silicon boride, SiB6, dispersed phase. Multiphase coatings of B+B4C+SiB6 and SiC+SiB6+SiB14 were also deposited by controlling the partial pressure of methane and boron trichloride. Non-equilibrium thermodynamic analysis qualitatively predicted the experimentally deposited multiphase coatings.

Role of high temperature chemistry in CVD (chemical vapor deposition) processing

Abstract

Predicting The Chemistry In Cvd Systems

AbstractTailoring the chemical and physical nature of CVD deposits for specific applications requires understanding and controlling the chemical processes occurring in a deposition reactor. This implies understanding the combined equilibrium, kinetic, and transport processes. The occurring processes produce composition gradients in the reactor which can cause variations in the deposition chemistry. Derived physical and mathematical models of possible mechanisms provide critical tools for manipulating and controlling the chemical and physical nature of CVD deposits. Examples of silicon boride deposition from SiH4, BC13, and H2 mixtures are used to illustrate thermochemical modeling techniques which utilize the concept of partial equilibrium. Predictions of limits on the chemical behavior, and comparison of these limits with experimental observations are used to hypotheses deposition mechanisms.

Comparative characteristics of phases in the B-Si and B-C systems

A systematic study was made of the technological properties of silicon boride powders: pycnometric density, morphology and size of particles depending on the conditions of synthesis. It is found that the B 4Si particles inherit the shape and size of particles of amorphous boron powder used as a raw material for the synthesis. The character of the interaction between the B 4Si powder, the aggressive liquid media and atmospheric oxygen was investigated at various temperatures and holding times. Techniques of chemical analysis were developed for fine-grained powders of the B-Si system. Comparison was made between the properties of silicon boride and boron carbide and the properties showed a significant resemblance.

A new method for growing crystals of SiB2.8–4 type silicon boride from a metallic melt

Abstract Crystals of “SiB2.8–4” silicon-boride have been grown in a copper melt, starting from boron and silicon powders. The crystals have been isolated and characterized by their diffraction pattern and infra-red absorption spectra.

Mécanisme de formation des couches borurées sur les aciers a haut carbone—I. Technique des pates

The boridisation of SAE 1095 steel by means of a paste of boron carbide and cryolite in an aqueous solution of methyl cellulose has been studied. An homogeneous boridised layer approximately 120 μm thick was obtained after 4 h treatment at 900°C. Two borides FeB and Fe 2B have been identified in this film as well as a continuum structure joining these phases to the matrix. This continuum itself is composed of three phases, namely iron silicon boride, aluminium boride and cementite in a dilute solid solution of silicon in α-iron. In the mechanism proposed the initiator of boron diffusion is ascribed to aluminium. The orientation texture along the 001 axis in the boridised layer and in the stages of formation of the transition zone has also been explained. These stages are consequent upon the saturation of the adjacent matrix in silicon. The specific volume differences between FeB and Fe 2B, and Fe 2B and the matrix cause the formation of residual compressive stresses which are responsible for both the texture orientation and some cracking in the middle of the boridised layer.

An Investigation of the Compound Silicon Boride (SiB6)

Single crystals of the compound silicon hexaboride have been prepared. Laue, rotation, and Weissenberg photographs indicate an orthorhombic cell with lattice parameters , , and . The pycnometer density is 2.43 g/cm3. The average electrical resistivity of single crystals, based on a probe technique, is 0.2 ohm‐cm. The melting point is 1950°C. has a Knoop hardness, with a 100‐g load, of 1910 kg/mm2.

Preliminary Investigations of the Silicon Boride, SiB6

ALTHOUGH there has been a good deal of experimental work on the borides of silicon dating back to Moissan1, current reports (ref. 2 and Parthe, E., M.I.T., private communication) have been somewhat confusing as to the exact phases present in the boron–silicon system. We now wish to report that we have prepared single crystals of silicon boride (SiB6) by fusion of the elements. The wet chemical analysis after removal of excess silicon agrees with stoichiometric values for the compound SiB6 as are shown by the following determinations: B—69.05, 70.16, 69.3 Si—29.43, 29.48, 30.4 The density as determined by pycnometer methods is 2.45 gm./c.c.