B-doped Silicon Research Articles

Potential barrier at grain boundaries in polycrystalline silicon: influence of grain-boundary character and copper/iron contamination

We used Kelvin probe force microscopy to measure the potential barrier associated with grain boundaries in B-doped p-type polycrystalline silicon. The potential barrier at non-contaminated grain boundaries varied approximately from 5 to 90 meV according to grain-boundary character. The random boundaries had a higher potential barrier than the coincidence boundaries. This barrier increased with increasing twist component of the grain boundary when the misorientation angle of two adjoining grains was almost the same. Copper and iron contamination caused a significant increase in the grain-boundary potential barrier, making the potential barrier depend more strongly on misorientation, probably because of anisotropy of grain-boundary segregation.

Thermodynamic and Kinetic Behavior of B and Na Through the Contact of B-Doped Silicon with Na2O-SiO2 Slags

Abstract Boron (B) is the most problematic impurity to be removed in the processes applied for the production of solar grade silicon. Boron removal from liquid silicon by sodium-silicate slags is experimentally studied and it is indicated that B can be rapidly removed within short reaction times. The B removal rate is higher at higher temperatures and higher Na2O concentrations in the slag. Based on the experimental results and thermodynamic calculations, it is proposed that B removal from silicon phase takes place through its oxidation at the slag/Si interfacial area by Na2O and that the oxidized B is further gasified from the slag through the formation of sodium metaborate (Na2B2O4) at the slag/gas interfacial area. The overall rate of B removal is mainly controlled by these two chemical reactions. However, it is further proposed that the B removal rate from silicon depends on the mass transport of Na in the system. Sodium is transferred from slag to the molten silicon through the silicothermic reduction of Na2O at the slag/Si interface and it simultaneously evaporates at the Si/gas interfacial area. This causes a Na concentration rise in silicon and its further decline after reaching a maximum. A major part of the Na loss from the slag is due to its carbothermic reduction and formation of Na gas.

Extended defects and precipitation in heavily B-doped silicon

The boron precipitation phenomenon in highly supersaturated boron-implanted silicon (10keV B+, 5×1015 ions/cm2) has been investigated by means of atom probe tomography, transmission electron microscopy and secondary ion mass spectrometry. We demonstrate that (001) dislocation loops observed by transmission electron microscopy in the region close to the boron projected range and boron precipitates ((001)-platelets) observed by atom probe are the same objects. For sufficiently high thermal budgets (900°C for 5h), boron precipitates close to the stoichiometric SiB3 composition have been observed, indicating that the precipitation induced by high-dose boron implantation can lead to the formation of a stable phase.

Effect of Nitrogen Doping on Vacancy State in Silicon Crystals Observed by Low-Temperature Ultrasonic Measurements

For the B-doped silicon crystals grown with and without N-doping, we measured the temperature dependence of the elastic constant in low-temperature region, to examine whether the N-doping annihilates the elastic softening caused by the gap-states of the isolated single vacancy. We have found that the elastic softening clearly observed for the N-free crystals is not observed for the N-doped ones, suggesting that the gap-states of the vacancies causing the elastic softening are destroyed by the N-doping. This is consistent with the model of Abe [T. Abe, J. Crystal Growth, 327 (2011) 1] in which the nitrogen molecule (N-N pair) occupies the vacancy to destroy its original gap-states. We have further observed that the N-doped silicon, which exhibits no softening in its as-grown state, exhibits the softening after the short-time annealing. This suggests that during the annealing the N-N pair is thermally activated to jump off the lattice site leaving the vacancy.

Active doping of B in silicon nanostructures and development of a Si quantum dot solar cell

Active doping of B was observed in nanometer silicon layers confined inSiO2 layers by secondary ion mass spectrometry (SIMS) depth profiling analysis and confirmed byHall effect measurements. The uniformly distributed boron atoms in the B-doped silicon layers of[SiO2 (8 nm)/B-dopedSi(10 nm)]5 films turned out to besegregated into the Si/SiO2 interfaces and the Si bulk, forming a distinct bimodal distribution by annealingat high temperature. B atoms in the Si layers were found to preferentiallysubstitute inactive three-fold Si atoms in the grain boundaries and thensubstitute the four-fold Si atoms to achieve electrically active doping. As aresult, active doping of B is initiated at high doping concentrations above1.1 × 1020 atoms cm − 3 and high activedoping of 3 × 1020 atoms cm − 3 could be achieved. The active doping in ultra-thin Si layers was implemented for siliconquantum dots (QDs) to realize a Si QD solar cell. A high energy-conversion efficiencyof 13.4% was realized from a p-type Si QD solar cell with B concentration of4 × 1020 atoms cm − 3.

Study of Nitrogen Effect on the Boron Diffusion during Heat Treatment in Polycrystalline Silicon/Nitrogen-Doped Silicon Thin Films

The present paper studies the boron (B) diffusion in nitrogen (N) doped amorphous silicon (a-Si) layer in original bi-layer B-doped polycrystalline silicon (poly-Si)/in-situ N-doped Si layers (NIDOS) thin films deposited by low pressure chemical vapor deposition (LPCVD) technique. The B diffusion in the NIDOS layer was investigated by secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR) analysis. A new extended diffusion model is proposed to fit the SIMS profile of the bi-layer films. This model introduces new terms which take into account the effect of N concentration on the complex diffusion phenomena of B atoms in bi-layer films. SIMS results show that B diffusion does not exceed one third of NIDOS layer thickness after annealing. The reduction of the B diffusion in the NIDOS layer is due to the formation of complex B–N as shown by infrared absorption measurements. Electrical measurements using four-probe and Hall effect techniques show the good conductivity of the B-doped poly-Si layer after annealing treatment.

Study of Nitrogen Effect on the Boron Diffusion during Heat Treatment in Polycrystalline Silicon/Nitrogen-Doped Silicon Thin Films

The present paper studies the boron (B) diffusion in nitrogen (N) doped amorphous silicon (a-Si) layer in original bi-layer B-doped polycrystalline silicon (poly-Si)/in-situ N-doped Si layers (NIDOS) thin films deposited by low pressure chemical vapor deposition (LPCVD) technique. The B diffusion in the NIDOS layer was investigated by secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR) analysis. A new extended diffusion model is proposed to fit the SIMS profile of the bi-layer films. This model introduces new terms which take into account the effect of N concentration on the complex diffusion phenomena of B atoms in bi-layer films. SIMS results show that B diffusion does not exceed one third of NIDOS layer thickness after annealing. The reduction of the B diffusion in the NIDOS layer is due to the formation of complex B–N as shown by infrared absorption measurements. Electrical measurements using four-probe and Hall effect techniques show the good conductivity of the B-doped poly-Si layer after annealing treatment.

Fe Ions Diffusion and Oxygen Complexes Evolution during Thermodonors Annealing in B-Doped Czochralsky Silicon Crystal

The great enhancement of the minority carrier recombination rate after optical activation of FeB pairs was used for investigation of interstitial Fe ions diffusion in p-type Czochralsky silicon by Surface Photo-Voltage Technique (SPV). Highly reproducible profiles of the Fe ions distribution were obtained after one-time thermodonors annealing process (650 °C, 30 min.) with Fe film evaporated on one side of silicon slab and successive layer by layer silicon slab etching. Evolution of Fe concentration distribution between both sides of initially inhomogeneously contaminated silicon slab in durable annealing processes was investigated. Gradual Fe concentration reduction near preliminary contaminated slab surface and Fe concentration increase near opposite surface were observed. Fe concentrations near both surfaces become equal after approximately 10-times annealing in agreement with diffusion rate obtained by the first method. Chaotic release and hiding of latent Fe were observed after 5-7 hours of annealing process. Two-exponential increase of other than Fe recombination centers concentration was observed during 650 °C annealing. Both exponential factors are found to be functions of interstitial oxygen concentration in the silicon ingot.

Inductively coupled plasma-assisted RF magnetron sputtering deposition of boron-doped microcrystalline Si films

An innovative custom-designed inductively coupled plasma-assisted RF magnetron sputtering deposition system has been developed to synthesize B-doped microcrystalline silicon thin films using a pure boron sputtering target in a reactive silane and argon gas mixture. Films were deposited using different boron target powers ranging from 0 to 350 W at a substrate temperature of 250 °C. The effect of the boron target power on the structural and electrical properties of the synthesized films was extensively investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and Hall-effect system. It is shown that, with an initial increase of the boron target power from 0 to 300 W, the structural and electrical properties of the B-doped microcrystalline films are improved. However, when the target power is increased too much (e.g. to 350 W), these properties become slightly worse. The variation of the structural and electrical properties of the synthesized B-doped microcrystalline thin films is related to the incorporation of boron atoms during the crystallization and doping of silicon in the inductively coupled plasma-based process. This work is particularly relevant to the microcrystalline silicon-based p– i– n junction solar cells.

Effect of the spin-orbit interaction and the electron phonon coupling on the electronic state in a silicon vacancy

The electronic state around a single vacancy in silicon crystal is investigated by using the Green's function approach. The triply degenerate charge states are found to be widely extended and account for extremely large elastic softening at low temperature as observed in recent ultrasonic experiments. When we include the LS coupling λSi on each Si atom, the 6-fold spin-orbital degeneracy for the V+ state with the valence +1 and spin 1/2 splits into Γ doublet groundstates and Γ8 quartet excited states with a reduced excited energy of O(λSi/10). We also consider the effect of couplings between electrons and Jahn-Teller phonons in the dangling bonds within the second order perturbation and find that the groundstate becomes Γ8 quartet which is responsible for the magnetic-field suppression of the softening in B-doped silicon.

Trans-R<sub>P</sub> Gettering and Out-Diffusion of Oxygen Implanted into Highly B-Doped Silicon

Implantation of 18O into highly B-doped and undoped silicon provides the possibility to investigate the effect of B-doping and to distinguish the processes of in-diffusion and out-diffusion of oxygen by profiling of 16O and 18O, respectively. The simultaneous in- and outdiffusion of oxygen was observed at 1000°C under oxidizing conditions. For silicon, heavily Bdoped to concentrations of 1019 B cm-3, oxygen tends to diffuse out toward the surface. Moreover, a fraction of the oxygen from both sources, implanted 18O and in-diffused 16O, also migrates deep into the substrate and is trapped far beyond the mean ion range RP in the depth of x 3RP at the so-called trans-RP gettering peak. In undoped silicon oxygen accumulation only takes place at vacancy-type defects introduced by ion implantation at a position shallower than RP. The mobility of oxygen implanted into B-doped Si is higher than for implantation into undoped Si. Highly mobile defects are suggested to be formed in B-doped silicon beside the common mobile interstitial oxygen, Oi, and the immobile SiOX precipitates. These I OXBY defects may involve selfinterstitials, I, and O and B atoms. The trans-RP peak appears due to the decay of these defects and the segregation of their constituents.

Radiative transitions in P- and B-doped silicon nanocrystals

The radiative transitions in P- and B-doped Si nanocrystals are investigated by means of first-principle calculations. Using a three-level model, we show that the radiative lifetimes and oscillator strengths of the transitions between the conduction and the impurity bands, as well as the transitions between the impurity and the valence bands are strongly affected by the impurity position. On the other hand, the direct conduction-to-valence band decay is practically unchanged due to the presence of the impurity. In addition, the emission intensity of P(B)-doped nanocrystals with impurities positioned in the surface (anywhere) is higher (lower) than for pure nanocrystals.

Ab initio study of electron-phonon coupling in boron-doped SiC

Density functional theory calculations have been used to study the electronic structure, lattice dynamics, and electron-phonon coupling in boron-doped silicon carbide in the cubic phase. Our results provide evidence that the recently discovered superconducting transition in boron-doped silicon carbide can be explained within a standard phonon-mediated mechanism. For the same doping rate, the coupling constant λ in B-doped SiC is very close to that of doped diamond and twice as large as that of B-doped silicon. However, doped silicon carbide differs from its diamond counterpart as most of the electron-phonon coupling originates from low energy vibrational modes.

Effects of Electron Correlation and Electron–Phonon Coupling on the Quantum State of a Silicon Vacancy

Recently, Goto et al. have discovered an anomalous elastic softening due to the vacancy in the crystalline silicon at low temperature by using the ultrasonic measurements. To investigate the quantum state of the silicon vacancy, we introduce a cluster model which includes both the Coulomb interaction between electrons in the dangling bond and the coupling between the electrons and Jahn–Teller phonons. We solve the model by using the numerical diagonalization method and find that the V 0 state with neutral charge shows a transition from S = 0 to S = 1 in the strong coupling regime. Remarkably, the V + state with S =1/2 becomes stable due to the trigonal phonon while unstable due to the tetragonal phonon. The obtained result with the trigonal phonon is consistent with the elastic softening in the B-doped silicon.

Effects of deposition pressure on the microstructural and optoelectrical properties of B-doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films grown by hot-wire chemical vapor deposition

We report on the effects of deposition pressure Pd on the growth and properties of the B-doped nanocrystalline silicon (nc-Si:H) thin films grown by hot-wire chemical vapor deposition (HWCVD) at very high hydrogen dilution of 98.8%. We found that the crystallinity of nc-Si:H or μc-Si:H films is not only determined by hydrogen dilution but also the concentration ratio of atomic H to SiH3 ([H]/[SiH3]) on the growing surface which is varied with deposition pressure Pd. Furthermore, there is a threshold of [H]/[SiH3] ratio which we name as overfull hydrogen (OH). When the [H]/[SiH3] ratio is lower than the threshold OH ([H]/[SiH3]<OH), the crystallinity of the nc-Si:H or μc-Si:H films increases with increasing [H]/[SiH3] ratio. But when the [H]/[SiH3] ratio is higher than the threshold OH ([H]/[SiH3]>OH), the crystallinity decreases with increasing [H]/[SiH3] ratio. Finally, the high conductivity of 4.22Scm−1 of the B-doped nc-Si:H thin film deposited at 15Pa is obtained.

Raman Scattering Studies of Electrically Active Impurities in in Situ B-Doped Silicon Nanowires: Effects of Annealing and Oxidation

The concentration of electrically active impurities in in situ boron (B)-doped silicon (Si) nanowires (SiNWs) synthesized by gold (Au)-catalyzed chemical vapor deposition (CVD) is studied by Raman spectroscopy. B-doped SiNWs exhibit an asymmetric Raman spectrum due to Fano resonance between discrete phonon Raman scattering and continuous electric Raman scattering caused by the excitation of holes in the valence band. To quantitatively evaluate the concentration of active B atoms from the asymmetric spectral shape, the spectra are fitted by a Fano resonance formula and asymmetry parameters are extracted. From the comparison of the asymmetry parameter with those obtained for reference samples, the concentration of active B atoms in SiNWs is estimated. The effects of thermal annealing in nitrogen and oxygen gases on the active B concentration are also studied. The annealing in nitrogen ambience for a short period significantly increases the concentration of active B atoms especially when the doping level is ...

First-principles study of heavily B-doped silicon

The energy and electronic properties of heavily B-doped silicon were investigated by density functional theory calculations. Our calculated electronic structures show that the impurity levels mix with valence band edge and the Fermi level locates in the valence band. It supports the metal–superconductor transition mechanism of heavily B-doped silicon, which is similar to that of diamond. A few differences from in diamond that the superconducting critical temperature T C is related to both B concentration and configurations, T C in silicon is only related to B concentration.

The influence of boron concentrations on structural properties in disorder silicon films

In this work we present a detailed structural of a series of B-doped hydrogenated microcrystalline silicon (μc-Si:H) films deposited by plasma-enhanced chemical vapor deposition (PECVD) and B-doped polycrystalline silicon (poly-Si) films produced by step-by-step laser crystallization process from amorphous silicon. The influence of doping on the structural properties and structural changes during the sequential crystallization processes were monitored by Raman spectroscopy. Unlike μc-Si:H films, that consist of a two-phase mixture of amorphous and ordered Si, partially crystallized sample shows a stratified structure with polycrystalline silicon layer at the top of an amorphous layer. With increasing doping concentration the LO-TO phonon line in poly-Si shift to smaller wave numbers and broadens asymmetrically. The results are discussed in terms of resonant interaction between optical phonons and direct intraband transitions known as a Fano resonance. In μc-Si:H films, on the other hand, the Fano effect is not observed. The increase of doping in μc-Si:H films suppressed the crystalline volume fraction, which leads to an amorphization in the film structure. The structural variation in both μc-Si:H and poly-Si films leads to a change in hydrogen bonding configuration.

Observation of Vacancy in High Purity Silicon Crystal Using Low-Temperature Ultrasonic Measurements

We have succeeded in direct observation of isolated vacancies in high purity silicon crystals grown by a floating zone (FZ) method using low-temperature ultrasonic measurements. The softening of elastic constants below 20 K down to 20 mK is observed in non-doped FZ silicon and B-doped FZ silicon. This softening is caused by an interaction of electric quadrupoles of triply degenerate vacancy orbital to elastic strains of ultrasonic waves. The low-temperature elastic softening measured by ultrasonic methods verifies existence of the isolated vacancies in Pv-region distributed in pure crystal of a Czockralski ingot. The ultrasonic measurement of the low- temperature softening is a faithful probe for vacancy evaluation in high purity silicon crystals in commercial base.

Direct observation of vacancy in silicon using sub-Kelvin ultrasonic measurements

We carried out sub-Kelvin ultrasonic measurements for observation of vacancies in crystalline silicon. The longitudinal elastic constants of non-doped and B-doped floating zone (FZ) silicon crystals in commercial base revealed low-temperature elastic softening below 20 K. The applied magnetic fields turns the softening of the B-doped FZ silicon to a temperature-independent behavior, while the fields up to 16 T at base temperature 20 mK make no effect on the softening of the non-doped FZ silicon. This result means that the vacancy accompanying the non-magnetic charge state V 0 in the non-doped silicon and the magnetic V + in the B-doped silicon is responsible for the low-temperature softening through the Jahn–Teller effect. The direct observation of the vacancy using the sub-Kelvin ultrasonic measurements advances point defects controlling in silicon wafers and semiconductor devices.