Boron-doped Crystalline Silicon Research Articles

Near-infrared and mid-infrared light emission of boron-doped crystalline silicon.

The bottleneck in achieving fully integrated silicon photonics lies in silicon-based light-emitting devices that are compatible with standard CMOS technology. Dislocation loops created by implanting boron into silicon and annealing represent an enticing strategy to transform highly inefficient silicon into a luminescent material. However, the emission at telecommunication wavelength suffers from the strong thermal quenching effect, resulting in low efficiency at room temperature. Here, we applied a new deep cooling process to address this issue. Interestingly, we find that electrons and holes recombine through defects emitting two photons, one in near infrared (NIR, 1.3∼1.6 µm) and the other in mid-infrared band (MIR, around 3.5 µm). The photoluminescence intensity at NIR increases three fold when the temperature increases from 77 K to 300 K. Furthermore, the NIR light emission of reverse biased silicon diodes was significantly enhanced compared to forward bias, emitting the maximum output power of 42 nW at 60 mA. The results offer new opportunities for the development of infrared light sources in integrated circuits.

22.8% efficient ion implanted PERC solar cell with a roadmap to achieve 23.5% efficiency: A process and device simulation study

Among all available photovoltaic (PV) technology options, c-Si based passivated emitter rear cell (PERC) has already proved its mettle by delivering more than 22% conversion efficiency at the industrial level. However, designing of PERC solar cell device remains a challenge owing to recombination and absorption losses, which eventually restrict the performance of the device toward achieving an Auger efficiency limit of 29.4%. The primary contributor to these losses is emitter recombination losses that limits the performance of PERC devices. The n+-emitter region in the proposed PERC devices is formed using ion implantation followed by a diffusion process. The impact of process variables such as dose (cm−2), energy (keV), diffusion time (s), and diffusion temperature (oC) on the PERC device performance is not much explored. Therefore, in an attempt to fulfill this research gap, the emitter region performance in terms of ion implantation of phosphorus, influence of local back surface field (BSF) doping concentration (1017 to 1019 cm−3), and back surface recombination velocity (SRV 101–107 cm/s) on PERC device performance followed by the loss analysis is examined in this work. An upright pyramid textured structure with constant height (5 μm) and industrial standard stacked dielectric anti-reflective coating layers, SiNX/SiO2 and Al2O3/SiNX at front and back side respectively is realized using different process statements such as deposit, etch, implantation, and diffusion with the help of a Silvaco TCAD software. Investigations reveal that the PERC device can deliver an efficiency of 22.8% with 150 μm boron-doped crystalline silicon (c-Si) wafer at an optimum phosphorous dose (5 × 1015 cm−2), BSF doping (1019 cm−3), and SRVn (107 cm/s). Short-circuit current density (JSC) of 40.8 mA/cm2, open-circuit voltage (VOC) of 0.686 V, and fill factor (FF) of 81.54% are obtained for the structure under consideration. A roadmap to achieve 23.5% conversion efficiency has also been reported by assuming a carrier selective contact at the backside with a surface recombination velocity of 10 cm/s.

Effect of Phosphorus Doping on Photoinduced Thermal Processes in Silicon Nanowires

We report on the effect of phosphorus doping of silicon nanowires (SiNWs) on the photoinduced heating processes. SiNWs samples were prepared by metal-assisted chemical etching of low boron-doped crystalline silicon (c-Si) wafers followed with thermo-diffusional doping with phosphorous (P) up to 1020 cm-3. We establish that the P-doping (n-type) results in effective heat conduction along SiNWs toward the c-Si substrate during laser heating. Partial phase transition in P-doped SiNWs under intense photoheating was detected by means of Raman spectroscopy and photoluminescence. The observed doping effects were explained by a contribution of charge carriers (electrons) to the heat distribution along SiNWs and partial screening of the crystal lattice potential. The obtained results can be useful for the development of new photonic and optoelectronic devices based on SiNWs.

Cavitation Induced by Janus-Like Mesoporous Silicon Nanoparticles Enhances Ultrasound Hyperthermia.

The presence of nanoparticles lowers the levels of ultrasound (US) intensity needed to achieve the therapeutic effect and improves the contrast between healthy and pathological tissues. Here, we evaluate the role of two main mechanisms that contribute to the US-induced heating of aqueous suspensions of biodegradable nanoparticles (NPs) of mesoporous silicon prepared by electrochemical etching of heavily boron-doped crystalline silicon wafers in a hydrofluoric acid solution. The first mechanism is associated with an increase of the attenuation of US in the presence of NPs due to additional scattering and viscous dissipation, which was numerically simulated and compared to the experimental data. The second mechanism is caused by acoustic cavitation leading to intense bubble collapse and energy release in the vicinity of NPs. This effect is found to be pronounced for as-called Janus NPs produced via a nano-stopper technique, which allow us to prepare mesoporous NPs with hydrophobic inner pore walls and hydrophilic external surface. Such Janus-like NPs trap air inside the pores when dispersed in water. The precise measurement of the heating dynamics in situ enabled us to detect the excessive heat production by Janus-like NPs over their completely hydrophilic counterparts. The excessive heat is attributed to the high intensity cavitation in the suspension of Janus-like NPs. The present work elicits the potential of specifically designed Janus-like mesoporous silicon NPs in the field of nanotheranostics based on ultrasound radiation.

Easy-to-apply methodology to measure the hydrogen concentration in boron-doped crystalline silicon

Hydrogen is a highly relevant impurity in crystalline silicon associated with a variety of effects particularly important in solar cell technology such as defect and surface passivation. It has been the subject of countless studies. Much of this however relies to a certain degree on speculation due to limitations in hydrogen measurement capability, making a direct quantitative correlation of these phenomena with the hydrogen content largely impossible so far. In this contribution, we apply recent advances in the understanding of hydrogen introduction and behaviour in silicon - in particular the conversion of quenched in dimeric hydrogen to boron hydrogen pairs (BH) - and introduce an easy-to-apply methodology to determine quantitatively the hydrogen concentration in bulk boron-doped silicon. The technique involves the measurement and analysis of changes to the bulk resistivity, as recorded by contactless eddy-current measurements, due to the formation of BH pairs during annealing at temperatures between 140 and 180 °C. To demonstrate the method, we apply it to boron-doped float-zone silicon wafers with SiNx:H coatings of different compositions, introducing different total hydrogen concentrations between 5 × 1014 and 1.3 × 1015 cm−3 into the silicon bulk during a fast-firing step. Without firing, the hydrogen content is below the detection limit of 3 × 1014 cm−3 for the 1 Ω cm test sample used.

An industrial solution to light-induced degradation of crystalline silicon solar cells

Boron-oxygen defects can cause serious light-induced degradation (LID) of commercial solar cells based on the boron-doped crystalline silicon (c-Si), which are formed under the injection of excess carriers induced either by illumination or applying forward bias. In this contribution, we have demonstrated that the passivation process of boron-oxygen defects can be induced by applying forward bias for a large quantity of solar cells, which is much more economic than light illumination. We have used this strategy to trigger the passivation process of batches of aluminum back surface field (Al-BSF) solar cells and passivated emitter and rear contact (PERC) solar cells. Both kinds of the treated solar cells show high stability in efficiency and suffer from very little LID under further illumination at room temperature. This technology is of significance for the suppression of LID of c-Si solar cells for the industrial manufacture.

Raman diagnostics of photoinduced heating of silicon nanowires prepared by metal-assisted chemical etching

Raman spectroscopy was applied to investigate laser-induced heating of silicon nanowires (SiNWs) formed by metal-assisted chemical etching of lightly boron-doped crystalline silicon (c-Si) wafers. Low-frequency shift of the Raman peak from 520.5 cm−1 to about 517 cm−1 for SiNWs with length of 40–65 µm under laser irradiation with wavelengths of 632.8 or 488 nm was observed, and it was explained by an increase in the average temperature of SiNWs on about 150 K for the laser intensity about 1 kW/cm2. The same photoinduced heating was confirmed by analyzing the ratio between the Stokes and anti-Stokes components of the Raman scattering. The obtained experimental data allow us to estimate the thermal conductivity coefficient of SiNW array ~0.1 W/(m K), which is three orders of magnitude smaller than that of c-Si. Furthermore, the Raman spectra of SiNWs under excitation with intensity above 0.2–0.5 kW/cm2 consisted of an additional low-frequency peak, which is related to an overheated subsystem of well spatially separated fine SiNWs up to 600–700 K. The observed strong photoinduced heating can be used for local laser-induced treatment of SiNWs and biomedical applications.

Kinetics of the boron-oxygen related defect in theory and experiment

The formation of boron-oxygen complexes in boron-doped crystalline silicon can lead to a severe reduction in the minority charge carrier lifetime. This strongly influences, e.g., solar cell efficiencies if the material is used for photovoltaic application. Recent investigations have shown that a recovery of the carrier lifetime can be achieved by a subsequent thermally enhanced reaction induced by charge carriers. A model of the reaction dynamics of the boron-oxygen complex by means of rate equations is presented in this paper. Following a mathematical description of the reactions involved, the consequences based on the calculations are presented and allow a prediction of the observable electrical parameters. The fundamental agreement with measured data is proven experimentally for different phenomena.

Imaging interstitial iron concentrations in boron-doped crystalline silicon using photoluminescence

Imaging the band-to-band photoluminescence of silicon wafers is known to provide rapid and high-resolution images of the carrier lifetime. Here, we show that such photoluminescence images, taken before and after dissociation of iron-boron pairs, allow an accurate image of the interstitial iron concentration across a boron-doped p-type silicon wafer to be generated. Such iron images can be obtained more rapidly than with existing point-by-point iron mapping techniques. However, because the technique is best used at moderate illumination intensities, it is important to adopt a generalized analysis that takes account of different injection levels across a wafer. The technique has been verified via measurement of a deliberately contaminated single-crystal silicon wafer with a range of known iron concentrations. It has also been applied to directionally solidified ingot-grown multicrystalline silicon wafers made for solar cell production, which contain a detectible amount of unwanted iron. The iron images on these wafers reveal internal gettering of iron to grain boundaries and dislocated regions during ingot growth.

A simulation model for the density of states and for incomplete ionization in crystalline silicon. II. Investigation of Si:As and Si:B and usage in device simulation

Building on Part I of this paper [Altermatt et al., J. Appl. Phys. 100, 113714 (2006)], the parametrization of the density of states and of incomplete ionization (ii) is extended to arsenic- and boron-doped crystalline silicon. The amount of ii is significantly larger in Si:As than in Si:P. Boron and phosphorus cause a similar amount of ii although the boron energy level has a distinctly different behavior as a function of dopant density than the phosphorus level. This is so because the boron ground state is fourfold degenerate, while the phosphorus ground state is twofold degenerate. Finally, equations of ii are derived that are suitable for implementation in device simulators. Simulations demonstrate that ii increases the current gain of bipolar transistors by up to 25% and that it decreases the open-circuit voltage of thin-film solar cells by up to 10mV. The simulation model therefore improves the predictive capabilities of device modeling of p-n-junction devices.

Study of pulsed laser-deposited phosphorus-doped carbon/p-silicon photovoltaic cell

A heterostructure is fabricated by depositing phosphorus (P)-doped camphoric (C 10H 16O) carbon thin film (n-type) on boron-doped crystalline silicon (p-type) substrate by pulsed laser deposition (PLD) technique. In order to dope, different amount of P was incorporated with the camphoric soot target and successful doping of P in carbon thin film is realized. Temperature-dependent conductivity data, transmittance-reflectance measurements in the UV–VIS–IR region and current density–voltage ( J–V) characteristics are analyzed to obtain different device parameters. With the obtained device parameters, C–V characteristics of the device is modeled. The analyses reveal P diffusion from the carbon layer into the Si region during the film deposition and thus the formation of a P–I–N device rather than a simple P–N junction device.

Time Evolution of the Resistance of Arsenic, Phosphorous, and Boron-Doped Crystalline Silicon

We activated 200 nm thick arsenic, phosphorous, and boron-doped crystalline silicon (c-Si) at 1100°C and examined the time dependence of the resistance of the layers with respect to subsequent processing steps at lower temperatures. The resistance of the doped c-Si layer does not change over time for a dose of For the higher dose of however, the resistance increases with time and finally saturates. The onset time for this resistance increase is independent of temperature in the high-temperature regime, while it depends on temperature in the low-temperature regime. Our proposed model explains the overall resistance increase of c-Si layers, doped with different impurities, by changing the corresponding parameters in function of the doping impurity. © 2002 The Electrochemical Society. All rights reserved.

Hydrogen molecules in boron-doped crystalline silicon

Boron-doped, float zone silicon has been hydrogenated at C and then quenched to room temperature. Infrared absorption measurements of the samples in their as-quenched state and following anneals at C reveal the vibrational mode from H-B pairs, together with the line that we have assigned to the vibrational mode of isolated hydrogen molecules . Annealing leads to irreversible increases in the concentrations of H-B pairs and decreases in the concentration of centres. The results imply that molecules diffuse to boron acceptors and that there is subsequent dissociation of these molecules with the formation of H-B pairs. The measurements confirm an earlier proposal that `hidden hydrogen' present in such samples is in the form of isolated molecules.

Precipitation of Boron in Highly Boron-Doped Silicon

The clustering of boron in highly boron-doped silicon and its influence on electrical deactivation are reported. Highly boron-doped crystalline silicon was fabricated as a starting material by solid phase epitaxy of boron-doped amorphous silicon films. Boron can be supersaturated in the crystallized samples annealed at a low temperature of about 600°C. A lot of precipitates, containing clustered boron, were observed in the samples annealed at high temperatures of about 1000°C. The chemical states and the atomic configuration of boron in samples annealed at various temperatures corresponded to the electrical deactivation of boron.

Thermally Assisted Tunneling of the B-H Complex in Silicon

Jump rate constants of atomic hydrogen and deuterium in boron-doped crystalline silicon are calculated using quantum transition-state theory, based on the path-integral centroid formalism. A break in the slope of the Arrhenius plot for the jump rate of hydrogen is obtained at $T\ensuremath{\sim}60$ K, indicating a crossover from thermally activated quasiclassical motion over a barrier to thermally assisted quantum tunneling, in good agreement with previous experimental results. For deuterium, no deviation from an Arrhenius law is found down to 30 K. It is shown that the defect complex undergoing quantum tunneling consists of hydrogen, boron, and the nearest silicon atoms.

Characterization of stain-etched porous silicon.

We have performed infrared absorption, continuous-wave photoluminescence, and nanosecond photoluminescence decay measurements to characterize stain-etched porous silicon. Stain-etched porous silicon samples were prepared by etching boron-doped crystalline silicon wafers in a solution of HF:${\mathrm{HNO}}_{3}$:${\mathrm{H}}_{2}$O with ratios of 1:5:10 by volume. The dependence of continuous-wave photoluminescence intensity upon temperature and excitation energy has been studied. We have also investigated the dependence of nanosecond photoluminescence decay upon temperature, excitation energy, and emission energy. Our results suggest that the photoluminescence in stain-etched porous silicon might be due to the presence of siloxene-type bonding configuration in an amorphous alloy of silicon, oxygen, and hydrogen.

Stress-induced hydrogen motion in Boron-doped crystalline silicon

Raman scattering is used to analyse the H-vibrations in passivated B-doped crystalline silicon under uniaxial stress. The observed line-splittings reveal a non-trigonal symmetry for H-B complexes, corresponding to off-bond-centered interstitial sites for hydrogen. The stress dependence observed for the line intensities indicates a stress-induced hydrogen redistribution among non-equivalent positions around a B-impurity.

Lattice relaxation due to hydrogen passivation in boron-doped silicon

The influence of hydrogen on the lattice constant of boron-doped crystalline silicon is studied by x-ray diffraction. It is found that hydrogen passivation of the boron acceptors is accompanied by a noticeable expansion of the passivated layer. A value of +(2.5±0.5)×10−24 cm3/atom is obtained for the expansion coefficient due to B-H complexes.

Impurity induced vibrations in light doped silicon

The local densities of states (LDOSs) are calculated by use of the recursion method for the lithium or phosphorous-compensated boron-doped crystal-line silicon. The impurity induced local and quasi-local vibrational modes are studied. Good agreement between experiments and calculation is achieved.