Porous silicon and aluminum co-gettering experiment in p-type multicrystalline silicon substrate

The lifetimes of non-equilibrium minority carriers, which bound with the diffusion length, are considered as two important parameters of the low-quality multicrystalline silicon (mc-Si) substrate. Its value defines the quality of the initial substrate. It is also subjected to change as a result of many high-temperature operations during the device fabrication. Therefore, it is necessary to incorporate certain processing steps that either improve or preserve the electronic quality of the mc-Si substrate. In this study, a novel porous silicon and aluminum co-gettering experiment has been applied as a beneficial approach to improve the electronic quality of the low-resistivity mc-Si substrates. Porous silicon layers were prepared by anodization of the n⁺ silicon region by a simple electrochemical etching process using an aqueous HF-based electrolyte, which leads to the creation of porous silicon microcavities. Besides making porous silicon and aluminum co-gettered samples, both phosphorous and aluminum alloy-gettered samples and reference samples were made. The gettering-induced lifetime enhancement in the test samples was monitored by measuring the lifetime/diffusion length of the test samples using two independent methods such as photoconductivity decay (PCD) measurement and the photocurrent generation method (PCM), respectively. The result in both the measurements has shown a reasonably good agreement with each other. Therefore, it is inferred that the applied co-gettering experiment has a synergetic effect to improve the lifetime of the mc-Si substrate.     

Properties of plasma enhanced chemical vapor deposited silicon nitride for the application in multicrystalline silicon solar cells

Hydrogenated films of silicon nitride (SiNx:H) were investigated by varying the deposition condition in plasma enhanced chemical vapor deposition (PECVD) reactor and annealing condition in infrared (IR) heated belt furnace to find the optimized condition for the application in multicrystalline silicon solar cells. By varying the gas ratio (ammonia to silane), the silicon nitride films of refractive indices 1.85-2.45 were obtained. Despite the poor deposition rate, silicon wafer with the film deposited at 450 °C showed the best minority carrier lifetime. The film deposited with the gases ratio of 0.57 showed the best peak of carrier lifetime at the annealing temperature of 800 °C. The performance parameters of cells fabricated by varying co-firing peak temperature also showed the best values at 800 °C. The multicrystalline silicon (mc-Si) solar cells fabricated in conventional industrial production line applying the optimized film deposition and annealing conditions on large area substrate (125 mm × 125 mm) was found to have the conversion efficiency of 15%.     

Purifying multicrystalline silicon by directional solidification method with induced electromagnetic field

Abstract

As an improved directional solidification (DS) method, the complex directional solidification (CDS) method is used for purifying and preparing multicrystalline silicon ingot in this experiment. The induced electromagnetic field is imposed to control refining and solidification process. An integral silicon ingot with the diameter of 130 mm, the length of 130 mm and the weight of 4 kg is successfully fabricated in a self-designed CDS furnace. Metallographic analyses reveal that the direction of the most grains is parallel to the axial of silicon ingot. Analyses proved that the distribution of impurities in the cross-section is more homogeneously, the distribution in axial is improved and the effective length of silicon ingot is increased. Theoretical calculations indicate that the effect of solidified rate on the removal of impurities is limited and the impurities can be removed effectively after more than two times directional solidification process.     

Efficiency improvement of multicrystalline silicon solar cells after surface and grain boundaries passivation using vanadium oxide

The aim of this work is to investigate the effect of vanadium oxide deposition onto the front surface of multicrystalline silicon (mc-Si) substrat, without any additional cost in the fabrication process and leading to an efficient surface and grain boundaries (GBs) passivation that have not been reported before. The lowest reflectance of mc-Si coated with vanadium oxide film of 9% was achieved by annealing the deposited film at 600 °C. Vanadium pentoxide (V₂O₅) were thermally evaporated onto the surface of mc-Si substrates, followed by a short annealing duration at a temperature ranging between 600 °C and 800 °C, under O₂ atmosphere. The chemical composition of the films was analyzed by means of Fourier transform infrared spectroscopy (FTIR). Surface and cross-section morphology were determined by atomic force microscope (AFM) and a scanning electron microscope (SEM), respectively. The deposited vanadium oxide thin films make the possibility of combining in one processing step an antireflection coating deposition along with efficient surface state passivation, as compared to a reference wafer. Silicon solar cells based on untreated and treated mc-Si wafers were achieved. We showed that mc-silicon solar cells, subjected to the above treatment, have better short circuit currents and open-circuit voltages than those made from untreated wafers. Thus, the efficiency of obtained solar cells has been improved.     

Visible luminescence in photo-electrochemically etched p-type porous silicon: Effect of illumination wavelength

The effect of low power density of ~ 5 μW/cm² monochromatic light of different wavelengths on the visible photoluminescence (PL) properties of photo-electrochemically formed p-type porous silicon (PS) has been investigated. Two-peak PL “red” and “green” is resolved in PS samples etched under blue-green wavelength illumination; 480, 533 and 580 nm. It is found that the weight of “green” PL has maxima for the sample illuminated with 533 nm wavelength. Whereas, PL spectra of PS prepared under the influence of red illumination or in dark does not exhibit “green” PL band, but shows considerable enhancement in the “red” PL peak intensity. Fourier transform infrared (FTIR) spectroscopic analysis reveals the relationship between the structures of chemical bonding in PS and the observed PL behavior. In particular, the PL efficiency is highly affected by the alteration of the relative content of hydride, oxide and hydroxyl species. Moreover, relative content of hydroxyl group with respect to oxide bonding is seen to have strong relationship to the blue PL. Although, the estimated energy gap value of PS samples shows a considerable enlargement with respect to that of bulk c-Si, the FTIR, low temperature PL and Raman measurements and analysis have inconsistency with quantum confinement of PS.     

Study on volatilization rate of silicon in multicrystalline silicon preparation from metallurgical grade silicon

The quantity of silicon lost during evaporation is greater than theoretical expectation during the purification of metallurgical grade silicon by vacuum evaporation. In this paper, silicon volatilization rates were measured for evaporation times of 30, 45 and 60 min at 1723, 1773 and 1823 K, respectively. Results indicate that volatilization rates determined in our experiments are one or two orders of magnitude greater than those from theoretical calculation. The equation for theoretical calculation was revised (ω = (2.23-6.30)× 10⁻¹ p_pa(M/T)^1/2) using silicon evaporation coefficient of 8.5-24. The details of the experimental set-up were found to be important and the mass of silicon evaporated in particular was found to be related to the water-cooling system. The carbon/graphite inserts also and the presence of trace amounts of oxygen in the vacuum furnace could reduce the content of silicon in gaseous phase and support the evaporation of silicon. It was found under certain conditions that there are two principal stages involved: 1) Formation of vapor-liquid equilibrium, 2) Maintenance of the established vapor-liquid equilibrium during the silicon evaporation process. It was found that silicon process losses can be reduced by shortening the time of the first stage.     

A technique for the fabrication of various multiparametric porous silicon samples on a single substrate

Porous silicon (PSi) samples generally have a uniform thickness and pore size according to specific anodization conditions, as the Si wafer is entirely immersed into hydrofluoric (HF) acid during the anodization process. In contrast, multiparametric (MP) PSi, as described in the present work, is fabricated by inserting a Si wafer gradually (or by stages) into a HF solution during the anodization process. Consequently, MP-PSi allows single layer fabrication with a pore-size and layer thickness gradient or various multilayers, on a single substrate. Therefore, MP-PSi can be readily used in sensor application areas to determine optimized detection conditions for various materials, such as gas, liquid, and bio-materials. MP-PSi layer with a lateral pore gradient distribution can also be used as size-exclusion matrix. In addition, the MP-PSi multilayer array is expected to open up application areas involving optical electronic nose systems.     

Some new results in porous silicon

Efforts have been made to see the effect of some standard microelectronic processing steps on porous silicon. Our diffusion experiments for making p-n junctions confirm that this material can withstand high temperatures of the order of 800°C to 1000°C. A new technique for photolithography has been suggested to obtain porous silicon in selected areas. Etch stop method to control the thickness of the porous layer and an organic protective layer for porous silicon have also been suggested. Models proposed by other workers to explain luminescence in porous silicon are not sufficient to explain many experimental observations. A hybrid model is suggested.     

多孔硅衬底在RFIC无源器件中的应用

介绍低阻硅基多孔硅作为低损耗衬底在射频集成电路（RFIC）无源器件上的应用，分别讨论了在RFIC中的传输线，移相器和电感等无源器件运用多孔硅衬底后性能和主要参数的改良变化，并从理论上分析变化的原因。多孔硅衬底大大降低了RFIC电路中传输线的损耗，使移相器的工作频率和相移量都发生变化。运用多孔硅衬底，回旋电感的最高Q增加了63％（16．8～27．4），最高响应频率增加了28％（15．2～19．4GHz）。结果表明多孔硅衬底在高频情况下完全能满足RFIC元件的要求。     

Raman scattering and photoluminescence study of porous silicon formed on n-type silicon

We report Raman scattering and photoluminescence studies on porous silicon film formed on n-type silicon. The Raman spectra over the sample surface exhibit considerable variation whereas the photoluminescence spectra are practically identical. Our results indicate that, well inside the film surface, it consists of spherical nanocrystals of typical diameter ≈ 100Å, while on the edge these nanocrystals are ? 300Å. We further observe that there is no correlation between the photoluminescence peak position and the nanocrystal diameter. This suggests that the origin of the photoluminescence is due to radiative recombination between defect states in the bulk as well as on the surface of the nanocrystal.     

Hydrogen in porous silicon — A review

This review is devoted to summarising the hydrogen-assisted properties and applications of porous silicon (PS). The role of hydrogen as an intermediate product in silicon porosification technology is accentuated. The regularities of hydrogen bonding in PS and its applications for hydrogen storage are listed. The models of hydrogen influence on luminescence and electrical properties of PS are analysed. The corresponding applications of PS for H₂ gas sensors and pH metres are illustrated. Hydrogen-assisted explosion and grafting of PS are discussed. Such a review can be useful for the tailoring of PS properties.     

Magneto-transport in porous silicon

Magnetoresistance (MR) measurements have been performed in the temperature range 100–300 K on macroporous porous silicon (P.S.) samples. P.S. layers have been prepared by the anodic dissolution of Si in HF acid. The MR has been found to be positive in P.S. for the temperature range 100–300 K and for the entire range of magnetic field (0–5 kG). However the magnitude of the positive MR is found to be much less than expected on the basis of free electron conduction. Also the value of n in the relation Δρ/ρ_o∞Bⁿ for the temperature range 100–300 K is found to be < 2, implying that there is a contribution of some phenomenon other than the free electron conduction to MR. The measured data suggest that it is the contribution of localized state conduction near the Fermi level and in the localized states near the band edges.     

Experimental study on hot melt splashes during electromagnetic continuous casting silicon

Hot melt splashes often take place during electromagnetic casting silicon, which break the stable pulling process and endanger the safety of equipments in the vacuum chamber. Investigations are carried out to analyze the causes of melt splashes and put forward measures to prevent the happening of them. Pores with different sizes are observed in perpendicular sections of the left ingots, their surface compositions are analyzed by EDS. The results indicate that they are rich in O, Al, Ca and Mg elements, which means that the driving pressures of hot melt splashes are mainly the vapor tension of SiO, Al, Ca and Mg. Calculated results show that the volatilization of those elements is inevitable, so the vapor excluding and pressure releasing are key issues for preventing hot melt splashes, both of which are affected by the height and top position of the liquid dome. Therefore, controlling of those parameters properly can efficiently prevent the hot melt splashes. The optimal dome top position is from 6 mm to 16 mm lower than the coil top position.     

Optical sensing of flammable substances using porous silicon microcavities

A porous silicon multilayer, constituted by a Fabry–Pèrot cavity between two distributed Bragg reflectors, is exposed to vapor of several organic species. Different resonant peak shifts in the reflectivity spectra, ascribed to capillary condensation of the vapor in the silicon pores, have been observed. Starting from experimental data, the layer liquid volume fractions condensed in the sensing stack have been numerically estimated. Values ranging between 0.27 (for ethanol) and 0.33 (for iso-propanol) have been found. Time-resolved measurements show that the solvent identification occurs in less then 10 s.     

多孔硅的微结构及其冷阴极的电子发射特性研究

采用电化学腐蚀法在n型单晶Si基底上制备多孔硅(PS)膜,利用场发射扫描电子显微镜(FE-SEM)分析其微结构特征,研究了硅基底材料、腐蚀电流密度和腐蚀时间等制备条件和工艺参数对PS微结构的影响。实验结果表明,轻掺杂n型Si基底上形成的PS膜(n--PS)中的孔较深且孔径较大,而重掺杂n型Si基底上的PS膜(n+-PS)中的孔分布较为密集,而且呈多分枝结构;腐蚀电流密度相同时,PS的膜厚和腐蚀时间成正比,而在较高的腐蚀电流密度下制备得到的PS膜在结构上更为疏松。制作了基于n+型硅基底的PS阴极,此阴极的阈值电压约为14 V,高于该电压后发射电流随着二极管电压提高而呈指数性增加趋势,而且发射电流对环境气压不敏感,即使在0.1 Pa的低真空中也没有明显的衰减。     

Hydrogen isotopic substitution experiments in nanostructured porous silicon

Nanostructured porous silicon is usually prepared by electrochemical anodization of monocrystalline silicon using a fluorine-rich electrolyte. As a result of this process, the silicon atoms conserve their original crystalline location, and many of the dangling bonds appearing on the surface of the nanostructure are saturated by hydrogen coming from the electrolyte. This work presents an IR study of the effects produced by partial substitution of water in the electrolytic solution by deuterium oxide. The isotopic effects on the IR spectra are analyzed for the as-prepared samples and for the samples subjected to partial thermal effusion of hydrogen and deuterium. We demonstrate that, although deuterium is chemically indistinguishable from hydrogen, it presents a singular behaviour when used in porous silicon preparation. We found that deuterium preferentially bonds forming Si-DH groups. A possible explanation of the phenomenon is presented, based on the different diffusivities of hydrogen and deuterium.     

Nanostructure, electrical and optical properties of p-type hydrogenated nanocrystalline silicon films

In this paper, p-type hydrogenated nanocrystalline (nc-Si:H) films were prepared on corning 7059 glass by plasma-enhanced chemical vapor deposition (PECVD) system. The films were deposited with radio frequency (RF) (13.56 MHz) power and direct current (DC) biases stimulation conditions. Borane (B₂H₆) was a doping agent, and the flow ratio η of B₂H₆ component to silane (SiH₄) was varied in the experimental. Films’ surface morphology was investigated with atomic force microscopy (AFM); Raman spectroscopy, X-ray diffraction (XRD) was performed to study the crystalline volume fraction X_c and crystalline size d in films. The electrical and optical properties were gained by Keithly 617 programmable electrometer and ultraviolet-visible (UV-vis) transmission spectra, respectively. It was found that: there are on the film surface many faulty grains, which formed spike-like clusters; increasing the flow ratio η, crystalline volume fraction X_c decreased from 40.4% to 32.0% and crystalline size d decreased from 4.7 to 2.7 nm; the optical band gap E_g^opt increased from 2.16 to 2.4 eV. The electrical properties of p-type nc-Si:H films are affected by annealing treatment and the reaction pressure.     

基于p型多孔硅的一维光子晶体的微结构分析

经由阳极氧化实现了p型硅基的60周期(120层)反射结构,并在此基础上成功制作了基于一维光子晶体的全反射镜,其中心反射波长位于1.6μm左右,反射率几乎为100%的禁带带宽约为0.25μm.