直流电化学腐蚀法制备多孔硅的表面形貌研究

采用正交实验，直流电化学腐蚀法制备多孔硅。用原子力显微镜对表面进行观察，研究电化学腐蚀参数对其表面形貌的影响。氢氟酸浓度（CHF）升高，使临界电流密度（JPS）增大，有利于多孔硅的形成。电流密度（J）增大，多孔硅的孔隙率和孔径随之变大，而其纳米粒径将变小。腐蚀时间（t）越长，孔径越大，孔越深。     

Porous silicon nanowires fabricated by electrochemical and laser-induced etching

Electrochemical and laser-induced etching processes were simultaneously used to synthesize the nanowires structure of porous silicon (PS). Surface morphology and structural properties of nanostructured silicon were characterized by using scanning electron microscopy (SEM) and atomic forces microscopy (AFM) images. Nanowires with dimensions of few nanometers were formed on the whole etched surface. The optical properties of silicon nanostructures were studied. Raman spectra were shifted and broadened relatively to 519.9 cm⁻¹ of PS prepared by electrochemical etching, and shifted to 517.2 cm⁻¹ for laser-induced etching process and to 508.9 cm⁻¹ for electrochemical and laser etching simultaneously. Blue shift luminescence was observed at 649.6 nm for PS produced by electrochemical etching, and at 629.5 nm for laser-induced etching. PS produced a blue shift at 626.5 nm using both etching procedures simultaneously. X-Ray diffraction (XRD) was used to investigate the crystallites size of the PS as well as to provide an estimate of the degree of crystallinty of the etched sample.     

Blue luminescence from porous layers produced by metal-assisted chemical etching on low-doped silicon

Photoluminescent porous layers were formed on highly resistive p-type silicon by a metal-assisted chemical etching method using K₂Cr₂O₇ as an oxidizing agent. A thin layer of Ag is deposited on the (1 0 0) Si surface prior to immersion in a solution of HF and K₂Cr₂O₇. The morphology of the porous silicon (PS) layer formed by this method as a function of etching time was investigated by scanning electron microscopy (SEM). It shows that the surface is formed by macropores filled with microporous silicon. The porous layers were characterized by backscattering spectrometry (BS) as a function of etching time in random and channelling mode. Channelling spectra show that the porous layer remains crystalline after etching. On the other hand, random and channelling spectra show that the deposited silver diffuses into the pore. Luminescence from metal-assisted chemically etched layers was measured. It was found that the PL intensity increases with increasing etching time. This behaviour is attributed to increase of the density of the silicon nanostructure. Finally, the PL spectra show two peaks of emission at 450 and 600 nm.     

Surface and optical characterization of the porous silicon textured surface

In the present studies, the structural and optical properties of the electrochemically etched PS layers are presented. The formation conditions under constant anodization current density was varied to get a variety of PS samples to analyze the structural and optical characteristics of the porous silicon layers and, then to correlate the resultant surface morphology with the etching process. The low-porosity PS layers thus formed on the silicon substrate have a refractive index value (n_ps = 1.9), which is an intermediate value between bulk silicon substrate (n_Si = 3.4) and air (n_air = 1.0). The results of diffused reflectance, surface morphology by atomic force microscopy (AFM), and Raman scattering measurements show that the resultant surface morphology of the PS layers consist of irregular and randomly distributed nanocrystalline Si structures. The reduction in reflection of the low porosity porous silicon layers is due to light scattering and light trapping of the incoming light by total randomization of the incoming light within the PS structure. The Fourier transform infrared (FTIR) measurements on the PS layer on Si substrate show that PS surface is characterized by chemical species like Si—H and Si—O etc., co-existing on the surface. The presence of hydrogen-related species on the PS layer can provide to some extent a surface passivation effect.     

Reactive-ion-etching (RIE) process in CF4 plasma as a method of fluorine implantation

We have investigated the concentration of fluorine in a newly formed film, which is located on the etched surface during modification of thermal silicon dioxide layer in reactive-ion-etching (RIE) system in CF₄ plasma. We try to find the correlation between parameters of the RIE process, depth and concentration of implanted fluorine ions, and finally, the thermal stability of fluorine ions incorporated into etched layer.During the RIE of silicon dioxide in fluorine plasma, on the etched surface a layer containing fluorine atoms is formed. This layer is very thin (about 1.5 nm) and has high concentration of fluorine ions. This concentration can be changed with r.f. power, CF₄ gas pressure and CF₄ flow. The suitable selection of etching parameters makes inspection of concentration and of the depth of fluorine ions incorporated into silicon possible, during the etching process. Unfortunately, ultra-shallow junction formed in this way does not show resistance to high temperature. So it is recommended only for low-budget technologies.     

Fluorine-doped SiO2 and CF low-k dielectrics obtained during RIE process in fluorine plasmas

We have investigated the effect of silicon dioxide reactive ion etching (RIE) parameters and the type of plasma on the concentration of fluorine and its chemical compounds, such as CF, SiF and SiOF, in the polymer layer that is formed during this process on the top of etched layer, and their thermal stability.The polymeric layer formed on the etched surface appeared to consist of fluorine and silicon fluoride (CF, SiOF and SiF). The thickness and chemical composition of polymer layer formed on the etched surface depends on the type of used fluorine plasma (CF₄ or CHF₃). Low-k layer formed during RIE in CHF₃ plasma consists of CF, SiOF and SiF species, whose intensity and thickness depend on the etching process parameters. For CF₄ plasma, polymer layer consists of SiOF and SiF species, whose intensity and thickness depend also on the etching process parameters. However, only for CHF₃ plasma it is possible to control the etching/deposition process dynamically by the adequate adjusting process parameters.In contrast to the CF/SiOF/SiF layer formed during RIE in CHF₃ plasma, the SiOF/SiF ultra-thin layer is not thermally stable and its thickness is too low for the intermetal dielectric (IMD) application.     

Characterization of nanoporous silicon layer to reduce the optical losses of crystalline silicon solar cells

Reduction of optical losses in crystalline silicon solar cells by surface modification is one of the most important issues of silicon photovoltaics. Porous Si layers on the front surface of textured Si substrates have been investigated with the aim of improving the optical losses of the solar cells, because an anti-reflection coating and a surface passivation can be obtained simultaneously in one process. We have demonstrated the feasibility of a very efficient porous Si AR layer, prepared by a simple, cost effective, electrochemical etching method. Silicon p-type CZ (100) oriented wafers were textured by anisotropic etching in sodium carbonate solution. Then, the porous Si layers were formed by electrochemical etching in HF solutions. After that, the properties of porous Si in terms of morphology, structure and reflectance are summarized. The structure of porous Si layers was investigated using SEM. The formation of a nanoporous Si layer on the textured silicon wafer result in a reflectance lower than 5% in the wavelength region from 500 to 900 nm. Such a surface modification allows improving the Si solar cell characteristics. An efficiency of 13.4% is achieved on a monocrystalline silicon solar cell using the electrochemical technique.     

多孔硅对单晶硅少子寿命影响状况的研究

以p型单晶硅片为研究对象,在单晶硅片表面采用化学腐蚀方法制备多孔硅层,通过实验选取制备多孔硅的最佳工艺条件,采用SEM观察多孔硅表面形貌,以及用微波光电导法测试少子寿命的变化情况。结果表明,在相同的腐蚀溶液配比条件下腐蚀11min得到的多孔硅层的表面形貌最好,孔隙率最大。在850℃下热处理150min时样品少子寿命的提高达到最大,不同腐蚀时间的样品少子寿命提高程度不同,腐蚀11min的样品少子寿命提高最大,约有10%左右。多孔层的形成伴随着弹性机械应力的出现,引起多孔层-硅基底界面处产生弹性变形,这有利于缺陷和金属杂质在界面处富集。另外,多孔硅仍具有晶体结构,但其表面方向上的晶格参数要比初始硅的晶格参数大,也有利于金属杂质向多孔层迁移。     

Preparation of Co-passivated porous silicon by stain etching

Co-passivated porous silicon (CPS) was prepared by stain etching. CPS samples prepared at different etching stages have different morphologies. All these morphologies are significantly different from those of conventional porous silicon (PS) etched in none cobalt-etching solution. The experimental results indicate that Co atoms only exist in a very thin layer on CPS surface, where Co atoms are well-distributed and Co atoms have hardly diffused into the substrate. Compared with the formation mechanism of conventional PS, there are two routes to generate holes in the formation of CPS, and there is NO₂ gas evolved from etching solution in a certain condition during the etching.     

Characterization of stain etched p-type silicon in aqueous HF solutions containing HNO3 or KMnO4

Stain etching of p-type silicon in hydrofluoric acid solutions containing nitric acid or potassium permanganate as an oxidizing agent has been examined. The effects of etching time, oxidizing agent and HF concentrations on the electrochemical behavior of etched silicon surfaces have been investigated by electrochemical impedance spectroscopy (EIS). An electrical equivalent circuit was used for fitting the impedance data. The morphology and the chemical composition of the etched Si surface were studied using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques, respectively. A porous silicon layer was formed on Si etched in HF solutions containing HNO₃, while etching in HF solutions containing KMnO₄ led to the formation of a porous layer and simultaneous deposition of K₂SiF₆ inside the pores. The thickness of K₂SiF₆ layer increases with increasing the KMnO₄ concentration and decreases as the concentration of HF increases.     

Ellipsometric study of the influence of chemical etching on thin porous silicon structures

The effect of chemical etching on Porous Silicon (PS) samples is studied and quantified by using variable angle spectroscopic ellipsometry (VASE). The main aim of this work is to assess the impact of such etching on the physical properties of electrochemically etched, thin PS antireflection coatings (ARC) for solar cell applications. In this study, detailed models of PS layers etched at constant current densities are created using a graded uniaxial Bruggeman Effective Medium Approximation (BEMA). Changes in porosity, thickness, and optical anisotropy of the PS samples due to chemical etching are determined as a function of etching time after PS formation. Three series of PS films, etched at three different current densities, are investigated. It is shown that significant changes in physical properties occur for chemical etching times longer than ~ 60 s. The anodic etching process for fabricating PS ARC structures can be performed in less than 10 s. Therefore, chemical etching does not lead to significant deviations from the intended PS structure and is not seen as a hindrance to accurate control of processes for fabricating thin PS ARCs.     

Effect of porous silicon stain etched on large area alkaline textured crystalline silicon solar cells

This work shows the effects of porous silicon stain etched on alkaline textured antireflection coatings of large area monocrystalline silicon solar cells. The texturization process has been produced by immersion of the silicon wafers in different carbonate-based solutions. The porous silicon layers were formed by stain etching in a HNO₃/HF aqueous solution before or after the texturization process. We study the effects of different alkaline and acidic solutions and the etching times on the solar cell parameters and the surface reflectance of the device. We have found that the average reflectance of the surface is lowered when the porous etching is combined with the texturization in the alkaline solution. However, the solar cell characteristics are not improved.     

Investigation of opto-electronic properties on gradient-porosity porous silicon layer

Electrochemical etching with step-gradient current was applied to form gradient-porosity porous silicon (PS) layer on n-Si substrate and Al/gradient-porosity. PS/n-Si structure was fabricated to extract its opto-electronic properties using reflectivity, spectral response, and scanning electron microscopy. A conventional single-layer PS etched with constant-current was also compared. Compared to the single-layer PS, the absorption wavelength of gradient-porosity PS is blue-shifted due to a smaller quantum size, hence a wider band-gap. Such a wider band-gap leads to a larger barrier-height in Al/gradient-porosity PS than that in Al/single-layer PS one. More dangling bonds are found on the surface of gradient-porosity PS owing to inhomogeneous etching, thus a poor electronic property, though it has a lower broadband antireflection than single-layer PS.     

高阻硅掩膜选择性生长多孔硅阵列

介绍一种在低阻P型硅衬底上用氢离子注入技术形成局部高阻硅掩膜,用电化学腐蚀选择性生长多孔硅微阵列的工艺流程。结果证明,用高阻硅掩膜选择性生长多孔硅具有很好的掩蔽效果,生成的多孔硅阵列的有序性和完整性良好。     

Metal-assisted electrochemical etching of silicon

In this paper the metal-assisted electrochemical etching of silicon is introduced. By electrochemical measurement and sequent simulation, it is revealed that the potential of the valence band maximum at the silicon/metal interface is more negative than that of the silicon/electrolyte interface. Accordingly, holes injected from the back contact are driven preferentially to the silicon/metal interface. Consequently, silicon below metal is electrochemically etched much faster than a naked silicon surface without metal coverage. Metals such as Ag and Cu have been utilized to catalyze the electrochemical etching. Feature sizes as small as 30 nm can be achieved by metal-assisted electrochemical etching. Meanwhile, the metal-assisted electrochemical etching method enables convenient control over the etching direction of non-(100) substrates, and facilitates the fabrication of orientation-modulated silicon nanostructures.     

Step voltage with periodic hold-up etching: A novel porous silicon formation

A novel etching method for preparing light-emitting porous silicon (PS) is developed. A gradient steps (staircase) voltage is applied and hold-up for different periods of time between p-type silicon wafers and a graphite electrode in HF based solutions periodically. The single applied staircase voltage (0–30 V) is ramped in equal steps of 0.5 V for 6 s, and hold at 30 V for 30 s at a current of 6 mA. The current during hold-up time (0 V) was less than 10 μA. The room temperature photoluminescence (PL) behavior of the PS samples as a function of etching parameters has been investigated. The intensity of PL peak is initially increased and blue shifted on increasing etching time, but decreased after prolonged time. These are correlated with the study of changes in surface morphology using atomic force microscope (AFM), porosity and electrical conductance measurements. The time of holding-up the applied voltage during the formation process is found to highly affect the PS properties. On increasing the holding-up time, the intensity of PL peak is increased and blue shifted. The contribution of holding-up the applied steps during the formation process of PS is seen to be more or less similar to the post chemical etching process. It is demonstrated that this method can yield a porous silicon layer with stronger photoluminescence intensity and blue shifted than the porous silicon layer prepared by DC etching.     

Fabrication and Characterization of Silicon (100) Membranes for a Multi-beam Superconducting Heterodyne Receiver

We fabricated silicon (100) membranes of 3 mm in diameter on the surface of silicon-on-insulator (SOI) substrates and investigated the characteristics of the membranes. The handle layer of one SOI substrate was etched using deep reactive ion etching process with the buried oxide (BOX) layer that remained together with the device layer. The BOX layer of the other SOI substrate was removed using C₄F₈-based plasma etching after the handle layer etching. The surfaces of both silicon (100) membranes were observed using the scanning white light interferometer system at room temperature. Both silicon (100) membranes have dome-like deformations. The silicon (100) membranes are effectively flattened by etching the BOX layer under the device layer. Both silicon (100) membranes were cooled from room temperature to 4 K by a Gifford–McMahon refrigerator. Wrinkles appeared on the surfaces of both silicon (100) membranes when the temperature dropped to about 200 K. However, the wrinkles disappeared below about 180 K. This phenomenon indicates the wrinkles at low temperature would depend on the properties of the silicon (100) of the device layers and independent of the properties of the BOX layers under the silicon (100) membranes.     

Thermal annealing behaviour of Si/SiO2 structures

The interaction of vacuum-deposited silicon films with the underlying oxide at annealing temperatures of 1050–1250°C was studied by means of low energy electron diffraction, Auger spectroscopy, IR spectroscopy, and optical and electron microscopy. The deposition of silicon onto SiO₂-covered silicon substrates and the annealing of the samples was carried out in ultrahigh vacuum. The results obtained revealed that the silicon and oxide layers are etched away by heat treatment. The dependence of the etching on the thickness of the silicon film, the thickness of the SiO₂ layer and the annealing temperature was determined. A qualitative model of the etching process is proposed.     

Absorption enhancement of near infrared in Te doped nanoporous silicon

In this paper, the optical properties of Te doped nanoporous silicon have been studied. The nanoporous silicon was fabricated by using alkaline etching and electrochemical anodization. The etched nanoporous silicon was injected with Te atoms by ion implantation. These nanostructures formed in electrochemical anodization directly affect the optical properties of nanoporous silicon such as reflectance, transmittance and absorptance. According to the optical measurement, the absorptance of the Te doped nanoporous silicon is over 80 % in the wavelength range from 250 to 1,100 nm. The absorptance of Te doped nanoporous silicon at wavelength longer than 1,100 nm is almost four times of that of untreated silicon, indicating that the ion implantation of Te element increases the NIR absorption of nanoporous silicon considerably.     

Effect of nanoporous silicon coating on silicon solar cell performance

The surface modification of silicon solar cells was used for improvement of photovoltaic characteristics of silicon solar cells. A screen-printed solar cell technology is used to fabricate n⁺-p silicon solar cell. Nanoporous silicon (PS) layer on n⁺-type Si wafers or on the frontal surface of (n⁺-p)Si solar cell was formed by electrochemical etching in HF-containing solution. The surface morphology, porosity, spectra of photoluminescence and reflectance of PS layers were analyzed. The photovoltaic characteristics of two silicon solar cell type with and without PS layer (PS/(n⁺-p)Si and (n⁺-p)Si cell) were measured and compared. The spectra of photosensitivity of cells were measured in the wavelength range of 300-1100 nm. An average reflection of the porous silicon layer, fabricated on a polished silicon surface, is decreased to 4%. A remarkable increment of the conversion efficiency by 20% have been achieved for PS/(n⁺-p)Si solar cell comparing to (n⁺-p)Si solar cell without PS layer. The results, related with improving of the performance of PS/(n⁺-p)Si solar cell, have been attributed to the effective antireflection and the wide-gap window role of nanoporous silicon on the silicon solar cell.