劉　琳　李志勝　胡慧東　宋維力(華北電力大學可再生能源學院，新能源電力系統(tǒng)國家重點實驗室，北京006；北京師范大學，能量轉(zhuǎn)換與存儲材料北京市重點實驗室，北京00875；北京科技大學新材料技術(shù)研究院，北京0008)

基于宏觀金屬輔助化學刻蝕制備硅納米線的研究

劉琳1,2,*李志勝1胡慧東1宋維力3
(1華北電力大學可再生能源學院，新能源電力系統(tǒng)國家重點實驗室，北京102206；2北京師范大學，能量轉(zhuǎn)換與存儲材料北京市重點實驗室，北京100875；3北京科技大學新材料技術(shù)研究院，北京100083)

分別利用鍍銀的硅襯底和鉑絲電極作為原電池反應(yīng)中的陰極和陽極，基于金屬輔助化學刻蝕采用宏觀原電池的方法制備硅納米線，深入研究了該法制備硅納米線陣列的機理。通過改變電連接、鍍銀、刻蝕參數(shù)、硅襯底和光照等實驗條件，系統(tǒng)地研究了所得硅納米線形貌與其對應(yīng)短路電流的關(guān)系，實驗發(fā)現(xiàn)短路電流與硅納米線長度有一定的對應(yīng)關(guān)系。文章中所提出的模型旨在從根本上解決金屬輔助化學刻蝕制備硅納米線的機理。最后對這種方法所具有的潛在應(yīng)用價值進行了展望和討論。

半導體；微結(jié)構(gòu)；電化學；硅納米線；金屬刻蝕

[Article]

www.whxb.pku.edu.cn

In principle,the fabrication of Si via MACE process is intrinsically microscopic galvanic corrosion,electrons flow from the Si anodes to the noble metal cathodes with the presence of an aqueous solution containing oxidant.In the recent work,many factors and improvements have been considered in this process18－21,but unfortunately,real-time corrosion current that is significant to understand the fundamental of the process which was absent in the strategies above.

On the other hand,the macro-galvanic cells,which could be formed when a noble metal is short-circuited to Si in the presence of an oxidant in the hydrofluoric acid(HF)solution,were firstly demonstrated by Kelly and co-workers22－26.Currently,Liu et al.14have fabricated SiNWs by combining the macro-galvanic cell with micro-galvanic cell via the MACE process,and investigated the relationship between the circuit current and SiNW length by changing the size of the cathode area.It is noteworthy that fabrication of SiNWs could be realized by the MACE using dissolved oxygen as the oxidizing agent with the presence of HF solution.

The follow-up work based on the combination of traditional MACE and anodic etching has been explored for achieving modified SiNWs27－31,and it is noted that an external electrical bias (current or voltage source)is needed in such process.Compared to the traditional MACE,the holes for accelerating etching in such combined method are provided by the external bias,which is greater than the contribution from reduction of the added oxidants. Usually,this combined method has been widely used for studying MACE mechanism or producing porous SiNWs.Hence,it should be emphasized that the combined method is different from the MACE aforementioned,where external electrical bias is not needed and the holes are solely provided by the reduction of the oxidants.The use of cathode materials in the MACE is known to fully utilize the oxygen in the etchant.

Up to date,there are still many phenomena and mechanisms that have not been well understood in the nanowire fabrication. For such purpose,here we utilize the MACE to fabricate SiNWs in the HF solution and take insight into the formation of SiNW arrays.In the setup of MACE,the catalytic Ag coating and Pt electrode are specifically employed as the cathode.Effects of electrical connection,Ag coatings,etching conditions,Si substrates,and light irradiations on the SiNWs are intensively studied based on the SiNW morphologies and related current densities recorded in the preparation.The results indicate that the control of the holes in the Si substrates via changing the conditions is the critical key in the kinetics of forming SiNWs.The corresponding mode for understanding the mechanism in the MACE has been proposed and discussed.

2　Experimental

2.1Materials

As listed in Table 1,four types of Si wafers,Si(100)wafers(ptype,boron-doped,2.0－8.0 Ω?cm),Si(100)wafers(p-type,borondoped,0.5－0.8 Ω?cm),Si(100)wafers(p-type,boron-doped, 0.003－0.009 Ω?cm),Si(100)wafers(n-type,phosphor-doped, 0.002－0.006 Ω?cm),were commercially available and used as the starting materials.HF(49%),H2O2(30%),H2SO4(98%),AgNO3(>99.9%),CH3COCH3(AR)and EtOH(AR)were purchased from Sigma-Aldrich.

2.2Nanowire synthesis

In a typical synthesis,Si substrates of 4 cm2area were firstly cleaned with acetone,ethanol and deionized water via ultrasonic condition,followed by being immersed into a boiling solution of H2SO4-H2O2(volume ratio 3:1)for 15 min.The Si sheets were rinsed with deionized(DI)water.The substrates were then dipped into an aqueous solution of 5%HF for a few tens of seconds before use.The as-cleaned Si pieces were coated with Ag nanoparticle films by immersing into the mixing solution of HF and AgNO3.The back side of the prepared Si substrates with the Ag nanoparticle films that were connected with copper plates was scratched with a eutectic InGa alloy to establish good electrical contact.The silicon wafer was attached to the cell with an O-ring for a window,and a part of(1 cm2)the test specimen was exposed to the solution.The Pt electrode was electrically connected to the Si substrate directly.Prior to each experiment,the cell body and the Pt electrode were carefully cleaned.After etching,the surface of Si was rinsed with deionized water for several times to remove the residual fluoride ions,followed by drying in the dark.All experiments were carried out at room temperature.

2.3Characterization

Surface morphology and microstructure of the as-prepared SiNWs were examined on a field emission scanning electron microscopy(SEM,HITACHI S-4800).Electrochemical data in the galvanic corrosion studies were obtained on the Zennium IM6 (Zahner)electrochemical workstation.In the three-electrode system,silicon,Pt,and silver chloride electrode(RHE)were used as the working electrode,counter electrode,and reference electrode,respectively.

Table 1　Silicon wafers used in this work

3　Results and discussion

3.1Mechanisms of the etching

The experimental setup is illustrated in Fig.1.Upon immersing into HF and AgNO3for Ag coating,the as-prepared Ag-coated Si substrate was exposed to the aqueous HF solution and meanwhile the other side was connected to the Pt electrode,as shown in Fig.1 (a).It is noted that the area of the Pt electrode immersed in the etchant remained exactly the same throughout all the experimentsin this work.In the reference setup,no electrical connection was applied between the Pt electrode and Si substrate,as shown in Fig.1(b).SEM images of the as-prepared SiNWs under different conditions were given in Fig.1(c,d),exhibiting that the electrical connection between the Pt electrode and Si substrate essentially promoted the growth of SiNWs.According to the well-recognized mechanism,it is suggested that redox reactions of the etching should follow Eqs.(1),(2)14

Fig.1　(a,b)Schemes of the galvanic cell for preparing SiNWs and(c,d)SEM images of the as-prepared SiNWs

Cathode reactions:

According to Eq.(1),electrical connection between the Pt electrode and Si substrate in term of increasing reaction area would substantially enhance electron consumption on the cathode (Fig.1(a)),which in turn promotes the kinetics for Si etching.As a consequence,generation of macroscopic galvanic cell could vastly facilitate the growth of SiNWs via MACE.On the basis of the etching mechanisms and characteristics,the effects of preparation conditions on the MACE would be systematically investigated.

3.2Effects of Ag coating on growing SiNWs

In order to investigate the effects of Ag coating,a reference sample was prepared under the same conditions(Table 2)except for using bare Si substrate in the MACE.Typical SEM images in Fig.2 demonstrate the top-down sectional and cross-sectional views of the porous Si from bare Si(Fig.2(a,c))and SiNW arrays from Ag-coated Si(Fig.2(b,d)).Apparently,the presence of Ag leads to rapid Si etching to form the SiNW arrays(with length up to 6.2 μm),which indicates that Ag plays a strong catalytic role in term of electron acceptor upon MACE(Fig.1),consistent with previous observation12,15.

Table 2 Silicon wafers with differentAg coating treatments

The electrochemical measurements of these two samples were plotted in Fig.2(e,f),exhibiting the current density－time and the potential－time curves,respectively.According to the curve in Fig.2(f),it is clearly seen that a macroscopic galvanic cell is constructed between the Si substrate and Pt electrode when both are electrically connected.Two pronounced effects were found via such connection.(1)Potential is sharply increased,and(2) electrons start to transfer between the Si substrate and Pt electrode (Fig.2(f)).As exhibited in Fig.2(e),a very small current density appeared between the Si substrate and Pt electrode in the setup with bare Si,indicating formation of porous Si(Fig.1(d)).In the setup with Ag-coated Si,on the contrary,a dramatically enhanced initial current density and a stable current density were observed after hundreds of minute(Fig.2(e)),which corresponds to the moment when electrical connection is established and potential is stabilized in the polarization process,respectively.Such polarization process is associated with the production of SiNW arrays (Fig.1(c)).The comparison also confirms that the presence of Ag tremendously boosts the Si etching rate in the HF solution,consistent with the results found in the SEM images.Additionally,it is note that the shift observed in the potential is in good agreementwith the galvanic cell theory21－25.In principle,a potential difference between Si and metals is assumed in the rest potential of the system when the metal electrode is connected to the Si electrode. Prior to electrical connection,the potential of the Ag-coated Si is higher than that of the bare Si,because the potential of the Agcoated Si refers to the rest potential of the bare Si and Ag.Upon electrical connection,the potential shift ofAg-coated Si was found to be much larger than that of the bare Si as expectation.

Fig.2　(a－d)SEM images of the as-prepared SiNWs and(e－f)results of the electrochemical testing

Further investigation was carried out via changing the immersing time(1,2,and 4 min)in the HF and AgNO3solution, followed by the same preparation conditions(listed in Table 2). Fig.3(a,c,e)exhibits the top-down sectional views of the SiNW arrays with various immersing time of 1,2,and 4 min in the HF and AgNO3solution,respectively.When the Ag coating time is shorter,it is observed that the length of the SiNW arrays is much shorter,but larger in the average diameter(Fig.3(b,d,f)).The results imply that the increasing Ag coatings are expected to essentially enhance the Si etching rate.In contrast to the current densities for Pt/Si galvanic couples with different Ag coating time (Fig.3(g)),the current densities are considerably higher for the Pt/ Si galvanic couples with longer Ag-coating time,also indicating that the highest Ag amount has the strongest catalytic activity for the formation of SiNWs.

For understanding the effects of the etching parameters on the SiNWs,the Ag-coated Si substrates(Si-1,2 min coating)were immersed into the 9 mol?L－1HF solution for different time,as listed in Table 3.Fig.4 shows the representative SEM images of the top-down sectional and cross-sectional views of the SiNW arrays.Compared to the sample without etching(Fig.4(a,b)),the Ag coating gradually sunk into the Si substrate once electrical connection was generated between the Ag-coated Si and Pt electrode in the HF solution.As the etching time stayed longer,the as-formed SiNWs were found to grow longer as anticipated(Fig.4 (c－h(huán))).In the early etching stage,Ag coatings could be maintained at the bottom of the SiNW arrays,as shown in Fig.4(d).With increasing etching time,the Ag coatings were found to break intoAg nanoparticles and SiNW arrays could be clearly observed (Fig.4(f,h)).Moreover,it is emphasized that the evolution of the Ag coating in the MACE here is almost the same as that of the traditional MACE12,15.The SiNWdiameter could be affected by the Ag shape,but it is difficult to control and evaluate the SiNW diameter in this paper32,33.

Fig.3　(a－f)Top-down sectional and cross-sectional SEM images of the SiNW arrays with various immersing time in the HF andAgNO3solution and(g)current density－time curves for the coupled Si and Pt electrodes in HF solution with differentAg-coating time

Table 3　Growth of SiNWs with different etching time

The concentration of the HF solution was also varied to study the concentration-dependent etching in the formation of the SiNWs.As listed in Table 4,the concentrations of the HF etchant were changed into 4,9,and 15 mol?L－1,and the other conditions remained the same.Fig.5(a－c)presents the cross-sectional SEM images of the prepared SiNW arrays,which indicates that the HF concentration possess the fastest etching rate,leading to the longest SiNWs(Table 4).As plotted in Fig.5(d),the current density－time curves also exhibit that the coupled Si and Pt electrodes in 15 mol?L－1HF solution deliver the largest current density among the three samples.Implication of the results suggests that the length of the as-formed SiNWs is due to the effect of fluorine anions within a certain concentration range.According to Eq.(2),the mechanism is associated with the fact that increasingfluorine anions participate in the oxidation reaction.As a consequence,the generated electrons are transferred onto the Pt surface and participate in the reduction reaction as given by Eq.(1).

Fig.4　Top-down sectional and cross-sectional SEM images of the SiNW arrays with different immersing time in 9 mol?L－1HF solution

Table 4　Growth of SiNWs with different concentrations in the HF etchant

The doping levels of the Si wafers used for achieving SiNWs also have great impacts on the morphology of the prepared SiNWs.According to Table 5,the Si substrates of different electrical resistivity were applied in the preparation and the other procedures stayed the same.SEM images shown in Fig.6(a－c)are the cross-sectional views of the SiNW arrays using various Si substrates,suggesting that the SiNW length ranges from 6.2 to 8.5 μmas the decrease in the electrical resistivity.As exhibited in Fig.6 (d),current densities were observed to enhance with the increase of the doping level,which is identical to the anodic etching for fabricating porous Si in the previous report34,35.This observation could be linked to the point that the consumption of holes during the Si etching plays an important role in the electrochemical reaction.Consequently,current density was found to be enlarged with increasing number of holes in the p-type Si substrates,which indicates the promotion of dissolving Si in the HF solution.

Fig.5　(a－c)Cross-sectional SEM images of the SiNW arrays with different HF concentrations in the etching and(d)current density－time curves for coupled Si and Pt electrodes in the HF solutions with different concentrations

In order to explore the doping influences of Si substrates on the prepared SiNWs,three different types of Si substrates were utilized and the conditions were listed in Table 6.Representative SEM images of the SiNWs are given in Fig.7(a－c),showing that the SiNWs grown on the heavily boron-doped Si possess larger length(Fig.7).Additionally,the current density－time curves in Fig.7(d)demonstrate that the setup based on heavily phosphor-doped Si delivers a larger current density than those based on ptype Si.The result here is completely different from the observation on the modestly doped n-type substrates in the previous work7.In general,a much larger cathode area or light irradiation is required to draw the equivalent current density on the modestly doped n-type Si.However,the current density－time characteristics are similar in both p-type and heavily doped n-type Si samples in this work,where the area of the Pt electrode was exactly the same.In the previous studies,on the other hand,surface breakdown effect was found to result in a larger current36.

Table 5　Silicon wafers for growing SiNWs

Fig.6　(a－c)Cross-sectional SEM images of the SiNW arrays prepared and(d)current density－time curves for coupled Si and Pt electrodes assembled with the Si substrates of various doping levels

Silicon wafer Si-1(boron doped,p-type,2.0－8.0 Ω?cm) Si-3(boron doped,p-type,0.003－0.009 Ω?cm) Si-4(phosphor doped,n-type,0.002－0.006 Ω?cm) Growth ofAg coating/min 2 2 2 9 9 9 HF etchant/(mol?L－1)　Etching time/h 2 2 2 SiNW length/μm 6.2 8.5 0.2

3.3Effects of light irradiation on the growing SiNWs In the previous work for etching SiNWs,utilization of lightirradiation has been proved to be an effective approach for enlarging the kinetics in growing SiNW7.In the MACE setup here, light-irradiation was introduced as an assistant condition in the preparation of SiNWs.For understanding the impact of light-irradiation on the SiNWs,n-type Si substrates(0.002－0.006 Ω?cm) were used with different irradiation conditions in the process (Table 7).According to the SEM images shown in Fig.8,the light from the front side has a strong effect on dissolving Si,with no pronounced SiNWs formed(Fig.8(c,d)).In the contrary,the light irradiation from the back side presents a more favorable condition for growing SiNWs,as exhibited in Fig.8(e,f).The current density－time curves are not provided here,because the large current density in the sample without light irradiation was very strong.Therefore,the changes in the current density under the light irradiation were insufficient to be monitored.

3.4More discussion

For overall understanding the electrochemical mechanisms of preparing the SiNW arrays in this work,a model of the macroscopic galvanic cell driven MACE process is shown in Fig.9.As illustrated in Fig.9(a),only one galvanic cell exists in the system before electrical connection and it is formed by the microscopic short-circuit galvanic cells between Si and Ag nanoparticles, which deliver weak corrosion in the Si.Upon electrical connection (Fig.9(b)),another galvanic cell based on the connected macroscopic short-circuit galvanic cell between Si and Pt is introduced into the system.In this work,the recorded current densities in the electrical connection systems refer to the electron flows from Si to Pt,which greatly contributes to the formation of SiNWs.Because of diffusion effect and electric field37－39,the left holes transfer to theAg-coated Si surface,which could be described as Eq.(3):

Fig.7　(a－c)Cross-sectional SEM images of the SiNW arrays and(d)current density－time curves for coupled Si and Pt electrodes prepared with the Si substrates of different types

Table 7　Conditions of light irradiation for growing SiNWs

Fig.8　Top-down sectional and cross-sectional SEM images of the SiNW arrays using n-type Si substrates with different irradiation conditions

Moreover,the light-irradiation effects from the front side and the back side are illustrated in Fig.9(c,d),respectively.The number of holes can be massively enlarged in the n-type Si under light irradiations.In the case of the light irradiations from the front side(Fig.9(c)),the generated holes are easy to transfer to the surface by the surface breakdown effect,leading to formation of SiNWs in term of dissolving Si.On the other hand,the generated holes are prone to transfer to theAg-coated valleys if the holes are generated from the back side(Fig.9(d)).In addition,the generatedholes that are close to the sidewalls or far from the metal areas may enhance the porosity of the SiNWs under the light irradiations either from the front side or back side.

Fig.9　(a,b)Schemes of the macroscopic galvanic cell driven MACE process for the SiNW preparation in HF aqueous solution and(c,d)light-irradiation effect on the heavily doped n-type Si

According to the above results,the overall MACE process mainly involves four steps.(1)Due to the catalytic activity of Ag, the oxidant is reduced at its surface;once the Si is electrically connected with Pt electrode,the oxidant is reduced at both the Pt and Ag surfaces.(2)The corrosion rate of the SiNWs depends on the hole concentration and its transfer direction,which is controlled by the rate of cathodic reactions and the types of Si substrates.(3)Due to the reduction of the oxidant,the generated holes are injected into the Si,and Si is subsequently oxidized and dissolved into HF.(4)The holes that diffuse to the off-metal areas or to the NW sidewall may shorten the SiNW length and enlarge porosity.

4　Conclusions

In summary,various SiNW arrays were produced by MACE in the HF solution via varying the process parameters.Significant factors including electrical connection,Ag coatings,etching conditions,Si substrates and light irradiations have been considered to investigate the corresponding impacts on the formation of the SiNWs.The generation of holes in silicon plays a vital role on the formation of SiNWs and the associated mechanism has been discussed.The results suggest that MACE holds various advantages of facile,effective and scalable features,promising a unique stage for large-scale synthesizing SiNWs in many fields.

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Insight into Macroscopic Metal-Assisted Chemical Etching for Silicon Nanowires

LIU Lin1,2,*LI Zhi-Sheng1HU Hui-Dong1SONG Wei-Li3
(1State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources,School of Renewable Energy, North China Electric Power University,Beijing 102206,P.R.China;2Beijing Key Laboratory of Energy Conversion and Storage Materials,Beijing Normal University,Beijing 100875,P.R.China;3Institute of Advanced Materials and Technology,University of Science and Technology Beijing,Beijing 100083,P.R.China)

To understand the principles of the fabrication of nanowire arrays using macroscopic metal-assisted chemical etching(MACE),Si nanowires(SiNWs)are synthesized using Ag-coated Si substrates and Pt electrodes by the macroscopic MACE.Analysis of the SiNW morphology coupled with the corresponding current density in the MACE process is applied to systematically investigate the effects of the electrical connection,Ag coating,etching conditions,Si substrates,and light irradiation on the formation of SiNWs.It is found that there is a certain relationship between the current density and the SiNW length.Amode is proposed to fundamentally understand the mechanisms of the preparation of SiNWs using MACE.Associated opportunities are also discussed.

Semiconductor;Microstructure;Electrochemistry;Silicon nanowire;Metal etching

1　Introduction

Over the past decades,nanomaterials have been tremendously considered as one of the most important materials in the electronics industry1－7.Recently,various efforts have been largely paid to fabricate Si nanostructures with applicable performance in the device applications,such as vapor-liquid-solid growth8,9,electrochemical etching10,11.Very recently,the metal-assisted chemical etching(MACE)technique has been well developed by Peng and coworkers12－14and is now widely utilized to fabricate Si nanowires (SiNWs)with exceptional morphologies and structures.Thus far,MACE is believed to be a low-cost,simple,reliable top-down fabrication technique for producing a variety of Si nanostructures15,and the nanostructures fabricated by MACE have demonstrated great potential application in various fields16,17.

December 30,2015;Revised:February 15,2016;Published on Web:February 18,2016.*Corresponding author.Email:liulin2014@ncepu.edu.cn;Tel:+86-10-61773932. The project was supported by the China Postdoctoral Science Foundation(2014M560934)and Fundamental Research Funds for the Central Universities,China(2015QN16).

O649

10.3866/PKU.WHXB201602183

中國博士后科學基金面上項目(2014M560934)和中央高?；究蒲袠I(yè)務(wù)費項目(2015QN16)資助