Materials Chemistry and Physics

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1 Mterils Chemistry nd Physis () 7 8 Contents lists ville t SieneDiret Mterils Chemistry nd Physis journl homepge: Chrteriztion of stin ethed p-type silion in queous HF solutions ontining HNO or KMnO A.S. Mogod, Y.H. Ahmd, W.A. Bdwy Deprtment of Chemistry, Fulty of Siene, Ciro University, Giz, Egypt rtile info strt Artile history: Reeived Mrh Reeived in revised form Deemer Aepted Deemer Keywords: Silion Ething AC impedne Surfe properties Stin ething of p-type silion in hydrofluori id solutions ontining nitri id or potssium permngnte s n oxidizing gent hs een exmined. The effets of ething time, oxidizing gent nd HF onentrtions on the eletrohemil ehvior of ethed silion surfes hve een investigted y eletrohemil impedne spetrosopy (EIS). An eletril equivlent iruit ws used for fitting the impedne dt. The morphology nd the hemil omposition of the ethed Si surfe were studied using snning eletron mirosopy (SEM) nd energy dispersive X-ry (EDX) tehniques, respetively. A porous silion lyer ws formed on Si ethed in HF solutions ontining HNO, while ething in HF solutions ontining KMnO led to the formtion of porous lyer nd simultneous deposition of K SiF inside the pores. The thikness of K SiF lyer inreses with inresing the KMnO onentrtion nd dereses s the onentrtion of HF inreses. Elsevier B.V. All rights reserved.. Introdution Intense sientifi nd tehnologil interests in porous silion fous on the remrkle high surfe re, enhned hemil retivity nd novel physil properties of this mteril. Biologil [ ] nd hemil [ 7] sensors hve een prepred on hemilly modified porous silion thin films. Porous silion n lso e used in optoeletronis [8], photovoltis [9], miromhining [,] nd in frition of diffusion memrnes []. Porous silion is typilly formed either y nodi ething in HF or y stin ething in HF/oxidizing gent []. The strutures, nd thus the properties of the porous silion films prepred y these methods, depend on the silion type nd the ething onditions used, suh s the hemil omposition of the ethnt nd the ething time [ 7]. In stin ething no externl is is required nd holes neessry for sustining the oxidtive dissolution of silion re supplied y HNO or other strong oxidnts. Porous lyers formed y stin ething oth on single nd polyrystlline Si posses similr struturl nd light-emitting properties to those otined y eletrohemil ething [8,9]. The homogeneous Si surfe n e stin ethed rther uniformly, nd the resulting porous silion hs een used s n nti-refletion oting on silion-sed solr ells []. Vrious oxidizers dded to HF solutions hve een tested for stin ething, exmples eing HNO, NNO, CrO [] nd Fe(III), Mn(VII) []. Anisotropi textured silion ws otined y stin Corresponding uthor. Tel.: + 7; fx: E-mil ddress: wd mogod@hotmil.om (A.S. Mogod). ething of Si in queous HF/HNO solutions t low ething rtes []. AC eletril impedne hrteriztion of porous silion llows to otin good informtion out the eletril ondutivity mehnisms, stility of mterils towrds mient onditions, nd the morphology of porous film through theoretil modeling of the impedne spetr []. Certin studies hve pplied this tehnique to porous silion s sustrte for polymers [], the ontt etween Au nd porous silion [], nd the use of porous silion s guest mtrix for metlli tions or for metlli oxides [7]. Stin ething of Si in HF solutions ontining oxidizing gents t open-iruit potentil is of interest euse mny industril ething proesses tke ple without potentil ontrol. The im of the present work is to study the eletrohemil impedne ehvior of p-si surfe stin ethed in queous HF solutions ontining HNO or KMnO (s oxidizing gent) s funtion of time nd onentrtions of HF nd oxidnts.. Experimentl Silion smples were prepred from oron-doped p-type (.. m) single rystl wfer of () orienttion mnuftured y Shin-Etsu Co. (Tokyo, Jpn). The Si smples were ut nd pressed ginst n O-ring seled in the wll of n eletrohemil ell, leving. m exposed to the eletrolyte. The k ontt ws mde vi In G lloy. Finlly, opper wire ws onneted to the In G t the surfe y silver pste. The eletrohemil ell used ws ylindril polytetrfluoroethylene ell 9 m high nd m in dimeter. The ounter eletrode ws Pt sheet of m re. Before the ething proess eh smple ws lened with etone for min then ethed in 7. M HF solution for few minutes to remove orgni impurities nd the ntive oxide lyer from the wfer. The ething solutions were prepred from nlytil grde hemil regents in deionized wter. The -8/$ see front mtter Elsevier B.V. All rights reserved. doi:./j.mthemphys...

2 A.S. Mogod et l. / Mterils Chemistry nd Physis () d Fig.. Nyquist impedne plots of p-si fter ething in 7. M HF ontining. M HNO : () for min, () for min, () for min, (d) for 8 min. oxidizing gents used in the present work were nitri id, HNO, nd potssium permngnte, KMnO. The eletrohemil impedne spetrosopi investigtion of p-si ws reorded during the ething proess in queous HF-oxidizing gent solutions t the open-iruit potentil using the IMd AMOS system (Zhner Eletrik GmH & Co., Kronh, Germny). The input signl ws mv pek to pek in the frequeny domin. Hz. Eh yle of impedne mesurements lsted out 8 s nd ws rried out t onstnt room temperture of 7 ± C. The morphology of the ethed Si surfe ws exmined y snning eletron mirosopy (Philips XL-). The hemil nlysis of the silion surfe fter ething ws performed y energy dispersive X-ry (EDX).. Results nd disussion.. Effet of ething time To study the effet of ething time on the eletrohemil hrteristis of the surfe of p-si in queous HF-oxidizing gent solutions the impedne mesurements were rried out fter different ething times, i.e.,, nd 8 min. The oxidizing gents used were HNO nd KMnO. The onentrtion of KMnO in these mesurements ws kept onstnt t. M nd the onentrtion of HF ws. M. A onentrtion of. M HNO hd negligile ething effet, so n ethnt solution of 7. M HF nd. M HNO ws used to produe the porous struture.... Effet of ething time on the p-si in queous HF/HNO solution The impedne response (in the Nyquist formt) of p-si in queous 7. M HF/. M HNO solution exhiits pitive loop t high frequenies nd smll indutive loop t low frequenies. This indutive loop ppers lerly fter long ething times. Previously, indutive loops hve lso een found during silion ething in HF solutions ner the open-iruit potentil [8,9]. From Fig. it n e oserved tht the impedne vlue dereses with inresing the ething time, implying n inrese of silion dissolution with time. A similr ehvior in the impedne of the n-si hs een oserved upon ddition of N CrO to the ethnt HF solution [9]. This oservtion is lso in line with previous impedne dt in tht eletril impedne of porous silion is muh lower thn tht of re Si []. The pitive loop n e ttriuted to the spe hrge pitne prllel with the surfe sttes lying in the foridden energy gp, while the indutive loop ppers relted to n tive surfe growth due to the formtion of porous silion []. Fig. shows the snning eletron photogrph of the silion surfe fter ething in n queous solution of 7. M HF/. M HNO for 8 min. From the figure it is ler tht the surfe of the ethed silion eomes porous. Therefore, the SEM is in good greement with the impedne results.

3 78 A.S. Mogod et l. / Mterils Chemistry nd Physis () 7 8 Fig.. () The snning eletron photogrph nd () the EDX nlysis of p-si surfe fter ething in 7. M HF ontining. M HNO for 8 min d Fig.. Nyquist impedne plots of p-si fter ething in. M HF ontining. M KMnO : () for min, () for min, () for min, (d) for 8 min.

4 A.S. Mogod et l. / Mterils Chemistry nd Physis () log(z/ohms m ) phse/deg log(z/ohms m ) phse/deg log(f/hz) log(f/hz) d 8 log(z/ohms m ) phse/deg log(z/ohms m ) phse/deg log(f/hz) log(f/hz) Fig.. Bode impedne plots of p-si fter ething in. M HF ontining. M KMnO : () for min, () for min, () for min, (d) for 8 min. Fig. shows typil EDX spetrum of p-si fter ething in n queous solution of 7. M HF/. M HNO for 8 min. The spetrum revels tht the elements present on the ethed surfe re minly Si with very smll mounts of oxygen nd fluorine.... Effet of ething time on the p-si in queous HF/KMnO solution Fig. shows the impedne mesurements for p-si in queous. M HF/. M KMnO solution t different ething times. Fig. represents Nyquist impedne plots whih show tht the impedne spetrum exhiits one pitive loop fter min. At high frequenies the impedne response is dominted y the spe hrge pitne of the Si/eletrolyte interfe, nd t middle frequenies the vlue of the pitne n e ttriuted to the presene of the surfe sttes. Suh surfe sttes my result from the modifitions of the surfe properties during the dissolution retion of silion [9]. As the ething proess is going on, the high frequeny prt of the impedne is enhned with time nd n dditionl high frequeny pitive loop in the impedne plot is reorded. The size of this loop inreses with inresing the ething time (Fig. ). The impedne ehvior, in this se, n e explined y dissolution of Si t the erly stges of the ething proess to form pores onto the silion surfe, therefter pssive lyer strts to deposit on the surfe. Bode plots of p-si in the sme ethnt solution in Fig. show the presene of two peks for the phse t the seleted frequeny rnge whih onfirm ourrene of two proesses t the p-si/solution interfe s indited y Nyquist plots. The surfe ws lso investigted y SEM nd EDX. Fig. shows the snning eletron photogrph of p-si ethed in. M HF/. M KMnO /H O solution. From this figure it is ler tht the silion surfe is overed with film (stin film). The EDX spetrum of the ethed smple (Fig. ) indites tht the elements present on the ethed surfe re minly Si, K nd F. The presene of the oserved mounts of F nd K on the surfe is ttriuted to depo- Fig.. () The snning eletron photogrph of p-si nd () the EDX nlysis of the p-si fter ething in queous. M HF solution ontining. M KMnO for 8 min.

5 8 A.S. Mogod et l. / Mterils Chemistry nd Physis () d Fig.. Nyquist impedne plots of p-si fter ething in. M HF ontining different onentrtions of HNO : (). M, (). M, (). M, (d). M. sition of lyer of K SiF on the silion surfe during the ething proess. This lyer ws previously oserved y X-ry diffrtion on the p-type silion during ething in. M HF/. M KMnO /H O solution []. Therefore, the results drwn from the presented impedne ehvior of the Si surfe ethed in. M HF/. M KMnO /H O solution gree well with those of SEM nd EDX mesurements. R s.. Effet of oxidizing gent onentrtion R Q... Effet of HNO onentrtion p-type silion ws ethed in. M HF solutions ontining different HNO onentrtions, i.e..,.,. nd. M. The impedne digrms of the ethed surfes in the different solutions re shown in Fig.. The digrms revel one pitive loop. To lrify the reltionship etween the impedne responses of the ethed Si surfe nd its morphology, the impedne dt hve een nlyzed using n equivlent iruit onsisting of minimum numer of elements whih fit the whole set of experimentl dt with fir preision (i.e. the disrepny etween experimentl dt R Fig. 7. Equivlent iruit used to fit the impedne dt of the ethed p-si in queous HF/HNO solutions. Q

6 A.S. Mogod et l. / Mterils Chemistry nd Physis () Z''/kohms m Z''/kohms m experimentl vlues simulted vlues Z'' (kohms m ) experimentl vlues simulted vlues Fig. 8. Nyquist impedne plots of p-si fter immersion in. M HF ontining different onentrtions of KMnO : (). M KMnO, (). M KMnO, (). M KMnO. nd theoretil fit is out.%). As result of the simultion we hve otined the equivlent iruit model shown in Fig. 7 whih is similr to tht otined in previous work []. The equivlent iruit model onsists of resistor R s orresponding to the ohmi drop in the eletrolyte, pir R Q, representing the elements of the doule lyer t the silion/eletrolyte interfe, where Q is onstnt phse element, nd the pir R Q is equivlent to the proess ourring t the silion surfe during the ething proess. The vlues of the prmeters of the eletril equivlent iruit for p-si in different onentrtions of HNO re summrized in Tle. As oserved from the dt in Tle, the vlue of the resistne, R, dereses, while tht of the onstnt phse element, Q, inreses s the onentrtion of HNO inreses, whih onfirms tht the rte of dissolution of the p-si inreses with inresing the HNO onentrtion. The dissolution proess leds to the formtion of the porous lyer.... Effet of KMnO onentrtion The impedne responses digrms of p-si ethed in. M HF ontining different onentrtions of KMnO fter ething for 8 min re shown in Fig. 8. The experimentl impedne dt were lso nlyzed using the equivlent iruit shown in Fig. 7. The vl- Tle The effet of HNO onentrtion on the vlues of the different fitting prmeters of the equivlent iruit model ssigned to the ething of p-si in. M HF solutions. Eletrolyti solution Fitting prmeters R ( m ) Q ( Fm ) R (k m ) Q ( Fm ) R s ( m ). M HNO M HNO M HNO M HNO

7 8 A.S. Mogod et l. / Mterils Chemistry nd Physis () Fig. 9. Nyquist impedne plots of p-si fter ething for 8 min in. M HNO solution ontining: (). M HF, (). M HF, () 7. M HF. ues of equivlent iruit elements re presented in Tle. From these dt it is ler tht the vlue of resistne, R, inreses nd tht of the onstnt phse element, Q, dereses s the onentrtion of KMnO inreses, inditing n inrese in the thikness of the pssive lyer of the slt, K SiF, formed on the surfe of silion. These results re in good greement to those otined previously y Nhm et l. []... Effet of HF onentrtion To optimize the onditions of the porous silion formtion, it ws essentil to investigte the effet of the onentrtion of the min ethnt, i.e. HF. Therefore, the effet of HF onentrtion on the ething of p-si in the presene of different oxidizing gents ws investigted. The onentrtion of HF ws hnged from. to. M keeping the onentrtion of oxidizing gent onstnt.... Effet of HF onentrtion on the ething of p-si in. M HNO The impedne mesurements were rried out fter ething of p-si in. M HNO solutions ontining different onentrtions of HF for 8 min nd the results re presented in Fig. 9. The impedne dt otined were nlyzed using the sme equivlent iruit model shown in Fig. 7 nd the prmeters vlues of this Tle The effet of KMnO onentrtion on the vlues of different fitting prmeters of the equivlent iruit model ssigned to p-si in. M HF solutions. Eletrolyti solution Fitting prmeters R ( m ) Q ( Fm ) R (k m ) Q ( Fm ) R s ( m ). M KMnO M KMnO M KMnO..8.8.

8 A.S. Mogod et l. / Mterils Chemistry nd Physis () Z''/kohms m experimentl vlues simulted vlues -Z''/kohms m d -Z''/kohms m Exprimentl vlues -Z''/kohms m Fig.. Nyquist impedne plots of p-si fter ething for 8 min in. M KMnO solution ontining: (). M HF, (). M HF, () 7. M HF, (d). M HF. equivlent iruit re listed in Tle. The resistne, R, dereses, while the vlue of Q inreses s the onentrtion of HF inreses, whih indites n inrese in the rte of dissolution of Si. The dissolution is ompnied y porous film formtion.... Effet of HF onentrtion on the ething of p-si in. M KMnO The impedne experiments were rried out in the sme wy s desried efore. The impedne plots responses of p-si in. M KMnO solutions ontining different onentrtions of HF re shown in Fig.. The prmeters vlues of the equivlent iruit (Fig. 7) used for the nlysis of the impedne dt re presented in Tle. It is ler from the results tht t low onentrtions of HF, silion dissolves with the formtion of K SiF on its surfe. With inresing onentrtion of HF the vlue of R dereses s result of n inrese in the rte of dissolution of the deposited slt. Two ler pitive loops were reorded t. M HF inditing the presene of two time onstnts representing the two proesses, pore formtion nd slt deposition onto the porous struture. Tle Effet of HF onentrtion on the vlues of the fitting prmeters to equivlent iruit model representing p-si in. M HNO solutions. Eletrolyti solution Fitting prmeters R ( m ) Q ( Fm ) R (k m ) Q ( Fm ) R s ( m ). M HF M HF M HF

9 8 A.S. Mogod et l. / Mterils Chemistry nd Physis () 7 8 Tle Effet of HF onentrtion on the vlues of the fitting prmeters of the equivlent iruit model representing the ething of p-si in. M KMnO solutions. Eletrolyti solution Fitting prmeters R ( m ) Q ( Fm ) R (k m ) Q ( Fm ) R s ( m ). M HF M HF M HF M HF Conlusions By mens of the impedne mesurements, the formtion proess of stin ethed porous silion lyer revels ontinuous dissolution of Si surfe nd simultneous formtion of porous silion t the porous/rystlline interfe. The thikness of the porous silion lyer inreses with inresing ething time to limiting vlue in the se of n ethnt onsisting of queous HF solution ontining HNO. In se of queous HF solution ontining KMnO the formtion of porous silion is followed y the deposition of the pssive lyer of the slt K SiF on the pores surfes. The impedne results indite tht the thikness of K SiF inreses with inresing ething time nd KMnO onentrtion, while it dereses s the onentrtion of HF inreses. This n e ttriuted to the dissolution of K SiF in the more onentrted HF solutions. Referenes [] V.S.Y. Lin, K.P.S. Dnil, M.J. Silor, M.R. Ghdiri, Siene 78 (997) 8. [] K.P.S. Dnil, D.P. Greiner, M.J. Silor, J. Am. Chem. (999) 79. [] V.M. Strodu, L.L. Fedorenko, A.P. Sisetskiy, N.F. Strodu, Sens. Atutors B 8 (999) 9. [] S.E. Lètnt, M.J. Silor, Adv. Mter. (). [] S. Zngooie, R. Bjorklnd, H. Arwin, Sens. Atutors B (997) 8. [] T. Tlierio, M. Dihn, E. Mssone, A. Fourn, A.M. gué, T. Bretgnon, B. Frisse, L. Montès, Sens. Atutors A (99). [7] S.E. Létnt, S. Content, T.T. Tn, F. Zenhusen, M.J. Silor, Sens. Atutors B 9 () 9. [8] L.T. Cnhm, Appl. Phys. Lett. 7 (99). [9] C. Levy-Clement, S. Bstide, Zeitshrift fur Physiklishe Chemie Bd. (999). [] T.E. Bell, P.T.J. Gennissen, D. DeMunter, M. Kuhl, J. Miromeh. Miroeng. (99). [] V.A. Melnikov, E.V. Astrov, T.S. Perov, V. Srigengn, J. Miromeh. Miroeng. 8 (8) 9. [] S. Cruz, A. Honig-d Orville, J. Muller, J. Eletrohem. So. () C8. [] A.G. Cullis, L.T. Cnhm, P.D.J. Clott, J. Appl. Phys. 8 (997) 99. [] M.I.J. Bele, J.D. Benjmin, M.J. Uren, N.G. Chew, A.G. Cullis, J. Cryst. Growth 7 (98). [] M.I.J. Bele, J.D. Benjmin, M.J. Uren, N.G. Chew, A.G. Cullis, J. Cryst. Growth 7 (98) 8. [] B. Gonzlez-Diz, R. Guerrero-Lemus, J. Mendez-Rmos, B. Diz-Herrer, V.D. Rodriguez, Sens. Atutors A (9) 97. [7] K.S. Khshn, A.A. Awd, M.A. Mohmed, J. Eng. Tehnol. 7 (9). [8] F. Nmvr, R.F. Pizzinotto, H. Yng, N. Klkorn, P. Mrusk, Mter. Res. So. Pro. 98 (99). [9] S. Liu, C. Plsule, S. Yi, S. Gngopdhyy, Phys. Rev. B9 (99) 8. [] L. Shirone, G. Sotgiu, F.P. Clifno, Thin Solid Films 97 (997) 9. [] V. Lehmnn, Eletrohemistry of Silion, Wiley-VCH, Germny,, p.. [] D. Mills, M. Nhidi, K.W. Kolnski, Phys. Sttus Solidi (). [] B. González-Diz, R. Guerrero-Lemus, N. Mrrero, C. Hernández-Rodriguez, F.A. Ben-Hnder, J.M. Mrtinez-Durt, J. Appl. Phys. 9 (). [] J. Ross Mdonld, W.B. Johnson, in: J. Ross Mdonld (Ed.), Impedne Spetrosopy, John Wiley nd Sons, New York, 988. [] V. Prkhutik, R. Fenollos, E. Mtveev, T.P. Nguyen, P.Le. Rendu, V.H. Trn, Synth. Metls () 9. [] F. Fonthl, T. Trifonov, A. Rodriguez, L.F. Mrsl, J. Pllres, Miroeletron. Eng. 8 () 8. [7] K. Ben Sd, M. Sdoun, H. Hmzoui, B. Bessis, Mter. Si. Eng. C 8 (8). [8] V. Prkhutik, J. Porous Mter. 7 () 97. [9] M. Sfi, J.N. Chzlviel, M. Cherkoui, A. Belidi, O. Gorohov, Eletrohim. At 7 () 7. [] D. Vnmekelergh, P.C. Serson, J. Eletrohem. So. (99) 97. [] F. Oznm, J.N. Chzlviel, A. Rdi, M. Etmn, J. Eletrohem. So. 9 (99) 9. [] K.S. Nhm, Y.H. Seo, H.J. Lee, J. Appl. Phys. 8 (997) 8.