Investigations of the pore formation in the lead selenide films using glacial acetic acid- and nitric acid-based electrolyte

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1 Zimin et l. Nnoscle Reserch Letters 2012, 7:338 NANO EXPRESS Open Access Investigtions of the pore formtion in the led selenide films using glcil cetic cid- nd nitric cid-sed electrolyte Sergey P Zimin 1*, Egor S Gorlchev 1,2, Viktor V Numov 2 nd Fedor O Skok 1 Astrct We report novel synthesis of porous PSe lyers on Si sustrtes y nodic electrochemicl tretment of PSe/CF 2 /Si(111) epitxil structures in n electrolyte solution sed on glcil cetic cid nd nitric cid. Electron microscopy, X-ry diffrctometry, nd locl chemicl micronlysis investigtion results for the porous lyers re presented. Averge size of the synthesized mesopores with pproximtely cm 2 surfce density ws determined to e 22 nm. The oserved phenomenon of the ctive selenium redeposition on the mesopore wlls during nodic tretment is discussed. Keywords: Porous semiconductors, Led selenide, Anodic electrochemicl tretment, Electrolyte, Mesopores PACS: Rm; Nr; Cf. Bckground Currently, there is new rpidly emerging direction of modern nnotechnology reserch in the friction of porous semiconductor compound mterils. Such nnostructured mterils cn e pplied in mny novel nd unique prcticl pplictions, such s optoelectronic devices sed on the quntum size effects due to the smll dimensions of the interpore mteril, or iomedicl compounds sed on storing nnoinclusions in the pore volume, nd mny more. Of course, porous Si [1] nd III-V nd II-VI compound semiconductors [2,3] to dte hve een studied rther extensively. On the other hnd, IV-VI mterils, in prticulr led chlcogenides PX (X = Te, Se, S), while eing extremely useful for thermoelectric nd optoelectronic pplictions [4,5], re not reserched t porous form lmost t ll. Among the resons for tht re the lck of the lrge monocrystlline wfer smples fitted for nodic cell tretment nd lso the sence of n estlished electrolyte etchnt solution. Recently, while eing le to e the first to demonstrte the pore formtion in led chlcogenide lyers nd study their properties [6-9], we lso found out tht * Correspondence: zimin@uniyr.c.ru 1 Microelectronics Deprtment, Yroslvl Stte University, Yroslvl , Russi Full list of uthor informtion is ville t the end of the rticle potssium hydroxide (KOH)-sed electrolyte, which ws originlly proposed y Norr [10,11] nd is most commonly used for the PX electropolishing, is not optiml for stle pore formtion in PSe, in contrst to PTe [9]. It ecme necessry to optimize nodic tretment conditions for the friction of well-defined pore morphology in led selenide lyers, nd such new nd more fruitful pproch is presented in this work. Methods Initil smples were high-qulity epitxil monocrystlline PSe films with thickness of 2.2 to 5.2 μm grown on CF 2 /Si(111) wfer sustrtes y moleculr em epitxy (MBE) in ETH, Zürich [5]. The thickness of the clcium fluoride uffer lyer ws 2 to 4 nm. Anodic electrochemicl tretment experiments were sed on technologicl pproch tht llows us to prepre porous led chlcogenide lyers on silicon wfer sustrtes using verticl-type electrochemicl cell [6]. The key tretment feture ws the electrolyte solution. For the discussed study, the first tch of the initil epitxil PSe films ws nodized using KOH-sed electrolyte solution (Norr electrolyte) [10] contining 20 g of KOH, 45 ml of distilled wter, 35 ml of glycerol, nd 20 ml of ethnol. Current density ws 2 to 8 ma cm 2, nd tretment durtion ws 10 to 20 min, performed t room 2012 Zimin et l.; licensee Springer. This is n Open Access rticle distriuted under the terms of the Cretive Commons Attriution License ( which permits unrestricted use, distriution, nd reproduction in ny medium, provided the originl work is properly cited.

2 Zimin et l. Nnoscle Reserch Letters 2012, 7:338 Pge 2 of 5 temperture. These film smples were further divided into two groups depending on the morphology type of their initil surfce: the first group hd typicl flt surfce for MBE-grown PX lyers on CF 2 /Si(111) with nnoterrces nd disloction exit pits with pproximtely 10 7 cm 2 density, while the second group hd peculir grnulr surfce (for detils, see [8]). The second tch of PSe films (contining 3% Sn, with flt initil surfce) ws nodized using solution of 10 ml of nitric cid (HNO 3 ), 10 ml of glcil (undiluted) cetic cid (CH 3 COOH), nd 40 ml of glycerol (first used y EH Tompkins nd GL Johnson [11] for the electropolishing of led selenide t high current densities), for which, y nlogy with porous silicon, we reduced the current density (1 ma cm 2 ) nd processing temperture (20 C) with durtion of 10 min. Scnning electron microscopy (SEM) studies were performed on Supr-40 (Crl Zeiss, Inc., Oerkochen, Germny); chemicl energy-dispersive X-ry spectroscopy (EDS) locl micronlysis ws crried out simultneously with SEM using n INCA-Energy spectrometer (Oxford Instruments, Aingdon, UK). Results nd discussion The first tch of porous PSe films ws fricted using the Norr electrolyte. We discussed these results in much detil in [8], nd here, we will give only the sic overview. The principl result ws tht the modified nodized lyer morphology strongly depended on the initil film surfce stte. Thus, PSe lyers with islnd-like morphology (Figure 1) for the films with flt initil surfce were otined. PSe nnoislnds with lterl size of 40 to 400 nm, height of 90 nm, nd verge seprting gps of 70 nm were synthesized on the surfce of the underlying unmodified PSe film. A peculir effect ws tht for the longer tretment durtions, when the entire PSe film ws etched wy, such isolted PSe nnoislnds were positioned directly on the Si sustrte. On the other hnd, using PSe films with grnulr initil surfce, it ws possile to fricte hierrchicl mesoporous surficil lyers (Figure 1). Such hierrchicl porous networks typiclly contined lrge open mcropores (100 to 250 nm) with smller secondry mcropores (50 to 75 nm) nd mesopores (15 to 20 nm) on the mcropore wlls. Notly, only modifiction of thin ner-surfce lyer with thickness of pproximtely 150 to 200 nm took plce during nodic tretment independently of its durtion. One importnt conclusion from this experimentl study ws tht, unfortuntely, in oth cses, the modified lyer occupied only thin surficil prt of the processed PSe film, nd formtion of well-defined pore network did not tke plce. It ecme ovious tht the min prolem for the results discussed ove ws the undesirly high etch rte of PSe tht did not llow the formtion of developed thick porous lyer due to n ctive mteril removl. Therefore, it ecme necessry to pply new electrolyte with low-speed electropolishing. We hve proposed to introduce to nodic tretment process n electrolyte solution of nitric cid, glcil cetic cid, nd glycerol (Tompkins nd Johnson solution). Here, nitric cid is the etching gent nd cetic cid is the wetting gent tht stilizes the mteril removl nd modifiction processes. Consequently, the nodic tretment resulted in the formtion of well-defined porous lyers, showing the success in using glcil cetic cid to limit the dissolution rte. Typicl SEM imges of surfce nd cross section of the PSe porous lyer with thickness of 750 nm on top of the unmodified PSe film re given in Figure 2. The verge size of the synthesized mesopores ws 22 nm, nd their density on the surfce ws pproximtely cm 2. The thickness of the underlying unmodified PSe film indicted tht the thickness of the PSe lyer tht ws etched wy ws equl to the thickness of the fricted porous lyer, due to the stilizing role of glcil cetic cid, in contrst to the Norr electrolyte tretment [8], when the thickness of porous modified PSe lyer ws typiclly n order of mgnitude smller thn the removed lyer thickness. Figure 1 SEM imges of the modified PSe lyers otined y nodic tretment with Norr electrolyte. () Islnd-like morphology for the smples with initil flt film surfce. () Hierrchicl porous film for the smples with initil grnulr film surfce.

3 Zimin et l. Nnoscle Reserch Letters 2012, 7:338 Pge 3 of 5 c Surfce Pores Porous lyer Initil film Figure 2 SEM imges of the surfce nd cross section of the porous PSe lyer. The surfce with norml view () nd with 70 tilt (), nd the smple cross section with 70 tilt (c). X-ry diffrctometry investigtions did not show formtion of ny new compounds for the nodized PSe/ CF 2 /Si(111) smples in ddition to the PSe cuic structure, indicting tht the doule-lyer PSe structure mteril is still essentilly monocrystlline PSe. EDS nlysis, however, showed tht the stoichiometry of PSe for the mesoporous lyers ws severely modified. The EDS chemicl mpping sed on the L α1 2 nd M α1 rys of the elements led nd selenium showed high chemicl heterogeneity etween the porous lyer nd the underlying unmodified lyer (Figure 3). For the initil film, the tomic rtio etween P nd Se toms on the surfce ws 0.96, nd for the porous lyer, it ecme 0.07, due to the extremely high selenium enrichment. We cn ssume tht during nodic etching, the positive led ions dissolve into the electrolyte, while the negtive selenium ions redeposit on the pore wlls. As result, the mtrix remins monocrystlline PSe. The ovementioned conclusion is supported y the SEM imges in ck-scttered electrons (Figure 4), which exposed the fct tht mny of the pores on the surfce re oviously closed due to the ctive redeposition of the products of the electrochemicl rections. More specificlly, we oserved drker res round the pores, which ecme more profound with the increse of the electron energy, mening tht the yield of the ck-scttered electrons for these res decreses. The reson for tht is the increse in the proportion of inelstic collisions of electrons with the smple surfce. This is possile if the toms re not rigidly rrnged, i.e., hve the ility to move under electron impct. Hence, the electrons will pss such lrge mount of energy to toms, while they themselves will lose it nd will not e detected s ck-scttered. Therefore, this process indictes the soft, morphous regions. We cn conclude tht for the res round the pores during nodic c 2 m 2 m 2 m Figure 3 SEM imge of the porous PSe smple nd EDS mps for Se nd P. SEM imge of the porous PSe smple cross section (smple tilt is 70 ) with the drk porous surficil lyer nd the underlying initil PSe film (); EDS mp for Se (), EDS mp for P (c).

4 Zimin et l. Nnoscle Reserch Letters 2012, 7:338 Pge 4 of nm 200 nm Figure 4 SEM imge nd imge in ck-scttered electrons of the surfce of the porous PSe lyer. SEM imge of the surfce of the porous PSe lyer () nd the imge in ck-scttered electrons with 10-kV high voltge () for the sme re. etching, tomic P leves the porous lyer for electrolyte nd the development of n morphous frile Se lyer occurs. It should e noted tht the deposition of chlcogen during electrochemicl processings of PX mterils is known effect, nd Se lyers were reported to e fricted electrochemiclly [11-14]. However, we re the first to oserve the effect of selenium redeposition for the electrochemicl synthesis of porous PX lyers. Thus, in [12], where Se thin films with PSe nnoclusters were fricted electrochemiclly in nitric cidsed solutions vi nodic dissolution of PSe, the nlysis of the corresponding processes showed the rection of PSe dissolution to P 2+ ions nd Se toms, which presumly tkes plce for the porous lyers s well. The Se lyer on the pore wlls cn potentilly e very eneficil for the prcticl pplictions of the porous PSe lyers due to its modified semiconductor properties nd high sensitivity in the visile rnge, nd morphous cotings in prticulr cn hve their own specific pplictions. At the sme time, the redeposited Se film cn serve s protective, stilizing lyer y nlogy with Te cotings. For exmple, in [15], PTe hs een coted with thin sulimed porous elementl tellurium lyer, in order to prevent surfce oxidtion nd cron sorption. Oviously, further experimentl reserch is required to lern the properties of Se-coted mesoporous PSe lyers fricted in our study, ut there re ll resons to elieve tht this nnostructured mteril holds much promise for the friction of stilized porous lyer-sed devices on silicon wfer sustrtes. Conclusions In this work, our results demonstrted the potentil of the developed novel technique for the synthesis of porous PSe y nodic electrochemicl tretment of PSe/ СF 2 /Si(111) epitxil structures in n electrolyte contining glcil cetic cid nd nitric cid, with the former wetting the surfce nd stilizing the growth of the porous lyer. SEM studies showed the presence of mesopores with surfce density of pproximtely cm 2 nd n verge size of 22 nm, while EDS mpping showed the enrichment of the pore wlls with selenium, nd the ltter effect cn e useful for the prcticl pplictions of PSe porous lyers in vrious devices, s well s for the fundmentl study of the electrochemicl processes during the porous PX mteril friction. Arevitions CH 3 COOH: Acetic cid; EDS: Energy-dispersive X-ry spectroscopy; HNO 3 : Nitric cid; KOH: Potssium hydroxide; MBE: Moleculr em epitxy; PSe: Led selenide; SEM: Scnning electron microscopy. Competing interests The uthors declre tht they hve no competing interests. Authors contriutions SPZ designed the reserch. SPZ, ESG, VVN, nd FOS performed the experimentl reserch. SPZ, ESG, nd VVN nlyzed the dt. SPZ nd ESG wrote the mnuscript. All uthors red nd pproved the finl mnuscript. Authors informtion SPZ is professor t the Microelectronics Deprtment, Yroslvl Stte University. ESG is principl engineer t the Microelectronics Deprtment, Yroslvl Stte University nd reserch ssocite t the Yroslvl Brnch of the Institute of Physics nd Technology of Russin Acdemy of Sciences. VVN is senior reserch fellow t the Yroslvl Brnch of the Institute of Physics nd Technology of Russin Acdemy of Sciences. FOS is student t the Microelectronics Deprtment, Yroslvl Stte University. Acknowledgements The uthors would like to thnk E Yu Buchin (Yroslvl Brnch of the Institute of Physics nd Technology RAS) nd VM Vsin (Yroslvl Stte University) for their vlule contriutions to the nodic electrochemicl tretment experiments, nd H Zogg (ETH, Zürich) for the provided epitxil PSe smples. Electron microscopy nd chemicl micronlysis investigtions were performed t the Center for Collective Use of Scientific Equipment Dignostics of Micro- nd Nnostructures (Yroslvl). This study ws finncilly supported y the Russin Foundtion for Bsic Reserch (RFBR) (grnts Bel_ nd Ukr_).

5 Zimin et l. Nnoscle Reserch Letters 2012, 7:338 Pge 5 of 5 Author detils 1 Microelectronics Deprtment, Yroslvl Stte University, Yroslvl , Russi. 2 Yroslvl Brnch of the Institute of Physics nd Technology of Russin Acdemy of Sciences, Yroslvl , Russi. Received: 25 April 2012 Accepted: 12 June 2012 Pulished: 22 June 2012 References 1. Silor MJ: Porous Silicon in Prctice. Preprtion, Chrcteriztion nd Applictions. Weinheim: Wiley-VCH; Föll H, Leisner M, Cojocru A, Crstensen J: Mcroporous semiconductors. Mterils 2010, 3: Lng S, Tiginynu IM, Monico E, Föll H: Porous II-VI vs. porous III-V semiconductors. Phys Stt Sol C 2011, 8: Dmitriev AV, Zvygin IP: Current trends in the physics of thermoelectric mterils. Phys Usp 2010, 53: Zogg H, Kellermnn K, Alchli K, Zimin D: Opticlly pumped ledchlcogenide mid-infrred emitters on silicon sustrtes. Infrred Phys Technol 2004, 46: Zimin SP, Bogoyvlensky EA, Buchin EY, Petrkov AP, Zogg H, Zimin D: Formtion of porous nnostructured led telluride films y n nodic electrochemicl etching method. Semicond Sci Technol 2009, 24 (105008): Zimin SP, Gorlchev ES, Numov VV, Buchin EY, Zogg H: Friction of porous nnostructured led chlcogenide semiconductors for modern thermoelectric nd optoelectronic pplictions. J Phys Conf Ser 2011, 291 (012023): Zimin SP, Vsin VM, Gorlchev ES, Buchin EY, Numov VV: Investigtions of PSe lyers fter nodic electrochemicl etching y scnning electron microscopy. Phys Stt Sol C 2011, 8: Zimin S, Vsin V, Gorlchev E, Numov V, Petrkov A, Shilov S: Friction nd study of porous PTe lyers on silicon sustrtes. Phys Stt Sol C 2011, 8: Norr MK: An electrolytic polish nd etch for led telluride. J Electrochem Soc 1962, 109: Norr MK: Polishes nd etches for tin telluride, led sulfide, led selenide, nd led telluride.: Nvl Ordnnce Lortory Report NOLTR; 1963: Streltsov EA, Poznyk SK, Osipovich NP: Photoinduced nd drk underpotentil deposition of led on selenium. J Electronlyticl Chem 2002, 518: Ivnov DK, Osipovich NP, Poznyk SK, Streltsov EA: Electrochemicl preprtion of led-doped morphous Se films nd underpotentil deposition of led onto these films. Surf Sci 2003, : Alnylioglu M, Demir U, Shnnon C: Electrochemicl formtion of Se tomic lyers on Au(111) surfces: the role of dsored selente nd selenite. J Electronlyticl Chem 2004, 561: Bettini M, Brndt G, Rotter S: Tellurium coting of PTe surfces. J Vc Sci Technol 1979, 16: doi: / x Cite this rticle s: Zimin et l.: Investigtions of the pore formtion in the led selenide films using glcil cetic cid- nd nitric cid-sed electrolyte. Nnoscle Reserch Letters :338. Sumit your mnuscript to journl nd enefit from: 7 Convenient online sumission 7 Rigorous peer review 7 Immedite puliction on cceptnce 7 Open ccess: rticles freely ville online 7 High visiility within the field 7 Retining the copyright to your rticle Sumit your next mnuscript t 7 springeropen.com