Electrodeposition of Bi 1Àx Sb x Thin Films

Transcription

1 Jurnal f The Electrchemical Sciety, C131-C /2003/150 3 /C131/9/$7.00 The Electrchemical Sciety, Inc. C131 Electrdepsitin f Bi 1Àx Sb x Thin Films P. M. Vereecken,*,a S. Ren, L. Sun, and P. C. Searsn z, * Department f Materials Science and Engineering, Jhns Hpkins University, Baltimre, Maryland 21218, USA Bismuth and antimny are semimetals with a rhmbhedral crystal structure and similar lattice parameters. Bi 1 x Sb x allys exhibit unusual electrnic prperties and are f interest fr thermelectric and magnetelectrnic devices. In this paper, we shw that Bi 1 x Sb x allys can be electrdepsited frm acidic chlride-based slutins and that a hmgenus slid slutin is btained acrss the entire cmpsitin range. We shw that bulk depsitin is preceded by mnlayer depsitin at ptentials negative t the equilibrium ptential. The depsitin/stripping reactin fr bismuth is mre reversible than fr antimny leading t a small increase in the bismuth cncentratin in films cmpared t the cncentratin in slutin at lw Sb III cncentratins. Thin films have a preferred 012 texture and the interplanar spacing is prprtinal t the film cmpsitin accrding t Vegard s rule. The grain size fr thin films increases frm abut 1 m fr pure Bi t several micrmeters fr pure Sb The Electrchemical Sciety. DOI: / All rights reserved. Manuscript submitted March 15, 2002; revised manuscript received September 11, Available electrnically January 31, Bismuth and antimny are semimetals with unusual thermal, electrical, and magnetic transprt prperties. Bth elements have a rhmbhedral crystal structure with similar lattice parameters and frm a slid slutin ver the entire cmpsitin range. Interest in these elements is largely due t their small carrier effective masses and hence high mbilities. 1,2 Bismuth exhibits very lw carrier cncentratins and lng carrier mean free paths 3 leading t a very large magnetresistance effect in bulk single crystals 4,5 r thin films grwn by mlecular beam epitaxy 6 r electrdepsitin Bi 1 x Sb x allys are semimetals ver mst f the cmpsitinal range but are semicnductrs fr 0.07 x 0.22, with a maximum band gap f abut 30 mev at abut x There have been relatively few studies f electrdepsitin f Bi, Sb, r BiSb thin films e.g., Ref Bth Bi and Sb can be depsited frm chlride r perchlrate slutins at sufficiently lw ph s that the frmatin f the xyanin can be avided. In this paper, we reprt n the depsitin f Bi 1 x Sb x ally thin films frm acidic chlride slutin. We shw that a hmgenus slid slutin can be frmed acrss the entire cmpsitin range. Experimental Electrchemical measurements were perfrmed under ambient cnditins in a three-electrde cell with a platinum cunter electrde and a Ag/AgCl 3M NaCl (aq) ) reference electrde U eq V saturated hydrgen electrde, SHE psitined clse t the wrking electrde using a Luggin capillary. The wrking electrde was a 200 nm thick plycrystalline gld film evaprated nt a Si 100 wafer with a 15 nm Cr adhesin layer. The electrde area was 0.2 cm 2. Electrical cntact t the gld films was made with cllidal silver paste Electrn Micrscpy Sciences. The slutin used fr depsitin f Bi 1 x Sb x allys was an aqueus slutin f (0.1 y) M BiCl 3 y M SbCl M HCl 0.03 M ethylenediaminetatracetic acid EATA with the SbCl 3 cncentratin, y, ranging between 0 and 0.1 M. Prir t each experiment the Au surface was etched with 10 vl % HNO 3 fr 30 s, rinsed with distilled, deinized DI water, rinsed with 2.4 M HCl, and finally rinsed with the (0.1 y) M BiCl 3 y M SbCl 3 slutin. The depsitin efficiency was determined by depsiting films at cnstant ptential fr 50 s and then stripping by scanning the ptential t 0.4 V at a scan rate f 10 mv s 1. The depsitin efficiency was determined frm the rati f the stripping charge t the depsitin charge fr slutins with 0% Sb, 18% Sb, 70% Sb, and 100% Sb. a Present address: IBM T. J. Watsn Research Center, Yrktwn Heights, New Yrk * Electrchemical Sciety Active Member. z searsn@jhu.edu Bi 1 x Sb x films fr physical characterizatin were electrdepsited at 7 ma cm 2 fr 200 s using a tw-electrde arrangement. The substrate was a 100 nm thick gld film sputter depsited n a Si 100 wafer with a presputtered Cr adhesin layer. The depsitin charge f 1.4 C cm 2 crrespnds t a film thickness f 1.0 m fr Bi and 0.88 m fr Sb, assuming 100% depsitin efficiency. The area f the film was 1.43 cm 2. After depsitin, the films were thrughly rinsed with 1 M HCl, rinsed with distilled, DI water, and finally dried in a nitrgen flw. X-ray diffractin XRD measurements were perfrmed n a Philips X pert X-ray diffractmeter using a Cu K surce. All scans were perfrmed with a step width f 0.01 and at a scan rate f 0.02 s 1. The instrument was calibrated befre each measurement using the Si 004 peak. The cmpsitin f the depsited films was determined frm wavelength dispersive X-ray spectrscpy WDS using a Jel 8600 Superprbe. Cmpsitins were btained using a 20 m diam beam taking the average f ten measurements at different lcatins n the film. Speciatin Figure 1 shws the cncentratin f the relevant Bi III and Sb III chlride cmplexes vs. the mle fractin f Sb in slutin fr a ttal metal in cncentratin Bi(III) Sb(III) f 0.1 M. The slutin cmpsitin was calculated frm the stability cnstants n fr BiCl 3 n n, 19 and SbCl 3 n n, 20 summarized in Table I. The details are prvided in the Appendix. The cncentratin f uncmplexed Bi 3 and Sb 3 ins is negligibly small; fr a 0.1 M BiCl 3 slutin, the Bi 3 cncentratin is M; fr a 0.1 M SbCl 3 slutin, the Sb 3 cncentratin is M. Figure 1b shws the percentage f each Bi III and Sb III cmplex with respect t the ttal Bi III r Sb III cncentratin vs. the mle fractin f Sb in slutin. This figure illustrates that the hexachlride and pentachlride cmplexes are the dminant bismuth species in slutin at abut 60 and 40%, respectively. The pentachlride, hexachlride, and tetrachlride cmplexes are the main antimny species at 50, 25, and 20%, respectively. In all cases, the percentages are almst independent f the Sb/Bi rati in slutin. Fr the case f antimny, even at lw ph, the cncentratin f the antimnyl catin SbO must als be cnsidered 21 SbO aq 2H aq 3e Sb s H 2 O The frmatin f the xyin frm uncmplexed Sb 3 ins and water is given by Sb 3 aq H 2 O SbO aq 2H aq 1 2

2 C132 Jurnal f The Electrchemical Sciety, C131-C related t the standard ptentials f the uncmplexed Bi 3 and Sb 3 ins, U M 3 /M, thrugh their respective stability cnstants n and at 298 K U 3 n MCln /M U M 3 /M RT 3F lg n U M 3 /M lg n 4 Reliable values f the standard ptentials fr the Bi 3 /Bi and Sb 3 /Sb cuples are nt knwn, since cmplexatin and hydrlysis generally results in very small cncentratins f uncmplexed Bi 3 and Sb 3 ins. 19,22,23 As a result, the Bi 3 /Bi and Sb 3 /Sb redx ptentials are btained indirectly thrugh measurements f their cmpunds and subsequent thermdynamic calculatins. Fr the Bi 3 /Bi equilibrium Bi aq 3e Bi s 5 values f the standard ptential reprted in the literature are in the range 0.20 t 0.32 V SHE. 19,21,22,24 Fr the BiCl 4 /Bi equilibrium BiCl 4 aq 3e Bi s 4Cl aq 6 the standard ptential U BiCl4 /Bi, calculated indirectly frm the xychlride cmpund, is 0.16 V SHE. 19,22,24 The standard ptential fr the Bi 3 /Bi cuple calculated frm the standard ptential f the tetrachlr cmplex and its stability cnstant 4 see Table I using Eq.4isU (Bi 3 /Bi) 0.28 V SHE. Fr the Sb 3 /Sb equilibrium Sb 3 aq 3e Sb s 7 Figure 1. a Cncentratin f relevant Bi III and Sb III chlride cmplexes in slutin vs. the mle percent f Sb III in slutin, fr a ttal metal in cncentratin Bi(III) Sb(III) f 0.1 M. b Percentage f Bi III and Sb III chlride cmplexes with respect t the ttal Bi III r Sb III cncentratin vs. the mle percent f Sb III in slutin. Nte that the cncentratins f Sb 3,Bi 3, and SbO are negligible. The equilibrium cnstant, K eq, fr Reactin 2 can be determined frm the standard ptential fr the Sb 3 /Sb redx cuple U Sb 3 /Sb 0.26 V SHE and the Nernst equatin fr Reactin 1 with U SbO /Sb V SHE K eq SbO H 2 Sb ml 2 L 2 Fr an H cncentratin f 2.4 M, the SbO cncentratin is M fr a 0.1 M SbCl 3 slutin. This cncentratin is negligibly small and can be neglected. The standard ptentials fr the bismuth chlride and antimny chlride cmplexes, dented as U 3 n MCIn /M with M Bi r Sb, are 3 values fr the standard ptential are nt available in the literature. Fr the antimny tetrachlride cmplex SbCl 4 aq 3e Sb s 4Cl aq the standard ptential U SbCl4 /Sb 0.17 V SHE. 19,23 Frm the value fr U SbCl4 /Sb and the stability cnstant 4 see Table I, the standard ptential fr uncmplexed Sb 3 ins determined frm Eq. 4isU Sb 3 /Sb 0.26 V SHE. The equilibrium ptentials fr bismuth and antimny fr any slutin cmpsitin can be determined either frm the cncentratin f uncmplexed M 3 ins r frm the cncentratin f the chlride cmplexes MCl 3 n n in slutin using the Nernst equatin. Fr the uncmplexed M 3 ins we btain U eq U M 3 /M lg M 3 Fr the chlride cmplexes MCl n 3 n we btain U eq U 3 n MCn /M lg MCl n 3 n Cl n The standard ptentials fr the different BiCl n 3 n and SbCl n 3 n species calculated frm Eq. 4 are summarized in Table II. The stan- Table I. Stability cnstants n MCl n nà3 Õ M 3 Cl À n. M Bi Sb

3 Jurnal f The Electrchemical Sciety, C131-C C133 Table II. Standard ptentials U 3Àn MCln ÕM V vs. SHE fr uncmplexed metal ins n 0 and metal chlride cmplexes fr 1 n 6 determined frm Eq. 4 and the stability cnstants given in Table I. MCl 3Àn n 3e À M ncl À. n 0 n 1 n 2 n 3 n 4 n 5 n 6 U n (BiCln /Bi) U n (SbCln /Sb) dard ptentials fr the chlride cmplexes f bth elements with n 2 are very clse indicating that bismuth and antimny will cdepsit as an ally even at lw verptentials. In summary, well defined values fr U M 3 /M and U 3 n MCln /M are nt available. In the literature, the standard ptentials fr the tetrachlr cmplexes are U BiCl4 /Bi 0.16 V SHE determined indirectly and U SbCl4 /Sb 0.17 V SHE. The standard ptentials fr the Bi 3 /Bi and Sb 3 /Sb cuples btained frm the tetrachlr cmplexes using the relevant stability cnstants are U Bi 3 /Bi 0.28 V SHE and U Sb 3 /Sb 0.26 V SHE. Results and Discussin Figure 2 shws current-ptential curves fr gld in (0.1 y) M BiCl 3 y M SbCl M HCl 0.03 M EDTA, with Sb III mle fractins frm 0% (y 0) t 100% (y 0.1 M). All slutins exhibit characteristic bulk depsitin and stripping peaks. The separatin between the peaks increases with increasing Sb III cncentratin in slutin indicating that depsitin and stripping becme mre irreversible with increasing Sb III cncentratin. The decrease in the peak current density with increasing Sb III cncentratin is als indicative f increasing irreversibility. 25 Underptential depsitin was bserved between the pen-circuit ptential OCP 0.5 V vs. Ag/AgCl and 0.1 V fr all slutins, but is nt cnsidered further here. The equilibrium ptentials fr the Bi III /Bi and Sb III /Sb cuples can be experimentally identified frm the ptential f zer current in the reverse scan. The standard ptentials can subsequently be determined using Eq. 9 and 10. Figure 3 shws the ptential f Figure 2. Current/vltage curves fr (0.1 y) M BiCl 3 y M SbCl M HCl 0.03 M EDTA, with Sb III mle fractins frm 0% (y 0) t 100% (y 0.1 M). The mle percent f Sb III in the slutin is indicated in the figure. The scan rate was 10 mv s 1. Figure 3. Zer-current ptential, U i 0, btained frm the reverse scan in current/vltage curves fr slutins with (0.1 y) M BiCl 3 y M SbCl 3 with y varying frm 0 t 0.1 M. The dashed lines shw the equilibrium ptentials fr the Bi III /Bi and Sb III /Sb cuples vs. the mle percent f Sb III in slutin calculated a frm the experimental equilibrium ptentials fr the pure cmpnents (U eq,bi 0.16 V; U eq,sb 0.12 V) and b frm the partial standard equilibrium ptentials U Bi U Sb U Bi1 x Sb x V Ag/AgCl determined frm a best fit f U i 0 fr the mixed slutins t U mix. The slid line crrespnds t the mixed ptential, U mix, fr the tw cmpnent mixed slutin calculated assuming a rule f mixtures fr the pure cmpnents in a and b.

4 C134 Jurnal f The Electrchemical Sciety, C131-C zer current in the reverse scan, U i 0, btained frm slutins with (0.1 y) M BiCl 3 y M SbCl 3. In 0.1 M BiCl 3 slutin, U i 0,Bi 0.16 V vs. Ag/AgCl (U eq,bi V vs. SHE. Fr the 0.1 M SbCl 3 slutin, U i 0,Sb 0.12 V vs. Ag/AgCl (U eq,sb V vs. SHE. Frm Eq. 9 it fllws that U Bi 3 /Bi 0.25 V SHE and U Sb 3 /Sb 0.24 V SHE, clse t the literature values btained frm the tetrachlride cmplexes. The equilibrium ptentials fr the Bi III /Bi and Sb III /Sb cuples in mixed slutins (U eq,bi and U eq,sb,) can nw be calculated fr all slutin cmpsitins frm the values f U M 3 /M using Eq. 9 and the speciatin data. The dependencies f U eq,bi and U eq,sb n Sb III cncentratin in slutin, calculated frm the standard ptentials fr pure Bi and Sb slutins, are shwn in Fig. 3a. Fr a simple rule f mixtures, the equilibrium ptential fr the tw cmpnent system is given by U mix 1 U eq,bi xu eq,sb 11 where 0 x 1. The variatin f U mix with slutin cmpsitin is als shwn Fig. 3a. It can be seen that there is abut a 20 mv difference between the experimentally measured ptentials fr the mixed slutins (U i 0,BiSb ) and U mix calculated frm the zer current ptentials fr the pure slutins. Hwever, fitting the experimental U i 0 values fr the mixed slutins t U mix by adjusting the equilibrium ptentials fr Bi and Sb (U eq,bi and U eq,sb ) resulted in a gd crrelatin between U i 0 and U mix, as demnstrated in Fig. 3b. The fit shwn in Fig. 3b was btained fr U Bi U Sb U Bi1 x Sb x 0.26 V SHE 12 where U Bix Sb 1 x is the standard ptential fr the ally. The rigin f the apprximately 20 mv difference between the equilibrium ptentials fr the mixed slutins and the equilibrium ptentials fr the pure slutins is nt knwn. Substitutin f the equilibrium ptentials int Eq. 11 using the Nernst equatin then yields an expressin fr the equilibrium ptential f the Bi x Sb 1 x ally. At 298 K U Bi1 x Sb x U Bi1 x Sb x lg Bi 3 1 x Sb 3 x 3 13 Equatin 13 represents the Nernst equatin fr the electrchemical frmatin f a Bi x Sb 1 x ally frm uncmplexed Bi 3 and Sb 3 ins 1 x Bi 3 xsb 3 Bi 1 x Sb x 14 The bulk depsitin peak seen in the current-ptential curves Fig. 2 was in all cases preceded by ne r tw smaller reductin peaks, as shwn in Fig. 4. Figure 4a shws the reductin peak P a preceding the main depsitin peak in 0.1 M SbCl 3 slutin and Fig. 4b shws the reductin peak P b preceding the main depsitin peak in 0.1 M BiCl 3 slutin. The ptential dependences f the depsitin (P bulk, P a, and P b ) peaks and stripping (P strip ) peaks n mle fractin f Sb III in slutin are shwn in Fig. 5. The ptentials f the prepeaks P a and P b were essentially independent f Sb III cncentratin. The peak current density assciated with P a was abut 0.5 ma cm 2, r abut tw rders f magnitude smaller than the current density f the bulk depsitin peak. This peak was present at all slutin cmpsitins except at lw Sb III cncentratin and is hence attributed t Sb III reductin. Prepeak P b immediately preceded the bulk depsitin peak but was nly bserved fr slutins with high Bi III cncentratin 70% Bi III and is attributed t the reductin f Bi III. The peak current density is this case was abut 5 macm 2, abut ne rder f magnitude larger than P a, and ne rder f magnitude smaller than the bulk depsitin peak. Figure 4. I-V curves fr a 0.1 M SbCl 3 slutin shwing prepeak P a and b 0.1 M BiCl 3 slutin shwing prepeak P b. The scan rate was 10 mv s 1. When the scan directin was reversed at a ptential slightly psitive t the nset f the bulk depsitin peak, a distinct stripping peak was bserved in the reverse scan, indicating that prepeaks P a and P b are assciated with depsitin. The area under the prepeaks was in the range mc cm 2, clse t the values fr a clse-packed 001 mnlayer f Bi 0.54 mc cm 2 and Sb 0.60 mc cm 2. Figure 5. The ptentials fr the bulk depsitin peak (P bulk ), prepeaks (P a and P b ), and stripping peaks (P strip ) vs. mle percent f Sb III in slutin. All data btained frm I-V curves at a scan rate f 10 mv s 1.

5 Jurnal f The Electrchemical Sciety, C131-C C135 Figure 6. Peak ptentials lg fr tp 0% Sb III, middle 50% Sb III, and bttm 100% Sb III. Figure 7. a Current density f the bulk depsitin peak vs. 1/2 fr 0% Sb III, 50% Sb III, and 100% Sb III. b Current density f the prepeak P a vs. fr 50% Sb III and 100% Sb III. c Current density f the prepeak P b vs. fr 0% Sb III ; the slid line crrespnds t 0.7. Als shwn is a plt f i p (U p U ea ) 2 vs. fr prepeak P b. The separatin between the bulk depsitin peak and the stripping peak is abut 90 mv fr the 0.1 M BiCl 3 slutin 0% Sb III and 250 mv fr the 0.1 M SbCl 3 slutin 100% Sb III. This illustrates the higher reversibility f the bismuth depsitin/stripping reactin and the lwer reversibility f the antimny depsitin/ stripping reactin. The bulk depsitin and stripping peaks are almst independent f ptential fr Sb III cncentratins 30%, shwing that the higher reversibility f the bismuth depsitin/ stripping reactin is maintained up t abut 30% Sb III. Fr slutin cmpsitins 30% Sb III, the peak separatin between the main reductin peak and the stripping peak becmes larger with increasing Sb III cncentratin due t the lwer reversibility f the depsitin/stripping reactin. Figure 6 shws the scan rate dependence f the ptentials assciated with the bulk depsitin peak, the prepeaks, and the striping peak fr 0.1 M BiCl 3 0% Sb III, 0.05 M BiCl M SbCl 3 50% Sb III, and 0.1 M SbCl 3 100% Sb III. The increasing reversibility f the metal in cuples with decreasing Sb III cncentratin are again clearly seen. Accrdingly, the scan rate dependence f the bulk depsitin peak decreased frm 30 mv/decade fr 100% Sb III t 20 mv/decade fr 50% Sb III t 10 mv/decade fr 0% Sb III. Figure 7 shws the dependence f the current density assciated with the bulk depsitin peak, the prepeaks, and the striping peak in slutins with 0% Sb III, 50% Sb III, and 100% Sb III, n scan rate. The peak current density assciated with the bulk depsitin peak is prprtinal t the square rt f the scan rate, as expected fr a diffusin-limited grwth prcess. The peak current density fr prepeak P a assciated with Sb depsitin was prprtinal t the scan rate, as shwn in Fig. 7b, cnsistent with mnlayer frmatin either by randm adsrptin r by irreversible tw-dimensinal 2D nucleatin and grwth. 25

6 C136 Jurnal f The Electrchemical Sciety, C131-C Figure 8. Depsitin efficiency fr films depsited at cnstant ptential in 0% Sb III, 18% Sb III, 70% Sb III, and 100% Sb III slutin. The scan rate dependence f the current density fr prepeak P b, assciated with Bi depsitin, exhibited an expnent f 0.7, i.e., between 0.5 and 1.0, as shwn in Fig. 7c. Such behavir is cnsistent with mdels fr reversible mnlayer frmatin by 2D nucleatin and grwth. 25 The peak current density i p and peak ptential U p fr this prcess are given by i p K 2 q a m 15 U p U a a K 16 where q m is the charge assciated with mnlayer depsitin, is the scan rate, U a is the equilibrium ptential fr adsrptin, and a is cnstant assciated with nucleatin. b The cnstant K is given by K lg N 0 N T k n k 2 g n 6 a Figure 9. Cncentratin f Sb in films vs. the mle percent f Sb III in slutin. In all cases, 1.4 C cm 2 films were depsited at a cnstant current density f 7 ma cm 2. The antimny cncentratin in the film was determined frm WDS measurements. A plt f (U p U nset ) 1 vs. lg fr P b was linear frm which we btained a Frm the slpe in Fig. 7c and taking a 0.45 we btain q m 0.68 mc cm 2, clse t the value fr a clsepacked 001 mnlayer f Bi 0.54 mc cm 2. Frm the analysis f the tw prepeaks, we cnclude that bulk depsitin is preceded by a 2D nucleatin and grwth prcess. Hwever, this 2D prcess ccurs at ptentials negative t the equilibrium ptential and hence is nt underptential depsitin. Grwth f Bi n GaAs 26 and Bi n Si 27 frm the vapr phase als prceeds thrugh 2D layer frmatin fllwed by 3D island grwth. An initial 2D prcess preceding 3D depsitin at ptentials negative t the equilibrium ptential has been reprted fr electrdepsitin f Ag n Ag. 28 Figure 8 shws the depsitin efficiency fr films depsited at cnstant ptential. In all cases the depsitin efficiency is between 95 t 100% fr ptentials negative t abut V Ag/AgCl. The pint at which the efficiency becmes clse t 100% crrespnds t ptentials between the pre-peak and the bulk depsitin peak in the current-ptential curves. where N 0 is the number f active sites, N T the ttal number sites per unit area, k n is the standard rate cnstant fr nucleatin, and k g is the standard rate cnstant fr grwth. Since K is dependent n scan rate, the peak ptential and peak currents are als dependent n scan rate. Substitutin f K frm Eq. 16 int Eq. 15 gives i p U p U a aq m 18 Fr a reversible prcess, the standard adsrptin ptential can be apprximated by the nset ptential fr depsitin. Figure 7c shws the scan rate dependence f i p (U p U ea ) 2 fr prepeak P b substituting the nset ptential fr the adsrptin ptential. The linear dependence f i p (U p U ea ) 2 n scan rate is cnsistent with Eq. 18. The slpe in Fig. 7c crrespnds t 0.368aq m. The parameter a can be btained frm the dependence f the peak ptential n scan rate. Cmbining Eq. 16 and 17 we btain U p U a a a 2,303 a lg N 0 k n k g 2 N T n 6 a a lg 19 b a 2 r 2 2 /nekt where r is the island radius, is the energy per unit area, n is the number f electrns, e is the charge n the electrn, k is the Bltzmann cnstant, and T is temperature. 25 Figure 10. XRD patterns fr a series f Bi 1 x Sb x displaced fr clarity. films. The curves are

7 Jurnal f The Electrchemical Sciety, C131-C C137 Figure 11. Crystal structure f Bi and Sb. a Tw rhmbhedral unit cells n hexagnal axes. Fr Bi: a nm, b The hexagnal primitive cell has atms n the crners f the parallelepiped as well as atms at the 2/3,1/3,1/3 and 1/3,2/3,2/3 psitins. Fr Bi: a b nm, c nm, 90, 120 ; fr Sb: a b nm, c nm, 90, 120. The 012 plane is als shwn. c Nrmal view t the 012 plane, illustrating the tw-fld symmetry in this plane. Figure 13. Scanning electrn micrscpe plan view images fr Bi 95 Sb 5, Bi 70 Sb 30, and Bi 10 Sb 90 films. Figure 12. The lattice spacing in the 012 directin determined frm the diffractin peak vs. the Sb cncentratin in the film. Figure 14. Crss-sectinal scanning electrn micrscpe image f a Bi 95 Sb 5 film illustrating a characteristic clumnar micrstructure.

8 C138 Jurnal f The Electrchemical Sciety, C131-C Thin film characterizatin. Figure 9 shws the mle fractin f Sb in films depsited at 7 ma cm 2 vs. the mle fractin f Sb III in slutin. The Sb cncentratin in the film is slightly lwer than predicted by the linear dependence at lw Sb III cncentratins in slutin 30% Sb III and slightly higher at larger Sb III cncentratins in slutin. The crssver at 30% Sb III crrespnds t the limit f Bi-dminated reversible depsitin and stripping 30% Sb III. At higher cncentratins f Sb III, the depsitin reactin is dminated by the mre irreversible Sb depsitin/stripping reactin see Fig. 5. Figure 10 shws XRD patterns fr a series f Bi 1 x Sb x films. In all cases, the films are highly textured exhibiting the characteristic 012 and 024 peaks fr the hexagnal crystal structure f Bi and Sb. Similar results have been reprted fr the electrdepsitin f Bi n Au 8 and n GaAs 9,10 frm acidic Bi(NO 3 ) 3 slutin. Bismuth and antimny have a rhmbhedral crystal structure. Fr cnvenience, hwever, the crystal structures are usually represented n hexagnal axes, with a trignal c axis alng the 001 directin and the twfld r binary a and b axes nrmal t the trignal directin and 120 apart frm each ther. 1 Figure 11a shws tw rhmbhedral unit cells n the hexagnal axes. The hexagnal primitive cell Bi: a b nm, c nm, 90, 120 ; Sb: a b nm, c nm, 90, 120 ) shwn in Fig. 11b has atms n the crners f the parallelepiped as well as atms at the 2/3,1/3,1/3 and 1/3,2/3,2/3 psitins. The 2D unit cells fr the hexagnal clsepacked 001 planes f Bi and Sb are diamnd shaped with sides f and nm, respectively. 1 The clse-packed 001 planes are arranged in bilayers alng the 001 directin 1 s that crystals are easily cleaved perpendicular t the trignal axis. As described previusly, the depsitin f a clse-packed 001 mnlayer crrespnds t 0.54 mc cm 2 fr Bi and 0.60 mc cm 2 fr Sb. The 012 plane is als shwn in Fig. 11b. Figure 11c shws a nrmal view t the 012 plane, illustrating the tw-fld symmetry in this plane, with tw layers f atms ne slightly abve and ne slightly belw the plane. The depsitin f an 012 mnlayer crrespnds t 0.45 mc cm 2 fr Bi and 0.50 mc cm 2 fr Sb. Figure 12 shws the lattice spacing in the 012 directin determined frm the diffractin peak. This figure illustrates that the lattice spacing increases mntnically frm Sb t Bi accrding t Vegard s rule, cnsistent with the frmatin f a hmgeneus slid slutin. Similar results have been reprted fr Bi 1 x Sb x films depsited at cnstant current. 18 Fr all cmpsitins the films were matte gray and exhibited faceted grains ranging frm abut 1 m in size fr pure Bi t several micrmeters fr pure Sb films. Figure 13 shws scanning electrn micrscpe plan view images fr Bi 95 Sb 5, Bi 70 Sb 30, and Bi 10 Sb 90 films. These images illustrate the prgressive increase in grain size with increasing Sb cntent. These results are cnsistent with the faster kinetics fr bismuth depsitin reactin which wuld be expected t lead t higher cluster density and hence smaller grain size. Figure 14 shws a crss-sectinal image f a Bi 95 Sb 5 film illustrating a characteristic clumnar micrstructure. Cnclusins Hmgeneus Bi 1 x Sb x allys can be electrdepsited frm acidic chlride-based slutins. Bulk depsitin is preceded by mnlayer depsitin at ptentials negative t the equilibrium ptential. The depsitin/stripping reactin fr bismuth is mre reversible than fr antimny leading t a small increase in the bismuth cncentratin in films cmpared t the cncentratin in slutin at lw Sb III cncentratins. Thin films have a preferred 012 texture and the interplanar spacing is prprtinal t the film cmpsitin accrding t Vegard s rule. The grain size fr thin films increases frm abut 1 m fr pure Bi t several micrmeters fr pure Sb. Acknwledgments This wrk was supprted by the JHU MRSEC NSF grant number DMR Jhns Hpkins University assisted in meeting the publicatin csts f this article. Appendix Fr the determinatin f speciatin, the fllwing species were cnsidered: Bi 3, BiCl 4, BiCl 5 2, BiCl 6 3,Sb 3, SbCl 3, SbCl 4, SbCl 5 2, SbCl 6 3, and Cl. The species BiCl 2, BiCl 2, BiCl 3, SbCl 2, and SbCl 2 were nt cnsidered since their stability cnstants are neglible. The stability cnstants fr bismuth and antimny are given by n BiCl n n 3 Bi 3 Cl n n * SbCl n n 3 Sb 3 Cl n A1 A2 The values fr the relevant stability cnstants are given in Table I. In additin t the equilibria fr the bismuth and antimny species we have five mass balance equatins 3c BiCl3 c HCl 3c SbCl3 Cl 4 BiCl 4 5 BiCl BiCl SbCl 3 4 SbCl 4 5 SbCl SbCl 6 3 c BiCl3 BiCl 4 BiCl 5 2 BiCl 6 3 Bi 3 4 Cl 4 5 Cl 5 6 Cl 6 c SbCl3 SbCl 3 SbCl 4 SbCl 5 2 SbCl 6 3 Sb 3 3 * Cl 3 4 * Cl 4 5 * Cl 5 6 * Cl 5 c BiCl3 c SbCl3 0.1 M c HCl 2.4 M A3 A4 A5 A6 A7 Substitutin f A-4, A-5, A-6, and A-7 int A-3 gives the relatinship between the chlride in cncentratin in slutin, Cl, and the ttal cncentratin f bismuth chlride, c BiCl3 and antimny chlride c SbCl3 added t the slutin Cl c SbCl3 4 4 Cl Cl Cl 6 4 Cl 4 5 Cl 5 6 Cl 6 c SbCl3 3 3 * Cl * Cl * Cl * Cl 6 3 * Cl 3 4 * Cl 4 5 * Cl 5 6 * Cl 6 0 A8 Fr each slutin cmpsitin (c SbCl3 ) the chlride cncentratin Cl was determined frm the zer intercept f Eq. A-8 with chlride cncentratin Cl. Subsequently, the cncentratins f free bismuth ins Bi 3 and free antimny ins Sb 3 were calculated frm Eq. 4 and 5. 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