_____~ ~ Synthesis, properties and performances of electrodeposited bismuth telluride films Pierre Magri, Clotilde Boulanger and Jean-Marie Lecuire Laboratoire d'Electrochimie des Matkriaux, URA CNRS 158, Uniuersitk de Metz, Ile du Saulcy, 57045 Metz Cedex, France Bismuth telluride alloy films of uniform thickness have been successfully prepared by electrodeposition from a solution containing Bi3+ and HTe02+ ions in 1 mol dm-3 nitric acid (pH=O) on stainless steel. The electrodeposited films are monophasic and exhibit a polycrystalline structure (R3rn).The film composition is dependent on the electrolyte composition and the current density. The electrical properties of the electrodeposited samples have been determined. The obtained films are n-type semiconductors with high carrier concentration.Thermoelectricity is the phenomenon which results from the direct conversion of heat into electrical energy (or vice versa). Since 1950, much effort has been devoted to the use of this phenomenon for applications in static coolers. The relative efficiency of a thermoelectrical material is measured in terms of the figure of merit, Z, which is defined by: 2=a2/p/l where a is the Seebeck coefficient, p the electrical resistivity and 2. the thermal conductivity. In order to improve values of 2,the material should be a good electrical conductor, a poor thermal conductor and should have a large thermoelectric power. Mildly degenerate semiconductors have the best combi- nations of these intrinsic properties.' Bismuth telluride (Bi,Te,) and its derivative compounds are considered to be the best materials for use in thermoelectric refrigeration at room temperature.',2 These materials are generally synthesised by directional crystallisation powder metallurgy process7,* or evaporation However, these tech- niques do not readily lend themselves to the production of large-area thermoelements. Electrochemical deposition may provide an alternative process to these classical methods.Furthermore, electrodeposition techniques have been success- fully employed for preparation of chalcogenide semiconductors, e.g. CdS, CdSe, CdTe, InSe, PbTe.13,14 With regard to bismuth chalcogenides, thin films of the sulfide (Bi2S3) have been prepared by anodisation of bismuth metal in polysulfide solu- tions'' and by direct electrodeposition from non-aqueous media.16 We have previously shown that an electrodeposition process leads to the formation of textured bismuth telluride films."7'* The process is based on the electroreduction of telluride ions, in the presence of bismuth salts, according to the reaction: 3TeIV+2 Bi"' +18 e --+Bi2Te3 Takahashi et al." have also obtained Bi-Te alloy films in potentiostatic conditions. The present work concerns in particular the definition of the optimum conditions for a galvanostatic process of bismuth telluride films, and the electrical characterisation of the syn- thesised compounds.Analytical Study under Voltamperometric Conditions Experimental details The reaction analysis was realised by voltamperometric tech- niques on a rotating platinum disk electrode using a classical three-electrode device in a thermostatted cell (25 "C) and under an inert atmosphere (argon HP).All the potentials were measured and expressed by reference to the aqueous KCl saturated calomel electrode (SCE). The auxiliary electrode was a platinum disk. The linear sweep voltammograms were obtained using a Tacussel PJT 24.1 potentiostat, IMT.l interface, Voltamaster 2 software and an IBM-compatible machine. Electrolytes The electrolytes were prepared in solution with deionized water. To ensure the stability and the solubility of bismuth(m) solutions, the selected solvent was 1 mol dmP3 aqueous HNO,. The Bi"' solutions were obtained by dissolution of Bi(N03),.5H20 (analytical grade).The concentrations were determined by EDTA volumetry.20 The TetV solutions were prepared from the reaction of nitric acid on high-purity elemental tellurium. Under these acidic conditions, tellurium was in the form of HTe02+ (telluryl ion). The concentrations were controlled by an oxido-reduction titration based on TeIV oxidation by a titrated CrV' solution and on a back titration of this reagent (in excess) against an iron@) solution.20 The Bi3 + and HTe02+ electrolyte mixtures were obtained from the above solutions, and the Bi3+ :HTe02+ ratios were varied to determined values. Results The aim of the analytical study was to investigate the behaviour of Bi3+, HTe02+ and mixtures of them during cyclic voltammetry.Bi3+ solution. The voltamperogram of the Bi3 + solution is shown in Fig. 1. During a cathodic exploration, the obtained curve shows a reduction wave at -150 mV due to the reduction of Bi3+ to bismuth metal. This wave presents a diffusion-limited current which increases with increasing Bi"' concen-tration. A shift towards more negative potentials causes solvent reduction from -350mV. The X-ray identification of the electrodeposited product after this cathodic scan confirms the formation of bismuth metal. The deposit is oxidisable by a reverse sweep. Indeed, an anodic scan gives evidence of a peak (E=-50 mV) which corresponds to bismuth anodic stripping. The anodic :cathodic charge ratio is 0.82.The cathodic charge excess is due to hydrogen evolution which was not observed if the cathodic scan was stopped at -250 mV. HTe02+ solution. The voltamperogram of the HTe02+ solution is shown in Fig. 2. In contrast with the bismuth behaviour, the tellurium system appears to be a slower system. J. Mater. Chem., 1996, 6(5), 773-779 773 6001 i 1 L 450 -a 30°-3.. \.r--150 0 -1SOl " " " " " ' " ' -500 0 500 loo0 EImV (vs SCE) Fig. 1 Voltamperogram in 1mol dm HNO, of a BI3+ solution, [BI3+] =3 5 x 10 mol dm Working electrode, Pt, surface area, 3 14 mm', rotation rate, 600 rpm, potential sweep rate, 60 mV min 6001 I 1 450 -300 -Te' +TP + 4e a5 150-J1+ I -600 -400 -200 0 200 400 600 800 lo00 EImV (vsSCE) Fig.2 Voltamperogram in 1rnol dm HNO, of an HTe02+ solution, [HTeOJ =3 5 x 10 mol dm Working electrode, Pt, surface area, 3 14 mm', rotation rate, 600 rpm, potential sweep rate, 60 mV min In this solution, the solvent reduction occurs at ca -350 mV The reduction process due to the deposition of elemental tellunum occurs at a potential similar to that of bismuth but the oxidation peak is observed at ca +600mV The syn- thesised film was removed from the electrode, ground and analysed by X-ray diffraction The obtained pattern shows that the product is crystalline tellurium In this case, the anodic ca-thodic charge ratio is 092 The cathodic charge excess is due to hydrogen evolution which was not observed if the cathodic scan was stopped at -250 mV The deposition potentials of each element are very similar, which suggests that codeposition should be possible and should lead to alloy formation after an appropriate thermal treatment Mixture solutions.We carried out similar experiments in solutions containing Bi3+ and Te4+ in different ratios The investigations led to proton reduction (-350 mV) and, for the reverse scans, to the oxidation of electrodeposited material In order to obtain Bi2Te3, the Bi3+ HTe02+ ratio was firstly fixed at 2 3 The voltammetric curve for such a solution (Fig 3) evidences only one signal in the form of a reduction wave with deposit formation = -50 mV), and one anodic peak (E= +400 mV) which is well defined The voltamperog- ram (Fig 4) for a solution corresponding to Bi3+ HTe02+ = 4 3 presents two successive reduction waves that are not well defined We can also see two anodic stripping peaks (El=+ 180 mV, E2= +400 mV) If the cathodic exploration is stopped at -200mV, the first peak disappears, and the predominant signal situated at +400mV remains For a solution containing an excess of tellurium (Bi3+ HTeO,' = 1 2), the current-potential curve (Fig 5) shows a cathodic wave A reversal of the potential scanning reveals not only the same signal (E = +400 mV) but also a shoulder which could 774 J Muter Chem , 1996,6(5), 773-779 -500 -250 0 250 4(Jo 500 EIrnV (vs SCE) Fig.3 Voltamperogram in 1 mol dm HNO, of a mixture containing [HTeO2+]=525x10 3,[Bi3f]=35~10 ,rnoldm ,,[Bi] [Te]= 2 3 Working electrode, Pt, surface area, 3 14mrn2, rotation rate 600 rpm, potential sweep rate, 60 mV min 500 250 0 a -250 s, -500 - -750 - -1000 -.c * I * * - * * ' , ' * , ' * - ' ' 050 025 0 0 25 050 0 75 EImV (vs SCE) Fig. 4 Voltamperogram in 1 mol dm HNO, of a mixture containing [HTeO2+]=525x10 ,, [Bi3+]=7x10 ,moldm [Bi] [Te]= 4 3 Working electrode, Pt, surface area, 3 14 mmz rotation rate, 600 rpm, potential sweep rate 60 mV min 46 04 -02 0 02 04 EImV (vsSCE) Fig. 5 Voltamperogram in 1 mol dm HNO, of a mixture containing [HTeO2+]=7x10 ,, [Bi3+]=35x10 ,rnoldm [Bi] [Te]= 2 4 Working electrode, Pt, surface area, 3 14mm2 rotation rate, 600 rpm, potential sweep rate, 60 mV min be attributable to the tellurium behaviour, as indicated previously These first results indicate that there is no codeposition of bismuth metal and elemental tellurium for an electrolyte composition corresponding to a Bi Te ratio of 2 3 because the curve does not present the specific anodic signals of each element The presence of one signal during reduction and oxidation is expected to be the signature of the electrochemical process according to the following reaction 3 HTe02++2 Bi3++18 e-+9 H++Bi2Te3+6 H20 In order to verify this hypothesis, we studied the anodic oxidation behaviour of Bi2Te, obtained by directional crystal- lisation The powdered sample synthesised in the S Scherrer Laboratory' (Laboratoire de Physique du Solide, INPL, Nancy) was attached to the section of a glassy carbon rod by means of a colloidal graphite slurry according to a previously described procedure.2' We investigated the anodic oxidation of the latter compound in 1mol dm-3 HNO, electrolyte.A comparison of this anodic voltammogram (Fig. 6) with that obtained with a deposit obtained under stoichiometric con- ditions (Fig. 3) shows a similarity in feature and in potential range. Furthermore, electrolysis at a fixed potential on the diffusion current limit was performed in a stoichiometric electrolyte on a platinum electrode to obtain a sufficient bulk. The elec- trodeposited product was ground and analysed by X-ray diffraction. The diffraction pattern shows that the product had good crystallinity and all diffraction lines could be indexed to the hexagonal rhombohedra1 structure of Bi2Te3 (space group R3m)(Fig.7). The lattice parameters of the hexagonal structure were determined from the observed reticular distance, using a least-squares method (Table 1). The obtained values were in Table 1 Lattice parameters of the Bi,Te, hexagonal structure source ah/A Ch/A ref. 22 4.3835( 5) 30.487(1) ref. 23 4.3850(2) 30.487(2) electrodeposited films 4.384(2) 30.11 (3) 0 250 rw 500 750 EImV (ws. SCE) Fig. 6 Anodic oxidation in 1 mol dm-3 HNO, of a directional crystallised Bi, 06Te2 94. Working electrode, Pt; surface area, 3.14 mm2; rotation rate, 600 rpm; potential sweep rate, 60 mV min-'. '9 N \Y good agreement with those observed by Francombe2, and Brebrick,, with, however, a much smaller c parameter, indica- tive of a composition other than Bi2Te3.Galvanostatic Synthesis The galvanostatic process is commonly used in the electroplat- ing industry. Therefore, we concentrated our efforts on a technique using a constant current to define the best parameters for galvanostatic cathodic deposition. Deposition conditions Electrodeposition was carried out at a constant temperature of 25 "C. The working electrodes (stainless-steel disk) were polished with carborundum paper and with diamond paste (1 pm size). An area of 2 cm2 was exposed for deposition. The electrolyte composition was imposed by the solubility of HTeO,' in nitric acid solvent (1 mol dm-3 HNO,) at pH =0.Under these acidic conditions, the HTe02 maximal concen- + tration is 5 x rnol dm-3 because insoluble TeO, precipi- tates at higher concentrations according to Pourbai~.~~ Therefore, the Bi3+ content of solution is adjusted to 3.33 x mol dm-3. Controls Samples were prepared after the electrodeposition by thorough rinsing in three steps [O.l mol dm-3 nitric acid solution (pH= l), deionized water and ethanol] followed by drying in air. The composition of products removed from the support was analysed using two techniques: electron probe microanalysis (CAMECA SX50) calibrated with tellurium (purity 99.9%) and bismuth (purity 99.9%) standards, or a volumetric method.25 The bismuth and tellurium microanalyses were performed in ten different sections of the samples.The stoichi- ometry corresponds to the average of these ten values and is calculated with a total atom number assigned as 5. Analyses were reproducible within & 1YO.The stoichiometry volumetric determination was elaborated in our laboratory according to previously reported procedure^.^^^^^ Analyses were reproduc- ible to 0.5-1.5Y0. h 0 \ h m0 .. . . . . . . . 0 m om NWcU4*4* 4 ti1.-E u)C B 1.-c i 1I i 11 I I 1 J I I 10.0 5.04.0 3.0 2.5 2.0 1.5 1.2 1.0 dhkl =Fig. 7 Reflecting XRD pattern obtained on a ground electrodeposited compound under potentiostatic conditions: Edeposlt -150mV J. Muter. Chem., 1996,6(5), 773-779 775 The phase identification and the estmation of lattice param- eters were carned out by X-ray diffraction using a curve detector (INEL, Co-Kcx radiation), and the morphology was studied using a scanning electron microscope (HITACHI model S 2500 LB) We realised the physical characterisation, after removing the films from their supports, through measurements of the electri- cal resistivity and the Hall effect in a direction perpendicular to the cleavage planesz5 [using the device from the S Scherrer Laboratory5 (Laboratoire de Physique du Solide, INPL, Nancy)] The Hall effect and resistivity measurements were made on rectangular samples over the temperature range 100-300K Current contacts were made on the underside of the sample by soldering gold spring wires (5 pm diameter) with a bismuth tin eutectic to minimise interfacial and thermoelectric effects Measurements of the Hall effect were conducted with the magnetic field parallel to the cleavage planes and the electric current and Hall voltage perpendicular to the cleavage planes, using a Van der Pauw technique 27 z8 To achieve an accuracy of 2% for resistivity values and 3% for Hall effect, twenty measurements were made for each sample Chemical composition, structure and morphology The optimal value of the applied current density was deter- mined using the Hull's cell method 29 The obtained films were regular, metallic, and pearl-grey for current densities varying from 03 to 12AdmP2, corresponding to a growth rate of 6-25 pm h-l At higher values, the material was black and did not adhere to the support electrode At lower values, the growth rate of films was very slow After electrodeposition, the current density dependence of the stoichiometry of the Bi-Te alloys was studied for films removed from the support Table 2 shows the obtained stoichi- ometry values From these expenmental values, two main features are observed First, the electrodeposited alloys always present an excess of tellurium in comparison with the Bi,Te3 composition In the Bi2Te3 structure, two major defaults are observed An excess of bismuth induces a substitution of Te for Bi accepting one electron For an excess of tellunum, there is a substitution of Bi for Te, with the loss of one electron3' In our case, where the tellurium atomic percentage is >6O%, Te,, defaults are evident, which induces an n-type conductivity Secondly, the tellunum atomic percentage decreases and tends towards the theoretical value in BizTe3 with increasing current density The optimal obtained value is 644 atom% tellurium, corresponding to a Bi, 78Te3 22 compound In order to reach the ideal value of Bi Te (=2 3), we can imagine two possibil- ities either an increase of the current density, or enrichment of the Bi3+ concentration in the solution The first possibility was eliminated because values higher than 1 2 A dm-2 are not compatible with a good matenal, as discussed above Also, we added Bi3+ to the electrolytes Two solutions were studied with Bi Te=3 3 and 4 3 Fig 8 indicates the atomic percent- age of tellunum evolution as a function of the current density for the different electrolyte compositions It can be seen that Table 2 Current density dependence of stoichiometry' current density/A dm Te (YO) Bi,Te, <O 17 69 8 B1151Te3 49 0 17-0 25 67 6 B1l 6ZTe3 38 0 25-0 35 66 0 B1l 70Te3 30 0 35-0 47 65 0 B1l 7STel 25 0 47-0 61 64 8 B1l 76Te3 24 0 6140 70 64 5 B1l 77STe3 225 0 70+0 79 64 6 B1l 77Te3 23 0 7940 90 64 5 B1l 77STe3 225 090-,105 64 4 B117*Te322 "Solution with Bi Te =2 3, Hull's cell IA=0 35 A, t = 30 min 776 J Muter Chem , 1996,6(5), 773-779 mooo0 25 050 075 1 00B"Te' current density/A dm-* Fig.8 Evolution of Te atomic percentage with current density and Bi Te ratio (Rso,)in electrolyte the tellurium percentage depends on the current density and that a high Bi Te furthers an alloy formation containing 62 4 atom% tellurium, 1 e Bi, 88Te3 12 For a current density below 12Adm and for three electrolyte compositions, the faradaic efficiencies were meas- ured for the real alloy stoichiometry from the mass increase of the electrode after deposition, and by comparing the exper- imental increase to that expected from Faraday's law Efficiencies were calculated for the synthesis of alloys using the theoretical mass and the real electron number used in the electrochemical process reaction, taking into account the alloy stoichiometry The results are collected in Table 3 Note that the efficiencies are found to be close to 100% These results reveal that the assumed deposition mechanism is indeed operat- ive and that the applied current density does not induce hydrogen formation The cathodically deposited materials were analysed by reflecting X-ray diffraction The patterns show a single phase, whatever the applied current density All the diffraction lines could be completely indexed on the basis of the hexagonal cell or an equivalent rhombohedra1 cell cor_responding to the structure of Bi2Te3 or its solid solution (R3m) The diffraction peaks remain sharp for all alloys with no detectable shoulders However, the intensity ratios of the peaks are not in good agreement with those obtained by X-ray diffraction on a ground product This fact indicates an orientational effect in the film growth, which we have studied in detail l7 The analysis of the film stoichiometries show an excess of tellurium This fact could suggest tellurium metal deposition, but there are no diffraction peaks corresponding to tellurium metal Furthermore, the possibility of the electroformation of an amorphous tellurium should be ruled out Indeed, galvanostatic deposition under the same conditions (current density, support) from an electrolyte containing only Te" ions led to the formation of crystallised tellurium films The experimental reticular distances are slightly different A least-squares fit was performed using all lines with O(hkl)> 20 O that could be indexed unambiguously The procedure yielded lattice parameters us tellurium percentage in the material (Table 4) The parameters vary smoothly, suggesting the mate- rial is a single phase and corresponds to a solid solution Moreover, the ah lattice parameter of electrodeposited films is Table 3 Electrodeposition efficiencies faradaic efficiency (YO) current density/A dm Bi Te=2 3 Bi Te=3 3 Bi Te=4 3 <O 17 92 49 95 24 95 37 0 17-+0 25 93 16 95 36 95 85 0 2540 35 93 22 96 14 96 90 0 35-0 47 94 24 96 70 97 30 0 47-0 61 95 20 97 34 98 50 0 70-0 79 95 46 97 56 98 80 0 904 05 95 38 98 30 99 20 Table 4 Hexagonal lattice parameters of electrodeposited Bi-Te alloys Te (%) 60" 4 3835( 5) 30.487( 1) 63 6 4 399( 2) 30.13( 3) 67 8 4.403 (6) 29.92( 4) 68 04 4.409(4) 29 97( 3) 68 5 4.41 1 (2) 29.90( 4) 'BiZTe3 single crystal, ref.22. Fig. 9 SE micrograph of a Bi,Te, film electrodeposited onto stainless steel from an HNO, electrolyte solution containing Bi :Te =4 :3 larger than that of a single crystal obtained by solid-state reaction, and the ah lattice parameter seems to increase with increasing tellurium content, while the ch parameterdecrease^.^^.^^ The reason for such a difference is certainly due to the substitution in bismuth planes by tellurium in our tellurium-enriched compounds. This leads to a more compact structure because the tellurium atom is smaller than the bismuth atom. The SEM image of the surface of the electrodeposited compound (Fig.9) shows a polycrystalline assembly of regular needles of length 1.5 pm for the face which is in contact with the electrolyte. This appearance persists up to a thickness of 0.1 mm. For the synthesis of thicker films, dendrites grow and prevent the formation of a regular film. After removing the film, the face which is in contact with the support electrode exhibits a uniform surface with metallic lustre. Transport properties The measurements were made to determine the influence of the thickness and the composition of samples at room tempera- ture, and over a temperature range of 100-300 K. Influence of the thickness. Three films series were prepared with a current density of 0.9 A dm-2 and for deposition times of 2, 3 and 4h.In these cases, the thicknesses of the films were 38, 56 and 74 pm respectively. Experimental values of electrical resistivity p, Hall coefficient R,, carrier concentration C, and Hall mobility pH were determined. The values obtained for the Bi, ,,Te3 23 stoichiometry series (Table 5) were representative of all series. First, note that the Hall coefficient is negative in all cases. In accordance with theory, a negative Hall coefficient induces a n-type semiconductor. Therefore, our electrodepos- ited compounds are n-type and these results confirm the excess of tellurium found by the stoichiometry measurements. Secondly, the values of resistivity and the Hall coefficient can be considered as constant, with no significant effect of thickness.This point implies that the electrodeposited alloy is homo-geneous and retains the same composition during its formation. Moreover, the film resistivity (cu. 11 pi2 m) is clearly lower than that of the single crystal (cu. 75 pl2 m).' Note that a low value is favourable for increasing the figure of merit 2 as regards its literal expression. If we compare the carrier concentrations, the film shows a higher concentration (C,=57 x lo1' ~m-~)than that of the single crystal (C, =0.7 x 10'' cm-'). However, these high values are certainly due to the significant presence of grain boundaries in the films. Influence of the composition. Films with the same thickness were investigated in the wide composition range 63.6< Te <70%.They were prepared under galvanostatic conditions and Table 6 gives the results. The variation of the resistivity value is slight compared with our predictions. The RH values and the Hall mobility increase when the tellurobismuthite stoichiometry is approached. However, the obtained RHvalues, corresponding to a donor density of ca. 2.0 x lo2' e- cmP3, do not correspond to those observed for a single crystal, and the Hall mobility values are larger than those observed for a single crystal. The significant differences are certainly due to the polycrystalline state of the films. Evolution vs. temperature. The resistivity variations for the Bi, ,,Te, 21 sample were studied between 100 and 300 K. The results are plotted in Fig.10. The electrical resistivity increases with increasing temperature and reaches a maximum value at 270K, in agreement with the results of Smirnov et uL3' and Satterthwaite and Ure.32 The RH coefficient [Fig. 1 l(u)] appears to be temperature independent as expected for a semiconductor with high carrier concentration. The carrier concentration C, [Fig. 11 (b)]decreases slightly when the tem- perature increases, as for a single crystal. The values are larger than those of the single crystal; this is caused by the high grain-to-grain connectivity and polycrystalline state. A decrease in the Hall mobility occurs with increasing tempera- ture [Fig. 11(c)]. An analysis of a theoretical mobility model (pH=pOT-X)was carried out where pH is the Hall mobility and T is the temperature.The x exponent was found to be Table 5 Influence of film thickness on electrical properties for Bi, 77Te, 23 carrier Hall mobility/ thickness/pm temperature/# resistivity/pZ2 m cm3 C-' concentration/lo-l9 cm-3 cm2 V-' s-l 38 301.3 11.92 -11.77 -53.10 -9.88 56 304.2 10.87 -10.84 -57.67 -9.97 74 303 8 10.27 -10.38 -60.43 -10.12 J. Muter. Chem., 1996, 6(5),773-779 777 Table 6 Influence of stoichiometry on electrical properties carrier Hall mobility/ Te (YO) Bi,Te, resistivity/jd2 m R,/10 3cm3C ~oncentration/l0'~cm cm2V 's 70 0 B1l SOTe, 50 1206 -4 20 -146 3 -3 54 69 2 B1l 54Te3 46 9 43 -6 37 -98 47 -6 75 65 4 B1l 73Te3 27 1168 -8 72 -71 76 -8 76 647 B1l 77Te3 23 11 09 -11 02 -57 06 -9 99 642 B1179Te3 21 12 33 -15 25 -40 99 -12 37 63 6 B1l82Te3 18 12 74 -19 26 -32 45 -15 12 equal to 0 5 This value does not correspond to any theoretical 11 1 model of diffusion carrier mechanism Note that the high defect concentration and the polycrystalline state induce a large grain boundary quantity and therefore mask the carrier diffusion E lo: xx mechanisms X n g 9-2-28-a w -> x x x x X X Conclusion This study shows that an electrochemical process has success-fully synthesised bismuth-tellunum alloys which possess ther- 7~"""""""'"''"'' 100 150 200 250 300 TfK Fig.10 Temperature dependence of resistivity for an electrodeposited B1179Te3 21 film 0 0 0 0 0 mo 0 -0 00 SO 50 100 150 200 250 300 350 -5 0.L -7 5 1-0'Y)-1001 o00 00>-125-0 cu W moelectric properties The analytical study of the reduction of Bi3+,HTeOzf and their mixtures has established the direct electroformation of non-stoichiometric bismuth tellunde compounds with an excess of tellurium The galvanostatic process may be a simple and economic method for the large-scalefabrication of thermo-electric convertors The electrodeposited samples are polycrystalline and their stoichiometries vary according to the solution composition and the applied current density Physico-chemicaland electrical characterisations showed that the film composition remains constant during the synthesis The electrodeposited compounds show n-type semiconducting behaviour with a significant car-rier concentration References 1 A Ioffe, Semiconductors Thermoelements and Thermoelectricity Cooling, Infosearch, London, 1957 2 B Yim and F Rosi, Solid State Electronics, 1972,15, 1121 3 Harmon, J Phys Chem ,1957,2,181 4 F Rosi, B Abeles and R Jensen, J Phys Chem Solids, 1959, 10,191 5 J P Fleunal, These de I'INPL, Nancy, 1988 6 T Caillat, M Carle, P Pierrat, H Scherrer and S Scherrer, J Phys Chem Solids, 1992,53,1121 7 T Ohta, T Uesugi, T Tokiai, N Nosaka and T Kajikawa, Proc 8th Intl Conf Thermoelectric Energy Conversion, ed S Scherrer and H Scherrer, INPL, Nancy, 1988, p 7 8 R Griot, G Brun and J C Tadenac, Proc 8th Intl Conf Thermoelectric Energy Conversion,ed S Scherrer and H Scherrer, INPL, Nancy, 1988, p 27 9 C Machet, P Lebon and A Septier, le vide-les couches minces, 1982,211,125 10 J Przyluski and K Borkowski, Proc 6th Intl Conf Thermoelectrics,Arlington, TX, 1986, p 100 11 Y Shing, Y Chang, A Mirshafii, L Hayashi, S Roberts, J Josefourier and N Tran, J Vac Sci Technol, 1983,171,903 12 E Charles, E Groubert and A Boyer, J Mater Sci Lett, 1988, 7,579 13 C Lokhande and S Pawar, Phys Status Solidi A, 1989,111,17 \6-150; 14 C De Mattei and R Feigelson, Electrochemistry of SemiconductorsI: and Electronics Process and Devices, ed J McHardy and=t-17 51 F Ludwig, Noyes, New Jersey, 1992, p 1 15 B Miller and A Heller, Nature (London),1976,262, 680-20 0* 50 100 150 200 250 300 350 16 A Baranski, W Fawcett and C Gilbert, J Electrochem Soc , 1983, TIK 130,2423 Fig.11 Temperature dependence of the Hall constant RH (a), carrier concentration C, (b) and Hall mobility pH (c) for an electrodeposited B1179Te3 21 film 17 18 H Chaouni, P Magri, J Bessieres, C Boulanger and J J Heizmann, Proc Icotom 10, Mater Forum, 1994, 157-162, 1371 P Magri, C Boulanger and J M Lecuire, Proc 13th Intl Conf 778 J Mater Chem , 1996,6(5), 773-779 Thermoelectrics,ed. 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Solid State, 1970,11,2681. 32 C. Satterthwaite and R. Ure, Phys. Rev., 1957, 108, 1164. Paper 5/06632H; Received 9th October, 1995 J. Muter. Chem., 1996, 6(5), 773-779 779