Thin Solid Films 518 (2010) 6550–6553

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Crystallization behavior and thermoelectric characteristics of the electrodepositedSb2Te3 thin films

Min-Young Kim, Tae-Sung Oh ⁎Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Republic of Korea

⁎ Corresponding author. Tel.: +82 2 320 1655; fax: +E-mail address: ohts@hongik.ac.kr (T.-S. Oh).

0040-6090/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.tsf.2010.03.052

a b s t r a c t

a r t i c l e i n f o

Available online 27 March 2010

Keywords:Sb2Te3AmorphousCrystallization temperatureSeebeck coefficientElectrodeposition

Crystallization behavior of the electrodeposited Sb2Te3 film was characterized and the effect of theamorphous-crystalline transition on the Seebeck coefficient was evaluated. The as-electrodeposited Sb2Te3film was amorphous and exhibited the Seebeck coefficient of 268–322 μV/K, which was much larger than thevalue of the crystalline Sb2Te3 film. When annealed at temperatures above 100 °C, the Seebeck coefficient ofthe Sb2Te3 film dropped significantly to 78–107 μV/K due to the amorphous-crystalline transition at 94 °C.The thermal stability of the electrodeposited Sb2Te3 film was improved by the addition of Cu, and thecrystallization temperature of the CuSbTe film increased up to 149.5 °C.

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1. Introduction

Sb2Te3 is a narrow band-gap semiconductor attracting muchattention for potential applications to thermoelectric energy-conver-sion devices [1–5], phase-change memories [6,7], optical data storagemedia [8,9], and ohmic contacts for CdS/CdTe thin-film solar cells [10].

Due to superior thermoelectric characteristics at around roomtemperature, various works have been performed to apply Sb2Te3 thinfilms to thermoelectric micro-devices such as thermal sensors andmicro-coolers [1–5]. Sb2Te3 film has been reported to have differentphase states, either crystalline or amorphous, depending on fabrica-tion processes. While the crystalline phase is formed by sputtering atroom temperature [7], electrochemical atomic layer deposition atroom temperature [11], electrodeposition at 100 °C [10,12], andmetal–organic chemical vapor deposition at 450 °C [3], the amorphousstate is obtained by evaporation or electrodeposition at roomtemperature [5,12,13]. The thermoelectric properties of Sb2Te3 film,such as Seebeck coefficient and electrical resistivity, vary with itsphase state [5]. With its higher Seebeck coefficient than that of thecrystalline state [5], the amorphous state of Sb2Te3 film would bebeneficial for some applications such as thin-film sensor devices. Asamorphous Sb2Te3 film undergoes transition to crystalline phase atelevated temperature [6–8], improvement of its thermal stability isdesirable to prevent crystallization during thin-film sensor processing.

Although Ge2Sb2Te5 film is most widely used for phase-changememory, rewritable compact disc, and digital versatile disc, it has adrawback of large reset current due to its low crystalline resistivityand high melting temperature [6,7]. Sb2Te3 film has been considered

as a candidate to replace Ge2Sb2Te5 film due to its low meltingtemperature and high crystallization speed [6,7]. However, amor-phous Sb2Te3 has poor thermal stability and it is crucial to increase itscrystallization temperature to ensure the data retention for phase-change memory applications [6,7].

Although various works have been carried out to increase thecrystallization temperature of the evaporated and sputtered Sb2Te3films by doping Ag, Cu, N, and Si [6–8], little work has been reportedfor the improvement of the thermal stability of an electrodepositedSb2Te3 film with the addition of foreign elements. While variousprocessing techniques can be used to fabricate Sb2Te3 thin films,electrodeposition is attractive because it is a rapid and inexpensiveprocess [11,12]. In this study, the crystallization behavior of theelectrodeposited Sb2Te3 film was characterized and the effect ofthe amorphous-crystalline transition on the Seebeck coefficient ofthe Sb2Te3 film was evaluated. The crystallization retardation of theamorphous Sb2Te3 film was also examined with Cu addition.

2. Experimental procedure

Electrodeposition of the Sb2Te3 films and the Cu-doped Sb–Te(CuSbTe) films of 2-μm thickness was performed at room tempera-ture. To make the Sb–Te solution, we successively dissolved TeO2 andSb2O3 into a mixed solution of 3.5 M perchloric acid and 0.35 Mtartaric acid at 160 °C with a Sb/(Sb+Te) mole fraction of 0.9 to forman aqueous solution containing 70-mM Sb–Te electrolyte. Theelectrodeposition solutions to process the CuSbTe films were madeby dissolving the appropriate amounts of CuSO4 5H2O into the 70-mMSb–Te electrodeposition solution at 160 °C. The Cu/(Sb+Te+Cu)mole percentages in the Cu–Sb–Te electrodeposition solutions werecontrolled in the range of 0–0.5 mol%. As a seed layer for electrode-position, we sputter-deposited a 1-μm-thick Ti onto a Si substrate. A

Fig. 1. Composition of the Sb2Te3 film as a function of the electrodeposition potential.Fig. 3. Seebeck coefficient of the Sb2Te3 film as a function of the annealing temperature.

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three-electrode electrochemical cell system was employed with a Ti/Si substrate as a cathode, a Pt mesh electrode as an anode, and an Ag/AgCl electrode as a reference electrode. The Sb2Te3 films wereelectrodeposited on Ti/Si substrates at a constant potential rangingfrom 10 mV to 30 mV relative to the Ag/AgCl reference electrode.Electrodeposition of the CuSbTe films was carried out at a constantpotential of 20 mV.

After electrodeposition, the films were isothermally annealed for1 h at temperatures between 100 °C and 300 °C. The heating rate forannealing process was fixed at 10 °C/min. In order to measure thecrystallization temperature, differential scanning calorimetry (DSC)was employed in argon atmosphere at a heating rate of 10 °C/min.Compositions of the as-deposited films were analyzed by energydispersive spectroscopy (EDS). Crystalline phases of the films beforeand after annealing treatment were characterized with an X-raydiffractometer. The Seebeck coefficients (α) of the films were mea-sured at room temperature by applying a temperature difference of20 °C at both ends of the films.

Fig. 2. X-ray diffraction pattern of the Sb2Te3 film electrodeposited at a constantpotential of (a) 10 mV, (b) 20 mV, and (c) 30 mV.

3. Results and discussion

Fig. 1 shows the compositions of the as-electrodeposited Sb2Te3films.Although the films electrodeposited in a solution with a Sb/(Sb+Te)ratio of 0.9, they possessed the compositions close to the Sb2Te3stoichiometry. This could be attributed to the fact that the reversiblepotential for the HTeO2

+/Te reaction is much more positive than that ofthe SbO+/Sb reaction [12,13]. The X-ray diffraction patterns in Fig. 2exhibit that the as-electrodeposited Sb2Te3 films were amorphous.Huang et al. also reported formation of amorphous Sb-Te films with Sbcontent ranging from 37 to 57 at.% with electrodeposition at roomtemperature [12].

The Seebeck coefficient of the Sb2Te3 film, electrodeposited at aconstant potential of 20 mV, is shown in Fig. 3 as a function of theannealing temperature. The as-electrodeposited Sb2Te3 film exhibitedthe Seebeck coefficient of 322 μV/K which was much higher thanvalues reported for the crystalline Sb2Te3 films grown by evaporation,MBE, and MOCVD [2–4]. The Seebeck coefficients of 268 μV/K and

Fig. 4. X-ray diffraction pattern of the Sb2Te3 film after annealing for 1 h at (a) 100 °C,(b) 150 °C, and (c) 200 °C.

Fig. 5. DSC curve of the Sb2Te3 film, exhibiting the crystallization temperature of 94 °C.

Fig. 7. X-ray diffraction pattern of the CuSbTe film electrodeposited in the electrolyteswith the Cu/(Sb+Te+Cu) mole percentage of (a) 0.1 mol%, (b) 0.3 mol%, (c) 2.9 mol%,and (d) 5 mol%.

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296 μV/K, similar to the value of the as-electrodeposited film in Fig. 3,were also obtained for the Sb2Te3 films electrodeposited at 10 mV and30 mV, respectively. The large Seebeck coefficients of the as-electrodeposited Sb2Te3 films could be attributed to noncrystallinityof the films. Seebeck coefficients as high as 750 μV/Kwere reported foramorphous Sb2Te3 films processed by RF magnetron sputtering [14].As shown in Fig. 3, however, the Seebeck coefficient of the Sb2Te3 filmdropped significantly by annealing treatment which caused theamorphous-crystalline transition of the films. As confirmed with theXRD analysis in Fig. 4, the Sb2Te3 film was crystallized by annealing attemperatures above 100 °C. A substantial decrease in the Seebeckcoefficient by amorphous-crystalline transition was also reported forthe evaporated Sb2Te3 films [5].

The DSC curve of the Sb2Te3 film, shown in Fig. 5, reveals that theSb2Te3 film was crystallized at 94 °C, which was similar to 97.4 °Cmeasured as the crystallization temperature of the evaporated Sb2Te3film [8]. The crystallization temperatures of amorphous Sb–Te filmshave been reported to vary with the Sb content ranging from 38.6% to70%; 97.4 °C for the evaporated Sb38.6Te61.4 film [8], 120 °C for theelectrodeposited Sb49Te51 film [12], and 135 °C for the Sb70Te30 film[9]. It seems that the crystallization temperature of an amorphous Sb–Te film becomes higher with increasing the Sb content of the film. Asthe Sb2Te3 phase has a very narrow range of compositions with an Sbcontent between 40 and 40.4 at.% [12], amorphous-crystallinetransition may become more difficult with more deviation of the

Fig. 6. Composition of the as-electrodeposited CuSbTe film as a function of the Cu/(Sb+Te+Cu) mole percentage in the Cu–Sb–Te electrolyte.

composition from the stoichiometry due to phase separation forwhich long-range atomic rearrangements are necessary [9].

To improve the thermal stability of the electrodeposited Sb2Te3film, the Cu-doped Sb2Te3 films were electrodeposited at a constantpotential of 20 mV. Fig. 6 shows the compositions of the as-electrodeposited CuSbTe films as a function of the Cu/(Sb+Te+Cu)mole percentage in the Cu–Sb–Te electrolytes. The Cu content of theCuSbTe film was changed from 6.1 to 23.4 at.% with varying the Cu2+

mole percentage in the electrolyte from 0.1 to 5 mol%. XRD patterns inFig. 7 illustrate that the CuSbTe films were amorphous, evenelectrodeposited in the electrolyte containing 5 mol% Cu2+ ions.Amorphous states of the CuSbTe films were also confirmed by DSCanalysis. The DSC curve for the CuSbTe film electrodeposited in theelectrolyte with 5 mol% Cu2+ ions clearly reveals an exothermic peakat 133.5 °C caused by the amorphous-crystalline transition of the film.

The crystallization temperature of the CuSbTe film is illustrated inFig. 8 as a function of the Cu/(Sb+Te+Cu) mole percentage in the

Fig. 8. Crystallization temperature of theCuSbTefilmasa functionof theCu/(Sb+Te+Cu)mole percentage in the Cu–Sb–Te electrolyte.

Fig. 9. Seebeck coefficient of the CuSbTe film as a function of the Cu/(Sb+Te+Cu)molepercentage in the Cu–Sb–Te electrolyte.

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electrodeposition solution. By increasing the mole percentage of Cu2+

ions in the electrolyte from 0 to 3 mol%, the crystallization tem-perature of the CuSbTe film increased from 94 °C to 149.5 °C and thenslightly decreased to 133.5 °C with further increase of the Cu2+

concentration in the electrolyte to 5 mol%. For the evaporated CuSbTefilms, the crystallization temperature was also reported to increasefrom 97.4 °C to 120.8 °C with increasing the Cu content of the filmfrom 0 to 8.4 at.% [8]. As shown in Fig. 8, the thermal stability of theelectrodeposited Sb2Te3 filmwas improvedwith addition of Cu, whichcould be explained with the confusion principle [9,15]. According tothe confusion principle, an alloy system consisting ofmore elements ismore resistant to crystallization due to a less chance to select a viablecrystal structure [9,15]. Phase separation would also play a role toincrease the crystallization temperature of the CuSbTe film [8,9]. Theamorphous CuSbTe film, formed by evaporation at room temperature,has been reported to undergo phase separation to Sb2Te3, CuTe, andCu7Te4 during crystallization, causing an increase in the crystallizationtemperature due to long-range atomic rearrangements [8].

Fig. 9 shows the Seebeck coefficients of the as-electrodepositedCuSbTe films as a function of the Cu/(Sb+Te+Cu) mole percentage inthe electrodeposition solution. When electrodeposited in an electrolytecontaining the Cu/(Sb+Te+Cu) mole percentage up to 0.7 mol%, theCuSbTe films exhibited the Seebeck coefficients of 340–350 μV/Kwhichwere much higher than values reported for the crystalline Sb2Te3 filmsprocessedbyevaporation,MBE, andMOCVD[2–4]. By increasing theCu/(Sb+Te+Cu) ion ratio in the electrolyte to 2.9 mol% and 5 mol%, theSeebeck coefficient of the CuSbTe film decreased to 270 μV/K and

158 μV/K, respectively. Based on the dependence of the Seebeckcoefficient upon the Cu/(Sb+Te+Cu) mole ratio in the electrolyte, itcould be suggested that Cu dopants were incorporated in the Sb2Te3matrix when the films were electrodeposited in the electrolytes withthe Cu/(Sb+Te+Cu)mole percentage up to 0.7 mol%. On the contrary,compositional separation into Sb–Te phase and Cu–Te phase wouldoccur for the films electrodeposited in the electrolytes containing theCu2+ mole percentages of 2.9 and 5 mol%, resulting in the decrease ofthe Seebeck coefficient.

4. Conclusion

The Sb2Te3 films electrodeposited in a solution with an Sb/(Sb+Te)ratio of 0.9 at potentials ranging from 10 mV to 30 mVwere amorphousand exhibited the compositions near the Sb2Te3 stoichiometry. The as-electrodeposited Sb2Te3 films exhibited high Seebeck coefficients of268–322 μV/K due to noncrystallinity of the films. With annealing attemperatures above 100 °C, the Seebeck coefficient of the Sb2Te3 filmdropped significantly to 78–107 μV/K due to the amorphous-crystallinetransition at 94 °C. The thermal stability of the amorphous Sb2Te3 filmwas improved by addition of Cu. The CuSbTe film, electrodeposited inthe electrolyte containing the Cu2+ concentration of 3 mol%, exhibitedthe crystallization temperature of 149.5 °Cwhichwasmuchhigher than94 °C of the Sb2Te3 film. When electrodeposited in an electrolytecontaining the Cu/(Sb+Te+Cu) mole percentage up to 0.7 mol%, theCuSbTe films exhibited the Seebeck coefficients of 340–350 μV/K higherthan 322 μV/K of the Sb2Te3 film.

Acknowledgement

This work was supported by the Seoul R&BD Program.

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