Pulsed laser deposition of Bi2Te3 thin films

ELSEVIER

Thin Solid Films 280 (1996) 61--66

Pulsed laser deposition of Bi2Te3thin films A. Dauscher, A. Thomy, H. Scherrer Laboratoire de Mdtallurgie Physique et Sciences des Matdriaux. URA 155 du CNRS, Ecole des Mines de Nancy, Parc de Saurupt, F-54042Nancy Cedex, France Received 21 March 1995; accepted 30 October 1995

Abstract A feasibility study of thin stoichiometric Bi2Te3 films by pulsed laser deposition (PLD) was performed, the difficulty arising from the difti~renees of vapour pressure between Te and Bi. The films were elaborated using a pulsed Nd:YAG laser under various experimental conditions and were characterized by electron microprobe, scanning electron microscopy, X-ray diffraction and secondary ion mass spectroscopy analyses. Congruent transfer of stoichiometry occurs from the target to the substrate on several cm2 and a good crystallinity can be achieved, even on glass substrates at room temperature, by combining convenient target to substrate facing and distance, respectively. Depletion in Te observed in some films may result from a laser beam-plume interaction that was put forward during elaboration of films on large scale substrates. Keywords: Laser ablation; Surface composition; Tellurium; X-ray diffraction

1. Introduction Pulsed laser depc~sition (PLD) has turned out to be a suitable method for thin film deposition of high-temperature superconductors. Recent reviews are devoted to this subject [ 1--4 ]. PLD has been applied to the elaboration of many other compounds such as. ferroelectrics, semiconductors and even polymers. As compared with other physical deposition techniques, PLD offers the following advantages: (i) easy use of the experimental set-up even if the laser-material interaction is a very complex !physical phenomenon, (ii) fast stoichiometric deposition o f complex materials, (iii) achievement of films of any kind of material even for those having a high melting point, (iv) achievement of well crystallised films even when working on substrates at room temperature, and finally (v) lowered ~cost because of the possibility of using small targets. The main disadvantage is the presence of droplets on the deposited films which is partly linked to the roughening of the tmget during the ablation process [5,6]. Incongruent transfer of stoichiometry from target to substrate has seldom been reported. In most cases, it happens either during deposition on heated substrates or with oxide compounds for which it is necessary to work under a low oxygen pressure in order to avoid oxygen depletion in the deposited film. The aim of this paper is to verify, when work0040-6090/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD10040-6090 ( 95 ) 0g ~ 21 -2

ing on substrates remaining at room temperature, the achievement of a congruent transfer of stoichiometry for a compound such as Bi2Te3 possessing components of very different vapour pressures and furthermore to check the crystallinity of the obtained films. Bi2Te3 is nowadays one oflhe most efficient thermoelectric bulk materials working at room temperature. Some recent theoretical studies tend to sh¢)w that the use of Bi2Te3 as thin films or in quantum well s,~perlattices could increase the performances of the material [ 7,8 ]. Another interest of thin films is their potential to be ased in the microfabrication of integrated thermoelectric devices such as sensors and captors. Thin films of Bi2Te3 have already been elaborated by several methods: sputter deposition [9,10], co-evaporation of the elements [ 11 ], flash evaporation [ 12-15], molecular beam epitaxy [ 16,17 ], chemical vapour deposition [ 18 ] and electrodeposition [ 19], but to) our knowledge never by PLD. All the previous studies show the difficulty to obtain stoichiometric deposition that can only be achieved in a small range of substrate temperature ( 52{~-580 K). In this paper, attention has been particularly focused on the elaboration of stoiehiometric Bi2Te3 thin films by PLD by the way of changing some deposition conditions in relation with laser parameters, target characteristics and target to substrate configuration.

62

A. Dauscher et al. / Thin Solid Films 280 (1996) 61-66

substrate heater&holder electrical thermocouple feedthr°ughl:~l[~] I: feedthr°ugh

2. Experimental 2.1. Elaboration

|

Bi2Te3 (space group R:~m) ingots were grown by the Bridgman method from 5N Bi and Te pure materials. The targets used for PLD were obtained from cutting the ingots with a wire saw parallel or perpendicular to their growth axis. They were then polished a t 6 lxm. They present as disks of 18 mm diameter and 10 mm thickness. The same target is used for the elaboration of several films. Nevertheless, it is re-polished at 6 p,m before each new deposition in order to avoid large droplet formation on the films and to eliminate the mierosegregations induced on the target during the ablation process (see Section 3.1 ). The target is brazed onto a cupper disk to better evacuate the calories. This set is mounted on a support rotated at 5 rpm. Glass substrates of dimensions 18 × 18 or 50 × 50 mm 2 were chemically etched in HCI solutions prior to being loaded in the ,vacuum system. They were placed parallel to the target in such a way that the centre of the target coincides with the centre of the substrate. A pulsed Nd:YAG (Quantel, YG 571C) was used for the ablation process, delivering a main wavelength at A = 1060 nm that can be transformed in A = 530 nm and A = 265 nm by two- and four-folding the frequency, respectively. The energy of the outcoming beam is monitored by an energy meter. The beam (pulse width, 10 ns) is directed onto the target via a prism and two mirrors. It is focalised at the entry of the vacuum chamber by a converging lens of 30 mm focal. The density of energy (or fluence) of the incident beam can be varied on the target by adjusting the lens to target distance. The laser beam intersects the target at an angle of 60 ° with respect to the normal. The beam can be scanned onto the target via an X,Y,Z table. Both rotation of the target and scanning of the beam preclude pit formation on the target and ensui¢ a more uniform ablation. The vacuum chamber is an UHV stainless-steel vessel pumped by an ionic pump to a base pressure better than 10- 5 Pa. A schematic view of the laser experimental set-up is reported in Fig. 1. The deposition parameters we kept constant were the repetition rate ( 10 Hz), the type of moving of the beam onto the target (linear, 12 mm large, rate of 2 m m s ~ ~), the rotating rate of the target (5 rpm) and the substrate temperature (295 K) while those that were changed were the wavelength, the fluence (2-100 J cm-2), the deposition duration ( 10-120 rain), the target to substrate distance (30-80 mm) and the crystallographic orientation of the target (parallel or perpendicular to the growth axis). In order to allow comparisons, reference ablation conditions were chosen: A, 265 nm; fluence, 10 J era- 2; target to substrate distance, 40 mm; duration of ablation process, 60 min; and Bi2Te3 targets cut parallel to their trigonal axis. Under these experimental conditions, the deposition rate was about 0.5 ,~ s- ~. All the films that were

gasinlet

z=dt ~ +

% ionicpump ~

i

T-1 I J. l / ~

~

dfastr°pening

~

suprasiwi l ndow

focusinglens laserbeam (scanningontothetargetviaanX,Y,Ztable) Fig. l. Schematic diagram of the pulsed laser deposition set-up.

obtained were adherent and mirror like. The films were not further annealed.

2.2. Characterization The composition of the targets was measured by means of a wavelength dispersive (WDS) microanalyser (Cameca, SX 50) before and after the ablation process, as well as the composition of the resulting films. Roughly 20 points were checked on 300 mm 2 surface film areas. The analyzed lines were Bi Mv at 2.58 keV, Te Lm at 4.34 keV and Si K at 1.84 keV. In order to determine the best analysis conditions, we kept constant the filament intensity (250 nA) and varied the acceleration voltage V within the 5-8 keV range. The best accuracy on the measures of bulk materials was obtained for V)7 keV. Nevertheless, such voltages were too high for the determination of the compositions of the films, especially if their thickness did not exceed 300 nm, because of the sat,aration of the Si signal outcoming from the glass substrate. The analyses were then performed under a voltage of 5 keV while knowing that the Te content will be overestimated. For instance, bulk Bi2Te3 targets present contents of 63 at.% Te under 5 keV while only 61% under 8 keV, for measures performed at the same place. In the results part, we have omitted the Si content and corrected the contents of Bi and Te so as to have Bi +Te-- 100% atomic content in order to obtain a better outlook of their real contents in the films. X-ray diffraction (XRD) studies (Philips, PF 1380) were carded out with Co Ka~ radiation. Two modes of analysis have been used, either under glancing incidence, allowing the detection of any crystallite being in the Bragg position or

A. Dauscher et al. / Thin Solid Fibns 280 (1996) 61-66

in the 0/20 mode where only the crystallites orientated parallel to the surface are detected. Thickness of the films, if less than 200 nm, was determined by small-angle X-ray scattering (Philips, PF 1380). Otherwise, it was estimated from comparisons of the Si signal detected during microprobe analyses. The surface morphology and the concentration profiles of Bi and Te in the films have been checked by scanning electron microscopy (SEM) (Jeol, JMS-820) and secondary ion mass spectroscopy (SIMS) (VG) depth profiling, respectively.

3. Results and discussion 3.1. Scanning electron microscopy

In Fig. 2 are reported two typical micrographs of film surfaces that were obtained from PLD of Bi2Te3 targets. When the laser fluence is !es~ than 20 J cm-2, micronic or submicronic droplets lie on a smooth surface (Fig. 2 ( a ) ) . The presence of greater particulates (7-8 l~m), looking more like fiat disks, is also observed. These two types of particulates

63

may certainly originate from target surface roughening as it has already been shown [5,12]. As a function of time, the laser irradiation produces defects on the target surface such as pits, columns or craters. Due to laser-induced thermal and mechanical shocks, they are dislodged and lead to the existence of molten micronic globules in the plume that will be quenched onto the substrate. Actually, the density of droplets decreases by diminishing the irradiation time. Moreover, the presence of large droplets disappears by decreasing the laser fluence. Fig. 2(b) shows a film surface obtained at a much greater fluence (100 J cm-2), for A = 5 3 0 nm. This surface is very rough. Here, the molten globules are not quenched smoothly as it was the case previously, but like splashes. It is true for globules of various sizes ranging from 3 to 15 lxm. So, it is not a size effect. We have to assume that the species produced with a high fluence have a much larger kinetic energy than those produced under a lower fluence. The roughness of the film is only due to the splashes that introduce the presence of micronic sized hillocks of material at their circumference. We have verified that all types of particulates present the same stoichiometry as their surroundings. 3.2. Microprobe analyses

Fig. 2. Scanningelectronmicrographsof Bi~Teyfilmsdepositedby means of (a) A=265 rim, laser fluence, 10 J cm-2, and (b) A=530 nm, laser fluence, 100 J cm-2 (target to substratedistance,40 mm; substratetemperature, 295 K).

The composition of the target was checked before and after the ablation process. The initial surface presents a homogeneous distribution in Bi and Te (Bi = 37 and Te = 63 at.%). The laser-treated surface (a circle of 12 mm diameter) has the same composition as the initial surface whereas the nontreated part presents an excess of the most volatile component which is Te (at.% T e - 6 6 ) . It is certainly due to its segregation induced by the heating but not the melting of the target near the laser irradiated zone. The first set of experiments, performed on substrates of 18 × 18 mm 2 size, gave very disappointing results about the homogeneity of composition in the elaborated films and even the reproducibility of the experiments. Compositions ranging from 68 to 37 at.% Bi have been found, including all the domains of existence of the Bi-Te system ( BiTTe3 to Bi2Te3). In the majority of cases, a large excess of Bi was observed. The change of any elaboration condition did not allow one to reach in a reproducible way a congruent transfer of stoichiometry on all the substrate area. Moreover, we never observed any gradual variation of composition for instance as a function of target to substrate distance, fluence or even wavelength. These results suggested the existence of another parameter that was not controlled. In order to know which could be this parameter and to have an idea of the homogeneity in the plume, some films were performed on large-scale 55 × 55 mm 2 glass substrates. The wavelength chosen was 530 nm in order to avoid some random loss energy effects of the laser beam observed when working in the UV range and that could distort the reproducibility of the experiments. A map of the compositions obtained on such great substrates is reported in Fig. 3. On the

64

A. Dauscher et al. / Thin Solid Films 280 (1996) 61--66

Bi

"re 61.3 38.7

64.6 35.4

47.3 52.7'

38.4 61.6

38.1 61.9

56.0 44.0

60.9 48.8 37.1 0 .1 ,.~ :.: ~. : ~ . ~ : ~ . ~ r"2 .,. .. 9

37.2 62.8

49.6 50.4

61.

37'.2

50.0 50.0

63.Oi!iiiiii}i}iiii~)iii~!~il)~)i:il;i)ii~i~38.2

36.6

36.2

43.4

61.8

63.4

58.2 41.8

69.6 30.4

49.7 50.3

39.0 61.0

37.6 62.4

7'.6

D middle of the 50X50 mm2 substrate corresponding to the place o! the 18)(18 ram2substmte and to the centre of the plume

Fig. 3. Map of compositions obtained on large-scale 55 × 55 mm2 glass ,,ubstrates (~ -- 530 nm; laser fluence, 5 J cm-2; target to substrate distance, 40 ram; substrate temperature, 295 K). Congruent transfer of stoichiometry from target to substrate is observed on the right hand side of the substrate.

right-hand side, the composition is close to the bismuth telluride stoichiometry, in good agreement with the target composition, on areas as large as 2 cm z. When displacing towards the left, the bismuth content is enhanced and a large number of composition appears. We think that these surprising features may be due to the interaction of the laser beam with the outcoming plume of the target, leading to a random composition on a part of the substrate, as it is shown schematically in Fig. 4. Actually, the bad compositions are obtained on the same side as the arriving beam. Moreover, as the centre of the target coincides with the centre of the substrate and as the beam is only scanned linearly onto the target, we exclude that the non-stoichiometry may be due to the fact that the substrate is not located at the centre of plume. The film thickness, even if not determined with precision, seems also to have no influence on the stoichiometry because it is clear that the films are thinner along both the right and left sides of the substrates by comparison with the middle line, as greater amounts of Si dlllpl~emenI o! 0111plume s~oeJing to ~ beam scenning onto the target v

laser beam

substrate

plume target

m bad stolchlometp/

were detected (middle, at.% Si = 0; sides, at.% Si about 8%). The interaction of the laser beam may come off either with long life particulates remaining near the target between two following pulses but more probably with particulates or gasphase material emitted during the shot itself. Two assumptions can be made on the effects of the beamplume interaction on the stoichiometry. First, it could act by changing the composition of the plume components, for instance by photodissociating big entities while creating a depletion in the most volatile element that could be ejected from the plume or even redeposited onto the target (excess of Te on the non-treated target surface). Second, it could enhance the kinetic energy of some species that will then be able to sputter Te atoms located at the surface of the films, leading to Te depletion. Such a feature explained the loss of Ga observed during PLD of Fe72SilTRu4Ga7 for fluences higher than the threshold fluence [ 5 ]. In our case, assumption 1 is prevailed over assumption 2 because: (i) no influence of the density of energy oil the film stoichiometry has been observed whereas it was the case in Ref. [5], (ii) the depletion in Te is only effective on a part of the film, and (iii) the particulates are quenched as droplets over all the surface and not partly as splashes as it would be observed if: the kinetic energy of the arriving species is high, as it has been seen in Section 3.1. Such a laser beam-plume interaction can also explain the erratic results obtained on the smaller substrates. As the form of the plume may certainly be '.argely dependent on the experimental conditions, even on small changes of them, the distribution in composition of the various films varies. With a fine plume, congruent transfer of stoichiometry occurs while with a larger plume, every composition may be obtained. We have shown that it is possible to achieve a congruent transfer of stoichiometry if the distribution of composition inside the plume is well known. This is easily achievable, as pointed out, on large-scale substrates. Smaller substrates have then to be located at the place where the good stoichiometry is obtained, place that do not coincide, in the case of BizTe3, to the middle of the plume. By comparison with other deposition techniques, PLD offers the advantage to obtain the BizTe3 stoichiometry on substrates at room temperature. All the techniques described in Section I lead to films presenting an excess of Te for films performed at room temperature. The Te/Bi ratio of 1.5 is only obtained within the substrate temperature range 520 K(T(580 K by molecular beam epitaxy [16,20] and flash evaporation techniques [ 11 ]. Higher substrate temperatures lead to the re-evaporation of Te and so in depletion of Te in the films.

3.3. SIMS analyses

[ ] stolchiomMryof the lauget

Fig. 4. Schematic view of the laser beam-plume interaction explainiag the differences of stoichiometry obtained on large-scale substrates (right-hand side, good stoichiometry; leR-hand side, bad stoichiometry). The centre of the substrate coincides with the centre of the target.

SIMS annlyses have been performed on different places of films elaborated on large scale substrates. The Bi/Te ratio is quasi-constant during depth analysis for film places where the stoichiometry is correct while it varies erratically else-

A. Dauscher et al. / Thin Solid Films 280 (1996) 61-66

300

65

4000 0 0

'~.

,~ 200-

01,-" ,-I0

~ ,- 0 ,.- ,,- N

"" 0

0 ¢,~

>, 2000 IR

c 100-

l} 4~

_fi!m!Bi37T.e.63) 0

I

20

I

40

I

60

I

I

80 100 20 (deg)

I

120

I

0

140

Fig. 5. XRD diffractionpatternsof a Bi~Tey filmof 34 nm thicknesstaken under glancing incidence and in the 0/20 mode (no attribution of the diffraction peaks becausethe compositionis not known). where. These results confirm the fact that a phenomenon, certainly linked to laser beam-plume interactions, renders erratic the deposition on given parts of the substrates.

3.4. XRD analyses XRD peaks have been obtained whatever the elaboration conditions of the films and therefore for any Bi~Tex composition. All the patterns include crystalline peaks, more or less fine and intense, as a function of experimental parameters but not as a function of stoichiometry. Although no general trends can be drawn from the results because crystallinity largely depends on film thickness, it can be pointed out that the increase of both target to substrate distance and shooting duration leads to an enhancement of the crystallinity of the films. Moreover, it is worthwhile to note the achievement of crystallized films by PLD for room temperature deposition conditions and furthermore on amorphous substrates, contrarily to films performed by other deposition techniques and for which further annealings are required. Interesting features have been observed at the beginning of film growth. In Fig. 5 are reported two diffraction patterns of a Bi~Te r film of 34 nm thickness (shooting duration, 10 min), one taken under glancing incidence and the other in the 0/20 mode. Peaks appear only in the 0/20 mode showing that all the crystallites are oriented parallel to the surface. Moreover, the growth takes place in two preferred directions that unfortunately cannot be determined because the two peaks may correspond to several Bi~-Tey define compounds. In that case, the nominal composition is not known because the film is to thin to be analyzed by electron microprobe, rendering then difficult the peak attribution. In Fig. 6 are reported the diffraction patterns obtained with a Bi2Te3 target and with a film whose composition is close to that of the target. The film is polycrystalline and all the lines of the diffractogram belong to the Bi2Te3 structure, according to the JCPDS-ICDD files. The diffraetogram also shows a halo that can be attributed to the glass substrate. Some lines are not present but would certainly appear after annealing treatments as it has already been observed on films elaborated by other deposition techniques [ 10,14,16]. The (015) and

2'o

4'o

6'o

8'o

. . . . . . . .

16o

14o

'"

14o

20 (deg) Fig. 6. Comparison of the diffraction patterns of a Bi2Te3 bulk target (taken in its growth direction) and a film of same stoichiometry obtained by PLD.

(10.10) are generally the preferred growth planes of all Bi2Te3 films but the relative intensities of the peaks vary as a function of the deposition technique of the films and the substrate temperature. The films prepared by PLD present however (006) planes that are generally only observed after an annealing. It confirms the higher degree ot crystallinity obtained at room temperature by PLD in comparison w;,th other elaboration methods. Our films show no marked growth plane. Nevertheless, as the XRD pattern of the bulk material has been performed, like the film, in the growth direction of the Bi2Te3 ingot and as the XRD pattern of the target is completely different in the direction perpendicular to the growth direction, we can say that PLD induces a growth in the same direction as a bulk material elaborated by a Bridgman method. 4. Conclusion The first attempts to elaborate bismuth telluride films by PLD have been presented. We have shown that the challenge of reproducing the stoichiometry of the target, induced by the great difference of vapour pressure between bismuth and tellurium, can be achieved under convenient experimental conditions. The stoichiometry can be easily achieved, even on substrates at room temperature contrarily to other deposition techniques, when the substrate is correctly placed with regard to the target. It is obtained on the substrate side where there is no interaction between the laser beam and the expanding plume, on areas greater than several cm 2. Such areas are largely sufficicat to perform thermoelectric measurements, the required dimensions being less than 1 cm 2. The other elaboration conditions seem to have only a minor influence on the obtained stoichiometry. The films are crystallized, at room temperature and on amorphous substrates, and grow in the same direction as a bulk material would do, with nevertheless a preferential growth direction at the beginning of the process. The greater the target to substrate distance, the better is the crystallinity of the films. Studies as a function of substrate temperature and/or further annealing will, however, be of interest, especially with regard to crystallinity and thermoelectric properties.

66

A. Dauscher et al. / Thin Solid Films 280 (1996) 61-66

Acknowledgements T h e authors are grateful to P. Cordier for assistance in laser e x p e r i m e n t s , to J.P. H a e u s s l e r for the electron m i c r o p r o b e analyses, to L, H e n n e t for the X R D analyses and to S. W e b e r for the SIMS analyses.

References [ 1] D. Biiuerle, Appl. Phys., A48 (1989) 527. [2] R.E. Muenchausen and X.D. Wu, in D.B. Chrisey and G.K. Hubler (eds.), Pulsed Laser Deposition of Thhz Films, Wiley, New York, 1994, p. 357. [3] X.X. Xi, in D.B. Chrisey and G.K. Hubler (eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994, p. 381. [4] Q. Li, in D.B. Chrisey and G.K. Hubler (eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994, p. 535. [5] E. Van de Riet, C.J.C.M. Nillesen and J. Dieleman, J. Appl. Phys., 74 (1993) 2008. [6] R.K. Singh, D. Bhattacharya, S. Sharan, P. Tiwari and J. Narayan, J. Mater. Res., 7 (1992) 2639. [7] L.D Hicks and M.S. Dresselhaus, Phys. Rev. B, 47 (1993) 12727.

[ 8 ] L.D. Hicks, T.C. Harmann and M.S. Dresselhaus, Appl. Phys. Lett., 63 (1993) 3230. [9] Y.H. Shing, Y. Chang, A. Mirshafii, L. Hayashi, S.S. Roberts, J.Y. Josefowiez and N. Tran, J. Vac. Sci. Technol. A, 1 (1983) 503. [10] H. Noro, K. Sato and H. Kagechika, J. Appl. Phys., 73 (1993) 1252. [ 11 ] J. George and B. Pradeep, SolidState Commun., 56 (1985) 117. [ 12] F. ViSlkleih,V. Baier, U. Dillner and E. Kessler, Thin Solid Fihns, 187 (1990) 253. [ 13] N.G. Patei and P.G. Patel, SolidState Electron., 35 ~1992) 1269. [ 14] T. Tanaka, M. Sakai, F. Kiya, Y. Ogawa, K. Mukasa, N. Sasa and J. Nagao, in K. Matsuura (ed.), Proc. Xll lnt. Conf. on Thermoelectrics, Institute of Electrical Engineers of Japan, Tokyo, 1994, p. 355. [ 151 ll-Ho Kim and Dong-Hi Lee, in K. Matsuura (ed.), Proc. XliInt. Conf. on Thermoelectrics, Institute of Electrical Engineers of Japan, Tokyo, 1994, p. 328. [161 E. Charles, E. Groubert and A. Boyer, J. Mater. Sci. Lett., 7 (1988) 575. [ 17] A. Boyer and E. Ciss6, Mater. Sci. Eng. B, 13 (1992) 103. [ 18] R. Venkatasubramanian, M.L. Timmons and J.A. Hutchby, in K. Matsuura (ed.), Proc. Xll h~t. Conf. on Thermoelectrics, Institute of Electrical Engineers of Japan, Tokyo, 1994, p. 322. [ 19] P. Magri, C. Boulanger and J .M. Lecuire, in B. Mathiprakasam (eds.), Proc. Xlll Int. Conf. on Thermoelectrics, AlP Press, New York, 1995, p. 277 [20] A. Mzerd, D. Sayah, J.C. Tadenac and C. Boyer, J. Mater. Sci. Lett., 13 (1994) 301.