Applied Surface Science ~2 (19t)2) 11)5-114 North-! Iolland

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Epitaxial growth of ultrathin films of bismuth: an atomic force microscop~ study

J i n g J ing , P .N. H e n r i k s e n *, H.T . C h u a n d H o n g W a n g Physic~ IhT~artment. The I_'nit cr~io" rJl'.4kron. .-Iknm. 0tl 44325. US,4

Received 2;"; Fcbrual2.' 1092: acccpled ft~r publication 4 June 1992

Uhrathin films of bismuth have been grov.n cpitaxially onto mica substratcs by vapor deposition, and atomic fi)rce micro~,copv has been used to intagc the grox~th surface to delcrmine optimum cimdilions for fabricating films having the le;IM variatitms in thickness and largesl possible grain size. ]he films alv.ays grim ;'rt}m Ihree-dimensitmal islantls which coalesce to fi)rm cl3'stallites ~itl} their trigonal axis perpendicular to lhc mica surface. ('13Mallites as k,.~, micr,~metcrs squared with monatomic stepped topolog~ Luld an rmr, surface lOtlghner, r, on the order of [.1i nm have been obserx'et~, For mica substralcs, it has been fimnd that cleavage conditions, substrate preheating, substratc temperature during deposition. Bi deposition rate. and annealing temperature all affccl ml¢lcation ;rod gro~lh ot Bi films.

I. Introduction

In surface science ;uld meta ls physics, one imagines large single crystals which are free of dc lec t s and with uniformly smooth surfaces. Al- though CD'stals with perfect surfaces arc difficult to produce in the htboratory, they arc des i red since most theore t ica l predict ions, with which one wishes It) comp;irc measuremen t s . ;ire based on ideal systems. Of ten fl~r surface :;tudies it is not always pract ical to cut and polish crystals grown from the melt . thcrcG)rc, processes for producing cpi taxial films of high crystal l ini ty and with uni- form surhiccs ;ire of u tmost impor tance in pure and appl ied research. The growth process, in itself, is of fundamenta l interest because meta l films do not usual ly grow layer-by-layer over large surface areas (Van dc r Mcrwe mechanism) , but ins tead grow from island format ions which then coalt2scc to form cont inuous s t rucl t l res ( V o l m e r - W e b e r mechan ism) [I]. The objective of this work has bccn to grow ul t ra th in epi taxial films of bis- muth with uniform thickness on mica substratcs . In our case, these films are used for q u a n t u m .dTe

+ I-o ~qltml ct}rresptmdence shnuhl be atltlrc',sed. Phtme 21t~- q72-fil)54. Fax 21fi-972-53()1.

effect m e a s u r e m e n t s [2]. but they have appl ica- t ions in molecu la r adsorp t ion p h e n o m e n a [3], non l inear opt ics [4] and may serve as subst ra tes for addi t iona l s tudies.

To date . epi taxial b ismuth films have becn grown on e i the r BaF, [5.6] or mica [2.7] sub- strafes. Nahm et al. [5] repor ted that when bis- muth films were grown on BaF, substrates , the growth of an amorphous or polyerystal l ine layer about 90 nm preceded the onset of epitaxy. To the best of our knowledge, u l t ra thin ( th ickness < 50 nm) cpi taxial films of b ismuth with reason- ably smooth surfaces have never been grown on

e i the r BaF, or mica. The cryslal l ine s t ructure and surface morphol-

ogy of a film can bc severely affected by the growth parameters : the subs t ra te temperature. rate of film deposition, annealing temperature and duration, etc. In this communication we re- port the use of an atomic force microscope (AFM) to evalua te the t )pt imum condi t ions for cpi taxial film growth. The A F M allows one to ab ta in topo- graphical images of the film growth .surface which ,:an be analyzed to evalua te the effect of growth parameters . Such an approach is still in the regime of " 'art-based technology" [8]. but ca~: be benefi- cial in developing epitaxial film g ro~ th as it "'sci-

106 !i;tg .ling et aL / EpittL~ial grou'th of ultrathin fihns of hi.~muth

Fig. 1. AFM images of the surface of mica: (a) 10 × 10/zm 2 image of mica surface cleaved by the described method. (b) 10 × I0 /zm-" image of mica surface cleaved by using an adhesive tape, (c) image with atomic resolution.

Jing Jing et aL / l-pitfa'ial gnm'th of tdmtthin fihns of bismuth 107

Fig. I (continued).

ence-bascd technology". The experimental proce- dure for film fabrication and the AFM arc de- scribed in section 2. The optimum combination of growth parameters, as related to this research, is presented in section 3, Also included is a compar- ison of films, grown with, respectively, the opti- mum and off-optimum conditions. A brief sum- mary is given in section 4.

2. Experimental

2.1. &m~ple preparation

High-purity (99.9999%) bismuth was thermally evaporated onto a freshly cleaved ruby mica sub- strate in a liquid-nitrogen-trapped oil-diffusion- pumped vacuum chamber. The pressure was _< 1.0 x I() -7 Tort before, and never rose above 5.0 × 10-7 Torr during an evaporation. Two side- by-side mica substrates were used during each growth; one served as a control and was never heated during the entire procedure. The surface

of the control was always imaged with the AFM for a comparison of the effect of parameters under consideration, and to ascertain that there were no "hidden" or uncontrolled parameters which affected growth over the duration of all measurements. The second substrate was pre- heated to various temperatures and for extended periods to monitor the influence of surface des- orption and thermal conditioning of the substratc before growth commenced. A substrate heater [9] was used to control the substrate temperature with an accuracy of +5°C, and the heater current versus substrate temperature was calibrated against a chromel-constantan thermocouple, The preheating temperatures ~f the substrate ranged from 100 to 160°C with an interval of 10°C. The substrate was kept at the preheated temperature during the film deposition. Film deposition at the room temperature without preheating the sub- strate was also made for comparison. Both the rate of deposition and film thickness were moni- tored and controlled with a quartz crystal mi- crobalance. The deposition rate ranged from 0.5

11)8 Jing Jing et al. / l-pihtriul gron'th o]'tdtrtlthin flhns o]'bl.~muth

Fig. 2. AFM surface image of a 30 nm bismuth film (suhstrate temperature 14I]°C. deposition rate [).5 nm/s. annealing temperature 16D°C. annealing duration 1 h): (ul top view. (b) three-dimensional view.

Jing Jing et aL / l-pittLrial growth

tO t.7 n m / s for films of thickness of 30 through 300 nm. After the desired thickness was reached, the deposition was stopped and the film was either annealed at a temperature ranging from 100 to 185°C or left to cool without annealing. The annealing duration ranged from 45 min to > 2 h. After annealing, the sample was left in the chamber to cool for at least 30 min before venting with dry nitrogen, thus completing the fabrication process.

2.2. A tomic force microscopy

A commercial AFM, manufactured by Digital Instruments, Inc. (Nanoscope I1), was used to obtain topographical images of the growth sur- faces [10]. The AFM utilizes an optical lever in combination with a microfabricated Si3N 4 tip and cantilever, and images were obtained by scanning the tip over the surface with constant force.

o]'tdtrathitt films o[ hisnutth 109

3. Results and discussion

After imaging the surfaces of many films grown under various conditions, it is concluded that both pre- and post-growth factors, as well as growth parameters, affected the final results of film growth. For films ranging in thickness from 30 to 300 nm the prescription for high-quality films is as follows. A wooden or metal blade should be used for cleavage apd the mica sheets should be separated slowly. After separation of the mica sheets commences, it is helpful to add a drop of deionized water between the layers to promote separation along a particular layer. (Lift- ing a layer of mica with adhesive tape usually damages the underlying surface and provides a poor substrate for epitaxial growth.) The mica substrate should be preheated to 140°C for at least 2 h before film deposition starts, and kept at this temperature during the deposition. The rate

Fig. 3. AFM surface image of a 30 nm bismuth film (substrate lemperature 140°C. deposition rate 0.5 nm/s. annealing temperature 185°C. annealing duration I h).

110 Jing Jing et al. / Epitetrial gr~.,wth of u/trath#l fi/ms oJ'bismuth

of deposition should be as close to 0.5 ~lm/s as possible (minor fluctuations are unavoidable). The annealing temperature should be 160 +_ 5°C, and the annealing time should be no less than an hour.

Figs. la and lb show the AFM images of mica surfaces prepared by the n 'ethod mentioned above and by the use of an adhesive tape, respec- tively. It can be seen that the ~urface uniformity and smoothness is much better ~f the former. Fig. lc shows the topography of the surface in fig. la on atomic scale, showing he hexagona l / triangular structure which is suitable for growing epitaxial bismuth films along the trigonal direc- tion. The effect of the different cleavage of mica on the overgrown film has not been specifically studied. The rms surface roughness of a bismuth film was found, however, to be smaller when the smoother mica substrate was used.

Preheating the substrates to 140°C is probably required to remove surface-bouna OH groups introduced in cleavage, and the elevated temper- ature during deposition provides enough interra- cial energy for the Bi atoms to diffuse across the surface while maintaining a nucleation rate higher than the lateral spreading rate. The slightly higher annealing temperature is then required to re- move stresses and defects caused by the high nucleation rate. Figs. 2 and 3 show images of 30 nm films grown under opt imum conditions as stated above with the exception of annealing. Fig. 2 shows an image of a film annealed at 160 ° for 60 min: (a) is a topview, (b) is a three-dimensional view. The surface is s tepped with smooth planes and a r m s roughness of 0.82 nm. The triangular structured islands seen on the topographic image (taken on the thousand nm scale, as shown in fig. 2) are likely the "macroscopic" accumulation of

Fig. 4. AFM surface image of a 50 nm bismuth film (substrate temperature 140°C. deposition rate 1.7 nm/s. annealing temperature t600C, annealing duration I h).

Jing Jing et al. / Epitaxial growth of uhrathin fibns of bisnutth

the microscopic hexagonal lattice cells in the trig- onal planes (planes normal to the trigonal axis) of the rhomhohedral bismuth crystal. In a previous work [11], the hexagonal lattice structure in a bismuth film has been observed with AFM when imaging the surface with resolutions on the atomic scale, Diffraction pattern with a transmission electron microscope showing the hexagonal pat- tern has also been observed previously [2]. Thus, the crystallites in bismuth films grown on mica are perpendicular to the trigonal axis and the film growth is epitaxial. Fig. 3 shows the results of annealing at 185°C for 60 min. This image shows crystallites which ate irregular in size and orien- tation, and the surface roughness has increased to a r m s value of 1.88 nm. As a result of many growths, annealing has been found to be the most important factor in producing smooth films in which islands coalesce to form continuous planes with the fewest steps and surface defects (see fig.

2). The best surfaces were produced by annealing at 160°C; annealing at temperatures 25°(2 above or below this value produced noticeably rougher surfaces and with considerably more defects.

Higher deposition rates followed by annealing do not produce better-quality films. Fig. 4 shows the result of a 50 nm film grown with a deposition rate of 1.7 n m / s (substrate temperature 140~C and annealing temperature 160°C). The grains are irregular in size and shape, and have little triangular faceting. The rms surface roughness is 2.52 rim, which is greater than that of fig. 2.

Fig. 5 shows a series of four images of a 300 nm thick film: (a) is an image of 10 x 10 /~m 2 area, (b) is a 5 x 5 t~m 2 area, (c) is a 1.6 × 1.6 /.tm 2 area, and (d) is a 1.2 × 1.2/*m 2 image. The surface of this film has highly ordered triangular structures, and the average grain size is about the same as that of the 30 nm film grown under the same conditions (fig. 2). In general, the stepped

Fig. 5. AFM surface image of a 30(~ nm bismuth film (substrate temperature 140°C. deposition rate 0.5 nm/s. annealing temperature 160°C. annealing duration ! h): (a) image of I0 x I0 pm" area. (b) .q x 5 p.m z area. (e) 1.6 x 1.6 pm 2 area and (d)

1.2 x 1.2 g, ra-" image.

112 Jing Jing ct aL / Epittttinl growlh of tdtrnth#l fihns of bismuth

surface is a common feature of bismuth films. The surface is made up of an array of triangular terraces where each additional layer is smaller than the underlying layers. As a result, the sur- face of thick films terminates in an array of pyramidal apexes which can be clearly seen in figs. 5c and 5d.

Based on these observations, we can tenta- tively conclude that the bismuth films start with the nucleation and formation of small islands, possibly in t r iangular /hexagonal shapes, each is- land grows layer by layer. The coalescence of the islands may result in a larger triangular grain if the boundaries match crystallographically per- fectly or crystal defects like twins, stacking faults, grain boundaries, etc. are resulted. The higher density of the defects in films makes thin films less single-crystalline than the bulk crystal. A study of the crystal defects in epitaxial bismuth films on mica will be reported separately.

Films grown under the above conditions have been used for galvanomagnetie measurements . Results from 12 films in the thickness range from 30 to 300 nm show a systematic and explicable dependence of magnetoresistance and Hall resis- tance on film thickness [12]. Because of the rea- sonably good single-crystallinity, Shubnikov-de Haas oscilla:ions in the magnetoresistance have also been c,bserved. On the other hand, films grown with conditions other than the opt imum combination showed galvanomagnetic properties that exhibited little systematic dependence on either the lilm thickness or the magnetic field.

4. Summary

Atomic force microscopy has been used as an aid in determining opt imum conditions for grow- ing epitaxial films of bismuth on mica in the

Fig. 5 (continued).

Jing Jin1~ el al. / EpiftLrial growth of ullmthin films of bi.*nltlth

30-300 nm thickness range. It has been found that substrate cleavage, preheating, substrate temperature during deposition, deposition rate. and annealing temperature and duration all af-

feet epitaxial growth. Epitaxial bismuth films have a stepped surface consisting of equilateral trian- gular terraces which is a result of the trigonal axis being normal to the substrate surfaces. Over the

Fir.. 5 (cominu-d).

Jing Jing et aL / Lpitaxial gn,a'th of ultra{bin films of bismuth

r a n g e o f t h i c k n e s s e s i n c l u d e d in this work , t h e a v e r a g e r m s s u r f a c e r o u g h n e s s d e c r e a s e d as t h e f i lm th i cknes s i nc rea se s .

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[7] N. Garcia. Y.H. Kao and M. Strongin. Phys. Rev. B 5 (1972) 2029.

[8] W.A. Tiller. The Science of Crystallization: Microscopic lqterfacial Phenomena (Cambridge University Press. Cambridge. 1991) p. 6.

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