Satyatn, P V

Sekar, K

Sundaravel, B

IACS Institutional Repository

Indian J. Phys. 70A (6 ), 681-685 (1996)IJP A— an international journalC h an nelin g  s tu d ies  on self-assem bled  A u4S i islands on B r-p assiva ted  S i ( l l l )  surfacesB .  S u n d a r a v e l ,  K .  S e k a r ,  P .  V .  S a t y a t n ,  G .  K u r i ,B .  R o u t ,  S .  K .  G h o s e ,  D .  P .  M a h a p a t r a  a n d  B .  N .  D e vI n s t i t u t e  o f  P h y s i c s ,  S a c h i v a l a y a  M a r g ,B h u b a n e s w a r  -  7 5 1 0 0 5 ,  I n d i aA b s tra c t  : Growth of epitaxial gold silicide inlands have been observed when an Au thin film, deposited on fir-passivated vicinal (4° misoriented) S i( lll)  substrate, was annealed around the Au -  S t eutectic temperature. The epitaxial silicide islands were observed over a thin uniform layer of gold silicide. Epitaxy of these islands and the thin layer underneath has been determined using Rutherford backseattering and channeling techniques. Both the gold silicide layer and the islands have been found to be epitaxial indicating a Stranski-Krastanov growth.Keywords : gold silicide, epitaxy, Rutherford backscattering and channeling.P A C S  N o s . :  6 8 . 3 5 . F x ,  6 8 . 5 5 . - a ,  6 1 . 8 0 . M ,  6 1 . 8 0 . M k1 . I n t r o d u c t i o nStudies of metal-silicon thin film reactions, kinetics of the reactions and prop­erties of the phases formed, have been stimulated by the technological im­portance of these systems. Metallic silicides, which arc formed by the reac­tion of a metal species with silicon, are finding a variety of applications in VLSI technology. Growth of an epitaxial overlayer could be a homoepitaxial growth (overlayer same as the substrate) or a heteroepitaxial growth (over­layer different from the substrate). Heteroepitaxial growth produces strained epitaxial layers due to lattice mismatch between the substrate and the over­layer. Strained epitaxial layers have interesting properties and are important in semiconductor devices [1]. For the growth of a silicide from a metal film on a silicon substrate, it is important that the diffusion across the interface is not hindered. The presence of an oxide layer at the interface is known to hinder the diffusion across the interface [2 , 3]. It has been shown that the diffusion behaviour is comparable in the cases where a metal film is deposited© 1996IACS682 B Sundaravel et alon a chemically prepared fir-passivated 5 t ( l ll)  substrate or an atomically clean 5 t(lll)(7  x 7) reconstructed substrate [3,4]. fir-adsorption in this case inhibits substrate surface oxidation.Growth of epitaxial gold silicide islands have been observed when an Au  thin film, deposited on fir-passivated vicinal (4° misoriented) S t( lll)  sub­strate, was annealed around the A u  — S i  eutectic temperature [5]. The fir- passivated substrates were prepared by a chemical method and A u  film de­position and annealing were performed under high vacuum. The composition and the general features of growth of the silicide islands are comparable to those obtained under ultra high vacuum conditions [6]. Energy dispersive x- ray (EDX) analysis results show the composition of the silicide islands to be A u 4S i .  From the EDX analysis the heights of our observed islands were esti­mated to be ~  1 f im . We have measured the island thickness for gold silicide islands on another sample with comparable initial A u  thickness by secondary ion mass spectrometry (SIMS). These silicide islands show a constant thick­ness of 1.23 /im and an abrupt silicide/silicon interface [7]. Also a concomitant growth of fractal patterns and nearly spherical islands of gold have been ob­served [8]. The islands grow in the shape of the equilateral triangle, reflecting the symmetry of the (111) substrate upto a critical size beyond which the sym­metry of the structure is broken resulting in shape transition from triangle to trapezoid [5]. Such shape transition has recently been identified to be one of the ways of relieving strain from the sytem undergoing epitaxial growth [9]. The island edges are aligned along the three equivalent {llO} directions of the S i(lll)  substrate. The elongated islands, instead of growing along three equivalent {110} directions, grow only along one preferential direction which has been attributed to the vicinality of the substrate [6].The alignment of the silicide islands with a specific crystallographic ori­entation of the substrate indicates that the islands are epitaxial in nature. However this is an indirect evidence of epitaxy. Rutherford backscattering spectrometry(RBS)/channeling is a good tool to characterize the crystalline nature of a material. With this technique it is very easy to find out whether an overlayer is in epitaxy with the underlying substrate. In RBS, we detect the number of backscattered incident ions and their energy using a surface barrier detector. When any of the crystallographic axis of the substrate is aligned with the incident beam direction, there will be a drastic reduction in the number of backscattered ions. This reduction in the number of backscattered ions, is a measure of the crystalline quality of the sample under study. More reduction corresponds to better crystalline quality. This is the basic principle behind this powerful non-destructive technique. There are books [10] and review ar­ticles [11] containing the detailed description of this technqiue. Here we haveemployed the combined RBS/channeling technique to probe the epitaxial na- ture of gold silicide islands on the Si substrate. Thus in this we show the direct evidence of epitaxy.2. E x p e r im e n ta lWe used a non-UHV method to obtain the gold silicide island structures on silicon substrates. The method involves gold evaporation onto a bromine- passivated 5 t ( l l l )  polished wafer (n-type, Si-doped, 0.005-0.02 ftem ) sub­strate and subsequent annealing. The details of the substrate preparation have been discussed elsewhere [3, 4]. B r adsorption on the S i( lll)  surfaces inhibits the surface oxidation process. A detailed x-ray photoelectron spec­troscopic characterization of the B r-treated Si surfaces has been published elsewhere [12].In the present work a 120 nm thin Au film was evaporated from a W  basket onto a Br-passivated S t ( l l l)  substrate at room temperature in high vacuum (3 x 10-4  Pa). Then the sample was annealed at 360°C, that is around the Au -  S i eutectic temperature (363°C), for 20 minutes in high vacuum. Upon annealing the Si substrate contained a very thin layer of Au4Si (~  40A) and on top of this uniform layer, in addition to the triangular and trapezoidal gold silicide island structures, stray gold was present. The stray unreacted gold was removed by dipping the sample in aqua regia for 2 minutes. This process left the gold silicide islands unaffected. The aqua regia etched sample was taken for the RBS/channeling studies. RBS/channeling measurements were made using a 2 MeV H e+ beam from the 3 MV tandem Pelletron accelerator in our laboratory. The experimental set up has been described elsewhere [4,11].3 . R e s u l t s  a n d  d is c u s s io n sRBS measurements were made on as-deposited samples, i.e., on the Au film de­posited on a jBr-adsorbed 5 t ( l l l )  surface as well as on annealed samples. The spectrum for the as-deposited sample (not shown here, but shown in Ref. [6]) indicates that the Au-film is laterally uniform. The thickness of the film has been determined from the RBS spectrum. The crystallinity of Au4Si islands has been determined with Rutherford backscattering spectrometry (RBS) and channeling measurements along [111] direction (which is closer to the surface normal of the substrate) as well as along the [110] axial direction using 2 MeV He+ beam. The size of the ion beam was 1.5 mm while the island size was of the order of microns to tens of microns.Fig. 1. shows the backscattering spectra for the He'*’ beam incident from a random direction and also for the beam aligned along the [111] crystallographic *008 of the substrate. A tiny peak appearing around higher channel number or energy (around ch. no. 1160), is due to Au from a thin uniform layer ofChanneling studies on self-assembled Au4Si islands etc 6g3684 B Sundaravel et alA u AS i .  (Here this peak signal is blown up by a factor of 10 for clarity). It is clear from this tiny peak that the thickness of the uniform A u 4 S i  layer is very small, around ~  40 A  (obtained from RBS simulation). Tailing towards the low energy side is due to the presence of about a micron thick gold silicide islands. Appearance of tails in an RBS spectrum could be due to real diffusion or due to non-uniformity of the surface of the sample. The spectrum for the aligned case (along [111] direction), shows a reduction in the backscattered signal in the tailing region though not prominent. This confirms that the A u 4 S i  islands are epitaxial. There is little reduction in the backscattering yield from the thin uniform layer.50004000^ 3 0 0 0 :P™4♦2^  20001000Aligned [ Hi ]  Random1X10 1 ? r. *1 ;f )§06 466 666 1 800 1000 1200Channel No.§06 46b' ebb' * so o  1006 i 200Channel No.Fig. 1 . RBS spectrum for the ran­dom and aligned condition for the goldsilicide/5t sample aligned along [1 1 1 ] direction of the S i Bubstrate.Fig. 2 RBS spectrum for the ran­dom and aligned condition for the goldsilicide/5t sample aligned along [110] direction of the S i substrate.Fig. 2 shows the backscattering spectra for the He+ beam incident from a random direction and also for the beam aligned along the [110] crystallo­graphic axis of the substrate. In this case one can notice a clear reduction in the backscattering yield, from the thin uniform layer as well as from the tailing region (from the islands). It is obvious from these two figures that there is reduction in the backscattering yield of gold in the aligned condition from the thin uniform layer as well as from the islands along both [111] and [110] directions. This confirms the crystallinity and alignment of the gold sili­cide islands. Greater reduction in the backscattering yield is seen in aligned spectrum along the [110] axis compared to the [111]. This implies that Au4Si islands have better lattice matching with the $*(111) substrate along [110] direction. This is in good agreement with the results of Green and Bauer[13]. They have shown for an ultrathin silicide layer, using low energy electron diffraction that either the [01] or the [10] direction of the gold silicide layer685is parallel to the [110] direction of the S i(lll) substrate. In the present work we have demonstrated that on the S i( lll)  surface, there is a thin uniform layer of epitaxial silicide on top which there are epitaxial silicide islands. This layer-plus-island growth conforms to the Stranski-Krastanov growth mode.4. ConclusionsRBS/channeling techniques have been used to characterize the epitaxial nature of the gold silicide islands on Br-passivated vicinal S i( lll)  substrate. The silicide islands, grown over a thin uniform layer of gold silicide, and the thin uniform layer were found to be in epitajgr with the underlying Si substrate. Although indirect evidence of epitaxy was obtained in previous works [5, 6], application of the combined RBS/channeling technique has provided direct evidence of epitaxy.R eferen ces[1] T. P. Pearsall, Critical Reviews in Solid State and Materials Sciences 15 551 (1989).[2] T Nakahara S Ohkura F Shoji T Hanawa and K Oura Nucl. Inst, and Meth. B45 467 (1990).[3] K Sekar P V Satyam G Kuri D P Mahapatra and B N Dev Nucl. Instr. and Meth. B71 308 (1992).[4] K Sekar P V Satyam G Kuri D P Mahapatra and B N Dev Nucl. Instr. and Meth. B73, 63 (1993).[5] K Sekar G Kuri P V Satyam B Sundaravel D P Mahapatra and B N Dev Phys. Rev. B51 (1995) 14330.[6] K Sekar G Kuri P V Satyam B Sundaravel D P Mahapatra and B N Dev Surf. Sci. 339 96 (1995).[7] B Sundaravel K Sekar P V Satyam B N Dev S Bera and S L Narasimhan P Chakraborty and F Caccavale (to be published).[8] K Sekar G Kuri P V Satyam B Sundaravel D P Mahapatra and B N Dev Solid. State. Comm. 96 871 (1995).[9] J Tersoff and R M IVomp Phys. Rev. Lett. 70 2782 (1993).[10] For example, W K Chu J Mayer M A Nicolet Backscattenng Spectrometry (New York: Academic) (1978).[11] K Sekar P V Satyam G Kuri B Sundaravel D P Mahapatra and B N Dev Indian. J. Phys. 6 8 A 1 (1994) and references therein.[12] K Sekar G Kuri D P Mahapatra B N Dev J V Ramana Sanjiv Kumar and V S Raju Surf. Sci. 302 25 (1994).[13] A K Green and E Bauer J. Appl. Phys. 47 (1976), 1284.Channeling studies on self-assembled AujSi islands etc70A (6)-5

Channeling Studies on Self-Assembled Au4Si Islands on Br-Passivated Si (111) Surfaces

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Channeling Studies on Self-Assembled Au4Si Islands on Br-Passivated Si (111) Surfaces

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