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LEED-AES study of the AuSi(100) system
LEED-AES STUDY OF THE Au-S@ 00,) SYSTEM
Received 19 September 1978; manuscript received in final form 14 Novcrrnbsr 1978
The system A$%[ $00) has been sttzdied using LEEX) and AES. Au films grow as Artfl1f) tt S~~~~~~hatig six ~~u~~y rotated ~~~~~a~~~§ at low dep&tim t~~~~~~~~r~~ bekswWC after the formation of intermediate gold sificide layers. CrystaWe g&d silicide thin layers are formed on the Au(l11) fXm after heat treatment at 10%400°C. Two types ofsifkide LEEI? pattern observed seem to have no correlatiun with crystallographic data reparted on quenched alloy films. Heat treatment over 450°C leads to agglomeration of the film, producing a series of Au-induced superstructures. Heat treatment of the Au film over 1000°C regenerates the clean Si surface accompanied with many etch pits.
of
Most structural investigations themetallic f&n on silicon have been concerned with the (111) surface of the substrate, amang which the system Au/Si( 111) is the one most thoroughly studied. Le Lay et al. did a series of detailed studies on this system [l-4], and established the appearance condition of a given superstructure among various ones leading to the 2D phase diagram for lower eoverages (coverage is used instead of ~o~~e~trat~on). As for the goId silicide, on& an ~o~hous or disordered couture was detected on thi gold films of a few tens of an rS deposited on the ($11) fs] and on the (100) [6] sur faces at around room temperature; heating these films at high temperature (about 5OO-700°C) did not lead to the crystals lized silicide but to superstructures. Green and Bauer [S] showed that enough thickness was necessary to obtain well crystallized silicides by moderate annealing. They obtained three types of silicide LEED pattern, and assigned a unit mesh for one phase, but could not go further owing to the lack structural information of the bulk. We have &ready reported f6f supe~~~~tures fcxmed on the flQO> surf;tce but not giving the sihcide. Tn the preser~t work, we co&d produce crystaltine surface s&ides on thick Au fifms deposited on Si( 3OO), and assigned unit meshes for them, which were different from the silicide grown on the Au/%( 115) system 151, though the underlying Au(lll> face was common in both cases. In addition, the heat
of
cleaning process for this system was also studied. 202
K. Oura, T. Hanawa, LEED-AES study of Au-Si(lO@ system
203
2. Experimental A three-grid LEED-AES optics was used, the base pressure of the work chamber was 3 X lo-” Torr. Gold of 99.99% purity was evaporated from a tungsten basket heater under a vacuum of 1 X lop9 Torr at a rate about 5 A/min, which was estimated indirectly by measuring the thickness of the Au film deposited on a glass plate placed midway between the source and the substrate. A conventional method of Ar ion etching and AES monitering was applied to obtain the depth profile of the sample. To estimate the etching rate of the film, a given thickness of Au film deposited on a NESA glass was sputter etched under a given condition and its removal time was measured by AES. In the present work, an Ar ion beam of 500 eV energy and current density of 1.8 X lo-’ A/cm2 produced an etching rate of 4 A/min. The substrate was a mirror-polished, 2-3 $2 cm, P-doped Si(lO0) wafer (5 X 25 X 0.5 mm), which was etched with hydrofluoric acid and rinsed with distilled water just before installing it into the LEED system. The specimen was resistively heated and its temperature was measured by a fine Pt-PtRh thermocouple. To avoid alloying at high temperatures, the thermocouple was spot welded to a small piece of Ta foil pressed on the back face of the sample; a systematic error expected in such a system was corrected by comparing the thermocouple reading and the optical pyrometer reading at higher temperatures. Repeated flashing of the specimen at 1200°C produced a well-de~ned Si(lOO)-(2 X 1) LEED pattern, where only a trace of carbon was detected by AES. The argon ion bombardment-~eal~g method was also tried for cleaning, but no difference was observed in the LEED pattern and or in the Auger spectrum. LEED-AES observations were made at around room temperature to follow the change of the surface caused by deposition and annealing, and a series of scanning electron microscopic observations was made on specimens removed from the UHV system to supplement the LEED-AES analysis.
3. Result 3.1. Low energy electron diffmction When Au was deposited onto clean Si surfaces at around room temperatures, the (2 X 1) pattern gradually dissolved into a strong background, but a very diffuse and weak ring became detectable at the exposure of about 30 A, the spacing of the ring corresponding neither to Si nor to Au. At this stage the Si LW (92 eV) Auger peak split into 90 and 95 eV peaks, suggesting the formation of gold silicide layer [5-83. The diffuse ring became gradually invisible and new diffuse spots arranged on a circle appeared as the Au deposit increased, these spots became somewhat sharper at higher exposure: LEED patterns obtained at exposures of 60 and 300 A are
204
K. Oura, T. Hanawa, LEED-AES
study of Au-Sill
00) system
Fig. 1. LEED patterns obtained from Au films deposited onto Si(100) at room temperature: (a) clean Si(100)<2 X 1) (49 eV); (b) 60 A Au film (49 eV, the sample is tilted to the left side and the (00) spot cannot be seen in the pattern); (c) 300 A Au film (57 eV); (d) schematic illustration of the pattern shown in (c).
in fig. 1. The spot pattern was analysed as due to Au(ll1) (1Si(100) having six azimuthally rotated orientations, among which the dominant ones (strong spots) were Au[Oll] I] Si[Oll] and Au[Ol l] II Si[Oil] and the rest were rotated by +lO” from the dominant orientations; these orientations were illustrated in fig. Id. Auger spectra taken from specimens having thick Au films showed no Si peaks but only Au peaks. Heat treatment of these specimens at around 150°C for a few minutes was enough to reduce the Au( 111) spots considerably in the increased background. Further heating at around 350°C for a few minutes produced new LEED patterns of spots or arcs or rings as shown in fig. 2. However, the condition leading to a specific LEED pattern was not found out. Anyways, the fact that Auger spectra for these specimens show the doublet peaks of Si characteristic of silicide indicates that these LEED patterns are due to domains of gold silicide. Since any given LEED pattern may have occasional extinction of some diffraction spots, it is safe to synthesize several of them to avoid misinterpretation. The completed pattern is shown
K. Oura, T. Hanawa, LEED-AES study of Au-Sill 00) system
Fig. 2. LEED patterns from crystalline gold silicides formed after heating thick Au films (300500 A) at 350°C for a few minutes: (a) spot pattern (72 ev), (b) arc pattern (35 eV), (c) ring pattern A (34 eV), (d) ring pattern B (48 eV).
shown in fig. 3, where circles passing through spots are drawn for convenience. Interpretation of this figure was difficult, but the superposition of hexagonal unit meshes (four orientations) and rectangular ones (six orientations) shown in fig. 4 was found to have the least failure; only very weak spots marked by open circles in fig. 3 were remained unidentified. The size of the hexagonal unit mesh was measured as a = 4.3 + 0.2 8, and that of the rectangular one a = 8.1 + 0.3 A and b = 9.0 + 0.3 A. According to this interpretation every reciprocal lattice point (hk) having k = even for the rectangular mesh must be lost. Atomic arrangement inside the unit mesh satisfying this extinction rule was difficult to derive. We observed two types of ring pattern, A and B, where A was just the same as the rotated pattern, and B was not. But B was most likely to the pattern of “gold silicide I” formed on Si( 111) reported by Green and Bauer [S]. All arc patterns were found to have spacings of the ring pattern A but not B. The gold silicide patterns described above persisted up to about 400°C. Heating
206
K. Oura, T. ~u~awa, LEED-AES study
of Au-S(lOO)
system
siLO111
0.1A-’ Fig. 3. Completed LEED pattern obtained after synthesizing the silicide spot patterns at various electron beam energies (25-85 eVf.
t Si [oil]
(b) Fig. 4. Unit meshes derived for the gold silicide pattern shown in fig. 3: (a) hexagonal unit mesh; (b) rectangular unit mesh.
K. Oura, T. Hanawa, LEED-AES study of Au-Si(lO0) system
201
over 400°C
produced a series of Au-induced superstructures as reported in the previous paper [6,17]. A few minutes heating in a temperature range of 450700°C produced Si(lOO)-c(8 X 2) which converted into a structure Si(lOO)(d26 X 1) at 75O”C, and (d26 X 1) changed to (426 X 3) after heating over 8OO’C. Heat treatment above 1000°C regenerated the (2 X 1) pattern, the transition took place abruptly after an induction period determined by the amount of deposited Au. Though repeated regeneration was possible, the background of the LEED pattern was found to increase with repetition. The temperatures cited above do not mean definite transition temperatures but indicate a temperature at which a rapid transition takes place after a few minutes heating. In addition, at the beginning of the appearance of c(8 X 2), the diffraction spots were almost hidden by a strong background, the temperatures in the range of 600-750°C being necessary to produce a well-defmed c(8 X 2) as reported before [6,17]. 3.2. Auger electron spectroscopy Deposition of Au at room temperature caused a rapid decrease of the characteristic Auger peak at 92 eV of clean Si accompanied with the appearance of two peaks at 90 and 95 eV which has been interpreted as due to the gold silicide [5-81; the exposure of 20 A was enough to complete the conversion. After exposure over about 100 a, the double peak disappeared and only Au peaks were developed, of which a broad peak at about 95 eV was interpreted as being due to the Au OW peak from the observation intentionally made on an Au film deposited onto a Ta
Cd)’
kd ’
(e) ’
(f)
92 ELECTRON
ENERGY
(e’f)
Fig. 5. Auger spectra taken at successive stages of the deposition at room temperature and the following heat treatment: (a) clean Si; (b) 30 A Au film; (c) 300 A Au film; (d) after heat treatment the specimen (c) at 150°C for 2 min; (e) 3OO”C, 2 min; (f) SOO”C, 3 min.
a
5
15
10 HEATING
TIME
AT 1oooC
25
20 ( MIN
)
Fig. 6. Changes of Auger peak heights of Au (70 eV) and Si(92 eV) peaks during heat treatment of the 100 A film at 1000°C.
sheet. Heat treatment of an Au film having enough thickness (300-500 A) between 100 and 400°C caused the regeneration of the double peak, which changed to a single peak at 92 eV (Si LVV) after heating over 450°C. In the case of very thin ftims, the double peak could persist up to 300°C over which the conversion into the single peak took place. The intensity of the single peak increased with the temperature, and reached the originaf value after heating over lOOO*C. The above-stated features of the Auger spectra are illustrated in fig. 5. Auger peak heights of both Au 70 eV and Si 92 eV during heat treatment at 1000°C are plotted against time together with the LEED pattern in fig. 6, where correspondence between the plateau of the curve and the (J24 X 3) LEED pattern is evident. To make this process clearer, SEM observations were made on specimens, taken at successive stages of the annealing. Figs. 7a and 7b suggest that the (426 X 3) pattern comes from a flat area between the Au islands, which were identified by the accompanied X-ray microprobe analysis. The increased background in the initial stage of the appearance of t”26 X 3) can be interpreted as due to the high density of the small Au islands dispersed on the flat area. It was difficult to take samples for SEM observation at the stage of just changing to the clean SWface, but heating of the (d26 X 3) specimen was found to reduce the density of both larger and smaller Au islands, while AES levels of both Au and Si did not change. It was also found that the regenerated clean surface was accompanied with
K. Oura, T. Hana~a, LEED-A/B
study ofAu-S~ii~O~ system
Fig I. .S cannti ng electron micrographs taken at various stages of UHV heat tre atr nen t: ( a) 100 A AU fil m 1000’‘C, 1 min (initial stage of the (J26 X 3) structure); (b) 120 A AIu fi h 1000”c, 18: mii01[&la1 stage of the (426 X 3) structure); (c) 300 A Au film, 1000’ C. 80 mi n (regenerai :ed Cl ean Si surface}.
’
Fig. 8. ~ep~-co~~o~~~~~ profife for a Au fitm on s&lo@] after heat treatment at 250°C for a few minutes. The peak heights of Au (70 eV) and Si (90 or 32 etr) have been normalized to their pure states, respectively. Sputtering rate for pure Au films is about 4 A/min.
etch pits as shown in fig. 7c; the surfaces of both the initial clean Si and the silicide showed no contrast in the SEM pattern, whatsoever. Fig. 8 shows the depth-composition profile for a silicide sample. The normalized peak heights of two Auger peaks are shown: a Au peak at 70 eV, and a Si peak which varies in cha~cter~tic energy from 90 to 92 eV while sputtering through the AujSi interface. To simplify the analysis, the data of fig. 8 are divided into four regions. In region A, the double peak characteristic of silicide was observed at 90 and 95 eV. Upon entering region B, the double peak was hardly visible. In region C, the double peak reappeared, while in the hatched region it gradually converted into the single peak at 92 eV and hence no Si peak height was plotted in the figure. In region D, the Si single peak was dominantly detected. In addition, no change in the ch~acteristic shape and energy for the Au Auger peak at 70 eV was observed in regions A-D. These experimental results suggest that silicide layers are formed both on the outer surface of the Au film and in the Au-Si interface. The outer silicide layer is very thin, while the other is much thicker. The AU peak which persists even after long sputtering periods (region D) can be interpreted as due to the atomic mixing caused either by the so-called knock-on effect [18] of Ar ion beam or by diffuseness of the interface itself. The minute concentration of Si possibly resting in the Au film (region B) cannot be determined in the present study.
4. Discussion 4.1. Gold silicide The present study revealed the formation of thin gold silicide on both sides of Au(ll1) fdms deposited on Si(lO0) surfaces. This feature and its stable region
K. Oura, T. maraca, LBED-AES study of Au-Si(100)
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400°C) are the same as in the case of the Au-Si(1 11) system reported by Green and Bauer (GB) [.5]; however, the detailed structures seem to be different from each other for the following reasons: (1) In the present study, the complex pattern is analysed as composed of six orientations of a rectangular mesh (a = 8.1 2 0.3 8, b = 9.0 zt 0.3 A) and four orientations of a hexagonal mesh (a = 4.3 f 0.2 a), while GB derive nine orientations of the rectangular mesh (a = 7.35 8, b = 9.35 a) and no hexagonal mesh. (2) In both cases, derivation of the rect~~lar unit mesh was impossible without assuming missing spots; however, the extinction rules are different from each other as shown in fig. 9. The crystallographic data of gold silicide studied so far are summarized in table 1, which strongly suggests the existance of many structures according to the formation condition. Unit meshes of surface silicides derived in this study and also by GB seem to have no correlation with the unit cells found in the quenched Films. The composition of the silicide was estimated from the ratio of Auger peak heights of Au (70 eV) and Si (90 eV) corrected against their pure states; the result of about 15 at% of Si is comparable to that of GB and may correspond to the formula AusSi. (below
4.2. Oriented overgrowth of Au The observation that six definite orientations of Au( 111) II Si( 100) appear after a ring pattern of the silicide has been formed at room temperature (less definite orientation of Au{1 11) films on Si(100) surfaces has been reported in the X-ray work by Fisher and Wissman [14,X5]), seems to suggest that the orientating effect of the substrate propagates through the thin silicide layer, though the detailed mechanism is not known. It is interesting to note that different silicides are formed on the (111) planes of an Au film according to whether they are grown on the (111) face of Si or on the (100) face. This may suggest that the difference in imperfection and/or strain in the Au film affects the structure of the silicide.
212
K. Oura, T. Hanawa, LEED-AES
study of Au-Sill
00) system
K. Oura, T. Hanawa, LEED-AES
study of.&-Sill
00)
system
213
FILM THICKNESS (monolayer) Fig. 10. Schematic illustration of the surface structure of the Au/%( 100) system.
4.3. S~perst~~tures The condition of the appearance of the superstructures is schematically represented in fig. 10, together with that for silicide and Au(lll). Coverages of Au for the completion of c(8 X 2), (d26 X 1) and (426 X 3) were not determined exactly, but a 0.5 monolayer for c(8 X 2) and a 1 monolayer for (426 X 3) were estimated from the exposure assuming a sticking coefficient of unity. The reason why the notation (d26 X 1) was used instead of (5 X 1) was given in a previous paper [6], In the figure, the temperatures distinguishing three superstructures do not mean definite transition temperatures but indicate a temperature at which a rapid transition takes place, higher temperature being necessary for specimens of thicker Au deposit. 4.4. Regeneration of clean Si surface The initial very slight decrease of the Auger signal of Au followed by the sudden rapid decrease at high temperatures, as shown in fig. 6, can be easily interpreted if we adapt the idea that Au is mainly evaporated through a 2D adsorbed gas state equilibrated with Au islands on the surface of the (426 X 3) as long as Au islands exist. In other words, the 2D gas and the underlying adlayer of (426 X 3) will evaporate very quickly as soon as Au islands are consumed out; the latter stage seems to correspond to the case of Au-Si(1 11) system as reported by Le Lay et al. f 11. Change of LEED patterns during the course of the above-stated rapid transition was observable only for very thin deposits. Heat treatment of a few monolayers of Au film at 850°C successively produced (426 X 3), (d26 X l), c(8 X 2), and finally (2 X 1). The absence of small Au islands near the larger island shown in fig. 7 suggests the easy movement of the small Au islands or atoms, which supports the above-stated evaporation mechanism. Also, the formation of etch pits found in the regenerated clean Si(100) surface can be understood considering the rapid diffusion of Si atoms [ 161 through Au islands; the crystalline shaped deposit on the
Au islands identified.
found
in figs. 7a and 7b may be segregated
Si, though it cannot
be
5. Summary We have studied the structure of Au films deposited on Si(lOtJ) clean surfaces using LEED and AILS. The experimental results suggest: (1) Au flms grow as rotated orientations at low deposition Au(l11) li Si( 100) with six azimuthally temperatures below about 50°C after the formation of intermediate layers of gold silicide. (2) Crystalline gold silicide thin layers are formed on. the Au( 111) film after heat treatment in the ragne of IOO-35O’C. The unit meshes assigned to the silicide are found to have no correlation with crystallographic data reported on the quenched Au-Si alloy films. (3) The gofd silicide is stable up to 400°C. Heat treatment at higher temperatures (above 450°C) leads to a~omeration of the film, producing a series of gold-induced superstmctures of Si(100). (4) Thermal desorption of Au films occurs over 1000°C and clean Si(2 X 1) surfaces are reproduced, while the regenerated clean surface is accompanied by etch pits. Acknowledgements The authors would like to acknowledge the contribution of the Welding Research Institute of Osaka University in taking scanning electron micro~a~hs. They would also like to thank Mr. K. Sato for his assistance in the experiment. This work was partly supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Japan. References fl] $2] [3] [4] [S] [6] [7] [S] f9)
G. LeLay, M. Mannewille and R. Kern, Surface Sci. 65 (1977) 261, G. LeLay and J.P. Faurie, Surface Sci, 69 (1977) 295. G. LeLay, G. Quentel, J.P. Faurie and A. Masson, Thin Solid Films 35 (1976) 273. G. LeLay, G. Quentel, J.P. Faurie and A. Masson, Thin Solid Films 35 (1976) 289. A.K. Green and E. Bauer, J. Appl. Phys. 47 (1976) 1284. K. Oura, Y. Mtlkino and T. Hanawa, Japan. J. Appl. Phys. 15 (1976) 737. T. Narusawa, S. Komiya and A. Hiraki, Appl. Phys. Letters 20 (1972) 272. A. Hiraki and M. Iwami, Japan. J. Appl. Phys. Suppl. 2, Pt. 2 (1974) 749. T.R. A~nt~a~srn~, H.L. Luo and W, Klement, Nature 210 (1966) X040. IlO] P. Frtxiecki, B.C. Giessen and N.J. Grant, Trans. AIME 233 (1965) 1438. fll] R.C. Kxuteaat, JX, Tien and D.E. Fornwalt, Met. Trans. 2 (1971) 1479. [12] GA, Anderson, J.L. Bestel, A.A. Johnson and 8. Post, Mater. Sci. Eng. 7 (1971) 83. i13] C. Suryanarayama and T.R. Anantharaman, Mater. Sci. Eng. 13 (1974) 73. [14] W. Fischer and P. Wissmann, 2. Naturfobrsch. 31a (1976) 183. [15] W. Fischer and P. Wissmann, Z. Naturforsch. 31a (1976) 190. [ 161 A. Hiraki, MA. Nicolet and J.W. Mayer, Appl. Phys. Letters 18 (1971) 178. [17] K. Oura and T. Hanawa, in: Proc. 7th Intern. Vacuum Con& and 3rd Intern. Conf. on Solid Surfaces (Vienna, 1977) p. A-2753. 1181 M. Kaminsky, Atomic and Ionic Impact Phenomena on Metal Surfaces (Springer, Berlin, 1965) p. 218.
Received 19 September 1978; manuscript received in final form 14 Novcrrnbsr 1978
The system A$%[ $00) has been sttzdied using LEEX) and AES. Au films grow as Artfl1f) tt S~~~~~~hatig six ~~u~~y rotated ~~~~~a~~~§ at low dep&tim t~~~~~~~~r~~ bekswWC after the formation of intermediate gold sificide layers. CrystaWe g&d silicide thin layers are formed on the Au(l11) fXm after heat treatment at 10%400°C. Two types ofsifkide LEEI? pattern observed seem to have no correlatiun with crystallographic data reparted on quenched alloy films. Heat treatment over 450°C leads to agglomeration of the film, producing a series of Au-induced superstructures. Heat treatment of the Au film over 1000°C regenerates the clean Si surface accompanied with many etch pits.
of
Most structural investigations themetallic f&n on silicon have been concerned with the (111) surface of the substrate, amang which the system Au/Si( 111) is the one most thoroughly studied. Le Lay et al. did a series of detailed studies on this system [l-4], and established the appearance condition of a given superstructure among various ones leading to the 2D phase diagram for lower eoverages (coverage is used instead of ~o~~e~trat~on). As for the goId silicide, on& an ~o~hous or disordered couture was detected on thi gold films of a few tens of an rS deposited on the ($11) fs] and on the (100) [6] sur faces at around room temperature; heating these films at high temperature (about 5OO-700°C) did not lead to the crystals lized silicide but to superstructures. Green and Bauer [S] showed that enough thickness was necessary to obtain well crystallized silicides by moderate annealing. They obtained three types of silicide LEED pattern, and assigned a unit mesh for one phase, but could not go further owing to the lack structural information of the bulk. We have &ready reported f6f supe~~~~tures fcxmed on the flQO> surf;tce but not giving the sihcide. Tn the preser~t work, we co&d produce crystaltine surface s&ides on thick Au fifms deposited on Si( 3OO), and assigned unit meshes for them, which were different from the silicide grown on the Au/%( 115) system 151, though the underlying Au(lll> face was common in both cases. In addition, the heat
of
cleaning process for this system was also studied. 202
K. Oura, T. Hanawa, LEED-AES study of Au-Si(lO@ system
203
2. Experimental A three-grid LEED-AES optics was used, the base pressure of the work chamber was 3 X lo-” Torr. Gold of 99.99% purity was evaporated from a tungsten basket heater under a vacuum of 1 X lop9 Torr at a rate about 5 A/min, which was estimated indirectly by measuring the thickness of the Au film deposited on a glass plate placed midway between the source and the substrate. A conventional method of Ar ion etching and AES monitering was applied to obtain the depth profile of the sample. To estimate the etching rate of the film, a given thickness of Au film deposited on a NESA glass was sputter etched under a given condition and its removal time was measured by AES. In the present work, an Ar ion beam of 500 eV energy and current density of 1.8 X lo-’ A/cm2 produced an etching rate of 4 A/min. The substrate was a mirror-polished, 2-3 $2 cm, P-doped Si(lO0) wafer (5 X 25 X 0.5 mm), which was etched with hydrofluoric acid and rinsed with distilled water just before installing it into the LEED system. The specimen was resistively heated and its temperature was measured by a fine Pt-PtRh thermocouple. To avoid alloying at high temperatures, the thermocouple was spot welded to a small piece of Ta foil pressed on the back face of the sample; a systematic error expected in such a system was corrected by comparing the thermocouple reading and the optical pyrometer reading at higher temperatures. Repeated flashing of the specimen at 1200°C produced a well-de~ned Si(lOO)-(2 X 1) LEED pattern, where only a trace of carbon was detected by AES. The argon ion bombardment-~eal~g method was also tried for cleaning, but no difference was observed in the LEED pattern and or in the Auger spectrum. LEED-AES observations were made at around room temperature to follow the change of the surface caused by deposition and annealing, and a series of scanning electron microscopic observations was made on specimens removed from the UHV system to supplement the LEED-AES analysis.
3. Result 3.1. Low energy electron diffmction When Au was deposited onto clean Si surfaces at around room temperatures, the (2 X 1) pattern gradually dissolved into a strong background, but a very diffuse and weak ring became detectable at the exposure of about 30 A, the spacing of the ring corresponding neither to Si nor to Au. At this stage the Si LW (92 eV) Auger peak split into 90 and 95 eV peaks, suggesting the formation of gold silicide layer [5-83. The diffuse ring became gradually invisible and new diffuse spots arranged on a circle appeared as the Au deposit increased, these spots became somewhat sharper at higher exposure: LEED patterns obtained at exposures of 60 and 300 A are
204
K. Oura, T. Hanawa, LEED-AES
study of Au-Sill
00) system
Fig. 1. LEED patterns obtained from Au films deposited onto Si(100) at room temperature: (a) clean Si(100)<2 X 1) (49 eV); (b) 60 A Au film (49 eV, the sample is tilted to the left side and the (00) spot cannot be seen in the pattern); (c) 300 A Au film (57 eV); (d) schematic illustration of the pattern shown in (c).
in fig. 1. The spot pattern was analysed as due to Au(ll1) (1Si(100) having six azimuthally rotated orientations, among which the dominant ones (strong spots) were Au[Oll] I] Si[Oll] and Au[Ol l] II Si[Oil] and the rest were rotated by +lO” from the dominant orientations; these orientations were illustrated in fig. Id. Auger spectra taken from specimens having thick Au films showed no Si peaks but only Au peaks. Heat treatment of these specimens at around 150°C for a few minutes was enough to reduce the Au( 111) spots considerably in the increased background. Further heating at around 350°C for a few minutes produced new LEED patterns of spots or arcs or rings as shown in fig. 2. However, the condition leading to a specific LEED pattern was not found out. Anyways, the fact that Auger spectra for these specimens show the doublet peaks of Si characteristic of silicide indicates that these LEED patterns are due to domains of gold silicide. Since any given LEED pattern may have occasional extinction of some diffraction spots, it is safe to synthesize several of them to avoid misinterpretation. The completed pattern is shown
K. Oura, T. Hanawa, LEED-AES study of Au-Sill 00) system
Fig. 2. LEED patterns from crystalline gold silicides formed after heating thick Au films (300500 A) at 350°C for a few minutes: (a) spot pattern (72 ev), (b) arc pattern (35 eV), (c) ring pattern A (34 eV), (d) ring pattern B (48 eV).
shown in fig. 3, where circles passing through spots are drawn for convenience. Interpretation of this figure was difficult, but the superposition of hexagonal unit meshes (four orientations) and rectangular ones (six orientations) shown in fig. 4 was found to have the least failure; only very weak spots marked by open circles in fig. 3 were remained unidentified. The size of the hexagonal unit mesh was measured as a = 4.3 + 0.2 8, and that of the rectangular one a = 8.1 + 0.3 A and b = 9.0 + 0.3 A. According to this interpretation every reciprocal lattice point (hk) having k = even for the rectangular mesh must be lost. Atomic arrangement inside the unit mesh satisfying this extinction rule was difficult to derive. We observed two types of ring pattern, A and B, where A was just the same as the rotated pattern, and B was not. But B was most likely to the pattern of “gold silicide I” formed on Si( 111) reported by Green and Bauer [S]. All arc patterns were found to have spacings of the ring pattern A but not B. The gold silicide patterns described above persisted up to about 400°C. Heating
206
K. Oura, T. ~u~awa, LEED-AES study
of Au-S(lOO)
system
siLO111
0.1A-’ Fig. 3. Completed LEED pattern obtained after synthesizing the silicide spot patterns at various electron beam energies (25-85 eVf.
t Si [oil]
(b) Fig. 4. Unit meshes derived for the gold silicide pattern shown in fig. 3: (a) hexagonal unit mesh; (b) rectangular unit mesh.
K. Oura, T. Hanawa, LEED-AES study of Au-Si(lO0) system
201
over 400°C
produced a series of Au-induced superstructures as reported in the previous paper [6,17]. A few minutes heating in a temperature range of 450700°C produced Si(lOO)-c(8 X 2) which converted into a structure Si(lOO)(d26 X 1) at 75O”C, and (d26 X 1) changed to (426 X 3) after heating over 8OO’C. Heat treatment above 1000°C regenerated the (2 X 1) pattern, the transition took place abruptly after an induction period determined by the amount of deposited Au. Though repeated regeneration was possible, the background of the LEED pattern was found to increase with repetition. The temperatures cited above do not mean definite transition temperatures but indicate a temperature at which a rapid transition takes place after a few minutes heating. In addition, at the beginning of the appearance of c(8 X 2), the diffraction spots were almost hidden by a strong background, the temperatures in the range of 600-750°C being necessary to produce a well-defmed c(8 X 2) as reported before [6,17]. 3.2. Auger electron spectroscopy Deposition of Au at room temperature caused a rapid decrease of the characteristic Auger peak at 92 eV of clean Si accompanied with the appearance of two peaks at 90 and 95 eV which has been interpreted as due to the gold silicide [5-81; the exposure of 20 A was enough to complete the conversion. After exposure over about 100 a, the double peak disappeared and only Au peaks were developed, of which a broad peak at about 95 eV was interpreted as being due to the Au OW peak from the observation intentionally made on an Au film deposited onto a Ta
Cd)’
kd ’
(e) ’
(f)
92 ELECTRON
ENERGY
(e’f)
Fig. 5. Auger spectra taken at successive stages of the deposition at room temperature and the following heat treatment: (a) clean Si; (b) 30 A Au film; (c) 300 A Au film; (d) after heat treatment the specimen (c) at 150°C for 2 min; (e) 3OO”C, 2 min; (f) SOO”C, 3 min.
a
5
15
10 HEATING
TIME
AT 1oooC
25
20 ( MIN
)
Fig. 6. Changes of Auger peak heights of Au (70 eV) and Si(92 eV) peaks during heat treatment of the 100 A film at 1000°C.
sheet. Heat treatment of an Au film having enough thickness (300-500 A) between 100 and 400°C caused the regeneration of the double peak, which changed to a single peak at 92 eV (Si LVV) after heating over 450°C. In the case of very thin ftims, the double peak could persist up to 300°C over which the conversion into the single peak took place. The intensity of the single peak increased with the temperature, and reached the originaf value after heating over lOOO*C. The above-stated features of the Auger spectra are illustrated in fig. 5. Auger peak heights of both Au 70 eV and Si 92 eV during heat treatment at 1000°C are plotted against time together with the LEED pattern in fig. 6, where correspondence between the plateau of the curve and the (J24 X 3) LEED pattern is evident. To make this process clearer, SEM observations were made on specimens, taken at successive stages of the annealing. Figs. 7a and 7b suggest that the (426 X 3) pattern comes from a flat area between the Au islands, which were identified by the accompanied X-ray microprobe analysis. The increased background in the initial stage of the appearance of t”26 X 3) can be interpreted as due to the high density of the small Au islands dispersed on the flat area. It was difficult to take samples for SEM observation at the stage of just changing to the clean SWface, but heating of the (d26 X 3) specimen was found to reduce the density of both larger and smaller Au islands, while AES levels of both Au and Si did not change. It was also found that the regenerated clean surface was accompanied with
K. Oura, T. Hana~a, LEED-A/B
study ofAu-S~ii~O~ system
Fig I. .S cannti ng electron micrographs taken at various stages of UHV heat tre atr nen t: ( a) 100 A AU fil m 1000’‘C, 1 min (initial stage of the (J26 X 3) structure); (b) 120 A AIu fi h 1000”c, 18: mii01[&la1 stage of the (426 X 3) structure); (c) 300 A Au film, 1000’ C. 80 mi n (regenerai :ed Cl ean Si surface}.
’
Fig. 8. ~ep~-co~~o~~~~~ profife for a Au fitm on s&lo@] after heat treatment at 250°C for a few minutes. The peak heights of Au (70 eV) and Si (90 or 32 etr) have been normalized to their pure states, respectively. Sputtering rate for pure Au films is about 4 A/min.
etch pits as shown in fig. 7c; the surfaces of both the initial clean Si and the silicide showed no contrast in the SEM pattern, whatsoever. Fig. 8 shows the depth-composition profile for a silicide sample. The normalized peak heights of two Auger peaks are shown: a Au peak at 70 eV, and a Si peak which varies in cha~cter~tic energy from 90 to 92 eV while sputtering through the AujSi interface. To simplify the analysis, the data of fig. 8 are divided into four regions. In region A, the double peak characteristic of silicide was observed at 90 and 95 eV. Upon entering region B, the double peak was hardly visible. In region C, the double peak reappeared, while in the hatched region it gradually converted into the single peak at 92 eV and hence no Si peak height was plotted in the figure. In region D, the Si single peak was dominantly detected. In addition, no change in the ch~acteristic shape and energy for the Au Auger peak at 70 eV was observed in regions A-D. These experimental results suggest that silicide layers are formed both on the outer surface of the Au film and in the Au-Si interface. The outer silicide layer is very thin, while the other is much thicker. The AU peak which persists even after long sputtering periods (region D) can be interpreted as due to the atomic mixing caused either by the so-called knock-on effect [18] of Ar ion beam or by diffuseness of the interface itself. The minute concentration of Si possibly resting in the Au film (region B) cannot be determined in the present study.
4. Discussion 4.1. Gold silicide The present study revealed the formation of thin gold silicide on both sides of Au(ll1) fdms deposited on Si(lO0) surfaces. This feature and its stable region
K. Oura, T. maraca, LBED-AES study of Au-Si(100)
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400°C) are the same as in the case of the Au-Si(1 11) system reported by Green and Bauer (GB) [.5]; however, the detailed structures seem to be different from each other for the following reasons: (1) In the present study, the complex pattern is analysed as composed of six orientations of a rectangular mesh (a = 8.1 2 0.3 8, b = 9.0 zt 0.3 A) and four orientations of a hexagonal mesh (a = 4.3 f 0.2 a), while GB derive nine orientations of the rectangular mesh (a = 7.35 8, b = 9.35 a) and no hexagonal mesh. (2) In both cases, derivation of the rect~~lar unit mesh was impossible without assuming missing spots; however, the extinction rules are different from each other as shown in fig. 9. The crystallographic data of gold silicide studied so far are summarized in table 1, which strongly suggests the existance of many structures according to the formation condition. Unit meshes of surface silicides derived in this study and also by GB seem to have no correlation with the unit cells found in the quenched Films. The composition of the silicide was estimated from the ratio of Auger peak heights of Au (70 eV) and Si (90 eV) corrected against their pure states; the result of about 15 at% of Si is comparable to that of GB and may correspond to the formula AusSi. (below
4.2. Oriented overgrowth of Au The observation that six definite orientations of Au( 111) II Si( 100) appear after a ring pattern of the silicide has been formed at room temperature (less definite orientation of Au{1 11) films on Si(100) surfaces has been reported in the X-ray work by Fisher and Wissman [14,X5]), seems to suggest that the orientating effect of the substrate propagates through the thin silicide layer, though the detailed mechanism is not known. It is interesting to note that different silicides are formed on the (111) planes of an Au film according to whether they are grown on the (111) face of Si or on the (100) face. This may suggest that the difference in imperfection and/or strain in the Au film affects the structure of the silicide.
212
K. Oura, T. Hanawa, LEED-AES
study of Au-Sill
00) system
K. Oura, T. Hanawa, LEED-AES
study of.&-Sill
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system
213
FILM THICKNESS (monolayer) Fig. 10. Schematic illustration of the surface structure of the Au/%( 100) system.
4.3. S~perst~~tures The condition of the appearance of the superstructures is schematically represented in fig. 10, together with that for silicide and Au(lll). Coverages of Au for the completion of c(8 X 2), (d26 X 1) and (426 X 3) were not determined exactly, but a 0.5 monolayer for c(8 X 2) and a 1 monolayer for (426 X 3) were estimated from the exposure assuming a sticking coefficient of unity. The reason why the notation (d26 X 1) was used instead of (5 X 1) was given in a previous paper [6], In the figure, the temperatures distinguishing three superstructures do not mean definite transition temperatures but indicate a temperature at which a rapid transition takes place, higher temperature being necessary for specimens of thicker Au deposit. 4.4. Regeneration of clean Si surface The initial very slight decrease of the Auger signal of Au followed by the sudden rapid decrease at high temperatures, as shown in fig. 6, can be easily interpreted if we adapt the idea that Au is mainly evaporated through a 2D adsorbed gas state equilibrated with Au islands on the surface of the (426 X 3) as long as Au islands exist. In other words, the 2D gas and the underlying adlayer of (426 X 3) will evaporate very quickly as soon as Au islands are consumed out; the latter stage seems to correspond to the case of Au-Si(1 11) system as reported by Le Lay et al. f 11. Change of LEED patterns during the course of the above-stated rapid transition was observable only for very thin deposits. Heat treatment of a few monolayers of Au film at 850°C successively produced (426 X 3), (d26 X l), c(8 X 2), and finally (2 X 1). The absence of small Au islands near the larger island shown in fig. 7 suggests the easy movement of the small Au islands or atoms, which supports the above-stated evaporation mechanism. Also, the formation of etch pits found in the regenerated clean Si(100) surface can be understood considering the rapid diffusion of Si atoms [ 161 through Au islands; the crystalline shaped deposit on the
Au islands identified.
found
in figs. 7a and 7b may be segregated
Si, though it cannot
be
5. Summary We have studied the structure of Au films deposited on Si(lOtJ) clean surfaces using LEED and AILS. The experimental results suggest: (1) Au flms grow as rotated orientations at low deposition Au(l11) li Si( 100) with six azimuthally temperatures below about 50°C after the formation of intermediate layers of gold silicide. (2) Crystalline gold silicide thin layers are formed on. the Au( 111) film after heat treatment in the ragne of IOO-35O’C. The unit meshes assigned to the silicide are found to have no correlation with crystallographic data reported on the quenched Au-Si alloy films. (3) The gofd silicide is stable up to 400°C. Heat treatment at higher temperatures (above 450°C) leads to a~omeration of the film, producing a series of gold-induced superstmctures of Si(100). (4) Thermal desorption of Au films occurs over 1000°C and clean Si(2 X 1) surfaces are reproduced, while the regenerated clean surface is accompanied by etch pits. Acknowledgements The authors would like to acknowledge the contribution of the Welding Research Institute of Osaka University in taking scanning electron micro~a~hs. They would also like to thank Mr. K. Sato for his assistance in the experiment. This work was partly supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Japan. References fl] $2] [3] [4] [S] [6] [7] [S] f9)
G. LeLay, M. Mannewille and R. Kern, Surface Sci. 65 (1977) 261, G. LeLay and J.P. Faurie, Surface Sci, 69 (1977) 295. G. LeLay, G. Quentel, J.P. Faurie and A. Masson, Thin Solid Films 35 (1976) 273. G. LeLay, G. Quentel, J.P. Faurie and A. Masson, Thin Solid Films 35 (1976) 289. A.K. Green and E. Bauer, J. Appl. Phys. 47 (1976) 1284. K. Oura, Y. Mtlkino and T. Hanawa, Japan. J. Appl. Phys. 15 (1976) 737. T. Narusawa, S. Komiya and A. Hiraki, Appl. Phys. Letters 20 (1972) 272. A. Hiraki and M. Iwami, Japan. J. Appl. Phys. Suppl. 2, Pt. 2 (1974) 749. T.R. A~nt~a~srn~, H.L. Luo and W, Klement, Nature 210 (1966) X040. IlO] P. Frtxiecki, B.C. Giessen and N.J. Grant, Trans. AIME 233 (1965) 1438. fll] R.C. Kxuteaat, JX, Tien and D.E. Fornwalt, Met. Trans. 2 (1971) 1479. [12] GA, Anderson, J.L. Bestel, A.A. Johnson and 8. Post, Mater. Sci. Eng. 7 (1971) 83. i13] C. Suryanarayama and T.R. Anantharaman, Mater. Sci. Eng. 13 (1974) 73. [14] W. Fischer and P. Wissmann, 2. Naturfobrsch. 31a (1976) 183. [15] W. Fischer and P. Wissmann, Z. Naturforsch. 31a (1976) 190. [ 161 A. Hiraki, MA. Nicolet and J.W. Mayer, Appl. Phys. Letters 18 (1971) 178. [17] K. Oura and T. Hanawa, in: Proc. 7th Intern. Vacuum Con& and 3rd Intern. Conf. on Solid Surfaces (Vienna, 1977) p. A-2753. 1181 M. Kaminsky, Atomic and Ionic Impact Phenomena on Metal Surfaces (Springer, Berlin, 1965) p. 218.