Influence of ion bombardment on the interaction of Sb with the Si(100) surface

Surface

INFLUENCE OF ION BOMBARDMENT WITH THE Si(100) SURFACE

Science 181 (1987) 596-603 North-Holland, Amsterdam

ON THE INTERACTION

OF Sb

S.A. BARNETT Department Luborutov,

of Materials Science, the Coordinated Science Laboratory,, and the Mutericrls Research University of Illinois, 1101 West Sprmgfield Avenue, Urhanu, IL 61801, USA

H.F. WINTERS IRM Almtiden Research Center, 650 Hurry Road, San Jose, CA 95120, USA and J.E. GREENE Deportment Luhorutoty, Received

of Mutenals Scrence, the Coordmuted Scrence Laboratory. and the Materiuls Research lJniversit_v of Illinois, 1101 West Springfield Avenue, Urhona, IL 61801, USA 18 July 1986; accepted

for publication

17 October

1986

One monolayer (ML) of Sb, which is the saturation coverage at the substrate temperature T, = 425°C used in these experiments, was adsorbed on clean Si(100) surfaces and subsequently bombarded with 2 keV Ar+ ions incident at an angle of 21” and a flux JAr = 1.7 x lOI cm-’ s -I. Thermally-stimulated desorption (TSD) was then used to measure the remaining Sb coverage 0 as a function of ion dose D (0 to 2.3 X lOi cm-*) and to determine the effect of ion irradiation on the Sb surface binding energy. In the absence of ion bombardment, only one TSD peak was observed which, at a heating rate of 10°C s-i, occurred at ri = 89O“C. In agreement with previous results, the peak was fit with a binding energy Et = 2.33 eV. Ion irradiation resulted in the formation of an additional, higher temperature, TSD peak at T2 = 1005OC which was fit with E, = 2.6 eV. The Sb coverage 19~ at the T2 site increased with increasing ion dose, reached a maximum of 0 = 0.08 ML at D = 8.5 X lOi cm -*, and then gradually decreased. A simple model, which includes terms for ion-bombardment-induced sputtering and trapping processes, was used to calculate Sb coverages on both the T, and T2 sites as a function of D and shown to be in good agreement with experimental results, The sputtering yield S from the two sites was found to be - 0.08 atoms/ion and the trapping yield a2 of Sb adatoms from the T, into the T, site was 0.015 atoms/ion. The low value of S was due primarily to the poor mass matches between Ar/Sb and Si/Sb. Ion bombardment carried out during Sb deposition on Si(100) with JSh, = 1.5 X lOi cm ’ s 1 and the same value of JAr resulted in an increase in the saturation coverage from 1 to 1.6 ML. TSD peaks were observed at both the T, and T2 positions and an additional broad low-energy TSD peak was obtained at T, = 710°C. The coverage of Sb bound at the T, site was - 0.25 ML.

Ion/surface interaction effects are of current research interest in such diverse areas as sputter cleaning of surfaces, sputter etching for compositional 0039-6028/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

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profiling, sample preparation for transmission electron microscopy, and lithographic plasma etching. In addition, a variety of vapor-phase film growth technologies such as sputter deposition, plasma-enhanced chemical vapor deposition (PE-CVD), and accelerated-beam molecular-beam epitaxy (MBE) all rely on ion bombardment of the growing film to modify film composition, dopant distribution, and/or morphology [l]. Examples of ion/surface interactions during film growth include the use of low-energy collisional-mixing during sputter deposition in order to grow thermodynamically metastable phases [2] such as (III-V)(,_,,(IV,), alloys [3] and the use of ionized and accelerated dopant beams during MBE of Si and GaAs to increase the dopant incorporation probability by several orders of magnitude while reducing profile broadening due to surface segregation [4]. Another aspect of ion-assisted film growth which has received considerable attention is the use of low-energy ion bombardment of the substrate to modify nucleation kinetics leading to low-temperature epitaxy [5]. One mechanism which has been proposed for altering the nucleation rate is the production of non-random preferential binding sites at surface defects produced by ion irradiation [6,7]. The key quantities which one would like to know in order to model processes such as those listed above are sputtering yields S, trapping yields (Y, and, in the last example given above, adatom surface binding energies E. Unfortunately, most of the materials systems which are technologically interesting are also chemically complex and the parameters S, (Y, and E are not available and not easily calculated. For example, a calculation of the partial sputtering yield of a given species from an alloy depends upon the local surface potential (i.e. the local surface chemistry) as well as the partitioning of momentum transfer between the different lattice species [S]. In this paper, we describe the results of experiments using thermally stimulated desorption (TSD) to measure S, OL,and E after ion bombardment of a well-defined two-component system, Sb adsorbed on clean Si(100) surfaces. Previous experiments [9] using a combination of modulated-beam mass spectrometry and TSD showed that Sb, is dissociatively chemisorbed on clean Si(100) surfaces at all temperatures investigated, from 100 to 1025°C. TSD spectra, Sb surface lifetime 7, and saturation coverage 0,,, measurements (all coverages are referenced to the surface site density of unreconstructed Si(lOO), 6.8 X 1014 cm-‘) were used to determine the Sb binding energy E as a function of 8. At the substrate temperature, T, = 425 “C, employed in the present experiments, d,,, = 1 ML, E = 2.33 eV, and T is essentially infinite (> lo6 s) on the time scale of the present experiments. The ion irradiations were carried out with 2 keV Ar+ ions incident at an angle of 21” and a flux JAr = 1.7 X 1014 cme2 s-i. The ion dose D ranged from 0 to 2.3 X 1016 cm-‘. TSD spectra taken after ion bombardment showed, in addition to the peak at Tl = 890°C (i.e. El = 2.33 ev), a higher temperature peak T, at 1005°C. Integration of the area under the peaks demonstrated that

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the Sb coverage 0t in the Tl site decreased exponentially with increasing ion dose while 8, in the T2 site increased up to a maximum of - 0.08 ML at D = 3.5 X 1015cm-’ and then slowly decreased at higher values of D. These results were fit with a model which gave a sputtering yield S = 0.08 atoms/ion and a trapping yield (Ye= 0.015 atoms/ion. Experiments were also carried out in which the samples were bombarded during the exposure at 425°C. In this case, e,,, was observed to increase and, in addition to the T, and T2 peaks, a new broad lower energy peak at T3= 710°C was observed. The maximum coverage of Sb bound at the T, site was - 0.25 ML. The ultrahigh vacuum molecular-beam apparatus used in the present experiments has been described in detail previously [9] and only the essential features will be briefly discussed here. The chamber was evacuated with three 100-170 / s-l turbomolecular pumps and routinely reached pressures of the order of 5 X lo-” Torr (6.7 X lo-* Pa) after baking at 200°C. The molecular beam was provided by an Sb-charged effusion cell aimed at an RF induction heated [lo] Si(100) wafer. Both the incident beam and the antimony flux desorbing from the Si target could be modulated, the former via a mechanical shutter with a time constant of - 0.3 s and the latter through an electromechanical chopper. The desorbed flux was detected using a mass spectrometer positioned at the end of a drift tube such that its ionization chamber was in direct line of sight to the sample. Cracking patterns for Sb, as a function of electron energy in the ionizer were determined previously [9]. However, in these experiments the only desorbing species were Sb monomers. The effusion cell, sample surface, and mass spectrometer were aligned so as to prevent specular reflections into the ionizer. Prior to initiating the experiments, the graphite effusion cell was outgassed in vacuum for several hours at - 600°C filled with 99.9999% pure Sb powder, and then outgassed again at 460°C in order to remove contamination due to air exposure of the Sb powder. The cell temperature was maintained at 450°C during these experiments to provide an Sb, beam flux Jsr,, of 1.5 X 1013 crnd2 s-l as monitored by a calibrated quartz-crystal microbalance. Highly polished (lOO)-oriented Si wafers were mounted on a sample carousel using Ta hooks threaded through 0.25 mm diameter holes drilled near the edge of the samples. The Ta hooks were glued to the wafers using a high-temperature ceramic cement [ll] and chromel-alumel thermocouples were spotwelded directly to the Ta hooks at the point where they encountered the sample. An optical pyrometer was used to show that temperature differences between the Ta hook and the sample were small. Final sample preparation consisted of Art bombardment for 600 s using a double-grid electron-impact ion source to provide a 10 PA cm- 2, 2 keV ion beam, followed by an anneal at 1025°C for 10 s. The ion flux was monitored using a Faraday cup. The experimental procedure employed was to first adsorb 1 ML of Sb on a clean Si(100) surface and then irradiate the sample, maintained at T,= 425°C

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TSD

Sb/SI (100) E,,= 2 keV dT/dt = 10°C s-1 ---

Measured Calculated

D = 1.04 x KFcm-2 _-- & -D=l5x1016cm-2 500

.

ae) 700

Temperature

1

900

PC)

(d) __

-_

1

1100

Fig. 1. Thermally stimulated desorption (TSD) spectra obtained from adsorbed Sb (initial at a dose D of (a) 0 cm -*, coverage = 1 ML) on Si(100) following 2 keV Ar + ion bombardment (b) 1.7~10” cm-*. (c) 5.1~10’~ cme2, (d) l.OX1O’6 cmm2, and (e) 1.5X1O’6 cm-’ The substrate temperature during adsorption and ion irradiation was T, = 425°C and the heating rate during TSD was dT/dr =lO°C s-l

with 2 keV Ar+ ions at a beam flux Jh = 1.7 X 1014 cmP2 s-l for times ranging from 0 to 130 s. The beam was rastered to provide a uniform current density over the entire sample as determined by a movable Faraday cup. Following each ion irradiation, TSD spectra were obtained as the sample temperature was linearly ramped from 425 to 1025°C. The sample was then maintained at T, = 1025°C (r < 0.1 s) [9] for - 10 s in order to ensure a clean, well-annealed surface for the next experiment. Surface reproducibility was checked every few runs by readsorbing 1 ML of-Sb and, without ion irradiation, taking a TSD spectrum and comparing the result with the original spectrum shown in fig. la. Only a single symmetric peak at Tl = 890°C was observed in TSD spectra from unirradiated samples. The T, peak shown in fig. la was well described by the classic expression for first-order desorption from a single binding energy site [12], dB,/dt where fitting dance 8 l,sat =

= Y,B, exp( -Ei/kaq),

(1)

vi is the preexponential factor and k, is the Boltzmann constant. In fig. la we have used E, = 2.33 eV and vt = 2.2 x 10” s-l in accorwith previous results [9]. The integrated area under the peak was 1( k 0.03) ML as expected. After several measurements cycles, corre-

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sponding to a total accumulated ion dose of >, 4 x 1016 cmP2 (i.e. 250 s of ion irradiation), the area under the T, TSD peak began to increase due to irreversible surface morphological changes. Thus all results presented below were from samples which had received less than 4 x 1016 cmP2 total exposure. Figs. lb-le show TSD spectra obtained following ion doses of 1.7 x 1015, 5.1x 1015, 1.0 x 1016 and 1.5 X 1016cmP2, respectively, corresponding to irradiation times of 10, 30, 60, and 90 s. The intensity of the Tl peak at 890°C was found to decrease with increasing D and a higher temperature peak appeared at T2= 1005°C. Although the total ion dose was larger than the initial Sb coverage, the integrated area under the curves in figs. lb-le did not decrease appreciably. This indicates that the net sputtering yield of Sb adatoms on Si was low. For example, after 10 s of irradiation, D = 1.7 X 1015 cme2, 19decreased from 1 to 0.8 ML resulting in an overall sputtering yield of 0.08 atoms/ion. The TSD spectra in figs. lb-le, as well as spectra obtained at other doses, were fit with eq. (l), using the values for v1 and E, given above, and an equivalent expression for the T, site assuming v2 = vt. The spectra calculated with E, = 2.6 eV, also shown in figs. lb-le, exhibited good agreement with the experimental data in all cases. The trapping of Sb into the higher binding energy T2 site was due to recoil collisions. That is, for favorable collision geometries, incident Ar atoms imparted a fraction of their forward momentum to surface Sb atoms, driving them into higher binding energy sites. A similar effect has recently been reported by Jorke et al. [13] who observed enhanced Sb doping concentrations in MBE Si films grown under conditions in which the steady state Sb surface coverage during deposition was large due to segregation and a fraction of the Si flux was ionized and accelerated to bombard the growing film. Fig. 2 shows measured Sb coverages in both the r, and T2 states in the present experiments as a function of ion dose. The Sb coverage 19~ in the T, site decreased exponentially with increasing D while 8, increased, reached a maximum of 0.08 ML at D = 8.5 x 1015 cm-*, and then decreased slowly. The rate of change of 8, as a function of D can be expressed as N,(d@,/dr)

= -6$&S,

+ ~z>,

(2)

where N, is the surface atom site density, 6.8 X 1014cme2, S, is the sputtering yield from the Tl site, in atoms/ion, and CQ is the probability in atoms/ion of trapping a T, adatom into the T2 site. A similar expression can be written to describe the rate of change of the adatom population f?,, N,(dWdt)

=J/,r(4(~2

-hs2),

(3)

in which S, is the sputtering yield of Sb from the T2 site. Since E, and E, are different by only - 10% and S varies inversely with the first power of the surface binding energy of the sputtered species [14], S, = S,. Eqs. (2) and (3)

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I

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TSD Sb/Sl(lOO) 4,:

2keV

J&r=17 x ld4 cm-as-r

Fig. 2. Sb coverage 0( 1) in binding sites corresponding to Et = 2.33 eV and E, = 2.6 eV on a Si(100) surface after 2 keV Ar+ irradiation at an ion current density of JAr = 1.7,~ lOI cm-* s- ‘. The initial coverage in the Et = 2.33 eV site was 1 ML. Data points were obtained by integration under thermally stimulated desorption spectra such as shown in fig. 1. The calculated curves were obtained by solving eqs. (2) and (3) using a sputtering yield S = 0.08 atoms/ion and a trapping yield from the 2.33 eV to the 2.6 eV binding energy site of a = 0.015 atoms/ion.

were used to fit the 8i and 8, versus D data and the calculated results, also shown in fig. 2, exhibited good agreement when S, = S, = 0.08 atoms/ion and CX*= 0.015 atoms/ion. The exponential decrease in 19~with increasing D was due to the fact that the sputtering rate term in eq. (2) was proportional to B, while the observed maximum in 19, occurred because of a competition between sputtering and trapping rates. Initially, when 8i was large, - 1 ML, the trapping rate into the T2 site dominated the loss of T2 species by sputtering. However, as 8, decreased and 8, increased, the sputtering rate from the T, site became larger than the trapping rate resulting in a maximum in 0,. The low sputtering yield for adsorbed Sb on Si can be qualitatively understood based on the Winters-Sigmund theory [15] in which it is postulated that the total sputter cross section u is composed of three partial cross sections. ui is the cross section for “direct knock-off” which in this case corresponds to an adsorbed Sb atom being reflected from the Si substrate after being struck by an incoming ion. u2 is the “reflected ion” sputter cross section in which the incoming ion is reflected from the substrate and collides with an Sb atom on the way out. Since the masses of Sb and Ar are both greater than that of Si, their reflection coefficients are small and ui and a, are therefore also small. us is the cross section for cascade sputtering in which energy is transferred from the incoming ion to Si atoms which in turn transfer energy to Sb atoms as part of a collision sequence. The cascade sputtering probability is also expected to be small due to the poor mass match.

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,

I

I

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/

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I

TSD Sb/Si(lOO) E,,=2keV JAr = 1.7 x 1014cm-2s1

T =425”C t=60s dT/dt qlOoC s-1 Measured

I 500

I T:mierature

I

I

900 (“Cl

I 111

Fig. 3. Thermally stimulated desorption (TSD) spectrum obtained from a sample in which thermal Sb, and 2 keV Ar+ ion fluxes were incident with current densities of 1.5X1O’3 and 1.7~10’~ cm -2 s-1 , respectively, on a clean Si(lO0) surface maintained at T, = 425°C. The total dosing time was t = 60 s and the heating rate during TSD was d T/dr = 10 o C s 1

Additional experiments were carried out in which the Sb, and Ar+ beams were simultaneously incident upon the clean Si(100) surface maintained at T, =425OC. Jsb, and JAr were again 1.5 x 1013 and 1.7 X 1014 cmP2 s-l, respectively. The TSD spectra, as exemplified by the typical result shown in fig. 3, had the same general shape as in the previous experiments, exhibiting both Ti and T, peaks. In addition, however, there was a broad low-energy peak centered near T3 = 71O“C. The coverages in the Tl and T2 sites were 8, = 1.2 ML and 8, = 0.16 ML and those portions of the spectra were again well described, as shown in fig. 3, by first-order desorption with E, = 2.33 eV and E, = 2.6 eV. The coverage of the T3 site was found to be 19~= 0.25 ML. The increase in the coverage of the original Tl site above 1 ML, the saturation coverage in the absence of ion irradiation, and the appearance of the new T3 site were both due to ion-bombardment-induced surface damage which was not annealed out at T, = 425OC. The Si surface was disordered by sputtering and collisional displacements resulting in an increase in the total density of adsorption sites and hence in the total coverage. The disordered surface also contained new lower-energy sites. The shape of the broad T, peak could not be well fit assuming a simple first-order desorption process with a single binding-energy site. Instead, the ion bombardment appears to have introduced new low-energy sites with a range of binding energies as might be expected from a disordered surface. Trapping into the T,( E, = 2.6 eV) site during Sb deposition with simultaneous Ar+ irradiation also contributed to the increased saturation coverage. Eq. (3) was used to estimate the coverage in the T2 site due to trapping assuming a constant coverage 0t = 1.2 ML in the T, site and the sputtering and trapping yields obtained in the experiments described above. Neglecting

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effects due to ion bombardment of chemisorbed Sb on T3 sites, the calculated result was 6, = 0.17 ML in good agreement with the measured value, @, = 0.16 ML. In summary, we have demonstrated that thermally stimulated desorption can be used as a quantitative probe of ion/surface interaction effects which play an important, and sometimes controlling, role during film deposition and etching. Ar+ ion irradiation of adsorbed Sb on Si(100) surfaces was shown to lead to an additional higher binding energy site T, due to trapping by recoil collisions from the Tl site. Both the Sb sputtering yield and the trapping yield were determined from measurements of 8, and 0, as a function of ion dose. Initially, when the coverage in the Tl site was large (0, = 1 ML), the trapping rate of Sb adatoms into the T, site dominated the loss of T2 species by sputtering. However, as 0i decreased, the sputtering rate from the T, site became larger than the trapping rate and a maximum in 0, was obtained at a critical ion dose. The use of simultaneous irradiation during Sb deposition resulted in the formation of an additional lower-energy T3 site. The authors gratefully acknowledge the financial support of the Joint Services Electronics Program and the Semiconductor Research Corporation and the technical assistance of Dean Pearson during the course of this research.

References [l] See, for example, J.E. Greene and S.A. Bamett, J. Vacuum Sci. Technol. 21 (1982) 285, and references therein. [2] See, for example, J.E. Greene, J. Vacuum Sci. Technol. A2 (1984) 427, and references therein. [3] S.A. Bamett, B. Kramer, L.T. Romano, S.I. Shah, M.A. Ray, S. Fang and J.E. Greene, in: Layered Structures, Epitaxy, and Interfaces, Eds. J.M. Gibson and L.R. Dawson (North-Holland, Amsterdam, 1985) p. 285. [4] See, for example, J.E. Greene, S.A. Bamett, A. Rockett and G. Bajor, Appl. Surface Sci. 22/23 (1985) 520, and references therein. (51 T. Narusawa, S. Shimizu and S. Komiya, J. Vacuum Sci. Technol. 16 (1979) 366; S. Shimizu and S. Komiya, J. Vacuum Sci. Technol. 18 (1981) 765. [6] E. Krikorian and R.J. Sneed, Astrophys. Space Sci. 65 (1979) 129. [7] G.E. Lane and J.C. Andersen, Thin Solid Films 26 (1975) 5; 57 (1979) 277. [8] H.H. Anderson, in: Symposium on the Physics of Ionized Gases, Ed. B. Cobic (Boris Kidric Institute of Nuclear Sciences, Belgrad, Yugoslavia, 1981); J.W. Cobum, Thin Solid Films 64 (1979) 371. (91 S.A. Bamett, HF. Winters and J.E. Greene, Surface Sci. 151 (1985) 67. [lo] H.F. Winters, J. Schlaegel and D. Home, J. Vacuum Sci. Technol. 15 (1978) 1605. [ll] S.A. Cohen, T.O. Seqwick and J.L. Speidell, Mater. Res. Sot. Symp. Proc. 23 (1984) 321. [12] P.A. Redhead, Vacuum 12 (1962) 203. [13] H. Jorke, H.J. Herzoz and H. Kibbel, Appl. Phys. Letters 47 (1985) 511. [14] P. Sigmund, Phys. Rev. 184 (1969) 383. [15] H.F. Winters and P. Sigmund, J. Appl. Phys. 45 (1986) 4760.