Nuclear Instruments and Methods in Physics Research B 120 (1996) 226-229

ELSEVIER

Beam Interactions wlth Materials & Atoms

Surface regrowth of Sb ion implanted Si( 100)

G. Pet6 a2 * , V. Schiller a, N.Q. IWinh a, J. Gyulai a, J. Kanski

a KFKI Research Institute for Materials Science, P.O. Box 49, Budapest, Hungary b Department of Physics, Chalmers University of Technology, 41296 Giiteborg, Sweden

b

Abstract

Thermal regrowth of a SitlOO) surface, damaged by 80 keV Sb implantation, was monitored by angular resolved photoemission (ARUPS), Rutherford backscattering (RBS) and channelling. It was found that regrowth in UHV at 650°C does not result in a well ordered surface. Annealing at higher temperatures (700-l 100°C) results in densities of surface

defects of (2.5 f 0.4) X 10” at./cm2. A well ordered Si(100)2 X 1 reconstructed surface can be formed only after removal

of a 10 nm thick layer by Ne ion bombardment, and heat treatment at 600°C. These observations can be explained by the formation of a surface layer with misoriented domains simultaneously with the solid phase epitaxy.

1. Introduction

Ion implantation doping depends critically on good control of the regrowth during processing. Regrowth of radiation damaged Si by various thermal treatments has

been the subject of many extensive investigations using different experimental methods (RBS, channelling, TEM, XTEM, double crystal X-ray diffraction, optical measure- ments, electron spin resonance, deep level transient spec- troscopy, etc.) [ 1- 121. These investigations were focused on the regrowth of the entire damaged layer. Although such studies have a long history, research in this field remains very active, mostly because detailed control of the process is increasingly important with device miniaturisa-

tion. It is accepted that the regrowth is a solid phase epitax-

ial process, localised at the interface between damaged and undamaged regions [ 1,5]. However, while the quality of the regrown layer is known to depend on many parameters [6-121, comparatively little attention has been paid to the surface properties of the regrown layers, e.g. the surface relaxation or reconstruction. Yet the near surface structure may be of significant technological importance, for exam-

ple in the case of shallow junctions. Our earlier investigations on the sheet resistance of

P-implanted Si(100) showed that P atoms in the surface region were displaced from substitutional to interstitial positions upon annealing at 1000-l 100°C in Ar atmo- sphere. If the highly resistive layer is incorporated in the

* Corresponding author. Fax + 36 I 155 0694; e-mail peto@rl.atki.kfki.hu.

subsequently grown thermal oxide, this effect is eliminated

[ 131. Similar conclusions were drawn earlier from Ruther- ford backscattering (RBS) measurements on Sb implanted Si(100) [14]. These results suggest that a defect-rich layer may be generated at the surface during thermal regrowth,

even at very high temperatures. In this paper we investi- gate the regrowth of Sb implanted Si(lOO), emphasising the surface characteristics.

2. Experiment

A freshly HF-etched Si(100) sample was implanted with 80 keV antimony ions at a dose of lOI at./cm2 at a rate of 1 PA/cm’. The implantation was carried out in an oil-free vacuum of 10m3 Pa. The implantation damaged layer of the sample (S) was regrown by radiative heating in ultrahigh vacuum at 650°C for 30 min. A 10 nm thick surface layer was removed in situ by 1 keV Ne ion bombardment and the sample was annealed again at 600°C for 10 min. The use of Ne was motivated by the need to minimise the sputtering-induced structural damage. Ac- cording to previous experience [ 151 regrowth of Ne- sputtered Si(100) surfaces takes place at a significantly lower temperature than that of Ar sputtering (580°C vs. 650°C). An ex situ furnace annealing for 30 min was carried out in argon atmosphere at temperatures in the

range 700- 1100°C (Table I >. A reference implanted sample (R) was subjected to the

same processing, with the exception of the 650°C UHV annealing and Ne-sputtering. The surface electron structure of the regrown samples was probed with angle resolved ultraviolet photoemission (ARUPS), with the purpose of

0168-583X/96/$15.00 Copyright 0 1996 Published by Elsevier Science B.V. All rights reserved PII SO1 68-583X(96)0051 4-9

G. Pet15 et uI./Nucl. Instr. und Meth. in Phys. Rex B 120 (1996) 226-229 221

Table 1 reconstruction. The spectrum is free of any 0 or C contam- Annealing conditions ination originated emission.

Treatment

Etching, HF 20%

Implantation, Sb 80 keV, lOI at./cm’

UHV heat treatment at 650°C

Ne sputter etching (10 LeV)

Heat treatment at 650°C in Ar

Heat treatment at 700°C in Ar

Heat treatment at 800°C in Ar

Heat treatment at 900°C in Ar

Heat treatment at 1000°C in Ar

Heat treatment at 1100°C in Ar

Sample

s R

Yes Yes

Yes Yes

Yes No

Yes No

No Yes

Yes Yes

Yes Yes

Yes Yes

Yes Yes

Yes Yes

This result can be explained by assuming that misori-

ented grains develop at the surface during the solid phase epitaxy.

It is known that the damaged surface layer formed by

80 keV Sb implantation of Si(100) is completely regrown

by solid phase epitaxy under annealing conditions similar to those employed here [ 1.51, although some defects were

detected near the surface by TEM and double crystal X-ray diffraction [19]. The apparent conflict between ARUPS

and other data can be resolved by assuming that random

surface crystallisation proceeds parallel with the crystalline

amorphous interface solid phase epitaxy, ultimately pre- venting the epitaxial regrowth from reaching the surface.

using well-known photoemission data from the 2 X l- reconstructed Si( 100) surface as reference [ 16,171. The photoemission spectra were obtained with a VG-400 elec- tron spectrometer using He1 radiation (21.2 eV). The anal-

yser was operated in the constant pass energy mode, with a resolution of 0.15 eV. The Fermi level position was deter-

mined by photoemission from the metal sample holder.

The angle of light incidence was 45”. and the photoelec- trons were collected in the plane of incidence. The base pressure in the analytical chamber was 2 X 10m9 Pa, rising

to about 2 X lo-’ Pa during the photoemission measure- ments due to He flow from the light source. The sample temperature was measured with a thermocouple spot

welded onto the sample holder, with an estimated accuracy of 50°C.

This hypothesis of a non-regrown surface layer can be

tested by removing this layer by means of a method which

can be assumed to cause minimum damage to the surface.

In a previous work we have shown that a surface subject to Ne ion bombardment [16] regrows with a significantly milder thermal treatment than used for Sb implanted

Si(100) (curve A, Fig. 1). The surface represented by spectrum A of Fig. 1 was therefore sputtered with 500 eV

Ne ions, removing a layer of approximately 10 nm, and

annealed for 5 min at 600°C. Note that this represents a significantly lower thermal dose than the preceding dose. The result of this treatment is shown as spectrum B in Fig.

1. We find here a spectrum characteristic of a well ordered clean Sit 100)2 X 1 surface, including a sharp dangling- bond surface state peak at 0.7 eV binding energy, and well developed bulk interband excitations [16,17]. Since the Ne ion sputtering was the only difference in the experimental

After outgassing at 3OO”C, the sample (S) was cleaned by Ar ion bombardment and the regrowth was carried out

as described above. Besides the photoemission studies, both samples (S)

and (R) were investigated by RBS and sheet resistance measurements after the mentioned annealing in Ar atmo- sphere. RBS and channelling techniques with 1 MeV He

ions were used to determine the radiation damage. The detector was placed so as to monitor ions scattered through 97” (i.e. with a glancing exit angle of 7”). In this geometry the depth resolution for damage detection was better than 5 nm [ 141.

Si( 100) normal emissmn

:’ A ,:’

,, .... ,,_.... “. .._,.

,..’ “.. .._,,...” .:.”

3. Results and discussion

A normal emission photoelectron spectrum from the Sb implanted, Ar sputtered and annealed surface is shown in Fig. 1 (spectrum A). This spectrum is very different from that of implantation induced a-Si [ 181, as well as from that of a well ordered Si(lOO>Z X 1 surface [ 16,171. Some similarities are observed with spectra from polycrystalline Si, but the shoulder at 0.7 eV binding energy can be correlated to the surface state peak of the Si(100)2 X 1

1 , I 1 I I 1, / / I ( I I / I 11

0.0 2.0 4.0 6.0 8.0 10.0

Binding energy tel. E ,(eV)

Fig. 1. Normal photoemission from Sb implanted and annealed

Si( 100): (A) after annealing at 650°C for 30 min in UHV, and (B)

after removal of an approximately IO nm thick surface layer by

500 eV Ne ions and annealing at 600°C for 5 min.

228 G. Pet5 et al./Nucl. Instr. andbfeth. in Phys. Res. B 120 (1996) 226-229

conditions leading to spectrum B, the difference between these spectra can be unambiguously ascribed to the re- moval of the surface layer.

The formation of a defected surface layer was investi-

gated by further comparative experiments. Sample S with

the fully regrown surface (see curve B of Fig. I), and a

reference sample (R), annealed ex situ in Ar atmosphere for 30 min at 65O”C, were further annealed in Ar atmo-

sphere at 700, 800,900, 1000 and 1 lOO”C, in each case for

30 min. The sheet resistivity dependence on annealing

temperature for these samples is shown in Fig. 2. It is seen that for sample type S the sheet resistance remained un-

changed in the 700-900°C range, and only a slight in-

crease was observed at higher temperatures. The sheet resistivity of sample R was initially lower than that of S,

which can then be associated with the removal of a surface layer, including interstitial Sb atoms, but increased markedly in the temperature range 700-1000°C. At higher

temperatures (1100°C) the increase of the sheet resistivity

was even more pronounced. The annealing dependence of the sheet resistivity can

be explained by the changes in the concentmtion of substi-

tutional Sb. As already mentioned, in the case of P-im- planted Si, a displacement of P atoms from substitutional

to interstitial sites has been found during annealing 1131. A similar displacement can therefore be expected for Sb atoms, even though the effect should perhaps be less pronounced in this case due to the larger size of the Sb atoms. Assuming that the surface defects act as a sink for the Sb dopants, this observation would be in good agree-

ment with a previous study [ 141, in which Sb enrichment in the surface region was reported after annealing at 900°C.

The temperature independent sheet resistivity for Si can be explained by the Ne ion bombardment removing a

defected surface layer.

150 - 1 I I 1 A /

I

z /

I /

E

/ OS AR .f

g 100 - I ‘; I ‘co / E I

/

z / I

iFI I 50 - I

4 0 -- _- -- I I

600 700 800 900 1000 1100 1200 Annealing temperature ["Cl

Fig. 2. The sheet resistance dependence on the annealing tempcra- ture for sample type S, after removal of a 10 nm thick surface layer, and sample type R, the as-implanted surface, as measured with a four-point probe.

Energy [MeV] 0.8 1.0 1.2 1.4

2000 , , a I I , I . I , I I , I

1500 keV 4HeC ANALYSIS 8=97” H random = 3000

to

2 , 1000

2

500

0 150 200 250 300

Channel number

Fig. 3. Rutherford backscattering spectra for samples S and R after the last annealing at 1100°C.

The surface defect density arising during regrowth of ion implantation damaged Si( 100) can be monitored di-

rectly by RBS. Fig. 3 shows the RBS channelled spectra from samples S and R after annealing at 1100°C for 30 min. The spectrum of virgin Si is also included. As one can see the surface peak is larger for R than for S and also for virgin Si. The broadening of the Si surface peak in the channeled RBS spectrum could be due to oxygen (native oxide) and carbon on the surface of the sample. However,

both oxygen and carbon are almost the same for sample S and R. With the oxygen concentration (4.4 + 0.4, 2.4 &- 0.4 and 3.7 + 0.4 X 10’5/cm2 for virgin Si, S and R samples,

respectively) the silicon atoms included in defects can be calculated. On the other hand, as carbon has very little effect on the surface peak area, carbon in this case cannot

be the source of the difference between S and R. Taking into account the amount of silicon in the surface

oxide of each sample the Si atoms included in the surface defects were (0.5 5 0.4) X 1015/cm2 for S and (2.5 + 0.4) x 10”/cm2 for R.

On the magnified Sb edge of the channelled spectra in Fig. 3 we can only see Sb in the surface region of R (i.e. interstitial Sb) while Sb does not appear in sample S (i.e. substitutional Sb). The depth profile of interstitial Sb and that of surface damage evaluated from the channelled spectrum of sample R are shown together in Fig. 4. One can see that the Sb distribution coincides with the defect distribution, which is an indication for the migration of the substitutional Sb atoms during high temperature annealing.

While the well-known solid phase epitaxial regrowth is

obviously not questioned by our experimental observa- tions, the formation of a defected surface layer is in conflict with the idea of complete regrowth. The develop- ment of a defected surface layer does, however, appear to be a natural consequence of the lower coordination of

G. Pet6 et ul./Nucl. lmtr. und Meth. in Phys. Res. B 120 (19961226-229 229

‘“c”‘o’o - 0.008

s 2 y 10 E - 0.006 7 E ’ 0 2

- 0004 $

:

10 20 30 40

Depth[nm]

Fig. 4. The defect density (

interstitial Sb (*J for sample R

) and the distribution of

surface atoms. Starting with a disordered surface, it is

reasonable to expect incoherent local development of crys-

talline order at the surface. simultaneously with the epitax-

ial interfacial regrowth. Observations of the preferred

amorphous-crystalline surface have been made previously for the amorphous Fe-B-Si system [20]. Another aspect, which may be significant in this process, is the possibility

that the amorphous state induced by Sb implantation has a higher energy than that of “normal” amorphous Si. Such

a situation would probably increase the tendency for ran- dom crystallisation upon annealing.

We should mention, however, that uncompleted re- growth (defected surface layer) has also been observed by RBS for an implanted, but not amorphised, surface [21].

4. Conclusion

The present experiments show that Sb implanted Si( 100) does not regrow fully according to the well-known solid phase epitaxy. We find that the regrowth is limited by the

formation of an approximately 10 nm thick defected layer. Even after annealing at a relatively high temperature ( I 100°C) the surface defects cannot be totally removed. It is suggested that the formation of this defected surface is due to an amorphous-amorphous or amorphous-crystal- line transition at the surface, occurring at a lower tempera-

ture than needed for the solid phase epitaxy.

Acknowledgements

This work was supported by the Swedish Natural Sci-

ence Research Council and OTKA grant 2963.

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