applii surface science

ELSEVIER Applied Surface Science 103 (I YY6) 45Y-463

Thermal regrowth of Sit 100) damaged bombardment

G. Pet6 “. . J. Kanski

by Ne, Ar, and Xe ion

h

Abstract

Thermally induced regrowth of Sit 100) after ion sputterin g with 3 keV Ne, Al-. and Xe ions has been studied with angle

resolved photoemission. Significant differences are found in the regrowth. depending on the kind of ions used for the

sputtering. While the Ne sputtered surface was fully recovered after annealing at 750°C. the Ar and Xe treated surfaces

remained disordered after such treatment. It is concluded that Ne ion bombardment induces a different defect system than

that obtained after bombardment with Ar or Xe ions at the same energy.

1. Introduction

The surface cleaning of Si single crystals is a crucial technological problem in many contexts. e.g.

preparation of substrates for MBE or silicide growth.

A common method for this purpose is Ar ion bom- bardment. As this treatment results in a heavily

damaged (amorphized) surface, supplementary an-

nealing is required to restore a single crystal surface. It has been shown that a Si surface cleaned by Ar ion bombardment can not be regrown at the temperature of amorphous-crystalline transition (600°C). but a much higher temperature. well above 800°C is needed

to generate a well ordered 3 X 1 reconstructed SLIT- face [I .I?]. By using Ne instead of Ar in the sputter-

ing process. it is possible. however. to produce a clean and ordered Si surface at lower annealing

temperature (720°C) [3]. Clear correlations between ion beam induced damages and the relative weight of

the ion beam species have also been found in studies of implantation treated surfaces. with beam energies in the range of 100 keV [4-l I].

The purpose of this paper is to investigate the regrowth characteristics of Sit 100) after bombard-

ment with Ne. Ar. and Xe at an energy typical for surface cleaning purposes.

2. Experiment

The experiments were performed in a VG ADES

400 angle resolving photoelectron system equipped with low energy electron diffraction (LEED) and an AI/ME X-ray source to supplement the noble gas discharge UV source. The sample was a well ori- ented Si(100) wafer (Wacker) I X I cm’ in size. It was clamped with a thin Ta-spring to a Mo-holder.

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460 G. Pet& .I. Kanski/Applied Sulfate Science 103 (19961459-463

containing filaments for radiative heating and a spot

welded thermocouple. A clean and well ordered two-domain Si(10012 X 1 surface was obtained in the conventional way by means of repeated cycles of

500 eV Ar-ion sputtering and annealing. The sample

holder was well outgassed prior to the experiments,

and the vacuum remained in the low lo- ‘” Torr

range during the annealings. Special attention was paid to check the impurity

levels of carbon and oxygen, as these could in principle be of critical importance for the surface

regrowth process. The most sensitive probe for such contamination was the valence band spectra them-

selves, as the excitation cross sections for 0 2p and

C 2p states are very large for 21 eV photon energy. The ion bombardment was performed with a stan-

dard 3 keV ion gun (Varian) at 60” angle of inci-

dence, with the vacuum chamber back-filled with the

sputter-gas to a pressure of 5 X 1O-5 Tot-r. Since the thermocouple was not in direct contact with the

sample, the absolute temperatures quoted below are estimated to be accurate only within about 50°C.

Therefore, rather than quoting temperature values, we use in the present study the term ‘thermal dose’

to quantify the combined effect of heating current and duration of annealing. As the experiments re- ported here are of a comparative nature, these uncer- tainties do not influence the conclusions of the study.

Before each of the treatments with a different gas

a clean and well ordered surface was prepared by 2 h 500 eV Ar sputtering and annealing, to give identical

starting conditions. The surface destruction repre- senting the first step in the actual experiment was made at 3 keV ion energy and a duration of 30 min.

3. Results and discussion

The photoemission data obtained directly after Ne, Ar and Xe ion bombardments are shown in Fig. 1. In the case of Ar the peak at 9.2 eV binding energy reflects the Ar 3p state from atoms embedded in Si. The structureless broad peak at O-5 eV is characteristic for amorphous Si [12]. A similar spec- trum is obtained for the Ne bombarded surface, except that the peak at 9.2 eV is absent. Some structures in the range 5-8 eV binding energy are visible. We associate these structures with a weak

I” I”““‘,” I”““’

;.--c /‘

Q :--_i’

;: ,’

i ,:::..zL Xe

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!i AI

.e” B

,, ,,.‘*.. “k.,._ \,.., >~-.d’ - ,>”

“\. ,;,_.P’

.z ,,--..\_

.’ ,,,, I’._ .,,/ F.’

; ,: ,;/ -. _/

.I’ Ne ,,*,--’

_e’-‘.*, -~~;_/i~‘-

,,,--

p ,’ : ..’ ,./

“S. ‘L_ ,,_\ _^ ,,,,, :/*.-

_/..,-

8 : 6 --’ . .:’ E, ” : :

) __.: ;

I : __.i

1 / 1 J / I 1 1 I

0.0 2.0 4.0 6.0 8.0 10.0

Binding energy (eV)

Fig. 1. UPS spectra of Ne, Ar and Xe ion bombarded Sif 100). The

inset shows Xe 5p emission from a surface exposed to 100 eV Xe

iOIlS.

surface contamination during the Ne treatment. Ne is

not very efficient for surface cleaning, which means that contamination due to an increasing background

pressure during the 30 min Ne sputtering is accumu- lated. For the Xe bombarded sample the O-5 eV binding energy range is again similar to the spectrum

of amorphous Si. Some structures are also observed in the 5-9 eV binding energy range, partly due to

emission from the Xe 5p doublet. As the intensity

ratio of the two components deviates strongly from

the branching ratio of 1.54 reported for atomic Xe [ 131 (the high binding energy component is clearly

stronger than the low energy one), a supplementary

experiment was performed to examine whether this structure may partly be due to Si states. In previous studies of implantation damaged Si we could indeed observe a peak in this energy range, which we associated with the formation of a new amorphous phase [5]. To test this, the surface was first sputtered

with 3 keV Ar, and then exposed to a 100 eV Xe beam. The result is shown in the inset in Fig. 1. In this case we find peaks at 7.1 eV and 5.8 eV binding energies. The energy separation between the two peaks corresponds well to the spin-orbit splitting of Xe 5p. Moreover, their intensity ratio agrees well with the above mentioned branching ratio. It is clear that the emission in the 5-9 eV binding energy range from the 3 keV Xe ion bombarded Si can not be ascribed only to the Xe 5p states. The additional

G. PetG. J. Kanski /Applied Surjace Science 103 C 19961459-463 461

B ;:

.~,‘..~.~. .:’

I >I -*._ 2 ‘,, .-. _..\-r:. . ..-‘f _ _.- ;A

- .3”,..:,_, :, ,,. ..”

‘2. %’ ‘-.. .,. ,...” ,;.“.-’

,. -, ,:- “, ..,,,-. ,\I ,_ ,..“.%.”

-.. ., :’

-. ..- ,.,,....

Ne

I-L.--l IL-t.1 -L__L I 1 -L-L! 0.0 2.0 40 6.0 8.0 10.0

Binding energy (eV)

Fig. 2. Photoemission spectra from Ne ion bombarded SKIOO)

after 30 mitt annealings with 5.5 A and 5.7 A heating currents (A

and B. respectively). Both spectra are recorded in normal emis-

sion.

emission is associated with states in amorphized Si, in analogy with the mentioned results on implanta-

tion induced a-Si. The effects of annealings are shown in Figs. 2-4.

It is seen that in the Ne case the spectrum is little affected by a heating current of 5.5 A, but with an increase to 5.7 A (giving around 550°C) clear signs

of crystallization are seen. Interestingly enough, the main spectral change occurs at the uppermost part of

_/ ,’ ..”

;*. ?...*.:_ ,’

;: -\,,

.4.** ,.’ :._

/.-

_.I :

Al

A

I .~~--_-----GL-L.,L>1 18, /1I,.NI

0.0 2.0 4.0 6.0 8.0 10.0

Bindmg energy (eV)

Fig. 3. Photoemission spectra from Ar ion bombarded Si(100)

after 30 min annealings with 5.5 A, 5.7 A and 6.2 A heating

currents (A, B, and C. respectively). All spectra recorded in

normal emission.

Xe

0.0 2.0 4.0 6.0 80 10.0

Bindmg energy kV)

Fig. 4. Same as Fig. 3 but with Xe ion bombarded surface.

the spectrum, where a surface state peak is devel-

oped. Below this peak the spectrum is rather feature- less, which we take as an indication of poor crys- tallinity.

The annealing process for the Ar bombarded sur-

face is shown in Fig. 3. We note immediately that the same thermal dose which resulted in crystalliza-

tion of the Ne treated surface is insufficient to induce a corresponding modification in this case. Even at

higher heating current (6.0 Al no sign of crystalliza- tion is found. The surface state peak starts to appear only with a heating current of 6.2 A (approximately 650°C).

A similar crystallization process is found for the Xe bombarded sample, as can be seen in Fig. 4. The

same minimal thermal dose as in the case of Ar is needed for the appearance of surface state emission, which is initially clearly broadened.

Let us mention that while the Ar 3p derived

emission is continuously decreasing during the an- nealing, reflecting a gradual outdiffusion of the em- bedded Ar, emission in the Xe 5p range shows a different behavior. The broad peak at 6 eV disap- pears at 5.7 A heating, while the peak at 7.1 eV is

observed until the onset of crystallization around 6.2 A heating current. This supports our conclusion that the 7.1 eV peak contains some contribution from the Xe 5p but mostly reflects a Si state associated with the new amorphous structure.

It is clear that different thermal doses are required for crystallization of the amorphized Si indicates.

: .._..“” I

Fig. 5. Normal emission spectra from Ne. Ar and Xc ion bon-

barded Sic 100) after 30 min annealin_r at 7.5 A heating current.

One trivial cause for these differences could be different degrees of contamination, Such effects can

in principle never be excluded, but as we find no signs of contamination at least for the Ar and Xe- treated surfaces, we ascribe the differences to differ-

ent structural properties in Si. induced by Ne. Ar,

and Xe ion bombardments. The thermally induced single crystal regrowth is

not an abrupt process as a function of temperature. but proceeds gradually in the temperature range

550-750°C. In Fig. 5 the effect of the annealing at 7.5 A (approximately 750°C) is shown for the three

sputtering cases. The bulk state emission within the range 1-5 eV binding energies [ 1,2] is more pro-

nounced in all three cases than they were at lower thermal doses. but they are much better defined for the Ne sputtered surface than for the other two. As

the surface state peak appears at an early stage, it can be concluded that the continuous regrowth is mainly

a bulk process. The progress of this solid phase epitaxy may be understood in terms of mosaic or grain-like structure formation at the first stage of the regrowth process. similarly to the mosaic structure formation during the interface regrowth of Sb im- planted Sic1 11) [14]. Following this. the epitaxial regrowth involves a continuous displacement of atoms within the grains to the regrowing single crystal. This kind of regrowth has been reported recently for thin metallic films [ 1 s]. Our suggestion for the grain formation in Ar and Xe ion bombarded

Si( 100) is in agreement with the STM data published

for Ar ion bombarded Si(100) [16].

4. Summary

Ne, Ar and Xe ion bombardment amorphizes the

Si(lO0) surface. Regrowth of these amorphous sur- face layers is different. probably because their amor- phous structures are different. The regrowth is a continuous process as a function of the thermal dose.

The surface epitaxy of the ion bombarded layer precedes the regrowth of the bulk. Bombardment

with Ne ions induces less damage than with Ar or

Xe at the same energies and requires. therefore. less

postannealing. The difference between the amorphous states in-

duced by Ne and Ar/Xe low energy ion bombard- ment can be the explanation of the better recrystal-

lized Si reported in a previous study [3], though the effect of Xe in comparison with that of Kr [3] is in contradiction with the mechanisms discussed here.

Acknowledgements

This work was supported by the Hungarian

Academy of Sciences via OTKA grant 2963 and by the Swedish Natural Science Research Council.

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