ESDIAD with Cl+ and Cl− ions

surface science ELSEVIER

Surface Science 356 (1996) 75-91

The adsorption and bonding of chlorine at silicon (100) investigated using ESD/ESDIAD with C1 ÷ and C1- ions Q. Guo, D. Sterratt, E.M. Williams ,,1 IRC in Surface Science, The University of Liverpool, P.O. Box 147, Liverpool L69 3BX, UK Received 26 July 1995; accepted for publication 13 December 1995

Abstract The adsorption of chlorine on Si(100) 2 × 1 surface has been studied using electron stimulated desorption (ESD) and electron stimulated desorption ion angular distribution (ESDIAD) in conjunction with AES and gas uptake techniques. ESDIAD and ESD measurements were performed on negative as well as positive atomic chlorine species, and the responses with the different polarity of charged species are not seen as complementary. Gas uptake at the surface proceeds initially with a high sticking probability with the atomic chlorine resulting from dissociation not being limited to single dimer sites. ESDIAD studies with positive CI ÷ ions reveal normal and off-normal beams associated with symmetric and asymmetric dimers, with relative contributions depending on surface coverage and temperature. Transformations between bonding configurations seen in positive ion ESDIAD are linked with lateral interactions in the adsorbate layer, and their influence is also evident in the forms of the ion yields of both polarity of species with changing coverage. Negative chlorine ions exhibit a predominance of emission around the surface normal, and are produced via a dipolar dissociation process. Missing atom defect sites with an associated high electron density are postulated as playing a central role in their production. The desorption of positive chlorine ions follows mainly from a two-hole, one-electron (2hle) repulsive state initiated by the ionisation of the C1 3s level.

Keywords: Angle resolved DIET; Chemisorption; Chlorine; Electron stimulated desorption (ESD); Low index single crystal surfaces; Silicon; Solid-gas interfaces

1. Introduction The study of the interaction between chlorine and silicon surfaces is an important subject in regard to the operation of dry etching processes in VLSI technology [ 1] and is currently the focus of much scientific interest. The application of surface science techniques has undoubtedly enhanced the understanding of many fundamental processes * Corresponding author. Fax: +44 151 708 0662; e-mail: [email protected] 1 Also at Department of Electrical Engineering and Electronics, The University of Liverpool.

relating to the geometric and electronic structure of the chlorine-silicon interface [2-14]. Compared with ESDIAD (electron stimulated desorption ion angular distribution) of F ÷ ions from Si(100), where only off-normal emission associated with tilted Si-F bonds is observed [ 11,15], the findings of ESDIAD with C1÷ ions are more complicated with both normal and off-normal emission having been reported [11,12,14]. This is also reflected in X-ray adsorption spectroscopy (XAS) 1-9] and angle resolved photoemission results [3] where normal as well as off-normal Si-C1 bond directions have been proposed. As will be demonstrated in the present paper, the conditions of chlorine cover-

0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PH S 0 0 3 9 - 6 0 2 8 ( 9 6 ) 0 0 0 1 1 - 8

76

Q. Guo et al./ Surface Science 356 (1996) 75-91

age and substrate temperature do influence the relative contributions of the different bonds, and it would appear that the particular features in the bonding of chlorine arise from the stronger repulsive interaction amongst adsorbates. This paper serves to bring together the findings in our laboratory over a range of experimental conditions and techniques applied to the study of chlorine adsorption at Si(100), and elaborates upon features of our research which have been previously reported only in outline form [14,16,17]. It is of particular interest to link the findings of positive ion ESDIAD with observations of angular distributions of negative C1- ions, as well as to compare the mechanisms of ion formation for both charged species. This we believe represents the first study of the angular distribution of negative ions with chlorine adsorbed at silicon substrates. From the standpoint of desorption mechanisms, recent photon-stimulated desorption studies of C1 ÷ ions from the C1-Si(lll) 1,18,19] and C1-Si(100) interfaces [ 19] have suggested that photo-excitation of the C1 3s level triggers the process. In ESD of the C1-Si(100) interface we have found a similar threshold around the C1 3s level, in addition, however, to a lower threshold corresponding to the Si-C1 a bond excitation. A two-hole one-electron (2hle) final state created by Auger transitions involving valence electrons is proposed as the main channel for positive ion desorption. The desorption of C1- ions is identified mainly with polar dissociation processes, and these are foreseen as occurring predominantly at defect sites which have been shown to be associated with enhanced electron density.

2. Experimental Measurements were performed using the ESD/ ESDIAD facility in our laboratory, previously described elsewhere I-11,14], which incorporates a quadrupole mass spectrometer (Hiden, model HAL IDP) containing a time-of-flight facility for ion energy determination, with an ESDIAD optic similar to that developed by Madey and co-workers at NIST [15]. The Si(100) sample could be resistively heated and its temperature measured using

a Chromel-Alumel thermocouple fitting in a tantalum pouch attached to the back of the sample. The sample could be cooled to a lower limit of 120 K using liquid nitrogen. Controlled doses of high purity chlorine gas (99.9%, Merck Ltd.) were admitted directly to the silicon surface via a doser tube and the initial sticking probability could be measured using a gas uptake approach similar to that of King and Wells 1,20] whereby the change in partial gas pressure is monitored as the surface is moved to intercept the gas flux. The non-uniform distribution of the gas flux from the doser used in our work allowed only the initial sticking coefficient to be specified. Positive ion ESDIAD experiments were performed with a 50 V sample bias to both compress the ion trajectory and to enhance the image quality at the phosphor screen. The electron beam energy at the sample surface was typically 250 eV. The images were normally taken at the lowest sample temperatures, although the degradation in image quality with temperature up to room temperature was usually insignificant. LEED was performed using the same imaging system by reversing the bias on the channel plates and with the sample at earth potential. The ordered LEED patterns served to identify the azimuthal directions of desorbed ions relative to the registry of atoms of the silicon substrate. The quadrupole mass spectrometer in use as a surface ion probe, could be employed for either positive or negative ions, moreover, in the negative ion mode it also filtered out the large number of background electrons and so allowed the time-offlight technique to be carried over for energy determination with negative ions. The procedure followed for determining the angular distributions of negative ions was by using the surface ion probe in an angle-by-angle approach whereby the sample was rotated with respect to the fixed axis of the mass spectrometer. The sample bias was set at a low value of - 3 V so as to minimise any distortion of the ion trajectory while imparting some acceleration of the desorbed ions towards the mass spectrometer. Electron beam energies in the region 200-250 eV were employed. The correctness of the method was checked by duplicating distributions of positive ions both by the discrete, angle-byangle approach and by using the integrated

Q. Cruoet al./Surface Science 356 (1996) 75-91

ESDIAD optic. The acceptance angle of the mass spectrometer was + 5° and the sample itself was orientated with the [011] direction of the silicon substrate aligned with the rotational axis of the sample holder. In this manner the experiment aimed to provide a representation of the polar angle of negative ion desorption across the plane containing the silicon dimers. The cleanliness of the sample was checked by AES as well as by monitoring the contribution of the residual impurity ions, H ÷ and F ÷ . It is well known that large F ÷ ion signals can be detected in ESD even when the fluorine concentration on the surface is well below the detection limit of AES. Flashing the sample to > 1000 K was found to be effective in reducing both H ÷ and F ÷ ion signals in ESD, and the response typical of the behaviour of these ion signals with thermal annealing is shown in Fig. 1. The H ÷ ion signal decreased monotonically with temperature while the F ÷ ion signal shows an initial increase followed subsequently by depletion at temperatures > 800 K. The initial increase of F ÷ ion signal with temperature points to the diffusion of fluorine from the bulk. The most likely source of hydrogen is dissociated water from the residual gas. The level of the

77

background contributed by these foreign ions could be kept below 10% of the chlorine ion signal, even lower if the Si(100) surface was covered with a monolayer of chlorine.

3. R e s u l t s

3.1. The adsorption kinetics The initial sticking probability with chlorine gas admitted to the clean surface at room temperature was determined to be 0.9+0.1. The procedure using the gas uptake method involved the specification of changes in chlorine partial pressures (pressures <1 x 10 - 9 mbar), and the uncertainty in the determination arose from the inevitable problem of the measurement of low pressure of an active gas. The CI(LMM)/Si(LMM) Auger ratios measured following chlorine dosing at various sample temperatures up to 550 K are shown in Fig. 2. At 120 K the Auger ratios increase linearly with exposure up to a value close to unity, followed by a slower rise to a maximum value of 1.3.

1 . 4 --

10

•

;\

h~

1.2[]

F+ 1.00

•~ 0 . 8 < 0.6-

~

0.4

4- n

I

0.2 2-

•

I

//

a

0.0 I

400

I

I

600 800 Temperature (K)

I

I000

Fig. I. ESD signals of residual F + and H + ions from an initially clean Si(100) surface after isolation in UHV for 1 h.

,/ I

0.0

0.5

I

I

1.0 1.5 Exposure to C12 (L)

2.0

Fig. 2. The change of CI(LMM)/Si(LMM) Auger ratio (peakto-peak heights) with increasing chlorine exposure at different sample temperatures. 120 K (¢,), 200 K ( n ) and 550 K (D).

78

Q. Guo et al./Surface Science 356 (1996) 75-91

Temperature is adjudged to play a significant role only for C1/Si ratios greater than around 0.6. At the higher Auger ratios, the slope decreases with increasing temperature and correspondingly higher chlorine exposure is needed to attain saturation. Based on the study, the AES ratio of 1.3 was defined to represent 1 ML coverage. These findings are in general agreement with the results of Gao et al. [12] who employed a doser with a calibrated flux output. It is of interest to note that Auger ratios as high as 1.9 were previously found in our laboratory when using chlorine gas generated from an electrochemical cell [ 11 ], with the same CMA analyser for Auger analysis as at present. The suspicion is that excited and/or atomic species contributed to the output gas flux from the cell, possibly compounded by the incomplete collimation of the output flux.

3.2. ESDIAD of positive chlorine ions Positive chlorine ion ESDIAD images from Si(100) saturated with ~1 ML of chlorine are shown in Fig. 3. The images at the screen were captured using a CCD camera and transferred to a computer where the intensities were plotted as raised contour plots. In each case the contours span the range from zero intensity to the maximum following a linear scale. The perspective viewing angle in Figs. 3(a), 3(b) and 3(c) is around 85 °, corresponding to a view some 5 ° away from the normal to the image plane. The (2× 1) LEED pattern from the clean reconstructed Si(100) surface, forming the basis of the labelling of the crystal axes in the figure, remained unchanged with the chlorine adsorption and over the sequence of patterns illustrated. The ESDIAD image after adsorption to saturation coverage with dosing at 150 K, Fig. 3(a), appears as a broad emission peak with some degree of azimuthal distribution, giving a diamond-shaped outline. Annealing to higher temperatures enhances the features corresponding to the off-normal emissions, Fig. 3(b), and eventually over the range 400-670 K the normal emission intensity becomes progressively lower as four well resolved lobes appear as shown in Fig. 3(c). These are aligned with the [011] and [011] azimuth directions of the Si(100) surface. The same pattern

as is in Fig. 3(c) is shown with a perspective angle of 65 ° in Fig. 3(d) where the contrast between the intensities of the emergent lobes and that of the central depression can be seen more clearly without undue loss of the overall shape of the pattern. Annealing to temperatures higher than 770K caused a decrease of chlorine coverage, as adjudged by AES, with desorption of adsorbate presumably as SIC12[12]. However, the four-lobe emission pattern stayed unchanged until the C1-Si Auger ratio dropped to below 0.6, at which point, the four-lobe form was replaced by the sharper normal emission feature shown in Fig. 3(e). No new ESDIAD patterns appeared with further decrease of chlorine coverage; what can be seen from Fig. 3(f) is the slight narrowing of the normal distribution. The ESDIAD patterns shown here serve mainly to identify the change of bonding geometries with different experimental conditions, thus the emission intensity itself is not scaled. This aspect of ESD and ion yield from the interface will be examined later in the text. When the Si(100) sample is covered by ~0.25 ML of chlorine at 150K, the ESDIAD pattern is dominated by normal emission, but with again some evidence of a decoration from offnormal emission as illustrated in Fig. 4(a). Annealing of the 0.25 ML surface to 650 K only slightly enhances the feature of the off-normal emission, Fig. 4(b), in clear contrast to the strong emergence of the off-normal beams obtained at the 1 ML starting coverage (Figs. 3(c) and 3(d)). The ESDIAD pattern was also found to be dependent on the history of the overlayer. With the sample containing 0.5 ML of chlorine formed at 150 K, the ESDIAD pattern consisted of a broad normalcentred image as shown in Fig. 5(a), which then upon annealing to 650 K transformed to yield a more diamond-shaped image, Fig. 5(b). Adsorbing chlorine directly at 650 K to 0.5 ML coverage also resulted in a very similar pattern as shown in Fig. 5(b). However, when starting from a Si(100) surface with 1.0ML chlorine and depleting through thermal desorption to the same 0.5 ML coverage, as adjudged by AES, the ESDIAD image proved to be sharper and with very little azimuthal dependence, Fig. 5(c). With chlorine re-adsorbed at 150 K on this surface, the saturation coverage

Q. Guo et al./ Surface Science 356 (1996) 75-91

(a)

(c)

(e)

79

(b)

(d)

(t3

Fig. 3. Contour plots of ESDIAD patterns obtained with sequential treatment of the surface following an initial chlorine coverage of 1 ML. (a) Adsorption at 150 K without annealing. (b) Surface annealed to 400 K. (c) Surface annealed to 670 K. (d) As in (c) with viewing angle changed from 85° to 65°. (e) Surface annealed to 770 K causing the chlorine coverage to decrease to 0.5 ML (viewing angle 65°). (f) Surface annealed to 770 K for a longer time causing the coverage to decrease further to 0.2 ML (viewing angle 65°).

was f o u n d to be significantly lower at a r o u n d 0.85 M L . T h e a s s o c i a t e d E S D I A D pattern, Fig. 5(d), d e m o n s t r a t e d the r e - e s t a b l i s h m e n t of the b r o a d e r n o r m a l d i s t r i b u t i o n with s o m e i n d i c a t i o n also of an a z i m u t h a l dependence.

3.3. Negative ion E S D I A D B o t h positive a n d negative chlorine ion E S D I A D d e t e r m i n e d using the a n g l e - b y - a n g l e a p p r o a c h are shown in Fig. 6. T h e s a m p l e was

80

(2, Guo et al./Surface Science 356 (1996) 75-91

(a)

(b)

Fig. 4. Contour plots of ESDIAD patterns with 0.2 ML of adsorbed chlorine (viewing angle 85°). (a) Following adsorption at 150 K. (b) As (a) but with sample annealed to 670 K prior to capturing the image.

CI+ / O~0

e-

/o / °"°'°-O'OjO

(a)

(b)

..o

\ 0

,o

°k o/e o,..O..ONo

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o/

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o

"o,,

--

CI

O,-O.

(a)

_= -40 0 40 Emission angle (degree)

/-O\o CI*

e-

(c)

(d)

Fig. 5. Contour plots of ESDIAD patterns with 0.5 ML of adsorbed chlorine (viewing angle 85°). (a) Following adsorption at 150 K. (b) As (a) but with sample annealed to 670 K before taking the image. (c) Surface with 0.5 ML of chlorine derived by annealing to 770 K the surface formed initially with dosing to 1.0 ML at 150 K. (d) Following re-adsorption at 150 K to saturation coverage of the interface created in (c).

prepared by dosing to 0.8 ML followed by annealing to 650 K so as to enhance the off-normal emission in CI ÷ ion ESDIAD seen earlier. The data for positive ions with this surface condition (Fig. 6(a)) show the expected off-normal emission, although with some asymmetry in the distribution.

//\\

£

/°

o

/

e-

/

(b)

I -40 ; 4'0 Emission angle (degree) Fig. 6. Ion angular distributions of both C1÷ and CI- ions obtained by the discrete, angle by angle approach. (a) Surface with 0.8 ML of chlorine formed after annealing to 650 K. (b) Following additional annealing at 770 K to reduce the chlorine coverage from 0.8 ML to 0.2 ML.

Q. Guo et al./ Surface Science 356 (1996) 75-91

This arises from the presence in the vacuum chamber of metal components on one side of the sample which somewhat distorts the electric field due to the 3 V bias between the sample and the aperture plate of the mass spectrometer. The yield of negative CI- ions, in general, represented a few percent of the positive ion yield and this relatively low signal level was the main factor in limiting the response with the integrated ESDIAD optic. As is seen in the diagram (Fig. 6(a)), the spatial form for the negative species is centred around the surface normal with a corresponding F W H M of the angular distribution of + 27 °. This normal emission is maintained as the sample is annealed to 770 K (Fig. 6(b)) decreasing the coverage to 0.2ML, although the F W H M of the angular distribution contracts to -t-20°. The contraction suggests the possibility of some off-normal emission contributing to the desorption peak prior to the thermal treatment. The emergence in Fig. 6(b) of a normally orientated, positive ion pattern, following annealing to 770 K, is consistent with the earlier

81

findings with the ESDIAD optic. The F W H M of both negative and positive species in the normally orientated pattern attained by annealing is similar.

3.4. ESD of positive and negative ions An insight into the mechanisms of ion desorption formed an essential feature of the research, and for this reason we undertook systematic observations of the threshold and the energy dependence of ion yields, together with observations of the kinetic energy distributions of the ions. The study in the main was conducted with one monolayer chlorine coverage. For the near-threshold energy region, the response of ion signal normalised to electron beam current with increasing electron energy is depicted in Fig. 7. Positive C1 ÷ ions were detected at electron energy above 15 eV with thresholds both at 15 eV and more strongly at 22 eV; the inset in Fig. 7(a) showing the derivative of ion yield, highlights the onsets of signal in the low energy region. A low energy threshold in the region of

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(a)

/

m/

/

300-

/ hum/m~m 2(3(3-

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Electron energy (eV)

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/

400

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/

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I

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15

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I

I

20 25 30 Electron Energy / eV

l

35

I

40

10

15

i I

i

I

20 25 30 Electron energy (eV)

I

35

40

Fig. 7. The dependence of the ion yields on electron energy near the low energy thresholds, (a) CI + ions and (b) C1- ions. Chlorine coverage ~ 1 ML.

Q. Guo et al. /Surface Science 356 (1996) 75-91

82

20 eV was found for the negative chlorine ions as shown in Fig. 7(b). The forms of the ion yields at electron energies well above the low energy thresholds are shown in Fig. 8 where the feature of the stronger positive ion signal may be noted from the relative sensitivity factors of the two ion signals. Positive ion yields are seen to increase nearly monotonically with energy up to about 150 eV at which point ion production begins to decrease slowly. The negative ion yield is more complex with the appearance of two peaks at ~ 80 eV and at ~160eV. It is of interest to note that the relatively low (~ 2%) ratio of negative to positive ion yield could be influenced by the condition of the surface prior to chlorine adsorption. With the surface lightly sputtered by Ar ÷ ion bombardment and subsequently annealed to 350 K before exposure to chlorine, thus imparting some minor surface damage, the ratio could be increased three fold to around 6%. The kinetic energy distributions for both positive and negative ions obtained from time-of-flight

5000 -

Cl+

4000 -

/

6 t~

/ D

! =

/

= 3000-

e-

[]

•

2000

_o

1000 D !

0

Io 0

5

I

I

I

100 150 200 Electron energy (eV)

I 250

300

Fig. 8. The dependence of ion yields on electron energy in the region well above threshold. Chlorine coverage ~ 1 ML.

measurements with the surface at monolayer chlorine coverage are shown in Fig. 9. The peak in the negative ion distribution occurs at about 1.5 eV while the positive ions have a most probable energy at 1 eV. Within the uncertainty due to contact potential differences ~ +0.5 eV it is difficult to distinguish between the two values. Careful measurements with the positive species, particularly using low sample bias <2 eV, so as to enhance the instrumental resolution, indicated a second peak feature at around 0.5 eV. Its production was linked in a complex manner with the precise sample conditions, however, its relatively weak strength and energy position did not permit any clear characterisation. Fig. 10 shows the change of positive ion yield with chlorine coverage up to one monolayer under customary conditions of low current probing with ESD. The coverage was attained in two ways. On the one hand, adsorption was carried out continuously at room temperature, only being interrupted briefly for the assessment of the ion yield and for monitoring the CI(LMM)/Si(LMM) Auger ratio, as an indication of each stated coverage. The response of ion yield in this fashion is seen to increase with coverage, pass through a maximum at around 0.4 ML, and thereafter to decrease. In the second format, the Si(100) surface was first dosed to 1.0 ML with chlorine and the coverage attained by progressive thermal desorption. Again the chlorine coverage was monitored by measuring the CI(LMM)/Si(LMM) Auger ratio. The results obtained by the two approaches are quite different, pointing to irreversible behaviour at the interface. The ESDIAD study with C1÷ ions had earlier indicated that conditions were not simply a reflection on coverage, but depended critically on the history of the overlayer. Interestingly, the onset of the growth in ion yield when the coverage driven by thermal depletion decreases to around 0.6 ML, falls in the range where the four-lobe to singlelobe transition occurs in the positive ion ESDIAD. The variation of the negative ion yield with coverage, again obtained by both adsorption and desorption methods, is shown in Fig. 11. This is similar to the behaviour of positive ions except that the maxima in ion yield with the negative

Q. Guo et aL /Surface Science 356 (1996) 75-91

83

(a) (b)

+,.

I

r..)

0.0

I

I

I

I

I

0.5

1.0

1.5

2.0

2.5

3.0

0.0

I

i

]

I

I

0.5

10

1.5

2.0

2.5

Ion kinetic energy (eV)

3.0

Ion kinetic energy (eV)

Fig. 9. Kinetic energy distributions of (a) C1÷ ions and (b) C1- ions. Chlorine coverage ~ 1 ML, electron beam energy 250 eV.

3000

e-

r"

2500

180 160-

2000 .--->.

¢9

.9

140 -

1500 120 -

r"

_o

0

1000

100-

[] [] !

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80-

500 P

60-

[3

0.0

0.2

0.4 0.6 Coverage (ML)

0.8

1.0

Fig. 10. The C1÷ ion yield as a function of chlorine coverage. Coverage derived by adsorption at room temperature ([~), and by thermal desorption of an initial 1 ML covered surface ( I ) .

species occur at a slightly (~ 7%) lower coverage. This could reflect the larger size and hence greater proximity effects for the negative species.

40O I

0.0

0.2

I

I

0.4 0.6 Coverage (ML)

I

0.8

1.0

Fig. 11. The C1- ion yield as a function of chlorine coverage at the surface, corresponding to the data in Fig. 10 for positive ions. Chlorine coverage obtained by adsorption at room temperature (O) and by thermal desorption of an initial 1 ML covered surface (O).

Q. Guo et al. /Surface Science 356 (1996) 75-91

84

3.5. Secondary electron emission The production and release of secondary electrons at solid surfaces presents an agency which can modulate the ion yield in the region above the threshold, arising from desorption induced by the secondaries in their own right. Measurements of secondary electron emission from Si(100) were carried out over a range of primary beam energies (>60eV) and the integrated yield is plotted in Fig. 12. Data were obtained by measuring the sample drain current with zero and with + 15 V bias, the latter setting being taken to represent the upper-energy, cut-off of the secondary electrons. Following this procedure, the secondary electron yield is identified as the difference between the two current readings normalised with respect to the current with + 15 V bias. The secondary electron yield is seen to increase continuously with electron energy up to around 150 eV, with the contribution of Si 2p ionisation giving rise to an inflection in the response in the region of 100 eV.

4. Discussion

the Cl-Si(100) interface points to the presence of tilted Si-C1 bonds on the surface. This can be interpreted as arising from chlorine atoms populating both the dangling bonds at symmetric dimer sites as illustrated in Fig. 13(a) (hereafter referred to as type I bonding in the text). Whether the dimers on clean Si(100) are symmetric or buckled is still an unresolved issue [21,22], but the addition of chlorine atoms on both ends of a dimer would be expected to form a symmetric configuration. The overall four-fold symmetry in the ESDIAD pattern follows from the existence of two equivalent and orthogonal domains of dimers at the Si(100) 2 x 1 reconstructed surface. The bonding configuration is the same as that of fluorine [11,15] and of hydrogen in the monohydride form [23] and seems to be a rather general feature of the adsorption of atoms which form single bond with silicon atoms at the Si(100) surface. The normal emission in ESDIAD points to another type of bonding configuration with the C1-Si bond axis close to the surface normal. An adsorption geometry consisting of one chlorine atom sitting on an asymmetric dimer has been proposed by Purdie et al. from

4.1. Surface structure inferred from ESDIAD The strong off-normal desorption features observed in ESDIAD with CI ÷ ions after annealing 1.0Si 2s Si 2p

0.8"

(a)

/

/

*~ 0.6-

(b)

¢*

¥

~ 0.4-

0.2-

I

50

I

(c)

I

100 150 200 Primary electron energy (eV)

250

Fig. 12. Secondary electron yield at the silicon (100) substrate as a function of primary electron beam energy.

(d)

Fig. 13. Models for the bonding geometry of chlorine on Si(100) 2 x 1 surface. (a) Type I symmetric dimer, (b) type II asymmetric dimer, (c) bonding at isolated silicon atoms and (d) bridge bonding at dimers.

Q. Guo et al./ Surface Science 356 (1996) 75-91

XAS studies [10] and is shown in Fig. 13(b). This bonding geometry is a candidate for the normal emission mode in our work and will be referred to as type II bonding in the discussion which follows. The C1-Si bond axis may well be slightly (<5 °) off-normal since the inherent wagging (frustrated rotation) of the bond and the distribution in ion kinetic energy, allied to the sample bias voltage, both contribute to the angular spread in ESDIAD and thus limit the angular resolution. Seen in terms of ion desorption from either end of dimers situated in orthogonal arrays, the overall appearance would be of normal bonding. The formation of type II chlorine does not necessarily require the existence of buckled dimers on the clean Si(100) surface as a prerequisite, since the bonding of a single chlorine atom at one end of a symmetric dimer site would likely induce dimer buckling in view of the break down of symmetry at the adsorption site. When adsorbing at or below room temperature, individual chlorine atoms can adopt either a type I or type II configuration, therefore giving overall the observed diamond-shaped form of pattern. The high sticking probability of chlorine at the clean Si(100) surface indicates that adsorption does not occur preferentially at defect sites. A chlorine molecule can either dissociate leaving two chlorine atoms at one dimer site, type I chlorine, or with the chlorine atoms at near dimer sites give rise to a type II adsorbate. The concentration of the off-normal type I chlorine would be expected to increase with coverage, leaving type II chlorine as minority species scattered within islands of type I as the coverage approaches 1 ML. The transformation of the high coverage ESDIAD pattern with annealing, without incurring a diminution in coverage, is seen as a manifestation of the transformation of type II to type I bonding. This transformation does not involve any change of surface energy since the number of dangling bonds on the surface is not altered, but is likely driven by the tendency to reduce strain energy by minimising the number of asymmetric dimer sites on the surface. The ESD cross section for type II chlorine is anticipated to be higher since its orientation is closer to the surface normal, thus the reneutralisation probability for escaping CI ÷ ions is lower. It is therefore

85

expected that a type II to type I transformation would reduce the total ion yield for the same coverage of chlorine at the surface. Annealing the surface without incurring any change of coverage always showed a decrease in ion yield which is consistent with this view. The reduction was most pronounced at ~ 0.5 ML, with an average finding ~ 20%. One-dimensional intensity profiles of the contour plots of Figs. 3(a) and 3(c) taken along the [011] direction, are shown in Figs. 14(a) and 14(b), respectively. Fig. 14(b) corresponds mainly to the type I configuration while Fig. 14(a) contains contributions from a mixture of type I and type II bonding. An attempt at the depiction of a profile unique to type II, derived by subtracting the profiles (a) and (b), is displayed in Fig. 14(c). In all cases, the analysis of polar angles was performed using the measured value of most probable kinetic energy of the ions. The polar angle for the type I

+

+

u

u -40 0 40 Angle (degree)

-40 0 40 Angle (degree)

(a) (b)

+

÷

(.)

-40 0 40 Angle (degree) (0

-~o

;

20

Angle (degree) (d)

Fig. 14. One-dimensional intensity profiles taken along the [011] direction. (a) Chlorine coverage around 1ML by adsorption at 150 K. (b) As (a) with surface annealing to 670 K. (c) The difference profile found by subtracting (a) and (b). (d) Chlorine coverage 0.2 ML obtained by thermal desorption of a surface initially formed by dosing to 1 ML at 150 K.

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Q. Guo et al./ Surface Science 356 (1996) 75-91

bonding is determined as 25+5 ° which is in agreement with the result also for the symmetric dimer bonding configuration by Gao et al. [ 12]. The coverage dependence of the transformation of type II to type I is surprising in that the asymmetric-dimer, type II chlorine appears to be more stable at lower coverages with no evidence of the type II to type I transformation being driven to completion. At coverage lower than 0.5 ML, the normal emission in ESDIAD always has the higher intensity even after annealing close to the desorption limit. This may be due to repulsive forces between the adsorbed chlorine atoms which acts as a barrier for two chlorine atoms to move on to a single dimer site. Strong repulsion between chemisorbed halogen atoms on metals has been reported on a number of occasions [24,25]. It is interesting to note that in the case of fluorine, only off-normal ion emissions are seen for all coverages (even extremely low coverages) [11,15]. This is probably due to the difference in ionic radii; both C1 and F at Si(100) will reflect their electronegative charge character, and the radius of the fluorine negative ion at 1.33 ~. compares with the larger chlorine negative ion at 1.81 .~ [26]. As a consequence the chlorine at the surface should feel much stronger repulsion which would, therefore, tend to stabilise the type II configuration at low coverages. The evidence for the asymmetric type II bonding from the XAS study of Purdie et al. [10] was obtained from a Si(100) surface with 0.25 ML of chlorine, and is in good agreement with our ESDIAD results at this coverage which showed a well-defined normal pattern. The observed four to one-lobe transition when the coverage is reduced by thermal desorption from ~ 1 ML to around 0.5 ML, accompanied by a distinct increase in ion yield at this stage (Fig. 10), is consistent with a transition at the surface. This most probably involves the replacement of type I chlorine by the normally orientated type II form. A one-dimensional intensity profile of the normal emission associated with the depleted surface (0.2 ML), seen earlier in contour form in Fig. 3(f), is presented in Fig. 14(d). The FWHM of the profile at around 30° is similar to that of the profile of type II in Fig. 14(c), found by subtracting the type I from the combined type I and II profiles.

The transformation between type I and II forms induced by thermal depletion of adsorbate would contribute to the irreversibility in the form of the ion yield with coverage. When adsorbing at or below room temperature, a mixture of type I and type II chlorine would always be expected, even with relatively low coverages. However, with the coverage changed by thermal desorption, the chlorine atoms are able to diffuse on the surface at the higher annealing temperature, thus increasing the likelyhood of populating type II bonding sites with their higher C1 ÷ ion yield. There is also the possibility that with thermal desorption as SiClz from the surface, the preponderance of single silicon atoms sites may be amplified which could contribute a further candidate for increased in ion emission normal to the surface (Fig. 13(c)). Previously both the type I and type II of C1 bonding have been proposed from independent experiments [3,10,14] as well as a bridge bond configuration which is shown in Fig. 13(d) [12]. The present assignment of the off-normal emission in ESDIAD to the type I chlorine bonding is the same as that adopted by Gao et al. [12] and as noted earlier is comparable to the results of F ÷ ion ESDIAD. They interpreted the low temperature normal emission pattern in ESDIAD as due to the bridge bond configuration. Being energetically less stable than the terminally bonded configuration, the authors argue that a transformation occurs with increase in sample temperature. In our experiment, the transformation from normal to offnormal emission is far less prevalent below halfmonolayer coverages, which would not appear to conform at least with a simple bridge to terminal bond transition. It would, rather, seem from the present study that the repulsive interactions between adsorbed species play the dominant role. It is also relevant to note that the kinetic energy distribution of C1 ÷ ions is independent of changes in the ESDIAD pattern, which suggests that the local chlorine-silicon bond is not grossly altered with the change of bonding geometries. Further, the type II to type I chlorine transformation as envisaged in the present study represents a second order process and is thus expected to occur over a relatively wide temperature range, in keeping with the experimental observations. It has been

Q. Guo et al./Surface Science 356 (1996) 75-91

seen that the diffusion of silicon atoms at the reconstructed Si(100) surface is non-isotropic and almost one-dimensional in character, occurring primarily along dimer rows [27]. A similar behaviour for chlorine would contribute to an enhanced range of temperatures for transformation. 4.2. Mechanisms of ESD 4.2.1. Positive ions For convenience, the energy levels appropriate to a discussion of the C1-Si(100) system are collected in Fig. 15 which draws upon results from previous photo-emission studies [9], XPS studies on SiHaC1 [28] and theoretical calculations [29]. The C1 3s level with a binding energy of 24 eV lies close to the observed threshold of positive ion generation at 22 eV found in the present work; allowing the ionising electron to terminate within the conduction band at the surface, the minimum energy for ionisation of the C1 3s level would be 19.8 eV. In consequence of the empty C1 3s level, Auger transitions involving valence electrons can occur giving rise to valence band excitation. Such a process is illustrated in Fig. 15 where the filling a C1 3s level by a t r bonding electron is depicted as exciting a further a electron. The energy released by the tr electron in the transition to the CI 3s level is around 11 eV which is not sufficient to raise the other tr electron to the vacuum level. However, this electron can stay in the tr* antibonding level

0 3.5 eV 4.3 eV 5.5 eV

Vacuum Level ..--------~....................... / _ ~

11.2 eV 13.2 eV

Px Py nonbonding state obonding orbital

~ l

c *Antibonding CBMin VBMax

" 1

18 eV 24 eV

t ~

_-_--

~

Si3s C1 3s

Fig. 15. Electronic energy levels at a chlorine covered Si(100) surface. The process leading to the formation of a two-hole one-electron repulsive (2hle) state following ionization of the C1 3s level is depicted.

87

forming a two-hole one-electron (2hle) state from which desorption ensues. The antibonding electron will finally become attached to either the desorbing CI ÷ ion, creating a desorbed neutral chlorine atom, or stay with the Si + ion at the surface, yielding the desorbed C1÷ ion in association with the screened surface Si ÷ ion. The effective screening of the Si ÷ ion is the main factor responsible for the very low kinetic energy of the C1÷ ions [ 16]. Recent photon stimulated desorption (PSD) measurement with chlorine species at S i ( l l l ) [18] as well as at Si(100) [19] have similarly proposed a threshold related to the ionisation of the C1 3s level. It is clear that excitation or ionisation of the C1 (Px, Pr) non-bonding electrons is ineffective in liberating C1÷ ions, since ions are first detected only with electron energy higher than 15 eV while the ionisation of these non-bonding electrons is already underway at 11 eV. The initial energy threshold at 15 eV does, however, points to the possible role of tr electron ionisation in C1÷ ion desorption. At this energy, excitation of the Si 3s level is also possible and Auger transitions involving the a bonding electron and the non-bonding electrons Px Pr can produce a 2hle state. The configuration of the final state leading to desorption would appear to be the same for all threshold processes since the kinetic energy position does not shift with primary electron energy. The inflection in the response of the secondary electron yield with primary electron beam energy at the silicon 2p edge identifies the onset of a channel for secondary electron production. A similar feature is not prominent in C1÷ ion yield at this energy (Fig. 8), rather the ongoing increase in ion yield with energy up to ,-~ 150 eV is considered mainly as reflecting the behaviour of the crosssection for excitation of the C1 3s level. There is a role for secondary electrons in C1÷ ion desorption, however, which could be amplified in our measurements by extending the angle of incidence of the electron beam to near grazing incidence. Previous PSD results in our laboratory also point to the role of secondary electrons in ESD of C1÷ ions near a Si 2p edge [30], but with no evidence of a direct desorption process arising from the initial hole ionisation. Our observations taken with those of Durbin et al. [ 19] indicate that C1÷ ion desorp-

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tion follows only with the creation of a hole at the chlorine end of the Si-C1 bond. 4.2.2. Negative ions The production of negative chlorine ion is customarily identified with dissociative attachment (DA) and dipolar dissociation (DD) processes. The absence of a resonance behaviour at low energy (< 10 eV) detracts from the operation of a dissociative attachment process in our ease. The threshold energy does, however, fall in the regime of dipolar dissociation processes and, accordingly, the desorption would ensue from an excited (Si-C1)* complex which breaks up into its charged components. The measurement of the angular distribution of the negative species revealed that the clear majority of the C1- ions desorb in a direction along the surface normal. Off-normal components of the kind associated with type I bonding contribute at most ~10%, thus compared with C1 + ion ESDIAD, it is clear that the production of C1- ions from the type I configuration is quite inefficient. This is most likely linked with a high neutralisation probability of the C1- ion at this site by electron transfer to the surface. Interestingly, the difference in emission patterns detracts from the notion whereby negative ions are produced as a result of electron charge transfer from the surface to an outgoing CI + ion species (probably even to an outgoing neutral species from this site). Alternatively, the pattern of emission along the surface normal could be taken to indicate emission from the type II configuration. The increase of the C1-/C1 + ion ratio with surface roughening, however, suggest that the desorption of C1- ions is specially related to features of defect sites. Three types of defects have been identified in STM studies [31], namely, a missing dimer, two adjacent missing dimers, and a missing atom/ broken dimer site. The latter, contributing around a few percent of sites at the surface, was found to be the most common. In its electrical character the location was found to be metallic over a lateral dimension of 6 A, unlike the other two which were found to be semiconducting. The enhanced electron density of states at this site would certainly contribute to an increased polarisation in the Si-C1 bond, thereby enhancing the negative charge environ-

ment of the chlorine. As a consequence, the efficiency for dissociation into polar fragments would likely be enhanced, but in addition the higher electron density in the vicinity would suppress the chance of reneutralisation of negative ions. The action is similar to that whereby alkali metal adsorption, and the ensuing charge donation to the surface, leads to a distinct increase in negative ion production [32]. It is envisaged [17] that the bonding geometry at this defect site on silicon involves chlorine atop the non-missing Si atom of the incomplete dimer, with the remaining unpaired electrons of the silicon skewed to bind with the silicon atoms in the underlayer (directly beneath the missing atom). Thereby the C1-Si bond at the defect site is brought towards the surface normal, which as with the type II bonding would account for the observed normal pattern of emission of the C1- ions. The energy dependence of the negative ion yield has been seen to be complex, and what appears as a bimodal response is interpreted as arising from the contribution of primary and secondary electrons from the surface. Similar bimodal features have been observed in ESD studies of positive and negative ion desorption at SiOz/Si surfaces [33]. The excitation with primary electrons at the Si(100) substrate is onset from the threshold at ~ 20 eV and peaks at around 90 eV (in the manner of excitation of possibly the Si 3s level with increasing energy). Beyond 90 eV and particularly with the traversal of the Si 2p level at ~ 100 eV, the main source of excitation is foreseen as secondary electrons. As demonstrated in the present study of secondary electron emission, the yield increases with increasing energy up to ~170eV, roughly corresponding to the maximum in negative ion production. A charge transfer in the manner of a harpooning action [34] can possibly be envisaged, involving the transfer of an electron from the defect site to an escaping neutral at the point of separation where the affinity level of the chlorine coincides with high-lying, occupied electron states at the defect site. The scale of distance would be expected to be large by usual inter-atomic distances, in keeping with the harpooning process, and the kinetic energy of the ion would be largely determined by the movement of the neutral chlo-

Q. Guo et al./Surface Science356 (1996) 75-91

rine atom along the potential energy surface (Si-C1) before the electron transfer occurs. The electronegative charge character of chlorine at the surface would, however, appear to make a dipolar dissociative process more likely. 4.2.3. The dependence of ion yields on coverage

The fact that both negative and positive ion signals behave similarly with increasing coverage emphasises that the adsorption, proceeding with a large initial sticking coefficient, occurs randomly at the surface. The behaviour of the ion yields with changes in coverage, induced by thermal desorption, suggests that depletion also occurs rather randomly. Both adsorption and desorption routes for inducing coverage changes feature a maximum in the ion yield. From the standpoint of the positive species, part of the explanation is linked to the conversion from normal type II to off-normal type I bonding. The exact quantitative relationship between the desorption cross sections of the two configurations is, of course, not known, thus the scale of the relative coverage is uncertain. The only quantitative measure as noted earlier in the discussion is of the decrease in intensity ~20% when the surface is annealed to promote the appearance of the off-normal modes. For the negative ions, however, the predominance of normal emission for all coverages defies a parallel view of ion yield changes due to transition between bonding modes. An alternative explanation for the form of the changing ion yield addresses the role of lateral interaction in the adsorbed layer. Starting with low coverage, with correspondingly large distance between neighbouring chlorine atoms, the excitation leading to ion desorptions is localised. As the coverage increases, the concomitant growth in the ion yield arises from the higher concentration of chlorine atoms on the surface. With continued adsorption and consequently a reduced adsorbate-adsorbate distance, overlapping of states begins to occur, possibly similar to that reported with the formation of a twodimensional band structure for bromine on Cu(100) [35] and chlorine on Ag(100) [36]. This kind of two-dimensional band structure would reduce the lifetime of ionic species formed at the surface in view of the associated delocalisation of

89

electrons. A similar behaviour with a maximum in ion yield as a function of coverage has been observed in other systems [37-39] which has been interpreted in terms of the increased probability of neutralisation due to resonant charge exchange between neighbouring molecules [37,38]. This explanation is consistent with the concept of delocalisation of electrons, although, while the space charge exchange process is valid for both long and short adsorbate distances, the notion of a twodimensional band is mainly suitable to describe an ordered adsorbate system with small interadsorbate distances. The behaviour of ion currents accompanying thermal induced desorption of adsorbed chlorine fits within the framework of a random depletion process occurring in the presence of lateral interactions. If the depletion were to occur solely from edges of agglomerations of adsorbate, then the ion yield would decrease directly starting from high coverage. On the other hand if desorption takes place randomly, the process will contribute to a general increase in the average distance between chlorine atoms, while also possibly exposing more missing or single silicon atom sites following desorption as SIC12. Both yields would thus increase with decreasing coverage, pass through a maximum and eventually fall at low coverage. The general features of both the ion signals seen with decreasing coverage are consistent with these descriptions. On the basis of lateral interactions and the enhanced delocalisation of electrons, the larger C1- ion would be affected at relatively lower coverage, and there is indeed an indication of this trend as already noted in the text.

5. Conclusions

ESDIAD studies with C1 ÷ ions show that chlorine adsorbs dissociatively on the Si(100) surface forming two types of bonding geometries: Type I. Two chlorine atoms bound at each end of a symmetric dimer with the bond angle tilted away from the surface normal at an angle of 25 + 5°. Type H. A single chlorine atom bound at the

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up atom position of a buckled dimer with the dangling bond of the down atom unsaturated. Above 0.5 ML, a transformation from type II to type I can take place as a result of thermal activation. Below 0.5 ML, type II chlorine bonding is stabilised, due to repulsive interactions within the overlayer. Lateral interactions are also very much in evidence in studies of the yields of ions accompanying coverage changes. ESDIAD with negative C1- ion species is dominated by normal emission for all stages of surface preparation and chlorine coverage. The production of negative ions from the type I chlorine is ineffective due most probably to the high chance of neutralisation. The weak correlation between the ESDIAD patterns with positive and negative species conflicts with a notion of negative ion formation due to electron charge transfer to an outgoing positive ion species. Missing atom defect sites associated with regions of high electron density are postulated as constituting the main source of negative ion production. Negative ions are formed through a dipolar dissociation process and their yield is seen to be correlated with the response of secondary electron emission from the surface. Positive ions are liberated predominantly by a mechanism involving ionisation of the C1 3s level followed by Auger transitions of valence electrons to create a two-hole, one-electron state. The low kinetic energy of the ions is interpreted as being due to the effective screening of the silicon ions at the substrate which greatly reduces the Coulomb repulsion. Excitation of the Si 2p level while contributing to the secondary electron yield, does not present a channel for ion desorption, rather it would appear that desorption ensues with the creation of a hole at the chlorine end of the surface bond.

Acknowledgements This work was supported by the EPSRC (UK) and one of us (D.S.) is further indebted to Hiden Analytical for support through a CASE Studentship.

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