Chlorination and photodesorption on Si(100) and Si(111)

applied surface science ELSEVIER

Applied Surface Science 79/80 (1994) 72-78

Chlorination and photodesorption on Si(100) and Si(111) C.M. P a u l s e n - B o a z , T . N . R h o d i n *, C . C . L a n g School of Applied and Engineering Physics and Department of Chemistry, Clark Hall 217, Cornell University, Ithaca, N Y 14853, USA (Received 13 October 1993; accepted for publication 22 December 1993)

Abstract

The surface parameters controlling the dynamics of SiCIx (x = 1-4) chlorination and photodesorption by near UV irradiation from doped Si(lll) and Si(100) have been studied as a function of crystallography and carrier concentration. Significant variations in desorption yield were found for the Si(100) and Si(ll 1) samples with type and level of the sample doping (specific conditions: (1) low chlorine pressures, 1 × 10 6 Torr, and (2) low laser fluence, 30 mJ/cm2). The SiCI, (x = 1-4) desorption yields are best interpreted in terms of a mechanism in which photoexcitation couples the effect of carrier doping level with surface chlorine coverage. The dependence of the yields on these two variables is analyzed in terms of a model which balances the surface charge production of holes with the surface chlorine coverage.

1. Introduction Photoinitiated reactions on semiconductor surfaces provide for the achievement of highly controlled etching and deposition, hence much effort has been expended in investigating the underlying mechanisms. Differences in photoreactivity with dopant type and concentration are well documented [1-4]. The observed relationships have been interpreted utilizing concepts of band bending and its effect upon carrier mobility. As elucidated by Houle [5], with the presence of an electron withdrawing adsorbate, both the conduction and valence bands in an n-type semiconductor bend upwards at the surface. With illumination by p h o t o n s in the ultraviolet range

* Corresponding author. Fax: (+ 1) 607 255 7658.

electron-hole pairs are formed. Due to the band bending, holes then migrate towards the surface while electrons flow away from it. In the case of p-type doping the bands bend in the opposite direction and the electrons migrate to the surface. Photodesorption of a surface species is initiated by a hole; hence n-type doping should in general enhance photodesorption and p-type depress it. This qualitative explanation has matched the observed photoreactions on some s u r f a c e / adsorbate systems such as XeF 2 [1], and NO [2] on silicon crystals.

2. Analysis While under some experimental conditions the C12/silicon system has also evidenced this dopant trend [3,4], we have previously reported results on

0169-4332/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 1 6 9 - 4 3 3 2 ( 9 4 ) 0 0 0 3 1 - U

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C.M. Paulsen-Boaz et al. /Applied Surface Science 79/80 (1994) 72-78

the U V photoinitiated chlorination reaction of silicon [6,7] which were inconsistent with this prediction. In our experiments a pulsed excimer laser of 248, 308 or 351 n m irradiated silicon (111) samples of various d o p a n t types and concentrations for 1 0 - 2 0 ns at a 10 Hz repetition rate. Fluences were typically 3 0 - 6 0 m J / ( c m 2 pulse). Chlorine was provided by an effusive b e a m with an effective pressure over the surface from 10 -7 to 10 -6 Torr. A quadrupole mass analyzer ( Q M A ) , synchronized with the laser pulse, was used to collect time-of-flight ( T O F ) spectra of the various silicon chlorides. T h e extent of SiC1 x p h o t o d e s o r p t i o n was f o u n d to be n + ~ p + > n (Fig. 1). As we were unable to monitor p h o t o d e s o r b e d chlorine, due to its large background, we initially interpreted the discrepancy between these results and the predicted d o p a n t trend as due to a variation in the relative a m o u n t of chlorine atoms desorbed with sample doping. T h e r e is a m o r e reasonable explanation. In applying this prediction, the assumption is that the availability of

p h o t o g e n e r a t e d charge carriers limits the extent of photodesorption; however, u n d e r many experimental conditions the adsorbate concentration can also be a significant factor. This affects the results in two ways. (1) If the adsorption of chlorine is m o r e facile on the p - d o p e d samples than the n-type, it could c o u n t e r balance the difference in hole mobility should the experimental conditions be such that the samples are never saturated. (2) If the conditions are such that the samples do reach saturation prior to the laser pulse, then if chlorine saturation coverage is greater for p-type than for n-type silicon (this has recently b e e n seen by Y a r m o f f [8]) it would also counter the hole mobility trend. Published values of the initial sticking probability [9] ( ~ 0.1 at room t e m p e r a t u r e ) and of chlorine exposures necessary for surface saturation of u n d o p e d silicon (20 L) [9] strongly suggest that, given the chlorine pressures used, in the time between laser pulses only a small fraction of the a m o u n t necessary for saturation can be adsorbed. H e n c e it is

0 0

o

A

O

% c TM

l

•

\XX

._~

0

200

400 Microseconds

600

800

Fig. 1. TOF scans showing the dopant effect at 351 and 248 nm on Si(lll). The curve for 308 nm would be indistinguishable from the baseline. Similar TOF scans were obtained for all heavily doped samples. Product levels were significantly reduced for lightly doped samples. Data was collected at the minimum fluence necessary to detect products at each wavelength. The laser pulse was initiated at t = 0 and signal intensities are counts per 1000 laser pulses.

C.M. Paulsen-Boaz et al. /Applied Surface Science 7 9 / 8 0 (1994) 72-78

74

very likely that the chlorine coverage is never near saturation and the first scenario is applicable. A detailed accounting of the chlorine coverage t h r o u g h o u t the experiment is necessary for a complete analysis. Within a relatively short interval after the start of pulsed irradiation, a constant a m o u n t of products is f o r m e d per pulse. H e n c e a steady state e q u a t i o n can be written equating the a m o u n t of chlorine d e s o r b e d during a laser pulse with the a m o u n t adsorbed b e t w e e n pulses. As the laser pulse time is 1 0 - 2 0 ns and the time b e t w e e n pulses 0.1 s, the laser pulse is treated as a delta function. Defining 02, the chlorine coverage immediately prior to the laser pulse and 0t, the coverage immediately following the pulse, the fraction of chlorine desorbed per pulse, D=(O2-Ot)/O

2.

(1)

Since the repetition rate is held constant, we may treat the a m o u n t of chlorine a d s o r b e d b e t w e e n pulses as a function, A, i n d e p e n d e n t of time. T h e second equation defining coverage is: 02 = 01 + A .

(2)

If the chlorine coverage remains m u c h less than the saturation coverage, A is well described as proportional to a sticking coefficient (s) times the chlorine pressure over the surface ( P ) with k being the proportionality constant: A = ksP.

®s

SINGLE PULSE ..........

STEADY STATE

( 1- D )®

O2 ®1

o

o:l

0:2

0:3

0:4

Seconds

Fig. 2. Comparison of chlorine coverage with time in the laser induced chlorination reaction of silicon for the two experimental conditions. Single pulse, in which the surface is initially saturated with chlorine 0 = 0s. The chlorine beam is then shut off and the laser pulses occur at 0.1, 0.2 and 0.3 s. The fraction of chlorine desorbed per pulse is defined as D. Steady state, in which there is a continuous flux of chlorine concurrent with the pulsed laser irradiation. The laser pulses occur at time 0 and every 0.1 s before and after. The chlorine coverage shown is that which exists after the system has been given time to equilibrate. A maximum coverage of 0 z is reached immediately prior to the laser pulse, a minimum coverage of 01 immediately following. The fraction of chlorine desorbed by the laser pulse is again D.

(3)

This assumption will be shown to be consistent with the results of our experiments. If we then hold constant the wavelength and fluence of the p h o t o n s and the chlorine pressure, both A and D will only vary with the doping and crystal orientation of the silicon sample. Examination of the Eqs. (1) and (2) indicates why the a m o u n t of d e s o r b e d species m e a s u r e d n e e d not simply reflect the d o p a n t trend as predicted by the b a n d b e n d i n g model. T h e a m o u n t of chlorine r e m o v e d by a laser pulse is the p r o d u c t DO 2, and while D would reflect this model, 0 2 is d e t e r m i n e d by the equilibrium established by the interaction between A and D. In general w h e n a continuous adsorbate b e a m is c o m b i n e d with a pulsed laser

for reaction at a surface A and D must be d e t e r m i n e d independently. O n e means of establishing the desorption efficiency D, as defined in the previous section, is to do an experiment in which the continuous presence of chlorine gas is eliminated. By preventing adsorption between laser pulses what is m e a s u r e d at the n t h pulse is proportional to: D(1 -D)"-'O,

(4)

where 0 is the initial chlorine coverage. Taking the ratio of the results of subsequent pulses provides for the direct calculation of D. A comparison of the chlorine coverage during the two types of experiments is shown in Fig. 2.

C.M. Paulsen-Boaz et al. /Applied Surface Science 79/80 (1994) 72-78

75

3. Experimental Two S i( lll) samples of heavy n-doping (0.001 1)- cm), and heavy p-doping (0.009 l ) . cm) as well as three Si(100) samples of heavy n (0.004 l-l. cm), p (0.3 ~ . cm), and heavy p-doping (0.002 1~. cm) were placed in an ultrahigh vacuum chamber (base pressure 2 × 101° Torr). The chamber was equipped with a differentially pumped QMA for monitoring gas phase products, a cylindrical mirror analyzer for Auger and X-ray photoelectron spectroscopy (XPS), and an argon ion gun for cleaning the surfaces. Samples were first cleaned with argon ions of 500 eV energy then etched via the laser beam and chlorine gas (248 n m , ~ 75 mJ/(cm 2 pulse), ~ 1 h) until the ion damaged layer was removed. By etching the silicon samples in this manner, the surface present in the experiments with concurrent photon and chlorine beams (hereafter referred to as "steady state") is reproduced for use in experiments in which the surface is first dosed then subsequently irradiated ("single pulse"), avoiding the distinct possibility that the values of D or the saturation coverage would differ between the two types of experimental conditions. For the single pulse experiments, samples were saturated by exposures to > 100 L of chlorine gas. The chamber was then allowed to pump back down and each sample exposed to a series of five laser pulses of 248 nm radiation (30 mJ/(cm 2 pulse)). The production of CI and each of the SiC1x species was recorded for each laser pulse. Results of one set of scans is shown in Fig. 3. After correcting for isotopic abundances, the weighted sum of the observed desorption products: Y'~SiC1x + C1

(5)

x

at each pulse was used to calculate desorption efficiencies of chlorine for the samples (Table 1). One possible uncertainty is whether the fraction of chlorine desorbed from a saturated surface is representative of the fraction desorbed during a steady state experiment, in which the surface is far from saturated. The ratios of products produced in the two types of experiments were used

~.._~_

=.%%',_ .%~

-

11

Pulse 1

Pulse 2

.=

=,.Jl•

a

~

It

Pulse 3

Pulse 4

. . . . . .

Pulse 5

Microseconds Fig. 3. Data collected by a single pulse experiment. The silicon sample surface was initially saturated with chlorine. At each laser pulse the entire surface was irradiated by a 30 m J / c m 2, 248 u m excimer laser beam. No chlorine gas was present to adsorb upon the sample between laser pulses. The experiment was repeated for each of the masses of interest: 35 (CI), 63 (SiCI), 98 (SIC12) and 133 (SiCI3). Mass 168 (SiCI 4) yielded no detectable signal under these experimental conditions.

Table 1 Desorption efficiency and product ratios Sample

D

CI

SiC1

SiCI 2

SiCI 3

(111) n ÷ p+ (100) n ÷ p p+

0.5 0.4 0.5 0.4 0.2

18 32 8.8 7.2 25

1 1 1 1 1

4.9 3.2 3.0 2.8 1.6

4.9 2.5 4.8 4.9 0.4

D is the fraction of chlorine desorbed per laser pulse (248 nm, 30 m J / c m 2 ) . This amount was determined from the time integrated intensity of the single pulse experiment T O F scans after correcting for cracking and isotopic abundances. The a m o u n t of chlorine desorbed per pulse is calculated as in Eq. (5). The product ratios shown are normalized to SiCI = 1. These are the ratios calculated from the first laser pulse in the single shot experiments. This is in agreement with the steady state experiments.

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C.M, Paulsen-Boaz et al. /Applied Surface Science 7 9 / 8 0 (1994) 72-78

as a check. We find that the ratio produced by the first two or three laser pulses in a pulsed laser experiment match, within noise levels, the product ratios of a steady state experiment. However, subsequent laser pulses produce ratios which differ increasingly; hence D was calculated from the data generated by the initial two laser pulses. Steady state experiments were performed on the same samples utilizing identical laser conditions and varying chlorine pressures. To obtain reasonable signal-to-noise typically a thousand signal averages were taken.

0

"~

0

750' n+ 0

0

0

._~ 5oo.

&

Production of silicon chlorides in the steady state experiments for the Si(100) samples exhibited the same trend previously seen in the Si(111) work: heavily doped samples of n- and p-type produced similar amounts of photodesorption products. These amounts were much greater than for lightly doped samples. When these same samples were examined by single pulse experiments, the dopant trend altered to that predicted by the band bending model as measured by the total amount of photodesorbed species:

~

~

~

~

1o

Chlorine Pressure xlO -7 torr

Fig. 4. Results of pressure variation upon the steady state data. The slopes of the variously doped Si(100) samples are proportional to the sticking coefficient for the adsorption of chlorine on that sample. As the ratio between the various desorption species was constant over this pressure range, the time integrated signal of the mass 98 peak was taken as representative of the total SiCI~ production. The ratio of the sticking coefficients of the p+ to the n + Si(100) sample is 1.6.

(6)

The heavily n-doped samples were much more reactive than the lightly doped samples which in turn reacted more readily then the heavily pdoped samples. This is a clear indication that chlorine coverage limitations significantly alter the apparent dopant trend in a steady state type experiment. The absolute value of the sticking coefficients cannot be obtained from these two experimental techniques; however, by combining Eqs. (1)-(3), the relative sticking coefficients can be calculated from the steady state experiments response to changes in chlorine pressure: DO 2 = ksP.

p+ 1000-

250"

4. R e s u l t s

C1 + Y'~SiC1 x .

125(

(7)

Realizing that D O 2 is the amount of chlorine desorbed as calculated by Eq. (5), a plot of chlorine desorbed versus pressure has a slope propor-

tional to the sticking coefficient, s, for a given sample. As k is constant for all samples, the ratio of these slopes for differing samples is the ratio of their respective sticking coefficients. Since the product ratios remained constant over the experimental pressure range, a plot of any one of the desorption species should yield this same slope. Fig. 4 shows these m e a s u r e m e n t s for the mass 98 peak from the Si(100) samples. The ratio of the sticking coefficients p + / n + is 1.6. We can also calculate the chlorine coverage during a steady state experiment by comparing the yield per signal average in the steady state experiments with the yield of the first pulse in the single pulse experiments for a given sample. As the former is proportional to 02 and the latter to 0s, the saturation coverage, we can estimate what fraction of

C.M. Paulsen-Boaz et al. /Applied Surface Science 79/80 (1994) 72-78

saturation coverage is reached by the variously doped Si(100) surfaces immediately prior to the laser pulse under the steady state experimental conditions: for the p + sample Oz/O s is 0.02 while for the n + sample 0~/0 S is 0.005. These results are consistent with the assumption of Eq. (3).

5. Discussion / conclusions The relationship between sample doping and photodesorption can now be seen to be a function of experimental conditions: by varying the flux of chlorine available to the surfaces any of three dopant relationships can be observed. (1) When surfaces are very far from saturation (produced by either large desorption efficiencies, which can be increased by increases in laser fluence, or a combination of low chlorine pressure and little dark time between laser pulses) chlorine coverage and not the number of photogenerated holes reaching the surface is the limiting factor in the photodesorption reaction. It is the variation in chlorine sticking coefficients with sample doping which determines the trend: p + > n+. (2) At the other extreme, a saturated surface (produced by either very small desorption efficiencies or large chlorine pressures or dark times) has the number of holes reaching the surface as the limiting factor and the trend becomes that predicted by the band bending model where the hole mobility to the surface is: n +> p +. (3) In the ambiguous middle region neither the number of holes reaching the surface nor the surface chlorine coverage is limiting and we find the enhanced mobility of holes to the n + surface offset by the enhanced adsorption of chlorine on the p+: n +--- p+. The question then becomes if, given our irradiation conditions producing desorption of typically half the available surface chlorine, it would ever be possible to guarantee a saturated or nearly so surface. At a repetition rate of 10 Hz this would require an exposure of roughly 10 L or a pressure on the order of 10 -4 Torr. Without the use of pulsed molecular beams this is unworkable for examining photogenerated e l e c t r o n / h o l e pair reactions as chlorine gas has a high cross section for adsorption of U V radiation at excimer

77

wavelengths and the reaction mechanism changes to one involving gas phase chlorine radicals [4,10]. The single pulse experiments are more valuable in that they reflect more directly the trend in photogenerated charge carrier mobility with sample doping, requiring only the additional information of the variation in saturation coverage with sample doping for a complete analysis. The question remains of why heavily n-doped samples absorb chlorine less readily than the p-doped samples. The answer likely lies again in band bending. For the cleavage surfaces of both Si(100) and S i ( l l l ) there exist surface states [11,12] which pin the Fermi level in the band gap at the surface. For the heavily n-doped samples the Fermi level is higher in the surface than in the bulk creating band bending near the surface which causes a flow of electrons to the surface. As a result when a silicon sample becomes more heavily n-doped an increasing negative charge accumulates on the surface [13]. This charge likely inhibits the approach and adsorption of chlorine on heavily n-doped samples.

Acknowledgements Research and facilities support from Professor T.R. Cool is sincerely appreciated as well as consultations with Professor Wilson Ho. Primary funding was received from N S F - D M R 87-05680 and secondary support from the Materials Science Center, N S F - D M R 88-18558-AO2. Professor J. Yarmoff is thanked for providing a prepublication report of his chemisorption results.

References [1] F.A. Houle, J. Chem. Phys. 79 (1983) 4237; 80 (1984) 4851. [2] Z. Ying and W. Ho, Phys. Rev. Lett. 60 (1988) 57. [3] W. Sesselman and T.J. Chuang, J. Vac. Sci. Technol. B 3 (1985) 1507. [4] H. Okano, Y. Horiike and M. Sekine, Jpn. J. Appl. Phys. 24 (1985) 68. [5] F.A. Houle, Appl. Phys. A 41 (1986) 315.

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C.M. Paulsen-Boaz et aL /Applied Surface Science 7 9 / 8 0 (1994) 72-78

[6] T.N. Rhodin, C. Paulsen-Boaz and W.L. O'Brien, Surf. Sci. 283 (1993) 109. [7] C. Paulsen-Boaz, T.N. Rhodin and W.L. O'Brien, J. Vac. Sci. Technol. B 10 (1992) 216. [8] J.A. Yarmoff, personal communication. [9] J.V. Florio and W.D. Robertson, Surf. Sci. 18 (1969) 398. [10] R. Kullmer and D. Bauerle, Appl. Phys. A 43 (1987) 227.

[11] P. Martensson, A. Cricenti and G.V. Hansson, Phys. Rev. B 33 (1986) 8855. [12] J.E. Ortega and F.J. Himpsel, Phys. Rev. B 47 (1993) 2130. [13] G.V. Hansson and R.I.G. Uhrberg, Surf. Sci. Rep. 9 (1988) 197.