Adsorption kinetics of SiH4, Si2H6 and Si3H8 on the Si(111)-(7×7) surface

Surface Science 195 (1988) 307-329 North-Holland, Amsterdam

307

ADSORPTION KINETICS OF Sill4, Si2H 6 AND S i 3 H s ON THE St(Ill)-(7 X 7) SURFACE Stephen M. GATES IBM T.£ Watson Research Center, Yorktown Heights, N Y 10598, USA Received 15 May 1987; accepted for publication 5 October 1987

The rates and mechanisms of chemisorption on the Si(111)-(7 x 7) surface have been investigated under UHV conditions for Sill4, Si 2H6, and Si3H s. Temperature programmed desorption of H 2 has been used to quantitate adsorption of the silanes following calibrated exposures. A dramatic enhancement of the adsorption rate occurs with one St-St bond in the molecule. A second St-St bond has no effect on ~u~O~vuon rate on the clean surface, but increases the reactivity with a hydrogen covered surface. The reactive sticking coefficient, S R, for sflane is less than 0.001 near zero coverage on St(Ill)-(7 x7). S R (Stria) is independent of temperature from 25 to 275°C. Reactive sticking coefficients of 0.474-0.1 for higher silanes Si2H 6 and Si3H s are observed at low coverage for 25 ° C surface temperature. The higher silanes adsorb on the clean surface through molecular precursor states, as evidenced by coverage independent sticking coefficients and by negative activation energies for adsorption. At low coverage, Si 2D6 and Si 2H6 exhibit the same sticking coefficient, so that r~o deuterium kinetic isotope effect is observed for the adsorption of disilane. This observation is consistent with Si-Si bond breaking as the rate limiting step for conversion of the molecular precursor to chemisorbed species. The surface residence time of molecular Sill 4 is inferred to be very shcrt, relative to the residence times of molecular Si 2H 6 and SiaH s. Sill4 is relatively unreactive due to its short residence time and because adsorption requires Si-H bond scission.

1. Introduction Defining the chemisorption and reaction pathways of stable silicon hydride molecules, Si,,H2,,+2, on well characterized silicon surfaces is essential to achieving a fundamental understanding of the surface chemistry of silicon. Knowledge of quantities such as reactive sticking coefficients and surface decomposition rates for these molecules also contributes to o u r ability to understand and control chemical vapor deposition (CVD) processes. The compiex mechanisms of Sill 4 decomposition under thermal or plasma excitation involve reactions at walls which are related to the surface reactions studied here. Once the surface reactions of silicon hydride molecules on sificon surfaces are well characterized, fundamental questions regarding thin film epitaxial growth from these molecules may be experimentally addressed, and realistic modelling of silicon CVD processes may proceed.

308

S.M. Gates / Silicon hydride o n Si(lll)-(7x 7)

Silane, Sill 4, is a common CVD precursor for the deposition of thin film silicon and silicon based insulators such as oxide and nitride. Higher silanes, Si,Hz,,+ 2 (n = 2, 3, ... ) were initially observed as products of Sill 4 pyrolysis in the fundamental work of Ptu"aeH and Walsh, who readily detected H2, Si2H 6 and Si3H s when Sill 4 was heated near 4 0 0 ° C under static conditions [1]. Film growth rate measurements and kinetic modelling studies of Si CVD at low temperatures (500-725 °(2) are consistent with a model in which higher silanes are gas phase intermediates and are significant links between SiH4(g) and Si(s) under CVD conditions [2,3]. The higher silanes have been observed in the effluent gas of CVD flow reactors [2] and may be formed by insertion reactions such as (2) and (3), following homogeneous Sill4 dissociation to silylene, reaction (1), SiH4(g) ~ :SiH2(g) + H2(g), :SiH2(g) + SiS4(g) --, Si2H6(g), :SiH2(g) + Si2H6(g) ~ Si3Hs(g ).

(1) (2) (3)

The extensive literature on modelli g CVD processes has been recently reviewed [4] and higher silanes are gas phase intermediates in many of the current models. The surface rcactivities of silane, disilane and trisilane are compared in this work. The low reactivity of silane restricts this UHV study to the 0.01-0.1 monolayer regime for Sill4, while the higher silanes are conveniently studied from 0.001 to 2 monolayers coverage. The very low coverage regime of chemisorption accessible for the higher silanes allows the basic molecule plus Si surface interaction to be studied in some detail. At intermediate and high coverages, the surface is partially covered with SiHx species, the surface structure is poorly defined, and only a qualitative reactivity comparison is made. Dramatic differences are seen comparing silane (no Si-Si bonds) and disilane (one Si-Si bond). Both similarities and differences are s ~ e ,zomparing disilane and trisilane (two Si-Si bonds).

2. Experimental devils Mono-, di-, and trisiiane were adsorbed on the clean Si(111)-(7 x 7) reconstracted surface, as monitored in situ by low energy electron diffraction (LEED), in a stainless steel ultrahigh vacuum (UHV) chamber with a base pressure of 1 × 10 -1° Torr. All samples were cleaved from a single Si(111) wafer, antimony doped to 0.0.06 ~ cm resistivity. Two wafer sections were clamped back to back, so that oniy polished (111) faces were exposed. The sample temperature was measured by an infrared pyrometer through a CaF 2 window in the UHV chamber. Resistive sample heating and linear heating

S.M. Gates / Silicon hydride on Si( l 11)-(7 × 7)

309

arnps were controlled by a personal computer executing a feedback program, vhich used the pyrometer signal as input and controlled a programmable rower supply. A UTI 100C quadrupole mass spectrometer (unshielded) with a ine of sight to the surface measured the H 2 or D2 partial pressure as a 'unction of sample temperature. The surfaces were cleaned by heating to 1050°C in UHV. Slow cooling ~rodueed the (7 × 7) reconstructed surface, and only samples exhibiting sharp 7 x 7) LEED patterns were used. The maximum temperature used for TPD, 100 ° C, is 400 ° C below the (7 x 7) to (1 × 1) surface phase transition. The "7 x 7) surfaces were repeatedly cooled to 25 ° C and heated to 700 ° C with 9rdy slight degradation of the LEED pattern. Silane (Airco) and disilane (Matheson) were purchased, whereas trisflane ~,as synthesized. The silanes were purified by trap to trap distillation, and Cd2/MS analysis was used to determine the purity of each gas. The Sill4 was 99.5% pure (principal impurity disilane -0.4%) and the Si2H 6 was 98.9% pure (principal impurity sflane). The trisilane contained 2% each of Sill4 and Si2H6, approximately 1% Si4Hlo, and contained 0.05% non-silicon hydride impurities. It will be shown that the very reactive disilane impurity ha the diane limits the range of reactive sticking coefficient measurable for silane on silicon surfaces, when surface H atoms are used to measure silane adsorption. A calibrated effusive doser facing the sample at a distance of 2.5 cm was used for gas exposures, with all filaments and ion pumps off during the exposures. The absolute flux effused out of the doser was measured using a calibrated volume, d P / d t measurements, and gas effusion laws. The computations of Campbell and Valone [5] were used to calculate the fraction of the effused flux which is intercepted by the crystal surface as (kl ___9.02. The error in this calculated fraction, + 20%, is the dominant contribution to error in the absolute reactive sticking coefficient, S R. Relative values of S R comparing the three molecules studied under identical experiment conditions are subject to an error estimated at + 5%. The absolute number of adsorbed H atoms due to a particular Si,H2,+.~ exposure was measured using the H 2 temperature programmed desorpfiun (TPD) area, calibrated to an internal standard. The saturation coverage of H atoms produced by adsorption of H atoms (l~sing a hot filament doser and H 2 gas) on Si(111)-(7 × 7) is c~.8× 1014 cm -2, as determined by nuclear microana~y~ ___1-.--'~ [O].r"'~ ~'~n~ 1r~1__ H 2 TPD area from this we;i oeimecl H atom coverage was measured after every 3 to 5 Si,H,~+2 TPD e×periments, and was the interna~ standard. So~ was evaluated rigorously from the slopes of adsorption plots only at room temperature in the very low coverage regime (fig. 6). Exposure of the surface to Si,H2,+2 gases results in formation of surface silicon hydride species, SiH~,(a), where Si from the surface and from the gas are not distinguishable. The hydrogen is formalized here as surface H atoms for book keeping purposes. The follewmg equations indicate the H atom

S.M. Gates / Silicon hydride on Si(l l l)-(Tx 7)

310

counting scheme used to calculate the number of adsorbed Sill 4, Si2H6, or Si 3H 8 molecules:

(4) (5) (6)

SiH4(g) ~ Si(a) + 4 H(a), si2n6(g ) ~ 2 Si(a) + 6 n ( a ) , Si3Ha(g ) ~ 3 Si(a) + 8 n ( a ) .

This quantitation method for adsorption of all three molecules assumes that no H2(g) or SiHx(g) species desorb from the surfr ,e during the exposure. This assumption is supported by the fact that all surface temperatures used for gas exposure are below the observed thermal desorption thresholds of H 2 and Sill 4 from this surface [7,8], but this ~.~s,~,mption is yet to be experimentally verified.

3. Results a.~d interpretation 3. I. Adsorpti :-'zrates of Si, H2, + e measured using H 2 TPD data 3.1.1. TPD from a standard surface: S i ( I l l ) - ( 7 x 7) plus saturation H atoms H E TPD data from the Si(111)-(7 x 7) surface exposed to H atoms, and to Sill 4 are compared in fig. 1. Curve A in fig. 1 corresponds to the coverage at saturation of H atoms on this surface (produced by hot filament dissociation of H2). Culbertson and coworkers have previously found this coverage to be 9.8 x 1014 H atoms cm -2 by nuclear rnicroanalysis [6]. This H 2 TPD area is then an absolute calibration for the surface coverage of H atoms desorbed in TPD experiments performed under identical conditions following silane exposure. The peak temperature and relative heights of the fll and /[~2 peaks agree with previously reported H 2 TPD data of Schulze and Henzler [7]. t

Z

'

I

'

~(=SK/s SiN4 B

I

I

'

'

rs=TG=25°C .__.~__._.~~

-~'~f

I

I

.2

SATURATION H ATOMS[~~ I

300

,

_n

L

I

~

4OO

,

,

1

500

,

, ~

TEMPERATURE (%1

1

~

,

600

Fig. 1. TPD of H 2 from Si(111)-(7×7) plus saturation coverage of H atoms (curve A), and 1.9x10 ~7 cm - z silane (curve B). Resulting H atom coverage from Sill 4 is 2 . 7 × 1 0 ~4 cm -z, approximately 1 / 4 monotayer. Gas exposures are made at 25 ° C gas and surface temperatures.

S.M. Gates / Silicon hydride on Si(l l l)-(7× 7)

311

The fll desorption peak with rpeak~-540°C has been attributed [7] to surface monohydride decomposition: 2 Sill(a)--, H2(g) + 2 Si(a).

(7)

The f12 peak (Toeak = 425 o C) was attributed [7] to surface dihydride decoroposition: SiH2(a) ~ Hz(g) + Si(a), 2 SiH2(a) --, H2(g) + 2

Sill(a).

(8) (9)

During the temperature program through the/32 peak (above roughly 325 ° C) the following reaction may also occur after empty Si sites have become available as products of reaction (8): Si(a) + SiH2(a ) ~ 2 Sill(a).

(10)

Surface Sill 3 groups were previously postulated to decompose in the vicinity of 350°C [7], based on a weak H z TPD feature at this temperature. This assignment was not certain, and no such feature was seen in this work.

3.1.2. TPD from silicon hydride complexes on Si(111)-(7× 7) In fig. 1, both the fll and/~2 TPD peaks are seen at the same temperatures for tlus surface exposed to H atoms and to Sill4. H- TPD data from the Si(lll)-(7 × 7) surface exposed to H atoms, and Si2H 6 and Si3H s are compared in fig. 2, where exposures resulting in similar H atom coverages were selected. Again the fll and 132 desorption peaks are observed at the same temperatures, as emphasized by the vertical dashed hnes. The figure captions state the different exposures to SiH4, Si2H 6, =nd Si3H 8 used in figs. I and 2, and the resulting H atom coverages. Note that a faster time scale (more sensitive X-a~s) was used for fig. 1 (Sill4 data). Figs. 1 and 2 demonstrate that H2 desorption from the decomposition of adsorbed SiHx(a) species on the Si(lll) surface is characterized by desorpfion maxima at appr~ "Jrnately 425 °C (/32) and 540°C (ill) independent of the gas phase origin of the SiHx(a ) species. Subsequently, it will be shown that 2 SiH3(a ) groups are formed by Si-Si bond scission in the initial chemisorption of disilane. The fact that no low temperature H z desorptior, features due to SiH3(a) decomposition are seen in fig. 2 or in fig. 7 irldicates that the initial step in SJH3(a) decomposition does not evolve HE. In contrast, Sill(a) decomposition, reaction (7), is desorption rate !irrfited and evolves H 2. SiHE(a) decomposk.;on, reactions (8), (9) and (10) may or may not evolve H z depending on the surface coverage of SiH2(a) and Si(a). The essential conclusion is that the relative population, s of Sill(a), SiHE(a), and SiH3(a ) on a Si surface cannot be deduced from the H2 TPD shape. The absolute number of H atoms on the surface can be measured by proper calibration of the H 2 TPD area, and this information allows quantkao

112

S.M. Gates /Siliconhydrideon Si(l l l)-(7x T) I

o') I-

'

I

'

I

'

I

'

!

~i3H 8

x3

:3

{d n,-

"in

I/

<3

//~

I

300'

SATURATION

H ATOMS

I I I I I I I

500

700 TEMPERATURE (*C)

Fig. 2. T P D of H2 from Si(ll 1)-(7 x 7) plus saturation coverage of H atoms (curve A), 1.9 x 10 a4 cm -2 trisilane (curve B), and 2.9 x 10 ]5 cm -2 disilane (curve C). Resulting H atom coverage from Si3H8 is 4 . 8 x 1 0 ]4 cm -2 and from Si2H 6 is 4.5 x l 0 1 4 cm -2. Gas exposures are made at 2 5 ° C gas and surface temperatures.

rive adsorption kinetic measurements to be made for silicon hydride molecules.

3.1.3. S R compared for Sill4, Si2H 6 and S i s H s Once the adsorbed H atoms have been quantified, reactions (4)-(6) (see section 2) allow the number of adsorbed S i , H 2 , + 2 molecules to be measured subject to the assumptions and procedures stated in section 2. Fig. 3 summafizes a large number of calibrated H 2 T P D experiments of the form illustrated in figs. 1 and 2. It compares the reactivity of silane, disilane, and tfisilane as a function of exposure at room temperatures on Si(111)-(7 × 7), spanning 4 decades of exposure. Three trends are apparent. First, the sequence of reactivity trisiiane > disiiane >> siiane follows the number of Si-Si bonds in the molecules. Second, a dramatic difference is seen between silane and the higher sitanes. Measurable adsorption (0.01 mono!ayer) of silane requires roughly ten munolayers exposure , while measmable adsorption of di- or trisilane requires only - 0.0! monolayer exposure. SR(Si2H6) is at least 1000 times SR(SiH4) in the low coverage limiL Third, trisilane continues to react with the surface after 2.5 x ]0 a4 cm -2 molecules of trisilane have been

S.M. Gates / Silicon hydride on Sq 111)-(?x 7)

313

adsorbed, and the resulting H atom coverage is 2 x 1015 cm -2, twice the saturation coverage produced by adsorption of dissociated H 2. The silane used in this work contained a disilane impurity of 0.4%. This is near the detection limit of the gas chromatograph/mass spectrometer used, and must be viewed as an estimate. The sticking coefficient of 0.47 for disilane times the 0.004 concentration times 2 / 3 (for the ratio of H atoms in Sill4 versus SiEHt) places an upper l i ~ t on the measurable Sos for silane of > 0.001, when enly surface H atoms are measured. It is noteworthy that Sill4 purity has not been reported or discussed in previous surface science studies [14-16] of S i l l , adsorption and decomposition, except by Joyce [17].

3.2. Activation energies for Sill4, Si2H6 and SisH s adsorption Experiments probing the great differences in reactivity depicted in fig. 3 begin with examination of the temperature dependence of adsorptic,n. When the Si surface temperature was varied for activation energy measurements, the reactive sticking coefficient, S R, was evaluated for a single constant exposure 40.0

I

I

I

I

x 1013 35.0 COMPARATIVE 30.0 t.3

O IJJ f13 Cr

o 03 a <

03 iJJ E) U

ADSORPTION OF Sill 4,

Si2H 6, AND Si3H 8 ON Si(1 1 1)-(7X7)

h

Ts

=

T6

=

25

°C

25.0

~,,,e ,t,~e

20.0

SJ

D

s~

0

,...J

15.0

,,--I

o

J

10.0

m

Si3H8

f•t

~

-'

t

"

..- ,,,,.,e'

Si2P~6

m .....

5.0-

0.0 3 101

A 10 14

! 10 !5'

.....

1 IO16

EXPOSUhE (molecules cm.

_.2~ 101 7 -2

10 ~s

)

Fig. 3. Adsorption of silane, disilane, and tfisilane on Si(lll)-(7 x7). Y-axis: molecules adsorbed cm -2, measured by H2 TPD area. X-arJs: (log scale) molecules exposure cm -2, from effusive gas doser. Surface and gas temperatures equal 25 ° C.

S.M. Gates / Silicon hydride on Si( l l l )-(T x 7)

314

as S R= (molecules adsorbed/molecules exposure), and care was taken to be in the low coverage regime where coverage is proportional to exposure. The activation energy for chemisorption of Sill4 on Si(lll)-(7 × 7) was evaluated using a fixed Sill a exposure, 1.2 × 1017 Sill4 molecules cm -2, at 2 5 ° C gas temperature. The surface temperature was varied from 50 to 285 ° C. The results appear in fig. 4, where zero activation energy is observed. These data indicate that Sill 4 adsorption is not activated. Also these data suggest that silane adsorption does not occur through a molecular precursor, SiH4(a). A precursor having a very small adsorption energy (well depth) much less than k T (0.6 to 1 kcal/mol) is consistent with the data. The activation energy for disilane adsorption was evaluated using one exposure of 4.2 × 1013 c m - 2 at 2 5 ° C gas temperature. The surface temperature was varied from about 130 to 375°C, and the results appear in fig. 5. In this temperature range, it is seen that greater adsorption of disilane occurs the

1 0 -3

I

I

I

9 8

2 8 5 °C

50 °C

I

I

7 A

Z 0

6 a

o

Q @

C~ @

Z ¢,.3 I-" £0

4

(.9 © ..J

3

2 0.0015

Sill a

I

EA = 0

I....

0.0020

I

0.0025 1/T

(K

0.0030 -I

.... 0,0035

)

Fig. 4. Arrhenius plot for adsorption of silane on Si(1~1)-(7×7). The sticking coefficient, S = molecules adsorbed/molecules exposure, was measured as a function of surface temperature, using the H 2 TPD area and stoickiometric equation (4) (see text). The fixed gas exposure at 25 o C gas temperature is 1.2×1017 Sill4 molecules cm -2, and the resulting H atom coverage is - 2 . 2 x 10 ~4 H atoms cm -2.

S.M. Gates / Silicon hydride on Si(111)-(7× 7) I

I

315

I

365 °C

130 °C

I

I

uu (J

TRISILANE

u IJL Ill

EA = - 4.9 Kcal mole \

-1

0 ¢j

p

/

\//"

(3 z v

u_ !--

°/p

O3

(.9 0 ,_1

Z

DISILANE E A =

-

2.6 Kcal mole -1

10 -1

0.0010

I

i

i

0.0015

0.0020 1/T (K -1 )

0.0025

0.0030

Fig. 5. Arrhenius plots for adsorption of disilane and tfisilane on Si(111)-(7×7). The sticking coefficient, S = molecules adsorbed/molecules exposure, was measured as a function o~ surmce temperature, using the H2 TPD area and stoicbSometric equations (5) or (6), depending on the molecule (see text). The fixed gas exposures at 25 °C gas temperature arid the resulting H atom coverages for each gas are for disilane: 4.2×1013 Si2H 6 molecules cm -2, (0.4-1.0)xlO TM H atoms cm -2 and for trisilane: 3.2x1013 Si3H 8 molecules cm -2, (0.3-1.2)x 10 ~4 H atoms cm -2.

lower the surface temperature, giving a negative apparent activation energy for adsorption of - 2 . 6 kcal/rnol. Also appearing in fig. 5 are similar data for trisilane, evaluated using an exposure of 3.2 × 10 a3 cm -2, and yielding - 4 . 9 kcal/mol activation energy. The negative apparent activation energies indicate that adsorption proceeds through a precursor state and are interpreted as the numerical differences between activation energies for two elementary reaction steps (see section 4). 3. 3. Low coverage reactiv#y of disi&ne and trisi[ane on d~e dean surface

A qualitative comparison has been made of the surface reacti,~fities of SiI't4, SizH6, and Si3H 8 as a function of exposure. The disparate adsorption kinetics seen in fig. 3 are primarily due to structure and reactivity differences in the 3 silicon hydride molecules. The adsorption kinetics are influenced by a sec-

S.M. Gates / Silicon hydride on Si(l l l)-(7x 7)

316

ondary surface effect, the different H atom surface coverages and different populations of Si(a), Sill(a), SiH2(a), and SiH3(a ). These surface effects are eliminated by examining adsorption in the lh:nit of zero coverage, where a quantitative comparison of the molecular reactivities with a common well defined surface is possible. The very low coverage regime (0-2% of a monolayer) is considered in fig. 6), where molecules adsorbed are plotted as a function of exposure for disilane and trisilane, and also for deuterated ais0ane Si2D 6. The solid line is drawn through the tdsil,'me data points. T h e slope, 0.47, equai~ the zero coverage reactive sticking coefficient, Soa, of trisilane at 25 ° C surface temperature with an estimated uncertainty of _+20%, or 0.1. S0a for trisilane and disilane in the zero coverage limit are the same within experimental error. Reactive sticking coefficients for Si2H 6 and Si2D 6 are compared in fig. 6 in order to explore the role of S i - H bond breaking in the chemisorption of disflane. Within experimental error, S0a(Si2H6)= Soa(Si2D6) and no deuterium kinetic isotope effect (DKIE) is observed. The mechanisitic implications for SoR(Si-I-I) = S0a(Si-D) are considered in section 4. 5.0

I

I

I

I

x 10 )3 Si2D 6 , Si2H 6 AND Si3H 8 ON Si(111)-(7X7): 4.0I

£3

a UJ tn c~ © £/3 £3 < ..J

VERY LOW COVERAGE

REGIME

-

T s = T G = 25 °C

3.0

I

2.0

O

o

A

O

Si3H 8

&

Si2H 6

o

Si2D 6

.c

1.0

oy ~ . . _ 0.0 ~ . . ,,...

o

t 1.0

I ~ EXPOSURE

I

I

"~ (molecu!es

'~..V

cm.

;3.tJ

-2

"

x 10 ~3

Fig. 6. Adsorption of Si3H s, Si2H 6 and Si2D 6 on Si(lll)-(7x7), in the 0.02 monolayer regime. F-axis: molecules adsorbed cm- 2 measured by H 2 TPD area. X-axis: molecules exposure cm- 2 from molecular beam doser. Surface a n d gas temperatures equal 25 o C. The solid line is d r a w n through the trisilane data and has slope of 0.47 equal to Soa, the zero coverage reactive sticking coefficient.

S.M. Gates / Silicon hydrideon Si(l l l)-(7× 7)

317

D 2 TPD areas were measured to quantitate Si2D 6 adsorption and were calibrated to the D2 TPD area from a saturation coverage of D atoms. The data of fig. 6 are limited to the 0.02 monolayer regime. At this low D atom coverage, H D T P D areas were compared following Si2D6 exposure and following the saturation exposure to D atoms. The H D areas were the same to within 2% when expressed as a fraction of the D 2 area. Therefore, the D 2 areas were used without correcting for D atoms which appeared as H D due to reaction with H2 in the mass spectrometer ionizer.

3.4. Adsorption of S i z H 6 and Si 3H s at intermediate and high coverage Fig. 7 compares four H 2 T P D curves measured following exposure o t Si(111)-(7 x 7) to Si2H 6 at room temperature (curves B-E) with the internal standard curve A from saturation coverage of H atoms. At the fight side of the figure disilane exposures are shown which span over three decades. Through I'

I'1

:SK/s

TcT¢2:C

~M Si2H6

OLECULES ¢m "2 1'4xi017

~o p-

xl

Q: 2"gxlOtS-- x2 13. <3

5.9xmOg3

SATURATION H ATOMS

300

500 700 TEMPERATURE(°C)

Fig. 7. TPD of H 2 from Si(111)-(7×7) plus saturation, coverage of H atoms (curve A), and increasing exposure to disilane(curves B through E). Gas exposures are made at 25 °C gas and surface temperatures, and are listed m the figure.

118

S.M. Gates / Silicon hydride on Si(l l l).(Tx 7)

25.0

I

I

I

I

I

I

x 1013 ADSORPTION

AT T s = T G = 25 °C

20.0 -

(3J u

m rr

3H8 15.O

o ~3 < _J

10.O

_J

O

5.O t

0.0 0.0

SigH6

I

I

I

I

I

I

5.0

10.O

15.0

20.O

25.0

30.0

EXPOSURE (molecules cm.

-2

)

35.0 x 1015

Fig. 8. Adsorption of Si2H 6 (triangles) and Si3H 8 (sofid circles) on Si(l11)-(7x7). Y-axis: molecules adsorbed cm -2, measured by H 2 TPD area and stoichiometric equations (5) or (6), depending on the molecule (see text). X-ards: molecules exposure cm-2, from calibrated effusive doser. Surface and gas temperatures equal 25 o C.

this full range of disilane exposures, the/~2 desorption peak (around 425 ° C) appears as a poorly resolved shoulder on the leading edge of the main fla peak. Exposures lower than roughly 5 × 1013 SiEH 6 molecules cm -a yield only/31 desorption (data not shown). The areas under several H 2 TPD curves measured following disilane exposure (such as curves B - E in fig. 7) have been calibrated to the standard area (curve A) and the results are plotted using triangles ha fig. 8. The low coverage region is also shown with expanded scales in the top panel of fig. 9. The slope of the fines in these figures at a given point is the reactive stickJns, coefficient, S R"

The data exl~bit a constant slope in the low coverage r e , m e , indicated by the dashed fine in the top panel of fig. 9. Steted in other words, the reactive sticking coefficient is independent of surface coverege. This is standard behavior indicative of a mobile molecular precursor to chen'fisorption [9] and is expected based on the negative activation energy for disilane adsorption (fig. 5). As seen in fig. 9 (top), after roug~fiy i0 ~4 Si2H 6 molecules C m - 2 e~:posure,

S.M. Gates / Silicon hydride on e;tl l 1)-(7X 7) 8.0xlO 13

I

I

I

I

I

I

319

I

LOW COVERAGE ADSORPTION 6.0

/ 4.0 -

/A

g

I

!

E U

C3 Iz,I CO n,. 0 (/3

A

1' I

i

t

Si2H s

2.0

0.0 8.0xlO 13

[

I

I

1

I

I

I

I

I

I

I

I

I

I

O3 LU .J

UJ -J O

Q

6.0

e

/

4.0 -

/

/ 2.0

_

I

/e

•

Si3H 8

P P

0.0 ~' 0.0

I

I i.O

I

I Z.O

I

I

I

3.0

4.0xiO 14

EXPOSURE (molecules cm -2) Fig. 9. Adsorption of Si 2H6 (top, triangles) and Si 3Hs (bottom, solid circles) on Si(111)-(7 x 7), in the low coverage regime. Y-axis: molecules adsorbed cm -2, measured by H2 T P D area. X-axis: molecules exposure cm -2, from calibrated effusive doser. Surface and gas temperatures equal 25°C. The dashed lines have slope equal 1~ 0.47. The arrows indicate exposures used for Arrhenius plots in fig. 5.

S R drops dramatically. A continuous decrease by roughly a factor of ten is then seen for S R in fig. 8 from roughly 1014 to 10 a6 Si2H 6 molecules cm -2 exposure, as the surface sites become filled with adsorbed SiHx species. Fig. 10 compares five H 2 TPD curves measured following expusarc of the Si(111)-(7 × 7) surface to Si3H s at room temperature (curves B - F ) with the internal standard (curare A) from saturation coverage of H atoms. At the right side of the figure. . .t.r. i. s. .i .l.a.n. e e x p n . ~ a r e ~ nr~ ~hcvarn ~vh;rh vr='~en*~-'el!,, ,-,,,~,.,, ~,.~, ~h,-,~,,,.,. decades. The areas under several H z TPD curves measured following ;cisi~ane exposure (such as curves B - F in fig. 10) have been calibrated to the area under A (fig. 10) and the results are summarized in fig. 8 using solid circles and also in the bottom panel of fig. 9. It is convenient to consider the data in three

;20

S.M. Gates / Silicon hydride on Si(l l l).(7x 7)

dT I' I'" 1 ' I' I

_ _

~ I~I~/~,

"It" --'1" - ~ t ¢ 0 C

dt

A

or) l..Z ::3

cd <~ Cxl


13.

t

~ ~ / _

l:l,l~ill 500 5OO

SATURATION

7OO

TEMPERATURE(°C) Fig. 10. TPD of ~'~2 from Si(111)-(7x7) plus saturation coverage of H atoms (curve A), and increasing exposure to tfisilane (curves B through F). Gas exposures are made at 25 ° C gas and surface temperatures, and are listed in the figure.

exposure regimes. At low tfisilane exposures, the r2 desorpfion peak (around 425 ° C) appears as a poorly resolved shoulder on the leading edge of the main fll peak and ~2 grows roughly in proportion to ~1 (fig. 10). A constant slope in the bottom panel of fig. 9 for the low coverage regime is observed (indicated by the dashed line) because the reactive sticking coefficient is independent of ~uria~:e coverage. This kinetic behavior again indicates the presence o~ a mobile molecular precursor to chemisorption [9] as expected based on the negative activation energy for trisilane adsorption (fig. 5) and on the similarity between di- and trisilane adsorption (fig. 6). The intermediate regime is characterized by a coverage dependent S R, indicated by the solid circles in fig. 8. Growth of the fi2 feature at high trisilane exposures (curves E and F, fig. 10) signals the onset of the high exposure regime and is peculiar to trisitane.

S.M. Gates / Silicon hydride o n Si(l;l)-(7x 7)

321

Three conclusions may be drawn by inspection of fig. 10. First, the silicon hydride surface species formed from Si3H 8 exhibit an H2 desorption peak at - 4 2 5 ° C (/32) characteristic of a dihydride species even in the low coverage limit (curve B). This is in contrast to H atom, Sill4 and Si2H 6 behavior at very low coverage. Second, at high Si3H s exposures the /32 peak height approaches that of/~x, again in contrast to the H atom, Sill 4 and Si2H 6 data. Third and most significant, the H2 TPD areas under curves E and F are substantially greater than the H 2 area from the saturation H atom surface (curve A) indicating that Si3Hs adsorption does not saturate or drop to an undetectable level with exposures on the order of 1017 cm -2.

4. Discussion

4.1. General Recent electronic and geometric Structure studies [11,12] of the (7 × 7) reconstructed Si(111) surface make this a well defined surface for chemisorption studies. Measurements of surface H atom coverage following calibrated exposure of the Si(111)-(7 >< 7) surface to Si,,H2,,+2 gases with n = 1, 2 and 3 have revealed distinct similarities and differences in the adsorption kinetics of these three molecules. Silane is characterized by a low reactive sticking coefficient (S R less than 0.001 at 25°C) that is independent of surface temperature. The higher silanes exhibit high reactive sticking coefficients ( S R = 0.5 at 25 ° C) that decrease with increasing surface temperature (negative apparent activation energies for adsorption). Quantitative comparison of disilane and trisilane is reliable at low exposure. High exposures of the surface to silanes in UHV (for example see the Sill4 measurements shown here) cause both an increase in H z partial pressure the UHV chamber and a decrease in the QMS electron multipfier gain. These two effects do not reproducibly offset each other. H2 TPD areas are therefore less reproducible in the 1017 molecules cm -2 exposure regime, and these measurements are presented for qualitative comparison [10]. Data presented here for S i z H 6 and Si3H s adsorption in the 1017 molecules c m -2 exposure regime must also be viewed qualitatively due to the uncertain surface structure involved.

4.2. Chemisorption of silane Fig. 11 depicts a schematic potential energy diagram governing the disilane plus Si(l 11)-(7 × 7) interaction using a sofid line, and the silane potential using a dashed fine. The cherrfisorption of Sill 4 on Si(lll)-(7 × 7) is characterized by zero apparent activation energy for adsorption (fig. 4, 2 5 - 2 7 5 ° C surface temperature), indicating that neither an activation barrier nor a measurable molecular precursor well is present. Buss and Ho have measured the overall

S.M. Gates / Silicon hydride o n Si(l l l)-(7× 7)

~22

SCHEMATIC POTENTIAL ENERGY DIAGRAM FOR Si ( I l l ) - (TX?') AND SILANE . . . .

>. E 141 Z LU i ¢t

;,%7"- -%

E.-t/-"

,,'"

t-Z

,S o.

DISTANCE FROM SURFACE

Fig. 11. Schematic potential energy diagram of disilane (solid line) and silane (dashed line) plus 8i(111)-(7x7). Energy differences are for disilane adsorption only, and refer to eqs. (13)-(15) in the text.

rate of deposition of Si from Sill 4 as a fancfion of surface temperature and have extracted activation energies for individual steps. Their data analysis is also consistent with a near zero activation energy (0-3 kcal/mol) for the initial chemisorption step [13]. An Sill 4 molecule impinging on this surface either scatters off the repulsive part of the potential with high probability, or dissociatively chemisorbs with low probability. Sill4 adsorption may occur via reaction (11). Reaction (12) is also possible, as proposed by Farnaarn and Olander [14]: S i H 4 ( g ) + S i ( a ) --~ S i H 3 ( a ) + S i l l ( a ) ,

(11)

SiH4(g) + Si(a) ~ 2 SiSz(a ). (12) Description o.f silane adsorption as a competition between scattering and dissociative chemisorpfion draws on the work of Faarnam and Olander using a modulated Sill4 beam and higher temperature Si(111) surfaces. These authors found the surface residence time for molecular silane on this surface to be less than the detection fin,At (few microseconds) and independent of surface temperature for aata sets at 25, 900 and 1175°C. The sticking coefficient of Sill4 on the Si(111)-(1 × 1) surface was found to be 0.08, and independent of

S.M. Gates / Sificon hydride on Si(l l l).(7x 7)

323

surface temperature from 900 to 1175 o C. The surface species in Sill4 decomposition were also investigated [14]. Earlier surface science studies of silane chemisorption also used the (111) silicon surface, but measured kinetic parameters for the overall growth reaction of silicon film from silane, rather than the initial chemisorption step. Henderson and Helm [15], Farrow [16], and Joyce and coworkers [17] all agree on an activation energy of 17-20 kcal/mol for the production of H2(g) and the growth of silicon film from Sill,, on S i ( l l l ) in the temperature range 800-1100°C. Viewed together, these three investigations span about seven decades of silane pressure. It is clear that this activation energy is due to surface processes in the decomposition mechanism that follow the chemisorption of silane.

4.3. Chemisorption of disilane Chemisorption of disilane and trisilane is ve.ry different from silane. A dior tfisilane molecule impinging on the surface adsorbs through a molecular precursor adsorption state with a few kcal/mol adsorption energy ("well depth"). Two pieces of evidence for this state are tbe negative activation energies for chemisorpfion (fig. 5) and the constant S0g values observed at low coverage (fig. 9). A simplified picture of this state is a physisorbed molecule with free mobility in two dimensions and hindered mobility in the t~rd dimension (two-dimensional ga~). Lacking experimental data, we assume refit adsorption probability from gas phase to precursor [9]. Using disilane as the specific example, consider the following reactions:

Si2H6(g ) ~ Si2H6(a),

(13)

E l , - E~,

(14)

Si H0(g), Si2H6(a ) ~ 2 SiH3(a),

E*

(15)

Ee,

-

(16)

Si2H6(a ) --. Si2Hs(a ) + H(a).

Reaction (13) is assumed to proceed at the gas colfision rate at all temperatures studied here. S0g = 0.5 at 25°C means that the precursor has equal probabififies of desorbing, reaction (14), or chemisorbing dissociatively (15), prcwided

th~

uni¢ zd.~o_rptlcm n r n h ~ b i l l t v

t n nre~nr.~cw ~

e.nrree.#

A~ the.

surface temperature is increased, the rate of reaction (14) increases faster than the rate of (15) (assun~ng the rate of (13) is independent of temperature)° Evidence for reaction (15) rather than (16) is discussed below. Energy differences represent the activation energies for selected steps and refer to fig. 11. The data of this work measvre only those molecules that chen~sorb according to the following net reaction (the sum of (13) and (15)):

si

6(g)

2 siH3(a),

e*

-

324

S.M. Gates / Silicon hydride o n Si(l l l )-(7× 7)

The apparent activation energies ( E A = - 2 . 6 kcal/mol for disilane and E A = - 4 . 9 kcal/mol for trisilane) measured in fig. 5 are for the net reaction (17). Based on the above mechanism, the apparent activation energy is the difference E * - E G . The molecular precursor well depths, E p - E G, must be greater than or equal to the apparent activation energies. As discussed below, trisilane is presumed to chemisorb by a similar mechanism (eqs. (18)-(20)). Assuming the activation energy for Si-Si bond scission (E * - Ep) is the same for di- and trisilane, we suggest that the trisilane precursor well depth is roughly 2.3 keal/mol larger than the disilane well depth. A more complete picture of the potential energy surfaces for di- or trisilane plus Si(lll)-(7 x 7) awaits molecular beam and computational investigation. Adsorption studies in the low coverage limit probe the fundamental molecule-surface interaction without the complications of coverage dependent effects. We have therefore made quantitative studies involving temperature dependence and deuterium labelling only at low surface coverage of H atoms. Changes in the surface structure and in the strength of molecule-surface interaction due to H atoms on the surface are eliminated by focussing on the 0-2% monolayer regime, the zero coverage limit. It is shown in fig. 6 that Sort is the same for Si2H 6 and Si2D 6 within experimental error. The SiED6 and SizH 6 comparison suggests that Si-H bond breaking occurs after the rate determining step in disilane chernisorption. Substitution of deuterium for hydrogen bonded to a carbon atom can significantly decrease the rate of C - H bond scission steps, known as the deuterium kinetic isotope effect (DKIE). The DKIE is a well established tool for the study of bond breaking processes and mechanisms [18] with a firm basis in statistical mechanics [] c~]. This effect has recently proved to be an incisive probe of bond scission steps in molecular decomposition on metal surfaces [20,23]. The absence of a DKIE in Si2H 6 chemisorption is consistent with the mechanism of reactions (13)-(15), where Si-Si bond scission, reaction (15), is rate 1/..... rag. Additional evidence for this mechanism has been obtained using surface vibrational HREELS spectroscopy and a molecular state of disilane E observed at 80 K surface temperature [22]. A mechanism consisting of reactions (13), (14) and (16) would be expected to yield a measurable DKIE, so that SoR would be smaller for SizD 6 than for Si2H6° It is noteworthy that all evidence for the disilane dissociation mechanism of reactions (13)-(15) pertains to very low coverage, and the mechanism may change ov.ce the s ---~ . . .coverage of ra atoms. u , ~. c contains a. substantial inspection of H 2 TPD curve shapes following Si2H 6 exposures (fig. 7) shews that the/3 z desorption peak remah~s a poorly resolved shoulder on the leading edge of the fl~ peak throughout the low to high exposure range. Expressed relative to the fl~ peak height, the/3~ shoulder height increases from roughl 3 1 / 6 (curves B, C) to rougbAy 1/3 (curve E) as the disilane exposure is increased. TbSs qualitatively indicates an increase in surface Sill 3 and Sill 2 .

•

.

S.M. Gates / Silicon hydride on Si(111)-(7×7)

325

groups among the SiHx(a) population at higher disilane exposures. As noted in section 3.1, further interpretation of these TPD curve shapes to deduce SiHx(a) stoichiometry is not rigorous. The reactive sticking coefficient of disilane is 0.47 + 0.1 near zero coverage at room temperature and drops dramatically after roaghly 1014 Si2H 6 molecules cm -2 exposure, as seen in fig. 9 (top panel). A continuous decrease is seen in S rt from roughly 10~4 to 10 ~6 Si2H6 molecules cm -2 exposure, as the surface sites become filled with adsorbed Sill x species (fig. 8). S R drops to about 0.05 in the plateau region of fig. 8. Fig. 9 (top panel) snows that precursor adsorption kinetics (constant sticking coefficient) are limited to the low coverage regime. The departure from precursor kinetics occurs when roughly 3 × 1013 Si2H 6 molecules cm -2 have been adsorbed, and the total H atom coverage is about 1.8 × 1014 cm -2. This is approximately 1 / 5 of a monolayer of H atoms, expressed as H / S i atom and counting only the original first layer surface Si atoms.

4.4. Chemisorption of trisilane It was shown in fig. 6 that S0R for tdsilane and disilane are equal in the zero coverage limit. The adsorption activation energies for these two molecules are both small and negative (fig. 5). The mechanism of the following reactions is postulated for tdsilane adsorption in the zero coverage llafit, based on the similarities seen for di- and trisilane adsorption kinetics: Si3Hs(g ) -~ Si3H8(a),

(18)

Si3Hs(a ) --, Si3Hs(g),

(19)

sigHs(a)

(20)

sim3(a) + Si2 s(a).

Figs. 8-10 summarize our Si3H s adsorption studies. Three distinct regimes of tfisilane adsorption are clearly seen in these data. The low coverage regime is characterized by a constant sticking coefficient (S R--- 0.47 + 0.1) and precursor adsorption kinetics (fig. 9, bottom panel). The intermediate regime, 1014 to 2 )( 1015 Si3H s molecules cm -2, involves a coverage dependent S R. Both fll and/32 H2 desorption features are observed, with t2 ha constant proportion to fll (curves C and D, fig. 10). Finally, above 1016 Si3H 8 molecules cm -2 exposure a distinct high exposure regime of Si3Hs adsorption is seen in curves E and F of fig. 10. The high exposure regin~ is characterized by a total H atom surface coverage 1.5 to 2 times gr~:ater than the saturation coverage from H atom exposure (areas under curves A, E and F, fig. 10). Abundant H z desorpfion at 425°C (P2) is also seen in curves E and F of fig. 10. At very high coverage; the flz desorpfion peak reaches a height almost equal to that of the B~ peak. This is in sharp contrast to the /32 behavior for H atoms and for d;~silane. A

326

S.M. Gates / Silicon hydride o n Si(111)-(7x 7)

pronounced increase in surface Sil-I 3 and Sill 2 groups among the SiHx(a ) population at high trisilane exposures is indicated. Si3H s adsorption results in multilayer growth of hydrogenated silicon species on a room temperature surface with expe~ures on the order of 100 monolayers of room temperature gas.

4. 5. Implications for precursor/surface interactions The comparison made here for Si(111)-(7 x 7) plus three silicon hydride molecules gives insight into the molecule plus semiconductor surface interaction which results in a mobile precursor to chemisorption. Even at temperatures of 100-300°C, the higher silanes become weakly adsorbed molecules while silane does not to a measurable extent. An abrupt change between n -- 1 (surface scattering) and n = 2 (trapping into precursor state) is observed for SinH2n+2 adsorption mechanisms. A similar abrupt change has also been reported between n = 1 and n = 2 for CnH2,+2 adsorption on Ni(100) by Hamza and Madix, who measured angular distributions of scattered molecules using molecular beams with n = 1 to 4 [23]. Trapping followed by desorption was observed for ethane, while direct inelastic scattering dominated the methane data. Considering the molecules, we suggest that a stronger interaction is possible for the higher sflanes primarily due to the greater polarizability of larger molecules in a homologous series [24]. A second proposed source of weak molecule/surface interaction is overlap between the highest occupied molecular orbital (HOMO) of the molecular and empty Si surface states. A weak induced dipole interaction occurs at a few A distance between a surface and the incoming molecule. The response of the molecule's valence electrons to the surface electric field determines the strength of this interaction. This response is most familiar in the case of noble gas physisorption on surfaces, where physisorption energies follow the rank order of atom polarizab~ifies (Xe > Kr > Ar > Ne > He) [24]. The refractive index is an approximate measure of molecular polarizab~ity. Refractive k~dices have been measured for tfisilane and the higher sUanes [25] (as well as many homologous series of organic molecules), and re~u!ar!y increase wffh increasing chain length through a homologous ¢cfies. Qualitatively, we relate the greater polarizability of di- and tfisilane compared to silane to the vertical ionization potential (IP) of each molecule as a gas, and these quantities are listed in table I. ~|ectrons which occupy the HOMO of disflane (or tfisilane) are less tightly bound to the molecule skeleton by 1.7 eV (or 2.5 eV) compared to those which occupy the silane HOMO [26,27]. The valence electrons of disflane or trisilane exhibit a greater response to the surface electric field, resulting in a larger induced dipo!e moment and a stronger molecule/surface interaction. The effect is more pronounced for

S.M. Gates / Silicon hydride on Si(l l l)-(7x 7)

327

Fable 1 Molecule

Ionization potential gas phase ~) (eV)

HOMO energy relative to E F b) (eV)

HOMO energy relative to empty adatom states c) (eV)

gill4

12.4 10~7 9.9

7.8 6.1 5.3

8.3 6.6 5.8

Si2H 6

Si3H 8

~) Refs. [26,271. b) Work function of S i ( l l l ) . ( 7 x 7 ) -- 4.63 eV [281. o Ref. [121.

trisilane. The angular distributions of scattered C,,H 2,, + 2 molecules measured by Hamza and Madix [23] are also explained by increasing polarizability through a homologous series. Ethane and the higher alkanes are sufficiently polarizable to become bound as molecuiar precursors on Ni(100) at 125 °C surface temperature, while methane is not and undergoes direct scattering from the surface. Tdsilane is similar to disilane in terms of the departure from precursor adsorption kinetics as surface H atom coverage increases. Precursor adsorption kinetics (constant S g) are seen in fig. 9 (bottom panel) for leas than about 3 x 10 ~3 Si3H 8 adsorbed molecules c m -2. The resultant H atom coverage (about 2.4 × 1014 c m - 2 ) is roughly 1 / 4 monolayer. Both di- and trisilane precursor adsorption mechanism are disrupted at roughly 1 / 5 - 1 / 4 monolayer of H atoms, as seen in fig. 9. This coincides with the number of adatom dangling bonds per unit cell of the Si(111)-(7 × 7) structure (12 adatoms/49 atoms per unit cell, - 1 / 4 monolayer adatoms) [11,12]. Participation of adatom surface states in the molecule/surface interaction which binds the molecular precursors is supported by these data. A second contribution to the molecule/surface interaction, in addition to molecular polarization, may be molecule to surface electron donation, specifically donation into the empty adatom surface states. The strength of this donation is also related to the molecule IP values shown in table 1. Electron donation from the molecular HOMO to the surface is more effective for the higher silane. Consider the energy difference (fight column, table 1) between the respective HOMO's of each molecule and empty surface states located on the adatoms of the (7 × 7) structure. The empty adatom states lie 0.5 eV above E F (4.1 eV below the vacuum), as determined by current imaging tunnding spectroscopy with the STM [12]. These states are available to accept electron dens-~ty froro the 5i-Si bonding HOMO's of the higher siianes. This electron donation interaction is much weaker for silane because the energy mSsmatch is greater by 1.7 eV (Sill4 compared tc Si2H 6) to 2.5 eV (Sill4 compared to Si3Hs). Donation from the filled adatom surface states near E F into low lying empty states of the molecules has also been considered. Pronounced dif-

328

S.M. Gates / Silicon hydride o n Si(lll)-(7× 7)

ferences are not expected in the strength of molecule/surface interactions considering silane versus the higher sflanes based on surface to molecule electron donation [29].

5. Summary H 2 TPD has been used to study the adsorption kinetics of Sill4, Si2H 6 and Si3Hs on Si(111)-(7 x 7), as a function of surface temperature. The following conclusions have been reached: (1) Di- and trisilane are at least 1000 times more reactive than silane on the clean surface. (2) Di- and trisilane adsorb through molecular precursor states on the clean surface, but no evidence is seen for a precursor state in the silane case. (3) The dissociation of the di- and trisilane molecular precursors to chemisorbed SiHx species occurs by Si-Si bond scission with the low frequency Si-Si stretch as reaction coordinate. (4) Trisilane grows a multilayer of SiHx(a) species on the 2 5 ° C surface at 25 o C gas temperature. (5) The abrupt change in absorption mechanism between silane and the higher silanes is attributed to molecular polarizabilities, to molecule to surface electron donation, and to the presence of Si-Si bonds.

AcLaowledgements The author gratefully acknowledges B.A. Scott and J. Jasinski for helpful discussions, J.E. Demuth for the use of the UHV apparatus, and D.B. Beach and R.D. Estes for purification and analysis of the silanes.

References [1] [2] [3] [4] [5] [6]

J.H. Purnell and R. Walsh, Proc. Roy. Sec. (London) A293 (1966) 543. C.H.J. van den Brekel and L.J.M. BoUen, J. Crystal Growth 54 (1981) 310. B.S. Meyerson, B.A. Scott and R. Tsuei, Chemtronics I (1987) 155. D.W. Hess, K.F. Jenson aad T.J. Aanderson, Rev. Chem. Eng. 3 (1985) 97. C. CampbeI1 and S. Valone, J. Vacuum Sci. Technol. A3 (1985) 408. R.J. Culbertson, L.C. Feldman, P,J. Silverman and R. Haight, J. Vacuum Sci. Technol. 20 (1982) 868. [7] G. Schulze and M, Henz2er, Surface Sci. 124 (1983) 336. [8] S.M. Gates, unpublished results. [9] D.A. King and M.G. Wells, Pro~. Roy. Soc. (London) A379 (1974) 245. [10] Quantitative Sill4 s~udies ere intended for future studies with a differentially pumped QMS due to the excessive exposure required

S.M. Gates / Silicon hydride on Si(l l l).(7x 7)

329

[11] K. Takayanagi, Y. Tanishiro, M. Takahashi and S. Takahashi, J. Vacuum Sci. Technol. A3 (1985) 1502. [12] R.J. Hamers, R.M. Tromp and J.E. Demuth, Phys. Rev. Letters 56 (1986) 1972. [13] R. Buss and P. Ho, private communication. [14] M.K. Farnaam and D.R. Olander, Surface Sci. 145 (1984) 390. [15] R.C. Henderson and R.F. Helm, Surface Sci. 30 (1972) 310. [16] R.F.C. Farrow, J. Electrochem. Soc. 121 (1974) 899. [17] B.A. Joyce, R.R. Bradley and G.R. Booker, Phil. Mag. 15 (1967) 1167, and references therein. [18] F.H. Westheimer, Chem. Rev. 61 (1961) 265. [19] L Melander and W.H. Saunders, Reaction Rates of Isotopic Molecules (Wiley, New York, 1980). [20] S.M. Gates, ].N. Russell, Jr. and J.T. Yates, Jr., Surface Sci. 146 (1984) 199. [21] S.M. Gates, J.N. Russell, Jr. and J.T. Yates, Jr., Surface Sci. 174 (1986) 111. [22] R. Imbihl, J.E. Demuth, S.M. Gates and B.A. Scott, in preparation. [23] A.V. Hamza and R.J. Madix, Surface Sci. 179 (1987) 25. [24] Y.K. Syrkin and M.E. Dyatkina, Structt~re of Molecules and the Chemical Bond (Interscience, New York, 1950). [25] Gmelin Handbook of Inorganic Chemistry, Silicon, 8th ed. (Springer, Berlin, 1982). [26] H. Bock, W. Ensslin, F. Feher and R. Freund, J. Am. Chem. Soc. 98 (1976) 668. [27] U. Itoh, Y. ToyosKima, H. Onuki, N. Washida and T. Ibuki, J. Chem. Phys. 85 (1986) 4867. [28] G. Hollinger and FJ. Himpsel, J. Vacuum Sci. Technol. A1 (1983) 640. [29] All of the low lying excit.ed states of di- and trisilane have recently been assigned to Rydberg states of Si 4s, 4p and 4<1 character [27]. Similar Rydberg assignments for Sill4 are in the literature [25]. The eltergies w;.'th respect to the vacuum and the Rydbe~g character of all these states is approximately the same for all three molecules.