Surface Sczence 166 (1986) 129-140 129 North-Holland, Amsterdam

A N G L E - R E S O L V E D T I M E - O F - F L I G H T S P E C T R A O F N E U T R A L P A R T I C L E S D E S O R B E D F R O M LASER IRRADIATED CdS

A. N A M I K I , T. K A W A I and K. I C H I G E

Department of Electrwal Engineering and Electromcs, Toyohasht Unwerstty of Technology, Tempaku, Toyohasht 440, Japan

Recewed 8 July 1985, accepted for pubhcatlon 5 September 1985

Angle-resolved tlme-of-fhght spectra of neutral particles desorbed from laser irradiated CdS have been investigated with a pulse mass counting method Three quantlues to characterize the desorptlon dynanucs - the desorptlon flux, the mean kinetic energy and the speed ratio - depend remarkably on the incident laser power and the ejection polar angle 0. The desorptlon flux is strongly peaked forward deviating from the cosine distribution. For low laser power, the desorbmg S 2 follows the same Maxwelllan velocity distribution over the polar angles below 50 ° For high laser power, the Maxwell distribution is hmlted to a narrow cone around the polar axis and with increasing polar angle the spectra of the velocity distribution become broader than the stmulated Maxwelhan one With increasing laser power the mean kinetic energy increases for 0 < 40 °, while for 0 > 400 ~t decreases abnormally For these apparent breakdowns of the Maxwell law and Knudsen law, the dynarmc behavlour of desorbmg particles obeys a non-equdlbrlum thermal mechanism, wbach may involve a solid-gas phase transformation

1. Introduction

Laser- induced desorpt ion of surface atoms is one of the e lementary processes of laser-sol id interaction. It is very impor tan t to reveal its mechanism espe- cially for senuconductors , not only because of the scientific interest in the behaviour of an e lec t ron-hole (EH) plasma [1] bu t also the technological apphcat ion to device processing. The scientific interest is main ly concerned with e l e c t r o n - p h o n o n coupling in the high density EH plasma, which might induce chemical reactions in the bulk involving a phase t ransi t ion as predicted theoretically [2]. There have appeared two models on the mechanism for laser- induced desorpt ion from semiconductors: One is the thermal mel t ing model based on rapid energy conversion from the laser-generated EH plasma to the lattice, followed by heating, melting, and then desorbing of surface atoms [3,4]. The other is the so-called non- thermal model in which dissociat ion followed by desorpt ion of the surface atoms is driven by the EH pairs [5] or by the localization of two holes [6]. When an EH plasma exists, the crystal b o n d is weakened and hence it may be easy to reduce the enthalpy for desorpt ion [7]. In previous studies on ni t rogen laser- induced desorpt ion in CdS, we have

0039-6028/86/$03 .50 © Elsevier Science Publishers B.V. (Nor th -Hol land Physics Publishing Division)

130 A Namlkl et al / Spectra of neutral partldea desorbed from (dS

admitted rather an eclectical mechanism which revolves both thermal and non-thermal processes, ~.e., the laser excitation of the electromc system under- goes a thermal actwation process to induce desorption [8]. Thus, laser desorp- tion from CdS comprises at least three steps, such as laser heating of the surface, dissociation of surface constituents in coexistence with the EH plasma, and then leaving the surface.

Experiments on tlme-of-flaght (TOF) spectra for desorbmg particles have been carried out in various semiconductors at fixed desorptlon angles [4,8-11 ]. Usually, observed TOF spectra are distributed m the thermal energy region m accordance with the presumed laser-heating. Certainly, m the fmal stage of desorption processes which determines the dynamical behaviour of the outgo° lng flux thermal effects must be involved. Stritzker and co-workers [4] have tried to estimate the surface temperature when desorption occurs by assuming that particles desorbing from S1 and GaAs are in thermal equihbrium with the solid surface and hence the desorption flux follows a Maxwellian velocity distribution (Maxwell law). But this assumption is not always necessarily true, at least, for compound semiconductors [9]. Even in purely thermal desorptlon phenomena such as associative desorption of hydrogen from metal surfaces [12,13] or rare gas atoms from cooled surfaces [14], It has been proven that the desorption dynamics does not necessarily obey the Maxwell law. In fact, recently, in a dynamic pulse mass counting study on mtrogen laser-induced desorption, we also found some evidence of the apparent deviation from thermal equilibrium; namely, the effective translational temperature of the outgoing flux from the surfaces of CdS [8] and GaAs [11] changes depending on the kind of desorbing particles, and also on the polar angles at 0 ° and 45 °.

The dynamic behaviour of the outgoing flux must be determined by the final process of leaving the surface. Angle-resolved TOF distributions of the desorption flux can give many reformation from a microscopic point of view regarding the interaction of the desorbing particles with a solid surface.

In this paper, we report the polar-angle-resolved TOF spectra in nitrogen laser-irradiated CdS. We find a clear breakdown of the Maxwell law and Knudsen law. The mean kinetic energy of the desorbing particles changes depending on the polar angle and also on the injected laser power. The desorption flux in the cone around the polar axis gains kinetic energy quasi-lin- early to the laser power, but a peculiar decrease in kinetic energy with increasing laser power ~s found for grazing angles above 40 °. This abnormal behaviour cannot be interpreted fully with the model proposed for the thermal desorption of hydrogen from metal surfaces [12-15]

2. Experimental

A CdS single crystal was attached onto a sample holder suspended m an ultrahigh-vacuum chamber. The high vacuum (2 × 10 - l ° Torr) was to avoid

A. Namtkt et al / Spectra of neutral parttcles desorbed from CdS 131

A3 fl ¢D ~ ~ O . .O ~

I 410 J J 0 0 2J0 60 80 0 (de9.)

Fig 1 Correction factor r/(0) for the desorptlon yield as a function of the polar angle 0

surface contamination and to ensure signal accumulation free from back- ground noise in the dynamic pulse mass counting measurement. The polar angle was vaned by means of rotation of the sample about the holder axis. Excitation light from a pulsed nitrogen laser (5 ns, 337 nm) was focused onto the sample surface with a quartz lens. TOF spectra were measured with a quadrupole mass spectrometer (QMS), a multi-channel analyzer (MCA) and a laser power dxscriminator. The total arrangement of the experiment was reported previously [8,11]. A rmcrocomputer was newly added to derive physical quantities such as mean kinetic energy, desorptlon yields, etc., from the data stored in the MCA. The substrate temperature before laser irradiation was kept at room temperature, about 300 K.

Undesirable change of laser power density occurred inevitably from rotation of the sample and introduces some ambiguity m the laser power density for each angle. Since the desorptlon process is a non-linear process for the laser photon flux, a suitable correction treatment ~s needed in evaluating the desorption yield. To do this we measured the absolute erosion profile using an electro-mechanical stylus instrument for various polar angles and made the curve of correction factor 7(0) to evaluate the relative desorption yield as shown in fig. 1. The variation in ~/(0) is well limited within 1 < 7(0) ~< 2. This means that on the average a variation of + 2 m J / c m 2 in the laser power density exists in the polar angle scan of TOF spectra considenng the curves of desorption yield versus laser power (fig. 10). The ambiguity in the average kinetic energy E" is limited at most within +0.5 × 10 -20 J / c m 2 or +200 K for desorptlon into a direction perpendicular to the plane of the surface, where the largest error is expected. Desorption yield Y(0), mean kinetic energy E(0) , and speed ratio S(O) whose definitions are gwen in section 3, were measured first by changing the polar angle O for fixed laser power. Observed trends in the change of these quantities were then tested by changing the laser power for fixed polar angle. Both measurements were always consistent within experi- mental error as will be presented in section 3.

The quantities to describe the dynamic behaviour of desorblng particles are E(O), Y(O) and S(O). To evaluate these values we need an average velocity and an average squared velocity. Theses quantities were calculated from the

132 A Namtkt et al / Spectra of neutral parttcles deaorbed from CdS

raw data of desorpuon flux as follows. Since the lomzaUon efficiency for desorblng atoms and molecules by electron bombardment is reversely propor- tional to their velooty, the real desorption TOF curve, dn(t) /dt versus t, should be obtained after multiplying d n 0 ( t ) / d t by their velocity v (t), where d n 0 ( t ) / d t is the apparent uncorrected desorptton flux counted for each 10/zs by the MCA. Therefore, Y(0) can be calculated from

l dno(t,)At ' (1) Y ( O ) = n ( O ) ~ t , dt, "

!

where t , represents the time of the z th channel, l the fltght dtstance and At, the width of each channel of the MCA. Following the defimtion of the way of averaging, the average velocity ~(0) and the average squared velocity ~ ( 0 ) were calculated as follows,

l 2 dn0( t , ) At, ~(0) = ~/(0) Y'~ (2)

, t 2, dt, r ( o ) "

v-~(O) = * / ( O ) ~ 13 d n ° ( t ' ) At, t) dr, r ( o ) " (3)

These values are used to evaluate E(0 ) and S(O) m section 3.

3. Results and discussion

In desorbmg particles, atormc sulfur S, dlatomlc sulfur S 2 and atomic cadmium Cd are detected; the respectwe mass spectra obtained by pulse mass counting techmque are shown in fig. 2. Two mare bands centered at m/c = 112 and 114 are due to the existence of Cd isotopes. The highly asymmetric bands

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Fig 2 Mass spectrum measured by the pulse mass counting method The intensity m the ordinate is uncorrected

A Namlkl et al / Spectra ofneutralpart:cles desorbedfrom CdS 133

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Fig 3. Uncorrected TOF spectra of S 2 for various polar angles The lnodent laser power is 42 m J / c m 2

Fig 4 Uncorrected TOF spectra of Cd for various polar angles The mo d en t laser power ts 42 m J / c m 2

having tads to lower mass numbers involve also other Cd isotopes. They are not resolvable because of the intentionally lowered mass resolution to obtain a large counting rate. Taking the detection efficiency of the QMS into considera- tion - it decreases as the particle to be detected becomes heavier - the total yield of Cd is almost equal to that of sulfur. Therefore, laser-induced desorp- tion occurs rather congruently [8]. This is quite different from GaAs, in which laser irradiation induces Ga-rich precipitates on the surface [4,11].

Figs. 3 and 4 show TOF spectra, i.e. the desorptlon flux dno(t)/dt versus t, of S 2 and Cd, respectively, for a relatively higher laser power of 42 m J / c m 2. With increasing polar angle O, the peak of the TOF spectra shifts to a slower time region, with a broader spectrum for both S 2 and Cd.

If the desorbing particles have a Maxwellian velocity distribution the desorption flux, dno(t)/dt, as a function of time t can be written as,

dn 0 (t)/dt = const × t -4 e x p ( - ml2/2kTt . . . . / 2 ) , (4)

where k is the Boltzmann constant, m the mass of particle, l the flight distance, and Ttran s an effective translational temperature of the desorbing flux. Representative spectra at high and low power regions for S 2 were tried to fit with eq. (4) by applying the least-mean-squares method with a chi-square (X 2) test. The results are shown in fig. 5. The quality of the fit for desorption at

134 A Namtkl et al / Spectra of neutral particles de,orbed from CdS

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Fig 5 Fitting the Maxwelhan velocity distribution to the measured TOF spectra of S 2 for high

and low laser powers at 0 = 0 ° and 45 ° Dots and crosses are experimental The solid hnes are slmulauon curves of eq (4) by a least-mean-squares method with a reduced chl-square X~(u = 60) test Values obtained for T t .... at 0 = 0 ° are 1548 K (X~ = 1 41) and 2600 K (X~ = 1 87), for laser powers of 20 and 42 m J / c m 2 respectively At 0 = 45 °, these are 1387 K (X 2 = 0 7) and 430 K (X~ = 7 1) for laser powers of 20 and 42 m J / c m 2 respectively

0 = 0 ° IS g o o d w ] t h a r e d u c e d X~ wel l b e l o w 2.0 for t he f r e e d o m u = 5 0 - 6 0 for

b o t h h i g h a n d l ow la se r p o w e r s . O n t he c o n t r a r y , for h i g h l a se r p o w e r a t

8 = 45 ° t he d e v i a t i o n is s e r ious w i t h a b r o a d e r d l s m b u t l o n t h a n a s i m u l a t e d

M a x w e l h a n one , r e s u l t i n g in a ve ry l a rge X2, v a l u e of a b o u t 7. N a m e l y , t h e

M a x w e l l d i s t r i b u t i o n d o e s n o t h o l d for h i g h l a se r p o w e r a t l a rge ang les . O n t he

o t h e r h a n d , a r e l a t ive ly g o o d fit c a n b e sti l l a d m i t t e d for low la se r p o w e r . O n e

s h o u l d n o t e t he p e c u h a r fac t t h a t a t 0 = 45 ° the p e a k of t he T O F s p e c t r u m

sh i f t s to t he s l o w e r t i m e r e g i o n as the l a se r p o w e r i n c r e a s e s , w h i l e a t 0 = 0 ° ~t

sh i f t s to t he f a s t e r t i m e r e g i o n w h i c h is the n o r m a l t r e n d to b e e x p e c t e d . In t h e

fo l lowing , we give a m o r e p r ec i s e a n d q u a n t i t a t i v e e v a l u a t i o n a b o u t t h e m e a n

e n e r g y a n d t he e x t e n t o f d e v i a t i o n f r o m a M a x w e l l d i s t r i b u t i o n , f o c u s i n g o u r

a t t e n t i o n to t h e i r d e p e n d e n c e o n the i n c i d e n t l ase r e n e r g y a n d t he e j e c t i o n

p o l a r angle .

A Namlkt et al / Spectra ofneutralparttcles desorbedfrom CdS 135

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Fig 6. Speed ratio S(O) of molecular sulfur S 2 versus polar angle for high and low laser powers S(O) is defined by eq (5) m the text

Fig 7 Mean klneUc energy spectra of S, S 2 and Cd for high laser power. E(O) = m~/2 The right ordinate is scaled with ff~(O)/2k m units of Kelvan

For the intuitive quantification of the deviation of the desorption flux from the Maxwell distribution it is very advantageous to utilize the so-called speed ratio S [13], which can be defined as,

where the bar means the average. For S = 1 the TOF spectrum coincides with the Maxwellian velocity distribution, for S = 0 it is purely monochromatic, for S < 1 and S > 1, respectively, it is narrower and broader than the Maxwellian one. Fig. 6 shows the curves of S(O) for S 2 v e r s u s polar angle for the representative high and low laser powers. In the case of low laser power S(O) is almost equal to one throughout the range of angles examined here; this means that the desorption flux comprises particles which have a Maxwellian velocity distribution determined by a certain effective translational temperature Ttr~n s- On the other hand, in the case of high power laser irradiation, S(8) increases from one tending to two with increasing 8 from 0 ° to 60 °, while in the narrow cone around the polar axis S(O) is still limited around one, indicating that the particles desorbing toward the normal direction always have a Maxwellian velocity distribution. When the polar angle is fixed at 0 = 45 ° a systematic increase of S(45 °) is recognized with increasing laser power from 20 to 45 rnJ /cm 2, implying that the variation of S(O) in fig. 6 is not an artifact arising from the sample rotation.

136 A Narmkt et al / Spectra of neutral parncles desorbed from CdS

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Fig 8 Variation of E'(0) versus angle 0 for S 2 for various values of the modent laser power

The mean kinetic energy E ( 0 ) of S 2, S and Cd is plot ted in fig. 7 for a high laser power of 35.5 m J / c m 2. When the T O F spectra comprise a Maxwellian ve loo ty distribution, i.e., eq. (4), the mean kinetic energy can be related to Tt . . . . which determines the temperature of the system of desorbing particles as,

E ( 0 ) = 2kTt ..... . (6)

So the right ordinate of fig. 7 is scaled with E ( O ) / 2 k . One can note the facts that the energy spectra are strongly peaked at O = 0 °, and that the absolute peak values are &fferent among the particles. These facts in&cate that we cannot evaluate directly the surface temperature f rom E ( 0 ) or T t . . . . even when the velocity distribution is a Maxwellian.

The rejected laser power affects strongly the shape of E ( 0 ) curves as shown in fig. 8 for the case of S 2. The apparent peak observable for high power laser irradiation diminishes as the laser power decreases, the curve becoming flat. This fact, together with S(O) in fig. 6, indicates that at low laser power the desorpt ion spectra have an almost unique Maxwellian velocity distr ibution for a wide range of angles between 0 ° and 60 °. One should note that at 0 > 40 ° E ( 0 ) decreases abnormal ly with increasing laser power, while at O < 40 ° it increases with laser power. It can be conf i rmed that this abnormal behaviour at 0 > 40 ° is really due to the laser power effect, and not to an artifact arising f rom sample rotation, by measuring the desorpt ion yields versus laser power at fixed polar angle as shown in fig. 9. At 0 = 0 ° and 15 °, E ( 0 ) increases quasi-linearly with increasing laser power, but at 0 = 45 ° it decreases with increasing laser power.

Fig. 10 show desorpt ion yields integrated over the desorpt ion time and also

A Namtkt et al / Spectra ofneutralpartwles desorbedfrom CdS 137

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corrected for the ionization efficiency arising from the velocity. The desorption flux is strongly peaked perpendicular to the plane of the surface, strongly deviating from a cosine distribution (i.e., Knudsen law). Fig. 11 shows the desorptlon yield versus laser power independently measured from fig. 10 for fixed angles of O = 0 °, 15 ° and 45 °. As the laser power decreases the Y(O) curves tend to coming closer to each other, but the desorptlon flux still deviates strongly from the cosine distribution even at low laser power. Since the desorption yield is extremely small for laser powers below 18 mJ/cm 2 (i.e., the threshold for desorption), we cannot make sure whether the desorption yield follows the Knudsen law at the lower limit of the laser power.

The present experimental results on Y(O), E (0 ) and S(O) are schematically

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Fig 10 Distribution of the desorptlon flux for S 2 and C d

138 A Namlkl et a/ / Spectra of neutral partzcles desorbed from CdS

I 0 E

E

~ 10 5

._J UJ

10 4 Z 0

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I 310 I , J ,i 20 40 50 LASER POWER(mJ/cm2)

Fig 11 Laser power dependence of desorptlon yield Desorptlon yields are corrected following eq (1) (see the text)

summarized in fig 12 for typical low and high laser powers, compared with those expected from the Maxwel l and Knudsen laws which are applicable for desorpt ion in thermal equlhbrlum. Apparently, the Knudsen law does not hold for any incident laser power (fig 11). On the other hand, the Maxwel l distribution in E ( 0 ) and S ( 0 ) ts seemingly acceptable for low laser power. For high laser power, however, E ( 0 ) and S(O) begin to depart from the Maxwel l

$2

CdS~ Y(e)m(o) E(B)/~.(0) s(e)

(a) LOW LASER POWER

(b). HIGH LASER POWER

Fig 12 Schematic representations of Y(O), L'(O) and S(O) normalized at 0 = 0 ° Dashed hnes represent the thermal eqmhbrmm, l e, Knudsen law for Y(O) and Maxwell law for E'(0) and s(o)

A Namlkt et al / Spectra ofneutralparttcles desorbedfrom CdS 139

distribution, the velocity distribution becomes broader than the Maxwellian one.

Despite the apparent breakdown of the Maxwell law and the Knudsen law, we consider here that, at least, the final stage of laser-induced desorption still obeys the thermal (not melting) mechanism, based on the followmg facts: (1) The mean energy E(8 ) appears, more or less, in the expected thermal energy region attainable with laser heating between a few hundred and a few thousand degrees Kelvm. (2) Y(0) depends strongly on the substrate temperature before laser irradiation [8]. The Arrhenius plot for the temperature estimated by the calculation of a thermal diffusion equation results in a simple activation process [8]. (3) The Maxwell distribution (i.e., S(6) = 1) which must be a result of statistical dynamics, is valid over almost all angles 0 _< 50 ° for low laser power and, at least, in a narrow solid angle around the polar axis even for high laser power.

With these measurements on Y(O), ff?(0) and S(0), and especially their dependence on laser power, we are not yet in a position to present a comprehensive model to explain those values theoretically. We can now only speculate in a rather general way. As stated previously [8], laser-reduced desorptlon stems from at least three stages, i.e., laser heating of the lattice via rapid energy conversion from the EH plasma to the lattice, dissociation of surface atoms with the aid of the EH plasma and then desorption of the dissociated atoms. Dimerizatlon reactions may play a role as the motive force to proceed bond breakage under EH plasma. Since there exists a close similarity in dynamic behaviour of desorbing particles between the present laser-induced desorption in CdS and the purely thermal associative desorption of hydrogen molecules from metal surfaces [13], the final step of desorption may proceed via an adparticle state, at least for low laser power. But the residence time of adatoms must be suffioently short considering the fact that a fair amount of atomic sulfur can desorb escaping from the dlmerlzation reactions. On the other hand, the deviation from the Knudsen law and Maxwell law at high laser power, or especially the gradual increase of S(O) with increasing 0 seems to indicate that there exasts a certain interaction of the desorbing particle with the substrate or with adjacent desorbing particles. In fact, for low laser power, it was found that on the average a fraction of the first monolayer desorbs, while for high laser power a few layers desorb in the same time event of desorptlon [8]. Hence, upon increase of the incident laser power the dissociated atom begins to interact with adjacent dissociating atoms and also to feel the varying substrate. Thus, this may involve the problem of the statistical thermodynamics of the solid-gas phase transformation for high laser power. The systematic changes of E(O) and S(O) with the laser power may be reasonably understood by the continuous change of the phase transformation from a one-atomic process at low laser power to a many-atormc process at high laser power.

140 4 Nanukt et al / Spectra of neutral parttcle~ desorbed from CdS

T h e fact that E ' ( 0 > 40 °) dec reases a b n o r m a l l y wi th inc reas ing i nc iden t

laser p o w e r m c o n t r a d i c t i o n to the e x p e c t e d increase of the subs t ra te t e m p e r a -

ture by laser hea t i ng is puzzl ing . Th is fact m a y no t be s imp ly u n d e r s t a n d a b l e

wi th the usua l d e s o r p t i o n theor ies d e v e l o p e d for n o n - e q u l l i b r m m t h e r m a l

d e s o r p t l o n p h e n o m e n a [12-15] . T h u s this p e c u h a r fact m a y be a g o o d clue to

m a k i n g a c o m p r e h e n s i v e theore t i ca l m o d e l to desc r ibe the gas d y n a m i c s in the

s o l i d - g a s phase t r a n s f o r m a t i o n .

Ackowledgements

T h e au tho r s express thei r s incere thanks to P ro fes so r s Y. Y a s u d a , T.

N a k a m u r a and A. Y o s h l d a for the i r e n c o u r a g e m e n t t h r o u g h o u t this work . Th is

w o r k was pa r t ly s u p p o r t e d by a G r a n t - m - A i d for sc ient i f ic research f r o m the

M i m s t r y of E d u c a t i o n , Sc ience and C u l t u r e o f J a p a n a n d also by the I sh tda

F o u n d a t i o n .

References

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J A ,~an Vechten and R Tsu, Phys Letters 74A (1979) 422 [3] J M. Llu, R Yen, H Kurz and N Bloembergen, Appl Phys Letters 39 (1981) 755 [4] B Stmzker, A Pospleszczyk and J A Tagle, Phys Rev Letters 47 (1981) 356

A Posp~eszczyk, M.A Haruh and B Smtzker, J Appl Phys 54 (1983) 3176 [5] J A van Vechten, m Cohesive Propemes of Semiconductors under Laser Irradiation, Ed

L D Laude (Nijhoff, The Hague, 1983) [6] N Itoh and T Nakayama, Phys Letters 92A (1982) 471 [7] M Wautelet, Surface Scl 133 (1983) L437 [8] A Namlkl, H Fukano, T Kawal, Y Yasuda and T Nakamura, J Ph'cs Soc Japan 54 (1985)

3162 [91 T, Nakayama, Surface Scl 133 (1983) 101

[10] A Namlkl, K Watabe, H Fukano, S Nlshlgakl and T Noda, Surface Scl 128 (1983) L243 [11] A Namlkl, T Kawal, Y Yasuda and T Nakamura, Japan J Appl Phys 24 (1985) 270 [12] W van Wdhgen, Ph~s Letters 28A (1968) 80 [13] G Comsa, R David and K D Renduhc, Ph~s Rev Letters 38 (1977) 775,

G Comsa and R David, Surface Scl 117 (1982) 77 [14] P Taborek, Phys Rev Letters 48 (1982) 1737 115] J Reyes, I Romero and F O Goodman, J Chern, Phys 79 (1983) 5906