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Vacancy-initiated laser sputtering from semiconductor surfaces
Nuclear Instruments and Methods m Physics Research B 82 (1993) 310-316 North-Holland
B Beam Interactions with Materials & Atoms
Vacancy-initiated laser sputtering from semiconductor surfaces Noriaki Itoh, Akiko Okano, Ken Hattori, Jyun'ichi Kanasaki and Yasuo Nakai Department of Physws, Facul~. of Scwnce, Nagoya Untverstty, Furo-cho, Chtkusa, Nagoya 464-01. Japan Recewed 7 December 1992 and in rewsed form 16 February 1993
We review recent experimental results of laser ablation of semiconductors and of submonolayer-sens~twlty measurements of laser-pulse-reduced particle emissions from cleaned surfaces of GaP and GaAs It ts pointed out that the parhcle emission yield below the ablation threshold decreases as irradmtlon is repeated, whde that above the ablation threshold increases, leading finally to ablahon The ablation processes are discussed on the basis of the experimental observation of the subthreshold emissions, which are ascribed to be due to breaking of bonds of weakly bonded atoms around adatom- and step-type defects on surfaces due to cascade electromc excitation It ~s shown that existing experimental results on laser ablation can be explained on the bas~s of the following mechamsm: the ablation is lmtlated by laser-induced bond breaking of weakly bonded atoms around the vacancies, which leads to evolution of vacancy clusters not only m the original topmost surface layer but also m the original tuner layers It ~s emphasized that only weakly bonded atoms around vacancies and vacancy clusters can be sputtered, but yet massive erosion of the surface is reduced
1. Introduction P a r t i c l e - i n d u c e d s p u t t e r i n g i n d u c e d by ions is hom o g e n e o u s in its n a t u r e : e a c h a t o m o n the surface is capable of b e i n g s p u t t e r e d . T h e solid surfaces are r e c e d e d at a macroscopic d e g r e e by irradiation, emitting a masswe a m o u n t of c o n s t i t u e n t atoms. T h u s t h e process is distinguished from particle emission or desorption in which a limited n u m b e r of a t o m s is emitted. D e s o r p t l o n is often related to t h e emission of atoms a d s o r b e d on the surface, b u t we relate it to the emission of a limited n u m b e r of c o n s t i t u e n t a t o m s such as those a r o u n d defects. S p u t t e r i n g by ions is k n o w n to b e i n d u c e d by two processes: ballistic collisions a n d electronic collisions of c o n s t i t u e n t a t o m s with i n c i d e n t particles a n d recoiled a t o m s [1]. In the former, a n a t o m n e a r the surface is ejected by deposition of kinetic energy, if it exceeds a t h r e s h o l d energy. Since the kinetic energy can be t r a n s f e r r e d to any a t o m s o n the surface, the s p u t t e r i n g i n d u c e d by elastic collisions is r e g a r d e d as homogeneous. E l e c t r o n i c s p u t t e r i n g can b e i n d u c e d if electronic excitation energy is c o n v e r t e d to atomic energy, leading to atomic emissions from the surfaces. Such conversion of energy occurs only in the n o n m e t a l l i c solids,
Correspondence to N Itoh, Department of Physics, Faculty of Science, Nagoya Unwerslty, Furo-cho, Chlkusa, Nagoya 46401, Japan
since an energy of the o r d e r of t h e b a n d - g a p energy can be t r a n s f e r r e d in n o n m e t a l l i c solids as a q u a n t u m , while the electronic excitation energy is divided into small pieces by e l e c t r o n - p h o n o n scattering m metals. Since the primary excitation of n o n m e t a l l i c sohds produces e l e c t r o n - h o l e pairs or excitons of which the wave functions are of the Bloch type, the primary excitation energy is delocahzed. Thus, as a r g u e d by several a u t h o r s [2], localization of electronic excitation should p r e c e d e sputtering. Self-trapping of holes a n d excltons in alkali halides a n d alkaline e a r t h fluoride is c o n s i d e r e d to be o n e of the localization processes leading to sputtering T h e s p u t t e r i n g arising from selft r a p p i n g in t h e s e crystals is Induced only in the halogen sublattice, since the top of the valence b a n d consists of the h a l o g e n orbitais a n d h e n c e it is the h a l o g e n sublattice t h a t is modified by hole or exciton locahzatlon [3] Because of the selective emission of h a l o g e n atoms, the surface is e n r i c h e d by alkali, if the t e m p e r a t u r e is below t h e t h e r m a l vaporization t e m p e r a t u r e . Vaporization of m e t a l a t o m s along with electronic s p u t t e r i n g of the h a l o g e n sublattice leads to sputtering of alkali halides u n d e r the condition w h e r e only electronic excitation is induced. This type of electronic sputtering, which is observed in a limited n u m b e r of n o n m e t a l h c solids like alkali halides, is r e g a r d e d as h o m o g e n e o u s . Sputtering by lasers is called laser ablation. It has b e e n a r g u e d often t h a t laser ablation arises from heatlng [4]. T h e m a j o r p h e n o m e n o l o g i c a l difference between electronic sputtering discussed above a n d laser a b l a t i o n ~s that the yield is p r o p o r t i o n a l to the n u m b e r
0168-583X/93/$06.00 © 1993 - Elsevier Science Pubhshers B V All rights reserved
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N Itoh et al / Laser-reduced sputtermg of compound semtconductors
of excitations in the electronic sputtering, while it is a superlinear function of the number of excitations in the laser ablation. The processes of laser ablation differ for materials in which linear electronic sputterlng is observed and those in which conventional ionizIng radiation does not cause sputtering, like semiconductors [5]. In the former type of materials, the accumulation of the damage induced by electronic transitions on the surface causes nonlinear sputtering [6]. However, m the latter type of materials, the atomic emission due to electronic transitions is not induced by conventional ionizing radiation and the emission can occur only under intense irradiation. In this paper we deal with laser ablation of nonmetallic solids in which no electronic sputtering IS observed under conventional ionizing radiation. The materials covered in this paper are typically semiconductors, although the same concept can be applied to some insulators, such as M g O and Z n O [7]. Emphasis of the paper is on the electromc processes which can cause laser ablation It is argued that laser ablation in this group of materials is initiated by defects: only atoms around vacancies and vacancy clusters are emitted for laser fluences above the ablation threshold, and yet massive emission of atoms from the surface is induced
2. Summary of experimental results of laser ablation of semiconductors Photo-induced emission from semiconductor surfaces has been detected only by intense laser pulses. Usually a sharp threshold laser fluence is observed and the yield increases rapidly as the laser fluence increases above the threshold [8-14], as shown in fig. 1 As we shall see later, the ablation threshold is considered to be the manifestation of a strong dependence of the yield on laser fluence, we use this terminology for convenience. Irradiation with laser pulses above the ablation thresholds leads to destruction of L E E D patterns [15], StolChlometry change and often to evolution of morphology change [16], which can be detected with several techniques. Emissions of neutral atoms have been found to dominate near the ablation thresholds. The yield of ions increases much faster than that of atoms as the laser fluence increases further from the ablation threshold [11] The velocity distribution of emitted atoms shows almost a Maxwell distribution [9-11]; the temperature derived by fitting the measured distribution to the Maxwell distribution is higher than the temperature of the specimen but lower than the melting point. Furthermore, Ichige et al. [10] have observed that the effective temperature obtained by the Maxwell fit depends on the direction of emission. According to
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a recent analysis by Kelly and Dreyfus [17], the angular dependence of the effective temperature is explained in terms of formation of a Knudsen layer above the specimen. Thus, it appears that the velocity distribution does not provide us useful information on the ablation processes. Because of the yield-fluence dependence for neutrals and ions, as described above, it is believed that the emission of neutral atoms is the primary process and ions are produced by secondary processes. Ichige et al. [10] have pointed out a clear correlation between the laser ablation threshold and the bond strength of solids. According to their compilation, as shown in fig. 2, the ablation threshold is smaller as the bond strength is smaller. The ablation threshold is the largest for silicon, for which it has been argued that melting of the surface layers is the cause of ablation or sputtering [18]. The correlation between the threshold and bond strength, however, suggests that the laser ablation of compound semiconductors, for which the ablation threshold IS smaller than that for S1, is of electronic origin. Ichige et al. ascribed this relation to a phase change induced under dense electronic excitation as suggested earher by Van Vechten [19,20]. We discuss this point later.
3. Laser-induced bond breaking Before discussing the process of laser ablation, we present experimental observations which evidence that the bond of atoms constituting defects on the surface
N Itoh et al / Laser-induced sputtermg of compound semiconductors
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can be broken by electronic excitation. Experiments described below are highly sensitivity measurements of emission of Ga ° atoms from semiconductor surfaces exhibiting a clear L E E D pattern, induced by irradiation with laser pulses of 10 ns below the ablation threshold [13,14,21]. The high-sensitivity detection of a range of 10 -5 monolayer has been m a d e using resonance ionization spectroscopy (RIS). The experimental results shown in this paper are mainly on GaP, but similar results have been obtained for G a A s [22] and SI [23]. Irradiation of the same spot of a specimen of GaP and G a A s below the ablation threshold is found to induce a change in the yield, typically as described in fig. 3, first rapidly and then slowly. Furthermore, the specimen, irradiated with laser pulses until the rapidly decaying component disappears and further with Ar ions to an extent to which the L E E D pattern still remains, exhibits an additional laser-Induced burst when the irradiation with the laser pulses is repeated. A similar rapidly decaying component appears after deposition of Ga atoms of 10 4 - 1 0 - 2 monolayers. These results indicate clearly that the rapidly decaying component originates from a small number of defects. Since the defects responsible for this component are ehmlnated by laser irradiation, they have been considered to be adatom-type. All types of defects which modify structures substantially after removal of a Ga atom fall in this category. The results imply that bonds of atoms constituting these defects are broken by laser irradiation. Since the magnitude of the slowly decaying component changes from specimen to specimen, the emission in this stage has been ascribed to defects. Most plausible candidates which may give rise to the defects
responsible for this stage are kink sites, for which elimination of an atomic pair [24] leaves a defect of the same structure. We tentatively call this type of defects step-type. The reason for the slow decay is considered to be the elimination of kink sites by interaction with other defects. The excitation spectra for emissions of rapidly and slowly decaying components have been measured for GaP [25] and G a A s [26]. The results reflect clearly the surface and bulk electronic structures of these materials. For GaP the surface band-gap energy is smaller than the bulk band-gap energy. Furthermore, in this energy range, the bulk band-to-band transition is indirect, while the surface band-to-band transition is direct On the other hand, the bulk and surface band-gap energies for G a A s are nearly the same and, contrary to GaP, the surface band-to-band transition near the band-gap energy is indirect and the bulk band-to-band transition is direct. Studies of the excitation spectrum for GaP indicate that the emission yield decreases drastically as the photon energy increases across the bulk band-gap energy For GaAs, it has been found that the yield increases as the photon energy increases not only across the bulk band-gap energy but also across the energy of the direct transitions from the surface valence band to the bulk and the surface conduction bands. The results described above indicate clearly that the e l e c t r o n - h o l e pairs generated in the surface states are most effective in inducing defect-imtiated emission. The reduction of the yield for G a P upon crossing the bulk band-gap energy can be ascribed to the phonon scattering of two-dimensional (2D) e l e c t r o n - h o l e ( e - h ) pairs localized on the surface on three-dimensional
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(3D) e - h pairs, of which the number is less numerous than the 2D e - h pairs because of the indirect nature of the transition For GaAs, it appears also that the contribution of 2D e - h pairs to defect-initiated emissions dominates that of the 3D e - h pairs in view of the strong e n h a n c e m e n t of the yield upon crossing the energies of the direct transitions involving the surface states. The yield of these defect-initiated particle emissions has been found to be a superlinear function of the fluence A typical result shown in fig. 4 indicates the yxeld-fluence relations for the rapidly and slowly decaying components by curves 1 and 2, respectively. Clearly the relations follow power functions. It has been pointed out that the power indices are larger for defects around which atoms are more strongly bonded [27]. As pointed out in section 1, a crucial step for the bond breaking is the localization of e l e c t r o n - h o l e pairs Evidently 2D e l e c t r o n - h o l e pairs can be localized more efficiently by adatom- and step-type defects. The superhnear relation indicates that localization alone is not sufficient for bond breaking which leads to particle emission, as in alkali halides. Two possible mechanisms have been suggested: the negative-U localization [28,29] and cascade excitation [14,30]. Stabilization by the negatave-U interaction occurs if the electron lattice interaction energy upon two-hole localization is larger than the on-site Coulomb interaction energy before relaxation [31]. According to the cascade excitation model, the localization of an e l e c t r o n - h o l e pair induces a metastable state, in which the lattice around the defect
is highly distorted but yet no atomic emission is induced. The metastable state, which usually has a lifetime longer than the pulse width (10 ns) and a localized level in the band gap, can be excited again during the same laser pulse. The e x o t e d metastable state will form another metastable state, which can be excited during the same pulse It is suggested that the metastable excited state after several cascade excitations is of an anti-bonding nature for a weakly bonded atom (WBA) around the defect. Thus if the anti-bonding excited state is reached during a laser pulse, emission of an atom results. A semi-empirical quantum mechanical calculation for the bond breaking by electronic excitation was performed for the Ga adatom on GaP [30]. Fig. 5 shows the adiabatic potential energy surface for the ground state ($3), at a single excited state ($6), at an ionized state (S 4) and at an excited state of the ionized state ($5). The ordinate of the figure is the total energy and the abscissa is the position of the Ga atom with respect to the surface Evidently the state S 5 is antibonding. Thus if an adatom traps a hole and subsequently is excited, the emission of the atom is induced Experimentally, it has been shown that the yield of the emission from adatoms is a power function of the laser intensity with a power index of 1.7. Thus the result of calculations is consistent with the experimental results. Higher power indices have been obtained for the kink sites experimentally. The p h e n o m e n a described in this section should be observed directly using a scanning tunneling microE~ev;
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314
N. Itoh et al / Laser-mduced sputtermg of compound semtconductors
scope (STM). O b s e r v a t i o n of e m i t t e d atoms, however, has indicated clearly t h a t defect-initiated emission of a t o m s from s e m i c o n d u c t o r surfaces i n d e e d takes place due to b o n d b r e a k i n g of weakly b o n d e d a t o m s ( W B A s ) n e a r t h e defects.
4. Vacancy-initiated bond breaking and laser sputtering As we have argued, a defect on surfaces can act as a localization site of an e l e c t r o n - h o l e pair, a n d f u r t h e r cascade excitation of a thus g e n e r a t e d m e t a s t a b l e excited state w e a k e n s the b o n d of a W B A a r o u n d a defect on t h e surface a n d finally leads to emission. In this section, we show that, if the laser intensity is suff~clently high to emit a W B A n e a r a vacancy, laser s p u t t e r i n g or ablation results. A c c o r d i n g to this model, only the W B A s a r o u n d vacancies can b e s p u t t e r e d by laser pulses of a fluence n e a r the a b l a t i o n threshold. thus the s p u t t e r i n g process is essentially h e t e r o g e n e o u s A r g u m e n t s in this section can b e applied to any sohds in which electronic s p u t t e r i n g at low excitation intensity ~s not effective. First we describe the process quantitatively and t h e n show t h e results of M o n t e Carlo calculation briefly H e r e we take a m o n a t o m i c sohd as an example. E h m i n a t i o n of a W B A from a vacancy forms a divacancy, the W B A s a r o u n d which are c o n s i d e r e d to be b o n d e d less strongly t h a n those a r o u n d a vacancy. Similarly, emission of a W B A a r o u n d a dlvacancy forms a trivacancy, which possesses W B A s less strongly b o n d e d . T h u s d u r i n g irradiation with a laser pulse of a fluence which can cause emission of W B A s a r o u n d vacancies, the n u m b e r n w of W B A s which can be e m i t t e d increases. It has b e e n shown [21] t h a t n w is an e x p o n e n t i a l function of irradiation time t w h e n the multiplicity m of the vacancy clusters is low (an mcrease of m by 1 increases n w at least by 1 w h e n m IS close to 1), a n d t h a t n w is p r o p o r t i o n a l to t w h e n rn b e c o m e s large ( h e r e the form of the clusters was ass u m e d to b e a circle). A M o n t e Carlo simulation [32] for a trigonally b o n d e d monatom~c lattice, o n the ass u m p t i o n t h a t t h e s t r e n g t h of the W B A s does not c h a n g e as m increases, shows t h a t the above r e l a t i o n is satisfied a n d t h a t t h e switch from e x p o n e n t i a l to linear occurs for m = 1.5. So far we have d~scussed the lateral evolution of the surface damage. A n o t h e r point for which a q u a n t l t a twe description should be m a d e is the v e m c a l evolution of the surface damage. As the multlphc~ty of vacancy clusters on the t o p m o s t surface layer increases, the vacancies t h a t have b e e n located in the second layer are now exposed to t h e t o p m o s t layer. T h u s the evolution of vacancy clusters starts to occur m the second layer. Similarly the evolution of vacancy clus-
ters will p r o c e e d to d e e p e r layers once a W B A a r o u n d a single vacancy is emitted. T h u s the s p u t t e r i n g of the bulk of a solid can b e induced. T h e sputtering described above can eliminate all c o n s t i t u e n t atoms m the surface layers b u t originates from h e t e r o g e n e o u s emission of W B A s a r o u n d vacancies. A M o n t e Carlo calculation has b e e n m a d e to simulate this vacancy-initiated s p u t t e r i n g [32] This calculation takes into account t h a t the b o n d b r e a k i n g of W B A s itself IS a s u p e r l i n e a r function. T h e o b t a i n e d yield vs fluence curve follows a power function with a p o w e r of a b o u t 10. T h e e x p e r i m e n t a l results of the y i e l d - f l u c n c e r e l a t i o n o b t a i n e d at a s u b m o n o l a y e r sensltivlty m d l c a t e a power d e p e n d e n c e of 15 [14]. In v~ew of the a s s u m p t i o n s e m p l o y e d m the calculation, we may conclude t h a t the M o n t e Carlo calculation can simulate the laser-induced sputtermg. So far we have dealt with m o n a t o m l c sohds W e p r e s u m e t h a t the results are substantially s~milar for multi-atomic sohds. Taking G a A s as an example, we consider first t h a t s p u t t e r i n g is initiated by an As vacancy. A G a atom can be r e g a r d e d as the W B A a r o u n d an As vacancy. T h u s a G a atom a r o u n d an As vacancy ~s emitted, forming a G a - A s dlvacancy. Similarly, an As a t o m a r o u n d a G a atom ~s e m i t t e d forming a G a - A s dlvacancy. T h e ablation t h r e s h o l d is the smaller of t h e t h r e s h o l d s for emission of a G a atom a r o u n d an As vacancy a n d of an As atom a r o u n d a G a vacancy. W h a t follows after f o r m a t i o n of G a - A s dlvacanc~es ~s first t h e emission of a G a a t o m or an As atom a r o u n d the divacancy. As we discussed before, it ~s logical to p r e s u m e t h a t the b o n d i n g s t r e n g t h s of atoms a r o u n d a dwacancy are not larger t h a n that of the respective atoms a r o u n d a vacancy of the opposite atoms. A f t e r emission of e~ther a G a or As a t o m a r o u n d a dlvacancy, an emission of a c o m p l e m e n t a r y atom follows. T h u s the vacancy cluster is c o n s i d e r e d to grow while the stolchlometry is m a i n t a i n e d , at least in the mitml stage of vacancy evolution. As the vacancy clusters grow, coagulation of metals at the b o u n d a r y of clusters may be i n d u c e d a n d the sto~chlometry may not necessarily be m a i n t a i n e d W e note f u r t h e r t h a t the m o d e l described above does not assume any critical process t h a t induces laser ablation, yet the strong fluence d e p e n d e n c e gives rise to a p p a r e n t t h r e s h o l d laser fluence. T h e strong fluencc d e p e n d e n c e originates from the s u p e r h n e a r d e p e n d e n c e of e m i s s m n yield of W B A s on the fluence and from t h e increase of W B A s d u r i n g laser ~rradiahon
5. Summary T h e vacancy-lmtlated sputtering described m this p a p e r can explain several features of laser sputtering. First of all, t h e strong laser fluence d e p e n d e n c e can b e
N Itoh et aL / Laser-mduced sputtermg of compound semiconductors 4
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described. Secondly, the laser ablation threshold fluence has been shown to be strongly d e p e n d e n t upon the lonicity of solids or the bond strength, as shown in fig. 2 It is presumed that the strength of the bond of the W B A s around a vacancy in various sohds is strongly correlated to the lonicity Thirdly, the vacancy-initiated model is in accordance with the experimental result (fig. 6 [14]) that the yield of Ga emissions from GaP, detected by sensitwities of 10 -5 monolayer, increases by repeating irradiation with laser pulses above the ablation threshold, while it decreases below the threshold as shown in fig. 2. The present model is based on the bond breaking due to cascade excitation of defect sites. The model is completely different from that suggested by Ichige et al. [10]. According to these authors, a phase change on the surface layer is mduced under dense electronic excitation, leading to sputtering. T h e vacancyoriginated model is based on the suggestion made by Hattori et al. [14] that an e l e c t r o n - h o l e pair is locallzed on a vacancy and weakens the bond of a WBA, producing a metastable state, which can be multiply excited leading to the emission of a weakly bonded atom. In this model, it is assumed that the excitation energy is localized on a defect site and excited further. The phase change model assumes that the lattice in the surface layer becomes deformable when the density of e l e c t r o n - h o l e pairs becomes sufficiently high. Evidently the degree of the lattice weakening on a specific point as much higher when an e l e c t r o n - h o l e pair is "'localized" than when the density is "sufficiently high". F u r t h e r m o r e the bond breaking of adatoms by cascade excitation has been proved for G a P [14], G a A s [22] and
315
Si [23]. Thus we consider that the vacancy-originated model is more plausible. Further experiments are needed to prove the vacancy-initiated laser-induced sputtering. O n e of the crucial experiments will be to observe using STM whether vacancy clusters are indeed formed above the ablation threshold. The laser ablation dealt with in this paper is of electronic nature: The ablation threshold is determined in terms of the electronic process, while the ablation often is referred to as plasma formation or massive ejection of particles. We note, however, that once the "electronic" ablation threshold, namely the condition of breaking the bonds of W B A s surrounding a vacancy, is satisfied, the surface morphology is changed rapidly during a laser pulse, increasing the absorption coefficient for laser beams and also the probability of laser-induced bond breaking. Thus It is very conceivable that the "electronic" ablation threshold dealt with in the present paper is substantially the same as that detected by massive ejection of atoms or by macroscopic surface damage. The concept of the vacancy-originated laser-induced sputtering suggests that the ablation threshold becomes much higher if the surface is perfect, or free from defects. It is of interest to investigate the correlation between the vacancy concentration and the emission yield slightly above the ablation threshold. The number of vacancies may be reduced by careful treatment of the surface after deposition of small amounts of component atoms. It has been suggested that a combination of filhng vacancies with component atoms and ejection of adatoms and steps by laser irradiation can produce defect-free surfaces [33]. Further works on the control of defects on surfaces and its influence on laser-induced emission and ablation are of interest.
References [1] R Behrisch fed ), Sputtering by Particle Bombardment I, II (Springer, Berlin, 1981, 1983) [2] K Tanimura and N Itoh, Nucl. Instr and Meth. B46 (1990) 207; Y Toyozawa, Physica B + C 117/118 (1983) 23 [3] R T. Wllhams, Opt. Eng. 28 (1989) 1024. [4] M yon Allmen, Laser-Beam Interactions with Materials (Springer, Berlin, 1987). [5] R F Haglund and N Itoh, Laser Ablation, Principle and Applications, ed J C. Miller (Springer, Berlin, 1992) in press. [6] J Relf, Opt Eng 28 (1989) 1122 [7] N Itoh, Nucl Instr. and Meth B27 (1987) 155 [8] J.M Molson and M Bensoussan, J. Vac. Sci. Technol 21 (1982) 315, Surf Scl 126 (1983) 294, Appl. Surf. Scl 20 (1984) 84 [9] A. Namlki, T Kawai and K Ichige, Surf Sci 166 (1986) 129
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[10] K Ichlge, Y Matsumoto and A Namiki, Nucl Instr and Metb B33 (1988) 820 [11] T Nakayama, H Ichlkawa and N Itoh, Surf Scl 123 (1982) L693, T Nakayama, Surf Scl 133 (1983) 101, T Nakayama and N. Itoh, in Desorptlon Induced by Electronic Transitions I, eds W Brenlg and D. Menzel (Springer, Berhn, 1985) p 237 [12] T Yamamoto, K Hattorl, Y Nakal and N Itoh, Radiat Eft. Def. Solids 109 (1989) 213, Y. Nakai, K Hattorl and N Itoh, Appl Phys Lett 56 (1990) 1980, Y Nakal, K Hatton and N Itoh, Desorptlon Induced by Electronic Transitions, DIET IV, eds G. Betz and P Varga (Springer, Berlin, 1990) p 143 [13] K. Hattori, Y Nakal and N Itoh, Surf. Sci 227 (1990) Ll15; K. Hattori, A Okano, Y. Nakai, N. Itoh and H F Haglund, Jr, J Phys Condensed Matter 3 (1991) 7001 [14] K Hattorl, A Okano, Y Nakal and N Itoh, Phys Rev. B45 (1992) 8424. [15] Y. Kumazakl, Y. Nakal and N Itoh, Surf. Sci 184 (1987) L445; Y. Kumazakl, Y Nakal and N Itoh, Phys Rev Lett 59 (1987) 2883 [16] T. de Jong, Z L Wang and F W Saris, Phys Lett. A90 (1982) 147, A F Tuhnov, K K Bourdelle, N G Chechenin, P K Kashkarov and A I. Efimova, Nucl Instr and Meth 33 (1988) 844, A.E Wllner, D.V Podlesmk, H.H Gilgen and R.M Osgood, Jr, Appl. Phys. Lett. 53 (1988) 1198, A Sakalas and S. Zhukauskas, Sohd State Commun. 79 (1989) 363, V M . Donnelly and T R . Hayes, Appl Phys Lett 57 (1990) 701
[17] R Kelly and R W Dreyfus, Nucl Instr and Meth B32 (1988) 341 [18] B Strltzker, A. Posplezcyk and J A Tagle, Phys. Rev Lett 47 (1981) 356 [19] J A van Vechten, in: Cohesive Properties of Semiconductors under Laser Irradiation, ed L D Laude (Nohoff, The Hague, 1983) p 49 [20] M Combescot and J Bok, Phys Rev Lett 48 (1982) 1413 [21] Y Nakai, K Hattorl, A Okano, N Itoh and R F Haglund, Jr, Nucl Instr and Meth B58 (1991) 452 [22] J Kanasakl, A.Y Matsuura, Y Nakal, N Itoh and R F Haglund, Jr., Appl Phys Lett, in press [23] K Kanasakl, I K Yu, Y Nakai and N Itoh, Jpn J Appl Phys, in press [24] We presume that emission of a Ga atom from a kink site leaves a weakly bonded As atom, of which emission follows, leaving a weakly bonded Ga atom. The smaller of the weakly bonded Ga or As atom at kink sites governs the emission process keeping the Stolchlometry (see ref [3]) [25] A Okano, K Hattorl, Y Nakal and N Itoh, Surf Scl 258 (1991) L671 [26] J. Kanasakl, A Okano, K Hattorl, Y Nakal and N Itoh, Phys Rev. Lett, in press [27] C K. Ong, G S Khoo, K Hattorl, Y Nakal and N Itoh, Surf Sci 259 (1991) L787 [28] N Itoh and T Nakayama, Phys Lett A92 (1982) 471 [29] H Suml, Surf Scl 248 (1991) 382. [30] G.S Khoo, C K Ong and N Itoh, Phys Rev B 47 (1993) 2031 [31] P W Anderson, Phys Rev Lett 34 (1975)953 [32] A Okano, A Y Matsuura, K Hattorl and N Itoh, J Appl Phys, in press [33] N Itoh, Y Nakal, K Hattorl, C K Ong and G S Khoo, Appl Phys Lett 60 (1992)3271.
B Beam Interactions with Materials & Atoms
Vacancy-initiated laser sputtering from semiconductor surfaces Noriaki Itoh, Akiko Okano, Ken Hattori, Jyun'ichi Kanasaki and Yasuo Nakai Department of Physws, Facul~. of Scwnce, Nagoya Untverstty, Furo-cho, Chtkusa, Nagoya 464-01. Japan Recewed 7 December 1992 and in rewsed form 16 February 1993
We review recent experimental results of laser ablation of semiconductors and of submonolayer-sens~twlty measurements of laser-pulse-reduced particle emissions from cleaned surfaces of GaP and GaAs It ts pointed out that the parhcle emission yield below the ablation threshold decreases as irradmtlon is repeated, whde that above the ablation threshold increases, leading finally to ablahon The ablation processes are discussed on the basis of the experimental observation of the subthreshold emissions, which are ascribed to be due to breaking of bonds of weakly bonded atoms around adatom- and step-type defects on surfaces due to cascade electromc excitation It ~s shown that existing experimental results on laser ablation can be explained on the bas~s of the following mechamsm: the ablation is lmtlated by laser-induced bond breaking of weakly bonded atoms around the vacancies, which leads to evolution of vacancy clusters not only m the original topmost surface layer but also m the original tuner layers It ~s emphasized that only weakly bonded atoms around vacancies and vacancy clusters can be sputtered, but yet massive erosion of the surface is reduced
1. Introduction P a r t i c l e - i n d u c e d s p u t t e r i n g i n d u c e d by ions is hom o g e n e o u s in its n a t u r e : e a c h a t o m o n the surface is capable of b e i n g s p u t t e r e d . T h e solid surfaces are r e c e d e d at a macroscopic d e g r e e by irradiation, emitting a masswe a m o u n t of c o n s t i t u e n t atoms. T h u s t h e process is distinguished from particle emission or desorption in which a limited n u m b e r of a t o m s is emitted. D e s o r p t l o n is often related to t h e emission of atoms a d s o r b e d on the surface, b u t we relate it to the emission of a limited n u m b e r of c o n s t i t u e n t a t o m s such as those a r o u n d defects. S p u t t e r i n g by ions is k n o w n to b e i n d u c e d by two processes: ballistic collisions a n d electronic collisions of c o n s t i t u e n t a t o m s with i n c i d e n t particles a n d recoiled a t o m s [1]. In the former, a n a t o m n e a r the surface is ejected by deposition of kinetic energy, if it exceeds a t h r e s h o l d energy. Since the kinetic energy can be t r a n s f e r r e d to any a t o m s o n the surface, the s p u t t e r i n g i n d u c e d by elastic collisions is r e g a r d e d as homogeneous. E l e c t r o n i c s p u t t e r i n g can b e i n d u c e d if electronic excitation energy is c o n v e r t e d to atomic energy, leading to atomic emissions from the surfaces. Such conversion of energy occurs only in the n o n m e t a l l i c solids,
Correspondence to N Itoh, Department of Physics, Faculty of Science, Nagoya Unwerslty, Furo-cho, Chlkusa, Nagoya 46401, Japan
since an energy of the o r d e r of t h e b a n d - g a p energy can be t r a n s f e r r e d in n o n m e t a l l i c solids as a q u a n t u m , while the electronic excitation energy is divided into small pieces by e l e c t r o n - p h o n o n scattering m metals. Since the primary excitation of n o n m e t a l l i c sohds produces e l e c t r o n - h o l e pairs or excitons of which the wave functions are of the Bloch type, the primary excitation energy is delocahzed. Thus, as a r g u e d by several a u t h o r s [2], localization of electronic excitation should p r e c e d e sputtering. Self-trapping of holes a n d excltons in alkali halides a n d alkaline e a r t h fluoride is c o n s i d e r e d to be o n e of the localization processes leading to sputtering T h e s p u t t e r i n g arising from selft r a p p i n g in t h e s e crystals is Induced only in the halogen sublattice, since the top of the valence b a n d consists of the h a l o g e n orbitais a n d h e n c e it is the h a l o g e n sublattice t h a t is modified by hole or exciton locahzatlon [3] Because of the selective emission of h a l o g e n atoms, the surface is e n r i c h e d by alkali, if the t e m p e r a t u r e is below t h e t h e r m a l vaporization t e m p e r a t u r e . Vaporization of m e t a l a t o m s along with electronic s p u t t e r i n g of the h a l o g e n sublattice leads to sputtering of alkali halides u n d e r the condition w h e r e only electronic excitation is induced. This type of electronic sputtering, which is observed in a limited n u m b e r of n o n m e t a l h c solids like alkali halides, is r e g a r d e d as h o m o g e n e o u s . Sputtering by lasers is called laser ablation. It has b e e n a r g u e d often t h a t laser ablation arises from heatlng [4]. T h e m a j o r p h e n o m e n o l o g i c a l difference between electronic sputtering discussed above a n d laser a b l a t i o n ~s that the yield is p r o p o r t i o n a l to the n u m b e r
0168-583X/93/$06.00 © 1993 - Elsevier Science Pubhshers B V All rights reserved
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N Itoh et al / Laser-reduced sputtermg of compound semtconductors
of excitations in the electronic sputtering, while it is a superlinear function of the number of excitations in the laser ablation. The processes of laser ablation differ for materials in which linear electronic sputterlng is observed and those in which conventional ionizIng radiation does not cause sputtering, like semiconductors [5]. In the former type of materials, the accumulation of the damage induced by electronic transitions on the surface causes nonlinear sputtering [6]. However, m the latter type of materials, the atomic emission due to electronic transitions is not induced by conventional ionizing radiation and the emission can occur only under intense irradiation. In this paper we deal with laser ablation of nonmetallic solids in which no electronic sputtering IS observed under conventional ionizing radiation. The materials covered in this paper are typically semiconductors, although the same concept can be applied to some insulators, such as M g O and Z n O [7]. Emphasis of the paper is on the electromc processes which can cause laser ablation It is argued that laser ablation in this group of materials is initiated by defects: only atoms around vacancies and vacancy clusters are emitted for laser fluences above the ablation threshold, and yet massive emission of atoms from the surface is induced
2. Summary of experimental results of laser ablation of semiconductors Photo-induced emission from semiconductor surfaces has been detected only by intense laser pulses. Usually a sharp threshold laser fluence is observed and the yield increases rapidly as the laser fluence increases above the threshold [8-14], as shown in fig. 1 As we shall see later, the ablation threshold is considered to be the manifestation of a strong dependence of the yield on laser fluence, we use this terminology for convenience. Irradiation with laser pulses above the ablation thresholds leads to destruction of L E E D patterns [15], StolChlometry change and often to evolution of morphology change [16], which can be detected with several techniques. Emissions of neutral atoms have been found to dominate near the ablation thresholds. The yield of ions increases much faster than that of atoms as the laser fluence increases further from the ablation threshold [11] The velocity distribution of emitted atoms shows almost a Maxwell distribution [9-11]; the temperature derived by fitting the measured distribution to the Maxwell distribution is higher than the temperature of the specimen but lower than the melting point. Furthermore, Ichige et al. [10] have observed that the effective temperature obtained by the Maxwell fit depends on the direction of emission. According to
20
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a recent analysis by Kelly and Dreyfus [17], the angular dependence of the effective temperature is explained in terms of formation of a Knudsen layer above the specimen. Thus, it appears that the velocity distribution does not provide us useful information on the ablation processes. Because of the yield-fluence dependence for neutrals and ions, as described above, it is believed that the emission of neutral atoms is the primary process and ions are produced by secondary processes. Ichige et al. [10] have pointed out a clear correlation between the laser ablation threshold and the bond strength of solids. According to their compilation, as shown in fig. 2, the ablation threshold is smaller as the bond strength is smaller. The ablation threshold is the largest for silicon, for which it has been argued that melting of the surface layers is the cause of ablation or sputtering [18]. The correlation between the threshold and bond strength, however, suggests that the laser ablation of compound semiconductors, for which the ablation threshold IS smaller than that for S1, is of electronic origin. Ichige et al. ascribed this relation to a phase change induced under dense electronic excitation as suggested earher by Van Vechten [19,20]. We discuss this point later.
3. Laser-induced bond breaking Before discussing the process of laser ablation, we present experimental observations which evidence that the bond of atoms constituting defects on the surface
N Itoh et al / Laser-induced sputtermg of compound semiconductors
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can be broken by electronic excitation. Experiments described below are highly sensitivity measurements of emission of Ga ° atoms from semiconductor surfaces exhibiting a clear L E E D pattern, induced by irradiation with laser pulses of 10 ns below the ablation threshold [13,14,21]. The high-sensitivity detection of a range of 10 -5 monolayer has been m a d e using resonance ionization spectroscopy (RIS). The experimental results shown in this paper are mainly on GaP, but similar results have been obtained for G a A s [22] and SI [23]. Irradiation of the same spot of a specimen of GaP and G a A s below the ablation threshold is found to induce a change in the yield, typically as described in fig. 3, first rapidly and then slowly. Furthermore, the specimen, irradiated with laser pulses until the rapidly decaying component disappears and further with Ar ions to an extent to which the L E E D pattern still remains, exhibits an additional laser-Induced burst when the irradiation with the laser pulses is repeated. A similar rapidly decaying component appears after deposition of Ga atoms of 10 4 - 1 0 - 2 monolayers. These results indicate clearly that the rapidly decaying component originates from a small number of defects. Since the defects responsible for this component are ehmlnated by laser irradiation, they have been considered to be adatom-type. All types of defects which modify structures substantially after removal of a Ga atom fall in this category. The results imply that bonds of atoms constituting these defects are broken by laser irradiation. Since the magnitude of the slowly decaying component changes from specimen to specimen, the emission in this stage has been ascribed to defects. Most plausible candidates which may give rise to the defects
responsible for this stage are kink sites, for which elimination of an atomic pair [24] leaves a defect of the same structure. We tentatively call this type of defects step-type. The reason for the slow decay is considered to be the elimination of kink sites by interaction with other defects. The excitation spectra for emissions of rapidly and slowly decaying components have been measured for GaP [25] and G a A s [26]. The results reflect clearly the surface and bulk electronic structures of these materials. For GaP the surface band-gap energy is smaller than the bulk band-gap energy. Furthermore, in this energy range, the bulk band-to-band transition is indirect, while the surface band-to-band transition is direct On the other hand, the bulk and surface band-gap energies for G a A s are nearly the same and, contrary to GaP, the surface band-to-band transition near the band-gap energy is indirect and the bulk band-to-band transition is direct. Studies of the excitation spectrum for GaP indicate that the emission yield decreases drastically as the photon energy increases across the bulk band-gap energy For GaAs, it has been found that the yield increases as the photon energy increases not only across the bulk band-gap energy but also across the energy of the direct transitions from the surface valence band to the bulk and the surface conduction bands. The results described above indicate clearly that the e l e c t r o n - h o l e pairs generated in the surface states are most effective in inducing defect-imtiated emission. The reduction of the yield for G a P upon crossing the bulk band-gap energy can be ascribed to the phonon scattering of two-dimensional (2D) e l e c t r o n - h o l e ( e - h ) pairs localized on the surface on three-dimensional
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313
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(3D) e - h pairs, of which the number is less numerous than the 2D e - h pairs because of the indirect nature of the transition For GaAs, it appears also that the contribution of 2D e - h pairs to defect-initiated emissions dominates that of the 3D e - h pairs in view of the strong e n h a n c e m e n t of the yield upon crossing the energies of the direct transitions involving the surface states. The yield of these defect-initiated particle emissions has been found to be a superlinear function of the fluence A typical result shown in fig. 4 indicates the yxeld-fluence relations for the rapidly and slowly decaying components by curves 1 and 2, respectively. Clearly the relations follow power functions. It has been pointed out that the power indices are larger for defects around which atoms are more strongly bonded [27]. As pointed out in section 1, a crucial step for the bond breaking is the localization of e l e c t r o n - h o l e pairs Evidently 2D e l e c t r o n - h o l e pairs can be localized more efficiently by adatom- and step-type defects. The superhnear relation indicates that localization alone is not sufficient for bond breaking which leads to particle emission, as in alkali halides. Two possible mechanisms have been suggested: the negative-U localization [28,29] and cascade excitation [14,30]. Stabilization by the negatave-U interaction occurs if the electron lattice interaction energy upon two-hole localization is larger than the on-site Coulomb interaction energy before relaxation [31]. According to the cascade excitation model, the localization of an e l e c t r o n - h o l e pair induces a metastable state, in which the lattice around the defect
is highly distorted but yet no atomic emission is induced. The metastable state, which usually has a lifetime longer than the pulse width (10 ns) and a localized level in the band gap, can be excited again during the same laser pulse. The e x o t e d metastable state will form another metastable state, which can be excited during the same pulse It is suggested that the metastable excited state after several cascade excitations is of an anti-bonding nature for a weakly bonded atom (WBA) around the defect. Thus if the anti-bonding excited state is reached during a laser pulse, emission of an atom results. A semi-empirical quantum mechanical calculation for the bond breaking by electronic excitation was performed for the Ga adatom on GaP [30]. Fig. 5 shows the adiabatic potential energy surface for the ground state ($3), at a single excited state ($6), at an ionized state (S 4) and at an excited state of the ionized state ($5). The ordinate of the figure is the total energy and the abscissa is the position of the Ga atom with respect to the surface Evidently the state S 5 is antibonding. Thus if an adatom traps a hole and subsequently is excited, the emission of the atom is induced Experimentally, it has been shown that the yield of the emission from adatoms is a power function of the laser intensity with a power index of 1.7. Thus the result of calculations is consistent with the experimental results. Higher power indices have been obtained for the kink sites experimentally. The p h e n o m e n a described in this section should be observed directly using a scanning tunneling microE~ev;
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314
N. Itoh et al / Laser-mduced sputtermg of compound semtconductors
scope (STM). O b s e r v a t i o n of e m i t t e d atoms, however, has indicated clearly t h a t defect-initiated emission of a t o m s from s e m i c o n d u c t o r surfaces i n d e e d takes place due to b o n d b r e a k i n g of weakly b o n d e d a t o m s ( W B A s ) n e a r t h e defects.
4. Vacancy-initiated bond breaking and laser sputtering As we have argued, a defect on surfaces can act as a localization site of an e l e c t r o n - h o l e pair, a n d f u r t h e r cascade excitation of a thus g e n e r a t e d m e t a s t a b l e excited state w e a k e n s the b o n d of a W B A a r o u n d a defect on t h e surface a n d finally leads to emission. In this section, we show that, if the laser intensity is suff~clently high to emit a W B A n e a r a vacancy, laser s p u t t e r i n g or ablation results. A c c o r d i n g to this model, only the W B A s a r o u n d vacancies can b e s p u t t e r e d by laser pulses of a fluence n e a r the a b l a t i o n threshold. thus the s p u t t e r i n g process is essentially h e t e r o g e n e o u s A r g u m e n t s in this section can b e applied to any sohds in which electronic s p u t t e r i n g at low excitation intensity ~s not effective. First we describe the process quantitatively and t h e n show t h e results of M o n t e Carlo calculation briefly H e r e we take a m o n a t o m i c sohd as an example. E h m i n a t i o n of a W B A from a vacancy forms a divacancy, the W B A s a r o u n d which are c o n s i d e r e d to be b o n d e d less strongly t h a n those a r o u n d a vacancy. Similarly, emission of a W B A a r o u n d a dlvacancy forms a trivacancy, which possesses W B A s less strongly b o n d e d . T h u s d u r i n g irradiation with a laser pulse of a fluence which can cause emission of W B A s a r o u n d vacancies, the n u m b e r n w of W B A s which can be e m i t t e d increases. It has b e e n shown [21] t h a t n w is an e x p o n e n t i a l function of irradiation time t w h e n the multiplicity m of the vacancy clusters is low (an mcrease of m by 1 increases n w at least by 1 w h e n m IS close to 1), a n d t h a t n w is p r o p o r t i o n a l to t w h e n rn b e c o m e s large ( h e r e the form of the clusters was ass u m e d to b e a circle). A M o n t e Carlo simulation [32] for a trigonally b o n d e d monatom~c lattice, o n the ass u m p t i o n t h a t t h e s t r e n g t h of the W B A s does not c h a n g e as m increases, shows t h a t the above r e l a t i o n is satisfied a n d t h a t t h e switch from e x p o n e n t i a l to linear occurs for m = 1.5. So far we have d~scussed the lateral evolution of the surface damage. A n o t h e r point for which a q u a n t l t a twe description should be m a d e is the v e m c a l evolution of the surface damage. As the multlphc~ty of vacancy clusters on the t o p m o s t surface layer increases, the vacancies t h a t have b e e n located in the second layer are now exposed to t h e t o p m o s t layer. T h u s the evolution of vacancy clusters starts to occur m the second layer. Similarly the evolution of vacancy clus-
ters will p r o c e e d to d e e p e r layers once a W B A a r o u n d a single vacancy is emitted. T h u s the s p u t t e r i n g of the bulk of a solid can b e induced. T h e sputtering described above can eliminate all c o n s t i t u e n t atoms m the surface layers b u t originates from h e t e r o g e n e o u s emission of W B A s a r o u n d vacancies. A M o n t e Carlo calculation has b e e n m a d e to simulate this vacancy-initiated s p u t t e r i n g [32] This calculation takes into account t h a t the b o n d b r e a k i n g of W B A s itself IS a s u p e r l i n e a r function. T h e o b t a i n e d yield vs fluence curve follows a power function with a p o w e r of a b o u t 10. T h e e x p e r i m e n t a l results of the y i e l d - f l u c n c e r e l a t i o n o b t a i n e d at a s u b m o n o l a y e r sensltivlty m d l c a t e a power d e p e n d e n c e of 15 [14]. In v~ew of the a s s u m p t i o n s e m p l o y e d m the calculation, we may conclude t h a t the M o n t e Carlo calculation can simulate the laser-induced sputtermg. So far we have dealt with m o n a t o m l c sohds W e p r e s u m e t h a t the results are substantially s~milar for multi-atomic sohds. Taking G a A s as an example, we consider first t h a t s p u t t e r i n g is initiated by an As vacancy. A G a atom can be r e g a r d e d as the W B A a r o u n d an As vacancy. T h u s a G a atom a r o u n d an As vacancy ~s emitted, forming a G a - A s dlvacancy. Similarly, an As a t o m a r o u n d a G a atom ~s e m i t t e d forming a G a - A s dlvacancy. T h e ablation t h r e s h o l d is the smaller of t h e t h r e s h o l d s for emission of a G a atom a r o u n d an As vacancy a n d of an As atom a r o u n d a G a vacancy. W h a t follows after f o r m a t i o n of G a - A s dlvacanc~es ~s first t h e emission of a G a a t o m or an As atom a r o u n d the divacancy. As we discussed before, it ~s logical to p r e s u m e t h a t the b o n d i n g s t r e n g t h s of atoms a r o u n d a dwacancy are not larger t h a n that of the respective atoms a r o u n d a vacancy of the opposite atoms. A f t e r emission of e~ther a G a or As a t o m a r o u n d a dlvacancy, an emission of a c o m p l e m e n t a r y atom follows. T h u s the vacancy cluster is c o n s i d e r e d to grow while the stolchlometry is m a i n t a i n e d , at least in the mitml stage of vacancy evolution. As the vacancy clusters grow, coagulation of metals at the b o u n d a r y of clusters may be i n d u c e d a n d the sto~chlometry may not necessarily be m a i n t a i n e d W e note f u r t h e r t h a t the m o d e l described above does not assume any critical process t h a t induces laser ablation, yet the strong fluence d e p e n d e n c e gives rise to a p p a r e n t t h r e s h o l d laser fluence. T h e strong fluencc d e p e n d e n c e originates from the s u p e r h n e a r d e p e n d e n c e of e m i s s m n yield of W B A s on the fluence and from t h e increase of W B A s d u r i n g laser ~rradiahon
5. Summary T h e vacancy-lmtlated sputtering described m this p a p e r can explain several features of laser sputtering. First of all, t h e strong laser fluence d e p e n d e n c e can b e
N Itoh et aL / Laser-mduced sputtermg of compound semiconductors 4
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described. Secondly, the laser ablation threshold fluence has been shown to be strongly d e p e n d e n t upon the lonicity of solids or the bond strength, as shown in fig. 2 It is presumed that the strength of the bond of the W B A s around a vacancy in various sohds is strongly correlated to the lonicity Thirdly, the vacancy-initiated model is in accordance with the experimental result (fig. 6 [14]) that the yield of Ga emissions from GaP, detected by sensitwities of 10 -5 monolayer, increases by repeating irradiation with laser pulses above the ablation threshold, while it decreases below the threshold as shown in fig. 2. The present model is based on the bond breaking due to cascade excitation of defect sites. The model is completely different from that suggested by Ichige et al. [10]. According to these authors, a phase change on the surface layer is mduced under dense electronic excitation, leading to sputtering. T h e vacancyoriginated model is based on the suggestion made by Hattori et al. [14] that an e l e c t r o n - h o l e pair is locallzed on a vacancy and weakens the bond of a WBA, producing a metastable state, which can be multiply excited leading to the emission of a weakly bonded atom. In this model, it is assumed that the excitation energy is localized on a defect site and excited further. The phase change model assumes that the lattice in the surface layer becomes deformable when the density of e l e c t r o n - h o l e pairs becomes sufficiently high. Evidently the degree of the lattice weakening on a specific point as much higher when an e l e c t r o n - h o l e pair is "'localized" than when the density is "sufficiently high". F u r t h e r m o r e the bond breaking of adatoms by cascade excitation has been proved for G a P [14], G a A s [22] and
315
Si [23]. Thus we consider that the vacancy-originated model is more plausible. Further experiments are needed to prove the vacancy-initiated laser-induced sputtering. O n e of the crucial experiments will be to observe using STM whether vacancy clusters are indeed formed above the ablation threshold. The laser ablation dealt with in this paper is of electronic nature: The ablation threshold is determined in terms of the electronic process, while the ablation often is referred to as plasma formation or massive ejection of particles. We note, however, that once the "electronic" ablation threshold, namely the condition of breaking the bonds of W B A s surrounding a vacancy, is satisfied, the surface morphology is changed rapidly during a laser pulse, increasing the absorption coefficient for laser beams and also the probability of laser-induced bond breaking. Thus It is very conceivable that the "electronic" ablation threshold dealt with in the present paper is substantially the same as that detected by massive ejection of atoms or by macroscopic surface damage. The concept of the vacancy-originated laser-induced sputtering suggests that the ablation threshold becomes much higher if the surface is perfect, or free from defects. It is of interest to investigate the correlation between the vacancy concentration and the emission yield slightly above the ablation threshold. The number of vacancies may be reduced by careful treatment of the surface after deposition of small amounts of component atoms. It has been suggested that a combination of filhng vacancies with component atoms and ejection of adatoms and steps by laser irradiation can produce defect-free surfaces [33]. Further works on the control of defects on surfaces and its influence on laser-induced emission and ablation are of interest.
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