- Email: [email protected]
Excimer laser induced crystallization and oxidation of amorphous Cr thin films
640
Applied Surface Science36 (1989) 6'10-.647 North-Holland, Amsteldam
E X C | M E R L A S E R l N D U C E D C R Y S T A L L I Z A T I O N AND O X I D A T I O N O F A M O R P H O U S Cr T H I N F I L M S I. U R S U , M.L BIILTEGA, M. D I N E S C U , I.N. M I H A I L E S C U , N. P O P E S C U - P O G R I O N , L. R~BCO Central Institute of Physics, Bucharest, POB MG-6, Romania
A.M. P R O K H O R O V , V.I. K O N O V and V.N. T O K A R E V Institute of General Physics, Moscow, USSR
Received 2 June 1988; accepted for publication 23 July 1988
TEM and TED results are reported on the crystallizationand oxidation of 20 and 30 nm thick amorphous Cr films supported on electron microscopecopper grids when submitted in air to the action of a single excimer laser pulse (~ = 0.308 gin) of 20 ns width (FWHM). The energy incident on the target was varied using calibrated attenuators ensuring, in spots of 0.7 to 0.9 mm diameter, laser intensities ranging from 0.6 × 10s to 1.2 × 106 W cm- 2. Crystallizationphenomena are noticed beginning from 0.6×105 W cm -2. Crystallizationand oxidation of the thin film not involving full deterioration are seen to occur for laser spot intensities ranging from 0.17 × 106 to 0.27 x 106 W cm- 2. The copper grid meshes(~ ~ 120/Lm) which support the chromium thin film, channel the incident laser beam so that at mesh level in the irradiated spot, temperature gradients develop from the center of the mesh towards its borders. Thus at grid mesh level in the irradiated area, a sequenceof microstructurescould be noticed beginning with a-Cr small crystals followed by a-Cr and Cr304 small crystals, Cr304 and o-Cr203 small crystals and finally large a-Cr203 crystals.
1. Introduction We have previously studied [1-5] the crystallization/recrystallization and oxidation of thin amorphous/polycrystalline Cr films, < 80 n m thick supported on electron microscope grids, under the action of low power (a few W) cw CO2 laser irradiation by T E M and T E D techniques. We have shown that depending on the power densities and dwell times, after completion o f tile crystallization/recrystallization process, the oxidation proceeds with the, formation of a spinel-like T-Cr~O3, which is thereafter accompanied and finally replaced by the formation of stable a - C r 2 0 3 crystals. In this work we report T E M and T E D investigations on the crystallization and oxidation ih air o f 20 and 30 n m thick amorphous Cr films supported on eiectron microscope (EM) copper grids under the action of a single laser pulse, 29 ns in width ( F W H M ) , delivered from an excimer laser (;k = 0.308 pm). 0169-4332/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
I. Ursu et al. / Excimer laser induced crystallization of Cr thin films
641
2. Experimental procedure Amorphous Cr thin films, 20 and 30 nm thick were deposite~l onto freshly air-cleaved KC1 substrates using a standard RF, sputtering apparatus. Details on the deposition conditions are given in our previous papers [1-5]. 'l"h.e films were removed from the KC1 substrates by immersion in distilled water and floated off onto EM copper grids. These films are referred to as free standing (FS) samples [6] even though they are supported on the copper grids (i.e. networks of circular holes 120/~m in diameter). Each of these Cr thin films supported on copper grids was exposed in air and at room temperature to a single pulse produced by an excimer laser able to generate energies of - 1 J within a pulse duration of 7 --- 20 ns. The e~:ergy incident on the target was varied using calibrated attenuators placed in the path of the beam. Targets were irradiated at normal incidence with a single Gaussian pulse. The correlation of the irradiation spot dimension and incident energy made it possible to obtain intensities in the range of (0.06-1.2) × 106 W cm -2, high enough to induce modifications in the thin film, but not as high as to cause complete removal of the material. The structural trar~sformations produced on the amorphous Cr thin films as a re~alt of one single z0 ns pulse of the e×ckmer laser were investigated by bright transmission electron microscopy (BTEM) and transmission electron diffraction (TED) performed with the aid of a TEM 200 CX electron microscope working at 200 kV.
3. Results and discussion The TEM and TED data reveal that at the lowest kntensity used in our expegment, namely 0.6 × 105 W cm -2, under the action of one pulse, only a fine crystallization of the amorphous Cr thin films occurs without any deterioration or oxidation of the thin film (fig. lb). Crystallization and oxidation o f the thin film not involving fir.!1 deterioration is seen to occur for laser spot intensities ranging from 0.17 × 106 to 0.27 × 106 W cm -2 (figs. 2a I and 2a:). At the highest laser spot intensity, that is 1.2 × 106 W cm -2, crystallization and oxidation of the thin film are associated with serious deterioration of the film where very little is left in a few meshes in the irradiated zone (figs. 2b I and 2b2). The TEM and TED investigations performed on the amorphous Cr samples irradiated with single pulses at laser spot intensities ranging from 0.17 × -6 to 0.27 × 106 W cm -2, also revealed that the EM copper grids acted as a thermal sink generating thermal gradients between the center and the mesh edge. As a consequence, despite the large area of the irradiation spot ( ~ 5 × l0 s ~m 2) and the urdfornuty of the incident energy, the effects of the single laser pulse ax-e
642
L Ursu et al. / Excimer laser induced crystallization of Cr thin films
Fig. 1. Electron microgtaphs and corresponding eleclr,~n diffraction patterns of the 30 nm thick Cr film (a) before and (b) after laser irradiation (~ = 3.308/tin; 20 ns, 0.6 × 105 W cm-2) a-Cr (CJ~. clearly visible in the corresponding area of the EI~ copper grid mesh ( - 7.8 x 1 0 3 # m z) (fig. 2). As in our previous experiments performed on free-standing a m o r p h o u s 55wt%Cr-45wt%Ni films [7] u n d e r the action of the laser irradi~tio~ at the level of the meshes in the irradiated area, we observed the formation of various
L Ursu et al. / Excimer laser induced crystallization of Cr thin films
643
Fig. 2. Electron micrographs of the 20 nm thick Cr film after laser irradiation (~ = 0.308 ~m, 20 ns) (a) 0.2 × 106 W c m - 2 and (b) 1.2 x 106 W c m - 2
Fig. 3. Electron micrograph and corresponding electron diffraction pattern of the 20 nm thick Cr film aftt.r laser irradiation (h = 0.308 ~am; 20 ns, 0 . 2 x lq 6 W c m - 2 ) . TILe corre;ponding thin film areas are situated very close to the coppel mesh edge: a - C r (~), C r f l ~ (O).
Fig. 4. Electron micrograph ,rod corresponding electron diffraction pa~.em of Me 30 nm thick Cr film after laser irradiation (A = 0.308 pm, 20 ns. ~.2x106 W cm-2). The corresponding thin film areas are situated near to the copper mesh edge: a-Cr (O), Cr304 (O), a-Cr203 (*).
1. Ursu et al. / Excimer laser induced crystallization of Cr thin films
645
structures suggesting that different degrees of heating had been reached in the respective areas. A sequence of typical structures that develop from the b o r d e r towards the center of the copper grid mesh for excimer laser irradiated a m o r p h o u s Cr thin films is presented in the following electron mierographs
Fig. 5. Electren micrographs and corresponding electron diffraction patterns of the 20 nm thick Cr film after laser irradiation (4 = 0.308/.tin, 20 ns, 0.2::<106 W cm-2). The corresponding thin film areas eze situated towards the centel of the copper mesh Cr304 (tD), a-Cr203 ( * ).
646
L Ursu et al. ,/Excimer laser induced crystallization of Cr thin fibrte
Fig. 6. Electron micrograph and corresponding electron diffraction paticcn of the 30 nm thick Cr film after laser irradiation (h = 0.308 #m, 20 ns, 0.2 × 106 W cm -2). The thin film areas are situated by the center of a copper mesh and o.,.,tlae border of a rupture as a rule: a-Cr,O3 ( * ). and corresponding electron diffraction images (figs. 3-6). Electron micrographs and corresponding electron diffraction pattern from a zone situated nearest to the copper mesh edge showed that after one pulse, a cermet-type structure developed which consisted of very small (30-50 nm) a-Cr crystals and Cr304 mic:ocrystals (fig. 3). The electron microscopy and corresponding ~leetron 6iffl'action images (fig. 4) for a zone situated next tc that in fig. 3 show lhat as mentioned earlier, higher temperatures were gradually reached, ~hich is :apparent from an increase in the diffraction line intensity and in the ~-Cr and Cr304 crystallite dimensions, as well as from the emergence of ~-CrEO3 crystallites. The electron microscopy and corresponding electron diffraction images figs. 5 and 6) for the successive zones towards the center of the g,'id exhibit ncreased oxidation and crystallization levels, respectively, indicating that ncreasingly higher temperatures have been reached. Along the same line, we lote the gradual disappearance of the diffraction rings corresponding to less ~xygen-rich oxides Cr304 (fig. 5a as compared to fig. 5b) and an increase in ~-Cr203 crystalli.tes (fig. 5 as compared to fig. 6). The ot-Cr203 crystallites, vhose largest dimensions range from 200 to 800 nm (figs. 2b 2 and 6), are of rregular polygonal forms and a bfigh density of pores (either gas-filled or lollow) is randomly distributed in the grains of along grain boundaries. The )resence of the pores and voids filled with gaseous impurities is yet more
L Ursu et aL / Excimer laser induced crystallization of Cr thin films
647
evidence t h a t high t e m p e r a t u r e s a n d large cooling rates higher t h a n 10 4 K s - i [8] are reached in these zones. tt is w o r t h m e n t i o n i n g that, in a d d i t i o n to the role o f E M c o p p e r grids in c h a n n e l l i n g the incident laser flux a n d in generating t h e r m a l g r a d i e n t s b e t w e e n the edge a n d c e n t e r o f the grid mesh, the h e a t released f r o m the crystallization a n d especially the o x i d a t i o n reaction is also partly respolxsible for the different t e m p e r a t u r e s d e v e l o p i n g in d i f f e r e n t z o n e s [9,10].
4. Conclusions Lateral differences in the crystallization a n d o x i d a t i o n o f 20 a n d 30 n m thick a m o r p h o u s C r films s u p p o r t e d o n E M c o p p e r grids subjec!ed to the action o f a single laser pulse 20 n s in w i d t h ( F W H M ) delivered f r o m a n e x c i m e r !.aser ( h = 0.308 # m ) at p o w e r densities r a n g i n g f r o m 0.6 x l 0 s to 1.2 x 106 W c m -2 in s p o t s ha~fing a d i a m e t e r o f - 0,7 to 0.9 m m are revealed by T E M a n d T E D investigations.
References [1] M.I. Birjega, C.A. Constantin, M. Dinescu, l.Th. Florescu, LN. Mihailescu, L. Nanu, N. Popescu-Pogrion and C. Sarbu, MRS Pro,:. VoL 29, 1984, Laser Controlled Chemical Processing of Surfaces, Eds. A.W. Johnson, D.J. Ehrlich and H.R. Schlossherg, p. 289. [2] M.I. Birjega, I. Zberea and N. Popescu-Pogrion, Rev. Roumaine Phys. 30 (1985) 763. [3] M.1. Birjega, M. Dineseu, I.N. Mihaileseu, L. Nanu, C.A. Constantin, I.Th. Florescu, N. Popescu-Pogrion and C. Sarbu, Phys. SLatus Solidi (a) 95 (1986) 423. [4] M.I. Birjega, C.A. Constantin, M. Dinescu, 1.Th. FIoreseu. LN. Mihailescu, L Nanu, N. Popeseu-Pogrion, C. Sarbu and I. Zberea, Rev. Roumaine Phys. 31 (1986) 177. [5] M.1. Birjega, L. Nanu, I.N. Mihailescu, M. Dinescu, N. Popescu-Pogrion and C. Sarbu, Opt. Acta 33 (1986) 1073. [6] R. Andrew, L. Baufay, A. Pigeolet and L.D. Lande, J. Appl. Phys. 53 (1982) 4862. [7] 1. Ursu, M.I. Birjega, C.A. Constantin, M. Dinescu, I.N, Mihailescu, N. Popescu-Pogrion, I. Ketskemety and E. Szill. MRS Proc. Vol. 74, 1986, Beam Solid Interactions ,and Transient Processes, Eds. M.O. Thompson, s.T. Picraux and J.S. Williams, p. 197. [8] A.M, Chaplanov and E.L Toclutsky. Thin Solid Films 116 (1984) 117. [9] L.D. Laude, M, Wantelet and R. Andrew, Appl. Phys. A 40 (1986) 133. [10] I.W. Boyd, Optically Enhanced Oxidation, NATO ASI "Interfaces under Laser Radiation", Maratea, lialy, J~'ly 1986.
Applied Surface Science36 (1989) 6'10-.647 North-Holland, Amsteldam
E X C | M E R L A S E R l N D U C E D C R Y S T A L L I Z A T I O N AND O X I D A T I O N O F A M O R P H O U S Cr T H I N F I L M S I. U R S U , M.L BIILTEGA, M. D I N E S C U , I.N. M I H A I L E S C U , N. P O P E S C U - P O G R I O N , L. R~BCO Central Institute of Physics, Bucharest, POB MG-6, Romania
A.M. P R O K H O R O V , V.I. K O N O V and V.N. T O K A R E V Institute of General Physics, Moscow, USSR
Received 2 June 1988; accepted for publication 23 July 1988
TEM and TED results are reported on the crystallizationand oxidation of 20 and 30 nm thick amorphous Cr films supported on electron microscopecopper grids when submitted in air to the action of a single excimer laser pulse (~ = 0.308 gin) of 20 ns width (FWHM). The energy incident on the target was varied using calibrated attenuators ensuring, in spots of 0.7 to 0.9 mm diameter, laser intensities ranging from 0.6 × 10s to 1.2 × 106 W cm- 2. Crystallizationphenomena are noticed beginning from 0.6×105 W cm -2. Crystallizationand oxidation of the thin film not involving full deterioration are seen to occur for laser spot intensities ranging from 0.17 × 106 to 0.27 x 106 W cm- 2. The copper grid meshes(~ ~ 120/Lm) which support the chromium thin film, channel the incident laser beam so that at mesh level in the irradiated spot, temperature gradients develop from the center of the mesh towards its borders. Thus at grid mesh level in the irradiated area, a sequenceof microstructurescould be noticed beginning with a-Cr small crystals followed by a-Cr and Cr304 small crystals, Cr304 and o-Cr203 small crystals and finally large a-Cr203 crystals.
1. Introduction We have previously studied [1-5] the crystallization/recrystallization and oxidation of thin amorphous/polycrystalline Cr films, < 80 n m thick supported on electron microscope grids, under the action of low power (a few W) cw CO2 laser irradiation by T E M and T E D techniques. We have shown that depending on the power densities and dwell times, after completion o f tile crystallization/recrystallization process, the oxidation proceeds with the, formation of a spinel-like T-Cr~O3, which is thereafter accompanied and finally replaced by the formation of stable a - C r 2 0 3 crystals. In this work we report T E M and T E D investigations on the crystallization and oxidation ih air o f 20 and 30 n m thick amorphous Cr films supported on eiectron microscope (EM) copper grids under the action of a single laser pulse, 29 ns in width ( F W H M ) , delivered from an excimer laser (;k = 0.308 pm). 0169-4332/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
I. Ursu et al. / Excimer laser induced crystallization of Cr thin films
641
2. Experimental procedure Amorphous Cr thin films, 20 and 30 nm thick were deposite~l onto freshly air-cleaved KC1 substrates using a standard RF, sputtering apparatus. Details on the deposition conditions are given in our previous papers [1-5]. 'l"h.e films were removed from the KC1 substrates by immersion in distilled water and floated off onto EM copper grids. These films are referred to as free standing (FS) samples [6] even though they are supported on the copper grids (i.e. networks of circular holes 120/~m in diameter). Each of these Cr thin films supported on copper grids was exposed in air and at room temperature to a single pulse produced by an excimer laser able to generate energies of - 1 J within a pulse duration of 7 --- 20 ns. The e~:ergy incident on the target was varied using calibrated attenuators placed in the path of the beam. Targets were irradiated at normal incidence with a single Gaussian pulse. The correlation of the irradiation spot dimension and incident energy made it possible to obtain intensities in the range of (0.06-1.2) × 106 W cm -2, high enough to induce modifications in the thin film, but not as high as to cause complete removal of the material. The structural trar~sformations produced on the amorphous Cr thin films as a re~alt of one single z0 ns pulse of the e×ckmer laser were investigated by bright transmission electron microscopy (BTEM) and transmission electron diffraction (TED) performed with the aid of a TEM 200 CX electron microscope working at 200 kV.
3. Results and discussion The TEM and TED data reveal that at the lowest kntensity used in our expegment, namely 0.6 × 105 W cm -2, under the action of one pulse, only a fine crystallization of the amorphous Cr thin films occurs without any deterioration or oxidation of the thin film (fig. lb). Crystallization and oxidation o f the thin film not involving fir.!1 deterioration is seen to occur for laser spot intensities ranging from 0.17 × 106 to 0.27 × 106 W cm -2 (figs. 2a I and 2a:). At the highest laser spot intensity, that is 1.2 × 106 W cm -2, crystallization and oxidation of the thin film are associated with serious deterioration of the film where very little is left in a few meshes in the irradiated zone (figs. 2b I and 2b2). The TEM and TED investigations performed on the amorphous Cr samples irradiated with single pulses at laser spot intensities ranging from 0.17 × -6 to 0.27 × 106 W cm -2, also revealed that the EM copper grids acted as a thermal sink generating thermal gradients between the center and the mesh edge. As a consequence, despite the large area of the irradiation spot ( ~ 5 × l0 s ~m 2) and the urdfornuty of the incident energy, the effects of the single laser pulse ax-e
642
L Ursu et al. / Excimer laser induced crystallization of Cr thin films
Fig. 1. Electron microgtaphs and corresponding eleclr,~n diffraction patterns of the 30 nm thick Cr film (a) before and (b) after laser irradiation (~ = 3.308/tin; 20 ns, 0.6 × 105 W cm-2) a-Cr (CJ~. clearly visible in the corresponding area of the EI~ copper grid mesh ( - 7.8 x 1 0 3 # m z) (fig. 2). As in our previous experiments performed on free-standing a m o r p h o u s 55wt%Cr-45wt%Ni films [7] u n d e r the action of the laser irradi~tio~ at the level of the meshes in the irradiated area, we observed the formation of various
L Ursu et al. / Excimer laser induced crystallization of Cr thin films
643
Fig. 2. Electron micrographs of the 20 nm thick Cr film after laser irradiation (~ = 0.308 ~m, 20 ns) (a) 0.2 × 106 W c m - 2 and (b) 1.2 x 106 W c m - 2
Fig. 3. Electron micrograph and corresponding electron diffraction pattern of the 20 nm thick Cr film aftt.r laser irradiation (h = 0.308 ~am; 20 ns, 0 . 2 x lq 6 W c m - 2 ) . TILe corre;ponding thin film areas are situated very close to the coppel mesh edge: a - C r (~), C r f l ~ (O).
Fig. 4. Electron micrograph ,rod corresponding electron diffraction pa~.em of Me 30 nm thick Cr film after laser irradiation (A = 0.308 pm, 20 ns. ~.2x106 W cm-2). The corresponding thin film areas are situated near to the copper mesh edge: a-Cr (O), Cr304 (O), a-Cr203 (*).
1. Ursu et al. / Excimer laser induced crystallization of Cr thin films
645
structures suggesting that different degrees of heating had been reached in the respective areas. A sequence of typical structures that develop from the b o r d e r towards the center of the copper grid mesh for excimer laser irradiated a m o r p h o u s Cr thin films is presented in the following electron mierographs
Fig. 5. Electren micrographs and corresponding electron diffraction patterns of the 20 nm thick Cr film after laser irradiation (4 = 0.308/.tin, 20 ns, 0.2::<106 W cm-2). The corresponding thin film areas eze situated towards the centel of the copper mesh Cr304 (tD), a-Cr203 ( * ).
646
L Ursu et al. ,/Excimer laser induced crystallization of Cr thin fibrte
Fig. 6. Electron micrograph and corresponding electron diffraction paticcn of the 30 nm thick Cr film after laser irradiation (h = 0.308 #m, 20 ns, 0.2 × 106 W cm -2). The thin film areas are situated by the center of a copper mesh and o.,.,tlae border of a rupture as a rule: a-Cr,O3 ( * ). and corresponding electron diffraction images (figs. 3-6). Electron micrographs and corresponding electron diffraction pattern from a zone situated nearest to the copper mesh edge showed that after one pulse, a cermet-type structure developed which consisted of very small (30-50 nm) a-Cr crystals and Cr304 mic:ocrystals (fig. 3). The electron microscopy and corresponding ~leetron 6iffl'action images (fig. 4) for a zone situated next tc that in fig. 3 show lhat as mentioned earlier, higher temperatures were gradually reached, ~hich is :apparent from an increase in the diffraction line intensity and in the ~-Cr and Cr304 crystallite dimensions, as well as from the emergence of ~-CrEO3 crystallites. The electron microscopy and corresponding electron diffraction images figs. 5 and 6) for the successive zones towards the center of the g,'id exhibit ncreased oxidation and crystallization levels, respectively, indicating that ncreasingly higher temperatures have been reached. Along the same line, we lote the gradual disappearance of the diffraction rings corresponding to less ~xygen-rich oxides Cr304 (fig. 5a as compared to fig. 5b) and an increase in ~-Cr203 crystalli.tes (fig. 5 as compared to fig. 6). The ot-Cr203 crystallites, vhose largest dimensions range from 200 to 800 nm (figs. 2b 2 and 6), are of rregular polygonal forms and a bfigh density of pores (either gas-filled or lollow) is randomly distributed in the grains of along grain boundaries. The )resence of the pores and voids filled with gaseous impurities is yet more
L Ursu et aL / Excimer laser induced crystallization of Cr thin films
647
evidence t h a t high t e m p e r a t u r e s a n d large cooling rates higher t h a n 10 4 K s - i [8] are reached in these zones. tt is w o r t h m e n t i o n i n g that, in a d d i t i o n to the role o f E M c o p p e r grids in c h a n n e l l i n g the incident laser flux a n d in generating t h e r m a l g r a d i e n t s b e t w e e n the edge a n d c e n t e r o f the grid mesh, the h e a t released f r o m the crystallization a n d especially the o x i d a t i o n reaction is also partly respolxsible for the different t e m p e r a t u r e s d e v e l o p i n g in d i f f e r e n t z o n e s [9,10].
4. Conclusions Lateral differences in the crystallization a n d o x i d a t i o n o f 20 a n d 30 n m thick a m o r p h o u s C r films s u p p o r t e d o n E M c o p p e r grids subjec!ed to the action o f a single laser pulse 20 n s in w i d t h ( F W H M ) delivered f r o m a n e x c i m e r !.aser ( h = 0.308 # m ) at p o w e r densities r a n g i n g f r o m 0.6 x l 0 s to 1.2 x 106 W c m -2 in s p o t s ha~fing a d i a m e t e r o f - 0,7 to 0.9 m m are revealed by T E M a n d T E D investigations.
References [1] M.I. Birjega, C.A. Constantin, M. Dinescu, l.Th. Florescu, LN. Mihailescu, L. Nanu, N. Popescu-Pogrion and C. Sarbu, MRS Pro,:. VoL 29, 1984, Laser Controlled Chemical Processing of Surfaces, Eds. A.W. Johnson, D.J. Ehrlich and H.R. Schlossherg, p. 289. [2] M.I. Birjega, I. Zberea and N. Popescu-Pogrion, Rev. Roumaine Phys. 30 (1985) 763. [3] M.1. Birjega, M. Dineseu, I.N. Mihaileseu, L. Nanu, C.A. Constantin, I.Th. Florescu, N. Popescu-Pogrion and C. Sarbu, Phys. SLatus Solidi (a) 95 (1986) 423. [4] M.I. Birjega, C.A. Constantin, M. Dinescu, 1.Th. FIoreseu. LN. Mihailescu, L Nanu, N. Popeseu-Pogrion, C. Sarbu and I. Zberea, Rev. Roumaine Phys. 31 (1986) 177. [5] M.1. Birjega, L. Nanu, I.N. Mihailescu, M. Dinescu, N. Popescu-Pogrion and C. Sarbu, Opt. Acta 33 (1986) 1073. [6] R. Andrew, L. Baufay, A. Pigeolet and L.D. Lande, J. Appl. Phys. 53 (1982) 4862. [7] 1. Ursu, M.I. Birjega, C.A. Constantin, M. Dinescu, I.N, Mihailescu, N. Popescu-Pogrion, I. Ketskemety and E. Szill. MRS Proc. Vol. 74, 1986, Beam Solid Interactions ,and Transient Processes, Eds. M.O. Thompson, s.T. Picraux and J.S. Williams, p. 197. [8] A.M, Chaplanov and E.L Toclutsky. Thin Solid Films 116 (1984) 117. [9] L.D. Laude, M, Wantelet and R. Andrew, Appl. Phys. A 40 (1986) 133. [10] I.W. Boyd, Optically Enhanced Oxidation, NATO ASI "Interfaces under Laser Radiation", Maratea, lialy, J~'ly 1986.