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Surface morphology and structure modification of silicon layers induced by nanosecond laser radiation
applied surface science ELSEVIER
Applied Surface Science 86 (1995) 353-358
Surface morphology and structure modification of silicon layers induced by nanosecond laser radiation A.V. Demchuk *, V.A. Labunov Minsk Radioengineering lnstil~ae, P. Brouka 6, Minsk 220600, Belarus Received 27 May 1994; accepted for publication 20 September 1994
Abstract New kinds of surface structures were observed during nanosecond pulsed-laser recrystallization of silicon layers. We observed, depending on the crystalline structure of the surface (amorphous or polycrystalline), after the local melting of the surface the formation of dome-shaped regions, and a dendritic surface structure at epitaxial recrystallization of the silicon layer. Also after the etching, we revealed circular surface structures on the boundary of the transition from fine-grained to large-grained recrystallized structures connected with the diffraction on the generated optical surface heterogeneities.
1. Introduction
2. Experiment
The use of laser radiation as high-energy source for solid-state surface processing gives great opportunities for modifying surface properties. The potential of this method has not been fully investigated to date. At present a great many investig~itions have been carried out on ion-implanted silicon with different time ranges of exposure of laser radiation [1,2]. During nanosecond (and shorter) pulsed-laser recrystallization the initial structure of the irradiated surface (amorphous, poly- or mono-crystalline) can significantly affect this process. In this paper we present experimental results of the surface morphology and structure modification of silicon layers depending on the initial crystalline structure of the surface and report new kinds of surface structure formation in this process.
The investigations are performed on polycrystalline silicon (poly-Si) layers of 0.45 /xm thickness obtained by LPCVD (Low Pressure Chemical Vapour Deposition) and CVD methods on (111) and (100) monosilicon wafers. The layers obtained by CVD methods were doped by P+ up to a dose of 102o c m -3 during their deposition. One part of the samples was implanted with 100 keV P+ to a dose of 2 × 1015 cm -a and the near-surface layer on the order of 0.1 /.cm of these samples was fully amorphized. The samples were exposed to 50-ns single-pulse radiation at fundamental (1.06 /xm) and doubled (0:53/xm) frequencies of a Nd-glass laser with the spot diameter equal to 5 mm and a spatial uniformity better than 90% (without hot spots) with fluences of 0.1-5 J / c m 2 controlled to an accuracy of _+5%. The treatment was carried out in air at room temperature in the condition of normal incidence of laser radiation on the surface.
* Corresponding author. Fax: +7-0172-310914.
0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD] 01 6 9 - 4 3 3 2 ( 9 4 ) 0 0 3 8 3 - 1
354
A. V. Demchuk, V.A. Labunov /Applied Surface Science 86 (1995) 353-358
The morphology and structure of the surface were studied by optical microscopy, Pt-carbon replica electron microscopy and electronography. The investigation of the dynamics of the liquid-phase development was performed by time-resolved measurement of the reflectivity of a probe laser.
and the sizes of these regions. When the surface is fully molten, a •e-grained structure is formed with the size of grains on the order of 0.05/xm (Fig. lc). With further increase in fluence the melting front penetrates deeper and deeper into the silicon layer, when it reaches the substrate a large-grained structure is formed with the size of grains on the order of 0.5/zm (Fig. id). During the treatment of non-amorphized poly-Si layers with fluences within the range of local melting the dome-shaped melt regions are not formed. At the same time, we observed the formation of dendritic structures in the regime of epitaxial crystallization (Fig. 2). During the laser irradiation of these samples at ~--1.06 /xm epitaxial crystallization occurs for fluences between 2.7 and 5 J/cm ~. The formation of the dendritic structures is observed for fluences between 2.7 and 3.2 J/cm a irrespective of the orienta-
3. Results
For fluences above the melting threshold of the surface, where partial (local) melting takes place [3-5], some separate dome-shaped regions with sizes on the order of 0.1 /zm appeared on the amorphized surface samples (Figs. la and lb). With increasing fluences within the range of local melting (0.4-0.9 J/cm ~ at h = 1.06 /zm and 0.2-0.5 J / c m 2 at 3. = 0.53 /,m) we observed an increase in the number
(a)
(c)
(b)
lp, m
(d)
Fig. 1. Electron micrographs of the surface replicas of a poly-Si layer with amorphized surface before (a) and after recrystallization by pulsed (z i = 50 ns) radiation with wavelength h = 1.06 /xm and fluence: 0.5 J/cm 2 (b), 1 J/cm 2 (c) and 1.4 J/crn'- (d).
A.V. Demchuk, V.A. Labunov/Applied Surface Science 86 (1995) 353-358
tion of the monocrystalline substrate and any impurity presence in the poly-Si layer. At first the large dendritic structures are formed (Fig. 2a), and then, with increasing fluence, fine dendritic structures are formed (Fig. 2d). It is necessary to note that in these regimes of laser treatment twins are observed in the electronograms of the recrystallized surface [6]. For fluences between 3.2 and 5 J / c m a the dendrites vanish and an epitaxial layer with a smooth surface is formed. For fluences above 5 J / c m a the irradiated surface was damaged. In the regions of a re-exposure to the laser (at a repeated recrystallization) the dendritic structure observed initially is transformed into a new dendritic structure with regular surface relief (Fig. 2b). With laser irradiation at ,~ -- 0.53/xm the changes of morphology and structure of the surface were similar to the one described above.
355
When the irradiated samples are etched in the structure-sensitive Sirtl solvent, the dendritic structure becomes more pronounced (Figs. 2c and 2d). Also a great number of crystalline structure defects are revealed in the epitaxial recrystallized layer with a smooth surface [% It is significant that a large block-growth of crystal (with block sizes equal to 1 /xm) is revealed on the boundary of the epitaxial recrystallized layer after etching of the samples. Unlike the samples with the amorphized surface, laser epitaxial recrystallization growth of a solid monocrystalline layer is observed and the dendritic structures are not revealed. The investigations of the decorated and etched bevels of the surface show that the growth of the dendritic structures is initiated on the phase boundary, while the melting front penetrates into the substrate. With increasing laser fluences the melting front penetrates deeper and deeper
(a)
20 pm
(b)
25 #m
(c)
~ 2~m 0
(d)
~ 2pro 0
Fig. 2. Dendritic structures on the surface of the epitaxial recrystallized poly-Si layer (with non-amorphized surface) induced by nanosecond pulsed-laser radiation (3. = 1.06 /zm, ~'i = 50 ns, • = 2.8 J / c m 2 (a, c) and 3.2 J / c m 2 (b, d); (b) - in the region of a re-exposure to the laser, (c) and (d) - after etching during 20 s).
356
A.V. Demchuk, V.A. Labunov /Applied Surface Science 86 (1995) 353-358
into the substrate and the dendritic structures disappear. Etching of irradiated samples in structure-sensitive Sirtl solvent, revealed other surface structures which were not observed by optical microscopy before etching. In the regime of local melting linear and circular surface periodic structures (SPS) are revealed with the spacing equal to the wavelength of the incident light as described previously [2,8-10]. The formation of this SPS is connected with the interference of incident and scattered light due to high inhomogeneity of the surface optical properties in the regime of local surface melting [2,8,9]. At the same time, during the etching of the amorphized surface samples a new kind of surface structure (SS) of a circular type is revealed on the boundary of the transition from fine-grained to the largegrained structure (Fig. 3) where the melting front reaches the substrate [11]. The formation of these SS was observed during the laser treatment both at 3. = 1.06 and 0.53 `am [12]. But, at the laser treatment with 3. = 1.06 >m the SS formation of this type is more pronounced. During the laser treatment with 3.~ = 1.06 `am, the radius (rt) of these circular SS was equal to 2 `am and with 3.2 = 0.53 /zm their radius (r 2) was equal to 1.4-1.5 ,am and the following relation is fulfilled:
r~/r2 =
~3.t/3.a = V~" = 1.4.
(a)
16 ~m
4. Discussion The formation of the dome-shaped melt regions may be explained by the spontaneous collection of the melt nuclei in drops during the local melting of the surface [13]. The surface of amorphous silicon is less wetted by its own melt than the crystalline surface. Our estimations, taking into account this surface effect, show that on the amorphous silicon surface the size of the critical melt nucleus (at which the spontaneous collection of the melt nuclei in the drops occurs) is less than that on the crystalline surface and during the local melting of the surface induced by nanosecond pulsed-laser radiation the conditions of the melt nuclei collection in the drops are realized only on the amorphous silicon surface [14]. During millisecond pulsed-laser recrystallization the sizes of liquid nuclei increase and a spontaneous collection of the local molten areas in the drops takes place on the poly-Si surface [15]. With increasing fluence the density of nuclei in the local molten regions grows. At a certain stage of their evolution, there are some solid-phase circular areas (with rather small dimensions) in the molten layer. This surface state may be imagined in the form of a "screening plane" (it is the molten areas where most part of the laser radiation is reflected and the other one is absorbed in a thin shallow region of the melt) in which "circular holes" (it is the solid areas
(b)
10 p.m
Fig. 3. Surface periodic structures (b) revealed by etching for 10 s on the poly-Si layer (with amorphized surface) recrystallized by nanosecond pulsed-laser radiation (3. = 1.06 /zm, r i = 50 ns, • = 1.4 J / c m 2) on the boundary of the transition from fine-grained to large-grained structure; (a) before etching.
A. V. Demchuk, V,A. Labunov/Applied Surface Science 86 (1995) 353-358
with a rather large absorption depth) exist. Probably, under these conditions the circular SS are formed (Fig. 3b). The connection of the parameters of these SS with a wavelength of incident radiation points to the diffraction mechanism of their formation. Thus, in accordance with Fresnel's principle, when the radii of these "holes" are equal to the radii of the m-Fresnel region (rm, when m < 10), Fresnel diffraction on the "circular hole" takes place [16]. The radius of the m-Fresnel region for a planar wave is equal to rm= ~-~
,
where b is the distance from the "screening plane" to the observation plane. It is necessary to note the following typical features of these circular SS. During the etching the hills are formed in the centre of these SS, which corresponds to the maximum of the radiation intensity (the "hole" shows a non-even number of the Fresnel regions) and subsequently, pits are formed due to the reduction of the intensity in this region (the "hole" shows an even number of the Fresnel regions). This confirms the diffraction mechanism of the formation of these SS. The SS formation on the boundary of the transition from the fine-grained to the large-grained structures is connected to the effect of rapid structure change at small changes of the laser fluence in this region. Evidently, at the smaller fluences these SS are not formed in spite of the existence of this local radiation diffraction. The formation of the dendritic structures is connected with the peculiarities of the phase transition induced by nanosecond pulsed-laser radiation in a heterogeneous system such as poly-Si. One of the main peculiarities of this regime of laser treatment is the very high value of the temperature gradient on the melt-crystal phase boundary and consequently the crystal growth velocity reaches a high value (several meters per second) [1,2]. It is necessary to note that during millisecond pulsed-laser treatment these dendritic structures are not formed irrespective of the crystalline structure of the irradiated surface
[151. The formation of the dendritic structures in these conditions is evidence of the disturbance of the crystallization front plane. As is known, this is a
357
reason for the formation of twins during high-velocity crystal growth [17]. The spatial non-uniformity of the crystallization front is caused by the non-uniformity of the melting front of the poly-Si layer due to the increase of the absorption on the grain boundaries [18]. When the melting front penetrates into the monocrystalline substrate, the influence of the initially non-uniform heating field in a poly-Si layer is eliminated step by step, and as a result of it, the dendritic structures are not formed. When an amorphous silicon surface is treated, a lot of radiation is absorbed in this layer (the absorption coefficient in amorphous silicon is higher than that of poly-Si), where the melting process is just initiated. The heating of the underlying poly-Si layer and its melting is generally determined by diffusion processes, which are less sensitive to structural nonuniformities. During millisecond pulsed-laser treatment the temperature is equilibrated along the poly-Si layer due to the heat diffusion and this excludes the formation of such structures.
5. Conclusions
Nanosecond pulsed-laser-radiation-induced surface modification features of silicon layers have been studied depending on the initial crystalline structure of the surface. It has been shown that in the regimes of local melting molten dome-shaped melt regions are being formed on the amorphous surface (unlike on a crystalline surface) as the result of a spontaneous collection of the local molten areas in the drops. After recrystallization of the poly-Si layers induced by nanosecond laser radiation, the circular surface structures are revealed on the boundary of the transition from fine-grained to large-grained recrystallized structures by etching in Sirtl solvent. For the radii of surface structures r 1 and r 2 formed under radiation with wavelength h 1 an~____~, the following relation is fulfilled: r l / r 2 = ] h l / A 2 . The mechanism of surface structure formation is suggested to be connected to the diffraction of laser radiation on the optical surface heterogeneities of the recrystallized layer.
358
A.V. Demchuk, VA. Labunov /Applied Surface Science 86 (1995) 353-358
In near-thresh01d regimes of epitaxial recrystallization of a poly-Si layer with a non-amorphized surface dendritic structures are formed irrespective of any impurity presence, the monosilicon substrate orientation and the radiation wavelength. The formation of these dendritic structures during nanosecond pulsed-laser treatment at a high velocity of crystal growth is explained by spatial non-uniformity of the melting front due to optical non-uniformity of the recrystallized structure.
References [1] I.B. Khaibulin and L.S. Smirnov, Fiz. Tekh. Poluprovodn. 19 (1985) 569 (in Russian). [2] M. von Allmen, Laser-Beam Interactions with Materials (Springer, Berlin, 1987). [3] A.M. Pristrem, A.V. Demchuk and N.I. Danilovich, Soy. Phys.-Tech. Phys. 31 (1986) 717. [4] G.E. Jellison, D.H. Lowndes, D.N. Nashburn and R.F. Wood, Phys. Rev. B 34 (1986) 2407.
[5] S.R. Stiffler, M.O. Thompson and P.S. Peercy, AppI. Phys. Lett. 56 (1980) t025. [6] A.V. Demchuk and V.A. Labunov, Zh. Tekh. Fiz. 59 (1989) t46 (in Russian). [7] A.V. Demchuk and V.A. Labunov, Poverldm. Fiz. Khim. Mekhan. 7 (1991) 80 (in Russian). [8] J.F. Young, J.E. Sipe, J.S. Preston and H.M. Driel, Appl. Phys. Lett. 41 (1982) 261. [9] J.F. Young, J.E. Sipe and H.M. DrM, Phys. Rev. B 30 (1984) 2001. [10] A.V. Demehuk and V.A. Labunov, Izv. RAN Set. Fiz. 56 (1992) 124 (in Russian). [11] A.V. Demchuk and V.A. Labunov, Poverkhn. Fiz. Khim. Mekhan. 5 (1991) 85 (in Russian). [12] A.V. Demchuk and V.A. Labunov, Pis'ma Zh. Tekh. Fiz. 16 (1990) 18 (in Russian). [13] A.V. Demchuk and V.A. Labunov, Zh. Tekh. Fiz. 61 (1991) 141 (in Russian). [14] A.V. Demchuk and V.A. Labunov, Poverkhn. Fiz. Khim. Mekhan. 6 (1991) 78 (in Russian). [15] A.V. Demchuk and V.A. Labunov, Poverkhn. Fiz. Khim. Mekhan. 2 (1991) 103 (in Russian). [16] E. Hecht, Optics (Addison-Wesley, Reading, MA, 1987). [17] R.A. Laudise, The Growth of Single Crystals (Prentice-Hall, Englewood Cliffs, NJ, 1970). [18] F. Catalina, C.N. Afonso, R. Serna and C. Ortiz, Mater. Sci. Eng. B 14 (1992) 37.
Applied Surface Science 86 (1995) 353-358
Surface morphology and structure modification of silicon layers induced by nanosecond laser radiation A.V. Demchuk *, V.A. Labunov Minsk Radioengineering lnstil~ae, P. Brouka 6, Minsk 220600, Belarus Received 27 May 1994; accepted for publication 20 September 1994
Abstract New kinds of surface structures were observed during nanosecond pulsed-laser recrystallization of silicon layers. We observed, depending on the crystalline structure of the surface (amorphous or polycrystalline), after the local melting of the surface the formation of dome-shaped regions, and a dendritic surface structure at epitaxial recrystallization of the silicon layer. Also after the etching, we revealed circular surface structures on the boundary of the transition from fine-grained to large-grained recrystallized structures connected with the diffraction on the generated optical surface heterogeneities.
1. Introduction
2. Experiment
The use of laser radiation as high-energy source for solid-state surface processing gives great opportunities for modifying surface properties. The potential of this method has not been fully investigated to date. At present a great many investig~itions have been carried out on ion-implanted silicon with different time ranges of exposure of laser radiation [1,2]. During nanosecond (and shorter) pulsed-laser recrystallization the initial structure of the irradiated surface (amorphous, poly- or mono-crystalline) can significantly affect this process. In this paper we present experimental results of the surface morphology and structure modification of silicon layers depending on the initial crystalline structure of the surface and report new kinds of surface structure formation in this process.
The investigations are performed on polycrystalline silicon (poly-Si) layers of 0.45 /xm thickness obtained by LPCVD (Low Pressure Chemical Vapour Deposition) and CVD methods on (111) and (100) monosilicon wafers. The layers obtained by CVD methods were doped by P+ up to a dose of 102o c m -3 during their deposition. One part of the samples was implanted with 100 keV P+ to a dose of 2 × 1015 cm -a and the near-surface layer on the order of 0.1 /.cm of these samples was fully amorphized. The samples were exposed to 50-ns single-pulse radiation at fundamental (1.06 /xm) and doubled (0:53/xm) frequencies of a Nd-glass laser with the spot diameter equal to 5 mm and a spatial uniformity better than 90% (without hot spots) with fluences of 0.1-5 J / c m 2 controlled to an accuracy of _+5%. The treatment was carried out in air at room temperature in the condition of normal incidence of laser radiation on the surface.
* Corresponding author. Fax: +7-0172-310914.
0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD] 01 6 9 - 4 3 3 2 ( 9 4 ) 0 0 3 8 3 - 1
354
A. V. Demchuk, V.A. Labunov /Applied Surface Science 86 (1995) 353-358
The morphology and structure of the surface were studied by optical microscopy, Pt-carbon replica electron microscopy and electronography. The investigation of the dynamics of the liquid-phase development was performed by time-resolved measurement of the reflectivity of a probe laser.
and the sizes of these regions. When the surface is fully molten, a •e-grained structure is formed with the size of grains on the order of 0.05/xm (Fig. lc). With further increase in fluence the melting front penetrates deeper and deeper into the silicon layer, when it reaches the substrate a large-grained structure is formed with the size of grains on the order of 0.5/zm (Fig. id). During the treatment of non-amorphized poly-Si layers with fluences within the range of local melting the dome-shaped melt regions are not formed. At the same time, we observed the formation of dendritic structures in the regime of epitaxial crystallization (Fig. 2). During the laser irradiation of these samples at ~--1.06 /xm epitaxial crystallization occurs for fluences between 2.7 and 5 J/cm ~. The formation of the dendritic structures is observed for fluences between 2.7 and 3.2 J/cm a irrespective of the orienta-
3. Results
For fluences above the melting threshold of the surface, where partial (local) melting takes place [3-5], some separate dome-shaped regions with sizes on the order of 0.1 /zm appeared on the amorphized surface samples (Figs. la and lb). With increasing fluences within the range of local melting (0.4-0.9 J/cm ~ at h = 1.06 /zm and 0.2-0.5 J / c m 2 at 3. = 0.53 /,m) we observed an increase in the number
(a)
(c)
(b)
lp, m
(d)
Fig. 1. Electron micrographs of the surface replicas of a poly-Si layer with amorphized surface before (a) and after recrystallization by pulsed (z i = 50 ns) radiation with wavelength h = 1.06 /xm and fluence: 0.5 J/cm 2 (b), 1 J/cm 2 (c) and 1.4 J/crn'- (d).
A.V. Demchuk, V.A. Labunov/Applied Surface Science 86 (1995) 353-358
tion of the monocrystalline substrate and any impurity presence in the poly-Si layer. At first the large dendritic structures are formed (Fig. 2a), and then, with increasing fluence, fine dendritic structures are formed (Fig. 2d). It is necessary to note that in these regimes of laser treatment twins are observed in the electronograms of the recrystallized surface [6]. For fluences between 3.2 and 5 J / c m a the dendrites vanish and an epitaxial layer with a smooth surface is formed. For fluences above 5 J / c m a the irradiated surface was damaged. In the regions of a re-exposure to the laser (at a repeated recrystallization) the dendritic structure observed initially is transformed into a new dendritic structure with regular surface relief (Fig. 2b). With laser irradiation at ,~ -- 0.53/xm the changes of morphology and structure of the surface were similar to the one described above.
355
When the irradiated samples are etched in the structure-sensitive Sirtl solvent, the dendritic structure becomes more pronounced (Figs. 2c and 2d). Also a great number of crystalline structure defects are revealed in the epitaxial recrystallized layer with a smooth surface [% It is significant that a large block-growth of crystal (with block sizes equal to 1 /xm) is revealed on the boundary of the epitaxial recrystallized layer after etching of the samples. Unlike the samples with the amorphized surface, laser epitaxial recrystallization growth of a solid monocrystalline layer is observed and the dendritic structures are not revealed. The investigations of the decorated and etched bevels of the surface show that the growth of the dendritic structures is initiated on the phase boundary, while the melting front penetrates into the substrate. With increasing laser fluences the melting front penetrates deeper and deeper
(a)
20 pm
(b)
25 #m
(c)
~ 2~m 0
(d)
~ 2pro 0
Fig. 2. Dendritic structures on the surface of the epitaxial recrystallized poly-Si layer (with non-amorphized surface) induced by nanosecond pulsed-laser radiation (3. = 1.06 /zm, ~'i = 50 ns, • = 2.8 J / c m 2 (a, c) and 3.2 J / c m 2 (b, d); (b) - in the region of a re-exposure to the laser, (c) and (d) - after etching during 20 s).
356
A.V. Demchuk, V.A. Labunov /Applied Surface Science 86 (1995) 353-358
into the substrate and the dendritic structures disappear. Etching of irradiated samples in structure-sensitive Sirtl solvent, revealed other surface structures which were not observed by optical microscopy before etching. In the regime of local melting linear and circular surface periodic structures (SPS) are revealed with the spacing equal to the wavelength of the incident light as described previously [2,8-10]. The formation of this SPS is connected with the interference of incident and scattered light due to high inhomogeneity of the surface optical properties in the regime of local surface melting [2,8,9]. At the same time, during the etching of the amorphized surface samples a new kind of surface structure (SS) of a circular type is revealed on the boundary of the transition from fine-grained to the largegrained structure (Fig. 3) where the melting front reaches the substrate [11]. The formation of these SS was observed during the laser treatment both at 3. = 1.06 and 0.53 `am [12]. But, at the laser treatment with 3. = 1.06 >m the SS formation of this type is more pronounced. During the laser treatment with 3.~ = 1.06 `am, the radius (rt) of these circular SS was equal to 2 `am and with 3.2 = 0.53 /zm their radius (r 2) was equal to 1.4-1.5 ,am and the following relation is fulfilled:
r~/r2 =
~3.t/3.a = V~" = 1.4.
(a)
16 ~m
4. Discussion The formation of the dome-shaped melt regions may be explained by the spontaneous collection of the melt nuclei in drops during the local melting of the surface [13]. The surface of amorphous silicon is less wetted by its own melt than the crystalline surface. Our estimations, taking into account this surface effect, show that on the amorphous silicon surface the size of the critical melt nucleus (at which the spontaneous collection of the melt nuclei in the drops occurs) is less than that on the crystalline surface and during the local melting of the surface induced by nanosecond pulsed-laser radiation the conditions of the melt nuclei collection in the drops are realized only on the amorphous silicon surface [14]. During millisecond pulsed-laser recrystallization the sizes of liquid nuclei increase and a spontaneous collection of the local molten areas in the drops takes place on the poly-Si surface [15]. With increasing fluence the density of nuclei in the local molten regions grows. At a certain stage of their evolution, there are some solid-phase circular areas (with rather small dimensions) in the molten layer. This surface state may be imagined in the form of a "screening plane" (it is the molten areas where most part of the laser radiation is reflected and the other one is absorbed in a thin shallow region of the melt) in which "circular holes" (it is the solid areas
(b)
10 p.m
Fig. 3. Surface periodic structures (b) revealed by etching for 10 s on the poly-Si layer (with amorphized surface) recrystallized by nanosecond pulsed-laser radiation (3. = 1.06 /zm, r i = 50 ns, • = 1.4 J / c m 2) on the boundary of the transition from fine-grained to large-grained structure; (a) before etching.
A. V. Demchuk, V,A. Labunov/Applied Surface Science 86 (1995) 353-358
with a rather large absorption depth) exist. Probably, under these conditions the circular SS are formed (Fig. 3b). The connection of the parameters of these SS with a wavelength of incident radiation points to the diffraction mechanism of their formation. Thus, in accordance with Fresnel's principle, when the radii of these "holes" are equal to the radii of the m-Fresnel region (rm, when m < 10), Fresnel diffraction on the "circular hole" takes place [16]. The radius of the m-Fresnel region for a planar wave is equal to rm= ~-~
,
where b is the distance from the "screening plane" to the observation plane. It is necessary to note the following typical features of these circular SS. During the etching the hills are formed in the centre of these SS, which corresponds to the maximum of the radiation intensity (the "hole" shows a non-even number of the Fresnel regions) and subsequently, pits are formed due to the reduction of the intensity in this region (the "hole" shows an even number of the Fresnel regions). This confirms the diffraction mechanism of the formation of these SS. The SS formation on the boundary of the transition from the fine-grained to the large-grained structures is connected to the effect of rapid structure change at small changes of the laser fluence in this region. Evidently, at the smaller fluences these SS are not formed in spite of the existence of this local radiation diffraction. The formation of the dendritic structures is connected with the peculiarities of the phase transition induced by nanosecond pulsed-laser radiation in a heterogeneous system such as poly-Si. One of the main peculiarities of this regime of laser treatment is the very high value of the temperature gradient on the melt-crystal phase boundary and consequently the crystal growth velocity reaches a high value (several meters per second) [1,2]. It is necessary to note that during millisecond pulsed-laser treatment these dendritic structures are not formed irrespective of the crystalline structure of the irradiated surface
[151. The formation of the dendritic structures in these conditions is evidence of the disturbance of the crystallization front plane. As is known, this is a
357
reason for the formation of twins during high-velocity crystal growth [17]. The spatial non-uniformity of the crystallization front is caused by the non-uniformity of the melting front of the poly-Si layer due to the increase of the absorption on the grain boundaries [18]. When the melting front penetrates into the monocrystalline substrate, the influence of the initially non-uniform heating field in a poly-Si layer is eliminated step by step, and as a result of it, the dendritic structures are not formed. When an amorphous silicon surface is treated, a lot of radiation is absorbed in this layer (the absorption coefficient in amorphous silicon is higher than that of poly-Si), where the melting process is just initiated. The heating of the underlying poly-Si layer and its melting is generally determined by diffusion processes, which are less sensitive to structural nonuniformities. During millisecond pulsed-laser treatment the temperature is equilibrated along the poly-Si layer due to the heat diffusion and this excludes the formation of such structures.
5. Conclusions
Nanosecond pulsed-laser-radiation-induced surface modification features of silicon layers have been studied depending on the initial crystalline structure of the surface. It has been shown that in the regimes of local melting molten dome-shaped melt regions are being formed on the amorphous surface (unlike on a crystalline surface) as the result of a spontaneous collection of the local molten areas in the drops. After recrystallization of the poly-Si layers induced by nanosecond laser radiation, the circular surface structures are revealed on the boundary of the transition from fine-grained to large-grained recrystallized structures by etching in Sirtl solvent. For the radii of surface structures r 1 and r 2 formed under radiation with wavelength h 1 an~____~, the following relation is fulfilled: r l / r 2 = ] h l / A 2 . The mechanism of surface structure formation is suggested to be connected to the diffraction of laser radiation on the optical surface heterogeneities of the recrystallized layer.
358
A.V. Demchuk, VA. Labunov /Applied Surface Science 86 (1995) 353-358
In near-thresh01d regimes of epitaxial recrystallization of a poly-Si layer with a non-amorphized surface dendritic structures are formed irrespective of any impurity presence, the monosilicon substrate orientation and the radiation wavelength. The formation of these dendritic structures during nanosecond pulsed-laser treatment at a high velocity of crystal growth is explained by spatial non-uniformity of the melting front due to optical non-uniformity of the recrystallized structure.
References [1] I.B. Khaibulin and L.S. Smirnov, Fiz. Tekh. Poluprovodn. 19 (1985) 569 (in Russian). [2] M. von Allmen, Laser-Beam Interactions with Materials (Springer, Berlin, 1987). [3] A.M. Pristrem, A.V. Demchuk and N.I. Danilovich, Soy. Phys.-Tech. Phys. 31 (1986) 717. [4] G.E. Jellison, D.H. Lowndes, D.N. Nashburn and R.F. Wood, Phys. Rev. B 34 (1986) 2407.
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