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Laser induced ablation and epitaxial growth of SnSe
126
Thin Solid Films, 241 (1994) 126 128
Laser induced ablation and epitaxial growth of SnSe R . T e g h i l a, A . G i a r d i n i - G u i d o n i b, A. M e l e b, S. P i c c i r i l l o b, G . P i z z e l l a b a n d V. M a r o t t a ° aDipartimento di Chimica, Universitft della Basilicata, Via N. Sauro 85, Potenza (Italy) bDipartimento di Chimica, Universitgt "La Sapienza", P. le Aldo Moro 5, Rome (Italy) ~lstituto Materiali Speciali CNR, Tito, Potenza (Italy)
Abstract The ablation technique was applied to thin film deposition from SnSe powders. A mechanism for epitaxial growth as characterized by X-rays and scanning electron microscope analysis is suggested. Plume analysis of the ablated material is also reported.
1. Introduction Laser ablation methods have been successfully applied to thin film deposition of semiconductors, superconductors and refractory materials [1]. This technique is particularly suitable for fabricating layers consisting of one or more regions of various substrates with a desired profile. It is a very simple method which can be extended to various material systems without causing any damage to the substrate-film interface. The ejected material obtained by laser evaporation of a solid target can be analyzed by several techniques. Emission analysis of the plume at the solid-gas interface [2] and mass spectrometry of the ions produced are reported [3]. The composition and distribution of neutral and ionic particles emitted from targets of polycrystalline powders of mixtures of VI group elements and various metals have been determined by these two methods. This paper reports the epitaxial growth of SnSe on several substrates, as obtained by laser-assisted ablation. Thin films of various thicknesses were characterized in terms of surface morphology and optical properties, and using X-ray diffraction analysis. Several experiments were carried out to study the effects of laser power, deposition rate, and geometry between the substrate and target on the quality of the deposits obtained.
(,~ = 532 nm) or by an excimer K r F laser (2 = 248 nm). A multiport vacuum chamber equipped with holders for the target and the heated substrate was used.
Thin film deposition. Laser evaporation and deposition of samples of SnSe were carried out in the vacuum chamber with a residual pressure of less than 10 -3 Pa. SrTiO3(100), MgO(100) and glass were used as substrates, placed at a distance of 2.5 cm from the target. The substrate size was 0.5 x 0.5 cm 2 and the laser spot was between 0.3 and 1 mm in diameter. The films were deposited at substrate temperature of 293 K and 423 K. X-ray and scanning electron microscope analysis. The X-ray diffraction patterns were recorded with a Siemens X-ray diffractometer using Cu K s radiation. A Cambridge scanning electron microscope (SEM, 70/~ resolution) coupled to EDS microanalysis L I N K was used for structural and composition characterization. Plume analysis. Measurements of space and time dependent emission spectra of the plume were obtained by an optical multichannel analyzer OMA III equipped with a 30 cm monochromator. The system was operated either with a 15 ms exposure time or gated by a pulse generator with a typical gate width of 100 ns.
2. Experimental details
3. Results and discussion
The tin selenide, SnSe, employed in this work was a highly pure 99.99% commercial product. Pellets of 1.3 cm diameter were prepared and used directly for analysis of the laser-induced plume and for thin film deposition. The SnSe specimens under rotation were irradiated with a frequency doubled N d - Y A G laser
The X-ray diffraction spectrum of an SnSe film obtained by laser N d - Y A G ablation of a polycrystalline pellet of SnSe and deposited onto SrTiO3 at 423 K is shown in Fig. 1. The diffraction diagramm indicates preferential growth with the (100) crystallographic plane of the orthorombic phase. X-ray spectra of the
0040-6090/94/$7.00 SSDI 0040-6090(93)03727-3
(~'~ 1994
Elsevier Sequoia. All rights reserved
R. Teghil et al. / Ablation of SnSe
127
400
%
200
I
a
20
800
.11
A
40
6"0 28
Fig. 1. X-ray diffraction pattern of SnSe thin film on SrTiO3(100) as obtained by laser ablation: * SrTiO3 peaks.
films deposited onto MgO are quite similar. Thin films on glass substrates are not crystalline, as determined by X-ray analysis. Films grown at room temperature were strongly oriented with the (011) plane parallel to the substrate surface. The effect of substrate temperature on the thin film crystallization of SnSe was verified for thermally vacuum deposited thin films on glass substrates. At high substrate temperature the intensities of (100) reflections increase whereas the others diminish leading to an oriented film [4]. Under our experimental conditions the results obtained for growth on (100) at 423 K are the same as those obtained by vacuum deposition at the same temperature. The results are different at r o o m temperature. Deposition by thermal vacuum evaporation leads to randomly oriented films. The X-ray diffraction patterns are very close to those of the starting material. In contrast, laser deposited films are strongly oriented on the (011) crystallographic plane. This could be due to the fact that in vacuum evaporation the temperature of the substrate plays a larger role in the rearrangement of the film. In pulsed laser deposition the high energy of the ejected particles [5] is sufficient to allow rearrangements even at lower temperatures. A typical SEM picture of SnSe deposited onto MgO is shown in Fig. 2. The film appears to have a fine smooth structure. The presence of a few grains with sizes ranging from 1 to 10 ~m can be noticed. The composition measured at different positions is quite homogeneous and the result was 1:1 SnSe, the same as the SnSe target. The thickness of the film measured at the edge provides a rough estimate of the deposition rate of about 0.35 ~tm min -~. Figure 3 shows a SEM picture of the film edge. It is interesting to note a structure perpendicular to the substrate plane. This texture could be an indication of the growth process of the particles or constituent parts
Fig. 2. SEM picture of an SnSe film surface.
Fig. 3. SEM picture of an SnSe film edge.
of the film. It shows the steps or stages of formation which starts from single atoms arriving at the substrate surface, through nucleation and coalescence and ending in a continuous crystalline layer. The optical spectra of the laser-produced plume from SnSe at a distance of 5 m m from the target show atom and ion emission peaks together with a large band. Several strong lines which are attributed to Sn ÷ ions are present. Other peaks of lower intensities were identified as excited Se + ions. The large band which peaks around 533 nm is assigned to the SnSe A - - X I E system [6]. It was found that the trend of the intensity of the tin ion emission line at 380.08 nm as a function of laser energy is linear. It is worth mentioning that the onset of emission at 380.34 nm and 393.36 nm as measured from excited Sn ÷ and Se ÷ ions occurs within 100 ns after the laser pulse. The time resolved spectra also show, at about 1300 ns, a delayed emission. These two time dependent emissions have already been observed and attributed to laser and collisional excitation of ablated species respectively [7]. These results could be explained alternatively by the fact that prompt and delayed emissions are both due to laser ejected
128
R. Teghil et al. / Ablation of SnSe
species which are formed by two different mechanisms of ablation.
4. Conclusions The preliminary results reported in this paper indicate that the technique of pulsed lasser ablation for depositing thin films of semiconductors has several advantages which can be summarized as follows: (1) the simplicity of the experimental set-up; (2) the technique is well suited to the growth of refractory semiconductor films [8]; (3) the preparation of films should not contain heteroatom impurities; (4) deposition of thin films with accurately predetermined thickness by control of the deposition rate; (5) laser ablation may be more effective for the orientation of the film. The results of the present investigation may be improved by fabricating film with better adhesion to the substrate. Higher temperature deposition seems to provide films which are more adherent. A more exhaustive study of the ablation and deposition parameters is in progress on this and other semiconductor materials.
Acknowledgments The authors wish to thank Dr. Daniela Ferro for SEM pictures and useful discussion. This work has been supported financially by CNR Progetto Finalizzato Chimica Fine.
References 1 F. Beech and I. W. Boyd, Photochemical Processing of Electronic Materials, Academic Press, New York 1992, p. 387. 2 A. Giardini-Guidoni, A. Morone, M. Snels, E. Desimoni, A. M. Salvi, R. Fantoni, W. C. Berdem and M. Giorgi, Appl. Surf Sci., 46 (1990) 321. 3 A. Mele, A. Giardini-Guidoni, G. Pizzella, R. Teghil and S. Piccirillo, in P. Jena et al. (eds.), Physics and Chemistry of Finite Systems: From Clusters to Crystals, Vol. II, Kluwer, Dordrecht, 1992, p. 1109. 4 T. Subba Rao, B. K. Samantaray and A. K. Chaudhuri, Z. Krystallogr., 175 (1986) 37. 5 J. Dielman, E. van de Riet and J. S. C. Kools, Jpn. J. Appl. Phys., 31 (1992) 1964. 6 R. W. B. Pearse and A. G. Gaydon, The Identification of Molecular Spectra, Chapman and Hall, London, 1976. 7 J. P. Zheng, Z. Q. Huang, D. T. Shaw and H. S. Kwok, Appl. Phys. Lett., 54 (1989) 280. 8 K. Seki, X. Xu, H. Okabe, J. M. Frye and J. B. Halpern, Appl. Phys. Left., 60(18) (1992) 2234.
Thin Solid Films, 241 (1994) 126 128
Laser induced ablation and epitaxial growth of SnSe R . T e g h i l a, A . G i a r d i n i - G u i d o n i b, A. M e l e b, S. P i c c i r i l l o b, G . P i z z e l l a b a n d V. M a r o t t a ° aDipartimento di Chimica, Universitft della Basilicata, Via N. Sauro 85, Potenza (Italy) bDipartimento di Chimica, Universitgt "La Sapienza", P. le Aldo Moro 5, Rome (Italy) ~lstituto Materiali Speciali CNR, Tito, Potenza (Italy)
Abstract The ablation technique was applied to thin film deposition from SnSe powders. A mechanism for epitaxial growth as characterized by X-rays and scanning electron microscope analysis is suggested. Plume analysis of the ablated material is also reported.
1. Introduction Laser ablation methods have been successfully applied to thin film deposition of semiconductors, superconductors and refractory materials [1]. This technique is particularly suitable for fabricating layers consisting of one or more regions of various substrates with a desired profile. It is a very simple method which can be extended to various material systems without causing any damage to the substrate-film interface. The ejected material obtained by laser evaporation of a solid target can be analyzed by several techniques. Emission analysis of the plume at the solid-gas interface [2] and mass spectrometry of the ions produced are reported [3]. The composition and distribution of neutral and ionic particles emitted from targets of polycrystalline powders of mixtures of VI group elements and various metals have been determined by these two methods. This paper reports the epitaxial growth of SnSe on several substrates, as obtained by laser-assisted ablation. Thin films of various thicknesses were characterized in terms of surface morphology and optical properties, and using X-ray diffraction analysis. Several experiments were carried out to study the effects of laser power, deposition rate, and geometry between the substrate and target on the quality of the deposits obtained.
(,~ = 532 nm) or by an excimer K r F laser (2 = 248 nm). A multiport vacuum chamber equipped with holders for the target and the heated substrate was used.
Thin film deposition. Laser evaporation and deposition of samples of SnSe were carried out in the vacuum chamber with a residual pressure of less than 10 -3 Pa. SrTiO3(100), MgO(100) and glass were used as substrates, placed at a distance of 2.5 cm from the target. The substrate size was 0.5 x 0.5 cm 2 and the laser spot was between 0.3 and 1 mm in diameter. The films were deposited at substrate temperature of 293 K and 423 K. X-ray and scanning electron microscope analysis. The X-ray diffraction patterns were recorded with a Siemens X-ray diffractometer using Cu K s radiation. A Cambridge scanning electron microscope (SEM, 70/~ resolution) coupled to EDS microanalysis L I N K was used for structural and composition characterization. Plume analysis. Measurements of space and time dependent emission spectra of the plume were obtained by an optical multichannel analyzer OMA III equipped with a 30 cm monochromator. The system was operated either with a 15 ms exposure time or gated by a pulse generator with a typical gate width of 100 ns.
2. Experimental details
3. Results and discussion
The tin selenide, SnSe, employed in this work was a highly pure 99.99% commercial product. Pellets of 1.3 cm diameter were prepared and used directly for analysis of the laser-induced plume and for thin film deposition. The SnSe specimens under rotation were irradiated with a frequency doubled N d - Y A G laser
The X-ray diffraction spectrum of an SnSe film obtained by laser N d - Y A G ablation of a polycrystalline pellet of SnSe and deposited onto SrTiO3 at 423 K is shown in Fig. 1. The diffraction diagramm indicates preferential growth with the (100) crystallographic plane of the orthorombic phase. X-ray spectra of the
0040-6090/94/$7.00 SSDI 0040-6090(93)03727-3
(~'~ 1994
Elsevier Sequoia. All rights reserved
R. Teghil et al. / Ablation of SnSe
127
400
%
200
I
a
20
800
.11
A
40
6"0 28
Fig. 1. X-ray diffraction pattern of SnSe thin film on SrTiO3(100) as obtained by laser ablation: * SrTiO3 peaks.
films deposited onto MgO are quite similar. Thin films on glass substrates are not crystalline, as determined by X-ray analysis. Films grown at room temperature were strongly oriented with the (011) plane parallel to the substrate surface. The effect of substrate temperature on the thin film crystallization of SnSe was verified for thermally vacuum deposited thin films on glass substrates. At high substrate temperature the intensities of (100) reflections increase whereas the others diminish leading to an oriented film [4]. Under our experimental conditions the results obtained for growth on (100) at 423 K are the same as those obtained by vacuum deposition at the same temperature. The results are different at r o o m temperature. Deposition by thermal vacuum evaporation leads to randomly oriented films. The X-ray diffraction patterns are very close to those of the starting material. In contrast, laser deposited films are strongly oriented on the (011) crystallographic plane. This could be due to the fact that in vacuum evaporation the temperature of the substrate plays a larger role in the rearrangement of the film. In pulsed laser deposition the high energy of the ejected particles [5] is sufficient to allow rearrangements even at lower temperatures. A typical SEM picture of SnSe deposited onto MgO is shown in Fig. 2. The film appears to have a fine smooth structure. The presence of a few grains with sizes ranging from 1 to 10 ~m can be noticed. The composition measured at different positions is quite homogeneous and the result was 1:1 SnSe, the same as the SnSe target. The thickness of the film measured at the edge provides a rough estimate of the deposition rate of about 0.35 ~tm min -~. Figure 3 shows a SEM picture of the film edge. It is interesting to note a structure perpendicular to the substrate plane. This texture could be an indication of the growth process of the particles or constituent parts
Fig. 2. SEM picture of an SnSe film surface.
Fig. 3. SEM picture of an SnSe film edge.
of the film. It shows the steps or stages of formation which starts from single atoms arriving at the substrate surface, through nucleation and coalescence and ending in a continuous crystalline layer. The optical spectra of the laser-produced plume from SnSe at a distance of 5 m m from the target show atom and ion emission peaks together with a large band. Several strong lines which are attributed to Sn ÷ ions are present. Other peaks of lower intensities were identified as excited Se + ions. The large band which peaks around 533 nm is assigned to the SnSe A - - X I E system [6]. It was found that the trend of the intensity of the tin ion emission line at 380.08 nm as a function of laser energy is linear. It is worth mentioning that the onset of emission at 380.34 nm and 393.36 nm as measured from excited Sn ÷ and Se ÷ ions occurs within 100 ns after the laser pulse. The time resolved spectra also show, at about 1300 ns, a delayed emission. These two time dependent emissions have already been observed and attributed to laser and collisional excitation of ablated species respectively [7]. These results could be explained alternatively by the fact that prompt and delayed emissions are both due to laser ejected
128
R. Teghil et al. / Ablation of SnSe
species which are formed by two different mechanisms of ablation.
4. Conclusions The preliminary results reported in this paper indicate that the technique of pulsed lasser ablation for depositing thin films of semiconductors has several advantages which can be summarized as follows: (1) the simplicity of the experimental set-up; (2) the technique is well suited to the growth of refractory semiconductor films [8]; (3) the preparation of films should not contain heteroatom impurities; (4) deposition of thin films with accurately predetermined thickness by control of the deposition rate; (5) laser ablation may be more effective for the orientation of the film. The results of the present investigation may be improved by fabricating film with better adhesion to the substrate. Higher temperature deposition seems to provide films which are more adherent. A more exhaustive study of the ablation and deposition parameters is in progress on this and other semiconductor materials.
Acknowledgments The authors wish to thank Dr. Daniela Ferro for SEM pictures and useful discussion. This work has been supported financially by CNR Progetto Finalizzato Chimica Fine.
References 1 F. Beech and I. W. Boyd, Photochemical Processing of Electronic Materials, Academic Press, New York 1992, p. 387. 2 A. Giardini-Guidoni, A. Morone, M. Snels, E. Desimoni, A. M. Salvi, R. Fantoni, W. C. Berdem and M. Giorgi, Appl. Surf Sci., 46 (1990) 321. 3 A. Mele, A. Giardini-Guidoni, G. Pizzella, R. Teghil and S. Piccirillo, in P. Jena et al. (eds.), Physics and Chemistry of Finite Systems: From Clusters to Crystals, Vol. II, Kluwer, Dordrecht, 1992, p. 1109. 4 T. Subba Rao, B. K. Samantaray and A. K. Chaudhuri, Z. Krystallogr., 175 (1986) 37. 5 J. Dielman, E. van de Riet and J. S. C. Kools, Jpn. J. Appl. Phys., 31 (1992) 1964. 6 R. W. B. Pearse and A. G. Gaydon, The Identification of Molecular Spectra, Chapman and Hall, London, 1976. 7 J. P. Zheng, Z. Q. Huang, D. T. Shaw and H. S. Kwok, Appl. Phys. Lett., 54 (1989) 280. 8 K. Seki, X. Xu, H. Okabe, J. M. Frye and J. B. Halpern, Appl. Phys. Left., 60(18) (1992) 2234.