- Email: [email protected]
A method to grow silicon crystallites on glass
Journal of Crystal Growth 198/199 (1999) 420—424
A method to grow silicon crystallites on glass Torsten Boeck*, Thomas Teubner, Klaus Schmidt, Peter-Michael Wilde Institute of Crystal Growth, Rudower Chaussee 6, D-12489 Berlin, Germany
Abstract In our laboratory a method has been developed for low temperature growth of silicon on glass from metallic solutions [1]. This technique is based on creating pointlike nucleation centres using natural coalescence phenomena of the metallic solvent for masking the substrate. Thus, uncontrolled spontaneous nucleation can be avoided and locally defined selective growth of silicon crystallites seeded by the Si saturated metallic solution droplets occurs. The nucleation of silicon on the substrate surface and the growth of crystallites must be initiated and maintained by a steady temperature gradient. The material transport is governed by a vapour—liquid—solid (VLS) mechanism. As a first result of this artificial nucleus selection principle silicon crystallites have been grown in dimensions of 10 lm. Size and distribution of the solvent droplets as well as the morphology of the grown silicon crystallites have been characterized by SEM and optical microscopy. The crystallites show good adhesion on glass and are sufficiently regular arranged. The focused ion beam (FIB) method followed by X-ray microanalysis has been used to identify silicon crystallites still encapsulated by the solution. 1999 Elsevier Science B.V. All rights reserved. PACS: 68.55.!a; 81.15.!z; 81.15.Jj; 81.15.Lm; 81.30.Mh; 81.65.Cf Keywords: Solution growth; Silicon on glass; Solar cells; Artificial nucleation; Solvent droplets patterning
1. Introduction The preparation of perfect crystals and crystalline layers on amorphous substrates is an insufficiently solved problem up to now. This concerns the experimental results as well as the theoretical treatment of nucleation and growth processes. However, many applications require amorphous
* Corresponding author. Tel.: #49 30 6392 3051; fax: #49 30 6392 3003; e-mail: [email protected].
substrates. The use of glass, e.g., is compelling as a substrate of thin silicon layers for TFTs in active matrix LCDs. Especially in large area devices such as solar cells, commercial aspects even exclude the application of crystalline substrates, desired from a crystallographic point of view. Thus, the use of low cost amorphous substrates like glass is aimed. From the literature many attempts to form polycrystalline silicon layers on foreign substrates are known. In most processes silicon is first deposited by CVD or PVD techniques as an amorphous layer and is then crystallized by different thermal
0022-0248/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 2 3 8 - X
T. Boeck et al. / Journal of Crystal Growth 198/199 (1999) 420–424
processes. Examples are laser annealing [2], rapid thermal annealing (RTA) [3] or electron beam crystallisation [4]. An interesting approach to produce polycrystalline thin films is the selective nucleation-based epitaxy (SENTAXY) [5,6]. For the formation of large crystalline grains long annealing times and temperatures above 1000°C are necessary. Besides the energy consumption, the high level of impurities incorporated into the crystalline layer and the limited choice of applicable substrate materials are highly disadvantageous in these high temperature processes. For this reason, solution growth techniques working at temperatures far below the melting point of the material to be crystallized are of increasing interest. Due to the lack of a crystallographic lattice of the substrate, no epitaxial intergrowth of substrate and deposit is possible. Therefore, we developed a two step procedure. Initially, silicon nuclei are formed in small solution droplets. These nuclei will be enlarged by applying steady state VLS-growth conditions. Good adhering, texturized crystalline deposits are formed. In a second step the deposited crystallites act as nucleation centres which can be grown epitaxially to closed polycrystalline layer structures. For this process the temperature difference method (TDM), as a modification of conventional LPE, can be used.
2. Experiments Fig. 1 illustrates the performance of the experiments in a strongly simplified picture. During the whole process uncontrolled nucleation at the glass substrate must be avoided. Therefore, the surface should be extremely smooth and homogeneous. AFM measurements are applied to examine the results of the cleaning procedure. At the beginning of the growth process a metallic solvent layer is deposited on the purified glass substrate. Appropriate coating methods are evaporation by resistance heating or electron beam evaporation. In order to enable nucleation at preferred sites in a simple way, i.e. without photolithographic structuring, the natural coalescence behaviour of thin metallic films at temperatures above their melting point is utilized. Heating the
421
Fig. 1. Schematic presentation of the growth principle using metal droplets as nucleation centres to deposit silicon crystallites on glass.
glass to such temperatures results in the formation of solvent droplets via coalescence phenomena by surface tension minimization. In a next step, silicon is evaporated to deposit on the substrate, patterned with the statistically distributed droplets. During evaporation, the sample has to be heated at a temperature higher than the eutectic temperature of silicon and the metal. The typical temperature range is 550—600°C. Silicon dissolves in the liquid metal droplets and, maintaining the evaporation process, an excess of silicon begins to precipitate as crystallites. A higher temperature at the surface of the droplets causes a concentration gradient, mediated by a higher solubility of silicon in the “upper” range. Therefore, a thermodynamic potential for the material transport exists. Lowering the temperature at the substrate by an active cooling of the backside supports the crystallization of silicon on the glass surface. The growth of only one crystallite within each droplet is essential to apply this nuclei selection
422
T. Boeck et al. / Journal of Crystal Growth 198/199 (1999) 420–424
method. Preconditions are appropriate droplet dimensions as well as an optimized silicon deposition rate and substrate temperature. To enhance the adhesion of the crystallites on glass it is advantageous to coat the glass with an intermediate layer. More details of the growth process are given in Ref. [1].
3. Results The droplet formation is presented in Fig. 2. The sizes and arrangement of the nucleation centres generating droplets are sufficiently regular. Typical values of droplet dimensions are 3 lm, while the mean distances are about 12 lm. To identify growing silicon crystallites still encapsulated by the solution the focused ion beam (FIB) method has been applied (Fig. 3). Due to the ion bombardment the external parts of droplets have been successively removed in FIB technology. In this way, details of the internal structure become visible. Local X-ray microanalyses at the tilted samples in an analytical scanning electron microscope revealed that pure silicon crystallites have been grown inside the droplets. From Fig. 4 it is obvious that as a result of this growth technique silicon single crystallites with well developed facets have been grown from the droplets. Some crystallographic +1 1 1, facets are
Fig. 2. SEM image of solvent droplets on glass.
Fig. 3. Focused ion beam cutting of a solvent droplet. An encapsulated silicon crystallite is visible in the centre attached with the glass surface.
visible. The crystals are still surrounded by residual melt solution. Light microscopy (Fig. 5) shows the uniform large area distribution of silicon crystallites on glass.
4. Discussion In our method transport of material from the source occurs by physical deposition methods from the gas phase (PVD), nucleation and growth are realized within the droplets from the liquid phase. By this combination of transport and growth mechanism it is possible to link a decisive advantage of PVD techniques — the large area deposition of material without any difficulties — with the advantages of crystallization from a liquid solution near the equilibrium. Solution crystallization methods are characterized by the possibility to prepare perfect single crystals and layers, respectively. Moreover, the employment of a solvent allows the growth far below the melting point of the substance to be crystallized and therefore allows the use of glass compatible temperatures. A further advantage is the cleaning effect obtainable for many impurities (distribution coefficient(1). Applying liquid solutions with large volumes (some cm) already a small lateral temperature
T. Boeck et al. / Journal of Crystal Growth 198/199 (1999) 420–424
423
Fig. 4. Silicon single crystallites surrounded by residual melt solution.
fections of crystal growth. The problem of heat and material transport instabilities, being very sensitive to external fluctuation in large volume solutions, can be solved by the local separation of the liquid phase in solvent droplets with a diameter in the range of 10 lm. For a well patterned deposition of solvent droplets the application of the bubble jet printer principle could be an interesting approach in future.
Fig. 5. Low magnification image reveals uniform large area distribution of silicon crystallites on glass. The mark corresponds to 100 lm.
inhomogeneity in the range of 0—1 K cm\ results in hydrodynamic instabilities caused by lateral density inhomogeneity. The consequences are imper-
Acknowledgements The authors are indebted to W. Ho¨ppner for FIB investigations. This work has been supported by “Deutsche Forschungsgemeinschaft” under contract No. Bo 1129/2-1 and “Bundesministerium fu¨r Bildung, Wissenschaft, Forschung und Technologie” under contract No. 13N7048/7.
424
T. Boeck et al. / Journal of Crystal Growth 198/199 (1999) 420–424
References [1] T. Boeck, K. Schmidt, A. Braun, Verfahren zur Herstellung von kristallinen Schichten, 196 31 551.4, PCT/DE 96/01439, 26 July 1996. [2] H. Tanabe, K. Sera, K. Nakamura, K. Hirata, K. Yuda, F. Okumura, NEC Res. Dev. 35 (3) (1994). [3] N. Duhamel, B. Loisel, in: H.P. Strunk, J.H. Werner, B. Fortin, O. Bonnaud (Eds.), Solid State Phenomena, Polycrystalline Semiconductors III, vols. 37 and 38, Scitec Publications Ltd., Switzerland, 1994, p. 535.
[4] T. Reindl, T. Fuska, C. Walz, H. Cerva, R. Lemme, W. Kruehler, M. Pauli, J. Mueller, in: H.P. Strunk, J.H. Werner, B. Fortin, O. Bonnaud (Eds.), Polycrystalline Semiconductors III, vols. 51 and 52, Scitec Publications Ltd., Switzerland, 1996, p. 295. [5] K. Yamagata, T. Yonehara, Appl. Phys. Lett. 61 (1992) 2557. [6] T. Yonehara, in: McConnell (Ed.), First NREL Conf., Future Generation Photovoltaic Technologies, CP404, 1-56396-704-9, The American Institute of Physics, New York, 1997.
A method to grow silicon crystallites on glass Torsten Boeck*, Thomas Teubner, Klaus Schmidt, Peter-Michael Wilde Institute of Crystal Growth, Rudower Chaussee 6, D-12489 Berlin, Germany
Abstract In our laboratory a method has been developed for low temperature growth of silicon on glass from metallic solutions [1]. This technique is based on creating pointlike nucleation centres using natural coalescence phenomena of the metallic solvent for masking the substrate. Thus, uncontrolled spontaneous nucleation can be avoided and locally defined selective growth of silicon crystallites seeded by the Si saturated metallic solution droplets occurs. The nucleation of silicon on the substrate surface and the growth of crystallites must be initiated and maintained by a steady temperature gradient. The material transport is governed by a vapour—liquid—solid (VLS) mechanism. As a first result of this artificial nucleus selection principle silicon crystallites have been grown in dimensions of 10 lm. Size and distribution of the solvent droplets as well as the morphology of the grown silicon crystallites have been characterized by SEM and optical microscopy. The crystallites show good adhesion on glass and are sufficiently regular arranged. The focused ion beam (FIB) method followed by X-ray microanalysis has been used to identify silicon crystallites still encapsulated by the solution. 1999 Elsevier Science B.V. All rights reserved. PACS: 68.55.!a; 81.15.!z; 81.15.Jj; 81.15.Lm; 81.30.Mh; 81.65.Cf Keywords: Solution growth; Silicon on glass; Solar cells; Artificial nucleation; Solvent droplets patterning
1. Introduction The preparation of perfect crystals and crystalline layers on amorphous substrates is an insufficiently solved problem up to now. This concerns the experimental results as well as the theoretical treatment of nucleation and growth processes. However, many applications require amorphous
* Corresponding author. Tel.: #49 30 6392 3051; fax: #49 30 6392 3003; e-mail: [email protected].
substrates. The use of glass, e.g., is compelling as a substrate of thin silicon layers for TFTs in active matrix LCDs. Especially in large area devices such as solar cells, commercial aspects even exclude the application of crystalline substrates, desired from a crystallographic point of view. Thus, the use of low cost amorphous substrates like glass is aimed. From the literature many attempts to form polycrystalline silicon layers on foreign substrates are known. In most processes silicon is first deposited by CVD or PVD techniques as an amorphous layer and is then crystallized by different thermal
0022-0248/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 2 3 8 - X
T. Boeck et al. / Journal of Crystal Growth 198/199 (1999) 420–424
processes. Examples are laser annealing [2], rapid thermal annealing (RTA) [3] or electron beam crystallisation [4]. An interesting approach to produce polycrystalline thin films is the selective nucleation-based epitaxy (SENTAXY) [5,6]. For the formation of large crystalline grains long annealing times and temperatures above 1000°C are necessary. Besides the energy consumption, the high level of impurities incorporated into the crystalline layer and the limited choice of applicable substrate materials are highly disadvantageous in these high temperature processes. For this reason, solution growth techniques working at temperatures far below the melting point of the material to be crystallized are of increasing interest. Due to the lack of a crystallographic lattice of the substrate, no epitaxial intergrowth of substrate and deposit is possible. Therefore, we developed a two step procedure. Initially, silicon nuclei are formed in small solution droplets. These nuclei will be enlarged by applying steady state VLS-growth conditions. Good adhering, texturized crystalline deposits are formed. In a second step the deposited crystallites act as nucleation centres which can be grown epitaxially to closed polycrystalline layer structures. For this process the temperature difference method (TDM), as a modification of conventional LPE, can be used.
2. Experiments Fig. 1 illustrates the performance of the experiments in a strongly simplified picture. During the whole process uncontrolled nucleation at the glass substrate must be avoided. Therefore, the surface should be extremely smooth and homogeneous. AFM measurements are applied to examine the results of the cleaning procedure. At the beginning of the growth process a metallic solvent layer is deposited on the purified glass substrate. Appropriate coating methods are evaporation by resistance heating or electron beam evaporation. In order to enable nucleation at preferred sites in a simple way, i.e. without photolithographic structuring, the natural coalescence behaviour of thin metallic films at temperatures above their melting point is utilized. Heating the
421
Fig. 1. Schematic presentation of the growth principle using metal droplets as nucleation centres to deposit silicon crystallites on glass.
glass to such temperatures results in the formation of solvent droplets via coalescence phenomena by surface tension minimization. In a next step, silicon is evaporated to deposit on the substrate, patterned with the statistically distributed droplets. During evaporation, the sample has to be heated at a temperature higher than the eutectic temperature of silicon and the metal. The typical temperature range is 550—600°C. Silicon dissolves in the liquid metal droplets and, maintaining the evaporation process, an excess of silicon begins to precipitate as crystallites. A higher temperature at the surface of the droplets causes a concentration gradient, mediated by a higher solubility of silicon in the “upper” range. Therefore, a thermodynamic potential for the material transport exists. Lowering the temperature at the substrate by an active cooling of the backside supports the crystallization of silicon on the glass surface. The growth of only one crystallite within each droplet is essential to apply this nuclei selection
422
T. Boeck et al. / Journal of Crystal Growth 198/199 (1999) 420–424
method. Preconditions are appropriate droplet dimensions as well as an optimized silicon deposition rate and substrate temperature. To enhance the adhesion of the crystallites on glass it is advantageous to coat the glass with an intermediate layer. More details of the growth process are given in Ref. [1].
3. Results The droplet formation is presented in Fig. 2. The sizes and arrangement of the nucleation centres generating droplets are sufficiently regular. Typical values of droplet dimensions are 3 lm, while the mean distances are about 12 lm. To identify growing silicon crystallites still encapsulated by the solution the focused ion beam (FIB) method has been applied (Fig. 3). Due to the ion bombardment the external parts of droplets have been successively removed in FIB technology. In this way, details of the internal structure become visible. Local X-ray microanalyses at the tilted samples in an analytical scanning electron microscope revealed that pure silicon crystallites have been grown inside the droplets. From Fig. 4 it is obvious that as a result of this growth technique silicon single crystallites with well developed facets have been grown from the droplets. Some crystallographic +1 1 1, facets are
Fig. 2. SEM image of solvent droplets on glass.
Fig. 3. Focused ion beam cutting of a solvent droplet. An encapsulated silicon crystallite is visible in the centre attached with the glass surface.
visible. The crystals are still surrounded by residual melt solution. Light microscopy (Fig. 5) shows the uniform large area distribution of silicon crystallites on glass.
4. Discussion In our method transport of material from the source occurs by physical deposition methods from the gas phase (PVD), nucleation and growth are realized within the droplets from the liquid phase. By this combination of transport and growth mechanism it is possible to link a decisive advantage of PVD techniques — the large area deposition of material without any difficulties — with the advantages of crystallization from a liquid solution near the equilibrium. Solution crystallization methods are characterized by the possibility to prepare perfect single crystals and layers, respectively. Moreover, the employment of a solvent allows the growth far below the melting point of the substance to be crystallized and therefore allows the use of glass compatible temperatures. A further advantage is the cleaning effect obtainable for many impurities (distribution coefficient(1). Applying liquid solutions with large volumes (some cm) already a small lateral temperature
T. Boeck et al. / Journal of Crystal Growth 198/199 (1999) 420–424
423
Fig. 4. Silicon single crystallites surrounded by residual melt solution.
fections of crystal growth. The problem of heat and material transport instabilities, being very sensitive to external fluctuation in large volume solutions, can be solved by the local separation of the liquid phase in solvent droplets with a diameter in the range of 10 lm. For a well patterned deposition of solvent droplets the application of the bubble jet printer principle could be an interesting approach in future.
Fig. 5. Low magnification image reveals uniform large area distribution of silicon crystallites on glass. The mark corresponds to 100 lm.
inhomogeneity in the range of 0—1 K cm\ results in hydrodynamic instabilities caused by lateral density inhomogeneity. The consequences are imper-
Acknowledgements The authors are indebted to W. Ho¨ppner for FIB investigations. This work has been supported by “Deutsche Forschungsgemeinschaft” under contract No. Bo 1129/2-1 and “Bundesministerium fu¨r Bildung, Wissenschaft, Forschung und Technologie” under contract No. 13N7048/7.
424
T. Boeck et al. / Journal of Crystal Growth 198/199 (1999) 420–424
References [1] T. Boeck, K. Schmidt, A. Braun, Verfahren zur Herstellung von kristallinen Schichten, 196 31 551.4, PCT/DE 96/01439, 26 July 1996. [2] H. Tanabe, K. Sera, K. Nakamura, K. Hirata, K. Yuda, F. Okumura, NEC Res. Dev. 35 (3) (1994). [3] N. Duhamel, B. Loisel, in: H.P. Strunk, J.H. Werner, B. Fortin, O. Bonnaud (Eds.), Solid State Phenomena, Polycrystalline Semiconductors III, vols. 37 and 38, Scitec Publications Ltd., Switzerland, 1994, p. 535.
[4] T. Reindl, T. Fuska, C. Walz, H. Cerva, R. Lemme, W. Kruehler, M. Pauli, J. Mueller, in: H.P. Strunk, J.H. Werner, B. Fortin, O. Bonnaud (Eds.), Polycrystalline Semiconductors III, vols. 51 and 52, Scitec Publications Ltd., Switzerland, 1996, p. 295. [5] K. Yamagata, T. Yonehara, Appl. Phys. Lett. 61 (1992) 2557. [6] T. Yonehara, in: McConnell (Ed.), First NREL Conf., Future Generation Photovoltaic Technologies, CP404, 1-56396-704-9, The American Institute of Physics, New York, 1997.