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The crystallisation of deep amorphous wells in silicon produced by ion implantation
Nuclear Instruments and Methods in Physics Research B 175±177 (2001) 164±168
www.elsevier.nl/locate/nimb
The crystallisation of deep amorphous wells in silicon produced by ion implantation A.C.Y. Liu b
a,*
, J.C. McCallum a, J. Wong-Leung
b
a Micro-Analytical Research Centre, School of Physics, University of Melbourne, Vict. 3010, Australia Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National University, ACT 0200, Australia
Abstract The crystallisation of deep amorphous wells is studied. These model systems are formed by high energy self implantation through a mask into silicon. The amorphised regions have an aspect ratio opposite to that employed in previous experiments. At elevated temperatures crystallisation proceeds inwards with the amorphous-phase being transformed through both lateral and vertical solid-phase epitaxy (SPE). Complementary information is obtained from performing plan view and cross-sectional transmission electron microscopy analyses. It is discovered that the recovery of the amorphous material is governed by the dynamics of solid-phase epitaxy and that unique secondary structures result from the heat treatment. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 68.55.Ln; 61.72.Nn; 61.72.Ff; 61.50.Ks Keywords: Lateral solid phase epitaxy; Ion implantation; Transmission electron microscopy; Amorphous silicon
1. Introduction Amorphous silicon may be transformed to pristine crystal in the solid-phase using a crystalline seed as a template. In lateral solid-phase epitaxy (SPE) in which the growth direction coincides with the plane of the ®lm, experiments have formerly been con®ned to radial growth from a
*
Correspopnding author. Fax: +61-3-9347-4783. E-mail addresses: [email protected] (A.C.Y. Liu), [email protected] (J.C. McCallum), [email protected] (J. Wong-Leung).
seed. In these it has been determined that the maximum growth rate occurs in the [0 1 0] direction in (1 0 0) wafers [1]. Slow growth is intimately related to the ease with which {1 1 1} facet planes may form. While experiments with an inverse sample geometry have been performed, we ®nd the results of these ambiguous due to the shallowness of the amorphous volumes and the species' used to amorphise [2±4]. We present our ®ndings on the crystallisation of deep amorphous volumes constrained in two dimensions by a crystalline substrate. The behaviour of such structures may be pertinent if MeV implants into select areas gain application in the semiconductor industry.
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 3 5 - 4
A.C.Y. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 164±168
165
2. Experiment Lightly p-doped (5±10 X (1 0 0)) silicon wafers undergo an implantation regime to amorphise the regions exposed through a mask. Eight implants were performed in the range 100 keV to 8 MeV at liquid nitrogen temperature with ¯uences of 1 1015 =cm2 . The mask consists of a nickel grid with aperture dimensions of 1±2 lm and a period of 12:5 lm. The samples thus consist of an array of amorphous volumes, each with a lateral size of 1±2 lm and a vertical extent of 5 lm. We apply the term amorphous silicon wells to these structures. Anneals were performed in air on a hot stage at a temperature of 620°C for durations of 7 and 20 min, respectively. In®ltration of hydrogen due to surface oxidation is expected to in¯uence only the ®rst 1:5 lm of the crystallisation [5]. One sample underwent a subsequent treatment at 900°C for 1 h. The steps in this process are enumerated in Fig. 1. Preparation of electron transparent regions for transmission electron microscope (TEM) studies employed standard techniques. Final polishing was achieved by acid etching and ion milling for the plan view (P-V) and cross-sectioned (X-sectional) samples, respectively. All TEM analyses were performed on a Philips EM 430 microscope at an operating voltage of 300 kV. 3. Results 3.1. P-V TEM While P-V studies were conducted on samples at a range of dierent stages in their thermal processing, results were displayed for only the sample which underwent a 620°C anneal for 7 min. The full study is presented elsewhere [6]. A zone axis bright ®eld (BF) image of an amorphous well from this sample is seen in Fig. 2(a). This single image articulates many aspects of the process being reviewed. Firstly, the well is only partially crystallised at this stage. A selected area diraction pattern (SADP) obtained from the central region (not shown) was typical of the diuse rings that arise
Fig. 1. The process of making an amorphous well. A multienergy implant through a mask amorphises select regions of the crystal. Lateral and vertical SPE reincorporate this volume into the surrounding crystalline phase. Shown as an inset is an optical image of the nickel grid used as a mask.
from amorphous diamond structure materials. The original position of the lateral amorphous±crystalline interface is marked by a dense band of defects. These defects may originate from a similar mechanism to that which gives rise to the endof-range loops observed in vertical SPE. This is, that the implant pro®le is widened due to lateral straggling of the high energy ions. Regions just outside the lateral interface are thus rich in interstitials or perhaps interstitials are ejected from the crystallising amorphous region, and these give rise to dislocation loops during crystallisation. The current lateral interface forms a square which is not parallel to the original lateral interface. This suggests that facets have formed due to the existence of a favoured growth direction. This direction must be independent of the original mask edge alignment since this was arranged to diverge from any major cleavage plane. A comparison of the image to the SADP obtained from the crystal
166
A.C.Y. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 164±168
outside the defective area (displayed in Fig. 2(b)) allows the prominent growth directions to be identi®ed as the [0 1 0] directions. This con®rms earlier experiments in lateral SPE. Large dislocations are seen to have grown into the recovered crystal from the original lateral interface. The mechanism generating this dense network is not obvious at this point.
Fig. 2. (a) Plan-view zone axis BF image of a partially annealed well with the SADP obtained from the crystal shown in (b) to facilitate identi®cation of fast growth directions. The central portion of the well is yet amorphous.
3.2. X-sectional TEM Information complementary to that revealed by P-V TEM was obtained through examining crosssectioned samples. These were cleaved along {0 1 1} planes and examined from a vantage perpendicular to this in the [0 1 1] direction. This meant that in a given area of thin material many adjacent sections of wells were visible yielding a lot of varied information. Only select images are presented here and the reader is referred elsewhere for supplementary information [6]. Fig. 3 displays a zone axis BF image of a well from a sample annealed at 620°C for 20 min. The surface in this and the subsequent ®gure is indicated by an arrow. In this sample, no trace of amorphous material could be detected indicating complete recovery of the amorphous volume. SADP's taken from within the area shown identify the recovered crystal as single-crystal silicon with the same orientation as the substrate. The material is highly defective. It is dicult to characterise the defects seen from a cursory examination since we are not examining them normal to their plane, as may be imagined from the exercise
Fig. 3. XTEM zone axis BF image of a fully crystallised well. The large defective structures are related to the misalignment of converging fast growth fronts.
A.C.Y. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 164±168
of tracing sections through the P-V image of Fig. 2. Moreover, the network is prohibitively dense. A comparison of this image with the P-V shown just prior is useful in attempting to explain the origin of the more prominent of the defects. For instance the large core dislocation that runs vertically from the surface most plausibly arises from competition between the fast growth fronts identi®ed in the P-V. At some point these four fronts will converge and the misalignment of these, or the injection of interstitials ahead of the interfaces will result in a large planar fault. Another area where such dislocations may arise is at the bottom of the well where the meeting of lateral and vertical growth fronts (both of the fast [0 1 0] variety) would engender a defect of the same kind. This is borne out in the image. The dislocations which grow into the crystal from the lateral interface intersect the large core dislocation. These may be a consequence of the discrepancy between the original lateral interface position and the major growth directions. Dislocations may arise as the fast growth directions attain prominence. To monitor the evolution of these defects structures with higher temperature annealing a sample was prepared for XTEM which had been subjected to an additional anneal of 900°C for 1 h. A zone axis BF image of the secondary defect structures detected in this sample is shown in Fig. 4. It is clear from the image that the density of defects has decreased. However, the large dislocation resulting from the multiplicity of competing fast growth fronts persists. Moreover, the end-ofrange dislocation loops are more prominent, suggesting that the threading type dislocations that are intersecant with the core defect may disassociate to form these. Unlike previous experiments [4] our work has allowed the identi®cation of secondary defect structures intrinsic to the crystallisation of amorphous volumes with this geometry. The prominent faults stem from the accommodation of fast growth directions and the convergence of these during crystallisation.
167
Fig. 4. XTEM zone axis BF image from a sample which was treated with a subsequent high temperature anneal. Many of the defects seen in the previous image persevere.
4. Conclusions Lateral SPE has been observed in a system with the opposite aspect ratio to those studied previously. The dynamics of these amorphous wells with heat treatment are complex and interesting. Lateral and vertical SPE contribute to the recovery of the amorphous volumes. An analysis of the secondary defect structures has been presented. During crystallisation faceting occurs on the anticipated planes. This process is dictated by the dynamics of SPE regarding favoured growth directions. We ®nd that the rate of lateral SPE is maximum in [0 1 0] directions con®rming earlier work. The original lateral amorphous±crystalline interface is evident after heat treatment as a band of dislocations. The competition between perpendicular fast growth fronts produces a large dislocation, which tolerates a further heat treatment at a higher temperature.
168
A.C.Y. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 164±168
Acknowledgements
References
We wish to acknowledge the use of the Philips 430 microscope at the Electron Microscopy Unit at the Australian National University and the expertise of the sta there. All implantations were performed at the Electronic Materials Engineering department in the Research School of Physical Sciences and Engineering at the Australian National University. Professor J.S. Williams is thanked for his interest.
[1] H. Yamamoto, H. Ishiwara, S. Furukawa, Jpn. J. Appl. Phys. 25 (1986) 667. [2] C. Nieh, L. Chen, Nucl. Instr. and Meth. B 55 (1991) 831. [3] M. Tamura, H. Horiuchi, K. Kawamoto, Nucl. Instr. and Meth. B 37±38 (1989) 329. [4] M. Horiuchi, M. Tamura, S. Aoki, Nucl. Instr. and Meth. B 37±38 (1989) 285. [5] G.L. Olson, J.A. Roth, in: D.J.T. Hurle (Ed.), Handbook of Crystal Growth, Elsevier, Amsterdam, 1994, Chapter 7. [6] Journal of Materials Research, submitted.
www.elsevier.nl/locate/nimb
The crystallisation of deep amorphous wells in silicon produced by ion implantation A.C.Y. Liu b
a,*
, J.C. McCallum a, J. Wong-Leung
b
a Micro-Analytical Research Centre, School of Physics, University of Melbourne, Vict. 3010, Australia Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National University, ACT 0200, Australia
Abstract The crystallisation of deep amorphous wells is studied. These model systems are formed by high energy self implantation through a mask into silicon. The amorphised regions have an aspect ratio opposite to that employed in previous experiments. At elevated temperatures crystallisation proceeds inwards with the amorphous-phase being transformed through both lateral and vertical solid-phase epitaxy (SPE). Complementary information is obtained from performing plan view and cross-sectional transmission electron microscopy analyses. It is discovered that the recovery of the amorphous material is governed by the dynamics of solid-phase epitaxy and that unique secondary structures result from the heat treatment. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 68.55.Ln; 61.72.Nn; 61.72.Ff; 61.50.Ks Keywords: Lateral solid phase epitaxy; Ion implantation; Transmission electron microscopy; Amorphous silicon
1. Introduction Amorphous silicon may be transformed to pristine crystal in the solid-phase using a crystalline seed as a template. In lateral solid-phase epitaxy (SPE) in which the growth direction coincides with the plane of the ®lm, experiments have formerly been con®ned to radial growth from a
*
Correspopnding author. Fax: +61-3-9347-4783. E-mail addresses: [email protected] (A.C.Y. Liu), [email protected] (J.C. McCallum), [email protected] (J. Wong-Leung).
seed. In these it has been determined that the maximum growth rate occurs in the [0 1 0] direction in (1 0 0) wafers [1]. Slow growth is intimately related to the ease with which {1 1 1} facet planes may form. While experiments with an inverse sample geometry have been performed, we ®nd the results of these ambiguous due to the shallowness of the amorphous volumes and the species' used to amorphise [2±4]. We present our ®ndings on the crystallisation of deep amorphous volumes constrained in two dimensions by a crystalline substrate. The behaviour of such structures may be pertinent if MeV implants into select areas gain application in the semiconductor industry.
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 3 3 5 - 4
A.C.Y. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 164±168
165
2. Experiment Lightly p-doped (5±10 X (1 0 0)) silicon wafers undergo an implantation regime to amorphise the regions exposed through a mask. Eight implants were performed in the range 100 keV to 8 MeV at liquid nitrogen temperature with ¯uences of 1 1015 =cm2 . The mask consists of a nickel grid with aperture dimensions of 1±2 lm and a period of 12:5 lm. The samples thus consist of an array of amorphous volumes, each with a lateral size of 1±2 lm and a vertical extent of 5 lm. We apply the term amorphous silicon wells to these structures. Anneals were performed in air on a hot stage at a temperature of 620°C for durations of 7 and 20 min, respectively. In®ltration of hydrogen due to surface oxidation is expected to in¯uence only the ®rst 1:5 lm of the crystallisation [5]. One sample underwent a subsequent treatment at 900°C for 1 h. The steps in this process are enumerated in Fig. 1. Preparation of electron transparent regions for transmission electron microscope (TEM) studies employed standard techniques. Final polishing was achieved by acid etching and ion milling for the plan view (P-V) and cross-sectioned (X-sectional) samples, respectively. All TEM analyses were performed on a Philips EM 430 microscope at an operating voltage of 300 kV. 3. Results 3.1. P-V TEM While P-V studies were conducted on samples at a range of dierent stages in their thermal processing, results were displayed for only the sample which underwent a 620°C anneal for 7 min. The full study is presented elsewhere [6]. A zone axis bright ®eld (BF) image of an amorphous well from this sample is seen in Fig. 2(a). This single image articulates many aspects of the process being reviewed. Firstly, the well is only partially crystallised at this stage. A selected area diraction pattern (SADP) obtained from the central region (not shown) was typical of the diuse rings that arise
Fig. 1. The process of making an amorphous well. A multienergy implant through a mask amorphises select regions of the crystal. Lateral and vertical SPE reincorporate this volume into the surrounding crystalline phase. Shown as an inset is an optical image of the nickel grid used as a mask.
from amorphous diamond structure materials. The original position of the lateral amorphous±crystalline interface is marked by a dense band of defects. These defects may originate from a similar mechanism to that which gives rise to the endof-range loops observed in vertical SPE. This is, that the implant pro®le is widened due to lateral straggling of the high energy ions. Regions just outside the lateral interface are thus rich in interstitials or perhaps interstitials are ejected from the crystallising amorphous region, and these give rise to dislocation loops during crystallisation. The current lateral interface forms a square which is not parallel to the original lateral interface. This suggests that facets have formed due to the existence of a favoured growth direction. This direction must be independent of the original mask edge alignment since this was arranged to diverge from any major cleavage plane. A comparison of the image to the SADP obtained from the crystal
166
A.C.Y. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 164±168
outside the defective area (displayed in Fig. 2(b)) allows the prominent growth directions to be identi®ed as the [0 1 0] directions. This con®rms earlier experiments in lateral SPE. Large dislocations are seen to have grown into the recovered crystal from the original lateral interface. The mechanism generating this dense network is not obvious at this point.
Fig. 2. (a) Plan-view zone axis BF image of a partially annealed well with the SADP obtained from the crystal shown in (b) to facilitate identi®cation of fast growth directions. The central portion of the well is yet amorphous.
3.2. X-sectional TEM Information complementary to that revealed by P-V TEM was obtained through examining crosssectioned samples. These were cleaved along {0 1 1} planes and examined from a vantage perpendicular to this in the [0 1 1] direction. This meant that in a given area of thin material many adjacent sections of wells were visible yielding a lot of varied information. Only select images are presented here and the reader is referred elsewhere for supplementary information [6]. Fig. 3 displays a zone axis BF image of a well from a sample annealed at 620°C for 20 min. The surface in this and the subsequent ®gure is indicated by an arrow. In this sample, no trace of amorphous material could be detected indicating complete recovery of the amorphous volume. SADP's taken from within the area shown identify the recovered crystal as single-crystal silicon with the same orientation as the substrate. The material is highly defective. It is dicult to characterise the defects seen from a cursory examination since we are not examining them normal to their plane, as may be imagined from the exercise
Fig. 3. XTEM zone axis BF image of a fully crystallised well. The large defective structures are related to the misalignment of converging fast growth fronts.
A.C.Y. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 164±168
of tracing sections through the P-V image of Fig. 2. Moreover, the network is prohibitively dense. A comparison of this image with the P-V shown just prior is useful in attempting to explain the origin of the more prominent of the defects. For instance the large core dislocation that runs vertically from the surface most plausibly arises from competition between the fast growth fronts identi®ed in the P-V. At some point these four fronts will converge and the misalignment of these, or the injection of interstitials ahead of the interfaces will result in a large planar fault. Another area where such dislocations may arise is at the bottom of the well where the meeting of lateral and vertical growth fronts (both of the fast [0 1 0] variety) would engender a defect of the same kind. This is borne out in the image. The dislocations which grow into the crystal from the lateral interface intersect the large core dislocation. These may be a consequence of the discrepancy between the original lateral interface position and the major growth directions. Dislocations may arise as the fast growth directions attain prominence. To monitor the evolution of these defects structures with higher temperature annealing a sample was prepared for XTEM which had been subjected to an additional anneal of 900°C for 1 h. A zone axis BF image of the secondary defect structures detected in this sample is shown in Fig. 4. It is clear from the image that the density of defects has decreased. However, the large dislocation resulting from the multiplicity of competing fast growth fronts persists. Moreover, the end-ofrange dislocation loops are more prominent, suggesting that the threading type dislocations that are intersecant with the core defect may disassociate to form these. Unlike previous experiments [4] our work has allowed the identi®cation of secondary defect structures intrinsic to the crystallisation of amorphous volumes with this geometry. The prominent faults stem from the accommodation of fast growth directions and the convergence of these during crystallisation.
167
Fig. 4. XTEM zone axis BF image from a sample which was treated with a subsequent high temperature anneal. Many of the defects seen in the previous image persevere.
4. Conclusions Lateral SPE has been observed in a system with the opposite aspect ratio to those studied previously. The dynamics of these amorphous wells with heat treatment are complex and interesting. Lateral and vertical SPE contribute to the recovery of the amorphous volumes. An analysis of the secondary defect structures has been presented. During crystallisation faceting occurs on the anticipated planes. This process is dictated by the dynamics of SPE regarding favoured growth directions. We ®nd that the rate of lateral SPE is maximum in [0 1 0] directions con®rming earlier work. The original lateral amorphous±crystalline interface is evident after heat treatment as a band of dislocations. The competition between perpendicular fast growth fronts produces a large dislocation, which tolerates a further heat treatment at a higher temperature.
168
A.C.Y. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 164±168
Acknowledgements
References
We wish to acknowledge the use of the Philips 430 microscope at the Electron Microscopy Unit at the Australian National University and the expertise of the sta there. All implantations were performed at the Electronic Materials Engineering department in the Research School of Physical Sciences and Engineering at the Australian National University. Professor J.S. Williams is thanked for his interest.
[1] H. Yamamoto, H. Ishiwara, S. Furukawa, Jpn. J. Appl. Phys. 25 (1986) 667. [2] C. Nieh, L. Chen, Nucl. Instr. and Meth. B 55 (1991) 831. [3] M. Tamura, H. Horiuchi, K. Kawamoto, Nucl. Instr. and Meth. B 37±38 (1989) 329. [4] M. Horiuchi, M. Tamura, S. Aoki, Nucl. Instr. and Meth. B 37±38 (1989) 285. [5] G.L. Olson, J.A. Roth, in: D.J.T. Hurle (Ed.), Handbook of Crystal Growth, Elsevier, Amsterdam, 1994, Chapter 7. [6] Journal of Materials Research, submitted.