- Email: [email protected]
Si(001) island formation
Physica E 13 (2002) 974 – 977
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E ect of phosphorus on Ge=Si(0 0 1) island formation T.I. Kaminsa; ∗ , G. Medeiros-Ribeiroa; b , D.A.A. Ohlberga , R. Stanley Williamsa a Quantum
Science Research, Hewlett-Packard Laboratories, 1501 Page Mill Road, M=S 1123, Palo Alto CA 94304-1126, USA b Laboratorio Nacional de Luz Sincrotron, Campinas SP 13083-970, Brazil
Abstract Adding a phosphorus-containing species during chemical vapor deposition of Ge islands on Si(0 0 1) modi2es the island sizes and shapes, primarily by changing the surface energies and the relative surface energies of di erent surface facets. Three distinct island shapes occur, but the island types and their sequence of formation di er from those found with undoped Ge islands. The addition of phosphorus decreases the size of the multifaceted “domes”—the island shape that has a favored island size, providing an additional method for controlling the islands. The largest islands have a steep pyramidal structure not seen for undoped islands. ? 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanostructures; Self-assembly; Germanium; Doping; Chemical vapor deposition
1. Introduction Self-assembled Ge islands on Si(0 0 1) have been extensively studied [1,2]. Similar results have been obtained for islands deposited by chemical vapor deposition (CVD) and for those deposited by physical vapor deposition in ultra-high vacuum. For undoped Ge the 2rst few monolayers form a uniform wetting layer. As more Ge is added, the increasing stress causes the additional Ge to condense into islands with three di erent characteristic shapes determined by the amount of Ge deposited (Fig. 1a): {1 0 5} faceted pyramids are most numerous for low coverages ( ∼ 6 eq ML); multi-faceted “domes” for intermediate coverages ( ∼ 11 eq ML); and large,
∗ Corresponding author. Tel.: +1-650-857-5470; +1-650-236-9885. E-mail address: [email protected] (T.I. Kamins).
fax:
defective islands (“super-domes”) above about 15 eq ML. Typical volumes for pyramids and domes are 5000 and 1:5 × 104 nm3 , respectively. The shape of an island is determined by the energies of the surfaces, edges, and interfaces, along with the strain energy associated with the atoms in the volume of the island. Adding other species to an island, especially on its surface, can change the energies and kinetics and, consequently, the tendency of islands to form [3–8]. Some species, such as lead, facilitate island formation [3] while others, such as antimony, suppress island formation [4]. The shapes of the islands can also be strongly modi2ed by the added species [5]. Phosphorus is strongly adsorbed on Si and probably on Ge [9], and its concentration is likely to be higher near the surface than in the bulk. Consequently, the surface energies of the islands are expected to change when phosphorus is added, possibly modifying the shapes of the islands formed. This paper describes the inIuence of phosphorus on the island shapes and their
1386-9477/02/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 0 2 ) 0 0 2 4 8 - 5
T.I. Kamins et al. / Physica E 13 (2002) 974 – 977
975
5
10
4
Area (nm2)
10
Pyramids 3
10
Domes Minidomes
2
10
1
10
10
Fig. 1. Perspective atomic-force micrographs of (a) undoped and (b) phosphorus-doped Ge islands on Si(0 0 1); 11 eq ML of Ge were deposited in each case. (1 m × 1 m surface area.)
distributions when phosphorus is added either during the deposition or during an annealing treatment after deposition.
2. Experimental method The Ge islands were deposited onto 150-mmdiameter, Si(0 0 1) wafers in a lamp-heated, singlewafer reactor with the wafer sitting on a support with moderate thermal mass. After baking a wafer at ◦ 1150 C in H2 , a Si bu er layer was deposited using ◦ SiH2 Cl2 in H2 at 1080 C. Germanium and phosphorus ◦ were subsequently deposited at approximately 600 C by adding GeH4 and PH3 to the H2 ambient. Selected ◦ wafers were annealed at 600 C in a H2 or H2 =PH3 ambient before exposing the wafers to air. After removing the samples from the deposition chamber, they were measured using atomic-force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The thicknesses of the Ge layers were measured using Rutherford backscattering spectroscopy (RBS). Additional thick layers were deposited so that the phosphorus concentration could be determined by secondary ion mass spectrometry (SIMS).
1
10
2
3
4
10 10 Volume (nm3)
10
5
6
10
Fig. 2. Surface area vs. volume of doped and undoped islands. Points lying on a line of slope 23 correspond to the same island shape. The minidomes of the doped layers lie on the same line as the undoped domes, but are smaller. The pyramids lie on a di erent line, corresponding to their di erent shape.
3. Results and discussion When PH3 is added to the CVD deposition environment, three island types form, as for undoped layers, but the three types of islands are signi2cantly di erent from the three island types found on undoped layers, as shown in Fig. 1b. The smallest islands are very low precursor islands (“mounds”). The second island type appears to be a small dome-shaped structure (“mini-dome”). At intermediate thicknesses a third type of island is seen; layers ∼ 11 eq ML thick are dominated by large pyramids (“super-pyramids”) not seen in undoped layers. The correlation between selected island types in undoped layers and in doped layers is shown in Fig. 2, where the surface area of each island from a number of representative layers is plotted against the volume of that island. The points corresponding to islands with the same shape lie on a single line with slope 23 , while islands of di erent shapes lie on parallel lines with di erent intercepts. The pyramids of undoped layers clearly lie on one line, while the domes from undoped layers lie on another line. The mini-domes found in the doped layers appear to lie on the same line as the domes, suggesting that they are related shapes. However, the doped mini-domes are considerably smaller than the undoped domes. A typical volume of a doped
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T.I. Kamins et al. / Physica E 13 (2002) 974 – 977
Fig. 3. AFM contour plots of doped super-pyramids, showing the rectangular bases of the super-pyramids.
mini-dome is ∼ 6000 nm3 , considerably smaller than the 1:5 × 104 nm3 found for undoped domes, but comparable to the 5000 nm3 of the undoped pyramids. Unlike the undoped layers, where the domes do not appear until more than 8 eq ML of Ge are deposited, the minidomes appear for thinner layers (∼ 6 eq ML), only slightly above the threshold of island formation. The smallest type of island found in the doped layers (“mounds”) is diMcult to analyze quantitatively because their heights are only marginally greater than the measured background roughness; consequently, the uncertainty in the reference level used to calculate the island volume signi2cantly a ects the result. Within this uncertainty, the volume of the doped mounds is found to be 300 –400 nm3 , again considerably smaller than the 5000 nm3 found for undoped pyramids. In the 11 eq ML doped layers, a signi2cant amount of the Ge is contained in large “super-pyramids”, which have their bases aligned along 1 1 0 directions [10]. The “super-pyramids” are bounded by {1 1 3} facets near their tops and steeper {1 1 1} facets near their bases. Fig. 3 shows a plan-view AFM contour plot of a doped layer dominated by super-pyramids, illustrating the characteristic rectangular contours of this type of island. In contrast, the undoped super-domes do not have extensive {1 1 1}
facets, but do have a number of other facets with bases aligned along near-0 0 1 directions. Because of these additional facets, the bases of super-domes appear approximately octagonal, rather than rectangular. Thus, the {1 1 1} facets are favored when phosphorus is present, and {0 0 n} facets are favored for undoped Ge surfaces; i.e., phosphorus on the Ge surface changes the ratio of the {1 1 1} surface energy to the {0 0 n} surface energy. Although the super-pyramids and super-domes appear to be related shapes, numerous super-pyramids are present in layers containing only 11 eq ML of phosphorus-doped Ge, while very few super-domes appear in undoped layers until the amount of Ge deposited signi2cantly exceeds 14 eq ML. Thus, related shapes are again present in doped and undoped layers, but less Ge is needed to obtain the structure when the layers are doped. We attribute the di erent behavior of undoped and doped structures to the modi2cation of the surface energies of the bounding facets by the phosphorus at the surface of the Ge islands. The amount of phosphorus included in the volume of the island is small (¡ 0:2%) compared to the amount of Ge, so the volume energy does not vary appreciably as phosphorus is added. However, phosphorus probably adsorbs strongly on the surface [9], so the surface energy is expected to be signi2cantly di erent for doped and for undoped islands. For the pyramids and the minidomes of similar volume, phosphorus apparently increases the ratio of the surface energy of {1 0 5} facets to that of the steeper facets bounding the domes. For the similarly shaped mini-domes and domes, the ratio of surface to volume increases for smaller islands. A lower surface energy allows a larger relative surface area, and therefore favors smaller islands of the same shape, consistent with the observed decrease in island size for doped layers. The e ect of adsorbed species on the surface of Ge on Si(0 0 1) has been observed previously for other species, such as Sb and In [5]. The doped Ge islands are usually bounded on top by a (0 0 1) plane, with the importance of this plane varying for di erent dopants. For phosphorus in the present experiments, no (0 0 1) plane is visible at the top of the super-pyramids. For the undoped layers, the volume distribution of pyramids decreases monotonically with increasing island size; however, the domes have a maximum in
T.I. Kamins et al. / Physica E 13 (2002) 974 – 977
their volume distribution. Obtaining such a favored island size is key to achieving a narrow size distribution. A pyramid is completely bounded by {1 0 5} facets. Hence, the interface area, surface area, and edge length are proportional, and cannot vary independently. For a dome, several crystal facets are present, and the amount of each can vary within some limits. Consequently, there is no longer a strict proportionality between the surface area, the edge length, and the interface area. Taller islands can form to accommodate increasing volume with a modest increase in surface and interface areas, providing an additional degree of freedom. Similarly to the undoped domes, the mini-domes in a doped layer also have a maximum in the volume distribution, as is reasonable because of their related shapes. Thus, the islands (mini-domes) in a ∼ 6 eq ML doped layer have a maximum in the volume distribution, while the similar-size islands (pyramids) in an undoped layer of the same thickness do not, allowing a narrower island size distribution in smaller doped islands compared to undoped islands. In addition to phosphorus changing the island shapes during deposition, the presence of phosphorus during annealing after deposition also can modify the shapes of the islands. When doped islands are annealed in H2 , the phosphorus in the islands initially retards structural rearrangement. As the phosphorus leaves the islands by di usion or evaporation, the island shapes change toward those associated with undoped islands. The detailed shape of the islands and their evolution on annealing can best be seen in the largest islands (doped “super-pyramids” and undoped “super-domes”), on which the facets are better resolved by the AFM measurements. During annealing in a H2 ambient for 2 h, the initial shape of the super-pyramids with their rectangular contours (Fig. 3) evolves toward that of the super-domes, with their characteristic octagonal contours, formed by similarly annealing undoped domes. It is likely that phosphorus can evaporate or di use from the surfaces of the doped islands as they are annealed, changing the relative surface energies of the various crystal facets so that the island shapes approach those of similarly annealed undoped islands.
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4. Summary Adding a phosphorus-containing species during deposition of Ge islands on Si(0 0 1) modi2es the island shapes, primarily by changing the surface energies and the relative surface energies of di erent surface planes. In particular, adding phosphorus-containing species decreases the size of domes—the island shape that has a favored island size—so that the favored size is smaller in doped layers than in undoped layers, providing an additional method for controlling the islands. In addition, large pyramidal islands form in doped layers while they are not seen in undoped layers.
Acknowledgements The authors would like to thank D. Le orge, C. Nauka, M. Wong, A. Phillips, N. Shamma, and D. Gomez for experimental assistance and HewlettPackard’s former ULSI Laboratory for the use of deposition equipment. References [1] R. Stanley Williams, et al., Acc. Chem. Res. 32 (1999) 425. [2] G. Medeiros-Ribeiro, A.M. Bratkovski, T.I. Kamins, D.A.A. Ohlberg, R. Stanley Williams, Science 279 (1998) 353. [3] H. Hibino, N. Shimizu, K. Sumitomo, Y. Shinoda, T. Nishioka, T. Ogino, J. Vac. Sci. Technol. A 12 (1994) 23. [4] J.M.C. Thornton, A.A. Williams, J.E. Macdonald, R.G. van Silfhout, J.F. van der Veen, M. Finney, C. Norris, J. Vac. Sci. Technol. B 9 (1991) 2146. [5] D.J. Eaglesham, F.C. Unterwald, D.C. Jacobson, Phys. Rev. Lett. 70 (1993) 966. [6] C.K. Seal, D. Samara, S.K. Banerjee, Appl. Phys. Lett. 71 (1997) 3564. [7] O.G. Schmidt, C. Lange, K. Eberl, O. Kienzle, F. Ernst, Appl. Phys. Lett. 71 (1997) 2340. [8] D.J. Bottomley, M. Iwami, Y. Uehara, S. Ushioda, J. Vac. Sci. Technol. A 17 (1999) 698. [9] T. Kikuchi, et al., in: C.L. Claeys, et al. (Eds.), Proceedings of the First International Symposium on ULSI Process Integration, Proc. Vol. PV99-18, Electrochemical Society, 1999, p. 147. [10] T.I. Kamins, D.A.A. Ohlberg, R. Stanley Williams, Appl. Phys. Lett. 78 (2001) 2220.
www.elsevier.com/locate/physe
E ect of phosphorus on Ge=Si(0 0 1) island formation T.I. Kaminsa; ∗ , G. Medeiros-Ribeiroa; b , D.A.A. Ohlberga , R. Stanley Williamsa a Quantum
Science Research, Hewlett-Packard Laboratories, 1501 Page Mill Road, M=S 1123, Palo Alto CA 94304-1126, USA b Laboratorio Nacional de Luz Sincrotron, Campinas SP 13083-970, Brazil
Abstract Adding a phosphorus-containing species during chemical vapor deposition of Ge islands on Si(0 0 1) modi2es the island sizes and shapes, primarily by changing the surface energies and the relative surface energies of di erent surface facets. Three distinct island shapes occur, but the island types and their sequence of formation di er from those found with undoped Ge islands. The addition of phosphorus decreases the size of the multifaceted “domes”—the island shape that has a favored island size, providing an additional method for controlling the islands. The largest islands have a steep pyramidal structure not seen for undoped islands. ? 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanostructures; Self-assembly; Germanium; Doping; Chemical vapor deposition
1. Introduction Self-assembled Ge islands on Si(0 0 1) have been extensively studied [1,2]. Similar results have been obtained for islands deposited by chemical vapor deposition (CVD) and for those deposited by physical vapor deposition in ultra-high vacuum. For undoped Ge the 2rst few monolayers form a uniform wetting layer. As more Ge is added, the increasing stress causes the additional Ge to condense into islands with three di erent characteristic shapes determined by the amount of Ge deposited (Fig. 1a): {1 0 5} faceted pyramids are most numerous for low coverages ( ∼ 6 eq ML); multi-faceted “domes” for intermediate coverages ( ∼ 11 eq ML); and large,
∗ Corresponding author. Tel.: +1-650-857-5470; +1-650-236-9885. E-mail address: [email protected] (T.I. Kamins).
fax:
defective islands (“super-domes”) above about 15 eq ML. Typical volumes for pyramids and domes are 5000 and 1:5 × 104 nm3 , respectively. The shape of an island is determined by the energies of the surfaces, edges, and interfaces, along with the strain energy associated with the atoms in the volume of the island. Adding other species to an island, especially on its surface, can change the energies and kinetics and, consequently, the tendency of islands to form [3–8]. Some species, such as lead, facilitate island formation [3] while others, such as antimony, suppress island formation [4]. The shapes of the islands can also be strongly modi2ed by the added species [5]. Phosphorus is strongly adsorbed on Si and probably on Ge [9], and its concentration is likely to be higher near the surface than in the bulk. Consequently, the surface energies of the islands are expected to change when phosphorus is added, possibly modifying the shapes of the islands formed. This paper describes the inIuence of phosphorus on the island shapes and their
1386-9477/02/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 0 2 ) 0 0 2 4 8 - 5
T.I. Kamins et al. / Physica E 13 (2002) 974 – 977
975
5
10
4
Area (nm2)
10
Pyramids 3
10
Domes Minidomes
2
10
1
10
10
Fig. 1. Perspective atomic-force micrographs of (a) undoped and (b) phosphorus-doped Ge islands on Si(0 0 1); 11 eq ML of Ge were deposited in each case. (1 m × 1 m surface area.)
distributions when phosphorus is added either during the deposition or during an annealing treatment after deposition.
2. Experimental method The Ge islands were deposited onto 150-mmdiameter, Si(0 0 1) wafers in a lamp-heated, singlewafer reactor with the wafer sitting on a support with moderate thermal mass. After baking a wafer at ◦ 1150 C in H2 , a Si bu er layer was deposited using ◦ SiH2 Cl2 in H2 at 1080 C. Germanium and phosphorus ◦ were subsequently deposited at approximately 600 C by adding GeH4 and PH3 to the H2 ambient. Selected ◦ wafers were annealed at 600 C in a H2 or H2 =PH3 ambient before exposing the wafers to air. After removing the samples from the deposition chamber, they were measured using atomic-force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The thicknesses of the Ge layers were measured using Rutherford backscattering spectroscopy (RBS). Additional thick layers were deposited so that the phosphorus concentration could be determined by secondary ion mass spectrometry (SIMS).
1
10
2
3
4
10 10 Volume (nm3)
10
5
6
10
Fig. 2. Surface area vs. volume of doped and undoped islands. Points lying on a line of slope 23 correspond to the same island shape. The minidomes of the doped layers lie on the same line as the undoped domes, but are smaller. The pyramids lie on a di erent line, corresponding to their di erent shape.
3. Results and discussion When PH3 is added to the CVD deposition environment, three island types form, as for undoped layers, but the three types of islands are signi2cantly di erent from the three island types found on undoped layers, as shown in Fig. 1b. The smallest islands are very low precursor islands (“mounds”). The second island type appears to be a small dome-shaped structure (“mini-dome”). At intermediate thicknesses a third type of island is seen; layers ∼ 11 eq ML thick are dominated by large pyramids (“super-pyramids”) not seen in undoped layers. The correlation between selected island types in undoped layers and in doped layers is shown in Fig. 2, where the surface area of each island from a number of representative layers is plotted against the volume of that island. The points corresponding to islands with the same shape lie on a single line with slope 23 , while islands of di erent shapes lie on parallel lines with di erent intercepts. The pyramids of undoped layers clearly lie on one line, while the domes from undoped layers lie on another line. The mini-domes found in the doped layers appear to lie on the same line as the domes, suggesting that they are related shapes. However, the doped mini-domes are considerably smaller than the undoped domes. A typical volume of a doped
976
T.I. Kamins et al. / Physica E 13 (2002) 974 – 977
Fig. 3. AFM contour plots of doped super-pyramids, showing the rectangular bases of the super-pyramids.
mini-dome is ∼ 6000 nm3 , considerably smaller than the 1:5 × 104 nm3 found for undoped domes, but comparable to the 5000 nm3 of the undoped pyramids. Unlike the undoped layers, where the domes do not appear until more than 8 eq ML of Ge are deposited, the minidomes appear for thinner layers (∼ 6 eq ML), only slightly above the threshold of island formation. The smallest type of island found in the doped layers (“mounds”) is diMcult to analyze quantitatively because their heights are only marginally greater than the measured background roughness; consequently, the uncertainty in the reference level used to calculate the island volume signi2cantly a ects the result. Within this uncertainty, the volume of the doped mounds is found to be 300 –400 nm3 , again considerably smaller than the 5000 nm3 found for undoped pyramids. In the 11 eq ML doped layers, a signi2cant amount of the Ge is contained in large “super-pyramids”, which have their bases aligned along 1 1 0 directions [10]. The “super-pyramids” are bounded by {1 1 3} facets near their tops and steeper {1 1 1} facets near their bases. Fig. 3 shows a plan-view AFM contour plot of a doped layer dominated by super-pyramids, illustrating the characteristic rectangular contours of this type of island. In contrast, the undoped super-domes do not have extensive {1 1 1}
facets, but do have a number of other facets with bases aligned along near-0 0 1 directions. Because of these additional facets, the bases of super-domes appear approximately octagonal, rather than rectangular. Thus, the {1 1 1} facets are favored when phosphorus is present, and {0 0 n} facets are favored for undoped Ge surfaces; i.e., phosphorus on the Ge surface changes the ratio of the {1 1 1} surface energy to the {0 0 n} surface energy. Although the super-pyramids and super-domes appear to be related shapes, numerous super-pyramids are present in layers containing only 11 eq ML of phosphorus-doped Ge, while very few super-domes appear in undoped layers until the amount of Ge deposited signi2cantly exceeds 14 eq ML. Thus, related shapes are again present in doped and undoped layers, but less Ge is needed to obtain the structure when the layers are doped. We attribute the di erent behavior of undoped and doped structures to the modi2cation of the surface energies of the bounding facets by the phosphorus at the surface of the Ge islands. The amount of phosphorus included in the volume of the island is small (¡ 0:2%) compared to the amount of Ge, so the volume energy does not vary appreciably as phosphorus is added. However, phosphorus probably adsorbs strongly on the surface [9], so the surface energy is expected to be signi2cantly di erent for doped and for undoped islands. For the pyramids and the minidomes of similar volume, phosphorus apparently increases the ratio of the surface energy of {1 0 5} facets to that of the steeper facets bounding the domes. For the similarly shaped mini-domes and domes, the ratio of surface to volume increases for smaller islands. A lower surface energy allows a larger relative surface area, and therefore favors smaller islands of the same shape, consistent with the observed decrease in island size for doped layers. The e ect of adsorbed species on the surface of Ge on Si(0 0 1) has been observed previously for other species, such as Sb and In [5]. The doped Ge islands are usually bounded on top by a (0 0 1) plane, with the importance of this plane varying for di erent dopants. For phosphorus in the present experiments, no (0 0 1) plane is visible at the top of the super-pyramids. For the undoped layers, the volume distribution of pyramids decreases monotonically with increasing island size; however, the domes have a maximum in
T.I. Kamins et al. / Physica E 13 (2002) 974 – 977
their volume distribution. Obtaining such a favored island size is key to achieving a narrow size distribution. A pyramid is completely bounded by {1 0 5} facets. Hence, the interface area, surface area, and edge length are proportional, and cannot vary independently. For a dome, several crystal facets are present, and the amount of each can vary within some limits. Consequently, there is no longer a strict proportionality between the surface area, the edge length, and the interface area. Taller islands can form to accommodate increasing volume with a modest increase in surface and interface areas, providing an additional degree of freedom. Similarly to the undoped domes, the mini-domes in a doped layer also have a maximum in the volume distribution, as is reasonable because of their related shapes. Thus, the islands (mini-domes) in a ∼ 6 eq ML doped layer have a maximum in the volume distribution, while the similar-size islands (pyramids) in an undoped layer of the same thickness do not, allowing a narrower island size distribution in smaller doped islands compared to undoped islands. In addition to phosphorus changing the island shapes during deposition, the presence of phosphorus during annealing after deposition also can modify the shapes of the islands. When doped islands are annealed in H2 , the phosphorus in the islands initially retards structural rearrangement. As the phosphorus leaves the islands by di usion or evaporation, the island shapes change toward those associated with undoped islands. The detailed shape of the islands and their evolution on annealing can best be seen in the largest islands (doped “super-pyramids” and undoped “super-domes”), on which the facets are better resolved by the AFM measurements. During annealing in a H2 ambient for 2 h, the initial shape of the super-pyramids with their rectangular contours (Fig. 3) evolves toward that of the super-domes, with their characteristic octagonal contours, formed by similarly annealing undoped domes. It is likely that phosphorus can evaporate or di use from the surfaces of the doped islands as they are annealed, changing the relative surface energies of the various crystal facets so that the island shapes approach those of similarly annealed undoped islands.
977
4. Summary Adding a phosphorus-containing species during deposition of Ge islands on Si(0 0 1) modi2es the island shapes, primarily by changing the surface energies and the relative surface energies of di erent surface planes. In particular, adding phosphorus-containing species decreases the size of domes—the island shape that has a favored island size—so that the favored size is smaller in doped layers than in undoped layers, providing an additional method for controlling the islands. In addition, large pyramidal islands form in doped layers while they are not seen in undoped layers.
Acknowledgements The authors would like to thank D. Le orge, C. Nauka, M. Wong, A. Phillips, N. Shamma, and D. Gomez for experimental assistance and HewlettPackard’s former ULSI Laboratory for the use of deposition equipment. References [1] R. Stanley Williams, et al., Acc. Chem. Res. 32 (1999) 425. [2] G. Medeiros-Ribeiro, A.M. Bratkovski, T.I. Kamins, D.A.A. Ohlberg, R. Stanley Williams, Science 279 (1998) 353. [3] H. Hibino, N. Shimizu, K. Sumitomo, Y. Shinoda, T. Nishioka, T. Ogino, J. Vac. Sci. Technol. A 12 (1994) 23. [4] J.M.C. Thornton, A.A. Williams, J.E. Macdonald, R.G. van Silfhout, J.F. van der Veen, M. Finney, C. Norris, J. Vac. Sci. Technol. B 9 (1991) 2146. [5] D.J. Eaglesham, F.C. Unterwald, D.C. Jacobson, Phys. Rev. Lett. 70 (1993) 966. [6] C.K. Seal, D. Samara, S.K. Banerjee, Appl. Phys. Lett. 71 (1997) 3564. [7] O.G. Schmidt, C. Lange, K. Eberl, O. Kienzle, F. Ernst, Appl. Phys. Lett. 71 (1997) 2340. [8] D.J. Bottomley, M. Iwami, Y. Uehara, S. Ushioda, J. Vac. Sci. Technol. A 17 (1999) 698. [9] T. Kikuchi, et al., in: C.L. Claeys, et al. (Eds.), Proceedings of the First International Symposium on ULSI Process Integration, Proc. Vol. PV99-18, Electrochemical Society, 1999, p. 147. [10] T.I. Kamins, D.A.A. Ohlberg, R. Stanley Williams, Appl. Phys. Lett. 78 (2001) 2220.