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Kinetics of Si and Ge nanowires growth through electron beam evaporation.

Artoni P, Pecora EF, Irrera A, Priolo F - Nanoscale Res Lett (2011)

Bottom Line: Moreover, Si NWs growth requires a higher evaporated fluence before the NWs become to be visible.These differences arise in the different kinetics behaviors of these systems.The authors investigate the microscopic growth mechanisms elucidating the contribution of the adatoms diffusion as a function of the evaporated atoms direct impingement, demonstrating that adatoms play a key role in physical vapor deposition (PVD) NWs growth.

View Article: PubMed Central - HTML - PubMed

Affiliation: MATIS IMM-CNR, Via Santa Sofia 64, I-95123 Catania, Italy. alessia.irrera@ct.infn.it.

ABSTRACT
Si and Ge have the same crystalline structure, and although Si-Au and Ge-Au binary alloys are thermodynamically similar (same phase diagram, with the eutectic temperature of about 360°C), in this study, it is proved that Si and Ge nanowires (NWs) growth by electron beam evaporation occurs in very different temperature ranges and fluence regimes. In particular, it is demonstrated that Ge growth occurs just above the eutectic temperature, while Si NWs growth occurs at temperature higher than the eutectic temperature, at about 450°C. Moreover, Si NWs growth requires a higher evaporated fluence before the NWs become to be visible. These differences arise in the different kinetics behaviors of these systems. The authors investigate the microscopic growth mechanisms elucidating the contribution of the adatoms diffusion as a function of the evaporated atoms direct impingement, demonstrating that adatoms play a key role in physical vapor deposition (PVD) NWs growth. The concept of incubation fluence, which is necessary for an interpretation of NWs growth in PVD growth conditions, is highlighted.

No MeSH data available.


Related in: MedlinePlus

SEM images of Si NWs and Ge NWs. (a) Low-magnification SEM images of sample of Si NWs. The bottom inset shows a higher magnification of a Si NW. The top inset is a cross-sectional SEM image of the sample showing the substrate and the 2D Si layer on top of it. (b) Low-magnification SEM images of Ge NWs. The bottom inset shows a Ge NW. In the top inset, the cross section of the sample is shown, and the Si substrate, the 2D Ge layer, and some NWs are visible
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Figure 1: SEM images of Si NWs and Ge NWs. (a) Low-magnification SEM images of sample of Si NWs. The bottom inset shows a higher magnification of a Si NW. The top inset is a cross-sectional SEM image of the sample showing the substrate and the 2D Si layer on top of it. (b) Low-magnification SEM images of Ge NWs. The bottom inset shows a Ge NW. In the top inset, the cross section of the sample is shown, and the Si substrate, the 2D Ge layer, and some NWs are visible

Mentions: Figure 1 shows the low-magnification SEM images of typical samples of Si (a) and Ge (b) NWs. In particular, these were prepared after evaporation of a Si fluence of 1.75 × 1018 atoms cm-2 (Figure 1a) or a Ge fluence of 1.00 × 1018 atoms cm-2 (Figure 1b). The bottom insets of Figure 1a, b show high-magnification images of Si and Ge NWs samples, respectively. The growth temperature was set at 480°C in both cases. Both Si and Ge NWs are clearly visible with the Au droplet standing on top of them. The growth direction of these NWs is (111) (they are perpendicular to the substrate), since these growth parameters lead to a major percentage of (111) NWs, while other crystallographic directions are observed at different growth temperatures or evaporated fluences, as has already been demonstrated earlier [14,15]. A key issue of the NWs growth by EBE is the competition between the axial growth and the planar growth of a layer all over the sample. In fact, the evaporated atoms reaching the heated substrate from the vapor phase can directly impinge on the gold droplet or interact with the overall substrate, becoming adatoms. Depending on the substrate temperature, they can diffuse on the surface of the sample, and if they are not so far from the Au droplet, then they can diffuse along the NW sidewall eventually reaching the metal/semiconductor interface contributing effectively to the axial growth. On the other hand, the adatoms stop when they form more than one stable bonding with the surface atoms, contributing to the growth of a planar layer. A film is clearly visible both in Si and Ge NWs samples growing on top of the substrate. A cross-sectional SEM images of Si and Ge NWs samples are shown in the top inset of Figure 1a, b, respectively: the Si and Ge layer on top of the Si substrate is visible, and the Si and Ge NWs overcome this layer. Such a competition between the planar versus the axial growth has been modeled by Dubrovskii et al. [19] and it has been observed in the NWs growth both by MBE [20,21] and EBE [14,22,23]. In particular, the presence of a dip around the NWs clearly demonstrates that the atoms missing from the planar layer act as a sort of reservoir contributing to the axial growth of the NWs. The surface area of this dip is named as the "collecting area." Only atoms impinging inside this area can potentially contribute to the NWs axial growth. For an effective contribution, these adatoms should not be desorbed from the substrate, or be adsorbed (in this way, they would contribute to the planar layer growth), and finally they have to be able to reach the growing NW up to the metal/semiconductor interface. The relevant role played by kinetic processes for the NWs growth in PVD techniques is evident as well as the thermodynamic constraints. It has been recently demonstrated for Si grown by EBE, by investigating the role of oxygen contaminations in relation to the adatoms surface diffusivity [18].


Kinetics of Si and Ge nanowires growth through electron beam evaporation.

Artoni P, Pecora EF, Irrera A, Priolo F - Nanoscale Res Lett (2011)

SEM images of Si NWs and Ge NWs. (a) Low-magnification SEM images of sample of Si NWs. The bottom inset shows a higher magnification of a Si NW. The top inset is a cross-sectional SEM image of the sample showing the substrate and the 2D Si layer on top of it. (b) Low-magnification SEM images of Ge NWs. The bottom inset shows a Ge NW. In the top inset, the cross section of the sample is shown, and the Si substrate, the 2D Ge layer, and some NWs are visible
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Related In: Results  -  Collection

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Figure 1: SEM images of Si NWs and Ge NWs. (a) Low-magnification SEM images of sample of Si NWs. The bottom inset shows a higher magnification of a Si NW. The top inset is a cross-sectional SEM image of the sample showing the substrate and the 2D Si layer on top of it. (b) Low-magnification SEM images of Ge NWs. The bottom inset shows a Ge NW. In the top inset, the cross section of the sample is shown, and the Si substrate, the 2D Ge layer, and some NWs are visible
Mentions: Figure 1 shows the low-magnification SEM images of typical samples of Si (a) and Ge (b) NWs. In particular, these were prepared after evaporation of a Si fluence of 1.75 × 1018 atoms cm-2 (Figure 1a) or a Ge fluence of 1.00 × 1018 atoms cm-2 (Figure 1b). The bottom insets of Figure 1a, b show high-magnification images of Si and Ge NWs samples, respectively. The growth temperature was set at 480°C in both cases. Both Si and Ge NWs are clearly visible with the Au droplet standing on top of them. The growth direction of these NWs is (111) (they are perpendicular to the substrate), since these growth parameters lead to a major percentage of (111) NWs, while other crystallographic directions are observed at different growth temperatures or evaporated fluences, as has already been demonstrated earlier [14,15]. A key issue of the NWs growth by EBE is the competition between the axial growth and the planar growth of a layer all over the sample. In fact, the evaporated atoms reaching the heated substrate from the vapor phase can directly impinge on the gold droplet or interact with the overall substrate, becoming adatoms. Depending on the substrate temperature, they can diffuse on the surface of the sample, and if they are not so far from the Au droplet, then they can diffuse along the NW sidewall eventually reaching the metal/semiconductor interface contributing effectively to the axial growth. On the other hand, the adatoms stop when they form more than one stable bonding with the surface atoms, contributing to the growth of a planar layer. A film is clearly visible both in Si and Ge NWs samples growing on top of the substrate. A cross-sectional SEM images of Si and Ge NWs samples are shown in the top inset of Figure 1a, b, respectively: the Si and Ge layer on top of the Si substrate is visible, and the Si and Ge NWs overcome this layer. Such a competition between the planar versus the axial growth has been modeled by Dubrovskii et al. [19] and it has been observed in the NWs growth both by MBE [20,21] and EBE [14,22,23]. In particular, the presence of a dip around the NWs clearly demonstrates that the atoms missing from the planar layer act as a sort of reservoir contributing to the axial growth of the NWs. The surface area of this dip is named as the "collecting area." Only atoms impinging inside this area can potentially contribute to the NWs axial growth. For an effective contribution, these adatoms should not be desorbed from the substrate, or be adsorbed (in this way, they would contribute to the planar layer growth), and finally they have to be able to reach the growing NW up to the metal/semiconductor interface. The relevant role played by kinetic processes for the NWs growth in PVD techniques is evident as well as the thermodynamic constraints. It has been recently demonstrated for Si grown by EBE, by investigating the role of oxygen contaminations in relation to the adatoms surface diffusivity [18].

Bottom Line: Moreover, Si NWs growth requires a higher evaporated fluence before the NWs become to be visible.These differences arise in the different kinetics behaviors of these systems.The authors investigate the microscopic growth mechanisms elucidating the contribution of the adatoms diffusion as a function of the evaporated atoms direct impingement, demonstrating that adatoms play a key role in physical vapor deposition (PVD) NWs growth.

View Article: PubMed Central - HTML - PubMed

Affiliation: MATIS IMM-CNR, Via Santa Sofia 64, I-95123 Catania, Italy. alessia.irrera@ct.infn.it.

ABSTRACT
Si and Ge have the same crystalline structure, and although Si-Au and Ge-Au binary alloys are thermodynamically similar (same phase diagram, with the eutectic temperature of about 360°C), in this study, it is proved that Si and Ge nanowires (NWs) growth by electron beam evaporation occurs in very different temperature ranges and fluence regimes. In particular, it is demonstrated that Ge growth occurs just above the eutectic temperature, while Si NWs growth occurs at temperature higher than the eutectic temperature, at about 450°C. Moreover, Si NWs growth requires a higher evaporated fluence before the NWs become to be visible. These differences arise in the different kinetics behaviors of these systems. The authors investigate the microscopic growth mechanisms elucidating the contribution of the adatoms diffusion as a function of the evaporated atoms direct impingement, demonstrating that adatoms play a key role in physical vapor deposition (PVD) NWs growth. The concept of incubation fluence, which is necessary for an interpretation of NWs growth in PVD growth conditions, is highlighted.

No MeSH data available.


Related in: MedlinePlus