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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.


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Schematic picture of the Si NWs (left-hand side) and of the Ge NWs growth on Si substrate (right-hand side), in different fluence regimes. Color scale refers to the evolution of the growth as a function of the evaporated fluence.
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Figure 5: Schematic picture of the Si NWs (left-hand side) and of the Ge NWs growth on Si substrate (right-hand side), in different fluence regimes. Color scale refers to the evolution of the growth as a function of the evaporated fluence.

Mentions: Figure 5 shows the schematic picture of the Si (left-hand side) and of the Ge NWs (right-hand side) growth on a Si substrate. Color scale refers to the evolution of the growth as a function of the evaporated fluence, as indicated in the scale bar. The top panel refers to the first stages of the growth, corresponding to an evaporated fluence, named Φ1, at which Si NWs are still not observable outside the planar layer, while Ge NWs have started to grow with their maximum possible axial rate. In other words, Φ1 is higher than the Ge incubation fluence ΦcGe and less than the Si incubation fluence ΦcSi, i.e., in the range between 0.25 and 1.75 × 1018 cm-2. It is clear that Si axial rate is equal to the planar one, but the gold droplets are still active as they have not been covered and they are visible from the top of the sample. On the other hand, Ge adatoms are contributing to the planar layer also, but as they can move on the surface faster than Si adatoms, the Ge incubation fluence has been reached, and we observe very tall Ge NWs despite the low evaporated fluence, and the dip around the NW just being formed. The picture represents this stage. The Ge adatoms path from the dip to the liquid eutectic interface is indicated by arrows. The width of the dip is correlated to the Ge adatoms mean diffusion length, RcGe. The bottom panel refers to the subsequent stages, in which both Si and Ge NWs are growing. This occurs at evaporated fluences higher than the Si and Ge incubation fluences but less than the respective saturation fluences, named, ΦsatSi and ΦsatGe. Strong differences are observable. In fact, the picture clearly depicts what we discussed about the growth rate measurements in Figure 3. Si NWs are growing with an axial rate which increases with increasing evaporated fluence (note the color scale in the picture) so that the Si NWs length strongly increases at the later stages only. Actually, the total Si NWs length is lower than that of Ge NWs. The dip in this case is also visible, and it is continuously used as a reservoir for the growth. Its width, being determined by the Si adatoms diffusion length RcSi, is narrower than that of Ge. In the fluence regime that are now being analyzed, the Ge axial growth rate is decreasing with increasing evaporated fluence. In fact, the Ge NW is so tall that Ge adatoms cannot reach the gold droplet, because of their finite diffusion length. Therefore, the contribution of the adatoms for the growth is reduced, and adatoms are favored to contribute to the planar layer growth. As a consequence, the Ge NWs length measured outside the planar layer saturates. At the final stage, the collecting area has been totally filled by the adatoms. If the diffusion mean length could be similar for Si and Ge, then NWs should grow in the same regime. Actually, this condition requires either a Ge growth temperature less than the eutectic one or a Si growth temperature so high that desorption process would be dominant.


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

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

Schematic picture of the Si NWs (left-hand side) and of the Ge NWs growth on Si substrate (right-hand side), in different fluence regimes. Color scale refers to the evolution of the growth as a function of the evaporated fluence.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211214&req=5

Figure 5: Schematic picture of the Si NWs (left-hand side) and of the Ge NWs growth on Si substrate (right-hand side), in different fluence regimes. Color scale refers to the evolution of the growth as a function of the evaporated fluence.
Mentions: Figure 5 shows the schematic picture of the Si (left-hand side) and of the Ge NWs (right-hand side) growth on a Si substrate. Color scale refers to the evolution of the growth as a function of the evaporated fluence, as indicated in the scale bar. The top panel refers to the first stages of the growth, corresponding to an evaporated fluence, named Φ1, at which Si NWs are still not observable outside the planar layer, while Ge NWs have started to grow with their maximum possible axial rate. In other words, Φ1 is higher than the Ge incubation fluence ΦcGe and less than the Si incubation fluence ΦcSi, i.e., in the range between 0.25 and 1.75 × 1018 cm-2. It is clear that Si axial rate is equal to the planar one, but the gold droplets are still active as they have not been covered and they are visible from the top of the sample. On the other hand, Ge adatoms are contributing to the planar layer also, but as they can move on the surface faster than Si adatoms, the Ge incubation fluence has been reached, and we observe very tall Ge NWs despite the low evaporated fluence, and the dip around the NW just being formed. The picture represents this stage. The Ge adatoms path from the dip to the liquid eutectic interface is indicated by arrows. The width of the dip is correlated to the Ge adatoms mean diffusion length, RcGe. The bottom panel refers to the subsequent stages, in which both Si and Ge NWs are growing. This occurs at evaporated fluences higher than the Si and Ge incubation fluences but less than the respective saturation fluences, named, ΦsatSi and ΦsatGe. Strong differences are observable. In fact, the picture clearly depicts what we discussed about the growth rate measurements in Figure 3. Si NWs are growing with an axial rate which increases with increasing evaporated fluence (note the color scale in the picture) so that the Si NWs length strongly increases at the later stages only. Actually, the total Si NWs length is lower than that of Ge NWs. The dip in this case is also visible, and it is continuously used as a reservoir for the growth. Its width, being determined by the Si adatoms diffusion length RcSi, is narrower than that of Ge. In the fluence regime that are now being analyzed, the Ge axial growth rate is decreasing with increasing evaporated fluence. In fact, the Ge NW is so tall that Ge adatoms cannot reach the gold droplet, because of their finite diffusion length. Therefore, the contribution of the adatoms for the growth is reduced, and adatoms are favored to contribute to the planar layer growth. As a consequence, the Ge NWs length measured outside the planar layer saturates. At the final stage, the collecting area has been totally filled by the adatoms. If the diffusion mean length could be similar for Si and Ge, then NWs should grow in the same regime. Actually, this condition requires either a Ge growth temperature less than the eutectic one or a Si growth temperature so high that desorption process would be dominant.

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