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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

Measured volume of the entire NW (blue column); measured volume due to the contribution of the Si or Ge diffusing adatoms (red columns); difference between the overall volume and the part ascribed to the adatoms (green columns). The calculated volume V2D which should be filled by a completely planar layer after an evaporation of such a fluence is reported in the graph with the dashed line. All data are normalized to this value.
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Figure 4: Measured volume of the entire NW (blue column); measured volume due to the contribution of the Si or Ge diffusing adatoms (red columns); difference between the overall volume and the part ascribed to the adatoms (green columns). The calculated volume V2D which should be filled by a completely planar layer after an evaporation of such a fluence is reported in the graph with the dashed line. All data are normalized to this value.

Mentions: It is demonstrated that surface adatoms diffusion has a relevant role on the NWs growth, determining the collecting area and consequently the axial growth rate. Temperature and evaporated fluence dependences support this model. On the other hand, in the typical description of the VLS mechanism, the main role is ascribed to the atoms impinging on the Au droplet, then to those diffusing into it and reaching the liquid interface. In order to quantify, which is the effective role of the two processes (direct impingement vs adatoms diffusion form the surface) in the PVD techniques, both in the cases of Si and Ge evaporations, a specific experiment that can evaluate the volume of the dip around the NWs is performed. The dip is a sort of reservoir such that the atoms missing in this volume have been consumed for the NWs growth, thus contributing to its total volume. In particular, through FIB cross sections of single Si (and Ge) NWs were locally performed, both of them being prepared at a growth temperature of 480°C; the evaporated fluence has been chosen such that the thickness of the planar layer is constant. In particular, half of the NW and the surrounding grown layer were vertically cut till the Si wafer substrate to make visible a section of the dip around the NW. The volume of this dip was measured, corresponding to the evaporated adatoms contribution to the axial growth. Furthermore, the entire volume of the NWs was measured. Since the densities of Si and Ge are different, and since the measured NWs have different radius, data are analyzed to make direct comparison possible. Both the NW and the dip volumes to the volume of a cylinder having the same radius of the NW and the same height of the 2D planar layer, named V2D, were normalized. In this study, the total and the adatoms contributions to the NW growth were obtained, which are reported in Figure 4 with blue and red columns, respectively, for both Si and Ge. In the inset of the figure, a schematic picture of the experiment is depicted. A section of the NW is drawn, and the measured volumes (of the dip and of the NW) are colored according to the column in the graph. To complete the description, it is necessary to quantitatively evaluate the contribution of the atoms which directly impinge on the Au droplet and are adsorbed into the liquid interface through the catalyst. With this purpose in view, the difference between the total NW volume and the volume of the dip was calculated. The properly normalized difference is reported in the green columns, and it represents the direct impingement contribution. The height of the green column has to be compared with the volume V2D which should be filled by a completely planar layer after an evaporation of such a fluence. This volume refers to a 2D planar layer grown under the same conditions without the presence of the gold droplet. This calculated value is reported in the graph with the dashed line.


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

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

Measured volume of the entire NW (blue column); measured volume due to the contribution of the Si or Ge diffusing adatoms (red columns); difference between the overall volume and the part ascribed to the adatoms (green columns). The calculated volume V2D which should be filled by a completely planar layer after an evaporation of such a fluence is reported in the graph with the dashed line. All data are normalized to this value.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 4: Measured volume of the entire NW (blue column); measured volume due to the contribution of the Si or Ge diffusing adatoms (red columns); difference between the overall volume and the part ascribed to the adatoms (green columns). The calculated volume V2D which should be filled by a completely planar layer after an evaporation of such a fluence is reported in the graph with the dashed line. All data are normalized to this value.
Mentions: It is demonstrated that surface adatoms diffusion has a relevant role on the NWs growth, determining the collecting area and consequently the axial growth rate. Temperature and evaporated fluence dependences support this model. On the other hand, in the typical description of the VLS mechanism, the main role is ascribed to the atoms impinging on the Au droplet, then to those diffusing into it and reaching the liquid interface. In order to quantify, which is the effective role of the two processes (direct impingement vs adatoms diffusion form the surface) in the PVD techniques, both in the cases of Si and Ge evaporations, a specific experiment that can evaluate the volume of the dip around the NWs is performed. The dip is a sort of reservoir such that the atoms missing in this volume have been consumed for the NWs growth, thus contributing to its total volume. In particular, through FIB cross sections of single Si (and Ge) NWs were locally performed, both of them being prepared at a growth temperature of 480°C; the evaporated fluence has been chosen such that the thickness of the planar layer is constant. In particular, half of the NW and the surrounding grown layer were vertically cut till the Si wafer substrate to make visible a section of the dip around the NW. The volume of this dip was measured, corresponding to the evaporated adatoms contribution to the axial growth. Furthermore, the entire volume of the NWs was measured. Since the densities of Si and Ge are different, and since the measured NWs have different radius, data are analyzed to make direct comparison possible. Both the NW and the dip volumes to the volume of a cylinder having the same radius of the NW and the same height of the 2D planar layer, named V2D, were normalized. In this study, the total and the adatoms contributions to the NW growth were obtained, which are reported in Figure 4 with blue and red columns, respectively, for both Si and Ge. In the inset of the figure, a schematic picture of the experiment is depicted. A section of the NW is drawn, and the measured volumes (of the dip and of the NW) are colored according to the column in the graph. To complete the description, it is necessary to quantitatively evaluate the contribution of the atoms which directly impinge on the Au droplet and are adsorbed into the liquid interface through the catalyst. With this purpose in view, the difference between the total NW volume and the volume of the dip was calculated. The properly normalized difference is reported in the green columns, and it represents the direct impingement contribution. The height of the green column has to be compared with the volume V2D which should be filled by a completely planar layer after an evaporation of such a fluence. This volume refers to a 2D planar layer grown under the same conditions without the presence of the gold droplet. This calculated value is reported in the graph with the dashed line.

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