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Fabrication of self-assembled Au droplets by the systematic variation of the deposition amount on various type-B GaAs surfaces.

Sui M, Li MY, Kim ES, Lee J - Nanoscale Res Lett (2014)

Bottom Line: Under an identical growth condition, the self-assembled Au droplets of mini to supersizes are successfully synthesized via the Volmer-Weber growth mode.Depending on the DA, an apparent evolution is clearly observed in terms of the average height (AH), lateral diameter (LD), and average density (AD).In addition, accompanied with the dimensional expansion, the AD of Au droplets drastically swings on 2 orders of magnitudes from 3.2 × 10(10) to 4.2 × 10(8) cm(-2).

View Article: PubMed Central - HTML - PubMed

Affiliation: College of Electronics and Information, Kwangwoon University, Nowon-gu Seoul 139-701, South Korea.

ABSTRACT
The fabrication of self-assembled Au droplets is successfully demonstrated on various GaAs (n11)B, where n is 2, 4, 5, 7, 8, and 9, by the systematic variation of the Au deposition amount (DA) from 2 to 12 nm with subsequent annealing at 550°C. Under an identical growth condition, the self-assembled Au droplets of mini to supersizes are successfully synthesized via the Volmer-Weber growth mode. Depending on the DA, an apparent evolution is clearly observed in terms of the average height (AH), lateral diameter (LD), and average density (AD). For example, compared with the mini Au droplets with a DA of 2 nm, AH of 22.5 nm, and LD of 86.5 nm, the super Au droplets with 12-nm DA show significantly increased AH of 316% and LD of 320%, reaching an AH of 71.1 nm and LD of 276.8 nm on GaAs (211)B. In addition, accompanied with the dimensional expansion, the AD of Au droplets drastically swings on 2 orders of magnitudes from 3.2 × 10(10) to 4.2 × 10(8) cm(-2). The results are systematically analyzed with respect to the atomic force microscopy (AFM) and scanning electron microscopy (SEM) images, energy-dispersive X-ray spectrometry (EDS) spectra, cross-sectional line profiles, Fourier filter transform (FFT) power spectra, and root-mean-square (RMS) roughness as well as the droplet dimension and density summary, respectively.

No MeSH data available.


Related in: MedlinePlus

Au droplet evolution on GaAs (211)B induced by the systematic variation of the Au DA. (a) 2 nm, (b) 3 nm, (c) 4 nm, (d) 6 nm, (e) 9 nm, and (f) 12 nm. Au droplets are presented with AFM top views of 3 × 3 μm2 and 1 × 1 μm2.
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Figure 2: Au droplet evolution on GaAs (211)B induced by the systematic variation of the Au DA. (a) 2 nm, (b) 3 nm, (c) 4 nm, (d) 6 nm, (e) 9 nm, and (f) 12 nm. Au droplets are presented with AFM top views of 3 × 3 μm2 and 1 × 1 μm2.

Mentions: Figure 2 shows the self-assembled Au droplets on GaAs (211)B by the systematic variation of the Au DA from 2 to 12 nm with subsequent annealing at 550°C. Figure labels indicate the related DAs. AFM top views (3 × 3 μm2) of the corresponding samples are shown in Figure 2a,b,c,d,e,f along with enlarged 1 × 1 μm2 images below. The line profiles in Figure 3a-1,b-1,c-1,d-1,e-1,f-1 show the cross sections indicated by the white lines in the 1 × 1 μm2 images of Figure 2. The Fourier filter transform (FFT) power spectra shown in Figure 3a-1,b-1,c-1,d-1,e-1,f-1 are transformed from each AFM image. Figure 4a,b summarizes the average height (AH) and the lateral diameter (LD) of the self-assembled Au droplets, and Figure 4c,d shows the average density (AD) of the corresponding samples as well as the RMS surface roughness (Rq) as a function of the DA. The self-assembled Au droplets were fabricated based on the Volmer-Weber growth mode, thus resulting in the initial appearance of round dome-shaped droplets at 2 nm as in Figure 2a [32-34,38]. Once sufficient thermal energy for the surface diffusion is supplied, Au adatoms can be driven to diffuse. As a result of the binding energy between Au adatoms (Ea) being greater than the binding energy between Au adatoms and surface atoms (Ei), the Au droplets can be nucleated from the thin Au film during surface diffusion [39,40]. After the nucleation, nuclei can grow by absorbing nearby adatoms inward as well as merging with other smaller nuclei and thus can form into gradually larger round dome-shaped droplets. After systematic annealing with 2-nm deposition as shown in Figure 2a, dense Au droplets of round dome shapes were synthesized with an AH of 22.5 nm and LD of 86.5 nm, and the AD was 3.2 × 1010 cm-2 as plotted in Figure 4. When the DA was increased to 3 nm as shown in Figure 2b, the size of droplets was increased by × 1.38 to 31.1 nm for the AH and by × 1.23 to 106.5 nm for the LD as plotted in Figure 4a,b. Meanwhile, the corresponding AD was shapely decreased by × 3.08 from 3.2 × 1010 cm-2 to 1.04 × 1010 cm-2 as plotted in Figure 4c. Then at the 4-nm DA, the size of Au droplets was increased by × 1.44 to 44.9 nm for the AH and × 1.33 to 142.4 nm for the LD, and the AD was 3.9 × 109 cm-2 which was decreased by × 2.66. Then the trend, namely increased size along with the decreased density, was continuously maintained with the increased DA for 6 to 12 nm, and notably, at 6-nm DA as seen in Figure 2d, droplets began to show slightly irregular shapes without any preferential direction as evidenced by the round FFT power spectrum in Figure 3d-1. The LD measurements were performed along the shorter diameter. When the DA increased from 6 to 12 nm, the AH was further increased from 52.5 to 71.1 nm, the LD was increased from 186.2 to 276.8 nm, and the corresponding AD was dropped to 4.2 × 108 cm-2. Overall, with the DA variation from 2 to 12 nm, the AH of the self-assembled Au droplets was increased by × 3.16 from 22.5 to 71.1 nm and the LD was increased by × 3.20 from 86.5 to 276.8 nm as shown in Figure 4a,b. Meanwhile, the corresponding AD was decreased by nearly 2 orders from 3.2 × 1010 to 4.2 × 108 cm-2. The size of droplets can be increased with decreased density when more amount of material is provided. This evolution of size and density can be a conventional behavior, and it also can be observed with other self-assembled nanostructures [41-44]. The diffusion length (lD) can be defined as (where D is the surface diffusion coefficient and τ is the residence time), and the D has a strong proportional dependency on the substrate temperature (D ∝ Tsub). Then, driven by a high Tsub, the lD can be significantly increased. In a thermodynamic equilibrium system, nanostructures tend to increase their dimensions by absorbing nearby adatoms to lower the surface energy until reaching the equilibrium in order to keep the energy of the whole system in the lowest state. Therefore, when more adatoms exist within the lD, the increased dimensions of droplets can be expected. In terms of the uniformity, the color pattern of the FFT power spectrum represents the frequency of the height with a directionality. The FFT spectrum with the 2-nm DA in Figure 3a-1 showed a round shape due to the round shape of the droplets. With the 3-nm DA, a smaller core of the FFT pattern was observed due to the reduced height frequency associated with the reduced density in Figure 3b-1 as well as the AFM image in Figure 2b. Then, the FFT patterns in Figure 3c-1,d-1,e-1,f-1 with the increased DAs became smaller and smaller as the frequency of the height became narrower and uniform. In addition, flat tops of droplets were observed with the line profiles of the DAs of 9 and 12 nm in Figures 3e,f and 5e,f. This is in strong contrast with the round dome-shaped droplets at lower DAs. In the case of Si with the increased Au deposition amount, lateral growth of Au nanostructures occurred even with as low as approximately 5-nm DA and finally resulted in the formation of a merged Au layer at approximately 20-nm DA [45]. However, in this experiment, the droplets were still maintained even above 12-nm DA (not shown here). Although it is not very logical to compare GaAs and Si directly due to the different growth conditions such as temperature, from this result, it can be expected that the binding energy between Au adatoms and surface atoms (Ei) is weaker on GaAs surfaces than on Si (111). In other words, with increased DAs, droplets with lateral dimension expansion (coalescence) would require much higher DAs. In terms of the surface roughness (Rq) during the DA variation from 2 to 3 nm, the Rq was increased from 6.22 to 11.63 nm along with the expansion of the droplet dimensions as shown in Figure 4d. With the gradually increased DAs, the Rq in Figure 4d showed an increasing trend accompanied with increased droplet dimensions, 6.22 nm for the 2-nm DA and 11.63 for the 3-nm DA, and gradually increased to 24.37 nm at the 9-nm DA. Then, the Rq was saturated and showed a decreasing trend from there, likely due to the dominance of density decrease over the dimensional increase. Figure 6 shows the EDS spectra of the surface elemental characterization and the related SEM images of 4- and 12-nm samples. Generally, the resulting EDS spectra showed similar spectra for Ga and As with 4- and 12-nm DA as expected. The Kα1 peaks of Ga and As at 9.243 and 10.532 keV and Lα1 peaks of Ga and As at 1.096 and 1.282 keV were observed in Figure 6a,b. However, likely caused by the variation of the DA and the interaction volume of Au with the X-ray, the Au peaks show obvious difference in peak counts as seen in Figure 6a,b. For example, the Mα1 peak at 2.123 keV of the 12-nm sample showed a peak count value of approximately 22,000 while only approximately 5,000 for 4 nm. Also, the Lα1 peak at 9.711 keV showed a clear difference between 4 and 12 nm as shown in Figure 6a-2,b-2.


Fabrication of self-assembled Au droplets by the systematic variation of the deposition amount on various type-B GaAs surfaces.

Sui M, Li MY, Kim ES, Lee J - Nanoscale Res Lett (2014)

Au droplet evolution on GaAs (211)B induced by the systematic variation of the Au DA. (a) 2 nm, (b) 3 nm, (c) 4 nm, (d) 6 nm, (e) 9 nm, and (f) 12 nm. Au droplets are presented with AFM top views of 3 × 3 μm2 and 1 × 1 μm2.
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Figure 2: Au droplet evolution on GaAs (211)B induced by the systematic variation of the Au DA. (a) 2 nm, (b) 3 nm, (c) 4 nm, (d) 6 nm, (e) 9 nm, and (f) 12 nm. Au droplets are presented with AFM top views of 3 × 3 μm2 and 1 × 1 μm2.
Mentions: Figure 2 shows the self-assembled Au droplets on GaAs (211)B by the systematic variation of the Au DA from 2 to 12 nm with subsequent annealing at 550°C. Figure labels indicate the related DAs. AFM top views (3 × 3 μm2) of the corresponding samples are shown in Figure 2a,b,c,d,e,f along with enlarged 1 × 1 μm2 images below. The line profiles in Figure 3a-1,b-1,c-1,d-1,e-1,f-1 show the cross sections indicated by the white lines in the 1 × 1 μm2 images of Figure 2. The Fourier filter transform (FFT) power spectra shown in Figure 3a-1,b-1,c-1,d-1,e-1,f-1 are transformed from each AFM image. Figure 4a,b summarizes the average height (AH) and the lateral diameter (LD) of the self-assembled Au droplets, and Figure 4c,d shows the average density (AD) of the corresponding samples as well as the RMS surface roughness (Rq) as a function of the DA. The self-assembled Au droplets were fabricated based on the Volmer-Weber growth mode, thus resulting in the initial appearance of round dome-shaped droplets at 2 nm as in Figure 2a [32-34,38]. Once sufficient thermal energy for the surface diffusion is supplied, Au adatoms can be driven to diffuse. As a result of the binding energy between Au adatoms (Ea) being greater than the binding energy between Au adatoms and surface atoms (Ei), the Au droplets can be nucleated from the thin Au film during surface diffusion [39,40]. After the nucleation, nuclei can grow by absorbing nearby adatoms inward as well as merging with other smaller nuclei and thus can form into gradually larger round dome-shaped droplets. After systematic annealing with 2-nm deposition as shown in Figure 2a, dense Au droplets of round dome shapes were synthesized with an AH of 22.5 nm and LD of 86.5 nm, and the AD was 3.2 × 1010 cm-2 as plotted in Figure 4. When the DA was increased to 3 nm as shown in Figure 2b, the size of droplets was increased by × 1.38 to 31.1 nm for the AH and by × 1.23 to 106.5 nm for the LD as plotted in Figure 4a,b. Meanwhile, the corresponding AD was shapely decreased by × 3.08 from 3.2 × 1010 cm-2 to 1.04 × 1010 cm-2 as plotted in Figure 4c. Then at the 4-nm DA, the size of Au droplets was increased by × 1.44 to 44.9 nm for the AH and × 1.33 to 142.4 nm for the LD, and the AD was 3.9 × 109 cm-2 which was decreased by × 2.66. Then the trend, namely increased size along with the decreased density, was continuously maintained with the increased DA for 6 to 12 nm, and notably, at 6-nm DA as seen in Figure 2d, droplets began to show slightly irregular shapes without any preferential direction as evidenced by the round FFT power spectrum in Figure 3d-1. The LD measurements were performed along the shorter diameter. When the DA increased from 6 to 12 nm, the AH was further increased from 52.5 to 71.1 nm, the LD was increased from 186.2 to 276.8 nm, and the corresponding AD was dropped to 4.2 × 108 cm-2. Overall, with the DA variation from 2 to 12 nm, the AH of the self-assembled Au droplets was increased by × 3.16 from 22.5 to 71.1 nm and the LD was increased by × 3.20 from 86.5 to 276.8 nm as shown in Figure 4a,b. Meanwhile, the corresponding AD was decreased by nearly 2 orders from 3.2 × 1010 to 4.2 × 108 cm-2. The size of droplets can be increased with decreased density when more amount of material is provided. This evolution of size and density can be a conventional behavior, and it also can be observed with other self-assembled nanostructures [41-44]. The diffusion length (lD) can be defined as (where D is the surface diffusion coefficient and τ is the residence time), and the D has a strong proportional dependency on the substrate temperature (D ∝ Tsub). Then, driven by a high Tsub, the lD can be significantly increased. In a thermodynamic equilibrium system, nanostructures tend to increase their dimensions by absorbing nearby adatoms to lower the surface energy until reaching the equilibrium in order to keep the energy of the whole system in the lowest state. Therefore, when more adatoms exist within the lD, the increased dimensions of droplets can be expected. In terms of the uniformity, the color pattern of the FFT power spectrum represents the frequency of the height with a directionality. The FFT spectrum with the 2-nm DA in Figure 3a-1 showed a round shape due to the round shape of the droplets. With the 3-nm DA, a smaller core of the FFT pattern was observed due to the reduced height frequency associated with the reduced density in Figure 3b-1 as well as the AFM image in Figure 2b. Then, the FFT patterns in Figure 3c-1,d-1,e-1,f-1 with the increased DAs became smaller and smaller as the frequency of the height became narrower and uniform. In addition, flat tops of droplets were observed with the line profiles of the DAs of 9 and 12 nm in Figures 3e,f and 5e,f. This is in strong contrast with the round dome-shaped droplets at lower DAs. In the case of Si with the increased Au deposition amount, lateral growth of Au nanostructures occurred even with as low as approximately 5-nm DA and finally resulted in the formation of a merged Au layer at approximately 20-nm DA [45]. However, in this experiment, the droplets were still maintained even above 12-nm DA (not shown here). Although it is not very logical to compare GaAs and Si directly due to the different growth conditions such as temperature, from this result, it can be expected that the binding energy between Au adatoms and surface atoms (Ei) is weaker on GaAs surfaces than on Si (111). In other words, with increased DAs, droplets with lateral dimension expansion (coalescence) would require much higher DAs. In terms of the surface roughness (Rq) during the DA variation from 2 to 3 nm, the Rq was increased from 6.22 to 11.63 nm along with the expansion of the droplet dimensions as shown in Figure 4d. With the gradually increased DAs, the Rq in Figure 4d showed an increasing trend accompanied with increased droplet dimensions, 6.22 nm for the 2-nm DA and 11.63 for the 3-nm DA, and gradually increased to 24.37 nm at the 9-nm DA. Then, the Rq was saturated and showed a decreasing trend from there, likely due to the dominance of density decrease over the dimensional increase. Figure 6 shows the EDS spectra of the surface elemental characterization and the related SEM images of 4- and 12-nm samples. Generally, the resulting EDS spectra showed similar spectra for Ga and As with 4- and 12-nm DA as expected. The Kα1 peaks of Ga and As at 9.243 and 10.532 keV and Lα1 peaks of Ga and As at 1.096 and 1.282 keV were observed in Figure 6a,b. However, likely caused by the variation of the DA and the interaction volume of Au with the X-ray, the Au peaks show obvious difference in peak counts as seen in Figure 6a,b. For example, the Mα1 peak at 2.123 keV of the 12-nm sample showed a peak count value of approximately 22,000 while only approximately 5,000 for 4 nm. Also, the Lα1 peak at 9.711 keV showed a clear difference between 4 and 12 nm as shown in Figure 6a-2,b-2.

Bottom Line: Under an identical growth condition, the self-assembled Au droplets of mini to supersizes are successfully synthesized via the Volmer-Weber growth mode.Depending on the DA, an apparent evolution is clearly observed in terms of the average height (AH), lateral diameter (LD), and average density (AD).In addition, accompanied with the dimensional expansion, the AD of Au droplets drastically swings on 2 orders of magnitudes from 3.2 × 10(10) to 4.2 × 10(8) cm(-2).

View Article: PubMed Central - HTML - PubMed

Affiliation: College of Electronics and Information, Kwangwoon University, Nowon-gu Seoul 139-701, South Korea.

ABSTRACT
The fabrication of self-assembled Au droplets is successfully demonstrated on various GaAs (n11)B, where n is 2, 4, 5, 7, 8, and 9, by the systematic variation of the Au deposition amount (DA) from 2 to 12 nm with subsequent annealing at 550°C. Under an identical growth condition, the self-assembled Au droplets of mini to supersizes are successfully synthesized via the Volmer-Weber growth mode. Depending on the DA, an apparent evolution is clearly observed in terms of the average height (AH), lateral diameter (LD), and average density (AD). For example, compared with the mini Au droplets with a DA of 2 nm, AH of 22.5 nm, and LD of 86.5 nm, the super Au droplets with 12-nm DA show significantly increased AH of 316% and LD of 320%, reaching an AH of 71.1 nm and LD of 276.8 nm on GaAs (211)B. In addition, accompanied with the dimensional expansion, the AD of Au droplets drastically swings on 2 orders of magnitudes from 3.2 × 10(10) to 4.2 × 10(8) cm(-2). The results are systematically analyzed with respect to the atomic force microscopy (AFM) and scanning electron microscopy (SEM) images, energy-dispersive X-ray spectrometry (EDS) spectra, cross-sectional line profiles, Fourier filter transform (FFT) power spectra, and root-mean-square (RMS) roughness as well as the droplet dimension and density summary, respectively.

No MeSH data available.


Related in: MedlinePlus