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Growth Interruption Effect on the Fabrication of GaAs Concentric Multiple Rings by Droplet Epitaxy.

Somaschini C, Bietti S, Fedorov A, Koguchi N, Sanguinetti S - Nanoscale Res Lett (2010)

Bottom Line: WE PRESENT THE MOLECULAR BEAM EPITAXY FABRICATION AND OPTICAL PROPERTIES OF COMPLEX GAAS NANOSTRUCTURES BY DROPLET EPITAXY: concentric triple quantum rings.A significant difference was found between the volumes of the original droplets and the final GaAs structures.By means of atomic force microscopy and photoluminescence spectroscopy, we found that a thin GaAs quantum well-like layer is developed all over the substrate during the growth interruption times, caused by the migration of Ga in a low As background.

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ABSTRACT
WE PRESENT THE MOLECULAR BEAM EPITAXY FABRICATION AND OPTICAL PROPERTIES OF COMPLEX GAAS NANOSTRUCTURES BY DROPLET EPITAXY: concentric triple quantum rings. A significant difference was found between the volumes of the original droplets and the final GaAs structures. By means of atomic force microscopy and photoluminescence spectroscopy, we found that a thin GaAs quantum well-like layer is developed all over the substrate during the growth interruption times, caused by the migration of Ga in a low As background.

No MeSH data available.


AFM 2 × 2 micron images of Sample A (a) and B (b). Insets show a 3D-magnified picture of a single structure. Cross sectional height profiles of a single Ga droplet and concentric triple quantum ring (c)
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Figure 1: AFM 2 × 2 micron images of Sample A (a) and B (b). Insets show a 3D-magnified picture of a single structure. Cross sectional height profiles of a single Ga droplet and concentric triple quantum ring (c)

Mentions: In Fig. 1a and 1b, we show the AFM images of Sample A, where the growth was stopped just after the deposition of Ga and Sample B, where the entire procedure was performed. The Ga supply clearly resulted in the formation of nanometer-sized, nearly hemispherical Ga droplets. Their average diameter and height were around 80 and 35 nm, respectively, while the density was estimated to be around 8 × 108 cm−2. At the end of the procedure, clear CTQRs structures with good rotational symmetry appeared, with inner, middle and outer ring diameters of around 80, 140 and 210 nm, respectively, while heights were around 7 nm for the inner rings, 4 nm for middle rings and 3 nm for the outer rings. These structures showed an elongation of around 11% along the [0–11] direction, which might be due to the anisotropic diffusion of Ga on GaAs (001) surface [15]. It is worth noticing that the inner rings showed nearly the same diameter to that of the original Ga droplet and that the density of the final GaAs structures was equal to that of the original droplets, confirming that all Ga droplets transformed into GaAs triple rings at the end of the procedure. As already discussed in Ref. [14], the formation of the inner rings comes from the crystallization of the droplets edge, thus explaining the identity between droplets and inner rings diameters. On the contrary, middle and outer rings appear caused by the subsequent As supplies, as a result of the interplay between As adsorption and Ga migration from the droplets. Fig. 1c shows the cross-sectional height profile for Sample A (black line) and B (red line) obtained from the AFM images. It is important to point out that there is a significant difference between the number of Ga atoms initially supplied, corresponding to the equivalent amount of 10 MLs, and the number of Ga atoms, evaluated to be equivalent to around 3–4 MLs, inside the final structure. This difference suggests that only a fraction of the initially supplied Ga atoms effectively concur to the formation of the 3D nanostructures, while the other part, estimated to be around 6–7 MLs, might be consumed in another process. The reason for this discrepancy might be found considering our experimental procedure for the formation of CTQRs. As mentioned before, the three main steps of the growth are performed at different temperatures: 350°C for the droplets formation, 250°C for the first As supply and 300°C for the second As supply. To establish the thermal equilibrium of the substrate, we observed 1 h growth interruption times after each change of the substrate temperature. During these waiting times, a portion of the Ga atoms stored in the droplets might be consumed to form a 2D GaAs thin layer all over the substrate. We believe this phenomenon to be caused by a slow 2D crystallization of Ga atoms diffusing from the droplets, even in the absence of an intentional As supply. Indeed, an As background pressure of around 1 × 10−9 Torr is present during the whole procedure, thus providing the unintentional As pressure needed for the partial crystallization of Ga atoms. As we recently found in similar systems, a slow GaAs crystallization all over the substrate might take place in case of very low As supply to the Ga droplets [16]. In these conditions of very low As flux, the surface mobility of Ga atoms is so large that an uniform layer of GaAs might be formed all over the substrate surface. In a capped sample, embedded in an Al0.3Ga0.7As barrier, this layer can act as a quantum well, confining carriers and eventually being optically active. In order to check the optical quality of CTQRs and to confirm the presence of a thin quantum well-like GaAs layer all over the substrate coming from the unintentional crystallization of a certain amount of Ga atoms during the procedure, we performed PL investigations. The same structure of Sample B was therefore grown on another sample (Sample C) and embedded in an Al0.3Ga0.7As barrier layer. Figure 2 shows the PL spectra of Sample C excited at 15 K by a green laser (λexc = 532 nm) with a power density Pexc = 10 W/cm2 and recorded by a Peltier-cooled CCD camera. In the region where the emission from quantum-confined GaAs structures is expected, two peaks, respectively named Peak 1 at 1.55 eV and Peak 2 at 1.76 eV, appeared. A calculation on the electronic structure for the CTQRs was performed in the framework of the effective mass approximation [17-20], allowing us to attribute Peak 1 to the emission of the localized states within the CTQRs. On the other hand, on the basis of the same theoretical predictions, Peak 2 can be safely assigned to a 2D GaAs quantum well (QW), which appeared all over the substrate during the procedure. As already discussed, only a fraction of the total supplied Ga is effectively crystallized to form the Triple Rings, while the remaining 6–7 MLs of Ga atoms concur to the formation of a 2D layer of GaAs, as described above. Within the effective mass approximation, a 6–7 MLs-thick GaAs QW is expected to emit at 1.76 eV, in excellent agreement with the observed Peak 2 feature. We believe that the presence of this 2D layer might be a general feature in the samples grown with our multiple steps DE, by observing growth interruption times.


Growth Interruption Effect on the Fabrication of GaAs Concentric Multiple Rings by Droplet Epitaxy.

Somaschini C, Bietti S, Fedorov A, Koguchi N, Sanguinetti S - Nanoscale Res Lett (2010)

AFM 2 × 2 micron images of Sample A (a) and B (b). Insets show a 3D-magnified picture of a single structure. Cross sectional height profiles of a single Ga droplet and concentric triple quantum ring (c)
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Figure 1: AFM 2 × 2 micron images of Sample A (a) and B (b). Insets show a 3D-magnified picture of a single structure. Cross sectional height profiles of a single Ga droplet and concentric triple quantum ring (c)
Mentions: In Fig. 1a and 1b, we show the AFM images of Sample A, where the growth was stopped just after the deposition of Ga and Sample B, where the entire procedure was performed. The Ga supply clearly resulted in the formation of nanometer-sized, nearly hemispherical Ga droplets. Their average diameter and height were around 80 and 35 nm, respectively, while the density was estimated to be around 8 × 108 cm−2. At the end of the procedure, clear CTQRs structures with good rotational symmetry appeared, with inner, middle and outer ring diameters of around 80, 140 and 210 nm, respectively, while heights were around 7 nm for the inner rings, 4 nm for middle rings and 3 nm for the outer rings. These structures showed an elongation of around 11% along the [0–11] direction, which might be due to the anisotropic diffusion of Ga on GaAs (001) surface [15]. It is worth noticing that the inner rings showed nearly the same diameter to that of the original Ga droplet and that the density of the final GaAs structures was equal to that of the original droplets, confirming that all Ga droplets transformed into GaAs triple rings at the end of the procedure. As already discussed in Ref. [14], the formation of the inner rings comes from the crystallization of the droplets edge, thus explaining the identity between droplets and inner rings diameters. On the contrary, middle and outer rings appear caused by the subsequent As supplies, as a result of the interplay between As adsorption and Ga migration from the droplets. Fig. 1c shows the cross-sectional height profile for Sample A (black line) and B (red line) obtained from the AFM images. It is important to point out that there is a significant difference between the number of Ga atoms initially supplied, corresponding to the equivalent amount of 10 MLs, and the number of Ga atoms, evaluated to be equivalent to around 3–4 MLs, inside the final structure. This difference suggests that only a fraction of the initially supplied Ga atoms effectively concur to the formation of the 3D nanostructures, while the other part, estimated to be around 6–7 MLs, might be consumed in another process. The reason for this discrepancy might be found considering our experimental procedure for the formation of CTQRs. As mentioned before, the three main steps of the growth are performed at different temperatures: 350°C for the droplets formation, 250°C for the first As supply and 300°C for the second As supply. To establish the thermal equilibrium of the substrate, we observed 1 h growth interruption times after each change of the substrate temperature. During these waiting times, a portion of the Ga atoms stored in the droplets might be consumed to form a 2D GaAs thin layer all over the substrate. We believe this phenomenon to be caused by a slow 2D crystallization of Ga atoms diffusing from the droplets, even in the absence of an intentional As supply. Indeed, an As background pressure of around 1 × 10−9 Torr is present during the whole procedure, thus providing the unintentional As pressure needed for the partial crystallization of Ga atoms. As we recently found in similar systems, a slow GaAs crystallization all over the substrate might take place in case of very low As supply to the Ga droplets [16]. In these conditions of very low As flux, the surface mobility of Ga atoms is so large that an uniform layer of GaAs might be formed all over the substrate surface. In a capped sample, embedded in an Al0.3Ga0.7As barrier, this layer can act as a quantum well, confining carriers and eventually being optically active. In order to check the optical quality of CTQRs and to confirm the presence of a thin quantum well-like GaAs layer all over the substrate coming from the unintentional crystallization of a certain amount of Ga atoms during the procedure, we performed PL investigations. The same structure of Sample B was therefore grown on another sample (Sample C) and embedded in an Al0.3Ga0.7As barrier layer. Figure 2 shows the PL spectra of Sample C excited at 15 K by a green laser (λexc = 532 nm) with a power density Pexc = 10 W/cm2 and recorded by a Peltier-cooled CCD camera. In the region where the emission from quantum-confined GaAs structures is expected, two peaks, respectively named Peak 1 at 1.55 eV and Peak 2 at 1.76 eV, appeared. A calculation on the electronic structure for the CTQRs was performed in the framework of the effective mass approximation [17-20], allowing us to attribute Peak 1 to the emission of the localized states within the CTQRs. On the other hand, on the basis of the same theoretical predictions, Peak 2 can be safely assigned to a 2D GaAs quantum well (QW), which appeared all over the substrate during the procedure. As already discussed, only a fraction of the total supplied Ga is effectively crystallized to form the Triple Rings, while the remaining 6–7 MLs of Ga atoms concur to the formation of a 2D layer of GaAs, as described above. Within the effective mass approximation, a 6–7 MLs-thick GaAs QW is expected to emit at 1.76 eV, in excellent agreement with the observed Peak 2 feature. We believe that the presence of this 2D layer might be a general feature in the samples grown with our multiple steps DE, by observing growth interruption times.

Bottom Line: WE PRESENT THE MOLECULAR BEAM EPITAXY FABRICATION AND OPTICAL PROPERTIES OF COMPLEX GAAS NANOSTRUCTURES BY DROPLET EPITAXY: concentric triple quantum rings.A significant difference was found between the volumes of the original droplets and the final GaAs structures.By means of atomic force microscopy and photoluminescence spectroscopy, we found that a thin GaAs quantum well-like layer is developed all over the substrate during the growth interruption times, caused by the migration of Ga in a low As background.

View Article: PubMed Central - HTML - PubMed

ABSTRACT
WE PRESENT THE MOLECULAR BEAM EPITAXY FABRICATION AND OPTICAL PROPERTIES OF COMPLEX GAAS NANOSTRUCTURES BY DROPLET EPITAXY: concentric triple quantum rings. A significant difference was found between the volumes of the original droplets and the final GaAs structures. By means of atomic force microscopy and photoluminescence spectroscopy, we found that a thin GaAs quantum well-like layer is developed all over the substrate during the growth interruption times, caused by the migration of Ga in a low As background.

No MeSH data available.