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Improved infrared photoluminescence characteristics from circularly ordered self-assembled Ge islands.

Das S, Das K, Singha RK, Manna S, Dhar A, Ray SK, Raychaudhuri AK - Nanoscale Res Lett (2011)

Bottom Line: The formation of circularly ordered Ge-islands on Si(001) has been achieved because of nonuniform strain field around the periphery of the holes patterned by focused ion beam in combination with a self-assembled growth using molecular beam epitaxy.The photoluminescence (PL) spectra obtained from patterned areas (i.e., ordered islands) show a significant signal enhancement, which sustained till 200 K, without any vertical stacking of islands.The origin of two activation energies in temperature-dependent PL spectra of the ordered islands has been explained in detail.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics and Meteorology, Indian Institute of Technology Kharagpur, Kharagpur 721302, India. physkr@phy.iitkgp.ernet.in.

ABSTRACT
The formation of circularly ordered Ge-islands on Si(001) has been achieved because of nonuniform strain field around the periphery of the holes patterned by focused ion beam in combination with a self-assembled growth using molecular beam epitaxy. The photoluminescence (PL) spectra obtained from patterned areas (i.e., ordered islands) show a significant signal enhancement, which sustained till 200 K, without any vertical stacking of islands. The origin of two activation energies in temperature-dependent PL spectra of the ordered islands has been explained in detail.

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SEM images of Ge islands grown on a FIB pre-patterned region with (a) smaller (approx. 160 nm) and (b) larger (approx. 500 nm) spacing of holes. The circles drawn in Figure 1a show the ordering of islands along the circular periphery. The inset in (b) shows the array of islands in higher magnification.
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Figure 2: SEM images of Ge islands grown on a FIB pre-patterned region with (a) smaller (approx. 160 nm) and (b) larger (approx. 500 nm) spacing of holes. The circles drawn in Figure 1a show the ordering of islands along the circular periphery. The inset in (b) shows the array of islands in higher magnification.

Mentions: Microscopic analysis has been carried out in patterned as well as the unpatterned substrates to compare the nature of growth of Ge nanoislands. These experiments have been primarily done at different alloy compositions and growth conditions, where previous studies [11,12] have shown that it is possible to constrain island growth to occur only at the energetically favored edges. Figure 1a shows the atomic force microscopy (AFM) image of the unpatterned regions. From Figure 1b, it is clear that islands distribution is nearly bimodal in unpatterned area. The smaller islands have an average diameter of approx. 65 nm and height approx. 7 nm, whereas the larger ones are approx. 95 nm in diameter and approx. 18 nm in height. Many researchers observed clear bimodal distribution in the epitaxial growth of Ge on Si [13,14]. Medeiros-Ribeiro et al. [1] showed an energy diagram predicting the existence of two energy minima for the different island shapes at fixed volumes. Ross et al. [14] reported a bimodal distribution attributed to the coarsening process during growth, which leads to a shift in the island size distribution with time. Figure 2a shows the scanning electron microscopy (SEM) image of the sample where Ge islands were grown in the patterned region for 100-nm pit depths. Typically, the holes are of about 120 nm in diameter with a spacing of around 160 nm. It is clear that the islands have nucleated around the periphery of the holes in a circular fashion. This nature of island formation in a circular fashion is present around almost all the holes. Figure 2b shows the SEM micrograph of the grown islands on FIB-patterned substrate with higher pitches (about 500 nm). The preferential circular organization of Ge QDs is more pronounced in this case, as the pitch is large compared to the hole sizes. Therefore, the lateral ordering of islands on patterned substrates has been found to be dependent on the pitch of the holes. Figure 3a,b represents the size distribution of the Ge nano-islands on patterned substrate with 160-and 500-nm pitch, respectively. From Figure 3a,b, it is clear that there is a wide size distribution of Ge islands on patterned substrate. The patterned substrate consists of pits and unpatterned area in between the pits, which leads to a large variation of strain field along the surface. The variation of stain field leads to a wider size distribution.


Improved infrared photoluminescence characteristics from circularly ordered self-assembled Ge islands.

Das S, Das K, Singha RK, Manna S, Dhar A, Ray SK, Raychaudhuri AK - Nanoscale Res Lett (2011)

SEM images of Ge islands grown on a FIB pre-patterned region with (a) smaller (approx. 160 nm) and (b) larger (approx. 500 nm) spacing of holes. The circles drawn in Figure 1a show the ordering of islands along the circular periphery. The inset in (b) shows the array of islands in higher magnification.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211512&req=5

Figure 2: SEM images of Ge islands grown on a FIB pre-patterned region with (a) smaller (approx. 160 nm) and (b) larger (approx. 500 nm) spacing of holes. The circles drawn in Figure 1a show the ordering of islands along the circular periphery. The inset in (b) shows the array of islands in higher magnification.
Mentions: Microscopic analysis has been carried out in patterned as well as the unpatterned substrates to compare the nature of growth of Ge nanoislands. These experiments have been primarily done at different alloy compositions and growth conditions, where previous studies [11,12] have shown that it is possible to constrain island growth to occur only at the energetically favored edges. Figure 1a shows the atomic force microscopy (AFM) image of the unpatterned regions. From Figure 1b, it is clear that islands distribution is nearly bimodal in unpatterned area. The smaller islands have an average diameter of approx. 65 nm and height approx. 7 nm, whereas the larger ones are approx. 95 nm in diameter and approx. 18 nm in height. Many researchers observed clear bimodal distribution in the epitaxial growth of Ge on Si [13,14]. Medeiros-Ribeiro et al. [1] showed an energy diagram predicting the existence of two energy minima for the different island shapes at fixed volumes. Ross et al. [14] reported a bimodal distribution attributed to the coarsening process during growth, which leads to a shift in the island size distribution with time. Figure 2a shows the scanning electron microscopy (SEM) image of the sample where Ge islands were grown in the patterned region for 100-nm pit depths. Typically, the holes are of about 120 nm in diameter with a spacing of around 160 nm. It is clear that the islands have nucleated around the periphery of the holes in a circular fashion. This nature of island formation in a circular fashion is present around almost all the holes. Figure 2b shows the SEM micrograph of the grown islands on FIB-patterned substrate with higher pitches (about 500 nm). The preferential circular organization of Ge QDs is more pronounced in this case, as the pitch is large compared to the hole sizes. Therefore, the lateral ordering of islands on patterned substrates has been found to be dependent on the pitch of the holes. Figure 3a,b represents the size distribution of the Ge nano-islands on patterned substrate with 160-and 500-nm pitch, respectively. From Figure 3a,b, it is clear that there is a wide size distribution of Ge islands on patterned substrate. The patterned substrate consists of pits and unpatterned area in between the pits, which leads to a large variation of strain field along the surface. The variation of stain field leads to a wider size distribution.

Bottom Line: The formation of circularly ordered Ge-islands on Si(001) has been achieved because of nonuniform strain field around the periphery of the holes patterned by focused ion beam in combination with a self-assembled growth using molecular beam epitaxy.The photoluminescence (PL) spectra obtained from patterned areas (i.e., ordered islands) show a significant signal enhancement, which sustained till 200 K, without any vertical stacking of islands.The origin of two activation energies in temperature-dependent PL spectra of the ordered islands has been explained in detail.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics and Meteorology, Indian Institute of Technology Kharagpur, Kharagpur 721302, India. physkr@phy.iitkgp.ernet.in.

ABSTRACT
The formation of circularly ordered Ge-islands on Si(001) has been achieved because of nonuniform strain field around the periphery of the holes patterned by focused ion beam in combination with a self-assembled growth using molecular beam epitaxy. The photoluminescence (PL) spectra obtained from patterned areas (i.e., ordered islands) show a significant signal enhancement, which sustained till 200 K, without any vertical stacking of islands. The origin of two activation energies in temperature-dependent PL spectra of the ordered islands has been explained in detail.

No MeSH data available.


Related in: MedlinePlus