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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.

No MeSH data available.


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Schematic band alignment in Ge/Si heterointerface for (a) larger (height 18 nm) and (b) smaller (height 7 nm) islands.
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Figure 5: Schematic band alignment in Ge/Si heterointerface for (a) larger (height 18 nm) and (b) smaller (height 7 nm) islands.

Mentions: where Egap,Si is the bulk Si band gap, ΔEv is the valence band offset of Ge on Si, and Econf is the confinement energy which strongly depends upon the height of the nanoislands. Figure 5 schematically represents that the confinement energy (Econf) for smaller islands (height 7 nm) is larger than that for larger islands (height 18 nm) because of quantum size effect. As all the islands are assumed to have same germanium content, the ΔEv is same for both types of islands. Hence, the EPL position is blue shifted for smaller islands versus larger ones grown on unpatterned substrates. Ge islands grown on patterned substrates (500-nm pitch) exhibit PL peaks at 0.691 and 0.655 eV along with a broad luminescence in the range of 0.710-0.850 eV, as shown in Figure 4b. The 0.691 and 0.655 eV PL peaks are assigned to NP and TO phonon-related emissions from ordered islands, respectively, which are around 75 nm in diameter and 17 nm in height. The broad luminescence band in the range of 0.710-0.890 eV observed from the patterned area compared to 0.761 eV PL peak for unpatterned one, is ascribed to the large-size variation and compositional fluctuations within the smaller islands for the former sample. Temperature-dependent PL measurements show that the PL signal from unpatterned sample quenches at a temperature higher than 45 K, whereas it exists up to 200 K for the patterned sample (500-nm pitch). Therefore, the circular ordering of Ge islands plays an important role to sustain the PL signal at a much higher temperature. We have observed enhanced PL form the highly ordered Ge nanoislands (for 500-nm pitch-pattern sample) only. The PL improvement is attributed to lateral coupling between Ge islands. From AFM and SEM images, we have calculated the inter-dot distance among the islands. For unpatterned sample, the average inter-dot distance among the Ge islands is 60 nm, whereas for patterned sample, the average inter-dot distance is 30 nm for 500-nm-pitch sample and is 47 nm for 160-nm pitch pattern. Owing to smaller inter-dot distance and improved circular ordering, the lateral coupling for 500-nm pitch sample is more dominant. Coupling between QDs can occur either (i) via a Coulomb-related interaction, such as the dipole-dipole interaction or the resonant Förster transfer process; or (ii) via a particle tunneling process, whereby the electron or hole or both can move from one dot to the other [18,19]. The actual coupling process that takes place for a given system of QDs primarily depends on the individual dot parameters, the uniformity of the dots, and the inter-dot barrier potential properties, such as the material band gap and thickness [20]. At relatively large inter-dot separations, Coulomb coupling is more likely to occur than single particle tunneling. However, at nanoscale separations, electron/hole tunneling becomes increasingly probable. Clearly, a full configuration model, such as that presented by Bester et al. [20], is necessary for a detailed understanding of such coupling mechanisms. For our case, particle tunneling is the dominant process for lateral coupling due to smaller inter-dot distance. In this case, the particle is a hole due to type-II band alignment of Ge islands on Si. The phonon-assisted hole-tunneling process is confirmed by the PL thermal-quenching activation energy (Eb ~ 38 meV). It may be noted that the high PL quenching temperature in this study is only due to lateral ordering without any vertical stacking of islands.


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)

Schematic band alignment in Ge/Si heterointerface for (a) larger (height 18 nm) and (b) smaller (height 7 nm) islands.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 5: Schematic band alignment in Ge/Si heterointerface for (a) larger (height 18 nm) and (b) smaller (height 7 nm) islands.
Mentions: where Egap,Si is the bulk Si band gap, ΔEv is the valence band offset of Ge on Si, and Econf is the confinement energy which strongly depends upon the height of the nanoislands. Figure 5 schematically represents that the confinement energy (Econf) for smaller islands (height 7 nm) is larger than that for larger islands (height 18 nm) because of quantum size effect. As all the islands are assumed to have same germanium content, the ΔEv is same for both types of islands. Hence, the EPL position is blue shifted for smaller islands versus larger ones grown on unpatterned substrates. Ge islands grown on patterned substrates (500-nm pitch) exhibit PL peaks at 0.691 and 0.655 eV along with a broad luminescence in the range of 0.710-0.850 eV, as shown in Figure 4b. The 0.691 and 0.655 eV PL peaks are assigned to NP and TO phonon-related emissions from ordered islands, respectively, which are around 75 nm in diameter and 17 nm in height. The broad luminescence band in the range of 0.710-0.890 eV observed from the patterned area compared to 0.761 eV PL peak for unpatterned one, is ascribed to the large-size variation and compositional fluctuations within the smaller islands for the former sample. Temperature-dependent PL measurements show that the PL signal from unpatterned sample quenches at a temperature higher than 45 K, whereas it exists up to 200 K for the patterned sample (500-nm pitch). Therefore, the circular ordering of Ge islands plays an important role to sustain the PL signal at a much higher temperature. We have observed enhanced PL form the highly ordered Ge nanoislands (for 500-nm pitch-pattern sample) only. The PL improvement is attributed to lateral coupling between Ge islands. From AFM and SEM images, we have calculated the inter-dot distance among the islands. For unpatterned sample, the average inter-dot distance among the Ge islands is 60 nm, whereas for patterned sample, the average inter-dot distance is 30 nm for 500-nm-pitch sample and is 47 nm for 160-nm pitch pattern. Owing to smaller inter-dot distance and improved circular ordering, the lateral coupling for 500-nm pitch sample is more dominant. Coupling between QDs can occur either (i) via a Coulomb-related interaction, such as the dipole-dipole interaction or the resonant Förster transfer process; or (ii) via a particle tunneling process, whereby the electron or hole or both can move from one dot to the other [18,19]. The actual coupling process that takes place for a given system of QDs primarily depends on the individual dot parameters, the uniformity of the dots, and the inter-dot barrier potential properties, such as the material band gap and thickness [20]. At relatively large inter-dot separations, Coulomb coupling is more likely to occur than single particle tunneling. However, at nanoscale separations, electron/hole tunneling becomes increasingly probable. Clearly, a full configuration model, such as that presented by Bester et al. [20], is necessary for a detailed understanding of such coupling mechanisms. For our case, particle tunneling is the dominant process for lateral coupling due to smaller inter-dot distance. In this case, the particle is a hole due to type-II band alignment of Ge islands on Si. The phonon-assisted hole-tunneling process is confirmed by the PL thermal-quenching activation energy (Eb ~ 38 meV). It may be noted that the high PL quenching temperature in this study is only due to lateral ordering without any vertical stacking of islands.

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