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Excitation Intensity Driven PL Shifts of SiGe Islands on Patterned and Planar Si(001) Substrates: Evidence for Ge-rich Dots in Islands.

Brehm M, Grydlik M, Hackl F, Lausecker E, Fromherz T, Bauer G - Nanoscale Res Lett (2010)

Bottom Line: For randomly nucleated SiGe/Si(001) islands, a significantly stronger blue-shift of the PL spectra as a function of the excitation intensity is observed when compared to islands grown on patterned substrates side by side within the same run in a solid source molecular beam epitaxy chamber.We ascribe this different PL behavior to the much larger inhomogeneity of the Ge distribution in islands on planar substrates when compared to islands grown on pit-patterned ones, as observed previously. 3D band-structure calculations show that Ge-rich inclusions of approximately 5 nm diameter at the apex of the islands can account for the observed differences in the PL spectra.The existence of such inclusions can be regarded as a quantum dot in an island and is in agreement with recent nano-tomography experiments.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Semiconductor and Solid State Physics, University of Linz, Altenbergerstrasse 69, 4040 Linz, Austria.

ABSTRACT
For randomly nucleated SiGe/Si(001) islands, a significantly stronger blue-shift of the PL spectra as a function of the excitation intensity is observed when compared to islands grown on patterned substrates side by side within the same run in a solid source molecular beam epitaxy chamber. We ascribe this different PL behavior to the much larger inhomogeneity of the Ge distribution in islands on planar substrates when compared to islands grown on pit-patterned ones, as observed previously. 3D band-structure calculations show that Ge-rich inclusions of approximately 5 nm diameter at the apex of the islands can account for the observed differences in the PL spectra. The existence of such inclusions can be regarded as a quantum dot in an island and is in agreement with recent nano-tomography experiments.

No MeSH data available.


a and b AFM micrographs of islands grown on planar Si(001) (5.5 ML at TGe = 700°C). The color scale represents the local surface slope with respect to (001). Every color corresponds to a certain facet of the SiGe domes (blue: {105}, yellow: {113}, red: {15 3 23}). c–f: Calculated energy level scheme for islands grown on pit-patterned Si(001) substrates without regions of Ge enrichment (d, f) and islands grown on planar Si(001) substrates containing Ge-rich regions (c, e) as described in the text. The stronger Ge composition gradient of the islands grown on planar substrates compared to the ones grown on pit-patterned substrates [7-9] leads to a confining potential at the top of the island. The strong shift of the island signal on planar substrates at moderate Pexc as observed in Figs. 1c, 2 and 3 is explained by the decreased density of states (indicated by the horizontal lines) in this confining potential
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Figure 4: a and b AFM micrographs of islands grown on planar Si(001) (5.5 ML at TGe = 700°C). The color scale represents the local surface slope with respect to (001). Every color corresponds to a certain facet of the SiGe domes (blue: {105}, yellow: {113}, red: {15 3 23}). c–f: Calculated energy level scheme for islands grown on pit-patterned Si(001) substrates without regions of Ge enrichment (d, f) and islands grown on planar Si(001) substrates containing Ge-rich regions (c, e) as described in the text. The stronger Ge composition gradient of the islands grown on planar substrates compared to the ones grown on pit-patterned substrates [7-9] leads to a confining potential at the top of the island. The strong shift of the island signal on planar substrates at moderate Pexc as observed in Figs. 1c, 2 and 3 is explained by the decreased density of states (indicated by the horizontal lines) in this confining potential

Mentions: We suggest instead that the intra-dot Ge concentration inhomogeneity as reported in [7-9] is responsible for the observed large blue-shift with Pexc in the randomly nucleated islands. Ross et al. and Montalenti et al. have shown that domes evolve from pyramids by exploiting a complex shape transformation [18,19] via asymmetric Ge accumulation at the islands apex, finally, resulting in the formation of the steeper {113} and {15 3 23} facets characteristic for domes. This evolution can be clearly seen in the AFM micrographs given in Fig. 4a, 4b for which the scan size is 500 × 500 nm2 and 2 × 2 μm2, respectively. The scans were taken on a sample where 5.5 ML of Ge were deposited at 700°C. At this coverage, the shape transition from pyramids to domes can be best observed [15]. The color coding represents the local surface slope with respect to the (001) surface. In order to highlight the shape transition, we have chosen the color scale in such a way that every of the four different facets of a dome-shaped island corresponds to a different color. The high index facet {15 3 23} that has an inclination with respect to (001) of 33.6° is red, while the {113}-facets (25.2°) are yellow, the 11.3° {105}-facets blue and the (001) surface white. The asymmetric growth of the steeper facets can be directly correlated with Ge enrichment on one side of the island as described in Ref. [8]. We want to point out that such strong enrichment effects are not observed for islands on pit-patterned substrates.


Excitation Intensity Driven PL Shifts of SiGe Islands on Patterned and Planar Si(001) Substrates: Evidence for Ge-rich Dots in Islands.

Brehm M, Grydlik M, Hackl F, Lausecker E, Fromherz T, Bauer G - Nanoscale Res Lett (2010)

a and b AFM micrographs of islands grown on planar Si(001) (5.5 ML at TGe = 700°C). The color scale represents the local surface slope with respect to (001). Every color corresponds to a certain facet of the SiGe domes (blue: {105}, yellow: {113}, red: {15 3 23}). c–f: Calculated energy level scheme for islands grown on pit-patterned Si(001) substrates without regions of Ge enrichment (d, f) and islands grown on planar Si(001) substrates containing Ge-rich regions (c, e) as described in the text. The stronger Ge composition gradient of the islands grown on planar substrates compared to the ones grown on pit-patterned substrates [7-9] leads to a confining potential at the top of the island. The strong shift of the island signal on planar substrates at moderate Pexc as observed in Figs. 1c, 2 and 3 is explained by the decreased density of states (indicated by the horizontal lines) in this confining potential
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Figure 4: a and b AFM micrographs of islands grown on planar Si(001) (5.5 ML at TGe = 700°C). The color scale represents the local surface slope with respect to (001). Every color corresponds to a certain facet of the SiGe domes (blue: {105}, yellow: {113}, red: {15 3 23}). c–f: Calculated energy level scheme for islands grown on pit-patterned Si(001) substrates without regions of Ge enrichment (d, f) and islands grown on planar Si(001) substrates containing Ge-rich regions (c, e) as described in the text. The stronger Ge composition gradient of the islands grown on planar substrates compared to the ones grown on pit-patterned substrates [7-9] leads to a confining potential at the top of the island. The strong shift of the island signal on planar substrates at moderate Pexc as observed in Figs. 1c, 2 and 3 is explained by the decreased density of states (indicated by the horizontal lines) in this confining potential
Mentions: We suggest instead that the intra-dot Ge concentration inhomogeneity as reported in [7-9] is responsible for the observed large blue-shift with Pexc in the randomly nucleated islands. Ross et al. and Montalenti et al. have shown that domes evolve from pyramids by exploiting a complex shape transformation [18,19] via asymmetric Ge accumulation at the islands apex, finally, resulting in the formation of the steeper {113} and {15 3 23} facets characteristic for domes. This evolution can be clearly seen in the AFM micrographs given in Fig. 4a, 4b for which the scan size is 500 × 500 nm2 and 2 × 2 μm2, respectively. The scans were taken on a sample where 5.5 ML of Ge were deposited at 700°C. At this coverage, the shape transition from pyramids to domes can be best observed [15]. The color coding represents the local surface slope with respect to the (001) surface. In order to highlight the shape transition, we have chosen the color scale in such a way that every of the four different facets of a dome-shaped island corresponds to a different color. The high index facet {15 3 23} that has an inclination with respect to (001) of 33.6° is red, while the {113}-facets (25.2°) are yellow, the 11.3° {105}-facets blue and the (001) surface white. The asymmetric growth of the steeper facets can be directly correlated with Ge enrichment on one side of the island as described in Ref. [8]. We want to point out that such strong enrichment effects are not observed for islands on pit-patterned substrates.

Bottom Line: For randomly nucleated SiGe/Si(001) islands, a significantly stronger blue-shift of the PL spectra as a function of the excitation intensity is observed when compared to islands grown on patterned substrates side by side within the same run in a solid source molecular beam epitaxy chamber.We ascribe this different PL behavior to the much larger inhomogeneity of the Ge distribution in islands on planar substrates when compared to islands grown on pit-patterned ones, as observed previously. 3D band-structure calculations show that Ge-rich inclusions of approximately 5 nm diameter at the apex of the islands can account for the observed differences in the PL spectra.The existence of such inclusions can be regarded as a quantum dot in an island and is in agreement with recent nano-tomography experiments.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Semiconductor and Solid State Physics, University of Linz, Altenbergerstrasse 69, 4040 Linz, Austria.

ABSTRACT
For randomly nucleated SiGe/Si(001) islands, a significantly stronger blue-shift of the PL spectra as a function of the excitation intensity is observed when compared to islands grown on patterned substrates side by side within the same run in a solid source molecular beam epitaxy chamber. We ascribe this different PL behavior to the much larger inhomogeneity of the Ge distribution in islands on planar substrates when compared to islands grown on pit-patterned ones, as observed previously. 3D band-structure calculations show that Ge-rich inclusions of approximately 5 nm diameter at the apex of the islands can account for the observed differences in the PL spectra. The existence of such inclusions can be regarded as a quantum dot in an island and is in agreement with recent nano-tomography experiments.

No MeSH data available.