Fabrication of Strain-Relaxed Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>x</Subscript>/Si(001) Buffer Layers of Low Surface Roughness

The results are presented of the fabrication of strain-relaxed graded Si_{1 − x} Ge_x/Si(001) buffer layers with a maximum Ge fraction of about 0.25 that have a low density of threading dislocations (<10⁶ cm⁻²) and low surface roughness. The buffer layers are grown by atmospheric-pressure hydride CVD. It is found that chemical mechanical polishing can reduce their surface roughness to a level comparable with that of the original Si(001) substrates. It is shown that the polished buffer layers can serve as substrates for MBE-grown SiGe/Si heterostructures.__________Translated from Mikroelektronika, Vol. 34, No. 4, 2005, pp. 243–250.Original Russian Text Copyright © 2005 by Vostokov, Drozdov, Krasil’nik, Kuznetsov, Novikov, Perevoshchikov, Shaleev.     

Comparative analysis of photoluminescence and electroluminescence of multilayer structures with self-assembled Ge(Si)/Si(001) island

Comparative studies of the photoluminescence and electroluminescence of multilayer structures with self-assembled Ge(Si)/Si(001) islands are carried out. The luminescence signal from the islands is observable up to room temperature. Annealing of the structures induces a shift of the luminescence peak to shorter wavelengths. The shift is temperature dependent, making possible controllable variations in the spectral position of the luminescence peak of the Ge(Si) islands in the range from 1.3 to 1.55 μm. The enhancement of the temperature quenching of photoluminescence of the islands with increasing annealing temperature is attributed to the decrease in the Ge content in the islands during annealing and, as a result, to a decrease in the depth of the potential well for holes in the islands. The well-pronounced suppression of the temperature quenching of electroluminescence of the Ge(Si) islands in the unannealed structure with increasing pumping current is demonstrated.     

Luminescence of Spatially Ordered Self-Assembled Solitary Ge(Si) Nanoislands and their Groups Incorporated into Photonic Crystals

Semiconductors - The luminescence properties of arrays of spatially ordered self-assembled solitary Ge(Si) nanoislands and their groups, including those embedded in two-dimensional photonic...     

Effect of silicon spacer thickness on the electroluminescence of multilayer structures with self-assembled Ge(Si)/Si(001) islands

The results of investigation of the electroluminescence of multilayer p-i-n structures with Ge(Si)/Si(001) self-assembled islands are presented. The nonmonotonic dependence of the room-temperature intensity of the electroluminescence signal from islands on the Si spacer thickness is revealed. The highest electroluminescence signal intensity is observed for structures with a Si spacer thickness of 15?C20 nm. The significant decrease detected in the electroluminescence signal from the islands in structures with thick Si spacers (>20 nm) is explained by the formation of defect regions in them. The observed decrease in the electroluminescence signal in structures with thin Si layers is associated with a decrease in the Ge fraction in the islands in these structures, which is caused by enhanced Si diffusion into islands with increasing elastic strains in the structure.     

Transition from the two- to three-dimensional growth of Ge films upon deposition onto relaxed SiGe/Si(001) buffer layers

The critical thickness of the two-dimensional growth of Ge on relaxed SiGe/Si(001) buffer layers different in Ge content is studied in relation to the parameters of the layers. It is shown that the critical thickness of the two-dimensional growth of Ge on SiGe buffer layers depends on the lattice mismatch between the film and the substrate and, in addition, is heavily influenced by Ge segregation during SiGe-layer growth and by variations in the growth-surface roughness upon the deposition of strained (stretched) Si layers. It is found that the critical thickness of the two-dimensional growth of Ge directly onto SiGe buffer layers with a Ge content of x = 11–36% is smaller than that in the case of deposition onto a Si (001) substrate. The experimentally detected increase in the critical thickness of the two-dimensional growth of Ge with increasing thickness of the strained (stretched) Si layer predeposited onto the buffer layer is attributed to a decrease in the growth-surface roughness and in the amount of Ge located on the surface as a result of segregation.     

Special features of the formation of Ge(Si) islands on the relaxed Si1−xGex/Si(001) buffer layers

The results of studying the growth of self-assembled Ge(Si) islands on relaxed Si_1?xGe_x/Si(001) buffer layers (x≈25%), with a low surface roughness are reported. It is shown that the growth of self-assembled islands on the buffer SiGe layers is qualitatively similar to the growth of islands on the Si (001) surface. It is found that a variation in the surface morphology (the transition from dome-to hut-shaped islands) in the case of island growth on the relaxed SiGe buffer layers occurs at a higher temperature than for the Ge(Si)/Si(001) islands. This effect can be caused by both a lesser mismatch between the crystal lattices of an island and the buffer layer and a somewhat higher surface density of islands, when they are grown on an SiGe buffer layer.     

On the radiative recombination and tunneling of charge carriers in SiGe/Si heterostructures with double quantum wells

For SiGe/Si(001) epitaxial structures with two nonequivalent SiGe quantum wells separated by a thin Si barrier, the spectral and time characteristics of interband photoluminescence corresponding to the radiative recombination of excitons in quantum wells are studied. For a series of structures with two SiGe quantum wells different in width, the characteristic time of tunneling of charge carriers (holes) from the narrow quantum well, distinguished by a higher exciton recombination energy, to the wide quantum well is determined as a function of the Si barrier thickness. It is shown that the time of tunneling of holes between the Si_0.85Ge_0.15 layers with thicknesses of 3 and 9 nm steadily decreases from ~500 to <5 ns, as the Si barrier thickness is reduced from 16 to 8 nm. At intermediate Si barrier thicknesses, an increase in the photoluminescence signal from the wide quantum well is observed, with a characteristic time of the same order of magnitude as the luminescence decay time of the narrow quantum well. This supports the observation of the effect of the tunneling of holes from the narrow to the wide quantum well. A strong dependence of the tunneling time of holes on the Ge content in the SiGe layers at the same thickness of the Si barrier between quantum wells is observed, which is attributed to an increase in the effective Si barrier height.     

Effect of tensile-strained Si layer on photoluminescence of Ge(Si) self-assembled islands grown on relaxed SiGe/Si(001) buffer layers

The results of studying the photoluminescence of the structures with Ge(Si) self-assembled islands embedded into tensile-strained Si layer are reported. The structures were grown on smooth relaxed Si_{1 ? x} Ge_x/Si(001) (x = 0.2–0.3) buffer layers. The photoluminescence peak found in the photoluminescence spectra of the studied structures is related to the indirect (in real space) optical transition between the holes localized in the Ge(Si) islands and electrons localized in the tensile-strained Si layers under and above an island. It is shown that one can efficiently control the position of the photoluminescence peak for a specified type of structure by varying the thickness of the strained Si layers. It is found that, at 77 K, the intensity of the photoluminescence signal from the heterostructures with Ge(Si) self-assembled islands contained between the tensile-strained Si layers exceeds by an order of magnitude the intensity of the photoluminescence signal from the GeSi structures with islands formed on the Si(001) substrates.