Effect of growth temperature on photoluminescence of self-assembled Ge(Si) islands confined between strained Si layers

The effect of growth temperature on photoluminescence is studied for structures with Ge(Si) islands grown on relaxed SiGe/Si(001) buffer layers and confined between strained Si layers. It is shown that, with decreasing growth temperature in the range from 700 to 630°C, the photoluminescence peak associated with the islands shifts to lower energies, which is due to the increase in Ge content in the islands and to suppression of degradation of the strained Si layers. The experimentally observed shift of the photoluminescence peak to higher energies with decreasing temperature from 630 to 600°C is attributed to the change in the type of the islands from domelike to hutlike in this temperature range. This change is accompanied by an abrupt decrease in the average height of the islands. The larger width of the photoluminescence peak produced by the hut islands in comparison with the width of the peak produced by the domelike islands is interpreted as a result of a wider size dispersion of the hutlike islands.     

Effect of parameters of Ge(Si)/Si(001) self-assembled islands on their electroluminescence at room temperature

The electroluminescence (EL) of multilayered p-i-n structures with the self-assembled Ge(Si)/Si(001) islands are investigated. It is found that the structures with islands grown at 600°C have the highest intensity of the electroluminescence signal at room temperature in the wavelength range of 1.3–1.55 μm. The annealing of structures with the Ge(Si) islands leads to an increase in the EL-signal intensity at low temperatures and hampers the temperature stability of this signal, which is related to the additional Si diffusion into islands during annealing. The found considerable increase in the electroluminescence-signal intensity with the thickness of the separating Si layer is associated with a decrease in the elastic stresses in the structure with an increase in this layer’s thickness. The highest EL quantum efficiency in the wavelength range of 1.3–1.55 μm obtained in investigated structures amounted to 0.01% at room temperature.     

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.     

Special features of the photoluminescence of self-assembled Ge(Si)/Si(001) islands grown on a strained Si1−x Gex layer

The effect of the predeposition of strained Si_{1 ? x} Ge_x layers (x ≤ 20%) on photoluminescence (PL) of self-assembled Ge(Si)/Si(001) islands is studied. A shift of the PL peak related to dome-shaped islands (domes) to lower energies, with respect to the PL peak related to pyramidal islands is observed; this shift is related to a much larger height of the domes compared to that of pyramids. It is found that, as the Ge content in the Si_{1 ? x} Ge_x layer (x) becomes higher than 0.1, two separate peaks appear in the broad PL band related to the islands; these peaks are attributed to the zero-phonon and phonon-assisted optical transitions in the islands. The appearance of these transitions is caused by a change of the TO-phonon type involved in radiative recombination: a TO_Ge-Ge phonon is replaced by a TO_Si-Ge phonon with a shorter wavelength.     

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.     

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.     

Photoluminescence line width of self-assembled Ge(Si) islands arranged between strained Si layers

The effect of variations in the strained Si layer thicknesses, measurement temperature, and optical excitation power on the width of the photoluminescence line produced by self-assembled Ge(Si) nanoislands, which are grown on relaxed SiGe/Si(001) buffer layers and arranged between strained Si layers, is studied. It is shown that the width of the photoluminescence line related to the Ge(Si) islands can be decreased or increased by varying the thickness of strained Si layers lying above and under the islands. A decrease in the width of the photoluminescence line of the Ge(Si) islands to widths comparable with the width of the photoluminescence line of quantum dot (QD) structures based on direct-gap InAs/GaAs semiconductors is attained with consideration of diffusive smearing of the strained Si layer lying above the islands.     

Elastic strain and composition of self-assembled GeSi nanoislands on Si(001)

The growth of self-assembled Ge islands on Si(001) surface and changes in the island structure parameters in the course of subsequent annealing were studied. Island structures possessing a small (～6%) scatter with respect to lateral dimensions and heights of the islands were obtained. The Raman spectra and X-ray diffraction data show evidence that silicon dissolves in the islands. The atomic fraction of Si in the resulting Si_xGe_1?x solid solution was determined and the elastic strain in the islands was measured. It was found that annealing of the heterostructures with islands is accompanied by increasing Si fraction in the islands, which leads to changes in the island shape and size.     

Epitaxially Grown Monoisotopic Si,Ge, and Si<Subscript>1–<Emphasis Type="Italic">x</Emphasis> </Subscript>Ge<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript> Alloy Layers: Production and Some Properties

The technology of the growth of Si, Ge, and Si_1–xGe_x layers by molecular-beam epitaxy with the use of a sublimation source of monoisotopic ³⁰Si or ²⁸Si and/or gas sources of monogermane ⁷⁴GeH₄ is demonstrated. All of the epitaxial layers are of high crystal quality. The secondary-ion mass spectroscopy data and Raman data suggest the high isotopic purity and structural perfection of the ³⁰Si, ²⁸Si, ⁷⁴Ge, and ³⁰Si_1–x⁷⁴Ge_x layers. The ³⁰Si layers doped with Er exhibit an efficient photoluminescence signal.     

Time-resolved photoluminescence from self-assembled Ge(Si) islands in multilayer SiGe/Si and SiGe/SOI structures

The results of a study of the spectral and temporal characteristics of the photoluminescence (PL) from multilayer structures with self-assembled Ge(Si) islands grown on silicon and “silicon-on-insulator” substrates in relation to temperature and the excitation-light wavelength are presented. A substantial increase in island-related PL intensity is observed for structures with Ge(Si) islands grown on silicon substrates upon an increase in temperature from 4 to 70 K. This increase is due to the diffusion of nonequilibrium carriers from the silicon substrate into the active layer with the islands. In this case, a slow component with a characteristic time of ～100 ns appears in the PL rise kinetics. At the same time, no slow component in the PL rise kinetics and no rise in the PL intensity with increasing temperature are observed for structures grown on “silicon-on-insulator” substrates, in which the active layer with the islands is insulated from the silicon substrate. It is found that absorption of the excitation light in the islands and SiGe wetting layers mainly contributes to the excitation of the PL signal from the islands under sub-bandgap optical pump conditions.     

Polarized photoluminescence of nc-Si–SiO x nanostructures

The effect of photoluminescence polarization memory in nc-Si–SiO _x light-emitting structures containing Si nanoparticles (nc-Si) in an oxide matrix is for the first time studied. The polarization properties of continuous and porous nanostructures passivated in HF vapors (or solutions) are studied. It is established that the polarization memory effect is manifested only after treatment of the structures in HF. The effect is also accompanied by a shift of the photoluminescence peak to shorter wavelengths and by a substantial increase in the photoluminescence intensity. It is found that, in anisotropic nc-Si–SiO _x samples produced by oblique deposition in vacuum, the degree of linear photoluminescence polarization in the sample plane exhibits a noticeable orientation dependence and correlates with the orientation of SiO _x nanocolumns forming the structure of the porous layer. These effects are attributed to the transformation of symmetrically shaped Si nanoparticles into asymmetric elongated nc-Si particles upon etching in HF. In continuous layers, nc-Si particles are oriented randomly, whereas in porous structures, their preferential orientation coincides with the orientation of oxide nanocolumns.     

Optically active centers in Si/Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>:Er heterostructures containing Er<Superscript>3+</Superscript> ions

The basic types of optically active erbium centers that make the major contribution to the photo-luminescence signal from the Si/Si_{1 ? x} Ge _x :Er heterostructures with the Ge content from 10 to 30% are analyzed in detail. It is shown that the origin of the optically active centers containing Er³⁺ ions correlates with the molar composition of the Si_{1 ? x} Ge _x :Er layer and the content of oxygen impurity in the layer. The major contribution to the photoluminescence signal from the Si/Si_{1 ? x} Ge _x :Er heterostructures with the Ge content below 25% is made by the well-known centers containing Er³⁺ ions and oxygen. An increase in the Ge content in the Si_{1 ? x} Ge _x :Er layer (x ≥ 25%) yields the formation of a new type of centers, specifically, the Gecontaining optically active erbium centers unobserved in the Si-based structures previously.     

Effect of chemical treatment on photoluminescence spectra of SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> layers with built-in Si nanocrystals

The effect of chemical treatment in saturated vapors of ammonia and acetone on the spectral composition and intensity of photoluminescence in porous SiO _x films containing Si nanocrystals (nc-Si) is studied. The porosity of the SiO _x films is provided by oblique vacuum deposition of thermally evaporated silicon or silicon monoxide on polished silicon substrates. The kinetics of adsorption of the vapors is monitored by variations in the frequency of a quartz oscillator on which the films to be studied are deposited. As a result of chemical treatment followed by high-temperature annealing of the SiO _x films at the temperature 950°C, a new band, absent from the as-prepared films, appears in the photoluminescence spectrum at shorter wavelengths. The peak position and intensity of the band depend, correspondingly, on the composition of the film and on the time duration of the treatment. It is found that the new photoluminescence band is quenched upon exposure to laser radiation at the wavelength 488 nm. The quenching is more pronounced at the band peak. The possibility of controlling the characteristics of photoluminescence of the porous structures by chemical treatment is shown.     

SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> layer formation during plasma sputtering of Si and SiO<Subscript>2</Subscript> targets

Deposition of SiO _x layers of variable composition onto silicon wafers was performed by co-sputtering of spaced Si and SiO₂ targets in argon plasma. Coordinate dependences of the thickness and refractive index of separately deposited Si and SiO₂ layers and the SiO _x layer grown during co-sputtering of targets were determined using optical techniques. It was shown that the SiO _x layer composition is not equal to a simple sum of thicknesses of separately deposited Si and SiO₂ layers. The coordinate dependences of the Si and SiO₂ layer thicknesses were calculated. To fit the calculated and experimental data, it is necessary to assume that no less than 10% of silicon is converted to dioxide during co-sputtering. A comparison of the coordinate dependences of the IR absorbance in SiO₂ and SiO _x layers with experimental ellipsometric data confirmed the presence of excess oxygen in the SiO _x layer. Taking into account such partial oxidation of sputtered silicon, composition isolines in the substrate plane were calculated. After annealing of the SiO _x layer at 1200°C, photoluminescence was observed in a wafer area predicted by calculations, which was caused by the formation of quantum-size Si nanocrystallites. The photoluminescence intensity was maximum at x = 1.78 ± 0.3, which is close to the composition optimum for ion-beam synthesis of nanocrystals.     

Interaction of self-assembled Ge islands and adjacent Si layers grown on unpatterned and patterned Si(001) substrates

For use in electronic devices, self-assembled Ge islands formed on Si(001) must be covered with an additional Si layer. Chemically vapor deposited Si layers initially grow very rapidly over Ge islands because of the catalytic effect of Ge on the reaction of the Si-containing gas. The edges of the Si features covering Ge “pyramids” are rotated by 45° with respect to the edges of the Ge pyramids because of the different mechanisms orienting the Ge islands and the Si features. When multiple layers of islands are formed, the in-plane ordering of the Ge islands depends on the thickness of the Si interlayer separating the island layers. When selective Si is grown on a patterned Si wafer to form the underlying structure for the Ge islands, the position of the islands is influenced by the detailed shape of the Si near the edges, which in turn depends on the thickness of the selectively deposited Si, the pattern size, and the amount of surrounding oxide.     

Ge based nanostructures for electronic and photonic devices

Self-assembled Ge_xSi_1−x islands were grown on Si(0 0 1) substrates by solid source molecular beam epitaxy. Two different morphological shapes with different sizes were evolved by tuning the growth time at a constant deposition temperature. Micro-Raman analysis was carried out to investigate the composition, intermixing and strain of resultant islands. The observed broad infra-red photoluminescence signal from grown samples was associated with radiative recombination of holes confined in the Ge islands and electrons localized in the Si buffer layer. The PL peak position and intensity were found to be influenced by the islands size and intermixing of Si and Ge. The electrical properties of the islands were studied through photoexcited I-V characteristics and current imaging using conducting mode atomic force microscopy.     

Confocal Raman microscopy of self-assembled GeSi/Si(001) Islands

The method of confocal Raman microscopy is used for the first time to study the spatial distribution of elemental composition and elastic strains in self-assembled GexSi_{1 − x} /Si(001) islands grown by the method of sublimation molecular-beam epitaxy in the GeH₄ ambient. The lines related to vibrational modes Si-Si, Ge-Ge, and Si-Ge are identified in the Raman spectra measured in the regions with dimensions <100 nm on the surface of the samples. The spatial distribution of the Ge atomic fraction x in the Ge _x Si_{1 − x} alloy and of the elastic strain ɛ (averaged in depth over the island layer) have been calculated from the maps of the Raman shifts of the corresponding lines over the sample surface. The dependences of x and ɛ on the islands’ growth temperature and on the nominal thickness of the deposited Ge layer have been studied.     

Ultra-Thin Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Dislocation Blocking Layers for Ge/Strained Si CMOS Devices

We demonstrate ultra-thin (<150 nm) Si_1−x Ge _x dislocation blocking layers on Si substrates used for the fabrication of tensile-strained Si N channel metal oxide semiconductor (NMOS) and Ge P channel metal oxide semiconductor (PMOS) devices. These layers were grown using ultra high vacuum chemical vapor deposition (UHVCVD). The Ge mole fraction was varied in rapid, but distinct steps during the epitaxial layer growth. This results in several Si_1−x Ge _x interfaces in the epitaxially grown material with significant strain fields at these interfaces. The strain fields enable a dislocation blocking mechanism at the Si_1−x Ge _x interfaces on which we were able to deposit very smooth, atomically flat, tensile-strained Si and relaxed Ge layers for the fabrication of high mobility N and P channel metal oxide semiconductor (MOS) devices, respectively. Both N and P channel metal oxide semiconductor field effect transister (MOSFETs) were successfully fabricated using high-k dielectric and metal gates on these layers, demonstrating that this technique of using ultra-thin dislocation blocking layers might be ideal for incorporating high mobility channel materials in a conventional CMOS process.     

Wannier-stark effect in Ge/Si quantum dot superlattices

Deep level transient spectroscopy (DLTS) measurements were performed to study electron emission from quantum states in a 20-layer Ge quantum-dot superlattice (QDSL) in a Ge/Si p-n heterostructure. It was established that the changes in the DLTS spectra depend heavily on the magnitude of the applied reverse bias U _r . Three regions of the reverse bias U _r were identified, corresponding to the manifestation of the three modes of the Wannier-Stark effect: Wannier-Stark ladder mode, Wannier-Stark localization, and nonresonant Zener tunneling mode. Furthermore, it was found that the appearance of DLTS peaks for all three modes is associated with electron emission from deep-level defects via Wannier-Stark localized states arising as a result of the splitting of the electron miniband of the Ge/Si QDSL.     

Photoreflectance Spectroscopy Study of LT-GaAs Layers Grown on Si and GaAs Substrates

The mechanical strains and densities of surface charge states in GaAs layers grown by low-temperature (LT) molecular-beam epitaxy on Si(100) and GaAs(100) substrates are investigated by photoreflectance spectroscopy. Lines corresponding to the fundamental transition (E _g ) and the transition between the conduction band and spin-orbit-split valence subband (E _g + Δ _SO ) in GaAs are observed in the photoreflectance spectra of Si/LT-GaAs structures at 1.37 and 1.82 eV, respectively. They are shifted to lower and higher energies, respectively, relative to the corresponding lines in GaAs/LT-GaAs structures. Comparing the spectra of the Si/LT-GaAs and GaAs/LT-GaAs structures, it is possible to estimate mechanical strains in LT-GaAs layers grown on Si (by analyzing the spectral-line shifts) and the density of charge-carrier states at the GaAs/Si heterointerface (by analyzing the period of Franz–Keldysh oscillations).