Three-dimensional SiGe nanostructures grown on Si using molecular beam epitaxy exhibit photoluminescence (PL) in the important spectral range of 1.3–1.6 μm. At a higher level of photo-excitation, thermal quenching of the PL intensity is suppressed and the previously accepted type II energy band alignment at Si/SiGe cluster hetero-interfaces no longer controls radiative carrier recombination. Instead, a dynamic type I energy band alignment governs the strong decrease in carrier radiative lifetime and further increase in the luminescence quantum efficiency. In contrast to the strongly temperature dependent and slow radiative carrier recombination found in bulk Si, Auger mediated PL emanating from the nanometer-thick Si layers is found to be nearly temperature independent with a radiative lifetime approaching 10−8 s, which is comparable to that found in direct band gap III-V semiconductors. Such nanostructures are thus potentially useful as CMOS compatible light emitters and in optical interconnects.

Extended abstract of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007

The annular dark field (ADF) image contrast of a 0.92% tensile strained GaN0.045As0.955 layer on GaAs substrate was studied with a scanning transmission electron microscope (STEM) as a function of ADF detector inner semi-angles ranging from 28 mrad to 90 mrad. The GaN0.045As0.955 layers were brighter than the surrounding GaAs for the values of ADF detector semiangle up to 65 mrad, and the measured contrast decreased with increasing ADF detector inner semi-angle. For a 37 nm thick specimen, the GaN0.045As0.955 intensity is about 13% higher than that of GaAs in the 28 mrad ADF detector inner semi-angle. Multislice simulations show that the displacement around substitutional N atoms plays an important role in the observed ADF-STEM contrast, while the contribution to the contrast due to misfit strain between GaN0.045As0.955 and GaAs is small.

We review recent advances in the growth of Si1−xGex islands and Ge dots on (001) Si. We first discuss the evolution of the island morphology with Si1−xGex coverage and the effect of growth parameters or post-growth annealing on the shape of the islands and dots. We outline some of the structural and optical properties of Si1−xGex islands and assess progress in the determination of their composition and strain distribution. Finally, we discuss various approaches currently being investigated to engineer Si1−xGex quantum dots and in particular to control their size, density, and spatial distribution. For example, we show how C pre-deposition on Si (001) can influence the nucleation and growth of Ge islands.

Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005

We review progress in the growth of Si1-xGex islands and Ge dots on (001) Si. We discuss the evolution of the island morphology with Si1-xGex coverage, and the effect of growth parameters or post-growth annealing on the shape of the islands and dots. We outline some of the structural, and optical properties of Si1-xGex islands and review recent progress in the determination of their composition and strain distribution. We discuss various approaches currently being investigated to engineer Si1-xGex quantum dots and in particular control their size, density and spatial distribution. For example, we show how C pre-deposition on Si (001) can influence nucleation and growth of Ge islands.

We report a study of the surface morphology and microstructure of Si epitaxial layers grown by MBE on (001) Si at temperatures at which epitaxy breakdown is observed. For films grown in the 400 - 450 °C temperature range the epitaxy breakdown is very sluggish and characterized by a columnar growth and the formation of surface cusps. We have used atomic force microscopy to study the shape, size and distribution of those surface cusps. Surface cusps are of square shape with sides predominantly oriented along <110> directions and are typically of 50 nm size and 5 nm depth. The cusps can be very regular in size and their surface density (typically of 109-1010 cm-2) is dependent on the growth temperature. The epitaxial Si in this temperature regime exhibits a residual strain of the order of -5 × 10−5 in the growth direction. Photoluminescence (PL) from cusped Si films is characterized by a broad PL at low energy possibly due to impurities incorporation at low growth temperatures. We have observed that Ge self-assembled dots can be grown on cusped surfaces. Large area AFM measurements reveal that surface cusps are “decorated” by clusters of large dome-like Ge dots, while a lower density of smaller dome and pyramid shape islands are seen away from the cusps.

We produce a novel form of amorphous silicon through ultra-high-vacuum molecular beam epitaxy. By depositing silicon atoms onto a fused quartz substrate at temperatures between 98 and 335°C, we obtain a silicon-based material that lacks the characteristic periodicity of crystalline silicon but nevertheless has 98% of its density. The impurity content of this material is studied through infrared and secondary ion mass spectroscopies. The primary impurity found is oxygen, with hydrogen and carbon atoms also being found at trace levels. The Raman spectra of the amorphous silicon films are measured and the results, as they relate to the presence of disorder, are interpreted. We also use this molecular beam epitaxy method to fabricate a number of amorphous silicon superlattices, comprised of thin layers of amorphous silicon separated with even thinner layers of SiO2. The optical properties of the films and superlattices are contrasted.

We have deposited a novel form of amorphous silicon through molecular beam epitaxy in an ultra-high vacuum. In particular, by depositing silicon atoms onto an optical quality fused quartz substrate at room temperature we have obtained a silicon-based material that lacks the periodicity that characterizes crystalline silicon but nevertheless has 98% of the density. Spectroscopic studies reveal that there are only trace amounts of hydrogen and other impurity atoms in this novel form of amorphous silicon, this contrasting dramatically with the case of conventional amorphous silicon. The Raman and optical spectroscopic properties of this form of amorphous silicon are contrasted with those of conventional amorphous silicon and sputtered amorphous silicon, and conclusions, regarding the amount of disorder, are drawn. Finally, the device implications of this novel form of amorphous silicon are discussed.

We present optical and microstructural characterization of nanocrystalline silicon superlattices (nc-Si SLs). Our samples have better than 5 % Si nanocrystal size distribution and a long range order along the direction of growth provided by periodically alternating layers of Si nanocrystals and SiO2. Flat and chemically abrupt nc-Si/SiO2 interfaces with a roughness of < 4Å are confirmed by transmission electron microscopy (TEM), Auger elemental microanalysis, X-ray small angle reflection, and low-frequency Raman scattering. Photoluminescence (PL) in our structures has been studied in details including time-resolved and steady-state PL spectroscopy in a wide range of temperature, excitation wavelength and power. Resonantly excited PL spectra show phonon steps proving that the PL originates in Si nanocrystals. Electrical measurements show signature of phonon-assisted tunneling proving low defect density nc-Si/SiO2 interface.

The index of refraction of pseudomorphic layers grown on Si has been determined at wavelengths λ=1330 nm and λ= 1550 nm, for Ge concentrations between x=0.01 and x=0.1. The refractive index values were obtained from mode profile measurements on a series of Si1−xGex waveguides. The index of refraction is significantly larger for light polarized parallel to the growth direction than for light polarized in the plane of the epilayer. This birefringence is consistent with the anisotropic index change predicted using photoelelastic theory, given the biaxial strain present in the pseudomorphic Si1−xGex layers.

Understanding has advanced significantly in the interpretation of the thermal release of self interstitials into a silicon lattice from a region, e.g. ion implanted, which is rich in point defects. Progress has been accelerated through the use of delta doped layer structures, especially of boron doping type, where the transient enhanced diffusion (TED) of the dopant marks the arrival of the self interstitial species from a physically separate damage region. Frequently observed in such TED experiments is an immobile, electrically inactive, boron peak on top of the spreading diffusion profile. This signature boron peak has been attributed to the Si-interstitial driven clustering of boron. It has been observed that this feature occurs for high boron concentrations and implant fluences; however, a complete understanding of the clustering mechanism has remained elusive.

We present new data from experiments where the self implant fluence has been varied to allow the intensity of the transient flux to be adjusted and where repeat distances in the MBE structure have been adjusted to test for trapping. This has led to some clarifications to the current picture of boron clustering during TED. These results seem to be important in the context of the search for engineering control over dopant redistribution in Si during post-implantation annealing.

Amorphous Si/SiO2 superlattices with periodicities from 2 to 5 nm have been grown on (100) Si wafers by several different techniques: molecular beam epitaxy, magnetron sputtering, and plasma enhanced chemical vapor deposition (PECVD). With the first two methods little or no hydrogen was incorporated during growth and visible photoluminescence (PL) was obtained at wavelengths from 520 to 800 nm. The shift in the PL peak position with Si layer thickness is consistent with quantum confined emission. Annealing the sputtered superlattices at temperatures up to 1100°C produced a very bright red PL that is similar in intensity to that found in porous Si. The PL was also considerably enhanced by deposition on aluminum-coated glass substrates. For large numbers of periods (e.g., 425) the PL was strongly modulated in intensity owing to optical interference within the superlattice. Similar quantum-confined PL was also observed in the PECVD grown superlattices, where the amorphous Si layers were heavily hydrogenated. The blue-red cathodoluminescence observed from sputtered superlattices is due primarily to defects in the SiO2 layers.

Delta-doped boron marker layers in silicon have been used to test further the relationship between B transient enhanced diffusion (TED) and the flux of silicon interstitials released during the annealing stage following self implantation. We present new data which address a number of questions raised by the present models. We show that in our experiments bulk trapping of interstitials is significant only for low implant fluences (˜1012 cm2). The origin of the observed diffusion-like profiles for the interstitial flux is instead found to lie in local trapping within the δ-doped layers themselves. Boron trapped in immobile clusters may be associated with Si interstitials in approximately a 1:1 ratio; nevertheless this trapping contribution alone may not entirely account for the observed gradient. We suggest that some part of the observed TED response with depth is attributable to local trapping of silicon interstitials within the boron doped layers.

A study of the interface chemical and physical abruptness of Si-Ge heterostructures grown on (001) Si by molecular beam epitaxy under atomic hydrogen exposure is reported. Atomic hydrogen (AH) was produced by the dissociation of molecular hydrogen interacting with a hot tungsten filament. Secondary-ion mass spectroscopy (SIMS) of structures made of alternating Ge (0.5 nm)/Si (40 nm) layers demonstrated that AH can effectively suppress Ge surface segregation. The segregation length was reduced from 1.5 nm to about 0.5 nm in films grown at a hydrogen partial pressure of −5 × 10-3 Pa and cell temperature of 2140 °C with an estimated cracking efficiency of ~5%. However, the high hydrogen background pressure had detrimental effects on the physical sharpness of the interfaces. This was evidenced by comparing the interface quality of Si/Ge atomic layer superlattices grown with and without AH exposure. X-ray reflectivity and Raman spectroscopy revealed a significant increase of the interface roughness, although the periodic character and the good crystallinity of the structures were preserved.

Delta-doped boron marker layers in silicon have been used to test further the relationship between B transient enhanced diffusion (TED) and the flux of silicon interstitials released during the annealing stage following self implantation. We present new data which address a number of questions raised by the present models. We show that in our experiments bulk trapping of interstitials is significant only for low implant fluences (∼1012 cm −2). The origin of the observed diffusion-like profiles for the interstitial flux is instead found to lie in local trapping within the δ-doped layers themselves. Boron trapped in immobile clusters may be associated with Si interstitials in approximately a 1:1 ratio; nevertheless this trapping contribution alone may not entirely account for the observed gradient. We suggest that some part of the observed TED response with depth is attributable to local trapping of silicon interstitials within the boron doped layers.

Two competing strain relaxation mechanisms, namely misfit dislocation generation and surface roughening, have been extensively studied using the GexSi1-x/Si (x< 0.5) system as an example. A predictive model has been developed which accurately describes the nature of misfit dislocation nucleation and growth under non-equilibrium conditions. Using optical and electron microscopy, coupled with a refined theoretical description of dislocation nucleation, it is shown that strain relieving dislocations are readily generated at low misfits with a characteristic activation energy barrier regardless of the growth technique employed (i.e. MBE, RTCVD and UHVCVD). Secondly we have studied the alternative elastic strain relaxation mechanism involving surface undulation; x-ray diffraction, electron and atomic force microscopy have been used to characterize GexSi1-x/Si (x<0.5) structures grown by UHVCVD and MBE at relatively higher temperatures. A theoretical model has been used to model the critical thickness for surface wave generation. The conditions governing the interplay between dislocation formation and surface buckling are described in terms of a "morphological instability diagram".

We report the growth and characterization of amorphous Si (a-Si)/SiO2 superlattices on (100) Si. The a-Si layers (thickness varying from 1 to 10 nm) were vacuum-deposited at room temperature by molecular beam epitaxy while the SiO2 layers (1 nm) were grown by an ex-situ UV-ozone treatment. This procedure was repeated six times to produce periodic multilayer structures. The chemical modulation of these structures was confirmed by transmission electron microscopy and depth profiling using Auger electron spectroscopy. X-ray specular reflectivity showed that the structures have a well defined periodicity. The a-Si layers have a density approaching (>95 %) that of c-Si and an interfacial roughness that increases with the a-Si layer thickness. The Raman spectrum from the Si layers of all samples shows broad peaks near 150, 310 and 470 cm−1 that are typical of a-Si. On annealing at high temperatures, the three Raman bands decrease in intensity, while the 470 cm−1 band also shifts to higher frequency and becomes narrower. After annealing for 30 s at 1100 °C, the a-Si bands are weak and the 470 cm−1 band is merging with the c-Si 520 cm−1 line, indicating that partial re-crystallization of the Si layers has occurred. The room temperature light emission properties of these nanostructures in the green to red wavelength region is reported. The luminescence shifts to longer wavelength with increasing a-Si layer thickness, consistent with a quantum confinement mechanism.

Several Si1-xGex/Si heterostructures were grown at 525°C using a commercially available UHV-CVD reactor. Layers with a germanium fraction ranging from 0.15 to 0.5 were examined by means of cross-sectional transmission electron microscopy and atomic force microscopy. Surface waves were found in layers with a thickness above a critical value which decreases rapidly as the Ge fraction is increased. Both experimental and modeling results show that surface waves are generated before misfit dislocations for Ge fractions above 0.3.