P. M. Mooney

Measurements of alloy composition and strain in thin GexSi1−x layers

The utility of Raman spectroscopy for the simultaneous determination of composition and strain in thin GexSi1−x layers has been investigated. Using data from the literature and new data for the strain shift of the Si‐Si phonon mode presented here, we show how Raman spectra provide several different means of measuring composition and strain in samples as thin as 200 A. We demonstrate that for largely relaxed layers with compositions near x=0.30, Raman scattering can measure the composition, x, with an accuracy of ±0.015 and the strain, e, with an accuracy ±0.0025. The accuracy of the alloy composition obtained from Raman spectra is comparable or, in the case of very thin layers, superior to that measured by other techniques such as x‐ray diffraction, electron microprobe, and Auger electron spectroscopy.

Nucleation of dislocations in SiGe layers grown on (001)Si

A comprehensive x‐ray‐diffraction study of the variation of the tilt angle between a Si1−xGex layer and the (001) Si substrate is presented. Such measurements provide the basis of a new method for determining the nucleation activation energy of misfit dislocations. A detailed model, independent of the particular relaxation mechanism, is derived which relates the tilt angle to the nucleation activation energy on the different slip systems and to the density of misfit dislocations. The model has been applied to the modified Frank–Read mechanism observed in graded samples. Relaxation occurs in such samples for strain in the range 0.002≤e≤0.006 with an activation energy of about 4 eV. The critical thickness for growth of a strained layer is shown to be smaller when the substrate is miscut than when it is well oriented.

Facet formation in strained Si1−x Gex films

Abstract The surface morphology of epitaxial (001) Si1−x Gex films, subject to biaxial strain, is studied by atomic force microscopy (AFM). Distinct facets are observed, oriented on {105}, {311}, and {518} crystal faces. The tiled arrangement of facets resembles a mosaic. We find that the growth sequence begins with the shallow {105} facets, followed by the appearance of steeper facets. After strain relaxation, the morphology coarsens and facets become less distinct. The existence of discrete facets produces a kinetic barrier to strain-induced roughening; and we show that increasing this barrier (by growing at reduced strain or reduced temperature) leads to a flatter surface morphology.

Evolution of strain relaxation in step‐graded SiGe/Si structures

Strain relaxation in a series of step‐graded SiGe/Si structures has been quantitatively investigated by high‐resolution x‐ray diffraction measurements. We show that beyond a critical thickness, dislocations nucleate continuously as layers with higher Ge mole fraction are added to the structure and that the mismatch strain at which nucleation occurs is therefore essentially constant. It had been found empirically that a lower growth temperature is required to suppress roughening of layers with higher Ge mole fraction, even in graded structures. We prove that this is not because the strain increases, but rather because of the lower melting temperature of layers with higher Ge content.

Effect of thermal processing on strain relaxation and interdiffusion in Si/SiGe heterostructures studied using Raman spectroscopy

The effect of thermal annealing on Si/SiGe heterostructures is studied using Raman spectroscopy. The structures consisted of Si on relaxed Si0.8Ge0.2 where the top Si thickness was 20–30 nm. Micro-Raman spectroscopy with 488 nm incident radiation revealed no significant shift in the strained Si peak position with thermal annealing at temperatures up to 1100 °C for 30 s. However, the intensity of the Si peak was systematically reduced with increasing thermal processing, a result which is attributed to interdiffusion at the Si/SiGe interface resulting in an apparent thinning of the Si cap layer.

Strain relaxation and threading dislocation density in helium-implanted and annealed Si1−xGex/Si(100) heterostructures

Strain relaxation and threading dislocation densities in Si1−xGex (0.15<x<0.30) produced by He implantation and annealing have been investigated using x-ray diffraction and transmission electron microscopy. The degree of strain relaxation is very sensitive to the SiGe layer thickness; only small differences in strain relaxation are obtained when the helium dose and energy are varied over a relatively wide range. In contrast, the threading dislocation density is strongly influenced by the implantation dose and depth. A composite parameter, the He dose in the SiGe layer (He(SiGe)), calculated from He profiles simulated using the program Stopping and Range of Ions in Matter (SRIM2000), correlates well with the threading dislocation density. We have found that to achieve a low threading dislocation density, <5×107 cm−2, He(SiGe) must be less than 1×1015 cm−2.

Observation of local tilted regions in strain-relaxed SiGe/Si buffer layers using x-ray microdiffraction

The microstructure of strain-relaxed Si1−xGex/Si films that relaxed by different dislocation nucleation mechanisms has been investigated using x-ray microdiffraction with a diffracted beam footprint of 1 μm×5μm. Intensity variations in the x-ray microtopographs of samples having step-graded intermediate layers, which relaxed by dislocation multiplication, are due to the presence of local tilted regions which are larger in area than the diffracted x-ray beam. In contrast, microtopographs of uniform composition layers, which relaxed by surface roughening and subsequent random dislocation nucleation, show little intensity contrast as the local tilted regions in these samples are much smaller than the diffracted x-ray beam. The difference in microstructure arises from the different distributions of 60 ° misfit dislocations in these two types of samples.

Near band‐edge photoluminescence in relaxed Si1−xGex layers

Near band‐gap photoluminescence was observed at low temperatures from relaxed Si1−xGex layers with 0.17<x<0.32 grown on Si(001) by ultrahigh vacuum chemical vapor deposition. The luminescence from undoped samples was dominated at low temperature and low excitation densities by recombination of excitons bound to alloy fluctuations exhibiting the smallest full width at half‐maximum, 2.44 meV, reported for relaxed epitaxial Si1−xGex layers. Excitons bound to phosphorous and boron were also observed as was free exciton recombination.

X-ray diffraction analysis of SiGe/Si heterostructures on sapphire substrates

Si/Si1−xGex heterostructures on improved silicon-on-sapphire substrates were grown epitaxially by ultrahigh vacuum chemical vapor deposition for application as p-channel field effect transistors. High-resolution triple-axis x-ray diffraction was used to analyze these structures quantitatively and to evaluate their thermal stability. Outdiffusion of Ge from the strained Si1−xGex quantum well was observed after annealing at 850 °C. The amount of outdiffusion was comparable to that observed in Si1−xGex structures on bulk Si wafers.

DEFECT STATES IN STRAIN-RELAXED SI0.7GE0.3 LAYERS GROWN AT LOW TEMPERATURE

Two shallow hole traps dominate the deep level transient spectroscopy (DLTS) data for strain-relaxed Si0.7Ge0.3 layers grown on Si(100) by ultrahigh vacuum chemical vapor deposition at temperatures ⩽560 °C. The trap energy levels are at Ev+0.06 and Ev+0.14 eV and trap concentrations are ⩽5×1014 cm−3 in relaxed layers having threading dislocation densities of 2–4×107 cm−2. A logarithmic dependence of the filling rate indicates that these traps are associated with extended defects and this is confirmed by their absence in a sample having no dislocations. The annealing temperature of the DLTS peaks is consistent with the interpretation of these traps as states of defect complexes at dislocations, rather than intrinsic dislocation states or isolated defect complexes. The trap concentrations are proportional to the oxygen concentration in the film, suggesting that oxygen may be a constituent of the defect complex.