In this growth process a new strain relief mechanism operates, whereby the SiGe epitaxial layer relaxes without the generation of threading dislocations within the SiGe layer. This is achieved by depositing SiGe on an ultrathin silicon on insulator (SOI) substrate with a superficial silicon thickness less than the SiGe layer thickness. Initially, the thin Si layer is put under tension due to an equalization of the strain between the Si and SiGe layers. Thereafter, the strain created in the thin Si layer relaxes by plastic deformation. Since the dislocations are formed and glide in the thin Si layer, no threading dislocation is ever introduced in to the upper SiGe material, which appeared dislocation free to the limit of the cross sectional transmission electron microscopy analysis. We thus have a method for producing very low dislocation, relaxes SiGe films with the additional benefit of an SOI substrate.

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Strain compensation is an important aspect of heterostructure engineering. In this letter, we discuss the synthesis of pseudomorphic Si1−yCy and Si1−x−yGexCy alloy layers on a silicon (100) substrate by molecular beam epitaxy using solid sources and the controlled strain compensation that results from the introduction of the ternary system. The introduction of C into substitutional sites in the crystal lattice is kinetically stabilized by low-temperature growth conditions (400–550 °C) against thermodynamically favored silicon-carbide phases. The lattice constant in Ge is about 4% larger than in Si, whereas in diamond it is 52% smaller. Consequently, the compressive strain caused by 10.8% Ge in a pseudomorphic Si1−xGex alloy can be compensated by adding about 1% carbon into substitutional lattice sites of the film assuming Vegard’s law of linear change of the lattice constant in the alloy as a function of the composition. Using x-ray diffraction, we observe a partial strain compensation in Si0.75−yGe0.25Cy alloys on Si depending on the amount of carbon in the layer, with no observable misfit dislocation generation. The Raman spectra from Si1−yCy and Si1−x−yGexCy alloys show a substitutional carbon vibration mode at about 600 wavenumbers. No indication of silicon-carbide precipitation is observed in transmission electron microscopy and Raman spectroscopy.

Wittmer et al. Reply:In our first paper [l] we reported that converting Pd to Pd2Si induced diffusion of buried marker layers in the underlying silicon substrate. We further reported that the broadening, as determined by secondary ion mass spectroscopy (SIMS) profiling, appeared asymmetric. As we discussed in Ref. [l], while there is no theoretical problem with observing dopant diffusion induced by a surface reaction process, the asymmetric nature of our SIMS profiles is not readily explained by conventional theory.In our second paper [2] we tried to determine if surface topography induced by SIMS sputtering could be solely responsible for profile broadening. To this end, we used two approaches: (l) We made stylus profiling measurements (Alpha Step) in the SIMS craters and imaged the surface topography of the SIMS craters by crosssectional scanning electron microscopy (SEM). The topography information from these two techniques was then compared with corresponding SIMS profiles to ascertain whether or not there was a clear correlation between surface topography and SIMS profile shape. (2) We performed Rutherford backscattering (RBS) measurements on Sb-doped samples in order to obtain information on profile broadening without using the suspect SIMS sputter profiling technique. Our conclusions were as follows: (1) We were unable to discern any clear correlation between the apparent roughness of the surfaces and the deviation of the SIMS profiles from their expected Gaussian shapes. In one case, we observed that the as-grown sample had a rougher surface (after SIMS profiling) than a sample reacted with Pd, even though the sample on which silicidation took place showed a SIMS profile with a large deviation from a Gaussian shape and the as-grown sample had a narrow and symmetric SIMS profile.(2) RBS measurements indicated that Sb buried marker layers were broader after silicidation, though the RBS resolution is not good enough to tell whether the profile is truly asymmetric.Ronsheim and Tejwani [3] repeated our work in an attempt to resolve the question of asymmetric broadening of the measured SIMS profiles. By changing the energy of the analyzing beam to reduce its mixing depth they have been able to remove the asymmetrical broadening of the B marker layer. The fact that the asymmetry of the profiles is affected so strongly by such changes in SIMS conditions is convincing evidence that the asymmetric profiles are an artifact of the SIMS profiling technique. Ronsheim and Tejwani conclude further that the distortion in SIMS profile shapes results from topography induced by the SIMS sputtering process and is not related to a low-temperature diffusion phenomenon. This, of course, is the most logical presumption. However, as we stated in Ref.[2] and reiterate here, in our observations of a large number of samples we could not unambiguously establish a correlation between surface topography and profile shape. This means that the appearance of surface roughness by SEM or standard stylus profiling is not a reliable indicator...

Single-phase SnxGe1−x alloys with x up to 0.3 have been grown by molecular beam epitaxy. X-ray diffraction measurements indicate the layers to have the diamond crystal structure. The metastability of the alloys is apparent as increases in the growth temperature, layer thickness, or Sn composition cause phase separation of the Sn into a noncubic (white or β-Sn) form. Rutherford backscattering spectrometry and reflection high-energy electron diffraction measurements indicate that the initial stages of growth are complicated. The first several hundred angstroms of growth are compositionally graded, with the Sn incorporation rate increasing with film thickness. Thereafter, the alloy composition remains constant, determined by flux composition, until a critical thickness for phase separation is reached (≂2000 Å for x=0.3).

Si0.5Ge0.5/Si superlattices and thick Si0.5Ge0.5 layers grown on (100), (111), and (110) Si surfaces by molecular-beam epitaxy (MBE) exhibit different growth morphologies and defect structures. The best morphology is achieved on (100) surfaces at low temperatures (∼400 °C), while thin and defect-free SiGe layers grown at higher temperatures (∼600 °C) tend to exhibit undulated surfaces due to the mismatch strain. Strained SiGe layers grown on (111) and (110) surfaces are much more susceptible to twin formation. SiGe layers grown on (100) surfaces at low temperatures exhibit a long-range order along the 〈111〉 directions. Our results indicate that such ordering occurs only in thick and relaxed SiGe layers but not in thin SiGe layers strained in a SiGe/Si superlattice structure. No ordering was observed in SiGe layers grown on (111) and (110) surfaces.