A selective-ion-implantation technique was developed for introducing uniaxial strain into Si/Ge heterostructures. Laterally selective ion implantation with a stripe pattern was carried out into a Si substrate, followed by SiGe overgrowth in the whole region. Large strain relaxation of SiGe occurred selectively only in the ion-implanted area. This largely relaxed SiGe was found to considerably affect the strain state of the neighboring strained SiGe in the unimplanted area, resulting in the realization of a highly asymmetric strain state, that is, uniaxial strain. This result indicates that this technique has a high potential to realize high-mobility Si/Ge channels with uniaxial strain.

We investigated the line width dependence of anisotropic strain states in SiGe layers grown on selectively ion-implanted Si substrates with stripe patterns. It was found that strain relaxation of the SiGe layers on the unimplanted region occurred only along the stripe pattern and that the increase in the ion-implanted region provided larger anisotropy of the strain state. From the observation of one-directional surface steps, it was concluded that the uniaxially strained SiGe layer was formed by plastic deformation caused by selective misfit dislocation generation, which means that the anisotropic strain induced in the SiGe film is stable and good for device applications.

We systematically studied on ion dose, energy, and species dependencies of strain relaxation ratios for SiGe buffer layers fabricated by ion implantation technique where the epitaxial growth of SiGe layers was carried out on Si or Ar ion preimplanted Si substrates. For Si+ implantation, we found that there was an optimal ion-implantation condition to effectively enhance strain relaxation of the SiGe layers, that is, relaxation ratios increased with the ion dose but reduced remarkably when it exceeded a certain critical dose (∼1×1015 cm−2). The drop of relaxation also occurred as the implantation energy increased. Based on simulations and transmission electron microscopy (TEM) observations, it was concluded that end-of-range (EOR) defects generated by Si+ implantation crucially caused formation of high-density misfit dislocations at the heterointerface, and the observed complicated results were well understood in terms of the position of EOR defects from the heterointerface. We confirmed this conclusion by observing that relaxation ratios monotonically increased as the EOR defects position from the heterointerface was decreased by means of surface etching. On the other hand, for Ar+ implantation, relaxation ratios were seen to increase monotonically with the increase in ion dose without any drop even in the high dose region. Void-related defects formed around projected range of ion implantation were thought to dominate strain relaxation of the SiGe layers differently from Si+ implantation case. This difference in the relaxation mechanism between Si+ and Ar+ implantation was also found in and confirmed by TEM and atomic force microscopy observations.

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Very thin SiGe relaxed buffer layers whose Ge composition was higher than 40% were fabricated by utilizing ion implantation method. Strain relaxation ratio of 70% with respect to Si was obtained. Rms roughness of the sample with Si+ implantation was only 0.45 nm in spite of high Ge composition and was much smaller than that of the sample without ion implantation. It was confirmed by cross-sectional transmission electron microscope (XTEM) observation that few threading dislocations existed in the Si0.53Ge0.47 layer. P-type modulation doped strained Ge channel structure formed on the Si0.53Ge0.47 buffer showed hole Hall mobilities as high as 16500 and 1450 cm2/(V s) at low and room temperatures, respectively. These results indicate that ion implantation method is promising for realization of high-performance strained channel heterostructure devices based on high Ge composition SiGe substrates.