Dislocation-Free Elastically Strain-Sharing Si(110) Nanomembranes
The drive for high-performance, low-power consumption semiconductor devices has resulted in novel materials modifications to enhance charge carrier mobility. Strain engineering of Si(001) has been well established to improve electron mobility, however the hole mobility improvement is not as great, thus new materials modifications are necessary to enhance hole majority carrier devices (p-type). Due to the anisotropic nature of the Si electronic band structure (Feng Chen), (110)-oriented Si provides the necessary boost in hole mobility over the conventional (001)-oriented Si. Although the hole mobility in Si(110) is less than the electron mobility in Si(001), strain engineering can improve the hole mobility in Si(110).
Conventional techniques for inducing tensile strain in Si involve the epitaxial growth of Si on relaxed SiGe substrates. In this conventional method, challenges inherent in growth and relaxation of SiGe(110) always result in substrates with very high dislocation densities and anisotropic strain relaxation, both of which transfer to the strained Si layer. Thus the advantages in mobility created from strain are negated by charge carrier scattering from defects.
We fabricate strained Si(110) by using our elastic strain sharing nanomembrane (NM) technique. This technique (shown in Figure 1), originally developed for Si(001), involves elastic strain sharing between layers of Si/SiGe/Si(110). Elastic strain sharing results in dislocation-free strained Si. First, the SiGe(110) and top Si(110) layers are pseudomorphically grown with solid-source molecular beam epitaxy on the Si(110) template layer of SOI(110). Initially, all the strain in the system (compressive) is in the larger lattice constant SiGe layer. Next, the buried oxide layer is etched away to release the Si(110) NM from the host substrate. During the release process, in the now free-standing NM, some of the compressive strain in the SiGe layer transfers as tensile strain to the Si layers. Upon release, the NMs are readily transferred to other substrates (See Hybrid-Orientation Technology or Nanomembrane Fabrication).
The strain in the Si(110) layers is uniform and can be engineered: variations in Ge composition and relative thicknesses between the Si and SiGe layers control the amount of strain transferred to the Si layers. Although (110)-oriented Si is crystallographically anisotropic, the biaxial strain transferred from the SiGe(110) to the Si(110) layers is isotropic. This is because SiGe and Si have the same crystal structure, and therefore similar mechanical properties. The uniformity and isotropy of the biaxial strain was confirmed by measuring in-plane lattice constants with high-resolution off-axis x-ray diffraction. We have been able so far to achieve biaxial strains up to ~0.65%. We are currently investigating the potential for mobility enhancement in trilayer strained Si(110) NMs (Hyuk Ju Ryu).
Figure 1. Release and transfer process. (a) Grow pseudomorphic layers of Si(1-x)Gex and Si on thin template layer of SOI(110). (b) Lithographically pattern etch holes. (c) Release NM from handling substrate by etching away SiO2 with HF. Typical Si(110) NM trilayer structures consist of 8-15nm Si/40-60nm Si.70-.90Ge.30-.10/8-15nm Si. The optical micrograph show wrinkles in the NM from expansion due to strain sharing during release. (d) Transfer Si(110) NM to another host substrate by floating in water. The Si(110) NM is flat upon transfer to most substrates.
Author: Debbie Paskiewicz
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