Strain-Induced Diffusion in Strained Sige/Si Heterostructures
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STRAIN-INDUCED DIFFUSION IN STRAINED SiGe/Si HETEROSTRUCTURES Y.S. Lim1, J.Y. Lee1, H.S. Kim2, and D.W. Moon3 1
Dept. of Mater. Sci. & Eng., KAIST, 337-1 Gusung-dong, Yusung-ku, Daejon 305-701, Korea 2 SiGe Device Team, ETRI, 161 Kajong-dong, Yusung-ku, Daejon 305-350, Korea 3 Surface Analysis Group, KRISS, 1 Doryong-dong, Yusung-ku, Daejon 305-340, Korea
ABSTRACT Diffusivity of a strained heterostructure was theoretically investigated, and general diffusion equations with strain potential were deduced. There was an additional diffusivity by the strain potential gradient as well as by the concentration gradient. The strain-induced diffusivity was a function of concentration, and its temperature dependence was formulated. The activation energy of the strain-induced diffusivity was measured by high-resolution transmission electron microscopy. This result can be generally applied for the investigation of the diffusion in strained heterostructures.
INTRODUCTION Strained Si1-xGex/Si heterostructures have been investigated extensively by many research groups for various device applications, such as heterojunction bipolar transistor, field effect transistor, and optoelectronic devices. In these structures, a Si1-xGex layer is grown under compressive stress on a Si substrate. The strain-induced band offset at the heterointerface leads to the formation of a two-dimensional hole gas in the strained layer, and a substantial enhancement of the hole mobility over that in bulk silicon was observed [1]. However, the prerequisite for a high carrier mobility is atomically abrupt heterointerfaces [2]. Therefore, the interdiffusion under thermal stress is of great concern. In a Si1-xGex/Si heterostructure, the strain due to the lattice mismatch in the epilayer plays an important role in interdiffusion. In the literature, many evidences of the enhanced-diffusion by strain in strained epilayers have been observed [3-6]. Moreover, the surface migration by strain gradient in Si1-xGex islands was also reported [7]. However, although many authors have reported the strain effects on the diffusion in a P11.2.1
strained heterostructure, the qualitative relation between the strain and the diffusivity has not yet been proposed. In this work, a theoretical model of the interdiffusion of a strained SiGe/Si heterostructure is proposed and a strain-induced diffusivity is formulated by defining a strain potential gradient. The activation energy of the strain-induced diffusivity is measured by high-resolution transmission electron microscopy (HRTEM), and the experiments are consistent with the theoretically expected results. This model can give general information on the enhanced-diffusion, and can be generally applied for various strained heterostructures.
THEORETICAL STUDY A strained heterostructure has a self-strain energy, and the energy induces an elastic deformation of the heteroepitaxial layer. The strain energy of the as-deposited film is given by [8] 1+ν E Str = 2 G 1−ν
2 ε o z o .
(1)
where G is a shear modulus, ν is a
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