Mechanism of Stress-Enhanced Solid-Phase Epitaxy
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MECHANISM OF STRESS-ENHANCED SOLID-PHASE EPITAXY T.K.CHAKI State University of New York, Department of Mechanical Engineering and Center for Electronic & Electro-optic Materials, Buffalo, NY 14260 ABSTRACT Enhancement of solid-phase epitaxial growth (SPEG) due to hydrostatic pressures and bending stresses is explained by stress-enhanced mobility of point defects in the amorphous solid. The crystallization is by the adjustment of atomic positions in the vicinity of the crystallization/amorphous (c-a) interface due to self-diffusion in the amorphous phase, assisted by a free energy decrease equal to the difference in free energies between the amorphous and crystalline phases. Due to a mismatch in the bulk moduli between the amorphous and crystalline phases, the application of a hydrostatic pressure can develop tensile stresses in the amorphous layer near the c-a interface. Non-hydrostatic stresses in the amorphous layer enhance the mobility of point defects in the amorphous layer and, therefore, an enhancement of the SPEG rate. In the cases of both hydrostatic pressure and bending, the enhancement occurs in the tensile side, indicating that vacancy-like mechanism is predominant in SPEG. INTRODUCTION In semiconductors such as Si, Ge and GaAs, an amorphous layer produced by ion-implantation can be crystallized epitaxially parallel to the crystalline/amorphous (c-a) interface by annealing at elevated temperatures. The phenomenon, known as solid-phase epitaxial growth (SPEG), occurs with appreciable rates in Si above 500 0 C [1,2], in Ge in the range 300 - 400'C [3] and in GaAs at 200 - 400'C [4,5]. SPEG can be induced at much lower temperatures when the amorphous layer is bombarded by energetic ions [6-10]. SPEG can be enhanced by applications of hydrostatic pressures [11] as well as by bending stresses [12]. The mechanism of SPEG has not been properly understood. The role and nature of defects controlling SPEG have remained controversial. Spaepen and Turnbull [13] suggested that formation of ledges at the c-a interface could cause SPEG in Si and Ge. Narayan [14] proposed that the c-a interface migrates due to jumping of atoms across the interface under a driving energy equal to the free energy difference between the crystalline and amorphous phases. In explaining ion-induced SPEG, Chaki [15] proposed that the point defects produced in the amorphous layer by ion bombardMat. Res. Soc. Symp. Proc. Vol. 237. 01992 Materials Research Society
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ment increase self-diffusivity in the amorphous phase, thereby enhancing SPEG rate. The investigation of SPEG with hydrostatic pressures applied during the growth is very important, since it may shed light on the nature of defects involved in SPEG. The pressure dependence of SPEG of Si (100) surface in the temperature range of 530 - 550'C can be expressed by an activation volume of -0.28Qsi [11], where Qsi is the atomic volume of Si. The activation volume for SPEG of Ge(100) in the temperature range of 300 - 365°C is -0.
4
5QGe [11], where QGe is the atomic volume of Ge. The
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