• PDF / 524,446 Bytes
• 10 Pages / 420.48 x 639 pts Page_size
• 36 Downloads / 257 Views

DOWNLOAD

REPORT

---

EFFECT OF FREE SURFACES AND INTERFACES ON DOPANT DISTRIBUTION PROFILES K. JAGANNADHAM AND J. NARAYAN Department of Materials Science and Engineering North Carolina State University, Raleigh, North Carolina 27695 ABSTRACT The drift force exerted by a free surface, a coherent or a We noncoherent interface on dopant atoms has been analyzed. have considered a coherent or a noncoherent interface present in an epilayer of finite thickness. The difference in the shear modulus of the matrix and of the second phase is responsible for the configurational energy of a dopant atom in a coherent two-phase medium. On the other hand, the hydrostatic component of stress associated with a misfit dislocation in a noncoherent interface gives rise to a first-order size interaction. The drift forces are responsible for dopant diffusion towards a free surface or an interface. The diffusion equation including the drift term is solved using an eigen function expansion method with appropriate boundary conditions. The concentration profiles obtained from the analysis of the diffusion equation are in qualitative agreement with those obtained experimentally. The attractive drift force gives rise to a peak in the dopant concentration at the interface followed by a minimum near the interface. These calculations and the observed concentration profiles enable us to evaluate the interaction energy term of the dopants with free surfaces and interfaces and the dilatation associated with the point defects. INTRODUCTION Dopant ion implantation to control the n-type or p-type behavior of semiconductor materials is a routine procedure employed in semiconductor technology. Annealing always follows the implantation step so that damage created during implantation is recovered with simultaneous diffusion resulting in a change in dopant concentration profiles. An accurate control of dopant atom profiles is essential for desired device characteristics. Dopant diffusion in ion implanted crystals takes place in the presence of drift forces which have several different origins. A free surface or an interface is a source of internal stress which exerts a drift force on the dopant atom. In this paper, we present experimental results of boron dopant distribution in silicon wafers that were implanted and thermally annealed. Modeling of dopant atom interaction with free surfaces, coherent and noncoherent interfaces in the presence of epilayers of finite thickness is presented. The results are used to calculate the drift force acting on a dopant atom. Further, the diffusion of the dopant atoms in the presence of the drift force is formulated in terms of the classical diffusion equation and the solution given using the eigen function expansion method. The concentration profiles of the boron dopant atoms in the presence of a free surface are in good agreement with the experimental observations. EXPERIMENTAL RESULTS Silicon single crystals were implanted with l1 B+ ions and annealed using pulsed laser irradiation [1]. The laser annealed samples were free from extended def