Enhanced diffusion in Si and Ge by light ion implantation

Abstract

This paper presents theoretical and experimental results of investigations of impurity‐atom diffusion enhancement in Si and Ge crystals under light ion bombardment. Using existing ion range and displacement mechanism theories, a continuity equation for bombardment‐generated defects is written and solved to give an expression for the steady‐state excess defect concentration as a function of depth in the crystal. With the aid of existing diffusion theory, this is interpreted as a spatially varying atomic diffusivity which is a function of the bombarding beam parameters, temperature, the atomic species, and an experimentally measurable defect diffusion length. This spatially varying diffusivity, inserted in an appropriate diffusion equation, determines the evolution of the distribution of any impurity species diffusing by a defect‐related mechanism. Some numerical solutions of this equation and some analytic solutions of interest are given, and a comparison of theory and experiment is presented. Among the results of this comparison are (i) defect diffusion length measurements of 0.27 μ in p‐type Ge, and 0.086 μ in n‐type Si, (ii) a measured yield of approximately 25 mobile defects/proton in either Ge or Si, mostly clustered at the end of the proton's track. A simple method is presented for the experimental measurement of the required defect diffusion length. Impurity concentration profiles of the proton‐enhanced diffusion of P₃₁ in Ge and B in Si were measured, and experimental checks of several theoretical predictions were carried out. There are some indications of a diffusivity saturation at high beam currents in Si which may result from the generation of immobile defect clusters. Impurity diffusivity enhancement by a factor of greater than 10⁵ was observed in Si. Several physical parameter values of interest are calculated. Transmission electron microscopy measurements were undertaken to observe the effects of the bombardment on crystalline order in Si, and V‐I and C‐V characteristics of p‐n diodes produced using enhanced diffusion of B in Si were examined. The uses of bombardment‐enhanced diffusion as a device fabrication tool are discussed, and some unresolved topics are indicated.