“Up-hill” diffusion of Zinc in the III–V semiconductors

When radio-tracer zinc is diffused into a single crystal of one of the III–V semiconductors using the isoconcentration technique, a peculiar “up-hill” region is sometimes observed in the diffusion profile. The zinc appears to be diffusing from a region of low concentration to one of high concentration, in contradiction to Fick's law. An analysis is given of the situation in which a radioactive impurity diffuses into a semiconductor already homogeneously doped with the same impurity. The results of the analysis are compared with experimental results and good agreement is found. The problem can be considered as a simple case of two elements diffusing simultaneously and interacting with each other.