Question

A sensitive infra-red detector is made by vapor depositing alternate 13 nm thick layers of HgTe and CdTe on a substrate. It is found that the detector loses its sensitivity due to interdiffusion after 55 hr at 162 ^oC or after 250 hr at 110 ^oC.

Calculate an activation energy for the interdiffusion process and estimate the life of device at 25 ^oC

Estimate the interdiffusion coefficient at 162 ^oC

Answer 1

To determine the binary interdiffusion coefficient:

Darken’s equation: D = xA D_B^C+ xB D_A^C    DA C , DA C are in general functions of (or dependent on) composition:

D_A^C = DA { 1 + dlny_A/dlnx _A } = DA{ 1 + d lny_B / dln x_B }

DB^C = DB { 1 + dlnyB/dlnx B } = DB{ 1 + d lnyA/ dln xA}

Therefore, D is difficult to measure. It can only be determined (estimated) by graphical or numerical method (but no analytical solution)

The experimental procedure usually used for determining D follows: form a diffusion couple anneal with a given time measure composition profile, which can be done by electron probe microanalysis (EPMA) or other solid composition analysis method like