The determination of small variations in the stoichiometry of undoped, semi-insulating gallium arsenide can be achieved by using constant current coulometry. Samples taken from a wafer are etched in HF, dissolved in NaOH-peroxide solution, then treated with a citric acid buffer. To convert the arsenic to arsenic(III), sulphurous acid is added. After removal of excess SO₂, the arsenic concentration of the solution is measured by coulometric generation of iodine at a platinum electrode. The dissolution in alkaline peroxide, and the controlled pH and coordinating properties of the citrate buffer, ensure that arsenic losses are decreased to an insignificant level. This method was routinely applied to the determination of arsenic in gallium arsenide wafers and gives a reproducibility of ±0.002% relative.

Source: Analytica Chimica Acta

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