Artefacts in iodine ion milling of some compound semiconductors

Iodine ion milling was developed to overcome the deficiencies of using argon as a milling gas for certain compound semiconductors. Here it is shown that iodine gas introduces strong artefacts in some materials such as Zn3As2, a material occurring at some ZnSe/GaAs interfaces and that even efficient specimen cooling does not prevent this effect. Argon ion milling by contrast leaves the Zn3As2 layer intact. However, deliberate amorphisation of the Zn3As2 by iodine enables a clearer detection of its presence and to distinguish better the coverage of very thin ZnSe epilayers. These effects can be accounted for by simple thermodynamic calculations and enable the possibility for prediction as to whether reactive milling agents such as iodine will cause problems in specimen preparation. These results also indicate that iodine molecules rather than atoms are the principle milling species.