The chemical composition of the surface of InP samples etched in Cl2 and Cl2/Ar inductively coupled plasma (ICP) is analyzed using ex-situ x-ray photoelectron spectroscopy (XPS). Comparison between ex-situ and in-situ XPS measurements shows that the stoichiometry of the etched surface can be retrieved from the ex-situ analysis provided that an adapted procedure is used. This allows for investigating the evolution of the surface stoichiometry as a function of etching parameters. The sample temperature is found to play a determining role in the top surface composition during etching. An abrupt switch from a rough and In-rich surface to a smooth and significantly P-rich surface is observed above a critical temperature and is found to depend only weakly upon the other etching parameters such as direct current bias or pressure. Ex-situ XPS measurements are used to estimate the thickness of the phosphorus layer identified on the top surface as ∼1 nm, which is consistent with the value previously derived using in-situ XPS. Finally, the stoichiometry of the InP etched sidewalls is analyzed selectively using dedicated microscale periodic patterns. The surface P-enrichment of the etched sidewalls is found to be very similar to that of the bottom etched surface. The presence of the phosphorus top layer may have an impact on the sidewall passivation mechanism during anisotropic ICP etching of InP-based heterostructures using Cl2-containing plasma chemistry.

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Ref. 33 shows that the direct chemisorption of chlorine on In atoms is an exothermic process without barrier (which is not the case for the chemisorption on P atoms) and that the transfer of adsorbed Cl atoms from P atoms to In atoms and vice-versa is largely in favor of Cl–P→Cl–In. Since the probability of direct desorption of PCl is also calculated to be low (high desorption energy calculated in any cases), the net result is a preferential adsorption of Cl on In atoms on the surface. It is also calculated in the case of low chlorination that the direct desorption of InCl is the most probable reaction (desorption energy in the range of 19–32 kcal/mol), while P removal proceeds via P2(/P4) direct desorption (37 kcal/mol).

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